Engineering topics

Autrey, Thomas

Publications and source records attributed to Autrey, Thomas.

31 records · Page 2

DOE OSTI2011

Control of hydrogen release and uptake in amine borane molecular complexes: Thermodynamics of ammonia borane, ammonium borohydride, and the diammoniate of diborane

Molecular complexes of Lewis acid-base pairs can be used to activate molecular hydrogen for applications ranging from hydrogen storage for fuel cells to catalytic hydrogenation reactions. In this paper, we examine the factors that determine the thermodynamics of hydrogen activation of a Lewis acid-base pair using the pedagogical examples of ammonia borane (NH3BH3, AB) and ammonium borohydride ([NH4][BH4], ABH2). At ambient temperatures, ABH2 loses hydrogen to form the Lewis acid-base complex AB, suggesting that free energy drives the reaction to release hydrogen. However, direct measurement of the reaction enthalpy is not straightforward given the complex decomposition pathways leading to the formation of the diammoniate of diborane ([NH3BH3NH3][BH4], DADB). In this work, we compare two approaches for deriving the thermodynamic relationships among AB, DADB, and ABH2.

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DOE OSTI2011

Characterization and Mechanistic Studies of the Dehydrogenation of NHxBHx Materials

In this review we cover the recent developments providing insight into the chemical and physical properties for a series of hydrogen-rich NBHx materials that are of interest as energy storage media for fuel cell power applications. These materials, ammonium borohydride (ABH2, [NH4][BH4]; 240 g H2/kg; 165 g/liter), ammonia borane (AB, [NH3BH3]; 196 g H2/kg; 147 g/liter) and diammoniate of diborane (DADB, [NH3BH2NH3][BH4]; 196 g H2/kg; ca. 151 g/liter), release hydrogen by a series of moderately exothermic reaction pathways. The advantage of these materials is that hydrogen release is kinetically controlled and occurs at relatively low temperatures and moderate pressures. The challenges are devising economical pathways to regenerate the fully charged hydrogen storage materials off board and understanding and controlling the formation of volatile impurities that decrease the purity of the hydrogen available for polymer electrolyte membrane fuel cell applications. The focus of this review is on the solid phase ABH2, AB and DADB materials to complement the coverage of AB in other recent review articles [1-2]. Additional discussion is given on the decomposition products of these materials, polyaminoborane (PAB, [NH2BH2]n) and polyiminoborane (PIB, [NHBH]n). The article is organized into 3 sections, (i) Synthesis and stuctural characterization; (ii) Kinetics and thermodynamics of hydrogen release and (iii) Outstanding challenges for breakthroughs. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

08 HYDROGEN↗

DOE OSTI2011

Reversible Dehydrogenation of Magnesium Borohydride to Magnesium Triborane in the Solid State Under Moderate Conditions

Thermal decomposition of magnesium borohydride, Mg(BH4)2, in the solid state was studied by a combination of PCT, TGA/MS and NMR spectroscopy. Dehydrogenation of Mg(BH4)2 at 200 °C, results in the highly selective formation of magnesium triborane, Mg(B3H8)2. This process is reversible at 250 °C under 120 atm H2. Dehydrogenation at higher temperature, > 300 °C, produces a complex mixture of polyborane species. Solution phase 11B NMR spectra of the hydrolyzed decomposition products reveals the formation of the B3H8 anion, boric acid from hydrolysis of the unstable polyboranes (BnHx) (n = 3-11, x >8), and the closoborane B12H12 dianion as a minor product. A BH condensation mechanism involving metal hydride formation is proposed to explain the limited reversible hydrogen storage in magnesium borohydride.

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DOE OSTI2010

Growth of Crystalline Polyaminoborane through Catalytic Dehydrogenation of Ammonia Borane on FeB Nanoalloy

Tremendous effort has been devoted to the study of complex and chemical hydrides for hydrogen storage in the past decade [1, 2] . Ammonia Borane (NH 3 BH 3 , AB) with a hydrogen content of 19.6 wt % has received significant attention [3-5] . Methods to improve the kinetics of the step-wise dehydrogenation of AB are diverse including the uses of mesoporous frameworks [6] , catalysts [7-16] , and additives [17] . It was reported that when dissolving in organic solvents AB released hydrogen readily in the presence of transition metal catalysts through the formation of M∙∙∙HBH 2 NH 3 complex (where M is Ir, Ru, or Ni etc.) [8-10] ; Lewis or Brønsted acids, on the other hand, react with AB in solution to form the initiating species (BH 2 NH 3 ) + [11] , which may have the similar function as the [BH 2 (NH 3 ) 2 ] + BH 4 - (DADB) in the dehydrogenation of solid AB [17, 18] . However, comparatively little study has been reported on the catalytic dehydrogenation of AB in solid form. Other important but less investigated aspects in the solid-state reaction are the characterizations of functional catalytic species and products from the step-wise dehydrogenation.

