Probing Electrode Losses in All-Vanadium Redox Flow Batteries with Impedance Spectroscopy

We report on single-electrode electrochemical impedance spectroscopy studies of an all-vanadium redox battery using a dynamic hydrogen reference electrode. The negative electrode, comprising the V2+/V3+ couple, contributes approximately 80% of the total cell overpotential during discharge. The impedance spectra measured at the negative electrode exhibit high-frequency, semicircular arcs which correspond to the double layer capacitance in parallel with a faradaic resistance. The faradaic resistance decreases in magnitude with increasing polarization. Integration of the current-dependent faradaic resistance quantifies the fraction of the overvoltage that is attributed to the kinetic limitations of the charge transfer reaction.