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

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NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 265 (2007) 213–216 www.elsevier.com/locate/nimb

Development of sulfonated FEP–Nafion hybrid proton exchange membranes for PEFC Y. Sato *, K. Fujii, N. Mitani, A. Matsuura, T. Kakigi, F. Muto, J. Li, A. Oshima, M. Washio Advanced Research Institute for Science and Engineering, Waseda University, 3-14-9, Okubo, Shinjuku, Tokyo 169-0072, Japan Available online 14 September 2007

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 Nafion112 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.  2007 Elsevier B.V. All rights reserved. PACS: 61.82.Pv Keywords: PEFC; PEM; Radiation grafting; Electrochemical impedance spectroscopy (EIS)

1. 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 [2–6,9]. Although ion exchange capacities (IEC) of obtained PEMs were 1.3–2.2 times higher than that of Nafion112, their cell performance were same or lower than Nafion112 [2,5]. The low cell performance should have been caused by a poor interface between the PEM and electrodes because *

Corresponding author. Tel./fax: +81 5286 2917. E-mail address: [email protected] (Y. Sato).

0168-583X/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.08.088

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 partialfluorinated 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

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water uptake, IEC, polarization performance and electrochemical impedance. 2. Experiments 2.1. Preparation for the PEMs Poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP, FLON INDUSTRY CO., LTD.) with thickness of 25 lm 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 ð%Þ ¼

WgWI  100; WI

where Wg and WI are the weight of the sample after and before grafting, respectively. The grafted membranes were sulfonated with chlorosulfonic acid in carbon tetrachloride (1:100, v/v) at room temperature for 24 h. Subsequently, they were soaked in deionised water at room temperature and treated in 1 M HCl for 24 h to obtain acid form membranes. Obtained sulfonated FEP (s-FEP) were milled to powder with a mixer mill MM301 (Retsch Co., Ltd.) under 1800 rpm, 10 min. The average diameter of obtained particles measured by scanning electron microscope was 22 lm. Then, s-FEP powder was dispersed in 5 wt% Nafion dispersion (Du Pont Co., Ltd.) and 1-propanol. 1-Propanol was used to decrease the viscosity of the solution and prevent the PEM from being cracked. The amount of s-FEP mixed with Nafion dispersion was 56 wt%. Finally, hybrid membranes (FN) were obtained by drying the solution at 80 C for 1 h and at room temperature for 24 h. Nafion112 membrane was purchased from DuPont Co., Ltd. which was used as a reference. 2.2. Membrane characterization The ion exchange capacities (IEC) of FN, s-FEP and Nafion112 were determined by acid–base titration method with 716 DMS Titrino automatic titrator (Metrohm herisau Co., Ltd.). The water uptake of the PEMs was determined by the difference between the wet and the dry mass of the membranes. 2.3. Fabrication of membrane electrode assembles (MEAs) and the cell performance Carbon electrodes with 1 mg cm2 Pt loaded were purchased from ElectroChem, Inc. They were cut into 2 cm · 2 cm, coated with Nafion dispersion 0.2 mg cm2

for the anode and the cathode, respectively, and subsequently dried at 80 C for 1 h. FN, s-FEP and Nafion112 with an active area of 1 cm2, were stored in deionized water for 1 h to obtain good adhesion between the PEM and electrodes [5]. All the MEAs were prepared by hotpressing at 110 C under 8 MPa for 3 min. Fuel cell tests were performed with humidified H2 and dry O2 supplied under 0.2 MPa, and both gas flow rates were 50 ml min1. Characterization of the MEAs was determined by polarization performance and electrochemical impedance spectroscopy (EIS; Hokuto Denko Co., Ltd.). EIS was measured by the four-electrode frequency response analyzer method, and was taken at dc current density of 500 mA cm2 with ac frequency ranging from 100 kHz to 0.1 Hz [2]. Ohmic resistance (Rohm) contains the resistance of electrode, separator and a PEM and induces resistance polarization. A PEM has serious influence to determine Rohm. Charge transfer resistance (Rct) induces diffusion and activation polarization. The ionic conductivity (IC) was calculated from the Rohm and membranes thickness. 3. 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 Nafion112. DOG of s-FEP was 39%. IEC values of FN, s-FEP and Nafion112 were 1.6 meq g1, 2.0 meq g1 and 0.9 meq g1, respectively. IEC value of FN is about 1.7 times higher than that of Nafion112. Water uptake of FN, s-FEP and Nafion112 was about 78%, 29%, and 16%, respectively. FN revealed the highest water uptake among them. It would be explained by the making process of FN. As FN was fabricated by a casting method, the molecular chain of FN may be isolated without any physical or chemical interaction. Thus, the motion of molecular chain would have increased and it enabled FN to swell up and to obtain water easily. Fig. 1 shows the results of the cell performance test where the cell temperature was 60 C. The values of open circuit voltage (OCV) and power density of maximum and 500 mA cm2 are shown in Table 2. Cell performance of the MEA based on the s-FEP is better than that of Nafion112 at a high current density area, but worse than that of Nafion112 at current density of less than 1000 mA cm2. These results should be caused by polarization as described below. Generally, cell performance is degraded by ohmic, activation and diffusion polarization. Activation polarization takes place at a low current density, while diffusion polarization takes place at a high current density [10]. Thus, s-FEP would decrease diffusion polarization by its high IEC value compared with Nafion112. On the other hand, the apparent activation polarization of s-FEP would have been caused by the poor interface between the s-FEP and electrodes.

