Enhanced conductivity and stability via comb-shaped polymer anion exchange membrane incorporated with porous polymeric nanospheres

Journal Pre-proof Enhanced conductivity and stability via comb-shaped polymer anion exchange membrane incorporated with porous polymeric nanospheres Shuai Zhang, Xiuling Zhu, Yajie Wang, Xueqiang Gao, Pinyang Liu, Xinyu Wang PII:

S0376-7388(19)32364-6

DOI:

https://doi.org/10.1016/j.memsci.2019.117750

Reference:

MEMSCI 117750

To appear in:

Journal of Membrane Science

Received Date: 30 July 2019 Revised Date:

9 December 2019

Accepted Date: 13 December 2019

Please cite this article as: S. Zhang, X. Zhu, Y. Wang, X. Gao, P. Liu, X. Wang, Enhanced conductivity and stability via comb-shaped polymer anion exchange membrane incorporated with porous polymeric nanospheres, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2019.117750. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

Enhanced conductivity and stability via comb-shaped polymer anion exchange membrane incorporated with porous polymeric nanospheres 1

H

Graphic abstract

1

Enhanced conductivity and stability via comb-shaped polymer

2

anion exchange membrane incorporated with porous polymeric

3

nanospheres

4

Shuai Zhanga, Xiuling Zhu*a, Yajie Wanga, Xueqiang Gaob, Pinyang Liua, Xinyu Wanga

5 6

a State Key Lab of Fine Chemicals, Department of Polymer Science & Materials, Dalian

7

University of Technology, Dalian 116024, P R China

8

b Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese

9

Academy of Sciences, Dalian, China

10 11 12 13 14 15 16 17 18 19 20

Corresponding authors

21

Tel.: + 86-411-84986095

22

Fax: + 86-411-84986095

23

E-mail address: [email protected]

24 25

1

Abstract

2

The

3

poly(biphenyl-alkylene)s (PB-g-PipVBC) were synthesized by super-acid catalyzed Friedel-Crafts

4

polycondensation and atom transfer radical polymerization (ATRP). To further construct ion

5

transport channels and improve ion conductivity, the composite membranes were fabricated via

6

doping the pre-designed ionic porous polymeric nanospheres. The resulting composite membrane

7

loaded with 1wt% nanospheres showed high chloride conductivity of 65.6 mS·cm-1 at 80

8

benefiting from the ordered ion conductive channel and ionic nanoaggregates. Meanwhile, the

9

composite membranes doped with nanospheres all exhibited acceptable alkaline stability and

comb-shaped

benzyl

piperidinium

cations

functionalized

aryl-ether

bonds-free

,

10

maintained above 40% of the original ion conductivity after soaking in 1 M NaOH at 80

for

11

1000 h, and the oxidative durable remaining mass retained above 75% for 1 h in 80

12

reagent. Among them, the composite membrane loaded with 3wt% nanospheres possessed the

13

highest alkaline stability (66.3% of the original hydroxide conductivity after storage in 1 M NaOH

14

for 1000 h), which was ascribed to morphology and the absence of alkaline labile aryl-bonds in

15

polymer backbone. Moreover, it exhibited a peak power density of 77.3 mW·cm-2 at 143.4

16

mA·cm-2 in a direct borohydride fuel cell, which was higher than that of Nafion®211 (67.4

17

mW·cm-2 at 134.8 mA·cm-2). Therefore, the poly(biphenyl-alkylene)s and ionic polymeric

18

nanospheres grafting with benzyl piperidinium cations by ATRP have great potential for designing

19

anion exchange membranes in diverse applications.

Fenton’s

20 21

Keywords: super-acid catalyzed Friedel-Crafts polycondensation; nanospheres; atom transfer

22

radical polymerization; anion exchange membrane; alkaline stability

23

1

1

1. Introduction

2

The shortage of energy has become a hot issue worldwide, and the emergence of clean energy

3

is a good solution for this problem. Among them, fuel cells are considered as a promising

4

approach which have attracted more and more attentions. Compared with the proton exchange

5

membranes-based fuel cells, the anion exchange membranes fuel cells (AEMFCs) are popular

6

among the field of ion exchange membranes due to the usage of nonprecious metal catalysts and

7

higher oxygen reduction kinetics [1-3]. Specially, the more efficient water management is

8

introduced into anion exchange membrane fuel cells (AEMFCs) [4, 5]. Additionally, AEMs have

9

great potential in application of direct sodium borohydride/hydrogen peroxide fuel cells (DBHFCs)

10

due to their high energy density and long-term stability [6, 7]. However, the insufficient chemical

11

stability and hydroxide ion conductivity are also the major dilemma for the widespread application

12

of AEMFCs and DBHFCs. To address these issues, developing AEMs with high ionic conductivity

13

and durability become indispensable requirement for obtaining membranes with excellent

14

performance [8, 9].

15

Generally, various aromatic polymer and cationic species are employed to construct AEMs.

16

For example, the quaternary ammonium [10], imidazolium [11], guanidinium [12], sulfonium [13],

17

phosphonium [14] and metallonium cations [15] were tethered to aromatic polymers for

18

developing high-performance AEMs [16]. Among them, vinylbenzyl quaternary ammoniums were

19

usually used as monomers for preparing radiation-grafting AEMs, polyethylene-based AEMs and

20

atom transfer radical polymerization-grafting AEMs [17-19]. For example, Biancolli et al.

21

reported ETFE-based AEMs with different head-group, which exhibited high power density

22

ranging among 1-2 W·cm-2. Furthermore, they pointed out that the benzyl-N-methylpiperdinium

23

might be a focus in future research [20]. Lin et al. developed the AEMs with N-methylpiperdine

24

functionalized poly(vinylbenzyl chloride) and polybenzimidazole. The results displayed the 10.7%

25

of conductivity loss after alkaline treatment, which can be ascribed to usage of cycloaliphatic

26

cation [21]. In addition, Marino et al. investigated the alkaline stability of 26 different quaternary

27

ammonium cations, which revealed the alkaline stability of N-benzyl-N-methylpiperidinium was

28

higher

29

N-methylpiperdiniums as monomers for ATRP-graft AEMs is a practicable method for

30

constructing alkaline durable AEMs.

than

bezyltrimethylammonium

[16].

