Direct modification of polyketone resin for anion exchange membrane of alkaline fuel cells

Journal of Colloid and Interface Science 556 (2019) 420–431

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Direct modification of polyketone resin for anion exchange membrane of alkaline fuel cells Yi-Cun Zhou, Ling Zhou, Chang-Ping Feng, Xiao-Tian Wu, Rui-Ying Bao, Zheng-Ying Liu, Ming-Bo Yang, Wei Yang ⇑ College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, Sichuan, People’s Republic of China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 27 May 2019 Revised 21 August 2019 Accepted 24 August 2019 Available online 24 August 2019 Keywords: Anion exchange membrane Polyketone Ionic channel Hydroxide conductivity

⇑ Corresponding author. E-mail address: [email protected] (W. Yang). https://doi.org/10.1016/j.jcis.2019.08.086 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

a b s t r a c t A kind of side-chain type anion exchange membranes (AEMs) with high ionic conductivity and good comprehensive stability was prepared via direct modification of commercial engineering plastic polyketone with diamines through Paal-Knorr reaction and quaternization reaction. It was found that the amount of diamine can effectively tune the microphase morphology and properties of the prepared quaternized functionalized-polyketone anion exchange membranes (QAFPK-AEMs). The tensile strength was increased from 18.6 MPa to 38.6 MPa, and the ion exchange capacity (IEC) was increased from 1.11 mmol/g to 2.71 mmol/g depending on the amount of added diamine. The QAFPK-1-6-AEM with the IEC of 1.43 mmol/g showed the highest hydroxide conductivity of 65 mS/cm at 25 °C and 96.8 mS/ cm at 80 °C. The high ionic conductivity was achieved through the establishment of effective ionic channels, and it maintained 70% of the initial ionic conductivity after the 192 h treatment in 2 mol/L KOH (aq) at 80 °C. Moreover, a peak power density of 129 mW/cm2 was achieved when the assembled single cell with QAFPK-1-6-AEM was operated at 50 °C. Thus, the prepared QAFPK-AEMs showed great potential applications for the anion exchange membrane fuel cells (AEMFCs). Ó 2019 Elsevier Inc. All rights reserved.

Y.-C. Zhou et al. / Journal of Colloid and Interface Science 556 (2019) 420–431

1. Introduction Energy and environmental issues are undoubtedly huge challenges for the whole world. To address these issues, it is vital to develop new energy storage/conversion systems and devices [1], in which fuel cells are a powerful tool. However, the ion exchange membranes (IEMs), a key component of membrane electrode assemblies (MEAs) in fuel cells, is still defective [2,3]. Polyfluorinated ionomers such as NafionÒ have been widely studied and applied in the proton-exchange membrane fuel cells (PEMFCs). But PEMFCs are difficult to accomplish large-scale industrial applications owing to the high cost and the use of noble metal (such as platinum) as catalyst under strong acid conditions [4]. Thus, anion exchange membrane fuel cells (AEMFCs) have attracted a great attention, because it has shown advantages including higher reaction kinetics at the cathodes and the use of non-precious metal catalyst under high pH conditions [5,6]. The preparation methods of anion exchange membranes (AEMs) mainly include direct polymerization, grafting method and chemical modification [7–10]. The cationic functional groups such as phosphonium, sulfonium, pyridinium [11], guanidinium [12] and imidazolium [13,14] can be created on the preformed membranes. The quaternary ammonium (QA) based AEMs are still extensively studied on account of the low-cost and the simpleness [15–17]. However, the AEMs suffer from low ionic conductivity and poor physical and chemical stability [3,18,19]. In order to improve the performance of AEMFCs, the ionic conductivity is so important that many researchers have been trying to improve it. From the understanding of the ionic conduction mechanism in NafionÒ [20,21] and recently reported researches [22–24], it is found that the hydroxide (OH
) conductivity of AEMs is determined by the ionic transport channels induced by the cationic functional groups and the polymer backbone. Due to the hydrophilic-hydrophobic distinction between the hydrophobic polymer backbone and the hydrophilic cationic groups, ionic clusters can form [25,26]. Many ionic clusters interacted with each other to form the ionic channels [7,8,13]. Therefore, it is an important goal in the AEMFCs field to efficiently establish the ionic channels via an inexpensive raw material to prepare the AEMs [27]. Polyketone (PK) [28,29], an engineering plastic that has been used in many fields, is synthesized by the complete alternating copolymerization of olefins (composition of ethylene and propylene) and carbon monoxide using palladium catalyst [30,31]. PK exhibits great mechanical properties, excellent abrasion resistance and chemical resistance. In addition, the 1, 4-dicarbonyl in PK endows it a high reactivity, enabling PK to easily react with many functional groups such as diamine [32–34]. The 1, 4-dicarbonyl can react with diamine via Paal-Knorr reaction. A series of studies on the modification of PK [33,35–37] indicates that Paal-Knorr reaction is of great significance for the multi-functional applications of PK [38]. The introduction of functional cationic groups has high selectivity and flexibility for the kind of a diamine. The fabrication of PK-based AEMs is more convenient and less toxic than the modification of the benzene rings in polymers such as polyphenylene oxide and polysulfone [9,12,39]. Moreover, this approach not only allows the design of macromolecules containing QA groups, but also provides the possibility for the regulation of membrane morphology through varying the ratio of the introduced diamine. Aiming at producing AEMs via a simple chemical modification of commercial PK resin, this work directly modified PK resin via the Paal–Knorr reaction and the quaternization reaction to prepare a membrane with good ionic conductivity and mechanical properties. In the Paal–Knorr reaction, a part of the 1, 4-dicarbonyl in PK chains react with diamine, which creates many N-substituted pyrrole units in the PK backbone. Due to the statistical characteristics of the Paal–Knorr reaction, the modified PK has a similar main-

