Experimental investigations on thermally induced energy storage characteristics of hydrogel thermo-electrochemical cells for low-grade heat recovery

Hydrogel thermo-electrochemical cells (also called thermocells or thermogalvanic cells) represent a novel heat-to-power technology driven by temperature difference for harvesting low-grade waste heat. Recently it has been found that hydrogel thermocells possess certain energy storage function with high energy storage density during the thermoelectric conversion process, indicating great potential for harvesting low-grade waste heat. However, the mechanisms behind their energy storage process remain unclear. Most existing studies focus only on materials, which restricts further exploration of their energy storage characteristics. To address this knowledge gap, we prepared typical hydrogel thermocells consisting of graphite electrodes and polyacrylamide (PAAm) hydrogel with K₃[Fe(CN)₆]/K₄[Fe(CN)₆] redox pair electrolytes. We then built a test bench to investigate the energy storage characteristics of hydrogel thermocells under various parametric conditions, including the effects of electrode temperature difference, electrode average temperature, shelving duration, redox pair ion concentration and hydrogel thickness. The above experiments indicate that the energy storage characteristic of hydrogel thermocells originate from the aggregation of ions near the two electrodes during thermoelectric conversion process, but charge storage is achieved by the gel structure to hinder the ion transport and concentration equilibrium after the removing of the temperature difference. The thermally induced energy storage performance is greatly affected by above parameters, for example, the areal capacitance and the energy storage coefficient both decreased by about 57.5 % when the temperature difference of the electrode increases from 10 °C ∼ to 30 °C. This research provides guidance for designing and optimizing energy storage performance of hydrogel thermocells, filling an important void in the field.