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 ([Im1,4]), N-butyl-N-methylpyrrolidinium ([Pyr1,4]) and N-butyl-N-methylpiperidinium ([Pip1,4]). 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 [Im1,4], [Pyr1,4] and [Pip1,4], respectively) than the widely studied IL containing the bis(trifluoromethylsulfonyl)imide anion, namely [Pyr1,4][Tf2N] (2.7 mS cm-1 at 25 °C). Of the three ILs investigated, [Pip1,4][C(CN)3] presents the widest electrochemical stability window, 3.0 V, while [Pyr1,4][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, [Pyr1,4][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 [Pyr1,4][Tf2N] counterpart only delivered 3.0 W h kg-1 when operated at similar power.
Highlights
Electrochemical double-layer capacitors (EDLCs), or supercapacitors, are energy storage devices that can deliver charge at high rates, typically discharging over a few milliseconds up to several minutes.[1,2,3,4] The main difference between EDLCs and batteries is their charge storage mechanism; where batteries store energy through electrochemical reactions, EDLCs rely on the electrostatic interaction between electrode surfaces and electrolyte ions, i.e. the double-layer.[4]
As no structural changes occur during cycling, the cycle life of EDLCs are substantially greater than batteries, often reported as being in excess of 100 000 cycles
We found that the [C(CN)3] containing ionic liquids (ILs) have high ionic conductivity and that the EDLC using [Pyr1,4][C(CN)3] electrolyte stores more energy when operating at higher power than its [Tf2N] containing analogue
Summary
Electrochemical double-layer capacitors (EDLCs), or supercapacitors, are energy storage devices that can deliver charge at high rates, typically discharging over a few milliseconds up to several minutes.[1,2,3,4] The main difference between EDLCs and batteries is their charge storage mechanism; where batteries store energy through electrochemical reactions, EDLCs rely on the electrostatic interaction between electrode surfaces and electrolyte ions, i.e. the double-layer.[4]. We found that the cyano-based ILs present higher ionic conductivity (9.4, 8.7 and 4.2 mS cmÀ1 at 25 1C for [Im1,4], [Pyr1,4] and [Pip1,4], respectively) than the widely studied IL containing the bis(trifluoromethylsulfonyl)imide anion, namely [Pyr1,4][Tf2N] (2.7 mS cmÀ1 at 25 1C).
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