Mechanistic Insights into KCl-Induced Water Disordering for Efficient Ammonia Electrosynthesis

Abstract

Abstract

The electrocatalytic nitrite reduction reaction (eNO2-RR) presents an extraordinary surge in clean yet green ammonia (NH3) synthesis due to mild operation and its promising compatibility with renewable energy sources. Nevertheless, the sluggish NO2- reaction kinetics and the competing hydrogen evolution reaction (HER) remain challenging for efficient NO2--to-NH3 electrosynthesis. Herein, we reveal a strong dependence of H2O structure on KCl concentration in the electrolyte and decouple the KCl-induced water disordering mechanism for efficient NH3 electrosynthesis. This disrupts the hydrogen-bond network of H2O and increases the proportion of K+-H2O species, thereby transforming an ordered proton-hopping network into a disordered environment. This ordering-to-disordering transition increases the activation entropy and lowers the activation enthalpy, leading to a reduction in the apparent free energy and enabling a 2.9-fold enhancement in NH3 production compared to the conventional electrolyte. The KCl-water electrolyte exhibits an exceptional NH3 yield rate of 111.3 mg cm-2 h-1, a Faradaic efficiency of 99.6% at -0.5 V vs RHE, and stable operation for over 1000 h at 1 A cm-2. Multiscale theoretical simulations further confirm that the K+ ions suppress the HER by elevating the water dissociation barrier from 0.1 to 0.7 eV, while the Cl- ions lower the *NO-to-*NOH hydrogenation barrier, thereby promoting the eNO2-RR kinetics. Moreover, the proposed KCl-induced H2O disordering mechanism for efficient eNO2-RR is universal, delivering consistent performance gains across various catalysts.