Porous Carbons: Syntheses and Applications

By Feiyu Kang, Michio Inagaki and Hiroyuki Itoi

Carbon materials form pores ranging in size and morphology, from micropores of less than 1nm, to macropores of more than 50nm, and from channel-like spaces with homogenous diameters in carbon nanotubes, to round spaces in various fullerene cages, including irregularly-shaped pores in polycrystalline carbon materials. The large quantity and rapid rate of absorption of various molecules made possible by these attributes of carbon materials are now used in the storage of foreign atoms and ions for energy storage, conversion and adsorption, and for environmental remediation. Porous Carbons: Syntheses and Applications focuses on the fabrication and application of porous carbons. It considers fabrication at three scales: micropores, mesopores, and macropores. Carbon foams, sponges, and 3D-structured carbons are detailed. The title presents applications in four key areas: energy storage, energy conversion, energy adsorption, including batteries, supercapacitors, and fuel cells and environmental remediation, emphasizing the importance of pore structures at the three scales, and the diffusion and storage of various ions and molecules. The book presents a short history of each technique and material, and assesses advantages and disadvantages. This focused book provides researchers with a comprehensive understanding of both pioneering and current synthesis techniques for porous carbons, and their modern applications.

• Presents modern porous carbon synthesis techniques and modern applications of porous carbons
• Presents current research on porous carbons in energy storage, conversion and adsorption, and in environmental remediation
• Provides a history and assessment of both pioneering and current cutting-edge synthesis techniques and materials
• Covers a significant range of precursor materials, preparation techniques, and characteristics
• Considers the future development of porous carbons and their various potential applications

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 4, 2021
• ISBN: 9780128221532

Feiyu Kang

Feiyu Kang received his PhD from The Hong Kong University of Science and Technology in 1997. He is honorary editorial advisory board of international journal CARBON, Joint Chairmen of international symposiums: CARBON2002 (Beijing), Carbon2011 (Shanghai) and 15th International Symposium on Intercalation Compounds (ISIC15), Coordinators of international research projects: Professor M. Inagaki (NSFC-JSPS) and Professor I. Mochida (JST-MOST). Prof. Kang has investigated graphite and carbon materials since 1988. His research interest includes nano-carbon materials, graphite producing process, porous carbon and nuclear graphite. Prof. Kang had published more than 200 scientific papers and 3 books.

Book preview

Porous Carbons

Syntheses and Applications

Michio Inagaki

Professor Emeritus, Hokkaido University, Hamamatsu, Shizuoka-ken, Japan

Hiroyuki Itoi

Aichi Institute of Technology, Toyota, Aichi-ken, Japan

Feiyu Kang

Graduate School at Shenzhen, Tsinghua University, Shenzhen, Guangdong Province, China

Table of Contents

Cover image

Title page

Copyright

Preface

Acknowledgments

Chapter 1. Introduction

1.1. Carbon materials

1.2. Pores in carbon materials

1.3. Identification and evaluation of pores in carbons

1.4. Purposes and construction of this book

1.5. Abbreviations of technical terms employed

Chapter 2. Syntheses of porous carbons

2.1. Microporous carbons

2.2. Mesoporous carbons

2.3. Macroporous carbons

2.4. Hierarchically porous carbons

Chapter 3. Porous carbons for energy storage and conversion

3.1. Rechargeable batteries

3.2. Supercapacitors

3.3. Hybrid cells

3.4. Fuel cells

3.5. Hydrogen storage

3.6. Storage of methane and methane hydrate

3.7. Thermal energy storage

Chapter 4. Porous carbons for environment remediation

4.1. Adsorption

4.2. Gas separation

4.3. Capacitive deionization

4.4. Electromagnetic interference shielding

4.5. Sensing

Chapter 5. Concluding remarks and prospects

5.1. Concluding remarks

5.2. Constraint and reaction space in carbons, pores

Index

Copyright

Elsevier

Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands

The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom

50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States

Copyright © 2022 Elsevier Inc. All rights reserved.

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions.

This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein.

Library of Congress Cataloging-in-Publication Data

A catalog record for this book is available from the Library of Congress

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library

ISBN: 978-0-12-822115-0

For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Glyn Jones

Editorial Project Manager: Naomi Robertson

Production Project Manager: Kamesh Ramajogi

Cover Designer: Victoria Pearson

Typeset by TNQ Technologies

Preface

One of characteristics of carbon materials is the formation of pores with a wide range of sizes and morphologies, from micropores with sizes of less than 1 nm to macropores with sizes of more than 50 μm, and from channel-like pores with homogeneous diameters in porous carbons and carbon nanotubes to round pores in porous carbons and fullerene cages, including irregular-shaped pores in polycrystalline carbon materials. Large quantities and rapid adsorption of different gas and liquid molecules have been utilized for the remediation of our circumstances since prehistorical times and nowadays for storage of foreign atoms and ions for energy storage and conversion. In addition, porous carbons can keep various functional groups on their surfaces, which can enable the storage of electrochemically active materials, such as Li+, Na+, etc., and the removal of the environmental pollutants, such as Pb²+, Hg⁰, etc., through chemical interaction.

The authors have published three books on carbon materials in a series entitled Materials Science and Engineering of Carbons: Fundamentals, Advanced Materials Science and Engineering of Carbons, and Materials Science and Engineering of Carbons: Characterization. A book focused on graphene and its related materials was also published by the same publishers, entitled Graphene: Preparations, Properties, Applications and Prospects. Porous carbons are among the most important themes in discussing their of the fabrication and applications of carbon materials in these books. In the first book (Fundamentals), their fabrication (synthesis and preparation) processes are described mainly on the bases of fabrication techniques, such as activation, etc., together with their applications in specific fields, such as energy storage and environment remediation. In the second book (Advanced), the focus is on novel fabrication techniques, such as template carbonization, carbon nanofibers via electrospinning and carbon foams, and applications in electrochemical capacitors, lithium-ion batteries, and adsorption. The characterization techniques of carbon materials are discussed on gas adsorption techniques for the evaluation of pore structures and the determination of electrochemical performances of various carbon materials in the third book (Characterization). In the fourth book (Graphene), graphene and its related materials are discussed, and the importance of the surface functional groups on reduced graphene oxides is demonstrated to develop new applications.

