Effect of temperature on irreversible and reversible heat generation rates in ionic liquid-based electric double layer capacitors

Abstract

This study reports for the first time, isothermal calorimetric measurements of the instantaneous heat generation rate at each electrode of ionic liquid-based electric double layer capacitors (EDLCs) at different temperatures. Indeed, EDLCs generate reversible and irreversible heat during normal operation and the properties of ionic liquids are known to depend strongly on temperature. Here, the EDLC cell consisted of two identical activated carbon electrodes separated by a mesh separator submerged in ionic liquid-based electrolyte consisting of 1 M N-butyl-n-methylpyrrolidinium bis(trifluoromethane sulfonyl)imide (Pyr14TFSI) dissolved in propylene carbonate (PC). The instantaneous heat generation rate at each electrode was measured at 20 °C, 40 °C, and 60 °C under galvanostatic cycling using an in operando calorimeter. The potential window was limited to 1 V to compare with results from similar devices using aqueous or organic electrolytes. The time-averaged irreversible heat generation was attributed to Joule heating and decreased with increasing temperature due to the associated decrease in internal resistance. Furthermore, the reversible heat generation rates at the positive and negative electrodes were mostly exothermic during charging due to ion adsorption and endothermic during discharging due to ion desorption. They increased slightly with increasing temperature as a result of increasing charge storage. The reversible heat generation rate at the positive electrode was slightly larger than that at the negative electrode. In fact, an endothermic dip was observed in the reversible heat generation rate at the negative electrode at the beginning of charging caused by overscreening effect and/or desolvation of Pyr14+ cations as they enter the activated carbon pores.