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

The Oxygen Reduction Reaction on Graphene from Quantum Mechanics: Comparing Armchair and Zigzag Carbon Edges

Using density functional theory (PBE-D2 flavor), we report the mechanism for the oxygen reduction reaction (ORR) on graphene sheets. We find that ORR starts with OO chemisorbing onto the carbon edges, rather than the basal plane face, which is not energetically favorable. The carbon edges were described as one-dimensional periodic graphene ribbons with both armchair and zigzag edges. We calculated the binding energies of the ORR products (OO, OOH, O, OH, HOH, HOOH) for the zigzag and armchair edges, examining both the Langmuir–Hinshelwood (LH) and Eley–Rideal (ER), to understand how OO is reduced. For the armchair edge, we calculate an onset potential of 0.55 V vs reversible hydrogen electrode (RHE), which corresponds to −0.22 V at pH 13 in agreement with experiments. We find that the rate-determining step (RDS) to form peroxide (a 2e^– process) is hydrogenation of adsorbed OO with a barrier of 0.92 eV. The process to make water (a 4e^– process) was found to be unfavorable at the onset potential but becomes more favorable at lower potentials. Thus, undoped carbon catalysts prefer the 2e^– mechanism to form peroxide, rather than the 4e^– process to form water, which agrees with experiment. The predictions open the route for experimental studies to improve the sluggish ORR on carbon catalysts

First-Principles Modeling of Ni₄M (M = Co, Fe, and Mn) Alloys as Solid Oxide Fuel Cell Anode Catalyst for Methane Reforming

In this study, we used quantum mechanics (QM) to investigate steam reforming of methane on Ni-alloy catalyst surfaces and to examine the effect of anode material modifications on the catalytic processes in a solid oxide fuel cell (SOFC). The conventional Ni anode suffers from coking, coarsening, and sulfur poisoning because of the decomposition of hydrocarbon fuels, Ni particle agglomeration at high operating temperature, and impurities contained in fuels. Ni-electrode surface modification, such as alloying Ni with other metals (e.g., Fe and Cu), is probably the most practical and promising way of developing SOFC anodes tolerant to coking and sulfur poisoning. According to experimental data, Ni₄Fe shows a good catalytic performance and excellent long-term stability as an SOFC anode catalyst. We have performed QM calculations of segregation energy for various surface structures of five-layer Ni₄M slabs (M = Co, Fe, and Mn) and found that Ni atoms show segregation preference for the surface layer and the most favorable Ni₄M surface structure has two M atoms in the 2nd layer and one M atom in the 3rd and in the 4th layer (the numbering starts from the bottom layer). This structure was used for our further QM calculations of binding energies for CH_<i>x</i>, C, and H. We find that the Ni₄M­(111) surfaces bind CH_<i>x</i> species weaker (by 1–10 kcal/mol) than pure Ni, and the binding energy of C is always ∼10 kcal/mol lower for the Ni₄M alloys compared to pure Ni. This is consistent with improved catalytic characteristics of certain Ni-based alloys compared to pure Ni obtained in experiment. Reaction energy barriers for methane decomposition on the Ni₄M­(111) catalyst surfaces were calculated as well. On the basis of these results, the rate-determining step for the methane decomposition was found to be the CH → C + H reaction. Our results predict that Ni₄Fe and Ni₄Mn have both better activity and better coking resistance and can be considered as candidates for an SOFC anode catalyst suitable for the CH₄ fuel reforming

ReaxFF Reactive Force-Field Modeling of the Triple-Phase Boundary in a Solid Oxide Fuel Cell

In our study, the Ni/YSZ ReaxFF reactive force field was developed by combining the YSZ and Ni/C/H descriptions. ReaxFF reactive molecular dynamics (RMD) were applied to model chemical reactions, diffusion, and other physicochemical processes at the fuel/Ni/YSZ interface. The ReaxFF RMD simulations were performed on the H₂/Ni/YSZ and C₄H₁₀/Ni/YSZ triple-phase boundary (TPB) systems at 1250 and 2000 K, respectively. The simulations indicate amorphization of the Ni surface, partial decohesion (delamination) at the interface, and coking, which have indeed all been observed experimentally. They also allowed us to derive the mechanism of the butane conversion at the Ni/YSZ interface. Many steps of this mechanism are similar to the pyrolysis of butane. The products obtained in our simulations are the same as those in experiment, which indicates that the developed ReaxFF potential properly describes complex physicochemical processes, such as the oxide-ion diffusion, fuel conversion, water formation reaction, coking, and delamination, occurring at the TPB and can be recommended for further computational studies of the fuel/electrode/electrolyte interfaces in a SOFC

Finding Correlations of the Oxygen Reduction Reaction Activity of Transition Metal Catalysts with Parameters Obtained from Quantum Mechanics

