Side-Chain Grafting-Modified Sulfonated Poly(ether ether ketone) with Significantly Improved Selectivity for a Vanadium Redox Flow Battery

Abstract

A sulfonated poly(ether ether ketone) (SPEEK) proton exchange membrane with side-chain grafted sulfamic acid (SA) was designed. The introduction of SA with an amphoteric functional group achieves the purpose of preventing the migration of vanadium ions in the case of a lossless proton conductive group (−SO3H), which is caused by the Donnan repulsion of a protonated −NH– group to a vanadium ion. At the same time, acid–base pairs and hydrogen bonds are formed between the sulfonated side-chain grafted secondary amine (−NH−) group and the sulfonic (−SO3H) group, which hinders the penetration of vanadium ions. The results show that the prepared SPEEK-SA-50 membrane has high proton conductivity (0.099 S cm–1) and vanadium ion selectivity (1.8 × 105 S min cm–3), and its selectivity is significantly improved compared with SPEEK (4.9 × 104 S min cm–3) and the Nafion 212 membrane (4.8 × 104 S min cm–3). Under the condition of 80 mA cm–2, the Coulombic efficiency (CE) of the vanadium redox flow battery with the SPEEK-SA-50 membrane is 95.45%, and the energy efficiency (EE) is 76.11%, which is better than that of the Nafion 212 membrane (CE: 86.39%; EE: 68.26%).

Review and Perspectives of Sulfonated Poly(ether ether ketone) Proton Exchange Membrane for Vanadium Flow Batteries

“…Aiming at the challenge of improving the proton conductivity and decreasing VO 2+ permeability of the SPEEK membrane, researchers have adopted various methods to achieve high-performance membranes with improved mechanical properties and chemical stability (Figure ). − The methods of modification mainly include covalent grafting modification of SPEEK from the view of the material itself and preparation of a SPEEK-base composite membrane to improve the connectivity of proton transmission sites and transmission channels from the view of promoting proton transmission.…”

“…Therefore, the modification of the SPEEK membrane should focus on improving the VFB battery performance and chemical stability. Xi et al 108 compared the VFB performance with SPEEK-SA series membranes, Nafion 212, and the pristine SPEEK membrane. The SPEEK-SA-50 membrane exhibited the highest CE, VE, and EE (Figure 17a−c), which is due to the N atoms of the SPEEK-SA membrane acting as proton donors and acceptors.…”

“…Strategies to improve SPEEK performance. Reproduced with permission from refs − . Copyright 2019, 2022, 2023, American Chemical Society, Copyright 2017, 2019, Elsevier.…”

Anion Exchange Membranes Based on Bis-Imidazolium and Imidazolium-Functionalized Poly(phenylene oxide) for Vanadium Redox Flow Battery Applications

“… 10 , 11 Nevertheless, the high vanadium permeability of sulfonated poly(arylene ether) membranes remains unresolved. 12 − 14 …”

“…Vanadium redox flow batteries (VRFBs) have attracted great attention for their long service life, operational flexibility, and environmental friendliness. − An important component of VRFBs, the ion-exchange membrane (IEM), is used to separate the anode of VRFBs from their cathode. , The low vanadium permeability, high conductivity, and robust chemical stability are required for an ideal IEM in the application of VRFBs . Currently, Nafion membranes are commonly used as IEMs in VRFBs owing to their high conductivity. − However, the high vanadium permeability of Nafion significantly hinders its commercialization in this field. , Recently, sulfonated poly­(arylene ether) has been widely used as an IEM material for VRFBs because of its high conductivity and good chemical stability. , Nevertheless, the high vanadium permeability of sulfonated poly­(arylene ether) membranes remains unresolved. − …”