Effect of carbon addition on hydrogen storage behaviors of Li–Mg–B–H system

Various carbon additives were mechanically milled with LiBH₄/MgH₂ composite and their hydrogen storage behaviors were investigated. It was found that most of the carbon additives exhibited prominent effect on the host material. Among the various carbon additives, purified single-walled carbon nanotubes (SWNTs) exhibited the most prominent effect on the kinetic improvement and cyclic stability of Li–Mg–B–H system. Results show that LiBH₄/MgH₂ composite milled with 10 wt.% purified SWNTs additive can release nearly 10 wt.% hydrogen within 20 min at 450 °C, which is about two times faster than that of the neat LiBH₄/MgH₂ sample. On the basis of hydrogen storage behavior and structure/phase investigations, the possible mechanism involved in the property improvement upon carbon additives was discussed.     

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.     

Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst

CeF₃ as a catalyst is first added to activated carbon (AC) by ball milling under low rotation speed. Then the treated AC was used as the scaffold to confine LiBH₄ by melt infiltration process. The combined effects of confinement and CeF₃ doping on the hydrogen storage properties of LiBH₄ are studied. The experimental results show that LiBH₄ and CeF₃ are well dispersed in the AC scaffold and occupy up to 90% of the pores of AC. Compared with pristine LiBH₄, the onset dehydrogenation temperature for LiBH₄-AC and LiBH₄-AC-CeF₃ decreases by 150 and 190 °C, respectively. And the corresponding dehydrogenation capacity increases from 8.2 wt% to 13.1 wt% for LiBH₄-AC and 12.8 wt% for LiBH₄-AC-CeF₃, respectively. The maximum dehydrogenation speed of LiBH₄-AC and LiBH₄-AC-CeF₃ is 80 and 288 times higher than that of pristine LiBH₄ at 350 °C. And LiBH₄-AC andLiBH₄-AC-CeF₃ show good reversible hydrogen storage properties. On the during 4th dehydrogenation cycle, the hydrogen release capacity of LiBH₄-AC and LiBH₄-AC-5 wt% CeF₃ reaches 8.1 and 9.3 wt%, respectively.     

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.     

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.     

Interface effects in NaAlH4–carbon nanocomposites for hydrogen storage

For practical solid-state hydrogen storage, reversibility under mild conditions is crucial. Complex metal hydrides such as NaAlH₄ and LiBH₄ have attractive hydrogen contents. However, hydrogen release and especially uptake after desorption are sluggish and require high temperatures and pressures. Kinetics can be greatly enhanced by nanostructuring, for instance by confining metal hydrides in a porous carbon scaffold. We present for a detailed study of the impact of the nature of the carbon–metal hydride interface on the hydrogen storage properties. Nanostructures were prepared by melt infiltration of either NaAlH₄ or LiBH₄ into a carbon scaffold, of which the surface had been modified, varying from H-terminated to oxidized (up to 4.4 O/nm²). It has been suggested that the chemical and electronic properties of the carbon/metal hydride interface can have a large influence on hydrogen storage properties. However, no significant impact on the first H₂ release temperatures was found. In contrast, the surface properties of the carbon played a major role in determining the reversible hydrogen storage capacity. Only a part of the oxygen-containing groups reacted with hydrides during melt infiltration, but further reaction during cycling led to significant losses, with reversible hydrogen storage capacity loss up to 40% for surface oxidized carbon. However, if the carbon surface had been hydrogen terminated, ∼6 wt% with respect to the NaAlH₄ weight was released in the second cycle, corresponding to 95% reversibility. This clearly shows that control over the nature and amount of surface groups offers a strategy to achieve fully reversible hydrogen storage in complex metal hydride-carbon nanocomposites.     

