Enhanced Proton Conductivity of Nafion Hybrid Membrane under

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Enhanced Proton Conductivity of Nafion Hybrid Membrane under Different Humidities by Incorporating MetalOrganic Frameworks With High Phytic Acid Loading Zhen Li, Guangwei He, Bei Zhang, Ying Cao, Tiantian Zhou, Hong Wu, and Zhongyi Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/am502236v • Publication Date (Web): 03 Jun 2014 Downloaded from http://pubs.acs.org on June 9, 2014

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ACS Applied Materials & Interfaces

4

Enhanced Proton Conductivity of Nafion Hybrid Membrane under Different Humidities by Incorporating Metal-Organic Frameworks With High Phytic Acid Loading

5

Zhen Li,†,‡ Guangwei He,†,‡ Bei Zhang,†,‡ Ying Cao, Hong Wu,†,‡ Zhongyi

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Jiang,*,†,‡ Zhou Tiantian,†,‡

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†

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School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072,

9

China

1 2 3

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

10

‡

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Tianjin 300072, China

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* To whom correspondence should be addressed. Fax: +86 22 2350 0086; Tel: +86 22

13

2350 0086; E-mail: [email protected];

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ABSTRACT

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

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In this study, phytic acid (myo-inositol hexaphosphonic acid) was first immobilized

16

by MIL101 via vacuum-assisted impregnation method. The obtained phytic@MIL101

17

was then utilized as a novel filler to incorporate into Nafion to fabricate hybrid proton

18

exchange membrane for application in PEMFC under different relative humidities

19

(RHs), especially under low RHs. High loading and uniform dispersion of phytic acid

20

in MIL 101(Cr) were achieved as demonstrated by ICP, FT-IR, XPS and 1

ACS Paragon Plus Environment


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EDS-mapping. The phytic@MIL101 was dispersed homogeneously in the Nafion

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matrix when the filler content was less than 12 %. Hybrid membranes were evaluated

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by proton conductivity, mechanical property, thermal stability and so forth.

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Remarkably, the Nafion/phytic@MIL hybrid membranes showed high proton

5

conductivity at different RHs, especially under low RHs, which was up to 0.0608 S

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cm-1 and 7.63×10-4 S cm-1 at 57.4 % RH and 10.5 % RH (2.8 and 11.0 times higher

7

than that of pristine membrane), respectively. Moreover, the mechanical property of

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Nafion/phtic@MIL hybrid membranes was substantially enhanced and the thermal

9

stability of membranes was well preserved.

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KEYWORDS: Metal-organic framework; MIL101; Nafion; phytic acid, Proton exchange membrane; Proton conductivity

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INTRODUCTION

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Proton exchange membrane fuel cell (PEMFC) is one of the most promising energy

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source for various applications.1-3 Current PEMFCs often confront some serious

15

challenges such as catalyst poisoning, low catalyst activity and complex heat

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management when operated at low temperature, which could be surmounted by

17

increasing the operating temperature of PEMFCs.1,2,4 However, as the heart of

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PEMFCs, the proton exchange membranes (PEMs) often suffer from sharply declined

19

proton conductivity at elevated temperature because of undesirable dehydration.5-7 For

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instance, Nafion membrane is the most widely used polyelectrolyte membrane and the

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proton conductivity of Nafion drops off by orders of magnitude with relative humidity 2


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(RH) decreasing. Great efforts have been dedicated to developing PEMs with

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excellent proton conductivity under low RHs, as represented by the following three

3

approaches. The first approach is the introduction of anhydrous proton-conduction

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groups into membrane, such as phosphonic acid,8,9 imidazole6 and ionic liquids.10 The

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second approach is the incorporation of hydrophilic materials to enhance the binding

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capacity of water of membranes.11,12 The third approach is the exploitation of new

7

polyelectrolytes with reduced sensitivity to RH such as block copolymer.13,14 The

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introduction of phosphate groups into membranes combining the first two approaches

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is deemed quite promising because of the unique superiority of phosphate group such

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as high charge carrier concentration, high proton conductivity with low dependence

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on moisture, excellent water retention property and good thermal stability.15,16

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Furthermore, phosphate group could act as both proton acceptor and donor by

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self-dissociation, which is beneficial to forming dynamic hydrogen bond networks

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and increasing the density of protons at low RHs.17 Compared with sulfonic acid and

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imidazole groups, phosphate groups exhibit the highest proton conductivity under low

16

RHs.18

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For stable and long-term utilization of phosphate groups, phosphate group donating

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molecules should be in immobilized form rather than in free form, which required to

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find a suitable match between supporter and phosphate group donating molecules

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(guest molecules).The immobilization of guest molecules on supporters has been

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widely explored in many fields like enzyme catalysis,19 heterogeneous catalysis.20,21

