Microelectrode designs diffusion layer

This chapter deals with the design and use of electrochemical cells for cases in which the dimensions that define the electrode area (e.g., diameter, length, width) are large relative to the dimensions of the diffusion layer. The design of electrochemical cells for use with microelectrodes (i.e., electrodes with dimensions on the order of micrometers) may require consideration of different characteristics and are considered in Chapter 12. 

This expression is the sum of a transient tenu and a steady-state tenu, where r is the radius of the sphere. At short times after the application of the potential step, the transient tenu dominates over the steady-state tenu, and the electrode is analogous to a plane, as the depletion layer is thin compared with the disc radius, and the current varies widi time according to the Cottrell equation. At long times, the transient cunent will decrease to a negligible value, the depletion layer is comparable to the electrode radius, spherical difhision controls the transport of reactant, and the cunent density reaches a steady-state value. At times intenuediate to the limiting conditions of Cottrell behaviour or diffusion control, both transient and steady-state tenus need to be considered and thus the fiill expression must be used. Flowever, many experiments involving microelectrodes are designed such that one of the simpler cunent expressions is valid. 

In the sample generation/tip collection (SG-TC) mode, a redox species is generated at the sample and collected at the tip (Fig. 5b). Again, the current intensities of both tip and sample are measured. Initially, this mode of operation was designed to determine concentration profiles in the difiusion layer generated at the sample [23, 24]. However, in principle, any electrochemically active species generated at the sample can potentially be studied in the SG-TC mode. Ideally, a small microelectrode should not perturb much the free diffusion of species generated at the sample. 

---