Destabilized Metal Hydrides

Project Lead: Ian Robertson
Project Lead: Ian Robertson,

University of Illinois at Urbana-Champaign
Telephone: (217) 333-1440
Email: ianr@uiuc.edu

This project is devoted to significantly extending the limits of hydrogen storage technology for practical transportation applications. To meet the capacity goals set forth by the DOE for hydrogen storage systems, solid-state materials consisting of light elements are being developed. Although light-metal hydrides have compelling gravimetric and volumetric capacity advantages, two major hurdles that must be overcome before those materials are acceptable for on-board reversible hydrogen storage applications. First, in contrast to the delocalized metallic bonding that characterizes transition metal hydrides, the chemical bonds in light metal hydrides are predominately covalent, polar covalent, or ionic. These bonds are often strong, resulting in unacceptably high thermodynamic stability and therefore, low equilibrium hydrogen pressures. Second, the high directionality of the covalent/ionic bonds in these systems leads to large activation barriers for atomic motion, resulting in prohibitively slow hydrogen sorption kinetics and, therefore, limited reversibility.

We are addressing the thermodynamics challenge through the use of hydride destabilization strategies in which alloy or compound formation in the dehydrogenated state reduces the overall energy for the reacting system, resulting in an increase in equilibrium pressure. Although a system that meets all of the thermodynamic requirements has not yet been demonstrated, hydride destabilization provides a compelling approach to developing a materials system that meets those goals. In addition to the experimental efforts being conducted in this project to investigate the changes in thermodynamics that accompany destabilization of binary and complex hydrides, theory and modeling efforts by project team members are being used to identify new systems and predict how destabilization will alter the thermodynamic properties of those systems.

The slow hydrogen sorption kinetics in light-metal systems poses an even more challenging problem. The project team is exploring approaches that employ nanoscale materials to reduce diffusion distances, thereby increasing rates for hydrogen exchange. Hydrogen sorption kinetics are being investigated in nanoscale systems formed by both "top-down" (e.g. mechanical attrition by energetic ball-milling) and "bottom-up" (e.g. gas condensation, chemical vapor synthesis) processes, and combinatorial methods are being used to evaluate the effects of nanoscale catalysts on the hydrogen exchange rates in wide range of destabilized hydrides. Basic studies of hydrogen transport and phase formation in nanoscale light metal hydrides are being conducted using thin-film model systems, and methods for incorporating metal hydride reactants into nanoporous "scaffold" structures (e.g. carbon aerogels) are being investigated. For example, Figure 1 shows thermogravimetric analysis (TGA) data that illustrate reduction in the dehydrogenation temperature of LiBH4 when the material is confined within nanoporous carbon scaffold structures with different pore sizes. Studies are currently being conducted to optimize the scaffold and to incorporate hydride destabilization agents and catalysts into the framework. In all aspects of this project, the experimental work is being supported by advanced diagnostic capabilities to probe material structure, phase formation dynamics, and hydrogen sorption behavior in the destabilized hydride systems.

Thermogravimetric analysis

Figure 1. Thermogravimetric analysis of LiBH4 dehydrogenation. Data are shown for LiBH4 mixed with nonporous graphite, and incorporated into 13 and 26 nm pore size carbon aerogels and activated carbon.

A summary of the participants and the R&D activities in the Destabilized Hydride project is provided in Table 1. The project comprises 11 organizations within the MHCoE with expertise in experimental studies, theory and modeling, and materials and process diagnostics. The team is currently focusing on three major areas: 1) new destabilized systems, 2) kinetics, and 3) advanced diagnostics and characterization.

Table 1. Participants and research activities in Project A, Destabilized Hydrides

New Destabilized Systems

HRL Labs

Greg Olson, John Vajo

LiBH4-based destabilized systems

U. Pittsburgh, Carnegie-Mellon U., U. Illinois

Karl Johnson(Pitt) David Sholl (CMU) Duane Johnson (UIUC)

Theory and modeling (mainly DFT): thermodynamics and new systems

NIST

Ursula Kattner

Phase diagram calculations (CALPHAD)

Caltech, JPL

Channing Ahn, Bob Bowman

Ca alanate systems

Kinetics

HRL Labs

Greg Olson, John Vajo

Nanostructured materials, scaffold systems, sorption behavior of nanoscale materials

Stanford U.

