Hydrogen Release from Mixtures of Lithium Borohydride and Lithium Amide: A Phase Diagram Study

Development of continuous measurement system for hydrogen and impurity gases using detector tube

A relatively accurate, inexpensive, simple, and continuous quantification system for hydrogen and impurity gas(es) using a detector tube was developed in this study. Additionally, different detector tubes can be applied to measure different types of gases in a wide range from ppm order to % level. We optimized this system and evaluated its accuracy as well as the behavior of released H2 and impurity (NH3) gases from a hydrolysis of ammonia borane using a Pt/Al2O3 catalyst. The accuracy of hydrogen quantitation achieved by this system was comparable to that of commercial mass flow meters, and the accuracy of ammonia quantitation was 10% or 5% relative standard deviation, which depends on the detector tube. The concentration of released NH3 was evaluated by image analysis with a time-lapse video of the detector tube and succeeded in analyzing from ppm to % order. The H2 and NH3 release behaviors agreed with pH, and the percentage of reaction was estimated by NMR measurement of the reacted solution. These results confirmed the accuracy of this system.

Ammonia-assisted fast Li-ion conductivity in a new hemiammine lithium borohydride, LiBH4·1/2NH3

Hemiammine lithium borohydride, LiBH₄·1/2NH₃, is characterized and a new Li⁺ conductivity mechanism is identified. It exhibits a Li⁺ conductivity of 7 × 10^-4 S cm^-1 at 40 °C in the solid state and 3.0 × 10^-2 S cm^-1 at 55 °C after melting. The molten state of LiBH₄·1/2NH₃ has a high viscosity and can be mechanically stabilized in nano-composites with inert metal oxides and other hydrides making it a promising battery electrolyte.

Borohydride-Scaffolded Li/Na/Mg Fast Ionic Conductors for Promising Solid-State Electrolytes

Borohydride solid-state electrolytes with room-temperature ionic conductivity up to ≈70 mS cm^-1 have achieved impressive progress and quickly taken their place among the superionic conductive solid-state electrolytes. Here, the focus is on state-of-the-art developments in borohydride solid-state electrolytes, including their competitive ionic-conductive performance, current limitations for practical applications in solid-state batteries, and the strategies to address their problems. To open, fast Li/Na/Mg ionic conductivity in electrolytes with BH₄ ^- groups, approaches to engineering borohydrides with enhanced ionic conductivity, and later on the superionic conductivity of polyhedral borohydrides, their correlated conductive kinetics/thermodynamics, and the theoretically predicted high conductive derivatives are discussed. Furthermore, the validity of borohydride pairing with coated oxides, sulfur, organic electrodes, MgH₂ , TiS₂ , Li₄ Ti₅ O₁₂ , electrode materials, etc., is surveyed in solid-state batteries. From the viewpoint of compatible cathodes, the stable electrochemical windows of borohydride solid-state electrolytes, the electrode/electrolyte interface behavior and battery device design, and the performance optimization of borohydride-based solid-state batteries are also discussed in detail. A comprehensive coverage of emerging trends in borohydride solid-state electrolytes is provided and future maps to promote better performance of borohydride SSEs are sketched out, which will pave the way for their further development in the field of energy storage.

Improved Dehydrogenation Performance of Li-B-N-H by Doped NiO

In order to improve the dehydrogenation properties of the Li-B-N-H system, a flower-like NiO was successfully synthesized using the hydrothermal method. The effect of the NiO on the dehydrogenation properties of the LiBH4-2LiNH2 system was studied. The results showed that the dehydrogenation properties of the LiBH4-2LiMH2 system were significantly enhanced by doping with NiO. The composite doped with 5 wt. % NiO exhibited optimal hydrogen storage properties. It released about 10.5 wt. % hydrogen below 300 °C, and the onset dehydrogenation temperature was only 90 °C, 110 °C lower than that of LiBH4-2LiNH2. The isothermal dehydrogenation experiment indicated that the LiBH4-2LiNH2-5 wt. % NiO composite released 8.8 wt. % hydrogen within 15 min at 150 °C. Structural analysis revealed that the as-prepared NiO was reduced to metallic Ni, which worked as an active catalytic species in the remainder of the dehydrogenation process. The Mass Spectrometer (MS) analyses showed that the doped NiO inhibited the content of NH3 released in the process of the dehydrogenation of LiBH4-2LiNH2-NiO.

