First-Principles Prediction of Thermodynamically Reversible Hydrogen Storage Reactions in the Li-Mg-Ca-B-H System

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.

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.

First-Principles Elucidation of Initial Dehydrogenation Pathways in Mg(BH4)2

Complex borohydrides such as Mg(BH₄)₂ offer one of highest capacities to chemically store hydrogen for onboard applications; however, it suffers greatly from kinetic constraints that prevent realization of full capacity and reversibility. Understanding these kinetic limitations solely from experiments is extremely challenging due to the unusual complexity of various competing elemental reaction steps involved during the de/rehydrogenation reaction. This work aims to map out the energetics associated with initial dehydrogenation of Mg(BH₄)₂ from first-principles simulations and to identify the preferred reaction pathways. Our calculations suggest the rate-limiting step during BH₄^--B₃H₈^- conversion is the formation of the B₂H₇^- intermediate. We further emphasize and clarify that the B₃H₈^- and H^- intermediates, formed during initial Mg(BH₄)₂ decomposition, appear as molecular species that are embedded in the Mg-BH₄-Mg matrix as evidenced in the nuclear magnetic resonance measurements and not as bulk MgH₂ and Mg(B₃H₈)₂ as previously assumed in theoretical predictions of the thermodynamics.

Analysis of Intermediates and Products from the Dehydrogenation of Mg(BH4)2

The thermodynamic properties of key compounds Mg(B₃H₈)₂, MgB₂H₆, MgB₁₀H₁₀, Mg(B₁₁H₁₄)₂, Mg₃(B₃H₆)₂, and MgB₁₂H₁₂, proposed to be formed in the release of hydrogen from magnesium borohydride Mg(BH₄)₂ and the uptake of hydrogen by MgB₂, have been investigated using solid-state density functional theory (DFT) calculations. More accurate tretment of the cell-size effects with respect to the entropies was also investigated in order to improve the accuracy of the thermodynamic properties of complex borohydrides. We find that the zero-point energy corrections can lower the electronic energies of reaction by 20-30 kJ/(mol H₂) for these intermediates, while adding the thermal and entropy contibutions results in a total decrease of up to ∼50 kJ/(mol H₂). Although our treatment lowers the calculated formation energy of Mg(B₃H₈)₂, it is still too high to explain the experimental observation of B₃H₈^-. We discuss possible reasons for this disparity and propose that the formation of B₃H₈^- and H^- in a disordered amorphous phase has a large energy difference compared to the phase-separated Mg(B₃H₈)₂ and MgH₂ considered in calculations. A comparison of the experimental and NMR chemical shifts calculated within a DFT approach for known species Mg(BH₄)₂, Mg(B₃H₈)₂, Mg(B₁₁H₁₄)₂, MgB₁₀H₁₀, and MgB₁₂H₁₂ provides validation for predicting the chemical shifts of the other compounds which are yet to be confirmed experimentally. These include MgB₂H₆ and the proposed trianion species Mg₃(B₃H₆)₂ that both have favorable thermodynamics for reversible hydrogen storage in Mg(BH₄)₂ without the formation of MgH₂ as a coproduct which could phase separate and inhibit rehydrogenation.

First-principles study of closo-dodecaborates M2B12H12 (M = Li, Na, K) as solid-state electrolyte materials