2

Therefore,

developing

vinylbenzyl

1

Another important factor for the chemical stability of AEMs is the alkaline durable polymer

2

backbones. A variety of polymer backbones have been investigated as AEMs materials, including

3

polysulfone [22, 23], poly(phenylene oxide) (PPO) [24, 25], poly(arylene ethers) [26, 27],

4

poly(styrene)s [28, 29], poly(phenylene)s [30, 31] and poly(olefin)s [32, 33]. However, the

5

arylene ether bonds in AEMs are susceptible to hydroxide ions, which will impair the alkaline

6

stability of AEMs [9, 34, 35]. For example, Chen et al. reported a polysulfone-based AEMs

7

functionalized by hexamethylenetetramine. These membranes retained 86% of the initial

8

hydroxide conductivity after storage in 1 M KOH at 60

9

prepared a series of poly(arylene ether sulfone) block copolymer tethered with benzyl-quaternary

for 168 h [36]. Zhang et al. also

10

ammonium groups, which exhibited the hydroxide conductivity of 86.3 mS·cm-1 at 80

11

remained about 84% of hydroxide conductivity after storage in 1 M NaOH at 80

12

In order to enhance the alkaline stability, PPO-based AEMs are introduced by bromination and

13

quaternization. Xu’s group investigated PPO-based membranes by grafting dual hydrophobic

14

chains. The resulting membranes exhibited high hydroxide conductivity of 61 mS·cm-1 at 30

15

and maintained 83.5% of the original ion exchange capacity (IEC) after storage in 2 M NaOH for

16

1000 h [38]. However, these types of membranes almost used toxic reagents such as

17

chloromethylation or benzylic bromination reagents, which possess carcinogenicity and easily

18

cause gelation. To further improve the alkaline stability of AEMs, the aromatic ether bonds-free

19

polymer backbones were developed as AEMs. Bae’s group explored the poly(biphenyl alkylene)s

20

as AEMs, which did not contain alkaline labile C-O bonds in polymer backbone. The AEMs

21

showed high hydroxide ion conductivity of 120 mS·cm-1 at 80

22

structure in 1 M NaOH at 80

23

chloromethylation reagents [39, 40]. Thus it can be seen that the poly(biphenyl alkylene)s have

24

great potential for developing AEMs.

and

for 144 h [37].

and no change in chemical

. Furthermore, the whole procedure did not involve

25

In addition, constructing effective ion conductivity channel is a key factor to achieve high

26

hydroxide ion conductivity. Xu’s group developed a series of comb-shaped copolymers as AEMs

27

by atom transfer radical polymerization (ATRP) [41, 42]. Most of the resultant membranes

28

exhibited high hydroxide ion conductivity. Among them, the highest hydroxide ion conductivity

29

reached 55 mS·cm-1 at 30

30

for 25 days [43]. Therefore, the ATRP could provide an effective approach to functionalize

and retained 80% of original IEC after soaking in 2 M NaOH at 60

3

1

membranes. On the other hand, the ionic functionalized nanospheres synthesized by ATRP are

2

doped into membranes for fabricating the interconnected ionic channels. He et al. took the

3

core-shell nanoarchitecture composed of SiO2/quaternary ammonium functionalized polystyrene

4

as filler for doping into nonionic polymer membranes. The resulting membranes displayed high

5

hydroxide conductivity of 188.1 mS·cm-1 at 80

6

about 77% of the initial hydroxide conductivity after storage in 1 M NaOH at 60

7

which revealed the relatively poor alkaline durability due to the usage of polysulfone as polymer

8

backbone. Hence, it was benefit to construct good microphase separation and improve ion

9

conductivity through by doping the ionic nanoparticles into membranes.

[44]. However, these membranes only remained for 480 h,

10

Inspired by aryl ether-free polymer and predesigned ionic nanoaggregates, the bromomethane

11

tethered poly(biphenyl-alkylene)s were synthesized by super acid catalyzed Friedel-Crafts

12

polycondensation and denoted as PB-Br, and then, PB-Br was used as macroinitiator for grafting

13

with piperidinium functionalized 4-vinylbenzyl chloride (PipVBC) to obtain the PB-g-PipVBC

14

copolymer through ATRP (Scheme 1). Subsequently, the pre-synthesized porous nanospheres

15

were coated by PipVBC via ATRP and doped into the PB-g-PipVBC copolymer to fabricate the

16

composite AEMs (Scheme 2). Different amounts of PipVBC were used to prepare the

17

PB-g-PipVBC copolymer and the optimum copolymer was selected as pristine polymer for

18

constructing composite membranes. Specifically, the preparation of comb-shaped copolymers

19

avoids using the chloromethylation reagents and do not contain the aryl-ether bonds in polymer

20

backbone. In addition, the ionic porous polymeric nanospheres were used as filler to construct

21

interconnected hydroxide-conducting channels and microphase separation morphology. The

22

composite membranes showed enhanced hydroxide conductivity, alkaline stability and mechanical

23

properties.

4

1 2

Scheme 1 The synthesis process of PB-Br by super-acid catalyst polycondensations and

3

comb-shaped PB-g-PipVBC copolymer by ATRP.

4 5

Scheme 2 Schematic diagram for fabricating the PB-msphere composite membranes (m represents

6

different percentages of nanospheres mass in composite membranes).

7

2. Experimental section

8

2.1 Materials and methods

9

2.1.1 Materials

10

4-vinylbenzyl chloride (VBC, 90%, stabilized with p-tert-butylcatechol, 2-nitro-p-cresol) and

11

divinylbenzene (DVB, 80%, stabilized with p-tert-butylcatechol) were purchased from Energy

12

Chemical and purified by a basic alumina column to remove the inhibitor. Biphenyl (99%),

13

3-bromo-1,1,1-trifluoroacetone (98%), N-methylpiperidine (98%), pentamethyldiethylenetriamine

14

(PMDETA, 98%), trifluoroacetic acid (TFA), trifluoromethanesulfonic acid (TFSA, 99%),

15

2,2'-Azobis(2-methylpropionamide) dihydrochloride (V-50, 98%), sodium tungstate dihydrate 5

1

(Na2WO4·2H2O), silver nitrate (AgNO3) and copper (I) bromide (CuBr, 99.5%) were obtained

2

from

3

N-Dimethylformamide (DMF), methanol, acetone and dimethyl sulfoxide (DMSO) were all of

4

analytical grade and used as received.

5

2.1.2 Synthesis of piperidinium functionalized 4-vinylbenzyl chloride (PipVBC)

Energy

Chemical

and

used

as

received

without

further

purification.

N,

6

A solution of N-methylpiperidine (7.8 mL, 0.064 mol) in 30 mL acetone was added into

7

two-neck round bottom flask. The VBC (9.05 mL, 0.064 mol) in 10 mL acetone was dropped into

8

the above solution during two hours. The reaction was kept at room temperature until the

9

formation of white precipitate. Then, the precipitate was filtered and washed by acetone for three

10

times. The product was dried under vacuum at 60

overnight. The chemical structure of the

11

product was characterized by H-nuclear magnetic resonance (1H-NMR) as shown in Figure S1.