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Some principles could be summarized for choosing ideal supporters and guest 3



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molecules. 1) Large amount of active sites in guest molecules and high load capacity

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of supporters. 2) Good chemical stability, thermal stability for guest molecules and

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high mechanical stability for supporters as well. 3) Modest interaction between guest

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molecules and supporters. 4) Easy availability of active sites of guest molecules

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within the supporters. Based on above principles, phytic acid (myo-inositol

6

hexakisphosphate, Figure 1) and MIL101 (Figure 2) were employed as guest

7

molecules (phosphate group donation molecule) and supporter in this study,

8

respectively. Phytic acid is obtained from plants with unique properties such as the

9

highest phosphate groups content (except phosphoric acid), excellent iron-chelating

10

ability, good chemical and thermal stabilities.22-24 The molecular diameter of phytic

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acid is ~1.1 nm, which is similar to the window diameter of cavities in MIL101,

12

indicating that phytic acid could be confined by those cavities effectively. Meanwhile,

13

there are abundant coordinatively unsaturated metal sites (CUS) in MIL101, which

14

could be chelated by phytic acid, resulting in moderate interaction between phytic

15

acid and MIL101.25,26 It can be expected that phytic acid could be immobilized in

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MIL101 with a low leakage ratio. As a kind of porous coordination polymer, MIL101

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possesses open-framework structure, which ensures the availability of active sites in

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phytic acid for rapid proton transfer. In addition, MIL101 is endowed with potential

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of high load capacity for phytic acid because of its unique characters like gigantic

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pore volume of 1.5~1.9 cm3 g-1, huge Langmuir surface area of ~5900±300 m2 g-1

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and good stability.27

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Herein, we impregnated phytic acid into MIL101 by vacuum-assisted method 4


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(VAM) and the corresponding Nafion/MIL101 hybrid membranes were prepared

2

using phytic@MIL101 as the novel filler. Phytic acid was dispersed homogeneously

3

in MIL101 and the loading content of phytic acid in MIL101 was up to about 12

4

wt. %. The Nafion/ MIL101 hybrid membranes showed good proton conductivity at

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different RHs, especially under low RHs, which was 1100% higher than that of

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pristine membrane at 10.5 % RH.

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Figure 1. Molecule structure of phytic acid

2

4

3

5

1 2

3

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1) Cell of 2)Smaller MIL101 cages

3)Bigger cages

4)Pentagonal 5)Hexagonal windows windows

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Figure 2. Structure of MIL101: MIL101 is constructed by two types of rigid

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quasi-spherical cavities with diameter of 2.9 nm for small and 3.4 nm for big. The

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small cavities are only constructed by 1.2 nm pentagonal windows, and the big

13

cavities owns are constructed both the same pentagonal windows and 1.5 nm 5



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hexagonal windows.27

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MATERIALS AND METHODS

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Synthesis, purification of MIL101 and impregnation of phytic acid into

4

MIL101. MIL101(Cr) was synthesized and purified as described in previous study.27,

5

28

6

solid was separated, which was a mixture of MIL101, crystalline terephthalic acid and

7

few chromic nitrate. DMF was employed to dissolve the residual crystalline

8

terephthalic acid and chromic nitrate at room temperature and MIL101 was left,

9

which was named as raw-MIL101. Raw-MIL101 was treated by DMF for 12 h 3

10

times under reflux to remove terephthalic acid in their pores, and treated by 1M

11

ammonium fluoride for 12 h 3 times under reflux. For reducing the difficulty of

12

vacuum-pumping in next step, the raw-MIL101 was then treated by EtOH and

13

dichloromethane for 12 h under reflux to replace the high boiling point solvents by

14

low one. The product was named as pure-MIL101.

MIL101 was synthesized by hydrothermal method. When the reaction finished,

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Phytic acid was impregnated into MIL101 via VAM by the following steps.4 Firstly,

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pure-MIL101 was treated in a Schlenk tube under vacuum at 120 oC for 12 h to

17

remove the trace of residual dichloromethane and trapped air in pores. Secondly, the

18

Schlenk tube was cooled to 80 oC. Water (20 ml) and phytic acid aqueous solution (20

19

ml) were added into the Schlenk tube under vacuum. Thirdly, vacuum was removed

20

and the mixture was stirred for 24 h. Phytic acid could be pressed into the cavities of

21

MIL101 by the pressure difference between the cavities and atmosphere. Fourthly, the 6


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treated pure-MIL101 was centrifuged from the mixture, washed by water until the pH

2

of supernatant liquid was neutral, and dried at 45 oC under vacuum until constant

3

weight. Finally, a celadon powder could be obtained, which was designed as

4

phytic@MIL101.

