Modulating the Electron Mediators for Spatially Separated H ₂ and O ₂ Evolutions in Photocatalytic Water Splitting

A fundamental obstacle in photocatalytic overall water splitting lies in the simultaneous evolution of H 2 and O 2 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) 2 Cl 2 ]Cl, through ligand functionalization. Electron‐donating groups (‐OCH 3 , ‐CH 3 ) 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‐CH 3 ) 2 Cl 2 ]Cl, exhibits enhanced electron transfer and water oxidation activity on BiVO 4 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‐CH 3 ) 2 Cl 2 ]Cl electron mediator, the work demonstrated the spatial separation of hydrogen and oxygen evolution reactions in particulate photocatalytic water splitting.