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The hydrogen desorption and absorption reactions of the complex metal hydride NaAlH4 are disproportionation processes, and the kinetics can be improved by adding a few mol% of Ti compounds, although the catalytic mechanism, including the location and state of Ti, remains unknown. In this study, we aimed to reveal the generating mechanism of catalytic Al–Ti alloy in NaAlH4 with TiCl3 using quantum multiprobe techniques such as neutron diffraction (ND), synchrotron X-ray diffraction (XRD), anomalous X-ray scattering (AXS), and X-ray absorption fine structure (XAFS). Rietveld refinements of the ND and XRD, profiles before the first desorption of NaAlD(H)4–0.02TiCl3 showed that Al in NaAlD(H)4 was partially substituted by Ti. On the other hand, Ti was not present in NaAlH4, and Al–Ti nanoparticles were detected in the XRD profile after the first re-absorption. This was consistent with the AXS and XAFS results. It is suggested that the substitution promotes the formation of a highly dispersed nanosized Al–Ti alloy during the first desorption process and that the effectiveness of TiCl3 as an additive can be attributed to the dispersion of Ti.

Complex light metal hydrides are promising candidates for efficient, compact solid-state hydrogen storage. (De)hydrogenation of these materials often proceeds via multiple reaction intermediates, the energetics of which determine reversibility and kinetics. At the solid-state reaction front, molecular-level chemistry eventually drives the formation of bulk product phases. Therefore, a better understanding of realistic (de)hydrogenation behavior requires considering possible reaction products along all stages of morphological evolution, from molecular to bulk crystalline. Here, we use first-principles calculations to explore the interplay between intermediate morphology and reaction pathways. Employing representative complex metal hydride systems, we investigate the relative energetics of three distinct morphological stages that can be expressed by intermediates during solid-state reactions: i) dispersed molecules; ii) clustered molecular chains; and iii) condensed-phase crystals. Our results verify that the effective reaction energy landscape strongly depends on the morphological features and associated chemical environment, offering a possible explanation for observed discrepancies between X-ray diffraction and nuclear magnetic resonance measurements. Our theoretical understanding also provides physical and chemical insight into phase nucleation kinetics upon (de)hydrogenation of complex metal hydrides.

To overcome the important challenges of facilitating dehydrogenation in the complex metal hydride, LiBH4, the structural and chemical effects were considered using the strain (−3% − +3%) and five dopants (M = Na, K, Al, F, or Cl). The desorption energies of a hydrogen molecule decreased with increasing tensile strain on the LiBH4(010) surface. The tensile strain was useful for promoting the dehydrogenation process by weakening the B-H interactions. Among the dopants examined, the most favorable dopant to enhance dehydrogenation was Al. The ranking of dopants for hydrogen release was Al > Cl > F > Na > K. Remarkably, codoping of Al and Cl was more effective for hydrogen release than the single doping of Al or Cl with the lowest hydrogen desorption energy. These methods that destabilize metal hydrides are practical for tuning the hydrogen release of LiBH4 hydrides. These studies will provide efficient means for designing excellent hydrides for hydrogen release.

Molten mixtures of ytterbium and lithium metals can be used as fluxes for the synthesis of metal hydrides. Reactions of indium and lithium hydride in Yb/Li flux produce Yb~51In13H27 (cubic, Im-3; a = 16.218(6) A, Z = 2). Yb~51In13H27 is an analog of previously reported Ca54In13H27 which was grown from Ca/Li flux. The compound features In@Yb12@H20@Yb30 Bergman-like clusters at the corners and body centers of the cubic lattice. The clusters are connected by a hydride site and disordered ytterbium sites. Magnetic susceptibility measurements indicate the ytterbium is divalent; the compound shows diamagnetic behavior with a Curie tail at lower temperatures attributed to impurities such as traces of Yb3+ that form on crystal surfaces due to slight oxidation.

The complex metal hydride materials have been researched and reported as an effective hydrogen storage material with the gravimetric hydrogen storage capacity of around 5-7 wt%. In this paper, the 20 mg/cm3 of lithium alanate solution prepared at 4 degree C and 25 degree C were dip coated on glass substrate. The post-annealing time was varied at 0 s, 1800 s and 3600 s. Phase and grain size were investigated using the X-ray powder diffraction. The hydrogen storage capacity and hydrogen desorption temperature were analyzed using thermogravimetric analysis. The lithium aluminium hydroxide hydrate and lithium hexahydroaluminate were observed on the deposited lithium alanate thin films when using the lithium alanate solution prepared at 4 degree C while only the lithium hexahydroaluminate was detected on the deposited lithium alanate thin films when the lithium alanate solution was prepared at 25 degree C. It was observed that the deposited lithium alanate thin films with lithium aluminium hydroxide hydrate have lower hydrogen storage capacity than the lithium alanate thin films without lithium aluminium hydroxide hydrate even the lithium alanate thin films with lithium aluminium hydroxide hydrate have smaller grain size. The hydrogen storage capacity and hydrogen desorption temperature of the deposited lithium alanate thin films were improved with longer annealing time.

