Recyclable hydrogen storage system composed of ammonia and alkali metal hydride

Ammonia (NH₃) reacts with alkali metal hydrides MH (M = Li, Na, and K) in an exothermic reaction to release hydrogen (H₂) at room temperature, resulting that alkali metal amides (MNH₂) which are formed as by-products. In this work, hydrogen desorption properties of these systems and the condition for the recycle from MNH₂ back to MH were investigated systematically. For the hydrogen desorption reaction, the reactivities of MH with NH₃ were better following the atomic number of M on the periodic table, Li < Na < K. It was confirmed that the hydrogen absorption reaction of all the systems proceeded under 0.5 MPa of H₂ flow condition below 300 °C.     

Efficient and highly rapid hydrogen release from ball-milled 3NH3BH3/MMgH3 (M = Na,K, Rb) mixtures at low temperatures

The stoichiometric reactions of ammonia borane (NH₃BH₃, AB) and selected alkali or alkaline-earth metal hydrides produce metal amidoboranes, which possess dehydrogenation property advantages over their parent AB. However, the losses of hydrogen capacity and chemical energy in the preparation process make metal amidoboranes less energy-effective for hydrogen storage application. In the present study, by combining the M⁺–Mg²⁺ double cations remarkably lowers the reactivity of the alkali metal hydrides toward AB. As a result, the starting Mg-based ternary hydrides MMgH₃ (M = Na, K, Rb) and AB phases are largely stable in the mechanical milling process, but transform to the corresponding mixed-cation amidoboranes in the subsequent heating process. Importantly, when the post-milled 3AB/MMgH₃ mixtures are isothermally heated at above 60 °C using water bath, the formation and decomposition processes of the mixed-cation amidoboranes can be favorably combined, giving rise to rapid and efficient dehydrogenation performances at the mild temperatures (60–80 °C). The results acquired may provide a generalized reactions coupling strategy for designing and synthesis other potentially efficient hydrogen storage system.     

Dehydrogenation reactions of mechanically activated alkali metal hydrides with CO2 at room temperature

Reactions between alkali metal hydrides MH (M = Li, Na, or K) and carbon dioxide (CO₂) at room temperature were systematically investigated for the first time. It was found that the raw alkali metal hydrides did not react with CO₂ under static-pressure conditions at room temperature, but the mechanically activated alkali metal hydrides reacted with CO₂ and released large amounts of hydrogen (H₂). Under the same ball-milling conditions, the order of reactivity of the alkali metal hydrides with CO₂ was KH > NaH > LiH. The particle size of the activated alkali metal hydrides had a large influence on the reactivity of the alkali metal hydrides with CO₂. During the reactions, CO₂ was reduced by alkali metal hydrides, generating elemental carbon, alkali metal oxides and H₂, and it was consumed by alkali metal oxides, forming carbonates.     

Amidoboranes and hydrazinidoboranes: State of the art,potential for hydrogen storage,and other prospects

Following the seminal B–N–H compound ammonia borane NH₃BH₃, a series of derivatives have been developed as possible chemical hydrogen carriers. First, alkali and alkaline earth derivatives of ammonia borane, i.e. amidoboranes M(NH₂BH₃)_n with n = 1 or 2, emerged. Then, hydrazine borane, which is actually a derivative of ammonia borane, was re-discovered and considered as a precursor of alkali and alkaline earth derivatives, i.e. hydrazinidoboranes M(N₂H₃BH₃)_n. A number of B–N–H ionic salts were, in this way, synthesized and reported. The present review provides the general background (e.g. syntheses, crystallographic structures, thermal stability, dehydrogenation properties, and decomposition mechanisms) relating to these alkali and alkaline earth amidoboranes and hydrazinidoboranes, and carries on two objectives. The first objective is to discuss the potential of these materials (for the application for which they have been primarily developed over the past decades, namely hydrogen storage) and the challenges they are facing. It is concluded that the light alkali amidoboranes, namely the lithium, sodium and potassium amidoboranes) should be further developed owing to better dehydrogenation properties in terms of the onset temperature of dehydrogenation, the extent of dehydrogenation, and the purity of the hydrogen released. They should, however, be further developed with the aim of upscaling, something that has not yet been achieved. Achieving high technological readiness levels is thereby the main challenge ahead. The second objective of the present review is to explore alternative uses because, as things stand, none of the developed materials have the expected features for hydrogen storage. We suggest that any of the discussed derivatives are potentially solid-state reducing agents, energetic materials, and/or precursors of boron nitride-based ceramics, bearing in mind that for this last use, if the targeted material is porous, it could be regarded as a reversible hydrogen sorbent. It is thus important to maintain efforts in the development of the current derivatives as well as in the discovery of novel ones.     

