Facilitating Proton Transport in Nafion-Based Membranes at Low

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Facilitating Proton Transport in Nafion-based Membranes at Low Humidity by incorporating Multi-functional Graphene Oxide Nanosheets Xueyi He, Guangwei He, Anqi Zhao, Fei Wang, Xunli Mao, Yongheng Yin, Li Cao, Bei Zhang, Hong Wu, and Zhongyi Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06424 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017

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ACS Applied Materials & Interfaces

Facilitating Proton Transport in Nafion-based Membranes at Low Humidity by incorporating Multi-functional Graphene Oxide Nanosheets Xueyi He a,b, Guangwei He

a,b

, Anqi Zhao

a,b

, Fei Wang a,b, Xunli Maoa,b, Yongheng

Yina,b, Li Caoa,b, Bei Zhanga,b, Hong Wu a,b, Zhongyi Jiang a,b* a

Key Laboratory for Green Chemical Technology of Ministry of Education, School of

Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. b

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),

Tianjin, 300072, China. ABSTRACT Nafion, as state-of-the-art solid-electrolyte for proton exchange membrane fuel cells (PEMFCs), suffers from drastic decline in proton conductivity with decreasing humidity, which significantly restricts the efficient and stable operation of the fuel cell system. In this study, the proton conductivity of Nafion at low relative humidity (RH) was remarkably enhanced by incorporating multi-functional graphene oxide (GO) nanosheets as multi-functional fillers. Through surface-initiated atom transfer radical polymerization of sulfopropyl methacrylate (SPM) and poly(ethylene glycol) methyl ether methacrylate, the copolymer-grafted GO was synthesized and incorporated into Nafion matrix, generating efficient paths at the Nafion-GO interface for proton conduction. The Lewis basic oxygen atoms of ethylene oxide (EO) units and sulfonated acid groups of SPM monomers served as additional proton binding and *

Corresponding author: Tel: +86-22-27406646; Fax: +86-22-23500086; E-mail address: [email protected]

ACS Paragon Plus Environment


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release sites to facilitate the proton hopping through the membrane. Meanwhile, the hygroscopic EO units enhanced the water retention property of the composite membrane, conferring dramatic increase in proton conductivity under low humidity. With 1 wt% filler loading, the composite membrane displayed the highest proton conductivity of 2.98 × 10-2 S cm-1 at 80 oC and 40% RH, which was 10 times higher than that of recast Nafion. Meanwhile, the Nafion composite exhibited a 135.5% increase in peak power density at 60 oC and 50% RH, indicating its great application potential in PEMFCs.

KEYWORDS: Nafion-based composite membrane, graphene oxide nanosheets; sulfonate, ethylene oxide units, low humidity, proton conductivity

1.

INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) have attracted enormous attention

as promising energy conversion devices for transportation and portable power applications owing to their rapid startup, high efficiency and power density with low emission levels.

1-2

. Serving as the proton conductor and gas barrier (O2, H2), proton

exchange membrane (PEM) plays a critical role in PEMFC and also governs the efficient and stable operation of the fuel cell system

3-4

. Nafion, as state-of-the-art

solid-electrolyte, is widely used in PEMFCs due to superior chemical-mechanical stability and proton conductivity under wet conditions 5-6. The proton conductivity of Nafion relies heavily on the water content/state in the membrane. At high levels of hydration under wet conditions, the well-connected water structure in the membrane 2


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renders highly efficient proton transport via the vehicular and structural diffusion mechanism. With decreasing humidity, however, the collapse of the interconnected hydrophilic channels leads to noteworthy decrease of proton conductivity in the sulfonated acid based system 7-8. The humidification becomes an essential step for the state-of-art ionomers, which significantly increases the complexity of the water management system and narrows the operation range of the ionomers

9-10

. It is thus

desirable to develop solid electrolytes with sufficient proton conductivity under low humidity conditions (i.e., 100 oC), deterring the complete collapse of the interconnected hydrophilic channels for proton conduction. Except for acting as

4


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humidifying agent, the EO units are favorable for anhydrous proton transport via proton solvation and rearrangement of the chain segments ascribed to the Lewis basic oxygen atoms and flexible segments

27-28

. Ghosh et al.29 directly utilized PEGylated

polymer as proton conducting electrolyte and analyzed the dependence of proton conductivity on PEG content. The experimental results showed elevated ion mobility of electrolyte with increasing PEG content, enlightening the contribution of EO segments to anhydrous proton conduction based on the Grothuss mechanism. Given the positive effect of EO segments on proton conduction, sulfopropyl methacrylate

(SPM)

and

poly(ethylene

glycol)

methyl

ether methacrylate

(PEGMEMA) were copolymerized, generating versatile polymer brushes on graphene oxide (GO) nanosheets by surface-initiated atom transfer radical polymerization (SI-ATRP). The copolymer-grafted GO nanosheets were blended with Nafion to render rapid transport channels for protons through the composite membrane. Simultaneously acting as humidifying agents and bearing proton binding sites, the flexible EO units were assumed to keep the filler surface hydrated under low RHs and facilitate the proton transport at the organic-inorganic interface via segment movement. The SPM monomers afforded effective proton release sites and therefore reduced the energy barrier for proton hopping. Consequently, the incorporation of the copolymer-grafted GO nanosheets imparted additional amount of proton binding and release sites and enhanced the water retention property of the Nafion-based system, which efficiently facilitated the proton conduction of the composite membrane over a wide range of humidity.

5



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Scheme 1. Preparation of copolymer-grafted GO nanosheets

2.