ConspectusThe fixation of dinitrogen to ammonia is critically important for the biogeochemical cycle on earth. Ammonia also holds promise as a sustainable energy carrier. Tremendous effort has been devoted to the development of green processes and advanced materials for ammonia synthesis and decomposition under milder conditions, and encouraging progress has been made.The reduction of dinitrogen to ammonia needs electrons and protons, which hydridic hydrogen H- could supply. Polarized, electron-rich NxHy intermediates, on the other hand, can be stabilized by alkali or alkaline earth metal cations to lower kinetic barriers in the transformation. The inherent properties of alkali/alkaline earth metal hydrides (denoted as AH) endow them with a unique function in ammonia synthesis.In this Account, recent efforts in the exploration of alkali or alkaline earth metal hydrides (denoted as AH), amides, and imides (denoted as ANH hereafter) for ammonia synthesis and decomposition reactions will be summarized and discussed. We begin with an introduction to the chemistry of A with N2, NH3, and H2, highlighting the interconversion between AH and ANH that has profound implications on the formation and decomposition of NH3. We then present our finding on the strong synergistic effect between ANH and transition metals (TM) in ammonia decomposition catalysis, which stimulated our subsequent research on AH for ammonia synthesis. We discuss the effect and function mechanism of AH in the thermocatalytic and chemical looping ammonia synthesis processes. In the thermocatalytic process, AH cooperates with both early and late TM forming either composite catalysts with two active centers or complex metal hydride catalysts with electron- and hydrogen-rich ionic centers facilitating ammonia synthesis with high activities at lower temperatures. Very interestingly, AH levels the catalytic performances of TMs and intervenes in the energy-scaling relations of TM-only catalysts. Moreover, ANH serves as a new type nitrogen carrier effectively mediating ammonia synthesis via a low-temperature chemical looping process, in which N2 is fixed by AH forming ANH. Subsequently, ANH is hydrogenated to ammonia and AH. Late TMs have a strong catalytic effect on the chemical looping process. The unique interplay of A, N, TM, and H- offers plenty of opportunities for achieving dinitrogen conversion under mild conditions, while further efforts are needed to address the challenges in the fundamental understanding and practical application.

The present paper reports the effect of Graphene Nanoribbons (GNRs) for improving the dehydrogenation of MXH 4 , where M = Na , Li and X = Al , B through density functional calculations. The rectangular Armchair GNRs and ZigZag GNRs with width up to 7.57 A & 10 A and length of 15 A &10 A respectively were considered for the investigation. The effect of dopant and defect was introduced by doping with different dopants ( B , N , Al , Cl and F) and introducing vacancy defect respectively. The MXH 4 interacts with the surface atoms of GNRs with binding energy from 0.31–0.89 eV with AGNR and from 0.26 eV to −1.42 eV with ZGNR respectively. Charge transfer of 0.12–0.26 e take place from GNR surface to MXH 4 molecule resulting in significant decrease in hydrogen release energy ( E HRE ). Doped and defective AGNR lowers E HRE of MXH 4 sharply by 19–56% whereas ZGNR decreases E HRE by 18–70% respectively. The results agree qualitatively with the available experimental result and outline the importance of nano carbon as a catalyst for improving the performance of light complex metal hydride systems.

Adding a secondary complex metal hydride can either kinetically or thermodynamically facilitate dehydrogenation reactions. Adding Mg2FeH6 to LiBH4 is energetically favoured, since FeB and MgB2 are formed as stable intermediate compounds during dehydrogenation reactions. Such “hydride destabilisation” enhances H2-release thermodynamics from H2-storage materials. Samples of the LiBH4 and Mg2FeH6 with a 2:1 molar ratio were mixed and decomposed under three different conditions (dynamic decomposition under vacuum, dynamic decomposition under a hydrogen atmosphere, and isothermal decomposition). In situ synchrotron X-ray diffraction results revealed the influence of decomposition conditions on the selected reaction path. Dynamic decomposition of Mg2FeH6–LiBH4 under vacuum, or isothermal decomposition at low temperatures, was found to induce pure decomposition of LiBH4, whilst mixed decomposition of LiBH4 + Mg and formation of MgB2 were achieved via high-temperature isothermal dehydrogenation.

