Sulfonated Poly(ether Ether Ketone)/Sulfonated Covalent Organic Framework Composite Membranes with Enhanced Performance for Application in Vanadium Redox Flow Batteries

In this study, a series of sulfonated poly(ether ether ketone) (SPEEK)-based ionic exchange membranes were prepared by introducing sulfonated Schiff base network-type (SSNW-1) covalent organic frameworks, which exhibited enhanced properties for application in vanadium redox flow batteries (VRFBs). Compared with the pure SPEEK and Nafion 212 membranes, the SPEEK/SSNW-x% composite membranes showed lower vanadium permeability owing to the size screening of SSNW-1. The vanadium permeability of the SPEEK/SSNW-x% composite membranes decreased with an increase in the SSNW-1 content; the lowest vanadium permeability of 4.02 × 10–9 cm2 s–1 was observed with SPEEK/SSNW-3%. However, the vanadium permeability of the composite membranes increased when the SSNW-1 content exceeded 3 wt % because of the aggregation of SSNW-1. In addition, the SPEEK/SSNW-x% composite membrane exhibited good thermal stability and mechanical properties. The VRFB assembled with the SPEEK/SSNW-3% composite membrane showed a Coulombic efficiency (CE) of 99.8% at a current density of 80 mA cm–2. In contrast, the values with the pure SPEEK and Nafion 212 membranes under the same conditions were 95.4 and 90.4%, respectively. The VRFBs assembled with the SPEEK/SSNW-3% membrane also exhibited higher voltage efficiency (VE) (85.9%) than that of Nafion 212 (85.0%) and SPEEK (82.6%). Finally, the capacity retention rate of VRFB assembled with the SPEEK/SSNW-3% composite membrane was observed to be superior to that with the pure SPEEK and Nafion 212 membranes. Hence, this study provides a direction for preparing membranes with enhanced performance for VRFB applications.