Tuning transition metals layered-electroplated on bimetallic MCu crystallites (M = Fe, Co, Ni, and Zn) to boost ammonia yield in electrocatalytic reduction of nitrate wastewaters.

Nitrate-containing wastewaters have been recognized as an important source for recovering valuable ammonia. This work targets integrating a series of transition metals (M = Fe, Co, Ni, and Zn) onto Cu crystallites through a layered-plating method. The strategy to promote the nitrate reduction reaction (NORR) involves tuning M surfaces in specific ratios for the hydrogenation of nitrogenous species on MCu electrodes. Electrochemical analysis and operando Raman spectra identified that a solid-state CuO-to-Cu transition acted as the primary mediator, while its high corrosion resistance protected the M metals or metal oxides from inactivation in nitrate-to-ammonia pathways. Among bimetals, FeCu was the best combination, with the order of performance in constant potential electrolysis, FeCu > NiCu > CoCu > ZnCu. The collaboration of Cu and M in deoxygenating nitrate and subsequently hydrogenating NO at respective overpotentials is key to enhancing ammonia yield. Nitrate removal (96 %), NH selectivity (93 %), and Faradaic efficiency (92 %) were optimized on FeCu electrode at -0.6 V (vs. RHE). A steady yield as high as 14,080 μg h mg was achieved at 30 mA cm using a real water sample (NO ∼ 500 mg-N L, pH 4) as the input stream, continuously operated for 96 h.