12

2.1.3 Preparation of nanospheres and piperidinium functionalized nanospheres

13

The nanospheres were made up of poly(4-chloromethyl styrene) and DVB, which were

14

prepared according to the reported literature [45]. The VBC (12 mL) and divinylbenzene were

15

added into deionized water (250 mL) in the three-neck round bottom flask equipped with

16

mechanical stirring and N2 atmosphere. After the solution was fully mixed, an aqueous solution of

17

V-50 (0.2 g in 2 mL deionized water) was added into the above mixture and kept at 75

18

Finally, the product was obtained after centrifugation and dried under vacuum at 60

19

and denoted as PCMS.

for 4 h. overnight,

20

Preparation of the crosslinked nanospheres: The PCMS was immersed in dichloroethane

21

(50 mL) overnight. 3.7 g of FeCl3 in 10 mL dichloroethane was added into the above mixture and

22

kept at 80

23

and denoted as xPCMS.

for 24 h. The product was filtered, washed and dried under vacuum at 60

overnight,

24

Synthesis of piperidinium functionalized nanospheres: xPCMS (0.3 g), PMDETA (0.24 g),

25

PipVBC (1.5 g) and DMF (20 mL) were added into Schlenk flask under N2 atmosphere. The

26

solution was bubbled with N2 atmosphere and conducted by three freeze-pump-thaw cycles to

27

remove oxygen. Then, CuBr (0.12 g) was added into the mixed solution. The polymerization

28

reaction was conducted at 80

29

and exposure to air. The product was collected by centrifugation and washed with 0.01 M HCl

30

aqueous solution. Finally, the product was dried under vacuum at 80

for 12 h. Subsequently, the reaction was stopped under cool water

6

overnight, and denoted as

1

xPCMS-g-PipVBC.

2

2.1.4 Synthesis of bromomethane tethered poly(biphenyl-alkylene)s

3

Briefly, the mixture of 1-bromo-3,3,3-trifluoroacetone (5.5 mmol), biphenyl (5 mmol) and

4

dichloromethane (4.8 mL) were placed in a three-neck flask and kept in ice bath. 4.3 mL of TFSA

5

was added into the above solution, and kept the reaction at 0

6

warmed to room temperature and kept at this temperature for 24 h. Finally, the gel-like mass was

7

obtained and washed thoroughly with methanol. Subsequently, the product was dissolved in DMF

8

and precipitated in methanol. The white fiber-like product can be obtained from precipitate.

9

Finally, the white fiber was dried under vacuum at 60

for 1 h. Following the reaction was

overnight, and denoted as PB-Br. The

10

intrinsic viscosity ([η]) of PB-Br is 0.675 dL·g-1 in DMAC at 25

11

Ubbelohde viscometer.

12

2.1.5 Synthesis of AEMs

as determined by an

13

Synthesis of PB-g-nPipVBC copolymers: PB-g-nPipVBC copolymers (n represents

14

different percentages of PipVBC) were prepared by ATRP using PB-Br as macroinitiator and

15

PipVBC as monomers. In brief, PB-Br (0.6 g) dissolved in DMF (20 mL) and a certain amount of

16

PipVBC dissolved in methanol (1 mL) were mixed together. Then, the mixture was placed in

17

Schlenk flask and degassed by N2 bubbling for 1 h. Subsequently, CuBr (0.12 g) and PMDETA

18

(0.24 g) were added into the mixture under N2 atmosphere, and three freeze-pump-thaw cycles

19

were performed to remove oxygen. The polymerization reaction was performed at 110

20

The final PB-g-nPipVBC copolymers were obtained after filtration and washing. The products

21

were dried under vacuum at 80

22

for 12 h.

overnight.

Preparation of AEMs: 0.2 g of PB-g-3PipVBC was dissolved in 5 mL of DMSO. An

23

appropriate amount of xPCMS-g-PipVBC was dispersed into the copolymer solution at 60

, and

24

then the mixture was stirred for 1 h. Subsequently, the mixture was distributed uniformly via

25

ultrasonic-dispersion for 10 h. The homogenous dispersion was casted on a Petri dish and allowed

26

to remove the solvent at 60

27

deionized water, and denoted as PB-msphere (m represents different percentages of nanospheres

28

mass in composite membranes). The thickness of membranes was controlled at 35-50 µm.

29

2.2 Membrane characterization

30

2.2.1 Characterization of chemical composition of AEMs

overnight. The AEMs were obtained by soaking the membranes in

7

1

The chemical structure of AEMs was characterized by Fourier transform infrared 1

2

spectroscopy (FT-IR) and

H-NMR. The FT-IR spectra was performed on Nexus Euro

3

spectrometer in the range of 400-4000 cm-1 (32 scans at 4 cm-1 resolution). The 1H-NMR was

4

conducted on a Bruker instrument at 500 MHz with D2O or DMSO-d6 as solvent and

5

tetramethylsilane as an internal standard. The X-ray photoelectron spectroscopy (XPS, Thermo

6

Fish, ESCALAB XI+) was used to evaluate the chemical structure of AEMs after alkaline

7

treatment.

8

2.2.2 Morphology characterization

9

The morphology of polymeric nanospheres and AEMs was observed by field emission

10

scanning electron microscopy (FE-SEM, JSM-7001F). The cross section of prepared AEMs was

11

cut by freeze-fractured in liquid nitrogen before measurement. The microphase structure of

12

prepared AEMs was recorded on transmission electron microscopy (TEM, JEOL JEM-2000EX).

13

The AEMs were immersed in 1 M Na2WO4 solution at 30

14

water before tests.

15

2.2.3 Mechanical and thermal properties

for 48 h, and washed with deionized

16

The membranes were tailored to 40 mm × 10 mm sheets. Subsequently, the tensile strength

17

and tensile elongation of the membrane were measured by Instron Model 1122 with the tensile

18

rate of 10 mm·min-1 at 52% relative humidity.

19

The thermal properties of AEMs were characterized by thermogravimetric analysis (TGA,

20

Netzsch 209C). Before test, all the membranes were dried under vacuum at 80

21

heat treatments were performed from 25 to 800

22

atmosphere.

23

2.2.4 Water uptake and swelling ratio

at a heating rate of 10

overnight. The ·min-1 under N2

24

The water uptake (WU) of AEMs is directly connected with hydroxide ion conductivity.

25

Before test, the prepared AEMs were immersed in 1 M NaCl aqueous solution. Following, these

26

membranes were soaked in 1 M NaOH for 24 h and washed by deionized water to remove the

27

excess of NaOH solution. To characterize WU, the AEMs in OH- form was dried under vacuum at

28

80

29

were immersed in deionized water for 12 h at 20, 40, 60 and 80

30

obtained by quickly wiping the residual water of wet membranes and weighing the weight of the

until the weight of AEMs has no change and recorded as WOH-. Subsequently, these AEMs

8

, respectively. The W′OH- was

1

membranes. The WU was finally calculated as: × 100%........................................................................................................(1)

2

=

3

The swelling ratio (SR) is an important parameter to evaluate dimension stability of AEMs.

4

The length of dry membranes was measured and recorded as Ld. Then, these samples were soaked

5

in deionized water for 12 h at 20, 40, 60 and 80

6

was measured and recorded as Lw. The SR was calculated as:

7 8

=

, respectively, and the length of wet membranes

× 100%.................................................................................................................(2)

2.2.5 IEC and ionic conductivity

9

The IEC of AEMs was measured by Mohr’s titration method. The membranes in Cl- form

10

were immersed in 25 mL of 0.2 M NaNO3 aqueous solution for 24 h. Next, the solution was

11

titrated using K2CO4 as indicator and the volume of the consumed 0.01 M aq. AgNO3 was

12

recorded as VAgNO3. The IEC was calculated as: .

