Understanding the Solid Electrolyte Interphase in the Lithium-Mediated Nitrogen Reduction Reaction

Ammonia stands as one of the most vital chemical commodities worldwide, with its importance poised to grow even further in the future. For more than a century, its production has relied on the Haber-Bosch process. Currently the lithium mediated nitrogen reduction reaction (Li-NRR), during which lithium is plated electrochemically from an organic electrolyte to react with nitrogen, remains the only viable electrochemical strategy for ammonia production. Improvements in selectivity and stability have been predominantly attributed to the solid electrolyte interphase (SEI) layer, however its exact function and composition is mostly unknown.

By means of synchrotron grazing incidence wide angle X-ray scattering (GI WAXS) the SEI layer formed in tetrahydrofuran (THF) based electrolytes was studied operando to elucidate its role in the Li-NRR. The experiments showed that, depending on the electrolyte salt, the SEI layer can stabilize plated lithium by limiting the proton transport to the electrode, which is critical in limiting the competing hydrogen evolution reaction. Insights into the reaction mechanism were gained, as reaction intermediates towards competing reactions but also towards ammonia formation could be observed.

A new electrochemical cell for operando synchrotron measurements was developed to improve on the previous setup that limited faradaic efficiencies. The new cell enabled electrolyte flow, allowing for continuous nitrogen saturation of the electrolyte, enabling faradaic efficiencies up to 37%. Incorporating a gas diffusion electrode on the anode enabled the hydrogen oxidation reduction, preventing oxidative electrolyte decomposition. This improved cell closer resembles scaled-up flow cell systems and allows synchrotron operando measurements under high-performance conditions.

Utilizing this new setup the diglyme/LiBF₄ system was studied. Higher faradaic efficiencies allowed for the clear identification of LiNH₂ as the most stable reaction intermediate towards ammonia formation. Clear differences in SEI species compared to THF based electrolytes were observed and give insights into better performance of diglyme based electrolytes.