The present book, Porous Carbons: Syntheses and Applications, is focused on the syntheses and applications of porous carbons, the pore structures of which depend strongly on the precursors and synthesis conditions, temperatures and durations of carbonization and activation, activation reagents, templates, etc. The syntheses are explained by dividing the synthesis techniques into activation, template-assisted carbonization, and precursor-designing on the resultant microporous, mesoporous, and macroporous carbons. In the section on macroporous carbons, so-called carbon foams composed from exfoliated graphite and carbon nanofibers are included. The applications are discussed by dividing them into two chapters, one on the applications for energy storage and conversion, and another on the applications for environment remediation. In the applications for energy storage and conversion, rechargeable batteries (lithium-ion, sodium-ion and potassium-ion batteries, lithium-sulfur and lithium-oxygen batteries) and electric double-layer capacitors (supercapacitors), together with their hybridization, and fuel cells are discussed, in addition to the storage of energy sources (hydrogen and methane) and thermal energy. The applications for environment remediation are discussed by dividing them into adsorption, gas separation, capacitive deionization, electromagnetic interference shielding, and sensing. The section on adsorption is composed of adsorptive removal of environment pollutants (including inorganic and organic species), water vapor adsorption, CO2 capture, metal ions trapping, and oil sorption. The section on sensing is discussed by separating sensors into chemical sensors, mechanical sensors (strain sensors), and biosensors. In the final chapter, concluding remarks are presented on syntheses and applications by emphasizing the pore structure of the carbon and the presence of surface functional groups. In addition, the possibility of some porous carbons, including carbon nanotubes and fullerenes, presenting constraint and reaction spaces is proposed by showing some experimental results.

It would be a source of great pleasure for the authors if the content of this book was to provide useful information to readers, to enable readers to gain a thorough understanding of the syntheses and applications of porous carbons and also to form new ideas on the carbon materials. For readers' convenience, it is recommended to consult the three books mentioned above which are published by Tsinghua University Press and Elsevier. These books will supply the fundamental knowledge on carbon materials and provide a broad understanding of the range of topics discussed in this book.

Acknowledgments

The authors would like to express their sincere thanks to the people who kindly provided the data and figures for this book, their origins being mentioned in the caption of figures and tables as reference numbers, with the names of contributing persons and journals published being presented in the list of the references. The authors also thank all of those people at Elsevier and also at Tsinghua University Press who helped in the publication of this book.

Chapter 1: Introduction

Abstract

Firstly, the purposes and construction of this book are discussed, together with a brief explanation of the fundamentals of the carbon materials: classification, structure and nanotextures, and carbonization and graphitization of carbon materials. In addition, the characteristics of pores and a brief explanation of the identification and evaluation of pores are presented.

Keywords

Anisotropic layers; Carbon materials; Mesopores; Micropores; Nanotextures

1.1. Carbon materials

Carbon materials are one of most important and essential materials for realizing and maintaining a low-carbon society. Various carbon materials are currently being investigated, such as the electrode materials of lithium-ion rechargeable batteries, electrochemical capacitors (supercapacitors), fuel cells, and solar batteries, which are important devices for creating, saving, and storing sustainable energies. Also, they serve as materials to contribute to environmental remediation, such as adsorbents for pollutants, sensor elements for toxic gases, and electrodes for capacitive desalination of water. Carbon nanotubes and graphenes have contributed greatly to accelerating the development of nanodevices. Some carbon materials are considered to be promising materials for biomedical applications, mainly because of their biocompatibility, such as drug-delivery carriers, cell culturing beds, and biosensors. In these carbon materials, pores with various sizes and morphologies play important roles in the improvement and realization of functional materials and their applications.

Before discussing porous carbons, the fundamentals of carbon materials science are briefly explained here, emphasizing the diversity of carbon materials in structure and textures in various scales. On the detailed explanation and discussion on carbon materials, the readers of this book are referred to fundamental books written by the current authors (M.I. and F.K.) [1,2].

1.1.1. Classification of carbon materials

Numerous kinds of carbon materials have been synthesized and widely used in many fields of industry. These carbon materials are proposed to be classified on the basis of chemical bonds of constituent carbon atoms using sp³, sp², and sp hybrid orbitals [3]. The sp² hybrid bonding of carbon atoms results in two structures: flat layers composed of six-membered carbon rings, which have so far been represented by graphite but now are typified by graphene, and curved layers created by introducing five-membered carbon rings into six-membered rings, as occurs in fullerenes. Carbon layers composed of sp² orbitals, both flat and curved, are intrinsically anisotropic and have π-electron clouds on both sides of the layer, and these anisotropic layers create the broad diversities in the structure and properties of the carbon materials. Carbon nanotubes can be placed between fullerene and graphene, because the tips of the tube include five-membered rings (fullerene-like) and its wall is composed of six-membered rings (graphene-like) though it is rolled up. A classification of carbon materials based on hybrid bonds is presented, together with the diversities of the materials, in Fig. 1.1.

Due to the anisotropic nature and the presence of π-electron clouds in the carbon materials composed mainly of sp² hybrid orbitals, the number of carbon layers stacked in parallel has a strong influence on their properties. The importance of the number of stacked layers has been pointed out on carbon nanotubes and fullerenes, and now on graphene. In addition to the layer number stacked, the stacking regularity of layers and the size of anisotropic layers widen the diversity of carbon materials. An infinite number of large-sized layers stacked with regularity has been called graphite, which is crystalline and strongly anisotropic in structure, and as a consequence anisotropic in properties. In contrast, random aggregation of the units of irregularly stacked small layers results in so-called amorphous carbon, which is isotropic in properties with high mechanical strength and high hardness. In between crystalline graphite and noncrystalline amorphous carbon, various graphite-related materials having structures with different degrees of graphitization (proportion of regular stacking) and layer sizes have been produced in industries and used as important industrial materials.

Figure 1.1 Classification and diversity of carbon materials.

1.1.2. Structure and nanotexture of carbon materials

Most carbon materials are composed of flat layers using sp² hybrid orbitals and consist of small units of layers stacked in parallel, which are called basic structural unit (BSU) or crystallite, as shown by the lattice fringe TEM image on an aggregation of some BSUs and the schematic illustration on a BSU in Fig. 1.2A and B, respectively. In the unit, two kinds of stacking regularity of layers coexist: random and regular stacking. The latter is graphitic stacking (usually written as AB stacking) with interlayer spacing d 002 of 0.3354   nm (the same as natural graphite), while the former is stacking with a slightly larger d 002 as 0.342   nm, called turbostratic stacking. These BSUs are strongly anisotropic in bonding nature, with strong covalent bonding using sp² hybrid orbital along the layer and weak van der Waals bonding due to the interaction between π-electron clouds of the layers.

Figure 1.2 Basic structural unit (BSU) of carbon materials: (A) lattice fringe image and (B) schematic illustration of a unit.

