Densely quaternized anion exchange membranes synthesized from Ullmann coupling extension of ionic segments for vanadium redox flow batteries

Abstract

Membranes with high ion conductivity and selectivity are important for vanadium redox flow batteries. Herein, densely quaternized anion exchange membranes based on quaternary ammonium functionalized octa-benzylmethyl-containing poly(fluorenyl ether ketone)s (QA-OMPFEKs) were prepared from the (i) condensation polymerization of a newly developed octa-benzylmethyl-containing bisphenol monomer via Ullmann coupling, (ii) bromination at the benzylmethyl sites using N-bromosuccinimide, and (iii) quaternization of the bromomethyl groups using trimethylamine. The QA-OMPFEK-20 with an ion exchange capacity (IEC) of 1.66 mmol g⁻¹ exhibited a higher SO₄²⁻ conductivity (9.62 mS cm⁻¹) than that of the QA-TMPFEK-40 (4.82 mS cm⁻¹) at room temperature, which had a slightly higher IEC of 1.73 mmol g⁻¹ but much lower QA density. The enhanced SO₄²⁻ conductivity of QA-OMPFEK-20 was attributed to the ion-segregated structure arising from the densely anchored QA groups, which was validated by SAXS observation. Furthermore, the QA-OMPFEK-20 showed much lower VO²⁺ permeability (1.24×10⁻¹⁴ m² s⁻¹) than QA-TMPFEK-40 (5.40×10⁻¹³ m² s⁻¹) and Nafion N212 (5.36×10⁻¹² m² s⁻¹), leading to improved Coulombic and energy efficiencies in Vanadium redox flow batteries (VRFBs). Therefore, the Ullmann coupling extension is a valuable approach for the development of high performance anion exchange membranes for VRFBs.

摘要

离子传导率和选择性是全钒液流电池隔膜的两项核心性能指标. 本文基于乌尔曼偶联反应设计合成含八苯甲基的双酚单体, 然后依次将其聚合、 溴甲基化、 季铵化制得一系列密集季铵化型阴离子交换膜(QA-OMPFEKs). 产物QA-OMPFEK-20 (离子交换容量IEC=1.66 mmol g⁻¹)的室温SO⁴²⁻传导率为9.62 mS cm⁻¹, 明显高于参照样品QA-TMPFEK-40 (IEC = 1.73 mmol g⁻¹)的4.82 mS cm⁻¹. 这归因于QA-OMPFEK-20经小角X射线散射所证实的离子聚集型结构. 此外, QA-OMPFEK-20的钒离子渗透率(1.24×10⁻¹⁴ m² s⁻¹)明显低于QA-TMPFEK-40 (5.40×10⁻¹³ m² s⁻¹)和Nafion N212 (5.36×10⁻¹² m² s⁻¹), 从而使其全钒液流电池的库伦效率和能量效率最高. 可见, 乌尔曼偶联反应是合成高性能全钒液流电池阴离子交换膜的有效路径.