Orderly branched anion exchange membranes bearing long flexible multi-cation side chain for alkaline fuel cells

Journal of Membrane Science 589 (2019) 117247

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Orderly branched anion exchange membranes bearing long flexible multication side chain for alkaline fuel cells

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Xue Lang Gao, Qian Yang, Hong Yue Wu, Qi Hui Sun, Zhao Yu Zhu, Qiu Gen Zhang, Ai Mei Zhu, Qing Lin Liu* Department of Chemical & Biochemical Engineering, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Orderly branched structure Anion exchange membranes Phase separation

Poly(arylene ether ketone) (PEK) based anion exchange membranes (AEMs) with different structures were successfully prepared in this work by polycondensation, bromination and Menshutkin reaction. The orderly branched AEMs (OBAEMs) have an obvious microphase separation morphology and well-connected ion cluster region, as confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The OBAEMs with IEC of 1.81−2.01 meq g-1 showed superior ionic conductivity (93.3−119 mS cm-1) over the random branched AEM (RBAEM) (78.9 mS cm-1) and the side-chain AEM (SAEM) (67.8 mS cm-1). Meanwhile, the maximum power density of a single cell using the OBAEMs (130.8 mW cm-2) is much higher than that using the RBAEM (84.1 mW cm-2) and the SAEM (63.2 mW cm-2). Besides this, the OBAEMs presented reasonable alkaline stability after exposed to a harsh alkaline environment at 80 °C for 500 h. All those results pointed out that the AEMs with orderly branched structure holds a promising future for application in fuel cells.

1. Introduction As a kind of promising energy conversion device, polymer electrolyte fuel cells (PEFCs) have attracted considerable concentration for their excellent energy efficiency, portability and friendly emissions. Generally, PEFCs can be classified into two types, proton exchange membrane fuel cells (PEMFCs) and anion exchange membrane fuel cells (AEMFCs). PEMFCs have been widely explored attributed to the high proton conductivity, desirable alkaline and dimensional stability of proton exchange membranes (PEMs) [1-3]. However, the practical application of PEMFCs is still seriously hindered by the high fuel permeability of the PEMs and consumption of precious metal catalysts such as platinum [4]. On the contrary, AEMFCs are gradually becoming the focus of researchers for their numerous advantages, including the improved oxygen reduction kinetics, the reduced corrosion oxidation in alkaline media and the use of non-noble metal catalyst [3-4]. Meanwhile, AEMs are responsible for transporting OH− between the two poles of AEMFCs and play a crucial role in the fuel cell performance [5]. Hitherto, a wide range of polymer backbones and cationic groups have been investigated for the AEM applications. Poly(ether sulfone) (PES) [6-7], poly(phenylene oxide) [8-9] and PEK [10-11] have been currently applied in the preparation of polymer membranes due to their

*

excellent chemical resistance and thermal ability. Various cationic groups including quaternary ammonium (QA) [12-13], imidazolium [14-15], piperidinium [16-17] and spiral quaternary ammonium [1819] are available for preparation of AEMs for fuel cells. Although great efforts have been taken to make high-performance AEMs, the application of AEMs still lags behind PEMs due to the insufficient ionic conductivity and poor alkaline stability. To overcome these obstacles, researchers are trying to design suitable polymer backbone structures in anticipation of forming well-constructed ion channels to improve the membrane performance. Up to now, various polymer backbone structures have been explored as the AEM materials such as side-chain [4,20], comb-shaped [21-22], crosslinked [6,23], interpenetrated [2425], block [5,26] and branched polymers [27-28]. It is worth mentioning that the branched polymers have received plenty of attention in many fields owing to the unwinding and good solubility [29]. Matsumura et al. [30] reported series of branched PES based PEMs with sulfonic acid groups that were synthesized using a novel trifunctional branching agent. The branched PEMs with significant phase-separated morphology showed higher proton conductivity than Nafion membranes. Liu et al. [31] reported that a random branched polystyrene based AEM (QHPVI-OH) displayed desirable dimensional stability (the limited swelling ratio only increased

Corresponding author. E-mail address: [email protected] (Q.L. Liu).

https://doi.org/10.1016/j.memsci.2019.117247 Received 13 April 2019; Received in revised form 10 June 2019; Accepted 3 July 2019 Available online 05 July 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.