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chain structure with other random copolymers such as quaternized poly (2,6-dimethyl phenylene oxide)s [40]. In this work, we adopted a suitable catalyst to accelerate the reaction rate at room temperature [41] and investigated the effect of the amount of introduced diamines on the structure and properties of the modified PK membranes. Owing to the difference in the hydrophilicity-hydrophobicity between the introduced Nsubstituted pyrrole units and the aliphatic polymer backbone, microphase separation occurs in the membranes. The ionic cluster domains formed during the phase separation act as the main pathway for anionic conduction [23,42]. Systematical characterizations, including chemical structure characterization by high resolution solid-state 13C NMR spectra (SSNMR) and Fourier transform infrared spectroscopy (FTIR), ionic conductivity characterization by AC impedance, microphase separation structure characterization by atomic force microscopy (AFM) and small angle X-ray scattering (SAXS), were performed. The thermal stability, mechanical properties and alkaline resistance of the quaternized functionalizedpolyketone anion exchange membranes (QAFPK-AEMs) were examined. Finally, the single cell performance of QAFPK-AEMs in a H2/O2 alkaline fuel cell was also measured. 2. Experimental section 2.1. Materials The commercial Polyketone (PK) resin (M630A, Mw = 100,000 g/ mol) was from Hyosung Co. Ltd (Seoul, Korea). 1, 2Propanediamine (DAP) and 1, 1, 1, 3, 3, 3-hexafluoro-2-propanol (HFIP) was purchased from Aladdin Reagent Co. Ltd (Shanghai, China). Bismuth (III) nitrate pentahydrate (BNP) and methyl iodide (MeI) were purchased from Adamas-beta Reagent Co. Ltd (Shanghai, China). Sodium hydroxide (NaOH), potassium hydroxide (KOH), ethanol and dichloromethane (DCM) were obtained from Haihong Chemical Reagents Co. Ltd (Chengdu, China). All the reagents were used as received without further purification. 2.2. Preparation of functionalized-polyketone (FPK) precursor In the preparation of QAFPK-AEMs, 1 g PK was added to a mixed solvent of DCM and HFIP (with a mass ratio of 7/3) of 30 mL. A clear polymer solution was obtained under magnetic stirring at room temperature. Then, 86.8 mg BNP was added into the solution and BNP was uniformly dispersed in the polymer solution after sonication treatment. A certain amount of 1, 2-propanediamine (DAP) was added into the solution with a pipette and the reaction solution was then kept at 35 °C for 15 h under high-speed stirring. As the reaction progressed, the color of the solution gradually changed from pink to orange-red, following the reaction mechanism (R1) showed in Fig. 1(a). Here, the amount of diamine added was changed. As shown in Table. 1, the product was designated as FPK-1-n (n = 8, 6, 5, 4 and 3), where ‘‘1-n” was used to indicate the molar ratio of the added diamine with 1, 4-dicarbonyl groups. For instance, FPK-1-6 means that the added diamine can theoretically react with one-sixth of the 1, 4-dicarbonyl in the PK chains. 2.3. Membrane casting After the reaction (R1) was completed, the reaction solution was allowed to stand at 0 °C for 10 min, and the BNP at the bottom of the solution was removed and this operation was repeated for 3–4 times. Afterwards, the solution was poured into a mold of Teflon for casting. The FPK membrane was obtained after natural evaporation for 36 h in a fume hood at room temperature. The thickness of FPK membrane was controlled from 60 to 80 lm by

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Fig. 1. (a) Synthesis of QAFPK-1-n-AEM (n = 8, 6, 5, 4 and 3) via the Paal-Knorr reaction (R1) and the quaternization reaction (R2). (b) Photographs and SEM images of the surface of QAFPK-1-n-AEMs (n = 8, 6 and 5).

2.5. Measurements and characterizations

Table 1 Elemental composition of the samples. Samples

C%

H%

O%

N%

X%

Z

PK FPK-1-8 FPK-1-6 FPK-1-5 FPK-1-4 FPK-1-3

64.36 62.64 64.36 64.4 64.41 67.43

6.73 9.02 7.03 6.93 6.81 7.43

28.87 22.56 22.55 21.41 19.46 16.49

/ 3.28 4.44 5.26 5.89 7.90

/ 13.88 19.10 22.91 25.86 35.69

/ 0.16 0.24 0.30 0.35 0.56

X represents the molar proportion of the converted 1, 4-carbonyl groups (N-substituted pyrrole units). Z (x/y) means the molar ratio of N-substituted pyrrole units to 1, 4-carbonyl groups in the polymer backbone.

changing the amount of the casting solution. The casting membrane was immersed in a mixed solution of dichloromethane and ethanol for 24 h to remove the unreacted DAP and residual HFIP, and then a transparent yellow precursor membrane was obtained. 2.4. Fabrication of quaternized functionalized-polyketone anion exchange membranes (QAFPK-AEMs) The precursor membrane was washed for 4 to 5 times with deionized water, and the dried membrane was obtained by lyophilization to remove any residual water. The dried membrane was placed in a closed opaque reactor and a sufficient amount of methyl iodide was added into the reactor. The quaternization reaction was carried out at room temperature for 24 h, following the reaction mechanism (R2) shown in Fig. 1(a). Finally, the membranes obtained from the reaction (R2) was thoroughly washed with ethanol to remove residual methyl iodide. The obtained membrane was designated as QAFPK-1-n-AEMs (n = 8, 6, 5, 4 and 3) according to the molar ratio of the added diamine with 1, 4dicarbonyl groups as mentioned above.

