High ionic thermopower in flexible composite hydrogel for wearable self-powered sensor

Ionic-conductive hydrogel sensors are widely used in wearable electronics, and biomedical monitoring. Meanwhile, the hydrogel can use the heat continuously released from human body to generate thermal voltage by relying on the thermal diffusion effect and achieving thermoelectric conversion. It is the most effective solution to realize self-powered supply obtaining energy from environmental waste heat. However, the low thermoelectric output power of hydrogel restricts their applications. Herein, a highly flexible composite hydrogel with ultrahigh thermoelectric output power is designed, wherein hydrogel containing NaCl is prepared by radical polymerization and metal ion complexation, in which the CaCl2 provide the second crosslinking network. Consequently, the optimized hydrogel has excellent stretchability and can withstand up to 1500% tension. The sensitivity of the hydrogel sensor is up to 7.01 in the range of 600%–1500%, which has excellent stability and reversibility. Furthermore, the fast response time of the hydrogel sensor was 12.8 ms. The ionic thermovoltage and power density observed in this study are 34.27 mV K−1 and 730 mW m−2, respectively. The results demonstrated that the ionic-conductive hydrogels with excellent ionic thermovoltage and the ultrahigh power density may be a potential candidate to realize the self-powered performance of hydrogel wearable sensor.