Highly conductive and robust composite anion exchange membranes by incorporating quaternized MIL-101(Cr)

Accepted Manuscript Article Highly conductive and robust composite anion exchange membranes by incorporating quaternized MIL-101(Cr) Xueyi He, Mingyue Gang, Zhen Li, Guangwei He, Yongheng Yin, Li Cao, Bei Zhang, Hong Wu, Zhongyi Jiang PII: DOI: Reference:

S2095-9273(17)30040-3 http://dx.doi.org/10.1016/j.scib.2017.01.022 SCIB 47

To appear in:

Science Bulletin

Received Date: Revised Date: Accepted Date:

6 December 2016 13 January 2017 17 January 2017

Please cite this article as: X. He, M. Gang, Z. Li, G. He, Y. Yin, L. Cao, B. Zhang, H. Wu, Z. Jiang, Highly conductive and robust composite anion exchange membranes by incorporating quaternized MIL-101(Cr), Science Bulletin (2017), doi: http://dx.doi.org/10.1016/j.scib.2017.01.022

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Article Received: 06-Dec-2016 Revised: 13-Jan-2017 Accepted: 17-Jan-2017

Highly conductive and robust composite anion exchange membranes by incorporating quaternized MIL-101(Cr) Xueyi Hea, b, Mingyue Ganga, b, Zhen Lic, Guangwei Hea, b, Yongheng Yina, b, Li Caoa, b, Bei Zhanga, b, Hong Wua, b, Zhongyi Jianga, b * a

Key Laboratory for Green 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. c

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084,

China.

*

Corresponding author

E-mail address: [email protected]

1

Abstract With well-defined channels and tunable functionality, metal-organic frameworks (MOFs) have inspired the design of a new class of ion-conductive compounds. In contrast to the extensive studies on proton-conductive MOFs and related membranes attractive for fuel cells, rare reports focus on MOFs in preparation of anion exchange membranes. In this study, chloromethylated MIL-101(Cr) was prepared and incorporated into chloromethylated poly (ether ether ketone) (PEEK) as a multifunctional filler to prepare imidazolium PEEK/imidazolium MIL-101(Cr) (ImPEEK/ImMIL-101(Cr)) anion exchange membrane after synchronous quaternization. The successful synthesis and chloromethylation of MIL-101(Cr) were verified by transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy while the enhanced performance of composite membranes in hydroxide conductivity,

mechanical

strength

and

dimensional

stability

were

evaluated

by

alternating-current impedance, electronic stretching machine and measurement of swelling ratio. Specifically, incorporating 5.0 wt% ImMIL-101(Cr) afforded a 71.4% increase in hydroxide conductivity at 20 oC, 100% RH. Besides, the composite membranes exhibited enhanced dimensional stability and mechanical strength due to the rigid framework of ImMIL-101(Cr). At room temperature and the ImMIL-101(Cr) content of 10 wt%, the swelling ratio of the ImPEEK/ImMIL-101(Cr) was 70.04% lower while the tensile strength was 47.5% higher than that of the pure membrane. Keywords

MIL-101(Cr); poly (ether ether ketone); chloromethylation; quaternization;

composite anion exchange membranes

2

1

Introduction

Metal-organic frameworks (MOFs) as an emergent class of organic-inorganic materials are comprised of ordered networks from organic bridging ligands and inorganic metal cations. The large surface area, low crystal density as well as tunable pore size and functionalities of MOFs make them possible for specific applications such as storage or separation of small molecules [1-5]. Recently, MOFs, as intriguing class of ion conducting materials, have been broadly explored in energy relevant fields [6-11]. There are usually two ways to construct ion channels in MOFs [12-14]. One is to form hydrogen-bonding networks by framework decoration of hydrophilic groups or atoms. (–NH2 [15], –SO3H [16], –COOH [14], etc.), the other is to encapsulate ionic carriers (phytic acid [17], 1, 2, 4-triazole [18, 19], poly N-vinylimidazolium [20], tetrabutylammonium salts [10], etc.) into the framework to confer ion conductivity. However, fabricating defect-free membranes purely from these crystalline particles is still a grand challenge, which limits the utilization of MOFs as electrolyte materials [6]. Moreover, due to the resistance of ion transfer at grain boundaries of the crystal powder [12, 21-24], MOFs exhibit low ion conductivity under ambient conditions, which is several orders of magnitude lower than that of the polymer electrolytes [6, 12, 25]. Embedding MOFs as multi-functional fillers into polymer matrix is an effective way to make full use of ion-conducting MOF crystals and develop a novel class of ion conductors [17, 26-28]. Either through framework decoration of functional groups or encapsulation of ionic carriers, incorporation of these ion-conducting fillers increases ion concentration and optimizes the channel morphology/microenvironment [29] within the membrane, leading to enhanced ion conductivity of the mixed matrix. For example, Fe-MIL-101-NH2 was prepared and adhered to sulfonated poly (2, 6-dimethyl-1, 4-phenylene oxide) (SPPO) to prepare a composite membrane with high proton conductivity. Wu et al. [24] reported that the proton conductivity of the membrane reached 0.11 S cm–1 at 30 oC, 6 wt% MOFs loading. Both –OH and –NH2 groups on Fe-MIL-101-NH2 attributed to the performance elevation of the composite membrane than the pure membrane. Li et al. [17] reported the preparation and characterization of a composite membrane combining Nafion with phytic acid immobilized by MIL-101(Cr). The membrane exhibited high proton conductivity at low humidity, which was 7.63 × 10−4 S cm−1 at 10.5 % RH, 11 times higher than Nafion. Although a number of researches have been carried out to incorporate proton-conducting MOFs into polymer matrix to

