Carbon nanomaterials as catalysts for hydrogen uptake and release in NaAlH4

Utilizing Nanostructured Materials for Hydrogen Generation, Storage, and Diverse Applications

The rapid advancement of refined nanostructures and nanotechnologies offers significant potential to boost research activities in hydrogen storage. Recent innovations in hydrogen storage have centered on nanostructured materials, highlighting their effectiveness in molecular hydrogen storage, chemical storage, and as nanoconfined hydride supports. Emphasizing the importance of exploring ultra-high-surface-area nanoporous materials and metals, we advocate for their mechanical stability, rigidity, and high hydride loading capacities to enhance hydrogen storage efficiency. Despite the evident benefits of nanostructured materials in hydrogen storage, we also address the existing challenges and future research directions in this domain. Recent progress in creating intricate nanostructures has had a notable positive impact on the field of hydrogen storage, particularly in the realm of storing molecular hydrogen, where these nanostructured materials are primarily utilized.

Nickel Nanoparticles Decorated on Glucose-Derived Carbon Spheres as a Novel, Non-Palladium Catalyst for Epoxidation of Olefin

Carbon spheres supporting nickel nanoparticles (NPs), generated by the integration of hydrothermal and microwave irradiation techniques, catalyzed the epoxidation of 1-octene, cyclooctene, styrene, allyl alcohol, and cyclohexene. The average particle sizes of the carbon spheres (CSs) and nickel oxide species immobilized on the CSs were 240 nm and 26 nm, respectively. The fabricated composites incorporating nickel NPs showed higher activity in the cyclohexene epoxidation process. The cyclohexene conversion was enhanced by raising the Ni loading to 10%. Within 14 h, the cyclohexene conversion had grown to 98%. This robust catalytic activity can be attributed to the efficient distribution of Ni species on the CSs, the facile lowering of the surface, and the development of uniformly nanosized species. The composite exhibited good recyclability across at least five cycles (which is not a simple task involving nickel-nanoparticle-based catalysts that are employed in water), and no nickel species leached into the solution, making the total system environmentally benign and cost-effective.

Surfactant Induced Synthesis of LiAlH4 and NaAlH4 Nanoparticles for Hydrogen Storage

LiAlH4 and NaAlH4 are considered to be promising hydrogen storage materials due to their high hydrogen density. However, their practical use is hampered by the lack of hydrogen reversibility along with poor kinetics. Nanosizing is an effective strategy to enable hydrogen reversibility under practical conditions. However, this has remained elusive as the synthesis of alanate nanoparticles has not been explored. Herein, a simple solvent evaporation method is demonstrated to assemble alanate nanoparticles with the use of surfactants as a stabilizer. More importantly, the roles of the surfactants in enabling control over particle size and morphology was determined. Surfactants with long linear carbon chains and matching the hard character of alanates are more prone to lead to the formation of small particles of ~10 nm due to steric hindrance. This can result in significant shifts in the temperature for hydrogen release.

Rapid access to ordered mesoporous carbons for chemical hydrogen storage

Ordered mesoporous carbon materials offer robust network of organized pores for energy storage and catalysis applications, but suffer from time-consuming and intricate preparations hindering their widespread use. Here we report a new and rapid synthetic route for a N-doped ordered mesoporous carbon structure through a preferential heating of iron oxide nanoparticles by microwaves. A nanoporous covalent organic polymer is first formed in situ covering the hard templates of assembled nanoparticles, paving the way for a long-range order in a carbonaceous nanocomposite precursor. Upon removal of the template, a well-defined cubic mesoporous carbon structure was revealed. The ordered mesoporous carbon was used in solid state hydrogen storage as a host scaffold for NaAlH 4 , where remarkable improvement in hydrogen desorption kinetics was observed. The state-of-the-art lowest activation energy of dehydrogenation as single step was attributed to their ordered pore structure and N-doping effect.

