Highly stretchable, low-hysteresis, and antifreeze hydrogel for low-grade thermal energy harvesting in ionic thermoelectric Supercapacitors

Efficient harvest of low-grade waste heat in the environment through thermoelectric materials represents a promising strategy to address the current energy crisis. In practical applications, the mechanical properties of thermoelectric materials are crucial for their lifespan and operational environment. In this study, we designed a series of ionic thermoelectric hydrogels, PAM/CMC-xLiCl, with high mechanical properties. These hydrogels are formed by a dual network structure composed of polyacrylamide (PAM) and sodium carboxymethyl cellulose (CMC), with LiCl serving as the conductive material. At room temperature, the optimal Seebeck coefficient and ionic conductivity of the hydrogels can reach 2.96 mV K⁻¹ and 36.51 mS cm⁻¹, respectively. Based on the physical entanglement, hydrogen bonding, and chemical crosslinking of the polymer chains within the ionic hydrogels, which demonstrate outstanding mechanical properties (elongation at break >1300 %, fracture toughness >1700 kJ m⁻³), maintaining 95 % resilience under a large strain of 400 %. Furthermore, due to the hydration properties of LiCl, the ionic hydrogels exhibit excellent freeze resistance and the capability to absorb moisture for self-regeneration upon drying. Lastly, the fabricated ionic thermoelectric supercapacitor can generate a thermoelectric voltage of 0.182 V under a ΔT of 12 K, with a power density of 6.68 mW m⁻², showing promising prospects for application in waste heat harvest fields.