Kinetics of Aqueous Polysulfide Solutions: Part III . Investigation of Homogeneous and Electrode Kinetics by the Rotating Disk Method

Abstract

The steady-state current density-overpotential relationships for cobalt and platinum rotating disk electrodes in aqueous polysulfide electrolyte (1.3M total S21M S°, and pH = 12) were determined over a temperature range of 25°-77°C. A model developed to take into account the rate of the homogeneous dissociation of S42-to S2~ as well as a finite heterogeneous electrochemical reaction rate yielded parameters consistent with the results of previous transient experiments (in the low overpotential region). At higher overpotentials in the cathodic region a second cathodic reaction was evident. This second cathodic reaction was less prominent when the current densities were lower (as on Pt electrodes) or when the S2“ concentrations were higher (as temperature increased). The addition of 13 mole percent dimethylformamide (DMF) to the aqueous polysulfide solution was found to increase significantly the current density, at a given temperature and overpotential, by increasing the equilibrium concentration of S2~. In the range of overpotentials studied, additions of DMF also caused the first cathodic reaction wave to become much more prominent. Although independent quantitative data are lacking for the DMF-water mixtures, the use of reasonable values for k{and io/v allows the quantitative prediction of the observed cathodic behavior. © 1987, The Electrochemical Society, Inc. All rights reserved.