Closo-dodecaborates M₂B₁₂H₁₂ are considered among the potential candidates for solid-state electrolyte materials due to their high ionic conductivities. It has been demonstrated that the reorientation of the icosahedral anion B₁₂H₁₂^2- plays a key role in high cation motion. However, this category of B_nH_n materials is still not well established with respect to their structural, thermodynamic and diffusion properties. In the present work, the electronic, vibrational and thermodynamic properties of M₂B₁₂H₁₂ (M = Li, Na, K) structures are reported using first-principles calculations. The results of structural and electronic properties show that these structures have an insulator character with a large band gap of 5.75, 5.63 and 5.59 eV, respectively, for Li₂B₁₂H₁₂, Na₂B₁₂H₁₂ and K₂B₁₂H₁₂. The thermodynamic stabilities of these systems are confirmed by their phonon calculation results. The primary quantities, such as heat capacity, vibrational entropy and volume variation at finite temperatures, are determined using the quasi-harmonic approximation in order to provide an input for the Gibbs free energy assessment. The calculated enthalpy of formation of the Li₂B₁₂H₁₂ structure at 0 K and the proposed one at 300 K are found to be -127.31 and -740.44 kJ mol^-1 per H₂, respectively. The migration energy barrier of various cations in each system is calculated to be 0.7 (Li⁺), 1.16 (Na⁺) and 1.25 eV (K⁺), where the lowest energy barrier corresponds to the lithium ion migration in Li₂B₁₂H₁₂. Additionally, the molecular dynamics simulation of M₂B₁₂H₁₂ (M = Li, Na, K) structures demonstrated that these structures are stable above room temperature, except for the Li₂B₁₂H₁₂ structure at 600 K, where the most stable is Na₂B₁₂H₁₂. Finally, the temperature effect on icosahedral anion reorientation in each structure is elucidated as a function of temperature and cation type.

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 comprehensive study on lithium-based reactive hydride composite (Li-RHC) as a reversible solid-state hydrogen storage system toward potential mobile applications

Reversible solid-state hydrogen storage is one of the key technologies toward pollutant-free and sustainable energy conversion. The composite system LiBH₄–MgH₂ can reversibly store hydrogen with a gravimetric capacity of 13 wt%. However, its dehydrogenation/hydrogenation kinetics is extremely sluggish (∼40 h) which hinders its usage for commercial applications. In this work, the kinetics of this composite system is significantly enhanced (∼96%) by adding a small amount of NbF₅. The catalytic effect of NbF₅ on the dehydrogenation/hydrogenation process of LiBH₄–MgH₂ is systematically investigated using a broad range of experimental techniques such as in situ synchrotron radiation X-ray powder diffraction (in situ SR-XPD), X-ray absorption spectroscopy (XAS), anomalous small angle X-ray scattering (ASAXS), and ultra/small-angle neutron scattering (USANS/SANS). The obtained results are utilized to develop a model that explains the catalytic function of NbF₅ in hydrogen release and uptake in the LiBH₄–MgH₂ composite system.

Superb dehydrogenation/hydrogenation kinetic enhancement of the LiBH₄–MgH₂ reactive hydride composite system by addition of NbB₂ nano-particles as nucleation agents for MgB₂.

Thermodynamic Hydricity of Small Borane Clusters and Polyhedral closo-Boranes

Thermodynamic hydricity (HDA^MeCN) determined as Gibbs free energy (ΔG°[H]⁻) of the H⁻ detachment reaction in acetonitrile (MeCN) was assessed for 144 small borane clusters (up to 5 boron atoms), polyhedral closo-boranes dianions [B_nH_n]²⁻, and their lithium salts Li₂[B_nH_n] (n = 5–17) by DFT method [M06/6-311++G(d,p)] taking into account non-specific solvent effect (SMD model). Thermodynamic hydricity values of diborane B₂H₆ (HDA^MeCN = 82.1 kcal/mol) and its dianion [B₂H₆]²⁻ (HDA^MeCN = 40.9 kcal/mol for Li₂[B₂H₆]) can be selected as border points for the range of borane clusters’ reactivity. Borane clusters with HDA^MeCN below 41 kcal/mol are strong hydride donors capable of reducing CO₂ (HDA^MeCN = 44 kcal/mol for HCO₂⁻), whereas those with HDA^MeCN over 82 kcal/mol, predominately neutral boranes, are weak hydride donors and less prone to hydride transfer than to proton transfer (e.g., B₂H₆, B₄H₁₀, B₅H₁₁, etc.). The HDA^MeCN values of closo-boranes are found to directly depend on the coordination number of the boron atom from which hydride detachment and stabilization of quasi-borinium cation takes place. In general, the larger the coordination number (CN) of a boron atom, the lower the value of HDA^MeCN.

