Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V20: Focus Session: Physics of Energy Storage Materials V -- Thermal Storage and Conventional Hydrides |
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Sponsoring Units: FIAP/DMP GERA/DCOMP Chair: Donald J. Siegel, University of Michigan Room: D168 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V20.00001: Origin of the Diverse Melting Behaviors of Aluminum Nanoclusters with Around 55 Atoms Joongoo Kang, Su-Huai Wei, Yong-Hyun Kim Microscopic understanding of thermal behaviors of metal nanoparticles is important for nanoscale catalysis and thermal energy storage applications. Using first-principles molecular dynamics simulations, we reveal the microscopic origin of the diverse melting behaviors of Al$_{N}$ clusters with N around 55 [1,2]. The conceptual link between the degree of symmetry (e.g., T$_{d}$, D$_{2d}$ and C$_{s})$ and solidity of atomic clusters is quantitatively demonstrated through the analysis of the configuration entropy. The size-dependent, diverse melting behaviors of Al clusters originate from the reduced symmetry (T$_{d} \quad \to $ D$_{2d} \quad \to $ Cs) with increasing the cluster size. In particular, the sudden drop of the melting temperature and appearance of the dip at N = 56 are due to the T$_{d}$-to-D$_{2d}$ symmetry change, triggered by the surface saturation of the tetrahedral Al$_{55}$ with the T$_{d}$ symmetry.\\[4pt] [1] G. A. Breaux, C. M. Neal, B. Cao, and M. F. Jarrold, Phys. Rev. Lett. \textbf{94}, 173401 (2005).\\[0pt] [2] J. Kang, S.-H. Wei, and Y.-H. Kim, J. Am. Chem. Soc. (in press). [Preview Abstract] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V20.00002: Azonbenzene-functionalized carbon nantubes as a high energy density solar thermal fuel Alexie Kolpak, Engin Durgan, Jeff Grossman Solar thermal fuels, which store energy from the sun in the chemical bonds of molecules, are a fascinating energy storage prospect: in principle they are 100\% renewable, produce no emissions or by-products, are easily transportable in the form of liquids or powders, and can be recharged by the sun without any special equipment. However, adaptation of solar fuels as a viable, low-cost, large-scale means of energy storage will require the discovery of new materials other than the one known case based on Ruthenieum that can perform the process over many cycles with no degradation. Here we discuss a novel approach to the design of solar fuels based on photoswitchable molecules covalently bonded to carbon nanotubes (CNTs). Using density functional theory, we examine the potential for maximizing the energy density via a combination of steric and intermolecular interactions between metastable azobenzene photoisomers and a CNT substrate. In addition, we investigate how a tuning parameter unique to the nanoparticle/molecule geometry --- the packing density of the molecules on the nanotube --- can be varied to produce significant, controlled changes in the transition pathway and barriers via ordered molecule-molecule interactions, potentially leading to new classes of nanoparticle-based solar fuels. [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V20.00003: A high volume, high throughput volumetric sorption analyzer Y.C. Soo, M. Beckner, J. Romanos, C. Wexler, P. Pfeifer, P. Buckley, J. Clement In this talk we will present an overview of our new Hydrogen Test Fixture (HTF) constructed by the Midwest Research Institute\footnote{http://www.mriresearch.org} for The Alliance for Collaborative Research in Alternative Fuel Technology\footnote{http://all-craft.missouri.edu} to test activated carbon monoliths for hydrogen gas storage. The HTF is an automated, computer-controlled volumetric instrument for rapid screening and manipulation of monoliths under an inert atmosphere (to exclude degradation of carbon from exposure to oxygen). The HTF allows us to measure large quantity (up to 500 g) of sample in a 0.5 l test tank, making our results less sensitive to sample inhomogeneity. The HTF can measure isotherms at pressures ranging from 1 to 300 bar at room temperature. For comparison, other volumetric instruments such as Hiden Isochema's HTP-1 Volumetric Analyser can only measure carbon samples up to 150 mg at pressures up to 200 bar. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V20.00004: Synthesis of Li$_{2}$MgIr and LiMgIrH$_{6}$: Guidance from DFT Jan Herbst, James Salvador, Martin Meyer Formation of Li$_{2}$MgIr was suggested by theoretical modeling of Li$_{2}$MgX systems and their hydrides with density functional theory (DFT). Verifying our DFT results, we have synthesized Li$_{2}$MgIr and determined its crystal structure and hydrogen sorption behavior. The phase crystallizes in the cubic $P\bar {4}3m$ space group and is isostructural to the known ternary Li$_{2}$MgSi. Its reaction with hydrogen proceeds according to Li$_{2}$MgIr + $\textstyle{7 \over 2}$H$_{2}\to $ LiMgIrH$_{6}$ + LiH. The hydride LiMgIrH$_{6}$ also features $P\bar {4}3m$ symmetry; its detailed crystal structure is established via a combination of x-ray diffraction and DFT analyses. A metal $\to $ insulator transition accompanies formation of the hydride. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V20.00005: Thermodynamics of MgH$_{2}$ hydrogen storage materials: nanoparticle size and topological structure effects Jason Reich, LinLin Wang, Duane Johnson Via plane-wave-based Density Functional Theory calculations, we investigate H-desorption from (110) rutile MgH$_{2}$, a surface step, and surfaces of nanoscale Mg$_{30}$XH$_{62}$ clusters having catalytic dopants (X=Mg, Ti, or Fe). All calculated desorption enthalpies are endothermic, in contrast to results in the literature,\footnote{Larsson, P.; Araujo, C. M.; Larsson, J. A.; Jena, P.; Ahuja, R. \textit{P Natl Acad Sci USA} 2008, $105$, 8227} and no particle size effect is found for desorption of H singly, doubly, or triply-bonded to metal atoms, indicating only local bond energy is relevant. In contrast to recent results, we show that exothermic results are not obtained when initial cluster structures are carefully relaxed globally via simulated annealing, in which amorphous structures are found to be favored. A topological feature is identified that offers potential utility for using nanostructured MgH$_{2}$ as a hydrogen-storage solution. [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V20.00006: Hydrogen desorption from MgH$_{2}$(110) surface with transition-metal catalyst: a DFT study of energetics and barriers Lin-Lin Wang, Duane D. Johnson Transition-metal (TM) catalysts are widely used in hydrogen-storage materials to increase hydrogen absorption and desorption kinetics. Using density functional theory calculations, we elucidate the catalytic effect of Ti on H-desorption from MgH$_{2}$(110) surface. Kinetic energy barriers of different reaction pathways of hydrogen desorption are calculated via nudged-elastic-band method. We find that Ti dopant is effective in reducing kinetic barriers, in agreement with experimental observations. We also find that magnetic degrees of freedom must be carefully included to describe the change of magnetic states during catalytic-enhanced desorption. As vacancy migration barriers are lower than desorption barrier, bulk diffusion of H inherently feeds into the favorable surface desorption mechanism. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V20.00007: Tuning the Hydrogen Storage in Magnesium Alloys Suleyman Er, Gilles A. de Wijs, Geert Brocks We investigate the hydrogen storage properties of promising magnesium alloys. MgH$_{2}$ (7.6 wt {\%} H) would be a very useful storage material if the (de)hydrogenation kinetics can be improved and the desorption temperature is markedly lowered. Using first principles calculations, we show that hydrides of Mg-transition metal (TM) alloys adopt a structure that promotes faster (de)hydrogenation kinetics, as is also observed in experiment [1]. Within the lightweight TMs, the most promising alloying element is titanium. Alloying Mg with Ti alone, however, is not sufficient to decrease the stability of the hydride phases, which is necessary to reduce the hydrogen desorption temperature [2]. We find that adding aluminium or silicon markedly destabilizes Mg-Ti hydrides and stabilizes Mg-Ti alloys. Finally, we show that controlling the structure of Mg-Ti-Al(Si) system by growing it as multilayers, has a beneficial influence on the thermodynamic properties and makes it a stronger candidate for hydrogen storage [3].\\[4pt] Ref: [1] S. Er \textit{et al.}, Phys. Rev. B, \textbf{79}, 024105 (2009). [2] S. Er \textit{et al.}, J. Phys.: Condens. Matter, \textbf{22}, 074208 (2010). [3] S. Er \textit{et al.}, J. Phys. Chem. Lett., \textbf{1}, 1982 (2010). [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V20.00008: Neutron, Thermodynamic, and Modeling Studies of Hydrogen Interaction with MgO(100) John Larese, L.L. Daemen, J. Ollivier, T. Seydel, E. Cruz Silvia, B. Sumpter We report our investigations of thermodynamic, neutron diffraction, inelastic and quasielastic neutron scattering (INS and QENS) studies of hydrogen adsorbed onto MgO(100). Guided by our volumetric adsorption measurements, we used INS and QENS to probe the dynamics of the adsorbed H2 molecules. Our structural studies indicate that near monolayer completion the intermolecular distance of the H2 molecules on MgO are $\sim $20{\%} more