Nitrogen-Doping Through Two-Step Pyrolysis of Polyacrylonitrile on Graphite Felts for Vanadium Redox Flow Batteries

We report a facile method for the nitrogen-doping process using the same precursor material for graphite felt electrodes, polyacrylonitrile (PAN). To utilize the nitrogen content in PAN, two steps of thermal treatment of PAN-coated graphite felts are performed; a PAN solution is coated on a graphite felt, and the sample is oxidized at 280 °C under the ambient atmosphere and carbonized at 900 °C under N2, consecutively. Through the two-step pyrolysis, nitrogen is successfully doped on the graphite felts, and the concentration of PAN solution is controlled to enhance the performance of vanadium redox flow batteries (VRFBs). With 4 wt % of PAN coating solution, the electrode electrocatalytic activity is enhanced compared to that of a conventional electrode, and the voltage efficiency increases, resulting in higher energy efficiency under the various current densities. Especially at high current densities above 100 mA/cm2, the optimized nitrogen-doped electrode shows about 5% higher voltage and energy efficiencies and a higher long-term stability in terms of efficiencies and capacity retention. This nitrogen-doping process with the same precursor for the electrode offers potential for employing nitrogen-doping on the conventional electrode materials in an inexpensive way.