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

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 SnO2–x 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 SnO2–x 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 SnO2–x/BiVO4 photoanode can also inhibit H2O2 decomposition into O2 under either electrocatalysis or photocatalysis conditions for continuous H2O2 accumulation. Overall, the SnO2–x/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.