New carbon materials

Carbon blacks with an appropriate pre-treatment for functionalization of the surface have been shown as the most suitable support for PEMFG catalysts in practical applications. Nevertheless, there are several barriers that the use of this kind of support cannot resolve  

Corrosion problems from the different issues related to the durability of the MEAs in the PEMFCs, corrosion of the carbon support is one of the most significant in terms of improving the stability of the electrodes. Carbon blacks are relatively stable with respect to other kind of carbons, but still their behaviour is not ideal for long-term use of FCs. 

Reduction of noble metal loading a significant reduction has been achieved since the late 1990s, but an approximately fivefold reduction of the amount of platinum is required for large-scale application, due to both cost and Pt supply considerations. This objective seems to be difficult to attain by using carbon blacks as support. This is particularly important for the cathode, since the limitation on the kinetics of the ORR requires higher loadings of Pt. 

With the objective of dealing with these issues, the use of alternative carbon materials as supports for electrocatalysts has been investigated. Corrosion problems could be the most important limitation for commercialization of FCs, since there are several limits of durability that should be guaranteed for practical use. Carbon is electrochemically unstable at potentials above 0.207 V in acidic electrolyte  

Carbon can be also consumed by the heterogeneous water-gas reaction  

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A classic definition of electrochemical ultracapacitors or supercapacitors summarizes them as devices, which store electrical energy via charge in the electrical double layer, mainly by electrostatic forces, without phase transformation in the electrode materials. Most commercially available capacitors consist of two high surface area carbon electrodes with graphitic or soot-like material as electrical conductivity enhancement additives. Chapter 1 of this volume contains seven papers with overview presentations, and development reports, as related to new carbon materials for this emerging segment of the energy market. 

In the last paper, A. Lewandowski et al. of Poland, examines the role of ionic liquids as new electrolytes for carbon-based supercapacitors. Although not directly addressing the role of new carbon materials (the area of major focus of this book), this interesting theoretical work seeks to optimize electrolyte media, which is in contact with carbon electrodes. 

The future remains bright for the use of carbon materials in batteries. In the past several years, several new carbon materials have appeared mesophase pitch fibers, expanded graphite and carbon nanotubes. New electrolyte additives for Li-Ion permit the use of low cost PC based electrolytes with natural graphite anodes. Carbon nanotubes are attractive new materials and it appears that they will be available in quantity in the near future. They have a high ratio of the base plane to edge plain found in HOPG. The ultracapacitor application to deposit an electronically conductive polymer on the surface of a carbon nanotube may be the wave of the future. 

M. Inagaki and Hishiyama, New Carbon Material, Gohodo, (1994) (Japanese). 

Graphite intercalation compounds are prospective precursors of the new carbon materials, which are in high demand by the market today. 

T. H. Ko, Y. K. Liao, and C. H. Liu. Effects of graphitization of PAN-based carbon fiber cloth on its use as gas diffusion layers in proton exchange membrane fuel cells. New Carbon Materials 22 (2007) 97-101. 

The synthesis of nanostructured carbon using aliphatic alcohols as selfassembling molecules has demonstrated that this strategy can be extended beyond metal oxide-based materials [38]. Recently, we have reported the synthesis of a novel carbon material with tunable porosity by using a liquid-crystalline precursor containing a surfactant and a carbon-yielding chemical, furfuryl alcohol. The carbonization of the cured self-assembled carbon precursor produces a new carbon material with both controlled porosity and electrical conductivity. The unique combination of both features is advantageous for many relevant applications. For example, when tested as a supercapacitor electrode, specific capacitances over 120 F/g were obtained without the need to use binders, additives, or activation to increase surface area [38]. The proposed synthesis method is versatile and economically attractive, and allows for the precise control of the structure. 

N. Ohta, H. Nozaki, K. Nagaoka, K. Hoshi, T. Tojo, T. Sogabe, and M. Inagaki, New Carbon Materials... 

M. Inagaki and Hishiyama, New Carbon Material, Gohodo, (1994) (Japanese). Denchigijutu, (No 6) 1994-(No. 10) 1999 Battery Committee Japan (Japanese). 

