Role of hydrocarbon deposits in the enhanced performance of direct-oxidation SOFCs

We have examined the changes that occur in the performance of solid oxide fuel cells (SOFCs) with Cu-ceria-yttria-stabilized zirconia anodes at 973 K following exposure to various hydrocarbon fuels, including methane, propane, n-butane, n-decane, and toluene. For cells with Cu contents of 20 wt % or less, large increases were observed in the power densities for operation in H-2 after the anode had been exposed to any of the hydrocarbons except methane. The increased performance is completely reversible upon oxidation of the anode and subsequent reduction in H-2. The enhancement decreases with increasing Cu content, implying that the deposits improve the connectivity of the metallic phase in the anode. Impedance spectra taken on cells before and after exposure to hydrocarbon fuels confirm that the conductivity of the anode improves after exposure. Temperature-programmed oxidation and weight changes were used to show that the deposits that enhance performance correspond to similar to1 wt % of the anode and are probably not graphitic. Measurements of the open-circuit voltages in hydrocarbon fuels suggest that equilibrium is established with partial oxidation products and that the chemical structure of the deposits change upon current flow. Finally, the implications of these results for operation of SOFC on hydrocarbons without added steam and with low copper contents are discussed. (C) 2003 The Electrochemical Society.