Cathode reaction oxygen electrode process

General Description of Cathode Reaction and Polarization 7.3.1 Oxygen Electrode Process... 

It is not appropriate here to consider the kinetics of the various electrode reactions, which in the case of the oxygenated NaCl solution will depend upon the potentials of the electrodes, the pH of the solution, activity of chloride ions, etc. The significant points to note are that (a) an anode or cathode can support more than one electrode process and b) the sum of the rates of the partial cathodic reactions must equal the sum of the rates of the partial anodic reactions. Since there are four exchange processes (equations 1.39-1.42) there will be eight partial reactions, but if the reverse reactions are regarded as occurring at an insignificant rate then... 

Although Table 2.16 shows which metal of a couple will be the anode and will thus corrode more rapidly, little information regarding the corrosion current, and hence the corrosion rate, can be obtained from the e.m.f. of the cell. The kinetics of the corrosion reaction will be determined by the rates of the electrode processes and the corrosion rates of the anode of the couple will depend on the rate of reduction of hydrogen ions or dissolved oxygen at the cathode metal (Section 1.4). 

Corrosion is a mixed-electrode process in which parts of the surface act as cathodes, reducing oxygen to water, and other parts act as anodes, with metal dissolution the main reaction. As is well known, iron and ferrous alloys do not dissolve readily even though thermodynamically they would be expected to, The reason is that in the range of mixed potentials normally encountered, iron in neutral or slightly acidic or basic solutions passivates, that is it forms a layer of oxide or oxyhydroxide that inhibits further corrosion. 

In general, in an electrolytic process, oxygen is evolved at the anode, and hydrogen at the cathode. If these electrodes are in different compartments, with a suitable electrolyte we may expect to have reactions of oxidation taking place in the anode compartment and reactions of reduction in the cathode compartment. In inorganic chemistry, the more successful electrolytic preparations are chiefly those of oxidation while in organic chemistry, reactions of both oxidation and reduction are often successful. In inorganic chemistry, reactions of reduction are usually effected in simple ways. The several units of the necessary apparatus are connected as shown in Fig. 10. 

In the case of the ternary eutectic Li2C03-Na2C03-K2C03 at 605°C saturated with pure C02 (p02 = 6), the anodic limit is about 0.27 V vs. the oxygen electrode however, when saturated with Li20 (pO2- = 0) this melt is reported to exhibit an anodic limit of only -0.23 V [5]. The cathodic limit of ternary eutectic carbonate melts with p02 = 2 to 6 is about -1.9 to -2.1 V [5]. The reduction process produces elemental carbon according to the reaction... 

At a high cathodic potential (region II), a sharp transition is observed at the potential referred to as ET. The authors demonstrate that the sudden increase of the electrode kinetics could not be attributed to the sole electrochemical reduction of the electrode material, nor to the electrolyte reduction. They conclude that after the transition, the main electrode process is still an oxygen electrode reaction with a major change of mechanism, leading to the onset of an important electrocatalytic effect. This assertion is sustained by the analysis of ... 

Apart from electrokinetic phenomena, the electrofiltration process may also be affected by the electrochemical reactions that occur at the electrodes. A typical cathodic process and anodic process in aqueous systems are the formation of hydrogen gas at the cathode and oxygen at the anode ... 

During ECM, electrochemical dissolution of anode and cathodic evolution of hydrogen proceeds on the electrodes (the WP and TE, respectively). Along with these basic reactions, parallel reactions proceed concurrently, for example, oxygen anodic evolution, cathodic reduction of nitrate ions, if NaNC>3 electrolyte is used. It is important to note that electrochemical reactions in a narrow IEG result in gas evolution. The temperature of the electrolyte in the IEG and the void fraction increase as the electrolyte flows along the gap. This leads to a variation in the electrolyte conductivity that has an effect on the distributions of current and metal dissolution rate over the WP surface. The electrode processes and the processes in... 

The auxiliary electrode used is usually electrochemically inert, yet capable of maintaining rapid cathodic reactions to maintain the desired polymerization rate at the working electrode surface. During deposition at anodic potentials, the auxiliary electrode is exposed to (sometimes extreme) negative potentials. The increase in pH (due to the reduction of dissolved oxygen in the solution and to the reduction of water itself) may interfere with the polymerization process hence, the positioning of the auxiliary electrode is of paramount importance. To prevent an increase in pH, easily reduced species such as copper or silver salts may be added. This provides a more facile electrode reaction at the cathode and prevents electrode reactions that result in a change in pH. 

A fuel cell (Fig. 9.7) can be described as a kind of accumulator in which hydrogen is introduced at one electrode, the anode, and oxygen at the other, the cathode. By a chain of chemical reactions, differing from case to case, water is eventually formed. This process is accompanied by the creation of a voltage difference between the anode and the cathode. When the electrodes are connected, for example by a light bulb or an electric appliance, an electric current is generated. The nature of the reactions depends on the choice of the electrolyte (the medium in which the electrochemical reactions take place). 

The dominating process depends on the band structure of the oxide, the film thickness, and the electrode potential. Cathodic reactions usually take place via the CB, while anodic processes at high potentials (e.g. oxygen evolution) obey the VB-mechanism. Figure 13 shows some Tafel diagrams for ETR at passive iron... 

The rate of corrosion process will depend on the conductivity of electrolyte and the difference of potential between the anode and cathode. Particularly the oxygen access, necessary for the cathodic reaction, can be the factor limiting the rate of corrosion [98]. Simultaneously, as a result of corrosion current, the polarization of electrodes occurs (their potentials increase in respect to the equilibrium potential values) and the dynamically maintained potential value has the deciding effect on the corrosion rate. In the case of steel in paste environment strong polarization of anodic microareas occurs, which increase anodic potential, decreasing the difference of potential in respect to cathode therefore, as it results from the curves in E -pH system, the passivation of steel due to the oxides film occurs [98]. 

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