In Situ Characterization of Kinetics, Mass Transfer, and Active Electrode Surface Area for Vanadium Redox Flow Batteries

Engineering the electrochemical reactor of a vanadium redox flow battery (VRFB) is critical to deliver sufficiently high power densities to achieve cost-effective, grid-scale energy storage. Understanding and ultimately alleviating the cell-level resistive losses in VRFBs fundamentally depend on the ability to accurately measure the electron and mass transfer rates as a function of applied potential and interpret the results in the context of VRFB operation. In this study, an in situ electroanalytical technique of electrochemical reaction in porous electrodes is proposed by a symmetrical cell design for VRFB. For both V2+/V3+ and VO2+/VO2 (+) redox couples, the polarization curves at different flow rates are acquired on the symmetrical flow cell. The high-frequency resistance is also obtained by electrochemical impedance spectroscopy at open circuit. The ohmic, kinetic, and mass transfer resistance are obtained by deconvoluting the total polarization curve. Corresponding key parameters (i.e., membrane conductivity, reaction rates, and mass transfer coefficients) are obtained along with the specific surface area of porous electrode. The full-cell simulations using extracted key parameters are in excellent agreement with experimental full-cell tests at different applied currents. This novel in situ electroanalytical technique provides an invaluable approach to characterize the performance of electrolyte and electrode in redox flow batteries.