Design of an Efficient Nitrate Reduction Electrocatalyst via Active Site Identification and Optimization in Fe3O4

Abstract

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.

Environmental implication

Nitrate pollution represents a significant environmental challenge confronting the global community. The traditional Haber-Bosch process, as the cornerstone technology for ammonia synthesis, has been instrumental in supporting modern agriculture and industrial development. However, its high energy consumption, substantial carbon emissions, and dependence on fossil fuels are at odds with the principles of sustainable development. Electro-catalytic nitrate reduction technology provides a novel solution to these challenges. This study identified the active sites of Fe₃O₄ and developed a FeCo₂O₄ nitrate reduction catalyst that exhibited exceptional performance in the nitrate reduction reaction. This catalyst not only holds considerable promise for mitigating nitrate pollution but also has the potential to advance the development of green ammonia synthesis and sustainable nitrogen cycling, thereby exerting profound impacts on environmental protection and energy sustainability.