A.c. impedance techniques

The a.c. impedance technique [33,34] is used to study the response of the specimen electrode to perturbations in potential. Electrochemical processes occur at finite rates and may thus be out of phase with the oscillating voltage. The frequency response of the electrode may then be represented by an equivalent electrical circuit consisting of capacitances, resistances, and inductors arranged in series and parallel. A simplified circuit is shown in Fig. 16 together with a Nyquist plot which expresses the impedance of the system as a vector quantity. The pattern of such plots indicates the type and magnitude of the components in the equivalent electrical network [35]. 

D. E. Williams, C.C. Naish, Introduction to the A.C. Impedence Technique and its Applica tion to Corrosion Problems, HL 85/1066 (C14), Materials Development Division, AERE Harwell, 1985. 

There have been a few reports on the application of the Marcus theory to true ET systems. Cheng and Schiffrin [37] employed the a.c. impedance technique to determine the rate constants for some ET systems including the LuPc2... 

McHardy, J., J.M. Baris, and R. Stonehart, Investigation of hydrophobic porous electrodes. I. Differential capacitance by a low frequency a. c. impedance technique. Journal of Applied Electrochemistry, 1976. 6 pp. 371-376... 

Fig. 1.116 Circuit diagram of model used for a.c. impedance techniques.

A.c. impedance. Measurements of the frequency variation of impedance allow separation of the change transfer resistance from the contributions to the total impedance of the environment resistance, surface films, adsorbed layers, etc. Robust instruments utilising a two-frequency technique have been developed . 

Recent developments in the mechanisms of corrosion inhibition have been discussed in reviews dealing with acid solutions " and neutral solu-tions - . Novel and improved experimental techniques, e.g. surface enhanced Raman spectroscopy , infrared spectroscopy. Auger electron spectroscopyX-ray photoelectron spectroscopyand a.c. impedance analysis have been used to study the adsorption, interaction and reaction of inhibitors at metal surfaces. 

Electrochemical tests This group includes the various electrochemical tests that have been proposed and used over the last fifty or so years. These tests include a number of techniques ranging from the measurement of potential-time curves, electrical resistance and capacitance to the more complex a.c. impedance methods. The various methods have been reviewed by Walter . As the complexity of the technique increases, i.e. in the above order, the data that are produced will provide more types of information for the metal-paint system. Thus, the impedance techniques can provide information on the water uptake, barrier action, damaged area and delamination of the coating as well as the corrosion rate and corroded area of the metal. However, it must be emphasised that the more comprehensive the technique the greater the difficulties that will arise in interpretation and in reproducibility. In fact, there is a school of thought that holds that d.c. methods are as reliable as a.c. methods. 

Developments in electrochemical methods since 1976 for measurement of corrosion have been rapid. Research and development has produced several new techniques, e.g. a.c. impedance and electrochemical noise. These methods require corrosion expertise for both operation and interpretation. Industry generally prefers instrumentation that can be operated by process... 

Fig. 5.2 Variation of conductivity with mol% Ru02 in the coating. The measurement technique and the final firing temperatures are noted below. The symbols are = 350-600°C/ direct resistance measurement (Ref. [3]) O = 450°C/a.c. impedance (Ref. [4]) A = 400°C/ direct resistivity measurement (Ref. [5]).

However, as mentioned previously, gas-diffusion electrodes usually deviate substantially from traditional electrochemical—kinetic behavior, often being limited by multiple rate-determining factors and/or changes in those factors with overpotential or other conditions. In attempting to analyze this type of electrode, one of the most influential experimental techniques to take hold in the solid-state electrochemical literature in the last 35 years is electrochemical impedance spectroscopy (EIS)—also know as a.c. impedance. As illustrated in Figure 6, by measuring the sinusoidal i— response as a function... 

With the noise techniques, both analogue and digital, no externally applied signal is required, and measurement of the fluctuations around the free corrosion potential provides all the information. Hie noise technique is useful in that it allows a fairly rapid estimation of the electrochemical impedance of the system being studied, whereas, with for instance, a.c. lnpedance techniques, very often the minimum frequency studied is still not low enough to provide sufficient information to allow an accurate estimation of the impedance. 

Conventional electrochemical techniques including CV [1-7], current-scan po-larography [13-15,34,35] and a.c. impedance method [36,37] have so far been used for the study of ET at O/W 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. 

Potential modulation techniques are used frequently in electrochemistry. The most well-known potential modulation electrochemical technique is a.c. impedance spectroscopy, in which current modulation caused by a potential modulation is analyzed. The potential modulation technique has also been used for in-situ IR spectroscopy (EMIRS and SNIFTIRS), but its use was aimed only to subtract the solution background and to enhance the S/N ratio of the spectram. If the IR signal caused by a potential modulation is analyzed, some information on electrode dynamics could be obtained as in a.c. impedance spectroscopy. 

Fig. 26.3. H-difTusion coefficient measured on TSA.28H2O by the PFG-NMR technique , (a) Echo attenuation as a function of applied magnetic field gradient showing separation of the contributions from the mother liquor and the solid, (b) The resultant diffusion coefficient as a function of the reciprocal temperature the diffusion coefficient calculated from the proton conductivity measured by a.c.-impedance spectroscopy is given for comparison (see Chapters 29-31).

It is clear that PFG-NMR sees any protonic diffusion regardless of whether the proton is part of a neutral (e.g. H2O) or charged species (e.g. H" ", HjO, OH ) whereas a.c.-impedance spectroscopy is sensitive only to displacement of ionic charge. The application of both techniques therefore frequently allows one to identify the proton (or protonic species) as the charge carrier, to quantitatively determine its diffusion coefficient (conductance) and sometimes even to deduce the underlying conduction mechanism (see Chapter 31). 

Progress in the understanding of superionic conduction is due to the use of various advanced techniques (X-ray (neutron) diffuse scattering, Raman spectroscopy and a.c.-impedance spectroscopy) and-in the particular case of protons - neutron scattering, nuclear magnetic resonance, infrared spectroscopy and microwave dielectric relaxation appear to be the most powerful methods. A number of books about solid electrolytes published since 1976 hardly mention proton conductors and relatively few review papers, limited in scope, have appeared on this subject. Proton transfer across biological membranes has received considerable attention but is not considered here (see references for more details). 

This historical account of the development of ideas and experiments on charge transfer in electrochemistry should not conclude without reference to the Faraday Discussion(28) in 1947, held at the University of Manchester. This discussion marked an important turning point in electrode kinetics towards more modern and quantitative analyses of electrode process mechanisms and utilization of relatively new (for that time) techniques, e.g. a.c. impedance studies in the papers by.Randles (37) and by Ershler (38). it also brought together many European electrochemists, following the war years, during which little scientific intercourse had taken place on fundamental aspects of electrochemistry. 

The development during the last three decades of laboratory methods for investigation of the kinetics of electrode reaction[44,45,46] e.g. of relaxation techniques (potential step, current step, A.C. impedance methods, etc.), of cyclic voltammetry and, most recently of the use of electrodes of very small dimensions, has led to a marked increase in our level of understanding of these processes. The value of these techniques has been greatly enhanced by the development of computer based methods of data analysis. 

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