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Anion Conductive Triblock Copolymer Membranes with Flexible Multication Side Chain Chen Xiao Lin, Hong Yue Wu, Ling Li, Xiu Qin Wang, Qiu Gen Zhang, Ai Mei Zhu, and Qing Lin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03757 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

TOC

Triblock copolymer

Multication side chain

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Anion Conductive Triblock Copolymer Membranes with Flexible Multication Side Chain Chen Xiao Lina,b, Hong Yue Wua, Ling Lia, Xiu Qin Wanga, Qiu Gen Zhanga, Ai Mei Zhua, Qing Lin Liua,* a

Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Department of

Chemical & Biochemical Engineering, The College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China. b

Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Xiamen University,

Xiamen 361005, P. R. China. *Corresponding author: Q.L. Liu, E-mail: [email protected], Tel: 86-592-2188072, Fax: 86-592-2184822.

1


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Abstract To achieve highly conductive and stable anion exchange membranes (AEMs) for fuel cells, novel triblock copolymer AEMs bearing flexible side chain were synthesized. The triblock structure and flexible side chain are responsible for the developed hydrophilic/hydrophobic phase separated morphology and well-connected ion conducting channels, as confirmed by transmission electron microscopy (TEM). As a result, the triblock copolymer AEMs with flexible side chain (ABA-TQA-x) demonstrated considerably higher conductivities, up to 130.5 mS cm-1 at 80 oC, than the AEMs with monocation side chain (ABA-MQA). Furthermore, the long alkyl spacer between the backbone and quaternary ammonium groups, as well as long inter-cation spacer limits the water swelling of the membranes to some degree, resulting in good alkaline stability. The ABA-TQA-44 membrane retained 84.7% and 83.1% of its original conductivity and ionic exchange capacity (IEC), respectively, after immersed in a 1 M aqueous KOH solution at 80 oC for 480 h. Furthermore, the peak power density of a H2/O2 single cell using ABA-TQA-44 is 204.6 mW cm-2 at a current density of 500 mA cm-2 at 80 o

C.

KEYWORDS: Anion exchange membranes, fuel cells, flexible multication side chain, triblock copolymers, ion conduction

2



1. Introduction Fuel cells are regarded to be a prospective electrochemical appliance that can convert chemical energy directly into electric energy with high efficiency (40%−70%).1 In the past decades, there have been growing attention in anion exchange membrane fuel cells (AEMFCs) because they exhibit lower fuel permeability and higher oxygen reduction reaction kinetics than proton exchange membrane fuel cells (PEMFCs).2 In addition, PEMFCs rely heavily on precious metal catalysts (such as Pt), resulting in high cost of the fuel cell devices. For AEMFCs, non-precious metals, such as Ag, Fe and Ni, are available as catalysts, and can greatly lower the cost of devices.3-5 Nevertheless, there are obstacles to the practical applications of AEMFCs. In particular, AEMs as a critical component of AEMFCs are still a weakness owing to their low conductivity and poor chemical stability at operating temperature and at high pH. In recent years, cationic polymers based on quaternary ammonium (QA),6-8 imidazolium,9-11 guanidinium,12-14 phosphonium,15-17 sulfonium18 and metal ions19,20 have been extensively used to prepare AEMs. Although great efforts have been taken to develop high performance AEMs, the conductivity of AEMs still lags behind owing to the lower mobility of hydroxide ions than protons. The key to hydroxide ions transport in AEMs is believed to be conductive nanochannels, through which hydrated hydroxide ions can conduct efficiently in the membrane.21,22 Several strategies have been developed to fabricate nanochannels in AEMs, and thus enhance hydroxide conductivity.22,23 These approaches include design of AEMs with microphase-separated morphology, and incorporation of functionalized nanofillers into polymer matrices. Of those, the formation of microphase-separated morphology in AEMs has been proved as an important approach for enhancing conductivity since the hydrophilic and hydrophobic moieties in the AEMs will drive the cationic polymer to self-assemble 3


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and form highly efficient ion nanochannels.24 Recently, block copolymer AEMs are claimed to be effective for fabricating well-developed microphase-separated morphology. The Miyatake’s group25 prepared multiblock copolymer AEMs with obvious hydrophilic/hydrophobic segregation morphology, resulting in a high conductivity of 144 mS cm-1 (80 oC). Strasser et al. reported that multiblock copolymers AEMs bearing spirocyclic ammonium groups showed a high conductivity of 102 mS cm-1 at 80 oC, which is ascribed to its obvious microphase-separated structure.26 Zhang et al. developed a series of poly(arylene ether sulfone) block copolymer AEMs with a high conductivity of 86.3 mS cm-1 at 80 oC due to the well-connected nanochannels resulting from the block structure.27 However, the high conductivity of block AEMs is often accompanied with high water uptake, leading to a high swelling and poor mechanical property. Therefore, the challenge in developing high performance AEM materials lies in the design of hydroxide conducting nanochannels with low water uptake and swelling. Recently, Hickner’s group prepared a series of polyphenylene oxide (PPO)-based AEMs with multiple quaternary ammonium (MQAPPO) in the side chain. Compared with main-chain-type AEMs (BTMA40), MQAPPO has a high ionic conductivity, small swelling and slow degradation.28 Previously, our group has demonstrated that AEMs with multi-imidazolium side chain result in confined swelling even at elevated temperature, but the AEMs degrade heavily under alkaline condition due to the unstable imidazolium groups in which the conductivity decreased by 31.9% after the AEMs being dealt with 1 M NaOH for 468 h.29 Inspired by the advantages of block copolymer structure and multication side chain, we aimed to further design a novel copolymer with block structure as well as multi-QA side chain to improve the conductivity and constrain swelling. Although there have been a few publications on AEMs based on multiblock copolymer and 4