A novel porous Mg scaffold was synthesised and melt-infiltrated with LiBH4 to simultaneously act as both a confining framework and a destabilising agent for H2 release from LiBH4. This porous Mg scaffold was synthesised by sintering a pellet of NaMgH3 at 450 °C under dynamic vacuum. During the sintering process the multi-metal hydride, decomposed to Mg metal and molten Na. The vacuum applied in combination with the applied sintering temperature, created the ideal conditions for the Na to vaporise and to gradually exit the pellet. The pores of the scaffold were created by the removal of the H2 and Na from the body of the NaMgH3 pellet. The specific surface area of the porous Mg scaffold was determined by the Brunauer–Emmett–Teller (BET) method and from Small-Angle X-ray Scattering (SAXS) measurements, which was 26(1) and 39(5) m2 g−1 respectively. The pore size distribution was analysed using the Barrett–Joyner–Halenda (BJH) method which revealed that the majority of the pores were macropores, with only a small amount of mesopores present in the scaffold. The melt-infiltrated LiBH4 was highly dispersed in the porous scaffold according to the morphological observation carried out by a Scanning Electron Microscope (SEM) and also catalysed the formation of MgH2 as seen from the X-ray diffraction (XRD) patterns of the samples after the infiltration process. Temperature Programmed Desorption (TPD) experiments, which were conducted under various H2 backpressures, revealed that the melt-infiltrated LiBH4 samples exhibited a H2 desorption onset temperature (Tdes) at 100 °C which is 250 °C lower than the bulk LiBH4 and 330 °C lower than the bulk 2LiBH4/MgH2 composite. Moreover, the LiH formed during the decomposition of the LiBH4 was itself observed to fully decompose at 550 °C. The as-synthesised porous Mg scaffold acted as a reactive containment vessel for LiBH4 which not only confined the complex metal hydride but also destabilised it by significantly reducing the H2 desorption temperature down to 100 °C.

Hydrogenases are among the fastest H2 evolving catalysts known to date and have been extensively studied under in vitro conditions. Here, we report the first mechanistic investigation of an [FeFe]-hydrogenase under in vivo conditions. Functional [FeFe]-hydrogenase from the green alga Chlamydomonas reinhardtii is generated in genetically modified Escherichia coli cells, by addition of a synthetic cofactor to the growth medium. The assembly and reactivity of the resulting semi-synthetic enzyme was monitored using whole-cell electron paramagnetic resonance as well as Fourier-transform infrared spectroscopy. Through a combination of gas treatments, pH titrations and isotope editing, we were able to corroborate the physiological relevance of a number of proposed catalytic intermediates, including reactive iron-hydride species. We demonstrate the formation of the so-called hydride state in vivo. Moreover, two previously uncharacterized redox species are reported herein, illustrating the complex metal hydride chemistry of [FeFe]-hydrogenase.

Experimental and theoretical studies were conducted to investigate the pyrophoricity and water-reactivity risks associated with employing sodium alanate (NaAlH4) complex metal hydride in on-board vehicular hydrogen (H2) storage systems. The ignition and explosivity of NaAlH4 upon exposure to oxidizers in air or water were attributed to the spontaneous formation of stable hydroperoxyl intermediates on the NaAlH4 surface and/or H2 production, as well as the large driving force for NaAlH4 conversion to favorable hydroxide products predicted by atomic and thermodynamic modeling. The major products from NaAlH4 exposure to air: NaAl(OH)4, gibbsite and bayerite Al(OH)3, and Na2CO3 observed by XRD, were identified to be formed by surface-controlled reactions. The reactivity risks were significantly minimized, without compromising de-/re-hydrogenation cyclability, by compacting NaAlH4 powder into wafers to reduce the available surface area. These core findings are of significance to risk mitigation and H2 safety code and standard development for the safe use of NaAlH4 for on-board H2 storage in light-duty vehicles.

Nanoscale CeAl4 was directly synthesized by the thermal reaction between CeH2 and nano-aluminum at 300 °C. Then nano CeAl4-doped sodium alanate (NaAlH4) was synthesized by ball milling NaH/Al with 0.04CeAl4 under hydrogen atmosphere at room temperature, and the catalytic efficiency of nanoscale CeAl4 for hydrogen storage of NaAlH4 was systematically investigated. It is shown that CeAl4 can effectively improve the dehydrogenation properties of sodium alanate system. The 0.04CeAl4-doped NaAlH4 system starts to release hydrogen below 80 °C, completes dehydrogenation within 10 min at 170 °C, and exhibits good cycling de/hydrogenation kinetics at relatively lower temperature (100–140 °C). Apparent activation energy of the dehydrogenation of NaAlH4 can be effectively reduced by addition of CeAl4, resulting in the decrease in desorption temperatures. Moreover, by analyzing the reaction kinetics of nano CeAl4-doped NaAlH4 sample, both of the decomposition steps are conformed to a two-dimensional phase-boundary growth mechanism. The mechanistic investigations gained here can help to understand the de-/rehydrogenation behaviors of catalyzed complex metal hydride systems. Nanoscale CeAl4 was successfully synthesized and doped into NaAlH4 system ball milling NaH/Al + 0.04CeAl4. The addition of CeAl4 can effectively improve the dehydrogenation properties of NaAIH4, which starts to release hydrogen below 80 °C, completes dehydrogenation within 10 min at 170 °C, and exhibits good cycling de/hydrogenation kinetics at relatively lower temperature.