prepare

proton

exchange

membranes,

quite

few

of

them

are

related

to

hydroxide-conducting composite membranes. In particular, it has not been reported that modified MOFs are embedded into an anion-conductive polymer matrix to enhance the hydroxide conductivity of the composite membranes.

3

In this study, a novel composite anion exchange membrane (AEM) was prepared by embedding quaternized MIL-101(Cr) into a polymer matrix. MIL-101(Cr) was chosen as the filler with rigid framework, thermal stability and bigger surface area than most MOF materials [30, 31]. Moreover, a large amount of organic ligands-terephthalic acid enabled the introduction of sufficient functional groups through appropriate chemical modifications [16, 32-34]. Herein, a facile chloromethylation method was applied to functionalize MIL-101(Cr) [35]. Poly (ether ether ketone) (PEEK) was chosen as the polymer matrix due to its good mechanical strength, chemical and thermal stability and was also functionalized to endow the membrane with high hydroxide conductivity [36-39]. The chloromethylated MIL-101(Cr) and PEEK were mixed together and synchronously quaternized by 1-methylimdazole to prepare imidazolium PEEK/ imidazolium MIL-101(Cr) (ImPEEK/ImMIL-101(Cr)) membranes. The composite membranes exhibited enhanced hydroxide conductivity without accompanying a decrease in mechanical strength and dimensional stability. Besides, compared with the pure membrane, higher methanol resistance, thermal and alkaline stability were acquired in composite membranes, which were desirable attributes for fuel cell applications. 2

Materials and methods

2.1 Materials and chemicals Terephthalic acid (analytical reagent (AR), 99.0 wt%) was supplied by J&K Scientific Ltd. Chromium (III) nitrate nonahydrate (AR) was purchased from Strem Chemicals Inc. Hydrofluoric acid (AR, 40 wt%) and concentrated sulfuric acid (AR, 98.2 wt%) were obtained from Tianjin Jiangtian Chemical Scientific Ltd. Aluminium chloride hexahydrate (metal basis, 99.9 wt%) , methoxyacetyl chloride (97.0 wt%) and nitromethane (chromatically pure (GC), 99.0 wt%) were purchased from Sigma-Aldrich Co. Ltd. Poly(ether ether ketone) (Victrex® PEEK, grade 381G) was purchased from Nanjing Yuanbang Engineering Plastics Co. Ltd. Tetrahydrofuran (AR) and N,N-dimethyl formamide (DMF, AR) were supplied by Tianjin Guangfu Technology Development Co. Ltd. All the reagents were used as received without further purification. Other reagents not mentioned were commercially available with analytical pure degree and used as received. Deionized water was used throughout this study.

2.2 Synthesis of chloromethylated MIL-101(Cr) 2.2.1 Synthesis and purification of MIL-101(Cr) MIL-101(Cr) was synthesized via a hydrothermal method, as described in our previous study [17, 20, 28]: 1.66 g terephthalic acid, 4.00 g chromium (III) nitrate nonahydrate, 0.413 mL hydrofluoric acid and 50 mL deionized water were transferred into a hydrothermal synthesis 4

reactor. The mixture was kept stirring for 8 h at 220 oC. Thus, a turbid liquid was obtained and the product was separated by centrifugation. Sufficient amount of DMF was used to wash the product until the supernatant fluid was colorless. After drying under vacuum at 40 oC, crude MIL-101(Cr) was obtained. The product needed further purification to remove the residual terephthalic acid and chromium (III) nitrate. Crude MIL-101(Cr) was dispersed in DMF and kept refluxing and stirring for 12 h at 80 oC, followed by rinsing with ethanol for 3 h at 40 oC. After centrifugation and drying under vacuum for several hours, pure MIL-101(Cr) power could be obtained.