Journal of Membrane Science 589 (2019) 117247

X.L. Gao, et al.

with water repeatedly and purification by anhydrous DMSO, a brown product was obtained with a yield of 76.09% (1.94 g).

by 0.3% from 30 to 80 °C) and improved ionic conductivity of 85.1 mS cm-1. Ge et al. [27] prepared a range of random branched poly (vinyl benzyl chloride) AEMs with excellent ionic conductivity of 123 mS cm-1 and exceptional alkaline stability (a negligible decrease in conductivity after immersed in a 1 M sodium hydroxide solution for 20 days) at 80 °C. Recently, our group [28] has reported a set of AEMs based on block hyperbranched PEKs with long side chains showing a high ionic conductivity up to 122.5 mS cm-1 (IEC = 2.21 meg q-1) and a maximum power density of nearly 190 mW cm-2 at 80 °C. Although some branched structures have been studied, unfortunately, researches on the performance of ion conductive membranes with different branched structures using the same monomer are still in its infancy. Inspired by this, in this work we are motivated to investigate the effects of different branched structures produced by placing the branching agent in different positions of main chain on the properties of AEMs. Therefore, a series of AEMs including orderly branched, random branched and side-chain structure were successfully synthesized by polycondensation, bromination and Menshutkin reaction. Particularly, to prevent crosslinking and improve the performance of AEMs, the degree of branching was set to 8%. Meanwhile, long flexible multi-cation side chains were introduced to facilitate obvious hydrophilic/hydrophobic phase separation and improve the ionic conductivity [32]. TEM, AFM and small angle X-ray scattering (SAXS) were taken to observe the microphase separated morphology of AEMs. The ionic conductivity, mechanical property, dimensional stability, thermal stability and alkaline stability of the AEMs were investigated. The performance of a single cell with the as-prepared AEMs was studied systematically. The effect of IEC on the properties of OBAEMs was investigated in detail.

2.2.3. Synthesis of side-chain poly (arylene ether ketone)-50 (SPEK-50), orderly branched poly (arylene ether ketone)-X (OBPEK-X) and random branched poly (arylene ether ketone)-50 (RBPEK-50) The polymerization and branching degrees were set at X% (X = 100 × THQ/FBP) and 8% (branching agent/(BPZ + THQ)), respectively. The synthetic routes for SPEK-50, RBPEK-50 and OBPEK-X (X = 50, 55 and 60) are illustrated in Scheme 2. 2.2.3.1. Synthesis of SPEK-50. A typical polycondensation reaction for synthesizing SPEK-50 is shown in Scheme 2A. BPZ (2.68 g, 10.00 mmol), THQ (1.24 g, 10.00 mmol), FBP (4.36 g, 20.00 mmol), K2CO3 (6.08 g, 44.00 mmol), DMAc (50 mL) and toluene (25 mL) were added into a 150 mL flask equipped with a Dean-Stark trap, a condenser, a N2 inlet and a magnetic stirrer. Firstly, the reaction mixture was heated at 145 °C for 4 h. After the azeotropic agent was removed, the reaction mixture was continued heating to 175 °C for 16 h. When the reaction system was cooled to 160 °C, BPZ (0.13 g, 0.50 mmol) and DMAc (3 mL) were carefully introduced into the reaction mixture to trigger blocking reaction for 4 h. Finally, the resulting mixture was precipitated in methanol and washed repeatedly. A white fibrous polymer (SPEK-50) was gained after drying in a vacuum oven at 80 °C overnight (yield: 98.9%) 2.2.3.2. Synthesis of OBPEK-X. The OBPEK-X was synthesized from SPEK-X and branching agent Compound 1 via condensation polymerization as shown in Scheme 2B. Taking the synthesis of OBPEK-50 as an example, typically, SPEK-50 (2.00 g, 0.09 mmol), Compound 1 (0.29 g, 0.43 mmol), K2CO3 (0.25 g, 1.77 mmol), DMAc (20 mL) and toluene (10 mL) were carefully charged into a 100 mL flask, follow by stirring at 145 °C for 4 h and 175 °C for 16 h. Following precipitation, purification and drying treatment the same as for the synthesis of SPEK-50, a pale fibrous polymer (OBPEK-50) was obtained (yield: 97%).

2. Experimental 2.1. Materials 4, 4′-Difluoro-diphenylmethanone (FBP, 99%) and bisphenol Z (BPZ, 98%) were supplied from TCI Reagent. Methylhydroquinone (THQ, 98%), N-bromosuccinimide (NBS), 4-fluorobenzoyl chloride (98%), N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA), 1,6-dibromohexane (DBH), 1,3,5-triphenyl benzene (TPB, 98%), trimethylamine (TMA, 25% in H2O), iron (III)-chloride (FeCl3, 99%) and benzoyl peroxide (BPO, 99%) were bought form J&K Chemical Ltd. N, N-dimethylformamide (DMF, AR), N, N-dimethylacetamide (DMAc, AR), CDCl3 (99%), toluene (AR) and anhydrous dimethyl sulfoxide (DMSO, with 4 Å molecular sieves) were obtained from Aladdin Reagent. Potassium carbonate (K2CO3, 99%), deionized (DI) water and other reagents were obtained commercially and used as received.

