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

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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: O<sub>2</sub> β†’ 2O<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.00 eV), O<sub>ad</sub> + H<sub>2</sub>O<sub>ad</sub> β†’ 2OH<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.50 eV), OH<sub>ad</sub> + H<sub>ad</sub> β†’ H<sub>2</sub>O<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.24 eV), in which OH<sub>ad</sub> is formed by the hydration of surface O<sub>ad</sub>. For the gas phase (hydrophilic phase of Nafion), we find that the favored step for activation of the O<sub>2</sub> is H<sub>ad</sub> + O<sub>2ad</sub> β†’ HOO<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.30 eV) β†’ HO<sub>ad</sub> + O<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.12 eV) followed by O<sub>ad</sub> + H<sub>2</sub>O<sub>ad</sub> β†’ 2OH<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.23 eV), OH<sub>ad</sub> + H<sub>ad</sub> β†’ H<sub>2</sub>O<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.14 eV). This suggests that to improve the efficiency of ORR catalysts, we should focus on decreasing the barrier for O<sub>ad</sub> hydration while providing hydrophobic conditions for the OH and H<sub>2</sub>O formation steps
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