An empirical study of the gas–solid reaction of carbon dioxide (CO2) with alanates is presented. This investigation was triggered by reports of hazards related to the reaction of lithium aluminum hydride with carbon dioxide, together with the recent emergence of alanates as potential hydrogen storage materials. Furthermore, the reduction of CO2 by hydrides is an alternative to the conventional CO2 reduction employing hydrogen gas in combination with a catalyst. Experimentally this work was carried out by studying the decomposition of alanate samples in a carbon dioxide atmosphere with thermogravimetric and ex situ IR spectroscopic techniques. It is shown that alanates react with CO2 at atmospheric pressure in two distinct temperature regions, yielding methane, hydrogen gas, and metal oxides as the major products. The experimental findings allowed us to postulate a mechanism for the complex metal hydride reduction of CO2, involving alane as a highly reactive intermediate. It is suggested that nucleophilic ...

Sodium aluminum hydride (NaAlH4) was directly synthesized by ball milling NaH/Al co-doped with CeCl3 + KH under a hydrogen pressure of 3 MPa at room temperature. Out of various samples corresponding to xNaH/Al + 0.02CeCl3 + yKH (x + y = 1; y = 0, 0.02, 0.04 mol%) composites, the composite with y = 0.02 exhibits the optimum de/hydrogenation properties. It shows that the addition of KH can effectively improve the dehydrogenation properties of second step reaction of NaAIH4 system. The composite with y = 0.02 starts to release hydrogen from 87 °C and completes dehydrogenation within 20 min at 170 °C, with good cycling de/hydrogenation kinetics at relatively lower temperature (100–140 °C). After ball milling, the CeCl3 precursor can be changed into CeH2 catalytic active component in the first several de/hydrogenation cycles. Apparent activation energy of the second decomposition step of NaAIH4 system can be effectively decreased by addition of KH, resulting in the decrease of desorption temperatures. Based on the microstructure analyses combined with hydrogen storage performances, the improved dehydrogenation properties of sodium aluminum hydride system are ascribed to the lattice volume expansion of Na3AlH6 during the dehydrogenation process resulted from the addition of KH. Moreover, by analyzing the reaction kinetics of CeCl3 + KH co-doped sample, both of the decomposition steps of composite with y = 0.02 were conformed to the two-dimension phase-boundary growth mechanism. The mechanistic investigations gained here could help to understand the de/rehydrogenation behaviors of catalyzed complex metal hydride systems.

Complex metal hydride is a promising hydrogen storage material for automobile applications due to its reversible storage capacity. The presence of transition metal halide is found to improve significantly the kinetics of H2 adsorption and desorption processes. Experimental studies have indicated the formation of distorted MAl3 phase where M = Sc, Ti, Zr. In this study, a first-principles density functional study has been performed to investigate the hydrogen interaction and saturation on stable tetrahedral MAl3 clusters [M = Li, Sc, Ti, and Zr] by employing spin-polarized hybrid and non-local density functionals. On saturation, the first H2 molecule undergoes chemisorption in the transition metals while further loading results in physisorption with the Kubas-type H2 interaction. Activation energy barrier for the H2 dissociation over the cluster is calculated to be ∼0.2 eV for the transition metals. Effect of external electric field on the MAl3H4 cluster with molecular H2 is studied which leads to polarization of physisorbed H2 and the cluster. The results offer an explanation for catalysts role in improving the kinetics of H2 sorption process in complex metal hydrides.

Zintl phase compounds constitute a unique class of compounds composed of metal cations and covalently bonded multiply charged cluster anions. Potential applications of these materials in solution chemistry and thermoelectric materials have given rise to renewed interest in the search for new Zintl ions. Up to now these ions have been mostly composed of group 13, 14, and 15 post-transition metal elements and no Zintl ions composed of all transition metal elements are known. Using gradient corrected density functional theory we show that the 18-electron rule can be applied to design a new class of Zintl-like ions composed of all transition metal atoms. We demonstrate this possibility by using Ti@Au12(2-) and Ni@Au6(2-) di-anions as examples of Zintl-like ions. Predictive capability of our approach is demonstrated by showing that FeH6(4-) in an already synthesized complex metal hydride, Mg2FeH6, is a Zintl-like ion, satisfying the 18-electron rule. We also show that novel Zintl phase compounds can be formed by using all transition metal Zintl-like ions as building blocks. For example, a two-dimensional periodic structure of Na2[Ti@Au12] is semiconducting and nonmagnetic while a one-dimensional periodic structure of Mg[Ti@Au12] is metallic and ferromagnetic. Our results open the door to the design and synthesis of a new class of Zintl-like ions and compounds with potential for applications.

Recent experiments show that carbon nanomaterials, such as carbon nanofibers, may be used to catalyze the dehydrogenation and rehydrogenation of hydrogen storage materials, such as the benchmark complex metal hydride NaAlH4. However it is not clear how the carbon material can accomplish the dissociation (or recombination) of H2. In this work we investigate the dissociation of H2 on Aln (n = 2, 4, and 6) clusters supported by coronene and graphene substrates using density functional theory (DFT), where coronene and graphene are taken as models for nanographitic surfaces. In our calculations, we account for van der Waals interactions by adapting the correlation part of the PBE exchange-correlation functional with the Grimme and Langreth corrections, and we use NEB to calculate the minimum energy reaction path for the dissociation of H2. Analysis of the minimum barrier reaction paths and the associated dissociation barriers of H2 on Aln clusters interacting with the modeled carbon surfaces shows that the inv...

