Surface regeneration of sulfur-poisoned Ni surfaces under SOFC operation conditions predicted by first-principles-based thermodynamic calculations

The surface regeneration or de-sulfurization process of a sulfur-poisoned (i.e. sulfur-covered) nickel surface by O2 and H2O has been studied using first-principles calculations with proper thermodynamic corrections. While O2 is more effective than H2O in removing the sulfur atoms adsorbed on nickel surface, it readily reacts with the regenerated Ni surface, leading to over-oxidization of Ni. Thus, H2O appears to be a better choice for the surface regeneration process. In reality, however, both O2 and H2O may be present under fuel cell operating conditions. Accordingly, the effects of the partial pressures of O2 [pO2] and H2O [pH2O] as well as the ratio of pO2/pH2O on the regeneration of a sulfur-covered Ni surface without over-oxidization at different temperatures are systematically examined to identify the best conditions for regeneration of Ni-based SOFC anodes under practical conditions.