Boosting Negative Thermopower of Chitosan Hydrogel via Bio-Inspired Anisotropic Porous Structure

Thermoelectric materials, as key materials for realizing efficient conversion of thermal and electrical energy, are crucial for renewable energy utilization and efficient energy management. However, materials with high negative thermoelectric coefficients are relatively rare. Herein, inspired by the structure and function of plant stem which is capable of blocking heavy metal ions, chitosan/CuCl₂ hydrogel (ChCu) with a huge negative thermoelectric coefficient is reported. The ChCu displayed lamellar porous structure, which is constructed synergistically by freeze-casting technique and complexation between Cu²⁺ and chitosan. In a ChCu hydrogel subjected to a temperature gradient, most of the Cu²⁺ is immobilized within the chitosan matrix by complexation, while the thermal migration of the unbound Cu²⁺ is further intercepted by the special layered porous structure. On the contrary, Cl⁻ migrates unhindered to the cold end and accumulates, which realizes selective migration and distribution of ion/counterion. As a result, ChCu exhibits a thermoelectric coefficient as high as -23.8 mV K⁻¹, and can respond rapidly with a thermal voltage of 4.0 mV under a small temperature difference (ΔT = 0.3 K). This work reveals the significant influence of the polymer aggregate structure on the thermal diffusion of ions, providing an innovative strategy in designating thermoelectric materials with high-performance, high-efficiency and environmentally friendly.