Electrode processes modes

Recent research development of hydrodynamics and heat and mass transfer in inverse and circulating three-phase fluidized beds for waste water treatment is summarized. The three-phase (gas-liquid-solid) fluidized bed can be utilized for catalytic and photo-catalytic gas-liquid reactions such as chemical, biochemical, biofilm and electrode reactions. For the more effective treatment of wastewater, recently, new processing modes such as the inverse and circulation fluidization have been developed and adopted to circumvent the conventional three-phase fluidized bed reactors [1-6]. 

The above considerations concern a reversible electrodic process, ox + ne red as instead of 20-100 hz in the sinusoidal technique a fixed frequency of 225 Hz is normally used in the square-wave mode, the chance of irreversibility in the latter becomes greater, which then appears as asymmetry of the bellshaped I curve. Such a phenomenon may occur more especially when the complete i versus E curve is recorded on a single drop, a technique which has appeared useful51 in cases of sufficient reversibility. 

Electrode processes are conveniently classified according to the nature of the final product1 and its formal mode of formation, since then the interplay between nucleophile(s) or electrophile(s), substrate, and loss or addition of electron(s) is best expressed. It is upon our ingenuity to choose the correct combination of electrolyte components that the practical success of an electrochemical reaction rests, and therefore the rather formalized classification system to be outlined and exemplified below is the logical point of departure into the maze of mechanistic intricacies of electrode processes. 

First, however, we shall dissect the concepts of direct and indirect electrode processes, since mechanistic problems are implicated in this mode of classification. 

The increased rate of mass transport associated with shrinking electrode size means that electrode processes which appear electrochemically reversible at large electrodes may show quasi- or irreversible electrode kinetics when examined using both steady-state and transient mode microelectrode methods. The latter represents a powerful approach for the determination of fast heterogeneous electrode kinetics. Rate constants in excess of lcms have been reported (Montenegro, 1994). 

The field required for breakdown to occur and the mode of breakdown depend on doping level. As the dopant concentration increases, the width of the space charge layer decreases and the probability of tunneling increases rapidly so that Z mr breakdown becomes more likely than avalanche breakdown. Zener breakdown is, in general, involved in the electrode processes on p and n materials under a reverse bias. 

In electrochemical transducers, changes of various electrochemical quantities are utilized, which are associated with electrode processes in the presence of analyte (amperometry, various modes of voltammetry, voltohmmetry, coulometry) or with changes of electrical properties of medium connected with the presence of analyte (potentiometry, conductivity, capacitance, dielectric permittivity, inductance). 

There is some scope for diversifying the range of mechanisms to which SECM can be applied. In particular, all three modes considered in this chapter could prove useful in the study of the catalytic EC mechanism and its variants (38), which describe a diversity of significant electrode processes. It may also be possible to study preceding chemical reactions with SECM, using the interelectrode gap to vary mass transport to the tip via hindered diffusion. The range of accessible rate constants and sensitivity with which measurements could be made would, however, be unlikely to compete with alternative methods. 

Unwin P R and Bard A J 1991 Scanning electrochemical microscopy—theory and application of the feedback mode to the measurement of following chemical-reaction rates in electrode processes J. Phys. Chem. 95 7814... 

In conclusion, UVAds/NIR spectroelectrochemistry in both transmission and reflection mode are extremely useful techniques that yield a wealth of complementary data additional to those obtained in pure electrochemical voltammetric experiments. Especially when based on computer simulation models, this data may be used to unravel the kinetics and thermodynamics of complex electrode processes. 

Notwithstanding, the experimental simplicity of chronopotentiometry may still make it a first-choice electroanalytical technique for higher-temperature molten systems. Furthermore, in the current-reversal mode, it is one of only a few diagnostic techniques for assessing the classical reversibility of an electrode process. A recent review has surveyed many of its applications in this context, and so, bearing in mind the authors own interests, it seems appropriate to use examples of chronopotentiometric studies to illustrate some of the present aspects of current interest. 

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