Complex Hydride Compounds with Enhanced Hydrogen Storage Capacity

The United Technologies Research Center (UTRC), in collaboration with major partners Albemarle Corporation (Albemarle) and the Savannah River National Laboratory (SRNL), conducted research to discover new hydride materials for the storage of hydrogen having on-board reversibility and a target gravimetric capacity of ≥ 7.5 weight percent (wt %). When integrated into a system with a reasonable efficiency of 60% (mass of hydride / total mass), this target material would produce a system gravimetric capacity of ≥ 4.5 wt %, consistent with the DOE 2007 target. The approach established for the project combined first principles modeling (FPM - UTRC) with multiple synthesis methods: Solid State Processing (SSP - UTRC), Solution Based Processing (SBP - Albemarle) and Molten State Processing (MSP - SRNL). In the search for novel compounds, each of these methods has advantages and disadvantages; by combining them, the potential for success was increased. During the project, UTRC refined its FPM framework which includes ground state (0 Kelvin) structural determinations, elevated temperature thermodynamic predictions and thermodynamic / phase diagram calculations. This modeling was used both to precede synthesis in a virtual search for new compounds and after initial synthesis to examine reaction details and options for modifications including co-reactant additions. The SSP synthesis method involved high energy ball milling which was simple, efficient for small batches and has proven effective for other storage material compositions. The SBP method produced very homogeneous chemical reactions, some of which cannot be performed via solid state routes, and would be the preferred approach for large scale production. The MSP technique is similar to the SSP method, but involves higher temperature and hydrogen pressure conditions to achieve greater species mobility. During the initial phases of the project, the focus was on higher order alanate complexes in the phase space between alkaline metal hydrides (AmH), Alkaline earth metal hydrides (AeH2), alane (AlH3), transition metal (Tm) hydrides (TmHz, where z=1-3) and molecular hydrogen (H2). The effort started first with variations of known alanates and subsequently extended the search to unknown compounds. In this stage, the FPM techniques were developed and validated on known alanate materials such as NaAlH4 and Na2LiAlH6. The coupled predictive methodologies were used to survey over 200 proposed phases in six quaternary spaces, formed from various combinations of Na, Li Mg and/or Ti with Al and H. A wide range of alanate compounds was examined using SSP having additions of Ti, Cr, Co, Ni and Fe. A number of compositions and reaction paths were identified having H weight fractions up to 5.6 wt %, but none meeting the 7.5 wt%H reversible goal. Similarly, MSP of alanates produced a number of interesting compounds and general conclusions regarding reaction behavior of mixtures during processing, but no alanate based candidates meeting the 7.5 wt% goal. A novel alanate, LiMg(AlH4)3, was synthesized using SBP that demonstrated a 7.0 wt% capacity with a desorption temperature of 150°C. The deuteride form was synthesized and characterized by the Institute for Energy (IFE) in Norway to determine its crystalline structure for related FPM studies. However, the reaction exhibited exothermicity and therefore was not reversible under acceptable hydrogen gas pressures for on-board recharging. After the extensive studies of alanates, the material class of emphasis was shifted to borohydrides. Through SBP, several ligand-stabilized Mg(BH4)2 complexes were synthesized. The Mg(BH4)2*2NH3 complex was found to change behavior with slightly different synthesis conditions and/or aging. One of the two mechanisms was an amine-borane (NH3BH3) like dissociation reaction which released up to 16 wt %H and more conservatively 9 wt%H when not including H2 released from the NH3. From FPM, the stability of the Mg(BH4)2*2NH3 compound was found to increase with the inclusion of NH3 groups in the inner-Mg coordination sphere, which in turn correlated with lowering the dimensionality of the Mg(BH4)2 network. Development of various Ak Tm-B-H compounds using SSP produced up to 12 wt% of H2 desorbed at temperatures of 400°C. However, the most active material can only be partially recharged to 2 wt% H2 at 220-300°C and 195 bar H2 pressure due to stable product formation. While gravimetric & volumetric targets are feasible, reversibility remains a persistent challenge

Reversed surface segregation in palladium–silver alloys due to hydrogen adsorption, Surface Science 602

a b s t r a c t It is well known that silver segregates to the surface of pure and ideal Pd-Ag alloy surfaces. By first-principles band-structure calculations it is shown in this paper how this may be changed when hydrogen is adsorbed on a Pd-Ag(1 1 1) surface. Due to hydrogen binding more strongly to palladium than to silver, there is a clear energy gain from a reversal of the surface segregation. Hydrogen-induced segregation may provide a fundamental explanation for the hydrogen or reducing treatments that are required to activate hydrogen-selective membrane or catalyst performance

