Studies On Oscillatory Electrodissolution Of Several Metallic Anodes

Electrochemical oscillations are spatiotemporal phenomenna underconditions of keeping far away from equilibrium. They are often found inthe processes of electrodissolution and electrodeposition of metals,electrocatalytic oxidation of small organic molecules, semiconductorelectrode and membrane, and so on. Electrochemical oscillatory systemsare suitable as model systems, for current or potential in these systems iseasily controlled and measured with low noise and short period. Thestudy of electrochemical oscillations has important scientific meaning forrecognizing the oscillatory phenomena, developing the kinetics ofelectrode processes and enriching the research contents ofnon-equilibrium thermodynamics and nonlinear dynamics. Our group hasproposed a physical model, i.e., bistability coupled with positive andnegative feedbacks, and a universal criterion of crossing cycle based onthis model for the oscillatory electrochemical systems from the viewpointof the electrode processes. It has been proved that the crossing cyclecriterion is more intuitionistic and less time-consuming. It has a widerrange of application than the negative impedance-based criterion.We have investigated the electrochemical behavior and oscillatoryphenomena of cobalt, lead and lead-tin alloy in sulphuric acid systems. Crossing cycle criterion, together with modern instruments and analytictechniques such as Raman spectroscopy, SEM, and XRD have beenemployed to study the oscillatory systems. The main contents aresummarized as follows:1. Potential or current oscillations involving periodic oxygenevolution have been observed for the first time during theelectrodissolution of static cobalt electrode in sulfuric acid with a higherconcentration of iodide. The passive film will form on the Co surfaceduring its electrooxidation, and the oxide of higher value could bereduced to lower value by I^-. Oxygen evolving not only arousesconvection to resume the iodide near surface but also destroys passivefilm mechanically. Oscillations are affected jointly by both of masstransfer processes and surface changing, i.e. alternate consumption andresumption of iodide as well as periodic breakage of surface film byoxygen evolving. Experimental results of agitation inflicting to theoscillatory system show that the oscillations are controlled by the masstransfer process. The electrooxidation of C0304 from CoO and itsreduction to CoO by I^- were detected by the ex situ Raman spectroscopy.2. Potential oscillations which involves the interfacial phasetransitions have been observed tbr the first time during theelectrooxidation of Pb electrode in sulfuric acid in the presence of SCN^-.The new oscillations appear in the phase transition region of PbSO₄-PbO₂ accompanied by periodic oxygen evolution. Experimental results ofRaman spectroscopy and XRD show that the PbO₂ film formed byelectrooxidation can be reduced chemically to PbSO₄ by SCN^-. Therefore,the oscillatory phenomena found here could be ascribed to the alternateformation and reduction of PbO₂ films on the surface. Oxygen evolutioncan promote the formation of PbO₂ and vice versa. The oscillatorymechanism has been further validated both by replacing SCN^- withanother source of S^2- from thiourea and by replaying the oscillations onan electrodeposited PbO₂ film.3. Electrochemical behavior of lead and lead-tin alloys has beeninvestigated during their electrooxidation in sulphuric acid systemscontaining sulfite or cystine. Potential oscillatory phenomena have beenobserved corresponding to the crossing cycle. Periodic oxygen evolutionoccurs during oscillations. Vigorous agitation stops the oscillations,implying that mass transfer processes play important role. Theoscillations could be ascribed to the consumption and resumption ofcys/SO₃^2- by oxidation through PbO₂ that formed during theelectrooxidation of lead or lead-tin alloy and by the forced convectionfrom periodic oxygen evolution, respectively. The tin doping effect oflead on the potential oscillations has been investigated comparing withthat on pure lead. The results show that the oscillations begin at a lowercurrent by doping lead with small proportion of tin, but the increasing of tin will depress the potential oscillations.