The aggregation of these anisotropic nano-sized BSUs gives different textures to the particles, due to the different schema of preferred orientation of anisotropic BSUs, i.e., planar, axial, point, and random orientations, as illustrated together with some representative carbon materials [3,4] in Fig. 1.3. The aggregation of BSUs in different schema is called nanotexture. By planar orientation, films and platelets of highly oriented graphite are produced and some coke particles are principally composed by the planar orientation of BSUs. By axial orientation, fibrous carbon materials are produced from different precursors, such as carbon nanotubes and vapor-grown carbon fibers with a coaxial mode of orientation, and some mesophase-pitch-based carbon fibers with radial mode. By point orientation, a variety of carbon spheres are produced, as represented by various-sized fullerene particles and different nano-sized carbon blacks with concentric mode and mesophase spheres with radial mode. The particles composed of these oriented nanotextures are still anisotropic. In addition, random aggregation of small BSUs occurs in so-called glass-like carbon (glassy carbon), of which the particles are isotropic in nature.

1.1.3. Carbonization and graphitization

Most carbon materials used in industry are produced from organic precursors, such as pitches, biomasses, and organic polymers, via heat treatment at high temperatures in inert atmosphere, pyrolysis, carbonization, and graphitization processes. In Fig. 1.4, changes in chemical, crystallographic, and electronic band structures with heat treatment temperature are illustrated.

Figure 1.3 Nanotextures in carbon materials on the bases of preferred orientation of BSUs.

Figure 1.4 Schematic illustration of the changes in chemical, crystallographic, and electronic band structures with heat treatment temperature for an organic carbon precursor.

The carbonization process proceeds after pyrolysis of organic precursors at the temperature range from 800°C up to 2000°C, in which the BSUs are formed and their basic aggregation scheme (nanotexture) is established through polycondensation of six-membered carbon rings, accompanying the emission of foreign atoms, oxygen, hydrogen, and nitrogen, as gases. This process is the most important in the production of various carbon and graphite materials, because the nanotexture of most carbon materials is established during this process and the nanotexture governs the development of the crystalline structure in carbon materials during the following graphitization process. During pyrolysis and carbonization processes, a high amount of shrinkage occurs due to the large amount and rapid emission of gas species, associated with cracking in the resultant carbon particles in many cases. Therefore, the process of carbonization is usually performed separately from that of graphitization.

Above 2000°C, the change in crystalline structure, in other words, the development of graphite structure, occurs mainly. The development of the graphite structure is evaluated by different techniques, including X-ray diffraction (XRD), electromagnetic property measurements, Raman spectroscopy, high-resolution transmission electron microscopy, etc. In BSUs formed during carbonization, turbostratic stacking with an interlayer spacing of about 0.342   nm is randomly changed to graphitic regular stacking with a spacing of 0.3354   nm (the spacing in the graphite crystal) with increasing heat treatment temperature (HTT) above 2000°C, which is evaluated as the decrease in average interlayer spacing d 002 by XRD, associated with the growth of BSU sizes (crystallite sizes) along the a and c axes, L a and L c, respectively. The change in d 002 with HTT depends greatly on the materials after carbonization (carbon materials). In Fig. 1.5, the changes in these parameters with HTT are shown for various carbon materials. In a needle-like coke with planar orientation scheme d 002 decreases quickly to approach the value of graphite crystal (0.3354   nm) and L c and L a grow rapidly. In glass-like carbon with a random orientation scheme, in contrast, almost no decrease in d 002 and no appreciable growth in L c and L a even after 3000°C treatment are observed, i.e., there is no development of graphite structure. Carbon blacks with a point orientation scheme exhibit intermediate behaviors, large-sized thermal black showing more structure improvement than small-sized furnace black.

Diffraction peaks of XRD for carbon materials are classified into three groups, 00l, hk0, and hkl, mainly due to the strong anisotropy of BSUs and the coexistence of two interlayer spacings, as shown in Fig. 1.2B. Diffraction peaks with the indices 00l give averaged interlayer spacing d 002, which decreases gradually with increasing HTT from more than 0.344   nm for small and highly defective layers to 0.3354   nm for graphitic stacking through about 0.342   nm for turbostratic stacking, in other words, with improving crystallinity of carbon, as shown in Fig. 1.5A. In turbostratic stacking of layers, there is no three-dimensional regularity in stacking, i.e., no l-index is defined. Therefore, their diffraction peaks are expressed as hk for carbon materials mainly consisting of turbostratic stacking of layers, although they are indexed as hk0 for the graphitic structure with three-dimensional regular stacking. Regular and random stackings, graphitic and turbostratic, are clearly demonstrated in diffraction profiles of hk0 and hk peaks, as 100 and 10 peaks. The 10 peak for turbostratic structure shows a characteristic asymmetric profile and is modulated by the appearing 101 peak, together with its sharpening and improving in symmetry with increasing HTT. The 100 peak for a graphitic structure is sharp and symmetrical, associated with the 101 peak due to the formation of three-dimensional stacking regularity. The peaks with the indices of hkl are caused by the graphite structure and so the 112 peak is often selected as an indication of the formation of graphite structure, because the 112 peak is not overlapped with other peaks although the 101 peak is often overlapped with the 100 peak. Sharpening of these diffraction peaks, 002, 004, 100, 110, and 112, with increasing HTT provides the information on the growth of crystallite sizes, average size along the c-axis (L c), that along the a-axis (L a), as shown in Fig. 1.5B and C, respectively, and the size of three-dimensional graphite crystallites.

Figure 1.5 Changes in XRD parameters with HTT on various carbon materials: (A) d 002, (B) L c measured from 002 diffraction peak, and (C) L a from 110 peak.

Since most carbon particles are anisotropic, except glass-like carbon, their aggregation into a block gives different textures in a larger scale, which may be called microtexture and macrotexture. One of the examples of macrotexture is illustrated for the carbon fiber-reinforced composites in Fig. 1.6. These large-scale textures must be controlled for practical applications, although the evaluation technique for these large-scale textures has not been established yet.

1.2. Pores in carbon materials

All carbon materials contain pores, with the exception of highly oriented graphite and graphite crystal, because they are polycrystalline products of thermal decomposition of organic precursors, such as various resins and pitches. During their pyrolysis and carbonization, a large amount of decomposition gases is formed in a wide range of temperatures, as shown in Fig. 1.4. Since the gas evolution behavior of organic precursors is strongly dependent on both the precursors used and the heating conditions, such as heating rate, pressure, etc., carbon materials contain a large amount of pores consisting of a wide range of sizes and morphologies. The pores in solids, including carbon materials, are classified on the bases of sizes, origins, and states, as summarized in Table 1.1.

Figure 1.6 Different schema of the arrangements of carbon fibers for reinforcing the composites.