High resolution solid-state 13C NMR spectra (SSNMR) were obtained on a Bruker Avance III 500 MHz spectrometer (Bruker, Germany) working at 125.76 MHz for 13C frequency, equipped with a standard 4 mm magic angle spinning (MAS) triple resonance probe [43]. Fourier transform infrared spectroscopy (FTIR) spectra were recorded using a Nicolet 6700 FTIR spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) from 400 cm
1 to 4000 cm
1 with a resolution of 4 cm
1. The morphology of the membranes was observed on a field-emission scanning electron microscopy (SEM, JEOL JSM-5900LV, Japan) operating at an accelerating voltage of 20 kV. Tapping mode atomic force microscopy (AFM) images were recorded with atomic force microscope (AFM, Multimode 8, Bruker Corporation). Small angle X-ray scattering was performed at a laboratory-based Xeuss 2.0 (Xenocs, Sassenage, France) at room temperature [44]. The thermal stability of the membranes was characterized under nitrogen in the temperature range from 30 °C to 800 °C at the heating rate of 10 °C min
1 by a thermogravimetric analyzer (TGA, NETZSCH-TG209F1, Germany). The differential scanning calorimetry (DSC) profiles were recorded with a DSC Q20 (TA Instruments, Milford, MA, USA). The mechanical properties was performed on a universal material testing machine (Instron5967, USA). The rate of extension was 5 mm/ min. At least five samples were measured for each testing and the average results were recorded. Based on ISO standard 151051: 2007, the oxygen permeability coefficient (PO2) of QAFPKAEMs was measured on a VAC-V2 film permeability testing machine (Labthink instrument, Jinan, China) at room temperature with 50% relative humidity. 2.5.1. Elemental analyses In order to determine the molar ratio (x) of the reacted 1, 4dicarbonyl units in PK, elemental analyses were performed with

Y.-C. Zhou et al. / Journal of Colloid and Interface Science 556 (2019) 420–431

an automatic element analyzer (Elementar Vario EL, GmbH, Hanau, Germany) and the molar ratio was calculated by the following formula [35]:

NMw2 x ¼ MN n þ NðMw2 - Mw1 Þ

ð1Þ

where N was the nitrogen content per gram obtained by elemental analysis, MN was the atomic mass of nitrogen, n was the number of nitrogen atoms in the repetitive unit of the functionalizedpolyketone, Mw1 was the molecular weight of a converted 1, 4dicarbonyl unit (149.8 g/mol for FPK) and Mw2 was the molecular weight of non-converted 1, 4-dicarbonyl (113.7 g/mol for PK). 2.5.2. Linear swelling ratio, water uptake, ion exchange capacity and hydration number For the measurement of the linear swelling ratio (LSR) and water uptake (WU), QAFPK-AEM (1 cm  4 cm) was completely immersed in deionized water at room temperature for 24 h firstly. Then, the water remaining on the surface of the membrane was dried with an absorbent paper. The mass change and dimensional change of the membranes were quickly measured, and the water uptake (WU) and linear swelling ratio (LSR) of the membranes were calculated using Eqs. (2) and (3):

WU ¼

Ww
Wd  100 % Wd

ð2Þ

where Ww and Wd were the mass of the membrane in the hydrated state and the dry state, respectively.

LSR ¼

Lw
Ld  100 % Ld

VAg ðmlÞ  0:1 mol=L WCl- ðgÞ

10  WU ð%Þ 18  IEC

ð6Þ

where L was the distance between the platinum black electrodes, R was the membrane resistance, d and W were the thickness and width of the membranes, respectively. 2.5.4. Alkaline stability To characterize the alkaline stability of the membranes, the QAFPK-AEMs were immersed in a 2 mol/L KOH aqueous solution at 80 °C for different soaking time. Then, the membranes were repeatedly washed with deionized water to remove the residual KOH. The ionic conductivity of the membranes was measured again and compared with its initial value. 2.5.5. Single fuel cell performance The single fuel cell performance of QAFPK-AEMs was performed at 50 °C. The catalyst (HPT020, 20% mass Pt/C, Shanghai Hesen Co. Ltd, China) was first sonicated with glycerol in ethyl acetate solution to obtain a catalyst ink. The catalyst ink was sprayed onto the surface of the hydrophobic treated carbon paper (HCP120, Shanghai Hesen Co. Ltd, China) by a spray gun, and the final Pt loading of the hydrophobic carbon paper was 0.4 mg/cm2. Then, two sheets of carbon paper and QAFPK-AEM were heat pressed at 80 °C and 3 MPa for 5 min to form the membrane electrode assemble (MEA). The single cell performance of MEA was tested by the fuel cell testing system (FCTS, ARBIN Instruments, USA) at 50 °C and 100% relative humidity. The flux rates of hydrated H2 and O2 were set as 150 mL/min with 100% relative humidity. 3. Results and discussion

ð4Þ

where VAgþ was the amount of AgNO3 solution consumed during the titration, and WCl
was the mass of the membrane in the Cl- form. The hydration number (k) of membranes can be calculated with equation (5):

k¼

L R  W  d

ð3Þ

where Lw and Ld were defined as the membrane’s length which is the size of the long side of a 1 cm  4 cm rectangular samples under hydration and dry conditions, respectively. The ion exchange capacity (IEC) of the membranes was measured by using a typical titration method. The QAFPK-AEMs (1 cm  4 cm) were immerged in a 1 mol/L NaCl aqueous solution at room temperature for 24 h. After drying at 60 °C under vacuum for 12 h, the mass of the membranes was recorded. The dried membrane was immersed in a 0.5 mol/L Na2SO4 aqueous solution for an additional 24 h to release Cl- from the membranes. Finally, this solution was titrated with a 0.1 mol/L AgNO3 aqueous solution in which K2CrO4 was used as an indicator. The IEC was calculated using equation (4):

