Dissociation of Hydrogen and Formation of Water at the (010) and (111) Surfaces of Orthorhombic FeNbO4

Abstract

The orthorhombic structure of FeNbO₄, where the Fe and Nb cations are distributed randomly over the octahedral 4c sites, has shown excellent promise as an anode material in solid oxide fuel cells. In this study, we have used calculations based on the density functional theory with a Hubbard Hamiltonian and long-range dispersion corrections (DFT+U-D2) to explore the adsorption and dissociation of H₂ molecules and the formation reaction of water at the (010) and (111) surfaces. We have generated pristine surfaces with random distributions of cations from a 2×2×2 quasi-random orthorhombic bulk structure. Specifically, we have considered various terminations for the (010) and (111) surfaces with different ratios of Fe and Nb cations in the exposed layers. The top and sub-surface layers of the (010) surface move in opposite directions after relaxation, whereas the relaxed layers of the (111) surface shift outward by no more than 2.5 %. Simulations of the surface properties confirmed that the bandgaps are significantly reduced compared to the bulk material. We found that the hydrogen molecule prefers to dissociate at the O bridge sites of the (010) and (111) surfaces, especially where these are coordinated to Fe cations, thereby forming two hydroxyl groups. We have investigated the water formation reaction and found that the energy barriers for migration of the H ions are generally lower for the Fe/Nb−O sites than for the O−O site. Overall, our simulations predict that after dissociation, the H atoms tend to remain stable in the form of O_layer-H groups, whereas a larger barrier needs to be overcome to achieve the formation of water. Future work will focus on potential surface modifications to reduce further the barrier of migration of the dissociated H ions, especially from the oxygen bridge sites.