Light-absorbing semiconductor electrodes decorated with electrocatalysts are key components of photoelectrochemical energy conversion and storage systems. Efforts to optimize these systems have been slowed by an inadequate understanding of the semiconductor (SC) and electrocatalyst (EC) interface. Experiments to directly measure the interface behavior are challenging because conventional photoelectrochemical techniques do not measure the electrocatalyst potential during operation. We developed dual working electrode (DWE) photoelectrochemistry to address this limitation. A second electrode is attached to the catalyst layer to control and probe current/voltage independent from that of the semiconductor.

The DWE investigations on a model system TiO₂ reveal that electrolyte-permeable ECs form an “adaptive” junction with the semiconductor. This talk will focus on studies of the interface of the visible-light-absorbing semiconductor α-Fe₂O₃ and the electrocatalyst a-Ni₈₀Fe₂₀O_x with DWE photoelectrochemistry. We discover that the potential at the SC|EC interface changes in situ with the oxidation level of the EC. Additionally, the activation of EC film is vital to the charge transport between SC and EC. A fully-activated amorphous Ni(Fe)OOH film can harvest the vast majority of photo-generated holes from α-Fe₂O₃ electrodes.