Cathodic oxygen reduction

If the potential of a metal surface is moved below line a, the hydrogen reaction line, cathodic hydrogen evolution is favored on the surface. Similarly a potential below line b, the oxygen reaction line, favors the cathodic oxygen reduction reaction. A potential above the oxygen reaction line favors oxygen evolution by the anodic oxidation of water. In between these two lines is the region where water is thermodynamically stable. 

The second approach, followed by Vayenas et al39 is direct measurement of Ntpb and N n using cyclic voltammetry, as in aqueous electrochemistry,49 and measuring the height, Ip, or the area fldt of the cathodic oxygen reduction peak (Fig. 5.28a). Then Ntpb can be estimated from ... 

Systematic studies of cathodic oxygen reduction, unlike those of its anodic evolution, were only started in the 1950s when required for the realization of fuel cells. The large polarization of this reaction is one of the major reasons that the efficiency of the fuel cells developed so far is not very high. 

Similar size effects have been observed in some other electrochemical systems, but by far not in all of them. At platinized platinum, the rate of hydrogen ionization and evolution is approximately an order of magnitude lower than at smooth platinum. Yet in the literature, examples can be found where such a size effect is absent or where it is in the opposite direction. In cathodic oxygen reduction at platinum and at silver, there is little difference in the reaction rates between smooth and disperse electrodes. In methanol oxidation at nickel electrodes in alkaline solution, the reaction rate increases markedly with increasing degree of dispersion of the nickel powders. Such size effects have been reported in many papers and were the subject of reviews (Kinoshita, 1982 Mukerjee, 1990). 

A distinct answer has been found, however, with respect to the influence of crystallographic orientation of pyrolytic graphite on the rates of various reactions. It could be shown that the rate of cathodic oxygen reduction at the basal plane of graphite is much lower than at surfaces with edge orientation (Morcos and Yeager, 1970). To the contrary, the rates of simple redox reactions hardly depend on face orientation. 

Complex Base-Metal Oxides Complex oxide systems include the mixed oxides of some metals which have perovskite or spinel structure. Both the perovskites and the spinels exhibit catalytic activity toward cathodic oxygen reduction, but important differences exist in the behavior of these systems. 

An example for a compound of the perovskite type is LaNiOj. In other com-ponnds of the perovskite type, nickel may be replaced by cobalt or iron, and lan-thannm in part by alkaline-earth metals, an example being Lag 8Sro2Co03. The activity of perovskites toward cathodic oxygen reduction is low at room temperature but rises drastically with increasing temperature (particularly so above 150°C). In certain cases the activity rises so much that the equilibrium potential of the oxygen electrode is established. 

At the N4 complexes, cathodic oxygen reduction has been studied in the greatest detail. These systems are of great practical value inasmuch as these complexes are practically the only nonplatinum catalysts that can be used for oxygen reduction in acidic solutions. 

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,... 

Fuel cells (continued) metal catalysis, cathodic oxygen reduction, 40 127... 

Cyclic voltammetry was also used under conditions of ethylene oxidation.31 The rate of carbon dioxide production was seen to vary with the potential of the cell as would be expected from a system exhibiting NEMCA. Cyclic voltammetry was used to estimate the coverage of oxygen under working conditions by comparing the cathodic oxygen reduction peak with the peak obtained in the absence of reaction. 

Anodic hydrogen oxidation and cathodic oxygen reduction in different types of fuel cells... 

Cathodic hydrogen evolution is the counterreaction to anodic chlorine evolution. It is expected to be superseded by the energy-saving cathodic oxygen reduction (61) only after a decade of further development and only if energy prices continue to increase. 

V. Electrocatalysis of Cathodic Oxygen Reduction and Anodic Hydrogen Oxidation in Fuel Cells... 

Anodic hydrogen oxidation and even more cathodic oxygen reduction is kinetically hampered at low temperature, so that anodic hydrogen oxidation in AFCs, PEMFCs, and PAFCs demands catalysts of highest activity, that is, platinum metals and platinum in particular. Also Raney nickel is used in... 

Also for cathodic oxygen reduction in low-temperature fuel cells, platinum is indispensible as a catalyst whereas the cathodic electrocatalysts in MCFCs and SOFCs are lithiated nickel oxide and lanthanum-manganese per-ovskite, respectively. Appleby and Foulkes in the Fuel Cell Handbook (101) reviewed the fundamental work as well as the technologically important publications covering electrocatalysis in fuel cells till 1989. 

Electrocatalyzed oxygen reduction proceeds always in the adsorbed state. Figure 17 depicts the three different modes of oxygen adsorption on a metal or metal oxide surfaces, which are supposed to be of relevance for cathodic oxygen reduction (113). So-called Griffith adsorption (I), which has been observed by Gland and co-workers (114) on Pt(l 11) surfaces due to interac-... 

Pyrochlores (general stoichiometryt A2B2C>60 ), whose structure is composed of a cubic A206 framework of corner-shared octahedra interpenetrated by a A20 subarray of corner-shared O A tetrahedra, are presently discussed as potential catalysts for cathodic oxygen reduction (125). 

Fig. 22, Comparison of current voltage curves of cathodic oxygen reduction in alkaline electrolyte (a) PTFE-bonded nonactived soot cathode, (b) PTFE-bonded Pt-activated soot cathode, (c) Silflon cathodes consisting essentially of submicrometer PTFE particles covered by a 0.1-fim Ag layer.

The electrode kinetics of cathodic oxygen reduction at Pt in contact with the acidic polymer is enhanced by a factor of at least 10 compared to aqueous sulfuric or phosphoric acid at temperatures of 50 to 80°C (759) because the sulfonic acid groups interact adsorptively less with Pt than S04 2 or P04 ions, leaving the greater part of the Pt surface free for adsorption of O2. 

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