Role of Metal Electronegativity in the Dehydrogenation Thermodynamics and Kinetics of Composite Metal Borohydride-LiNH2 Hydrogen Storage Materials

The composites of M(BH₄) _n-LiNH₂ (1/2 n molar ratio, n = 1 or 2, M = Ca, Mg, Li) were synthesized by liquid ball milling. Samples were characterized by X-ray diffraction, thermogravimetry-differential thermal analysis-mass spectroscopy (TG-DTA-MS), and kinetic models (Achar differential/Coats-Redfern integral method). The higher-electronegativity metal M in M(BH₄) _n-4LiNH₂ (M = Ca, Mg) samples not only enables [BH₄]^- group to release easily, so as to facilitate the interaction of [BH₄]^- and [NH₂]^- groups, but also restrains the NH₃ release and slightly decreases the onset dehydrogenation temperature concluded by TG-MS. Moreover, in stage 1 (200-350 °C), the kinetics performances of M(BH₄) _n-4LiNH₂ (M = Ca, Mg) samples are distinctly improved, that is, the activation energies of them are reduced by ca. 30% compared to those of sample LiBH₄-2LiNH₂. The outstanding contribution of the replacement of M(BH₄) _n with high-electronegativity metal ion is to both improve the kinetics performance by changing the kinetics mechanism and decrease the temperature range of the initial dehydrogenation region.

Clarifying the dehydrogenation pathway of catalysed Li4(NH2)3BH4-LiH composites

The effect of different metal oxides (Co₃O₄ and NiO) on the dehydrogenation reaction pathways of the Li₄(NH₂)₃BH₄-LiH composite was investigated. The additives were reduced to metallic species i.e. Co and Ni which act as catalysts by breaking the B-H bonds in the Li-B-N-H compounds. The onset decomposition temperature was lowered by 32 °C for the Ni-catalysed sample, which released 8.8 wt% hydrogen below 275 °C. It was demonstrated that the decomposition of the doped composite followed a mechanism via LiNH₂ and Li₃BN₂ formation as the end product with a strong reduction of NH₃ emission. The sample could be partially re-hydrogenated (∼1.5 wt%) due to lithium imide/amide transformation. To understand the role of LiH, Li₄(NH₂)₃BH₄-LiH-NiO and Li₄(NH₂)₃BH₄-NiO composites were compared. The absence of LiH as a reactant forced the system to follow another path, which involved the formation of an intermediate phase of composition Li₃BN₂H₂ at the early stages of dehydrogenation and the end products LiNH₂ and monoclinic Li₃BN₂. We provided evidence for the interaction between NiO and LiNH₂ during heating and proposed that the presence of Li facilitates a NH_x-rich environment and the Ni catalyst mediates the electron transfer to promote NH_x coupling.

The [BN2]3− anion – a carbon dioxide isosteric building unit for a large family of complex nitridoborate structures

The crystal chemistry of nitridoborates with the CO2 analogous [BN2]3− ion is reviewed. Such nitridoborates form with the alkali and alkaline earth metals as well as with divalent europium. Also quaternary compounds with mixed cations along with nitridoborate nitrides, oxides, halides and hydrides are discussed. The spectroscopic (IR, Raman, solid state NMR and Mössbauer spectroscopy) and magnetic behavior as well as optical properties are discussed in the light of structure-property relationships.

Metal borohydrides and derivatives – synthesis, structure and properties

A comprehensive review of metal borohydrides from synthesis to application.

Mechanistic insights into the remarkable catalytic activity of nanosized Co@C composites for hydrogen desorption from the LiBH4–2LiNH2 system

Weakened B–H bonding and enhanced electrostatic H^δ+⋯H^δ− interactions due to charge redistribution explain the lowered dehydrogenation temperatures of Co-catalysed Li–B–N–H systems.

