Investigation to ionic thermoelectric materials and principles

Abstract

Ionic thermoelectrics have arisen great interest in recent years. Flexible ionic thermoelectric materials with large thermopowers (~10 mV K^-1) show great promise for ultrasensitive thermal detection in wearable devices and heat harvesting. However, the lack of effective n-type ionic thermoelectric materials seriously hinders their applications. Though more materials are reported with larger thermopowers up to tens of millivolts per Kelvin, there is still lack of exploration to underlying mechanisms of ionic thermoelectric effects. We begin with a report on giant and bidirectionally tunable thermopowers within an ultrawide range from -15 to +17 mV K^-1 in solid ionic-liquid-based ionogels. Particularly, a record high negative thermopower of -15 mV K^-1 is achieved in the ternary ionogel, rendering it among the best n-type ionic thermoelectric materials under the same condition. A novel thermopower regulation strategy through ion doping to selectively induce ion aggregates to enhance ion-ion interactions is proposed. These selective ion interactions are found decisive in modulating the sign and magnitude of the thermopower in the ionogels. A prototype wearable device integrated with 12 p-n pairs is demonstrated with a total thermopower of 0.358 V K^-1, showing promise for ultrasensitive thermal detection. To make ionic thermoelectric materials more fit for wearable devices, large and tunable p- and n-type thermopowers are reported in stretchable and self-healing ionogels made of a fluorocarbon elastomer poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), an ionic liquid 1-ethy1-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI) and a salt acting as the thermopower regulator. Particularly, a self-healing n-type ionic thermoelectric material is developed for the first time. The thermopower of the resultant ionogels varies from + 30 mV K^-1 to -21 mV K^-1 at 90% relative humidity. Meanwhile, the ionogels exhibit excellent stretchability up to 1700% and appropriate Young’s modulus of around 0.26 MPa. Both the mechanical and thermoelectric properties can self-heal spontaneously after damage. Taking advantage of the large p and n-type thermopowers, the stretchable self-healing ionogels are integdrated as a flexible thermoelectric detector with 10 pairs of p- and n-type legs on copper electrodes, showing a total thermopower of 0.25 V K^-1 operating with a low temperature difference of 1-2 K. Regard to the underlying principles, we systematically investigated some typical ionic thermoelectric materials and experimentally proved that ion thermodiffusion (or the Soret effect) contributes little to the large ionic thermoelectric voltage, but the electric double layers caused by temperature-induced differential ion and water adsorption generate the thermovoltage. Possible ion transport channels along the temperature gradient were cut off during the thermopower measurement and it was found the thermovoltage was less influenced compared to the pristine condition. Different species of the electrodes and hydrophilicity of the electrodes surface have a significant impact on the electric double layers, which further affects the thermopower. This research may shed a light on the future development of high-performance ionic thermoelectrics.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Baoling HUANG (Supervisor)