×

13

=

14

The ionic conductivity was measured by electrochemical impedance spectroscopy in the

…………………………………………………………………………....(3)

15

deionized water between 20 and 80

16

China) at 10 mV and 1 to 105 Hz. The ionic conductivity was measured by two-probe technique

17

for measurement of in-plane conductivity and calculated as:

18

by employing a CHI660C workstation (Chenhua, Shanghai,

!

= "×#………………………………………………………………………………………..(4)

19

where l (cm) is the length between two platinum electrodes, A (cm2) is the cross-sectional

20

area of membranes, and R (Ω) is the membranes resistance measured by electrochemical

21

impedance spectroscopy.

22

2.2.6 Oxidative stability and alkaline stability measurement

23 24 25

The oxidative stability of AEMs was evaluated by the changes in weight of membranes after soaking in Fenton’s reagent (4 ppm FeSO4 and 3% H2O2) at 80

The alkaline stability was monitored by changes in OH- form and IEC of membrane after

26

alkaline treatment in 1 M aq. NaOH at 80

27

2.2.7 Fuel cell test

28

for 1 h.

for 1000 h.

To evaluate the performance of prepared membranes, the prepared membranes in OH- form 9

1

were used as separator for constructing the DBHFCs. The anode was consist of a Pt foil (0.25 cm2,

2

Gaossunion) and 30 mL of 1 M NaBH4 in 4 M NaOH solution. Here, the cathodic side was made

3

of a Pt foil (5 cm2) and 30 mL of 5 M H2O2 in 1.5 M HCl solution. The fuel cells were assembled

4

by two fuel tanker and the prepared AEMs with an active area of 1.8 cm2, as shown in Figure S2.

5

The fuel cell test was conducted in linear sweep voltammetry at room temperature.

6

3. Results and discussion

7

3.1 Characterization of porous polymeric nanospheres

8

The

porous

polymeric

nanospheres

were

prepared

via

surfactant-free

emulsion

9

polymerization and ATRP with 4-chloromethylstyrene and PipVBC as monomers and DVB as

10

precrosslinker according to the previous reporter [45]. FT-IR spectra provide overwhelming

11

evidence of successful grafting of PipVBC from xPCMS, where the new adsorption peak around

12

937 cm-1 corresponds to C-N of piperidinium in Figure 1. Furthermore, the amount of N-atom in

13

xPCMS-g-PipVBC is 1.09% as determined by elemental analysis. Therefore, the polymeric

14

spheres are successfully modified with PipVBC monomers.

Trasmittance / %

xPCMS-g-PipVBC C-N

xPCMS

PCMS

3500

15 16

3000

2500

2000

1500

Wavenumber / cm-1

1000

Figure 1 FT-IR spectra of PCMS, xPCMS and xPCMS-g-PipVBC nanospheres.

17

The morphology of spheres is observed by SEM. As shown in Figure 2a, the PCMS

18

nanospheres exhibit a regular spherical shape and the diameter is 587 nm as calculated by the

19

SEM images (Figure S3a). After achieving hyper-cross-linking, the xPCMS keeps the spherical

20

shape (Figure 2b), while the diameter decreases to 547 nm as shown in Figure S3b. However, the

21

xPCMS-g-PipVBC exhibits agglomerate and irregularly spherical particles after grafting with

22

PipVBC monomers (Figure 2c). Results indicate that the introduction of PipVBC changes the

23

morphology of xPCMS and prove that the grafting reaction is finished successfully. In addition,

24

the pore structure of nanospheres was characterized by nitrogen adsorption analysis. The PCMS 10

1

nanospheres exhibit a Brunaure-Emmett-Teller (BET) surface area as low as 0.542 m2·g-1,

2

indicating that PCMS possesses nonporous property. The BET surface area of xPCMS-g-PipVBC

3

is 127.5 m2·g-1, which is superior to that of PCMS. Furthermore, as shown in Figure S4, the

4

dimeter of micropores is around 0.58 nm for xPCMS-g-PipVBC. Results suggest that PipVBC

5

have successfully grafted from xPCMS and formed micropore structure. Therefore, the

6

xPCMS-g-PipVBC would adsorb more polymer chains and is beneficial for performance of the

7

composite membranes due to the existence of micropores.

8 9 10

Figure 2 SEM images of (a) PCMS, (b) xPCMS and (c) xPCMS-g-PipVBC nanospheres. 3.2 Synthesis and characterization of membranes

11

The AEMs were prepared using PB-Br as backbones and PipVBC as monomers. The

12

amounts of monomer that react with PB-Br polymer played an important role in performance of

13

AEMs. Hence, different amounts of PipVBC were used in reaction and the performance of AEMs

14

was investigated. To ensure the good performance of AEMs, the detailed conditional optimization

15

experiment of PB-g-nPipVBC was provided in Supporting Information. Here, the

16

PB-g-3PipVBC was chosen as pristine materials to form the AEMs due to its good properties.

17

1

H-NMR spectrum is used to investigate the chemical composition of PipVBC. As shown in

18

Figure S1, the signals appear at 7.59 and 7.49 ppm are ascribed to the aromatic protons. The

19

signals located at 6.82, 5.96 and 5.43 ppm are assigned to the three-olefin proton. The signal at

20

4.45 ppm is attributed to the methylene close to benzene ring. The signals at 3.30, 1.94, 1.73 and

21

1.61 ppm are corresponding to the methylene of piperidinium ring. Additionally, the signal located

22

at 2.94 ppm is owing to the methyl groups. All results suggest the successful syntheses of PipVBC.

23

The 1H-NMR spectra of PB-Br and PB-g-3PipVBC are shown in Figure 3. The signals located at

24

7.76 and 7.43 ppm are ascribed to the aromatic protons belonging to the polymer backbone. The

25

signal at 6.57 ppm in the spectrum of the graft copolymer is attributed to the benzene ring of

26

PipVBC. Additionally, the signals appear at 3.36, 2.93, 1.85 and 1.64 ppm are assigned to the 11

1

methylene and methyl protons of piperidinium ring. The signal at 1.23 ppm is due to the

2

methylene of side group attached to the polymer backbone. Therefore, the results reveal that

3

PipVBC is successfully introduced to PB-Br. The graft amounts are estimated according to the

4

equation (6):

5

$%& '( $'

=

)* "+'

……………………………………………………………………………….(6)

6

where A, B, and F represent the integral areas of “a”, “b” and “f” peaks in Figure 3(a),

7

respectively. As shown in Figure S5(a), the amount of grafting is 20%, 42% and 58% for

8

PB-g-3PipVBC, PB-g-5PipVBC and PB-g-6PipVBC, respectively. After ATRP reaction, the signal

9

at 4.68 ppm (denoted as “l+k”) corresponds to the methylene or methine protons close to benzene

10

rings. The peak intensity increases with increasing amount of monomer due to introduction of

11

-CH2- units in PB-g-nPipVBC copolymers. The increasement amount of -CH2- units is 1.24, 1.48

12

and 1.76 for PB-g-3PipVBC, PB-g-5PipVBC and PB-g-6PipVBC, respectively. This result reveals

13

that the PB-g-nPipVBC copolymers are successfully prepared via a “grafting from” approach.

