Session L36: Focus Session: Hydrogen Storage II: Chemical Hydride and Complex Metal Hydride Materials II

Effects of point defects and impurities on kinetics in hydrogen storage materials

First-principles calculations are playing an important role in developing a fundamental understanding of the physics and chemistry of hydrogen storage materials. In order to accurately describe the mechanisms of hydrogen uptake and release that are at the core of the hydrogen storage process, it is necessary to consider the addition or removal of individual hydrogen atoms. We have performed supercell calculations to model addition or removal of hydrogen, corresponding to the formation of hydrogen interstitials or vacancies. While the concepts discussed here are general, they will be illustrated with detailed results for sodium alanate, a viable hydrogen storage material. The calculations are based on density functional theory in the generalized gradient approximation, using the projector-augmented-wave approach. We find that hydrogen-related point defects are the dominant defect species involved in (de)hydrogenation of sodium alanate. These defects are positively or negatively charged, and hence their formation energies are Fermi-level dependent - an important feature that has not been recognized in past studies. This dependence enables us to explain why small amounts of transition-metal additives drastically alter the kinetics of dehydrogenation. The rate-limiting step for hydrogen release is the creation of charged hydrogen-related defects, while transition-metal additives (such as Ti) act as electrically active impurities that lower the formation energy of these defects. Comparisons with experimental measurements that confirm the proposed mechanisms will be discussed. The ideas outlined here suggest improved preparation methods for complex hydrides through enhanced control of the addition of small concentrations of impurities.   

Density Functional Theory Based Kinetic Monte Carlo Approach for Understanding Atomistic Mechanisms for Reversible Hydrogen Storage in Metal Hydrides: Application to Alane Formation on Ti Doped Al Surfaces

We describe a density functional kinetic Monte Carlo approach enabling us to study and simulate the steady-state situation of dissociative adsorption of hydrogen along with diffusion and reaction of Al and H atoms leading towards the formation of alane species on Ti-doped Al surfaces. In the first step, density functional theory is used in conjunction with the nudged elastic band/drag method to obtain the energetics of the relevant atomistic processes of Al and H diffusion and their reactions on Al surfaces with different concentration of dopant Ti atoms. Subsequently, the kinetic Monte Carlo method is employed, which accounts for the spatial distribution, fluctuations, and evolution of chemical species at Ti-doped Al surfaces under steady-state conditions. This DFT-based KMC approach provides an insight into the kinetics of alanes at technologically relevant pressure and temperature conditions. Our computed production rates of AlH$_{3}$ on Al surfaces are in agreement with experimental data. We also obtained temperature programmed desorption spectra of different alane species, which is agreeing well with experiments.   

Alanes formation on the Al(111) surface

Alane clusters (Al$_{x}$H$_{y})$ are believed to be the ubiquitous intermediates in hydrogen storage reactions for a wide variety of alanates (LiAlH$_{4}$, NaAlH$_{4})$ currently considered for hydrogen storage. The formation and behavior of alanes at surfaces appear to control and limit the efficiency of hydrogen storage. In particular, hydrogen adsorption on the Al(111) surface leads to the coexistence of several adsorbed species, the concentration of which is affected by the step density, the surface coverage and the temperature. We combine density functional theory (DFT) and surface infra-red (IR) absorption spectroscopy to uncover the mechanisms for alane formation on Al(111) surfaces. At low coverage, DFT predicts a two-fold bridge site adsorption for atomic hydrogen, consistent with previous Electron Energy Loss Spectroscopy measurements. At higher coverage, the formation of small chemisorbed AlH$_{3}$ occurs at the step edges. With increasing coverage AlH$_{3}$ is extracted from the step edge and becomes highly mobile on the terraces in a weakly bound state. This mobility is the key factor leading to the growth of larger alanes through AlH$_{3}$ oligomerization. For these large alanes, previous Thermal Programmed Desorption studies are discussed and compared to the thermal stability observed in IR.   

Watching the dehydrogenation of alane (AlH3) in a TEM

Alane (AlH3) is a promising candidate for on-board hydrogen storage applications. Its theoretical gravimetric capacity is 10.1 percent and decomposition is achieved with modest heating (60-200 deg C). We studied the dehydrogenation of alane, insitu, in a TEM. Alane powder was loaded into the TEM and heated at 80 deg C. We were able to `watch' the dehydrogenation of the alane to aluminum. Electron diffraction and dark fiend images are used to show how and where the aluminum crystallites grow. Although crystalline aluminum phases were successfully identified, some of the sample remained amorphous. We will discuss the nature of the amorphous material and present images clearly identifying the nature of the aluminum crystallites.   

Hydrogen Multicenter Bonds on Small Metal Clusters

We investigate the saturation of hydrogen on metal clusters employing \textit{ab initio} calculations. Our calculations reveal that energetically the most preferred configuration of the hydrogen saturated metal clusters exhibit hydrogen multicenter bonds. The strength of these hydrogen multicenter bonds can be modulated either by changing the extent of hydrogen saturation or using different metal clusters. In the context of hydrogen storage materials, our calculations indicate that early first-row transition metals have the best propensity to form hydrogen multicenter bonds. The relevance of this work in the context of hydrogenation and dehydrogenation kinetics of complex metal hydrides will be discussed.   

The absorption of hydrogen by Nb thin films capped with Pd studied by transmission of visible light

Samples of Nb films with thickness between 2.5 to 14 nm were deposited on glass and capped by a continuous 6 nm Pd film in a sputtering system. The light transmission and reflection, in the visible range (400 to 1000 nm), were measured when the sample was exposed to different hydrogen pressure up to 75 Torr. Experiment on continuous pure Pd film and Nb film for were done for comparison. The relative change in transmission for Nb at 74 Torr of hydrogen is about 5 percent but the saturation occurs after 2500 s. A sample of 14 nm Nb capped by Pd shows a 7 percent increased in transmission at the same pressure but the saturation time is reduced to 50 s, same as pure Pd. Change in the kinetics of hydrogen absorption by Nb capped with Pd indicates that the rate limiting step in the absorption process by pure Nb is located on the Nb surface.   

Theoretical Analysis on X-ray Absorption Spectra of Ti compounds as Catalysts in Lithium Amide-Imide reactions

Solid-state storage is conceptually efficient approach for on-board vehicular hydrogen storage. In this context, light-element materials such as lithium amide LiNH$_{2}$ and lithium imide Li$_{2}$NH have been attracted much attention due to their high gravimetric densities of hydrogen. Recently, various transition-metal compounds have been examined with ball-milling technique for exploring catalysts to improve the hydrogen storage and desorption kinetics, and it is found that a small amount (1mol\%) of titanium compounds revealed a superior effect in hydrogen desorption reaction LiNH$_{2}$ + LiH ${\to}$ Li$_{2}$NH + H$_{2}$ [1]. However, these catalysis mechanism and role of Ti in the reaction remain unanswered. Isobe $et$ $al$. have carried out measurements of X-ray absorption spectroscopy(XAS) at Ti $K$-edge to see the electronic states of Ti recently [2]. In this paper, we calculate the electronic structure of Ti metal and its compounds, and obtained theoretical spectra to compare with the measured spectra by using first-principles calculations based on the all-electron FLAPW method. We discuss chemical bonding and local geometry of catalytically active states in the reaction. [1] T. Ichikawa, S. Isobe, N. Hanada and H. Fujii, J. of Alloys and Comp. 365, 271 (2004) . [2] S. Isobe, T. Ichikawa, Y. Kojima and H. Fujii, J. of Alloys and Comp. 446-447, 360 (2007).