Constructing Ionic Liquid-Filled Proton Transfer Channels within

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Constructing Ionic Liquid-Filled Proton Transfer Channels within Nanocomposite Membrane by Using Functionalized Graphene Oxide Wenjia Wu, Yifan Li, Pingping Chen, Jin-dun Liu, Jingtao Wang, and Haoqin Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09642 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 22, 2015

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ACS Applied Materials & Interfaces

Table of Contents

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Constructing Ionic Liquid-Filled Proton Transfer Channels within Nanocomposite Membrane by Using Functionalized Graphene Oxide Wenjia Wu, Yifan Li, Pingping Chen, Jindun Liu, Jingtao Wang,* Haoqin Zhang School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, P. R. China S Supporting Information

ABSTRACT: Herein, nanocomposite membranes are fabricated based on functionalized graphene oxides (FGOs) and sulfonated poly(ether ether ketone) (SPEEK), followed by being impregnated with imidazole-type ionic liquid (IL). The functional groups (acidic group or basic group) on FGOs generate strong interfacial interactions with SPEEK chains and then adjust their motion and stacking. As a result, the nanocomposite membranes possess tunable interfacial domains as determined by free volume characteristic, which provide regulated location for IL storage. The stored ILs act as hopping sites for water-free proton conduction along the FGO-constructed interfacial channels. The microstructure at SPEEK-FGO interface governs the IL uptake and distribution in nanocomposite membrane. Different from GO and vinyl imidazole functionalized GO (VGO), the presence of acidic (–SO3H) groups confers the p-styrenesulfonic acid functionalized GO (SGO) incorporated nanocomposite membrane loose interface and strong electrostatic attraction with imidazole-type IL, imparting an enhanced IL uptake and anhydrous proton conductivity. Nanocomposite membrane containing 7.5% SGO attains the maximum IL uptake of 73.7% and hence the anhydrous conductivity of 21.9 mS cm-1 at 150 oC, more than 30 times of that of SPEEK control membrane (0.69 mS cm-1). In addition, SGOs generate electrostatic 1


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attractions to the ILs confined within SGO-SPEEK interface, affording the nanocomposite membrane enhanced IL retention ability. KEYWORDS: functionalized graphene oxide, ionic liquid uptake and distribution, interfacial microstructure, nanocomposite membrane, anhydrous proton conductivity

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1. INTRODUCTION Development of highly conductive membrane that works under anhydrous condition is one of the most urgent demands for proton exchange membrane fuel cell (PEMFC) technology.1 PEMFC operated under elevated temperatures (above 100 oC) and anhydrous conditions has been the focus of interest, due to the numerous advantages of high tolerance of catalyst to CO, simplified heat/water management, and improved electrode kinetics.2-4 However, a fatal building block is the dificiency of water-independent high-performance proton exchange membrane (PEM). According to the transfer mechanism, protons can only be transported via short-distance ( SP-VGO-X-IL > SP-GO-X-IL > SP-IL (Figure 7b). The IL leaching would reduce the amount of proton hopping sites and thus decrease the proton conduction ability of the membrane. Figure 7c showed the anhydrous proton conductivities of the IL-filled membranes after IL leaching. SP-IL with the constant IL retention of 9.9% achieved the conductivity of 2.57 mS cm-1 at 150 oC. By comparison, nanocomposite membranes achieved enhanced anhydrous conductivities due to the higher amount of ILs in membrane, especially in the SPEEK-filler interfacial domains. For instance, the conductivities after IL leaching of SP-GO-X-IL, SP-VGO-X-IL, and SP-GO-X-IL were elevated to 3.37, 3.60, and 4.71 mS cm-1, respectively. Lin et al. used imidazolium functionalized SiO2 and mesoporous SiO2 to enhance the anhydrous conductivity and IL retention of polymer membrane, which attained the maximum conductivities of 10 and 12 mS cm-1 (160 oC) with the IL retentions of about 5.8% and 8.0%, respectively.47,48 By comparison, the corresponding values obtained in this study were acceptable, probably attributed to the long-range interfacial pathways formed by 2-D FGO/GO. 24