IEC ðmmol=gÞ ¼

r¼

423

ð5Þ

2.5.3. Hydroxide conductivity The hydroxide conductivity of the QAFPK-AEMs was obtained with a four-electrode AC impedance spectroscopy, which was measured on an electrochemical workstation (CHI 660E, Shanghai Chenhua). The potentiostat control mode was used when testing the AC impedance spectrum over a frequency range from 0.1 MHz to 1 Hz with a perturbation voltage amplitude of 10 mV. The membrane and electrode were placed in a Teflon mold and measured in a thermal chamber of deionized water with a distance of 1 cm between the platinum black electrodes. The ionic conductivity (r) was calculated using Eq. (6):

3.1. Chemical structures and basic properties of QAFPK-AEMs As shown in Fig. S1, the uniform, customizable, tough QAFPKAEMs were fabricated after the two-step reaction (Paal-Knorr reaction and quaternization reaction). In the first-step reaction (R1: Paal-Knorr reaction), the loading of DAP was varied and the obtained membranes were designated as FPK-1-n (n = 8, 6, 5, 4 and 3) according to the molar ratio of the added diamine with 1, 4-dicarbonyl groups in PK. This is a typical Paal-Knorr reaction [45] to chemically modify the PK resin. The different amount of reactive functional groups (the pyrrole with amino side chain) was introduced into the PK backbone. In order to obtain the conversion degree of 1, 4-dicarbonyl in the PK, FPK was characterized by elemental analysis (Table 1). The quaternization of FPK was achieved by quaternization reaction (R2) to obtain the final membrane QAFPK-1-n-AEMs (n = 8, 6, 5, 4 and 3). The chemical composition of QAFPK-AEMs was characterized and analyzed by using FTIR and SSNMR. As shown in Table 1, with increasing loading of DAP in the Paal– Knorr reaction (R1), the molar proportion of N element in the polymer gradually increased. After getting the molar proportion of the nitrogen element, the ratio of the N-substituted pyrrole units introduced in the polymer can be calculated with Eq. (1). The molar proportion of the N-substituted pyrrole units increased from 13.9% to 35.7% by changing the ratio of the loading diamine in PK from 1/8 to 1/3. Since the reactive structure units have a statistically distributed characteristics, the ratio of the N-substituted pyrrole units will have a significant effect on movement, interaction and stacking of the modified polymer chains. FTIR and SSNMR were employed to elucidate the chemical structures of the QAFPK-AEMs. As shown in Fig. 2, the infrared transmission spectra of the QAFPK-AEMs and PK was compared. Firstly, with the increase of the molar proportion of Nsubstituted pyrrole units, the intensity of the carbonyl peak at

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Fig. 2. FTIR transmission spectra of PK and QAFPK-1-n (n = 8, 6, 5, 4 and 3).

1692 cm
1 was significantly reduced [46], corresponding to the consumption of the 1, 4-dicarbonyl groups and the formation of N-substituted pyrrole units. Plenty of bands were presented in the spectrum of QAFPK-AEMs owing to the N-substituted pyrrole units. For instance, the peak located at 756 cm
1 was associated with the out-of-plane vibration of CAC in pyrrole groups, the peak at 1510 cm
1 was corresponding to the C@C stretching vibration, and the peak at 3100 cm
1 was associated with the sp2 vibration (CAH) of pyrrole [33,38]. In addition, there were also peaks for functional groups containing N atom in the range of 1600 cm
1 to 1000 cm
1, which can also prove the presence of pyrrole units [33]. The CAN stretching vibration of the pyrrole rings was detectable at 1351 cm
1 and 1427 cm
1, and the peak at 1019 cm
1 was attributed to the CAN stretching vibration of the side groups [38]. After quaternization reaction, the peak of N-CH3 stretching vibration appeared at 1355 cm
1 [47]. As shown in Fig. 1(a), the main difference between QAFPK-1-n and FPK-1-n was the appearance of some new peaks owing to the quaternization reaction. Apart from these new peaks, the infrared absorption peaks of PK itself were completely preserved, which showed that the chemical structure of the original PK chain backbone was not significantly dam-

Fig. 3. (A)

13

aged. As shown in Fig. S1, the color of the QAFPK-AEMs became darker as the molar proportion (x) of the N-substituted pyrrole units increases. This is because the pyrrole units as a heterocyclic component has a strong absorption of visible light. This phenomenon also corresponded to the peak enhancement of the pyrrole rings in Fig. 2. Although diamines were employed in this study, the steric hindrance of the two primary amines differed greatly such that their reactivity was greatly different. It is considered that the primary amine which reacted with a large amount of 1, 4-dicarbonyl is that having a small steric hindrance, and the reaction of primary amine having a large steric hindrance with 1, 4-dicarbonyl can be ignored. As shown in Fig. S3, the chemical structure of the precursor membrane was characterized. The apparent broad peak near 3433 cm
1 corresponds to the stretching vibration of the primary amine (ANH2), and the peak near 1629 cm
1 corresponds to the bending vibration of the amine group [47]. As the amount of diamine introduced increases, the peak intensity increases significantly. The results indicate that a large amount of primary amine was present in FPK and prove the molecular structure designed in Fig. 1. Due to the lack of effective deuterated reagents for PK, FPK and QAFPK, solid-state nuclear magnetic resonance (SSNMR) was used to analyze the chemical structure of the PK, FPK and QAFPK. As shown in Fig. 3(A), the signal (a) with the chemical shift between 210.4 and 218.6 ppm was assigned to the typical carbonyl carbon in the polymer backbone [32]. The peak (d) from 36.1 ppm to 43.9 ppm in the spectrum of all samples corresponded to the methylene carbon (ACH2A) in PK [32,48]. These two peaks (a and d) representing the basic characteristics of PK backbone appeared in the results of all samples. Two distinct new peaks appeared in the spectra of QAFPK-1-n and FPK-1-n, owing to the formation of the N-substituted pyrrole units. One of the peaks was the characteristic pyrrole-related band from 131.3 ppm to 139.8 ppm which was assigned to -NAC@ (b) of pyrrole-rings [37]. The other peak was the representative resonance from 106.2 ppm to 115.2 ppm which was assigned to AHAC@ (c) of pyrrole-rings. These two peaks (b and c) were the most direct evidence of the presence of the N-substituted pyrrole units in FPK-1-n and QAFPK-1-n (n = 8, 6 and 5). Furthermore, during the preparation of QAFPK-AEMs, the quaternization reaction (R2) was carried out after Paal-Knorr reaction (R1), leading to the appearance of the band from 51.3 ppm to 58.2 ppm corresponding to ANACH3 (e), which proved the effective quaternization [48], as shown in

