Oxygen reduction reaction steps

A striking example of the interaction of solution velocity and concentration is given by Zembura who found that for copper in aerated 0-1 N H2SO4, the controlling process was the oxygen reduction reaction and that up to 50°C, the slow step is the activation process for that reaction. At 75 C the process is now controlled by diffiision, and increasing solution velocity has a large effect on the corrosion rate (Fig. 2.5), but little effect at temperatures below 50 C. This study shows how unwise it is to separate these various... 

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 ... 

Hartnig C, Koper MTM. 2002. Molecular dynamics simulation of the first electron transfer step in the oxygen reduction reaction. J Electroanal Chem 532 165-170. 

Fig. 43. Double-logarithmic plot of the electrode polarization resistance versus the microelectrode diameter measured with impedance spectroscopy (ca. 800 °C) at (a) a cathodic dc bias of -300 mV, and (b) at an anodic dc bias of +300 mV. In (b) the first data point of the 20-pm microelectrode is not included in the fit. (c) Sketch illustrating the path of the oxygen reduction reaction for cathodic bias, (d) Path of the electrochemical reaction under anodic bias the rate-determining step occurs close to the three-phase boundary.

If a reaction path is predominant and one of its elementary step is rate determining, the Tafel slope will be characteristic of this step. Thus, electrochemical measurements performed on rotating disk electrode can provide a first solution to elucidate the mechanistic behavior of oxygen reduction reaction on a catalyst. Par-sons has demonstrated that, in a several step electrode process,... 

Calvo and Balbuena examined the structure and reactivity of Pd-Pt nanoclusters with 10 atoms in the oxygen reduction reaction. In contrast with what is expected in a periodic slab calculation, they found that mixed states with randomly distributed Pd atoms in a Pt7Pd3 cluster was more stable than an ordered cluster structure due to more eflective charge transfer in the mixed state. They found that increasing the concentration of Pd in the surface favors formation of the OOH species in the first step of the reaction, but Pt atoms were needed to promote the second stage of the oxygen reduction reaction. They report that due to charge transfer eflhcts the Pd atoms have an intermediate reactivity between pure Pd and Pt, and in the mixed cluster the Pd atoms the Pd atoms act more similarly to Pt than in the ordered cluster. 

It becomes evident from this last expression that E°22 will shift toward more positive values as i , or equivalently, the magnitude of the rate constant of association k2 is increased. It is conceivable that the half-wave potential for the oxygen reduction reaction can be shifted by as much as 100 mV from E rf however, larger shifts are not likely, as a subsequent electron transfer step may become rate-limiting. 

In order to explain the experimental results for the effect of fr, on the electrocatalytic activity, we discuss the reaction mechanism. As a typical case, we consider the oxygen reduction reaction at the Pt cathode. By means of various experimental techniques, it has been well recognized that ERZ for the Pt/zirconia interface is restricted to the portion around the physical TPB. The cathodic reaction consists of tire following elementary steps ... 

The second step in the procedure requires the working electrode to be anodicaUy polarized, yielding one of the dashed Hnes shown in Fig. 3.6. The electrode is then cathodicaUy polarized, and the other dashed Hne from Fig. 3.6 is obtained. The anodic polarization usuaUy results in the oxidation of the metal species, whUe the reaction resulting from cathodic polarization depends on the medium. In an aerated solution, the oxygen reduction reaction may be the prime cathodic reaction, while, in the case of deaerated aqueous solutions, hydrogen reduction could be the dominant reaction. In Fig. 3.6, the redox reaction is represented by a general reaction... 

Iron hydroxide ion formation, Fe(OFI), depends on solution pFl, that is, the availability of OH ions. As bivalent Fe ions are formed through Eq. (12.6), OH ions are transported from the bulk to the surfice to maintain electroneutrality. Flydroxide ions are also produced via the cathodic oxygen reduction reaction, causing an increase in surface pH. At high pH, the formation of adsorbed [Fe(OH)]ads on the iron surface becomes more favorable than bivalent Fe ions, Eq. (12.6). The electrode potential tends to shift into a more anodic direction to accommodate the formation of Fe(OH)". As time increases, the Fe(OH) concentration at the surface increases. In the next step, Fe(OH) oxidizes to ferric oxide at e° = —0.084Vvs.SCE, resulting in a barrier oxide layer ... 

The oxygen reduction reaction can proceed by two pathways in aqueous electrolytes. The first one, the so-called direct pathway, involves releasing four electrons per oxygen molecule to yield HgO. The indirect pathway involves releasing two electrons to yield hydrogen peroxide (HgOg) that in successive steps can produce water. Two further pathways, which are combinations of the above, can be envisaged. The series pathway implies sequential two or... 

The oxygen reduction reaction is a multi-electron process involving numerous steps and intermediate species. As stated above, ORR may proceed via four or two electron transfer in aqueous acidic medium. The most relevant reactions pathways and their thermodynamic electrode potentials in acidic medium are shown below ... 

As in the case of H2 oxidation, the oxygen reduction reactions (ORR) on the cathode are also assumed to take place in a multi-step manner. The adsorption of O2 on the cathode surface is followed by the dissociation into two O atoms, and the surface diffusion to the three-phase boundary region. The O atoms take part in a number of electron transfer steps, reducing O to 0 . The rate limiting process, however, has not yet been identified conclusively. The overall oxygen reduction reaction and the incorporation of the ions into the electrolyte can be written in Kroger-Vink notation as... 

Platinum has a myriad of practical uses, especially in the field of electrochemistry where it is used as a catalyst and as a reference electrode. In particular, platinum is the most active known pure metal for the oxygen reduction reaction (ORR) in which O2 is split and combined with protons to form H2O. This is an important step in low-temperature fuel cells (polymer electrolyte fuel cells, direct methanol fuel cells, etc.) as it often is what limits the total fuel cell efficiency. Furthermore, platinum is rather expensive with the materials cost of its use in fuel cells being roughly half of the total fuel cell cost. Consequently, a great deal of effort is made in order to optimize its use. 

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