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Preparation of membranes. The membranes were prepared by solution-casting

6

method. The received Nafion solution was evaporated at 60 oC under vacuum for 24 h

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to obtain dry Nafion resin. The obtained Nafion resin (0.2 g) was redissolved in

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DMAc (4 mL) and a desired amount of phytic@MIL101 or pure-MIL101 was

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dispersed into the solution homogeneously. The mixture was cast onto a glass plate,

10

kept at 80 oC for 12 h, and then kept at 120 ºC for another 4 h. Through immersing the

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glass plate in water, the membranes were peeled off.

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Recasting Nafion membrane must be activated before used. All membranes were

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soaked in aqueous oxyful (3 wt %), rinsed in water and soaked in sulfuric acid

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(aqueous solution, 1 mol L-1 ), successively, and each aforementioned operation was

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carried out at 80 oC for 1 h. Subsequently, the membranes were washed by water until

16

the pH was neutral. The hybrid membranes were named after Nafion/phytic@MIL-X

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or Nafion/pure-MIL-X, where phytic@MIL or pure-MIL referred to the filler in

18

hybrid membranes, respectively, and X referred to the weight percentage of

19

phytic@MIL101 or pure-MIL101 relative to Nafion. Nafion pristine membrane was

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prepared and activated by the same procedures for comparison purpose and named

21

after pristine Nafion membrane.

22

Leakage ratio of phytic acid in dynamic condition. The leakage ratio of phytic 7



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acid was tested in periodic-replacement water at 80 oC. Phytic@MIL101 (0.05g) was

2

immersed in 10 mL deionized water at 80 oC for 4.5 days, and then phytic@MIL101

3

was centrifuged from supernatant liquid. The obtained phytic@MIL101 was

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immersed in 10 mL fresh deionized water at 80 oC for another 4.5 days and the

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supernatant liquid was tested by ICP to detect the content of phosphorus element. The

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aforementioned procedures were in loop execution. The leakage ratio of phytic acid

7

was calculated by the following equations (Eqs. (1) and (2)).

8

mn = V

ωn × M Phy

(1)

6M p n

(∑ m j ) 9

Ln =

j =1

(2)

0.05ω p

10

Where ωn (wt. %, g ml-1) is the content of phosphorus element of supernatant

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liquid separated after the nth circle, V is the volume of water (10 ml), Mphy (g mol-1)

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and Mp (g mol-1) are relative molecular mass of phytic acid and relative atomic mass

13

of phosphorus, respectively, mn (g) is the leakage of phytic acid in the nth circle, Σ is a

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summation notation and is employed to calculate the total leakage of phytic acid after

15

the nth circle, ωp (wt. %, g g-1) is the phytic acid content of phytic@MIL101

16

determined by ICP as described in section characterizations of MIL101, Ln (%) is

17

leakage ratio of phytic acid after the nth circle.

18

Proton conductivity of membranes. To explore the potential application of

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membranes for PEMFCs at low RHs, proton conductivity of membranes (σ, S cm-1) at

20

different temperatures and different RHs were investigated using AC impedance

21

spectroscopy. 8


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1 2

Proton conductivity at different temperatures was obtained by the same way as reported in our previous study.29

3

Proton conductivity at different RHs was obtained in the same method except the

4

test condition was controlled at 80 oC and a series of RHs. A two point platinum probe

5

cell was placed in a stainless steel container (cubage =1.5 L) at 80 oC. Humidity was

6

controlled by the following saturated salt solution in a chamber placed in the stainless

7

steel container: LiCl ( RH = 10.5 % ), KAc ( RH = 15.1 % ), MgCl2 ( RH = 26.1 % ),

8

K2CO3 ( RH = 41.1 % ), and NaNO2 ( RH = 57.4 % ). Each sample was equilibrated

9

in controlled environment for at least 6 h until the impedance of sample was constant.

10

The proton conductivity was obtained by the same way as that at 100% RH.

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RESULTS AND DISCUSSION

12

Characterization of MIL101. Morphology of MIL101 was observed by FESEM

13

and FETEM, and the results were shown in Figure 3 and Figure 4, respectively.

14

Approximate octahedral crystals with a uniform size of about 450 nm could be

15

observed. By comparing Figure 2, it could be concluded that the morphology of

16

octahedral crystals was not altered by the purification process and degasification

17

process, but was altered by the impregnation of phytic acid. FETEM images revealed

18

the morphology of MIL101. The octahedral shape of MIL101 was not changed after

19

purification process, but horns of MIL101 were slightly damaged by impregnation of

20

phytic acid afterwards.

9



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1 2

Figure 3. FESEM images of raw-MIL101 (a), pure-MIL101 (b) and phytic@MIL101

3

(c).

