Interfacial Water Transport Measurements in Nafion Thin Films Using a Quartz-Crystal Microbalance

Abstract

Understanding the transport of water in a polymer electrolyte membrane, for example, Nafion, is important for optimizing and predicting fuel cell performance. It had been shown that the interfacial resistance could be responsible for a significant portion of the net water transport. In this study, water vapor sorption experiments were done on thin ionomer films (0.03À3 μm thick) by supporting them on a quartz-crystal microbalance (QCM). The high mass resolution of the QCM enables determination of the membrane water content (λ) of the thin films. A slight depression of the water content was observed in very thin films. Meanwhile, study on thin ionomer films minimizes the contribution of internal water diffusion in the film and allows for simplification of the analyses. Interfacial mass-transfer and mechanical relaxation coefficients were determined as a function of water content and temperature. Empirical equations for the mass-transfer coefficient governing absorption and desorption, each as a function of temperature and ionomer water content, were proposed.

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

“…However, recent studies on the underlying knowledge of the structure/function relationships of ionomer thin-films in the catalyst layers are beginning to show that such an assumption is poor as discussed in more detail below. 157,[298][299][300][301][302][303][304][305] The kinetic expressions, equations 58 and 60 for the HOR and ORR, respectively, result in a transfer current between the electronicconducting (1) and ionic-conducting (2) phases (see equation 10), which is related to the current density in the two phases through equation 23 (neglecting double-layer charging)…”

Resistor-Network Modeling of Ionic Conduction in Polymer Electrolytes

“…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].…”

Effect of humidity on the interaction of CO2 with alkaline anion exchange membranes probed using the quartz crystal microbalance

“…The QCM has proved to be a versatile in-situ mass monitoring technique for the study of polymer interactions at a solid interface, offering nanogram mass resolution [27][28][29]. For this study, the commercial AAEM ionomer (Tokuyama, Japan) is drop cast onto the QCM surface creating a composite resonator, thus allowing the QCM to be operated as a true insitu sensor…”

“…This investigation uses thin film (65 ± 4 nm) drop cast ionomers for each study to ensure that the deposited mass is within the QCM's operable range [29], and to minimise contribution from internal water diffusion from the bulk when operating through the investigative range of relative humidities [28,32,33].…”