Synthesis of Polyethylene-Based Proton Exchange Membranes

Feb 17, 2012 - Containing PE Backbone and Sulfonated Poly(arylene ether sulfone) ... Energy Institute, The Pennsylvania State University, University P...
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Synthesis of Polyethylene-Based Proton Exchange Membranes Containing PE Backbone and Sulfonated Poly(arylene ether sulfone) Side Chains for Fuel Cell Applications Hyung Kyu Kim,† Min Zhang,† Xuepei Yuan,† Serguei N. Lvov,†,‡,§ and T. C. Mike Chung†,* †

Department of Materials Science and Engineering, ‡Department of Energy and Mineral Engineering, and §Electrochemical Technologies Program, EMS Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: This paper discusses a new class of proton exchange membranes (PEMs) that are based on a wellcontrolled polyolefin graft copolymer containing a polyethylene (PE) backbone and several sulfonated poly(arylene ether sulfone) (s-PAES) side chains. The chemistry involves a graft-onto reaction between high molecular weight PE with few pendent benzyl bromide groups and poly(arylene ether sulfone) (PAES) with two terminal phenol groups. The resulting PE-g-PAES graft copolymer, with predetermined backbone molecular weight, graft density, and graft length, was solution-cast into uniform film (thickness 20−40 μm), followed by a heterogeneous sulfonation reaction of PAES side chains to obtain the desired PE-g-s-PAES PEMs with a high sulfonation level. The unique combination of hydrophobicity, semicrystallinity, and high molecular weight of the PE backbone offers PEM with a stable (nonswellable) matrix. The embedded hydrophilic sPAES proton-conductive domains show only moderate water uptake, even with a high ion exchange capacity (IEC >3 mmol/g in the s-PAES domains). Compared to Nafion 117, most PE-g-s-PAES PEMs show similar hydration numbers (λ 1.4) show significantly smaller water swelling. In fact, the water uptake of PE-g-s-PAES PEM is strongly dependent on the PE backbone molecular weight, even in such a high molecular weight range (390k vs 220k g/mol). Comparing similar IEC values (s-B-1 vs s-E-3; 1.64 vs 1.67), the B set graft copolymers with a 390k g/mol PE backbone show less than half of the overall water swelling than those in the E set graft copolymers with a 220 K g/mol PE backbone. Evidently, the water pressure in the PE-g-s-PAES with a high

Figure 6. A comparison of strain−stress curves for a set of (a) PE-co-pMS, (b) PE-co-p-MS−Br, (c) PE-g-PAES, and (d) PE-g-s-PAES during the PEM preparation.

elongation. The high Tg PAES in the PE-g-PAES graft copolymer abruptly increases tensile strength and modulus and decreases the elongation, resembling a high performance engineering plastic. After the subsequent heterogeneous sulfonation reaction, the sulfonated PE-g-s-PAES PEM film absorbs some water (discussed later), however it still maintains very good mechanical strength with similar stiffness with PE-gPAES film and high tensile strength (∼35 MPa). Table 5 compares the mechanical properties of three graft copolymer sets, with various PE backbone molecular weights (390k, 320k, and 250k g/mol), before and after sulfonation. Table 5. Summary of Mechanical Properties of the Polymers during the PEM Preparation sample PE-co-p-MS PE-co-p-MS− Br Nafion117 PE-g-PAES (run B-4) PE-g-s-PAES (run s-B-4) PE-g-PAES (run C-3) PE-g-s-PAES (run s-C-3) PE-g-PAES (run C-1) PE-g-s-PAES (run s-C-1)

structure informationa (390k/0/0/ 0) (390k/0/0/ 0) − (390k/20k/ 63/0) (390k/20k/ 63/1.54) (320k/20k/ 63/0) (320k/20k/ 63/1.56) (250k/20k/ 64/0) (250k/20k/ 64/1.60)

tensile strength [MPa]

Young’s modulus [MPa]

elongation at break [%]

16.7 ± 1.0

230 ± 40

256 ± 31

11.4 ± 0.6

150 ± 30

171 ± 19

13.7 ± 1.5 48.0 ± 7.3

120 ± 10 1750 ± 150

208 ± 13 4.3 ± 0.2

34.7 ± 5.0

1470 ± 130

5.8 ± 0.3

34.5 ± 4.7

1320 ± 130

3.2 ± 0.2

25.7 ± 2.3

1120 ± 120

6.6 ± 0.7

27.4 ± 2.4

1090 ± 120

3.1 ± 0.2

24.7 ± 2.3

960 ± 80

5.2 ± 0.3

a

(PE backbone molecular weight/PAES side chain molecular weight/ PAES wt %/degree of sulfonation).

