Graphene-based 3D Macrostructures for Clean Energy and Environmental Applications [1] 9781839160011

Table of contents :
Cover
Half Title
Chemistry in the Environment Series
Graphene-based 3D Macrostructures for Clean Energy and Environmental Applications
Copyright
Preface
Contents
1. Engineering the Architecture of 3D Graphene-based Macrostructures
1.1 Introduction
1.2 Graphene Aerogels
1.2.1 Sol–Gel Hydrogels, Freeze-drying, Gelation Methods
1.2.2 Template Methods
1.2.2.1 Soft Templating
1.2.2.2 Hard Templating
1.3 Graphene Aerogel Composites
1.3.1 Polymeric Graphene Aerogels (PGA)
1.3.2 Metal-doped Graphene Aerogels (MDGAs)
1.3.3 Carbon Nano Tube/Graphene Aerogels (CNT/GA)
1.3.4 Fullerene/Graphene Aerogels
1.4 3D Printing Methods of Graphene Aerogels
1.4.1 Direct Ink Writing (DIW)
1.4.2 Inkjet
1.4.3 Freeze Gelation
1.4.4 Casting
1.4.5 Projection Micro-stereolithography (PµSL)
1.4.6 Fused Deposition Modelling (FDM)
1.4.7 Laser-based Methods
1.4.8 Other Methods
1.5 Conclusion
Acknowledgements
References
2. Structure–Property Relationships in 3D Graphene-based Macrostructures
2.1 Introduction
2.2 Structure–Property Relationship in 3D GBMs
2.2.1 3D Graphene Networks
2.2.2 Graphene Fibres and Tubes
2.2.3 Vertical Graphene Sheets
2.2.4 Graphene Cages
2.2.5 3D Porous Graphene Films
2.3 Conclusions
Acknowledgements
References
3. Flexible 3D Graphene- based Electrodes for Ultrahigh Performance Lithium Ion Batteries
3.1 Introduction
3.2 Flexible 3D Graphene-based Cathode Materials
3.2.1 Intercalation-type Cathode Materials Based on 3D GBMs
3.2.2 Conversion-type Cathode Materials Based on 3D GBMs
3.3 Flexible 3D Graphene-based Anode Materials
3.3.1 Intercalation-type Anode Materials Based on 3D GBMs
3.3.2 Conversion-type Anode Materials Based on 3D GBMs
3.3.3 Alloying-type Anode Materials Based on 3D GBMs
3.4 Summary and Outlook
References
4. 3D Graphene-based Materials for Enhancing the Energy Density of Sodium Ion Batteries
4.1 Introduction
4.2 Sodium Ion Batteries and their Ion Storage Mechanism
4.2.1 Operating Principle
4.2.2 Battery Performance Against Reaction Mechanics
4.2.3 Advantages of Nanostructured Materials on Ion Exchange Mechanisms
4.2.4 Graphitic Materials for Electrode Design
4.2.5 Advantages of the 3D Graphene Nanostructure
4.3 Synthesis of 3D Graphene-based Electrodes
4.3.1 Template-assisted Method
4.3.2 Self-assembly Methods
4.3.3 Emerging Novel Methods
4.3.3.1 Electrochemical Exfoliation and Deposition
4.3.3.2 3D Printing
4.3.3.3 Blowing Synthesis
4.3.3.4 Supercritical Carbon Dioxide (CO2) Fluid
4.4 Applications of 3D Graphene Materials in SIBs
4.4.1 Application as Anodes
4.4.1.1 Graphene Aerogel (GA)
4.4.1.2 Graphene Foam (GF)
4.4.1.3 3D Porous Graphene
4.4.1.4 3D Graphene Coated Anodes
4.4.2 Application as Cathodes
4.5 Conclusions and Future Perspectives
References
5. Ultrafast Charging Supercapacitors Based on 3D Macrostructures of Graphene and Graphene Oxide
5.1 Introduction
5.2 Graphene for Double Layer and Pseudocapacitive Type Devices
5.2.1 Combining or Substituting AC with Graphene
5.2.2 Graphene Foams
5.2.3 Graphene Papers and 3D Films
5.2.4 Graphene-based Fibres for Supercapacitors
5.3 Recent Advances in GBM for LICs
5.3.1 Graphene-based Cathode Materials for LICs
5.3.2 Graphene-based Anode Materials for LICs
5.4 Concluding Remarks
References
6. 3D GBM-supported Transition Metal Oxide Nanocatalysts and Heteroatom-doped 3D Graphene Electrocatalysts for Potential Application in Fuel Cells
6.1 Introduction
6.2 Methods for Synthesis of 3D G
6.2.1 Self- assembly Methods
6.2.2 Template Strategies
6.2.3 Supercritical CO2 Method
6.2.4 Indirect Freezing and Electrochemical Synthesis
6.3 Heteroatom-doped 3D G
6.3.1 Nitrogen-doped 3D G (N-3D G)
6.3.1.1 The Progress in N-doped Carbon Materials
6.3.1.2 The Active Sites for ORR in N-doped Carbon Materials
6.3.2 B, P, S-doped 3D GBMs
6.4 3D G- supported Transition Metal Macrocyclic Compounds
6.5 3D Transition Metal, N Codoped Graphene (3D M-Nx/G)
6.6 3D GBM-supported Transition Metal Oxide Catalysts
6.6.1 Single Metal Oxide Catalysts
6.6.2 Spinel-type Oxide Catalysts
6.6.3 Pyrochlore-type and Perovskite-type Oxide Catalysts
6.7 Conclusion
List of Abbrevations
References
7. 3D Graphene-based Scaffolds with High Conductivity and Biocompatibility for Applications in Microbial Fuel Cells
7.1 Introduction
7.2 Graphene-dispersed Laser-ablated 3D Carbon Micropillars
7.2.1 Electrode Synthesis
7.2.2 Surface Morphology
7.2.3 Electrochemical Characterizations
7.2.3.1 Cyclic Voltammetry (CV)
7.2.3.2 Linear Sweep Voltammetry (LSV)
7.2.3.3 Electrochemical Impedance Spectroscopy (EIS)
7.2.4 Biocompatibility of the Electrode
7.3 Graphene Aerogel (GA)-based 3D Electrodes
7.3.1 High Capacitative 3D GA Anodes
7.3.1.1 Electrode Fabrication
7.3.1.2 Biocompatibility of the Electrode
7.3.2 GA-modified 3D Graphite Fiber Brush (GFB) Electrode
7.3.2.1 Electrode Synthesis
7.3.3 Nitrogen-doped Graphene Aerogel Electrode (N- GA)
7.3.3.1 Electrode Preparation
7.3.3.2 Bacterial Colonization (Biocompatibility)
7.3.4 3D Pt NP/GA Composite
7.4 3D Graphene Foams
7.4.1 Macroporous Graphene/Multi-walled CNTs (MWCNTs)/FeO Foams
7.4.1.1 Electrode Fabrication
7.4.1.2 Physiochemical Characterization of G/MWCNTs/Fe3O4 Foam
7.4.2 Flexible 3D Graphene-Ni Foam
7.4.2.1 Preparation of Electrode
7.5 3D Macroporous-monolithic Graphene Modified with Polyaniline (PANI)
7.6 3D Graphene Macroporous Scaffold
7.7 3D Graphene Sponges
7.7.1 Macroporous Flexible 3D Graphene Sponge
7.7.1.1 Synthesis of Electrode and SEM Analysis
7.8 Graphene Sponge (GS)-SS Composite
7.8.1 Synthesis and Characterization of GS
7.8.2 Synthesis of GS-SS, Mechanism, and EIS
7.9 Additional Graphene-modified 3D Scaffolds
7.9.1 Chitosan/Vacuum-stripped 3D Graphene Scaffold
7.9.2 3D Graphene Nanosheets
7.10 Conclusions and Outlook
References
8. Highly Efficient Dye-sensitized Solar Cells with Integrated 3D Graphene-based Materials
8.1 Introduction
8.1.1 Dye-sensitized Solar Cells
8.1.2 Cell Architecture and Working Mechanism
8.1.3 Electron Transport and Recombination Kinetics
8.2 Graphene and 3D Graphene-based Materials (3D GBMs)
8.2.1 Synthesis Methods
8.2.2 Hybrid Graphene-based Composites for Counter Electrodes
8.2.3 Graphene Integrated Wide Bandgap Semiconductor Photoanodes
8.3 Biomolecular Dyes for Naturally Sensitized Photoanodes
8.3.1 Sources and Chemical Structure of Bio-sensitizers
8.3.2 Pigment Bandgap and Bio-DSSCs Performance
8.3.3 Graphene-based Naturally-sensitized DSSCs
8.4 Conclusion
References
9. Fuelling the Hydrogen Economy with 3D Graphene-based Macroscopic Assemblies
9.1 Introduction
9.2 Hydrogen Evolution by 3D GBMs
9.2.1 Electrochemical Process for H2 Generation through H2O Reduction by 3D Graphene Foam
9.2.2 Photochemical Process for H2 Generation from the Degradation of H2O Catalysed by Graphene Hydrogels (GHs)
9.2.3 Hydrolysis of NH3BH3 for H2 Generation Over Modified 3D Graphene Materials
9.2.4 Hydrolysis of NH3BH3 for H2 Generation Mediated by Modified 3D GHs
9.3 Hydrogen Storage
9.3.1 Hydrogen Storage on Pillared Carbon Materials
9.3.2 Reversible Hydrogen Storage on Carbon Material Composition (SiC/G)
9.4 Conclusion
List of Abbreviations
Acknowledgements
References
10. Harvesting Solar Energy by 3D Graphene-based Macroarchitectures
10.1 Introduction
10.2 Basis of Solar-thermal Conversion and Transport
10.2.1 Solar Absorption
10.2.2 Thermal Transfer
10.2.3 Thermal to Steam Generation
10.3 Development of 3D GBMs for Efficient SSG
10.3.1 3D Graphene
10.3.2 3D GO/rGO
10.3.3 Hybrid Materials
10.3.3.1 Graphene-metal Materials
10.3.3.2 Graphene-carbon Materials
10.3.3.3 Graphene-organic Materials
10.3.3.4 Others
10.4 Development of Photothermal Solar Evaporation Systems
10.4.1 Direct Contact System
10.4.2 Indirect Contact System
10.4.2.1 2D Water Channel
10.4.2.2 1D Water Channel
10.4.2.3 Others
10.4.3 Isolation Evaporation System
10.5 Current Technologies for Enhanced SSG
10.5.1 Light Harvesting
10.5.1.1 Micro Optimization
10.5.1.2 Macro Optimization
10.5.1.3 Others
10.5.2 Thermal Management
10.5.2.1 Thermal Insulation
10.5.2.2 Evaporation Enthalpy
10.5.2.3 Others
10.6 Applications Associated with SSG
10.6.1 Seawater Desalination
10.6.2 Sewage Purification
10.6.2.1 Heavy Metals in Waste Water
10.6.2.2 Organic Dye Waste Water
10.6.2.3 Oily Waste Water
10.6.3 Electricity Generation
10.6.4 Sterilization
10.6.5 Intelligent Water Evaporation
10.7 Conclusion and Outlook
10.7.1 Conclusion
10.7.2 Outlook
10.7.2.1 Standardized Photothermal Measurements and Evaluation
10.7.2.2 Understanding the Fundamentals of Photothermal Conversion in 3D GBMs
10.7.2.3 Exploring New Designs of 3D GBMs
10.7.2.4 Exploring New Functional Applications
References
11. 3D Graphene-based Macrostructures as Superabsorbents for Oils and Organic Solvents
11.1 Introduction
11.2 GBMs for Oils and Organic Solvents Removal: A Structure–Properties–Performance Paradigm
11.2.1 Properties of the Contaminants
11.2.2 Properties of 3D GBMs
11.3 Performance of 3D GBMs in Oils and Organic Solvents Removal
11.3.1 Self-assembled 3D GBMs for Oils and Organic Solvents Removal
11.3.2 3D GBMs Processed by Chemical Vapour Deposition (CVD)
11.3.3 Organic Foams Coated with GBMs
11.3.4 Solar Heated 3D GBMs for Crude Oil Removal
11.4 Regeneration and Reuse of GBMs
11.5 Conclusion and Future Outlook
References
12. Fast and Efficient Removal of Existing and Emerging Environmental Contaminants by 3D Graphene-based Adsorbents
12.1 Introduction
12.2 3D GBAs for Water Treatment
12.2.1 Heavy Metals
12.2.2 Organic Contaminants
12.2.3 Dyes
12.2.4 Emerging Contaminants
12.2.4.1 Pharmaceutical and Personal Care Products
12.2.4.2 Endocrine-disrupting Chemicals
12.3 3D GBAs for Air Purification
12.3.1 Volatile Organic Compounds
12.3.2 Particulate Matter and Viruses
12.4 Regeneration and Reuse
12.5 Concluding Remarks
References
13. Freestanding Photocatalytic Materials Based on 3D Graphene for Degradation of Organic Pollutants
13.1 Introduction and Scope
13.1.1 Scope and Organization of this Chapter
13.2 3D GBMs Systems
13.2.1 Template-based 3D GBMs
13.2.2 Template-free 3D GBMs
13.3 Degradation of Organic Pollutants
13.4 Conclusions and Outlook
List of Abbreviations
Acknowledgements
References
14. 3D Graphene-based Macroassemblies for On-site Detection of Environmental Contaminants
14.1 History of 3D Graphene
14.2 Synthesis of 3D Graphene
14.3 Gas Sensors Based on 3D Graphene
14.4 3D GBM-based Biosensors
14.5 3D GBM-based Soil Sensors
14.6 Conclusion
Acknowledgements
References
15. Graphene-based Macroassemblies as Highly Efficient and Selective Adsorbents for Postcombustion CO2 Capture
15.1 Introduction
15.2 3D GMAs for Postcombustion Carbon Capture
15.3 Improving CO2 Capture Performance of 3D GMAs
15.4 Conclusion
References
16. Artificial Photosynthesis by 3D Graphene-based Composite Photocatalysts
16.1 Introduction
16.2 Methodologies for Synthesis of 3D Graphene-based Composites
16.2.1 Spray-based Aerosol Routes
16.2.1.1 Basics of Aerosol Routes
16.2.1.2 Fundamentals of Graphene Crumpling
16.2.1.3 Aerosol Processing of Crumpled GBCs
16.2.1.3.1 Encapsulation with Pre-synthesized Cargo Particles
16.2.1.3.2 In Situ Formation of Cargo Particles within CGO
16.2.1.3.3 Post-mixing
16.2.2 Wet Chemistry Methods
16.3 Graphene-based Composites for CO2 Photoreduction
16.3.1 Basic Principles of CO2 Photoreduction
16.3.2 Typical CO2 Photoreduction Analysis Systems
16.3.3 Design of 3D GBCs for CO2 Photoreduction
16.3.3.1 Aerosol-processed Crumpled GBCs
16.3.3.2 Wet Chemistry Generated GBCs
16.3.3.2.1
Simple Mixing
16.3.3.2.2
Designing Hierarchical Structures
16.3.3.2.3
In Situ Coating or Growth
16.3.4 The Roles of Graphene in CO2 Photoreduction
16.4 Summary and Outlook
Acknowledgements
References
Subject Index

Citation preview

Graphene-­based 3D Macrostructures for Clean Energy and Environmental Applications

Chemistry in the Environment Series Editor-­in-­chief:

Dionysios D. Dionysiou, University of Cincinnati, USA

Series editors:

Rajasekhar Balasubramanian, National University of Singapore, Singapore Triantafyllos Kaloudis, Athens Water Supply and Sewerage Company ­ (EYDAP S.A.), Greece Rafael Luque, University of Cordoba, Spain

Titles in the series:

1: Graphene-­based 3D Macrostructures for Clean Energy and Environmental Applications

How to obtain future titles on publication:

A standing order plan is available for this series. A standing order will bring delivery of each new volume immediately on publication.

For further information please contact:

Book Sales Department, Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge, CB4 0WF, UK Telephone: +44 (0)1223 420066, Fax: +44 (0)1223 420247 Email: [email protected] Visit our website at www.rsc.org/books

Graphene-­based 3D Macrostructures for Clean Energy and Environmental Applications Edited by

Rajasekhar Balasubramanian

National University of Singapore, Singapore Email: [email protected] and

Shamik Chowdhury

Indian Institute of Technology Kharagpur, India Email: [email protected]

Chemistry in the Environment Series No. 1 Print ISBN: 978-­1-­83916-­001-­1 PDF ISBN: 978-­1-­83916-­248-­0 EPUB ISBN: 978-­1-­83916-­344-­9 Print ISSN: 2516-­2624 Electronic ISSN: 2516-­2632 A catalogue record for this book is available from the British Library © The Royal Society of Chemistry 2021 All rights reserved Apart from fair dealing for the purposes of research for non-­commercial purposes or for private study, criticism or review, as permitted under the Copyright, Designs and Patents Act 1988 and the Copyright and Related Rights Regulations 2003, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry or the copyright owner, or in the case of reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of ­ Chemistry at the address printed on this page. Whilst this material has been produced with all due care, The Royal Society of ­ Chemistry cannot be held responsible or liable for its accuracy and completeness, nor for any consequences arising from any errors or the use of the information contained in this publication. The publication of advertisements does not constitute any endorsement by The Royal Society of Chemistry or Authors of any products advertised. The views and opinions advanced by contributors do not necessarily reflect those of The Royal Society of Chemistry which shall not be liable for any resulting loss or damage arising as a result of reliance upon this material. The Royal Society of Chemistry is a charity, registered in England and Wales, Number 207890, and a company incorporated in England by Royal Charter (Registered No. RC000524), registered office: Burlington House, Piccadilly, London W1J 0BA, UK, Telephone: +44 (0) 20 7437 8656. For further information see our website at www.rsc.org Printed in the United Kingdom by CPI Group (UK) Ltd, Croydon, CR0 4YY, UK

Preface With escalating world population, unsustainable consumption of fossil fuels, increased energy demand, global climate change and rapid environmental degradation, energy and environmental issues are receiving considerable attention worldwide in the context of sustainable development. In order to address these interconnected challenges, the development of clean energy technologies and environmental remediation techniques has intensified in recent years. By virtue of its enormous specific surface area, outstanding electrochemical stability and superior mechanical properties, two-­dimensional (2D) graphene holds significant promise for a range of energy and environmental applications. However, just as any other carbon allotrope, graphene as a bulk material tends to form irretrievable agglomerates due to strong van der Waals interactions between the individual graphene sheets. This agglomeration leads to incompetent utilization of isolated graphene layers for practical applications. In order to overcome this restacking issue, the integration of 2D graphene nanosheets into three-­dimensional (3D) macrostructures, and ultimately into a functional system, has emerged as an innovative approach in recent years. The unification of graphene macromolecules into 3D macrostructures not only prevents their restacking, but also largely translates the intriguing characteristics of individual graphene sheets into the resulting monoliths, thereby improving their application potential. The 3D graphene-­based macrostructures (3D GBMs), such as sponges, foams, hydrogels, and aerogels manifest extraordinary nanoscale effects due to their superlative properties, novel functionalities, structural integrity and interconnected porosity. Furthermore, owing to their intense porosity, these 3D GBMs can serve as ideal scaffolds for functionalization with heteroatoms, functional polymers,

  Chemistry in the Environment Series No. 1 Graphene-based 3D Macrostructures for Clean Energy and Environmental Applications Edited by Rajasekhar Balasubramanian and Shamik Chowdhury © The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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inorganic nanostructures, as well as a whole range of topologically different carbon architectures. The change in the geometrical configuration of 3D GBMs in turn leads to the conceptualization of original material systems with unique properties and novel functionalities. As a consequence, 3D GBMs are being extensively synthesized and rigorously explored for a wide range of potential applications in clean energy technologies (such as batteries, supercapacitors, fuel cells, solar cells, water splitting devices and hydrogen storage) and environmental remediation methods (wastewater treatment, water purification, air pollution control and artificial photosynthesis). In fact, in the last seven years, over 1000 research articles have been published with a particular focus on fabricating high performance 3D GBMs for energy production and storage as well as environmental remediation applications. As such, a comprehensive and up-­to-­date synthesis of the current knowledge pertaining to 3D GBMs explored for sustainable energy and environmental applications is highly desirable. The consolidation of fundamental knowledge and practical applications of 3D GBMs would promote further advances in this rapidly evolving cross-­disciplinary research field of current global interest. With this goal in mind, we invited well-­known experts in the area of 3D GBMs from nine nations across the globe to share their key research outcomes in this book. We believe that this book will be useful to emerging researchers and senior scientists who are interested in gaining deep insights into various aspects of 3D GBMs from multidisciplinary perspectives and in applying these materials to tackle global energy and environmental challenges in a sustainable manner. Specifically, this book will make a strong appeal to chemists, chemical engineers, material scientists and engineers, environmental scientists and engineers, and energy specialists. The book is organized into 16 chapters. The first two chapters deal with the fundamental properties and architectures of 3D GBMs and their practical significance. Specifically, Chapter 1 explores the various types of graphene-­ based aerogels reported-­to-­date, and explains how their architecture influences their ultimate performance. Chapter 2 provides a fundamental understanding of the structure–property relationship of 3D GBMs to precisely tune their physicochemical properties and expand their application potential. The next 14 chapters are put together in two sections. Section 1 focuses on sustainable energy applications (Chapters 3 to 10) while section 2 deals with environmental remediation applications (Chapters 11 to 16). In particular, Chapter 3 summarizes the recent advances in the design and fabrication of 3D GBMs-­based high performance foldable and stretchable electrodes for applications in lithium ion batteries. Chapter 4 provides an overview of the significant progress achieved on 3D graphene-­based anodes and cathodes for application in sodium ion batteries. Chapter 5 presents the latest developmental status in 3D GBMs-­based supercapacitors with unprecedented performance. Chapter 6 brings together the recent progress in the development of 3D GBM-­supported transition-­metal oxide nanocatalysts and

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heteroatom doped 3D graphene electrocatalysts for potential application in fuel cells. Chapter 7 collates the applications of 3D graphene-­based scaffolds with high conductivity and biocompatibility in microbial fuel cells (MFCs). In addition, it discusses the key scientific and technological challenges in using them to improve the performance of MFCs. Chapter 8 describes the synthesis of 3D GBMs through bottom-­up strategies and their potential in improving the overall performance of dye sensitized solar cells. Chapter 9 presents a systematic, updated summary of the current status on the application of 3D GBMs in hydrogen production and storage. Chapter 10 provides a broad overview of the latest development in 3D GBMs-­mediated solar steam generation for potential applications in sterilization of waste and seawater desalination. Chapter 11 collates the current state-­of-­the-­art on the development and application of ultralight and mechanically resilient 3D GBMs for the selective absorption of a broad variety of oils and organic solvents, with an emphasis on underlying mechanisms. Chapter 12 critically reviews the recent advances in the development of novel graphene and graphene oxide-­based 3D macrostructures for fast and efficient removal of a variety of pollutants from water and air, with a special focus on interaction mechanisms with contaminant molecules. Chapter 13 summarizes the recent advances in the rational design of 3D GBM-­based photocatalysts and highlights their applications in photocatalytic environmental remediation, with an emphasis on the corresponding reaction mechanisms and pollutant transformation pathways. Chapter 14 introduces the basic principles of sensor design and explores the application of flexible 3D GBM-­based sensors for the on-­site detection of various classes of chemical pollutants and biological contaminants in various environmental matrices. Chapter 15 summarizes the most recent advances in 3D GBM-­mediated CO2 adsorption, and describes the numerous surface modification schemes that are actively pursued to enrich the CO2 adsorption capacity of 3D GBMs. Finally, Chapter 16 provides a systematic overview of the recent progress in the development and application of 3D GBM-­based photocatalysts for CO2 reduction to solar fuels. We are indeed grateful to all Lead as well as Contributing Authors for sharing their valuable expertise in various aspects of 3D GBMs, without which this book would not have been possible. We also thank the RSC editorial team, especially Dr Helen Armes and Mr Lewis Pearce, for their constructive feedback, logistical support and constant encouragement.

Contents Chapter 1 E  ngineering the Architecture of 3D Graphene-­based Macrostructures  S. Chandrasekaran, M. R. Cerón and M. A. Worsley

1.1 Introduction  1.2 Graphene Aerogels  1.2.1 Sol–Gel Hydrogels, Freeze-­drying, Gelation Methods  1.2.2 Template Methods  1.3 Graphene Aerogel Composites  1.3.1 Polymeric Graphene Aerogels (PGA)  1.3.2 Metal-­doped Graphene Aerogels (MDGAs)  1.3.3 Carbon Nano Tube/Graphene Aerogels (CNT/GA)  1.3.4 Fullerene/Graphene Aerogels  1.4 3D Printing Methods of Graphene Aerogels  1.4.1 Direct Ink Writing (DIW)  1.4.2 Inkjet  1.4.3 Freeze Gelation  1.4.4 Casting  1.4.5 Projection Micro-­stereolithography (PµSL)  1.4.6 Fused Deposition Modelling (FDM)  1.4.7 Laser-­based Methods  1.4.8 Other Methods  1.5 Conclusion  Acknowledgements  References 

  Chemistry in the Environment Series No. 1 Graphene-­based 3D Macrostructures for Clean Energy and Environmental Applications Edited by Rajasekhar Balasubramanian and Shamik Chowdhury © The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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1 1 4 4 9 15 15 17 19 21 22 22 25 26 27 27 31 31 33 35 35 35

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Chapter 2 S  tructure–Property Relationships in 3D Graphene-­based Macrostructures  Kimal Chandula Wasalathilake and Cheng Yan

2.1 Introduction  2.2 Structure–Property Relationship in 3D GBMs  2.2.1 3D Graphene Networks  2.2.2 Graphene Fibres and Tubes  2.2.3 Vertical Graphene Sheets  2.2.4 Graphene Cages  2.2.5 3D Porous Graphene Films  2.3 Conclusions  Acknowledgements  References 

Chapter 3 F  lexible 3D Graphene-­based Electrodes for Ultrahigh Performance Lithium Ion Batteries  Faxing Wang

3.1 Introduction  3.2 Flexible 3D Graphene-­based Cathode Materials  3.2.1 Intercalation-­t ype Cathode Materials Based on 3D GBMs  3.2.2 Conversion-­t ype Cathode Materials Based on 3D GBMs  3.3 Flexible 3D Graphene-­based Anode Materials  3.3.1 Intercalation-­t ype Anode Materials Based on 3D GBMs  3.3.2 Conversion-­t ype Anode Materials Based on 3D GBMs  3.3.3 Alloying-­t ype Anode Materials Based on 3D GBMs  3.4 Summary and Outlook  References 

Chapter 4 3  D Graphene-­based Materials for Enhancing the Energy Density of Sodium Ion Batteries  Shaikh Nayeem Faisal, Luba Shabnam, Shazed Aziz, Md Habibullah Dalal, Md Monirul Islam, Mahbub Hassan and Mohammad Saiful Islam

4.1 Introduction  4.2 Sodium Ion Batteries and their Ion Storage Mechanism  4.2.1 Operating Principle  4.2.2 Battery Performance Against Reaction Mechanics 

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4.2.3 Advantages of Nanostructured Materials on Ion Exchange Mechanisms  4.2.4 Graphitic Materials for Electrode Design  4.2.5 Advantages of the 3D Graphene Nanostructure  4.3 Synthesis of 3D Graphene-­based Electrodes  4.3.1 Template-­assisted Method  4.3.2 Self-­assembly Methods  4.3.3 Emerging Novel Methods  4.4 Applications of 3D Graphene Materials in SIBs  4.4.1 Application as Anodes  4.4.2 Application as Cathodes  4.5 Conclusions and Future Perspectives  References 

Chapter 5 U  ltrafast Charging Supercapacitors Based on 3D Macrostructures of Graphene and Graphene Oxide  Michael R. Horn, Suaad A. Alomari, Jennifer MacLeod, Nunzio Motta and Deepak P. Dubal

5.1 Introduction  5.2 Graphene for Double Layer and Pseudocapacitive Type Devices  5.2.1 Combining or Substituting AC with Graphene  5.2.2 Graphene Foams  5.2.3 Graphene Papers and 3D Films  5.2.4 Graphene-­based Fibres for Supercapacitors  5.3 Recent Advances in GBM for LICs  5.3.1 Graphene-­based Cathode Materials for LICs  5.3.2 Graphene-­based Anode Materials for LICs  5.4 Concluding Remarks  References 

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Chapter 6 3  D GBM-­supported Transition Metal Oxide Nanocatalysts and Heteroatom-­doped 3D Graphene Electrocatalysts for Potential Application in Fuel Cells  139 Chen Wang, Zhongfang Li, Likai Wang, Xueliang Niu, Shenzhi Zhang and Yuepeng Liu

6.1 Introduction  6.2 Methods for Synthesis of 3D G  6.2.1 Self-­assembly Methods  6.2.2 Template Strategies  6.2.3 Supercritical CO2 Method  6.2.4 Indirect Freezing and Electrochemical Synthesis 

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6.3 Heteroatom-­doped 3D G  6.3.1 Nitrogen-­doped 3D G (N-­3D G)  6.3.2 B, P, S-­doped 3D GBMs  6.4 3D G-­supported Transition Metal Macrocyclic Compounds  6.5 3D Transition Metal, N Codoped Graphene (3D M-­Nx/G)  6.6 3D GBM-­supported Transition Metal Oxide Catalysts  6.6.1 Single Metal Oxide Catalysts  6.6.2 Spinel-­t ype Oxide Catalysts  6.6.3 Pyrochlore-­t ype and Perovskite-­t ype Oxide Catalysts  6.7 Conclusion  List of Abbrevations  References 

Chapter 7 3  D Graphene-­based Scaffolds with High Conductivity and Biocompatibility for Applications in Microbial Fuel Cells  Ashish Yadav and Nishith Verma

7.1 Introduction  7.2 Graphene-­dispersed Laser-­ablated 3D Carbon Micropillars  7.2.1 Electrode Synthesis  7.2.2 Surface Morphology  7.2.3 Electrochemical Characterizations  7.2.4 Biocompatibility of the Electrode  7.3 Graphene Aerogel (GA)-­based 3D Electrodes  7.3.1 High Capacitative 3D GA Anodes  7.3.2 GA-­modified 3D Graphite Fiber Brush (GFB) Electrode  7.3.3 Nitrogen-­doped Graphene Aerogel Electrode (N-­GA)  7.3.4 3D Pt NP/GA Composite  7.4 3D Graphene Foams  7.4.1 Macroporous Graphene/Multi-­walled CNTs (MWCNTs)/FeO Foams  7.4.2 Flexible 3D Graphene-­Ni Foam  7.5 3D Macroporous-­monolithic Graphene Modified with Polyaniline (PANI)  7.6 3D Graphene Macroporous Scaffold  7.7 3D Graphene Sponges  7.7.1 Macroporous Flexible 3D Graphene Sponge 

149 149 155 157 159 161 161 162 167 169 170 171

179 179 181 182 183 183 186 187 187 188 189 190 191 191 193 195 196 197 197

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7.8 Graphene Sponge (GS)-­SS Composite  7.8.1 Synthesis and Characterization of GS  7.8.2 Synthesis of GS-­SS, Mechanism, and EIS  7.9 Additional Graphene-­modified 3D Scaffolds  7.9.1 Chitosan/Vacuum-­stripped 3D Graphene Scaffold  7.9.2 3D Graphene Nanosheets  7.10 Conclusions and Outlook  References 

Chapter 8 H  ighly Efficient Dye-­sensitized Solar Cells with Integrated 3D Graphene-­based Materials  Hisham A. Maddah, Anmole Jhally, Vikas Berry and Sanjay K. Behura

8.1 Introduction  8.1.1 Dye-­sensitized Solar Cells  8.1.2 Cell Architecture and Working Mechanism  8.1.3 Electron Transport and Recombination Kinetics  8.2 Graphene and 3D Graphene-­based Materials (3D GBMs)  8.2.1 Synthesis Methods  8.2.2 Hybrid Graphene-­based Composites for Counter Electrodes  8.2.3 Graphene Integrated Wide Bandgap Semiconductor Photoanodes  8.3 Biomolecular Dyes for Naturally Sensitized Photoanodes  8.3.1 Sources and Chemical Structure of Bio-­sensitizers  8.3.2 Pigment Bandgap and Bio-­DSSCs Performance  8.3.3 Graphene-­based Naturally-sensitized DSSCs  8.4 Conclusion  References 

Chapter 9 F  uelling the Hydrogen Economy with 3D Graphene- ­based Macroscopic Assemblies  Wingkei Ho and Jinliang Lin

198 198 198 201 201 201 201 202 205

205 207 207 209 211 211 213 217 221 223 224 226 227 228 237

9.1 Introduction  237 9.2 Hydrogen Evolution by 3D GBMs  239 9.2.1 Electrochemical Process for H2 Generation through H2O Reduction by 3D Graphene Foam  239

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9.2.2 Photochemical Process for H2 Generation from the Degradation of H2O Catalysed by Graphene Hydrogels (GHs)  9.2.3 Hydrolysis of NH3BH3 for H2 Generation Over Modified 3D Graphene Materials  9.2.4 Hydrolysis of NH3BH3 for H2 Generation Mediated by Modified 3D GHs  9.3 Hydrogen Storage  9.3.1 Hydrogen Storage on Pillared Carbon Materials  9.3.2 Reversible Hydrogen Storage on Carbon Material Composition (SiC/G)  9.4 Conclusion  List of Abbreviations  Acknowledgements  References 

Chapter 10 H  arvesting Solar Energy by 3D Graphene-­based Macroarchitectures  Xianbao Wang, Zhenzhen Guo, Fang Yu and Xin Ming

10.1 Introduction  10.2 Basis of Solar-­thermal Conversion and Transport  10.2.1 Solar Absorption  10.2.2 Thermal Transfer  10.2.3 Thermal to Steam Generation  10.3 Development of 3D GBMs for Efficient SSG  10.3.1 3D Graphene  10.3.2 3D GO/rGO  10.3.3 Hybrid Materials  10.4 Development of Photothermal Solar Evaporation Systems  10.4.1 Direct Contact System  10.4.2 Indirect Contact System  10.4.3 Isolation Evaporation System  10.5 Current Technologies for Enhanced SSG  10.5.1 Light Harvesting  10.5.2 Thermal Management  10.6 Applications Associated with SSG  10.6.1 Seawater Desalination  10.6.2 Sewage Purification  10.6.3 Electricity Generation  10.6.4 Sterilization  10.6.5 Intelligent Water Evaporation  10.7 Conclusion and Outlook  10.7.1 Conclusion  10.7.2 Outlook  References 

242 243 245 247 250 250 251 252 252 252 257 257 259 259 260 263 264 264 266 267 269 270 271 274 274 275 277 279 279 280 283 284 285 288 288 288 289

Contents

xv

Chapter 11 3  D Graphene-­based Macrostructures as Superabsorbents for Oils and Organic Solvents  296 Nariman Yousefi

11.1 Introduction  11.2 GBMs for Oils and Organic Solvents Removal: A Structure–Properties–Performance Paradigm  11.2.1 Properties of the Contaminants  11.2.2 Properties of 3D GBMs  11.3 Performance of 3D GBMs in Oils and Organic Solvents Removal  11.3.1 Self-­assembled 3D GBMs for Oils and Organic Solvents Removal  11.3.2 3D GBMs Processed by Chemical Vapour Deposition (CVD)  11.3.3 Organic Foams Coated with GBMs  11.3.4 Solar Heated 3D GBMs for Crude Oil Removal  11.4 Regeneration and Reuse of GBMs  11.5 Conclusion and Future Outlook  References 

Chapter 12 F  ast and Efficient Removal of Existing and Emerging Environmental Contaminants by 3D Graphene-­based Adsorbents  Haitao Wang, Mingmei Li, Dongpeng Zhang, Guoquan Liu and Sihui Zhan

12.1 Introduction  12.2 3D GBAs for Water Treatment  12.2.1 Heavy Metals  12.2.2 Organic Contaminants  12.2.3 Dyes  12.2.4 Emerging Contaminants  12.3 3D GBAs for Air Purification  12.3.1 Volatile Organic Compounds  12.3.2 Particulate Matter and Viruses  12.4 Regeneration and Reuse  12.5 Concluding Remarks  References 

296 297 298 298 301 301 302 303 306 307 307 309

313

313 314 315 320 321 324 327 327 328 329 330 331

Chapter 13 F  reestanding Photocatalytic Materials Based on 3D Graphene for Degradation of Organic Pollutants  M. Ussia, V. Privitera and S. C. Carroccio

337



337 340 341 341 343

13.1 Introduction and Scope  13.1.1 Scope and Organization of this Chapter  13.2 3D GBMs Systems  13.2.1 Template-­based 3D GBMs  13.2.2 Template-­free 3D GBMs 

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13.3 Degradation of Organic Pollutants  13.4 Conclusions and Outlook  List of Abbreviations  Acknowledgements  References 

Chapter 14 3  D Graphene-­based Macroassemblies for On-­site Detection of Environmental Contaminants  Kriengkri Timsorn and Chatchawal Wongchoosuk

14.1 History of 3D Graphene  14.2 Synthesis of 3D Graphene  14.3 Gas Sensors Based on 3D Graphene  14.4 3D GBM-­based Biosensors  14.5 3D GBM-­based Soil Sensors  14.6 Conclusion  Acknowledgements  References 

Chapter 15 G  raphene-­based Macroassemblies as Highly Efficient and Selective Adsorbents for Postcombustion CO2 Capture  Shamik Chowdhury and Rajasekhar Balasubramanian

15.1 Introduction  15.2 3D GMAs for Postcombustion Carbon Capture  15.3 Improving CO2 Capture Performance of 3D GMAs  15.4 Conclusion  References 

Chapter 16 A  rtificial Photosynthesis by 3D Graphene-­based Composite Photocatalysts  Zan Zhu, Jianping Chen and Wei-­Ning Wang

16.1 Introduction  16.2 Methodologies for Synthesis of 3D Graphene-­based Composites  16.2.1 Spray-­based Aerosol Routes  16.2.2 Wet Chemistry Methods  16.3 Graphene-­based Composites for CO2 Photoreduction  16.3.1 Basic Principles of CO2 Photoreduction  16.3.2 Typical CO2 Photoreduction Analysis Systems 

345 359 360 361 361 367 367 369 371 376 377 380 380 380

384 384 387 390 392 393 396 396 398 399 405 407 407 413

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16.3.3 Design of 3D GBCs for CO2 Photoreduction  16.3.4 The Roles of Graphene in CO2 Photoreduction  16.4 Summary and Outlook  Acknowledgements  References 

Subject Index 

415 422 424 425 425 432

Chapter 1

Engineering the Architecture of 3D Graphene-­based Macrostructures S. Chandrasekarana, M. R. Ceróna and M. A. Worsley*a a

Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, 7000 East Ave. L-­367, Livermore, CA, USA *E-­mail: [email protected]

1.1  Introduction Carbon is one of the most abundant elements on Earth with unique mechanical, thermal and electrical properties. Carbon has raised a lot of interest in the scientific community because of its intriguing properties and structural variability. The possibility of forming three different strong covalent bonds (with sp1, sp2 and sp3 hybridization) makes carbon a very promising element for both material scientists and engineers. Carbon can be found in different allotropic forms depending on its hybridization and crystalline structure (Figure 1.1). For instance, the first non-­ amorphous allotropic form of carbon discovered was graphite. In graphite, the carbon atoms are arranged in a hexagonal structure through sp2 hybridization (Figure 1.1b). In diamond, the carbon atoms have an sp3 hybridization forming four tetrahedral bonds with the four nearest neighbors to create diamond cubic unit cells (Figure 1.1c). In 1985, fullerenes or buckyballs were discovered by Kroto et al. as an unexpected result of investigating particles found in space.1 The 0D fullerene carbon allotrope forms a hollow cage of   Chemistry in the Environment Series No. 1 Graphene-­based 3D Macrostructures for Clean Energy and Environmental Applications Edited by Rajasekhar Balasubramanian and Shamik Chowdhury © The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

1

Chapter 1

2

carbon atoms connected by single and double bonds, resulting in twelve pentagons, where each pentagon is surrounded by five hexagons (Figure 1.1d).2 Soon after the discovery of fullerenes, carbon nanotubes (CNTs)3–5 and carbon nano-­onions (CNOs)6 were reported in 1991 and 1992, respectively (Figure 1.1e and f). CNOs consist of multi-­layered spherical or polyhedral shaped closed carbon shells with a structure resembling that of an onion (Figure 1.1f). The 1D CNTs consist of a tube with diameters generally in the nanometer range and are broadly distinguished by the number of concentric walls that make up the tube (Figure 1.1e). Generally, there are two different types of CNTs, single wall carbon nanotubes (SWCNTs) and multi-­wall carbon nanotubes (MWCNTs). Unrolling a SWCNT results in the last carbon allotrope discovered: graphene (Figure 1.1g). The so-­called 2D graphene describes a single layer of carbon taken from a 3D graphite block, first observed using electron microscopy in 1962.7 Forty-­t wo years later graphene was “rediscovered”, isolated,

Figure 1.1  Allotropic  forms of carbon. (a) Amorphous carbon. (b) Graphite. (c) Diamond. (d) Fullerenes. (e) Nanotubes. (f) Nano-onions.

Engineering the Architecture of 3D Graphene-­based Macrostructures 8

3

and characterized by Novoselov et al. Graphene has a special set of properties, such as high electrical and thermal conductivity,9 enormous specific surface area (1168 m2 g−1),9 and larger nonlinear diamagnetism than graphite,10 which set it apart from other allotropes of carbon. It is approximately 100 times stronger than steel, yet with a much lower density, having one of the largest strength-­to-­weight ratios observed.11 Given these impressive properties, graphene and graphene-­based materials have great potential in numerous applications12 such as energy storage,13–17 nanoelectronics,18,19 sensors,20,21 catalysis,22,23 and composites,24–26 among others.27 However, in many cases we do not observe the full theoretical potential of the synthesized composites.28 These sub-­par outcomes have been attributed to poor graphene dispersion due to π–π stacking interactions between several graphene sheets. One of the best strategies to overcome poor graphene dispersion, while keeping the intrinsic properties of graphene, is breaking the symmetry of the single sheets by introducing oxygen functional groups in the backbone. In this context, graphene oxide (GO) is widely used as a precursor of graphene composites because it is economical to fabricate on a large scale and easy to process.29 Today GO is produced in large quantities through chemical exfoliation of graphite, known as the Hummers' method.30,31 The technique is based on the principle of oxidizing graphite by treatment in a mixture of strong acids (e.g. H2SO4) and oxidizing agents (e.g., NaNO2, KMnO4) for 2 hours at 45 °C. Then the reaction mixture is washed in an ice-­bath with H2O2 to remove the residual KMnO4. This process introduces several oxygen functional groups such as epoxides, hydroxyl groups, and carboxylic acid, among others (Figure 1.2),32 making GO hydrophilic and easily dispersible in water via ultra-­ sonication. The oxidized carbons (i.e., sp3 carbons) and lattice defects of GO can be later eliminated by a reduction step (e.g., chemical, or thermal) to recover the graphene-­like properties (i.e., sp2 carbons).

Figure 1.2  Surface  groups on graphene oxide.

4

Chapter 1

However, like any other carbon allotrope, graphene as a bulk material has a strong propensity to form irretrievable agglomerates due to strong π–π interactions among individual graphene sheets. This leads to inadequate exploitation of isolated graphene layers for practical applications. In order to overcome this problem, the integration of 2D graphene nanosheets into 3D macrostructures, and ultimately into a functional system, such as aerogels has been recognized as a progressively critical approach during the past five years. Their intensively interconnected networks, enormous surface area, intense porosity, remarkable sturdiness, and superior graphene building blocks endow a plethora of exciting features that make them extremely suitable for a broad range of clean energy and environmental applications. However, in order to truly harness their potential, one must understand how the design and assembly of these 3D graphene networks impact their final properties. In this chapter, we explore the various types of graphene-­based aerogels reported to date and how their architecture impacts their ultimate performance.

1.2  Graphene Aerogels Graphene aerogels (GA) were first synthesized in 2010 by two independent groups, Xu et al.33 and Worsley et al.34 Both groups used GO as the precursor but different gelation processes (e.g., covalent vs. noncovalent). Here we describe the different methods to synthesize GAs.

1.2.1  Sol–Gel Hydrogels, Freeze-­drying, Gelation Methods Xu and co-­workers reported a hydrothermal route involving a 180 °C treatment in a pressure vessel for 12 hours to simultaneously reduce and gel an aqueous GO suspension.33 For the hydrothermal route, gelation occurs upon reduction because the electrostatic repulsion is eliminated due to loss of oxygen functionalities in GO. Local regions on the GO sheets then become hydrophobic and are prone to noncovalent bonding (e.g., π–π stacking) with reduced regions on nearby sheets forming a physically crosslinked gel. The properties of the aerogel achieved by freeze-­drying were highly correlated to the starting GO concentration of the suspension. If the concentration was below 1 mg mL−1, no gel was formed. The duration of the hydrothermal treatment also proved to be significant in determining the properties of the aerogel. Both the maximum pressure and treatment time determined the degree of reduction, which impacted density, conductivity, and crosslinking (Figure 1.3).33 Following this seminal report, which demonstrated the properties of graphene sheets in an aerogel form, a number of related studies were inspired.35,36 Tang et al. reported a noble metal promoted self-­assembly of GO gels.37 The method entailed using glucose to reduce a metal salt (e.g., chlorides of Au, Ag, Pd, Ir, Rh, or Pt, etc.) dissolved in an aqueous GO suspension, to metal nanoparticles which created strong cross-­links between the

Engineering the Architecture of 3D Graphene-­based Macrostructures

5

Figure 1.3  (a)  Photographs of a 2 mg mL−1 homogeneous GO aqueous dispersion

before and after hydrothermal reduction at 180 °C for 12 h; (b) photographs of a strong SGH allowing easy handling and supporting weight; (c–e) SEM images with different magnifications of the SGH interior microstructures; (f) room temperature I–V curve of the SGH exhibiting Ohmic characteristic, inset shows the two-­probe method for the conductivity measurements. Reproduced from ref. 33 with permission from American Chemical Society, Copyright 2010.

