Performance of the zinc-bromine redox flow battery is correlated to the surface properties of the positive electrode. Herein, we have modified the graphite felt electrode by thermal treatment and plasma treatment under oxygen and nitrogen atmospheres. These treatments induced specific changes to the electrode surface. Under the experimental conditions, thermal treatment increased overall surface area whereas, the O₂ and N₂ plasma increased the defects in the felt, by generating oxygen functional groups as well as N doping respectively. By combining both thermal and O₂ plasma treatment, an increase in both surface area, as well as oxygen functional groups, were achieved. The performance of the cell was influenced by the surface area and surface functional groups. Most importantly surface oxygen functionalization proved to be a decisive factor for the enhanced performance especially when the concentration of organic polybromide phase is higher. A better charge transfer/adhesion between the electrode and polybromide phase is expected through halogen/hydrogen bonding between the complexed Br and oxygen/proton of the phenolic or carboxyl functional groups.