AC Impedance Measurements

EIS was carried out 0.1 M Na2S04 + 0.1 N NaCl solution on as received MS and WS and impedance spectra interpretation was performed after numerically fitting and using equivalent circuit. The impedance plots are given in Figs. 3.50 and 3.51 and EIS parameters are given in Table 3.24. 

EIS was carried out in SAEJ solution exposed for 48 h on MS and WS after exposing in humid SO2 environment for 9 months (m.) The impedance plots are given in Fig. 3.52 along with EIS parameters in Table 3.25. 

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Nonfaradaic components associated with the uncompensated resistance between reference electrodes (7 ) and the double layer capacitance (Qi) can be accurately determined by AC impedance measurements. In this technique, a small AC potential perturbation is superimposed to the DC bias, and the resulting AC current is measured as a function of the frequency of modulation. The simplest circuit considered for a polarizable... 

Another parameter essential for quantitative applications of micropipettes is the internal ohmic resistance, R. It is largely determined by the solution resistance inside the narrow shaft of the pipette, and can be minimized by producing short (patch-type) pipettes. The micropipette resistance has been evaluated from AC impedance measurements. Beattie et al. measured the resistance of micropipettes filled with aqueous KCl solutions (0.01, 0.1, and 1 M) [18b]. The value obtained for a 3.5/am-radius pipette was within the range from 10 to 10 As expected, the tip resistance was inversely proportional to the concentration of KCl in the filling solution. In ref. 18b, the effect of pipette radius on the tip resistance was evaluated using a constant concentration of KCl. The pipette resistance varied inversely with the tip radius. The iR drop was found to be 4.5-8 mV for the pipette radii of 0.6 to 19/rm when 10 mM KCl was used. 

The rotating hemisphere electrode has been used to investigate the effect of AC on the electrodissolution and deposition reactions of zinc in zinc chloride [25] and copper in acid copper sulfate solutions [55], AC was found to increase the rate of nucleation and produce more uniform deposit on the zinc electrode. The corrosion of an iron rotating hemisphere in dilute sulfuric acid was investigated by Haili [31] using the AC impedance measurement. 

The IV measurements on molecules and monolayers have been carried out almost exclusively using direct current (DC) frequency-dependent alternating current (AC) impedance measurements have rarely been performed, even though a rich spectroscopy may reveal itself, if the IV measurements were followed as a function of frequency v. 

AC impedance measurements were also made in bulk paints. A Model 1174 Solartron Frequency Response Analyser (FRA) with a Thompson potentiostat developed ac impedance data between 10 KHz and 0.1 Hz at the controlled corrosion potential The circuit has been described in the literature( ). 

Flash Rusting (Bulk Paint and "Wet" Film Studies). The moderate conductivity (50-100 ohm-cm) of the water borne paint formulations allowed both dc potentiodynamic and ac impedance studies of mild steel in the bulk paints to be measured. (Table I). AC impedance measurements at the potentiostatically controlled corrosion potentials indicated depressed semi-circles with a Warburg diffusion low frequency tail in the Nyquist plots (Figure 2). These measurements at 10, 30 and 60 minute exposure times, showed the presence of a reaction involving both charge transfer and mass transfer controlling processes. The charge transfer impedance 0 was readily obtained from extrapolation of the semi-circle to the real axis at low frequencies. The transfer impedance increased with exposure time in all cases. 

Simplicity and reliability of operation make AC impedance measurements attractive as a technique in the evaluation of coating integrity. As opposed to classical salt spray test, analysis times are shorter with the AC impedance technique and quantitative data are obtained permitting relevant mechanistic Information to be derived. Impedance test methods are likely to find many applications in the resolution of unsolved practical problems ( .) ... 

AC Impedance measured using a few specimens anodlcally phosphated and painted with Tremclad or Marlnox paint are shown Table II and Figures 3 and 4. Cyclovoltammetry Indicated a notable fall... 

TABLE II. RESULTS OF AC IMPEDANCE MEASUREMENTS MADE ON SAMPLES WITH THICKER COATINGS... 

AC Impedance measurements taken on the same specimen at different temperatures In the range 25-90 C are shown In Table III. A specimen with no surface treatment other than mechanical polishing shows Cdi 40vF cm" at 25 C but the value Increases appreciably with Increasing temperature. The values of R t for different specimens ( 28, 29) show a systematic decrease with Increasing temperature whereas the values of C i show a systematic Increase. Figures 5 and 6 show the evolution of Impedance plots as a function of temperature. In addition to the variation In the values of Rgt Cjj, It Is noticed that the Warburg-... 

AC Impedance measurements enable the determination of charge transfer resistance and double layer capacitance and other parameters related to coated systems. 

Although not dealt with in this chapter, AC impedance measurements (sometimes called electrochemical impedance spectroscopy) are important in studying electrode dynamics. Generally in this method, a sinusoidal voltage (10 2 to 105 Hz) is applied to the cell, the phase angle and the amplitude of the response current are measured as a function of... 

