Demonstration of Scalable Water Splitting into H₂ and O₂ by a Flow-Type Photocatalysis-Electrolysis Hybrid System Using a Highly Stable Photocatalyst

Water splitting via a photocatalysis-electrolysis hybrid system has been investigated as a potentially scalable and economically feasible means of producing renewable H₂. However, there are no reports demonstrating a scalable system for stoichiometric water splitting using an efficient and stable photocatalyst, and the key operating conditions for efficiently driving the entire system have not been established. Herein, we address the issues required to efficiently drive the entire system of a Cs⁺, Fe²⁺, and H⁺ ion-modified WO₃ (denoted as H-Fe-Cs-WO₃) photocatalyst fixed reactor combined with a polymer electrolyte membrane (PEM)-type electrolyzer. In electrochemical H₂ production using Fe²⁺, the current density improved as the concentration of both H⁺ and Fe²⁺ increased, and we determined the optimum conditions for a hybrid system using high concentrations of HClO₄ and Fe(ClO₄)₃, which differ from those reported for photocatalysis alone. No performance deterioration of the H-Fe-Cs-WO₃ photocatalyst was observed even after light irradiation for more than 10 000 h under strong acidic conditions. The accumulated Fe²⁺ ions were extremely stable and did not oxidize even when exposed to air for more than two months. As for the stepwise operation that takes advantage of the characteristics of the hybrid system, the contribution factor of the photocatalyst in the photocatalysis-electrolysis hybrid system for H₂ evolution (CP@STH_ap) under an applied bias was estimated to be 0.24%, which is a value comparable to that of the solar-to-chemical (STC) conversion efficiency (0.31%). The efficiency difference (0.07%) corresponds to the overpotential of the electrolytic reaction and indicates that water splitting via the photocatalysis-electrolysis hybrid system proceeds efficiently at a small overpotential of 0.06 V (∼11.6 kJ mol^-1).