Facile preparation of novel cardo Poly(oxindolebiphenylylene) with pendent quaternary ammonium by superacid-catalysed polyhydroxyalkylation reaction for anion exchange membranes

Journal Pre-proof Facile preparation of novel cardo Poly(oxindolebiphenylylene) with pendent quaternary ammonium by superacid-catalysed polyhydroxyalkylation reaction for anion exchange membranes Rong Ren, Shuomeng Zhang, Hamish Andrew Miller, Francesco Vizza, John Robert Varcoe, Qinggang He PII:

S0376-7388(19)30828-2

DOI:

https://doi.org/10.1016/j.memsci.2019.117320

Reference:

MEMSCI 117320

To appear in:

Journal of Membrane Science

Received Date: 22 March 2019 Revised Date:

14 June 2019

Accepted Date: 30 July 2019

Please cite this article as: R. Ren, S. Zhang, H.A. Miller, F. Vizza, J.R. Varcoe, Q. He, Facile preparation of novel cardo Poly(oxindolebiphenylylene) with pendent quaternary ammonium by superacid-catalysed polyhydroxyalkylation reaction for anion exchange membranes, Journal of Membrane Science (2019), doi: https://doi.org/10.1016/j.memsci.2019.117320. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Facile Preparation of Novel Cardo Poly(oxindolebiphenylylene) with Pendent

Quaternary

Ammonium

by

Superacid-catalysed

Polyhydroxyalkylation Reaction for Anion Exchange Membranes Rong Rena, Shuomeng Zhanga, Hamish Andrew Millerb,*, Francesco Vizzab, John Robert Varcoec,*, Qinggang Hea,d* a

College of Chemical and Biological Engineering, Zhejiang University, Hangzhou,

Zhejiang 310027, China b

Institute of Chemistry of Organometallic Compounds, ICCOM-CNR, Polo

Scientifico Area CNR, 50019 Sesto Fiorentino, Italy c

Department of Chemistry, The University of Surrey, Guildford, Surrey GU2 7XH,

UK d

Ningbo Research Institute, Zhejiang University, Ningbo, Zhejiang 315100, China

Corresponding author. E-mail: [email protected] (Q. He), [email protected] (H. A. Miller), [email protected] (J. R. Varcoe)

Abstract: Novel anion exchange membranes (AEMs) with C-C polymer backbones, cardo fragments, flexible alkyl side chains and cyclic quaternary ammonium (QA) groups were designed and synthesized. Cardo poly(oxindolebiphenylylene) polymers with pendent cyclic QAs of different length (POXIA-Br) were prepared by a 1

superacid-catalysed polyhydroxyalkylation reaction and subsequent quaternion functionalization. This preparation process has the advantages of mild reaction conditions, simple operation, low cost and environmental friendliness. The morphology, water uptake (WU), ion conductivity, stabilities and the fuel cell performance were all investigated thoroughly. With the extention of the side chain length, both the microphase separation scale and the ionic conductivity increased. AEMs with longer pendent QAs exhibited decreased WU and better alkaline stability performance than did AEMs with shorter pendent QAs. POXIH-OH (in OH- form) with a hexyl side-chain spacer showed a maximal conductivity of 73.6 mS cm-1 at 80 °C. There was no degradation observed for POXIH-OH except for loss of 9.5% of QA groups after 1200 h in 1 M NaOH at 80 °C. In addition, the AEM fuel cell test performed a high peak power density of 476 mW cm−2 at 60 °C.

Keywords: anion exchange membrane; cardo polymer; polyhydroxyalkylation reaction; alkaline stability; fuel cells

1. Introduction As an environmentally friendly and efficient electrochemical technology, alkaline anion exchange membrane fuel cells (AEMFCs) have received considerable interest [1-3]. Anion exchange membrane (AEM) acts as a solid state electrolyte for transporting hydroxide ions and separator for the cathode and anode in AEMFCs. Currently, the development of AEMs combined with high conductivity and long-term 2

