Measuring density and viscosity of vanadium electrolytes: An exhaustive database with multivariate polynomial fits

The variation of the physical properties of vanadium electrolytes during vanadium redox flow batteries (VRFB) operation is known to have a significant impact on the flow of the electrolytes both in the cells and in the tanks. This study presents extensive accurate measurements of the density and viscosity of vanadium electrolytes for VRFBs spanning a four-dimensional parameter space, including State of Charge (SoC), total vanadium concentration, total sulfate concentration, and temperature. The experimental results reveal different trends in the density variations of the posolyte and negolyte. Specifically, while the density of the posolyte slightly increases with SoC, that of the negolyte decreases more significantly. Furthermore, temperature exerts a linear influence on the density of both electrolytes. The analysis also reveals complex and non-linear dependencies between viscosity and the parameters under study, with more pronounced variations being observed at higher vanadium concentrations. In particular, the viscosities of both electrolytes are seen to decrease with SoC, the viscosity of the negolyte being consistently higher than that of the posolyte. We also present multivariate regression fits accurately capturing the variations of electrolyte properties, aiming to provide valuable insights into the dynamic behavior of vanadium electrolytes and enable more accurate physics-based mathematical models.