The Role of Ion Solvation in Lithium Mediated Nitrogen Reduction

Abstract

Since its verification in just 2019, there have been numerous high-profile papers reporting improved efficiency of the lithium-mediated electrochemical nitrogen reduction system to make ammonia. However, the literature lacks a cohesive investigation systematically linking bulk electrolyte properties to electrochemical performance and Solid Electrolyte Interphase (SEI) properties. In this study, we vary electrolyte salt concentration and observe a transition from an unstable working electrode potential to working electrode potential stability and peak in Faradaic efficiency of 7.8 ± 0.5 % at 0.6 M LiClO4. The behaviour is linked to the formation of Solvent Separated Ion Pairs in the electrolyte through Raman spectroscopy. Time of Flight Secondary Ion Mass Spectrometry and X-Ray Photoelectron Spectroscopy reveal a more inorganic, and therefore more stable, SEI layer with increasing salt concentration. A drop in Faradaic efficiency is seen at concentrations higher than 0.6 M LiClO4, which is attributed to a combination of a loss in nitrogen solubility and diffusivity as well as increased SEI conductivity as measured by Electrochemical Impedance Spectroscopy.

Non aqueous Li-mediated nitrogen reduction: Taking control of potentials

“…Variable activity coefficients concentration, eventually affecting activity coefficients of Li-ions making the LiFePO4/Li + equilibrium potential not solely concentration-dependent (Figure 4Figure 4b, right). 5,17,36 Since the solvation environment of Li-ions is so crucial to the performance and stability of Limediated ammonia synthesis systems, 5,9 this tool opens avenues in the fast screening of alternative electrolytes. However, we recommend that experimentalists testing different electrolytes should assess variations in LiFePO4 potential before making comparative conclusions.…”

“…1 The lithium-mediated ammonia synthesis, initially proposed by Tsuneto et al, 2 allows the splitting of the N2 bond by direct dissociation on metallic Li; much evidence suggests that the selectivity is due to the formation of a solid electrolyte interphase over the active surface (Figure 1). [3][4][5] Despite several breakthroughs in performance, [6][7][8][9] the current understanding of this system is still largely limited, notably by the nature of experimental setups. To date, researchers in the field have mostly been using Pt or Ag wires as pseudo-references in conventional 3-electrode systems.…”

“…It therefore makes sense to use a material that can equilibrate with Li-ions to meet criterium (i). As much as the research community draws inspiration from Li-ion battery science to understand and optimise this reaction, 5,7,8 one can design a reference electrode accordingly. In this regard, battery intercalation materials such as lithium iron phosphate (LiFePO4) and lithium titanate (Li4Ti5O12) stand out.…”

“…For most of this work, LiNTf2 1M was chosen as a model electrolyte for its stable working electrode potential during electrolysis and for its high conductivity. 5,9 The Ferrocene-Ferrocenium redox couple acts as an internal reference redox system, 26 with a defined one electron redox equilibrium (Figure 2a, insert). In every test condition, a cyclic voltammogram of the electrolyte was recorded within the Ferrocene-Ferrocenium redox voltage range, at a 50mV.s -1 rate.…”

The origin of overpotential in lithium-mediated nitrogen reduction

“…Lithium is electrodeposited onto the working electrode in-situ and catalyses the reaction. The most common solvent is tetrahydrofuran (THF) and the most common proton donor is ethanol respectively, with different lithium salts yielding varied performance for N 2 reduction 8 . LiClO 4 , originally used by Tsuneto et al, is generally outperformed by fluorinated salts such as LiNTf 2 [9][10][11] .…”