Table 1.1

The classification of pores based on their sizes was proposed by IUPAC (International Union for Pure and Applied Chemistry). As illustrated in Fig. 1.7, pores are usually classified into three types: macropores (>50   nm), mesopores (2–50   nm), and micropores (<2   nm) [5,6]. Micropores are further divided into supermicropores with a size of 0.7–2   nm and ultramicropores of less than 0.7   nm. The terms nanopores and nanoporous carbons are often used to show the presence of micropores and mesopores in the carbon.

Figure 1.7 Classification of pores in carbon materials on the bases of sizes.

Pores are also classified into two types, intraparticle and interparticle pores, on the bases of their origins. The intraparticle pores are further classified into two subtypes, intrinsic and extrinsic. The former owes its origin to the crystal structure, and a typical example is the pores in zeolite crystals. The graphite gallery between neighboring hexagonal carbon layers can be a slit-shaped pore with a width of 0.3354   nm, which can accept various atoms, ions, and even molecules, with the insertion of foreign species into interlayer spaces of graphite often being called intercalation. Therefore, the graphite gallery is an intrinsic intraparticle pore. Into the graphite gallery, which is widened by the intercalation of Cs ions (binary intercalation compound), hydrogen and some hydrocarbon molecules can be intercalated to form ternary intercalation compounds, with this intercalated gallery being an extrinsic intraparticle pore. In most activated carbons, a large amount of pores with various sizes of nanometer scale, micropores and mesopores, are formed because of random orientation of crystallites, which are rigid interparticle pores. In exfoliated graphite, which consists of worm-like particles, large pores in and among worm-like particles are formed, which are flexible intraparticle and interparticle pores, respectively. These pores can be easily deformed by compression and by sorption of heavy oil.

In addition, pores are classified on the bases of their states, either open or closed. In order to identify the pores by gas adsorption, they must be exposed to the adsorbate gas, in other words, the pores must be open for adsorbate gas molecules and have sufficient size to accommodate gas molecules. If some pores are too small to accept absorbate gas molecules, they cannot be recognized as pores by the gas adsorption technique. These pores are often called latent pores to differentiate them from structurally closed pores. Closed pores are not necessarily small in size.

Porous carbon materials are often classified into microporous, mesoporous, and macroporous carbons, whether the principal pores are micropores, mesopores, or macropores, as indicated in Fig. 1.7. Macroporous carbons are often called carbon foams, and sometimes carbon sponges. However, it has to be pointed out that these pores coexisted in most of the carbons without any intention. For example, micropores exist in the walls of mesopores and also between two mesopores as the windows to interconnect two mesopores. In most carbon foams, micropores, and even mesopores, often exist in the walls of macropores. Although the words nanopores and nanoporous carbons are used in some of the literature without definitions, they are used to indicate pores having nanoscale sizes in most cases, i.e., micropores and mesopores including small-sized macropores, as shown on the last line in Fig. 1.7. Recently hierarchical pore structures have attracted attention, mainly because the coexistence of micropores, mesopores, and macropores is desired for some applications of porous carbons.

In this book, the classification of porous carbons into four classes is employed: microporous carbons, mesoporous carbons, macroporous carbons (or carbon foams), and hierarchically porous carbons. The last class, hierarchically porous carbons, is limited to the carbons in which three kinds of pores, micropores, mesopores, and macropores, intentionally coexist and their structures (i.e., size and volume) are independently controlled, because most porous carbons are more or less hierarchically porous.

1.3. Identification and evaluation of pores in carbons

Pores in carbon materials have been identified and evaluated by different techniques depending mostly on their size. The techniques to evaluate pores in carbon materials are summarized in Fig. 1.8. Pores with nanometer sizes, i.e., micropores and mesopores, are identified by the analyses of gas adsorption–desorption isotherms mostly using nitrogen gas at 77   K. The fundamental theories, instruments, measurement practices, analysis procedures, and many results obtained so far by gas adsorption measurements have been reviewed in different publications [7]. X-ray small-angle scattering has an advantage to identify the latent pores, including closed pores, though gas adsorption can detect only open pores which can accept gas molecules. For macropores, mercury porosimetry has been frequently applied [7]. Identification of intrinsic intraparticle pores, including interlayer space between hexagonal carbon layers in the case of carbon materials, is carried out by X-ray diffraction (XRD), while extrinsic intraparticle and rigid interparticle pores, including latent pores, are identified by small-angle X-ray scattering (SAXS). Direct observation of pores on the surface of carbon materials has been reported by using microscopy techniques coupled with an image-processing technique, e.g., scanning tunneling microscopy/atomic force microscopy (STM/AFM) with transmission electron microscopy (TEM) for micropores and mesopores, and scanning electron microscopy (SEM) with optical microscopy for macropores.

Figure 1.8 Pores in carbon materials and their identification techniques.

On the bases of these characterization techniques, the explanations focusing on the key issues for the measurements and analyses were presented in a separate book [8].

1.3.1. Gas adsorption

The pore structure of solids has usually been measured using physical adsorption of various gases. For the convenience of the measurements, nitrogen adsorption has often been used at a liquid nitrogen temperature of 77   K [9,10]. There are many reviews and books on pore structure determination by nitrogen gas adsorption focusing on carbon materials [7].

The isotherms of adsorption and desorption were classified by their shapes into six types by IUPAC (International Union of Pure and Applied Chemistry), as shown in Fig. 1.9A [5]. The type I isotherm is typical for microporous solids, especially for activated carbons, where micropore filling occurs at a low relative pressure P/P 0, and adsorption completes below 0.5 in most cases. In Fig. 1.10A, N2 adsorption–desorption isotherms for some microporous carbons are shown, where an abrupt increase in N2 adsorption at very low P/P 0 (below 0.05) is characteristic for the presence of micropores. The type IV isotherm is typical for mesoporous carbons, being characterized by a hysteresis, which is classified by IUPAC into four kinds from H1 to H4, as shown in Fig. 1.9B. Fig. 1.10B shows representative N2 isotherms with clear H2 hysteresis and an abrupt increase in N2 adsorption at low P/P 0, suggesting the coexistence of micropores with mesopores. As shown in Fig. 1.10C, isotherms of some macroporous carbons (carbon foams) show very small adsorption of N2 even at a high P/P 0 of 0.8 and an abrupt increase in N2 adsorption above P/P 0 of 0.9. In Fig. 1.10C, pore size distribution curves are inserted, to show the existence of macropores with about 50   nm sizes.

Figure 1.9 Classification of gas adsorption isotherms by IUPAC [5].

For the analysis of these adsorption–desorption isotherms, various methods have been proposed: BET (Brunauer–Emmett–Teller) method, α s plot, BJH (Barrett–Joyner–Halenda) method, DR (Dubinin–Radushkevich) plot, DH (Horvath–Kawazoe) method, t plot, and DFT (density functional theory) method, together with theoretical calculation based mainly on the Grand Canonical Monte-Carlo method. They have been applied on the isotherms measured on various carbon materials.

