Efficient Photoelectrochemical Hydrogen Evolution Using Pseudocapacitive NiOx/Si Junction with Misaligned Energy Levels

Photoelectrochemical (PEC) water splitting performed by an electrocatalyst integrated with a semiconducting photoelectrode is advantageous with improvements in both charge-transfer kinetics and interface energetics because of the electrocatalyst/semiconductor junction. In general, interface energetics has been considered to arise from differences in the intrinsic electronic energy levels between the electrocatalyst and the semiconductor. Here, we demonstrated that when a NiOx thin film with porous and nanocrystalline structures is integrated with a Si photoelectrode, the interface energetics is developed by an electrochemical energy level extrinsically formed by the pseudocapacitive surface reaction (a redox reaction of NiOx for electrochemical charge storage). This new type of junction, named a pseudocapacitive NiOx/Si junction, revealed two intriguing features: the interface energetics is dynamically changed as charging/discharging progresses, and the developed electrochemical energy level and the electronic energy level of Si are abnormally misaligned under equivalent circuit conditions. With these features, the open circuit potential (Voc) of the PEC device was determined by the degree of misalignment (i.e., the electrochemical energy level). The electrochemical energy level was maximized by ∼1 V through the insertion of a SiO2 interfacial layer thick enough to suppress discharge and 1 h of PEC operation for sufficient charging by the transfer of light-induced electrons. As a result, the highest Voc of ∼1 V, surpassing the theoretical limit of 0.85 V in Si photovoltaics, was achieved. This finding demonstrated a new paradigm for self-powered PEC reactions.