Carbon nanotexture

Electrophysical properties of quasi-graphitic carbons are directly related to their structure and chemical state of their surface [34,80,81,83-87]. Hence, the above conclusions [71,82] bring one to the understanding of the effect of bulk (i.e., substructural) properties of the carbon matrix on the state of the supported metal. Thus, the electrophysical approach to describing the stability of supported metal particles to coalescence is based on the collective effect of substructural properties of carbons, nanotexture, and the chemical state of their surface. 

High Resolution Transmission Electron Microscopy (HRTEM, Philips CM20, 200 kV) was applied to get structural and nanotextural information on the fibers, by imaging the profile of the aromatic carbon layers in the 002-lattice fringe mode. A carbon fiber coated with pyrolytic carbon was incorporated in epoxy resin and a transverse section obtained by ultramicrotomy was deposited on a holey carbon film. An in-house made image analysis procedure was used to get quantitative data on the composite. 

The HRTEM observation of the cross section of a coated fiber showed that the core is constituted of aromatic layers highly misoriented, whereas they are preferentially oriented in parallel for the thin coating pairs of stacked layers form mainly Basic Structural Units (BSUs) in which the average interlayer distance is smaller than between the aromatic layers in the bulk of the fiber. Since the nanotexture is more dense for the pyrolytic carbon than for the fiber itself, it acts as a barrier which prevents the diffusion of the large solvated lithium ions to the core of the fiber, allowing the passivation layer to be less developed after this treatment. Hence, the major amount of lithium inserted is involved in the reversible contribution therefore this composite material is extremely interesting for the in-situ 7Li NMR study of the reversible insertion. 

As it will be explained below, the nanotexture of carbon materials is principally established during the carbonization process. Moreover, nanotexture has determining effects on the structural changes at high temperatures and also on the properties of carbons. Therefore, carbonization is the most important process for producing carbon materials. 

Cokes are also the product of liquid-phase carbonization of pitches, as mentioned above. The nanotexture of the resultant cokes can be changed by applying a shear stress during liquid-phase carbonization, giving the so-called needle-like cokes, which are now important raw materials in the production of large-sized graphite electrodes for metal refining [93],... 

The recent developments in science and technology require a more exact control of structure/ nanotexture and properties of various materials, including carbon materials. In order to meet the requirements for carbon materials, various novel carbonization processes have been proposed. In relation to electrochemistry, the following processes have to be mentioned template method, polymer blend method, defluorination of fluorinated hydrocarbons, and carbonization of organic aerogels [99],... 

FIGURE 2.27 Classification of nanotexture for carbon materials in graphite family. 

An axial orientation of the layers is found in various fibrous carbon materials, in other words, a fibrous morphology is possible because of this axial orientation scheme. In carbon fibers, the coaxial and radial alignments of the layers along the reference axis (i.e., fiber axis) are possible (Figure 2.27). In Figure 2.31, the variety in the nanotexture of different carbon fibers is illustrated [129] and examples are shown in Figure 2.32. 

FIGU RE 2.31 Nanotextures in the sections along and perpendicular to the fibre axis of various carbon fibers. 

FIGURE 2.33 Schematic illustration of nanotextures in carbon nanofibers. 

FIGU RE 2.37 Models of nanotexture for small-sized carbon black, mesophase sphere, and carbon spherule. 

Various treatments, such as high-temperature treatment, intercalation of different foreign species, and doping by boron and nitrogen, are frequently applied to carbons in order to modify and improve their nanotexture and properties. 

FIGURE 2.39 (a) 002 lattice fringe image of sugar coke and (b) nanotexture model for glass-like carbon. 

FIGURE 2.40 Influence of HTT on the interlayer spacing d002 and crystallite size La of carbon materials with different nanotextures. 

