To clarify transport mechanisms of ions and water molecules in perfluorosulfonated ionomer membranes, various membranes, such as one Nafion, two Aciplex, and four Flemion types, having different equivalent weight values (EW) were examined. H-, Li-, and Na-form samples were prepared for each membrane by immersion in 0.03 M HCl, LiCl, and NaCl aqueous solutions, and their properties in the fully hydrated state were investigated systematically. The water content of the membranes showed the tendency that the size and/or the number of ionic cluster region increases with decreasing EW value and the Li-form membranes have the most largely expanded ionic cluster regions. The ionic conductivity of the H-form membranes was considerably higher than that of the Li-and Na-form membranes. It was suggested that the proton in the membranes transports by the hopping mechanism and the Li + and Na + ions by the vehicle mechanism. In addition, the ionic conductivity of all membranes increased with increasing water content within the same kinds of membranes. Although the cation concentration in the membranes is insensitive to the EW value, the cation mobility increased with decreasing EW value. This means that the increased mobility of the carrier cations is the major factor to enhance the ionic conductivities due to the expansion of the ionic cluster regions. The water transference coefficients for the Li-and Na-form membranes showed higher values than those of the H-form membranes, while the water permeabilities gave the inverse tendency. This means that the water molecules in the Li-and Na-form membranes interact with the Li + and Na + cations more strongly than the proton when the cations move in the membranes, and as a result the diffusion of the water molecules is reduced. All of the above results were in good agreement with the results of the self-diffusion coefficients of the water molecules and the Li + cation in the membranes measured by pulsed field gradient NMR.

To clarify the mechanisms of transport of ions and water molecules in perfluorosulfonated ionomer membranes for fuel cells, the temperature dependence of their transport behaviors was investigated in detail. Two types of Flemion membranes having different equivalent weight values (EW) were utilized along with Nafion 117 as the perfluorinated ionomer membranes, and H-, Li-, and Na-form samples were prepared for each membrane by immersion in 0.03 M HCl, LiCl, and NaCl aqueous solutions, respectively. The ionic conductivity, water self-diffusion coefficient (D(H)(2)(O)), and DSC were measured in the fully hydrated state as a function of temperature. The ionic conductivity of the membranes was reflected by the cation transport through the intermediary of water. Clearly, H(+) transports by the Grotthuss (hopping) mechanism, and Li(+) and Na(+) transport by the vehicle mechanism. The differences of the ion transport mechanisms were observed in the activation energies through the Arrhenius plots. The D(H)(2)(O) in the membranes exhibited a tendency similar to the ionic conductivity for the cation species and the EW value. However, no remarkable difference of D(H)(2)(O) between H- and the other cation-form membranes was observed as compared with the ionic conductivity. It indicates that water in each membrane diffuses almost in a similar way; however, H(+) transports by the Grotthuss mechanism so that conductivity of H(+) is much higher than that of the other cations. Moreover, the D(H)(2)(O) and DSC curves showed that a part of water in the membranes freezes around -20 degrees C, but the nonfreezing water remains and diffuses below that temperature. This fact suggests that completely free water (bulk water) does not exist in the membranes, and water weakly interacting with the cation species and the sulfonic acid groups in secondary and higher hydration shells freezes around -20 degrees C, while strongly binding water in primary hydration shells does not freeze. The ratio of freezing and nonfreezing water was estimated from the DSC curves. The D(H)(2)(O) in the membranes was found to be influenced by the ratio of freezing and nonfreezing water. DFT calculation of the interaction (solvation) energy between the cation species and water molecules suggested that the water content and the ratio of freezing and nonfreezing water depend strongly on the cation species penetrated into the membrane.

As a typical degradation of the proton exchange membranes ͑PEMs͒ for fuel cells, formation of hydrogen peroxide ͑H 2 O 2 ͒ on the cathode surface has been presented to be a key issue which leads to the decomposition of PEM. Using perflurosulfonated ionomeric membranes with different equivalent weights ͑EW = 900, 1000, and 1100͒ as test samples, degradation of PEM was investigated systematically in practical fuel cell usage conditions ͑e.g., 80°C͒ during the progress of H 2 O 2 treatment. Membranes were characterized for proton conductivity by ac impedance technique, pulsed-field-gradient spin-echo NMR, Fourier transform infrared spectroscopy, thermogravimetric analysis ͑TGA͒, and extensile experimentation. Durability studies over a period of 1 month operation revealed evident membrane degradation ascribed to the decomposition of sulfonic acid groups in pendant side chains. The products of cross-linked S-O-S ͑condensation sulfonates͒ were strongly demonstrated by IR spectroscopy as a result of long H 2 O 2 treatment times, which suggests oxidation provoked by H 2 O 2 . Proton conductivity and the water self-diffusion coefficient decreased significantly due to the loss of water inside the membranes. TGA revealed further changes in the membrane morphology, where the onset and decomposition temperatures of the membranes changed upon exposure to H 2 O 2 . Membranes with high EW showed a faster decomposition rate than the other ones, whereas the mass loss step showed the reverse case. Although the membranes still retained their bulk physical properties in that they remained flexible and plastic, the tensile analysis showed decreased tensile strength and increased elongation-to-break accompanied by an increased Young's modulus, which suggests a mechanically weaker membrane after exposure to H 2 O 2 .

No abstract

To clarify the transport mechanisms of alcohols and proton in perfluorosulfonated ionomer (PFSI) membranes for fuel cells, four membranes having different equivalent weight (EW) values were examined. Membranes were immersed in methanol, ethanol, and 2-propanol to prepare a total of 12 samples, and membrane swelling, mass (alcohol and proton) transports, and interactions between alcohols and proton were investigated systematically in the fully penetrated state. The membrane expansion fraction theta and alcohol content lambda increased with decreasing the EW value for all the samples. The self-diffusion coefficients (D's) of the alkyl group and of OH (including protons) were measured separately by the pulsed-gradient spin-echo (PGSE)-NMR method and the D's also increased with decreasing the EW value. These results implied that the alcohols penetrate into the hydrophilic regions of the PFSI membranes and diffuse through the space expanded by the alcohols. The ionic cluster regions formed by the alcohols resemble those induced by water in the water swollen membrane, where protons dissociated from sulfonic acid groups transport through the regions together with water molecules. The D values decreased with increasing the molecular weight of alcohols. This trend was supported by activation energies Ea estimated from the Arrhenius plots of D in the temperature range from 30 to -40 degrees C. The PGSE-NMR measurements also revealed that protons move faster than the alkyl groups in the membranes. The proton transport by the Grotthuss (hopping) mechanism was facilitated by the increase of the alcohol content and the decrease of the molecular weight. This result was also supported by the experimental results of proton conductivity kappa and mobility u(H(+)). Density functional theory (DFT) calculations of the interaction energy DeltaE(int) between proton and alcohol (including OH) showed that the /DeltaE(int)/ increases with increasing the molecular weight of alcohols, which is in a inverse relationship with the kappa and u(H(+)) values. The proton transport depends strongly on the DeltaE(int) in the membranes.