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

Abstract

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 AgO_x/NiO_x 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 (η_inj) and the bulk charge separation efficiency (η_sep) of BiVO₄ photoanodes were improved by the composite AgO_x/NiO_x electrocatalyst. Photo-assisted electrochemical impedance spectroscopy (EIS) was employed to illustrate the significantly reduced surface charge transfer resistance of the BiVO₄/AgO_x/NiO_x 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/AgO_x/NiO_x photoanode is higher than those of BiVO₄/AgO_x and BiVO₄/NiO_x, 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/AgO_x/NiO_x photoanode. The possible working mechanism is that one component of the composite AgO_x/NiO_x 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.