Symposium Organizers
Kwiseon Kim National Renewable Energy Laboratory
Mark van Schilfgaarde Arizona State University
Vidvuds Ozolins University of California-Los Angeles
Gerd Ceder Massachusetts Institute of Technology
Vikas Tomar Purdue University
Y2: Battery and Fuel Cell Materials II
Session Chairs
Wednesday AM, April 07, 2010
Room 3006 (Moscone West)
9:30 AM - **Y2.1
First-principles Calculations of Li Migration in Li Battery Electrode Materials.
Kristin Persson 1
1 EETD, LBNL, Berkeley, California, United States
Show AbstractWe present ab initio studies of Li motion in two materials, commonly used for Li battery electrodes. Layered Li(Ni⅓,Mn⅓,Co⅓)O2, is an interesting candidate for Li battery cathode applications, especially if the Co is partially substituted for cheaper Al. While most reports agree that Al substitution decreases the electronic conductivity, raises the intercalation voltage, and increases the stability of the material against oxygen evolution, it is still unclear how the Li mobility is affected. Li migration in the layered material is influenced by several factors such as Li slab space, cation ordering and interlayer mixing. We present ab initio calculations of Li diffusivity as a function of Al content and slab spacing in the layered material, which elucidates the intrinsic effect of the Al substitution in the bulk material. For anode applications, we study Li motion in graphite. Traditionally, carbons are considered relatively low rate material for Li intercalation, especially at low temperature1. The bulk low-temperature Li-carbon phase diagram is well established as a sequence of staged ordered compounds, but Li diffusion in the different stages has not been firmly established. To clarify these matters we present a comprehensive study of Li thermodynamics and kinetics in bulk graphite as a function of Li concentration, ordering, temperature and graphene layer stacking. Interestingly, our kinetic Monte Carlo simulations show that intra-layer Li motion in bulk graphite is intrinsically very fast, even at high Li concentrations, suggesting the possibility of designing carbonaceous anode materials with high rate capability.1 Huang et al, J Phys. Chem. Sol., 67, 1228, 2006.
10:00 AM - Y2.2
A High-throughput Computational Search for New Li-ion Battery Cathode Materials.
Geoffroy Hautier 1 , Anubhav Jain 1 , Charles Moore 1 , Hailong Chen 1 , Robert Doe 1 , Christopher Fischer 1 , Byoungwoo Kang 1 , Jae Chul Kim 1 , Denis Kramer 1 , Xiaohua Ma 1 , Tim Mueller 1 , Kristin Persson 1 , Gerbrand Ceder 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractDuring the last few decades, improvements in ab initio computational methods have allowed researchers to rationalize and even predict essential Li-ion electrode properties such as voltage, diffusivity, or thermodynamic stability. When coupled with exponential rises in computational power available to research groups, these developments provide the opportunity for a large-scale computational search for new Li-ion battery cathode materials. Tens of thousands of novel materials can be generated and computationally screened for battery performance, focusing experiments on the most promising candidates and expanding the coverage of new chemical spaces. In this talk, we will present how such a project can be set up, highlighting the challenges of automating data generation, management and analysis as necessitated by the scale of a high-throughput computational project. In addition, preliminary results on experimentally confirmed new Li-ion battery materials predicted by our high-throughput screening approach will be presented. Besides the straightforward search in our computational database for ground-breaking materials, we will also show how such a large pool of computational data can be used to gain chemical knowledge and design better cathode materials.
10:15 AM - Y2.3
The Effect of Al Substitution in Lithium Transition Metal Oxides from First-principles.
Michael Kocher 1 , Kristin Persson 1 , Quentin Ramasse 1
1 , Lawrence Berkeley National Lab, Berkeley, California, United States
Show AbstractFirst-principles methods are a useful tool for investigating potential electrode materials to be used in batteries. In this work, we performed first-principles calculations on several lithium transition metal oxides used as cathode materials in Li-ion batteries. Specifically, we studied the layered LiCoO2 and LiMn1/3Ni1/3Co1/3O2, and the spinel, LiMn1.5Ni0.5O4. By substituting Al for Co, or Ni in these materials, two fundamental changes are observed. First, the Al substitution for the transition metal decreases the number of states near the Fermi level. This will result in a decrease in the electrical conductivity of the material, which is consistent with experimental observations. Secondly, the Al substitution changes the number of unoccupied O p states, which suggests that there is an increase in the iconicity of the overall metal-oxygen bonding. This increased ionicity of the Al-O bonding, relative to the Tm-O bonding, increases the Li intercalation voltage and increases the thermal stability of the material. In addition to investigating the changes in the electronic structure, we also calculated the Li diffusion barriers in LiCo1-xAlxO2 to understand the effect of Al substitution on the Li diffusivity. Lastly, we compare the electronic density of states determined from first-principles calculations to edges measured from Electron Energy Loss Spectroscopy (EELS).
10:30 AM - Y2.4
Comparison of Lithium Intercalation Potentials of Transition Metal Compounds Obtained from Hybrid and Gradient-corrected Density Functional Calculations.
Vincent Chevrier 1 , Shyue Ping Ong 1 , Rickard Armiento 1 , Gerbrand Ceder 1
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractWe present a comparison of the calculated redox potentials of a number of transition metal compounds, including the olivine LixMPO4 (M=Mn, Fe Co, Ni), layered LixMO2 (x=Co, Ni) and spinel-like LixM2O4 (M=Mn). Redox potentials were calculated using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid density functional and the Perdew-Burke-Ernzerhof (PBE) gradient-corrected density functional. First-principles calculations within the local density approximation (LDA) or generalized gradient approximation (GGA), though very successful, are known to underestimate redox potentials for transition metal compounds. Fei et al. [1] have previously demonstrated that this inaccuracy can be attributed to the lack of cancellation of electron self-interaction errors in LDA/GGA, and that the DFT+U method with a self-consistent evaluation of the U parameter can satisfactorily reproduce the experimental lithium intercalation potentials of a number of transition metal compounds. The recent development of functionals incorporating a fraction of the Hartree-Fock (HF) exchange presents an alternative route by which contributions to the exchange energy missing from conventional semi-local density functionals can be included. We will show that lithium intercalation potentials obtained with the HSE06 hybrid functional compare favorably to those obtained with the GGA+U method, with the important advantage of not requiring the choice of a value for the on-site Coulomb repulsion interaction parameter.1. F. Zhou, M. Cococcioni, C. A. Marianetti, D. Morgan, and G. Ceder, Phys. Rev. B, 70, 235121 (2004)
10:45 AM - Y2.5
First-principle Calculations of Li Migration in LiCoO2 Using GGA+U Approach.
Hiroki Moriwake 1 , Akihide Kuwabara 1 , Craig Fisher 1 , Rong Huang 1 , Yumi Ikihara 1 , Yuichi Ikuhara 1 2 , Hideki Oki 3
1 Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya, Japan, 2 , The University of Tokyo, Tokyo Japan, 3 , Toyota Motor, Toyota Japan
Show AbstractFirst-principle calculations of Li migration in LiCoO2 have been carried out using Nudged Elastic Band (NEB) method in comparison with two extreme Li concentrations, fully discharged state (LiCoO2) and fully charged state (CoO2). All calculations were carried out with Projector Augmented Wave (PAW) methods based on the density functional (DFT) theory. The Generalized Gradient Approximation (GGA) has been used as exchange-correlation functional. Electron correlation was treated with GGA+U approach. Our calculation results revealed that depending on the Li concentration in LiCoO2, Li migration path and migration energies were varied significantly. In the fully discharged state (LiCoO2), Li ions migrate straight path with activation energy of 0.8 eV. On the other hand, in fully charged state (CoO2), Li migrates through tetrahedron site which located between two Li sites, with activation energy of 0.4 eV. By valence charge density analysis, spatial distribution radius of Li 2s orbital is estimated to be 0.7 Å. This large difference in Li migration path and migration energies should be attributed to their local crystal structures. In the case of fully discharged state (LiCoO2), two Li sites, which is located vicinity of tetrahedron site are occupied by Li ions, so that, migrate Li ion can not be situated tetrahedron site. By precise charge density analysis, migrate Li ions can form Li+ + e- pair.
11:30 AM - **Y2.6
Multi-scale Multi-dimensional Lithium-ion Battery Model.
Gi-Heon Kim 1 , Kandler Smith 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractSuccessful market acceptance of hybrid electric vehicles (HEVs), achieving better vehicle efficiency by combining electric motor and battery into conventional internal combustion engine powertrain, and growing concern for global environmental issue are accelerating electrification of our transportation system. Next generation plug-in hybrid vehicles (PHEVs) and battery electric vehicles (EVs) may also enable vehicles to be powered by diversified energy sources including electricity generated from clean, renewable resources. Lithium-ion batteries are the most favorable for near-term inclusion in those advanced electric vehicles due to their demand for energy- and power-dense storage. But in order to increase viability of such vehicle technologies toward sustainable transportation, the capacity, rate capability, durability, thermal stability and cost of the batteries must be further improved. Lithium-ion battery physics, such as kinetics, phase transition, ion transport, electronic current distribution, energy dissipation and heat transport happen over a wide range of time and length scales and closely interact. Therefore, the requirements for lithium-ion batteries for next generation electrified vehicles must be addressed over various length and time scales in which physical and chemical processes are occurring; from atomic variations to vehicle interface controls (Figure 1). There have been various model-based investigations focusing on different scales of battery physics to promote theoretical understanding beyond what is possible from experiment alone. NREL researchers have pursued to develop a model that incorporates physics of lithium-ion battery in all scales with bridging of the existing multi-scale models or by direct integration of them. An integrated multi-scale model would expand knowledge on interplay of different scale battery physics and show a pathway toward expediting the process of advanced battery system developments enabling green mobility technologies.This talk will discuss existing model approaches, and introduce the recent progress and study with NREL’s multi-scale multi-dimensional (MSMD) model for linking the microscopic lithium-diffusion dynamics and charge transfer kinetics with the macroscopic transport of electron and heat.
