Nafion^® Membranes: Molecular Diffusion, Proton Conductivity and Proton Conduction Mechanism

Abstract

H-PFG-NMR diffusion and proton conductivity data for NAFION® 117 is presented as a function of temperature and water content. From this, H2O diffusion on a molecular scale is concluded to be similar to the one in bulk water. But “geometrical” restrictions of diffusion lead to a decreasing pre-exponential factor, i.e. decreasing macroscopic diffusion coefficient with decreasing water content. It is shown that proton conductivity is correlated with water diffusion for low degrees of hydration (vehicle mechanism). With increasing hydration there is an increasing contribution from “structure diffusion” culminating in an amplification factor of A = 2.5 for fully hydrated protonic NAFION® 117 at room temperature. The bonding of water in NAFION® as well as the local environment (hydration) of the acidic proton are concluded to be similar as in acidic aqueous solutions.

Proton conducting composite membranes based on poly(1-vinyl-1,2,4-triazole) and nitrilotri (methyl triphosphonic acid)

“…Polymer electrolyte membranes (PEM) have attracted much attention with the development of perfluorinated sulfonic membranes due to their application in various electrochemical devices such as fuel cells [1,2]. The commercialization of Nafion by DuPont in the late 1960s helped to demonstrate the potential interest in terrestrial applications [3,4].…”

Ionic Conductivity of an Extruded Nafion 1100 EW Series of Membranes

“…Kreuer et al 33 have shown that at reduced water contents ( Ͻ 12) the proton mobility in the hydrophilic nanopores of Nafion is very similar to the mobility of water. At higher water levels ͑such as a value of 22͒ proton hopping was much more significant and the ratio of proton to water movement increased to 2.5.…”

“…Kreuer et al 33 further stressed the significance of the phase separation in Nafion in providing a better-connected hydrophilic network within the ionomer for enhanced proton diffusion. Based on a comparison with a homogeneously sulfonated polyaromatic ionomer, which does not show phase separation, it was shown that the phase separation in the Nafion membrane produced an order of magnitude enhancement in both proton and water mobility.…”

“…Based on a comparison with a homogeneously sulfonated polyaromatic ionomer, which does not show phase separation, it was shown that the phase separation in the Nafion membrane produced an order of magnitude enhancement in both proton and water mobility. 33 Clearly, the membrane pretreatment is important in determining both the water uptake and the proton conductivity of Nafion membranes.…”

Proton conducting plastic crystal electrolytes based on pivalic acid

“…Despite the ability of the electrolytes to deliver good properties at moderate and near-ambient temperatures, many technical problems can arise from operating at lower temperatures [1]. The primary problem that these membranes suffer from is the dependence of their properties (conductivity, mechanical strength, etc.)…”