GO sheets. The essential role of the noble metal nanoparticles was evidenced by dissolving the metal in aqua regia, which led to the degradation of the aerogel. High electrical conductivities and good mechanical strength were also reported.37 Xu et al. reported the use of DNA to cross-­link GO sheets. In this case, a solution of double-­stranded DNA (dsDNA) was added to a GO suspension and heated to 90 °C for 5 minutes.38 The elevated temperature lead to an unwinding of the dsDNA to single-­stranded DNA (ssDNA) chains, which made noncovalent bonds between GO sheets. Despite relying primarily on physical crosslinking, these GO/DNA gels showed remarkable chemical resistance and mechanical strength. Shi et al. used glutathione to simultaneously serve as the cross-­linker and reducing agent, yielding GAs that were doped with nitrogen and sulfur.39 In addition to initiating gelation via noncovalent and chemical cross-­ linkers, many researchers have leveraged the abundant chemical functionality native to GO to induce self-­assembly of GO suspensions.40,41 Strong bases, such as ammonium hydroxide, can induce self-­assembly of GO suspensions with crosslinking analogous to that found in resorcinol-­formaldehyde (RF) sol–gel chemistry. Nuclear magnetic resonance (NMR) spectroscopy

Chapter 1

6 3

techniques reveal the appearance of sp carbon as well as –CH2– and –CH2O– cross-­linkers after gelation of the GO suspension in the presence of ammonium hydroxide at 85 °C, indicating some covalent bonding between GO sheets during gelation (Figure 1.4).42 Further, a number of chemical reducing agents can aid in the self-­ assembly process similar to that observed using hydrothermal treatment by Xu et al.33 Broadly speaking, the self-­assembly mechanism consists of the clustering of partially reduced GO sheets as their hydrophilicity decreases. The sheets assemble randomly as the reduced GO clusters are formed and water is excluded from the hydrophobic reduced GO gel, resulting in volume shrinkage. A number of chemical reagents, such as NaHSO3, Na2S, ethylenediamine, ammonia, hydroiodic acid, and hydroquinone, have been used to drive GO gelation.43,44 As many chemical reducing agents are hazardous to chemical workers and/or the environment, there have been several studies focused on “green” reducing agents, such as ascorbic acid. Zhang et al.,45 and many other researchers, have used ascorbic acid (i.e., vitamin C) to initiate GO gelation,46–48 resulting in well-­formed aerogels with improvements in electrical and mechanical properties. In addition to being non-­hazardous, ascorbic acid is also a mild reductant, and as such no gaseous products are evolved (which tend to disrupt or completely destroy the integrity of the gel). Ji et al. utilized carbohydrates as both reductant and morphology orienting agents in GA synthesis.35 Alternate “green” reductants that have also been reported include tannic acid, dopamine, and amino acids.49–52

Figure 1.4  13  C NMR spectra and picture of GO powder, GO after initial gelation, and 3D graphene macroassembly. Reproduced from ref. 42 with permission from the Royal Society of Chemistry.

Engineering the Architecture of 3D Graphene-­based Macrostructures

7 −3

In another work, GAs with densities of less than 3 mg cm were assembled via a one-­pot method at the oil-­water interface of a GO emulsion (Figure 1.5).53 This emulsion was prepared using a cyclohexane/water mixture in the presence of sodium bisulfite under ultrasonication. The GO emulsion gelled at 70 °C over 12 hours via the gradual removal of the oxygen functionalities. Here, sodium bisulfite served as both reductant and a co-­emulsifier due to the salt effect.54 Using this method, a cellular pore morphology was formed, which enhanced the mechanical robustness of the aerogel. Finally, gamma ray irradiation was used by He et al. for self-­assembly of porous honeycomb GAs.55

Figure 1.5  GA  fabrication and characterizations. (a) Scheme illustrating the syn-

thesis process of GA from the assembly of GO at oil-­water interface and the subsequent chemical reduction. (b) Whole view of the GA derived from the emulsion with GO concentration of 2.00 mg mL−1, revealing that the bulk of aerogel is entirely composed of cellular-­like pores. (c) The image shows the closely linked pores with polyhedral morphology. (d) A hexagonal pore shares the boundary with other six adjacent pores, ensuring the firm bridge of the connection. (e) The ultrathin and wrinkled wall. (f) UV-­vis spectra of the aqueous suspensions of GO and GA. (g) Raman spectra of GO and GA. (h) C 1s XPS spectra of GA. Scale bars 150 µm (b), 50 µm (c), 8 µm (d) and 500 nm (e). Reproduced from ref. 53, https://doi.org/10.1038/srep25830, under the terms of the CC BY 4.0 license, http://creativecommons.org/licenses/by/4.0/.

8

Chapter 1

Thermal treatment, chemical reagents, and hydrothermal reduction in an autoclave can be used to reduce GO aerogels to GAs.56 Hydrazine, borohyrides, aluminum hydrides, and hydrohalic acids are the most commonly used chemical reagents. Sudeep et al. reported a controlled reduction process to reduce GO gel covalently bonded with resorcinol-­gluteraldehyde using hydrazine monohydrate vapor at 50 °C under vacuum for 12 hours.57 The reduced GO had an electrical conductivity of 3.4 S m−1 and exhibited good adsorption capacity for CO2 storage. Tang and co-­workers used magnesium vapor to reduce GO aerogels.58 The freeze-­dried GO aerogel was heated in an ampoule with magnesium powder at 700 °C for 5 hours. After the magnesiothermic reaction, the reduced GO was decorated with MgO nanoparticles, washed with acid, and freeze-­dried again. The final GA retained the original morphology with densities as low as 1.1 mg cm−3 and had an electrical conductivity of 27 S m−1.58 Mi et al. reported 3D highly compressible, elastic, anisotropic, cellulose/graphene aerogels (CGAs) prepared by bidirectional freeze-­drying (Figure 1.6).59 When the GO content was further increased to 40%, both pore size and aspect ratio decreased, which might be due to the nucleation effect of GO dominating and inducing the formation of smaller ice crystals. The bidirectionally aligned porous structure gave the as-­prepared GA outstanding compression properties and was able to recover 99.8% and 96.3% when compressed to 60% and 90% strain, respectively. The combined physical properties of a low density of 5.9 mg cm−3 and a high surface area of 47.3 m2 g−1 synergistically led to a remarkable absorption capacity of 80–197 times of its own weight.

Figure 1.6  (a)  Schematic illustration of the bidirectional freeze-­drying fabrication process of MCGA. The CNF/GO solution was frozen in the y and z directions and freeze-­dried to obtain CGA. Then the CGA was grafted with DDTS using the CVD method to introduce a hydrophobic coating. (b) Temperature gradient simulation in the freezing process using the COMSOL Multiphysics software. (c) Digital images show the solution freezing process via bidirectional freezing. Reproduced from ref. 59 with permission from Elsevier, Copyright 2018.

Engineering the Architecture of 3D Graphene-­based Macrostructures

9

The other common reduction method is thermal treatment in inert gas. Thermal annealing is one of the most effective methods of achieving high electrical conductivity in the GA. Annealing at 800–1100 °C under inert gas produces aerogels with conductivities of ∼100 S m−1.34 Nonetheless, when using even higher thermal annealing temperatures (1500 to 2500 °C), additional improvements in the crystallinity of the graphene sheets are realized. These improvements are clearly reflected in the Raman spectra, oxidative thermal stability, electrical conductivity, and mechanical properties of these GAs (Figure 1.7).60,61 For example, the electrical conductivity can be increased 5–6 times the values recorded at lower temperature anneals. This work shows the critical role of crystallinity in determining the physicochemical properties of GA. Thermal reduction in a furnace is most common, but in some work other means are used. For instance, Hu and co-­workers reported the synthesis of ultralight GAs by microwave irradiation. Those aerogels showed densities as low as 3 mg cm−3 and yet the structure fully recovered without any fracture after 90% compression.62

1.2.2  Template Methods Templating is a process where an ordered or relevant structure (i.e., the template) having a length scale of micro-­ or nanometers is filled with another material and the template is subsequently removed, thereby leaving an imprint of the template on the filled material.63 In the field of aerogels, templating is mainly used to control the pore size distribution and morphology as these two parameters determine the physical properties of an aerogel. The templating method is classified into two types; (1.2.2.2) hard templating, which uses a rigid material with a stable structure,64,65

Figure 1.7  Electrical  conductivity of GMA vs. annealing temperature. Inset: Raman spectra for GMA after annealing at 2500 °C and zoom-­in HRTEM images of GMA annealed at 2500 °C. Reproduced from ref. 60 with permission from American Chemical Society, Copyright 2014.

10

Chapter 1

and (1.2.2.1) soft templating, which is a cooperative self-­assembly process based on inter-­and intramolecular interactions between the surfactant and guest species.66

1.2.2.1 Soft Templating Mesoporous carbon microspheres/graphene composites (MCMG) were synthesized in situ via a soft template method by Chen et al. using cetyltrimethylammonium bromide (CTAB) as the structure-­directing agent, and an aqueous mesophase pitch (AMP) and GO as the carbon sources.67 The authors claimed that the negatively charged GO becomes positively charged upon addition of CTAB in the pH range 7–13, and observed a strong electrostatic interaction between the GO/CTAB composites and the AMP molecules at pH 12. Subsequently, the CTAB/AMP micelles with a spherical shape were formed between the GO sheets through self-­assembly.67 An interface-­induced co-­assembly process was adapted by Liu and co-­ workers to fabricate a composite of ordered mesoporous carbon/graphene aerogel (OMC/GA).68 They employed a strategy where GA acts as a macroporous substrate and a triblock copolymer F127 as a soft template, where resol is the carbon source. The macroporous graphene network was covered by highly ordered mesoporous carbon of 9.6 nm diameter and the orientation of the mesopores was tuned by varying the ratio of the components. The resol-­F127 monomicelles, when mixed with GA, gradually deposited on the macropore walls of GA via non‐covalent interactions, such as hydrogen‐bonds, amphiphilic interactions, and π–π interactions, to obtain such ordered structures.68 An all‐solid‐state supercapacitor (ASSS) based on OMC/ GA with vertical mesopores exhibited an outstanding specific capacitance (44.3 F g−1) at 5 mV s−1, and high power density with fast charge/discharge rate (≈3545 W kg−1 in less than 3.6 s). Zhang et al. proposed a versatile technique based on soft bubble templating and freezing to fabricate 3D bubble‐derived graphene foams (BGFs) and 2D bubble‐derived graphene porous membranes (BGPMs).69 This technique can be extended to assemble other nanomaterials as building blocks into macroscopic configurations. Hierarchical structures of well‐aligned macroscopic spherical pores were formed by templating of bubble clusters, and random minor pores from ice templating. The volume ratio of bubble clusters to the GO dispersion, the concentration of GO dispersion, freezing rate, and size of bubbles controlled the final architecture of the graphene sheets. The authors proved that the optimized volume ratio to prepare a stable mixture is 1 : 1, and the concentration of GO dispersion needs to be higher than 5 mg mL−1. Flexible sensors made with BGF/polydimethylsiloxane (PDMS) composite exhibited excellent resistance change to a compressive strain of 30%.69 An emulsion templating technique was employed to synthesize porous materials by forming an emulsion of GO containing hexane droplets by Li et al.70 Hexane droplets with diameters in the range of several tens of micrometers to about 200 µm were dispersed homogeneously in a GO dispersion.

Engineering the Architecture of 3D Graphene-­based Macrostructures

11

Barg and co-­workers reported a novel self-­assembly strategy for the fabrication of chemically modified graphene cellular networks (CMG-­CNs) via a multi-­step soft/hard template mechanism that combined emulsion and ice templating.71 GO acts as a surface-­active amphiphile, self-­assembling at the interface between the oil droplets and the water phase, and stabilizing the GO emulsion for several months (Figure 1.8).

1.2.2.2 Hard Templating Zhang et al. reported a double-­layer templated graphene (DTG) with two non-­stacked graphene layers, separated by numerous mesosized protuberances extending from the graphene layers. MgAl-­layered double oxides (MgAl-­LDO) were used as templates for the chemical vapor deposition (CVD) – a mediated synthesis of the novel DTG materials.72 After the deposition of the graphene layer, the products were purified through hydrothermal reactions with sodium hydroxide and hydrochloric acid. The samples were then filtered, washed, and freeze-­dried to yield a 3D porous double-­layer graphene. The authors also reported that the defect-­rich DTG samples were hard carbons [i.e., not graphitizable and exhibited high ORR current even after heat treatment at 1600 °C (Figure 1.9)]. Another common technique to obtain 3D graphene macroscopic structures with a foam-­like network, i.e., graphene foam (GF), is template-­directed CVD.73 A porous 3D interconnected nickel foam is used as a scaffold and

Figure 1.8  The  viscoelastic properties of the GO emulsion system developed in this

work enables its extrusion through micro needles resulting in GO emulsion wires that maintain their shape (straight, curved or spirals) (a) and can be further processed by the approach described in this paper. In (b–d) details of rGO-­CN wire and internal cellular microstructure after thermal treatment at 1000 °C in Ar/H2 atmosphere. GO emulsions prepared by the emulsification of 65 vol% decane in 1 wt% GO suspensions containing 1.2 wt% organic additives (1 : 1, PVA:sucrose). The wires are several centimeters long and down to 200 µm in diameter. Scale bars, 200 µm (a), 300 µm (b), 20 µm (c) and 10 µm (d). Reproduced from ref. 71, https://doi.org/10.1038/ncomms5328, under the terms of the CC BY 4.0 license, http://creativecommons.org/licenses/by/4.0/.

Chapter 1

12

methane is introduced as a carbon source which decomposes at 1000 °C under ambient pressure. Graphene films later precipitate on the surface of nickel foam. Due to the difference in thermal expansion coefficient, the films have ripples and wrinkles. A polymethyl methacrylate (PMMA) layer is coated on the graphene-­Ni foam before etching away the Ni to avoid the collapse of the graphene network during the etching process. The Ni template is removed by thermo-­chemical etching. The PMMA layer is later dissolved in hot acetone to yield a monolith of continuous and interconnected 3D graphene networks (Figure 1.10).

Figure 1.9  Scheme  for the synthesis of (a) DTG and (b) rGO. Reproduced from ref. 72 with permission from Elsevier, Copyright 2016.

Figure 1.10  (a,  b) CVD growth of graphene films (Ni–G, b) using a nickel foam

(Ni foam, a) as a 3D scaffold template. (c) An as-­grown graphene film after coating a thin PMMA supporting layer (Ni–G-­PMMA). (d) A GF coated with PMMA (GF-­PMMA) after etching the nickel foam with hot HCl (or FeCl3/HCl) solution. (e) A free-­standing GF after dissolving the PMMA layer with acetone. (f) A GF/PDMS composite after infiltration of PDMS into a GF. All the scale bars are 500 µm. Reproduced from ref. 73 with permission from Springer Nature, Copyright 2011.

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13 −3

The free-­standing graphene foam has a density of 5 mg cm which corresponds to 99.7% porosity with a surface area of 850 m2 g−1 and when infiltrated with polydimethylsiloxane (PDMS), the electrical conductivity of this foam/composite is as high as 10 S cm−1 for 0.5 wt% loading. Hence, the PDMS/GF composites can be used as stretchable conductors due to their excellent electromechanical stability.74 The use of polystyrene (PS) beads or spheres as hard sacrificial templates to create an ordered porous architecture in graphene foams is also common. PS latex spheres of 280 nm diameter were assembled with GO to build up a sandwich-­t ype composite film, followed by heat removal and simultaneous reduction of GO. The 3D GF exhibited a high specific surface area of 402.5 m2 g−1. PS microspheres were uniformly wrapped by crinkled GO sheets due to electrostatic interaction. The highly oriented laminar and macroporous structure of the free-­standing GF was preserved after removal of the template via calcination at 800 °C (Figure 1.11).74 Fang and co-­workers reported a low-­cost iron oxide hard template strategy to create highly wrinkled graphene film (HWGF) with a hierarchical pore structure. The hierarchical porosity and high packing density were achieved by capillary compression in the presence of Fe3O4 nanoparticles and the generated HWGF exhibited a surface area of 383 m2 g−1 along with a

Figure 1.11  SEM  images of the (a) surface and (b) a cross-­section of a GO/PS com-

posite film; (c) low and (d) high-­magnification SEM images of the cross-­section of a 3DGF; (e) low magnification SEM images of surface of a 3DGF; (f) high-­magnification TEM images of 3DGF. Reproduced from ref. 74 with permission from Elsevier, Copyright 2016.

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Figure 1.12  The  schematics of the fabrication strategy of HWGF. Reproduced from ref. 75 with permission from the Royal Society of Chemistry.

high capacitance of 242 F g−1 at low current densities (Figure 1.12).75 Fe3O4 nanoparticles (NPs) were homogeneously embedded between the stacked graphene nanosheets, which is attributed to steric hindrance effect, endowing graphene sheets with highly wrinkled morphology.75 In addition, commercially available polymer foams (such as melamine foam) have been used as supporting frameworks to fabricate graphene oxide/ graphite nano-­platelets GO/GNP composite aerogels with conductive channels. However, unlike other techniques, the melamine foam framework was carbonized to form conductive networks with a homogeneous covering of reduced GO/GNP sheets.76 Powder metallurgy templating combined with CVD annealing is another route that has been used by Wang et al. to prepare free-­standing nitrogen-­ doped graphene foams (NGFs).77 The authors used melamine as a precursor, which acted both as a carbon and nitrogen source. Melamine and Ni powders were evenly mixed, ground, and pressed. Here, the Ni powder served as both a template and as a catalyst to form the 3D porous structure. The pellets were then annealed in a CVD furnace with H2/Ar flowing under negative pressure. During annealing, the melamine decomposed into carbon and nitrogen atoms which permeated into the Ni particles and deposited on the surface to form a 3D network. The Ni template was later removed by pickling, cleaning, and drying the structure to obtain a self-­supported N-­doped graphene foam.77 A continuous microporous 3D GF was synthesized by Lu et al. by means of combining porous metals through the reduction of metallic salts and CVD (Figure 1.13).78 For the synthesis of metallic salts, iron and nickel chloride were used as precursors, pressed into pellets and subjected to hydrogen reduction in a one-­step CVD process at a temperature range of 600–1000 °C. The authors found that by increasing the reduction temperature from 600 °C to 900 °C, the thickness of Ni ligaments increased from 0.5 µm to 3 µm, and the size of pores was in the range of 0.5 to 5 µm. Further increasing the temperature to 1000 °C, methane or other hydrocarbon gases were introduced

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15

Figure 1.13  Schematic  illustration of the synthesis of micron-­porous graphene foam from metallic salt precursor. The overall process contains three steps: thermal reduction of metallic salts, CVD growth of graphene in porous metal template and removal of the metal template. The photos show a nickel chloride chip, micron-­porous Ni chip, graphene coated micron-­porous Ni chip and micron-­porous graphene foam chip of 13 mm diameter, respectively, made in each step. Reproduced from ref. 78 with permission from Elsevier, Copyright 2019.

as the carbon source. The as-­generated carbon atoms diffused and dissolved into the Ni ligaments because of the high solubility of carbon in Ni. Upon cooling, the dissolved carbon atoms segregated and precipitated onto the surface of Ni ligaments, which was followed by graphene nucleation and propagation over the Ni ligaments.78

1.3  Graphene Aerogel Composites The physicochemical properties of GAs are strongly dependent on how they are assembled. Mechanical properties can vary widely depending on whether the cross-­links between the sheets are physical or chemical. Electrical properties depend on low resistance connections between graphene sheets. The introduction of electrochemically active or catalytic molecules can add functionality. The following section will explore how incorporating different elements into the carbon matrix of GAs can impact its structure and functionality.

1.3.1  Polymeric Graphene Aerogels (PGA) One of the most important, inexpensive, and useful synthetic methods to obtain GAs was reported in 1989 and is based on the resorcinol-­formaldehyde (RF) method for producing carbon aerogels (CAs).79 In 2011, Worsley et al. synthesized 3D graphene assemblies by adding different concentrations of RF

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29

reactants to a 10 mg mL GO suspension. The RF units preferentially nucleated and grew on the surface of GO sheets, covalently bonding them together. A lower RF content produced a graphene assembly with a higher sp2 carbon content and higher degree of exfoliation contrary to the gels with a higher RF content. Changing the synthetic parameters and RF contents resulted in a wide range of surface areas (600 to 1200 m2 g−1), pore volume, and pore size.80 Lim et al. drastically reduced the gelation time of GA from several days to 1–2 hours by crosslinking RF and GO using hydrochloric acid (HCl) as a catalyst and acetonitrile as the solvent.81,82 The GO was suspended in acetonitrile instead of water, followed by the addition of RF and HCl. The GO-­RF gels were supercritically dried and carbonized at 1000 °C to obtain a GA with similar surface area and porosity of the RF-­derived CA (Figure 1.14).81 Recently, Scaffaro et al. synthesized an ultralight graphene-­based aerogel (GPA) by coupling GO and an amino terminated polyethylene glycol (PEGNH2) by carbodiimide (EDC) in an aqueous environment, followed by freeze-­drying (Figure 1.15).83 The GPA showed an ultralight and highly porous (99.7%) network with good mechanical properties. Furthermore, cytocompatibility and hemolysis assays of the GPA exhibited no toxicity in vitro at the tested doses.83 A multimodal pore graphene/carbon aerogel was reported by Zhang et al.84 The hierarchical aerogel was synthesized via one-­step carbonization of graphene crosslinked polyimide (PI) aerogel, avoiding the use of harmful formaldehyde (Figure 1.16). The incorporation of graphene sheets into carbon aerogels reduced the pore size while increasing the amount of micro-­ and mesopores. The as-­prepared graphene/carbon aerogel showed a high specific surface area of 998.7 m2 g−1 and specific capacitance of 178.1 F g−1 in 6 M KOH at a current density of 1 A g−1, which is much higher than that of pure carbon aerogels (193.6 m2 g−1 and 104.2 F g−1).84

Figure 1.14  Diagram  of the GO-­RF aerogel preparation process. Reproduced from ref. 81 with permission from Elsevier, Copyright 2015.

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Figure 1.15  Schematics  of the pathway followed to synthesize GO-­PEG aerogel

(GPA). Reproduced from ref. 83 with permission from Elsevier, Copyright 2016.

1.3.2  Metal-­doped Graphene Aerogels (MDGAs) To explore new functionalities of GAs, such as electrocatalysis or electrode fabrication, researchers have investigated the use of transition metal ions for 3D graphene assembly.85,86 Chen et al. synthesized a graphene/CeO2 aerogel using a one-­step in situ electrochemical method.87 The MDGA was synthesized by freeze-­drying a graphene/CeO2 colloidal solution, which in turn

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Figure 1.16  Schematic  illustration of the preparation of graphene/carbon aero-

gels. Reproduced from ref. 84 with permission from the Royal Society of Chemistry.

was obtained via electrochemical exfoliation of a graphite anode and in situ deposition of CeO2 nanoparticles on the resulting graphene sheets using ammonium sulfate and cerium nitrate salts as electrolytes (Figure 1.17). An increase in the concentration of cerium salts in the electrolyte enhanced the Faradaic reactivity of the graphene/CeO2 hybrid aerogels.87 Wei et al. prepared Ni-­doped graphene/carbon cryogels (NGCCs) using a Ni2+ catalyst and adding resorcinol and formaldehyde (RF) to a GO suspension.88 The Ni2+ catalyst improved the crosslinking between GO and RF, strengthening the cryogel. Freeze-­drying and carbonization under an inert atmosphere yielded the Ni-­doped aerogel. Ni2+ ions were reduced to Ni particles during the carbonization process and thus embedded in the interconnected structures.88 Molybdenum disulfide (MoS2) has been used for the hydrothermal synthesis of MoS2-­GA hybrids. Hou et al. prepared a MoS2/nitrogen-­doped GA to study the application of the aerogel as a catalyst for hydrogen evolution in microbial electrolysis cells. The authors observed a significantly higher hydrogen production rate (0.19 m3 H2 m−3 d−1) compared to pristine MoS2 nanosheets and N-­GAs.89 Worsley et al. infiltrated a GA with ammonium thiomolybdate (ATM), which upon thermal reduction resulted in MoS2 sheets layered on graphene sheets in the GA. This MoS2-­GA hybrid exhibited a very large surface area (ca. 700 m2 g−1) and retained the native conductivity of the GA (1.12 S cm−1). With 50 wt% MoS2, the aerogel proved to be an efficient hydrogen evolution catalyst with a low overpotential.90 Zhang et al. combined defect-­rich MoS2 nanosheets and conductive graphene nanosheets (GNS) to obtain a hybrid MoS2-­GA with outstanding electrochemical performance as anodes for lithium ion batteries.91 Tadyszak et al. reported the synthesis and characterization of transition metal ion (TMi) doped partially reduced GO aerogels using VCl3, CrCl3, FeCl2·4H2O, CoCl2, NiCl2, and CuCl2 chlorides as reducing agents (Figure 1.18).92 The authors studied the influence of different TMis on the oxygen

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Figure 1.17  Schematic  drawing shows in situ electrochemical route to fabricate the

aerogel electrode materials including graphene and CeO2. (a) Graphite electrode was electrochemical exfoliated in mixing electrolyte including (NH4)2SO4/Ce(NO3)3 and (NH4)2SO4/(NH4)2Ce(NO3)6. Then, the colloidal solution of graphene/CeO2 composite was obtained. After freeze-­drying process, graphene/CeO2 aerogel was obtained. (b) The detailed reactions on electrochemical exfoliation of graphite and in situ deposition of CeO2 on graphene sheets. Reproduced from ref. 87 with permission from Elsevier, Copyright 2015.

concentration and specific surface area of the derived aerogel, concluding that VCl3 possesses the strongest reducing properties, resulting in the formation of the densest aerogel with the lowest oxygen content and lowest specific surface area.92 Chu et al. synthesized Ni, Co, and Mn doped SnS2-­GAs using metal chlorides and thioacetamide as precursors.93 All the metal-­doped SnS2-­ GAs showed improved electrochemical performance compared to SnS2. Mn-­SnS2-­GA exhibited almost three times higher specific capacitance than SnS2 (523.51 F g−1 at the scan rate of 5 mV s−1) and excellent cycling stability (98.57% capacitance retention after 2000 cycles at 10 A g−1).93

1.3.3  Carbon Nano Tube/Graphene Aerogels (CNT/GA) The first CNT aerogel was synthesized from aqueous-­gel precursors in 2007 via critical-­point drying and freeze-­drying.94 However, the mechanical integrity of pure CNT aerogels relied solely on van der Waals interactions,94 which opened the possibility of investigating ways to increase their mechanical properties.

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For example, Bryning et al. reinforced the CNT aerogel's network by adding 1 wt% of polyvinyl alcohol (PVA). Although a decrease in the conductivity of the CNT aerogel was observed, the authors achieved a significant increase in the mechanical properties (Figure 1.19).94 Worsley et al., in an effort to increase the conductivity and the mechanical properties of CNT aerogels, changed the traditional polymer binder for a conductive binder by adding a CNT dispersion to a resorcinol-­formaldehyde (RF) solution before gelation. SEM images of the carbonized CNT-­carbon aerogel showed CNTs uniformly dispersed within the carbon aerogel matrix.95 However, since the CNTs concentration was low compared to the concentration of RF, the improvements in the mechanical and electrical properties were modest.94 A further reduction in RF concentration to 4 wt% and an increase in CNT to 2 wt% resulted in a CNT aerogel that simultaneously exhibited high electrical conductivity, mechanical stiffness, and super-­compressibility.96 Since then several reports have shown enhanced performance of CNT aerogels, though the recent focus has been on CNT-­ graphene hybrid aerogels.29,97 Sui et al. synthesized a CNT/graphene hybrid aerogel (CNT/GA) through heat treatment of aqueous suspensions of GO and CNT with dissolved vitamin C as a reducing agent, followed by supercritical CO2 drying.98 The CNT/ GAs were investigated for the desalination of brackish water as capacitive deionization (CDI) electrodes, showing high removal capability for dyes and heavy metal ions including Pb2+ and Ag+. The hybrid aerogels exhibited a high conductivity of 7.5 S m−1, a large BET (Brunauer–Emmett–Teller) surface area of 435 m2 g−1 with a hierarchically porous structure, and a desalination capacity of 633.3 mg g−1 for a 35 g L−1 NaCl solution.98 Wang et al. synthesized a GA by hydrothermal treatment of GO in the presence of dopamine and FeCl3. The GA served as a template for the in situ growth of CNTs to obtain a hybrid CNT/GA with enhanced surface area and hierarchical meso-­and micro-­scale pores.99 This synthetic approach resulted in CNTs distributed within the layers of the GA (Figure 1.20), increasing the hydrophobicity, thermal stability, and oleophilicity towards organic compounds. The low-­density CNT/graphene aerogel exhibited selective adsorption of organics and oils from water.99

Figure 1.18  Optical  images of (a) TMi-­doped prGO hydro-­ and (b) prGO reference aerogel compared to 1 PLN coin (∅ = 23 mm, 5 g). Reproduced from ref. 92 with permission from Springer Nature, Copyright 2018.

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Figure 1.19  Images  of aerogels. (a) Macroscopic pieces of 7.5 mg mL−1 CNT aero-

gels. Pristine CNT aerogel (left) appears black, whereas the aerogel reinforced in a 1 wt% PVA bath (right) is slightly gray. (b) Three PVA-­ reinforced aerogel pillars (total mass = 13.0 mg) supporting 100 g, or ca. 8000 times their weight. (c) This scanning electron microscopy (SEM) image of a critical-­point-­dried aerogel reinforced in a 0.5 wt% PVA solution (CNT content = 10 mg mL−1) reveals an open, porous structure. (d) This high-­magnification transmission electron microscopy (TEM) image of an unreinforced aerogel reveals small-­diameter CNTs arranged in a classic filamentous network. Reproduced from ref. 94 with permission from John Wiley and Sons, Copyright 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

1.3.4  Fullerene/Graphene Aerogels Fullerenes are characterized by their high electron affinity.100,101 C60 for instance is capable of storing up to 6 electrons in its triply degenerated lowest unoccupied molecular orbital (LUMO), which corresponds to 0.1 electrons per carbon.100,102,103 For comparison, graphene can store ∼0.01–0.02 electrons per carbon within the electrochemical stability window of water.104–107 Cerón et al. integrated fullerenes in graphene aerogels to improve the electrochemical activity of fullerenes by taking advantage of the high electrical conductivity and surface area of GAs.108 The gravimetric current density of GA electrodes was increased upon physisorption of C60 and C60 monoadduct, which provided additional acceptor states in the form of the low lying LUMOs

Chapter 1

22

Figure 1.20  Electronic  microscope images showing the structural differences of

samples (a–c) SEM images of RGO and CNT/RGO aerogel. (d–f) TEM images of RGO and CNT/RGO aerogels. Reproduced from ref. 99 with permission from Elsevier, Copyright 2017.

of C60 and its derivatives. The hybrid GA-­C60 electrode showed ∼50% higher gravimetric peak current density than the pristine GA. Functionalization of GA with C60-­monoadduct doubled the gravimetric peak current density of the GA electrode (Figure 1.21).108 Further optimization of this hybrid system can be achieved by covalently bonding fullerene derivatives to the graphene backbone, thus providing higher electrochemical stability.

1.4  3D Printing Methods of Graphene Aerogels GAs have been used in several 3D printing methods; here we summarize some of the most recent examples.

1.4.1  Direct Ink Writing (DIW) Direct ink writing (DIW), also known as robocasting, is an extrusion-­based technique that involves the extrusion of ink through a fine nozzle, which is programmed to follow a toolpath that allows the construction of a 3D structure. The DIW technique employs a three-­axis motion stage to assemble 3D structures by robotically extruding a continuous “ink” filament through a micronozzle at room temperature in a layer-­by-­layer scheme. The prerequisite for this method is to design gel-­based viscoelastic ink materials possessing

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23

Figure 1.21  Cyclic  voltammogram of GA-­C60 monoadduct (red line) compared

with pristine GA (blue line). Calculated binding configuration of graphene-­C60 monoadduct hybrid with the addend of the fullerene close to the graphene. Reproduced from ref. 108 with permission from American Chemical Society, Copyright 2019.

shear thinning behavior to facilitate extrusion flow under pressure and a rapid pseudoplastic-­to-­dilatant recovery resulting in shape retention after deposition. This technique was first adapted by Zhu and co-­workers to form a 3D periodic microlattice of GAs.109 Zhu et al. fabricated high concentrations of aqueous GO suspensions (20–40 mg mL−1 GO) which exhibited shear thinning properties but lacked the stiffness to support its own weight while printing. To further enhance the stiffness and viscosity, hydrophilic silica particles were added to the ink. The 3D aerogels must remain wet during printing so that the liquid can be removed either by freeze-­drying or supercritical drying to prevent the collapse of pores under ambient conditions. Therefore, the printing process is carried out in the presence of an organic solvent (isooctane) immiscible with the GO ink. After subsequent gelation at 80 °C followed by supercritical drying, the DIW aerogel is thermally reduced under an inert atmosphere at 1050 °C to recover the graphene properties. The leftover silica particles are removed by etching in the presence of hydrofluoric acid. The physical properties of the 3D printed GA are like those of the bulk GA (Figure 1.22).109 The DIW aerogels had large surface areas (up to 1100 m2 g−1) and pore volumes (2–4 cm3 g−1), and carbon: oxygen ratios above 20. The electrical conductivity of the DIW aerogels varied from 87 to 278 S m−1. Further, the aerogel exhibited super-­compressibility of up to 90% of the compressive strain. The Young's modulus vs. density of the bulk and printed GAs obeyed the power-­scaling law (E ∝ ρ2.5), indicating that the failure mechanism was mainly bending dominated for these cellular materials. Interestingly, the engineered microlattice displayed higher (almost twice) Young's modulus for a given density when compared to bulk GAs. The electrical resistance of

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Figure 1.22  Log–log  plots of (a) apparent viscosity as a function of shear rate and (b) storage and loss modulus as a function of shear stress of GO inks with and without silica fillers. (c) Schematic of the fabrication process. Following the arrows: fumed silica powders and catalyst (that is, (NH4)2CO3 or R–F solution) were added into as-­prepared aqueous GO suspensions. After mixing, a homogeneous GO ink with designed rheological properties was obtained. The GO ink was extruded through a micronozzle immersed in isooctane to prevent drying during printing. The printed microlattice structure was supercritically dried to remove the liquid. Then, the structure was heated to 1050 °C under N2 for carbonization. Finally, the silica filler was etched using HF acid. The in-­plane centre-­to-­centre rod spacing is defined as L, and the filament diameter is defined as d. Reproduced from ref. 109, https://doi.org/10.1038/ncomms7962, under the terms of the CC BY license, http://creativecommons.org/licenses/by/4.0/.

the printed GA only slightly decreased under cyclic compression, confirming structural resilience. Highly stretchable aerogels were reported by Guo et al., by reinforcing GO inks with multi-­walled carbon nanotubes. Aerogels with a 200% elongation through hierarchical synergistic assembly were printed using the DIW method.110 Zhu et al. also applied DIW GA for energy storage applications, such as supercapacitors. Through the addition of graphite nano-­platelets (GNP) the electrical resistance of the DIW GA was lowered to ensure sufficient rate capability and capacitance of the DIW electrode.111

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25

The DIW GA electrode significantly outperformed its bulk GA counterpart and provides an example of how one can use 3D printed electrodes to overcome mass transport limitations and boost energy storage performance.112 Yao and Chandrasekaran et al. further exploited the advantage of 3D printed graphene electrodes as a conductive scaffold/current collector to increase the mass loading of MnO2, a pseudo-­capacitive material. The capacitive performance of the 3D electrode is not limited by ion diffusion even at extremely high mass loadings, which is impossible for conventional bulk electrodes. Most importantly, these findings validate the concept of “printing” practically feasible pseudo-­capacitor electrodes and devices.113 Jiang et al. also showed that printed GA can be structurally resilient and exhibit extraordinary capacitive rate and cycle performances.114 Responsive graphene inks were fabricated by García-­Tuñon and Barg et al., using GO sheets chemically modified with a branched polymer as a precursor. Structures with low concentrations of GO could be printed with high resolution (100 µm) by converting the ink to a ‘pseudo-­gel’ when the pH is 8, making it printable via DIW.115

1.4.2  Inkjet Zhang et al. achieved a low-­density 3D printed GA (10 mg cm−3) by combining drop-­on-­demand inkjet printing with freeze-­casting of GO suspensions.116 The technique involves rapidly freezing aqueous GO suspensions on a cold sink held at a temperature of −25 °C. Unlike the DIW method, low-­viscosity Newtonian fluids can be used for drop-­on-­demand inkjet printing. As the printing progresses, every new layer is deposited onto an already frozen layer, which upon contact melts and the low-­viscosity ink fills the voids between the layers and is re-­frozen again as the whole structure is still in contact with the cold sink. This ensures good adhesion between the layers because of interlayer diffusion. The authors observed that the 2D GO sheets are well aligned along the freezing direction in the 3D printed GA. The 3D printed structure exhibited good structural integrity due to the bonding between the layers when characterized under in situ compression up to 50% strain (Figure 1.23).116 The printed GAs possessed conductivities of 2–15 S m−1 as the density increased from 0.5 to 10 mg cm−3 and were electrically resilient when compressed multiple times. The relationship between Young's modulus vs. density of the printed GA exhibited a lower scaling index of 1.4 unlike the conventional monoliths which gave a value of 2.5. The authors attributed this to the designed 3D macroscopic hollow structures. The electromechanical properties were studied by monitoring the resistivity change as a function of compressive strain. 3D printed aerogels also exhibited remarkable super-­ elasticity over a temperature range from −100 °C to 300 °C.116

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Figure 1.23  3D  printing graphene aerogel (GA). (a–f) The 3D GA printing process.

(a) 3D printing setup. (b) 3D printing of ice support. (c) 3D printing of GO suspension. (d) Immersing printed ice structure into liquid nitrogen. (e) Freeze-­drying. (f) Thermally reduced to 3D ultralight GA on catkin. (g) 3D GA architecture, left: 2.5 structure and right: 3D architecture with overhang structures. (h) GAs with various wall thickness. Reproduced from ref. 116 with permission from John Wiley and Sons, Copyright 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

1.4.3  Freeze Gelation 3D printing of pristine GAs using room temperature freeze gelation (RTFG) was first introduced by Lin et al.117 Like DIW,118 RTFG involves extrusion of ink through a micronozzle assisted by a programmed tool path to build a 3D structure.117 In RTFG, freeze gelation at room temperature is possible because the ink consists of graphene, not GO, suspended in an organic solvent, such as camphene or phenol, whose melting point is above 20 °C. High vapor pressure solvents were selected to facilitate sublimation at room temperature. The final architecture of the 3D printed GA mimics the traditional DIW, but the microstructure is determined by the solvent used. For instance, rGO inks prepared with phenol as a solvent resulted in aerogels with lamellar, directional based microstructures as observed for aqueous suspensions.119 On the other hand, camphene-­based inks had rough interfaces more like

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27

metals, and since they solidify at room temperature, give the GAs a more random pore morphology (Figure 1.24).117 The printed GAs do not contain any chemical cross-­linkers to boost their mechanical properties and hence are reinforced with polymers. However, the use of pristine graphene yields aerogels with large surface areas (up to 700 m2 g−1), high electrical conductivities (up to 9 S cm−1) and densities of 20 mg cm−3. The RTFG aerogels also showed promising performance as electrochemical double-­layer capacitor electrodes with energy densities as high as 27 W h kg−1 and power densities up to 21 kW kg−1, which is among the highest reported for 3D printed aerogels. Freeze-­casting of GO was adapted by Wang et al.,120 to form radial and centrosymmetric structure within GAs. Through controlled formation of ice crystals in aqueous GO dispersions, aerogels with aligned channels and predetermined pore sizes were obtained.

1.4.4  Casting Complex shaped 3D graphene lattices were fabricated via the casting of GO suspensions in a 3D printed polymer mold by Zhang et al.121 In this study, GO/ethylenediamine (EDA) ink is poured into a hollow polymer architecture. The polymer mold with the desired wall thickness is generated through projection micro-­stereolithography. To obtain the 3D GAs, the GO/EDA ink is hydrothermally reduced, freeze-­dried and the polymer template removed via thermal etching (Figure 1.25). These hydrophobic GA lattices had densities as low as 1.6 mg cm−3, BET surface areas of ca. 15 m2 g−1 and electrical conductivities of 11–81 S m−1. When compressed, the unit cells of GAs exhibited elastic bending and compression deformation until reaching an elastic strain of 4%. Beyond that structure showed local yielding and fracture propagation until the whole structure collapsed. This work also investigated the chemical sensing capabilities of the 3D GAs and stated that these aerogels have a sensitive detecting response to acetone. When tested for other organic solvents, the detection sensitivity varied from 5% to 22%. In addition, the GA lattices showed potential sensitive chemiresistor properties for organic solvents, absorption capacity toward solid organics, such as asphalt, and good cycling stability.

1.4.5  Projection Micro-­stereolithography (PµSL) As noted above, many techniques have been used to print 3D aerogels with moderate structural control; however, they have all failed to directly demonstrate a truly arbitrary design space primarily due to the limits of the printing technique (e.g., toolpath requirement, casting, and serial writing). Thus, many of the 3D printed aerogels are still limited in their design and minimum feature size (>100 µm). To address this issue, Hensleigh et al. reported a process to 3D print GAs with essentially any desired architecture and resolutions an order-­of-­magnitude finer than any previously reported using a light-­based

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Figure 1.24  (a)  Schematic of the RTFG process. (b, c) Scanning electron microscopy (SEM) images of phenol‐based and camphene‐based aerogel structures cooled using liquid N2, an ice/water mixture, and at room temperature. Reproduced from ref. 117, https://doi.org/10.1002/ adma.201602393, under the terms of the CC BY 4.0 license, https://creativecommons.org/licenses/by/4.0/.

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Figure 1.25  The  schematic of 3D graphene lattices fabrication within hollow polymer architectures template Reproduced from ref. 121 with permission from American Chemical Society, Copyright 2018.

3D printing technique called projection micro-­stereolithography (PµSL).122 The key breakthrough of this technique was the development of photocurable GO resins that (i) rapidly solidify by light-­initiated polymerization, (ii) have strong light absorption to maintain small (µm-­scale) layer thicknesses, and (iii) have sufficiently low viscosity to allow dipping and recoating for the layer-­by-­layer processing (Figure 1.26).122 The resin is a dilute (1 wt%) GO dispersion with a small amount of photocurable acrylates (12 wt%) and photo-­initiator (2 to 4 wt%). The GO suspension consisted of crosslinked GO particles (XGO) made by ultrasonically dispersing a GO hydrogel monolith. It was shown that the crosslinked GO in the XGO resin led to higher surface area aerogels than simply using neat GO flakes in the GO suspension. The acrylates and initiator are the photoactive elements that allow PµSL printing by forming a temporary “green” structure that traps the XGO in the desired 3D architecture. The majority of the resin is solvent, N,N-­dimethylformamide (DMF) as it provides a stable GO suspension, and solubilizes the acrylates and photo-­initiator. The green structures are kept in solvent until dried either by supercritical or freeze-­drying processes to maintain surface area. Pyrolysis of the “green” structures eliminates the majority of the photopolymer and reduces the GO, yielding the complex hierarchical 3D micro-­architected graphene (MAG) assemblies (Figure 1.27).122 Using this technique, 3D printed GAs could be extended to gyroid and octet-­truss geometries with features as small as 10 µm. Specifically, MAG octet-­truss geometries exhibited much improved elastic moduli at low densities in addition to high surface areas and good electrical conductivities. MAG's essentially unlimited design space, high surface area and electrical conductivity opens the path to exploring mesoscale architectures for advanced GA applications including catalysis and separation platforms, tunable thermal conductivity, and fluid flow.122 Similarly, Feng and Li et al. successfully fabricated graphene reinforced poly nanocomposite via digital light processing (DLP), showing the great potential of current photocurable resins. 3D complex structures including a jawbone with a square architecture as well as gyroid scaffold for bone tissue engineering applications were printed.123

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Figure 1.26  Scheme  of resin synthesis. GO is first crosslinked (XGO) into a hydro-

gel monolith then dispersed by sonication into a gel fragment dispersion. The addition of acrylates and photo-­initiator creates the “XGO resin” and allows PuSL 3D printing, followed by drying and pyrolysis to the final microarchitected graphene (MAG). Reproduced/Adapted from ref. 122 with permission from the Royal Society of Chemistry.

Figure 1.27  (a)  MAG aerogel supported by a single strawberry blossom filament; (b, c) SEMs of MAGs showing porous nature of the struts; (d, e) Optical and SEM images of MAG gyroid showing intricate overhanging pore structures; (f) Zoomed in image of (g) showing the porous nature of the walls. Reproduced from ref. 122 with permission from the Royal Society of Chemistry.

