Heat generation in electric double layer capacitors with neat and diluted ionic liquid electrolytes under large potential window between 5 and 80 °C

This study investigates the effect of temperature on the heat generation and the associated electrochemical phenomena occurring in IL-based EDLCs. The EDLCs consisted of two identical activated carbon electrodes with neat Pyr14TFSI or Pyr14TFSI diluted in propylene carbonate (PC) as electrolytes. The instantaneous heat generation rate at each electrode was measured by isothermal calorimetry between 5 and 80 °C under constant current cycling and potential window of 2.5 V. First, the instantaneous heat generation rate was similar at each electrode in neat IL. However, it was smaller at the negative electrode in diluted IL and featured endothermic dips growing with increasing temperature >40 °C due to overscreening effects, ion desolvation, and/or decomposition of PC. The irreversible heat generation was similar in each half-cell and decreased with increasing temperature due to the reduction in internal resistance, particularly with neat IL. The irreversible heat generation exceeded Joule heating in all cases, especially at high temperature and low current. This was attributed to ion desorption and charge redistribution in the porous electrodes. Finally, the reversible heat generation for both electrolytes was larger at the positive than at the negative electrode due to the difference in anion and cation sizes.