The hydrogen-bromine redox flow battery's (H2 Br2 RFB) advantages of high energy capacity, high round-trip conversion efficiency and low cost, make it an optimal candidate for large-scale energy storage systems. The crossover of bromide species through the membrane degrades the performance of the H2 Br2 RFB by poisoning the catalyst responsible for the hydrogen evolution and oxidation reactions. Herein we propose the new concept of a selective catalyst coating layer that mitigates the effect of bromide crossover. The polymerization of dopamine on the catalyst surface yields a nanoscale conformal polydopamine layer which acts as a semi-permeable barrier to block bromide species. The H2 Br2 RFB with the coated catalyst shows a low capacity fading of 6% at 300 mA cm−2 after exposure to 4.5 M charged electrolyte for 2 h. Even the beginning of life polarization curves show the benefit of catalyst coating with a high peak power of ∼550 mW cm−2. Hence, the catalyst coating opens a way to solve the crossover issue in H2 Br2 RFB technology.