2.2.3.3. Synthesis of RBPEK-50. RBPEK-50 (Scheme 2C) was synthesized by one-pot method. Compound 1 (0.54 g, 0.80 mmol), FBP (2.18 g, 10.00 mmol), BPZ (1.34 g, 5.00 mmol), THQ (0.64 g, 5.00 mmol), K2CO3 (2.76 g, 20.00 mmol), DMAc (25 mL) and toluene (10 mL) were put into a 100 mL flask. By the same procedure as described for the synthesis of OBPEK-50, a pale fibrous polymer (RBPEK-50) was made (yield: 98.7%). 2.2.4. Bromination of the OBPEK-X, RBPEK-50 and SPEK-50 By adjusting the amount of NBS and BPO, brominated orderly branched PEK-X (Br-OBPEK-x), random branched PEK-50 (Br-RBPEK50) and side-chain PEK-50 (Br-SPEK-50) were synthesized, separately. Taking the synthesis of brominated OBPEK-50 as example, OBPEK-50 (1.00 g, 1.34 mmol) was first dissolved in CH2Cl4 (50 mL) in a 100 mL single port flask, then BPO (0.06 g, 0.19 mmol) and an excess amount of NBS (0.60 g, 3.37 mmol) were added as the radical initiator and the brominating reagent, respectively. After kept at 85 °C for 5 h under stirring, the resulting mixture was cooled down and precipitated in methanol. Finally, a yellowish-white precipitate Br-OBPEK-50 was obtained after washing with methanol repeatedly and drying at 80 °C under vacuum overnight.

2.2. Preparation of AEMs 2.2.1. Synthesis of the ionic liquid 1-Bromo-N, N, N-trimethylhexane-6-aminium bromide (MQA) and 1-(N′, N′-dimethylamino)-6, 12-(N, N, N-trimethylammonium) dodecane bromide (BQA) were synthesized according to the previous report [32]. The synthesis process was described in detail in Supplementary Information (Scheme S1, Figs. S1 and S2). 2.2.2. Synthesis of monomer (compound 1) Compound 1 (Scheme 1) was synthesized from 4-fluorobenzoyl chloride and TPB by a method as reported in the literature with modification [30]. Typically, FeCl3 (0.56 g, 3.50 mmol), TPB (1.00 g, 3.26 mmol) and 4-fluorobenzoyl chloride (1.71 g, 10.77 mmol) were added into a 50 mL Schlenk-tube. Prior to reaction, the bottle was subjected to freeze-pump-thaw cycles. Then, nitrobenzene (5 mL) was slowly injected into the mixture, followed by stirring at 110 °C for 3 h. The resulting mixture was cooled down and poured into a mixture of methanol/HCl (1/0.03, v/v) to obtain a precipitate. After washing it

2.2.5. Synthesis of BQA-OBPEK-X, BQA-RBPEK-50 and BQA-SPEK-50 and membrane formation The BQA-OBPEK-X, BQA-RBPEK-50 and BQA-SPEK-50 were synthesized by the Menschutkin reaction of brominated polymers with a certain amount of BQA, separately. The synthesis of BQA-OBPEK-50 was taken as an example. Br-OBPEK-50 (0.50 g) was firstly dissolved in DMF (10 mL) at 80 °C to yield a 5 wt % brownish yellow solution, then 2

Journal of Membrane Science 589 (2019) 117247

X.L. Gao, et al.

Scheme 1. Synthesis of monomer (compound 1).

Scheme 2. Synthesis of the polymers.

water more than 24 h before measurements. Moreover, the membranes used for the subsequent tests were all in OH− form.

BQA (0.30 g) was charged into the system and stirred at 80 °C for 8 h. The transparent solution obtained by centrifugation was poured onto a clean glass plate, and dried under vacuum at 60 °C for 24 h to obtain a 30~50 μm thick membrane. The obtained membrane was subsequently stripped and soaked in a 1 M sodium hydroxide solution for 24 h to exchange ions. Finally, the as-prepared AEMs were immersed in DI 3

Journal of Membrane Science 589 (2019) 117247

X.L. Gao, et al.

electrochemical workstation (Versa STAT 4, USA). The temperature was raised from 30 to 80 °C and kept constant for 1 h every 10 °C during the measurement. The ionic conductivity (σ, mS cm-1) of the AEMs is calculated as follows

2.3. Characterizations 2.3.1. 1H and 13C NMR The 1H NMR spectra of the synthesized Compound 1, BQA, OBPEKX, RBPEK-50, SPEK-50, brominated polymers and BQA-polymers and 13 C NMR of Compound 1 were recorded with a Bruker advance III spectrometer (Switzerland, 500 MHz) using CDCl3, D2O or DMSO‑d6 as the solvent.

