Anodes oxygen evolution

FIGURE 15.5 Polarization curves for anodic oxygen evolution at a platinum electrode in perchloric acid solutions with various concentrations (1) 1.34 (2) 3 (3) 5 (4) 9.8 M. 

In the reaction following the second pathway, the 0-0 bond is not broken while the first two electrons are added it is preserved in the HjOj produced as an intermediate, and breaks in a later step, when the hydrogen peroxide is reduced or cat-alytically decomposed. An analog for this pathway does not exist in anodic oxygen evolution. 

Anodic oxygen evolution (also anodic chlorine evolution, in solutions containing chlorides)... 

When the layer has electronic in addition to ionic conductivity, the electrochemical reaction will be partly or completely pushed out to its outer surface. In addition, other electrochemical reactions involving the solution components, particularly anodic oxygen evolution, can occur on top of the layer. 

M. Faraday was the first to observe an electrocatalytic process, in 1834, when he discovered that a new compound, ethane, is formed in the electrolysis of alkali metal acetates (this is probably the first example of electrochemical synthesis). This process was later named the Kolbe reaction, as Kolbe discovered in 1849 that this is a general phenomenon for fatty acids (except for formic acid) and their salts at higher concentrations. If these electrolytes are electrolysed with a platinum or irridium anode, oxygen evolution ceases in the potential interval between +2.1 and +2.2 V and a hydrocarbon is formed according to the equation... 

Aluminum is produced according to the Hall-Heroult process [42-44]. At the cathode, AlxFy species are reduced and lead to liquid aluminum. As the electrolysis proceeds, the metal from the aluminum oxide precipitates at the bottom of the cell. At the anode, oxygen evolution takes place producing carbon dioxide/monoxide and hence resulting in current and performance losses [42-44]. 

Figure 10-32 shows the polarization curves for both the anodic oxygen evolution at an n-type anode and the cathodic hydrogen evolution at a p-type cathode. The anodic current (solid curve, n-SC ) of the photoexcited n-type anode occurs in the range of potential more cathodic (more negative) than the rai of potential for the anodic current (dashed curve n-SC ) of a p-type anode of the same semiconductor as the photoexcited n-type anode and the cathodic current (solid curve, p-SC ) of the photoexcited p-type cathode occurs in the range of potential more anodic (more positive) than the range of potential for the cathodic current (dashed curve, n-SC ) of an n-type cathode of the same semiconductor as the photoexcited p-type cathode. 

Fig. 11-10. Anodic polarization curves observed for metallic iron, nickel, and chromium electrodes in a sulfuric acid solution (0.5 M H 2SO 4) at 25°C solid curve = anodic metal dissolution current dot-dash curve s anodic oxygen evolution current [Sato-Okamoto, 1981.]...

Likewise, the electrochemical reactions of oxygen, which is the electrochemical anodic oxygen evolution and cathodic reduction on one hand and... 

The Ru02 anode is at a clear advantage. Without treating the details of the electrocatalytic reaction—which have been dealt with by other authors (11, 23-25)—the mechanism is likely to be similar to that described by Krasil shchikov (11) [Eqs. (7a)-(7c)] for anodic oxygen evolution. The same type of mechanism is also operative with anodic 02 evolution at Ru02 anodes. 

Fig. 7. Current voltage curves of the anodic oxygen evolution at different temperatures in 30 wt% KOH at nickel anodes with coatings containing cobalt oxides (a) La0 5Co -.EaC), (b) Nin2, CouxLaO , (c) Co,04, and, for comparison, (d) RuO..

A completely novel approach to technical electrolysis for anodic oxygen evolution from alkaline solution is the use of amorphous metals, i.e. chilled melts of nickel/cobalt mixtures whose crystallization is prevented by the addition of refractory metals like Ti, Zr, B, Mo, Hf, and P (46-51). For this type of material, enhanced catalytic activity in heterogeneous catalysis of gas-phase reactions has been observed (51). These amorphous metals are shown to be more corrosion resistant than the respective crystallized alloys, and the oxides being formed at their surfaces often exhibit a higher catalytic activity than those formed on ordered alloys, as shown by Kreysa (52-54). 

Fig. 9. Current voltage curves of anodic oxygen evolution in 30 wt% KOH at 90°C from nickel and nickel anodes coated in situ by (a) C03O4, (b) Fe,Ov, and (c) a mixture of both.

Fig. 11. Temperature dependence of current voltage curves of the anodic oxygen evolution from 30 wt% KOH at nickel anodes coated by mixtures of cobalt and iron oxyhydrates.

Electrocatalysis of the Anodic Oxygen Evolution by Raney-Nickel Coatings... 

Traditionally, anodic oxygen evolution from acid solution—particularly from aqueous electrolytes containing sulfuric acid—has been performed at lead anodes that are passivated and stabilized against corrosion by a selfforming coating of Pb02. 

The coin has its reverse, however. The broadening of the potential window that is often bordered by the solvent electrochemical decomposition potentials (e.g. cathodic hydrogen evolution and anodic oxygen evolution from water) is due to an increase in reactions overvoltage. This may be caused by a diamond s lower electrochemical activity, as compared with the glassy carbon and like electrode materials. On the whole, this conclusion is corroborated by the kinetics studies on diamond electrodes... 

Another source of anodic oxygen evolution iB a direct discharge of hydroxyl ions according to equationj... 

The above proposed mechanism of photo-oxidation of water at titanium dioxide, including the sequence of reactions (32), (33), (38) and (39), presents some analogy with the mechanism of anodic oxygen evolution at a palladium electrode, in acid solutions, involving electrochemical oxidation of the metal into unstable palladium dioxide, Pd02, and its subsequent decomposition ... 

FIGURE 22.24 Anodic polarization curves for passivation and transpassivation of metallic iron and nickel in 0.5 kmol m-3 sulfuric acid solution with inserted sketches for electronic energy diagrams of passive films [32] /ip = passivation potential, TP = transpassivation potential, fb = flat band potential, /Fe = anodic dissolution current of metallic iron, Nl = anodic dissolution current of metallic nickel, and io2 — anodic oxygen evolution current. 

Figure 24 shows some examples in an energetic diagram. It shows the case in which an oxide can be oxidized before anodic oxygen evolution (CU2O to CuO), since the conditions (1) and (3) are full filled. In case of Ni(OH)2, Uox > Uq2> but the oxygen evolution is strongly hindered, while the oxidation is fast. Hence, the oxide oxidation takes place in spite of the fact that rule (2) is not fulfilled. 

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