An Amphiphilic Proton Donor Decouples Acidity from Interfacial Reactivity for Lithium-Mediated Ammonia Synthesis

Abstract Electrochemical ammonia synthesis via a lithium-mediated nitrogen reduction reaction (Li-NRR) faces a fundamental selectivity challenge in which proton donors that accelerate productive protonation simultaneously promote parasitic hydrogen evolution. Here, we demonstrate that 2,2,2-trifluoroethanol (TFE), an amphiphilic alcohol, mitigates this limitation by decoupling thermodynamic acidity from kinetic reactivity. Despite its high acidity, TFE suppresses parasitic reactions through rapid interfacial passivation associated with its fluorinated structure, while modulating the microstructure of a fluoride-rich solid electrolyte interphase (SEI). Blending TFE with ethanol further optimizes proton activity and hydrogen bonding, yielding a thin, uniform SEI enriched in highly crystalline LiF with reduced interfacial resistance. The optimized electrolyte achieved a Faradaic efficiency of 59.0% in a continuous-flow reactor, maintained stable performance under potential-cycling, and reached 82.8% in a high-pressure batch reactor. This study highlights proton-donor engineering as a key strategy to modulate the SEI nanostructure for selectivity in Li-NRR.