In this study, significantly active hollow core mesoporous shell (HCMS) carbon catalysts are synthesized for the hydrogen-bromine flow battery cathode electrode. As an alternative to cathode catalysts such as Pt/C or carbon black, high surface area HCMS carbons are synthesized and its effect on hydrogen-bromine flow battery performance has been investigated for grid-scale energy storage for the first time. HCMS carbon is synthesized by template replication of the solid core mesoporous shell silica spheres. By changing the amount of TEOS (1 mL, 6 mL, 10 mL) used during silica template formation, carbons with different core/shell structures have been produced (HCMS1, HCMS6, HCMS10). Structural characterizations of the synthesized cathode catalysts have been performed with SEM, TEM, and N-2 adsorption analyses. Electrochemical characterization of catalysts has been performed by cyclic voltammetry analyses and flow battery performance tests. 0.50 W/cm(2) peak power density obtained with HCMS1 carbon is promising compared to Pt-based catalysts and commercial carbons. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.