Carbon microtexture

The pores of the silica template can be filled by carbon from a gas or a liquid phase. One may consider an insertion of pyrolytic carbon from the thermal decomposition of propylene or by an aqueous solution of sucrose, which after elimination of water requires a carbonization step at 900°C. The carbon infiltration is followed by the dissolution of silica by HF. The main attribute of template carbons is their well sized pores defined by the wall thickness of the silica matrix. Application of such highly ordered materials allows an exact screening of pores adapted for efficient charging of the electrical double layer. The electrochemical performance of capacitor electrodes prepared from the various template carbons have been determined and are tentatively correlated with their structural and microtextural characteristics. 

Figure 1 presents the plot of the BET specific surface area vs the irreversible capacity measured for graphite samples milled in different atmospheres and sometimes post-treated by pyrolytic carbon deposition. The experimental values are quite scarce and, contrarily to several claims [7-9], there is not any linear dependence between these two parameters. It seems that the linearity would exist only for samples from the same family with comparable microtextures. 

Disordered carbons usually exhibit a multiscale organization (structure, microtexture, texture)4. Structurally, they are made of more or less distorted polyaromatic layers, nanometric in size. The spatial association or the layers, from the nanometric to the micrometric scales, gives rise to different microtextures (lamellar, porous, concentric, fibrous, etc.) forming the carbons skeleton4. The multiscale organization is the fingerprint of the kind of precursor and of the formation conditions (temperature, pressure, strains, time, etc.) met either in laboratory experiments or in Nature, and is directly related with numerous properties. 

Carbon is unique among chemical elements since it exists in different forms and microtextures transforming it into a very attractive material that is widely used in a broad range of electrochemical applications. Carbon exists in various allotropic forms due to its valency, with the most well-known being carbon black, diamond, fullerenes, graphene and carbon nanotubes. This review is divided into four sections. In the first two sections the structure, electronic and electrochemical properties of carbon are presented along with their applications. The last two sections deal with the use of carbon in polymer electrolyte fuel cells (PEFCs) as catalyst support and oxygen reduction reaction (ORR) electrocatalyst. 

Concerning the microtextures of carbons derived from petroleum products, we find that a range of intermediate textures exist, between non-graphitizing and graphitizing carbons. The mechanism of graphitization is important to understand because the worth of carbonaceous products, among which are refinery residues, is a function of the ability to graphitize. 

Finally, the determination of both microtexture and graphitization degree is useful to explain, and even to predict, the physical properties of carbons (32). 

On the basis of the results of investigations on a large number of commercial carbon blacks synthesized under various conditions it was established [60] that the microtexture of all the materials could be described in terms of the characteristics represented in Fig. 1. Depending on the production method of a material its parameters L, Li, and y may vary. A decrease in y and an increase in L give evidence for the ordering of the particle structure. Figure 2(a) displays a schematic view of a section of a carbon black particle, where individual crystallites are visible [60]. The surface of each of the crystallites visible in this figure has a turbostratic structure. 

FIG. 4 Microtextural particularities of the structure of microporous activated carbons heat-treated at a low temperature (a) and partially graphitized (b) in compliance with the data of Ref. 74. 

Characterization of Structure and Microtexture of Carbon Materials by Magnetoresistance Technique, Yoshihiro Hishiyanui, Yiitaka Kahiiragi, and Michio Inagaki... 

Toupin M., Brousse T., Belanger D. Influence of microtexture on the charge storage properties of chemically synthesized manganese dioxide, Chem Mater 2002 14 3946-52. Wu N.L. Nanocrystalline oxide supercapacitors. Mater Chem Phys 2002 75 6-11. Frackowiak E., Metenier K., Bertagna V., Beguin F. Supercapacitor electrodes from multiwalled carbon nanotubes. Appl Phys Lett 2000 77 2421-3. 

Yoshida, A., Kaburagi, Y., and Hishiyama, Y. (1991). Microtexture and magnetoresistance of glass-like carbons. Carbon, 29, 1107-11. 

Rouzaud, J.N. and Oberlin, A. (1989). Structure, microtexture, and optical properties of anthracene and saccharose-based carbons. Carbon, 27, 517-29. 

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