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

摘要

Hematite (α-Fe _(2) O _(3) ) 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.