(653c) Evolution of the Solid-Electrolyte Interphase Formed during the Lithium-Mediated Electrochemical Ammonia Synthesis Reaction

Since the advent of the Haber-Bosch process in 1908, ammonia (NH₃) has become one of the most ubiquitous chemicals worldwide. However, the demanding conditions (350-450 °C and 100-200 bar) required for Haber-Bosch have accounted for 1.4% of global CO₂ emissions.¹ Thus, there is incentive to develop cheaper, cleaner alternative NH₃ production methods.

A particularly promising approach in this arena is the lithium-mediated electrochemical ammonia synthesis (LiMEAS) method, in which electroplated Li metal acts as a reducing agent to convert nitrogen gas (N₂) to lithium nitride (Li₃N), which is then protonated to form NH₃.² The most commonly employed electrolyte for such a system is LiClO₄ in 99 vol% tetrahydrofuran (THF) and 1 vol% ethanol (which acts as the proton donor).³ When using this electrolyte, one can achieve ~5% Faradaic efficiency towards NH₃, with most of the remaining charge going towards the undesired hydrogen evolution reaction (HER).⁴ In order to improve selectivity towards NH₃, a precise tailoring of the solid-electrolyte interphase (SEI) formed on the cathode will be necessary. In this work, we elucidate the composition of the SEI throughout the course of a LiMEAS experiment while using the standard THF-based electrolyte. We find that lithium ethoxide (LiEtO) is the major constituent of the SEI, and the amount of LiEtO grows throughout the course of the experiment. We additionally quantify other minor constituents in the SEI, such as lithium chloride and lithium propoxide, and we discuss rational electrolyte design principles to impart an SEI layer which can promote nitridation while suppressing HER.

References

1. Capdevila-Cortada, M. Nature Catalysis 2, 1055–1055 (2019).

2. Fichter, Fr., Girard, P. & Erlenmeyer, H. Helvetica Chimica Acta 13, 1228–1236 (1930).

3. Tsuneto, A., Kudo, A. & Sakata, T. Chemistry Letters 22, 851–854 (1993).

4. Schwalbe, J. A. et al. ChemElectroChem 7, 1542–1549 (2020).