Bent-twisted block copolymer anion exchange membrane with improved conductivity

Author’s Accepted Manuscript Bent-twisted block copolymer anion exchange membrane with improved conductivity Kuibo Zhang, Shoutao Gong, Baolin Zhao, Yanxiang Liu, Naeem Akhtar Qaisrani, Lingdong Li, Fengxiang Zhang, Gaohong He www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(17)31363-7 https://doi.org/10.1016/j.memsci.2017.12.044 MEMSCI15815

To appear in: Journal of Membrane Science Received date: 13 May 2017 Revised date: 22 November 2017 Accepted date: 16 December 2017 Cite this article as: Kuibo Zhang, Shoutao Gong, Baolin Zhao, Yanxiang Liu, Naeem Akhtar Qaisrani, Lingdong Li, Fengxiang Zhang and Gaohong He, Benttwisted block copolymer anion exchange membrane with improved conductivity, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2017.12.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Bent-twisted block copolymer anion exchange membrane with improved conductivity Kuibo Zhang, Shoutao Gong, Baolin Zhao, Yanxiang Liu, Naeem Akhtar Qaisrani, Lingdong Li, Fengxiang Zhang* and Gaohong He* State Key Laboratory of Fine Chemicals, School of Chemical and Petroleum Engineering, Dalian University of Technology 2 Dagong Road, Liaodongwan New District, Panjin, China *Correspondence: [email protected], [email protected] Abstract Inadequate hydroxide conductivity of anion exchange membrane (AEM) is one of the obstacles restricting practical application of anion exchange membrane fuel cell. In this work, we report the design, fabrication and properties of AEMs based on bent-twisted non-coplanar block copolymer. The bent-twisted polymer backbone creates high free volume size so that hydroxide ion transport is facilitated. The free-volume effect can also facilitate microphase separation due to enhanced chain segment motion. With the free-volume holes working synergistically with microphase-separation enabled ion channels, the membrane showed a high conductivity (35 mS cm-1 at room temperature) at a relatively low ion exchange capacity (IEC, 1.25 meq g-1); it also exhibited a good dimension stability, the swelling ratio being 4.7 % at room temperature and kept virtually unchanged with temperature because of relatively low IEC. A H2/O2 fuel cell employing the fabricated AEM yielded a high power density (262 mW cm−2 at 50 oC). Our work opens up a new route to fabrication of highly conductive AEM with relatively low IEC. Keywords: anion exchange membrane; bend-twist backbone; free volume; conductivity; fuel cell 1. Introduction Alkaline anion exchange membrane fuel cells (AEMFCs) show great promise as a next-generation energy conversion technology [1]. Their high-pH operating medium alleviates catalyst corrosion [2-4], and makes the oxygen reduction reaction more facile than in proton exchange membrane fuel cell, thus allowing the use of non-precious electrocatalysts and making the technology less expensive than proton exchange membrane fuel cell [5, 6]. In addition, the use 1

of AEM is beneficial for minimizing fuel crossover to the cathode because the direction of fuel diffusion across the membrane is opposite to that of hydroxide ion transport [7, 8]. In an AEMFC, the AEM functions as a solid electrolyte to enable hydroxide ion transport from the cathode to the anode, where the hydroxide ions undergo an electrochemical reaction with the fuel. In order to achieve a good fuel cell performance, AEMs should be sufficiently conductive, and possess good chemical and mechanical robustness. In terms of conductivity, although many research and development efforts have been made, it is still low relative to that of proton exchange membranes such as Nafion [9, 10] with comparable ion exchange capacity (IEC). One straightforward approach to conductivity improvement is to increase IEC, but this often leads to high swelling of the membrane and accelerated backbone scission [11]. This issue can be addressed by constructing a proper cross-linked structure and/or by the use of highly basic cations such as guanidinium [12-14] and phosphonium [15]; high basicity of cations allows easier dissociation of the cation-hydroxide ion pairs and thus leads to high conductivity at a relatively lower IEC. Manipulating membrane morphology or constructing wide and continuous ion transport channels in the membrane is another effective way to promote ion transport [16]. Different strategies, such as side-chain grafting [17, 18] and block copolymerization [19-21] have been employed to produce ion clusters via micro-phase separation so that wide channels can be formed. However, at a low IEC (for robustness consideration), micro-phase separation may not be prominent enough, and therefore, it is necessary to create new ion transport pathways as a supplement to microphase-separation enabled ion channels. Supplementary pathways may be constructed via a method different from the traditional cation-based mechanisms. For instance, Lee et al. [22] reported smart hydrophobic layer coated ion exchange membranes; this layer “breaks” into nano-cracks upon membrane hydration and the nanocracks become smaller automatically for water conservation when the membrane is dehydrated; these cracks allow facile ion transport so that the surface coated membrane showed higher conductivity than the pristine one. Free volume holes in the polymers of intrinsic microporosity (PIM) [23] may also function as ion transport pathways. PIM chains, which are highly rigid and twisted, do not pack densely so that micropores may be 2

formed. Making use of this feature, Xu et al. obtained highly conductive AEMs [24]. While PIM is effective to create plentiful free volume holes and thus boost AEM conductivity, there are relatively limited PIM structures that can be used for AEM fabrication; meanwhile, the large amount of free volume associated with the typical PIM structures may undermine the membrane robustness. Therefore, it is necessary to explore new modes of creating and utilizing free volume effect for AEM conductivity enhancement. In our previous work, we reported that the polysulfone AEM containing isopropylene groups at the backbone showed a higher conductivity than that without such a group although the two membranes have the same cation (imidazolium) and similar IEC values [25]. Such a difference is due to the free volume imparted by the isopropylene groups, which are non-coplanar relative to their neighboring benzene rings and thus result in less compact chain packing. However, the isopropylene associated free volume effect is insufficient and the AEM’s ion transport still relied on a medium level of IEC (ca. 1.7 mmol/g). Further to our previous work, we herein report a new AEM with high free volume for conductivity improvement. Specifically, we design and synthesize a bend-twist structured block copolymer containing imidazolium cation (NCBP-Im); it consists of poly(phthalazinone ether) and fluorenyl-containing poly(aryl ether) blocks, the former being bent and the latter twisted (Fig. 1). Both the bent and the twisted blocks contribute to free volume so that the ion transport resistance in the resulting AEM can be reduced significantly; the enhanced free volume also facilitates microphase separation due to easier chain segment motion. Therefore, the bent-twisted non-coplanar AEM allows more facile hydroxide ion transport, and exhibits a relatively high conductivity (35 mS cm-1, room temperature) at a low IEC (ca. 1.25 mmol/g). Twisted unit

F

F F

F

R2

R2

O

F

F F

F

R2

R2

Bent unit

F O

O n

F

FF

N N F

FF

O F

FF

F

F

FF

F

F O

O N N

R2=H or

N

OHN

NCBP-Im

R2 m p

3

Fig. 1. The bend-twist structured non-coplanar block copolymer containing imidazolium cation (NCBP-Im) for AEM fabrication. 2. Experimental 2.1 Materials Phenolphthalein, decafluorobiphenyl (DFBP), Bisphenol A, hydrazine hydrate (40

%),

1,2-dimethylimidazole,

4,4'-(9-fluorenylidene)diphenol

(BHPF),4,4'-(hexafluoroisopropylidene)diphenol, chloromethyl methyl ether (CMME) and hydroxylamine hydrochloride were provided by Aladdin Reagents. DFBP

and

BHPF

were

N,N´-dimethylacetamide

recrystallized

(DMAc),

immediately

before

1-methyl-2-pyrrolidone

use.

