Role of lattice oxygen content and Ni geometry in the oxygen evolution activity of the Ba-Ni-O system

Abstract The activity of oxides towards the oxygen evolution reaction (OER) is usually tuned by changing the transition metal components and/or the surface facets. An important yet less studied feature is the repercussion of the lattice oxygen content of the oxide in the active site's coordination and catalytic performance. This is illustrated here for the Ba-Ni-O system. We synthesized two oxides with Ni in different coordination and oxidation states, namely BaNiO 2 with Ni 2+ in square-planar positions, and BaNiO 2.78(2) with Ni 3+ and Ni 4+ in octahedral positions. We show that the square-planar configuration of Ni endows BaNiO 2 with high intrinsic OER activity, comparable to the best catalysts in the literature. DFT indicates that progressively lowering the lattice oxygen content from BaNiO 3 to BaNiO 2 increases the Ni sites' affinity for the reaction intermediates, thereby lowering the OER overpotential. Thus, oxygen content is an important parameter in oxide catalysts, as it modulates the coordination, orbital splitting, oxidation number, and catalytic activity of the active sites.