σ=

where L (cm) is the distance between the cathode and the anode, A (cm2) represents the cross-sectional area of the sample, and R (Ω) denotes the membrane's resistance.

2.3.2. Microphase structure The TEM images were acquired on an electron microscope (JEM2100, Japan, 200 kV). AFM (DI, Bruker Co.) was employed to examine the morphology of the AEMs in a tapping mode under RT. The samples for TEM and AFM characterization were prepared according to the report [28]. SAXS (Anton Paar, Austria) was also used to further investigate the microphase separation of the AEMs at RT.

2.4.4. Mechanical property and thermal stability The mechanical property of the AEMs was measured on a universal testing machine (Instron 3343). Prior to test, the AEM samples in bone shape (20 mm × 2 mm) were immerged in DI water for 24 h. Thermal stability of the AEMs was evaluated by using a thermogravimetric analyzer (TG209F1, Netzsch, Germany) under an N2 atmosphere with a temperature ranging from 30 to 800 °C.

2.3.3. Gel permeation chromatograph (GPC) The molecular weights of the synthesized OBPEK-X, SPEK-50 and RBPEK-50 were measured via a GPC system (Waters, MA, USA) equipped with a Waters 1515 isocratic HPLC pump, three Styragel columns (Waters HT4, HT5E and HT6) and a Waters 2414 refractive detector. HPLC-grade tetrahydrofuran (THF) was used as the eluent standards.

2.4.5. Alkaline stability The alkaline stability of the AEMs was evaluated by immersing in alkaline media at 80 °C for a certain time. The remaining ionic conductivity of the AEMs was monitored during the immersion. The change in chemical structure of the AEMs before and after the alkaline resistance test was characterized via 1H NMR. The AEMs are insoluble in THF, so their molecular weight cannot be determined using GPC. Instead, the viscosity of the AEMs before and after the alkaline stability test was measured via a Ukrainian-style viscometer to characterize the change in the molecular weight. The AEM samples were dissolved in DMF to make a 1.830 g dL-1 solution. The intrinsic viscosity of the solution was recorded (DMF was used as the standard).

2.4. Measurements 2.4.1. Water uptake (WU) and swelling ratio (SR) Before measurements, the as-prepared AEM was dried at 60 °C for 48 h, and immersed into DI water at a preset temperature (30, 60 and 80 °C) for more than 48 h. After that, the membrane was fetched out and the water in its surface was wiped with tissue paper quickly. The mass and length of fully hydrated membrane were recorded. The WU of the AEM is calculated by

2.4.6. Single cell performance The membrane electrode assembly (MEA) was made by the method [33]. Firstly, Pt/C catalyst (40% wt% Pt, Johnson Matthey, UK) was mixed with ethanol and the ionomer solution by ultrasonic dispersion to prepare a homogeneous catalyst ink at RT. Subsequently, the catalyst ink was sprayed on both sides of the membrane to fabricate the catalystcoated membrane (CCM). After that, the catalyst ink was repeatedly loaded into carbon paper to prepare the anode and the cathode. Finally, the 410~440 μm thick MEA was constructed by sandwiching the CCM between the two electrodes. The performance of a single cell with three kinds of AEMs was evaluated on a system (Kunshan Sunlaite, China) at 80 °C with 100% humidified pure hydrogen and oxygen gases at 100 mL min-1.

Mw − Md Md

WU =

where Md and Mw represent the mass of the dry and wet AEMs, respectively. The SR of the membranes is calculated by:

SR =

l wet − ldry ldry

where ldry and lwet are the length of the dry and wet AEMs, respectively. The hydration number (λ) around each ionic group can be calculated as follows

λ=

L AR

WU × 10 IEC × 18

3. Results and discussion 3.1. Synthesis and characterization of BQA

2.4.2. Ionic exchange capacity (IEC) The IEC of the AEMs was measured by the back-titration method. Dry AEMs were soaked into a 0.1 M hydrochloric acid solution for 24 h, followed by back-titrating with a 0.1 M sodium hydroxide solution. The calculation of IEC is described as follows

IEC =

BQA was made by two-step method via the Menshutkin reaction. (Scheme S1). For the preparation of MQA, a TMA gas was slowly bubbled into a solution consisting of excess 1,6-dibromohexane (DBH) and THF under stirring to effectively prevent the di-quaternized byproduct formation [34]. Chemical structure of the MQA was confirmed using 1H NMR in Fig. S1, in which the peak area ratio (4.52:1.00) of the quaternary ammonium (QA) group (-N+-CH3) (3.07 ppm) to the alkyl bromide group (Br–CH2-) (3.48 ppm) is close to the theoretical value (4.50:1.00), indicating the successful preparation of the MQA and the selective quaternization of MQA at one end only. For the preparation of BQA, a MQA solution was added dropwise into an excess TMHDA solution to suppress the formation of bis-substituted by-product [32]. The 1H NMR of BQA is exhibited in Fig. S2, compared with Fig. S1, the peak at 3.48 ppm assignable to the alkyl bromide group (Br–CH2-) disappeared. Instead, there are two new

