Ion-Migrated Mechanistic Insights into Electrocatalytic Nitrogen Reduction at Electrified Cu(111)/Water Interface

Abstract

Modulation of the electric double layer (EDL) by different alkali metal cations (AMCs) is a powerful yet poorly understood strategy for steering the selectivity of the electrochemical nitrogen reduction reaction (eNRR) over the dominant hydrogen evolution reaction (HER) for ammonia synthesis. This study unveils a transformative mechanism for steering the selectivity of the electrochemical nitrogen reduction reaction (eNRR) over the dominant hydrogen evolution reaction (HER) at the charged Cu(111)/water interface using constant-potential ab initio molecular dynamics. This study examines the alkali metal cations (Li+, Na+, and K+) modulated proton dynamics near the electrified interface at –0.5 V vs RHE. The core discovery goes beyond the traditional spectator-ion paradigm by establishing a dynamic solvation descriptor. The results demonstrate that while Li+ remains trapped in the outer Helmholtz plane (OHP) due to a rigid hydration shell, K+ undergoes spontaneous partial desolvation, migrating into the inner Helmholtz plane (IHP). This dynamic relocation provides direct electrostatic stabilization of the critical *NNH intermediate, lowering its activation barrier to 0.973 eV—a pathway inaccessible with Li+ or Na+. Furthermore, the presence of K+ in the IHP induces a catastrophic disruption of the interfacial hydrogen-bond network. By forcing an H-down water orientation, K+ physically deconstructs the proton-transfer pathways required for the competing HER. While Li+ and Na+ maintain multiple water orientations that promote hydrogen bonding, the steric gating effect of K+ effectively suppresses HER. These atomistic insights are consistent with experimental findings showing high NH3 Faradaic efficiency in KOH at –0.5 V vs RHE. By correlating cation desolvation kinetics with proton steric gating, this work identifies hydration-shell lability and local-field effects as the fundamental drivers of selectivity, offering a sophisticated roadmap for designing electrified liquid interfaces for sustainable ammonia synthesis.