Complementary enhancement between noble metal-like Fe3C and Cu0/Cu+ sites to fulfil highly selective wide-potential nitrate electroreduction to ammonia

Mild electrocatalytic nitrate reduction reaction (NO3RR), driven by renewable electricity, is regarded as a desirable strategy for green ammonia synthesis and simultaneous removal of nitrogen-containing environmental pollutants. In view of different supply voltages from renewable energy sources, developing cost-effective and efficient electrocatalysts with a wide operating potential window is very meaningful for practical application. However, currently reported catalysts usually need to introduce noble metals to synergistically achieve wide-potential selective ammonia synthesis from nitrate. In this work, we present for the first time a dual-transition-metal electrocatalyst (Fe₃C-CuO_x@NC, x = 0, 1) with wide-potential-adaptability for highly selective nitrate reduction to ammonia. Such Fe₃C-CuO_x@NC with spatially separated CuO_x and noble-metal-like Fe₃C nanoparticles encapsulated with nitrogen-doped graphitized carbon, exhibits outstanding performance in NO3RR with desirable NH₃ Faraday efficiency of more than 90% over a wide potential ranging from −0.2 V vs. RHE to −0.6 V vs. RHE, comparable to the reported noble metal catalysts. Different from common tandem catalysis, the wide-potential high ammonia selectivity of Fe₃C-CuO_x@NC is dominantly ascribed to the complementary enhancement between CuO_x and Fe₃C, fully supported by results of experiments and density function theory calculations. CuO_x exhibit highly intrinsic nitrate reduction to nitrite to compensate for the slow potential determination step (*NO₃ → *NO₃H) of Fe₃C, while Fe₃C, besides behaving like noble metals to supply adequate active hydrogens, has both good adsorption and reduction abilities for nitrite species to ammonia. Moreover, Fe₃C partially stabilizes active Cu⁰/Cu⁺ sites, and the unique carbon-layer encapsulation structure effectively prevents the agglomeration and corrosion of metal nanoparticles during the electrocatalysis, thus maintaining good cyclic stability. The Zn-NO₃⁻ battery assembled with Fe₃C-CuO_x@NC can reach a high power density of 5.2 mW cm⁻² at a potential of 1.0 V vs. Zn, with an NH₃ Faraday efficiency of 92.4% at a current of 8.0 mA, proving its potential practical application. This advance provides unique insights into complementary catalysis mechanisms on multiple metal sites in NO3RR, and offers a reference for the design of other transition metal electrocatalysts matching with renewable electricity.