EXPERIMENTAL SECTION 2.1

Materials. Nafion 117 and Nafion solution (5 wt%) were provided by

DuPont. Flake graphite was purchased from Qingdao Risheng graphite Ltd. 3-Aminopropyl-triethoxysilane (APTES, > 98.0%) and 2-Bromo-2-methylpropionyl bromide (BiBB, > 98.0%) were supplied by TCI (Shanghai) Development Co., Ltd. Ammonia solution (25% ~ 28%), triethylamine (TEA, 99%) and Dimethylformamide (DMF, 99.8%) were purchased from Aladdin (Shanghai). 2,2’-Bipyridine (99%), Copper (II) chloride (98%) and Copper (I) chloride (98%) were purchased from J&K Scientific Co. Ltd, among which Copper(I) chloride was purified by stirring in acetic acid for 12 h, washing with ethanol and dimethyl ether and drying under vacuum overnight

30

. 3-Sulfoporpyl methacrylate potassium salt (98%) and PEGMEMA

(average Mn ≈ 300) were obtained from Sigma-Aldrich. PEGMEMA was passed through a small basic alumina column before copolymerization. Other reagents were used as received without further purification. Deionized water was utilized through the entire experiment. 2.2

Synthesis of Polymer-grafted GO Nanosheets. GO nanosheets were 6


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synthesized by a typical Hummer’s method. As shown in Scheme 1, GO was first functionalized with APTES using ammonia as the catalyst. Then, BiBB was added to the aqueous dispersion of functionalized GO nanosheets to attach the ATRP initiator to the GO surface (catalyzed by TEA). The preparation method of initiator functionalized GO (GO-Br) was consistent with the procedures reported in the literature 31. GO-Br (50 mg) was homodispersed in the mixed solution of methanol (6 mL) and water (4 mL). Then, 2, 2’-Bipyridine (63 mg), 3-Sulfoporpyl methacrylate potassium salt (1.6 g) and PEGMEMA (0.4 g) were dissolved in the aqueous mixture by stirring and ultrasonication. After degassing (10 min) and back-filled with nitrogen twice, CuCl (14 mg) and CuCl2 (8 mg) were quickly added to the system. The reactor was degassed and back-filled with nitrogen for another three times before sealing the gas inlet. The reaction was carried out at 40 oC with continuous stirring for 48 h and terminated by exposure to air. The resulting contents were washed with large amount of water and ethanol and finally freeze-dried for 48 h. The copolymer-grafted GO was designated as GO-poly(SPM-co-PEGMEMA). For comparison, GO-polySPM was synthesized following the same procedures aforementioned, during which 2.0 g 3-Sulfoporpyl methacrylate potassium salt was added to the aqueous dispersion of functionalized GO as the single monomer. 2.3

Preparation of the Composite Membranes. Prior to membrane preparation,

the polymer-grafted GO was kept in a sulfuric acid solution (1 M) for 24 h and rinsed with large sum of water to exchange the K+ to H+ ions. After freeze-drying, the

7



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copolymer-grafted GO was dispersed in 2 mL DMF via alternating stirring and ultrasonication for 12 h. The origin solvent of Nafion solution (6 mL) was evaporated (60 oC) and replaced by DMF (4 mL). After complete dissolution of Nafion, the two solutions were mixed together, followed by stirring and ultrasonication for another 12 h. The homogeneous solution was cast onto a clean glass plate, dried at 80 oC for 12 h and heated up to 120 oC for 6 h to obtain the composite membrane. The membrane was then immersed in hydrogen peroxide solution (3%), deionized water and sulfuric acid solution (1 M) successively for 1 h in each case at 80 oC. Finally, residual acid was rinsed off from the membrane by a large sum of water. The composite membranes were designated as Nafion/GO-poly (SPM-co-PEGMEMA)-X, where X referred to the mass ratio of GO-poly (SPM-co-PEGMEMA) to Nafion. With a mass ratio of 1%, GO and GO-polySPM were also impregnated into the Nafion ionomer following the same protocol as GO-poly (SPM-co-PEGMEMA) to prepare composite membranes for comparison. The composite membranes were designated as Nafion/GO-1.00 and Nafion/GO-polySPM-1.00, respectively. 2.4

Characterization. The chemical composition of polymer-grafted GO was

characterized by Fourier transform infrared spectroscopy (FTIR, BRUKER Vertex 70) and X-ray photoelectron spectroscopy (XPS, GENESIS EDAX) with Al Kα radiation. The morphologies of functionalized GO nanosheets were observed by transmission electron microscopy (TEM, Tecnai G2 F20) and field emission scanning electron microscopy (FESEM, Nanosem 430). The crystal structure of GO-based nanosheets was investigated by X-ray diffraction (XRD, Rigaku D/max 2500 v/pc) with a

8


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scanning rate of 5o min-1. Small-angle X-ray scattering (SAXS) was utilized to delineate the morphological features of the Nafion-based membranes. 1H nuclear magnetic resonance (NMR, Varian Unity Inova, 500 MHz) spectroscopy was used to investigate the composition of the copolymer (in D2O). The thermal stability of functionalized GO and composite membranes was characterized by thermal gravimetric analysis (TGA, NETZSCH-TG209 F3) under N2 atmosphere from 40 to 800 oC (heating rate: 10 oC min-1). FESEM was utilized to examine the cross-section morphology of the Nafion-based membranes. . The mechanical performance of the membranes at dry state was investigated by electronic stretching machine (Yangzhou Zhongke WDW-02) with a strain rate of 10 mm min-1. 2.5