In the case of intercalation reactions, the nanotexture of the host carbon materials has a strong effect. The intercalation of sulfuric acid into natural graphite can proceed at room temperature in concentrated sulfuric acid with a small amount of oxidant, such as nitric acid. The resulting intercalation compound is commonly used in industry for the preparation of exfoliated graphite [32], However, in order to intercalate sulfuric acid into carbon fibers, electrolysis is needed [145,146], Potassium as a vapor, on the other hand, can be easily intercalated in various carbon materials, even in low-temperature-treated carbon fibers [147],... 

FIGURE 2.41 Scanning electron micrographs and structural parameters of mesophase-pitch-based carbon fibers with different cross-sectional nanotextures. 

In this chapter, the structures and textures of carbons at different scales are explained. The carbon materials are classified into four families, diamond, graphite, fullerene, and carbyne on the basis of hybridized sp3, flat sp2, curved sp2, and sp orbitals used, respectively. Each family has its own characteristic diversity in structure and also in the possibility of accepting foreign species. The formation of these carbon materials from organic precursors (carbonization) is shortly described by dividing the process into three phases (gas, solid, and liquid), based on the intermediate phases formed during carbonization. The importance of nanotexture, mainly due to the preferred orientation of the anisotropic BSU in the graphite family, i.e., planar, axial, point, and random orientation schemes, is particularly emphasized. 

Inagaki, M. and Kang, F. Nanotexture development in carbon materials. In Carbon Materials Science and Engineering, Beijing, China Tsinghua University Press, 2006 47. 

Novel, inexpensive synthesis routes for producing materials with precisely controlled nanotexture must be developed to improve the performance of batteries and electrochemical capacitors, as well as to enable new electrochemical applications of carbons. Two alternatives, carbide-derived carbon (CDC) and templated carbon, have shown a promise to offer the requisite control necessary to push device performance to the next level and will be explored in this chapter. 

As first shown by Boehm in 1975 [41] on CDC synthesized from TaC and SiC, and later for most carbide precursors [33], the resulting carbons have type I isotherms in the Brunauer classification, which are indicative of microporous carbon having pore sizes less than 2nm and relatively high surface areas up to 2000m2/g [37,39,42], The pore size of CDC can be tailored by the selection of carbide precursors with different spatial distributions of carbon atoms in the initial carbide lattice, changing the nanotextural ordering in the CDC by varying the synthesis temperature, and posttreatment in a... 

Frackowiak, E., Lota, G., Machnikowski, J., Vix-Guterl, C., and Beguin, F. Optimization of supercapacitors using carbons with controlled nanotexture and nitrogen content. Electrochim. Acta. 51, 2006 2209-2214. 

Beguin, F. and Frackowiak, E. Nanotextured carbons for electrochemical energy storage. In Nanomaterials Handbook, edited by Y. Gogotsi, pp. 713-737. Boca Raton, FL CRC Press, 2006. 

In order to satisfy the industrial demand, the performance of supercapacitors must be improved and new solutions should be proposed. The development of new materials and new concepts has enabled important breakthroughs during the last years. In this forecast, carbon plays a central role. Due to its low cost, versatility of nanotextural and structural properties, high electrical conductivity, it is the main electrode component. Nanoporous carbons are the active electrode material, whereas carbon blacks or nanotubes can be used for improving the conductivity of electrodes or as support of other active materials, e.g., oxides or electrically conducting polymers. 

This chapter intends to discuss the fundamental role played by carbons, taking particularly into account their nanotexture and surface functionality. The general properties of supercapacitors are reviewed, and the correlation between the double-layer capacitance and the nanoporous texture of carbons is shown. The contribution of pseudocapacitance through pseudofaradaic charge transfer reactions is introduced and developed for carbons with heteroatoms involved in functionalities able to participate to redox couples, e.g., the quinone/hydroquinone pair. Especially, we present carbons obtained by direct carbonization (without any further activation) of appropriate polymeric precursors containing a high amount of heteroatoms. 

Many forms of carbon materials do exist with a continuous variation in the structure/nanotexture and properties between the graphite (sp2 hybrid orbitals) and diamond (sp3 hybrid orbitals) limiting structures. 

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