Carbon Alloys part 14 pot

Electrochemical Functions 439 processes without dissociation in acidic solutions (pH < 7). In basic solutions, lactones dissociate to form carboxylic groups which also may undergo reversible one-electron transfer reactions. 2.2 Oxidative Intercalation Reactions Graphite forms intercalation compounds with various atoms, ions, and molecules. Graphite intercalation compounds of H,SO, are a typical example produced by anodic oxidation of graphite [8]. When the intercalation compound of graphite thus formed is reduced, the original graphite is reformed via reversible redox processes [ 1,8]. When graphite intercalation compounds of H,SO, are further oxidized by using a higher anodic potential in concentrated H,SO, solution, the oxidation processes are irreversible and the reduced product is quite different from the original graphite [1,9]. Breakage of structure is suggested by a noted decrease in the La crystallite size as measured by Raman spectroscopy. Such reversible intercalation and irreversible oxidation mechanisms are schematically shown in Fig. 6 [l]. Extensive oxidation of graphite removes electrons from the aromatic n: system, resulting in the formation of aliphatic bonds in the graphite plane [l]. As a result, such an oxidized graphite has a significantly lower electrical conductivity than the original graphite [l]. 3 Electrochemical Behavior of Various Carbons Many types of carbons have been used as electrodes, such as HOPG, GC, CF, etc. The properties of the carbon electrode vary greatly because of differences in size and orientation of crystallites, which are characterized by La, Lc, or do,, calculated from X-ray diffraction and Raman scattering measurements. In this section the electro- chemical behavior of several carbon materials are described in terms of their structures [l-31. Such a discussion is useful to select or control variables of carbon materials when using them as electrodes for special purposes. Reversible intercalation 1 I Further oxidation t e Reduction =+ OH0 OHOH Damaged graphite Irreversible intercalation Fig. 6. Schematic structural image of reversible and irreversible oxidative intercalation of graphite [1,9]. 440 Chapter 26 3.1 Highlj Ordered Pyrolytic Graphite Highly ordered pyrolytic graphite (HOPG) has a highly developed crystal structure and shows extensive anisotropy. The well-defined ordered structure of HOPG is suitable to estimate electrochemical behavior of ideal basal or edge planes. A clean basal plane (graphene layer), with a low defect concentration, is easily prepared by cleavage, laser activation, or electrochemical activation [l]. The basal plane is extremely inert as an electrode (for electron transfer) in contrast to the edges of the planes (edge planes) as reported by cyclic voltammetry measured over cleaved basal and edge planes of HOPG in aqueous solution containing Fe(CN)c3"-" [1,10]. The observed electric double layer capacitance of a basal plane is only 5% of that of an edge plane [1,10]. Therefore, the main conclusion derived from comparison of edge and basal planes of HOPG is the extreme anisotropy of such electrochemical behavior as electron transfer and capacitance [1,10]. HOPG forms intercalation compounds with various elements, molecules, and ions by electrochemical methods as well as chemical methods. Recently, intercalation compounds with lithium have been extensively studied as anode materials for the lithium-ion secondary battery. 3.2 Glassy Carbon Glassy carbon (GC) is a vitreous carbon with isotropic properties. GC is impermeable to gases and liquids. In addition, it may be polished, exposed to high vacuum, heated, and chemically modified. Thus, GC can be conveniently made into electrodes with such shapes as disks and rods. A highly porous reticulated vitreous carbon (RVC) has also been used in a variety of electrochemical measurements [l]. 3.3 Carbon Fiber Some pitch-based carbon fibers (CF) have an anisotropic structure because hexa- gonal planes (graphene layers) prefer to orientate along the fiber axis. There are different types of orientation as seen (by SEM) in cross-sections of fibers such as onion, radial, and/or random [ 11. Different electrochemical responses can be expected from fiber ends vs. fiber sides because of different anisotropic presentations. A wide range of carbon fibers are available, from different sources and with different values of HTT and hence with different properties producing different electro- chemical behaviors. Carbon fibers are especially applied as microelectrodes for in vivo analysis of bio-related materials. 