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1.4.6  Fused Deposition Modelling (FDM) FDM is the most common 3D-­printing technique used to create complex 3D objects layer-­by-­layer by extruding molten plastic filaments (or metal wires) through a nozzle, while the nozzle moves along the three axes via a computer-­controlled mechanism such as DIW. This is typically a low precision technique which needs surface finishing treatment after extrusion and hardening, but offers other advantages, such as low fabrication costs and large-­scale printing. The two most widely used filaments in FDM are acrylonitrile-­butadiene-­styrene (ABS) and polylactic acid (PLA). Wei et al. first reported 3D printing of a graphene/ABS and graphene/PLA composite using this technique.124 Graphene-­ABS composites were prepared with different graphene loadings and were further extruded into 17.5 mm diameter filaments (Figure 1.28). These filaments were extruded through a 0.4 mm diameter nozzle onto a platform held at 80 °C. When the graphene loading exceeded 5.6 wt%, inhomogeneity and aggregation of graphene flakes resulting in clogging of the nozzle being observed. The highest graphene-­loaded printable composite (5.6 wt%) possessed an electrical conductivity of 1.05 × 10−3 S m−1. Foster et al. 3D printed free-­standing electrodes using graphene-­based mPLA filaments in a conventional FDM printer.125 The authors also built a 3D printed solid-­state supercapacitor (3D-­SC) to evaluate the potential of this 3D printable graphene filament for energy storage. Utilizing two 3D printed discs with only 8% graphene loading and a solid electrolyte sandwiched between the discs, a fully free-­standing supercapacitor could be created. A columbic efficiency of 85% was observed after 120 cycles, with an irreversible capacity reaching 3.69 mA h g−1 with respect to the weight of the 3D printed electrodes.125 Another promising application of these 3D printed electrodes was the hydrogen evolution reaction (HER) where a low overpotential for HER onset was observed even after the 1000th scan, thus making it the most beneficial electrode towards the HER of all the carbon-­based electrodes examined.125 Pumera et al. also reported a simple activation method for graphene/polymer 3D printed electrodes by a combined solvent and electrochemical route on a graphene/PLA filament.126

1.4.7  Laser-­based Methods There are a number of laser-­based techniques that have been used to 3D print carbon structures. Laser induced graphene (LIG) is formed by irradiating a carbon source with a laser, which photothermally converts the carbon to sp2-­hybridized carbon.127 3D LIG foams are fabricated by preparing layers of LIG through the irradiation of polyimide (PI) film. Initially, the PI film is exposed to a CO2 laser to form the first layer of LIG. The LIG layer is then coated with ethylene glycol (EG) which acts as a binding agent, before the next PI film layer is deposited. The sandwiched layers are then lased, and the

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Figure 1.28  Composite  preparation and 3D printing (a) Graphite flakes. (b, c) Dis-

persions of GO and ABS in NMP solvent. (d, e) Homogeneous mixture of GO-­ABS in NMP before and after chemical reduction by hydrazine hydrate at 95 °C for 1 h. Brownish GO-­ABS turned into black G-­ABS suspension during chemical reduction.(f) G-­ABS coagulations obtained after isolation (e) with water. (g) G-­ABS composite powder after washing and drying. (h) Schematic illustration of fused deposition modelling 3D printing process. Inset is the graphene-­based filament winding on a roller. The filament was deposited through a nozzle onto a heated building plated, whose temperature was set at 80 °C. (i) A typical 3D printed model using 3.8 wt% G-­ABS composite filament, scale bar: 1 cm. Reproduced from ref. 124, https://doi.org/10.1038/ srep11181, under the terms of the CC BY 4.0 license, http://creativecommons.org/licenses/by/4.0/.

process is repeated to build macroscale foams of LIG. After the process is complete to the desired height, the printed foam is dried at 200 °C to evaporate the remaining EG (Figure 1.29). Alternatively, the foam can be dried in a vacuum at 600 °C to remove any excess polymer residue.127 Sha et al. used selective laser melting (SLM) to fabricate a Ni/sucrose scaffold using a CO2 laser, where sucrose serves as a carbon precursor and Ni acts as a catalyst and template to form a free-­standing 3D GF.128 In situ synthesis of free-­ standing 3D GFs was successfully modeled by manually placing a mixture of Ni and sucrose onto a platform and then using a commercial CO2 laser to convert the Ni/sucrose mixture into 3D GFs, the Ni metal catalyzing the graphene growth from the sucrose carbon source. This technique combines powder metallurgy templating with 3D printing techniques (Figure 1.30).128 The 3D printed GFs show high porosity (∼99.3%) and low density (∼0.015 g cm−3).

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Figure 1.29  (a)  Optical images of the 3D printable graphene/PLA. (b) The 3D printing process. (c) A variety of printed 3DEs used throughout this study. Corresponding (d) SEM, (e) Raman, (f) XPS analysis of the printed 3DE. Reproduced from ref. 125, https://doi.org/10.1038/srep42233, under the terms of the CC BY 4.0 license, http://creativecommons.org/ licenses/by/4.0/.

The GFs have an electrical conductivity of ∼8.7 S cm−1, a remarkable storage modulus of ∼11 kPa, and a high damping capacity of ∼0.06. The printed 3D GFs showed minimal shrinkage of 20% in width. Shrinkage can be tuned by varying the size of Ni precursor and by selecting different carbon precursors. The two critical parameters in this technique are the laser duty cycle and the rastering speed. High quality graphene was obtained by using a fixed raster speed and high duty cycle.

1.4.8  Other Methods Apart from the well-­known techniques above, other methods such as stamping, templating, and laminated object manufacturing are also used to form graphene-­based electrode materials in 3D patterns. For example, Zhang et al. demonstrated rapid production of flexible micro-­super capacitors (MSC) through a scalable, low‐cost stamping strategy, wherein the authors combined 3D printed stamps and 2D titanium carbide or carbonitride inks (Ti3C2Tx and Ti3CNTx, also known as MXenes)129 The viscous, aqueous MXene inks (24 mg mL−1) are brushed onto 3D printed stamps of various electrode designs and printed on paper substrates to produce the different coplanar MSC electrodes. After deposition of the PVA/H2SO4 electrolyte on the electrodes, the

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Figure 1.30  (a)  Schematic of in situ synthesis of 3D graphene foam (GF) using a sim-

ulated 3D printing process. (b) Photographs of the 3D printed GF before and after dissolving the Ni. Reproduced from ref. 128, https://pubs. acs.org/doi/10.1021/acsnano.7b01987, with permission from American Chemical Society, Copyright 2017. Further permissions requests related to the material excerpted should be directed to the ACS.

Figure 1.31  (a–d)  Fabrication of all‐MXene‐based micro‐supercapacitors using the stamping strategy. Reproduced from ref. 129 with permission from John Wiley and Sons, Copyright 2018 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

MSCs exhibited good aerial capacitance. For scaling up the production of supercapacitors, the authors demonstrated a roll-­to-­roll compatible method that could create dozens of MSCs in seconds (Figure 1.31).129 Binder jet printing has also been used by researchers to 3D print thick graphene electrodes (300 µm) for supercapacitors.130 Thermally reduced GO

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powders densified by adding acetone are spread on the feed bed and the motion-­controller was pre-­adjusted to a layer height of 100 µm. The dimensional resolution of these structures is limited by the penetration of injected binder to adjacent powders to ∼1 mm. Supercapacitor electrodes decorated with palladium particles printed using this technique demonstrated high gravimetric and areal capacitance values of 260 F g−1 and 700 mF cm−2, respectively.130

1.5  Conclusion The development of advanced multifunctional materials for energy and environmental applications is becoming increasingly important from the perspective of sustainable development. Attributing to their structural integrity and interconnected porosity, graphene aerogels manifest extraordinary nanoscale effects that result in new material systems with superlative properties and novel functionalities. With advancement in processing techniques, rational tuning of the materials properties of such graphene-­ based 3D macrostructure is the key to enhancing their energy and environmental performance. The extraordinary properties of these new materials will further stimulate the next generation of technologies in the fields of energy storage, filtration and separation, catalysis and sensors, among others.

Acknowledgements This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-­AC52-­07NA27344.

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109. C. Zhu, T. Y.-­J. Han, E. B. Duoss, A. M. Golobic, J. D. Kuntz, C. M. Spadaccini and M. A. Worsley, Nat. Commun., 2015, 6, 6962. 110. F. Guo, Y. Jiang, Z. Xu, Y. Xiao, B. Fang, Y. Liu, W. Gao, P. Zhao, H. Wang and C. Gao, Nat. Commun., 2018, 9, 881. 111. C. Zhu, T. Liu, F. Qian, T. Y.-­J. Han, E. B. Duoss, J. D. Kuntz, C. M. Spadaccini, M. A. Worsley and Y. Li, Nano Lett., 2016, 16, 3448–3456. 112. S. Chandrasekaran, B. Yao, T. Liu, W. Xiao, Y. Song, F. Qian, C. Zhu, E. B. Duoss, C. M. Spadaccini, Y. Li and M. A. Worsley, Mater. Horiz., 2018, 5, 1166–1175. 113. B. Yao, S. Chandrasekaran, J. Zhang, W. Xiao, F. Qian, C. Zhu, E. B. Duoss, C. M. Spadaccini, M. A. Worsley and Y. Li, Joule, 2019, 3, 459–470. 114. Y. Jiang, Z. Xu, T. Huang, Y. Liu, F. Guo, J. Xi, W. Gao and C. Gao, Adv. Funct. Mater., 2018, 28, 1707024. 115. E. García-­Tuñon, S. Barg, J. Franco, R. Bell, S. Eslava, E. D'Elia, R. C. Maher, F. Guitian and E. Saiz, Adv. Mater., 2015, 27, 1688–1693. 116. Q. Zhang, F. Zhang, S. P. Medarametla, H. Li, C. Zhou and D. Lin, Small, 2016, 12, 1702–1708. 117. Y. Lin, F. Liu, G. Casano, R. Bhavsar, I. A. Kinloch and B. Derby, Adv. Mater., 2016, 28, 7993–8000. 118. J. A. Lewis, Adv. Funct. Mater., 2006, 16, 2193–2204. 119. L. Qiu, J. Z. Liu, S. L. Y. Chang, Y. Wu and D. Li, Nat. Commun., 2012, 3, 1241. 120. C. Wang, X. Chen, B. Wang, M. Huang, B. Wang, Y. Jiang and R. S. Ruoff, ACS Nano, 2018, 12, 5816–5825. 121. Q. Zhang, F. Zhang, X. Xu, C. Zhou and D. Lin, ACS Nano, 2018, 12, 1096–1106. 122. R. M. Hensleigh, H. Cui, J. S. Oakdale, J. C. Ye, P. G. Campbell, E. B. Duoss, C. M. Spadaccini, X. Zheng and M. A. Worsley, Mater. Horiz., 2018, 5, 1035–1041. 123. Z. Feng, Y. Li, C. Xin, D. Tang, W. Xiong and H. Zhang, C – J. Carbon Res., 2019, 5, 25. 124. X. Wei, D. Li, W. Jiang, Z. Gu, X. Wang, Z. Zhang and Z. Sun, Sci. Rep., 2015, 5, 11181. 125. C. W. Foster, M. P. Down, Y. Zhang, X. Ji, S. J. Rowley-­Neale, G. C. Smith, P. J. Kelly and C. E. Banks, Sci. Rep., 2017, 7, 42233. 126. M. P. Browne, F. Novotný, Z. Sofer and M. Pumera, ACS Appl. Mater. Interfaces, 2018, 10, 40294–40301. 127. D. X. Luong, A. K. Subramanian, G. A. L. Silva, J. Yoon, S. Cofer, K. Yang, P. S. Owuor, T. Wang, Z. Wang, J. Lou, P. M. Ajayan and J. M. Tour, Adv. Mater., 2018, 30, 1707416. 128. J. Sha, Y. Li, R. Villegas Salvatierra, T. Wang, P. Dong, Y. Ji, S.-­K. Lee, C. Zhang, J. Zhang, R. H. Smith, P. M. Ajayan, J. Lou, N. Zhao and J. M. Tour, ACS Nano, 2017, 11, 6860–6867. 129. C. Zhang, M. P. Kremer, A. Seral-­Ascaso, S.-­H. Park, N. McEvoy, B. Anasori, Y. Gogotsi and V. Nicolosi, Adv. Funct. Mater., 2018, 28, 1705506. 130. A. Azhari, E. Marzbanrad, D. Yilman, E. Toyserkani and M. A. Pope, Carbon, 2017, 119, 257–266.

Chapter 2

Structure–Property Relationships in 3D Graphene-­ based Macrostructures Kimal Chandula Wasalathilakea and Cheng Yan*a a

School of Mechanical, Medical and Process Engineering, Queensland University of Technology, Brisbane, Australia *E-­mail: [email protected]

2.1  Introduction Graphene, a monolayer of graphite, has received tremendous attention from the global scientific community because of its enormous specific surface area (2630 m2 g−1),1 excellent mechanical flexibility (Young's modulus of 1.0 TPa),2 superior electrical conductivity (200 S m−1), and outstanding thermal conductivity (2000–5000 W m−1 K−1),3,4 ever since its first successful isolation by Andre Geim and Konstantin Novoselov in 2004.5,6 As a result of its unique properties, graphene has emerged as an attractive candidate for a wide range of interesting and important applications in sensors,7,8 catalysis,9,10 medicine,11,12 environmental remediation,13,14 energy storage devices15–19 and so on. Consequently, various synthesis methods have been developed to produce high-­quality graphene, including epitaxial growth of graphene on metal or SiC substrates,20,21 chemical vapour deposition (CVD),22–24 chemical reduction,25,26 thermal reduction,27,28 electrochemical synthesis,29,30 and liquid phase exfoliation.31,32 However, due to strong π–π stacking and van der   Chemistry in the Environment Series No. 1 Graphene-­based 3D Macrostructures for Clean Energy and Environmental Applications Edited by Rajasekhar Balasubramanian and Shamik Chowdhury © The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

41

42

Chapter 2

Waals interactions, graphene sheets tend to form irreversible agglomerates or even restack to form graphite, hindering their many potential applications by reducing the accessible surface area as well as ion diffusion rates.33 Besides, the giant delocalized π electron system of pristine graphene makes it chemically inert, which in turn results in poor compatibility in end-­use systems, sluggish ion mobility, and weak reactivity.34 To overcome these limitations, extensive research efforts have been devoted to transforming graphene into self-­supporting 3D macrostructures so that undesirable restacking of graphene sheets could be prevented while retaining its inherent properties.35,36 Up to now, 3D GBMs with a variety of architectures have been developed, such as graphene networks, graphene fibres, graphene tubes, vertical graphene sheets, graphene cages and porous graphene films. A series of reviews have been published focusing on the preparation methods, properties, and application of 3D GBMs. However, dedicated reviews emphasizing on the fundamental understanding of structure–property relationships of 3D GBMs are relatively scarce. Therefore, in this chapter we summarize the recent progress in the development of 3D GBMs from a structural viewpoint, while highlighting the importance of pore size and morphology towards enhancing the overall performance of these materials.

2.2  Structure–Property Relationship in 3D GBMs 2.2.1  3D Graphene Networks 3D graphene networks including graphene hydrogels, aerogels, foams and sponges are the most frequently used 3D GBMs with favourable properties for various applications. The mass production of graphene networks is mostly carried out by self-­assembly methods. In self-­assembly approach, 2D graphene nanosheets form multiple ordered structures with interconnected networks via non-­covalent bonds, such as hydrogen bonds, van der Waals forces, electrostatic forces, hydrophobic–hydrophilic interactions, and π–π stacking interactions. As a classic example, Shi et al.37 prepared a self-­assembled graphene hydrogel (GH) by heating a GO dispersion in a Teflon-­lined autoclave at 180 °C for 12 hours (Figure 2.1a). The hydrothermally reduced GO had a well-­defined 3D interconnected porous network (Figure 2.1b). The 3D framework of GH was assembled by partial overlapping of flexible graphene sheets due to π–π stacking interactions. The pore sizes of the networks were in the range of submicrometre to several micrometres and the pore walls consisted of thin layers of stacked graphene sheets. The as-­prepared GH showed a high compressive elastic modulus of 290 kPa and a good electrical conductivity of 0.5 S m−1 due to the restoration of conjugated GO sheets during hydrothermal reduction, which formed the strong cross-­links of the 3D structure. Later, the same research group reported highly conductive GHs with conductivities of 1.3–3.2 S m−1, which were chemically reduced by hydrazine hydrate and hydrogen iodide, further removing residual oxygenated groups.38 The additional treatment of GHs

Structure–Property Relationships in 3D Graphene-­based Macrostructures

43

Figure 2.1  (a)  Photographs of a homogeneous GO dispersion and resulting hydrogel after hydrothermal synthesis. (b) Scanning electron microscopy (SEM) image of the hydrogel microstructure. Reproduced from ref. 37 with permission from American Chemical Society, Copyright 2010. (c) Image showing the compressibility of the aerogel. (d) Stress–strain curves of the aerogel at various densities at 10th cycle. Reproduced from ref. 40, with permission from the Royal Society of Chemistry. (e) High magnification SEM of graphene hydrogel. (f) Compressive stress–strain curves of graphene hydrogels. Reproduced from ref. 41 with permission from Elsevier, Copyright 2018.

with reducing agents had less impact on the pore structure; nevertheless, hydrazine hydrate treated GH expanded slightly due to gas release during reduction, while hydrogen iodide treated GH became more compact due to improved non-­covalent interactions. Qiu et al.39 developed ultralight and highly compressible graphene aerogels via an ethylenediamine (EDA)-­ mediated process, which led to the simultaneous functionalization and the reduction of GO as well as the assembly of a hydrogel with limited stacking. Although the highly porous graphene structure showed a low density of 3 mg cm−3, it could fully recover without fracture even after 90% compression. The improved mechanical properties could be attributed to the presence of 1D wrinkles along the surface of the assembled graphene sheets. Yu et al.40 prepared highly compressive graphene aerogels (GAs) by heating a mixture of GO and EDA at 80 °C for 24 hours, followed by freeze-­drying. The aerogels with densities ranging from 4.4 mg cm−3 to 7.9 mg cm−3, demonstrated good compressibility in air and organic liquids (Figure 2.1c and d). The electrical resistance of the aerogel varied under compression and was found to be proportional to the strain, as a better contact was made between graphene sheets due to the enhanced density. Recently, our group developed a pH assisted hydrothermal method to synthesize GHs with different 3D porous structures and found that despite having a low C/O ratio, the hydrogels prepared

44

Chapter 2

under acidic conditions give rise to high electrical conductivities due to the highly interconnected 3D porous structure providing conductive pathways41 (Figure 2.1e). Furthermore, GHs synthesized under acidic pH have high mechanical strength due to the compact dense structure consisting of thick pore walls and highly crosslinked 3D structure influenced by strong π–π interactions (Figure 2.1f). Furthermore, the introduction of polymer molecules as crosslinking agents enhance the mechanics of GO aerogels remarkably. As an example, Zhan et al.42 fabricated polyethylenimine (PEI)-­crosslinked GAs with large specific area (∼850 m2 g−1) and outstanding mechanical properties (20 MPa Young's modulus and 1 MPa yield strength) (Figure 2.2a–c). The GAs annealed at high

Figure 2.2  (a)  Schematic synthesis of La3+ and PEI-­crosslinked GO aerogels. (b) SEM image of PEI-­crosslinked GO aerogel. (c) Compression stress– strain curve of PEI-­crosslinked GO aerogel. Reproduced from ref. 42 with permission from John Wiley & Sons, Copyright 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Photograph of graphene aerogel supporting 2 kg counterpoise without deformation. (e) Compressive stress–strain curves of graphene aerogels. Reproduced from ref. 45, https://doi.org/10.1038/s41598-­017-­01601-­x, under the terms of the CC-­By 4.0 license, http://creativecommons.org/licenses/by/4.0/. (f) SEM image of graphene covered Ni nanowires after CVD. Resistance variation of graphene foam on (g) the degree of stretching and (h) stretching/releasing cycles. Reproduced from ref. 52 with permission from Elsevier, Copyright 2014.

Structure–Property Relationships in 3D Graphene-­based Macrostructures

45

temperature (2500 °C) resulted in enhanced thermal stability and improved electrical conductivity (∼550 S m−1) due to the high crystallinity and the low defect density.43 With the increase of annealing temperature from 1050 to 2500 °C, the aerogels showed resistance to oxidation with a nearly 200 °C improvement of the maximum oxidation temperature.43 Yan et al.44 prepared a series of GAs by mild reduction via various reducing agents and observed that the density and degree of reduction are the key factors for the electrical conductivity. By heat treating at 1500 °C and supercritical ethanol drying, Han and co-­workers prepared GAs with high BET surface area of 440.8 m2 g−1 45 (Figure 2.2d and e). The supercritical drying from solvents circumvents the formation of destructive differential capillary stresses of conventional evaporative drying by transferring the solvent into its supercritical state.46 Due to the high drying efficiency of supercritical ethanol, the as-­prepared aerogels showed homogeneously interconnected pores with sizes of less than 2 µm. Recently, various CVD methods have being developed to produce 3D graphene networks with low defect content and high quality.47–50 Cheng et al.51 reported a template directed CVD technique to fabricate 3D graphene foam (GF) using nickel (Ni) foam. In the macroscopic structure, the graphene sheets were seamlessly interconnected and the charge carriers could move rapidly with a small resistance. The flexible GF exhibited a high electrical conductivity of 10 S cm−1 and a specific surface area of ∼850 m2 g−1. Jung et al.52 synthesized a freestanding 3D GF using 3D Ni nanowire foam as a catalyst for CVD at a relatively low temperature of 670 °C (Figure 2.2f–h). The resulting interconnected porous structure displayed superior electrical conductivity (17.5 S cm−1) and excellent stretching, bending and folding stability. In template-­assisted techniques, the properties of 3D graphene networks highly depend on the morphology and the pore structure of the template. Most of the foam/foil and powder templates consist of relatively large pore sizes in the range of hundreds of micrometres and, as a result, the as-­obtained 3D graphene structures do not have enough bonding density. Therefore, recent attempts were made to develop 3D graphene networks with controlled morphology via CVD techniques using 3D-­printed templates.53,54 Ding et al.54 employed a 3D-­printed sacrificial silica template with complex-­ designed structure to fabricate a crack-­free and phase-­pure bicontinuous GF with an excellent electrical conductivity of 2.39 S cm−1 and a large surface area of 994.2 m2 g−1.

2.2.2  Graphene Fibres and Tubes Graphene fibres are generally made by controlled assembly of GO sheets into fibre-­t ype structures via hydrothermal treatment of a GO dispersion in a confined container55 or wet-­spinning of a concentrated GO liquid.56–61 Light and flexible graphene fibres, with strength comparable to carbon nanotube yarns, were fabricated by a facile one-­step dimensionally-­confined hydrothermal strategy from GO suspension.55 A GO suspension was injected into

46

Chapter 2

a glass pipeline of 0.4 mm in inner diameter and baked at 230 °C for 2 hours to produce graphene fibres with a uniform diameter of ∼33 µm. Although derived from GO sheets, the hydrothermally prepared graphene fibres had a tensile strength of up to 420 MPa and an electrical conductivity of ∼10 S cm−1. Gao et al.56 employed giant GO liquid crystals to achieve highly-­ordered, continuous graphene fibres by wet-­spinning, followed by wet-­drawing and ion-­ cross-­linking. The introduction of divalent ionic cross linkers (Ca2+ and Cu2+) improved the tensile strength from 184.6 MPa to 364.6 MPa and 274.3 MPa, respectively. After chemical reduction by hydroiodic acid, the tensile strength of Ca2+ crosslinked graphene fibre further improved up to 501.5 MPa. Furthermore, the graphene fibres showed excellent electrical conductivities in the range of 3.8–4.1 × 104 S m−1, about 40% higher than that of graphene fibres fabricated from small graphene sheets (2.5 × 104 S m−1),57 which can be attributed to the large size of graphene sheets together with their regular alignment in fibres. Graphene tubes consist of a tube-­like structure, which is quite similar to CNTs, although possess a larger diameter. Sacrificial templates are generally required to fabricate graphene tubes with walls composed of stacked graphene layers.62–65 Thong et al.62 reported a method to synthesize graphene tubes by CVD on Ni nanowire templates using ethylene at temperature around 750 °C. The diameter and the length of the tubes depend on the structure of the template, while the wall thickness depends on the number of graphene layers, which can be controlled by the growth time. Lin et al.63 proposed a 3D architecture of graphene tubes, grown on anodic aluminium oxide (AAO) templates, using a versatile ambient CVD technique (Figure 2.3a–c). According to transmission electron microscopy (TEM) imaging, the diameter of the isolated graphene tubes were estimated to be 80–90 nm and demonstrated an electrical conductivity of 950 S m−1 along the tube direction and 4.3 S m−1 along the direction vertical to channel. In another study, metre-­long graphene microtubings (µGTs) were developed with a tunable diameter of 40–150 µm based on Cu wires via hydrothermal reduction.65 A glass pipeline was used to define the fibre shape of the resultant sample, while the Cu wire provided the core for the subsequent formation of tubular structure. The as-­prepared µGTs were flexible and mechanically stable (tensile strength of up to 180 MPa), which could be effectively shaped into various geometries with controlled morphology on demand. Wu et al.66 prepared nitrogen-­doped graphene/graphene tube nanocomposite by heat treating a nitrogen/carbon precursor, dicyandiamide (DCDA), in the presence of iron species. The same group prepared bamboo-­like nitrogen-­doped graphene tubes (N-­GTs) with large diameters (100–200 nm) through a high‐temperature graphitization process of DCDA along with the subsequent deposition of Pt nanoparticles.67 The N-­GTs remarkably modified the geometry and electronic aspects of deposited Pt particles, as well as provided a large amount of complementary non-­ precious oxygen reduction reaction (ORR) active sites, thereby improving the ORR activity of Pt catalysts.

Structure–Property Relationships in 3D Graphene-­based Macrostructures

47

Figure 2.3  SEM  image of (a) surface and (b) cross-­section of graphene coated

Al2O3. (c) Thermal conductivity and sheet resistance of 3D graphene tubes. Reproduced from ref. 63 with permission from John Wiley & Sons, Copyright 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Synthesis process and structure of vertical graphene sheets. Reproduced from ref. 73 with permission from John Wiley & Sons, Copyright 2018 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. (e) SEM image of graphene-­encapsulated silicon microparticle. (f) TEM image of an individual graphene-­encapsulated silicon microparticle. (g) High-­resolution TEM image of the graphene layered structure. (h) TEM image of the hollow graphene cage after etching of silicon. Reproduced from ref. 84 with permission from Macmillan Publishers Ltd, Copyright 2016.

2.2.3  Vertical Graphene Sheets Vertical graphene (VG) sheets also referred to as “carbon nanowalls” are nanoscale flakes of 1–20 layers of graphene sheets that are typically oriented vertically on a substrate.68–70 Wu et al.71 first reported the well-­controlled growth of VG sheets on various types of substrates including silicon, stainless steel, Cu and GaAs under microwave plasma-­enhanced chemical vapour deposition (PECVD) conditions. The top edges of the wall-­like structures were composed of either folded double layers or unfolded single layers suggesting that some of the nanowalls were hollow shells with nanometre scale spacings. In another study, Hori et al.72 fabricated VG sheets on Si, SiO2

48

Chapter 2

and sapphire substrates via radio-­frequency PEVCD method assisted by H radical injection. The carbon walls grown on Si(100) substrate had a thickness of 10–30 nm with a height around 600 nm, and as the growth time was increased, vertical nanowalls combined with each other, resulting in the formation of linked nanowalls like a maze. Yu et al.73 reported the vertical growth of graphene sheets by a thermal CVD method, and the 3D structure was composed of densely arranged and interconnected edges exposed on the surface (Figure 2.3d). The pore size of the interspace between VG sheets were in the range of 20 to 100 nm, which were well below the pore sizes of 3D porous graphene reported in literature.74,75 The resulting structure demonstrated a high electrical conductivity of 3.4 × 104–1.2 × 105 S m−1, electromagnetic shielding of 60 932 dB cm2 g−1, and superhydrophobicity and superoleophilicity. VG sheets grown on a wide variety of substrates have been successfully employed in supercapacitors,76 Li-­ion batteries,77 fuel cells,78 surface analysis tools,79,80 sensors81 and light absorbers.82

2.2.4  Graphene Cages Many efforts have been made to construct graphene cages with controlled interior void spaces and porosity in recent years.83–85 The flexibility, mechanical integrity and high electrical conductivity of graphene cages offer remarkable properties for energy storage devices. Cui et al.84 used conformally synthesized multilayered graphene cages to encapsulate Si microparticles (∼1–3 µm) and employed it as an anode in Li-­ion batteries (Figure 2.3e–h). Si particles were initially coated with Ni so that it served as both the catalyst for graphene growth and the sacrificial layer to provide void space between graphene cage and Si. The multilayered structure of cages exhibited a wavy structure due to conformal graphene growth along the grains of deposited Ni on Si. The mechanically strong graphene cages could buffer the volume change of Si during galvanostatic cycling, allowing the particles to expand freely while retaining the electrical conductivity. In another study, a sulfur-­templated shrinkage strategy was used to prepare graphene cages encapsulated tin oxide, with high-­density and well-­defined void spaces85 (Figure 2.4a and b). By adjusting the sulfur content in the composite, the pore size could be changed from 3 nm to 15 nm. When used as an anode in Li-­ion batteries, graphene caged tin oxide composite delivered an ultrahigh volumetric capacity of 2123 mA h cm−3 along with good cycling stability. Wrinkled graphene cages fabricated via low-­temperature graphene synthesis using spiky nickel powder as a template, were used to host Li metal in Li-­ion batteries.86 The unique structure demonstrated improved mechanical stability, better Li ion conductivity and excellent solid electrolyte interphase for continuous robust Li metal protection.

2.2.5  3D Porous Graphene Films Van der Waals forces and π–π interactions cause GO and rGO sheets to stack together and form graphic structure, resulting in a significant loss of surface area. The reformation of the graphitic structure limits their large-­scale

Structure–Property Relationships in 3D Graphene-­based Macrostructures

49

Figure 2.4  (a)  High resolution TEM image of SnO2@graphene cage hybrid prepared from sulfur template. (b) Cycling performance of SnO2@graphene cage hybrid at a current density of 100 mA g−1. Reproduced from ref. 85, https://doi.org/10.1038/s41467-­017-­02808-­2, under the terms of the CC BY 4.0 license http://creativecommons.org/licenses/by/4.0/. (c) High magnification SEM image of 3D porous chemically modified graphene (CMG) film. (d) Specific capacitance of CMG film. Reproduced from ref. 100 with permission from American Chemical Society, Copyright 2012. (e) High resolution TEM image of silicon encapsulated graphene composite (Si@rGO). (f) Cycling performance of different anodes at a current density of 0.5 A g−1. (g) Long-­term cycling performance of Si@ rGO at a current density of 2.5 A g−1 for 1000 cycles. Reproduced from ref. 17 with permission from American Chemical Society, Copyright 2020.

application in energy storage devices due to poor ion accessibility. One effective strategy to overcome this limitation and retain the high surface area is to produce 3D porous graphene films by incorporating spacer materials between graphene sheets. CNTs,87,88 polymers,89,90 metal oxides,91,92 metal chlorides,93,94 metal nanocrystals,95,96 metal organic frameworks97,98 have been widely used as spacer materials to fabricate porous graphene films. Li et al.99 demonstrated that graphene sheets could remain largely separated in a solvated state, providing a highly open pore structure to allow electrolyte solution to easily reach the individual sheets. Despite being highly swollen by water, the film exhibited a good electrical conductivity with a sheet resistivity of 1860 Ω sq−1 due to the interconnected structure of individual sheets. Recently, polystyrene (PS) nanospheres,100,101 PMMA spheres102,103 and silica particles104 have been used as templates to prepare graphene films with highly-­ordered macroporous structures. Choi et al.100 developed 3D porous chemically modified graphene (CMG) films by using PS colloidal particles as a sacrificial template (Figure 2.4c and d). Due to the well-­defined interconnected pore networks with a uniform pore size of ∼2 µm, CMG films showed a high electrical conductivity of 1204 S m−1. After functionalizing with MnO2, the composite demonstrated a specific capacitance of 389 F g−1 at 1 A g−1 when employed as a supercapacitor electrode due to its large surface

Chapter 2

50 2

−1

area (194.2 m g ), which had the ability to facilitate rapid ion transport. Moreover, using an electrostatic self-­assembly method, our group developed a hierarchical nanostructure of Si nanoparticles, encapsulated inside graphene bubble films, and further anchored in a 3D graphene macroporous network as an anode material for Li-­ion batteries17 (Figure 2.4e–g). Due to the unique structure of the graphene bubble film which smoothly wrapped the Si nanoparticles with void spaces, the composite electrode demonstrated excellent electrochemical performance and structural stability. In addition, molecular dynamics simulations confirmed that graphene bubble film could effectively control the stress build-­up near the silicon surface. Apart from the aforementioned contents, Table 2.1 summarizes some of the 3D GBMs in terms of pore geometry and physiochemical properties.

2.3  Conclusions 3D GBMs with excellent physiochemical properties have proved highly promising in numerous energy and environmental applications in recent years. Their unique interconnected framework with exceptionally high surface area, ultrahigh porosity, high electrical and thermal conductivity and remarkable mechanical stability offer a great opportunity to explore graphene across a broad range of diverse applications. Self-­assembly and CVD are the basic methods to prepare 3D GBMs with specific porous structures. Although numerous strategies have been demonstrated to fabricate 3D GBMs with specific morphologies, still there are some technical challenges that need to be addressed. Since the properties of 3D GBMs directly rely on the porous framework, the precise control of pore size and porosity is extremely important to obtain the most favourable physiochemical properties for a specific application. However, according to most of the reported 3D GBMs, their porosity and pore size distribution have not been properly controlled. Self-­assembly methods have shown a lot of potential in mass production of 3D GBMs; however, the resulting materials do not possess a uniform pore size distribution and proper connectivity of pore channels. Furthermore, controlling the thickness of stacked graphene layers in 3D graphene structures built from the self-­assembly of GO sheets is quite difficult. Therefore, CVD could be considered as an effective technique to produce high-­quality graphene with well-­controlled pore structures; however, extreme conditions of temperature and pressure hamper the possibility of large-­scale production. To develop 3D GBMs with highly crystalized and tailorable graphene layers, more advanced templates and processes are still required. With regard to the large-­scale production of high-­quality 3D graphene structures for practical applications, feasible low-­cost techniques should be implemented. In conclusion, the precise control of surface area, pore size distribution, connectivity of pores, and improving electrical conductivity and mechanical stability of 3D GBMs, could be considered as the major technical challenges that need to be addressed in future research endeavours.

Table 2.1  Pore  geometry and physiochemical chemical properties of 3D GBMs. Structure

Pore size

Surface area (m2 g−1)

3D porous graphene

50–100 µm

500–600

N-­doped graphene framework

0.5–10 µm

280 ∼560

Graphene network Graphene/Fe2O3 aerogel MoSe2/graphene foam TiO2/graphene aerogel MnO2/graphene foam

10–13 and 50–120 nm 1.5–90 nm 4–6 nm and 0.1–5 µm 200–500 µm

316 496 204 392

Sn/graphene cages Fe2O3/graphene aerogel NiCo2O4/graphene sponge Nanoporous graphene foam CNT/graphene aerogel

0.5–36 nm 3–4 nm 2–5 nm and 50–500 µm 32.5 nm 1–100 nm

365 212 194 851 435

3D porous graphene 3D porous graphene 3D porous graphene

4–100 nm 3–5 µm 2–5 nm and 0.05–1 µm

344 151 1810

3D porous graphene

1–30 nm

1005

Co3O4/graphene film Graphene foam

0.1–2 µm 2–5 µm

142

Graphene cryogel Graphene fibre

3–20 nm 3.4–100 nm

195–244 884

Graphene hydrogel film

2–70 nm

414

Properties

Reference −1

Electrical conductivity: 60 000 S m LIB cathode rate performance: 109 mA h g−1 at 10C Electrical conductivity: 1000–1400 S m−1 Capacitance: 484 F g−1 at 1 A g−1 Electrical conductivity: 1200 S m−1 Electrical adsorption capacities: Cd2+ ∼434 mg g−1, Pb2+ ∼882 mg g−1, Ni2+ ∼1683 mg g−1, Cu2+ ∼3820 mg g−1 Capacitance: 151.2 F g−1 at 10 A g−1 LIB anode capacity: 650 mA h g−1 at 0.1C LIB anode capacity: 605 mA h g−1 at 0.59C Electrical conductivity: 5500 S m−1 Capacitance: 130 F g−1 at 0.1 mg cm−2 LIB anode capacity: 1245 mA h g−1 at 0.2 A g−1 LIB anode capacity: 1340 mA h g−1 at 100 mA g−1 Capacitance: 778 F g−1 at 1 A g−1 LIB anode capacity: 750 mA h g−1 at 300 mA g−1 Electrical conductivity: 2.8–7.5 S m−1 Desalination capacity: 633.3 mg g−1 in 35 000 mg L−1 NaCl Li–O2 cathode capacity: 5978 mA h g−1 at 3.2 A g−1 Electrical conductivity: 0.7–24.8 S m−1 Electrical conductivity: 1000 S m−1 Capacitance: 178 F g−1 at 1 A g−1 Electrical conductivity: 1 S m−1 Capacitance: 250 F g−1 and 1 A g−1 LIB anode capacity: 1108 mA h g−1 at 50 mA g−1 Electrical conductivity: 500–800 S m−1 Tensile strength: 3.2 MPa, Young's modulus: 7–40 MPa Capacitance: 110 F g−1 at 0.5 A g−1 Electrical conductivity: 133 S m−1 Electrical conductivity: 2600–4900 S m−1 Specific tensile strength: 188 kN m kg−1, Compression modules: 3.3 MPa Electrical conductivity: 192 S m−1 Capacitance: 186 F g−1 at 1 A g−1

105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125

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Chapter 2

Acknowledgements The financial support from the Australian Research Council (ARC) under Discovery Project (DP180102003) is greatly appreciated.

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Chapter 3

Flexible 3D Graphene-­based Electrodes for Ultrahigh Performance Lithium Ion Batteries Faxing Wang* Department of Chemistry and Food Chemistry, Technische Universität Dresden, Dresden 01062, Germany *E-­mail: faxing.wang@tu-­dresden.de, [email protected]

3.1  Introduction Lithium (Li) was discovered from a mineral (LiAlSi4O10) by a young Swedish chemist, Johan August Arfwedson, in 1817. Thereafter, his mentor (Jöns Jakob Berzelius) coined the word “lithium”.1 The low standard redox potential of Li/ Li+(−3.05 V vs. standard hydrogen electrode (SHE)) and the smallest atomic weight (6.95) among metals make Li an ideal anode for batteries with high energy density and high voltage.2,3 However, Li is so reactive that it can react violently when exposed to water or air. Therefore, to use Li as an anode, non-­ aqueous electrolytes and suitable cathode materials are mandatory. In 1958, William Harris suggested the use of carbonate-­based organic electrolytes for Li stripping/plating in his doctoral thesis.4 In the 1960s, initial efforts on cathode materials focused on graphite fluoride, metal sulfides, and metal

  Chemistry in the Environment Series No. 1 Graphene-­based 3D Macrostructures for Clean Energy and Environmental Applications Edited by Rajasekhar Balasubramanian and Shamik Chowdhury © The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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oxides. A Li primary battery using MnO2 cathode and Li anode in carbonate-­ based organic electrolyte was soon made commercially available by a Japanese company in 1975. Although Li primary batteries cannot compete with aqueous alkaline batteries and lead-­acid batteries in the market due to the high manufacturing costs and serious safety issues, the success of Li primary batteries has led researchers to make rechargeable Li batteries. In the 1970s, two scientists (Stanley Whittingham and Michel Armand) in Robert Huggins' group, proposed the concept of “electrochemical intercalation” based on “Host–Guest” chemistry.5,6 In 1976, Stanley Whittingham reported the first rechargeable Li ion batteries (LIBs) (Figure 3.1a), which was composed of Li metal anode and titanium disulfide (TiS2) cathode in organic LiClO4 electrolyte.7 Li ions can intercalate/deintercalate into/from the TiS2 crystal lattice during the discharge/charge processes. The operating voltage of the first LIB was, however, low (only 2.5 V vs. Li/Li+). Inspired by Whittingham's work, John Goodenough demonstrated LixCoO2 as a new layered cathode material for Li ion storage,8 which had a high operating voltage of up to 4 V. He believed that the low voltage of the metal sulfide (MS2) cathode was due to an overlap of d-­band of

Figure 3.1  (a)  Schematic illustration of the structure and working principle of the first LIBs. (b) Schematic electronic structure of sulfide and oxide cathodes with d-­band of Co3+/2+ and p-­band of the nonmetal (S and O). (c) Schematic illustration of the structure and working principle of the commercial LIBs based on the “rocking chair” model. (d) Schematic of the gravimetric and volumetric energy densities of several commercial rechargeable batteries.

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59 2−

the high valent metal (M ) with the p-­band of the nonmetal S (Figure 3.1b). Such an overlap may lead to an introduction of holes into the S2− 3p band and the formation of molecular ions (such as S22−), resulting in the inaccessibility of high oxidation states of Mn+ in LixMS2. Conversely, the location of the top of O2− 2p band is much below the top of the S2− 3p band (Figure 3.1b), making the higher valent state accessible in oxides. Despite the development of several cathode materials for Li ion storage, the commercialization of LIBs was still hindered for many years because suitable anode materials were still lacking. Early attempts for the commercialization of LIBs were unsuccessful owing to the direct use of Li metal as the anode. Typically, Li dendrites are always formed on the Li metal anode surface during the cycles. The excessive growth of dendrites at the anode can pierce the separator and cause a short circuit in LIBs. To avoid Li dendrite growth, scientists then turned their attention to the exploration of intercalation-­t ype anode materials for Li ion storage based on the early dual-­intercalation concept for batteries reported in 1938 by Walter Rüdorff and Ulrich Hofmann.9 In 1980, Michel Armand proposed the dual-­intercalation concept for LIBs,10 which was then experimentally demonstrated as the first “rocking chair”-­ type LIBs using LixWO2 anode and LiyTiS2 cathode by Bruno Scrosati.11 These “rocking chair”-­t ype LIBs work through the transport of Li ions between the layered anode and cathode materials without using Li metal. However, most of the intercalation materials known at the time had potentials close to each other. Thus, the early “rocking chair”-­t ype LIBs with two intercalation electrodes had a very low operating voltage ( Co2+ > Ni2+ ≈ Cu2+.110 The researchers also found that the ORR activity of the macrocyclic compounds is related not only to their properties and pyrolysis temperature but also to the supporter for the immobilization of macrocyclic molecules. Based on the reports, Vulcan XC-­72 active carbon usually used to be the supporter of macrocyclic compounds for ORR, owing to better electrical conductivity, high adsorption and specific surface area.111–114 In recent years, as novel carbon materials such as carbon nanotubes, porous carbon, graphene, and heteroatom-­doped graphene become more widely studied, the preparation of ORR catalysts by using these carbon materials to immobilized MPcs and MPys has attracted more attention.115–119 Basiuk et al. used single-­walled CNTs (SWNTs) to immobilize the MPcs via the CVD method to prepare the ORR electrocatalysts.120 The interactions between MPcs studied and nanotube sidewalls can obtain high binding energy, the prepared catalyst exhibits excellent catalytic ORR preformance.120 Li et al. prepared the four different FePcs combined with rGO, mesoporous carbon vesicle (MCV), and ordered mesoporous carbon (OMC) (Figure 6.16a).121 Among these carbon matrixes, the OMC has a large specific surface area and most surface active sites, which can disperse FePc molecules uniformly. The FePc/OMC exhibits the 4e− pathway in ORR in acid or alkaline electrolyte, which possesses higher catalytic ORR activity, better durability and superior stability toward methanol than the Pt/OMC catalyst (Figure 6.17b and c).121 In addition, the MPcs supported on 2D graphene exhibit high ORR performance, owing to the unique π–π interactions between graphene and MPcs. Cui et al. used graphene as the carrier to support the iron tetracumylphenoxy phthalocyanine (FePc(CP)4) and utilized the strong π–π interactions between graphene and FePc(CP)4 to further improve its catalytic ORR performance and

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Figure 6.16  (a)  Illustration of the preparations of the FePc/rGO, FePc/MCV, and

FePc/OMC as metal-­free catalysts for ORR. (b) CV curves of FePc and FePc/carbon materials in O2-­saturated 0.1 M KOH. (c) LSV curves of the Pt/OMC, FePc, and FePc/carbon materials in O2-­saturated 0.1 M KOH at 5 mV s−1. Reproduced from ref. 121 with permission from Elsevier, Copyright 2014.

durability.122 Graphene has high electron conductivity, flexibility and specific surface area, and thus it can be seen that graphene possesses special advantages as the carrier of MPcs. However, the 2D G is prone to accumulation due to π–π interactions and aggregation by van der Waals forces, thus losing its excellent properties. The strategy of building 3D G is one of the solutions to this problem. Sun et al. fabricated the 3D G via pyrolysis of the coal tar pitch coated MgO template (30 nm) to support for macrocyclic compounds CoPc, and the CoPc/3D-­G exhibited high catalytic ORR performance.12 The 3D G effectively avoids the issues of accumulation and aggregation and retains the good properties of graphene. The 3D interpenetrating pore structure greatly enhances the mass transfer function of catalysts. And above all, the π–π interactions between 3D G and MPcs increase the electronic cloud density and electron delocalization energy, thus the energy difference between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) would be smaller and the catalytic activity and selective of catalysts could be improved. In addition, owing to the π–π interactions, the bond between 3D G and MPcs is stronger and the durability of the catalysts is improved.

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Figure 6.17  (a)  HAADF STEM image, (b) EELS spectrum, (c–f) EELS mapping of

C, Fe, N and overlaid Fe and N of the area marked by yellow square in (a) for Fe@C-­FeNC-­2. (g) Experimental Fourier transforms at the Fe K-­edge of EXAFS data of three Fe@C-­FeNCs, and FePc and iron foil as references. (h) The deconvoluted N 1s spectra of FePc and three Fe@C­FeNC catalysts. (i) The Fe2p narrow scan spectra of FePc and three FeC-­FeNCs catalysts. ( j) Time-­dependent steady-­state ORR polarization curves of SCN− poisoned Fe@C-­FeNC-­2 measured in 0.1 M KOH. Reproduced from ref. 136 with permission from American Chemical Society, Copyright 2016.

