The Effectiveness of Silver and Gold in Catalytic Homogenous and Heterogenous Borohydride Hydrolysis - a DFT Study

Energy demands, and environmental aspects raised the need to study hydrogen-carrying material such as borohydride for the practical usage of hydrogen as a cleaner and more efficient fuel. A proper understanding of the hydrogen generation mechanism is a key requirement for the designing of efficient catalysts, as the non-catalytic hydrolysis of borohydride in non-acidic media is a slow process. The hydrolysis mechanism of borohydride varies considerably using homogeneous and heterogeneous catalysts. A comparison of the hydrolysis mechanism of borohydride using gold and silver as homogenous and heterogeneous catalysts is given in this review. Unexpectedly, with gold catalyst, Au+ or Au(111), only two steps of hydrolysis occur and BH(OH)2 is produced, while with silver catalyst, Ag+ or Ag(111), the hydrolysis can proceed to completion.

Reorientational Dynamics in Y(BH4)3·xNH3 (x = 0, 3, and 7): The Impact of NH3 on BH4- Dynamics

The reorientational dynamics of Y(BH4)3·xNH3 (x = 0, 3, and 7) was studied using quasielastic neutron scattering (QENS) and neutron spin echo (NSE). The results showed that changing the number of NH3 ligands drastically alters the reorientational mobility of the BH4- anion. From the QENS experiments, it was determined that the BH4- anion performs 2-fold reorientations around the C2 axis in Y(BH4)3, 3-fold reorientations around the C3 axis in Y(BH4)3·3NH3, and either 2-fold reorientations around the C2 axis or 3-fold reorientations around the C3 axis in Y(BH4)3·7NH3. The relaxation time of the BH4- anion at 300 K decreases from 2 × 10-7 s for x = 0 to 1 × 10-12 s for x = 3 and to 7 × 10-13 s for x = 7. In addition to the reorientational dynamics of the BH4- anion, it was shown that the NH3 ligands exhibit 3-fold reorientations around the C3 axis in Y(BH4)3·3NH3 and Y(BH4)3·7NH3 as well as 3-fold quantum mechanical rotational tunneling around the same axis at 5 K. The new insights constitute a significant step toward understanding the relationship between the addition of ligands and the enhanced ionic conductivity observed in systems such as LiBH4·xNH3 and Mg(BH4)2·xCH3NH2.

Potassium Decahydrido-closo-Decaborane Urea Complex as a Potential Solid-State Electrolyte for Potassium Metal Batteries

All-solid-state potassium metal batteries have been considered promising candidates for large-scale energy storage because of abundance and wide availability of K resources, elimination of flammable liquid organic electrolytes, and incorporation of high-capacity K metal anode. However, unideal K-ion conductivities of most reported K-ion solid electrolytes have restricted the development of these batteries. Herein, a novel K2B10H10·CO(NH2)2 complex is reported, forming by incorporating urea into K2B10H10, to achieve an enhanced K-ion conductivity. The crystal structure of K2B10H10·CO(NH2)2 was determined as a monoclinic lattice with the space group of C2/c (No. 15). K2B10H10·CO(NH2)2 delivers an ionic conductivity of 2.7 × 10-8 S cm-1 at 25 °C, and reaching 1.3 × 10-4 S cm-1 at 80 °C, which is about 4 orders of magnitude higher than that of K2B10H10. One possible reason is the anion expansion in size due to the presence of dihydrogen bonds in K2B10H10·CO(NH2)2, resulting in an increase in the K-H bond distance and the electrostatic potential, thereby enhancing the mobility of K+. The K-ion conductivity is also higher than those of most hydridoborate-based K-ion conductors reported. Besides, K2B10H10·CO(NH2)2 reveals a wide electrochemical stability window and remarkable interface compatibility with K metal electrodes, suggesting a promising electrolyte for all-solid-state K metal batteries.

Theoretical prediction of nanosizing effects and role of additives in the decomposition of Mg(BH4)2

The energetic transition towards renewable resources is one of the biggest challenges of this century. In this context, the role of H2 is of paramount importance as a key source of energy that could substitute traditional fossil fuels. This technology, even if available in several manufactures, still needs to be optimized at all levels (production, storage and distribution) to be integrated on a larger scale. Among materials suitable to store H2, Mg(BH4)2 is particularly interesting due to its high content of H2 in terms of gravimetric density. Nanosizing effects and role of additives in the decomposition of Mg(BH4)2 were studied by density functional theory (DFT) modelling. Both effects were analyzed because of their contribution in promoting the decomposition of the material. In particular, to have a quantitative idea of nanosizing effects, we used thin film 2D models corresponding to different crystallographic surfaces and referred to the following reaction: Mg(BH4)2 → MgB2 + 4H2. When moving from bulk to nanoscale (2D models), a remarkable decrease in the decomposition energy (10-20 kJ mol-1) was predicted depending on the surface and the thin film thickness considered. As regards the role of additives (Ni and Cu), we based our analysis on their effect in perturbing neighboring borohydride groups. We found a clear elongation of some B-H bonds, in particular with the NiF2 additive (about 0.1 Å). We interpreted this behavior as an indicator of the propensity of borohydride towards dissociation. On the basis of this evidence, we also explored a possible reaction pathway of NiF2 and CuF2 on Mg(BH4)2 up to H2 release and pointed out the major catalytic effect of Ni compared to Cu.

Producing Alkali Metal Hydrides from Hydroxides

In this study, a novel method for producing different alkali metal hydrides (NaH, KH, RbH, and CsH) from their corresponding metal hydroxides (NaOH, KOH, RbOH, and CsOH) is presented. For the production of NaH from NaOH, a variety of metallic reducing agents (Mg, Al, Si, CaH2, Cr, Mn, and Sr) were investigated. The reactions took place in an autoclave reactor with paraffin oil at 250 °C and 14 bar of H2 pressure. Splitting the process into two steps (metal formation and hydrogenation) simplified the separation and purification for the produced metal hydride. Moreover, the study explores the potential for this method of NaH production to be used for NaBH4 production and regeneration for hydrogen export applications. This approach offers an alternative, cost-effective method for producing NaH.

