Nitrogen-Doping Through Two-Step Pyrolysis of Polyacrylonitrile on Graphite Felts for Vanadium Redox Flow Batteries
Sang Jun Yoon, Sangwon Kim, Dong Kyu Kim, Duk Man Yu, Rolf Hempelmann, Young Taik Hong, Soonyong So
Abstract
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.