Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q6: Hydrogen Storage Materials |
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Sponsoring Units: DCOMP Chair: Mei-Yin Chou, Georgia Institute of Technology Room: Ballroom C2 |
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q6.00001: First-Principles Prediction of Crystal Structures, Reaction Pathways, and Intermediate Products in Hydrogen Storage Reactions Invited Speaker: Practical hydrogen storage for mobile applications requires materials that exhibit high hydrogen densities, low decomposition temperatures, and fast kinetics for absorption and desorption. Unfortunately, no reversible materials are currently known that possess all of these attributes. Here we present an overview of our recent efforts aimed at developing a first-principles computational approach to the discovery of novel hydrogen storage materials. We have developed computational tools which enable accurate prediction of decomposition thermodynamics, crystal structures for unknown hydrides, and thermodynamically preferred decomposition pathways. We present examples that illustrate each of these three capabilities. Specifically, we focus on recent work on crystal structure and dehydriding reactions of (i) borohydride materials, such as Ca(BH$_{4})_{2}$ and Mg(BH$_{4})_{2}$, (ii) amidoboranes and their decomposition products, and (iii) mixtures of complex hydrides. \\[4pt] [1] J. Yang, A. Sudik, C. Wolverton, and D. J. Siegel, Chem. Soc. Rev. \textbf{39}, 656 (2010). \\[0pt] [2] D. E. Farrell, D. Shin, and C. Wolverton, Phys. Rev. B \textbf{80}, 224201 (2009). \\[0pt] [3] C. Weidenthaler, A. Pommerin, M. Felderhoff, W. Sun, C. Wolverton, B. Bogdanovic, and F. Schuth, J. Amer. Chem. Soc. \textbf{131}, 16735 (2009). \\[0pt] [4] A. R. Akbarzadeh, C. Wolverton, and V. Ozolins, Phys. Rev. B \textbf{79}, 184102 (2009). \\[0pt] [5] Y. S. Lee, Y. Kim, Y. W. Cho, D. Shapiro, C. Wolverton, and V. Ozolins, Phys. Rev. B \textbf{79}, 104107 (2009). \\[0pt] [6] V. Ozolins, E. H. Majzoub, and C. Wolverton, J. Amer. Chem. Soc. \textbf{131}, 230 (2009). \\[0pt] [7] V. Ozolins, E. H. Majzoub, and C. Wolverton, Phys. Rev. Lett. \textbf{100}, 135501 (2008). \\[0pt] [8] Y. Zhang, E. Majzoub, V. Ozolins, and C. Wolverton, Phys. Rev. B 82, 174107 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q6.00002: Towards predictor based design of thermodynamic and kinetic properties of complex materials for hydrogen storage Invited Speaker: A calculational approach for the design of new complex materials for hydrogen storage with favorable thermodynamic stability and enhanced diffusion kinetics is presented. By combining density functional theory (DFT) calculations on stable crystal structures and local coordination models with database methods, we perform large-scale screening studies to determine a number of potential alloys/mixtures with favorable thermodynamic stabilities and identify simple descriptors for subsequent materials prediction. Predictors for the kinetic properties of the materials are derived from combining materials screening with path techniques and harmonic transition state theory (TST) to indentify materials parameters, e.g. the hydrogen binding energy, which correlate with the macroscopic diffusion rates. These predictors are then used to design new alloy/mixture compositions and ratios to favor structures with optimal diffusion kinetics. We present results from binary and ternary alkali-transition metal borohydrides and Perovskite based hydrogen permeable membranes, as well as results from studies of binary and mixed metal ammines. Results from the modeling of pathways and rates of dynamical processes involved in the ab-/desorption mechanisms will also be presented and compared to quasi elastic neutron scattering data. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q6.00003: Theory of molecular hydrogen sorption for hydrogen storage Invited Speaker: Molecular hydrogen (H$_{2})$ sorption has the advantage of fast kinetics and high reversibility. However, the binding strength is often too weak to be operative at near room temperatures. Research into such hydrogen sorption materials has branched into the study of pure van der Waals (vdW) physisorption and that of weak chemisorption (known to exist in the so-called Kubas complexes). In either case, however, theoretical tools to describe such weak interactions are underdeveloped with error bars that often exceed the strength of the interaction itself. We have used quantum-chemistry (QC) based approaches to benchmark the various available DFT methods for four classes of weak chemisorption systems [Sun et al., Phys. Rev. B \textbf{82}, 073401 (2010)]. These involve complexes containing Li, Ca, Sc, and Ti with increased strength of H$_{2}$ binding from predominantly vdW to mostly Kubas-like. The study reveals that most of the DFT functionals within the generalized gradient approximation underestimate the binding energy, oppose to overestimating it. The functionals that are easy to use yet yielding results reasonably close to those of accurate QC are the PBE and PW91. I will also discuss the effort of implementing vdW interaction into the currently available density functional methods [Sun, J. Chem. Phys. \textbf{129}, 154102 (2008)]. The rationale is that while the true vdW is an electron-electron correlation, a DFT plus classical dispersion approach may be too simple and unnecessary within the DFT. A local pseudopotential approach has been developed to account for the core part of the polarizability of the elements. Applications to a number of benchmark systems yield good agreement with QC calculations. The application of this method and the QC methods to vdW hydrogen binding will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:39PM |
Q6.00004: Point-defect-mediated dehydrogenation of alane Invited Speaker: For the engineering of better hydrogen storage materials a systematic understanding of their hydrogen sorption kinetics is crucial. Theoretical studies on metal hydrides have indicated that in many cases point defects control mass transport and hence hydrogen uptake and release. Manipulating point-defect concentrations thus allows control over hydrogen sorption kinetics, opening up new engineering strategies. However, in some cases the relevance of kinetic limitations due to point defects is still under debate; kinetic inhibition of hydrogen sorption has also been attributed to surface effects, e.g. oxide layers or low recombination rates. We present a systematic analysis of the dehydrogenation kinetics of alane (AlH3), one of the prime candidate materials for hydrogen storage. Using hybrid-density functional calculations we determine the concentrations and mobilities of point defects and their complexes. Kinetic Monte Carlo simulations are used to describe the full dehydrogenation reaction. We show that under dehydrogenation conditions charged hydrogen vacancy defects form in the crystal, which have a strong tendency towards clustering. The vacancy clusters denote local nuclei of Al phase, and the growth of these nuclei eventually drives the AlH3/Al transformation. However, the low concentration of vacancy defects limits the transport of hydrogen across the bulk, and hence acts as the rate-limiting part of the process. The dehydrogenation is therefore essentially inactive at room temperature, explaining why AlH3 is metastable for years, even though it is thermodynamically unstable. Our derived activation energy and dehydrogenation curves are in excellent agreement with the experimental data, providing evidence for the relevance of bulk point-defect kinetics. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 2:15PM |
Q6.00005: Kinetics of hydrogen transport in metal hydrides, crystalline alloys, and amorphous metals Invited Speaker: The diffusion of hydrogen is critical in the kinetics of hydrogen uptake and release in metal hydrides and in membrane-based approaches to hydrogen purification. First principles calculations have become a valuable counterpart to experimental methods to study hydrogen diffusion. Examples will be presented of using first principles calculations to understand hydrogen diffusion in a diverse range of solid materials, including metal hydrides in their bulk state and near interfaces, crystalline alloys for membrane applications, and amorphous metals for membrane applications. [Preview Abstract] |
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