Vanadium Redox Flow Battery Using an N-Doped Porous Carbon-Coated Positive Electrode Derived from Zeolitic Imidazolate Framework-8-Coated Graphite Felt

The poor electrochemical activity and low wettability of graphite felt (GF) electrodes significantly limit the energy efficiency and power of vanadium redox flow batteries (VRFBs). To solve these problems, we developed an N-doped porous carbon-coated electrode by the carbonization of zeolitic imidazolate framework-8- (ZIF-8-) coated GF. By controlling the carbonization temperature, the effects of temperature on the structural morphology, pore structure, and amount and type of functional groups of the electrode were confirmed. In addition, the effect of the change in the pore structure and the amount and type of functional groups on the electrocatalytic activity for the vanadium redox reaction was demonstrated. The synthesized N-doped porous carbon-coated electrode, which showed the best electrochemical activity, had a sufficient amount of nitrogen functional groups with a high ratio of graphitic N and the highest surface area and pore volume among all the samples, providing abundant active sites for the VO2+/VO2+ redox reaction. The VRFB fabricated using prepared electrode exhibited energy and voltage efficiencies of 70.44% and 73.44%, respectively, at 200 mA cm-2, which were 11.77% and 11.08% higher than those of the bare GF electrode. Furthermore, the modified battery operated effectively at a high current density of 350 mA cm-2. The energy efficiency exhibited excellent stability over 500 charge/discharge cycles at 250 mA cm-2. Hence, this study presents a promising method for the development of high-performance VRFB electrodes.