bis-QA-functionalized side chain,30 the strategy for utilizing triblock copolymers with tri-cation side chain in their terminal block is expected to be useful for AEMs. In this work, we prepared, for the first time, a kind of novel triblock copolymer with tri-QA side chain to improve the AEM performance. A series of triblock copolymer AEMs made up of poly(ether sulfone) and multi-QA functionalized poly(phenylene oxide) moieties were prepared (Scheme 1). Three QA groups located in one side chain will enhance the local ion concentration in favor of fabricating microphase-separated morphology. Furthermore, the long flexible methylene spacers between the QA groups and backbone, as well as between the QA groups themselves will result in a reasonable swelling and good alkaline tolerance.31 Thus, our strategy for preparing AEMs from triblock copolymers combining multi-QA side chain is expected to enhance conductivity, mitigate water swelling as well as improve the chemical stability of the AEMs. Herein, AEMs with various poly(ether sulfone) segment lengths were prepared successfully. The water uptake, swelling ratio, hydroxide conductivity, thermal stability, mechanical properties, alkaline stability and fuel cell performance of the AEMs were investigated. 2. Experimental Section 2.1 Materials Bis(4-fluorophenyl) sulfone (FPS) (99.0%), 6-bromohexanoyl chloride (BHC) (97%), triethylsilane (TES) (98%), hexafluorobenzene (HFB) (99%) and N,N,N',N'-tetramethyl-1,6-hexanediamine (TMHDA) (98%) were purchased from Tokyo Chemical Industry Co. Ltd. 2,6-Dimethylphenol (DMP) (99%), trimethylamine (30% in water), aluminum chloride (AlCl3) (AR), 1,6-dibromohexane (97%), N-methyl-2-pyrrolidone (NMP) (99.8%), 4,4'-(hexafluoroisopropylidene) diphenol (BPHF) (98%), trifluoroacetic acid (TFA) (99%), N,N,N',N'-tetramethylethylenediamine (TMEDA) (AR), potassium 5


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carbonate

(K2CO3)

(99%),

cyclohexane

(AR),

dimethyl

sulfoxide

(AR)

(DMSO)

and

N,N-dimethylacetamide (DMAc) (99.8%) were obtained from Aladdin Chemistry Co. Ltd and used as received. All other chemicals were obtained from Sinopharm Chemical Reagent Co. Ltd. and used without further treatment. The deionized water was degassed before use. 2.2 Preparation of Anion Exchange Membranes Scheme 1 shows the route for synthesis of ABA-TQA-x. The procedure for synthesis of di-quaternary ammonium (DQA) salt,32,33 fluorine-terminated mono-telechelic oligomers (PPO-F),34 hydroxyl-terminated telechelic oligomers (PES-OH-x, x=44, 61 and 90, where x represents the polymerization degree of PES-OH oligomers), triblock copolymers (ABA-x), triblock copolymers bearing flexible side chain (ABA-COBr-x and ABA-CH2-x) and triblock copolymers bearing multi-QA side chain (ABA-TQA-x) is provided in Supporting Information. 2.3 Characterization The chemical structure of the products was analyzed by 1H NMR (Bruker Avance III 500 MHz spectrometer). Molecular weights (Mn and Mw) of the as-synthesized copolymers were determined via gel permeation chromatography (GPC) (Waters, USA) using tetrahydrofuran as the eluent with a flow rate of 1.0 mL min-1 and standard polystyrene samples for calibration. The aggregation behavior of the copolymer in the solvent was determined by 2D 1H-1H Overhauser effect spectroscopy (NOESY). A small angle X-ray scattering diffractometer (Anton Paar) was applied to study the micro-morphology of the membranes. Transmission electron microscopy (TEM) images were recorded on an electron

microscope (JEM-2100) with an accelerating voltage of 200 kV. Before observation, the membranes were immersed in a 1 M Na2WO4 solution to exchange ions at RT for 48 h. Then, the membrane samples were washed with deionized water to remove residual Na2WO4 and dried at 60 oC overnight. 6



The dried membranes were embedded in epoxy resin, sectioned to approximately 60 nm thickness and collected on copper grid before observation.

7


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O S O

CF3 HO

F

+

OH CF3

BPHF

DMAc K2CO3

CF3 HO

FPS 145oC 20 h O S O

O CF3

F

CF3 O

OH x

CF3

PES-OH-x CH3

F

O y CH3

F

PPO-F CH3

F

NMP K2CO3 F Cyclohexane 110 oC 4 h F 140 oC 48 h

F CF3

O CH3

F

O y

F

O S O

O CF3

F

F

Br

O O

CH3

O y

F

O S O

O CF3

F

F

H3C

H3C

F

F

H3C

O

CF3 O

ABA-COBr-x

y

O CF3

x

O F

F

y O

H3C

Trifluoroacetic acid Triethylsilane Dichloromethane 100 oC 48 h

Br CH3

F

CH3

F CF3 O

y

F

F

CH3

Br-

y F

O CF3

F

H3C

Br

CF3 F

O

F O

F

O CF3

F

O y

O x

H3C

NMP 60 oC 24 h

N+

DQA F

F

CF3

ABA-CH2-x

N+ Br

Br

O S O

O CF3

N

CH3

Br

F

O

N+

F

O CF3

F CF3

F

Dichloromethane AlCl3 RT 5 h

O

CH3

O x

ABA-x Cl

F CF3

O S O

ABA-TQA-x

F

CF3 O x

O CF3

H3C O y

F

F

H3C

Br-

Br-

N+

BrN+

Br-

N+

Br- N+

N+ Br-

Scheme 1 The route for synthesis of ABA-TQA-x. 8



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2.4 Ionic Exchange Capacity The experimental IEC value of the AEMs was measured by Mohr titration. The membranes in Br− form was dried under vacuum at 80 oC overnight. Subsequently, 0.1 g of the sample was immersed in a 0.5 M aqueous NaNO3 solution at RT for 48 h to release Br− ions from the membrane sample to solution. A certain amount of solution was taken out and titrated with a AgNO3 solution using potassium chromate as the indicator.

