Electronic Structure of the (Undoped and Fe-Doped) NiOOH O2 Evolution Electrocatalyst

Abstract

A DFT study was carried out on the atomic and electronic structure of bulk β-NiOOH, with and without substitution of Ni by Fe, examining different stackings and proton distributions in the constituent sheets. The energies of different NiOOH configurations, evaluated at the DFT + U level including dispersion interactions, have very similar values for different stackings and proton distributions, justifying the observed fact that NiOOH is usually obtained in highly disordered form and supporting the assumption that local structures might depend on the synthesis method. Ni ions seem to prefer centrosymmetric coordinations, even if this implies inhomogeneous numbers of OH groups around them; these inhomogeneities can even induce disproportionation of Ni3+ into Ni2+ and Ni4+. The electronic structure, computed with a hybrid DFT functional able to give accurate band gaps for different materials, always shows semiconducting character, with band gaps having widths varying in the 0.8–1.5 eV range and edges formed by Ni 3d states. A metallic character, anticipated by some earlier DFT calculations, is never found. Substitution of Ni by Fe seems to be preferred at sites coordinated with lower numbers of OH groups; this can induce electron transfer from Fe to Ni, that is, formation of Fe4+ and Ni2+, accompanied in some cases by proton jumps across the intersheet space. Some local-level environments, the presence of which might depend on the material synthesis method used, can however stabilize Fe3+, explaining conflicting literature data on the preferred redox state of Fe when included in NiOOH. Overall, the system seems to facilitate electron and proton movement across the material, which can help electrocatalytic O2 evolution processes.