4 5

Figure 4. FETEM images of raw-MIL101 (a), pure-MIL101 (b) and phytic@MIL101

6

(c).

7

XRD was employed to determine the crystal structures of raw-MIL101,

8

pure-MIL101 and phytic@MIL101. XRD patterns of MIL101 were shown in Figure 5

9

together with the simulated XRD pattern of MIL101, which were simulated from

10

the .cif file supplied by Cambridge crystallographic data centre (CCDC). The

11

comparison between patterns in Figure 5 revealed that a pure phase MIL101 was

12

synthesized successfully and the crystal structure of MIL101 was unaffected after

13

purification, degasification, and impregnation processes.

14

The successful impregnation of phytic acid was confirmed by EDX, XPS and

15

FT-IR. The EDX result verified the presence of a considerable amount of phosphorus

16

element (P). The spectra of FT-IR demonstrated the presence of phytic acid. The 10


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1

peaks at 1142 and 1058 cm-1 could be corresponded to the stretching vibrations of

2

−P=O and phosphate ester group (P-O-C), respectively.15 New peaks appearing at 611

3

cm-1 reveal the existence of chelation between phytic acid and CUSs.30 The peak at

4

1690 cm-1 could be relevant to the flexural vibration of bonded water (H-O-H),30

5

indicating that water molecules were bound by phytic acid. XPS was performed to

6

determine P content in surface layer of MIL101 and the oxidation state of P. The peak

7

of binding energy at around 131.5 eV indicated the pentavalent-oxidation state of P

8

and the presence of P-O bond. The content of P element in surface-layer of MIL101

9

(4~10 nm depth) could be calculated by the peak area ratio, which was 3.61 wt.%.

10

The content of P element in entire MIL101 was measured by ICP and the results was

11

3.69 wt.%, indicating 11.54 wt.% of phosphate groups was immobilized in MIL101.

12

The spectra corresponding to these configurations can be found in the Supporting

13

Information.

14

The distribution of phytic acid in MIL101 was observed by the result of

15

EDS-mapping and inferred by the comparison between the result of ICP and XPS.

16

The result of EDS-mapping in Figure 5 revealed the distribution of P in

17

phytic@MIL101, with the distribution of carbon(C), chromium (Cr) and oxygen (O)

18

as comparison. It could be clearly observed that P was of uniform distribution, which

19

was further confirmed by the similar P content in the entire MIL101 (3.69 wt. %,

20

detected by ICP) and in the surface layer of MIL101 (3.61 wt. %, detected by XPS).

21

11



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1 2

Figure 5. Elemental distribution of phytic@MIL101 probed by EDS-mapping: (a)

3

FETEM imagine of phytic acid; (b) Distribution of carbon; (c) Distribution of

4

chromium; (d) Distribution of oxygen ; (e) Distribution of phosphorus ; (f) STEM

5

image display HAADF detector.

6

Leakage ratio of phytic acid from phytic@MIL101. The leakage ratio of phytic

7

acid was calculated to evaluate phytic acid retention property of phytic@MIL101 in

8

water at 80 oC, as shown in Figure 6. The leakage ratio of phytic acid increased over

9

time while the leakage rate of phytic acid decreased. The functional relationship

10

between leakage ratio and time fitted a power function with a formula

11

of y = 0.01984 x0.31758 , where y was the leakage ratio and x was time. The fitting curve

12

showed a high coefficient of determination of 0.99751, indicating that the fitting was

13

reasonable. It could be deduced from the leakage ratio formula that the leakage ratio

14

was 12.920%, 26.844% and 30.534% after 1, 10 and 15 years, respectively. In

15

comparison, H3PO4@MIL101 was prepared in previous literature and about 95% of

16

acid leaked out after 20 minutes.31 The low leakage ratio can be attributed to two

17

factors. i) The similar diameter between phytic acid and the windows in MIL101. ii)

18

Chelation between phytic acid and CUSs. As to low RHs, the leakage ratio was

19

significantly lower than that in water under identical temperature. Hence, 12


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phytic@MIL101 is suitable for the application in PEMFC (whose service life span is

2

about ten years). 0.12

a 0.10

Leakage ratio (%)

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Leakage-rate Fitting curve

0.08

0.06

Fitting function: y = 0.01984 x

0.04

0.31758

0.02

0.00 0

3 4

10

20

30

40

50

60

70

Time (day)

Figure 6. The leakage ratio curve of phytic acid

5

Thermal stability of MIL101 and membranes. TGA of MIL101 was performed

6

to investigate their thermal stability, as shown in Figure 7. The decomposition process

7

of pure-MIL101 could be divided into three sections. The trace of weight loss

8

between 50 and 150 oC was caused by the bound water and residual solvent like

9

free-water and DMF. The weight was constant at the temperature range of 150 to 300

10

o

11

stable until 300 oC, and then the weight decreased sharply from 300 to 500 oC, which

12

was due to the decomposition of skeleton of MIL101.27 Because of the nonvolatility

13

of CrOx, the weight remained nearly constant in last section above 550 oC.

