Voltammetry at a Stationary Electrode

This method is one of the most used to characterize active masses. It quickly provides useful information about potential range of activity, capacity, cyclability, and kinetics. The result is a current versus potential (or versus time). Sweep voltammetry is easily conducted with commercially available potentiostat-galvanostat. Common sweep rates are in the range of 0.001-100 mV/s and common current densities from 0.01 to 10mA/cm2. Cyclic voltammetry is usually applied for estimating the reversibility of the electrochemical reaction. 

FI GU RE 1.6 First and second CVs of composite C-V2Os electrodes. (From Koltypin, M., et al., J. Electrochem. Soc., 154, A605, 2007. With permission.) 

Carbons for Electrochemical Energy Storage and Conversion Systems 

Increasing the sweep rate increases the sensitivity and the precision of the capacity measurement, but time-dependent phenomena may interfere with the measurement at higher sweep rate, especially resistive (in relation with the current intensity), electron transfer kinetics, and diffusion phenomena. Capacity measurements and redox potentials determination should be independent of the sweep rate. 

If the electrochemical reaction is controlled by diffusion, the peak height is proportional to the square root of the sweep rate. Integrating the current versus time may still give good approximations of the specific capacity if the current at the end of the sweep is sufficiently low. Forward and reverse current peaks are observed at different potentials, the difference depending on electron transfer kinetics. 

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As discussed in Section 2 material may reach the electrode surface by diffusion or convection. In cyclic voltammetry at a stationary electrode, and assuming that migration can be neglected, diffusion is the sole form of mass transport. However, material may additionally be transported to the electrode by convection. This genre of voltammetry, where convection is a dominant form of mass transport, is described as hydrodynamic voltammetry. The focus in Section 4 will be on the use of rotating disc and channel electrodes in studies... 

While the treatment of adsorption at the DME generally follows that for linear sweep voltammetry at a stationary electrode, it is complicated by the growth of the drop with time and the continuous exposure of fresh surface. In this case, the rate of mass transfer of reactant and product (see Section 13.5.3) and the rate of adsorption can affect the height of the adsorption wave. Although the first explanation of adsorption in voltam-metric methods and the explanation of prewaves and postwaves arose from the classic studies by Brdicka (48, 49), dc polarography is not the method of choice in the study of adsorption. Only a brief discussion will be given here more detailed treatments have appeared (33, 44, 50). 

The voltammetric method, developed at ORNL for determination the [U(1V)]/[U(111)] ratio in the MSBR fuel salt [47], is based on measuring of the difference between the redox potential of the melt Feq and 1/2, the voltammetric equivalent of the standard redox potential of the U(1V)AJ(111) couple, at [U(IV)] S> [U(Ill)]. In conditions of linear voltammetry, at a stationary electrode and a reversible charge transfer of the melt-soluble oxidized and reduced forms of uranium, is approximately equal to the polarographic half-wave potential 1/2 and corresponds to the potential in the voltammogram, at which the current accounts for 85.2% of the peak current. The relationship between eq, 1/2 and [U(lV)]/[U(in)] ratio is given by the Nernst equation ... 

The application of this technique (even in its various modes such as cyclic voltammetry) to other electrodes has already been mentioned in the description of LSV at the dme [Section 3.3.1.2.1(5)]. Especially with stationary electrodes LSV becomes fairly simple, under the conditions of sufficient solubility of ox and red, because of the constant and undisturbed electrode surface at an inert electrode the residual faraday current can be adequately eliminated by means of "J compensation (cf., Fig. 3.23) or by subtractive [cf., Section 3.3.1.2.1(3)] and derivative59 [cf., Section 3.3.1.2.1(4)] voltammetry at a stationary mercury electrode (e.g., HMDE), in addition to the residual faradaic current,... 

Polarography is the measurement of the current flowing at a dropping mercury electrode as the potential applied to this electrode is changed. Voltammetry is the measurement of the current flowing at a stationary electrode as the potential applied to this electrode is changed. 

Stirred-solution voltammetry utilizes current-voltage relationships that are obtained at a stationary electrode immersed in a stirred solution. In order to understand this aspect of electrochemistry, it is extremely useful to consider a typical current-voltage curve (voltammogram) in terms of the concept of concentration-distance profiles presented in the preceding section. The discussion will consider the potential, rather than the current, as the controlled variable. 

The main SECM program is involved in acquiring data as a function of position. However, the SECM program is enhanced by the ability to perform voltammetry (i vs. E) or other time-based routines at a stationary electrode. A DAC on the ADC board can generate a voltage sweep or step. Alternately, a trigger pulse starts an external generator. 

Figure 7.3.4 Schematic experimental arrangement for tast polarography. Staircase voltammetry is carried out at a stationary electrode with the same system except the drop knocker.

