Electrochemical Impedance Spectroscopic Investigation of Vanadium Redox Flow Battery

The kinetics of redox reactions relevant to vanadium redox flow battery (VRFB) is investigated using voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a three-electrode configuration and in a VRFB setup (two-electrode configuration). Impedance spectra are recorded in the VRFB setup with equimolar concentration of V 5+ /V 4+ and V 3+ /V 2+ solutions as positive and negative electrolytes, respectively (full-cell; asymmetric feed system); with an equimolar V 5+ /V 4+ electrolyte on both sides (symmetric feed system 1); and also with V 3+ /V 2+ electrolyte on both sides (symmetric feed system 2). Impedance of the full-cell VRFB (recorded with asymmetric feed system) is comparable to the sum of the half of the impedance of symmetric feed systems (of V 5+ /V 4+ and V 3+ /V 2+ electrolytes) at open circuit potential (OCP). Impedance and voltammograms recorded in the three-electrode configuration using Vulcan XC-72 modified rotating disk electrode, and the impedance recorded in the two-electrode full-cell configuration unequivocally confirms that the V 5+ /V 4+ redox reaction limits the VRFB performance. The optimal performance of the VRFB with the in situ treated carbon felt compressed to 47% is ∼389 and ∼336 mW cm −2 with 5 and 25 cm 2 cell area, respectively.