Potential sweeps steady state experiments

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. 

The scan rate, u = EIAt, plays a very important role in sweep voltannnetry as it defines the time scale of the experiment and is typically in the range 5 mV s to 100 V s for nonnal macroelectrodes, although sweep rates of 10 V s are possible with microelectrodes (see later). The short time scales in which the experiments are carried out are the cause for the prevalence of non-steady-state diflfiision and the peak-shaped response. Wlien the scan rate is slow enough to maintain steady-state diflfiision, the concentration profiles with time are linear within the Nemst diflfiision layer which is fixed by natural convection, and the current-potential response reaches a plateau steady-state current. On reducing the time scale, the diflfiision layer caimot relax to its equilibrium state, the diffusion layer is thiimer and hence the currents in the non-steady-state will be higher. 

The time factor in stepwise potentiostatic or potentiodynamic polarisation experiments is very important, because large differences can be caused by changes in the scanning rate. Since the steady state depends on the particular system and conditions of exposure, no set rule exists for the magnitude or frequency of potential changes. Chatfield etal. have studied the Ni/H2S04 system and have shown how becomes more passive with increase in sweep rate. 

In a typical voltammetric experiment, a constant voltage or a slow potential sweep is applied across the ITIES formed in a micrometer-size orifice. If this voltage is sufficiently large to drive some IT (or ET) reaction, a steady-state current response can be observed (Fig. 1) [12]. The diffusion-limited current to a micro-ITIES surrounded by a thick insulating sheath is equivalent to that at an inlaid microdisk electrode, i.e.,... 

A steady state is independent of the details of the experiment used in attaining it. Thus, under conditions where a steady state is attained, e.g., under convective conditions in an - electrochemical cell, the application of a constant current leads to a constant potential and similarly the application of a constant potential leads to the same constant current. Voltammetric steady states are most commonly reached using linear potential sweeps (or ramps) in a single or cyclic direction at a UME or RDE. A sigmoidally shaped current (l)-potential (E) voltammogram (i.e., a steady-state voltammogram) is recorded in the method known as steady-state voltammetry as shown in the Figure. Characteristics of the... 

More detailed studies are required in order to check whether the above correlation, established on the basis of relatively rapid potential sweep measurements, holds also for the steady-state photocurrents, i.e., in the situation when the Ti02 surface becomes covered with the peroxo-titanate species. These should also include water photocleavage experiments onto titanium dioxide powders loaded with some of the catalysts investigated by Contractor and Bockris. The difficulty, associated with the fact that most of... 

Experiments are performed at an RRDE with the following dimensions r = 0.20 cm, r2 = 0.22 cm, T3 = 0.32 cm. A disk voltammogram (/q vs. Eq) is to be recorded for a rotation rate of 2000 rpm. What maximum potential sweep rate should be employed to prevent non-steady-state effects from occurring What is the transit time with this electrode ... 

The polarization curve can also be obtained by the potential sweep method, employing a potentiostat and a function generator at low sweep rates of about 20mVmin . However, preliminary experiments at various sweep rates are recommended to obtain a quasi-steady-state polarization curve, which is close to the true steady-state curve. 

There are in fact two slightly different types of non-steady state technique. In the first an instantaneous perturbation of the electrode potential, or current, is applied, and the system is monitored as it relaxes towards its new steady state chronoamperometry and chronopotentiometry are typical examples of such techniques. In the second class of experiment a periodically varying perturbation of current or potential is applied to the system, and its response is measured as a function of the frequency of the perturbation cyclic and a.c. voltammetry are examples of this type of approach. In both cases the rate of mass transport varies with the time (or frequency), and by obtaining data over a wide range of these variables and by using curve fitting procedures, kinetic parameters are obtainable. Pulse techniques will be discussed later in this chapter, whilst sweep methods are described in Chapter 6 and a.c. methods in Chapter 8. 

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 the case of continuous CO oxidation CO gets re-adsorbed on the smface of the electrode after the electrode surface is cleaned upon deep anodic excursion, which leads to a steady state due to continuous CO supply [79]. Recorded RDE polarization emv es show dependence on the potential sweep rate and the electrode rotation rate (not only in the region of diffusion control). Onset for CO oxidation is always higher in the experiments of continuous CO oxidation than in CO stripping voltammetry measurements due to self-poisoning effect. For complete overview of theory of electrocatalytic oxidation of CO the reader is referred to couple excellent existing reviews [77, 79],... 

Choosing a proper potential sweep rate (v) in pipette voltammetry is essential for attaining a steady-state and sufficiently low charging current. Computer simulations and experiments showed that ion diffusion on either side of the nano-ITIES reaches a steady state dnring a potential cycle at a moderate The related dimensionless parameter g= a lAD.,) z,FvlRT) com-... 

When the sweep rate is very low, in the range of v = (0.1—5) mVs , measurement is conducted under quasi-steady-state conditions. The sweep rate plays no role in this case, except that it must be slow enough to ensure that the reaction is effectively at steady state along the course of the sweep. This type of measurement is widely used in corrosion and passivation studies, as we shall see, and also in the study of some fuel cell reactions in stirred solutions. Reversing the direction of the sweep should have no effect on the current-potential relationship, if the sweep is slow enough. Deviations occur sometimes as a result of slow formation and/or reduction of surface oxides or passive layers. Because the sweep rate is slow, the potential is often swept only in one direction, and the experiment is then referred to as linear sweep voltammetry (LSV). 

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