In-situ electrochemical formation of nickel oxyhydroxide (NiOOH) on metallic nickel foam electrode for the direct oxidation of ammonia in aqueous solution

A nickel foam-supported Ni(OH)₂/NiOOH electrode, synthesized in-situ at a specific electrode overpotential, was used to study the oxidation of ammonia in aqueous solution. Results of voltammetric analysis showed the formal potential of Ni(OH)₂/NiOOH transition at +0.6 V (vs. Hg/HgO, pH 11) at which the current profile was improved by electron transfers of NH₃ in the electrolyte. Selectivity of NH₃ conversion to NO₃ ⁻ and N₂ was evaluated by batch constant current experiments. Electrochemical parameters, including solution pH (6–12), temperature (20–40 °C), current density (0.2–3 mA cm⁻²), and initial NH₃-N concentration (20–450 mg-L⁻¹), that may affect ammonia oxidation toward nitrogen selectivity were studied. At constant current density of 1.5 mA cm⁻²A, ammonia removal reached 98.5% and NO₃ ⁻ was the major product at initial NH₃-N concentration of 50 mg-L⁻¹ in 7 h. By contrast, N₂ evolution dominated at low current density (<1 mA cm⁻²) and high initial NH₃-N concentration (i.e., >100 mg-L⁻¹). A surface steady-state approach, with NH₃ deprotonation as the rate-limiting step, provided the reaction pathways of NH₃ conversion to molecular nitrogen byproduct.