Leonard DICK1, Timo STETTNER2, Yuxin LIU2, Siqi LIU2, Barbara KIRCHNER1, Andrea BALDUCCI2
1Clausius Institute of Physical and Theoretical Chemistry, University of Bonn, Bonn, Germany
2Institute for Technical Chemistry and Environmental Chemistry Friedrich Schiller University, Jena, Germany
Ionic liquids (ILs) inherently have interesting properties like a large electrochemical stability window, low flammability and a low vapor pressure, which are desirable properties for electrolytes in energy storage devices. However, the limited transport properties of pure ILs have been an obstacle to make them favorable electrolytes for power devices. Various approaches have been undertaken to tackle this problem, mainly by the introduction of an additional solvent, hoping to combine the beneficial properties of solvent and IL in the mixture.
Herein, the synthesis of a new protic ionic liquid (PIL), 1-methylpyrrolidinium tetrafluoroborate (PyrH1BF4), containing 0.8\% of water is described and its performance in an electric double-layer capacitor (EDLC) is characterized. Employing such PILs with traces of water as electrolytes is beneficial, as the IL bears an available proton, which can form a hydrogen bond network together with the present water in the system. Therefore the vehicular transport present in a regular IL is accompanied by a Grotthuss like proton hopping mechanism, enhancing the overall charge mobility in the system.
Various properties and the electrochemical stability of the electrolyte system are derived experimentally and show promising results such as high operating voltages, high conductivity and high ionicity. Using molecular dynamics simulations, the structure inside the liquid is revealed, showing that for these low concentrations the water molecules are dispersed throughout the ionic liquid and do not form larger aggregates. Instead, the water molecules are placed between a cation and anion, resulting in solvent separated ion pairs and the formation of a hydrogen bond network within the system.
With these results, PyrH1BF4 with 0.8% water can be considered as an interesting novel electrolyte for high performance electrical devices, representing a way to realize safe and high performance EDLCs.