Water Uptake and Swelling Ratio. The water uptake and swelling ratios of

the membranes were calculated based on the following formulas:

Water uptake (wt%) =

Area swelling (%) =

W1 − W0 × 100% W0

A1 − A0 × 100% A0

(1)

(2)

The weights (W0, g) and surface areas (A0, cm2) of the samples were measured after drying the membranes in an 80 oC oven until constant weight. Then, the samples were immersed in water at a certain temperature for 24 h. The weights (W1, g) and surface areas (A1, cm2) of the wet samples were measured again after wiping off the water from the membrane surface. The vapor phase water uptake measurement was also carried out by placing the samples in a thermo-hygrostat at certain humidity for 24 h. The vapor phase water uptake could also be obtained through measuring the weights

9



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of the dry and hydrated samples based on equation 1. 2.6

Ion Exchange Capacity and Proton Conductivity. A classical titration

method was utilized to measure the ion exchange capacity of the samples. Before titration, certain weight of dry sample (W0, g) was soaked in 2 M NaCl solution for 36 h. then, aqueous NaOH (a mol L-1, VOH L) was used to titrate the solution to an end point of pH = 7. The ion exchange capacity (IEC, mmol g-1) of the sample could be calculated according to the following relationship:

IEC =

1000 × a ×VOH W0

(3)

The proton conductivity of the membranes was measured by two-probe AC impedance spectroscopy method based on Princeton Parstat 4000 Electrochemical Workstation over 100 KHz to 1 Hz frequency range. The measurement was carried out under different temperatures and humidity conditions, during which the sample was clamped between the electrodes and put in a thermo-hygrostat. Measured in horizontal direction, the conductivity of the sample could be calculated according to equation 4:

σ=

l AR

(4)

Where σ (S cm-1) is the proton conductivity of the sample; l (cm) the distance between the electrodes; A (cm2) and R (Ω) represent the cross-section area and Ohmic resistance of the sample. The proton conductivity is thermally activated and could be further depicted by the following equation:

10


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ln σ = −

Ea 1 . + ln σ 0 R T

(5)

As a result, the activation energy for proton conduction (Ea, kJ mol-1) could be estimated from the slop of the Arrhenius curve (ln σ vs. 1/T). 2.7

Membrane-Electrode Assembly (MEA) and Single Fuel Cell Test. Pt/C

(60%, Johnson Matthey Co.) was mixed with Nafion solution, water and isopropanol to prepare the catalyst slurry. After ultrasonication for 30 min, the obtained mixture was painted onto the carbon papers with catalyst loading of 0.10 mg/cm2 for anode and 0.25 mg/cm2 for cathode. The polymer electrolyte was sandwiched between the two electrodes and the assembly was hot-pressed at 130 oC for 10 min. The obtained MEA (active area: 6.25 cm2) was then tested in a fuel cell device at 60 oC under 150 mL min-1 flow rate of fully humidified H2 and 200 mL min-1 of O2 with 0.1 MPa back-pressure. 3.

RESULTS AND DISCUSSION 3.1

Characterization of Functionalized GO Nanosheets. XPS was carried out

to investigate the surface composition of functionalized GO nanosheets (Figure 1). The characteristic Br 3d peak of GO-Br at 71.2 eV (Figure S1a) and the appearance of S 2p signal in GO-polySPM and GO-poly(SPM-co-PEGMEMA) at 168.3 eV (Figure S1b and S1c) indicated successful bromination and polymeric functionalization of GO. Further evidence could be obtained from the difference in C 1s high resolution spectra. Compared with the typical C 1s spectrum of GO (Figure 2a), the energy peak at 287.2 eV (C-O-C) disappeared in the C 1s spectrum of GO-Br (Figure 2b) and new peaks appeared at 288.0 eV, 286.5 eV and 285.3 eV attributable to O=C-N, C-O/C-N and

11



C-Br species

32

, which implied successful attachment of the initiator on the GO

surface. GO-polySPM and GO-poly(SPM-co-PEGMEMA) showed similar C 1s spectra in Figure 2c and 2d, which could be curve-fitted into three peak components corresponding to C=O, C-O/C-N and C-C species 33. In addition, an obvious increase in the percentage of the peak area from 23.4% to 51.4% could be observed at 286.0 eV, arising from the copolymerization of the PEGMEMA monomer in GO-poly(SPM-co-PEGMEMA). The atomic C/O ratios on the surface of the nanosheets also agreed with the difference in the grafted polymer based on the atomic concentrations of C and O calculated from the XPS results (Table S1, 1.49 for GO-polySPM and 1.85 for GO-poly(SPM-co-PEGMEMA) ). O 1s C 1s GO-poly(SPM-co-PEGMEMA)

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

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GO-polySPM

K 2p S 2p

GO-Br

N 1s Si 2p

GO

Br 3d 1200

1000

800

600

400

200

Binding Energy (eV)

Figure 1. XPS results of GO-based nanosheets

12


0

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(b)

(a) 287.2 (C-O-C)

284.7 (C-C)

286.5 (C-O/C-N)

Intensity (a.u.)

Intensity (a.u.)

285.4 (C-OH)

288.6 (C=O)

288.0 (O=C-N)

284.6 (C-C)

292

290

288

286

284

282

288.9 (O=C-O)

280

292

290

Binding Energy (eV)

285.3 (C-Br)

288

286

284

282

280

Binding Energy (eV)

(c)

(d)

Intensity (a.u.)

