Halide Free M(BH4)2 (M = Sr, Ba, and Eu) Synthesis, Structure, and Decomposition

Complex Metal Borohydrides: From Laboratory Oddities to Prime Candidates in Energy Storage Applications

Despite being the lightest element in the periodic table, hydrogen poses many risks regarding its production, storage, and transport, but it is also the one element promising pollution-free energy for the planet, energy reliability, and sustainability. Development of such novel materials conveying a hydrogen source face stringent scrutiny from both a scientific and a safety point of view: they are required to have a high hydrogen wt.% storage capacity, must store hydrogen in a safe manner (i.e., by chemically binding it), and should exhibit controlled, and preferably rapid, absorption-desorption kinetics. Even the most advanced composites today face the difficult task of overcoming the harsh re-hydrogenation conditions (elevated temperature, high hydrogen pressure). Traditionally, the most utilized materials have been RMH (reactive metal hydrides) and complex metal borohydrides M(BH4)x (M: main group or transition metal; x: valence of M), often along with metal amides or various additives serving as catalysts (Pd2+, Ti4+ etc.). Through destabilization (kinetic or thermodynamic), M(BH4)x can effectively lower their dehydrogenation enthalpy, providing for a faster reaction occurring at a lower temperature onset. The present review summarizes the recent scientific results on various metal borohydrides, aiming to present the current state-of-the-art on such hydrogen storage materials, while trying to analyze the pros and cons of each material regarding its thermodynamic and kinetic behavior in hydrogenation studies.

Toward Opto-Structural Correlation to Investigate Luminescence Thermometry in an Organometallic Eu(II) Complex

Lanthanide-based luminescent materials have unique properties and are well-studied for many potential applications. In particular, the characteristic 5d → 4f emission of divalent lanthanide ions such as Eu^II allows for tunability of the emissive properties via modulation of the coordination environment. We report the synthesis and photoluminescence investigation of pentamethylcyclopentadienyleuropium(II) tetrahydroborate bis(tetrahydrofuran) dimer (1), the first example of an organometallic, discrete molecular Eu^II band-shift luminescence thermometer. Complex 1 exhibits an absolute sensitivity of 8.2 cm^-1 K^-1 at 320 K, the highest thus far observed for a lanthanide-based band-shift thermometer. Opto-structural correlation via variable-temperature single-crystal X-ray diffraction and fluorescence spectroscopy allows rationalization of the remarkable thermometric luminescence of complex 1 and reveals the significant potential of molecular Eu^II compounds in luminescence thermometry.

Metal Borohydrides beyond Groups I and II: A Review

This review consists of a compilation of synthesis methods and several properties of borohydrides beyond Groups I and II, i.e., transition metals, main group, lanthanides, and actinides. The reported properties include crystal structure, decomposition temperature, ionic conductivity, photoluminescence, etc., when available. The compiled properties reflect the rich chemistry and possible borohydrides’ application in areas such as hydrogen storage, electronic devices that require an ionic conductor, catalysis, or photoluminescence. At the end of the review, two short but essential sections are included: a compilation of the decomposition temperature of all reported borohydrides versus the Pauling electronegativity of the cations, and a brief discussion of the possible reactions occurring during diborane emission, including some strategies to reduce this inconvenience, particularly for hydrogen storage purposes.

Synthesis, Crystal Structures and Thermal Properties of Ammine Barium Borohydrides

Ammine metal borohydrides show large compositional and structural diversity, and have been proposed as candidates for solid-state ammonia and hydrogen storage as well as fast cationic conductors. Here, we report the synthesis method of ammine barium borohydrides, Ba(BH4)2·xNH3 (x = 1, 2). The two new compounds were investigated with time-resolved temperature-varied in situ synchrotron radiation powder X-ray diffraction, thermal analysis, infrared spectroscopy and photographic analysis. The compound Ba(BH4)2·2NH3 crystallizes in an orthorhombic unit cell with space group symmetry Pnc2, and is isostructural to Sr(BH4)2·2NH3, forming octahedral [Ba(NH3)2(BH4)4] complexes, which are connected into a two-dimensional layered structure, where the layers are interconnected by dihydrogen bonds, N–Hδ+⋯−δH–B. A new structure type is observed for Ba(BH4)2·NH3, which crystallizes in an orthorhombic unit cell with space group symmetry P212121, forming a three-dimensional framework structure of [Ba(NH3)(BH4)6] complexes. The structure is built from distorted hexagonal chains, where NH3 groups form dihydrogen bonds to the nearby BH4−-groups within the chain. Ba(BH4)2·2NH3 is unstable at room temperature and releases NH3 in two subsequent endothermic reactions with maxima at 49 and 117 °C, eventually reforming Ba(BH4)2. We demonstrate that the thermal stability and composition of the gas release for the ammine alkaline earth metal borohydrides can be correlated to the charge density of the metal cation, but are also influenced by other effects.

