Mixed‐Tetravalent Cs2RumPt1−mX6 (X = Cl−, Br−)‐Based Vacancy‐Ordered Halide Double Perovskites for Enhanced Solar Water Oxidation

Vacancy‐ordered halide double perovskites (VOPs) are reported to be suitable for photoelectrochemical (PEC) water splitting, due to their panchromatic visible absorption and stability in electrolyte. In this work, ruthenium‐based VOP, Cs 2 RuX 6 (X = Cl − , Br − ), is reported for PEC water splitting, and the material is found to be extremely stable in harsh pH media, between pH1 and pH11. The PEC devices made with these materials show good photogenerated hole transport to electrolyte; however, the photovoltage remains low due to high recombination. To enhance the performance, Ru 4+ is partially substituted with Pt 4+ tetravalent cation, where the latter is shown to possess better photovoltage. The mixed‐tetravalent compound, Cs 2 Ru m Pt 1− m X 6 , leads to high PEC water oxidation current at m = 0.5, compared to their end members. The mixed‐tetravalent compounds show a synergic effect of reduced charge‐transfer resistance from the material to the electrolyte and increased photovoltage leading to increased PEC performance.