Analyzing the Soret Coefficient Using Time Resolved FTIR-ATR

A new method for measuring the Soret coefficient in opaque and translucent samples is reported. Time-resolved Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) is used to measure the change in concentration near the heated ATR crystal surface. A temperature gradient is imposed on the sample via a water-bath cooled anvil in contact with the opposite surface of the sample. In addition to quantifying the steady-state Soret coefficient, the transient thermal diffusion coefficient (related to mass-diffusion driven by the concentration gradient) can be quantified with this technique. The sample used to demonstrated the technique is a polymer electrolyte composed of lithium bis-trifluoromethanesulfonylimide (LiTFSI) salt in a block copolymer polystyrene-poly(ethylene oxide) (SEO). This study varies equilibrium salt concentrations and examines different temperature gradients, analyzing the effect these parameters have on the magnitude of the Soret coefficient. The Soret effect can be utilized in thermogalvanic cells, by re-harvesting wasted heat from most power generating systems. Thermogalvanic cells are a way to utilize a heat sink to create electrical energy, having the temperature gradient drive the system. Specifically, solid state polymer electrolytes are of interest in these cells as they offer benefits such as low flammability, high safety, and improved energy density, compared to common liquid electrolytes. Thus, application of the Soret effect in thermogalvanic cells is a clear way to greatly increase global energy efficiency, saving money for companies, and decreasing the global environmental footprint. To gain insight as to direction of diffusion in solid-state polymer electrolytes, and make this cell a reality, FTIR-ATR measurements provide an unprecedented data accuracy for modeling. The FTIR makes the correlation between the infrared absorbance of the presented salt and its boundary concentration in the polymer through the Beer Lambert law, which has been proven true for salts in previous work. This technique overcomes many previous limitations to experimental techniques measuring the Soret Coefficient, making the FTIR-ATR an effective and repeatable instrument to use for collecting diffusion coefficients.