Improvement of Photoelectrocatalytic Activity and Stability of WO₃ for Oxygen Photoevolution Reaction by Loading of Brownmillerite‐Type Ca₂FeCoO₅ as a Cocatalyst

Brownmillerite‐type Ca 2 FeCoO 5 (CFCO), whose crystal structure is categorized into an oxygen‐deficiency‐ordered perovskite, is one of the most effective catalysts for the oxygen evolution reaction (OER). Herein, CFCO is applied as a cocatalyst of WO 3 for the oxygen photoevolution reaction under visible light illumination. CFCO is synthesized by a sol−gel method and calcination at 873 K and loaded on a WO 3 electrode by an electrophoretic deposition method. The photocatalytic activity of WO 3 for the oxygen photoevolution reaction is improved by loading an appropriate amount of CFCO to reach a close level of RuO 2 /WO 3 and Co−Pi/WO 3 . Furthermore, photocorrosion of WO 3 is suppressed by CFCO loading, leading to high stability. Because both the lower edge of the conduction band and the upper edge of the valence band of CFCO are higher than those of WO 3 , only photoexcited holes at WO 3 move to CFCO and easily react with water on the CFCO surface, leading to acceleration of the OER and suppressing of photocorrosion of WO 3 .