Water transport in liquid and vapour equilibrated Nafion™ membranes

Abstract

Water transport across Nafion membranes was investigated under activity gradients at atmospheric pressure. The activity gradients across the membrane were controlled by exposing one side of the membrane to dry gas under laminar flow, while maintaining liquid or vapour equilibrium with water on the other side of the membrane. The measurements were made under steady state conditions. A dual chamber cell was designed with a gated valve to control boundary and initial conditions for the membrane measurements. The experimental results presented here were used in a vaporization-exchange model reported previously [C.W. Monroe, T. Romero, W. Mérida, M. Eikerling. A vaporization-exchange model for water sorption and flux in Nafion, J. Membr. Sci. 324 (2008)], which showed that sorption across the gas–membrane interface is the limiting transport process for thickness below 300 μm. We hypothesize that the water transport can be represented as the sum of series resistances across the membrane bulk and interfaces. The experimental implementation required new hardware and techniques with precise control over the membrane–gas boundary conditions. Under isothermal conditions, the water transport across liquid-equilibrated (LE) membranes was larger than the corresponding transport under vapour-equilibrated (VE) conditions. Non-isothermal experiments showed that the dehydration at the membrane surface increases the resistance to water transport. These results were interpreted under the assumption that regions of distinct water content will experience diffusion rate variations. Our analysis included consideration of the sorption across the interfaces, and the diffusivity dependency on water content.