C MAS NMR spectra of PK, FPK-1-8, FPK-1-6 and FPK-1-5. (B)

13

C MAS NMR spectra of QAFPK-1-8, QAFPK-1-6 and QAFPK-1-5.

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Fig. 3(B). Therefore, the 13C MAS NMR spectra of the FPK and QAFPK well supported the given chemical structures. After determining the chemical structure of QAFPK-AEMs, the parameters of the QAFPK-AEMs need to be discussed. The values of IEC, WU, LSR and k, the basic performance parameters, for the QAFPK-AEMs are summarized in Table 2. As shown in Fig. 1(b) and more details in Fig. S2, the membrane surface of QAFPKAEMs was compact with no voids. The smooth surface facilitates a better fit to the electrode material in the fuel cells. It can be seen that the increasing content of the introduced functional structure units led to IEC increasing. Due to the hydrophilic nature of the quaternary ammonium groups, WU and LSR also increased with the increasing N-substituted pyrrole units. Although the specific values of WU and LSR were relatively low, the water uptake (WU) increased synergistically as the IEC increasing, which in turn leads to the limited change in the hydration number (k).

Table 2 IEC, WU, LSR and k of the QAFPK-AEMs at room temperature 25 °C. Membranes

IEC

QAFPKm-1-8 QAFPKm-1-6 QAFPKm-1-5 QAFPKm-1-4 QAFPKm-1-3

1.11 1.43 1.96 2.09 2.71

titr

[mmol/g]

WU (wt %)

LSR (%)

k

5.26 9.52 11.11 12.73 15.26

7.20 7.43 9.44 11.56 13.18

2.63 3.70 3.15 3.38 3.12

425

3.2. Ionic conductivity and phase morphology of QAFPK-AEMs 3.2.1. Ionic conductivity The efficient delivery of hydroxide ion can be achieved via the interconnected ionic channels in AEMs [49]. However, the formation of ion channels depends on the effect of polymer molecular chain stacking and mobility. The design of chemical structure is significant for the formation of ionic channels [23,40,49]. Therefore, in our work, the molecular chains in the QAFPK-AEMs were tuned by changing the amount of introduced diamine. Then, QAFPK-1-n-AEMs (n = 8, 6, 5, 4 and 3) with different ionic conductivity and microphase structure can be obtained. As shown in Fig. 4(a), the PK resin membrane was a complete ionic insulator. When the diamine was introduced into PK, the ionic conductivity of QAFPK-AEMs increased rapidly. The hydroxide conductivity of QAFPK-1-n-AEM (n = 8, 6 and 5) was all larger than 20 mS/cm at 25 °C, which can satisfy the ionic conductivity requirement of AEMFC. With the temperature increasing, the hydroxide conductivity of QAFPK-AEMs showed an increasing tendency in all membranes. It was because the increase in temperature enhances the kinetics of hydrated anion ions and also increases the mobility of the polymer chains, thereby accelerating ionic conduction [24,50]. At the same temperature, as the ratio of introduced N-substituted pyrrole units increased, there was an increase first and then a decrease in the hydroxide conductivity for the QAFPK-AEMs. The largest hydroxide conductivity reached 96.8 mS/cm for the QAFPK-1–6-AEM at 80 °C. The AC impedance

Fig. 4. (a) The hydroxide conductivity of the QAFPK-1-n-AEM (n = 8, 6, 5, 4 and 3) as a function of temperature. (b) Arrhenius-like plots of the QAFPK-AEMs at different temperatures. (c) The hydroxide conductivity of the QAFPK-AEMs as a function of IEC at the different temperature. (d) SAXS profiles of the QAFPK-AEMs measured at room temperature.