3.4 Carbon Paste Carbon paste is a mixture of graphite with an organic liquid of low volatility. A major Electrochemical Functions 44 1 advantage of carbon paste is the ease of renewal of a surface so providing a fresh surface unaffected by electrode history. An electrode prepared from carbon paste has an extremely low oxidative background current when compared with those of plati- num and graphite [l]. The development of carbon paste has led to the widespread use of voltammetry for studying organic reaction mechanisms and the development of electrochemical detectors for liquid chromatography [ 11. 4 Application of Carbon Electrodes Carbon materials have unique characteristics superior to those of metals. Thus, vari- ous carbon electrodes are used extensively in industry and are available commercially. This section describes recent applications of carbon electrodes, especially the lithium rechargeable battery, the electric double layer capacitor and sensors. 4.1 Lithium Rechurgeable Battery The success of lithium rechargeable batteries arises from the use of carbon materials as a lithium reservoir at the negative electrode [11-141. Reversible intercalation of lithium into a carbon host lattice avoids the problem of lithium dendrite formation coming from the decomposition of electrolyte organic solvent. An objective of the Carbon Alloys project is to control the “atomic” spaces of carbon to form a hetero- atomic alloy. The development of high performance anodes for lithium rechargeable batteries with a controlled carbon microstructure is a research target for the Carbon Alloys project. Several research groups are actively engaged in this Carbon Alloys project. Of these, Endo and Kim have studied correlations between the micro- structural parameters and electrochemical properties of various carbon materials; their results are described in Chapter 25. Another important objective of Carbon Alloys is control of electronic states of carbon by introduction of heteroatoms (doping). Such modification by doping has been investigated recently for carbon anodes of lithium rechargeable batteries. The doping methods include: (i) co-deposition by CVD of carbon and foreign atoms, (ii) pyrolysis of organic molecules containing foreign atoms, and (iii) chemical treatment of the carbon [14]. Boron is known to enter into the carbon lattice by substitution. For example, by CVD from benzene and boron trichloride, carbons containing up to 25 at% boron can be prepared [14]. Carbon doped with boron is reported to show greater reversible capacity for lithium than pure carbon [ 141. In addition, an increase in a cell voltage of about 0.5 V is reported [14]. The effect of boron doping in carbon can be interpreted qualitatively as follows [14]: Boron has three valence electrons and an acceptor character. In a rigid-band model, the Fermi Ievel is lowered by the addition of boron which allows more lithium (electron donor) to be intercalated. In addition, the presence of boron strengthens the chemical bond between the lithium and boronxarbon host. As a result, the potential of the lithium in boron-substituted carbon increases compared with pure carbon. Nitrogen is another element which can 442 Chapter 26 be incorporated, by substitution, into carbon, either alone or together with boron [ 141. Unfortunately, results reported for nitrogen-containing carbons differ from each other at the moment. New approaches have been proposed in order to improve performance of carbon anodes for lithium rechargeable batteries. For example, carbons with controlled porosity were prepared from polymeric precursors by using inorganic templates [15]. Control of surface states of carbon is very important to enhance lithium insertion from solvents. A composite electrode made up of ultrafine silver particles supported on graphite has been reported to provide a higher volumetric specific capacity and longer cycle life than conventional graphite electrodes [16,17]. The formation of thin, passivating surface films is clarified to prevent co-intercalation of solvent molecules together with lithium ions, which causes exfoliation of the graphite [ll-141. The search for new solvents, salts, and additives, in which graphites behave reversibly due to unique surface chemistry, has been extensively studied [ 11-14]. 