Effect of SWNTs on the reversible hydrogen storage properties of LiBH4–MgH2 composite

Single-walled carbon nanotubes (SWNTs) were mechanically milled with LiBH₄/MgH₂ mixture, and examined with respect to its effect on the reversible dehydrogenation properties of the Li–Mg–B–H system. Experimental results show that the addition of SWNTs results in an enhanced dehydriding rate and improved cyclic stability of the LiBH₄/MgH₂ composite. For example, the LiBH₄/MgH₂ composite with 10 wt% purified SWNTs additive can release nearly 10 wt% hydrogen within 20 min at 450 °C, with an average dehydriding rate over 2 times faster than that of the neat LiBH₄/MgH₂ sample. Based on the results of phase analysis and a series of designed experiments, the mechanism underlying the observed property improvement was discussed.     

Nanoconfined 2LiBH4–MgH2–TiCl3 in carbon aerogel scaffold for reversible hydrogen storage

Nanoconfinement of 2LiBH₄–MgH₂–TiCl₃ in resorcinol–formaldehyde carbon aerogel scaffold (RF–CAS) for reversible hydrogen storage applications is proposed. RF–CAS is encapsulated with approximately 1.6 wt. % TiCl₃ by solution impregnation technique, and it is further nanoconfined with bulk 2LiBH₄–MgH₂ via melt infiltration. Faster dehydrogenation kinetics is obtained after TiCl₃ impregnation, for example, nanoconfined 2LiBH₄–MgH₂–TiCl₃ requires ∼1 and 4.5 h, respectively, to release 95% of the total hydrogen content during the 1st and 2nd cycles, while nanoconfined 2LiBH₄–MgH₂ (∼2.5 and 7 h, respectively) and bulk material (∼23 and 22 h, respectively) take considerably longer. Moreover, 95–98.6% of the theoretical H₂ storage capacity (3.6–3.75 wt. % H₂) is reproduced after four hydrogen release and uptake cycles of the nanoconfined 2LiBH₄–MgH₂–TiCl₃. The reversibility of this hydrogen storage material is confirmed by the formation of LiBH₄ and MgH₂ after rehydrogenation using FTIR and SR-PXD techniques, respectively.     

Processing analysis of the ternary LiNH2–MgH2–LiBH4 system for hydrogen storage

In this article, we investigate the ternary LiNH₂–MgH₂–LiBH₄ hydrogen storage system by adopting various processing reaction pathways. The stoichiometric ratio of LiNH₂:MgH₂:LiBH₄ is kept constant with a 2:1:1 molar ratio. All samples are prepared using solid-state mechano-chemical synthesis with a constant rotational speed, but with varying milling duration. Furthermore, the order of addition of parent compounds as well as the crystallite size of MgH₂ are varied before milling. All samples are intimate mixtures of Li–B–N–H quaternary hydride phase with MgH₂, as evidenced by XRD and FTIR measurements. It is found that the samples with MgH₂ crystallite sizes of approximately 10 nm exhibit lower initial hydrogen release at a temperature of 150 °C. Furthermore, it is observed that the crystallite size of Li–B–N–H has a significant effect on the amount of hydrogen release with an optimum size of 28 nm. The as-synthesized hydrides exhibit two main hydrogen release temperatures, one around 160 °C and the other around 300 °C. The main hydrogen release temperature is reduced from 310 °C to 270 °C, while hydrogen is first reversibly released at temperatures as low as 150 °C with a total hydrogen capacity of ∼6 wt.%. Detailed thermal, capacity, structural and microstructural properties are discussed and correlated with the activation energies of these materials.     

Effects of MoS2 addition on the hydrogen storage properties of 2LiBH4–MgH2 systems

A 2LiBH₄–MgH₂–MoS₂ composite was prepared by solid-state ball milling, and the effects of MoS₂ as an additive on the hydrogen storage properties of 2LiBH₄–MgH₂ system together with the corresponding mechanism were investigated. As shown in the TG–DSC and MS results, with the addition of 20 wt.% of MoS₂, the onset dehydrogenation temperature is reduced to 206 °C, which is 113 °C lower than that of the pristine 2LiBH₄–MgH₂ system. Meanwhile, the total dehydrogenation amount can be increased from 9.26 wt.% to 10.47 wt.%, and no gas impurities such as B₂H₆ and H₂S are released. Furthermore, MoS₂ improves the dehydrogenation kinetics, and lowers the activation energy (E_a) 34.49 kJ mol⁻¹ of the dehydrogenation reaction between Mg and LiBH₄ to a value lower than that of the pristine 2LiBH₄–MgH₂ sample. According to the XRD test, Li₂S and MoB₂ are formed by the reaction between LiBH₄ and MoS₂, which act as catalysts and are responsible for the improved hydrogen storage properties of the 2LiBH₄–MgH₂ system.     

