Non-Fickian Diffusion of Water in Nafion

Abstract

Understanding the dynamics of water in solid-state polymer electrolytes (e.g., Nafion) is important for a variety of applications ranging from membrane-based water purification to hydrogen fuel cells. In this study, the dynamics of water in Nafion was investigated at both low and high humidities with time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy; a technique that provides a molecular fingerprint of both the diffusant and the polymer simultaneously in real time. At low humidities (0-22% RH), an extended initial time lag resulted in non-Fickian behavior, where dynamic infrared data provided evidence for a reaction between water and sulfonic acid. A diffusion-reaction model was developed and predicted this anomalous behavior, where the time lag was a function of water content. At high humidities (0-100% RH), a slow approach to steady state resulted in non-Fickian behavior, where dynamic infrared data provided evidence of water-induced relaxation in the polymer backbone. A diffusion-relaxation model was developed and regressed well to both the polymer relaxation and water diffusion data, where only one fitting parameter was used for each data set to determine both a relaxation time constant and diffusion coefficient. This approach differs significantly from previous work on non-Fickian behavior in glassy polymers, which, consisted of regressing gravimetric data to models with a minimum of six fitting parameters. Not only do the diffusion coefficients from these two models compare well with Fickian diffusion coefficients from experiments with small water concentration gradients, but also the results in this study provide physical insight into the transport mechanisms of water and relaxation phenomena in solid-state polymer electrolytes.

Mechanism of Ionization, Hydration, and Intermolecular H-Bonding in Proton Conducting Nanostructured Ionomers

“…The MIR region has been already highly documented in Nafion and other PFSA membranes, 10,[16][17][18][19][20][21][22]43 but the FIR spectra were never reported in the literature. 3D plots of a typical hydration sequence from dry to 100%RH are shown in Figure 1 The 3D plots evidence several distinct mechanisms identified through the appearance or disappearance of characteristic bands in both the FIR and MIR.…”

Influence of the Electrolyte Film Thickness on Charge Dynamics of Ionic Liquids in Ionic Electroactive Devices

“…14, 19, 21, 26, 30, 31 Our earlier studies have shown that 40 wt% [C 2 mim][TfO] uptake is above the critical IL uptake where the conductivity of the ionomer/IL membrane exhibits much higher conductivity (~ 9 × 10 −6 S/cm) than that of the pure ionomer. 14, 32, 33 It should also be noted that the Aquivion membrane without ILs exhibits very low ionic conductivity which makes it unsuitable for studying the charge dynamics here.…”

Fast Water Diffusion and Long-Term Polymer Reorganization during Nafion Membrane Hydration Evidenced by Time-Resolved Small-Angle Neutron Scattering

“…34,36 Yet, as recently highlighted by Hallinan et al, most of the results were obtained using experimental cells subjected to a low flow of humidified gas, which does not provide a constant water concentration at the membrane surface, as required to properly use Fick's model. 36,37 Consequently, in these studies the water uptake should be controlled by the interfacial transport resistance, biasing the D m measurements, as first evidenced by Satterfield and Benziger. 35 The influence of interfacial effects can be overcome by forcing a constant boundary condition during the sorption process, i.e., by contacting the membrane with liquid water.…”