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

Abstract

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.