Oxygen electrode reactions

The situation in alkaline solutions is very different. In this case, many materials that exhibit little adsorption of oxygenated material show reasonable rates at high pH. Carbons are good examples. In general, these materials often have low Tafel slopes (typically 3RT/2F), indicating a chemical step following a preliminary one-electron transfer. The reaction on these materials is pH independent, which implies a process of the type  

Other surfaces, typically platinum, show pH-dependent processes which have been variously explained. Damjanovic and Brusic believe that the rds is similar to reaction (93) under Temkin conditions, with a Tafel slope or RT/F  

volcano relations should also arise as a function of redox potential. 

One side of one is shown, with the correct slope, in Ref. 129 for oxygen reduction in alkaline solution. For the macrocyclics, optimum catalysis demands optimum spin crossover and electronic conductivity is of the charge transfer complex type in thin layers via charged reaction inter-mediates. In acid solutions, rates are lower on the macrocyclics, but some recent work on face-to-face cobalt porphyrins is exciting.  

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These subsystems profoundly affect the fuel cell system performance. As an example, the inherently slow air (oxygen) electrode reaction must be acceler-... 

Damjanovic, A. Mechanistic Analysis of Oxygen Electrode Reactions 5... 

In the case of aqueous solutions containing dissolved particles (solutes), a number of localized electron levels associated with solute particles Eirise in the mobility gap of aqueous solutions as shown in Fig. 2-34. These localized electron levels of solutes may be compared with the localized impiuity levels in semiconductors. In electrochemistry, the electron levels of the solutes of general interest are those located within the energy range from - 4 eV to - 6 eV (around the electron levels of the hydrogen and oxygen electrode reactions) in the mobility gap. 

These levels of interfacial redox electrons are connected with the hydrogen and oxygen electrode reactions. As noted in Sec. 5.1.2, the electron level of adsorbate particles is broadened by contact adsorption and undergoes the Franck-Condon level splitting due to a difference in adsorption energy between the oxidized particle and the reduced particle on the interface of semiconductor electrodes as shown in Fig. 5-59. 

As the adsorption affinity of redox particles on the electrode interface increases, the hydrated redox particles is adsorbed in the dehydrated state (chemical adsorption, contact adsorption) rather than in the hydrated state (ph3 ical adsorption) as shown in Fig. 7-2 (b). Typical reactions of redox electron transfer of dehydrated and adsorbed redox particles on electrodes are the hydrogen and the oxygen electrode reactions in Eqns. 7-6 and 7-7 ... 

Fig. 10-14. Energy levels and polarization curves (current vs. potential) for anodic transfer ofphotoexdted holes in oxygen reaction (2 HgO. -t- 4h O24 4 H. ) on a metal electrode and on an n-type semiconductor electrode j = anodic reaction current ep(02 20)- Fermi level of oxygen electrode reaction dCpi, = gain of photoenergy q = potential for the onset of anodic photoexdted ox en reacti . 4 pi, (=-Ae.. le) = shift of potential for the onset of anodic oxygen reaction from equilibrium oxygen potential in the negative direction due to gain of photoenergy in an n-type electrode Eib = flat band potential of an n-type electrode.

Trasatti, S. (1990) Electrode kinetics and electrocatalysis of hydrogen and oxygen electrode reactions. 1. Introduction, in Electrochemical Hydrogen Technologies (ed. 

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. 

Diagnostic plots for heterogeneous catalytic electrode reactions at the RRDE have many features in common with those for simple parallel reactions [178]. This type of analysis is important in the investigation of the oxygen electrode reaction where non-electrochemical surface processes can occur. 

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 ... 

The oxygen electrode reaction with accompanying standard potentials at 25°C may be represented as... 