2.2.2 Chloromethylation of MIL-101(Cr) The chloromethylation of MIL-101(Cr) was conducted as follows: MIL-101(Cr) powder (0.30 g), aluminium chloride hexahydrate (0.57 g), methoxyacetyl chloride (0.12 g) and nitromethane (24.00 g) were mixed together and refluxed with continuous stirring for 5 h at 100 oC. Then, the solid product was collected by centrifugation, washed with boiling water and THF and activated at 100 oC under air.

2.3 Chloromethylation of PEEK Chloromethyl octyl ether (CMOE) was chosen as the chloromethylation reagent with lower toxicity instead of commonly used chloromethyl methyl ether [40-42] and synthesized by bubbling hydrogen chloride into the mixture of n-octane, paraformaldehyde, chloroform and anhydrous calcium chloride [43]. Operated under –10 oC, 4.00 g PEEK was dissolved in 240 mL concentrated sulfuric acid under mechanical stirring for 4 h. After complete dissolution, 80 mL CMOE was added to the solution and the reaction carried on for a certain time. The obtained solution was poured into excessive water to precipitate the polymer product, followed by rinsing with water and drying under vacuum at 40 oC. The solid product with different reaction time (X, min) was designated as CMPEEK-X (chloromethylated PEEK, X = 20, 40 or 60).

2.4 Preparation of composite membranes ImPEEK/ImMIL-101(Cr) membranes were prepared by solution-casting method. 0.40 g CMPEEK was dissolved in 3.000 mL DMF and stirred for 3 h. Then, well-dispersed MIL-101(Cr) in DMF solution and 0.500 mL 1-methylimdazole were added to the CMPEEK solution and stirred vigorously overnight. The mixture was cast onto glass plates, dried at 60

5

o

C for 12 h and another 12 h at 80 oC. After cooling down, the membranes were immersed in

NaOH solution (1.0 mol L–1) for 48 h and washed thoroughly with deionized water until the pH of the filtrate reached neutral. The composite membranes were designated as ImPEEK/ImMIL-Y (Y = 1.0, 2.5, 5.0, 7.5 or 10.0), where Y referred to the mass fraction of the filler relative to the polymer matrix. The synthetic and ion exchange process of imidazolium PEEK and MIL-101(Cr) was illustrated in Scheme 1, respectively.

Scheme 1 Synthesis of (a) imidazolium PEEK and (b) imidazolium MIL-101(Cr) 2.5 Instrumental characterizations The morphology and crystal structure of MIL-101(Cr) and chloromethylated MIL-101(Cr) (CM MIL-101(Cr)) were confirmed by transmission electron microscopy (TEM, Tecnai G220 S-TWIN) and X-ray diffraction (XRD, Rigaku D/max 2500 v/pc). Fourier transform infrared (FTIR, BRUKER Vertex 70) spectra was used to define the chemical structure of MIL-101(Cr) and CM MIL-101(Cr) in the 4000-400 cm–1 range. The chloromethylation degrees of PEEK were calculated based on the 1H nuclear magnetic resonance (NMR, Varian Unity Inova, 500 MHz) spectrum (DMSO-d6 ). A field emission scanning electron microscope (FESEM, Nanosem 430) was used to obtain the cross-section morphologies of composite membranes after the samples were freeze-fractured in liquid nitrogen and then sputtered with gold. Differential scanning calorimetry (DSC, 204 F1 NETZSCH) measurement was carried out under nitrogen atmosphere. The samples were first preheated from 20 to 150 °C at a heating 6

rate of 10 °C min–1, then cooled to 90 °C at a cooling rate of 15 °C min–1 and reheated to 260 °C. Thermal gravimetric analysis (TGA, NETZSCH-TG209 F3) was conducted under nitrogen atmosphere from 40 to 800 °C (heating rate: 10 °C min–1) in order to analyze the thermal stability of the filler and the membranes. The mechanical strengths of the composite membranes were tested by an electronic stretching machine (Yangzhou Zhongke WDW-02) with the strain rate of 10 mm min–1.

2.6 Water uptake and area swelling Samples were vacuum-dried at 80 °C for 12 h, measured and immersed in water at different temperatures before fully hydrated. The wet samples needed a quick measurement after the excess water was wiped off on the surface. Each sample should be measured for three times with an error within 5.0%. Average values were obtained and used to calculate the water uptake and swelling ratios of the samples based on the following equations: water uptake (wt%) =

area swelling (%) =

Wwet − Wdry Wdry

Awet − Adry Adry

(1)

×100,

(2)

×100,

where Wdry (g), Wwet (g), Adry (cm2 ) and A wet (cm2) represent the weights and surface areas of the dry and wet membranes, respectively.

