Thermodynamics of sulfur poisoning in solid oxide fuel cells revisited: The effect of H2S concentration, temperature, current density and fuel utilization

Thermodynamics of sulfur poisoning in SOFCs is revisited in the present study, aiming at contributing to the understanding of the effects of different operating parameters on deactivation by sulfur. Ni-S Gibbs energy diagram shows that, whenever sulfur chemical potential is increased, the catalyst poisoning becomes greater, due to increase in nickel sulfide activity, which is a strictly increasing function of sulfur coverage on Ni surface. For studying current density effect, simulations are carried out in the range of 0 (OCV) - 1 A cm−2, at 1123 K, considering methane as a fuel. At 10 ppm H2S, an increase in current density from 0 (OCV) to 0.5 A cm−2 results in a slight increase in the nickel sulfide activity from 3.0 × 10−6 to 1.2 × 10−5; however, at 1 A cm−2, nickel sulfide activity is 25.000 times higher than at 0.5 A cm−2. At 1 A cm−2, H2 and CO are almost entirely converted to H2O and CO2 by electrochemical reaction. Therefore, the effect of current density becomes remarkable when fuel utilization approaches 100%. These theoretical findings are corroborated by the recent experimental evidences related to the detrimental influence of current density on SOFC performance during sulfur poisoning.