Novel solvates M(BH₄)₃S(CH₃)₂ and properties of halide-free M(BH₄)₃ (M = Y or Gd)

Alkali metal alkoxyborate ester salts; a contemporary look at old compounds

Research into the use of sodium tetraalkoxyborate salts for different chemical applications including synthetic catalysis, hydrogen storage, or battery applications has been investigated, however, understanding of the structural, thermal and electrochemical properties of these salts has been lacking since the 1950s and 1960s. A review of the synthesis, as well as a thorough characterization using 1H NMR, 11B NMR, 13C{1H} NMR, FTIR, XRD, in situ XRD, DSC-TGA, RGA-MS, TPPA, and EIS has newly identified polymorphic phase changes for Na[B(OMe)4], K[B(OMe)4], Li[B(OMe)4], Na[B(OEt)4], Na[B(OBu)4], and Na[B(OiBu)4]. The crystal structure of K[B(OMe)4] was also solved in I41/a (a = 22.337(2) Å, c = 7.648(3) Å, V = 3815.6(4) Å3, ρ = 1.128(1) g cm-3). Ionic conductivity of the different salts was analyzed, however it was found that the compounds with longer alkyl chains had no measurable ionic conductivity compared to the shorter chained samples, Na[B(OMe)4] and K[B(OMe)4] with 9.6 × 10-8 S cm-1 and 1.6 × 10-7 S cm-1, at 114 °C respectively.

Divalent closo-monocarborane solvates for solid-state ionic conductors

Li-ion batteries have held the dominant position in battery research for the last 30+ years. However, due to inadequate resources and the cost of necessary elements (e.g., lithium ore) in addition to safety issues concerning the components and construction, it has become more important to look at alternative technologies. Multivalent metal batteries with solid-state electrolytes are a potential option for future battery applications. The synthesis and characterisation of divalent hydrated closo-monocarborane salts - Mg[CB₁₁H₁₂]₂·xH₂O, Ca[CB₁₁H₁₂]₂·xH₂O, and Zn[CB₁₁H₁₂]₂·xH₂O - have shown potential as solid-state electrolytes. The coordination of a solvent (e.g. H₂O) to the cation in these complexes shows a significant improvement in ionic conductivity, i.e. for Zn[CB₁₁H₁₂]₂·xH₂O dried at 100 °C (10^-3 S cm^-1 at 170 °C) and dried at 150 °C (10^-5 S cm^-1 at 170 °C). Solvent choice also proved important with the ionic conductivity of Mg[CB₁₁H₁₂]₂·3en (en = ethylenediamine) being higher than that of Mg[CB₁₁H₁₂]₂·3.1H₂O (2.6 × 10^-5 S cm^-1 and 1.7 × 10^-8 S cm^-1 at 100 °C, respectively), however, the oxidative stability was lower (<1 V (Mg²⁺/Mg) and 1.9 V (Mg²⁺/Mg), respectively). Thermal characterisation of the divalent closo-monocarborane salts showed melting and desolvation, prior to high temperature 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.

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, Polymorphism and Thermal Decomposition Process of (n-C4H9)4NRE(BH4)4 for RE = Ho, Tm and Yb

In total, three novel organic derivatives of lanthanide borohydrides, n-But₄NRE(BH₄)₄ (TBAREB), RE = Ho, Tm, Yb, have been prepared utilizing mechanochemical synthesis and purified via solvent extraction. Studies by single crystal and powder X-ray diffraction (SC-XRD and PXRD) revealed that they crystalize in two polymorphic forms, α- and β-TBAREB, adopting monoclinic (P2₁/c) and orthorhombic (Pnna) unit cells, previously found in TBAYB and TBAScB, respectively. Thermal decomposition of these compounds has been investigated using thermogravimetric analysis and differential scanning calorimetry (TGA/DSC) measurements, along with the analysis of the gaseous products with mass spectrometry (MS) and with analysis of the solid decomposition products with PXRD. TBAHoB and TBAYbB melt around 75 °C, which renders them new ionic liquids with relatively low melting points among borohydrides.

