Elsevier

European Polymer Journal

Volume 124, 5 February 2020, 109463
European Polymer Journal

Influences of non-ionic branches on the properties of the anion exchange membranes based on imidazolium functionalized poly (2, 6-dimethyl-1, 4-phenylene oxide)

https://doi.org/10.1016/j.eurpolymj.2019.109463Get rights and content

Highlights

  • Modifying of the anion exchange membranes with non-ionic branches.

  • The hydrophilicity of grafted branches affects the morphology of the membranes.

  • Membranes with a low hydration coefficient but a high conductivity are obtained.

  • Improved alkaline stability of the membranes by alkyl ether side chains.

Abstract

Different nonionic groups were, respectively, grafted onto the backbone of 1,2,4,5-tetramethylimidazolium functionalized poly(2,6-dimethyl-1,4-phenyleneoxide) to fabricate anion exchange membranes (AEMs). The presence of the nonionic branches significantly decreased the water swelling of the AEMs by 145% compared to that of the original imidazolium functionalized PPO (Im-PPO) with a comparable hydration level (λ) of around 20. The membrane possessing side chains of the 2-ethoxyethanamine exhibited a hydroxide conductivity of 64 mS cm−1 at 80 °C, while that Im-PPO only showed a conductivity of 48 mS cm−1 under the same conditions. It is found that the presence of ether-containing side chains facilitated formation of more obvious hydrophilic-hydrophobic nanoscopic domains according to the images of atomic force microscope (AFM), and the results of small angle X-ray scattering (SAXS) of the membranes. As a result, the AEM grafted with 2-ethoxyethanamine or 3-methoxypropylamine exhibited higher tensile stress at break and lower methanol permeability than that the pristine Im-PPO membrane. In addition, the ether-containing branches allowed the AEMs to exhibit high alkaline stability. A conductivity of about 54 and 47 mS cm−1 was retained after immersed this kind of AEMs in 1 M KOH at 60 °C for 300 and 750 h, respectively.

Introduction

As one of the power generation devices converting chemical energy into electricity, the anion exchange membrane fuel cell (AEMFC) possesses advantages of fast oxygen reduction kinetics in alkaline medium, low fuel permeability, and usably earth-abundant catalysts like nickel and silver [1], [2], [3]. However, compared to the polymer electrolyte of proton exchange membranes (PEMs), the anion exchange membrane (AEM) is urgently needed to improve its ionic conductivity and durability from points of view of its development and practical applications [4], [5], [6]. To solve these problems, great efforts have been made to design and optimize the structure of both polymer backbones and functional cations. For instance, thermally and chemically stable polymers of poly(vinylbenzyl chloride)-based copolymers [7], polyolefins [8], and fluorene-based polymers [9] have been employed as matrix materials for membrane preparation. Zhang et al. [10] designed tetra-pyrrolidinium modified block polymer of poly(arylene ether sulfone)s and achieved a conductivity of 68 mS cm−1 at 80 °C. After exposed to 1 M NaOH at 60 °C for 16 days, the membrane still exhibited a conductivity of 57 mS cm−1 at 80 °C. In addition, various functional groups such as quaternary ammonium [11], [12], imidazolium [13], [14], phosphonium [15], guanidinium [16], [17], and metal-based cations [18], [19] have been used to modify the polymers for both hydroxide ion conduction and possible high tolerance to the nucleophilic attack of hydroxide ions. Among these quaternary cations, the imidazolium has been widely used since it could bring on the polymer with relatively high hydroxide conductivity [20], [21]. Moreover, the imidazolium ring is easy to be structurally modified via grafting for a reasonable high stability against the attack of hydroxide ions [22]. For example, Yan and co-workers have demonstrated that the sterically bulky substituents at C2 or/and N3 positions could improve alkaline stability of the imidazolium based AEMs by prevention of ring-opening mechanism [23], [24].

Generally, the ionic conductivity of the membrane electrolyte is related to its cationic concentration, water content and micro-phase structure [25]. For instance, it is reported that the conductivity of the poly(arylene ether sulfone)s-based AEMs was increased from 12 to 72 mS cm−1 at room temperature as an increase in the ion exchange capacity (IEC) from 1.34 to 2.61 mmol g−1 [26]. It is essential to the AEMs to have enough cation groups for ion conduction [27]. However, the improvement on the conductivity via this way might bring on poor mechanical property of the AEMs due to the excessive water swelling [28]. It is well known that crosslinking of the polymer backbone is the most used strategy to reinforce the mechanical strength of the polymer electrolyte membrane [21], [29], [30], [31]. However, the enhanced dimensional and mechanical stability via crosslinking are often at the expense of ionic conductivity since the chemical crosslinking might occupy cationic functional sites of the polymer [32].