6.5  3  D Transition Metal, N Codoped Graphene   (3D M-­Nx/G) It is known that transition metal-­carbon (M-­N-­C) catalysts are initially prepared by pyrolysis of transition metal macrocyclic compounds. In M-­N-­C catalysts, the metal-­N4 center is usually regarded as the active site for ORR.123,124 Although the ORR activity and durability of M-­N-­C increase after pyrolysis, the catalyst sintering could lead to the surface area and the catalytic activity of catalysts to decrease. This kind of catalyst catalyzes the ORR process

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mainly through the 2e transfer pathway and is likely to form the intermediate product H2O2, which can poison the catalyst.125 Yeager was the first researcher to report the preparation of M-­N-­C catalysts for ORR by pyrolysis of the non-­macrocyclic compound at high temperature.126 Gupta et al. used the cost-­effective N-­containing polymer polyacrylonitrile (PAN) and iron salt to substitute the expensive transition metal macrocyclic compounds to prepare non-­precious metal, non-­macrocyclic compound electrocatalysts for ORR.127 Since then, many researchers studying macrocyclic catalysts have turned to the research of transition metal non-­macrocyclic catalysts. They developed different forms of transition metal, nitrogen and carbon precursors and applied them to the preparation of carbon-­supported transition metal-­nitrogen coordinations (M-­Nx/C, M = Fe, Co) catalysts. In recent years, the simple and inexpensive precursors (PANI,128,129 PPY,130,131 phenanthroline,132,133 and polyquaternium) are usually pyrolyzed to replace the high-­cost transition metal macrocycle compounds (phthalocyanine and porphyrin compounds). The high-­temperature pyrolysis of transition metals (Fe, Co, Ni, et al.), nitrogen and carbon precursors has become a conventional method to obtain M-­Nx/C catalysts with high oxygen reduction catalytic properties. Although the M-­Nx/C catalysts prepared by this traditional pyrolysis method possess high catalytic ORR activity, there are also disadvantages such as low specific surface area, less pore structure and less exposed active sites. Thus, preparing the M-­Nx/C catalysts to possess high N-­containing, the large specific surface area, high electrical conductive and more exposed active sites is still the current research hotspot. Among the carbon-­based materials, graphene possesses a large specific surface area and flexibility, thus the M-­Nx graphene is one of the most promising non-­precious metal catalysts. Wang et al. used FeCl3·6H2O as the metal precursor, PANI as the C and N precursor, H-­MMT as the nanoreactor to synthesize the 2D Fe, N codoped graphene with high bifunctional ORR/OER performance.128 They proved that the Fe-­Nx site in Fe/N-­G was the main active sites for ORR. The present results show that 2D G-­like materials are easy to stack layer by layer, reducing the number of active sites. The 3D M-­Nx/G not only retains its intrinsic properties but also avoids the accumulation of layers of graphene to provide a large specific surface area, electronic transmission channel and more exposed active sites, and further improve the catalytic performance. In addition, the synergistic action between metal and N makes the catalyst show excellent catalytic ORR performance. Wang et al. used high N-­containing water-­soluble sewage agent polyacrylamide (PAM) as C and N-­precursors, FeCl3·6H2O as metal-­precursors, NaCl as the template via the green synthetic route to fabricate the bifunctional 3D Fe/N-­G electrocatalysts for the cathode of the Zn-­air batteries, and the overvoltage between OER and ORR (ΔE = Ej=10 − E1/2) is 0.771 V which is better than that of the 20 wt% Pt/C and RuO213. The addition of Fe ions all coordinated with the N to form Fe-­Nx from the XRD, HR-­TEM and XPS, and proved that the Fe-­Nx coordination site is the main active center by XPS and electrochemical tests.13 The active sites of Fe-­Nx/C materials have always been controversial. Hu et al. and Dodelet et al.

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reported that the encapsulated Fe3C nanoparticles were activated the surrounding graphitic layers making the outer surface of the carbon active toward ORR.134,135 Hu et al. synthesized a high active FeNC ORR catalyst containing Fe-­Nx coordination sites and Fe/Fe3C to investigate the active centers for ORR. The EXAFS analysis together with two control electrochemical experiment tests indicates the Fe@C nanoparticles can dramatically promote the activity of the neighboring Fe-­Nx sites for catalyzing ORR (Figure 6.17).136 Although the real active sites and whether Fe boosts the catalytic ORR activity of Fe-­Nx/C materials are still controversial, the Fe-­Nx/C materials remain one of the most promising catalysts to replace precious metal catalysts.

6.6  3  D GBM-­supported Transition Metal Oxide Catalysts Transition metal oxides have become a research hotspot owing to the advantages of abundant reserves, cost-­effectiveness, simple preparation and being more environmentally friendly. Transition metals have many valence states and various structures, which have a great influence on the electrocatalytic performance of catalysts. At present, transition metal oxides include single metal oxides, pyrochlore-­t ype oxides, perovskite oxides and spinel oxides.

6.6.1  Single Metal Oxide Catalysts Single metal oxides mainly include MnOx,137 CoOx,138 TiOx, FeOx, WOx, CeOx and possess a good development prospect in the field of fuel cell cathode catalysts. In particular, the Mn-­based oxides catalysts have widely concerned researchers owing to the advantages of abundant raw materials, cost-­ effectiveness and non-­toxicity. The Mn element has various valent states and Mn-­based oxides have various structures, such as MnO2, Mn2O3, Mn3O4, and Mn5O8. Due to the special octahedral structure and excellent ORR performance, MnO2 has become a research hotspot. In recent years, the research direction of MnO2 mainly focused on the influence of crystal structure, microstructure and particle size on catalytic ORR performance. Meng et al. investigated the effects of four most distinct structures MnO2 (α-­MnO2, AMO, β-­MnO2, δ-­MnO2) on the electrocatalytic OER and ORR activity in alkaline media, which follow the order: α-­MnO2 > AMO > β-­MnO2 > δ-­MnO2.139 In addition, the microstructures of MnO2 also have a great influence on the electrocatalytic activity, which includes nanoparticles, nanorods, nanowires, nanospheres, nanoflowers and nanotubes. Selvakumar et al. used theoretical calculation to prove that α-­MnO2 nanowire possesses higher catalytic ORR performance than either α-­MnO2 nanoparticles or α-­MnO2 nanotubes, owing to the large surface area of α-­MnO2 nanowire.140 Jiang et al. synthesized α-­MnO2 nanowire catalysts via hydrothermal reaction, which possesses better electrocatalytic ORR performance than MnO2.141 The α-­MnO2 nanowire was used for the air-­cathode of Mg-­air batteries and exhibits better performance – the

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peak power can be 96 mW cm The morphology of MnO2 is closely related to the catalytic ORR activity. Selvakumar et al. synthesized five types of α-­MnO2 nanostructures via hydrothermal reaction.142 The electrocatalytic activities are dependant on the five types of α-­MnO2 nanostructures and follow an order: nanowire > nanorods > nanotube > nanoparticles > nanoflower.142 The 1D nanostructure of α-­MnO2 nanorods can form a framework structure between α-­MnO2 nanorods to improve the ORR active sites and enhance the oxygen transfer. However, the poor conductivity of α-­MnO2 nanorods limits the electrocatalytic activity. To further improve the performance of α-­MnO2, carbon-­based materials such as carbon black, CNT, 2D G and 3D G are used to support the α-­MnO2 to increase the active sites and conductivity. Yue et al. prepared MnO2 nanorods with different supporting materials such as carbon black and multiwalled carbon nanotube (MWCNT) by the hydrothermal method for Mg-­air fuel cells.143 The MnO2 nanorods with MWCNT supporters (MnO2/MWCNT) have better ORR performance and the Mg-­air fuel cell with the MnO2/MWCNT cathode exhibits 70.47 mW cm-­2.143 Since then, Zhang et al. used hydroxyl nanotubes as the supporter to support the α-­MnO2 catalyst via the coprecipitation method.144 The hydroxyl carbon nanotubes (CNTs-­OH), which can improve the dispersity of the α-­MnO2 on the surface of CNTs-­OH, making the α-­MnO2 and CNTs-­OH combine firmly. MnO2 forms Mn–O–C bond with the hydroxyl group during heat treatment, which can improve ORR catalytic activity and stability of the catalyst.144 3D G has attracted extensive attention owing to the intrinsic superior properties of graphene, large specific surface area and more active sites, thus used 3D G as a supporter is a hotspot.26,145 Zhang et al. recently reported a novel work that used the carbon black, GO and 3D G as the supporters to support the α-­MnO2 nanorods to synthesize the α-­MnO2/C, α-­MnO2/rGO and α-­MnO2/3D G, respectively.26 The electrochemical tests showed that α-­MnO2/3D G nanorods possess higher ORR performance and durability than other catalysts and 20 wt% Pt/C. The Mg-­air fuel cell based on α-­MnO2/3D G exhibits a peak power density of 106.2 mW cm−2 (Figure 6.18).26 To sum up, α-­MnO2/3D G can uniformly deposit on the surface of 3D G, due to 3D G having more defects and a large specific surface area. In addition, the α-­MnO2 firmly combined with 3D G in the same plane is beneficial to promote electrical conductivity. The π bond of 3D G interacts with the molecular orbitals of α-­MnO2, which strengthens α-­MnO2 nanorods on the surface of 3D-­G to improve ORR performance.14

6.6.2  Spinel-­type Oxide Catalysts The chemical formula of a spinel-­t ype oxide is AB2O4, where the A-­site is a metal ion of +2 valence state, mainly including Mn, Co, Ni, Ca, Fe and Zn et al. The B-­sites are the metal ions of +3 valence states mainly including Co, Mn, Ni, Fe, Al and Cr.146,147 Metal ions in positions A and B occupy tetrahedral and octahedral sites in different proportions, respectively. According to the

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Figure 6.18  (a)  TEM and (b) HRTEM images of α-­MnO2/3D-­G. (c) STEM image and

EDX elemental mapping of (d) Mn, (e) O and (f) Mn, O and h. (g) LSV curves of α-­MnO2/3D-­G, α-­MnO2/rGO, α-­MnO2/C and 20 wt% Pt/C. (h) Discharge performances of α-­MnO2/3D-­G, α-­MnO2/rGO and α-­MnO2/C. Reproduced from ref. 26 with permission from Elsevier, Copyright 2019.

distribution of metal ions, spinel can be divided into normal spinel structure, anti-­spinel structure and mixed spinel structure. In order to distinguish the different structures more accurately, the chemical formula of spinel-­t ype oxide is A1–λBλ(AλB2–λ)X4. The ions of A1–λBλ lie in tetrahedral voids, and the ions of AλB2–λ lie in octahedral sites. When λ = 0, it is a normal spinel structure; when λ= 1, it is an inverse spinel structure. When 0 < λ < 1, it is a complex spinel structure. As shown in Figure 6.19a, MgAl2O4 has a typical normal spinel structure, with Mg2+ occupying the tetrahedral void and Al3+ occupying

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Figure 6.19  Representative  structures of (a) a normal spinel (MgAl2O4), (b) an

inverse spinel (NiFe2O4), and (c) a complex spinel (CuAl2O4) in different styles and views. The green and purple polyhedra correspond to octahedral and tetrahedral metal occupation sites, respectively. Representative A, B, and O defect sites in spinel AB2O4 have been illustrated in panel a. (d) Normal spinel (MgAl2O4) with (111), (311), and (400) view directions. Reproduced from ref. 148 with permission from American Chemical Society, Copyright 2017.

the octahedral site. The chemical formula of the anti-­spinel structure can be expressed as B(AB)X4. NiFe2O4 and is an inverse spinel structure; Ni2+ ions and half of the Fe3+ ions occupy the octahedral site, another half of the Fe3+ takes up the tetrahedral site, The formula can be expressed as Fe(NiFe)O4. The complex spinel structure is defined as the intermediate between normal spinel and inverse spinel. Taking CuAl2O4 as an example in Figure 6.19c, Cu2+ and Al3+ occupy both tetrahedral and octahedral sites. The chemical formula is (Cu1–λAlλ (CuλAl2–λ)O4).148 The spinel-­t ype oxides possess the advantages of cost-­effective, simple preparation and adjustable structure and morphology, which have attracted the wide attention of researchers to design and synthesize a variety of spinel-­ type oxides catalysts in the field of fuel cells, metal-­air batteries and the

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electrolysis of water. Complex spinel oxides have higher electron conductivity and can reduce the activation energy of electron migration. In addition, lattice defects in the spinel structure facilitate the doping and substitution of metal ions, which can realize the adjusting of electrocatalytic activity and promote the ORR activity of the catalysts.149 Li et al. synthesized the spinel Co3–xMxO4 with different crystalline structures through the adjustment and control of the structure.150 Through investigating the ORR performance of the different structures of Co3–xMxO4 with different rations of Co/Mn, Li et al. indicated that the cubic spinel CoMnO has a high Mn content which is beneficial to improve the ORR activity.150 Wu et al. studied the effect of a Co-­Fe-­ based spinel structure on electrocatalytic ORR performance.151 They tuned a Co-­Fe based spinel structure from its normal to the inverse and then back to its normal as increasing the Fe content (Figure 6.20), and indicated that the inverse a Co[FeCo]O4 spinel possesses the highest ORR activity among all spinel structures in alkaline electrolyte. DFT results proved that the dissimilarity effect of Fe and Co atoms at the octahedral sites can modulate the adsorption energy, and the inverse spinel structures possess has

Figure 6.20  The  spinel structures of Co[Co2]O4, Co[FeCo]O4, and Co[Fe2]O4 and

the corresponding oxygen adsorptions and ORR activities (Fe green, Co blue, absorbed O magenta, lattice O red) Reproduced from ref. 151 with permission from John Wiley and Sons, Copyright 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

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longer O–O bonds. Xu et al. prepared a 3D hierarchical porous spinel CoFe2O4 hollow nanospheres via the hydrothermal method.152 The multistage channel hollow nanosphere structure is favorable for oxygen transfer and easy to form the active sites for a three-­phase reaction and improve the ORR performance.152 Devaguptapu et al. prepared three types of NiCo2O4 catalysts with significantly distinct morphologies using template-­free, soft and hard templates by the hydrothermal method (Figure 6.21).153 They systematically investigated the effect of the morphology of NiCo2O4 on the corresponding catalytic ORR/OER performance.153 In addition, the other method to improve the electronic conductivity, active sites and surface area of the spinel-­t ype oxides catalysts is to combine the spinel-­t ype oxides with other highly conductive materials including CNT, GO, graphene and 3D G, etc. Tong et al. prepared a mesoporous NiCo2O4 nanoplate array on 3D G foam with more active sites and accessible surface area, which increases the ion diffusion and electron transfer to improve the catalytic ORR activity.154 In recent years, Zhang et al. have reported a series of spinel-­t ype oxides array on 3D G (Figure 6.22). First, they prepared a NiCo2O4 with spinel structure through control of the ratio of Ni and Co.15 The interconnected hierarchical 3D G was synthesized with the CaCO3 template, which can produce CO2 during the pyrolysis. The spinel NiCo2O4 was supported on

Figure 6.21  Three  morphologies of NiCo2O4 catalysts synthesized from (a) template-­ free, (b) P-­123 soft template, and (c) SiO2 hard-­template methods, respectively. The SEM images are presented from low to high magnifications. Reproduced from ref. 153 with permission from American Chemical Society, Copyright 2017.

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Figure 6.22  Schematic  illustration of the α-­MnO2/3D G, NiCo2O4/3D G, MnCo2O4/3D G and CoFe2O4/3D G preparation from Zhang et al.14,15,25,26

the hierarchical 3D G (NiCo2O4/3D G), the hierarchical 3D G not only has more defects to allow NiCo2O4 nanoparticles to deposit evenly on its surface but also can enhance the mass transfer channel. Meanwhile, the interaction between the π bonds of 3D G and the d orbital of spinel oxides can facilitate NiCo2O4 nanoparticles to bind tightly to graphene to improve the catalytic ORR activity and durability.15 After that, in order to further improve the catalytic ORR performance of the spinel-­t ype oxides, Li et al. used +2 valence Mn to instead of +2 valence Ni to adjust the position of Mn and Co in the space of tetrahedron and octahedron to adjust the crystal structure of the Mn-­Co spinel-­t ype oxides (MnCo2O4).14 The MnCo2O4 was supported on the hierarchical 3D G (MnCo2O4/3D G) and possesses higher ORR performance.14 In addition, Zhang et al. prepared CoFe2O4 catalysts supported on different supports (CoFe2O4/C, CoFe2O4/CNTs, CoFe2O4/3D G) to investigate the effect of the supports on the catalysts.25 Owing to the hierarchical interpenetrating channels, large surface area, more active sites and mass transfer channels of 3D G, the conductivity of the CoFe2O4 is increased and the CoFe2O4/3D G catalyst possesses the highest electrocatalytic performance.25

6.6.3  Pyrochlore-­type and Perovskite-­type Oxide Catalysts Pyrochlore-­t ype oxide catalysts are novel inorganic materials with octahedral structures, which belong to the vertical centripetal crystal systems, and their generic chemical formula is A2B2O7.155 Figure 6.23 shows the

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Figure 6.23  The  8-­coordinate A-­site is presented in green while the 6-­coordinate B-­site is presented in blue. The O atoms are shown as red spheres. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Reproduced from ref. 156 with permission from Elsevier, Copyright 2013.

structural diagram of the pyrochlore-­t ype oxides.156 The A-­site positive ions are +3 valence state, including Ca, Na, Ba and Fe, etc. The B-­site positive ions are +4 valence state, including Zr, Ta, Nb and Ru et al. The irregular shape of pyrochlore-­t ype oxides possesses more defects, which is beneficial to enhance the electronic conductivity to improve the electrocatalytic activity. The perovskite-­t ype oxide chemical formula is ABO3.157,158 As shown in Figure 6.24, the A-­site is mostly alkali and rare-­earth metals, mainly including Sr Ba, La Ca, etc., located in the hole of the octahedron. The B-­site is mostly occupied by transition metal ions, mainly including Ni Co Cr Fe, etc., located at the vertex of the octahedron.159 Perovskite-­t ype oxide is considered a promising cathode oxygen reduction catalyst because of its stable structure and high electrocatalytic activity, good electronic conductivity, strong oxidation resistance and low cost. Recent studies have shown that the ORR catalytic performance of catalysts can be improved by doping or replacing A-­site or B-­site elements with other elements and controlling the atomic valence of the corresponding components. Zhu et al. reported a facile silver nanoparticle-­decorated perovskite oxide (Sr0.95Ag0.05Nb0.1Co0.9O3–δ) for low-­ temperature solid oxide fuel cells (LT-­SOFCs).160 Ag nanoparticles that are strongly bonded to the surface of the Sr0.95Ag0.05Nb0.1Co0.9O3–δ could improve the catalytic ORR activity and durability. The Sr0.95Ag0.05Nb0.1Co0.9O3–δ possesses a lower area-­specific resistance value, reflecting a higher catalytic ORR activity.160 Lu et al. reported La0.8Sr0.2MnO3 (LSM) perovskite nanorods as high active electrocatalysts fabricated via a soft template method.161 LSM perovskite nanorods have a microporous structure, resulting in more defects and higher specific surface products. The results showed that the LSM perovskite nanorods possess excellent catalytic activity and stability, which are better than the commercial Pt/C catalyst.

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Figure 6.24  Schematic  diagram of perovskite. Reproduced from ref. 159 with permission from John Wiley and Sons, Copyright 2017 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

6.7  Conclusion An excellent, cost-­effective ORR catalyst needs to have advantages of more defects, exposed active sites, oxygen adsorption–desorption ability, enhanced mass transfer and to be able to cleave the O=O bond. 3D GBMs materials have been regarded as promising catalysts to replace precious metal catalysts, owing to the intrinsic properties of graphene and the advantages of large specific surface area, high mechanical strength, better electron conductivity and enhanced mass transfer. Recently, much research on ORR catalysts revolves around 3D G, such as the heteroatom-­doped 3D G, 3D G supported transition metal macrocyclic compound, 3D M-­Nx/G, (M: Fe, Co, etc.) and 3D G supported transition metal oxide nanocatalysts. The heteroatom can significantly modulate the characteristics and functions of 3D G materials, and its low preparation cost and high catalytic activity have aroused the extensive interest of researchers. Among the heteroatom-­ doped 3D G, N-­3D G is a research hotspot at present. Identifying its active sites and how to promote the content of active sites and controlled the pore size and specific surface area of 3D G to enhance mass transfer are the directions of future development. In the 3D G supported transition metal macrocyclic compound materials, the π–π interactions between 3D G and MPCs increase the electronic cloud density and electron delocalization energy, thus the energy difference between the LUMO and the HOMO would be smaller and the catalytic activity and selective of catalysts could be improved. Through pyrolyzing the transition metal macrocyclic compounds can synthesize the M-­N-­C catalysts, the M-­Nx sites are regarded as the main active centers for ORR. The 3D M-­Nx/G materials are prepared by pyrolyzing the simple and inexpensive nitrogen

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and transition precursors replaced the high-­cost transition metal macrocycle compounds. The cost-­effective 3D M-­Nx/G materials have the intrinsic properties of 3D G, and are rich in M-­Nx active sites that greatly enhance ORR catalytic activity, which could improve the commercial progress of the fuel cells. At present, the 3D M-­Nx/G catalysts contained M-­Nx active sites exhibit excellent ORR catalytic performance in alkaline conditions, superior to commercial Pt/C. However, the alkaline fuel cells (AFC) are still in their infancy, and the proton exchange membrane fuel cells (PEMFC) with acidic electrolytes are developing rapidly. Therefore, the development of 3D M-­Nx/G cathode catalysts in the future should tend to maintain high ORR activity in acidic conditions. The transition metal oxides include single metal oxides, pyrochlore-­t ype oxides, Perovskite oxides and spinel oxides. 3D G with multistage channels has a higher specific surface area, and the interpenetrating structure of multistage channels can provide an enhanced mass transfer channel, which is conducive to improving the ORR catalytic performance of the catalyst. In future research, the regulation of spinel morphology, structure and interaction with the carrier should be carried out to further improve the ORR performance. To sum up, 3D G provides numerous possibilities for applications due to its unique and superior mechanical, electrical, and physical properties. At present, many researchers have applied 3D graphene-­based macrostructures (3D GBMs) materials to the preparation of cathode catalysts for the fuel cells. Based on their cost-­effective, high ORR activity and durability, 3D GBMs can be effective substitutes for Pt/C as cathode catalysts in fuel cells.

List of Abbrevations 3D GBMs 3D graphene-­based macrostructures ORR Oxygen reduction reaction 3D G Three-­dimensional graphene 3D M-­Nx/G 3D transition metal/N codoped graphene G Graphene 2D Two-­dimensional GO Graphene oxide SEM Scanning electron microscope CVD Chemical vapor deposition PMMA Poly(methyl methacrylate) N-­3D G N-­doped 3D G OPD o-­phenylenediamine rGO Reduce GO CNT Carbon nanotube CNF N-­doped carbon nanofiber VA-­NCNTs Vertically aligned nitrogen-­containing carbon nanotubes DFT Density functional theory SSM Sacrificial support method

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PANI Polyaniline PPY Polypyrrole MMT Montmorillonite B-­3DrGO Boron-­doped 3D reduced graphene oxide B-­THF Borane–Tetrahydrofuran B–N(C2H5)3 Borane–triethylamine Py Porphyrins Pc Phthalocyanines SWNTs Single-­walled CNTs MCV Mesoporous carbon vesicle OMC Ordered mesoporous carbon LUMO Lowest unoccupied molecular orbital HOMO Highest occupied molecular orbital PAN Polyacrylonitrile M-­Nx/C Carbon-­supported transition metal-­nitrogen coordinations PAM Polyacrylamide MWCNT Multiwalled carbon nanotube CNTs-­OH Hydroxyl carbon nanotubes

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Chapter 7

3D Graphene-­based Scaffolds with High Conductivity and Biocompatibility for Applications in Microbial Fuel Cells Ashish Yadav*a and Nishith Verma*b a

Department of Chemical Engineering, Indian Institute of Technology Roorkee, Uttarakhand, India; bDepartment of Chemical Engineering, Indian Institute of Technology, Kanpur, Uttar Pradesh, India *E-­mail: [email protected]; [email protected]

7.1  Introduction Microbial fuel cell (MFC) technology offers an environmentally benign and sustainable solution to wastewater treatment in addition to the generation of bioenergy. It directly converts the chemical energy of biodegradable substances to electrical energy in the presence of exoelectrogenic microorganisms. The microorganisms catalytically oxidize the biodegradable organic substrate at the anode, yielding protons and electrons. The protons diffuse then through a proton exchange membrane while electrons pass through

  Chemistry in the Environment Series No. 1 Graphene-based 3D Macrostructures for Clean Energy and Environmental Applications Edited by Rajasekhar Balasubramanian and Shamik Chowdhury © The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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an external circuit to reach the cathode. Electrochemical reduction reaction occurs at the cathode in the presence of oxygen to form water. The present thrust in the MFC technology is on the fabrication and enhancement of its performance. In this context, researchers have focused on identifying several types of cell configurations and exploring different electrodes (both anode and cathode), bioactive microorganisms, and electrolytes.1 Moreover, electrical energy harvested from an MFC can be used in different bioelectrochemical systems for the production of value-­added chemicals, such as hydrogen (H2) in microbial electrolytic cells (MECs) or water desalination in microbial desalination cells (MDCs).2 However, the main bottlenecks of this technology are low energy output and high fabrication cost. Therefore, recent research is geared towards exploring various types of electrodes which can enhance power generation. The anode material plays a prominent and vital role in the performance of MFCs. The anode should be highly conductive (low electrical resistance), biocompatible, chemically stable, mechanically resilient, and possess high porosity. Biocompatibility is an important aspect as the formation and adhesion of biofilm occur at the anode, which in turn is responsible for the metabolism of the exoelectrogenic microorganisms and electron transfer. With regard to electron transfer mechanism, the transfer occurs via two ways: (i) use of external mediators, such as thionine,3 neutral red,4 methyl blue3 and benzyl viologen3 and (ii) mediator-­less electron transfer through electrochemically active microorganisms. External mediators, however, add to the total cost and are sometimes toxic. Therefore, electrochemically active microbes which facilitate mediator-­less electron transfer are preferred in MFCs. In this context, several electrochemically active microorganisms have been used in MFCs namely, Shewanella putrefaciens,5 Aeromonas hydrophila,6 and Geobacteraceae sulfurre,5 etc. The most extensively used microorganism listed in the literature is Escherichia coli (E. coli). There are several reasons associated with the use of E. coli. The electron transfer in the case of E. coli is facilitated via two mechanisms: (i) extracellular electron transfer initiated by excretion of redox compounds, and (ii) electron transport through pili/cytochromes.7,8 Another important aspect of the MFC technology is the choice of cathode material. Oxygen reduction reaction (ORR) occurs at the cathode. Therefore, a suitable electrocatalyst is required to lower the overpotential for the ORR. The most commonly used cathode catalyst in MFC is Pt. Various electrode materials have been explored for their potential application in MFCs. Amongst them, carbon-­based materials have attracted tremendous interest, owing to their high electrical conductivity, chemical stability, and biocompatibility. Diverse forms of carbon have been used as electrodes in MFCs, namely, carbon felt,9 cloth,10 carbon nanotubes (CNTs),11–13 paper,14 brush,15 and mesh.16 These materials are either directly used in MFCs or dispersed with metal or metal-­oxide nanoparticles (NPs). Examples of noble metals used are Au,17,18 Pt,11,12,14 and Ru,9 and those of transition metals are Ni,19,20 Al,16 Mn,21,22 and W.23 Carbon nanomaterials dispersed with metal NPs are particularly preferred because they possess a large surface area,

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small charge transfer resistance and high catalytic activity towards ORR at the cathode. This in turn results in a high power density. Over the last decade, one of the promising carbon materials that has been extensively explored is graphene. Graphene has been used in various energy applications such as supercapacitors, fuel cells, flexible and wearable electronics, sensors, batteries, and solar cells, amongst others.24,25 The primary reasons for its diverse applications are its high surface area, excellent electrical and thermal conductivity, remarkable mechanical strength, and outstanding electrocatalytic properties. Graphene is a sp2 hybridized 2D layer of carbon atoms arranged in a hexagonal lattice structure. The delocalized π-­electrons of graphene sheets results in high carrier mobility. In view of the above merits, graphene-­based electrodes have been used in MFCs with enhanced power generation. Of late, the focus of researchers has shifted towards processing two-­ dimensional (2D) structures into self-­standing 3D scaffolds. Such 3D modification plays a prominent role in improving the performance of the anode materials. Various forms of 3D scaffolds have been used as electrode material in MFC application, such as graphene-­based Ni foam,26 CNT-­based sponge,27 graphene-­based aerogels,28 graphene foams,29 and graphene-­based stainless steel (SS) fibers.30 The graphene/carbon-­based 3D scaffolds enhance the electroconductivity in the synthesized matrix and facilitate electron transfer to the anode surface. The advantage of transforming 2D materials to 3D macrostructures is to attain a high surface to volume ratio, thereby creating roughness on the electrode surface. This in turn results in greater formation and adhesion of the biofilm at the anode and enhancing the interaction between anode and microorganisms. Furthermore, it has been well documented in the literature that there is a limited biofilm formation at the anode surface if a flat 2D surface is used rather than a 3D surface.31,32 The current chapter, therefore, specifically focuses on the applications of 3D graphene-­based macrostructures (GBMs) as electrode materials in MFCs. It addresses the state-­of-­the-­art development in 3D GBMs for MFC applications, their synthesis and fabrication schemes, advantages over their peers, performance assessment in terms of power generation, as well as associated technological challenges for large-­scale commercialization.

7.2  G  raphene-­dispersed Laser-­ablated 3D Carbon Micropillars Graphene-­based electrodes have been extensively used in MFCs. Similar 3D GBMs have also found applications in other bioelectrochemical systems, such as MECs. MECs are used for the production of H2 (a clean and renewable energy source) from organic substrates. The principle concept is similar to that of MFC technology with a slight modification, i.e., it requires a supply of an external voltage in addition to anaerobic conditions in the cathode chamber. An advantage of producing H2 from MEC technology is that a lower

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voltage (0.4–0.6 V) is required as compared to that required for water electrolysis (∼1.2–1.4 V). Therefore, the H2 evolution reaction (HER) overpotential is much smaller in the case of MEC than that required for water electrolysis. A recent study pertaining to the use of 3D GBMs in MEC application has focused on the synthesis of reduced graphene oxide (rGO)-­dispersed carbon film.24 Ni NPs were incorporated in the prepared carbon matrix to enhance HER. 3D micropillars were then engraved on metal-­carbon composite film using laser ablation methodology. The purpose of engraving the film to create 3D micropillars was to augment the exposure of dispersed Ni NPs and rGO. The laser ablation also enhanced surface to volume ratio, thereby creating surface roughness, conducive for microbial colonization and adhesion.

7.2.1  Electrode Synthesis The electrode fabrication procedure has been schematically presented in Figure 7.1. The preparation is based on suspension polymerization reaction, using phenol as the monomer, formaldehyde as a solvent and triethylamine as the basic catalyst. The cross-­linking agent and the binder for the reaction were hexamethylenetetramine and polyvinyl alcohol, respectively. The reaction mixture was heated to an optimized temperature (∼60 °C), followed by the addition of nickel nitrate salt and graphene oxide (GO) (synthesized using standard Hummer's method) to the reaction mixture at the onset of

Figure 7.1  A  schematic illustration of the synthesis of Ni-­rGO-­PC-­L and its appli-

cation in MECs. Reprinted from ref. 24 with permission from Elsevier, Copyright 2019.

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gel formation. The prepared polymeric mixture was cast in Teflon molds of the required shape and dried to synthesize the polymeric mass. The polymeric material was carbonized at high temperature (∼900 °C) under an inert atmosphere (using N2 gas) to convert GO to rGO, in addition to conversion of NiNO3 to NiO. This was followed by H2-­reduction at a temperature determined by temperature-­programmed reduction analysis, to transform NiO to Ni NPs. Thus, a carbon film composite consisting of Ni NPs and rGO was prepared. Fabrication of 3D micropillars was carried out on the prepared carbon film (Ni-­rGO-­PC-­L) using an Epilog laser instrument (30 W). A pulsed ytterbium source was used for fiber laser engraving. Laser parameters, viz., speed, dots per inch, and power were optimized to operate the instrument in raster mode for creating 3D micropillars. A square cross-­sectional pattern of the micropillars of dimensions 500 µm × 500 µm and a height of ∼100 µm was fabricated. An equidistant spacing of 500 µm was maintained between the micropillars.

7.2.2  Surface Morphology The surface morphology of the electrode material is examined using scanning electron microscopy (SEM). The SEM images showed GO as a flake-­like structure (see Figure 7.2a and a'). The presence of Ni NPs, dense flakes of rGO and carbon were clearly visible in the SEM images of the carbonized and H2-­reduced film (Ni-­rGO-­PC) (See Figure 7.2b and b'). SEM images of laser-­ ablated electrode (Ni-­rGO-­PC-­L) clearly show 3D micropillars having dimensions of ∼500 µm × 500 µm, with the same interspacing, i.e., 500 µm (See Figure 7.2c and c'). An assessment of the biocompatibility and the impact of the transformation of 2D to 3D electrodes was made by performing the SEM analysis of Ni-­rGO-­PC and Ni-­rGO-­PC-­L electrodes after their use in MEC test (See Figure 7.2d and d'). It was clearly evident that a dense and uniform microbial colonization appeared in the 3D electrode, corroborating the fact that the creation of the 3D structure created roughness in the electrode leading to enhanced biofilm formation and adhesion.

7.2.3  Electrochemical Characterizations 7.2.3.1 Cyclic Voltammetry (CV) The electrochemical response of the prepared electrode was assessed by performing CV analysis. A three-­electrode assembly was used for analysis, consisting of the working electrode, reference electrode (Ag/AgCl), and counter electrode (Pt). No significant redox peaks were observed in the base material (electrodes without Ni and rGO) (PC) (see Figure 7.3a). The electrodes consisting of Ni NPs and/or rGO showed much higher capacitive as well as Faradaic currents. The redox peaks were, therefore, more significant in the Ni NPs-­ and rGO-­containing electrodes. The laser-­ablated 3D micropillar-­containing electrode showed the highest capacitive currents, in addition to significantly

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Figure 7.2  SEM  images of (a and a') GO, (b and b') Ni-­rGO-­PC, (c and c') Ni-­rGO-­ PC-­L, (d) biofilm formation on Ni-­rGO-­PC and (d') Ni-­rGO-­PC-­L. Reprinted from ref. 24 with permission from Elsevier, Copyright 2019.

distinct redox peaks. It is evident that the laser-­ablated electrodes showed peak current values twice of those for the unablated electrodes (Ni-­rGO-­PC), ascribed to the 3D micropillars providing an increased biofilm formation and enhancing the interaction between the microorganisms and electrode. Also, laser ablation enhanced the exposure of dispersed Ni NPs and rGO within the material to the electrolyte.

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Figure 7.3  (a)  CV and (b) LSV analysis, and (c–c') Nyquist plots for the EIS analysis

of the electrodes. Reprinted from ref. 24 with permission from Elsevier, Copyright 2019.

The Brunauer–Emmett–Teller (BET) surface area analysis reveals that the prepared electrode possesses a hierarchical porous structure. It is well established in the literature that micro-­mesoporosity enhances the growth of biofilm and diffusion of electrolytes at the electrode surface.33,34 Therefore, the analysis further corroborated that the laser-­ablated samples facilitated the biofilm formation and growth, as was evident from the CV analysis. Moreover, microbial colonization was augmented by secretion of adhesions on pili and fimbriae of E. coli. Also, electron transfer occurred from the c-­t ype cytochrome of E. coli to the electrode surface in the presence of the metal NPs and carbon nanomaterials.20,35

7.2.3.2 Linear Sweep Voltammetry (LSV) LSV analysis was carried out to assess the efficacy of the electrodes towards HER. The data clearly show that overpotential for HER decreased and current density increased as the electrodes were modified with Ni NPs and rGO (see Figure 7.3b). It is well evident from the documented studies that Ni NPs

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act as an electrocatalyst towards HER. For comparison, experiments were also conducted employing a standard Pt/C electrode. It is noteworthy to mention here that the laser-­ablated electrode (Ni-­rGO-­PC-­L) exhibited lower overpotential and higher current density than Pt/C electrode, highlighting the importance of the 3D structure in addition to the roles of Ni NPs and rGO towards enhancing the MEC performance. The LSV data for different electrodes were also in agreement with the CV data.

7.2.3.3 Electrochemical Impedance Spectroscopy (EIS) EIS analysis was conducted to gain insight into the internal resistances related to MEC, viz., solution resistance (Rs), charge transfer resistance (Rct) and the Warburg diffusion resistance (W). Nyquist plots were used to determine the values of the resistances (see Figure 7.3c and c'). The analysis revealed Rs, Rct, and W values of 4.5, 1.8 and 10 Ω, respectively on a Ni-­rGO-­PC-­L electrode, which were the lowest in comparison to the resistances recorded for all other electrodes. Moreover, a 1.5 to 4-­fold reduction in the internal resistances was observed for the laser-­ablated electrode in comparison with the unablated electrode. The lower values of the internal resistances corroborate to a lower HER overpotential and higher current generation in MEC.

7.2.4  Biocompatibility of the Electrode The biocompatibility of the electrode material is vital in MFC applications. An antibacterial test was, therefore, performed to determine the biocompatibility of the synthesized electrode, using E. coli. An antibacterial plate count technique was utilized. A control and broth containing the prepared electrode were assessed for the antibacterial effects, if any, for 72 h. It was observed at the completion of the test that the number of colony-­forming units (CFU) in the control (approximately 108 CFU mL−1) was similar to that in the broth dipped with the electrode, confirming that the electrode material was biocompatible with the anolyte solution. Furthermore, sufficient literature is available to showcase that Ni or carbon-­based electrodes have been amply utilized in fuel cell applications without any inhibitory effects caused to the microorganism-­laden anolyte solutions.20,38 A performance evaluation of the electrode towards H2 production in MEC revealed a maximum production rate of 4.84 ± 0.24 m3 m−3 d−1 and a yield of 2.96 ± 0.14 mol H2 mol−1 substrate at an external applied voltage of 1 V. The combined synergistic contributions of Ni NPs, rGO and 3D carbon micropillars were responsible for high H2 production rate and yield. Ni NPs acted as electrocatalyst for HER at the cathode and facilitated electron transport. rGO enhanced the electroconductivity of the prepared electrode. The 3D micropillars induced roughness on the flat 2D surface and enhanced surface to volume ratio, facilitating biofilm formation. The exposure of dispersed Ni NPs and rGO was augmented by modification of the 2D surface to a 3D structure. The synthesized electrode exhibited better performance than the

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conventionally used Pt/C electrode. Therefore, it can potentially serve as a substitute to the uneconomical noble metal-­incorporated electrodes presently utilized in fuel cell applications. A similar method of fabricating 3D micropillars was used in another study pertaining to MFC application.31 The electrodes were prepared as per the aforementioned procedure with slight modification. Open circuit potential of ∼0.746 V, current density of ∼17 046 mA m−2, and maximum power output of 2496 mW m−2 were generated using the synthesized electrode in MFC. The performance of the electrode was higher than most of the electrodes used in the literature for MFC application.

7.3  Graphene Aerogel (GA)-­based 3D Electrodes 7.3.1  High Capacitative 3D GA Anodes 3D porous graphene aerogels (GAs) are promising candidates for applications in MFCs. In a recent study, GAs were synthesized using the hydrothermal reduction method and used as anodes in MFC.39 The prepared electrode showed high performance, owing to high capacitance, sufficient biocompatibility, and enhanced microbial growth and adhesion on the electrode. The study also introduced the idea of a long-­term electricity generation stability test to assess the performance of the electrode.

7.3.1.1 Electrode Fabrication GA was synthesized from a GO precursor, as per the Hummer's method with some modification. Briefly, GO was dispersed in deionized water, ultrasonicated, and then centrifuged. The supernatant liquid was discarded. The suspension of GO was dried. An optimized composition of the GO suspension was mixed with ascorbic acid to facilitate chemical reduction. The mixture was then heated to form a 3D graphene hydrogel. The hydrogel was washed multiple times using deionized water to remove any impurities, followed by freeze-­drying to synthesize GA. A schematic representation of the configuration and the use of GA as an anode in MFC is presented in Figure 7.4.

7.3.1.2 Biocompatibility of the Electrode Biocompatibility of the electrode, which plays a vital role in power generation by MFCs, was demonstrated by conducting the SEM analysis. SEM images showcase dense growth of the biofilm on the porous electrode surface as well as within the pores. To highlight the contrast in biofilm formation, SEM images of the bare carbon paper with biofilm were also captured. It was clear that microbial growth on the plain carbon paper was much less than in the 3D porous GA. This was attributed to the rough surface which provided a greater number of active sites for biofilm adhesion, thereby confirming good biocompatibility of the electrode. Moreover, the presence of hydrophilic

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Figure 7.4  Schematic  illustration of porous GA with microorganisms and extracellular electrons (left) and the fundamental configuration of microbial fuel cells (right). Reprinted from ref. 39 with permission from Elsevier, Copyright 2018.

functional groups at the electrode surface further enhanced microbial film formation.40,41 In addition, electroconductive nanowires present at the bacterial surface facilitated electron transfer. Thus, the more biofilm formation, the higher the electron transfer. The MFC based on the 3D GA-­based electrode showed maximum output voltage and power density of 0.488 V and 2381.44 mW m−3, respectively. The long-­term electricity generation stability of 100 h was achieved, indicating high capacitive characteristics of the synthesized 3D porous GA anode electrode. The specific capacitance of 3670 F m−2 was measured for the 3D GA electrode in comparison to 10 F m−2 for the bare carbon paper electrode. The efficacy of the electrode was attributed to porous 3D configuration, thereby signifying its potential in commercialization and large-­scale MFC applications.

7.3.2  GA-­modified 3D Graphite Fiber Brush (GFB) Electrode A unique configuration of 3D-­3D structured electrode material has been synthesized using the freeze-­drying technique.42 In this study, the anode consisted of GFB modified with graphene oxide aerogel (GOA). An 18-­month

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Figure 7.5  Schematic  diagram for the fabrication of the GOA-­GFB electrode. Reprinted from ref. 42, with permission from Elsevier, Copyright 2016.

assessment of the synthesized bioanode was conducted to gain insight into the successful operation and performance of the electrode in MFCs.

7.3.2.1 Electrode Synthesis A 3D graphite fiber brush (GFB) consisting of a spiral structure was taken as a substrate. GO was prepared from graphite powder using a modified Hummer's technique. The 3D-­3D structured electrode was synthesized by immersing GFB in the GO suspension at an optimized concentration. The dispersed mixture was subjected to ultrasonication, drying and vacuum freeze-­drying under a liquid N2 atmosphere to create a 3D-­3D configuration-­based GOA-­ GFB electrode. A pictorial representation of the synthesis procedure is given in Figure 7.5. This study, therefore, presents a facile and one-­step synthesis of a 3D-­3D structured electrode. The performance measurements of the electrode when used in MFCs showed ∼400–fold higher power generation than that in the bare GFB electrode. The MFC exhibited a maximum power density of ∼54 W m−3 after 18 months of operation, which was much higher than the MFCs reported in the literature. The higher performance of 3D-­3D structured electrode over the long term was attributed to the increased accessible specific surface area, relatively better extracellular electron transfer, and higher bacterial loading.

7.3.3  Nitrogen-­doped Graphene Aerogel Electrode (N-­GA) Recent studies of fuel cells have extensively focused on heteroatom doping, such as nitrogen (N). N-­doping is known to enhance the transfer of electrons, generated at the anode, via the external circuit. A study along a similar line has been reported focusing on the synthesis of N-­GA.43 Such 3D N-­doped matrix showed high electrochemical performance, owing to 3D macroporous structure facilitating bacterial loading both at the exterior as well as interior portion of the electrode, along with enhanced electroconductivity due to N-­doping. The BET surface area analysis revealed a hierarchical porosity

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consisting of micro-­, meso-­, and macropores. This further corroborated the fact that a high bacterial formation occurred on such an electrode surface, as is amply reported in the literature.

7.3.3.1 Electrode Preparation A facile technique based on the hydrothermal method was used for the preparation of N-­GA. In this method, GO suspension was mixed with concentrated ammonium hydroxide solution and heated in an autoclave reactor. This resulted in the formation of N-­doped graphene hydrogel. The hydrogel was treated with deionized water to remove any impurities, followed by freeze-­drying to yield a 3D network of N-­GA.

7.3.3.2 Bacterial Colonization (Biocompatibility) The biocompatibility of the electrode was corroborated from SEM images. A distinct difference was observed between the bacterial colonization on N-­GA and bare carbon cloth electrode. The dense growth of biofilm on N-­GA was ascribed to the high surface area of the 3D electrode and N-­doping. N-­doping introduced N-­containing functional groups such as amine, which imparted positive charge to the graphene sheets.44,45 Therefore, microbes with negative charges adhered on the surface of such electrodes, owing to electrostatic attractive forces, resulting in an enhanced bacterial colonization and power generation in MFC. Yang et al. described a simple and facile technique to prepare 3D porous N-­GA electrode.42 MFC with N-­GA as electrode exhibited maximum power density and open circuit potential of ∼225 W m−3 and ∼0.69 V, respectively. The high surface area of the 3D electrode, i.e., both interior and exterior portions provided access to microbial colonization. Moreover, the hydrothermal preparation technique can be easily commercialized and scaled-­up for MFC applications.

7.3.4  3D Pt NP/GA Composite The synthesis of 3D GA decorated with Pt NPs as a free-­standing anode for MFC application is described by Zhao et al.46 This study, for the first time, used 3D Pt NP/GA-­based MFC to operate an electrical device (timer). The 3D macroporous structure increased the immobilization of microbes. The Pt NPs facilitated electron transport. A suspension of GO was mixed with an ammoniacal solution, followed by ultrasonication and heating to form GA. The mixture was washed multiple times with deionized water and freeze-­dried to form a 3D network of GA. The synthesized 3D GA was dispersed in ethylene glycol and chloroplatinic acid. The mixture was subjected to microwave irradiation and washed with deionized water. The washed materials were dried in a vacuum oven to synthesize a 3D Pt NP/GA. The high performance of a Pt NP/GA anode-­based MFC was

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evident from the measured current and power densities of 4.88 A m and 1460 mW m−2, respectively, which were much higher than the other GA and carbon-­based electrodes reported in the literature.

7.4  3D Graphene Foams 7.4.1  M  acroporous Graphene/Multi-­walled CNTs (MWCNTs)/ FeO Foams An interconnected 3D network of graphene sheets modified with MWCNTs and Fe3O4 nanospheres (G/MWCNTs/Fe3O4) was synthesized via a one-­pot methodology using a combination of solvothermal and freeze-­drying techniques.47 Numerous factors such as macroporous structure (3D form), good electroconductivity, enhanced affinity to microbial adhesion, and efficient extracellular electron transfer contributed to the high performance of the electrode when used as an anode in MFCs.

7.4.1.1 Electrode Fabrication A schematic representation of the synthesis steps for 3D G/MWCNTs/Fe3O4 foam and its application in MFC is illustrated in Figure 7.6. It involves a combination of hydrothermal and cryogenic treatments. A mixture of MWCNTs, FeCl3·6H2O and GO-­dispersed ethylene glycol solution was ultrasonicated in

Figure 7.6  Fabrication  of the 3D macroporous G/MWCNTs/Fe3O4 foams and their

use as MFC anode materials. Reprinted from ref. 47 with permission from American Chemical Society, Copyright 2016.