14

Phytic@MIL101 displayed a similar decomposition process as prue-MIL101 except

15

for the following details. The weight loss of about 4.4 % was more obvious than that

16

of prue-MIL101 at the temperature range from 100 to 150 oC. Considering the boiling

17

point of phosphoric acid and inositol (which were products of the decomposition of

18

phytic acid) was 158 °C (decomposition) and 225-227 °C, the weight loss was not

C because the water and residual solvent exhausted. The pure-MIL101 remained

13



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1

ascribed to the decomposition or volatilization of phytic acid, but ascribed to the loss

2

of bound water. The existence of large amount of bound water in phytic@MIL101

3

benefited from the extensive hydrogen bonding of phytic acid with water. The large

4

amount of bound water also implied good proton conductivity of phytic@MIL101

5

under low RHs. No another obvious weight loss was found below 250 oC, indicating

6

that the phytic acid could not decomposed below 250 oC in MIL101. The total weight

7

loss of phytic@MIL101 was 2.8 % higher than that of prue-MIL101, which was

8

caused by the volatilization of bound water. Thus, phytic@MIL101 can not be

9

decomposed below 300 oC, indicating that phytic@MIL101 was stable enough to bear

10

the application conditions of PEMFCs.

11

TGA of membranes was carried out to investigate the influence of incorporation of

12

MIL101 on the thermal stability of membranes. It could be concluded that that the

13

thermal decomposition process of membranes was scarcely affected by the

14

incorporation of MIL101, indicating that the thermal stability of membranes was

15

maintained. The total weight loss decreased with the increasing content of MIL101,

16

and the weight loss of hybrid membranes was slower than that of pristine membrane,

17

which was caused by that CrOx was non-volatile and it could act as a barrier retarding

18

the escape of other volatile substances.32,33

14


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Page 15 of 30

110

120

a

20

b

100

100

b 2 a 1 P

90 80

Tg (%)

80

Tg (%)

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


70

60

40

60

pure-MIL101 phytic@MIL101

50

10

20

P a b 1 2

500

600

500

600

700

800

pristine Nafion membrane Nafion/phytic@MIL-8 Nafion/phytic@MIL-12 Nafion/pure-MIL-8 Nafion/pure-MIL-12

40 0

100

200

1

300

400

500

600

700

100

200

o

300

400

700

800

o

Temperaure ( C)

Temperatue ( C)

2

Figure 7. Thermal stability analysis of MIL101 and membranes: (a) The TGA curve

3

of raw-MIL101 and phytic@MIL101; (b) The TGA curve of membranes.

4

Characterization of membranes. The morphology of membranes (Figure 8) was

5

observed by probing cross-sections of membranes using FESEM. The membranes

6

were dense without obvious defect. Because the Nafion polymer chains were very

7

flexible, the MIL101 was wrapped tightly by the chains of Nafion, resulting in good

8

compatibility between MIL101 and Nafion matrix. Both pure-MIL101 and

9

phytic@MIL101 were uniformly dispersed in hybrid membranes at the filler contents

10

below 12 wt. %. However, serious agglomeration occurred at the filler content of 16

11

wt %.

12

13 14

Figure 8. FESEM images of the cross-section of membranes: (a) pristine Nafion

15

membrane;

(b)

Nafion/pure-MIL-8;

(c)

Nafion/pure-MIL-12;

15


(d)


1

Nafion/phytic@MIL-8; (e) Nafion/phytic@MIL -12; (f) Nafion/phytic@MIL-16.

2

FR-IR of membranes was carried out to determine the interactions between Nafion

3

matrix and phytic@MIL (Figure 9). Peaks at 1628 and 3559 cm-1 (related to the

4

S-O-H groups in Nafion) were shifted to low wave number (1612 and 3478 cm-1,

5

respectively.), indicating that hydrogen bonding was formed between sulfuric group

6

and phytic acid after the incorporation of phytic@MIL.

1628

3559

Transmittence

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 16 of 30

3405 1612 pristine Nafion membran Nafion/pure-MIL-12 Nafion/phytic@MIL-12 4000

3500

3000

2500

8

2000

1500

1000

-1

7

Wave number (cm )

Figure 9. The FT-IR spectrum of membranes.

