Defying Thermodynamics: Stabilization of Alane Within Covalent Triazine Frameworks for Reversible Hydrogen Storage.

The highly unfavorable thermodynamics of direct aluminum hydrogenation can be overcome by stabilizing alane within a nanoporous bipyridine-functionalized covalent triazine framework (AlH3 @CTF-bipyridine). This material and the counterpart AlH3 @CTF-biphenyl rapidly desorb H2 between 95 and 154 °C, with desorption complete at 250 °C. Sieverts measurements, 27 Al MAS NMR and 27 Al{1 H} REDOR experiments, and computational spectroscopy reveal that AlH3 @CTF-bipyridine dehydrogenation is reversible at 60 °C under 700 bar hydrogen, >10 times lower pressure than that required to hydrogenate bulk aluminum. DFT calculations and EPR measurements support an unconventional mechanism whereby strong AlH3 binding to bipyridine results in single-electron transfer to form AlH2 (AlH3 )n clusters. The resulting size-dependent charge redistribution alters the dehydrogenation/rehydrogenation thermochemistry, suggesting a novel strategy to enable reversibility in high-capacity metal hydrides.

Cationically Substituted Bi0.7Fe0.3OCl Nanosheets as Li Ion Battery Anodes.

Cation substitution of Bi3+ with Fe3+ in BiOCl leads to the formation of ionically layered Bi0.7Fe0.3OCl nanosheets. The synthesis follows a hydrolysis route using bismuth(III) nitrate and iron(III) chloride, followed by postannealing at 500 °C. Room temperature electrical conductivity improves from 6.11 × 10-8 S/m for BiOCl to 6.80 × 10-7 S/m for Bi0.7Fe0.3OCl. Correspondingly, the activation energy for electrical conduction reduces from 862 meV for pure BiOCl to 310 meV for Bi0.7Fe0.3OCl. These data suggest improved charge mobility in Bi0.7Fe0.3OCl nanosheets. Density functional theory calculations confirm this behavior by predicting a high density of states near the Fermi level for Bi0.7Fe0.3OCl. The improvement in electrical conductivity is exploited in the electrochemical performance of Bi0.7Fe0.3OCl nanosheets. The insertion capacity of Li+ ions shows an increase of 2.5×, from 215 mAh·.g-1 for undoped BiOCl to 542 mAh·g-1 for Bi0.7Fe0.3OCl after 50 cycles at a current density of 50 mA·g-1. Thus, the direct substitution of Bi3+ sites with Fe3+ in BiOCl results in nanosheets of an ionically layered ternary semiconductor compound which is attractive for Li ion battery anode applications.

First-principles calculated decomposition pathways for LiBH4 nanoclusters.

We analyze thermodynamic stability and decomposition pathways of LiBH4 nanoclusters using grand-canonical free-energy minimization based on total energies and vibrational frequencies obtained from density-functional theory (DFT) calculations. We consider (LiBH4)n nanoclusters with n = 2 to 12 as reactants, while the possible products include (Li)n, (B)n, (LiB)n, (LiH)n, and Li2BnHn; off-stoichiometric LinBnHm (m ≤ 4n) clusters were considered for n = 2, 3, and 6. Cluster ground-state configurations have been predicted using prototype electrostatic ground-state (PEGS) and genetic algorithm (GA) based structural optimizations. Free-energy calculations show hydrogen release pathways markedly differ from those in bulk LiBH4. While experiments have found that the bulk material decomposes into LiH and B, with Li2B12H12 as a kinetically inhibited intermediate phase, (LiBH4)n nanoclusters with n ≤ 12 are predicted to decompose into mixed LinBn clusters via a series of intermediate clusters of LinBnHm (m ≤ 4n). The calculated pressure-composition isotherms and temperature-pressure isobars exhibit sloping plateaus due to finite size effects on reaction thermodynamics. Generally, decomposition temperatures of free-standing clusters are found to increase with decreasing cluster size due to thermodynamic destabilization of reaction products.

Reversible hydrogen storage by NaAlH4 confined within a titanium-functionalized MOF-74(Mg) nanoreactor.

We demonstrate that NaAlH(4) confined within the nanopores of a titanium-functionalized metal-organic framework (MOF) template MOF-74(Mg) can reversibly store hydrogen with minimal loss of capacity. Hydride-infiltrated samples were synthesized by melt infiltration, achieving loadings up to 21 wt %. MOF-74(Mg) possesses one-dimensional, 12 Å channels lined with Mg atoms having open coordination sites, which can serve as sites for Ti catalyst stabilization. MOF-74(Mg) is stable under repeated hydrogen desorption and hydride regeneration cycles, allowing it to serve as a "nanoreactor". Confining NaAlH(4) within these pores alters the decomposition pathway by eliminating the stable intermediate Na(3)AlH(6) phase observed during bulk decomposition and proceeding directly to NaH, Al, and H(2), in agreement with theory. The onset of hydrogen desorption for both Ti-doped and undoped nano-NaAlH(4)@MOF-74(Mg) is ∼50 °C, nearly 100 °C lower than bulk NaAlH(4). However, the presence of titanium is not necessary for this increase in desorption kinetics but enables rehydriding to be almost fully reversible. Isothermal kinetic studies indicate that the activation energy for H(2) desorption is reduced from 79.5 kJ mol(-1) in bulk Ti-doped NaAlH(4) to 57.4 kJ mol(-1) for nanoconfined NaAlH(4). The structural properties of nano-NaAlH(4)@MOF-74(Mg) were probed using (23)Na and (27)Al solid-state MAS NMR, which indicates that the hydride is not decomposed during infiltration and that Al is present as tetrahedral AlH(4)(-) anions prior to desorption and as Al metal after desorption. Because of the highly ordered MOF structure and monodisperse pore dimensions, our results allow key template features to be identified to ensure reversible, low-temperature hydrogen storage.

