Solar-driven electrochemical NH3 splitting into H2 and N2 on BiVO4-based photoanodes

Abstract

From the viewpoints of energy and the environment, NH₃ splitting into N₂ and H₂ is an important challenge in chemistry, and we have found that BiVO₄ is a highly promising base photoanode material for it. Semiconductor films (TiO₂ and BiVO₄) were formed on a fluorine-doped tin oxide (FTO) electrode, and three-electrode photoelectrochemical (PEC) cells using them as the photoanodes were fabricated. The photocurrent in the TiO₂/FTO photoanode cell is saturated at ∼0.1 mA cm⁻² in the range of electrode potential (E) more positive than +0.3 V vs. the standard hydrogen electrode (SHE) in an electrolyte solution containing NH₃ (pH 11) under illumination of simulated sunlight (AM 1.5, 100 mW cm⁻², one sun). In contrast, the photocurrent in the BiVO₄/FTO photoanode cell increases with increasing anodic polarization to reach 1.78 mA cm⁻² at E = +1 V vs. SHE. This PEC cell produces H₂ from NH₃ with a selectivity of 92% under the same irradiation conditions, and the incident photon-to-current conversion efficiency reaches 11.7% at a wavelength of excitation light = 365 nm. Provision of an SnO₂ interlayer between the BiVO₄ and FTO films effectively suppresses the recombination at the interface to enhance the photocurrent under weak anodic polarization at E < +0.5 V vs. SHE.