Rethinking Metal-Mediated Electrochemical Ammonia Synthesis through Full-Cell Thermodynamics

Abstract

Dissociative, metal-mediated electrochemical nitrogen reduction can achieve high selectivity under mild conditions, but its energy cost is often obscured by cathode-only accounting. Here, we develop a full-cell thermodynamic framework for metal-mediated ammonia electrosynthesis by decomposing the cycle into mediator generation, nitrogen incorporation, and protonation. We show that, once nitrogen incorporation and protonation are made downhill, the dominant lower bound on reversible electrical work is set by the mediator equilibrium potential. For Li-mediated nitrogen reduction, cathode-only Li generation already imposes a floor of approximately 9.12 eV per NH3, while full-cell accounting with anodic water oxidation raises the system-level cost further. Highly reducing mediators such as Li, Ca, and Mg preserve dissociative nitride chemistry but exceed practical energy benchmarks. The resulting design map identifies lower-energy routes through electrolyte-driven potential shifts, alloy or intercalation buffering, electrochemical nitridation, and non-nitride intermediates.