As AEMs generally are operated at temperatures below the water boiling point of 100 °C, construction of appropriate express way for ion conduction with lubrication of water molecules are attractive for enhanced performance of the AEMs. It has been found that design of different side chains could bring on appropriate amphiphilic phase-separated structure and reach high ion conductivity of the AEMs [33], [34], [35], [36]. For example, Zhuang et al. [37] constructed a highly efficient ion-aggregating structure by grafting alkyl side chains of different lengths to the polysulfone backbones. The membrane possessing a six-carbon alkyl side chain exhibited a hydroxide conductivity of 108 mS cm−1 at 80 °C, which is obviously higher than that of its pristine membrane, i.e., 41 mS cm−1.

Herein, we prepared the AEMs by grafting different non-ionic side chains, including amylamine (AA), 4-phenylbutylamine (PhA), 2-ethoxyethanamine (EOA) and 3-methoxypropylamine (MOA), onto poly(phenylene oxide) (PPO) backbones and 1,2,4,5-tetramethylimidazolium as functional cationic groups. The lone pair electrons of oxygen atom allow the electron-rich ether groups to show favorable interactions with the cationic groups and water molecules. It is expected that the ether containing side chains of EOA and MOA could assist the formation of water-rich ion transport domains, which might benefit creation of appropriate microphase distribution of the AEMs for high ionic conductivity and reasonable chemical and mechanical stabilities in the alkaline conditions. In addition, AA and PhA with different hydrophilicity were grafted onto the polymer backbones, separately, to investigate the influence of the nonionic branches on the properties of the AEMs as well. All the hydrogen atoms in the grafted imidazolium ring were substituted with methyl groups in order to achieve the AEMs a high alkaline stability [38], [39]. Comprehensive characterization and investigations were made to better understand the effects of the nonionic side branches on the performance of the AEMs.

Section snippets

Materials and reagents

The polymer PPO, 2,2′-azobis-isobutyronitrile (AIBN), and N-bromosuccinimide (NBS) were purchased from Sigma-Aldrich. The 1,2,4,5-tetramethylimidazole (TMIm) and various amines including AA, PhA, EOA and MOA were purchased from TCI Development Co. Ltd. Anhydrous methanol, anhydrous ethanol, N-methyl-2-pyrrolidone (NMP), and N,N-dimethylacetamide (DMAc) were obtained from Tianjin Yongda Chemical Reagent Co. Ltd. The AIBN was recrystallized from the anhydrous methanol before use, and all the

Synthesis and characterization of AEMs

The 1H NMR spectra of PPO, BPPO, PPO-AA, Im-PPO and S-PPO membranes are shown in Fig. 1. For BPPO, the characteristic peak at 2.1 ppm is assigned to the protons of methyl groups (Hc,d and Hg), and the one at 4.3 ppm is attributed to the protons of bromomethyl groups (Hh). The degree of bromination of the BPPO could be calculated according to the ratio of integral areas of these two peaks [42]. Hereinafter, a bromination degree of 20%, which could be controlled by the addition molar ratio of NBS

Conclusions

Novel anion exchange membranes possessing different nonionic branches are fabricated based on imidazolium functionalized poly (2, 6-dimethyl-1, 4-phenylene oxide). Four kinds of side chains including amylamine, 4-phenylbutylamine, 2-ethoxyethanamine and 3-methoxypropylamine have been successfully grafted onto the polymer structure, respectively, according to 1H NMR and FTIR spectra. The side-chain structure can effectively improve the dimensional stability and control the water uptake and

CRediT authorship contribution statement

Ruiying Wan: Conceptualization, Methodology, Data curation, Validation, Writing - original draft, Investigation, Writing - review & editing. Dengji Zhang: Investigation, Data curation. Shaoshuai Chen: Software. Niya Ye: Validation. Yunfei Yang: Validation. Ronghuan He: Writing - review & editing.

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.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (grant No. 51572044).

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

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