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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.
Sha et al. (Thu,) studied this question.