Novel electrolytes and system designs for thermo-electrochemical cells

Abstract

Low-grade waste heat is ubiquitous and a byproduct of all energy conversion mechanisms. Thermo-electrochemical cells (thermocells) directly convert temperature difference to electromotive force with no moving parts. It is an inexpensive technology to harvest waste heat. However, low power conversion efficiency due to ohmic, interfacial charge transfer and mass transfer resistances has limited its use in commercial applications. This work explores improving electrochemical properties of the standard electrolyte and implementation of thermocells in applications providing liquid cooling. In the initial part of this dissertation, poly (3,4-ethylenedioxythiophene)− poly (styrenesulfonate) was used to form a composite having percolated networks of dispersed carbon nanotubes. The composite was dispersed in the standard electrolyte of thermocells aqueous potassium ferri/ferrocyanide and the mixture was characterized using electrochemical impedance spectroscopy. Analysis of the impedance spectra showed a 10 % increase in ohmic conductivity and about a 5-fold decrease in interfacial charge transfer resistance in the composite electrolyte, which is caused by addition of the charge carriers, interfacial polarization and improved contact at the electrode/electrolyte interface. The enhancement of properties in the composite electrolyte increases the power of a thermo-electrochemical cell by about 30 %. In addition, thermal characterization using transient plane source technique showed that the thermal conductivity decreased by about 5 % for the new composite electrolyte, leading to ZT enhancement by about 15 %. Thermocells low efficiencies, restrict there use as a standalone energy harvesting device. To this end, flow thermo-electrochemical cell (fTEC) has been invented, fabricated and implemented specifically for data centers and it can be extended to wherever liquid cooling heat sinks are employed. The current sole purpose of heat sinks is to provide cooling, fTEC enable supplementing the existing purpose with energy harvesting and temperature monitoring for entire liquid flow line. Initial unoptimized design in term of electrode material and configuration produced power density of 0.05 W/m2 and provided heat transfer coefficient of 450 W/m2 K. Carbon nanotube buckypaper electrodes are used to improve the performance of fTEC. The use of carbon nanotube buckypaper electrode provided a larger surface area and faster kinetics for redox reaction. Optimization of the electrode material and configuration, improved the power density of fTEC to 0.36 W/m2. The original contribution of this work are three fold. Firstly, composite of poly (3,4-ethylenedioxythiophene)− poly (styrenesulfonate) and carbon nanotube was developed that improved the interfacial charge transfer resistance, ohmic conductivity and thermal conductivity of the benchmark electrolyte used for thermocells. Secondly, thermocell having flowing electrolyte was identified, fabricated and implemented. The fTEC provides cooling, energy harvesting and temperature monitoring for liquid cooling applications. Thirdly, the design of the fTEC is improved by taking advantage of the higher temperature available and using electrodes providing larger surface area and faster charge transfer kinetics.