High frequency impedance

Results are similar for films deposited on YSZ however, there appears to be a difference between films deposited on ceria vs YSZ in terms of interfacial electrochemical resistance. As shown previously in Figure 6c, LSC films on YSZ often exhibit a second high-frequency impedance associated with oxygen-ion exchange across the electrode/electrolyte interface.That this difference is associated with the solid—solid interface has been confirmed by Mims and co-workers using isotope-exchange methods. As discussed in greater detail in sections 6.1—6.3, this interfacial resistance appears to result from a reaction between the electrode and electrolyte, sometimes detected as a secondary phase at the interface. 

A notable exception may be Sr-doped ceria. As reported by Liu and Wu, LSCF electrodes on 10 mol % Sr-doped ceria exhibit a significant high-frequency impedance arc in air at 650—750 °C, which is comparable in frequency (but somewhat smaller) to that observed for identically processed and tested LSCF electrodes on 8 mol % YSZ. One explanation may be found in the thermodynamic data for the Sr— Ce—O system, which shows that the solubility of SrO in Ce02 is less than 10 mol % at these temperatures.Thus, precipitation of SrCeOs at the LSCF/ceria interface is favored, depending on the exact firing conditions and A/B ratio of the perovskite. Oddly, the authors did not consider this possibility. 

The addition of an electromagnetic shield to the body of the tip can greatly reduce the stray capacitance. This is particularly useful for fast-scan voltammetry [54] and high frequency impedance experiments [55],... 

The curves showing the frequency dependence of loss functions [tan 5, G"(g)), or / (to)] permit the detection in the frequency domain, at temperatures just slightly above the glass transition temperature, of a prominent absorption or a process. The unavailability of experimental devices to measure mechanical viscoelastic functions at high frequencies impedes the detection of a fast process or P relaxation in the high frequency region. This latter process is usually detected in the glassy state at low frequencies. 

With reference to the two-electrode method (Fig. 6.21), why is the low-frequency impedance equal to 2Rp + Rs and the high-frequency impedance equal to Rs ... 

Despas et al. [63] described the use of high-frequency impedance measurements to study the reactivity of sifica gel and Stober silica with respect to molecular and ionic bases of various sizes and strengths in aqueous solution. The experimental setup has been previously described [64]. With this technique it is possible to distinguish between silanol and silanolate groups. Stober silica shows a different behavior compared to silica gel due to the microporosity present in the former. 

The locus diagram (Fig. 8) reveals that the impedance characteristic is completely different from the dry case. The addition of the humidity lowers the apparent impedance (i.e., the modulus of the impedance) at the lower frequencies. In contrast, the high-frequency impedance is all but unchanged by the addition of humidity. 

Sanyo introduced electrolytic capacitors in 1983 based on tetracyanoquinodimethane (TCNQ). Since that times several other capacitors have been introduced into the marketplace. Polypyrrole, polyaniline and polythiophene have all been used. These capacitors range in values from 0.1 pF to 200 pF and have low equivalent series resistance and high frequency impedance with good reliability and lifetimes. 

In the case of blocking electrodes, the impedance increases to infinity as the frequency approaches zero. In such cases, approximation with the Voigt circuit is not appropriate. When a high-frequency impedance is finite, the easiest way to verify the Kramers-Kronig compliancy is to fit the impedances to the admittance representation of the circuit containing a ladder of (RC) element series (Fig. 13.4) [575]. In addition, capacitance, Cq, or inductance can be added in parallel. 

Fig. 16.3 Setup for high-frequency impedance measurements WEI working electrode, WE2 high-surface-area Ft electrode. Potential is applied and ctnrent measured by FRA one electrode is polarized by potentiostat (From Ref. [686], copyright (2012), with permission fiom Elsevier)...

Hence the high-frequency impedance of a reduced nonconducting polymer and the low-frequency capacitance of an oxidized polymer both depend linearly on layer thickness (via the N factor). The linear dependence of capacitance on thickness is of considerable diagnostic importance in model verification, since we usually expect the total capacitance of a thin film to be inversely proportional to film thickness. We can explain the prediction in Eqn. 438 by noting that low-frequency capacitance is confined to pore walls, which has its major axis orientated at right angles to the electrode surface. Thus the deeper the pore, the more capacitors there are in parallel in the pore wall and consequently the greater the total capacitance. 

Figure 8.23 to Figure 8.25 show two full-range impedance plots and the high frequency parf of all three. We note that the high frequency impedance... 

An electrical equivalent circuit model, composed of a resistor and a capacitor which were connected in series, was adopted to represent the soymilk-electrode system. High frequency impedance measurements presented an increase in resistance and constant value in reactance for different coagulation times. This resistance is attributed to the bulk resistance of soymilk mixture and changed minimally when the coagulation process is finished [50]. 

Gabrielli, C., Huet, F., Keddam, M., Rousseau, P, Vivier, V. Scanning electrochemical microscopy imaging by means of high-frequency impedance measurements in feedback mode. J. Phys. Chem. B 2004, 108, 11620-11626. 

Fast screening of materials for their catalytic activity has gained popularity in the area of polymer electrolyte fuel cells. The method involves preparing a combinatorial library of various candidate catalysts of different composition and a quick evaluation of a key electrochemical parameter such as open-circuit potential or high-frequency impedance. This enables a quick screening of an array of materials to identify... 

Figure 5 Ranges of some dielectric relaxation spectroscopy techniques (a) conventional transient methods (b) newer transient methods (c) low frequency impedance bridge (d) conventional impedance bridges and analyzers (e) high frequency impedance analyzers and time domain spectroscopy (f) microwave cavities, transmission lines ...

---