Atomic Perspective about the Reaction Mechanism and H2 Production during the Combustion of Al Nanoparticles/H2O2 Bipropellants

The combination of Al nanoparticles (ANPs) and hydrogen peroxide (H₂O₂) can serve as environmentally friendly bipropellants and maximize the energetic benefits through harnessing heat release and chemical energy stored in H₂. This work presents an atomic insight into the combustion mechanism of ANPs/H₂O₂. Two main paths, including the ANPs oxidation by H₂O₂ to produce H₂ and Al oxides, and the catalytic decomposition of H₂O₂ on ANP surface to generate O₂ and H₂O, are confirmed to maintain the combustion. OH and HOO radicals as well as H₂O, O₂, H₂, and Al oxides are detected as dominant intermediates and products therein. It is evidenced that higher temperature, smaller ANP size, and higher H₂O₂ concentration enhance the combustion. Moreover, atomic details show that the H desorption from ANPs/Al clusters is a critical step for both H₂ production and ANP oxidation. In addition, microexplosion that has been confirmed in hot and dense O₂ is not observed in H₂O₂, even with a high concentration, possibly due to a slower heat release. Besides, the observed excellent specific impulse of the ANP/H₂O₂ bipropellants could be attributed to the considerable H₂ production, instead of heat release. This work is expected to present an overall atomic perspective about the combustion mechanism of ANP/H₂O₂ bipropellants.

Design of Nanomaterials for Hydrogen Storage

The interaction of hydrogen with solids and the mechanisms of hydride formation experience significant changes in nanomaterials due to a number of structural features. This review aims at illustrating the design principles that have recently inspired the development of new nanomaterials for hydrogen storage. After a general discussion about the influence of nanomaterials’ microstructure on their hydrogen sorption properties, several scientific cases and hot topics are illustrated surveying various classes of materials. These include bulk-like nanomaterials processed by mechanochemical routes, thin films and multilayers, nano-objects with composite architectures such as core–shell or composite nanoparticles, and nanoparticles on porous or graphene-like supports. Finally, selected examples of recent in situ studies of metal–hydride transformation mechanisms using microscopy and spectroscopy techniques are highlighted.

Alanates, a Comprehensive Review

Hydrogen storage is widely recognized as one of the biggest not solved problem within hydrogen technologies. The slow development of the materials and systems for hydrogen storage has resulted in a slow spread of hydrogen applications. There are many families of materials that can store hydrogen; among them, the alanate family can be of interest. Basic research papers and reviews have been focused on alanates of group 1 and 2. However, there are many alanates of transition metals, main group, and lanthanides that deserve attention in a review. This work is a comprehensive compilation of all known alanates. The approaches towards tuning the kinetics and thermodynamics of alanates are also covered in this review. These approaches are the formation of reactive composites, double cation alanates, or anion substitution. The crystallographic and X-ray diffraction characteristics of each alanate are presented along with this review. In the final sections, a discussion of the infrared, Raman, and thermodynamics was included.

Synthesis and Applications of Graphdiyne-Based Metal-Free Catalysts

The development of carbon materials offers the hope for obtaining inexpensive and high-performance alternatives to substitute noble-metal catalysts for their sustainable application. Graphdiyne, the rising-star carbon allotrope, is a big family with many members, and first realized the coexistence of sp- and sp² -hybridized carbon atoms in a 2D planar structure. Different from the prevailing carbon materials, its nonuniform distribution in the electronic structure and wide tunability in bandgap show many possibilities and special inspirations to construct new-concept metal-free catalysts, and provide many opportunities for achieving a catalytic activity comparable with that of noble-metal catalysts. Herein, the recent progress in synthetic methodologies, theoretical predictions, and experimental investigations of graphdiyne for metal-free catalysts is systematically summarized. Some new perspectives of the opportunities and challenges in developing high-performance graphdiyne-based metal-free catalysts are demonstrated.

Nanostructured Metal Hydrides for Hydrogen Storage

Knowledge and foundational understanding of phenomena associated with the behavior of materials at the nanoscale is one of the key scientific challenges toward a sustainable energy future. Size reduction from bulk to the nanoscale leads to a variety of exciting and anomalous phenomena due to enhanced surface-to-volume ratio, reduced transport length, and tunable nanointerfaces. Nanostructured metal hydrides are an important class of materials with significant potential for energy storage applications. Hydrogen storage in nanoscale metal hydrides has been recognized as a potentially transformative technology, and the field is now growing steadily due to the ability to tune the material properties more independently and drastically compared to those of their bulk counterparts. The numerous advantages of nanostructured metal hydrides compared to bulk include improved reversibility, altered heats of hydrogen absorption/desorption, nanointerfacial reaction pathways with faster rates, and new surface states capable of activating chemical bonds. This review aims to summarize the progress to date in the area of nanostructured metal hydrides and intends to understand and explain the underpinnings of the innovative concepts and strategies developed over the past decade to tune the thermodynamics and kinetics of hydrogen storage reactions. These recent achievements have the potential to propel further the prospects of tuning the hydride properties at nanoscale, with several promising directions and strategies that could lead to the next generation of solid-state materials for hydrogen storage applications.