Structural Diversity and Trends in Properties of an Array of Hydrogen-Rich Ammonium Metal Borohydrides

Metal borohydrides are a fascinating and continuously expanding class of materials, showing promising applications within many different fields of research. This study presents 17 derivatives of the hydrogen-rich ammonium borohydride, NH₄BH₄, which all exhibit high gravimetric hydrogen densities (>9.2 wt % of H₂). A detailed insight into the crystal structures combining X-ray diffraction and density functional theory calculations exposes an intriguing structural variety ranging from three-dimensional (3D) frameworks, 2D-layered, and 1D-chainlike structures to structures built from isolated complex anions, in all cases containing NH₄⁺ countercations. Dihydrogen interactions between complex NH₄⁺ and BH₄^- ions contribute to the structural diversity and flexibility, while inducing an inherent instability facilitating hydrogen release. The thermal stability of the ammonium metal borohydrides, as a function of a range of structural properties, is analyzed in detail. The Pauling electronegativity of the metal, the structural dimensionality, the dihydrogen bond length, the relative amount of NH₄⁺ to BH₄^-, and the nearest coordination sphere of NH₄⁺ are among the most important factors. Hydrogen release usually occurs in three steps, involving new intermediate compounds, observed as crystalline, polymeric, and amorphous materials. This research provides new opportunities for the design and tailoring of novel functional materials with interesting properties.

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.

Mechanochemistry of Metal Hydrides: Recent Advances

This paper is a collection of selected contributions of the 1st International Workshop on Mechanochemistry of Metal Hydrides that was held in Oslo in May 2018. In this paper, the recent developments in the use of mechanochemistry to synthesize and modify metal hydrides are reviewed. A special emphasis is made on new techniques beside the traditional way of ball milling. High energy milling, ball milling under hydrogen reactive gas, cryomilling and severe plastic deformation techniques such as High-Pressure Torsion (HPT), Surface Mechanical Attrition Treatment (SMAT) and cold rolling are discussed. The new characterization method of in-situ X-ray diffraction during milling is described.

Two new derivatives of scandium borohydride, MSc(BH4)4, M = Rb, Cs, prepared via a one-pot solvent-mediated method

Two new derivatives of scandium borohydride, MSc(BH₄)₄, M = Rb, Cs, were prepared via two different synthetic methodologies - mechanochemical and solvent-mediated. The latter led to products free from the commonly present halide contamination, as evidenced by powder X-ray diffraction, FTIR spectroscopy and TGA/DSC/MS. The rubidium derivative crystallizes in an orthorhombic unit cell of the Pbcm space group in the structure which can be derived from ht-CrVO₄, while CsSc(BH₄)₄ adopts a monoclinic (P2₁/c) unit cell which has monazite (CePO₄) as a structural aristotype. Thermal decomposition of the samples obtained using the two methods was compared, evidencing the influence of lithium chloride on the decomposition reactions as well as chemical identity of the decomposition products. Uncontaminated MSc(BH₄)₄ salts decompose thermally yielding nearly pure hydrogen with the maximum decomposition rate at 230 °C and 235 °C, for M = Rb and Cs, respectively. Among the by-products of the solvent-mediated synthesis, a new cubic crystalline phase of M₃ScCl₆, M = Rb, Cs, has been detected.

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.

