As a thermodynamic driving force obtained from sunlight, the open-circuit potential (OCP) in photoelectrochemical cells is typically limited by the photovoltage ( Vph). In this work, we establish that the OCP can exceed the value of Vph when an electrolyte-permeable NiO x thin film is employed as an electrocatalyst in a Si photocathode. The built-in potential developed at the NiO x/Si junction is adjusted in situ according to the progress of the NiO x hydration for the hydrogen evolution reaction (HER). As a result of decoupling of the OCP from Vph, a high OCP value of 0.75 V (vs reversible hydrogen electrode) is obtained after 1 h operation of HER in an alkaline electrolyte (pH = 14), thus outperforming the highest value (0.64 V) reported to date with conventional Si photoelectrodes. This finding might offer insight into novel photocathode designs such as those based on tandem water-splitting systems.
Keywords: NiOx; photoelectrochemical cells; semiconductor junction; silicon; water splitting.