C1 V1 − C2 V2 Md

where C1 (M) and V1 (mL) denote the concentration and the volume of the hydrochloric acid solution, respectively, and C2 and V2 are the concentration and the volume of the sodium hydroxide solution consumed in the titration, respectively. 2.4.3. Ionic conductivity Before the measurement, AEMs were placed in DI water at ambient temperature for 4 h. Then the resistance of AEMs was recorded on an 4

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reaction between BQA and the brominated polymers. The 1H NMR spectrum of BQA-OBPEK-50 was interpreted as an example (Fig. S7b). Compared with the 1H NMR spectrum of Br-OBPEK-50 (Fig. S7a), the new peaks emerged at 1.30 (He), 1.69 (Hd) and 3.31 (Hc) ppm in Fig. S7b are associated with the methylene protons and the new peaks appeared at 3.06 ppm (Hb and Hf) belong to the methyl protons of BQA. Moreover, the characteristic peak of the bromomethyl groups at 4.50 ppm (Ha, Fig. S7a) shifts to 4.63 (Ha’, Fig. S7b) ppm, and is associated with the methylene protons of BQA. All those results indicate the success of the Menschutkin reaction. The schematic diagram of the three kinds of AEMs is displayed in Fig. 1.

peaks emerged at 2.16 and 2.98 ppm, which are ascribed to the methyl of tertiary amine and new methyl of the QA group, respectively. Further, the peak area ratio of the methyl group of the tertiary amine (2.16 ppm) to the methyl group of the QA group (3.07 ppm) and the methyl group of new QA group (2.98 ppm) was 1.00:1.53:1.02, which is very close to the theoretical value (1.00:1.5:1.00). Those results indicate that the desired BQA was obtained and the quaternization of BQA selectively occurred at one end only. 3.2. Synthesis and characterization of monomer (compound 1) Compound 1 was synthesized in one step by Friedel-Crafts reaction (Scheme 1) and characterized by 1H NMR (Fig. S3) and 13C NMR (Fig. S4). The peaks at 7.38−7.47 ppm (H5 in Fig. S3) is assignable to protons of the benzene ring near the fluorine end group. The peaks at 7.51−7.60 (H2, H4), 7.85−7.92 (H3) and 8.05−8.17(H1) ppm (Fig. S3) are assignable to the aromatic protons. According to the peaks shown in Fig. S3, the peak area ratio of H1:H3:H2&H4:H5 is 1.00:1.93:3.93:1.96, and is very close to the theoretical value of 1.00:2.00:4.00:2.00, indicating that the desirable Compound 1 was successfully prepared. Meanwhile, 13C NMR also demonstrated the successful synthesis of Compound 1 (Fig. S4).

3.6. Morphology TEM was adopted to investigate the microstructure of the as-prepared AEMs. As depicted in Fig. 2 (a-c), the light areas represent the hydrophobic regions formed by the aggregation of the aromatic polymer backbones, while the darker regions are ascribed to the hydrophilic domains probably consisting of water molecules and the ion cluster of QA groups [21,35]. Particularly, BQA-OBPEK-50 has the most distinct microphase separation. This is attribute to the presence of a unique orderly branched structure resembling a block structure in the OBAEM. As a result, the hydrophilic and hydrophobic segments in the AEM are readily self-assembled to induce microphase separation [33]. The microphase separation structure of AEMs was further observed by AFM. As shown in Fig. 2 (d-f), BQA-OBPEK-50 has the largest dark domains. This result is in accordance with the TEM observation. Moreover, as displayed in Fig. 2 (f-h), an increase in the IEC of OBAEMs tended to result in aggregation of larger ionic domains, which is crucial for ion transport [36,37]. The morphology and the distance between ionic domains were further explored by SASX [5,38-39]. Fig. 3 shows the distinct peaks (qmax) of BQA-SPEK-50, BQA-RBPEK-50, BQA-OBPEK-50, BQA-OBPEK55 and BQA-OBPEK-60 located at 1.65, 0.85, 0.75, 0.62 and 0.55 nm-1, respectively. Their corresponding average interdomain spacing (d) calculated from the Bragg's equation (d = 2π/qmax) is 3.8, 7.4, 8.4, 10.1 and 11.4 nm, respectively. Those results indicate that the ionic domain size of the OBAEMs has the largest value. The domain size increases with IEC, which is consistent with the TEM and AFM phase images.