Bruce Clemens

Thin film model system, nanoscale structure and kinetics modeling

Intematix

Darshan Kundaliya, Xiongfei Shen and Jonathan Melma

Combinatorial studies, nanoparticle synthesis, catalysis screenin

U. Utah

Zak Fang, H.Y. Sohn

High energy ball-milling; nanoparticles by chemical vapor synthesis

Caltech

Channing Ahn

Nanoparticles by gas condensation

U. Hawaii

Craig Jensen

Novel catalysts

Advanced Characterization

NIST

Terry Udovic

Neutron-based material diagnostics

Caltech/JPL

Channing Ahn, Bob Bowman

TEM, NMR

Stanford U.

Bruce Clemens

Synchrotron x-ray diffraction

U. Illinois

Ian Robertson

in situ TEM

Sandia Natl Labs

Ewa Ronnebro

High pressure P-C-T measurements

Some selected recent presentations and publications on work conducted by team members in the MHCoE Destabilized Hydrides Project are given below.

Sudhakar V. Alapati, J. Karl Johnson, and David S. Sholl, "Identification of Destabilized Metal Hydrides for Hydrogen Storage Using First Principles Calculations", Journal of Physical Chemistry B, 110, 8769-8776 (2006).

R.C. Bowman, Jr., S.-J. Hwang, C. C. Ahn, A. Dailly, J. J. Vajo, T. J. Udovic, M. Hartman, and J. J. Rush, "Studies of Thermodynamics and Phases Produced in the Destabilized LiH-Si System", invited presentation: Nordic Energy Research Meeting, Krusenberg, Sweden, 17-18 June 2005.

R. C. Bowman, Jr., S-J. Hwang, C. C. Ahn, A. Dailly, M. R. Hartman, T. J. Udovic, J. J. Rush, and J. J. Vajo, "Reversibility and Phase Compositions of Destabilized Hydrides Formed from LiH", Spring 2006 Materials Research Society Meeting, San Francisco, CA, April, 2006.

B.M. Clemens, "Nanostructures for Hydrogen Storage," invited presentation: Fall 2005 Materials Research Society, Boston, MA, December 2005.

A.F. Gross, J.J. Vajo, S.L. Skeith, and G.L Olson, "Enhanced Hydrogen Storage Properties of Metal Hydrides using Nanoporous Carbon Scaffolds" atAmerican Chemical Society Meeting, Atlanta, GA (March 27-31, 2006).

M. R. Hartman, T. J. Udovic, J. J. Rush, R. C. Bowman, Jr., J. J. Vajo, and C. C. Ahn "Neutron Scattering Investigations of a Destabilized LiH:Si System for Hydrogen Storage Applications," Fall 2005 Materials Research Society Meeting, Boston, MA, December, 2005.

J. Karl Johnson, S. V. Alapati, B. Dai, D.S. Sholl "Computational Study of Metal Hydride Destabilization" invited presentation: American Physical Society March Meeting, Baltimore, MD, March, 2006.

U. R. Kattner, "A Thermodynamic Database for Metal-Hydrogen Systems," Plenary presentation: TMS 135th Annual Meeting, San Antonio, TX, March 2006.

J.J. Vajo, "Destabilization of Strongly Bound Hydrides for Hydrogen Storage Applications" Invited presentation at Gordon Research Conference Hydrogen-Metal Systems, Waterville, ME, July, 2005.

J.J. Vajo, T.T. Salguero, A.F. Gross, S.L. Skeith, and G. L. Olson, "Kinetics and Thermodynamics of Destabilized Hydride Systems", Invited presentation: Spring 2006 Materials Research Society Spring Meeting, San Francisco, CA, April, 2006.

John J. Vajo and Gregory L. Olson, "Hydrogen Storage in Destabilized Chemical Systems", Invited "Viewpoint Set" article, to be published in Scripta Materialia.

N. Zarkevich, A. S. Alapati, D.D. Johnson, "First-principles prediction, and proper comparison to experiment, of enthalpies and van't Hoff plots of complex metal-hydride destabilized hydrogen-storage reactions," submitted to J. Phys. Chem. B (Letters), May 2006.


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