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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.

To improve the radical oxidative stability, a series of covalently cross-linked blend membranes have been prepared from a sulfonated poly(sulfide sulfone) with 80% degree of sulfonation (SPSSF80) and a polybenzimidazole with pendant amino groups (H 2 N-PBI) using glycidyloxypropyltrimethoxysilane (KH-560) and bisphenol A diglycidyl ether (BADGE) as cross-linkers. The resulting cross-linked membranes show increased tensile strength but slightly decreased elongation at break compared with the plain SPSSF80. The radical oxidative stability of the blend membranes is significantly improved due to the synergic action of the covalent cross-linking and the presence of the PBI component. For example, the crosslinked membrane with the composition of SPSSF80/H 2 N-PBI/KH-560 ¼ 7/1/3 started to break into pieces after being soaked in Fenton's reagent for 98 min, which is about 4 times longer than that (20 min) of the plain SPSSF50 (degree of sulfonation ¼ 50%). The covalent cross-linking is also essential to suppress membrane swelling and to enhance membrane water stability. The KH-560-cross-linked blend membranes tend to show higher proton conductivities at low relative humidities than the BADGE-cross-linked one due to the hydrophilic silica network in the former. At fully hydrated state, the cross-linked membranes generally show high proton conductivities comparable with that of Nafion112 ® .

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