Recent quantum chemical studies of the ORR mechanism

A large number of studies have focused on the chemisorption of molecular oxygen on well-defined Pt and Pt alloys. Chemisorption is the first step in the ORR process, and the structure, bonding type and energy are key elements for understanding trends in ORR activity as function of electrode composition and structure. 

At an electrode potential of U = V, the ORR is running with a high reaction rate. This situation would correspond to a short-circuited fuel cell where all elementary reaction steps are highly exothermic. At an electrode potential of U=1.23 V, where the chemical potential of the electrons is shifted by 1.23 eV, both protonation and electron transfer steps are activated. Process (26) or (28) are therefore rate-limiting under these conditions. 

This theoretical framework accounts for the concept of surface poisoning by adsorbed OH and at low overpotentials (values of U close to 1.23 V) high OH and coverages result from the reduced activation barriers associated with the backward reactions of steps (26) and (28) according to 

The dependence of the overall ORR reaction rate on the surface adsorption energy of Pt-OH and Pt— species, and consequently on their rate of formation, 

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