To facilitate a less empirical approach to developing improved catalysts, it is important to correlate catalytic performance to surrogate properties that can be measured or predicted accurately and quickly, allowing experimental synthesis and testing of catalysts to focus on the most promising cases. Particularly hopeful is correlating catalysis performance to the electronic density of states (DOS). Indeed, there has been success in using just the center of the d-electron density, which in some cases correlates linearly with oxygen atom chemisorption energy, leading to a volcano plot for catalytic performance versus “d-band center”. To test such concepts we calculated the barriers and binding energies for the various reactions and intermediates involved in the oxygen reduction reaction (ORR) for all 12 transition metals in groups 8–11 (Fe–Cu columns). Our results show that the oxygen binding energy can serve as a useful parameter in describing the catalytic activity for pure metals, but it does not necessarily correlate with the d-band center. In addition, we find that the d-band center depends substantially on the calculation method or the experimental setup, making it a much less reliable indicator for ORR activity than the oxygen binding energy. We further examine several surfaces of the same pure metals to evaluate how the d-band center and oxygen binding energy depend on the surface

Density Functional Theory Study of Pt₃M Alloy Surface Segregation with Adsorbed O/OH and Pt₃Os as Catalysts for Oxygen Reduction Reaction

Using quantum mechanics calculations, we have studied the segregation energy with adsorbed O and OH for 28 Pt₃M alloys, where M is a transition metal. The calculations found surface segregation to become energetically unfavorable for Pt₃Co and Pt₃Ni, as well as for the most other Pt binary alloys, in the presence of adsorbed O and OH. However, Pt₃Os and Pt₃Ir remain surface segregated and show the best energy preference among the alloys studied for both adsorbed species on the surface. Binding energies of various oxygen reduction reaction (ORR) intermediates on the Pt(111) and Pt₃Os­(111) surfaces were calculated and analyzed. Energy barriers for different ORR steps were computed for Pt and Pt₃Os catalysts, and the rate-determining steps (RDS) were identified. It turns out that the RDS barrier for the Pt₃Os alloy catalyst is lower than the corresponding barrier for pure Pt. This result allows us to predict a better ORR performance of Pt₃Os compared to that of pure Pt

Using Photoelectron Spectroscopy and Quantum Mechanics to Determine d‑Band Energies of Metals for Catalytic Applications

The valence band structures (VBS) of eight transition metals (Fe, Co, Ni, Cu, Pd, Ag, Pt, Au) were investigated by photoelectron spectroscopy (PES) using He I, He II, and monochromatized Al Kα excitation. The influence of final states, photoionization cross-section, and adsorption of residual gas molecules in an ultrahigh vacuum environment are discussed in terms of their impact on the VBS. We find that VBSs recorded with monochromatized Al Kα radiation are most closely comparable to the ground state density of states (DOS) derived from quantum mechanics calculations. We use the Al Kα-excited PES measurements to correct the energy scale of the calculated ground-state DOS to approximate the “true” ground-state d-band structure. Finally, we use this data to test the d-band center model commonly used to predict the electronic-property/catalytic-activity relationship of metals. We find that a simple continuous dependence of activity on d-band center position is not supported by our results (both experimentally and computationally)

DFT Study of Oxygen Reduction Reaction on Os/Pt Core–Shell Catalysts Validated by Electrochemical Experiment

Proton exchange membrane fuel cells (PEMFCs) have attracted much attention as an alternative source of energy with a number of advantages, including high efficiency, sustainability, and environmentally friendly operation. However, the low kinetics of the oxygen reduction reaction (ORR) restricts the performance of PEMFCs. Various types of catalysts have been developed to improve the ORR efficiency, but this problem still needs further investigations and improvements. In this paper, we propose advanced Os/Pt core–shell catalysts based on our previous study on segregation of both bare surfaces and surfaces exposed to ORR adsorbates, and we evaluate the catalytic activity of the proposed materials by density functional theory (DFT). Quantum mechanics was applied to calculate binding energies of ORR species and reaction energy barriers on Os/Pt core–shell catalysts. Our calculations predict a much better catalytic activity of the Os/Pt system than that of pure Pt. We find that the ligand effect of the Os substrate is more important than the lattice compression strain effect. To validate our DFT prediction, we demonstrate the fabrication of Os/Pt core–shell nanoparticles using the underpotential deposition (UPD) technique and succeeding galvanic displacement reaction between the Pt ions and Cu-coated Os nanoparticles. The Os/Pt/C samples were evaluated for electrocatalytic activities toward the ORR in acidic electrolytes. The samples with two consecutive UPD-displacement reaction cycles show 3.5 to 5 times better ORR activities as compared to those of commercially available Pt/C. Our results show good agreement between the computational predictions and electrochemical experimental data for the Os/Pt core–shell ORR catalysts