14

FT-IR spectra is also used to characterize the chemical structure of prepared AEMs. As

15

shown in Figure 3b, the typical bands at 1733, 1601 and 1500 cm-1 are assigned to the C=C

16

stretching vibration of benzene ring [46]. The bands at 937 and 1035 cm-1 are ascribed to the C-N

17

bond of piperidinium, revealing the pristine membrane is successfully prepared. Additionally, the

18

bands at 1214 and 650 cm-1 are attributed to the wagging vibration of -CH2Br and stretching

19

vibration of C-Br bonds, respectively [47]. The broad peak around 3400 cm-1 is owing to the O-H

20

groups of absorbed water from the environment. Furthermore, the FT-IR spectra of different

21

amounts of PipVBC functionalized membranes exhibit similar results as shown in Figure S5b.

22

These results confirm the successful preparation of the composite membranes.

12

(a)

~

(b)

~

n

l+k

h

g

i j c,d,e

PB-g-PipVBC

Transmittance / %

a b

f

PB-5sphere

H2O DMSO-d6

PB-3sphere

PB-1sphere

PB-g-3PipVBC

PB-Br

8

1

6

4

2

3500

3000

2500

2000

1500

1000

Wavenumber / cm-2

Chemical shift / ppm

2

Figure 3 The 1H-NMR spectras of PB-g-PipVBC and PB-Br (a) and FT-IR spectra of composite

3

membranes (b)

4

3.3 Membrane morphology

5

The morphology of prepared AEMs affects the mechanical properties and ionic conductivity.

6

Hence, SEM and TEM were used to study the microphase morphology of the membranes. The

7

surface and cross-section morphology of prepared membranes was characterized by SEM and

8

shown in Figure 4. The PB-g-3PipVBC membrane (Figure 4a) shows craters surface, which is

9

due to the electrostatic ionic complexation and high evaporation rate of the solvent [48, 49]. After

10

doping 1% of nanospheres into the PB-g-3PipVBC membrane, the PB-1sphere membrane exhibits

11

a smooth and dense surface, which is beneficial for avoiding the gas permeation (Figure 4b).

12

When the amount of nanospheres is 3%, the surface of PB-3sphere membrane becomes rough and

13

irregular (Figure 4c). Moreover, cracks occur on the surface of PB-5sphere membrane when the

14

amount of nanospheres is as high as 5%, as shown in Figure 4d. The spherical xPCMS-g-PipVBC

15

is clearly observed in the surface of PB-5sphere membrane, which is caused by the introduction of

16

more nanospheres. Meanwhile, they also bring about generation of crack in the composite

17

membranes. Furthermore, the cross-section of the prepared membranes was also investigated by

18

SEM as shown in Figure 5. PB-g-3PipVBC membrane shows uniform and wrinkle surface

19

(Figure 5a). After doping into the membranes, some nanospheres are clearly observed on the

20

cross-section, indicating the good compatibility of composite membranes (Figures 5b-d). All the

21

results reveal that the membranes doped with low content of nanospheres have more uniform 13

1

morphology and compatible structure.

2 3

Figure 4 SEM images of (a) PB-g-3PipVBC, (b) PB-1sphere, (c) PB-3sphere, and (d)

4

PB-5sphere.

14

1 2

Figure 5 The cross-section SEM images of (a) PB-g-3PipVBC, (b) PB-1sphere, (c) PB-3sphere,

3

and (d) PB-5sphere.

4

Additionally, the microphase structure of AEMs was characterized by TEM. Figure 6

5

exhibits the distinct microphase separation, in which the bright regions represent hydrophobic

6

phase contained in the polymer backbone. Meanwhile, the dark regions represent hydrophilic

7

phase assigned to side chain and piperidinium functionalized nanospheres [50]. The TEM image

8

of PB-g-3PipVBC membrane (Figure 6a) shows obvious microphase separation and lack of

9

long-range order structure. When nanospheres are added into the membranes, the TEM image of

10

PB-1sphere (Figure 6b) displays more continuous ion conductivity channels than that of

11

PB-g-3PipVBC membrane. With the increasing of doping nanospheres, the ion conductivity

12

channels are destroyed. Meanwhile, it causes slight decline of ion conductivity of the membrane

13

due to aggregate of ionic cluster in PB-3sphere membranes (Figure 6c). Furthermore, when the

14

amount of doping nanospheres increases to 5%, more nanospheres and inapparent microphase

15

morphology are observed in Figure 6d, leading to the lower ion conductivity of the PB-5sphere

16

membrane compared with others. Hence, PB-g-3PipVBC and PB-1sphere membranes possess 15

1

well-developed morphology according to the TEM images.

2 3

Figure 6 TEM images of (a) PB-g-3PipVBC, (b) PB-1sphere, (c) PB-3sphere, and (d)

4

PB-5sphere.

5 6

3.4 Thermal stability and mechanical properties of the membranes The thermal stability of AEMs determines the operating temperature range for fuel cells.

7

Figure 7a records TGA curves of AEM samples from room temperature to 800

8

mass loss below 200

9

The second mass loss stage in the range of 260-350

. Generally, the

is ascribed to evaporation of water and residual solvent in the samples. is attributed to the degradation of

10

piperidinium groups. The degradation of side chain occurs in the range of 350-450

, which is

11

due to the degradation of styrene segment synthesized via ATRP. The mass loss above 450

12

assigned to the degradation of polymer backbone. In addition, DTG curves of the samples are also

13

shown in Figure 7a. The DTG curves of all samples contain four peaks, indicating their well

14

thermal stability below 200

15

suitable for application in fuel cells.

is

. Therefore, all membranes have good thermal stability and are

16

1

The mechanical properties of AEMs were characterized by tensile strength and elongation at

2

break as shown in Figure 7b. The tensile strength of all membrane ranges from 17.7 to 50.3 MPa

3

and the elongation at break is in the range of 2%-6%. Among them, the PB-1sphere shows the

4

highest tensile strength and PB-g-3PipVBC has maximum value of elongation at break. With the

5

increasement of doping nanospheres, the mechanical properties of composite membranes become

6

worse, due to the incompatibility between nanospheres and polymers. However, the small amount

7

of nanospheres can improve the tensile strength of composite membranes, which is consistent with

8

the SEM images. Results reveal that the composite membranes possess good mechanical

9

properties when the content of doping nanospheres is low.

