Nonlinear Phenomena during the Electrochemical Oxidation of Sulfur Compounds

Sulfur compounds have important applications in biological function and industry, such as DNA sequencing and energy production. From the perspective of fundamental research on macroscopic reaction kinetics, sulfur compounds have also a great deal of interest due to its ability to exhibit unique dynamical behaviors. This thesis focuses on uncovering nonlinear dynamics/phenomena during the electrochemical oxidation of two distinct sulfur compounds, bisulfite and L-cysteine. Chapter 2 demonstrated that the dominant S(IV) electroactive species in acidic media, dissolved sulfur dioxide, could exhibit a surface area-dependence of the onset of potential and current oscillations. Furthermore, electrochemical impedance spectroscopy was able to identify the oscillator as the HN-NDR type. The electro-oxidation of bisulfite in neutral media resulted in significantly different dynamical behaviors and was the focus in Chapter 3. In KCl electrolytes, sustained oscillations arose despite not exhibiting a NDR in the j-E curve. The competition between the oxidation of bisulfite and chloride ions were proposed to be the major driving force behind the observed oscillatory behavior. Preliminary characterization using impedance spectroscopy suggested that the instabilities may be of a possible new type of electrochemical oscillator. Lastly, the electrochemical oxidation of L-cysteine at a Pt electrode was explored in acidic, neutral, and alkaline conditions, where a well-defined negative branch was observed in the polarization curve and was able to exhibit bistability. Moreover, in a sufficiently high solution pH, the deprotonation of the -NH2, -SH and -COOH groups caused the emergence of current oscillations within a narrow range of parameters. In addition, This thesis also demonstrated the formation of sulfur elements as well as cysteine polymers, suggesting the great relevance of this nonlinear kinetics research to the field of material sciences, where the fabrication of various materials, such as different allotropes of sulfur, and cystine, may be feasible