stability in the alkaline operating environment, is an intensive field of materials research [4]. Regarding to thedevelopment of AEMs, novel high performance functional polymers are desirable. Among the promising structures put forward in the literature, cyclic cardo groups can reduce the intermolecular forces and the crystallinity of polymers and thus exhibit specific properties [5]. Due to their improved properties, not only with regard to thermal, mechanical and chemical stability but also in terms of the phase separation and ion conductivity performance [6], polymers containing cyclic side groups in the repeating units, named cardo polymers, have become a preferable subject for the AEM [7]. 9,9-Diarylfluorene-based cardo functional polyelectrolytes have been employed as AEMs and exhibit excellent fuel cell performance [8-12]. However, most of the methods used to functionalize fluorene-based polymers have been only limited to chloromethylation and subsequent quaternization on the benzene rings, for which one is unable to accurately control the location and degree of grafting functionalization. In addition, the usage of a toxic regent and the unstable benzyl trimethylamine in the alkaline condition restricts the application of this method [13, 14]. As another common cardo monomer, phenolphthalein is commercially available and easily functionalized. Liu [15-17] and Zhang [18-22] synthesized a cardo monomer with a pendant tertiary amine by aminolysis reaction of phenolphthalein and N,N-disubstituted

diamine,

N,N-dimethyl-1,3-propanediamine

such

as

and

N,N-dimethylethane-1,2-diamine,

1-(2-Aminoethyl)piperidine.

Cardo

polyelectrolytes with pendent quaternary ammoniums (QAs) were obtained by 3

condensation polymerization and followed quaternization of the tertiary amine. The AEMs based on these cardo polyelectrolytes with pendent QAs exhibited promoted stability and conductivity, due to the flexible side chain and the cardo structure that facilitated the formation of microphase separation and connected ionic domains. Although the functionalization of the polymer avoided the chloromethylation process, the limited length of the pendent alkyl and finite functional groups resulting from the lack of variety of N,N-disubstituted diamines made it difficult to achieve optimal performance for the AEMs. In addition, in these aryl ether-containing polymers, nucleophilic aromatic substitution of hydroxide anion would cleave the C-O bonds [23]. To improve the alkaline stability of the AEM, one should take into account not only the configuration of polymer and the stability of the QAs but also the polymer backbone [24, 25]. In this study, in order to take advantage of the cardo polymer structure and ameliorate the performance of AEMs, a C-C polymer backbone, a cardo structure, flexible alkyl side chain and cyclic QAs were simultaneously incorporated into AEMs. For this purpose, new reactive bromoalkyl-indoline-2,3-dione monomers with a pendent

bromoalkyl

were

synthesized.

Cardo

poly(oxindolebiphenylylene)

(POXIA-Br, where A indicates the side chain length, and E-ethyl, B-butyl, H-hexyl, Br denotes the Br- formed AEM) with pendent cyclic QAs were prepared by superacid-catalysed

polyhydroxyalkylation

reaction

followed

by

quaternized

functionalization. The morphology, water uptake, ion conductivity, thermal and alkaline stabilities of the AEMs were investigated. Because of reliable overall 4

properties, POXIH-OH (OH- formed AEM) was chosen for testing in single fuel cells, and was found to exhibit excellent performance. 2. Experimental 2.1. Materials 1,2-Dibromoethane, isatin, 1,4-dibromobutane, biphenyl, 1,6-dibromohexane, trifluoromethanesulfonic acid (TFSA) were purchased from the J&K company. All other inorganic salts and solvents were used as received. 2.2. Synthesis of 1-bromoalkyl indoline-2,3-dione (OXIA) OXIA was synthesized by the N-alkylation of isatin with dibromoalkyl, where A denotes the bromoalkyl chain connected to the isatin, which could be E (ethyl), B (butyl) and H (hexyl). The typical synthetic method used for OXIH is described below: isatin (2.94 g, 20 mmol) was dissolved in 10 mL N,N-Dimethylformamide (DMF) and the solution was added dropwise to a stirred suspension of sodium hydride (60 wt%, 1.2 g, 30 mmol) in DMF (10 mL) during 20 min at 0 °C under an inert atmosphere of nitrogen. 1,6-dibromohexane (9.74 g,40 mmol) was added to the suspension at the same temperature. The contents were stirred overnight at room temperature, and thereafter, quenched with ice water to yield a red-coloured semisolid. After extraction with dichloromethane, the collected organic layer was evaporated and purified by flash column chromatography in petroleum ether/dichloromethane (70:30) as eluent to yield pure OXIH as a red viscous liquid. Yield, 56%. 2.3. Synthesis of poly(bromoalkyl oxindolebiphenylylene) (POXIA) A superacid-catalysed polyhydroxyalkylation method was used for the preparation 5

