Freeze-Tolerant and Transparent Eutectogel with High Conductivity and Long-Lasting Usability for Low-Grade Heat Harvesting

Abstract

Ionic hydrogels and ionogels have been widely utilized as ionic thermoelectric (iTE) materials for converting low-grade heat to usable electricity. However, developing iTE gels that possess environmental tolerance, long-lasting usability, and biofriendliness remains challenging. Herein, we present a highly transparent polymer eutectogel comprising a green eutectic solvent (ES) along with a lithium salt. This eutectogel exhibits a high ionic conductivity of 20.2 mS cm^–1 and an ionic Seebeck coefficient of 9.7 mV K^–1. Thanks to the excellent freeze-resistant property of ES, the ionic conductivity can still reach 7.6 mS cm^–1 even at −20 °C. Additionally, the interaction between the polymer network and ES prevents crystallization within as-prepared eutectogel, instead resulting in a glass transition at −114 °C, slightly lower than a glass transition temperature of ES at −113 °C. Furthermore, as-prepared eutectogel demonstrates exceptional long-term solvent retention, showing no weight loss when exposed to an ambient environment (∼23 °C, ∼60% RH) for 7 days and maintaining 90% of its weight after being placed in an oven for 1 day (50 °C, ∼30% RH with strong air circulation). As-prepared eutectogel also shows excellent stability of TE performance over a wide range of relative humidity. The homemade device utilized as-prepared eutectogel shows the potential to directly power some small electronic devices (such as light-emitting diodes and timers) and to recover wasted heat generated by solar panels. Our results provide a foundation for the development of biofriendly TE gel-like materials that exhibit outstanding environmental tolerance and long-lasting usability.