A comparative study of LiBH4-based composites with metal hydrides and fluorides for hydrogen storage

A comparative study was performed on four LiBH₄-based hydrogen storage composites 2LiBH₄ + MgX₂ and 6LiBH₄ + CaX₂ (X = H and F). The composites with fluorides and those with corresponding hydrides exhibited similar hydrogen storage properties. The dehydrogenation of 2LiBH₄ + MgF₂ demonstrated a strong dependence on the hydrogen back pressure, similar to that of 2LiBH₄ + MgH₂. The reversible hydrogen storage of 2LiBH₄ + MgF₂ was achieved under a back pressure of 5 bar at 450 °C. Dehydrogenation under lower H₂ pressures resulted in the production of Mg and thus a partial reversibility. In contrast, both 6LiBH₄ + CaH₂ and 6LiBH₄ + CaF₂ revealed reversible hydrogen storage properties regardless of the hydrogen back pressure. The structural difference between MgB₂ and CaB₆ may account for the observed differences in hydrogen storage properties of the Mg- and Ca-containing LiBH₄ reactive composites.     

NEXAFS study of 2LiF–MgB2 composite

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, LiBH_4−xF_x compounds might be present during hydrogen absorption–desorptions and play important role on their kinetics and reversibility.     

Small hydrogen storage tank filled with 2LiBH4MgH2 nanoconfined in activated carbon: Reaction mechanisms and performances

De/rehydrogenation performances and reaction pathways of nanoconfined 2LiBH₄MgH₂ into activated carbon (AC) packed in small hydrogen storage tank are proposed for the first time. Total and material storage capacities upon five hydrogen release and uptake cycles are 3.56–4.55 and 2.03–3.28 wt % H₂, respectively. Inferior hydrogen content to theoretical capacity (material capacity of 5.7 wt % H₂) is due to partial dehydrogenation during sample preparation and incomplete decomposition of LiBH₄ as well as the formation of thermally stable Li₂B₁₂H₁₂ upon cycling. Two-step dehydrogenation of MgH₂ and LiBH₄ to produce Mg and MgB₂+LiH, respectively is found at all positions in the tank. For rehydrogenation, reversibility of MgH₂ and LiBH₄ proceeds via different reaction mechanisms. Although isothermal condition (T_set = 350 °C) and controlled pressure range (e.g., 30–40 bar H₂ for hydrogenation) are applied, temperature gradient inside the tank and poor hydrogen diffusion through hydride bed, especially in the sample bulk are detected. This results in alteration of de/rehydrogenation pathways of hydrides at different positions in the tank. Thus, further development of hydrogen storage tank based 2LiBH₄MgH₂ nanoconfined in AC includes the improvement of thermal conductivity of materials and temperature control system as well as hydrogen permeability.     

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.     

Destabilization of LiBH4 by MH2 (M = Ce, La) for hydrogen storage: Nanostructural effects on the hydrogen sorption kinetics

Destabilization of LiBH₄ by addition of metal hydrides or borohydrides is a powerful strategy to develop new promising hydrogen storage systems. In this study, we compare the destabilization behavior of the LiBH₄ by addition of MH₂ (M = La, Ce). A notable improvement in the hydrogen desorption temperature, the rate and the weight percentage of hydrogen released is observed for LiBH₄-MH₂ with respect to LiBH₄. Formation of LaB₆ and CeB₆ after dehydriding of the composites is proved by PXRD. Remarkable hydrogen storage reversibility of LiBH₄-MH₂ composites is confirmed under moderate conditions: 400 °C and 6.0 MPa of hydrogen pressure for 4 h without catalyst. The LiBH₄-LaH₂ composite exhibits improved hydrogen desorption performance compared with LiBH₄-CeH₂ composite, but the hydrogen storage reversibility is inferior. Notably, the LiBH₄-CeH₂ nanocomposite produced by in situ formation of CeH₂ from Ce(BH₄)₃-LiH displays excellent hydrogen storage properties. The addition of ZrCl₄ as a catalyst improves dehydriding kinetics. The mechanism underlying the enhancement in the LiBH₄-MH₂ composites is also discussed. Our study is the first work about reversible hydrogen storage in LiBH₄-LaH₂.     