2.7 Ion exchange capacity and hydroxide conductivity The ion exchange capacity (IEC) of each sample was determined by a back-titration method. Before titration, samples were dried at 60 °C overnight until constant weight (m, g) and then soaked into a certain volume (V HCl , mL) of standard HCl solution (cHCl, mol L–1) at room temperature for 24 h. With phenolphthalein as the indicator, the residual solution was titrated using standard NaOH aqueous solution (cNaOH, mol L–1). The volume of the NaOH used for titration was designated as VNaOH (mL). The IEC values (mmol g–1 ) can be calculated as the following formula: IEC =

cHClVHCl − cNaOHVNaOH . m

(3)

Membrane impedance was acquired using an electrochemical workstation (CompactStat, IVIUM Tech.) by two-probe AC impedance method in a frequency range from 100 kHz to 10 Hz. Before testing, samples were immersed in NaOH solution (1.0 mol L–1) for 24 h, then soaked and washed with deionized water several times. The impedance of the membrane was tested at different temperatures in an attemperator under 100% RH. According to the Nyquist 7

plot, the hydroxide conductivity of the membrane could be calculated via the following relationship:

σ=

l , AR

(4)

where σ (S cm–1) represents the ion conductivity of the membrane; l (cm) the distance between two electrodes; A (cm2 ) the cross section area of the membrane and R (Ω) the membrane impedance derived from the high frequency intercept with the Re(z)-axis on Nyquist plot. Using coordinate transformation theory, Arrhenius curves were plotted based on the variation of hydroxide conductivity with temperature. According to the following equation, the activation energy (Ea) of hydroxide conductivity could be calculated from the slop of the Arrhenius curves, lnσ = −

Ea 1 . +lnσo . R T

(5)

2.8 Alkaline stability After testing the hydroxide conductivity at room temperature, samples were soaked in 2 mol L–1 NaOH solution at 50 and 80 o C respectively for 48 h and then washed with deionized water several times until the pH of the supernatant was neutral. Hydroxide conductivity was tested again at room temperature and compared with the value obtained before. The ratio of hydroxide conductivity reflected the alkaline stability of the membrane.

3

Results and discussion

3.1 Characterization of polymer matrix The chloromethylation of PEEK was confirmed by the 1H NMR spectrum according to the peaks at ~4.7 ppm assigned to the protons of –CH2Cl. As was shown in Fig. 1a, the chloromethylation degrees of CMPEEK-X (X=20, 40 or 60) increased with increasing reaction time and calculated to be 0.66, 0.78 and 0.93 respectively via the following equation [44]:

CD =

2 A(Hd ) , A(Hc )

(6)

where A(Hd) and A(Hc) represent the peak area of Hd and Hc (~7.8 ppm). The increasing degree of chloromethylation elevated the solubility of the polymer: CMPEEK-40 and CMPEEK-60 both had a high solubility while the dissolving process of CMPEEK-20 was time-consuming. Besides, compared with CMPEEK-40, the membrane swelling of CMPEEK-60 was quite serious after quaternization. As a result, CMPEEK-40 was chosen as

8

the polymer matrix with the optimum chloromethylation degree and denoted as CMPEEK in the subsequent sections. Since the chloromethyl groups in CMPEEK were completely functionalized after quaternization (Fig. 1b), the theoretical IEC value of ImPEEK could be calculated according to the following equation [42]: IEC =

1000 × D , 288.3 +112.1D

(7)

where D refers to the chloromethylation degree of CMPEEK, and the theoretical IEC value of ImPEEK was calculated to be 2.07 mmol g–1.

Ha2 Ha3

(a) *

O

Hb Hc

O C

O

Hc Hb O

x

Ha1 CH2 Hb Hc Cl H d Hc

Hb Hc

Ha Ha

Hc Hb

O

Ha Ha

O C

Hb Hc

Hd

(b)

Hc Hb

H2 C Cl

*

Hc Hb

y

CMPEEK Hd

Ha,Hb Hd

CMPEEK-60

Hd' H2 C

He N

Hf

N CH3

Hf

CMPEEK-40 ImPEEK He

CMPEEK-20 9

8

7

6

5

4

9

7

6

5

4

Chemical shift (ppm)

Chemical shift (ppm)

Fig. 1

8

Hd'

(Color online) (a) 1H NMR spectra of CMPEEK samples. (b) 1H NMR spectra of

CMPEEK and ImPEEK 3.2 Characterization of MIL-101(Cr) and CM MIL-101(Cr) It was shown in Fig. 2 that MIL-101(Cr) crystals had octagonal structures and a uniform size of about 400 nm before and after chloromethylation, demonstrating that the nanostructure of MIL-101(Cr) was not affected by chloromethylation.

