Synthesis and Physicochemical Properties of Fluorinated Ionic Liquids with High Nitrogen Gas Solubility

Electrosynthesis of ammonia, especially at ambient temperature and pressure, could provide a facile and efficient renewable energy transportation and storage process. However, the nitrogen reduction reaction (NRR) often exhibits low faradaic efficiencies due to (i) the low solubility of nitrogen gas (N-2) in water and (ii) the hydrogen evolution reaction that is prominent in the same range of potentials in aqueous systems. Ionic liquids (ILs) have been shown to overcome these issues to some extent. In this work, we describe the synthesis and characterization of a family of phosphonium-based ILs with highly fluorinated anions, which are shown to display high N-2 solubilities. Their thermal properties were examined, with perfluorosulfonate-based ILs showing high decomposition temperatures in comparison to a low, two-step decomposition process found with perfluorocarboxylate-based ILs. Their transport properties, including viscosity and ionic conductivity, were fitted to the Vogel-Tammann-Fulcher (VTF) equation over a wide temperature range, and the VTF parameters are described. The electrochemical window for all of the synthesized ILs extend past the reduction potentials required for N-2 reduction. Thus, these high N-2 solubility ILs show scope as nonaqueous electrolytes for the electrochemical NRR.