The Oxygen Electroreduction Reaction

At the cathode, oxygen electroreduction occurs according to the overall reaction (in acid medium)  

The kinetics of this reaction are relatively slow (io from 10 to lO A cm- referred to the geometric surface area, depending on the degree of dispersion of the platinum catalyst), which is the main cause of the high 

The production of HjOj, either as an intermediate in the four-electron reduction of Oj to water, or as a reaction product. In the latter case the thermodynamic equilibrium potential of the reaction, i.e., 

Observance of a mixed potential of about 1.0 V (instead of the equilibrium thermodynamic reversible potential Ec= 1.23 V vs. SHE) due to the formation of surface oxides at the platinum electrode, according to different electrode reactions  

In all these cases, the electrode potential will be determined by a mixed reaction resulting from the reduction of oxygen and the oxidation of the 

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Ab initio modeling of charge transfer process in the oxygen electroreduction reaction (OER) has achieved some notable stepwise progresses in complexity and in proximity to realistic systems. Initial models only involved a few Pt and reactant atoms. Later, a catalyst slab replaced the Pt cluster. The... 

With the aim of using them in an H2/O2 PEMFC, Lai et al. prepared composite films of Nafion- and PAni-containing Pt particles as electrocatalysts for the oxygen electroreduction reaction [19]. Films cast from a solution of PAni and Nafion containing chemically reduced Pt displayed good electrocatalytic activity compared to films without PAni. 

FIGURE 1.10 Current intensity versus electrode potential voltammetric profile for platinum single crystals, Pt(100) (cooled in air) with blue lines, Pt(110) with black lines and Pt(lll) in red lines, in 0.5 M sulfuric acid solution run at 0.05 V s 1 at room temperature. A difference with respect to the early single crystal preparation method by Will [18] is that the ones reported here are those obtained by Clavilier early in 1980. After 50 years of the first paper by Will on the use of single crystals for the oxygen electroreduction reaction and his finding of the given preferred activities on (111) and (100) planes, the industry still uses polycrystalline surfaces. 

Effect of Crystallographic Orientation in the Oxygen Electroreduction Reaction.64... 

The electrocatalysis of the oxygen electroreduction reaction has been studied since the early days of electrochemistry and surface science, and since the importance of corrosion technology and fuel cells was realized. In the early 1960s, it was proved that the reaction was not structure sensitive [92] however, the problem of the preparation of clean and atomic ordered single crystal surfaces still remained. 

Nowadays, it has been demonstrated that the reaction is indeed structure sensitive with a multielectron transfer process that involves several steps and the possible existence of several adsorption intermediates [93-96]. The main advantage that we have with the new procedures with respect to cleanliness is that we have well-ordered surfaces to study a complex mechanism such as the oxygen electroreduction reaction [96-99]. In aqueous solutions, the four-electron oxygen reduction appears to occur by two overall pathways a direct four-electron reduction and a peroxide pathway. The latter pathway involves hydrogen peroxide as an intermediate and can undergo either further reduction or decomposition in acid solutions to yield water as the final product. This type of generic model of a reaction has been extensively studied since the early 1960s by different authors [100-108]. 

Wang, Y. Balbuena, P. B. Ab initio Molecular Dynamics Simulations of the Oxygen Electroreduction Reaction on a Pt(lll) Surface in the Presence of Hydrated Hydronium (H3O) + (H20)2 Direct or Series Pathway J. Phys. Chem. B 2005,109,14896 14907. 

Zinola, C. F. Triaca, W. E. Arvia, A. J. Kinetics and mechanism of the oxygen electroreduction reaction on faceted platinum-electrodes in trifluoromethane-sulfonic acid solutions. J. Appl. Electrochem. 1995, 25, 740-754. 

Stolbov S (2012) Nature of the selenium submonolayer effect on the oxygen electroreduction reaction activity of ru(0001). J Phys Chem C 116 (12) 7173-7179. 

These mechanisms could explain the fact that, on such surfaces, the kinetics of the oxygen electroreduction reaction into hydrogen peroxide is higher in an alkaline medium than in an acid medium. Indeed, if the S-O2 species is likely to be stable in an alkaline medium, the free energy for the formation of this species is certainly higher in an acid medium. The formation of such an entity must involve the simultaneous transfer of an electron and a proton, which affects the kinetics of the process. 

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