Fig. 2 TEM images of (a) MIL-101(Cr) and (b) CM MIL-101(Cr)

9

XRD patterns in Fig. 3 reflected the crystal structure of MIL-101(Cr) and CM MIL-101(Cr). The pattern of MIL-101(Cr) was consistent with the .cif file obtained from Cambridge Crystallographic Data Centre (CCDC) database, which indicated the successful synthesis of MIL-101(Cr). The characteristic peaks of CM MIL-101(Cr) were in accordance with the pattern of MIL-101(Cr), demonstrating that the original crystal structure of MIL-101(Cr) was unchanged after chlorometylation.

Fig. 3 (Color online) XRD patterns of MIL-101(Cr)s and CM MIL-101(Cr) Fig. 4 showed the FTIR spectrum of MIL-101(Cr) compared with the sample after chloromethylation. In the spectrum of CM MIL-101(Cr), apart from the intensified peaks at 1406, 1506 and 1623 cm–1, new peaks appeared at 670, 1260 and 2930 cm–1, which verified the successful chloromethylation of MIL-101(Cr). The peak at 670 cm–1 was assigned to the stretching vibration of C-Cl. The peak at 1260 cm–1 was related to the reinforced C-H in-plane bending vibration on benzene ring after –CH2Cl substitution [45] and the peak at 2930 cm–1 corresponded to the vibration of C-H in –CH2Cl [46].

10

Transmittance

MIL-101(Cr)

CM MIL-101(Cr)

1260

1623

2930

670

1506 1406

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 4 (Color online) The FTIR spectra of MIL-101(Cr) and CM MIL-101(Cr) TGA and derivative thermal gravimetric (DTG) curves of MIL-101(Cr) and CM MIL-101(Cr) were shown in Fig. 5. MIL-101(Cr) displayed three stages of degradation [28]: the first stage of degradation was from 50 to 200 oC due to the loss of water; the weight loss between 200 and 375 oC was arisen from the residual solvent evaporation; a sharp weight loss appeared in the last stage from 375 to 585 oC, referring to the framework degradation of MIL-101(Cr). At the temperature over 585 oC, the organic linkers of MIL-101(Cr) were under further oxidation until complete decomposition and the residual Cr2O3 was oxidized to CrOx, during which no obvious weight loss could be observed. Similar to MIL-101(Cr), CM MIL-101(Cr) exhibited a three-stage degradation and had identical characteristic temperature at each stage. However, the remaining mass fraction of CM MIL-101(Cr) was lower than that of MIL-101(Cr) in temperature range of 450–800 o C, which could be interpreted by the higher content of organic moieties in CM MIL-101(Cr) via –CH2Cl introduction [47]. The results indicated that both MIL-101(Cr) and CM MIL-101(Cr) had a good potential to be applied in ion exchange membranes for fuel cells.

11

100

CM MIL-101(Cr) MIL-101(Cr)

90

0.3

Weight (%)

o

-1

0.1

80

Weight loss rate ( C )

0.2

0.0

70

-0.1 60 -0.2 50 -0.3 40 30

-0.4

100

200

300

400

500

600

700

-0.5 800

ο

Temperature ( C)

Fig. 5 TGA and DTG curves of MIL-101(Cr) and CM MIL-101(Cr) 3.3 Characterization of composite membranes

Fig. 6 SEM cross-section images of (a) ImPEEK, (b) ImPEEK/ImMIL-1.0, (c) ImPEEK/ImMIL-2.5,

(d)

ImPEEK/ImMIL-5.0,

(e)

ImPEEK/ImMIL-7.5

and

(f)

ImPEEK/ImMIL-10.0 The morphologies of the pure and composite membranes were observed by SEM and shown in Fig. 6. With good compatibility, ImMIL-101(Cr) was uniformly distributed in the ImPEEK 12

matrix, which was verified by the homogeneous, defect-free cross-section morphologies of the composite membranes. However, slight aggregation of ImMIL-101(Cr) could be observed over 1.0 wt% ImMIL-101(Cr) loading. TGA was carried out to evaluate the thermal stability of the pure and composite membranes. Fig. 7 revealed a three-stage thermal degradation of the pure membrane: the first stage was from 40 to 145 oC, resulting from the evaporation of water (free and bound water) and residual solvent; in the second stage from 145 to 355 oC, the weight loss was derived from the thermal decomposition of imidazolium salt; the third stage above 355 oC was mainly caused by the thermal degradation of the polymer matrix [48]. Composite membranes experienced similar stages of thermal degradation compared with the pure membrane. Besides, due to the addition of ImMIL-101(Cr), another stage of weight loss appeared in the composite membranes at around 500 oC, which was caused by the complete decomposition of organic ligands in MIL-101(Cr). It could be concluded that the composite membrane had a better thermal stability than the pure membrane below 500 o C. In contrast, higher percent of weight loss could be observed above 500 oC in composite membranes with increasing loading of fillers due to ImMIL-101(Cr) decomposition. It should be noted that all membranes were stable until 155 oC, meeting the practical application requirements for fuel cells. (a) 100

