Electrochemical characterization of electrodes in the electrochemical Bunsen reaction of the sulfur–iodine cycle

In the electrochemical Bunsen reaction, SO2 is oxidized to H2SO4 at the anode while I2 is reduced to HI at the cathode. Both electrodes were electrochemically characterized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The effects of H2SO4 concentration in the anolyte, as well as HI concentration and I2/HI molar ratio in the catholyte, were explored. The cyclic voltammograms of platinum electrode shift with rising scan rate, indicating the irreversibility of two electrode reactions. The equivalent circuit for the cathode reaction impedance consists of an ohmic resistance of the solution, in series with a parallel combination of a charge transfer resistor and a constant phase element, yet the impedance spectra for the anode reaction can be modeled using a parallel combination of a charge transfer resistor and a constant phase element. The electrode reaction kinetics was also analyzed using the exchange current density (j0) and the standard reaction rate constant (k0). The results indicate that a high electrode reaction rate in the cell can be obtained for a HI concentration of 8 mol/kgH2O and an I2/HI molar ratio of 0.5 in the catholyte and a H2SO4 concentration of 13 mol/kgH2O in the anolyte.