The BET method [11] is commonly used for carbon materials by determining the surface area S BET, which is convenient to compare the pore structures among different carbon materials. S BET is calculated from the slope and the intercept of the linear relation of 1/{W[(P 0/P)–1]} against P/P 0 (BET plot), where W is the amount of adsorbed gas at P/P 0. Porous carbons containing mesopores together with micropores, which are rather common in porous carbons, show composite isotherms of types I and IV. They show an abrupt increase in N2 adsorption at low P/P 0 below 0.1 and hysteresis at P/P 0 above 0.5, providing certain errors in surface area calculated by the BET method. S BET is frequently applied to all kinds of carbon materials to give a relative measure of their pore structures.

An α s plot [12,13] is based on the comparison of isotherms between the sample and the reference (nonporous solid) and is often used for carbon materials. A typical plot with analysis procedures is schematically shown in Fig. 1.11: W in the unit of mg/g is plotted against α s, which is nitrogen adsorption of the reference at each P/P 0 after normalizing at P/P 0 of 0.4. Often it shows the deviations upward from the line passing through the origin, f-swing (filling swing) below α s of 0.5, and c-swing (cooperative filling) over α s of 0.5–1.0. The f-swing and c-swing are mainly due to the strong adsorption into micropores and quasi-micropore condensation, respectively. External surface area S ext is calculated from the slope A and total surface area S total from the slope B in α s plot using the respective equations in Fig. 1.11, and microporous surface area S micro is obtained as a balance. The values of surface area thus calculated are those subtracted from the effect of strong potential field in micropores and so-called SPE surface areas (SPE: subtracting pore effect). From the intercept W 0, micropore volume V micro can be obtained. These pore structure parameters are the reliable information on microporous and mesoporous carbons. In t plot analysis, the parameter t is the thickness of adsorbed layer on the reference at the corresponding P/P 0.

Figure 1.10 Representative N2 adsorption-desorption isotherms for (A) microporous, (B) mesoporous, and (C) macroporous carbons.

Figure 1.11 Illustration of α s plot of N2 adsorption at 77   K with the calculation procedure for the pore structure parameters.

Analysis of isotherms by the BJH method [14] is used to determine the size distribution of mesopores assuming that micropores are absent from the sample and only mesopores are present with homogeneous morphology. However, the BJH method has frequently been applied to activated carbons containing a large amount of micropores and to calculate V micro as well as mesopore volume V meso.

In the DR method [15], Gaussian-type distribution of pore sizes is assumed. For four activated carbons having different pore structures, N2 adsorption–desorption isotherms observed, their DR plots, and α s plots are shown in Fig. 1.12A–C, respectively [16]. DR plots show a linear portion at low P/P 0 range, revealing the presence of micropores, of which the back extrapolation gives V micro. In Table 1.2, V micro values calculated from DR and α s plots, V micro(DR) and V micro(α s), are listed together with S BET. The activated carbons having high S BET, AX21 and JF517, show a marked deviation from the linearity at P/P 0 below 0.05 and give quite different values of V micro calculated from DR and α s plots, suggesting that these carbons have a wide range of micropore sizes and that the evaluation of V micro via the DR plot seems not to be appropriate.

The HK method [17] assumes the presence of slit-shaped micropores between graphite layers to calculate the size distribution of micropores by using semiempirical formulae. The DFT method is based on the simulation of adsorption isotherms by theoretical calculation and has the advantage of obtaining the pore size distribution in a wide range of sizes from micropores to mesopores. The results of the DFT analysis depend strongly on the calculation parameters, such as interaction parameter between gas molecules and that between gases and solids.

The analysis of pore structure in carbon materials by CO2 adsorption around room temperature was recommended to complement the characterization by N2 adsorption at 77   K [18,19]. The studies using CO2 adsorption at 298   K up to 4   MPa on activated carbon fibers [18] and at 273   K down to subatmospheric pressure on five activated carbons having different pore structures [19] demonstrated that it has a special relevance for the characterization of activated carbons with narrow micropores which are not accessible to N2 at 77   K. In addition, it is more convenient to use the temperature 0°C (273   K) as the adsorption temperature instead of 298   K because of high uncertainty of the density of the adsorbed CO2 at high temperature. In Table 1.3, V micro determined by the DR method using N2 at 77   K, V micro(N2), is compared to that using CO2 at 273   K, V micro(CO2), on carbon materials with high V micro, except nonactivated carbon fibers (CF) [20]. On nonactivated CF, no micropores are detected by N2 adsorption but some micropores are detected by CO2 adsorption, demonstrating CO2 is adsorbed into the pores which N2 cannot get into (the pores having a size less than 0.4   nm). V micro(CO2) gives smaller values than V micro(N2), probably because CO2 is adsorbed selectively into narrow micropores and V micro(N2) includes large micropores (supermicropores).

Figure 1.12 Comparison between DR and α s plot analyses on activated carbons; (A) N2 adsorption–desorption isotherms measured at 77   K, (B) DR plots, and (C) α s plots [16].

Table 1.2

Table 1.3

1.3.2. Mercury porosimetry

Pore structures of macroporous solids were often studied using a mercury porosimeter. For macroporous carbons, particularly carbon foams derived from exfoliated graphite (EG) and reduced graphene oxide (rGO), however, the application of a mercury porosimeter needs special care to prevent collapsing of fragile foams by heavy mercury. A new U-type dilatometer was proposed [21], as illustrated by comparing with the conventional N-type dilatometer in Fig. 1.13A, and pore size distributions measured by these two dilatometers are shown in Fig. 1.13B. By using a conventional N-type dilatometer, only pores having sizes up to 60   μm are detected in an EG foam, whereas the pores larger than 60   μm, which are reasonably supposed to be the interparticle pores, are collapsed during the intrusion of mercury and are not detected. By using a U-type dilatometer, the pores having much larger sizes up to 600   μm can be reproducibly detected. It has to be pointed out, however, that there is a certain risk of overlooking larger macropores, more than a few hundred, even using this U-type dilatometer.

Figure 1.13 Mercury porosimetry: (A) illustrations of two types of dilatometers and (B) pore size distributions of an exfoliated graphite foam measured by two dilatometers [21].

1.3.3. Microscopy techniques and image processing

To identify the pores in carbon materials, various microscopic techniques are applied, as shown in Fig. 1.8, from nano-sized pores by tunneling microscopic techniques (STM and AFM) to millimeter-sized pores by an optical microscopy technique. Most microscopic data are presented as images in different scales. In order to obtain the quantitative data from these images, the assistance of image processing techniques is essential.

