Tuning transition metals layered-electroplated on bimetallic MxCu1-x crystallites (M = Fe, Co, Ni, and Zn) to boost ammonia yield in electrocatalytic reduction of nitrate wastewaters.

ABSTRACT

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 (NO3-RR) involves tuning M surfaces in specific ratios for the hydrogenation of nitrogenous species on MxCu1-x electrodes. Electrochemical analysis and operando Raman spectra identified that a solid-state Cu2O-to-Cu0 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, Fe0.36Cu0.64 > Ni0.73Cu0.27 > Co0.34Cu0.66 > Zn0.64Cu0.36. The collaboration of Cu and M in deoxygenating nitrate and subsequently hydrogenating NOx at respective overpotentials is key to enhancing ammonia yield. Nitrate removal (96 %), NH3 selectivity (93 %), and Faradaic efficiency (92 %) were optimized on Fe0.36Cu0.64 electrode at -0.6 V (vs. RHE). A steady yield as high as 14,080 µg h-1 mg-1 was achieved at 30 mA cm-2 using a real water sample (NO3- ∼ 500 mg-N L-1, pH 4) as the input stream, continuously operated for 96 h.