Dirhodium(II,II)/NiO Photocathode for Photoelectrocatalytic Hydrogen Evolution with Red Light

A new Rh₂(II,II) dimer has been synthesized and anchored onto a NiO photocathode. The dirhodium complex acts as both the sensitizer to inject holes into NiO and as catalyst for the production of hydrogen. The single-molecule design circumvents limitations of the conventional multicomponent approach with separate sensitizer and catalyst, thus simplifying the hydrogen production pathway and reducing energy losses associated with additional intermolecular charge transfer steps. The Rh₂(II,II) complex absorbs strongly from the ultraviolet throughout the visible range and tails into the near-IR to ∼800 nm, permitting absorption of a significantly greater portion of the solar irradiance as compared to traditional dyes used in dye-sensitized solar cells and photoelectrosynthesis cells. The irradiation of the Rh₂-NiO photoelectrode with 655 nm light (53 mW cm^-2) results in a photocurrent that reaches 52 μA cm^-2 at -0.2 V vs Ag/AgCl in the presence of p-toluenesulfonic acid (0.1 M), with Faradaic efficiencies of H₂ production up to 85 ± 5% after 2.5 h without photoelectrode degradation. This work presents the first single-molecule photocatalyst, acting as both the light absorber and catalytic center on NiO, able to generate hydrogen from acidic solutions with red light when anchored to a p-type semiconductor, providing a promising new system for solar fuel production.