Potential-Sweep Experiments

The resulting voltammogram thus has a sigmoidal (wave) shape. By increasing the stirring rate (U), the diffusion layer thickness becomes thinner, according to 

Initially it was assumed that no solution movement occurs within the diffusion layer. Actually, a velocity gradient exists in a layer, termed the hydro-dynamic boundary layer (or the Prandtl layer), where the fluid velocity increases from zero at the interface to the constant bulk value (U). The thickness of the hydrodynamic layer SH is related to that of the diffusion layer 

FIGURE 1-3 Planar (a) and spherical (b) diffusional fields at spherical electrodes. 

FIGURE 1-5 Concentration profiles for two rates of convection transport low (curve a) and high (curve b). 

In this section we consider experiments in which the current is controlled by the rate of electron transfer (i.e., reactions with sufficiently fast mass transport). The current-potential relationship for such reactions is different from those discussed (above) for mass transport-controlled reactions. 

FIGURE 1-6 The hydrodynamic boundary (Prandtl) layer. Also shown (as dotted line), is the diffusion layer. 

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The range of potentials over which potential sweep experiments are carried out in aqueous solutions is from around 0.00 to 1.5 V on the NHS. This is to avoid hydrogen evolution on the negative side and Oz evolution on the positive side (pH 0). If then, one had a steady current, the limitingly slow sweep rate would be one that covers 1500 mV in 27 s. On this basis, the lowest sweep rate should be about 50 mV s"1. 

Residual currents, also referred to as background currents, are the sum of faradaic and nonfaradaic currents that arise from the solvent/electrolyte blank. Faradaic processes from impurities may be practically eliminated by the careful experimentalist, but the nonfaradaic currents associated with charging of the electrode double layer (Chap. 2) are inherent to the nature of a potential sweep experiment. Equation 23.5 describes the relationship between this charging current icc, the double-layer capacitance Cdl, the electrode area A, and the scan rate v ... 

Potential sweep experiments (linear sweep voltammetry and cyclic voltammetry)... 

Fig. 30. I/U plots obtained during CO oxidation on a rotating Pt (110) electrode in 0.1 M HCIO4 electrolyte solution (CO saturated) for different scan rates (a) during potential sweep experiments (to = lOOrpm) and (b) during current scans. (Reproduced with permission from M. T. M. Koper, T. J. Schmidt, N. M. Markovik, and P. N. Ross, J. Phys. Chem. B 105 (2001) 8381, (2001) American Chemical Society.)...

The problem, in the case of linear potential sweep experiments, is... 

Current-potential measurements, in the dark and under illumination of the semiconductor working electrode, are extremely useful for first defining the charge-transfer behavior across the interface before more sophisticated experiments are undertaken. The irradiation can be either continuous or intermittent (chopped) the latter mode has the distinct advantage that both the dark and light behavior can be examined in the same scan [55, 58]. Even some dynamic information can thus be extracted under the nominally steady-state conditions typical of a cyclic or linear potential sweep experiment. Another useful steady-state experiment is photocurrent spectroscopy (performed at a fixed DC potential) [55], although this can also be dynamically performed via IMPS (see below). Such measurements not only yield the so-called photoaction spectrum of the semiconductor electrode, but also afford information on surface recombination and surface state activity at the interface as discussed below. 

A denotes the starting material that is oxidised (or reduced) at the electrode (e.g. Ru(bpy)3+ oxidation or MV2+ reduction on a Pt electrode) and it is subsequently reformed by a catalyst in a pseudo first order reaction that concomitantly forms a product P. P, in our case, is 02 (or H2). On a planer electrode, from the limiting current in the case of pure catalytic control (i.e. at remote enough a potential after the peak in a potential sweep experiment or at t - in an experiment where the potential is fixed sufficiently past the E° of the redox couple), kca, can be calculated using the expression,... 

For a potential step experiment at a stationary, constant-area electrode, the charging current dies away after a time equivalent to a few time constants (/ uCd) (see Section 1.2.4). Since the potential is continuously changing in a potential sweep experiment, a charging current, /c always flows (see equation 1.2.15) ... 

Analysis of experimental linear potential sweep experiments according to (6.7.19) or the equivalent expression for a totally irreversible one-step, one-electron reduction S << 1),... 

Potential sweep experiments were performed on film nickel hydroxide electrodes in an effort to determine the redox mechanism. The over-all reaction for a-nickel hydroxide was found to be ... 

MAC] Mac Arthur, D. M., The hydrated nickel hydroxide electrode potential sweep experiments, J. Electrochem. Soc., 117, (1970), 422-426. Cited on pages 118, 343. 

The role of the reference electrode in electrochemical studies is to provide a fixed potential which does not vary during the experiment. In most cases, the potential of the reference electrode relative to an agreed standard, for example to the normal hydrogen electrode, is required. In other cases it is only necessary for the reference electrode to remain at the same potential during the experiment, for example during a linear polarization resistance or a potential sweep experiment. 

Firstly, if the current vs. potential curve generated in a potential sweep experiment is of the irreversible type, and corresponds only to the electron transfer step, then rate constant information can be extracted from the behavior by standard procedures. Secondly, and of more interest, if a chemical step is coupled before or after it, then the magnitude of currents observed will depend on the ratio of the rate constants for the electrochemical and chemical steps, and the relation between the rate constant of the ratecontrolling step and the time scale of the experimental measurement, i.e., the potential sweep rate. 

Fig. 6.3 - Concentration-distance profiles for the electroactive species, 0, in the reaction O e R during a linear potential sweep experiment. The curves...

Hadzi-Jordanov S, Angerstein-Kozlowska H, Conway BE (1975) Surface oxidation and H deposition at ruthenium electrodes Resolution of component processes in potential-sweep experiments. J Electroanal Chem Interfacial Electrochem 60 359-362... 

Since we are considering a potentiodynamic linear potential sweep experiment, then we have... 

In a potential sweep experiment (see Chapter 11) the output is the current, which accounts for both the faradaic and non-faradaic processes. Since the scan rate, v [V sec ] is known, it is possible to calculate the non-faradaic current, inf [A] ... 

Potential sweep experiments involving adsorbed species... 

Quantitatively, the voltammetric response of reactions involving adsorption is ranark-ably sensitive and without any special effort or experimental precautions, the voltanuno-gram indicates the reduction or oxidation of a very small number of molecules, as low as a fraction of a monolayer. Qualitatively, the position and shape of the peaks give a signature, the voltammetric fingerprint, of the surface processes (Figure 11.9). Overall, this makes potential sweep experiments highly suited to the study of redox processes with adsorption steps. 

In a linear potential sweep experiment performed on a RDE, the potential of the working electrode is scanned from a potential where no reaction occurs to a potential that causes a reaction to occur. A limiting current is achieved when the overpotential is high enough so that the reaction rate is determined by the mass transport rate of the reactant at a given electrode rotation rate. The surface concentration of the reactant drops to zero, and a steady mass transport profile is attained as C/L, where L is the diffusion layer thickness. At a fixed electrode rotation rate, L does not change, and thus C/L does not change. Therefore, a steady-state diffusion-controlled current is achieved, described by the Levich equation ... 

In either the ECE or DISP 1 case, the generation of FH" is rate-limiting. The current is therefore controlled by the transport of F to the surface and its protonation, but not by the exact mechanism by which the second electron is transferred. This is because in a potential sweep experiment it is impossible to probe the potential region between the reduction potential of FH and the reduction potential of F, under conditions where FH is present in solution. 

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