Interfacial Charge Transfer Pathways in Photoelectrochemical H₂ Evolution by a Single-Component Molecular Catalyst on...

Dye-sensitized photoelectrosynthesis cells traditionally combine a photosensitizing dye to harvest light and a catalyst to generate chemical fuels on a semiconductor. In this work, a photoactive catalyst capable of both light absorption and fuel formation, [Cp*Ir(4,4'-Y₂-bpy)Cl][Cl] (Cp* = pentamethylcyclopentadienyl, bpy = 2,2'-bipyridine, Y = CH₂PO₃H₂), is anchored to a mesoporous tin-doped indium oxide (ITO) electrode and facilitates photoelectrochemical H₂ evolution in water without the need for additional photosensitizers or sacrificial reductants. Spectroelectrochemistry indicates a single-site H₂ evolution mechanism involving charge injection to ITO, in contrast to the bimetallic mechanism observed in solution. Cyclic voltammetry and variable-potential chronoamperometry under illumination probe competing pathways via interfacial electron transfer between the Ir hydride excited state and the conductive ITO electrode. A Marcus theory framework provides reorganization energies for competing interfacial electron transfer pathways from potential-dependent quantum yield measurements. The light-driven H₂ evolution catalysis on conducting oxide proceeds with high Faradaic efficiency and current densities comparable to photoelectrodes utilizing p-type semiconductors. By uncovering the principal electron transfer pathways that govern photocatalytic efficiency, this study establishes design principles for hybrid molecular photoelectrocatalysts.