Impedance electrode

Impedance Some of the errors arising from the use of linear polarisation resistance led to interest and development in a.c. systems.An early development used a fixed a.c. frequency and a commercial instrument was produced in the UK. Inaccuracies still occurred, however, and were due to the electrode impedance which is fequency dependent. Electrode reactions have a capacitance component, in addition to resistance, resulting in a requirement to measure the impedance. However, the total impedance comprises values for the reaction, solution, diffusion and capacitance. Measurements at different frequency are more reliable, particularly where high solution resistances occur. Simplifications for industrial monitoring have been developed consisting of two measurements, i.e. at a high (10 kHz) and low frequency (0-1 Hz). The high-frequency measurement can identify the... 

The measurement of the electrode impedance has also been ealled Faradaie impedanee method. Since measurements are possible by applying either an electrode potential modulated by an AC voltage of discrete frequeney (which is varied subsequently) or by applying a mix of frequencies (pink noise, white noise) followed by Fourier transform analysis, the former method is sometimes called AC impedance method. The optimization of this method for the use with ultramicroelectrodes has been described [91Barl]. (Data obtained with these methods are labelled IP.)... 

FIGURE 12.15 Electrode impedance with kinetic (a), diffusional (b), and combined (c) reaction control (W is the Warburg impedance). 

The use of impedance electrochemical techniques to study corrosion mechanisms and to determine corrosion rates is an emerging technology. Electrode impedance measurements have not been widely used, largely because of the sophisticated electrical equipment required to make these measurements. Recent advantages in microelectronics and computers has moved this technique almost overnight from being an academic experimental investigation of the concept... 

The impedance can be measured in two ways. Figure 5.23 shows an impedance bridge adapted for measuring the electrode impedance in a potentiostatic circuit. This device yields results that can be evaluated up to a frequency of 30 kHz. It is also useful for measuring the differential capacity of the electrode (Section 4.4). A phase-sensitive detector provides better results and yields (mostly automatically) the current amplitude and the phase angle directly without compensation. 

Kinetic parameters can also be obtained by using the zero-point method as described earlier.40 The advantage of this method is that the values of a and k° can be deduced independent of the determination of values of the double-layer capacitance, electrode impedance, and potential difference across the electrode/solution... 

CN ions. Anodic dissolution of silver electrode in cyanide solutions and also the behavior of Ag at potentials preceding dissolution have been studied applying electrode impedance measurements [381]. At potentials of anodic dissolution, the process was represented by the equivalent circuit with two parallel branches. 

Guidelli and associates showed (236) by electrode impedance measurements that acrylonitrile is coadsorbed, into an adsorbate layer of these cations,... 

Figure 9.1 Equivalent circuit of an electrochemical cell. A, Auxiliary electrode R, reference electrode W, working electrode Rc, compensated resistance R , uncompensated resistance Rr, reference electrode impedance Zf, faradaic impedance Cdl, doublelayer capacitance.

In (5.27), the second term is the real part of the electrode impedance ZRe and the third term is its imaginary part Z. ... 

This impedance was considered as a parallel connection of impedances of the non immersed conductor and the immersed electrode. Thereby, both the active (Rs) and the reactive (Xs) component of electrode impedance were defined as... 

Fig. 3. The frequency dependence of Pt L electrode impedance active component. L 1-0.5M K2S04 2- 0.1M K3Fe(CN)6 / 0.1M K4Fe(CN)6 3- 0.5M H2S04.

Fig. 5. The frequency dependence of Pt 0.5M H2S04 electrode impedance reactive component at relative length of extended conductor electrode parties equal to 1- 1.0 2- 0.8 3-0.64 4- 0.52 5- 0.40.

Fig. 6. The dependence of the specific value of Pt 0.1M K3Fe(CN)6 /0.1M K4Fe(CN)6 electrode impedance active component depending on the relative length of the extended...

