Pyridinium-based protic ionic liquids as electrolytes for RuO2 electrochemical capacitors

A series of pyridinium-based room temperature protic ionic liquids has been synthesized and their key properties, including dynamic viscosity, density, ionic conductivity, glass transition temperature, and potential window on Pt and on glassy carbon have been determined. In order to evaluate the PILs performance as electrolytes in metal-oxide based electrochemical capacitor, the specific capacitance (from 40 to 50 F g−1) and the specific charge (from 23 to 48 C g−1) of a thermally prepared RuO2 electrode have been measured. This class of protic ionic liquids was found to exhibit very low viscosities (from 2.77 to 87.9 cP) along with conductivities ranging from 1.10 to 11.25 mS cm−1, which are properties of the highest importance for applications in energy-storage devices. Structure–property relationships, including the effect of the alkyl chain length of the cation's substituent and of its position, are reported and should improve the success of the rational design of protic ionic liquids for specific applications in the future.