The zinc/bromine flow battery is considered as one of the most suitable candidates for the large-scale electrical energy storage attributed to its nature of high energy density and low cost. However, the relatively low power density determined by the working current density of 20 mA cm−2 limits its performance and application. The relatively low working current density is caused by the insufficient electrochemical activity of positive electrode and the high internal resistance of cell. The formation of Brads is the rate-determined step to the positive electrode reaction. Thus the activated carbon with the specific surface area of 2314 m2 g−1, compared with the carbon felt, is a much more suitable and competitive choice. The activated carbon coated membrane is first designed to decrease the internal resistance. Demonstrated by the cycle voltammetry, AC impedance and polarization distribution investigations, both the positive electrode overpotential and the cell internal resistance decrease obviously attributed to the high electrochemical activity layer closing to the membrane. Thus the energy efficiency of 75% is achieved at charge–discharge current density of 40 mA cm−2, which means a nearly two-fold increase in power density.