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an ice bath followed by the addition of sodium acetate to synthesize a stable suspension. The suspension was heated in an autoclave reactor. The hydrothermal treatment resulted in the reduction of Fe3+ to Fe3O4 (nanospheres) in the mixture of sodium acetate, used as an alkali source, and ethylene glycol, used as the reducing agent. The prepared 3D structure was washed several times with deionized water, followed by freeze-­drying to synthesize a 3D macroporous structure of G/MWCNTs/Fe3O4 hybrid foam. The electrode was fabricated by sticking the synthesized 3D foams on both sides of a SS mesh, using a carbon paint as the conductive glue.

7.4.1.2 Physiochemical Characterization of G/MWCNTs/Fe3O4 Foam Morphology of the synthesized 3D foam was examined using SEM and transmission electron microscopy (TEM) (see Figure 7.7a and b). An interconnected network of macropores is clearly evident in the SEM image. The Fe3O4

Figure 7.7  SEM  (a) and TEM (b) images of 3D macroporous G/MWCNTs/Fe3O4 foams.

(insets) The high-­magnification images. (c) XRD patterns of MWCNTs (I), Fe3O4 (II), and 3D macroporous G/MWCNTs/Fe3O4 foams (III). (d) XPS survey spectrum of 3D macroporous G/MWCNTs/Fe3O4 foams, the inset corresponding to the Fe 2p spectrum. Reprinted from ref. 47 with permission from American Chemical Society, Copyright 2016.

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nanospheres reside on both sides of graphene sheets, whereas MWCNTs occupy the intercalation cavity between the sheets. The TEM image corroborates that Fe3O4 nanospheres and MWCNTs were uniformly dispersed over the surface of graphene sheets. X-­ray diffraction (XRD) analysis (see Figure 7.7c) reveals all characteristic peaks of Fe3O4 and MWCNTs in the 3D composite. X-­ray photoelectron spectroscopy (XPS) analysis (see Figure 7.7d) shows C 1s, Fe 2p and O 1s bands in the prepared 3D material. The magnified spectrum shows two additional peaks which correspond to Fe 2p3/2 and Fe 2p1/2 peaks of Fe3O4. Such novel 3D graphene-­modified foams provide a promising alternative for anode materials in MFCs due to long-­term electrode stability and high power generation (maximum power density of 882 W m−3). Besides providing a high surface area for enhanced bacterial colonization, such electrodes retain their activity over a long duration, owing to enhanced metabolism of iron-­reducing bacteria in the presence of Fe3O4.

7.4.2  Flexible 3D Graphene-­Ni Foam A conducting scaffold of 3D Ni foam dispersed with rGO was synthesized and used as anode in MFC.26 The electrode showed high performance than other carbon-­based electrodes and plain Ni foam discussed in the literature, which can be ascribed to high effective surface area, better microbial compatibility, and efficient electron transfer through highly conductive Ni foam.

7.4.2.1 Preparation of Electrode Ni foam substrate was introduced in an autoclave, initially filled with GO suspension. The mixture was heated to an optimized temperature and allowed to cool. It was washed with deionized water and air-­dried. It was then subjected to annealing at high temperature under a H2 atmosphere to enhance its electrical conductivity. The prepared rGO-­Ni foam composite was attached to a Ti wire using a conductive silver epoxy. Figure 7.8a presents a pictorial representation of the synthesis of rGO-­Ni foam using hydrothermal method. SEM analysis was performed to get an insight into the morphology of the synthesized electrode. A continuous 3D scaffold structure of Ni foam with large pores, having sizes between 100 and 500 µm, was clearly visible in the SEM image (Figure 7.8b). The SEM image of the rGO-­Ni composite, post hydrothermal treatment in an autoclave, followed by annealing under H2 atmosphere was also captured (Figure 7.8c). It is evident from the image that rGO was uniformly dispersed over the surface of Ni foam. The mechanical flexibility of the electrode is showcased by the digital image (Figure 7.8d). The electrode can be easily bent, rolled into a cylindrical

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Figure 7.8  (a)  A schematic diagram illustrates the preparation of rGO-­Ni anode.

(b,c) SEM images and digital pictures (insets) of plain nickel foam and rGO-­Ni foam. Scale bars are 200 µm. (d) Digital picture of a curved rGO-­Ni foam. Inset: rGO-­Ni foam rolled up into a cylindrical shape. (e) Digital picture of a 25 cm × 20 cm rGO-­Ni foam. Reprinted from ref. 26 with permission from the Royal Society of Chemistry.

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form, and used in different shapes without any mechanical failure. A digital image of the (25 cm × 20 cm) fabricated electrode is also presented, showing the efficacy of the synthesis procedure to be used for large-­scale MFC applications (Figure 7.8e). The synthesized 3D rGO-­Ni foam anode electrode could pave the way for the scale-­up of the MFC technology, considering that the electrode is flexible and can have different shapes, with ease of handling. Also, a high power density (661 W m−3) was generated using a 3D rGO-­Ni-­based MFC, which is the highest value reported in the literature for MFCs using a pure strain of Shewanella oneidensis MR-­1.

7.5  3  D Macroporous-­monolithic Graphene Modified with Polyaniline (PANI) Synthesis of a 3D macroporous anode electrode consisting of conductive, flexible, and free-­standing monolithic graphene foam dispersed with PANI is reported for the first time by Yong et al.29 The electrode performs better than planar carbon electrodes, which can be ascribed to a highly conductive framework of the material and 3D interface, providing better biofilm formation and electron transfer. The Ni foam substrate was subjected to chemical vapor deposition (CVD) to synthesize 3D graphene foam. PANI was deposited on the surface of the graphene foam using polymerization reaction, with aniline used as the monomer and ammonium persulphate used as the catalyst. The polymerization reaction yielded a 3D PANI/graphene foam composite. SEM images of the PANI/graphene 3D network and plain carbon cloth post-­MFC operation using S. oneidensis MR-­1 bacterial strain are presented at low and high magnifications in Figure 7.9a–c and Figure 7.9d–f, respectively. A dense growth of the bacterial film is evident on the exterior surface of both electrodes, viz., PANI/graphene and carbon cloth (Figure 7.9b and e). However, insignificant biofilm formation was observed at the interior surface of the plain carbon cloth (Figure 7.9c and f ). The images confirm the efficacy of 3D graphene-­based networks in enhancing the biofilm formation, both at the exterior as well as interior surfaces of the electrode. Moreover, PANI electrostatically interacted with negatively charged bacterial film, considering that it possessed a positive charge in a neutral medium.48 The PANI/graphene-­based anode showed higher power density than other graphene-­based anode electrodes and carbon-­based electrodes with/without PANI (see Table 7.1). This study, therefore, reports a 3D graphene-­based composite modified with conducting polymer (PANI) as an effective anode material and a potential option for usage in large-­scale MFC applications.

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Figure 7.9  SEM  images of graphene/PANI (a–c) and carbon cloth (d–f) electrodes

after 60 h incubation in MFC with Shewanella oneidensis MR-­1 cell suspension. With a higher magnification, images b and e were taken at the electrode surface while images c and f were focused on the electrode interior. Reprinted from ref. 29 with permission from American Chemical Society, Copyright 2012.

Table 7.1  Maximum  power density outputs of the anode materials. Reprinted from ref. 29 with permission from American Chemical Society, Copyright 2012.

Electrode

Power density (mW m−2)

Specific power density (mW g−1)

Carbon cloth (CC) CC/PANI Carbon felt (CF) CF/PANI Nickel foam Nickel foam/PANI Graphene foam Graphene/PANI foam

158 323 10.8 145 21.9 70.8 12.8 768

1.16 2.37 0.09 1.20 0.05 0.17 4.26 256

7.6  3D Graphene Macroporous Scaffold A miniaturized MFC with a 3D graphene scaffold as the anode was fabricated by Ren et al.49 The electrode possessed high conductivity, surface area, and biocompatibility with a thick formation of biofilm (∼150–200 µm).

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The synthesis involved the use of Ni foam as the 3D scaffold substrate. The CVD technique was used to grow graphene on the substrate surface. The composite material was treated with polymethyl methacrylate (PMMA) solution and heated. Ni was leached using a hydrochloric acid solution. Finally, PMMA was removed by dissolving the composite in acetone to yield a free-­standing 3D graphene scaffold. The study claimed to have achieved the highest power density reported till now, i.e., 11 220 W m−3, which is almost 3-­fold higher than those reported in the literature. It exhibited a comparable power density with generally available power sources such as Li-­ion batteries, Ni-­cadmium batteries, etc., showcasing its potential to be commercialized and used for renewable power applications.

7.7  3D Graphene Sponges 7.7.1  Macroporous Flexible 3D Graphene Sponge A simple and facile technique for the preparation of flexible 3D graphene sponge using an ice template is described by Chen et al.25 The study is based on the fact that the rate of cooling controls the shape and structure of the crystal during freezing. A slow cooling rate facilitates the formation of large-­sized crystals, resulting in the formation of a macroporous flexible 3D graphene sponge. Such a flexible matrix finds applications in the field of flexible electronics, owing to its high conductivity, porous structure, and lightweight characteristics.

7.7.1.1 Synthesis of Electrode and SEM Analysis An aqueous suspension of GO was mixed with a reducing agent, sodium bisulfite, and heated to prepare a hydrogel. The hydrogel was treated with deionized water, followed by freeze-­drying to remove water and form GAs. A schematic representation of the synthesis of the 3D porous graphene structure and SEM images for different rates of cooling are shown in Figure 7.10. Highs rate of cooling results in the formation of small ice crystals rendering the isotropic ice template structure, viz., graphene foam. On the other hand, a slow rate of cooling produces large ice crystals, leading to the formation of an anisotropic ice template, viz., graphene sponge. The study concluded that a macroporous and flexible 3D graphene sponge can be synthesized by regulating the cooling rate of the formation of ice crystals. Remarkable flexibility of the electrode was corroborated by the fact that it was able to regain its original shape if subjected to a deformation of 50%. A high power density of 427 W m−3 was achieved using the fabricated electrode as the anode in MFCs.

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Figure 7.10  Schematic  diagram of the formation process of 3D porous graphene structures induced by ice template at different growing rates of ice crystals (a and d). Cross-­sectional SEM images of graphene foam (b and c) and graphene sponge (e) prepared by liquid nitrogen freezing (quick) and −10 °C treatment in a refrigerator (slow), respectively. Surface of graphene film of graphene sponge aligned by graphene sheets (f). Reprinted from ref. 25 with permission from the Royal Society of Chemistry.

7.8  Graphene Sponge (GS)-­SS Composite A 3D conductive network of GS was synthesized by graphene nanosheets using a coating technique.50 To enhance the electrical conductivity, a stainless steel (SS) current collector was used to fabricate the GS-­SS composite.

7.8.1  Synthesis and Characterization of GS The electrode was synthesized by coating synthetic sponges with graphene. A dispersed solution of graphene nanopowder was prepared and coated on the sponge via a simple dipping and drying method. A pictorial representation of the sponge before and after graphene coating is presented in Figure 7.11a. The SEM image of the prepared electrode clearly shows a 3D porous network (Figure 7.11b). The durability of the electrode was tested by the Scotch tape test (Figure 7.11c) and flush water (Figure 7.11d). In both tests, no particles of graphene were visible, corroborating the stability of the electrode.

7.8.2  Synthesis of GS-­SS, Mechanism, and EIS The GS-­SS composite was synthesized by sticking GS on both sides of a SS mesh using a conducting carbon paste (See Figure 7.12a). A pictorial mechanism showcasing a decrease in ohmic resistance owing to the incorporation of SS in GS is presented in Figure 7.12b. The mechanism corroborating the

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Figure 7.11  Graphene  sponge (GS) composite electrode. (a) Schematic of the plain

sponge (left) with three-­dimensional (3D) open structure and the GS composite (right) with conformal graphene coating. (b) Scanning electron microscope (SEM) image of the G-­S showing the macroscale porous structure and the graphene surface (inset). (c) Scotch tape test: a piece of Scotch tape was attached to the GS composite and then peeled off. (d) Water flush: the GS composite was flushed with water (∼100 mL per second) for 10 minutes. Reprinted from ref. 50 with permission from the Royal Society of Chemistry.

decrease in resistance is supported by the EIS analysis. The Nyquist plots using three electrodes, namely SS, GS, and GS-­SS, are illustrated in Figure 7.12c. It was found that resistance decreased from 180 Ω in the GS electrode to 22 Ω in the GS-­SS electrode, which can be ascribed to the introduction of SS as a current collector in the material. The 3D GS-­SS electrode, therefore, provides a potential option to be used as an anode for large-­scale application in MFC. The synthesis technique provides a facile, simple, and inexpensive approach for the fabrication of electrodes.

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Figure 7.12  G-­S  with a stainless steel (SS) current collector (G-­S-­SS). (a) Schematic

of the G-­S-­SS composite electrode (right) vs. the G-­S composite electrode (left). (b) Schematic showing the electron pathways in the G-­S electrode with (right) and without (left) an SS current collector. (c) Nyquist curves of the electrochemical impedance spectroscopy (EIS) tests for different electrodes. The x-­intercepts on the Nyquist curves are ∼14 Ω for SS, ∼22 Ω for G-­S-­SS, and ∼180 Ω for G-­S, respectively. Reprinted from ref. 50 with permission from the Royal Society of Chemistry.

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7.9  Additional Graphene-­modified 3D Scaffolds 7.9.1  Chitosan/Vacuum-­stripped 3D Graphene Scaffold He et al. describe a unique freeze-­drying technique (ice segregation induced self-­assembly) for the preparation of a 3D network of chitosan-­modified graphene scaffolds.51 The prepared composite possessed hierarchical porosity consisting of macro-­ and meso/micropores. The macroporous structure facilitated the colonization of microorganisms on the interior portion of the 3D graphene scaffold, whereas the meso/micropores enhanced the surface area of the graphene scaffold for electron transport. The combined synergetic effect of the hierarchical porosity resulted in the enhanced performance of the 3D chitosan-­dispersed graphene scaffold as the anode in MFCs. GO was heated in a vacuum to form vacuum-­stripped graphene powder. The powder was dispersed in a chitosan solution to form a uniformly dispersed suspension. The solution was dipped in liquid N2 using a syringe to form a frozen sample. It was then subjected to freeze-­drying to form chitosan/vacuum-­stripped 3D graphene scaffold as the anode. The synthesized electrode recorded a power output of 1530 mW m−2, which is ∼78-­fold higher than those reported for carbon cloth anodes.

7.9.2  3D Graphene Nanosheets 3D graphene nanosheets were synthesized and used as an efficient cathode in MFCs.52 The synthesis involved the use of the sacrificial support method in which amorphous silica (sacrificial template) was incorporated in a graphene matrix and was later selectively etched to yield porous graphene nanosheets. Briefly, a GO suspension was mixed with silica, ultrasonicated and then dried. The synthesized powder was ball-­milled and subjected to H2-­reduction at a high temperature. This was followed by etching the silica substrate using hydrofluoric acid and then neutralizing its pH. The dried composite was pyrolyzed at a high temperature under an inert (N2) atmosphere to form the desired cathode composite. The novel 3D porous graphene nanosheets, when used as the cathode in an air-­breathing MFC, exhibited a power output of ∼2 W m−2. Thus, the prepared cathode showed good electrocatalytic activity for ORR in MFCs.

7.10  Conclusions and Outlook The chapter presents a description pertaining to 3D graphene-­based scaffolds such as graphene-­based micropillars, graphene aerogels, graphene foams, and graphene sponges. It focuses on the synthesis of such 3D macrostructures, in addition to highlighting the factors contributing to the enhanced performance of the electrodes when used in MFCs. Different techniques such as laser ablation, hydro-­ and solvothermal, freeze-­drying, ice template, and

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sacrificial support methods were used to transform a 2D surface to 3D macrostructures. Biocompatibility, which is a vital aspect in the performance of MFCs, was elucidated and it was found that graphene-­based electrodes possess excellent biocompatibility in MFC applications. Physiochemical characterizations of 3D electrodes were also highlighted and presented. 3D modification, besides enhancing the surface to volume ratio (roughness) for an enhanced microbial colonization in comparison to its counterpart 2D surface, also facilitates extracellular electron transport. The 3D graphene-­based electrodes provide high conductivity and porosity for high power generation in MFCs. Some studies presented the synthesis of flexible electrodes, which is an important and exciting aspect in flexible electronic applications and from the viewpoint of ease in handling in fuel cells. It is evident that the present area of research (3D modification of electrode) has paved a totally different way in the field of MFC technology over the past few years. The graphene-­based 3D scaffolds indeed present a promising future and research field for further investigations related to commercialization and scale-­up of the MFC technology.

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Chapter 8

Highly Efficient Dye-­sensitized Solar Cells with Integrated 3D Graphene-­based Materials Hisham A. Maddaha,b, Anmole Jhally b, Vikas Berry b and Sanjay K. Behura*c,d a

Department of Chemical Engineering, King Abdulaziz University, Rabigh 21911, Saudi Arabia; bDepartment of Chemical Engineering, University of Illinois at Chicago, 929 West Taylor Street, Chicago, IL 60607, USA; c Department of Chemistry and Physics, University of Arkansas at Pine Bluff, 1200 N. University Drive, Pine Bluff, AR 71601, USA; dDepartment of Mathematics and Computer Science, University of Arkansas at Pine Bluff, 1200 N. University Drive, Pine Bluff, AR 71601, USA *E-­mail: [email protected]

8.1  Introduction Radiation from the sun provides the Earth with abundant, free, and environmentally friendly solar energy, with power of approximately 1.8 × 1011 MW.1–3 Harnessing solar power via photovoltaics (PV) allows us to efficiently convert the incident photons to excitons for generating electricity.1 Nevertheless, first-­generation silicon-­based solar cells are expensive and can be potentially replaced by cost-­effective second-­generation multi-­crystalline Si4–9 or third-­generation photovoltaic (PV) systems, including organic/inorganic

  Chemistry in the Environment Series No. 1 Graphene-based 3D Macrostructures for Clean Energy and Environmental Applications Edited by Rajasekhar Balasubramanian and Shamik Chowdhury © The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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206 10–19

perovskits, cadmium telluride (CdTe), copper indium gallium selenide (CIGS),20–27 organic tandem,28–34 quantum dot cells,35–40 and dye-­sensitized solar cells (DSSCs).41–51 Among the wide range of third-­generation PV systems, DSSCs have attracted broad interest as a promising inexpensive solar cell technology. DSSCs use specialized materials for specific cell functions including photon absorption, charge separation, and charge transport. A photon enters the solar cell through a transparent electrode and can be absorbed by a sensitizer, thus exciting an electron. This electron can be injected into the conduction band of a neighbouring semiconductor, which diffuses therein. The diffused electron can perform work, given to an external load, and flow to the cathode where it is transferred to an electrolyte or a hole conductor. The electrolyte can then transfer an electron to the sensitizer, regenerating it and ultimately completing the circuit. To optimize these devices and achieve enviable power conversion efficiencies (PCEs), researchers have explored ways to maximize the light-­harvesting efficiency and minimize electron losses due to parasitic resistances.52 To date, significant progress has been made towards the development of high performance DSSCs through: (i) material selection, (ii) engineering wide bandgap semiconductor-­based photoanodes (e.g., TiO2, ZnO, SnO2, TiO2/ ZnO, SnO2/ZnO, ZnO/SnO2, SnO2/TiO2, TiO2/ZnO/TiO2), (iii) designing efficient counter electrodes (e.g., C, Pt, C/Pt, graphene quantum dots (GQDs)), and (iv) employing electrolytes in either liquid-­state (e.g., iodide/triiodide redox) or solid-­state (e.g., cobalt complexes). Moreover, the use of toxic-­free natural sensitizers (e.g., β-­Carotene) extracted from carotenoids and protein complexes53 and/or perovskite semiconductors (for cosensitization) hold the promise to enhance visible-­light absorption in DSSCs, instead of using the very toxic and expensive54 commercial dyes including organic dyes,47 ruthenium dyes,55 and platinum dyes.56 DSSCs can become even more efficient through incorporation of graphene sheets. Graphene-­based materials (e.g., chemical vapor deposited (CVD) graphene and chemically exfoliated reduced graphene oxide (rGO)) have been studied as transparent conductors, in the semiconducting layer, and even as the sensitizer itself for the photoanode of a DSSC.52,57–59 Furthermore, graphene sheets have been used to optimize the efficiency of nearly every main component of a DSSC, which can improve the overall performance and/ or reduce the cost of such devices.60 The 3D graphene-­based materials (3D GBMs) can be integrated as 3D-­graphene/TiO2 nanocomposites, which are appropriate photoanodes for DSSCs, owing to their high transparency, excellent electron transport mobility, and remarkable electrochemical activities, which also makes them appropriate for application in counter electrodes.61 The outstanding electrical conductivity of graphene nanosheets and the large surface area (arising from interconnected porous networks) make 3D GBMs promising anode/cathode materials for building DSSCs with enhanced photovoltaic performance.62

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8.1.1  Dye-­sensitized Solar Cells The idea of DSSCs was initiated in 1991 by O'Regan and Grätzel,41 upon being inspired by natural photosynthesis and photography processes.63 Molecular pigments found in plants and fruits absorb visible-­light energy to excite free electrons residing in conjugated-­bond structures. DSSCs convert solar energy into electrical energy via a pigmented thin-­film semiconductor material that absorbs photons for generating excitons.41 Since 1991, the PCE of DSSCs has improved from 7.1% to 13% (2014) by employing different Ru-­based sensitizers and photoanode or cathode structures.64 Cosensitization of TiO2-­solid-­ state DSSCs with perovskite semiconductors (CH3NH3PbX3) have also been found to be promising for increasing the cell performance up to 15%.65 Electricity production costs in DSSCs are below $0.5 per W, owing to their inexpensive materials and components, whereas silicon solar cells costs around $2.7–3.57 per W.1,44,66,67 This gives a great advantage for DSSCs systems to be utilized as potential photovoltaic molecular machines, based on the sensitization of a wide bandgap semiconductor (e.g., TiO2, ZnO, and SnO2) photoanode for electron excitation, injection, and current generation.63 Large surface area nanoporous semiconductors can absorb pigments and create a monolayer of dye molecules, ultimately generating electron–hole pairs upon visible-­light illumination; charge separation occurs in femtoseconds at the semiconductor/dye interface resulting in the injection of an electron from the dye molecules into the conduction band of the semiconductor.45,68

8.1.2  Cell Architecture and Working Mechanism Conventional DSSCs consist of four main components: (i) photoanode, (ii) photosensitizer, (iii) electrolyte, and (iv) counter electrode (cathode).67 The four components work synergistically to convert visible sunlight energy into electricity (photons-­to-­electrons) starting from the photosensitizers being sensitized for electron excitation/injection.63,66,67,69–71 Typical materials of the key components and their role in electron excitation and transport are shown in Table 8.1 giving us the opportunity to understand the charge transfer mechanisms for PCE enhancement. Low sheet resistance (15 to 40 Ω/■) of transparent conductive oxide (TCO) glass sheets [as indium-­doped tin oxide (ITO) or fluorine-­doped tin oxide (FTO)] is desired to allow easy electron transport from TiO2 to ITO and thereby to the counter electrode for current generation. High optical transmittance of ITO (>85%) for visible-­ light (400 to 700 nm) makes it a promising candidate for the photoanode architecture in DSSCs. Moreover, utilization of thin-­film semiconductors, fabricated from crystallization and annealing of TiO2 nanoparticles at 100– 500 °C, is critically important to reduce series and/or interfacial charge resistance and therefore facilitate charge transfer and conduction. Covalently bonded monolayer dye molecules onto the semiconductor exposed surface (nanoparticles) would improve photon absorption and electron–hole pair

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Table 8.1  Components  of a typical DSSC: Key components and their roles in electron excitation, separation, transport, and current generation.

S/N

Component

Typical DSSC

i

Photoanode electrode

ITO/TiO2/Dye

ii

Counter electrode

ITO/Pt (or) ITO/C

iii

Photosensitizer (part of the photoanode)

Commercial metal Ru-­dyes (e.g., N3, N719, and N749)

iv

Redox electrolyte

[I−/I3− ] redox couple

Role in charge generation and transfer Charge conduction of received dye electrons and charge separation/ transport at TiO2/dye. Electron collection of photoanode electrons. Electron excitation, injection, and charge transfer to the semiconductor. Dye regeneration by reduction/oxidation.

Figure 8.1  Schematic  representation of a typical DSSC: (A) Donor-­π-­acceptor (D-­π-­A) structure of an organic dye in DSSCs with a wide bandgap semiconductor photoanode; (B) Typical components and architecture including photosensitizer for electron injection, photoanode electrode for charge separation/transport, counter electrode for electron collection, and redox electrolyte for dye neutralization, Reproduced from ref. 74 with permission from Elsevier, Copyright 2020.

generation under standard solar illumination (AM 1.5). Using a charge transport medium (redox electrolyte) applied between the two electrodes is necessary for uninterrupted electron transport and dye neutralization. Liquid or solid (gel) electrolytes regenerate missing electrons of the oxidized dye molecules by donating the received electrons from the cathode electrode (i.e., oxidation/reduction of iodide and triiodide [I−/I3−] redox couple).63,66,67,69–74 A schematic representation of a typical DSSC system with its main components is shown in Figure 8.1. To understand how current is being generated within DSSCs, the working mechanism, operation cycle, and electron transport from visible-­light excitation can be summarized as follows:63,67,69,72,73    (a) Dye excitation: Absorption of incident photon energy excites dye molecules from their ground state (S) to a higher energy state (S*); this is equivalent to movement of dye electrons from their highest occupied molecular orbital (HOMO) to their lowest unoccupied molecular

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orbital (LUMO) in the dye organic molecular structure, forming electron–hole pairs. (b) Electron injection: Once a dye molecule is excited (S*), the excited electron from S* transports to the donor segments via the dye-­acceptor segments (in the D-­π-­A structure) and is injected into the conduction band of the semiconductor (e.g., TiO2); the electron excitation ensures charge separation and allows continuous electron injection and charge flow through the photoanode structure towards the cathode charging an external load in every completed cycle. (c) Oxidized dye regeneration: An electron donation from iodide (I3−) in the redox electrolyte [I−/I3−] regenerates the oxidized dye (S+) for repeatable electron excitation/injection. (d) Electrochemical reduction: At the cathode side, collection of transported photoanode electrons occurs while the redox mediator gets regenerated from reduction of triiodide (3I−).

8.1.3  Electron Transport and Recombination Kinetics To facilitate electron transport in DSSCs, electron recombination with either oxidized dye molecules (e1) or electrolyte species (e2) should be minimized.72,73 The timescale of electron transfer at the interfacial contacts determines the controlling recombination reactions.75 Typically, photogenerated electrons recombine within 10 µs,76,77 where the electron transfer occurs within 10 fs and 1 ns at the semiconductor/dye and dye/electrolyte interfaces, respectively. Due to the faster electron injection rates, recombination dynamics in DSSCs are believed to be controlled by the slow transport of reduced electrolyte electrons.72 The operation cycle and mechanisms of electron transport in DSSCs with typical reactions at anode, cell, and cathode, and expected recombination are illustrated and explained in Figure 8.2A–C. If an excited electron is lost across the semiconductor/dye interface, the electron follows the (e1) path and recombines with available holes in the oxidized dye molecules under visible-­light illumination. However, the lost electrons across the dye/electrolyte interface follow the (e2) path and recombine with available holes in the oxidized redox electrolyte under dark current situations and/or no visible-­light illumination. Recombination occurs at the phase contacts when electrons are being transferred from ITO to the electrolyte, electrolyte to dye, dye to the semiconductor (TiO2), TiO2 to ITO, and so on. Most importantly, recombination dynamics at the TiO2/electrolyte and the TiO2/dye interfaces, as shown in Figure 8.2A and E, are very common and crucial in controlling the electron excitation/injection.70,72,78,79 There should be a reasonable back electron transfer blockage from the semiconductor, where injected electrons from dye-­acceptor segments must remain in the semiconductor with exclusively forward transport required for reducing recombination rates and facilitating electron injection. The large surface area of TiO2 films interfaced with dyes, containing many hydroxyl and carboxyl anchoring groups, provide efficient charge injection and forward electron transfer at the TiO2/dye and TiO2/ITO interfaces.72,80

Figure 8.2  DSSC  Operation Cycle and Reactions: (A) DSSCs complete operation

cycle and recombination dynamics; (B) A closer look at electron excitation and transport in DSSCs; (C) Anode (TiO2) and cathode (Pt/C) typical redox reactions in a DSSC (electron flow depends on light intensity and trapping-­detrapping effect of the surface); Charge regeneration mechanism in (A) and (B): (a) photons energy excite electrons from HOMO to LUMO levels and generate excitons within the dye molecules to (b) inject excited electrons into the conduction band of the semiconductor which initiate charge separation/transport of electrons from photoanode electrode to cathode electrode for current generation while (c) electrolyte ensures continuous current generation by neutralizing dye molecules through (d) redox reactions mechanism, with (e1) and (e2) describing possible recombination within DSSCs; (D) Diode model for DSSCs; (E) TiO2 photoanode compact layer in DSSCs. Reproduced from ref. 74 with permission from Elsevier, Copyright 2020.

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There is always a competition between recombination and regeneration reactions arising from oxidation/reduction of both dye molecules and electrolyte redox species. Further, possible recombination reactions compete with each other where lost electrons are drifted back to recombine with either oxidized dye molecules or oxidized redox species in the electrolyte.73 Moreover, it has been discussed that electron lifetime can be tuned by changing the deposited dye amounts onto the photoanode structure. Slight increases in the organic dye amounts may result in a prominent enhancement in generated electron lifetimes, yielding lowered recombination rates and improved photocurrents. Ruthenium metal-­based dyes utilized in DSSCs show high photocurrent and decent open-­circuit voltage (Voc) with less electron recombination at the dye/electrolyte or dye/semiconductor interfaces, due to plenty of available carboxyl groups. Conversely, organic pigments extracted from natural sources such as plants, carotenoids, and protein complexes show excess interfacial electron recombination and low photocurrents,81,82 which may be improved by the incorporation of 3D GBMs in the anode structure. Another approach to mitigate interfacial recombination at the photoanode interfaces includes photoanode passivation or insulation using SiO2, Al2O3, and ZrO2 layers, which would create a metal-­oxide barrier, preventing back electron transport to the electrolyte.70,83

8.2  G  raphene and 3D Graphene-­based Materials   (3D GBMs) Pristine graphene is an atomically-­thick layer of sp2-­hybridized carbon arranged in a hexagonal crystal structure.52 Graphene-­based materials, with their exceptional electrical, optical, and mechanical properties, have been previously incorporated into each aspect of a DSSC.52 Interestingly, this amazing material can be derived from graphite (via a top-­down approach), which is economical and naturally abundant.84 Wang et al. (2012)85 utilized graphene-­ based composites in DSSCs owing to their excellent conductivity and high electrocatalytic activity. The hexagonal “honeycomb” two-­dimensional lattice with sp2 carbon atoms86 possesses high carrier mobility (200 000 cm2 V−1 s−1),87 high specific surface area (2600 m2 g−1),88 and high optical transparency (97.7%);89 making graphene a promising material to be utilized as an electrode for efficient and practical DSSCs. Besides the superlative properties of 2D graphene, 3D GBMs (e.g., aerogels, hydrogels, sponges) are characterized by interconnected networks/channels providing large specific surface area, high electrochemical stability, and excellent mechanical strength90 (Figure 8 3).

8.2.1  Synthesis Methods Graphene sheets have been typically produced either by mechanical exfoliation via repeated peeling of highly ordered pyrolytic graphite (HOPG) or by chemical oxidation of graphite (top-­down approach).91 In the past few years, a number of approaches have been established to fabricate 3D interconnected

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Figure 8.3  (A)  Schematic illustration of the different steps for fabricating reduced

graphene oxide (rGO) aerogel. Reproduced from ref. 102 with permission from John Wiley & Sons, Copyright 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. (B) Synthesis of graphene foam (GF) and CVD growth of graphene films (Ni–G) integrated with PDMS. Reproduced from ref. 94, with permission from Macmillan Publishers Ltd, Copyright 2011. (C) Illustration of the fabrication process of ultralight graphene aerogel (ULGA). Reproduced from ref. 100, with permission of John Wiley & Sons, Copyright 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. (D) Aqueous suspension of graphene oxide (GO) hydrogel to graphene aerogel before and after heating and mixing with l-­ascorbic acid and freeze-­drying. Reproduced from ref. 101 with permission from the Royal Society of Chemistry. (E) Illustration for the mechanism of chemical conversion of amorphous porous carbon to 3D graphene. Reproduced from ref. 98 with permission from American Chemical Society, Copyright 2012. (F) Schematic of the fabrication process for graphene aerogel from mixing GO suspension with silica powders and R–F catalyst forming GO ink templated on SiO2. Reproduced from ref. 97 with permission from Macmillan Publishers Ltd, Copyright 2015.

structures of graphene (e.g., ice template, wet chemistry assembly, self-­ gelation, freeze casting, chemical vapor deposition (CVD), and in situ unzipping of carbon nanotubes sponge).92 For most of the methods, freeze-­drying or supercritical drying is essential to inhibit capillary-­force-­driven structural collapse of 3D graphene networks during drying.92 The synthesis of 3D GBMs mainly starts with using graphene oxide (GO) as precursors, following either of the two mechanisms discussed in the literature, self-­assembly approaches and template-­directed approaches,93–102 resulting in 3D graphene macroarchitectures with different structures and properties. Bai et al.103 reported

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213

that 3D assembly of GO sheets in water is possible by adding polyvinyl alcohol (PVA) as a cross-­linker, forming a pH-­sensitive supramolecular hydrogel. Hydrogen bonding between GO sheets and PVA chains is believed to be responsible for the formation of the hydrogel.103 Recently, single-­stranded DNA was also found to be a good cross-­linker for preparing GO/DNA composite hydrogel, in which π-­π interaction was the dominant driving force.104 Similarly, hydrogels based on chemically converted graphene (CCG) have also been reported, reflecting that GO and CCG are good gelators. The GO-­based hydrogels were prepared by acidification or adding small organic molecules, polymers, or ions as cross-­linkers103 (Table 8.2).

8.2.2  H  ybrid Graphene-­based Composites for Counter Electrodes Counter electrodes (CEs) in DSSCs play a critical role in determining the cell efficiency depending on their abilities to collect electrons coming from the photoanode through the external circuit. A number of parameters should be investigated for the selection of optimal and scalable CEs: (1) sheet resistance; (2) catalytic activity; (3) chemical stability; and (4) cost.67,85,105–109 For an optimized cell, the CE sheet resistance must be as low as possible to facilitate electron transport, the CE catalytic activity should be very high for effective chemical reduction of redox species, and the CE materials must possess a noble-­like behaviour as in Pt electrodes for corrosion stability.110–113 However, Pt is highly expensive for large-­scale applications due to its scarcity. As such, the development of Pt-­free CEs should be pursued to have graphene-­ based cost-­effective and scalable CEs.85 Velten et al.114 utilized spun multi-­walled carbon nanotube (MWCNT) sheets with graphene flakes as CEs achieving a maximum PCE of 7.55% in iodide-­based DSSCs; the MWCNTs provided high electrical conductivity whereas graphene flakes ensured low charge transfer resistance (Rct), as shown in Figure 8.4A–C. Choi et al.115 fabricated MWCNTs/graphene heterostructure on SiO2/Si, which was then transferred onto FTO for utilization as a CE interfaced with TiO2 and N719 dye yielding 3% PCE under AM1.5 (Figure 8.4D and E). Negatively charged graphene oxide films were also electrochemically reduced (ERGO) via a layer-­by-­layer (LBL) assembly against positively charged poly(diallyldimethylammonium chloride) (PDDA), as shown in Figure 8.4F by Xu et al.,116 for preparing highly efficient and durable graphene-­based [PDDA@ERGO] CEs (9.5–7.6%) for long-­time operation (>1000 h). Chang et al.117 prepared a novel hybrid nanostructure based on 3D graphene nanosheets@ZnO nanorods (GNs@ZnO) via hydrothermal and spin coating processes on FTO for use as CEs (Figure 8.4G–I). ZnO nanorods prevented graphene nanosheets aggregation and allowed electrolyte penetration into CEs, exposing large surface area and active defective sites of graphene nanosheets for triiodide reduction. The 3D networks significantly improved the CE electrocatalytic activity due to lower peak separation (Epp) of

S/N

Method

1

CVD Approach

2

3

4

5 6 7 8 9

Nickel foams are heated and annealed, then a thin PMMA layer is used. Next, Nickel/PMMA is baked and dissolved in a hot acetone bath to obtain free-­standing graphene foams (GFs). Ice Template Graphene oxide (GO) solution is mixed with ascorbic acid, then placed in a boiling water bath. Next, the solution is immersed in a dry ice bath to freeze, then thawed at room temperature. The obtained gel is then sequentially subjected to dialysis in water, freeze-­drying, and thermal annealing. Emulsion GO is prepared and organic additives are added. Next, the aqueous nanocarbon suspensions are Template emulsified with a hydrophobic phase. The nanocarbon emulsion is unidirectionally frozen and bulk nanocarbon monoliths are obtained by freeze-­drying. The nanocarbon monoliths are thermally reduced to produce the final reduced GO (rGO) monoliths. SiO2 Template GO powder is produced then gelled into GO ink. The sol–gel mixture consists of an aqueous solution of resorcinol (R), formaldehyde (F) and sodium carbonate catalyst (C). 3D periodic micro lattices were assembled by patterning an array of parallel (rod-­like) filaments in a meander line-­like pattern. After gelation, the wet GO gels are removed, washed, and dried. Lithographical A bottom antireflection coating is spun onto silicon wafers and baked. A thin resist layer of NR 7 Template is deposited and a thick resist layer (6 µm) of NR7-­6000P is deposited. An interference pattern is formed, and the beam is expanded and split. Finally, the samples are baked and rinsed. Hydrothermal A suspension of GO is prepared by sonication involving a noble-­metal salt and glucose. The mixture Reduction is then treated hydrothermally, washed, then freeze-­dried. Cross-­linking GO dispersion is mixed uniformly with ethylenediamine and freeze-­dried. After freeze-­drying, funcAgent tionalized graphene aerogel (FGA) is produced and exposed to microwave irradiation (MWI). The MWI restores interaction in the cross-­linking sites, which tightly bonds the sheets together. Reducing Agent The aqueous mixture of GO is heated with l-­ascorbic acid without stirring. Polymer Aqueous GO solution is prepared then freeze-­dried into rGO aerogel. Next, PVA is dissolved then Assembly Hypophosphorous acid and Iodine are added. The suspension undergoes reaction then is washed and treated with a glutaraldehyde solution. Finally, the solution is freeze-­dried. Gelation GO is prepared from natural graphite powder by a modified Hummers method and purified by dialysis. To prepare GO hydrogels, a certain volume GO dispersion is mixed with a solution of acid or other cross-­linkers. The blend is then shaken to form a hydrogel.

Reference 94 95

96

97

98 99 100 101 102 103

Chapter 8

10

Experiment

214

Table 8.2  Synthesis  methods for 3D GBMs via template-­directed approach (1 to 5) or assisted/induced self-­assembly mechanism (6 to 10).

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AOX/ARE redox pair, and higher reduction peak current density (JRE) (Figure 8.4J), which enhanced electrolyte reduction for high short-­circuit current density (Jsc) and low Rct, resulting in PCE of 8.12% comparable to Pt-­based CE of 8.82% (Figure 8.4K).117 In another work, Casaluci et al.118 spray-­coated chemically exfoliated graphene ink on FTO as an alternative to Pt CEs for large-­ area DSSCs modules (43.2 cm2), achieving a PCE of 3.5% (Figure 8.4M and L). Wang et al.119 integrated polyaniline (PANI) nanoparticles into graphene sheets, via in situ polymerization, forming a graphene/polyaniline nanocomposite as a potential CE for DSSCs, with 6.09% PCE achieved from the enhancement of CE electrocatalytic performance (Figure 8.4N). Dodoo-­Arhin et al.120 fabricated graphene-­based CE using stable inkjet printable graphene ink chemically exfoliated from graphite for natural and ruthenium-­based DSSCs; dye extracts of Caesalpinia pulcherrima carotenoids interfaced with inkjet-­graphene CE exhibited PCE of 0.9% (attributed to better intermolecular dye interactions), whereas N719 dye showed a relatively high PCE of 3.0% (4.4% when using Pt CEs) (Figure 8.4O). Yue et al.121 prepared Pt/graphene hybrid films as CEs for DSSCs using Pt nanoparticles and electrochemical deposition techniques, where cyclic voltammetry and other electrochemical measurements confirmed the hybrid films had higher conductivity and better electrocatalytic activity towards triiodide reduction than that of pristine Pt electrodes, achieving a high PCE of 7.88% (Figure 8.4P). Kaniyoor and Ramaprabhu122 identified a low transfer resistance of 11.7 Ω cm2 that is close enough to Pt (6.5 Ω cm2) when using thermally exfoliated graphene (TEG) films; system efficiency of 2.8% for TEG was also comparable to that of Pt-­based DSSCs (3.4%). Further, Kavan et al.123 demonstrated that defected graphene structures containing oxygen or –NHCO– groups increase active sites and enhance the electrocatalytic activity of graphene CEs. Under 1-­sun illumination, DSSCs modified with carbon-­based CEs showed high PCEs of 6.67, 3.9, 4.5, and 7.7% for CEs of graphite,124 activated carbon,125 single-­walled carbon nanotubes (SWCNTs)126 and MWCNTs,127 respectively.85 The combination of graphene sheets with other carbon materials, Pt, transition metal sulfides, and nitrides provide fast electron diffusion and transport at the electrode–electrolyte interface for system efficiencies up to 7.66, 7.5, and 5.7% for graphene/Pt, graphene/MWCNTs, and Ni12P5/graphene, respectively.85 Li et al.128 used graphene-­NiS2 as a CE to achieve a PCE of about 8.55%. Nitrogen-­doped graphene and/or metal-­free CEs have been extensively investigated in recent works,129–133 where Xue et al. (2015)133 fabricated a highly efficient nitrogen-­doped graphene nanoribbons (N-­GNRs) with a surface area of 751 cm2 g−1 for disulfide/thiolate redox-­mediated DSSCs. Incorporation of single metal active sites (Mn, Fe, Co, Ni, and Cu) to the nitrogen atoms doped in graphene leads to composite CEs (e.g., CoN4/ graphene nanosheets) with superior activity, stability, and appropriate adsorption energy as those excellent properties observed in highly stable/ expensive metal electrodes (Pt, Au, and Ag).134

216

Chapter 8

Figure 8.4  (A)  Schematic of CNTs/graphene-­based DSSCs; (B) J–V curves of DSSCs using Pt, graphene, or MWCNTs; (C) SEM image of CNTs/graphene composite with inset showing carbon bridges. Reproduced from ref. 114 with permission from IOP Publishing, Copyright 2012. (D) Device processing flowchart: (a) GMWNT synthesis, (b) GMWNT lift-­off, (c) GMWNT transfer, (d) Pt interface; (E) J–V characteristics of DSSCs with different CEs. Reproduced from ref. 115 with permission from Elsevier, Copyright 2011. (F) Fabrication procedure of [PDDA@ERGO] films. Reproduced from ref. 116 with permission from Macmillan Publishers Ltd, Copyright 2013. (G) Preliminary structure and fabrication

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217

62,135

3D GBM-­based composites are even more promising for building DSSCs with enhanced CEs properties. Wang et al.107 synthesized a 3D honeycomb-­like structured graphene (HSG) on a conductive FTO glass to use HSG/FTO as a CE. HSG enhanced the catalytic performance of the cell which resulted in achieving a high efficiency of 7.8% that is comparable to Pt/FTO CEs.62,107 Tang et al.135 integrated 3D GBMs with rGO to fabricate a 3D/2D graphene-­based CE showing excellent photovoltaic performance as high as 9.79% in DSSCs. The interconnected networks of the 3D GBM are believed to provide fast electron transport while the rGO diminished contact resistance at the graphene/electrolyte interface.135,136 Sahito et al.106,137 incorporated cotton fabrics in rGO structure to fabricate a low cost, lightweight, Pt-­ and metal-­free, flexible, cotton-­based graphene textile CE for DSSCs. The synthesis approach was simple and quick, involving common dip/dry techniques for adsorption of rGO on cotton fabrics. The novel fiber-­based graphene electrode showed excellent bending flexibility, high electrical conductivity, and decent electrocatalytic activity towards reduction of I3− with a conversion efficiency of 2.52% (Table 8.3).

8.2.3  G  raphene Integrated Wide Bandgap Semiconductor Photoanodes The integration of graphene-­based materials into the wide bandgap semiconductor within the photoanode structure leverages charge transport of excited electrons from dye-­acceptor moieties, which can be attributed to increased electrode conductivity for easy electron injection. For instance, GQDs were applied on TiO2 films in the photoanode with an optimal controlled amount of 1.7 × 10−4 mol cm−2 by soaking in the GQDs solution at 60 °C for 24 h, which enhanced both charge transfer and cell total current density. The GQDs-­modified-­photoanode DSSCs showed a maximum Jsc of 14.07 mA cm−2 (and PCE of 6.1%), which were approximately 31% and

of ZnO-­GNs CE for DSSCs; (H) Top-­view SEM image of ZnO nanorods on FTO; (I) HRTEM image of GNs@ZnO nanorods with inset showing crumpled GNs sheets; (J) Cyclic voltammograms (CVs) for GNs, ZnO, GNs@ZnO, and Pt CEs at a scan rate of 50 mV s−1; (K) J–V curves of DSSCs using different CEs. Reproduced from ref. 117 with permission from Elsevier, Copyright 2015. (L) Spray coating graphene ink; (M) Optical image of graphene-­based CE. Reproduced from ref. 118 with permission from the Royal Society of Chemistry. (N) J–V curves for graphene/ PANI CE based cells with CE SEM image in the inset. Reproduced from ref. 119, with permission from Elsevier, Copyright 2012. (O) J–V curves for graphene ink CE based cells, Reproduced from ref. 120, with permission from Elsevier, Copyright 2016. (P) CVs for Pt and Pt/GN CEs at a scan rate of 10 mV s−1 with Pt/GN SEM image in the inset, Reproduced from ref. 121 with permission from Elsevier, Copyright 2013.

