High-performance and stable poly(carbazole)-based anion exchange membrane containing rigid-flexible coupled side-chains for fuel cells (2023)


Fuel cell is a late-model energy technology equipment, which have the advantages of clean energy, wide fuel range, high efficiency and so on [[1], [2], [3], [4], [5]]. Among many types of fuel cells, anion exchange membrane fuel cells (AEMFCs) have the significant advantage of achieving faster oxygen reduction reaction kinetics. In addition, the ability to use cheap non-precious metal catalysts could enable the AEMFCs to be effectively implemented the transformation of energy and power under the condition of low cost [6,7]. Which is expected to replace traditional proton exchange membrane fuel cells. However, as a core component of the AEMFC, the commercialization of the AEM is still limited because of its poor ionic conductivity and base stability, which are two major problems that need to be solved urgently for the application in AEMFCs [[8], [9], [10]].

It is generally believed that the conductivity of the AEM is closely associated with its IEC, and it is a resultful way to increase ion conductivity by enhancing IEC. However, the AEM with too large IEC is usually accompanied by the problem of excessive water absorption and swelling [11,12]. Therefore, in recent years, some researchers have focused on improving ionic conductivity by inducing the microphase separation structure to form high-speed ion transport channels for the AEM. Among them, the introduction of side-chain structure to main-chain backbone is an effective way to construct good microphase separation due to their hydrophilic and hydrophobic differences. Gao et al. [13] designed a new AEM with rigid side-chains, showing a conductivity of 108mScm−1 and excellent dimensional stability due to its large free volume. However, the flexible side-chain has better movement ability than the rigid side-chain, which is easy to make its tethered ion groups gather to build a more obvious hydrophilic channel. Liu et al. [14] synthesized a new AEM with dense flexible alkyl side-chains, and obtained ionic conductivity of 126mScm−1 due to distinct microphase separation structure formed by the dense flexible side-chains. These studies indicate that the side-chain type AEM can usually construct better microphase separation structures than the main-chain ones, among them, rigid side-chains and flexible ones have their respective advantages [13,[15], [16], [17], [18]], so it is speculated that combination of the two kinds of side-chains may achieve much better comprehensive performance.

The alkaline stability is another crucial parameter of the AEMs, which determines the runing time of AEMFCs, because the polymer backbone is easily degraded under alkaline conditions for a long time [[19], [20], [21], [22], [23]]. Current research has shown that polymers containing ether bonds in the main-chain are more susceptible to hydroxide ion attack [[24], [25], [26], [27], [28]]. Therefore, the AEM with ether-free backbone has attracted widespread attention of researchers in recent years [[29], [30], [31], [32], [33]]. Wang et al. [34] prepared a new AEM with poly(aryl piperidinium) as the backbone, where its ionic conductivity only decreased by 3% and its structure did not change significantly after alkaline stability test. Lee and Chen et al. reported a variety of poly(aryl piperidinium) type AEMs, all of which also showed excellent alkaline stability [[35], [36], [37], [38], [39], [40], [41]]. The above studies reveal that the construction of the AEMs without ether main-chain can effectively improve the alkaline stability.

In this paper, in order to prepare an innovative AEM with outstanding conductivity and excellent alkaline stability for fuel cells, a series of novel and applicable poly(carbazole)-based copolymers tethered with three different cationic groups at rigid-flexible coupled side-chains were synthesized and studied. The various properties including water absorption (WU), dimensional changes, ionic conductivity, mechanical properties and stability of these new poly(carbazole)-based AEMs were investigated and compared with the reported AEMs. Meanwhile, the relationship between the properties and the structure of the AEMs, as well as the rules of different cations affecting the performance of the AEMs were in-depth discussed. Finally, a membrane electrode assembly was fabricated using the AEMs and set into a single-cell for test and evaluation.

Section snippets

Materials and reagents

Carbazole, 3,5-dimethoxybenzyl bromide, tribromoborane (BBr3), 1,6-dibromohexane, 1-bromohexane, trimethylamine (TMA) aqueous solution (40%), 1-methylpiperidine (PIP), 1, 2-dimethylimidazole (DMI) and methanesulfonic acid (MSA) were supplied by Shanghai Aladdin Biochemical Technology Co., Ltd. Trifluoromethanesulfonic acid (TFSA) was supplied by Energy Chemical Co. Ltd, while 1,1,1-trifluoroacetone was supplied by Aldrich Ltd. All the solvents were purchased from Sinopharm Chemical Reagent.



Structural analysis

Carbazole is a stable heterocyclic structure with two benzene rings and a nitrogen-containing five-membered ring [45,46], and it is easy to introduce other functional groups due to the presence of N–H bond. Therefore, a carbazole derivative with ionizable alkyl bromine was designed and synthesized as monomer 1, meanwhile, N-hexylcarbazole was also prepared to act as the hydrophobic unit of the poly(carbazole)-based copolymers, route of the synthesis is shown in Scheme S1. The monomer 1 needs to


In conclusion, a series of novel poly(carbazole)-based AEMs possessing three different pendent cationic groups tethered at the end of rigid-flexible coupled side-chains was developed. The poly(carbazole)-based AEMs having titrimetric IEC of 2.03–2.21 mequiv. g−1, and exhibiting the high σ of 133.7–151.3mScm−1 at 80°C. The AEMs tethering densely alkyl side-chains with pendant cationic groups were beneficial for the aggregation of multiple hydrophilic regions and the construction of a good

CRediT authorship contribution statement

Ning Xie: Validation, Formal analysis, Investigation, Writing – review & editing. Tao Wang: Investigation. Shenghua Du: Investigation. Qiang Weng: Writing – review & editing. Kai Zheng: Investigation. Tong Zhang: Investigation. Xingming Ning: Writing – review & editing. Pei Chen: Methodology, Supervision. Xinbing Chen: Conceptualization, Methodology, Writing – original draft, Project administration. Zhongwei An: Methodology, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


The authors thank for the financial support by National Nature Science Foundation of China (52273186, 51873100 and 62105194), Sanqin scholars innovation teams in Shaanxi Province, China, and International Science and Technology Cooperation Project of Shaanxi Province, China (2021KW-20).

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© 2023 Elsevier B.V. All rights reserved.

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