12:00 PM - Y2.7
A Generalized Multi-dimensional Mathematical Model for Charging and Discharging Processes in a Supercapacitor.
Sreekanth Pannala 1 , Srikanth Allu 1 , Badri Velamur Asokan 2 , William Shelton 1
1 , Oak Ridge National Laboratory, Oak Ridge , Tennessee, United States, 2 , Exxon Mobil, Houston, Texas, United States
Show AbstractWe present the development of a unified formulation across the electrode-electrolyte-electrode system - using a rigorous volume averaging approach typical of multiphase formulation1, 2. Unlike previously used methods for supercapacitors based on a segregated formulation 3 with intermediate boundaries, by having a unified, single-domain approach, complex geometries are naturally incorporated with the numerical algorithms guaranteeing stability and convergence. The same formulation also applies to 1D, 2D and 3D geometries irrespective of the complexities associated with dimensionality as well as with electrode/electrolyte spatial arrangements. In addition, the formulation accounts for any spatio-temporal variation of the different properties such as electrode/void volume fractions and anisotropic conductivities. This generality will aid in upscaling of local material properties directly into the system-level model without ad hoc modification such as Bruggeman’s coefficients or tortuousity factors to account for local effects using global constants. The resulting governing equations are solved using finite volume and finite element techniques. We are going to present results from this study on a standard supercapacitor and show that we can reproduce the cell level behavior. In addition, we conduct an investigation of the effect of spatial variability of the void volume fraction on the performance of multidimensional supercapacitor configurations. We analyze the cell level performance and relate it to the detailed spatio-temporal variations of charge and discharge behavior in the electrodes. We will also note show how one can apply this formulation to batteries and how one could include detailed electrochemistry, thermal energy and structural mechanics to construct a fully-integrated model to study electrochemical storage devices.
References:
1.A. Prosperetti, in Computational Methods for Multiphase Flow, edited by A. Prosperetti and G. Tryggvason (Cambridge University Press, Cambridge, 2007).
2.C. Y. Wang, W. B. Gu and B. Y. Liaw, J. Electrochem. Soc. 145 (10), 3407-3417 (1998).
3.M. W. Verbrugge and P. Liu, J. Electrochem. Soc. 152 (5), D79-D87 (2005).
12:15 PM - Y2.8
Ab initio Molecular Dynamics Simulations of the Initial Stages of Solid-electrolyte Interphase Formation on Lithium Metal and Li-intercalated Graphite Anodes.
Kevin Leung 1
1 Surface and Interface Science, Sandia National Labs, Albuquerque, New Mexico, United States
Show AbstractThe decomposition of ethylene carbonate (EC) during the initial growth (and post-film-cracking regeneration) of solid-electrolyte interphase (SEI) films at the solvent-graphitic anode interface is critical to lithium ion battery operations. Ab initio molecular dynamics techniques are applied to study the decomposition of ethylene carbonate (EC) in liquid EC, at the interface between liquid EC and lithium-intercalated graphite, and at the EC-lithium metal interface. In liquid EC, an excess electron can cause EC ring opening reactions in surprisingly short, picosecond timescales. Carbon edge terminations at LiC6-EC interfaces have large effects at this stage of growth. With C=O decorated edges, we observe fast electron transfer to the solvent and EC ring-opening reactions to form multiple products including CO32- and OC2H4O2-, which are either found in SEI or are chemical precursors to SEI components. Similar reactions are observed at the lithium metal-EC interface. Our explicit treatment of liquid EC/electrode interfaces suggests that achievable experimental conditions can lead to EC decomposition via multiple pathways.
12:30 PM - Y2.9
Mechanism of Ultrafast (dis)Charging of Li Ion Batteries by Heterogeneous Doping of LiFePO4.
Stefan Adams 1
1 Materials Sience and Engineering, National University of Singapore, Singapore Singapore
Show AbstractHigh-energy density batteries are the key component of plug-in hybrid electric vehicles and also required for the integration of renewable energy sources into the power grid. While Li ion batteries (LIBs) are closest to meeting requirements in terms of energy density, storage efficiency and cyclability, a major challenge is still to charge and discharge them safely at an acceptably fast rate. Here we computationally explore the effect of surface modification of the low cost high safety cathode material LiFePO4 to understand the experimentally observed high charge and discharge rate capabilities. Enhanced ion transport in nanostructured solids attracted significant interest both for its practical applications and from a scientific point of view, because transport behavior changes qualitatively when the spacing between adjacent interfaces becomes comparable to the thickness of the space charge layer. Still, the influence of local structure variations on the charge transport properties is not well understood at an atomic level. The mechanism of the experimentally observed drastic enhancement of the value and dimensionality of the ionic conductivity in different nanoscale heterostructures is studied, with an emphasis on LixFePO4. By means of molecular dynamics simulations with a dedicated forcefield and our bond valence pathway analysis the experimentally observed ultrafast Li+ transport in surface modified LixFePO4-d and the related LixMNO4 phases (M=Fe, Mn… ; N=P, Si...) can be reproduced and explained as a mesoscopic multiphase effect due to heterogeneous doping, i.e. the Li+ redistribution in the vicinity of the interface between LixFePO4 and a pyrophosphate glass surface layer. Over the usual working temperature range of LIBs Li+ ion conductivity in the surface modified LiFePO4 phase is enhanced by 2-3 orders of magnitude, while the mesoscopic multiphase effect practically vanishes for T>700K. For the bulk phase the simulations reproduce the experimental conductivitie and the activation energy of 0.57eV. A layer-by-layer analysis of Li+ ion mobility in structurally relaxed multilayer systems with periodic boundary conditions indicates a continuous variation of the Li+ ion mobility with the distance from the interface with the maximum mobility close to the interface, but Li+ diffusion rate remains enhanced (compared to bulk values) even at the center of the simulated cathode material crystallites. The bond valence (BV) pathway analysis is utilized to study correlations between conductivity enhancement and microstructure. Details of the dynamic ion transport pathways in the local structure models are extracted by our transport pathway analysis to series of instantaneous configurations from the MD trajectories. The ion mobility variation can be related to the extension of unoccupied accessible pathway regions. Moreover the influence of local strains, misfit dislocations is quantified.
12:45 PM - Y2.10
Diffusion-induced Fracture of Si-based Anodes and LiFePO4-based Cathodes in Lithium-ion Batteries.
Yifan Gao 1 , Min Zhou 1
1 Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractNano-sized materials promise significant performance enhancement for lithium ion batteries. For example, Si nanowires have been successfully used as an anode material which can undergo volume expansion of up to 400% without mechanical failure, therefore, significantly increasing the capacity of anodes in Li ion batteries [1]. It has also been reported that nano-sized LiFePO4 cathode particles with a fast ion-conducting surface phase can lead to ultra-fast charging speeds due to improved diffusion kinetics [2].
One of the most important physical processes underlying these technological developments is the coupling between mass diffusion and mechanical deformation. For most solid cathode/anode materials, anisotropy (as in the case of C6 and LiFePO4) and large deformation (as in the case of silicon) play very important roles. So far, no theory for mass diffusion that accounts for both anisotropy and finite deformation is available. Here, such a theoretical framework is developed based strictly on thermodynamic principles. While analytical solutions are obtained for geometrically simple cases, finite element implementation of this theory requires the use of quadratic elements to account for the strain gradient effect. Simulations of the lithium ion charge/discharge cycles of Si nanowire anodes have revealed factors dominating the instability of the coupled deformation and mass diffusion process. The coupling between the diffusion kinetics and the tri-axial stress state right at LiFePO4/FePO4 boundaries is investigated. The results demonstrate the effect of LiFePO4 flake thickness on charge/discharge speed and on material stability in regard to fracture initiation.
1 Chan, C.K. et al., High-performance lithium battery anodes using silicon nanowires. Nature Nanotechnology 3 (1), 31-35 (2008).
2 Kang, B. & Ceder, G., Battery materials for ultrafast charging and discharging. Nature 458 (7235), 190-193 (2009).
Y3: Electrical and Chemical Energy Storage Materials
Session Chairs
Wednesday PM, April 07, 2010
Room 3006 (Moscone West)
2:30 PM - **Y3.1
Theory of Pseudocapacitive and Hysteretic Energy Storages.
Yong-Hyun Kim 1
1 Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Show AbstractAs the characteristic length of the energy storage material decreases to nanometer scale, energy storage mechanisms becomes different from those of macroscopic counterparts. An example is the pseudocapacitive energy storage, which exhibits mixed characters of Faradaic redox and capacitive electrostatic reactions [1]. Also, hydrogen storage exhibits unusual hysteresis [2]. Recently we have developed theories of pseudocapacitive energy storage in anatase TiO2 [3] and hysteretic hydrogen storage in small-pore metal-organic frameworks (MOFs) [4], based on the analyses of results of first-principles calculations.In pseudocapacitive energy storage [3], an approach is developed to calculate electrochemical capacitance of nanoscale materials as a function of open-circuit voltage (Voc). From our simulation results, an anatase TiO2 nanosheet is a hybrid of electrochemical double-layer capacitor and Faradaic battery with exhibiting sub-surface redox reactions. At low Voc, the system behaves as capacitor with the formation of electric double layers at the surface. As Voc increases above a threshold, lithium intercalation into subsurface region takes place, assisted by the surface electric double layers. The subsurface intercalation is the origin of pseudocapacitance. Our findings provide a coherent picture of how a transition from pure capacitors to batteries or pseudo-capacitors occurs in these nanostructured materials.In hysteretic hydrogen storage [4], we have developed a theory of the enhanced adsorption hysteresis of H2 molecules, based on first-principles calculations and simulated adsorption/desorption isotherms. We used the MOF Co(1,4-benzenedipyrazolate) [Co(BDP)] as a model system, in accordance with the experiment [2]. We demonstrate that the small-pore system would exhibit hysteretic H2 adsorption under changes of external pressure, if the H2 gas should mainly diffuse along the small-pore channels of < 1 nm. On contrary, open-channel porous materials such as activated carbon and metal-(1,4-bezendicarboxylate) [M(BDC), M = Cr, Al] do not show hysteresis upon hydrogen storage, because fast equilibration occurs by exchanging H2 between storage materials and the gas reservoir. Our detailed analysis of transient adsorption dynamics reveals that the hysteretic H2 adsorption is intrinsic in the diffusion-controlled small-pore systems. The hysteretic H2 adsorption opens a new possibility for H2 storage, i.e., the kinetics-based H2 trapping at rather moderate pressures.[1] T. Brezesinski, J. Wang, J. Polleux, B. Dunn, and S. H. Tolbert, J. Am. Chem. Soc. 131, 1802 (2009).[2] H. J. Choi, M. Dincă, and J. R. Long, J. Am. Chem. Soc. 130, 7848 (2008).[3] J. Kang, S.-H. Wei, K. Zhu, and Y.-H. Kim, submitted (May 2009).[4] J. Kang, S.-H. Wei, and Y.-H. Kim, submitted (October 2009).
