Quasi-solid state nanoparticle/(ionic liquid) gels with significantly high ionic thermoelectric properties

Abstract

Ionic conducting materials such as polymer electrolytes and ionic liquids can have a thermovoltage higher than those of electronic thermoelectric materials by several orders of magnitude, while their conductivity is much lower than those of the latter. Hence, their heat-to-electricity conversion efficiency is not high enough for practical application. The heat conversion efficiency of ionic conducting materials depends on the ionic figure of merit (ZT_i). Here, we demonstrate quasi-solid state ionogels made of ionic liquids and SiO₂ nanoparticles. They can have high thermovoltage and ionic conductivity. The optimal ionic ZT_i value can be up to 1.47 with an ionic Seebeck coefficient of 14.8 mV K⁻¹, ionic conductivity of 4.75 × 10⁻² S cm⁻¹, thermal conductivity of 0.21 W m⁻¹ K⁻¹ and ionic power factor of 1040.4 μW m⁻¹ K⁻² at room temperature. This ZT_i value is almost twice the previous highest ZT_i value for ionic conductors used for heat-to-electricity conversion in the literature.