Effect of organic solvents and electrode materials on electrochemical reduction of sulfur

Abstract

The electrochemical reaction of sulfur has been studied through cyclic voltammetry (CV) method in six organic solvents including new solvent systems such as dimethoxyethane and diglyme. For the first time, the CV curves of sulfur are classified on the basis of the shape of the second reduction step. The overall shapes of CV curves, the peak potentials, and the potential shifts with scan rate were variable with the solvent used. These results apparently indicate that the solvent molecules are directly involved in the overall electrode reactions. Following reaction mechanism based on the successive one-electron reduction (2 x 1e(-)), which was proposed for the first redox waves of sulfur in our previous study, was verified both by digital simulation and by curve fitting to the CV curves obtained at glassy carbon (GC) and platinum (Pt) electrodes: S-8c + e(-) -> S-8c(-) (E-1 degrees', k(1)degrees, alpha(1)) S-8c(-) + e(-) -> S-8c(2-) (E-2 degrees', k(2)degrees, alpha(2)) S-8c(2-) -> S-81(2-) (K-3, k(f3)) S-81(2-) -> S-6(2-) + 1/4S(8c) (K-4, k(f4)) S-6(2-) -> 2S(3)(-) (K-5, k(f5)) According to the mechanism, we obtained useful information such as formal potentials and standard rate constants from the first redox waves through digital simulation technique. The estimated standard rate constants (k(1)degrees and k(2)degrees) of the two redox couples, i.e., S-8c/S-8c(-) and S-8c(-)/S-8c(2-), at glassy carbon electrodes were much larger than those at platinum electrodes. We think this feature probably arose from the much higher affinities of sulfur to glassy carbon electrodes. Overall results indicate that the electrode reactions of sulfur are significantly affected by the electrode material as well as the nature of solvent.