Physical and Electrochemical Properties of Ionic Liquids Based on Quaternary Phosphonium Cations and Carboxylate Anions As Electrolytes

Ionic liquids (ILs) have been developed for a wide variety of electrochemical applications due to their unique physicochemical properties such as favorable solubility of organic and inorganic compounds, relatively high ionic conductivity, no measurable vapor pressure, high thermal stability, low flammability, etc. Another advantage of ILs is a designability of the ion structures. Although many kinds of ILs have been already investigated, phosphonium-based ILs have rarely been proposed. We have previously designed and synthesized the phosphonium ILs together with typical anion species such as bis(trifluoromethylsulfonyl)amide (TFSA) and bis(fluorosulfornyl)amide (FSA) anions.1,2 On the other hand, interests in ILs consisting of carboxylic acid based anions have been increasing in recent years. In this work, we synthesize the ILs based on quaternary phosphonium cations (tetraethylphosphonium, P2222 +, and tetrabutylphosphonium, P4444 +) in combination with carboxylate anions such as formate (HCO2 -), acetate (MeCO2 -) and propionate (EtCO2 -), characterizing the physicochemical and electrochemical properties of the ILs as a new family of IL-based electrolyte. The preparation of the phosphonium ILs was carried out by neutralization reactions of the precursor phosphonium hydroxides with the carboxylic acids. The obtained ILs were dried under high vacuum for at least 10 h at 60◦C. We also prepared the corresponding ammonium salts by the same procedure for comparison. The physicochemical and electrochemical properties such as conductivity (ac impedance method), viscosity (a cone-plate type viscometer), density, melting point (DSC), thermal decomposition temperature (TGA) and electrochemical window (linear sweep voltammetry using a conventional three-electrode cell) were measured under dry argon atmosphere. Table 1 summarizes the physicochemical properties of the ILs obtained in this work. The almost compounds were low-melting white solids; however only P4444-HCO2 became a liquid at ambient temperature. The conductivity and viscosity of P4444-HCO2 depended on temperature, showing the Arrhenius-type convex curve behaviors. This means a typical electrolyte behavior generally observed in IL media. P4444-HCO2 showed the lowest viscosity and highest conductivity in the three P4444-based phosphonium salts, which might be attributed to the anion sizes of the three phosphonium salts. It should be noted that the thermal decomposition temperatures of the phosphonium salts were much higher than those of the corresponding ammonium salts, revealing the high thermal stability of the carboxylic anion based phosphonium ILs (Fig. 1). In the electrochemical measurements, it was found that P4444-HCO2 had a high cathodic stability similar to those of the TFSA- and FSA-anion based phosphonium ILs. On the other hand, relatively low anodic potential window up to 0.7 V vs. Ag/Ag+ was observed, which seems likely to be due to the anodic oxidation of formate anion. References 1. K. Tsunashima and M. Sugiya, Electrochem. Commun., 9, 2353 (2007). 2. K. Tsunashima, A. Kawabata, M. Matsumiya, S. Kodama, R. Enomoto, M. Sugiya and Y. Kunugi, Electrochem. Commun., 13, 178 (2011). Figure 1