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.