Enhanced hydrogen generation by hydrolysis of LiBH4 doped with multiwalled carbon nanotubes for micro proton exchange membrane fuel cell application

LiBH₄ has a high hydrogen storage capacity and could potentially serve as a superior hydrogen storage material. However, during the hydrolysis process for hydrogen generation, the agglomeration of the hydrolysis product of LiBH₄ limits its full utilization. In order to completely release the stoichiometric amount of H₂ from LiBH₄ hydrolysis, multiwalled carbon nanotubes (MWCNTs) were doped with LiBH₄ by mechanical milling. The results show that MWCNT carried LiBH₄ can slowly react with water vapor at room temperature which is 25 °C lower than the reaction temperature of neat LiBH₄. Agglomeration can be avoided when the addition of MWCNTs exceeds 7 wt.%, which results in a complete hydrolysis process. The total hydrogen capacity is 7.5 wt.%. The enhanced hydrolysis of LiBH₄ can be attributed to the MWCNTs which increased the contact areas between LiBH₄ and water and created gas channels for hydrogen diffusion. The performance of a micro proton exchange membrane fuel cell connected to MWCNT-doped LiBH₄ powder packed-bed reactor was examined. The result demonstrates that doping with MWCNTs enhanced the hydrogen generation of LiBH₄ hydrolysis. MWCNT-doped LiBH₄ can be applied as hydrogen source of fuel cells.     

A computational study of LiBH4 clusters and enhancement of their hydrogen storage by excess electrons

The electronic structures and energies of neutral and anionic (LiBH₄)_x clusters (x = 1 – 5) have been systematically studied by using density functional theory with the B3LYP/6‐311++G(d, p) level. For investigating the importance of excess electrons on hydrogen storage capacity, the interactions between hydrogen atoms and the anionic (LiBH₄)_x clusters are also examined. The calculated formation energies of the anionic clusters show that the anionic clusters have a high thermal stability. It is found that hydrogen atoms are adsorbed on the anionic (LiBH₄)_x clusters chemically with adsorption energies in the range of ?69.13 – ?153.73 kcal/mol. The hydrogen storage capacity can be improved from 18.51% to 19.26 – 22.12% in weight percent depending on the size of various anionic (LiBH₄)_x clusters. Our calculation results show that the existence of excess electrons on the (LiBH₄)_x clusters can enhance the hydrogen storage capacity. The Mulliken charge analysis was performed to illustrate the interactions between H atoms and the anionic (LiBH₄)_x clusters. Copyright © 2016 John Wiley & Sons, Ltd.     

Hydrogen storage performance of LiBH4+1/2MgH2 composites improved by Ce-based additives

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.     

Synthesis and hydrogen storage properties of lithium borohydride amidoborane complex

A series of mixtures of LiAB/LiBH₄ with different molar ratios were prepared and their hydrogen storage properties were investigated in this study. Among them, a new structure was found in the LiAB/LiBH₄ sample with a molar ratio of 1/1. It is of orthorhombic structure and composed of alternative layers of LiAB and LiBH₄. It shows similar hydrogen desorption behaviors of LiAB–LiBH₄ and LiAB–0.5LiBH₄. For use in hydrogen storage, high hydrogen capacity and low operation temperature are demanded, thus, the dehydrogenation properties of LiAB–0.5LiBH₄ were subsequently measured. Three steps of desorption were observed during the heating process, with a total release of 11.5 wt% H₂ at 500 °C. The reaction path was identified using a combined investigation of XRD and ¹¹B solid state NMR. Dehydrogenation kinetic analyses show that the complex has lower activation energy (61 ± 4 kJ mol⁻¹ H₂) than that of LiAB (71 ± 5 kJ mol⁻¹ H₂). It is likely that dehydrogenation process was promoted due to the presence of LiBH₄.     