Decomposition pathway of KAlH4 altered by the addition of Al2S3

Altering the decomposition pathway of potassium alanate, KAlH4, with aluminium sulfide, Al2S3, presents a new opportunity to release all of the hydrogen, increase the volumetric hydrogen capacity and avoid complications associated with the formation of KH and molten K. Decomposition of 6KAlH4-Al2S3 during heating under dynamic vacuum began at 185 °C, 65 °C lower than for pure KAlH4, and released 71% of the theoretical hydrogen content below 300 °C via several unknown compounds. The major hydrogen release event, centred at 276 °C, was associated with two new compounds indexed with monoclinic (a = 10.505, b = 7.492, c = 11.772 Å, β = 122.88°) and hexagonal (a = 10.079, c = 7.429 Å) unit cells, respectively. Unlike the 6NaAlH4-Al2S3 system, the 6KAlH4-Al2S3 system did not have M3AlH6 (M = alkali metal) as one of the intermediate decomposition products nor were the final products M2S and Al observed. Decomposition performed under hydrogen pressure initially followed a similar reaction pathway to that observed during heating under vacuum but resulted in partial melting of the sample between 300 and 350 °C. The measured enthalpy of hydrogen absorption (ΔHabs) was in the range -44.5 to -51.1 kJ mol-1 H2, which is favourable for moderate temperature hydrogen applications. Although, the hydrogen capacity decreases during consecutive H2 release and uptake cycles, the presence of excess amounts of aluminium allow for further optimisation of hydrogen storage properties.

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.

Synthesis, structure and properties of bimetallic sodium rare-earth (RE) borohydrides, NaRE(BH4)4, RE = Ce, Pr, Er or Gd

Formation, stability and properties of new metal borohydrides within RE(BH₄)₃-NaBH₄, RE = Ce, Pr, Er or Gd is investigated. Three new bimetallic sodium rare-earth borohydrides, NaCe(BH₄)₄, NaPr(BH₄)₄ and NaEr(BH₄)₄ are formed based on an addition reaction between NaBH₄ and halide free rare-earth metal borohydrides RE(BH₄)₃, RE = Ce, Pr, Er. All the new compounds crystallize in the orthorhombic crystal system. NaCe(BH₄)₄ has unit cell parameters of a = 6.8028(5), b = 17.5181(13), c = 7.2841(5) Å and space group Pbcn. NaPr(BH₄)₄ is isostructural to NaCe(BH₄)₄ with unit cell parameters of a = 6.7617(2), b = 17.4678(7), c = 7.2522(3) Å. NaEr(BH₄)₄ crystallizes in space group Cmcm with unit cell parameters of a = 8.5379(2), b = 12.1570(4), c = 9.1652(3) Å. The structural relationships, also to the known RE(BH₄)₃, are discussed in detail and related to the stability and synthesis conditions. Heat treatment of NaBH₄-Gd(BH₄)₃ mixture forms an unstable amorphous phase, which decomposes after one day at RT. NaCe(BH₄)₄ and NaPr(BH₄)₄ show reversible hydrogen storage capacity of 1.65 and 1.04 wt% in the fourth H₂ release, whereas that of NaEr(BH₄)₄ continuously decreases. This is mainly assigned to formation of metal hydrides and possibly slower formation of sodium borohydride. The dehydrogenated state clearly contains rare-earth metal borides, which stabilize boron in the dehydrogenated state.

Hydrogenation properties of lithium and sodium hydride – closo-borate, [B10H10]2− and [B12H12]2−, composites

The hydrogen absorption properties of metal closo-borate/metal hydride composites are studied under high hydrogen pressures.

Metal borohydrides and derivatives – synthesis, structure and properties

A comprehensive review of metal borohydrides from synthesis to application.

The influence of LiH on the rehydrogenation behavior of halide free rare earth (RE) borohydrides (RE = Pr, Er)