3:00 PM - Y3.2
Synthesis and Computation in Low-coordinate Transition Metal Composites for Kubas-type Hydrogen Storage.
David Antonelli 1
1 Sustainable Energy Research Center, University of Glamorgan, Pontypridd United Kingdom
Show AbstractWhile satisfactory technology exists for hydrogen production and fuel cell implementation, there is currently no efficient means of storing hydrogen in such a way as to conveniently use it as fuel for on-board and stationary applications. For this reason chemical adsorbents have been developed that hold hydrogen under a wide variety of conditions. These fall into two categories, hydrides, and physisorption materials, however neither class has led to a practical solution to hydrogen storage. Hydrides require too much heat to liberate hydrogen and release too much heat when hydrogen is absorbed, necessitating the use of cumbersome designs to circumvent this problem. Physisorption materials must be cooled to 77 K to function effectively and this cuts drastically into the energy efficiency of the system. For these reasons a new class of storage materials must be developed if hydrogen is ever to see the light of day as a fuel. The US Department of Energy (DOE) has predicted that materials with 20-30 KJ/mol binding enthalpies would be ideal for room temperature applications (this compares to >70 kJ/mol for hydrides and > 10 kJ/mol) for physisorption materials). To date the Kubas interaction (a non-dissociative binding of hydrogen between a hydride and physisorption in bond strength) has not yet been exploited as a sole means of hydrogen binding in a system, even though this is the only type of hydrogen binding that functions within this ideal 20-30 kJ/mol range. Recent results from the Antonelli group have demonstrated that the Kubas interaction can be exploited in hydrogen storage systems based on silica supported Ti(III) fragments in low coordination numbers (ie. 3-4). These were the first systems developed that function in the 20-30 KJ/mol target zone recommended by the DOE. The Ti fragments, comprising only 5 wt% of the system, were able to retain over 40% of their activity at room temperature and 80 atm. (1.7 H2/Ti as compared to 3.5 H2/Ti) without saturation or any kinetic barrier. While these model systems fall far short of the DOE storage targets they suggest that accessible 3-4 coordinate Ti(III) centers fabricated into an extended solid consisting of a much larger proportion of Ti by weight could surpass the DOE storage goals. This talk will explore efforts to synthesize extended solids based on the Kubas interaction as well as computations on the silica-supported benzyl Ti system that help understand the factors influencing hydrogen storage performance of these and related materials.
3:15 PM - Y3.3
First-principles Calculations on Alkali Atom Doping to Enhance the Storage Capacity of Materials.
Hiroshi Mizuseki 1 , Natarajan Venkataramanan 1 , Ryoji Sahara 1 , Yoshiyuki Kawazoe 1
1 , Institute for Materials Research, Tohoku Univ., Sendai, Miyagi, Japan
Show AbstractDoping with alkali metal elements increases the hydrogen storage capacity of many materials. Inspired by these findings, we have explored the hydrogen storage properties of lithium doped Metal Organic Frameworks (MOFs) and BN fullerenes. Binding energy of alkali atoms on BN fullerenes were identical to C60. However, the binding on BN fullerene occurs at the bridge site near the tetragonal site. Alkali adsorption can be adsorbed to a maximum of six sites. Each Li atom was found to hold up to 3 hydrogen molecules. In the case of the MOF materials, Li-doping significantly improves the hydrogen uptake. Each Li atom doped was found to hold three H2 molecules firmly due to the charge induced dipole interaction. The most stable position for the Li atom was found to be on the benzene ring, forming a Li-benzene complex and each benzene ring was able to hold two Li atoms [1]. A part of this work has been supported by New Energy and Industrial Technology Development Organization (NEDO) under “Advanced Fundamental Research Project on Hydrogen Storage Materials”. 1) N. S. Venkataramanan, R. Sahara, H. Mizuseki, and Y. Kawazoe, Int. J. Mol. Sci. 10, 1601 (2009).
3:30 PM - Y3.4
Metallacarboranes as Metal Organic Frameworks Linkers for Enhanced Hydrogen Storage.
Abhishek Singh 1 2 , Arta Sadrzadeh 1 2 , Boris Yakobson 1 2
1 Department of Mechanical Engineering and Materials Sciences, Rice University, Houston, Texas, United States, 2 Department of Chemistry, Rice University, Houston, Texas, United States
Show AbstractUsing first principles calculations we demonstrate the high hydrogen storage capacity of metallacarboranes1. The transition metal (TM) atoms in the metallacarborane can bind up to 5 H2’s with the average binding energy of ~0.4 eV/H, which lies within the reversible adsorption range. Among the first row TM atoms Sc and Ti are found to be maximize the H2 storage (~8 wt%) on the metallacarborane. Used as a linker in MOF design and being integral part of the MOF cage, TMs do not suffer from the clustering problem, which has been a biggest hurdle for the success of TM decorated graphitic materials. Furthermore, the presence of carbon atom in the cage can link the metallacarboranes to form the metal organic frameworks, which will sorb hydrogen via both Kubas interaction as well as physical Van de Waals type of binding.References:1. A. K. Singh, A. Sadrzadeh, and Boris I. Yakobson, submitted.
3:45 PM - Y3.5
Periodic Planewave Density Functional Theory Studies of Small Molecule Interactions in Metal-Organic Frameworks.
Taku Watanabe 1 , Seda Keskin 2 1 , David Sholl 1
1 School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Department of Chemical and Biological Engineering, Koc Univeristy, Istanbul Turkey
Show AbstractMetal-organic frameworks (MOFs) are known to have high porosity within their structures that are ideal for separation and storage of gas molecules. We investigated the interaction of small molecules such as CO, CO2, CH4, NO, H2O, etc. with a number of MOF structures using a combination of quantum chemistry calculations and molecular simulations. Several MOFs including Cu-hfipbb were found to exhibit very high diffusion barriers for CH4 but not for CO2, indicating the possibility of high kinetic selectivity. We also investigated the chemisorption of small molecules on the open metal sites in CuBTC and CPO-27-Ni. Our results show clear preferences in the binding modes of these molecules, suggesting the existence of interesting adsorption-storage-delivery cycles. The possibility of functionalization and catalytic reactions via open metal sites in MOFs will also be discussed.
4:00 PM - Y3: Hydrogen
BREAK
4:30 PM - Y3.6
Hydrogen Desorption from Lithium Hydride and Ammonia as Hydrogen Storage System.
Takayuki Ichikawa 1 , Aki Yamane 1 , Kozo Hoshino 2 1 , Yoshitsugu Kojima 1
1 , Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima, Hiroshima, Japan, 2 , Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima Japan
Show AbstractIn order to distribute hydrogen practically, the system of gravimetrically and volumetrically condensed hydrogen has to be developed, which should work under a moderate condition. As one of the candidates of the condensed hydrogen states, we have focused on ammonia (NH3) because it has a high hydrogen (H2) density of 18 mass% and is easily liquefied by compression of about 1 MPa at room temperature. Recently, we have succeeded in generating an H2 gas at room temperature by the gas-solid reaction between NH3 and lithium hydride (LiH)[1, 2]. This reaction was known as one of two elementary steps in the solid-solid reaction with H2 desorption, LiNH2 + LiH → Li2NH + H2[3, 4]. In the previous report, we suggested that the model in which H2 molecules are randomly formed from four equivalent H2 atoms in hypothetical LiNH4 produced by the reaction between LiH and NH3 according to the laws of probability[4]. In this work, we demonstrate a process of the reaction between a Li2H2 cluster and NH3 based on ab initio molecular-dynamics (MD) simulation. The H2 molecule is formed by two H atoms, one of which is from the LiH cluster and the other is from the NH3, and each H atom of H2 is not equivalent both in geometry and in charge during the reaction; first an Hδ- atom in Li2H2 and an Hδ+ atom in NH3 atom approach each other by Coulomb interation and then make a dimer, and atomic charges of both H atoms get close to neutral at the moment of dimerisation.Acknowledgment This work is supported by the Grants of the NEDO project ’Advanced Fundamental Research on Hydrogen Storage Materials’ in Japan.References [1] Y. Kojima et al., J. Mater. Res. 24 (2009) 2185 [2] Y. Yamamoto et al., Int J Hydrogen Energy in press. [3] T. Ichikawa et al., J. Phys. Chem. B 108 (2004) 7887[4] S. Isobe et al., J. Phys. Chem. B 109 (2005) 14855
4:45 PM - Y3.7
Computationally Affordable Quantum Chemistry in the Condensed Phase.