An improved method for the synthesis and formation mechanism of M2B10H14 based on the reactions of B10H14 with MNH2BH3 (M = Na, K)

An efficient method for the synthesis of M2B10H14 (M = Na and K) has been developed. The two possible formation mechanisms of the B10H142- anion are proposed, in which the NH2BH3- anion acts as a proton abstractor and a hydride donor. Furthermore, the B10H13- and B10H15- intermediates were detected.

Three-Center M-H-B Bonds Are Strong Field Interactions. Synthesis and Characterization of M(CH2NMe2BH3)3 Complexes of Titanium, Chromium, and Cobalt

We describe new compounds of stoichiometry M(CH2NMe2BH3)3 (M = Ti, Cr, and Co), each of which contains three chelating boranatodimethylaminomethyl (BDAM) ligands. In all three compounds, the BDAM anion, which is isoelectronic and isostructural with the neopentyl group, is bound to the metal center at one end by a metal-carbon σ bond and at the other by one three-center M-H-B interaction. The crystal structures show that the d1 titanium(III) compound is trigonal prismatic (or eight-coordinate, if two longer-ranged M···H interactions with the BH3 groups are included), whereas the d3 chromium(III) compound and the d6 cobalt(III) compounds are both fac-octahedral. The Cr and Co compounds exhibit two rapid dynamic processes in solution: exchange between the Δ and Λ enantiomers and exchange of the terminal and bridging hydrogen atoms on boron. For the Co complex, the barrier for Δ/Λ exchange (ΔG⧧298 = 10.1 kcal mol-1) is significantly smaller than those seen in other octahedral cobalt(III) compounds; DFT calculations suggest that Bailar twist and dissociative pathways for Δ/Λ exchange are both possible mechanisms. The UV-vis absorption spectra of the cobalt(III) and chromium(III) species show that the ligand field splittings Δo caused by the M-H-B interactions are unexpectedly large, thus placing them high on the spectrochemical series (near ammonia and alkyl groups); their nephelauxetic effect is also large. The DFT calculations suggest that these properties of M-H-B interactions are in part a consequence of their three-center nature, which delocalizes electron density away from the metal center and reduces electron-electron repulsions.

The Preparation, Characterization, and Pressure-Influenced Dihydrogen Interactions of Tetramethylphosphonium Borohydride

Tetramethylphosphonium borohydride was synthesized via an ion metathesis reaction in a weakly-coordinating aprotic environment. [(CH3)4P]BH4, in contrast to related [(CH3)4N]+ compounds which tend to crystallize in a tetragonal system, adopts the distorted wurtzite structure (P63mc), resembling some salts containing analogous ions of As and Sb. [(CH3)4P]BH4 decomposes thermally in several endo- and exothermic steps above ca. 240 °C. This renders it more stable than [(CH3)4N]BH4, with a lowered temperature of decomposition onset by ca. 20 °C and solely exothermic processes observed. Raman spectra measured at the 0-10 GPa range indicate that a polymorphic transition occurs within 0.53-1.86 GPa, which is further confirmed by the periodic DFT calculations. The latter suggests a phase transition around 0.8 GPa to a high-pressure phase of [(CH3)4N]BH4. The P63mc phase seems to be destabilized under high pressure by relatively closer dihydrogen interactions, including the C-H…H-C contacts.

Structural insight into the magnesium borohydride - ethylenediamine solid-state Mg-ion electrolyte system

A highly complex crystal structure of stoichiometric Mg₅(en)₆(BH₄)₁₀ was solved from single crystal synchrotron X-ray diffraction and confirmed by neutron powder diffraction (NPD) on isotopically substituted Mg(en)_1.2(¹¹BD₄)₂. We highlight the role of the amorphous Mg(BH₄)₂ in the reactivity of the Mg(BH₄)₂-en system and characterized a previously overlooked phase, Mg(en)₂(BH₄)₂.

High Yield Autoclave Synthesis of pure M2B12H12 (M = Na, K)

Metal dodecaborates (M_xB₁₂H₁₂) are a versatile class of materials used in polymer chemistry and cancer treatment and are promising candidates as electrolytes for solid-state batteries. However, a general and scalable approach has not yet been developed for producing high-purity B₁₂H₁₂^2- derivatives. In this work, we report a simple, efficient, and environmentally benign solvothermal method to prepare diffraction and ¹¹B NMR pure Na₂B₁₂H₁₂ (85% yield) and K₂B₁₂H₁₂ (84% yield). This new synthetic approach is based on the use of the borane dimethyl sulfide complex (DMS·BH₃) and borohydrides (NaBH₄, KBH₄) heated at different temperatures in diglyme in an autoclave. It was found that high-purity Na₂B₁₂H₁₂·diglyme solvate is obtained via an intermediate formation of B₃H₈^-, B₉H₁₄^-, and B₁₁H₁₄^-, which are all soluble in diglyme. Heating under vacuum is shown to be efficient for removing the coordinated diglyme, allowing the formation of unsolvated Na₂B₁₂H₁₂. Autoclave synthesis starting from KBH₄ directly yields solvent-free K₂B₁₂H₁₂, and ball-milling KBH₄ prior to the synthesis enabling us to significantly improve the final yield. The new synthetic method paves the way for large-scale synthesis of M_xB₁₂H₁₂ derivatives, enabling to envisage a wider scope of practical applications.

