Fluorinated poly(fluorenyl ether)s with linear multi-cationic side chains for vanadium redox flow batteries
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Published online 28 August 2020 | https://doi.org/10.1007/s40843-020-1421-3
Fluorinated poly(fluorenyl ether)s with linear multicationic side chains for vanadium redox flow batteries 1
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Yu Chen , Yanyan Li , Bingshu Wang , Meijin Lin , Zailai Xie and Dongyang Chen ABSTRACT The cationic group distribution along the polymeric backbones of anion exchange membranes (AEMs) has significant influence on their microscopic morphology and anion conductivity. To develop high-performance AEMs for vanadium redox flow batteries (VRFBs), a series of poly (fluorenyl ether) samples bearing di- and tri-quaternary ammonium side chains with similar ion exchange capacities (IECs) were synthesized by grafting cationic alkyl chains with tertiary amine-containing poly(fluorenyl ether) precursors. The experimental results indicate that the introduction of the multi-cationic side chains facilitates the formation of microphase-separated morphologies and enhances anion conductivity. Moreover, the number of spacer atoms between the quaternary ammonium groups on the side chains affects the water uptake of the membranes, thus complicating the relationship between the density of cationic group distribution and anion conductivity. The poly(fluorenyl ether)s with dicationic side chains and six spacing atoms (DQA-PFE-C6) showed the highest anion conductivity. A VRFB assembled with DQA-PFE-C6 exhibited a maximum power density of −2 −2 239.80 mW cm at 250 mA cm , which is significantly higher than a VRFB assembled with Nafion 212. Therefore, side chain engineering is an effective chemical approach to enhance the properties of AEMs for VRFB applications. Keywords: anion exchange membranes, side chain engineering, phase separation, Coulombic repulsion, vanadium redox flow batteries
INTRODUCTION Vanadium redox flow batteries (VRFBs) have been considered for large-scale stationary energy plants to power cities or remote locations where electrical outages or shortages occur [1–4]. The separator membrane is a key component of VRFBs as it affects the Coulombic efficiency, voltage efficiency, energy efficiency, power den1 2
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sity, and cycling life of the batteries. Three categories of separators have been previously evaluated to conduct charge-carrier ions and block vanadium ions, namely proton exchange membranes (PEMs) [5–7], anion exchange membranes (AEMs) [8–10], and nanofiltration membranes [11–13]. Compared with PEMs and nanofiltration membranes, AEMs are receiving increasing attention because of their low vanadium ion permeability due to the Coulombic repulsion between the cationic groups of the membranes and the vanadium cations. However, the sulfate charge carrier for AEMs in VRFBs is large, leading to relatively low conductivity. Therefore, it is critically important to increase the anion (chargecarrier) conductivity of AEMs for use in high-performance VRFBs. The anion conductivity of AEMs is mainly affected by the ion exchange capacity (IEC) and ion distribution within the membrane [14–16]. The IEC is related to the degree of f
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