Polarization oxygen evolution

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

The polarization curves for the oxygen evolution reaction are more complex than those for hydrogen evolution. Usually, several Tafel sections with different slopes are present. At intermediate CD their slope b is very close to 0.12 V, but at low CD it sometimes falls to 0.06 V. At high CD higher slopes are found at potentials above 2.2 V (RHE) new phenomena and processes are possible, which are considered in Section 15.6. 

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. 

FIGURE 15.9 Anodic polarization curves recorded at a platinum electrode in the region of high anodic potentials in the presence of acetate ions (1) total current (2) partial current of oxygen evolution (3) partial current of oxidation of adsorbed species. 

Fig. 10-28. Polarization curves for cell reactions of photoelectrolytic decomposition of water at a photoezcited n-type anode and at a metal cathode solid curve M = cathodic polarization curve of hydrogen evolution at metal cathode solid curve n-SC = anodic polarization curve of oxygen evolution at photoezcited n-type anode (Fermi level versus current curve) dashed curve p-SC = quasi-Fermi level of interfadal holes as a ftmction of anodic reaction current at photoezcited n-type anode (anodic polarization curve r re-sented by interfacial hole level) = electrode potential of two operating electrodes in a photoelectrolytic cell p. sc = inverse overvoltage of generation and transport ofphotoezcited holes in an n-type anode.

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.]...

Figure 4. Polarization curves of carbon corrosion and oxygen evolution reactions based on measured carbon corrosion kinetics for Pt/Vulcan and Pt/Graphitized-Vulcan and oxygen evolution kinetics for Pt/C catalysts. The upper horizontal dotted line denotes a current density equivalent to oxygen crossover through membrane from cathode to anode.

Fig. 1. Polarization curve of metals with active, passive and (a) transpassive potential range including oxygen evolution (b) passive potential range going directly to oxygen evolution (c) continuing passivity for valve metals to very positive potentials. Pitting between critical pitting lim and inhibition potential fsj in the presence of aggressive anions and inhibitors.

Fig. 27a. Potentiodynamic polarization Curve of Fe5Cr in 0.5 M H2SO4 with potential ranges of hydrogen evolution, active dissolution (Cr2+), passivity (Cr3+), transpassivity (C Cb2-), and oxygen evolution [69].

Under Ti02 electrode polarization slightly anodic from the Hatband potential (Vft), a cathodic current superimposed to the anodic photocunent (transient behaviour) can be observed (Fig. 1). This catohdic effect is attributed to the recombination with 6cb of holes trapped at surface species (mainly OH° radicals and H202 molecules) photogenerated at intermediate steps of oxygen evolution (Salvador, 1985). 

The long-term stability of the material is an essential aspect, particularly when a periodical polarity reversal is applied. Using metallic or nonstable materials which could be corroded (mainly under oxygen evolution conditions) induce undesired long-term pollutions in treated waters and moreover generate additional costs linked to frequent replacement of the electrodes. 

Passivation potential — Figure 1. Polarization curves of three metals in 0.5 M H2SO4 with active dissolution, a passive potential range, and transpassive dissolution and/or oxygen evolution at positive potentials Ep(Cr) = -0.2 V, -Ep(Fe) FP(Ni) = 0.6 V [i]... 

However, as indicated in the text of this chapter, there are many other ways of generating such radical cations, occasionally leading, under oxygen atmosphere, to reaction products similar to those obtained in the photoinduced oxygenation pathway. Although, in many instances, the different modes of evolution of radical cation intermediates, dictated by reaction conditions (solvent polarity, oxygenated or inert atmosphere, the presence or absence of other intermediates, nucleophiles,... 

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