Germanium hydrides as an efficient hydrogen-storage material operated by an iron catalyst

The use of metal hydrides such as NaBH₄ as hydrogen-storage materials has recently received substantial research attention on account of the worldwide demand for the development of efficient hydrogen-production, -storage, and -transportation systems. Here, we report the quantitative production of H₂ gas from a germanium hydride, Ph₂GeH₂, mediated by an iron catalyst at room temperature via dehydrogenative coupling, concomitant with the formation of (GePh₂)₅. Of particular importance is that Ph₂GeH₂ can be facilely recovered from (GePh₂)₅ by contact with 1 atm of H₂ or PhICl₂/LiAlH₄ at 0 °C or 40 °C, respectively. A detailed reaction mechanism for the iron-catalyzed dehydrogenative coupling of Ph₂GeH₂ is proposed based on the isolation of four intermediate iron species.

Paving the Way to the Fuel of the Future-Nanostructured Complex Hydrides

Hydrides have emerged as strong candidates for energy storage applications and their study has attracted wide interest in both the academic and industry sectors. With clear advantages due to the solid-state storage of hydrogen, hydrides and in particular complex hydrides have the ability to tackle environmental pollution by offering the alternative of a clean energy source: hydrogen. However, several drawbacks have detracted this material from going mainstream, and some of these shortcomings have been addressed by nanostructuring/nanoconfinement strategies. With the enhancement of thermodynamic and/or kinetic behavior, nanosized complex hydrides (borohydrides and alanates) have recently conquered new estate in the hydrogen storage field. The current review aims to present the most recent results, many of which illustrate the feasibility of using complex hydrides for the generation of molecular hydrogen in conditions suitable for vehicular and stationary applications. Nanostructuring strategies, either in the pristine or nanoconfined state, coupled with a proper catalyst and the choice of host material can potentially yield a robust nanocomposite to reliably produce H₂ in a reversible manner. The key element to tackle for current and future research efforts remains the reproducible means to store H₂, which will build up towards a viable hydrogen economy goal. The most recent trends and future prospects will be presented herein.

Na2B11H13 and Na11(B11H14)3(B11H13)4 as potential solid-state electrolytes for Na-ion batteries

Solid-state sodium batteries have attracted great attention owing to their improved safety, high energy density, large abundance and low cost of sodium compared to the current Li-ion batteries. Sodium-boranes have been studied as potential solid-state electrolytes and the search for new materials is necessary for future battery applications. Here, a facile and cost-effective solution-based synthesis of Na₂B₁₁H₁₃ and Na₁₁(B₁₁H₁₄)₃(B₁₁H₁₃)₄ is demonstrated. Na₂B₁₁H₁₃ presents an ionic conductivity in the order of 10^-7 S cm^-1 at 30 °C, but undergoes an order-disorder phase transition and reaches 10^-3 S cm^-1 at 100 °C, close to that of liquids and the solid-state electrolyte Na-β-Al₂O₃. The formation of a mixed-anion solid-solution, Na₁₁(B₁₁H₁₄)₃(B₁₁H₁₃)₄, partially stabilises the high temperature structural polymorph observed for Na₂B₁₁H₁₃ at room temperature and it exhibits Na⁺ conductivity higher than its constituents (4.7 × 10^-5 S cm^-1 at 30 °C). Na₂B₁₁H₁₃ and Na₁₁(B₁₁H₁₄)₃(B₁₁H₁₃)₄ exhibit an oxidative stability limit of 2.1 V vs. Na⁺/Na.

Fast magnesium ion conducting isopropylamine magnesium borohydride enhanced by hydrophobic interactions

New materials for the next generation of electrochemical energy storage devices such as batteries are of extreme importance. Here we investigate the structure, ionic conductivity and thermal properties of isopropylamine magnesium borohydride based composites with different compositions, Mg(BH₄)₂·x(CH₃)₂CHNH₂, x = 0.5, 0.9, 1.25, 1.5, 1.75, 2.5, 3.1. Three new compounds are discovered, x = 1, 2, and 3 and the monoclinic structure of Mg(BH₄)₂·2(CH₃)₂CHNH₂ (P2₁/c) is investigated in detail. This structure consists of neutral complexes [Mg(BH₄)₂((CH₃)₂CHNH₂)₂] di-hydrogen bonded to form layers and these layers are connected by hydrophobic interactions via the isopropyl moieties. The orthorhombic unit cell of Mg(BH₄)₂·(CH₃)₂CHNH₂ was also determined, a = 9.78, b = 12.17 and c = 17.24 Å. In general, the samples are thermally stable up to 50 °C where they started to become softer, and at 70 °C isopropylamine release and melting started. The highest Mg²⁺ ionic conductivity was that of Mg(BH₄)₂·1.5(CH₃)₂CHNH₂, σ(Mg²⁺) = 2.7 × 10^-4 S cm^-1 at 45 °C, with an activation energy of E_A = 1.22 eV. Furthermore, reversible stripping/plating of Mg was displayed at 45 °C, with an oxidative stability of 1.2 V vs. Mg/Mg²⁺. The addition of MgO nanoparticles (75 wt%) improves the mechanical and thermal stability, and decreases the activation energy, to E_A = 0.56 eV. Thereby the Mg²⁺ conductivity is increased at low temperature. This suggests that the hydrophobic interactions contribute to the high ionic conductivity in the solid state, which opens a new avenue for design and discovery of electrolyte materials.

