Normal pulse voltammograms

Figure 17.6 (a) Reverse normal pulse voltammogram, (b) current measured prior to the analysis pulse in (a), and (c) normal large-amplitude pulse voltammogram at a platinum electrode for a 13.4 mM solution of Ti(IV) in the 60-40 mol% AlCl3-l-methyl-3-ethylimidazolium chloride melt at 25°C. [From Ref. 68, with permission.]... 

Figure 17.7 Reverse normal-pulse voltammogram pulse train used to produce wave (a) in Figure 17.6.

Fig. 3.20 Normal Pulse Voltammograms for the reduction of 6.9 x 10 4 M tetracyanoquino-dimethane (TCNQ) in acetonitrile with 0.10 NBu4NPF6 at 293 K (platinum disc electrode with diameter 0.31 cm). Pulse duration 0.050 s. Lines are simulations with the following input parameters Ef = —0.107 V, t Ef = —0.551 V, kJ = 104 cm s-1, a = 0.5, a% = 0.35, k = 6.5 x 10-3 cm s-1, diffusion coefficients of neutral, anion, and di-anion are 1.44 x 10 5, 1.35 x 10 5, and 9.1 x 10 6 cm2 s-1, respectively. Reproduced from reference [38] with permission...

Before studying the influence of the different kinetic parameters on the single potential step or normal pulse voltammograms corresponding to these three reaction mechanisms, it is of great interest to point out some features of these curves, which can be directly deduced from the equations presented in the previous sections corresponding to the dependence of the limiting current and of the half-wave potential with the characteristic parameters when diffusion coefficients of species B and C are assumed equal. 

As an example of this type of study, Fig. 2 shows some back-step corrected normal pulse voltammograms obtained by Wightman s group [4] for three different substances injected into brain tissue. It can be seen that quite well-defined waves are obtained, whereas the respective cyclic voltammograms would be poorly defined. 

The left side of (7.3.8) is the height of the whole reverse pulse voltammogram, which is found now to be the same as the height of the normal pulse voltammogram taken with the same timing characteristics. 

At successive potential pulses the current increases accordingly until the concentration of the substrate at the electrode surface approaches zero. Then the current reaches a plateau. A typical experimental normal pulse voltammogram is presented in Fig. II.2.5. [Tris-(9,10-dioxo-l-anthryl)]trisaminoacetyl amine (TDATAA) has been taken as an illustration. A corresponding cyclic voltammogram is added to the figure to serve as a reference. Apparently, TDATAA is reduced in two one-electron steps. It is easier to draw this conclusion in the case of NPV, since both waves are easily found to be of very similar height. 

Normal pulse voltammetry is very sensitive to adsorption of both the substrate and the product at the electrode surface. If the substrate is adsorbed, a peak-shaped voltammogram is obtained. The stronger the adsorption or the lower the concentration of the substrate or the shorter the pulse time, the better defined is the peak. This is illustrated in Fig. II.2.6 by the normal pulse voltammograms of imida-zoacridinone. This compound, at -0.4 V, is strongly adsorbed at the mercury surface, therefore the two consecutive reduction steps result in the formation of two peaks instead of two waves. 

Fig. II.2.6 Normal pulse voltammograms at a hanging mercury drop electrode in 5 x 10 M imidazoacridinone (substrate adsorbs at electrode surface). Pulse time a 2, b 5, c 10, d 25, e 50, and/100 ms. Instrumental drop time 1 s. (Adapted from [8] with permission)...

A typical shape of a reverse pulse voltammogram is presented in Fig. II.2.8. The corresponding normal pulse voltammogram is added to the figure as a reference. The DC part of the RP voltammogram, /dc, is positive as is the normal pulse current, tNP (Ox + ne Red). The reverse pulse current, Irp, is of opposite sign, since the product of the electrode reaction is oxidized now. 

These comparisons show that our PEDOT/PSS supported catalysts are as active for methanol oxidation as the best polymer supported catalysts r >orted in the literature. However, a question that must be answered is vdiether polymer supported catalysts can provide superior performance to commercially available carbon supported catalysts. To answer this, the PEDOT/PSS supported catalyst used for the experiments in Fig. 5, was compared with a commercial 0 -TEK) carbon supported Pt-Ru alloy catalyst Fig. 6 shows normal pulse voltammograms at 60° C, while Fig. 7 shows the results of constant potential experiments (at 22 °C) over a much longer time period. In both experiments, and over all timescales studied, the carbon supported catalyst delivers currents that are as much as 10 times higher than for the polymer supported catalyst. The only conditions under which the polymer supported catalyst is superior are at short times and high potentials, which are not relevant to fuel cell operation. 

Figure 5. Normal Pulse voltammogram (20 s step time 10 s pulse width) for methanol (IMin IM H O/aq)) oxidation at glassy carbon rotating (1500 rpm) disc eleclrwies coated with 35%Pi-Ru (0.10 mg cmr 1.3 1 Pt.Ru) on PEDOT/PSS in a Nafion matrix.

Fig. 11. Differential normal pulse voltammograms obtained from various solutions as indicated with pretreated carbon fiber electrodes. —0.2 V, Xq 0.5 s, AE 30 mV (from ref. [55]).

The electron transport by electron self-exchange can be regarded as a kind of diffusion process, and therefore represented by the diffusion coefficient. The apparent diffusion coefficient, D pp, for the charge transport in the polymer film was obtained from the slope of the cathodic limiting current in the normal pulse voltammogram (him) vs (x the sampling time) plots by using the Cottrell equation (Eq. (10)),... 

For the heterogeneous electron transfer between the electrode and the redox sites confined in the film, the standard rate constant, k°, is derived from the current-potential relationship in the normal pulse voltammograms by using Eq. (11)... 

Fig. 14. Modified plots of normal pulse voltammograms for the oxidation of (A) W(CN)J" and (B) IrCl " incorporated into a protonated PVP film. Sampling time (t) (1)1, (2)2, (3)4, (4) 10 ms. Other conditions are the same as in Fig. 13...

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