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DOE OSTI2010

The diammoniate of diborane: Crystal structure and hydrogen release

[(NH3)2BH2]+[BH4]- is formed from the room temperature decomposition of NH4+BH4-, via a NH3BH3 intermediate. Its crystal structure has been determined and contains disordered BH4- ions in 2 distinct sites. Hydrogen release is similar to that from NH3BH3 but with faster kinetics.

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DOE OSTI2010

An Investigation of the Structural Phase Transition of Ammonia Borane

A detailed anelastic spectroscopy study of the structural phase transition of ammonia borane was conducted for the first time. The transformation from the tetragonal high temperature phase into the orthorhombic low temperature one is detected on cooling around 220K by a huge drop of the elastic modulus and a spike of the elastic energy dissipation.We find clear indications of a hysteresis, which led us to conclude that the transition is of first-order. The kinetics of the transitionwas investigated in detail. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

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DOE OSTI2010

Theoretical Investigations on the Formation and Dehydrogenation Reaction Pathways of H(NH2BH2)nH (n=1-4) Oligomers: Importance of Dihydrogen Interactions (DHI)

The H(NH2BH2)nH oligomers are possible products from dehydrogenation of ammonia borane (NH3BH3) and ammonium borohydride (NH4BH4), which belong to a class of boron-nitrogen-hydrogen (BNHx) compounds that are promising materials for chemical hydrogen storage. Understanding the kinetics and reaction pathways of formation of these oligomers and their further dehydrogenation is essential for developing BNHx-based hydrogen storage materials. We have performed computational modeling using density functional theory (DFT), ab initio wavefunction theory, and Car-Parrinello molecular dynamics (CPMD) simulations on the energetics and formation pathways for the H(NH2BH2)nH (n=1-4) oligomers, polyaminoborane (PAB), from NH3BH3 monomers and the subsequent dehydrogenation steps to form polyiminoborane (PIB). Through transition state searches and evaluation of the intrinsic reaction coordinates, we have investigated the B-N bond cleavage, the reactions of NH3BH3 molecule with intermediates, dihydrogen release through intra- and intermolecular hydrogen transfer, dehydrocoupling/cyclization of the oligomers, and the dimerization of NH3BH3 molecules. We discovered the formation mechanism of H(NH2BH2)n+1H oligomers through reactions of the H(NH2BH2)nH oligomers first with BH3 followed by reactions with NH3 and the release of H2, where the BH3 and NH3 intermediates are formed through dissociation of NH3BH3. We also found that the dimerization of the NH3BH3 molecules to form c-(NH2BH2)2 is slightly exothermic, with an unexpected transition state that leads to the simultaneous release of two H2 molecules. The dehydrogenations of the oligomers are also exothermic, typically by less than 10 kcal/(mol of H2), with the largest exothermicity for n=3. The transition state search shows that the one-step direct dehydrocoupling cyclization of the oligomers is not a favored pathway because of high activation barriers. The dihydrogen bonding, in which protic (HN) hydrogens interact with hydridic (HB) hydrogens, plays a vital role in stabilizing different structures of the reactants, transition states, and products. The dihydrogen interaction (DHI) within the -BH2(η2-H2) moiety accounts for both the formation mechanisms of the oligomers and for the dehydrogenation of ammonia borane. Support was provided from the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division and from the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Chemical Hydrogen Storage Center of Excellence. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

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DOE OSTI2010

Decomposition Pathway of Ammonia Borane on the Surface of nano-BN

Ammonia borane (AB) is under significant investigation as a possible hydrogen storage material. While many chemical additives have been demonstrated to have a significant positive effect on hydrogen release from ammonia borane, many provide additional complications in the regeneration cycle. Mechanically alloyed hexagonal BN (nano-BN) has been shown to facilitate the release of hydrogen from AB at lower temperature, with minimal induction time, less exothermically, and inert nano-BN may be easily removed during any regeneration of the spent AB. The samples were prepared by mechanically alloying AB with nano-BN. Raman spectroscopy indicates that the AB:nano-BN samples are physical mixtures of AB and h-BN. The release of hydrogen from AB:nano-BN mixtures as well as the decomposition products were characterized by 11 B magic angle spinning (MAS) solid state NMR, TGA/DSC/MS with 15 N labeled AB, and solution 11 B NMR spectroscopy. The 11 B MAS solid state NMR spectrum shows that diammonate of diborane (DADB) is present in the mechanically alloyed mixture, which drastically shortens the induction period for hydrogen release from AB. Analysis of the TGA/DSC/MS spectra using 15 N labeled AB shows that all the borazine (BZ) produced in the reaction comes from AB and that increasing nano-BN surface area results in increased amounts of BZ. However, under high temperature, 150°C, isothermal conditions, the amount of BZ released was the same as for neat AB. High resolution transmission electron microscopy (HRTEM), selected area diffraction (SAD), and electron energy loss spectroscopy (EELS) of the initial and final nano-BN additive provide evidence for crystallinity loss but not significant chemical changes. The higher concentration of BZ observed for low temperature dehydrogenation of AB:nano-BN mixtures versus neat AB is attributed to a surface interaction that favors the formation of precursors which ultimately result in BZ. This pathway can be avoided through isothermal heating at temperatures >150°C.

