Ionic Coordination and Hierarchical Architecture Enable Record n-Type Thermoelectric Efficiency in Soft Hydrogels

Ionic thermoelectric materials combine large thermopower, quasi-solid-state behavior, mechanical flexibility, and intrinsic stability, offering a low-cost route for harvesting low-grade heat energy. While p-type ionogels based on ionic liquids have achieved excellent performance, efficient n-type analogues remain challenging. Here, we design a poly(vinyl alcohol) (PVA)-based n-type ionogel by incorporating poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), ionic liquid, and zinc bromide. Upon annealing, strong coordination between Zn²⁺ ions and hydroxyl/sulfonic groups, together with the formation of anion-rich clusters via preferential ion association, promotes rapid anion transport and enhances the Eastman entropy change. Simultaneously, the hierarchically layered ionic architecture suppresses phonon propagation and enhances carrier selectivity, resulting in a desirable balance between high ionic conductivity and low thermal conductivity. Consequently, the hydrogel achieves a remarkable thermopower of -128 mV K^-1, an ultrahigh power factor of 16.7 µW cm^-1 K^-2, and an excellent thermoelectric figure of merit of 1.1 at room temperature. This work establishes a universal strategy for designing high-performance n-type ionic thermoelectric materials and opens avenues for flexible and sustainable heat-to-electricity conversion.