A Mechanistic Analysis of Phase Evolution and Hydrogen Storage Behavior in Nanocrystalline Mg(BH4)2 within Reduced Graphene Oxide

Magnesium borohydride (Mg(BH₄)₂, abbreviated here MBH) has received tremendous attention as a promising onboard hydrogen storage medium due to its excellent gravimetric and volumetric hydrogen storage capacities. While the polymorphs of MBH-alpha (α), beta (β), and gamma (γ)-have distinct properties, their synthetic homogeneity can be difficult to control, mainly due to their structural complexity and similar thermodynamic properties. Here, we describe an effective approach for obtaining pure polymorphic phases of MBH nanomaterials within a reduced graphene oxide support (abbreviated MBHg) under mild conditions (60-190 °C under mild vacuum, 2 Torr), starting from two distinct samples initially dried under Ar and vacuum. Specifically, we selectively synthesize the thermodynamically stable α phase and metastable β phase from the γ-phase within the temperature range of 150-180 °C. The relevant underlying phase evolution mechanism is elucidated by theoretical thermodynamics and kinetic nucleation modeling. The resulting MBHg composites exhibit structural stability, resistance to oxidation, and partially reversible formation of diverse [BH₄]^- species during de- and rehydrogenation processes, rendering them intriguing candidates for further optimization toward hydrogen storage applications.

Tuning LiBH4 for Hydrogen Storage: Destabilization, Additive, and Nanoconfinement Approaches

Hydrogen technology has become essential to fulfill our mobile and stationary energy needs in a global low–carbon energy system. The non-renewability of fossil fuels and the increasing environmental problems caused by our fossil fuel–running economy have led to our efforts towards the application of hydrogen as an energy vector. However, the development of volumetric and gravimetric efficient hydrogen storage media is still to be addressed. LiBH₄ is one of the most interesting media to store hydrogen as a compound due to its large gravimetric (18.5 wt.%) and volumetric (121 kgH₂/m³) hydrogen densities. In this review, we focus on some of the main explored approaches to tune the thermodynamics and kinetics of LiBH₄: (I) LiBH₄ + MgH₂ destabilized system, (II) metal and metal hydride added LiBH₄, (III) destabilization of LiBH₄ by rare-earth metal hydrides, and (IV) the nanoconfinement of LiBH₄ and destabilized LiBH₄ hydride systems. Thorough discussions about the reaction pathways, destabilizing and catalytic effects of metals and metal hydrides, novel synthesis processes of rare earth destabilizing agents, and all the essential aspects of nanoconfinement are led.

Hydrogen Desorption in Mg(BH4)2-Ca(BH4)2 System

Magnesium borohydride, Mg(BH4)2, and calcium borohydride, Ca(BH4)2, are promising materials for hydrogen storage. Mixtures of different borohydrides have been the subject of numerous researches; however, the whole Mg(BH4)2-Ca(BH4)2 system has not been investigated yet. In this study, the phase stability and the hydrogen desorption were experimentally investigated in the Mg(BH4)2-Ca(BH4)2 system, by means of XRD, ATR-IR, and HP-DSC. Mg(BH4)2 and Ca(BH4)2 are fully immiscible in the solid state. In the mechanical mixtures, thermal decomposition occurs at slightly lower temperatures than for pure compounds. However, they originate products that cannot be identified by XRD, apart from Mg and MgH2. In fact, amorphous phases or mixtures of different poorly crystalline or nanocrystalline phases are formed, leading to a limited reversibility of the system.

