Mitsuru Higa

Cross-Linked Sulfonated Polyimide Membranes for Polymer Electrolyte Fuel Cells

The SO 2 cross-linked membranes of sulfonated polyimides (SPIs) bearing sulfophenoxy side groups were prepared and evaluated as polymer electrolyte membranes for polymer electrolyte fuel cells (PEFCs). They maintained a high mechanical strength and proton conductivity after aging in water at 130°C for 500 h, indicating their high water stability. PEFCs with the SPI membranes showed high performances at 90°C under a high feed-gas humidity of 84% relative humidity (RH), and also fairly high performances even at a low feed-gas humidity of 30% RH due to the back-diffusion of water formed at the cathode. The PEFCs were operated under a constant current density of 0.5 A/cm 2 at 90°C and 84% RH for 1600 h without any reduction in cell performance. There was no change in the Fourier transform infrared spectra for the SPI membranes before and after the durability test. These results indicate that they have a high durability in PEFC operation. The cross-linked SPI membranes have a high potential for PEFCs at higher temperatures above 80°C.

An experimental study of ion permeation in multicomponent ion systems as a function of membrane charge density

Abstract The membrane potential and the permeability coefficient of ions in various kinds of electrolyte systems are investigated as a function of membrane charge density. The theoretical predictions in the previous paper [J. Membrane Sci., 49 (1990) 145] agree with the experimental data using membranes made from poly(vinyl alcohol) (PVA) and poly(styrenesulfuric acid) (PSSA) mixtures, and PVA and poly(allylamine) (PAAM) mixtures. The results show that the counterion with the highest valence is the most important for ion permeation phenomena in a multicomponent ion system ion system and that uphill transport of a bivalent counterion occurs because its concentration gradient in a charged membrane is in the opposite direction to its external concentration gradient.

Changing permselectivity between halogen ions through anion exchange membranes in electrodialysis by controlling hydrophilicity of the membranes

To change the permselectivity between halogen ions through anion exchange membranes in electrodialysis, the anion exchange membranes were modified: impregnation of compounds which have ether bonds into the membrane and formation of a cationic polysoap layer on the membrane surface, led to control of the hydrophilicity of the membrane. Evaluation was performed by measurement of the transport numbers of fluoride, bromide and iodide ions relative to chloride ions. The permeation of the less hydrated bromide anions decreased and that of the strongly hydrated fluoride anions increased relative to chloride ions by impregnating compounds containing ether bonds into the membranes, which was a consequence of the increase in the hydrophilicity of the membranes. Notably, permeation of chloride ions through the membrane exceeded that of bromide ions in the presence of ethylene glycols, which is the opposite behaviour of those of conventional anion exchange membranes. On the contrary, to examine the effect of decreasing the hydrophilicity on the permselectivity between halogen ions, a hydrophobic layer [poly(N-dodecyl 4-vinylpyridinium bromide)] was formed on the desalting side of the membrane surface. Although fluoride ions permeated only with difficulty through the membrane owing to formation of the layer, permeation of bromide and iodide ions was enhanced by the layer. In particular, the transport number of iodide ions relative to chloride ions reached 35.5 (without the layer 4.8). Controlling the hydrophilicity of the anion exchange membranes is an effective method to change the permeation behavior of halogen ions through anion exchange membranes.

Change in permselectivity between sulfate and chloride ions through anion exchange membrane with hydrophilicity of the membrane

The transport number of sulfate ions relative to chloride ions through an anion exchange membrane was measured by electrodialyzing a sodium sulfate and sodium chloride mixed solution containing diethylene glycol after immersing the membrane in diethylene glycol because the membrane had good affinity to ethylene glycol and diethylene glycol. The transport number of sulfate ions relative to chloride ions increased in the presence of diethylene glycol in the membrane. This is due to the increase in the uptake of sulfate ions in the membrane compared with chloride ions, not to the change in a ratio of mobilities of both anions.

Electrodialytic transport properties of cation exchange membranes in the presence of cyclodextrins

Electrodialysis of mixed salt solutions, sodium chloride and sodium sulfate, and sodium chloride and sodium nitrate, was carried out in the presence of α-cyclodextrin using commercial anion-exchange membranes. It was confirmed by several methods that the compound existed in the membrane matrix when the membrane had been immersed in its aqueous solution, though the molecular weight of α-cyclodextrin is relatively high. In electrodialysis, sulfate ions, large and strongly hydrated anions, easily permeated through the membranes and nitrate ions, less hydrated anions, permeated with difficulty through the membranes in the presence of α-cyclodextrin. Because α-cyclodextrin is a hydrophilic compound, which has many ether and alcoholic groups, the hydrophilicity of the anion-exchange membranes is thought to increase. Thus, sulfate ions easily permeate and nitrate ions permeate with difficulty. This proves that the hydrophilicity of the anion-exchange membranes controls permselectivity between anions through the membranes.

