Highly activated phosphate-derived oxyhydroxide overlayer for enhancing photoelectrochemical ammonia oxidation-boosted hydrogen evolution

Ammonia (NH₃) oxidation reaction (AOR) has received great interest in recent years as an effective method to enable highly sustainable hydrogen (H₂) storage and conversion. Here, we demonstrate an intensively enhanced photoelectrochemical (PEC) AOR-boosted H₂ production with a highly activated bismuth-vanadium oxyhydroxide overlayer, derived from an overlaid thin phosphate layer, on BiVO₄ (BVO) photoanode. A simple electrochemical (EC) activation is applied on a phosphate-modified bismuth vanadate (Pi-BVO) film to achieve the bimetallic oxyhydroxide overlayer on BVO (a-Pi-BVO) photoanode, which provides a high activity for enhancing the AOR kinetics. We confirm that the a-Pi-BVO photoanode follows pseudo-first order kinetics and demonstrates a 10-fold increase in PEC AOR kinetics, compared to pristine BVO at 0.77 V vs. reversible hydrogen electrode (RHE) in a dilute (500 ppm) NH₃ aqueous media. In our solar-assisted overall ammonia electrolysis cell system with 2-electrode configuration, composed of PEC ammonia oxidation and EC hydrogen evolution reactions, a-Pi-BVO photoanode shows a 6.5-fold enhancement in H₂ production compared to pristine BVO photoanode. This study demonstrates that the simple surface activation of the Pi-BVO photoanode enables highly active PEC AOR for efficient H₂ production under dilute ammonia conditions, which is a promising strategy with applicability to practical wastewater applications.