N-type and P-type series integrated hydrogel thermoelectric cells for low-grade heat harvesting

Low-grade heat is abundant and ubiquitous, but it is generally discarded due to the lack of cost-effective recovery technologies. Ion thermoelectric cells are an affordable and straightforward approach of converting low-grade heat into usable electricity for sustainable power. Despite their potential, ion thermoelectric cells face challenges such as limited Seebeck coefficient and required series integration. Here, we demonstrate that the N-type and P-type conversion of ion thermoelectric cells can be achieved through the phase transition of temperature-sensitive hydrogel containing the triiodide/iodide redox couple. Through the strong interaction between the hydrophobic region of the hydrogel and triiodide, the hydrophobic side selectively captures triiodide and the hydrophilic side repels triiodide, raising the concentration difference of triiodide and thereby increasing the Seebeck coefficient. Specifically, the Seebeck coefficient of the N-type ion thermoelectric cells is 7.7 mV K^-1, and the Seebeck coefficient of P-type ion thermoelectric cells is -6.3 mV K^-1 (ΔT = 15 K). By connecting 10 pairs of the N-type and P-type ion thermoelectric cells, we achieve a voltage of 1.8 V and an output power of 85 μW, surpassing the reported triiodide/iodide-based ion thermoelectric cells. Our work proposes a phase transition strategy for the N-P conversion of ion thermoelectric cells, and highlights the prospect of series integrated hydrogel ion thermoelectric cells for low-grade heat harvesting.