Electrochemical oscillations during the anodic oxidation of sulfide

Abstract

Sulfide is most often encountered as a waste product of the pulp and paper, petroleum, and mining industries. Organic decomposition is another major source of sulfide. During my M. Sc. program, I have studied electrochemical oxidation o f aqueous sulfide on Pt electrodes as well as on IrO2-based electrodes using a number of electrochemical methods (e.g., cyclic voltammetry, cyclic and linear galvanic voltammetry, differential capacitance, chronoamperommetry, chronopotentiommetry, galvanostatic technique, and electrochemical impedance spectroscopy (EIS)) and surface analytical techniques such as scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Investigating electrochemical oscillations plays an important role in nonlinear dynamic studies. During the oxidation of sulfide on a platinum electrode we observed current oscillations and two distinct potential oscillations (Oscillation a and Oscillation P) as well as bistability features. Two peaks are observed in the CV curve when scanning the potential from -0.8 to +1.8 V. The small peak is located in the potential range between -0.5 and 0.0 V, while the large one is located between 0.6 and 1.4 V. The current oscillations occur within the large peak potential range and are likely caused by the periodic formation and removal of platinum oxide and sulfur deposits. Our EIS studies show that both Oscillation a and Oscillation p can be classified as hidden negative differential resistance (HNDR) oscillators. The formation and removal of sulfur on the Pt surface, switching the direct oxidation of S2-/HS~ to polysulfides off and on, are responsible for Oscillation a , which occurs at low current densities (below 10 mA/cm2). Oscillation P appears at high current densities and it may be due to the synergic effect of sulfur formation/removal combined with oxygen evolution on the Pt oxide surface. The well-defined bistability features are caused by the change of the electrode surface states; in the low potential region sulfide is oxidized on a platinum surface, while in the high potential region platinum oxides are formed and sulfide is oxidized on a platinum oxide surface.