Stabilizing calcium nitride for efficient, long-term electrochemical ammonia synthesis.

Electrochemical ammonia synthesis at ambient conditions via calcium-mediated nitrogen fixation holds considerable promise but is impeded by fundamental gaps such as poor gas-liquid interface stability, sluggish hydrogen oxidation reaction (HOR) kinetics, and instability of the critical intermediate calcium nitride. To systematically address these barriers, we i) introduced a high surface-area Ni-based anode specifically selected for enhancing HOR kinetics and minimizing solvent oxidation; ii) substituted the conventionally used tetrahydrofuran solvent with dimethoxyethane (DME) to significantly improve chemical stability; and iii) developed a tailored flow cell configuration to enhance gas-liquid mass transport and stabilize reaction intermediates. Employing in-situ Raman spectroscopy and X-ray photoelectron spectroscopy, we provided direct evidence of stabilized calcium nitride formation, elucidating the crucial roles of solvent stability and electrode composition in sustaining reactive intermediates. As a result of these combined innovations, our system demonstrates substantial performance improvements, achieving a Faradaic efficiency (FE) of 34.35 ± 1.76% in short-term tests and sustaining ~20% FE over extended continuous operation (~56 h). At elevated current densities, the improved gas-liquid interface stability enables robust ammonia production, reaching partial current densities of approximately 219 mA cm-2 at ~29% FE. Isotope-labeling studies with 15N2 confirmed the direct electroreduction of N2, while kinetic analyses underscored the impact of anode material selection on HOR efficiency and overall electrochemical stability. These insights establish critical mechanistic understanding and clear design principles for future calcium-mediated electrochemical nitrogen fixation systems, enabling stable, efficient, and selective ammonia synthesis.