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

Abstract

High thermopower and fire-safety ionic thermoelectric hydrogel is crucial for the development of new energy storage device 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, the 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 being 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 applications in supercapacitors.