Theoretical Insights on the Synergy and Competition between Thermochemical and Electrochemical Steps in Oxygen Electroreduction

It is widely accepted that electrode potentials determine the reaction rate of the electrocatalytic oxygen reduction reaction (ORR) for proton-coupled electron transfer processes (O 2 → OOH* → O* → OH* → H 2 O). However, we found that each of the electrochemical steps can be replaced with a thermochemical step. With an ab initio electrochemical model, the rate of electrochemical steps is compared with thermochemical ones. Microkinetic calculations confirm that the ORR reaction rate over Pt(111) is limited by both thermochemical O 2 * dissociation and electrochemical OH* protonation at a high electrode potential but dominated only by the thermochemical O* + H 2 O + * → 2OH* at a low electrode potential. A complete reaction phase diagram (RPD) was established to study the optimal reaction pathway and activity trend over a series of catalysts. Again, thermochemical steps were found to be the G RPD -limiting step at a low electrode potential, explaining well a set of experiment results. The activity optimum was found to be shifted at different potentials, in agreement with the varying Tafel slopes observed from experiments, which is significantly affected by thermochemical steps.