In situ polarization study of zinc–cerium redox flow batteries

Abstract

An in situ investigation of the sources of performance loss during discharge of a zinc-cerium redox flow battery (RFB) has been carried out. Polarization and electrochemical impedance spectroscopy (EIS) measurements on a bench-scale zinc-cerium RFB are combined to determine the overpotentials due to kinetic and ohmic effects and provide an estimate of the mass transfer losses. In order to further evaluate the source of these overpotentials, the contribution of each half-cell is evaluated by insertion of Ag/AgCl reference electrodes at each of the two inlets to the battery. Measurements reveal that most of the losses are due to the kinetic overpotential of the Zn/Zn2+ half-cell at low and intermediate current densities. Additionally, in situ kinetic analysis reveals that the exchange current density is ~7.4×10−3A cm−2 for Zn/Zn2+ oxidation and ~24.2×10−3A cm−2 for Ce4+ reduction. The difference in these values supports the conclusion that the slower kinetics at the negative electrode limits the performance at low and intermediate current densities. The effects of an alternative mixed sulfonate/chloride electrolyte to facilitate the kinetics of the Zn/Zn2+ redox couple and electrolyte flow rate to enhance the mass transfer at the positive electrode are also investigated through in situ polarization experiments.