2.5 Water Uptake and Swelling Ratio

To measure the water uptake (WU) and swelling ratio (SR), the AEMs in hydroxide form were dried at 80 oC in a vacuum oven overnight to measure the weight (mdry) and length (Ldry). Next, the AEMs were immersed in deionized water under the protection of nitrogen at 20, 40, 60 and 80 oC for 24 h to obtain the weight (mwet) and length (Lwet). The WU and SR are calculated by

WU =

m wet − m d ry m d ry

× 100 %

(1)

where mwet and mdry represent the mass of the wet and dry membranes, respectively.

SR =

Lw et − Ldry Ldry

× 100%

(2)

where Lwet and Ldry represent the length of the wet and dry membranes, respectively. Furthermore, the number of absorbed H2O molecules near each QA group (λ) can be calculated by

λ=

WU(%)×10 IEC ×18

(3)

2.6 Hydroxide Conductivity

Hydroxide conductivity was measured on a VersaSTAT 4 electrochemical workstation (Princeton 9


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Applied Research, USA) at frequencies ranging from 0.1 Hz to 1 MHz. The impedance (R, kΩ) of the membranes was measured at a given temperature under fully hydrated condition. The measurements were carried out in a sealed container under the protection of nitrogen atmosphere. The hydroxide conductivity (σ, mS cm-1) of the AEMs is obtained by

σ=

L AR

(4)

where L (cm) is the distance between the two reference electrodes, and A (cm2) is the cross-sectional area of the membrane sample.

2.7 Mechanical Property and Thermal Stability

The mechanical properties were examined using an Instron 3343 universal testing machine. The membrane samples were cut into a dumbbell-shape with a gauge area of 5.0 mm× 2.0 mm prior to test. The stretching rate was controlled to be 0.2 mm s-1. The thermal stability of the membrane samples was examined by thermogravimetric analysis (TGA) on a SDT-Q600 device (TA instruments, USA). A small amount of sample was heated from 30 to 800 oC under a nitrogen atmosphere at a heating rate of 10 oC min-1. The membrane samples were dried in a vacuum oven at 100 oC for 24 h before test.

2.8 Alkaline Stability The AEM samples were exposed to a 1 M aqueous KOH solution at 80 oC to study the alkaline stability. The conductivity and IEC value of the membrane samples were recorded during the test. Moreover, the alkaline stability was also investigated by 1H NMR.

2.9 Single Cell Performance

Catalyst-coated membrane (CCM) was used for fabricating the electrodes. The electrocatalyst ink 10



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was made up of 0.25 g of ABA-TQA-44 ionomer, 0.8 g of Pt/C (40%wt Pt, Johnson Matthey) catalyst, 3.3 g of ethanol, 4.3 g of deionized water and 5 mL of DMSO. The catalyst ink was ultrasonically homogenized for 0.5 h before spraying onto the both sides of the AEMs to prepare CCM. The active area of the electrodes was 4.0 cm2 and the loading content of Pt was 0.5 mg cm-2 for both the anode and cathode. Membrane electrode assembly (MEA) was prepared by sandwiching the CCM between two pieces of carbon paper (Toray TGP-H-060, Japan). A TE201 fuel cell evaluation system (Kunshan Sunlaite, China) was applied for testing the fuel cell performance of the MEA at 80 oC. Fully humidified H2 and O2 were supplied to the anode and cathode of the single cell, respectively. The flow rate of H2/O2 was controlled to be 100 mL min-1. For the durability experiment, the current density of the single cell was setting at a constant value of 100 mA cm2, during which the change in cell voltage was recorded.

3. Results and Discussion

3.1 Synthesis and Characterization of Copolymers

The

target

copolymers

(ABA-TQA-x)

were

synthesized

via

nucleophilic

substitution

polycondensation reaction, coupling reaction, Friedel-Crafts reaction, reduction reaction and Menschutkin reaction, as shown in Scheme 1. The products contained various midblock segment lengths are termed as ABA-TQA-44, ABA-TQA-61 and ABA-TQA-90, separately, where x is the polymerization degree of the PES-OH-x precursor.

MQA was prepared by the Menschutkin reaction between trimethylamine and 1,6-dibromohexane, as described in our previous report.32 DQA was made via the Menschutkin reaction between an excess of TMHDA and MQA.33 As demonstrated in the 1H NMR spectrum of DQA (Fig. S1), the 11


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characteristic peaks at 2.11, 3.03 and 3.09 ppm originate from the methyl protons in tertiary amine groups, middle QA groups and terminal QA groups of DQA chain, respectively. Furthermore, the integration ratios of H12 (3.1 ppm) to H8 (3.0 ppm) and H1 (2.1 ppm) are 1.51 and 1.01, respectively, and are in line with the theoretical data, confirming the successful synthesis of DQA.

PPO-OH oligomers were synthesized via oxidative polymerization. The PPO-OH oligomers can precipitate from methanol because of their low solubility in a strong-polar solvent.34 In order to prepare

fluorine-terminated

mono-telechelic

PPO-F

free

of

by-product,

an

excess

of

hexafluorobenzene was used to avoid inter-oligomer coupling reaction. Fig. S2 shows the 1H NMR spectra of PPO-OH and PPO-F oligomers. The peaks in the region between 6.0 and 8.0 ppm correspond to the protons from the benzene ring. The spectrum of PPO-OH shows a characteristic peak at 4.24 ppm assignable to the proton resonance from the terminal hydroxyl groups (H5), and this peak disappeared after end-capping reaction with hexafluorobenzene, as shown in Fig. S2(b). By comparing the integral area of the characteristic peaks arising from H5 (4.24 ppm) and the methyl groups H4 (2.12 ppm) in Fig. S2(a), the number average molecular weight and polymerization degree of PPO-OH are calculated to be 1.9 kg mol-1 and 15.8, respectively. Fig. S3 shows the

13

C NMR

spectrum of the PPO-F. The chemical shifts of the peaks are consistent with the chemical structure of the PPO-F, further confirming the successful synthesis of PPO-F.