Complex metal borohydrides: multifunctional materials for energy storage and conversion

With the limited supply of fossil fuels and their adverse effect on the climate and the environment, it has become a global priority to seek alternate sources of energy that are clean, abundant, and sustainable. While sources such as solar, wind, and hydrogen can meet the world's energy demand, considerable challenges remain to find materials that can store and/or convert energy efficiently. This topical review focuses on one such class of materials, namely, multi-functional complex metal borohydrides that not only have the ability to store sufficient amount of hydrogen to meet the needs of the transportation industry, but also can be used for a new generation of metal ion batteries and solar cells. We discuss the material challenges in all these areas and review the progress that has been made to address them, the issues that still need to be resolved and the outlook for the future.

New amide-chloride phases in the Li-Al-N-H-Cl system: formation and hydrogen storage behaviour

New amide-chloride phases were successfully synthesized by mechanical milling of the LiNH2-AlCl3 mixture at a molar ratio of 1 : 0.11 and further heating at 150 °C under argon (0.1 MPa) or under hydrogen pressure (0.7 MPa). Powder X-ray diffraction measurements as a function of milling time increase revealed that the milling of the LiNH2-0.11AlCl3 mixture results in the formation of a FCC solid solution with an excess of LiNH2. Subsequent heating of the LiNH2-0.11AlCl3 sample ball milled for 5 hours at 150 °C under argon or under hydrogen induces the appearance of an amide-chloride phase isostructural with cubic Li4(NH2)3Cl. This Li-Al-N-H-Cl phase transforms progressively into the trigonal phase after prolonged heating at 300 °C under hydrogen pressure. The thermal behaviour of the amide-chloride without and with LiH addition displays dissimilar decomposition pathways. The decomposition of amide-chloride alone involves the formation of ammonia and hydrogen from 120 to 300 °C. Conversely, the amide-chloride material in the presence of LiH only releases hydrogen avoiding the emission of ammonia. The resultant material is able to be rehydrogenated under moderate conditions (300 °C, 0.7 MPa H2), providing a new reversible hydrogen storage system.

Effective participation of Li4(NH2)3BH4 in the dehydrogenation pathway of the Mg(NH2)2–2LiH composite

The presence of Li₄(NH₂)₃BH₄ in the MgNH₂–LiH composite enhances the hydrogen sorption kinetics and its cycling stability.

Hydrogen Storage Materials for Mobile and Stationary Applications: Current State of the Art

One of the limitations to the widespread use of hydrogen as an energy carrier is its storage in a safe and compact form. Herein, recent developments in effective high-capacity hydrogen storage materials are reviewed, with a special emphasis on light compounds, including those based on organic porous structures, boron, nitrogen, and aluminum. These elements and their related compounds hold the promise of high, reversible, and practical hydrogen storage capacity for mobile applications, including vehicles and portable power equipment, but also for the large scale and distributed storage of energy for stationary applications. Current understanding of the fundamental principles that govern the interaction of hydrogen with these light compounds is summarized, as well as basic strategies to meet practical targets of hydrogen uptake and release. The limitation of these strategies and current understanding is also discussed and new directions proposed.

The role of native defects in the transport of charge and mass and the decomposition of Li4BN3H10

We propose an atomistic mechanism for the decomposition of Li₄BN₃H₁₀ in which the cogeneration of NH₃ gas is associated with self-diffusion of negatively charged hydrogen vacancies.

Mobility and dynamics in the complex hydrides LiAlH4 and LiBH4

The dynamics and bonding of the complex hydrides LiBH4 and LiAlH4 have been investigated by vibrational spectroscopy. The combination of infrared, Raman, and inelastic neutron scattering (INS) spectroscopies on hydrided and deuterided samples reveals a complete picture of the dynamics of the BH4- and AlH4 anions respectively as well as the lattice. The straightforward interpretation of isotope effects facilitates tracer diffusion experiments revealing the diffusion coefficients of hydrogen containing species in LiBH4, and LiAlH4. LiBH4 exchanges atomic hydrogen starting at 200 degrees C. Despite having an iso-electronic structure, the mobility of hydrogen in LiAlH4 is different from that of LiBH4. Upon ball-milling of LiAlH4 and LiAlD4, hydrogen is exchanged with deuterium even at room temperature. However, the exchange reaction competes with the decomposition of the compound. The diffusion coefficients of the alanate and borohydride have been found to be D approximately equal 7 x 10(-14) m2 s(-1) at 473 K and D approximately equal 5 x 10(-16) m2 s(-1) at 348 K, respectively. The BH4 ion is easily exchanged by other ions such as I- or by NH2-. This opens the possibility of tailoring physical properties such as the temperature of the phase transition linked to the Li-ion conductivity in LiBH4 as measured by nuclear magnetic resonance and Raman spectroscopy. Temperature dependent Raman measurements on diffusion gradient samples Li(BH4)1-cIc demonstrate that increasing temperature has a similar impact to increasing the iodide concentration c: the system is driven towards the high-temperature phase of LiBH4. The influence of anion exchange on the hydrogen sorption properties is limited, though. For example, Li4(BH4)(NH2)3 does not exchange hydrogen easily even in the melt.