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.     

A new strategy to remarkably improve the low-temperature reversible hydrogen desorption performances of LiBH4 by compositing with fluorographene

LiBH₄ is a promising hydrogen storage material for its large capacity. However, high desorption temperature, sluggish kinetics and demanding rehydrogenation severely hinder its practical use. Surface functional groups of graphene in many cases are treated as effective approaches to obtain some kinds of excellent properties of energy storage materials. In the current work, a new facile and effective strategy to improve the reversible hydrogen desorption properties of LiBH₄ is proposed by composing with functionalized graphene to form the LiBH₄–fluorographene composite. The fluorographene (FG) nanosheets are successfully exfoliated from fluorographite (FGi) and composed with LiBH₄. It is demonstrated that the FG can remarkably improve the hydrogen desorption thermodynamics, kinetics and reversibility of LiBH₄ via reactant destabilization method. An extremely fast hydrogen desorption process with a high capacity of 8.2 wt.% at 148.1 °C is achieved in the LiBH₄–50FG composite. Further research reveals that the enhancement actually roots in the strengthened interfacial interaction between LiBH₄ and exfoliated FG. Moreover, it is confirmed that the LiBH₄–40FG composite exhibits a significantly enhanced reversible hydrogen desorption capacity of 7.2 wt.% and LiBH₄ is regenerated. Such enhanced reversible hydrogen desorption properties are ascribed to the strengthened interfacial interactions between LiBH₄ and FG with large surface, as well as the formation of LiH_xF_1?x phase.     

Enhanced dehydrogenation properties of LiBH4 compositing with hydrogenated magnesium-rare earth compounds

LiBH₄ is regarded as a promising hydrogen storage material due to its high hydrogen density. In this study, the dehydrogenation properties of LiBH₄ were remarkably enhanced by doping hydrogenated Mg₃RE compounds (RE denotes La, Ce, Nd rare earth metals), which are composed of nanostructured MgH₂ and REH_2+x (denoted as H − Mg₃RE). For the LiBH₄ + H − Mg₃La mixture, the component LiBH₄ desorbed 6 wt.% hydrogen even at a relatively low temperature of 340 °C, far lower than the desorption temperature of pure LiBH₄ or the 2LiBH₄ + MgH₂ system. This kinetic improvement is attributed to the hydrogen exchange mechanism between the H − Mg₃La and LiBH₄, in the sense that the decomposition of MgH₂ and LaH_2+x catalyzed the dehydrogenation of LiBH₄ through hydrogen exchange effect rather than mutual chemical reaction requiring higher temperature and hydrogen pressure. However, prior to fast hydrogen release, the hydrogen exchange effect suppressed the dehydriding of MgH₂ and elevated its desorption temperature. It is expected to strengthen the hydrogen exchange effect by compositing the LiBH₄ with other nanosized metal hydrides and to obtain better dehydrogenation properties.     

Structure of nanoconfined LiBH4 from first principles B NMR chemical shifts calculations

Nanoconfinement of metal borohydrides is a promising route for improvement of their kinetic and thermodynamic properties related to hydrogen storage. Since the atomic-scale properties of nanoconfined LiBH₄ remain under debate we propose a model of such a structure. Within extensive density functional theory calculations we consider continuous transformation of the lithium borohydride equilibrium structures from 3-dimensional to 1-dimensional. Such structures consist of single linear chains, linear ribbons and 3-dimensional chains, and together with the low temperature bulk phase they reproduce ¹¹B NMR spectra observed in LiBH₄ confined in porous carbon. The present model suggest that nanoconfined LiBH₄ consists of a dense bulk-like region and loosely distributed chains and ribbons of Li–BH₄. Even with the low effective density of loosely distributed LiBH₄ they occupy region with thickness as small as a quarter of a pore diameter.