284.7 (C-C)

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


286.0 (C-O/C-N) 288.6 (O=C)

290

288

286

284

282

280

288.8 (O=C)

290

Binding Energy (eV)

284.7 (C-C)

286.0 (C-O/C-N)

285

280

Binding Energy (eV)

Figure 2. C 1s XPS spectra of (a) GO, (b) GO-Br, (c) GO-polySPM, and (d) GO-poly(SPM-co-PEGMEMA)

The FTIR spectra identified the functional groups in GO-based nanosheets (Figure 3). Apart from a broad strong adsorption between 3700 and 3023 cm-1 (stretching vibration of O-H), characteristic peaks at 1728 cm-1, 1627 cm-1and 1040 cm-1 could be observed in the GO sample, respectively assigning to the stretching vibrations of C=O, C=C and C-O-C bonds

34

. For GO-Br nanosheets, the attachment of the initiator

generated new adsorption bands at 2930 cm-1 and 1400 cm-1 corresponding to the stretching vibrations of C-H and C-N (in amide species) bonds

35

and peaks at 1130

cm-1 and 1043 cm-1 relating to the stretching vibrations of Si-O and C-O in C-O-Si groups

36

. As for GO-polySPM and GO-poly(SPM-co-PEGMEMA), polymeric

13



functionalization of GO was evidenced by the pronounced antisymmetric and symmetric vibrational adsorption of sulfonic acid groups

37

at 1190 cm-1 and 1040

cm-1 as well as the intensive adsorption at 1720 cm-1 assigning to the C=O vibration. Moreover,

the

intensity

of

the

peak

at

cm-1 was

1040

stronger

in

GO-poly(SPM-co-PEGMEMA) than that in GO-polySPM, which was probably due to the simultaneous vibration of sulfonic acid groups and C-O-C bonds.

GO

1728

GO-Br

Transmittance

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 14 of 35

1040 1627

GO-polySPM

2930

1400

1043 1130

1720

GO-poly(SPM-co-PEGMEMA)

1040

1190

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Figure 3. FTIR spectra of GO-based nanosheets

In the XRD patterns (Figure 4), the decoration of functional sites (-Br) increased the interlayer distance of GO nanosheets from 0.81 nm to 1.18 nm corresponding to the decreased diffraction angle from 10.96o to 7.46o. For GO-polySPM and GO-poly(SPM-co-PEGMEMA), the grafted polymers facilitated the exfoliation of GO nanosheets and thus led to a broad amorphous diffraction peak centered at around 22o. 14


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o

2θ = 10.96 d = 0.81 nm

GO

o

2θ = 7.46 d = 1.18 nm

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


GO-Br

GO-polySPM


5

10

15

20

25

30

35

40

2θ (degree) Figure 4. XRD patterns of GO-based nanosheets

TGA was used to investigate the thermal property and composition of GO-based nanosheets (Figure 5). All materials exhibited a major weight loss corresponding to the decomposition of organic components. The difference in the decomposition temperature

(140

o

C

for

GO

and

250

o

C

for

GO-polySPM

and

GO-poly(SPM-co-PEGMEMA ) suggested the effectiveness of polymer coating for enhancing the thermal stability of graphene nanosheets

38

. The mass ratios of the

grafted polymers could be estimated from the weight loss (form 40 oC to 600 oC) of corresponding samples compared with that of GO-Br. The polymer-grafted GO samples exhibited similar weight loss of ca. 59.0 wt% while the GO-Br sample had a total weight loss of ca. 42.2 wt%, which revealed a polymer graft content of about 16.8 wt% in GO-polySPM and GO-poly(SPM-co-PEGMEMA)

15


35

. In terms of the


copolymer-grafted GO nanosheets, the percentage of PEGMEMA in the polymer was further estimated to be 17.7 mol% based on the 1H NMR spectrum (Figure S2). (b)

(a) 100

0.0

-0.5

-1

DTG ( C )

80

60

o

Weight (%)

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

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40

20

-1.0

GO GO-Br GO-polySPM GO-poly(SPM-co-PEGMEMA)

-1.5

0 100

200

300

400

500

600

GO GO-Br GO-polySPM GO-poly(SPM-co-PEGMEMA) 100

200

300

o

Temperature ( C)

400

500

600

o

Temperature ( C)

Figure 5. (a) TGA and (b) derivative thermal gravimetric (DTG) curves of GO-based nanosheets

The morphological difference of the nanosheets was directly presented in the TEM images shown in Figure 6 (SEM images were illustrated in Figure S3). Compared with the pristine image of GO, the polymer-grafted GO showed blurred dark images with folded morphology on the fringe of the nanosheet (marked by the black arrow), which implied the occurrence of agglutination between grafting chains 39.

16


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Figure

6.

TEM

images

of

(a)

GO,

(b)

GO-Br,

(c)

GO-polySPM

and

(d)


3.2

Characterization of the Membranes. The cross-section morphologies of

the Nafion-based membranes were illustrated in Figure 7. Compared with the smooth cross-section surface of recast Nafion, obvious wrinkles could be observed in the cross-section images of the composite membranes due to the incorporation of the flake-like fillers (marked by the yellow arrows in Figure 7b~7d). With increasing GO-poly(SPM-co-PEGMEMA) loading, rougher morphologies were presented and disordered stacking of the fillers could be observed in Figure 7e and 7f (marked by yellow circles), which might cause partial defect in the composite membranes. (b)

(a)

(c)

4 µm (e) (e)

(d)

(d)

(f)

44 µm µm Figure

7.

