n-Type Ionic Thermoelectric Hydrogels for Low-Grade Heat Harvesting and Motion Monitoring

Abstract With the rapid development of flexible electronics and wearable devices, ionic thermoelectric hydrogels (i-TEHs) have attracted extensive attention in low-grade heat harvesting and wearable sensing applications. However, most existing ionic thermoelectric systems still face challenges, including insufficient n-type thermoelectric performance, poor mechanical flexibility, and limited processing compatibility for integrated device fabrication. To address these challenges, n-type i-TEHs were fabricated via electrohydrodynamic (EHD) printing combined with in situ photopolymerization. The hydrogels were constructed using acrylamide (AM) and 2-acrylamido-2-methyl-1-propanesulfonate (AMPS) as copolymer monomers, LiCl as the conductive salt, and [EMIM][BF4] as the ionic liquid modifier. The effects of LiCl concentration and ionic liquid content on the microstructure, mechanical properties, ionic conductivity, Seebeck coefficient, and power factor were systematically investigated. The optimized APC1.5-IL20 hydrogel achieves a high n-type Seebeck coefficient of −9.22 mV·K–1, an ionic conductivity of 64.09 mS·cm–1, and a power factor of 544.82 μW·m–1·K–2, while exhibiting excellent stretchability (448.46%), fast response (100 ms), and a high gauge factor (3.28). Moreover, the EHD-printed hydrogel demonstrates stable voltage output under various temperature gradients and monitoring capability for human motions, including finger, wrist, elbow, and knee movements. This study not only provides a feasible strategy for the preparation of high-performance n-type i-TEHs by EHD printing but also broadens their applications in low-grade heat recovery and wearable sensing systems.