of POXIA, and POXIH was described as an example. TFSA (10 mL), biphenyl (1.542 g, 10 mmol) and CH2Cl2 (10 mL) were stirred in a 50 mL three-neck flask under N2 atmosphere. An ice-water bath was applied to cool down the mixed solution to 0 °C. The 1-(6-bromohexyl)indoline-2,3-dione (3.10 g, 10 mmol) was added quickly to the mixture. The ice-water bath was removed after 10 min and the reaction was quenched 3 h later by pouring the high viscous reaction solution into methanol. The obtained white fibre was filtered and dried overnight under vacuum at 40 °C. Yield, 96%. 2.4. Synthesis of quaternary ammonium-functionalized POXIA-Br 1-Methylpiperidine was selected as the functional agent for the quaternization of the POXIA. POXIA (300 mg) was dissolved in 6 mL N-methyl pyrrolidone (NMP), N-methyl piperidine (1.0 mL) was added subsequently. The reaction mixture was heated at 50 °C for 12 h. After that, 6 mL methanol was added to dilute the solution and precipitated in anhydrous ether. The precipitation was washed thoroughly with ether to obtain QA functionalized copolymers and dried at 60 °C for 24 h under vacuum to yield POXIA-Br (>95% yield). 2.5. Membrane casting of ion-functionalized polymers AEMs were prepared by dissolving the above QAs functionalized POXIA-Br (0.5 g in Br- anion form) in NMP (5 wt%) followed by casting on a glass plate (0.1 m × 0.1 m). AEMs with thickness of ~50 µm were formed after drying at 60 °C for 24 h. More experimental details are reported in the Supporting Information.

3. Results and Discussion 6

Scheme 1. Synthesis route for POXIA-Br, where A denotes the alkyl side chain: E (n=1), B (n=3), H (n=5).

The synthetic route for POXIA-Br is shown in Scheme 1, where bromoalkyl substituted indolinone monomers (OXIA) are synthesized first by the N-alkylation of isatin with dibromo alkyl in the presence of NaH/DMF according to the literature [26]. To obtain different side chain lengths, 1,2-dibromoethane, 1,4-dibromobutane and 1,6-dibromohexane were employed to prepare 1-(4-bromoethyl)indoline-2,3-dione (OXIE),

1-(4-bromobutyl)indoline-2,3-dione

(OXIB)

and

1-(6-bromohexyl)indoline-2,3-dione (OXIH), respectively (1H NMR spectra are shown in Fig. S1-S3). The 1H NMR for OXIH was taken as a typical example to elucidate peak assignment. Two triplets at 3.72 and 3.40 ppm, integrating for two protons of -NCH2- and -CH2Br, respectively, indicated the presence of the 6-bromohexyl group at the nitrogen of the indole moiety. The remaining methylenes of the hexyl chain appeared as multiplets at 1.47-1.90 ppm, and the aromatic protons of the indole moiety appeared at 6.8-8 ppm. 1H NMR data for OXIE and OXIB are shown in Fig. S2 and Fig. S3, which are similar to that obtained for OXIH; all the 7

peaks were clearly defined. POXIA with high viscosity was successfully synthesized within 3 h via superacid-catalysed polyhydroxyalkylation of bromoalkyl substituted indolinone and biphenyl. As shown in Fig. S4-S6, the 1H NMR for POXIA was nearly identical to that for the monomer except for two new signals in 7-8 ppm (a and b) assigned to biphenyl protons. The molecular weights and polydispersity index of the POXIA were evaluated using a gel permeation chromatograph (shown in Table S1), and all of them exhibited high molecular weights. 1-Methylpiperidine was selected as the quarternization agent because AEMs with such cycloaliphatic QAs reported exhibited high alkaline stabilities when hydrated [27]. Recently, poly(isatin biphenylene) with different pendent cationic groups have been used as AEMs, and piperidium AEMs showed highest alkaline stability and good fuel cell performance [28]. A series of polyelectrolytes with pendent QAs (POXIA-Br) were synthesized by reaction of 1-methylpiperidine with POXIA. The chemical structures for POXIA-Br were characterized by 1H NMR (Fig. S7 and Fig. S8). The two peaks at 2.92 ppm and 3.22 ppm were attributed to the protons of -CH3 and -CH2directly connected to the N atom in the QAs. The efficiency of quaternization could be calculated from the integral ratio of the QA methyl protons at 2.9 ppm with the aromatic protons at 7-8 ppm, which verified that the quaternization reaction was nearly complete (Fig. S7). It is worth noting that for POXIE-Br, there are two less pronounced peaks at 5.1 ppm and 5.9 ppm (the protons of -N-CH=CH2), which means a base-catalyzed elimination reaction occurs in the side chain [29]. Therefore, in order 8

to mitigate the decomposition caused by the electron withdrawing nature of the amide, we need to separate the QAs from the polymer backbone via a longer side chain. Transparent and flexible membranes (Fig. S9) with thickness of approximate 50 µm were obtained by dissolving POXIA-Br in NMP and followed casting and evaporation. Membrane morphology plays a significant role in the construction of ion-conducting channels and the resulting conductivity of the AEMs [30-32]. Small angle

