Organic and Carbon Gels: From Laboratory Synthesis to Applications [1st ed.] 978-3-030-13896-7;978-3-030-13897-4

Table of contents :
Front Matter ....Pages i-xiv
Organic and Carbon Gels: From Laboratory to Industry? (Ana Arenillas, J. Angel Menéndez, Gudrun Reichenauer, Alain Celzard, Vanessa Fierro, Francisco José Maldonado Hodar et al.)....Pages 1-26
Organic and Carbon Gels Derived from Biosourced Polyphenols (Ana Arenillas, J. Angel Menéndez, Gudrun Reichenauer, Alain Celzard, Vanessa Fierro, Francisco José Maldonado Hodar et al.)....Pages 27-85
Properties of Carbon Aerogels and Their Organic Precursors (Ana Arenillas, J. Angel Menéndez, Gudrun Reichenauer, Alain Celzard, Vanessa Fierro, Francisco José Maldonado Hodar et al.)....Pages 87-121
Fitting Carbon Gels and Composites for Environmental Processes (Ana Arenillas, J. Angel Menéndez, Gudrun Reichenauer, Alain Celzard, Vanessa Fierro, Francisco José Maldonado Hodar et al.)....Pages 123-147
Carbon Gels for Electrochemical Applications (Ana Arenillas, J. Angel Menéndez, Gudrun Reichenauer, Alain Celzard, Vanessa Fierro, Francisco José Maldonado Hodar et al.)....Pages 149-189
Back Matter ....Pages 191-195

Citation preview

Advances in Sol-Gel Derived Materials and Technologies Series Editors: Michel A. Aegerter · Michel Prassas

Ana Arenillas · J. Angel Menéndez Gudrun Reichenauer · Alain Celzard Vanessa Fierro Francisco José Maldonado Hodar Esther Bailόn-Garcia · Nathalie Job

Organic and Carbon Gels From Laboratory Synthesis to Applications

Advances in Sol-Gel Derived Materials and Technologies

Series editors Michel A. Aegerter Michel Prassas

More information about this series at http://www.springer.com/series/8776

The International Sol-Gel Society (ISGS) Dear Readers, The International Sol-Gel Society (ISGS) was established in 2003 as an international, interdisciplinary, not-for-profit organization whose primary purpose and objective is to promote the advancement of sol-gel science and technology. ISGS’s aims are both to represent the particular needs and aspirations of the international sol-gel community and to support this sol-gel community. The society’s mission is threefold: • To coordinate the promotion of sol-gel science and technology in the scientific and industrial community • To foster communication between researchers from different fields and geographical regions through the organization of conferences and the publication and circulation of technical papers • To encourage education, training, and research in the field of sol-gel science and technology To achieve these purposes, ISGS convenes the biannual International Sol-Gel Conference in many parts of the world. The 19th edition of this International Conference was held in Liège, Belgium, from 3 to 8 September 2017. These conferences play an important role in the education, federation, and dissemination of scientific knowledge to people working in related fields. To initiate young researchers and engineers into the sol-gel field, an introductory short course was planned and organized by the local organizers of the conference. The topic of the short course was “carbon gels,” an important and emerging area in both scientific and industrial fields. This book is a summary of the lectures given in the course and thus enables readers to learn the fundamentals and applications of “carbon gels”. I wish you very pleasant and educative reading! Masahide Takahashi President of the International Sol-Gel Society http://www.isgs.org

Ana Arenillas • J. Angel Menéndez Gudrun Reichenauer • Alain Celzard Vanessa Fierro Francisco José Maldonado Hodar Esther Bailόn-Garcia • Nathalie Job

Organic and Carbon Gels From Laboratory Synthesis to Applications

Ana Arenillas Instituto Nacional del Carbón (INCAR-CSIC) Oviedo, Spain

J. Angel Menéndez Instituto Nacional del Carbón (INCAR-CSIC) Oviedo, Spain

Gudrun Reichenauer Bavarian Center for Applied Energy Research Wuerzburg, Germany

Alain Celzard Institut Jean Lamour University of Lorraine Épinal, France

Vanessa Fierro Institut Jean Lamour French National Centre for Scientific Research Épinal, France

Francisco José Maldonado Hodar Department of Inorganic Chemistry University of Granada Granada, Spain

Esther Bailόn-Garcia Department of Inorganic Chemistry University of Granada Granada, Spain

Nathalie Job Department of Chemical Engineering University of Liège Liège, Belgium

ISSN 2364-0030     ISSN 2364-0049 (electronic) Advances in Sol-Gel Derived Materials and Technologies ISBN 978-3-030-13896-7    ISBN 978-3-030-13897-4 (eBook) https://doi.org/10.1007/978-3-030-13897-4 © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG. The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Foreword

Sol-gel science and technology is now firmly established as a rich interdisciplinary domain, in which approaches for preparing materials that span the range from purely inorganic systems to hybrids with properties suitable for a breathtaking array of applications are now well established. Historically, sol-gel processing was shaped by a combination of physics and chemistry, taking advantage of “Chimie Douce” processes and complementary characterization tools, to prepare (mainly) metallic oxides at low temperatures. This pioneering work has been extended significantly during the past few decades, with organic moieties and biologically active species being incorporated within sol-gel-derived systems through control at the molecular level. These developments have led to an explosion in the availability of new hybrid materials with novel properties which are being exploited across a wide variety of application fields. In parallel with developments in traditional sol-gel science and technology, purely organic (or carbon) gels have also been intensively developed to obtain versatile materials which are applied in many areas, including supported catalysis, separation science, and environmental remediation. When dried under appropriate conditions, such gels are chemically inert and stable and exhibit excellent thermal insulation characteristics. They can be transformed into highly porous and low-­ density aerogels, which exhibit good mechanical strength and high flexibility. Carbon gels are easily obtained from organic precursors (e.g., formed by resorcinol-­ formaldehyde) pyrolyzed under inert atmosphere. They also exhibit good electrochemical conductivity and are being used as capacitors and supercapacitors, as well as efficient solar energy collectors. It is evident that there is now an opportunity to take advantage of the relative maturity and expertise within the carbon gel and the “classical” sol-gel communities to exploit synergies that might lead to the potential development of new hybrid materials. This book, which is entitled Organic and Carbon Gels: From Laboratory Synthesis to Applications, is intended to serve as a bridge between these two communities, to stimulate new ideas and collaborations leading to new classes of high-­ performance materials and new application domains that could not otherwise be envisaged. We hope that the dissemination of such knowledge will stimulate interest v

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Foreword

in contributing to the rich palette of opportunities that exist at the (largely unexplored) interface between these two cognate domains. Michel Wong Chi Man CNRS Senior Researcher Prof, International Open Laboratory Gunma University Initiative for Advanced Research (GIAR) Maebashi City, Japan

Preface

Sol-Gel 2017, the 19th International Sol-Gel Conference, was organized from September 3rd to 8th in Liège, Belgium. The great success of the Sol-Gel Conference series over the past 35 years attests that sol-gel science and technology is an extraordinarily multidisciplinary research area. However, while these Sol-Gel conferences have succeeded in connecting many actors of the sol-gel technology in the fields of inorganic chemistry and materials, the carbon gel community is usually not so well represented. In order to try to bridge the gap between the inorganic gels and the carbon gel research fields, it was decided to organize a 1-day workshop devoted to carbon gels as an introduction to the conference. We are very pleased to present this book, which gathers chapters presenting carbon gels from the points of view of the five lecturers of the workshop, each being a renowned specialist in a specific field related to those materials. On the one hand, carbon gels represent a promising class of materials in high added value applications using carbon materials. They present many assets, like the possibility to accurately tailor their structure, porosity, and surface composition as well as to easily dope them with numerous species. The ability to obtain them in custom shapes, such as powder, beads, monoliths, or impregnated scaffolds, opens the way toward numerous applications: catalysis, adsorption, electrochemical energy storage, etc. On the other hand, the feasibility of design synthesis and manufacturing processes from an economic and environmental perspective remains a crucial question. The present book covers most of these aspects. Chapter 1 deals with the scale-up of the synthesis of classical carbon gels in order to transfer the technology from lab to industry. Chapter 2 presents variations of the synthesis procedure in order to move from fossil resources to renewable carbon precursors while preserving carbon gel’s exceptional properties. In Chap. 3, the characterization of these materials is debated: indeed, applying characterization techniques to these materials is not always straightforward, and the reader will be made aware of the specific difficulties encountered with these materials. Finally, Chaps. 4 and 5 will provide an overview of the main applications of carbon gels in catalysis and electrochemistry. vii

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Preface

Through the workshop and the present book, we hope to encourage new collaborations that bridge the gap between inorganic and organic sol-gel science. Let’s bet that the present intense international research in the field will make it possible. We wish you a rewarding lecture. Nathalie Job, Vice-Chair Benoît Heinrichs, Chair Local Organizing Committee of Sol-Gel 2017 Christelle Alié Cédric Calberg Stéphanie Lambert Alexandre Léonard Rudi Cloots Frédéric Boschini Oviedo, Spain Oviedo, Spain Wuerzburg, Germany Épinal, France Épinal, France Granada, Spain Granada, Spain Liège, Belgium

Ana Arenillas J. Angel Menéndez Gudrun Reichenauer Alain Celzard Vanessa Fierro Francisco José Maldonado Hodar Esther Bailόn-Garcia Nathalie Job

Contents

1 Organic and Carbon Gels: From Laboratory to Industry?����������������    1 1.1 Introduction��������������������������������������������������������������������������������������    1 1.1.1 What Are Organic and Carbon Gels?������������������������������������    2 1.1.2 The Sometimes Misleading Nomenclature of the Gels ����������������������������������������������������������������������������    2 1.1.3 Advantages and Disadvantages of the Organic and Carbon Gels Compared with Other Porous Materials ������������������������������������������������������������������    4 1.2 The Synthesis Process of the Organic Gels��������������������������������������    6 1.2.1 Acidic and Basic Routes: The Two Main Mechanisms��������    7 1.2.2 The Effect of pH ������������������������������������������������������������������    9 1.2.3 The Effect of Dilution����������������������������������������������������������    9 1.2.4 The Effect of the Ratio of the Monomers ����������������������������   11 1.2.5 The Effect of the Grade of the Monomers����������������������������   12 1.2.6 The Complexity of Dealing with Interdependent Variables: Combined Effects������������������������������������������������   13 1.2.7 The Role of Additives ����������������������������������������������������������   14 1.3 Post-Synthesis Treatments of Organic Gels��������������������������������������   15 1.3.1 Thermal Stabilization������������������������������������������������������������   15 1.3.2 Passivation Against Moisture������������������������������������������������   16 1.4 Thermal Treatments for Obtaining Carbon Gels������������������������������   17 1.4.1 Carbonization������������������������������������������������������������������������   18 1.4.2 Activation Process����������������������������������������������������������������   18 1.4.3 Ordering Structure����������������������������������������������������������������   19 1.5 A General Overview of the Versatility of These Materials ��������������   20 1.5.1 Potential Uses of Organic Gels ��������������������������������������������   20 1.5.2 Potential Uses of Carbon Gels����������������������������������������������   21 1.6 The Prospects for the Industrial Production of the Organic and Carbon Gels��������������������������������������������������������   21 1.7 Conclusions��������������������������������������������������������������������������������������   22 References��������������������������������������������������������������������������������������������������   23 ix

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2 Organic and Carbon Gels Derived from Biosourced Polyphenols������   27 2.1 Introduction��������������������������������������������������������������������������������������   28 2.2 Biosourced Polyphenols and Related Materials ������������������������������   29 2.2.1 Introduction to Plant Polyphenols����������������������������������������   29 2.2.2 Tannins����������������������������������������������������������������������������������   30 2.2.3 Lignin������������������������������������������������������������������������������������   34 2.2.4 Polyphenol-Based Carbons��������������������������������������������������   37 2.3 Gels Derived from Condensed Tannins��������������������������������������������   40 2.3.1 General Information About the Synthesis of Polyphenol-­Based Gels����������������������������������������������������   40 2.3.2 Tannin-Resorcinol-Formaldehyde (TRF) and Tannin-­­Formaldehyde (TF) Organic and Carbon Aerogels and Cryogels��������������������������������������   41 2.3.3 Tannin-Formaldehyde Organic and Carbon Xerogels����������   47 2.3.4 Alternative Tannin-Based Carbon Gels��������������������������������   53 2.4 Mixed Gels Derived from Natural Polyphenols��������������������������������   55 2.4.1 Tannin-Soy-Formaldehyde (TSF) Gels��������������������������������   55 2.4.2 Tannin-Lignin-Formaldehyde (TLF) Organic Gels��������������   59 2.5 Brief Overview of Properties and Applications��������������������������������   63 2.5.1 Mechanical Properties����������������������������������������������������������   63 2.5.2 Thermal Properties����������������������������������������������������������������   65 2.5.3 Applications as Electrodes for Supercapacitors��������������������   66 2.6 Recent Developments ����������������������������������������������������������������������   69 2.6.1 One-Step Microwave-Assisted Synthesis and Drying����������   69 2.6.2 Organic and Carbon Xerogel Microspheres�������������������������   71 2.6.3 Elastic Gels with Tunable Properties������������������������������������   73 2.7 Conclusion����������������������������������������������������������������������������������������   75 References��������������������������������������������������������������������������������������������������   76 3 Properties of Carbon Aerogels and Their Organic Precursors ����������   87 3.1 Introduction��������������������������������������������������������������������������������������   88 3.2 Organic Versus Carbon Aerogels������������������������������������������������������   90 3.3 Properties of Organic and Carbon Aerogels ������������������������������������   91 3.3.1 Aerogel Structure������������������������������������������������������������������   94 3.3.2 Aerogel Backbone Connectivity ������������������������������������������  105 3.3.3 Pore Connectivity of Aerogels����������������������������������������������  113 3.4 Composites����������������������������������������������������������������������������������������  115 3.5 Summary and Conclusions ��������������������������������������������������������������  116 References��������������������������������������������������������������������������������������������������  116 4 Fitting Carbon Gels and Composites for Environmental Processes ��������������������������������������������������������������������������������������������������  123 4.1 Statement of the Problem������������������������������������������������������������������  124 4.2 The Starting Point in the Materials Design��������������������������������������  125 4.3 The Sol-Gel Synthesis as a Powerful Tool����������������������������������������  126

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4.4 About the Performance of Carbon Gels in Environmental Processes��������������������������������������������������������������  132 4.4.1 Removing Pollutants by Specific Adsorbents and Molecular Sieves������������������������������������������������������������  132 4.4.2 Removing Pollutants by Catalyzed Processes����������������������  136 4.5 Conclusions and Perspectives ����������������������������������������������������������  141 References��������������������������������������������������������������������������������������������������  142 5 Carbon Gels for Electrochemical Applications ������������������������������������  149 5.1 Introduction��������������������������������������������������������������������������������������  149 5.2 Conductivity of Carbon Gels������������������������������������������������������������  152 5.3 Supercapacitors ��������������������������������������������������������������������������������  154 5.3.1 General Principle������������������������������������������������������������������  154 5.3.2 Carbon Gels in Supercapacitor Electrodes ��������������������������  156 5.4 Batteries��������������������������������������������������������������������������������������������  159 5.4.1 General Principle������������������������������������������������������������������  159 5.4.2 Carbon Gels in Li-Ion Battery Negative Electrodes ������������  161 5.4.3 Carbon Gels in Na-Ion Battery Electrodes ��������������������������  167 5.5 Fuel Cells������������������������������������������������������������������������������������������  168 5.5.1 General Principle������������������������������������������������������������������  168 5.5.2 Electrocatalysts with Carbon Gels as Supports��������������������  170 5.5.3 Membrane-Electrode Assemblies from Carbon Gel-­Supported Catalysts����������������������������������  177 5.5.4 Durability of Pt and Pt-M Catalysts Supported on Carbon Gels����������������������������������������������������  180 5.6 Conclusions��������������������������������������������������������������������������������������  181 References��������������������������������������������������������������������������������������������������  182 Index������������������������������������������������������������������������������������������������������������������  191

About the Authors

Ana  Arenillas  holds a doctorate degree in Chemical Engineering from the University of Oviedo, Spain (1999). She is research scientist at INCAR-CSIC in Spain and head of the research group Microwaves and Carbon for Technological Applications (MCAT, http://www.incar.csic.es/mcat). Her research activity is focused on carbon materials, especially organic and carbon xerogels, and their use in solving energy and environmental issues. She is coauthor of more than 200 peer-­ reviewed papers, chapters, and patents and is co-founder of SME Xerolutions Ltd. (http://www.xerolutions.com). J.  Angel  Menéndez  graduated from the University of Oviedo, Spain, where he received his M.Sc. in Chemistry and Ph.D. in Chemical Engineering in 1988 and 1994, respectively. He then worked as a research assistant at Penn State University, USA (1995–1996). In 1997, he joined INCAR-CSIC, Spain, where he currently works as a scientific researcher. His research activity is mainly focused on carbon materials and the use of microwave heating as applied to industrial processes. He leads various research projects in these fields. Díaz is author or coauthor of more than 200 scientific publications and 10 patents. He is founding editor of the GEC Bulletin and co-founder of SME Xerolutions Ltd. (http://www.xerolutions.com). Gudrun Reichenauer  obtained her Ph.D. in Physics at the University of Würzburg, Germany. She pursued her career as a research associate first at the Bavarian Center for Applied Energy Research (ZAE Bayern) and the Physics Department of the University of Würzburg (1993–1998) and then at Princeton University and Princeton Institute for the Science and Technology of Materials (NJ, USA, 1999–2000). Since 2000, she has led the Nanomaterials Group at ZAE Bayern. Her scientific interests lie in the identification of nanoeffects for use in energy-related fields, synthesis of well-scalable nanomaterials for applications in energy technology, and development of novel or adjusted methods for fast and artifact-free characterization of nanoporous materials.