Compressed hydrogen production via reaction between liquid ammonia and alkali metal hydride

Ammonia NH₃ is recognized as one of the attractive hydrogen H₂ carriers because it has a high hydrogen content of 18 mass% and it is easily liquefied under about 1 MPa of pressure at a room temperature. NH₃ can react with alkali metal hydrides and generate H₂ even at room temperature, resulting that metal amides are formed as reaction products. The H₂ generation is exothermic reaction, and it is not effectively prevented by H₂ partial pressure in a closed system as thermodynamic properties. In this work, we demonstrated the production of compressed H₂ by the reaction between liquid NH₃ and lithium hydride LiH in a closed pressure vessel, where liquid NH₃ would realize better kinetic properties for the reaction with metal hydride than gaseous NH₃. Actually, more than 12 MPa H₂ was obtained within several hours.     

Alkali metal silanides α-MSiH3: A family of complex hydrides for solid-state hydrogen storage

The alkali metal silanides α-MSiH₃ appear to be a promising family of complex hydrides for solid-state hydrogen storage. Herein the structural, energetic and electronic properties of α-MSiH₃ silanides (M = Li, Na, K, Rb, Cs) and MSi Zintl phases are systematically investigated for the first time by using first-principles calculations method based on density functional theory. The structural parameters of α-MSiH₃ and MSi including lattice constants and atomic positions are determined through geometry optimization. The obtained results are close to the experimental data analysed from X-ray and neutron powder diffraction. The calculations of formation enthalpy show that α-KSiH₃, α-RbSiH₃ and α-CsSiH₃ silanides are easier to be synthetized relative to α-LiSiH₃ and α-NaSiH₃, which interprets well the lower thermostabilities of experimental α-LiSiH₃ and α-NaSiH₃. Nevertheless, LiSi, KSi and CsSi phases are easier to be formed relative to NaSi and RbSi. The calculations of hydrogen desorption enthalpy reveal that the dehydrogenation abilities of α-MSiH₃ silanides along the decomposition path of α-MSiH₃→MSi + H₂ are gradually enhanced in the order of α-CsSiH₃, α-RbSiH₃, α-KSiH₃, α-NaSiH₃ and α-LiSiH₃, which may be originated from their decreasing thermostabilities. From a comprehensive point of view including hydrogen storage capacity, thermostability and dehydrogenation ability, α-KSiH₃ (～4.29 wt%) is identified as the most promising alkali metal silanide for reversible hydrogen storage. Analysis of electronic structures indicates that a significant charge transfer leads to positively charged M ions and negatively charged SiH₃ complex, which constitutes the ionic bonding between them. The bonding within SiH₃ complex not only involves the covalent hybridization between Si (3s) (3p) and H (1s) orbitals, but also exhibits some ionic bond characteristics due to the partial charge transfer from Si to H. The covalent bonding interactions between H and Si atoms within SiH₃ mainly dominate the thermostabilities and dehydrogenation properties of α-MSiH₃ silanides.     

Selective hydrogen absorption from gaseous mixtures by BCC Ti-V alloys

Hydrogen absorption behaviour of the BCC Ti-V alloys has been studied as related to their potential application for the selective hydrogen absorption from mixtures with active gases containing CO and steam at high temperatures. Alloys and their hydrides were characterised using Scanning Electron Microscopy (SEM), Thermal Desorption Spectroscopy (TDS), Temperature Programmed Reaction (TPR) and in situ synchrotron X-ray powder diffraction (SR-XRD). BCC Ti_0.8V_0.2 and Ti_0.9V_0.1 were able to absorb 3.95 wt.% H by forming a FCC type hydride γ−(Ti,V)H₂. Nanoparticles of Pd and Pd/Pt were electroless deposited on the surface of the hydrides to catalyse hydrogen absorption. TPR tests showed that such alloys were capable of absorbing hydrogen even when substantial amounts of CO and H₂O were present in the gas stream. Nanoparticles of Pd/Pt provide a better performance as compared to Pd alone. In situ SR-XRD has been used to probe the mechanisms of hydrogenation and dehydrogenation.     