Fig. 22 a, b. Electrode impedance spectrograph obtained from a platinum electrode (a Value b phase)... 

Figure 41. Impedance of an SrTi03 single crystal (a) and an SrTiO bicrystal (b). In both cases reversible YE CiriOe. electrodes were used,230 such that electrode impedances are absent. Reprinted from J. Jamnik, X. Guo, and J. Maier, Appl. Phys. Lett., 82 (2003) 2820-2822. Copyright 2003 with permission from the American Institute of Physics.

Figure 54 shows a two-point experiment on an AgBr bicrystal using Ag electrodes. The low-frequency response is solely due to the internal interface. (Figure 35 shows another example for SrTi03.) The distinction with regard to electrode impedance is possible via four point measurements but also via an evaluation of the capacitance as described in Section III.2. 

Evidently, the adequate description of electrode impedance in the case of the actual contact between the surface-modified semiconductor and electrolyte represents a very complicated problem owing to the appearance of some hardly measured parameters... 

Hence, in simple cases each bulk layer, each grain boundary plane, and both electrodes of the brick layer model sample, can be represented by separate RC elements (Fig. 7b). The RC elements of the n bulk layers can be combined to a single RC element with the -fold resistance and the 1 / -fold capacitance of a single layer. The n — 1 grain boundary impedances can also be summed, as can the two electrode impedances, and hence the model sample corresponds to a series connection of three RC elements (Fig. 7c) with... 

The impedance spectroscopy is most promising for electrochemical in situ characterization. Many papers have been devoted to the AB5 type MH electrode impedance analysis [15-17]. Prepared pellets with different additives were used for electrochemical measurements and comparing. Experimental data are typically represented by one to three semicircles with a tail at low frequencies. These could be described to the complex structure of the MH electrode, both a chemical structure and porosity [18, 19] and it is also related to the contact between a binder and alloy particles [20]. The author thinks that it is independent from the used electrolyte, the mass of the electrode powder and the preparing procedure of electrode. However, in our case the data accuracy at high frequencies is lower in comparison with the medium frequency region. In the case, the dependence on investigated parameters is small. In Figures 3-5, the electrochemical impedance data are shown as a function of applied potential (1 = -0.35V, 2 = -0.50V and 3 = -0.75V). 

In LEIS measurements, the working electrode is under potential control in a three-electrode cell. The pseudoreference electrode pair is then brought close to the sample surface to measure the local AC current density. A key assumption behind LEIS is that in the potential field near a working electrode surface, the AC solution current density is proportional to the local electrode impedance, and at any given measurement frequency, to, the current density in solution is... 

For the sake of simplicity usually the charging and the Faraday processes are treated independently, however, it is justified only in certain cases. This approximation is valid if a high excess of supporting electrolyte is present, i.e., practically only nonreacting ions build up the double layer at the solution side. In modelling the electrode - impedance almost always an - equivalent circuit is used in that the - double-layer impedance and the -> faradaic impedance are in parallel which is true only when these processes proceed independently. 

The -> impedance of a planar capacitive electrode immersed in a resistive solution conforms to that of a serial R-C circuit. If the geometry of the capacitive electrode is some fractal, then the electrode impedance, Z(o>), is a - CPE, Z = const ( to) n, and the exponent n - depending on the actual form of the fractal - is some function of Df. 

By the successive substitution of values for T. and Ck into the equation, the total electrode impedance Z can be obtained. 

In principle, three-electrode EIS measurement is capable of separating the contributions from the anode and the cathode [61], In a three-electrode EIS experiment, as shown in Figure 5.43, the potentiostat, with three electrode probes, is applied to measure signals between the working electrode (WE) and the reference electrode (RE). The counter electrode (CE) is used to collect the induced current from the WE. For a fuel cell, if the anode serves as a WE, the cathode will serve as a CE, and vice versa. In this way, the individual electrode impedances can be determined independently. 

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