Circuitry similar to that presented in Figure 8.13b has been used to analyze cells with impedances ranging from 102 to 1011 Q with 1% accuracy and resolution better than 1 part in 104 over a frequency range of 0.005 Hz to 10 kHz [14]. The technique has been especially useful for studies of the reaction kinetics of moderately fast chemical reactions. Kadish et al. [15] used phase-selective techniques to make ac impedance measurements to evaluate reference electrodes for use in nonaqueous solvents. Recent decreases in the cost of integrated function modules such as analog multipliers, oscillators, and phase-locked loops make this type of phase-selective instrumentation more accessible than ever. 

This method, first proposed by Smyrl [65] to determine the Schmidt number from ac impedance measurements, was furtherly extended by Tribollet et al. [66] to the EHD impedance. By taking a Taylor expansion of W to the first order, they showed d (Re (IV))... 

Figure 19. Predicted carbon loss distribution along anode flow-field channel over a complete H2/air-front start—stop cycle using the pseudo-capacitance model in comparison with one-dimensional, normalized mass activity from Fig. 17. The pseudo-capacitance value used in the model is obtained from AC-impedance measurements as described in references (42, 43).

Fiquid pumping or dispensing was achieved on a Si-Pyrex chip based on electrochemical generation of gas bubbles (H2 and 02) at Pt electrodes [309,391]. To ensure accuracy in the dispensing volume, an AC impedance measurement (at 200 kHz) of gas volume using interdigital electrodes was adopted. This method is... 

Figure 3.20. AC impedance measurement by TDI RBL 488 load bank and Solartron 1260 [21]. (Reproduced by permission of ECS—The Electrochemical Society, from Tang Y, Zhang J, Song C, Liu H, Zhang J, Wang H, Mackinnon S, Peckham T, Li J, McDermid S, Kozak P. Temperature dependent performance and in situ AC impedance of high-temperature PEM fuel cells using the Nafion-112 membrane.)...

Although EIS offers many advantages for diagnosing fuel cell properties, clear difficulties exist for applying impedance methods and fitting the data to the model to extract the relevant electrochemical parameters. The limitations of the EIS technique derive from the several requirements required to obtain a valid impedance spectrum, because the accuracy of EIS measurement depends not only on the technical precision of the instrumentation but also on the operating procedures. Theoretically, there are three basic requirements for AC impedance measurements linearity, stability, and causality. 

FFT techniques can measure multiple-frequency AC impedance at the same time as a stimulus waveform containing multiple frequencies. FFT performs very fast AC impedance measurements and can function as a complement to the single sine wave technique. The stimulus waveform, which is constructed by multiple sine waves, should also have as low an amplitude as possible to avoid or decrease the harmonic distortion effects at the non-linear regime. However, if the amplitude of the stimulus wave is too low, each single sine wave will be much lower and may be distorted by noises. Fortunately, some commercially available analyzers have the option of selecting the frequencies for AC impedance measurements. Usually, stimulus frequencies are selected in such a way that each is not coincident with the main harmonics of the lower frequencies. As a result, the main components of the harmonic distortion from each frequency in the stimulus waveform do not interfere with the other stimulus frequencies. 

Raposa G (2004) Performing ac impedance measurements on fuel cells. Elektron 21 15-16... 

The results also suggest that through AC impedance measurements, the performance drops caused by individual processes such as electrode kinetic resistance, membrane resistance, and mass transfer resistance can be correlated to either reduction or improvement in cell performance. If individual impedances are known, the contribution to the change in performance can be identified, which is very important in the design and optimization of high-temperature MEA catalyst layer components, structure down-selection, and MEA architecture. 

Figure 20.5 shows the temperature dependence of the ionic conductivity for zwitterion 10 with and without an equimolar amount of lithium salts. Neat zwitterion 10 shows low ionic conductivity of about 10 S cm at even 200°C from the ac impedance measurement. This is because there are no mobile ions in the system. However, zwitterion 10, which is mixed with an equimolar amount of lithium salt. 

Fig.20 Proton conductivity of P4VP(MSA)i.o(PDP)i.o-fc-PS recorded during heating at 5 °C/min based on AC impedance measurements extrapolated to zero frequency. The cartoons show the dimensionality transitions occurring upon heating from one-dimensional slabs to two-dimensional lamellae to one-dimensional cylinders. The order-disorder transitions at ca. 100 °C and the order-order transition at ca. 150 °C observed by SAXS (Fig. 17) are distinctly reflected in the conductivity [46]...

Ac impedance measurements of the compound sintered at 700 °C were performed in air in the temperature range of 200-600 °C. The Arrhenius plots of the ionic conductivity are shown in Figure 4. The relationship between conductivity and the reciprocal of the absolute temperature is linear over the temperature range measured. 

Carpio and coworkers [4] supported this hypothesis via a potentiodynamic study of a set of HN03-containing slurries. The corrosion currents and potentials under both the static and the dynamic conditions were practically the same. This is consistent with the fact that there was no native copper oxide film formed because of the presence of these slurries. As a matter of fact, the corrosion currents decreased slightly upon abrasion of the copper surface. The contact between the metal surface and the abrasive pad may have limited the mass transport of chemicals to and from the copper surface. This was verified via an AC impedance measurement that showed the importance of the systems mass transport. It was also concluded that in a dissolution-controlled process, mechanical abrasion would not enhance the chemical corrosion rate or reduce the mass transport of reactants and/or products in the system. 

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