Dehydrogenation and low-pressure hydrogenation properties of NaAlH4 confined in mesoporous carbon black for hydrogen storage

For hydrogen to be successfully used as an energy carrier in a new renewable energy driven economy, more efficient hydrogen storage technologies have to be found. Solid-state hydrogen storage in complex metal hydrides, such as sodium alanate (NaAlH₄), is a well-researched candidate for this application. A series of NaAlH₄/mesoporous carbon black composites, with high NaAlH₄ content (50–90 wt%), prepared via ball milling have demonstrated significantly lower dehydrogenation temperatures with intense dehydrogenation starting at ∼373 K compared to bulk alanate's ≥ 456 K. Dehydrogenation/hydrogenation cycling experiments have demonstrated partial hydrogenation at 6 MPa H₂ and 423 K. The cycling experiments combined with temperature-programmed dehydrogenation and powder X-ray diffraction have given insight into the fundamental processes driving the H₂ release and uptake in the NaAlH₄/carbon composites. It is established that most of the hydrogenation behavior can be attributed to the Na₃AlH₆ ↔ NaH transition.     

The effect of Na addition on the first hydrogen absorption kinetics of cast hypoeutectic Mg–La alloys

With superior properties of Mg such as high hydrogen storage capacity (7.6 wt% H/MgH₂), low price, and low density, Mg has been widely studied as a promising candidate for solid-state hydrogen storage systems. However, a harsh activation procedure, slow hydrogenation/dehydrogenation process, and a high temperature for dehydrogenation prevent the use of Mg-based metal hydrides for practical applications. For these reasons, Mg-based alloys for hydrogen storage systems are generally alloyed with other elements to improve hydrogen sorption properties. In this article, we have added Na to cast Mg–La alloys and achieved a significant improvement in hydrogen absorption kinetics during the first activation cycle. The role of Na in Mg–La has been discussed based on the findings from microstructural observations, crystallography, and first principles calculations based on density functional theory. From our results in this study, we have found that the Na doped surface of Mg–La alloy systems have a lower adsorption energy for H₂ compared to Na-free surfaces which facilitates adsorption and dissociation of hydrogen molecules leading to improvement of absorption kinetic. The effect of Na on the microstructure of these alloys, such as eutectic refinement and a density of twins is not highly correlated with absorption kinetics.     

Substituted heterocycles as new candidates for liquid organic hydrogen carriers: In silico design from DFT calculations

A new set of compounds based on N- and S-heterocycles were investigated through Density Functional Theory (DFT) for their use as liquid organic hydrogen carriers (LOHCs). The hydrogenated forms of these compounds could release hydrogen within the most important technical requirements in mobile and stationary applications. In this work, the potential of the 1H-pyrrole/tetrahydro-1H-pyrrole and thiophene/tetrahydrothiophene pairs as possible leader structures to synthesize more sustainable LOHCs from costless oil-refining and oil-hydrotreating by-products is shown. According to DFT-M06-HF results, the 3-allyl-1H-pyrrole/3-allyl-tetrahydro-1H-pyrrole pair presented an adequate theoretical hydrogen storage capacity (3.6 %wt H) and a high theoretical dehydrogenation equilibrium yields (% ε_d = 67.8%) at 453 K. Therefore, this pair is recommended for hydrogen storage stationary applications. On the other hand, the 2-(thiophen-2-yl)-1H-pyrrole/2-(2,3-dihydrothiophen-2-yl)tetrahydropyrrole pair proved to be suitable for both mobile and stationary applications; the storage capacity of this pair was 3.9 %wt H and the theoretical dehydrogenation equilibrium yields at 453 K (% ε_d = 28.1%) was considered moderate.     

Enthalpy analysis of Ce–Mg–Ni–H formation based on extended miedema theory: Investigation of selected Ce2MgNi2–H2

In this paper, an extended Miedema's model is constructed to illustrate its applicability to estimating the solid-solution enthalpies of Ce–Mg–Ni–H hydrides, adopting the range of an optimized stoichiometry alloy in the contour map of solid-solution state enthalpy. Ce₂MgNi₂ alloy is designed to investigate its hydrogen storage properties, and its main phase is confirmed with X-ray diffraction characterizations. The alloy shows a good activation ability and the pressure component temperature plateau is extremely flat. The formation enthalpy of Ce₂MgNi₂–H₂ is calculated with the extended Miedema theory, with the least enthalpy value of ?59.1 kJ/mol for the corresponding hydrogen content of 1.64 wt %. Both experimental and theoretical data of the hydrogen-containing alloy confirm that the thermodynamic enthalpy of the quaternary Ce₂MgNi₂–H₂ is consistent with that of the experimental results. When calculating the formation enthalpy of hydrogen and metal, the enthalpy of the elastic contribution between metal and hydrogen was considered, generally improving the versatility and accuracy of the calculation. Moreover, the extended Miedema's model is used to predict the hydrogen storage performance.     