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Alain Celzard  graduated with a degree in Chemical Physics in 1992 and received his Ph.D. in materials science in 1995 in Nancy, France. Since 2005, he has been a full-time professor at ENSTIB engineering school (Epinal). In 2010, he was appointed junior member of the Institut Universitaire de France. His scientific interests lie with disordered, porous, and related materials, ranging from composites and nanoporous adsorbents to macroporous solid foams through gels, with applications in catalysis, depollution, energy, and gas storage. Vanessa Fierro  graduated in Chemistry at the University of Zaragoza, Spain, and holds a doctorate degree in Sciences from the same university (1998). After working several years as a researcher in France (IFP Energies Nouvelles – Solaize and IRCE – Villeurbanne) and then in Spain (URV  – Tarragona), she is now CNRS research director and head of the Bio-Sourced Materials research group at Jean Lamour Institute. Her present research deals with the preparation, characterization, and applications of porous solids. She is coauthor of more than 200 peer-reviewed papers. Francisco José Maldonado Hodar  obtained his Ph.D. in Chemistry with distinction from the University of Granada, Spain, in 1993, where he was a member of the group of research in Carbon Materials and received an Extraordinary Doctorate Award. In 2012, he returned to the university as a full professor in the Inorganic Chemistry Department. His primary research interest is in the areas of carbon materials and heterogeneous catalysis for the purpose of developing materials with fitted physicochemical properties to be used as adsorbents, molecular sieves, and heterogeneous catalysts in processes relating to environmental protection, clean energy, and fine chemistry. He is coauthor of more than 130 highly cited manuscripts (h=34), as well as book chapters and patents. Esther  Bailόn-Garcia  obtained her Ph.D. in Chemistry with distinction in the area of materials design for catalytic and photocatalytic applications from the University of Granada, Spain, in 2015. The quality of her work has been recognized through the awarding of the Young Researchers Award from the Spanish Carbon Group and the Extraordinary Doctorate Award from the University of Granada. After postdoctoral stays at the University of Trieste, Italy, and the Instituto Superior Técnico of Lisbon, Portugal, she earned a postdoctoral position (Juan de la Cierva fellow) at the University of Alicante, Spain, in the Department of Inorganic Chemistry, where she currently works on materials design for environmental and clean energy applications. Nathalie Job  received her Ph.D. in Chemical Engineering from the University of Liège, Belgium, in 2006. Her thesis work, dedicated to carbon gels for applications in heterogeneous catalysis, evolved toward carbon-supported electrocatalysts for fuel cells during her postdoctoral fellowship (F.R.S.-FNRS). Since 2014, she has held the position of associate professor in the Department of Chemical Engineering (NCE group) at the University of Liège. Her research now deals with electrochemical devices such as fuel cells, batteries, and supercapacitors from materials synthesis to the building of complete systems.

Chapter 1

Organic and Carbon Gels: From Laboratory to Industry?

Abstract  Since the first report on organic gels based on the polycondensation of resorcinol with formaldehyde presented by Pekala in 1989, the number of publications, on both organic gels and carbon gels has experimented an enormous increase to the point where nowadays are published every year more than a hundred papers covering topics ranging from variations in the synthesis to the potential applications of this vast family of porous materials. This is due to the fact that, by controlling the synthesis conditions, it is possible to obtain materials with a suitable porosity for a specific application and even also with predetermined chemical properties, something that is practically impossible to achieve with any other porous materials. However, even after almost 30 years of continuous researching at laboratory scale, their industrial production and commercialization are still marginal compared with that of competitive materials. This chapter summarizes how the physicochemical properties of organic and carbon gels can be designed by controlling all the variables involved in the synthesis process. The chapter also addresses the most challenging problem of their mass production, i.e., scaling-up of production methods currently used in the labs. Keywords  Organic gel · Carbon gel · Gelation · Porous structure · Applications · Industrial production

1.1  Introduction The continuous technological developments have led to an increase in efforts to find new materials with improved properties. Carbon materials have extraordinary and probably the most versatile properties due to the peculiar characteristics of the carbon element and the variety of carbon–carbon bonds [1]. However, carbon materials from natural sources have certain disadvantages such as the presence of impurities content, variability between batches, and the lack of control of their properties. Consequently attention has now shifted to synthetic carbon materials with a high purity, controllable chemistry, and the possibility they offer of being able to design

© The Author(s), under exclusive licence to Springer Nature Switzerland AG 2019 A. Arenillas et al., Organic and Carbon Gels, Advances in Sol-Gel Derived Materials and Technologies, https://doi.org/10.1007/978-3-030-13897-4_1

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1  Organic and Carbon Gels: From Laboratory to Industry?

their final properties. Nevertheless, the fact cannot be ignored that these synthetic carbon materials need to be obtained by means of as quick, as simple, and as a low cost process as possible, in order to reach the level of mass production and implementation. Of these synthetic carbon materials, polymers have attracted a lot of attention, and in particular carbon gels, which are highly porous materials, composed of primary particles that are interconnected to create a three-dimensional network structure.

1.1.1  What Are Organic and Carbon Gels? On the basis of synthesis of inorganic silica gels, Pekala presented the first organic gel by applying sol-gel methodology in 1989 [2]. Stated in simple terms, the monomers, initially dissolved in a reaction media, initiate the polymerization reaction and a suspension of colloidal solid particles is formed (sol). These particles, also called clusters or nodules of polymeric material, tend to grow and interconnect to form an incipient polymeric network. As a consequence the liquid gradually densifies into a gel, until finally forming a polymeric and crosslinked solid structure. When the solvent, i.e., the reaction medium, is removed, the result is a solid organic polymer, called organic gel, formed by the sol-gel synthesis process. This material is not thermally stable, and a large amount of oxygen exists in its chemical composition. Therefore, it cannot be called a carbon material. Only after a heat treatment, where most of the heteroatoms are lost and the chemical composition of the material is based mainly on the carbon element, can it be called a carbon gel. Although both types of materials, organic and carbon gels, have very controlled chemistries and porosities, they exhibit very different chemical and physical properties. The organic gel has a large amount of carbon and oxygen in its composition and numerous oxygen surface functional groups. These materials are therefore characterized by a very low electrical and thermal conductivity, a highly hydrophilic character, and a low thermal stability. In contrast, carbon gels can display similar mesoporosity and have a high thermal stability. They are composed almost exclusively of highly condensed sp2 carbon, which confers them a hydrophobic character and a much higher thermal and electrical conductivity than organic gels. From these descriptions of organic and carbon gels it follows that they are suited to different applications, as will be detailed below.

1.1.2  The Sometimes Misleading Nomenclature of the Gels The chemical variables which control the polymerization reaction determine the final properties of the organic and carbon gels. However, the way the solvent is removed can also affect the porous properties due to surface tensions and may cause the polymeric structure to collapse. Traditionally, three types of drying method can

1.1 Introduction

3

Fig. 1.1  The three most common pathways for removing the solvent

be found in the literature, supercritical drying, freeze-drying, and subcritical drying. The differences that distinguish them are the operating conditions (i.e., temperature and pressure) used to eliminate the solvent (see Fig. 1.1 for the use of water as the reaction medium). The most common method to eliminate the solvent found in the scientific literature is supercritical drying [3]. CO2 is exchanged for the solvent under supercritical conditions (i.e., high pressure and temperature), and then CO2 is eliminated as a gas, thereby minimizing the surface tensions. When water is used as the solvent, the first solvent exchange must take place between the water and an organic solvent, due to the high solubility of CO2 in water. In the case of cryogenic drying, the solvent is first frozen and then eliminated by sublimation; in this way the high liquid--vapor surface tension is also eliminated. If water is used as the reaction medium, it is also necessary to perform the solvent exchange before the freeze-drying, in order to avoid the formation of ice crystals inside the polymeric structure, which would lead to the uncontrolled formation of megalopores or voids. The third method for eliminating the solvent is by direct evaporation. In this case, there is a liquid--vapor interphase and therefore surface tensions occur with the subsequent collapse of the structure. However, we have demonstrated [4] that, under controlled operating conditions, the shrinkage of the gel structure can be minimized, and high porous materials with a controlled structure can be obtained. For this reason, subcritical drying is the most straightforward and consequently the most up-scalable alternative for producing gels on a large scale.

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1  Organic and Carbon Gels: From Laboratory to Industry?

Traditionally, the organic and carbon gels obtained by different drying methods receive different names: aerogels, cryogels, and xerogels for the gels dried by ­supercritical, cryogenic, or evaporation procedures, respectively. However, according to IUPAC recommendations from 2007 for terms relating to the structure and processing of sols, gels, and networks [5], the term aerogel should be used to refer to any gel comprised of a microporous solid in which the dispersed phase is a gas, i.e., a porous gel, independently of the methodology used for its synthesis. Xerogel is defined as the network formed by the removal of all swelling agents from the gel, like the compact macromolecular structure of rubber. In their recommendations, there is no mention of the term cryogel. However, in the scientific literature, there is some confusion in the terminology when aerogel is used to refer to any dried gel with a porous structure [6] and to a gel dried under supercritical conditions, while xerogels are used to refer to gels dried by simple evaporation and also to a dense polymeric material with a low porosity [7]. Also misleading is the terminology regarding how the compound added to modify the pH of the precursor solution should be defined (see further Sect. 1.3). A lot of works in the literature refer this compound as catalyst [8], probably to reflect the notion that it is used to promote and accelerate the polymerization reaction. Strictly speaking however it is not a real catalyst, as it is not an independent compound in the reaction and it cannot be removed after the synthesis process. Other authors refer to these compounds as boosters [9], because of their augmentative effect on the kinetics of the polymerization reaction. However, many studies show that the modification of the pH of the reaction medium influences not only the speed of the reaction, but also the mechanism of the polymerization reactions (i.e., see Sect. 1.1.3 for mechanisms) [10]. Furthermore, these compounds cannot be recovered after the synthesis process and are incorporated into the polymeric structure. For this reason, many scientific works indicate that the nature of these compounds modifies the porous structure when they are incorporated, independently of the pH of the initial precursor solution [11–13]. Therefore, the proper way to refer to these compounds would be as additives that modify the polymerization reaction.

1.1.3  A  dvantages and Disadvantages of the Organic and Carbon Gels Compared with Other Porous Materials Organic and carbon gels are synthetic materials with very controlled chemical composition and pore structures. The ability to design and tailor a material with a high degree of purity and with a pore-size distribution suited to a certain application confers to these materials a clear advantage versus active carbons and other porous carbon materials. The synthesis process is, however, sometimes complex, especially under supercritical or cryogenic conditions. Only in the case of controlled evaporation that requires mild operating conditions, does the scalability of synthesizing organic and carbon gels seem more feasible than that offered by synthetic materials

5

1.1 Introduction

based on templating techniques [14] or other multistep processes involving very expensive raw materials (i.e., MOFs) [15]. From the chemical point of view, organic and carbon gels are mainly composed of carbon, oxygen, and hydrogen, but they can incorporate any targeted heteroatom considered necessary for an additional application. The advantage afforded by these gels is that the heteroatom can be incorporated either after the synthesis process (i.e., incorporated onto the surface of the gel structure) like any other carbon material, but also during the synthesis process by adding the heteroatom to the reaction medium, which causes the heteroatom to be incorporated directly into the gel structure itself. However, the main advantage of the gels is that their pore-size distribution may be designed by modifying the chemical variables involved during the synthesis process. They have a polymeric structure in which the monomers tend to react and form interconnected clusters or nodules of a certain size. The mesopores (i.e., pores between 2 and 50 nm) and macropores (i.e., pores wider than 50 nm) are formed by the space between the nodules of the polymer, whereas the microporosity (i.e., pores with a size of 95 wt%). The elimination of volatiles and labile matter leads to the formation of microporosity in the polymeric nodules, while the nanostructure designed and formed during the synthesis process usually remains intact [46]. Figure  1.12 shows that the carbonized gel has a similar mesoporosity than that of the former organic gel; however, the variation, after carbonization under nitrogen atmosphere at 800 °C, mainly affects the micropore volume and hence the BET surface area. Therefore, the meso/ macroporosity can be controlled during the polymerization process and the microporosity during the heat post-treatment, which is an enormous advantage for the design and control of the porous properties of carbon gels. It is also worth mentioning that sometimes the carbonization or the heat treatments are performed with reactive gases, not only for purposes of carbonization but also for introducing heteroatoms in a controlled way (i.e., nitrogen, sulfur, etc.) [65].

1.4.2  Activation Process Certain applications require very large surface areas and it is necessary to apply a particular type of carbonization in order to enhance the development of micropores in the nodules of the polymer. Activation processes can increase the surface area to above 2000 m2/g. The activation process can be performed during or after the carbonization step (i.e., with the organic or with the carbon gel). During activation the volume of narrow mesopores created during the synthesis of the organic gel can also

Fig. 1.12  Examples of the different thermal treatments and their influence on the porosity (i.e., BET surface area, mesopore volume, and mean pore size) of the resultant carbon gels

1.4  Thermal Treatments for Obtaining Carbon Gels

19

be augmented (see Fig.  1.12), but the main effect is on the microporosity of the carbon gels, with a great increase of the specific surface area. The temperature, time of activation, and the activating agents are some of the variables that need to be optimized in order to obtain a well-developed pore structure of the carbon gels. Basically, there are two types of activation methods: physical activation and chemical activation [66]. Physical activation is the most widely used method due to the simplicity of the process and the reduction of costs. This treatment consists in heating the material under a reactive gas which may be steam, CO2, or low proportions of air in an inert gas, or a combination of them [66]. Chemical activation involves mixing the sample with an activating compound and heating the mixture to promote a reaction between them. This method requires subsequent washing steps to remove any unreacted chemical agent or residual product [67]. Examples of activating agents for this chemical activation are phosphoric acid, alkali metal carbonates, or alkali hydroxides [8, 11, 68].

1.4.3  Ordering Structure Usually the thermal treatments produce condensation of the carbonaceous structure of the gels to certain extent, being more important as the operating temperature is increased (i.e., and especially when the operating temperature is higher than 1000 °C) [69]. The condensation reactions lead to a more ordered structure that has two main consequences for the final properties of the carbon gels. On the one side the electrical and thermal conductivity of the carbon gels is increased considerably, and on the other hand the defects in the polymeric nodules are substantially reduced. These defects and voids are responsible for the development of microporosity; therefore high temperature treatments produce a reduction of the microporosity and the specific surface area of the carbon gels (see Fig. 1.12). The meso or macroporosity designed during the synthesis is however preserved. Summarizing, during the synthesis (i.e., polymerization reactions), the meso and macroporosity of the organic and carbon gels can be tailored and the post-­treatments will hardly modify these properties at all. The microporosity developed during the synthesis of organic gels can also be modified by different heat treatments. However, it has to be taken into account that the chemical structure of the material will also be modified by the increase in temperature. The microporosity and surface area can be moderately increased by mild-treatment only by means of an increase in temperature (i.e., carbonization processes up to 800 °C), while, if highly microporous materials are needed, activation processes with activating agents are needed. In contrast, if the target is to increase the structural order and, therefore, both the thermal and electrical conductivity, high temperature treatments (>1000 °C) are required.

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1  Organic and Carbon Gels: From Laboratory to Industry?

Fig. 1.13  The potential and actual applications of organic and carbon gels

1.5  A General Overview of the Versatility of These Materials Both organic and carbon gels with a large variety of porous textures and surface functionalities can be obtained, which renders them very versatile materials with use in a wide range of applications (Fig. 1.13).

1.5.1  Potential Uses of Organic Gels Organic gels are essentially porous synthetic polymers, whose pores can be modulated in size, ranging from mesopores (2–50 nm) to macropores with sizes of up to microns. Very different compounds can be accommodated (either absorbed or adsorbed) because of their porosity and their rich surface chemistry, composed mainly of hydroxyl surface groups, able to attach to different molecules. Such materials can undoubtedly be used in many different applications, at least in theory. However, until now this potential has been practically unexploited. Practically no attempt of commercialization of these materials has been made and only a few reports on some of their potential uses have been published. Among these applications, its use as a desiccant with a large capacity for water removal at high relative humidities is the most appealing [70]. Interestingly, the hydroxyl groups can be passivated against water adsorption by means of relative simple treatments, which transform the organic xerogel into a hydrophobic one [63], or even a super hydrophobic one [17]. Such materials, with the appropriate porosity, can then be used as thermal insulators [71, 72]. Given that carbon gels can be synthesized with relatively large pores of different sizes and with narrow pore distributions, they also were tested as molecular sieves for the purification of biomolecules [73] or as biocatalyst supports [74]. These are only a few examples of the tremendous, but underexploited, potential of these materials.

