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TECHNICAL PAPERS

Preparation of Highly Stable Ion Exchange Membranes by Radiation-Induced Graft Copolymerization of Styrene and Bis(vinyl phenyl)ethane Into Crosslinked Polytetrafluoroethylene Films

[+] Author and Article Information
Tetsuya Yamaki1

Quantum Beam Science Directorate,  Japan Atomic Energy Agency (JAEA), 1233 Watanuki, Takasaki, Gunma 370-1292, Japanyamaki.tetsuya@jaea.go.jp

Junichi Tsukada, Ryoichi Katakai

Faculty of Engineering,  Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan

Masaharu Asano, Masaru Yoshida

Quantum Beam Science Directorate,  Japan Atomic Energy Agency (JAEA), 1233 Watanuki, Takasaki, Gunma 370-1292, Japan

1

Corresponding author.

J. Fuel Cell Sci. Technol 4(1), 56-64 (Apr 11, 2006) (9 pages) doi:10.1115/1.2393305 History: Received December 12, 2005; Revised April 11, 2006

We prepared novel ion exchange membranes for possible use in polymer electrolyte fuel cells (PEFCs) by the radiation-induced graft copolymerization of styrene and new crosslinker bis(vinyl phenyl)ethane (BVPE) into crosslinked polytetrafluoroethylene (cPTFE) films and subsequent sulfonation and then investigated their water uptake, proton conductivity, and stability in an oxidizing environment. In contrast to the conventional crosslinker, divinylbenzene (DVB), the degree of grafting of styrene∕BVPE increased in spite of high crosslinker concentrations in the reacting solution (up to 70mol%). Quantitative sulfonation of the aromatic rings in the crosslinked graft chains resulted in the preparation of membranes with a high ion exchange capacity that reached 2.9meqg. The bulk properties of the membranes were found to exceed those of Nafion membranes except for chemical stability. The emphasis was on the fact that the BVPE-crosslinked membranes exhibited the higher stability in the H2O2 solution at 60°C compared to the noncrosslinked and DVB-crosslinked ones, as well as decreased water uptake and reasonable proton conductivity. These results are rationalized by considering the reactivity between styrene and the crosslinker, which is an important factor determining the distribution of the crosslinks in the graft component. In the case of BVPE, the crosslinks at a high density were homogeneously incorporated even into the interior of the membrane because of its compatibility with styrene while the far too reactive DVB led to a crosslink formation only near the surface. The combination of both the cPTFE main chain and BVPE-based grafts, i.e., a perfect “double” crosslinking structure, is likely to effectively improve the membrane performances for PEFC applications.

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Figures

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Figure 1

Chemical structure of the two crosslinkers used in this study: (a) DVB and (b) BVPE

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Figure 2

Process for the preparation of our PEM by the radiation-induced graft polymerization of styrene∕crosslinker into the cPTFE film

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Figure 3

Degree of grafting versus time curves for the styrene grafting in the presence of 5mol% BVPE into the cPTFE films. This figure compares the results of the styrene and styrene∕DVB grafting during an early stage of <8h.

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Figure 4

Influence of the BVPE concentration on the degree of grafting during a longer reaction time: (a) 5, (b) 50, (c) 60, and (d)100mol%.

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Figure 5

A plot of experimental IEC as a function of the degree of grafting of styrene, styrene∕DVB, and styrene∕BVPE. The solid line represents the theoretical values of the IEC, which are calculated on the basis of the assumption that the ratio of the sulfonic acid group to the aromatic ring was equal to unity.

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Figure 6

Distribution of sulfur in the transverse plane of the ion exchange membranes with IECs of (b) 2.0 and (c)1.1meqg−1, which are prepared from styrene∕BVPE-grafted films with the degree of grafting of 35% and 20%, respectively. (d) shows the result of the styrene-grafted, sulfonated membrane at an IEC of 1.3meqg−1. (a) A representative SEM image of the membrane with concentration curve (b).

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Figure 7

Relationship between the water uptake and IEC in the membranes with and without crosslinkers

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Figure 8

Semi-log plots of the proton conductivity as a function of 1000∕T, where T is the temperature in the unit of Kelvin. The membrane with an IEC of 2.0meqg−1 was obtained by the styrene∕BVPE grafting at a crosslinker concentration of 50mol%. For comparison, the results of the styrene-grafted, sulfonated membranes with the same ionic content as well as Nafion 117.

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Figure 9

Chemical stability of the (a) styrene, (b) styrene∕DVB (5mol%), (c) styrene∕BVPE (5mol%) and (d) styrene∕BVPE (50mol%) grafted ion exchange membranes in the 3% aqueous H2O2 solution at 60°C. The IEC of all these membranes was set at 2.0meqg−1.

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