Thermo-Electrochemical Redox-Mediated Decoupling of Water Splitting for Improving H₂ Production

Water splitting driven by renewable energy is a promising avenue for hydrogen production. However, the high energy consumption of water electrolysis results in an unsatisfactory efficiency for large-scale commercialization. In addition, due to the intermittent supply and low power density of renewable energy, the produced hydrogen and oxygen gases in the electrolyzer might cross through the membrane, producing an explosive hydrogen/oxygen mixture. Herein, we propose a thermo-electrochemical redox-mediated decoupling system for water splitting. In the system, the redox mediator can be used as a temperature-dependent mediator for decoupling water splitting and thermopower conversion, through which low-grade heat can be converted into thermopower for improving H2 production. The results show that the [Fe(CN)6]3–/4– mediator exhibits reversible redox couples between the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), and a satisfactory thermopower of 1.9 mV/K points toward good application prospects for decoupling water splitting and thermo-electric conversion. As a proof-of-concept, the system is demonstrated to be feasible for decoupling H2 and O2 production using the [Fe(CN)6]3–/4– mediator, and low-grade heat is converted into thermopower to enhance water splitting, suggesting the dual role of the thermo-electrochemical redox in the decoupling of water splitting and low-grade heat conversion. With an increase in the temperature difference from 0 to 30 °C, the current density of water splitting exhibits a 70% increase from 41.2 to 70.0 mA/cm2 at 2.5 V, with a ∼1% conversion efficiency of heat to hydrogen. These results indicate that the as-proposed system can harvest low-grade heat to facilitate the decoupling performance of water splitting through liquid-state thermo-electrochemical conversion.