Cyclic voltammetry microdisc

This book is aimed at students, researchers and professors in the broad area of electrochemistry who wish to simulate electrochemical processes in general, and voltammetry in particular. It has been written as a result of numerous requests over recent years for assistance in getting started on such activity and aims to lead the novice reader who has some prior experience of experimental electrochemistry at least, from a state of zero knowledge to being realistically able to embark on using simulation to explore nonstandard experiments using cyclic voltammetry, microdisc electrodes and hydrodynamic electrodes such as rotating discs. 

Table 6.1 Linear sweep and cyclic voltammetry characteristics associated with the four categories (see text), where 5 is the size of the diffusion zone, Rb is the microdisc radius, d is the center-to-center separation, /p is the peak current, lum is the limiting current, and V is the scan rate [35].

An implementation of a basic microdisc program for one-electron cyclic voltammetry written in C- —p using the methods developed in this chapter is given in Appendix B. 

As discussed for the case of (hemi)spherical microelectrodes in Chapter 4, the response in cyclic voltammetry at microdiscs varies from a transient, peaked shape to a steady-state, sigmoidal one as the electrode radius and/or the scan rate are decreased, that is, as the dimensionless scan rate, a = Y r lv/TZTD, is decreased. The following empirical expression describes the value of the peak current of the forward peak for electrochemically reversible processes [11] ... 

As with the isolated microdisc simulations in Chapter 9, we here consider the simulation of the cyclic voltammetry of a simple fully reversible one-electron reduction. For an array, since each unit cell is identical, the concentrations of the electroactive species will necessarily be the same on either side of the cell boundary and there can be no flux of electroactive material across the boundary. After using the diffusion domain approximation, this boundary is at a distance r = Vd, therefore... 

Fig. 5 Theoretical limitations on ultrafast cyclic voltammetry. The shaded area between the slanted lines represents the radius that a microdisc must have if the ohmic drop is to be less than 15 mV and distortions due to nonplanar diffusion account for less than 10% of the peak current.

Fig. 6 Effect of scan rate on the cyclic voltammetry of 1.0 mM ferrocene at a 6.5-tim gold microdisc where the supporting electrolyte is 0.1 M tetrabutyl ammonium perchlorate in acetonitrile, (a) Scan rate is 0.1 V s and (b) scan rate is 10 V s . (Reproduced with the permission of the American Chemical Society from J. O. Howell and R. M. Wightman, Anal. Chem. 1984, 56, 524.)...

Alden JA, Hutchinson F, Compton RG (1997) Can cyclic voltammetry at microdisc electrodes be approximately described by one-dimensional diffusion J Phys Chem B 101 949-958... 

Lee HI, Beriet C, Ferrigno R, Girault HH (2001) Cyclic voltammetry at a regular microdisc electrode array. J Electroanal Chem 502 138-145... 

Guo, J. and Linder, E. (2009) Cyclic voltammetry at shallow recessed microdisc electrode theoretical and experimental study. J. Electroaml. Chem., 629,180. 

When diffusion layers overlap by a large amount, an overall planar response will be expected, but with a characteristic area equivalent to the total array surface area rather than just the electroactive surface area. Hence, the Case 4 current will be (1/ ) times larger than the Case 1 current. This will occur when X(jiff d where d is the separation of the individual microdiscs. Therefore, Case 4 behaviour arises at t 0.1 s. This will therefore be the dominant behaviour for cyclic voltammetry at normal scan rates at this particular array. With chronoamperometry, short timescales are accessible and so Case 3 behaviour may also be observed. 

Most of the practical and theoretical work on cyclic voltammetry has been based on the use of macroscopic sized inlaid disc electrodes. For this type of electrode, planar diffusion dominates mass transport to the electrode surface (see Fig. II. 1.13 a). However, reducing the radius of the disc electrode to produce a microdisc electrode leads to a situation in which the diffusion layer Aickness is of the same dimension as the electrode diameter, and hence the diffusion layer becomes non-planar. This non-linear or radial effect is often referred to as edge effect or edge diffusion . 

Cyclic voltammograms can be presented in an alternative format to that shown in Fig. 5 by using a time rather than potential axis, as shown in Fig. 8. The equivalent parameters in steady-state voltammetric techniques are related to a hydrodynamic parameter (e.g. flow-rate, rotation speed, ultrasonic power) or a geometric parameter (e.g. electrode radius in microdisc voltammetry). 

T. J. Davies, S. Ward-Jones, C. E. Banks, J. del Campo, R. Mas, F. X. Mnnoz, and R. G. Compton. The cyclic and linear sweep voltammetry of regnlar arrays of microdisc electrodes Fitting of experimental data, J. Elec-troanal. Chem. 585, 51-62 (2005). 

Davies TJ, Ward-Jones S, Banks CE, del Campo J, Mas R, Munoz FX, Compton RG (2(X)5) The cyclic and linear sweep voltammetry of regular arrays of microdisc electrodes fitting of experimental data. J Electroanal Chem 585 51-62... 

Davies TJ, Compton RG (2005) The cyclic tmd linetir sweep voltammetry of regular and random arrays of microdisc electrodes theory. J Electroanal Chem 585 63-82... 

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