Hydroxyl-terminated PES-OH-x with polymerization degree of 44, 61 and 90 was made from FPS and BPHF via nucleophilic substitution polycondensation reaction, as described previously.34 The polymerization degree is controlled by the FPS to BPHF molar ratio. Fig. S4 shows the 1H NMR spectrum of PES-OH-44. The number-average molecular weight (Mn) of PES-OH-44 was calculated to be 24.6 kg mol-1 by comparing the integral area of H4 (7.94 ppm) and H8 (6.86 ppm). Furthermore, the 12



Mn of PES-OH-61 and PES-OH-90 is calculated to be 33.9 and 49.9 kg mol-1, respectively, and is in line with the experimental data from GPC measurements, as shown in Table S1.

Triblock copolymers ABA-x were prepared via a coupling reaction between the PES-OH-x and PPO-F oligomers. Fig. S5(a) shows the 1H NMR spectrum of the ABA-44 copolymers. The characteristic peaks between 7.0 and 8.0 ppm correspond to proton resonance from the benzene rings on the midblock of ABA-x. The peaks at 2.1 and 6.5 ppm are attributed to the protons from the methyl groups and benzene rings in poly(phenylene oxide) segments. Furthermore, the integration ratio of H6 (7.9 ppm) to H1 (6.5 ppm) is 1.00: 0.36, and is in line with the data from the feed ratio of PES-OH-x to PPO-F oligomers, indicating successful synthesis of ABA-44. There is the possibility of crosslinking of PPO-F and PES-OH-x oligomers since the PPO-F oligomers have multi-fluorine groups. However, no obvious increase in Mn of the copolymers after coupling reaction was observed by GPC, as demonstrated in Table S1, indicating that the undesirable crosslinking reaction between the PPO-F and PES-OH-x oligomers was avoided. The Mn of ABA-44, ABA-61 and ABA-90 was measured to be 28.7, 38.4 and 54.1 kg mol-1, respectively, and is approximately equal to that calculated from the feed ratio of PPO-F to PES-OH-x oligomers.

The pendent alkyl chains are introduced into the poly(phenylene oxide) segments via Friedel-Crafts reaction. The 1H NMR spectrum of ABA-COBr-44 is shown in Fig. S5(b). New peaks are observed at chemical shifts of 1.5−3.5 ppm corresponding to the proton resonance of -CH2- groups in the pendent alkyl chain. The peak between H8 and H9 is assignable to the proton resonance of H2O from CDCl3. The integral area ratio of H1 (6.1 ppm) to H6 (7.9 ppm) is 0.18 after Friedel-Crafts acylation, and this value is a half of that from ABA-44, indicating that a half of the benzene proton in the poly(phenylene oxide) segments was replaced by alkyl chain via Friedel-Crafts acylation. Thus, the degree of alkyl 13


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bromination is nearly 100%. Additionally, the ketone groups in ABA-COBr-44 are reduced with triethylsilane and trifluoroacetic acid according to our previous report.35 The 1H NMR spectrum presented in Fig. S6 indicates the successful synthesis of ABA-CH2-44 by the appearance of a new signal at 2.8 ppm arising from the first -CH2- groups attached to the poly(phenylene oxide) segments. The signal from the -CH2- groups (H8) attached to the ketone groups before reduction reaction shifts from 3.0 to 1.9 ppm. This further suggest the successful reduction reaction.

The ABA-TQA-x was made via the Menschutkin reaction between ABA-CH2-x and DQA in NMP at 60 oC (Scheme 1). An excess of DQA was used to prepare ABA-TQA-x. The molar ratio of DQA to -Br groups in ABA-CH2-x is 3.0. Fig. S7 shows the 1H NMR spectrum of ABA-TQA-44. The new characteristic peak at 3.1 ppm originating from the -N+(CH3)2- and the terminal -N+(CH3)3 protons in the side chain indicates the successful synthesis of ABA-TQA-44. The characteristic peaks between 1.0 and 2.0 ppm are assignable to the -CH2- protons. All the chemical shifts in the spectra suggest the successful Menschutkin reaction. Herein, AEMs with various IECs were obtained by synthesizing triblock copolymers with various hydrophobic segment lengths. The IEC measured by Mohr titration is in line with that calculated from the 1H NMR spectra, further suggesting the successful Menschutkin reaction. As listed in Table 1, the IEC of the membranes is measured to be 1.27−1.93 meq g-1.

Herein, triblock copolymer AEMs bearing only one QA group in the terminal of the side chain (ABA-MQA) are prepared for comparison. The synthetic route of ABA-MQA is similar to that of ABA-TQA-x. However, the x-length of the poly(ether sulfone) segments is 22 and the quaternization reagent used for the synthesis of ABA-MQA is a trimethylamine solution (30 wt% in methanol). The 1

H NMR spectrum in Fig. S8 indicates the successful synthesis of ABA-MQA. Table 1 IEC, WU, SR and λ of the ABA-TQA-x membranes. 14



IEC (meq·g-1)

WU (%)c

WU (%)d SR (%)c

Membrane Cal. a

a

Page 16 of 38

Exp. b

20 oC

λe

20 oC

ABA-TQA-90

1.28

1.27

30.3f0.5

7.3f0.4

8.4f0.4

13.3

ABA-TQA-61

1.63

1.60

46.9f0.4

11.6f0.8

13.6f0.5

16.3

ABA-TQA-44

1.94

1.93

58.6f0.5

14.8f0.7

18.9f0.4

16.9

ABA-MQA

1.36

1.32

42.1f0.8

11.1f0.4

11.7f0.6

17.7

calculated from 1H NMR spectra, b measured by Mohr titration, c OH− form, d Br− form; e measured at

20 oC, λ, average number of H2O close to each QA group at 20 oC. The WU and SR of the AEMs were measured at 20 oC.

