| The direct operation of solid oxide fuel cells (SOFCs) on hydrocarbon fuels is desired since it could reduce power plant size, weight and complexity. The primary challenge is to find effective means through which anode-coking could be suppressed or avoided. Throughout the research, conventional Ni-anode supported SOFCs were employed because they provide high power densities and are being actively developed for commercial applications. Various strategies were used to reduce or avoid anode-coking during the SOFC operation.; Firstly, air or CO2/H2O was added to hydrocarbon fuels, such that coking was less thermodynamically favorable, and the resulting internal partial oxidation or dry/steam reforming reactions provided H 2 and CO to the fuel cell. For example, for low hydrocarbons like propane, coke-free operation was achieved on 8% yttrium-stabilized zirconia (YSZ) electrolyte SOFCs via internal partial oxidation, yielding stable and high power densities, e.g. 0.7 W·cm-2 at 790°C.; Secondly, a novel design for hydrocarbon fueled SOFCs was proposed, i.e. a separate supported catalyst (Ru-CeO2) layer was placed against the anode side. The catalyst layer provided good catalytic activity for the hydrocarbon reforming reactions, while the nickel-based anode was retained to provide excellent electrochemical activity for the oxidation of the hydrogen and carbon monoxide reforming products. For heavy hydrocarbons like iso-octane, the catalyst layer was crucial far allowing stable cell operation without coking. The lack of coking at the Ni-YSZ anode can be explained by reforming at the Ru-Ceria catalyst layer, which eliminated most of the hydrocarbon species before the fuel reached the anode. A key element of this strategy was the choice of a catalyst metal, Ru, that promotes hydrocarbon reforming but does not itself cause coking.; Thirdly, reduced-temperature SOFCs with thin samarium-doped Ceria (SDC) electrolytes were developed; these devices have potentially improved stability since the coking rate is significantly reduced at low temperatures. The SDC electrolyte SOFCs were successfully operated on propane, iso-octane and methanol via internal partial oxidation and/or dry reforming at 400--600°C, with or without the catalyst layer. This low operating temperature promises faster thermal cycling and less thermal energy in heatup, thus making SOFCs more amenable to portable and transportation applications. |
