Crucial role of side-chain functionality in anion exchange membranes: Properties and alkaline fuel cell performance

https://doi.org/10.1016/j.memsci.2021.119172Get rights and content

Highlights

  • Side chain functionality has significant impact on the performance of the AEMs for fuel cells.

  • PPO-C-nQA membrane displayed higher conductivity and superior alkaline stability than PPO-mQA membrane.

  • PPO-C-1CQA membrane had a highest peak power density of 141.3 mW cm−2 of AEMFC.

Abstract

Side-chain functionality is critical to achieve highly conductive and chemically stable anion exchange membrane (AEM) materials. Herein, two series of quaternized poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) having different lengths (n or m) of cationic side-chains were designed and prepared, including PPO-C-nQA with a triazole-containing linker and PPO-mQA without triazole groups in the side chains. The effect of the side-chain functionality on the properties of AEMs has been systematically investigated. Long cationic side chains in PPO-mQA membranes induced the formation of obvious microphase-separated morphology. However, PPO-C-nQA membranes with a triazole-containing linker showed a considerable higher hydroxide conductivity than that of PPO-mQA membranes, due to the developed hydrogen-bond networks between triazole and water/hydroxide. High retention of conductivity and low degree of crosslinking were observed for the AEMs with longer side-chains (PPO-C-3QA, PPO-C-4QA, PPO-6QA, and PPO-7QA) in the alkaline stability testing under 10 M NaOH at 80 °C for 250 h, indicating their excellent stability. Moreover, a single H2/O2 AEMFC with these side-chain type AEMs demonstrated that PPO-C-1QA with highest conductivity exhibited a peak power density of 141.3 mW cm−2 at a current density of 320 mA cm−2.

Introduction

Recently, the rapid development of anion exchange membranes (AEMs) has promoted their practical integration into various advanced electrochemical devices, such as fuel cells, water electrolyser, and flow batteries [1,2]. Particularly, anion exchange membrane fuel cells (AEMFCs) have been regarded as one of the most promising fuel cell technologies as they offer many advantages as compared to their acidic counterparts. Under alkaline conditions, the utilization of inexpensive non-noble metal-based electrocatalyst and enhanced oxygen reduction reaction kinetics can be realized, enabling a low-cost and high-performance fuel cell device [3]. Moreover, replacing the liquid KOH electrolyte with solid polymeric AEMs can alleviate CO2 poisoning during long-term operation in traditional alkaline fuel cells [4].

As a key component in AEMFCs, it is essential to prepare anion exchange membrane with high hydroxide conductivity and chemical stability, achieving a high power density and long-term durability AEMFC [5,6]. A variety of polymer backbones has been functionalized with cationic groups as AEM materials, such as poly(olefin)s [7], polystyrene [8], and aromatic polymers including poly(phenylene oxide)s [9], poly(arylene ether)s [10], poly(phenylene)s [11], and poly (ether sulfone) [12]. Although the ionic conductivity of AEMs has been improved greatly in recent years, AEMs mostly displayed inferior ionic conductivity as compared to state-of-the-art proton exchange membranes (such as Nafion), owning to the low mobility of hydroxide ions. Incorporating more cationic groups on the polymer backbone could increase the ionic conductivity of AEMs to some extent, but more hydrophilic functional groups may reversely lead to much water absorption, excessive swelling, and subsequent poor mechanical properties [13]. Another approach to increase the conductivity of AEMs is constructing ionic transport highway in membranes. By designing anion-conductive polymers with block [14], comb/grafted [15], or ion-clustered [16] architecture, microphase-separated morphology can be induced, thus promoting the ion conduction in AEMs. On the other hand, due to the strong nucleophilic and basic working conditions, functional organic cations in AEMs may undergo multiple degradation reactions at elevated temperatures including Hofmann elimination, SN2 nucleophilic substitution, or ylide formation, resulting in the loss of ion conductivity [17,18]. Therefore, numerous high stable cationic groups, such as sterically-protected imidazolium [19], benzimidazolium [20], N-spirocyclic quaternary ammonium (QA) [21], phosphonium [22] et al., were designed and tethered onto polymer backbones to produce durable AEMs under alkali treatment. Holdcroft and coworkers have investigated the chemical stability of a series of arylimidazolium and bis-arylimidazolium, and subsequently prepared sterically protected poly(arylimidazolium) AEMs, which possessed high ion-exchange capacity and exceptional alkaline stability [19]. With a combination of ether-bond-free aryl polymer and stable piperidinium cations, a family of poly(aryl piperidinium) (PAP) polymers was reported as both AEMs and ionomers that showed exceptional chemical stability even in 1 M KOH for 2000 h at 100 °C [23,24].