A new carbon material, Tetracarbon , was developed on the basis of sp -hybridized highly oriented carbon films. Figure 11.1(g) indicates that Tetracarbon consists of densely packed linear chains of sp -hybridized carbon, oriented perpendicularly to the substrate surface. An analogy could be velvet, where linear carbon chains project perpendicularly to the substrate surface. 

On the basis of the TEM, AFM, STM, and Raman spectroscopy results the atomic structure of the new carbon material is found to correspond to two-dimensional ordered linear sp -hybridized carbon. 

According to the proposed model the new carbon material has a multilayered structure consisting of densely packed linear carbon chains. The chains are connected with each other by correlated chain bends. 

The results on examination of the electric conductivity and cold emission, as well as the mechanical and biological properties of the new carbon material, clearly show it can be employed in a broad field of applications, such as microelectronics, machine-building, etc. The excellent biological properties of Tetracarbon make it the best material for many applications in medicine. 

Ordered mesoporous carbons (OMCs) are new carbon materials that were developed over the last ten years. Their mesopores have a defined width with a very narrow pore size distribution. This sets them aside from older nanoporous carbons, such as activated carbons or activated carbon fibers. The last two classes of carbons are produced from various carbon-containing materials by carbonization followed by partial oxidation (activation). To a certain degree, the pore structure of these materials can be controlled by the carbonization and activation conditions. However, it is not possible to produce purely mesoporous activated carbons or activated carbon fibers. Furthermore, these materials generally exhibit a broad pore size distribution [1, 2]. 

Nevertheless, it is hardly possible to assess the development within the last two decades and to perceive its impact on the multitalented carbon s perspectives in chemistry, material science, and physics, without an understanding of its long-known modifications, mainly graphite and diamond. Hence, the first chapter summarizes the essential facts on the classical modifications and their properties, for only a solid comprehension of basic concepts and principles enables us to understand the properties of new carbon materials and to develop new ideas. 

Proceeding from the facts presented in this chapter, we will now discuss the new carbon materials, starting with the fullerenes. These have first been discovered in 1985, many years after their theoretical prediction. 

Carbon nanotubes are one of the most important classes of new carbon materials. Distinctions are made between single- and multiwalled as well as between zig-zag, armchair, and chiral nanotubes. The structure is characterized by the descriptors n and m. These structural parameters allow for a prediction of the electric conductivity. Only armchair nano tubes n,n) and such species with m-n = iq are electric conductors. Any other nanotube is semiconducting. These statements have been established from symmetry considerations and from determining the band structure by way of the zone-folding method. There are different approaches to the production of single- and multiwalled nanotubes. Important methods of preparation are ... 

After having made the acquaintance of fuUerenes and of single- and multiwalled carbon nanotubes, the question arises on the existence of multiwalled fullerenes. Such carbon cages concentrically arranged one inside another are also called carbon onions. In comparison to other new carbon materials, they have by far been studied less. Chiefly this is because only small amounts of those are available. Still they represent an interesting structural variant of carbon. This chapter deals with their structure, different methods of preparation, and first results regarding their properties. 

Contrary to the new carbon materials presented so far, diamond films are a product that is already being employed with many variants in large scale applications. Especially the coating of tools and fast turning parts represents a considerable market, but there are also electronic uses and synthetic optical windows on offer. Some of the important fields of appHcation are presented in more detail below. 

Since the discoveiy of carbon nanotube (CNT) in the beginning of the 1990 intensive research has been conducted to qualify its electric conductivity and mechanical properties for use in several industrial applications. These new carbon materials are actually synthesized by using different processes such as... 

Two important classes of new carbon materials, carbon nanotnbes/nanofibers and carbon aerogels/xerogels/cryogels are reviewed and discnssed in the next two chapters. These materials exhibit interesting properties that can be exploited in many applications, particnlarly in catalysis. 

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