Aqueous eutectic hydrogel electrolytes enable flexible thermocells with a wide operating temperature range

Abstract

Thermocells (TECs) can directly convert thermal energy into electricity via the thermogalvanic effect of redox ions. The employment of hydrogel electrolytes allows for the facile fabrication of flexible quasi-solid-state TECs with low health hazard, easy scalability and eco-friendliness. In contrast, conventional hydrogel electrolytes cannot adapt to a sufficiently wide working temperature range, as they inevitably freeze under sub-zero temperatures and dry out under elevated temperatures, resulting in loss in both mechanical flexibility and ion transport capability. Herein, a hydrogel electrolyte using low-concentration redox ions with excellent freeze-tolerance and self-humidifying capabilities is rationally designed by regulating the hydration effect, affording a flexible quasi-solid-state aqueous TEC system that can continually work under a wide temperature range (−15 °C to 70 °C). It also demonstrates long-term environmental stability without the need for encapsulation or packaging. The colligative properties of the hydrogel electrolyte can suppress ice crystallization, and in-depth molecular dynamics simulations reveal that a vital underlying mechanism is the strong coordination effect of Li⁺ ions with water molecules over a wide range of temperatures. The TEC designed in this work shows high adaptability to temperature fluctuations and environmental changes, and offers a promising route to promote low-grade heat harvesting under extreme environmental conditions.