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Facile construction of poly(arylene ether)s-based anion exchange membranes bearing pendent N-spirocyclic quaternary ammonium for fuel cells
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Facile construction of poly(arylene ether)s-based anion exchange membranes bearing pendent N-spirocyclic quaternary ammonium for fuel cells Chenxiao Lin a,b,c, Demei Yu c, Jixia Wang b, Yan Zhang b, Dong Xie a, Faliang Cheng a,**, Shiguo Zhang b,* a
Guangdong Engineering and Technology Research Center for Advanced Nanomaterials, School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan, 523808, PR China b College of Materials Science and Engineering, Hunan University, Changsha, 410082, PR China c School of Science, Xi’an Jiaotong University, Xi'an, 710049, PR China
highlights Novel
graphical abstract
side-chain-type
AEMs
bearing N-spirocyclic cations were prepared. The synthesized procedure was performed at mild condition. The
AEM
demonstrated
developed
well-
micro-phase
separation. The AEM exhibited high conductivity at low water uptake. High retention rate of conductivity was
achieved
after
alkaline
treatment.
article info
abstract
Article history:
Anion exchange membrane (AEM) fuel cells have received significant attention due to their
Received 17 April 2019
low fuel permeability and the use of non-platinum catalysts. However, the development of
Received in revised form
AEMs with robust chemical stability and high conductivity is still a great challenge. Herein,
29 July 2019
we prepare a new type of partially fluorinated backbone bearing pendent N-spirocyclic
Accepted 12 August 2019
quaternary ammonium (QA) cations via a facile Williamson reaction, which displays great
Available online 6 September 2019
potential for fuel cells. The integration of the two substructures (a fluorinated moiety into a polymer backbone and a pendent cation structure) is beneficial for the fabrication of a well-
Keywords:
defined micro-phase separation structure, thereby facilitating the construction of a highly-
Anion exchange membrane
efficient ion transporting pathway. Correspondingly, the resulting AEM (PAENQA-1.0),
N-spirocyclic quaternary
despite its a relatively low ionic exchange capacity (0.93 meq g1) demonstrates a
ammonium
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Cheng), [email protected] (S. Zhang). https://doi.org/10.1016/j.ijhydene.2019.08.092 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Micro-phase separation structure
conductivity of 63.1 mS cm1 (80 C). Meanwhile, the constrained ring conformation of N-
Robust alkaline stability
spirocyclic QA results in improved stability of the AEMs.
Fuel cell
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells are a prospective technology for portable devices and electric vehicles owing to their high efficiency, low emissions, and high power density. Based on differences in ion transportation, fuel cells can be classified into as proton exchange membrane fuel cells (PEMFCs) or anion exchange membrane fuel cells (AEMFCs). Compared with PEMFCs, AEMFCs are attracting attention owing to their low fuel permeability, enhanced oxygen reduction kinetics, a variety of available fuels and the use of non-platinum catalysts (e.g., Fe, Co and Ni) [1e4]. However, most anion exchange membranes (AEMs) cannot meet the requirements for practical application and large-scale commercialization of AEMFCs. Developing AEMs with high hydroxide conductivity and robust alkaline stability at elevated temperatures is one of the major challenges in the development of AEMFCs [5,6]. Recently, a variety of polymer backbones, including poly(phenylene oxide)s (PPOs) [7,8], poly(ether sulfone)s [9,10], poly(ether ketone)s [11,12], polystyrene [13,14], polybenzimidazole [15,16], and polymers of intrinsic microporosity (PIM) [17,18], have been investigated for developing high-performance AEMs. Among the developed AEMs, the quaternary ammonium (QA) group is the most commonly used cation owing to its easy preparation. However, conventional QA groups are prone to attack by OH ions via nucleophilic substitution and/or b-H Hofmann elimination under highly basic conditions, resulting in performance degradation of the AEMs [19]. Over the past several years, two major approaches have been applied to improve the alkaline stability of AEMs: 1) design of QA cations with large steric hindrance, strong electron donors or conformation restriction [20e24]; and 2) development of alternative cations such as imidazolium [10,24,25], guanidinium [26,27], phosphonium [28], sulfonium [29,30], and metal-complex [31,32]. Recently, N-spirocyclic QA-containing AEMs have attract attention since the constrained ring conformation increases the transition state energy of the degradation reaction, resulting in high stability of the cations [22,24,33e35]. As reported by Thanh et al., N-spirocyclic QA ionenes demonstrated excellent alkaline stability such that no degradation was detected even after storage in 1 M KOD/D2O at 80 C for more than 1500 h [24]. However, these ionenes are soluble in water, which cannot be used directly for AEMs. Although a blend approach by mixing ionenes with polybenzimidazole is feasible, this will sacrifice the ionic content of the AEMs and lead to a lower conductivity. Therefore, a breakthrough is necessary to develop N-spirocyclic QA-containing AEMs that are insoluble in water. AEMs with high ionic conductivity at operating temperature are desired for fuel cell applications. The conductivity of
the membrane is associated with ionic exchange capacity (IEC) and micro-morphology [36,37]. Increasing the IEC value is beneficial for enhancing the conductivity to some extent. However, this often results in a high water uptake (WU) and poor dimensional stability. Fabricating a microphase separated structure inside the membrane is regarded as a promising method for enhancing the conductivity since the formation of microphase separated morphology is critical for fabricating a highly efficient ion transport pathway [38]. It is claimed that AEMs with a side chain or block structure favor the formation of a microphase separation and drive the cationic groups to aggregate together, which can lead to fast conduction of OH. The group of Xu prepared a side-chaintype PPO with a hydrophilic side chain, which had the obvious hydrophilic/hydrophobic microphase separation [39]. The as-prepared AEMs showed the highest hydroxide conductivity of 45 mS cm1 at room temperature (RT). The multiblock copolymer with a quaternized hydrophilic block synthesized by the group of Li demonstrates well-organized phase separated morphology, resulting in an improvement in conductivity relative to the random AEMs [40,41]. However, block copolymer AEMs often have high WU. Additionally, the process for the synthesis of a block copolymer is complex and the precise control of micro-morphology is difficult due to the polydispersity of each block [42,43]. Recently, AEMs with a fluorine moiety have attracted great attention. Zhu et al. found that the fluoro-containing AEMs demonstrated much lower WU compared with similar AEMs without fluorine [13]. Since introducing fluorine moiety into AEMs is favored for improving the hydrophobicity of the backbone, which facilitates greater microphase separation and constrain the waterswelling behavior of the AEMs [44,45], this provides design rules for developing high-performance AEMs. In this work, we prepared a new class of poly(arylene ether) (PAE) AEMs bearing pendent N-spirocyclic QA groups via a facile Williamson reaction, in which the cationic groups are tethered at the end of a flexible side chain that is intended to enhance the local mobility of the cations and weaken the electron-withdrawing effect on the backbone [46,47]. The Williamson reaction is simple and easy to conduct. Meanwhile, free of strong electron-withdrawing (such as eSO2e or eCOe linkage), the designed backbone should have good chemical stability [48]. Furthermore, introducing fluorinated moiety into the PAE backbone is expected to strengthen the hydrophobic of the backbone and facilitate the formation of a micro-phase separation structure, which seems to be essential for constructing a high-efficient ion transporting pathway [45]. This configuration is expected to improve the alkaline stability and conductivity of the AEMs. In addition, the relationship between structure, morphology and physicochemical properties of the AEMs are investigated.
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Experimental Materials 9,9-Bis(4-hydroxyphenyl)fluorine (BHPF, 98.0%), decafluorobiphenyl (DFBP, 98.0%), 6-bromohexanoyl chloride (BHC, 97.0%), 4-(3-hydroxyphenyl)piperidine (HPP, 98.0%), a,a0 dibromo-o-xylene (DBX, 98.0%) and triethylsilane (98%) were obtained from TCI (Tokyo, Japan). N,N-dimethylacetamide (DMAc, 99.8%), N-methyl-2-pyrrolidone (NMP, 99.8%), trifluoroacetic acid (99%), N,N-diisopropylethylamine (DIPEA, 99.5%), dimethyl sulfoxide (DMSO, 99.8%), K2CO3 (99%), AlCl3 (AR) and 4,4-(hexafluoroisopropylidene)diphenol (HFDP, 98.0%) were obtained from Aladdin (Shanghai, China). The other chemicals used were obtained from Sinopharm Chemical Reagent Co. Ltd. and were used without further purification unless noted.
Preparation of AEMs Scheme 1 presents the synthetic route for PAE containing pendent N-spirocyclic QA (PAENQA-x, x ¼ 0.6, 0.8, 1.0), where x refers to the molar ratio of BHPF to DFBP. Firstly, PAE-x was synthesized via nucleophilic polycondensation of BHPF, DFBP and HFDP. Considering PAE-0.8 as an example, a mixture of BHPF (2.8034 g), DFBP (3.3412 g), HFDP (0.6725 g), K2CO3 (2.7642 g), DMAc (50 mL) and toluene (5 mL) were added into a flask equipped with a condenser. The reaction was performed under N2 atmosphere at 80 C for 12 h. Next, the high-viscous mixture was poured slowly into methanol, resulting in the formation of a polymer solid. PAE was obtained by washing these solids thoroughly with deionized water, and drying in vacuo at 60 C for 24 h. PAEKCBr-x was produced though the FriedeleCrafts reaction between PAE-x and BHC, as described in the literature [49]. The procedure for synthesis of PAEKCBr-0.8 is presented as follows. PAE-0.8 (1.000 g) was dissolved in dichloromethane (40 mL) to fabricate a homogeneous solution. Next, BHC (1.331 g) and AlCl3 (1.039 g) were placed into the solution. The mixture was then stirred at RT for 5 h. PAEKCBr-0.8 was obtained by pouring the resulting solution into methanol, washing with an excess of methanol, and drying in vacuo at 60 C, successively. The procedure for the ketone reduction reaction of PAEKCBr-x is described as follows. PAECOBr-0.8 (1.000 g), dichloromethane (60 mL), CF3COOH (0.85 mL) and triethylsilane (0.81 mL) were loaded into a 150 mL flask. The mixture was heated to boiling during the reaction process. After 24 h, the resulting mixture was poured into methanol. The product was filtered and washed thoroughly with methanol and PAECBr-0.8 was dried in vacuo at 60 C for 24 h. The synthetic route for N-spirocyclic QA containing eOH groups (NQA) is explained as follows. HPP (2.0000 g, 11.3 mmol), DIPEA (2.9210 g) and DBX (5.9568 g, 22.6 mmol) were dissolved in NMP (40 mL) to form a homogeneous solution. Next, the reaction was performed at 60 C for 24 h. The mixture was then precipitated in diethyl ether. NQA was obtained through filtration and washed thoroughly
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with diethyl ether followed by drying under a vacuum overnight at RT. PAENQA-x was synthesized by a Williamson reaction between PAECBr-x and NQA, as shown in Scheme 1. Considering PAENQA-0.8 as an example, PAECBr (1.0000 g) was dissolved in DMSO (30 mL) to form a homogeneous solution. NQA (0.6000) and K2CO3 (0.2864 g) were then added into the solution and the mixture was stirred at 90 C for another 24 h. Subsequently, the mixture was poured into acetone to produce a precipitate. After filtration and washing with deionized water, the obtained precipitate (PAENQA-0.8) was obtained by vacuum drying at 60 C overnight. The general procedure for preparing AEMs is described as follows. First, 0.5 g of PAENQA-x was dissolved in DMSO (10 mL) at RT. This solution was filtrated by a syringe filter and cast onto a glass dish which was then put in vacuo at 60 C for at least 24 h. The obtained AEMs were peelled off from the glass dish and soaked in 1 M a.q. KOH solution for 48 h. To eliminate the remaining alkalis, the AEMs were soaked in degassed deionized water (DDW) for 24 h and washed thoroughly with DDW.
Characterization and measurements 1
H-NMR spectra were collected on an Avance II 400-MHz spectrometer (Bruker Corp., US). Atomic force microscopy (AFM) experiments were carried out on a DI Multimode V microscope (Bruker Corp.) at ambient temperature. Small angle X-ray scattering (SAXS) characterization was carried out on a SAXSess-MC2 X-ray diffractometer (Anton Paar, Austria). The molecular weight of the polymers was measured through a gel permeation chromatography (GPC) system (Waters Corp., US) using tetrahydrofuran as the eluent and polystyrene as the standard. A thermal stability test was conducted using an STA449F5 thermogravimetric analyzer (Netzsch, Germany) under a N2 flow at a heating rate of 10 C min1, and all the membrane samples were pre-dried under a vacuum at 80 C for 24 h. The equipment used for the measurement of mechanical properties was a universal materials strength testing machine (Intstron, U.S). The membrane samples were provided in a gauge area of 20 mm 0.2 mm and with a crosshead speed of 0.2 mm s1. The IEC of the PAENQA-x AEMs was measured by the titration method. Typically, the as-prepared AEMs in bromide form were soaked in 100 mL of 0.5 M NaNO3 for 48 h in order to replace the Br ions with NO 3 ions. Accordingly, the resulting solution was titrated with 0.05 M AgNO3 solution using potassium chromate as the indicator. The WU and swelling ratio (SR) of the AEMs in OH form were measured after equilibration at a certain temperature. A piece of membrane was dried in vacuo at 80 C for 24 h and then equilibrated in DDW for another 24 h. The weight and length of the AEMs were collected before and after soaking in DDW. The WU of the AEMs was calculated as: WU ¼
mwet mdry 100% mdry
(1)
where, mwet and mdry are the mass of wet and dry membranes, respectively.