O Grady et al. [227] studied the reduction and evolution of oxygen on RuO on titanium and reported that the cathodic reaction involves the formation of peroxide in solution. From y 1/2 vs. rotating disk electrodes, the authors concluded that the reaction 02/H202 was in equilibrium at the surface [227], Miles et al. [428] also studied the oxygen electrode reactions on several metal oxides (Ir, Ru, Pd, and Rd)... 

Although the thermodynamic potential for the oxygen electrode reaction [i.e. 1.229 V (RHE)] is lower than that of the chlorine electrode, in practice the kinetics of the chlorine electrode reactions are substantially faster than... 

Of the various models which have been proposed for the oxygen electrode reactions, the model of Presnov and Trunov [341, 345] for oxygen reduction at semiconductor oxide electrodes deserves special mention. This model is based on concepts of coordination chemistry and local interaction of surface cation d-electrons with HO", H20, and 02 acceptor species in solution. 

The major dehciency of the oxygen electrode reaction is its low exchange current density (about 10 A/cm on a smooth surface) in acid electrolytes on even the best-known electrocatalyst (a platinum-chromium alloy). This value is about six orders of magnitude lower than that for the hydrogen electrode reaction in the same electrolyte. The reaction is about three orders of magnitude faster on smooth platinum or nickel oxide surfaces in an alkaline medium as compared to acid. The... 

A. Damjanovic, Mechanistic analysis of oxygen electrode reactions, in Modem Aspects of Electrochemistry, No. 5 (Eds. B. E. Conway, J. O. M. Bockris), Plenum Press, New York, 1969, pp. 369-483, Chapter 5. 

Because electrochemical kinetic data are available only for the hydrogen electrode reaction (HER, H2/H+), the oxygen electrode reaction (OER, O2/H2O), and the hydrogen peroxide electrode reaction (HPER, H2O2/H2O), only H2, O2, and H2O2 are considered as the redox species in the MPM. The redox reactions of interest are therefore written as follows [35] ... 

M. Kleitz and T. Kloidt, Dessemondt, Conventional oxygen electrode reaction facts and models, in F.W. Poulsen, J.J. Bentzen, T. Jacobsen, E. Skou, M.J.L. OstergSrd (Eds.), High Temperature Electrochemical Behaviour of Fast Ion and Mixed Conductors. Rise National Laboratory, Denmark, 1993, pp. 89-116. 

FIGURE 22.6 Electron energy diagrams for an n-type semiconductor electrode in the dark (a) and in the photoexcited state (b) 8s = band edge level at the interface, eF(H+ — Fermi level of the normal hydrogen electrode reaction, Sp /ifeo) = Fermi level of the normal oxygen electrode reaction, Asph = photo potential, p p = quasi-Fermi level of photoexcited holes, and nsF = quasi-Fermi level of photoexcited electrons (nsF eF for n-type semiconductors). 

Furthermore, as we saw in a foregoing section, photoexcitation produces in a semiconductor electrode electron-hole pairs and introduces a photo-potential, which reduces the space charge potential in the semiconductor. With an n-type semiconductor in contact with a corroding metal, photoexcitation raises the Fermi level up to the flat band level of the semiconductor, thus shifting the corrosion potential in the less positive direction toward the flat band potential of the n-type oxide as shown in Figure 22.35c. Photoexcitation therefore will shift the corrosion potential in the less positive (more cathodic) direction and the corrosion will then be suppressed. With some n-type oxides such as titanium oxide, photoexcitation brings the interfacial quasi-Fermi level, peF, down to a level lower than the Fermi level, F(redox> of the oxygen electrode reaction ... 

The oxygen electrode reactions taking place at difierent pH values are as follows ... 

Damjanovic A. Mechanistic analysis of oxygen electrode reactions. Mod Asp Electrochem 1969 5 369-483. 

Mauvy F, Bassat J M, Boehm E, Manaud J P, Dordor P and Grenier J C (2003), Oxygen electrode reaction on Nd2Ni04+5 cathode materials impedance spectroscopy study , Solid State Ionics, 158,17-28. 

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