Simultaneous measurement and ODE-modeling of ion- and water permeability through ion exchange membranes

The determination of diffusion coefficients in solid materials as a measure of particle mobility is of great scientific interest. This applies both to desired diffusion, such as in fuel cell- and dialysis membranes, and to undesired diffusion in insulating materials. In both cases, diffusion is a measure of performance. Especially for solvated ions, the permeability of the solvent must also be considered. In this work, a measurement method for the simultaneous determination of coupled ion diffusion and water permeability in polymer membranes is presented. For this purpose, measurement methods for the determination of concentration-, volume- and membrane-potential changes are integrated into a single, miniaturized concentration cell. A system of ordinary differential equations is developed for the general description of the coupled transport processes. This makes it possible to model the interdependent solute concentration and solvent volume changes. The coefficients of interest are defined by fitting the model parameters to the measured data. To evaluate the new combined measurement setup, a freestanding ion exchange membrane with largely known properties is used. The membrane of choice is a Nafion® NR211 membrane, since Nafion® is one of the most investigated PEM in literature and allows a comparison of computational and experimental results with existing data.