SEM

4 µm

4 µm

4 µm

4 µm

4 µm

micrographs

of

cross-sections:

(a)

recast

Nafion

and

Nafion/GO-poly(SPM-co-PEGMEMA)-X composite membranes: (b) X=0.25, (c) X=0.50, (d) X=1.00, (e) X=1.50 (f) X=2.00

17



The thermal property of Nafion-based membranes was investigated by TGA and displayed in Figure 8. Both the recast Nafion and the composite membranes exhibited a three-staged process of thermal degradation 40-41: the first stage from 40 oC to 220 oC was mainly attributed to the loss of water; the second stage started at around 300 oC was associated with the desulfonation process of Nafion as well as the thermolysis of grafted polymers on the fillers; the third stage above 400 oC was related to the backbone decomposition of Nafion. It should be noted that the addition of the fillers induced a steeper weight loss of the Nafion composites than that of recast Nafion at the second stage, which was consistent with the major weight loss of the fillers from 300 oC to 400 oC. In general, the incorporation of GO-poly(SPM-co-PEGMEMA) had little influence on the thermal stability of the composite membranes. All membranes were thermally stable below 250 oC, which well met the requirement for PEMFC operation. 100

80

Weight (%)

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

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60

40

20

Recast Nafion Nafion/GO-poly(SPM-co-PEGMEMA)-0.25 Nafion/GO-poly(SPM-co-PEGMEMA)-0.50 Nafion/GO-poly(SPM-co-PEGMEMA)-1.00

0

Nafion/GO-poly(SPM-co-PEGMEMA)-1.50 Nafion/GO-poly(SPM-co-PEGMEMA)-2.00

200

400

600

800

o

Temperature ( C)

Figure 8. TGA curves of recast Nafion and Nafion/GO-poly (SPM-co-PEGMEMA) composite 18


Page 19 of 35

membranes

The rigid framework of the fillers slightly enhanced the tensile strength and decreased the flexibility of the composite membranes. As shown in Figure 9, the tensile strength of the Nafion-based membranes reached the maximum at 1.50 wt% GO-poly (SPM-co-PEGMEMA) loading and then decreased with increasing content of

the

fillers.

The

decreased

tensile

strength

of

the

Nafion/GO-poly

(SPM-co-PEGMEMA)-2.00 might associated with the partial defect in the composite membrane caused by the aggregation of the fillers. The elongation at break of the composite membranes decreased with increasing GO-poly (SPM-co-PEGMEMA) content and reached the minimum at 217.1%, 17.8% lower than that of the pure membrane. However, the composite membranes were still flexible enough for practical application in PEMFCs. As a result, with even dispersion of the fillers, the composite membranes would exhibit higher mechanical performance compared with the pure membrane. 30 Recast Nafion Nafion/GO-poly(SPM-co-PEGMEMA)-0.25 Nafion/GO-poly(SPM-co-PEGMEMA)-0.50 Nafion/GO-poly(SPM-co-PEGMEMA)-1.00 Nafion/GO-poly(SPM-co-PEGMEMA)-1.50 Nafion/GO-poly(SPM-co-PEGMEMA)-2.00

20

Stress (MPa)

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


10

0 0

40

80

120

160

200

240

Strain (%)

19


280


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

Figure 9. Stress-strain curves of recast Nafion and Nafion/GO-poly (SPM-co-PEGMEMA) composite membranes at room temperature.

3.3

Water Uptake, Area Swelling and Morphological Features of the

Membranes. Both the water uptake and area swelling of the Nafion/GO-poly (SPM-co-PEGMEMA) membranes were tested under fully hydrated state (100% RH) as shown in Figure 10. With increasing temperature, the enhanced chemical potential drove more external water molecules to attach onto the ionic groups inside the membrane, which accordingly led to the increase in water uptake and area swelling of the membranes5. Figure 10a showed remarkable enhancement in water uptake of the composite membranes due to the addition of the fillers. The large surface area of GO nanosheet as well as hydrophilic polymer segments on its surface allowed the formation of hydrophilic channels at the Nafion-GO interface and obviously enhanced the water-absorbing capacity of the composite membranes. Similar trend could be observed in area swelling of the membranes as shown in Figure 10b. However, attributed to the rigid framework of GO nanosheets, the composite membranes showed slight increase (< 6%) in area swelling despite obvious enhancement in water absorption, which suggested that the composite membranes were dimensional stable for practical use in fuel cell applications.

20


Page 20 of 35

Page 21 of 35

(a) 70

o

(b)

30 C o 60 C

60

50 o

30 C o 60 C

45 40

50

Swelling ratio (%)

Water uptake (wt%)

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


40 30

35 30 25 20 15

20

10 10

5 0

0

0

0.25

0.50

1.00

1.50

0

2.00

0.25

Filler content (wt%)

0.50

1.00

1.50

2.00

Filler content (wt%)

Figure 10. (a) Water uptake and (b) area swelling of recast Nafion and Nafion/GO-poly (SPM-co-PEGMEMA) composite membranes at 100% RH.

