Boosting Electrochemical Ammonia Synthesis from Nitrate via Electrolyte Cation-Tuned NiFe-LDH Catalysts with Low-Spin Fe ³⁺ 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/mg_cat.. 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.