High Nitrogen Gas Solubility and Physicochemical Properties of [C₄mpyr][eFAP]-Fluorinated Solvent Mixtures

Electrochemical ammonia synthesis at ambient temperature and pressure shows promise as a route to a carbon-free chemical fuel for renewable energy transportation and storage. However, before this process can be industrialized, issues with the low faradaic efficiencies for the nitrogen reduction reaction must be overcome. This stems from the low nitrogen (N₂) solubility in commonly used aqueous electrolytes and the hydrogen evolution reaction that is usually the more dominant process therein. On the other hand, fluorinated ionic liquid (IL)-based electrolytes can dissolve large amounts of N₂ and reduce the extent of the hydrogen evolution reaction. In this work, the IL 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate [C₄mpyr][eFAP] was studied in binary mixtures with three fluorinated solvents: trifluorotoluene, 1H,1H,2H-heptafluorocyclopentane, and 1H,1H,5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, all of which were found to have high N₂ solubility. The salt-solvent mixtures were found to be miscible in all proportions. The N₂ solubility followed an increasing trend with increasing volume fraction of the fluorinated solvent. The interactions within these binary mixtures were investigated in terms of their volumetric properties, revealing negative values of excess molar volume that were also correlated with higher than expected measured viscosity ratios. The thermodynamic properties of solvation in relation to the role of free volume and the effects of fluorous domains are discussed. At compositions around χ₂ ≈ 0.2, the mixtures showed a maximum in ionic conductivity and this was examined in terms of ionicity. This study demonstrates the scope for the design of fluorinated IL-based electrolytes for electrochemical nitrogen reduction, both in terms of N₂ solubility and mass transport properties.