Influence of the Water Content on the Diffusion Coefficients of Li⁺ and Water across Naphthalenic Based Copolyimide Cation-Exchange Membranes

Abstract

The transport of lithium ions in cation-exchange membranes based on sulfonated copolyimide membranes is reported. Diffusion coefficients of lithium are estimated as a function of the water content in membranes by using pulsed field gradient (PFG) NMR and electrical conductivity techniques. It is found that the lithium transport slightly decreases with the diminution of water for membranes with water content lying in the range 14 < λ < 26.5, where λ is the number of molecules of water per fixed sulfonate group. For λ < 14, the value of the diffusion coefficient of lithium experiences a sharp decay with the reduction of water in the membranes. The dependence of the diffusion of lithium on the humidity of the membranes calculated from conductivity data using Nernst−Planck type equations follows a trend similar to that observed by NMR. The possible explanation of the fact that the Haven ratio is higher than the unit is discussed. The diffusion of water estimated by 1 H PFG-NMR in membranes neutralized with lithium decreases as λ decreases, but the drop is sharper in the region where the decrease of the diffusion of protons of water also undergoes considerable reduction. The diffusion of lithium ions computed by full molecular dynamics is similar to that estimated by NMR. However, for membranes with medium and low concentration of water, steady state conditions are not reached in the computations and the diffusion coefficients obtained by MD simulation techniques are overestimated. The curves depicting the variation of the diffusion coefficient of water estimated by NMR and full dynamics follow parallel trends, though the values of the diffusion coefficient in the latter case are somewhat higher. The WAXS diffractograms of fully hydrated membranes exhibit the ionomer peak at q = 2.8 nm−1, the peak being shifted to higher q as the water content of the membranes decreases. The diffractograms present additional peaks at higher q, common to wet and dry membranes, but the peaks are better resolved in the wet membranes. The ionomer peak is not detected in the diffractograms of dry membranes.

Transport Properties of Perfluorosulfonate Membranes Ion Exchanged with Cations

“…PFG NMR has been widely used to characterize the water diffusion coefficient in polymer membrane systems ,,,− and it is used here in a similar fashion. The dependence of D H 2 O on the water content for five types of PFSA membranes is shown in Figure b.…”

Molecular Dynamics Simulations of Depth Distribution of Spin-Labeled Phospholipids within Lipid Bilayer

“…23,24,25,26 Fluorescence techniques combined with MD simulations have been used successfully for studying the effect of hydration 27,28 and solvent penetration 29,30 on membrane properties. 31,32,33 …”

Ionic Conductivity, Diffusion Coefficients, and Degree of Dissociation in Lithium Electrolytes, Ionic Liquids, and Hydrogel Polyelectrolytes

“…On the other hand, Figure illustrates the variation of the molar conductivity of lithium with the concentration of various electrolytes in water, specifically lithium acetate, lithium triflate, and lithium chloride (data from ref ), and the corresponding values found in the lithium hydrogel networks. It is observed that there is a great reduction of the molar conductivity of lithium in the networks compared to that in solution.…”

“…Figure 8. Variation of the molar conductivity of lithium with the concentration of various electrolytes in water, specifically lithium acetate (black squares), lithium triflate (green triangles), and lithium chloride35 (red circles), and the corresponding values found in the lithium hydrogel networks (blue triangles).…”