An electrochemical study on the positive electrode side of the zinc–cerium hybrid redox flow battery

In this study, the electrochemical behaviour of the Ce(III)/Ce(IV) redox couple in methanesulfonic acid medium on various electrode substrates was investigated as a function of temperature. Carbon composite electrodes as well as Pt and Pt-Ir coated electrodes were studied for their suitability in carrying out the Ce3+/Ce4+ redox reaction. Cyclic voltammetry in 0.8 mol dm-3 cerium and 4.5 mol dm-3 methanesulfonic acid solution showed that elevated temperatures favoured the Ce3+/Ce4+ reaction on the various platinum and platinum-iridium coated substrates as well as on carbon composite surfaces. The latter electrodes showed better kinetics than the metal coatings but deteriorated badly under the high positive potentials required for the cerium reaction. The exchange current density (io), obtained through Tafel extrapolation, polarisation resistance and electrochemical impedance spectroscopy measurements, increased with temperature over the range 25°C to 60°C. The Pt-Ir coatings gave the largest io at 60°C and appear best suited for use as the positive electrode in the Zn-Ce redox flow battery.