Concentrating solar–thermal power (CSP) with thermal energy storage (TES) represents an attractive alternative to conventional fossil fuels for base-load power generation. Sodium alanate (NaAlH4) is a well-known sodium-based complex metal hydride but, more recently, high-temperature sodium-based complex metal hydrides have been considered for TES. This review considers the current state of the art for NaH, NaMgH3−x F x , Na-based transition metal hydrides, NaBH4 and Na3AlH6 for TES and heat pumping applications. These metal hydrides have a number of advantages over other classes of heat storage materials such as high thermal energy storage capacity, low volume, relatively low cost and a wide range of operating temperatures (100 °C to more than 650 °C). Potential safety issues associated with the use of high-temperature sodium-based hydrides are also addressed.

Sodium alanate (NaAlH4) is a promising complex metal hydride due to its reasonable hydrogen storage capacity (7.4 wt %). However, the pristine NaAlH4 suffers from the inherent limitations of unfavorable thermodynamics (high desorption temperature), slow kinetics, and poor reversibility. In the present work, the efficacy of Nb2O5, TiO2, and Cr2O3 nanoparticles in ameliorating hydrogen sorption properties of NaAlH4 was evaluated. The use of Nb2O5 and TiO2 displayed superior catalytic effects in terms of enhancing dehydriding/rehydriding kinetics and reducing the dehydrogenation temperature of NaAlH4. Isothermal volumetric measurements at 150 °C revealed that kinetics of hydrogen desorption with Nb2O5 and TiO2 were almost 11–12 times that of pristine NaAlH4. The apparent activation energy as well as enthalpy of dehydrogenation were considerably lowered by addition of Nb2O5 and TiO2 nanopowders. Moreover, the pronounced enhancement on hydrogen capacity arising upon adding Nb2O5 and TiO2 was observed to persis...

Li-N-H system, as a representative of metal-N-H system, is regarded as one of the most promising hydrogen storage materials. Motivated by the ammonia mediated mechanism of Li-N-H system, the effect of MgCl2 (which is an effective NH3 sorbent) on this syst

Carbonaceous nanomaterials utilized as scaffolds, catalysts, and additives in conjunction with complex metal hydrides have shown remarkable hydrogen sorption properties. Our studies have confirmed fullerene-C 60 is an excellent catalyst for temperature induced hydrogen desorption for both NaAlH 4 and LiAlH 4 . Fullerene-containing complex metal hydride composites comprised of fullerene-C 60 with NaAlH 4 or LiAlH 4 desorbed hydrogen at elevated temperature and go onto form alkali metal fullerides and aluminum metal as final products. The as-prepared composites exhibit rapid hydrogen desorption at onset temperatures of 130 °C and 150 °C, and released hydrogen content of 5.9 and 2.2 wt.% (LiAlH 4 and NaAlH 4 , respectively) relative to the composite. The resultant alkali metal fulleride containing composites have been characterized and are capable of reversible hydrogen storage. A series of desorption/absorption experiments on the Na–C 60 and Li–C 60 based composites demonstrate a 1.5 wt.% and a 1.2 wt.% reversible capacity, respectively. The complex metal hydride-C 60 systems were characterized by PCT, XRD, FT-IR, and TGA-RGA and demonstrate the formation of fulleride material similar to traditional hydrofullerenes which appear to be responsible for the observed reversible hydrogen storage.

High energy ball milling up to at least 1h does not lead to the transformation of undoped LiAlH 4 as well as LiAlH 4 doped with n-Ni into Li 3 AlH 6 . In a DSC test the melting of LiAlH 4 is completely eliminated by the addition of 5 wt% n-Ni incorporated by high energy ball milling. The LiAlH4 + 5 wt% n-Ni system processed by ball milling desorbs ∼4.8 wt% H 2 at 120 °C within 2000 s. At 120 °C within a 5000 s time interval the desorption reaction occurs fully in a solid state in one step according to the following reaction: LiAlH 4 → 1/3Li 3 AlH 6 + 2/3Al + H 2 . This reaction is completely non-volatile. At 140 °C and higher temperatures the desorption reaction of ball milled LiAlH 4 + 5 wt% n-Ni occurs fully in a solid state most likely in two steps according to the following reactions: (1) LiAlH 4 → 1/3Li 3 AlH 6 + 2/3Al + H 2 and (2) 1/3Li 3 AlH 6 → LiH + 1/3Al + 0.5H 2. However, XRD patterns after reaction (2) do not show the presence of LiH but instead, they show the presence of LiOH. It is possible that LiH is transformed rapidly into LiOH by a mechanism which is now under investigation. When stored for a long time at room temperature the heavily ball milled LiAlH 4 + 5 wt% n-Ni system gradually degrades losing H 2 capacity.

