Diffusion-convection process electron transfer kinetics

Normally, if the electron transfer kinetics of oxidant reduction in Reaction (5-1) is very fast so that the diffusion-convection process could not catch up the speed of this electron-transfer process, the oxidant s surface concentration is quickly exhausted to zero, and the obtained Levich plot of /dc,o vs according to Eqn (5.14a) will be a straight line. From the slope (= 0.62nFDQ r / CQ) of the straight line, the parameter either as n, Do, v, or Cq can be estimated if the other three are known. 

Usual conditions for LSV or CV experiments require a quiet solution in order to allow undisturbed development of the diffusion layer at the electrode. Some groups, however, have purposely used the interplay between diffusion and convection in electrolytes flowing in a channel or similar devices [23]. In these experiments (see also Chapter 2.4), mass transport to the electrode surface is dramatically enhanced. A steady state develops [54] with a diffusion layer of constant thickness. Thus, such conditions are in some way similar to the use of ultramicroelectrodes. Hydro-dynamic voltammetry is advantageous in studying processes (heterogeneous electron transfer, homogeneous kinetics) that are faster than mass transport under usual CV or LSV conditions. A recent review provides several examples [22]. 

RDE is a commonly used technique for investigating the ORR in terms of both the electron transfer process on electrode surface and diffusion—convection kinetics near the electrode. To make appropriate usage in the ORR study, fundamental understanding of both the electron transfer process on electrode surface and diffusion—convection kinetics near the electrode is necessary. In this chapter, two kinds of RDE are presented, one is the smooth electrode surface, and the other is the catalyst layer-coated electrode. Based on the electrochemical reaction 0 + ne R), the RDE theory, particularly those of the diffusion—convection kinetics, and its coupling with the electron-transfer process are presented. The famous Koutecky—Levich equation and its... 

In Chapter 6, the importance of RRDE fundamentals and practical usage in ORR study is emphasized in terms of both the electron transfer process on electrode surface, diffusion-convection kinetics near the electrode, and the ORR mechanism, particularly the detection of intermediate such as peroxide. One of most important parameters of RRDE, the collection efficiency, is deeply described including its concept, theoretical expression, as well as experiment calibration. Its usage in evaluating the ORR kinetic parameters, the apparent electron transfer, and percentage of peroxide formation is also presented. In addition, the measurement procedure including RRDE preparation, current—potential curve recording, and the data analysis are also discussed in this chapter. 

This first chapter to Volume 2 Interfadal Kinetics and Mass Transport introduces the following sections, with particular focus on the distinctive feature of electrode reactions, namely, the exponential current-potential relationship, which reflects the strong effect of the interfacial electric field on the kinetics of chemical reactions at electrode surfaces. We then analyze the consequence of this accelerating effect on the reaction kinetics upon the surface concentration of reactants and products and the role played by mass transport on the current-potential curves. The theory of electron-transfer reactions, migration, and diffusion processes and digital simulation of convective-diffusion are analyzed in the first four chapters. New experimental evidence of mechanistic aspects in electrode kinetics from different in-situ spectroscopies and structural studies are discussed in the second section. The last... 

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