Undoped and Ni-Doped CoO_x Surface Modification of Porous BiVO₄ Photoelectrodes for Water Oxidation

Surface modification of photoanodes with oxygen evolution reaction (OER) catalysts is an effective approach to enhance water oxidation kinetics, to reduce external bias, and to improve the energy harvesting efficiency of photoelectrochemical (PEC) water oxidation. Here, the surface of porous BiVO₄ photoanodes was modified by the deposition of undoped and Ni-doped CoO_x via nitrogen flow assisted electrostatic spray pyrolysis. This newly developed atmospheric pressure deposition technique allows for surface coverage throughout the porous structure with thickness and composition control. PEC testing of modified BiVO₄ photoanodes shows that after deposition of an undoped CoO_x surface layer, the onset potential shifts negatively by ca. 420 mV and the photocurrent density reaches 2.01 mA cm^-2 at 1.23 vs V_RHE under AM 1.5G illumination. Modification with Ni-doped CoO_x produces even more effective OER catalysis and yields a photocurrent density of 2.62 mA cm^-2 at 1.23 V_RHE under AM 1.5G illumination. The valence band X-ray photoelectron spectroscopy and synchrotron-based X-ray absorption spectroscopy results show the Ni doping reduces the Fermi level of the CoO_x layer; the increased surface band bending produced by this effect is partially responsible for the superior PEC performance.