Quasi-solid conductive gels with high thermoelectric properties and high mechanical stretchability consisting of a low cost and green deep eutectic solvent

Abstract

Ionic thermoelectric (TE) materials such as ionic hydrogels and ionogels consisting of ionic liquids have emerged as next generation TE materials because of their quasi-solid states and high TE properties, particularly high ionic Seebeck coefficient. However, ionic hydrogels can have a severe stability problem owing to water evaporation, and the ionic liquids of ionogels are very expensive and are usually not biocompatible or even toxic. Here, we report the high TE and mechanical properties of eutectogels made of elastomeric water-borne polyurethane (WPU) and choline chloride (ChCl):ethylene glycol (EG). ChCl:EG is a deep eutectic solvent with high ionic conductivity and non-volatility like ionic liquids, and it has advantages of low cost and environment friendliness in comparison with ionic liquids. The eutectogels can exhibit a high ionic Seebeck coefficient of 19.5 mV K⁻¹, a high ionic conductivity of 8.4 mS cm⁻¹ and a low thermal conductivity of 0.20 W m⁻¹ K⁻¹ at a relative humidity of 90%. In addition, they can have a high mechanical stretchability of up to 216% and a good tensile strength of 1.2 MPa. They can be used as the active materials in ionic TE capacitors for heat harvesting. This is the first time that the TE properties of the gels consisting of a deep eutectic solvent have been reported.