Oxygen Reduction Reaction ORR

Oxygen reduction in biosystems is carried out by cytochrome oxidase using two iron heme groups and two copper centers. Overall, it is a four-electron process that reduces oxygen to water, as it is important in living systems to minimize the production of hydrogen peroxide and reactive oxygen species (ROS). 

In the chemical industry, more than a megaton of hydrogen peroxide is produced yearly in a biphasic reaction scheme known as the anthraquinone auto-oxidation process, where reduced anthraquinone is used to reduce oxygen to H2O2 and where the anthraquinone is reduced again by hydrogen on a palladium catalyst. 

One unique property of the ORR with porphyrins adsorbed at ITTES is that the reaction mechanisms depend strongly on aggregation and self-assembly. Whereas, isolated 

Of course, it is possible to reduce oxygen at ITIES functionalized with platinum nanoparticles floating at the interface. 

As mentioned in the introduction of this chapter, water formation from hydrogen and oxygen is one of the most important reactions occurring in fuel cells. At the cathode, four protons react with molecular oxygen to form two water molecules (see Eq. 1). Although possible intermediates only consist of hydrogen and oxygen, the exact reaction mechanism is still unknown. A realistic electrochemical system (such as a fuel cell) is an extremely complex system where catalytic reactions occur in a multicomponent environment and under conditions of finite temperature, pressure, and electrode potential. 

We now discuss the idealized case of platinum catalyzed hydrogen combustion in oxygen  

To study pathways for reaction (10), we considered all possible intermediates separately H, H2, O, O2, OH, OOH, H2O2, H2O. For eaeh eompound, we evaluated stable surface sites and binding energies on Pt(lll), modeled by a finite 35 atom three-layer cluster surfaee model (see Fig. 2), and then obtained barriers 

Two major pathways ean be distinguished. First, ean first dissoeiate on the surfaee generating two atoms and then react with atoms to form water. Seeond, the Of moleeule ean react 

Starting with moleeular H2 in gas phase and the plain Pt surfaee (Ptss eluster), we find adsorption of H2 on the surface is effeetively barrierless. Adsorption lowers the energy of the whole system by 0.66 eV, or 0.33 eV per I/2 H2. This value is twice 

Bouroushian, Electrochemistry of Metal Chalcogenides, Monographs in Electrochemistry, DOI 10.1007/978-3-642-03967-6 6, 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

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In this section, we summarize the kinetic behavior of the oxygen reduction reaction (ORR), mainly on platinum electrodes since this metal is the most active electrocatalyst for this reaction in an acidic medium. The discussion will, however, be restricted to the characteristics of this reaction in DMFCs because of the possible presence in the cathode compartment of methanol, which can cross over the proton exchange membrane. 

As with the phase diagrams and Pourbaix diagrams, the theoretical standard hydrogen electrode also allows us to calculate the relative energies of intermediates in electrochemical reactions. As an example, we investigate the oxygen reduction reaction (ORR). We look at the four proton and electron transfer elementary steps ... 

This allows a direct influence of the alloying component on the electronic properties of these unique Pt near-surface formations from subsurface layers, which is the crucial difference in these materials. In addition, the electronic and geometric structures of skin and skeleton were found to be different for example, the skin surface is smoother and the band center position with respect to the metallic Fermi level is downshifted for skin surfaces (Fig. 8.12) [Stamenkovic et al., 2006a] owing to the higher content of non-Pt atoms in the second layer. On both types of surface, the relationship between the specific activity for the oxygen reduction reaction (ORR) and the tf-band center position exhibits a volcano-shape, with the maximum... 

In order to establish a clear strategy, we have examined the properties of Pt-based catalysts for both the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) systematically and comprehensively using various techniques, the results of which complement each other. We have also developed a standard method to evaluate the real activity. In this chapter, we summarize our recent research results. 

Baranton S, Coutanceau C, Gamier E, Leger JM. 2006. How does a-FePc catalysts dispersed onto high specific surface carbon support work towards oxygen reduction reaction (orr) J Electroanal Chem 590 100-110. 

Sometime probably two billion years before humans became interested in efficient catalysts for four-electron, four-proton reduction of O2 to H2O, the so-called oxygen reduction reaction (ORR),... 

Carbon is unique among chemical elements since it exists in different forms and microtextures transforming it into a very attractive material that is widely used in a broad range of electrochemical applications. Carbon exists in various allotropic forms due to its valency, with the most well-known being carbon black, diamond, fullerenes, graphene and carbon nanotubes. This review is divided into four sections. In the first two sections the structure, electronic and electrochemical properties of carbon are presented along with their applications. The last two sections deal with the use of carbon in polymer electrolyte fuel cells (PEFCs) as catalyst support and oxygen reduction reaction (ORR) electrocatalyst. 

The electrocatalytic activity of the nanostructured catalysts was investigated for electrocatalytic reduction of oxygen and oxidation of methanol. Several selected examples are discussed in this section. The results from electrochemical characterization of the oxygen reduction reaction (ORR) are first described. This description is followed by discussion of the results from electrochemical characterization of the methanol oxidation reaction (MOR). 

Concentrating on the operation of the so-called membrane electrode assembly (MEA), E includes irreversible voltage losses due to proton conduction in the PEM and voltage losses due to transport and activation of electrocatalytic processes involved in the oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) ... 

The most important electrokinetic data pertinent to fuel cell models are the specific interfacial area in the catalyst layer, a, the exchange current density of the oxygen reduction reaction (ORR), io, and Tafel slope of ORR. The specific interfacial area is proportional to the catalyst loading and inversely proportional to the catalyst layer thickness. It is also a strong function of the catalyst layer fabrication methods and procedures. The exchange current density and Tafel slope of ORR have been well documented in refs 28—31. 

These kinetic expressions represent the hydrogen oxidation reaction (HOR) in the anode catalyst layer and oxygen reduction reaction (ORR) in the cathode catalyst layer, respectively. These are simplified from the general Butler-Volmer kinetics, eq 5. The HOR... 

The phase behavior of water is directly relevant to the oxygen reduction reaction (ORR O2 + 4H + 4e 2H2O), another frequently studied... 

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