Oxygen reduction mechanism

FIG. 3 Scheme of 2 1 1 electron oxygen reduction mechanism at the c h ochrome c oxidase active site. (From Refs. 15,17-19, and 24.)... 

Oxygen Reduction Oxygen reduction often plays a major role in corrosion, in particular in neutral and alkaline electrolytes. The oxygen-reduction mechanism is called either direct or indirect, depending on the details of the reaction. The indirect reduction mechanism includes the formation of intermediate products (4). 

Oxygen (air) is reduced at the cathode catalytic surface. It is generally accepted that the first electron transfer is the rate determining step (rds) in a four-electron oxygen reduction mechanism in acid medium, yielding water as the final product [3, 13, 28, 39, 40], but some disagreement still exists with respect to the reaction mechanism. The overall reduction reaction is ... 

Oxygen Reduction Mechanism in Acid Medium on Pt and Pt-Alloys... 

Tripkovic V, Skulason E, Siahrostami S, Norskov JK, Rossmeisl J (2010) The oxygen reduction mechanism on Pd(lll) from density functional theory calculations. Electrochim Acta 55 7975-7981... 

Sanchez-Sanchez CM, Rodriguez-Lopez J, Bard AJ (2008) Scaiming electrochemical microscopy. 60. Quantitative calibration of the SECM substrate generation/tip collection mode and its use for the study of the oxygen reduction mechanism. Anal Chem 80(9) 3254-3260... 

While it may be relatively easy to understand the interaction between O2 (a diradical in its fundamental state) and the 3dz orbital of the L4Fe catalytic site (see the Energy Scheme in Fig. 10.21), it is not so easy to understand the interaction mechanism and reaction path followed during the reduction reaction of O2 on N-doped carbons in acid medium. Several calculations were performed by various groups to determine the precise origin of the reduction capabilities and the oxygen reduction mechanism on these N-based catalysts. They will now be reviewed. 

This chapter focuses on the theoretical modeling studies of ORR catalysts for PEMFC. Theoretical methods, such as density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulation, are presented. Current understanding of ORR mechanism in acidic medium is briefly discussed. Recent theoretical investigations on oxygen reduction electrocatalysts, such as Pt-based catalysts, non-Pt metal catalysts (Pd, Ir, CuCl), and non-precious metal catalysts (transitional metal macrocyclic complexes, conductive polymer materials, and carbon-based materials), are reviewed. The oxygen reduction mechanisms catalyzed by these catalysts are discussed based on the results. 

Oxygen Reduction Mechanism on Metal-Free Catalysts... 

When a corrosion cell is formed between an active anode and a passive cathode (see Chap. 7), oxygen reduction takes place on the passive film. The rate of oxygen reduction depends on the electronic properties of the film in a complicated way, the reason being that the oxygen reduction mechanism involves several steps. Figure 6.28 [30] illustrates the fact that the reduction rate on a passive oxide films depends not only on the film properties, but also on the nature of the oxidizing agent. 

Figure 1 shows electrochemical oxygen reduction mechanism in alkaline system. Oxygen reduction in alkaline system is cmisidered to proceed by two overall pathways. One is the direct 4-electron pathway (O2 + 4e + 2H2O = 40H ), and the other one is the peroxide pathway (O2 + 2e + H2O = H02 + OH ), which produces hydrogen peroxide as an intermediate product. The H02 is further reduced to OH by either electrochemical reaction or catalytical decomposition reaction. The reactions are dependent on the kind of electrocatalysts [4]. 

The oxygen reduction mechanism of the electrocatalyst is usually examined by the rotating ring-disk electrode technique as shown in Fig. 2. Cathodic electrochemical oxygen reduction occurs on the disk electrode attached with electrocatalyst in the alkaline aqueous electrolyte containing dissolved oxygen. While, the... 

Anion-Exchange Membrane Fuel Cells, Oxide-Based Catalysts, Fig. 1 Oxygen reduction mechanism in alkaline system... 

It is well known that the catalysts, Pt-family, transition-metal oxides, LaB03(B Mn, Co)-based perovskite-type oxide [5], metal-nitride, metal-sulfide, and organometallic compounds, show direct 4-electron oxygen reduction mechanism. 

Nishina T, Uchida I, Selman JR (1994) Gas electrode reactions in molten carbonate media V. Electrochemical analysis of the oxygen reduction mechanism at a fully immersed gold electrode. J Electrochem Soc 141 1191-1198... 

Figure 1.19. Oxygen reduction mechanism on noble catalysts [52]. (Reprinted from Journal of Power Sources, 152(2), Wang Bin, Recent development of non-platinum catalysts for oxygen reduction reaction, 1-15, 2005, with permission from Elsevier.)...

Currently, several air-side contamination models have been published in the literature, ranging from simple empirical and adsorption models to general kinetic models. These models have been applied to simulate and predict SO2, NO2, NH3, and toluene contamination. The kinetic model is a very general one based on the associative oxygen reduction mechanism. It takes into account contaminant reactions, such as surface adsorption, competitive adsorption, and electrochemical oxidation, and has the capability of simulating and predicting both transient and steady state cell performance. The model can be applied to other cathode contaminants, e.g., SO2 and NO2. 

Deng H, Peljo P, Momotenko D, Cortes-Salazar F, Stockmann TJ, Kontturi K, Opallo M, Girault HH (2014) Kinetic differentiation of buLk/interfacial oxygen reduction mechanisms at/near liquid/liquid interfaces using scanning electrochemical microscopy. J Electroantd Chem 732 101-109... 

---