X-ray

scattering

(SAXS)

was

used

for

understanding

the

microphase-segregation of the AEMs. As shown in Fig. 1a, all three POXIA-Br AEMs with different side chain lengths demonstrate obvious so-called ionomer peaks, which manifest the formation of microphase separation [33]. By increasing the side chain length, the q value of ionomer peaks for the AEMs tends to decrease. The d-spacing in the AEMs, recognized as the distance between the ionic domains, is calculated from the Bragg's equation: d = 2π/q [34]. The d-spacing was calculated to be 25.0, 34.6, and 54.2 nm for POXIE-Br, POXIB-Br and POXIH-Br, respectively, which verified that longer pendent alkyl side chains were more liable to induce the formation of characteristic microphase-separation.

Fig. 1. (a) SAXS profiles for POXIA-Br AEMs, AFM (b) phase image and (c) 9

topographical height image for POXIH-Br AEMs. The surface morphology of the AEMs was further characterized by atomic force microscopy (AFM). AFM phase and topographical height images of POXIA-Br AEMs showed clear hydrophilic/hydrophobic phase separation and appropriate correspondence (Fig. 1b, 1c). In Fig. 1c, the bright regions correspond to the hydrophobic polymer matrix while the dark regions correspond to hydrophilic domains [35, 36]. The microphase separation was mainly the result of the self-assembly of the hydrophilic side chain QAs and the hydrophobic backbone in POXIA-Br. Furthermore, the side-chain-type configuration and cardo structure were beneficial to the aggregation of ions for enhancing the hydrophilic/hydrophobic phase separation [37, 38]. The distance between the ionic domains in POXIA-Br AEMs expanded with the increasing length of the side chains (Fig. 1b, 1c and Fig. S10). These results are consistent with that of the SAXS. In addition, the surface roughness of these AEMs is only a few nanometres, which means all AEMs were flat and smooth at the nanometre scale. Both SAXS and AFM test confirm the formation of closely connected ionic channels and a distinct phase-separated morphology for all AEMs, which implies that the cardo structure and side-chain configuration may contribute to ion-cluster formation, because the large free-volume size facilitates chain-segment motion [39]. It is well known that microphase-separation plays significant roles in the conductivity and other properties, such as water uptake (WU) behaviour. As a medium for conducting ions, moderate water in AEM is beneficial for the 10

construction of efficient ion-conducting pathways [13]. Ion exchange capacity (IEC) was closely relevant to WU. The titration IECs for POXIB-Br and POXIH-Br (1.89 and 1.78 meq/g) were consistent with the theoretical IECs (1.93 and 1.83 meq/g, calculated from complete conversion) and the 1H NMR IECs (1.92 and 1.83 meq/g, calculated from the functional degree based on 1H NMR spectra, Table S2). Nevertheless, the IEC of POXIE-Br was identified by titration to be as low as 1.65 meq/g; one could not obtain the IEC from 1H NMR because of the poor solubility of POXIE-Br, which may be because of the crosslink polymerization of N-vinyl in POXIE. Therefore, POXIB-Br exhibited the highest IEC values, and we discuss the WU below. The WU and swelling ratios of POXIA-OH AEMs were measured from 30 to 80 °C (Fig. 2a and 2b). It was clearly observed that both WU and swelling ratios increased with increasing temperature as expected. AEMs with higher IEC exhibit higher WU and swelling ratios as a result of higher ion content. Due to the low IEC and possible crosslink, POXIE-OH displayed the lowest WU (15.1% at 30 °C and 31.2% at 80 °C) and swelling ratios (3.1% at 30 °C and 4.6% at 80 °C). POXIH-OH displayed lower WU and swelling ratios than POXIB-OH because of the lower IEC, increased hydrophobicity and larger microphase separation caused by the relatively longer alkyl side chain.