3.3. Synthesis and characterization of polymers OBPEK-X (X = 50, 55 and 60), RBPEK-50 and SPEK-50 containing THQ were produced via polycondensation (Scheme 2). The difference in the chemical structure of SPEK-50, RBPEK-50 and OBPEK-50 was examined by 1H NMR (Fig. S5). SPEK-50 (Fig. S5a) was taken as an example, the characteristic peaks at 1.55−1.60 ppm (H4-5) and 2.3 ppm (H3) are associated with the cyclohexylidene moiety of BPZ. The peak at 2.22 ppm (H11) is ascribed to the methyl groups of THQ (benzylmethyl). Meanwhile, the peaks appeared at 7.81−7.83 ppm (H7) and 6.97−7.05 ppm (H1, H6, H8-10) are assignable to the protons of the benzene rings adjacent the ketone groups and the ether bonds, respectively. All those characteristic peaks reveal the successful synthesis of the desired SPEK-50. As for RBPEK-50 (Fig. S5b) and OBPEK-50 (Fig. S5c), apart from the same proton peaks as SPEK-50, new peaks emerged at 7.89−7.92 ppm (H12−16, Fig. S5b) and 7.91−7.95 ppm (H12’−H16’, Fig. S5c) are associated with protons of the branching agent. This suggests that the branching agent was successfully introduced into the random branched and orderly branched polymers. Further, the molecular weight of the OBEPK-X, RBPEK-50 and SPEK50 polymers is in the range of 22.6–73.5 kDa (Table S1), as determined by GPC (GPC curves are shown in Fig. S6). This indicates all the polymers displaying enough molecular weight for membrane formation.

3.7. IEC, WU and SR analysis As an important feature of AEMs, IEC plays an important role in ionic conductivity and is closely related to WU and SR. As depicted in Table 1, the IEC of the AEMs is within the scope of 1.81−2.03 meq g-1 calculated from the 1H NMR spectra and 1.79−2.01 meq g-1 from the back-titration. The similar IEC values acquired from the both methods indicate almost complete synthesis of the target materials. The WU and SR have profound effects on the ionic conductivity and dimensional stability of the AEMs. As listed in Table 1, the WU of the AEMs increased from 24.5% to 66.6% at 30 °C and from 70.8% to 123.4% at 80 °C, while the SR increased from 8.1% to 15.2% at 30 °C and from 19.3% to 31.1% at 80 °C. As can be seen, the order of WU of the AEMs at similar IECs is: OBAEM > RBAEM > SAEM, while the SR is: RBAEM > SAEM > OBAEM. This result may be attributed to the bulky rigid orderly branched structure of the OBAEM, which could hinder the close packing of the polymer chains, increase the inter-chain spacing of membrane and restrict the movement of the backbone, thus creating an additional free volume within the AEMs to promote WU and to inhibit SR [40,41]. Meanwhile, the well-distributed long alkyl multication side chain within the OBAEMs can allow water molecules to keep away from the polymer backbone, also preventing excessive swelling of the AEMs [15,39,42]. In addition, as expected, the WU and SR of the OBAEMs at various temperatures increase with increasing IEC, as depicted in Fig. 4. However, BQA-OBPEK-60 with the highest IEC of 2.01 meq·g-1 still displays higher dimensional stability than some previously

3.4. Bromination of polymers Chemical structure of the brominated polymers was confirmed by H NMR. Fig. S7a exhibits the 1H NMR spectrum of brominated OBPEK50 (Br-OBPEK-50). Compared with the 1H NMR of OBPEK-50 (Fig. S5c), the peak appeared at 2.22 ppm (H11) associating with the benzylmethyl is significantly weakened after bromination reaction, and the new characteristic peak emerged at 4.5 ppm (Ha) is related to the methylene proton of bromomethyl. These results reveal the successful synthesis of Br-OBPEK-50. Further, the degree of bromination (DB) of those brominated polymers can be calculated from the integral area ratio of the brominated benzyl peaks to the brominated benzyl peaks and the remained benzylmethyl peaks, and is listed in Table 1. 1

3.5. Membrane preparation and characterization As depicted in Scheme 2, the AEMs were made via the Menschutkin 5

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Table 1 IEC, DB, WU, SR and λ of the AEMs. Membrane

IEC (meq·g-1) Theor.

BQA-SPEK-50 BQA-RBPEK-50 BQA-OBPEK-50 BQA-OBPEK-55 BQA-OBPEK-60 a b

a

1.81 1.84 1.82 1.93 2.03

WU (%) Exp.

b

1.79 ± 0.05 1.82 ± 0.06 1.81 ± 0.03 1.90 ± 0.07 2.01 ± 0.05

SR (%)

DB

30 °C

80 °C

30 °C

80 °C

RT

24.5 34.7 45.7 53.6 66.6

70.8 90.6 97.8 108.5 123.4

8.7 15.2 8.1 10.1 11.3

27.8 31.1 19.3 21.4 25.0

68.5% 67.8% 69.6% 68.2% 65.6%

λ

6.4 9.3 13.7 15.4 18.2

Calculated from 1H NMR spectra. Measured by Mohr's titration.

