Oxygen overpotential

Oxygen overpotential is about 0.4-0.5 volt at a polished platinum anode in acid solution, and is of the order of 1 volt in alkaline solution with current densities of 0.02-0.03 A cm-2. As a rule the overpotential associated with the deposition of metals on the cathode is quite small (about 0.1-0.3 volt) because the depositions proceed nearly reversibly. 

Owing to high oxygen overpotential and high oxygen radicals formation efficiency, mainly OH radicals ([407] and references given therein), and lead dioxide electrodes are used as... 

Metal Hydrogen overpotential, V Exchange current density for hydrogen evolution, A cm 2 Oxygen overpotential, V... 

According to Bowden, 20 X 10 7 coulombs per apparent sq. cm. are required for each 100 mv. of hydrogen overpotential and 37 x 10 7 coulombs for the same amount of oxygen overpotential, on polished platinum. This indicates a capacity of 20 microfarads per apparent sq. cm. for the double layer producing the overpotential at a cathode and 37 pf. 

Fig. 31. Oxygen overpotential at 1 A cm on different metals as a function of calculated M—OH bond strength. O, Data of Riietshi and Delahay (324). (From Ref. 325.)...

In spite of their low solubility ( 5 x 10 M litre), HFeOJ ions diffuse to the positive electrode and are oxidized to solid FeOOH causing further dissolution of iron and its continous transfer to the positive electrode. The process is irreversible, the potential of the nickel electrode being too positive, even during discharge, for the reduction of trivalent iron. Further decrease of capacity is caused by the lowering of oxygen overpotential on the nickel oxide in the presence of FeOOH. The self-discharge and iron transfer processes are somewhat inhibited by additives to the electrode (sulfur) or electrolyte (e.g., lithium and sulfide ions, or hydrazine sulfate). 

Lead Alloying lead with silver, tin, or cobalt often improves the corrosion resistance of lead anodes. In many cases, the surface of a lead anode is acmaUy lead dioxide [58]. Pb02 electrodes are stable in sulfate media at low pH and the oxygen overpotential is high, but the material has poor mechanical properties and corrodes in HCl. 

Oxygen overpotential (vs. Hg/Hg2S04) as a function ofSb content [17]. 

Comparison of the anodic and cathodic Tafel constants shows that when a = 0.5, a = — a -, b = — b. Tables 3.2 and 3.3 list values of the Tafel constants for cathodic hydrogen evolution at T= 20 it 2 ° C on different metals and the effect of electrode materials and solution composition on oxygen overpotential [21]. The Tafel equation has been confirmed for numerous cathodic and anodic reactions, and its use is illustrated in the examples and case studies that follow. 

Table 3.3 The Effect of Electrode Material and Solution Composition on Oxygen Overpotential [21]...

Figure 3.3 shows the oxygen overpotential dependence on pH. Assuming that the applied potential of platinum remains at 0.515 V vs. SCE, the oxygen reduction reaction occurs only when the pH is less than or equal to 8. 

A cathodic current of 3.03 X 10 A/cm should be appHed to the iron electrode to shift the Fe/Fe electrode potential to its reversible value of —0.449 V vs. SHE, where the electrode is at equhibrium as shown in Fig. 3.10. The effect of electrode material and solution composition on oxygen overpotential is given in Table 3.3. 

The overpotentials at the anode qAnode (oxygen overpotential) and cathode qcathode (hydrogen overpotential), also referred to as charge transfer overpotentials, result from the inhibition of electron transport in the separate electrochemical reactions (see Fig. 11.2). In order for current to flow through the electrolysis cell, the resistance polarization must also be overcome. It is caused by the ohmic resistance of the ceU (electrolytes, separator and electrodes). The ohmic voltage drop can be calculated from the current density i in A cm and the surface-specific resistance R of the ceU in Q cm. ... 

It was noted above that one should seek electrode materials where the overpotential for chlorine evolution is as low as possible but where there is a considerable oxygen overpotential. In addition, for diaphragm and membrane cells there is the requirement for cathodes with a low hydrogen overpotential. Catalysis of hydrogen evolution was discussed in Chapter 1 and it was shown that strong catalysis is promoted by metals where the free energy of adsorption of hydrogen atoms has an intermediate value. This allows mechanisms such as... 

The polarization corresponding to this reaction is called oxygen overpotential. Overpotential may also occur with Cl or Br discharge, but the values at a given current density are much smaller than those for O2 or H2 evolution. 

The potential of a platinum anode at which oxygen evolves in an electrolyte of pH 10 is 1.30 V with respect to the saturated calomel electrode. What is the value of oxygen overpotential ... 

On further gradual change of the potential, the current density remains at the above low value, and the corrosion product is now Fe. At about 1.2 V, the equilibrium oxygen electrode potential is reached, but oxygen is not evolved appreciably until the potential exceeds the equilibrium value by several tenths of a volt (oxygen overpotential). The increased current densities in the region labeled transpassive represent O2 evolution plus Fe + formation. 

Platinum anodes have a limited operational range of oxidation potentials and thus attention has focused on Sn02-coated titanium materials. The tin oxide material, when doped with Sb (approximately 5%) to impart the appropriate electrical conductivity, has oxygen overpotentials some 600 mV greater than those of platinum. Tin oxide gives higher oxidation efficiencies to those of platinum, lead dioxide, ruthenium and iridium oxide (DSA) electrodes and is reported to be stable to corrosion during anodic oxidation. 

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