Markedly enhanced hydrogen production in wastewater via ammonia-mediated metal oxyhydroxides active sites on bifunctional electrocatalysts

The ability to substitute oxygen evolution reaction (OER) in water splitting with ammonia oxidation reaction (AOR) represents an important endeavor in producing high-purity hydrogen with the lowered energy consumption. Due to the reduced overpotential of AOR over OER and preventing the hydrogen and oxygen mixing and thus possible explosion. Herein, we report the substitution of slow-kinetics OER with AOR to effectively boost hydrogen evolution reaction (HER) of wastewater electrolysis, thereby rendering energy saving as well as decontamination of ammonia (NH3) in wastewater. Three-dimensional CoS@NiCu electrodeposited on Ni foam is employed as self-supporting bifunctional electrocatalysts for both AOR and HER. The XPS and in-situ Raman studies reveal the generation of electrocatalytically active cobalt oxyhydroxide (CoOOH) on CoS@NiCu during the course of AOR. Interestingly, the density functional theory (DFT) calculation unveils that NH3 preferentially adsorbing on the surface of electrocatalyst prolongs the Co-S bond length, thus promoting the bond cleavage and accelerating the formation of active CoOOH species. Moreover, the rate-determining step of AOR according to the Gerischer-Mauerer (G-M) mechanism only has a 1.79 eV energy barrier to overcome. The electrochemical impedance spectroscopy investigation suggests that the AOR enables an enhanced interfacial charge transfer. As such, the hydrogen evolution rate of the AOR-HER system reaches 41.9 μmol h−1, representing 3.2-fold increase in hydrogen production over the conventional OER-HER water splitting. This study highlights a promising perspective of integrating AOR with HER to synergize efficient hydrogen production.