Fig. 1. Schematic diagram of the AEMs.

Fig. 2. TEM phase images of the (a) BQA-SPEK-50, (b) BQA-RBPK-50 and (c) BQA-OBPEK-50 AEMs, the AFM phase images (1 μm × 1 μm) of the (d) BQA-SPEK-50, (e) BQA-RBPEK-50, (f) BQA-OBPEK-50, (g) BQA-OBPEK-55 and (h) BQA-OBPEK-60 AEMs.

the well-developed microphase separation in the OBAEM. Meanwhile, the ionic conductivity of the OBAEMs steadily increased from 93.3 to 119.0 mS cm-1 at 80 °C by increasing the IEC of the OBAEMs from 1.81 to 2.01 meq g-1. This is mainly due to the increase in the number of hydrophilic segments in the AEMs. Arrhenius plots of the as-prepared AEMs are shown in Fig. 5b. Conversely, unlike BQA-SPEK-50 exhibiting higher apparent activation energy (Ea) of 18.96 kJ mol-1, the BQA-OBPEK-X has an Ea in a lower range of 18.46−14.95 kJ mol-1, which is lower than some data in the report [5,44-47]. This indicates that less energy is required for transporting hydroxide ions in the OBAEMs.

reported AEMs [28,43]. To describe the WU of AEMs more practically, the average water absorbability of each ionic group for AEMs (designated as λ) calculated is shown in Table 1. As we can see, λ for the asprepared AEMs is within the range of 6.4–18.2 at 30 °C. 3.8. Ionic conductivity The temperature-dependent ionic conductivity of the AEMs is plotted in Fig. 5a. The BQA-SPEK-50, BQA-RBPEK-50 and BQA-OBPEK50 AEMs show conductivities of 23.5, 28.3 and 33.4 mS cm-1 at 30 °C, and 67.8, 78.9 and 93.3 mS cm-1 at 80 °C, respectively. The results reveal that the order of ionic conductivity of those three kinds of AEMs at similar IEC values is: OBAEM > RBAEM > SAEM. This is attributed to 6

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worth noting that the TS and YM of the OBAEMs reduce with the increment of IEC, while the EB shows a countertrend. This is mainly attributed to the introduction of more hydrophilic segments that has been found to increase the plasticization effect of water [48]. Thermal stability of the AEMs was tested by TGA from 35 to 800 °C under an N2 atmosphere. As depicted in Fig. 6b, all the AEMs display three weight loss stages. The first weight loss stage up to 270 °C is associated with the evaporation of the residual solvent and water molecules within the AEMs. Further degradation occurred from 270 to 420 °C is associated with the degradation of cations and side chains. The third stage above 430 °C is probably attributed to the decomposition of the polymer backbone. In general, all the as-prapared AEMs possess good thermal stability and are desirable for AEMFCs applications [42]. Fig. 3. SAXS profiles of the as-prepared AEMs.

3.10. Alkaline stability Apart from excellent ionic conductivity and mechanical property, the long-term alkaline stability of AEMs under a harsh alkaline environment is considered as a vital role for AEMFCs. The as-prepared BQA-OBPEK-50, BQA-OBPEK-60, BQA-RBPEK-50 and BQA-SPEK-50 were used to examine their alkaline stability by monitoring their ionic conductivity change after immersing in a 1 M sodium hydroxide solution at 80 °C for 500 h. As shown in Fig. 7, the ionic conductivity of the AEMs shows an obvious decline in the initial 200 h. This dramatic decrease is mainly due to the degradation of QA groups attacked by hydroxide ions in a harsh alkaline surrounding. This has been reported as the nucleophilic substitution, Hofmann elimination or nitrogen ylide formation [49-51]. During the next 300 h test, the remained ionic conductivity of the tested BQA-OBPEK-50, BQA-OBPEK-60, BQARBPEK-50 and BQA-SPEK-50 AEMs is about 82.3%, 78.4%, 75.2% and 72.2%, respectively. The chemical structure of the AEMs before and after the alkaline stability test was identified by 1H NMR to further investigate the alkaline stability. As shown in Figs. S8, S9 and S10, all the peaks of BQAOBPEK-50, BQA-OBPEK-60, BQA-RBPEK-50 and BQA-SPEK-50 are well-distributed after 500 h test, and the integral area of peaks Hb and Hf (the methyl protons on the BQA) over the original sample decreased by 17.0%, 21.3%, 22.2% and 28.9%, respectively. This is similar with the decrease in the ionic conductivity. All the results demonstrate that the OBAEMs show the highest alkaline stability among the three kinds of AEMs. The reason is that the well-defined ordered morphology of the OBAEMs can suppress swelling and reduce the attack on the backbone by hydroxide ions [52]. Meanwhile, there is a steric hindrance to the interaction between the QA groups of the OBAEMs and the hydroxide ions. These are responsible for the desirable alkaline stability of OBAEMs. Besides, the alkaline stability of the OBAEMs has a slight decrease with increasing IEC (Fig. 7) because the attack on the cationic groups by the hydroxide ions would increase accordingly. However, BQA-OBPEK-60 still exhibits good alkaline stability [15,45,53]. In other