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Scheme 1 e Synthetic route for PAENQA-x polymer.
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The SR of the AEMs was calculated as: SR ¼
Lwet Ldry 100% Ldry
(2)
where, Lwet and Ldry are the length of wet and dry membranes. The hydroxide conductivity of the PAENQA-n membranes was characterized using the electrochemical impedance spectroscopy approach in a frequency range of 1 MHze100 mHz. The hydroxide conductivity of the AEMs was determined in the temperature range of 20e80 C. The hydroxide conductivity was calculated as: s¼
L AR
(3)
where L (cm) is the distance between the two electrodes; R (U) is the resistance of the membrane; and A (cm2) is the crosssectional area of the membrane sample. The stability of the PAENQA-1.0 membranes against alkalis was tested using a KOH solution (1 M) at 80 C. Accordingly, the membranes were soaked in the KOH solution (1 M) for 480 h, during which changes in IEC value, conductivity and chemical structure were recorded. The oxidative stability of the PAENQA-x AEMs were tested by soaking the membrane sample into Fenton’s reagent (3% H2O2 solution þ 4 ppm Fe2þ) at 80 C for 12 h. The weight of the membrane sample was measured before and after the test.
Single cell performance To prepared membrane electrodes for the AEMFC performance, 0.4 g of Pt/C (40 wt% content of Pt) was mixed with 2.2 g deionized water and 1.5 g ethanol, followed by dispersing in 5 wt% solution of the PAENQA-1.0 membrane and sonicating for 30 min. The prepared mixture was painted onto the both sides of the membrane to fabricate the catalyst-coated membrane (CCM) (active area: 2 cm 2 cm) using a N2 spray gun. The Pt loading in the catalyst layer was calculated to be 0.5 mg cm2. The membrane electrode assembly (MEA) was fabricated by assembling CCM and two pieces of TGP-H060 carbon paper (Toray, Japan) together. Next, the as-prepared MEA was assembled into the cell. The single cell performance was measured at 60 C with a flow rate of 100 mL min1 for both H2 and O2 at 100% relative humidity.
Results and discussion Synthesis and characterization of copolymers PAENQA-x copolymers were synthesized according to the synthetic route demonstrated in Scheme 1. The PAE-x copolymer was first synthesized from BHPF, DFBP, and HFDP via nucleophilic condensation polymerization. The as-synthesized PAE-x copolymers are still soluble in solvents (such as CHCl3), indicating that no crosslinking reaction occurred during the polymerization reaction. The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of PAE0.6 (Mn ¼ 45 kg mol1, Mw ¼ 72 kg mol1), PAE-0.8 (Mn¼ 52 kg mol1, Mw ¼ 81 kg mol1) and PAE-1.0 (Mn ¼ 54 kg mol1, Mw ¼ 86 kg mol1) are presented in Table S1, which indicate
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that the molecular weight of the polymers is high enough for fabricating membranes. As demonstrated in Fig. S1a, the peaks ranged from 6.7 ppm to 7.9 ppm, and are attributed to the signals from benzene, excepting for the peak at 7.26 ppm which belongs to the proton resonance from the deuterated reagent. The long flexible side chain was attached to the PAE-x backbone via a FriedeleCrafts reaction. The successful synthesis of PAEKCBr-0.8 was confirmed by the 1H NMR spectrum, as demonstrated in Fig. S1b. Compared with the spectrum of the PAE-0.8 polymer structure presented in Fig. S1a, five new peaks (H6, H9eH12) at 1.5e3.5 ppm appeared after the FriedelCrafts reaction. These new peaks are attributed to the signals from eCH2e groups attached to the backbone. It is noted that ketones in the side chain may take part in the degradation reaction under alkaline condition, resulting in a lower stability of the AEMs [49,50]. Thus, the ketone groups in the side chain of PAEKCBr-x was reduced to methylene groups using triethylsilane and trifluoroacetic acid. As displayed in Fig. S1c, the appearance of the new peak H17 (2.6 ppm), assigned to the eCH2e groups close to the benzene rings of the backbone and the shift of peak H6, from 2.9 ppm (Fig. S1b) to 1.6 ppm, indicated the successful reduction of ketone groups in the side chain. HPP was employed in the cycloquaternization reaction with DBX to synthesize NQA. According to a study reported by the group of Jannasch [51], the cyclic amine in HPP would first react with one of the benzyl bromide group in DBX and form a tertiary amine. Next, the tertiary group will further react with the other adjacent benzyl bromide groups to form the desired N-spirocyclic QA via an intramolecular Menshutkin reaction. As displayed in the 1H-NMR spectrum of NQA in Fig. S1d, the signals around 5.0 ppm and 3.7 ppm are ascribed to the methylene groups adjacent to the N atom. The signals from the protons of the benzene rings were found between 6.6 ppm and 7.6 ppm. Furthermore, the peaks ranging from 1.8 ppm to 3.0 ppm are ascribed to the signals from cycloaliphatic methylene/methine protons. The above discussion therefore confirmed the successful synthesis of NQA. The attachment of NQA to the end of the side chain of the backbone was accomplished by the facile, mild Williamson reaction between PAECBr-x and NQA. Fig. S2 demonstrates the 1 H NMR spectrum of PAENQA-0.8. The appearance of peaks (H17eH26) corresponds to the protons from NQA, which confirms the successful Williamson reaction. Accordingly, the other two PAENQA-x samples (x ¼ 0.6 and 1.0) were synthesized to study the effects of ionic content on the properties and performance of the PAENQA-x AEMs. All of the obtained PAENQA-x polymers were found to be insoluble in water.
Morphology of the membranes It is noted that the conductivity of the AEMs increases with increasing IEC owing to the increased ionic group density and higher water content. Nevertheless, a higher IEC of an AEM often results in a decreased dimensional and mechanical properties. In order to obtain AEMs with high conductivity, fabrication of well-connected ion transporting pathway in the membrane is proposed [38,52,53]. According to our previous results [47], AEMs with a hydrophilic side chain is favorable for forming a micro-separation structure, which seems to be
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essential for constructing a well-connected ion transporting pathway. Herein, the hydrophobic partially fluorinated PAE backbone is incompatible with a hydrophilic functionalized side chain, thus driving the PAENQA polymer to selfsegregation and forming a micro-separation structure. As displayed in Fig. 1, morphology of the PAENQA AEMs analyzed by tapping-mode AFM demonstrate distinct micro-phase separation, wherein the darker and brighter areas correspond to hydrophilic and hydrophobic domains, respectively. As anticipated, the size of the hydrophilic domain increases with an increase of the ionic content and PAENQA-1.0 AEM demonstrated a larger ionic domain than PAENQA-0.8 and PAENQA-0.6 analogs. This suggests that the combination of a fluorinated moiety into a polymer backbone and pendent cation structure is beneficial to the fabrication of wellconnected ion transporting pathways in the AEMs, thereby affecting the conductivity of the membranes. The morphology of the as-prepared AEMs was further investigated by SAXS, as shown in Fig. S3. All the PAENQA-x AEMs demonstrate obvious scattering peaks, which is an indicative of microphase separation. According to the Bragg equation (d ¼ 2p/q), the d-spacing of the three AEMs are calculated to be 52.4, 37.0 and 28.6 nm for PAENQA-1.0, PAENQA-0.8 and PAENQA-0.6, respectively. The d-spacing of PAENQA-1.0 is higher than that of other two AEMs. The reason is that the higher IEC tend to aggregate easily to form wider ion domains.
Water uptake and swelling behavior To meet the requirements for fuel cell applications, a relatively low WU and SR are expected, thus, a higher ion content can be introduced [54]. Herein, the WU and SR of the AEMs were investigated with temperatures ranging from 20 C to 80 C, considering their potential applications at higher temperatures. Fig. 2 demonstrates the WU and SR results of the as-prepared PAENQA-x AEMs. As anticipated, the WU and SR of PAENQA-x increased with increasing IEC values owing to the higher content of ionic groups in the AEMs. Membrane PAENQA-0.6 shows the lowest WU of 6.5% at 20 C and 9.3% at 80 C due to its low IEC values. This membrane also has the lowest SR of 3.2% and 4.6% at 20 C and 80 C, respectively. Membrane PAENQA-1.0 with the highest IEC values reached the highest WU of 13.1% and 19.5% at 20 C and 80 C, respectively. This membrane also reached the highest SR of
8.6% (80 C). The reason for the relatively low WU and SR of the PAENQA-x AEMs is most likely due to the side-chain-type structure which is effective for constraining water absorption and enhancing dimensional stability [19,55].
Hydroxide conductivity Conductivity is a core parameter for the applications of AEMs in fuel cells. To investigate the ion transport behavior of the PAENQA-x AEMs, the temperature-dependent OH conductivity was measured. As expected, the hydroxide conductivity increased with increasing temperature and IEC values, as shown in Fig. 3a. Membrane PAENQA-0.6 displays the lowest conductivity of 30.1 mS cm1 at 80 C due to its low IEC value and WU, while PAENQA-1.0 achieves the highest conductivity of 63.1 mS cm1 at 80 C. It was found that the hydroxide conductivity of PAENQA-1.0 and PAENQA-0.8 is higher than that of commercial Tokuyama AHA membranes that have similar IEC values. Furthermore, the conductivity of the AEMs at 95 C was also measured, as shown in Fig. 3a. As anticipated, the PAENQA-1.0 exhibited a high conductivity of 75.3 mS cm1 at 95 C. The reason for this is that the sidechain-type PAENQA-x membranes have a well-developed phase separation structure, which is beneficial for fabricating “highway” for ion conduction. Moreover, the hydroxide conductivity demonstrated an Arrhenius-like behavior in the temperature range of 20e80 C (Fig. 3b). The determined apparent activation energies (Ea) for PAENQA-0.6, PAENQA-0.8 and PAENQA-1.0 are 14.70, 14.90 and 13.69 kJ mol1, respectively. These values are comparable to those of reported AEMs (10e20 kJ mol1) [36,56e58], indicating that PAENQA-x AEMs have a similar OH transport mechanism as the reported AEMs [56,59]. It is mentioned that the carbonation process may occur during the period of ion exchanging [60]. Using a KOH-free technique for measuring the hydroxide conductivity reported by the group of Dekel [61], the conductivity may be higher than the values reported in the standard measurements used in this work. Fig. 3c and d presents the relationship between hydroxide conductivity and IEC/WU for PAENQA-x AEMs, together with the results for other reported AEMS for comparison. It is found that the performance of PAENQA-x AEMs is comparable to the reported block polymer AEMs [62e64] and side-chain-type polymer AEMs [35,65e69]. Interestingly, PAENQA-1.0 AEM
Fig. 1 e AFM phase images of PAENQA-x (x ¼ 0.6, 0.8 and 1.0) AEMs.
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Fig. 2 e The water uptake and swelling ratio of the PAENQA-x AEMs as a function of IEC value.
Fig. 3 e (a) Hydroxide conductivity of PAENQA-1.0 AEMs as a function of temperature, (b) their Arrhenius plots, (c) hydroxide conductivity (80 C) versus IEC value and (d) hydroxide conductivity (80 C) versus water uptake (80 C). The water uptake of X3Y5-1 and PES-60-IL are measured at RT and 30 C, respectively.
exhibits a higher conductivity and lower WU than most reported AEMs which have IEC values comparable to PAENQA1.0 AEM. These results can be explained by the formation of micro-phase separation structures in PAENQA-1.0 AEM (as confirmed by AFM), which seems to be essential for constructing highly efficient ion transporting pathways, resulting in a relatively high hydroxide conductivity with a lower IEC value. This would further certify that introduction of pendent cationic groups in the side chain is beneficial for promoting an improvement of conductivity.
