Kinetics of Water Sorption in NafionThin Films − Quartz Crystal Microbalance Study

Abstract

Quartz crystal microbalance measurements are used to investigate the water sorption in the cast Nafion films exposed to water vapor. Equilibrium and kinetic sorption data are reported for H-and Na-form Nafion films of a thickness ca. 0.02 µm. Water sorption isotherms obtained agree well with the literature data for Nafion117. However, the water diffusion coefficients evaluated from the initial sorption and desorption rates by using the theory of diffusion in a plane sheet are ca. 7 orders of magnitude lower than those obtained from sorption measurements in ca. 200 µm thick Nafion117 membranes. It is concluded that the kinetics of water sorption (desorption) in the partially hydrated Nafion exposed to water vapor is controlled by the first-order process of water transfer in and out of the nanoscopic hydrophilic regions (ion clusters) for membranes thinner than 10 µm. On the other hand, the linear diffusion is the rate-determining step of water transport in thick Nafion membranes and in fully hydrated thin Nafion films.

Resistor-Network Modeling of Ionic Conduction in Polymer Electrolytes

“…As 2 and 3 layers of nodes are used in the simulations, the effective conductivity behavior quickly approaches that of the 3-D case. This is also in qualitative agreement with the existing experimental observations [76][77][78][81][82][83][84][85][86][87][88], which show conductivity approaching the bulk value for thicknesses above 100 nm or so.…”

“…Finally, it is of interest to examine possible proton conduction through interfacial structures or Nafion thin-films that exist within PEFC catalyst layers, where these thin films form a 3D network throughout the layer in which protons must travel [80]. These thin-films demonstrate reduced conductivity and water uptake [76][77][78][81][82][83][84][85][86][87][88] and can limit PEFC performance at low catalyst loadings as discussed recently [89], and the modeling of such transport in thin-films is of active interest [1]. It is well known that when a polymer is confined to thicknesses comparable to its characteristic domain size, its properties and morphology differ from the analogous bulk materials [90].…”

Measurement of water uptake in thin-film Nafion and anion alkaline exchange membranes using the quartz crystal microbalance

“…The QCM has been used for a range of investigations, including polymer interactions [30], and more recently in the study of the Nafion ionomer water uptake [4,11,12]. The water uptake results presented by these investigations vary and only consider the frequency shift of the QCM, so limiting the extent of information accessible using the technique.…”

“…The study of thin film (o1000 nm) Nafion ionomers has been explored in a limited capacity and remains somewhat ambiguous [4,11,12]. Characterisation work presented by Wood et al [13] Karan and co-workers [14,15] as well as Dishari and Hickner [16] have shown the presence of an ultra-thin o 55 nm hydrophilic layer developed on self-assembled thin film Nafion ionomers.…”

“…Studies on Nafion have described water uptake trends and some have commented on the so-called Schroeder's Paradox [4,11,12]. However, to date, most QCM studies have limited the analysis to a simple consideration of the frequency of operation as the key metric of water uptake and overlooked viscoelastic effects, loading mechanisms and subsequent swelling of the interfacial layer upon hydration.…”