The Significance of the Local Structure of Cobalt-based Catalysts on the Photoelectrochemical Water Oxidation Activity of BiVO4

Abstract The local structures of the water oxidation catalysts play an important role in reaction kinetics and the performance of the photoanodes. In this study, we deposited cobalt-based catalysts on nanoporous BiVO4 with controlled thicknesses by atomic layer deposition (ALD). Despite the similar oxidation states of cobalt in all depositions, different water oxidation activities in neutral pH conditions were observed. A dramatic photocurrent raise, lowered kinetic overpotential, and smaller charge transfer resistance across the photoanode/electrolyte interface were achieved when a uniform ultrathin Co(OH)2 layer was formed on BiVO4. Photocurrent density for water oxidation showed a 95% enhancement at 0.6 V vs. RHE when the catalyst was in the form of Co(OH)2, while an 80% increase was obtained for CoO. Ideal coordination of Co(OH)2 on hydroxylated BiVO4 surface assists the charge transfer between the electrolyte and BiVO4 without increasing surface recombination. The results of this study emphasize the importance of controlling the local structure of the catalysts in the performance of the water splitting photoanodes.