3.2 Morphology 1

H-1H NOESY was used to study the self-assembly behavior of the functionalized polymers in a

solution since the cross-peaks in NOESY spectra can reveal as to which protons are close to each other in space.35,36 Herein, the aggregation behavior of ABA-TQA-44 was investigated in a solvent. As demonstrated in Fig. 1, the protons belong to poly(ether sulfone) segments exhibit obvious cross-peaks with each other (H5-H4, H5-H3, H6-H3 and H6-H4), suggesting that the midblock aggregated together via intermolecular interaction. However, there is no cross-peaks between the protons in the poly(phenylene oxide) and poly(ether sulfone) segments, indicating that the functionalized poly(phenylene oxide) segment is incompatible with the poly(ether sulfone) segment, and this will result in self-assembly of the copolymers in the solvent. Furthermore, the cross-peaks of H1-H2,8,9,10,11,14,15, H2-H7-11,13-15,

17,

H8-H7,9-17, H9,10,14-H12,13,16,17, H12-H16, H11,15-H9-10,12-14,16-17,

H12,16-H7-11,13,14,17 and H13,17-H7,14-16 exhibit that the poly(phenylene oxide) segments aggregated together and participated in self-assembly. It is noted that the self-assembly of the copolymers in a 15


Page 17 of 38

solution is pertinent to the membrane morphology.34,37,38 Thus, we can conclude that the copolymers will form microphase-separated morphology via self-assembly after membrane fabrication.

1

13

2

3 4

56

7 8 9 10 11 12

14 15 16

12,16,H2O

17 6

5

4,3

13,17

11,15 9,10,14 1

28

7

2 8

7

13,17 DMSO 9,10,14 11,15

DMSO

4,3

6 5

1

12,16,H2O

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 1 1H-1H NOESY spectrum of ABA-TQA-44.

Herein, we used SAXS and TEM to investigate the morphology of the AEMs. Fig. 2 shows the SAXS plots of the membranes. As seen, the AEMs bearing multi-QA side chain show distinct peaks at 0.25, 0.32 and 0.43 nm-1 for ABA-TQA-44, ABA-TQA-61 and ABA-TQA-90, respectively, corresponding to a d-spacing of 25.1, 19.6 and 14.6 nm, using the Bragg’s law (d=2π/q). On the basis

16



of the “ionic cluster” model, the calculated d-spacing represents the size of ionic cluster. With increasing the ratio of ionic segment, increased ionic groups will take part in self-assembly and form larger ionic cluster resulting in larger d-spacing.39 However, the AEM bearing one QA group in the terminal of the side chain (ABA-MQA) shows very weak (if any) characteristic peaks. These findings demonstrate that the triblock copolymer AEMs bearing multi-QA side chain are efficient for fabricating microphase-separated morphology during membrane formation. As shown in the 1H-1H NOESY spectrum, the microphase-separated morphology is stemmed from the incompatibility between the functionalized hydrophilic poly(phenylene oxide) and hydrophobic poly(ether sulfone) segments, which drive the copolymers to self-assemble and fabricate nanoscale areas.

-1

0.25 nm

ABA-TQA-44 0.32 nm-1

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

ABA-TQA-61 -1

0.43 nm

ABA-TQA-90

ABA-MQA

0.4

0.8

1.2

1.6

2.0

-1

q (nm ) Fig. 2 SAXS plots of the AEMs.

TEM observations were carried out for all the membranes stained with tungstate ions. As demonstrated in Fig. 3, the dark areas represent the stained hydrophilic regions which are mainly made up of functionalized poly(phenylene oxide) segments, while bright areas represent the hydrophobic regions which are mainly comprised of poly(ether sulfone) segments. The ABA-TQA-x (x=44, 61 and 90) membranes show microphase-separated morphology with well-connected ion-conducting 17


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nanochannels. Increasing the ion content or decreasing the length of poly(ether sulfone) segments facilitates the formation of hydrophilic/hydrophobic phase separated morphology and thus promotes the fabrication of ion clustering areas. The size of hydrophilic domains in ABA-TQA-44, ABA-TQA-61 and ABA-TQA-90 is about 15, 12.5 and 7.5 nm, respectively, and is somewhat larger and more connected than that in ABA-MQA.

Fig. 3 TEM images of (a) ABA-MQA, (b) ABA-TQA-90, (c) ABA-TQA-61 and (d) ABA-TQA-44.

It is found that the d-spacing of the AEMs calculated from the SAXS result is different from that from the TEM result. Since the calculated d-spacing is on the basis of the “ionic cluster” model,39 while the TEM images show the practical size of the ionic domain. The larger ionic domains can be ascribed to the block copolymer structure and introduction of multi-QA side chain in the ABA-TQA-x membranes that facilitate the aggregation of the ionic groups and fabricate well-developed ion conducting channels. Above all, SAXS and TEM results show that microphase-separated morphology 18



Page 20 of 38

is formed in the AEMs, and this morphology would have a great influence on the conductivity of the AEMs.25,40

3.3 Water Uptake (WU) and Swelling Ratio (SR)

WU plays a critical role in promoting ionic conduction through developed ion conducting channels. The WU of the AEMs in OH− form is measured between 20 and 80 oC under fully hydrated condition and plotted as a function of temperature (Table 1 & Fig. S9(a)). As anticipated, the water uptake of the ABA-TQA-x membranes increases with the IEC or temperature. For instance, the WU of o

ABA-TQA-90 (IEC=1.27 meq g-1) is 30.3% at 20 oC and 51.7% at 80 C. ABA-TQA-44 (IEC=1.93 meq g-1) shows the highest WU of 58.6% at 20 oC and 88.4% at 80 oC. It is found that ABA-TQA-90 (IEC=1.27 meq g-1) exhibits much lower WU than ABA-MQA with a similar IEC of 1.32 meq g-1. The presence of long hydrophobic alkyl spacers between the cations in the side chain of ABA-TQA-x may be responsible for the lower WU of ABA-TQA-90.41 In addition, the WU of the AEMs in Br− form was also measured at 20 oC. As shown in Table 1, the WU of the AEMs in Br− form ranged from 7.3% to 14.8%, and is much lower than that of AEM in OH− form, indicating that the AEMs in OH− form are more prone to absorb water. Moreover, the number of water molecules close to each QA group (designated as λ) is calculated to be 13.3−16.9 for the ABA-TQA-x membranes.

