Synergistic Surface Engineering of BiVO₄ Photoanodes for Improved Photoelectrochemical Water Oxidation

Surface engineering of BiVO₄ photoanodes is effective and feasible for photoelectrochemical (PEC) water splitting. To achieve superior PEC performance, however, more than one surface engineering method is usually indispensable, for which a positive synergistic effect is vital and thus highly desired. Herein, it is reported that the incorporation of borate moieties into ultrathin p-type NiO_x catalysts can induce the reconfiguration of surface catalytic sites to form new highly active species, in addition to enhanced fast charge separation and transfer. The photocurrent density of BiVO₄ photoanodes is enhanced from 1.49 to 5.76 mA cm^-2 at 1.23 V versus reversible hydrogen electrode (RHE) under AM 1.5G illumination, which is achieved by successive modifications of NiO_x and borate moieties. It is found that BO₃ groups anchored to Ni atoms by replacing the surface hydroxyl sites of NiO_x catalysts not only increase the relative ratio of Ni³⁺ species to facilitate charge transfer but also provide efficient active sites for H₂O molecule adsorption and oxidation reactions. This work demonstrates the positive synergistic effect of these two surface engineering methods and provides an effective pathway to construct highly efficient and stable photoanodes for PEC water splitting.