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: O_2 → 2O_(ad) (E_(act) = 0.00 eV), O_(ad) + H_2O_(ad) → 2OH_(ad) (E_(act) = 0.50 eV), OH_(ad) + H_(ad) → H_2O_(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_2O_(ad) → 2OH_(ad) (E_(act) = 0.23 eV), OH_(ad) + H_(ad) → H_2O_(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 H_2O formation steps

Mechanism for Degradation of Nafion in PEM Fuel Cells from Quantum Mechanics Calculations

We report results of quantum mechanics (QM) mechanistic studies of Nafion membrane degradation in a polymer electrolyte membrane (PEM) fuel cell. Experiments suggest that Nafion degradation is caused by generation of trace radical species (such as OH^●, H^●) only when in the presence of H_2, O_2, and Pt. We use density functional theory (DFT) to construct the potential energy surfaces for various plausible reactions involving intermediates that might be formed when Nafion is exposed to H_2 (or H^+) and O_2 in the presence of the Pt catalyst. We find a barrier of 0.53 eV for OH radical formation from HOOH chemisorbed on Pt(111) and of 0.76 eV from chemisorbed OOH_(ad), suggesting that OH might be present during the ORR, particularly when the fuel cell is turned on and off. Based on the QM, we propose two chemical mechanisms for OH radical attack on the Nafion polymer: (1) OH attack on the S–C bond to form H_2SO_4 plus a carbon radical (barrier: 0.96 eV) followed by decomposition of the carbon radical to form an epoxide (barrier: 1.40 eV). (2) OH attack on H_2 crossover gas to form hydrogen radical (barrier: 0.04 eV), which subsequently attacks a C–F bond to form HF plus carbon radicals (barrier as low as 1.00 eV). This carbon radical can then decompose to form a ketone plus a carbon radical with a barrier of 0.86 eV. The products (HF, OCF_2, SCF_2) of these proposed mechanisms have all been observed by F NMR in the fuel cell exit gases along with the decrease in pH expected from our mechanism

The effect of different environments on Nafion degradation: Quantum mechanics study

Degradation of the Nafion electrolyte in Proton Exchange Membrane Fuel Cells (PEMFCs) limits the lifetime, motivating development of materials that resist degradation. The mechanism for degradation of Nafion under fuel cell conditions remains uncertain. Studies of Nafion degradation in concentrated OH• environments, such as Fenton or vapor HOOH tests, show that the main chain significantly degrades in these conditions. However it has not been established whether this applies to fuel cell conditions. We have used quantum mechanics (Density Functional Theory with the B3LYP and M06 functionals) to determine the mechanism of Nafion degradation under both concentrated OH• and fuel cell conditions. These studies confirm that under concentrated OH• conditions Nafion degrades when peroxide radicals attack end groups (–COOH, –CF ═ CF2, –CF_2H); followed by degradation of Nafion along the polymer main chain, as proposed previously. However we find that under fuel cell conditions, Nafion degradation occurs along the polymer side chain starting with H• attacking the side chain groups such as the sulfonic acid, –SO_3^−. We find that it is easier for OH• to attack the main chain than H•, while vice versa, it is easier for H• radical to attack the side chain than OH•

Mechanism for Oxygen Reduction Reaction on Pt_3Ni Alloy Fuel Cell Cathode

We use quantum mechanics, density functional theory at the PBE level, to predict the binding-site preferences and reaction barriers for all intermediates involved in the oxygen reduction reaction (ORR) on the low energy surface of Pt_3Ni alloy. Here we calculate that the surface layer is Ni depleted (100% Pt) while the second layer is Ni enriched (50% Pt) as shown by experiment. Even though the top layer is pure Pt, we find that the sublayer Ni imposes strong preferences in binding sites for most intermediates, which in turn strongly influences the reaction barriers. This strong preference leads to a strong site dependence of the barriers. Considering water as the solvent, we predict that, at low coverage of O_(ad) and OH_(ad), the barrier for the rate-determining step is 0.81 eV, whereas, at high coverage, this barrier decreases to 0.43 eV. It can be compared to a barrier of 0.50 eV for pure Pt, explaining the improved ORR rate for the Pt_3Ni alloy. We report the results both for gas phase and for aqueous phase environments

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 (<i>E</i>_act = 0.00 eV), O_ad + H₂O_ad → 2OH_ad (<i>E</i>_act = 0.50 eV), OH_ad + H_ad → H₂O_ad (<i>E</i>_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 (<i>E</i>_act = 0.30 eV) → HO_ad + O_ad (<i>E</i>_act = 0.12 eV) followed by O_ad + H₂O_ad → 2OH_ad (<i>E</i>_act = 0.23 eV), OH_ad + H_ad → H₂O_ad (<i>E</i>_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