Fig. S9(b) shows the SR of the AEMs at elevated temperatures. Although the SR of the AEMs increases with increasing temperature, but increasing the number of cations in the side chain can improve the dimensional stability of the membranes.28,41 For the AEMs with multi-QA side chain, such as ABA-TQA-90, the swelling ratio is 11.7% at 30 oC and 15.1% at 80 oC, and is lower than that of AEMs with monocation side chain (ABA-MQA) at the same temperature. The difference in the

19


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swelling between ABA-TQA-90 and ABA-MQA could be ascribed to the fact that the increased charge density in the side chain can enhance the intermolecular interaction of the hydrophilic segments of the backbone and thus prevent swelling.28,42 Furthermore, the hydrophobic alkyl spacers between cationic groups may be also responsible for the lower swelling of ABA-TQA-90.

3.4 Hydroxide Conductivity

High hydroxide conductivity is critical for high current density cell outputs. In general, the conductivity of AEMs is required to be higher than 100 mS cm-1 at 80 oC for the practical application.43 Fig. 4(a) demonstrates the relationship of hydroxide conductivity of the AEMs and temperature. As expected, the conductivity of the ABA-TQA-x membranes increases with decreasing the length of hydrophobic poly(ether sulfone) segments because of the increased IEC. For example, ABA-TQA-44 (IEC=1.94 meq g-1) achieves the highest hydroxide conductivity of 58.7−130.5 mS cm-1 at the temperature range from 20 to 80 oC, while ABA-TQA-90 (IEC=1.28 meq g-1) exhibits the lowest conductivity of 31.5−86.1 mS cm-1 at 20−80 oC. Moreover, the hydroxide conductivity of ABA-TQA-44 and ABA-TQA-61 is greater than 100 mS cm-1 at 80 oC, indicating that the ABA-TQA-x membranes hold promise for fuel cell applications. The ABA-TQA-x AEMs with multi-QA side chain (IEC=1.28−1.94 meq g-1) are found to demonstrate higher conductivity than ABA-MQA (IEC=1.36 meq g-1, 18.1−55.8 mS cm-1, 20−80 oC ). This is because the ABA-TQA-x membranes have a well-developed microphase-separated morphology (as confirmed by TEM) that fabricates high efficient ion conducting pathway in the membrane. The hydroxide conductivity follows Arrhenius behavior in the temperature range from 20−80 oC (Fig. 4(b)). The apparent activation energies of ABA-TQA-44, ABA-TQA-61 and ABA-TQA-90 are 11.62, 11.86 and 14.25 kJ mol-1, respectively, and are on par with the reported results.7,10,44,45 Additionally, the conductivity of the 20



AEMs in Br− form was also measured as a function of temperature. As shown in Fig. S10, the Br− conductivity of the ABA-TQA-x membranes ranged from 15.2 to 60.2 mS cm-1 at 20−80 oC, and is lower than that of the hydroxide form AEMs. 150

0

ABA-TQA-44 ABA-TQA-61 ABA-TQA-90 ABA-MQA

(a)

100

(b)

-1 Log σ (S cm-1)

-1

50

ABA-TQA-44 ABA-TQA-61 ABA-TQA-90 ABA-MQA -1

Ea=11.86 kJ mol

-2

Ea=11.62 kJ mol

-1

-3 -1

Ea=14.25 kJ mol

-4

20

40

60

-1

Ea=16.13 kJ mol

2.8

80

2.9

3.0

o

3.1

3.2 -1

Temperature ( C)

1000/T (K

)

Fig. 4 (a) Hydroxide conductivity and (b) Arrhenius plots of the AEMs. ABA-TQA-44

(a)

ABA-TQA-61

125

ABA-TQA-90

ABA-MQA -1

0

σ (mS cm )

Hydroxide conductivity (mS cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

MM-PES-1.5-1 ref. 46 QA-(PS140-PDVPPA90-PS140) ref. 47

100

SEBS-CH2-QA-1.5 ref. 48 PMP-TMA-41 ref. 49 PPO-7QPi-1.7 ref. 50

75

50PPOC6N ref. 51 C18BQAPPO-3 ref. 52 PPO7Q6Q-1.9 ref. 31 PPO7Q6Q6Q-1.9 ref. 31

50

PAES-12.5-IMPPO ref. 53

1.0

1.5

2.0 2.5 3.0 -1 IEC (meq g )

21


3.5

4.0

3.3

3.4

Page 23 of 38

ABA-TQA-44

(b)

ABA-TQA-61

125

ABA-TQA-90

ABA-MQA MM-PES-1.5-1 ref. 46 -1

σ (mS cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


QA-(PS140-PDVPPA90-PS140) ref. 47

100

SEBS-CH2-QA-1.5 ref. 48 PMP-TMA-41 ref. 49 PPO-7QPi-1.7 ref. 50

75

50PPOC6N ref. 51

C18BQAPPO-3 ref. 52 PPO7Q6Q-1.9 ref. 31

PPO7Q6Q6Q-1.9 ref. 31

50

PAES-12.5-IMPPO ref. 53

40

80

120 160 Water uptake (%)

200

240

Fig. 5 (a) Hydroxide conductivity (σ) of the reported AEMs as a function of IEC at 80 oC. (b) relationship of σ and water uptake of the reported AEMs at 80 oC.

For most AEMs, the hydroxide conductivity is pertinent to the IEC and water uptake. AEMs absorbing plenty of water or having a high IEC normally exhibit a high conductivity.40 Here, we compare the conductivity and IEC/water uptake of the ABA-TQA-x membranes with reported AEMs. As demonstrated in Fig. 5(a) & (b), the ABA-TQA-x exhibit higher hydroxide conductivity than reported AEMs with a similar IEC. Furthermore, ABA-TQA-44 and ABA-TQA-61 exhibit lower IEC and water uptake, and higher conductivity than most reported block copolymer AEMs46-48, monocation side chain AEMs49-51, multication side chain AEMs 31,52,53. Introduction of multi-QA side chain into the ABA-TQA-x membranes is believed to contribute strongly to the high conductivity and low water uptake. Additionally, the triblock structure and multi-QA side chain induce the AEMs to utilize water molecules more efficiently for conducting hydroxide ions. These results indicate that our strategy for preparing AEMs from multi-QA side chain and block structure is effective at enhancing conductivity as well as mitigating water swelling.