1.6  The Prospects for the Industrial Production of the Organic…

21

1.5.2  Potential Uses of Carbon Gels The carbonization of organic xerogels transforms them into carbon xerogels. In general, carbonization preserves the mesoporosity of organic xerogels and adds new micropores that are formed by the elimination of volatile matter. Sometimes the formation of micropores is amplified by including an activation process. Therefore, carbon gels are hierarchical porous carbons that have micro and mesopores or, in some cases, macropores. Carbonization and activation eliminates most of the surface groups, although new oxygen-containing groups may be formed by a process of (re)oxidation upon atmosphere exposure. Consequently, carbon gels contain, in general, few surface groups, unless they are subjected to specific treatments with the aim of incorporating them. To sum up what has been described, the potential uses of these materials cover all those of activated carbon, i.e., liquid and gas phase adsorption [75], catalyst supports [44], electrodes [76, 77], etc. However, both the porosity and, to some extent, the surface chemistry of the carbon gels can be tailored for a given application, which represents a tremendous advantage over common activated carbons. Moreover, for use as electrodes, which is perhaps the most widely known and studied application of carbon gels, they offer the advantage of having a very low inorganic matter content and a much higher electrical conductivity than active carbons [78, 79]. Several examples of the great versatility of these carbon gels and their potential uses are presented in the following chapters. However, the reality is that the commercialization of carbon gels is much less advanced than that of activated carbons. The next section speculates on the reasons for this.

1.6  T  he Prospects for the Industrial Production of the Organic and Carbon Gels Although there is an abundant and increasing amount of scientific literature describing the various methods of producing organic or carbon gels, the number of patents granted so far has been relatively small [80]. Moreover, a search through the Internet has revealed that there are relatively few companies that either produce these materials or market them in their product catalogs; as a general rule, it is only possible to acquire small quantities of such materials at relatively high prices. To date the market has been limited exclusively to carbon gels and no more than two or three companies are in a position to supply these materials in great quantities (i.e., tons), mainly for the energy storage industry. Interestingly, the potential applications of organic gels appear to be unknown to the markets.

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1  Organic and Carbon Gels: From Laboratory to Industry?

Of the possible factors that have restricted the launch of the mass production of carbon gels, perhaps the most important is the fact that the manufacturing methods described above are complicated to scale up due to their complexity and the long synthesis times required making the process very expensive. In addition, carbon gels have to compete, in many of their applications, with active carbons that are generally cheaper to produce. The price of high purity activated carbons ranges between 30 and 60 US$/kg while carbon gels cost between 40 and 80 US$/kg. However, carbon gels tend to have higher performances than active carbons. For example, in the case of supercapacitors, if prices are compared per farad instead of per kg, activated carbons cost between 8 and 11 US$/kF compared to 7–9 US$/kF for carbon gels. Comparison in terms of power is even more favorable for carbon gels. Other competitors like graphene or graphene derivatives perform perhaps better than carbon gels in some applications, but their price is at present much higher than that of carbon gels: between 100 and 500 US$/kg for low-end or high-end graphene, respectively. To date these high prices do not generally compensate for the higher performance exhibited in some applications. In summary, the characteristics of carbon gels, which can be tailored for specific applications, make them highly versatile materials, with a far superior performance to that of other similar materials. However, their mass marketing first requires a simplification of the production methods to facilitate the industrial scaling of the production processes and to reduce the production costs. Development of methodologies for the production of organic and carbon gels easy to scale up is the first step towards their mass commercialization. The need to use monomers that do not have great impact on the final cost of the material and to conform to environmental regulations that are each year more restrictive is another important factor to take into account for the widespread production of these materials. These factors that might appear as disadvantages at first sight should be considered by the scientific community as an opportunity for investigating new methodologies and precursors.

1.7  Conclusions The synthesis of organic and carbon gels involves a large number of variables including the type and amount of selected monomers, the pH of the precursor mixture and the compound used to set it, the amount and type of solvent to use, the need to use additives or not, and the different operating conditions involved in each step of the production process, including post-synthesis treatments. Predicting the influence that these variables will have on the characteristics of the resulting material (mainly its nanoporous texture) so as to be able to control the process is not an easy task, since most of the variables are interdependent. However, statistical methods can contribute to determine the most appropriate conditions for obtaining a material with the desired properties with a relatively high degree of precision, making organic and carbon gels unique bespoke porous materials. The number of potential applications of such materials is enormous covering practically all the applications

References

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of any porous material. The fact is, however, that the industrial commercialization of these materials is relatively marginal compared to that of other nanoporous materials. The main reason for this is that the processes developed so far for removing the solvent without causing the porosity to collapse require either harsh temperatures and/or pressures or very long process times that may last for days. The focus, therefore, should be directed at clearing this bottleneck by employing simpler and easier technologies, in order to reduce process costs and allow organic and carbon gels to be marketed on a large scale. Acknowledgements  The authors gratefully acknowledge the financial support of the Ministerio de Economía Industria y Competitividad of Spain, FDER funds, and the Research State Agency (Project CTQ2017-87820-R).

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39. C.I. Merzbacher, S.R. Meier, J.R. Pierce, et al., Carbon aerogels as broadband non-reflective materials. J. Non-Cryst. Solids 285, 210–215 (2001) 40. J. Laskowski, B. Milow, L. Ratke, Subcritically dried resorcinol-formaldehyde aerogels from a base-acid catalyzed synthesis route. Microporous Mesoporous Mater. 197, 308–315 (2014) 41. R. Brandt, R. Petricevic, H. Probstle, et al., Acetic acid catalyzed carbon aerogels. J. Porous. Mater. 10, 171–178 (2003) 42. S. Morales-Torres, F.J. Maldonado-Hodar, A.F. Perez-Cadenas, et al., Textural and mechanical characteristics of carbon aerogels synthesized by polymerization of resorcinol and formaldehyde using alkali carbonates as basification agents. Phys. Chem. Chem. Phys. 12, 10365– 10372 (2010) 43. N. Rey-Raap, J.A. Menéndez, A. Arenillas, RF xerogels with tailored porosity over the entire nanoscale. Microporous Mesoporous Mater. 195, 266–275 (2014) 44. C.  Moreno-Castilla, F.J.  Maldonado-Hodar, Carbon aerogels for catalysis applications: an overview. Carbon 43, 455–465 (2005) 45. I. Matos, S. Fernandes, L. Guerreiro, et al., The effect of surfactants on the porosity of carbon xerogels. Microporous Mesoporous Mater. 92, 38–46 (2006) 46. N. Job, R. Pirard, J. Marien, et al., Porous carbon xerogels with texture tailored by pH control during sol-gel process. Carbon 42, 619–628 (2004) 47. S.J. Taylor, M.D. Haw, J. Sefcik, et al., Gelation mechanism of resorcinol-formaldehyde gels investigated by dynamic light scattering. Langmuir 30, 10231–10240 (2014) 48. N.  Rey-Raap, J.A.  Menéndez, A.  Arenillas, Simultaneous adjustment of the main chemical variables to fine-tune the porosity of carbon xerogels. Carbon 78, 490–499 (2014) 49. F. Wang, L.F. Yao, J. Shen, et al., The effect of different ratio in carbon aerogel on pore structure in ambient dry. Adv. Mater. Res. 941-944, 450–453 (2014) 50. L. Zubizarreta, A. Arenillas, J.A. Menéndez, et al., Microwave drying as an effective method to obtain porous carbon xerogels. J. Non-Cryst. Solids 354, 4024–4026 (2008) 51. N. Rey-Raap, A. Arenillas, J.A. Menéndez, A visual validation of the combined effect of pH and dilution on the porosity of carbon xerogels. Microporous Mesoporous Mater. 223, 89–93 (2016) 52. F.J.  Maldonado-Hodar, M.A.  Ferro-Garcia, J.  Rivera-Utrilla, et  al., Synthesis and textural characteristics of organic aerogels, transition-metal-containing organic aerogels and their carbonized derivatives. Carbon 37, 1199–1205 (1999) 53. I.D. Alonso-Buenaposada, N. Rey-Raap, E.G. Calvo, et al., Effect of methanol content in commercial formaldehyde solutions on the porosity of RF carbon xerogels. J. Non-Cryst. Solids 426, 13–18 (2015) 54. I.D.  Alonso-Buenaposada, L.  Garrido, M.A.  Montes-Morán, et  al., An underrated variable essential for tailoring the structure of xerogel: the methanol content of commercial formaldehyde solutions. J. Sol-Gel Sci. Technol. 83, 478–488 (2017) 55. M.A. Worsley, J.H. Satcher Jr., T.F. Baumann, Influence of sodium dodecylbenzene sulfonate on the structure and properties of carbon aerogels. J. Non-Cryst. Solids 356, 172–174 (2010) 56. N. Rey-Raap, A. Szczurek, V. Fierro, et al., Advances in tailoring the porosity of tannin-based carbon xerogels. Ind. Crop. Prod. 82, 100–106 (2016) 57. N. Rey-Raap, A. Szczurek, V. Fierro, et al., Towards a feasible and scalable production of bio-­ xerogels. J. Colloid Interface Sci. 456, 138–144 (2015) 58. F.J. Maldonado-Hodar, C. Moreno-Castilla, J. Rivera-Utrilla, et al., Catalytic graphitization of carbon aerogels by transition metals. Langmuir 16, 4367–4373 (2000) 59. N. Job, S.D. Lambert, A. Zubiaur, et al., Design of Pt/carbon xerogel catalysts for PEM fuel cells. Catalysts 5, 40–57 (2015) 60. K. Guo, H. Song, X. Chen, et al., Graphene oxide as an anti-shrinkage additive for resorcinol-­ formaldehyde composite aerogels. Phys. Chem. Chem. Phys. 16, 11603–11608 (2014) 61. M. Canal-Rodríguez, A. Arenillas, N. Rey-Raap, et al., Graphene-doped carbon xerogel combining high electrical conductivity and surface area for optimized aqueous supercapacitors. Carbon 118, 291–298 (2017)

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1  Organic and Carbon Gels: From Laboratory to Industry?

62. I.D. Alonso-Buenaposada, A. Arenillas, et al., On the desiccant capacity of the mesoporous RF-xerogels. Microporous Mesoporous Mater. 248, 1–6 (2017) 63. I.D. Alonso-Buenaposada, M.A. Montes-Morán, J.A. Menéndez, et al., Synthesis of hydrophobic resorcinol-formaldehyde xerogels by grafting with silanes. React. Funct. Polym. 120, 92–97 (2017) 64. A.H.  Moreno, A.  Arenillas, E.G.  Calvo, et  al., Carbonization of resorcinol-formaldehyde organic xerogels: effect of temperature, particle size and heating rate on the porosity of carbon xerogels. J. Anal. Appl. Pyrolysis 100, 111–116 (2013) 65. M. Enterria, J.L. Figueiredo, Nanostructured mesoporous carbons: tuning texture and surface chemistry. Carbon 108, 79–102 (2016) 66. C. Lin, J.A. Ritter, Carbonization and activation of sol-gel derived carbon xerogels. Carbon 38, 849–861 (2000) 67. L.  Zubizarreta, A.  Arenillas, J.P.  Pirard, et  al., Tailoring the textural properties of activated carbon xerogels by chemical activation with KOH.  Microporous Mesoporous Mater. 115, 480–490 (2008) 68. F.L. Conceicao, P.M. Carrott, M.M.L. Carrott, New carbon materials with high porosity in the nm range obtained by chemical activation with phosphoric acid of resorcinol-formaldehyde aerogels. Carbon 47, 1874–1877 (2009) 69. M. Wiener, G. Reichenauer, Microstructure of porous carbons derived from phenolic resin-­ impact of annealing at temperatures up to 2000°C analyzed by complementary characterization methods. Microporous Mesoporous Mater. 203, 116–122 (2015) 70. J.A. Menéndez, A. Arenillas, I. Díaz, et al., Use of an organic xerogel as a desiccant, Patent WO2017149189 71. A. Arenillas, J.A. Menéndez, N. Rey-Raap, et al., Use of an inorganic xerogel as heat insulator, Patent WO2017153624 72. A.  Demilecamps, M.  Alves, A.  Rigacci, et  al., Nanostructured interpenetrated organic-­ inorganic aerogels with thermal superinsulating properties. J. Non-Cryst. Solids 452, 259–265 (2016) 73. F. Svec, Y. Lv, Advances and recent trends in the field of monolithic columns for chromatography. Anal. Chem. 87, 250–273 (2015) 74. L.A. Ramirez-Montoya, A. Concheso, I.D. Alonso-Buenaposada, et al., Protein adsorption and activity on carbon xerogels with narrow pore size distributions covering a wide mesoporous range. Carbon 118, 743–751 (2017) 75. B.S. Girgis, I.Y. Sherif, A.A. Attia, et al., Textural and adsorption characteristics of carbon xerogel adsorbents for removal of Cu (II) ions from aqueous solutions. J. Non-Cryst. Solids 358, 741–747 (2012) 76. E.G. Calvo, F. Lufrano, P. Staiti, et al., Carbon xerogel and manganese oxide capacitive materials for advanced supercapacitors. Int. J. Electrochem. Sci. 6, 596–612 (2011) 77. M. Mirzaeian, P.J. Hall, Preparation of controlled porosity carbon aerogels for energy storage in rechargeable lithium oxygen batteries. Electrochim. Acta 54, 7444–7451 (2009) 78. M. Canal-Rodríguez, J.A. Menéndez, A. Arenillas, Performance of carbon xerogel-graphene hybrids as electrodes in aqueous supercapacitors. Electrochim. Acta 276, 28–36 (2018) 79. N. Rey-Raap, E.G. Calvo, J.M. Bermúdez, et al., An electrical conductivity translator for carbons. Measurement 56, 215–218 (2014) 80. N. Rey-Raap, A. Arenillas, J.A. Menéndez, Carbon gels and their applications: a review of patents, in Submicron Porous Materials, ed. by P. Bettotti, (Springer, New York, 2017), pp. 25–52

Chapter 2

Organic and Carbon Gels Derived from Biosourced Polyphenols

Abstract  This chapter presents the most recent updates about sol-gel chemistry of phenolic molecules and the corresponding materials: xerogels, cryogels, and aerogels. The structure and properties of the latter, whether in the organic or carbon forms, are detailed and actual and potential applications are reported. After an introduction about plant polyphenols in general, the focus is mainly given to condensed (flavonoid) tannins, shown to be the most relevant raw material for preparing resins, and hence gels. Lignin is considered as well, despite its lower reactivity and its less reproducible character, because of its industrial importance. Details about the nature and the properties of the carbon that can be obtained by pyrolysis of crosslinked polyphenols are also given. Tannin-formaldehyde resins and mixed formulations associating resorcinol, soy protein, lignin, phenol, or surfactant are then discussed in terms of reactivity and ability to produce highly porous gels, depending on the experimental conditions of synthesis (dilution, pH, amount of crosslinker, etc.) and drying (subcritical, supercritical, or lyophilization). The porous structure of those materials is also explained in relation to gelation time and mechanical properties of the corresponding hydrogels. Derived carbons gels, including N-doped, formaldehyde-free materials, and activated carbon gels, are also considered. Mechanical and thermal properties of organic gels, as well as electrochemical properties of carbon gels, are next introduced. Finally, recent developments including one-step microwave synthesis of xerogels, carbon xerogel microspheres having the characteristics of carbon molecular sieves, and elastic gels behaving as rubber springs with tunable elastic properties, all biosourced and tannin-based, are presented. Keywords  Tannin · Lignin · Gelation · Glassy carbon · Supercapacitors

© The Author(s), under exclusive licence to Springer Nature Switzerland AG 2019 A. Arenillas et al., Organic and Carbon Gels, Advances in Sol-Gel Derived Materials and Technologies, https://doi.org/10.1007/978-3-030-13897-4_2

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2  Organic and Carbon Gels Derived from Biosourced Polyphenols

2.1  Introduction As emphasized and thoroughly developed in the other chapters, resorcinol-­ formaldehyde formulations are by far the most popular precursors of organic gels and hence of carbon gels derived from them by pyrolysis in inert atmosphere. Resorcinol indeed combines several advantages. First, it leads to high-quality thermosetting resins, thanks to its ability to react with formaldehyde, even at room temperature. Second, those resins are quite reproducible since resorcinol is a simple synthetic molecule, and can therefore be obtained at the highest possible purity. Finally, gels with very well-controlled properties can be produced, whose properties only depend on the ingredients of the formulation: amount of catalyst (or pH), dilution and nature of the solvent, amount of formaldehyde and nature, and concentration of its stabilizer. Once the organic gel is formed, the porous structure is also determined by the conditions of drying, e.g., supercritical, subcritical, or freeze-­drying. As a result, resorcinol-formaldehyde gels are extremely versatile materials [1–8]. However, a serious disadvantage of resorcinol is its cost. This is the reason why phenol has been suggested as a five times-cheaper precursor of organic and carbon gels. Phenol is also very versatile from the point of view of the reactions it may do, depending on the pH and on the amount and the nature of the crosslinker, leading to either resole or novolac resins (having a formaldehyde to phenol molar ratio higher and lower than 1, respectively). However, in the present case, only resoles are relevant for preparing carbon gels, since novolacs are thermoplastic and therefore the porous structure obtained after drying of the organic gel would not survive the pyrolysis step. Anyway, and whereas the polycondensation reactions involving resorcinol or phenol with formaldehyde are different, the resultant gels obtained after drying and, for resoles only, after pyrolysis, can present quite developed porous textures [9–12]. In a more general way, not only phenol can be used but also its counterparts bearing various kinds of moieties such as the isomers of cresol and xylenol. Moreover, resorcinol is just a special case of simple phenolic molecule, and the functionality of the latter appears to be one more degree of freedom for making gels. And indeed, cresols were successfully proved to be an alternative to resorcinol for producing organic and carbon gels [13]. However, not all crosslinkers can be used for getting resoles, and hence carbon gels, since some combinations of aldehydes and substituted phenols may lead to novolacs only. Another way of producing less expensive organic and derived carbon gels is to substitute part of the resorcinol by a cheaper phenolic molecule. This strategy is inspired by what is known for adhesives, for which decreasing the cost of the final product while maintaining its properties is critical. Assuming that there is a continuum between a cold-set adhesive and a chemical gel, considering that the chemistry is the same and that the gel only differs from the hard adhesive by its degree of dilution in a solvent, it has been postulated that gels can be prepared from any cold-set adhesive formulation [14]. For instance, phenol–resorcinol–formaldehyde formulations were successfully used for preparing adhesives for wood, but their cost has

2.2  Biosourced Polyphenols and Related Materials

29

been further decreased by limiting the incorporation of resorcinol; this has been made possible by adding molecules able to branch polymer chains, thanks to several sites reacting with the aldehyde during or after the preparation of the phenol–formaldehyde resin [15]. Such molecules are traditionally nitrogenated species like melamine, aniline, or urea. The latter was indeed tested to produce “blue glue,” due to its characteristic color, from which organic and carbon gels with well-developed porous textures were prepared [16]. Nevertheless, despite the aforementioned advantages, phenol suffers reactivity typically 10–15 times lower than that of resorcinol. Additionally, phenol is even more toxic than resorcinol. Indeed, according to the US standard NFPA 704 (Standard System for the Identification of the Hazards of Materials for Emergency Response) [17], phenol is ranked 4 in terms of health, i.e., “a very short exposure might cause death or major residual injury,” whereas resorcinol is ranked “only” 2, i.e., “intense or continued but not chronic exposure might cause temporary incapacitation or possible residual injury.” Therefore, research efforts were made in the recent past for finding other alternatives to classical phenolic compounds, being at the same time very cheap, reactive, and poorly or even non-toxic. Biosourced polyphenols constitute a very broad family of compounds able to match those goals, and the present chapter is devoted to the synthesis and to the characterization of organic and carbon gels derived from such raw materials. The special cases of condensed tannins and lignin will be considered in detail, as justified in the next section.

