Impedance measurement active circuit

Fast detection of the electro-acoustic impedance is a condition for successful kinetic studies. Soares [64] introduced a circuit to measure both resonant frequency and damping resistance R, though not as fast as simple active oscillator methods mainly used for resonant frequency measurement. Most active circuits operate in the series frequency a>s although some oscillators are designed to operate in the parallel frequency wp , which is slightly higher and very susceptible to the value of Ca. 

The electrochemical activation was carried out in successive reduction and oxidation in 1 M sulfuric acid. The corresponding potentials were -t-2.40 V (oxidation) and -0.35 V (reduction) vs. reference (Ag/AgCl). After each step 10 cyclic voltammograms were recorded followed by an impedance measurement. Both methods were initiated by an open circuit measurement for 1200 s. 

Usually the impedance is measured as a ftmction of the frequency, and its variation is characteristic of the electrical circuit (where the circuit consists of passive and active circuit elements). An electrochemical cell can be described by an equivalent circuit. Under appropriate conditions, i.e., at well-selected cell geometry, working and auxihary electrodes, etc., the impedance response will be related to the properties of the working electrode and the solution (ohmic) resistance. 

The anode of a Ni-Cd battery typically consists of a mix of Cd and CdO powders with the addition of a conductive additive (acetylene black). The impedance of the anode-particle surface is determined by the activated adsorption of OH anions first on the metal surface, with subsequent conversion into Cd(OH)2 and hydrated CdO layers (Duhirel et al. [1992])). Reaction products are also present in a partly dissolved Cd(OH)3" state. The activated adsorption mechanism of the anode reaction, as well as porous structure of the electrode, makes it appropriate to use for its analysis the equivalent circuit shown in Figure 4.5.14. It was shown by Xiong et al. [1996], by separate impedance measurements on the anode and cathode, that most of the impedance decrease during discharge is due to the anode, as the initial formation of a Cd(OH)Jrate limiting step of the reaction. The... 

However, it is important to note that the Ret values are not directly related to the susceptibility to corrosion of the different alloys and composites. They are related to the rate of charge transfer reactions that give rise to the formation of a passive layer on the surface of the samples (the impedance measurements are at open circuit potential only). The protective characteristics of these passive films depend on the preparation conditions of the alloys, the distribution of elements in the alloy and the presence on the surface of active sites for adsorption of chloride ion. 

A different approach to impedance measurements of electrodes in CP conditions was assumed by Juchniewicz and Jankowski (1993). They elaborated a quantitative method, allowing the determination of the partial anodic current of polarized electrodes. They proposed an electric equivalent circuit of a polarized electrode describing the impedance characteristic of an electrode as a function of the applied potential. It has been assumed that on the electrode one activation controlled anodic reaction and one cathodic reaction with mixed activation-diffusion control proceeds. The adopted electric equivalent circuit is presented in Fig. 8-11. 

Immittance — In alternating current (AC) measurements, the term immittance denotes the electric -> impedance and/or the electric admittance of any network of passive and active elements such as the resistors, capacitors, inductors, constant phase elements, transistors, etc. In electrochemical impedance spectroscopy, which utilizes equivalent electrical circuits to simulate the frequency dependence of a given elec-trodic process or electrical double-layer charging, the immittance analysis is applied. 

FIGURE 4.17 Equivalent electrical circuit for electrochemical oxygen sensor a at the absence of polarization and b polarization of the solid electrolyte-electrode interface. (From Zhuiykov, S., In-situ" diagnostics of solid electrolyte sensors measuring oxygen activity in melts by developed impedance method, Meas. Sci. Technol. 17 (2006) 1570-1578. With permission.)... 

The results of the present work may be applicable for diagnostics of oxygen sensors at more complicated applications, such as measurement of oxygen activity in liquid sodium, lithium, or lead-bismuth heat carriers for atomic power plants. Corrosion and mass transfer in nonisothermal lead-bismuth circuits with temperatures of a heat carrier of 300-500°C do usually occur at a concentration of dissolved O2 of 10 - 10 mass %. The proposed impedance method is developed for determining the level and the character of polarization at the electrolyte-electrode interface, which ensures a continuous oxide protection of materials against corrosion by means of zirconia sensors in all tanperature regimes of exploitation of liquid-metal circuits. 

In homogeneous corrosion systems (active dissolution, passive state) where the same electrochemical reactions occur over the whole surface, the interrupter and ac technique can be successfully applied and the same value for the ohmic resistance is measured by both techniques. Problems arise in localized corrosion systems, where small active areas coexist with a large passive surface and the impedance of the active areas (pits) is short circuited by the surrounding passive surface. 

The circuits in Fig. 32 are suited for impedances not exceeding 1 MQ. If very small electrodes are used, e.g., for measuring of single cells, their impedance values can exceed 1,(XX) MQ. Although shielding can prevent excessive noise, care should be taken of parasitic elements, especially of stray capacitances. Moreover, if active front ends in the immediate vicinity of the electrodes are used, they may heat up the chamber. 

For illustration, in Figure 4.5.58 impedance spectra measured over the whole performance range of a PEFC from open circuit to full load were presented. The nature of the impedance spectra varied with cell potential, between open circuit potential (1024 mV) and 840 mV an exponential potential dependency of the impedance due to the high activation energy of the oxygen reduction reaction is observed in the low frequency range (/< 1 Hz). 

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