The Oxygen Reduction Reaction ORR

The ORR is one important reaction at the cathode of a fuel cell system, and the overall reaction is generally written as  

This multi-electron charge transfer reaction (1.23 V/SHE) depends on the electrolyte nature and the catalysts in form of a well defined surface or faceting nanoparticles, and represents a substantial cathodic overpotential loss of ca. 300 mV on the best catalyst, Pt. This means that its four-electron ORR kinetics is slow with an exchange cnrrent density between 10 to lO ttiA cm depending on the natnre of the exposed surface and electrolyte. The consequence is the generation of intermediate species of the overall reaction (3) that can be depicted by a series of reactions, namely  

Although this simple picture predicts activities of solely metal centers towards the ORR, thermodynamically speaking, it is clear 

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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. 

Koffi RC, Coutanceau C, Gamier E, Leger JM, Lamy C. 2005. Synthesis, characterization and electrocatalytic behaviour of non-alloyed PtCr methanol tolerant nanoelectrocatalysts for the oxygen reduction reaction (ORR). Electrochim Acta 50 4117-4127. 

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. 

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

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... 

The capability of SECM to detect and to image regions with different catalytic activities is well known [120-122]. So far, this technique has been applied to studies of mainly two electrocatalytic reactions, the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR), which have important implications for fuel cells. Unlike reversible redox mediators usually employed in SECM experiments, the kinetics of oxygen and hydrogen reactions are strongly dependent on the catalytic activity of the substrate surface. 

A final example of electrochemical kinetics will consider a return of the Pt WE from before but now exposed to a neutral (pH 7.2) solution into which oxygen is bubbled. The kinetics of the oxygen reduction reaction (ORR) will be studied. The data generated might appear as shown in Fig. 24. The reversible potential for the ORR in pH 7 solution, according to the Nernst equation, is... 

Figure 16 shows the steady-state limiting current density, ilim, for the oxygen reduction reaction (ORR) on pure Al, pure Cu, and an intermetallic compound phase in Al alloy 2024-T3 whose stoichiometry is Al20Cu2(Mn,Fe)3 after exposure to a sulfate-chloride solution for 2 hours (43). The steady-state values for the Cu-bearing materials match the predictions of the Levich equation, while those for Al do not. Reactions that are controlled by mass transport in the solution phase should be independent of electrode material type. Clearly, this is not the case for Al, which suggests that some other process is rate controlling. 

For many years, the oxygen reduction reaction (ORR) on Pt and Pt-based alloys has been studied extensively by experimental " and computational methods, and it has been shown that the formation of a Pt-skin layer is accompanied with a Pt-depleted layer underneath for many Pt-3d alloys. Notably, several studies of O adsorption on Pt-skin surfaces have revealed that the binding strength is weaker than that on the pure Pt(lll) surface and this may facilitate the removal of adsorbed O, therefore in-... 

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