Oxygen Hydration Mechanism for the Oxygen Reduction Reaction at Pt and Pd Fuel Cell Catalysts

We report the reaction pathways and barriers for the oxygen reduction reaction (ORR) on platinum, both for gas phase and in solution, based on quantum mechanics calculations (PBE-DFT) on semi-infinite slabs. We find a new mechanism in solution: O₂ → 2O_ (ad) (E_ (act) = 0. 00 eV), O_ (ad) + H₂O_ (ad) → 2OH_ (ad) (E_ (act) = 0. 50 eV), OH_ (ad) + H_ (ad) → H₂O_ (ad) (E_ (act) = 0. 24 eV), in which OH_ (ad) is formed by the hydration of surface O_ (ad). For the gas phase (hydrophilic phase of Nafion), we find that the favored step for activation of the O₂ is H_ (ad) + O_ (2ad) → HOO_ (ad) (E_ (act) = 0. 30 eV) → HO_ (ad) + O_ (ad) (E_ (act) = 0. 12 eV) followed by O_ (ad) + H₂O_ (ad) → 2OH_ (ad) (E_ (act) = 0. 23 eV), OH_ (ad) + H_ (ad) → H₂O_ (ad) (E_ (act) = 0. 14 eV). This suggests that to improve the efficiency of ORR catalysts, we should focus on decreasing the barrier for Oad hydration while providing hydrophobic conditions for the OH and H₂O formation steps.