The effect of current density on H 2S-poisoning of nickel-based solid oxide fuel cell anodes

Abstract

Sulphur-containing impurities can have a damaging impact on nickel-based SOFC anode performance even at sub-ppm concentrations, but the electrochemical mechanism of this interaction is not fully understood. In this work, three-electrode cells of Ni-Ce 0.9Gd 0.1O 1.95/YSZ/(La 0.8Sr 0.2)MnO 3− x have been used to obtain new electrochemical data on the sulphur poisoning behaviour of Ni-based SOFC anodes operating at different current densities in the temperature range 700–750 °C. The three-electrode arrangement enabled direct measurement of anode overpotential, with concurrent impedance measurement to provide detail into the electrochemical processes occurring at the anode during sulphur poisoning. The initial, stepwise degradation on exposure to 0.5 ppm H 2S caused an increase in anode polarization resistance, which was almost entirely recoverable on removal of H 2S. Operation at higher current density was found to result in a smaller increase in anode polarization resistance. It is proposed that this initial poisoning behaviour is caused by adsorbed sulphur inhibiting surface diffusion of H atoms to active sites. Exposure to 1 ppm and 3 ppm levels of H 2S led to an observed secondary degradation which was also recoverable on removal of sulphur. This degradation was caused by an increased ohmic resistance, and was more severe at higher temperatures. The authors discuss possible explanations for this behaviour.