Solvation and Stability in Lithium-Mediated Nitrogen Reduction

The lithium-mediated method of electrochemical nitrogen reduction, pioneered by Tsuneto et al1 then verified by Andersen et al2, is currently the sole paradigm capable of unequivocal electrochemical ammonia synthesis. Such a system could allow the production of green, distributed ammonia for use as fertiliser or a carbon-free fuel. However, despite great improvements in Faradaic efficiency and stability since just 20193, fundamental understanding of the mechanisms governing nitrogen reduction and other parasitic reactions is lacking. Lithium Ion Battery (LIB) research can provide insight; since both lithium-mediated electrochemical ammonia synthesis and LIBs utilise an organic solvent and lithium salt, both form a Solid Electrolyte Interphase (SEI), which is electronically insulating but ionically conducting, at the electrode surface. In LIBs, this is necessary to stabilize and cycle low potential materials4. In lithium-mediated ammonia synthesis, the SEI could also have a critical role in controlling the access of protons and other key reactants to the catalytically active sites and promoting greater selectivity toward nitrogen reduction to ammonia5. While some characterisation of the SEI has been carried out for the lithium-mediated nitrogen reduction system6, the literature still lacks holistic studies which aim to carefully characterise the bulk electrolyte and SEI components and link them to system performance. In this work we use insight from battery science to tackle a significant stability problem in lithium-mediated nitrogen reduction. The traditional electrolyte employed by Tsuneto et al. was 0.2 M LiClO4 in a 99:1 tetrahydrofuran to ethanol mix. While this system can produce ammonia, the working electrode potential becomes more negative over time. Our initial investigations show that this problem stems from an unstable SEI which becomes increasingly organic. Simply by raising the concentration of LiClO4 in the electrolyte, we vastly improve stability, as shown in figure 1(a), and boost Faradaic efficiency. Bulk electrolyte salt solvation properties are investigated through Raman spectroscopy, as shown in figure 1(b). Here we observe the emergence of a shoulder at around 930 cm-1 with increasing LiClO4 concentration, which we assign to the emergence of Contact-Ion-Pairs (CIPs) through comparison to Density Functional Theory calculations. These CIPs mean that perchlorate anion degradation products are more abundant in the formed SEI, as shown in our X-Ray Photoelectron Spectroscopy and Time-of-Flight Secondary Ion Mass spectrometry results. This more inorganic SEI protects the electrolyte against further degradation, preventing the working electrode drift to more negative potentials. We then link this behaviour to a peak observed in the Faradaic efficiency of ammonia synthesis at 0.6 M LiClO4 by also considering decreasing N2 solubility and diffusivity, as well as a more ionically conductive SEI, in an increasingly concentrated electrolyte. We also present never-before seen cross-sectional images of the SEI using cryogenic Focussed Ion Beam milling and Scanning Electron Microscopy, further aiding understanding of how salt solvation affects the morphology of the formed SEI and system performance. Our results emphasise the need to consider SEI properties in electrolyte design for lithium-mediated nitrogen reduction, as well as the need to balance desirable SEI properties with desirable bulk electrolyte properties. Tsuneto, A., Kudo, A. & Sakata, T. Efficient Electrochemical Reduction of N 2 to NH 3 Catalyzed by Lithium . Chemistry Letters vol. 22 851–854 (1993). Andersen, S. Z. et al. A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements. Nature 570, 504–508 (2019). Westhead, O., Jervis, R. & Stephens, I. E. L. Is lithium the key for nitrogen electroreduction? Science. 372, 1149–1150 (2021). Peled, E. & Menkin, S. Review—SEI: Past, Present and Future. J. Electrochem. Soc. 164, A1703–A1719 (2017). Singh, A. R. et al. Electrochemical Ammonia Synthesis—The Selectivity Challenge. ACS Catal. 7, 706–709 (2017). Li, K. et al. Enhancement of lithium-mediated ammonia synthesis by addition of oxygen. Science. 1597, 1593–1597 (2021). Figure 1