Focused impedance measurements

Sometimes more specialized results can be achieved by using more than four electrodes Rabbani et al. (1999) combined two orthogonal four-electrode systems to achieve a more localized zone of measurement sensitivity in what he called focused impedance measurements. Kwon et al. (2012) showed that a system using 16 miniature electrodes was able to outperform the four-electrode system in sensitivity and impedanee estimation. With this method, they were also able to recover anisotropic properties from the measured object. 

After this step, the understanding of microwave electrochemical mechanisms deepened rapidly. G. Schlichthorl went to the laboratory of L. Peter to combine potential-modulated microwave measurements with impedance measurements, while our efforts focused on laser pulse-induced microwave transients under electrochemical conditions. It is hoped that the still relatively modest knowledge provided will stimulate other groups to participate in the development of microwave photoelectrochemistry. 

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 steady-state and dynamic properties of the Au(lll)/aqueous electrolyte interface were investigated experimentally with a variety of electrochemical and structure sensitive methods. Examples are impedance measurements [13], electroreflectance [14], LEED [14,15] SHG [16], in-situ surface X-ray scattering [17] and STM [18-20] experiments. In the following paragraph we will only focus on some essential results of our in-situ STM and SEIRAS studies [21,22]. 

In a first step we determined the impedimetric behavior of NETs. With some constrains, neutrophils in cell medium model a wound environment. We used commercial interdigitated electrode arrays (Roche xCelligence) equipped with wells for cell culture. This disposable device is meant to have no chemical interaction between the gold electrodes and the standard cultivation medium during impedance measurements. The electrodes were connected via a self-made adapter to an impedance analyzer (ScioSpec IX-3). The experiments were carried out with the standard settings of the device at 12.5 mV amplitude at frequencies from 100 Hz to 1 MHz. First, we measured cell cultures, where we focused on the reaction of the... 

With increasing interest in time-resolved impedance measurements but also with the demand of parallel measurements, fast methods based on time domain approach move more and more into the focus. Although time and frequency domain are well defined, they are often not clearly presented. Especially, when the impedance spectrum changes with time, a joint analysis in terms of time and frequency dependence is often accompanied by uncertainties in wording. 

Standard test procedures are defined within ASTM standards ASTM G 59, Practice for Conducting Potentiodynamic Polarization Resistance Measurements G 5, "Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements G 106, Practice for Verification of Algorithm and Equipment for Electrochemical Impedance Measurements and G 102, Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements. Each of these methods describes a standard procedure or practice for the test method. A complete discussion of the technologies is beyond the scope of the current text. For the current text, the focus is on the application of the most simple and most widely used of these techniques, the polarization resistance measurement, ASTM G 59. The parameters discussed are, however, applicable concerns for all electrochemical tests. 

Impedance measurement can be considered a third way to evaluate electrochemical sensors besides potentiometry and amperometry. Electrochemical impedance studies in a narrower sense deal with phenomena at the electrode surface. The overall impedance of a chemosensor also includes effects of charge carrier properties far from the electrode. This was visualized by equivalence circuits presented in Chaps. 2 and 5. By individual experimental design, the study can be focused more on processes at the electrode surface or otherwise on ion properties in homogeneous solution. Even the variation of the dielectric constant in a layer will affect the overall impedance. If impedimetry is designed only to acquire data corresponding to ionic properties or value of the dielectric constant, it is not really an electrochemical method, in a strict sense. 

One limit of behavior considered in the models cited above is an entirely bulk path consisting of steps a—c—e in Figure 4. This asymptote corresponds to a situation where bulk oxygen absorption and solid-state diffusion is so facile that the bulk path dominates the overall electrode performance even when the surface path (b—d—f) is available due to existence of a TPB. Most of these models focus on steady-state behavior at moderate to high driving forces however, one exception is a model by Adler et al. which examines the consequences of the bulk-path assumption for the impedance and chemical capacitance of mixed-conducting electrodes. Because capacitance is such a strong measure of bulk involvement (see above), the results of this model are of particular interest to the present discussion. 

Finally, transient absorption measurements were deemed necessary to confirm the photoproducts in 21a,b and 22a,b. Due to overlapping absorptions of Cso, oFL and ZnP, which would impede a clear analysis, we have focused first on the selective excitation of ZnP. To this end, transient absorption spectra of the reference compounds (19 and 20a,b) reveal the instantaneous formation of the ZnP singlet excited state with maxima at 460 and 800 nm and minima at 565 and 605 nm. Furthermore, an isosbestic point at 500 nm as it develops on a time scale of 3000 ps reflects the intersystem crossing process at which end the triplet excited state of ZnP stands. The latter includes maxima at 530, 580 and 640 nm (Fig. 9.57a). Equally important is the fact that the decay of the singlet excited state matches the formation of the triplet excited state kinetics (Fig. 9.57b). 

This set of experiments has focused on the use of two nondestructive electrochemical techniques to measure polarization resistance and thereby estimate the corrosion rate. In addition, the effects of scan rate and uncompensated ohmic resistance were studied. Three main points should have been made by this lab (1) Uncompensated ohmic resistance is always present and must be measured and taken into account before Rp values can be converted into corrosion rates, otherwise an overestimation of Rv will result. This overestimate of Rp leads to an underestimate of corrosion rate, with the severity of this effect dependent upon the ratio Rp/Ra. (2) Finite scan rates result in current shunted through the interfacial capacitance, thereby decreasing the observed impedance and overestimating the corrosion rate. (3) Both of these errors can be taken into account by measuring Ra via EIS or current interruption and by using a low enough scan rate as indicated by an EIS measurement in order to force the interfacial capacitance to take on very large impedance values in comparison to Rp. 

Such measurements tended to focus attention on the vertical dimension. After so many hours staring down a wire, oceanographers were naturally inclined to feel that the answers lay directly below. But recently emphasis has shifted. Physical studies have demonstrated the strong impedance to... 

Experimental systems used for electrochemical measurements should be selected to take maximum advantage of well-imderstood phenomena such as mass transfer so as to focus attention on the less-understood phenomena such as electrode kinetics. For example, the study of electrochemical reactions in stagnant environments should be avoided because concentration and temperature gradients give rise to natural convection, which has an effect on mass transfer that is difficult to characterize. It is better to engage in such experimental investigations in systems for which mass transfer is well defined. To simplify interpretation of the impedance data, the electrode should be uniformly accessible to mass transfer. 

In recent years, several technologies such as white noise stimulation, hydrodynamic focusing and trapping arrays have been implemented within single cell impedance microfluidic cytometry to achieve broadband spectroscopy, improvement in sensitivity and continuous time course measurements. 

The measurement of the cell characteristics in a bridge yields values of R and Cb that in series are equivalent to the whole cell impedance, including the contributions from Rq and Q, which are often not of interest in studies focused on the faradaic process. In general, one desires to separate the faradaic impedance from Rq and Q. It is possible to do so by considering the frequency dependencies of R and Cb, or by evaluating Rq and from separate experiments in the absence of the electroactive couple. Techniques for making such determinations are considered in Section 10.4. For the moment, let us assume that the faradaic impedance, expressed as the series combination R and Cg, is evaluable from the total impedance (see Figure 10.1.14). 

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