Synergistic Cu/Cu₂O/Co₃O₄ catalyst with crystalline-amorphous interfaces for efficient electrochemical nitrate reduction to ammonia

The electrocatalytic nitrate reduction to ammonia (ENRA) is a useful technology to convert aqueous nitrate contamination into recyclable ammonia. However, most current research only focuses on the inherent properties of materials, such as d-band centers when designing catalysts, while the characteristics of materials producing active species are often ignored. Herein, a novel Cu/Cu₂O/Co₃O₄ catalyst with crystalline-amorphous interfaces was synthesized through a straightforward reduction method for efficient use in ENRA. The results from transmission electron microscopy (TEM) analysis demonstrated good adherence of crystalline Cu/Cu₂O to the amorphous Co₃O₄ surface in Cu/Cu₂O/Co₃O₄ catalyst. X-ray photoelectron spectroscopy confirmed good electron transfer between Cu and Co elements. The enhanced mechanism studied by active species trapping experiments and electron paramagnetic resonance (EPR) revealed the crucial role of atomic hydrogen (atomic H*) as an active substance in the ENRA process. Density functional theory (DFT) calculations demonstrated that Cu/Cu₂O/Co₃O₄ could facilitate the N-containing intermediate adsorption and the formation of *NOH intermediate is the rate-limiting step. The synergism between Cu and Co elements enhanced ENRA performance by promoting the generation of atomic H*, resulting in notable Faradaic efficiency and ammonia yield of 91.5 % and 5.7 mg/h/mg_cat in neutral solution, and 99.5 % and 19.8 mg/h/mg_cat in alkaline solution, respectively. Overall, new insights into the performance improvement mechanism of synergistic effect in ammonia synthesis from nitrate reduction were provided, promising for future nitrate decontamination from wastewater.