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

Abstract

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 2 → 2O ad ( E act = 0.00 eV), O ad + H 2 O ad → 2OH ad ( E act = 0.50 eV), OH ad + H ad → H 2 O 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 O 2 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 + H 2 O ad → 2OH ad ( E act = 0.23 eV), OH ad + H ad → H 2 O 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 H 2 O formation steps.