Understanding Hydrogenation Chemistry at MgB2 Reactive Edges from Ab Initio Molecular Dynamics

Solid-state hydrogen storage materials often operate via transient, multistep chemical reactions at complex interfaces that are difficult to capture. Here, we use direct ab initio molecular dynamics simulations at accelerated temperatures and hydrogen pressures to probe the hydrogenation chemistry of the candidate material MgB₂ without a priori assumption of reaction pathways. Focusing on highly reactive (101̅0) edge planes where initial hydrogen attack is likely to occur, we track mechanistic steps toward the formation of hydrogen-saturated BH₄^- units and key chemical intermediates, involving H₂ dissociation, generation of functionalities and molecular complexes containing BH₂ and BH₃ motifs, and B-B bond breaking. The genesis of higher-order boron clustering is also observed. Different charge states and chemical environments at the B-rich and Mg-rich edge planes are found to produce different chemical pathways and preferred speciation, with implications for overall hydrogenation kinetics. The reaction processes rely on B-H bond polarization and fluctuations between ionic and covalent character, which are critically enabled by the presence of Mg²⁺ cations in the nearby interphase region. Our results provide guidance for devising kinetic improvement strategies for MgB₂-based hydrogen storage materials, while also providing a template for exploring chemical pathways in other solid-state energy storage reactions.

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

The number of rare earth (RE) starting materials used in synthesis is staggering, ranging from simple binary metal-halide salts to borohydrides and "designer reagents" such as alkyl and organoaluminate complexes. This review collates the most important starting materials used in RE synthetic chemistry, including essential information on their preparations and uses in modern synthetic methodologies. The review is divided by starting material category and supporting ligands (i.e., metals as synthetic precursors, halides, borohydrides, nitrogen donors, oxygen donors, triflates, and organometallic reagents), and in each section relevant synthetic methodologies and applications are discussed.

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.

Fast Room-Temperature Mg2+ Conductivity in Mg(BH4)2·1.6NH3-Al2O3 Nanocomposites

Design of new functional materials with fast Mg-ion mobility is crucial for the development of competitive solid-state magnesium batteries. Herein, we present new nanocomposites, Mg(BH₄)₂·1.6NH₃-Al₂O₃, reaching a high magnesium conductivity of σ(Mg²⁺) = 2.5 × 10^-5 S cm^-1 at 22 °C assigned to favorable interfaces between amorphous state Mg(BH₄)₂·1.6NH₃; inert and insulating Al₂O₃ nanoparticles; and a minor fraction of crystalline material, mainly Mg(BH₄)₂·2NH₃. Furthermore, quasi-elastic neutron scattering reveals that the Mg²⁺-ion mobility in the solid state appears to be correlated to relatively slow motion of NH₃ molecules rather than the fast dynamics of BH₄^- complexes. The nanocomposite is compatible with a metallic Mg anode and shows stable Mg²⁺ stripping/plating in a symmetric cell and an electrochemical stability of ∼1.2 V. The nanocomposite has high mechanical stability and ductility and is a promising Mg²⁺ electrolyte for future solid-state magnesium batteries.

Molecular Adsorption of H2 on Small Neutral Silver-Copper Bimetallic Nanoparticles: A Search for Novel Hydrogen Storage Materials

In the search for novel hydrogen storage materials, neutral silver-copper bimetallic nanoparticles up to the size of eight atoms (Cu _m Ag _n : m + n ≤ 8) have been computationally studied. Density functional theory with the B3LYP exchange-correlation functional and the combined basis sets of LanL2DZ and aug-cc-pVQZ were used in all of the calculations. H₂ adsorption studies on the most stable cluster geometries of all of the neat and heterogeneous entities found that 12 potential candidates, CuAg₄, Cu₆, Cu₅Ag, Cu₄Ag₂, Cu₃Ag₃, Cu₂Ag₄, CuAg₆, Cu₅Ag₃, Cu₄Ag₄, Cu₃Ag₅, Cu₂Ag₆, and CuAg₇, fall within the recommended physisorption range of -18 to -6 kJ mol^-1. A correlation in the behavior of binding energy, vibrational frequency, average bond distance, highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap, and chemical hardness with H₂ adsorption was observed. This analysis further revealed that the H₂ adsorption to the cluster was either a parallel or a perpendicular alignment. The analysis of the electron configuration of each atom in the cluster and the H₂ molecule and the charge transfer analysis of these 12 clusters also showed that the physisorption in the perpendicular mechanism is due to an induced dipole interaction, while that in the parallel mechanism is due to a weak ionic interaction. The clusters identified with perpendicular adsorption, CuAg₄H₂, Cu₆H₂, Cu₃Ag₃H₂, and Cu₂Ag₄H₂, polarized the H₂ molecule but had no charge transfer with the H₂ molecule and those identified with parallel adsorption, Cu₅AgH₂, Cu₄Ag₂H₂, CuAg₆H₂, Cu₅Ag₃H₂, Cu₄Ag₄H₂, Cu₃Ag₅H₂, Cu₂Ag₆H₂, and CuAg₇H₂, pulled the electrons from the H₂ molecule and had charge transfer with the H₂ molecule. The shapes of the frontier molecular orbital diagrams of the HOMO and LUMO also followed this observation.

Boron Hydrogen Compounds: Hydrogen Storage and Battery Applications

About 25 years ago, Bogdanovic and Schwickardi (B. Bogdanovic, M. Schwickardi: J. Alloys Compd. 1-9, 253 (1997) discovered the catalyzed release of hydrogen from NaAlH4. This discovery stimulated a vast research effort on light hydrides as hydrogen storage materials, in particular boron hydrogen compounds. Mg(BH4)2, with a hydrogen content of 14.9 wt %, has been extensively studied, and recent results shed new light on intermediate species formed during dehydrogenation. The chemistry of B3H8-, which is an important intermediate between BH4- and B12H122-, is presented in detail. The discovery of high ionic conductivity in the high-temperature phases of LiBH4 and Na2B12H12 opened a new research direction. The high chemical and electrochemical stability of closo-hydroborates has stimulated new research for their applications in batteries. Very recently, an all-solid-state 4 V Na battery prototype using a Na4(CB11H12)2(B12H12) solid electrolyte has been demonstrated. In this review, we present the current knowledge of possible reaction pathways involved in the successive hydrogen release reactions from BH4- to B12H122-, and a discussion of relevant necessary properties for high-ionic-conduction materials.