Theoretical description of alkali metal closo-boranes - towards the crystal structure of MgB12H12

Solid state closo-borane salts of alkali metals have very high ionic conductivity. This makes them interesting for practical applications as solid state electrolytes, and has triggered extensive research efforts. Improvement and understanding of their properties require accurate theoretical description of their static and dynamical properties. In this work, we report accuracy assessment of density functional theory in the description of solids with B12H122- anions. We show that these aromatic anions interact via weak dispersive forces. For that reason, non-local exchange-correlation functionals give better description of structural properties and phonons in Li2B12H12 and Na2B12H12. Numerically efficient semi-local methods provide satisfactory results when applied in structure volumes obtained in a non-local method. An extensive structural search for stable crystalline phases of MgB12H12 predicts a new denser lattice with C2/c symmetry that is stabilized by van der Waals interactions. These structures might be discovered as anhydrous MgB12H12 in high pressure experiments, avoiding the amorphous state at ambient pressures.

Theoretical study of hydrogen storage in metal hydrides

Adsorption, absorption and desorption energies and other properties of hydrogen storage in palladium and in the metal hydrides AlH₃, MgH₂, Mg(BH₄)₂, Mg(BH₄)(NH₂) and LiNH₂ were analyzed. The DFT calculations on cluster models show that, at a low concentration, the hydrogen atom remains adsorbed in a stable state near the palladium surface. By increasing the hydrogen concentration, the tetrahedral and the octahedral sites are sequentially occupied. In the α phase the tetrahedral site releases hydrogen more easily than at the octahedral sites, but the opposite occurs in the β phase. Among the hydrides, Mg(BH₄)₂ shows the highest values for both absorption and desorption energies. The absorption energy of LiNH₂ is higher than that of the palladium, but its desorption energy is too high, a recurrent problem of the materials that have been considered for hydrogen storage. The release of hydrogen, however, can be favored by using transition metals in the material structure, as demonstrated here by doping MgH₂ with 3d and 4d-transition metals to reduce the hydrogen atomic charge and the desorption energy.

Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds

Two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. Yet, only a few dozen 2D materials have been successfully synthesized or exfoliated. Here, we search for 2D materials that can be easily exfoliated from their parent compounds. Starting from 108,423 unique, experimentally known 3D compounds, we identify a subset of 5,619 compounds that appear layered according to robust geometric and bonding criteria. High-throughput calculations using van der Waals density functional theory, validated against experimental structural data and calculated random phase approximation binding energies, further allowed the identification of 1,825 compounds that are either easily or potentially exfoliable. In particular, the subset of 1,036 easily exfoliable cases provides novel structural prototypes and simple ternary compounds as well as a large portfolio of materials to search from for optimal properties. For a subset of 258 compounds, we explore vibrational, electronic, magnetic and topological properties, identifying 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors.

Synthesis, stability and Li-ion mobility of nanoconfined Li2B12H12

This communication presents the first synthesis of nanoconfined Lithium closo-borate, Li₂B₁₂H₁₂, using nanoporous SiO₂ as scaffold. The yield of Li₂B₁₂H₁₂ is up to 94 mol%. The as-synthesized nanoconfined Li₂B₁₂H₁₂ exhibits a structural transition around 380 °C and conversion to H-deficiency Li₂B₁₂H_12-x at 580 °C.

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.

Elucidating the mechanism of MgB2 initial hydrogenation via a combined experimental–theoretical study

The initial hydrogenation of MgB₂ occurs via a multi-step process, which can result in the direct production of [BH₄]⁻ complexes.

Controllable decomposition of Ca(BH4)2 for reversible hydrogen storage

The formation of CaB₆ from the thermal decomposition of Ca(BH₄)₂ goes along two distinct routes, i.e. via CaB₂H₆ or elemental boron as a reaction intermediate, depending on temperature.