0.6

70

0.5

Weight / %

0.4

60

0.3

50

0.2 40

Tensile strength / MPa

80

100

60

Deriv. Weight / %/oC

90

PB-g-3PipVBC 0.8 PB-1sphere PB-3sphere 0.7 PB-5sphere

(b) Tensile strength Elongation at break

60

50

50

40

40

30

30

20

20

10

10

0

0

Elongation at break %

(a)

0.1 30 0.0 100 200 300 400 500 600 700 800

Temperature / oC

PB

10 11 12

BC here here here PipV PB-1sp PB-3sp PB-5sp 3 g -

Figure 7 (a) TGA curves and (b) mechanical properties of the composite membranes. 3.5 Water uptake, swelling ratio of membranes and IEC

13

The WU and SR are important performance of AEMs. The reasonable WU and SR could

14

promote ion conductivity. As shown in Figure 8a, the WU of composite membranes shows a

15

temperature dependence. Generally, the WU of AEMs is in the range of 13.8%-62.8% at 20

16

The PB-5sphere shows remarkably high WU of 62.8%-103.3% between 20 and 80

17

PB-1sphere possesses the lowest WU of 13.8%-25.1% among the four kinds of membranes. This

18

phenomenon is related to morphology of the membranes as shown in Figure 4. The surfaces of

19

PB-g-3PipVBC and PB-5sphere membranes are rough and crack, leading to more water into the

20

membranes. However, the surfaces of PB-1sphere and PB-3sphere membranes are smooth, which

21

prevent water into the membranes. Furthermore, part of piperidinium groups in the membrane are 17

.

. The

1

replaced by xPCMS-g-PipVBC nanospheres, causing IEC changement of the composite

2

membranes. At lower amount of doping xPCMS-g-PipVBC nanospheres, the IEC of composite

3

membranes decreases to 1.46 and 1.48 mmol·g-1 for PB-1sphere and PB-3sphere membranes,

4

respectively. In contrast, the IEC of PB-5sphere reaches 1.61 mmol·g-1, which is close to the IEC

5

of PB-g-3PipVBC membrane (1.80 mmol·g-1). Therefore, the higher IEC of composite

6

membranes can absorb more water and achieve higher WU. On the other hand, the SR is affected

7

by WU. The SR of composite membranes shows similar tendency with WU, as shown in Figure

8

7b. The SR of all composite membranes is below 20%, and the PB-5sphere membrane exhibits the

9

highest SR of 19.1% at 80

. Result illustrates that the xPCMS-g-PipVBC doped into membranes

10

could decrease the SR of membranes. Also, the low SR could maintain good mechanical

11

properties of the membranes, which ensure the performance of fuel cells.

(a)

(b) 24

PB-g-3PipVBC PB-1sphere PB-3sphere PB-5sphere

100

Swelling ratio / %

Water uptake / %

120

80 60 40

20

PB-g-3PipVBC PB-1sphere PB-3sphere PB-5sphere

16 12 8 4

20 0 20 30 40 50 60 70 80 12 13 14

20 30 40 50 60 70 80

Temperature / oC

Temperature / oC

Figure 8 Temperature dependence of (a) WU and (b) SR of the AEMs in OH- form. 3.6 Ion conductivity

15

The ion conductivity is a key factor for the performance of fuel cells, and high ion

16

conductivity is helpful to reduce the resistance of membrane electrode assembly. Figure 9 shows

17

the ion conductivity of the membranes measured from 20 to 80

18

As expected, the ion conductivity of all membranes exhibits strong dependence on temperature. At

19

low temperature (20

20

similar values, which is close to 15 mS·cm-1. This indicates that the xPCMS-g-PipVBC

21

nanospheres have good ion conduction ability, which benefits from the continuous ion channels

under full hydration condition.

), the Cl- conductivity of all PB-msphere composite membranes shows

18

1

[44]. The hydroxide conductivities of all membranes display the range of 15.3-31.0 mS·cm-1 at 20

2

under full hydration condition. It appears that the hydroxide conductivities of all membranes are

3

about 2 times than the corresponding chloride conductivities at low temperature. The result is

4

agreed with the reported results [51]. However, the hydroxide conductivities of all membranes

5

cannot follow this law at high temperature, which is due to that the hydroxide groups are affected

6

by CO2. The as-prepared membranes in hydroxide form can react with CO2 under high

7

temperature and ambient air [52, 53]. Therefore, the real values of hydroxide conductivities of all

8

membranes should be higher than those of measured. Even so, PB-1sphere membrane shows the

9

highest hydroxide conductivity from 31.0 to 86.3 mS·cm-1 with the temperature from 20 to 80

.

10

The hydroxide conductivity of PB-5shpere membrane shows the lowest value among the three

11

types of composite membranes. It can be explained that the morphology decides the ion

12

conductive rate, and the smooth surface is more helpful to enhance the hydroxide conductivity.

13

Moreover, PB-5sphere membrane possesses higher WU, which can be concluded that excessive

14

WU leads to the dilution of ionic groups [51, 54]. In brief, all the fabricated membranes exhibit

15

acceptable ion conductivity compared with the previous reports (Figure 13a). For better

16

understanding the mechanism of hydroxide conductivity for all membranes, the activation energy

17

(Ea) of PB-msphere composite membranes is calculated according to Arrhenius equation (Ea = -b

18

× R), where R is the gas constant (8.314 J·mol-1·K-1) and b is the linear slope. As shown in Figure

19

S8, the Ea of all membranes ranges from 13.55 to 15.35 kJ·mol-1, which is similar to other reports

20

[55, 56]. Results reveal that the composite membranes have water-facilitated OH- conduction

21

mechanism similar to other hydrated AEMs [57].

OH- conductivity / mS⋅⋅cm-1

Cl- conductivity / mS⋅⋅cm-1

(a) PB-g-3PipVBC PB-1sphere PB-3sphere PB-5sphere

70 60 50 40 30 20 10 0

PB-g-3PipVBC PB-1sphere PB-3sphere PB-5sphere

90 80 70 60 50 40 30 20 10

20

22

(b)

30

40

50

60

70

Temperature / oC

80

20

19

30

40

50

60

70

Temperature / oC

80

1

Figure 9 Temperature dependence of (a) Cl- conductivity and (b) OH- conductivity of the

2

composite membranes.