compressed than the closest packed bulk solid plane. The melting of this compressed solid takes place at temperatures above the bulk triple point, whereas most other 2D films melt at about 70{\%} of the triple point. Using INS, the motion of the adsorbed hydrogen is examined as a function of film thickness. For rotational motions, we use the ortho-to-para transition as a guide and find that the rotational barrier for H2 adsorbed on MgO is shifted to lower energy at low surface coverage. These results are compared with modeling for additional insight into the microscopic processes that underpin the observed behavior. This work partially supported by the U.S. DOE, BES under contract DE-AC05-00OR22725 with ORNL managed and operated by UT-Battelle, LLC, the NSF under grant DMR-0412231 and a grant from the University of TN, JINS. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V20.00009: Enhanced Hydrogen Storage Properties of Magnesium Nanotrees by Glancing Angle Deposition Mehmet Cansizoglu, Tansel Karabacak Magnesium has a high hydrogen storage capacity of 7.6 wt {\%}. In addition it is one of the most abundant low cost materials in nature. However, absorption/desorption of hydrogen in Mg mainly suffer from slow kinetics. In this study, we investigate the hydrogen storage properties of Mg ``nanotrees with nanoleaves'' fabricated by glancing angle deposition (GLAD) method and compare to those of conventional thin films of Mg. A recently developed quartz crystal microbalance (QCM) gas absorption/desorption technique was used for hydrogen storage measurements on our thin film and nanostructured coatings. Storage experiments were performed at temperatures between 100-300 $^{\circ}$C, and at 30 bars of H$_{2}$ pressure. Our results reveal that Mg nanotrees have significantly faster kinetics and lower absorption temperatures for hydrogen storage compared to Mg thin films. The enhancement in absorption properties is believed to be due to decreased diffusion lengths, favorable crystal orientations for diffusion of hydrogen, and resistance to surface oxidation of Mg nanotrees. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V20.00010: Neutron spectroscopy of gamma-MgH2 Alexander Kolesnikov, Vladimir Antonov, Vadim Efimchenko, Garrett Granroth, S.N. Klyamkin, A.V. Levchenko, Michael Sakharov, Yang Ren, Timmy Ramirez-Cuesta Under ambient conditions, magnesium dihydride exists in two forms, alpha-MgH2 (the most stable modification) and gamma-MgH2 (a less stable modification). The alpha-phase partly transforms to gamma-MgH2 in the course of ball-milling and under high pressure and temperature. Due to the high hydrogen content of 7.6 wt.{\%}, MgH2 has been intensively studied as a prospective material for hydrogen storage. By exposing of alpha-MgH2 to a pressure of 5 GPa and temperature 840 K, we prepared a sample, in which about 60{\%} of the alpha-MgH2 was transformed to gamma-MgH2. We have measured inelastic neutron scattering (INS) spectra of both the high pressure treated MgH2 and starting alpha-MgH2, and extracted the spectrum for gamma-MgH2. The differences between the INS spectra and their agreement with the first-principles calculations for these compounds will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V20.00011: The chemical potential of hydrogen in Mg-films and metal-doped carbon nanostructures Mina Yoon, Hanno Weitering, Zhenyu Zhang We use first-principles density functional theory to study the binding mechanism of hydrogen to nanoscale systems. We investigate the performance of the exchange-correlation functional in describing the interaction between hydrogen and metal systems and the importance of the vibrational contribution in the formation enthalpy. In ultrathin Mg films the stability of hydrides is much lower than in the corresponding bulk systems and it can be modified by metal alloying. We calculate the chemical potential of hydrogen in Mg films for different dopant species and film thicknesses while including all vibrational degrees of freedom. By comparing the chemical potential with that of free hydrogen gas at finite temperature and pressure, we construct a hydrogenation phase diagram and identify the conditions for hydrogen absorption/desorption. The vibrational contribution to the chemical potential of hydrogen becomes more prominent for dihydrogen adsorption to metals, where its significance dramatically changes depending on the binding characteristics. This feature is illustrated by the example of metal-doped nanocarbon systems. [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V20.00012: Computational search for hydrogen storage materials: accuracy and alloys Lucas Wagner, Eric Majzoub, Mark Allendorf, Jeffrey Grossman Metal