Capacitive charging

In the second case, when the circuit has a low p.f, or carries a capacitive charging current a condition which may occur in the following cases (for ease of analysis we have classified them into two categories) ... 

On I./2 kV networks the MVAr loading is light, as most of the p.f. is controlled at the distribution level and the capacitive charging MVAr demand is low (Table 24.2, column 9). The charging MVAr is normally not compensated because, on load, more than this is olTsei hy the load p.f. 

Calcium fluoride, solid electrolyte, 92, 420 Capacitance charging current, 2... 

The discussion on the capacitive charging of conductive polymers continues 362 - 364) poiiowing Feldberg s hypothesis the capacitive charge should be proportional to the amount of oxidizable film whereby it is assumed that oxidation of the film occurs at one defined redox potential All available experimental findings are unequivocal evidence that the latter assumption is wrong. In a recent... 

The most commonly used waveform for in vivo voltammetric measurements is square-wave. This involves the application of a potential pulse to the working electrode for a fixed time at fixed intervals. The current is measurai at the end of the potential pulse to minimize capacitive charging current contributions. This waveform is shown in Fig. 15 A. 

As the field of electrochemical kinetics may be relatively unfamiliar to some readers, it is important to realize that the rate of an electrochemical process is the current. In transient techniques such as cyclic and pulse voltammetry, the current typically consists of a nonfaradaic component derived from capacitive charging of the ionic medium near the electrode and a faradaic component that corresponds to electron transfer between the electrode and the reactant. In a steady-state technique such as rotating-disk voltammetry the current is purely faradaic. The faradaic current is often limited by the rate of diffusion of the reactant to the electrode, but it is also possible that electron transfer between the electrode and the molecules at the surface is the slow step. In this latter case one can define the rate constant as ... 

In addition, from the analysis of the ac impedance spectra obtained from the previously pitted surfaces under open circuit conditions at room temperature in 0.5 M Na2S04 solution51 and from the in-situ pitted surfaces in Cl ion-containing thiosulfate solution at various applied potentials and solution temperatures,52 they verified that dr>ss is inversely proportional to the CPE exponent for capacitive charging process (Eq. 30)51 and the depression parameter for charge transfer process.52... 

In a CV measurement, the current output always contains two components the Faradaic current, /F, due to the reaction of the redox species and the capacitive charging current, /c, which results from the charging of the electrode double layer and the diffusion layer. (This diffusion layer contains all charged and polar species in the solution and therefore differs from that of the redox species.) While /F changes linearly with vm as determined by diffusion, Ic is directly proportional to v as shown below, where CD is the total electrode capacitance and q the added capacitance charge ... 

This means that with increasing scan rate or lowering the solution concentration, the effect of lc will increase. Because a peak-shaped CV can only be obtained at a sufficiently high scan rate, the effect of electrode capacitance charging limits the CV application in low-concentration solutions. SWV has been developed to overcome this problem and to increase the quantitative accuracy of voltammetric techniques. The concentration for recording a SW voltammogram can be as low as a hundredth of that for recording a CV. 

Another advantage of SWV over CV can be seen when dealing with a separate multi-electron transfer reaction. The CV current wave of each or each group of electrons always contains the contribution from the previous electron transfer, particularly the diffusion-controlled current. Separating currents from different electron transfers can be tedious, if not impossible. It can be even worse when we have to take into account the capacitive charging current. Since both capacitive and diffusion-controlled currents are absent or at least minimized on the 7net vs E curve of an SW voltammogram, current waves from each electron transfer are much better resolved and more accurate information can be obtained. 

In some cases, the kinetics of the redox charge— discharge reactions can proceed almost as quickly and reversibly as EDL charging. Thin film redox electrodes, based on the lithium intercalation/inser-tion principle such as Li4Ti50i2, exhibit high reversibility and fast kinetics. The Ru02 materials deposited on carbon show pseudo-capacitive charge—... 

In these experiments, Au(lll) electrode was immersed in 0.1 M KOH solutions with 10 pM, 100 pM and 1 mM CioSH and kept at the constant potential of -FO.l V for a certain time. After that, the potential was scanned to more negative values in order to record the cathodic peak current. These and other experiments have proved that the saturated coverage is obtained after a sufficiently long holding time. The same applies to the cathodic peak potential also. Sumi and Uosald [125] have found that the maximum reaction charge is equal to 103 ( 5%) pC cm . Two components contribute to the measured total charge the first is produced in reaction (1), and the second is the capacitance charge... 

Sottomayor etal. [273] have investigated adsorption of n-hexanol on Au(lll) electrode from 0.2 M NaCl04 aqueous solution, applying chronocoulometry to measure dynamic capacitance and capacitive charge. They have found that in this case, adsorption may be described by the Frumkin isotherm and have determined the corresponding parameters. The obtained results were discussed and... 

Adsorption of -hexanol on Ag(lOO) and Ag(llO) from aqueous 0.05 M KCIO4 solutions has been studied by Foresti et al. [59], who performed capacitive charge measurements and compared the results with those obtained for Ag(lll). The calculated adsorption free energy (at the potential of maximum adsorption) was = —17.7 kJ mol for Ag(lOO),... 

In Section (8.6.5) it was argued that the value of the sweep rate must be less than that which would cause the capacitative charging current to be significant compared with the total current and greater than a value that in the potential range of the sweep would cause 8t to reach a value at which natural convection would cause the breakdown of the relation 8t = VjiDt. 

Fig. 9.23 Same results shown in Figure 9.22 with expanded timebase for load decrease transient. This figure more clearly shows the short term capacitive charging for the double layer.

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