The True Role of Ethanol in Lithium-Mediated Ammonia Synthesis as revealed by Cryo-Electron Microscopy

Ammonia is one of the highest value industrial chemicals, with 50% of global agriculture using industrially produced ammonia in fertiliser. However, the industrial method to make ammonia, the Haber-Bosch process, is extremely environmentally damaging. The requirement for extreme operating conditions and reliance on methane derived hydrogen results in an enormous energy and CO₂ penalty¹. A better method of ammonia production would be distributed, powered by renewable energy, and carbon free. Electrochemical ammonia synthesis via the Nitrogen Reduction Reaction (NRR) could provide a solution. The only rigorously verified method of electrochemical ammonia synthesis is the lithium-mediated nitrogen reduction system^1,2. This system utilises a lithium salt and organic electrolyte to catalyse the reaction between dinitrogen gas and a non-aqueous proton donor, which is often ostensibly sacrificial ethanol. While great strides have been made in terms of Faradaic efficiency, stability and activity^1,3, there are still numerous unanswered questions about exactly why this system is the sole example of efficient electrochemical ammonia synthesis.<br/><br/>One aspect of the lithium-mediated paradigm that could make it unique is the formation of the Solid Electrolyte Interphase (SEI) on the surface of the working electrode¹. This electronically resistive but ionically conductive SEI is formed from the decomposition of the organic electrolyte and lithium salt on the application of the highly reductive lithium plating potential, analogous to that formed in lithium-ion batteries¹. Recent work⁴ reveals that the chemical makeup of the SEI is critical for system stability and controlling the access of reactants (Li⁺, N₂ and protons) to the electrode surface. The role of ethanol as a proton donor has been recently disputed and it is proposed that it may in fact play a greater role in NRR-SEI formation^5,6. It would be beneficial to be able to study NRR-SEI morphology changes with ethanol concentration via electron microscopy. However, the NRR-SEI is highly beam and air sensitive, making both analysis in and transport to the microscope complex. Therefore, a specialised air-free transfer system and cryogenic analysis is required.<br/><br/>While previous cryo-microscopy investigations suggest changes in NRR-SEI makeup with and without ethanol⁶, we present a systematic study of the effect of changing ethanol concentration on NRR-SEI morphology via cryo-Scanning Electron Microscopy and cryo-Focussed Ion Beam milling, and propose how NRR-SEI morphology may affect Faradaic selectivity. Such work also paves the way for the future investigation of the SEI via cryo-Transmission Electron Microscopy. In addition, we present a novel method of cryogenic specimen preparation for Atom Probe Tomography (APT)⁷. APT will allow us to examine low atomic number elements, such as Li⁸, as well as the effect of ethanol concentration on reactant transport through the SEI.<br/><br/>References<br/>1. Westhead, O., Jervis, R. & Stephens, I. E. L. <i>Science (1979)</i> <b>372</b>, 1149–1150 (2021).<br/>2. Tsuneto, A., Kudo, A. & Sakata, T. <i>Journal of Electroanalytical Chemistry</i> <b>367</b>, 183–188 (1994).<br/>3. Du, H.-L. <i>et al.</i> <i>Nature</i> <b>609</b>, 722–727 (2022).<br/>4. Westhead, O. <i>et al.</i> <i>J Mater Chem A Mater</i> (2023) doi:10.1039/D2TA07686A.<br/>5. Westhead, O. <i>et al.</i> <i>Faraday Discuss</i> (2022) doi:10.1039/D2FD00156J.<br/>6. Steinberg, K. <i>et al.</i> <i>Nat Energy</i> (2022) doi:10.1038/s41560-022-01177-5.<br/>7. Douglas, J. O., Conroy, M., Giuliani, F. & Gault, B. <i>ArXiv</i> (2022) doi:https://doi.org/10.48550/arXiv.2211.06877.<br/>8. Kim, S.-H. <i>et al.</i> <i>J Mater Chem A Mater</i> <b>10</b>, 4926–4935 (2022).