Design of an efficient nitrate reduction electrocatalyst via active site identification and optimization in Fe₃O₄

Identifying the active sites of an electrocatalyst is crucial for understanding and improving the electrocatalytic performance of electrocatalyst. Fe₃O₄, as an efficient electrocatalyst for the electrocatalytic NO₃^- reduction reaction (eNO₃^-RR), has been extensively investigated while the genuine active site is unclear. In this study, we demonstrated that the eNO₃^-RR activity of Fe₃O₄ is not influenced by the geometric position of Fe ions but is highly dependent on the valence state of iron. Specifically, eNO₃^-RR is more likely to occur on Fe²⁺ sites compared to Fe³⁺ sites. The FeCo₂O₄ catalyst, synthesized by substituting inert Fe³⁺ with highly active Co³⁺, exhibits exceptional performance. At an electrode potential of -0.6 V vs. RHE, the Faradaic efficiency for ammonia production reached 98.90 %, with an ammonia generation rate of 29.20 mg h^-1 cm^-2. Furthermore, when utilized as the cathode material in a Zn-NO₃^- battery, the peak power density reaches 4.7 mW cm^-2. Our study not only elucidates the correlation between the valence state of Fe ions in Fe₃O₄ and the intrinsic activity of eNO₃^-RR, but also demonstrates that the FeCo₂O₄ catalyst synthesized via cation substitution exhibits superior performance. This finding offers a novel and effective strategy for the rational design of high-performance spinel electrocatalysts.