Electrochemical technique

In Part II, we deal with various electrochemical techniques and show how they are applicable in non-aqueous solutions. In this chapter, we give an overview of electrochemical techniques, from the principles of basic techniques to some recent developments. It will help readers from non-electrochemical fields to understand the latter chapters of Part II. Many books are available to readers who want to know more about electrochemical techniques [1], In particular, the excellent book by Bard and Faulkner [la] provides the latest information on all important aspects of electroanalytical chemistry. 

In order to give an overview of the many electrochemical techniques, it is convenient to classify them. Table 5.1 is an example of such a classification. All electro- 

A Methods that electrolyze the electroactive species under study completely Electrogravimetry Coulometry 

Polarography and Voltammetry (DC, AC, SW, pulse methods for each) Amperometry Chronopotentiometry, Polarography and Voltammetry at the interface between two immiscible electrolyte solutions (ITIES) 

Empirically it has been demonstrated that Si (Oehrlein et al., 1981 Pearton et al., 1984b), Ge (Pearton et al., 1984a), and GaAs (Chevallier et al., 1985) can be hydrogenated in simple two electrode (externally biassed) electrochemical cells. In most of these cases, exposures of many minutes were involved cell currents were monitored during treatment but could not be directly used to calculate H influx because of the competing evolution of gaseous H2  

In virtually all of the simple immersion and two electrode experiments carried out so far, in-diffused H has been detected at the 1016/cm3 level or less. There has been no demonstration that large densities ( 1018/cm3) of defects can be passivated by these methods, and where plasma and electrochemical treatments have been directly compared, the former have been found to be more effective (Tavendale et al., 1986). In contrast to plasma techniques, the electrolyte boiling point limits the temperature range of electrochemical methods, although several hundred degrees Celsius can be utilized for electrolytes like H3P04. 

To summarize this section, the electrochemical hydrogenation experiments performed to date have yielded substantial penetration of low levels of hydrogen, but show promise for practical utilization despite temperature limitations and complications such as material removal. As with plasma and ion beam methods, the surface of this subject has barely been scratched. 

SERS due to pyridine on Au electrode surfaces appears to arise from the adsorption of pyridine in or on surface carbon present after the oxidation-reduction cycle [25,26], Anodically roughened Ag electrode surfaces, which were subsequently cathodically cleaned, exhibited no SERS from pyridine. This confirms that the SERS-active phase is carbon-pyridine and not pyridine alone. In ultrahigh vacuum, SERS can be induced in pyridine by coadsorbing pyridine with CO [27], The effect depends on the type of silver surface and involves shifts in the peak positions and intensities of some of the vibrational modes. SERS peaks were not observed at 2100 cm 1 at the position of the C O stretching mode of CO. A possible interpretation is that surface complexes are formed between pyridine and CO molecules at the active or hot sites on the silver surface. 

A combination of electrochemical methods and SERS is used to detect chlorinated hydrocarbons in aqueous solutions [28], Electrochemistry prepares the surface of a copper electrode for SERS and concomitantly concentrates the analyte on the surface of the electrode, possibly by electrophoretic processes. Detection sensitivity of 1 ppm for trichloroethylene, for example, was achieved. 

We will discuss the response of essentially seven cell types which are designed to measure a mixed conductor which conducts 

Analogous cells for a cationic mixed conductor (conductivity via electrons and cations, e.g., Ag2S) can be formulated (AglAgI instead of 02, Ptl YSZ Ag or an Ag-alloy instead of 02, Pt). In this case sealing is not necessary however, the component activity cannot be tuned so simply. 

We will consider these cells, primarily the oxygen cells, under open circuit conditions and under load (or even short-circuit condition). In the transient and in the steady state it is not necessary to treat them all in detail, since (as outlined below) cells with one selectively blocking electrode and those with two of the same kind show far-reaching similarities (compare cell 3 with cell 4 and cell 5 with cell 6). The same is true if we compare cells with electrodes that are selectively blocking for electrons with cells that are specifically blocking for ions (compare cell 3 with cell 5 and cell 4 with cell 6) it is easy to show that the relations are symmetrical as regards the indices e and O2 (see below and Appendix l).21011 

We will see that in the steady state of the blocking cells, we can extract partial conductivities, and from the transients chemical diffusion coefficients (and/or interfacial rate constants). Cell 7 combines electronic with ionic electrodes here a steady state does not occur but the cell can be used to titrate the sample, i.e., to precisely tune stoichiometry. Cell 1 is an equilibrium cell which allows the determination of total conductivity, dielectric constant or boundary parameters as a function of state parameters. In contrast to cell 1, cell 2 exhibits a chemical gradient, and can be used to e.g., derive partial conductivities. If these oxygen potentials are made of phase mixtures212 (e.g., AO, A or AB03, B203, A) and if MO is a solid electrolyte, thermodynamic formation data can be extracted for the electrode phases. 