9

Water uptake and area swelling of membranes. Figure 10 displayed that pristine

10

Nafion membrane showed a high area swelling from 25.68 % to 59.62 % with the

11

increased temperature from 20

12

phytic@MIL101 or pure-MIL101 restrained the area swelling of membranes

13

effectively. For instance, the highest decrement ratios of area swelling reached

14

61.76 % and 68.26 % after the incorporation of phytic@MIL101 and pure-MIL101 at

15

20 oC, respectively. The variation tendency of water uptake was in accordance with

16

that of area swelling. Apparent activation energies of area swelling and water uptake

17

(Eas and Eau) were derived from their Arrhenius plots, as shown in Table 1. Both Eas

o

C to 80

o

C. The incorporation of either

16


Page 17 of 30

1

and Eau of hybrid membranes were higher than those of pristine Nafion membrane. It

2

was attributed to the fact that the framework of MIL101 is rigid, which could not

3

swell in water.34,35 In addition, MIL101 constrained the swelling of polymer matrix

4

via constraining motions of polymer chains, as confirmed by the result of DSC.

5

Similarly, it was difficult for MIL101 to absorb as much water as polymers by

6

swelling.35 As a result, all hybrid membranes showed higher resistance to area

7

swelling and water uptake than pristine Nafion membrane. As the filler content

8

increasing, more rigid MIL101 was introduced into membranes, hybrid membranes

9

exhibited increased Eas and Eau as well as further decreased area swelling and water

10

uptake. Compared with Nafion/pure-MIL hybrid membranes, Nafion/phytic@MIL

11

hybrid membranes showed lower Eau and Eas. This was caused by that the hydrogen

12

bonding could form between phytic acid and other hydroxy compounds, which was

13

not only from the sulfuric acid groups in Nafion but also from water molecules. Temperature (K)

Temperature (K) 310.0

320.0

330.0

340.0

290.0

350.0

a

50

-1.5 20 -2.0

Nafion/phytic@MIL-4 Nafion/phytic@MIL-8 Nafion/phytic@MIL-12 Nafion/phytic@MIL-16 pristine Nafion membrane Nafion/pure-MIL-8 Nafion/pure-MIL-12

-2.5

-3.0 3.4

3.3

3.2

3.1

3.0 -3

2.9

310.0

320.0

330.0

340.0

350.0 60

b

50 40 30

ln(Water uptake)

30

300.0

-1.0

40

-1.0

14

-0.5

60

Water uptake (%)

-0.5

300.0

-1.5 20 -2.0

Nafion/phytic@MIL-4 Nafion/phytic@MIL-8 Nafion/phytic@MIL-12 Nafion/phytic@MIL-16 10 pristine Nafion membrane Nafion/pure-MIL-8 Nafion/pure-MIL-12

-2.5

10

-3.0 2.8

3.4

-1

3.3

3.2

3.1

3.0 -3

1/Temperature x10 ( K )

2.9

Water uptake (%)

290.0

ln(area swelling)

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


2.8

-1

1/Temperaturex10 (K )

15

Figure 10. Arrhenius plot of area swelling (a) and water uptake (b) of membranes

16

Table 1. Apparent activation energy of area swelling and water uptake of membranes activation energy

Pristine

Nafion/phytic@MIL-X

17


Nafion/pure-MIL-X


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

(kJ mol-1) Eas Eau

11.88 ±0.30 9.94 ±0.25

Page 18 of 30

X=4

X=8

X=12

X=16

X=8

X=12

16.15 ±0.40 13.12 ±0.33

16.55 ±0.41 13.48 ±0.34

16.81 ±0.42 13.87 ±0.35

15.95 ±0.39 12.49 ±0.31

16.94 ±0.42 13.91 ±0.35

17.45 ±0.44 14.51 ±0.36

1

Activation energy of proton conductivity of membranes. Proton conductivity

2

is a key parameter for a PEM determining fuel cell performance. Proton conductivity

3

at different temperatures was measured to evaluate the influence of incorporated

4

MIL101 on activation energy of proton conductivity, as illustrated in Figure 11. It

5

could be concluded that there was positive correlation between proton conductivity

6

and temperature, indicating that proton conduction was a thermally activated process.

7

All the hybrid membranes showed higher proton conductivity compared with pristine

8

Nafion membrane. Nafion/Phytic@MIL-12 possessed the highest proton conductivity,

9

which was about 45 % higher than pristine Nafion membrane in whole temperature

10

range. Activation energy of pristine Nafion membrane was 16.96 kJ mol-1. The

11

incorporation of MIL101 decreased the activation energy of hybrid membranes,

12

which was decreased to about 15 kJ mol-1 for Nafion/phytic@MIL hybrid membranes

13

and 16 kJ mol-1 for Nafion/pure-MIL hybrid membranes, suggesting the resistance of

14

proton transfer was decreased by incorporating MIL101. The reduced activation

15

energy can be interpreted as follows.

