Diffusion-convection layer near electrode surface

Diffusion—Convection Layer Near the Electrode Surface 173... 

As we discussed above, the RDE theory is based on the convection kinetics of the electrolyte solution. If the electrode rotating rate is too small, meaning that the solution flow rate is too slow, it will be difficult to establish the meaningful diffusion-convection layer near the electrode surface. In order to make meaningful measurement, there is a rough formula that can be used to obtain the limit of electrode rotating rate (wiow) ... 

We assume that the concentration distribution within the diffusion—convection layer can be treated in the similar way to that described in Chapter 2, and then the concentration distribution of the oxidant near the electrode surface can be schematically expressed in Figure 5.2. Thus, the diffusion—convection current density (ioc.o) can be expressed in a similar form to those Eqns (2.57) and (2.58) ... 

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... 

Similar processes for producing conducting polymeric films of benzene and its derivatives had been studied earlier [2-4], Necessary conditions for the successful realization of these processes are the use of a platinum electrode and a polar solvent in the presence of catalysts (Lewis acids) and thermostatting of the reactor at -75°C. A poly(para)phenylene polymerizate of the linear structure H-(-C6FLr)n-H with the degree of polymerization n, which varies between 3 and 16, is formed. Forced convection of monomeric molecules facilitates the polymerization reaction in the diffusion layer near the electrode and the formation of a dense film on the electrode surface and prevents the formation of poly(para)phenylene in the bulk. 

When an electrochemical process takes place at the electrode surface, a concentration gradient develops near the surface, resulting in diffusion as an additional mode of mass transport. The liquid layer in which the transport by diffusion is comparable to the convectional motion is called the diffusion boundary layer, and its approximate thickness S is given by Eq. (86), corresponding to approximately 5% of Sq... 

We saw above that the concentration gradient at an electrode will be linear with respect to the spatial coordinate perpendicular to the electrode surface if the anode/cathode cell were operated at a constant current density and if the fluid velocity were zero. In actuality, there will always be some bulk liquid electrolyte stirring during current flow, either an imposed forced convection velocity or a natural convection fluid motion due to changes in the reacting species concentration and fluid density near the electrode surface. In electrochemical systems with fluid flow, the mass transfer and hydrodynamic fluid flow equations are coupled and the solution of the relevant differential equations is often a formidable task, involving complex mathematical and/or numerical solution techniques. The concept of a stagnant diffusion layer or Nemst layer parallel and adjacent to the electrode surface is often used to simplify the analysis of convective mass transfer in... 

Electrokinetic effects are not usually of importance in the type of electrochemical experiments considered in this monograph, because the electric fields at the walls of the glass cells are small and significant convection is not induced. Although electrohydrodynamic flow can be induced by the interaction of an electric field with the diffuse layer near an electrode surface, the fields in the diffuse layer near the electrode surface are not sufficiently large in most electrochemical experiments to produce measurable fluid flow. However experiments in which very large fields are intentionally applied can produce... 

Unfortunately, the case presented in Figure 2.10 is ideal, not necessary to reflect the real situation. For a practical electrode, both diffusion and convection processes coexist. Even inside the diffusion layer, there is some degree of convection—that is, the solution within the diffusion layer is not static. Therefore, the diffusion layer thickness should be determined by both the diffusion and convection processes. Fortunately, using mathematical modeling, the reactant concentration distribution profile near the electrode surface has been found to be similar to tbat shown in Figure 2.9, from which the effective (or equivalent) diffusion-layer thickness can also be defined in tbe same way as Eqn (2.51). Eor a detailed expression about tbis diffusion layer thickness induced by both diffusion and convection process, we will give more discussion in Chapter 5. 

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