Proton Donors Influence Nitrogen Adsorption in Lithium-Mediated Electrochemical Ammonia Synthesis

Lithium-mediated electrochemical ammonia synthesis (LiMEAS) has recently shown promise towards efficient electrochemical ammonia production. This process relies on the formation of a lithium nitride film which is sub- sequently protonated to release ammonia. Designing the electrolyte for this technology requires the selection of a proton donor. In this work, we perform a first-principles analysis to investigate the initial step of nitride formation considering 30 different proton donors (PD). As a baseline, modeling nitrogen on a lithium surface without a PD, we observe that N2 does not spontaneously dissociate on the lithium surface. However, explicitly introducing a PD into the system results in five unique recurring nitrogen config- urations on the lithium slab: (1) embedded, (2) adsorbed, (3) standing, (4) buried, and (5) transferred states. We show that these PD-induced states possess an elongated N-N bond and adsorb more strongly on lithium. Using charge analysis, we show that the charge transferred onto these states strongly correlates with the change in their bond length, a crucial parameter for nitrogen dissociation. These results suggest a more involved role of the PD in the initial stages of nitride formation, and motivate greater consideration for their impact on the LiMEAS pathway.