A Cu/Fe₃O₄@CN tandem catalyst for efficient ammonia electrosynthesis from nitrate reduction

Electrochemical nitrate reduction reaction (ENO₃RR) is a green technology for value-added ammonia production meanwhile treating waste water at ambient conditions. However, it remains a great challenge to construct efficient catalysts due to the complex multiple proton and electron transfer process. Herein, we report a tandem catalyst Cu/Fe₃O₄@CN composed of N-doped carbon layer coated Cu/Fe₃O₄ heterostructure toward highly efficient electrocatalytic nitrate reduction to ammonia production in alkaline medium. The catalyst can achieve an ammonia Faradaic efficiency and ammonia yield rate as high as 96.57 % and 22104.15 μg h^-1 mg_cat^-1 at the optimal potential of -0.9 V vs. RHE in 0.1 M KOH. The outstanding ENO₃RR performance outperforms the most of reported transitional metal-based ENO₃RR electrocatalysts. More importantly, Cu/Fe₃O₄@CN also exhibits excellent stability even under large current density of 200 mA cm^-2, and long-time durability for continuous electrolysis for 36 h. The experimental and theoretical calculations verify the tandem catalysis mechanism of NO₃^- → NO₂^- → NH₃ through the synergism of the two components Cu and Fe₃O₄ in Cu/Fe₃O₄@CN. Specifically, Cu has strong NO₃^- adsorption capacity which can be reduced into NO₂^-, while Fe₃O₄ can boost water dissociation to generate abundant active *H, ensuring the smooth hydrogenation process while suppressing the occurrence of competitive hydrogen evolution reactions (HER). The heterostructure formation between Cu and Fe₃O₄ also significantly reduces the energy barrier of the rate-determining step (*NO → *NOH), which results in the high performance of Cu/Fe₃O₄@CN.