11

Fig. 2. (a) Water uptake, (b) swelling ratio, (c) ionic conductivity of the POXIA-OH as a function of temperature, and (d) Arrhenius plot of ionic conductivity vs. 1000/T. Ionic conductivity is the critical property of AEMs. As shown in Fig. 2c, the hydroxyl conductivity of all AEMs increased with increasing temperature. It should be noted that even the POXIH-Br was not among the AEMs with highest IEC, POXIH-OH exhibits the highest conductivity of 73.6 mS·cm-1 at 80 °C due to the long side chains. The trade-off issue between the conductivity and WU is solved with the longer hydrophobic hexyl side chain [31]. In addition, all AEMs exhibited good Arrhenius type temperature dependence for the hydroxide anion conductivity (Fig. 2d). The hydroxide conductivity activation energies (Ea) were calculated from the formula: Ea=-b×R, where R is the gas constant (8.314 J mol-1 K-1) and b is the slope of the ln (σ) vs 1000/T curves). The hydroxide 12

conductivity Ea for POXIE-OH, POXIB-OH and POXIH-OH were 12.7 10.7 and 10.5 kJ/mol, respectively, similar to that found for side chain aromatic polymer based AEMs reported [40]. The value of Ea for POXIE-OH is higher than that for POXIB-OH and POXIH-OH, indicating that the transportation of hydroxide ion in POXIE-OH membranes is more sensitive to temperature, and that the barrier that must be overcome is relatively higher. POXIH-OH shows the lowest Ea and maximum conductivity, which suggests that extending the spacers between polymer backbone and the QAs may be beneficial to enhance the local mobility of ionic groups [41]. It further validates the significant contribution of interconnecting morphology to the ionic conductivity. Stability is important for application in electrochemical devices displaying long-term performance. Thermal stability of the POXIA-Br was determined by thermal gravimetric analysis (TGA). TGA curves for the POXIA-Br AEMs were recorded at 50 to 700 °C (Fig. S11). All AEMs exhibited similar weight loss during the decomposition process. The first loss stage from 200 to 250 °C is assigned to the degradation of the QA groups. While the thermal decomposition of the alkyl groups was corresponding to the loss stage from 300 to 400 °C and the POXIH-Br exhibited more weight loss than the other two polymers because of longer alkyl side chains. The last weight loss stage above 500 °C is attributed to the thermal degradation of the aromatic polymer backbone. TGA results confirm that the cardo POXIA-Br AEMs have an excellent thermal stability. Furthermore, the stability of AEMs in the alkaline condition is considerable. In 13

AEMs, conductive OH− can disintegrate not only the head groups but also the polymer backbone [23, 33, 42]. As seen in the 1H NMR spectra (Fig. 3) for POXIB-Br and POXIH-Br after the alkaline stability test, two new signals appeared at 5.7 ppm (1 proton) and 4.9 ppm (2 protons), which indicated the formation of CH2=CH- at the terminal of the side chain degrading via Hofmann ring opening elimination [14]. Only 16.7% and 9.5% of the QAs groups were decomposed for POXIB-Br and POXIH-Br, respectively, as calculated from the integral ratios. Nevertheless, there was no discernible change in the 7-8 ppm region corresponding to aromatic protons, which means no detectable decomposition of the polymer backbone. However, for POXIE-Br, after alkaline treatment, the membrane became insoluble in common solvent (DMSO, DMF, DMAc), which meant the generation of cross-linked structures after alkaline degradation.

Fig. 3. 1H NMR spectra for (a) POXIB-Br and (b) POXIH-Br in DMSO-d6 before and after a stability test with 1 M NaOH at 80 °C for 1200 h. As shown in Table S2, the decrease in value for the titrated IECs was consistent with the calculated 1H NMR values, which means there was no decomposition in the 14

polymer backbone after alkaline treatment except for the QA groups. The POXIH-Br AEM exhibited higher alkaline stability than the AEMs with shorter side chain length (POXIE-Br or POXIB-Br), which was consistent with other side chain AEMs reported [43, 44]. The better alkaline stability of POXIH-Br AEM could originate from the steric effect of the alkyl groups in the side chains, which mitigated the decomposability of the QA groups in the alkaline environment [44]. Besides, mechanical properties (Fig. S12) and swelling ratios (Fig. S13) of the AEMs before and after alkaline treatment were tested. In the mechanical test, for POXIE-Br, the tensile strength increased and elongation at break decreased observably after alkaline treatment, which is a significant feature of cross-linking. Meanwhile, the swelling ratios of POXIE-OH sharply decreased, due to the result of the reduced IEC and the crosslink reaction [45]. However, for POXIB-OH and POXIH-OH, the swelling ratios decreased slightly, which is related to the reduced IECs caused by the degradation of QAs. In addition, both the strain strength and elongation at break of POXIB-Br and POXIH-Br exhibit slight changes before and after the alkaline stability testing, which confirms the stability of the polymer backbone. Due to high conductivity and good overall properties, POXIH-OH was chosen for AEMFC testing. PtRu/C and Pt/C catalysts (Pt and PtRu metal loadings of 0.4 mg cm-2) were used to prepare the anode and cathode, respectively, by spray coating the catalyst inks (containing 20 wt% of a radiation-grafted ETFE powder ionomer and using propanol/water) onto Toray paper (non-teflonated, TGP-H-60) [46, 47]. This 15

