Impedance data

Hitzig et al. have produced a simplified model of the aluminium oxide layer(s) to explain impedance data of specimens prepared under different layer formation and sealing conditionsThe model also gives consideration to the formation of active and passive pits in the oxide layer. Shaw et al. have shown that it is possible to electrochemically incorporate molybdenum into the passive film which, as previously noted, improves the pitting resistance. 

Electroreflectance data for pc-Cu579 confirm that the capacity minimum at E- -0.2 to -0.3 V (SCE) is due to the oxidation of the electrode surface. According to impedance data,564,565 as for pc-Ag and pc-Au,63 67 74 roughness factor for a pc-Cu electrode is approximately 2, which has been explained by the high surface inhomogeneity of the electrode surface. 

Using impedance data of TBN+ adsorption and back-integration,259,588 a more reliable value of <7 0 was found for a pc-Cu electrode574,576 (Table 11). Therefore, differences between the various EffM) values are caused by the different chemical states and surface structures of pc-Cu electrodes prepared by different methods (electrochemical or chemical polishing, mechanical cutting). Naumov etal,585 have observed these differences in the pzc of electroplated Cu films prepared in different ways. 

A diffuse-layer minimum in C,E curves has not been found with electrodes kept 3 min at E = -0.74 V, i.e., at a potential close to the rest potential of Fe.728 Complete cathodic reduction at <<-0.74 V (SCE) is not achieved since a diffuse-layer minimum is not found for cathodically reduced electrodes. This effect has been explained by the oxidation of Fe. According to impedance data, strong specific adsorption of Cl anions at renewed Fe electrodes occurs since a very large shift of Eosq takes place going from KF to KC1 solutions. 

An interesting special application has been proposed by Schlichthorl and Peter.31,41 It aims at deconvolution of electrochemical impedance data to separate space charge and surface capacitance contributions. The method relies on detection of the conductivity change in the semiconductor associated with the depletion of majority carriers in the space charge region via potential-modulated microwave reflectivity measurements. The electrode samples were n-Si(lll) in contact with fluoride solution. 

Impedance spectroscopy is best suited for the measurement of electronic conductivities in the range 10 -7to 10 2S cm 1.145 In principle, it is perhaps the best method for this range, but it is often difficult to interpret impedance data for conducting polymer films. The charge-transfer resistance can make measurements of bulk film resistances inaccurate,145 and it is often difficult to distinguish between the film s ionic and electronic resistances.144 This is even more of a problem with chronoamperometry146 and chronopotentiometry,147 so that these methods are best avoided. 

Electrochemical impedance spectroscopy techniques record impedance data as a function of the frequency of an applied signal at a fixed potential. A large frequency range (65 kHz-1 mHz) must be investigated to obtain a complete impedance spectrum. Dowling et al. and Franklin et al. demonstrated that the small signals required for EIS do not adversely affect the numbers, viability, and activity of microorganisms within a biofilm. EIS data may be used to determine the inverse of the corrosion... 

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. 

In particular, the coupling between the ion transfer and ion adsorption process has serious consequences for the evaluation of the differential capacity or the kinetic parameters from the impedance data [55]. This is the case, e.g., of the interface between two immiscible electrolyte solutions each containing a transferable ion, which adsorbs specifically on both sides of the interface. In general, the separation of the real and the imaginary terms in the complex impedance of such an ITIES is not straightforward, and the interpretation of the impedance in terms of the Randles-type equivalent circuit is not appropriate [54]. More transparent expressions are obtained when the effect of either the potential difference or the ion concentration on the specific ion adsorption is negli-... 

The impedance data were fitted to candidate electrical circuits using the non-linear weighted least-squares fitting program "EQIVCT" developed by Boukamp ( ). Graphical analysis was utilized to furnish reasonable first guesses of the circuit parameters for input to EQIVCT. 

There are several ways to present the Faradaic impedance data obtained at an electrode immobilized with an immunocomplex in the presence of a redox probe. For example, ZIm is plotted vs ZRe as a function of decreasing frequency to obtain a... 

To further understand and characterise the oxide deactivation process, a.c. impedance studies were carried out, primarily with a 30 at.% Ru/Ti electrode, at various stages during deactivation. These data were compared to those obtained for freshly formed Ru/Ti oxide films, ranging in Ru content from 5 to 40 at.%. Impedance data were collected at the oxide OCP (approximately 0.9 V versus SCE) in fresh NaCI solutions. Under these conditions, no chlorine reactions can occur and the OCP is defined by the equilibria of the redox states on the Ru oxide surface. Deactivation was generally accomplished by square-wave potential cycling, using overpotentials versus the chlorine/chloride potential of 1.59 to — 0.08 V (60 s cycle-1) in 5 M NaCI + 0.1 M HC1 solutions at room temperature. 

To understand what impedance is, learn its nomenclature, and understand the methodology involved in obtaining impedance data. 

Having introduced the concept of impedance, Z (and its nomenclature), the methodology involved in obtaining impedance data was outlined. It was seen that the most powerful way to interpret impedance data of an electroanalytical nature was to plot the imaginary impedance Z" (as y ) against the real impedance Z (as jc ), thus constructing a so-called Nyquist plot. 

The technique of constructing an equivalent circuit for impedance analysis represents the exception to the general rule that a chosen model can be almost certain to be correct. It is all too easy to compile an equivalent circuit which fits the impedance data, but is altogether wrong. In fact, many practitioners would say that impedance studies are so susceptible to this fitting to a bogus model that another technique should always be applied as a form of validation . It is much more unlikely for two techniques to fit a particular model, and the latter still be wrong ... 

Figure 10.2 shows data, both experimental and simulated, for hexacyanofer-rate(ii) ion at a gold disc electrode. The Warburg line at 45° indicates a diffusion process (see Section 8.2.2). The continuous line represents impedance data simulated by the Eco Chimie GPES software (as described below). Here, the fit between theory and experiment is seen to be very good. 

An adjunct within the GPES software allows for the manipulation of impedance data, thus leading to the rapid assembly of an equivalent circuit for mimicldng experimental data. 

It should also be mentioned that capacitors were then added in parallel with the resistors in equivalent circuit elements because the frequency-dependent experimental electrical impedance data had a component that was 90° out of phase with the resistor. 

Inconsistency of performance with a bulk path at low vacancy concentration. A quantitative comparison between predictions of the Adler model and impedance data for LSC shows the poorest agreement (underprediction of performance) at low temperatures, high F02. and/or low Sr content. These are the conditions under which the bulk vacancy concentration (and thus also the ionic conductivity and surface exchange rate of oxygen with the bulk) are the lowest. These are exactly the conditions under which we would expect a parallel surface path (if it existed) to manifest itself, raising performance above that predicted for the bulk path alone. Indeed, as discussed more fully in section 5, the Adler model breaks down completely for LSM (a poor ionic conductor at open-circuit conditions), predicting an... 

Surface path at low overpotential. Qualitative and quantitative analysis of impedance data, tracer studies, as well as various studies of thin-film electrodes suggest that under low-overpotential LSM operates primarily via a surface-mediated mechanism (like Pt). This conclusion appears to be consistent with the properties of LSM, which is fully oxygen stoichiometric under ambient Pq. However, little is known about how far the active region of reduction extends beyond the solid/solid interface (via surface diffusion) or the relative importance of chemical steps (on the LSM surface) vs electrochemical kinetics at the solid/solid interface. 

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