4.2 Electrochemical Double-Layer Capacitor Another objective of the Carbon Alloys project is to control surface space of carbon to form new functional interface. The storage of electrical energy based on the separa- tion of charged species (supporting electrolyte) by adsorption on electrode surfaces in electrolytic double layers is simple compared with rechargeable batteries. Such de- vices are called electrochemical double-layer capacitors (EDLCs). The EDLC has been considered as one of the most attractive rechargeable power devices because of its excellent high-rate charge/discharge ability with high energy density and with long cycle life compared with the common rechargeable battery [ 181. Essentially all elect- rode/electrolyte interfaces form electric double-layer (i.e., capacitance); however, EDLCs work as devices when no Faradaic reactions proceed over the potential range of operation. Because capacitance is proportional to amount of adsorbed ions on electrodes, electrochemically inert materials of high specific surface area can be utilized as electrodes in EDLC. Thus, the electrode must be designed to have (1) high specific surface areas; (2) good intra-and inter-particle conductivity in porous materials; and (3) good electrolyte accessibility to intra-pore surface area [ 18-20]. Carbon materials, such as activated carbons and ACF cloths, are promising electrode materials for EDLC, since they can be produced inexpensively with high surface areas. However, carbon materials are far from being electrochemically inert and surface chemical reactions may occur during polarization to introduce functional groups. Various types of oxygen functional groups are present on carbon surface (see Figs. 1 and 2) and may be further introduced by the application of voltage during operation of EDLCs devices, especially overcharging. Such oxygen functional groups on carbon cause some electrochemical reactivity and affect polarization, point of zero charge, and wettability with electrolyte solution (hydrophilic) [20]. The wettability, estimated by the contact angle measurements, increases by an increase in oxygen content and determines accessible porosity of aqueous electrolyte solutions, which Electrochemical Functions 443 might influence capacitance values [20]. Electrochemical oxidation of an active carbon fiber (ACF) electrode showed an enhancement of electrode capacitance presumably by improved wettability [20]. Many powder carbon materials inherently have dangling bonds, which are associated with free-radical behavior. It can also influence the self-discharge characteristics [20]. In order to remove such oxygen functional groups, heat-treatments at elevated temperatures in a vacuum, nitrogen, or hydrogen and sometimes in water vapor are known to be effective [20]. These heat-treatments are expected to open pore structures and increase the degree of crystallization to decrease inter-particle contact resistance. Various surface treat- ments of carbon materials are reported recently. For example, a cold plasma generated at low temperature has been used to modify the chemical and physical properties of surfaces of carbon materials, such as pore size and functionality distribution, without changing bulk properties [20]. In the case of the graphite electrode, certain intercalation processes occur on charging in which various species or ions in the electrolyte become injected into the interlayer space. Although such intercalation processes are essentially reversible at defined voltages, application of highly excessive voltage may cause carbon electrodes to physically fail (Fig. 6). The use of organic electrolytes in capacitors has been of interest due to high operating voltages [18-201. A disadvantage of organic electrolyte system is their low power densities due to higher resistivity than that of an aqueous system. Unlike lithium rechargeable batteries, where the electrode passivation occurs due to electro- lyte decomposition, the use of organic electrolytes in the EDLC is potentially less restricted. Next generation EDLCs should supply higher energy and power densities than those achieved previously. Recently, the transition between supercapacitors and batteries has been discussed, which is called “pseudo-capacitances’’ (Le., two- dimensional reversible Faradaic surface reactions) [ZO]. Applications of new types of carbons with unique pore structures are proposed; polyacenic semiconductor (PAS) materials prepared from pyrolytic treatment of phenol-formaldehyde resin [21], open-cell aerogel [22] and xerogel[23], foam carbon materials, and carbon nanotubes which have narrow distributions of pore sizes, highly accessible surface area, low resistivity, and high stability [24]. 4.3 Sensor An electrochemical sensor is a device that quantitatively detects a particular chemical species as an oxidation or reduction current [3,25,26]. An electrochemical sensor has advantages such as simple measurement procedure, short response time, and suffi- cient sensitivity and selectivity. Although chemical sensor systems, utilizing chemical reactions to convert target species to detectable ones (e.g., by UV-vis spectroscopy), have attained quite high sensitivity, they are not feasible to in situ measurements because of indirect detection of target species. On the other hand, an electrochemical sensor system can easily monitor changes in concentration with time. In order to 444 Chapter 26 establish high performance electrochemical sensors, modifications to electrode surface should be carried