Square wave voltammetry is normally carried out at a stationary electrode such as an HMDE, and involves the waveform and measurement scheme shown in Figure 7.3.13. As in other forms of pulse voltammetry, the electrode is taken through a series of measurement cycles however there is no renewal of the diffusion layer between cycles. In contrast to NPV, RPV, and DPV, square wave voltammetry has no true polarographic mode. The waveform can be viewed as a special case of that used for DPV (Figure 7.3.9), in which the preelectrolysis period and the pulse are of equal duration, and the pulse is opposite from the scan direction. However, the interpretation of results is facilitated by considering the waveform as consisting of a staircase scan, each tread of which is superimposed by a symmetrical double pulse, one in the forward direction and one in the... 

This conclusion is very important because it implies that all relations and all qualitative conclusions presented in Section 10.5.1 also hold for linear sweep ac voltammetry of reversible systems at a stationary electrode. 

If a stationary electrode is used, such as platinum, gold, or glassy carbon, the technique is called voltammetry. One useful voltammetric technique is called stripping voltammetry, in which the product of a reduction is deposited on the surface on purpose and then stripped off by an oxidizing potential— a potential at which the oxidation of the previously deposited material occurs. This technique can also use a mercury electrode, but one that is held stationary. 

The technique of cyclic voltammetry is conveniently applied at these stationary electrodes [46], ITie electrode potential is scanned with time between two limits in a triangular fashion depicted in Figure 1.9. The scan rate can be between mV s ... 

Ebel et al. have used a microliter vessel in the voltammetry and polarographic determination of small sample volumes of chlorpromazine [166]. The concentration of cells in glass or PTFE was described for use with a dropping-mercury electrode (sample volume 180 pL), a rotating disc electrode (sample volume 1 mL), or a stationary vitreous-carbon electrode (sample volume 80 pL). Chlorpromazine was determined using oxidative voltammetry at a 3 mm vitreous-carbon or a rotating electrode. 

Fig. 7 Cyclic voltammetry of O2 in pyridine, at a stationary mercury drop electrode. Reference electrode aqueous SCE scan rate 45 mV s potential scale from —0.6 to —1.05 V (Reproduced with permission from Ref. 35).

The tast method is designed for a periodically renewed electrode, so it is not even conceptually applicable to a stationary electrode, such as a Pt disk or an HMDE. However, staircase voltammetry (35), based on closely related concepts, can find use at such electrodes. The experiment is outlined in Figure 7.3.2. The sampling time and cycle duration are no longer limited by the growth and fall of a mercury droplet hence one can vary them over a wide range. Times as short as microseconds are possible. One also has the freedom to vary A considerably. This parameter defines the density of current samples along the potential axis, thus it controls the resolution of the voltammetry. Of... 

Figure 3. Cyclic voltammetry with a stationary Pt working electrode in 0.05 M AgN03 and 3.25 M HNO3 at 21.5°C. The potential scan rate was 100 mV/sec. Potentials were measured relative to aPt reference in pure 3.25 M HNO3. The Ag(I)/Ag(II) redox couple is visible between 0.5 and 1.0 V.

Various means of trace accumulation and determination are used in stripping voltammetry. In the simplest case the analyte is reduced to the metal and accumulated as an amalgam at a stationary mercury electrode (HMDE or TMFE). The determination step proceeds in the reverse direction to the accumulation and is based on the anodic stripping of the metal (reoxidation). This process of ASV is also known as inverse voltammetry and can be illustrated as follows ... 

Masui M, Sayo H,Tsuda Y (1%8) Anodic oxidation of amines. Part I. Cyclic voltammetry of aliphatic amines at a stationary glassy-carbon electrode. J Chem Soc B 1968 973-976... 

To overcome some of the problems associated with aqueous media, non-aqueous systems with cadmium salt and elemental sulfur dissolved in solvents such as DMSO, DMF, and ethylene glycol have been used, following the method of Baranski and Fawcett [48-50], The study of CdS electrodeposition on Hg and Pt electrodes in DMSO solutions using cyclic voltammetry (at stationary electrodes) and pulse polarography (at dropping Hg electrodes) provided evidence that during deposition sulfur is chemisorbed at these electrodes and that formation of at least a monolayer of metal sulfide is probable. Formation of the initial layer of CdS involved reaction of Cd(II) ions with the chemisorbed sulfur or with a pre-existing layer of metal sulfide. 

In practical terms, large-scale cracking in the produced films, detrimental to photoelectric applications, was the main drawback of the above method. In order to prevent the appearance of cracks, propylene carbonate (PC) has been used as a solvent, with encouraging results [51]. The mechanism of electrodeposition of CdS in PC solutions containing Cd(II) ions and elemental sulfur has been studied by performing cyclic voltammetry at stationary Pt and Au electrodes [52]. 

At the beginning of Section 3.3 a distinction was made between voltammetric techniques with non-stationary and stationary electrodes the first group consists of voltammetry at the dme or polarography, already treated, and voltammetry at hydrodynamic electrodes, a later subject in this section however, we shall now first consider voltammetry at stationary electrodes, where it is of significance to state whether and when the analyte is stirred. 

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