Spontaneous Photoelectrolysis of HBr and HI

Spontaneous and efficient photoelectrolysis of HBr into hydrogen and bromine is reported for light of wavelengths to 950 nm in a p‐n photoelectrolysis cell composed of a p ‐ InP photocathode with noble metal catalyst islands on the surface and an n ‐ MoSe 2 or an n ‐ WSe 2 photoanode. Photoelectrolysis of HI with the same photoelectrodes yields hydrogen and iodine and simultaneously produces electrical power. The efficiency of the photoelectrolysis is calculated by two methods, one based on the stored free energy and the other based on electrical power saved. The efficiencies for HBr photoelectrolysis by 632.8 nm light are 7.8%, if calculated by the first method, and 11.2%, if calculated by the second.