They have the same PAES side chains, similar high PAES content (63 wt %), and a degree of sulfonation (number of sulfonic acids per arylene ether sulfone unit in the s-PAES side chains). Three references, including PE-co-p-MS, PE-co-p-MS− Br, and Nafion 117 random copolymers, were also measured and compared side-by-side. In general, all PE-g-PAES and PE-g2467

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Table 6. Summary of IEC, Water Uptake, Hydration Number, and the in-Plane and through-Plane Conductivities of Two Sets of PE-g-s-PAES PEMS and Nafion 117 sample

IEC [mmol/g of PE-g-s-PAES]

IEC [mmol/g of s-PAES]

DS valuea

water uptake [%]

hydration numberb [λ]

in-plane conductivity [mS/cm]

through-plane conductivity [mS/cm]

s-B-1 s-B-2 s-B-3 s-B-4 s-E-1 s-E-2 s-E-3 s-E-4 s-E-5 Nafion117

1.67 1.88 2.05 2.16 1.03 1.40 1.64 1.78 1.91 0.91

3.63 3.42 3.42 3.43 3.68 3.59 3.49 3.36 3.29 −

1.60 1.52 1.52 1.54 1.62 1.60 1.56 1.50 1.48 −

34 42 48 55 43 65 80 93 104 24

11 12 13 14 23 26 27 29 30 15

64 80 86 94 28 39 61 70 78 77

99 129 155 167 31 40 83 104 144 81

a Degree of sulfonation: no. of sulfonic acid per arylene ether sulfone unit in the s-PAES side chains. bHydration number: no. of H2O molecules per sulfonic acid by 1H NMR.

Figure 7. (Left) Comparison of water uptake (%) and hydration number (λ) and (right) in-plane and through-plane conductivities vs IEC values for two sets of PE-s-PAES PEMs (B and E) and Nafion117.

IEC value is so high that some extra high molecular weight and high crystallinity PE polymer is needed to contain the water overswelling. In set B (the 390k case), although the water uptake still linearly increases with the IEC value, the hydration number only shows a moderate increase. With regards to PE-gs-PAES PEM (s-B-4) and its IEC value (more than double that of Nafion 117), the hydration number for each acid (λ = 14) in PE-g-s-PAES is nearly the same as that of Nafion 117 (λ = 15). The high molecular weight PE backbone, with the combination of high hydrophobicity and crystallinity, forms an exceptionally strong hydrophobic matrix (water prohibited zone) in PE-g-sPAES PEM to resist water swelling in the hydrophilic phases even with very high acid content. It may form some wellprotected (robotic) hydrophilic ion-conductive channels in PEg-s-PAES PEMs, which are not very sensitive to acid contents. Therefore, it is possible to continuously increase the IEC value and proton conductivity (set B), without the penalty of the dilution effect, due to excessive water swelling that also weakens the membrane strength and stability. Figure 7 (right) compares in-plane and through-plane conductivities for the same two sets of PE-g-s-PAES PEMs and Nafion 117. The conductivity essentially proportionately increases with the IEC value. The PE molecular weight appears

