Density functional simulation of the BaZrO3 (011) surface structure

The atomic structure and charge redistribution of different terminations of BaZrO3 (011) surfaces have been studied using density functional simulations. We found that the O-terminated (011) flat surface had the smallest cleavage energy among (011) surfaces, but this value was still twice as large as for the formation of a pair of complimentary (001) surfaces. The density functional calculations allowed us to estimate the excess surface Gibb's free energy and to compare stability of different (011) surfaces as a function of chemical environment. In addition, we compared stability of BaZrO3 (011) surfaces with respect to BaZrO3 (001) surfaces. Within boundaries, where BaZrO3 does not decompose, only the Ba- and O-terminated (011) surfaces appeared to be stable. However, if (001) surfaces are also taken into consideration, the BaO-terminated (001) surface is the only stable surface among all considered (001) and (011) surfaces

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

Theoretical Study of Solvent Effects on the Platinum-Catalyzed Oxygen Reduction Reaction

We report here density functional theory (DFT) studies (PBE) of the reaction intermediates and barriers involved in the oxygen reduction reaction (ORR) on a platinum fuel cell catalyst. Solvent effects were taken into account by applying continuum Poisson−Boltzmann theory to the bound adsorbates and to the transition states of the various reactions on the platinum (111) surface. Our calculations show that the solvent effects change significantly the reaction barriers compared with those in the gas-phase environment (without solvation). The O_2 dissociation barrier decreases from 0.58 to 0.27 eV, whereas the H + O → OH formation barrier increases from 0.73 to 1.09 eV. In the water-solvated phase, OH formation becomes the rate-determining step for both ORR mechanisms, O_2 dissociation and OOH association, proposed earlier for the gas-phase environment. Both mechanisms become significantly less favorable for the platinum catalytic surface in water solvent, suggesting that alternative mechanisms must be considered to describe properly the ORR on the platinum surface

Origin of low sodium capacity in graphite and generally weak substrate binding of Na and Mg among alkali and alkaline earth metals

It is well known that graphite has a low capacity for Na but a high capacity for other alkali metals. The growing interest in alternative cation batteries beyond Li makes it particularly important to elucidate the origin of this behavior, which is not well understood. In examining this question, we find a quite general phenomenon: among the alkali and alkaline earth metals, Na and Mg generally have the weakest chemical binding to a given substrate, compared with the other elements in the same column of the periodic table. We demonstrate this with quantum mechanics calculations for a wide range of substrate materials (not limited to C) covering a variety of structures and chemical compositions. The phenomenon arises from the competition between trends in the ionization energy and the ion–substrate coupling, down the columns of the periodic table. Consequently, the cathodic voltage for Na and Mg is expected to be lower than those for other metals in the same column. This generality provides a basis for analyzing the binding of alkali and alkaline earth metal atoms over a broad range of systems

Proton diffusion pathways and rates in Y-doped BaZrO_3 solid oxide electrolyte from quantum mechanics

We carried out quantum mechanical calculations (Perdew-Becke-Ernzerhof flavor of density functional theory) on 12.5% Y-doped BaZrO_3 (BYZ) periodic structures to obtain energy barriers for intraoctahedral and interoctahedral proton transfers. We find activation energy (E_a) values of 0.48 and 0.49 eV for the intraoctahedral proton transfers on O–O edges (2.58 and 2.59 Å) of ZrO_6 and YO_6 octahedra, respectively, and E_a=0.41 eV for the interoctahedral proton transfer at O–O separation of 2.54 Å. These results indicate that both the interoctahedral and intraoctahedral proton transfers are important in the BYZ electrolyte. Indeed, the calculated values bracket the experimental value of E_a=0.44 eV. Based on the results obtained, the atomic level proton diffusion mechanism and possible proton diffusion pathways have been proposed for the BYZ electrolyte. The thermal librations of BO6 octahedra and uncorrelated thermal vibrations of the two oxygen atoms participating in the hydrogen bond lead to a somewhat chaotic fluctuation in the distances between the O atoms involved in the hydrogen bonding. Such fluctuations affect the barriers and at certain O–O distances allow the hydrogen atoms to move within the hydrogen bonds from one potential minimum to the other and between the hydrogen bonds. Concertation of these intra- and inter-H-bond motions results in continuous proton diffusion pathways. Continuity of proton diffusion pathways is an essential condition for fast proton transport

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

Explanation of Dramatic pH-Dependence of Hydrogen Binding on Noble Metal Electrode: Greatly Weakened Water Adsorption at High pH

Hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) are both 2 orders slower in alkaline electrolyte than in acidic electrolyte, but no explanation has been provided. The first step toward understanding this dramatic pH-dependent HOR/HER performance is to explain the pH-dependent hydrogen binding to the electrode, a perplexing behavior observed experimentally. In this work, we carried out Quantum Mechanics Molecular Dynamics (QMMD) with explicit considerations of solvent and applied voltage (U) to in situ simulate water/Pt(100) interface in the condition of under-potential adsorption of hydrogen (H_(UPD)). We found that as U is made more negative, the electrode tends to repel water, which in turn increases the hydrogen binding. We predicted a 0.13 eV increase in hydrogen binding from pH = 0.2 to pH = 12.8 with a slope of 10 meV/pH, which is close to the experimental observation of 8 to 12 meV/pH. Thus, we conclude that the changes in water adsorption are the major causes of pH-dependent hydrogen binding on a noble metal. The new insight of critical role of surface water in modifying electrochemical reactions provides a guideline in designing HER/HOR catalyst targeting for the alkaline electrolyte