Design Rules for Selective N2 Reduction Through Electrolyte Chemical Potentials

Metal-mediated nitrogen reduction offers a route to decentralised, low-carbon ammonia synthesis, but its selectivity is poorly understood and often attributed to the formation of a solid-electrolyte interphase (SEI). Here we show that ammonia selectivity in lithium-mediated nitrogen reduction is governed by the relative chemical potentials of lithium and proton carriers in the electrolyte, rather than by SEI composition. By independently tuning lithium-ion and proton-carrier chemical potentials across a range of salts, solvents and proton sources, we find that ammonia, hydrogen and unreacted lithium yields collapse onto a single descriptor defined by the difference between the Li + /Li and H + /H 2 potentials. Post-mortem analysis of electrode deposits shows that SEI formation correlates with lithium chemical potential but does not determine selectivity; instead, SEI growth primarily suppresses both N 2 reduction and H 2 evolution rates through passivation. These results explain previously observed bell-shaped selectivity trends and provide a thermodynamic framework for designing electrolytes and redox mediators for selective and energy-efficient ammonia synthesis.