The Counter Electrode

The counter (auxiliary) electrode is used in the three-electrode system only. In this system, the current flows between the working and the counter electrode. Either a piece of platinum foil or a platinum or titanium wire is usually employed as the counter electrode. Carbon rods are also used. It is recommended that the area of the counter electrode is substantially larger than that of the working electrode. If this condition is met, the counter electrode should not affect the current measurement due to, e.g. passivation, deactivation and blocking of the surface. 

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The situation in figure C2.8.5(b) is different in that, in addition to the mechanism in figure C2.8.5(a), reduction of the redox species can occur at the counter-electrode. Thus, electron transfer tlirough the layer may not be needed, as film growth can occur with OH species present in the electrolyte involving a (field-aided) deprotonation of the film. The driving force is provided by the applied voltage, AU. 

Potentiometric measurements are made using a potentiometer to determine the difference in potential between a working or, indicator, electrode and a counter electrode (see Figure 11.2). Since no significant current flows in potentiometry, the role of the counter electrode is reduced to that of supplying a reference potential thus, the counter electrode is usually called the reference electrode. In this section we introduce the conventions used in describing potentiometric electrochemical cells and the relationship between the measured potential and concentration. 

The results of several studies were interpreted by the Poole-Erenkel mechanism of field-assisted release of electrons from traps in the bulk of the oxide. In other studies, the Schottky mechanism of electron flow controlled by a thermionic emission over a field-lowered barrier at the counter electrode oxide interface was used to explain the conduction process. Some results suggested a space charge-limited conduction mechanism operates. The general lack of agreement between the results of various studies has been summari2ed (57). 

Product Recovery. Comparison of the electrochemical cell to a chemical reactor shows the electrochemical cell to have two general features that impact product recovery. CeU product is usuaUy Uquid, can be aqueous, and is likely to contain electrolyte. In addition, there is a second product from the counter electrode, even if this is only a gas. Electrolyte conservation and purity are usual requirements. Because product separation from the starting material may be difficult, use of reaction to completion is desirable ceUs would be mn batch or plug flow. The water balance over the whole flow sheet needs to be considered, especiaUy for divided ceUs where membranes transport a number of moles of water per Earaday. At the inception of a proposed electroorganic process, the product recovery and refining should be included in the evaluation to determine tme viabUity. Thus early ceU work needs to be carried out with the preferred electrolyte/solvent and conversion. The economic aspects of product recovery strategies have been discussed (89). Some process flow sheets are also available (61). 

In Fig. 3-25 the locational dependence of t/, and is shown together. For practical applications and because of possible disturbance by foreign fields (e.g., stray currents) and t/g are less amenable to evaluation than f/g, which can always be determined by a point of inflection between two extreme values [50]. Furthermore, it should be indicated by Fig. 2-7 that there is a possibility of raising the sensitivity by anodic polarization which naturally is only applicable with small objects. In such cases care must be particularly taken that the counter electrode is sufficiently far away so that its voltage cone does not influence the reference electrodes. 

Figure 3.6-1 The electrochemical window of 76-24 mol % [BMMIM][(CF3S02)2N]/Li [(Cp3S02)2N] binary melt at a) a platinum working electrode (solid line), and b) a glassy carbon working electrode (dashed line). Electrochemical window set at a threshold of 0.1 mA cm. The reference electrode was a silver wire immersed in 0.01 m AgBp4 in [EMIM][BF4] in a compartment separated by a Vicor frit, and the counter-electrode was a graphite rod.

Reference electrode (RE) and potentiostatic setpoint are fed to the inverting and noninverting input of an operational amplifier. The counter-electrode (CE) is connected to the output of the operational amplifier. I (EC) electrochemical current. 

Fig. 19.39 Schematic representation of reactions during (a) controlled potential and (b) conventional corrosion tests in acidic chloride solutions. In (a) charge balance must be maintained by migration of Cl" ions, since the cathodic reaction occurs elsewhere at the counter-electrode. In (b) the anodic and cathodic sites are in close proximity, and charge balance is maintained without migration of Cl" ions from the bulk solution (after France and Greene )...

Both the counter and the reference electrodes are essential for fundamental NEMCA studies. They need not be of the same material with the catalyst. The counter electrode-solid electrolyte interface does not have to be polarizable. In fact, it is advantageous when it is not, because then most of the applied potential difference ends up as overpotential at the catalyst and not at the counter electrode. 

When a current I flows in an electrochemical cell, such as the one shown in Fig. 4.1, between the catalyst, or working electrode (W) and the counter electrode (C), then the potential difference Uwc deviates from its open-circuit value U c. The electrochemical cell overpotential t Wcis then defined from ... 

As already shown in Figures 5.10 and 5.11 the equality Aconstant current is applied at t = 0 between the catalyst and the counter electrode and one follows the time evolution of UWr by a voltmeter and of

[Pg.223]

At t=0 a constant anodic current I=5mA is applied between the Pt catalyst film and the counter electrode. The catalyst potential, Urhe, reaches a new steady state value Urhe=1.18 V. At the same time the rates of H2 and O consumption reach, within approximately 60s, their new steady-state values rH2-4.75T0 7 mol/s, ro=4.5T0 7 mol/s. These values are 6 and 5.5 times larger than the open-circuit catalytic rate. The increase in the rate of H2 consumption (Ar=3.95T0 7 mol H2) is 1580 % higher than the rate increase, (I/2F=2.5T0 8 mol/s), anticipated from Faraday s Law. This shows clearly that the catalytic activity of the Pt catalyst-electrode has changed substantially. The Faradaic efficiency, A, defined from ... 

This implies that Electrochemical Promotion or NEMCA is an electrochemically controlled metal-support interaction. It also implies that metal-support interactions on these supports can be viewed as a self-driven wireless NEMCA system, such as the one explored by Cavalca, Haller and Vayenas for the CH3OH oxidation system under catalyst-counter electrode short-circuit conditions where gaseous 02 replenishes O2 in the YSZ support at the vicinity of the counter electrode.24... 

Potentiostat-Galvanostat, for applying fixed values of potential between the catalyst and the reference electrode or fixed value of current between the catalyst and the counter electrode. 

We have previously considered the mechanism of electrospray ionization in terms of the charging of droplets containing analyte and the formation of ions as the charge density on the surface of the droplet increases as desolvation progresses. The electrospray system can also be considered as an electrochemical cell in which, in positive-ion mode, an oxidation reaction occurs at the capillary tip and a reduction reaction at the counter electrode (the opposite occurs during the production of negative ions). This allows us to obtain electrospray spectra from some analytes which are not ionized in solution and would otherwise not be amenable to study. In general terms, the compounds that may be studied are therefore as follows ... 

If an anodically colored electrochromic material (e.g., Ir02) is used as one electrode in the device in Eig. 33.1fi and a cathodically colored (e.g., WO3) is used as the other electrode, a much larger change in transmission per charge supplied can be seen compared to the case when only one electrode is electrochromic. Also, the use of an intercalation material as the counter electrode may be advantageous for the device shown in Eig. 33.1a, as it can minimize undesired reactions on the counter electrode. 

Figure 10. Tunnelling of a single electron from an electrode into an intermediate island causing a Coulomb blockade. If the electrostatic energy is large enough, transport to the counter electrode happens.

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