Water in Protic Ionic Liquids: Properties and Use of a New Class of Electrolytes for Energy-Storage Devices

In this work, the properties of water-in-PIL (PIL=protic ionic liquid) electrolytes are reported based on 1-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr(H4)TFSI). Taking advantage of experimental and theoretical investigations, it is shown that the amount of water inside the electrolyte has a dramatic effect on the viscosity, conductivity, density, cation-anion interplay, and electrochemical stability of Pyr(H4)TFSI. The impact of water on the properties of this ionic liquid also affects its use as an electrolyte for electrochemical double-layer capacitors (EDLCs). It is shown that the presence of water improves the transport properties of Pyr(H4)TFSI, with a beneficial effect on the capacitance retention of the devices in which these electrolytes are used. However, at the same time, water reduces the operative voltage of EDLCs containing this PIL as the electrolyte and, furthermore, it has a strong impact on the inactive components of these systems. To suppress this latter problem, and to realize EDLCs with high stability, the use of inactive components stable in aqueous environment appears necessary.