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image of QAFPK-1–6-AEM as a function of temperature was depicted in Fig. S4. It indicates that the microstructure formed in QAFPK-1-6-AEM may be the most favorable for ion transport compared to other samples. In order to understand the difference in the ionic conductivity between the prepared QAFPK-1-n-AEMs (n = 8, 6, 5, 4 and 3) from the mass transfer dynamics point of view. The activation energy (Ea) of the hydroxide conduction in the QAFPKAEMs was estimated by the Arrhenius-like behavior, as shown in Fig. 4(b). As the molar ratio of the introduced diamine increased (1/8, 1/6, 1/5, 1/4 and 1/3), the activation energy (Ea) of the hydroxide conduction in the QAFPK-AEM also showed a trend of increasing first and then decreasing. The lowest Ea (5.82 kJ/mol) was achieved for QAFPK-1-6-AEM, and the Ea values of the QAFPK-1-n-AEM (n = 8, 5, 4 and 3) were 14.49, 14.36, 13.98 and 12.93 kJ/mol, respectively. The results indicate that hydroxide ion has the lowest mass transfer resistance in QAFPK-1-6-AEM. Furthermore, this trend of increasing first and then decreasing was more intuitive in the case of ionic conductivity change with IEC, as plotted in Fig. 4(c). Generally, the ionic conductivity in AEMs is positively correlated with IEC. However, in the QAFPKAEMs, as the IEC increases, the ionic conductivity increased at the early stage and then decreased. We deemed that the molar ratio of the introduced diamine has a significant effect on formation of ionic channels in the QAFPK-AEMs. Different nano-sized ionic channels were formed in the QAFPK-AEMs owing to the micro-phase separation. It is the distinction in nano-sized ionic channels that causes the increasing first and then decreasing in the ionic conductivity as the IEC increases. The reasons for this difference in ionic channels will be discussed in the next section. 3.2.2. Phase morphology In order to analyze the feature of the ion channels between different membranes, the morphology of the QAFPK-AEMs was examined by SAXS and AFM. Firstly, as illustrated in SAXS profiles Fig. 4 (d), the ionomer peak was observed in all the QAFPK samples, but the shape of the peaks was quite different. For QAFPK-1-n-AEMs (n = 8, 6 and 5), the ion peak was relatively strong, while for QAFPK-1-n-AEMs (n = 4 and 3), the ion peak was rather weak. According to the model based on ion clusters in ionomer, the size of the ionic cluster formed in the AEMs can be characterized by the d value (d = 2p/q) [44,51]. The d can be calculated through q values corresponding to the peak position, which is associated with the periodic structure of the ionic clusters. It can be observed that as the molar ratio of introduced diamine increases, the size of the ionic cluster is gradually increasing, from 7.9 nm to 12.1 nm. It demonstrates that QAFPK-1-n-AEMs (n = 4 and 3) can form ionic clusters larger than QAFPK-1-n-AEMs (n = 8, 6 and 5). However, the ion clusters in QAFPK-1-n-AEMs (n = 4 and 3) are isolated from each other and QAFPK-1-n-AEMs (n = 8, 6 and 5) have more interconnected ion clusters capable of forming efficient ionic transport channels. This conclusion can be drawn from the variation of the intensity and position of the peaks in Fig. 4(d). The results of SAXS are in accordance with the results of ionic conductivity. Differential scanning calorimetry (DSC) of the QAFPK-AEMs was performed to analyze the effect of the functional chain structures on the chain mobility of the membranes (Fig. 5). It can be seen that with the increase of the molar ratio of the introduced rigidsegment (N-substituted pyrrole unit, the increase of the Z value), the glass transition temperature (Tg) increases from 44.2 °C to 73.9 °C, showing that the mobility of the molecular chains in QAFPK-AEMs gradually declines. The weaken mobility of the molecular chains in the QAFPK-AEMs hinders the interaction between the hydrophilic quaternary ammonium groups, which in turn changes the microphase morphology of the ionic channels in the QAFPK-AEMs. Taking QAFPK-1-6-AEM with a Tg of 46.9 °C as an example, the molecular chains show better mobility and suf-

Fig. 5. DSC first temperature rise curve of QAFPK-1-n-AEMs (n = 8, 6, 5, 4 and 3) at 10 °C/min.

ficient IEC, which enables the formation of excellent interconnected ionic channels and the high ionic conductivity in QAFPK1-6-AEM. To manifest the morphology of microphase separation more visually, the tapping mode atomic force microscopy (AFM) was performed and the results were presented in Fig. 6 and Fig. S5. In the AFM phase images, the dark regions were recognized as the hydrophilic ionic transport channels and the bright regions were the hydrophobic polymer backbones [7,8,25,49,52]. As shown in Fig. 6, before quaternization reaction, the FPK-1-n (n = 8 and 6) membranes can form the microphase separation, because the amine in the side chain of FPK-1-n is basically hydrophilic and can form a hydrogen bond with water. The quaternization treatment further strengthens the nano-sized microphase separation driven by the hydrophilic–hydrophobic discrimination. These results indicate that the precursor of AEM with modified PK facilitates the formation of high-efficiency ionic transport channels in the QAFPK-AEMs. Moreover, in the Fig. S5, PK film shows a uniform surface morphology, while QAFPK-1-n-AEMs (n = 8, 6 and 5) all show a fine nanophase separation morphology. Particularly, the ionic channels in QAFPK-1-6-AEM exhibit better interconnectivity compared with those of QAFPK-1-8-AEM and QAFPK-1-5-AEM. These results are in good agreement with the results of ionic conductivity and SAXS. The structure determines the performance, and it is the ionic transport channels constructed owing to the microphase separation that contribute to high ionic conductivity. It is generally agreed that the ion transport modes in solid polyelectrolytes are Grotthuss shuttling mechanism, vehicle mechanism and surface mechanism. The formation of ion channels in the polyelectrolyte can enhance the surface mechanism. Thus, the high ionic conductivity can be obtained by forming higher interaction ion channels (eg. QAFPK-1-6-AEM) in QAFPK-AEMs samples. The difference of ionic channels studied by SAXS and AFM in QAFPK-1-n-AEMs (n = 8, 6, 5, 4 and 3) can be understood via the mechanism description given in Fig. 7. The formation of the ionic channels in AEMs is owing to the interaction of the hydrophilic functional groups and the hydrophobic molecular backbone [23,25,49]. The presence of hydrophilic functional groups imparts ion exchange capacity (IEC) to the molecular backbone, and the introduction of functional groups in our membranes alters the mobility of the hydrophobic molecular backbone. A balance between the improved IEC and the reduced mobility of the molecular chains should be reached to facilitate the formation of the effective ionic channels. As shown in Fig. 7, QAFPK-1-n-AEMs (n = 8, 6 and 5) combines a relatively high level IEC with the

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Fig. 6. AFM phase images at room temperature. (a), (b) and (c) were FPK-1-6 (2 lm  2 lm), QAFPK-1-6 (2 lm  2 lm) and QAFPK-1-6 (500 lm  500 lm), respectively. (d), (e) and (f) were FPK-1-8 (2 lm  2 lm), QAFPK-1-8 (2 lm  2 lm) and QAFPK-1-8 (500 nm  500 nm), respectively. The dark areas indicate the hydrophilic channels and the light areas indicate the hydrophobic regions.

