A pyrrolidinium nitrate protic ionic liquid-based electrolyte for very low-temperature electrical double-layer capacitors

This study describes the use of the pyrrolidinium nitrate ([Pyrr][NO 3 ]) protic ionic liquid (PIL) in a mixture with gamma butyrolactone (γ-BL) as an electrolyte for carbon-based supercapacitors with an operating voltage of 2.0 V and at very low temperature. Thermal and transport properties of this electrolyte were firstly evaluated from −40 °C to 80 °C. The evolution of conductivity with the addition of γ-BL rendered it possible to determine the optimal composition for electrochemical application, with a molar fraction of γ-BL of 0.6. This mixture shows a Newtonian behavior with a low viscosity value of 5 mPa s at 25 °C, and exhibits high conductivity values of up to 65 mS cm −1 at 80 °C. At the same time, exceptional residual conductivity was measured for this composition at −40 °C (9 mS cm −1 ), thanks to the superionic character of pyrrolidinium nitrate PIL. Electrochemical characterization of this electrolyte demonstrated, at first, a passivation on the aluminum collector, secondly good cycling performances with an activated carbon electrode from 50 °C to −40 °C with capacitance up to 132 F g −1 at room temperature and a wide voltage window (2.0 V). Finally at very low temperature (−40 °C), this system demonstrates an unprecedented combination of high specific capacitance (up to 117 F g −1 ), and rapid charging-discharging even at high current density, which is very promising for the progress of energy storage systems with environmentally friendly electrolytes at such very low temperatures. This study describes the use of the pyrrolidinium nitrate ([Pyrr][NO 3 ]) protic ionic liquid (PIL) in a mixture with gamma butyrolactone (γ-BL) as an electrolyte for carbon-based supercapacitors with an operating voltage of 2.0 V and at very low temperature.