Thermal stability, mechanical properties, alkaline stability and oxidative stability Thermal stability is also an important property that significantly affects the long-term stability of the AEMs. As demonstrated in Fig. 4a, all AEMs demonstrate similar TGA curves under a nitrogen atmosphere. The weight loss before 100 C is ascribed to the evaporation of absorbed water. PAENQA-x AEMs show a remarkably high thermal decomposition temperature of 300 C. The thermal decomposition
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Fig. 4 e (a) TGA traces and (b) mechanical properties of the PAENQA-x AEMs.
temperature of the PAENQA-x AEMs is not only above the operating temperature of the fuel cells, but also much higher than that of conventional QA-functionalized AEMs [17,46,47,57]. The excellent thermal stability of PAENQA-x AEMs can be ascribed to the high aromaticity and rigidity of the functionalized polymers [33]. The mechanical properties of the wet AEMs were investigated at ambient condition and it was found that the tensile strength increased while elongation at break decreased with decreasing IEC value. This is mainly attributed to the plasticization and presence of higher content of water in AEMs with high IEC, which make the membrane more flexible [70]. As seen in Fig. 4b, the membrane PAENQA-0.6 which possesses the lowest IEC value (0.68 meq g1), demonstrates the highest tensile strength (22.6 MPa) but the lowest elongation at break (6.7%) due to the rigid structure of the AEM. In contrast, the PAENQA-1.0 membrane with an IEC value of 0.93 meq g1 reveals the highest elongation at break (11.5%), and minimum tensile strength (14.5 MPa). The high tensile strength (14.5e22.6 MPa) and appropriate elongation at break (6.7e11.5%) provide PAENQA-x AEMs with good mechanical properties for easy MEA construction. The lack of AEMs with robust alkaline stability is currently constraining the practical application of AEMs in fuel cells. To investigate the alkaline stability of the AEMs in this work, PAENQA-1.0 membrane samples were soaked in 1 M aq. KOH at 80 C for 480 h, during which the conductivity and IEC values were monitored. As seen in Fig. 5, the IEC and conductivity decreased rapidly in the initial 200 h. This may be mainly attributed to the morphological changes in the membranes. Similar phenomenon can be found in the literature [71]. After 200 h, the IEC and conductivity tended to be stable. The reason may be that the morphology of the membrane hardly changed after 200 h. Furthermore, the as-prepared AEMs have good alkaline stability, and thus hardly any decline in IEC and conductivity was observed after 200 h. The hydroxide conductivity and IEC of PAENQA-1.0 membrane after 480 h soaking are still 22.1 mS cm1 and 0.87 meq g1, respectively, demonstrating the excellent stability of the pendent N-spirocyclic QA with a high remaining rate of IEC (93.6%) and conductivity (94.0%). On the other hand, only 40% of the hydroxide conductivity remained for conventional QAbased AEMs (e.g., QPAF-1) after soaked the membrane in 1 M aq. KOH at 80 C for 288 h, because of the degradation of
benzyl-trimethyl ammonium [72]. It is worth pointing out that the water concentration will affect the alkaline stability of the membrane during the stability test [73]. A combination of alkaline and low hydration environment will result in an increase of chemical degradation of cationic groups [74e77]. In an operating AEMFC, the water concentration will decrease with increasing current density. In order to reveal the practical stability of the AEMs, our further work will focus on investigating the alkaline stability of the as-prepared AEMs under a low hydration level and alkaline condition. Unlike most reported AEMs that become insoluble after alkaline stability test [69,78,79], the PAENQA-1.0 membrane is still soluble in organic solvents which enabled us to further investigate the changes in chemical structure. Fig. S3 shows the 1H NMR spectrum of PAENQA-1.0 before and after the alkaline stability test. The signal around 3.7 ppm assigned to eCH2e proton close to the N atom slightly decreased after the test, indicating robust alkaline stability of PAENQA-1.0. As noted by Chu et al. [80], the proposed degradation pathway of N-spirocyclic quaternary ammonium was provided in Fig. S4. The three small new peaks around 0.86, 1.24, and 1.52 ppm were attributed to the proton signals from the degradation product. The superior stability of PAENQA-1.0 is believed to result from the constrained ring conformation of N-spirocyclic QA which increases the transition state energy of the
Fig. 5 e Hydroxide conductivity and IEC of PAENQA-1.0 as a function of time.
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Fig. 6 e (a) Performance of the H2eO2 fuel cell based on PAENQA-1.0 and Tokuyama AHA membrane at 60 C, and (b) Durability test of the single cell based on PAENQA-1.0 at 60 C under a constant current density of 100 mA cm¡2.
degradation reaction [22,24]. Furthermore, as noted by Dekel et al., the water molecules increased in the aqueous alkali ex situ tests, and this condition may result in a good alkaline stability [73]. Above all, our strategy of grafting N-spirocyclic QAs at the terminal of the side chain is effective for enhancing the alkaline stability. The oxidative stability was also studied by immersing the membrane in Fenton’s reagent at 80 C for 12 h, and the results were presented in Table S2. It was found that the residual weight of PAENQA-x decreased with increasing IEC. The reason is that higher water uptake in the AEMs often lead to the higher exposure of the membranes to reactive radicals in Fenton’s reagent [81,82]. It can be seen that the PAENQA-x AEMs remain above 80% of their initial weight after the test, indicating the as-prepared AEMs have good oxidative stability.
Fuel cell performance In order to evaluate the single cell performance of these AEMs, PAENQA-1.0 (50 mm) was chosen as an electrolyte membrane considering its excellent comprehensive performance. Fig. 6a illustrates the polarization and power density curves of the MEA at 60 C. A high open circuit voltage (OCV) of 1.01 V was achieved for the PAENQA-1.0 membrane, and this value is comparable to the reported AEMFCs under similar conditions [10,58,83,84], indicating that the as-prepared AEM is able to block gas fuel permeation effectively. The maximum power density (Pmax) of the fuel cell reached to 138.6 mW cm2 at a current density of 450 mA cm2. Thus, this result demonstrates the potential candidates of AEMs bearing pendent Nspirocyclic QA for fuel cells, although the Pmax of the fuel cell using PAENQA-1.0 is not as high as the values reported by Omasta et al. and Wang et al. [85,86], but still higher than that using a Tokuyama AHA membrane (40.2 mW cm2) under the same testing conditions. The reason for this may be that the high frequency resistance (HFR) of the cell based on PAENQA1.0 (ca. 0.4 U cm2) is lower than that of Tokuyama AHA (ca. 1.0 U cm2) (Fig. S4). It is believed that further refinement of the catalyst layer, MEA structure and operating condition could result in improved performance [85,87]. In addition, the durability test was also applied to evaluate the stability of the PAENQA-1.0 membrane in the single cell. As seen in Fig. 6b,
the voltage of the single cell was monitored under a constant current density of 100 mA cm2. It turned out that the voltage decreased slowly from ~0.5 V to ~0.45 V during a testing time of 12 h. The voltage decline is lower than that of some AEMFCs reported in a review under similar condition [88], indicating reasonable stability of the single cell. The decline in voltage may be attributed to the degradation of the AEMs during run time.
Conclusions In summary, a new type of partially fluorinated PAE-based AEMs bearing pendent N-spirocyclic QA were synthesized by consecutive steps involving nucleophilic substitution polymerization, FriedeleCrafts reaction, a ketone reduction reaction and a Williamson reaction. A well-defined micro-phase separation structure and a well-connected ion transporting pathway were accomplished via the combination of the partially fluorinated backbone and pendent cation structure. Therefore, enhanced conductivity and a low SR are observed for the PAENQA-x AEMs. The resulting AEMs also demonstrate good thermal stability, good oxidative stability and robust alkaline stability. The conductivity of PAENQA-1.0 membrane only decreased by ~6% after storage in 1 M a.q. KOH solution at 80 C for 480 h. This work demonstrates the feasibility of the rational design of AEM structure to enhance the hydroxide conductivity and alkaline stability for application to AEMFCs. Nevertheless, the conductivity of the AEMs can be further improved via increasing the IEC value. Further work will concentrate on developing AEMs having high conductivity via increasing the IEC and taking advantage of the configuration considered above.
Acknowledgment This work was supported by China Postdoctoral Science Foundation (2019M653657), Research Start-up Funds of DGUT (196100040018), National Natural Science Foundation of China (grant No. 21775022, 21872046 and 51772089), Guangdong
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Provincial Key Platform and Major Scientific Research Projects for Colleges and Universities (No. 2015KCXTD029), the Youth 1000 Talent Program of China, the Outstanding Youth Scientist Foundation of Hunan Province (Grant No. 2018JJ1009), Provincial Science and Technology Innovation Platform and Talent Plan - Changsha, Zhuzhou and Xiangtan High-level Talents Accumulation Project (Grant No. 2017XK2023).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.092.
references
[1] Hossen MM, 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;375:214e21. [2] Kabir S, Lemire K, Artyushkova K, Roy A, Odgaard M, Schlueter D, et al. Platinum group metal-free NiMo hydrogen oxidation catalysts: high performance and durability in alkaline exchange membrane fuel cells. J Mater Chem 2017;5:24433e43. [3] Zhuang Z, Giles SA, Zheng J, Jenness GR, Caratzoulas S, Vlachos DG, et al. Nickel supported on nitrogen-doped carbon nanotubes as hydrogen oxidation reaction catalyst in alkaline electrolyte. Nat Commun 2016;7:10141e8. €a € rik M, Kook M, Puust L, Saar R, [4] Ratso S, Kruusenberg I, Ka et al. Highly efficient transition metal and nitrogen co-doped carbide-derived carbon electrocatalysts for anion exchange membrane fuel cells. J Power Sources 2018;375:233e43. [5] Serov A, Zenyuk IV, Arges CG, Chatenet M. Hot topics in alkaline exchange membrane fuel cells. J Power Sources 2018;375:149e57. [6] Gottesfeld S, Dekel DR, Page M, Bae C, Yan Y, Zelenay P, et al. Anion exchange membrane fuel cells: current status and remaining challenges. J Power Sources 2018;375:170e84. [7] Ran J, Fu C, Ding L, Cao P, Xu T. Dual hydrophobic grafted chains facilitating quaternary ammonium aggregations of hydroxide conducting polymers: a theoretical and experimental investigation. J Mater Chem 2018;6:5714e23. [8] Ran J, Ding L, Yu D, Zhang X, Hu M, Wu L, et al. A novel strategy to construct highly conductive and stabilized anionic channels by fluorocarbon grafted polymers. J Membr Sci 2018;549:631e7. [9] Yan J, Zhu L, Chaloux BL, Hickner MA. Anion exchange membranes by bromination of tetramethylbiphenol-based poly(sulfone)s. Polym Chem 2017;8:2442e9. [10] Pan Y, Zhang Q, Yan X, Liu J, Xu X, Wang T, et al. Hydrophilic side chain assisting continuous ion-conducting channels for anion exchange membranes. J Membr Sci 2018;552:286e94. [11] Yan X, Gao L, Zheng W, Ruan X, Zhang C, Wu X, et al. Longspacer-chain imidazolium functionalized poly(ether ether ketone) as hydroxide exchange membrane for fuel cell. Int J Hydrogen Energy 2016;41:14982e90. [12] Li L, Lin CX, Wang XQ, Yang Q, Zhang QG, Zhu AM, et al. Highly conductive anion exchange membranes with long flexible multication spacer. J Membr Sci 2018;553:209e17. [13] Zhu M, Zhang X, Wang Y, Wu Y, Wang H, Zhang M, et al. Novel anion exchange membranes based on quaternized diblock copolystyrene containing a fluorinated hydrophobic block. J Membr Sci 2018;554:264e73.
