Direct electrocatalytic oxidation

This chapter starts by discussing the range of possible fuels for SOFCs a brief discussion on the possibility of using renewable fuels in SOFCs is also included. The remainder of the chapter is devoted to approaches in fuel processing in SOFCs, and some of the issues and problems inherent in such fuel processing. The possibility of direct electrocatalytic oxidation of the hydrocarbon fuels at the anode is also discussed. The chapter concludes with a brief consideration of future prospects. 

The major problem with direct electrocatalytic oxidation of the hydrocarbon fuel at the anode is the marked tendency towards carbon deposition via hydrocarbon decomposition (Eqs. (1) and (2)). It is extremely difficult to avoid carbon deposition in the absence of a co-fed oxidant. However, some recent studies have reported anodes which show considerable promise for the direct electrocatalytic oxidation of hydrocarbons [29,68,69]. The conditions under which these anodes can be used may present problems for their widespread application, whilst their long-term durability with respect to carbon deposition must be established. Electrically conducting oxides have also been proposed in recent years as having potential for use as anodes for the direct electrocatalytic oxidation of hydrocarbons [67.70,74]. 

Recent studies have also identified some alternative anodes, one being copper-based and incorporating significant quantities of ceria in addition to YSZ [68,114] and the other adding yttria-doped ceria to nickel and YSZ [69], both of which have been reported to show considerable promise for the direct electrocatalytic oxidation of the hydrocarbon fuels, without the need for any co-fed oxidant. However, the conditions under which such anodes can be used for direct hydrocarbon oxidation may be a problem for their widespread application, whilst their long-term performance in terms of deactivation resulting from carbon deposition remains to be investigated. 

The electrocatalytic oxidation of methanol has been widely investigated for exploitation in the so-called direct methanol fuel cell (DMFC). The most likely type of DMFC to be commercialized in the near future seems to be the polymer electrolyte membrane DMFC using proton exchange membrane, a special form of low-temperature fuel cell based on PEM technology. In this cell, methanol (a liquid fuel available at low cost, easily handled, stored, and transported) is dissolved in an acid electrolyte and burned directly by air to carbon dioxide. The prominence of the DMFCs with respect to safety, simple device fabrication, and low cost has rendered them promising candidates for applications ranging from portable power sources to secondary cells for prospective electric vehicles. Notwithstanding, DMFCs were... 

C. Lamy, E. M. Belgsir, and J.-M. Leger, Electrocatalytic oxidation of aliphatic alcohols Application to the direct alcohol fuel cell (DAFC), J. Appl. Electrochem. 31, 799-809 (2001). 

Oxidation of Alcohols in a Direct Alcohol Fuel Cell The electrocatalytic oxidation of an alcohol (methanol, ethanol, etc.) in a direct alcohol fuel cell (DAFC) will avoid the presence of a heavy and bulky reformer, which is particularly convenient for applications to transportation and portable electronics. However, the reaction mechanism of alcohol oxidation is much more complicated, involving multi-electron transfer with many steps and reaction intermediates. As an example, the complete oxidation of methanol to carbon dioxide ... 

Fig. 35 Cyclic voltammograms for the electrocatalytic oxidation of NADPH by 2 10 M P2W16V2, in pH 8 buffer (50 mM TRIS + 0.5 M Na2S04 + H2SO4) the scan rate was 2 mV s , the working electrode was glassy carbon (3 mm diameter disk), the reference electrode was SCE. (a) The excess parameter values for NADPH were y = 10 and y = 20 respectively (b) Comparison of the catalytic process (y = 20) by P2W16V2 with the direct oxidation of NADPH on the glassy carbon electrode (taken from Ref 185).

This section addresses the role of chemical surface bonding in the electrochemical oxidation of carbon monoxide, CO, formic acid, and methanol as examples of the electrocatalytic oxidation of small organics into C02 and water. The (electro)oxidation of these small Cl organic molecules, in particular CO, is one of the most thoroughly researched reactions to date. Especially formic acid and methanol [130,131] have attracted much interest due to their usefulness as fuels in Polymer Electrolyte Membrane direct liquid fuel cells [132] where liquid carbonaceous fuels are fed directly to the anode catalyst and are electrocatalytically oxidized in the anodic half-cell reaction to C02 and water according to... 

Jones, Anne K. Sillery, Emma Albracht, Simon P. J. Armstrong, Fraser A. Direct comparison of the electrocatalytic oxidation of hydrogen by an enzyme and a platinum catalyst. Chemical Communications (Cambridge, UK) 2002 (8) 866-867. 

Platinum, ruthenium and PtRu alloy nanoparticles, prepared by vacuum pyrolysis using Pt(acac)2 and Ru(acac)3 as precursors, were applied as anode catalysts for direct methanol oxidation . The nanoparticles, uniformly dispersed on multiwaUed carbon nanotubes, were all less than 3.0 nm in size and had a very narrow size distribution. The nanocomposite catalysts showed strong electrocatalytic activity for methanol oxidation, which can... 

As mentioned above, direct methanol oxidation and reformate tolerance represent two very challenging but significantly different electrocatalytic issues. This is despite the fact that poisoning by CO (or similar Ci moieties) is one of the critical aspects for both fuels. Binary catalysts such as PtSn, PtMo or PtRu offer superior performance but the precise reason for this is not known. At least 3 different mechanisms have been proposed, whereby the alloying M element ... 

The 3 H2 provides six electrons on electrocatalytic oxidation, the same as produced by a hypothetical direct oxidation ... 

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