Oxygen, electroreduction

Oxygen can undergo two different reduction reactions, one following a four-electron process to form water as presented in Eq. 9.2, but also a second one following a two-electron process as shown in the following equation  

Although thermodynamic data suggest a high instability of hydrogen peroxide in an acid medium, the kinetics of its decomposition is very low in aqueous solution. Tarasevich et at. proposed that the more hkely reaction mechanism for its decomposition involved a redox disproportionation reaction  

Under these conditions, the transfer of several electrons leading to unstable intermediates involves an increase of the free energy for the breaking of the 0-0 bond, which could explain the low kinetics of H2O2 decomposition. 

In the mechanism proposed by Morcos and Yeager, the two first steps are identical to those presented above but the last one becomes  

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

Figure 14. Oxygen electroreduction at different transition metal macrocy-clic electrocatalysts. Comparison with the behavior of a Pt electrode ( ).

Fernandez JL, Walsh DA, Bard AJ. 2005b. Thermodynamic guidelines for the design of bimetallic catalysts for oxygen electroreduction and rapid screening by scanning electrochemical microscopy. M-Co (M Pd, Ag, Au). J Am Chem Soc 127 357-365. 

Li X, Gewirth AA. 2005. Oxygen electroreduction through a superoxide intermediate on Bi-modified Au surfaces. J Am Chem Soc 127 5252-5260. 

Coutanceau C, Crouigneau P, Leger JM, Lamy C. 1994. Mechanism of oxygen electroreduction at pol3fpyrrole electrodes modified by cobalt phthalocyanine. J Electroanal Chem 379 389-397. 

Shukla A, Neergat M, Parthasarathi B, Jayaram V, Hegde MS. 2001. An XPS study on binary and ternary alloys of transition metals with platinized carbon and its bearing upon oxygen electroreduction in direct methanol fuel cells. J Electroanal Chem 504 111-119. 

Maillard F, Martin M, Gloaguen F, Leger JM. 2002. Oxygen electroreduction on carbon-sup-ported platinum catalysts. Particle-size effect on the tolerance to methanol competition. Electrochim Acta 47 3431-3440. 

Zinola CF, AM Castro Luna, Arvia AJ. 1994. Temperature dependence of kinetic parameters related to oxygen electroreduction in acid solutions on platinum electrodes. Electrochim Acta 39 1951-1959. 

Kim J, Gewirth A. 2006. Mechanism of oxygen electroreduction on gold surfaces in basic media. J Fhys Chem B 110 2565-2571. 

The organometallic complexes with d-metals are considered as promising electrocatalysts for oxygen electroreduction in air-metal electrochemical cells. Obviously, the first idea was to employ the catalytic mechanism of the oxygen reduction with porphyrin-like metal complexes [1] found in living beings (Figure 1). 

The electrochemical intercalation of Li was studied for carbon electrodes modified by the 2Co-Ni complex, which showed the best effect in the reaction of oxygen electroreduction. Galvanostatic charge-discharge technique (PC governed automatic bench) in 2016 coin type cells was used for this purpose. 

According to the results obtained from rotating ring disk experiments and the determination of Tafel slopes, it was concluded that the mechanism of oxygen electroreduction on an Ag/C catalyst is similar to that on a Pt/C catalyst at high potentials, that is, interesting potentials for fuel cell applications. Lee et al. also showed that a 30 wt% Ag electrode displayed the same electroactivity towards the ORR as a 10 wt% electrode and that the activity could be enhanced by doping Ag with Mg [115],... 

The change in transition states with electrode potential is observed, for instance, in the complex 4-electron reaction of oxygen electroreduction on several electrode materials, the mechanism of which may also change with pH. At low negative overpotentials, the rds is reaction (124) with reaction (123) in pre-equilibrium. 

Oxygen Electroreduction Reaction with an Emphasis on Charge Transfer at Metal/Water Interface... 

A generic quantum mechanics description of the oxygen electroreduction and charge transfer in EDL is as follows. 

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... 

An ambitious review of carbon applications in chemical power sources (see also Section 5.3.5) was offered by Fialkov [94], It is disconcerting, however, that the author discusses the influence exerted by... surface properties without citing even one of the well-known—or well-cited or more recent—studies on carbon surface chemistry. And yet, in conjunction with the use of carbon in air (oxygen) electrodes, he speculates that the oxygen electroreduction kinetics depend on... the degree to which side faces of carbon crystallites are developed because base groups are formed there and presumably interact (e.g., with HjOj) in the following manner ... 

One of informative methods of an directions of the mechanism of oxygen electroreduction is the method of a disk rotating electrode with a ring. 

Figure 6. The exchange current density of oxygen electroreduction from acetylene black AD-100(1), initial nanodispersed diamond(2), diamond ASM 1/0,5(3), initial nanodispersed diamond after palladium presipitation (4).

The following scheme sumarizes the main ways of oxygen electroreduction at a n-Ti02 electrode ... 

Tafalla, D. and Salvador, P. 1987. Mechanisms of charge transfer at the semiconductor-electrolyte interface oxygen electroreduction at naked and platinized n-Ti02 electrodes. Ber. Bunsenges. Phys. Chem., 91,475M79. 

The model of Presnov and Trunov [341,345] is based on the transition state theory and modern ideas on electron transfer at electrodes, and allows a quantitative calculation of the rate of oxygen electroreduction based on... 

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