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Influence of the Water Content on the Diffusion Coefficients of Li<sup>+</sup> and Water across Naphthalenic Based Copolyimide Cation-Exchange Membranes
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 <sup>1</sup>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 <i>q</i> = 2.8 nm<sup>–1</sup>, the peak being shifted to higher <i>q</i> as the water content of the membranes decreases. The diffractograms
present additional peaks at higher <i>q</i>, 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