Ion and pore fluid transport properties of a Nafion ® membrane separating two electrolyte solutions Part I. Kinetics of the proton and alkali metal cation transport

Abstract

A theoretical model is developed to describe the impedance of two electrolyte solutions separated by a fixed-site ion exchange membrane. The total impedance is the sum of the solution and membrane impedances, each of which splits into a resistance and a Warburg impedance owing to the diffusion—migration ion transport in the solution. Kinetics of the ion transport across the membrane solution interface is taken into account by using Schlögl's theory of membrane permeation. It is shown that the pore fluid flow can be responsible for the coupling between the kinetic and transport impedances. As a result, the Warburg coefficient of the membrane impedance is predicted to decrease when the ratio of the rate of the pore fluid flow (membrane permeability) and the ion transport in the membrane bulk and/or across the membrane|electrolyte interface increases. Impedance analysis can thus provide an insight into the pore fluid transport properties of an ion exchange membrane. Equilibrium impedance measurements are used to analyse the transport properties of a Nafion ® 117 membrane in the presence of alkali metal cations and protons. The Warburg coefficient of the membrane impedance decreases in the sequence Li +> Na +> K +> Rb + ≈ Cs + > H + and even becomes negative for last four ions, which indicates an increasing relative contribution of the pore fluid flow. An evaluation of the permeability coefficient of the Nation ® membrane from the impedance measurements confirms that in the presence of Na + or Cs + ions the pore diameter is about 5 or 3 times smaller, respectively than in the presence of protons.