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

We investigated the use of fluorine free ionic liquids (ILs) containing the tricyanomethanide anion ([C(CN)]) as an electrolyte in electrochemical double-layer capacitors (EDLCs). Three cations were used; 1-butyl-3-methylimidazolium ([Im]), N-butyl-N-methylpyrrolidinium ([Pyr]) and N-butyl-N-methylpiperidinium ([Pip]). 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 at 25 °C for [Im], [Pyr] and [Pip], respectively) than the widely studied IL containing the bis(trifluoromethylsulfonyl)imide anion, namely [Pyr][TfN] (2.7 mS cm at 25 °C). Of the three ILs investigated, [Pip][C(CN)] presents the widest electrochemical stability window, 3.0 V, while [Pyr][C(CN)] is stable up to 2.9 V and its [TfN] analogue can operate at 3.5 V. Despite operating at a lower voltage, [Pyr][C(CN)] EDLC is capable of delivering up to 4.5 W h kg when operating at high specific power of 7.2 kW kg, while its [Pyr][TfN] counterpart only delivered 3.0 W h kg when operated at similar power.