Redox-Flow-Battery-Inspired Decoupled Water Electrolysis: Synergistic Integration of the Ce3+/Ce4+ Redox Couple and Sb-SnO₂ Interfaces

Decoupled water electrolysis (DWE) enables spatial−temporal separation of HER and OER via redox mediators for high-purity green hydrogen, but conventional solid-state mediators suffer from limited capacity, restricting large-scale application. To address this, we proposed a redox-flow-battery (RFB)-inspired DWE system integrating a Ce³⁺/Ce⁴⁺ liquid mediator and Sb-doped SnO₂ (Sb-SnO₂) electrodes. The Sb/Sn = 3:10 electrode showed optimal catalytic activity owing to its highest O_ad/O_L and Sb⁵⁺/Sb³⁺ ratios, accelerating Ce³⁺/Ce⁴⁺ redox while suppressing parasitic oxygen evolution. In batch operation, stable H₂/O₂ production voltages (1.74/0.44 V) were achieved at 50 mA cm⁻², with H₂/O₂ volume ratio approaching theoretical 2:1. We further constructed a continuous DWE prototype coupled with photovoltaic (PV) cells for spatially separated simultaneous H₂/O₂ production. The integrated system reached a maximum solar-to-hydrogen (STH) efficiency of 11.26% under AM 1.5G illumination. Outdoor tests confirmed the system’s robustness under fluctuating solar input. This work provides a scalable, efficient route for green hydrogen production, promising to advance sustainable solar-to-hydrogen conversion.