Poly(phenylene) block copolymers bearing tri-sulfonated aromatic pendant groups for polymer electrolyte fuel cell applications

Novel poly(tri-sulfonated phenylene)-block-poly(arylene ether sulfone) copolymers (PTSP-b-PAESs) were synthesized by Ni(0)-catalyzed copolymerization of 2,5-dichloro-3′-sulfo-4′-((2,4-disulfo)phenoxy)-benzophenone and chlorobenzophenone-endcapped oligo(arylene ether sulfone). Their physical properties, morphology and polymer electrolyte fuel cell (PEFC) performance were investigated compared to those of poly(mono-sulfonated phenylene)-block-poly(arylene ether sulfone) and the corresponding random copolymers. They had a low ion exchange capacity (IEC) of 1.1–1.2 meq. g−1 and showed very low water uptake and in-plane dimensional change in water. They exhibited a more well-defined microphase-separated structure composed of hydrophilic and hydrophobic domains, where the hydrophilic domains were well-connected to each other to form the channels for proton conduction, than the mono-sulfonated one. This led to the relatively high proton conductivity under low relative humidities. The corresponding random copolymers exhibited a homogeneous morphology and much lower proton conductivity in spite of a high IEC of 2.0–2.1 meq. g−1. Even under the low humidification of 30% RH at 90 °C and 0.2 MPa, they exhibited high PEFC performance and durability; for example, a cell voltage of 0.69 V at a load current density of 0.5 A cm−2 and a maximum output of 0.73 W cm−2, which was comparable to that of the mono-sulfonated one with a much higher IEC of 1.8 meq. g−1 and much higher than those of the corresponding random copolymers. PTSP-b-PAESs have high potential as polymer electrolyte membranes for fuel cell applications.

Electrodialytic Transport Properties of Anion-Exchange Membranes Prepared from Poly(vinyl alcohol) and Poly(vinyl alcohol-co-methacryloyl aminopropyl trimethyl ammonium chloride).

Random-type anion-exchange membranes (AEMs) have been prepared by blending poly(vinyl alcohol) (PVA) and the random copolymer-type polycation, poly(vinyl alcohol-co-methacryloyl aminopropyl trimethyl ammonium chloride) at various molar percentages of anion-exchange groups to vinyl alcohol groups, Cpc, and by cross-linking the PVA chains with glutaraldehyde (GA) solution at various GA concentrations, CGA. The characteristics of the random-type AEMs were compared with blend-type AEMs prepared in our previous study. At equal molar percentages of the anion exchange groups, the water content of the random-type AEMs was lower than that of the blend-type AEMs. The effective charge density of the random-type AEMs increased with increasing Cpc and reached a maximum value. Further, the maximum value of the effective charge density increased with increasing CGA. The maximum value of the effective charge density, 0.42 mol/dm3, was obtained for the random-type AEM with Cpc = 4.2 mol % and CGA = 0.15 vol %. A comparison of the random-type and blend-type AEMs with almost the same Cpc showed that the random-type AEMs had lower membrane resistance than the blend-type ones. The membrane resistance and dynamic transport number of the random-type AEM with Cpc = 6.0 mol % and CGA = 0.15 vol % were 4.8 Ω cm2 and 0.83, respectively.

Ionic partition equilibrium in a charged membrane immersed in a mixed ionic solution

In partition equilibria of Ca2+, K+ and Cl– between a swollen charged membrane and an aqueous bath of mixed KCl and CaCl2 electrolytes, the equilibrium concentration of the bivalent ion in the membrane decreases to a minimum value as its concentration in the bath increases. The value of the concentration minimum decreases with increasing KCl : CaCl2 concentration ratio in the bath. Simulations and analysis have been performed on the basis of Donnan distribution theory.

Ionic transport against its concentration gradient across bipolar membranes

Countertransport of ions is defined as the transport of an ion against its own concentration gradient driven by the third driving electrolyte added to the systems. The countertransport was simulated for K+ and Ca2+ ions in dialysis systems that consist of two electrolyte solutions divided by a bipolar membrane as a function of the mobility of the driving ions, the charge density and thickness of the negative and positive components of the membrane. The simulations show that the countertransport occurs in the direction either of or opposite to the concentration gradient of the driving electrolyte depending on the values of the above parameters and on the bipolar direction, and that the direction of the countertransport can alter in a run under certain conditions. The permselectivity of the membrane for ionic valence depends on the bipolar direction.

Design and Preparation of a Novel Temperature-Responsive Ionic Gel. 3. Valence Selective Control of Transport Modes of Ions in Response to Temperature

We propose a novel model dialysis system that can valence-selectively control the transport modes of ions in response to temperature change. In a dialysis system consisting of an anionic gel membrane and mixed solutions containing a driving electrolyte and electrolytes with uni-, bi-, and trivalent cations, the dependence of the charge density of the gel and the valence of the ions on the transport modes of the ions through the gel membrane was investigated by computer simulations. The simulations show that the system has four transport types in the transport modes of the cations according to their valence [downhill (transport along their own concentration gradient in the system) and uphill (transport against their own concentration gradient)] in response to the charge density changes: (A) downhill transport of all the cations; (B) uphill transport of trivalent cations, downhill transport of the other cations; (C) uphill transport of bi- and trivalent cations, downhill transport of univalent cations; and (D) uphill transport of all the cations except for the driving cations. To examine the prediction of the simulations, a temperature-responsive anionic gel membrane was prepared from a modified poly(vinyl alcohol) (PVA) containing 2 mol % of sulfonic acid groups and another modified PVA prepared by in situ polymerization of N-isopropylacrylamide in a PVA solution. Permeation experiments in a dialysis system consisting of the membrane and mixed electrolyte solutions of NaCl, LiCl, CaCl2, and LaCl3 indicate that the system valence-selectively controls the transport modes of the cations in response to temperature change as predicted in our simulations.