Cathode reactions oxygen electrode

A fuel, e.g., hydrogen, methane, butane, methanol, etc., makes intimate contact with an anode or fuel electrode, and oxygen (usually in air) makes intimate contact with a cathode or oxygen electrode. Half-cell reactions occur at each electrode, and their sum is the overall reaction. Several types of fuel cell exist, each characterized by aparticular type of electrolyte. 

If the cathodic reaction (oxygen reduction) takes place at the limiting current, then the resistance at the electrode-solution interface of the cathode, / pjj (Figure 7.4), behaves as a non-linear electrical element that limits the current (current limiter). Indeed, at the limiting current plateau one has = (dE/dT)n = oo. The anode corrosion rate Vj depends, in this case, only on the value of limiting current density at the cathode, t lji that is determined by prevailing mass transport conditions. Neglecting the currents 1 i and 4 n, we find ... 

Cathode—the electrode of an electrolytic cell where reduction takes place. During corrosion, this is the area at wliich metal ions do not enter the solution. During cathodic reactions, cations take up electrons and discharge them, hence reducing oxygen. That is, there is a reduction from a higlier to a lower state of valency. 

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... 

It is so universally applied that it may be found in combination with metal oxide cathodes (e.g., HgO, AgO, NiOOH, Mn02), with catalytically active oxygen electrodes, and with inert cathodes using aqueous halide or ferricyanide solutions as active materials ("zinc-flow" or "redox" batteries). The cell (battery) sizes vary from small button cells for hearing aids or watches up to kilowatt-hour modules for electric vehicles (electrotraction). Primary and storage batteries exist in all categories except that of flow-batteries, where only storage types are found. Acidic, neutral, and alkaline electrolytes are used as well. The (simplified) half-cell reaction for the zinc electrode is the same in all electrolytes ... 

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. 

Electrode A is called the anode because the anodic reaction is favored over the cathodic reaction. In a fuel cell, the anodic oxidation of H2 is favored. The corresponding reaction at the cathode, electrode B, is the cathodic oxygen reduction reaction,... 

There existed oxidation-reduction reactions with the same reaction speed on the sulphide mineral surface in water. One is the self-corrosion of sulphide mineral. Another is the reduction of oxygen. If the equilibrium potential for the anodic reaction and the cathodic reaction are, respectively, E and, and the mineral electrode potential is E, the relationship among them is as follows ... 

E and E, . represent the equilibrium potential of mineral anodic dissolution and cathode reduction of oxygen, respectively. represents the mineral mixed potential in certain system. and Zg are current density of anodic and cathode reaction, respectively. When the discharge is the controlled step of electrode reaction, according to electrochemistry theory, the equation can be described as following ... 

Eor the purpose of modeling, consider a planar SOEC divided into anode gas channel, anode gas diffusion electrode, anode interlayer (active electrode), electrolyte, cathode interlayer (active electrode), cathode gas diffusion electrode, and cathode gas channel. The electrochemical reactions occur in the active regions of the porous electrodes (i.e., interlayers). In an SOFC, oxidant reduction occurs in the active cathode. The oxygen ions are then transported through the electrolyte, after which oxidation of the fuel occurs in the active anode by the following reactions. 

Wendt, H. and Plzak, V. (1990) Electrode kinetics and electrocatalysis of hydrogen and oxygen electrode reactions. 2. Electrocatalysis and electrocatalysts for cathodic evolution and anodic oxidation of hydrogen, in Electrochemical Hydrogen Technologies (ed. H. Wendt), Elsevier, Amsterdam, Chapter 1. 2. 

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. 

The electrode reactions are comprised of the oxidation of hydrogen on Ihe anode (the negative electrode) to hydrated protons with the release ol electrons and on the cathode (the positive electrode) the reaction of oxygen with protons la I unn water vapor with the consumption of electrons. Electrons llow from the anode through the external load to the cathode and the circuit is closed by ionic current transport through the electrolyte. In an acid cell, the current is carried by protons. 

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 ... 

As a first example of the use of reaction mechanism graphs, consider the electrochemistry of molten carbonate fuel cell (MCFC) cathodes. These cathodes are typically nickel-oxide porous electrodes with pores partially filled with a molten carbonate electrolyte. Oxygen and carbon dioxide are fed into the cathode through the vacant portions of the pores. The overall cathodic reaction is 02 + 2C02 + 4e / 2C03=. This overall reaction can be achieved through a number of reaction mechanisms two such mechanisms are the peroxide mechanism and the superoxide-peroxide mechanism, and these are considered next. 

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