Ceria-based anodes for the direct oxidation of methane in solid oxide fuel cells

In this study the performance of CeO2 and Rh supported on CeO2 as anodes in solid oxide fuel cells (SOFC) was investigated. Experiments were conducted using a model SOFC consisting of an electrolyte disk of yttria-stabilized zirconia with thin films of samaria-doped ceria as both the anode and cathode. The current-voltage characteristics of the cell were measured for H-2, CO, and CH4 fuels as a function of the thickness of the CeO2 anode and the Rh loading. For H-2 as the fuel, it was found that the cell performance was largely independent of the anode design, suggesting that, for this fuel, reaction on the anode was not limiting. In contrast, for CH4, it was observed that the maximum current density produced by the cell was highly dependent on both the CeO2 film thickness and the Rh loading. This suggests that for CH4 the catalytic properties of the anode are important for good performance. Since during the CH4 experiments only negligible amounts of H2O were produced (the fractional conversion was maintained below 10(-5)), this study also demonstrated that it is possible to oxidize CH4 electrochemically in a SOFC without prior steam reforming. The implications of these results to the design of practical SOFCs are discussed.