Ning Xie,Jinchao Han,Haowei Kang,Yiting Liu,Qiang Weng,Xingming Ning,Pei Chen,Xinbing Chen,Zhongwei An
Anion exchange membranes (AEMs) are crucial components of anion exchange membrane fuel cells (AEMFCs), and the development of highly efficient and stable AEMs is fundamental for advancing the practicality of AEMFCs. In this study, we propose the concept of “combining rigidity and flexibility” to design a series of poly(carbazole)-based AEMs with unique dual side chains. One side chain is a flexible dialkoxy chain, while the other consists of a rigid aromatic structure attached to two flexible alkyl chains. The resulting AEMs exhibit a remarkably high ionic conductivity, ranging from 120.2 to 175.7 mS cm–1 at 80 °C, along with an ion exchange capacity of 1.64–2.19 mequiv g–1. These properties are attributed to significant microphase separation facilitated by the strong interaction between the dual side chains. Furthermore, these AEMs demonstrated excellent alkaline stability. Even after being immersed in NaOH solution for 1080 h, their conductivity remains above 92%. This exceptional stability against hydroxide ions is attributed to the large electrostatic potential of the dialkoxy side chain, as verified by density functional theory calculations. Importantly, when incorporated into a hydrogen–oxygen fuel cell, the AEMs enable the fuel cell to achieve a large power density of 605 mW cm–2 at 70% relative humidity. This remarkable performance underscores their potential application prospects in the field of alkaline fuel cells. As a result, our findings contribute to the ongoing advancement of the AEMFC technology.
The ion separation performance of polymer membrane can be significantly enhanced by constructing suitable and stable ion channels within the dense matrix. In this work, we fabricated four anion exchange membranes (AEMs) based on a type of distorted poly(aryl ether sulfone) bearing dual-side-chains and explored the influence of these dual-side-chains by tuning the ratio of the two on the separation performance of Cl−/F− and Cl−/SO42−. Small-angle X-ray scattering analysis indicated that the as-prepared AEMs in wet state possessed sub-nanometer ion channels with a size of 0.334 nm, which resulted from the synergetic modification of the distorted main chains and dual-side-chains. In addition, the stability of the ion channels could be verified by a very low swelling ratio of 5.3%. The AEM with the smallest ion channel demonstrated superior perm-selectivity (Cl−/SO42−) of 52.21 during the electrodialysis (ED) process, compared to other AEMs, such as ACS AEM, which showed a selectivity of 18.63. In addition, the optimum AEM (PPH-sIm10−X-5NX) also exhibited selectivity for Cl−/F− due to its dense structure and hydrophobic side chains. By using free volume and electrostatics-driven simulations, it was inferred that the ion-blocking effect of the dense membrane structure on the transport of larger ions was effective. As a result, this dual-side-chain polymer with a distorted backbone is conducive to constructing stable sub-nanometer ion channels in AEMs for mono-/bi-valent anion resource recovery.
Anion exchange membranes (AEMs) usually suffer from the “trade-off” between ion conduction and stability. In this work, a series of dual-side-chain-grafted poly(phenylene oxide) AEMs were synthesized to explore how the side-chain architectures influence membrane performance. Our investigations suggest that the AEM containing both a long hydrophobic extender (attached to the cation’s central atom) and a hydrophilic additional side chain (beside the cation) show high hydroxide conductivity (21.3 mS/cm at 30 °C) and good chemical and dimensional stability (90.5% conductivity retention after 1 M NaOH treatment at 60 °C for 528 h). In addition, when a tri-cation side chain and a hydrophobic side chain are incorporated simultaneously, the resulting AEM shows further improved conductivity and stability (50.0 mS/cm at 30 °C; 90.6% conductivity retention after 1 M NaOH treatment at 60 °C for 528 h); it also shows excellent electrochemical performance when applied in a fuel cell and an electrodialyzer, accomplishing a peak power density of 506 mW/cm2 and a current efficiency of 96.11%, respectively. Our side-chain manipulation strategy provides a new and effective pathway to balance the ionic conductivity and stability of AEMs.
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