High-graphitization, large-surface area, and porous carbon nanofiber: A superior bi-functional electrode for vanadium redox flow battery

[Display omitted] •One-step activation of electrospun fibers by ferric ammonium citrate.•FAC can simultaneously realize the graphitization and etching of electrode.•Carbon nanofiber as bifunctional electrode can enhance VRFB performance greatly.•Highly porous, graphitic, and oxygen-functionalized carbon nanofiber was obtained.•Modified electrode exhibits superior kinetics for both vanadium redox reactions. In this paper, a vanadium redox flow battery carbon nanofiber electrode (CNF-700–4) with high graphitization, large specific surface area and high porosity was prepared by introducing ferric ammonium citrate (FAC). During carbonization process, FAC was employed to improve the graphitization degree, form microporous structure, and realize heteroatom doping. The effects of FAC content and calcination temperature on the electrochemical properties of carbon nanofiber electrode was systematically studied. CNF-700–4 exhibited excellent electrocatalytic activity and reversibility towards VO2+/VO2+ and V2+/V3+ redox couples among all electrodes. The electrochemical performance was mainly improved from three aspects: electron transport, charge transfer and diffusion processes. The higher degree of graphitization, abundant porous structures, and types of functional groups can improve the conductivity of the electrodes and provide active sites for reactions. The cell using CNF-700–4 exhibited excellent adaptability and good durability during charge–discharge tests. Compared with pristine cell, the cell using CNF-700–4 has a 10% higher energy efficiency and an improved discharge capacity of 224 mA h at 200 mA cm−2.