Fe4(OAc)10[EMIM]2: Novel Iron-Based Acetate EMIM Ionic Compound

We synthesized and characterized a novel iron(II) aceto EMIM coordination compound, which has a simplified empirical formula Fe4(OAc)10[EMIM]2, in two different hydration forms: as anhydrous monoclinic compound and triclinic dihydrate Fe4(OAc)10[EMIM]2·2H2O. The dihydrate compound is isostructural with recently reported Mn4(OAc)10[EMIM]2·2H2O, while the anhydrate is a superstructure of the Mn counterpart, suggesting the existence of solid solutions. Both new Fe compounds contain chains of Fe2+ octahedrally coordinated exclusively by acetate groups. The EMIM moieties do not interact directly with the Fe2+ and contribute to the structural framework of the compound through van der Waals forces and C-H···O hydrogen bonds with the acetate anions. The compounds have a melting temperature of ∼94 °C; therefore, they can be considered metal-containing ionic liquids. Differential thermal analysis indicates three endothermic transitions associated with melting, structural rearrangement in the molten state at about 157 °C, and finally, thermal decomposition of the Fe4(OAc)10[EMIM]2. Thermogravimetric analyses indicate an ∼72 wt % mass loss during the decomposition at 280-325 °C. The Fe4(OAc)10[EMIM]2 compounds have higher thermal stability than their Mn counterparts and [EMIM][OAc] but lower compared to iron(II) acetate. Temperature-programmed desorption coupled with mass spectrometry shows that the decomposition pathway of the Fe4(OAc)10[EMIM]2 involves four distinct regimes with peak temperatures at 88, 200, 267, and 345 °C. The main species observed in the decomposition of the compound are CH3, H2O, N2, CO, OC-CH3, OH-CO, H3C-CO-CH3, and H3C-O-CO-CH3. Variable-temperature infrared vibrational spectroscopy indicates that the phase transition at 160-180 °C is associated with a reorientation of the acetate ions, which may lead to a lower interaction with the [EMIM]+ before the decomposition of the Fe4(OAc)10[EMIM]2 upon further heating. The Fe4(OAc)10[EMIM]2 compounds are porous, plausibly capable of accommodating other types of molecules.

Solid-State Hydrogen Storage for a Decarbonized Society

Humanity is confronted with one of the most significant challenges in its history. The excessive use of fossil fuel energy sources is causing extreme climate change, which threatens our way of life and poses huge social and technological problems. It is imperative to look for alternate energy sources that can replace environmentally destructive fossil fuels. In this scenario, hydrogen is seen as a potential energy vector capable of enabling the better and synergic exploitation of renewable energy sources. A brief review of the use of hydrogen as a tool for decarbonizing our society is given in this work. Special emphasis is placed on the possibility of storing hydrogen in solid-state form (in hydride species), on the potential fields of application of solid-state hydrogen storage, and on the technological challenges solid-state hydrogen storage faces. A potential approach to reduce the carbon footprint of hydrogen storage materials is presented in the concluding section of this paper.

Improving Reproducibility in Hydrogen Storage Material Research

Research into new reversible hydrogen storage materials has the potential to help accelerate the transition to a hydrogen economy. The discovery of an efficient and cost-effective method of safely storing hydrogen would revolutionise its use as a sustainable energy carrier. Accurately measuring storage capacities - particularly of novel nanomaterials - has however proved challenging, and progress is being hindered by ongoing problems with reproducibility. Various metal and complex hydrides are being investigated, together with nanoporous adsorbents such as carbons, metal-organic frameworks and microporous organic polymers. The hydrogen storage properties of these materials are commonly determined using either the manometric (or Sieverts) technique or gravimetric methods, but both approaches are prone to significant error, if not performed with great care. Although commercial manometric and gravimetric instruments are widely available, they must be operated with an awareness of the limits of their applicability and the error sources inherent to the measurement techniques. This article therefore describes the measurement of hydrogen sorption and covers the required experimental procedures, aspects of troubleshooting and recommended reporting guidelines, with a view of helping improve reproducibility in experimental hydrogen storage material research.

Additive Destabilization of Porous Magnesium Borohydride Framework with Core-Shell Structure

Design of interfaces with thermodynamic and kinetic specificity is of great importance for hydrogen storage from both an applied and fundamental perspective. Here, in order to destabilize the metal hydride and protect the dehydrogenated products from oxidizing, a unique core-shell structure of porous Mg(BH₄ )₂ -based framework with a thin layer (no more than 5 nm) of MgCl₂ additives on the surface, has been proposed and synthesized via a wet-chemical method. The local structure and electronic state of the present complex system are systematically investigated to understand the correlation between the distribution of additives and dehydrogenation property of Mg(BH₄ )₂ . A significant improvement is achieved for hydrogen desorption with chlorides: initial hydrogen release from MgCl₂ decorated γ-phase Mg(BH₄ )₂ particles commences at 100 °C and reaches a maximum of 9.4 wt% at 385 °C. Besides the decreased decomposition temperature, an activation barrier of about 76.4 kJ mol^-1 lower than that of Mg(BH₄ )₂ without MgCl₂ is obtained. Moreover, MgCl₂ decoration can also prevent the whole decomposed system (both Mg- and B- elements) from oxidizing, which is a necessary condition to reversibility.

A safe and efficient synthetic method for alkali metal octahydrotriborates, unravelling a general mechanism for constructing the delta B3 unit of polyhedral boranes

A safe and efficient synthetic method for MB₃H₈ (M = Na, K, Rb and Cs) has been developed with excellent yields by directly reacting the corresponding MBH₄ with the dimethyl sulfide borane complex (DMS·BH₃). A general mechanism for constructing B₃H₈^-, a basic unit for building polyhedral boranes, has been unravelled.