16

MIL101 was a kind of Lewis acid due to the abundant CUSs.26 Large number of

17

hydroxyl groups and H+ can be created by hydrolyzing the CUSs, which were capable

18

of increasing membrane acidity and facilitating the construction of dynamic hydrogen

19

bonding networks for proton transfer in pores of MIL101, leading to decreased 18


Page 19 of 30

1

activation energy.36-38 As to phytic@MIL101, the acidity of phosphate groups is

2

stronger than CUSs, and the acidity of Nafion/phytic@MIL hybrid membrane can be

3

further enhanced for facile proton transport. In addition, since the high hydrogen

4

bonding energy of phosphate groups, phytic acid possessing high density of phosphate

5

groups could exhibit strong interaction with water molecules and sulfuric acid, which

6

helped to constructed dynamic hydrogen-bonded networks to facilitate the proton

7

conductivity

8

phytic@MIL101 showed lower activation energy and exhibited higher proton

9

conductivity than both pristine Nafion membrane and Nafion/pure-MIL hybrid

by

Grotthuss

mechanism.22,24

Thus,

the

incorporation

of

membranes. Temperature (K)

Temperature (K) 320.0

330.0

340.0

350.0

360.0

310.0

370.0 380.0 390.0

320.0

330.0

340.0

350.0

360.0

370.0 380.0 390.0

-1.0 30

30

-1.2

15 -1.8 -2.0

10

-2.2

pristine Nafion membrane Nafion/phytic@MIL -4 Nafion/phytic@MIL -8 Nafion/phytic@MIL -12 Nafion/phytic@MIL -16

-2.4 -2.6 -2.8 3.2

3.1

3.0

2.9

2.8 3

2.7

5

ln(Proton Conductivity)

-1.6

25

-1.4

-1

20

-2

-1.4

Proton Conductivity x10 (S cm )

25

-1

-1.2

20

-1.6 15 -1.8 -2.0

10

-2.2 -2.4

pristine Nafion membrane Nafion/MIL-8 Nafion/MIL-12

-2.6 -2.8 3.2

2.6

3.1

3.0

2.9

2.8 3

-1

2.7

12

Figure 11. Arrhenius plot of proton conductivity membranes

13

Table 2. Apparent activation energy of proton conductivity of membranes activation energy

Pristine

(kJ mol-1) Ea

16.96 ±0.42

2.6

-1

1/Temperature x10 (K )

1/Temperature x10 (K )

11

5

-2

310.0 -1.0

Proton Conductivity x10 (S cm )

10

ln(Proton Conductivity)

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/phytic@MIL-X

Nafion/pure-MIL-X

X=4

X=8

X=12

X=16

X=8

X=12

15.15 ±0.38

15.08 ±0.37

15.14 ±0.38

16.39 ±0.41

16.29 ±0.40

15.81 ±0.39

14

Proton conductivity of membranes in different RHs. The proton conductivity

15

dependence on RH of all the membranes was illustrated in Figure 13. Pristine Nafion 19



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

1

membrane displayed the highest sensitivity to the RH, and the proton conductivity

2

declined from 0.156 S cm-1 at 100 % RH to 6.36×10-5 S cm-1 at 10.5 % RH. The

3

proton conductivity of hybrid membranes was enhanced by incorporating

4

phytic@MIL101 and the enhancement increased along with decreased RH.

5

Nafion/phytic@MIL-12 exhibited the best proton conductivity of 0.228 S cm-1 at

6

100 % RH, 0.0608 S cm-1 at 57.4 % RH and 7.63×10-4 S cm-1 at 10.5 % RH, which

7

was 0.46, 2.8 and 11.0 times higher than that of pristine membrane, respectively. On

8

the contrary, the incorporation of pure-MIL101 decreased the proton conductivity of

9

hybrid membrane under reduced RH. As discussed in the section activation energy of

10

proton conductivity of membranes, the proton conductivity of Nafion/pure-MIL

11

hybrid membranes showed higher proton conductivity that pristine Nafion membrane

12

at 100 % RH, which was due to the construction of dynamic hydrogen bonding

13

networks in pure-MIL101 afforded by hydrolyzing the CUSs. As the RH decreased, it

14

is difficult for the CUSs to be fully hydrolyzed, resulting in weakened proton

15

conduction capacity of pure-MIL101. The remarkable enhancement of proton

16

conductivity by the incorporation of phytic@MIL-X can be interpreted as below. (i)

17

Numerous phosphate groups were immobilized in the regular frameworks of MIL101,

18

rendering continuous channels for facile proton transport. (ii) Phosphate group

19

possesses a less proton-conduction dependence on water, and exhibit the best proton

20

conductivity under low RHs in comparison with sulfuric acid and imidazole

21

groups.16,18 (iii) As to phosphate proton conductor, high mobility of phosphate groups

