Realization of Hydrogel Electrolytes with High Thermoelectric Properties: Utilization of the Hofmeister Effect

Ionic thermoelectric materials, renowned for their high Seebeck coefficients, are gaining prominence for their potential in harvesting low-grade waste heat. However, the theoretical underpinnings for enhancing the performance of these materials remain underexplored. In this study, the Hoffmeister effect was leveraged to augment the thermoelectric properties of hydrogel-based ionic thermoelectric materials. A series of PAAm-x Zn(CF₃SO₃)₂, PAAm-x ZnSO₄, and PAAm-x Zn(ClO₄)₂ hydrogels were synthesized, using polyacrylamide (PAAm) as the matrix and three distinct zinc salts with varying anion volumes to impart the Hoffmeister effect. Exceptionally, the most cost-effective ZnSO₄ yielded the highest ionic Seebeck coefficient among the hydrogels, with PAAm-1 ZnSO₄ achieving a remarkable value of -3.72 mV K^-1. To elucidate the underlying mechanism, we conducted an innovative analysis correlating the Seebeck coefficient with the zinc ion transfer number. Additionally, the hydrogel materials demonstrated outstanding mechanical properties, including high elongation at break (>1400% at its peak), exceptional resilience (virtually no hysteresis loops), and robust fatigue resistance (overlapping cyclic tensile curves). This work not only advances the understanding of ionic thermoelectric materials but also showcases the potential of hydrogels for practical waste heat recovery applications.