Fig. 4. The WU and SR of the BQA-OBPEK-X (X = 50, 55 and 60) AEMs at 30, 60 and 80 °C.

3.9. Mechanical property and thermal stability The mechanical property of the AEMs is crucial for their application in AEMFCs. As can be seen from Fig. 6a, the tensile strength (TS) of the as-prepared AEMs is in the range from 48.4 to 71.3 MPa, elongations at break (EB) from 6.9% to 17.0%, and Young's modulus (YM) from 0.58 to 1.69 GPa. Those results demonstrate that the as-prepared AEMs exhibit good mechanical properties to be applied in AEMFCs. It's also

Fig. 5. (a) Ionic conductivity of the AEMs and (b) Arrhenius plots of the AEMs. 7

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Fig. 6. (a) The mechanical properties of the AEMs, including TS, EB and YM, a-c means BQA-OBPEK-X (X = 60, 55 and 50), and d and e means BQA-RBPEK-50 and BQA-SPEK- 50, respectively, and (b) the TGA curves of the AEMs with the inset showing DTG curves.

condition [54-56]. At the same time, a maximum power density (Pmax) can be achieved using BQA-OBPEK-50 (103.5 mW cm-2), RBAEM (84.1 mW cm-2) and SAEM (63.2 mW cm-2) at a similar IEC. By increasing IEC of the OBAEMs, the Pmax displayed a growth trend up. The superior single cell performance with OBAEMs may attribute to the higher conductivity. Also, the peak power density of BQA-OBPEK-60 is found to be much higher than that of commercial Tokuyama AHA AEM, indicating that the prepared AEMs have a application prospect. Furthermore, the high frequency resistance (HFR) of the single cell during the test is shown in Fig. S11. It is found that the HFR of the MEA containing BQA-OBPEK-60 seems to be a constant value of 0.352 Ω cm-2 and is much lower than that of the MEA containing the Tokuyama AHA (1.030 Ω cm-2), BQASPEK-50 (0.562 Ω cm-2) and BQA-RBPEK-50 (0.463 Ω cm-2) AEMs. Those results demonstrate that the OBAEMs show great potential for application in AEMFCs.

Fig. 7. Alkaline stability of the AEMs after immersion in a 1 M sodium hydroxide solution at 80 °C.

words, the OBAEMs show reasonable alkaline resistance. Furthermore, the viscosity of the AEM solution is an indicative of the molecular weight. As displayed in Table S2, all the AEMs showed an insignificant change in the intrinsic viscosity before and after the alkaline stability test. This suggests a negligible polymer backbone degradation during the alkaline stability test.

4. Conclusions In summary, three types of PEK-based AEMs containing long alkyl multi-cation side chains were successfully synthesized. The OBAEMs show the most well-connected and clearly phase-segregated morphology, as noted by TEM, AFM and SAXS characterization. As a result, the OBAEMs possess superior ionic conductivity over the RBAEM and SAEM with a similar IEC. This is probably that ion conductive channels could easily be formed in the orderly branched structure OBAEMs to promote the transportation of hydroxide ions. Furthermore, with increasing the IEC values, the OBAEMs could reach a high ionic conductivity of 119.0 mS cm-1 at 80 °C and exhibit reasonable alkaline stability under a harsh alkaline environment. A single cell with the OBAEMs has a maximum power density of 130.8 mW cm-2 at 80 °C. These results point out that the orderly branched structure is beneficial for improving the AEM performance.

3.11. Single cell performance A single cell was operated to characterize the comprehensive performance of the AEMs. As shown in Fig. 8, the open circuit voltage of a single cell equipped with BQA-SPEK-50, BQA-RBPEK-50, BQA-OBPEK50 and BQA-OBPEK-60 is 0.963, 0.964, 0.986 and 0.997 V, respectively. The results are similar to some literature under the same testing

Acknowledgements The authors thanks for the financial support from the National Natural Science Foundation of China of China (grant nos. 21576226 & 21878252).

Appendix A. Supplementary data Fig. 8. Cell voltage and power density curves of the hydrogen single cell with the AEMs at 80 °C.

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.117247. 8

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