Gregory Beran 1 , Ali Sebetci 1 , Kaushik Nanda 1
1 Chemistry, University of California, Riverside, Riverside, California, United States
Show AbstractClathrates and other molecular inclusion compounds are promising materials for the storage of hydrogen fuel and other gases, but careful selection of the host lattice species and/or guest promoters is necessary to obtain molecular inclusion materials that combine high storage density with stability near ambient temperatures and pressures. Theory could play an important role in designing such materials, but computationally affordable techniques capable of describing the full range of important intermolecular interactions in these extended systems are required.In this context, we will discuss our recently developed hybrid quantum/classical many-body interaction (HMBI) model for studying systems of interacting molecules. Unlike standard quantum/classical models, which partition a system spatially, HMBI partitions a system based on the importance of the intermolecular interactions. Leading interactions are treated quantum mechanically, while less important and computationally expensive interactions are approximated classically. The force field parameters are almost entirely predicted on-the-fly, eliminating the need for extensive manual parameterization. HMBI's spatially homogeneous formalism makes it suitable for studying molecular inclusion compounds and other systems where the important chemistry is dispersed over a large region. HMBI requires orders of magnitude less computational effort than fully quantum mechanical techniques, and the computations can be run in parallel on hundreds of processors with very high efficiency. Finally, the HMBI approximation introduces only small errors compared to fully quantum mechanical treatments.
5:00 PM - Y3.8
Accurate ab initio Energy Gradients in Chemical Compound Space.
Anatole von Lilienfeld 1
1 , Sandia National Labs, Albuquerque, New Mexico, United States
Show AbstractDue to the combinatorial nature of chemical compound space even computationally a systematic screening of materials for interesting properties is beyond any current capacity. Consequently, when it comes to properties that require first principles calculations with atomistic resolution, optimization algorithms must find the ideal compromise between convergence and number of compounds "visited". Analytical gradients in chemical space promise significant speedup in predicting properties of compounds without the need to ``visit'' them. I will present analytical potential energy difference derivatives, based on the Hellmann-Feynman theorem, for any pair of iso-electronic compounds. The energies not being a monotonic function between compounds, these derivatives are insufficient to predict the right trends of the effect of alchemical mutation. Quantitative estimates without additional self-consistency calculations can be made when the Hellmann-Feynman derivative is multiplied with a linearization coefficient that is defined for a reference pair of compounds. These results suggest that accurate predictions can be made regarding any molecule's energetic properties as long as energies and gradients of three other molecules have been provided. Presented numerical evidence includes predictions of electronic eigenvalues of saturated and aromatic molecular hydrocarbons.
5:15 PM - Y3.9
Modeling of Hydrogen Releasing Mechanism in Complex Metal Hydride – Catalyst Systems.
Qizhen Li 1
1 , University of Nevada, Reno, Reno, Nevada, United States
Show AbstractThe metal hydride LiBH4 is one of the most promising hydrogen storage materials because of its extremely high hydrogen content. The biggest obstacle of the usage of LiBH4 is the high decomposing temperature for hydrogen releasing. This work explores computationally the hydrogen releasing mechanism for the complex metal hydride – catalyst systems to identify the possible systems with the ideal hydrogen releasing temperatures. The research has significant impacts on realizing the application of hydrogen as an energy source in all categories of transportation vehicles.
Symposium Organizers
Kwiseon Kim National Renewable Energy Laboratory
Mark van Schilfgaarde Arizona State University
Vidvuds Ozolins University of California-Los Angeles
Gerd Ceder Massachusetts Institute of Technology
Vikas Tomar Purdue University
Y5: Photovoltaic Materials - Organic and Thin-Film
Session Chairs
Thursday PM, April 08, 2010
Room 3006 (Moscone West)
2:30 PM - **Y5.1
Energy Transfer and Charge Transport Simulations in Conjugated Dendrimers.
Muhammet Kose 1 , Kwiseon Kim 2 , Peter Graf 2
1 Chemistry and Molecular Biology, North Dakota State University, Fargo, North Dakota, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractWe present here a joint experimental and theoretical investigation of exciton diffusion and charge transport in phenyl-cored thiophene dendrimers. Experimental exciton diffusion lengths of the dendrimers vary between 8 and 17 nm, increasing with the size of the dendrimer. A theoretical methodology is developed to estimate the exciton diffusion lengths for conjugated small molecules in a simulated amorphous film. The theoretical approach exploits Fermi’s Golden Rule to estimate the energy transfer rates for a large ensemble of bimolecular complexes in random relative orientations. Electronic coupling calculations with delocalized transition densities revealed efficient coupling pathways in the bulk of the material, but did not result in strong couplings between the chromophores that were calculated for more- localized transition densities. The molecular structures of dendrimers seem to be playing a significant role in the magnitude of electronic coupling between chromophores. The chemical structure of the chromophore, the shape of the transition densities, and the exciton lifetime are found to be the most important factors in determining the size of the exciton diffusion length in amorphous films of conjugated materials. The theoretical methodology for charge transport exploits molecular mechanics, quantum mechanical calculations, and Monte-Carlo simulations to predict the time-of-flight measurement mobilities in denrimer films. The simulations show that both hole and electron mobilities increase with the size of dendrimer, and that the former is larger than latter in all dendrimers. Internal reorganization energies are inversely correlated with the mobilities. Our simulations also indicate that the dendrimers have small density of states for energetic disorder (<60 meV) and both hole and electron mobilities possess weak electric field dependence. We examine the influence of external reorganization energy as well as the possible trap sites on charge transport in these materials. The implications of the presented results will also be discussed to identify the structure-property relationships for charge transport in conjugated polymers.
3:00 PM - Y5.2
Atomistic Investigation of the P3HT Adhesion on Nanostructured Titania.
Claudio Melis 1 2 , Alessandro Mattoni 2 , Luciano Colombo 1 2
1 Dept of Physics, University of Cagliary , Monserrato (CA) Italy, 2 , SLACS CNR-INFM, Monserrato (CA) Italy
Show AbstractPolymer based hybrid nanostructures (e.g. P3HT/titania, P3HT/ZnO ) have emerged as promising systems for photovoltaics, since they can in principle combine the good formability of polymers and the good trasport properties and thermal stability of the inorganic metaloxide. A strong link of the polymer (where light is absorbed) to the inorganic substrate (where electrons are accepted) is necessary to give rise to an efficient photoconversion.Accordingly, the theoretical understanding of the interface structure at the atomic scale (i.e. the actual interatomic distances, the overlap of the electronic density between polymer and substrate, the covalent versus electrostatic nature of the bonding) is of great relevance to improve the properties of such hybrid materials. In order to represent the structural complexity associated to nanostructured films and polymer distortions, we make use of models including up to ten thousands atoms. By using molecular dynamics (MD) simulations we are able to extensively explore the attraction basin between the polymer and the titania, as well as to calculate the adhesion energy as a function of its curvature and roughness. Since the system size here investigated falls out of reach of a systematic first principle calculation, the interatomic forces are derived from model potentials (MP).The description of interatomic forces in hybrids is challenging. A general model potential for the hybrid system is not available, while there exist reliable potentials for titania or polymers, separately. Here we combine such existing force fields by adding long range Coulomb and dispersive interactions to model interactions across the metaloxide-polymer boundary.The model is validated against experiments and first-principles calculations. By using the above theoretical framework we study the adhesion of poly(3-hexylthiophene) on nanostructured titania surfaces (consisting of spherical nanocaps on top of a (110) rutile surface) and we study the adhesion of an oligothiophene as a function of their local curvature and roughness. In the limit of a perfect planar surface, the maximum adhesion energy is calculated to be as large as 0.6 eV per monomer, and it corresponds to the oligothiophene oriented along the -110 direction of the surface. Deformations of the polymer are observed due the incommensurability between the titania and the polymer lattice parameters. When the surface is nanostructured, the adhesion of the polymer is affected by the local morphology and a nonmonotonic dependence on the surface curvature is observed. The atomistic results are explained by a simple continuum model that includes the strain energy of the polymer and its electrostatic interaction with the local surface charge.
3:15 PM - **Y5.3
Understanding and Prediction of Materials Properties for Energy Conversion and Storage.
Jeffrey Grossman 1
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractClassical and quantum mechanical calculations are employed to understand important microscopic mechanisms in energy conversion and storage materials and interfaces. Our goal is to predict key properties that govern the efficiency in these materials, including structural and electronic effects, interfacial charge separation, electron and hole traps, excited state phenomena, band level alignment, and binding energies. Examples of our work in the areas of photovoltaics, thermoelectrics, hydrogen storage, and solar fuels will be presented.
3:45 PM - Y5.4
An Efficient Method for the Computation of Hamaker Constants of Nanoparticales.
Yan Zhao 1 , Hou Hou 1 , Eric Hanson 1 , Jiannan Dong 2 , David Corti 2 , Elias Franses 2
1 Commercial Print Engine Lab, HP Labs, Palo Alto, California, United States, 2 School of Chemical Engineering, Purdue University, West Lafayette, Indiana, United States
Show AbstractOrganic photovoltaics constitute one of the solar cell technologies to fulfill energy and environmental sustainability. Printing of functional organometallic nanoparticles has been sorted as one of the potential routes to achieve low-cost, large area flexible solar cells. Typically, these nanoparticles are first suspended in a carrier liquid, and then jetted onto a substrate. To ensure uniformity and reproducibility of device performance, good dispersion stability of these nanoparticles is a pre-requisite. Classical theory such as the DLVO theory could possibly predict dispersion stability but relies critically on the accuracy of the Hamaker constants (A) of the materials, which are difficult to obtain experimentally. Although the Hamaker constant could be calculated from the Lifshitz’s continuum theory, it requires detailed molecular and macroscopic information on the dielectric or optical properties of the material over a wide frequency range. These can pose a challenge for many nanoparticles. We show in this paper an efficient computational approach to calculate Hamaker constants of CuPc. Our model employs time dependent density functional theory (TDDFT) to calculate the London dispersion coefficients of the building blocks of CuPc nanoparticle. A semi-empirical correction factor a to the standard Hamaker constant equation is then used to calculate the Hamaker constants from the London dispersion coefficients and the densities of the nanoparticles. The TDDFT scheme has been validated for predictions of the dispersion coefficients of five molecules (H2O, NH3, CO2, C6H6, and pentane) and for predictions of the static dipole polarizabilities of three organometallic compounds (TiCl4, OsO4, and Ge(CH3)4). The Hamaker constants of water, pentane, decane, hexadecane, polystyrene, and poly(methyl methacrylate) have been used to benchmarking the proposed model. The model is then finally used to predict the non-retarded Hamaker constants (A11) of CuPc pigments (nanoparticles of ~ 90 nm diameter). Using the experimentally derived density of 1.56 g/cm3 for a commercially available β-CuP, A11(β) =13.5 × 10-20 J was obtained. Its corresponding effective Hamaker constant in water (A121) is calculated to be 3.1 × 10-20 J.
