Professor Ouyang Jian-yong National University of Singapore: a high thermoelectric properties of ion-based solid ion gel

The thermoelectric materials can be used to directly convert thermal energy into electrical energy. Therefore they have important implications for sustainable human development. efficient thermoelectric materials should have high thermal voltage, high electrical conductivity and low thermal conductivity. The thermoelectric material is a conventional electron or hole carriers as electronic materials. In recent years, ion thermoelectric material (e.g., polymer electrolytes and ionic liquids) Since a voltage having a specific heat of several orders of magnitude higher electron thermoelectric material received widespread attention. However, the ion conductivity of the thermoelectric material is far lower than the latter, so that its practical application is greatly limited. Ion-thermal conversion efficiency of the thermoelectric conversion material depends on the thermoelectric figure of merit ion (ZTi). Recently, NUS Prof. Jian-yong Ouyang TF developed a composed ionic liquids and silica (SiO2) nanoparticles plasma-based solid gel . The gel ion and an ion having a high Seebeck coefficient (14.8 mV / K), high ion conductivity (4.75 × 10 -2 S / cm), and a high power factor ( 1040.4 μW / m / K 2 ) . Meanwhile, thanks to the low thermal conductivity of the ionic gel (0.21 W / m / K)), which is a room temperature ionic ZTi up to 1.47. The ZTI almost twice the highest figure of merit of the thermoelectric reported ions, it is the highest ion ZTi. In this paper three ionic liquid composites and SiO2 nanoparticles of different sizes, the ionic gels have a typical configuration thereof gel fluid properties. Interaction gel derived SiO2 behavior Nanosolid network and an ionic liquid phase of the particles formed. EMIM-DCA ionic liquid as an example, the ionic liquid C≡N bond due to their high polarity and tend to interact with the hydroxyl groups of the SiO2 surface, thus easy to form a stable solid-like gel structure.

FIG. 1. a chemical structural formula of three ionic liquids, b is the composite system with different amounts of SiO2 composition EMIM-DCA / ion gel photograph.

Studies have shown that a small amount of SiO2 nanoparticles can significantly increase the ionic conductivity of ionic gels, in an amount of 20%, The optimal ionic conductivity of up to 4.75 × 10-2 S / cm. This is due to the interaction between the Lewis acid group and the ionic liquid surface of the nanoparticle SiO2 lead ions. In one aspect of this interaction contributes to the positive and negative ions from the ionic liquid solution; while promoting the formation of more free space, thereby establishing a high-speed ion transport channel. Accordingly, the ion mobility of the ions in the gel is effectively enhanced. However, excessive agglomeration of nanoparticles easily hinder ion transport path, thus further adding SiO2 decreases the ionic conductivity.

FIG. 2. a pure EMIM-DCA and EMIM-DCA / (20% SiO2) ionogel relationship Voc, and the temperature difference of the thermal voltage of T. Ion ion gel Seebeck coefficient b for three different ILs SiO2 / ionic liquid mass percent dependence. C ZTi change with SiO2 / mass percentage of the ionic liquid.

ion diffusion at a temperature gradient of the change in enthalpy and entropy of a system of high ionic Seebeck coefficient of the ionic liquid can be understood by thermodynamics. Since the thermal voltage proportional to the difference in thermal swimming mobilities of positive and negative ions, elevated temperatures will increase the mobility difference between positive and negative ions, thereby enhancing the thermal voltage. Compared to the electrophotographic type thermoelectric materials, the ionic liquid / SiO2 ion gel having better thermoelectric properties. EMIM-DCA / (20% SiO2) gel ion plasma power factor 1040μW / m / K2, much higher than other ionic conductor. Meanwhile, ionic gelation temperature ZTi up to 1.47, preferably approximately twice the highest reported value of plasma thermoelectric material. They also demonstrate the use of these ionic thermoelectric materials of thermoelectric conversion. These ionic type thermoelectric materials used to construct the thermoelectric ionic supercapacitor (ITESC), can be converted into heat energy. This is the first report of an ionic liquid-based solid and SiO2 nano-particles made of ion gel having excellent ionic thermoelectric properties. Meanwhile, the ion gel may be used to build ITESC efficient thermoelectric conversion and use. This work provides a new way and to explore new thermoelectric conversion ion ion thermoelectric materials. Professor Ouyang Jian-yong is the corresponding author of this work, Dr. He Xu is the first author, Dr. Cheng Hanlin is co-corresponding author of the work. References: Xu He, Hanlin Cheng, Shizhong Yue, Jianyong Ouyang. Quasi-Solid State Nanoparticle / (Ionic Liquid) Gels with Significantly High Ionic Thermoelectric Properties. Journal of Materials Chemistry A, 2020,