New insights into sulfur poisoning behavior of Ni-YSZ anode from long-term operation of anode-supported SOFCs

Abstract

The performance characteristics of high-performance Ni-YSZ (yttria-stabilized zirconia) anode-supported solid oxide fuel cells (SOFCs) are examined under typical fuel cell operating conditions over a period of ∼3000 h when hydrogen contaminated with different concentrations of H₂S was used as the fuel. Analyses reveal some new insights into the sulfur poisoning behavior of a Ni-YSZ anode. First, an initial drop in power output upon exposure to H₂S-contaminated fuels is more dramatic in an anode-supported cell (with small cell resistance) than in electrolyte-supported cell (with large cell resistance), creating an illusion that a high-performance cell appears to be less sulfur tolerant than a low-performance cell. Second, the transition time for this rapid performance drop to cease is much longer for an anode-supported cell (with a thicker anode) than an electrolyte-supported cell (with a thinner anode). Third, the degree of sulfur poisoning (or the cell resistance increase due to sulfur poisoning) diminishes with operating cell current density, suggesting that the water produced at the active sites on anode surface from electro-oxidation of hydrogen (or oxygen ions) may promote the oxidation and removal of adsorbed sulfur. This effect becomes less pronounced at higher concentration of H₂S, but still significant at ∼1 ppm of H₂S. Fourth, the subsequent slow degradation in performance after the rapid performance drop upon initial exposure to H₂S can be avoided, indicating that the previously reported slow degradation is unlikely the inherent behavior of a Ni-YSZ anode, but associated with other complications. This further implies that sulfur poisoning may be reversible and the performance drop could be fully recovered when the fuel is switched back to clean hydrogen.