Temporal Instabilities in Cathodic Processes at Liquid and Solid Electrodes
Experimental examples and theoretical models of the oscillatory and mutistable behavior in cathodic processes of various species, studied at both liquid (mercury) and solid electrodes, are described. The following processes with the N-NDR characteristics
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Temporal Instabilities in Cathodic Processes at Liquid and Solid Electrodes
4.1 4.1.1
Electroreduction of Peroxodisulfate Ions The N-NDR Region as a Double Layer Effect
In this chapter, examples of temporal instabilities (oscillations and multistability) reported for cathodic processes carried out at both liquid and solid electrodes are described. With respect to liquid ones, the oscillations of the current associated with the electrode processes occurring at mercury have been quite intensively studied for several decades, especially when classical polarography was still the most popular experimental technique in electrochemistry. Therefore, studies of polarographic oscillations greatly contributed to the understanding of the mechanisms of electrochemical oscillations. A big advantage of mercury electrode is its smooth, uniform, and easily renewable surface and a high overvoltage for hydrogen evolution at negative potentials. The structure of the mercury–liquid electrolyte solution is thus much easier to define than, e.g., the structure of the passive layer on corroding solid electrodes. Hence, not only the general source of instability, but also a more detailed (electro)chemical mechanism of oscillations or multistability can be suggested. Based on the characteristics of the polarographic systems one can excellently show the different origins of the negative differential resistance in electrode processes: the potential-dependent repulsion of charged reactant particles from the reaction site in the double layer, the adsorption of an inhibitor of a charge-transfer process, or the desorption of a catalyst of this process (see Sect. 2.1.4). Such studies were later extended for solid electrodes, provided that the region of potential corresponding to instabilities was positive enough to avoid hydrogen evolution. One of the model electrode reactions that served for numerous studies of mechanisms and bifurcation scenarios of dynamic instabilities was the electroreduction of S2O82 ions in which the negative differential resistance (N-NDR region) was caused by repulsive interaction of these anions with negatively charged electrode surface. This Frumkin effect was first reported as early as 1933 [1]. In order to understand the detailed origin of this effect, one has to take into account the spatial distribution of the electric potential in the double M. Orlik, Self-Organization in Electrochemical Systems I, Monographs in Electrochemistry, DOI 10.1007/978-3-642-27673-6_4, # Springer-Verlag Berlin Heidelberg 2012
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4 Temporal Instabilities in Cathodic Processes at Liquid and Solid Electrodes
layer, varying from fM at the metal electrode surface up to fs—the potential in the bulk of the solution. The bulk potential fs is usually considered a reference potential, with respect to which other potentials are expressed, or in other words, fs is then assumed to be of zero value. The distance at which the potential profile develops in the solution depends on the concentration of ionic salt which can be introduced intent
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