Superior electrocatalytic performance of porous, graphitic, and oxygen-functionalized carbon nanofiber as bifunctional electrode for vanadium redox flow battery

Herein, a one-step method is applied to prepare porous, graphitic, and oxygen-functionalized carbon nanofiber as bifunctional electrode for vanadium redox flow battery (VRFB). Potassium ferrate is used to increase microporous structure, graphitization degree, and oxygen-containing group of carbon nanofiber synchronously at high temperature. This method is less time-demanding, highly efficient, and mild compared with typical two-step strategy (potassium hydroxide and ferric chloride treatment). Carbon nanofiber is modified at different mass ratio of carbon nanofiber and potassium ferrate (1:1, 1:3, and 1:5). The carbon nanofiber treated by one-step method with mass ratio of 1:3 (CNF-PF-3) demonstrates the best electrocatalysis towards V3+/V2+ redox reaction. Moreover, CNF-PF-3 also exhibits outstanding electrocatalysis towards VO2+/VO2+ redox reaction. The electrochemical performance of carbon nanofiber is improved by accelerating ion diffusion, electrochemical reaction, and electron transfer, systematically. It is due to an obvious increase in surface area, conductivity, active sites, and hydrophilcity for carbon nanofiber. The cell using CNF-PF-3 as bifunctional electrode presents the best energy storage performance compared with the cell using CNF-PF-3 at one side and pristine cell. Energy efficiency of the cell using CNF-PF-3 as bifunctional electrode reaches 75.5% at 100 mA cm−2, which is 9.9% higher than that of pristine cell.