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Collectively, these data implied that the anhydrous conductivity was IL loading amount and distribution controllable for IL-filled nancomposites, and incorporating functionalized GO could simultaneously elevate IL uptake and IL retention by forming nano-confined domains. Table S1 also indicated that the creation of continuous interfacial channels should be one promising approach to store ILs and then to transport protons when compared with the approaches in literature. 4. CONCLUSIONS In summary, we attempted a facile approach to enhance IL uptake and retention of polymer membrane by incorporating functionalized GO species for efficient anhydrous proton conduction. Within the nanocomposite membrane, the GO species constructed numerous interfacial domains, which provided additional space for IL storage. Helped by the high aspect ratio and ultrahigh surface area of the fillers, the stored

ILs

formed

long-range

transfer

pathways

and

therefore

endowed

nanocomposite membrane with enhanced anhydrous proton conduction ability. Varying the functional groups could efficiently tune the interfacial microstsructure and thus the IL uptake and distribution, which determined the conduction ability of IL-filled composites. In addition, the nano-confined effect of interfacial domains offered nanocomposite membrane enhanced IL retention ability through capillary force, and the ability would be further strengthened at the presence of strong attraction from the groups on channel surface. Collectively, the high IL storage and retention ability conferred enhanced anhydrous conductivity and promising long-time operation stability on the nanocomposite membrane. While more efforts should be devoted to 25


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avoiding any loss of ILs from membrane to ensure the practical application in fuel cell, for example highly cross-linking of membrane surface, surface coating by inorganic materials, etc.

■ ASSOCIATED CONTENT S Supporting Information

Evaluation of free volume characteristics. FTIR spectra and TGA curves of the membranes. Anhydrous conductivities and IL retention abilities of some other membranes in literatures.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: 86-371-63887135. Fax: 86-371-63887135. Notes The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

We gratefully acknowledge the financial supports from National Natural Science Foundation of China (21506232 and U1304215), and China Postdoctoral Science Foundation (2014T70687).

■

REFERENCES

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Scheme 1. Synthesis process of the nanocomposite membrane.

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Scheme 2. The schematic representation of synthesis process and chemical structure of FGO.

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d

Transmittance (a.u.)

GO 1736 1386 1628

VGO

745

SGO 1568

1447

2923 3500

3000

1250-1000 2500

2000

1500

Wave number (cm-1)

169.6

402.4

a b c

80

SGO 396

400

404

408

164

168

172

VGO

Weight (%)

Intensity (a.u.)

1000

100

e

176

3

S 2p

O 1s N 1s

GO

GO VGO SGO

500

f

b

60

c a

40

C 1s 20

600

500

400

300

200

100

100

0

200

SGO 2θ = 10.3°

VGO 2θ = 10.6°

GO 2θ = 9.8°

20

400

500 o

g

10

300

Temperature ( C)

Bind energy (eV)

Relative intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

2θ (degree)

36


600

700


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Figure 1. Characterization of GO, VGO, and SGO: TEM images of GO (a), VGO (b), and SGO (c), FTIR spectra (d), XPS spectra (e), TGA curves (f), and WXRD patterns (g).

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Figure 2. SEM images of the cross-section of (a) SP, (b) SP-IL, (c) SP-GO-5%, (d) SP-GO-5%-IL, (e) SP-VGO-5%, (f) SP-VGO-5%-IL, (g) SP-SGO-5%, and (h) SP-SGO-5%-IL.

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a b c d c

e f

e f g h

b g h

0.20

0.25

0.30

SP SP-GO-5% SP-VGO-5% SP-SGO-5%

0.35

SP-IL SP-GO-5%-IL SP-VGO-5%-IL SP-SGO-5%-IL

0.40

-1

0.45

0.50

b

a Relative intensity (a.u.)

d a Relative intensity (a.u.)

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SP SP-GO-5% SP-VGO-5% SP-SGO-5% SP-IL SP-GO-5%-IL SP-VGO-5%-IL SP-SGO-5%-IL 10

0.55

20

30

2θ (degree)

q (nm )

Figure 3. (a) SAXS patterns and (b) WXRD patterns of the membranes.

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SP-GO-X SP-VGO-X SP-SGO-X

70

IL uptake (%)

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60

50

40

0

2.5%

5.0%

7.5%

10.0%

Filler content

Figure 4. IL uptakes of the membranes (Treating temperature: 55 oC; Treating time:72 h).

