Unravelling the Contribution of Kinetics and Mass Transport Phenomena to Impedance Spectra in Vanadium Redox Flow Batteries: Development and Validation of a 1D Physics-Based Analytical Model

Vanadium redox flow battery technology can support the spread of energy storage in stationary applications and allow higher penetration of renewables in the electric grid. Currently, its market competitiveness is hindered by low power density, which stems from complex interplay between kinetic and mass transport losses. The quantitative interpretation of experimental observations should rely on physics-based models, which allow a consistent comparison of different operative conditions. In this work, a fast analytical physics-based 1D model of the impedance of vanadium flow battery is presented and validated with respect to experimental data. The model, made available online at http://mrtfuelcell.polimi.it, employs a macro-homogeneous approach and considers losses due to kinetics, reactant distribution within the electrode (Sigracet® SGL 39 AA carbon paper), convection in flow channel and vanadium transport to electrode surface. Additionally, analytical expressions of contributions to impedance of single physical phenomena are derived through an asymptotical analysis. The results show that, at negative electrode, transport of ions to active surface is the limiting phenomenon at lower flow rates, while at higher flow rates depletion of reactants within electrode becomes critical together with charge transfer processes. At positive electrode, the main contribution to performance loss is the vanadium transport to electrode surface.