Flexible and non-volatile redox active quasi-solid state ionic liquid based electrolytes for thermal energy harvesting

The conversion of low-grade thermal energy to electricity using thermoelectrochemical cells (TECs) is
a promising route to sustainable electricity production. Developing a leak-free and flexible electrolyte is
an important requirement for increasing the safety of these energy harvesting devices. Here, the addition
of polymers to non-volatile redox active ionic liquid electrolytes has been investigated. Polyvinylidene
difluoride (PVDF) or poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) was added to an ionic
liquid electrolyte, consisting of a [Co(bpy)3]
2+/3+ redox couple in 1-ethyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]), to produce either soft gels or free-standing films
depending on the polymer content. These quasi-solid state electrolytes have a Seebeck coefficient as
high as the liquid electrolyte. The effect of gelation, with different amounts of polymer, on the diffusion
coefficient of the redox species is reported. Finally, the first thermal energy harvesting performance of
a non-volatile, flexible redox active quasi-solid state electrolyte is demonstrated. The quasi-solid state
electrolytes allow use of a much thinner electrolyte layer while still maintaining more than half of the
power output of the best ionic liquid electrolyte TEC for the same temperature gradient.