State of Charge (SoC) of the Vanadium and Other Redox Flow Batteries: Identification of the Electrode and Bipolar Plate Contributions

In this paper attention is directed toward one of the key measures of performance, State of Charge (SoC) for vanadium and other battery chemistries. Explored are its simple definition, as well as a more complete examination of just what is meant by this terminology. Developed is a more sophisticated exposition of how to calculate it properly, including spatial distributions, and what consequences arise from various simplifications. Open Circuit voltage through the Nernst equation, is related to the SoC. Non-equilibrium overpotential is also related to the concentrations when treated as averaged field variables. Other chemistries examined include iron-chromium, all-iron, and hydrogen-bromine. For the first time, a rigorous decomposition of the spatial variation of the electrode and bipolar plate contributions is identified, allowing new formulas to be obtained. This may allow optimization of redox flow battery performance. Finally, an expression is found for the open circuit voltage cost going between no bipolar plates and their incorporation into the redox flow battery.