Voltage Scanning, Rate

Figure 6.9 Cyclic voltammetry of a Pt(llO) rotating disk electrode in a CO-saturated 0.1 M HCIO4 solution, (a) Influence of the voltage scan rate, (b) Influence of the disk rotation rate.

Moreover, each of the chemical and electrochemical reactions can have different reaction rates and reversibilities. All of them are reflected in cyclic voltammograms. If we measure cyclic voltammograms of an electrode reaction, changing parameters such as potential range, voltage scan rate, temperature, electrode material and solution composition, and analyze the voltammograms appropriately, we can obtain information about the electrode reaction. However, except for cases where the electrode process is very simple, it is not easy to analyze the cyclic voltammograms appropriately. 

The low-temperature electrochemistry technique is useful in the study of electrode reactions involving unstable products or intermediates. Lowering the temperature by 30-40 °C decreases the reaction rate of the unstable species to one-tenth of the original value. It is equivalent to a ten-fold increase in the voltage scan rate. Figure 8.22 shows the effect of temperature on the cyclic voltammo-gram for the oxidation of 1,2,3,6,7,8-hexahydropyrene. At ambient temperatures, it does not give a re-reduction peak Fiowever, at -60 °C, reversible oxidation and rereduction waves are observed. The techniques of low-temperature electrochemistry... 

In LSV experiments at stationary electrodes, there can be unwanted effects due to natural convection forced convection and a uniformly accessible electrode obviate this problem. The minimum voltage scan rate at which LSV effects appear (i.e. steady-state assumptions fail) will depend on the electrode kinetics and flow parameters. We can immediately identify two extreme situations. 

Figure 2. Current-voltage characteristics of the cell cond. glass/Pblg + RbAgJs/ RbAgsls/Ag + RbAgJg/cond. glass under chopped illumination (200 mW/cm2) at 25°C (voltage scan rate 25 mV/s)...

Fig. 11.1. Cyclic Voltammetry on Pt electrodes in 1.0 M H2S04. Voltage scan rate, vs = 100 mV s-1. Variation of potential of positive scan reversal basic curve curve in the presence of 11WEPN. Voltages of scan reversal UH (1) 1.4 V, (2) 1.6 V, (3) 1.7 V. Shown is the sixth cycle after starting the experimental sequence. Freshly prepared Pt electrode for the first sequence was at the most positive potential of scan reversal. T = 20 °C, N2 purging. (Reprinted from B. Wermeckes and F. Beck, Electrochim. >4cfa30 1491, copyright 1985, with permission from Elsevier Science.)...

H. Voltage Scan Rate, AC Frequency or Hold Time During Potential or Current Step... 

Capacitive effects cause hysteresis in small amplitude cyclic voltammogram current density-potential plots (16,31-34). Hysteresis in the current density-applied potential plot is brought about by combinations of high voltage scan rate, large... 

In separate but parallel efforts Gabrielli (32), Macdonald (31), and Townley (33) each discussed the choice of voltage scan rates for accurate Rp determination from small-amplitude cyclic voltammograms. 

In these equations a = Rs + Rp,b = RSRPC, t is time, v = sweep rate, and x = 1/RSC + 1 IRVC. Time in Eq. (19) can be equated to the sweep rate since t = A pP/v. All three equations include a term that is independent of voltage scan rate and a second term that depends on voltage scan rate. The scan-rate-dependent term becomes negligible at low scan rates. Macdonald (31) and Townley (33) separately derived the current response of the standard three-element electrical equivalent circuit (Fig. 3a) to a small-amplitude triangular voltage excitation... 

Implicit in the above is the notation that current-voltage curves measured at macroelectrodes for all but fast voltage scan rates are characterized by a mass transport limited current plateau rather than a current peak as in linear sweep voltammetry at a planar electrode of larger than micro dimensions. 

Fig. 17. Slow cyclic current-voltage curves for natural graphite CPP, A = 0.5 cm, in 18 m H2SO4 voltage scan rate =0.1 mV/s, potential range...

---