Solid-State Hydrogen Storage Systems and the Relevance of a Gender Perspective

This paper aims at addressing the exploitation of solid-state carriers for hydrogen storage, with attention paid both to the technical aspects, through a wide review of the available integrated systems, and to the social aspects, through a preliminary overview of the connected impacts from a gender perspective. As for the technical perspective, carriers to be used for solid-state hydrogen storage for various applications can be classified into two classes: metal and complex hydrides. Related crystal structures and corresponding hydrogen sorption properties are reviewed and discussed. Fundamentals of thermodynamics of hydrogen sorption evidence the key role of the enthalpy of reaction, which determines the operating conditions (i.e., temperatures and pressures). In addition, it rules the heat to be removed from the tank during hydrogen absorption and to be delivered to the tank during hydrogen desorption. Suitable values for the enthalpy of hydrogen sorption reaction for operating conditions close to ambient (i.e., room temperature and 1–10 bar of hydrogen) are close to 30 kJ·molH2−1. The kinetics of the hydrogen sorption reaction is strongly related to the microstructure and to the morphology (i.e., loose powder or pellets) of the carriers. Usually, the kinetics of the hydrogen sorption reaction is rather fast, and the thermal management of the tank is the rate-determining step of the processes. As for the social perspective, the paper arguments that, as it occurs with the exploitation of other renewable innovative technologies, a wide consideration of the social factors connected to these processes is needed to reach a twofold objective: To assess the extent to which a specific innovation might produce positive or negative impacts in the recipient socioeconomic system and, from a sociotechnical perspective, to explore the potential role of the social components and dynamics in fostering the diffusion of the innovation itself. Within the social domain, attention has been paid to address the underexplored relationship between the gender perspective and the enhancement of hydrogen-related energy storage systems. This relationship is taken into account both in terms of the role of women in triggering the exploitation of hydrogen-based storage playing as experimenter and promoter, and in terms of the intertwined impact of this innovation in their current conditions, at work, and in daily life.

Experimental and computational characterization of phase transitions in CsB3H8

Metal hydroborates are versatile materials with interesting properties related to energy storage and cation conductivity. The hydrides containing B₃H₈^- (triborane, or octahydrotriborate) ions have been at the center of attention for some time as reversible intermediates in the decomposition of BH₄^- (3BH₄^-↔ B₃H₈^- + 2H₂), and as conducting media in electrolytes based on boron-hydride cage clusters. We report here the first observation of two phase transitions in CsB₃H₈ prior to its decomposition above 230 °C. The previously reported orthorhombic room temperature phase (here named α-CsB₃H₈) with the space group Ama2 changes into a new phase with the space group Pnma at 73 °C (here named β-CsB₃H₈), and then into a face-centered cubic phase, here named γ-CsB₃H₈, at 88 °C. These phases are not stable at room temperature thus requiring in situ measurements for their characterization. The phase transitions and decomposition pathway of CsB₃H₈ were studied with in situ synchrotron powder X-ray diffraction (SR-PXD), in situ and ex situ vibrational spectroscopies (Raman and FTIR), and differential-scanning calorimetry combined with thermo-gravimetric analysis (DSC-TGA). The structure determination was validated by vibrational spectroscopy analysis and modeling of the periodic structures by density functional methods. In γ-CsB₃H₈, a significant disorder in B₃H₈^- positions and orientations was found which can potentially benefit cation conducting properties through the paddle mechanism.

Mechanochemical synthesis and structural analysis of trivalent lanthanide and uranium diphenylphosphinodiboranates

Phosphinodiboranates (H₃BPR₂BH₃^-) are a class of borohydrides that have merited a reputation as weakly coordinating anions, which is attributed in part to the dearth of coordination complexes known with transition metals, lanthanides, and actinides. We recently reported how K(H₃BP^tBu₂BH₃) exhibits sluggish salt elimination reactivity with f-metal halides in organic solvents such as Et₂O and THF. Here we report how this reactivity appears to be further attenuated in solution when the ^tBu groups attached to phosphorus are exchanged for R = Ph or H, and we describe how mechanochemistry was used to overcome limited solution reactivity with K(H₃BPPh₂BH₃). Grinding three equivalents of K(H₃BPPh₂BH₃) with UI₃(THF)₄ or LnI₃ (Ln = Ce, Pr, Nd) allowed homoleptic complexes with the empirical formulas U(H₃BPPh₂BH₃)₃ (1), Ce(H₃BPPh₂BH₃)₃ (2), Pr(H₃BPPh₂BH₃)₃ (3), and Nd(H₃BPPh₂BH₃)₃ (4) to be prepared and subsequently crystallized in good yields (50-80%). Single-crystal XRD studies revealed that all four complexes exist as dimers or coordination polymers in the solid-state, whereas ¹H and ¹¹B NMR spectra showed that they exist as a mixture of monomers and dimers in solution. Treating 4 with THF breaks up the dimer to yield the monomeric complex Nd(H₃BPPh₂BH₃)₃(THF)₃ (4-THF). XRD studies revealed that 4-THF has one chelating and two dangling H₃BPPh₂BH₃^- ligands bound to the metal to accommodate binding of THF. In contrast to the results with K(H₃BPPh₂BH₃), attempting the same mechanochemical reactions with Na(H₃BPH₂BH₃) containing the simplest phosphinodiboranate were unsuccessful; only the partial metathesis product U(H₃BPH₂BH₃)I₂(THF)₃ (5) was isolated in poor yields. Despite these limitations, our results offer new examples showing how mechanochemistry can be used to rapidly synthesize molecular coordination complexes that are otherwise difficult to prepare using more traditional solution methods.