[14] Xue J, Liu L, Liao J, Shen Y, Li N. UV-crosslinking of polystyrene anion exchange membranes by azidated macromolecular crosslinker for alkaline fuel cells. J Membr Sci 2017;535:322e30. [15] Li S, Zhu X, Liu D, Sun F. A highly durable long side-chain polybenzimidazole anion exchange membrane for AEMFC. J Membr Sci 2018;546:15e21. [16] Hu M, Ni J, Zhang B, Neelakandan S, Wang L. Crosslinked polybenzimidazoles containing branching structure as membrane materials with excellent cell performance and durability for fuel cell applications. J Power Sources 2018;389:222e9. [17] Ishiwari F, Sato T, Yamazaki H, Kondo JN, Miyanishi S, Yamaguchi T, et al. An anion-conductive microporous membrane composed of a rigid ladder polymer with a spirobiindane backbone. J Mater Chem 2016;4:17655e9. [18] Yang Z, Guo R, Malpass-Evans R, Carta M, McKeown NB, Guiver MD, et al. Highly conductive anion-exchange € ger’s base polymers. membranes from microporous Tro Angew Chem Int Ed 2016;55:11499e502. [19] Li NW, Yan TZ, Li Z, Thurn-Albrecht T, Binder WH. Comb-shaped polymers to enhance hydroxide transport in anion exchange membranes. Energy Environ Sci 2012;5:7888e92. [20] Zhu M, Zhang M, Chen Q, Su Y, Zhang Z, Liu L, et al. Synthesis of midblock-quaternized triblock copolystyrenes as highly conductive and alkaline-stable anion-exchange membranes. Polym Chem 2017;8:2074e86. [21] Zhang M, Liu J, Wang Y, An L, Guiver MD, Li N. Highly stable anion exchange membranes based on quaternized polypropylene. J Mater Chem 2015;3:12284e96. [22] Marino M, Kreuer K. Alkaline stability of quaternary ammonium cations for alkaline fuel cell membranes and ionic liquids. ChemSusChem 2015;8:513e23. [23] Gu S, Cai R, Luo T, Chen Z, Sun M, Liu Y, et al. A soluble and highly conductive ionomer for high-performance hydroxide exchange membrane fuel cells. Angew Chem Int Ed 2009;48:6499e502. [24] Pham TH, Olsson JS, Jannasch P. N-spirocyclic quaternary ammonium ionenes for anion-exchange membranes. J Am Chem Soc 2017;139:2888e91. [25] Liu D, Xu M, Fang M, Chen J, Wang L. Branched comb-shaped poly(arylene ether sulfone)s containing flexible alkyl imidazolium side chains as anion exchange membranes. J Mater Chem 2018;6:10879e90. [26] Xue B, Dong X, Li Y, Zheng J, Li S, Zhang S. Synthesis of novel guanidinium-based anion-exchange membranes with controlled microblock structures. J Membr Sci 2017;537:151e9. [27] Chen Y, Tao Y, Wang J, Yang S, Cheng S, Wei H, et al. Combshaped guanidinium functionalized poly(ether sulfone)s for anion exchange membranes: effects of the spacer types and lengths. J Polym Sci Part A: Polym Chem 2017;55:1313e21. [28] Tang H, Li D, Li N, Zhang Z, Zhang Z. Anion conductive poly(2,6-dimethyl phenylene oxide)s with clicked bulky quaternary phosphonium groups. J Membr Sci 2018;558:9e16. [29] Zhang B, Gu S, Wang J, Liu Y, Herring AM, Yan Y. Tertiary sulfonium as a cationic functional group for hydroxide exchange membranes. RSC Adv 2012;2:12683e5. [30] Hossain MA, Jang H, Sutradhar SC, Ha J, Yoo J, Lee C, et al. Novel hydroxide conducting sulfonium-based anion exchange membrane for alkaline fuel cell applications. Int J Hydrogen Energy 2016;41:10458e65. [31] Tianyu Z, Shichao X, Anisur R, Emir D, Peng Y, Parasmani P, et al. Cationic metallo-polyelectrolytes for robust alkaline anion-exchange membranes. Angew Chem Int Ed 2018;57:2388e92.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 5 6 5 e2 6 5 7 6
[32] Kwasny MT, Zhu L, Hickner MA, Tew GN. Thermodynamics of counterion release is critical for anion exchange membrane conductivity. J Am Chem Soc 2018;140:7961e9. [33] Pham TH, Olsson JS, Jannasch P. Poly(arylene alkylene)s with pendant N-spirocyclic quaternary ammonium cations for anion exchange membranes. J Mater Chem 2018;6:16537e47. [34] Strasser DJ, Graziano BJ, Knauss DM. Base stable poly(diallylpiperidinium hydroxide) multiblock copolymers for anion exchange membranes. J Mater Chem 2017;5:9627e40. [35] Olsson JS, Pham TH, Jannasch P. Poly(N,Ndiallylazacycloalkane)s for anion-exchange membranes functionalized with N-spirocyclic quaternary ammonium cations. Macromolecules 2017;50:2784e93. [36] Zhang X, Shi Q, Chen P, Zhou J, Li S, Xu H, et al. Block poly(arylene ether sulfone) copolymers tethering aromatic side-chain quaternary ammonium as anion exchange membranes. Polym Chem 2018;9:699e711. [37] Varcoe JR, Atanassov P, Dekel DR, Herring AM, Hickner MA, Kohl PA, et al. Anion-exchange membranes in electrochemical energy systems. Energy Environ Sci 2014;7:3135e91. [38] Shin DW, Guiver MD, Lee YM. Hydrocarbon-based polymer electrolyte membranes: importance of morphology on ion transport and membrane stability. Chem Rev 2017;117:4759e805. [39] Zhu Y, Ding L, Liang X, Shehzad MA, Wang L, Ge X, et al. Beneficial use of rotatable-spacer side-chains in alkaline anion exchange membranes for fuel cells. Energy Environ Sci 2018;11:3472e9. [40] Li X, Liu Q, Yu Y, Meng Y. Synthesis and properties of multiblock ionomers containing densely functionalized hydrophilic blocks for anion exchange membranes. J Membr Sci 2014;467:1e12. [41] Tanaka M, Fukasawa K, Nishino E, Yamaguchi S, Yamada K, Tanaka H, et al. Anion conductive block poly (arylene ether) s: synthesis, properties, and application in alkaline fuel cells. J Am Chem Soc 2011;133:10646e54. [42] Li N, Wang C, Lee SY, Park CH, Lee YM, Guiver MD. Enhancement of proton transport by nanochannels in combshaped copoly (arylene ether sulfone)s. Angew Chem Int Ed 2011;123:9324e7. [43] Li Q, Liu L, Miao Q, Jin B, Bai R. Hydroxide-conducting polymer electrolyte membranes from aromatic ABA triblock copolymers. Polym Chem 2014;5:2208e13. [44] Wang XQ, Lin CX, Liu FH, Li L, Yang Q, Zhang QG, et al. Alkalistable partially fluorinated poly(arylene ether) anion exchange membranes with a claw-type head for fuel cells. J Mater Chem 2018;6:12455e65. [45] Kim E, Lee S, Woo S, Park S-H, Yim S-D, Shin D, et al. Synthesis and characterization of anion exchange multiblock copolymer membranes with a fluorine moiety as alkaline membrane fuel cells. J Power Sources 2017;359:568e76. [46] Dang H-S, Weiber EA, Jannasch P. Poly (phenylene oxide) functionalized with quaternary ammonium groups via flexible alkyl spacers for high-performance anion exchange membranes. J Mater Chem 2015;3:5280e4. [47] Lin CX, Huang XL, Guo D, Zhang QG, Zhu AM, Ye ML, et al. Side-chain-type anion exchange membranes bearing pendant quaternary ammonium groups via flexible spacers for fuel cells. J Mater Chem 2016;4:13938e48. [48] Amel A, Zhu L, Hickner M, Ein-Eli Y. Influence of sulfone linkage on the stability of aromatic quaternary ammonium polymers for alkaline fuel cells. J Electrochem Soc 2014;161:F615e21. [49] Liu L, Ahlfield J, Tricker A, Chu D, Kohl PA. Anion conducting multiblock copolymer membranes with partial
[50]
[51]
[52]
[53]
[54]
[55]
[56]
[57]
[58]
[59]
[60]
[61]
[62]
[63]
[64]
[65]
[66]
[67]
[68]
26575
fluorination and long head-group tethers. J Mater Chem 2016;4:16233e44. Hibbs MR. Alkaline stability of poly (phenylene)-based anion exchange membranes with various cations. J Polym Sci, Part B: Polym Phys 2013;51:1736e42. Pham TH, Jannasch P. Aromatic polymers incorporating bis-N-spirocyclic quaternary ammonium moieties for anion-exchange membranes. ACS Macro Lett 2015;4:1370e5. Li N, Guiver MD. Ion transport by nanochannels in ioncontaining aromatic copolymers. Macromolecules 2014;47:2175e98. He G, Li Z, Zhao J, Wang S, Wu H, Guiver MD, et al. Nanostructured ion-exchange membranes for fuel cells: recent advances and perspectives. Adv Mater 2015;27:5280e95. He Y, Zhang J, Liang X, Shehzad MA, Ge X, Zhu Y, et al. Achieving high anion conductivity by densely grafting of ionic strings. J Membr Sci 2018;559:35e41. Li N, Leng Y, Hickner MA, Wang CY. Highly stable, anion conductive, comb-shaped copolymers for alkaline fuel cells. J Am Chem Soc 2013;135:10124e33. Zhu L, Yu X, Hickner MA. Exploring backbone-cation alkyl spacers for multi-cation side chain anion exchange membranes. J Power Sources 2018;375:433e41. Yang Q, Li L, Lin CX, Gao XL, Zhao CH, Zhang QG, et al. Hyperbranched poly(arylene ether ketone) anion exchange membranes for fuel cells. J Membr Sci 2018;560:77e86. Mandal M, Huang G, Kohl PA. Anionic multiblock copolymer membrane based on vinyl addition polymerization of norbornenes: applications in anion-exchange membrane fuel cells. J Membr Sci 2019;570e571:394e402. Chen D, Hickner MA. Ion clustering in quaternary ammonium functionalized benzylmethyl containing poly(arylene ether ketone)s. Macromolecules 2013;46:9270e8. Noga Z, MW E, DD R. The effect of ambient carbon dioxide on anion-exchange membrane fuel cells. ChemSusChem 2018;11:1136e50. Ziv N, Dekel DR. A practical method for measuring the true hydroxide conductivity of anion exchange membranes. Electrochem Commun 2018;88:109e13. Lin CX, Wu HY, Li L, Wang XQ, Zhang QG, Zhu AM, et al. Anion conductive triblock copolymer membranes with flexible multication side chain. ACS Appl Mater Interfaces 2018;10:18327e37. Hao J, Gao X, Jiang Y, Zhang H, Luo J, Shao Z, et al. Crosslinked high-performance anion exchange membranes based on poly(styrene-b-(ethylene-co-butylene)-b-styrene). J Membr Sci 2018;551:66e75. Liu L, Huang G, Kohl PA. Anion conducting multiblock copolymers with multiple head-groups. J Mater Chem 2018;6:9000e8. Wang XQ, Lin CX, Zhang QG, Zhu AM, Liu QL. Anion exchange membranes from hydroxyl-bearing poly(ether sulfone)s with flexible spacers via ring-opening grafting for fuel cells. Int J Hydrogen Energy 2017;42:19044e55. Liu W, Liu L, Liao J, Wang L, Li N. Self-crosslinking of combshaped polystyrene anion exchange membranes for alkaline fuel cell application. J Membr Sci 2017;536:133e40. Lin CX, Wang XQ, Hu EN, Yang Q, Zhang QG, Zhu AM, et al. Quaternized triblock polymer anion exchange membranes with enhanced alkaline stability. J Membr Sci 2017;541:358e66. He Y, Ge X, Liang X, Zhang J, Shehzad MA, Zhu Y, et al. Anion exchange membranes with branched ionic clusters for fuel cells. J Mater Chem 2018;6:5993e8.
26576
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 5 6 5 e2 6 5 7 6
[69] Lin CX, Zhuo YZ, Lai AN, Zhang QG, Zhu AM, Ye ML, et al. Side-chain-type anion exchange membranes bearing pendent imidazolium-functionalized poly(phenylene oxide) for fuel cells. J Membr Sci 2016;513:206e16. [70] Yuk J, Lee S, Nugraha AF, Lee H, Park S-H, Yim S-D, et al. Synthesis and characterization of multi-block poly(arylene ether sulfone) membranes with highly sulfonated blocks for use in polymer electrolyte membrane fuel cells. J Membr Sci 2016;518:50e9. [71] Dang H-S, Jannasch P. Anion-exchange membranes with polycationic alkyl side chains attached via spacer units. J Mater Chem A 2016;4:17138e53. [72] Ono H, Kimura T, Takano A, Asazawa K, Miyake J, Inukai J, et al. Robust anion conductive polymers containing perfluoroalkylene and pendant ammonium groups for high performance fuel cells. J Mater Chem 2017;5:24804e12. [73] Dekel DR, Amar M, Willdorf S, Kosa M, Dhara S, Diesendruck CE. Effect of water on the stability of quaternary ammonium groups for anion exchange membrane fuel cell applications. Chem Mater 2017;29:4425e31. [74] Diesendruck CE, Dekel DR. Water-A key parameter in the stability of anion exchange membrane fuel cells. Cur. Opin. Electrochem. 2018;9:173e8. [75] Dekel DR, Willdorf S, Ash U, Amar M, Pusara S, Dhara S, et al. The critical relation between chemical stability of cations and water in anion exchange membrane fuel cells environment. J Power Sources 2018;375:351e60. [76] Willdorf-Cohen S, Mondal AN, Dekel DR, Diesendruck CE. Chemical stability of poly(phenylene oxide)-based ionomers in an anion exchange-membrane fuel cell environment. J Mater Chem A 2018;6:22234e9. [77] Fan J, Willdorf-Cohen S, Schibli EM, Paula Z, Li W, Skalski TJG, et al. Poly(bis-arylimidazoliums) possessing high hydroxide ion exchange capacity and high alkaline stability. Nat Commun 2019;10:2306. [78] Mahmoud AMA, Elsaghier AMM, Otsuji K, Miyatake K. High hydroxide ion conductivity with enhanced alkaline stability of partially fluorinated and quaternized aromatic copolymers as anion exchange membranes. Macromolecules 2017;50:4256e66.
[79] Ono H, Miyake J, Shimada S, Uchida M, Miyatake K. Anion exchange membranes composed of perfluoroalkylene chains and ammonium-functionalized oligophenylenes. J Mater Chem 2015;3:21779e88. [80] Chu X, Liu L, Huang Y, Guiver MD, Li N. Practical implementation of bis-six-membered N-cyclic quaternary ammonium cations in advanced anion exchange membranes for fuel cells: synthesis and durability. J Membr Sci 2019;578:239e50. [81] Chen X, Lu¨ H, Lin Q, Zhang X, Chen D, Zheng Y. Partially fluorinated poly(arylene ether)s bearing long alkyl sulfonate side chains for stable and highly conductive proton exchange membranes. J Membr Sci 2018;549:12e22. [82] Lin C, Liu X, Yang Q, Wu H, Liu F, Zhang Q, et al. Hydrophobic side chains to enhance hydroxide conductivity and physicochemical stabilities of side-chain-type polymer AEMs. J Membr Sci 2019;585:90e8. [83] Akiyama R, Yokota N, Otsuji K, Miyatake K. Structurally welldefined anion conductive aromatic copolymers: effect of the side-chain length. Macromolecules 2018;51:3394e404. [84] Tuli SK, Roy AL, Elgammal RA, Tian M, Zawodzinski TA, Fujiwara T. Effect of morphology on anion conductive properties in self-assembled polystyrene-based copolymer membranes. J Membr Sci 2018;565:213e25. [85] Omasta TJ, Park AM, LaManna JM, Zhang Y, Peng X, Wang L, et al. Beyond catalysis and membranes: visualizing and solving the challenge of electrode water accumulation and flooding in AEMFCs. Energy Environ Sci 2018;11:551e8. [86] Wang L, Brink JJ, Liu Y, Herring AM, Ponce-Gonzalez J, Whelligan DK, et al. Non-fluorinated pre-irradiation-grafted (peroxidated) LDPE-based anion-exchange membranes with high performance and stability. Energy Environ Sci 2017;10:2154e67. [87] Hao J, Jiang Y, Gao X, Lu W, Xiao Y, Shao Z, et al. Functionalization of polybenzimidazole-crosslinked poly(vinylbenzyl chloride) with two cyclic quaternary ammonium cations for anion exchange membranes. J Membr Sci 2018;548:1e10. [88] Dekel DR. Review of cell performance in anion exchange membrane fuel cells. J Power Sources 2018;375:158e69.
