Photoenhanced Water Electrolysis in Separate O₂ and H₂ Cells Using Pseudocapacitive Electrodes

Water electrolysis has received much attention in recent years as a means of sustainable H₂ production. However, many challenges remain in obtaining high-purity H₂ and making large-scale production cost-effective. This study provides a strategy for integrating a two-cell water electrolysis system with solar energy storage. In our proposed system, CuO-Cu(OH)₂/Cu₂O was used as a redox mediator between oxygen and hydrogen evolution components. The system not only overcame the gas-mixing issue but also showed high gas generation performance. The redox reaction (charge/discharge) of CuO-Cu(OH)₂/Cu₂O led to a significant increase (51%) in the initial rate of H₂ production from 111.7 μmol h^-1 cm^-2 in the dark to 168.9 μmol h^-1 cm^-2 under solar irradiation. The effects of light on the redox reaction of CuO-Cu(OH)₂/Cu₂O during water electrolysis were investigated by in situ X-ray absorption and photoemission spectroscopy. These results suggest that surface oxygen vacancies are created under irradiation and play an important role in increased capacitance and gas generation. These findings provide a new path to direct storage of abundant solar energy and low-cost sustainable hydrogen production.