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The reactive hydride composite (RHC) LiBH4–MgH2 is regarded as one of the most promising materials for hydrogen storage. Its extensive application is so far limited by its poor dehydrogenation kinetics, due to the hampered nucleation and growth process of MgB2. Nevertheless, the poor kinetics can be improved by additives. This work studied the growth process of MgB2 with varying contents of 3TiCl3·AlCl3 as an additive, and combined kinetic measurements, X-ray diffraction (XRD), and advanced transmission electron microscopy (TEM) to develop a structural understanding. It was found that the formation of MgB2 preferentially occurs on TiB2 nanoparticles. The major reason for this is that the elastic strain energy density can be reduced to ~4.7 × 107 J/m3 by creating an interface between MgB2 and TiB2, as opposed to ~2.9 × 108 J/m3 at the original interface between MgB2 and Mg. The kinetics of the MgB2 growth was modeled by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation, describing the kinetics better than other kinetic models. It is suggested that the MgB2 growth rate-controlling step is changed from interface- to diffusion-controlled when the nucleation center changes from Mg to TiB2. This transition is also reflected in the change of the MgB2 morphology from bar- to platelet-like. Based on our observations, we suggest that an additive content between 2.5 and 5 mol% 3TiCl3·AlCl3 results in the best enhancement of the dehydrogenation kinetics. [ABSTRACT FROM AUTHOR]

The hampered kinetics of reactive hydride composites (RHCs) in hydrogen storage and release, which limits their use for extensive applications in hydrogen storage S1and energy conversion, can be improved using additives. However, the mechanism of the kinetic restriction and the additive effect on promoting the kinetics have remained unclear. These uncertainties are addressed by utilizing versatile transmission electron microscopy (TEM) on the LiBH4-MgH2 composite under the influence of the 3TiCl3·AlCl3 additives. The formation of the MgB2 phase, as the rate-limiting step, is emphatically studied. According to the observations, the heterogeneous nucleation of MgB2 relies on different nucleation centers (Mg or TiB2 and AlB2). The varied nucleation and growth of MgB2 are related to the in-plane strain energy density at the interface, resulting from the atomic misfit between MgB2 and its nucleation centers. This leads to distinct MgB2 morphologies (bars and platelets) and different performances in the dehydrogenation kinetics of LiBH4-MgH2. It was found that the formation of numerous MgB2 platelets is regarded as the origin of the kinetic improvement. Therefore, to promote dehydrogenation kinetics in comparable RHC systems for hydrogen storage, it is suggested to select additives delivering a small atomic misfit. [ABSTRACT FROM AUTHOR]

The Lithium–Boron Reactive Hydride Composite System (Li-RHC) (2 LiH + MgB 2 /2 LiBH 4 + MgH 2) is a high-temperature hydrogen storage material suitable for energy storage applications. Herein, a comprehensive gas-solid kinetic model for hydrogenation is developed. Based on thermodynamic measurements under absorption conditions, the system's enthalpy ΔH and entropy ΔS are determined to amount to −34 ± 2 kJ∙mol H 2 −1 and −70 ± 3 J∙K−1∙mol H 2 −1, respectively. Based on the thermodynamic behavior assessment, the kinetic measurements' conditions are set in the range between 325 °C and 412 °C, as well as between 15 bar and 50 bar. The kinetic analysis shows that the hydrogenation rate-limiting-step is related to a one-dimensional interface-controlled reaction with a driving-force-corrected apparent activation energy of 146 ± 3 kJ∙mol H 2 −1. Applying the kinetic model, the dependence of the reaction rate constant as a function of pressure and temperature is calculated, allowing the design of optimized hydrogen/energy storage vessels via finite element method (FEM) simulations. [Display omitted] • Experimental determination of hydrogenation thermodynamic parameters. • Critical analysis of the reported hydrogenation thermodynamic data. • Development of a comprehensive semi-empirical absorption kinetic model. • One-dimensional interface-controlled reaction upon hydrogenation. [ABSTRACT FROM AUTHOR]