Solid State Hydrogen Storage in Alanates and Alanate-Based Compounds: A Review

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.

In Situ Synthesis of 3D Flower-Like Nanocrystalline Ni/C and its Effect on Hydrogen Storage Properties of LiAlH4

Lithium alanate (LiAlH₄ ) is of particular interest as one of the most promising candidates for solid-state hydrogen storage. Unfortunately, high dehydrogenation temperatures and relatively slow kinetics limit its practical applications. Herein, 3D flower-like nanocrystalline Ni/C, composed of highly dispersed Ni nanoparticles and interlaced carbon flakes, was synthesized in situ. The as-synthesized nanocrystalline Ni/C significantly decreased the dehydrogenation temperature and dramatically improved the dehydrogenation kinetics of LiAlH₄ . It was found that the LiAlH₄ sample with 10 wt % Ni/C (LiAlH₄ -10 wt %Ni/C) began hydrogen desorption at approximately 48 °C, which is very close to ambient temperature. Approximately 6.3 wt % H₂ was released from LiAlH₄ -10 wt %Ni/C within 60 min at 140 °C, whereas pristine LiAlH₄ only released 0.52 wt % H₂ under identical conditions. More importantly, the dehydrogenated products can partially rehydrogenate at 300 °C under 4 MPa H₂ . The synergetic effect of the flower-like carbon substrate and Ni active species contributes to the significantly reduced dehydrogenation temperatures and improved kinetics.

Confined NaAlH4 nanoparticles inside CeO2 hollow nanotubes towards enhanced hydrogen storage

NaAlH₄ has been widely regarded as a potential hydrogen storage material due to its favorable thermodynamics and high energy density. The high activation energy barrier and high dehydrogenation temperature, however, significantly hinder its practical application. In this paper, CeO₂ hollow nanotubes (HNTs) prepared by a simple electrospinning technique are adopted as functional scaffolds to support NaAlH₄ nanoparticles (NPs) towards advanced hydrogen storage performance. The nanoconfined NaAlH₄ inside CeO₂ HNTs, synthesized via the infiltration of molten NaAlH₄ into the CeO₂ HNTs under high hydrogen pressure, exhibited significantly improved dehydrogenation properties compared with both bulk and ball-milled CeO₂ HNTs-catalyzed NaAlH₄. The onset dehydrogenation temperature of the NaAlH₄@CeO₂ composite was reduced to below 100 °C, with only one main dehydrogenation peak appearing at 130 °C, which is 120 °C and 50 °C lower than for its bulk counterpart and for the ball-milled CeO₂ HNTs-catalyzed NaAlH₄, respectively. Moreover, ∼5.09 wt% hydrogen could be released within 30 min at 180 °C, while only 1.6 wt% hydrogen was desorbed from the ball-milled NaAlH₄ under the same conditions. This significant improvement is mainly attributed to the synergistic effects contributed by the CeO₂ HNTs, which could act as not only a structural scaffold to fabricate and confine the NaAlH₄ NPs, but also as an effective catalyst to enhance the hydrogen storage performance of NaAlH₄.

High-Performance Hydrogen Storage Nanoparticles Inside Hierarchical Porous Carbon Nanofibers with Stable Cycling

An effective route based on space-confined chemical reaction to synthesize uniform Li₂Mg(NH)₂ nanoparticles is reported. The hierarchical pores inside the one-dimensional carbon nanofibers (CNFs), induced by the creation of well-dispersed Li₃N, serve as intelligent nanoreactors for the reaction of Li₃N with Mg-containing precursors, resulting in the formation of uniformly discrete Li₂Mg(NH)₂ nanoparticles. The nanostructured Li₂Mg(NH)₂ particles inside the CNFs are capable of complete hydrogenation and dehydrogenation at a temperature as low as 105 °C with the suppression of ammonia release. Furthermore, by virtue of the nanosize effects and space-confinement by the porous carbon scaffold, no degradation was observed after 50 de/rehydrogenation cycles at a temperature as low as 130 °C for the as-prepared Li₂Mg(NH)₂ nanoparticles, indicating excellent reversibility. Moreover, the theoretical calculations demonstrate that the reduction in particle size could significantly enhance the H₂ sorption of Li₂Mg(NH)₂ by decreasing the relative activation energy barrier, which agrees well with our experimental results. This method could represent an effective, general strategy for synthesizing nanoparticles of complex hydrides with stable reversibility and excellent hydrogen storage performance.

