Experimental and theoretical studies of the LiBH4-LiI phase diagram

The hexagonal structure of LiBH4 at room temperature can be stabilised by substituting the BH4- anion with I-, leading to high Li-ion conductive materials. A thermodynamic description of the pseudo-binary LiBH4-LiI system is presented. The system has been explored investigating several compositions, synthetized by ball milling and subsequently annealed. X-ray diffraction and Differential Scanning Calorimetry have been exploited to determine structural and thermodynamic features of various samples. The monophasic zone of the hexagonal Li(BH4)1-x(I)x solid solution has been experimentally defined equal to 0.18 ≤ x ≤ 0.60 at 25 °C. In order to establish the formation of the hexagonal solid solution, the enthalpy of mixing was experimentally determined, converging to a value of 1800 ± 410 J mol-1. Additionally, the enthalpy of melting was acquired for samples that differ in molar fraction. By merging experimental results, literature data and ab initio theoretical calculations, the pseudo-binary LiBH4-LiI phase diagram has been assessed and evaluated across all compositions and temperature ranges by applying the CALPHAD method.

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.

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.

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.

Effect of Different Amounts of TiF3 on the Reversible Hydrogen Storage Properties of 2LiBH4-Li3AlH6 Composite

Hydrogen is a potential green alternative to conventional energy carriers such as oil and coal. Compared with the storage of hydrogen in gaseous or liquid phases, the chemical storage of hydrogen in solid complex hydrides is safer and more effective. In this study, the complex hydride composite 2LiBH4-Li₃AlH₆ with different amounts of TiF₃ was prepared by simple ball-milling and its hydrogen storage properties were investigated. Temperature programmed desorption and differential scanning calorimetry were used to characterize the de/rehydrogenation performance, and X-ray diffraction and scanning electron microscopy (SEM) were used to explore the phase structure and surface topography of the materials. The dehydrogenation temperature decreased by 48°C in 2LiBH4-Li₃AlH₆ with 15 wt% TiF₃ composites compared to the composite without additives while the reaction kinetics was accelerated by 20%. In addition, the influence of hydrogen back pressure on the 2LiBH4-Li₃AlH₆ with 5 wt% TiF₃ composite was also investigated. The results show that hydrogen back pressure between 2.5 and 3.5 bar can improve the reversible performance of the composite to some extent. With a back pressure of 3.5 bar, the second dehydrogenation capacity increased to 4.6 wt% from the 3.3 wt% in the 2LiBH₄-Li₃AlH₆ composite without hydrogen back pressure. However, the dehydrogenation kinetics was hindered. About 150 h, which is 100 times the time required without back pressure, was needed to release 8.7 wt% of hydrogen at 3.5 bar hydrogen back pressure. The SEM results show that aluminum was aggregated after the second cycle of dehydrogenation at the hydrogen back pressure of 3 bar, resulting in the partial reversibility of the 5 wt% TiF₃-added 2LiBH4-Li3AlH₆ composite.

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

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

Complex Hydrides for Energy Storage, Conversion, and Utilization

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

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.

Exploring Ternary and Quaternary Mixtures in the LiBH4 -NaBH4 -KBH4 -Mg(BH4 )2 -Ca(BH4 )2 System

Binary combinations of borohydrides have been extensivly investigated evidencing the formation of eutectics, bimetallic compounds or solid solutions. In this paper, the investigation has been extended to ternary and quaternary systems in the LiBH₄ -NaBH₄ -KBH₄ -Mg(BH₄ )₂ -Ca(BH₄ )₂ system. Possible interactions among borohydrides in equimolar composition has been explored by mechanochemical treatment. The obtained phases were analysed by X-ray diffraction and the thermal behaviour of the mixtures were analysed by HP-DSC and DTA, defining temperature of transitions and decomposition reactions. The release of hydrogen was detected by MS, showing the role of the presence of solid solutions and multi-cation compounds on the hydrogen desorption reactions. The presence of LiBH₄ generally promotes the release of H₂ at about 200 °C, while KCa(BH₄ )₃ promotes the release in a single-step reaction at higher temperatures.

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.

Metal borohydrides and derivatives – synthesis, structure and properties

A comprehensive review of metal borohydrides from synthesis to application.

Phase diagrams of the LiBH4–NaBH4–KBH4 system

The LiBH₄–NaBH₄–KBH₄ system was explored combining experimental and theoretical techniques to establish phase diagrams and thermodynamic properties in all temperature and composition ranges.

Thermal decomposition of sodium amide, NaNH2, and sodium amide hydroxide composites, NaNH2–NaOH

Composites of NaNH₂ and the omnipresent NaOH have a lower melting temperature and form a non-stoichiometric solid solution, Na(OH)_1−x(NH₂)_x, during heating.