A Mesoporous Tungsten Oxynitride Nanofibers/Graphite Felt Composite Electrode with High Catalytic Activity for the Cathode in Zn‐Br Flow Battery

Abstract High electrochemical polarization during a redox reaction in the electrode of aqueous zinc–bromine flow batteries largely limits its practical implementation as an effective energy storage system. This study demonstrates a rationally‐designed composite electrode that exhibits a lower electrochemical polarization by providing a higher number of catalytically‐active sites for faster bromine reaction, compared to a conventional graphite felt cathode. The composite electrode is composed of electrically‐conductive graphite felt (GF) and highly active mesoporous tungsten oxynitride nanofibers (mWONNFs) that are prepared by electrospinning and simple heat treatments. Addition of the 1D mWONNFs to porous GF produces a web‐like structure that significantly facilitates the reaction kinetics and ion diffusion. The cell performance achieves in this study demonstrated high energy efficiencies of 89% and 80% at current densities of 20 and 80 mA cm −2 , respectively. Furthermore, the cell can also be operated at a very high current density of 160 mA cm −2 , demonstrating an energy efficiency of 62%. These results demonstrate the effectiveness of the mWONNF/GF composite as the electrode material in zinc–bromine flow batteries.