Oxygen reduction reaction Tafel slope

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. 

In practice, in all fuel cells that involve the utilization of 02 from air (Section 13.4.5), the oxygen reduction reaction [Eqs. (13.4) and (13.24)] is always rate determining for terrestrial applications. One can seejust how important it is to attempt to develop electrocatalysts for the cathodes of fuel cells on which the enhanced current density is high and Tafel slope is low and the efficiency of energy conversion, therefore, maximal. The direct relation of the mechanism of oxygen reduction and the associated Tafel parameters to the economics of electricity production and transportation is thus clearly seen. 

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,... 

The x-shapes [Eqs. (23.17-23.19)] allow us to construct a model for this process [14]. Our main assumption will be that the Tafel slope of the reaction in Eq. (23.22) is close to the Tafel slope of the oxygen reduction reaction (ORR) running in the electrode. This means that the shape of the overpotential which drives the VCH reduction follows the shape of the ORR. The equihbrium potential of the VCH reduction differs from that of the ORR however, this difference only shifts the overpotential as a whole along the potential axis. Hence the rate of Cr2 O3 deposition also peaks at the electrolyte interface and the thickness of the domain of fast Cr deposition is equal to the thickness of the ORR conversion domain f. ... 

Wang JX, Uribe FA, Springer TE, Zhang JL, Adzic RR (2008) Intrinsic kinetic equation for oxygen reduction reaction in acidic media the double Tafel slope and fuel cell applications. 

Figure 2.18. Tafel plots for ORR in 0.5 M H2SO4 on different catalysts bjcdi Tafel slope at low current density bhcd Tafel slope at high current density [47]. (With kind permission from Springer Science+Business Media Journal of Applied Electrochemistry, Electrocatalytic hehaviour for oxygen reduction reaction of small nanostructured erystaUme himetaUic Pt-M supported catalysts, 36, 2006, 1143-1149, A. Stassi, Figure 9, Springer.)...

Figure 2.28. Tafel slopes for oxygen reduction reaction on Pt/C and Pt-W2C/C electrocatalysts [75]. (Reprinted with permission from J Phys Chem B 2005 109 22705-9. Copyright 2005 American Chemical Society.)...

The relationship between temperature and performance is complex. An increased temperature drives increased kinetic activity for the oxygen reduction reaction (ORR) however, the reversible cell voltage decreases with increasing temperature. The Tafel slope increases in the low current density... 

In this notation, anodic current is positive, while cathodic current is negative. As the later section on oxygen reduction will show, the Tafel slope can change with overpotential. This is because the Butler-Volmer law only applies to outer-sphere reactions. Although it can describe electrode reactions, the equation does not account for repulsive interactions of the adsorbates or changes in the reaction mechanism as potential is changed. 

The lower the slope, the lower the activation loss for the O2 reduction reaction in acid media, the observed slope is 0.060 V/ decade and it can be obtained theoretically by assuming the oxygen discharge step as the slowest one (the rate determining step, the r.d.s. ( 7)). For hydrogen the mechanism is different and the r.d.s. is the atomic dissociation and in this case a Tafel slope of 0.030... 

In particular, Yeager and co-workers (42) have described a = a 4- j3 T for the reduction of oxygen on platinum in concentrated phosphoric acid in the temperature range 25—250°C with cathodic Tafel slopes nearly independent of temperature (a — 0.08 and j3 = 0.0012/K). The effect of electrode potential on the reaction kinetics is mainly through the entropy of activation [43]. 

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

Equation (47) can lead to the reaction mechanism from the Tafel slope for the ORR on the considered catalytic system. The determination of oxygen reduction pathways have been successful-... 

It should be noted that the Stern-Geary equation is dominated by the smaller of the Tafel slopes. Often in corrosion, the cathodic reaction is diffusion limited oxygen reduction, so be = 00. In that case, the corrosion rate is given by ... 

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