Understanding characteristic electrochemical impedance spectral data of redox flow batteries with multiphysics modeling

Electrochemical impedance spectroscopy (EIS) is a robust characterization method to probe prevalent (electro)chemical processes in an electrochemical system. Despite its extensive utilization in fuel cell research, the application of EIS in redox flow battery systems particularly for simplified two-electrode full-cell configurations is more limited. Herein we attempt to strengthen the understanding of characteristic EIS data of vanadium redox flow batteries by a combination of equivalent circuit modeling with a validated Multiphysics model analyzed under hydrodynamic conditions in frequency domain. Following a highlight of system linearity and stability concerns for EIS in redox flow batteries, we specifically use our combinatory approach to investigate the effects of different cell component properties on observed galvanostatic EIS spectra and accompanying fitted equivalent circuit element parameters. For the investigated two-electrode full-cell flow battery configuration with the same electrode material on both sides, the EIS spectral data is observed to be dominated by different mass or charge transport processes at different ends of the spectrum. Sensitivity analyses of both obtained EIS spectral data and fitted circuit elements parameters show that electrode morphological properties, membrane porosity, and electrolyte inflow conditions predominantly define the EIS spectral data. Insights from the type of analyses performed herein can facilitate flow battery cell/stack diagnostics and targeted performance improvement efforts.