Development of a Thermodynamic Model for the Open Circuit Voltage of All-Vanadium Redox-Flow-Batteries

• Applicant: Professor Dr.-Ing. Maximilian Kohns
• Subject Area: Technical Thermodynamics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 464239829

The all-vanadium redox flow battery (AVRFB) is a promising technology for electrochemical energy storage and can make an important contribution to a successful energy turnaround. AVRFBs use aqueous electrolyte solutions containing sulfuric acid and vanadium species of different oxidation states. These electrolyte solutions are pumped through the two half-cells of an electrochemical cell in which redox reactions take place. Thereby, the battery can store energy or deliver power. A key parameter of the AVRFB is the cell voltage. Its description starts at electrochemical equilibrium, i.e. the currentless state in which the open circuit voltage is applied to the cell. This state is described by the Nernst equation. However, since the activity coefficients of the species in the AVRFB electrolyte solutions are unknown, the Nernst equation cannot be rigorously evaluated at the present state of research. This results in great uncertainties in the development of models for the AVRFB. The aim of the present project is to develop a thermodynamic model for the activity coefficients in the AVRFB electrolyte solutions so that the Nernst equation becomes applicable. For this purpose, extensive measurements of the activity with the isopiestic method as well as measurements of the open circuit voltage are carried out. Based on this experimental data set, the thermodynamic model is developed. The thermodynamic model will be used to determine the solubility products of the various vanadium salts from published solubility data. Finally, a systematic optimization of the electrolyte composition with respect to the open circuit voltage is carried out, imposing the solubilities as a boundary condition.

• DFG Programme: Research Grants