2LiH + MgB 2 composite doped with TiO 2 (Li-RHC-Ti) is employed with a two-fold purpose: hydrogen purification under a H 2 –CO (0.1 mol%) mixture and CO methanation. Upon dynamic cycling under CO–H 2 mixture, hydrogen release curves display a quite stable amount of pure hydrogen of about 10 wt%, short release times of around 60 min, and minor degradation. Gas analysis by Fourier transform infrared spectroscopy (FTIR) after a thermal dehydrogenation process of MgH 2 and LiBH 4 under CO evidence the conversion of CO to CH 4. Li-RHC-Ti dehydrogenated under CO shows the simultaneous formation of CH 4 , CH 3 OH, and B(CH 3) 3 in the gas phase. X-ray powder diffraction (XRPD) and FTIR characterizations of the solid phases of Li-RHC-Ti after both H 2 –CO mixture and CO interactions demonstrate the formation of MgO, LiBO 2, and HCOO− species. Li-RHC-Ti acts as a hydrogen source and promoter for the CO conversion. Reaction pathways are proposed based on experimental results and equilibrium composition calculations. Image 1 • Li-RHC (2LiH + MgB 2 /2LiBH 4 + MgH 2) composite doped with TiO 2 (Li-RHC-Ti). • Dual ability of Li-RHC-Ti to purify CO–H 2 and to reduce this CO into CH 4. • Hydrides-CO systems for the preparation of gas mixtures based on CH 4. • Fast and stable dynamic cycling under a H 2 -CO mixture releasing 10 wt% H 2. • Formation of MgO, LiBO 2 and HCOO− after Li-RHC-Ti and CO interactions. [ABSTRACT FROM AUTHOR]

In recent years, the use of selected additives for improving the kinetic behavior of the system 2LiH + MgB2 (Li-RHC) has been investigated. As a result, it has been reported that some additives (e.g., 3TiCl3·AlCl3), by reacting with the Li-RHC components, form nanostructured phases (e.g., AlTi3) possessing peculiar microstructural properties capable of enhancing the system’s kinetic behavior. The effect of in-house-produced AlTi3 nanoparticles on the hydrogenation/dehydrogenation kinetics of the 2LiH + MgB2 (Li-RHC) system is explored in this work, with the aim of reaching high hydrogen storage performance. Experimental results show that the AlTi3 nanoparticles significantly improve the reaction rate of the Li-RHC system, mainly for the dehydrogenation process. The observed improvement is most likely due to the similar structural properties between AlTi3 and MgB2 phases which provide an energetically favored path for the nucleation of MgB2. In comparison with the pristine material, the Li-RHC doped with AlTi3 nanoparticles has about a nine times faster dehydrogenation rate. The results obtained from the kinetic modeling indicate a change in the Li-RHC hydrogenation reaction mechanism in the presence of AlTi3 nanoparticles.

The Mg10YNi alloy was hydrogenated and then coupled with LiBH4 to form LiBH4/Mg10YNi-H reactive hydride composite. The results indicate that thermal dehydrogenation stability of LiBH4 can be remarkably reduced by combining with Mg10YNi hydride. The starting and ending temperatures for hydrogen desorption from the LiBH4/Mg10YNi-H composite are approximately 275 and 430 ºC, respectively. Dehydrogenation of the LiBH4/Mg10YNi-H composite proceeds mainly in two steps with a total reaction of 12LiBH4 + 2.5Mg10YNiH20 → 24Mg + MgNi2.5B2 + 2.5YB4 + 12LiH + 43H2. After rehydrogenation at 450 ºC under 9 MPa hydrogen pressure, the LiBH4/Mg10YNi-H composite starts to release hydrogen around 260 ºC, and as much as approximately 5.2 wt.% of hydrogen can be desorbed during the second dehydrogenation process.