We determined the stability of the LiBH4/CeH2 composite system by dynamic pcT (pressure, composition, temperature) measurements by using different constant hydrogen flows and by extrapolating ln(pdes/p0) linearly to equilibrium at zero flow. During desorption, the reaction 6LiBH4 + CeH2 → 6LiH + CeB6 + 10H2 occurs, leading to a theoretical hydrogen capacity of the destabilized system of 7.4 mass %. Within the model used and by applying the Van ˈt Hoff equation, the following thermodynamic parameters were determined for the desorption: enthalpy of reaction ΔrH = (58 ± 3) kJ mol−1 H2 and entropy of reaction ΔrS = (113 ± 4) J K−1 mol−1 H2, leading to a decomposition temperature Tdec = (240 ± 32) °C at a hydrogen pressure of p0 = 1.01325 bar, compared with ΔrH = 74 kJ mol−1 H2 and ΔrS = 115 J K−1 mol−1 H2 (Tdec = 370 °C) for pure LiBH4.1

The effect of divalent metal-dopants, Mg, Ca, and Zn, on the stability of LiBH4 is studied by using the first-principles calculations and CALPHAD (CALculation of PHAse Diagram) modeling. The ground states of Mg1/2BH4, Ca1/2BH4, and Zn1/2BH4 are shown to be I 4 ¯ m 2 , F2dd, and I 4 ¯ m 2 , respectively, through first-principles calculations. Positive enthalpy of mixing between Li and the alloying element is predicted, indicating unfavorable solubility of alloying elements in LiBH4 and thus offering possibility to decrease the stability of LiBH4. The ionic sublattice model of (Li+, M2+, Va)1(BH4−)1 is adopted for the metal substituted LiBH4 phase. It is observed that the addition of Mg or Zn has limited effect as the decomposition temperature is between those of LiBH4 and M1/2BH4 for Mg and Zn substitutions. LiBH4 is destabilized with magnesium borides or LiZn4 formation but its decomposition temperature is higher than that of M1/2BH4. On the other hand, the addition of Ca significantly reduces the H2 releasing temperature due to the formation of highly stable CaB6.

Metal hydrides are likely candidates for the solid state storage of hydrogen. NaAlH4 is the only complex metal hydride identified so far that combines favorable thermodynamics with a reasonable hydrogen storage capacity (5.5 wt %) when decomposing in two steps to NaH, Al, and H2. The slow kinetics and poor reversibility of the hydrogen desorption can be combatted by the addition of a Ti-based catalyst. In an alternative approach we studied the influence of a reduced NaAlH4 particle size and the presence of a carbon support. We focused on NaAlH4/porous carbon nanocomposites prepared by melt infiltration. The NaAlH4 was confined in the mainly 2-3 nm pores of the carbon, resulting in a lack of long-range order in the NaAlH4 structure. The hydrogen release profile was modified by contact with the carbon; even for ∼10 nm NaAlH4 on a nonporous carbon material the decomposition of NaAlH4 to NaH, Al, and H2 now led to hydrogen release in a single step. This was a kinetic effect, with the temperature at which the hydrogen was released depending on the NaAlH4 feature size. However, confinement in a nanoporous carbon material was essential to not only achieve low H2 release temperatures, but also rehydrogenation at mild conditions (e.g., 24 bar H2 at 150 °C). Not only had the kinetics of hydrogen sorption improved, but the thermodynamics had also changed. When hydrogenating at conditions at which Na3AlH6 would be expected to be the stable phase (e.g., 40 bar H2 at 160 °C), instead nanoconfined NaAlH4 was formed, indicating a shift of the NaAlH4TNa3AlH6 thermodynamic equilibrium in these nanocomposites compared to bulk materials.

The recent advances on the effects of microstructural refinement and various nano-catalytic additives on the hydrogen storage properties of metal and complex hydrides obtained in the last few years in the allied laboratories at the University of Waterloo (Canada) and Military University of Technology (Warsaw, Poland) are critically reviewed in this paper. The research results indicate that microstructural refinement (particle and grain size) induced by ball milling influences quite modestly the hydrogen storage properties of simple metal and complex metal hydrides. On the other hand, the addition of nanometric elemental metals acting as potent catalysts and/or metal halide catalytic precursors brings about profound improvements in the hydrogen absorption/desorption kinetics for simple metal and complex metal hydrides alike. In general, catalytic precursors react with the hydride matrix forming a metal salt and free nanometric or amorphous elemental metals/intermetallics which, in turn, act catalytically. However, these catalysts change only kinetic properties i.e. the hydrogen absorption/desorption rate but they do not change thermodynamics (e.g., enthalpy change of hydrogen sorption reactions). It is shown that a complex metal hydride, LiAlH 4 , after high energy ball milling with a nanometric Ni metal catalyst and/or MnCl 2 catalytic precursor, is able to desorb relatively large quantities of hydrogen at RT, 40 and 80 °C. This kind of behavior is very encouraging for the future development of solid state hydrogen systems.

