Low energy density resulting from the limited solubility of conventional electroactive species in electrolyte solutions has been one of the most critical barriers in the viability of redox flow batteries. Here we report a novel positive electrolyte that consists of two redox couples, viz. Fe3+/Fe2+ and Br2/Br−. It is shown that both the capacity and energy density of this positive electrolyte are, respectively, as high as 804AhL−1 and 827WhL−1. Salient findings from the electrochemical characterizations of the electrolyte are as follows: (i) the two redox couples are reversible on the graphite electrode, in particular the presence of Br− further improves the reversibility of Fe3+/Fe2+; (ii) the anodic peak current in the cyclic voltammetry increases linearly with the concentration of Fe2+ and Br−, indicating that the oxidation of these ions follows a first-order reaction; (iii) the diffusion coefficients of Br− and Fe2+ are 14.16×10− 6 and 3.11×10− 6 cm2 s− 1, respectively; and (iv) the reversibility and mass transfer of the reactive species improves with an increase in temperature.