Elucidating the Impact of Doping and Strain on the Electrochemical Oxidation of Ammonia over β-M@NiOOH(0001) (M = Fe, Co, and Cu): A Comprehensive Theoretical Investigation

Abstract

Designing effective Pt-free catalysts and understanding their mechanism of the electrochemical ammonia oxidation reaction (AOR) are critical for hydrogen production. In this study, we theoretically investigated the AOR mechanism underlying N₂ formation over the β-NiOOH(0001) and β-M@NiOOH(0001) (M = Fe, Co, and Cu) surfaces to explore the roles of doping and strain on the AOR. The enhancement in the β-NiOOH(0001) catalyst activity observed with the doping effect was essentially attributed to the upshifted ε_d, which strengthened the key intermediate E_ads(NH₂). The strain effect enabled high activity in catalysts by enhancing E_ads(NH₂) through increased amounts of electron transfer from the strain-applied surface to adsorbed NH₂, and all potential-determining steps were NH₃* deprotonation on the catalysts. In addition, an analysis of nitrogen-containing products indicated that compressive strain-applied β-NiOOH(0001) surfaces with high catalytic activity and N₂ selectivity can be superior to those of Pt-free AOR catalysts. The relation between the limiting potential and E_ads(NH₂) exhibited a volcano curve.