Doping structure and degradation mechanism of polypyrrole-Nafion® composite membrane for vanadium redox flow batteries

A HPPY-N212 composite membrane was prepared by the in situ polymerization of pyrrole on Nafion® 212 substrate membrane, followed by sulfuric acid treatment. The proton-acid doping structure endowed the HPPY-N212 membrane with enhanced conductivity as well as reduced vanadium ion permeability. These unique properties enabled vanadium redox flow battery (VRFB) fabricated with HPPY-N212 membrane to exhibit better coulombic, voltage and energy efficiency than that with N212 membrane under current densities of 60-150 mA cm^-2. However, a gradual decay of voltage and energy efficiency occurred during the charge-discharge cycles. The efficiency decay resulted from the irreversible damage to PPY doping structure caused by the over-oxidation during charge-discharge cycles. These investigations help better understand the structure-performance relationships and open up exciting opportunities for the development of new high-performance membranes for VRFBs.