(NMP),

1,1,2,2-tetrachloroethane (TCE), toluene, n-butyl alcohol, ethanol, concentrated H2SO4 (98%), acetic acid, potassium hydroxide, potassium carbonate and stannic chloride were of reagent grade. DMAc, NMP and toluene were purified by distillation before use. Synthesis of 4-(4-hydroxylphenyl)-2,3-phthalazin-1(2H)-one (DHPZ). To a round bottom flask were added 100 mL aqueous KOH (10 wt %), 7.0 g phenolphthalein and 1.71 g hydroxylamine hydrochloride. The mixture was stirred at 80 °C for 2 h and transferred into 150 ml aqueous acetic acid (1.0 M), giving a bright yellow emulsion, to which was added KOH (1.0 M) until the bright yellow color disappeared, and the mixture was stirred for another 20 min. Next, 14 mL ethanol was added, followed by drop-wise addition of acetic acid aqueous solution (1.0 M) until the pH < 7. The precipitate formed was washed with DI water and dissolved in 140 mL hot H2SO4 (10 wt

%);

the

mixture

was

refluxed

for

3

h,

and

upon

cooling,

a

2-(4-hydroxy)benzoylbenzoic acid (HBA) solid resulted; it was filtered, washed with ice cold water and dried. Subsequently, 24.2 g HBA and 120 mL butanol were introduced to a three-necked round bottomed flask equipped with a dropping funnel and a condenser. The mixture was refluxed, and 10 mL hydrazine hydrate (40 wt %) was added drop-wise. The mixture was stirred at 120 °C for 3 h and then cooled to room temperature. The precipitate obtained (DHPZ) was filtered, washed with anhydrous ethanol, purified by recrystallization from DMAc, washed again with anhydrous ethanol, and finally vacuum dried at 80 oC for 48 h. 2.2 NCBP-Im synthesis and AEM fabrication (Scheme 1) 4

2.2.1 Synthesis of fluorine-terminated poly(aryl ether) (FPAE) oligomer. A three-necked round-bottomed flask equipped with a Dean-Stark trap was charged with DFBP (10.2 mmol), BHPF (10 mmol), K2CO3 (20 mmol), DMAc (50 mL), and toluene (10 mL) under a nitrogen atmosphere. The polymerization reaction was allowed to proceed at 60 oC for 5 h, after which toluene was removed, temperature was elevated to 80 oC and lasted for 20 h. After cooling, the mixture was added into 500 mL water/ethanol mixture (v/v =2:1) and soaked for 24 h, and then washed with absolute alcohol, vacuum dried at 50 oC for 24 h. 2.2.2 Synthesis of hydroxyl-capped poly(phthalazinone ether) (HPPE) oligomer. DFBP (10 mmol), DHPZ (10.2 mmol), potassium carbonate (21 mmol), DMAc (40 mL), and toluene (10 mL) were mixed in a three-necked round-bottomed flask equipped with a Dean-Stark trap under nitrogen flow. The mixture was stirred and refluxed at 40 oC for 5 h, after which toluene was removed, the temperature was raised to 90 oC and kept for 4 h. Upon cooling, the mixture was added into 300 mL water/ethanol mixture (v/v =1:1). The oligomer precipitate obtained was soaked and washed with a water/ethanol mixture, and finally vacuum dried at 40 oC for 24 h. 2.2.3 Coupling of FPAE and HPPE. A three-necked round-bottomed flask equipped with a Dean-Stark trap was charged with the above obtained FPAE oligomer (20 mmol) and HPPE oligomer (20 mmol), potassium carbonate (40 mmol), DMAc (40 mL), and toluene (10 mL) under nitrogen flow. The mixture was heated at 40 oC for 5 h, during which toluene was removed gradually, followed by temperature elevation to 50 oC and lasting for 10 h. After cooling to room temperature, the mixture was transferred into 500 mL water/ethanol mixture (v/v =1:1). The resulting block copolymer (NCBP) precipitate was soaked in a water/ethanol mixture for 24 h, and then filtered, washed with water/ethanol mixture and then vacuum dried at 60 oC for 24 h. 2.2.4 Chloromethylation of NCBP. In a round-bottomed flask equipped with a condenser, 2 g NCBP was dissolved with 60 mL TCE, and then 22.26 mg SnCl 4 was added; upon complete dissolution, 0.40 g CMME was added, temperature elevated to 50 oC and maintained for a certain time period for chloromethylation degree control. Next, the mixture was added into 300 mL absolute alcohol, and

5

the precipitate obtained was stirred overnight, washed with absolute alcohol and vacuum dried at 60 oC. The product is chloromethylated NCBP (cm-NCBP). 2.2.5 Quaternization of NCBP and AEM preparation. A cm-NCBP solution (10 %, w/v) was made with NMP, where a stoichiometric amount of 1,2-dimethylimidazole was added; the mixture was stirred at 50 oC for 10 h, and then cast on a glass substrate and dried on the hotplate at 40 oC for 24 h and at 60 oC for another 24 h. The membrane was peeled off, washed with DI water and soaked in a 1 M NaOH solution for 48 h. The obtained AEM (NCBP-Im) was washed with DI water and soaked in deionized water to prevent CO2 contamination. The dry membrane thickness was ca. 40 μm.

Scheme 1 Synthetic route of NCBP-Im. 2.3 Control experiments 6

Control

1:

imidazolium

non-coplanar

homopolymer

(NCHP-Im)

AEM

fabrication (Scheme 2) (1) A round-bottomed flask equipped with a Dean-Stark trap was charged with DFBP (3.34 g, 10 mmol), BHPF (10 mmol), potassium carbonate (20 mmol), DMAc (50 mL), and toluene (10 mL) under nitrogen flow. The polymerization proceeded at 70 oC for 8 h; toluene was removed, and temperature was elevated to 100 oC and kept for 12 h; upon cooling, the mixture was added into 400 mL water/ethanol (v/v =1:1). The polymer (NCHP) precipitate was soaked in a water/ethanol mixture for 48 h, filtered and vacuum dried at 60 oC for 24 h. (2) To a round-bottomed flask equipped with a condenser, 2 g NCHP and 60 mL 1,1,2,2-tetrachloroethane (TCE) were introduced, followed by addition of 13.0 mg SnCl4 and 0.24 g CMME; the mixture was refluxed at 50

o

C,

chloromethylation degree controlled by reaction time. The resulting mixture was

transferred

into

alcohol,

stirred

overnight;

the

chloromethylated

homopolymer (cm-NCHP) precipitate was filtered, washed with alcohol and vacuum dried at 50 oC. (3) A 10 % (w/v) NMP solution of cm-NCHP was made, to which a stoichiometric amount of 1,2-dimethylimidazole was added; the mixture was stirred at 40 oC for 10 h, cast on a glass plate, and dried. The resulting membrane was peeled and soaked in 1 M NaOH at room temperature for 36 h. The obtained NCHP-Im membrane was washed and stored in DI water.

Scheme 2 Synthetic route of NCHP-Im. Control 2: imidazolium coplanar block polymer (CBP-Im) AEM fabrication (Scheme 3) 7

(1) A three-necked round-bottomed flask with a Dean-Stark trap was charged with DFBP (3.41 g, 10.2 mmol), Bisphenol A (2.28 g, 10.0 mmol), K2CO3 (2.76 g, 20 mmol), DMAc (50 mL), and toluene (10 mL). The reaction proceeded at 100 oC for 10 h, toluene removed, temperature elevated to 130 oC and lasting for 15 h. After cooling, the mixture was transferred into 500 mL water/ethanol (v/v =2:1) and stirred for 24 h; the oligomer precipitate was washed with alcohol and vacuum dried at 50

o

C for 24 h. (2) DFBP (10.0 mmol),

4,4'-(hexafluoroisopropylidene) diphenol (10.2 mmol), K2CO3 (20.4 mmol), DMAc (50 mL), and toluene (10 mL) were mixed in a round-bottomed flask equipped with a Dean-Stark trap. The mixture was refluxed at 75 oC for 23 h while toluene was removed gradually; after cooling, the mixture was precipitated in water/ethanol (v/v =1:1); the resulting oligomer was soaked in water/ethanol for 48 h, filtered and vacuum dried at 55 oC for 15 h. (3) A round-bottomed flask equipped with a Dean-Stark trap was charged with the above obtained oligomers (each 30 mmol), K2CO3 (60 mmol), DMAc (60 mL), and toluene (15 mL). The mixture was refluxed at 60 oC for 10 h, toluene removed, temperature elevated to 70 oC and kept for 15 h. Upon cooling, the mixture was transferred into 500 mL water/ethanol (v/v =1:1), and the resulting block copolymer precipitate was soaked in a water/ethanol mixture for 24 h, filtered, washed with water/ethanol mixture and then vacuum dried at 50 oC for 24 h. (4) 5 g dried copolymer was dissolved in 150 mL TCE in a round-bottomed flask equipped with a condenser; SnCl4 (0.125 mmol) was added, followed by 0.60 g CMME, and the mixture was stirred and refluxed at 50

o

C (chloromethylation degree controlled by time). The mixture was

transferred into alcohol and stirred overnight; the precipitate (cm-CBP) was filtered, washed with alcohol and vacuum dried at 50 oC. (5) A cm-CBP solution (10 %, w/v) was made with NMP, to which a stoichiometric amount of 1,2-dimethylimidazole was added, and the mixture was stirred at 50 oC for 6 h, then cast on a glass substrate and dried on the hotplate for solvent removal. The resulting membrane was peeled and treated with 1 M NaOH solution at room temperature for 24 h. The obtained CBP-Im membrane was washed and stored in DI water.