Image processing techniques have successfully been applied to SEM micrographs for quantitative characterization of pore structure in exfoliated graphite (EG) [22]. In most cases, either the bulk density or the exfoliation volume has been evaluated for EGs, which have low bulk density and are very fragile. EG consists of at least three kinds of pores, i.e., pores inside the work-like particles, crevice-like pores on the worm-like particles (these two are intraparticle pores), and pores formed by complicated entanglement of these fragile worm-like particles (interparticle pores). The techniques to prepare the fractured surface of these fragile foams, including worm-like particles of EG, were proposed: a simple cleavage of worm-like particles [23] and an impregnation of paraffin followed by cutting [24]. With the assistance of image processing on the cross-sections, quantitative characterization of pore structure became possible and a series of works has been performed [22]. In Fig. 1.14A and B, an SEM image of a cross-section prepared by cleaving a worm-like particle of EG and the distribution of cross-sectional area of pores inside the particles determined from more than 7000 pores are shown, respectively. One of the advantages of this technique is to be able to observe large size pores, which cannot be identified by gas adsorption and mercury porosimetry.

Figure 1.14 (A) SEM image of the cross-section of a worm-like particle and (B) distribution of the cross-sectional area of the pores [22].

STM observation is possible to observe not only pores of very small sizes but also the shape and fractal dimension along the pore surfaces through image processing, although only the entrances of the pores are observed and a large number of observations are necessary. STM analysis was performed on the surfaces of glass-like carbon spheres carbonized at different atmospheres, including oxidizing atmosphere [25,26]. An STM image on the surface of a carbon sphere and an example of a contour map around a pore are shown in Fig. 1.15A and B, respectively. From the observations on a large number of pores, a pore size distribution is determined, as shown in Fig. 1.15C. In Table 1.4, the results obtained from STM image analysis are compared with those from gas adsorption. The results of the STM characterization show quite good correspondence to those of the gas adsorption analysis; the density of micropores with a size of 0.5–1.5   nm (number per 1   μm² area of the surface) measured on four different spheres corresponds to the BET surface area (S BET) determined by N2 adsorption at 77   K.

In transmission electron micrographs taken on a thin section of carbon materials with sufficiently high magnification, pores look white because the electron beam passes through and pore walls look black because of scattering of the electron beam. The quantitative analysis of these micrographs with the aid of image processing gives information on the pore size distribution and also smoothness of pore walls (fractal dimension). Detailed studies have been carried out mainly on activated carbon fibers (ACFs) [27,28]. A TEM micrograph (bright-field image) is converted to a power spectrum as a curve showing a change in brightness with distance, which is considered to reveal the pore size distribution. The area under the power spectrum curve corresponds to the relative pore volume. An original bright-field image exhibits a good correspondence to the bimodal image obtained by image processing on various carbons. In Fig. 1.16, the power spectrum obtained from TEM image is shown with pore size distribution determined by gas adsorption for three ACFs with different S BET. Since the power spectrum is expressed in reciprocal space, the distance in real space indicated on the abscissa increases on the left-hand side. Therefore, pore size distributions are plotted in the same manner. Taking into account that the magnification of TEM observation for this analysis does not give the information on the distance more than 5   nm, relatively good correspondence was obtained between the power spectrum from the TEM observation and pore size distribution from N2 gas adsorption. The distribution estimated from the TEM image is a little broader than that from gas adsorption, which is supposed to be due to the fact that three-dimensional averaging is performed in the former, but the minimum value of pore parameters is detected in the latter.

Figure 1.15 STM image of the surface of a carbon sphere (A), contour map for a pore (B), and pore size distribution (C) [26].

Table 1.4

Figure 1.16 Power spectrum determined by TEM analysis and pore size distribution determined by gas adsorption for three activated carbon fibers [28].

Optical microscopic images of isotropic high-density graphite blocks were analyzed by image processing and provided quantitative information on their macropores, which made discussion on the dependences of various properties on pore structure possible [29].

1.4. Purposes and construction of this book

The purpose of this book is to provide a basic and thorough understanding of the synthesis, structure, properties, and applications of porous carbon materials. As briefly explained above, carbon materials have a wide range of diversity in structure, textures, and properties, which mainly depend on the precursors and their conversion conditions to carbon materials, i.e., temperature, heating rate, atmosphere, pressure, etc. Even if a precursor, for example a pitch, is selected, the product obtained by pyrolysis, carbonization with a gentle heating rate, associated with flowing at the stage of liquid has graphitizing nature, such as needle-like cokes, which is used in graphite electrode production in industry, while the same pitch pyrolyzed with rapid heating is less graphitizing as a conventional coke. If its pyrolysis is performed in an oxidative atmosphere, the resulting carbon is nongraphitizing. To understand and discuss the experimental results on carbon materials, detailed information on the conditions of their preparation is essential. In this book, therefore, the experimental results associated with the keys to the preparation conditions of the carbon materials published in various journals have been collected as much as possible.

In this book, porous carbons are explained by their syntheses, structures, textures, properties, and applications. Their structures, textures, and properties are tightly related to their applications. The field of applications of porous carbons is divided into two areas: energy storage/conversion and environment remediation. In these applications, carbon nanotubes and nanofibers, as well as reduced graphene oxides, are included for comparison with porous carbons.

This book consists of the following five chapters.

In Chapter 1 Introduction, a brief explanation of carbon materials is presented with a classification based on the chemical bonds between carbon atoms, sp, sp², and sp³. The carbon materials with sp² hybrid bonding are explained on the basis of nanotextures to show the position of porous carbons in whole carbon materials. In addition, a brief explanation of the identification and evaluation of pores is presented, although for detailed theories and procedures of these techniques readers are referred to specific books.

In Chapter 2 Syntheses of porous carbons, the procedure for synthesis of porous carbons is summarized by dividing them into microporous, mesoporous, and macroporous carbons, with a separate section on hierarchically porous carbons which have attracted attention recently. They are explained on the basis of synthesis procedures, i.e., activation, template-assisted carbonization, precursor design including polymer blend, molecular design, organic-frameworks, carbon aerogels, and ionic liquids. For the synthesis of macroporous carbons, carbonization with blowing and exfoliation of graphite is described, in addition to template-assisted and precursor design processes.

In Chapter 3 Porous carbons for energy storage and conversion, the applications of porous carbons are described by dividing them into rechargeable batteries, supercapacitors, hybrid cells, fuel cells, hydrogen storage, and methane storage. In the section on rechargeable batteries, lithium-ion, lithium-sulfur, and sodium-ion batteries are included. In the section on hybrid cells, the cells composed of an electrode based on the Faradaic reaction and another electrode based on adsorption are explained. Hydrogen and methane are important future energy sources and thus their storage is discussed. In addition, the storage of thermal energy using porous carbons is also summarized.

