Rational design of electrocatalysts for simultaneously promoting bulk charge separation and surface charge transfer in solar water splitting photoelectrodes

For a photoanode, a large overpotential for water oxidation, which limits the solar-to-hydrogen efficiency, may be caused by slow water oxidation kinetics and the recombination of photo-induced electrons and holes. Herein, taking flat and thin BiVO4 photoanodes as an example, the composite AgOx/NiOx electrocatalyst was found to promote not only the water oxidation kinetics, but also the bulk charge separation. As a result, the surface charge injection efficiency (eta(inj)) and the bulk charge separation efficiency (eta(sep)) of BiVO4 photoanodes were improved by the composite AgOx/NiOx electrocatalyst. Photo-assisted electrochemical impedance spectroscopy (EIS) was employed to illustrate the significantly reduced surface charge transfer resistance of the BiVO4/AgOx/NiOx sample at the interface between the photoanode surface and the electrolyte. Analysis of the surface potential changes obtained from photo-assisted Kelvin probe force microscopy (KPFM) revealed that the surface photovoltage (SPV) of the BiVO4/AgOx/NiOx photoanode is higher than those of BiVO4/AgOx and BiVO4/NiOx, representing its large band bending region for bulk charge separation. The open circuit photovoltage (OCP) measurements also demonstrated the superior charge separation ability of the BiVO4/AgOx/NiOx photoanode. The possible working mechanism is that one component of the composite AgOx/NiOx electrocatalyst may stabilize the high valence states of the other metal ions, which is beneficial for the formation of water oxidation active sites and the extension of the band bending region.