Flexible graphite bipolar plates for vanadium redox flow batteries

The effects of polytetrafluoroethylene (PTFE) additives on expanded graphite bipolar plates (BPs) for vanadium redox flow batteries (VRFB) are investigated. Pure expanded graphite plates have immense potential for use in low-cost, rapid, and continuous fabrication of high performance VRFBs. However, pure expanded graphite BPs suffer from severe swelling in vanadium electrolytes over short periods of time, exhibiting decreased efficiency of 2.3% within 35 charge/discharge cycles. In contrast, expanded graphite BPs containing 6 and 10 wt% PTFE do not experience swelling or efficiency loss during cycle tests. The prevention of swelling is a key factor contributing to the use of expanded graphite plates for VRFB stacks. Microstructural analysis reveals that our fabrication method provides uniform PTFE distribution. PTFE polymers suppress swelling by providing coverage of surface cracks, repelling aqueous solutions due to their hydrophobicity, and more importantly, providing binding forces between expanded graphite layers. PTFE additives increase electrical resistivity, but improve flexibility, allowing for the fabrication of very thin plates. Thin flexible BPs exhibit low areal electrical resistance and thus the ohmic voltage loss of the BPs with 6 wt% PTFE is less than 1.2 mV (<0.1% of the VRFB nominal voltage of 1.25 V) even at a high current density of 0.2 A cm−2, which is negligible compared with other factors.