DSSCs with different graphene-­based counter electrodes.a

Cell configuration −

−

FTO/TiO2/N719/Graphene/[I /I3 ]/FTO FTO/TiO2/N719/Graphene/[I−/I3−]-­(AN-­50)/FTO ITO/TiO2/N719/[I−/I3−]/[Graphene/PEDOT-­PSS]/ITO FTO/TiO2/N719/[I−/I3−]/[Graphene/PANI]/FTO FTO/TiO2/N719/[I−/I3−]/[Graphene/PEDOT]/FTO FTO/TiO2/N719/[I−/I3−]/[Graphene/Pt]/FTO FTO/TiO2/N719/[I−/I3−]/[Graphene/Pt]/FTO FTO/TiO2/N719/[Graphene/Ni12P5]/[I−/I3−]/FTO FTO/TiO2/N719/[Graphene/Ni12P5]/[I−/I3−]/FTO ITO/TiO2/N719/[Graphene/MWCNTs]/[I−/I3−]/ITO FTO/TiO2/N719/[Graphene/MWCNTs]/[I−/I3−]/FTO FTO/TiO2/N719/[I−/I3−]/[Graphene-­(HSG-­12 h)]/FTO ITO/TiO2/N719/[I−/I3−]-­(Z946)/[PDDA@ERGO]/ITO ITO/TiO2/N719/[I−/I3−]-­(Z952)/[PDDA@ERGO]/ITO ITO/TiO2/N719/[I−/I3−]-­(Z946)/[PDDA@ERGO]/Pt/ITO FTO/TiO2/N719/[GNs@ZnO]/[I−/I3−]-­(BMII)/FTO FTO/TiO2/N3/NDG/[I−/I3−]/FTO FTO/TiO2/N719/Graphene-­(0.15 wt%)/[I−/I3−]/FTO

Voc (V)

Jsc (mA cm−2)

FF (%)

η (%)

Reference

0.74 0.54 0.72 0.68 0.77 0.71 0.79 0.74 0.70 0.72 0.75 0.77 0.69 0.65 0.69 0.76 0.69 0.75

16.99 14.30 12.96 13.28 12.60 15.20 12.06 12.86 12.88 8.95 16.05 27.20 18.77 15.19 18.11 21.70 15.76 15.46

0.54 0.65 0.48 0.67 0.63 0.71 0.67 0.61 0.52 0.70 0.63 0.37 0.74 0.76 0.74 0.67 0.64 0.68

6.81 5.69 4.50 6.09 6.26 7.66 6.35 5.70 4.70 4.46 7.55 7.80 9.54 7.66 9.14 8.12 7.01 7.88

138 139 140 119 141 142 143 144 144 140 114 145 116 116 116 117 146 121

218

Table 8.3  Performance  comparison of various DSSCs: Efficiency and photovoltaic parameter values observed in previously designed

a

 ANI: Polyaniline; PEDOT: Poly(3,4-­ethylenedioxythiophene); MWCNTs: Multi-­walled carbon nanotubes; HSG: honeycomb‐structured graphene; P PDDA: Poly(diallyldimethylammonium chloride); ERGO: Electrochemically reduced graphene oxide; GNs: Graphene nanosheets; NDG: Nitrogen-­doped graphene.

Chapter 8

Highly Efficient Dye-sensitized Solar Cells

219

20%, respectively, higher than the current density and efficiency observed in DSSCs devices without GQDs (Figure 8.5A and B). This is attributed to the enhanced photoexcitation response from GQDs along with the dye molecules, allowing more electron injection into TiO2. It is worth mentioning that GTP-­1, GTP-­2, GTP-­3, and GTP-­4, from Figure 8.5B, refer to 0.025 g, 0.05 g, 0.075 g, and 0.1 g of added GQDs into TiO2 photoelectrode.147 Fang et al.148 used a ball-­milling method to prepare G-­P25(TiO2) photoanode electrodes involving the addition of different volumes of GO to P-­25, which increased the rutile contents with high GO amounts reaching a maximum rutile phase at GO = 4.5 mL. A very high interface recombination resistance was observed in the fabricated DSSCs when using G-­P25, resulting in PCE of 5.09%, due to the increase in both rutile contents and the porosity achieved by graphene addition (Figure 8.5C–E).148 Wang et al.61 utilized graphene-­ doped TiO2 films (graphene/TiO2 composites) in DSSCs which showed a vast improvement in current density by 52%, yielded in enhancing PCE by 55% (from 1.79% to 2.78% dependent on the content of GO). An optimal GO content was obtained at around 0.8 wt% which was believed to maximize photogenerated cell current density due to the perfect ratio between dye molecules and graphene content for efficient visible-­light harvesting, electron injection, forward transport, and extended electron lifetime (Figure 8.5F–I).61 Sun et al.84 prepared graphene-­TiO2 nanocomposite photoanodes by heterogeneous coagulation between TiO2 P25 nanoparticles and Nafion-­ coated graphene for strong interfacial binding and attachment of deposited graphene. The high graphene theoretical specific surface area (2630 m2 g−1) ensures ideal interfacial contact and strong electrostatic attraction between graphene and TiO2 even at small amounts of added graphene. The incorporation of only 0.5 wt% of graphene (i.e., graphene-­to-­P25 ratio = 1 : 200) demonstrated a significant PCE of 4.28% and current density of 8.38 mA cm−2, which enhanced efficiency by 59% and current density by 66% as compared to cells without graphene (Figure 8.5J–L).84 Deposited graphene platelets yielded increasing dye molecule adsorption and significantly increased electron lifetime from the provided rapid electron pathways. Lim et al.149 integrated reduced graphene oxide (rGO) into TiO2 to create (rGO– TiO2) nanocomposite photoanodes which achieved high DSSCs efficiency of 5.83%. It was found that 0.5 mg would be the optimal rGO content for TiO2 in order to have high photon absorption as well as reduced back electron transport, boosting charge collection (Figure 8.5M and N).149 When a compact layer of TiO2 was deposited via aerosol-­assisted chemical vapour deposition (AACVD) between the ITO and the rGO–TiO2, the DSSCs generated an increased current density of 550% reaching Jsc = 13.43 mA cm−2 that was responsible for the highly observed PCE. Introduced compact layer in the photoanode further enhanced forward electron transfer from dye and/ or electrolyte to TiO2 and ITO owing to the extra charge transfer pathways provided by embedded rGO149 (Table 8.4).

220

Chapter 8

Figure 8.5  (A)  Configuration of GQDs modified DSSCs with PEG-­passivated GQDs structure; (B) J–V curves of DSSCs using different GQDs weights (cell area of 0.25 cm2). Reproduced from ref. 147 with permission from Elsevier, Copyright 2014. (C) SEM image of G-­P25 photoelectrode with GO = 4.5 mL; (D) J–V curves and (E) Nyquist plots of DSSCs with different GO contents for G-­P25. Reproduced from ref. 148 with permission from Elsevier, Copyright 2012. (F) Performance of TiO2/GO-­based DSSCs; (G) Jsc vs. GO content; (H) Morphology of 0.83 wt% GO/TiO2 composite; (I) UV/Vis absorbance spectra. Reproduced from ref. 61 with permission from American Chemical Society, Copyright 2012. (J) TEM image of the graphene dispersed by Nafion; (K) J–V characteristics of DSSCs with P25 and P25-­graphene; (L) Energy band matching diagram. Reproduced

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Table 8.4  Performance  comparison of various DSSCs: Efficiency and photovoltaic parameter values observed in previously designed DSSCs with different graphene-­based photoanodes.

Cell configuration

Voc (V) Jsc (mA cm−2) FF (%)

η (%)

Reference

FTO/TiO2/anthocyanin/ [I−/I3−]-­(BMII)/Pt/FTO FTO/GO/TiO2/[purple-­cabbage-­ dye]/[I−/I3−]-­(LiI+I2+PMII)/Pt/ FTO FTO/TiO2/GQDs/N719/ [I−/I3−]-­(LiI+I2+PMII)/Pt/FTO FTO/TiO2/GO/[I−/ I3−]-­(LiI+I2+PMII)/Pt/FTO FTO/TiO2/GO/N719/[I−/I3−]-­ (LiI+I2+TBP+MPN)/Pt/ FTO FTO/Graphene/TiO2/N719/ Electrolyte/Pt/FTO ITO/rGO-­TiO2/N719/ Iodolyte-­(Z-­100)/Pt/ITO ITO/TiO2/Graphene/N719/ [I−/I3−]/Pt/ITO FTO/TiO2/Graphene/N719/ [I−/I3−]-­(LiI+I2+PMII)/Pt/FTO FTO/TiO2/Graphene/N719/ [I−/I3−]/Pt/FTO

N/A

1.63

0.51

0.51

150

0.31

0.22

0.31

0.36

151

0.66

14.07

0.59

6.10

147

0.61

10.28

0.63

5.09

148

0.67

7.60

0.54

2.78

61

0.73

8.38

N/A

4.28

84

0.74

13.43

0.59

5.83

149

0.70

19.92

0.48

6.86

152

0.67

16.80

0.56

5.77

153

0.68

12.89

0.69

6.05

154

8.3  B  iomolecular Dyes for Naturally Sensitized Photoanodes The integration of natural dyes in DSSCs holds a potential towards fabricating low cost environmentally friendly cells operating with toxic-­free sensitizers.63,66,68,155 Different biomolecular sources as fruits, flowers, leaves and bacterial complexes can be used to extract anthocyanin, carotenoid, flavonoid, aurone, and chlorophyll, etc.156,157 A highly conjugated double bond structure is desired when selecting natural sensitizers to ensure visible-­ light absorption in the range 400–700 nm.54,158 Carotenoids are promising candidates for strong light absorption at 400–500 nm owing to their long

from ref. 84 with permission from American Institute of Physics, Copyright 2010. (M) J–V curves obtained for rGO–TiO2 nanocomposite-­ modified photoanodes with (a) 0.05, (b) 0.1, (c) 0.5, (d) 1, and (e) 2 mg of rGO content. (N) HRTEM image of rGO–TiO2. Reproduced from ref. 149 with permission from John Wiley & Sons, Copyright 2015 John Wiley & Sons, Ltd. (O) Electron transport/generation using GO/TiO2 modified-­ photoanode for DSSCs. Reproduced from ref. 151 with permission from Elsevier, Copyright 2014.

Chapter 8

222 159

chain length of more than seven conjugated π bonds. However, the highest observed efficiency in DSSCs using plant-­based carotenoid pigments is 3.27% and up to 9% due to their low HOMO/LUMO energies 8), where systems with few conjugations (n < 8) can only absorb UV-­light and high photon energy. Highly conjugated systems absorb low and/or visible-­light energy whereas molecular structures with fewer conjugated bonds absorb high energies.210–212 The blending of chlorophyll with carotenoids has shown much improvement in carotenoids sensitizing function in DSSCs since chlorophyll provides intensified visible-­light absorption capabilities with enhanced layer protection213,214 and formed radical cations.215 Protein complexes (LH2, BR, RC) and chlorophyll a combined with carotenoids showed a high DSSCs PCE of 0.16–0.57% and 4%, respectively. On the contrary, xanthophylls carotenoids showed a low PCE of 0.008–0.03% as compared with other biomolecular pigments. The use of co-­adsorbents boosted up PCE to 0.03% due to strong dye attachment enhancing charge transport and electron injection (Table 8.7).

Chapter 8

226

Table 8.7  PCE  comparison between various bio-­sensitized DSSCs when using different/common biomolecular pigments under AM1.5 radiation.a

Bacterial pigment

Jsc (µA cm−2) Voc (mV)

FF

η (%)

Reference

Chlorophyll a (PPB) + Carotenoids (Spx) Chromatophores PPCs (LH2) PPCs (RC) Light-­harvesting complex II (LHCII) Bacteriorhodopsin proteins and bacterioruberin carotenoids (BRs) Xanthophylls carotenoids (yellow) Xanthophylls carotenoids (red) Xanthophylls carotenoids (PURE orange) Xanthophylls carotenoids (RAW orange) Xanthophylls carotenoids (Cocktail) Lycopene carotenoids RC photosystem I trimer (PSI) Bacteriorhodopsin (BR) protein Bacteriorhodopsin (BR) protein

11 500

—

—

4

188

24.7 1460 1240 800

300 620 840 590

0.29 0.54 0.55 0.58

0.04 0.49 0.57 0.27

49 192 192 216

450

570

0.62

0.16

68

130

549

—

0.0323

54

200

435

—

0.0332

54

78

260

0.39

0.008

217

127

460

0.51

0.03

217

98

260

0.38

0.009

217

696 362

289 500

— 0.71

0.057 0.08

218 219

620

—

—

0.19

220

1008

—

—

0.49

220

a

Jsc = Short-­circuit current density; Voc = Open-­circuit voltage; FF = Fill factor; η = Incident photon-­to-­current efficiency (IPCE) = Quantum efficiency (QE).

8.3.3  Graphene-­based Naturally-sensitized DSSCs The integration of graphene-­based materials into the naturally sensitized photoanode, sensitized by chlorophyll, carotenoids, or other protein pigment complexes, extracted from plant or bacteria sources, can drastically improve DSSCs performance that is usually low without graphene.150,151 To the authors' knowledge, there are not many studies focused on the integration of graphene and/or 3D GBMs with natural sensitizers as an attempt to increase photoanode visible-­light sensitivity, which would enhance both electron injection and forward transport for more generated photocurrents. Graphene has been previously added to dye and titania in red cabbage anthocyanin-­based DSSCs which achieved a 2.4-­fold increase in PCE using graphene-­to-­titania dispersion with a 3 : 5 volumetric ratio.150

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Impedance spectroscopy revealed decreased charge transfer resistances with graphene added. The increased efficiency is attributed to the enhanced Jsc since co-­adsorbed graphene onto anthocyanin molecules and/or titania photoanode provide conductive electron pathways. These pathways ensure only forward electron transport developing photogenerated currents.150 Al-­Ghamdi et al.151 introduced pre-­s ynthesized GO, using the modified Hummers' method, into a purple cabbage naturally sensitized TiO2 photoanode for DSSCs as shown in Figure 8.5O.151 The introduction of GO improved the overall cell performance from 0.15% to 0.36%, which is believed to increase due to the enhanced visible-­light absorption capabilities from graphene and provided electron pathways for forward charge transport.151 Omar et al.221 created TiO2/rGO photoanodes sensitized with natural Roselle anthocyanin dyes. The incorporation of rGO leveraged easy electron movement from valence band to conduction band. The DSSCs with rGO-­ anthocyanin-­based-­photoanodes achieved a PCE 0.75% higher than that of N719-­based-­photoanodes, showing the promise behind using graphene-­ based photoanodes for naturally sensitized DSSCs.221 Moreover, Hug et al. discussed the importance of incorporating the photoanode with antireflective (AR) coating conductive polymers, based on graphene, which has higher melting temperatures that would boost the photoanode stability.66,141,222 The AR coating also minimizes absorbed solar heat and/or UV-­light by carotenoid-­ based DSSCs; hence, decreasing degradation of the photoanode active layers for high graphene-­based bio-­DSSCs performance even in high-­temperature or intense-­UV environments66,223.

8.4  Conclusion In conclusion, this chapter highlights the promising role of 3D GBMs as electrodes for the enhancement of photocurrent and charge carriers transport in DSSCs. Interconnected networks and channels in 3D GBMs provide extra electron pathways and large specific surface areas for improved electron transport and enhanced dye adsorption in the photoanode, respectively. Large-­scale processing of 3D GBMs based composite electrodes is advantageous for producing cost-­effective and large DSSCs modules. Additionally, employing environmental-­friendly sensitizers including biomolecular dyes with 3D GBMs for hybrid photoanode architectures will further facilitate exciton generation and electron forward transport owing to the highly conjugated dye double bonds structure and interconnected graphene channels. Further studies on charge carrier transport mechanism, lifetime of the charge carriers, dye–graphene interaction, graphene-­semiconductor adhesion, 3D GBMs composition, and photoanode electronic properties should be tackled for better understanding of advanced DSSCs.

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Chapter 9

Fuelling the Hydrogen Economy with 3D Graphene-­based Macroscopic Assemblies Wingkei Ho*a and Jinliang Lina,b a

Department of Science and Environmental Studies, The Education University of Hong Kong, Tai Po, New Territories, Hong Kong, China; b Zunyi Normal College, Department of Chemistry and Chemical Engineering, Ping'an Street No. 6, 563000, Zunyi, China *E-­mail: [email protected]

9.1  Introduction The development of energy usage technologies reflects the evolution of human civilisation to a certain extent. Wood and grass were first used as fuel thousands of years ago. Thereafter, oil, natural gas and coal were used as energy resources by ancient people in their daily life. Since the Industrial Revolution, fossil fuels have been extensively exploited. To date, severe environmental damage has resulted from the excessive combustion of fossil fuels. Moreover, fossil fuels are speculated to become exhausted in the near future considering the current combustion trends. Consequently, scientists have rigorously explored sustainable energy resources in recent years to alleviate both the energy and environmental crisis associated with fossil fuels. Various types of renewable energy sources, including hydropower, wind power, tidal energy, solar energy, biofuels and   Chemistry in the Environment Series No. 1 Graphene-based 3D Macrostructures for Clean Energy and Environmental Applications Edited by Rajasekhar Balasubramanian and Shamik Chowdhury © The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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hydrogen energy, have been developed. Amongst the various ‘green energy’ technologies developed by the scientific community, hydrogen energy is widely considered as a universal energy carrier and a high energy density fuel that can facilitate sustainable development in the foreseeable future. However, two challenges, namely the generation and storage of hydrogen, must be addressed before the large-­scale utilisation of hydrogen as an energy source can be fully realised. Hydrogen production is a climbing-­hill reaction that always involves considerable energy (e.g., thermal energy and electric energy) consumption, resulting in indirect fossil fuel usage. For example, most hydrogen sources are currently obtained through the decomposition of organic compounds (methanol, natural gas and naphtha), collection from high-­temperature processes (ammonia synthesis, gas refining, the chlor-­alkali process and brewing), water electrolysis or biochemical methods. Moreover, achieving appropriate (de)hydrogenation conditions and favourable reversibility remains challenging when storing hydrogen in condensed materials with high gravimetric and volumetric densities. To find an acceptable solution, considerable effort has been exerted to explore novel materials for high-­ efficiency energy conversion. With regard to hydrogen evolution, various materials, such as metal atoms, metal oxides, organic polymers, transition metal complexes and inorganic compounds, have been used as catalysts in photocatalytic and electrocatalytic reactions. Although many materials are capable of photocatalytically producing hydrogen, energy conversion efficiency remains low and considerably differs from those of practical applications. This is because the three crucial steps for the water-­splitting reaction (i.e. solar light harvesting, charge separation and transportation) and catalytic reduction and oxidation reactions are inefficient or not simultaneous.1 Apparently, 3D graphene-­based photocatalysts can solve these limitations. Many researchers who worked on hydrogen storage materials have reported that common low-­temperature hydrides for hydrogen storage can be grouped on the basis of stoichiometry as follows: AB5-­t ype (e.g., LaNi5), AB2-­t ype (e.g., Ti–Zr alloys), A2B-­t ype (e.g., Sb2Ti, Sn2Co) and AB-­t ype (e.g., Ti–Fe alloys).2–10 ‘A’ represents elements with high affinity for hydrogen, typically rare earth or alkaline earth metals (e.g., Ca, Ti, Y, Zr, Hf, La and Ce), and ‘B’ represents elements with low affinity for hydrogen, typically transition metals that form only unstable hydrides (e.g., Cr, Mn, Fe, Co and Ni). However, their combined hydrogen storage capacity is relatively low (below 2 wt%) because of crystal structure and unit cell volume limitations. A metal hydride is technically formed through a chemical reaction but acts similar to a physical storage method. Ammonia borane (NH3BH3), with a hydrogen content of ca. 19.6 wt%, is a promising hydrogen storage material that has elicited tremendous research effort (NH3BH3 + 2H2O = NH4+ + BO2− + 3H2). Carbon in its different forms has been widely used in various energy and environmental technologies. Sample applications include electrodes for electrochemical energy storage and conversion systems (e.g., supercapacitors,

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batteries and fuel cells), active materials for optoelectronic devices and solar cells, electrocatalysts and catalyst supports, filters and sorbents for water treatment, and hydrogen and methane storage.11–15 Especially, the graphene surface has been modified and dimension-­tailored functional graphene structures, including graphene quantum dots (0D), graphene fibres (1D), graphene sheets/films (2D) and graphene gels (3D), have been constructed to expand the application scope to hydrogen storage and production. Recent advances in hydrogen storage/evolution materials have focused on 3D graphene-­based materials (3DGBMs). 3DGBMs have attracted considerable attention in various research fields due to their unique properties. In particular, 3DGBMs have been considered promising candidates for hydrogen storage due to the theoretical prediction of improved gravimetric sorption on subnanometre slit pores formed by graphene planes and a large surface to bear numerous inherent and foreign active sites. The experimental results indicate that 3DGBMs may be promising materials due to their tuneable 3D hierarchical interconnected network, macroscopic monolithic and porous structure, large surface area, electrical conductivity and chemical stability.16–18 This chapter analyses the technical progress towards hydrogen energy utilisation and the development of 3D carbon materials. The specific research areas covered include hydrogen production mediated by 3DGBMs and the use of these materials in the hydrogen storage of vehicles and hydrogen generator. Each of the many aspects of hydrogen energy is discussed, and possible challenges are addressed. This chapter is a useful reference for emerging as well as established researchers with an interest in hydrogen energy.

9.2  Hydrogen Evolution by 3D GBMs 9.2.1  E  lectrochemical Process for H2 Generation through   H2O Reduction by 3D Graphene Foam Pure carbon materials with high surface area, large pore volume, and numerous active adsorption sites are expected to be excellent candidates as support for transition metals as well as adsorbent for hydrogen storage. 3D GBMs, such as foams, sponges and aerogels, are the major types of new-­generation porous carbon materials that can be used as robust matrices for hosting metals, metal oxides and active polymers in different applications,10–23 particularly in catalytic systems.24,25 These attractive materials exhibit low mass density, high surface area, continuously interconnected macroporous structures and excellent chemical and physical stability.26–29 Meanwhile, the chemical modification of the adsorbent surface with N-­based functional groups or heteroatom/transition metal doping are promising methods for enriching the binding energy states of H2.30 Thus, the development of efficient, economical, stable, long-­lasting and reusable metal catalysts to enhance the kinetic and thermodynamic features of hydrolysis reactions under mild conditions is crucial for practical applications.31–35

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Considering transport and purity, water electrolysis is proposed as the most appropriate hydrogen production process at present. Electrocatalytic water splitting is central to hydrogen energy. Electrochemical hydrogen evolution reaction (HER, 2H+ + 2e− → H2) from water splitting requires advanced catalysts with a high current density at low overpotential. Notably, 3DGBMs can reportedly replace Pt-­based electrodes in dye-­sensitised solar cells with the improving photo node and stability of the electrolyte solvent.36–39 Liao et al. developed a highly active and stable electrocatalyst via the in situ formation of MoS2 nanocomposite on 3D architectural graphene foam (GF) with nanometre-­scale pore size.1 The improvement was highlighted in the review.40 The MoS2/GF nanocomposites exhibited reformative HER activity with low overpotential and large apparent cathodic currents. Such enhanced catalytic activity results from the abundance of catalytic S edge sites, the increase in electrochemically accessible surface area and the unique synergic effects between the GF support and the active catalyst. Hydrogen evolution at the MoS2 catalytic edge sites is noticeable through the Volmer–Heyrovsky (rate-­determining step) mechanism. In detail, the overall HER reaction occurs via a rapid discharge step [Volmer reaction, eqn (9.1)] followed by an ion and atom reaction [Heyrovsky reaction, eqn (9.2)] in acidic media.   

Discharge reaction: H3O+ + Med + e− ⇌ H2O + M−Had

(9.1)

  



Ion and atom reaction: M−Had + H3O+ + e− ⇌ H2 + Med + H2O

(9.2)

  

In the search for complementary morphologies, the formation of graphene foam is frequently preferred over native 2D forms due to its higher available area. One of the possible applications of graphene foam is in chemical sensing. Graphene sensors are sensitive in detecting NOx; they can work at room temperature and are easily reused. All types of graphene require an electric shock to remove trapped gas molecules. GF may also be used in energy storage, such as in supercapacitors and batteries. The structure of GF offers the potential for storing massive amounts of energy, such as hydrogen. GF has a high surface area due to its porous nature, providing it with a high electrochemical capacitance. GFs have been successfully prepared using several routes, including chemical vapour deposition (CVD) methods and wet chemical approaches.1,40–43 Laser-­induced GF has also been prepared recently through a facile method.44 In the CVD fabrication of graphene, H2 and methane gas are introduced into a furnace that heats up to 1000 °C.42,43 In the furnace, a Ni or Cu film captures monolayer or multilayer graphene as the furnace heats up and methane decomposes. Instead of using a thin Ni sheet to capture graphene, Ni or Cu foam is used to create CVD GF. The metal foam captures graphene in a similar foam structure. Then, the metal skeleton is etched away, and a visible, porous 3D GF structure remains, as shown in Figure 9.1. This structure has a high surface area, introducing several novel applications of GF

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Figure 9.1  SEM  images of (a and c) graphene films on nickel foams and (b and d)

the resultant GFs. Reproduced from ref. 42 with permission from Macmillan Publishers Ltd, Copyright 2011.

whilst maintaining most of the properties of 2D graphene. The CVD processing of GF offers a cost-­effective route for engineering a new class of ultralight, highly conductive graphene-­based materials that exhibit exceptional mechanical strength, flexibility and elasticity. The electrical and structural properties of CVD-­produced GF are superior to those of other contenders, namely, chemically derived reduced graphene oxide (GO) and few-­layered graphene nanoplatelets. The latter two suffer from a high concentration of defects and poor interflake mechanical contact because unlike CVD GF, they require many separate sheets of graphene to connect with one another. By contrast, CVD processing simultaneously creates the entire GF. For the aforementioned conversion methods, either high-­temperature furnaces, extremely pure gases or large amounts of strong acids and oxidants are frequently required. The direct irradiation of polyimide (PI) plastic films by laser in the air using a commercial laser scribing system found in most machine shops recently converted PI into 3D porous graphene; this material is called laser-­induced graphene (LIG).44 This one-­step method does not require high-­temperature reaction conditions, solvents or subsequent

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treatments; it provides graphene with many five-­and seven-­membered rings. Ye et al. firstly introduced approaches developed for producing LIG and controlling morphology (either porous sheets or fibrils), porosity, composition and surface properties.45 The surfaces can be varied from being either superhydrophilic, with a 0° contact angle with water, to being superhydrophobic, with >150° contact angle with water. For the preparation of 3D GF, a roll-­to-­ roll method for fabricating in-­plane electronics and 3D printed LIG at the macroscopic scale is adaptable to the large-­scale production of LIG. Various applications of LIG ranging from renewable energy devices to water treatment platforms have been achieved using these types of materials. The electrodeposition method provides an effective and efficient approach for expanding the materials deposited on LIG beyond oxides.46 The case of LIG is presented as an example. Co–P and NiFe hydroxides were electrodeposited onto LIG surface to function as HER electrode and oxygen evolution reaction (OER) electrode, respectively.47 Two designs were reported in this work. The first design is fabricated on the two sides of a PI sheet; the second design is produced on a wood surface. Compared to materials electrodeposited on glassy carbon, the HER electrode made on the LIG surface has lower onset overpotentials and higher current densities at the same overpotentials due to its higher surface area and the metal-­enhanced synergistic effect. The HER and OER electrodes can deliver high current densities at low overpotentials, and the Tafel slopes reach 35 mV dec−1 and 78 mV dec−1, respectively. A durability test shows that the electrodes made on LIG surfaces are stable for catalysing overall water splitting.

9.2.2  P  hotochemical Process for H2 Generation from   the Degradation of H2O Catalysed by Graphene   Hydrogels (GHs) Although hydrogen energy can be obtained electrochemically, the massive electric energy requirement may contradict the concept of a sustainable development approach. Photocatalytic water splitting is considered a possible strategy for the clean, low-­cost and environment-­friendly production of H2 by utilising solar energy.48 Many materials are capable of photocatalytically producing hydrogen. However, energy conversion efficiency remains low and insufficient for practical applications because the three crucial steps for the water-­splitting reaction (i.e. solar light harvesting, charge separation and transportation) and catalytic reduction and oxidation reactions are inefficient or not simultaneous.49 Apparently, 3D graphene-­based photocatalysts can solve these limitations.17 A hydrogel can be formed if the swelling agent is water and the network component is a hydrophilic polymer.47 Chemically modified graphene (e.g., GO) is a 2D amphiphilic material with a unique edge-­to-­centre arrangement of hydrophilic and hydrophobic segments. On the basis of molecular structure and morphology, modified graphene sheets can function as polymer gelators because a 2D laminated structure with many conjugated domains

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facilitates contact with one another to form network junction points. GHs are formed by self-­assembling modified graphene sheets into a 3D network structure with a relatively low critical gelation concentration through hydrogen bonding, electrostatic interaction or π–π interaction. GHs can be further modified, such as through the hydrothermal reducing method, by mixing GO and ascorbic acid.50 The extended π conjugation in graphene sheets is advantageous for extended π stacking interactions between graphene sheets to form strong bindings, leading to highly robust cross-­links in GHs. Moreover, the partial overlapping of graphene sheets via hydrophobic and π–π interactions in 3D space benefits the formation of GHs. Chen et al. firstly reported a convenient hydrothermal process for the synthesis of N-­doped GH using organic amine and GO as precursors at low temperatures.51 Achievements in materials science are critical for the development of energy conversion and storage techniques. With their exceptional porous structure, large surface area, excellent electronic conductivity and high mechanical strength, 3DGBMs have been intensively explored for applications in energy storage and conversion systems.52 These fields include Li-­ion/Li batteries, supercapacitors, fuel cells, solar cells and hydrogen energy. An example is nanocomposite GH (NGH) consisting of a photostable TiO2 and Au nanostructure for photocatalytic H2 production under Xe arc lamp.17 A comparison was conducted amongst the photocatalytic activities of pure TiO2 nanorods, 2D rGO/TiO2 and NGH with different Au loadings under irradiation at different light wavelengths. The photocatalytic activity of NGH increased to 167–242 µmol h−1 g−1, outperforming TiO2 nanorods (∼156 µmol h−1 g−1) and 2D rGO/TiO2 (51 µmol h−1 g−1). The incident photons are absorbed by Au nanoparticles (NPs) via localised surface plasmon resonance excitation under visible light irradiation. The electrons generated from Au NPs are injected into the TiO2 conduction band, leading to the generation of holes in Au NPs. The holes are quenched by sacrificial electron donors. Upon ultraviolet–visible irradiation, TiO2 absorbs photons of energy greater than the bandgap, generating electron–hole pairs. In turn, all the excited electrons are transferred from the TiO2 conduction band to the graphene active sites to produce protons in the solution to form H2. The performance is outstanding because of the following reasons. (i) The NGH 3D framework with a large surface area has desirable pores that facilitate liquid access, diffusion and open structure for integrating functional nanomaterials (e.g., TiO2, Au). (ii) The rapid recombination of photogenerated electrons and holes is prevented by interconnected highly conductive electrical pathways.53

9.2.3  H  ydrolysis of NH3BH3 for H2 Generation Over Modified 3D Graphene Materials Given the volumetric and gravimetric issues in solid-­state storage, a feasible hydrogen storage system is the major bottleneck in its development. Considering its high hydrogen content, safe handling and stability, NH3BH3 is a highly attractive candidate as a hydrogen source54–58 that can be released

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55

through either pyrolysis or hydrolysis. Catalytic hydrolysis can produce 3 mol of hydrogen per mol of NH3BH3 at room temperature, presenting a high hydrogen capacity of up to 19.6 wt% of the starting materials (NH3BH3 and H2O); thus, it is an effective approach for hydrogen release from NH3BH3.55,60 At present, noble metal-­based materials (Rh, Pt or Ru) are considered the most effective catalysts for the catalytic hydrolysis of NH3BH3; however, their high cost and scarcity severely hinder their large-­scale applications.61–63 To date, various catalytic systems have been studied for hydrogen generation from the hydrolysis of NH3BH3. Amongst them, Pt exhibits the highest activity; however, efficient and economical catalysts must be developed for the practical application of this system, and kinetic properties must be improved by controlling particle size and increasing the surface area of the acquired metal catalysts. A good support material for catalysts should exhibit the following characteristics: (i) high surface area, (ii) strong affinity towards catalyst particles to immobilise them and ensure their good dispersion and (iii) good chemical stability under the operating conditions to maintain a stable catalyst structure.35 Hydrogen spillover is the dissociative chemisorption of hydrogen on metal and the subsequent migration of atomic hydrogen onto the adjacent surfaces of a support.64 The contact between metal NPs and the support plays a critical role in the spillover mechanism.65,66 As mentioned previously, the hydrogen adsorption and desorption processes of carbon materials is significantly influenced by chemical modification or doping.67 Theoretical and experimental studies have demonstrated that the substitutional doping of carbon materials can be used to modify their physical and/or chemical properties.68,69 In particular, numerous reports on N and B doping of carbon-­ based nanomaterials have emerged recently.70,71 N doping causes an increase in conductivity because N atoms donate additional electron density to the parent matrix.72,73 N doping promotes surface chemical activity in terms of polarity and basicity.74 Mahyari et al. presented a technologically sound and economically practicable material with good hydrogen generation catalysis at ambient or near-­ambient temperatures and pressures.35 This material is the first successful example of using 3DNGM-­supported metal catalysts for hydrogen generation from the catalytic hydrolysis of NH3BH3 in the numerous pores of 3DGBMs. 3DNGMs for immobilising the Ni NPs have been subsequently used in the catalytic hydrolysis of NH3BH3. To determine whether the support (i.e. 3DNGMs) of Ni NPs exerts a significant effect on their catalytic activity, Ni NPs were synthesised without 3DNGMs as support and examined in the catalytic hydrolysis of NH3BH3. With Ni NPs as the catalyst, complete hydrogen release under the same conditions required 35 min and increased significantly after every run. The results indicated that 3DNGMs as supports exert a considerable effect on catalytic activity due to their high efficiency for effectively immobilising Ni NPs, preventing them from aggregating, controlling their growth and increasing their specific surface area. Moreover, increasing the stability of a catalyst and facilitating the recovery process are

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Figure 9.2  Proposed  mechanism for the catalytic decomposition of NH3BH3. Reproduced from ref. 35 with permission from the Royal Society of Chemistry.

other advantages of using 3DNGMs as support for Ni NPs. In this regard, the judicious selection of support materials can help stabilise metal NPs and participate in gas adsorption. A plausible mechanism of the activation process is presented in Figure 9.2. The NH3BH3 molecules and the Ni particle surface interact to produce an activated complex species in the rate-­determining step, in which an attack by a H2O molecule readily results in the concerted dissociation of the B–N bond and the hydrolysis of the ensuing BH3 intermediate to form borate ion with the release of H2. In accordance with the literature, dehydrocoupling between NH3BH3 molecules occurs in the absence of H2O, producing new B–N bonds on the metal surface, most likely via a closely related intermediate.

9.2.4  H  ydrolysis of NH3BH3 for H2 Generation Mediated by Modified 3D GHs As mentioned previously (Section 9.2.1), GHs, as typical 3D graphene materials, exhibit advantages over various conventional materials and have attracted overwhelming attention from different fields. Modified GHs exhibit considerably superior properties with special functions, such as hydrogen generation. Men et al. reported the novel materials of P-­doped noble metal-­ free CoB NPs anchored on a 3D N-­doped GH, denoted as CoBP/NGH, which was successfully used as a catalyst towards the catalytic hydrolysis of NH3BH3 at room temperature.75 CoBP/NGH catalysts with different P doping levels were successfully prepared via a simple one-­pot coreduction approach and studied for catalytic hydrogen generation towards the hydrolysis of NH3BH3 at room temperature. N-­doped GH was used as support. NaBH4 was used as a B source and reductant, in which the metal precursors were reduced to form CoB NPs. In this work, CoBP/NGHs with different P doping levels were obtained by changing the amount of NaH2PO2 (Figure 9.3) Initially, Co0.85B0.15/NGHs were prepared to test the catalytic activity towards the hydrolysis of NH3BH3, and only a 2.5 equivalent of gas was obtained after more than 10 min, with

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Figure 9.3  Graphene  (a) catalytic performance and (b) durability tests of CoBP/ NGH with different molar amounts of P content. Reproduced from ref. 75 with permission from Elsevier, Copyright 2017.

a time-­of-­flight (TOF) value of 6.2 min−1. CoBP/NGH catalysts exhibited considerably enhanced catalytic activities with 100% hydrogen selectivity after P doping. In particular, Co0.79B0.15P0.06/NGH exhibited the highest catalytic activity amongst the tested catalysts, with a TOF value of 32.8 min−1, which is nearly five times higher than that of Co0.85B0.15/NGH without P doping and higher than those of most of the previously reported non-­noble metal-­based catalysts. These results highlight the synergistic effect between CoB and P in facilitating the hydrolysis of NH3BH3. For comparison, Co0.79B0.15P0.06/ NGH NPs without support materials or supported on GO were also prepared and their catalytic activities towards the hydrolysis of NH3BH3 were studied. Their catalytic activities were inferior to that of Co0.79B0.15P0.06/NGH. Moreover, catalytic activity and hydrogen selectivity were maintained well in the durability test. On the basis of the results and in accordance with the Arrhenius equation, Ea = 39.42 kJ mol−1, the catalytic activity is considerably lower than those of most reported Co-­based catalysts and even several noble metal-­ based catalyst All these results suggest that 3D N-­doped GHs are beneficial for the hydrolysis of NH3BH3. Similarly, a series of the N-­doped GHs anchored on different amounts of Co-­CeOx NPs (Co-­CeOx/NGH) were synthesised. After the catalysts were tested, N-­doped GH with an appropriate ratio of CeOx [Co-­(CeOx)0.91/NGH] exhibited desirable catalytic activity and 100% hydrogen selectivity, with a TOF value of 79.5 min−1, which is nearly 13 times higher than that of Co/ NGH and higher than most of the reported non-­noble metal-­based catalysts, including several noble metal-­based catalysts.76 Moreover, the synergistic effect between Co and Ce in facilitating hydrogen generation from the hydrolysis of NH3BH3 can be confirmed from the negligible activity of CeOx/rGO. A comparison of photocatalytic performance on Co-­(CeOx)0.91 anchored on different support materials was conducted to highlight the role of NGH. Co-­(CeOx)0.91/NGH exhibited the highest catalytic activity and 100% hydrogen

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selectivity, suggesting that N-­doped GH plays a critical role in accelerating hydrogen generation from the hydrolysis of NH3BH3 probably due to strong metal–support interaction.

9.3  Hydrogen Storage Many researchers have predicted that we will drive cars powered by hydrogen in the near future. These ‘green’ cars will only release water after the combustion of hydrogen, which will benefit the environment of our planet. However, a huge hurdle exists; that is, a safe method for storing hydrogen has not yet been found. Hydrogen storage is a key enabling technology for the advancement of hydrogen and related applications, including stationary power, portable power and transportation. Hydrogen has the highest energy per mass amongst all types of fuel; however, its low ambient temperature density results in a low energy per unit volume, requiring the development of advanced storage methods that have the potential for high energy density. Hydrogen can be stored physically as either gas or liquid. The storage of hydrogen as gas typically requires high-­pressure tanks (350–700 bar tank pressure). The storage of hydrogen as liquid requires cryogenic temperatures because the boiling point of hydrogen at one atmosphere pressure is −252.8 °C. This conventional storage system will need steel walls that are at least 3 inches thick, making them exceedingly heavy and large. Hydrogen can also be stored on the surfaces of solids (via adsorption) or within solids (via absorption). In summary, these challenges can be overcome by pursuing two strategic pathways: focusing on (1) compressed gas storage, cold or cryo-­compressed hydrogen storage and (2) materials-­based hydrogen storage technologies, including sorbents, chemical hydrogen storage materials and metal hydrides, as shown in Figure 9.4.77 Adsorbents can match or surpass the typical capacities of physical storage systems at lower pressures and with the potential to reduce cost. Since 1978, novel, highly active MgH2–Mg systems, which can be used in synthetic chemistry and as high-­temperature hydrogen storage materials, have been studied at the Max-­Planck-­Institut für Kohlenforschung in Mülheim an der Ruhr. Materials, such as organic compounds (ethylene glycol), metal hydrides and chemical hydrogen, have been explored as hydrogen storage.78 As shown in Figure 9.4, metal–organic frameworks are porous crystalline structures capable of storing gases, such as CO2, methane or H2, in tight spaces. A new material that operates at room temperature and atmospheric pressure at the flick of a switch was recently developed. It is made from a heavy metal (Rh) and its weight-­to-­fuel ratio is low (0.1%), but it can fill the time lag between a driver placing his/her foot on the accelerator and a metal hydride fuel tank reaching the desired temperature. This material is known as [Rh6(PH3)6H14]+, and it can be used to build future hydrogen tanks.79

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Figure 9.4  Illustration  of hydrogen storage (Image courtesy of US Department of Energy).

The functionality and performance of carbon materials can frequently be improved by fabricating them in the form of microstructures or nanostructures. Porous carbon materials for hydrogen storage have received extensive attention due to their large surface area, large pore volume, good chemical stability and easily tailored porosity.80 In particular, 3D carbon materials have recently attracted considerable attention due to their additional functionalities, such as high specific surface area and porosity, desired architecture for electrodes and current collectors, ease of doping and functionalisation. A set of pillared graphene carbon with various carbon nanotube (CNT) diameters is illustrated in Figure 9.5.81 Therefore, 3D carbon materials are used as electrodes for electrochemical energy storage and conversion systems, such as supercapacitors, batteries and fuel cells, active materials for optoelectronic devices and solar cells, electrocatalysts and catalyst supports, filters and sorbents for water treatment and hydrogen and methane storage.82–86 GO prepared using the Hummers method (H-­GO) exhibits a considerably stronger expansion of lattice due to swelling compared with GO prepared using the Brodie method (B-­GO); meanwhile, B-­GO demonstrates

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Figure 9.5  Configurations  of 3DGCs with different CNT diameters in (a) to (e). Reproduced from ref. 81 with permission from AIP Publishing, Copyright 2017.

better ordered structures with well-­defined layer-­by-­layer intercalation of solvents.87–89 Graphite and GO offer enormous possibilities for chemical modification to prepare materials with porous structures using pillaring molecules.90 Pillared GO (PGO) materials with high surface area can be useful in many applications, including gas storage and materials for supercapacitors and membranes.91–93 PGO has been considered a promising material for hydrogen storage applications due to theoretical predictions of improved gravimetric sorption in subnanometre slit pores formed by graphene planes.38 Graphite presents ∼3.35 Å spacing between graphene sheets. In accordance with theoretical studies, the optimal separation of graphene sheets for maximal hydrogen sorption is predicted at the level of 7–12 Å.94 However, these theoretical simulations merely postulate variations of interlayer distance, whilst experimental realisation requires some pillaring molecules that can keep graphene sheets separate from one another. The synthesis of pillared GO presented here involves three steps: (i) swelling (expansion) of GO structure in a solution of pillared molecules, (ii) insertion and attachment of pillaring molecules into GO interlayers at elevated temperatures and (iii) removal of solvent via evaporation whilst the pillaring molecules maintain the GO lattice in an expanded state, creating a porous structure.

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9.3.1  Hydrogen Storage on Pillared Carbon Materials GBMs with a 3D network nanostructure have been confirmed to exhibit outstanding hydrogen storage capacity based on theoretical predictions and experimental results.95 The adsorption of H2 on pillared carbon materials is discussed in this section. Initially, a novel pillared GO material was prepared via solvothermal reaction with tetrakis(4-­aminophenyl) methane (TKAm), denoted as GO/TKAm.96 The molecule has four amine groups, making the interlinking of GO planes probable, and a 3D shape that can be beneficial for the stability of pillared structures. Hydrogen sorption was measured for two samples at ambient and liquid N temperatures. The superior values of hydrogen sorption (1.66 wt% at 77 K and 0.25 wt% at 295 K) were obtained for the sample with an initial surface area of 660 m2 g−1 measured on GO/TKAm. Surface area values with a maximum of over 2000 m2 g−1 are theoretically possible. Hydrogen sorption was also verified for this sample using a gravimetric method, providing a value of 0.21 wt% at 295 K, which is in good agreement with the measurement made using a volumetric method. This value is also consistent with the theoretical estimation of hydrogen sorption for an ideal structure with a regular arrangement of pillars, which amounts to ∼5 wt% (saturation value at 77 K) and ∼0.5 wt% (300 K, 150 bar).

9.3.2  R  eversible Hydrogen Storage on Carbon Material Composition (SiC/G) The research results constitute the foundation for creating the functionality of the nanocomposite material ‘GraphRoll’ on the basis of polycrystalline graphene providing for the reversible storage of hydrogen (Figure 9.6), i.e. in

Figure 9.6  Guiding  idea of the nanocomposite material ‘Graphroll’ based on polycrystalline graphene providing the reversible storage of hydrogen. Reproduced from ref. 97 with permission from Elsevier, Copyright 2014.

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the installations provided for thermal and thermochemical processing using hydrogen as the process or hardening medium.97 In another case, non-­pomp compressors based on ‘GraphRoll’ are used for hydrogen recycling within high-­pressure gas hardening and hydrogen separation from after-­process gas mixtures in thermochemical processing operations.98 The innovation in this method is based on the fact that a nanomaterial is the nanocomposite of a spiral reel shape, with a spiral stroke ranging from 0.2 to 2, and a polycrystalline graphene with a size (grain diameter) of over 50 nm. The outer diameter of the reel is assumed to be within the range of 500 nm to 5 mm. Meanwhile, the width of the reel will be from 0.05 mm to 1000 mm. SiC NPs will be placed through functionality within the space between the succeeding wounds. The polycrystalline graphene will be coiled onto a core diameter ranging from 1 nm to 20 µm from a single graphene sheet or from multiple sheets ranging from 2 to 50 000. A structure obtained in this manner can store hydrogen with weight levels over 6.5%. Moreover, such a temperature can already be obtained by ‘pillaring’ at the level of the heterogenic growth of graphene flakes on SiC NPs. The theoretical analysis of the hydrogen contents supplemented by this study accounts for actual conditions and proves that the maximal contents of hydrogen within graphene can amount to even 6.0% weight at SiC levels amounting to an atomic 2% at a pressure of 10 MPa and a temperature of 250 K for interplane distances of 7 Å. In addition, when SiC content increases within the graphene structure, the theoretical capacity of hydrogen sorption slightly decreases from 6.58% (for 0% SiC presence) to 6.15 wt% H2 for 2% SiC under conditions of P = 5 MPa and T = 300 K.

