Oxygen reduction reaction species

Figure 6.15. Simplified schematic of the most important reaction pathways of the oxygen reduction reaction. The four-electron pathway results in the formation of water. The two-electron pathway forms hydrogen peroxide. Adsorption of molecular oxygen can form atomic oxygen (dissociative pathway) or form a superoxide species (associative pathway). The formation of Pt—OH and Pt— from water molecules represents the backward reactions of the later portion of the four-electron reduction pathway.

The DNS model can be deployed subsequently on the liquid water blocked CL structure pertaining to a saturation level for the evaluation of the hindered oxygen transport. In brief, the DNS model is a top-down numerical approach based on a fine-scale CFD framework which solves point-wise accurate conservation equations for species and charge transport in the CL with appropriate source terms due to the oxygen reduction reaction (ORR) directly on the CL microstructures.25-27 67 The conservation equations for proton, oxygen and water vapor transport, respectively, are given by 25-27 68... 

From the 3-D liquid water signature obtained from the two-phase LB simulation in the CL and representatively shown in Fig. 13, the pores occupied by liquid water are identified corresponding to a particular saturation level and these pores are rendered as dead pores. The idea is to generate a modified CL structure where the blocked pores do not take part in the oxygen reduction reaction (ORR) as well as produce extra resistance by reducing the effective porosity of the structure. With this virtual morphology of the liquid water blocked CL, the point-wise accurate species and... 

A detailed model for the oxygen reduction reaction at semiconductor oxide electrodes has been developed by Presnov and Trunov [341, 345, 346] based on concepts of coordination chemistry and local interaction of surface cation d-electrons at the oxide surface with HO, H20, and 02 acceptor species in solution. The oxygen reduction reaction is assumed to take place at active sites associated with cations at the oxide surface in a higher oxidation state. These cations would act as donor-acceptor reduction (DAR) sites, with acceptor character with respect to the solid by capture of electrons and donor electronic properties with respect to species in solution. At the surface, the long-range oxide structure is lost and short-range coordination by hydroxide ions and water molecules in three octahedral positions may occur [Fig. 16(b)], One hydroxide ion can compensate coulombically for the excess charge on surface M2+ cations with two coordinated water mole-... 

In the case of iron phthalocyanine catalyst dispersed on carbon, infrared spectroscopy measurements enabled to confirm the series mechanistic pathway for the ORR in the high overpotential domain (below 0.7 V vs RHE). Two intermediates species, 02 and H02 and two products, water (majority product) and hydrogen peroxide could be identified. However, since only few studies are led on the oxygen reduction reaction by infrared spectroscopy, a confident attribntion of the different vibrational transitions observed to a particnlar intermediate remains difficult. 

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. 

Methanol crossover is one of the major obstacles to prevent DMFC from commercialization. This effect, which is caused by diffusion of methanol through the membrane, reduces the cell efficiency. Competing with the oxygen reduction reaction on the cathode side, methanol adsorbs on the surface of Pt and is oxidized to CO2. The main poisoning species formed during the chemisorption and oxidation of methanol is carbon monoxide... 

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... 

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 ... 

More recent reviews on the electro catalysis of oxygen reduction in PEFC have been given in [23,49]. Surface chemistry of platinum, particularly the coverage of the metal surface with oxygen containing species above a potential of 750 mV vs. RHE caused difficulties in the kinetic description of the oxygen reduction reaction. 

---