Studies on Thermally Regenerative Electrochemical Cycles and Temperature Coefficient of Resistance Enhancement in Molybdenum Ditelluride

Abstract

Thermal energy can be used not only as an energy source but also as a signal for sensing. This dissertation studies temperature-driven electrical responses in two different systems. The first system converts low-grade heat into electrical energy using thermally regenerative electrochemical cycles (TRECs). The second system detects temperature variation through the temperature coefficient of resistance (TCR) of ultrathin two-dimensional (2D) molybdenum ditelluride (2H-MoTe2). The common objective is to obtain useful electrical responses from small temperature changes while identifying the loss and instability mechanisms that limit practical performance. The first part of this dissertation evaluates TRECs using three commercial supercapacitors: a lithium-ion capacitor (LIC), a pseudocapacitor, and an electric double-layer capacitor (EDLC). Since supercapacitors can lose stored charge during thermal cycling, self-discharge was explicitly included in the calculation of harvested work and temperature coefficient. A fitting method based on an exponential voltage decay model was used to separate the thermal voltage response from the self-discharge contribution. Under the assumption of 100% heat recuperation between 20 ℃ and 60 ℃, the resulting TREC efficiencies were 8.76 ± 0.02%, 10.2 ± 0.02%, and 10.4 ± 0.1% for the LIC, pseudocapacitor, and EDLC, respectively, with corresponding average temperature coefficients of -1.14, 0.84, and 0.39 mV/K. The LIC showed the largest specific work because of its large temperature coefficient and high operating voltage. The pseudocapacitor showed the lowest energy loss because of its weak self-discharge. The EDLC showed a high efficiency under the ideal heat recuperation assumption, but its small temperature coefficient and stronger self-discharge limited its practical work output. The second part investigates how convective heat transfer affects EDLC based TREC performance. A flow assisted TREC system was operated between 20 ℃ and 60 ℃ while varying the water flow rate from 56 to 184 mL/min. The results showed that the flow rate controls the thermal time constant of the EDLC. At higher flow rates, the heating and cooling periods became shorter, reducing charge loss during open circuit operation. The highest experimental net work was 81.3 μJ at 184 mL/min, corresponding to 34% of the Carnot efficiency. Numerical simulations extended the flow rate range down to 14 mL/min and showed that self-discharge becomes increasingly important below 56 mL/min, where the thermal time constant becomes comparable to the electrical leakage time constant. These results demonstrate that TREC performance depends on the competition between thermal voltage generation and charge leakage. The third part demonstrates continuous electrochemical power generation using thermally assisted capacitive mixing (CapMix) with flowable electrode capacitors. In this system, concentration difference and temperature difference were used simultaneously as driving forces. A single electrode flow capacitor (EFC) test confirmed that the temperature effect increased the generated voltage by a factor of 1.2 and the power density by a factor of 1.5 compared with concentration driven operation alone. The continuous two EFC system was then optimized by varying the activated carbon concentration, slurry salt concentration, and external load resistance. The maximum total power density reached 3.84 mW/m2 for a slurry containing 20 wt% active carbon and 30 g/L salt. Exergy analysis estimated a theoretical thermal conversion efficiency of 10.8% relative to the Carnot efficiency, while the experimental efficiency was limited by internal resistance and the narrow voltage window set by the capacitive Donnan potential (CDP). The final part focuses on TCR enhancement in ultrathin 2H-MoTe2 for resistance based thermal sensing. Surface oxidation by O2 reactive ion etching (RIE) was first used to form a surface layer related to MoOx and TeOx. For suspended oxidized 2H-MoTe2, additional vacuum annealing improved resistance stability and the TCR reached -1.63%/K at 300 K. For supported oxidized 2H-MoTe2, (3-aminopropyl)triethoxysilane (APTES) vapor treatment enhanced the drift corrected TCR to -1.38%/K at 300 K. These improvements were interpreted using a thermally activated transport model. The proposed mechanism is that modification of the oxidized surface layer and molecular functionalization can change the carrier concentration, Fermi level position, and apparent activation energy. The results also showed that TCR enhancement is accompanied by increased resistance, indicating that future sensor design should consider resistance stability and electrical noise as well as TCR. Overall, this dissertation provides experimental and analytical guidelines for improving temperature-driven electrical responses. In electrochemical heat harvesting, the results clarify how self-discharge, thermal time constant, internal resistance, and continuous flow operation determine the usable electrical output. In thermal sensing, the results show that surface and interface engineering can enhance the TCR of ultrathin 2H-MoTe2 while maintaining a thin sensing layer. These findings contribute to the development of low-grade heat harvesting systems and ultrathin resistance based thermal sensing materials.