Counterelectrodes reactions

Cathodic hydrogen evolution is also of some significance as a counterelectrode reaction in anodic organoelectrosyntheses as, for instance, in the Kolbe electrosynthesis of sebacic acid (63) or the anodic oxidation of toluenes to benzaldehydes (64). 

Besides the major features in electrolysis discussed here, there are smaller problems to be solved. It is, of course, easiest to consult an experienced electrochemist on such matters, but such a person may not always turn up when wanted, so the following items are briefly mentioned circuit, counterelectrode reaction, removal of oxygen before electrolysis, removal of impurities, detection of intermediates, integration of current, and workup. 

In divided cells the reaction at the counterelectrode is seldom a problem in aqueous solvents, oxygen evolution at the anode is convenient if chloride is desired as the anion, it is advisable to add ethanol to the anodic compartment as it reacts with the evolved chlorine. In oxidations, hydrogen evolution is a convenient counterelectrode reaction it may also be used in nonaqueous solvents [68-73]. In methylene chloride it has been noted that chloride ion formed by reduction of methylene chloride at the counterelectrode could diffuse to the anode compartment and participate in the follow-up reactions [426] this can be avoided by adding a little acetic acid to the cathode compartment. 

As Fig. 27 illustrates, there are basically three types of photoelectrochemical devices for solar energy conversion. The first type is regenerative in nature and the species that are photooxidized at the -type semiconductor electrode are simply re-reduced at the counterelectrode (Fig. 27a). Instead of an elec-trocatalytic electrode [291, 292] where the counterelectrode reaction occurs in the dark (this is the situation schematized... 

The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

Despite the potential impact of novel photosynthetic routes based on these developments, the most ambitious application remains in the conversion of solar energy into electricity. Dvorak et al. showed that photocurrent as well as photopotential response can be developed across liquid-liquid junctions during photoinduced ET reactions [157,158]. The first analysis of the output power of a porphyrin-sensitized water-DCE interface has been recently reported [87]. Characteristic photocurrent-photovoltage curves for this junction connected in series to an external load are displayed in Fig. 22. It should be mentioned that negligible photoresponses are observed when only the platinum counterelectrodes are illuminated. Considering irradiation AM 1, solar energy conversions from 0.01 to 0.1% have been estimated, with fill factors around 0.4. The low conversion... 

The photovoltaic effect is initiated by light absorption in the electrode material. This is practically important only with semiconductor electrodes, where the photogenerated, excited electrons or holes may, under certain conditions, react with electrolyte redox systems. The photoredox reaction at the illuminated semiconductor thus drives the complementary (dark) reaction at the counterelectrode, which again may (but need not) regenerate the reactant consumed at the photoelectrode. The regenerative mode of operation is, according to the IUPAC recommendation, denoted as photovoltaic cell and the second one as photoelectrolytic cell . Alternative classification and terms will be discussed below. 

The photopontential also approaches to zero when the semiconductor photoelectrode is short-circuited to a metal counterelectrode at which a fast reaction (injection of the majority carriers into the electrolyte) takes place. The corresponding photocurrent density is defined as a difference between the current densities under illumination, /light and in the dark, jDARK ... 

The potential which controls the photoelectrochemical reaction is generally not the photopotential defined by Eqs (5.10.20) and (5.10.21) (except for the very special case where the values of v, REdox and the initial Fermi energy of the counterelectrode are equal). The energy which drives the photoelectrochemical reaction, eR can be expressed, for example, for an n-semiconductor electrode as... 

Platinum-loaded Ti02 systems can be considered as a short-circuited photo-electrochemical cell where the Ti02 semiconductor electrode and metal Pt counterelectrode are brought into contact [159]. Light irradiation can induce electron-hole (e -h +) pair formation and surface oxidation and also reduction reactions on each Pt/Ti02 particle (Figure 4.11). These powder-based systems lack the advantage of... 

A new approach to improve the performance of solar devices using natural pigments is to employ carbon nanotube (CNT)-based counter-electrodes. As previously reported, the excited dye transfers an electron to Ti02 and so it acquires a positive charge. Then, the cationic molecule subtracts an electron from the counterelectrode which is transported by the electrolyte. This reaction is usually catalyzed by means of conductive and electrocatalytically active species for triiodide reduction of carbon coatings. CNTs have a high superficial area, which represents a very... 

In such devices the light-absorbing semiconductor electrode immersed in an electrolyte solution comprises a photosensitive interface where thermodynamically uphill redox processes can be driven with optical energy. Depending on the nature of the photoelectrode, either a reduction or an oxidation half-reaction can be light-driven with the counterelectrode being the site of the accompanying half-reaction. N-type semiconductors are photoanodes, p-type semiconductors are photocathodes, and... 

For example, the p-doping process of a typical heterocyclic polymer, say polypyrrole, can be reversibly driven in an electrochemical cell by polarising the polymer electrode vs a counterelectrode (say Li) in a suitable electrolyte (say LiC104-PC). Under these circumstances the p-doping redox reaction (9.15) can be described by the scheme ... 

Diffusion-Layer Model Let us consider again the general electrochemical reaction (6.6). Initially, at time before electrolysis, the concentration of the solution is homogeneous at all distances x from the electrode, equal to the bulk concentration of reactant Ox. In a more rigorous consideration, one would say that the concentration of the solution is homogeneous up to the outer Helmholtz plane (OHP), that is, up to x = xqhp-When a constant current is applied to the test electrodes and counterelectrodes such that the reaction... 

The preferred method for preparing STM tips is the dc dropoff method. The basic setup is shown in Fig. 13.1. It consists of a beaker containing an electrolyte, typically 2M aqueous solution of NaOH. A piece of W wire, mounted on a micrometer, is placed near the center of the beaker. The height of the W wire relative to the surface of the electrolyte can then be adjusted. The cathode, or counterelectrode, is a piece of stainless steel or platinum placed in the beaker. The shape and location of the cathode has very little effect on the etching process, which can be chosen for convenience. A positive voltage, 4 V to 12 V, is applied to the wire, which is the anode. Etching occurs at the air-electrolyte interface. The overall electrochemical reaction is (Ibe et al., 1990) ... 

This reaction occurs in about 10 ns when R is an iodide ion in the 0.5 M concentration range [5]. Diffusion of 2 through the nanocrystalline Ti02 film to the substrate Sn02 electrode and diffusion of the oxidized redox species, R +, through the solution to the counterelectrode allow both charge carriers to be transferred to the external circuit where useful work is performed. The transport of electrons [7,24-29] and redox species [30] will not be considered further except insofar as they relate to the interfacial processes that are the focus of this chapter. 

This reaction has been studied in some detail [2,4,31,32] and will be considered only briefly here. It is a remarkably slow process (microseconds to milliseconds) at short circuit and, thus, does not limit the short-circuit photocurrent density, Jsc. However, the rate of reaction (3) [33] and of the other recombination reactions increases as the potential of the substrate electrode becomes more negative [e.g., as the cell voltage charges from short-circuit (0 V) to its open-circuit photovoltage, Voc, (usually between —0.6 V and —0.8 V versus the counterelectrode)]. At open circuit, no current flows and the rate of charge photogeneration equals the total rate of charge recombination. 

The triiodide ion, I3-, formed by the reaction of oxidized Ru ( ) dye with the I- ion, is reduced back to the I" ion at the surface of the counterelectrode. In order to reduce the triiodide ion effectively, the counterelectrode must have a high electrocatalytic activity. Pt sputtered (5-10 Tg/cm2, or approximately 200 nm thickness) FTO glass or carbon material is usually used as the counterelectrode. 

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