First-principles calculated decomposition pathways for LiBH4 nanoclusters

We analyze thermodynamic stability and decomposition pathways of LiBH₄ nanoclusters using grand-canonical free-energy minimization based on total energies and vibrational frequencies obtained from density-functional theory (DFT) calculations. We consider (LiBH₄)_n nanoclusters with n = 2 to 12 as reactants, while the possible products include (Li)_n, (B)_n, (LiB)_n, (LiH)_n, and Li₂B_nH_n; off-stoichiometric Li_nB_nH_m (m ≤ 4n) clusters were considered for n = 2, 3, and 6. Cluster ground-state configurations have been predicted using prototype electrostatic ground-state (PEGS) and genetic algorithm (GA) based structural optimizations. Free-energy calculations show hydrogen release pathways markedly differ from those in bulk LiBH₄. While experiments have found that the bulk material decomposes into LiH and B, with Li₂B₁₂H₁₂ as a kinetically inhibited intermediate phase, (LiBH₄)_n nanoclusters with n ≤ 12 are predicted to decompose into mixed Li_nB_n clusters via a series of intermediate clusters of Li_nB_nH_m (m ≤ 4n). The calculated pressure-composition isotherms and temperature-pressure isobars exhibit sloping plateaus due to finite size effects on reaction thermodynamics. Generally, decomposition temperatures of free-standing clusters are found to increase with decreasing cluster size due to thermodynamic destabilization of reaction products.

Ca(BH4)2-Mg2NiH4: on the pathway to a Ca(BH4)2 system with a reversible hydrogen cycle

The Ca(BH4)2-Mg2NiH4 system presented here is, to the best of our knowledge, the first described Ca(BH4)2-based hydride composite that reversibly transfers boron from the Ca-based compound(s) to the reaction partner. The ternary boride MgNi2.5B2 is formed upon dehydrogenation and the formation of Ca(BH4)2 upon rehydrogenation is confirmed.

In situ characterization of the decomposition behavior of Mg(BH4)2 by X-ray Raman scattering spectroscopy

We present an in situ study of the thermal decomposition of Mg(BH4)2 in a hydrogen atmosphere of up to 4 bar and up to 500 °C using X-ray Raman scattering spectroscopy at the boron K-edge and the magnesium L2,3-edges. The combination of the fingerprinting analysis of both edges yields detailed quantitative information on the reaction products during decomposition, an issue of crucial importance in determining whether Mg(BH4)2 can be used as a next-generation hydrogen storage material. This work reveals the formation of reaction intermediate(s) at 300 °C, accompanied by a significant hydrogen release without the occurrence of stable boron compounds such as amorphous boron or MgB12H12. At temperatures between 300 °C and 400 °C, further hydrogen release proceeds via the formation of higher boranes and crystalline MgH2. Above 400 °C, decomposition into the constituting elements takes place. Therefore, at moderate temperatures, Mg(BH4)2 is shown to be a promising high-density hydrogen storage material with great potential for reversible energy storage applications.

Structure-dependent vibrational dynamics of Mg(BH4)2 polymorphs probed with neutron vibrational spectroscopy and first-principles calculations

Neutron vibrational spectroscopy and DFT calculations are used in order to gain deeper insights into the structure-dependent vibrational properties of Mg(BH₄)₂ polymorphs.

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.

Exploring PtSO4 and PdSO4 phases: an evolutionary algorithm based investigation

Metal sulfate formation is one of the major challenges to the emission aftertreatment catalysts. Unlike the incredibly sulfation prone nature of Pd to form PdSO4, no experimental evidence exists for PtSO4 formation. Given the mystery of nonexistence of PtSO4, we explore PtSO4 using a combined approach of an evolutionary algorithm based search technique and quantum mechanical computations. Experimentally known PdSO4 is considered for the comparison and validation of our results. We predict many possible low-energy phases of PtSO4 and PdSO4 at 0 K, which are further investigated in a wide range of temperature-pressure conditions. An entirely new low-energy (tetragonal P42/m) structure of PtSO4 and PdSO4 is predicted, which appears to be the most stable phase of PtSO4 and a competing phase of the experimentally known monoclinic C2/c phase of PdSO4. Phase stability at a finite temperature is further examined and verified by Gibbs free energy calculations of sulfates towards their possible decomposition products. Finally, temperature-pressure phase diagrams are computationally established for both PtSO4 and PdSO4.