SAXS was carried out to further investigate the nanostructure of the hydrated Nafion/GO-poly (SPM-co-PEGMEMA) and comparisons were made between the Nafion-based membranes. The peak position (q) of the SAXS curve reflected the mean spacing (d = 2π/q) between the ionic clusters42, which was highly related to the hydration number (λ= mol H2O/ mol SO3-) of the membrane(Table 1)5. In Figure 11, the Nafion/GO showed slightly decreased d-spacing from 4.14 to 3.95 nm compared to that of Nafion 117, reflecting the shrinkage of ionic clusters in the composite membrane. Owing to the great hydration capacity of GO-polySPM and GO-poly (SPM-co-PEGMEMA), the d-spacings of Nafion/GO-polySPM and Nafion/GO-poly (SPM-co-PEGMEMA) were shifted to larger values from 4.14 to 5.57 and 5.62 nm via rearrangement of the polymer conformation. The hydrophilic polymer segments on GO surface probably contributed to the nanoscale swelling of hydrophilic domains in the Nafion matrix, which further optimized the hydrophilic networks inside the composite membranes for proton conduction (Figure 12).

21



Table 1. IEC values, water uptake, swellings ratios and hydration numbers of Nafion 117 and Nafion-based composite membranes at 30 oC, 100% RH. Membrane Nafion 117 Nafion/GO-1.00 Nafion/GO-poly SPM-1.00 Nafion/GO-poly (SPM-co-PEGMEMA)-1.00

IEC (mmol g-1)

Water uptake (wt%)

Swelling ratio (%)

λ (H2O/SO3-)

0.92 ± 0.02 0.94 ± 0.02 1.02 ± 0.02

20.9 ± 1.8 20.8 ± 1.7 27.4 ± 2.1

17.2 ± 1.8 13.8 ± 1.4 19.3 ± 1.9

12.6 ± 1.8 12.1 ± 1.4 14.9 ± 1.5

1.00 ± 0.02

29.9 ± 2.3

20.7 ± 2.0

16.6 ± 1.6

Domain Spacing, d (nm) 12.56

9.42

6.28

3.14

Nafion 117 Nafion/GO-1.00 Nafion/GO-polySPM-1.00 Nafion/GO-polySPM-co-PEGMEMA-1.00 Relative Intensity

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 22 of 35

0.5

1

1.5

2

2.5

3

-1

q (nm )

Figure 11. SAXS curves of Nafion 117 and Nafion-based composite membranes at room temperature, 100% RH.

22


Page 23 of 35

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


Nafion matrix

H+

H+

H+

H+ H+

GO-poly (SPM-co-PEGMEMA) -SO3H

H+

-(CH2O)n

Figure 12. Proposed morphological description for the Nafion/GO-poly (SPM-co-PEGMEMA) membrane and proton transport through the hydrophilic domains (the blue lines) inside the composite system.

Figure 13 verified the superior water retention property of the GO-poly (SPM-co-PEGMEMA) under low humidity conditions. At 40% RH, the Nafion/GO-poly (SPM-co-PEGMEMA)-1.00 exhibited ~1.7 times the hydration number of Nafion 117 and ~1.3 times of Nafion/GO-polySPM-1.00. It could be concluded that the copolymerization of the PEGMEMA monomers enhanced the water retention property of the 2-D nanosheets owing to the superior hygroscopicity of the EO segments. The elevated hydration number (λ>3) of the Nafion/GO-poly (SPM-co-PEGMEMA)-1.00 suggested a larger water cluster around per sulfonic acid group, resulting in dissociation of the protons and formation of hydronium ions for proton diffusion through the membrane

43

. As a result, the Nafion/GO-poly

23



(SPM-co-PEGMEMA) was assumed to exhibit higher proton conductivity at low RHs compared with other Nafion-based membranes. 12 Nafion 117 Nafion/GO-1.00 Nafion/GO-polySPM-1.00 Nafion/GO-poly(SPM-co-PEGMEMA)-1.00

10

λ= [H2O]/[SO3]

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 24 of 35

8

6

4

2 40

50

60

70

80

90

RH (%)

Figure 13. Water uptake in PEMs at 30 oC. λ vs. RH for Nafion 117, Nafion/GO-1.00, Nafion/GO-poly SPM-1.00 and Nafion/GO-poly (SPM-co-PEGMEMA)-1.00.

3.4

Proton Conductivity of the Nafion-based Membranes. As a key

parameter for PEMs, the conductance of the membranes was investigated at different temperatures and humidity conditions. According to Figure 14, the incorporation of GO-poly (SPM-co-PEGMEMA) rendered the Nafion composites higher performance in proton conduction (such as elevated proton conductivity and decreased activation energy) compared with that of recast Nafion. As depicted in Figure 14a, at 80 oC, 100% RH conditions, the proton conductivity increased from 1.84 × 10-1 S cm-1 of the pure

membrane

to

2.29

×

10-1

S

cm-1

of

the

Nafion/GO-poly

(SPM-co-PEGMEMA)-1.00, while the activation energy decreased from 13.65 kJ 24


Page 25 of 35

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


mol-1 to 7.62 kJ mol-1 (Table S2). The low-energy-barrier paths for proton conduction in the Nafion/GO-poly (SPM-co-PEGMEMA) was probably attributable to the following reasons: (1) the polySPM segments afforded additional proton release sites to the Nafion matrix (Table S3); (2) the hydrophilic fillers increased the hydration number of the membrane, rendering unrestricted paths for proton diffusion; (3) the Lewis basic oxygen atoms in flexible EO segments could form hydrogen bond with protons and thus provided additional hopping sites for proton transfer. The conductivity of the membranes was expected to increase with increasing content of the fillers. However, when the filler content was over 1 wt%, the aggregation of the GO-poly (SPM-co-PEGMEMA) led to increased resistance for proton conduction in the composite membranes. Partially confined within the aggregated nanosheets, the additional proton binding and release sites might be less efficient in proton conduction. Moreover, as efficient proton channels, the decreased interface between the filler and the matrix might also cause the decrease in proton conductivity of the composite membranes. To evaluate the potential of the membrane for fuel cell application, it is of great importance to explore the effect of humidity on membrane conductance at relatively high temperature conditions. As shown in Figure 14b, the Nafion/GO-poly (SPM-co-PEGMEMA) composites exhibited relatively little change in proton conductivity with decreasing humidity in contrast to the sharp decline of recast Nafion (a total reduction of 98.5%). The highest performance of the composite membranes was at 1 wt% GO-poly (SPM-co-PEGMEMA) loading with a proton conductivity of

