Electrochemical Investigation and Modeling of Ion- and Water Transport Through Polymer Membranes

Abstract

Ion exchange membranes are essential for energy applications such as fuel cells or redox flow batteries. Ion-mobility and -selectivity are the key features of such membranes. In interaction with aqueous electrolytes another important feature is the water/solvent transport. The determination of the describing coefficients for the ion permeability, transport number and water permeability is crucial to make systems comparable or to develop membranes with improved performance. This work presents a measurement method for the simultaneous determination of coupled ion- and water permeability in polymer membranes. For this purpose, measurement methods for the determination of concentration-, volume- and membrane-potential changes are integrated into one miniaturized concentration cell. A freestanding ion exchange membrane with largely known properties is then used to evaluate the new combined measurement setup. The membrane of choice is a Nafion 211 membrane, since Nafion is one of the most investigated PEM in literature and allows a comparison of the results with existing data. For the general description of the coupled transport processes, a system of ordinary differential equations is developed. Furthermore, a FEM model is developed in COMSOL Multiphysics®, which specifically simulates the ion distribution within the membrane and the resulting electrical fields.