Near-Complete Suppression of Oxygen Evolution for Photoelectrochemical H₂O Oxidative H₂O₂ Synthesis

Abstract

Solar energy-assisted water oxidative hydrogen peroxide (H₂O₂) production on an anode combined with H₂ production on a cathode increases the value of solar water splitting, but the challenge of the dominant oxidative product, O₂, needs to be overcome. Here, we report a SnO_2-<i>x</i> overlayer coated BiVO₄ photoanode, which demonstrates the great ability to near-completely suppress O₂ evolution for photoelectrochemical (PEC) H₂O oxidative H₂O₂ evolution. Based on the surface hole accumulation measured by surface photovoltage, downward quasi-hole Fermi energy at the photoanode/electrolyte interface and thermodynamic Gibbs free energy between 2-electron and 4-electron competitive reactions, we are able to consider the photoinduced holes of BiVO₄ that migrate to the SnO_2-<i>x</i> overlayer kinetically favor H₂O₂ evolution with great selectivity by reduced band bending. The formation of H₂O₂ may be mediated by the formation of hydroxyl radicals (OH·), from 1-electron water oxidation reactions, as evidenced by spin-trapping electron paramagnetic resonance (EPR) studies conducted herein. In addition to the H₂O oxidative H₂O₂ evolution from PEC water splitting, the SnO_2-<i>x</i>/BiVO₄ photoanode can also inhibit H₂O₂ decomposition into O₂ under either electrocatalysis or photocatalysis conditions for continuous H₂O₂ accumulation. Overall, the SnO_2-<i>x</i>/BiVO₄ photoanode achieves a Faraday efficiency (FE) of over 86% for H₂O₂ generation in a wide potential region (0.6-2.1 V vs reversible hydrogen electrode (RHE)) and an H₂O₂ evolution rate averaging 0.825 μmol/min/cm² at 1.23 V vs RHE under AM 1.5 illumination, corresponding to a solar to H₂O₂ efficiency of ∼5.6%; this performance surpasses almost all previous solar energy-assisted H₂O₂ evolution performances. Because of the simultaneous production of H₂O₂ and H₂ by solar water splitting in the PEC cells, our results highlight a potentially greener and more cost-effective approach for "solar-to-fuel" conversion.