齋藤 守弘

As a typical degradation of the proton exchange membranes (PEMs) for fuel cells, formation of hydrogen peroxide (H2O2) 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 1100) as test samples, degradation of PEM was investigated systematically in practical fuel cell usage conditions (e.g., 80 degrees C) during the progress of H2O2 treatment. Membranes were characterized for proton conductivity by ac impedance technique, pulsed-field-gradient spin-echo NMR, Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA), 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 sulfonates) were strongly demonstrated by IR spectroscopy as a result of long H2O2 treatment times, which suggests oxidation provoked by H2O2. 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 H2O2. 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 H2O2. (C) 2006 The Electrochemical Society.

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.

To clarify the interaction between a Lewis acid and anionic species of the supporting salt incorporated in a polymer electrolyte, we designed a novel solid polymer electrolyte based on Mg salt complexes of poly(ethylene glycol) (PEO) chains cross-linked by a borate ester group as a Lewis acid and examined the ionic conduction mechanism of the electrolyte. Mg(ClO4)(2), Mg(CF3SO3)(2), and Mg[(CF3SO2)(2)N](2) were used as the Mg salt. To change the concentration of the Lewis acid in the polymer electrolyte, two different lengths of PEG chains, which were cross-linked by borate ester group, were used. By estimating the transport number of the Mg2+ cation (t(Mg2+)) of the electrolytes, it was found that the borate ester group interacts with anions with the consequence that t(Mg2+) increases with increasing concentration of borate ester group. By measuring Raman spectra for the electrolyte containing Mg(ClO4)(2) salt, it was also found that the concentration of the free ClO4- anion increased with the increasing concentration of the borate ester group in the polymer electrolyte, which implied that the relative proportion of free Mg2+ carrier ion also increased. The order of the t(Mg2+) value was Mg(ClO4)(2) > Mg(CF3SO3)(2) much greater than Mg[(CF3SO2)(2)N](2). The change in the total energy due to the interaction between the PEG-borate ester and each anion species using ab initio calculation is in good agreement with the results of the t(Mg2+) and Raman spectra. These results indicate that the borate ester group as a Lewis acid interacts with hard anions Of ClO4- or CF3SO3- more strongly than the soft anion of (CF3SO2)(2)N- to enhance the degree of dissociation of the salt and trap the anion in the polymer electrolyte.

In order to investigate the influence of Lewis acidity on the ionic conductivity of poly(ethylene glycol) (PEG) poly(ethylene oxide) (PEO)-type polymer electrolyte, we designed a new solid polymer electrolyte based on Mg(ClO4)(2) salt complexs of PEO chains cross-linked by a boric acid (BO3) group as a Lewis acid. To change the concentration of the Lewis acid point in the polymer electrolyte, the length of the PEO repeating unit was controlled. The ionic conductivity of the polymer electrolyte increased with increasing the length of the PEO chain and with decreasing the concentration of the cross-linking point by the boric acid group. By measuring the glass transition temperature (T-g) of the polymer electrolytes using differential scanning calorimetry, it was found that the Tg increases with decreasing the concentration of the cross-linking point. By investigating the temperature dependence of the ionic conductivity using William-Landel-Ferry-type equation and measuring Raman spectra of the polymer electrolytes, it became clear that the concentration of the carrier ion in the polymer electrolyte increased with increasing the concentration of the Lewis acid point by the boric acid group. Furthermore, the transference number of the Mg2+ cation (t(Mg2+)) increased with increasing the concentration of the boric acid group. By the ab initio calculation for the PEG-borate ester as a model molecule to investigate on the Lewis acidity of boric acid group, it was supported that the boric acid group may act as a Lewis acid to enhance the solubility of the salt in the polymer electrolyte by interacting with and trapping the ClO4- anion of the salt. (C) 2003 The Electrochemical Society.