4:30 PM - Y5.5
Optimizing Transparency in the [Cu2S2][Sr3Sc2O5] Structure: Predicting New p-type TCOs Using Hybrid Density Functional Theory.
David Scanlon 1 , Graeme Watson 1
1 School of Chemistry, Trinity College Dublin, Dublin, Leinster, Ireland
Show AbstractTransparent conducting oxides are vital components in Solar cells, flat panel displays etc.[1] Although most of the industry standard TCOs are n-type (e.g. SnO2:F, In2O3:Sn and ZnO:Al), the development of p-type TCOs has proved very challenging.[2] Since the discovery of concomitant p-type conductivity and transparency in thin films of CuAlO2,[3] the development of p-type TCOs has enjoyed an explosion of interest. This has resulted in the identification of other copper delafossite materials with p-type TCO properties (CuMO2, M – Ga, In, B, Sc, Y, Cr).[4, 5] As these materials are all either limited by an indirect band gap or low conductivity, or both, alternative Cu(I)-based materials have been investigated. SrCu2O2 was also found to have p-type ability and transparency, even having a direct band gap, but with lower conductivities even than the delafossites.[6] Transparent p-n junctions using n-type ZnO and p-type SrCu2O2 have been fabricated by Hosono and co-workers, but these possible UV LEDs for solid state lighting, have suffered from extremely poor performance, due mostly to the poor p-type conductivity of SrCu2O2.[7]Layered oxychalcogenides have emerged from the field of Cu(I) based materials as alternative p-type TCOs, often possessing large optical bandgaps and good hole mobility due to favourable mixing between Cu d states and chalcogen p states at the VBM.[8] [Cu2S2][Sr3Sc2O5] has recently come to light as a promising candidate, having the highest undoped conductivity of any p-type TCO and a higher hole mobility than In2O3:Sn.[9] In this study we analyse the electronic structure and geometry of [Cu2S2][Sr3Sc2O5] using a screened hybrid density functional approach (HSE06).[10] We investigate the optimization of transparency in this material through cation substitution and discuss strategies to improve the conductivity of these materials.
[1] Gordon, R. G. MRS Bull. 2000, 25, 52–57.
[2] Thomas, G. Nature 1997, 389, 907.
[3] Kawazoe, H.; Yasakuwa, H.; Hyodo, H.; Kurita, M.; Yanagi, H.; Hosono, H. Nature 1997, 389, 939.
[4] Marquardt, M. A.; Ashmore, N. A.; Cann, D. P. Thin Solid Films 2006, 496, 146–156.
[5] Scanlon, D. O.; Walsh, A.; Watson, G. W. Chem. Mater. 2009, 21, 4568–4576.
[6] Kudo, A.; Yanagi, H.; Hosono, H.; Kawazoe, H. Appl. Phys. Lett. 1998, 73, 220–222.
[7] Ohta, H.; Hosono, H, Mater. Today, 2004, 7, 42-51
[8] Ueda, K.; Hiramatsu, H.; Hirano, M.; Kamiya, T.; Hosono, H. Thin Solid Films 2006, 496, 8–15.
[9] Liu, M. L.; Wu, L. B.; Huang, F. Q.; Chen, L. D.; Chen, I. W. J. Appl. Phys. 2007, 102, 116108.
[10] Scanlon, D. O.; Watson, G. W. Chem. Mater. 2009, In Press . DOI: 10.1021/cm902260b
4:45 PM - Y5.6
Organic Heterojunctions for Photovoltaic Applications: C60 Growth on Pentacene.
Rebecca Cantrell 1 , Paulette Clancy 1
1 , Cornell University, Ithaca, New York, United States
Show AbstractMaking reliable organic electronic devices from carbon-rich (organic) semiconductors challenges researchers worldwide. There are many hurdles, one of which is making highly crystalline thin films out of organic materials. The focus in this proposal is on organic-on-organic thin film growth, which is less studied than organic-on-metal thin film growth. Specifically, the C60/Pentacene organic heterojunction is a model system due to its applications for photovoltaic devices. In addition, C60 and pentacene both have relatively high crystallinities, and hence relatively large charge carrier mobilities. Experimental evidence shows promising electrical properties of photovoltaic devices made from C60 films grown on pentacene films. One of the main areas for improvement is in making the C60 films grow in a crystalline, two dimensional manner on pentacene. Molecular Dynamics has shown the diffusive behavior of up to a few C60 molecules on the bulk-phase pentacene surface; the surface diffusion coefficients of C60 on pentacene are relatively liquid-like. Molecular dynamics has also shown a very interesting anisotropic diffusion behavior of C60 on bulk-phase pentacene around room temperature. Kinetic Monte Carlo in combination with a continuum coalescence theory aid in the understanding of how multiple C60 molecules grow on pentacene. Evidence of interesting faceted grain shapes bring understanding to the C60/pentacene system. There are very little previous simulation studies of what happens at organic interfaces, and this paper will bring insight into the properties of such a system that would be otherwise hard to see experimentally or predict theoretically.
5:00 PM - Y5.7
Electronic Properties of Dye-sensitized Solar Cells from Long-range Corrected Density Functional Theory.
Bryan Wong 1
1 Materials Chemistry Department, Sandia National Laboratories, Livermore, California, United States
Show AbstractThe excited-state properties in a series of solar cell dyes are investigated with a long-range-corrected (LC) functional which provides a more accurate description of charge-transfer states. Using time-dependent density functional theory (TDDFT), the LC formalism correctly predicts a large increase in the excited-state electric dipole moment of the dyes with respect to that of the ground state, indicating a sizable charge separation associated with the electronic excitation. The performance of the LC-TDDFT formalism, illustrated by computing excitation energies, oscillator strengths, and excited-state dipole moments, demonstrates that the LC technique provides a consistent picture of charge-transfer excitations as a function of molecular size. In contrast, the widely-used B3LYP functional severely overestimates excited-state dipole moments and underestimates the experimentally observed excitations, especially for larger dye molecules. The results of the present study emphasize the importance of long-range exchange corrections in TDDFT for investigating the charge-transfer dynamics in solar cell dyes.
5:15 PM - Y5.8
Molecular Dynamics Simulations of Oligomer Film Stabilization Through Ion-beam Deposition.
Travis Kemper 1 , Donghwa Lee 1 , Simon Phillpot 1 , Susan Sinnott 1
1 Materail Science and Engineering , University of Florida, Gainesville, Florida, United States
Show AbstractIon-beam deposition is being used to stabilize conducting oligomer films for use in organic photovoltaic devises. The goal of this work is to prevent structural changes caused by chemical attack, disorder, or desorption, which degrade devise performance. The second-generation reactive empirical bond-order (REBO) potential has been successfully applied to the irradiation and modification of crystalline, polymer and nanostructures, such as carbon nanotubes. In this work the atomic-level processes involved in selective modification of oligomers for optoelectronic applications through hyperthermal energy particle deposition are explored to identify the mechanisms by which different polyatomic ions and radicals assist in the stabilization of oligomer films. In particular, oligmer films of polythiophene are bombarded with incident H, CH2, C2H and thiophene at incident energies of 4-50 eV. The results of classical molecular dynamics simulations with the REBO potential are compared to experimental findings, as well as linearly scaled density-functional theory molecular dynamics results. This work was supported by the NSF (CHE-0809376).
5:30 PM - Y5.9
Photomechanical Effects in Algae-Tropocollagen Systems Under Influence of Electric Field.
Devendra Dubey 1 , Vikas Tomar 1
1 Aeronautics and Astronautics, Purdue University, West Lafayette, Indiana, United States
Show AbstractEnergy production from biological materials such as algae holds good promises for energy needs of the future. However, radical pathways for efficient energy derivation from such materials are still under exploration. Algae is an autotrophic microorganism and shows wide range of photo-chemical and photo-mechanical behavior subject to change in its surrounding environment. Present study investigates such effects under the influence of electro-mechanical properties of biopolymers such as tropocollagen (TC). Tropocollagen is a slender large molecule (~ 300 nm) and is shown to have piezoelectric properties. Such behavior could be useful for generating nanomechanical actuations in sub-micron scale applications under the influence of electric field. For examining the effect of such TC behavior on the photomechanics of algae we are using both experimental and computational techniques. In this study, the alga Chlamydomonas Reinhadtii is used for its ease of availability and fast growth capability in various media. We will be reporting results from three dimensional atomistic analyses of the effect of electric field on TC and TC-algae systems. Along with this, we will also present the results of initial experimental investigations on the TC-Algae system for their electric field governed photomechanical effects.
5:45 PM - Y5.10
Modeling the Molecular Network Structure of Hybrid Sol-gel Materials.
Mark Oliver 1 , Reinhold Dauskardt 1
1 , Stanford University, Stanford, California, United States
Show AbstractTo study the nature of sol-gel-derived hybrid molecular networks, we have developed a set of computational tools to generate accurate model molecular networks and quantify their medium and long-range structures using the mathematics of graph theory. Using a molecular dynamics/simulated annealing approach and a new empirical interatomic potential, large hybrid molecular networks can be generated from a wide range of precursors. A unique feature of our approach is the ability to generate models with well-controlled condensation degrees, which not only allows for the systematic study of network condensation on structure and properties but enables the generation of highly realistic models through calibration with experimental techniques such as solid-state 29Si NMR that measure local chemical structure of condensed films. We have applied these tools to investigate the structure of hybrid films processed from small organosilane precursors for use as low-k layers and optical coatings as well as more complex films processed from epoxy-functionalized silanes and metal alkoxides that show potential for use in optical waveguides. We will discuss the utility of the mathematics of graph theory in quantifying various aspects of medium and long-range structures and in understanding the fracture properties of these films that are critical for mechanical reliability. The modeling approaches presented provide power tools for precursor selection and hybrid material design.