2.2  Biosourced Polyphenols and Related Materials 2.2.1  Introduction to Plant Polyphenols Polyphenols are extremely common in the plant kingdom. By definition, this family of compounds is based on the combination of several aromatic rings bearing a variable number of hydroxyl groups. Many polyphenols belong to the broad family of extractives, i.e., are secondary metabolites that concentrate in various parts of the plants, depending on the species or on the nature of the considered molecule. Therefore, they can be found in barks, trunks or stems, and roots, but also in fruits or flowers. Their reactivity originates from their structure and chemical nature, and all polyphenols do have specific action on the living beings. As a matter of fact, the key role of tannins, i.e., one widespread class of plant polyphenols, is to protect the plant against herbivores, bacteria, and fungi, among others. More generally speaking, all polyphenols are biologically active, and it is thus not surprising that this kind of molecules is so important in nutrition, medicine, and cosmetics. Whereas phenol is fundamentally toxic and carcinogenic, plant polyphenols fight, or protect from, various diseases. Examples of properties that have been evidenced are, by way of illustration and in a non-exhaustive manner, anti-oxidant, anti-mutagenic, anti-cancer, anti-fungal, anti-bacterial, vasoprotective, photo-protective, anti-­diabetic, and neuroprotective.

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Although a few other compounds should be classified apart, plant polyphenols are generally separated in 4 main groups: (1) Phenolic acids, further subdivided into hydroxybenzoic acids and hydroxycinnamic acids; (2) Stilbenoids, being hydroxylated derivatives of stilbene and whose most known representative compound is resveratrol; (3) Lignans, based on the combination of two monolignols units, also called phytoestrogens due to the structural similarity with some sex hormones; and (4) Flavonoids, which correspond to the broadest family of polyphenols, with several thousand compounds already described. This family comprises the following subclasses of compounds, all based on the same flavon basic structural unit: anthocyanidins, flavanols, flavones, isoflavones, flavonols, and flavonones. The more popular term “tannin,” also used in this paper, corresponds to molecules belonging to anthocyanidins and flavanols groups. However, other tannins exist and should not been mixed with those used for preparing the gels described herein.

2.2.2  Tannins 2.2.2.1  Structure and Properties of Tannins Tannins are mostly found in dicotyledonous plants [18]. They are classified on the basis of their chemical structure and reactivity. Formerly, they were divided into two classes: hydrolyzable and non-hydrolyzable or condensed tannins (see [19] and refs. therein), but due to many exceptions to this categorization, tannins were more recently sorted into 4 subclasses, see Fig. 2.1 [20]. Gallotannins and ellagitannins combine esters of simple or complex carbohydrates bonded with gallic acid and ellagic acids, respectively; most of them are monomers and hydrolyzable. Complex

Fig. 2.1  The four tannin subclasses (reprinted from [20], with permission from RSC)

2.2  Biosourced Polyphenols and Related Materials

a

HO OH

Resorcinol HO

7 6

B

1

8

A 5

9 10

O 2

4

3

OH

31

b HO

OH 7 6

OH

A

9 10

5

O 2 4

3

OH

Phloroglucinol HO

7 6

8

B

1

9

A 5 10

OH

OH

O 2 4

3

OH OH

Pyrogallol

Prodelphinidin (pine)

OH OH

Profisetinidin (quebracho)

Prorobinetinidin (acacia)

c

B

1

8

OH

d HO

7 6

8

9

A 5 10

OH

B

1

O 2 4

3

OH OH

Catechol

Procyanidin (pine)

Fig. 2.2  The four main flavonoid units present in condensed tannins: (a) prorobinetinidin; (b) profisetinidin; (c) prodelphinidin; and (d) procyanidin

tannins are built up from a catechin unit linked with either a gallotannin unit or an ellagitannin; they are partly hydrolyzable. Finally, condensed tannins are hardly hydrolyzable, and are naturally found as oligomers or polymers based on flavan-­ 3-­ol units. Depending on how those units are linked to each other and on the number of hydroxyl groups they bear, 4 molecules can be obtained, as shown in Fig. 2.2. These are: (a) prorobinetinidin, the main component of Mimosa (acacia mearnsii) bark extract; (b) profisetinidin, the major tannin found in quebracho (shinopsis balansae) tree; and (c) and (d) prodelphinidin and procyanidin, both found in Pine (pinus radiata) bark extract. Both (a) and (b) have a resorcinol-type A ring whereas their B ring is pyrogallol- and catechol-type, respectively. The same applies to (c) and (d) as for the B ring, but these molecules have in common a phloroglucinol-type A ring instead. The repeating units are mainly linked 4–6 in (a) and (b), and mostly 4–8 in (c) and (d) (see atom numbering in Fig. 2.2). Whereas the aforementioned oligomers embrace the major part of tannin extracts, the latter are quite impure if no additional separation step is carried out. For instance, mimosa tannin extract is commercialized with “only” 74% guaranteed condensed tannins, and generally contains 80–82% of actual phenolic flavonoid materials, 4–6% of water, 1% of amino and imino acids, the remainder being monomeric and oligomeric carbohydrates, in general broken pieces of hemicelluloses [21]. However, as it will be shown below, such raw material is quite relevant for producing high-quality gels. Condensed tannins are soluble in water, and are extracted by flowing a hot aqueous solution of sodium bisulfite, the latter improving the extraction yield. The solution is passed through barks or wood chips at counter current so that it is progressively enriched. Next, the solution is concentrated and finally spray-dried. At the end, a light-brown powder is obtained. In the case of mimosa extract, oligomers of molecular weight typically ranging from 500 to 3500 g/mol are recovered [22]. The main

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2  Organic and Carbon Gels Derived from Biosourced Polyphenols

industrial sources of condensed tannin at present are mimosa with an actual yearly production of 112,000 tons worldwide, and a maximum capacity of 220,000 tons/ year. The production of quebracho tannin is lower, about 80,000 tons/year worldwide, but is constantly increasing and will probably exceed that of mimosa in the coming years. The aforementioned tree species are not European, but condensed tannins can also be extracted in Europe from pine, for which the production potential is huge. Unfortunately, only a few small-scale extraction plants exist at present so that pine tannin is poorly available. As for hydrolyzable tannins, the main industrial source is chestnut, which is the third most important vegetable tannin used for leather production. The leather tanning industry has indeed been for long the main outlet of tannin production, although such traditional use started declining in the middle of the twentieth century whereas other applications appeared. The latter emerged in relation to the special reactivity of tannins. 2.2.2.2  Reactivity of Tannins Aldehydes are common crosslinkers of condensed tannins, thereby leading to thermosetting resins. The A ring is by far the most reactive one of the flavonoid unit [23]. Therefore, since the A-ring of both prorobinetinidin and profisetinidin is resorcinol-­type, mimosa and quebracho extracts have similar reactivity towards formaldehyde, only a little lower than that of resorcinol alone. In contrast, pine tannin is typically 6–7 times more reactive than mimosa and quebracho [24, 25], and hence their polymerization is a little less easy to control. Based on these facts, it is thus expected that condensed tannins can substitute resorcinol and even phenol in the preparation of resins, and this actually happened since the 1970s with the commercialization of tannin-formaldehyde adhesives for wood. The latter were successfully used, for instance, in the production of particleboard, plywood, and glulam [22]. The most efficient crosslinker of condensed tannins is formaldehyde, see Fig. 2.3a, but alternative, less volatile, and much less toxic aldehydes such as glyoxal and glutaraldehyde, or hexamine, can also be used [26–29]. Unlike resorcinol, for which the stoichiometric amount of formaldehyde to be used is well known, the molecular weight and composition of polyflavonoids, and hence the number of their reactive sites, may vary. Therefore, the amount of formaldehyde to be used for crosslinking those polyphenols is ill-defined. However, such amount is clearly below what is required for resorcinol, thus further enhancing the environment-­ friendly character of condensed tannins for producing resins. Besides, in either acidic or basic pH, or in the presence of some catalysts, autocondensation may occur, leading to polymer without the need of using any crosslinker, see Fig. 2.3b. Finally, condensed tannins are also prone to react with furfuryl alcohol [30–32], see Fig.  2.3c. As a conclusion, whether the final resins are furanic or not, all are ­thermoset and are quite relevant precursors of organic and carbon gels. Reactions of condensed tannins with isocyanates, polyurethanes, ammonia, poly(amine-esters),

2.2  Biosourced Polyphenols and Related Materials

33

Fig. 2.3  Frequent reactions occurring when preparing materials based on condensed tannins: (a) Crosslinking of flavonoid units by formaldehyde. (b) Autocondensation in alkaline conditions. (c) Possible products of the reaction of catechin, i.e., an equivalent monoflavonoid building block, with furfuryl alcohol. Reprinted from [42] with permission from RSC

34

2  Organic and Carbon Gels Derived from Biosourced Polyphenols OH tannin +

+ HCHO OH

base heat

flavonoid

flavonoid

CH2

flavonoid

HO

OH

CH2

CH2

CH2 HO

CH2 HO

OH CH2 flavonoid

OH n

Fig. 2.4  Co-reaction of resorcinol-formaldehyde and tannin-formaldehyde condensates for producing tannin-resorcinol-formaldehyde resin (reprinted from [103] with permission from John Wiley & Sons, Inc.)

and proteins were also successfully used to produce new materials [33–41]. Even more possibilities of reaction of condensed tannins have been reported, and have been detailed and discussed elsewhere [42]. As a result, condensed tannins are highly versatile raw materials from which a number of multifunctional solids have been prepared and described. Besides, they can also react with resorcinol in all proportions, leading to tannin-resorcinol-­formaldehyde (TRF) formulations. TRF can be obtained either by grafting resorcinol on a tanninformaldehyde resole or by simultaneous synthesis of resorcinol-­formaldehyde and tannin-formaldehyde condensates, see Fig. 2.4, thus further broadening the range of resultant materials that can be obtained [43]. Among tannin-­based solids that were reported in the past recent years [44, 45], one can cite rigid organic foams [24, 46– 70], carbon foams [71–85], ceramic foams [86, 87] and other foam-like cellular monoliths [88–91], activated carbons [92], hydrothermal carbons [21, 93–97] and derived nanostructured oxides [98], ordered mesoporous carbons [99, 100], porous carbon spheres [101] and, of course, gels [102], objects of the present chapter. In contrast, although hydrolyzable tannins can also react with aldehydes, the corresponding reaction rates are much lower, they lack macromolecular structure, and their nucleophilicity is also lower. As a consequence, even if they have been incorporated to the formulation of resins and derived materials [104], hydrolyzable tannins can never be used alone for that purpose, and are thus far less interesting for preparing gels; they will be no more considered in this chapter.

2.2.3  Lignin Lignin or, better, lignins are special polyphenols as these are crosslinked phenolic polymers whose composition, molecular weight, branching, crosslinking rate, kinds of crosslinks, etc. depend on the plant species and on the way they were extracted

2.2  Biosourced Polyphenols and Related Materials

35

Fig. 2.5  Model structure of lignin derived from: (a) hardwood and (b) softwood (reprinted from [105] with permission from RSC)

from plants. As a result, these are highly variable bioresources. In comparison, condensed tannins are commercial industrial products that are very reproducible, which is a rare quality for a biomass-based raw material. Lignins are thus quite complex, and only approximate models of their structure can be given, such as the one shown in Fig. 2.5. Whereas both based on phenyl propane type units, hardwood and softwood lignins mainly differ by the proportions of 3 primary monomers they

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2  Organic and Carbon Gels Derived from Biosourced Polyphenols

comprise and called monolignols: p-coumaryl, coniferyl, and sinapyl alcohols. For instance, softwood lignin contains much more coniferyl alcohols and is more branched than hardwood lignin [105]. If wood is considered as a biocomposite, then, as a first and very rough approximation, cellulose nanofibrils are the reinforcement, and lignin and hemicelluloses are the matrix. Therefore, since lignin is a 3D crosslinked polymer, it is insoluble and its extraction thus consists in cleaving some bonds to turn its macromolecular chains into soluble fragments. In other words, extracting lignin is always accompanied by some changes of molecular structure: decrease of molecular weight, functionalization of fragments, and introduction of impurities. As a consequence, for a same initial biomass, different lignins can be obtained, depending on the processes, among which are Kraft, sulfite, soda or thermomechanical pulping, organosolv, etc. Lignin is thus a quite complex raw material for being used as base of resins, but needs to be considered for at least 3 reasons: (1) it is the second biopolymer on Earth, only exceeded by cellulose as it represents roughly 15–30% of the wood composition; (2) it is an industrial product, available in huge amounts and poorly valorized as it mainly is the by-product of cellulose production; (3) it is 50% aromatic, and as such must be a source of chemicals for many applications and an interesting precursor of carbon materials. However, in addition to the difficulty of obtaining well-defined lignin with constant characteristics, lignin generally suffers a moderate reactivity. Crosslinking with aldehydes is possible, as seen in Fig. 2.6a, but is slow and occurs to a low extent. It is easier with epichlorohydrin, see Fig. 2.6b. Additionally, adding lignin to most phenolic resins generally decreases their mechanical properties, so that only limited amounts can be used for replacing expensive chemicals such as resorcinol. This is indeed how lignin will be used in the following, and no acceptable dry gels from pure lignin could ever be prepared so far, to the best of the authors’ knowledge.

Fig. 2.6  Crosslinking of lignin: (a) with formaldehyde (reproduced from [106] with permission from Elsevier Ltd) and (b) with epicholorohydrin (reproduced from [107] with permission from John Wiley & Sons, Inc.)