A Review of the MSCA ITN ECOSTORE—Novel Complex Metal Hydrides for Efficient and Compact Storage of Renewable Energy as Hydrogen and Electricity

Hydrogen as an energy carrier is very versatile in energy storage applications. Developments in novel, sustainable technologies towards a CO2-free society are needed and the exploration of all-solid-state batteries (ASSBs) as well as solid-state hydrogen storage applications based on metal hydrides can provide solutions for such technologies. However, there are still many technical challenges for both hydrogen storage material and ASSBs related to designing low-cost materials with low-environmental impact. The current materials considered for all-solid-state batteries should have high conductivities for Na+, Mg2+ and Ca2+, while Al3+-based compounds are often marginalised due to the lack of suitable electrode and electrolyte materials. In hydrogen storage materials, the sluggish kinetic behaviour of solid-state hydride materials is one of the key constraints that limit their practical uses. Therefore, it is necessary to overcome the kinetic issues of hydride materials before discussing and considering them on the system level. This review summarizes the achievements of the Marie Skłodowska-Curie Actions (MSCA) innovative training network (ITN) ECOSTORE, the aim of which was the investigation of different aspects of (complex) metal hydride materials. Advances in battery and hydrogen storage materials for the efficient and compact storage of renewable energy production are discussed.

Complex Hydrides for Energy Storage, Conversion, and Utilization

Functional materials are the key enabling factor in the development of clean energy technologies. Materials of particular interest, which are reviewed herein, are a class of hydrogenous compound having the general formula of M(XH_n )_m , where M is usually a metal cation and X can be Al, B, C, N, O, transition metal (TM), or a mixture of them, which sets up an iono-covalent or covalent bonding with H. M(XH_n )_m is generally termed as a complex hydride by the hydrogen storage community. The rich chemistry between H and B/C/N/O/Al/TM allows complex hydrides of diverse composition and electronic configuration, and thus tunable physical and chemical properties, for applications in hydrogen storage, thermal energy storage, ion conduction in electrochemical devices, and catalysis in fuel processing. The recent progress is reviewed here and strategic approaches for the design and optimization of complex hydrides for the abovementioned applications are highlighted.

Accurate Computational Thermodynamics Using Anharmonic Density Functional Theory Calculations: The Case Study of B-H Species

The thermal decomposition of boron-hydrogen compounds is complex and multistep and involves the formation of various intermediates. An accurate description of the thermodynamics of the reactants, products, and intermediates is required for an in-depth understanding of their reactivity. In this respect, we have proceeded to the accurate determination of the key thermodynamic functions (ΔH(T), S(T), and C _P (T)) of 44 isolated B-H molecular species involved in the decomposition of B-H solids, with the inclusion of anharmonic effects. An excellent agreement is observed with available experimental data. We report the analytic expressions of these functions obtained by fitting them with NASA functions in the 200-900 K temperature range. Because the vibrational spectra of these species are their fingerprints, we also report the predicted IR and Raman spectra. The calculated anharmonic spectra show an excellent agreement with experiments and allow for a clear-cut identification of fundamentals, combinations, and overtones.

Trends in Synthesis, Crystal Structure, and Thermal and Magnetic Properties of Rare-Earth Metal Borohydrides

Synthesis, crystal structures, and thermal and magnetic properties of the complete series of halide-free rare-earth (RE) metal borohydrides are presented. A new synthesis method provides high yield and high purity products. Fifteen new metal borohydride structures are reported. The trends in crystal structures, thermal behavior, and magnetic properties for the entire series of RE(BH₄) _x are compared and discussed. The RE(BH₄) _x possess a very rich crystal chemistry, dependent on the oxidation state and the ionic size of the rare-earth ion. Due to the lanthanide contraction, there is a significant decrease in the volume of the RE³⁺-ion with increasing atomic number, which correlates linearly with the unit cell volume of the α- and β-RE(BH₄)₃ polymorphs and the solvated complexes α-RE(BH₄)₃·S(CH₃)₂. The thermal analysis reveals a one-step decomposition pathway in the temperature range from 247 to 277 °C for all RE(BH₄)₃ except Lu(BH₄)₃, which follows a three-step decomposition pathway. In contrast, the RE(BH₄)₂ decompose at higher temperatures in the range 306 to 390 °C due to lower charge density on the rare-earth ion. The RE(BH₄)₃ show increasing stability with increasing Pauling electronegativity, which contradicts other main group and transition metal borohydrides. The majority of the compounds follow Curie-Weiss paramagnetic behavior down to 3 K with weak antiferromagnetic interactions and magnetic moments in accord with those of isolated 4f ions. Some of the RE(BH₄) _x display varying degrees of temperature-dependent magnetic moments due to low-lying excited stated induced by crystal field effects. Additionally, a weak antiferromagnetic ordering is observed in Gd(BH₄)₃, indicating superexchange through a borohydride group.