Available online at www.sciencedirect.com
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Facile construction of poly(arylene ether)s-based anion exchange membranes bearing pendent N-spirocyclic quaternary ammonium for fuel cells Chenxiao Lin a,b,c, Demei Yu c, Jixia Wang b, Yan Zhang b, Dong Xie a, Faliang Cheng a,**, Shiguo Zhang b,* a
Guangdong Engineering and Technology Research Center for Advanced Nanomaterials, School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan, 523808, PR China b College of Materials Science and Engineering, Hunan University, Changsha, 410082, PR China c School of Science, Xi’an Jiaotong University, Xi'an, 710049, PR China
highlights Novel
graphical abstract
side-chain-type
AEMs
bearing N-spirocyclic cations were prepared. The synthesized procedure was performed at mild condition. The
AEM
demonstrated
developed
well-
micro-phase
separation. The AEM exhibited high conductivity at low water uptake. High retention rate of conductivity was
achieved
after
alkaline
treatment.
article info
abstract
Article history:
Anion exchange membrane (AEM) fuel cells have received significant attention due to their
Received 17 April 2019
low fuel permeability and the use of non-platinum catalysts. However, the development of
Received in revised form
AEMs with robust chemical stability and high conductivity is still a great challenge. Herein,
29 July 2019
we prepare a new type of partially fluorinated backbone bearing pendent N-spirocyclic
Accepted 12 August 2019
quaternary ammonium (QA) cations via a facile Williamson reaction, which displays great
Available online 6 September 2019
potential for fuel cells. The integration of the two substructures (a fluorinated moiety into a polymer backbone and a pendent cation structure) is beneficial for the fabrication of a well-
Keywords:
defined micro-phase separation structure, thereby facilitating the construction of a highly-
Anion exchange membrane
efficient ion transporting pathway. Correspondingly, the resulting AEM (PAENQA-1.0),
N-spirocyclic quaternary
despite its a relatively low ionic exchange capacity (0.93 meq g1) demonstrates a
ammonium
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Cheng), [email protected] (S. Zhang). https://doi.org/10.1016/j.ijhydene.2019.08.092 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Micro-phase separation structure
conductivity of 63.1 mS cm1 (80 C). Meanwhile, the constrained ring conformation of N-
Robust alkaline stability
spirocyclic QA results in improved stability of the AEMs.
Fuel cell
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells are a prospective technology for portable devices and electric vehicles owing to their high efficiency, low emissions, and high power density. Based on differences in ion transportation, fuel cells can be classified into as proton exchange membrane fuel cells (PEMFCs) or anion exchange membrane fuel cells (AEMFCs). Compared with PEMFCs, AEMFCs are attracting attention owing to their low fuel permeability, enhanced oxygen reduction kinetics, a variety of available fuels and the use of non-platinum catalysts (e.g., Fe, Co and Ni) [1e4]. However, most anion exchange membranes (AEMs) cannot meet the requirements for practical application and large-scale commercialization of AEMFCs. Developing AEMs with high hydroxide conductivity and robust alkaline stability at elevated temperatures is one of the major challenges in the development of AEMFCs [5,6]. Recently, a variety of polymer backbones, including poly(phenylene oxide)s (PPOs) [7,8], poly(ether sulfone)s [9,10], poly(ether ketone)s [11,12], polystyrene [13,14], polybenzimidazole [15,16], and polymers of intrinsic microporosity (PIM) [17,18], have been investigated for developing high-performance AEMs. Among the developed AEMs, the quaternary ammonium (QA) group is the most commonly used cation owing to its easy preparation. However, conventional QA groups are prone to attack by OH ions via nucleophilic substitution and/or b-H Hofmann elimination under highly basic conditions, resulting in performance degradation of the AEMs [19]. Over the past several years, two major approaches have been applied to improve the alkaline stability of AEMs: 1) design of QA cations with large steric hindrance, strong electron donors or conformation restriction [20e24]; and 2) development of alternative cations such as imidazolium [10,24,25], guanidinium [26,27], phosphonium [28], sulfonium [29,30], and metal-complex [31,32]. Recently, N-spirocyclic QA-containing AEMs have attract attention since the constrained ring conformation increases the transition state energy of the degradation reaction, resulting in high stability of the cations [22,24,33e35]. As reported by Thanh et al., N-spirocyclic QA ionenes demonstrated excellent alkaline stability such that no degradation was detected even after storage in 1 M KOD/D2O at 80 C for more than 1500 h [24]. However, these ionenes are soluble in water, which cannot be used directly for AEMs. Although a blend approach by mixing ionenes with polybenzimidazole is feasible, this will sacrifice the ionic content of the AEMs and lead to a lower conductivity. Therefore, a breakthrough is necessary to develop N-spirocyclic QA-containing AEMs that are insoluble in water. AEMs with high ionic conductivity at operating temperature are desired for fuel cell applications. The conductivity of
the membrane is associated with ionic exchange capacity (IEC) and micro-morphology [36,37]. Increasing the IEC value is beneficial for enhancing the conductivity to some extent. However, this often results in a high water uptake (WU) and poor dimensional stability. Fabricating a microphase separated structure inside the membrane is regarded as a promising method for enhancing the conductivity since the formation of microphase separated morphology is critical for fabricating a highly efficient ion transport pathway [38]. It is claimed that AEMs with a side chain or block structure favor the formation of a microphase separation and drive the cationic groups to aggregate together, which can lead to fast conduction of OH. The group of Xu prepared a side-chaintype PPO with a hydrophilic side chain, which had the obvious hydrophilic/hydrophobic microphase separation [39]. The as-prepared AEMs showed the highest hydroxide conductivity of 45 mS cm1 at room temperature (RT). The multiblock copolymer with a quaternized hydrophilic block synthesized by the group of Li demonstrates well-organized phase separated morphology, resulting in an improvement in conductivity relative to the random AEMs [40,41]. However, block copolymer AEMs often have high WU. Additionally, the process for the synthesis of a block copolymer is complex and the precise control of micro-morphology is difficult due to the polydispersity of each block [42,43]. Recently, AEMs with a fluorine moiety have attracted great attention. Zhu et al. found that the fluoro-containing AEMs demonstrated much lower WU compared with similar AEMs without fluorine [13]. Since introducing fluorine moiety into AEMs is favored for improving the hydrophobicity of the backbone, which facilitates greater microphase separation and constrain the waterswelling behavior of the AEMs [44,45], this provides design rules for developing high-performance AEMs. In this work, we prepared a new class of poly(arylene ether) (PAE) AEMs bearing pendent N-spirocyclic QA groups via a facile Williamson reaction, in which the cationic groups are tethered at the end of a flexible side chain that is intended to enhance the local mobility of the cations and weaken the electron-withdrawing effect on the backbone [46,47]. The Williamson reaction is simple and easy to conduct. Meanwhile, free of strong electron-withdrawing (such as eSO2e or eCOe linkage), the designed backbone should have good chemical stability [48]. Furthermore, introducing fluorinated moiety into the PAE backbone is expected to strengthen the hydrophobic of the backbone and facilitate the formation of a micro-phase separation structure, which seems to be essential for constructing a high-efficient ion transporting pathway [45]. This configuration is expected to improve the alkaline stability and conductivity of the AEMs. In addition, the relationship between structure, morphology and physicochemical properties of the AEMs are investigated.
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Experimental Materials 9,9-Bis(4-hydroxyphenyl)fluorine (BHPF, 98.0%), decafluorobiphenyl (DFBP, 98.0%), 6-bromohexanoyl chloride (BHC, 97.0%), 4-(3-hydroxyphenyl)piperidine (HPP, 98.0%), a,a0 dibromo-o-xylene (DBX, 98.0%) and triethylsilane (98%) were obtained from TCI (Tokyo, Japan). N,N-dimethylacetamide (DMAc, 99.8%), N-methyl-2-pyrrolidone (NMP, 99.8%), trifluoroacetic acid (99%), N,N-diisopropylethylamine (DIPEA, 99.5%), dimethyl sulfoxide (DMSO, 99.8%), K2CO3 (99%), AlCl3 (AR) and 4,4-(hexafluoroisopropylidene)diphenol (HFDP, 98.0%) were obtained from Aladdin (Shanghai, China). The other chemicals used were obtained from Sinopharm Chemical Reagent Co. Ltd. and were used without further purification unless noted.
Preparation of AEMs Scheme 1 presents the synthetic route for PAE containing pendent N-spirocyclic QA (PAENQA-x, x ¼ 0.6, 0.8, 1.0), where x refers to the molar ratio of BHPF to DFBP. Firstly, PAE-x was synthesized via nucleophilic polycondensation of BHPF, DFBP and HFDP. Considering PAE-0.8 as an example, a mixture of BHPF (2.8034 g), DFBP (3.3412 g), HFDP (0.6725 g), K2CO3 (2.7642 g), DMAc (50 mL) and toluene (5 mL) were added into a flask equipped with a condenser. The reaction was performed under N2 atmosphere at 80 C for 12 h. Next, the high-viscous mixture was poured slowly into methanol, resulting in the formation of a polymer solid. PAE was obtained by washing these solids thoroughly with deionized water, and drying in vacuo at 60 C for 24 h. PAEKCBr-x was produced though the FriedeleCrafts reaction between PAE-x and BHC, as described in the literature [49]. The procedure for synthesis of PAEKCBr-0.8 is presented as follows. PAE-0.8 (1.000 g) was dissolved in dichloromethane (40 mL) to fabricate a homogeneous solution. Next, BHC (1.331 g) and AlCl3 (1.039 g) were placed into the solution. The mixture was then stirred at RT for 5 h. PAEKCBr-0.8 was obtained by pouring the resulting solution into methanol, washing with an excess of methanol, and drying in vacuo at 60 C, successively. The procedure for the ketone reduction reaction of PAEKCBr-x is described as follows. PAECOBr-0.8 (1.000 g), dichloromethane (60 mL), CF3COOH (0.85 mL) and triethylsilane (0.81 mL) were loaded into a 150 mL flask. The mixture was heated to boiling during the reaction process. After 24 h, the resulting mixture was poured into methanol. The product was filtered and washed thoroughly with methanol and PAECBr-0.8 was dried in vacuo at 60 C for 24 h. The synthetic route for N-spirocyclic QA containing eOH groups (NQA) is explained as follows. HPP (2.0000 g, 11.3 mmol), DIPEA (2.9210 g) and DBX (5.9568 g, 22.6 mmol) were dissolved in NMP (40 mL) to form a homogeneous solution. Next, the reaction was performed at 60 C for 24 h. The mixture was then precipitated in diethyl ether. NQA was obtained through filtration and washed thoroughly
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with diethyl ether followed by drying under a vacuum overnight at RT. PAENQA-x was synthesized by a Williamson reaction between PAECBr-x and NQA, as shown in Scheme 1. Considering PAENQA-0.8 as an example, PAECBr (1.0000 g) was dissolved in DMSO (30 mL) to form a homogeneous solution. NQA (0.6000) and K2CO3 (0.2864 g) were then added into the solution and the mixture was stirred at 90 C for another 24 h. Subsequently, the mixture was poured into acetone to produce a precipitate. After filtration and washing with deionized water, the obtained precipitate (PAENQA-0.8) was obtained by vacuum drying at 60 C overnight. The general procedure for preparing AEMs is described as follows. First, 0.5 g of PAENQA-x was dissolved in DMSO (10 mL) at RT. This solution was filtrated by a syringe filter and cast onto a glass dish which was then put in vacuo at 60 C for at least 24 h. The obtained AEMs were peelled off from the glass dish and soaked in 1 M a.q. KOH solution for 48 h. To eliminate the remaining alkalis, the AEMs were soaked in degassed deionized water (DDW) for 24 h and washed thoroughly with DDW.
Characterization and measurements 1
H-NMR spectra were collected on an Avance II 400-MHz spectrometer (Bruker Corp., US). Atomic force microscopy (AFM) experiments were carried out on a DI Multimode V microscope (Bruker Corp.) at ambient temperature. Small angle X-ray scattering (SAXS) characterization was carried out on a SAXSess-MC2 X-ray diffractometer (Anton Paar, Austria). The molecular weight of the polymers was measured through a gel permeation chromatography (GPC) system (Waters Corp., US) using tetrahydrofuran as the eluent and polystyrene as the standard. A thermal stability test was conducted using an STA449F5 thermogravimetric analyzer (Netzsch, Germany) under a N2 flow at a heating rate of 10 C min1, and all the membrane samples were pre-dried under a vacuum at 80 C for 24 h. The equipment used for the measurement of mechanical properties was a universal materials strength testing machine (Intstron, U.S). The membrane samples were provided in a gauge area of 20 mm 0.2 mm and with a crosshead speed of 0.2 mm s1. The IEC of the PAENQA-x AEMs was measured by the titration method. Typically, the as-prepared AEMs in bromide form were soaked in 100 mL of 0.5 M NaNO3 for 48 h in order to replace the Br ions with NO 3 ions. Accordingly, the resulting solution was titrated with 0.05 M AgNO3 solution using potassium chromate as the indicator. The WU and swelling ratio (SR) of the AEMs in OH form were measured after equilibration at a certain temperature. A piece of membrane was dried in vacuo at 80 C for 24 h and then equilibrated in DDW for another 24 h. The weight and length of the AEMs were collected before and after soaking in DDW. The WU of the AEMs was calculated as: WU ¼
mwet mdry 100% mdry
(1)
where, mwet and mdry are the mass of wet and dry membranes, respectively.
