Solar water oxidation using nickel-borate coupled BiVO₄ photoelectrodes

A naturally abundant nickel-borate (Ni-Bi) complex is demonstrated to successfully catalyze the photoelectrochemical (PEC) water oxidation of BiVO₄ electrodes at 1.23 V_RHE with nearly 100% faradaic efficiency for oxygen evolution. Ni-Bi is electrodeposited (ED) and photodeposited (PD) for varying times on BiVO₄ electrodes in the 0.1 M borate electrolyte with 1 mM Ni²⁺ at pH 9.2. Surprisingly, optimally deposited Ni-Bi films (ED-10 s and PD-30 min) display the same layer thickness of ca. 40 nm. Both Ni-Bi films enhance the photocurrent generation of BiVO₄ at 1.23 V_RHE by a factor of 3-4 under AM 1.5-light irradiation (100 mW cm^-2) along with ca. 250% increase in the incident and absorbed photon-to-current efficiencies. Impedance analysis further reveals that the charge transfer resistance at BiVO₄ is markedly decreased by Ni-Bi deposits. The primary role of Ni-Bi has been suggested to be a hole-conductor making photogenerated electrons more mobile and catalyzing a four-hole transfer to water through cyclic changes between the lower and higher Ni oxidation states. However, thick Ni-Bi films (>∼40 nm) significantly reduce the PEC performance of BiVO₄ due to the kinetic bottleneck and charge recombination. Under identical PEC conditions (0.1 M, pH 9.2), the borate electrolyte (good proton acceptor) is found to be better than nitrate (poor proton acceptor), indicative of a proton-coupled electron transfer pathway in PEC water oxidation.