Sulfur poisoning and regeneration of Ni-based anodes in solid oxide fuel cells

The effect of hydrogen sulfide (H2S) on the performance of nickel/yttria-stabilized zirconia (YSZ) cermet anode for solid oxide fuel cells has been studied under various operating conditions. In all cases, a small amount of H2S (ppm level) causes a sharp drop in cell performance within the first few minutes of exposure, followed by a gradual but persistent deterioration in performance for several days. The extent of anode degradation caused by sulfur poisoning increases with increasing H2S concentration, increasing cell voltage (i. e., closer to open-circuit voltage or decreasing cell current density), or decreasing cell operating temperature. The initial sharp degradation in performance is attributed to rapid adsorption of sulfur onto the Ni surface, which blocks the active sites for hydrogen adsorption and oxidation. However, the mechanism for the subsequent slow degradation is still not clear. Upon removal of H2S from the fuel stream, the anode performance can be recovered fully or partially, depending on operating conditions and duration of H2S exposure. The rate of the recovery process increases with operating temperature and cell current density. (c) 2006 The Electrochemical Society. All rights reserved.