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 [p_O2] and H₂O [p_{H2 O}] as well as the ratio of p_O2 / p_{H2 O} 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.