Synthesis of Nanoflower-Shaped MXene Derivative with Unexpected Catalytic Activity for Dehydrogenation of Sodium Alanates

Surface group modification and functionalization of two-dimensional materials in many cases are deemed as effective approaches to achieve some distinctive properties. Herein, we present a new nanoflower-shaped TiO₂/C composite which was synthesized by in situ alcoholysis of two-dimensional layered MXene (Ti₃C₂(OH_xF_1-x)₂) in a dilute HF solution (0.5 wt %) for the first time. Furthermore, it is demonstrated that it bestows a strong catalytic activity for the dehydrogenation of NaAlH₄. The results show that the NaAlH₄ containing 10 wt % A_0.9R_0.1-TiO₂/C (containing 90% anatase TiO₂ and 10% rutile TiO₂) composite merely took ∼85 min to reach a stable and maximum dehydrogenation capacity of ∼3.08 wt % at 100 °C, and it maintains stable after ten cycles, which is the best Ti-based catalyst for the dehydrogenation of NaAlH₄ reported so far. Theoretical calculation confirms that this C-doping TiO₂ crystals remarkably decreases desorption energy barrier of Al-H bonding in NaAlH₄, accelerating the breakdown of Al-H bonding. This finding raises the potential for development and application of new fuel cells.

Metal borohydrides and derivatives – synthesis, structure and properties

A comprehensive review of metal borohydrides from synthesis to application.

Acridinium Ester-Functionalized Carbon Nanomaterials: General Synthesis Strategy and Outstanding Chemiluminescence

In this work, three different kinds of acridinium ester (AE)-functionalized carbon nanomaterials, including AE-functionalized carbon nanoparticles (AE-CNPs), AE-functionalized graphene oxide (AE-GO), and AE-functionalized multiwalled carbon nanotubes (AE-MCNTs), were synthesized for the first time via a simple, general, and noncovalent strategy. AE molecules were assembled on the surface of carbon nanomaterials by electrostatic interaction, π-π stacking interaction, and amide bond. The synthesized AE-CNPs, AE-GO, and AE-MCNTs with 5.0 × 10(-8) mol·L(-1) of synthetic AE concentration, which was very low compared with other chemiluminescence (CL) reagents such as luminol, N-(aminobutyl)-N-(ethylisoluminol), and lucigenin at the concentration of 3.3 × 10(-4) to 5.0 × 10(-6) mol·L(-1) used for the synthesis of CL-functionalized nanomaterials, exhibited outstanding CL activity and good stability. It was found that carbon nanomaterials as nanosized platforms could efficiently immobilize AE molecules and facilitate the formation of OH(•) and O2(•-), leading to strong light emission. Moreover, the CL intensity of AE-GO was the highest, which was about 8.7 and 3.7 times higher than that of AE-CNPs and AE-MCNTs, respectively. This mainly resulted from a difference in the amount of adsorbed AE molecules on the surface of different carbon nanomaterials. Additionally, the prepared AE-CNPs demonstrated excitation-dependent fluorescence property and good fluorescence stability against photobleaching. On the basis of the excellent CL and special fluorescence properties of AE-CNPs, a dual-mode array strategy has been proposed for the first time and seven kinds of transition-metal ions could be successfully discriminated.

A first-principles study of the tuning effect of a Fe2O3 cluster on the dehydrogenation properties of a LiBH4 (001) surface

First-principles calculations were performed to investigate the effects of a Fe2O3 cluster on the structural, electronic and dehydrogenation properties of a LiBH4 (001) surface. O atoms interact with Li atoms to form Li-O bonds, corresponding to an experimentally found ternary Li-Fe oxide. The DOS results show that the coupling effect of the spin-unrestricted Fe d orbitals, especially the spin-up state of Fe d orbitals, plays a crucial role in the hybridizations of H s, B p, and Fe d orbitals. The Fe2O3 cluster will serve as the nucleation site of surface activation at the surface of LiBH4 to improve the dehydrogenation kinetics of LiBH4. The doping of the Fe2O3 cluster is advantageous to facilitate the release of a H2 molecule from not only the surface layer but also the inner layer of the LiBH4 (001) surface.