2.2  Biosourced Polyphenols and Related Materials

37

2.2.4  Polyphenol-Based Carbons As explained above, resins based on condensed tannins and lignins are crosslinked, highly aromatic, polymers. Three consequences are thus foreseen: 1. These resins are non-fusible, and therefore the structure of materials made from them is fully maintained upon pyrolysis, during which the resin is gradually converted into carbon. It is thus expected that no pore collapse but only shrinkage occurs when organic gels are carbonized, the shrinkage being related to the evolution of volatile matter and to the densification of the solid during heat treatment under inert atmosphere. 2. The carbon yield is high, only slightly lower than 50%, which is excellent when compared to what is obtained from most organic materials. Indeed, looking again at Figs. 2.2, 2.3, 2.4, 2.5, and 2.6, it is clear that a significant part of the raw material is already based on aromatic rings that will condense into more or less extended 2D-aromatic carbon domains. Most of the non-aromatic fraction will be released as gases during carbonization, contributing to an additional porosity in the final carbonaceous material. 3. The carbon that is recovered after pyrolysis of polyphenol-derived resin contains a significant amount of residual oxygen [108, 109], is highly disordered, and generally referred to as “vitreous,” “glassy,” or “glass-like” carbon. This material is indeed isotropic and uniformly shiny, hard, and brittle, and presents a conchoidal fracture with sharp edges when broken. Such features make it definitely different from graphite, characterized by “infinite” aromatic layers, perfectly parallel with each other and hence highly anisotropic, with carbon atoms arranged

raw 300°C 450°C Transmittance (arb. units)

600°C 750°C 900°C

4000

3400

2800 2200 1600 Wavenumber (cm–1)

1000

400

Fig. 2.7  FTIR spectra of tannin-formaldehyde organic aerogels submitted to various temperatures under inert atmosphere (reprinted from [114] with permission from Elsevier)

38

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

Fig. 2.8  Two models of the structure of glassy carbon: (a) Jenkins and Kawamura model (reprinted from [115] with permission from Elsevier); (b) Shiraishi model (reprinted from [116] with permission from Elsevier). The crystallite size along (La) and perpendicular (Lc) to the stacking of aromatic carbon layers is typically a few nm

in hexagonal packing. FTIR spectroscopy shows that 900 °C is a sufficient temperature for converting completely the phenolic resin into glassy carbon, see Fig. 2.7. The highly disordered structure of such carbon, most of the time modelled either as entangled ribbons (Jenkins and Kawamura model [110], see Fig.  2.8a) or as packed sheets forming closed nano-cells (Shiraishi model, see Fig.  2.8b [111]), explains the huge rigidity and the isotropic properties of glassy carbon, and why the latter can never be converted into graphite, even by extensive treatment at very high temperature. The lack of long-range ordering at the atomic scale makes any XRD pattern poorly informative, unless special treatment of the spectra is used [112], only presenting a broad (002) band and a significant diffusion background. Application of Bragg’s law to the position of the maximum of the (002) band leads to a interlayer spacing typically close to 0.44 nm, i.e., more than 30% higher than in graphite [113]. Much more informative is the first-order Raman spectrum (from 800 to 2000 cm−1), a typical example of which is given in Fig. 2.9. In disordered carbons derived from phenolic precursors after pyrolysis at 900–1000 °C, it presents two broadbands whose maxima are centered on wavenumbers around 1350 and 1595 cm−1, a shallow valley between them, and a shoulder at around 1200 cm−1 [117, 118]. Those bands, referred to as D and G bands, respectively, are attributed to the vibration of amorphous carbon (aC) and crystalline graphitic carbon (gC), respectively. Their similar intensity is only apparent, as the envelopes of the spectrum can generally be accounted for by three more components (D4, D3, and D2) appearing at around 1250, 1520, and 1600 cm−1, respectively. D4 and D2 are responsible for the left and right shoulders of the spectra, respectively, whereas D3 is required for filling the gap between D1 and G bands. D4 is related to a poor organi-

2.2  Biosourced Polyphenols and Related Materials

39

Fig. 2.9 First-order Raman spectrum of carbon xerogel microspheres based on tannin-­ formaldehyde resin (inset), and its corresponding deconvolution into D1, D2, D3, D4, and G bands (reprinted from [132] with permission from RSC)

zation of carbon, the width of D1 is related to both the crystallite size (La) and the contribution of edge planes, D3 appears when the crystallization degree is very low, the width of the G peak is proportional to the disorder produced at sp2 sites because of the presence of sp3 carbon, and D2 corresponds to the second first-order zone boundary phonon [119]. The I(D1)/I(G) ratio, I being the intensity of each peak), used as a graphitization indicator since the 1970s, is worth calculating and comparing with that of other similar carbons, either derived from other polyphenolic resins or treated at different temperatures. As we here deal with non-graphitizable carbons, I(D1)/I(G) is expected to be proportional to La2 [120]. A material presenting a higher value of I(D1)/I(G) obtained in similar conditions (especially with the same energy for the laser excitation) thus presents a higher short-range order crystallinity, i.e., has a more ordered carbon nanotexture. Carbon derived from polyphenols, whereas not as electrically conducting as graphitic carbon, nevertheless presents a far enough conductivity for using it as electrodes in electrochemical storage systems, s­ upercapacitors, for example [94, 121–124]. Indeed, with an estimated conductivity of 7000 S/m [125], polyphenol-based carbon is also perfectly relevant for producing lightweight systems for electromagnetic shielding [101, 126–129] or metal-free electrocatalysts for oxygen reduction reaction in fuel cells [130, 131].

40

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

2.3  Gels Derived from Condensed Tannins 2.3.1  G  eneral Information About the Synthesis of Polyphenol-­ Based Gels As for most other chemical organic gels, tannin-based gels can be prepared using water as the main solvent, but water-alcohol mixtures can also be used for improving the solubility of tannins. Hydro/alcogels are thus obtained. Such materials are exactly as phenolic gels are expected to be, i.e., these are reddish-brown or reddish-­ orange, semi-elastic solids, whose porosity is saturated by a continuous liquid phase. From this point of view, they perfectly fit the gel definition given by Brinker and Scherer [133]. As usual, the “art” of preparing dry gels consists in removing the solvent trapped inside the tenuous solid backbone while maintaining as much as possible the porosity. With this aim in view, aging the gels is mandatory. During this step, the gel is maintained in a hot environment while drying is prevented at the same time, for example, by keeping the materials in a sealed tube in an oven. Doing so, the polycondensation can go on and significantly strengthens the gel, connecting unreacted mono/oligomers to the giant polymeric cluster that appeared at the gel point. The latter therefore only marks the onset of gelation, but the time required for producing an acceptable dry gel is far higher than the gelation time. Indeed, whereas the latter can be of the order of a few minutes to a few hours, depending on the formulation, polyphenols generally require several additional days for being aged correctly. Once the 2 aforementioned steps are performed, i.e., gelation and aging, the 3rd, drying step can be carried out. Although the hydro/alcogel is now much stiffer than at the gel point, capillary forces are still definitely quite high at the nanometer scale, so that any back movement of the liquid/vapor interface in a pore submitted to evacuation induces a dramatic collapse, and hence a loss of porosity. One way of limiting the problem is using some surfactants so that the surface tension of the solvent to be evacuated decreases significantly. In these conditions, polyphenol-­ based xerogels, i.e., obtained by simple evaporation at room pressure, might be obtained with moderate shrinkage. However, most of the time and as detailed in the first chapter of this book, supercritical drying, especially using CO2 as supercritical fluid, but freeze-drying as well, are methods that are required for minimizing the shrinkage further. The former way bypasses the critical point so that the meniscus disappears, and thus the capillary forces vanish as well, whereas the latter method minimizes them while evaporating the solvent after it has been frozen. In contrast, simple evaporation (i.e., subcritical drying) in the absence of surfactant may induce a dramatic shrinkage, as shown in Fig. 2.10. CO2 is generally preferred as supercritical drying fluid because of its low critical point (cp) of 31 °C and 74 bars. However, phenolic resins are quite heat-resistant polymers (see the last subsection of this chapter), hence solvents having much higher critical points, especially much higher critical temperatures, were found to be relevant for supercritical drying without significant thermal degradation of the

2.3  Gels Derived from Condensed Tannins

41

Fig. 2.10  Mimosa tannin-formaldehyde gels prepared at pH 2 with a concentration of solid phase in water of 6 wt%: (a) hydrogels undergoing solvent exchange in dry ethanol; (b) corresponding aerogel after supercritical drying with CO2; (c) cryogel after freeze-drying with tert-butanol; and (d) xerogel after evaporation in room conditions. All the hydrogels were prepared in test tubes so that their initial diameter was 1 cm

gels. Ethanol (cp: 241 °C, 61.4 bars) and even more acetone (cp: 235 °C, 47 bars) are applicable [134]. Whatever the critical fluid to be used, water and residual chemicals present in the pores of the gels must always be exchanged with special care prior to drying. Additionally, once the porosity is filled with very pure solvent, the supercritical drying shall be performed by largely circumventing the critical point and, very important, by depressurizing the tank very slowly. It has indeed been shown that a too fast depressurization may ruin the efforts made for minimizing the shrinkage, especially in the case of supercritical organic solvents [135]. In the following, either acetone or CO2 was used for preparing aerogels. As for freeze-drying, leading to cryogels, exchanging water with dry ethanol and then with tert-butanol is highly recommended. The latter indeed induces low volume changes upon freezing, thereby limiting changes of the porous structure, unlike water, and also has a much higher vapor pressure than ice, hence reducing the drying time. Cryogels reported below were thus systematically exchanged with tert-­ butanol. The corresponding materials present shrinkage comparable to that of aerogels, but their porous structure is generally coarser, as seen in Fig. 2.10c.

2.3.2  T  annin-Resorcinol-Formaldehyde (TRF) and Tannin-­ Formaldehyde (TF) Organic and Carbon Aerogels and Cryogels In 2010, gelation of condensed tannin was mainly known through the studies already carried out for preparing cold-set adhesives. However, very few works had been performed at that time for obtaining true gels, i.e., with the aim of producing highly porous materials. Some of them were described for being used as hydrogels only,

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

42

especially for water treatment [136, 137], whereas others were prepared as precursors of porous carbon materials [138, 139]. Because of so scarce preliminary works, it was decided to start from the well-known behavior of resorcinol-formaldehyde (RF) formulations and to substitute 2/3 of the resorcinol (R) with mimosa tannin extract (T), based on the formulation of TRF cold-set adhesives for wood. Because of the ill-defined stoichiometry reported in the previous section, the formaldehyde amount was varied such that the TRF/[TRF + formaldehyde] weight ratio was either 0.5 or 0.6, and the pH was adjusted from 2 to 8. Such values were empirically derived from the experiments [140]. The gelation time was found to pass through a maximum, see Fig. 2.11a, a very common feature of phenolic resins. The slowest gelation was thus found to occur at pH 4–5, i.e., very close to the natural pH of mimosa tannin in water (~4.3), thereby proving the catalytic effect of acids or bases. MALDI-ToF and 13C-NMR studies unambiguously showed that not just a mix of flavonoid oligomers and RF species were obtained, but instead that various proportions of resorcinol oligomers reacted with flavonoid monomers and dimers. A few examples are shown in Fig.  2.11b. Cryogels were then produced after the usual aging step, followed by solvent exchange with tert-butanol and subsequent freeze-drying, see Fig. 2.11c.

(a)

200

Gelation time (min)

T RF0 .5 T RF0 .6

150

100

50

0

1

2

3

4

5

pH

6

7

8

9

Fig. 2.11 (a) Gelation time of tannin-resorcinol-formaldehyde (TRF) formulations and (b) a few examples of mixed resorcinol-flavonoid oligomers formed (reprinted from [140] and [43], respectively, with permission from Elsevier and Brill, respectively). (c) Resultant TRF cryogels and (d) tannin-formaldehyde aerogels after drying with supercritical acetone

2.3  Gels Derived from Condensed Tannins

43

In a second step, resorcinol was completely removed from the formulation, so that pure mimosa tannin-formaldehyde (TF) gels could be obtained. For that purpose, the T/F weight ratio was fixed at 1.35, a value again visibly optimal according to the experimental results, and the pH was adjusted from 3 to 8.5. Outside such a range, no gelation could indeed be obtained. After aging, the gels were recovered and submitted to supercritical acetone, and the latter was finally exchanged with nitrogen at 250 °C under 140 bars. GC-MS analysis of the liquid condensed at the outlet of the reactor revealed that acetone was transformed into various products, but only one aromatic compound (mesitylene) was found in very low amount, suggesting that the chemical structure of the gels was negligibly affected by the supercritical drying in acetone. Figure 2.11d shows the corresponding TF aerogels [114]. The same TF formulations were also used to prepare cryogels [121]. Organic TRF and TF cryogels, as well as organic TF aerogels, were next pyrolyzed at 900 °C for producing new carbon gels. The nodular structure, typical of phenolic carbon gels, was recovered, as well as the usual trend according to which higher pH leads to smaller nodules and vice versa, as clearly seen in Fig. 2.12. Their total porosity, BET area, and pore-size distributions were measured, and the corresponding results are presented in Fig. 2.13. The following trends, which will be confirmed below for many other polyphenol-based gels, can be observed: First, the porosity is the highest in the range of pH for which the gelation time was also the highest. This suggests that, at either too low or too high pH, at which polycondensation is quite fast, the resultant polymer network is the most compact. In other words, the most porous materials are obtained at the natural pH of tannin in water, i.e., close to 4. The changes of BET area, which are most sensitive to changes in the volumes of narrow pores, follow the same trend with one exception. TF carbon cryogels indeed present exactly the opposite behavior, which is explained by the poor mechanical properties of the corresponding organic precursors. The latter were indeed produced with a (too) high dilution of the TF resin, so that the most porous resultant organic gels (again those at pH 4–5) could not properly resist the

Fig. 2.12  SEM pictures at two different magnifications (left scale 1  mm, right scale 1  μm) of tannin-formaldehyde carbon aerogels prepared: (a) at pH 3.3 and (b) at pH 8.3 (after [114] with permission from Elsevier)

44

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

Fig. 2.13 (a) Porosity; (b) BET area; and (c)–(e) pore-size distributions calculated by application of the DFT method to nitrogen adsorption isotherms of: TF carbon aerogels (CATF), TF carbon cryogels (CCTF), and TRF carbon cryogels (CCTRF). The pH at which these materials were prepared is indicated in the legends (after [114, 121, 140] with permission from Elsevier)

drying step, and the narrowest porosity collapsed. It thus clearly appears that preparing gels having the highest porosity is a difficult task and requires finding a compromise between dilution of the resin (which should not be too low for producing enough porosity) and mechanical properties (which should be high enough for avoiding the too high shrinkage observed with too diluted gels). From this point of view, tannin-based gels are less easy to prepare than their resorcinol counterparts, probably because of not so closely packed polymer chains, due to steric hindrance induced by the big-sized tannin oligomers from which they are made. The pore-size distributions calculated by application of the DFT method to nitrogen adsorption isotherms, shown in Fig.  2.13c–e, suggest that playing with ­formulation, pH, and drying method allows obtaining very different porous structures: either fixed distributions but with various pore volumes (as in TF carbon aerogels: CATF) or more or less constant porous volumes but with shifted pore-size distributions (as in TRF carbon cryogels: CCTRF). As a final remark, the cost of a typical carbon aerogel derived from resorcinol-­ formaldehyde was attributed at 80% to the resin and at 4% to the solvent, the rest corresponding to energy, manpower, equipment, and facility [141]. Using a 30 times-cheaper phenolic molecule and very low amounts of medium-quality acetone, the final TF carbon aerogels were calculated to be at least 5 times less expensive

2.3  Gels Derived from Condensed Tannins

45

Fig. 2.14  Typical aspects (see legend at the right) of tannin-formaldehyde (TF) hydrogels as a function of pH and dilution. The inset shows a translucent (orange-red) TF aerogel obtained by supercritical drying with CO2 (reprinted from [142] with permission from IOP Publishing). N not gelled, PG physical gelation (orange-red), O opaque (reddish-brown), T translucent (orange-red), ST semi-translucent (burgundy), T(LR) translucent (light red), T(DR) translucent (dark red)

than their RF counterparts. They are thus probably the cheapest carbon aerogels ever [114]. Due to such versatility, it was decided to perform more systematic investigations of the mimosa tannin-formaldehyde system, exploring the effects of pH (from 2 to 10 by steps of 2 pH units) and dilution (the fraction of TF solid phase was varied from 4 to 40 wt%) at constant T/F ratio of 1.35 [142]. The kind of “phase diagram” shown in Fig.  2.14 was thus obtained. It reveals that hydrogels having different aspects can be obtained; especially, some of them are quite similar to their RF counterparts, being reddish-brown and translucent. It also evidences that two situations are encountered in extreme conditions of high pH and high dilution: either no gelation occurs (pH 10, 4  wt% of TF resin) or physical gelation is observed (pH 8, 4 wt% of TF resin on the one hand, or pH 10, 6 wt% of TF resin on the other hand). The latter gels indeed never formed in their sealed test tubes at 85 °C, but appeared upon cooling at room temperature. This phenomenon was fully reversible, and the materials melted and reappeared when heated and cooled, respectively. Such characteristic behavior of physical gels was, as far as the authors know, never observed before for phenolic gels. All the other ones were traditional chemical gels, i.e., were irreversible. The hydrogels of Fig. 2.14 were all converted into organic aerogels by supercritical drying with CO2. The porosity was calculated from the measured bulk density of the aerogels and from the known skeletal density of TF resin. The shrinkage was also measured. Finally, and despite nitrogen adsorption was not recommended in the third chapter of this book, the BET area was determined from such method. Even if the absolute values of the corresponding results might be questioned due to possible absorption of nitrogen in the polymer network [143], clear trends can be seen. Moreover, no distortion of nitrogen isotherms attributable to other phenomena than pure adsorption, i.e., absorption or pore collapse, could be observed for those materials. Most BET areas were higher than 500  m2/g, the highest one being

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

46

a

b

c 900

700 600 500 400 300

2

200

4 pH

BET surface area (m 2/g)

800

6 8 10 4

6

10

14

18

22

26

30

34

40

Mass ratio (%)

Fig. 2.15 (a) Total porosity; (b) volume shrinkage; and (c) BET area of tannin-formaldehyde organic aerogels as a function of mass fraction of solid phase and pH of the initial formulation (reprinted from [142] with permission from IOP Publishing)

880 m2/g. The latter was obtained at medium pH, as expected, and corresponds to an optimal dilution, again due to a compromise between high porosity (promoted by higher dilution) and moderate shrinkage (favored by enhanced mechanical properties at lower dilution). Also noteworthy is that the highest porosity (up to 97%) does not correspond to the highest surface area, since in most porous materials, the ­highest contribution to the total pore volumes comes from the biggest pores, which have a poor effect on surface area, related to the narrowest ones. All these results are gathered in Fig. 2.15. A few pore-size distributions (PSD) calculated by application of the Barrett-­ Joyner-­Halenda method to the desorption branch of the nitrogen isotherms are presented in Fig. 2.16. All gels proved to be both micro and mesoporous; the mesopore volume increased with pH (not very clearly seen from the only examples provided in Fig. 2.16, but the reader can refer to [142]), and passed through a maximum at 18 wt% of solid phase when the fraction of the latter increased. When the pH and the concentration of TF resin in the formulations increased, the PSD was shifted to

2.3  Gels Derived from Condensed Tannins

47

Fig. 2.16  Pore-size distributions of a few tannin-formaldehyde chemical aerogels (blue box) and physical aerogels (red box). The data in the legends are the wt% of resin and the pH of the initial formulation (reprinted from [142] with permission from IOP Publishing)

lower pore sizes. Notable exceptions to these trends are physical gels, probably due to a different polymer structure.