In Chapter 4 Porous carbons for environment remediation, the applications of porous carbons mainly for environment remediation fields are summarized by dividing them into adsorption, gas separation, capacitive deionization, electromagnetic interference shielding, and sensing. In these application fields, not only porous carbons, but also other carbon materials, such as nanotubes and graphene-related materials, are important, and an explanation of these carbon materials is included. In the section on adsorption, adsorption removals of various pollutants are explained, followed by descriptions of focused adsorbates, i.e., water vapor, carbon dioxide, heavy metals, and oils. In the section on sensing, the applications for chemical, mechanical, and biomedical sensors are summarized.

In Chapter 5 Concluding remarks and prospects, the synthesis processes of porous carbons are summarized by reviewing briefly their history, and the applications of porous carbons are discussed by focusing on the contributions of pores in carbon materials. As prospects for porous carbons, the new possibility of pores as a constraint and reaction space in carbon materials is discussed by referring to various experimental results.

1.5. Abbreviations of technical terms employed

Technical terms used frequently in this book are abbreviated in order to save space. They are summarized in Table 1.5. Expressions of polyimides and ionic liquids are summarized in Fig. 4.89 and Table 2.21 to explain their application for gas separation and their carbonization, respectively.

Table 1.5

References

1. Inagaki M, Kang F.  Materials science and engineering of carbon: fundamentals . 2nd ed. Tsinghua Univ. Press and Elsevier; 2014.

2. Inagaki M, Kang F, Toyoda M, Konno H.  Advanced materials science and engineering of carbon . Tsinghua Univ. Press and Elsevier; 2013.

3. Inagaki M.  New carbons - control of structure and functions . Elsevier; 2000.

4. Inagaki M. Microtexture of carbon materials.  TANSO . 1985;122:114–121 [in Japanese].

5. Sing K.S.W, Everett D.H, Haul R.A.W, Moscou L, Pierotti R.A, Rouquérol J, Siemieniewska T, IUPAC recommendations 1984. Reporting physisorption data for gas solid systems with special reference to the determination of surface area and porosity.  Pure Appl Chem . 1985;57:603–619.

6. Thommes M, Kaneko K, Neimark A.V, Olivier J.P, Rodriguez-Reinoso F, Rouquerol J, Sing K.S.W.Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution.  Pure Appl Chem . 2015;87(9–10):1051–1069.

7. Patrick J.W, ed.  Porosity in carbons: characterization and applications . London: Edward Arnold; 1995.

8. Inagaki M, Kang F, eds.  Materials science and engineering of carbon: characterization . Tsinghua Univ. Press and Elsevier; 2017.

9. Lowell S, Shields J.E, Thomas M.A, Thommes M.  Characterization of porous solids and powders: surface area, pore size and density . Kluwer Academic Puiblisher; 2004.

10. Rouquerol F, Rouqerol J, Sing K.S.W, Llewellyn P, Maurin G.  Adsorption by powders and porous solids Principles, methodology and applications . 2nd ed. Elsevier; 2014.

11. Brunauer S, Emmett P.H, Teller E. Adsorption of gases in multi-molecular layers.  J Am Chem Soc . 1938;60:309–319.

12. Kaneko K, Ishii C. Superhigh surface area determination of microporous solids.  Colloid Surf . 1992;67:203–212.

13. Kaneko K, Ishii C, Kanoh H, Hanzawa Y, Setoyama N, Suzuki T. Characterization of porous carbons with high resolution α s-analysis and low temperatures magnetic susceptibility.  Adv Colloid Interface Sci . 1998;76–77:295–320.

14. Barrett E.P, Joyner L.G, Halenda P.P. The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms.  J Am Chem Soc . 1951;73:373–380.

15. Dubinin M.M. The potential theory of adsorption of gases and vapors for adsorbents with energetically nonuniform surfaces.  Chem Rev . 1960;60:235–241. .

16. Carrott P.J.M, Roberts R.A, Sing K.S.W. Adsorption of nitrogen by porous and non-porous carbons.  Carbon . 1987;25:59–68.

17. Horvath G, Kawazoe K. Method for the calculation of effective pore size distribution in molecular sieve carbon.  J Chem Eng Jpn . 1983;16:470–475.

18. Cazorla-Amoros D, Alcañiz-Monge J, Linares-Solano A. Characterization of activated carbon fibers by CO2 adsorption.  Langmuir . 1996;12:2820–2824.

19. Cazorla-Amoros D, Alcañiz-Monge J, de la Casa-Lillo M.A, Linares-Solano A. CO2 as an adsorptive to characterize carbon molecular sieves and activated carbons.  Langmuir . 1998;14:4589–4596.

20. Lozano-Castello D, Cazorla-Amoros D, Linares-Solano A, Quinn D.F. Micropore size distributions of activated carbons and carbon molecular sieves assessed by high-pressure methane and carbon dioxide adsorption isotherms.  J Phys Chem B . 2002;106:9372–9379.

21. Nishi Y, Iwashita N, Inagaki M. Evaluation of pore structure of exfoliated graphite by mercury porosimeter.  TANSO . 2002;201:31–34 [In Japanese].

22. Inagaki M, Suwa T. Pore structure analysis of exfoliated graphite using image processing of scanning electron micrographas.  Carbon . 2001;39:915–920.

23. Inagaki M, Saji N, Zheng Y.-P, Kang F, Toyoda M. Pore development during exfoliation of natural graphite.  TANSO . 2004;215:258–264.

24. Zheng Y.P, Wang H.N, Kang F.Y, Wang L.-N, Inagaki M. Sorption capacity of exfoliated graphite for oils-sorption in and among worm-like particles.  Carbon . 2004;42:2603–2607.

25. Vignal V, Morawski A.W, Konno H, Inagaki M. Quantitative assessment of pores in oxidized carbon spheres using scanning tunneling microscopy.  J Mater Res . 1999;14:102–112.

26. Inagaki M, Vignal V, Konno H, Morawski A.W. Effects of carbonization atmosphere and subsequent oxidation on pore structure of carbon spheres observed by scanning tunneling microscopy.  J Mater Res . 1999;14:3152–3157.

27. Oshida K, Kogiso K, Matsubayashi K, Takeuchi K, Kobayashi S, Endo M, Dresselhaus M.S, Dresselhaus G.Analysis of pore structure of activated carbon fibers using high resolution transmission electron microscopy and image processing.  J Mater Res . 1995;10:2507–2517.

28. Endo M, Furuta T, Minoura F, Kim C, Oshida K, Dresselhaus G, Dresselhaus M.S.Visualized observation of pores in activated carbon fibers by HRTEM and combined image processor.  Supramol Sci . 1998;5:261–266.