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Table 1 Major properties of PEMs Membrane

Wet thickness (lm)

DOG (%)

IEC (meq g1)

Water uptake (%)

FN s-FEP Nafion112

48 49 58

– 39 –

1.6 2.0 0.9

78.2(±4.2) 28.8(±3.8) 16.0(±1.2)

1200

1.2

0.05

FN

0.8

800

0.6

600

0.4

400

0.2

200

FN s-FEP Nafion112

Im Z(ohm cm2)

Voltage (V)

1000

Power Density (mW/cm2)

s-FEP Nafion112

1

0

-0.05

-0.1 0 0

500

1000

1500

2000

2500

3000

0 3500

0

0.1

0.2

0.3

0.4

0.5

Re Z(ohm cm2)

Current density (mA/cm2) Fig. 1. Polarization curves of MEAs based on FN, s-FEP and Nafion112. Cell temperature: 60 C, H2/O2 gas flow: 50 ml min1, gas pressure: 0.2 MPa, H2 humidified, O2 dry.

Fig. 2. Electrochemical impedance spectra of MEAs based on FN, s-FEP, Nafion112. Current density: 500 mA2, cell temperature: 60 C.

Table 3 Electrochemical properties of PEMs Table 2 Cell performances of PEMs Membrane

FN s-FEP Naifon112

OCV (mV)

759 930 970

Power density (mW cm2) At 500 mA cm2

Maximum

325 310 320

861 491 467

The MEA based on FN shows the better performance than any other tested membranes. Power density at a current density of 500 mA cm2 of FN was 325 mW cm2. The maximum power density of obtained FN was 861 mW cm2, which is about 1.8 times higher than that of s-FEP and two times higher than that of Nafion112. This reveals that both diffusion and activation polarization of FN have been improved. In terms of OCV, Nafion112 shows the highest and FN shows the lowest despite its high power density. It is considered that the mobility of its molecular chain would increase gas crossover and that would decrease its OCV. The optimized thermal treatment above glass transition temperature of FN would prevent FN from gas crossover and decreasing OCV. Fig. 2 shows the results of EIS where the cell temperature was 60 C. The values of the ohmic resistance (Rohm) and the charge transfer resistance (Rct), obtained by fitting the model parameters to the experimental data are shown in Table 3. Rohm is the intersections on the real axis at high frequency while Rct is the diameter of the semicircle on the

FN s-FEP Naifon112

Thickness (lm)

Rohm (X)

Rct (X)

IC (mS cm1)

48 49 58

0.05 0.06 0.11

0.15 0.3 0.28

96 81 55

real axis. Rohm depends on the thickness of PEMs. Thus, Rohm of Nafion112 which is 1.2 times thicker than other membranes was the largest among them. Ionic conductivities (IC) of FN, s-FEP and Nafion112 where cell temperature was 60 C were 96 mS cm1, 81 mS cm1 and 55 mS cm1, respectively. The high IC value of FN should be caused by high IEC values and high water uptake which makes proton and water move easier. Rct of FN was 0.15 X, which is half of others. This indicates that high IC and improved interface properties between the PEM and electrodes lead FN to have the lowest Rct among the tested materials. 4. 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 g1, which is 1.5 times higher than that of Nafion112. Water uptake of FN was 78.2%, and is considerably high. It is explained that the casting method enhanced the molecular

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chain motion of FN and resulted in large water uptake amount. We measured the polarization curve and electrochemical impedance of FN, s-FEP and Nafion112 where the cell temperature was 60 C. Consequently, IC value of FN was the highest in the tested PEMs. This is attributed to the high IEC value and high water uptake. FN showed the lowest charge transfer resistance. This would be resulted from not only the high IC, but also the well-laminated interface between the PEM and electrodes. The lower charge transfer resistance would decrease both activation and diffusion polarization in the cell performance test. The maximum power density of FN was 861 mW cm2, which is 1.8 times higher than that of s-FEP. This is the highest cell performance among the tested PEMs.

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

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