Unassisted photoelectrochemical water splitting beyond 5.7% solar-to-hydrogen conversion efficiency by a wireless monolithic photoanode/dye-sensitised solar cell tandem device

Achieving the spontaneous evolution of hydrogen from photoelectrochemical (PEC) cells in water using solar light is a desirable but difficult goal. Here, we report a highly efficient wireless monolithic tandem device composed of bipolar highly transparent BiVO₄-sensitised mesoporous WO₃ films/Pt and a porphyrin-dye-based photoelectrode achieving 5.7% without any external bias. A sandwich infiltration process was used to produce a thin BiVO₄ layer coated onto mesoporous WO₃ films while preserving high transparency, enabling high photonic flux into the second dye-sensitised photoanode. In addition, the porphyrin-dye-sensitised photoanode with a cobalt electrolyte generated sufficient bias, realising highly efficient unassisted solar water splitting in the tandem cells. By combining the highly transparent BiVO₄-sensitised mesoporous WO₃ films with the state-of-the-art water oxidation catalyst and a single dye-sensitised solar cell with a high open circuit potential in a monolithic tandem configuration, an extraordinarily high solar-to-hydrogen (STH) conversion efficiency with spontaneous hydrogen evolution was obtained.