Correlating flat band and onset potentials for solar water splitting on model hematite photoanodes

Abstract

Hematite (α-Fe₂O₃) is a very promising material for solar water splitting that requires a high anodic potential to initiate the oxygen evolution reaction (OER). In this work, we explore the correlation between the downshift in flat band potential of hematite, V_fb, and in onset potential of OER, V_onset, caused by prolonged annealing. We observed a cathodic shift (i.e., towards lower potentials) of 200 mV of V_onset on model photoanodes consisting of ultra-thin hematite films, upon increasing the oxidation time during fabrication and without any further modifications. Detailed physical characterization, electrochemical impedance spectroscopy, and Mott–Schottky analysis revealed a quantitative correlation between the cathodic shift of V_onset and a lowering of V_fb. We identified a reduction in concentration of grain boundaries with increasing oxidation time, as the mechanism behind the observed shift of the V_fb. The approach presented here can be seen as a complementary strategy to co-catalysts and other post-fabrication treatments to lower V_onset. Moreover, it is generically applicable to photoelectrodes used to carry out oxidation and reduction half–cell reactions.