40


-1

-1

1.5

Proton conductivity (mS cm )

a


1.0

0.5

0.0

b

SP-IL SP-GO-2.5%-IL SP-GO-5%-IL SP-GO-7.5%-IL SP-GO-10%-IL

20

15

10

5

60

80

100

120

140

40

60

Temperature (°C)

c

15

-1

SP-IL SP-VGO-2.5%-IL SP-VGO-5%-IL SP-VGO-7.5%-IL SP-VGO-10%-IL

20

80

100

120

140

Temperature (°C)


-1

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0 40


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10

5

d

SP-IL SP-SGO-2.5%-IL SP-SGO-5%-IL SP-SGO-7.5%-IL SP-SGO-10%-IL

20

15

10

5

0

0 40

60

80

100

120

40

140

60

80

100

120

140

Temperature (°C)

Temperature (°C)

Figure 5. Temperature-dependent proton conductivities of (a) IL-free membranes, (b) SP-GO-XIL, (c) SP-VGO-X-IL, and (d) SP-SGO-X-IL.

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a

-4

b

-7

-5

-8

ln σ (S cm-1)

ln σ (S cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-6


-7

-8 2.4

2.6

-1

Ea= 24.95 KJ mol -1 Ea= 24.53 KJ mol -1 Ea= 23.77 KJ mol -1 Ea= 24.16 KJ mol 2.8

3.0

-9

SP-IL SP-GO-5%-IL SP-VGO-5%-IL SP-SGO-5%-IL

-10

-11

3.2

2.4

2.6

1000/T (K-1)

-1

Ea= 23.13 KJ mol -1 Ea= 19.81 KJ mol -1 Ea= 19.85 KJ mol -1 Ea= 19.65 KJ mol 2.8

3.0

3.2

1000/T (K-1)

Figure 6. The Arrhenius plots of proton conductivity of membranes: (a) IL-free membranes and (b) IL-filled membranes.

42



a

70

b

85

SP-IL (70%) SP-GO-5%-IL (70%) SP-VGO-5%-IL (70%) SP-SGO-5%-IL (70%)

25

IL retention (%)

80

IL loss (%)

75


70

65

20

15

10

0 0

40

80

120

160

200

240

0

40

Immersing time (min)

80

120

160

200

240

Immersing time (min)

c

-1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 46


4

3

2

1

0 40

60

80

100

120

140

Temperature (°C)

Figure 7. (a) IL loss and (b) IL retention values of the membranes as a function of time; (c) temperature-dependent anhydrous conductivities of the membranes after being immersed in water.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Densities and FFV values of the membranes determined by the buoyancy method. Sample

ρB (g cm-3)

FFV (%)

SP

1.359±0.005

0.223±0.007

SP-GO-2.5%

1.281±0.007

0.225±0.008

SP-GO-5%

1.206±0.009

0.231±0.008

SP-GO-7.5%

1.146±0.007

0.233±0.007

SP-GO-10%

1.110±0.008

0.219±0.006

SP-VGO-2.5%

1.271±0.006

0.221±0.005

SP-VGO-5%

1.179±0.006

0.229±0.007

SP-VGO-7.5%

1.094±0.007

0.239±0.007

SP-VGO-10%

1.045±0.005

0.230±0.006

SP-SGO-2.5%

1.255±0.006

0.227±0.007

SP-SGO-5%

1.158±0.009

0.236±0.007

SP-SGO-7.5%

1.079±0.008

0.241±0.008

SP-SGO-10%

1.026±0.007

0.233±0.005

44



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 46

Table 2. IL uptakes and anhydrous proton conductivities (120 oC) of as-prepared membranes. Sample

IL uptake (%)

σ (mS cm-1)

SP-IL

43.0±0.75

7.2±0.47

SP-GO-2.5%-IL

45.3±0.78

8.3±0.49

SP-GO-5%-IL

56.0±0.84

10.5±0.51

SP-GO-7.5%-IL

57.9±0.89

10.8±0.52

SP-GO-10%-IL

36.2±0.74

5.1±0.44

SP-VGO-2.5%-IL

39.7±0.78

7.4±0.48

SP-VGO-5%-IL

52.8±0.79

9.5±0.48

SP-VGO-7.5%-IL

66.5±0.90

13.6±0.49

SP-VGO-10%-IL

56.8±0.87

9.9±0.49

SP-SGO-2.5%-IL

50.0±0.75

9.7±0.49

SP-SGO-5%-IL

64.3±0.86

14.3±0.51

SP-SGO-7.5%-IL

73.7±0.91

17.2±0.52

SP-SGO-10%-IL

58.5±0.80

12.6±0.51

45


Constructing Ionic Liquid-Filled Proton Transfer Channels within

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