Oxygen reduction reaction exchange current density

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. 

Extrapolation of the linear portion of the curves to the standard oxygen reduction potential gives the corresponding exchange current density, a measure of the equilibrium rate of the reaction [35]. The results are given in Table I. Within... 

As discussed above, if the exchange current density of anode hydrogen oxidation is i nMc =0,1 Acni 2 and that of the cathode oxygen reduction reaction (ORR) is... 

The kinetics of the hydrogen reduction reaction have received a great deal of attention by many investigators. It may be noted, however, that - as in the case of oxygen - the exchange current density (i0)jj is very low on most metals so... 

The foregoing equations have resulted from the stated assumptions of the models employed. These models are examples of many models that have been proposed for electrochemical reactions. The equations are accepted here because of their simplicity of form and the fact that they do predict relationships between exchange current density, equilibrium half-cell potential, and concentration, which are frequently observed experimentally. The theories and resulting equations are obviously more complicated for surface reactions, such as the reduction of dissolved oxygen, 02 + 4H+ + 4e — 2H20. Theories for this reaction have proposed as many as eight individual steps. 

Fie. 3.18 Illustration of the effect of exchange current density on the polarization curve for oxygen reduction in aerated environments of pH = 0.56 and Pq2 = °-2 atm- Curves converge to the same diffusion limit and are identical when the hydrogen ion reduction is the dominant reaction. 

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