Self-Diffusion of Ions at Different Time Scales in a Porous and Charged Medium: The Nafion Membrane

Abstract

The self-diffusion coefficients of N(CH 3 ) 4 + ions in a cation-exchange membrane, the Nafion, have been measured at different time scales. The time range extends from the picosecond with neutron quasi-elastic scattering experiments to the second with NMR experiments and tens of seconds with radiotracer experiments. At short times, the values of self-diffusion coefficient in the membrane are equal to those in nonconfined solution; the diffusion process is governed by ion-solvent collisions. The NMR results present two different time scales: for short times (<1 s), the self-diffusion coefficient depends both on time and on electrolyte concentration; for long times, these two dependencies disappear. The radiotracer method confirms the NMR results at long time scales: the diffusion does not depend on electrolyte concentration.

Transport in Proton Conductors for Fuel‐Cell Applications: Simulations, Elementary Reactions, and Phenomenology

“…In this region, the water is bulklike (for not too low degrees of hydration) with local proton transport properties similar to those described for water in Section 3.1.1.1.1. Therefore, the transport properties are indeed a function of the considered length and time scales, 224,225 and the activation enthalpies of both proton mobility and water diffusion are similar to those of bulk water and only increase slightly with decreasing degree of hydration for intermediate water contents (Figure 9). 197,224,[226][227][228] Apart from the slight retardation of proton mobility (D σ ) and water diffusion (D H 2 O ) within the hydrophilic domain, the decrease in the transport coefficients with decreasing degree of hydration mainly reflects on the decreasing percolation within the waterlike domain.…”

The Chemical and Structural Nature of Proton Exchange Membrane Fuel Cell Properties

“…Their tools have primarily consisted of small-angle X-ray scattering (SAXS) and micro-SAXS and small-angle neutron scattering (SANS). Some work has also been performed with nuclear magnetic resonance (NMR) and radiotracer experiments [6,7].…”

Water Dynamics in Ionomer Membranes by Field‐Cycling NMR Relaxometry

“…As a consequence, the proton conduction mechanisms have to be investigated in a wide range of relevant correlation times, from the molecular level to the macroscopic scale. This requires the combination of complementary techniques [1]. The local molecular dynamics inside a porous network or nearby the interfacial region is usually probed at short times and distances (t < 10 ns, r < 5 nm) by quasi-elastic neutron scattering and neutron spin-echo spectroscopy.…”