Trace water increases Faradaic selectivity of Li mediated nitrogen reduction Matthew Spry 1 , Olivia Westhead 1 , Romain Tort 1 , Benjamin Moss 1 , Yu Katayama 2 ,
Magda Titirici 1 , Ifan E. L. Stephens 1 , Alexander Bagger 1 1 Imperial College London, UK, 2 Osaka University, Japan
Electrochemical ammonia synthesis under ambient conditions is a promising sustainable alternative to the highly carbon-intensive Haber-Bosch process. The lithium-mediated system is to date the only rigorously verified method of reducing dinitrogen that has been reproduced by multiple laboratories 1–7 . In most reports on this system, the electrolyte consists of a lithium salt (e.g. LiClO 4 , LiBF 4 ) in THF, with ethanol as a proton source. In earlier reports, water as a proton source was shown to be ineffective 1 , and its presence detrimental to selectivity 5 , thus largely dismissed as a contaminant. In this work, we demonstrate a significant trend in Faradaic efficiency in the region of 10-50 mM in electrolytes with different LiClO 4 concentrations, previously overlooked in the literature. Under 1 bar N2, we observe a maximum Faradaic efficiency of 28% with 36 mM water and 0.8 M LiClO 4 . To the best of our knowledge, this is the highest Faradaic efficiency achieved so far at ambient pressure, without the use of a gas diffusion electrode. We propose that water acts not as a proton source, but as a Solid-Electrolyte Interphase (SEI) modifier, We attribute this improvement in performance to the formation of Li2O in the SEI, which reduces the Li ion diffusivity of the SEI and suppresses excessive Li plating 8 , which is a parasitic side reaction. This work highlights the sensitivity of the Li-mediated system, and may suggest that with more rigorous and systematic optimisation of electrolyte parameters, highly efficient ammonia synthesis may be possible under ambient conditions. References 1. Tsuneto A, Kudo A, Sakata T. Lithium-mediated electrochemical reduction of high pressure N2 to NH 3. Journal of Electroanalytical Chemistry . 1994;367:183-188. 2. Andersen SZ, Čolić V, Yang S, et al. A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements. Nature . 2019;570(7762):504-508. 3. doi:10.1038/s41586-019-1260-xAndersen SZ, Statt MJ, Bukas VJ, et al. Increasing stability, efficiency, and fundamental understanding of lithium-mediated electrochemical nitrogen reduction. Energy Environ Sci . 2020;13(11):4291-4300. 4. doi:10.1039/d0ee02246bSchwalbe JA, Statt MJ, Chosy C, et al. A Combined Theory-Experiment Analysis of the Surface Species in Lithium-Mediated NH3 Electrosynthesis. ChemElectroChem . Published online 2020. 5. doi:10.1002/celc.201902124Lazouski N, Schiffer ZJ, Williams K, Manthiram K. Understanding Continuous Lithium-Mediated Electrochemical Nitrogen Reduction. Joule . 2019;3(4):1127-1139. 6. doi:10.1016/j.joule.2019.02.003Cherepanov P v., Krebsz M, Hodgetts RY, Simonov AN, Macfarlane DR. Understanding the Factors Determining the Faradaic Efficiency and Rate of the Lithium Redox-Mediated N2Reduction to Ammonia. Journal of Physical Chemistry C . 2021;125(21):11402-11410. 7. doi:10.1021/acs.jpcc.1c02494Du HL, Chatti M, Hodgetts RY, et al. Electroreduction of nitrogen with almost 100% current-to- ammonia efficiency. Nature . 2022;609(7928):722-727. 8. doi:10.1038/s41586-022-05108-yLi K, Andersen SZ, Statt MJ, et al. Enhancement of lithium-mediated ammonia synthesis by addition of oxygen. Science . 2021;374:1593-1597.
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