A Room-Temperature Excessively Concentrated Polyanionic Polymer Hydrogel Thermocell with High Absorbency, Retainability and Extensibility

Thermo-electrochemical cells (thermocells) with high Seebeck coefficients hold great potential for low-grade heat recovery and low-power electronic applications such as wearable devices and wireless sensors. Hydrogel-based thermocells, which eliminate leakage risks, offer greater convenience and flexibility compared to liquid thermocells in real-world applications. However, hydrogel thermocells still suffer from low power density and limited mechanical performance. Sodium polyacrylate (PANa) hydrogels, with their abundant carboxylate groups, reduce mass transfer resistance for [Fe(CN)6]3–/K4[Fe(CN)6]4– redox couples, while also outperforming widely studied polyacrylamide-based hydrogels in terms of absorption capacity and moisture retention. In light of this, we developed a polyanionic polymer hydrogel thermocell composed of sodium polyacrylate cross-linked with sodium alginate (PANa-SA), which demonstrates an extensibility of 314%, a swelling rate of 571% in 0.4 M equimolar K3[Fe(CN)6]/K4[Fe(CN)6] solutions, and a moisture retainability of 94% over 4 weeks. Furthermore, the PANa-SA hydrogel thermocell exhibits a 35.9% higher normalized power density Pmax/ΔT2 compared to that of conventional polyacrylamide-based hydrogel thermocells. Additionally, since thermocells with higher electrolyte concentrations tend to exhibit superior performance, we successfully achieved a stable 0.8 M excessively concentrated PANa-SA hydrogel thermocell at room temperature, delivering a 20% increase in Pmax/ΔT2. Moreover, we developed a quasi-stable 1.0 M hydrogel thermocell, which achieves a 134.5% increase in Pmax/ΔT2 compared to that of the standard 0.4 M sample. These findings present a universal and effective strategy for enhancing the output performance of hydrogel thermocells by introducing excessive electrolyte concentrations via a simple heated-solution immersion method.