A prospect for LiBH4 as on-board hydrogen storage

In contrast to the traditional metal hydrides, in which hydrogen storage involves the reversible hydrogen entering/exiting of the host hydride lattice, LiBH4 releases hydrogen via decomposition that produces segregated LiH and amorphous B phases. This is obviously the reason why lithium borohydride applications in fuel cells so far meet only one requirement — high hydrogen storage capacity. Nevertheless, its thermodynamics and kinetics studies are very active today and efficient ways to meet fuel cell requirements might be done through lowering the temperature for hydrogenation/dehydrogenation and suitable catalyst. Some improvements are expected to enable LiBH4 to be used in on-board hydrogen storage.

New B,N-hydrides: Characterization and Chemistry

Three different classes of boron and nitrogen containing light metal complex hydrides have been investigated, resulting from the reactions of LiNH2 with LiBH4, NaNH2 with NaBH4 and MHx (where M = Li, Na and Ca) with NH3BH3. A rich variety of new phases has been identified, which exhibit modified decomposition pathways and onset temperatures of hydrogen desorption as low as 40°C. In each case the composition of phases formed has been examined in detail and the products of thermal decomposition—solid and gaseous—have been determined.

Light metal borohydrides: crystal structures and beyond

Experimental structures of M(BH4)n, where M is a 2nd–4th period element, are reviewed with a particluar emphasize on crystal chemistry. It is shown that except certain cases, the BH4 group has a nearly ideal tetrahedral geometry. Correction of the experimentally determined H-positions allows to compare directly the results obtained by different diffraction techniques and by theoretical calculations. Analysis of coordination geometries for M and BH4, and of mechanisms of phase transitions in LiBH4, suggest that the directional BH4 … M interaction is at the origin of structural complexity of borohydrides. The ways to influence their stability by chemical modification are discussed. Study of structural evolution with temperature and pressure is shown to be the way to access fundamental information on structural stability of these systems.

Size effects on the hydrogen storage properties of nanoscaffolded Li3BN2H8

The use of Li3BN2H8 complex hydride as a practical hydrogen storage material is limited by its high desorption temperature and poor reversibility. While certain catalysts have been shown to decrease the dehydrogenation temperature, no significant improvement in reversibility has been reported thus far. In this study, we demonstrated that tuning the particle size to the nanometer scale by infiltration into nanoporous carbon scaffolds leads to dramatic improvements in the reversibility of Li3BN2H8. Possible changes in the dehydrogenation path were also observed in the nanoscaffolded hydride.

Local bonding and atomic environments in Ni-catalyzed complex hydrides

The local bonding and atomic environments in the Ni-catalyzed destabilized system LiBH4/MgH2 and the quaternary borohydride-amide phase Li3BN2H8, were studied by x-ray absorption spectroscopy. In both cases the Ni catalyst was introduced as NiCl2 and a qualitative comparison of the Ni K-edge near-edge structure suggests the Ni2+ is reduced to primarily Ni0 after ball milling. The extended fine structure of the Ni K edge indicates that the Ni is coordinated by approximately 3 boron atoms with an interatomic distance of approximately 2.1 A and approximately 11 Ni atoms in a split shell at around 2.5 and 2.8 A. These results, and the lack of long-range order, suggest that the Ni is present as a disordered nanocluster with a local structure similar to that of Ni3B. In the fully hydrogenated phase of LiBH4/MgH2 a small amount Mg2NiHx was also present. Surface calculations performed using density functional theory suggest that the lowest kinetic barrier for H2 chemisorption occurs on the Ni3B(100) surface.

