齋藤 守弘

To clarify the mechanisms of transport of ions and water molecules in perfluorosulfortated ionomer membranes for fuel cells, the temperature dependence of their transport behaviors was investigated in detail. Two types of Flernion 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 HCI, LiCl, and NaCl aqueous solutions, respectively. The ionic conductivity, water self-diffusion coefficient (D-H2O), 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-H2O 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-H2O 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 HI is much higher than that of the other cations. Moreover, the D-H2O and DSC curves showed that a part of water in the membranes freezes around -20 degreesC, 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 degreesC, 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-H2O 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.

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.

Influence of the conductive additive loading on the anode performance of Li-ion battery was investigated for the coated anode where graphitized mesophase carbon fiber (MCF) was used as an active material, and loaded with natural graphite (NG), acetylene black (AB) or Ketjen black (KB). Incorporation of the conductive additive was quite effective to improve not only the cycle life, but also the rate of the electrochemical reaction. In addition, we found that the initial irreversible charging capacity (IICC) of MCF was suppressed to a great extent. The effectiveness was dependent on the kind of material loaded. On the other hand, making a homogeneous slurry for coating was found to be a key factor for the performance improvement. The most preferable choice of the preparation condition gave rise to a dramatic improvement. In contrast to these advantages, the carbon blacks showed a large IICC inherent to the materials which is an issue to be solved for actual use. (C) 2000 Elsevier Science S.A. All rights reserved.