Potentiostatic current-time transients

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

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.)...

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... 

They presumably are repassivated pits. About 0.1% of the pits grow to a size of some few 100 mn with an elongated irregular shape parallel to the steps. These pits are seen as lepassivated, i.e., metastable pits and grow when current transients are observed in the potentiostatic current-time curves. 

Chronoamperometry Chronoamperometry involves the study of the variation of the current response with time under potentiostatic control. Generally the working electrode is stepped from a potential at which there is no electrode reaction to one corresponding to the mass-transport-limited current, and the resulting current-time transient is recorded. In double-step chronoamperometry, a second step inverts the electrode reaction and this method is useful in analysing cases where the product of the initial electrode reaction is consumed in solution by a coupled homogeneous chemical reaction. 

Diagnostic Relationships Between Current, Maximum Current, and Time. Scharifker and Hills (26) developed a theory that deals with the potentiostatic current transients for 3D nucleation with diffusion-controlled growth. According to this theory, the theoretical diagnostic relationship in a nondimensional form is given by... 

Fig. 29. Electrodeposition of Ag from 0.017 M AgCN + 0.92 M KCN + 0.11 M K2CO3 solution dimensionless analysis of experimental potentiostatic current transients (/, and tm are the current and time corresponding to the maximum on the current transient curve, respectively). Upper curve calculated for the instantaneous nucleation mechanism lower curve, for the progressive nucleation mechanism. Different symbols/experimental points relating to different potentials [136], Reproduced by permission of The Electrochemical Society, Inc.

Potential or current step transients seem to be more appropriate for kinetic studies since the initial and boundary conditions of the experiment are better defined unlike linear scan or cyclic voltammetry where time and potential are convoluted. The time resolution of the EQCM is limited in this case by the measurement of the resonant frequency. There are different methods to measure the crystal resonance frequency. In the simplest approach, the Miller oscillator or similar circuit tuned to one of the crystal resonance frequencies may be used and the frequency can be measured directly with a frequency meter [18]. This simple experimental device can be easily built, but has a poor resolution which is inversely proportional to the measurement time for instance for an accuracy of 1 Hz, a gate time of 1 second is needed, and for 0.1 Hz the measurement lasts as long as 10 seconds minimum to achieve the same accuracy. An advantage of the Miller oscillator is that the crystal electrode is grounded and can be used as the working electrode with a hard ground potentiostat with no conflict between the high ac circuit and the dc electrochemical circuit. 

Fig. 12.8. Potentiostatic current transient (d) for the reduction of Au/quartz modified electrode with Os,(bpy)2py-PAA hydrogel in 0.1 M KNO, 50 mM TRIS buffer of pH 7.2 and 0.1 M glucose, for a potential step from 0 V to 0.56 V V (SCE) (c) and simultaneous time evolution of XLf (a) and Rf (b) components of the quartz impedance at 10 MHz.

Gabrielli, C., Maurin, G., Perrot, H., Poindessous, G. and Rosset, R. (2002) Investigation of electrochemical calcareous scaling potentiostatic current- and mass-time transients. J. Electroanal. Chem. 538/539, 133-143. 

Schrebler et al. studied the nucleation and growth mechanisms for Re deposition on polyerystalline Au electrodes, from a bath containing 0.75 mM perrhenic acid and 0.1 M sodium sulfate at pH = 2. The potentiostatic step technique was simultaneously employed with measurements of mass changes in an electrochemical quartz-crystal microbalance. The mass vs. time transients were fitted with equations deduced from the current versus time relationships of the conventional nucleation and growth models. It was concluded that electrodeposition of Re started with progressive nucleation and two-dimensional growth, followed by two other contributions ... 

Current vs. time transients Membranes with 1.77 cm area were placed between two aqueous solution in an electrochemical cell designed for ionic transport measurements (76,78). Desired voltage steps were applied by a Solatron 1250 FRA through an EG G 363 potentiostat controlled by a nith XT computer. Current vs. time data were collected by the same computer using a Data Translation DT 2801 board. 

Figure 11a shows some potentiostatic current transients for iron in solutions with and without Q . Below the pitting potential, the two curves are coincident. Above the pitting potential, after a certain anodization time, the two curves are seen to deviate. 

The deposition can be conducted in potentiostatic or in galvanostatic conditions. In the former case, it is possible to monitor the time course of the deposition and the progressive filling of the pores by analyzing the time transient current. As shown in... 

Figure 8.17. Potentiostatic transient of C2H4 oxidation on Pt/Ce02. Rate and current responses to step changes in catalyst potential, UWR) are plotted against time. T = 500°C, p02 = 5.5 kPa, PC2H4= 1 5 kPa.71 Reproduced by permission of The Electrochemical Society.

For the individual types of transient measuring techniques, special names exist but their terminology lacks uniformity. The potentiostatic techniques where the time-dependent current variation is determined are often called chronoamperometric, and the galvanostatic techniques where the potential variation is determined are called chronopotentiometric. For the potentiodynamic method involving linear potential scans, the term voltammetry is used, but this term is often used for other transient methods as well. 

The principle of this method is quite simple The electrode is kept at the equilibrium potential at times t < 0 at t = 0 a potential step of magnitude r) is applied with the aid of a potentiostat (a device that keeps the potential constant at a preset value), and the current transient is recorded. Since the surface concentrations of the reactants change as the reaction proceeds, the current varies with time, and will generally decrease. Transport to and from the electrode is by diffusion. In the case of a simple redox reaction obeying the Butler-Volmer law, the diffusion equation can be solved explicitly, and the transient of the current density j(t) is (see Fig. 13.1) ... 

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