Effects of the Aqueous Environment on the Stability and Chemistry of β-NiOOH Surfaces

Abstract

Nickel oxyhydroxide (NiOOH)-based anodes are among the most promising materials for the electrocatalytic production of oxygen from water under alkaline conditions. We explore the stability of the low-index facets of the catalytically active β-NiOOH phase, namely the (0001), {101N} surfaces, and the as yet unexplored {112N} surfaces, via density functional theory with a Hubbard-U like correction on Ni. We find that their relative stabilities depend strongly on the coordination number of the exposed Ni (cnNi) and O (cnNi). In the vacuum, where passivation of the surface dangling bonds is limited, the stability order is as follows: (0001) > {101N} ≫ {112N}, noting that the coordination numbers for each phase are, respectively, cnNi = 6, 5, and 4, and cnO = 3–4, 2–3, and 2–3. In aqueous media, the order of stability is (0001) > {101N} ≈ {112N}, as the cnNi and cnO of the latter two surface types increase due to water coordination and dissociation. Water adsorption is found to be most favorable on the {1...