(b)

ImPEEK/ImMIL-2.5

90

ImPEEK/ImMIL-1.0

Weight loss rate ( C )

48

-1

ImPEEK/ImMIL-10.0

80

ImPEEK/ImMIL-7.5 ImPEEK/ImMIL-5.0

44 640

680

720

o

Weight (%)

0.0

ImPEEK

52

760

70

ImPEEK ImPEEK/ImMIL-1.0 ImPEEK/ImMIL-2.5 ImPEEK/ImMIL-5.0 ImPEEK/ImMIL-7.5 ImPEEK/ImMIL-10.0

60

50

100

200

300

400

-0.1

ImPEEK/ImMIL-7.5 ImPEEK ImPEEK/ImMIL-2.5 ImPEEK/ImMIL-10.0 ImPEEK/ImMIL-1.0 ImPEEK/ImMIL-5.0

-0.2

500

600

700

800

100

200

300

400

500

600

700

ο

Temperature ( C)

Temperature (°C)

Fig. 7 (Color online) (a) TGA and (b) DTG curves of the ImPEEK membrane and ImPEEK/ImMIL-101(Cr) composite membranes

13

800

ImPEEK/ImMIL-10.0 137.0 °C ImPEEK/ImMIL-7.5

-1

)

134.5 °C

DSC (mV mg

ImPEEK/ImMIL-5.0 129.5 °C ImPEEK/ImMIL-2.5 129.3 °C ImPEEK/ImMIL-1.0 121.8 °C ImPEEK 128.2 °C

110

120

130

140

150

160

Temperature (°C)

Fig. 8 (Color online) DSC curves of the ImPEEK membrane and ImPEEK/ImMIL-101(Cr) composite membranes Different glass-transition temperatures (Tgs) of the membranes were reflected by DSC curves shown in Fig. 8. With increasing ImMIL-101(Cr) content, the T gs of the composite membranes decreased slightly at first and then increased. The decrease in T g was resulted from the enhanced chain mobility by ImMIL-101(Cr) incorporation. However, when the ImMIL-101(Cr) content was over 1.0 wt%, the aggregation of ImMIL-101(Cr) reduced the interfaces between the filler and the polymer matrix, which weakened the enhancement in chain mobility caused by filler incorporation. As a result, Tgs kept rising with increasing content of ImMIL-101(Cr) in the composite membranes.

3.4 Dimensional stability of the membranes Fig. 9a showed a decreasing water uptake of the membranes with increasing amount of ImMIL-101(Cr), indicating lower hydroscopicity of the filler than that of the polymer matrix [25]. Moreover, attributed to the reduced water uptake as well as restricted movement of the polymer segments, membranes filled with rigid fillers displayed good swelling resistant performance, which effectively solved the “trade-off” problem between hydroxide conductivity and dimensional stability [49]. Particularly, at 50 o C, 100% RH, the hydroxide conductivity of the ImPEEK/ImMIL-5.0 was 0.036 S cm–1, 63.6% higher than that of the

14

ImPEEK while the swelling ratio was 28% lower than that of the pure membrane. (a)

60

(b) 20 °C 50 °C

20 °C 50 °C

25

Area swelling (%)

Water uptake (wt%)

50

40

30

20

15

10 20

5 10 0

2

4

6

8

10

0

2

ImMIL-101(Cr) content (wt%)

4

6

8

10

ImMIL-101(Cr) content (wt%)

Fig. 9 (a) Water uptake and (b) area swelling of the ImPEEK membrane and ImPEEK/ImMIL-101(Cr) composite membranes 3.5 Mechanical properties of the membranes 45

ImPEEK/ImMIL-10.0 ImPEEK/ImMIL-7.5 ImPEEK/ImMIL-5.0

40 35

ImPEEK/ImMIL-2.5 Stress (MPa)

30

ImPEEK/ImMIL-1.0 ImPEEK

25 20 15 10 5 0 0

10

20

30

40

50

Strain (%)

Fig.

10

(Color

online)

Stress-strain

curves

of

the

ImPEEK

membrane

and

ImPEEK/ImMIL-101(Cr) composite membranes at room temperature Consistent with the swelling resistance, the tensile strength of the membranes kept rising with increasing ImMIL-101(Cr) content. As was shown in Fig. 10, the tensile strength of the composite membranes reached the highest (35.4 MPa) at 10.0 wt% ImMIL-101(Cr) loading, which was 47.5% higher than that of the pure membrane (24.0 MPa). Meanwhile, due to the 15

rigid framework of ImMIL-101(Cr), the filler introduction significantly reduced the elongation at break of the membranes and therefore made the membranes more rigid.

