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

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

Influence of grain-refiner addition on the morphology of fe-bearing intermetallics in a semi-solid processed Al-Mg-Si alloy

© The Minerals, Metals & Materials Society and ASM International 2013The three-dimensional morphologies of the Fe-bearing intermetallics in a semisolid-processed Al-Mg-Si alloy were examined after extracting the intermetallics. α -AlFeSi and β-AlFeSi are the major Fe-bearing intermetallics. Addition of Al-Ti-B grain refiner typically promotes β-AlFeSi formation. β-AlFeSi was observed with a flat, plate-like morphology with angular edges in the alloy with and without grain refiner, whereas α -AlFeSi was observed as "flower"-like morphology in the alloy with grain refiner. © 2013 The Minerals, Metals & Materials Society and ASM International

Evolution of spin-wave excitations in ferromagnetic metallic manganites

Neutron scattering results are presented for spin-wave excitations of three ferromagnetic metallic $A_{1-x}A^{\prime}_{x}$MnO$_3$ manganites (where $A$ and $A^\prime$ are rare- and alkaline-earth ions), which when combined with previous work elucidate systematics of the interactions as a function of carrier concentration $x$, on-site disorder, and strength of the lattice distortion. The long wavelength spin dynamics show only a very weak dependence across the series. The ratio of fourth to first neighbor exchange ($J_4/J_1$) that controls the zone boundary magnon softening changes systematically with $x$, but does not depend on the other parameters. None of the prevailing models can account for these behaviors.Comment: Submitted to Phys. Rev. Let

USING KINETIC ISOMETRIC MID-THIGH PULL VARIABLES TO PREDICT D-I MALE SPRINTERS’ 60M PERFORMANCE

The purpose of the study was to determine the relationship of isometric mid-thigh pull kinetic variables including: peak force (PF), instantaneous force at 50, 90, 200 and 250 milliseconds (F@50, 90, 200 and 250 ms) rate of force development (RFD@ 50, 90, 200 and 250 ms) and impulse at 50, 90, 200, and 250 ms (IP @ 50, 90, 200 and 250 ms) to college male sprinters’ 60 m running performance. Eleven NCAA Division I male sprinters participated in the study that included two testing sessions. The first session included sprint testing and the second session included isometric mid-thigh pull strength assessment. The results from current study indicated that explosive force production variables (F@ 50 ms, RFD @ 50 and 90 ms, IP @ 90 and 200 ms) showed strong correlations with 60 m running time and maximal running velocity; while the MPF was not related to sprint variables

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چکیده مطالعۀ حاضر، با هدف قضاوت در خصوص کیفیت برنامههای کـارآفرینی و نقـش آن در اشـتغال مولـد ، در سـه مرحله، میزان توفیق هنرستانهای دخترانۀ شهر تهران را به عنوان یکی از زیرمجموعههـای نظـام مهـارت آمـوزی رسمی در سطح پیش از دانشگاه، مورد ارزیابی قرار داده است. در مرحله «پیشارزشیابی»، نشـانگ رها و معیارهـای قضاوت در سطوح «نامطلوب، نسبتاًمطلوب و مطلوب» تدوین شد. در مرحلۀ «ارزشیابی»، با استفاده از نشانگرهای تدوین شده، و تدوین ابزارهای مورد نیاز، دادهها از زیرجامعههای هنرجویان، هنرآموزان، مدیران، دانشآموختگـان و کارفرمایان گردآوری شد. نتایج نشان داد که میزان توفیق هنرستانهای دخترانۀ نواحی شمال و مرکز شهر تهـران در برنامههای کارآفرینی، در سطح «نامطلوب» و در هنرستانهای دخترانۀ نواحی جنوب، شرق و غرب شهر تهران در سطح «نسبتاً مطلوب» قرار دارد. به طور کلی و در مجموع کلیۀ نواحی مورد بررسی، میزان توفیق هنرستانهـای دخترانۀ شهر تهران در برنامههای کارآفرینی، در سطح «نامطلوب» ارزشیابی شد. در مرحلۀ «پسارزشیابی»، ضـمن استفاده از نتایج تحلیل محتوای سؤالات بازپاسخ پرسشنامۀ دانشآموختگان و کارفرمایان و مصاحبه با هنرجویـان، هنرآموزان، و کارشناسان نظام مهارتآموزی، علل نتایج بدست آمده در مرحله ارزشیابی، مورد بحث قرار گرفته و راهکارهای بهبود، پیشنهاد شد

The Efficiency Limit of CH3NH3PbI3 Perovskite Solar Cells

With the consideration of photon recycling effect, the efficiency limit of methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells is predicted by a detailed balance model. To obtain convincing predictions, both AM 1.5 spectrum of Sun and experimentally measured complex refractive index of perovskite material are employed in the detailed balance model. The roles of light trapping and angular restriction in improving the maximal output power of thin-film perovskite solar cells are also clarified. The efficiency limit of perovskite cells (without the angular restriction) is about 31%, which approaches to Shockley-Queisser limit (33%) achievable by gallium arsenide (GaAs) cells. Moreover, the Shockley-Queisser limit could be reached with a 200 nm-thick perovskite solar cell, through integrating a wavelength-dependent angular-restriction design with a textured light-trapping structure. Additionally, the influence of the trap-assisted nonradiative recombination on the device efficiency is investigated. The work is fundamentally important to high-performance perovskite photovoltaics.published_or_final_versio

Is a 20 Kg Load Sufficient to Simulate Fatigue in Squat Jumps?

Abstract available in the Annual Coaches and Sport Science College