8

Scheme 3 Synthetic route of CBP-Im. Control AEM

3: imidazolium functionalized non-coplanar random copolymer（NCRP-Im）

fabrication (Scheme 4)

(1) A round-bottomed flask equipped with a Dean-Stark trap was charged with DFBP (3.3748 g, 10 mmol), BHPF (1.7878 g, 5 mmol), DHPZ (1.2396 g, 5 mmol), potassium carbonate (3.2109 g, 23 mmol), DMAc (40 mL), and toluene (10 mL) under nitrogen flow. Polymerization proceeded at 50°C for 27 h; upon cooling, the mixture was added into 500 mL water/ethanol (v/v =1:1). The non-coplanar random copolymer (NCRP) precipitate was soaked in a water/ethanol mixture for 48 h, filtered and vacuum dried at 50 °C for 24 h. (2) To a round-bottomed flask equipped with a condenser, 1g NCRP and 40 mL TCE were introduced, followed by addition of 44.52 mg SnCl4 and 2.12g CMME; the mixture was refluxed at 50 °C, chloromethylation degree controlled by reaction time. The resulting mixture was transferred into alcohol and stirred overnight; the chloromethylated NCRP (cm-NCRP) precipitate was filtered, washed with alcohol and vacuum dried at 50 °C. (3) Stoichiometric amount of 1,2-dimethylimidazole was added to a 5 % (w/v) NMP

9

solution of cm-NCRP; the mixture was stirred at 50 °C for 12 h, cast on a glass plate, and dried. The resulting membrane was peeled and soaked in 1 M NaOH at room temperature for 24 h. The obtained AEM (NCRP-Im) was washed with DI water and stored in deionized water.

Scheme 4 Synthetic route of NCRP-Im. 2.4 Measurements 2.4.1 Nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). 1

H-NMR spectra of the oligomers and polymers were recorded on a Bruker AVANCE

spectrometer at 500 MHz using dimethyl sulfoxide-d6 and CDCl3 as the solvent and tetramethylsilane as the internal standard. Molecular weights were measured by GPC; the test was carried out on a Waters 1515 system, where chloroform was used as a solvent for the polymer or oligomer samples. 2.4.2 Differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and small angel X-ray scattering (SAXS). Glass transition temperature was determined with a Mettler DSC 822 differential scanning calorimeter under flowing nitrogen at a 10

heating rate of 10 oC min-1. TEM was performed on a JEOL JEM-2000EX microscope. Before testing, the membrane sample was stained by immersion in a PdCl2/HCl solution for 2 days, washed with water and then dried at room temperature for 12 h [26]. SAXS spectra of the membranes were recorded on a D/Max-2400 powder X-ray diffraction instrument. The intensity of the X-ray scattering was plotted versus q (defined as 4πsinθ/λ, where λ is the X-ray wavelength of 0.1541 nm); all membranes were immersed in DI water for 24 h before measurement. 2.4.3 Positron annihilation lifetime spectroscopy (PALS) and microporosity. The PALS study was performed following literature reported methods [27, 28]. The average radius of free-volume holes (assumed spherical) was obtained using eqn (1) and ortho-positronium (o-Ps) lifetime obtained with the spherical infinite potential-well model [29, 30].  2R  1 R 1   3   1   sin  3 2  R0 2  R0  1

1

(1)

In eqn (1), τ3 (in ns) is the o-Ps lifetime, R (in Å) is the hole radius, Ro = R +ΔR, where ΔR (=1.66 Å) is the electron layer thickness. The average size of the free-volume holes Vf = (4/3)πR3. A butanol intrusion method was used to estimate the microporosity of membranes. The porosity was calculated as Δm /(ρVdry ), where Δm is the difference between the sample mass before and after butanol uptake, ρ is density of butanol and Vdry is the volume of the dried membrane. 2.4.4 Mechanical and chemical stability. Mechanical property of the hydrated NCBP-Im membrane was measured using an SANS CMT8102 tester at a stretch rate of 5 mm min-1. Chemical stability was studied by treating the membrane in a 1 M NaOH solution at 80 oC for different time durations; the treated membrane was then soaked and washed with deionized water; its conductivity was measured in 20 oC degassed deionized water. Fourier transform infrared spectroscopy of the treated membrane was recorded using a Lambda 950 spectrometer in the wave number range of 400~4000 cm-1 with a resolution of 4 cm-1. 2.3.5 IEC, water uptake (WU) and swelling ratio (SR). For IEC measurement, a membrane sample was treated in a HCl standard solution (0.01 M), which was titrated with a standardized NaOH solution (0.01 M) after the treatment. IEC

11

calculation follows eqn. (2), where C1 and C2 are the concentration of the standardized HCl and NaOH solutions, V1 and V2 are the volume of the HCl and NaOH solutions, and Wdry is the weight of the dry membrane sample [31].

C1V1  C2V2 100% Wdry

IEC 

(2)

WU and SR measurements were carried out as follows. A membrane sample was soaked in deionized water at a given temperature for 24 h. Its surface water was wiped, weight and dimensions measured quickly. Then the sample was vacuum dried at 60 oC for 24 h, weight and dimensions measured again. WU is calculated by eqn (3) and SR by eqn (4) [32, 33]. In these equations, Wwet and Lwet are the weight and the average dimension of the hydrated sample; Wdry and Ldry are the weight and the average dimension of the dried sample. Lwet =(Lwet1Lwet2)1/2 and Ldry =(Ldry1Ldry2)1/2. Lwet1, Lwet2 and Ldry1, Ldry2 are the length and width of the hydrated and dry membranes, respectively.

WU 

SR 

Wwet  Wdry Wdry

Lwet  Ldry Ldry

100%

100%

(3)

(4)

2.3.6 Conductivity and fuel cell testing. A four-electrode AC impedance analyzer (frequency range of 1~105 Hz) was used to measure the in-plane hydroxide conductivity of the membranes in degassed deionized water. The hydroxide ion conductivity (σ) was calculated with eqn (5) [34], where L is the distance between the two potential electrodes, S the cross sectional area of the sample, and R the membrane resistance obtained from impedance analysis.



L SR

(5)

For fuel cell testing, a catalyst coated membrane (CCM) was prepared as follows. First, a catalyst ink was made by mixing Pt/C (70 %, JM Co.) and an anion conductive ionomer solution. The ink was then sprayed onto the fabricated membrane, and the resulting CCM was hot pressed between two gas diffusion layers (Toray-060) at 60 oC and 1 MPa for 3 min. Fuel cell test was carried out with 0.1 MPa H2 and O2 12

at 50 oC and the relative humidity was controlled at 100 %. 3. Results and discussion 3.1 Synthesis and structure The synthetic route of NCBP-Im is shown in Scheme 1 (see experiment), where fluorine capped poly(aryl ether) (FPAE) oligomer and hydroxyl capped poly(phthalazinone ether) (HPPE) oligomer were synthesized first; equimolar coupling reaction between the FPAE and HPPE gave a non-coplanar block polyarylether (NCBP). The molecular weight and molecular weight distribution of the synthesized oligomers and polymer are shown in Table 1. The chloromethylation reaction follows a Friedel-Crafts mechanism and occurs between chloromethyl methyl ether and the polymer catalyzed with a Lewis acid (e.g. SnCl4). Quaternization of the chloromethylated NCBP (cm-NCBP) was carried out by its reaction with dimethylimidazole [35]. The quaternized block copolymer (NCBP-Im) was made into an AEM by solution casting, and the resultant membrane was alkalized by treatment with aqueous NaOH. Table 1 Molecular weights of the oligomers and the block copolymer synthesized. Oligomer/Polymer

Mn (g mol-1)

Mw (g mol-1)

Mw/Mn

FPAE

3.2 × 103

5.4 × 103

1.69

HPPE

2.3 × 103

3.3 × 103

1.43

NCBP

2.2 × 104

4.1 × 104

1.86

The FPAE oligomer is characterized by 1H-NMR (Fig. 2a), where chemical shift of the hydroxyl groups is not detected, suggesting that the di-hydroxyl monomer has been reacted completely in the oligomer synthesis. The 1H-NMR spectrum for the HPPE oligomer (Fig. 2b) shows the chemical shifts for hydroxyl and amino groups at 12.9 ppm and 9.9 ppm [36], respectively; these groups allow the coupling reaction between HPPE and FPAE via a nucleophilic substitution mechanism.

