Efficient Photoelectrochemical Hydrogen Evolution Using Pseudocapacitive NiO_x/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 NiO_x 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 NiO_x for electrochemical charge storage). This new type of junction, named a pseudocapacitive NiO_x/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 (V_oc) 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 V_oc 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.