Covalent Functionalization of Boron Nitride Nanotubes via Reduction Chemistry

Boron nitride nanotubes (BNNTs) exhibit a range of properties that hold great potential for many fields of science and technology; however, they have inherently low chemical reactivity, making functionalization for specific applications difficult. Here we propose that covalent functionalization of BNNTs via reduction chemistry could be a highly promising and viable strategy. Through density functional theory calculations of the electron affinity of BNNTs and their binding energies with various radicals, we reveal that their chemical reactivity can be significantly enhanced via reducing the nanotubes (i.e., negatively charging). For example, a 5.5-fold enhancement in reactivity of reduced BNNTs toward NH2 radicals was predicted relative to their neutral counterparts. The localization characteristics of the BNNT π electron system lead the excess electrons to fill the empty p orbitals of boron sites, which promote covalent bond formation with an unpaired electron from a radical molecule. In support of our theoretical findings, we also experimentally investigated the covalent alkylation of BNNTs via reduction chemistry using 1-bromohexane. The thermogravimetric measurements showed a considerable weight loss (12-14%) only for samples alkylated using reduced BNNTs, suggesting their significantly improved reactivity over neutral BNNTs. This finding will provide an insight in developing an effective route to chemical functionalization of BNNTs.

The enhanced hydrogen storage of micro-nanostructured hybrids of Mg(BH4)2-carbon nanotubes

We report the facile preparation of micro-nanostructured hybrids of Mg(BH4)2-carbon nanotubes (denoted as MBH-CNTs) and their enhanced hydrogen desorption/absorption performance. The hybrids with Mg(BH4)2 loadings of 25 wt%, 50 wt% and 75 wt% are synthesized through a one-step solvent method by adjusting the ratios of Mg(BH4)2 and CNTs. The optimized MBH-CNTs with 50 wt% Mg(BH4)2 exhibit a nanosized layer coating of Mg(BH4)2 with the thickness of 2-6 nm on the surface of CNTs. The MBH-CNTs with 50 wt% Mg(BH4)2 start to release hydrogen at 76 °C, which shows a significant decrease of about 200 °C compared with that of pure Mg(BH4)2 (about 292 °C). Furthermore, 3.79 wt% of H2 can be desorbed from this sample within 10 min at the peak release temperature of 117 °C. Meanwhile, the dehydrogenated MBH-CNTs could take up 2.5 wt% of H2 at 350 °C under the hydrogen pressure of 10 MPa. The high chemical activity of nanosized Mg(BH4)2 and the catalytic effect of CNTs synergistically promote reversible hydrogen storage. The simple synthesis process and enhanced hydrogen desorption/absorption of MBH-CNT hybrids shed light on the utilization of Mg(BH4)2 on CNTs as efficient hydrogen storage materials.

NaBH4 in "Graphene Wrapper:" Significantly Enhanced Hydrogen Storage Capacity and Regenerability through Nanoencapsulation

A new high-capacity reversible hydrogen-storage material synthesized by the encapsulation of NaBH4 nanoparticles in graphene is reported. This approach effectively prevents phase agglomeration or separation during successive H2 discharge/recharge processes and enables rapid H2 uptake and release in NaBH4 under mild conditions. The strategy advanced here paves a new way for application in energy generation and storage.

Hydrogen Storage Materials for Mobile and Stationary Applications: Current State of the Art

One of the limitations to the widespread use of hydrogen as an energy carrier is its storage in a safe and compact form. Herein, recent developments in effective high-capacity hydrogen storage materials are reviewed, with a special emphasis on light compounds, including those based on organic porous structures, boron, nitrogen, and aluminum. These elements and their related compounds hold the promise of high, reversible, and practical hydrogen storage capacity for mobile applications, including vehicles and portable power equipment, but also for the large scale and distributed storage of energy for stationary applications. Current understanding of the fundamental principles that govern the interaction of hydrogen with these light compounds is summarized, as well as basic strategies to meet practical targets of hydrogen uptake and release. The limitation of these strategies and current understanding is also discussed and new directions proposed.

