Thermal Diffusion in Polymer Electrolyte Membranes

In most mixtures, a concentration gradient develops in response to a temperature gradient. Steady state is quantified by the Soret coefficient, which is the ratio of the thermal diffusion coefficient to the mutual diffusion coefficient. Polymer electrolytes are an interesting class of materials to study in this regard because they could potentially be used in thermogalvanic cells to convert temperature gradients to electricity. Soret coefficients were measured in thermogalvanic cells with lithium electrodes and a dry polymer electrolyte composed of poly(ethylene oxide) (PEO) and lithium bis‐trifluoromethanesulfonylimide (LiTFSI). Voltage and power were measured in response to temperature gradients. Voltage was used to calculate the concentration gradient (and thereby Soret coefficient). Interestingly, the magnitude of Soret coefficient was similar to that in small‐molecule electrolytes and significantly less than that in neutral polymer blends. Surprisingly, the Soret coefficient of this polymer electrolyte depends on salt concentration. In order to understand the concentration dependence, separate measurements of mutual and thermal diffusion of LiTFSI in PEO-based polymers was conducted with time-resolved infrared spectroscopy.

This work was supported by NSF and FSU.