Discovery of novel hydrogen storage materials: an atomic scale computational approach

Practical hydrogen storage for mobile applications requires materials that exhibit high hydrogen densities, low decomposition temperatures, and fast kinetics for absorption and desorption. Unfortunately, no reversible materials are currently known that possess all of these attributes. Here we present an overview of our recent efforts aimed at developing a first-principles computational approach to the discovery of novel hydrogen storage materials. Such an approach requires several key capabilities to be effective: (i) accurate prediction of decomposition thermodynamics, (ii) prediction of crystal structures for unknown hydrides, and (iii) prediction of preferred decomposition pathways. We present examples that illustrate each of these three capabilities: (i) prediction of hydriding enthalpies and free energies across a wide range of hydride materials, (ii) prediction of low energy crystal structures for complex hydrides (such as Ca(AlH(4))(2) CaAlH(5), and Li(2)NH), and (iii) predicted decomposition pathways for Li(4)BN(3)H(10) and destabilized systems based on combinations of LiBH(4), Ca(BH(4))(2) and metal hydrides. For the destabilized systems, we propose a set of thermodynamic guidelines to help identify thermodynamically viable reactions. These capabilities have led to the prediction of several novel high density hydrogen storage materials and reactions.

Using first principles calculations to identify new destabilized metal hydride reactions for reversible hydrogen storage

Hydrides of period 2 and 3 elements are promising candidates for hydrogen storage, but typically have heats of reaction that are too high to be of use for fuel cell vehicles. Recent experimental work has focused on destabilizing metal hydrides through mixing metal hydrides with other compounds. A very large number of possible destabilized metal hydride reaction schemes exist, but the thermodynamic data required to assess the enthalpies of these reactions are not available in many cases. We have used density functional theory calculations to predict the reaction enthalpies for more than 300 destabilization reactions that have not previously been reported. The large majority of these reactions are predicted not to be useful for reversible hydrogen storage, having calculated reaction enthalpies that are either too high or too low, and hence these reactions need not be investigated experimentally. Our calculations also identify multiple promising reactions that have large enough hydrogen storage capacities to be useful in practical applications and have reaction thermodynamics that appear to be suitable for use in fuel cell vehicles and are therefore promising candidates for experimental work.

Improved Hydrogen Release from LiB0.33N0.67H2.67 with Noble Metal Additions

The hydrogen release behavior of the quaternary hydride LiB(0.33)N(0.67)H(2.67) has been successfully improved through the incorporation of small quantities of noble metal. Adding 5 wt % Pd either as Pd metal particles or as PdCl(2) reduced the temperature T(1/2) corresponding to the midpoint of the hydrogen release reaction by DeltaT(1/2) = -43 degrees C and -76 degrees C, respectively. PtCl(2) and Pt nanoparticles supported on a Vulcan carbon substrate proved to be even more effective, with DeltaT(1/2) = -90 degrees C. The amount of NH(3) released during dehydrogenation is reduced compared to that from additive-free material, and, more importantly, at temperatures below 210 degrees C hydrogen is released with no detectable NH(3). In contrast to additive-free LiB(0.33)N(0.67)H(2.67), which melts completely above 190 degrees C and releases hydrogen from the liquid state only above approximately 250 degrees C, hydrogen release from LiB(0.33)N(0.67)H(2.67) + 5 wt % Pt/Vulcan carbon is accompanied by partial melting plus a cascade through a series of solid intermediate phases. Calorimetric measurements indicate that both additive-free and Pt-added LiB(0.33)N(0.67)H(2.67) release hydrogen exothermically, and hence the reverse reaction is thermodynamically unfavorable. By exposing partially dehydrogenated samples to high H(2) pressures at modest temperatures, fractional hydrogen uptake (roughly 15% of the released hydrogen) has been achieved. The mechanism by which noble metals promote hydrogen release is not known, but the behavior is consistent with that expected for a catalyst, including a large effect with small additions and saturation of the effect at low concentration.