Isotopic and Ion-Exchange Kinetics in the Nafion-117 Membrane

Abstract

The exchange kinetics of Cs + , Cu 2+ , and Eu 3+ ions in Nafion-117 were measured at different H + ion concentrations in equilibrating salt solutions. The exchange kinetics were found to be independent of the H + ion concentration in equilibrating salt solutions over the pH range of 5-2. The exchange kinetics were therefore used to evaluate self-diffusion coefficients of the metal ions using an analytical solution of Fick's second law. The self-diffusion coefficient of Cs + in Nafion-117 was found to be in agreement with the value obtained in our earlier work (1.94 × 10 -7 cm 2 s -1 ) based on the isotopic-exchange method in the absence of H + ions. This further confirmed that the exchange process in the Nafion-117 membrane represents the pure isotopic exchange in the said pH range. The self-diffusion coefficients of Cu 2+ and Eu 3+ were found to be 2.9 × 10 -7 and 4.5 × 10 -8 cm 2 s -1 , respectively. The smaller self-diffusion coefficient of Eu 3+ reflects its increased electrostatic interaction with the membrane matrix. The kinetics of forward (H (m)n+ ) and reverse (M (m) n+ h H (s) + ) exchanges of metal ions (Cs + , Cu 2+ , and Eu 3+ ) in the Nafion-117 membrane were also studied. The Nernst-Planck theory was examined to predict the kinetics of these exchanges. The theory was found to be reasonably accurate for predicting the kinetics of the reverse ion exchange but completely failed for the forward ion exchange. Contrary to the prediction of the theory, the kinetics of forward exchanges of Cs + , Cu 2+ , and Eu 3+ with H + ions in the Nafion-117 membrane were found to be independent of the metal ions. This shows that the self-diffusion coefficients of the counterions in Nafion-117 can be obtained only from the analysis of reverse exchange kinetics based on the Nernst-Planck equation.

An Isothermal Model for Predicting Performance Loss in PEMFCs from Balance of Plant Leachates

“…[35][36][37][38][39][40] Finally, an average coverage along the channel can be obtained by integration of the local coverage along the length of the channel, 0 < ξ < 1:…”

“…The diffusion coefficient of water vapor through the GDL was used for gas (i.e., water vapor+air+contaminant) diffusion through GDL, and the diffusion coefficients of methanol, ammonium and Na + in Nafion membrane were used to represent the diffusion of DEGEE, 2,6-DAT, and Na + in the ionomer and membrane, respectively, because of the lack of data. [35][36][37][38][39][40][49][50][51] Scaling from ex situ experiments to in situ predictions.-Our goal for the model is to predict voltage losses using ex situ measurements. For the Pt coverage, our data indicates that equilibrium is obtained (Figure 10 quickly in the liquid phase ex situ measurements performed with thinfilm electrodes (TFEs).…”

Vibrational Spectroscopy for the Characterization of PEM Fuel Cell Membrane Materials

“…An important advance in the study was learning that the elements in C could be predicted from the mass transport equation for diffusion in a cylindrical pore in the long time limit [57][58][59][60]. This diffusion equation is known to describe the transport of water and ions within Nafion membrane [58][59][60].…”

“…An important advance in the study was learning that the elements in C could be predicted from the mass transport equation for diffusion in a cylindrical pore in the long time limit [57][58][59][60]. This diffusion equation is known to describe the transport of water and ions within Nafion membrane [58][59][60]. The equation has the form where N is the number of species in the film undergoing transport at time t, N max is the maximum N attained after long measurement periods, and t is a time constant given by the ratio of the pore length L (usually taken as the membrane thickness), and the diffusion coefficient for analyte in the pore D (t ¼ L 2 /D) [57][58][59][60].…”

Strategies for Probing Nanometer-Scale Electrocatalysts: From Single Particles to Catalyst-Membrane Architectures

“…The spectra were examined by applying a least squares approach [7,8]. A diffusion equation that describes molecular transport within a cylindrical pore [9][10][11] was used to predict the time dependent uptake of water into the membrane [1,3]. The analysis revealed bands for vibrational modes for the more hydrophobic polymer functional groups (-CF 2 and -C-O-C-) changed as predicted by the diffusion equation, whereas the response of the prominent band for the asymmetric S-O stretching mode of the hydrophilic -SO 3 -group (near 1064 cm -1 (Fig.…”