Ensembles of microelectrodes

Fig. 7.35. Development of diffusion concentration profiles in ensembles of microelectrodes. Concentration distortions at very short times during chronoamperometry or fast sweep rates during (a) cyclic voltammetry, (b) intermediate times or sweep rates, and (c) long times or slow sweep rates. Voltam-metric responses are shown schematically. (Reprinted from B. R. Scharifker, Microelectrode Techniques in Electrochemistry, in Modem Aspects of Electrochemistry, Vd. 22, J. O M. Bockris, B. E. Conway, and R. E. White, eds., Plenum, 1992, p. 505.)...

Arrays are ensembles of microelectrodes that may consist of regularly, or irregularly, spaced assemblies of identical electrodes, ensembles of electrodes with identical... 

The best configuration of ensembles of microelectrodes is a collection of micro discs of equal size, arranged on a uniform grid, at equal distances from each other. Sometimes this is not feasible, and ensembles containing electrodes of nonuniform size and/or distance have been considered. Ensembles of strip microelectrodes have also been constructed, and the variation of current with time for such configurations has been evaluated. 

Note that the current density in this case is larger by a factor of (L/r) than at short times, when Eq. 44L applies. The development of the diffusion field at an ensemble of microelectrodes is shown schematically in Fig. 20L. 

This prevents water penetration and only the exterior of the electrode is exposed to electrolytes. When used with 30 nm sized carbon black powder, the conductive area of the electrode in contact with the electrolyte consists of less than 1 % of the geometric cross-sectional area of the electrode. This has been compared with an ensemble of microelectrodes. These CCEs showed an improvement of up to three orders of magnitude in terms of Faradaic signal-to-noise compared with glassy carbon electrodes [218]. In addition, the sol gel method of preparation affords a variety of structural configurations and it has been shown that CCEs can be produced by thick film and ink jet technology, which allows for their mass production [219]. 

Similarly, it was shown by Gilleadi16 that the development of a diffusion field near the surface of an ensemble of microelectrodes occurs in the four successive steps, assuming total diffusion control of the process. The ensemble of microelectrodes consists of microelectrodes placed on the inert surface at distances between their centers larger than their diameter. The first, planar diffusion to the microelectrodes the second, spherical diffusion with no overlap the third, spherical diffusion with substantial overlap and finally, total overlap, equivalent to planar diffusion to the whole surface. 

The use of microelectrodes is impractical, but the use of ensembles of microelectrodes can be a real option. The ensemble of microelectrodes consists of microelectrodes placed on the inert surface at distances between their centers larger than their diameter. 

Assuming homogeneously distributed, equal to each other, hemispherical grains of the catalyst on an inert substrate, in a way similar to that used by Gileadi16 to describe an ensemble of microelectrodes, the surface of the macroelectrode can be presented by an idealized model, as in Fig. 2, and the number of grains per square centimeter is given by ... 

E. Gileadi (Tel Aviv University, Israel) The use of ensembles of microelectrodes for trace analysis should be considered. Recent studies in our laboratory (H. Reller, E. Kirowa-Eisner and E. Gileadi, J.Electroanal. 

A further advantage of ensembles of microelectrodes is the low solution resistance associated with the small diameter of each electrode in the ensemble. This allows electrolysis in poorly conducting solution such as tap water. 

Reller H, Kirowa-Eisner E, Gileadi E (1982) Ensembles of microelectrodes. A digital simulation. J Electroanal Chem 138 65. 

Scharifker BR (1988) Diffusion to ensembles of microelectrodes. J Electroanal Chem 240 61-76... 

The fraction of the surface that is active is given by (d/Lf, where d is the diameter of each electrode and L is the distance between their centers. Designing such an ensemble of microelectrodes, one must compromise between the desire to make the ratio d/L as small as possible (to decrease the overlap between the diffusion fields of the individual electrodes), and the desire to make d/L as large as possible (to increase the total active area). Values of d/L in the range 0.03-0.1, corresponding to 0.1%- % of active area, seem to be a reasonable choice, as we shall see. 

The development of the diffusion field at an ensemble of microelectrodes is shown schematically in Figure 14.12. 

Figure 14.12 Development of the diffusion field near the surface of an ensemble of microelectrodes, (a) semi-infinite linear diffusion (b) spherical diffusion with no...

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