The Mg(BH 4 ) 2 -AlH 3 - x LiH ( x  = 0, 0.5, 1) novel ternary composites have been synthesized through mechanical ball milling, their favorable dehydrogenation properties and mechanism have been experimentally investigated. It can be found that during ball milling, a slight amount of Mg(BH 4 ) 2 would react with LiH and transform into LiBH 4 , while most Mg(BH 4 ) 2 remained stable after ball milling. Two major dehydrogenation steps can be detected in the Mg(BH 4 ) 2 -AlH 3 - x LiH composite, which belong to the decomposition of AlH 3 and Mg(BH 4 ) 2 , respectively. Remarkably, the onset hydrogen desorption temperature of Mg(BH 4 ) 2 -AlH 3 - x LiH composite can be reduced to 125.9 °C, with a total hydrogen desorption capacity of 10.3 wt%. It is worth emphasizing that the reversible dehydrogenation capacity is obviously improved to 3.8 wt%, compared to the Mg(BH 4 ) 2 -AlH 3 composite. This reversible capacity can be held for 3 cycles without obvious degeneration. Microstructural analysis on de/rehydrogenated composite has identified the regeneration of [BH 4 ] - in Mg(BH 4 ) 2 -AlH 3 -0.5LiH composite and revealed the role of LiH in the cycling hydrogen desorption and absorption processes. The formation of LiBH 4 and MgH 2 resulted from the rehydrogenation between MgAlB 4 and LiH could be helpful for the stability of reversible hydrogen storage performance. [ABSTRACT FROM AUTHOR]

The safest way to store hydrogen is in solid form, physically entrapped in molecular form in highly porous materials, or chemically bound in atomic form in hydrides. Among the different families of these compounds, alkaline and alkaline earth metals alumino-hydrides (alanates) have been regarded as promising storing media and have been extensively studied since 1997, when Bogdanovic and Schwickardi reported that Ti-doped sodium alanate could be reversibly dehydrogenated under moderate conditions. In this review, the preparative methods; the crystal structure; the physico-chemical and hydrogen absorption-desorption properties of the alanates of Li, Na, K, Ca, Mg, Y, Eu, and Sr; and of some of the most interesting multi-cation alanates will be summarized and discussed. The most promising alanate-based reactive hydride composite (RHC) systems developed in the last few years will also be described and commented on concerning their hydrogen absorption and desorption performance. [ABSTRACT FROM AUTHOR]

The development of materials showing hydrogen sorption reactions close to room temperature and ambient pressure will promote the use of hydrogen as energy carrier for mobile and stationary large-scale applications. In the present study, in order to reduce the thermodynamic stability of MgH2, Ni has been added to form Mg2NiH4, which has been mixed with various borohydrides to further tune hydrogen release reactions. De-hydrogenation/re-hydrogenation properties of Mg2NiH4-LiBH4-M(BH4)x (M = Na, K, Mg, Ca) systems have been investigated. Mixtures of borohydrides have been selected to form eutectics, which provide a liquid phase at low temperatures, from 110 °C up to 216 °C. The presence of a liquid borohydride phase decreases the temperature of hydrogen release of Mg2NiH4 but only slight differences have been detected by changing the borohydrides in the eutectic mixture. [ABSTRACT FROM AUTHOR]

The safest way to store hydrogen is in solid form, physically entrapped in molecular form in highly porous materials, or chemically bound in atomic form in hydrides. Among the different families of these compounds, alkaline and alkaline earth metals alumino-hydrides (alanates) have been regarded as promising storing media and have been extensively studied since 1997, when Bogdanovic and Schwickardi reported that Ti-doped sodium alanate could be reversibly dehydrogenated under moderate conditions. In this review, the preparative methods; the crystal structure; the physico-chemical and hydrogen absorption-desorption properties of the alanates of Li, Na, K, Ca, Mg, Y, Eu, and Sr; and of some of the most interesting multi-cation alanates will be summarized and discussed. The most promising alanate-based reactive hydride composite (RHC) systems developed in the last few years will also be described and commented on concerning their hydrogen absorption and desorption performance.