Hydrogen back pressure remarkably promotes the formation of metal boride during the dehydrogenation of 4LiBH4 + YH3, 6LiBH4 + CeH2 and 6LiBH4 + CaH2 composites, which seems to be a general phenomenon in LiBH4-based reactive hydride composites that enables mutual destabilization between LiBH4 and metal hydride. The formation of metal boride plays a crucial role in the reversible hydrogen storage properties of these composites. The dependence of the dehydrogenation behavior on hydrogen back pressure might be associated with the microstructural evolution of the dehydrogenation products formed by a solid−liquid reaction.

Thermophysical properties of the complex metal hydride system such as zirconium cobalt hydride, an intermetallic hydride compound, in a massive state were estimated by introducing a crystal lattice structure in a stepwise formation and applying a mixing rule for each property. Experimental data in rarity in metal hydride system was used to calculate and to correlate the consistency of the mixed thermal and physical properties of the complex atomic structure in a unit cell. As a result, the volume expansion of the ZrCoH x was greatly influenced by the hydrogen content and increased to a maximum range of 36% at ZrCoH 3 system, but no meaning in the thermal expansion in engineering concept. In consideration of the heat capacity the temperature effect due to the hydrogen―an interstitial heat quantity― in the metal complex formation was mainly attributed, but not much for the hydrogen content (H/ZrCo ratio). In the temperature range between 200K and 600K the heat capacity of hydrogen atom was taken into account to reveal a sharp discrepancy in its non-hydriding property, especially in the lower temperature range. Atomic hydrogen was expected to behave from a gas to a solid property in heat capacity in the temperature ranges from 600K to 200K.

Complex metal hydrides such as sodium aluminohydride (NaAlH4) and sodium borohydride (NaBH4) are solid-state hydrogen-storage materials with high hydrogen capacities. They can be used in combination with fuel cells as a hydrogen source thus enabling longer operation times compared with classical metal hydrides. The most important point for a wide application of these materials is the reversibility under moderate technical conditions. At present, only NaAlH4 has favorable thermodynamic properties and can be employed as a thermally reversible means of hydrogen storage. By contrast, NaBH4 is a typical non-reversible complex metal hydride; it reacts with water to produce hydrogen.

Hydrogenation of the intermetallic compound LaMg 2 Pd at 200 °C and 10 bar leads to a complex metal hydride of composition LaMg 2 PdH 7 . Its structure has orthorhombic symmetry and displays tetrahedral [PdH 4 ] 4- anions. The Pd-H bond distances as measured on the deuteride range from 1.71 to 1.78 A and the H-Pd-H bond angles from 95° to 122°. Three additional hydride anions H - occupy La 2 Mg 2 -type interstices having tetrahedral metal configurations. Band structure calculations suggest the hydride to be non-metallic and to have a band gap of ∼1.0eV. The compound desorbs hydrogen at 125 °C yielding a pressure of more than 1 bar absolute.

A novel metal borohydride ammonia-borane complex Ca(BH4)2·NH3BH3 is characterized as the decomposition product of the recently reported perovskite-type metal borohydride NH4Ca(BH4)3, suggesting that ammonium-based metal borohydrides release hydrogen gas via ammonia-borane-complexes. For the first time the concept of proton-hydride interactions to promote hydrogen release is applied to a cation-anion pair in a complex metal hydride. NH4Ca(BH4)3 is prepared mechanochemically from Ca(BH4)2 and NH4Cl as well as NH4BH4 following two different protocols, where the synthesis procedures are modified in the latter to solvent-based ball-milling using diethyl ether to maximize the phase yield in chlorine-free samples. During decomposition of NH4Ca(BH4)3 pure H2 is released, prior to the decomposition of the complex to its constituents. As opposed to a previously reported adduct between Ca(BH4)2 and NH3BH3, the present complex is described as NH3BH3-stuffed α-Ca(BH4)2.

Magnesium alanate Mg(AlH4)2 has been synthesized by mechanochemically activated metathesis reaction between NaAlH4 and MgCl2 without solvent, this metathesis reaction proceeded completely. The thermal decomposition behavior of solvent-free Mg(AlH4)2 has been analyzed by TG/MS and DSC. It has been confirmed that Mg(AlH4)2 decomposes in two steps: the first decomposition reaction Mg(AlH4)2 → MgH2 + 2Al + 3H2 is exothermic and starts at about 115 °C. The second one Mg H 2 + 2 Al → 1 2 A l 3 M g 2 + 1 2 Al + H 2 is endothermic in total and starts at about 240 °C, although the formation reaction of Al3Mg2 (3Al + 2Mg → Al3Mg2) is exothermic.

