Demonstration of\nScalable Water Splitting into H<sub>2</sub> and O<sub>2</sub> by a\nFlow-Type Photocatalysis-Electrolysis\nHybrid System Using a Highly Stable Photocatalyst

Water splitting via a photocatalysis-electrolysis hybrid\nsystem\nhas been investigated as a potentially scalable and economically feasible\nmeans of producing renewable H<sub>2</sub>. However, there are no\nreports demonstrating a scalable system for stoichiometric water splitting\nusing an efficient and stable photocatalyst, and the key operating\nconditions for efficiently driving the entire system have not been\nestablished. Herein, we address the issues required to efficiently\ndrive the entire system of a Cs<sup>+</sup>, Fe<sup>2+</sup>, and\nH<sup>+</sup> ion-modified WO<sub>3</sub> (denoted as H-Fe-Cs-WO<sub>3</sub>) photocatalyst fixed reactor combined with a polymer electrolyte\nmembrane (PEM)-type electrolyzer. In electrochemical H<sub>2</sub> production using Fe<sup>2+</sup>, the current density improved as\nthe concentration of both H<sup>+</sup> and Fe<sup>2+</sup> increased,\nand we determined the optimum conditions for a hybrid system using\nhigh concentrations of HClO<sub>4</sub> and Fe(ClO<sub>4</sub>)<sub>3</sub>, which differ from those reported for photocatalysis alone.\nNo performance deterioration of the H-Fe-Cs-WO<sub>3</sub> photocatalyst\nwas observed even after light irradiation for more than 10 000\nh under strong acidic conditions. The accumulated Fe<sup>2+</sup> ions\nwere extremely stable and did not oxidize even when exposed to air\nfor more than two months. As for the stepwise operation that takes\nadvantage of the characteristics of the hybrid system, the contribution\nfactor of the photocatalyst in the photocatalysis-electrolysis hybrid\nsystem for H<sub>2</sub> evolution (CP@STH<sub>ap</sub>) under an\napplied bias was estimated to be 0.24%, which is a value comparable\n to that of the solar-to-chemical (STC) conversion efficiency (0.31%).\nThe efficiency difference (0.07%) corresponds to the overpotential\nof the electrolytic reaction and indicates that water splitting via\nthe photocatalysis-electrolysis hybrid system proceeds efficiently\nat a small overpotential of 0.06 V (∼11.6 kJ mol<sup>–1</sup>).