13

2

1

3

F

F F

F

F

4

O

F F

F F

F

F F

F

O

F

n 6

5

F

F F

F

CDCl3

(a) 4 5 1

8.0

6

2,3

7.8

7.6

7.4

7.2

7.0

6.8

6.6

Chemical shift / ppm 5

6

4

HO 1

2 3

7

NN

O F

FF

5,6

F O

F

FF

NN

F

O m

(b)

H 8

2 3,4

7 8

1

13

12

11

10

9

8

7

Chemical shift / ppm

Fig. 2 The 1H-NMR spectrums of (a) FPAE oligomer and (b) HPPE oligomer. CDCl3 was used as the solvent for (a) and DMSO-d6 for (b). Fig. 3a gives the 1H-NMR for NCBP, where the chemical shifts of hydroxyl and amino group are absent, indicating that these groups have undergone a nearly complete reaction with the FPAE oligomer. Chemical structures of the chloromethylated NCBP (cm-NCBP) and quaternized NCBP (NCBP-Im) were analyzed by 1H-NMR. As shown in Fig. 3b, the chemical shift at ca. 4.5 ppm confirms successful chloromethylation of NCBP [37]; this signal disappears in the 1H-NMR of NCBP-Im (Fig. 3c), revealing that the chloromethyl groups have reacted almost completely during the quaternization reaction.

14

(a) F

F F

5 6 4 3

F

F

O

*

F

F F

CDCl3

F O

O

N N

n

1 2

F

FF

F

FF

O F

FF

F

F

FF

F

10 11 12 9

F O

O N N

7 8

2

m

4,5

p

1 3,10,11

6,9

7

8

12

13

12

11

10

9

8

7

Chemical shift / ppm

11 10 F

F F

F

(b)

R1

R1

F

F F

F

F

9

12

FF

F O

O

O

*

8

R1

N N

n

F

R1

FF

4

O F

FF

F

F

FF

F

1

F

5

3

6

O

R1=H or CH2Cl 7

O R1

2

N N

CDCl3

m p

4,5,9 3,11 1,2

10

8

7 12

6

8.5

8.0

7.5

7.0

6.5

6.0

5.5

5.0

4.5

4.0

Chemical shift / ppm

15

10 F

F F

R2

F

F F

F

FF

O

1 N

N N

n

6

R2

R2

F

DMSO-d6

F

O

2

R2=H or

F

8

7 O

*

(c)

9

R2

F

FF

O F

14 15 FF

F

11

F

13

16

O F

OH3 N

FF

F

H2 O

O R2

12 N N m

5

4

p

2 8,14,15

11,12 10,13

4,5,6 16

9

8.0

7.5

7.0

6.5

3

1

7 6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

Chemical shift / ppm

Fig. 3 1H-NMR spectrum of (a) NCBP (CDCl3 as solvent), (b) cm-NCBP (CDCl3 as solvent) and (c) NCBP-Im (DMSO-d6 as solvent). 3.2 Free volume property Free volume property of the NCBP-Im membrane in comparison with two counterparts were studied by DSC. The two counterpart membranes are non-coplanar homopolymer (NCHP-Im) and coplanar block polymer (CBP-Im), whose synthesis details are provided in Schemes 2 and 3 in the experimental section; the three membranes have similar IEC values (ca. 1.2 mmol g-1), which were achieved by careful control of the chloromethylation reaction conditions. The DSC curves in Fig. 4 indicate a lower glass transition temperature (Tg) of the NCBP-Im (125 oC) than that of NCHP-Im (151 oC) and CBP-Im (200 oC). Such a difference stems from the bend-twist non-coplanar architecture, which expands the free volume so that polymer segments can move more easily when heated. The broader endothermic peak observed in the NCBP-Im curve than in the other two also indicates the bend-twist effect, which leads to less compact chain packing so that lower thermal energy is needed for chain segment motion. The endothermic peak for CBP is much sharper and the heat flow is much higher; this observation means the CBP chains pack better and there may be a crystalline phase in the membrane. 16

Heat flow (W/g)

NCBP-Im

NCHP-Im CBP-Im

0

30

60

90

120

150

180

210

Temperature (oC)

Fig. 4 Differential scanning calorimetry traces of NCBP-Im, NCHP-Im and CBP-Im membranes with similar IEC (ca. 1.2 mmol/g). Positron annihilation lifetime spectrum (PALS) is a straightforward strategy for free volume study. Ortho-positronium (o-Ps) preferentially localises in the free-volume holes of polymers from which it annihilates, and its lifetime and intensity are related to free volume size [38, 39]. Fig. 5 shows increased o-Ps lifetime and free volume size of the NCBP-Im membrane with temperature. The profile of free-volume size versus temperature can be used to determine Tg [40], and in our case, the extrapolated straight sections of the free volume curve gave a Tg of ca. 125 oC, which agrees well with that determined from DSC (Fig. 4). From o-Ps lifetime, the free-volume sizes of different membranes (Vf) at room temperature were calculated and shown in Table 2. Clearly, the free volume size of NCBP-Im (ca. 1.232 nm3) is larger than that of the other two (0.817 and 1.098

3.4

3.5

3.2

3.0

3.0

2.5

2.8

2.0

2.6

1.5

2.4 1.0 2.2 2.0

Tg=125 oC -200

-100

0

100

200

Free volume (nm3)

o-Ps lifetime (ns)

nm3 for NCHP-Im and CBP-Im, respectively).

0.5 300

Temperature (oC)

Fig. 5 Positron annihilation lifetime spectra of NCBP-Im membrane. 17

Table 2 Free volume properties of different membranes at room temperature derived from PALS. Membrane

τ3 (ns)a

D (nm)b

Vf (nm3)c

NCBP-Im

2.336

1.33

1.232

NCHP-Im

2.177

1.16

0.817

CBP-Im

2.269

1.28

1.098

a: o-Ps lifetime; b: the diameter of free volume holes; C: the free volume size.

The bent-twisted, non-coplanar structure also produced intrinsic molecular micropores because of the higher free volume indicated by PALS. Such microporosity can be determined from a butanol intrusion experiment as shown in Fig. 6, where the butanol intrusion amount in the micropores of different membranes are given; from these data, microporosity of the NCBP-Im, NCHP-Im and CBP-Im membranes are estimated to be 4.43 %, 2.12 %, and 1.32 %, respectively. The micropores can function as extra pathways for hydroxide ion transport, but will not allow H2/O2 crossover in a fuel cell because the pores are filled with water in the fuel cell operating state; this will be seen from the fuel cell performance, where a high open circuit potential was obtained (to be shown in a later section of this paper).

Butanol intrusion (%)

15

NCBP-Im NCHP-Im CBP-Im 10

5

0

50

100

150

200

Time (h)

Fig. 6 Butanol intrusion behavior of CBP-Im, NCHP-Im and NCBP-Im membranes with similar IEC (ca. 1.2 mmol/g).

18

3.3 Morphology Morphology of the NCBP-Im membrane in comparison with CBP-Im and NCHP-Im is shown in the TEM images in Fig. 7. The two block copolymer AEMs (NCBP-Im and CBP-Im) show higher degree of microphase separation than the homopolymer one (NCHP-Im), as seen from their larger ion clusters. More importantly, the NCBP-Im membrane displays larger ion clusters than CBP-Im. The above comparison implies that the bend-twist backbone structure may have a contributing effect on ion cluster formation, probably because the larger free volume size associated with the bend-twist structure facilitates chain segment motion. Interestingly, even in the non-coplanar homopolymer AEM (NCHP-Im), clear ion clusters can be seen (Fig. 7b).