3

3.7 Alkaline stability

4

The styrene pendent with methylpiperidinium as cations are known to have acceptable

5

alkaline chemical stability [58]. Here, the benzylpiperidinium cations were introduced into the

6

membranes via ATRP. To investigate the alkaline stability of AEMs, the hydroxide conductivity,

7

thermal stability, mechanical properties, 1H-NMR, XPS and FT-IR measurement after storage in 1

8

M NaOH at 80

9

digital photography of membrane samples after alkaline treatment, displaying the intact and

were used to characterize the performance of AEMs. Figure S9 shows the

10

transparent membranes. Furthermore, the remaining hydroxide conductivity at 30

11

after alkaline treatment is showed in Figure 10a. Generally, the hydroxide conductivities of all

12

samples exhibit similar tendency after soaking in 1 M NaOH. Among them, PB-3sphere exhibits

13

the highest alkaline stability, which remains 66.3% of the original hydroxide conductivity after

14

test for 1000 h. However, the other three types of membranes show similar degradation ratio,

15

remaining about 40% of the original hydroxide conductivity. Results indicate that PB-g-3PipVBC

16

membrane possesses an acceptable alkaline stability and the appropriate amount of nanospheres in

17

composite membranes is beneficial to improve alkaline stability of AEMs. Furthermore, the

18

decline of hydroxide conductivity of membranes is closely related to the degradation of

19

piperidinium cations and morphology of membranes. Therefore, PB-1sphere and PB-3sphere have

20

better alkaline stability than the other two kinds of membranes, which is owing to the regular

21

morphology and compact structure.

0.6 0.4

-

σOH remaing / %

0.8

0.2

35

Tensile strength / MPa

PB-g-3PipVBC PB-1sphere PB-3sphere PB-5sphere

0.0

23

10

30 25

8

20

6

15 4 10 2

5 0

0

22

(b)

200

400

600

800

1000 PB-

Time / h

ipV g-3P

BC

e e e her her her 5sp 3sp 1sp PBPBPB-

Figure 10 The remaining (a) hydroxide conductivity and (b) mechanical properties of the 20

0

Elongation at break / %

(a) 1.0

full hydration

1

composite membranes after storage in 1 M NaOH at 80

for 1000 h.

2

To figure out the degradation mechanism of the membranes, 1H-NMR of PB-g-3PipVBC was

3

investigated after immersing in 1 M NaOH for 1000 h as shown in Figure 11a. Here, TFA was

4

added into the DMSO-d6 solution to eliminate the influence of water in solvent. As shown in

5

Figure 11a, the new signal located at 4.48 ppm appears in the spectra of PB-g-3PipVBC after

6

alkaline treatment, which is assigned to methylene between piperidine and benzene ring. It could

7

be caused by degradation of benzylpiperidinium according to the degradation pathway (i) and (ii)

8

as shown in Figure 11b. Furthermore, it is observed that the signals of methylene in piperidine

9

ring shift from 3.32 ppm to 3.25 ppm. This phenomenon is ascribed to the SN2 degradation of

10

benzylpiperidinium.

11

Similarly, XPS was also used to reveal the degradation pathway of PB-g-3PipVBC, as shown

12

in Figure 12 and Figure S10. The C 1s spectrum of PB-g-3PipVBC membrane can be fitted to

13

three types of carbon bonds, which contain C-C (284.09 eV), C-Br or C-N (285.68 eV), and C-F3

14

(292.01 eV) in Figure 12c [59, 60]. After alkaline treatment, it can be observed that new carbon

15

bonds appear at 284.9 eV (C=C or C=N) and 287.5 eV (C-F), which are assigned to vinylic

16

protons derived from degradation of benzylpiperidinium by Hofmann hydrogen elimination

17

reactions, as shown in Figure 11b (i). In addition, the N 1s spectrum of PB-g-3PipVBC

18

membranes is also deconvoluted into two peaks, which are assigned to C-N bond (399.46 eV) and

19

quaternary ammonium (401.8 eV). Compared with the proportion of quaternary ammonium before

20

and after alkaline treatment in Figure 12b and Figure 12d, it reveals that the content of

21

quaternary ammonium of PB-g-3PipVBC declines after alkaline treatment. The results indicate

22

that the benzylpiperidinium is degraded and attacked by OH- during soaking in alkaline solution.

23

Furthermore, the signals of F 1s and Br 3d are observed from the survey scan of PB-g-3PipVBC in

24

Figure S10. All the results exhibit the degradation of benzylpiperidinium leads to the performance

25

decline of prepared AEMs.

26

Based on the discussion above, Figure 11b shows the possible degradation mechanism for

27

membranes, that is, Hofmann elimination or substitution reaction. Hofmann elimination is

28

ascribed to the ring-opening elimination of piperidiniums, which leads to the loss of cations and

29

decline of hydroxide conductivity. The degradation pathway (ii) might be attributed to the

30

substitution reaction. Besides, the bonds located at 937 and 1050 cm-1 are assigned to C-N bond of 21

1

piperidinium as shown in Figure S11a, indicating no obvious structure change before and after

2

alkaline treatment. Moreover, the polymer backbone can keep its integrity after alkaline treatment.

3

The degradation pathways are caused by degradation of piperidinium rings, which might lead to

4

cationic loss. Therefore, the loss of hydroxide conductivity and IEC are assigned to degradation of

5

piperidinium cations.

~DMSO-d6

(a)

(b)

(i)

Alkaline treatment

(ii)

PB-g-3PipVBC

8 6

7 6 5 4 3 2 Chemical shift / ppm

1

7

Figure 11 (a) 1H-NMR spectra of PB-g-3PipVBC membrane after alkaline treatment and (b) the

8

possible degradation mechanism for composite membranes. Here, the 1H-NMR spectra were

9

recorded with DMSO-d6 solutions containing 5 vol% of TFA.

22

(a) PB-g-3PipVBC C 1s alkaline treatment

(b) PB-g-3PipVBC N 1s alkaline treatment

C-C/C-H

C-N C=C/C=N

C-Br/C-N C-F3

quaternary ammonium

C-F

(c) PB-g-3PipVBC C 1s

292

1

288

284

(d) PB-g-3PipVBC N 1s

280

408

404

400

396

392

Binding Energy / eV

Binding Energy / eV

2

Figure 12 XPS spectra of (a) C 1s and (b) N 1s of PB-g-3PipVBC samples after alkaline

3

treatment, (c) C 1s and (d) N 1s of PB-g-3PipVBC samples

4

Additionally, the mechanical properties of membranes were performed after alkaline

5

treatment. Figure 10b shows tensile strength in the range of 22.89-32.2 MPa and elongation at

6

break in the range of 4.14%-9.6%, indicating obvious decline of tensile strength for all membranes.

7

It is caused by degradation of the piperidinium rings. Figure S11b shows TGA curves of all

8

membranes after alkaline treatment, and no changes are observed for the tendency of mass loss.

9

Result reveals that the membranes can still tolerate the temperature above 243

after alkaline

10

treatment, indicating high thermal stability of prepared membranes. Among them, PB-g-3PipVBC

11

and PB-3sphere have better performance after alkaline treatment. Compared with other reported

12

membranes [43, 55, 61-65], the prepared composite membranes show better alkaline stability, as

13

listed in Table 1 and Figure 13b. In other word, the composite membranes possess enhanced

14

comprehensive performance, as shown in Table 1. Therefore, it provides promising perspective

15

for designing polymer backbone and cations.

