Electrolyte design of ionic thermoelectric systems for low-grade heat harvesting

Abstract

Efficient recovery of abundant waste heat, especially low-grade waste heat (< 100 ºC), has sparked intensive research for its potential in reducing the primary energy demand and alleviating the impacts of climate change caused by greenhouse gas emissions. The emerging technologies of ionic thermocells enable direct conversion of the low-grade heat into electricity, exhibiting satisfactory thermoelectric performance, including high thermopower, large power output, and decent energy conversion efficiency. This dissertation focuses on the electrolyte design in ionic thermocells from the perspective of additive-induced interactions. The research work herein is divided into two parts: (1) the exploration of an electrolyte modified by the chemical additives in thermocells for thermoelectric performance improvement, and (2) the development of a novel ionic polymer material with a designed structure for efficient anion diffusion. A new ionic thermocell system, GCTC, based on the thermogalvanic effect, in which ionic chemical additives of guanidine hydrochloride (GdnHCl) and cysteamine hydrochloride (CH) are introduced into 0.4 M ferri/ferrocyanide ([Fe(CN)6]3-/4-) electrolyte, is developed. The concentration gradient profiles of the redox ions between the electrodes are established via the thermosensitive crystallization effect of GdnHCl on [Fe(CN)6]4- and the chemical interaction effect between CH and [Fe(CN)6]3-. The interaction for [Fe(CN)6]4- caused an initial concentration imbalance between the redox ions, triggering the interaction involving [Fe(CN)6]3-, which synergistically enhances the thermovoltage generation. GCTC attains a thermopower value of 4.32 mV K-1 with the additives of 0.5 M GdnHCl and 50 mg mL-1 CH (0.5 M-GCTC), which diminishes the impact of precipitates-resulted cell orientation dependence, making it more adaptive to different practical scenarios. By introducing the ionic additives, the electrical conductivity was protected from being sacrificed for additives-induced thermopower enhancement. A maximum output power of 3.09 mW m-2 K-2 and a Carnot-relative efficiency of 5.50% at a temperature difference of 50 K are obtained, rendering it promising for commercial viability. Ionic thermocells based on the Soret effect are potential candidates for thermal-to-electrical energy conversion due to their large thermovoltage responses. A new n-type ionic polymer material, polyethylenimmonium trifluoromethanesulfonate (PEIFS), is developed. The ionic thermoelectric polymer film (ITEPF) based on PEIFS features a large disparity in the molecular weight and ionic size between the polycationic backbones and anions, which exhibit different diffusion capabilities. The underlying working mechanism governing the generation of thermovoltage via ionic diffusion is proposed on the basis of the relationship among the thermopower, the electrical conductivity, and the relative humidity. The formation of hydrogen bonds between the absorbed water molecules and the anions in the hygroscopic film helps the dissociation of the ion pairs, promoting the anionic thermodiffusion. The effect of different anions, including TfO-, BF4-, and Cl-, on the thermopower further corroborates the contribution of hydrogen bond formation to the ion diffusion-induced thermovoltage generation. The highest negative thermopower of -13.52 mV K-1 (equivalent to -16.69 mV K-1 after temperature calibration) is achieved by PEIFS-based ITEPF at a relative humidity of ~70%, which narrows the development gap between the p- and n-type ionic thermoelectric materials.