Mechanism of Molecular Oxygen Reduction at the Cathode of a PEM Fuel Cell: Non-Electrochemical Reactions on Catalytic Pt Particles

Various catalytic reactions in proton exchange membrane (PEM) fuel cells are discussed, and the effects of different steps, parallel pathways, and side reactions are analyzed for the oxygen reduction reaction (ORR) mechanism. A suitable mechanism table is formulated for the prominent pathway of ORR. The kinetics of the proposed nonelectrochemical reactions on platinum surfaces are then studied using the B3LYP density functional theory (DFT) with the Wadt and Hay relativistic ECPs and basis sets augmented with a 4f function on Pt. The reactions considered are O2(ads) ↔ O(ads) + O(ads) (1), O2H(ads) ↔ O(ads) + OH(ads) (2), H2O(ads) ↔ H(ads) + OH(ads) (3), OH(ads) + OH(ads) ↔ O(ads) + H2O(ads) (4), and OH(ads) + O(ads) ↔ O(ads) + OH(ads) (5). Calibration calculations are carried out for reaction 1 on a single Pt atom using the CASSCF/MRCI method with a cc-pVDZ/relativistic ECP basis set for Pt and the aug-cc-pVDZ basis set for O. Comparison with B3LYP DFT calculations shows that the latter method overestimates binding energies by more than a factor of 2, but the barrier heights with respect to reactants are accurate. This result is consistent with calculations for the outer minimum for molecular oxygen on Pt(111), where the computed binding energy is about a factor of 2 larger than what is found from experiments. We find that, while a Pt2 cluster gives qualitatively correct results, the results are strongly influenced by cluster size effects, and inclusion of nearest neighbor atoms in at least the top and second layers is necessary for accurate energetics. The rates computed within the conventional transition state theory plus a Wigner estimate for tunneling using barriers obtained from the largest clusters show that the second, fourth, and fifth reactions are most important on the surface of catalytic Pt particles. Solvation effects were investigated using a model consisting of cyclic (H2O)n structures and were found to be small.