The chemistry of proton carriers in high-performance lithium-mediated ammonia electrosynthesis

Electrochemical lithium-mediated nitrogen reduction can enable synthesis of ammonia from renewables in a distributed fashion on various scales, but its integration into electrolyser devices presents an ongoing challenge, in particular due to the lack of understanding of the interrelation between the performance and the proton transport parameters. Herein, we use a top-performance N-2 electroreduction system with lithium bis(trifluoromethylsulfonyl)imide electrolyte to correlate the reaction metrics to the properties of the proton carrier, focusing on alcohols, a phosphonium cation, tetrahydrofuran, a Bronsted acid, ammonium and water. We demonstrate that productive carriers require optimised electrolyte compositions, which define the interplay of key reaction steps. Through this understanding, we achieved ammonia electrosynthesis with the phosphonium cation and iso-propanol at performance metrics close to those provided by the ethanol benchmark. Critically, we demonstrate that the latter undergoes irreversible degradation through reaction with oxidised solvent, which is not the case for the more robust iso-propanol and phosphonium cation proton carriers.

Automated summary

Electrochemical lithium-mediated nitrogen reduction can be used to synthesize ammonia from renewables, but integrating it into electrolyzer devices is challenging due to the lack of understanding of the relationship between performance and proton transport parameters. In this study, a top-performance N-2 electroreduction system was used to investigate the correlation between reaction metrics and proton carrier properties, including alcohols, a phosphonium cation, tetrahydrofuran, a Bronsted acid, ammonium, and water. The study showed that optimized electrolyte compositions are required for productive carriers, and ammonia electrosynthesis with the phosphonium cation and iso-propanol achieved performance close to the ethanol benchmark. It was also found that ethanol undergoes irreversible degradation through reaction with oxidized solvent, unlike iso-propanol and phosphonium cation proton carriers.