Electrochemical characteristics of silver/nickel oxide (Ag/Ni) for direct ammonia oxidation and nitrogen selectivity in paired electrode system

Electrochemical engineering processes are emerging alternatives for ammonia removal owing to high efficiency and low waste product. Ag electrodes were synthesized by electroplating metallic silver nanoparticles (Ag NPs) on different substrates (Ni, Ti, and graphite) for the electrochemical oxidation of ammonia in aqueous solutions. Among tested electrodes, Ag/Ni₄₀₀ was the best in NH₃ oxidation and N₂ yield. Behaviors of the two reversible couples, AgO/Ag₂O and NiOOH/Ni(OH)₂, catalytically mediated the faradaic process were analyzed by voltammetry. The performance of direct electro-oxidation in terms of current efficiency and energy consumption was investigated in an asymmetric cell using Ag/Ni₄₀₀ anode and copper cathode (Cu/Ni). Parameters of batch constant current electrolysis included initial concentration and current density. The ratio of paired electrodes significantly affected the N₂ selectivity. The Ag/Ni₄₀₀-Cu/Ni electrode system was further applied for the treatment of a real opto-electronic wastewater containing NH₃ of around 250 mg-N/L and low concentration of Cl^- ion (<50 mg L^-1); under an electrode ratio of 2:1, N₂ yield reached to>95 % at 10 mA cm⁻².