Onset Reaction Mechanism of Cr and S Poisoning on Perovskite Oxide Surfaces

Perovskite oxides serve as oxygen electrode materials in solid oxide fuel and electrolysis cells. These compounds are susceptible to poisoning by volatile chromium and sulfur species in the gas environment. The reaction mechanism of chromium and sulfur poisoning on perovskite oxide surfaces as a function of surface chemistry has not been resolved to date. Understanding the role of different surface chemistries in this degradation mechanism can help to guide the engineering of more stable surfaces. In this study, we take a state-of-the-art perovskite oxide (ABO₃), La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF), as a model oxygen electrode material. We investigate the onset of poisoning reactions by CrO₃ and SO₂, and their activity on different surface terminations of LSCF by density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. We find that both CrO₃ and SO₂ molecules bind more strongly onto the AO-terminated surfaces than do the BO₂ surfaces. AO-terminated LSCF surfaces, especially the Sr sites, result in more strongly adsorbed species with reduced mobility at the surface. The adsorption of CrO₃ and SO₂ on Sr sites of an AO-terminated LSCF surface forms atomic coordinations similar to SrCrO₄ and SrSO₄, thereby serving as nucleation sites for the formation of these secondary phases. We find two physical traits, surface oxygen Bader charge and subsurface oxygen 2p-band center, that correlate with the distinctly different adsorption energies of these species on the AO- and BO₂-terminated surfaces. This indicates that the electrostatic interaction and charge transfer between the adsorbate and the surface play a major role in the onset of these poisoning reactions on perovskite oxides. The results reveal the role of surface chemistry in affecting the thermodynamics and the kinetics of CrO₃ and SO₂ reactions at perovskite oxide surfaces and inform effective strategies for mitigation.