Analysis of Concentration Overpotential in an All-Vanadium Redox Flow Battery

In electrochemical cells, transport of chemical species to/from the electrodes is limited by the mass transfer resistance between the electrode surface and the bulk electrolyte. This mass transfer resistance thus contributes to voltage losses, referred to as mass transport losses or concentration overpotential, compared to the reversible potential of cell. In this paper, we derived analytical expressions for estimating the mass transport losses in all-vanadium redox flow batteries. A step-by-step analysis allows us to relate the surface and bulk concentrations and then, identify the voltage losses due to mass transport from the Nernst equation and the Butler-Volmer kinetics. A zero-dimensional (0D) model with the derived expressions for the concentration overpotential is calibrated and validated with experimental data; good agreement is obtained. The mass transport losses from the Butler-Volmer kinetics are found to be approximately twice of that from the Nernst equation.