Dependence of Coulombic Interaction and Hydration on Ion Transport and Seebeck Coefficient in Anion-Exchanged Polyelectrolytes

Abstract

The conversion of thermal energy into electrical energy using thermoelectric materials offers opportunities for heat harvesting, heat-flux sensing, and electrochemical energy storage. Unlike conventional electronic thermoelectrics, ionic thermoelectric materials generate voltage through the thermodiffusion of mobile ions under a temperature gradient, known as the Soret effect. Here, we report polyionic systems based on cationic polymer backbones and mobile counterions that exhibit reproducible negative Seebeck coefficients under humid conditions (50% relative humidity). Two commercially available chloride-based polymers, polydiallyldimethylammonium chloride (PDADMAC) and poly(2-dimethylaminoethyl methacrylate) methyl chloride quaternary salt (MADQUAT), were modified through solution-processable anion exchange using lactate ions, acetate ions, perchlorate (ClO4–) ions, and hexafluoroantimonate (SbF6–) ions. Successful ion exchange was confirmed by X-ray Photoelectron Spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDAX). Anion chemistry strongly influenced thermoelectric performance, particularly in MADQUAT-derived systems, where larger, more hydrophobic, and weakly coordinating anions enhanced ionic conductivity and negative Seebeck coefficients. The (SbF6–)-exchanged MADQUAT achieved an ionic conductivity of 0.73 ± 0.14 S/cm, and a Seebeck coefficient of −5500 ± 300 μV/K, yielding a power factor of 2200 ± 300 μW/mK2. The corresponding PDADMAC system exhibited a conductivity of 0.59 ± 0.20 S/cm, a Seebeck coefficient of −4300 ± 600 μV/K, reaching a power factor of 1100 ± 300 μW/mK2. Density functional theory calculations elucidated humidity-dependent ion diffusion along MADQUAT chains, revealing the roles of counterion chemistry, hydration, and water-mediated electrostatic screening in modulating ion–polymer interactions and diffusion barriers. Additionally, custom-synthesized poly(vinylpyridine) derivatives with different functionalized side chains were investigated. Ether-containing side chains outperformed alkyl analogues, highlighting the importance of water-assisted ion transport and polymer-chain dynamics. Overall, this study establishes counterion selection, polymer–ion interactions, and hydration as key design parameters for developing high-performance ionic thermoelectric materials.