22



3.5 Thermal Stability and Mechanical Property

100

ABA-TQA-90 ABA-TQA-61 ABA-TQA-44 ABA-MQA

80 Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

60 40 20 0

200

400 600 o Temperature ( C)

800

Fig. 6 Thermal stability of the AEMs.

The thermal stability of ABA-TQA-x membranes in hydroxide form was tested under the protection of nitrogen. As demonstrated in Fig. 6, three weight loss stages are observed for the ABA-TQA-x. The first weight loss up to 100 oC is attributed to the evaporation of absorbed moisture. The second weight loss stage from 155 to 350 oC, is ascribed to the decomposition of cations and alkyl side chains. As noted by Dang et al., the decomposition temperature of quaternary ammonium groups is above 200 oC, and this value is higher than that of ABA-TQA-x membranes.31 This may be attributed to the difference in negative ions and pretreatment before collecting TGA data. The third stage (above 350 oC) results from the decomposition of the polymer backbone. One can thus conclude that the ABA-TQA-x membranes can meet the requirements for AEMFCs (50−80 oC).1

The mechanical properties of the ABA-TQA-x membranes in fully hydrated states at RT are demonstrated in Fig. 7. The tensile strength is in the range of 15.6−27.6 MPa, and the elongation at break is 10.6%−14.4%, indicating that the triblock copolymer AEMs with multi-QA side chain are 23


Page 25 of 38

strong and tough enough for fuel cells. It is observed that the elongation at break of the membranes increases with increasing IEC, and the tensile strength decreases with increasing IEC, mainly because of the plasticization of water.54

Tensile strength

40

Elongation at break

30

30

20

20

10

10

0

ABA-TQA-44 ABA-TQA-61

ABA-TQA-90

ABA-MQA

Elongation at break (%)

Tensile strength (MPa)

40

0

Fig. 7 Mechanical properties of the AEMs.

3.6 Alkaline Stability 2.5

80 ABA-TQA-44 ABA-TQA-90

(a)

ABA-TQA-61 ABA-MQA

2.0

60 40 20 0

ABA-TQA-44 ABA-TQA-90

(b) IEC (meq g-1)

Hydroxide conductivity (mS cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ABA-TQA-61 ABA-MQA

1.5 1.0 0.5

0

100

200

300

400

0.0

500

0

100

200

300

400

500

Time (h)

Time (h)

Fig. 8 The change in the (a) conductivity and (b) IEC of the AEMs during the alkaline stability test.

It is well-known that QA groups will suffer from degradation via nucleophilic substitution reaction, ylide formation, and/or Hofmann elimination under alkaline condition resulting in a conductivity decline.55 The long-term alkaline stability experiment is carried out by immersing the as-prepared 24



AEMs into a 1 M aqueous KOH solution at 80 oC, during which the IEC and hydroxide conductivity were monitored. The ABA-TQA-x membrane maintained tough and flexible throughout the stability test, demonstrating a good chemical stability of the backbone. As shown in Fig. 8, the hydroxide conductivity has a similar downtrend to the IEC, indicating that the conductivity decline is attributed to the cation degradation. The ABA-TQA-44, ABA-TQA-61 and ABA-TQA-90 remained 84.7%, 82.1% and 81.0% of their initial conductivity after 480 h, respectively. Meanwhile, the IEC of ABA-TQA-44, ABA-TQA-61 and ABA-TQA-90 decreased from 1.93 to 1.60 meq g-1, 1.60 to 1.30 meq g-1 and 1.28 to 1.03 meq g-1, respectively, remained about 80% of their initial values after 480 h. It is found that the alkaline stability of ABA-TQA-x is better than that of ABA-MQA. The ABA-MQA remained 72.9% and 72.7% of its initial conductivity and IEC, respectively, after the alkaline stability test. Furthermore, Parrondo et al. reported functionalized PPO AEMs bearing mono-quaternary ammonium groups via a hexyl spacer (TMA-C6-PPO).56 The structure of the TMA-C6-PPO is similar to the functionalized poly(phenylene oxide) segments in ABA-MQA. However, the TMA-C6-PPO remained only 67% of its initial IEC value after 30 days stability test (60 oC, 1 M KOH). The alkaline stability of the ABA-TQA-x membranes is better than that of TMA-C6-PPO, even at a higher temperature. Except for the testing time, there are two main reasons for the difference in stability. (1) The well-defined hydrophilic/hydrophobic phase-separated morphology of the as-prepared AEMs is favorable to the good solvation of OH− in a hydrophilic environment, which will result in a slower chemical reaction of cationic groups;42 (2) The long alkyl spaces between the backbone and QA groups as well as between the cations themselves are in favor of the good alkaline stability of the ABA-TQA-x membranes. 28,31

25


Page 26 of 38

Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

13

2

56

34

78 9 10 11 12

12,16,H2O DMSO

15 14 16 17

13,17

6

4,3 5

11,15 1

9,10,14

2,8

Before 3.78

1.00

After 3.14

1.00

9

8

7

6 5 4 3 Chemical shift (ppm)

2

1

Fig. 9 1H NMR spectra of ABA-TQA-44 before and after the alkaline stability test. Fig. 9 shows the 1H NMR spectra of ABA-TQA-44 before and after the stability test. As seen, the integral area of the peak at 3.0 ppm (H13 and H17, attributed to the proton resonance of methyl groups from cationic groups in the side chain) decreased slightly after 480 h over the original membrane, suggesting that the cationic groups degraded during the test. This confirms that the decline in conductivity and IEC is stemmed from the degradation of cationic groups. Two new signals appeared at 0.75 and 1.25 ppm after the stability test. These signals are ascribed to the degradation product of the cationic groups. Additionally, the peak at 3.0 ppm is split into two peaks. This new peak is attributed to trimethylalkyl or dimethyldialkyl ammonium groups, which may come from the degradation product of quaternary ammonium groups in the terminal of the side chain. The alkyl chain close to the first and second cations in the side chain will protect the cations from the attack of OH− due to the steric hindrance effect, while less alkyl chain is close to the cation in the terminal of the side chain. As 26



a result, the third cation may be prone to degrade.

