Current-time transients

Figure 3.38 (a) Current-time transient during the adsorption of methanol from solution at 0.330 V vs, Pd/H in 0.01 M CH.,OH + 0.5 M H2SOA. (b> Cyclic voltammograrn showing the oxidation of the adsorbate after replacing the solution by pure supporting electrolyte. 10 mV s, same solution as in (a). From Iwasita et at. (1987). 

Fig. 19. Sampled-current voltammogram constructed from the current-time transients that resulted from a series of potential-step experiments at a stationary Pt electrode in a 35.0 x 10 3 mol L-1 solution of Ni(II) in the 66.7 m/o AlCl3-EtMeImCl melt ( ) total current, ( ) partial current for the electrodeposition of Ni, (O) partial current for the electrodeposition of Al. The total current was sampled at 3 s after the application of each potential pulse. Adapted from Pitner et al. [47] by permission of The Electrochemical Society. 

Cyclic voltammetry (CV) curves were recorded on a Kipp and Zonen BD91 X-Y recorder and the current-time transients resulting from the potential step experiments were recorded digitally. The apparatus used included a PAR Model 173 potentiostat and an IBM XT computer... 

Figure 6. A plot of the logarithm of the square root of the initial slope of the current time transients determined in the double step experiments against the final potential for the three single crystal surfaces investigated.

Figure 13. Plot of current against time, (a), (b), and (c) Current-time transients on Pt(l 11) in 0.5M H2SO4, when CO was endorsed at potentials of 0.08.0.3, and 0.5 V(RHE) of curve 3 of (d), respectively. When CO was ptesent in the solution, curves I and 2 were observed. The sweep rate was 50 mV s". (From Ref. 24.)...

It can be seen that Eqs. (7.9) and (7.10) represent the same type of current-time transient, iocl. Thus, to distinguish between 2D growth (progressive nucleation) and 3D growth (instantaneous nucleation), it is necessary to perform additional optical microscopic or electron microscopic experiments. These experiments can provide information enabling one to distinguish between progressive nucleation [Eq. (7.9)] and instantaneous nucleation [Eq. (7.10)]. 

Figure 7.5. Theoretical current-time transients for instantaneous and progressive nucleation.

At higher overpotentials the nucleation rate increases faster than the step (Chapter 3) propagation rate, and the deposition of each layer proceeds with the formation of a large number of nuclei. This is the multinuclear multilayer growth. Armstrong and Harrison (13) have shown that initially, the theoretical current-time transient for the two-dimensional nucleation (Fig. 7.7) has a rising section, then passes through several damped oscillations, and finally, levels out to a steady state. 

Figure 7.7. Potentiostatic current-time transient for the metal deposition together with theoretical currents for individual layers (1-5). Two-dimensional progressive nucleation taking overlap into account. (From Ref. 13, with permission from the Electrochemical Society.)...

Figure 7.7 also shows the theoretical i-t transients for the formation of successive layers under conditions of progressive nucleation. The theoretical current-time transient for three-dimensional nucleation is shown in Figure 7.8. The difference between 2D and 3D nucleation (Fig. 7.7 and 7.8) is in the absence of damped oscillations in the latter case. A comparison between the theoretical and experimental transients for the 2D polynuclear multilayer growth is shown in Figure 7.9. 

Figure 7.10. Theoretical potentiostatic current-time transient, including the effect of overlap. (From Ref. 27, with permission from Elsevier.)...

Deposition of mercury at boron-doped diamond (BDD) and platinum electrodes has also been studied [33]. Deposition and oxidation of mercury was performed by cyclic voltammetry from the solution of 1 mM Hg2 ( 104)2 in 1 M Na l04. In order to learn more about this deposition, it was carried out also under chronoamperometric conditions. The results obtained are shown in Fig. 2 in the form of dimensionless current-time transients. Experimental curves obtained at two different overpotentials were compared with the theoretical curves calculated for instantaneous and progressive nucleation. A good agreement of experimental plots with the instantaneous nucleation mechanism was... 

Fig. 2 Comparison of the experimental dimensionless current-time transients for electrodeposition of mercury onto boron-doped diamond electrode with the theoretical transients for instantaneous (upper curve) and progressive (lower curve) nucleation overpotentials (x) 0.862 V and ( ) 0.903 V (from Ref 33).

The contours correspond to the RMS deviation between the experimental and simulated current-time transients... 

In a detailed investigation of the kinetic behaviour of bases generated from (fluoren-9-ylidene)methane derivatives the problem has been overcome by computer simulation of current-time transients expected for the extended mechanism (including reaction 5), The program used for the comparison of simulated and experimental curves allows both kp and k to vary independently until the RMS deviation between the two i/t curves is minimised. The equilibrium constant for reproportionation (kf/kj) is calculable from values of Ep, (1) and Ep (2). It is important to realise that there may be any number of pairs of values of k and k which can give a good fit between experimental and simulated i/t curves. 

It is possible to distinguish between these two modes of nucleation experimentally, such as by the use of potentiostatic current-time transients (discussed in Section 7.7). 

Figure 8.9. Potentiostatic current-time transients of a Pt electrode in electroless copper solution showing effect of NaCN E = -900 mV. (From Ref. 50, with permission of the Electrochemical Society.)...

For high values of k°r, very sharp decays of the current-time transients are observed, indicating the almost immediate electrochemical conversion of oxidized species (see solid lines corresponding to k°r = 100). Indeed, for k°t > 100, the faradaic conversion is so fast that the oxidized species disappears at the very first instants of the experiment and under these conditions 0p = 0. When k°r decreases, the observed currents also decrease, since the rate constant modulates the whole faradaic current. For k°t < 1, the current transients appear as quasi-linear, with current-time profile being shifted toward more negative potentials. Under these conditions, general equation (6.130) becomes identical to Eq. (6.134), corresponding to irreversible processes. 

Recently a series of dialkylpyrrolidinium (Pyr+) cations have been studied in our laboratory 7-9). These cations are reduced at relatively positive potentials and could be investigated electrochemically as low concentration reactants in the presence of (C4H9)4N+ electrolytes. Using cyclic voltammetry, polarography and coulometry, it was shown that Pyr+ react by a reversible le transfer. The products are insoluble solids which deposit on the cathode and incorporate Pyr+ and mercury from the cathode. Both the cation and the metal can be regenerated by oxidation. Quantitative analysis of current-time transients, from potential step experiments, showed that the kinetics of the process involve nucleation and growth and resemble metal deposition. 

Figure 5.28 Current response for a 10 pm radius mercury microelectrode immersed in a 5 pM solution of adriamycin, following a potential step from —0.700 to —0.350 V the supporting electrolyte is 1.0 M perchlorate at a pH of 4.5. The inset shows the semi-log plot for data between the marks on the current-time transient, with the time axis being referenced to the leading edge of the potential step. From R. J. Forster, Analyst, 121, 733-741 (1996). Reproduced by permission of The Royal Society of Chemistry...

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