Furthermore, it is also not necessary to discuss different excitations in detail as long as we restrict ourselves to the linear response regime. There it holds that the response to any excitation allows the calculation of the response to other excitations via the convolution theorem of cybernetics.213 In the galvanostatic mode, e.g., we switch the current on from zero to /p (or switch it off from 7p to zero) and follow IKj) as a response to the current step. The response to a sinusoidal excitation then is determined through the complex impedance which is given by the Laplace transform of the response to the step function multiplied with jm (j = V-I,w = angular frequency). 

Hydrogel undergoes drastic volumetric changes including volumetric transition due to changes in the enviromnent that include solution 

This diagram is schematic and, thus, omits reference electrode and counter electrode, which are necessary for actual measurements 

Linear polarization takes advantage of the linear region of the -/ curve, near the cttrrosion potential, to extract values of /f. Within 10-20 mV of the corrosion potential, current varies linearly with potential, with the slope equal to /fci At high values, R i is related inversely to the corrosion rate. 

Linear polarization techniques are often used to conduct an initial electrochemical characterization of a metal or alloy, prior to more complex investigations, torr is first determined relative to a reference eicctrxxle, usually the standard calomel electrode (SCE). A small potential is then applied and swept from about 20 mV below to 20 mV anodic to it. The current density is measured and is calculated. 

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Source Compiled from Cammann, K. Working with ion-Seiective Eiectrodes. Springer-Verlag Berlin, 1977 and Lunte, C. E. Heineman, W. R. "Electrochemical Techniques in Bioanalysis." In Steckham, E., ed. Topics in Current Chemistry, Vol. 143, Springer-Verlag Berlin, 1988, p. 8. - Abbreviations E = enzyme B = bacterial particle T = tissue. 

R. Baboian, ed.. Electrochemical Techniques in Corrosion Engineering, NACE, Houston, Tex., 1986. 

Active electrochemical techniques are not confined to pulse and linear sweep waveforms, which are considered large ampHtude methods. A-C voltammetry, considered a small ampHtude method because an alternating voltage <10 mV is appHed to actively couple through the double-layer capacitance, can also be used (15). An excellent source of additional information concerning active electroanalytical techniques can be found in References 16—18. Reference 18, although directed toward clinical chemistry and medicine, also contains an excellent review of electroanalytical techniques (see also... 

Electrochemical systems are found in a number of industrial processes. In addition to the subsequent discussions of electrosynthesis, electrochemical techniques are used to measure transport and kinetic properties of systems (see Electroanalyticaltechniques) to provide energy (see Batteries Euel cells) and to produce materials (see Electroplating). Electrochemistry can also play a destmctive role (see Corrosion and corrosion control). The fundamentals necessary to analyze most electrochemical systems have been presented. More details of the fundamentals of electrochemistry are contained in the general references. 

Y"et, corrosion engineering and science is no longer an empirical art dissecting a large corrosion problem into its basic mechanisms allows the use of quite sophisticated electrochemical techniques to accomplish satisfactory results. On that positive side, there is real satisfaction and economic gain in designing a component that can resist punishing seiwice conditions under which other parts fail. In some cases, we cannot completely prevent corrosion, but we can try to avoid obsolescence or the component due to corrosion. 

Electrochemical techniques have been used for years to study fundamental phenomenological corrosion reactions of metals in corrosive environments. Unfortunately, the learning curve in the reduction of these elec trochemical theories to practice has been painfully slow. However, a recent survey has shown that many organizations in the... 

The potentiodynamic polarization electrochemical technique can be used to study and interpret corrosion phenomena. It may also furnish useful information on film breakdown or repair. 

The use of impedance electrochemical techniques to study corrosion mechanisms and to determine corrosion rates is an emerging technology. Elec trode impedance measurements have not been widely used, largely because of the sophisticated electrical equipment required to make these measurements. Recent advantages in micro-elec tronics and computers has moved this technique almost overnight from being an academic experimental investigation of the concept itself to one of shelf-item commercial hardware and computer software, available to industrial corrosion laboratories. 

Use and Uimitations of Electrochemical Techniques A major caution must be noted as to the general, indiscriminate use of all electrochemical tests, especially the use of AC and EIS test techniques, for the study of corrosion systems. AC and EIS techniques are apphcable for the evaluation of very thin films or deposits that are uniform, constant, and stable—for example, thin-film protective coatings. Sometimes, researchers do not recognize the dynamic nature of some passive films, corrosion produc ts, or deposits from other sources nor do they even consider the possibility of a change in the surface conditions during the course of their experiment. As an example, it is note-... 

Warnings are noted in the literature to be careful in the interpretation of data from electrochemical techniques applied to systems in which complex and often poorly understood effects are derived from surfaces which contain active or viable organisms, and so forth. Rather, it is even more important to not use such test protocol unless the investigator fuhy understands both the corrosion mechanism and the test technique being considered—and their interrelationship. 

