Complex Anionic Materials

Ewa Ronnebro photo

Project Lead: Ewa Ronnebro,
Sandia National Laboratory

To advance hydrogen storage materials for use as on-board storage, this MHCoE project focuses on discovering new materials and synthesis routes. This is a collaborative effort between eleven partners of National Labs; Sandia National Laboratories (SNL), Oakridge National Laboratories (ORNL), National Institute of Standards and Technology (NIST), Universities; U. Nevada, U. Illinois, U. Utah, U. Hawaii, and companies including; Jet Propulsion Laboratories (JPL), Hughes Research Laboratories (HRL), Intematix, General Electrics (GE). The class of materials with the potential for highest hydrogen storage capacity is complex anionic materials, comprised of a complex anion stabilized in a cationic matrix. We are preparing these materials mainly by solid-state synthesis routes. as well as solution based routes. The theorists are guiding the experimental work by predicting potential stable materials.

There is a need to discover new metal hydrides since there are currently no hydrogen storage materials that meet the DOE's targets. Novel, light-weight, high-capacity metal hydrides are being synthesized as potential candidates for on-board materials. The discovery process involves preparation methods in the solid state; mainly ball milling and Sandia‚Äôs high-pressure sintering technique (P <2000 bar, T <500 °C). By utilizing different ball milling approaches in collaboration within MHCoE, we are able to control the size of the particles, which is crucial for creating diffusion paths for hydrogen. Our focus is on complex ternary hydrides since they are known to have higher hydrogen content than the intermetallic compound hydrides. The task is to find a matrix of cations (such as Li, Na, Mg, Ca) that stabilizes the anionic complex which will consist of a d-element (such as Sc, Ti, V) or a p-element (such as B, Al, Si) that bonds to hydrogen and thus forms a ternary or higher metal hydride. New compounds are investigated in collaboration with MHCoE partners by XRD, neutron diffraction, synchrotron X-ray, Raman, MAS-NMR and FTIR. New compounds are investigated with respect to desorption temperatures, hydrogen capacity and reversibility. Currently the occurrence of complex hydrides is investigated in a number of ternary systems. In FY07, Sandia is also initiating a rapid thermal processing technique to discover new materials.

Modeling techniques are used to predict the occurrence of new metal hydrides and guide the experimental effort. Several of the complex hydrides have been shown by first principles calculations to have dominant ionic bonding character between the cations and anion complexes, indicating that most of the cohesive energy of these compounds is electrostatic. A structure searching algorithm based on Monte Carlo techniques and minimizing the electrostatic energy is employed to search for new possible compounds based on MHx anions and a collection of corresponding cations. The algorithm is used to generate electrostatic ground state structures which will be investigated more extensively with full DFT calculations. If the DFT calculations indicate a promising material, we will attempt to synthesize it.

Moreover, we are also continuing the work that initially focused on the development of new doping and substitution methods for the alanates without a loss in hydrogen sorption kinetics. Our effort at Sandia to discover a bi-alkali alanate with superior properties to sodium alanate resulted in the compound K2LiAlH6, but unfortunately, it was more stable and released less hydrogen. The crystal structure is shown in Figure 1. In FY06, the work was expanded to include high-capacity metal borohydrides, such as Mg(BH4)2 (GE), Ca(BH4)2 (SNL) and transition metal borohydrides of Zn and Mn (U. Hawaii). These borohydrides are synthesized by different synthesis routes, utilizing the variety of capabilities within MHCoE, such as mechanical alloying, high-pressure sintering and solution based metathesis reactions. ORNL is exploring a new approach to utilize liquid and volatile metal borohydrides. Computation modeling assists in characterization of these novel high capacity M(BH4)x materials. We are in a collaborative effort to explore possibilities to make these materials reversible using a various of experimental analysis tools such as XRD, Raman, DSC, TGA, in-situ XRD, TEM, MAS-NMR and neutron spectroscopy. Both the surface and bulk material properties will be thoroughly investigated in order to elucidate methods that improve the kinetics and thermodynamic behavior of the novel materials. Eventually, work will begin on optimizing materials with respect to sorption properties by adding dopants or catalysts, substituting metals, and/or preparing mixed borohydrides.

Below is a list of all Project B partners including contact persons and capabilities.

New materials and synthesis routes

Solid state synthesis
Ewa Ronnebro, SNL: Materials discovery by high-pressure sintering and mechanical alloying

Zak Fang, U. Utah: Materials discovery by reactive milling (H2-atmosphere)

Craig Jensen, U. Hawaii: Mechanical alloying of transition metal borohydrides

Anthony McDaniel, SNL: Rapid Thermal Processing

Solution based synthesis
Timothy Boyle, SNL: Metasthesis of metal borohydrides

Gilbert Brown, ORNL: Synthesis of liquid and volatile metal borohydrides

Ewa Ronnebro, SNL: structural characterization by XRD and NPD, also TGA, DSC, PCT

Eric Majzoub, SNL: Raman spectroscopy, XRD

Terry Udovic, NIST: Neutron vibrational spectroscopy and neutron diffraction

Robert Bowman, JPL: MAS-NMR

Ian Robertson, U. Illinois: TEM-characterization of new materials as function of number of cycles

Danesh Chandra, U. Nevada: In-situ XRD of dehydriding/hydriding reactions and measuring of vapor pressure

Darshan Kundaliy, Intematix: Catalyst screening

J-C. Zhao, GE: structural characterization and catalyst screening of Mg(BH4)2.

Theory Modeling
Eric Majzoub, SNL: Monte Carlo method: enthalpy estimations and assist in crystal structure determinations

Duane Johnson, U. Illinois: Full phonon calculations and kinetics studies

Figure 1. The crystal structure of K2LiAlH6. Potassium is shown in pink, lithium in yellow and aluminum in green. The smallest red symbols represent hydrogen that forms octahedra around aluminum.

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