Improved reversible dehydrogenation properties of 2LiBH4–MgH2 composite by milling with graphitic carbon nitride

The 2LiBH₄–MgH₂ 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 (C₃N₄) as a metal-free additive was added to the 2LiBH₄–MgH₂ 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 C₃N₄ additive can eliminate the incubation period between two dehydrogenation steps and thus markedly enhance the dehydrogenation kinetics of the LiBH₄–MgH₂ composite. Further cyclic study found that the composite with C₃N₄ 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 C₃N₄ additive could react with both LiBH₄ and MgH₂ in heating process, and the resulting products may improve the reversible dehydrogenation property of the composite system.     

Improved hydrogen storage performance of the LiNH2–MgH2–LiBH4 system by addition of ZrCo hydride

Significant improvements in the hydrogen absorption/desorption properties of the 2LiNH₂–1.1MgH₂–0.1LiBH₄ composite have been achieved by adding 3wt% ZrCo hydride. The composite can absorb 5.3wt% hydrogen under 7.0 MPa hydrogen pressure in 10 min and desorb 3.75wt% hydrogen under 0.1 MPa H₂ pressure in 60 min at 150 °C, compared with 2.75wt% and 1.67wt% hydrogen under the same hydrogenation/dehydrogenation conditions without the ZrCo hydride addition, respectively. TPD measurements showed that the dehydrogenation temperature of the ZrCo hydride-doped sample was decreased about 10 °C compared to that of the pristine sample. It is concluded that both the homogeneous distribution of ZrCo particles in the matrix observed by SEM and EDS and the destabilized N–H bonds detected by IR spectrum are the main reasons for the improvement of H-cycling kinetics of the 2LiNH₂–1.1MgH₂–0.1LiBH₄ system.     

Study on the hydrogen storage properties and reaction mechanism of NaAlH4–MgH2–LiBH4 ternary-hydride system

In this paper, we report the hydrogen storage properties and reaction mechanism of NaAlH₄–MgH₂–LiBH₄ (1:1:1) ternary-hydride system prepared by ball milling. It was found that during ball milling, the NaAlH₄/MgH₂/LiBH₄ combination converted readily to the mixture of LiAlH₄/MgH₂/NaBH₄ and there is a mutual destabilization among the hydrides. Three major dehydrogenation steps were observed in the system, which corresponds to the decomposition of LiAlH₄, MgH₂, and NaBH₄, respectively. The onset dehydrogenation temperature of MgH₂ in this system is observed at around 275 °C, which is over 55 °C lower from that of as-milled MgH₂. Meanwhile, NaBH₄-relevant decomposition showed significant improvement, starts to release hydrogen at 370 °C, which is reduced by about 110 °C compared to the as-milled NaBH₄. The second and third steps decomposition enthalpy of the system were determined by differential scanning calorimetry measurements and the enthalpies were changed to be 61 and 100 kJ mol⁻¹ H₂ respectively, which are smaller than that of MgH₂ and NaBH₄ alone. From the Kissinger plot, the apparent activation energy, E_A, for the decomposition of MgH₂ and NaBH₄ in the composite was reduced to 96.85 and 111.74 kJ mol⁻¹ respectively. It is believed that the enhancement of the dehydrogenation properties was attributed to the formation of intermediate compounds, including Li–Mg, Mg–Al, and Mg–Al–B alloys, upon dehydrogenation, which change the thermodynamics of the reactions through altering the de/rehydrogenation pathway.     

Improved reversible dehydrogenation properties of LiBH4–MgH2 composite by tailoring nanophase structure using activated carbon

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.     