When separating the cationic groups from the rigid and hydrophobic backbones via a flexible linker, side-chain-type architecture is designed for AEMs, which not only endows the AEMs with excellent alkaline stability, but also may drive their self-assembly of ionic side-chains to form continuous ion-conducting channels. Pan et al. reported the synthesis of side-chain PPO-based AEMs with pendent alkyltrimethylammonium groups through a secondary amine group, and superior alkaline stability and high conductivity were observed for these AEMs having longer spacers between the aromatic polymer backbone and the cation [25]. Aryl ether-free polymers, including poly(olefin)s (SEBS) [26] and polyaromatics [11], were also functionalized with pendent alkyltrimethylammonium groups to afford high-performance AEMs. In addition to flexible alkyl linkers, Xu et al. utilized hydrophilic ethylene oxide spacers to prepare imidazolium-containing AEMs, and this unique side-chain function facilitated the formation of hydrophilic-hydrophobic microphase-separated structure and provided more sites for water and ion transport, ultimately leading to improved AEMFC performance [27]. Although there are different designs for side-chain-type AEMs, no parallel comparison has been made to understand how side-chain functionality affects the membrane properties as well as resulting alkaline fuel cell performance, probable due to the synthetic challenges involving the lack of proper synthetic method for precisely controlling the chemical structure of different side chains.

Jannasch and Dang attached alkyl side chains with terminal QA groups onto PPO backbones by lithiation followed by quaternization with trimethylamine (TMA) and investigated the effect of side chains on AEM properties [28,29]. Recently, we reported the preparation of another kind of side-chain-type AEMs by efficient Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction, known as “click chemistry”, between azide-functionalized polymer and alkynyl-based QAs compound [17]. We noted that triazole groups in the side chains was found to constitute the only difference between the “clicked” side-chain-type AEMs and “lithiated” ones by Jannasch. Moreover, by careful selection of clickable cationic groups, the nature and location of functional groups within the polymer backbones can be easily tuned by CuAAC reaction, enabling a systematic structure-property investigation. Thus, in this work, we conducted an in-depth comparison between two series of side-chain-type AEMs to explore the critical role of side-chain functionalities in the properties of AEMs. In particular, our goal is to identify the influence of the triazole groups and the side chain length on the AEMs properties and their performance in AEMFC. To this end, PPO was chosen as the backbone of AEM materials due to its high stability, excellent film-forming ability, and simple chemical modifications. As outlined in Scheme 1, PPO-C-nQA with different lengths of triazole-containing pendant QA cations were synthesized by CuAAC reaction, while triazole-free alkyl side chains with terminal QA cations were also incorporated on PPO by direct quaternization of bromide-functionalized PPO with TMA to produce PPO-mQA, where n and m indicated the length of side chains. Subsequently, a detailed investigation on the properties of AEMs was performed in aspect of water uptake, ionic conductivity, morphology, and alkaline stability, which was further correlated to the nature of side chains. Moreover, the impact of side chain configuration on the fuel cell performance was also discussed.

Section snippets

Experimental

Materials. Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was purchased from Sigma-Aldrich (Mn = 20000 g/mol). N,N,N′,N′,N″-pentamethyldiethylene-triamine (PMDETA), copper(I) bromide (CuBr), n-butyllithium (n-BuLi, 2.5 M in hexanes), trimethyl amine, 6-chlorohex-1-yne, propargyl bromide, 4-iodo but-1-yne, 5-chloropent-1-yne, 9-bora-bicyclo [3,3,1]nonane solution in THF, allylmagnesiumbromide, carbontetrabromide, and triphenylphosphine were obtained from Energy Chemical (Shanghai, China) and used

Synthesis and characterization of structural units and polymers

In order to better understand the effect of side-chain functionality on the properties of AEMs with pendent QA cations, we synthesized two series of cationic polymers by two well-established synthetic approaches, as shown in Scheme 1. Anion-conductive PPO-c-nQA polymers were prepared by efficient Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC) reaction. By using different clickable monomers, alkyl length between QA groups and PPO backbone can be effectively tuned, affording PPO-C-nQA

Conclusion

In summary, to investigate the crucial role of side chain functionality, we have designed and synthesized two different types of side-chain AEMs with structural variations in the length of alkyl spacers and the presence of 1,2,3-triazole linker. PPO-C-nQA membrane with a triazole-containing linker had higher water uptake as compared to PPO-mQA membrane with triazole-free alkyl linker, leading to the higher hydroxide conductivity of PPO-C-nQA membrane, as triazole groups in the side chains

Credit author statement

Xiaomeng Chu: Writing-Original Draft, Visualization, Supervision, Project administration, Funding acquisition, Formal analysis, Methodology.

Jiaye Liu: Writing - Review & Editing, Conceptualization, Methodology, Investigation.

Shasha Miao: Methodology, Formal analysis.

Lei Liu: Visualization, Validation, Reviewing and Editing, Funding acquisition.

Yingda Huang: Investigation, Data Curation, Reviewing and Editing.

Erjun Tang: Investigation, Data Curation.

Shaojie Liu: Investigation, Data Curation,

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

The financial support for this work was provided by the National Natural Science Foundation of China (21975002 and 21835005), Science and Technology Major Projects of Shanxi Province of China (No.20181102019), the Hundred Talents Program of the Shanxi Province, Anhui Provincial Natural Science Foundation (2008085QB58) and Natural Science Foundation of Hebei Province (CN) (B2020208069 and B2020208032). Special thanks to Dr. Xuchao Wang in Test Center of Tangshan Graphene Application Technology

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