3.7 Single Cell Performance 250

Tokuyama AHA ABA-MQA ABA-TQA-44

(b) 200 150

0.5

100 50

0.0

0

200

400

400

0.7

600

300 0.6

200 0.5

100 0.4

0 800

0

6

12

18

Current density (mA cm-2)

(a)

Power density (mW cm-2) Cell voltage (V)

1.0

Cell voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 38

0 24

Testing time (h)

Current density (mA cm-2)

Fig. 10 (a) Polarization curves and power density of a single cell at 80 oC using ABA-TQA-44, ABA-MQA and Tokuyama AHA as the electrolyte membranes with a H2/O2 flow rate of 100/100 mL min-1. (b) Durability test of the cell at 80 oC under a constant current density of 100 mA cm-2.

A H2/O2 single cell performance was examined to address the applicability of the AEMs. Fig. 10(a) demonstrates the performance of the single cell using ABA-TQA-44, ABA-MQA and Tokuyama AHA as the electrolyte membranes. The open circuit voltage of the cell based on ABA-TQA-44 is 1.05 V, and this value is close to the theoretical value (1.23 V), demonstrating that the ABA-TQA-44 membrane possess good gas barrier ability. The peak power density of the cell using ABA-TQA-44 is 204.6 mW cm-2 at a current density of 600 mA cm-2, and is much higher than that measured from the cell using ABA-MQA (71.0 mW cm-2) and commercialized Tokuyama AHA (52.0 mW cm-2) at the same condition. Additionally, the high frequency resistance (HFR) of cell based on ABA-TQA-44 is measured to be nearly constant value of ~0.3 Ω cm2 (Fig. S11), and is lower than that of ~0.7 Ω cm2 measured from the cell using ABA-MQA. The high current density cell output and low HFR of the cell using ABA-TQA-44 result from the high conductivity. It is noted that the performance of the fuel 27


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cells can be further enhanced by optimizing the electrode structure, catalyst, MEA fabrication method, operating condition and so on.57 Our further work will focus on optimizing these conditions.

The durability of ABA-TQA-44 was also evaluated in a single cell. The cell was operated under a constant current density of 100 mA cm-2, during which the cell voltages were recorded (Fig. 10(b)). It is observed that the cell voltage decreases slightly from 0.58 to 0.55 V during the testing time of 24 h, indicating good durability of ABA-TQA-44 for fuel cell applications. This is attributed to the good chemical stability of ABA-TQA-44 under alkaline condition. Above all, the higher power density and good durability indicate that the triblock copolymer AEMs bearing multi-QA side chain are promising electrolyte membranes for fuel cells.

4. CONCLUTIONS In summary, a series of triblock copolymer AEMs bearing multi-QA side chain were prepared via nucleophilic substitution, coupling reaction, Friedel-Crafts acylation, ketone reduction and quaternization reaction. TEM and SAXS revealed that a well-developed microphase-separated morphology was formed in the as-prepared AEMs due to the self-assembly of the copolymers. Consequently, the ABA-TQA-x with the highest IEC exhibited the highest conductivity of 130.5 mS cm-1 at 80 oC. This value can meet the requirement for the practical application of the AEMs. Furthermore, the introduction of long alkyl spacers between the backbone and QA groups, as well as between the cations themselves are responsible for the reasonable water swelling and robust alkaline stability. The conductivity and IEC of ABA-TQA-44 only decreased by 15.3% and 16.9%, respectively, after being tested in a 1 M aqueous KOH solution for 480 h. Therefore, one can conclude that the strategy for utilizing a triblock copolymer structure and multi-QA side chain is effective for improving conductivity, mitigating water swelling as well as enhancing the alkaline stability of the AEMs. 28



ASSOCIATED CONTENT Supporting Information Route for synthesis of the MQA, DQA, PPO-OH and PPO-F; procedure for synthesis of the DQA, oligomers and copolymers; NMR spectra of all the as-synthesized products; molecular weight of the oligomers and copolymers; water uptake, swelling ratio and Br− conductivity of the AEMs; cell resistance of the single cell. ACKNOWLEDGEMENTS Financial support from the National Nature Science Foundation of China (grant nos. 21576226, 21376194 & 21736009) is gratefully acknowledged. REFERENCES (1) Stamenkovic, V. R.; Strmcnik, D.; Lopes, P. P.; Markovic, N. M., Energy and Fuels From Electrochemical Interfaces. Nat. Mater. 2017, 16 (1), 57-69. (2) Kim, Y.; Moh, L. C. H.; Swager, T. M., Anion Exchange Membranes: Enhancement by Addition of Unfunctionalized Triptycene Poly(Ether Sulfone)s. ACS Appl. Mater. Interfaces 2017, 9 (49), 42409-42414. (3) Hossen, M. M.; Artyushkova, K.; Atanassov, P.; Serov, A., Synthesis and Characterization of High Performing Fe-N-C Catalyst for Oxygen Reduction Reaction (ORR) in Alkaline Exchange Membrane Fuel Cells. J. Power Sources 2018, 375, 214-221. (4) Zhuang, Z.; Giles, S. A.; Zheng, J.; Jenness, G. R.; Caratzoulas, S.; Vlachos, D. G.; Yan, Y., Nickel Supported on Nitrogen-Doped Carbon Nanotubes as Hydrogen Oxidation Reaction Catalyst in Alkaline Electrolyte. Nat. Commu. 2016, 7, 10141-10148. (5) Zeng, L.; Zhao, T. S.; An, L., A High-Performance Supportless Silver Nanowire Catalyst for 29


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