22

is essential to unleashing their proton conduction potential;8 In phytic@MIL101, 20


Page 20 of 30

Page 21 of 30

1

phytic acid was immobilized not by covalent bonds but by coordinated bonds and

2

cage effect, which has relatively high mobility to maintain its excellent proton

3

conductivity. (iv) One phytic acid molecule has six phosphate groups. Such a

4

molecule structure with high concentration of phosphate groups may render proton

5

transport rapid. 0

10

Nafion Nafion/phytic@MIL-4 Nafion/phytic@MIL-8 Nafion/phytic@MIL-12 Nafion/MIL-12

-1

10 -1

Proton conductivity (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


-2

10

0.24 0.22 0.2 0.18 0.16

-3

10

0.14

-4

10

0.12 0.1 90 92 94 96 98 100 102 104

-5

10

6 7

10

20

30

40

50

60

70

80

90

100

Relative humidity (%)

Figure 12. Proton conductivity of membranes under different RHs at 80 oC 。

8

CONCLUSIONS

9

In this study, we immobilized phytic acid in MIL101 in order to gain a stable and

10

long-term utilization of phosphate groups. Due to the similar diameter between phytic

11

acid and the windows in MIL101 as well as the chelation between phytic acid and

12

CUSs, the phytic@MIL101 exhibited an low phytic acid leakage ratio. Using

13

phytic@MIL101 as the novel filler, Nafion/phytic@MIL hybrid membranes were

14

prepared which displayed excellent proton conductivity under different RHs,

15

especially under low RHs. For instance, the proton conductivity was up to 0.0608 S

16

cm-1 and 7.63×10-4 S cm-1 at 57.4 % RH and 10.5 % RH (2.8 and 11.0 times higher 21



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

1

than that of pristine membrane), respectively. This remarkable enhancement in proton

2

conductivity was mainly ascribed to the unique properties of phytic acid and the good

3

match between phytic acid and MIL101: (i) With the regular frameworks of MIL101,

4

continuous channels could be formed by the immobilized phosphate groups for facile

5

proton transport. (ii) With a less proton-conduction dependence on water, phosphate

6

groups exhibited the high proton conductivity under low RHs. (iii) High mobility of

7

phosphate groups in the cavities of MIL101 ensured rapid proton conduction potential.

8

(iv) High concentration of phosphate groups of phytic acid rendered massive proton

9

transport. Moreover, mechanical and swelling-resistance properties of the hybrid

10

membrane were improved. The developed hybrid membrane showed great potential

11

for application in PEMFC.

12

ASSOCIATED CONTENT

13

Supporting Information

14

More experimental details and additional experimental data including spectrum of

15

EDX, FT-IR, XPS; the BET results of MIL101; DSC measurement of membranes;

16

and mechanical property of membranes. This material is available free of charge via

17

the Internet at http://pubs.acs.org.

18

AUTHOR INFORMATION

19

Corresponding Author

20

*E-mail: [email protected]. Fax: 86-22-27406646. Tel: 86-22-27406646.

22


Page 22 of 30

Page 23 of 30

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


1

Author Contributions

2

The manuscript was written through contributions of all authors. All authors have

3

given approval to the final version of the manuscript.

4

Notes

5

The authors declare no competing financial interest.

6

ACKNOWLEDGMENT

7

We gratefully acknowledge financial support from National Science Fund for

8

Distinguished Young Scholars (21125627), the National High Technology Research

9

and Development Program of China (2012AA03A611), and the Program of

10

Introducing Talents of Discipline to Universities (No. B06006).

11

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Advanced

Mesostructured

Hybrid

Silica-Nafion

Membranes

17

High-Performance PEM Fuel Cell. Chem. Mater. 2008, 20, 1710-1718.

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36 Park, C. H.; Lee, C. H.; Guiver, M. D.; Lee, Y. M. Sulfonated Hydrocarbon

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Membranes for Medium-Temperature and Low-Humidity Proton Exchange

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Membrane Fuel Cells (PEMFCs). Prog. Polym. Sci. 2011, 36, 1443-1498.

for

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37 Jeong, N. C.; Samanta, B.; Lee, C. Y.; Farha, O. K.; Hupp, J. T.

22

Coordination-Chemistry Control of Proton Conductivity in the Iconic 28


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51-54.

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38 Wu, B.; Lin, X.; Ge, L.; Wu, L.; Xu, T. A Novel Route for Preparing Highly

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Proton Conductive Membrane Materials with Metal-Organic Frameworks. Chem.

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Commun. 2013, 49, 143-145.

6

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SYNOPSIS TOC

Phytic acid MIL101 Nafion hybrid membrane Proton 3

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Enhanced Proton Conductivity of Nafion Hybrid Membrane under

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