25



2.98 × 10-2 mS cm-1 at 80 oC, 40% RH, which was ~10 times over 2.74 × 10-3 S cm-1 for the pure membrane. The elevated conductivity of the Nafion composites was probably ascribed to the presence of hygroscopic agents (EO segments) which served to enhance the water retention of the membrane under low humidity conditions. Besides, the Lewis basic sites in EO units could accept protons from sulfonic acid groups, which facilitated the proton dissociation process and meanwhile served as proton binding sites for proton hopping based on the Grotthuss mechanism. (b)

-1.6 0.1 -1

Proton Conductivity (S cm )

-1

Proton Conductivity lnσ (S cm )

(a) -1.4

-1.8 -2.0 -2.2 -2.4 -2.6

Recast Nafion Nafion/GO-poly(SPM-co-PEGMEMA)-0.25 Nafion/GO-poly(SPM-co-PEGMEMA)-0.50 Nafion/GO-poly(SPM-co-PEGMEMA)-1.00 Nafion/GO-poly(SPM-co-PEGMEMA)-1.50 Nafion/GO-poly(SPM-co-PEGMEMA)-2.00

0.01 Recast Nafion Nafion/GO-poly(SPM-co-PEGMEMA)-0.25 Nafion/GO-poly(SPM-co-PEGMEMA)-0.50 Nafion/GO-poly(SPM-co-PEGMEMA)-1.00 Nafion/GO-poly(SPM-co-PEGMEMA)-1.50 Nafion/GO-poly(SPM-co-PEGMEMA)-2.00

1E-3

-2.8 2.8

2.9

3.0

3.1

3.2

3.3

3.4

40

50

60

70

80

90

100

-1

1000/T (K )

RH (%)

(d)

(c) -3.5

-1

-1

Proton Conductivity (S cm )

-4.0

Proton Conductivity lnσ (S cm )

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 26 of 35

-4.5 -5.0 -5.5 -6.0 -6.5 -7.0 -7.5

Recast Nafion Nafion 117 Nafion/GO-1.00 Nafion/GO-polySPM-1.00 Nafion/GO-polySPM-co-PEGMEMA-1.00

-8.0

0.1

0.01 Recast Nafion Nafion 117 Nafion/GO-1.00 Nafion/GO-polySPM-1.00 Nafion/GO-poly(SPM-co-PEGMEMA)-1.00 1E-3

2.8

2.9

3.0

3.1

40

3.2

50

-1

60

70

80

90

100

RH (%)

1000/T (K )

Figure 14. (a) Temperature-dependent proton conductivity of recast Nafion and Nafion/GO-poly (SPM-co-PEGMEMA) composite membranes at 100% RH; (b) Humidity-dependent proton conductivity of recast Nafion and Nafion/GO-poly (SPM-co-PEGMEMA) composite membranes at 80 oC ; (c) Temperature-dependent proton conductivity of Nafion-based composite membranes at 40% RH with recast Nafion and Nafion 117 as a comparison; (d) Humidity-dependent proton

26


Page 27 of 35

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conductivity of Nafion-based composite membranes at 80 oC with recast Nafion and Nafion 117 as a comparison.

Compared

with

other

Nafion-based

membranes,

the

Nafion/GO-poly(SPM-co-PEGMEMA) showed its superiority in proton conduction under low humidity conditions (Figure 14c and 14d). At 40% RH, the Nafion/GO-poly(SPM-co-PEGMEMA)-1.00 exhibited the lowest Ea value (16.32 kJ mol-1 as listed in Table S5) according to the Arrhenius plots and ~1.8 times the proton conductivity of Nafion/GO-polySPM-1.00 and ~4.5 times of Nafion/GO-1.00 at 80 oC. Additional proton sources and hygroscopic segments were found to be more effective on conductivity enhancement than the inherent functional sites on GO nanosheets. Moreover, partial substitution of the SPM monomers with EO units would further promote the proton conduction through the interfacial channels. Due to the presence of certain amount of SPM monomers, the IEC values of the membrane slightly decreased but still remained at a relatively high level (Table 1). Meanwhile, the EO segments enhanced the hydration number of the composite membrane under a wide range of RH (Figure 13) and offered additional Lewis basic sites for proton hopping, simultaneously facilitating the proton transport based on the vehicle and Grotthuss mechanism. It should be noted that a high density of sulfonic acid groups at the interface also favored the proton conduction through the composite membranes under low hydration levels 44. At 80 oC, 40% RH, the Nafion/GO-polySPM-1.00 exhibited a proton conductivity of 1.69 × 10-2 S cm-1, which was 1.53 times higher than that of