Density functional theory studies on the electronic, structural, phonon dynamical and thermo-stability properties of bicarbonates MHCO(3), M = Li, Na, K.

The structural, electronic, phonon dispersion and thermodynamic properties of MHCO(3) (M = Li, Na, K) solids were investigated using density functional theory. The calculated bulk properties for both their ambient and the high-pressure phases are in good agreement with available experimental measurements. Solid phase LiHCO(3) has not yet been observed experimentally. We have predicted several possible crystal structures for LiHCO(3) using crystallographic database searching and prototype electrostatic ground state modeling. Our total energy and phonon free energy (F(PH)) calculations predict that LiHCO(3) will be stable under suitable conditions of temperature and partial pressures of CO(2) and H(2)O. Our calculations indicate that the [Formula: see text] groups in LiHCO(3) and NaHCO(3) form an infinite chain structure through O⋯H⋯O hydrogen bonds. In contrast, the [Formula: see text] anions form dimers, [Formula: see text], connected through double hydrogen bonds in all phases of KHCO(3). Based on density functional perturbation theory, the Born effective charge tensor of each atom type was obtained for all phases of the bicarbonates. Their phonon dispersions with the longitudinal optical-transverse optical splitting were also investigated. Based on lattice phonon dynamics study, the infrared spectra and the thermodynamic properties of these bicarbonates were obtained. Over the temperature range 0-900 K, the F(PH) and the entropies (S) of MHCO(3) (M =Li, Na, K) systems vary as F(PH)(LiHCO(3)) > F(PH)(NaHCO(3)) > F(PH)(KHCO(3)) and S(KHCO(3)) > S(NaHCO(3)) > S(LiHCO(3)), respectively, in agreement with the available experimental data. Analysis of the predicted thermodynamics of the CO(2) capture reactions indicates that the carbonate/bicarbonate transition reactions for Na and K could be used for CO(2) capture technology, in agreement with experiments.

Tuning metal hydride thermodynamics via size and composition: Li-H, Mg-H, Al-H, and Mg-Al-H nanoclusters for hydrogen storage.

Nanoscale Li and intermetallic Al-Mg metal hydride clusters are investigated as a possible hydrogen storage material using the high-level quantum Monte Carlo computational method. Lower level methods such as density functional theory are qualitatively, not quantitatively accurate for the calculation of the enthalpy of absorption of H(2). At sizes around 1 nm, it is predicted that Al/Mg alloyed nanoparticles are stable relative to the pure compositions and the metal composition can be tuned in tandem with the size to tune the hydrogen absorption energy, making this a promising route to a rechargeable hydrogen storage material.

Improved synthesis of bis(borano)hypophosphite salts.

A synthesis of the bis(borano)hypophosphite anion with various counterions has been developed to make use of more benign and commercially available reagents. This method avoids the use of potentially dangerous reagents used by previous methods and gives the final products in good yield. Details of the crystal structure determination of the sodium salt in space group Ama2 are given using a novel computational technique combined with Rietveld refinement.

Calcium borohydride for hydrogen storage: catalysis and reversibility.

We demonstrate a new solid-state synthesis route to prepare calcium borohydride, Ca(BH4)2, by reacting a ball-milled mixture of CaB(6) and CaH(2) in a molar ratio of 1:2 at 700 bar of H2 pressure and 400-440 degrees C. Moreover, doping with catalysts was found to be crucial to enhance reaction kinetics. Thermogravimetric analysis and differential scanning calorimetry revealed a reversible low-temperature to high-temperature endothermic phase transition at 140 degrees C and another endothermic phase transition at 350-390 degrees C associated with hydrogen release upon formation of CaB(6) and CaH(2), as was evident from X-ray diffraction analysis. Thus, since Ca(BH(4))(2) here is shown to be prepared from its anticipated decomposition products, the conclusion is that it has potential to be utilized as a reversible hydrogen storage material. The theoretical reversible capacity was 9.6 wt % hydrogen.

Crystal structure, Raman spectroscopy, and ab initio calculations of a new bialkali alanate K2LiAlH6.

A new bialkali alanate K2LiAlH6 was synthesized at 320-330 degrees C and 100-700 bar. It was structurally characterized by powder X-ray diffraction. It crystallizes in space group R3m (No. 166) with unit cell parameters a = 5.62068(8) and c = 27.3986(6) A. The Li and K cation sites are mutually exclusive, and Rietveld refinement finds no cation mixing. First-principles total energy calculations were performed for nine competing database structures of the stoichiometry A2BCX6, taken from fluoride and oxide compounds in the Inorganic Crystal Structure Database (ICSD). The relaxed structures were compared via their total energies and their agreement with experimental diffraction spectra. Two database structures K2LiAlF6 (R3m) and Cs2NaAlF6 (C2/m) were found to have the lowest total energies, but with the Rietveld method the K2LiAlF6 structure type was shown to be the most favorable. Ab initio total energy calculations support the validity of the structure determination. First-principles calculations also indicate that cation mixing is energetically unfavorable. Hydride properties such as plateau pressure are therefore more difficult to manipulate through alloying in this class of compounds.