Outstanding\nElectrode-Dependent Seebeck Coefficients\nin Ionic Hydrogels for Thermally Chargeable Supercapacitor near Room\nTemperature

Abstract

Thermoelectric\npower generation from waste heat is an important\ncomponent of future sustainable development. Ion-conducting materials\nare promising candidates because of their high Seebeck coefficients.\nThis study demonstrates that ionic hydrogels based on imidazolium\nchloride salts exhibit outstanding Seebeck coefficients of up to 10\nmV K^–1. Along with their relatively high ionic conductivities\n(1.6 mS cm^–1) and extremely low thermal conductivities\n(∼0.2 W m^–1 K^–1), these\nhydrogels have good potential for use in heat recovery systems. The\nvoltage behavior in response to temperature difference (stable or\ntransient) differs significantly depending on the metal electrode\nmaterial. We evaluated the electrode-dependent temperature sensitivity\nof the double layer capacitance of these hydrogels, which revealed\nthat the thermally induced polarization of ions at the interface is\none of the main contributors to the thermovoltage. Our results demonstrate\nthe potential capability for ion and metal interactions to be used\nas an effective baseline for exploring ionic thermoelectric materials\nand devices. The developed thermoelectric supercapacitor exhibits\nreversible charging–discharging behavior under repeated disconnecting–connecting\nof an external load with a constant temperature difference, which\noffers a novel strategy for heat-to-electricity energy conversion\nfrom steady-temperature heat sources.