Modulating the Electron Mediators for Spatially Separated H₂ and O₂ Evolutions in Photocatalytic Water Splitting

A fundamental obstacle in photocatalytic overall water splitting lies in the simultaneous evolution of H₂ and O₂ gases, which complicates gas separation. Decoupling hydrogen and oxygen evolution via a redox electron mediator offers an attractive route to overcome this limitation; however, its success critically depends on the development of electron mediators that satisfy both suitable redox potentials and rapid interfacial charge-transfer kinetics. Here, we demonstrate tunable redox potential in cobalt bipyridine complexes, [Co(bpy)₂Cl₂]Cl, through ligand functionalization. Electron-donating groups (-OCH₃, -CH₃) induce negative shifts in the redox potential, whereas electron-withdrawing substituents (-Cl) leads to positive shifts, yielding a broad potential range from 0.15 to 0.62 V versus NHE. The optimized electron mediator, [Co(bpy-CH₃)₂Cl₂]Cl, exhibits enhanced electron transfer and water oxidation activity on BiVO₄ photocatalyst. Coupled with selective assembling of Pt on the electron-rich {010} facets, an Pt-Cl interfacial charge-transfer channel was established, which accelerates electron transfer and promotes the adsorption/desorption of electron mediator. This integrated system achieves efficient photocatalytic water oxidation with an apparent quantum efficiency of up to 90% at 420 nm. Using [Co(bpy-CH₃)₂Cl₂]Cl electron mediator, the work demonstrated the spatial separation of hydrogen and oxygen evolution reactions in particulate photocatalytic water splitting.