Weak reactivity of proton donors with lithium promotes highly selective electrochemical ammonia synthesis

This study compares the performance of four alcohols as proton donors in lithium mediated electro-chemical ammonia synthesis. We have identified tert-butanol as a very efficient proton donor at an op-timum concentration in the near absence of mass transfer limitations. Tert-butanol helped to achieve up to 93% Faradaic efficiency and up to 246 nmol cm-2 s-1 ammonia production rate. To understand why tert-butanol outperformed other proton donors, a new methodology was developed in this work: i) all electrochemical experiments were performed in a cell specifically designed to minimize nitrogen and alcohol mass transport limitations to the electrode surface; ii) commercial lithium nitride was chemical-ly reacted with alcohols, with and without added lithium metal, allowing to assess chemical yields of ammonia, completely decoupled from mass transfer and electrochemistry; and iii) thermodynamic analysis was performed. The novel methodology used in this work revealed that weaker nucleophiles, such as tert-butanol, which are less reactive with lithium metal, promote very selective conversion of lithium nitride to ammonia, unaffected by the presence of lithium metal. On the other hand, alcohols that react more readily with lithium, showed lower Faradaic efficiency and chemical yields when metal-lic lithium was present. Finally, this study is not only the first to establish a correlation between chemi-cal yields and Faradaic efficiency, but it also highlights the need for a much broader screening of proton donors and deeper mechanistic understanding to confirm the validity of such a correlation.