Faradaic impedance measurement

A modification of faradaic impedance measurement is a.c. polarography, where a small a.c. voltage is superimposed on the voltage polarizing the dropping mercury electrode (Fig. 5.181). 

In the faradaic impedance measurements described above (Section 10.1.1), the impedance determined by the bridge is a whole-cell impedance and includes contributions from the counter electrode s interface. Processes at the counter electrode are not usually of interest hence the impedance at that interface is intentionally reduced to insignificance by employing a counter electrode of large area. 

Smyrl, W. H., "Digital Faradaic Impedence Measurements on Corroding Copper in Acid Solutions, Electrochemical Corrosion Testing, ASTM STP 727, 1981, pp. 198-214. 

The electrode reaction of Cr / CyDTA has been studied in detail by Tanaka et al. [lb]. They report a value of kob = 0.029 cm s K We have tried to estimate this kob in the pH-range 4 to 8 where E is independent of pH. Using cyclic voltammetry (CV) and faradaic impedance measurements we confirm that kob is smaller than 0.1 cm s and a (charge transfer coefficient) < 0.4. Up to now we are unable to get exact values for these constants because a chemical pre-equilibrium of unidentified origin interferes with the charge transfer process. Further work is underway to clarify this point. 

A number of other methods (chronopotentiometry [24], polarography with dropping electrolyte electrode [25], faradaic impedance measurement [26], current scan voltammetry [27] etc.) were also applied to the study of electrolysis at ITIES. 

Tokuda K, Gueshi T, Matsuda H (1979) Voltammetry at partially covered electrodes. Part III. Faradaic impedance measurements at model electrodes. J Electroanal Chem 102 41 8... 

Figure 9.16 Nyquist diagram for the Faradaic impedance measurements corresponding to the electrodes (a) Au (b) Au/TA (c) Au/TA/CA (d) Au/TA/CA/BSA and (e) Au/TA/GA/BSA/ Ab-MNP (Reproduced from [94], with permission from Elsevier)...

Figure 14.5 Nyquist plots vs. Z J for Faradaic impedance measurements in the presence of 0.1 mol/l phosphate buffer solution containing I.Ommol/IKj[Fe(CN)J/KJFe(CN)J (1 1 mixture) and 0.1 mol/l KNO at (a) bare Au electrode, (b) dithiol/Au, (c) nano-Au/dithiol/Au, (d) AMOZ-Ab/nano-Au/dithiol/Au, (e) BSA/AMOZ-Ab/nano-Au/dithiol/Au, and (f) AMOZ/BSA/AMOZ-Ab/nano-Au/dithiol/Au. The concentration of AMOZ was 1. Ox ICP ng/ml. The frequency range is from 10 to 10 Hz with a signal amplitude of lOmV. From Ref [24] with permission from Elsevier...

Transient measnrements (relaxation measurements) are made before transitory processes have ended, hence the current in the system consists of faradaic and non-faradaic components. Such measurements are made to determine the kinetic parameters of fast electrochemical reactions (by measuring the kinetic currents under conditions when the contribution of concentration polarization still is small) and also to determine the properties of electrode surfaces, in particular the EDL capacitance (by measuring the nonfaradaic current). In 1940, A. N. Frumkin, B. V. Ershler, and P. I. Dolin were the first to use a relaxation method for the study of fast kinetics when they used impedance measurements to study the kinetics of the hydrogen discharge on a platinum electrode. 

Under potentiostatic conditions, the photocurrent dynamics is not only determined by faradaic elements, but also by double layer relaxation. A simplified equivalent circuit for the liquid-liquid junction under illumination at a constant DC potential is shown in Fig. 18. The difference between this case and the one shown in Fig. 7 arises from the type of perturbation introduced to the interface. For impedance measurements, a modulated potential is superimposed on the DC polarization, which induces periodic responses in connection with the ET reaction as well as transfer of the supporting electrolyte. In principle, periodic light intensity perturbations at constant potential do not affect the transfer behavior of the supporting electrolyte, therefore this element does not contribute to the frequency-dependent photocurrent. As further clarified later, the photoinduced ET... 

The impedance data have been usually interpreted in terms of the Randles-type equivalent circuit, which consists of the parallel combination of the capacitance Zq of the ITIES and the faradaic impedances of the charge transfer reactions, with the solution resistance in series [15], cf. Fig. 6. While this is a convenient model in many cases, its limitations have to be always considered. First, it is necessary to justify the validity of the basic model assumption that the charging and faradaic currents are additive. Second, the conditions have to be analyzed, under which the measured impedance of the electrochemical cell can represent the impedance of the ITIES. 

Figure 34. Total impedance (2) and faradaic impedance (Zf) of unsintered porous LSM on YSZ measured in air at 850 °C. Electrolyte resistance (i ) has been subtracted from the total impedance, while both i e and the double-layer capacitance (Cdi) have been subtracted from Zp. (Reprinted with permission from ref 215. Copyright 1998 Elsevier.)...

Label-free detection of ligand-aptamer interaction was also demonstrated by means of impedance spectroscopy technique [52,53]. Simultaneously, Radi et al. [52] and Rodriguez et al. [53] reported application of Faradaic impedance spectroscopy (FIS) in detection of interaction of proteins with DNA aptamers. The detection method is based on the measurement of resistance in presence of redox mediator Fe(CN)6-In absence of target protein, the negatively charged aptamer repulse the redox mediator molecules from the sensor surface. In a paper by... 

The faradaic impedance, Zf, and the total impedance how to calculate Zf from experimental measurements... 

We have discussed in the above sections Faradaic impedance and the correlation between Faradaic impedance and kinetic parameters. In general, one desires to separate the Faradaic impedance from Rel and Cd. Now we will focus on the extraction of Zf and the kinetic parameters from direct impedance measurements. This is based on the transformation between equivalent circuits in series and equivalent circuits in parallel. 

The Faradaic impedance is linked in parallel to the double-layer capacitance Cdi and then to the solution resistance Re as illustrated in Figure 9.1. The double-layer capacitance is in general considered to be constant. It is frequently observed, however, that Cdi changes with the dc current at which the impedance measurements were carried out. TTie capacitance Cdi has been observed, for example, to increase with increasing current density. 

Fig. 20.6. Experimental set-up for the current or voltage measurements. (B) Schematic representation of two (electrode/solution) interfaces coupled in series through the solution. C j double-layer capacitance R, Faradaic impedance of each interface Rj,...

VanderNoot69has studied poorly separated faradaic and diffusional processes. He has found that a complex, nonlinear, least-squares regression is capable of extracting kinetic information from impedance measurements when the ratio of the charge-transfer process time constant tf=... 

Experimentally measured ac current or total admittances are functions of the electrode potential. Figure 17 presents the dependence of the total admittances of a process limited by the diffusion of electroactive species to and from the electrode and the kinetics of the charge-transfer process, on the electrode potential. Information on the kinetics of the electrode process is included in the faradaic impedance. It may be simply... 

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