Y6: Poster Session: Computational Approaches to Materials for Energy
Session Chairs
Friday AM, April 09, 2010
Salon Level (Marriott)
9:00 PM - Y6.1
The Theoretical Study of Stability-increasing Mechanisms of Succinonitrile in Lithium Ion Batteries.
Seung Bum Suh 1 , Maengeun Lee 1
1 R&D Center, Samsung SDI, Yongin-si, Gyeonggi-do, Korea (the Republic of)
Show AbstractSuccinonitrile(SN) is an additive used in lithium ion batteries to improve the stability of cathode active material. Using density functional theory, we have identified the mechanism by which SN enhances the cathode active material stability. When a lithium or an oxygen ion falls out of the cathode active material, the crystal structure develops a void defect, reducing the stability of the active material. Because of the low lithium ion concentration and higher probability of oxygen vacancy at the surface, the destruction of the active material occurs primarily at the surface. SN binds onto the active material surface, and effectively raises the binding strength with metallic and other atoms, hence raising its stability even in case of oxygen vacancy. This work explains the stability-increasing mechanisms of additives used in lithium ion batteries, and suggests a direction for design of new additives.
9:00 PM - Y6.11
AgSbO3, A Promising Material for Photoelectrochemical Water Splitting: A Hybrid Density Functional Theory Investigation.
Jeremy Allen 1 , Kristin Nilsson 1 , David Scanlon 1 , Graeme Watson 1
1 School of Chemistry, Trinity College Dublin, Dublin, Dublin, Ireland
Show AbstractThe efficient utilization of solar radiation to combat global energy and environmental concerns is one of the grand challenges for materials scientists worldwide. Photoelectrochemical (PEC) decomposition of water by visible light is a highly desirable hydrogen production method as we bid to move away from fossil-fuel dependence.[1] Low cost metal oxides, with their high stability in aqueous solutions represent ideal catalysts for solar driven hydrogen production.[2] Typically TiO2 is employed as the oxide photocatalyst due to its abundance, photocatalytic ability and low cost, however, efficiency is lost due to its large band gap.[3] Significant research is now being focussed on selecting ternary oxides with designer properties in an effort to replace TiO2 as the oxide photocatalysts in PEC.[4] Recently AgSbO3, which crystallizes in both the ilmenite and pyrochlore structures, has been identified as a candidate photocatalyst.[5. 6] We present state of the art screened hybrid density functional theory calculation of AgSbO3 in both crystal structures. Geometry, electronic structure and optical absorption will be discussed for both polymorphs. The potential PEC performance will be related to the electronic structure by examination of the nature of both the valence band and conduction band and the potential for trapping and hopping of electrons and holes.
[1] M. Z. Jacobson; W. G. Colella and D. M. Golden, Science, 2005, 308, 1901.
[2] M. Woodhouse; G. S. Herman; B. A. Parkinson, Chem. Mater. 2005, 17, 4318.
[3] A. Fujishima and K. Honda, Nature, 1972, 238, 37.
[4] A. Walsh; Y. Yan; M. N. Huda; M. M. Al-Jassim, and S. –H. Wei. Chem Mater. 2009, 21, 547
[5] T. Kato; N, Kikugawa, and J. Ye, Catal. Today, 2008, 131, 197
[6] J. Singh and S. Uma, J. Phys. Chem. C 2009, 113, 12483
9:00 PM - Y6.12
Understanding Performance and Corrosion Behavior of Photo-electrode in Terms of Energetics of Water-derived Radicals on Ga-V (V=N,P,As) and GaP:N (110) Surfaces: First-principles Study.
Woon Ih Choi 1 , Yong-Hyun Kim 2 , John Turner 1 , Kwiseon Kim 1
1 , National Renewable Energy Lab, Golden, Colorado, United States, 2 , Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Show AbstractHydrogen production using photo-electrochemical cells (PEC) is one of the promising technologies that can convert sunlight directly to energy carriers. Photo-electrode material in PEC is to be resistive to corrosion that occurs at the interface with water and hydrogen evolution rate on its surface reasonably fast as well. Experimentally it is known that GaN is more resistive to the corrosion than GaP and GaAs, when they are used for photo-cathodes. Also hydrogen gas evolution rate on GaN is slower than that of GaP and GaAs. However, understanding of microscopic mechanisms of these properties based on first-principles theories are absent up to date. Holes supplied from sunlight will detach the hydrogen atoms of H2O as protons, leaving energetic O, H, or OH radicals. Therefore energetics of water-derived radicals on photo-electrode surface is important factor which determine its performance. Based on first-principles electronic structure and total energy calculations, we have studied reactions of -H, -O and -OH on the (110) surface of photo-cathode Ga-V and GaP:N materials, where V is N, P, and As. Zero-point vibration energy and chemical potential of H2 and O2 gases are considered after static calculations. We have found that atomic oxygen on the GaN surface prefers being detached as O2 to forming Ga-O. On the other hand, GaP and GaAs surfaces can have a strong Ga-O bond, hindering formation of O2 gas and thus promoting surface corrosion. The trend can be understood in terms of electronegativity difference between O and N, P, and As. On GaP and GaAs surfaces hydrogen easily evolves as H2 gas but on GaN not. Doped nitrogen in GaP to improve corrosion resistivity are tend to be clustered specially on the outermost surface region. These surface nitrogen atoms are expected to protect the surface and at the same time reduce hydrogen evolution rate.
9:00 PM - Y6.13
A Dft Study of Atomic Structure and Electronic Properties of Highly Mismatched GaNAs Alloy.
Ebru Bakir 1 2 , Lin Wang Wang 3 , Wladek Walukiewicz 1
1 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Engineering Physics, University of Gaziantep, Gaziantep Turkey, 3 Computational Research Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractFirst-principles calculations are performed to investigate the atomic structure and electronic properties of GaNAs, a highly mismatched alloy system with large band gap bowing. Energy gap of GaNAs can be continuously changed from 1.5 eV to 3.5 eV making this material suitable for a variety of opto-electronic applications including low cost, high efficiency multijunction solar cells. Another interesting property of this material is the splitting of the conduction band into E+ and E- subbands. The origin of this splitting and dependence of band gap energy on x for large composition range is currently under investigation. Recent experiments showed that the GaN1-xAsx alloys with As concentration 0.17> x >0.8 are amorphous and those with compositions outside this range are crystalline [1]. To analyze this properties we performed ab-initio molecular dynamics (MD) to simulate the amorphous structure of GaN1-xAsx alloy system using 216 atom supercells. We then tested a method which uses the atomic structures of these 216 atom supercell to construct a large amorphous system with thousands of atoms. We investigate pair distribution function, local bonding topologies and their relationships to the electronic structure of the system.
9:00 PM - Y6.14
Nanostructured (Zr, Hf)N/(Sc, Y)N Metal/Semiconductor Superlattices for Thermoelectric Energy Conversion: A First-principles Study.
Bivas Saha 1 , Timothy Sands 2 , Umesh Waghmare 1
1 Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore India, 2 Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana, United States
Show AbstractWe present first-principles density functional theory based simulations to determine electronic structures, vibrational spectra and thermal properties of ZrN/ScN, HfN/ScN and (Zr, Hf)N/(Sc, Y)N metal/semiconductor superlattices and solid solutions aiming at (a) understanding the role of interfaces and epitaxial strain in controlling the transport of electrons and phonons, (b) developing models that can be employed in determination of such properties of these superlattices with periodicities longer than those of the short-period superlattices modelled here, for potential applications in the thermoelectric and thermionic solid state energy conversion. Our results show (a) large asymmetry of the electronic density of states and flattening of electronic bands along the cross-plane direction near the Fermi energy of these superlattices, (b) the presence of Schottky barriers with height of the order of 0.1-0.3 eV, essential for optimizing ZT in these material systems. Analysis of vibrational spectra and Boltzmann transport based calculations shows 10-100 fold reduction of lattice thermal conductivity along the growth direction of these superlattices with respect to their individual bulk components due to phonon filtering at the interfaces resulting from the mismatch in bulk phonon density of states. HfN based superlattices have been proven to be better in reducing lattice thermal conductivity due to Hf’s larger mass than Zr. Temperature dependence of lattice thermal conductivity also suggests significant contribution of optical phonons on lattice thermal conductivity at room temperature, with effect most pronounced in case of HfN based superlattices. Our work uncovers important features of metal/semiconductor superlattices necessary to understand their suitability for thermoelectric and thermionic applications.Email id:
[email protected] [email protected] [email protected]9:00 PM - Y6.15
HOMO/LUMO Gap in Nitrogen Enriched Carbon Materials.
