Studying Reduction in Solid Oxide Fuel Cell Activity with Density Functional Theory− Effects of Hydrogen Sulfide Adsorption on Nickel Anode Surface

Abstract

To gain insight into the degree by which sulfur-based contaminants poison the solid oxide fuel cell (SOFC) anode, we examine adsorption and dissociation of consecutive molecules of hydrogen sulfide on a nickel (111) surface. Preferred adsorption sites, energies, transition states, and kinetic barriers are calculated for the resulting species, *SHx (x = 0 − 2) and *H. Systematically larger amounts of adsorbed sulfur (0, 25, 50, 75, 100%) are calculated to determine the most energetically favorable sulfur surface coverage. The removal of existing sulfur surface atoms is studied to probe the irreversibility of the hydrogen sulfide adsorption reaction. The extent of molecular hydrogen adsorption at increasing surface sulfur coverages allows us to conclude that the presence of even 25% surface sulfur can reduce molecular hydrogen adsorption on the surface by half. Concurring with experimental data, our research demonstrates equilibrium coverage of 50% adsorbed sulfur on the surface. Due to the considerable exothermic nature of the hydrogen sulfide adsorption and dissociation reaction, partial irreversibility of the reaction is exhibited. This irreversibility proves challenging during attempts to remove surface sulfur and regain the original electrochemical activity.