Electrochemical techniques working electrodes

The electrochemical investigations of gold and silver have employed a range of electrochemical techniques, working electrodes, solvents and reference electrodes. In compiling the tables that appear in this chapter, a decision was made to report all potentials as... 

Analyte Sample Electrochemical technique Working electrode Pretreatment of sample Supporting electrolyte Studied concentration/ detection limit (DE) References... 

The dominant tendency of my studies has been not so much to obtain and describe organic compounds but... to penetrate their mechanisms.. . . For undertaking this kind of problem, the classic methods of organic chemistry are far from sufficient. Physicochemical procedures become more and more necessary. I have been led to use especially optical methods (the Raman effect and ultraviolet spectra) and electrochemical techniques (conductibility, electrode potentials, and especially polarography).. . . The notion of reaction mechanism led almost automatically to envisioning the electronic aspect of chemical phenomena. From 1927, and working in common with Charles Prevost, I have directed my attention on the electronic theory of reactions." 56... 

Experimental Methods for Studying Electrochromic Materials. Redox systems which are likely to show promise as electrochromic materials are first studied, either as an electroactive surface film or an electroactive solute, at an electrochemically inert working electrode, under potentiostatic or galvanostatic control (2). Traditional electrochemical techniques (23), such as cyclic voltammetry (CV), coulometry, and chronoamperometry, all partnered by in situ spectroscopic measurements as appropriate (24,25), are employed for characterization. Three-electrode circuitry is generally employed, with coimter and reference electrodes completing the electrical circuit (2). 

Returning now to stress corrosion cracking, it is evident that it is premature to expect calculation of the ECP in the coolant circuit. Of a SCWNPP, but it is possible to perform experiments under electrochemical control using the electrochemical techniques (reference electrodes, etc.) discussed above. However, none of the work reported to date has employed a reference electrode to moni-... 

Nowadays all over the world considerable attention is focused on development of chemical sensors for the detection of various organic compounds in solutions and gas phase. One of the possible sensor types for organic compounds in solutions detection is optochemotronic sensor - device of liquid-phase optoelectronics that utilize effect of electrogenerated chemiluminescence. In order to enhance selectivity and broaden the range of detected substances the modification of working electrode of optochemotronic cell with organic films is used. Composition and deposition technique of modifying films considerably influence on electrochemical and physical processes in the sensor. 

The possibility that adsorption reactions play an important role in the reduction of telluryl ions has been discussed in several works (Chap. 3 CdTe). By using various electrochemical techniques in stationary and non-stationary diffusion regimes, such as voltammetry, chronopotentiometry, and pulsed current electrolysis, Montiel-Santillan et al. [52] have shown that the electrochemical reduction of HTeOj in acid sulfate medium (pH 2) on solid tellurium electrodes, generated in situ at 25 °C, must be considered as a four-electron process preceded by a slow adsorption step of the telluryl ions the reduction mechanism was observed to depend on the applied potential, so that at high overpotentials the adsorption step was not significant for the overall process. 

Chemical and electrochemical techniques have been applied for the dimensionally controlled fabrication of a wide variety of materials, such as metals, semiconductors, and conductive polymers, within glass, oxide, and polymer matrices (e.g., [135-137]). Topologically complex structures like zeolites have been used also as 3D matrices [138, 139]. Quantum dots/wires of metals and semiconductors can be grown electrochemically in matrices bound on an electrode surface or being modified electrodes themselves. In these processes, the chemical stability of the template in the working environment, its electronic properties, the uniformity and minimal diameter of the pores, and the pore density are critical factors. Typical templates used in electrochemical synthesis are as follows ... 

Principles and Characteristics Voltammetric methods are electrochemical methods which comprise several current-measuring techniques involving reduction or oxidation at a metal-solution interface. Voltammetry consists of applying a variable potential difference between a reference electrode (e.g. Ag/AgCl) and a working electrode at which an electrochemical reaction is induced (Ox + ne ----> Red). Actually, the exper-... 

Phase-sensitive detection is not at all specihc for EPR spectroscopy but is used in many different types of experiments. Some readers may be familiar with the electrochemical technique of differential-pulse voltammetry. Here, the potential over the working and reference electrode, E, is varied slowly enough to be considered as essentially static on a short time scale. The disturbance is a pulse of small potential difference, AE, and the in-phase, in-frequency detection of the current affords a very low noise differential of the i-E characteristic of a redox couple. 

The deposition of metals has also been studied by a large number of electrochemical techniques. For the deposition of Cu2+, for example, it is reasonable to ask whether both electrons are transported essentially simultaneously or whether an intermediate such as Cu+ is formed in solution. Such questions, like those of the ECE problem discussed above, have usually been investigated by forced convection techniques, since the rate of flow of reactant to and away from the electrode surface gives us an important additional kinetic handle. In addition, by using a second separate electrode placed downstream from the main working electrode, reasonably long-lived intermediates can be transported by the convection flow of the electrolyte to this second electrode and detected electrochemically. 

