Session D6: Simulating Hydrogen Storage: From Current Challenges to Future Promises

Computational Study of Metal Hydride Destabilization

The safe and efficient on-board storage of hydrogen in fuel cell vehicles is one of the major road-blocks for utilization of hydrogen in transportation. This talk will illustrate the use quantum molecular modeling techniques for investigating atomic- level details of hydrogen storage in new materials. Metal hydrides of period 2 and 3 materials have high volumetric and gravimetric hydrogen storage capacities. However, these materials typically have very high heats of reaction, meaning that high temperatures are required to dissociate the hydrides. Likewise, hydrogenation reactions evolve very large quantities of energy, making thermal management during refueling a impractical. Recent experimental work has focused on chemical destabilization of metal hydrides as a means of decreasing the heats of reaction. We have carried out quantum mechanical calculations, using the electronic density functional theory (DFT) formalism, for various metal hydride systems. The heats of reaction for over 300 different reactions have been computed. We have compared our calculations with experimental and tabulated data where available and find reasonable agreement. Our calculations demonstrate the utility of DFT for screening reactions and for identifying promising materials for further computational and experimental studies. We have also studied the hydration of Mg$_2$Si, a destabilized hydride of MgH2. Experiments have failed to hydrogenate this material in the laboratory under high pressures of H$_2$. We examine adsorption of H2 and dissociation on the Mg$_2$Si(110) surface to see if kinetic limitations are responsible for the failure to observe hydrogenation of this material.   

The~potential~of~hydrogen~storage~in~hydrate~and~graphitic~systems

Many methods have been proposed for efficient storage of molecular$^{ }$hydrogen for fuel cell applications. Recently, it was found that molecular hydrogen can be stored in large quantity approaching the U.S. Department of Energy goals of$^{ }$6.5{\%} mass ratio in ice clathrate under high pressure and low temperature [1]. Attempts were made to increase the stability of the clathrate. Unfortunately, so far the modified hydrates failed to meet the elusive goal [2,3]. To understand the thermodynamic stability and storage capacity, hydrogen occupancy in clathrate hydrate was examined with a statistical$^{ }$mechanical model in conjunction with first-principles quantum$^{ }$chemistry calculations [4]. The theoretical approach is extended to graphitic systems [5]. It is shown that insufficiently accurate $^{ }$carbon--H$_{2}$ interaction potentials, together with the neglect$^{ }$and incomplete treatment of the quantum effects in previous$^{ }$theoretical investigations led to incorrect conclusions for$^{ }$the absorption capacity. A proper account of the contribution$^{ }$of quantum effects to the free energy and the equilibrium constant$^{ }$for hydrogen adsorption suggest that the U.S. Department of$^{ }$Energy specification can be approached in a graphite-based physisorption$^{ }$system. [1] W.L. Mao, H.-K. Mao, A.F. Goncharov, V.V. Struzhkin,$^{ }$Q Guo, Q., \textit{et al}. \textit{Science} 297, 2247--2249 (2002) [2] H. Lee, J. Lee, D.Y. Kim, J. Park, Y. Seo, H. Zeng, I.L. Moudrakovski, C.I. Ratcliffe, J.A. Ripmeester, \textit{Nature} 434, 743-746, (2005) [3] L.J. Florusse, C.J. Peters, J. Schoonman, K.C. Hester, C.A. Koh, S.F. Dec, K.N. Marsh, E. D. Sloan, \textit{Science}, 306, 469 -- 471 (2004) [4] S. Patchkovskii, J.S. Tse, \textit{Proc. Nat. Acad. Sci}., 100, 14645-14650 (2003) [5] S. Patchkovskii, J.S. Tse, S.N. Yurchenko, L. Zhechkov, T. Heine, G. Seifert, \textit{Proc. Nat. Acad. Sci}., 102, 10439-10444 (2005)   

Theory of Hydrogen Storage: A New Strategy within Organometallic Chemistry

As one of the most vigorous fields in modern chemistry, organometallic chemistry has made vast contributions to a broad variety of technological fields including catalysis, light emitters, molecular devices, liquid crystals, and even superconductivity. Here we show that organometallic chemistry in nanoscale could be the frontier in hydrogen storage. Our study is based on the notion that the 3d transition metal (TM) atoms are superb absorbers for H storage, as their empty d orbital can bind dihydrogen ligands (elongated but non-dissociated H$_{2})$ with high capacity at nearly ideal binding energy for reversible hydrogen storage. By embedding the TM atoms into a carbon-based nanostructures, high H capacity can be maintained. This presentation contains four parts. First, by comparing the conventional hydrogen storage media, e.g., metal hydrides and carbon-based materials, the general principles for designing hydrogen storage materials are outlined. Second, organometallic buckyballs are studied to demonstrate the novel strategy. The amount of H$_{2}$ adsorbed on a Sc-coated fullerene, C$_{48}$B$_{12}$ [ScH]$_{12}$, could approach 9 wt{\%}, with binding energies of 30-40 kJ/mol. Third, the method is applied to the transition-metal carbide nanoparticles that have been synthesized experimentally. The similar non-dissociative H$_{2}$ binding is revealed in our calculation, thereby demonstrating the resilience of the overall mechanism. Moreover, a novel self-catalysis process is identified. In the fourth part, transition-metal functionalization of highly porous carbon-based materials is discussed heuristically to foresee macroscopic media for hydrogen storage. Finally follows the summary and discussion of the remaining challenges to practical hydrogen storage. Work in collaboration with A. C. Dillon, Y.-H. Kim, M. Heben {\&} S. B. Zhang and supported by the U.S. DOE/EERE under contract No. DE-AC36-99GO10337.   

Materials for Hydrogen Storage: From Nanostructures to Complex Hydrides

The limited supply of fossil fuels, its adverse effect on the environment, and growing worldwide demand for energy has necessitated the search for new and clean sources of energy. The possibility of using hydrogen to meet this growing energy need has rekindled interest in the study of safe, efficient, and economical storage of hydrogen. This talk will discuss the issues and challenges in storing hydrogen in light complex hydrides and discuss the role of nanostructuring and catalysts that can improve the thermodynamics and kinetics of hydrogen. In particular, we will discuss how studies of clusters can help elucidate the fundamental mechanisms for hydrogen storage and how these can be applied in Boron Nitride and Carbon nanocages and how metallization of these nanostructures is necessary to store hydrogen with large gravimetric density. We will also discuss the properties of complex light metal hydrides such as alanates and magnesium hydrides that can store up to 18 wt {\%} hydrogen, although the temperature where hydrogen desorbs is rather high. Using first principles calculations, we will provide a fundamental understanding of the electronic structure and stability of these systems and how it is affected due to catalysts. It is hoped that the understanding gained here can be useful in designing better catalysts as well as hosts for hydrogen storage.   

First Principles predictions of Hydrogen Storage Materials

A grand challenge in materials technology is the development of materials capable of reversible storage of H$_{2}$ at ambient temperatures and pressures capable of mass densities greater than 6{\%} by weight. We report here the results of first principles calculations on several classes of materials including: \begin{itemize} \item Carbon-alkali based systems \item Metal oxide framework systems \item Metal alloy systems. \end{itemize} These simulations indicate that the DOE goals for 2010 are achievable in materials that could be manufactured today.