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Scheme 1 e Synthetic route for PAENQA-x polymer.
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The SR of the AEMs was calculated as: SR ¼
Lwet Ldry 100% Ldry
(2)
where, Lwet and Ldry are the length of wet and dry membranes. The hydroxide conductivity of the PAENQA-n membranes was characterized using the electrochemical impedance spectroscopy approach in a frequency range of 1 MHze100 mHz. The hydroxide conductivity of the AEMs was determined in the temperature range of 20e80 C. The hydroxide conductivity was calculated as: s¼
L AR
(3)
where L (cm) is the distance between the two electrodes; R (U) is the resistance of the membrane; and A (cm2) is the crosssectional area of the membrane sample. The stability of the PAENQA-1.0 membranes against alkalis was tested using a KOH solution (1 M) at 80 C. Accordingly, the membranes were soaked in the KOH solution (1 M) for 480 h, during which changes in IEC value, conductivity and chemical structure were recorded. The oxidative stability of the PAENQA-x AEMs were tested by soaking the membrane sample into Fenton’s reagent (3% H2O2 solution þ 4 ppm Fe2þ) at 80 C for 12 h. The weight of the membrane sample was measured before and after the test.
Single cell performance To prepared membrane electrodes for the AEMFC performance, 0.4 g of Pt/C (40 wt% content of Pt) was mixed with 2.2 g deionized water and 1.5 g ethanol, followed by dispersing in 5 wt% solution of the PAENQA-1.0 membrane and sonicating for 30 min. The prepared mixture was painted onto the both sides of the membrane to fabricate the catalyst-coated membrane (CCM) (active area: 2 cm 2 cm) using a N2 spray gun. The Pt loading in the catalyst layer was calculated to be 0.5 mg cm2. The membrane electrode assembly (MEA) was fabricated by assembling CCM and two pieces of TGP-H060 carbon paper (Toray, Japan) together. Next, the as-prepared MEA was assembled into the cell. The single cell performance was measured at 60 C with a flow rate of 100 mL min1 for both H2 and O2 at 100% relative humidity.
Results and discussion Synthesis and characterization of copolymers PAENQA-x copolymers were synthesized according to the synthetic route demonstrated in Scheme 1. The PAE-x copolymer was first synthesized from BHPF, DFBP, and HFDP via nucleophilic condensation polymerization. The as-synthesized PAE-x copolymers are still soluble in solvents (such as CHCl3), indicating that no crosslinking reaction occurred during the polymerization reaction. The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of PAE0.6 (Mn ¼ 45 kg mol1, Mw ¼ 72 kg mol1), PAE-0.8 (Mn¼ 52 kg mol1, Mw ¼ 81 kg mol1) and PAE-1.0 (Mn ¼ 54 kg mol1, Mw ¼ 86 kg mol1) are presented in Table S1, which indicate
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that the molecular weight of the polymers is high enough for fabricating membranes. As demonstrated in Fig. S1a, the peaks ranged from 6.7 ppm to 7.9 ppm, and are attributed to the signals from benzene, excepting for the peak at 7.26 ppm which belongs to the proton resonance from the deuterated reagent. The long flexible side chain was attached to the PAE-x backbone via a FriedeleCrafts reaction. The successful synthesis of PAEKCBr-0.8 was confirmed by the 1H NMR spectrum, as demonstrated in Fig. S1b. Compared with the spectrum of the PAE-0.8 polymer structure presented in Fig. S1a, five new peaks (H6, H9eH12) at 1.5e3.5 ppm appeared after the FriedelCrafts reaction. These new peaks are attributed to the signals from eCH2e groups attached to the backbone. It is noted that ketones in the side chain may take part in the degradation reaction under alkaline condition, resulting in a lower stability of the AEMs [49,50]. Thus, the ketone groups in the side chain of PAEKCBr-x was reduced to methylene groups using triethylsilane and trifluoroacetic acid. As displayed in Fig. S1c, the appearance of the new peak H17 (2.6 ppm), assigned to the eCH2e groups close to the benzene rings of the backbone and the shift of peak H6, from 2.9 ppm (Fig. S1b) to 1.6 ppm, indicated the successful reduction of ketone groups in the side chain. HPP was employed in the cycloquaternization reaction with DBX to synthesize NQA. According to a study reported by the group of Jannasch [51], the cyclic amine in HPP would first react with one of the benzyl bromide group in DBX and form a tertiary amine. Next, the tertiary group will further react with the other adjacent benzyl bromide groups to form the desired N-spirocyclic QA via an intramolecular Menshutkin reaction. As displayed in the 1H-NMR spectrum of NQA in Fig. S1d, the signals around 5.0 ppm and 3.7 ppm are ascribed to the methylene groups adjacent to the N atom. The signals from the protons of the benzene rings were found between 6.6 ppm and 7.6 ppm. Furthermore, the peaks ranging from 1.8 ppm to 3.0 ppm are ascribed to the signals from cycloaliphatic methylene/methine protons. The above discussion therefore confirmed the successful synthesis of NQA. The attachment of NQA to the end of the side chain of the backbone was accomplished by the facile, mild Williamson reaction between PAECBr-x and NQA. Fig. S2 demonstrates the 1 H NMR spectrum of PAENQA-0.8. The appearance of peaks (H17eH26) corresponds to the protons from NQA, which confirms the successful Williamson reaction. Accordingly, the other two PAENQA-x samples (x ¼ 0.6 and 1.0) were synthesized to study the effects of ionic content on the properties and performance of the PAENQA-x AEMs. All of the obtained PAENQA-x polymers were found to be insoluble in water.
Morphology of the membranes It is noted that the conductivity of the AEMs increases with increasing IEC owing to the increased ionic group density and higher water content. Nevertheless, a higher IEC of an AEM often results in a decreased dimensional and mechanical properties. In order to obtain AEMs with high conductivity, fabrication of well-connected ion transporting pathway in the membrane is proposed [38,52,53]. According to our previous results [47], AEMs with a hydrophilic side chain is favorable for forming a micro-separation structure, which seems to be
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essential for constructing a well-connected ion transporting pathway. Herein, the hydrophobic partially fluorinated PAE backbone is incompatible with a hydrophilic functionalized side chain, thus driving the PAENQA polymer to selfsegregation and forming a micro-separation structure. As displayed in Fig. 1, morphology of the PAENQA AEMs analyzed by tapping-mode AFM demonstrate distinct micro-phase separation, wherein the darker and brighter areas correspond to hydrophilic and hydrophobic domains, respectively. As anticipated, the size of the hydrophilic domain increases with an increase of the ionic content and PAENQA-1.0 AEM demonstrated a larger ionic domain than PAENQA-0.8 and PAENQA-0.6 analogs. This suggests that the combination of a fluorinated moiety into a polymer backbone and pendent cation structure is beneficial to the fabrication of wellconnected ion transporting pathways in the AEMs, thereby affecting the conductivity of the membranes. The morphology of the as-prepared AEMs was further investigated by SAXS, as shown in Fig. S3. All the PAENQA-x AEMs demonstrate obvious scattering peaks, which is an indicative of microphase separation. According to the Bragg equation (d ¼ 2p/q), the d-spacing of the three AEMs are calculated to be 52.4, 37.0 and 28.6 nm for PAENQA-1.0, PAENQA-0.8 and PAENQA-0.6, respectively. The d-spacing of PAENQA-1.0 is higher than that of other two AEMs. The reason is that the higher IEC tend to aggregate easily to form wider ion domains.
Water uptake and swelling behavior To meet the requirements for fuel cell applications, a relatively low WU and SR are expected, thus, a higher ion content can be introduced [54]. Herein, the WU and SR of the AEMs were investigated with temperatures ranging from 20 C to 80 C, considering their potential applications at higher temperatures. Fig. 2 demonstrates the WU and SR results of the as-prepared PAENQA-x AEMs. As anticipated, the WU and SR of PAENQA-x increased with increasing IEC values owing to the higher content of ionic groups in the AEMs. Membrane PAENQA-0.6 shows the lowest WU of 6.5% at 20 C and 9.3% at 80 C due to its low IEC values. This membrane also has the lowest SR of 3.2% and 4.6% at 20 C and 80 C, respectively. Membrane PAENQA-1.0 with the highest IEC values reached the highest WU of 13.1% and 19.5% at 20 C and 80 C, respectively. This membrane also reached the highest SR of
8.6% (80 C). The reason for the relatively low WU and SR of the PAENQA-x AEMs is most likely due to the side-chain-type structure which is effective for constraining water absorption and enhancing dimensional stability [19,55].
Hydroxide conductivity Conductivity is a core parameter for the applications of AEMs in fuel cells. To investigate the ion transport behavior of the PAENQA-x AEMs, the temperature-dependent OH conductivity was measured. As expected, the hydroxide conductivity increased with increasing temperature and IEC values, as shown in Fig. 3a. Membrane PAENQA-0.6 displays the lowest conductivity of 30.1 mS cm1 at 80 C due to its low IEC value and WU, while PAENQA-1.0 achieves the highest conductivity of 63.1 mS cm1 at 80 C. It was found that the hydroxide conductivity of PAENQA-1.0 and PAENQA-0.8 is higher than that of commercial Tokuyama AHA membranes that have similar IEC values. Furthermore, the conductivity of the AEMs at 95 C was also measured, as shown in Fig. 3a. As anticipated, the PAENQA-1.0 exhibited a high conductivity of 75.3 mS cm1 at 95 C. The reason for this is that the sidechain-type PAENQA-x membranes have a well-developed phase separation structure, which is beneficial for fabricating “highway” for ion conduction. Moreover, the hydroxide conductivity demonstrated an Arrhenius-like behavior in the temperature range of 20e80 C (Fig. 3b). The determined apparent activation energies (Ea) for PAENQA-0.6, PAENQA-0.8 and PAENQA-1.0 are 14.70, 14.90 and 13.69 kJ mol1, respectively. These values are comparable to those of reported AEMs (10e20 kJ mol1) [36,56e58], indicating that PAENQA-x AEMs have a similar OH transport mechanism as the reported AEMs [56,59]. It is mentioned that the carbonation process may occur during the period of ion exchanging [60]. Using a KOH-free technique for measuring the hydroxide conductivity reported by the group of Dekel [61], the conductivity may be higher than the values reported in the standard measurements used in this work. Fig. 3c and d presents the relationship between hydroxide conductivity and IEC/WU for PAENQA-x AEMs, together with the results for other reported AEMS for comparison. It is found that the performance of PAENQA-x AEMs is comparable to the reported block polymer AEMs [62e64] and side-chain-type polymer AEMs [35,65e69]. Interestingly, PAENQA-1.0 AEM
Fig. 1 e AFM phase images of PAENQA-x (x ¼ 0.6, 0.8 and 1.0) AEMs.
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Fig. 2 e The water uptake and swelling ratio of the PAENQA-x AEMs as a function of IEC value.
Fig. 3 e (a) Hydroxide conductivity of PAENQA-1.0 AEMs as a function of temperature, (b) their Arrhenius plots, (c) hydroxide conductivity (80 C) versus IEC value and (d) hydroxide conductivity (80 C) versus water uptake (80 C). The water uptake of X3Y5-1 and PES-60-IL are measured at RT and 30 C, respectively.
exhibits a higher conductivity and lower WU than most reported AEMs which have IEC values comparable to PAENQA1.0 AEM. These results can be explained by the formation of micro-phase separation structures in PAENQA-1.0 AEM (as confirmed by AFM), which seems to be essential for constructing highly efficient ion transporting pathways, resulting in a relatively high hydroxide conductivity with a lower IEC value. This would further certify that introduction of pendent cationic groups in the side chain is beneficial for promoting an improvement of conductivity.
