Hygroscopic Protic Ionic Liquids as Electrolytes for Electric Double Layer Capacitors

In this work we report about the synthesis and characterization of a novel protic ionic liquid (PIL), 1-methylpyrrolidinium tetrafluoroborate (Pyr(H1)BF(4)), and about its use as electrolyte in electric double layer capacitors (EDLCs). We show that Pyr(H1)BF(4) containing 0.8 percentage by weight of water is displaying very favorable transport properties and a large electrochemical stability. With the help of molecular dynamics simulations, it is revealed that at these low concentrations the water molecules are dispersed in the ionic liquid and do not form larger aggregates, while arrangements are formed where a water molecule is placed between an anion and cation, forming solvent separated ion pairs. Employing this PIL in EDLCs allows the realization of devices that display higher energy and power in the timeframe of interest for high power systems (few minutes to seconds) compared to those possible with conventional aprotic ionic liquids, even though these latter devices have much higher operative voltages. Considering these results, Pyr(H1)BF(4) can therefore be considered as a very interesting electrolyte for the realization of safe and high performance EDLCs.