powder ionomer has previously shown excellent performance in recent AEMFC testing [47-50]. As shown in Fig. 4(a), the open circuit voltage was 1.01 V, which means that the gas permeability of the AEMs is insignificant [41]. A high peak power density of 476 mW cm−2 was achieved under a current density of 912 mA cm−2. Although there are many issues that determine the performance, such as catalysts, ionomers and MEA fabrication procedures, due to the high performance of radiation-grafted ETFE powder ionomer and this side chain type cardo AEM, the power density from this result is comparable to those reported for cardo AEMs to date [15, 16, 22, 28, 51, 52]; most reported cardo AEMs were not proceeded with single cell test [9, 11, 20, 21, 37, 38]. The cell was subjected to a constant current discharge test at 400 mA cm-2 under the above conditions to evaluate the initial stability of the MEA containing the POXIH-OH membrane. The high frequency resistance (HFR) and cell voltage was monitored over a 2 h period as shown in Fig. 4(b). Initially the cell voltage increased accompanied by an increase in the cell HFR. From 30 to 120 minutes the cell voltage remains relatively stable as well as the HFR. To understand fully degradation phenomena and membrane stability, longer-term cell testing is necessary, as is the current focus of testing in our laboratories.

16

Fig. 4. (a) H2/O2 AEM-FC performance data for the POXIH-OH AEM at 60 °C. (b) Cell voltage and HFR variation during a constant current discharge test (400 mA cm-2).

4. Conclusion In conclusion, poly(oxindolebiphenylylene) (POXI)-based AEMs with pendant cyclic QAs of different lengths were synthesized by the superacid catalysed polyhydroxyalkylation of bromoalkyl substituted oxoindoles with biphenyl and subsequent quaternization. This preparation process involved a mild reaction condition, simple operation, low cost and environmentally friendly, which open up a new field for the development of novel AEMs. From SAXS and AFM images, it was found that all POXIA-Br AEMs showed a well-developed microphase separation and the separation scale was increased with increasing side-chain length. As the length of the side chain changed, consistent tendency was observed for the IEC and WU, whereas POXIE-Br showed the lowest IEC value AEM due to the elimination of the side chain. Moreover, because of a longer side chain, POXIH-OH AEM exhibited a 17

moderate WU and the largest conductivity, 73.6 mS·cm-1 at 80 °C. Meanwhile, the activation energy of ionic conductivity for POXIH-OH was the lowest, verifying that a longer side chain can enhance the local mobility of ionic groups. In addition, POXIH-OH AEM showed higher alkaline stability compared with AEMs with shorter side chain. With a longer hexyl side chain, both the conductivity and stability were optimal; therefore, POXIH-OH AEM was selected for an AEMFC and achieved a high peak power density of 476 mW cm−2 at 60 °C.

Acknowledgements Q. He acknowledges the financial support from the National Natural Science Foundation of China (No.21676241, No.U1732111), “The Recruitment Program of Global Youth Experts” from the Chinese government, and the “Hundred Talents Program” of Zhejiang University. The collaboration between the University of Surrey and ICCOM (CNR) teams was facilitated by funding awarded by the Royal Society’s international exchange scheme (grant IES\R3\170134). The authors also thank Ente Cassa di Risparmio di Firenze for funding (project EnergyLab) and the Italian Ministry MUIR for the PRIN 2018 Project (No. 2017YH9MRK).

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Highlights Novel cardo poly(oxindolebiphenylylene) AEMs were designed and fabricated. C-C backbone, flexible alkyl side chain and cyclic QAs were present in AEMs. The preparation process was simple, low cost and environmental friendly. POXIH-OH AEMs showed distinct phase separation, high stability and conductivity. The AEMFC based on POXIH-OH exhibted high performance.