High pressure polymorphism of LiBH4 and of NaBH4

The pressure-induced structural changes in LiBH4 and in NaBH4 have been investigated experimentally up to 290 GPa by coupling Raman spectroscopy, infrared absorption spectroscopy and synchrotron X-ray diffraction. This data set is also analysed in the light of Density Functional Theory calculations performed up to 600 GPa. The [BH4]- unit appears to be remarkably resistant under pressure. NaBH4 remains stable in the known Pnma γ-phase up to 200 GPa and calculations predict a transition to a metallic polymeric C2/c phase at about 480 GPa. LiBH4 is confirmed to exhibit a richer polymorphism. A new Pnma orthorhombic phase VI is found to be stable above 60 GPa and there are hints of a possible phase VII above 160 GPa. DFT calculations predict that two other high pressure LiBH4 phases should appear at about 290 and 428 GPa. A very slight solubility of H2 inside phases II, III and V of LiBH4 is observed. A NaBH4(H2)0.5 complex is predicted to be stable above 150 GPa.

Factors Affecting Hydrogen Adsorption in Metal-Organic Frameworks: A Short Review

Metal–organic frameworks (MOFs) have significant potential for hydrogen storage. The main benefit of MOFs is their reversible and high-rate hydrogen adsorption process, whereas their biggest disadvantage is related to their operation at very low temperatures. In this study, we describe selected examples of MOF structures studied for hydrogen adsorption and different factors affecting hydrogen adsorption in MOFs. Approaches to improving hydrogen uptake are reviewed, including surface area and pore volume, in addition to the value of isosteric enthalpy of hydrogen adsorption. Nanoconfinement of metal hydrides inside MOFs is proposed as a new approach to hydrogen storage. Conclusions regarding MOFs with incorporated metal nanoparticles, which may be used as nanoscaffolds and/or H₂ sorbents, are summarized as prospects for the near future.

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.

Iodine-Substituted Lithium/Sodium closo-Decaborates: Syntheses, Characterization, and Solid-State Ionic Conductivity

Solid-state electrolytes based on closo-decaborates have caught increasing interest owing to the impressive room-temperature ionic conductivity, remarkable thermal/chemical stability, and excellent deformability. In order to develop new solid-state ion conductors, we investigated the influence of iodine substitution on the thermal, structural, and ionic conduction properties of closo-decaborates. A series of iodinated closo-decaborates, M₂[B₁₀H_10-nI_n] (M = Li, Na; n = 1, 2, 10), were synthesized and characterized by thermal analysis, powder X-ray diffraction, and electrochemical impedance spectroscopy; the stability and ionic conductivity of these compounds were studied. It was found that with the increase of iodine substitution on the closo-decaborate anion cage, the thermal decomposition temperature increases. All M₂[B₁₀H_10-nI_n] exhibit an amorphous structure. The ionic conductivity of Li₂[B₁₀H_10-nI_n] is higher than that of the Li₂[B₁₀H₁₀] parent compound. An ionic conductivity of 2.96 × 10^-2 S cm^-1 with an activation energy of 0.23 eV was observed for Li₂[B₁₀I₁₀] at 300 °C, implying that iodine substitution can improve the ionic conductivity. However, the ionic conductivity of Na₂[B₁₀H_10-nI_n] is lower than that of Na₂[B₁₀H₁₀] and increases with the increase of iodine substitution, which could be associated with the increase of the electrostatic potential, mass, and volume of the iodinated anions. Moreover, Li₂[B₁₀I₁₀] offers a Li-ion transference number of 0.999, an electrochemical stability window of 3.3 V and good compatibility with the Li anode, demonstrating its potential for application in high-temperature batteries.

Hybrid Perovskites, Metal-Organic Frameworks, and Beyond: Unconventional Degrees of Freedom in Molecular Frameworks

ConspectusThe structural degrees of freedom of a solid material are the various distortions most straightforwardly activated by external stimuli such as temperature, pressure, or adsorption. One of the most successful design strategies in materials chemistry involves controlling these individual distortions to produce useful collective functional responses. In a ferroelectric such as lead titanate, for example, the key degree of freedom involves asymmetric displacements of Pb2+ and Ti4+ cations; it is by coupling these together that the system as a whole interacts with external electric fields. Collective rotations of the polyhedral units in oxide ceramics are another commonly exploited distortion, driving anomalous behavior such as negative thermal expansion-the counterintuitive phenomenon of volume contraction on heating. An exciting development in the field has been to take advantage of the interplay between different distortion types: generating polarization by combining two different polyhedral rotations, for example. In this way, degrees of freedom act as geometric "elements" that can themselves be combined to engineer materials with new and interesting properties. Just as the discovery of new chemical elements quite obviously diversified chemical space, we might expect that identifying new and different types of structural degrees of freedom to be an important strategy for developing new kinds of functional materials. In this context, the broad family of molecular frameworks is emerging as an extraordinarily fertile source of new and unanticipated distortion types, the vast majority of which have no parallel in the established families of conventional solid-state chemistry.Framework materials are solids whose structures are assembled from two fundamental components: nodes and linkers. Quite simply, linkers join the nodes together to form scaffolding-like networks that extend from the atomic to the macroscopic scale. These structures usually contain cavities, which can also accommodate additional ions for charge balance. In the well-established systems-such as lead titanate-node, linker, and extra-framework ions are all individual atoms (Ti, O, and Pb, respectively). But in molecular frameworks, at least one of these components is a molecule.In this Account, we survey the unconventional degrees of freedom introduced through the simple act of replacing atoms by molecules. Our motivation is to understand the role these new distortions play (or might be expected to play) in different materials properties. The various degrees of freedom themselves-unconventional rotational, translational, orientational, and conformational states-are summarized and described in the context of relevant experimental examples. The much-improved prospect for generating emergent functionalities by combining these new distortion types is then discussed. We highlight a number of directions for future research-including the design and application of hierarchically structured phases of matter intermediate to solids and liquid crystals-which serve to highlight the extraordinary possibilities for this nascent field.

