Electrocatalytic Properties of (100)-, (110)-, and (111)-Oriented NiO Thin Films toward the Oxygen Evolution Reaction

Oriented NiO films ((111), (100), and (110)), reactively direct current-sputtered on oriented platinum films on top of MgO substrates, were studied electrochemically on a rotating disk electrode for their performance toward the oxygen evolution reaction (OER). From the results, two electrochemical domains can be defined. At low overpotential (<1.55 V vs reversible hydrogen electrode), electrochemical performances are rather similar for all orientations. At higher overpotential (>1.55 V), the ranking is NiO(110) > NiO(111) > NiO(100). The observed differences were attributed to the fact that NiO(111) and NiO(100) produce an hydroxide phase with the same electrocatalytic properties, but the NiO(100) sample is covered less by the hydroxide and thus produces less active sites. Moreover, NiO(110) forms the more stable and catalytically most active hydroxide phase. We attribute the observed differences to the formation of the beta-NiOOH phase on the NiO(110) orientation, being more active than gamma-NiOOH, which is supposedly produced during the OER on the NiO(111) surface and, to a lesser extent, on the NiO(100) surface orientation.