Separating bulk and surface processes in NiOx electrocatalysts for water oxidation

Nickel oxide-based catalysts currently represent the state of the art in electrochemical water oxidation in alkaline pH. However, much of their functionality remains poorly understood, particularly regarding catalytically active sites and mechanism. Herein, we conduct a thickness dependent study of sputter deposited NiO(x)films by electrochemical impedance spectroscopy and spectroelectrochemistry in order to differentiate bulk oxidation from catalytic activation. We find that while catalytic activation occurs throughout the film bulk, only the upper <= 5 nm of these films are able to participate in the water oxidation reaction, a result that may be critical in the design of next generation co-catalysts to maximise performance and minimise light absorption losses.