Reversibility aspect of lithium borohydrides

Reversibility is one of the key features for any hydrogen storage material. Borohydrides such as LiBH₄ have been studied or proposed as candidates for hydrogen storage because of their high hydrogen contents (18.4 wt% for LiBH₄). Limited success has been made in reducing the dehydrogenation temperature. However, full reversibility has not been realized. It is found that the dehydrogenation mechanism of metal borohydrides differs signicantly from the well-known metal hydrides such as LaNi₅H₆ and MgH₂ that release hydrogen in a single decomposition step through a solid state transformation of crystalline structure. The dehydrogenation of lithium borohydrides involves solid–liquid–gas reactions. Some of the steps in the multiple step decomposition processes of metal borohydrides are not reversible. Furthermore, the decomposition also produces stable intermediate compounds that cannot be rehydrided easily. Lastly, the volatile gases, such as BH₃ and B₂H₆, evolved in decomposition of the transition metal borohydrides cause unrecoverable boron loss. Although our experiments show the partial reversibility of the doped LiBH₄, it was not sustainable during dehydriding–rehydriding cycles because of the accumulation of hydrogen inert species and boron loss. Doping with additives reduces the stability of LiBH₄, but it also makes LiBH₄ less reversible. It raises reasonable doubt on the feasibility of making metal borohydrides suitable for reversible hydrogen storage.     

The thermal decomposition of hydrogen sulfide over alkali metal sulfides and polysulfides

The decomposition of hydrogen sulfide to hydrogen and sulfur on alkali metal sulfides M₂S (M = Li, Na and K) and polysulfides M₂S_x (x = 2–4; M = Na and K) has been studied in a flow system at 400–800°C. Hydrogen yields were measured as a function of temperature in order to compare the effectiveness of the alkali metal (poly)sulfides in promoting the decomposition. It was found that both Na₂S and K₂S are rapidly sulfided by H₂S to give the corresponding disulfides M₂S₂, whereas Li₂S acts as a catalyst for the thermal decomposition of H₂S. Although Na₂S₂ was further sulfided by H₂S, sodium polysulfides were not effective in enhancing hydrogen yields and all polysulfides produced an amorphous product of approximate composition Na₂S_2.4–2.6. With potassium polysulfides the final product was K₂S₃ in all cases and the sulfidation of K₂S₂ by hydrogen sulfide resulted in an enhancement of hydrogen yields at 500–700°C.     

Light metal hydride-based hydrogen storage system: Economic assessment in Argentina

Motivated by the high potential for hydrogen production from renewable resources in Argentina, the economic feasibility of employing light complex metal hydrides as hydrogen storage materials for mobile applications in Argentina is explored for the first time. Three main costs are analyzed: green H₂, H₂ storage system based on Mg(NH₂)₂–LiH and storage tank. Considering the production of H₂ by electrolysis using wind energy, a cost of ~5 USD/kg H₂ is obtained. The cost of hydride matrix is crucial and competitive values are viable only if the synthesis route starts from Mg⁰ and Li⁰, allowing reducing the total hydride matrix cost from ~2200 to ~4900 USD. The cost of a modular configuration tank with 4 kg of H₂ capacity is estimated to be ~5300–6700 USD. A cost ratio higher than of 2:1 is obtained between storage systems based on amides and high pressure systems.     

