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Solid state hydrogen storage may have a strong impact on future storage of renewable energy. Here we explore possible synergy effect in multi-component systems based on the lithium borohydride magnesium nickel hydride reactive hydride composite (LiBH 4 –Mg 2 NiH 4). The composites of LiBH 4 –Mg 2 NiH 4 are prepared by ball milling of LiBH 4 with Mg 2 NiH 4 –MgNi 2 or Mg 2 NiH 4 –MgH 2 –Ni. Thermal analysis reveals that LiBH 4 –Mg 2 NiH 4 –MgNi 2 provides 20 °C lower onset dehydrogenation temperature compared to Mg 2 NiH 4 –MgH 2 –Ni. The Powder X-ray diffraction analysis of dehydrogenated samples indicates the similar dehydrogenation mechanisms producing the reversible phase MgNi 2.5 B 2 , with a hydrogen storage capacity of 3.6 wt% at T = 400 °C under 5 bar of hydrogen back pressure. The addition of transition metal halide additives (MnF 2 , NbF 5 , TiF 3 , ZnF 2 , and TiCl 3) further reduces dehydrogenation temperatures from 308 °C to 260–266 °C (Δ T = 42–48 °C and initiates the dehydrogenation process by destabilizing LiBH 4. Among all additives, ZnF 2 shows the best performance offering an improved hydrogen capacity (3.76 wt%), lower dehydrogenation temperature, and suppression of diborane gas formation. [Display omitted] • Investigation of synergy effects in multi-component systems based on LiBH 4 –Mg 2 NiH 4 for solid-state hydrogen storage. • Incorporation of MgNi 2 reduces the onset dehydrogenation temperature by approximately 20 °C. • Transition metal halides such as ZnF 2 further lower the dehydrogenation peak temperature by 42–48 °C. • ZnF 2 enhances hydrogen storage capacity to 3.76 wt% and suppresses diborane gas release. [ABSTRACT FROM AUTHOR]

• Simulation of hydrogen absorption in different metal hydride materials and designs. • Evaluation of energy demand and temperature variations at selected measuring points. • Tank design affects absorption properties of the studied medium-temperature hydride. • A rectangular tank design is best for fast hydrogen absorption with minimal heating. • Cylindrical coiled tanks optimize hydrogen absorption for medium-temperature hydrides. The efficient, space-saving and safe storage of hydrogen is a major challenge that needs to be overcome for enabling renewable energy systems. Metal hydrides are a possible solution. But the key challenge is the identification and development of the most promising metal hydride material as well as the ideal tank design for an efficient hydrogen absorption / desorption in terms of energy demand / storage losses and loading / unloading time. Against this background this paper aims to identify suitable combinations of medium and low-temperature metal hydride materials in combination with three different tank design concepts. The goal is to determine which material fits best for each combination and could thus be a suitable solution for a future implementation in stationary and mobile applications of metal hydride storage tanks. To achieve this goal a finite element method (FEM) modeling and simulation of materials and construction designs in COMSOL Multiphysics is realized. The results are analyzed in terms of hydrogen absorption rate, temperature profile over time, and the necessary energy demand for the overall storage process. The results show that for the low-temperature metal hydride investigated here, the tank design is of subordinate importance, allowing for more application-specific design. For medium-temperature metal hydrides, the investigated construction concepts show heterogeneous results. For fast hydrogen absorption and minimal external heating time, the suggested rectangular tank design might be a promising option, requiring only 28% / 29% of the heating energy of the cylindrical concepts. If the goal is to achieve the most complete hydrogen absorption, the base design concept investigated here, consisting of a cylindrical tank with metal hydride material rolled up in a spiral, is the most favorable solution; achieving a hydrogen loading of about 3.6 wt–% for the medium-temperature metal hydride. The low-temperature metal hydride achieves a total hydrogen absorption of around 1.4 wt-% in the optimum concept. For concepts with higher operating temperatures, preheating the storage tank before feeding in the hydrogen could improve the absorption process (only examined here). [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]

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]

[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]

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: 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]