The electrolyte in the vanadium flow battery (VFB) is one of the key materials in determining the energy density, which further limits its commercialization. A novel strategy of varying the proton concentration to improve electrolyte utilization is proposed to enhance the energy density of the VFB. The effects of proton concentration on the equilibrium potential and polarization have been investigated systematically. Superior electrolyte utilization at a proton concentration of 5 mol L−1 is achieved; this corresponds to a high volumetric energy density (22.5 Wh L−1) and volumetric capacity density (14.8 Ah L−1), which is favorable toward the commercial application of VFBs. This general strategy also inspires research on improving electrolyte utilization in other flow battery systems.