Cost-Effective Hierarchical Catalysts for Promoting Hydrogen Release from Complex Hydrides

Fe nanoparticles (∼10 nm), used to grow carbon nanotubes (CNTs), have an outstanding ability to catalyze the dehydrogenation of LiAlH4 . The CNTs help connect Fe and LiAlH4 and create microchannels among the composite, thus promoting the release of hydrogen. Inspired by these results, a supercritical-CO2 -fluid-assisted deposition technique is employed to decorate the Fe/CNTs with highly dispersed nanosized Ni (∼2 nm in diameter) for better performance. With the incorporation of 10 wt % of this hierarchical catalyst (Ni/Fe/CNTs), the initial dehydrogenation temperature of LiAlH4 is decreased from ∼135 to ∼40 °C. At 100 °C, this catalyzed LiAlH4 takes only ∼0.1 h to release 4.5 wt % hydrogen, which is more than 100 times faster than the time needed with pristine LiAlH4 . The dehydrogenation mechanism of the complex hydride is examined using in situ synchrotron X-ray diffraction.

Superhalogens: A Bridge between Complex Metal Hydrides and Li Ion Batteries

Complex metal hydrides and Li ion batteries play an integral role in the pursuit of clean and sustainable energy. The former stores hydrogen and can provide a clean energy solution for the transportation industry, while the latter can store energy harnessed from the sun and the wind. However, considerable materials challenges remain in both cases, and research for finding solutions has traditionally followed parallel paths. In this Perspective, I show that there is a common link between these two seemingly disparate fields that can be unveiled by studying the electronic structure of the anions in complex metal hydrides and in electrolytes of Li ion batteries; they are both superhalogens. I demonstrate that considerable progress made in our understanding of superhalogens in the past decade can provide solutions to some of the materials challenges in both of these areas.

Pseudo-binary electrolyte, LiBH4-LiCl, for bulk-type all-solid-state lithium-sulfur battery

The ionic conduction and electrochemical and thermal stabilities of the LiBH4-LiCl solid-state electrolyte were investigated for use in bulk-type all-solid-state lithium-sulfur batteries. The LiBH4-LiCl solid-state electrolyte exhibiting a lithium ionic conductivity of [Formula: see text] at 373 K, forms a reversible interface with a lithium metal electrode and has a wide electrochemical potential window up to 5 V. By means of the high-energy mechanical ball-milling technique, we prepared a composite powder consisting of elemental sulfur and mixed conductive additive, i.e., Ketjen black and Maxsorb. In that composite powder, homogeneous dispersion of the materials is achieved on a nanometer scale, and thereby a high concentration of the interface among them is induced. Such nanometer-scale dispersals of both elemental sulfur and carbon materials play an important role in enhancing the electrochemical reaction of elemental sulfur. The highly deformable LiBH4-LiCl electrolyte assists in the formation of a high concentration of tight interfaces with the sulfur-carbon composite powder. The LiBH4-LiCl electrolyte also allows the formation of the interface between the positive electrode and the electrolyte layers, and thus the Li-ion transport paths are established at that interface. As a result, our battery exhibits high discharge capacities of 1377, 856, and 636 mAh g(-1) for the 1st, 2nd, and 5th discharges, respectively, at 373 K. These results imply that complex hydride-based solid-state electrolytes that contain Cl-ions in the crystal would be integrated into rechargeable batteries.

CNT addition to the LiBH4–MgH2 composite: the effect of milling sequence on the hydrogen cycling properties

The effect of the addition of CNT to the 2LiBH₄ : MgH₂ system was studied. The enhanced kinetic behaviour disappeared after several absorption/desorption cycles.

N-Ethylcarbazole-doped fullerene as a potential candidate for hydrogen storage, a kinetics approach

Due to the suitable possibility of hydrogen storage in liquid organic hydrogen carriers (LOHCs), a systematic analysis of the chemisorption pathway of hydrogen on N-ethylcarbazole doped fullerene (NEC@C₆₀) has been presented.

