Ionic liquids containing tricyanomethanide anions: physicochemical characterisation and performance as electrochemical double-layer capacitor electrolytes

Abstract

We investigated the use of fluorine free ionic liquids (ILs) containing the tricyanomethanide anion ([C(CN)₃]) as an electrolyte in electrochemical double-layer capacitors (EDLCs). Three cations were used; 1-butyl-3-methylimidazolium ([Im_1,4]), N-butyl-N-methylpyrrolidinium ([Pyr_1,4]) and N-butyl-N-methylpiperidinium ([Pip_1,4]). Their physicochemical properties are discussed alongside with their performance as electrolytes. We found that the cyano-based ILs present higher ionic conductivity (9.4, 8.7 and 4.2 mS cm⁻¹ at 25 °C for [Im_1,4], [Pyr_1,4] and [Pip_1,4], respectively) than the widely studied IL containing the bis(trifluoromethylsulfonyl)imide anion, namely [Pyr_1,4][Tf₂N] (2.7 mS cm⁻¹ at 25 °C). Of the three ILs investigated, [Pip_1,4][C(CN)₃] presents the widest electrochemical stability window, 3.0 V, while [Pyr_1,4][C(CN)₃] is stable up to 2.9 V and its [Tf₂N] analogue can operate at 3.5 V. Despite operating at a lower voltage, [Pyr_1,4][C(CN)₃] EDLC is capable of delivering up to 4.5 W h kg⁻¹ when operating at high specific power of 7.2 kW kg⁻¹, while its [Pyr_1,4][Tf₂N] counterpart only delivered 3.0 W h kg⁻¹ when operated at similar power.