[Display omitted] • Development of a comprehensive semi-empirical dehydrogenation kinetic model. • H 2 –release from MgH 2 and LiBH 4 decompositions and MgB 2 + LiH formation. • One-dimensional interphase-controlled decomposition of MgH 2. • Autocatalytic LiBH 4 decomposition based on novel Prout-Tompkins model approach. • Integral and differential form of the combined dehydrogenation rate model. • Application of the differential form of the developed model for FE simulations. Among some promising candidates for high-capacity energy and hydrogen storage is the Lithium-Boron Reactive Hydride Composite System (Li-RHC: 2 LiH + MgB 2 /2 LiBH 4 + MgH 2). This system desorbs hydrogen only at relatively high temperatures and presents a two-step series of reactions occurring in different time scales: first, MgH 2 desorbs, followed by LiBH 4. Hitherto, the dehydrogenation kinetic behavior of such a system has been described for different temperatures at specific values of operative pressure. However, a comprehensive model representing its dehydrogenation kinetic behavior under different operative conditions has not yet been developed. Herein, the separable variable method is applied to develop a comprehensive kinetic model, including the two-step dehydrogenation series reaction. The MgH 2 decomposition is described with the one-dimensional interface-controlled reaction rate Johnson-Mehl-Avrami-Erofeyev-Kholmogorov (JMAEK) with a (P equilibrium /P operative) pressure functionality and an Arrhenius temperature dependence activation energy of 63 ± 3 kJ/mol H 2. The LiBH 4 decomposition is modeled applying the autocatalytic Prout-Tompkins model. A novel approach to describe the Prout-Tompkins t 0 parameter as a function of the operative temperature and pressure model is proposed. This second reaction step presented a (P equilibrium – P operative /P equilibrium)2 pressure dependence and an Arrhenius temperature dependence with activation energy 94 ± 13 kJ/mol H 2. The proposed approach is experimentally and computationally validated, successfully describing the decomposition kinetic behavior of MgH 2 and LiBH 4 under three-phase gas, liquid and solid environment and shows good agreement between experimental and modeled curves. [ABSTRACT FROM AUTHOR]

In this work, hydrogen storage properties and reaction mechanism of NaAlH 4 –Mg(BH 4 ) 2 (2:1) composites system with and without additive have been investigated. The secondary-hydride system was found to initiate a transformation of the NaAlH 4 /Mg(BH 4 ) 2 to Mg(AlH 4 ) 2 /NaBH 4 during ball milling, and there was a mutual destabilization among the hydrides. Three major dehydrogenation steps were observed in the systems, which corresponded to the decomposition of Mg(AlH 4 ) 2 , MgH 2 , and NaBH 4 . Temperature-programmed desorption results showed that the TiF 3 -doped NaAlH 4 –Mg(BH 4 ) 2 (2:1) composite sample started to release hydrogen at about 75 °C, which was 60 °C lower than that of undoped NaAlH 4 –Mg(BH 4 ) 2 (2:1) composite sample. In addition, the reaction pathway of the NaAlH 4 –Mg(BH 4 ) 2 (2:1) composite system, and the mechanisms that worked in this composite during the de/rehydrogenation process were determined by X-ray diffraction. The Kissinger analysis has shown that the apparent activation energy, E A , for decomposition of NaBH 4 in the NaAlH 4 –Mg(BH 4 ) 2 –TiF 3 composite reduced to 139.85 kJ/mol compared with 155.73 kJ/mol in NaAlH 4 –Mg(BH 4 ) 2 composite. It is believed that the enhancement of the de/rehydrogenation properties of undoped NaAlH 4 /Mg(BH 4 ) 2 was attributed to the formation of intermediate compounds, including Mg–Al and Mg–Al–B alloys, upon dehydrogenation, which changed the thermodynamics of the reactions by altering the de/rehydrogenation pathway. Meanwhile, as for the doped sample, the TiF 3 component played a catalytic role through the formation of Ti-containing and F-containing catalytic species, which might have promoted the interaction of Mg(AlH 4 ) 2 and NaBH 4 , and thus, further improved the dehydrogenation of the NaAlH 4 –Mg(BH 4 ) 2 (2:1) system. [ABSTRACT FROM AUTHOR]