Thermal stability, mechanical properties, alkaline stability and oxidative stability Thermal stability is also an important property that significantly affects the long-term stability of the AEMs. As demonstrated in Fig. 4a, all AEMs demonstrate similar TGA curves under a nitrogen atmosphere. The weight loss before 100 C is ascribed to the evaporation of absorbed water. PAENQA-x AEMs show a remarkably high thermal decomposition temperature of 300 C. The thermal decomposition
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Fig. 4 e (a) TGA traces and (b) mechanical properties of the PAENQA-x AEMs.
temperature of the PAENQA-x AEMs is not only above the operating temperature of the fuel cells, but also much higher than that of conventional QA-functionalized AEMs [17,46,47,57]. The excellent thermal stability of PAENQA-x AEMs can be ascribed to the high aromaticity and rigidity of the functionalized polymers [33]. The mechanical properties of the wet AEMs were investigated at ambient condition and it was found that the tensile strength increased while elongation at break decreased with decreasing IEC value. This is mainly attributed to the plasticization and presence of higher content of water in AEMs with high IEC, which make the membrane more flexible [70]. As seen in Fig. 4b, the membrane PAENQA-0.6 which possesses the lowest IEC value (0.68 meq g1), demonstrates the highest tensile strength (22.6 MPa) but the lowest elongation at break (6.7%) due to the rigid structure of the AEM. In contrast, the PAENQA-1.0 membrane with an IEC value of 0.93 meq g1 reveals the highest elongation at break (11.5%), and minimum tensile strength (14.5 MPa). The high tensile strength (14.5e22.6 MPa) and appropriate elongation at break (6.7e11.5%) provide PAENQA-x AEMs with good mechanical properties for easy MEA construction. The lack of AEMs with robust alkaline stability is currently constraining the practical application of AEMs in fuel cells. To investigate the alkaline stability of the AEMs in this work, PAENQA-1.0 membrane samples were soaked in 1 M aq. KOH at 80 C for 480 h, during which the conductivity and IEC values were monitored. As seen in Fig. 5, the IEC and conductivity decreased rapidly in the initial 200 h. This may be mainly attributed to the morphological changes in the membranes. Similar phenomenon can be found in the literature [71]. After 200 h, the IEC and conductivity tended to be stable. The reason may be that the morphology of the membrane hardly changed after 200 h. Furthermore, the as-prepared AEMs have good alkaline stability, and thus hardly any decline in IEC and conductivity was observed after 200 h. The hydroxide conductivity and IEC of PAENQA-1.0 membrane after 480 h soaking are still 22.1 mS cm1 and 0.87 meq g1, respectively, demonstrating the excellent stability of the pendent N-spirocyclic QA with a high remaining rate of IEC (93.6%) and conductivity (94.0%). On the other hand, only 40% of the hydroxide conductivity remained for conventional QAbased AEMs (e.g., QPAF-1) after soaked the membrane in 1 M aq. KOH at 80 C for 288 h, because of the degradation of
benzyl-trimethyl ammonium [72]. It is worth pointing out that the water concentration will affect the alkaline stability of the membrane during the stability test [73]. A combination of alkaline and low hydration environment will result in an increase of chemical degradation of cationic groups [74e77]. In an operating AEMFC, the water concentration will decrease with increasing current density. In order to reveal the practical stability of the AEMs, our further work will focus on investigating the alkaline stability of the as-prepared AEMs under a low hydration level and alkaline condition. Unlike most reported AEMs that become insoluble after alkaline stability test [69,78,79], the PAENQA-1.0 membrane is still soluble in organic solvents which enabled us to further investigate the changes in chemical structure. Fig. S3 shows the 1H NMR spectrum of PAENQA-1.0 before and after the alkaline stability test. The signal around 3.7 ppm assigned to eCH2e proton close to the N atom slightly decreased after the test, indicating robust alkaline stability of PAENQA-1.0. As noted by Chu et al. [80], the proposed degradation pathway of N-spirocyclic quaternary ammonium was provided in Fig. S4. The three small new peaks around 0.86, 1.24, and 1.52 ppm were attributed to the proton signals from the degradation product. The superior stability of PAENQA-1.0 is believed to result from the constrained ring conformation of N-spirocyclic QA which increases the transition state energy of the
Fig. 5 e Hydroxide conductivity and IEC of PAENQA-1.0 as a function of time.
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Fig. 6 e (a) Performance of the H2eO2 fuel cell based on PAENQA-1.0 and Tokuyama AHA membrane at 60 C, and (b) Durability test of the single cell based on PAENQA-1.0 at 60 C under a constant current density of 100 mA cm¡2.
degradation reaction [22,24]. Furthermore, as noted by Dekel et al., the water molecules increased in the aqueous alkali ex situ tests, and this condition may result in a good alkaline stability [73]. Above all, our strategy of grafting N-spirocyclic QAs at the terminal of the side chain is effective for enhancing the alkaline stability. The oxidative stability was also studied by immersing the membrane in Fenton’s reagent at 80 C for 12 h, and the results were presented in Table S2. It was found that the residual weight of PAENQA-x decreased with increasing IEC. The reason is that higher water uptake in the AEMs often lead to the higher exposure of the membranes to reactive radicals in Fenton’s reagent [81,82]. It can be seen that the PAENQA-x AEMs remain above 80% of their initial weight after the test, indicating the as-prepared AEMs have good oxidative stability.
Fuel cell performance In order to evaluate the single cell performance of these AEMs, PAENQA-1.0 (50 mm) was chosen as an electrolyte membrane considering its excellent comprehensive performance. Fig. 6a illustrates the polarization and power density curves of the MEA at 60 C. A high open circuit voltage (OCV) of 1.01 V was achieved for the PAENQA-1.0 membrane, and this value is comparable to the reported AEMFCs under similar conditions [10,58,83,84], indicating that the as-prepared AEM is able to block gas fuel permeation effectively. The maximum power density (Pmax) of the fuel cell reached to 138.6 mW cm2 at a current density of 450 mA cm2. Thus, this result demonstrates the potential candidates of AEMs bearing pendent Nspirocyclic QA for fuel cells, although the Pmax of the fuel cell using PAENQA-1.0 is not as high as the values reported by Omasta et al. and Wang et al. [85,86], but still higher than that using a Tokuyama AHA membrane (40.2 mW cm2) under the same testing conditions. The reason for this may be that the high frequency resistance (HFR) of the cell based on PAENQA1.0 (ca. 0.4 U cm2) is lower than that of Tokuyama AHA (ca. 1.0 U cm2) (Fig. S4). It is believed that further refinement of the catalyst layer, MEA structure and operating condition could result in improved performance [85,87]. In addition, the durability test was also applied to evaluate the stability of the PAENQA-1.0 membrane in the single cell. As seen in Fig. 6b,
the voltage of the single cell was monitored under a constant current density of 100 mA cm2. It turned out that the voltage decreased slowly from ~0.5 V to ~0.45 V during a testing time of 12 h. The voltage decline is lower than that of some AEMFCs reported in a review under similar condition [88], indicating reasonable stability of the single cell. The decline in voltage may be attributed to the degradation of the AEMs during run time.
Conclusions In summary, a new type of partially fluorinated PAE-based AEMs bearing pendent N-spirocyclic QA were synthesized by consecutive steps involving nucleophilic substitution polymerization, FriedeleCrafts reaction, a ketone reduction reaction and a Williamson reaction. A well-defined micro-phase separation structure and a well-connected ion transporting pathway were accomplished via the combination of the partially fluorinated backbone and pendent cation structure. Therefore, enhanced conductivity and a low SR are observed for the PAENQA-x AEMs. The resulting AEMs also demonstrate good thermal stability, good oxidative stability and robust alkaline stability. The conductivity of PAENQA-1.0 membrane only decreased by ~6% after storage in 1 M a.q. KOH solution at 80 C for 480 h. This work demonstrates the feasibility of the rational design of AEM structure to enhance the hydroxide conductivity and alkaline stability for application to AEMFCs. Nevertheless, the conductivity of the AEMs can be further improved via increasing the IEC value. Further work will concentrate on developing AEMs having high conductivity via increasing the IEC and taking advantage of the configuration considered above.
Acknowledgment This work was supported by China Postdoctoral Science Foundation (2019M653657), Research Start-up Funds of DGUT (196100040018), National Natural Science Foundation of China (grant No. 21775022, 21872046 and 51772089), Guangdong
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Provincial Key Platform and Major Scientific Research Projects for Colleges and Universities (No. 2015KCXTD029), the Youth 1000 Talent Program of China, the Outstanding Youth Scientist Foundation of Hunan Province (Grant No. 2018JJ1009), Provincial Science and Technology Innovation Platform and Talent Plan - Changsha, Zhuzhou and Xiangtan High-level Talents Accumulation Project (Grant No. 2017XK2023).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.092.
references
[1] Hossen MM, 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;375:214e21. [2] Kabir S, Lemire K, Artyushkova K, Roy A, Odgaard M, Schlueter D, et al. Platinum group metal-free NiMo hydrogen oxidation catalysts: high performance and durability in alkaline exchange membrane fuel cells. J Mater Chem 2017;5:24433e43. [3] Zhuang Z, Giles SA, Zheng J, Jenness GR, Caratzoulas S, Vlachos DG, et al. Nickel supported on nitrogen-doped carbon nanotubes as hydrogen oxidation reaction catalyst in alkaline electrolyte. Nat Commun 2016;7:10141e8. €a € rik M, Kook M, Puust L, Saar R, [4] Ratso S, Kruusenberg I, Ka et al. Highly efficient transition metal and nitrogen co-doped carbide-derived carbon electrocatalysts for anion exchange membrane fuel cells. J Power Sources 2018;375:233e43. [5] Serov A, Zenyuk IV, Arges CG, Chatenet M. Hot topics in alkaline exchange membrane fuel cells. J Power Sources 2018;375:149e57. [6] Gottesfeld S, Dekel DR, Page M, Bae C, Yan Y, Zelenay P, et al. Anion exchange membrane fuel cells: current status and remaining challenges. J Power Sources 2018;375:170e84. [7] Ran J, Fu C, Ding L, Cao P, Xu T. Dual hydrophobic grafted chains facilitating quaternary ammonium aggregations of hydroxide conducting polymers: a theoretical and experimental investigation. J Mater Chem 2018;6:5714e23. [8] Ran J, Ding L, Yu D, Zhang X, Hu M, Wu L, et al. A novel strategy to construct highly conductive and stabilized anionic channels by fluorocarbon grafted polymers. J Membr Sci 2018;549:631e7. [9] Yan J, Zhu L, Chaloux BL, Hickner MA. Anion exchange membranes by bromination of tetramethylbiphenol-based poly(sulfone)s. Polym Chem 2017;8:2442e9. [10] Pan Y, Zhang Q, Yan X, Liu J, Xu X, Wang T, et al. Hydrophilic side chain assisting continuous ion-conducting channels for anion exchange membranes. J Membr Sci 2018;552:286e94. [11] Yan X, Gao L, Zheng W, Ruan X, Zhang C, Wu X, et al. Longspacer-chain imidazolium functionalized poly(ether ether ketone) as hydroxide exchange membrane for fuel cell. Int J Hydrogen Energy 2016;41:14982e90. [12] Li L, Lin CX, Wang XQ, Yang Q, Zhang QG, Zhu AM, et al. Highly conductive anion exchange membranes with long flexible multication spacer. J Membr Sci 2018;553:209e17. [13] Zhu M, Zhang X, Wang Y, Wu Y, Wang H, Zhang M, et al. Novel anion exchange membranes based on quaternized diblock copolystyrene containing a fluorinated hydrophobic block. J Membr Sci 2018;554:264e73.
