β-NiOOH is An Active Electrocatalyst for the Ammonia Oxidation Reaction Predicted by Density Functional Theory

The electrocatalyzed ammonia oxidation reaction (AOR) is an ambient temperature and pressure process with potential applications for sustainable energy generation and waste ammonia treatment. In this work, we study the mechanism of ammonia oxidation towards N2, NO2– , and NO3– on the (0001) β-NiOOH surface and compare the computed free energy of reaction intermediates to results previously obtained on (0001) β-Ni(OH)2. NiOOH surfaces with a hydroxide vacancy favoured NH2–NH2 coupling and reduced the theoretical onset potential for N2, NO2–, and NO3– relative to the β-Ni(OH)2 surface. The surface with an oxygen vacancy favoured NH–NH coupling, and had lower onset potentials for N2 and NO2– formation relative to the β-Ni(OH)2 surface, while NO3– formation was slightly hindered due to its increased stabilization of the precursor adsorbate, *NO2. In general, the NiOOH surface had lower computed onset potentials compared to the Ni(OH)2 surface due to the destabilization of certain adsorbates which form thermodynamic sinks in the AOR pathway, leading to differences in the potential-determining step for the formation of N2 and NO2–. This work sheds light on the ammonia electrooxidation mechanism on β-NiOOH and predicts the material to be a much more active electrocatalyst compared to β-Ni(OH)2.