A novel strategy for reversible hydrogen storage in Ca(BH4)2

We report that decomposition pathway of Ca(BH4)2 can be efficiently controlled by reaction temperature. That is, it decomposes into CaB6 at a lower temperature range of 320 to 350 °C, but into amorphous boron at 400 to 450 °C. We identified the formation of CaB2H6 as the crucial intermediate step on the way to CaB6 that only forms at 320 to 350 °C.

The role of MgB12H12 in the hydrogen desorption process of Mg(BH4)2

The presence of MgB12H12 has often been considered as the major obstacle for the reversible hydrogen storage in Mg(BH4)2. This communication provides evidence that the MgB12H12 phase (or [B12H12](2-) monomer) does not exist in the decomposition products of Mg(BH4)2 at temperatures ranging from 265 to 400 °C, and thereby it will not act as a dead end.

Superhalogens: A Bridge between Complex Metal Hydrides and Li Ion Batteries

Complex metal hydrides and Li ion batteries play an integral role in the pursuit of clean and sustainable energy. The former stores hydrogen and can provide a clean energy solution for the transportation industry, while the latter can store energy harnessed from the sun and the wind. However, considerable materials challenges remain in both cases, and research for finding solutions has traditionally followed parallel paths. In this Perspective, I show that there is a common link between these two seemingly disparate fields that can be unveiled by studying the electronic structure of the anions in complex metal hydrides and in electrolytes of Li ion batteries; they are both superhalogens. I demonstrate that considerable progress made in our understanding of superhalogens in the past decade can provide solutions to some of the materials challenges in both of these areas.

Formation of stoichiometric CsFn compounds

Alkali halides MX, have been viewed as typical ionic compounds, characterized by 1:1 ratio necessary for charge balance between M(+) and X(-). It was proposed that group I elements like Cs can be oxidized further under high pressure. Here we perform a comprehensive study for the CsF-F system at pressures up to 100 GPa, and find extremely versatile chemistry. A series of CsFn (n ≥ 1) compounds are predicted to be stable already at ambient pressure. Under pressure, 5p electrons of Cs atoms become active, with growing tendency to form Cs (III) and (V) valence states at fluorine-rich conditions. Although Cs (II) and (IV) are not energetically favoured, the interplay between two mechanisms (polyfluoride anions and polyvalent Cs cations) allows CsF2 and CsF4 compounds to be stable under pressure. The estimated defluorination temperatures of CsFn (n = 2,3,5) compounds at atmospheric pressure (218°C, 150°C, -15°C, respectively), are attractive for fluorine storage applications.

Facile synthesis of anhydrous alkaline earth metal dodecaborates MB12H12 (M = Mg, Ca) from M(BH4)2

Thermal decomposition of MB₁₂H₁₂ (M = Mg, Ca) forms H-deficient monomers MB₁₂H_12−x containing icosahedral B₁₂ skeletons and is followed by the formation of (MB_yH_z)_n polymers.

Powered by DFT: Screening methods that accelerate materials development for hydrogen in metals applications

CONSPECTUS: Not only is hydrogen critical for current chemical and refining processes, it is also projected to be an important energy carrier for future green energy systems such as fuel cell vehicles. Scientists have examined light metal hydrides for this purpose, which need to have both good thermodynamic properties and fast charging/discharging kinetics. The properties of hydrogen in metals are also important in the development of membranes for hydrogen purification. In this Account, we highlight our recent work aimed at the large scale screening of metal-based systems with either favorable hydrogen capacities and thermodynamics for hydrogen storage in metal hydrides for use in onboard fuel cell vehicles or promising hydrogen permeabilities relative to pure Pd for hydrogen separation from high temperature mixed gas streams using dense metal membranes. Previously, chemists have found that the metal hydrides need to hit a stability sweet spot: if the compound is too stable, it will not release enough hydrogen under low temperatures; if the compound is too unstable, the reaction may not be reversible under practical conditions. Fortunately, we can use DFT-based methods to assess this stability via prediction of thermodynamic properties, equilibrium reaction pathways, and phase diagrams for candidate metal hydride systems with reasonable accuracy using only proposed crystal structures and compositions as inputs. We have efficiently screened millions of mixtures of pure metals, metal hydrides, and alloys to identify promising reaction schemes via the grand canonical linear programming method. Pure Pd and Pd-based membranes have ideal hydrogen selectivities over other gases but suffer shortcomings such as sensitivity to sulfur poisoning and hydrogen embrittlement. Using a combination of detailed DFT, Monte Carlo techniques, and simplified models, we are able to accurately predict hydrogen permeabilities of metal membranes and screen large libraries of candidate alloys, selections of which are described in this Account. To further increase the number of membrane materials that can be studied with DFT, computational costs need to be reduced either through methods development to break bottlenecks in the performance prediction algorithm, particularly related to transition state identification, or through screening techniques that take advantage of correlations to bypass constraints.