2.3.3  Tannin-Formaldehyde Organic and Carbon Xerogels Hydrogels derived from TF formulations and simply dried at room pressure lead to dramatic shrinkage (see again Fig. 2.10d), unless the surface tension is sufficiently decreased for minimizing the action of capillary forces during drying. With this fact in mind, a well-known triblock copolymer sold under the name Pluronic® F127, consisting of hydrophilic polyethylene oxide (PEO) chains and hydrophobic polypropylene oxide (PPO) chains, with the chemical formula HO(EO)~100(PO)~65(EO)~100H and a molecular weight of 12.6  kDa, was used. Formulations based on 20 wt% of TF resin and an amount of Pluronic just above its critical micellar concentration, at pH adjusted from 2 to 10 were prepared, gelled at 85 °C, aged for 5 days, removed from their sealed test tubes, and let for

48

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

Fig. 2.17  Xerogels based on tannin-formaldehyde formulations with Pluronic® F127 triblock copolymer at different pH indicated on the images: (a) top view (b) side view (reprinted from [144] with permission from Elsevier)

a

b

c Hydrophilic head

Hydrophobic tail

Fig. 2.18 (a) SEM image of XTFP6, an organic xerogel based on tannin-formaldehyde-Pluronic formulation prepared at pH 6; (b) pore-size distributions of organic xerogels prepared at pH 5, 6, and 7; (c) schematic view of a spherical micelle of Pluronic® F127 (reprinted from [144] with permission from Elsevier)

drying in a ventilated oven at 80 °C for 5 more days [144]. The resultant materials are presented in Fig. 2.17, where it can be seen that no shrinkage occurred at pH lower than 7 since the diameter of the xerogel was exactly that of the test tubes in which they were prepared. Higher pH gradually led to higher shrinkage and to more significant deformation. In terms of porous structure, such xerogels presented very big nodules, and especially at low pH, as usual. Figure 2.18a shows one example, as well as a few pore-­ size distributions deduced from mercury intrusion experiments. As expected, big

2.3  Gels Derived from Condensed Tannins

a

49

b

250

40

3 Peak pore size

Peak pore size (µm)

150 100 50 0

Pore volume

2.5

30 25

2

20 15

1.5

10 5

2

4

6 pH

8

10

12

0

1

2

3

4

5 pH

6

7

8

9

Hg pore volume (cm3 g–1)

Gelation time (min)

35 200

1

Fig. 2.19  Correlation between: (a) gelation time of tannin-formaldehyde-Pluronic formulations and (b) total pore volumes and peak mesopore size of the corresponding xerogels (reprinted from [144] with permission from Elsevier)

Fig. 2.20  Top view of tannin-formaldehyde xerogels prepared at different pH indicated on the picture. Top row: Pluronic/tannin (P/T) weight ratio = 2; bottom row: P/T = 0 (reprinted from [145] with permission from Elsevier)

nodules led to big pores of similar size, and at pH below 7, the porosity was indeed mainly macro. However, mesopores appeared at pH 7 which a diameter centered on 10 nm, see Fig. 2.18b, which is exactly the value expected for spherical micelles such as those pictured in Fig. 2.18c. Interestingly, and as already reported above, a nearly perfect correlation was again found between the gelation time, the total pore volume, and the position of the peak deduced from the mesopore-size distribution of all xerogels. The corresponding data are presented in Fig. 2.19. In a second step, the amount of surfactant was varied so as to obtain the following Pluronic/tannin weight fractions: P/T = 0, P/T = 1, and P/T = 2. The formulations were prepared in the same range of pH as before, and the (subcritical) drying process was also carried out in the same conditions. Examples of the corresponding xerogels are shown in Fig. 2.20, and it is clear that most of the porosity was lost in the absence of Pluronic, unlike what happened at P/T = 2, at least for the lowest values of pH. After pyrolysis, the total porosity was found to be as high as 85%; it increased with the amount of Pluronic and decreased when the pH increased. The highest BET area was close to 900 m2/g [145].

50

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

Fig. 2.21  Pore-size distributions deduced from both nitrogen adsorption (left part of the graphs) and mercury intrusion (right parts) of carbon xerogels derived from tannin-formaldehyde-­Pluronic® F127 formulations with P/T = 2 and prepared at: (a) pH 3; (b) pH 5; (c) pH 6, with a photo of the materials (reprinted from [145] with permission from Elsevier)

2.3  Gels Derived from Condensed Tannins

51

Pluronic® F127 not only acted as a surfactant, preventing shrinkage in acidic conditions, but also acted as a structuring agent after carbonization. Pyrolysis of xerogels indeed led to carbon gels whose porous structure was dramatically dependent on the amount of triblock copolymer. The combination of nitrogen adsorption and mercury porosimetry revealed that such gels were perfectly bimodal when prepared at a P/T ratio of 2, and Fig. 2.21 shows the effect of pH in these conditions. Whereas the microporosity was not affected, with a very narrow peak of distribution centered on 0.5 nm, the macroporosity was seen as a sharp peak whose center was shifted from above 10 μm to below 10 μm when the pH increased from 2 to 6. When the Pluronic/tannin ratio was increased from 0.5 to 2 at constant pH = 6, the pore-­ size distribution of the resultant carbon xerogels changed dramatically. A purely macroporous solid was obtained at F/T = 0.5, having a rather narrow peak centered on a pore diameter close to 10 μm, corresponding more or less to the size of the nodules, see Fig. 2.22a. At F/T = 1, the distribution was shifted to lower pore size, but was broad and multimodal. At F/T = 2, the distribution was again very narrow, but with a unique peak centered on 10 nm. Surprisingly, the corresponding carbon material investigated by TEM did not show any order whereas the pore size was so well-defined, see Fig. 2.22c. This is quite uncommon, as most carbon materials presenting a single mesopore size are generally very well ordered. Another, even more common, surfactant was also tested for limiting the shrinkage of tannin-formaldehyde (TF) xerogels: sodium dodecylsulfate (SDS). So as to observe the effect of the amount of crosslinker on the final porous texture of the gels, which is another important variable, the T/F ratio was varied from 0.6 to 2.6. In parallel, the concentration of SDS was adjusted from 0 to 20 wt%, and the weight fraction of solid resin was fixed at 25% at constant pH = 5.5. After gelation, curing, and drying, all carried out at the same temperature of 85 °C in a ventilated oven, organic xerogels were recovered, which were next pyrolyzed at 900 °C [146]. The total pore volumes and the average pore size of the resultant carbon xerogels are presented in Fig. 2.23. It can be seen that, as usual in most porous materials, a higher porosity corresponds to bigger pores, and vice versa. The figure also shows the effect of both formaldehyde and surfactant contents, the concentration of the latter having by far the highest impact. Especially, on optimum around 15  wt% of surfactant was observed. Increasing the concentration of SDS indeed first prevented more and more efficiently the shrinkage due to the repulsive forces among the tannin anions generated during the polymerization reaction, but an excessive amount of surfactant led to the formation of a poorly crosslinked polymeric structure and to a more heterogeneous material, as revealed by SEM pictures (not shown, but see [146]). The same kind of optimum has been also reported for resorcinol-formaldehyde gels [147]. As for the optimum of formaldehyde amount, it is due to a competition between poorly branched polymer clusters at high T/F ratio on the one hand (leading to weak structures that are prone to shrink upon drying) and to highly condensed structures at low T/F ratio on the other hand (thus being less porous).

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

0.0005 0.0004 0.0003

(a)

0.0002

0.0001 0.0000 100000

D ifferential pore volume (cm3g-1nm-1)

D ifferential pore volume (cm 3g-1nm-1)

52

10000

1000

100

10

1

10

1

10

1

Pore Width (nm)

0.008 0.007 0.006 0.005

(b)

0.004 0.003

0.002 0.001 0 100000

10000

1000

100

Pore Width (nm) Differential pore volume (cm3g-1nm-1)

0.06

0.05 0.04

(c)

0.03 0.02 0.01 0 100000

10000

1000

100

Pore Width (nm)

Fig. 2.22  Pore-size distribution determined by mercury intrusion of carbon xerogels prepared at pH 6 and at different Pluronic/tannin weight ratios: (a) P/T = 0.5; (b) P/T = 1; (c) P/T = 2. The microporosity only seen by adsorption is not shown (reprinted from [145] with permission from Elsevier)

2.3  Gels Derived from Condensed Tannins

53

Fig. 2.23 (a) Total pore volume and (b) average pore size of tannin-formaldehyde (TF) carbon xerogels, depending on the amounts of formaldehyde (expressed as T/F wt. ratios) and SDS (expressed as wt%) in the initial formulations (reprinted from [146] with permission from Elsevier)

2.3.4  Alternative Tannin-Based Carbon Gels Carbon gels are materials combining high porosity, excellent resistance to high temperature, chemical inertness towards solvents, and good electrical conductivity. Those characteristics make them suitable for applications in electrochemical storage and conversion (see the last chapter of this book), adsorption [148], or catalysis [149]. However, in a number of cases, the surface area may not be high enough so that an activation process becomes necessary. In the context of carbon science, activation consists in opening and developing the porosity of a carbon material. For that purpose, the latter needs to have a pre-­ existing porosity, otherwise activation is not effective. Activation can be done either “physically,” i.e., the carbon is carefully gasified by an oxidizing gas at high temperature, thereby producing pores, or “chemically,” i.e., the carbon or its precursor before pyrolysis is mixed with a chemical substance acting both as an oxidizer and as a desiccant, is thermally treated, and is washed afterwards. Activated carbon gels derived from polyphenols can be produced by both ways, but such classical methods are multisteps and may consume significant amounts of chemicals. A recent study reported the possibility of producing activated carbon gels derived from tannin by chemical activation with sodium and potassium hydroxides [150]. Instead of using the conventional method by which carbon gels are ground and either mixed with NaOH or KOH pellets of impregnated with concentrated solutions of the same, before heat treatment in an oven under inert gas flow, hydrogels were directly employed. Doing so, the preliminary drying and carbonizations steps

54

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

a

pH 2

pH 5

pH 8

b BET surface area (m2 g–1)

BET surface area (m2 g–1)

1500

1000

500

0

pH 2

pH 5

pH 8

2000

2000

0.5

2

8 –1

NaOH concentration (mol L )

1500

1000

500

0

0.5

2

8

KOH concentration (mol L–1)

Fig. 2.24  BET area of activated tannin-based carbon xerogels as a function of initial pH and concentration of aqueous solutions of: (a) NaOH; (b) KOH (reprinted from [150] with permission from Elsevier)

were not only avoided, but diluted alkali solutions were used, allowing substantial savings of time and chemicals. Indeed, in a classical chemical activation, typical alkali/carbon weight ratios range from 1 to 3 on dry basis, i.e., are more than one to two orders of magnitude higher than what was used in the new method. Activated carbon xerogels were thus obtained by exchanging the water contained in the pores of TF hydrogels with NaOH or KOH solutions having various concentrations, followed by drying at 40 °C and heat treatment at 750 °C. It can be seen in Fig. 2.24 that the resultant surface areas could be quite high. The effect of pH of the precursor hydrogels was not significant; the highest impact was obtained by changing the concentration and the nature of the alkali solution. The results in terms of surface area and pore volumes were quite comparable to those of highly activated carbons obtained by traditional chemical activation of carbon xerogels [150 and refs. therein], at least for KOH, which is a better activating agent than NaOH. An alternative and highly environment-friendly way of producing carbon gels is based on the discovery of gelation properties of aminated tannins submitted to hydrothermal conditions [21]. Whereas it was known that the flavonoid B-ring (see again Fig. 2.2) can be aminated [151], the reaction mechanisms were only elucidated recently. The dissolution of tannin in a concentrated aqueous solution of ammonia, followed by evaporation of the ammonia excess, leads to a mixture of oligomers presenting at the same time multiamination of several phenolic hydroxyl groups, heterocycle opening, and oligomerization and crosslinking through the formation of –N〓 bridges between flavonoid units [36, 95]. The formation of those bridges explains the gelation in hot pressurized water (180 °C, 10–15 bars) whereas no crosslinker is present. Figure 2.25 presents examples of such molecules, whose existence was confirmed by combination of 13C-NMR and MALDI-ToF studies. The resultant hydrogel that is recovered after hydrothermal treatment (Fig. 2.26a) is very interesting, since not only no formaldehyde was required for preparing it, but

2.4  Mixed Gels Derived from Natural Polyphenols

55

(a)

HO

NH2 O

N

HO

OH

OH OH

OH

OH

OH

OH

OH OH OH

N

OH

OH

OH

OH

N

OH OH

N

OH

(b) HO

HO OH

N

OH OH

HO

HO

HO OH

NH2

HO

NH2

HO

N

HO OH

N

OH OH

OH NH2

OH OH

HO

OH NH2

OH OH

HO

NH2 NH2

OH

Fig. 2.25  Examples of molecules evidenced by 13C-NMR and MALDI-ToF studies: (a) before and (b) after hydrothermal treatment of tannin dissolved in ammonia and next evaporated (reprinted from [36, 95] with permission from Elsevier)

it can be dried either by subcritical, supercritical, or freeze-drying, leading to N-doped xerogels, aerogels, and cryogels, respectively. Nitrogen-doping of carbon materials is especially relevant in applications such as electrodes for supercapacitors as it increases the electrical conductivity of the material, it improves its wettability by electrolytes, and it can provide additional contribution to total capacitance through Faradaic reactions [152] (see Sect. 2.5 of this chapter). After pyrolysis at 900 °C, the resultant carbon gels presented the typical nodular structures expected for this kind of materials. As shown in Fig. 2.26b, the nodules were quite small, around 20 nm, suggesting the existence of an important fraction of mesoporosity. The latter was indeed clearly observed in the pore-size distributions (PSDs) deduced from nitrogen adsorption experiments, see Fig.  2.26c. As usual for carbon gels, microporosity was also present. It is interesting to see that the PSDs were quite similar for all carbon gels, whether they were xero, aero, or cryogels. However, the highest pore volumes and the highest BET areas, close to 900 m2/g, were obtained with aerogels, see Fig. 2.26d [122].

2.4  Mixed Gels Derived from Natural Polyphenols 2.4.1  Tannin-Soy-Formaldehyde (TSF) Gels As postulated earlier, any cold-set adhesive can be converted into a gel, and hence to an aerogel, provided that a few changes in the formulation are carried out. One more study supported this postulate, by adapting a formaldehyde-free resin based

56

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

a

b

c

d 1.2 CA27

0.8

900

700

SBET (m 2 g -1)

Differential pore volume (cm

3

-1

g )

800

0.4 0.0 1.2

CC27

0.8

600 500 400 300 200

0.4

CA

100

CC

0

0.0

11%

1.2

18%

CX 27%

CX27

0.8 0.4 0.0 0.1

1

10

100

Pore Width (nm)

Fig. 2.26 (a) N-doped, tannin-based hydrogel obtained by hydrothermal treatment of evaporated aminated tannin at a concentration of 27 wt.% in water; (b) SEM (left row) and TEM (right row) pictures of the derived carbon xerogels, cryogels, and aerogels (CX, CC, and CA, respectively); (c) corresponding pore-size distributions of the same materials; and (d) BET area of carbon gels obtained at various concentrations of aminated tannin in water (reprinted from [122] with permission from Elsevier)

2.4  Mixed Gels Derived from Natural Polyphenols

a

b

NH2 NH2

DENATURATION

H H

Protein–CH2 Tannin – –

+ H2O

Protein–CH–Protein CH2–Tannin–CH2–Tannin Tannin–CH –

C

85 °C

–

90 °C

Protein–HCHOH

Protein–HCHOH + Tannin O

Protein–H

NH2

O Protein–H + C H H

Protein–NH2

NH2

90 °C

Tannin +

57

2

Fig. 2.27 (a) Probable reactions of soy proteins with tannin and formaldehyde; (b) resultant aerogel prepared at pH 6 (reprinted from [154] with permission from RSC)

on soy flour and lignin and developed as a glue for wood particleboard [153], and turning it into a formulation containing tannin and able to produce nice gels. Among the formulations that were tested, those that led to the best results were based on 30 wt% tannin + 70 wt% soy-formaldehyde resin. Gelation occurred above 14 wt% of dry matter and for pH ranging from 5.5 to 9. Although such gels contained formaldehyde, the content of the latter was much lower than anything reported before for formulations containing tannins, and the gels were indeed natural at the 91% level [154]. Figure 2.27a suggests the probable reactions that occurred, explaining why no gelation could be observed in the complete absence of formaldehyde. Once aged at 85  °C for 5  days, the tannin-soy hydrogels were dried with supercritical CO2, leading aerogels such as those shown in Fig. 2.27b. Despite tannin was not the main component, the gelation time of TSF formulations presented the—now usual—bell curve seen in Fig. 2.28a. Again, the BET area of the corresponding aerogels followed a very clear correlation with gelation time even if, as already explained in the second chapter of this book, nitrogen adsorption results might be questionable for organic gels. However, as explained in Sect. 2.3, no analysis of nitrogen adsorption–desorption curves would have been done if the latter had presented unusual shapes or non-closing hysteresis loops, which was never the case for any of the present materials. The corresponding pore-size distributions are given in Fig. 2.28b and were found to be rather broad and rather similar with each other, with a maximum centered on around 30–40 nm, without clear trend when the pH was changed. The pH mainly influenced the pore volume, which was the highest at pH 6, as expected from the surface area, reaching 480 m2/g. Looking at the nitrogen adsorption–desorption curves of this particular sample, i.e., the one shown in Fig. 2.27b, it can be seen that a considerable amount of nitrogen was adsorbed, leading to an outstanding mesopore volume of 2.3 cm3/g, see Fig. 2.28c. As far as the authors know, such value is the highest reported for organic aerogels, only exceeded by a few silica aerogels. SEM pictures revealed an extremely thin, fiber-like, porous structure, as seen in Fig. 2.28d. Due to the significant part of proteins in the compo-

58

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

a

b

c

d 1600

Adsorbed N2 volume (cm3/g)

1400

Vmeso = 2.3 cm3/g BET = 478 m2/g Density 0.21 g/cm3

1200 1000 800 600 400 200 0

0

0.2 0.4 0.6 0.8 Relative N2 pressure

1

Fig. 2.28 (a) Gelation time of tannin-soy-formaldehyde (TSF) formulations as a function of pH, and correlation with the BET area of the corresponding aerogels. (b) Pore-size distributions of TSF aerogels prepared at various pH. (c) Nitrogen adsorption (circles)—desorption (squares) isotherms of the sample prepared at pH 6. (d) SEM image of the same sample (reprinted from [154] with permission from RSC)

2.4  Mixed Gels Derived from Natural Polyphenols

59

sition of TSF gels, converting them into carbon gels always failed. The carbon yield was indeed too low, and no monolith with a preserved porous structure could be recovered [154].

