Rotating disk electrode hydrodynamic methods

These methods constitute the frame on which any particular method can be elaborated. Yet in practice, the experimental difficulty is that with standard apparatus, 5 /D cannot be varied over an extremely wide range. For example, with the rotating disk electrode (RDE), which is the most convenient steady-state method (with the exception of ultramicroelectrodes [109]), 8 depends on the rotation frequency w of the electrode (see Chapter 2). Yet to maintain correct hydrodynamic conditions w cannot be varied, with... 

Graphical methods can be used to extract information concerning mass transfer if the data are collected under well-controlled hydrodynamic conditions. The systems described in Chapter 11 that are imiformly accessible with respect to convective diffusion would be appropriate. The analysis would apply to data collected on a rotating disk electrode as a function of disk rotation speed, or an impinging jet as a function of jet velocity. 

To carry out theoretical studies of oxidation-reduction reactions, it is often of interest to know how kf. in Equation 2.5-6 is affected by the hydrodynamics of the system. A common method for obtaining a rigorous description of the hydrodynamic flow of stirred solution is based on measurements made with a rotating disk electrode (RDE). such as the one illustrated in Figure 25-21a and b. When Ihe disk electrode is rotated rapidly, the flow pattern shown by the arrows in the figure is set up. At the surface of the disk, the liquid moves out horizontally from the center of the de-% icc, which produces an upward axial flow to replenish Ihe displaced liquid. A rigorous treatment of the hydrodynamics is possible in this case and leads to the I.evich equation ... 

To estimate ko or Xo quantitatively we must resort to an electrochemical technique that operates under steady-state conditions and for which the diffusion layer thickness is well-defined and quantifiable. The technique of choice is the rotating disk electrode (RDE) method. " Both of the aforementioned criteria are valid for the RDE. The solution hydrodynamics are well-defined, and the diffusion layer thickness can be estimated quantitatively in terms of the rotation speed... 

It is possible to increase the mass transport by introducing a forced convection in which the analyte solution flows relative to a working electrode. The most popular method for creating such a relative movement is by rotating a working electrode. Such a method is called a rotating disk electrode (RDE). Therefore, RDE could also be called as a hydrodynamic working electrode, where the steady-state current is determined by solution flow rather than diffusion [143,144]. 

The rotating disk electrode is becoming one of the most powerful methods for studying both diffusion in electrolytic solutions and the kinetics of moderately fast electrode reaction because the hydrodynamics and the mass-transfer characteristics are well understood and the current density on the disk electrode is supposed to be uniform. Levich [179] solved the family of equations and provided an empirical relationship between diffusion limiting current (id) and rotation rate ( >) as shown in Eq. (9.42). In particular applications in fuel cells, the empirical relationship which is given by Levich was also used in linear scan voltammetry (LSV) experiment performed on a RDE to study the intrinsic kinetics of the catalyst [151,159,180-190]. However, it is more appropriate to continue the discussion later in detail in the LSV section. 

The methods developed for rotating hydrodynamics are easily extended to ring-disk electrodes. This extension is facilitated by the fact that one needs only to consider radial convection in addition to linear connection and diffusion to extend disk methods to ring disk problems. This does require some model development, however, because mass transport is now permitted in two directions, x and r. Thus a linear volume element array is no longer sufficient, and a two-dimensional volume element array is required. The linear axis of this array is identical to that developed above for linear diffusion and cpnvection. Thus linear distances are still expressed in volume element units,... 

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