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44103-G10
First Principles Study of Light Metal Complex Hydrides as Potential Hydrogen Storage Materials
Our overall goal is to develop a multiscale approach to model desorption and adsorption of hydrogen in complex metal hydrides. The pioneer work of Bogdanovic & Schwickardi stimulated great interests in light-metal complex hydrides as potential hydrogen-storage materials for transportation applications. Over the past ten years or so, tremendous efforts have been made toward improving hydrogen storage capacity and adsorption/desorption kinetics. While significant progress has been achieved in engineering the catalysts and preparing the hydrides, there remains a significant gap between the demand of practical applications and what this type of material can offer at present. Future advances in hydrogen storage will require a detailed understanding of the intrinsic hydrogen-metal bond strength and the effect of the local reaction environment. In this project, we use density-functional-theory quantum chemical calculations to study the structure of the complex hydrides and the interactions of hydrogen with metal atoms in these hydrides. The specific objectives of the our work are to: 1) understand the nature of hydrogen interactions in the alanate-based materials and determine how hydrogen diffuses and interacts with defects, 2) elucidate how the ionic and covalent bonding between hydrogen and the host atoms in complex hydrides can be changed by alloying and uncover the role of dopants in altering the nature of hydrogen bonding in these hydrides, 3) establish the relationship of size and shape of nanosized particles with the strength and nature of hydrogen interactions. In order to achieve these goals we plan to combine DFT calculations with molecular dynamics or kinetic Monte Carlo simulations to:
- Predict and study the positions of hydrogen atoms in the crystalline structure of alanates, LiAlH4, NaAlH4, Mg(AlH4)2, and LiBH4.
- Examine the effects of different surface structures of these hydrides on hydrogen release and uptake.
- Probe the effects of dopants such as TiCl3 on the structure and stability of the selected hydrides.
- Construct phase diagrams for these hydrides and establish the influence of environment on the stability of various surface structures.
- Calculate hydrogen binding energies and adsorption/desorption barriers at the surfaces of a crystal and finite-sized particle of the pure and doped hydrides.
- Elucidate both geometrical and electronic effects of dopants on the interaction of hydrogen in these hydrides and on hydrogen desorption/adsorption mechanisms.
Over the past year, one aspect we foucused on was DFT studies of pure crystalline phases of the metal complex hydrides. We calculated the crystal structures of different phases of LiAlH4, NaAlH4, Mg(AlH4)2, and LiBH4. We determined the stability of different crystal surfaces of these hydrides. We computed desorption/adsorption of hydrogen from/onto the surfaces of some of the hydrides and continue with the remaining hydrides. We developed a surface doping model during our study of Ti doped NaAlH4 and found that the surface interstitial sites are favorable sites for doped Ti. To understand the catalytic effect of doped Ti in NaAlH4 and to search for more efficient catalysts, we examined the structure and energetics by doping each 3d transition metal element in turn in NaAlH4(001). We found that the interstitial structure formed when doping Ti existed for all the 3d elements and was the most stable for Sc-Co. The structure and the stability trends of the TMAl3H12 interstitial structure, with TM representing the transition metals, correlate with the 18-electron rule. The complexes formed with Sc ~ V, Cr, and Mn ~ Ni can be classified as electron-deficient, electron-neutral, and electron-rich complexes, respectively. The intrinsic electronic structure of the complex is reflected in the corresponding hydrogen desorption energy from different positions of the complex structure with different transition metal elements.
Our studies of hydrogen desorption from Ti-doped NaAlH4 indicated that AlH3 as well as its polymerized derivatives may be important intermediates. Furthermore, this type of binary hydrides has been shown to exist in solid matrices and characterized with various spectroscopic techniques. We studied the M2H6 (with M=B, Al, and Ga) molecules as well as their dehydrogenation derivatives, M2Hx, with x varying from 0 to 5, by using B3LYP hybrid density functionals and a 6-311++G(2d,3p) basis set. Based on the optimized minimum energy structure at each x value, we determined reaction energies of sequential dehydrogenation reaction, M2H6 → M2Hx + (6-x)/2 H2, for all M2H6 molecules. Transition state analysis for M2H6 → M2H4 + H2 showed that the elementary step for this unimolecular pathway proceeded through a transition state with a similar structure for all three hex hydrides and resulted in the formation of an intermediate state that is less stable than the geometrical ground state of M2H4.