We produced a novel Mg2+ conducting polymer electrolyte and added a poly(ethylene glycol) (PEG)-borate ester as a new type plasticizer having a Lewis acidity and investigated the influence of the Lewis acidity of the PEG-borate ester to a solid polymer electrolyte containing Mg(ClO4)(2) salt. Adding the PEG-borate ester into the electrolyte shows the increase in the ionic conductivity of the polymer electrolyte. By measuring the glass transition temperature (T-g) of the polymer electrolytes using differential scanning calorimetry, it became clear that the mobility of the carrier ion increases with increasing the amount of the PEG-borate ester. By investigating the temperature dependence of the ionic conductivity using William-Landel-Ferry type equation and measuring Raman spectra of the polymer electrolytes, it was found that the concentration of the carrier ion increases with increasing the amount of the PEG-borate ester in the polymer electrolyte. Furthermore, by estimating the transference number of the Mg2+ cation and performing the ab initio calculation for the PEG-borate ester, it is suggested that the PEG-borate ester may enhance the degree of dissociation of the Mg salt in the polymer electrolyte to increase the ratio of the free ion, especially Mg2+, by interacting with and trapping the ClO4- anion of the salt as a Lewis acid. (C) 2003 The Electrochemical Society.

In order to prepare a new thermally stable polymer electrolyte for Li-ion batteries operating at high temperature, poly(ethylene oxide) (PEO) dimethacrylate with bis-phenol A (PDBE450) was used as a monomer. The thermal and electrochemical properties of the polymer electrolyte complexed with PEO-monomethacrylate (PDE400) and lithium bis-trifluoromethanesulfonimide (LiTFSI) was investigated. The present polymer was thermally stable up to near 300degreesC and the ionic conductivities were 2.6 x 10(4) S cm(-1) at 80degreesC and 1.1 x 10(-4) S cm(-1) at 60degreesC, respectively. The electrolyte was electrochemically stable up to about 4.3 V (vs. Li+/Li) at 80degreesC.

Highly graphitized carbon is the most widely utilized anode material for Li-ion secondary batteries because of its high reversibility and reliability. However, it has so far been believed that well graphitized carbon material gives no stable Li charge/discharge performance in a pure propylene carbonate (PC)-based electrolyte, since PC decomposes vigorously at the surface of the graphite during the charging process. Surprisingly, we found that a single graphitized carbon fiber revealed a sufficiently stable performance in pure PC containing 1 M LiClO4. The initial irreversible charging capacity not only in PC but in an EC-based electrolyte was found strongly dependent on the conductivity homogeneity of the electrode. (C) 2001 Published by Elsevier Science B.V.

The charging capacity of the anode of Li-ion secondary batteries is based on the insertion of Li ions into the internal structure of carbonaceous materials, Low temperature mesophase carbon gives rise to an extraordinary capacity up to 900 mAh/g which is over two times that of graphitic carbon now utilized. However, the serious issue of its low cyclability prevents its actual application. Ln an attempt to improve the low cyclability of a mesophase carbon fiber, C/C composites have been prepared with an epoxy or a phenolic resin. The cyclability was found to be considerably improved by making the fiber felt into a C/C composite. The cyclability is dependent on how each individual fiber is bound together by the binder carbon, as well as how the electrical conductivity of the consisting material is increased. Epoxy resin created difficulty in forming a smooth binding but it still gave an improved cyclability as compared to the unbound fiber. Phenolic resin of resol-type showed a very smooth binding, giving rise to a more stabilized cyclability. The importance of the binding of individual active material particles with an electroconductive material for obtaining a long cycle life has been shown. (C) 2000 Elsevier Science B.V. All rights reserved.

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.

For the purpose of improving the Li charge/discharge cycle performance of less graphitezed carbon for the anode of Li-ion secondary batteries, the active material was made into C/C composite. A carbon fiber felt prepared at lower temperatures were impregnated with epoxy resin and pyrolyzed at a desired temperature. The SEM picture of the C/C composite thus obtained showed that each individual fiber was bound together by the carbon formed by pyrolysis. The charge/discharge cycle test was performed in propylene carbonate containing 1 M (M = mol dm(-3)) LiCIO4. It was found that the cycleability was improved remarkably for the C/C composite.

Lithium dope/undope characteristics have been examined for the carbon/carbon composite(C/C) where meso-phase carbon fibers prepared at lower temperature were bound together with conductive carbon formed by the pyrolysis of ep oxy resin or petroleum pitch. The reaction rate, however, was very slow when the surface was unfavorable to accept Li ion doping reaction even though the internal composition was satisfactory. After an appropriate surface treatment the composite gave rise to a satisfactory charge/discharge behavior showing that dope/undope reactions were more rapid with the increasing amount of conductive materials.