Flexible Quasi‐Solid State Ionogels with Remarkable Seebeck Coefficient and High Thermoelectric Properties

Abstract Thermoelectric materials can be used to harvest low‐grade heat that is otherwise dissipated to the environment. But the conventional thermoelectric materials that are semiconductors or semimetals, usually exhibit a Seebeck coefficient of much less than 1 mV K⁻¹. They are expensive and consist of toxic elements as well. Here, it is demonstrated environmental benign flexible quasi‐solid state ionogels with giant Seebeck coefficient and ultrahigh thermoelectric properties. The ionogels made of ionic liquids and poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) can exhibit a giant Seebeck coefficient up to 26.1 mV K⁻¹, the highest for electronic and ionic conductors. In addition, they have a high ionic conductivity of 6.7 mS cm⁻¹ and a low thermal conductivity of 0.176 W m⁻¹ K⁻¹. Their thermoelectric figure of merit (ZT) is thus 0.75. The giant Seebeck coefficient is related to the ion‐dipole interaction between PVDF‐HFP and ionic liquids. Their application in ionic thermoelectric capacitors is also demonstrated for the conversion of intermittent heat into electricity. They are especially important to harvest the low‐grade thermal energy that is abundant on earth.