UCLA

Electrochemical capacitors (ECs) have drawn significant attention as electrical energy storage systems for high power applications thanks to their high cycle efficiency and long cycle life. However, under high current cycling, they can experience a significant amount of heat generation resulting in excessively high cell temperatures. Elevated temperatures, in turn, can lead to (i) increased self-discharge rates, (ii) accelerated aging of the device, and (iii) electrolyte decomposition and evaporation causing deterioration of their performance and lifetime. In this context, ionic liquid (IL) electrolytes are promising due to their excellent thermal stability over large operating temperature windows. However, most recent calorimetric studies have measured irreversible and reversible heat generation rates in electric double layer capacitors (EDLCs) consisting of activated carbon (AC) electrodes with organic and aqueous electrolytes at room temperature. This doctoral thesis investigates experimentally the heat generation rate in AC electrodes with ILs neat or diluted in an organic solvent electrolytes in the temperature range between 5 and 80 �C. First, a potential window of 1 V was used to compare with past calorimetric studies using aqueous or organic electrolytes. Then a more realistic potential window of 2.5 V was tested for the same temperature range. Endothermic dips were observed in the instantaneous heat generation rate at the negative electrode in diluted IL and grew with increasing temperature 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 increase in the electrolyte conductivity with temperature. The total irreversible heat generation was in good agreement with Joule heating for potential window of 1 V, as also observed with aqueous and organic electrolytes. However, the total irreversible heat generation exceeded Joule heating for potential window of 2.5 V, 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 increased with temperature and was larger at the positive than at the negative electrode due to the difference in anion and cation sizes.

Moreover, elevated temperatures associated with high heat generation rates are very concerning for flexible and wearable all-solid-state supercapacitors with gel electrolytes placed in direct contact with the users. Therefore, quantifying the amount of heat generation in such devices is essential for developing thermal management strategy in order to ensure user's comfort and safety. This thesis also investigates heat generation in flexible all-solid-state supercapacitor devices consisting of graphene petals grown on buckypaper electrodes with either conventional (non-redox) or redox-active gel electrolyte. The total irreversible heat generation was equal to Joule heating for both types of devices but was larger in the device with redox-active gel electrolyte due to its larger internal resistance. In addition, the reversible heat generation rate was different at the positive and negative electrodes in each device due to asymmetry in the charging mechanisms caused by a combination of electric double layer (EDL) formation, overscreening effect, and additional redox reactions in the redox-active gel electrolyte. This study further illustrates how in operando calorimetry can not only quantify the heat generation rate necessary to device thermal management but also provide insights into the electrochemical phenomena occurring during cycling of ECs.