[14] Xue J, Liu L, Liao J, Shen Y, Li N. UV-crosslinking of polystyrene anion exchange membranes by azidated macromolecular crosslinker for alkaline fuel cells. J Membr Sci 2017;535:322e30. [15] Li S, Zhu X, Liu D, Sun F. A highly durable long side-chain polybenzimidazole anion exchange membrane for AEMFC. J Membr Sci 2018;546:15e21. [16] Hu M, Ni J, Zhang B, Neelakandan S, Wang L. Crosslinked polybenzimidazoles containing branching structure as membrane materials with excellent cell performance and durability for fuel cell applications. J Power Sources 2018;389:222e9. [17] Ishiwari F, Sato T, Yamazaki H, Kondo JN, Miyanishi S, Yamaguchi T, et al. An anion-conductive microporous membrane composed of a rigid ladder polymer with a spirobiindane backbone. J Mater Chem 2016;4:17655e9. [18] Yang Z, Guo R, Malpass-Evans R, Carta M, McKeown NB, Guiver MD, et al. Highly conductive anion-exchange € ger’s base polymers. membranes from microporous Tro Angew Chem Int Ed 2016;55:11499e502. [19] Li NW, Yan TZ, Li Z, Thurn-Albrecht T, Binder WH. Comb-shaped polymers to enhance hydroxide transport in anion exchange membranes. Energy Environ Sci 2012;5:7888e92. [20] Zhu M, Zhang M, Chen Q, Su Y, Zhang Z, Liu L, et al. Synthesis of midblock-quaternized triblock copolystyrenes as highly conductive and alkaline-stable anion-exchange membranes. Polym Chem 2017;8:2074e86. [21] Zhang M, Liu J, Wang Y, An L, Guiver MD, Li N. Highly stable anion exchange membranes based on quaternized polypropylene. J Mater Chem 2015;3:12284e96. [22] Marino M, Kreuer K. Alkaline stability of quaternary ammonium cations for alkaline fuel cell membranes and ionic liquids. ChemSusChem 2015;8:513e23. [23] Gu S, Cai R, Luo T, Chen Z, Sun M, Liu Y, et al. A soluble and highly conductive ionomer for high-performance hydroxide exchange membrane fuel cells. Angew Chem Int Ed 2009;48:6499e502. [24] Pham TH, Olsson JS, Jannasch P. N-spirocyclic quaternary ammonium ionenes for anion-exchange membranes. J Am Chem Soc 2017;139:2888e91. [25] Liu D, Xu M, Fang M, Chen J, Wang L. Branched comb-shaped poly(arylene ether sulfone)s containing flexible alkyl imidazolium side chains as anion exchange membranes. J Mater Chem 2018;6:10879e90. [26] Xue B, Dong X, Li Y, Zheng J, Li S, Zhang S. Synthesis of novel guanidinium-based anion-exchange membranes with controlled microblock structures. J Membr Sci 2017;537:151e9. [27] Chen Y, Tao Y, Wang J, Yang S, Cheng S, Wei H, et al. Combshaped guanidinium functionalized poly(ether sulfone)s for anion exchange membranes: effects of the spacer types and lengths. J Polym Sci Part A: Polym Chem 2017;55:1313e21. [28] Tang H, Li D, Li N, Zhang Z, Zhang Z. Anion conductive poly(2,6-dimethyl phenylene oxide)s with clicked bulky quaternary phosphonium groups. J Membr Sci 2018;558:9e16. [29] Zhang B, Gu S, Wang J, Liu Y, Herring AM, Yan Y. Tertiary sulfonium as a cationic functional group for hydroxide exchange membranes. RSC Adv 2012;2:12683e5. [30] Hossain MA, Jang H, Sutradhar SC, Ha J, Yoo J, Lee C, et al. Novel hydroxide conducting sulfonium-based anion exchange membrane for alkaline fuel cell applications. Int J Hydrogen Energy 2016;41:10458e65. [31] Tianyu Z, Shichao X, Anisur R, Emir D, Peng Y, Parasmani P, et al. Cationic metallo-polyelectrolytes for robust alkaline anion-exchange membranes. Angew Chem Int Ed 2018;57:2388e92.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 5 6 5 e2 6 5 7 6
[32] Kwasny MT, Zhu L, Hickner MA, Tew GN. Thermodynamics of counterion release is critical for anion exchange membrane conductivity. J Am Chem Soc 2018;140:7961e9. [33] Pham TH, Olsson JS, Jannasch P. Poly(arylene alkylene)s with pendant N-spirocyclic quaternary ammonium cations for anion exchange membranes. J Mater Chem 2018;6:16537e47. [34] Strasser DJ, Graziano BJ, Knauss DM. Base stable poly(diallylpiperidinium hydroxide) multiblock copolymers for anion exchange membranes. J Mater Chem 2017;5:9627e40. [35] Olsson JS, Pham TH, Jannasch P. Poly(N,Ndiallylazacycloalkane)s for anion-exchange membranes functionalized with N-spirocyclic quaternary ammonium cations. Macromolecules 2017;50:2784e93. [36] Zhang X, Shi Q, Chen P, Zhou J, Li S, Xu H, et al. Block poly(arylene ether sulfone) copolymers tethering aromatic side-chain quaternary ammonium as anion exchange membranes. Polym Chem 2018;9:699e711. [37] Varcoe JR, Atanassov P, Dekel DR, Herring AM, Hickner MA, Kohl PA, et al. Anion-exchange membranes in electrochemical energy systems. Energy Environ Sci 2014;7:3135e91. [38] Shin DW, Guiver MD, Lee YM. Hydrocarbon-based polymer electrolyte membranes: importance of morphology on ion transport and membrane stability. Chem Rev 2017;117:4759e805. [39] Zhu Y, Ding L, Liang X, Shehzad MA, Wang L, Ge X, et al. Beneficial use of rotatable-spacer side-chains in alkaline anion exchange membranes for fuel cells. Energy Environ Sci 2018;11:3472e9. [40] Li X, Liu Q, Yu Y, Meng Y. Synthesis and properties of multiblock ionomers containing densely functionalized hydrophilic blocks for anion exchange membranes. J Membr Sci 2014;467:1e12. [41] Tanaka M, Fukasawa K, Nishino E, Yamaguchi S, Yamada K, Tanaka H, et al. Anion conductive block poly (arylene ether) s: synthesis, properties, and application in alkaline fuel cells. J Am Chem Soc 2011;133:10646e54. [42] Li N, Wang C, Lee SY, Park CH, Lee YM, Guiver MD. Enhancement of proton transport by nanochannels in combshaped copoly (arylene ether sulfone)s. Angew Chem Int Ed 2011;123:9324e7. [43] Li Q, Liu L, Miao Q, Jin B, Bai R. Hydroxide-conducting polymer electrolyte membranes from aromatic ABA triblock copolymers. Polym Chem 2014;5:2208e13. [44] Wang XQ, Lin CX, Liu FH, Li L, Yang Q, Zhang QG, et al. Alkalistable partially fluorinated poly(arylene ether) anion exchange membranes with a claw-type head for fuel cells. J Mater Chem 2018;6:12455e65. [45] Kim E, Lee S, Woo S, Park S-H, Yim S-D, Shin D, et al. Synthesis and characterization of anion exchange multiblock copolymer membranes with a fluorine moiety as alkaline membrane fuel cells. J Power Sources 2017;359:568e76. [46] Dang H-S, Weiber EA, Jannasch P. Poly (phenylene oxide) functionalized with quaternary ammonium groups via flexible alkyl spacers for high-performance anion exchange membranes. J Mater Chem 2015;3:5280e4. [47] Lin CX, Huang XL, Guo D, Zhang QG, Zhu AM, Ye ML, et al. Side-chain-type anion exchange membranes bearing pendant quaternary ammonium groups via flexible spacers for fuel cells. J Mater Chem 2016;4:13938e48. [48] Amel A, Zhu L, Hickner M, Ein-Eli Y. Influence of sulfone linkage on the stability of aromatic quaternary ammonium polymers for alkaline fuel cells. J Electrochem Soc 2014;161:F615e21. [49] Liu L, Ahlfield J, Tricker A, Chu D, Kohl PA. Anion conducting multiblock copolymer membranes with partial
[50]
[51]
[52]
[53]
[54]
[55]
[56]
[57]
[58]
[59]
[60]
[61]
[62]
[63]
[64]
[65]
[66]
[67]
[68]
26575
fluorination and long head-group tethers. J Mater Chem 2016;4:16233e44. Hibbs MR. Alkaline stability of poly (phenylene)-based anion exchange membranes with various cations. J Polym Sci, Part B: Polym Phys 2013;51:1736e42. Pham TH, Jannasch P. Aromatic polymers incorporating bis-N-spirocyclic quaternary ammonium moieties for anion-exchange membranes. ACS Macro Lett 2015;4:1370e5. Li N, Guiver MD. Ion transport by nanochannels in ioncontaining aromatic copolymers. Macromolecules 2014;47:2175e98. He G, Li Z, Zhao J, Wang S, Wu H, Guiver MD, et al. Nanostructured ion-exchange membranes for fuel cells: recent advances and perspectives. Adv Mater 2015;27:5280e95. He Y, Zhang J, Liang X, Shehzad MA, Ge X, Zhu Y, et al. Achieving high anion conductivity by densely grafting of ionic strings. J Membr Sci 2018;559:35e41. Li N, Leng Y, Hickner MA, Wang CY. Highly stable, anion conductive, comb-shaped copolymers for alkaline fuel cells. J Am Chem Soc 2013;135:10124e33. Zhu L, Yu X, Hickner MA. Exploring backbone-cation alkyl spacers for multi-cation side chain anion exchange membranes. J Power Sources 2018;375:433e41. Yang Q, Li L, Lin CX, Gao XL, Zhao CH, Zhang QG, et al. Hyperbranched poly(arylene ether ketone) anion exchange membranes for fuel cells. J Membr Sci 2018;560:77e86. Mandal M, Huang G, Kohl PA. Anionic multiblock copolymer membrane based on vinyl addition polymerization of norbornenes: applications in anion-exchange membrane fuel cells. J Membr Sci 2019;570e571:394e402. Chen D, Hickner MA. Ion clustering in quaternary ammonium functionalized benzylmethyl containing poly(arylene ether ketone)s. Macromolecules 2013;46:9270e8. Noga Z, MW E, DD R. The effect of ambient carbon dioxide on anion-exchange membrane fuel cells. ChemSusChem 2018;11:1136e50. Ziv N, Dekel DR. A practical method for measuring the true hydroxide conductivity of anion exchange membranes. Electrochem Commun 2018;88:109e13. Lin CX, Wu HY, Li L, Wang XQ, Zhang QG, Zhu AM, et al. Anion conductive triblock copolymer membranes with flexible multication side chain. ACS Appl Mater Interfaces 2018;10:18327e37. Hao J, Gao X, Jiang Y, Zhang H, Luo J, Shao Z, et al. Crosslinked high-performance anion exchange membranes based on poly(styrene-b-(ethylene-co-butylene)-b-styrene). J Membr Sci 2018;551:66e75. Liu L, Huang G, Kohl PA. Anion conducting multiblock copolymers with multiple head-groups. J Mater Chem 2018;6:9000e8. Wang XQ, Lin CX, Zhang QG, Zhu AM, Liu QL. Anion exchange membranes from hydroxyl-bearing poly(ether sulfone)s with flexible spacers via ring-opening grafting for fuel cells. Int J Hydrogen Energy 2017;42:19044e55. Liu W, Liu L, Liao J, Wang L, Li N. Self-crosslinking of combshaped polystyrene anion exchange membranes for alkaline fuel cell application. J Membr Sci 2017;536:133e40. Lin CX, Wang XQ, Hu EN, Yang Q, Zhang QG, Zhu AM, et al. Quaternized triblock polymer anion exchange membranes with enhanced alkaline stability. J Membr Sci 2017;541:358e66. He Y, Ge X, Liang X, Zhang J, Shehzad MA, Zhu Y, et al. Anion exchange membranes with branched ionic clusters for fuel cells. J Mater Chem 2018;6:5993e8.
26576
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 6 5 6 5 e2 6 5 7 6
[69] Lin CX, Zhuo YZ, Lai AN, Zhang QG, Zhu AM, Ye ML, et al. Side-chain-type anion exchange membranes bearing pendent imidazolium-functionalized poly(phenylene oxide) for fuel cells. J Membr Sci 2016;513:206e16. [70] Yuk J, Lee S, Nugraha AF, Lee H, Park S-H, Yim S-D, et al. Synthesis and characterization of multi-block poly(arylene ether sulfone) membranes with highly sulfonated blocks for use in polymer electrolyte membrane fuel cells. J Membr Sci 2016;518:50e9. [71] Dang H-S, Jannasch P. Anion-exchange membranes with polycationic alkyl side chains attached via spacer units. J Mater Chem A 2016;4:17138e53. [72] Ono H, Kimura T, Takano A, Asazawa K, Miyake J, Inukai J, et al. Robust anion conductive polymers containing perfluoroalkylene and pendant ammonium groups for high performance fuel cells. J Mater Chem 2017;5:24804e12. [73] Dekel DR, Amar M, Willdorf S, Kosa M, Dhara S, Diesendruck CE. Effect of water on the stability of quaternary ammonium groups for anion exchange membrane fuel cell applications. Chem Mater 2017;29:4425e31. [74] Diesendruck CE, Dekel DR. Water-A key parameter in the stability of anion exchange membrane fuel cells. Cur. Opin. Electrochem. 2018;9:173e8. [75] Dekel DR, Willdorf S, Ash U, Amar M, Pusara S, Dhara S, et al. The critical relation between chemical stability of cations and water in anion exchange membrane fuel cells environment. J Power Sources 2018;375:351e60. [76] Willdorf-Cohen S, Mondal AN, Dekel DR, Diesendruck CE. Chemical stability of poly(phenylene oxide)-based ionomers in an anion exchange-membrane fuel cell environment. J Mater Chem A 2018;6:22234e9. [77] Fan J, Willdorf-Cohen S, Schibli EM, Paula Z, Li W, Skalski TJG, et al. Poly(bis-arylimidazoliums) possessing high hydroxide ion exchange capacity and high alkaline stability. Nat Commun 2019;10:2306. [78] Mahmoud AMA, Elsaghier AMM, Otsuji K, Miyatake K. High hydroxide ion conductivity with enhanced alkaline stability of partially fluorinated and quaternized aromatic copolymers as anion exchange membranes. Macromolecules 2017;50:4256e66.
[79] Ono H, Miyake J, Shimada S, Uchida M, Miyatake K. Anion exchange membranes composed of perfluoroalkylene chains and ammonium-functionalized oligophenylenes. J Mater Chem 2015;3:21779e88. [80] Chu X, Liu L, Huang Y, Guiver MD, Li N. Practical implementation of bis-six-membered N-cyclic quaternary ammonium cations in advanced anion exchange membranes for fuel cells: synthesis and durability. J Membr Sci 2019;578:239e50. [81] Chen X, Lu¨ H, Lin Q, Zhang X, Chen D, Zheng Y. Partially fluorinated poly(arylene ether)s bearing long alkyl sulfonate side chains for stable and highly conductive proton exchange membranes. J Membr Sci 2018;549:12e22. [82] Lin C, Liu X, Yang Q, Wu H, Liu F, Zhang Q, et al. Hydrophobic side chains to enhance hydroxide conductivity and physicochemical stabilities of side-chain-type polymer AEMs. J Membr Sci 2019;585:90e8. [83] Akiyama R, Yokota N, Otsuji K, Miyatake K. Structurally welldefined anion conductive aromatic copolymers: effect of the side-chain length. Macromolecules 2018;51:3394e404. [84] Tuli SK, Roy AL, Elgammal RA, Tian M, Zawodzinski TA, Fujiwara T. Effect of morphology on anion conductive properties in self-assembled polystyrene-based copolymer membranes. J Membr Sci 2018;565:213e25. [85] Omasta TJ, Park AM, LaManna JM, Zhang Y, Peng X, Wang L, et al. Beyond catalysis and membranes: visualizing and solving the challenge of electrode water accumulation and flooding in AEMFCs. Energy Environ Sci 2018;11:551e8. [86] Wang L, Brink JJ, Liu Y, Herring AM, Ponce-Gonzalez J, Whelligan DK, et al. Non-fluorinated pre-irradiation-grafted (peroxidated) LDPE-based anion-exchange membranes with high performance and stability. Energy Environ Sci 2017;10:2154e67. [87] Hao J, Jiang Y, Gao X, Lu W, Xiao Y, Shao Z, et al. Functionalization of polybenzimidazole-crosslinked poly(vinylbenzyl chloride) with two cyclic quaternary ammonium cations for anion exchange membranes. J Membr Sci 2018;548:1e10. [88] Dekel DR. Review of cell performance in anion exchange membrane fuel cells. J Power Sources 2018;375:158e69.