out. For example, modification by electrocatalytic materials has been proposed to attain high sensitivity [27]. In order to attain high selectivity, an electrode surface is coated with films as well as modified with enzyme or mediator, which show specific affinity to target chemicals [27]. Carbon is the preferred electrode material for such modifications because it has reactive functional groups on the surface available for chemical modification. Clearly, preparation and pretreatment history of carbon electrodes strongly affect the performance as a sensor, such as electron transfer kinetics, background current, reproducibility, and adsorption properties. Among various carbon materials, carbon fibers (CF) or activated carbon fibers (ACF) are used extensively as microelectrodes which can be used in very small spaces and to establish chemical events occurring inside single biological cells, such as neurotransmitter in living brain tissue [ 1-3,25,26]. A microelectrode used in vivo is typically made by placing a single carbon fiber in the lumen of a small glass tube. The entire electrode, including the surrounding insulator, must be of micrometer dimensions in order to minimize disruption of the biological environment in which the electrodes are employed. The use of microelectrodes mounted on piezoelectric micropositioners allows the local concentration of electroactive substances to be mapped in two dimensions, referred to as scanning electrochemical microscopy. New candidates of carbon materials used in electroanalytical chemistry have been proposed. Chemically modified carbon paste electrodes incorporated with zeolite molecular sieves were constructed as sensor electrodes for voltammetric deter- mination of trace copper in aqueous solution [28]. An amperometric biosensor for the detection of ethanol vapor was fabricated on a flexible polyester substrate where conventional screen-printing technology was employed to fix alcohol dehydrogenase to the carbon working electrode [29]. Ultra-thin porous carbon films are employed as transducers for amperometric biosensors [30]. Such foam-like nanoscopic films combine the advantage of high enzyme loadings within the micropore hosts and large microscopic area with a small geometric area [30]. Conductive boron-doped diamond thin films have been reported to have superior properties for electrochemical sensors, such as low background current, a wide potential window, high resistance to deactivation, and insensitivity to dissolved oxygen [31]. There seems to be no limit to the diversity and versatility of carbon materials, and it is certain that electroanalytical performance will improve as the nature and prepara- tion of such materials continues to be better understood. References 1. R.L.M. McCreery, Carbon Electrodes: Structural Effects on Electron Transfer Kinetics. In: A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 17, pp. 221-374. Marcel Dekker, New York, 1991. 2. C.A. Leon and L.R. Radovic, Interfacial Chemistry and Electrochemistry of Carbon Sur- faces. In: P.A. Thrower (Ed.), Chemistry and Physics of Carbon, Vol. 24, pp. 213-310. Mar- cel Dekker, New York, 1994. Electrochemical Functions 445 3. R.L.M. McCreery and K.K. Cline, Carbon Electrodes. In: P.T. Kissinger and W.R. Heineman (Eds.), Laboratory Techniques in Electroanalytical Chemistry, pp. 293-332. Marcel Dekker, New York, 1996. 4. H. Tohara, Electrochemistry, 66: 368, 1998. 5. S.E. Slein and D.M. Golden, J. Org. Chem., 42: 839, 1977. 6. M. Voll and H.P. Boehm, Carbon, 9: 481,1971. 7. H.P. Boehm, Angew. Chem. Int. Ed. Engl., 5: 537,1966. 8. J.O. Besenhard and H.P. Fritz, Angew. Chem. Int. Ed. Engl., 22 950,1983. 9. Y. Maeda, Y. Okamoto and M. Inagaki, J. Electrochem. SOC., 132 2369,1985. 10. R. Rice and R.L. McCreery, Anal. Chem., 61: 1637,1989. 11. M. Noel and R. Santhanam, J. Power Sources, 72: 53,1998. 12. M. Broussely, P. Biensan and B. Simon, Electrochim. Acta, 45: 3, 1999. 13. D. Aurbach, B. Mavkovsky, I. Weissman, E. Levi and Y. Ein-Eli, Electrochim. Acta, 45: 67, 14. S. Flandrois and B. Simon, Carbon, 37: 165,1999. 15. R.E. Winans and KA. Carrado, J. Power Sources, 54: 11,1995. 16. H. Momose, H. Honbo, S. Takeuchi, K. Nishimura, T. Horiba, Y. Muranaka, Y. Kozono 17. J. Aragane, K. Matsui, H. Andoh, S. Suzuki, H. Fukuda, H. Ikeya, K. Kitaba and R. 18. A. Nishino, J. Power Sources, 60: 137-147, 1996. 19. A. Yoshida, Electrochimistry, 66 884891,1998. 20. B.E. Conway, Electrochemical Super Capacitors: Scientific Fundamentals and Technolog- ical Applications. Kluwer AcademicPlenum Publishers, New York, pp. 183-229, 1999. 21. S. Yata, E. Okamoto, H. Satake, H. Kubota, M. Fujii, T. Taguchi and H. Kinoshita, J. Power Sources, 60 207,1996. 22. S.T. Mayer, R.W. Pekala and J.L. Kaschmitter, J. Electrochem. SOC., 140: 446,1993. 