A.c. impedance

A.C. impedance (thickness conduction mechanisms and profiles compactness crystallinity)... 

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 . 

Instruments providing simultaneous measurement of a number of parameters on multi-element probes have been developed, including potential noise , galvanic coupling, potential monitoring, and a.c. impedance . 

Reported plant applications of a.c. impedance and electrochemical noise are rare, but include stainless steels in terephthalic acid (TA) plant oxidation liquors , nuclear fuel reprocessing , and fluegas desulphurisation (FGD) scrubber systems . 

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... 

D.Y. Wang, and A.S. Nowick, Cathodic and anodic polarization phenomena at platinum electrodes with doped Ce02 as electrolyte. II. Transient overpotential and A-C Impedance, /. Electrochem. Soc. 126(7), 1166-1172(1979). 

Figure 4. Variations of the a.c. impedance with cycling measured with C/Li(NiojCoo,2Mn0f02 cells at the open cell voltage of 3.7V...

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]).

The working electrodes (WEs) used in the CV and a.c. impedance experiments consisted of Ti wire substrates (99.99%, Johnson-Matthey, Inc., 2 mm diameter). The cross-sectional end of each bare Ti wire (0.0314 cm2) was polished with 600 grit paper, rinsed with acetone, dried, rinsed with methylene chloride, dried again and then rinsed with triply distilled water before oxide-coating deposition. 

An EG G PARC 273 Potentiostat/Galvanostat was used in both the electrolysis and the CV experiments, coupled with an HP 7044B X/Y recorder. A Solartron 1255 HF Frequency Response Analyzer and a Solartron 1286 Electrochemical Interface were employed for the a.c. impedance measurements, using frequencies from 0.1 to 65 kHz and a 10 mV a.c. amplitude (effective) at either the open circuit potential (OCP) or at various applied potentials. As the RE can introduce a time delay at high frequencies, observed as a phase shift owing to its resistance and capacitance characteristics, an additional Pt wire electrode was placed in the cell and was connected via a 6.8 pF capacitor to the RE lead [32-34]. 

A.C. impedance behaviour of fresh and deactivated Ru/Ti oxide electrodes... 

To further understand and characterise the oxide deactivation process, a.c. impedance studies were carried out, primarily with a 30 at.% Ru/Ti electrode, at various stages during deactivation. These data were compared to those obtained for freshly formed Ru/Ti oxide films, ranging in Ru content from 5 to 40 at.%. Impedance data were collected at the oxide OCP (approximately 0.9 V versus SCE) in fresh NaCI solutions. Under these conditions, no chlorine reactions can occur and the OCP is defined by the equilibria of the redox states on the Ru oxide surface. Deactivation was generally accomplished by square-wave potential cycling, using overpotentials versus the chlorine/chloride potential of 1.59 to — 0.08 V (60 s cycle-1) in 5 M NaCI + 0.1 M HC1 solutions at room temperature. 

It is well recognised [1, 36, 38, 39] that the electrochemical response (that is the a.c. impedance and cyclic voltammetry) of Ru/Ti oxides at the OCP is due to the pseu-docapacitive reaction ... 

Theoretical investigations have shown [35, 39-44] that the a.c. impedance, Zp(p), associated with Equation 5.2, on a porous electrode matrix, is given by the expression ... 

Thus, the polarisation data, cyclic voltammetric results and the a.c. impedance measurements all suggest that, when an Ru02/TiC>2 anode exhibits a high overpotential, this is a direct consequence of the surface depletion of Ru. This is also consistent with the estimated Re values of approximately 20 Q for the failed electrodes, in contrast to the known, much higher specific resistivity of Ti02 of... 

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... 

Theories and discussions on this case have been published with consideration of large-amplitude potential control [127—129], a.c. impedance at equilibrium potential [130], and a.c. polarography [131]. The problem is formulated as follows. 

Figure 3.15. Schematic representation of the correlation between fuel cell impedance and polarization curve. (Modified from [23], with kind permission from Springer Science+Business Media Journal of Applied Electrochemistry, Characterization of membrane electrode assemblies in polymer electrolyte fuel cells using a.c. impedance spectroscopy, 32(8), 2002, 859-63, Wagner N. Figure 4.)...

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