Rare earth (RE) metal borohydrides are receiving immense consideration as possible hydrogen storage materials and solid-state Li-ion conductors. In this study, halide free Er(BH4)3 and Pr(BH4)3 have been successfully synthesized for the first time by the combination of mechanochemical milling and/or wet chemistry. Rietveld refinement of Er(BH4)3 confirmed the formation of two different Er(BH4)3 polymorphs: α-Er(BH4)3 with space group Pa3[combining macron], a = 10.76796(5) Å, and β-Er(BH4)3 in Pm3[combining macron]m with a = 5.4664(1) Å. A variety of Pr(BH4)3 phases were found after extraction with diethyl ether: α-Pr(BH4)3 in Pa3[combining macron] with a = 11.2465(1) Å, β-Pr(BH4)3 in Pm3[combining macron]m with a = 5.716(2) Å and LiPr(BH4)3Cl in I4[combining macron]3m, a = 11.5468(3) Å. Almost phase pure α-Pr(BH4)3 in Pa3[combining macron] with a = 11.2473(2) Å was also synthesized. The thermal decomposition of Er(BH4)3 and Pr(BH4)3 proceeded without the formation of crystalline products. Rehydrogenation, as such, was not successful. However, addition of LiH promoted the rehydrogenation of RE hydride phases and LiBH4 from the decomposed RE(BH4)3 samples.

From M(BH4)3 (M = La, Ce) Borohydride Frameworks to Controllable Synthesis of Porous Hydrides and Ion Conductors

Rare earth metal borohydrides show a number of interesting properties, e.g., Li ion conductivity and luminescence, and the series of materials is well explored. However, previous attempts to obtain M(BH₄)₃ (M = La, Ce) by reacting MCl₃ and LiBH₄ yielded LiM(BH₄)₃Cl. Here, a synthetic approach is presented, which allows the isolation of M(BH₄)₃ (M = La, Ce) via formation of intermediate complexes with dimethyl sulfide. The cubic c-Ce(BH₄)₃ (Fm3̅c) is isostructural to high-temperature polymorphs of A(BH₄)₃ (A = Y, Sm, Er, Yb) borohydrides. The larger size of the Ce³⁺ ion makes the empty void in the open ReO₃-type framework structure potentially accessible to small guest molecules like H₂. Another new rhombohedral polymorph, r-M(BH₄)₃ (M = La, Ce), is a closed form of the framework, prone to stacking faults. The new compounds M(BH₄)₃ (M = La, Ce) can be combined with LiCl in an addition reaction to form LiM(BH₄)₃Cl also known as Li₄[M₄(BH₄)₁₂Cl₄]; the latter contains the unique tetranuclear cluster [M₄(BH₄)₁₂Cl₄]^4- and shows high Li-ion conductivity. This reaction pathway opens a way to synthesize a series of A₄[M₄(BH₄)₁₂X₄] (M = La, Ce) compounds with different anions (X) and metal ions (A) and potentially high ion conductivity.

Organic derivatives of Mg(BH4)2 as precursors towards MgB2 and novel inorganic mixed-cation borohydrides

A series of organic derivatives of magnesium borohydride, including Mg(BH4)2·1.5DME (DME = 1,2-dimethoxyethane) and Mg(BH4)2·3THF (THF = tetrahydrofuran) solvates and three mixed-cation borohydrides, [Cat]2[Mg(BH4)4], [Cat] = [Me4N], [nBu4N], [Ph4P], have been characterized. The phosphonium derivative has been tested as a precursor for synthesis of inorganic mixed-metal borohydrides of magnesium, Mx[Mg(BH4)2+x], M = Li-Cs, via a metathetic method. The synthetic procedure has yielded two new derivatives of heavier alkali metals M3Mg(BH4)5 (M = Rb, Cs) mixed with amorphous Mg(BH4)2. Thermal decomposition has been studied for both the organic and inorganic magnesium borohydride derivatives. Amorphous MgB2 has been detected among the products of the thermal decomposition of the solvates studied, together with organic and inorganic impurities.

