High-thermopower ionic thermoelectric hydrogels for low-grade heat harvesting and intelligent fire protection

Abstract

Developing ionic thermoelectric hydrogels with high thermopower and fire safety is crucial for the development of new energy storage devices but full of challenges. Herein, a high-thermopower and flame-retardant ionic hydrogel (HIG) with high ion selectivity was prepared via thermally initiated free-radical polymerization. It was found that the thermoelectric type of HIG transitioned from negative (n-type) to positive (p-type) as the sodium chloride content increased from 0.67 wt% to 1.00 wt%. Afterwards, an ionic thermoelectric pair (p–n junction) with superior thermoelectric performance was constructed. The Seebeck coefficients of n-HIG and p-HIG reached −1.95 mV K⁻¹ and 2.14 mV K⁻¹, respectively, while the p–n junction exhibited a much higher Seebeck coefficient of 3.74 mV K⁻¹. When burned, HIG rapidly generated a voltage exceeding 50 mV and triggered a fire alarm within 4 s, demonstrating excellent thermoelectric efficiency and sensitive early fire-warning capability. Besides, HIG exhibited excellent flame retardancy. Even a propane torch could not ignite HIG within 20 s. The limiting oxygen index (LOI) of wood coated with HIG (Wood@HIG) was higher than 90%, making it rapidly self-extinguishing. Notably, HIG also demonstrated good electrochemical performance and remarkable thermal charging energy storage capabilities, showing good potential for application in supercapacitors.