Electrode reactions, fuel cells platinum surface

The major limitation of present fuel cells is that the rates of the electrode reactions, especially the one in which oxygen is reduced, tend to be very small, and thus so is the output current per unit of electrode surface. Coating the electrode with a suitable catalytic material is almost always necessary to obtain usable output currents, but good catalysts are mostly very expensive substances such as platinum, so that the resulting cells are too costly for most practical uses. There is no doubt that if an efficient, low-cost catalytic electrode surface... 

Anodic reactions at Pt have been claimed to be dependent upon the surface state of the platinum. The Kolbe reaction is perhaps the best known case (for a review, see Conway and Vijh, 1967) for which a change in the surface composition has been held responsible and indeed necessary for the reaction to occur. Thus, at a low potential, < 0-8 V, acetate in aqueous solution is completely oxidized to carbon dioxide and water on pure platinum sites (i.e. we have in effect a fuel cell electrode). On raising the potential, PtO and adsorbed oxygen begin to cover the surface and oxygen evolution takes place in the range between 1-2- 1-8 V. A further increase in the... 

In the quest to improve fuel cell performance, the concept of fuel cell reactions requiring a three-phase interface was first proposed by Grove. In his initial experiment, he noticed that the reaction sped up when the three-phase area was large. In 1923, Schmid [7] developed the first gas diffusion electrode, which significantly increased the electrode active surface area and revolutionized fuel cell electrodes. The electrode contained a coarse-pore graphite gas-side layer and a fine porous platinum electrolyte layer. 

It is well known that catalyst support plays an important role in the performance of the catalyst and the catalyst layer. The use of high surface area carbon materials, such as activated carbon, carbon nanofibres, and carbon nanotubes, as new electrode materials has received significant attention from fuel cell researchers. In particular, single-walled carbon nanotubes (SWCNTs) have unique electrical and electronic properties, wide electrochemical stability windows, and high surface areas. Using SWCNTs as support materials is expected to improve catalyst layer conductivity and charge transfer at the electrode surface for fuel cell oxidation and reduction reactions. Furthermore, these carbon nanotubes (CNTs) could also enhance electrocatalytic properties and reduce the necessary amount of precious metal catalysts, such as platinum. 

The power available is limited by the relatively slow reduction of oxygen at the cathode surface, O2 + 4H" + 4e 2H2O this problem exists with any fuel cell that uses an oxygen electrode. At present, platinum seems to be the best catalyst, but even platinum is not nearly as good as we would like. The rate of the anodic reaction, H2 2H +2e, the oxidation of hydrogen at the platinum surface, is relatively rapid. However, it would be nice if we could use something less expensive than platinum as a catalyst. At higher temperatures, the reaction rates are faster and the cell performance is better. 

For example, mechanism 4 can be applied to hydrocarbon-oxygen fuel cells. The potential of the electrode, at which ethylene or any other organic compound is anodically oxidized, is momentarily increased to a value near that of oxide formation of the metal 101) (e.g., 0.9-1.0 volt vs NHE for a platinum electrode). The activation process may be explained on the basis that an intermediate, formed by the partial oxidation of the hydrocarbon, tends to accumulate on the surface with time and retards the reaction but is rapidly removed from the surface by oxidation to carbon dioxide by momentarily increasing the potential. An acceleration of the reaction results. The gains in power output are 50-100%. 

In this chapter, we provide a succinct review of some of the advances in the development and application of ab initio methods toward understanding the intrinsic reactivity of the metal and the influence of the reactive site and its environment. We draw predominantly from some of our own recent efforts. More specifically we describe (a) the chemistry of the aqueous-phase on transition metal surfaces and its influence on the kinetics and thermodynamics within example reaction mechanisms, and (b) computational models of the electrode interface that are able to account for a referenced and tunable surface potential and the role of the surface potential in controlling electro-catalytic reactions. These properties are discussed in detail for an example reaction of importance to fuel cell electrocatalysis methanol dehydrogenation over platinum(ll 1) interfaces [24,25]. 

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