Krzysztof Fic 1 , Grzegorz Lota 1 , Elzbieta Frackowiak 1
1 Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, Poznan Poland
Show AbstractActivated carbons are very attractive energy storage materials (supercapacitor electrodes, hydrogen sorbents) because of their electrochemical behavior and physicochemical stability. However, energy density of these materials is still relatively low. Several approaches are recently developed to improve specific energy of carbon-based materials such as enhancing their specific surface area and ultramicropore volume, adjusting the micro/meso ratio, modifying the surface chemistry. The effect of pore size on the capacitance seems to be complicated and also depends strictly on the type of electrolyte. Carbon materials with well distributed mesopores network can provide some kind of pathway for fast transportation of the electrolyte ions. It plays an important role for efficient electrochemical hydrogen storage as well as for high power supercapacitor. The other interesting way of carbon material improvement is introducing the nitrogen atoms as functional groups on the carbon surface and into carbon matrix. It is well known that nitrogen-enriched carbons derived e.g. from melamine and other N-rich precursors have an excellent capacitor performance in acidic and alkaline medium. Mechanism of this phenomena is partly elucidated, increase of capacitance may be caused by faradic reactions of nitrogen functional groups. Also in hydrogen storage realized by electrosorption, i.e. water decomposition on carbon electrode with hydrogen molecules generation and “in statu nascendi” sorption, nitrogen-enriched carbons reveal an increase of hydrogen capacity compared to traditional activated carbons. It may suggests that apart from faradic reactions connected with surface functionality also the material conductivity (electron density states) plays a very important role.In this study the energetic gap between Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) in a model structure of nitrogen-enriched carbon was investigated. To calculate this gap, which can be correlated with the material conductivity, two different methods were applied, i.e. solving of the Hartree – Fock equations and calculating a band gap from Koopman’s theorem (as ab initio approach) as well as solving of Density Functional Theory equations and calculation of a band gap from Janak’s theorem. In these models we considered a different nitrogen amount and type of N functional groups, their location and vicinity. The results suggest that there is an optimum of nitrogen content and there are also preferable nitrogen functional groups which can effectively increase conductivity of carbon material. It is noteworthy that these investigations are supported by experimental results, molecular calculations have been correlated with electrochemical data.
9:00 PM - Y6.16
Electric Potential Distribution Along ZnO Nanowires, Embedded in Epoxy Matrix.
Kasra Momeni 1 , Gregory Odegard 1 , Reza Yassar 1
1 Mechanical Engineering-Engineering Mechanics, Michigan Technological University, Houghton, Michigan, United States
Show AbstractThe electric potential distribution along a zinc oxide nanowire embedded in an epoxy matrix is modeled using continuum mechanics and Maxwell’s equations. Perturbation theory is used for decoupling the constitutive equations. Since the governing differential equation is singular at the center of the nanowire, a modified finite difference method is proposed to solve the governing differential equations. It is shown that a gradient of electric potential is along the axis of nanowire and has a maximum and minimum value at the extreme ends of the nanowire that are equal in magnitude but have opposite signs. The positive and negative voltages are separated by a zero electric potential at the middle of the nanowire. The nanocomposite system acts as a nanogenerator. It is shown that the electric potential is a strong function of shear stress at the interface of matrix nanowire. The proposed system and loading configuration, i.e. the nanowire-embedded in a matrix under tensile loading, can generate about 160% more electric potential than the nanowire in the bended configuration which is reported in the literature.
9:00 PM - Y6.17
Structure Prediction of Bottom-up Assemblies With a Field Theoretic Approach.
Kahyun Hur 1 , Richard Hennig 1 , Fernando Escobedo 2 , Ulrich Wiesner 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York, United States
Show AbstractBottom-up type assemblies of amphiphilic block copolymers (BCPs) with nanoparticles (NPs) have attracted much attention for the potentially superior properties of the resulting nanocomposite materials for various applications such as electronics, optics, batteries, solar cells, and fuel cells. The use of a BCP as a structure directing agent enables spatial control of the BCP and NP components on the nanometer length scale. However, limited understanding of the underlying physics has hampered progress in theoretical predictions of morphology. Reliable prediction of structure and quantitative analysis of properties by theoretical and simulation approaches may provide helpful insights to key experimental design criteria. In this work, we study BCP/NP self-assembly using a combination of self-consistent field theory (SCFT) and density functional theory (DFT). Long-range Coulomb and hard sphere interactions between NPs were implemented. We found that long-range Coulomb interactions lead a pure NP system to form an ordered lattice. Furthermore, we found that the same strategy applied to BCP/NP self-assembly predicts an ordered NP lattice within the microphase separated BCP nanostructure. Such materials with NP lattices are expected to provide high figures of merit for thermoelectric materials.
9:00 PM - Y6.18
Silicon Growth From the Melt: Modeling the Formation of Planar Faults by a Refined Lattice Monte Carlo Approach.
Johan Pohl 1 , Michael Mueller 2 , Albrecht Seidl 2 , Karsten Albe 1
1 Institute for Materials Science, TU Darmstadt, Darmstadt Germany, 2 , Wacker Schott Solar GmbH, Alzenau Germany
Show AbstractSilicon growth from the melt is very often accompanied by the formation of (111) twin boundaries and stacking faults. There is, however, no widely accepted picture about the conditions promoting the formation of planar defect structures. In the past lattice based Monte-Carlo methods have been successfully used for simulating the growth of ideal, single crystalline grains from the melt but did ignore the role of planar faults.I n this contribution we present an extension that allows to include stacking faults and twin boundaries in one of the equivalent four (111) directions in the diamond lattice. It is shown how to construct a suitable lattice and model Hamiltonian for this purpose. The model is parameterized for silicon growth using available data on the interface free energies. Results on the roughening transition and the growth velocity of parallel twin boundaries are presented.
9:00 PM - Y6.19
Quantum Chemical Molecular Dynamics Simulation of Oxidation Process on Clean Metal Surface in High Temperature Water.
Ken Suzuki 1 , Nishith Das 1 , Tetsuo Shoji 1 , Hideo Miura 1
1 , Tohoku University, Sendai Japan
Show AbstractRegarding the materials degradation in operating nuclear power reactors, stress corrosion cracking (SCC) of stainless steels and nickel based superalloys is an essentially critical issue concerning the safety of plant operation and plant life extension. The chemical reaction behavior of metal surfaces in high temperature water such as oxidation and/or anodic dissolution, and formation of oxide or protective film is a crucial process in crack propagation of SCC. Therefore, understanding the interaction of water with metal surfaces and the mechanism of subsequent oxidation is of fundamental interest to improve the understanding of SCC behavior. In addition, most of the proposed SCC mechanisms deal with mass transfer behavior through the material at the crack tip region, indicating that the atomic diffusion behavior strongly affects the crack propagation. In this study, we performed tight-binding quantum chemical molecular dynamics method to study the oxidation dynamics of Fe-Cr-Ni/H2O interface. The simulation enabled us to present a clear view of the chemical reaction dynamics. We found that water molecules dissociated to OH group and H atom on the surface. Further dissociation reaction OH→O+H was also observed. The number of OH groups increased during the simulation, and the complete dissociation of H2O to form adsorbed oxygen and H2 also occurred at the surface. The OH groups and adsorbed oxygen gradually moved closer to the surface and penetrated into the Fe-Cr-Ni crystal. This penetration seems to result from the weakening of metal bonds due to the formation of strong Fe-O and Cr-O bonds. Weakening of metal bonds enlarge the first layer spacing and enable the oxygen or OH group to migrate more easily into the bulk. Furthermore, it is known that both chemisorbed OH groups and oxygen atoms cause high electronic fields, therefore the system tries to lower the fields by either segregation of metal ions from the surface or penetration of OH and oxygen into the metal lattice. After the penetration, metal atoms moved outward and the surface structure became rough. We found that chromium atoms had high mobility compared to iron and nickel atoms, and this resulted in increase in the concentration of chromium at the interface. Penetrating oxygen atoms were selectively distributed around the chromium atoms, implying that oxygen species had the tendency to coordinate the chromium atoms. These results indicate that the spontaneous formation of chromium oxide film would take place on Fe-Cr-Ni clean surface at the beginning of the oxidation process.
9:00 PM - Y6.2
Comparison of Theoretical and Experimental Fe (2p) X-ray Absorption Spectra for LaSrFeNi-oxides.
Selma Erat 1 2 , Artur Braun 1 , Cinthia Piamonteze 3 , Zhi Liu 4 , Thomas Graule 1 5 , Ludwig Gauckler 2
1 , Empa, - Swiss Federal Laboratories for Materials Testing & Research, Dubendorf Switzerland, 2 Department for Nonmetallic Inorganic Materials, ETH-Zurich, Zurich Switzerland, 3 Swiss Light Source, Paul Scherrer Institute, Villigen Switzerland, 4 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 5 , Technische Universität Freiberg, Freiberg Germany
Show AbstractLa1-xSrxFe1-yNiy-oxides which are potentially cathode materials for Solid Oxide Fuel Cells (SOFCs) are known as thermally activated polaron conductors having ABO3-perovskite structure. The transport properties of the system are affected mainly by B-site (Fe/Ni) under the electrostatic potential created by A-site (La/Sr). In order to explain the transport properties of such a complex system we first work on the electronic structure of B-site (Fe). Soft x-ray absorption spectra were recorded at Fe L edge which mainly shows the transitions from initial state (2p^63d^n) to final state (2p^53d^n+1) with quite high resolution. The spectra split into L2(2p1/2 ) and L3 (2p3/2) due to core hole spin-orbit coupling and additionally split into t2g and eg levels due to crystal filed effect. We support our experimental spectra with Ligand Field Multiplet Calculation (LFMC). The calculations are made in octahedral symmetry and the best fit with the experimental spectra is obtained for 10Dq=1.85 eV. In contrast to that Charge Transfer Calculation (from 2p^63d^n to 2p^63d^n+1L, L denotes ligand hole) for Fe3+ and Fe4+ which are also made in octahedral symmetry with different exchange energy (10)-(-5) eV, in the step of 1 eV, doesn’t fit with the experimental spectra. Therefore, we conclude that charge transfer mainly occurred from oxygen (2p) to Ni (3d) and not to Fe (3d).
9:00 PM - Y6.20
3D Dislocation Dynamics Simulations of Micropillars Under Uniaxial Compression.
Caizhi Zhou 1 2 , Bulent Biner 2 , Richard LeSar 1 2
1 Department of Materials Science and Engineering, Iowa state university, Ames, Iowa, United States, 2 Ames Laboratory, Iowa State University, Ames, Iowa, United States
Show AbstractIn the past few decades, small-scale metallic structures have been widely utilized in microelectronic circuits, optical and magnetic storage media, micro-electro-mechanical systems (MEMS) and so on, owing to their excellent mechanical, chemical, or electrical properties. Recently, size-dependent deformation properties of single crystals have attracted much attention in the materials science community, in part because they are closely related to the reliability of such structures in technical applications. In our study, an experimental-like initial dislocation structures cut from larger deformed samples have been introduced into 3D dislocation dynamics simulations (DDS) to study the plasticity in small sizes. Application was made to single-crystal nickel samples with coated and free surfaces. Comparison of the simulation results and the experiments are excellent, finding essentially identical behavior. Examining details of the dislocation mechanism illuminates how surface boundary conditions play an important role in small-scale plasticity, leading to confined dislocation reactions and multiplications such as cross-slip. The mechanical response of smaller samples depends on a single or, at most, very few, active dislocation sources, and the scatter in magnitude of the saturation flow stress increases with decreasing sample size.
