Kinetics of the O2 Reduction Reaction

It is desirable to have the O2 reduction reaction occurring at potentials as close as possible to the reversible electrode potential (thermodynamic electrode potential) with a satisfactory reaction rate. The current-overpotential is given in Equation 2.1 [3]  

If the overpotential is large, the backward reaction is negligible and Equation 2.1 can be simplified as 

Exchange current density is an important kinetic parameter representing the electrochemical reaction rate at equilibrium. For an electrochemical reaction. 

Electrode material /catalyst ORR exchange current density, A.cm Electron transfer co- efficiency Electron transfer num. in rate determining step Measnrement conditions Ref. 

The exehange current density is related to the true electrode area and to the reaetant eoneentration (or partial pressure, for a gas), especially for ORR on the Pt eleetrode in fuel cells. The true electroactive area of Pt is significantly different from its geometric area, and the partial pressure of O2 is not 1 atm. Thus, the intrinsie exehange current density should be used, which is shown in Equation 2.6 [4]  

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