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

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.