AC impedance method

The measurement of the electrode impedance has also been ealled Faradaie impedanee method. Since measurements are possible by applying either an electrode potential modulated by an AC voltage of discrete frequeney (which is varied subsequently) or by applying a mix of frequencies (pink noise, white noise) followed by Fourier transform analysis, the former method is sometimes called AC impedance method. The optimization of this method for the use with ultramicroelectrodes has been described [91Barl]. (Data obtained with these methods are labelled IP.)... 

The lithium transference number (l,) of these organoboron polymer electrolytes was evaluated by combination of dc polarization and ac impedance methods, as reported by Evans et al44 (Table 1). The observed t+ at 30°C was 0.50-0.35, indicating that anions were significantly trapped in these systems. Owing to the stronger Lewis acidity of the alkylborane unit, alkylborane-type polymers showed relatively higher t+. 

After adding lithium salts, ionic conductivity was measured by the ac impedance method (Fig. 5). Lithium salt concentration was first optimized by measurements... 

J. M. Song, S. Y. Cha, and W. M. Lee. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method. Journal of Power Sources 94 (2001) 78-84. 

In the AC impedance method, a modulation of potential causes a modulation of concentration at the interface. As a consequence there is a modulation of the concentration gradient and hence of the current density. The modulation of concentration decays with distance away from the interface. Thus there is a characteristic penetration depth into the solution phase of the concentration wave, SAC, where SAc = (2Dlw)m. The time scale for chemical processes, either in the solution or at the interface, probed by the perturbation, is simply / >. If the concentration boundary... 

Figure 6.4. The polarization curves of fuel cells with electrodes that contain various PTFE content in the gas diffusion layer ( ) 10 ( ) 20 (A) 30 (+) 40 wt% [5]. (Reprinted from Journal of Power Sources, 94(1), Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method, 78-84, 2001, with permission from Elsevier and the authors.)...

Conventional electrochemical techniques including CV [1-7], current-scan po-larography [13-15,34,35] and ac. impedance method [36,37] have so far been used for the study of ET at OAV interfaces. However, recent development in microtechnology brought to an introduction of SECM in this field [16-23]. Our group has also developed a few devices for characterizing ET reactions at O/W interfaces [38,39]. In this section, new methodologies recently introduced in this field are described. 

Sone Y, Ekdunge P, Simonsson D (1996) Proton conductivity of Nafion 117 as measured by a four-electrode AC impedance method. J Electrochem Soc 143(4) 1254-1259... 

Electrochemical impedance spectroscopy (EIS) or ac impedance methods have seen a tremendous increase in popularity in recent years. Initially applied to the determination of the double-layer capacitance " and in ac polarography, " they are now used to characterize electrode processes and complex interfaces. This method studies the system response to the application of a periodic small-amplitude ac signal. The measurements are carried out at different ac frequencies and thus the name impedance spectroscopy was later adopted. Analysis of the system response contains information about the interface, its structure, and the reactions taking place there. Electrochemical impedance spectroscopy is now described in the... 

From reference 65, T = 22 2°C, concentration of compounds 1 mM. Rate constants measured by ac impedance method. Final column calculated according to (13.7.6). 

J. A. Gonzales, The determination of the corrosion rate of steel embedded in concrete by the polarization and AC impedance methods , in Corrosion Effect of Stray Currents and the Techniques for Evaluating Corrosion of Rebars in Concrete, ASTM STP 906, V. Chaker (Ed.), Philadelphia, 43-63, 1986. 

Sone et al. [34] reported conductivity data for Nafion 117 in the E-form (no heat treatment), measured using a four-electrode AC impedance method. Membranes used in fuel cells are typically heated during the manufacture of the membrane electrode assembly, and it may thus be more appropriate to fit to the data for a membrane in the N or S form however, fitting to the E-form data allows direct comparison with Thampan et al. [22] fit to their model. 

Electrochemical AC impedance methods are well suited for in situ monitoring of changes in surface roughness and capacitance [43), as discussed in Chapter 2.6 in... 

The resistance of membranes can be measured by AC impedance methods [85,86], using the four-point-probe technique. The test membrane is placed in a cell consisting of two Pt-foil electrodes, spaced 3 cm apart, to feed the current to a sample of 3 x 1 cm and two platinum needles placed 1 cm apart, to measure the potential drop (see Fig. 4.3.26). The cell is placed in a vessel maintained at constant temperature by circulating water. The impedance measurements are then carried out at 1-10 kHz using a frequency-response analyzer (e.g., Solatron Model 1255HF frequency analyzer). After ensuring that there are no parasitic processes (from the phase angle measurements, which should be zero), one can measure the resistance directly. The membrane resistance can also be obtained directly from the real part of the impedance (see typical data in Fig. 4.3.27). 

W.I. Archer, R.D. Armstrong, The application of ac impedance methods to solid electrolytes, in Electrochemistry A Specialist Periodical Report, ed. by G.J. Hills, D. Pletcher, H.R. Thirsk, vol. 7 (The Royal Society of Chemistry/The Chemical Society, Burlington House/London, 1980), pp. 157-202... 

It is therefore apparent that in passing from high to low freqnency in a system of this kind, there is an additional impedance dne to concentration polarization. Macdonald and HnU [1984] considered this effect on the electrical response of this type of system. Under many drcnmstances, the presence of concentration polarization might be confnsed with an interface impedance. At different ratios of mobiU-ties of anions and cations, either diffnsion-Iike response (finite-length transmission line behavior) or parallel capacitative-resistive behavior may appear. Ac impedance methods have been nsed to determine ionic transference nnmbers in polymeric electrolytes using this principle (Sorensen and Jacobsen [1982]). 

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