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

Abstract

This study describes the use of the pyrrolidinium nitrate ([Pyrr][NO₃]) 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.