Thermal characterization in electric double layer capacitor and graphite

Abstract

As technology advances rapidly, industries are adapting to global challenges such as the energy crisis by embracing electrification and renewable energy sources. This transition emphasizes the crucial role of thermal energy in our everyday lives. From harnessing solar power to managing industrial heat, thermal energy is essential for sustainability and progress. Through thermal energy harvesting, we extract useful energy from heat to reduce reliance on fossil fuels and move towards a more environmentally friendly future. Additionally, effective thermal management is vital for optimizing the performance of systems such as electronics, highlighting the importance of both thermal energy harvesting and management. Throughout my Ph.D. program, I've been deeply interested in exploring thermal challenges in two engineering fields: thermal energy harvesting and thermal management. Firstly, I initiated the topic of low-grade heat harvesting as my first project. This thermal harvesting was conducted based on electrochemical concepts using electrochemically capacitive systems called supercapacitor. As part of this project, the electric-field dependent heat capacity change was investigated. To do this, the thermal properties of aqueous electric double layer capacitors (EDLCs) with KOH electrolytes were measured using the in situ and operando 3ω hot-wire method. The reversible entropy and free energy changes in the EDLCs caused dynamic changes in the effective heat capacity of the electrodes, which were monitored real-time during charge and discharge operations. In an EDLC with a 6 M KOH electrolyte, the effective heat capacities of the positive and negative electrodes with a varying voltage from 0 to 1 V were estimated to decrease by approximately 9.14% and 3.91%, respectively. This polarization dependence may be attributed to the capacitance capability, which is determined by the size of the adsorbed hydrated ions. Regarding the effect of KOH concentration, the maximal capacitance and change in effective heat capacity were observed in the vicinity of 8 M concentration, and attributed to changes in the thermodynamic state by the capacitance effect. For the second project, I focused on studying thermal conduction in graphite, which is a widely-used thermal management material due to its superb thermal properties. Thermal conduction is classified into diffusive, hydrodynamic, and ballistic thermal transport modes. Among them, phonon hydrodynamic transport has been relatively underexplored because of the stringent material conditions required for its emergence and the experimental challenges associated with its observation. Phonon hydrodynamics show the collective motion of phonons, similar to a molecular fluid flow, which is caused by the dominance of momentum-conserving normal (N) scattering over resistive scatterings (e.g., isotope, boundary, and Umklapp (U) scatterings). In common materials, the hydrodynamic phonon transport only occurs in the extremely low and narrow cryogenic temperature ranges. Recent calculations have predicted that graphitic materials such as graphene and graphite are potential candidates to possess phonon hydrodynamic phenomena even at near-room temperature due to the extraordinarily prevailing N-scattering over U-scattering. Second sound, which is a temperature wave analogous to a pressure sound wave in fluid dynamics, is a phenomenological evidence to characterize the phonon hydrodynamic transport in the transient aspect. Up to date, second sound in bulk graphite was observed in a relatively high temperature range of 100–200 K through optical pump-probe techniques, which agreed with previous computational findings. Compared to bulk graphite, thin graphite may be more favorable to have a strong second sound because of the more significant ZA phonon contribution and the absence of cross-plane heat conduction. Also, the thickness effect on basal-plane phonon transport in thin graphite remains a controversial issue, necessitating further experimental and theoretical investigation to clearly elucidate its relationship. For second sound measurements, a modified time-domain thermoreflectance method was employed with spatially separated pump and probe beams. Furthermore, the experimental results were analyzed with calculations based on the inviscid heat equation model. Although the propagation speed of observed thermal wave peak was closely similar to the theoretical value of the second sound speed, the thermal wave could not be definitely classified as either a diffusive or second sound. Therefore, further careful investigation is necessary for the analysis of peaks related to ballistic, hydrodynamic, and diffusive modes.