Fig. 7 TEM images of (a) CBP-Im, (b) NCHP-Im and (c) NCBP-Im membranes with similar IEC (ca. 1.2 mmol/g). Fig. 8 shows the SAXS patterns of the above three membranes (hydrated), where the peaks suggest microphase separation; according to the literature [41-44], the peak position, qmax, is related with the hydrophilic aggregates (or ion clusters), and the Bragg distance calculated by d = 2π/q is recognized as the distance between such aggregates. Based on Fig. 8, estimated d values of 7.75 nm (at q = 0.81 nm-1), 6.16 nm (at q = 1.02 nm-1) and 9.97 nm (at q = 0.63 nm-1) were obtained for CBP-Im, NCHP-Im and the NCBP-Im, respectively. Larger d values (or smaller q) indicate more distinct and ordered ion clusters [45, 46]. So, the highest d value of NCBP-Im means better developed ion clusters than in the other two membranes. The SAXS and TEM observations both imply that the non-coplanar structure promotes ion cluster formation probably due to enlarged free volume, which can facilitate motion of chain segments.

19

Relative counts

12000

NCHP-Im NCBP-Im CBP-Im

9000

6000

q=0.81 nm-1

3000 q=1.02 nm-1 q=0.63 nm-1 0

0.4

0.6

0.8

1.0

1.2

1.4

Scattering vector q (nm-1)

Fig. 8 SAXS patterns of (a) NCHP-Im, (b) NCBP-Im and (c) CBP-Im membranes with similar IEC (ca. 1.2 mmol/g). 3.4 Hydroxide conductivity Hydroxide conductivity of the NCBP-Im membrane in comparison with NCHP-Im and CBP-Im is given in Fig. 9a. They all show increased conductivity with IEC; this is expected since higher IEC means more cations in the membranes. Conductivity variation with temperature is given in Fig. 9b, from which the activation energy of OH- transport is calculated to be 12.01, 13.16 and 15.49 kJ mol-1 for NCBP-Im, NCHP-Im and CBP-Im, respectively. These activation energy values were obtained from the equation Ea= -bR, where b is the slope of the regressed linear lnσ~1000/T plots as shown in Fig. 9b, and R = 8.314 J/(mol K)-1. The comparison of Ea indicates lower energy barrier for hydroxide ion transport in NCBP-Im than in the other two membranes due to the bend-twist effect. -2.6

(a)

NCBP-Im NCHP-Im CBP-Im

35

(b)

-3.0

30

-3.2

25

-3.4

20

-3.6

15

-3.8

10 5 0.8

NCBP-Im NCHP-Im CBP-Im

-2.8

ln (conductivity)

Hydroxide conductivity (mS cm-1)

40

-4.0 0.9

1.0

1.1

IEC (mmol g-1)

1.2

1.3

2.9

3.0

3.1

3.2

3.3

3.4

3.5

1000/T (1/K)

Fig. 9 Hydroxide conductivity of NCBP-Im, NCHP-Im and CBP-Im membranes as a function of (a) IEC (at room temperature) and (b) temperature (with similar IEC ca. 1.2 mmol/g). 20

It is noteworthy that the NCBP-Im membrane exhibits evidently higher conductivity than NCHP-Im and CBP-Im membranes at all IEC values (Fig. 9a). The conductivity of NCBP-Im reaches 35 mS cm-1 at room temperature and 70 mS cm-1 at 70 oC with relatively low IEC (1.25 mmol/g). These values are higher than those of literature reported AEMs with similar IEC [47-49], and our previous polysulfone AEM with isopropylene groups despite a higher IEC of the latter (1.75 mmol/g) [25]. A more comprehensive conductivity comparison with the most recently reported AEMs is presented in Table 3. The above comparisons demonstrate the advantageous hydroxide ion transport capability of the NCBP-Im membrane at a relatively low IEC, which is attributed to the micropores formed with the bent-twisted backbone, and the ion channels formed by the enhanced ion clustering. Table 3. Hydroxide ion conductivity comparison between NCBP-Im and some of most recently reported AEMs Membrane

IEC (mmol/g) Conductivity (ms/cm)

Ref.

NCBP-Im

1.25

35, at RT; 70 at 70°C

This work

Phenylic guanidinuium poly(p-phenylene-co-arylether ketone)

1.38

18, at 20 oC

[50]

Quaternary ammonium polysulfone

2.0

60, at 70 oC

[51]

Self-crosslinked comb-shaped polystyrene bearing quaternary ammonium

1.42

10.2, at 20 oC

[52]

Pyrrolidinium poly(arylene ether sulfone), QQBPES-2.4OH

2.07

68.0, at 80 °C

[53]

Quaternary ammonium poly(vinylbenzyl chloride) (c-AEM-18)

1.2

3.8, at 20 oC

[54]

N-methyl dipicolylamine quaternzied poly(phenyl oxide), MDPA-1

0.48

17.33 at 30 oC

[55]

Poly(arylene ether sulfone) with 1, 2-dimethylimidazolium

1.98

70, at 70 oC

[56]

Poly(phenylene oxide) with quaternary ammonium side chains

2.27

22.5, RT

[57]

43, at 30°C

[58]

Poly(terphenylene) with pendant 2.12 quaternary ammonium

21

Poly(phenylene oxide) with flexible long-chain multication cross-links, x(QH)3QPPO-40

3.59

110.2, at 80 °C

[59]

PSf–diallylpiperidinium

1.29

29, RT

[60]

Poly(phenylene oxide) copolymer with quaternary ammonium side chains (X40Y10-C16）

1.29

26.3, RT

[61]

Fig. 9a has shown that the NCBP-Im membrane is more conductive than NCHP-Im and CBP-Im. This indicates that the non-coplanar backbone effect and the block architecture effect both contribute to the improved conductivity of NCBP-Im. In order to find out which is more important, we further synthesized imidazolium functionalized, non-coplanar random copolymer (NCRP-Im, Scheme 4 in the Experimental section) membranes, and compared their conductivity with that of NCBP-Im. As can be seen from Table 4, the room-temperature conductivity of NCRP-Im can reach 11 - 18 mS/cm at a relatively low IEC (0.36 - 0.69 mmol/g). For a reasonable comparison with NCBP-Im, we define a new parameter, namely IEC normalized conductivity, and this parameter of NCRP-Im is higher than that of NCBP-Im (average 30.0 vs 22.1), implying that the bent-twisted backbone effect can promote hydroxide ion transport more significantly than the block architecture effect. We have also tried to synthesize NCRP-Im of higher IEC but failed probably because the NCRP backbone is more curved than NCBP and it is likely that intra-chain Friedel–Crafts alkylation reaction (between the chloromethyl group and the phenyl ring) can occur. Other reasons may also exist, but right now it is difficult to identify the real reason; we will work further on this point in our future study and have a more detailed investigation on structure-property relation of the NCRP-Im membrane. Table 4. IEC and conductivity of NCRP-Im and NCBP-Im membranes Membrane

NCRP-Im

IEC (mmol/g)a 0.36 0.40 0.49 0.69

Conductivity (mS/cm)a 11±0.8 13±0.9 15±0.8 18±1.0

IEC normalized conductivity 30.6 32.5 30.6 26.1

Mean value of IEC normalized conductivity 30.0

22

0.91 17±1.2 18.7 0.94 18±1.1 19.1 0.98 20±1.0 20.4 b NCBP-Im 22.1 1.03 24±0.9 23.3 1.13 26±1.3 23.0 1.25 35±1.2 28.0 a b Measured at room temperature; data being the same as in Fig. 9a 3.5 Water uptake, swelling ratio and mechanical properties Water uptake (WU) and swelling ratio (SR) of the NCBP-Im membrane are shown in Fig. 10a and b, respectively. The WU increased with IEC and temperature, and the room temperature WU for the membrane with an IEC of 1.25 mmol g-1 is 13.27 %, lower than literature reported results with the same or comparable IECs [62, 63]. The SR is only 4.56 % at room temperature, also lower than literature values with the same or similar IEC [62, 64]. Meanwhile, WU and SR values change little with temperature; this is probably related to the the “nano-pores” formed by free volume, which can function as the reservoir for water storage, and also cushion the dimension change at elevated temperatures. Such a dimension stability is good for practical application of the membrane in a fuel cell [65]. 6

