Thermogalvanic and Thermocapacitive Behavior of Superabsorbent Hydrogels for Combined Low-Temperature Thermal Energy Conversion and Harvesting

Around two-thirds of the energy generated by the society is lost as waste heat. Thermogalvanic cells can continuously convert thermal energy directly into electrical energy. Conversely, thermocapacitors can convert and store thermal energy as thermocapacitance. Here, we report two superabsorbent monolithic polymer hydrogel matrices designed through vessel-templated synthesis, which act as soft host materials for extremely high concentrations of redox-active ions, namely, [Fe(CN)6]3-/4- and Fe2+/3+. These highly charged superabsorbent hydrogels were found to improve both electrocatalysis and ohmic resistance of the hosted redox couples, preventing electrolyte leakage, and enable the ability to perform both thermogalvanic conversion and thermocapacitive storage. An unoptimized maximum thermogalvanic power density was observed at ca. 95 mW m-2 (δT of 20 K), on par with other reported gelled systems. An optimized thermocapacitance density of ca. 220 F cm-2 was achieved, which is 15-fold higher than the highest previously reported. These novel systems therefore present new possibilities in both the harvesting and storage of low-grade waste thermal energy.