Dissociation of hydrogen and formation of water at the (010) and (111) surfaces of orthorhombic FeNbO4.

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. We have used DFT+U-D2 calculations to explore the adsorption and dissociation of H2 molecules and the formation reaction of water at the (010) and (111) surfaces. 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 Olayer-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.