23. C. Lin, J.A. Ritter and B.N. Popov, J. Electrochem. SOC., 146 3639,1999. 24. C. Niv, E.K. Sichel, R. Hoch, D. Moy and H. Tennent, Appl. Phys. Lett., 70: 1480, 1997. 25. G.A. Evtugyn, H.C. Budnikov and E.B. Nikolskaya, Talanta, 46: 465,1998. 26. J. Anderson, L.A. Coury, Jr. and J. Leddy, Anal. Chem., 70: 519R, 1998. 27. A. Kitajima, T. Teranishi and M. Miyake, Electrochemistry, 69: 16,2001. 28. B. Chen, N-K. Guh, and L-S. Chia, Electrochim. Acta, 42: 595,1997. 29. J-K. Park, H-J. Yee, and S-T. Kim, Biosensors & Bioelectronics, 10 587, 1995. 30. J. Wang and Q. Chen, Anal. Chem., 66: 1988,1994. 31. T.N. Rau, I. Yagi, T. Miwa, D.A. Tryk and A. Fujishima, Anal. Chem., 71: 2506,1999. 1999. and H. Miyadera, J. Power Sources, 68: 208,1997. Ishikawa, J. Power Sources, 68: 13, 1997. 447 Chapter 27 Electric Double Layer Soshi Shiraishi Capacitors Department of Chemistry, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan Abstract: The electric double layer capacitance of porous carbon electrodes strongly depends on its pore size distribution. Mesoporous activated carbon fibers (ACFs), prepared by the carbonization and steam-activation of a phenolic resin fiber containing a small amount of activation catalyst, have many mesopores in addition to well-developed micropore structure. The double layer capacitance in propylene carbonate electrolytes was not proportional to the BET specific surface area for mesoporous ACFs. This is due to the low mobility of the ions in the narrow micropores. However, mesoporous ACFs showed higher double layer capacitance than conventional ACFs consisting of mainly micropores, especially, in the case of high current density measurements. This result suggests that the presence of mesopores promotes the formation of an effective double layer or the fast transfer of ions to the micropores. The property of the double layer capacitance for single walled carbon nanotubes (SWCNT) is also discussed from the viewpoint of a comparison with ACFs. Keywords: Capacitor, Electric double layer, Activated carbon fiber, Pore size distribution, Capacitance, Carbon nanotubes. 1 Introduction Electric double layer capacitor (EDLC) [1,2] is the electric energy storage system based on charge-discharge process (electrosorption) in an electric double layer on porous electrodes, which are used as memory back-up devices because of their high cycle efficiencies and their long life-cycles. A schematic illustration of EDLC is shown in Fig. 1. Recently, EDLCs have been proposed as the sub-power source for the hybrid electricvehicle because of its higher power density (larger than 1000 W kg-' or 1000 W I-') and fast charge-discharge ability. Because the energy density of EDLC is only several Wh kg-' or Wh 1-', much lower than that of rechargeable batteries, an improvement in the capacitance of EDLC is required. The energy density of EDLC can be expressed as follows: cv 2 E= 448 Chapter 27 Fig. 1. Schematic illustration of electric double layer capacitor: (a) charge state, (b) discharge state. where E is electric energy stored in the capacitor, C is capacitance, and Vis applied voltage. The above correlation suggests that stability to electrochemical decompo- sition of the electrolyte, that is the electrochemical window of the electrolyte, is a key factor in energy storage, because the energy of EDLC varies as the square of applied voltage. Therefore, EDLC with a non-aqueous electrolyte has essentially a higher energy density than the aqueous type EDLC because non-aqueous electrolytes have wider electrochemical windows (= 3 V) compared with aqueous electrolytes (= 1 V). The practical electrode materials for EDLC are porous carbons, such as activated carbons. The high capacitance (100-200 F g-') is derived from the high specific surface area (> 1000 m2 g-') in microporosity (pore width < 2 nm [3]). In general, it is believed that there is a proportional correlation between specific surface area and the electric double layer capacitance of activated carbons as based on the following equation [1,4,5]: where C is specific capacitance, E,, is permittivity in vacuum, E, is relative permittivity of the double layer, 6 is thickness of the double layer, and S is specific surface area. This equation (2) is derived from an ideal capacitor consisting of a solid dielectric layer between two parallel plate electrodes such as practical ceramic or film capacitors (condensers). However, some researchers have reported non-linearity of the double layer capacitance with surface area of carbon [6-141. This may be due to the dimensions of the ion or solvent in the electrolyte and the pore size distribution (PSD) of activated carbons. The PSD depends on the structure and type of carbon precursor and the preparation method of the porous carbon. Thus, the dependence of the double layer capacitance on PSD for activated carbon electrodes has to be investigated using various porous carbons with different pore size distribution. [...]