Spin-State Effects on the Thermal Dihydrogen Release from Solid-State [MH(η2-H2)dppe2]+ (M = Fe, Ru, Os) Organometallic Complexes for Hydrogen Storage Applications

Mössbauer spectroscopy, experimental thermodynamic measurements, and computational studies were performed to investigate the properties of molecular hydrogen binding to the organometallic fragments [MHdppe₂]⁺ (M = Fe, Ru, Os; dppe =1,2-bis(diphenylphosphino)ethane) to form the dihydrogen complex fragments [MH(η²-H₂)dppe₂]⁺. Mössbauer spectroscopy showed that the dehydrogenated complex [FeHdppe₂]⁺ adopts a geometry consistent with the triplet spin state, transitioning to a singlet state complex upon addition of the dihydrogen molecule in a manner similar to the previously studied dinitrogen complexes. From simulations, this spin transition behavior was found to be responsible for the strong binding behavior experimentally observed in the iron complex. Spin-singlet to spin-singlet transitions were found to exhibit thermodynamics consistent with the 5d > 3d > 4d binding trend observed for other transition metal dihydrogen complexes. Finally, the method for distinguishing between dihydrogen and dihydride complexes based on partial quadrupole splittings observed in Mössbauer spectra was confirmed, providing a tool for further characterization of these unique species for Mössbauer active compounds.

Hydrogen reversibility of LiBH4–MgH2–Al composites

Numerous intermediates are involved in the decomposition mechanism of LiBH₄–MgH₂–Al and the cyclic stability is evaluated.

Is Y2(B12H12)3 the main intermediate in the decomposition process of Y(BH4)3?

Dodecaborates, i.e. the [B12H12](2-) containing species, are often observed as main intermediates in the hydrogen sorption cycle of metal borohydrides, hindering rehydrogenation. In the decomposition process of Y(BH4)3, yttrium octahydrotriborate, i.e. Y(B3H8)3, rather than the stable Y2(B12H12)3, is formed as the main intermediate.

Prediction of thermodynamically reversible hydrogen storage reactions utilizing Ca-M(M = Li, Na, K)-B-H systems: a first-principles study

Calcium borohydride is a potential candidate for onboard hydrogen storage because it has a high gravimetric capacity (11.5 wt.%) and a high volumetric hydrogen content (∼130 kg m(-3)). Unfortunately, calcium borohydride suffers from the drawback of having very strongly bound hydrogen. In this study, Ca(BH₄)₂ was predicted to form a destabilized system when it was mixed with LiBH₄, NaBH₄, or KBH₄. The release of hydrogen from Ca(BH₄)₂ was predicted to proceed via two competing reaction pathways (leading to CaB₆ and CaH₂ or CaB₁₂H₁₂ and CaH₂) that were found to have almost equal free energies. Using a set of recently developed theoretical methods derived from first principles, we predicted five new hydrogen storage reactions that are among the most attractive of those presently known. These combine high gravimetric densities (>6.0 wt.% H₂) with have low enthalpies [approximately 35 kJ/(mol(-1) H₂)] and are thermodynamically reversible at low pressure within the target window for onboard storage that is actively being considered for hydrogen storage applications. Thus, the first-principles theoretical design of new materials for energy storage in future research appears to be possible.