3.6 IEC and hydroxide conductivity Table 1 listed the IEC values of the membranes with different ImMIL-101(Cr) content. The IEC of the pure membrane was 2.04 mmol g–1, which was in accordance with the theoretical value calculated by the 1H NMR spectrum. With ImMIL-101(Cr) incorporation, more imidazolium groups were introduced to the polymer matrix, which induced higher IEC values of the composite membranes. However, demonstrated by higher Tgs of the composite membranes with increasing amount of fillers, the occurrence of filler aggregation hindered the chain movement at ImMIL-101(Cr) content above 1.0 wt%, which affected the ion exchange process during IEC testing and rendered lower apparent IEC values. When the ImMIL-101(Cr) content was above 2.5 wt%, the aggregation of ImMIL-101(Cr) played more critical role than the addition of imidazolium groups, leading to a decreased apparent IEC values from 2.21 to 1.94 mmol g–1.

Table 1 IEC of the membranes at 20 °C ImMIL-101(Cr) content (wt%) 0 1.0 2.5 5.0 7.5 10.0

IEC (mmol g–1) 2.04 2.09 2.21 2.16 2.10 1.94

Table 2 Ea of the membranes ImMIL-101(Cr) content (wt%)

Ea (kJ mol–1)

5.80 5.10 5.54 5.56 5.97 6.17

0 1.0 2.5 5.0 7.5 10.0

Fig. 11a showed the variation of hydroxide conductivity versus temperature for membranes under fully hydrated conditions, implying a thermally activated process. In the whole temperature range, the hydroxide conductivity of the membranes reached the maximum at 5.0 16

wt% ImMIL-101(Cr) loading and then decreased with increasing ImMIL-101(Cr) content. At 60 oC, the hydroxide conductivity of the membrane increased up to 0.047 S cm–1 with the addition

of

ImMIL-101(Cr),

67.9%

higher

than

that

of

the

pure

membrane.

Temperature-conductivity correlation could be further described by Eq. (5) according to the Arrhenius plots shown in Fig. 11b. The slope of the plot was directly related to the activation energy for ion transport through the membrane, so different Ea values could be calculated as listed in Table 2. The results showed that when the ImMIL-101(Cr) content was below 7.5 wt%, the composite membranes exhibited lower Ea values than the pure membrane, indicating lower energy-barriers for ion transport. The lowest Ea appeared at 1.0 wt% ImMIL-101(Cr) loading, probably due to the fact that the even dispersion of ImMIL-101(Cr) made ImPEEK/ImMIL-101(Cr) interfaces fully exposed as channels for ion transport. As a result, filler aggregation was the primary reason to explain why Ea kept rising with increasing ImMIL-101(Cr) content above 1.0 wt% [50]. In accordance with the change of IEC aforementioned, the variation of hydroxide conductivity was under combined influence of imidazolium group introduction and ImMIL-101(Cr) aggregation, which led to different influence of ImMIL-101(Cr) content on hydroxide conductivity and activation energy of the membranes. In

addition,

a

comparison

in

hydroxide

conductivity

was

made

between

polyaryletherketone based membranes reported in this study and in literatures. The ImPEEK/ImMIL membrane exhibited higher performance in hydroxide conductivity than the majority of other polyaryletherketone based membranes as listed in Fig. 12. However, IEC should not be excluded from the comparison in membrane performance considering the fact that membranes generally exhibited enhanced hydroxide conductivity but reduced dimensional stability with higher IEC values. The ImPEEK/ImMIL showed higher conductivity but had lower IEC values compared with the membranes in the right circle, which indicated that more efficient channels were constructed in the ImPEEK/ImMIL for ion transport. It was difficult to make a direct comparison between the ImPEEK/ImMIL and the membranes in the left circle considering much higher IEC values and conductivity of the ImPEEK/ImMIL. In terms of the xQAPEEK membrane, it had a comparable performance in hydroxide conductivity to the ImPEEK/ImMIL. However, with much lower IEC values, the xQAPEEK exhibited higher swelling ratios (37.5 % at 50 oC) than the ImPEEK/ImMIL (17.4 % at 50 oC), which implied that the addition of ImMIL-101(Cr) could effectively solve the “trade-off” restriction between hydroxide conductivity and dimensional stability in PEEK based anion exchange membranes.

