Development of sulfonated FEP–Nafion hybrid proton exchange membranes for PEFC

doi:10.1016/j.nimb.2007.08.088

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Volume 265, Issue 1, December 2007, Pages 213-216

https://doi.org/10.1016/j.nimb.2007.08.088 Get rights and content

Abstract

The performance of polymer electrolyte fuel cell (PEFC) is affected by an interfacial property between a proton exchange membrane (PEM) and electrodes. Thus, to develop a well-laminated membrane electrode assembly (MEA), a hybrid PEM (FN) was fabricated by mixing a radiation grafted membrane (sulfonated FEP) with ionomer (Nafion^® dispersion) which is applied to coat the interface of the PEM and electrodes.

The obtained FN, sulfonated FEP and Nafion^®112 were characterized in terms of water uptake, ion exchange capacity (IEC), polarization performance and electrochemical impedance. FN showed high IEC and water uptake, which would induce the highest ionic conductivity (IC) among tested PEMs. In terms of FN, the interface between the PEM and electrodes should have been improved because FN showed the lowest charge transfer resistance than other tested PEMs. The high IC and improved interface between the PEM and electrodes resulted in the best cell performance of FN in tested PEMs.

Introduction

The development of a long-lasting and low cost proton exchange membrane (PEM) is required for the application of a polymer electrolyte fuel cell (PEFC). Perfluorosulfonate membranes such as Nafion^® (DuPont Co., Ltd.) are often used for PEMs owing to their chemical stability, however, they are still expensive [1]. Therefore, the partial-fluorinated sulfonic acid membranes have been fabricated by a radiation grafting method to obtain low cost PEMs [2], [3], [4], [5], [6], [9]. Although ion exchange capacities (IEC) of obtained PEMs were 1.3–2.2 times higher than that of Nafion^®112, their cell performance were same or lower than Nafion^®112 [2], [5].

The low cell performance should have been caused by a poor interface between the PEM and electrodes because chemical reaction takes place between catalysts and a PEM in hydrogen or oxygen [7]. Thus, by improving the interface between the PEM and electrodes, catalysts should be used more effectively to attain high cell performance.

Two approaches have been considered to improve the performance of a membrane electrode assembly (MEA). One is to apply hotpressing above glass transition temperature of the PEM, another is to use ionomer such as Nafion^® dispersion which coated the interface between the PEM and electrodes [8]. However, when we use a PEM prepared by a radiation grafting method, the different chemical nature of the PEM and the ionomer on the catalyst layers should induce delamination between the PEM and electrodes [3].

In this study, PEMs were prepared by mixing partial-fluorinated sulfonic acid membranes with Nafion^® dispersion to get a well-laminated interface between the PEM and electrodes with keeping high ion exchange capacity (IEC). The resulting PEMs were characterized in terms of water uptake, IEC, polarization performance and electrochemical impedance.

Section snippets

Preparation for the PEMs

Poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP, FLON INDUSTRY CO., LTD.) with thickness of 25 μm was used for the experiments. FEP were irradiated electron beam with the dose of 15 kGy under nitrogen atmosphere at room temperature with the electron accelerator, CURETRON^® (NHV Corp.) [2]. The irradiated samples were grafted with styrene at 80 °C for 2 h.

The degree of grafting (DOG) was calculated by the weight increase of the sample according to the following equation: $degree of grafting (%) = W_{g} -$

Results and discussion

The hybrid membrane (FN) was prepared by mixing the obtained s-FEP with Nafion^® dispersion. Table 1 shows the characteristic properties of the obtained PEMs and Nafion^®112. DOG of s-FEP was 39%. IEC values of FN, s-FEP and Nafion^®112 were 1.6 meq g⁻¹, 2.0 meq g⁻¹ and 0.9 meq g⁻¹, respectively. IEC value of FN is about 1.7 times higher than that of Nafion^®112. Water uptake of FN, s-FEP and Nafion^®112 was about 78%, 29%, and 16%, respectively. FN revealed the highest water uptake among them. It would

Conclusion

In order to get a well-laminated interface between a radiation garafted PEM and electrodes, we have fabricated a PEM by mixing sulfonated FEP powder with Nafion^® dispersion that coated the interface between a PEM and electrodes. IEC value of the fabricated PEM was 1.6 meq g⁻¹, which is 1.5 times higher than that of Nafion^®112. Water uptake of FN was 78.2%, and is considerably high. It is explained that the casting method enhanced the molecular chain motion of FN and resulted in large water uptake

Acknowledgments

This study is performed by the projects research of RISE, Waseda University, “the manufacturing of high functional fluorinated-polymer materials”, we would like to acknowledge our research group members. We would also like to appreciate Retsch Co., Ltd. milling of the obtained PEMs.

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Development of sulfonated FEP–Nafion hybrid proton exchange membranes for PEFC

Abstract

Introduction

Section snippets

Preparation for the PEMs

Results and discussion

Conclusion

Acknowledgments

Solid State Ionics

J. Power Sources

Eur. Polym. J.

J. Power Sources

Electrochim. Acta