Oxygen Reduction Catalysis at a Dicobalt Center: The Relationship of Faradaic Efficiency to Overpotential

The selective four electron, four proton, electrochemical reduction of O₂ to H₂O in the presence of a strong acid (TFA) is catalyzed at a dicobalt center. The faradaic efficiency of the oxygen reduction reaction (ORR) is furnished from a systematic electrochemical study by using rotating ring disk electrode (RRDE) methods over a wide potential range. We derive a thermodynamic cycle that gives access to the standard potential of O₂ reduction to H₂O in organic solvents, taking into account the presence of an exogenous proton donor. The difference in ORR selectivity for H₂O vs H₂O₂ depends on the thermodynamic standard potential as dictated by the pK_a of the proton donor. The model is general and rationalizes the faradaic efficiencies reported for many ORR catalytic systems.