Electrochemical ammonia oxidation on terephthalic acid framework (BDC-NH2) derived core-shell nickel oxide/nickel-enriched 2D carbon nanoflakes (NiO/Ni/CNF)

A terephthalic framework, Ni-BDC-NH2, was used to synthesize the nickel nanoparticles and nanoclusters on-site encapsulated within mesoporous carbon nanoflakes (NiO/Ni/CNF) for ammonia electrolysis. Ni2+ nodes recrystallized to form a core-shell oxide film covering the Ni metal, with the dispersity and particles size influenced by the pyrolysis temperature in a N2 atmosphere. Electrochemical characterization and Raman spectra identified the solid-state Ni(II)⇌Ni(III) transition as the electron mediator, while its lowered onset potential (+1.3 V vs. RHE), compared to that of oxygen evolution reaction (+1.7 V) enhanced the Faradaic efficiency. The performance of electrodes in ammonia oxidation was assessed, which revealed a N2 selectivity of at least 80 % under electrolytic conditions of current density = 0.5 – 1.5 mA cm−2, pH > 10, and initial NH3 level = 50–300 mg-N L−1 using NiO/Ni/CNF(700). An undivided cell equipped with NiO/Ni/CNF(700) anode and Cu/Ni cathode was utilized to treat a real wastewater (∼500 mg-N L−1, pH 4.5). Consequently, both the removal efficiency of NH3 and N2 selectivity surpassed 90 % in a continuous flow mode for 4 days, indicating the durability of electrode system.