High-Performance Flexible Thermoelectric Hydrogels via NaOH-Doped Deep Eutectic Solvent/Polymer Composites

The development of biocompatible, flexible thermoelectric materials for efficient low-temperature waste-heat harvesting remains a key challenge in energy conversion research. In this work, we present a strategy for fabricating high-performance thermoelectric devices by incorporating a deep eutectic solvent (DES) composed of choline chloride and ethylene glycol (ChCl:EG) into a sodium hydroxide-doped poly(vinyl alcohol) (PVA) matrix. Structural characterization using FTIR spectroscopy and EDX confirmed the homogeneous distribution of all components and ionic species within the polymer network. To gain molecular-level insight, molecular dynamics (MD) simulations were performed on the ChCl:EG/PVA/NaOH system over 298–358 K, revealing a clear temperature dependence of electrostatic potentials. Linear regression of these values yielded a Seebeck coefficient of −2.82 mV/K, consistent with experimental results and confirming n-type thermoelectric behavior driven by enhanced ionic mobility at elevated temperatures. This is the first demonstration of such a direct potential-based MD method in the field of ionic thermoelectrics. Experimentally, the gel exhibited an exceptional Seebeck coefficient of −2.34 mV/K at 360.3 K, outperforming earlier reports for ChCl:EG with ferricyanide/ferrocyanide (−1.67 mV/K) and for PVA/NaOH hydrogels (−1.00 mV/K). Temperature-dependent measurements revealed simultaneous increases in Seebeck coefficient and ionic conductivity, leading to improved power-factor performance. Sodium hydroxide acted as an effective dopant, maintaining both mechanical flexibility and chemical stability, thereby supporting practical application. This cost-effective and environmentally friendly approach offers a promising route for flexible thermoelectric devices suited to low-grade heat recovery.