Ultrafast voltammetry

When the electrochemical properties of some materials are analyzed, the timescale of the phenomena involved requires the use of ultrafast voltammetry. Microelectrodes play an essential role for recording voltammograms at scan rates of megavolts-per-seconds, reaching nanoseconds timescales for which the perturbation is short enough, so it propagates only over a very small zone close to the electrode and the diffusion field can be considered almost planar. In these conditions, the current and the interfacial capacitance are proportional to the electrode area, whereas the ohmic drop and the cell time constant decrease linearly with the electrode characteristic dimension. For Cyclic Voltammetry, these can be written in terms of the dimensionless parameters yu and 6 given by... 

Ultrafast voltammetry has a crucial role for investigating the kinetics of very fast reactions, although for very small sizes the kinetics would be masked since the system would be under diffusion control. 

The heterogeneous electron transfer dynamics of a diverse range of organic and inorganic species and also the dynamics and energetics of ultrafast heterogeneous electron transfer dynamics of immobilized electroactive species on an electrode surface have been investigated with ultrafast voltammetry under a wide variety of experimental conditions of timescale, temperature, solvent, and electrolyte (see for example Fig. 5.16, obtained from [54]). 

Howell, J.O., and Wightman, R.M. 1984. Ultrafast voltammetry and voltammetry in highly resistive solutions with microvoltammetric electrodes. Analytical Chemistry 56, 524-529. 

Minimization of time scale measurements Ultrafast undistorted cyclic voltammetry may be performed at ultramicroelectrodes using an ultrafast potentiostat allowing on-line ohmic drop compensation. 

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.

Rees NV, Dryfe RAW, Cooper JA, Coles BA, Compton RG, Davies SG, McCarthy TD (1995) Voltammetry under high-mass transport conditions -a high-speed channel electrode for the study of ultrafast kinetics. J Phys Chem 99 7096-7101... 

Figure 6.1.4.2 Theoretical limitations on ultrafast cyclic voltammetry. The shaded area between the slanted lines represents the radius that a microdisk 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, (a) Without iR drop compensation by positive feedback, and (b) with 90 and 99% ohmic drop compensation. The dotted area in (a) and (b) represent the regions where transport within the double layer affects the voltammetric response. Limits are indicative and correspond approximately to a 5-mM anthracene solution in acetonitrile, 0.3 M tetrafluoroborate as supporting electrolyte. [Reproduced by permission of Marcel Dekker from C. Amatore, Electrochemistry at Microelectrodes, I. Rubenstein, Ed., 1995, Chapter 4, p. 198.]...

The potential drop across the electrolyte solution is determined by the product of current intensity and ionic resistance. In a microelectrode, the ionic resistance is independent of the distance to the other electrode, which allows working in solutions with low ionic conductivity. In addition, the ionic resistance is proportional to the inverse of the radius of the electrode. Since the intensity is proportional to the radius for steady-state conditions, the iR drop is not dependent on the size of the microelectrode. However, at nonsteady-state conditions, that is, e.g., in ultrafast cyclic voltammetry, the intensity is proportional to the area, and thus the iR drop is proportional to the radius of the microelectrode. In other words, the iR drop decreases as the size of the microelectrode decreases under nonsteady-state conditions. 

Indeed, specially synthesized frans-vitamin D that immediately forms AR2 gave a reversible wave [203]. It was the first example where the cis- and frans-isomeriza-tion caused a slowing down of the preceding e stage. This stage r should be very fast in accordance with the formula ultrafast chemical reaction is the cause of slow electron transfer [204] (Sect. 9.2.2, Cathodic and Gas-Phase Reactions of the Bond Cleavage ). Thus, it could not be tracked by the high-speed cyclic voltammetry (CVA, v < 1000 V sec ) and by the HSCP [202], The major reaction product is dihydrovitamin D (III), but dihydrotachysterol (IV) is also formed, which is effective in hypercalcemia (Sect. 9.2.7, Dihydrotachysterol ). 

Amatore C, Maisonhaute E, Simonneau G (2000) Ultrafast cychc voltammetry performing in the few megavolts per second range without ohmic drop. Electrochem Commun 2(2) 81-84. doi 10.1016/sl388-2481(99XX)150-2... 

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