Exceptional n-Type Ionic Thermoelectric Hydrogels by Synergistic Hydrophobic and Coordination Interactions.

Abstract

Ionic thermoelectric (i-TE) materials show promise for flexible energy harvesting and self-powered sensing due to their high ionic Seebeck coefficients (S_i). However, achieving both high thermoelectric performance and mechanical stretchability, especially in n-type systems, remains a critical challenge. Herein, a poly(vinyl alcohol) (PVA)-based n-type i-TE hydrogel is presented that exhibits both large negative S_i (-38.6 mV K^-1) and excellent stretchability (382.5%) through synergistic effect of coordination and hydrophobic interactions. Leveraging the amphiphilic nature of PVA, its hydrophilic hydroxyl groups form coordination bonds with Cu²⁺ ions from copper chloride (CuCl₂), while its hydrophobic backbone interacts with water-soluble conjugated polymer (poly(3-(3'-N,N,N-triethylammonium-1'-propy-loxy)-4-methyl-2,5-thiophene chloride (PMNT). Meanwhile, hydrophobic associations also occur among PMNT chains due to strong π-π stacking and backbone hydrophobicity. This dual-interaction strategy approach enhances the thermophoretic difference between chloride anions (from both CuCl₂ and PMNT) and cations, enabling the giant ionic thermopower. Besides, the long-range hydrophobic interactions contribute to the overall mechanical robustness. The developed hydrogels are successfully implemented in self-powered temperature sensors and low-grade heat harvesting systems, such as powering a cooling fan by utilizing the waste heat from a working tablet. This work offers an alternative pathway toward developing high-performance i-TE materials for wearable electronics and low-grade heat harvesting applications.