Boosting Electrochemical Ammonia Synthesis from Nitrate via Electrolyte Cation-Tuned NiFe-LDH Catalysts with Low-Spin Fe3+ Active Sites

Ammonia (NH3) is a crucial basic raw material in agricultural and industrial production and is an ideal hydrogen carrier in future clean energy systems. The electrocatalytic synthesis of NH3 from nitrate (NO3RR) not only yields a green product but also addresses ecological pollution, serving as a mild alternative to the Haber–Bosch technique. Electrolyte cations can significantly influence the kinetics, selectivity, and energy barriers of electrocatalytic reactions by modulating the electrical double-layer structure, local electric fields, stability of reaction intermediates, and interfacial water structure. However, the influence of the electrode–electrolyte interface environment dominated by electrolyte cations is often overlooked. This study first prepared a NiFe-LS catalyst with low-spin active centers through orbital modulation and subsequently verified the effects of three different cations (Li+, Na+, and K+) on the NO3RR from aspects including charge density, intermediate interaction, and solvation effects. The results indicate that K+ has the most significant promoting effect on NH3 synthesis. The Faraday efficiency (FE) reaches a maximum of 90.60% at −0.6 V vs RHE (0.1 M K2SO4 + 500 ppm of NO3–), with a corresponding yield of 10911.11 μg/h/mgcat.. Mechanistic analysis indicates that the presence of K+ can promote the breakage of the N–O bond, reduce the reaction energy barrier, inhibit the hydrogen evolution reaction (HER), and increase both the interfacial electrostatic potential and the electron transfer from the catalyst to NO3–. This study elucidates the mechanism by which cations enhance the activity of the electrocatalytic interface and reveals the significance of the local reaction environment on the electrode surface for NO3RR, contributing to the further development of novel electrolytes and catalysts.