Swelling-Induced Cross-Linked Pyridine-Containing Membranes with High Stability and Conductivity for Vanadium Redox Flow Batteries

Vanadiumredox flow batteries (VRFB) have many advantages in overcomingthe volatility of renewable energy. However, membranes, one of thekey materials for VRFB, face crucial challenges, for example, membraneswith high conductivity tend to exhibit high swelling, low selectivity,and poor mechanical stability. In this work, new cross-linked membranes(C-PyPEKK) were prepared by blending a novel chloromethylated adamantane-containingpolymer (CAPEK) (with a bulky, rigid, twisted backbone) with a uniquepyridine-containing polymer (PyPEKK). The synergistic effect of cross-linkedstructures and bulky, rigidly twisted adamantane-containing backbonegives the membrane excellent membrane swelling resistance and selectivity.C-PyPEKK membranes were selectively swelled and formed wide ion transportchannels, enhancing conductivity. The resulting C-PyPEKK membranesexhibited low water uptake (<8.0%), a low swelling ratio (<13.0%)(the uncross-linked PyPEKK membrane showed a high swelling ratio of28.6%), and high mechanical strength (about 60 MPa). The C-PyPEKK-130membrane showed a superior conductivity of 52.83 mS cm(-1) and outstanding energy efficiencies (EEs) (90.75% at 80 mA cm(-2); 79.4% at 300 mA cm(-2)), which wassuperior to the Nafion212 membrane (EE = 81.87%, 80 mA cm(-2); 75.48%, 300 mA cm(-2)). The C-PyPEKK membranesshowed high cycling stability (3500 cycles) in VRFB. Moreover, theC-PyPEKK membranes were robust and showed good battery performanceafter being immersed in strongly acidic and oxidizing 1.5 M VO2 (+) in 3 M H2SO4 solutionsfor 9 months, showing excellent chemical stability. This work proposesa new preparation route for membranes with high conductivity, highstability, and excellent mechanical strength for potential VRFB applicationsfrom unique new membrane materials combined with cross-linked membranestructures and unique conduction enhancement strategies. C-PyPEKK membranes for vanadium redoxflow batteries areused to overcoming the volatility of renewable energy.