Raman spectroscopic observation of dehydrogenation in ball-milled LiNH2–LiBH4–MgH2 nanoparticles

In situ Raman spectroscopy was used to monitor the dehydrogenation of ball-milled mixtures of LiNH₂–LiBH₄–MgH₂ nanoparticles. The as-milled powders were found to contain a mixture of Li₄BN₃H₁₀ and Mg(NH₂)₂, with no evidence of residual LiNH₂ or LiBH₄. It was observed that the dehydrogenation of both of Li₄BN₃H₁₀ and Mg(NH₂)₂ begins at 353 K. The Mg(NH₂)₂ was completely consumed by 415 K, while Li₄BN₃H₁₀ persisted and continued to release hydrogen up to 453 K. At higher temperatures Li₄BN₃H₁₀ melts and reacts with MgH₂ to form Li₂Mg(NH)₂ and hydrogen gas. Cycling studies of the ball-milled mixture at 423 K and 8 MPa (80 bar) found that during rehydrogenation of Li₄BN₃H₁₀ Raman spectral modes reappear, indicating partial reversal of the Li₄BN₃H₁₀ to Li₂Mg(NH)₂ transformation.     

Comprehensive hydrogen storage properties and catalytic mechanism studies of 2LiBH4–MgH2 system with NbF5 in various addition amounts

In the present work, the role of NbF₅ addition amount in affecting the comprehensive hydrogen storage properties (dehydrogenation, rehydrogenation, cycling performance, hydrogen capacity) of 2LiBH₄–MgH₂ system as well as the catalytic mechanism of NbF₅ have been systematically studied. It is found that increasing the addition amount of NbF₅ to the 2LiBH₄–MgH₂ system not only results in dehydrogenation temperature reduction and hydriding–dehydriding kinetics enhancement but also leads to the de/rehydrogenation capacity loss. Compared with other samples, 2LiBH₄–MgH₂ doping with NbF₅ in weight ratios of 40:4 exhibits superior comprehensive hydrogen storage properties, which can stably release ∼8.31 wt.% hydrogen within 2.5 h under 4 bar H₂ and absorb ∼8.79 wt.% hydrogen within 10 min under 65 bar H₂ at 400 °C even up to 20 cycling. As far as we know, this is the first time that excellent reversibility as high as 20 cycles without obvious degradation tendency in both of hydrogen capacity and reaction rate has been achieved in the 2LiBH₄–MgH₂ system. The further experimental study reveals that the highly catalytic effects of NbF₅ on the 2LiBH₄–MgH₂ system are derived from the reaction between NbF₅ and LiBH₄, which provides a fundamental insight into the catalytic mechanism of NbF₅.     

Enhanced hydrogen reversibility of nanoconfined LiBH4–Mg(BH4)2

This study shows the hydrogen desorption kinetics and reversible hydrogen storage properties of 0.55LiBH₄–0.45Mg(BH₄)₂ melt-infiltrated in different nanoporous carbon aerogels with different BET surface areas of 689 or 2660 m²/g and pore volumes of 1.21 or 3.13 mL/g. These investigations clearly show a significantly improved hydrogen storage capacity after four cycles of hydrogen release and uptake for bulk 0.55LiBH₄–0.45Mg(BH₄)₂ and infiltrated in carbon aerogel and the high surface area scaffold, where 22, 36 and 58% of the initial hydrogen content remain after four cycles of hydrogen release and uptake, respectively. Nanoconfinement in high surface area carbon aerogel appears to facilitate hydrogen release illustrated by release of 13.3 wt% H₂ (93%) and only 8.4 wt% H₂ (58%) from bulk hydride in the first cycle using the same physical condition. Notably, nanoconfinement also appear to have a beneficial effect on hydrogen uptake, since 8.3 wt% H₂ (58%) is released from the high surface area scaffold and only 3.1 wt% H₂ (22%) from the bulk sample during the fourth hydrogen release.     

Compaction pressure influence on material properties and sorption behaviour of LiBH4–MgH2 composite

Among different Reactive Hydride Composites (RHCs), the combination of LiBH₄ and MgH₂ is a promising one for hydrogen storage, providing a high reversible storage capacity. During desorption of both LiBH₄ and MgH₂, the formation of MgB₂ lowers the overall reaction enthalpy. In this work, the material was compacted to pellets for further improvement of the volumetric hydrogen capacity. The influence of compaction pressure on the apparent density, thermal conductivity and sorption behaviour for the Li-based RHC during cycling was investigated for the first time. Although LiBH₄ melts during cycling, decrepitation or disaggregation of the pellets is not observed for any of the investigated compaction pressures. However, a strong influence of the compaction pressure on the apparent hydrogen storage capacity is detected. The influence on the reaction kinetics is rather low. To provide explanations for the observed correlations, SEM analysis before and after each sorption step was performed for different compaction pressures. Thus, the low hydrogen sorption in the first cycles and the continuously improving sorption for low pressure compacted pellets with cycling may be explained by some surface observations, along with the form stability of the pellets.     