Fig. 7. Schematic diagram of the formation mechanism of ionic channels in QAFPK-1-n-AEMs (n = 8, 6, 5, 4 and 3). (1) b and b0 represent that when the molar ratio of added diamine is moderate, the molecular chain can move sufficiently, and the interconnected ionic transport channels can be formed driven by the hydrophilic–hydrophobic discrimination, such as QAFPK-1-8, QAFPK-1-6 and QAFPK-1-5. (2) c and c0 represent that when the number of the N-substituted pyrrole units introduced in the molecular chain is too large, the mobility of the entire molecular chain is limited, and it is difficult to form effective ionic transport channels in a large amount, such as QAFPK-1-3 and QAFPK-1-4.

suitable mobility in the b-b0 process, leading to a high ionic conductivity. However, as the molar ratio of the introduced diamine increases in the c-c0 process, such as QAFPK-1-n-AEMs (n = 4 and 3), the molecular chains are difficult to move to form highly efficient ionic transport channels, resulting in low ionic conductivity. 3.3. IEC-normalized conductivity IEC is only related to the content of cationic functional groups on the polymer chain and not to the chemical structure of the poly-

mer and the microphase morphology of the membrane. In order to examine the role of the ion transport channels, the effect of IEC on ionic conductivity should be excluded. Therefore, the IECnormalized conductivity is used as a parameter describing the ion conduction efficiency to evaluate the merits of the ionic channels. Fig. 8 showed the comparison of IEC-normalized conductivity of the QAFPK-1-n-AEM (n = 8 and 6) in this work as a function of IEC at 30 °C with other recent AEMs in literatures [7,10,13,18,39,52–55]. It can be seen that the QAFPK-1-6-AEM shows very high IEC-normalized conductivity in all AEMs, and

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Fig. 8. IEC-normalized conductivity as a function of IEC for different AEMs at 30 °C.

performs very good even compared with mostly recent literature reports. It indicates that the ionic transport channels constructed in QAFPK-1-6-AEM has higher ionic transport efficiency. 3.4. Mechanical, thermal and chemical stability of QAFPK-AEMs As a significant part of MEA in the AEMFCs, the mechanical, thermal and chemical stability of the AEMs during processing,

transferring and application is important [52,55]. Fig. 9(a) showed the stress-strain curve of QAFPK-AEMs and PK, and the mechanical properties of them were shown in Fig. S6. The polyketone casting membrane exhibits good ductility but low strength. After introducing different amounts of N-substituted pyrrole units, the rigidity of the molecular chain of the polymer increases. The intermolecular force also increases due to the increase of the cationic functional group content. As a result, the QAFPK-AEMs show improved tensile strength from 18.4 MPa to 27.8 MPa, under the influence of rigid pyrrole chain and flexible aliphatic chain, as shown in Fig. 9(a). Among these membranes, when the content of the introduced Nsubstituted pyrrole units does not significantly decrease the mobility of the molecular backbone, strong interaction can be formed in QAFPK-AEMs. Therefore, QAFPK-1-6-AEM and QAFPK-1-8-AEM show both excellent tensile strength and good elongation at break. They exhibit sufficient mechanical stability to maintain the stability of the morphological structure in practical applications. The thermal behavior of QAFPK-AEMs measured by TGA in a N2 atmosphere was shown in Fig. 9(b). The PK resin itself shows good thermal stability, and the degradation of the main chain occurs at around 340 °C. During the degradation, the molecular chains of PK undergo intramolecular and intermolecular cyclization, which produces cyclopentenone, furan and oxidized structure in the PK [56,57]. The thermal degradation behavior of the modified QAFPK-AEMs is completely different. Since the QAFPK-AEMs have a certain IEC, the evaporation of the bound moisture in the AEMs occurs at about 100 °C. Due to the QA cations as the functional groups are exposed on one side of the molecular chains, the thermal stability of the QAFPK-AEMs is lower compared with that of PK resin. The weight loss that occurs between 120 °C and 270 °C

Fig. 9. Mechanical, thermal and chemical stability of QAFPK-AEMs. (a) Stress- strain curve of QAFPK-1-n-AEMs (n = 8, 6, 5, 4 and 3). (b) TG and DTG curves for all QAFPKAEMs under N2 atmosphere at 10 °C/min. (c) The alkaline resistance results of the samples tested at 80 °C, wherein the alkaline accelerated degradation testing was conducted by immersing the sample in 2 M KOH (aq) at 80 °C. (d) FTIR spectra of QAFPK-1-n-AEMs (n = 6 and 5) before (B) and after (A) alkaline stability testing in 2 M KOH (aq) at 80 °C for 192 h.

