Use of aqueous redox flow battery (RFB) holds great promise for grid-scale and sustainable energy storage. Engineering the electrochemical reactor of an aqueous redox flow battery is critical to deliver sufficiently high-power densities with reasonable cost. Cell-level resistive losses that reduce RFB power density are originating from ohmic, kinetic, or mass transfer limitations. Understanding and ultimately alleviating the cell-level resistive losses in RFBs 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 RFB operation. However, existing studies on electron and mass transfer rates are mostly performed in ex-situ conditions based on planar electrodes, so the obtained key parameters are difficult to correlate with a practical electrode performance in a battery. To address this limitation, an in-situ electroanalytical technique of electrochemical reaction in porous electrodes is proposed by a symmetrical cell design for RFBs. This technique was tested on vanadium redox flow battery and organic alkaline flow battery. For both redox couples in cells, 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 condition. The ohmic, kinetic, and mass transfer resistance are obtained by decomposing 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. With these key parameters extracted for both anolyte and catholyte, the simulated symmetrical cell polarization agrees well with the experimental symmetrical cell results. In addition, the simulated full-cell results 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.

In-situ electroanalytical technique to characterize reaction kinetics in aqueous redox flow batteries

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