Reversible hydrogen storage behavior of LiBH4–Mg(OH)2 composites

The dehydrogenation/hydrogenation properties of LiBH₄-xMg(OH)₂ were systematically investigated. The results show that the LiBH₄-0.3Mg(OH)₂ composite possesses optimal dehydrogenation properties: approximately 9.6 wt% of hydrogen is released via a stepwise reaction with an onset temperature of 100 °C. In the range of 100–250 °C, a chemical reaction between LiBH₄ and Mg(OH)₂ first occurs to give rise to the generation of LiMgBO₃, MgO and H₂. From 250 to 390 °C, the newly developed LiMgBO₃ reacts with LiBH₄ to form MgO, Li₃BO₃, LiH, B₂O₃ and Li₂B₁₂H₁₂ with hydrogen release. From 390 to 450 °C, the decomposition of LiBH₄ and Li₂B₁₂H₁₂ proceeds to release additional hydrogen and to form LiH and B. A further hydrogenation experiment indicates that the dehydrogenated LiBH₄-0.3Mg(OH)₂ sample can take up 4.7 wt% of hydrogen at 450 °C and 100 bar of hydrogen with good cycling stability, which is superior to the pristine LiBH₄.     

Improved hydrogen storage performance of MgH2–NaAlH4 composite by addition of TiF3

In a previous paper, it was demonstrated that a MgH₂–NaAlH₄ composite system had improved dehydrogenation performance compared with as-milled pure NaAlH₄ and pure MgH₂ alone. The purpose of the present study was to investigate the hydrogen storage properties of the MgH₂–NaAlH₄ composite in the presence of TiF₃. 10 wt.% TiF₃ was added to the MgH₂–NaAlH₄ mixture, and its catalytic effects were investigated. The reaction mechanism and the hydrogen storage properties were studied by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry (DSC), temperature-programmed-desorption and isothermal sorption measurements. The DSC results show that MgH₂–NaAlH₄ composite milled with 10 wt.% TiF₃ had lower dehydrogenation temperatures, by 100, 73, 30, and 25 °C, respectively, for each step in the four-step dehydrogenation process compared to the neat MgH₂–NaAlH₄ composite. Kinetic desorption results show that the MgH₂–NaAlH₄–TiF₃ composite released about 2.4 wt.% hydrogen within 10 min at 300 °C, while the neat MgH₂–NaAlH₄ sample only released less than 1.0 wt.% hydrogen under the same conditions. From the Kissinger plot, the apparent activation energy, E_A, for the decomposition of MgH₂, NaMgH₃, and NaH in the MgH₂–NaAlH₄–TiF₃ composite was reduced to 71, 104, and 124 kJ/mol, respectively, compared with 148, 142, and 138 kJ/mol in the neat MgH₂–NaAlH₄ composite. The high catalytic activity of TiF₃ is associated with in situ formation of a microcrystalline intermetallic Ti–Al phase from TiF₃ and NaAlH₄ during ball milling or the dehydrogenation process. Once formed, the Ti–Al phase acts as a real catalyst in the MgH₂–NaAlH₄–TiF₃ composite system.     

Material properties and empirical rate equations for hydrogen sorption reactions in 2 LiNH2–1.1 MgH2–0.1 LiBH4–3 wt.% ZrCoH3

2 LiNH₂–1.1 MgH₂–0.1 LiBH₄–3 wt.% ZrCoH₃ is a solid state hydrogen storage material with a hydrogen storage capacity of up to 5.3 wt.%. As the material shows sufficiently high desorption rates at temperatures below 200 °C, it is used for a prototype solid state hydrogen storage tank with a hydrogen capacity of 2 kWh_el that is coupled to a high temperature proton exchange membrane fuel cell. In order to design an appropriate prototype reactor, model equations for the rate of hydrogen sorption reactions are required. Therefore in the present study, several material properties, like bulk density and thermodynamic data, are measured. Furthermore, isothermal absorption and desorption experiments are performed in a temperature and pressure range that is in the focus of the coupling system. Using experimental data, two-step model equations have been fitted for the hydrogen absorption and desorption reactions. These empirical model equations are able to capture the experimentally measured reaction rates and can be used for model validation of the design simulations.     