External reflectance. The most commonly applied in situ IR techniques involve the external reflectance approach. These methods seek to minimise the strong solvent absorption by simply pressing a reflective working electrode against the IR transparent window of the electrochemical cell. The result is a thin layer of electrolyte trapped between electrode and window usually 1 to 50 pm. A typical thin layer cell is shown in Figure 2.40. 

The first application of the quartz crystal microbalance in electrochemistry came with the work of Bruckenstein and Shay (1985) who proved that the Sauerbrey equation could still be applied to a quartz wafer one side of which was covered with electrolyte. Although they were able to establish that an electrolyte layer several hundred angstroms thick moved essentially with the quartz surface, they also showed that the thickness of this layer remained constant with potential so any change in frequency could be attributed to surface film formation. The authors showed that it was possible to take simultaneous measurements of the in situ frequency change accompanying electrolysis at a working electrode (comprising one of the electrical contacts to the crystal) as a function of the applied potential or current. They coined the acronym EQCM (electrochemical quartz crystal microbalance) for the technique. 

Emersion of an electrode from electrolyte with its double layer intact is now a widely accepted phenomenon and technique. Not only is it a phenomenon which deserves careful consideration and study, but also a process which opens up a new set of experimental methods to the study of the electrochemical double layer. Electrode emersion involves the careful removal of an electrode from electrolyte under potentiostatic control, usually hydrophobically 11-5). When fairly concentrated electrolyte parts ("unzips") from the electrode surface during hydrophobic emersion, the double layer remains essentially intact on the electrode surface and no electrolyte outside the double layer remains. This phenctnenon is not due to the presence of organics or other impurities as seme have suggested. The emersion process works well with rigorously clean electrode surfaces (5). 

Lithography With the STM Electrochemical Techniques. The nonuniform current density distribution generated by an STM tip has also been exploited for electrochemical surface modification schemes. These applications are treated in this paper as distinct from true in situ STM imaging because the electrochemical modification of a substrate does not a priori necessitate subsequent imaging with the STM. To date, all electrochemical modification experiments in which the tip has served as the counter electrode, the STM has been operated in a two-electrode mode, with the substrate surface acting as the working electrode. The tip-sample bias is typically adjusted to drive electrochemical reactions at both the sample surface and the STM tip. Because it has as yet been impossible to maintain feedback control of the z-piezo (tip-substrate distance) in the presence of significant faradaic current (vide infra), all electrochemical STM modification experiments to date have been performed in the absence of such feedback control. 

In principle, the auxiliary electrode can be of any material since its electrochemical reactivity does not affect the behaviour of the working electrode, which is our prime concern. To ensure that this is the case, the auxiliary electrode must be positioned in such a way that its activity does not generate electroactive substances that can reach the working electrode and interfere with the process under study. For this reason, in some techniques the auxiliary electrode is placed in a separate compartment, by means of sintered glass separators, from the working electrode. 

It should be emphasized that this design of the three-electrode cell gives good results in the majority of cases. However, as mentioned, in fast electrochemical techniques in non-aqueous solvents, iRnc can assume values which compromise the accurate control of the potential of the working electrode and hence the achievement of reliable electrochemical data. In such cases one must employ electronic circuits which compensate for the resistance of the solution. 

Voltammetric techniques involve perturbing the initial zero-current condition of an electrochemical cell by imposing a change in potential to the working electrode and observing the fate of the generated current as... 

There are a few electrochemical techniques in which the working electrode is moved with respect to the solution (i.e. either the solution is agitated or the electrode is vibrated or rotated). Under these conditions, the thickness of the diffusion layer decreases so that the concentration gradient increases. Since the rate of the mass transport to an electrode is proportional to the concentration gradient (Chapter 1, Section 4.2.2), the thinning of the diffusion layer leads to an increase of the mass transport, and hence to an increase of the faradaic currents. 

Generally, irrespective of the technique for which they are used, electrochemical cells are constructed in a way which minimizes the resistance of the solution. The problem is particularly accentuated for those techniques which require high current flows (large-scale electrolysis and fast voltammetric techniques). When current flows in an electrochemical cell there is always an error in the potential due to the non-compensated solution resistance. The error is equal to / Rnc (see Chapter 1, Section 3). This implies that if, for example, a given potential is applied in order to initiate a cathodic process, the effective potential of the working electrode will be less negative compared to the nominally set value by a amount equal to i Rnc. Consequently, for high current values, even when Rnc is very small, the control of the potential can be critical. 

A technique for such measurements is the electrochemical quartz crystal microbalance (EQCM figure 14) [71]. Here, the working electrode is part of a quartz crystal oscillator that is mounted on the wall of the electrochemical cell and exposed to the electrolyte. The resonance frequency / of the quartz crystal is proportional to mass changes Am A/ Am. With base frequencies around 10 MHz, the determination of Am in the ng range is possible. 

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