The 2LiBH 4 –MgH 2 reactive hydride composite is a promising hydrogen storage system due to the combined high hydrogen capacity and relatively moderate reaction enthalpy. However, the sluggish de/rehydrogenation kinetics severely impedes its practical applications. In this study, graphitic carbon nitride (C 3 N 4 ) as a metal-free additive was added to the 2LiBH 4 –MgH 2 composite and examined with respect to the promoting effect on the hydrogen storage properties of the composite. Our study found that mechanically milling with small amount of C 3 N 4 additive can eliminate the incubation period between two dehydrogenation steps and thus markedly enhance the dehydrogenation kinetics of the LiBH 4 –MgH 2 composite. Further cyclic study found that the composite with C 3 N 4 additive exhibits improved cyclic dehydrogenation property although it also shows capacity loss upon cycling, particularly in the second cycle. Combined dehydrogenation property, phase analysis and a series of designed experiments suggested that the C 3 N 4 additive could react with both LiBH 4 and MgH 2 in heating process, and the resulting products may improve the reversible dehydrogenation property of the composite system. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we report a novel method of improving the reversible dehydrogenation properties of the 2LiBH4–MgH2 composite. Our study found that mechanically milling with small amount of Al powder can markedly shorten or even eliminate the problematic incubation period that interrupts the dehydrogenation steps of the 2LiBH4–MgH2 composite. But the resulting composite showed serious kinetics degradation upon cycling. In an effort to solve this problem, we found that combined usage of small amounts of Al and MgO enabled the 2LiBH4–MgH2 composite to rapidly and reversibly deliver around 9 wt% hydrogen at 400 °C under 0.3 MPa H2, which compares favorably with the dehydrogenation performance of the composites with transition-metal additives. A combination of phase/microstructural analyses and series of control experiments has been conducted to gain insight into the promoting effects of Al and MgO. It was found that Al and MgO additives act as precursor and promoter for the formation of AlB2 heterogeneous nucleation sites, respectively. [Copyright &y& Elsevier]

By directly doping transition metal boride (NbB2, ZrB2, and CeB6) nanoparticles with size of less than 10 nm into 2LiBH4/MgH2 composites, a significant catalytic enhancement is achieved. As the representative catalyzed system, the nanoNbB2‐doped composite can release ca. 9.0 wt% H2 in 15 min and reabsorb ca. 9.0 wt% H2 in 5 min, which show more than 30 and 20 times faster than undoped system. [ABSTRACT FROM AUTHOR]

Abstract: In this study, activated carbon (AC) was added to the 2LiBH4–MgH2 composite and examined with respect to its effect on the hydrogen storage properties of the system. Our study found that AC is an effective additive for promoting the reversible dehydrogenation of the 2LiBH4–MgH2 composite. A series of control experiments were carried out to optimize the sample preparation method, milling time and addition amount of AC. In comparison with the neat LiBH4–MgH2 system, the LiBH4–MgH2–AC composite prepared under optimized conditions exhibits enhanced dehydrogenation kinetics, improved cyclic stability and particularly, eliminated incubation period between the two dehydrogenation stages. A combination of phase/microstructure/chemical state analyses has been conducted to gain insight into the promoting effect of AC on the reversible dehydrogenation of the 2LiBH4–MgH2 system. Our study found that AC exerts its promoting effect via tailoring nanophase structure of the 2LiBH4–MgH2 composite. [Copyright &y& Elsevier]

Abstract: Synthesis of lithium borohydridofluorides may pave a new way to pursue improved hydrogen storage properties of LiBH4 as reversible hydrogen storage materials that fit the fuel cell application. The main products of the hydrogen absorption by 2LiF–MgB2 composite are MgF2 and LiBH4. In addition to them, LiBH4−x F x compounds might be present during hydrogen absorption–desorptions and play important role on their kinetics and reversibility. First results of Near Edge X-ray Absorption Fine Structure (NEXAFS) at the B K-edge (193 eV) for LiF–MgB2 composite with molar ratio (2:1) are presented. Obtained results indicate a formation of mixed borohydrides/borofluorides of the type of LiBH4−xFx, thus suggesting fluorine substituting for hydrogen. [Copyright &y& Elsevier]