2.4.2  Tannin-Lignin-Formaldehyde (TLF) Organic Gels

T/L mass ratio

+ lignin

As explained in Sect. 2.2.3, selecting the lignin is of highest importance for producing repeatable materials. In the following, LignoBoost™ lignin was used, kindly supplied by Innventia, Sweden. In the LignoBoost™ process, Kraft lignin from softwood is precipitated from black liquor by injection of CO2. The resultant material is next filtered, re-dispersed, acidified again, filtered once more, and finally washed. As a result, a much purer lignin than usual is obtained. More details can be found elsewhere [155]. Since lignin is only soluble in water in alkaline conditions, the pH was fixed at 10 for preparing tannin-lignin-formaldehyde (TLF) gels. The formulation of the latter used a T/L wt. ratio adjusted from 0.1 to 1, a (L + T)/F ratio adjusted from 0.8 to 2.5, and a total wt. fraction of solid phase fixed at 26% [156]. As usual, the aforementioned parameters were optimized by successive trials and errors. Tannin was found to be a key component for recovering gels of good quality, mechanically speaking. And indeed, to the best of the authors’ knowledge, no dry gel based on pure lignin could ever be prepared so far: either hydrogels for liquid phase applications (i.e., not in the dry state) were reported [157–159] or additives had to be used for strengthening the corresponding aerogels, e.g., by using cellulose nanofibers as reinforcement [160]. The idea of substituting phenolic molecules by cheap lignin comes from the need of decreasing the cost of resins in plywood production without significant decrease

dark brown or

1.00 0.67 0.43 0.25

light brown/opaque

Brown / opaque

black/ not translucent black/ translucent dark brown / semitranslucent

not gelled/precipitation

0.11 0.83

1

1.25 1.7 (L+T)/F mass ratio dry basis

2.5

+ formaldehyde

Fig. 2.29  Typical aspects of tannin-lignin-formaldehyde (TLF) hydrogels as a function of T/L and (L + T)/F mass ratios (on dry basis) (after [156] with permission from Elsevier)

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

60

a

b

c 450-550

1,2-1,4 550

1,4

1-1,2

250-350

0,8-1

150-250

1,2

0,6-0,8

450

50-150

250

S BET (m 2/g)

0,4-0,6 350

1

0,2-0,4 0,8 0,6

V meso (cm 3/g)

350-450

0,4 2,5

150

2,5

1,7

(L

) +T

s

0,67

m

ti o

T /L

0,25

1.4

0,67

0,83

0,43 0,25

T /L

o s ra ti mas

1

Differenal pore volume (cm 3 g–1)

0.8 T/L = 0.25 T/L = 0.43 T/L = 0.67 T/L = 1

1.2

(L+T)/F = 0.83

0.8 0.6 0.4 0.2 0

1

1

ra

ti o

0,43

r a ti o as s

s

ra 0,83

as

as

m

m

1

1

0

1,25

/F

)/F

Differenal pore volume (cm 3 g-1)

(L

+T

1,25

d

0,2

1,7 50

1

10

100

Pore width (nm)

1000

(L+T)/F = 1 (L+T)/F = 1.25 (L+T)/F = 1.7 (L+T)/F = 2.5

0.7 0.6

T/L = 0.25

0.5 0.4 0.3 0.2 0.1 0

1

10

100

1000

Pore width (nm)

Fig. 2.30  Tannin-lignin-formaldehyde (TLF) aerogels: (a) general aspect of organic aerogels based on two different tannin/lignin (T/L) ratios and obtained by supercritical drying with CO2. Pore texture parameters: (b) surface area; (c) mesopore volumes; (d) pore-size distributions of aerogels having either different T/L ratios at constant (L + T)/F = 0.83 (left) or different (L + T)/F ratios at constant T/L = 0.25 (right) (reprinted from [156] with permission from Elsevier)

of their mechanical properties. Lignin could indeed replace phenol in phenol-­ formaldehyde adhesives, for instance, but never completely [161 and refs. therein]. The same was found for lignin-based gels whose formulation was, again, adapted from that of cold-set adhesives. The visual aspect of the corresponding hydrogels can be found in Fig. 2.29. In the present experimental conditions, no gel was produced with too low amount of tannin. The pore texture parameters of the corresponding aerogels obtained by supercritical drying with CO2 are shown in Fig. 2.30. It can be seen that those materials present quite broad pore-size distributions. Although a significant proportion of

2.4  Mixed Gels Derived from Natural Polyphenols

a

b

320 LPF 1.25 LPF 1.7

Porosity (%)

280 260 240

90

0.9

80

0.8

0.5

200 0.2

30

0.8

1

1.2

1.4

1.6

Macropore volume (cm3/g)

0.4 0.3

0

0.2

0.4

P/L mass rao

c

0.6

Density ALPF x/1.25 Density ALPF x/1.7

50 40

0.6

Porosity ALPF x/1.25 Porosity ALPF x/1.7

60

220

0.4

0.7

70

Bulk density (g/cm3)

Gelaon me (min)

300

61

0.6

0.8

1

P/L mass rao

1.2

1.4

1.6

0.2

3.5 ALPF x/1.25

3

ALPF x/1.7 CLPF x/1.25

2.5

CLPF x/1.7

2 1.5 1 0.5 0

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

P/L mass rao

Fig. 2.31 (a) Gelation time of phenol-lignin-formaldehyde (PLF) formulations; and (b) total porosity and corresponding bulk density of PLF aerogels at various phenol/lignin (P/L) ratios (x) and for two (L + P)/F ratios of 1.25 and 1.7; (c) Macropore volumes of PLF aerogels and cryogels (reprinted from [162] with permission from Elsevier)

macropores was present, these aerogels were mainly mesoporous, with a rather high BET area. Figures 2.30b and 2.30c are almost similar to each other, suggesting that the major part of the surface area is due to mesoporosity. The nitrogen adsorption– desorption isotherms from which these results were obtained had totally usual shapes, so that no absorption/pore changes mechanisms during adsorption experiments could be suspected, see [156]. Figure 2.30d shows that more tannin, and/or less formaldehyde, led to higher porosity and to narrower pores. Moreover, comparison with Fig. 2.16 clearly evidences that lignin-tannin gels had broader pores than tannin gels. But adding lignin thus not only broadened but also decreased the porosity. Depending on the application, replacing tannin by lignin is thus possible but to a rather limited extent. A fine micronodular structure was again observed by SEM, indistinguishable from that of most other phenolic aerogels and quite similar to Fig. 2.26b (not shown). For further extending the range of gels based on lignin, phenol-lignin-­ formaldehyde (PLF) formulations were investigated. Just as before, the pH and the wt. fraction of solid phase were fixed at 10 and at 26%, respectively. The range of

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

62

Differenal pore volume (cm 3/g nm)

a

b 0.1

0.5 ALPF 0.25/1.7 ALPF 0.43/1.7 ALPF 0.67/1.7 ALPF 1/1.7 ALPF 1.5/1.7

0.4

0.3

0.06

0.2

0.04

0.1

0.02

0

0

10

20

30

40

50

60

0

70

Pore diameter (nm)

c

CLPF 0.25/1.7 CLPF 0.43/1.7 CLPF 0.67/1.7 CLPF 1/1.7 CLPF 1.5/1.7

0.08

0

10

20

30

40

50

60

70

Pore diameter (nm)

d

Fig. 2.32 (a, b) Pore-size distributions; and (c, d) representative SEM pictures of phenol-lignin-­ formaldehyde (PLF) organic gels at two magnifications: (a, c) aerogels (ALPF); and (b, d) cryogels (CLPF) (after [162] with permission from Elsevier)

phenol/lignin (P/L) ratio from 0.1 to 1.5 was tested for two values of (L + P)/F ratio, 1.25 and 1.7 [162]. The corresponding gelation times are shown in Fig.  2.31a: increasing the proportion of lignin dramatically increased the gelation time, until no gelation occurred after 5 days spent in the oven at 85 °C. Such behavior is the same as the one already observed for TLF formulations at too low P/L ratios. The hydrogels were then thoroughly exchanged with dry ethanol first, followed either by supercritical CO2 for producing aerogels or by tert-butanol for producing cryogels. And again, for both kinds of materials, the most porous materials were the ones for which the former hydrogels presented the highest gelation times, as seen in Fig. 2.31b for the case of aerogels. However, such high porosity was mainly due to

2.5  Brief Overview of Properties and Applications

63

Table 2.1 Compressive modulus and compressive strength of some tannin-formaldehyde hydrogels and aerogels, prepared at various wt% of solid phase and various pH (after [142]) Samples Hydrogels

Aerogels

TF4%-6 TF18%-6 TF40%-6 ATF18%-2 ATF18%-8 ATF40%-2 ATF40%-10

Density (g cm−3) 1.07 1.10 1.17 0.18 0.34 0.49 0.84

Modulus (MPa) 0.03 0.32 2.07 0.26 1.01 3.19 12.76

Compressive strength (MPa) 0.01 0.08 1.46 0.48 1.21 – 6.34

macroporosity, and the latter was higher for higher fractions of lignin, as seen in Fig. 2.31c. At this point of the chapter, it clearly appears that the bigger are the molecules from which the organic gels are made, the broader is the final porosity. Thus, the pores of lignin-based gels are wider than those of tannin-based gels, themselves wider than those of resorcinol-based gels. Therefore, mixing lignin with phenol should lead to gels with narrower pores than when lignin is co-reacted with tannin. This behavior is indeed observed in Fig. 2.32a, b, and a straightforward comparison with Fig. 2.30d shows that mesopores are definitely narrower than in the case of tannin-lignin gels. Furthermore, at constant amount of formaldehyde, more phenol leads to narrower pores. Interestingly, and as already seen in the case of hydrothermal carbon gels (Fig. 2.26c), very similar peak pore sizes were found for both aerogels and cryogels and no obvious difference was observed either from SEM studies, see Fig.  2.32c, d. However, the total porosity of aerogels was the highest [162]. Before closing this part, it must be said that gels derived from both TLF and PLF formulations could be successfully converted into carbon gels upon pyrolysis at 900 °C. However, their structural properties were not investigated in detail.

2.5  Brief Overview of Properties and Applications 2.5.1  Mechanical Properties As far as gels are concerned, mechanical properties generally imply compression tests. The latter require that flawless samples, with very well-defined geometry, can be selected for that purpose. Results about hydrogels derived from polyphenols are very scarce, but a few were reported for tannin-formaldehyde formulations, soaked in water during compression for avoiding drying [142]. The modulus was defined as the slope of the initial, linear part of the strain–stress curves, whereas the strength was taken at the point where the curves deviate from linearity by 0.2% strain, as suggested by Pekala et  al. [163] for characterizing weak and highly porous

64

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

Stress (MPa)

a

16

18%-2

14

18%-8

12

40%-2

10

40%-10

8 6 4 2 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Strain

c

3

1.7

modulus

2.5

1.2

strength

2

log (E, s )

log (Compressive Modulus (MPa))

b

0.7 0.2

2.59

1.5 1

-0.3

0.5

-0.8

0

2.24

-0.8

-0.6

-0.4 log (Density)

-0.2

0

-0.5 -0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

log (density)

Fig. 2.33 (a) Stress–strain compression curves of tannin-formaldehyde aerogels (the fraction of solid phase and the pH are given on the plot); and (b) corresponding modulus (reprinted from [142] with permission from IOP Publishing); (c) modulus and strength of tannin-formaldehyde-Pluronic aerogels submitted to compression (reprinted from [144] with permission from Elsevier)

materials. As expected, both modulus and strength increased with the concentration of resin, i.e., decreased at higher dilution. A few data are given in Table 2.1. Aerogels obtained after supercritical drying with CO2 presented two kinds of extreme behaviors: the lightest ones irreversibly collapsed when compressed and were thus plastic, whereas the densest ones suddenly broke into pieces and were thus brittle. Figure 2.33a shows such kind of behavior. The usual power law linking the modulus E to the bulk density ρ, well known for many porous materials, was recovered (see Fig. 2.33b):

E = c ρn,

(2.1)

where c is a prefactor, and n an exponent whose value depends on the porous structure. For TF aerogels, n = 2.54 was found, in agreement with values ranging from

2.5  Brief Overview of Properties and Applications

65

2.48 to 2.87 already reported for resorcinol-formaldehyde (RF) aerogels [163]. The same was observed for tannin-formaldehyde-Pluronic (TFP) xerogels, both for modulus and strength, see Fig. 2.33c. As for the compressive strength, Eq. (2.1) also applies, but with a possibly different exponent. The exponent for the strength was found to be 2.24, again in agreement with the values observed for RF aerogels, ranging from 2.07 to 2.60 [163].

2.5.2  Thermal Properties Whereas superinsulating properties were already observed for aerogels derived from various formulations, whether based on phenolic, cellulosic and, of course, silica precursors, no gel derived from polyphenols having thermal conductivity lower than that of air was ever reported so far. There might be several reasons for this. The first one is that such materials are intrinsically not superinsulating: due to their larger average pore sizes with respect to RF formulations, for instance, the Knudsen effect may be systematically ruined by the presence of macropores. The gels prepared until now may also have a too high density, since more porous materials become quite fragile and sometimes very difficult to handle, especially at the moment of measuring their physical properties. Another possible reason is related to the methodology of measurement. Determining a conductivity lower than that of air, whereas the study is carried out in air, is indeed never a trivial task. Many biases can occur and, additionally, as large samples as possible are required. The latter condition is never easily achieved as soon as supercritical drying is concerned, and combining small samples with a non-suitable measurement technique can lead to dramatically high errors. At the moment of writing this chapter, the present authors are actively working on the means of ascertaining whether polyphenol-based aerogels can be superinsulating or not. So far, measurements were carried out with the Hot Disk method, which proved inefficient for recovering the correct conductivity value of small samples of materials whose superinsulating thermal properties were guaranteed by other techniques, especially flowmeters, using larger samples. However, trends were published, such as those presented in Fig. 2.34. Even if the absolute values are possibly wrong, it is very clear that, on average, the thermal conductivity increases with density. But too high pore volume is useless for decreasing further the thermal conductivity, as seen in Fig. 2.34b, since those pores are definitely too broad. The minimum of conductivity in this case was obtained with the sample presenting the best compromise between low density and high fraction of very narrow pores. The latter two quantities are somewhat antagonistic, hence the difficulty of producing superinsulating materials. So far, and provided that the measured values obtained by the Hot Disk method were correct, the lowest thermal conductivity was for tannin-soy-formaldehyde aerogels: 0.033 W/m/K at a density of 0.25 g/cm3.

