Jana Heiß, Maximilian Kohns
All-vanadium redox flow batteries (AVRFBs) are promising energy storage systems. The electrolyte solutions used in AVRFBs are highly concentrated aqueous solutions of vanadium species and sulfuric acid, resulting in a complex physicochemical behavior that has eluded a thorough understanding to date. This work contributes to improving the understanding of the behavior of solutions that contain vanadium in the oxidation state V(IV), i.e., the vanadyl cation VO2+, in aqueous sulfuric acid. In a comprehensive experimental campaign, the activity of the solvent water is measured over a wide range of overall molalities of H2SO4 and VOSO4 using the isopiestic method, with aqueous H2SO4 as the reference. The isopiestic experiments yield isoactivity data that are used to develop a thermodynamic model based on the Pitzer framework. Starting from an established Pitzer model parametrization for the H2SO4-H2O subsystem from the literature, the parameters of the VO2+-anion interactions are adjusted to the experimental data gathered in this work. In addition to providing a sound representation of the solvent activity and related properties, the thermodynamic model allows for calculating the ionic speciation in the studied composition range, including concentrations relevant to practical AVRFB electrolyte solutions. The model calculations reveal that adding VOSO4 shifts the dissociation equilibrium of sulfuric acid - an aspect that is often neglected in the literature. As a further application of the model and as an assessment of its predictive and extrapolation capabilities, it is used to study the solubility limit of VOSO4 as a function of sulfuric acid concentration, showing good agreement with literature solubility data. Altogether, the experimental data and the thermodynamic model developed in this work supply missing thermodynamic input for electrolyte solution property prediction and improved AVRFB modeling.