23

(a)

100

70

(62)

σOH remaining / %

(64)

60 (63)

50 (43)

40

(61)

-

-

σOH / mS⋅⋅cm-1

(b)

30

90 (64)

80

(63)

(65) (55) (43)

70 (61)

60 50

20 (55)

10

(62)

40

(65)

1.5

2.0

2.5

IEC / mmol⋅⋅g

3.0

3.5

-1

400 500 600 700 800 900 1000

Time / h

1 2

Figure 13 (a) The relationship between IEC and hydroxide conductivity of reported membranes,

3

(b) alkaline stability of reported membranes. The number in brackets represents the order of

4

references; the black squares indicate PB-g-3PipVBC, red circles indicate PB-1sphere, green

5

triangles indicate PB-3sphere, and blue triangles indicate PB-5sphere.

6

3.8 Oxidative stability

7

Considering complicated environment in fuel cells, the oxidative stability of membranes is

8

evaluated by monitoring the mass loss of all membranes in Fenton’s reagent (4 ppm FeSO4 + 3%

9

H2O2) at 80

. As listed in Table 1, the mass loss of PB-g-3PipVBC, PB-1sphere and PB-3sphere

10

membranes exhibit values above 93% after soaking in Fenton’s reagent at 80

for 1 h, revealing

11

acceptable oxidative durability for all membranes. However, PB-5sphere shows 78.02% remaining

12

mass after oxide testing, which is caused by the incompatibility between polymer backbones and

13

nanospheres. Furthermore, all the membranes can maintain integrality after storage in Fenton’s

14

reagent at 80

15

membranes exhibit acceptable oxidative stability.

16

3.9 Fuel cell performance

for 6 h. Compared with other reports listed in Table 1, the prepared composite

17

The performance of DBHFC single cell is evaluated by using the PB-3sphere membrane and

18

Nafion®211 as separators. The polarization curves and power density curves of DBHFC single cell

19

are shown in Figure 14. The open circuit voltages (OCV) of PB-3sphere and Nafion®211 24

1

membranes are about 1.7 V, which is lower than the theoretical value of DBHFC (3.01 V). This

2

phenomenon is caused by the mixed potential, which is originated from simultaneous oxidation of

3

BH4- ions and reduction of H2O2 [69]. It is noteworthy that the peak power density (PPD) of

4

PB-3sphere membrane is 77.3 mW·cm-2 at 143.4 mA·cm-2. However, the PPD of Nafion®211 is

5

67.4 mW·cm-2 at 134.8 mA·cm-2, which is lower than that of PB-3sphere membrane. It could be

6

ascribed that PB-3sphere membrane has thinner thickness and good ion conductivity compared

7

with Nafion®211. Nevertheless, the resulting PPD of DBHFC using PB-3sphere exhibits

8

comparable value with other reports [7]. Furthermore, the performance of H2/O2 fuel cells for the

9

prepared AEMs will be investigated in our future work after optimization of condition of AEMFCs measurement.

Cell voltage / V

2.0

80

Nafion 211 PB-3sphere

Power density / mW⋅⋅cm-2

10

1.6

60

1.2

40

0.8

20

0.4 0 0

20

40

60

80

100

120

140

11

Current density / mA⋅⋅cm-2

12

Figure 14 Polarization and power density curves of DBHFCs using PB-3sphere and Nafion211

13

membranes at 25

25

Table 1 Comparison of the IEC, hydroxide conductivity, WU, SR, alkaline stability and oxidative stability of the proposed AEMs with other reported AEMs.

a

Samples

IEC (mmol·g-1)

IEC residue %

σOH-1 (mS·cm-1)

σOH-1 residue %

WU %

SR %

oxidative stability %

References

PB-g-3PipVBC

1.80

75.62

19.56c

43.92d (1000 h)

21.1c

7.2c

93.18d (1 h)

This work

PB-1sphere

1.46

76.46

37.62c

42.45d (1000 h)

13.8c

2.7c

94.12d (1 h)

This work

PB-3sphere

1.48

80.43

25.93c

66.31d (1000 h)

23.7c

4.3c

93.90d (1 h)

This work

PB-5sphere

1.61

78.53

24.88c

40.21d (1000 h)

62.8c

10.6c

78.02d (1 h)

This work

Im-SiO2/TA-PPO

3.31

-

35.0c

69.8a (500 h)

158c

8c

89.1 (200 h)

[61]

PAEK-MpOH

1.28

92

44c

-

68.4a

29.4a

-

[66]

QHPEEK

1.66

-

29c

75b (576 h)

46c

29c

-

[67]

QPES

1.80

85.6

29.2a

89a (480 h)

11.5c

5.4c

-

[68]

SEBS-Pi-73%

1.19

79.17

10.09c

79.28d (576 h)

59.13c

25.03c

-

[65]

PES-MPRD

1.42

85.6

20 (Br-)c

79.1a (672 h)

36.5c

17.2c

-

[55]

test temperature: 60

, b test temperature: 40

, c test temperature: 20

, and d test temperature: 80

; the number in brackets represents testing time.

1

4. Conclusion

2

In summary, a family of PB-based composite membranes were developed by grafting the

3

PipVBC onto nanospheres and PB-Br polymer via ATRP. The membranes are composed of

4

aryl-ether bonds-free PB-backbone, nanospheres and highly stable piperidinium cations. The

5

composite membranes show better performance as AEM compared with PB-g-3PipVBC

6

membrane. PB-1sphere membrane exhibits high chloride conductivity of 65.6 mS·cm-1 at 80

7

while PB-3sphere membrane possesses good alkaline stability, with 66.31% remaining of the

8

original hydroxide conductivity in 1 M NaOH at 80

9

PB-3sphere show more distinct microphase separation morphology than that of other membranes

10

observed by TEM, which is beneficial to improve ion conductivity and dimension stability.

11

Additionally, the PB-3sphere membrane exhibited a peak power density of 77.3 mW·cm-2 at 143.4

12

mA·cm-2 when was applied in DBHFCs test. Therefore, the prepared AEMs show promise for

13

application in DBHFCs and AEMFCs.

14

Acknowledgements

,

for 1000 h. Besides, PB-1sphere and

15

The authors of this work greatly appreciate the financial support from the National Key

16

Research and Development Program of China (2016YFB0101203), the National Natural Science

17

Foundation of China (No. 21875029) and the Major State Basic Research Development Program

18

of China (2012CB215500).

19

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Highlights: The free aryl-ether bonds poly(biphenyl-alkylene)s are synthesized; The ionic nanospheres based polystyrene are prepared by ATRP; The PB-3sphere show a peak power density of 77.3 mW·cm-2 at 143.4 mA·cm-2 in a direct borohydride fuel cell.

Author statement： ： Zhang Shuai: Conceptualization, Investigation, Formal analysis, Writing-Original draft preparation. Zhu Xiuling: Methodology, Resources, Writing-Reviewing and Editing, Project administration, Supervision, Funding acquisition. Wang Yajie: Investigation, Visualization. Gao Xueqiang: Investigation. Liu Pingyang: Visualization, Validation. Wang Xinyu: Data Curation

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.