hydride materials are among the strongest contenders for hydrogen storage, offering good weight and volume density. The main reason that these materials are not used now is that it is very challenging to find a material that is both light enough and has the proper binding to allow for easy absorption/desorption near room temperature. We will evaluate two routes to controlling the binding energy: particle size and alloy composition using the highly accurate quantum Monte Carlo method. We find that traditional methods of calculating the binding energy such as the Wulff construction and density functional theory should be applied with caution, as they can lead to misleading results. We will also report on the prospects for finding a sweet spot of size and alloy composition that has the correct binding energy for hydrogen storage applications. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V20.00013: Ultra-low diffusion barriers for the ${\rm AlH_3}$-related vacancies in $\gamma$-${\rm NaAlH_4}$ Feng Zhang, Yan Wang, Mei-Yin Chou It has been suggested that the diffusion of ${\rm AlH_3}$-related vacancies plays an essential role in the decomposition of ${\rm NaAlH_4}$, a prototypical material for hydrogen storage[1,2]. We find from first-principles calculations that the diffusion barrier for both the neutral ${\rm AlH_3}$ vacancy and the charged ${\rm AlH_4^-}$ vacancy in the newly proposed $\gamma$-phase of ${\rm NaAlH_4}$ [3] is only about 0.1 eV, much lower than the barrier for the diffusion of corresponding vacancies in the conventional $\alpha$-phase 0.5 eV, calculated with the same method. Possible schemes to facilitate the $\alpha\rightarrow\gamma$ phase transformation in order to improve the kinetics of the decomposition reaction of ${\rm NaAlH_4}$ will also be discussed.\\[4pt] [1] H. Gunaydin, K. N. Houk, and V. Ozoli\c{n}\v{s}, Proc Natl Acad Sci USA {\bf 105}, 3673 (2008).\\[0pt] [2] G.~B.~Wilson-Short, A.~Janotti, K.~Hoang, A.~Peles, and C.~G.~Van de Walle, Phys. Rev. B {\bf 80}, 224102 (2009).\\[0pt] [3] B.~Wood and N.~Marzari, Phys. Rev. Lett. {\bf 103}, 185901; {\bf 104}, 019901. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V20.00014: Defect-mediated Alane formation on Ti-doped Al(111) surfaces: a DFT study Aditi Herwadkar, Lin-Lin Wang, Duane D. Johnson Understanding of Alane (AlH$_{3})$ formation on Al surface remains elusive, including interpreting STM results under various conditions. Using density functional theory calculations, we study Alane formation on close-packed (111) and stepped surfaces with {\{}111{\}} and {\{}100{\}} microfacets of Al, with and without Ti as a catalyst. We find that Ti dopants act as catalyst in the formation of Alane on Al(111) via a vacancy-mediated mechanism. Additionally, we find the Alane formation energy at steps is 40{\%} less than that from the flat surface. We assess the energetics of various surface-defect configurations to understand the concerted roles that Ti dopants, surface vacancies, and step defects play in Alane formation. Work was supported in part by Department of Energy, Office of Basic Energy Science under contract DEFC36-05GO15064 (Sandia Metal-Hydride Center of Excellence), DE-FG02-03ER15476, DE-FG02-03ER46026, and DE-AC02-07CH11358 at the Ames Laboratory operated by Iowa State University. [Preview Abstract] |
Thursday, March 24, 2011 10:48AM - 11:00AM |
V20.00015: Molecular hydrogen interaction with Ti doped Al(111) surfaces Irinder Chopra, Santanu Chaudhuri, Jean-Francois Veyan, Yves Chabal Alanates are promising hydrogen storage materials, but have poor re-hydrogenation kinetics. Decomposition\footnote{\textit{J. Alloys Compd.} 1997, 253, 1.} of NaAlH$_{4}$ can be made reversible at reasonable temperatures and pressures by adding titanium. There is however little understanding of the role of Ti as a catalyst,\footnote{\textit{J Am Chem Soc} 2006, 128, (35), 11404-11415.} and no experimental evidence for H$_{2}$ dissociation on Ti-doped Al surfaces. Using CO as a probe molecule in conjunction with in-situ infrared absorption spectroscopy, we present unambiguous evidence for molecular hydrogen dissociation, chemisorptions and spill over on with Ti doped Al(111) surfaces. The optimum catalytic activity of the Ti-doped Al surface occurs for a Ti coverage of 0.1 monolayer. At high hydrogen coverage, no CO physisorption is observed, indicating that the dissociated hydrogen spill over from the catalytic active Ti site. CO molecules can be chemisorbed at the catalytic sites but do not spill over. These findings provide important information on the nature of the catalyst during the hydrogenation reactions. [Preview Abstract] |
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