A flow-type decoupled electrolysis system based on Fe²⁺/Fe³⁺ redox couple for hydrogen generation under the fluctuating solar irradiation

To overcome the challenges associated with conventional water electrolysis, such as high onset potential, gas crossover, and the limited stability under intermittent renewable energy sources, this study presents a flow-type decoupled electrolysis system based on the Fe²⁺/Fe³⁺ redox couple, enabling efficient, stepwise hydrogen production under low-voltage conditions. The Fe²⁺/Fe³⁺ redox couple was demonstrated to enable efficient electron transfer across spatially separated electrodes, highlighting its suitability for decoupled electrolysis configurations. Unlike traditional proton exchange membrane electrolyzer cells (PEMECs), our design maintains stable operation under variable voltage conditions and seamlessly integrates with intermittent solar energy generation. It achieved a high Faradaic efficiency of 98.2 % at a current density of 50 mA cm⁻², with an average hydrogen evolution reaction (HER) voltage of 0.67 V lower than PEMECs with the same structure. Notably, the corresponding solar-to-hydrogen efficiency could reach 20.76 % under the full solar spectrum and maintain 12.7 % under the cloudy conditions. Moreover, the system features a cost-effective design, with electrolyte expenses approximately 80 % lower than vanadium-based systems. Combined with its robust electrochemical stability and inherent scalability, this configuration offers a promising approach for decentralized, solar-driven hydrogen production.