Apparent oxygen reduction

SepaDB, VojnovicMV, VracarLj. M, Damjanovic A. 1986a. Apparent enthalpies of activation of electrodic oxygen reduction at platinum in different current density regions—1. Acid solution. Electrochim Acta 31 91-96. 

There are two ways of increasing the catalytic activity of an electrode in a fuel cell (a) by finding a better catalyst, which yields a higher value of the exchange current density, hence a lower overpotential at any chosen current density, and (b) by increasing the surface area of the electrode per unit apparent or geometrical surface area. Achievements in the former direction have not been spectacular so far, and platinum, perhaps combined with some other noble metals, is still the best eleclrocatalyst for oxygen reduction in acid solutions. 

Surprisingly, the chlorate concentration in the melt is reported to remain at the level (0.004 mole %) at which it was initially present in the lithium perchlorate. This would seem to make it improbable that chlorate is an intermediate in the reaction. However, the chlorate analyses published could correspond only to upper limits of experimental uncertainty. The reactions appear to be homogeneous and unaffected by substantial additions of lithium hydroxide or of lithium nitrate. There is at least an apparent rate reduction on addition of sodium nitrate or on passage of oxygen through the melt. Since these last rate measurements were made by chloride analysis (and not by spectrophotometry for nitrite or nitrate as in other cases), diversion of an intermediate may explain all or some of them. 

During the inhibited self-initiated autoxidation of methyl linoleate by a-Toc in solution, Niki and coworkers made the interesting observation that a-Toc acts as an antioxidant at low concentrations, but high concentrations (up to 18.3 mM) actually increased hydroperoxide formation due to a pro-oxidant effect. The pro-oxidant effect of a-Toc was observed earlier by Cillard and coworkers in aqueous micellar systems and they found that the presence of co-antioxidants such as cysteine, BHT, hydroquinone or ascor-byl palmitate inverted the reaction into antioxidant activity, apparently by reduction of a-To" to a-Toc . Liu and coworkers ° found that a mixture of linoleic acid and linoleate hydroperoxides and a-Toc in SDS micelles exhibited oxygen uptake after the addition of a-Toc. The typical ESR spectrum of the a-To" radical was observed from the mixture. They attributed the rapid oxidation to decomposition of linoleate hydroperoxides, resulting in the formation of linoleate oxy radicals which initiated reactions on the lipid in the high concentration of the micellar micro-environment. Niki and coworkers reported pro-oxidant activity of a-Toc when it was added with metal ions, Fe3+25i Qj. jjj (jjg oxidation of phosphatidyl choline liposomes. a-Toc was found... 

Apparently, Holdcroft and Funt were the first to disperse platinum microparticles into a conducting film of PPy in order to activate the oxygen reduction reaction [59]. Different methods of preparation of the PPy/Pt electrodes were developed leading to film... 

As far as the kinetics and mechanistic aspects of oxygen reduction on these non-noble metal electrocatalysts are concerned, it has been shown that these catalysts may reduce O2 to water with an apparent number of electrons transferred, n, that may reach values very close to 4. This is especially true for Fe-based electrocatalysts made either from Fe-N4 chelates or from cheaper Fe salt precursors. It seems also that the Fe-N2/C catalytic site, which is the most active site in catalysts obtained after a pyrolysis temperature > 800°C, is characterized by a low release of peroxide. Co-based catalysts release, on average, more peroxide than the corresponding Fe-based materials. Studies that were undertaken to decouple the direct 4-electron reduction of oxygen to water from the successive 2 X 2-electron reduction indicate that the direct 4-electron reduction path may be important for these catalysts. This result is in agreement with the quantum theoretical approach of Anderson and Sidik about a model of the pyrolyzed... 

Vonset measured in 0.5 M H2SO4 was 0.76 V vs. RHE, as shown in Fig. 10.18a, with an apparent number of electrons of n = 3.55 transferred during ORR. The authors suggested that quaternary nitrogen atoms are responsible for the oxygen reduction in this carbon nitride catalyst via a 4e process. 

Despite the apparent complexity of the hydrogen reaction scheme given above, it is still comparatively simple compared with many important elec-trocatalytic processes, particularly oxygen reduction and hydrocarbon oxidation. Only one intermediate (Hads) is postulated to be involved, though it occurs in a rather complex manner as either the product or reactant in two electrochemical steps, or as a reactant (in the cathodic direction) in a chemical step. This is clearly an oversimplification, since it has often been supposed that other intermediates, e.g., H2, may be involved as short-lifetime species. " In addition, other problems can arise, for example, the fact that several states of adsorbed hydrogen may occur on a number of metals. This is particularly apparent in the case of platinum. 

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