... 3 Double Layer Capacitance of Other Carbon Materials 3.1 Other Porous Carbons Double layer capacitances have been studied of other mesoporous carbons such as carbon aerogel [29], carbon xerogel [9], porous carbon prepared with metal oxide template [12] and highly porous carbons derived from defluorination of perfluoropolymer [13] Higher capacitance of mesoporous carbons and enhancements at high rates... carbons with high densities such as the PVDC carbons will contribute practically to improvements in energy densities for EDLC if they also show higher volumetric capacitance in organic electrolytes 3.2 Carbon Nunotubes The EDLC performance of carbon nanotubes (CNTs) has attracted much attention since their discovery [31-381 In particular, single-walled carbon nanotubes (SWCNT) are expected as new carbon. .. 2000 469 Chapter 29 Gas Separations with Carbon Membranes Katsuki Kusakabe and Shigeharu Morooka Department of Applied Chemistry, Kyushu University,Fukuoka 812-8581, Japan Abstract: This chapter describes recent developments of carbon membranes for gas separations Carbon membranes are prepared by carbonizingvarious polymeric materials The molecular sieving carbon membranes produced under optimized... membranes at elevated temperature improved permeation properties Carbon membranes with 0.4-1.5 nm diameter pores were used to separate hydrocarbons or hydrogen sulfide from hydrogen by surface diffusion mechanisms Keywords: Membrane, Gas separation, Micropore, Permeation, Carbonization 1 Properties of Carbon Membranes Carbon membranes, prepared by the carbonization of polymeric membranes under optimized conditions,... distributions in activated carbons and of ion sizes in electrolytes to bring about improvements of double layer capacitance In the future, pore size distributions of porous carbon electrodesmust be carefully controlled to increase both high gravimetric capacitance (F g-’) and volumetric capacitance (F cm”) Additionally, the double layer capacitance of new porous carbons such as carbon nanotubes, carbon nanofibers,... 454-462, 1996 19 S Shiraishi, H Kurihara and A Oya, Carbon Science, 1: 133-137,2001 20 S Shiraishi, H Kurihara and A Oya, Electrochemistry, 69: 440-443,2001 21 S Shiraishi, H Kurihara and A Oya, Proceedings of Carbon '01,24.3,2001 22 K Kaneko, C Ishii, M Ruike and H Kuwabara, Carbon, 30: 1075-1088,1992 23 D Dollimore and G.R Heal, J Applied Chem., 14 109-1 14, 1964 24 M.M Dubinin and H.F Stoeckli, J Colloid... mesopores have been prepared by blending an activation catalyst into a phenolic resin [17] or an isotropic pitch [MI The blending method by which catalyst metal particles with nanometer size are uniformly dispersed in the carbon matrix is a topic for Carbon Alloys These ACFs with mesopores and macropores are suitable to investigate their influence on double layer capacitances In this chapter, double layer capacitances... Electrochem Solid-State Lett., 4, A5-A8,2001 Electric Double Layer Capacitors 457 14 G Salitra, A Soffer, L Eliad, Y Cohen and D Aurbach, J Electrochem SOC., 147 : 24862493,2000 15 M Suzuki, Carbon, 32: 577-586,1994 16 S.K Ryu, High Temperature-High Pressures, 2 2 345-354,1990 17 A Oya, S Yoshida, J Alcaniz-Monge and A Linares-Solano, Carbon, 33: 1085-1090, 1995 18 H Tamai, K Kakii, Y Hirota, T Kumamoto, and... electrolyte and the high mobility of ions in pores Especially, it is surprising that the EDLC from a carbon aerogel has a gravimetric power density of 7.5 k kg-' [29] On the other hand, mesoporous carbons such as carbon aerogels W generally have low bulk densities, so the electrodes composed of mesoporous carbons as well as the mesoporous ACFs all have low bulk densities The lower electrode density usually... 593-598,2002 459 Chapter 28 Field Electron Emissions from Carbon Nanotubes Yahachi Saito”,Koichi Hata”and Sashiro Uemura” “Departmentof Electrical and Electronic Engineering, M e University, Tsu 514- 850ir.Japan i hIseElectronics Cop., Ise 516-1103, Japan Abstract: Field emission microscopy (FEM) has been used to investigate the field emission of electrons from carbon nanotubes (CNTs) and related fibers, viz., . Other Carbon Materials 3.1 Other Porous Carbons Double layer capacitances have been studied of other mesoporous carbons such as carbon aerogel [29], carbon xerogel [9], porous carbon. batteries with a controlled carbon microstructure is a research target for the Carbon Alloys project. Several research groups are actively engaged in this Carbon Alloys project. Of these,. chemical treatment of the carbon [14] . Boron is known to enter into the carbon lattice by substitution. For example, by CVD from benzene and boron trichloride, carbons containing up to 25