Cationic Comonomer Induced n-Type PNIPAAM-Based Ionic Thermoelectric Hydrogels with Enhanced Thermopower

Ionic thermoelectric hydrogels, capable of directly converting thermal energy into electrical energy, show promise for applications in self-powered, flexible, and wearable health monitoring. However, achieving ionic thermoelectric hydrogels with excellent mechanical strength and high thermopower remains a major challenge. In this study, an n-type ionic thermoelectric hydrogel has been prepared based on a poly(N-isopropylacrylamide-co-dimethylethyl trimethylammonium methacrylate) (p(NIPAAm-co-DMC)) hydrogel with [Fe(CN)6]3–/4–. When the temperature exceeds the lower critical solution temperature (LCST), p(NIPAAm-co-DMC) contracts, causing the aggregation of positively charged molecular chains and regulating the diffusion of [Fe(CN)6]3–/4–. At the hot end, [Fe(CN)6]3– is captured, while at the cold end, [Fe(CN)6]3– increases, reversing the redox reaction and switching the Seebeck coefficient to the n-type, with a maximum Se value of −4.9 mV K–1. Moreover, ionic interactions within the hydrogel enhance its mechanical properties, increasing the tensile strength by five times to 125 kPa. For application scenarios, the hydrogel battery can generate sensitive electrical responses to temperature, external stress, and joint-induced strain during movement. Therefore, this work provides a feasible strategy for developing a kind of cationic comonomer induced n-type PNIPAAM-based ionic thermoelectric hydrogels, which have potential application in wearable health monitoring device with multimodal signal detection capabilities.