17.5

IEC=1.25 IEC=1.03 IEC=0.91

12.5

10.0

7.5

5.0 20

30

40

50

60 o

Temperature ( C)

IEC=1.25 IEC=1.03 IEC=0.91

(b) Swelling ratio (%)

Water uptake (%)

(a) 15.0

70

5

4

3 20

30

40

50

60

70

o

Temperature ( C)

Fig. 10 (a) Water uptake and (b) swelling ratio of the NCBP-Im membranes with different IEC values as a function of temperature. The WU and SR of the NCRP-Im membranes at room temperature are shown in Table 5. At an IEC of 0.69 mmol/g, the membrane displays a WU of 15.82% and an SR of 7.49%, which are both higher than the results of NCBP-Im at IEC of 0.91 mmol/g (WU and SR being respectively as shown in Fig. 10). This observation, being consistent with the conductivity behavior shown in Table 4, may also indicate a more 23

pronouned bent-twisted backbone effect in NCRP-Im, and thus the membrane can absorb more water than the NCBP-Im membrane. Table 5. WU and SR of NCRP-Im membranes at room temperature IEC (mmol/g) 0.36 0.4 0.49 0.69

WU (%) 9.95±0.95 10.53±1.3 11.39±2.75 15.82±2.94

SR (%) 4.99±0.14 6.04±0.66 5.87±1.16 7.49±0.68

Despite a non-coplanar structure, the NCBP-Im-1.25 membrane is mechanically robust. As shown in Table 6, it displays a tensile strength of 28.5 MPa and an elongation-at-break of 24.7 %, better than the literature reported poly(aryl ether) AEM with a similar IEC. This is because the NCBP-Im membrane is swelling resistant as shown in Fig. 10b, and meanwhile, the ether group imparts chain flexibility so that the adverse effect of non-coplanar structure on chain interactions can be mitigated. Actually our membrane, different from a PIM (polymer of intrinsic microporosity) based AEM, shows medium microporosity based on Fig. 6, and therefore, a balance can be achieved between ion transport property and mechanical robustness. The above mechanical properties can insure successful fabrication of the membrane/electrode assembly for AEMFC testing, as is shown subsequently. Table 6 Mechanical properties of NCBP-Im and some reported membranes. Sample

IEC (mmol g−1)a

Tensile strength (MPa)b

Elongation at break (%)b

Ref

NCBP-Im

1.25 ± 0.03

28.5 ± 1.2

24.7 ± 0.4

This work

c

MPAES-Q-2

1.30 ± 0.00

6.9 ± 0.3

6.3 ± 1.3

[66]

c

QMPAEK-70

1.22

25.56

41.08

[67]

Nafion-117

0.91

21.1

370.6

[68]

a

IEC, determined by titration; b Measured at 25 °C, 100 % RH; c A main chain type of poly(aryl ether) AEM with quaternary ammonium cations.

3.6 Fuel cell performance and alkaline stability To assess the application potential of the membrane, A H2/O2 fuel cell was assembled with NCBP-Im (IEC=1.25 mmol/g). Polarization and power density curves of the fuel cell are shown in Fig. 11. The cell exhibited an open circuit

24

voltage (OCV) of 1.07 V, which means the membrane was able to block fuel crossover effectively. The maximum power density reached 262 mW cm−2 at a current density of 428 mA cm−2 at 50 oC; this is comparable to or better than the power output results reported in the most recent literature, which mostly fall within the range of 11-302 mW cm-2 for H2/O2 type of AEMFC [52, 54, 58, 59, 61]. The higher power density is attributed to high hydroxide conductivity of NCBP-Im membrane, which is closely related to its bent-twisted non-coplanar

1.2

300

1.0

250

0.8

200

0.6

150

0.4

100

0.2

50

0.0

0

100

200

300

400

500

600

700

800

Power density, mW cm-2

Cell voltage (V)

structure.

0 900

Current density ( mA cm-2) Fig. 11 Polarization and power density of an AEMFC at 50 oC with H2/O2 flow rate of 0.2/0.2 L/min. For chemical stability investigation, the NCBP-Im membrane (IEC = 1.25 mmol g-1) was treated with 1 M NaOH at 80 oC for 360 h. Conductivity (measured at room temperature) of the treated membrane was plotted versus the treatment time as depicted in Fig. 12a; the conductivity shows a rapid decay over the first 140 h, and became more stable in the following treatment period; after 360 h, the final conductivity at room temperature is ca. 20 mS cm-1, equivalent to 60 % of the initial value. The conductivity decline was caused by the partial ring-opening decomposition of the imidazolium cations, which can be clearly seen from the dwindled infrared absorption band at 1616 cm-1 and the new band appearing at 680 cm-1 (Fig. 12b) [69]. Such decomposition may be mitigated or alleviated by further modification of the 25

cation and chain structure, which will be investigated in our future work.

-1

Hydroxide conductivity (mS cm )

50

(b)

(a)

pristine alkali treated

Absorbance

40

30

20

10

0 0

50

100

150

200

250

300

350

1650

Time (h)

1500

900

750 -1

Wavenumber (cm )

Fig. 12 (a) Conductivity (measured at 25 oC) of the NCBP-Im membrane (IEC=1.25 mmol/g) after treatment with 1 M NaOH solution at 80 oC (before measurement, the alkali treated membrane was soaked for at least 24 h and washed copiously with deionized water). (b) ATR-FTIR spectra of the pristine and the alkali-treated NCBP-Im membranes (treatment time was 48 h). 4. Conclusions We designed and synthesized a bent-twisted, non-coplanar block copolymer bearing

1,2-dimethylimidazolium

cations

for

AEM

fabrication.

The

bent-twisted structure enhances free volume size and microporosity of the membrane so that hydroxide ion transport became more facile; this effect also promoted ion cluster formation because the enlarged free volume allows for easier chain segment motion. The fabricated NCBP-Im membrane showed a room temperature hydroxide conductivity of 35 mS cm-1 at a relatively low IEC (1.25 mmol/g); its swelling ratio was as low as 4.56 %, and tensile strength was 28.5 MPa at room temperature; a H2/O2 fuel cell empolying this membrane yielded a peak power density of 262 mW cm−2 at 50 oC. Our work demonstrates the contribution of free volume to AEM conductivity, and more importantly, discloses the bend-twist effect on free volume size enlargement. It provides a new possibility of fabricating low-IEC AEM with high conductivity and good robustness. Acknowledgment We gratefully acknowledge the financial supports from the National Key Research and Development Program of China (Grant no. 2016YFB0101203), the National Natural Science Foundation of China (Grant no. 21776042 and 26

21276252), the JH2014009)

State Key Laboratory of Fine Chemicals (Panjin) (Grant no.