29. Oshida K, Ekinaga N, Endo M, Inagaki M. Pore analysis of isotropic graphite using image processing of optical micrographs.  TANSO . 1996;173:142–147 [in Japanese].

Chapter 2: Syntheses of porous carbons

Abstract

The syntheses of porous carbons are summarized by dividing them into microporous, mesoporous, macroporous, and hierarchically porous carbons. They are described on the basis of synthesis procedures, i.e., activation, template-assisted carbonization, and precursor design, including polymer blend, carbon aerogels, etc. For the synthesis of macroporous carbons, carbonization with blowing, exfoliation of graphite, and assemblage of fibrous carbons are also described.

Keywords

Activation; Molecular design; Polymer blending; Reduced graphene oxide; Template-assisted carbonization

Nano-sized pores are created in carbon materials and their structures are modified mainly by oxidation under controlled conditions; the process involves the oxidative gasification of precursor carbons to CO and CO2 and is called activation. The products are called activated carbons, having played important roles since prehistoric times and they are now becoming increasingly important materials in various technological fields. Different activation processes have been employed using air, steam, ZnCl2, and KOH as activating reagents in order to develop micropores in carbon materials, in particular KOH activation, resulting in a very high surface area of more than 3000   m²/g. Since the 1990s, various techniques to control the pore structure in carbon materials without an activation process have also been proposed, mainly with the aim of more precise control of pore structure and avoidance of sacrificial loss of carbon atoms; template-assisted carbonization using zeolite, mesoporous silicas, MgO, etc. Blending of precursor polymers was successfully employed to control the pore structure in carbons (polymer-blending). In addition, proper selection of the precursor organics for carbonization was also proposed, such as defluorination of fluorine-containing organics and gelation of organics by selecting appropriate precursors.

In this chapter, porous carbons are divided into microporous, mesoporous, and macroporous carbons, based on the principal pores, although these three kinds of pores coexist in most practical porous carbons. The techniques for the creation and control of pore structure in carbon materials are reviewed on the basis of the classification of the fundamental processes into activation, template-assisted carbonization, and precursor design. The activation process has been employed for the synthesis of so-called activated carbons since prehistoric times and is divided into physical activation and chemical activation. Since activated carbon fibers have advantages for some application fields, they are discussed in a separate section. The template-assisted carbonization process is reviewed on the bases of template materials, which are different from the desired micropores, mesopores, and macropores. The precursor design process is further divided into polymer-blend, molecular design, and process design. In the polymer-blend process, two kinds of polymers—matrix-forming polymer and pore-forming polymer (labile polymer)—are intentionally mixed before carbonization. The selection of a labile polymer is important to control the pore structure. For macroporous carbons, some labile polymers are selected to blow the matrix-forming polymer by evolving gaseous species from pyrolysis products. In the molecular design process, precursor molecules are designed to have some pending functional groups on the carbon framework, the former giving pores and the latter resulting in the matrix of porous carbons. The process design includes the gelation of precursor polymers before carbonization for the creation of mesopores and the pyrolysis of carbon precursors under pressure for creating macropores in carbons. The concept for this chapter is summarized in Table 2.1.

2.1. Microporous carbons

2.1.1. Activation

2.1.1.1. Physical activation

The principal reaction in activation of carbons is oxidation and consequently it is always accompanied by weight loss of matrix carbon, with weight loss (burn-off) being often used as a parameter to characterize the activation degree.

Oxidation of carbonaceous materials was studied as a fundamental process for coal gasification using various oxidation reagents. In Fig. 2.1A, burn-off of the carbon (char) prepared from coal at 1000°C is plotted against the oxidation (activation) time at 405°C in O2/N2 mixed gases with different O2 contents [1], revealing that burn-off (in other words, activation degree) depends strongly on the O2 content in the atmospheric gas. It has been shown that these burn-off data could be unified to a single curve using a normalized time scale t/t 0.5, where t 0.5 is the time to reach 50% burn-off, as shown in Fig. 2.1B. Unification curves are discussed experimentally and theoretically [2,3].

The pore growth process through O2 activation was also demonstrated using glass-like carbon spheres [4–6]. Glass-like carbon spheres prepared from resole-type phenol-formaldehyde resin at 1000°C were selected, where activation by oxygen in dry air started from the surfaces of spheres because of their gas impermeability. Pore development in the carbons with activation is understood through the master curves for each pore parameter, which was obtained by shifting the experimental points at different activation temperatures along the oxidation time axis in logarithmic scale to be fitted with the experimental points observed at the reference temperature. In Fig. 2.2, the master curves obtained at the reference temperature of 400°C on glass-like carbon spheres are shown for oxidation yield, S BET and V total. Those for S micro, S meso, V micro, V ultra, V super, and V meso were also obtained successfully [6]. On the bases of these master curves for pore structure parameters, O2 activation of the glass-like carbon spheres is discussed. In Fig. 2.3, the changes in pore volumes for ultramicropores, supermicropores, and mesopores (V ultra, V super, and V meso, respectively) together with micropore volume ( V micro   =   V ultra   +   V super ) and the corresponding SEM images of the carbon sphere are shown as a function of the activation time at 400°C. At the start of activation, i.e., up to 10   h at 400°C, the main process is the formation of ultramicropores. From 10 to 60   h, the relative amount of ultramicropores decreases, while the volumes of supermicropores and mesopores increase with increasing oxidation time, suggesting the transformation of ultramicropores to supermicropores and then mesopores. Above 65   h, the volume of micropores decreases rapidly associated with a slight increase in the mesopore volume, thereby resulting in a decrease in the surface areas measured by the different methods employed. The principal process in the formation of micropores was supposed to be the opening of closed pores intrinsically created in the matrix carbon during carbonization [7].

Table 2.1

Figure 2.1 Burn-off in O2/N2 mixed gases with different O2 contents for a carbon prepared from coal at 1000°C: (A) burn-off versus oxidation time t at 405°C and (B) burn-off versus normalized parameter t/t 0.5 [1].

Figure 2.2 Master curves for O2 activation of glass-like carbon spheres: (A) oxidation yield, (B) BET surface area S BET, and (C) total pore volume V total [6].

Figure 2.3 Changes in pore volumes, ultramicropores, supermicropores, and mesopores together with the sum of ultramicropores and supermicropores (micropores), with activation time at 400°C. The corresponding SEM images of the carbon spheres are also shown [6].

The carbons, which were prepared from commercially available Saran (copolymer of vinylidene and vinyl chlorides) by carbonization at 600°C, followed by treating in H2 at 700°C to remove chlorine complexes, had very high surface areas of up to 3500   m²/g after steam activation [8]. The Saran-derived carbon activated at 950°C for 135   min with a yield of 15   wt.% exhibited a surface area