Enhanced Thermopower by Asymmetric Doping of Sodium Chloride in Ionogels

While ionic thermoelectric (i-TE) materials can exhibit high thermopower, most of them rely on the addition of a large amount of ionic liquids (IL). This study demonstrates significant improvements in the thermopower of ionogels composed of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and 1-ethyl-3-methylimidazolium chloride (EMIM:Cl) through the asymmetric doping of sodium chloride (NaCl). This improvement results from the coupling of the preintroduced ion concentration difference and temperature difference, which reduces the cation migration barrier in the direction of thermal diffusion, leading to significantly enhanced performance compared to symmetric doping. Therefore, we report that the PVDF-HFP/EMIM:Cl ionogel doped with NaCl achieves ionic thermopower values of 40.3 ± 2.5 mV K^-1 at 75 wt % IL and 24.4 ± 1.1 mV K^-1 at 33 wt % IL. Asymmetric ion doping opens new avenues for the development of low-cost, high-efficiency i-TE materials for applications such as low-grade heat harvesting and flexible wearable devices.