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can be attributed to the decomposition of the QA groups. With the increase of the content of QA groups, the initial decomposition temperature of the QAFPK-AEMs increases from 122 °C to 158 °C. It can be attributed to the fact that the N-substituted pyrrole units in the main chain can protect the QA groups. Taking QAFPK-1–6AEM as an example, the thermal behavior image of QAFPK-1-6 was shown in Fig. 9(b). DTG curves show that the initial decomposition of QA groups in QAFPK-1-6-AEM occurs at 128 °C, followed by the decomposition of the polymer backbone occurring at 280 °C. It shows a good thermal stability because the maximum operating temperature of AEMFCs does not exceed 100 °C. Considering the requirements for working under long-term alkaline conditions, the alkaline stability of QAFPK-AEMs is very crucial for the electrochemical applications [9,39,53]. Maintaining chemical stability is to ensure that the ionic conductivity of the AEMs does not decay greatly during working. Herein, we accelerated the degradation of the AEMs by immersing QAFPK-AEMs into 2 mol/L KOH (aq) at 80 °C. The ionic conductivity of QAFPK-AEMs was characterized after different aging time to reflect the alkaline stability, and FTIR was employed to analyze the stability of the chemical composition of QAFPK-AEMs after long-term alkaline treatment. As shown in Fig. 9(c), QAFPK-1-n (n = 8, 6 and 5) can maintain 60% or more of the initial ionic conductivity after alkaline treatment for 192 h. In particular, the QAFPK-1-6-AEM can maintain 70% of the original ionic conductivity under strong alkaline conditions and high temperature. Since the quaternary ammonium groups are easily degraded by the E2 Hofmann elimination and the SN2 substitution reaction, the ionic conductivity of QAFPK-AEMs is expected to decrease under strong alkaline conditions. In fact, it can be seen that the ionic conductivity of QAFPK-1-n (n = 8, 6 and 5) still maintains a relatively high value ( > 20 mS/cm) after degradation. The ionic conductivity of QAFPK-1-6 could still reach 67.8 mS/cm at 80 °C after 192 h alkaline treatment. In addition, Fig. 9(d) showed the FTIR spectra of QAFPK-1-nAEMs (n = 6 and 5) before and after the exposure to 2 M KOH (aq) at 80 °C for 192 h. The main peaks of N-substituted pyrrole units and quaternary ammonium groups are well preserved. For instance, the peaks at around 745 cm
1 and 1356 cm
1 are assigned to N-substituted pyrrole unit and quaternary ammonium group, respectively. However, due to degradation of a small amount of main-chain, some new peaks appear between 1620 cm
1 and 1500 cm
1. The QA groups in QAFPK-AEMs can be converted to a hydroxyl groups by the mechanism of SN2 substitution reaction, which leads to the presence of a distinct peak of hydroxide ion around 3425 cm
1. In general, the alkaline resistance of QAFPK-AEM can meet the requirements of using in alkaline fuel cells. In addition, the oxygen permeability coefficient of QAFPK-1-n-AEMs (n = 8, 6 and 5) was tested, as shown in Fig. S7. The QAFPK-AEMs showed relatively high gases barrier properties with permeability coefficients below 10
15 cm3 cm/cm2 s Pa. Therefore, the gas penetration does not occur in fuel cell applications. 3.5. Single fuel cell performance Among all the prepared QAFPK-AEMs, QAFPK-1-6-AEM and QAFPK-1-5-AEM show higher ionic conductivity and superior overall stability. Therefore, they were chosen to evaluate feasibility of it in fuel cells system. The power density and polarization curves were shown in Fig. 10. The QAFPK-1-5-AEM achieves a peak power density of 72.7 mW/cm2 at a current density of 212.9 mA/cm2. The peak power density of QAFPK-1-6-AEM reaches 129 mA/cm2 at a current density of 396 mA/cm2. The maximum power density of QAFPK-1-6-AEM is 1.8 times that of QAFPK-1-5-AEM. This is mainly because the ionic conductivity of QAFPK-1-6-AEM is much larger than that of QAFPK-1-5-AEM. In view of these results, this

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Fig. 10. The polarization and power density curve of a single cell using QAFPK-1-6AEM and QAFPK-1-5-AEM.

work demonstrates for the first time the feasibility of polyketone-based anion exchange membranes in alkaline polyelectrolyte fuel cell systems. 4. Conclusion This work prepared a kind of PK-based AEMs with high ionic conductivity via the modification of polyketone with diamine. Good dimensional stability, tensile strength (at least 18.6 MPa) and thermal stability (over 120 °C) were achieved in QAFPKAEMs. The addition of different diamines can regulate the performance of QAFPK-AEMs by affecting the molecular structure and microphase morphology. When the loading molar ratio of the diamine was 1/5, 1/6 and 1/8 of the 1, 4-dicarbonyl groups in PK, suitable ionic transport channels can be established driven by the hydrophilic-hydrophobic discrimination. The high ionic conductivity of QAFPK-AEMs was achieved via the efficient ionic channels. Among them, QAFPK-1-6-AEM (IEC = 1.43 mmol/g) showed good comprehensive performance. The ionic conductivity at 80 °C can reach 96.8 mS/cm, and it can maintain more than 70% of the initial ionic conductivity after the alkaline resistance testing. The IECnormalized conductivity used to comprehensively assess the efficiency of the ionic channels, indicates that the establishment of interacting ionic channels in AEMs is a key for obtaining high ionic conductivity. Moreover, the single fuel cell testing using QAFPK-16-AEM showed a maximum power density of 129 mW/cm2 at a current density of 396 mA/cm2 operating at 50 °C. This work proves the feasibility of the PK-based AEMs in AEMFCs for the first time. However, the technology of MEA needs to be further improved in order to achieve higher power density. Due to the high controllability of the chemical structure of modified PK and the simplicity of the preparation process, this work sheds light on the application potential of PK-based ion exchange membranes. Acknowledgment This work was supported by the National Natural Science Foundation of China (NNSFC grants 51873126, 51422305 and 51721091). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.08.086.

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