In Operando Calorimetric Measurements for Electrochemical Capacitors
Electrochemical capacitors (ECs) serve as promising electrical energy storage systems due to their potential to achieve both high energy and high power densities. ECs are usually cycled at high current densities resulting in significant amount of volumetric heat generation. This, in turn, may lead to excessive temperature rise that can reduce their lifetime and performance. This dissertation aims to experimentally measure the heat generation rate at electrodes of ECs with different electrodes materials under various charging and discharging conditions.
First, the design, fabrication, and validation of an in operando calorimeter are presented. The in operando calorimeter was able to measure the heat generation rate in each electrode of the EC cell separately. First, EDLC cells consisted of two typical activated carbon (AC) electrodes were investigated. The irreversible heat generation rate in each electrode was in excellent agreement with predictions for Joule heating. The reversible heat generation rate in the positive electrode was exothermic during charging and endothermic during discharging. By contrast, the negative electrode featured both exothermic and endothermic heat generation during both charging and discharging steps. Such asymmetric heating was attributed to asymmetry in the charging mechanism due to the overscreening effect caused by interactions between the anionic functional groups of carboxymethyl cellulose (CMC) binder and the cations at the negative electrode. Second, the effect of potential window on heat generation rate in EDLCs in ionic liquid electrolyte was investigated under galvanostatic cycling. The irreversible heat generation rate increased with increasing the potential window and exceeded Joule heating. This could be attributed to the effect of potential-dependent pore resistance. In addition, a further increase in the irreversible heat generation rate was observed at high potential window due to electrolyte degradation. The reversible heat generation rate increased with increasing potential window due to the increase in the amount of ion adsorbed/desorbed at the electrode/electrolyte interface.
Moreover, the thermal signature associated with the charge storage mechanisms in hybrid supercapacitors consisted of highly porous pseudocapacitive electrode and AC electrode was investigated under constant current cycling. Pseudocapacitive electrodes made of either molybdenum dioxide on reduced graphene oxide (MoO2-rGO) or manganese dioxide on graphene (MnO2-G) were synthesized to investigate heat generation associated with reversible redox reactions involving ion intercalation or fast surface redox reactions, respectively. The irreversible heat generation rate in the pseudocapacitive electrodes exceeded Joule heating. This was attributed to irreversible heat generation associated with redox reactions, polarization heating, and hysteresis in EDL formation and dissolution. Moreover, MoO2-rGO featured endothermic reversible heat generation during charging due to Li+ intercalation. Similarly, MnO2-G featured endothermic heat generation during charging due to non-spontaneous surface redox reactions.
Finally, successful separation of EDL current from faradaic current in pseudocapacitive electrodes will improve the heat generation analysis and provide insights into the physicochemical phenomena associated with each regime. Here, step potential electrochemical spectroscopy (SPECS) and multiple potential step chronoamperometry (MUSCA) methods for determining the respective contributions of EDL and faradaic reactions to charge storage in pseudocapacitive electrodes were theoretically validated.