9:00 PM - Y6.24
Theoretical Study of Gold Clusters Sandwich Compounds.
Luis Sansores 1 , Jesus Muniz 1
1 Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Mexico, DF, Mexico
Show AbstractA theoretical study of possible sandwich compounds consisting of a layer of [AuCl]3 between either two aromatic ligands [Au3Cl3Tr2]2+ (1) or two alkaline metals [Au3Cl3M2] with M=Li, Na, K, Rb, or Cs (2), is presented. Full geometry optimization was carried out using perturbation theory at MP2 level and DFT. Both compounds show great stability. Compound (1) is in agreement with the 18-electron rule and the bonding nature is due to the donation interaction coming from the Tr rings to the gold cluster and back-donation from the later to the former. For compounds (2) the bonding is more ionic in nature, the donation is almost negligible and a small back-donation from the [Au3Cl3]2- to the 2M+ cations is observed. For both compounds an aurophilic interaction between gold atoms is observed. Results presented include orbital analysis, bonding energies, aromatic character, NBO charge distribution and reactivity.
9:00 PM - Y6.4
Interaction of Protons in Zn-doped BaZrO3: A First Principles Study.
Yong-Chan Jeong 1 , Dae-Hee Kim 1 , Dae-Hyun Kim 1 , Jong-Sung Park 2 , Byung-Kook Kim 2 , Yeong-Cheol Kim 1
1 Department of Materials Engineering, Korea University of Technology and Education, Cheonan Korea (the Republic of), 2 Center for Energy Materials Research, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractAmong the perovskite oxide materials, rare earth doped barium zirconate (RE-BZO, RE-BaZrO3) has been researched as a proton conducting material for applications in a variety of electrochemical devices, including solid oxide fuel cells (SOFCs), electrolysis cells, and hydrogen pumps. Copper oxide (CuO), nickel oxide (NiO), and zinc oxide (ZnO) have been used as sintering aids to decrease the sintering temperature of BZO. We studied the interaction between the Zn atom and two protons in a 3×3×3 BZO superstructure. The substitution of a Zr atom with a Zn atom produces an O vacancy to meet the charge neutrality condition. The H and OH dissociated from the H2O molecule can be incorporated into an O atom and the O vacancy, respectively. The incorporated protons usually repel the nearby Zn atom. When the two protons bonded to the first nearest O atoms from the Zn atom, they were energetically the most favorable. The resulting hydration energy was in the range of -1.22 ~ -0.49 eV. The minimum hydration energy (-1.22 eV) was obtained when the two protons were close and, therefore, pushed the Zn atom further away. In this case, the distance decrease between the proton and the second nearest O atom might have decreased the total energy and compensated the energy increase due to the Zn atom displacement. When the proton transferred from the first to the second nearest O atom, the activation energy was in the range of 0.36 ~ 1.02 eV. When the hydration was low, the activation energy became high. Because this high activation energy hinders proton conduction in BZO, the amount of ZnO used as a sintering aid should be limited to minimize its adverse effect.
9:00 PM - Y6.5
Hydrogen Desorption from Metal Hydride and Ammonia: ab initio Molecular-dynamics Simulation.
Aki Yamane 1 , Fuyuki Shimojo 2 , Kozo Hoshino 3 , Takayuki Ichikawa 1 4 , Yoshitsugu Kojima 1 4
1 Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima Japan, 2 Graduate School of Science and Technology, Kumamoto University, Kumamoto Japan, 3 Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima Japan, 4 Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima Japan
Show AbstractIntroductionAmmonia (NH3), which liquefies at about 1 MPa of pressure, contains 18.1 mass% of hydrogen and can be regarded as one of the excellent hydrogen storage and transport materials.The hydrogen storage system of alkali metal hydride (MH, M=Li, Na, K) and NH3 desorbs H2 as MH+NH3→MNH2+H2 at room temperature with exothermic reaction, and the reverse reaction MNH2+H2→MH+NH3 also occurs at relatively low temperatures and pressures[1].For this reaction, Isobe et al.[2] suggested that the model in which hydrogen molecules are randomly formed from four equivalent hydrogen atoms in hypothetical LiNH4 produced by the reaction between LiH and NH3 according to the laws of probability. On the other hand, Kar et al.[3] performed high level electronic structure calculations for NH3 and gas-phase diatomic molecule LiH and showed that each reactant contributes one H atom to the product H2.Our purpose is to reveal the mechanism of reaction dynamics and to show the consistency with the experiment.MethodWe have performed ab initio molecular-dynamics (MD) simulation on a system of a Li2H2 cluster and an NH3 molecule at temperatures of 500, 700 and 1000 K. We set time step to be 0.48 fs and perform MD simulations up to 30000 steps (14.4 ps).ResultsFor MD simulations at 700 and 1000 K, H2 molecule desorbs within 14.4 ps. The H2 molecule is formed by two H atoms, one of which is from the LiH cluster and the other is from the NH3, and each H atom of H2 is not equivalent both in geometry and in charge during the reaction; first an Hδ- atom in Li2H2 and an Hδ+ atom in NH3 atom approach each other by Coulomb interation and then make a dimer, and atomic charges of both H atoms get close to neutral at the moment of dimerisation. However, at this stage, both H atoms are still charged and they become neutral after dozens to hundreds time steps.This H-atoms-from-each-reactant mechanism is consistent with the result from electronic structure calculation performed by Kar et al.[3] We also show that, by considering the reverse reaction, this H2 desorption model qualitatively reproduces the H2 desorption profiles obtained from the experiments by Isobe et al.[2], i.e., time dependence of the ratio of the desorbed H2, HD, and D2 from LiD and LiNH2.AcknowledgmentThis work is supported by the Grants of the NEDO project ’Advanced Fundamental Research on Hydrogen Storage Materials’ in Japan.References[1] Y. Kojima et al., J. Mater. Res. 24 (2009) 2185[2] S. Isobe et al., J. Phys. Chem. B 109 (2005) 14855[3] T. Kar et al., J. Mol. Str. (THEOCHEM) 857 (2008) 111
9:00 PM - Y6.7
Binding Energy Estimation of Hydrogen Storage Materialsby All-electron Mixed-Basis Program TOMBO.
Ryoji Sahara 1 , Hiroshi Mizuseki 1 , Kaoru Ohno 3 , Sluiter Marcel 2 , Yoshiyuki Kawazoe 1
1 , Institute for Materials Research, Tohoku University, Sendai Japan, 3 , Yokohama National University, Yokohama Japan, 2 , Delft University of Technology, Delft Netherlands
Show AbstractIn 2004, for automobile onboard storage system, the U.S. Department of Energy (DOE) set the target that hydrogen storage capacity should be higher than 6 wt% (45 kg/m3). Metal-Organic Frameworks (MOFs) are one of the promising candidates for hydrogen storage materials [1]. In the present study, we propose a simple model of MOFs that can expand hydrogen storage capacity by lithium cation doping and clarify the mechanisms of enhancing hydrogen adsorption energy. We use TOhoku Mixed-Basis Orbitals ab initio simulation package TOMBO [2, 3] developed by our research group, which enables us to study based on ”all-electron mixed-basis approach” with smaller number of plane waves. We found that the adsorption of Li atoms on benzene unit of MOF-5 improves hydrogen storage function properties of these systems by 1.74 wt% and makes binding energy much higher than the systems without lithium doping. This work has been supported by New Energy and Industrial Technology Development Organization (NEDO) under "Advanced Fundamental Research Project on Hydrogen Storage Materials". [1] O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi and J. Kim, Science. 300(2003) 1127-1129.[2] K. Ohno, K. Esfarjani and Y. Kawazoe, Computational Materials Science From ab initio to Monte Carlo Methods, Springer Series in Solid-State Sciences 129 (Springer-Verlag, Berlin, Heidelberg, 1999), pp. 42-46. [3] M. S. Bahramy, M.H.F. Sluiter, and Y. Kawazoe, Phys. Rev. B73 (2007), 045111.
9:00 PM - Y6.9
Reactivity of Pd Ensembles Towards CO Oxidation: A First-principles Study.
Bin Shan 1 , Neeti Kapur 1 , Jangsuk Hyun 1 , Kyeongjae Cho 2
1 Computational Nanoscience Division, Nanostellar Inc, Redwood City, California, United States, 2 Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas, United States
Show AbstractPdAu bimetallic nanoparticles have recently received great interest in the catalysis community due to its unique capability of catalyzing a number of important chemical reactions. Despite the general consensus that Pd ensembles play an important role in the reactivity and selectivity of reactions, the reactivity of specific microscopic Pd ensembles has not been thouroughly investigated. In this paper, we report first-principles study of reactivities of small Pd ensembles on Au (111) surface towards CO oxidation reaction. Despite the almost barrierless dissociation of oxygen molecule on Pd surface, the oxygen dissociation channel on small Pd ensemble is highly activated and act as the rate determining step in CO oxidation. The critical ensemble size for Pd to be reactive towards CO oxidation are Pd trimers. Pd monomers and dimers are not reactive due to the inability of efficient oxygen dissociation. For the elementary step of CO oxidation, activation barriers for both the Eiley-Rideal (ER) mechanism and Langmuir-Hinshelwood (LH) mechanism are substantially lower than that on the pure Pd surface. LH mechanism is still the preferred reaction channel for CO oxidation even on small Pd ensembles. These findings clarifies the reactivity of different ensembles and enables engineering of catalyst active sites by precise geometrical ensemble control.