Mechanistic Investigations of Heterogeneously Catalyzed Steam Gasification of Coke Precursors

Atomic modeling was conducted to mechanistically investigate the CsOH steam gasification catalyst coating that has been successfully demonstrated to eliminate coke deposits during high temperature fuel pyrolysis. This effective coke mitigation was interpreted from the atomic modeling results to be due to the multiple functionalities of the CsOH coating for blocking the underlying metal surface from catalyzing coke formation, preventing deposition of coke-forming precursors and products, and catalyzing the oxidation of coke precursors in the presence of water. The discovery that the CsOH(010) surface was only predicted to strongly interact with hydrocarbon radicals that bombard surfaces during hydrocarbon pyrolysis led to the creation of novel reaction mechanisms for the effective steam gasification of radical coke precursors. The CsOH(010) surface was predicted to locally rearrange and form vacancies to facilitate the decomposition and oxidation of adsorbed hydrocarbon radicals. Two CsOH(010) heterogeneously catalyzed steam gasification reaction mechanisms were proposed involving hydrocarbon radical and H₂O coreactants for the decomposition and oxidation of a methyl adsorbate. The first “H vacancy mechanism” oxidized a methyl radical through a bimolecular reaction with H₂O. In the second “Cs insertion mechanism,” the adsorbed methyl radical was oxidized directly by reduction of the CsOH surface. The resulting OH vacancy was refilled by H₂O dissociation, in order to restore the surface reaction site. This latter mechanism was more energetically downhill overall and had a modest rate-limiting energy barrier that could be easily overcome during fuel pyrolysis at high temperatures. These mechanisms are consistent with the experimentally observed stability of the CsOH coating, which functions as a true catalyst that is not consumed or dissolved over time. Observations of the CsOH coating behavior over a range of temperatures supported the hypothesis that effective coke mitigation functionality is the result of a dynamic balance between steam gasification of coke precursors arriving at the surface and the removal of already accumulated deposits

Catalyzed Nano-Framework Stablized High Density Reversible Hydrogen Storage Systems

A wide range of high capacity on-board rechargeable material candidates have exhibited non-ideal behavior related to irreversible hydrogen discharge / recharge behavior, and kinetic instability or retardation. This project addresses these issues by incorporating solvated and other forms of complex metal hydrides, with an emphasis on borohydrides, into nano-scale frameworks of low density, high surface area skeleton materials to stabilize, catalyze, and control desorption product formation associated with such complex metal hydrides. A variety of framework chemistries and hydride / framework combinations were investigated to make a relatively broad assessment of the method'Âs potential. In this project, the hydride / framework interactions were tuned to decrease desorption temperatures for highly stable compounds or increase desorption temperatures for unstable high capacity compounds, and to influence desorption product formation for improved reversibility. First principle modeling was used to explore heterogeneous catalysis of hydride reversibility by modeling H{sub 2} dissociation, hydrogen migration, and rehydrogenation. Atomic modeling also demonstrated enhanced NaTi(BH{sub 4}){sub 4} stabilization at nano-framework surfaces modified with multi-functional agents. Amine multi-functional agents were found to have more balanced interactions with nano-framework and hydride clusters than other functional groups investigated. Experimentation demonstrated that incorporation of Ca(BH{sub 4}){sub 2} and Mg(BH{sub 4}){sub 2} in aerogels enhanced hydride desorption kinetics. Carbon aerogels were identified as the most suitable nano-frameworks for hydride kinetic enhancement and high hydride loading. High loading of NaTi(BH{sub 4}){sub 4} ligand complex in SiO{sub 2} aerogel was achieved and hydride stability was improved with the aerogel. Although improvements of desorption kinetics was observed, the incorporation of Ca(BH{sub 4}){sub 2} and Mg(BH{sub 4}){sub 2} in nano-frameworks did not improve their H{sub 2} absorption due to the formation of stable alkaline earth B12H12 intermediates upon rehydrogenation. This project primarily investigated the effect of nano-framework surface chemistry on hydride properties, while the effect of pore size is the focus area of other efforts (e.g., HRL, Sandia National Laboratories (SNL) etc.) within the Metal Hydride Center of Excellence (MHCoE). The projects were complementary in gaining an overall understanding of the influence of nano-frameworks on hydride behavior