Theoretical Study of Halogenated B12H nX(12- n)2- (X = F, Cl, Br)

The closoborane and their derivatives have attracted high interest due to their superionic conductivity. Very recently, high ionic conductivities have been reported for compounds containing the closoborane ion B₁₂H₁₂^2-. In this work, we address halogen-substituted ions B₁₂H _nX_{(12- n)}^2- ( n = 0-3, 6, 9-12 and X = F, Cl, Br) using DFT calculations to probe the structures, the chemical stability, and the electrochemical stability, as well as spectroscopic properties in view of potential future applications. Considering the theoretical reaction n/12 B₁₂H₁₂^2- + (12- n)/12 B₁₂X₁₂^2- → B₁₂H _nX_{(12- n)}^2-, it appears that for X = Cl and Br the compounds with n = 6 are stabilized by about 100 kJ/mol. The calculation of the vertical detachment energy (which is indirectly related to the electrochemical stability) shows an increasing stability with increasing halogen content. These results suggest that, for practical applications, it is likely that a partially halogenated ion offers the best compromise. The calculations of vibrational properties and NMR chemical shifts also reveal several systematic trends, which are discussed and compared to available literature values.

From Metal Hydrides to Metal Borohydrides

Commencing from metal hydrides, versatile synthesis, purification, and desolvation approaches are presented for a wide range of metal borohydrides and their solvates. An optimized and generalized synthesis method is provided for 11 different metal borohydrides, M(BH₄) _n, (M = Li, Na, Mg, Ca, Sr, Ba, Y, Nd, Sm, Gd, Yb), providing controlled access to more than 15 different polymorphs and in excess of 20 metal borohydride solvate complexes. Commercially unavailable metal hydrides (MH _n, M = Sr, Ba, Y, Nd, Sm, Gd, Yb) are synthesized utilizing high pressure hydrogenation. For synthesis of metal borohydrides, all hydrides are mechanochemically activated prior to reaction with dimethylsulfide borane. A purification process is devised, alongside a complementary desolvation process for solvate complexes, yielding high purity products. An array of polymorphically pure metal borohydrides are synthesized in this manner, supporting the general applicability of this method. Additionally, new metal borohydrides, α-, α'- β-, γ-Yb(BH₄)₂, α-Nd(BH₄)₃ and new solvates Sr(BH₄)₂·1THF, Sm(BH₄)₂·1THF, Yb(BH₄)₂· xTHF, x = 1 or 2, Nd(BH₄)₃·1Me₂S, Nd(BH₄)₃·1.5THF, Sm(BH₄)₃·1.5THF and Yb(BH₄)₃· xMe₂S (" x" = unspecified), are presented here. Synthesis conditions are optimized individually for each metal, providing insight into reactivity and mechanistic concerns. The reaction follows a nucleophilic addition/hydride-transfer mechanism. Therefore, the reaction is most efficient for ionic and polar-covalent metal hydrides. The presented synthetic approaches are widely applicable, as demonstrated by permitting facile access to a large number of materials and by performing a scale-up synthesis of LiBH₄.

Synthesis, structure, and polymorphic transitions of praseodymium(iii) and neodymium(iii) borohydride, Pr(BH4)3 and Nd(BH4)3

In this work, praseodymium(iii) borohydride, Pr(BH4)3, and an isotopically enriched analogue, Pr(11BD4)3, are prepared by a new route via a solvate complex, Pr(11BD4)3S(CH3)2. Nd(BH4)3 was synthesized using the same method and the structures, polymorphic transformations, and thermal stabilities of these compounds are investigated in detail. α-Pr(BH4)3 and α-Nd(BH4)3 are isostructural with cubic unit cells (Pa3[combining macron]) stable at room temperature (RT) and a unit cell volume per formula unit (V/Z) of 180.1 and 175.8 Å3, respectively. Heating α-Pr(BH4)3 to T ∼ 190 °C, p(Ar) = 1 bar, introduces a transition to a rhombohedral polymorph, r-Pr(BH4)3 (R3[combining macron]c) with a smaller unit cell volume and a denser structure, V/Z = 156.06 Å3. A similar transition was not observed for Nd(BH4)3. However, heat treatment of α-Pr(BH4)3, at T ∼ 190 °C, p(H2) = 40 bar and α-Nd(BH4)3, at T ∼ 270 °C, p(H2) = 98 bar facilitates reversible formation of another three cubic polymorph, denoted as β, β' and β''-RE(BH4)3 (Fm3[combining macron]c). Moreover, the transition β- to β'- to β''- is considered a rare example of stepwise negative thermal expansion. For Pr(BH4)3, ∼2/3 of the sample takes this route of transformation whereas in argon only ∼5 wt%, and the remaining transforms directly from α- to r-Pr(BH4)3. The β-polymorphs are porous with V/Z = 172.4 and 172.7 Å3 for β''-RE(BH4)3, RE = Pr or Nd, respectively, and are stabilized by the elevated hydrogen pressures. The polymorphic transitions occur due to rotation of RE(BH4)6 octahedra without breaking or forming chemical bonds. Structural DFT optimization reveals the decreasing stability of α-Pr(BH4)3 > β-Pr(BH4)3 > r-Pr(BH4)3.

Metal borohydrides and derivatives – synthesis, structure and properties

A comprehensive review of metal borohydrides from synthesis to application.

Synthesis, structure and properties of new bimetallic sodium and potassium lanthanum borohydrides

New compounds, NaLa(BH₄)₄ and K₃La(BH₄)₆, are synthesized. NaLa(BH₄)₄ has a new structure type and has partial reversibility for hydrogen release.