Stretchable polyvinyl alcohol and sodium alginate double network ionic hydrogels for low-grade heat harvesting with ultrahigh thermopower

Recently, employing ionic thermoelectric (i-TE) materials is regarded as a promising strategy to harness low-grade waste heat due to their remarkable ionic Seebeck coefficient. By blending polyvinyl alcohol (PVA), sodium alginate (SA), and polyethylene glycol (PEG) and additional freeze-thaw method, PVA/SA/PEG hydrogel is developed. Via immersing the hydrogel in NaBF4 solutions with different concentrations, the TE and mechanical properties could be adjusted. PVA/SA/PEG/NaBF4-1.5 M hydrogel demonstrates exceptional mechanical properties, with tensile stress and strain up to 69 kPa and 114%, respectively. Moreover, PVA/SA/PEG/NaBF4-1.5 M hydrogel exhibits a high ionic conductivity of 31.4 mS/cm, a maximum ionic Seebeck coefficient of 66.7 mV/K, and an impressive power factor of 13.96 mW/m/K2. These outstanding performances originate from the synergistic effect of Manning's counterion condensation facilitated by SA and the crystal PVA chains. The prototype application of the PVA/SA/PEG/NaBF4-1.5 M hydrogel is demonstrated by a flexible ionic thermoelectric supercapacitor. With the external load resistance is 90 kΩ, the energy collected in one thermal cycle could achieve 4 mJ. This study introduces the exceptional stretchable PVA/SA/PEG/NaBF4 hydrogels with a record-high ionic Seebeck coefficient as promising i-TE materials for future TE application in low-grade waste heat.