High energy density hydrogen storage is a critical technology requirement in a hydrogen-based energy infrastructure. Although there are no current storage methods that meet desired energy density goals for vehicular hydrogen storage, complex metal hydride based systems are among the most promising. These materials form compounds with hydrogen under appropriate conditions and release hydrogen by thermal decomposition. The complex hydride, sodium alanate, is particularly useful due to its favorable reversibility. Thermal properties characterization of sodium alanate has been performed at Sandia National Laboratories to gain a detailed understanding of how complex hydrides will behave in a storage system. Thermal properties were investigated using the thermal probe method (ASTM D5334). Custom test hardware was designed and built to accommodate the complex decomposition and recombination of sodium alanate. Thermal conductivity and thermal wall resistance were determined by utilizing analytical and numerical data analysis methods. The thermal conductivity of sodium alanate was found to vary by more than 90% with changes in phase composition and hydrogen gas pressures between 1 and 100 atm. The quality of thermal contact between the alanate and the vessel wall was characterized numerically for various pressures and phase compositions. The contact resistance is high for all states, indicating poor contact between the material and the vessel wall.

Destabilization of LiBH4 for reversible hydrogen storage has been studied using MgH2 as a destabilizing additive. Mechanically milled mixtures of LiBH4 + (1/2)MgH2 or LiH + (1/2)MgB2 including 2-3 mol % TiCl3 are shown to reversibly store 8-10 wt % hydrogen. Variation of the equilibrium pressure obtained from isotherms measured at 315-400 degrees C indicate that addition of MgH2 lowers the hydrogenation/dehydrogenation enthalpy by 25 kJ/(mol of H2) compared with pure LiBH4. Formation of MgB2 upon dehydrogenation stabilizes the dehydrogenated state and, thereby, destabilizes the LiBH4. Extrapolation of the isotherm data yields a predicted equilibrium pressure of 1 bar at approximately 225 degrees C. However, the kinetics were too slow for direct measurements at these temperatures.

The role of the heteroatom on the stereochemistry and the relative rate for the complex metal hydride reduction of heteracyclohexanones and methoxy substituted cyclohexanones is explained by the difference in the nonbonding two-electron stabilization between the incipient σ†* bond and the anti-periplanar allylic σi bonds which were perturbed by the through-space and/or through-bond interaction with the remote heteroatom. A significant directive effect of the 2-axial hydroxyl group appears in the reduction of cyclohexanone with representative complex metal hydrides, while the 3-axial hydroxyl group exhibits a steric hindrance. The distance between the carbonyl carbon and the hydroxyl group which interacts with the hydride reagent is mainly responsible for such a difference. The key point of the extremely high directive effect appeared in the Na[B(OAc)3H] reduction for both 2- and 3-axial hydroxycyclohexanone is the formation of Na[B(OAc)2(OR)H], which is far more reactive than the parent hydride, by exchanging the acetate ion with the alkoxide. Chirality 14:110–120, 2002. © 2002 Wiley-Liss, Inc.

Thermodynamics and kinetics of the reversible dissociation of metal-doped NaAlH 4 as a hydrogen (or heat) storage system have been investigated in some detail. The experimentally determined enthalpies for the first (3.7 wt% of H) and the second dissociation step of Ti-doped NaAlH 4 (3.0 wt% H) of 37 and 47 kJ/mol are in accordance with low and medium temperature reversible metal hydride systems, respectively. Through variation of NaAlH 4 particle sizes, catalysts (dopants) and doping procedures, kinetics as well as the cyclization stability within cycle tests have been substantially improved with respect to the previous status [B. Bogdanovic, M. Schwickardi, J. Alloys Comp. 253–254 (1997) 1]. In particular, using combinations of Ti and Fe compounds as dopants, a cooperative (synergistic) catalytic effect of the metals Ti and Fe in enhancing rates of both de- and rehydrogenation of Ti/Fe-doped NaAlH 4 within cycle tests, reaching a constant storage capacity of ∼4 wt% H 2 , has been demonstrated. By means of 57 Fe Mossbauer spectroscopy of the Ti/Fe-doped NaAlH 4 before and throughout a cycle test, it has been ascertained that (1) during the doping procedure, nanosize metallic Fe particles are formed from the doping agent Fe(OEt) 2 and (2) already after the first dehydrogenation, the nanosize Fe particles with NaAlH 4 present are probably transformed into an Fe–Al-alloy which throughout the cycle test remains practically unchanged.

A significant directive effect of the 2-axial hydroxy group appeared in the LiAlH4, NaBH4, and Zn(BH4)2 reduction of cyclohexanone, while the 3-axial hydroxy group exhibited a steric hindrance. The distance between the carbonyl carbon and the hydroxy group which interacts with the hydride reagent is mainly responsible for such a difference. In the reduction of Na[B(OAc)3H], both the 2- and 3-axial hydroxycyclohexanones exclusively gave the products which were obtained by the hydride approaching from the side of the hydroxy group. The key point of this stereoselectivity is the formation of Na[B(OAc)2(OR)H], which is far more reactive than the parent hydride, by exchanging the acetate ion with the alkoxide.

The complex metal hydride reductions of 2-t-butyl-1,3-dioxan-5-one (1) and 3-oxoquinolizidine (3) are faster than those of the corresponding carbocyclic compounds, 4-t-butylcyclohexanone (2) and trans-2-decalone (4). The stereoselectivities were similar in the LiAlH4 reduction, but the heteracyclohexanones exhibited higher stereoselectivity with NaBH4. These facts are discussed in terms of the intramolecular orbital interaction.