Fabrication of nitrogen-doped graphite felts as positive electrodes using polypyrrole as a coating agent in vanadium redox flow batteries

A novel method for preparing nitrogen-doped graphite felts as positive electrodes in vanadium redox flow batteries was developed and studied. These materials were synthesized by directly coating a thin polypyrrole layer on the graphite felt surface followed by subsequent carbonization in the presence of Co (NGF-Co). Co played an important role in promoting the concentration of nitrogen functional groups that are active towards the VO2+/VO2+ redox couple. The physical and electrochemical properties of the nitrogen-doped carbons were characterized by X-ray photoelectron spectrometry, cyclic voltammetry, impedance spectroscopy and charge-discharge tests. These results indicate that synthesizing NGF-Co via this facile method substantially improved the electrochemical catalytic activity and reduced charge transfer resistance. It exhibited a 211% and 34% increase in discharge capacity over the pristine graphite felt and thermally oxidized graphite felt, respectively, at a high current density of 150 mA cm(-2). The enhanced performance was attributed to the abundant active nitrogen groups created on the graphite felt, which benefit the fast electrochemical kinetics of vanadium redox reactions.