27



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

the Nafion/GO-1.00 and 2.84 times higher than that of Nafion 117. In general, both the experimental results (Figure 14 and S4, Table S2, S4 and S5) and the comparisons with other Nafion-based membranes reported in literature (Table S6 and S7) showed desirable performance of the Nafion/GO-poly(SPM-co-PEGMEMA) in proton conduction under both high and low humidity conditions, which suggested the superiority of the copolymer-grafted GO nanosheets as efficient fillers to facilitate the proton transport through the membrane. 3.5

Single Fuel Cell Performance. The single fuel cell test of the Nafion-based

membranes was carried out at 60 oC and further demonstrated the advantageous water-conserving effect of the hygroscopic fillers. In Figure 15, recast Nafion and Nafion 117 showed drastic declines (> 50%) in peak power density with decreased humidity due to low water retention. However, the introduction of GO-based nanosheets increased the hydration level of the Nafion matrix under low RHs (Figure 13) and thus rendered 81.6% ~ 135.5% increase in peak power density of the composite membranes compared with that of recast Nafion (Table 2). Under both high and low humidity conditions, the Nafion/GO-poly(SPM-co-PEGMEMA)-1.00 displayed the best fuel cell performance among the composite membranes. The incorporation of GO-poly(SPM-co-PEGMEMA) rendered 28.9% and 88.6% enhancement respectively in peak power density of the Nafion composites compared with that of Nafion 117 under 100% and 50% RH. The enhanced performance of the composite membranes resulted from the reduced Ohmic loss at the Ohmic polarization region, which implied decreased Ohmic resistance of the composite

28


Page 28 of 35

Page 29 of 35

systems. Since all membranes were based on the Nafion matrix and MEAs were prepared by the same procedure, membrane resistance should be the major factor determining the Ohmic resistance of the system

45

. Accordingly, the single fuel cell

performance was highly related to the proton conductivity of the membranes at the same

operation

conditions

and

the

highly

conductive

Nafion/GO-poly(SPM-co-PEGMEMA) had a great potential for practical use in PEMFCs. (b) 1.0 1.0

150

200

0.9

0.8

175

0.6 100

0.5

75

0.4 0.3

Recast Nafion Nafion 117

50

0.2

Nafion/GO-1.00 Nafion/GO-polySPM-1.00

25

-2

100 Voltage (V)

125

Power Density (mW cm )

150 0.7

-2

0.8

0.6

0.4 50 Recast Nafion

0.2

Nafion 117 Nafion/GO-1.00 Nafion/GO-polySPM-1.00 Nafion/GO-poly(SPM-co-PEGMEMA)-1.00

0.0

Nafion/GO-poly(SPM-co-PEGMEMA)-1.00

0.1 0

100

200

300

400

500

600

700

Power Density (mW cm )

(a)

Voltage (V)

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


0 800

0

200

0

400

600

-2

-2

Current Density (mA cm )

Current Density (mA cm )

Figure 15. Single fuel cell performance of Nafion-based membranes operating (a) under 100% RH at 60 oC; (b) under 50% RH at 60 oC

Table 2. PDpeak and CDpeak of Nafion-based membranes at 60 oC 100% RH Membrane

PDpeak

a

CDpeakb

Nafion 117 162.0 384.0 Recast Nafion 135.4 320.0 Nafion/GO-1.00 178.8 432.0 Nafion/GO-poly SPM-1.00 196.9 496.0 Nafion/GO-poly (SPM-co-PEGMEMA)-1.00 208.8 496.0 a Peak power density(mW cm-2). b Peak current density. (mA cm-2)

4.

CONCLUSION

29


50% RH PDpeaka

CDpeakb

75.3 60.3 109.5 126.7 142.0

224.0 224.0 352.0 352.0 448.0


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

In this study, hygroscopic sulfonated GO nanosheets were synthesized through SI-ATRP and doped into the Nafion matrix to prepare novel composite membranes. The comparison between different Nafion composites verified the superiority of the copolymer-grafted GO nanosheets as multi-functional fillers, which rendered 10 times enhancement in proton conductivity (80 oC, 40% RH) and 135.5% increase in peak power density (60 oC, 50% RH) of the composite membrane compared with that of recast Nafion. Apart from the presence of SPM monomers as efficient proton release sites, the copolymerization of the EO units afforded a 76.3% increase in proton conductivity at 80 oC and 40% RH compared with that of Nafion/GO-polySPM. Except for acting as humidifying agent, the flexible EO units would render additional Lewis basic sites to interact with the sulfonic acid groups of the SPM monomers or membrane matrix, constructing low energy barrier paths for proton hopping under low humidity. This study manifests the great application potential of the highly conductive Nafion/GO-poly(SPM-co-PEGMEMA) in PEMFCs.

ASSOCIATED CONTENTS Supporting Information The supporting information is available free of charge on the ACS publications website. 1H NMR spectrum of poly(SPM-co-PEGMEMA), additional high-resolution XPS spectra and SEM images of GO-based nanosheets, atomic concentration of GO-based nanosheets measured by XPS, IEC of recast Nafion and the Nafion/GO-poly (SPM-co-PEGMEMA) composite, Temperature-dependent proton

30


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conductivity of Nafion-based membranes at 100% RH, Ea values of the Nafion-based membranes and comparisons in membrane conductivity with previously reported work are presented in the supporting information. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: 86-22-27406646. Tel: 86-22-27406646 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge financial support from the National Natural Science Foundation of China (21490583，21621004), National Science Fund for Distinguished Young Scholars (21125627) and the Program of Introducing Talents of Discipline to Universities (B06006). REFERENCES 1.

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Table of Contents

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Facilitating Proton Transport in Nafion-Based Membranes at Low

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