Investigation of anodic oxidation mechanism of Na2S solution on graphite electrode

Electrochemical behavior of aqueous sodium sulfide solution at graphite anode, under the condition of different concentrations of Na2S and different temperatures, has been investigated by cyclic voltammetry and chronopotentiometry. The results show that the anodic oxidation of aqueous sulfide solution is irreversible and that polysulfides are formed in rate-determining step of the anodic oxidation, With the increase of electrolytic temperature, potentials corresponding to anodic peaks shift to more negative and those corresponding to cathodic peaks shift to more positive, which indicates that the elevating of temperature is favorable to both oxidation of the sulfide and reduction of products polysulfides or elemental sulfur. Rate constants k(s) of standard electrode reaction for anodic oxidation of Na2S solution are between 10(-4.08) similar to 10(-4.35)cm . s(-1) at 70 degrees C. Chronopotentiometric studies show two potential peaks in time-potential curve for a smaller current density value. The potential peaks may result from a self-catalytic effect taking place due to both dissolution of elemental sulfur by S2-, HS- and S-x(2-), and transition reactions among polysulfides. The potential peaks disappear with the increasing of anodic current density.