9.4  Conclusion To date, different graphene-­based materials with a 3D network structure, including hydrogels, aerogels, pillared materials and nitrogen-­doped compositions, have been prepared using various synthesis strategies. These newly developed materials have successfully served as catalysts supports and adsorbents. The production of hydrogen from H2O and BH3NH3 via 3D GF, GHs and nitrogen-­doped graphene composition has been discussed. The results indicate that 3D carbon materials (3DCMs) can be used as robust matrices for accepting metals, metal oxides and active polymers for different applications, particularly in catalytic systems. These attractive materials exhibit low mass density, high surface area, continuously interconnected macroporous structures and excellent chemical and physical stability. Moreover, 3DCMs, such as pillared carbon materials, e.g. ‘GraphRoll’, have also been presented in the applications of hydrogen storage. The chemical modification of the adsorbent surface with N-­based functional groups or heteroatom doping and transition metal doping are promising approaches for raising the binding energy states of H2. Thus, the development of efficient, economical, stable, long-­lasting and reusable metal catalysts to further develop the kinetic and

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thermodynamic features of hydrolysis reactions and hydrogen storage under mild conditions provides a feasible approach for practical applications. In future research, the performance of hydrogen production and storage through 3DCMs will be improved via further tuning and modified processing. In addition, different types of building blocks can also be applied to the construction of 3D structures. This process may combine the advantages of these hybrid material systems. The chemical and physical interactions amongst different components should be controlled well, and new cross-­ linking molecules for hydrogen engineering should be identified.

List of Abbreviations 0D/1D/2D/3D 0 dimension/1 dimension/2 dimension/3 dimension GBMs Graphene-­based materials HER Hydrogen evolution reaction (2H+ + 2e− → H2) GF Graphene foam CVD Chemical vapour deposition GO Graphene oxide rGO Reduced graphene oxide PI Polyimide LIG Laser-­induced graphene OER Oxygen evolution reaction GH(s) Graphene hydrogel(s) NGH Nanocomposite graphene hydrogel NPs Nanoparticles CNT Carbon nanotube H-­GO Graphene oxide prepared using the Hummers method B-­GO Graphene oxide prepared using the Brodie method PGO Pillared graphene oxide TKAm Tetrakis(4-­aminophenyl) methane 3DCMs 3D carbon materials

Acknowledgements This work was financially supported by the Education University of Hong Kong through the Dean's Research Fund 2018/19 (Project: 04405 and 0439).

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Chapter 10

Harvesting Solar Energy by 3D Graphene-­based Macroarchitectures Xianbao Wang*a, Zhenzhen Guoa, Fang Yua and Xin Minga a

Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan 430062, China *E-­mail: [email protected]

10.1  Introduction Fresh water, a finite natural resource, is vital for the survival of various life forms on Earth.1,2 However, the ongoing freshwater crisis, on account of the irrational use of this resource, environmental pollution, and climate change, is affecting over three billion people across the globe.3 Hence, there is an urgent need for developing efficient, low cost and green technologies to provide freshwater supplies to water-­stressed regions. In the past decades, great efforts have been devoted to generating high-­quality freshwater generation from brines or even polluted water. To date, various water purification technologies have been well-­developed in this regard,

  Chemistry in the Environment Series No. 1 Graphene-based 3D Macrostructures for Clean Energy and Environmental Applications Edited by Rajasekhar Balasubramanian and Shamik Chowdhury © The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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including reverse osmosis (RO), multistage flash (MF), and multiple-­ effect distillation (MED).4–6 However, these technologies suffer from several drawbacks, such as high energy consumption, membrane fouling, and low salt rejection.7 In addition, high electricity input and complex infrastructure are inevitably required, which greatly limit their practical applications, especially in offshore areas, rural settings, or remote off-­grid regions. Recently, solar steam generation (SSG), utilizing clean and abundant solar energy,8 has opened up a new territory for producing fresh water.9,10 Three main strategies are used to improve the efficiency of SSG systems: (i) developing photothermal materials with high spectral absorption across the entire solar spectrum, including metallic materials,11–13 semiconductors14–20 and carbon-­based materials;21,22 (ii) conceiving thermal positioning methods that can ensure solar transformation into heat and maximize the use of heat;23 (iii) synthesizing highly hydrophilic and porous frameworks that allow sufficient water transportation to the surface of the absorber.24 In particular, 3D porous structures display outstanding photothermal performance owing to the reduction of heat loss and light reflection and/ or greater effective evaporation area and optical absorption. Nonetheless, photothermal conversion materials are supposed to be easily scaled up and possess cyclic stability under extreme conditions for practical applications. Graphene is a flat monolayer of sp2-­hybridized carbon atoms, firmly packed into a two-­dimensional (2D) honeycomb lattice. The optical and surface properties of graphene can be elegantly tuned via regulating its oxidation degree (e.g., GO: graphene oxide or rGO: reduced graphene oxide).25,26 These graphene-­based derivatives with large surface area and excellent hydrophilicity are suitable for assembling into 3D architectures, such as sponge, hydrogel, aerogel, etc.27–29 3D GBMs are one of the most attractive candidates for efficient SSG as they bring together the merits of broadband light absorption, light weight, excellent chemical stability and tunable thermal conductivity.30 Herein, we review the impressive developments in 3D graphene-­based photothermal materials optimization, photothermal structural design, and their practical applications. We present guidelines for fabricating 3D GBMs-­ based solar evaporators for achieving efficient solar light harvesting, optimal thermal management, and fast water transport. Firstly, we explore the mechanism of solar to thermal conversion. Secondly, the structural design of photothermal materials to maximize light absorption and produce thermal energy is discussed. Meanwhile, advanced configurations are introduced, including 2D/1D water channel structures. Thirdly, the applications in seawater desalination, wastewater purification, and energy generation by 3D GBM-­mediated SSG systems are also summarized. Finally, the existing challenges and future perspectives for developing high-­performance 3D GBMs solar evaporators are also discussed.

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10.2  B  asis of Solar-­thermal Conversion and Transport For an efficient solar evaporator, excellent optical and thermal properties are supposed to be the basic criterion. For optical properties, the solar absorber material must possess a high absorption coefficient for the purpose of harvesting most of the energy from solar radiation. The light absorbed should be converted to heat energy by the absorber as much as possible. In addition, energy loss should be minimized to take full advantage of the thermal energy and avert other means of heat loss, including conduction, convection, and radiation.31 Simultaneously, the inherent properties of the materials, such as durability, anti-­fouling property as well as cost are also critically important for practical applications.

10.2.1  Solar Absorption A pivotal factor of how well the absorber converts light into heat is determined by the light capture ability of the absorber. Intuitively, the solar absorbers need to have high solar absorption with minimal transmittance and thermal emittance across the full solar spectrum range of 300 nm to 2500 nm (ASTM G-­173, AM 1.5). A high overall solar absorptance of the material is thus desirable for high solar to thermal efficiency, high surface temperature, and high vapour generation rate. Solar absorptance is a measure of the ability of the material to absorb solar irradiation, and it is equal to the ratio of the total absorbed solar radiation to the incident radiation. The total solar absorptance for a given angle of incidence θ is acquired by contrasting the spectral absorptance of the material with the solar spectral irradiance distribution of the standard solar spectrum (AM 1.5), and integrating over the range of wavelengths in which solar radiation reaches the solar absorber surface: α(θ) is the total solar absorptance, θ is the incident angle of light measured from the surface normal of the solar absorber, and Qsun is the total solar irradiance. The solar absorptance, α(θ), is defined as:32   

max



α( ) =



min

max

1  R( ,  ) A  ,   d  max



 A   d 

 1  R  ,   A  ,   d 

min

Qsun

(10.1)

min   

where λmin, λmax are 0.3 µm and 2.5 µm, respectively. A (λ) is the wavelength-­ dependent solar spectral irradiance. R (θ, λ) is the total reflectance at the wavelength of λ. The numerator of this equation is the total absorbed solar energy. The denominator is the incident solar irradiance Qsun.

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The emissivity is the ratio of the total emissive power of the real surface to that of a blackbody at the same temperature. The emissivity is dependent on the nature and surface state of the materials. According to Kirchhoff's law of thermal radiation, the thermal emittance, ε (θ) is calculated with the following equation:33   

max



   

 1  R  ,   B  ,   d 

min

max





(10.2)

B  d 

min   

where λmin, λmax are 0.3 µm and 2.5 µm, respectively. B (λ) is the spectral radiance of a black body at the measured temperature, given by Plank's law. The range of solar absorptance is between the visible and near-­infrared regions of solar irradiance, but that of the thermal emittance is temperature-­ dependent and usually in the infrared regions, where the blackbody radiation dominates. Generally, the solar absorptance (A) of the absorber can be briefly calculated by the following equations:34   



A = 1 − R − T

(10.3)

  

where R is the reflectivity at the interface of the absorber, and T is the transmissivity of the absorber. Based on the above theory, how to maximize the solar absorptance while maintaining low thermal radiation and low heat convection is the critical factor to acquire a high temperature of the solar absorber. Previously, optical concentrators, such as parabolic troughs, heliostats and lenses, are utilized to concentrate the ambient solar flux to get a high temperature,35,36 but optical concentrators are expensive. Besides, they need extra support structures and access to electrical energy to track the sun.37 Typical spectrally selective absorbers, cermet absorbers or lithography-­free ultrathin multilayer absorbers have been recently developed for solar-­thermal energy application. Radiation heat losses to the ambient environment are minimized by the spectrally selective absorber materials and then strong sunlight concentration is achieved.38 However, the absorber is able to possess low thermal emittance and exhibit high light adsorption under the low optical concentrations for achieving highly effective solar water evaporation without other energy input, which remains a challenge.

10.2.2  Thermal Transfer For such a solar evaporator, the conversion of sunlight received into heat is also a significant process. To achieve high thermal efficiency, the absorber is required to capture sunlight and convert it into energy effectively.

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Heat loss is inevitable in the total heat flow of the solar evaporation system. The latent heat in vapour generation (qevap) takes away the majority of the generated thermal energy while the rest is consumed by conduction to underlying bulk water (qcond), convection with ambient air (qconv), and radiation to the environment (qrad). Figure 10.1 schematically illustrates the steady-­state heat transfer process in a typical 3D solar steam system. In equilibrium, the input and output thermal energy can be described as:39   



αqsolar = qevp + qconv + qrad + qcond

(10.4)

  

where α denotes solar absorbance of the absorber, and qsolar is the incident sunlight. The effective energy for evaporation (qevap) can be calculated as:40   



qevap = mevap(hLv + (C(Tv − Tl)))

(10.5)

  

where mevap is the evaporation rate, hLv is the latent heat of liquid–vapour phase change, C is the specific heat of water (4.2 kJ kg−1 K−1), Tv is the vapour temperature, and Tl is the bulk water temperature.

Figure 10.1  Schematic  diagram of photothermal conversion.

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The conductive heat loss to bulk water (qcond) can be calculated through the temperature gradient in the underlying water:41   

  

qcond 

A  k  (Tl 1  Tl 2 ) l

(10.6)

where k is the thermal conductivity of the absorber with water inside, A is the surface area of absorber facing the sun, Tl1 and Tl2 are the temperatures of the two different bodies (assuming steady-­state temperatures) and Δl is the thickness of the medium. The convective heat loss (qconv) to the adjacent environment can be calculated as:42   



qconv = Ah(Ta − T∞)

(10.7)

  

where h is the heat transfer coefficient, Ta is the top surface temperature of the absorber, and T∞ is the temperature of the surrounding fluid. Some researchers think that the adjacent temperature can be approximated as the steam temperature since the absorber is surrounded by a water layer and hot steam. The radiative heat loss (qrad) to the ambient environment can be calculated as:43   



  

4  qrad A  Ta4  Tambi ent 



(10.8)

where ε denotes the radiation emittance of the absorber surface, and σ is the Stefan Boltzmann constant (5.669 × 10−8 W m−2 k−4). Currently, controversies exist in the use of ambient temperature (Tambient) for assessing radiative heat losses. Some researchers believe that the ambient temperature may be considered as the temperature at the infinity, namely, the temperature of cooler surroundings while others believe that the ambient temperature is approximately equal to the vapour temperature concerning the existence of water layers and hot vapour above the absorber. The upper and lower limits of the ambient temperature values should be set with a combination of the two currently prevailing viewpoints, and the radiative heat loss should be in the range that is delimited by the maximum and minimum values calculated by using T∞ = Tambient and T∞ = Tvapor, respectively. As mentioned earlier, heat flows occur in the total heat transfer process through three main ways, and all of them depend on the temperature gradient. Heat conduction is usually limited via an appropriate insulating material. As long as the solar absorbers are at a higher thermal state than the surroundings, radiative heat loss still exists and evaporation proceeds. In order to improve the heating efficiency, the solar evaporator is supposed to possess minimal heat resistance to minimize heat transfer to all other areas. Often systems employ a wicking layer or channels to allow capillary action to draw water instantaneously to the surface of the solar absorber for effective steam generation and water evaporation. The system that can let the

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263

solar absorber transmit the heat immediately to its surface, should be further investigated to optimally reduce its operating temperature for directly improving the photothermal conversion efficiency.

10.2.3  Thermal to Steam Generation To obtain maximum vaporization efficiency, the solar absorber should convert solar energy into heat for the purpose of water vaporization. However, in practice, some heat tends to be lost to the bulk water and surroundings by heat conduction, convection and radiation. Therefore, the solar-­thermal conversion efficiency (η) is divided into the heating efficiency (ηheating) and the evaporation efficiency (ηheating).44 The η can be summarized as follows:   

η = ηheating + ηevaporation



(10.9)

  

To quantify the transient performance, the transient efficiency is defined as the total energy used for water vaporization, which is divided by the incoming solar energy during the illumination time interval:   

t





 m C evap

0

p

 T  H vap  d t

t

C

opt



(10.10)

IAproj d t

0

  

where Cp is the specific heat capacity of water (4.18 J g−1 K−1); ΔT is the difference between the vapour temperature and the ambient temperature; ΔHvap is employed to the latent enthalpy of vaporization for water at different evaporation temperature. Recent studies employed ΔHvap at the vapour temperature, which was measured in actual experimental conditions and was usually lower than the boiling temperature. Copt is the optical concentration, I is the nominal direct solar illumination (1 kW m−2), and Aproj is the projection area. It is sometimes substituted by:   





  

m  Cp T  H vap 

(10.11)

Copt IAproj t

where Δm (kg m−2) is the mass change of water vaporization during illumination time Δt (h). However, mevap = Δm/Δt, the eqn (10.11) can be rewritten as:   

  



mevap  Cp  T  H vap  Copt IAproj



(10.12)

As briefly mentioned, solar-­thermal conversion efficiency mainly consists of evaporation efficiency. How to obtain a high evaporation efficiency is the key to achieving efficient photothermal conversion. However, the evaporation

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efficiency depends on the superior evaporation rate and the low enthalpy of vaporization. In this case, the excellent evaporation rate is acquired by the novel solar-­driven evaporation systems. Recent studies10 are devoted to investigating the hydratable polymer network. It is determined by the proportion of intermediate water that is vaporized by less energy compared with bulk water, and thus affects the overall energy demand of steam generation. Various strategies to enhance light-­to-­heat conversion efficiency of various solar absorber materials should be further discussed for efficient solar water vaporization.

10.3  Development of 3D GBMs for Efficient SSG Owing to large surface areas, broadband light harvesting properties and excellent thermal stabilities, 3D GBMs have stood out as promising photothermal materials for efficient SSG (Table 10.1). In this section, the use of 3D GBMs in terms of materials can be classified into three categories: (1) 3D graphene, (2) 3D GO/rGO, (3) 3D graphene hybrid materials.

10.3.1  3D Graphene Solar steam evaporators have four essential conditions: extended light absorption, efficient photothermal conversion, adequate water supply and rapid escape of steam. For excellent photothermal performance and porous structure for water supply and steam escaping, 3D graphene45–51 has always played an important role for efficient SSG. In 2015, 3D porous N-­doped graphene52 was first developed using a chemical vapour deposition method with 1–2 µm pore channels for SSG by local heating (Figure 10.2a). The 3D porous N-­doped graphene grown at 950 °C showed low reflection (2–10%) and transparency (∼0.01%). The high evaporation rate of 1.50 kg m−2 h−1 could be achieved, which was 4.17 times higher than that of water under the same light irradiation of 1 kW m−2. Subsequently, preparing and designing different structures of 3D graphene aerogel were gradually involved to enhance SSG. Vertically aligned graphene sheets membrane53 (VA-­GSM) was prepared by freeze-­drying and thermal annealing. Compared with structure-­ disordered rGO foam and rGO film, the vertical structure accelerated the steam escaping in the vertical direction (Figure 10.2b) to achieve a higher evaporation rate of 1.62 kg m−2 h−1 with evaporation efficiency up to 86.5%. Further, the same group developed a highly vertically ordered pillar array of graphene-­assembled framework48 (HOPGF) by laser processing. The HOPGF could achieve an excellent water evaporation rate of 2.10 kg m−2 h−1 with steam escaping in all directions. In addition, the water supply optimization of 3D graphene itself is also an important link for effective water evaporation. The water supply optimization of 3D graphene is usually achieved by adjusting the hydrophilicity of the surface structure using a plasma method. Our group developed a modified graphene aerogel54 (MGA) with effective oxygen

Photothermal materials 3D graphene

3D GO/rGO

Graphene-­ metal Graphene-­ carbon

Graphene-­ organic

Power density Evaporation Absorption (%) (kW m−2) rate (kg m−2 h−1) Efficiency (%) Reference

Direct contact Direct contact Indirect contact Indirect contact Direct contact Direct contact Direct contact Direct contact Indirect contact Indirect contact Indirect contact Indirect contact Indirect contact Indirect contact Indirect contact Indirect contact Indirect contact Indirect contact Direct contact Direct contact Indirect contact Indirect contact Direct contact

∼90 ∼90 >93

Direct contact Indirect contact Direct contact Direct contact Direct contact Direct contact Direct contact Direct contact

92 96

∼86 80 >80 >90 95 95 99 99 99 >97 99 97.57

95.5 91 98 100 95

1 1 1 1 10 1 1 1 0.82 1 1 4 1 1 1 1 10 1 1 1 1 1 1

1.5 1.4 1.62 2.1

1.21 1.25 1.27 1.558

80 91.4 86.5 95 80% ∼87.04 88.6 ± 2.1 76.9 78 102 48 71.8 ± 3 89.1 83 89.7 80 90.8 91 82 86.8 85.6 87.5 90

52 79 53 48 47 49 46 54 58 80 60 59 81 82 55 83 57 84 11 62 64 63 66

1 10 1 1 10 1 1 1

1.622 11.8 1.4352 1.375 11.24 2.5 1.3542 2.02

83 83 86 88 81 95 93.8 91

65 69 71 73 70 74 77 78

1.3 1.27 1.2 2.0 1.48 0.47 1.47 1.31 1.778 1.45 13.5 1.78

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Others

N-­doped graphene Graphene foam VA-­GSM HOPGF Graphene aerogel Cross-­linked graphene N-­fGPs MGA GO leaf rGO-­silk-­fabric Functionalizing rGO rGO/MCE RGO/cotton fabric rGO/polystyrene foam Paper-­based rGO GO film GO-­chitosan/ZnO scaffold GO by laser irradiation Cu/G rGO/Ag GO/CNT GO/CB Graphene-­organic carbon materials GO/MWCNTs rGO/BNC : BNC Chitosan/rGO rGO-­PFS rGO/PU rGO/PVA MnO2/rGO-­PPy rGO-­Ag/SA @PU

System

Harvesting Solar Energy by 3D Graphene-based Macroarchitectures

Table 10.1  Essential  parameters for SSG with 3D GBMs.

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Figure 10.2  SEM  and TEM images of various 3D GBMs: (a) Porous N-­doped

graphene. Reproduced from ref. 52 with permission from John Wiley & Sons, Copyright 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Vertically aligned graphene. Reproduced from ref. 53 with permission from American Chemical Society, Copyright 2017. (c) Cu nanodot-­embedded N-­doped graphene urchin. Reproduced from ref. 11 with permission from Elsevier, Copyright 2018. (d) rGO/BNC : BNC. Reproduced from ref. 69 with permission from John Wiley & Sons, Copyright 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

implant by oxygen plasma. Contact angles test showed that MGA demonstrated fine hydrophilicity, while untreated graphene aerogel (GA) showed strong hydrophobicity. The water evaporation rate was increased with the presence of MGA, which was a nearly 25% increase compared with that of GA. Growth of nitrogen-­doped hydrophilic graphene nanopetals (N-­fGPs)46 on hydrophobic graphene foam (SGF) by a customized inductively coupled plasma-­enhanced chemical vapour deposition method is also an effective way to achieve efficient water supply. The N-­fGPs/SGF contributed to high solar evaporation efficiency of 88.6 ± 2.1%.

10.3.2  3D GO/rGO GO is not only an excellent photothermal material but also possesses good hydrophilicity for water supply with oxygen-­containing functional groups. However, a 3D GO with a network structure is generally difficult to stabilize

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in water due to its rich oxygen-­containing functional groups. Therefore, optimized local photothermal structures are designed. These structures usually consist of two parts: the support layer and the GO photothermal layer. GO usually is dipped, filtered or coated on the substrate to form a 3D structure floating on the water surface for interfacial SSG.55–58 The design of these structures will be highlighted in the fourth part. rGO has been proven to have better photothermal performance than GO. For example, the double-­ layer system rGO/MCE59 (the top rGO sheets as a light-­to-­heat conversion layer and the bottom mixed cellulose esters (MCE) membrane as a porous supporting layer) achieved a 1.1 times higher evaporation rate than GO/MCE. The rGO/MCE was prepared by a microwave irradiation method, contributing to higher light absorption with a higher specific surface area than GO/ MCE. However, the reduction of GO affects hydrophilicity and thus affects water supply capacity. Controlled reduction is also an important process for efficient SSG. The functionalized rGO60 using hydrophilic groups can improve by 10% the overall solar-­to-­steam efficiency at 1 sun compared to chemically rGO. Different structures of rGO-­based solar evaporators include paper-­based rGO, rGO-­silk-­fabric, etc. The rGO was developed usually using thermal annealing, laser reduction, chemical reduction, microwave irradiation, etc.

10.3.3  Hybrid Materials Besides graphene materials, hybrid materials have proven to possess excellent photothermal properties. By taking advantage of individual material's photothermal properties, these hybrid materials obtained complementary desired characteristics. Here, we mainly discuss several widely-­used 3D graphene-­based hybrid materials, including graphene-­metal materials, graphene-­carbon materials, graphene-­organic materials and others.

10.3.3.1 Graphene-­metal Materials The coupling of incident light and electron motion on the metal surface usually enhances the absorption of sunlight and photo-­to-­thermal conversion. Studies have shown that when gold nanoparticles are added to graphene oxide by a physical mixing method, both light absorption and photothermal efficiency are greatly improved, and as the gold nanoparticles increase, the evaporation rate is gradually increased.61 In addition, the surface temperature of rGO/Ag material62 prepared by reducing GO/Ag+ solution could reach 67 °C under 1 sun irradiation for 600 s, which was 10 °C higher than only that of rGO material. The water evaporation rate increased from 0.83 to 1.12 kg m−2 h−1 with Ag nanoparticles. In addition to precious metal materials, copper nanodots were also embedded in graphene for enhanced SSG. A Cu nanodot-­embedded N-­doped graphene urchin11 (Figure 10.2c) was developed via a space-­confined thermal decomposition of copper carbodiimide.

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The introduction of copper nanodots enhanced the absorption of graphene throughout the solar spectrum and photothermal efficiency was 15% higher than graphene.

10.3.3.2 Graphene-­carbon Materials In addition to graphene, other carbon-­based materials, such as carbon black, carbon nanotubes, organic carbon, and bio-­carbon, also exhibit excellent solar-­thermal efficiency. Therefore, the composites of graphene-­carbon materials have also received considerable attention for efficient SSG. Among them, GO-­carbon black (GO/CB)63 or GO-­carbon nanotube (GO/CNT)64 composites achieved light absorption of 99% and 97% with solar photothermal evaporation efficiency of 85.6% and 87.5%, respectively. rGO-­multiwalled carbon nanotubes (rGO/MWCNTs)65 achieved an evaporation efficiency of ∼83% under 1 kW m−2 illumination, which was higher than rGO (56%). Graphene-­organic carbon materials were prepared by dip-­coating, air drying and carbonization at different temperatures. The graphene-­organic carbon materials66 carbonized at 600 °C could achieve an evaporation efficiency up to 90% with an evaporation rate of 1.558 kg m−2 h−1 under 1 kW m−2 illumination. Bio-­carbon materials are usually from daily bio waste, such as eggshells and sacred lotus. A graphene-­eggshell membrane67 showed a benign light absorption of up to 99%. A graphene-­sacred lotus68 was developed by innovative one-­step plasma carbonization. It achieved a high evapo­ ration rate of 1.33 kg m−2 h−1 and evaporation efficiency up to ∼90% under 1 kW m−2 illumination. The reuse of such abandoned resources is conducive to the sustainable development of the resource and environment.

10.3.3.3 Graphene-­organic Materials During long-­term use, 3D graphene gradually shows its disadvantages, such as weak mechanical strength and insufficient flexibility. Therefore, loading graphene materials into porous and hydrophilic organic network frameworks facilitates prominently their long-­term use for water evaporation. The development of graphene-­organic materials has also become one of the mainstream developments in the field of SSG. Organic network frameworks commonly used include bacterial nanocellulose (BNC), plant fibres, polyurethane (PU) foams, polyvinyl alcohol (PVA) hydrogels, poly (N-­isopropylacrylamide) (PNIPAm) membrane, chitosan, polyacrylamid (PAM), sodium alginate, and the like. In 2016, rGO/BNC : BNC (Figure 10.2d) bilayered biofoam69 first achieved an evaporation efficiency of 83% at a power density of 10 kW m−2. After that, the functional rGO/PU foam70 achieved photothermal efficiency of ∼81% at 10 kW m−2. rGO/chitosan71 achieved a high evaporation rate of 1.43 kg m−2 h−1 under 1 sun (1 kW m−2). rGO/PAM hybrid cryogels72 achieved an evaporation rate up to 1.76 kg m−2 h−1 with an evaporation efficiency of 86.8% under 1 sun. rGO-­plant fibre sponge (rGO-­PFS)73

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269

was developed by a cake-­making strategy involving foaming, moulding, and baking. It exhibited a high solar steam efficiency of 88.8% under 1 sun illumination. After multiple cycles of experimentation, their complete structures and excellent photothermal performance were still maintained. What is more impressive is that an ultrahigh evaporation rate of 2.5 kg m−2 h−1 was achieved by rGO/PVA hydrogel74 under 1 sun irradiation, which was mainly due to the hydrogel network structure. In addition, an intelligent graphene/ PNIPAm membrane75 regulated the rate of water evaporation by switching its own microscopic pores at different light intensities. An evaporation rate of 1.66 kg m−2 h−1 could be achieved under 1 sun.

10.3.3.4 Others In the field of SSG, other hybrid materials involved graphene-­semiconductor materials, ternary graphene composites, multi-­graphene composites, and so on. For example, GO-­black TiO2 materials76 achieved an evaporation efficiency of 69.1%, which was 1.6 times higher than only GO. rGO/MnO2 monolithic aerogel treated by polypyrrole77 possessed an outstanding evaporation efficiency of 93.8% under 1 kW m−2 irradiation, which was 4.9 times that of pure water. Furthermore, it could maintain a certain mechanical strength of 0.08 Mpa under 30% strain. rGO-­Ag/SA@PU solar evaporator78 reached an evaporation rate of 2.02 kg m−2 h−1 with an efficiency of 91% under 1 sun irradiation. The synergic effect of combining graphene with other materials contributed to the improvement of optical and physical properties for excellent and long-­term SSG.

10.4  D  evelopment of Photothermal Solar Evaporation Systems Generally, in a solar evaporation system, solar light is harvested by solar evaporators and then converted to thermal energy to heat water for vapour generation. The development of a solar evaporation system can be easily divided into three phases:85 bottom heating evaporation system, volumetric evaporation system and interfacial evaporation system. For the bottom heating evaporation system, the heat is collected at the bottom of the system by the light absorbers (Figure 10.3a), but the steam is generated elsewhere, which causes inevitable heat loss due to the separation of heat and steam generation, which ultimately leads to very low evaporation efficiency of 30–45%. The volumetric evaporation system reduces heat loss to some extent by moving heat to the interior of the fluid (Figure 10.3b). The evaporation efficiency is increased but still limited because of the heating loss to bulk water. Meanwhile, the volumetric evaporation system faces many challenges, such as the reuse of photothermal materials, long-­term stability, and uniform dispersion.

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Figure 10.3  The  history of solar evaporation system for SSG: (a) Bottom heating evaporation system. (b) Volumetric evaporation system. (c) Interfacial evaporation system. Reproduced from ref. 85 with permission from Springer Nature, Copyright 2018.

Therefore, it is quickly replaced by a new concept of interfacial evaporation system (Figure 10.3c) for more efficient SSG. The interfacial evaporation system achieves a high surface temperature by heat localization at a water–air interface. Such interfacial evaporation system minimizes heat loss by selectively evaporating the surface water rather than bulk water and successfully achieves an evaporation efficiency of up to 90% under 1 sun illumination. 3D graphene-­based interfacial evaporation system has become one of the research priorities for efficient SSG. Here, we mainly review the recent development of structural designs for 3D graphene-­based interfacial evaporation systems.

10.4.1  Direct Contact System A direct contact solar evaporation system means that the photothermal materials are in direct contact with bulk water (Figure 10.4a). They can self-­float freely on the water surface without any supporting materials. Generally, 3D GBMs include porous graphene membranes, graphene aerogels, graphene-­ organic aerogels or foams. Among 3D GBMs, porous N-­doped graphene membrane52 is the first direct contact system used for efficient interfacial SSG. The N-­doped graphene membrane could self-­float on the water due to its porous structure and lightweight property. Meanwhile, its wetting property and open mesoscopic porosity allowed the capillary action for adequate water supply. It achieved high-­efficiency energy conversion from solar light to steam by heat localization. Apart from the self-­floating graphene membrane, traditional graphene aerogels or foams have been widely used in

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271

Figure 10.4  The  structure of photothermal solar evaporation systems: (a) Direct contact system. Reproduced from ref. 83, with permission from Proceedings of the National Academy of Sciences, (b and c) Indirect contact system with 2D water channel. Reproduced from ref. 80 from the Royal Society of Chemistry. Reproduced from ref. 83, with permission from Proceedings of the National Academy of Sciences. (d and e) Indirect contact system with 1D water channel. Reproduced from ref. 63, with permission from Elsevier, Copyright 2017. Reproduced from ref. 86 with permission from American Chemical Society, Copyright 2017.

direct contact evaporation systems for efficient SSG. The porous structure of graphene aerogels or foams provided water supply and steam escape paths, which are beneficial for high-­efficiency SSG. 3D cross-­linked graphene foam,49 VA-­GSM,53 copper nanodot-­embedded graphene aerogel,11 N-­fGPs foam51 was developed for efficient solar-­thermal harvesting and thermal-­ vapour conversion. A high evaporation rate of 2.6 kg m−2 h−1 with 87% evaporation efficiency was achieved by 3D cross-­linked graphene foam under 1 sun. Other work of similar structure, for example, hierarchical graphene foam grown on 3D Ni foam79 by one-­step plasma-­enhanced chemical vapour deposition achieved the evaporation efficiency exceeding 90% for seawater desalination. Also, graphene-­organic aerogels or foams have been reported for interfacial solar evaporation, in which organic aerogels or foams were featured as 3D network skeleton and photothermal materials were mixed in the network skeleton to form 3D integrated aerogels and foams. Graphene-­ organic aerogels or foams represented a distinct advantage for long-­term solar evaporation due to structural stability.

10.4.2  Indirect Contact System The direct contact system ensures efficient energy transfer and water supply but also has intrinsic thermal loss through bulk water. For minimizing heat loss to bulk water, an indirect contact system has been designed, which is

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usually called a bilayered solar evaporation system. The top layer is made of phothothermal materials for light harvesting and photo-­to-­thermal conversion. The bottom layer is used as supporting materials for the water supply, thermal management, and preventing photothermal materials from being dispersed in the bulk water. According to the different water supply methods, the indirect contact system is divided into a 2D water channel, 1D water channel and others.

10.4.2.1 2D Water Channel In 2016, the concept of a 2D water channel was first proposed by Zhu's group,83 in which polystyrene (PS) foam was used as a thermal insulator and supporting material; GO film was not directly contacted with bulk water to reduce heat loss (Figure 10.4b). The 2D water channel was designed by a layer of cellulose-­wrapped over the surface of PS foam. Water was pumped by capillary force through the 2D water channel on the sidewalls of cellulose and spread towards the centre of the surface very quickly. The device achieved an evaporation rate of 1.45 kg m−2 h−1 with 80% efficiency. Similarly, a rGO-­ silk-­fabric80 was designed with silk-­fabric as a 2D water channel and PS foam as a thermal management layer for efficient SSG (Figure 10.4c). The system exhibited remarkably high photothermal performance with an evaporation rate of 1.48 kg m−2 h−1 under 1 sun irradiation. Subsequently, the different bottom layer materials with 2D water channel raised research hotspots for efficient water supply. Hu's group developed an all-­in-­one evaporator with GO/CNT layer as light absorption materials and GO/nanofibrillated cellulose (GO/NFC) as a water uptake and transport layer by a 3D-­printed technology. The 3D-­printed all-­in-­one evaporator64 had a high solar steam efficiency of 85.6% under 1 sun illumination. The concept of 2D water channel structure is still continuing to date for efficient SSG.

10.4.2.2 1D Water Channel The 1D water channel structure of solar evaporators is designed to further reduce the water contact areas between photothermal materials and bulk water. Water supply regulation and thermal management are designed to achieve efficient water evaporation. A jellyfish-­like solar evaporation system was developed consisting of a porous GO/CB composites layer as a light absorption layer, aligned GO pillars as a 1D water channel and expanded polystyrene (EPS) foam as a thermal management layer for efficient SSG63 (Figure 10.4d). The linear 1D water channel in the GO pillars greatly decreased the direct contact area between the photothermal materials and bulk water, which could effectively prevent heat transfer to the bulk water thus enhance the heat utilization. The solar evaporator achieved an excellent energy conversion efficiency of 87.5% under 1 sun illumination. Similarly, a compact device was designed using rGO-­paper fibres as a light absorption layer,

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capillaries as a water supply pipeline and PS foam as a thermal insulator for efficient SSG (Figure 10.4e).86 Heat localization is enabled by the photothermal materials rGO due to its broad absorption of sunlight. The heat transfer was suppressed by the thermal insulator PS foam. Meanwhile, bulk water could continuously transport to the light absorption layer by capillary based on the capillary effect. The evaporation efficiency could reach 89.2% under 1 sun. In the same year, a 3D mushroom-­shaped solar evaporator was designed by Zhu's group.87 The solar evaporator enabled a high efficiency of up to 85% under 1 kW m−2 with 3D conical GO films as photothermal materials and a cotton rod as the 1D water channel. The common feature for 2D/1D water channel evaporation is to achieve heat localization by reducing the contact areas between the photothermal materials and bulk water. The concept of water channels usually included hydrophilic or capillary effects and a combination of the two. A thermal insulator was used to reduce thermal transfer to bulk water by separating the solar absorber from bulk water. Therefore, efficient water supply and thermal management are achieved.

10.4.2.3 Others Based on the principle of 2D/1D water channels, silicone aerogel, chitosan aerogel and bacterial banocellulose have also been widely used as supporting layers due to their hydrophibility, porous structure and thermal management properties. These materials combine water supply and thermal management, and since their inception, have been providing excellent performance for efficient water evaporation. The evaporation system with a paper-­based reduced graphene oxide composite membrane as a light absorption layer on top of a silicone-­based porous insulation layer achieved an evaporation efficiency of 89.7% under 1 sun.55 A double-­layered GO-­chitosan/ZnO scaffold device,57 in which GO aerogel layer serves as a photothermal material and chitosan/ZnO composite layer serves as a unidirectional water pump, exhibited a high solar energy conversion efficiency of 89.4% under 1 sun irradiation. Apart from the above-­mentioned man-­made organic aerogel, inspired by tree transpiration, wood-­based interfacial solar evaporation has also entered the public's field of view due to the natural hydrophilicity and the vertically aligned porous structure. Based on natural wood, a highly efficient SSG was demonstrated by Hu's group.4,88–90 The wood-­based bilayered structures were usually prepared by carbonizing and coating a surface. A GO coated wood91 exhibited a solar-­thermal efficiency of ∼83%. Wood has been considered as an excellent water supply and thermal management material for efficient SSG due to its abundance, hydrophilicity, low thermal conductivity. Subsequently, wood-­ like polyacrylonitrile foam is designed for efficient SSG. The polyacrylonitrile foam81 exhibited capillarity with aligned vertically oriented pores, therefore not only served as a thermal management layer but also allowed the underlying water to be continuously pumped to the photothermal layer for

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evaporation. With RGO-­silk-­fabric as top layer photothermal materials, the system exhibited an excellent photothermal performance of 1.47 kg m−2 h−1 under 1 sun and could produce clean water from simulated seawater meet the individual drinking needs.

10.4.3  Isolation Evaporation System To more strictly enhance thermal management, a photothermal reservoir was designed for highly efficient SSG without the need for contact with bulk water. The system consisting of RGO-­agarose-­cotton aerogel92 sheet can soak up water, which is sufficient for one day of solar evaporation, thus no external water supplement is required. Since the evaporation is in complete isolation from the bulk water, the photothermal material reduced the heat loss to bulk water. It achieved an extremely high evaporation rate of 4.0 kg m−2 h−1 under 1 sun, which is higher than both the direct contact system and indirect contact system. However, the high water content of the 3D structure still causes heat loss by heating the excess water, and the surface temperature of the solar absorber will decrease with the increase of water content, thereby severely limiting the evaporation rates. Herein, an injection control technique achieved an appropriate amount of capillary water supply in the 3D graphene foam,93 ensuring the micrometre-­sized pore channels for vapour escape and preventing heat loss to excess water, which achieved a solar-­thermal efficiency approaching 100%. Another, by introducing highly water-­absorptive materials,94 it absorbed atmospheric water at night and is used for water evaporation and purification of water during the day. This system provides an attractive pathway to extract water from air, to relieve the thirst of arid, land-­locked, and other areas where fresh water is seriously scarce.

10.5  Current Technologies for Enhanced SSG Efficient photothermal harvesting of solar evaporator has an important impact on high-­efficiency SSG. We viewed photothermal harvesting from light harvesting and thermal management. In terms of light harvesting technology, micro optimization refers to the regulation of the photothermal materials, such as the regulation of the morphology, the doping of the photothermal materials, etc. Macro optimization involves the design of 3D structures, generally an increase of light absorption areas. Thermal management technology can reduce heat loss by adding additional thermal insulation materials, which mainly reduces heat to bulk water transfer. In addition, the reduction in the enthalpy of the evaporation system can effectively accelerate the solar evaporation rate. Finally, combining other forms of heat to the solar evaporation system also effectively increases the solar evaporation rate, such as ambient heat, geothermal heat, latent heat of the evaporation unit, etc.

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10.5.1  Light Harvesting To ensure efficient light harvesting over all the solar spectrum, minimizing light transmittance and reflectance by micro-structure optimization is an important method for efficient SSG. Meanwhile, combining different photothermal materials with graphene is also an efficient way to enhance the light absorption performance due to the light sensitivity of different photothermal materials in different wavelengths. Macro-­structure optimization usually increases light harvesting by minimizing light reflectance or increasing light absorption areas. Other factors such as the acquisition and enhancement of solar light from different incident angles are also crucial by phototropism photothermal materials.

10.5.1.1 Micro Optimization The micro-structure optimization of solar evaporators plays an important role in enhancing the solar light absorption property. It has been shown that the reflection and transmission of incident solar light can be minimized by multiple internal reflections of hierarchical structure. Graphene foam was developed consisting of vertical graphene nanoplates (GNPs) array on 3D Ni foam79 (Figure 10.5a) for efficient SSG. The vertical graphene nanoplates not only increased the surface areas of light absorption but also increase the path of light and the internal refraction process, so solar light can be “trapped” within the material and absorbed instead of bouncing off or penetrating, thus reducing the reflection and transmission of solar light. The hierarchical graphene foam showed low transmittance (∼0%) and reflectance (∼5% in the visible region, ∼10% in the near-­infrared region, and 90% v/v), low density (typically 420 nm), irradiation time (5 h). Photocatalyst (0.15 g), 15 W mercury, Lamp with UV cutoff, irradiation time (6 h). 200-­Watt UV-­A lamp, irradiation time (2 h). Photocatalyst (25 mg), 300 W Xe lamp, irradiation time (20 h). Photocatalyst (30 mg), Xe lamp with a 420 nm cutoff filter, irradiation time (8 h). Photocatalyst (40 mg), 300 W Xe lamp, irradiation time (8 h). 1.0 wt% Ag/3.0 wt% RGO/CdS (5 mg), H2O (4 mL), TEOA (2 mL), Xe lamp (λ > 420 nm), irradiation time (1 h). 1.0 wt% Pt/2.0 wt% rGO/TiO2 (0.1 g), CO2 pressure (0.1 MPa), H2O (2.0 mL), light source (320–780 nm), irradiation time (8 h), reaction temperature (4 °C). 10 wt% Ag2CrO4/20 wt% GO/g-­C3N4 (100 mg), H2O (10 mL), Xe lamp with a 420 nm cutoff filter. GQD catalysts (0.1 g), H2O (1.0 mL), 300 W Xe lamp with a 420 nm cutoff filter. 300 W Xe lamp, H2O (4.0 mL), irradiation time (4 h).

Synthetic method

Reference

Hydrothermal

68

Solvothermal

112

Solution-­based

73

Hydrothermal

113

Hydrothermal

109

Electrochemical Solution-­based

72 114

Hydrothermal

115

Solution-­based

110

Solvothermal

116

Hydrothermal

117

Self-­assembly

111

Solvothermal

118

Hydrothermal

69

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rGO/MoS2/SnS2

Experimental details

408

Table 16.1  Selected  3D GBCs for CO2 photoreduction.

4.89 (CH4), 2.44 (CO)

GO/O–TiO2

0.49 (CH4), 1.86 (CO)

N-­rGO/TiO2d

50 (CO)

N-­GO/ZnGeON

3.56 (CH4)

rGO/Fe3O4/CuZnO

110.67 (CH3OH)

A-­G/CdSe

2.84 (CH4)

GO/TiO2 N-­GO/CeO2/Cu2+ GO/HNb3O8

27.4 (CH4), 70.8 (CO) 507.3 (CH3OH) 54.35 (CO)

rGO/CuO

53.41 (CH3OH)

N-­G/Fe3O4 Pt-­GO/TiO2

15.9 (CO) 0.28 (CH4)

GO/O–TiO2

0.28 (CH4)

rGO/pCN

1.39 (CH4)

rGO/Ag/CdS

0.91 (CO), 0.28 (CH4)

N-­G/AgBr/g-­C3N4

21.17(CH3OH), 51.28 (C2H5OH) 66.67 (CH3OH)

N-­G/Cu complex

Photocatalyst (4 mg), 100 W Xe lamp with an AM 1.5G filter. Xenon arc lamp with AM 1.5G filter, irradiation time (8 h). Photocatalyst (10 mg), 400 W Xenon lamps with a UV cutoff filter, irradiation time (8 h). 0.25 wt% N-­Graphene/ZnGeON (20 mg), 300 W Xe arc Lamp with a UV cutoff filter, irradiation time (3 h). Photocatalyst (100 mg), 20 W LED, irradiation time (24 h). Photocatalyst (50 mg), H2O (4 mL), 300 W Xe lamp with a 420 nm cutoff filter, irradiation time (4 h). 10 wt% GO/TiO2 (10 mg), 300 W Xe arc lamp. CeO2/NGO/Cu2+ (0.1 g), 250 W Xe lamp. Photocatalyst (10 mg), 300 W Xenon arc lamp, irradiation time (4 h). 5 wt% rGO/CuO (100 mg), 20 W LED light, irradiation time (24 h). 3 wt% Fe3O4/NG (0.1 g), 300 W Xenon arc lamp. Xenon arc lamp with a UV cutoff filter, irradiation time (6 h). 5 wt% GO/O–TiO2, 15 W daylight bulbs, irradiation time (6 h). 15 wt% rGO/pCN (100 mg), 15 W daylight lamp, irradiation time (10 h). ACG-­2 wt% (20 mg), H2O (4 mL), 300 W Xe arc lamp (λ > 420 nm), irradiation time (2 h). Photocatalyst (0.02 g), visible light irradiation (λ > 420 nm), irradiation time (5 h). Photocatalyst (100 mg), 20 W white cold LED light, irradiation time (24 h).