Kinetic Barriers and Microscopic Mechanisms of Noble Gas Adsorption by Nanoporous γ-Mg(BH4 )2 Obtained by Means of Sub-Second X-Ray Diffraction

Gas adsorption by porous frameworks sometimes results in structure "breathing", "pores opening/closing", "negative gas adsorption", and other phenomena. Time-dependent diffraction can address both kinetics of the guest uptake and structural response of the host framework. Using sub-second in situ powder X-ray diffraction, three intracrystalline diffusion scenarios have been evaluated from the isothermal kinetics of Ar, Kr, and Xe adsorption by nanoporous γ-Mg(BH₄ )₂ . These scenarios are dictated by two possible simultaneous transport mechanisms: diffusion through the intra- (i) and interchannel apertures (ii) of γ-Mg(BH₄ )₂ crystal structure. The contribution of (i) and (ii) changes depending on the kinetic diameter of the noble gas molecule and temperature regime. The lowest single activation barrier for the smallest Ar suggests equal diffusion of the atoms trough both pathways. Contrary, for the medium sized Kr we resolve the contributions of two parallel transport mechanisms, which tentatively can be attributed to the smaller barrier of the migration paths via the channel like pores and the higher barrier for the diffusion via narrow aperture between these channels. The largest Xe atoms diffuse only along 1D channels and show the highest single activation barrier.

Interplay between the Reorientational Dynamics of the B3H8 - Anion and the Structure in KB3H8

The structure and reorientational dynamics of KB3H8 were studied by using quasielastic and inelastic neutron scattering, Raman spectroscopy, first-principles calculations, differential scanning calorimetry, and in situ synchrotron radiation powder X-ray diffraction. The results reveal the existence of a previously unknown polymorph in between the α'- and β-polymorphs. Furthermore, it was found that the [B3H8]- anion undergoes different reorientational motions in the three polymorphs α, α', and β. In α-KB3H8, the [B3H8]- anion performs 3-fold rotations in the plane created by the three boron atoms, which changes to a 2-fold rotation around the C 2 symmetry axis of the [B3H8]- anion upon transitioning to α'-KB3H8. After transitioning to β-KB3H8, the [B3H8]- anion performs 4-fold rotations in the plane created by the three boron atoms, which indicates that the local structure of β-KB3H8 deviates from the global cubic NaCl-type structure. The results also indicate that the high reorientational mobility of the [B3H8]- anion facilitates the K+ cation conductivity, since the 2-orders-of-magnitude increase in the anion reorientational mobility observed between 297 and 311 K coincides with a large increase in K+ conductivity.

Recent Advances in Applications of Co-B Catalysts in NaBH4-Based Portable Hydrogen Generators

This review highlights the opportunities of catalytic hydrolysis of NaBH4 with the use of inexpensive and active Co-B catalysts among the other systems of hydrogen storage and generation based on water reactive materials. This process is important for the creation of H2 generators required for the operation of portable compact power devices based on low-temperature proton exchange membrane fuel cells (LT PEM FC). Special attention is paid to the influence of the reaction medium on the formation of active state of Co-B catalysts and the problem of their deactivation in NaBH4 solution stabilized by alkali. The novelty of this review consists in the discussion of basic designs of hydrogen generators based on NaBH4 hydrolysis using cobalt catalysts and the challenges of their integration with LT PEM FC. The potential of using batch reactors in which there is no need to use aggressive alkaline NaBH4 solutions is discussed. Solid-phase compositions or pellets based on NaBH4 and cobalt-containing catalytic additives are proposed, the hydrogen generation from which starts immediately after the addition of water. The review made it possible to formulate the most acute problems, which require new sci-tech solutions.

Trends in the Series of Ammine Rare-Earth-Metal Borohydrides: Relating Structural and Thermal Properties

Ammine metal borohydrides display extreme structural and compositional diversity and show potential applications for solid-state hydrogen and ammonia storage and as solid-state electrolytes. Thirty-two new compounds are reported in this work, and trends in the full series of ammine rare-earth-metal borohydrides are discussed. The majority of the rare-earth metals (RE) form trivalent RE(BH₄)₃·xNH₃ (x = 7-1) compounds, which possess an intriguing crystal chemistry changing with the number of ammonia ligands, varying from structures built from complex ions (x = 5-7), to molecular structures (x = 3, 4), one-dimensional chains (x = 2), and structures built from two-dimensional layers (x = 1). Divalent RE(BH₄)₂·xNH₃ (x = 4, 2, 1) compounds are observed for RE²⁺ = Sm, Eu, Yb, with structures varying from molecular structures (x = 4) to two-dimensional layered (x = 2, 1) and three-dimensional structures (Yb(BH₄)₂·NH₃). The crystal structure and composition of the compounds depend on the volume of the rare-earth ion. In all structures, NH₃ coordinates to the metal, while BH₄^- has a more flexible coordination and is observed as a bridging and terminal ligand and as a counterion. RE(BH₄)₃·xNH₃ (x = 7-5, 4) releases NH₃ stepwise during thermal treatment, while mainly H₂ is released for x ≤ 3. In contrast, only NH₃ is released from RE(BH₄)₂·xNH₃ due to the lower charge density on the RE²⁺ ion and higher stability of RE(BH₄)₂. The thermal stability of RE(BH₄)₃·xNH₃ increase with increasing cation charge density for x = 5, 7, while it decreases for x = 4, 6. For x = 3, the thermal stability decreases with increasing charge density, due to the destabilization of the BH₄^- group, making it more reactive toward NH₃. This research provides a large number of novel compounds and new insight into trends in the crystal chemistry of ammine metal borohydrides and reveals a correlation between the local metal coordination and the thermal stability.