Water electrolysis has received much attention in recent years as a means of sustainable H 2 production. However, many challenges remain in obtaining high-purity H 2 and making large-scale production costeffective. 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 /Cu 2 O was used as a redox mediator between oxygen and hydrogen evolution components. The system not only overcame the gasmixing issue but also showed high gas generation performance. The redox reaction (charge/discharge) of CuO-Cu(OH) 2 /Cu 2 O led to a significant increase (51%) in the initial rate of H 2 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 /Cu 2 O 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.