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

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), {101̅N} surfaces, and the as yet unexplored {112̅N} 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 (cn_Ni) and O (cn_Ni). In the vacuum, where passivation of the surface dangling bonds is limited, the stability order is as follows: (0001) > {101̅N} ≫ {112̅N}, noting that the coordination numbers for each phase are, respectively, cn_Ni = 6, 5, and 4, and cn_O = 3–4, 2–3, and 2–3. In aqueous media, the order of stability is (0001) > {101̅N} ≈ {112̅N}, as the cn_Ni and cn_O of the latter two surface types increase due to water coordination and dissociation. Water adsorption is found to be most favorable on the {112̅N} surfaces, giving rise to fivefold-coordinated Ni (Ni^5c) or Ni^6c from Ni^4c. Our work suggests that a plethora of facets are likely to coexist on β-NiOOH crystallites with water serving to equalize the stabilities of the different surfaces.