Figure shows the density of the posolyte (left) and negolyte (right) as a function of SoC for different temperatures and total vanadium concentrations. In all cases, the measurements (symbols) follow linear trends as highlighted by the empirical regressions (dotted lines). The density of the posolyte shows a slight increment with SoC, with ρ + SoC = 2.73 • 10 -3 g cm -3 > 0, independent from c V . The experimental results suggest a slight reduction in ρ + SoC with decreasing c V . But the reduction is so weak that it cannot be accurately captured thorugh regression fitting, as it falls within the same order of magnitude as the estimated error. Regarding the temperature dependence, the posolyte density is seen to decrease linearly with T , the value of ρ + T remaining virtually independent across all parameter space. In the context of VRFB operation, during charge the density of the posolyte should slightly increase due to the SoC increment, but, as the temperature tends to decrease , the overall density variation could be roughly cancelled. Since these effects are known to reverse during discharge, the posolyte should therefore show negligible density variations during the entire charge/discharge cycle.