and

Natural

Science

Foundation

of

Liaoning

Province

(2015020630). References [1] G. Merle, M. Wessling, K. Nijmeijer, Anion exchange membranes for alkaline fuel cells: A review, Journal of Membrane Science, 377 (2011) 1-35. [2] M. Fujimura, T. Hashimoto, H. Kawai, Small-angle X-ray scattering study of perfluorinated ionomer membranes. 2. Models for ionic scattering maximum, Macromolecules, 15 (1982) 136-144. [3] J.R. Varcoe, P. Atanassov, D.R. Dekel, A.M. Herring, M.A. Hickner, P.A. Kohl, A.R. Kucernak, W.E. Mustain, K. Nijmeijer, K. Scott, Anion-exchange membranes in electrochemical energy systems, Energy & environmental science, 7 (2014) 3135-3191. [4] M. Jacobson, W. Colella, D. Golden, Cleaning the air and improving health with hydrogen fuel-cell vehicles, Science, 308 (2005) 1901-1905. [5] K. Asazawa, K. Yamada, H. Tanaka, A. Oka, M. Taniguchi, T. Kobayashi, A Platinum‐Free Zero‐Carbon‐ Emission Easy Fuelling Direct Hydrazine Fuel Cell for Vehicles, Angewandte Chemie International Edition, 46 (2007) 8024-8027. [6] J. Sanabria-Chinchilla, K. Asazawa, T. Sakamoto, K. Yamada, H. Tanaka, P. Strasser, Noble metal-free hydrazine fuel cell catalysts: EPOC effect in competing chemical and electrochemical reaction pathways, Journal of the American Chemical Society, 133 (2011) 5425-5431. [7] G. Couture, A. Alaaeddine, F. Boschet, B. Ameduri, Polymeric materials as anion-exchange membranes for alkaline fuel cells, Progress in Polymer Science, 36 (2011) 1521-1557. [8] J.R. Varcoe, R.C. Slade, E.L.H. Yee, An alkaline polymer electrochemical interface: a breakthrough in application of alkaline anion-exchange membranes in fuel cells, Chemical Communications, (2006) 1428-1429. [9] K.A. Mauritz, R.B. Moore, State of understanding of Nafion, Chemical reviews, 104 (2004) 4535-4586. [10] S. Zhang, B. Zhang, D. Xing, X. Jian, Poly (phthalazinone ether ketone ketone) anion exchange membranes with pyridinium as ion exchange groups for vanadium redox flow battery applications, Journal of Materials Chemistry A, 1 (2013) 12246-12254. [11] C.G. Arges, V. Ramani, Two-dimensional NMR spectroscopy reveals cation-triggered backbone degradation in polysulfone-based anion exchange membranes, Proceedings of the National Academy of Sciences, 110 (2013) 2490-2495. [12] J. Wang, S. Li, S. Zhang, Novel hydroxide-conducting polyelectrolyte composed of an poly (arylene ether sulfone) containing pendant quaternary guanidinium groups for alkaline fuel cell applications, Macromolecules, 43 (2010) 3890-3896. [13] J. Cheng, G. Yang, K. Zhang, G. He, J. Jia, H. Yu, F. Gai, L. Li, C. Hao, F. Zhang, Guanidimidazole-quanternized and cross-linked alkaline polymer electrolyte membrane for fuel cell application, Journal of Membrane Science, 501 (2016) 100-108. [14] C. Qu, H. Zhang, F. Zhang, B. Liu, A high-performance anion exchange membrane based on bi-guanidinium bridged polysilsesquioxane for alkaline fuel cell application, Journal of Materials Chemistry, 22 (2012) 8203-8207. [15] S. Gu, R. Cai, Y. Yan, Self-crosslinking for dimensionally stable and solvent-resistant quaternary phosphonium based hydroxide exchange membranes, Chemical Communications, 47 (2011) 2856-2858. [16] D.W. Shin, M.D. Guiver, Y.M. Lee, Hydrocarbon-Based Polymer Electrolyte Membranes: Importance of Morphology on Ion Transport and Membrane Stability, Chemical Reviews, 117 (2017) 4759-4805. [17] N. Li, Y. Leng, M.A. Hickner, C.-Y. Wang, Highly stable, anion conductive, comb-shaped copolymers for

27

alkaline fuel cells, Journal of the American Chemical Society, 135 (2013) 10124-10133. [18] Q. Zhang, S. Li, S. Zhang, A novel guanidinium grafted poly (aryl ether sulfone) for high-performance hydroxide exchange membranes, Chemical Communications, 46 (2010) 7495-7497. [19] H.A. Kostalik IV, T.J. Clark, N.J. Robertson, P.F. Mutolo, J.M. Longo, H.c.D. Abruña, G.W. Coates, Solvent processable tetraalkylammonium-functionalized polyethylene for use as an alkaline anion exchange membrane, Macromolecules, 43 (2010) 7147-7150. [20] X. Li, Y. Yu, Q. Liu, Y. Meng, Synthesis and properties of anion conductive multiblock copolymers containing tetraphenyl methane moieties for fuel cell application, Journal of membrane science, 436 (2013) 202-212. [21] J. Zhou, J. Guo, D. Chu, R. Chen, Impacts of anion-exchange-membranes with various ionic exchange capacities on the performance of H 2/O 2 fuel cells, Journal of power sources, 219 (2012) 272-279. [22] C.H. Park, S.Y. Lee, D.S. Hwang, D.W. Shin, D.H. Cho, K.H. Lee, T.-W. Kim, T.-W. Kim, M. Lee, D.-S. Kim, Nanocrack-regulated self-humidifying membranes, Nature, 532 (2016) 480-483. [23] I. Rose, M. Carta, R. Malpass-Evans, M.-C. Ferrari, P. Bernardo, G. Clarizia, J.C. Jansen, N.B. McKeown, Highly permeable benzotriptycene-based polymer of intrinsic microporosity, ACS Macro Letters, 4 (2015) 912-915. [24] Z. Yang, R. Guo, R. Malpass‐Evans, M. Carta, N.B. McKeown, M.D. Guiver, L. Wu, T. Xu, Highly Conductive Anion‐Exchange Membranes from Microporous Tröger's Base Polymers, Angewandte Chemie, 128 (2016) 11671-11674. [25] B. Zhang, L. Li, G. He, F. Gai, F. Zhang, Imidazolium functionalized polysulfone electrolyte membranes with varied chain structures: a comparative study, RSC Advances, 6 (2016) 31336-31346. [26] J. Pan, S. Lu, Y. Li, A. Huang, L. Zhuang, J. Lu, High‐Performance Alkaline Polymer Electrolyte for Fuel Cell Applications, Advanced Functional Materials, 20 (2010) 312-319. [27] H. Chen, Y. Jean, L.J. Lee, J. Yang, J. Huang, Positron annihilation study in inorganic‐polymer nano‐ composites, physica status solidi (c), 6 (2009) 2397-2400. [28] Z. Wang, B. Wang, N. Qi, H. Zhang, L. Zhang, Influence of fillers on free volume and gas barrier properties in styrene-butadiene rubber studied by positrons, Polymer, 46 (2005) 719-724. [29] M. Eldrup, D. Lightbody, J. Sherwood, The temperature dependence of positron lifetimes in solid pivalic acid, Chemical Physics, 63 (1981) 51-58. [30] S. Tao, Positronium annihilation in molecular substances, The Journal of Chemical Physics, 56 (1972) 5499-5510. [31] C. Yang, S. Wang, W. Ma, S. Zhao, Z. Xu, G. Sun, Highly stable poly (ethylene glycol)-grafted alkaline anion exchange membranes, Journal of Materials Chemistry A, 4 (2016) 3886-3892. [32] G. Das, B.J. Park, H.H. Yoon, A bionanocomposite based on 1, 4-diazabicyclo-[2.2. 2]-octane cellulose nanofiber cross-linked-quaternary polysulfone as an anion conducting membrane, Journal of Materials Chemistry A, 4 (2016) 15554-15564. [33] W. Chen, X. Yan, X. Wu, S. Huang, Y. Luo, X. Gong, G. He, Tri-quaternized poly (ether sulfone) anion exchange membranes with improved hydroxide conductivity, Journal of Membrane Science, 514 (2016) 613-621. [34] X. Gong, X. Yan, T. Li, X. Wu, W. Chen, S. Huang, Y. Wu, D. Zhen, G. He, Design of pendent imidazolium side chain with flexible ether-containing spacer for alkaline anion exchange membrane, Journal of Membrane Science, 523 (2017) 216-224. [35] K.M. Hugar, H.A. Kostalik IV, G.W. Coates, Imidazolium cations with exceptional alkaline stability: A systematic study of structure–stability relationships, Journal of the American Chemical Society, 137 (2015) 8730-8737. [36] L. Cheng, L. Ying, J. Feng, C.Y. Wang, J.L. Li, Z. Xu, Novel heterocyclic poly (arylene ether ketone) s:

28

Synthesis and polymerization of 4‐(4′‐hydroxyaryl)(2H) phthalazin‐1‐ones with methyl groups, Journal of Polymer Science Part A: Polymer Chemistry, 45 (2007) 1525-1535. [37] M. Elangovan, S. Dharmalingam, A facile modification of a polysulphone based anti biofouling anion exchange membrane for microbial fuel cell application, RSC Advances, 6 (2016) 20571-20581. [38] Y. Kobayashi, W. Zheng, E. Meyer, J. McGervey, A. Jamieson, R. Simha, Free volume and physical aging of poly (vinyl acetate) studied by positron annihilation, Macromolecules, 22 (1989) 2302-2306. [39] J. Raj, C. Ranganathaiah, A free-volume study on the phase modifications brought out by e-beam and microwave irradiations in PP/NBR and PVC/SAN blends, Polymer Degradation and Stability, 94 (2009) 397-403. [40] Y. Jean, Positron annihilation spectroscopy for chemical analysis: a novel probe for microstructural analysis of polymers, Microchemical Journal, 42 (1990) 72-102. [41] M. Fujimura, T. Hashimoto, H. Kawai, Small-angle X-ray scattering study of perfluorinated ionomer membranes. 1. Origin of two scattering maxima, Macromolecules, 14 (1981) 1309-1315. [42] L. Gao, G. He, Y. Pan, B. Zhao, X. Xu, Y. Liu, R. Deng, X. Yan, Poly (2, 6-dimethyl-1, 4-phenylene oxide) containing imidazolium-terminated long side chains as hydroxide exchange membranes with improved conductivity, Journal of Membrane Science, 518 (2016) 159-167. [43] T. Gierke, G. Munn, F. Wilson, The morphology in nafion perfluorinated membrane products, as determined by wide‐and small‐angle x‐ray studies, Journal of Polymer Science: Polymer Physics Edition, 19 (1981) 1687-1704. [44] R. Wang, X. Yan, X. Wu, G. He, L. Du, Z. Hu, M. Tan, Modification of hydrophilic channels in nafion membranes by DMBA: mechanism and effects on proton conductivity, Journal of Polymer Science Part B: Polymer Physics, 52 (2014) 1107-1117. [45] A.N. Lai, Y.Z. Zhuo, C.X. Lin, Q.G. Zhang, A.M. Zhu, M.L. Ye, Q.L. Liu, Side-chain-type phenolphthalein-based poly (arylene ether sulfone nitrile) s anion exchange membrane for fuel cells, Journal of Membrane Science, 502 (2016) 94-105. [46] D. Guo, A.N. Lai, C.X. Lin, Q.G. Zhang, A.M. Zhu, Q.L. Liu, Imidazolium-Functionalized Poly (arylene ether sulfone) Anion-Exchange Membranes Densely Grafted with Flexible Side Chains for Fuel Cells, ACS Applied Materials & Interfaces, 8 (2016) 25279-25288. [47] A.H. Rao, S. Nam, T.-H. Kim, Comb-shaped alkyl imidazolium-functionalized poly (arylene ether sulfone) s as high performance anion-exchange membranes, Journal of Materials Chemistry A, 3 (2015) 8571-8580. [48] E.A. Weiber, P. Jannasch, Polysulfones with highly localized imidazolium groups for anion exchange membranes, Journal of Membrane Science, 481 (2015) 164-171. [49] Y. Yang, D.M. Knauss, Poly (2, 6-dimethyl-1, 4-phenylene oxide)-b-poly (vinylbenzyltrimethylammonium) diblock copolymers for highly conductive anion exchange membranes, Macromolecules, 48 (2015) 4471-4480. [50] B. Xue, X. Dong, Y. Li, J. Zheng, S. Li, and S. Zhang, Synthesis of novel guanidinium-based anion-exchange membranes with controlled microblock structures, Journal of Membrane Science 537 (2017) 151-159. [51] E. Kim, S. Lee, S. Woo, S.-H. Park, S.-D. Yim, D. Shin, and B. Bae, Synthesis and characterization of anion exchange multi-block copolymer membranes with a fluorine moiety as alkaline membrane fuel cells, Journal of Power Sources 359 (2017) 568-576. [52] W. Liu, L. Liu, J. Liao, L. Wang, and N. Li, Self-crosslinking of comb-shaped polystyrene anion exchange membranes for alkaline fuel cell application, Journal of Membrane Science 536 (2017) 133-140. [53] X. Dong, D. Lv, J. Zheng, B. Xue, W. Bi, S. Li, and S. Zhang, Pyrrolidinium-functionalized poly(arylene ether sulfone)s for anion exchange membranes: Using densely concentrated ionic groups and block design to improve membrane performance, Journal of Membrane Science 535 (2017) 301-311. [54] J. Xue, L. Liu, J. Liao, Y. Shen, and N. Li, UV-crosslinking of polystyrene anion exchange membranes by

29

azidated macromolecular crosslinker for alkaline fuel cells, Journal of Membrane Science 535 (2017) 322-330. [55] M. Irfan, E. Bakangura, N.U. Afsar, M.M. Hossain, J. Ran, and T. Xu, Preparation and performance evaluation of novel alkaline stable anion exchange membranes, Journal of Power Sources 355 (2017) 171-180. [56] D. Lu, D. Li, L. Wen, and L. Xue, Effects of non-planar hydrophobic cyclohexylidene moiety on the structure and stability of poly(arylene ether sulfone)s based anion exchange membranes, Journal of Membrane Science 533 (2017) 210-219. [57] J. Pan, J. Han, L. Zhu, and M.A. Hickner, Cationic Side-Chain Attachment to Poly(Phenylene Oxide) Backbones for Chemically Stable and Conductive Anion Exchange Membranes, Chemistry of Materials 29 (2017) 5321-5330. [58] W.-H. Lee, E.J. Park, J. Han, D.W. Shin, Y.S. Kim, and C. Bae, Poly(terphenylene) Anion Exchange Membranes: The Effect of Backbone Structure on Morphology and Membrane Property, ACS Macro Letters 6 (2017) 566-570. [59] J. Han, L. Zhu, J. Pan, T.J. Zimudzi, Y. Wang, Y. Peng, M.A. Hickner, and L. Zhuang, Elastic Long-Chain Multication Cross-Linked Anion Exchange Membranes, Macromolecules 50 (2017) 3323-3332. [60] D.J. Strasser, B.J. Graziano and D.M. Knauss, Base stable poly(diallylpiperidinium hydroxide) multiblock copolymers for anion exchange membranes, J. Mater. Chem. A, 2017, 5, 9627–9640 [61] C.Yang, L. Liu, X. Han, Z. Huang, J. Dong and N. Li, Highly anion conductive, alkyl-chain-grafted copolymers as anion exchange membranes for operable alkaline H2/O2 fuel cells, J. Mater. Chem. A, 2017, 5, 10301–10310 [62] N. Li, L. Wang, M. Hickner, Cross-linked comb-shaped anion exchange membranes with high base stability, Chemical communications, 50 (2014) 4092-4095. [63] X. Li, S. Cheng, L. Wang, Q. Long, J. Tao, G. Nie, S. Liao, Anion exchange membranes by bromination of benzylmethyl-containing poly (arylene ether) s for alkaline membrane fuel cells, RSC Advances, 4 (2014) 29682-29693. [64] M. Zhang, J. Liu, Y. Wang, L. An, M.D. Guiver, N. Li, Highly stable anion exchange membranes based on quaternized polypropylene, Journal of Materials Chemistry A, 3 (2015) 12284-12296. [65] Z. Luo, Y. Gong, X. Liao, Y. Pan, H. Zhang, Nanocomposite membranes modified by graphene-based materials for anion exchange membrane fuel cells, RSC Advances, 6 (2016) 13618-13625. [66] Y. Huang, W. Zhang, J. Wang, Z. Wei, Probing the sensory property of perylenediimide derivatives in hydrazine gas: core-substituted aromatic group effect, ACS applied materials & interfaces, 6 (2014) 9307-9313. [67] K. Shen, J. Pang, S. Feng, Y. Wang, Z. Jiang, Synthesis and properties of a novel poly (aryl ether ketone) s with quaternary ammonium pendant groups for anion exchange membranes, Journal of membrane science, 440 (2013) 20-28. [68] B. Lin, L. Qiu, B. Qiu, Y. Peng, F. Yan, A soluble and conductive polyfluorene ionomer with pendant imidazolium groups for alkaline fuel cell applications, Macromolecules, 44 (2011) 9642-9649. [69] S. Bondareva, V. Lisitskii, N. Yakovtseva, Y.I. Murinov, Hydrolysis of 1, 2-disubstituted imidazolines in aqueous media, Russian Chemical Bulletin, 53 (2004) 803-807.

Highlights:  A novel AEM with bend-twist muliblock backbone was designed and fabricated;  The bend-twist structure created high free volume as confirmed by DSC and PALS;  Free volume worked together with microphase separation to boost ion 30

transport;  The bend-twist structured AEM showed high conductivity and low swelling at low IEC.  A high fuel cell power density was obtained with the fabricated AEM.

31