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DOE OSTI2010

Synthesis, Structure and Dehydrogenation of Magnesium Amidoborane Monoammoniate

Magnesium amidoborane monoammoniate (Mg(NH2BH3)2 NH3) which crystallizes in a monoclinic structure (space group P21/a) has been synthesized by reacting MgNH with NH3BH3. Dihydrogen bonds are established between coordinated NH3 and BH3 of [NH2BH3] in the structure, promoting stoichiometric conversion of NH3 to H2. T. A. and A. K. acknowledge support from US DoE EERE CHS CoE. The high-field NMR experiments described here were carried out in the Environmental Molecular Science Laboratory, a national scientific user facility sponsored by the US Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2010

Characterization of a New Phase of Ammonia Borane

An uncharacterized phase of ammonia borane (AB) was previously identified while investigating its thermal decomposition. This phase was identified as a key intermediate for the release of hydrogen from AB prompting the current characterization study. The new phase was investigated extensively using in situ solid state MAS NMR spectroscopy, including 11B, 15N and 1H NMR. Single-pulse excitation, cross polarization and T1 relaxation experiments collectively demonstrate a significant increase in mobility for the new phase when compared to the room temperature crystalline phase of AB. However, the data are not completely consistent with a liquid phase. The data are also not consistent with a solid phase, but are most consistent with a phase best described as “liquid-like”. This implies a disruption of the long-range order which is primarily stabilized by the extensive dihydrogen bonding network, allowing increased motional freedom. X-ray diffraction and Raman thermal decomposition data of neat AB provide support for a disordered phase, as does data of AB immobilized into a mesoporous scaffold. These observations suggest that a breakdown of the extensive hydrogen bonding network is necessary before hydrogen can be released and will be an important factor in the practical use of AB as a hydrogen storage material. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

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DOE OSTI2010

Storing Hydrogen

Researchers have been studying mesoporous materials for almost two decades with a view to using them as hosts for small molecules and scaffolds for molding organic compounds into new hybrid materials and nanoparticles. Their use as potential storage systems for large quantities of hydrogen has also been mooted. Such systems that might hold large quantities of hydrogen safely and in a very compact volume would have enormous potential for powering fuel cell vehicles, for instance. A sponge-like form of silicon dioxide, the stuff of sand particles and computer chips, can soak up and store other compounds including hydrogen. Studies carried out at the XOR/BESSRC 11-ID-B beamline at the APS have revealed that the nanoscopic properties of the hydrogenrich compound ammonia borane help it store hydrogen more efficiently than usual. The material may have potential for addressing the storage issues associated with a future hydrogen economy. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

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DOE OSTI2010

Hydrogen Release Studies of Alkali Metal Amidoboranes

A series of metal amido boranes LiNH2BH3 (LAB), NaNH2BH3 (SAB), LiNH(Me)BH3 (LMAB), NaNH(Me)BH3 (SMAB), KNH(Me)BH3 (PMAB), and KNH(tBu)BH3 (PBAB) were synthesized, by solution phase methods, and the thermal release of H2 in the solid state was studied. Based on the observed trends in reaction rates of H > Me > tBu and the kinetic isotope effect, the mechanism of hydrogen release from MAB compounds was found to proceed through a bimolecular mechanism involving the intermediacy of a MH (M = Li, Na, or K). The mechanism of hydrogen release from metal amidoboranes, a metal ion assisted hydride transfer, is very different than the mechanism of hydrogen release from the parent compound ammonia borane (AB). The non-volatile products formed from MAB’s are significantly different than the products formed after hydrogen release from AB. The boron containing resulting from the release of one equivalent of hydrogen from the metal amidoboranes were characterized by MAS 11B NMR spectroscopy and found to contain both BH3 and sp2 hybridized BH groups, consistent with a general structural feature MN(R)=BHN(R)MBH3. This work was funded by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy as part of the Chemical Hydrogen Storage CoE at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the U.S. DOE by Battelle. MAS NMR studies were performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at PNNL.

08 HYDROGEN↗

DOE OSTI2010

Thermal Stability of Ammonia Borane: A Case Study for Exothermic Hydrogen Storage Materials

A model to determine the thermal stability of chemical hydrogen storage materials was developed to predict the stability of ammonia borane at 50-60oC, the extreme range of environmental temperatures for hydrogen storage materials in PEM fuel cell applications. Experimental data from differential scanning calorimetry measurements between 70 and 85oC were used to develop isothermal and adiabatic reactivity models that could be extrapolated down to temperatures of interest. Results of the analysis show that solid ammonia borane is metastable in the 50-60oC range, having stability against appreciable reaction on the order of a week at 60oC and months at 50oC. An adiabatic analysis is the most extreme case, and stability of many days under such extreme conditions indicates that solid ammonia borane may be suited to most hydrogen storage applications. This type of analysis is also applicable to other candidate hydrogen storage materials.

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