Development of measurement technique of mass diffusion coefficient of aqueous methanol solutions in polymer electrolyte membranes based on infrared Soret forced Rayleigh scattering method using single crystal diamond window in a sample cell

Abstract

We have developed an experimental apparatus for the measurement of diffusion coefficient of aqueous methanol solutions in a polymer electrolyte membrane (PEM) for direct methanol fuel cells (DMFCs), based on the infrared Soret forced Rayleigh scattering (IR-SFRS) method. Because mass transport phenomena in PEMs play significant roles, evaluation and understanding of the mass diffusion in PEMs is important. In the present study, we developed a new sample cell using a single crystal diamond window, which transmits both the heating laser with the wavelength of 9.714 m and the probing laser with the wavelength of 639 nm. The heating wavelength was selected from the emission range of a valuable-wavelength carbon dioxide laser to utilize the absorption of methanol due to CO stretching. By using a single crystal diamond window, the scattered light of the probing laser generated at the window material which causes unpreferable effects on detection signals was reduced to about 1/150, compared to a polycrystalline diamond window. To evaluate the validity of the measurement system, we carried out experiments on the aqueous methanol solutions at a temperature of T = 298.2 K. The mass diffusion coefficient of the aqueous methanol solution with the mole fraction of 0.7 measured in the present study agreed with the measured values by the diaphragm cell technique within the relative standard uncertainty of 8.9%. After the validation, we performed experiments on three concentrations of aqueous methanol solutions in a PEM of Nafion 117 at T = 298.2 K. The mass diffusion in Nafion was observed in several hundreds of milliseconds. We obtained the mass diffusion coefficient with the relative standard uncertainties of 2.5-5.3%, depending on the methanol concentration. The mass diffusion coefficient in Nafion 117 was smaller than that of bulk aqueous methanol solutions, and increased with the increase in the methanol mole fraction.Keywords : Diffusion coefficient, Mass transport in membranes, Aqueous methanol solutions, Polymer electrolyte membrane (PEM), Nafion, Direct methanol fuel cell (DMFC), Optical measurement Matsuura, Tokuda and Nagasawa, Journal of Thermal Science and Technology, Vol.13, No.1 (2018)