Material selection and optimization for highly stable composite bipolar plates in vanadium redox flow batteries

The design of a graphite-based polymer composite bipolar plate is systematically studied for the vanadium redox flow battery system by the compression molding method with different major and minor filler contents. The optimized composite bipolar plate (denoted as the f-GKB-80) composed of flake-type natural graphite (<80 mu m) and ketjenblack nanoparticles (<50 nm) exhibits excellent electrical conductivity of 114 S cm(-1) and flexural strength of 26 MPa at a total filler loading of 85 wt%. This result can be attributed to the well-developed conducting pathways between the natural graphite flakes that are effectively filled with the ketjenblack minor fillers. Furthermore, this sample is substantially stable even when in storage in highly oxidative V5+ electrolyte solution at 80 degrees C for a week. We believe this excellent stability is due to the well-established packing structures, which protect it from concentrated acid-based electrolytes. Significantly, the f-GKB-80 demonstrates enhanced rate capability stable cycling performance, including only a 0.87% decay in energy efficiency for 50 cycles compared with commercial graphite plates (2.5% decay in energy efficiency).