First-principles prediction of structural,electronic and optical properties of alkali metals AM4BN3H10 hydrides

To explore the high storage capacity of hydrogen storage material, the structural feature and hydrogenated mechanism of Li₄BN₃H₁₀ alkali metal hydrides are studied by using the first-principles calculations. The calculated result predicts that the Li₄BN₃H₁₀ and Rh₄BN₃H₁₀ are thermodynamic and dynamical stabilities according to the phonon dispersion and thermodynamic model. Essentially, the hydrogen storage mechanism of AM₄BN₃H₁₀ hydride mainly depends on the formation of [BH₄] group and [NH₂] group. Compared to LiBH₄, the hybridization between the B atom and H atom in [BH₄] group and between the N atom and H atom in [NH₂] group can store a lot of hydrogen. However, the dehydrogenation of AM₄BN₃H₁₀ hydride prefers to [BH₄] group rather than the [NH₂] group. The narrow band gap is beneficial to hydrogen release in AM₄BN₃H₁₀ hydride. Therefore, the Rb₄BN₃H₁₀ has better hydrogen release properties in comparison to the Li₄BN₃H₁₀ and Na₄BN₃H₁₀. Therefore, we believe that the AM₄BN₃H₁₀ hydride is a promising hydrogen storage material with the high hydrogen storage capacity.     

The application of Pettifor structure maps to binary metal hydrides

The aim of this paper is the application of Pettifor stability structure maps to MH_x binary metal (M) hydrides in accordance with their hydrogenation properties. The formation enthalpy (ΔH_f) of selected hydrides, their stoichiometry (MH_x) and hydrogen capacity (H_cap) were extracted from the literature, where these values were either measured or calculated using various models. A comparison of the enthalpy values obtained from experiments or calculations based on the Miedema, Born–Haber and Energetic models is also presented. An assessment of the effect of the ΔH_f values obtained by various methods on the structure stability maps was performed by plotting the enthalpy vs. the Mendeleev number M and the value of the chemical scale, χ. Moreover, based on two- and three-dimensional Pettifor plots, the enthalpy of hydride formation and hydrogen capacity of selected hydrides are presented with respect to the hydride stoichiometry, H_x, and the Mendeleev number M. Higher values of M indicate less hydride stability and, thus, larger stability variations. The metal hydrides with only one hydrogen atom (x = 1) and M from 0 to 21 exhibit stability that is adequate for practical applications. For M from 21 to 53, all hydrides exhibit high stability regardless of H_x. For M from 53 to 102, all hydrides exhibit large variations in stability. Thus, the hydrides with x = 1–3 exhibit either high stability or stability that is adequate for practical applications, and those with x = 4 are highly unstable hydrides.     

Theoretical prediction of structure,electronic and optical properties of VH2 hydrogen storage material

The vanadium hydrides have better hydrogen storage capacity in comparison to the other metal hydrides. Although the structure of VH₂ hydride has been reported, the structural stability, electronic and optical properties of VH₂ hydride are unclear. To solve these problems, we apply the first-principles method to study the structural stability, electronic and optical properties of VH₂ hydrides. Similar to the metal dihydrides, four possible VH₂ hydrides such as the cubic (Fm-3m), tetragonal (I4/mmm), tetragonal (P4₂/mnm) and orthorhombic (Pnma) are designed. The result shows that the cubic VH₂ hydride is a thermodynamic and dynamical stability. In particular, the tetragonal (I4/mmm) and the orthorhombic (Pnma) VH₂ hydrides are firstly predicted. It is found that these VH₂ hydrides show metallic behavior. The electronic interaction of V (d-state)-H (s-state) is beneficial to improve the hydrogen storage in VH₂ hydride. In addition, the formation of V–H bond can improve the structural stability of VH₂ hydride. Based on the analysis of optical properties, it is found that all VH₂ hydrides show the ultraviolet response. Compared to the tetragonal and orthorhombic VH₂ hydrides, the cubic VH₂ hydride has better storage optical properties. Therefore, we believe that the VH₂ hydride is a promising hydrogen storage material.     

Investigation on standard entropy change of metal hydrides and work function of metals

We focused on the work function of metals as an index of ion binding property between metals and hydrogen and analyzed the relation between the standard entropy change of metal hydrides and the work function of metals (alloys). The standard entropy change of saline hydrides and metallic hydrides roughly increased with the work function. It was indicated that hydrogen storage alloys consist of metal elements A and B whose work functions are small and large, respectively. The standard entropy changes of alloy-based metal hydrides with different composition ratio ΔS⁰ linearly increased with the work functions of the alloys W_c. W_c also linearly increased with the bulk modulus B_c. Then, it was found that ΔS⁰ was proportional to the standard heat of formation ΔH⁰ because dW_c/dB_c, dΔS⁰/dW_c and dΔH⁰/dB_c are constant.