Mg2FeH6–LiBH4 and Mg2FeH6–LiNH2 composite materials for hydrogen storage

Two composite hydrogen storage materials based on Mg₂FeH₆ were investigated for the first time. The Mg₂FeH₆–LiBH₄ composite of molar ratio 1:5 showed a hydrogen desorption capacity of 5.6 wt.% at 370 °C, and could be rehydrogenated to 3.6 wt.% with the formation of MgH₂, as the material was heated to 445 °C and held at this temperature. The Mg₂FeH₆–LiNH₂ composite of 3:10 molar ratio exhibited a hydrogen desorption capacity of 4.3 wt.% and released hydrogen at 100 °C lower then the Mg₂FeH₆–LiBH₄ composite, but this mixture could not be rehydrogenated. Compared to neat Mg₂FeH₆, both composites show enhanced hydrogen storage properties in terms of desorption kinetics and capacity at these low temperatures. In particular, Mg₂FeH₆–LiNH₂ exhibits a much lower desorption temperature than neat Mg₂FeH₆, but only Mg₂FeH₆–LiBH₄ re-absorbs hydrogen.     

Combined effects of hydrogen back-pressure and NbF5 addition on the dehydrogenation and rehydrogenation kinetics of the LiBH4–MgH2 composite system

It is well known that the dehydrogenation pathway of the LiBH₄–MgH₂ composite system is highly reliant on whether decomposition is performed under vacuum or a hydrogen back-pressure. In this work, the effects of hydrogen back-pressure and NbF₅ addition on the dehydrogenation kinetics of the LiBH₄–MgH₂ system are studied under either vacuum or hydrogen back-pressure, as well as the subsequent rehydrogenation and cycling. For the pristine sample, faster desorption kinetics was obtained under vacuum, but the performance is compromised by slow absorption kinetics. In contrast, hydrogen back-pressure remarkably promotes the absorption kinetics and increases the reversible hydrogen storage capacity, but with the penalty of much slower desorption kinetics. These drawbacks were overcome after doping with NbF₅, with which the dehydrogenation and rehydrogenation kinetics was significantly improved. In particular, the enhanced kinetics was observed to persist well, even after 9 cycles, in the case of the NbF₅ doped sample under hydrogen back-pressure, as well as the suppression of forming Li₂B₁₂H₁₂. Furthermore, the mechanism that is behind these effects of NbF₅ additive on the reversible dehydrogenation reaction of the LiBH₄–MgH₂ system is discussed.     

Catalytic effect of carbon nanostructures on the hydrogen storage properties of MgH2–NaAlH4 composite

The present investigation describes the hydrogen storage properties of 2:1 molar ratio of MgH₂–NaAlH₄ composite. De/rehydrogenation study reveals that MgH₂–NaAlH₄ composite offers beneficial hydrogen storage characteristics as compared to pristine NaAlH₄ and MgH₂. To investigate the effect of carbon nanostructures (CNS) on the de/rehydrogenation behavior of MgH₂–NaAlH₄ composite, we have employed 2 wt.% CNS namely, single wall carbon nanotubes (SWCNT) and graphene nano sheets (GNS). It is found that the hydrogen storage behavior of composite gets improved by the addition of 2 wt.% CNS. In particular, catalytic effect of GNS + SWCNT improves the hydrogen storage behavior and cyclability of the composite. De/rehydrogenation experiments performed up to six cycles show loss of 1.50 wt.% and 0.84 wt.% hydrogen capacity in MgH₂–NaAlH₄ catalyzed with 2 wt.% SWCNT and 2 wt.% GNS respectively. On the other hand, the loss of hydrogen capacity after six rehydrogenation cycles in GNS + SWCNT (1.5 + 0.5) wt.% catalyzed MgH₂–NaAlH₄ is diminished to 0.45 wt.%.     

Improved hydrogen storage properties of LiBH4 doped Li–N–H system

Remarkable improvement of hydrogen sorption properties of Li–N–H system has been obtained by doping with a small amount of LiBH₄. The starting and ending temperatures of hydrogen desorption shift to lower temperatures and the release of NH₃ is obviously restrained by 10 mol% LiBH₄ doping. The kinetics of hydrogen desorption and absorption of Li–N–H system became faster by the addition of LiBH₄. About 4 wt.% H₂ can be released within 30 min and ∼4.8 wt.% H₂ can be reabsorbed within 2 min by LiBH₄ doped sample at 250 °C, while only 1.44 wt.% H₂ is released and 2.1 wt.% is reabsorbed for pure Li–N–H system. The quaternary hydride (LiNH₂)_x(LiBH₄)_(1−x) formed by the reaction between LiBH₄ and LiNH₂ may contribute to the enhancement of the hydrogen sorption performances by yielding a ionic liquid phase and transferring LiNH₂ from solid state to molten state with a weakened N–H bond.