Highly Active and Self-Healing Co-Doped BiVO₄ Photoanode in Borate Buffer to Enhance Charge Separation and Water Oxidation Kinetics during Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting has become an attractive way to sustainable hydrogen production. Herein, a cobalt ion-doped BiVO4 photoanode (Co:BiVO4) is prepared by an electrodeposition-solution drop-casting strategy. Co:BiVO4 presents a three-dimensional coral sheet structure with Co3O4 particles on the surface. The surface will be covered with an ultrathin cobalt borate (CoBi) film after PEC testing in borate buffer. The photocurrent density of Co:BiVO4 reaches 4.45 mA/cm2 at 1.23 VRHE (2.46 times that of BiVO4). Co:BiVO4 also exhibits self-healing properties in borate buffer during the PEC test. The excellent PEC performance of the Co:BiVO4 photoanode is due to the synergy effect caused by Co doping. Some of the Co ions are doped into the BiVO4 lattice to increase the electronic conductivity. Excess Co ions form the Co3O4 co-catalyst on the BiVO4 surface to promote the water oxidation reaction. After PEC testing in borate buffer, the CoBi film covering the Co:BiVO4 surface acts as a co-catalyst to provide more active sites for water oxidation. The applied bias photon-to-current efficiency, charge separation, and injection efficiency of the Co:BiVO4 photoanode are 0.94%, 63.19%, and 87.73% (3.62, 2.39, and 2.95 times that of bare BiVO4). This work provides a wealth of insights into the influence of Co:BiVO4 on PEC water oxidation in borate buffer and demonstrates the self-healing function of the system.