66

b

0.14 0.12 0.1 0.08 0.06 0.04 0.02 0

0.2

0.4

0.6

0.8

1

Therm al conductivity (w/m/K)

Thermal conductivity (w/m/K)

a

2  Organic and Carbon Gels Derived from Biosourced Polyphenols

0.14

40%-10 0.12 0.1 0.08

18%-8

0.06

6%-2

40%-2 18%-2

4%-2

0.04 0

3 Density (g/cm )

3

6 9 12 15 Pore volume (cm3/g)

18

21

Thermal conducvity (mW/m/K)

c 65 ALPF

60

CLPF

55 50 45 40 35 0.2

0.25

0.3

0.35

0.4

0.45

0.5

3

Bulk density (g/cm ) Fig. 2.34  Thermal conductivity of tannin-formaldehyde aerogels versus: (a) bulk density; and (b) total pore volume (reprinted from [142] with permission from IOP Publishing); (c) Thermal conductivity of lignin-phenol-formaldehyde aerogels and cryogels (reprinted from [162] with permission from Elsevier)

2.5.3  Applications as Electrodes for Supercapacitors Among the many applications that organic or carbon gels may have (see again Sect. 2.1 of this chapter), only electrochemical applications have been investigated in-­ depth for carbon gels derived from polyphenols. This is especially the case for the hydrothermal, nitrogen-doped, carbon gels presented in Fig. 2.26 after pyrolysis at 900 °C. Supercapacitors develop high capacitances because of two possible different mechanisms for storing electrochemical energy: (1) a purely capacitive effect, which is related to the area available for adsorbing the ions of the electrolyte; and (2) Faradaic effects, by which electrons can be stored and released through redox

2.5  Brief Overview of Properties and Applications

67

a 800

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Fig. 2.35 (a) Cyclic voltammograms of N-doped carbon aerogels (CA), cryogels (CC), and xerogels (CX) at a scan rate of 2 mV/s (reprinted from [121] with permission from Elsevier); (b) specific capacitances per unit of mass of surface area of carbon xerogels (dashed red line), compared to those of activated carbons (AC), other carbon gels (CG), ordered mesoporous carbons (OMC), and other hydrothermal carbons (HTC) (reprinted from [94] with permission from Elsevier); (c) probable redox reactions contributing to pseudocapacitance through Faradaic processes (reprinted from [165] with permission from RSC)

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68

reactions involving chemical functions at the surface of the electrode. The former mechanism is thus highly dependent on the surface area, but even more on the pore width, which should be relevant with respect to the size of the ions. Indeed, extremely narrow pores in which ions cannot penetrate would lead to a high surface area but to a low capacitance. Additionally, those pores should be connected by rather broad canals giving access to them, and in which the ions can diffuse easily, i.e., without producing a too high internal resistance. The Faradaic mechanisms, leading to what is called pseudocapacitance, can be as important as the purely capacitive effects, and even have the main contribution if the number of redox moieties involving reversible redox processes in the considered potential window is high enough [164]. Therefore, microporous carbon materials presenting both a hierarchical porosity and N- and O-functionalities on their surface should have interesting properties for such application. Figure 2.35a shows typical cyclic voltammograms of N-doped carbon aerogels, cryogels, and xerogels obtained by the hydrothermal route (the same as those previously introduced in Fig. 2.26b, c), and derived from aminated tannin. Such cycles represent the changes of capacitance when the voltage is varied at a given scan rate, here 2 mV/s. In the case of purely capacitive behavior, the cycles would be rectangular, but actually are not since bumps can be observed, corresponding to redox reactions. The specific capacitances calculated from those experiments, whether expressed by mass unit or by unit of surface area, and compared to a number of values from the literature, are given in Fig. 2.35b. It can be seen that, whereas the surface area is not as developed as for many other porous carbons reported so far, the specific capacitance still is huge, suggesting the importance of Faradaic processes in this kind of electrochemical storage. Examples of such reactions are pictured in Fig. 2.35c. However, it appears that there is an optimum in terms of amount of both N- and O-functionalities, as seen in Fig.  2.36. Such moieties are thus beneficial to the 80

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Fig. 2.36  Normalized capacitance of the same carbon gels as in Fig. 35a and of other tannin-based materials, plotted as a function of the amounts of: (a) nitrogen and (b) oxygen functional groups (reprinted from [122] with permission from Elsevier)

2.6  Recent Developments

69

capacitance until they block the pores and therefore hinder the diffusion of ions throughout the porosity, hence the observed maxima. Despite the specific capacitances obtained with those carbon gels are among the highest ever reported so far at low scan rate, the electrochemical storage properties decrease significantly as soon as the scan rate is increased (not shown). This should be ascribed to the poor diffusion of the ions in the porosity of the materials, producing a decrease of the porosity explored by the electrolyte when the charging/discharging rate increases too much. Further improvements are thus needed to maintain a good capacitance at high scan rate, and mesostructuration of such materials, within the range 3–13 nm, should be an efficient way of achieving this goal [94].

2.6  Recent Developments 2.6.1  One-Step Microwave-Assisted Synthesis and Drying Two ways of producing xerogels were compared: (1) traditional 3-step synthesis and (2) microwave oven synthesis. For that purpose, the whole range of formulations from pure resorcinol-formaldehyde (RF) to pure tannin-formaldehyde (TF) through resorcinol-tannin-formaldehyde (RTF) was tested. The traditional method included gelation and curing at 85 °C for 3 days in sealed tubes, followed by drying in air at the same temperature for 2 more days in a ventilated oven. In contrast, microwave synthesis only required 3 h for gelation and curing, and 1–2 h for drying. In this work, the shrinkage of the gels was limited by the use of an anionic surfactant (sodium dodecylsulfate) at a concentration of 5 wt%. All formulations were adjusted at pH 5.5 and respected the following ratios: wt. fraction of solid phase 25%, R-T/F wt. ratio fixed at 1.2, R/T wt. ratio ranging from 0 to 100%. The main results are presented in Fig.  2.37, wherein RTF xerogels prepared with the two methods and different R/T ratios are compared [166]. It can be seen that the materials based on the highest amounts of tannin are rather similar, irrespective of the synthesis/drying method. This suggests that a quite fast production of xerogels can be achieved, even if the microwave synthesis still led to materials having a little less porosity than for the conventional way. It can also be noticed that tannin-rich xerogels were again those characterized by the widest pores, in complete agreement with what was already concluded earlier in this chapter. This difference of pore size is indeed related to that of the corresponding molecules, tannins being far bigger than resorcinol and therefore leading to larger clusters and hence to larger pores between clusters. In contrast, the resorcinol-rich formulations produced the most porous xerogels, and microwave synthesis was even more efficient from the point of view of total pore volume. Still, microwaves led to larger pores, as seen by SEM images in Fig. 2.38. All these results are therefore extremely encouraging, and even if microwave synthesis is not optimized yet, it was shown that highly porous materials can be obtained in a very short time. It is therefore a first step towards the scalable synthesis of xerogels derived from polyphenols.

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Fig. 2.37  Main characteristics of resorcinol-tannin-formaldehyde (RTF) xerogels prepared using either the conventional (C) or the microwave synthesis (MW), as a function of the wt% of tannin in the RTF formulation (reprinted from [166] with permission from Elsevier) (a) Pore volumes; (b) Average pore size; (c) Bulk density; and (d) Total porosity

Fig. 2.38  SEM images of xerogels based on 75% of resorcinol and 25% of tannin: (a) conventional synthesis (b) microwave synthesis (reprinted from [166] with permission from Elsevier)

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71

2.6.2  Organic and Carbon Xerogel Microspheres Instead of letting the (now usual) tannin-formaldehyde (TF) formulation gelling in a sealed glass tube, emulsion polymerization can be carried out by dispersing the aqueous phase in a stirred oil medium. In such conditions, resin microspheres can be formed whose size depends on the stirring speed and on the amount of surfactant (Tween 80®) used to stabilize the water/oil interface [132]. Thus, simply stirring a blend of sunflower oil and TF aqueous solution, with or without surfactant, produced small beads such as those shown in Fig. 2.39a. Once hard enough and non-­ sticky, they were recovered by centrifugation, washed with an aqueous solution of commercial (dishwashing) detergent, exchanged several times with dry ethanol, and left to dry in air in room conditions for 7 days. They were then pyrolyzed at 900 °C, leading to carbon xerogel microspheres. The latter are shown in Fig. 2.39b.

Fig. 2.39  Optical (color) and SEM (grey level) images of: (a) organic and (b) carbon xerogel microspheres, prepared with 40  wt% of solid phase in the formulation, in the absence of surfactant. Materials shown in (a) were prepared at a stirring speed of 200 rpm; those in (b) show the effect of stirring speed on the microsphere-size distribution (reprinted from [132] with permission from RSC)

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Figure 2.39b evidences that the higher was the stirring speed, the lower was the average size, up to 800 rpm. But even higher stirring speed did not change the size further. Adding surfactant produced smaller microspheres, but again the effect was limited so that no further decrease of size was observed above 5 vol% of Tween 80® with respect to oil. Those trends are clearly seen in Fig. 2.40a. A very interesting point is also the fact that, after detailed studies of nitrogen adsorption at −196 °C, it appeared that those carbon xerogel microspheres are characterized by extremely narrow micropore-size distribution, centered on 0.5  nm for most materials. Moreover, they present almost zero mesoporosity, see Fig. 2.40b. Such features are those of some carbon molecular sieves actually commercialized at very high cost, typically several hundred euros for a few grams only. In contrast, the present materials were mainly based on cheap and natural resources (tannin and sunflower oil), and their preparation process was quite simple. a

b

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Fig. 2.40 (a) Influence of stirring speed (left) and of surfactant content at 1200 rpm (right) on the mean diameters of the resultant carbon xerogel microspheres; (b) Corresponding pore-width distributions of carbon xerogel microspheres prepared either without surfactant at different stirring speeds (left) or at the same stirring speed of 1200 rpm but with different amounts of surfactant (right) (reprinted from [132] with permission from RSC)

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2.6.3  Elastic Gels with Tunable Properties The present chapter only considered so far highly porous materials, whose porosity was obtained by removing the solvent from hydro/alcogels. However, hydrogels as such may have very important applications without drying, as it is already well known for many years in the context of tissue engineering [167], drug delivery [168], or medicine in general [169], for instance. The present authors are not aware of hydrogels derived from polyphenols that have been successfully tested for such biomedical applications, but instead proposed rubber-like materials whose elastic properties could be tuned within a very broad range of harnesses, depending on the formulation [170]. The synthesis of such materials was quite simple, based only on tannin, water, para-toluenesulfonic acid (pTSA), and ethylene glycol, mixed in suitable proportions and let to gel at 85 °C in sealed glass tubes, as usual. But whereas samples prepared in the absence of ethylene glycol quickly turned hard and were true xerogels, the others were—and remained—more or less flexible, depending on the amount of ethylene glycol in the formulation. MALDI-ToF and 13C-NMR studies proved that, in acidic medium, ethylene glycol and its polymerization products were grafted on tannin oligomers, and that the latter also autocondensed. Thus, a number of molecules such as those presented in Fig. 2.41 could be conjectured, among others, some of them bearing long polyether chains. As a result, a loose macromolecular structure was obtained, in which polyethylene glycol behaved as an internal plasticizer. Adding more ethylene glycol in the formulations produced materials whose Shore A hardness progressively decreased. Shore A hardness (SAH) scale is the usual one as far as elastomers are concerned, and is directly related to elastic modu-

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Fig. 2.41  Examples of molecules evidenced by the combination of MALDI-ToF and 13C-NMR studies, carried out on gels based on tannin–water–pTSA–ethylene glycol formulations

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Not elastic

TEG(0)

Shoe heel

Tire tread

TEG(20) TEG(33)

Chewing-gum Gummy bear

TEG(47)

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Fig. 2.42  Gels prepared from tannin-based formulations in which the ((tannin  +  pTSA)/total) mass ratio was fixed at 28%, the tannin/pTSA mass ratio was 5/0.8, and the ethylene glycol/water mass ratio (in brackets) ranged from 0 to 60%. Common materials having the same Shore A hardness are also shown (after [170] with permission from RSC)

lus. In the present case, the full scale was covered by the range of available materials, from extra-soft (like soft mouse mat (SAH close to 5) or chewing gum (SAH less than 10)) to soft (like rubber band (SAH around 25)), medium soft (like pencil eraser (SAH around 45)), medium hard (like tire tread (SAH around 70)) to hard (like shoe heel (SAH around 80) or shopping cart wheel (SAH higher than 90)). This range of gels is shown in Fig. 2.42 together with representative materials having the corresponding equivalent SAH. Not only those materials have tunable elastic properties, but they present extremely high fire retardance, as shown in Fig.  2.43. This is indeed one more advantage of polyphenols, as the latter release a negligible amount of flammable gases when submitted to high temperatures, thereby preventing any propagation of the fire. Moreover, no fumes are emitted, and since tannin-based resins are not fusible, no dripping of flaming droplets occur. In fact, this kind of resin was already shown to absorb a significant amount of heat for converting it into a dense, poorly thermally conductive and impervious glass-like carbon. The very slow oxidation of the latter, when put directly in the flame of a burner, releases far less heat than what is required for burning it [45, 50, 171]. As a consequence, the materials auto-­ extinguish as soon as the burner is removed. In addition, these gels are even more retardant due to their content of water and ethylene glycol, having rather high heats of vaporization, 2.26 and 1.06 kJ/g, respectively. The last photo in Fig. 2.43 shows that, even if the gel shrank significantly after 5  min spent in the flame, its shape was fully maintained and it did not crack. Obviously, the elastic properties were totally lost after the fire test, and the material became hard and brittle. But with so superior fire retardance, a cheap and easy manufacturing process, and tunable elasticity, this kind of material might be used in applications requiring springs and bumpers.

2.7 Conclusion

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Fig. 2.43  Fire test performed on TEG(33) sample submitted to the flame of a propane burner at 1000 °C at times t = 0, 1 min, 2 min, 3.5 min, and 5 min (from left to right). Bottom: view of the same sample at 5 min just after removing the flame (reprinted from [172] with permission from IOP Publishing)

2.7  Conclusion Polyphenols are natural molecules that are really worth considering for preparing new materials. They indeed combine several advantages such as renewable character, low cost, poor or even absence of toxicity, and excellent reactivity. Such reactivity is directly related to their chemical structure, essentially based on aromatic rings bearing hydroxyl groups. Thus, polyphenols can be crosslinked with aldehydes, especially with formaldehyde, but using far lower amounts than what is required with their synthetic and harmful counterparts: phenol and resorcinol. As a result, the environment-friendly character of such raw material is further improved. High-quality resins can be easily prepared from polyphenols and, depending on their dilution in a solvent, the whole range from hard adhesives to soft hydro/alcogels can be obtained. Wet gels can then be dried according to the usual methods and lead to aerogels, cryogels, or xerogels, with very different porosities and surface areas. So far, the most relevant polyphenols for preparing highly porous gels were found to be condensed tannins. The latter also have the significant advantage of being a cheap and reproducible commercial product. Additionally, condensed tannins can be associated with resorcinol, phenol, lignin, and even soy proteins in mixed formulations presenting new features. As a general comment, polyphenol-­ based materials are by far more versatile than their synthetic counterparts.

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Their polyphenolic nature also makes them highly resistant to fire. When submitted to high temperatures, polyphenols-based resins condense and aromatize further, whereas they release at the same time mainly water vapor and a negligible volume of flammable gases. The significant amount of heat that must be brought for burning them is absorbed for converting them into a char, which prevents any flame propagation and gives the auto-extinguishing character of those resins. It is thus not surprising that crosslinked polyphenols proved to be excellent precursors of carbon materials, since they present a high carbon yield upon pyrolysis. The resultant, glass-like carbon can be tailored in terms of porous structure, and can be functionalized with various kinds of moieties. Those features, associated with a rather high electrical conductivity, make this kind of carbon a valuable raw material in electrochemical devices such as supercapacitors. Despite the aforementioned advantages, polyphenol-based gels still suffer drawbacks that require further studies and improvements. Especially, the corresponding aerogels are never as porous as their resorcinol-based counterparts, and their porosity is also much coarser. This is even truer as far as the gels are made from high molecular weight-oligomers/polymers. This might be the reason why, so far, no superinsulating polyphenol-based aerogel has been reported. Polycondensation and crosslinking of natural polyphenols, associated with a lower purity than what can be expected for synthetic molecules, is also more difficult to control, and the stoichiometry of the reactants is questionable as one deals with distributions of molecular weights. As a result, one cannot think in terms of mol% but of wt%, which is never satisfactory for chemists. Finally, except lignin which is very abundant but unfortunately of quite variable quality, the worldwide supply of condensed tannin is still limited. Thus, even if significant advances were made for getting gels with controllable properties, a lot of work remains for improving the properties, the pore textures, the repeatability, etc., of these valuable materials. Acknowledgements  The authors gratefully acknowledge the financial support of the CPER 2007–2013 “Structuration du Pôle de Compétitivité Fibres Grand’Est” (Competitiveness Fibre Cluster), through local (Conseil Général des Vosges), regional (Région Lorraine), national (DRRT and FNADT), and European (FEDER) funds.

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Chapter 3

Properties of Carbon Aerogels and Their Organic Precursors

Abstract  Aerogels are sol-gel derived porous solids with structural properties, such as porosity, pore size, pore and solid phase connectivity that can be tailored over a wide range to provide unique material properties for different fields of applications, such as filters and adsorbers, catalyst supports, electrodes for electrical energy storage, and materials for lightweight construction or thermal insulation. In this context, carbon aerogels and their organic precursor represent an important class of aerogels with very different physical properties at similar structural characteristics. This is due to the different intrinsic properties of the respective backbone components: At given meso- and macrostructure carbon aerogels are characterized by high thermal and electrical conductivity, significant mechanical brittleness, high porosity of the backbone phase related to micropores (