Open circuit voltage of vanadium redox flow batteries: Discrepancy between models and experiments

A major issue with the existing vanadium redox flow battery (VRFB) models is the inaccurate prediction of the open circuit voltage (OCV), which results in a discrepancy of 131 to 140 mV in predicted cell voltages when compared to experimental data. This deviation is shown to be caused by the incomplete description of the electrochemical double layers within the cell when calculating the OCV using the Nernst equation adopted from fuel cell literature. Here, we propose a more complete description of the Nernst equation, which accounts for two additional electrochemical mechanisms that exist in a VRFB, namely: i) the proton activity at the positive electrode due to the involvement of the protons in the redox reaction, and ii) the Donnan potential due to the proton concentration differences across the membrane. The complete form of the Nernst equation proposed herein accurately predicts the reported experimental data with an average error of 1.2%, showing a significant improvement over the incomplete Nernst equation (8.1% average error) currently used in VRFB models.