to be not particularly sensitive to the conductivity. However, good control of water swelling in PE-g-s-PAES (set B), with higher PE molecular weight, allows the preparation of PEMs with higher IEC values while still maintaining high mechanical strength. Therefore, the PEMs in the high molecular weight B set can reach very high proton conductivity. After the IEC value reaches 1.4, both in-plane and through-plane conductivities accelerate with higher slopes. This turning point (from the s-E2 sample in Table 6) implies that the morphology in PE-g-sPAES changes from not fully connected ionic domains to a complete network structure with many ionic channels across the PEM film. In fact, this PEM sample was originated from the PE-g-PAES graft copolymer (run E-2 in Table 3) that contains 33 vol % of PAES, which is consistent with the expected percolation threshold.59−61 It is a pleasant surprise to observe the through-plane conductivity of s-B-4 sample reaching 167 mS/cm (double that of Nafion 117) and significantly higher through-plane conductivity than in-plane conductivity in all PEg-s-PAES graft copolymers with percolated ionic channels (IEC value >1.4). The difference becomes larger as the IEC increases. All results indicate a significantly improved connectivity between ion channels, especially in the throughplane direction. Since the through-plane conductivity is directly 2468

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Figure 8. Comparison of water drops on PE-g-PAES, PE-g-s-PAES, and Nafion 117 PEMS.

backbone in the graft copolymer provides the stable separator function, which allows the incorporation of a high concentration of sulfonic acids (high IEC value) in the conductive sPAES domains for conduction function. In addition to the control of water swelling and maintaining good mechanical properties, the PE-based PEMs possess a thin PE hydrophobic surface layer that is reflected in the water contact angle and the anisotropic ion conductivity. Overall, this new class of PE-g-sPAES PEMs offer a desirable set of properties, including conductivity, water uptake, mechanical strength, and costeffectiveness for fuel cell applications.

relative to the fuel cell performance, this is a positive surprise, and it deserved a further study to understand the structure causing this anisotropy in conductivity. Surface Properties of PE-g-s-PAES PEMs. The unusual phenomenon of higher through-plane conductivity than inplane conductivity prompted us to investigate the surfaces of PE-g-s-PAES membranes. It is logical to speculate that the low surface energy of the PE chain in the graft structure may be favorable to diffuse and stay on the surface62,63 of the PEM membrane. To verify this assumption, a contact angle measurement was performed using a water drop on PEM surfaces. Figure 8 shows the photographs of the water drop on the membrane surface (vs time) for both PE-g-PAES (run B-4 in Table 3) and PE-g-s-PAES (sample s-B-4 in Table 5), as well as for Nafion 117. Upon the water drop coming in contact with the Nafion 117 PEM surface, the water drop immediately diffused into the matrix. After 3 min, the water drop almost disappeared from the PEM surface, with the contact angle at 38.03°. On the contrary, the water drop was quite stable on the PE-g-PAES surface with the contact angle change less than 1 degree from 100.34° to 99.97°, which is significantly higher than the 83.84° of pure poly(arylene ether sulfone). The corresponding sulfonated PE-g-s-PAES PEM also retained the water drop very well, only allowing the water drop to slowly diffuse into the matrix. Its contact angle changed from 98.41° to 86.61° after 3 min. Evidently, the surfaces of both PE-g-PAES and PE-g-s-PAES are significantly more hydrophobic than the surface of Nafion 117. A very thin layer of PE must cover most of their surfaces. The detailed contact angle comparison is presented in the Supporting Information (Table S1).

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ASSOCIATED CONTENT

S Supporting Information *

Determination of PAES molecular weight, TGA thermograms, FTIR spectra, DSC curves, and contact angle results. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of this work by an NSF Research Grant (CBET-1067554). S.N.L. expresses thanks for the support from the U.S. Department of Energy (Contract No. DE-FG07-07ID14886). We also would like to thank Albemarle Corp. and Solvay Polymers for their kind donation of MAO and high purity grade dichlorodiphenyl sulfone.

CONCLUSION

We have developed a new class of polyethylene-based proton exchange membranes (PE−PEMs) for fuel cell applications. The systematic study of the graft-onto (coupling) reaction between brominated PE and poly(arylene ether sulfone) allows us to prepare a broad range of graft copolymers with wellcontrolled molecular structures, including the PE backbone and PAES side chain molecular weights, and graft density. With the combination of selective and efficient sulfonation reaction on PAES side chains, we have prepared PE-based PEMs with very high IEC values (>3 mmol/g in sulfonated PAES domains) in addition to well microphase separated morphology with a highly hydrophobic and crystalline PE matrix and the imbedded continuous proton conductive channels. The high molecular PE

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