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 O_ad hydration while providing hydrophobic conditions for the OH and H₂O formation steps.