Abstract: The hydrogenation behavior of 3CaH2+4MgB2+CaF2 composite was studied by manometric measurements, powder X-ray diffraction, differential scanning calorimetry and attenuated total reflection infrared spectroscopy. The maximum observed quantity of hydrogen loaded in the composite was 7.0wt%. X-ray diffraction showed the formation of Ca(BH4)2 and MgH2 after hydrogenation. The activation energy for the dehydrogenation reaction was evaluated by DSC measurements and turns out to be 162±15kJmol−1 H2. This value decreases due to cycling to 116±5kJmol−1 H2 for the third dehydrogenation step. A decrease of ca. 25–50°C in dehydrogenation temperature was observed with cycling. Due to its high capacity and reversibility, this composite is a promising candidate as a potential hydrogen storage material. [Copyright &y& Elsevier]

Abstract: In this work, the dehydrogenation kinetics of the 2LiBH4 +MgH2 composite under different hydrogen back pressures was studied. The applied hydrogen back pressure significantly influenced the hydrogen release rate of the uncatalyzed composite. Higher hydrogen pressures enhanced the nucleation of MgB2, resulting in better dehydrogenation kinetics and cycle stability. The composite with a CuCl2 catalyst demonstrated significantly improved dehydrogenation kinetics because the nucleation of MgB2 was promoted by heterogeneous nuclei. However, similar effects of hydrogen back pressure on dehydrogenation kinetics were also observed for the CuCl2-catalyzed composite. The extraordinary results suggest that hydrogen back pressure plays an indispensable role in the formation of MgB2, which determines not only the reaction pathway but also the kinetics of dehydrogenation for the 2LiBH4 +MgH2 composite. [Copyright &y& Elsevier]

Abstract: LiBH4+1/2MgH2 is a promising reactive hydride composite for hydrogen storage. In the present study, three Ce-based additives were used as catalysts to enhance the hydrogen storage performance of LiBH4+1/2MgH2 composites. The composites with Ce additives demonstrated significantly improved dehydrogenation kinetics and cyclic stability compared with the pure composite. X-ray diffraction and scanning electron microscopy analyses clearly revealed the phase transitions and morphological evolution during the hydriding-dehydriding cycling. The composites with Ce-based additives displayed stable nanostructures, in contrast to the rapid microstructural deterioration in the uncatalyzed composite. The CeB6 formed in the composites had a particle size of 10 nm after five cycles. It may act as the nucleus for MgB2 formation during dehydrogenation and thus account for the structural and performance stability of the composites upon cycling. [Copyright &y& Elsevier]

In recent years, the use of selected additives for improving the kinetic behavior of the system 2LiH + MgB2 (Li-RHC) has been investigated. As a result, it has been reported that some additives (e.g., 3TiCl3·AlCl3), by reacting with the Li-RHC components, form nanostructured phases (e.g., AlTi3) possessing peculiar microstructural properties capable of enhancing the system's kinetic behavior. The effect of in-house-produced AlTi3 nanoparticles on the hydrogenation/dehydrogenation kinetics of the 2LiH + MgB2 (Li-RHC) system is explored in this work, with the aim of reaching high hydrogen storage performance. Experimental results show that the AlTi3 nanoparticles significantly improve the reaction rate of the Li-RHC system, mainly for the dehydrogenation process. The observed improvement is most likely due to the similar structural properties between AlTi3 and MgB2 phases which provide an energetically favored path for the nucleation of MgB2. In comparison with the pristine material, the Li-RHC doped with AlTi3 nanoparticles has about a nine times faster dehydrogenation rate. The results obtained from the kinetic modeling indicate a change in the Li-RHC hydrogenation reaction mechanism in the presence of AlTi3 nanoparticles. [ABSTRACT FROM AUTHOR]