Although fuel cells work efficiently with hydrogen as a fuel there are still some major problems related to hydrogen availability and acceptability. In particular, it is still extremely difficult to obtain clean/renewable hydrogen. In the short to medium term future, the only realistic fuels for fuel cells are hydrocarbon based, especially natural gas. In this article, we address the optimal use of hydrocarbon fuels in high temperature, solid oxide fuel cells. In conventional solid oxide fuel cells, the hydrocarbon fuel is typically reformed with large quantities of steam over nickel externally to the fuel cell. Internal reforming is a more attractive process that can occur at the Nickel/zirconia cermets that are generally used as fuel electrodes. The various complicated steps that occur during this process are reviewed and discussed, paying particular attention to the hydrocarbon cracking reactions that often occur at low steam levels. There are a number of new materials, such as those based on lanthanum chromate, that offer improved resistance to hydrocarbon cracking at low steam partial pressures. Of particular relevance; is the concept of gradual internal reforming, whereby steam generated during the initial oxidation reaction is used to drive further reforming reactions. A further process, direct oxidation can also occur at low steam partial pressures. This process offers the ultimate in thermodynamic efficiency, almost 100 percent theoretically, which renders it very attractive. Some very good success has been reported with copper ceria electrodes, with direct oxidation claimed at temperatures up to 700 °C. The performance of a number of new oxide electrodes with perovskite, fluorite and related structures is reviewed. A particularly important conclusion is the importance of mixed conductivity for optimising catalytic performance.
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Irvine et al. (2001) studied this question.