To obtain the corrosion current from Rp, values for the anodic and cathodic slopes must be known or estimated. ASTM G59 provides an experimental procedure for measuring Rp. A discussion or the factors which may lead to errors in the values for Rp, and cases where Rp technique cannot be used, are covered by Mansfeld in Polarization Resistance Measurements—Today s Status, Electrochemical Techniques for Corrosion Engineers (NACE International, 1992). 

Specifics on the type of biological attack. This must be done by some other method such as chemical analysis of the solution (plus consideration given to limitations to the use of these several electrochemical techniques for MIC studies, noted previously under Corrosion Testing Laboratoiy Tests and subsequent subsections). 

Electrochemical techniques, proposed for the synthesis of naphthaquinone, anisaldehyde, and benzaldehyde (Walsh and Mills, 1993). 

Walsh, F., and G. Mills (1993). Electrochemical Techniques for a Cleaner Environment. Chemistry and Industry (2 August), 576-79. 

Several electrochemical techniques have been devised for the study of fast reactions. These methods require that one of the species involved in the reaction of interest be electroactive, so that the reaction under study is coupled to an electrode... 

Palladium and gold Palladium electrodeposition is of special interest for catalysis and for nanotechnology. It has been reported [49] that it can be deposited from basic chloroaluminate liquids, while in the acidic regime the low solubility of PdCl2 and passivation phenomena complicate the deposition. In our experience, however, thick Pd layers are difficult to obtain from basic chloroaluminates. With different melt compositions and special electrochemical techniques at temperatures up to 100 °C we succeeded in depositing mirror-bright and thick nanocrystalline palladium coatings [10]. 

These results are quite interesting. The initial stages of Al deposition result in nanosized deposits. Indeed, from the STM studies we recently succeeded in making bulk deposits of nanosized Al with special bath compositions and special electrochemical techniques [10]. Moreover, the preliminary results on tip-induced nanostructuring show that nanosized modifications of electrodes by less noble elements are possible in ionic liquids, thus opening access to new structures that cannot be made in aqueous media. 

Cathodic protection (CP) is an electrochemical technique of corrosion control in which the potential of a metal surface is moved in a cathodic direction to reduce the thermodynamic tendency for corrosion. CP requires that the item to be protected be in contact with an electrolyte. Only those parts of the item that are electrically coupled to the anode and to which the CP current can flow are protected. Thus, the inside of a buried pipe is not capable of cathodic protection unless a suitable anode is placed inside the pipe. The electrolyte through which the CP current flows is usually seawater or soil. Fresh waters generally have inadequate conductivity (but the interiors of galvanized hot water tanks are sometimes protected by a sacrificial magnesium anode) and the conductivity... 

Electrochemical Techniques Although the linear polarisation resistance technique has moved beyond the infancy status attributed to it in the original material, its inherent limitations remain, i.e. it is a perturbation technique, sensitive to environmental conductivity and insensitive to localised corrosion. Two developments have occurred ... 

An overall assessment and guidance to electrochemical techniques has been published... 

Development of this technique by CAPCIS (UMIST, Manchester, UK), has led to an instrument system utilising several electrochemical techniques (d.c. and a.c.) from a multi-element probe. Electrochemical noise was able to operate in an acid-condensing environment with small amounts of liquid The combination of data using several electrochemical techniques enabled identification of the corrosion mechanism in this application. 

Mansfield, F., Don t Be Afraid of Electrochemical Techniques-But Use Them With Care , Corrosion, 27, 12, 856-868, December (1988)... 

Baboian, R., (ed.), Electrochemical Techniques for Corrosion , Corrosion, Sponsored by Unit Committee T-3L, NACE (1976)... 

While each container manufacturer has developed proprietary tests, most are based on electrochemical techniques. Corrosion in enameled ETP or TFS cans can be evaluated using one of the available procedures (28, 29, 30). Corrosion performance of plain tinplate cans can be estimated using the Progressive ATC Test developed by Kamm (6, 7). These tests should speed the development of new containers. 

Shipping analysis is an extremely sensitive electrochemical technique for measuring trace metals (19,20). Its remarkable sensitivity is attributed to the combination of an effective preconcentration step with advanced measurement procedures that generate an extremely favorable signal-to-background ratio. Since the metals are preconcentrated into the electrode by factors of 100 to 1000, detection limits are lowered by 2 to 3 orders of magnitude compared to solution-phase voltammetric measurements. Hence, four to six metals can be measured simultaneously in various matrices at concentration levels down to 10 10 i. utilizing relatively inexpensive... 

FIGURE 4-27 Classification of composite electrodes used in controlled-potential electrochemical techniques. (Reproduced with permission from reference 87.)... 

Electrochemical biosensors combine die analytical power of electrochemical techniques with the specificity of biological recognition processes. The aim is to... 

The measurement of corrosion current has provided, as is well known, a quite useful electrochemical technique for determining corrosion rates. However, contrary to homogeneous corrosion, pitting corrosion is a typical heterogeneous reaction on a metal surface, so that it is difficult to estimate the actual corrosion state from the usual corrosion current data. 

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