Salts of highly fluorinated weakly coordinating anions as versatile precursors towards hydrogen storage materials

We report the most recent results related to application of a metathetic pathway towards mixed-metal borohydrides. The synthetic protocol utilizes highly-fluorinated weakly coordinating anion salts as precursors. We discuss the technicalities related to the use of fluorine-rich anions as well as the improvements which are still needed to deliver high-purity materials with potential applications for hydrogen storage. The applicability of the method is expanded beyond the previously described complex borohydrides of alkali metal Zn or Y, towards the systems containing Mg(II), Sc(III), Mn(II), or Eu(III). We have prepared for the first time [Ph4P]2[Mn(BH4)4] and [Me4N]2[Mg(BH4)4], solved their crystal structures from powder x-ray diffraction, and used selected organic metal borohydride derivatives as precursors towards mixed-metal borohydrides (K2Mn(BH4)4, Rb3Mg(BH4)5, etc.). We have also prepared [Ph4P][Eu(BH4)4], which is the first derivative of Eu(III) in the homoleptic environment of borohydride anions.

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.

Ammine Calcium and Strontium Borohydrides: Syntheses, Structures, and Properties

A new series of solvent- and halide-free ammine strontium metal borohydrides Sr(NH3 )n (BH4 )2 (n=1, 2, and 4) and further investigations of Ca(NH3 )n (BH4 )2 (n=1, 2, 4, and 6) are presented. Crystal structures have been determined by powder XRD and optimized by DFT calculations to evaluate the strength of the dihydrogen bonds. Sr(NH3 )(BH4 )2 (Pbcn) and Sr(NH3 )2 (BH4 )2 (Pnc2) are layered structures, whereas M(NH3 )4 (BH4 )2 (M=Ca and Sr; P21 /c) are molecular structures connected by dihydrogen bonds. Both series of compounds release NH3 gas upon thermal treatment if the partial pressure of ammonia is low. Therefore, the strength of the dihydrogen bonds, the structure of the compounds, and the NH3 /BH4 (-) ratio for M(NH3 )n (BH4 )m have little influence on the composition of the released gasses. The composition of the released gas depends mainly on the thermal stability of the ammine metal borohydride and the corresponding metal borohydride.

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.

Manganese borohydride; synthesis and characterization

Three manganese borohydride polymorphs are synthesized in solution and found to be structural analogues of three magnesium borohydride polymorphs.

Flux-assisted single crystal growth and heteroepitaxy of perovskite-type mixed-metal borohydrides

Single crystals of mixed-metal perovskite-type borohydride KCa(BH₄)₃ are prepared by using an easily generalized flux melting procedure based on eutectic borohydride systems.

Structure and properties of complex hydride perovskite materials

Perovskite materials host an incredible variety of functionalities. Although the lightest element, hydrogen, is rarely encountered in oxide perovskite lattices, it was recently observed as the hydride anion H(-), substituting for the oxide anion in BaTiO3. Here we present a series of 30 new complex hydride perovskite-type materials, based on the non-spherical tetrahydroborate anion BH4(-) and new synthesis protocols involving rare-earth elements. Photophysical, electronic and hydrogen storage properties are discussed, along with counterintuitive trends in structural behaviour. The electronic structure is investigated theoretically with density functional theory solid-state calculations. BH4-specific anion dynamics are introduced to perovskites, mediating mechanisms that freeze lattice instabilities and generate supercells of up to 16 × the unit cell volume in AB(BH4)3. In this view, homopolar hydridic di-hydrogen contacts arise as a potential tool with which to tailor crystal symmetries, thus merging concepts of molecular chemistry with ceramic-like host lattices. Furthermore, anion mixing BH4(-)←X(-) (X(-)=Cl(-), Br(-), I(-)) provides a link to the known ABX3 halides.

Hydrogen storage materials: room-temperature wet-chemistry approach toward mixed-metal borohydrides

The poor kinetics of hydrogen evolution and the irreversibility of the hydrogen discharge hamper the use of transition metal borohydrides as hydrogen storage materials, and the drawbacks of current synthetic methods obstruct the exploration of these systems. A wet-chemistry approach, which is based on solvent-mediated metathesis reactions of precursors containing bulky organic cations and weakly coordinating anions, leads to mixed-metal borohydrides that contain only a small amount of "dead mass". The applicability of this method is exemplified by Li[Zn2(BH4)5] and M[Zn(BH4)3] salts (M=Na, K), and its extension to other systems is discussed.