An in-Situ FTIR Study of Lithium-Mediated Electrochemical Nitrogen Reduction

Abstract

Electrochemical ammonia synthesis under ambient conditions is a promising sustainable alternative to the highly carbon-intensive Haber-Bosch process. The lithium-mediated system1 is to date the only rigorously verified method of reducing dinitrogen that has been reproduced by several laboratories; nonetheless, its mechanism is not yet understood. It is generally accepted that lithium is key to the success of this system. Metallic lithium binds strongly to dinitrogen and can directly reduce it under ambient conditions. However, it is possible that the apparently unique ability of this system to reduce dinitrogen under ambient conditions lies in its formation of a Solid Electrolyte Interphase (SEI)2 on the working electrode, like those formed in lithium-ion batteries. This likely provides a kinetic barrier to the competing hydrogen evolution reaction by moderating, but not blocking, proton mobility, thus favouring nitrogen reduction.While some post-mortem characterisation of the SEI for this system has been published3, specific SEI forming reactions and the roles of SEI species in the success of the lithium-mediated system are not yet understood. Optimising this system requires optimisation of the properties of the SEI. It is therefore highly desirable to identify key components which govern its properties and develop a mechanistic understanding of their formation. Just as lithium-ion battery electrolytes are tailored to optimise SEI properties, a deeper understanding of the formation of SEI species from bulk electrolyte components in this system would then inform specific tailoring of the electrolyte to optimise selectivity towards nitrogen reduction. Some reports thus far1,4,5 have proposed that electrodeposited metallic lithium reduces dinitrogen1; others that metallic Li or a LiH/Li3N layer provides a catalytic surface for direct protonation5. However, to the best of our knowledge, no proposed mechanism is yet supported by operando spectroscopic evidence, and no operando characterisation of the SEI has been published.In this work, we present a series of studies using operando surface-enhanced infrared spectroscopy to investigate the mechanisms of the two key processes using a bespoke ATR cell. Firstly, we investigate mechanisms of SEI formation reactions in different electrolyte formulations at different applied potentials, thus identifying the key SEI components that may govern the system’s stability and selectivity, and the potentials at which they form. Secondly, we exploit the surface-enhancement effect to directly probe nitrogen reduction reaction intermediates on the working electrode. By detecting potential NH intermediates, we identify the mechanism of nitrogen reduction and the active sites, potentially informing the design of novel electrocatalysts with greater activity and selectivity. A. Tsuneto, A. Kudo and T. Sakata. Chemistry Letters 22, 851-854 (1993).O. Westhead, R. Jervis and I.E.L. Stephens. Science 372, 1149-1150 (2021)K. Li et al. Science 1597, 1593-1597 (2021).A. Bagger, H. Wan, I.E.L. Stephens and J. Rossmeisl. ACS Catalysis 11, 6596-6601 (2021)J. Schwalbe et al. ChemElectroChem 7, 1-9 (2020).