Owing to the high energy density and low cost, the zinc-bromine flow battery is regarded as one of the most promising candidates for large-scale energy storage. However, the widespread applications of this type of battery are hindered by its low power density, resulting primarily from the large polarization in the positive electrode. Here, we develop a highly active mesoporous carbon material for zinc-bromine flow batteries by carbonizing pomelo peel impregnated with nickel salts followed by acid washing. The nickel species not only acts as a catalyst for graphitization, but also etches the carbon surface to generate pores, both of which improve the electrochemical performance of the carbon material. It is demonstrated that the obtained carbon exhibits a remarkable activity toward Br2/Br− redox reactions, and enables the zinc-bromine flow battery to operate at 100 mA cm−2 with an energy efficiency of as high as 81.2%. By contrast, the batteries assembled with the carbon material obtained by directly carbonizing pomelo peel without nickel salts and the pristine graphite-felt electrode can only deliver energy efficiencies of 74.0% and 68.7%, respectively. In addition, no degradation is observed for 100 cycles, indicating the outstanding stability of the prepared carbon for zinc-bromine flow batteries.