Reactant transport near electrode surface

For a research on the electrode reaction mechanism and kinetics, particularly those of oxygen reduction reaction (ORR) (O2 + 4H+ + 4e -> 2H2O in acidic solution, or 02 + 2H20 + 4e -> 40H in alkaline solution), it is necessary to design some tools that could control and determine the reactant transportation near the electrode surface and its effect on the electron-transfer kinetics. A popular method, called the rotating disk electrode (RDE) technique has heen widely used for this purpose, particularly for the ORR. 

Suppose an electrochemical cell is operated at some potential in the plateau region of Fig. 3. Solute transported by the electrode surface is immediately converted, thereby creating a void of reactant immediately adjacent to the surface. A concentration gradient now exists out in the bulk solution, the concentration is the same as before the reaction, but near the surface it is zero. New analyte molecules diffuse from bulk solution to the surface to fill the void and are also reacted. Mathematically, the process is a simple application of Fisk s first law of diffusion, which states that the flux (J) at the electrode surface is given by the product of the diffusion coefficient (D) of the solute in the mobile phase and the concentration gradient at the electrode surface ... 

Kinetics of Reactant Mass Transport Near Electrode Surface 44... 

Reactant transportation in electrolyte near the electrode surface occurs by the following three different modes as illustrated in Figure 2.5. 

There are three modes of mass transport in an electrochemical system diffusion, migration, and convection. Diffusion is driven by the concentration gradient where the material transfer occurs from a high concentration to a low concentration. Diffusion is particularly significant near the electrode surface where conversion reaction occurs. Consequently, electrode has a lower reactant concentration than in bulk solution. Similarly, product concentration is higher near the electrode than further out into solution. The diffusion flux (J j mol/cm s) for species j in steady state is expressed for a constant viscosity solution using Pick s first law... 

Faradaic reactions are divided into reversible and irreversible reactions [9]. The degree of reversibility depends on the relative rates of kinetics (electron transfer at the interface) and mass transport. A Faradaic reaction with very fast kinetics relative to the rate of mass transport is reversible. With fast kinetics, large currents occur with small potential excursions away from equilibrium. Since the electrochemical product does not move away from the surface extremely fast (relative to the kinetic rate), there is an effective storage of charge near the electrode surface, and if the direction of current is reversed then some product that has been recently formed may be reversed back into its initial (reactant) form. 

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