17

(a)

0.04

-1

Hydroxide conductivity lnσ ( S cm )

-1

Hydroxide conductivity (S cm )

(b)

ImPEEK ImPEEK/ImMIL-1.0 ImPEEK/ImMIL-2.5 ImPEEK/ImMIL-5.0 ImPEEK/ImMIL-7.5 ImPEEK/ImMIL-10.0

0.05

0.03

0.02

-1.4

-1.6

-1.8

-2.0

0.01

20

30

40

50

ImPEEK ImPEEK/ImMIL-1.0 ImPEEK/ImMIL-2.5 ImPEEK/ImMIL-5.0 ImPEEK/ImMIL-7.5 ImPEEK/ImMIL-10.0 3.0

60

3.1

3.2

3.3

3.4

-1

Temperature (°C)

1000/T (K )

Fig. 11 (Color online) (a) Temperature-dependent hydroxide conductivity of the membranes at 100% RH; (b) Arrhenius plots for hydroxide conductivity of the membranes

-1

Hydroxide conductivity (mS cm )

40

30 [51]

MIPAEK [52] PAEK-TMA [53] PEEK-Q [40] CL-QDPEEKOH [40] QDPEEKOH ImPEEK/ImMIL-101(Cr) [42] Im/SPEEK [54] xQAPEEK [48] QAPEEKOH [55] GQ-PEEK [56] PEEK-AeImOH

20

10

0 1

2

3

4

5

-1

IEC (mmol g ) Fig. 12

(Color

online)

A comparison

in

hydroxide conductivity (30

ImPEEK/ImMIL-101(Cr) with other polyaryletherketone based AEMs in literatures

3.7 Alkaline stability

18

o

C)

of

90

Residual ratio of hydroxide conductivity (%)

o

80 C o 50 C

80 70 60 50 40 30 20 10 0

Im Im Im Im Im Im PE PE PE PE PE PE EK EK EK EK EK EK /Im /I m /Im /Im /Im M MI MI MI MI IL L-1 L L-2 L-1 -5. -7. 0 0.0 .5 .0 5

Fig. 13 Residual ratios of hydroxide conductivity after the alkaline stability test

Alkaline stability is an important indicator which determines the lifetime of AEMs for fuel cells [57, 58]. Herein, the alkaline stability of the composite membrane was evaluated in 2 mol L–1 NaOH solutions at 50 and 80 oC respectively for 48 h. As was shown in Fig. 13, the residual ratio of the hydroxide conductivity of the pure membrane was much lower than that of the composite membrane with 1.0–5.0 wt% ImMIL-101(Cr) content. It could be deduced that the addition of ImMIL-101(Cr) increased the steric hindrance around the ion-conducting groups and thus decreased the chance of attack by hydroxide ions [59]. Nevertheless, this effect could be weakened by the aggregation of ImMIL-101(Cr) which rendered a decrease in alkaline stability with increasing ImMIL-101(Cr) content above 5 wt%. After immersion in 2 mol L–1 NaOH at 80 o C, the ImPEEK based membranes showed significantly decreased conductivity, which indicated that the imidazolium groups faced rapid degradation at elevated temperatures. The comparison in alkaline stability of the composite membranes between 50 and 80 oC suggested that the ImPEEK/ImMIL-101(Cr) membrane might be a preferable choice for low-temperature AEM fuel cell devices.

4

Conclusions

Chloromethylated MIL-101(Cr) was synthesized via a facile method and incorporated into

19

CMPEEK to prepare novel composite AEMs after synchronous quaternization. The introduction of ImMIL-101(Cr) as fillers significantly enhanced the ion conductivity of the membrane. Higher IEC values of the composite membranes could be observed with more imidazolium groups inside. Meanwhile, the three-dimensional open-framework structure of ImMIL-101(Cr) enabled the creation of well-constructed ion channels, with lower energy barriers for anion transport than conventional porous fillers. As a result, the ImMIL-101(Cr) incorporation induced the formation of continuous channels in composite membranes, leading to a decrease in activation energy and an increase in hydroxide conductivity. The hydroxide conductivity of the composite membranes reached the highest at 5.0 wt% ImMIL-101(Cr) content, 71.4% higher than that of the pure membrane at 20 oC while the Ea value was 4.24% lower than that of the pure membrane with the same ImMIL-101(Cr) content. Moreover, ascribed to the low water uptake and rigid framework of ImMIL-101(Cr), the composite membranes exhibited lower water uptake, enhanced dimensional stability and mechanical strength compared with the pure membrane, solving the ubiquitous “trade-off” restriction between hydroxide conductivity and stability in AEMs. It should be noted that incorporation of anion-conductive MOFs significantly enhanced the performance of composite AEMs, manifesting the attractive prospects of MOFs in fuel cell applications.

20

Conflict of interest The authors declare that they have no conflict of interest.

Acknowledgments This research was supported by the National Science Fund for Distinguished Young Scholars (21125627), the National Natural Science Foundation of China (21490583) and the Program of Introducing Talents of Discipline to Universities (B06006).

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Graphical abstract

26