Solution-­based

119

Hydrothermal

120

Solvothermal

108

Solution-­based

121

Solution-­based

102

Solvothermal

122

Solvothermal Hydrothermal Coflocculation

123 124 125

Hydrothermal

126

Hydrothermal Solvothermal

127 128

Hydrothermal

129

Hydrothermal

130

Self-­assembly

131

Solution-­based

132

Solution-­based

133 409

(continued)

Artificial Photosynthesis by 3D Graphene-­based Composite Photocatalysts

GO/CsPbBr3 QDsc

3D GBCs

Product yield (µmol g−1 h−1)

GO/Ru complex rGO/TiO2/CdS CCG/BODIPYf

85.61 (CH3OH) 0.09 (CH4) 72.11 (HCOOH)

G/N–TiO2 GO/Ru complex

0.37 (CH4) 82.86 (CH3OH)

GO/Co complex

78.79 (CH3OH)

rGO/Cu2O

1.78 (CO)

GO/WO3

1.11 (CH4)

G/TiO2

8 (CH4), 16.8 (C2H6)

rGO/TiO2 rGO/ZnO

0.135 (CH4) 4.57 (CH3OH)

GO GO/TiO2

0.172 (CH3OH) 8.91 (CO), 1.14 (CH4)

SEG/TiO2g

2 (CH4)

SEG/TiO2g

4 (CH4)

20 W white cold LED light, irradiation time (24 h). 300 W Xenon arc lamp, irradiation time (10 h). 450 W Xenon lamp with a 420 nm cutoff filter, irradiation time (2 h). 15 W daylight lamp, irradiation time (10 h). Photocatalyst (0.1 g), 20 W LED light, irradiation time (48 h). Photocatalyst (100 mg), 20 W white cold LED light, irradiation time (48 h). 0.5% RGO/Cu2O (0.5 g), 150 W Xe lamp, irradiation time (20 h). Photocatalyst (0.1 g), 300 W Xe lamp, irradiation time (8 h). Photocatalyst (0.1 g), 300 W Xenon arc lamp, irradiation time (4 h) 15 W daylight bulbs, irradiation time (5 h). Photocatalyst (100 mg), 500 W Xe lamp, irradiation time (10 h). Photocatalyst (0.2 g), 300 W commercial halogen lamp Photocatalyst (0.01 g), 300 W xenon arc lamp, H2O (0.4 mL) 0.25 wt% SEG/TiO2, 60 W daylight bulb, irradiation time (3 h). 0.55 wt% SEG/P25, 60 W daylight bulb

r GO: reduced graphene oxide. QA: quinacridone. QDs: quantum dots. d N-­: N-­doing. e A-­: amine functionalized. f BODIPY: boron-­dipyrromethene, a light-­harvesting enzyme. g SEG: solvent exfoliated graphene. b c

Synthetic method

Reference

Solvothermal Solvothermal Solvothermal

134 135 136

Solvothermal Solvothermal

137 138

Solvothermal

139

Hydrothermal

140

Hydrothermal

141

Hydrothermal

142

Hydrothermal Hydrothermal

143 144

Solution-­based Hydrothermal

145 146

Solvothermal

147

Solvothermal

105 Chapter 16

a

Experimental details

410

Table 16.1  (continued)

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2,9,14,25–27,30–37,74–87

the fundamental mechanisms. In this chapter, a brief introduction of a widely accepted mechanism is given by taking a semiconductor-­ based heterogeneous photocatalyst as an example. In general, CO2 photoreduction occurs via a series of steps. As illustrated in Figure 16.10, to photocatalytically reduce CO2 in an aqueous system over a heterogenous photocatalyst, such as TiO2, the energy of the incident light (Ehv) should be equal to or higher than the bandgap (Eg) of the photocatalyst. In addition to Eg, the photocatalyst is also required to have its charge carriers with higher energy as compared to the CO2 redox potential.74 This means that the conduction band (CB) potential of the catalyst should be well above the reduction potential for a certain product at a specific pH. Meanwhile, the holes should be able to oxidize water to produce protons (H+). The energy levels or the band edges of the semiconductors in the CO2 photoreduction system are documented elsewhere.20,27,30,36,37,83 A list of possible reactions that may occur in CO2 photoreduction in an aqueous system are summarized in Scheme 16.1.2,9,14,25–27,30–37,74–87 The initial step is the generation of electron–hole (e−–h+) pairs (R1) (Figure 16.10i). After photoexcitation, ideally, the e−–h+ pairs are to be separated spatially, and transferred to redox active species across the interface (Figure 16.10ii). However, the lifetime of the e−–h+ pairs is only a few nanoseconds, but the e−–h+ recombination is faster, which generates heat causing energy loss (R2) (Figure 16.10iv).74,87 The fast charge recombination is thus considered as a major limiting step in CO2 photoreduction. On the other side, water is oxidized by holes to form oxygen and H+ (R3) (Figure 16.10iii). However, in addition to the desired reactions, there are also several competing reactions, such as the formation of H2O2 and H2, which consume H+ and e− (R4 and R5).88 As for photoreduction reactions (Figure 16.10iii),

Figure 16.10  Schematic  illustration of reaction pathways in CO2 photoreduction

in an aqueous system over a heterogeneous photocatalyst (e.g., TiO2). (i) Photoexcitation, (ii) charge separation, (iii) photoreduction, and (iv) recombination. CB: Conduction band; VB: Valance band. Reproduced from ref. 79 with permission from The Royal Society of Chemistry.

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researchers proposed that CO2 activation involves the formation of a negatively charged CO2•− species,89 which can be detected by infrared (IR)90 and electron paramagnetic resonance (EPR) spectroscopy.91 However, the single electron transfer to CO2 is highly endothermic (R6) because of the negative adiabatic electron affinity of CO2.74,92,93 Hence, a widely acceptable CO2 photoreduction pathway is through multi-­electron transfer (MET) as shown in Scheme 16.1. Various products could be formed through the MET reactions (R7-­R14) (Figure 16.10iii). Carbon monoxide (CO) and formic acid (HCOOH) are the most common products, as the reactions require only two H+ and two e−. It should be noted that the MET reactions listed in Scheme 16.1 are the simplest forms. Some products may be formed through multistep reactions. For example, methane (CH4) could be formed through R11 or other reactions as well (R12–R13).2,94 Besides the above desired reactions, backward reactions are also possible, making CO2 photoreduction even more complicated. For example, the strong oxidation power of the holes, protons, •OH radicals, or O2 could oxidize the intermediates and products to form CO2. CO2 photoreduction kinetics are influenced by many parameters, such as light intensity and wavelength, reaction temperature, reactant adsorption, product desorption, CO2 activation, and photocatalyst properties

Scheme 16.1  Possible  CO2 photoreduction reaction pathways.

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9,26,95

(e.g., surface area and crystallinity). In particular, charge recombination is widely considered a major limiting step. Slowing down the charge recombination rates has become a key research topic. Common strategies to enhance CO2 photoreduction performance include but are not limited to (1) choosing photocatalysts with appropriate bandgaps and band edges; (2) formation of heterojunctions to enhance interfacial charge transfer; (3) use of high surface area catalyst supports; (4) surface modifications via metal coating/ doping;96 (5) doping with non-­metals, such as nitrogen;97 and (6) sensitization with dyes or enzymes to mimic nature. This chapter will limit the discussion on the design of 3D GBCs as efficient photocatalysts for CO2 photoreduction by addressing the aforementioned limitations.

16.3.2  Typical CO2 Photoreduction Analysis Systems In addition to the basic principles, the development of CO2 photoreduction analysis systems is a vital engineering aspect. As illustrated in Figure 16.11, a typical analysis system includes a gas feeding instrument, a light source, a reactor, and a gas chromatograph-­mass spectrometer (GC-­MS). Currently, there are two major systems for CO2 photoreduction analysis. One is a batch system where the photocatalysts and CO2 gas are mixed with water in a sealed reactor, which is illuminated by a light source, as shown in Figure 16.11A.98 In this system, the product sample is taken from the batch reactor at a fixed time interval and fed into the GC-­MS for species identification and quantification. However, the low solubility of CO2 in water (0.03 M under ambient conditions) limits the CO2 photoreduction efficiency.85 Therefore, attempts in enhancing CO2 pressure or adding alkaline agents within the photoreactor have been made to improve the solubility of CO2 in water. Different from the batch system, the other analysis system consists of a continuous flow reactor, where a mixture of CO2 and H2O vapor is fed through (Figure 16.11B). This fluidized system is more feasible for future integration with coal-­fired power plants. In both systems, parameters and conditions that would change the catalytic performance are briefly summarized here: (1) Carbon source: Compressed CO2 gas with high purity is usually used, which is regulated by a mass flow controller (MFC).7 In some cases, CO2 mixed with fresh air and other gases is used to mimic the practical flue gas.99,100 (2) Light irradiation: The intensity and wavelength of the incident light are important parameters.101 Generally, ultraviolet (UV) and visible lights are used. Xenon (Xe) lamps are a common light source as a solar simulator because their unfiltered spectrum matches reasonably well to sunlight. Caution should be taken when it comes to visible light as some sharp atomic transitional peaks are characterized in the Xe spectrum in this range. Although accounting for ca. 50% of the solar spectrum,99 near-­infrared (NIR) light is rarely used because it generates heat, which may affect the CO2 photoreduction efficiency. As shown in Figure 16.11B, a water filter is usually used to remove the NIR light to rule out the possibility of heat from the lamp. Except for a Xe lamp, other light sources,

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Figure 16.11  Schematic  diagram of experimental set-­ups for CO2 photoreduction

in a batch system (A) and continuous flow system (B). Inset is the TEM image of a GBC photocatalyst. (A) Reproduced from ref. 98, with permission from John Wiley & Sons, Copyright 2017 John Wiley & Sons, Ltd.

such as laser and light-­emitting diodes (LEDs), are also used.102,103 (3) Photoreactor: Both batch and continuous photoreactors share similar features, where quartz windows are used to allow light irradiation. In the continuous reactor, both inlet and outlet are required for gaseous samples. It should be noted that before the light is turned on, the reactor should be purged with CO2 at a high flowrate for a while to eliminate the effects of air. (4) Analyte detection: The gaseous products are analyzed by GC, and different columns and detectors are selected based on the target analytes. For example, CH3OH and CH4 are often detected by flame ionization detector (FID),100 while CO and H2 are detected by thermal conductivity detector (TCD).104 To further identify the species of these gaseous products, a mass spectrometer is usually

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415

needed in connection with the GC system. In addition to the above parameters, the reaction time is also a crucial parameter in determining the CO2 photoreduction efficiency in both systems.

16.3.3  Design of 3D GBCs for CO2 Photoreduction To enhance the CO2 photoreduction efficiency, in addition to the optimization of analysis parameters, the design of efficient photocatalysts is of paramount importance. In this chapter, we focus on the design and synthesis of 3D GBCs. With its unique electronic and optical properties, graphene offers a broad range of opportunities to integrate with other functional materials to build up precisely tuned and optimized composite materials to enhance the efficiency of CO2 photoreduction.105 However, there are a number of challenges in the fabrication of 3D GBCs. For example, the strong van der Waals force between each individual graphene nanosheets causes severe restacking, which makes it difficult to take full advantage of superior properties of graphene.44,50 In most applications, GO instead of graphene is used for fabrication of GBCs. The subsequent reduction processes by using chemicals, heat, or light, however, will create inevitable surface defects on the rGO surface, which may compromise the electronic properties and hence the overall efficiency. Herein, we introduce several strategies to design and synthesize 3D GBCs for efficient CO2 photoreduction.

16.3.3.1 Aerosol-­processed Crumpled GBCs As mentioned in Section 16.2.1.2, to remediate the restacking issue, the aerosol routes were used to crumple the 2D graphene nanosheets into 3D balls. These crumpled balls can still maintain the large specific surface area of graphene without compromising the superior electrical properties. Further, the 3D crumpled structures are versatile matrices to encapsulate with other functional materials to form crumpled GBCs. In this section, a comprehensive discussion will be carried out on the crumpled GBC photocatalysts and their behaviors during CO2 photoreduction. A series of CGO/TiO2 composites were developed by Wang and co-­workers by using the aerosol method for CO2 photoreduction (see Figure 16.12A for an example).106,107 In this system, CO2 was primarily reduced to CO following a two-­electron and two proton reaction mechanism (Scheme 16.1, R8).58 As compared with pure TiO2, the CGO/TiO2 exhibited significantly improved efficiency with light irradiation, which is caused by the electron trapping effects of reduced GO nanosheets that inhibit e−–h+ recombination.106 The processing temperature of the CGO/TiO2 composite also had a large effect on the CO2 photoreduction performance, as shown in Figure 16.12B. In general, a lower synthesis temperature (e.g., 200 °C) favors the charge transfer since the CGO would preserve most of the functional groups, which helps the enhancement of CO2 photoreduction efficiency. On the other hand, at a higher synthesis temperature, more functional groups are removed from

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Figure 16.12  (A)  TEM image of CGO/TiO2, (B) CO yield versus irradiation time

using CGO/TiO2 prepared at different temperatures. (A) Reproduced from ref. 107 with permission from The Royal Society of Chemistry and (B) ref. 106 with permission from Mary Ann Liebert Inc., Copyright 2014.

the CGO surface, which creates massive surface defects and impeded the electron mobility of CGO, making the CGO/TiO2 composite less efficient as a photocatalyst.106 Another strategy to enhance the adsorption and activation of CO2 is to functionalize the surface with amine-­ groups.107,108 Nie et al. developed CGO-­amine-­TiO2 (CGOATI) composite via the aerosol route by introducing ethylenediamine (EDA) as an amine source in the precursor.107 After aerosolization and thermal reduction, the TiO2 nanoparticles were successfully incorporated into CGO. The optimal mass ratio of EDA to GO is determined to be 15 : 1, with the highest CO yield of 65 µmol g−1 h−1. The introduced amine group not only plays an important role in the CO2 adsorption but also promotes the photoreduction process by its intermediates. Specifically, during the CO2 adsorption, partial amine groups (e.g., –NH2, –NH−) have reacted with CO2 to form the alkylammonium carbamate species, which was confirmed by the Fourier transform infrared spectra. After photoreduction, the formed carbamate species were reduced to CO with the removal of oxygen-­containing groups from the GO and the amine groups were regenerated under the UV light. The above evolution of CGOATI was verified by XPS analysis (Figure 16.13). Two new peaks of pristine CGOATI at 287.4 eV (C(O)N) and 286.6 eV (C–N) showed up, corresponding to the ring–opening reaction between the amine group and epoxy groups from the partially reduced GO. In addition, the intensity of initial oxygen-­containing groups (e.g., hydroxyl at 285.9 eV, epoxy groups at 286.5 eV) are significantly reduced, indicating that the simultaneous reduction of rGO in the CO2 photoreduction process.

Artificial Photosynthesis by 3D Graphene-­based Composite Photocatalysts

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Figure 16.13  C1s  XPS spectra of (A) pristine CGOTI (TiO2/GO 20%, at 200 °C),

(B) CGOTI after CO2 photoreduction, (C) pristine CGOATI (TiO2/ GO 20%, EDA/GO 15 : 1, at 200 °C), (D) CGOATI after CO2 photoreduction. Reproduced from ref. 107, with permission from the Royal Society of Chemistry.

Based on the detailed analysis of surface chemistry of CGOATI composite at different periods in CO2 photoreduction, a roadmap of its photocatalytic mechanism was proposed, as shown in Figure 16.14. This study gives the idea that the crumpled GBCs towards CO2 photoreduction can be tuned by changing the doping components in the precursor. More work needs to be done on the other doping effects on the crumpled GBCs and exploration of the relevant CO2 photoreduction mechanism.

16.3.3.2 Wet Chemistry Generated GBCs In addition to crumpled GBCs, many other 3D GBCs have been synthesized via wet chemistry methods. Various materials were studied by coupling with 3D graphene, such as semiconductor metal oxides and sulfides, polymeric

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Chapter 16

Figure 16.14  Reaction  mechanism of the insertion of EDA on r-­GO, and CO2

adsorption and photoreduction on CGO/TiO2 nanocomposites. Reproduced from ref. 107 with permission from the Royal Society of Chemistry.

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carbon nitrides, aerogels, and metal–organic frameworks. Table 16.1 summarizes the recent work on the wet chemistry generated 3D GBCs for CO2 photoreduction in descending chronological order. The table includes several critical information, such as product yields, experimental details, and synthetic methods, attempting to concisely present recent advances in the past few years. In the following subsections, representative strategies by using wet chemistry methods to create 3D GBCs for CO2 photoreduction are introduced and discussed. 16.3.3.2.1  Simple Mixing.  Simple mixing of graphene nanosheets with other components has been widely used to fabricate 3D GBCs for CO2 photoreduction. This strategy normally involves stirring or ultrasonic mixing of these components in an aqueous solution to form the heterojunctions. For example, Seeharaj et al. prepared surface modified TiO2/rGO/CeO2 heterojunctions for photoconversion of CO2 to CH3OH and C2H5OH. Intense ultrasonic waves were utilized as the driving force for the exfoliation and intercalation to obtain the GO suspension. The metal oxides TiO2 and CeO2 were protonated with hydrochloric acid and then mixed with rGO under stirring to obtain the precipitated sample.109 Kumar et al. reported an rGO/ Ag2CrO4/Ag/BiFeO3 nano-­junction with high visible light absorption by simple mixing with different ratios of rGO solutions. Photocatalytic experiments were performed with a 300 W Xe lamp equipped with 400 and 800 nm cutoff filters for visible (380–780 nm) and near-­infrared light (>780 nm). The conversion rates were found to be 22.5 µmol g−1 h−1 and 4.75 µmol g−1 h−1 for CH4 and CO, respectively, which was 2.5 times as much as the sample without rGO component.110 Xu et al. also prepared the ternary GO/Ag2CrO4/g-­C3N4 Z-­scheme nanocomposite by stirring the aqueous mixture suspension for 12 h and the photocatalyst achieved an increase of CO2 reduction activity by 2.3 times than the bare g-­C3N4.111 16.3.3.2.2  Designing Hierarchical Structures.  Fabrication of 3D GBCs by designing hierarchical structures has also been reported. The high photocatalytic performance was achieved by controlling the morphologies and configuration of the GBCs and optimizing the ratio of each component. For instance, Liu et al. reported nanoengineered HNb3O8/graphene composite layers with improved photoconversion of CO2 to CO. The HNb3O8/graphene hybrid was fabricated via flocculation with HCl solution as a precipitant and then followed by a reduction treatment.125 Xiong et al. prepared GO/ TiO2 nanocomposite with coexposed (001) and (101) facets via a solvothermal method. During the solvothermal process, GO was reduced and TiO2 nanoparticles with coexposed (001) and (101) facets were uniformly dispersed on graphene.123 Figure 16.15 illustrates a design strategy for making hierarchical 3D GBCs for CO2 photoreduction. The formation of the ordered 2D/1D heterojunctions of graphene-­supported carbon nitride nanoarrays is shown in Figure 16.15A.

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Figure 16.15  3D  GBCs with designed morphology for CO2 photoreduction (A) For-

mation of ordered 2D/1D heterojunctions. (B) CO2 photoreduction property of bCN, CNNA, and CNNA/rGO at various amounts of rGO in wet CO2 gas. Reproduced from ref. 148 with permission from Elsevier, Copyright 2019.

The binary rGO/CNNA (carbon nitride treated with molten salt) in Figure 16.15B shows the best photocatalytic performance with a total CO2 conversion of 12.63 µmol g−1 h−1. The obtained hybrid material with regular and crystalline microstructure functions as a 2D/1D heterojunction and shows substantial improvements of light absorption, exciton splitting, charge transport as well as selective CO2 adsorption, which enables better CO2 photoreduction performance.148 In another example, a self-­assembled 3D graphene aerogel was used as the matrix to incorporate a few layers of MoS2 and TiO2 nanocrystals (see the fabrication process and example morphologies in Figure 16.9). The hierarchical porous GO/MoS2/TiO2 structure demonstrated the best performance with a CO2 conversion of 92.33 µmol g−1 h−1.69 The high CO2 photoconversion rate was attributable to the porous matrix which not only improved the conductivity but also provided efficient mass transport paths, increased physiochemical stability, and a large surface area. 16.3.3.2.3  In Situ Coating or Growth.  The in situ coating or growth strategy has been developed to synthesize uniquely integrated 3D GBCs. In this process, a metal salt solution is mixed with GO to form the graphene wrapped composite photocatalyst via electrostatic interaction or condensation reaction. The composite can also be modified with functional groups to achieve better performance. For example, Lee et al. reported an experimental method to wrap TiO2 nanoparticles with graphene nanosheets.149 Specifically, negatively charged GO suspension was mixed with positively charged amine-­functionalized TiO2 under stirring at pH 6 and followed by crystallization of TiO2 from amorphous to anatase phase. Zhao et al. reported the preparation of graphene wrapped Pt/TiO2 photocatalysts by three steps: synthesis of truncated octahedral

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bipyramid anatase TiO2 via the hydrothermal method, in situ growth of Pt nanoparticles via the gas bubbling assisted membrane reduction method, and self-­assembly of GO on the surface of Pt/TiO2 nanoparticles via surface condensation reaction.117 Cho et al. reported a two-­step method to obtain a hierarchical AG/CdS composite. First, GO was selectively deposited through electrostatic interaction with CdS nanoparticles.122 The composite was then grafted with amine functional groups, enabling CO2 activation. Figure 16.16 shows the CO2 photoreduction property of the AG/CdS composite. The CH4 and CO yield with varying graphene contents is shown in Figure 16.16A. The CH4 yield for AG/CdS was 2.84 µmol g−1 h−1, which was 23 times that observed for CdS.122 In Figure 16.16B, the rate of gas evolution and the CH4/CO ratio under CO2 at 1 bar are given. A lower gas evolution rate was observed for low-­pressure CO2 at 0.1 bar in Figure 16.16C. Figure 16.16D shows that AG/ CdS was able to maintain a high CO2 conversion rate of 84.7% over 10 cycles. Figure 16.16E shows the possible mechanism for CO2 photoreduction by AG/CdS. The graphene wrapped TiO2 nanoparticles have a significantly reduced bandgap of 2.8 eV, which accounts for the improved photocatalytic activity under visible light.

Figure 16.16  CO  2 photoreduction on the AG/CdS composite (A) CH4 and CO yield

with varying graphene contents under visible light. (B) Rate of gas evolution and CH4/CO ratio under CO2 at 1 bar (40 °C). (C) Rate of gas evolution under low-­pressure CO2 (0.1 bar, 40 °C); (D) Cyclic photoreduction of CO2 on AG/CdS and rGO/CdS; (E) Proposed mechanism of CO2 photoreduction on the AG/CdS. Reproduced from ref. 122 with permission from American Chemical Society, Copyright 2017.

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16.3.4  The Roles of Graphene in CO2 Photoreduction In general, the CO2 photoreduction mechanisms by using 3D GBCs as the photocatalysts are similar to the other heterogeneous photocatalysts. As discussed in Section 16.3.1, a major limiting factor in CO2 photoreduction is the fast charge recombination, which is the primary reason for incorporating highly conductive graphene into the photocatalyst system as the support to tackle this issue. Apart from the fast charge recombination, another reason for the low solar photoconversion efficiency of common wide-­bandgap semiconductors, such as TiO2, is its inability to absorb solar energy over a wide light spectrum. GBCs have been proved with the ability to synergistically improve adsorption of photocatalytic reactants, as well as increase light absorption range and facilitate charge transportation and separation. Figure 16.17 shows the possible roles of graphene in CO2 photoreduction by using 3D GBCs. In this system, graphene can serve as adsorbents, conductive supports, cocatalysts, photocatalysts, photostabilizer, or photosensitizer.150 The following paragraphs aim to provide a current understanding of the roles of graphene for boosting the reaction activity of CO2 photoreduction. Firstly, the improved catalytic ability of 3D GBCs can be partially attributed to the role of graphene as an absorbent due to its high specific area.151 The incorporation of graphene as structural support can markedly enlarge the specific surface area of the photocatalyst. The photocatalytic reduction of CO2 shows increased reaction activity due to a higher number of accessible active sites for adsorption and catalytic reactions.152 Chemically prepared graphene contains many functional groups, which act as anchoring sites to

Figure 16.17  The  role of graphene for enhancing the reaction activity of CO2

photoreduction. Reproduced from ref. 150 with permission from John Wiley & Sons, Copyright 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

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promote the growth of catalysts on its surface. The large surface area of graphene nanosheets helps to place photocatalysts uniformly and reduce the aggregation of photocatalysts. Moreover, the graphene nanosheets can work as a capping agent, thus inhibiting the growth of the nanoparticles. Therefore, the size of the photocatalyst is limited to achieve a higher surface area. Furthermore, graphene has a large 2D π-­conjugated structure, which can facilitate π–π conjugation interactions with delocalized π-­conjugated binding π43 of CO2 molecules.32 As a result, CO2 molecules are adsorbed and activated at a faster rate with this strong conjugation interaction in effect. Increasing conjugation interactions and available surface-­active sites cause easier CO2 adsorption and activation and then improve the photocatalytic CO2 reduction activity. Graphene has high conductivity and a suitable work function and thus serves as good conductive support for accepting, storing, and transporting photo-­induced electrons.40 This unique characteristic of graphene plays an important role in achieving extended lifetimes of photo-­induced electron–hole pairs and facilitating charge carrier separation and extraction.154 Coupling the photocatalysts with 3D graphene has been used to synthesize a variety of possible 3D GBCs with synergistic physicochemical effects. Another interesting strategy is to construct the Z-­scheme composite materials to enable high catalytic efficiency due to the low recombination rate of the photogenerated electron–hole pairs.155 Therefore, photocatalytic performance of 3D GBCs improves with prolonged charge carriers and better structural stability. Graphene can also be employed as effective cocatalysts for CO2 photoreduction due to its flexible electronic states and abundant catalytic sites. Active sites are oftentimes formed through defects, heteroatoms, metal clusters, and functionalized groups.30 The dangling bonds of the surface defects of cocatalysts are able to capture charge carriers, reactant molecules, and key intermediates, thus affecting photocatalytic activity and selectivity. The adsorption, activation, and subsequent CO2 photoreduction are closely correlated to the surface composition, phase, and structure of the cocatalysts so it is possible to accommodate photocatalytic activity with rationally designed surface-­active sites. In addition, graphene can restrict the possible photocorrosion of the catalyst and thus function as the photostabilizer. Photocorrosion damages the stability of the catalyst under light illumination and the corrosion can either be reductive or oxidative depending on whether it is caused by photogenerated electrons or holes.30 The conductive graphene is a good acceptor of photoexcited electrons and the photogenerated holes remain on the catalyst, thus greatly inhibiting the reductive corrosion. On the other hand, the photogenerated holes and hydroxyl radicals can oxidize graphene so the oxidative corrosion is suppressed significantly. Furthermore, graphene is beneficial for the photostability of catalysts because it can wrap on the photocatalyst to prevent the direct attack by active radical species. Consequently, the 3D GBCs possess good photostability for a long-­term CO2 photoreduction capability.

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Last but not least, the increased photocatalytic activity of 3D GBCs can be ascribed to the role of graphene as an effective photosensitizer by enhancing light absorption.156 Because graphene has zero bandgap and a dark color, it acts as the photosensitizer and absorbs the almost entire spectrum of sunlight, creating a photothermal effect by increasing the local temperature of the catalysts. The kinetic energy of the photocatalytic molecules increases by this photothermal effect, which is beneficial for catalytic reactions. In addition, electrons with high energy are more likely to be created and transferred to graphene other than going through recombination on the catalysts under light illumination.

16.4  Summary and Outlook Graphene has been proven to have remarkable properties, such as large specific surface area, high flexibility, and excellent conductivity, thus making it a promising material for CO2 photoreduction. Coupling the photocatalysts with 3D graphene not only improves the CO2 photoreduction activity, through its various roles as adsorbents, conductive supports, cocatalysts, photostabilizers, and photosensitizers, but also provides the composite photocatalysts with unique synergistic properties. For instance, electrons can transfer quickly throughout the network due to the high electrical conductivity of graphene and the reduced electron diffusion length in the network. The structural advantage of 3D graphene also decreases the aggregation of photocatalysts as well as increasing the adsorption of CO2 and offers many reactive photoreduction sites. Despite the substantial progress towards achieving more efficient and robust 3D GBCs with a deeper understanding of the mechanisms, several theoretical and fundamental issues of 3D GBCs must be addressed to realize the practical applications of 3D GBCs for CO2 photoreduction. For instance, high mass transfer resistance may arise from the collapse and shrinkage of pores and channels within the 3D GBCs. Moreover, defects in the graphene network lead to reduced electrical conductivity though defects can act as active sites for photoreduction of CO2. The effects of graphene and its derivatives as semiconductors and cocatalysts need to be better studied because the function of pristine graphene varies significantly from its derivatives in the photoreduction process. Further, in practical applications, GO instead of graphene is generally used, which is suspended in solvents and should be exfoliated and must be reduced to achieve better electronic properties, during which the performance of 3D GBCs may be compromised. On the whole, CO2 photoreduction by using 3D GBCs as the photocatalysts remains an interesting and growing research subject and a promising and revolutionary strategy for inspiring potential applications. It is desirable to gain an in-­depth fundamental understanding of achieving precisely controlled nanostructures as well as taking a systematic perspective to make this strategy plausible. Though engineering challenges still need to be solved,

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such as discovering reactive, stable, and inexpensive photocatalytic materials, we believe 3D GBCs will continue to develop and advance as promising strategies for CO2 photoreduction.

Acknowledgements The authors thank Dr Pratim Biswas, Dr John Fortner, Dr Yi Jiang, and Dr Yao Nie for their collaborative work in graphene projects at Washington University in St. Louis, as well as Dr Jiaxing Huang at Northwestern University and Dr Hee-­Dong Jang at Korean Institute of Geoscience and Mineral Resources for their insightful comments. Dr Wang also acknowledges the financial supports for CO2 photoreduction research from the American Chemical Society Petroleum Research Fund and the Virginia Commonwealth University Presidential Research Quest Fund.

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Subject Index acrylonitrile-­butadiene-­styrene (ABS), 31 activated carbons (AC), 116 adjacent environment, 262 aerosol-­processed crumpled ­ GBCs, 415–418 air purification particulate matter and ­ viruses, 328–329 volatile organic compounds (VOCs), 327–328 ambient environment, 262 ammonium thiomolybdate ­ (ATM), 18 antibacterial effects, 186 aqueous mesophase pitch ­ (AMP), 10 Arrhenius equation, 246 artificial photosynthesis CO2 photoreduction analysis systems, 413–415 basic principles, ­ 407–413 design of 3D GBCs, 415–421 roles of graphene, 422–424 3D graphene-­based ­ composites spray-­based aerosol routes, 399–405 wet chemistry methods, 405–407 atomic layer deposition (ALD), 345

bacterial nanocellulose (BNC), 268 bidirectional freeze-­drying ­ fabrication process, 8 bilayered solar evaporation system, 272 bio-­DSSCs performance, 224–226 black phosphorus (BP), 80 Boltzmann theory, 403 Brodie method (B-­GO), 248 cadmium telluride (CdTe), 206 capacitive deionization (CDI) ­ electrodes, 20 capillary compression, 127 carbon aerogels (CAs), 15 carbon, crystalline structure, 1–2 carbon nanofibers (CNFs), 61 carbon nanotubes (CNTs), 61, 368 cell architecture, 207–209 cellulose/graphene aerogels ­ (CGAs), 8 cellulose nanofiber-­GO hybrid ­ aerogel (CNF/GOA), 325 cetyltrimethylammonium bromide (CTAB), 10 chemically modified graphene ­ cellular networks (CMG-­CNs), 11 chemically modified graphene (CMG) films, 49 chemical vapor deposition ­ (CVD), 11, 41 chitosan, 268 Clausius–Clapeyron equation, 387 CNFs. See carbon nanofibers (CNFs) CNTs. See carbon nanotubes (CNTs) 432

Subject Index

CO2 capture performance, 390–392 copper indium gallium selenide (CIGS), 206 counter electrodes (CEs), 213–217 crumpled GO (CGO), 401 cyclic voltammetry (CV), 183–185 density functional theory (DFT), ­ 76, 150–152, 222 direct ink writing (DIW), 22–25, 99 double-­layer templated graphene (DTG), 11 dye excitation, 208–209 dye-­sensitized solar cells ­ (DSSCs), 206, 207 cell architecture, 207–209 components of, 208 dye excitation, 208–209 electrochemical reduction, 209 electron injection, 209 electron transport, 209–211 graphene and 3D graphene-­ based materials (3D GBMs) counter electrodes (CEs), 213–217 graphene integrated wide bandgap semiconductor photoanodes, 217–221 synthesis methods, 211–213 naturally sensitized ­ photoanodes, 221–223 bio-­DSSCs performance, 224–226 chemical structure, 223–224 graphene-­based naturally sensitized photoanodes, 226–227 pigment bandgap, 224–226 sources, 223–224 oxidized dye regeneration, 209 recombination kinetics, 209–211 working mechanism, 207–209

433

ECs. See emerging contaminants (ECs) electrical conductivity, 8, 13, 20 GMA vs. annealing ­ temperature, 9 electric double-­layer effect ­ (EDLE), 116 electricity generation, 283–284 electrochemical energy storage (EES) devices, 87 electrochemical exfoliation and deposition, 98–99 electrochemical impedance ­ spectroscopy (EIS), 186 electrochemically exfoliated graphene (EEG), 65 electrochemical reduction, 209 electrochemical synthesis, ­ 148–149 electrode preparation, 190 electron injection, 209 electron microscopy, 121, 402 electron paramagnetic ­ resonance (EPR) ­ spectroscopy, 412 electron transport, 209–211 emerging contaminants (ECs), 324–327 endocrine-­disrupting chemicals (EDCs), 326–327 environmental contaminants air purification particulate matter and viruses, 328–329 volatile organic ­ compounds ­ (VOCs), 327–328 regeneration and reuse, 329–330 water treatment, 314–315 dyes, 321–324 emerging contaminants (ECs), 324–327 heavy metals, 315–320 organic contaminants, 320–321

434

field emission scanning electron microscopy (FESEM), 351, 377 flame ionization detector (FID), 414 freeze gelation, 26–27 Fuchs' theory, 402 fused deposition modelling ­ (FDM), 31, 99 galvanostatic intermittent titration technique (GITT) analysis, 106 gelation methods, 4–9 Ge quantum dots (Ge-­QD), 79 GO-­carbon black (GO/CB), 268 GO-­carbon nanotube (GO/CNT), 268 graphene, 368 graphene aerogel composites carbon nano tube/graphene aerogels (CNT/GA), 19–21 fullerene/graphene aerogels, 21–22 metal-­doped graphene ­ aerogels (MDGAs), 17–19 polymeric graphene aerogels (PGA), 15–17 graphene aerogels (GAs), 94 3D printing methods casting, 27 direct ink writing ­ (DIW), 22–25 freeze gelation, 26–27 fused deposition ­ modelling (FDM), 31 inkjet, 25–26 laser-­based methods, 31–33 projection micro-­ stereolithography (PµSL), 27–30 stamping and ­ templating, 33–35 freeze-­drying, 4–9 gelation methods, 4–9 sol–gel hydrogels, 4–9 template methods, 9–10 hard templating, 11–15 soft templating, 10–11 graphene-­based aerogel (GPA), 16

Subject Index

graphene foam (GF), 11, 94 graphene hydrogel (GH), 42–43 graphene-­modified 3D scaffolds chitosan/vacuum-­stripped 3D graphene scaffold, 201 3D graphene nanosheets, 201 graphene nanoplates (GNPs) array, 275 graphene oxide, 3 surface groups on, 3 graphene quantum dots (GQDs), 94 graphene sponge (GS)-­SS composite characterization, 198 EIS, 198–200 mechanism, 198–200 synthesis, 198 graphite nano-­platelets (GNP), 24 heating efficiency, 263 heteroatom-­doped 3D G B, P, S-­doped 3D GBMs, 155–157 nitrogen-­doped 3D G (N-­3D G), 149–150 ORR, active sites for, 152–155 progress in, 150–151 heterogeneous photocatalysis, 339 H2 evolution reaction (HER), 182 Heyrovsky reaction, 240 highest occupied molecular orbital (HOMO), 208 highly ordered pyrolytic graphite (HOPG), 211 highly vertically ordered pillar array of graphene-­assembled ­ framework (HOPGF), 264 highly wrinkled graphene film (HWGF), 13, 14 Hummers' method, 3, 248 hydrogel, 190 hydrogen evolution H2 generation from the degradation ­ of H2O, 242–243 mediated by modified 3D GHs, 245–247

Subject Index

over modified 3D graphene materials, 243–245 through H2O reduction, 239–242 hydrogen storage, 247–249 on carbon material composition (SiC/G), 250–251 on pillared carbon ­ materials, 250 hydrogen evolution reaction ­ (HER), 31, 154 hydrophobic graphene foam ­ (SGF), 266 impedance spectroscopy, 227 indirect freezing, 148–149 inkjet, 25–26 intelligent water evaporation, 285–287 Kirchhoff's law of thermal ­ radiation, 260 laser induced graphene (LIG), ­ 31, 241, 242 light harvesting macro optimization, ­ 276–277 micro optimization, 275–276 linear sweep voltammetry (LSV), 185–186 lithium ion batteries (LIBs), 58 flexible 3D graphene-­based anode materials, 73–74 alloying-­t ype anode materials, 78–80 conversion-­t ype anode materials, 77–78 intercalation-­t ype ­ anode materials, ­ 74–77 flexible 3D graphene-­based cathode materials, 60 conversion-­t ype cathode materials, 69–73

435

intercalation-­t ype ­ cathode materials, 61–69 structure and working ­ principle, 58 lowest unoccupied molecular orbital (LUMO), 21, 208–209 low-­temperature solid oxide fuel cells (LT-­SOFCs), 168 mesoporous carbon microspheres/ graphene composites ­ (MCMG), 10 MgAl-­layered double oxides ­ (MgAl-­L DO), 11 microbial desalination cells ­ (MDCs), 180 microbial electrolytic cells ­ (MECs), 180 microbial fuel cells (MFC) biocompatibility of electrode, 186–187 electrochemical characterizations cyclic voltammetry (CV), 183–185 electrochemical ­ impedance spectroscopy (EIS), 186 linear sweep ­ voltammetry (LSV), 185–186 electrode synthesis, 182–183 GA-­based 3D electrodes GFB electrode, 188–189 high capacitative 3D GA anodes, 187–188 nitrogen-­doped graphene aerogel electrode ­ (N-­GA), 189–190 3D Pt NP/GA composite, 190–191 surface morphology, 183 micro-­super capacitors (MSC), 33 mixed cellulose esters (MCE) ­ membrane, 267 molybdenum disulfide (MoS2), 18

Subject Index

436

multiple-­effect distillation ­ (MED), 258 multistage flash (MF), 258 multi-­wall carbon nanotubes (MWCNTs), 2, 162, 191, 213 MXene-­based micro-­ supercapacitors, 34 nanocellulose (NC), 79 nano-­confinement effects, 128 N-­doped graphene membrane, 270 Ni-­doped graphene/carbon cryogels (NGCCs), 18 nitrogen-­doped graphene foams (NGFs), 14 nitrogen-­doped graphene tubes ­ (N-­GTs), 46 nitrogen phosphorous dual doped graphene aerogel (NPGA), 101 nuclear magnetic resonance (NMR) spectroscopy, 5 Nyquist plots, 199 oils and organic solvents properties of contaminants, 298 properties of 3D GBMs, 298–301 removal performance of 3D GBMs chemical vapour ­ deposition (CVD), 302–303 crude oil removal, 306 organic foams, 303–305 regeneration and reuse, 307 self-­assembled 3D ­ GBMs, 301–302 oily waste water, 283 1D water channel, 272–273 organic tandem, 206 organogel, 94 oxidized dye regeneration, 209 oxygen evolution reaction (OER), 154, 242 oxygen reduction reaction (ORR), 141

perovskite-­t ype oxide chemical ­ formula, 168 pharmaceutical and personal care products (PPCPs), 324–326 phosphorus (P), 80 photocatalytic materials organic pollutants, 345–358 template-­based 3D GBMs, 341–342 template-­free 3D GBMs, 343–345 physiochemical chemical ­ properties, 51 pigment bandgap, 224–226 pillared GO (PGO ), 249 pillared graphene synthesis of, 371 plant fibres, 268 plasma-­enhanced chemical vapour deposition (PECVD), 47, 48 PMMA spheres, 49 polyacrylamide (PAM), 160, 268 polyacrylonitrile (PAN), 160 polyaniline (PANI), 195–196 polydimethyl siloxane (PDMS), ­ 13, 73 polylactic acid (PLA), 31 poly (N-­isopropylacrylamide) ­ (PNIPAm) membrane, 268 polymethyl methacrylate (PMMA) layer, 12 polystyrene (PS), 13, 49, 301 polysulfide shuttle effect, 72 polyurethane (PU) foams, 268 polyvinyl alcohol (PVA) hydrogels, 20, 268 polyvinyldiene fluoride (PVDF), 301 pore geometry, 51 postcombustion carbon capture, 387–390 projection micro-­stereolithography (PµSL), 27–30 quantum dot cells, 206 red phosphorus (RP), 80 reduced graphene oxide (rGO), 123

Subject Index

resorcinol-­formaldehyde (RF) method, 5, 15, 20 reverse osmosis (RO), 258 rGO-­multiwalled carbon nanotubes (rGO/MWCNTs), 268 room temperature freeze gelation (RTFG), 26 sacrificial support method (SSM), 152 scanning electron microscopy (SEM), 21, 28, 43, 96, 183, 192 selective laser melting (SLM), 32 selective laser sintering (SLS), 99 semiconductor bandgap, 345 sewage purification, 280–281 heavy metals in waste water, 281 oily waste water, 283 organic dye waste water, 281–282 silica particles, 49 single wall carbon nanotubes (SWCNTs), 2, 368 sodium alginate, 268 sodium–graphite intercalation ­ compounds (Na–GICs), 91 sodium ion batteries anodes, applications graphene aerogel (GA), 101–103 graphene foam (GF), 103–104 3D graphene coated anodes, 105 3D porous graphene, 104–105 cathodes, application, ­ 106–107 ion storage mechanism advantages of 3D graphene ­ nanostructure, 93 battery performance against reaction mechanics, 89–90 electrode design, 91–93

437

nanostructured ­ materials, 90–91 operating principle, 88–89 synthesis, 93–94 blowing synthesis, 99–100 electrochemical exfoliation and deposition, 98–99 emerging novel methods, 98–101 self-­assembly methods, 97–98 supercritical carbon ­ dioxide (CO2) fluid, 101 template-­assisted method, 94–97 3D printing, 99 solar absorptance, 259, 260 solar steam generation (SSG) applications electricity generation, 283–284 intelligent water ­ evaporation, 285–287 seawater desalination, 279–280 sewage purification, 280–283 sterilization, 284–285 designs of 3D GBMs, 289 functional applications, 289 graphene-­carbon materials, 268 graphene-­organic materials, 268–269 isolation evaporation system, 274 photothermal conversion, 289 technologies light harvesting, 275–277 thermal management, 277–279 3D GO/rGO, 266–268 3D graphene, 264–266 2D water channel, 272–274

438

solar-­thermal conversion efficiency, 263 and transport solar absorption, 259–260 thermal to steam ­ generation, 263–264 thermal transfer, 260–263 solar-­thermal energy, 260 sol–gel hydrogels, 4–9 solid electrolyte interphase (SEI), 59, 89 spray-­based aerosol routes, ­ 399–405 basics of, 399–400 crumpled GBCs, 403 cargo particles within CGO, in situ formation, 404 encapsulation with ­ pre-­synthesized cargo particles, 403–404 post-­mixing, 404 graphene crumpling, 400–403 stainless steel (SS) current collector, 198 standard hydrogen electrode (SHE), 57 stereolithography (SLA), 99 sterilization, 284–285 structure–property relationship graphene cages, 48 graphene fibres, 45–47 graphene tubes, 45–47 3D graphene networks, 42–45 3D porous graphene films, 48–50 vertical graphene sheets, 47–48 supercapacitors advances, 129–130 3D films, 125–128 double layer and pseudocapacitive type devices, 117–125 graphene-­based anode ­ materials, 133–135 graphene-­based cathode ­ materials, 130–133

Subject Index

graphene-­based fibres, 128–129 graphene papers, 125–128 supercritical carbon dioxide (CO2) fluid, 101, 147 surfactant-­free emulsions (SFE), 283 surfactant-­stabilized emulsions (SSE), 283 tetrakis(4-­aminophenyl) methane (TKAm), 250 thermal conductivity detector (TCD), 414 thermal emittance, 260 thermal management evaporation enthalpy, 278–279 thermal insulation, 277–278 thermodynamic principle, 90 3D bubble-­derived graphene foams (BGFs), 10 3D GBM-­based biosensors, 376–377 3D GBM-­based soil sensors, 377–380 3D graphene foams flexible 3D graphene-­Ni foam preparation of electrode, 193–195 macroporous graphene/multi-­ walled CNTs (MWCNTs)/ FeO foams electrode fabrication, 191–192 physiochemical ­ characterization of, 192–193 3D graphene macroporous scaffold, 196–197 3D graphene sponges macroporous flexible 3D graphene sponge electrode, synthesis of, 197–198 SEM analysis, 197–198 time-­of-­flight (TOF) value, 246 total solar irradiance, 259 transition metal-­carbon (M-­N-­C) ­ catalysts, 159–161

Subject Index

transition metal ion (TMi), 18 transition metal macrocyclic ­ compounds, 157–159 transition metal oxide catalysts perovskite-­t ype oxide catalysts, 167–169 pyrochlore-­t ype oxide ­ catalysts, 167–169 single metal oxide catalysts, 161–162 spinel-­t ype oxide catalysts, 162–167 transition metal oxides (TMOs), 77 transition metal sulfides (TMSs), 77 transmission electron microscopy (TEM), 46, 192 transparent conductive oxide ­ (TCO), 207 2D bubble-­derived graphene porous membranes (BGPMs), 10 two-­dimensional (2D) honeycomb lattice, 258 2D water channel, 272 ultrahigh performance lithium ion batteries. See lithium ion batteries

439

valence band (VB), 345 vertically aligned graphene ­ (VAG), 79 volatile organic compounds (VOCs), 327–328 water treatment, 314–315 dyes, 321–324 emerging contaminants ­ (ECs), 324–327 heavy metals, 315–320 organic contaminants, 320–321 water vaporization, 263 wet chemistry generated GBCs, 418–421 wet chemistry methods, ­ 405–407 white phosphorus (WP), 80 World Health Organization ­ (WHO), 281 X-­ray diffraction (XRD), ­ 192–193 X-­ray photoelectron ­ spectroscopy (XPS) analysis, ­ 192–193, 351, 401