Metal/graphene nanocomposites synthesized with the aid of supercritical fluid for promoting hydrogen release from complex hydrides

With the aid of supercritical CO2, Fe-, Ni-, Pd-, and Au-nanoparticle-decorated nanostructured carbon materials (graphene, activated carbon, carbon black, and carbon nanotubes) are synthesized for catalyzing the dehydrogenation of LiAlH4. The effects of the metal nanoparticle size and distribution, and the type of carbon structure on the hydrogen release properties are investigated. The Fe/graphene nanocomposite, which consists of ∼2 nm Fe particles highly dispersed on graphene nanosheets, exhibits the highest catalytic performance. With this nanocomposite, the initial dehydrogenation temperature can be lowered (from ∼135 °C for pristine LiAlH4) to ∼40 °C without altering the reaction route (confirmed by in situ X-ray diffraction), and 4.5 wt% H2 can be released at 100 °C within 6 min, which is faster by more than 135-fold than the time required to release the same amount of H2 from pristine LiAlH4.

Nanoconfined NaAlH4: prolific effects from increased surface area and pore volume

Nanoconfinement is a promising technique to improve the properties of nanomaterials such as the kinetics for hydrogen release and uptake and the stability during cycling. Here we present a systematic study of nanoconfined NaAlH4 in nanoporous scaffolds with increasing surface area and pore volume and almost constant pore sizes in the range of 8 to 11 nm. A resorcinol formaldehyde carbon aerogel was CO2-activated under different conditions and provided aerogels with BET surface areas of 704, 1267 and 2246 m(2) g(-1) and total pore volumes of 0.91, 1.30 and 2.21 mL g(-1), respectively. Nanoconfinement of NaAlH4 was achieved by melt infiltration and (27)Al MAS NMR reveals that the respective scaffolds incorporate 68, 82 and 91 wt% NaAlH4, for the above-mentioned samples, while the remaining fraction decomposes to metallic Al indicating that increasing CO2-activation tends to facilitate the infiltration process. The frequencies for the (23)Na and (27)Al MAS NMR centerband resonances from NaAlH4 vary systematically for the infiltrated samples and are shifted towards higher frequency and become more narrow with increasing degree of CO2 activation of the scaffolds. This new effect is attributed to increasing interactions with conduction electrons from increasingly graphite-/graphene-like scaffolds. The bulk versus nanoconfined ratio of NaAlH4 was investigated using Rietveld refinement, revealing that the majority of added NaAlH4 is confined inside the nanopores. The hydrogen desorption kinetics decreased with increasing surface area and the hydrogen storage capacity is more stable and decreases less during continuous hydrogen release and uptake cycles. In fact, the available amount of hydrogen (2.7 wt% H2) was more than doubled compared to the nanoconfinement in the non-activated carbon aerogel (1.3 wt% H2). Furthermore, it was demonstrated that Ti-functionalization of the CO2-activated aerogels combines the high storage capacity with fast hydrogen release kinetics from NaAlH4 which fully decomposes into Na3AlH6 at T ≤ 100 °C.

Synthesis, characterization, and reversible hydrogen sorption study of sodium-doped fullerene

Herein is presented a novel, straightforward route to the synthesis of an alkali metal-doped fullerene as well as a detailed account of its reversible and enhanced hydrogen sorption properties in comparison to pure C60. This work demonstrates that a reaction of sodium hydride with fullerene (C60) results in the formation of a sodium-doped fullerene capable of reversible hydrogen sorption via a chemisorption mechanism. This material not only demonstrated reversible hydrogen storage over several cycles, it also showed the ability to reabsorb over three times the amount of hydrogen (relative to the hydrogen content of NaH) under optimized conditions. The sodium-doped fullerene was hydrogenated on a pressure composition temperature (PCT) instrument at 275 °C while under 100 bar of hydrogen pressure. The hydrogen desorption behavior of this sodium-doped fullerene hydride was observed over a temperature range up to 375 °C on the PCT and up to 550 °C on the thermogravimetric analysis (TGA). Powder x-ray diffraction verifies the identity of this material as being Na6C60. Characterization of this material by thermal decomposition analysis (e.g. PCT and TGA methods), as well as FT-IR and mass spectrometry, indicates that the hydrogen sorption activity of this material is due to the reversible formation of a hydrogenated fullerene (fullerane). However, the reversible formation of fullerane was found to be greatly enhanced by the presence of sodium. It was also demonstrated that the addition of a catalytic amount of titanium (via TiO2 or Ti(OBu)4) further enhances the hydrogen sorption process of the sodium-doped fullerene material.

Adsorption of hydrogen on neutral and charged fullerene: experiment and theory

Helium droplets are doped with fullerenes (either C60 or C70) and hydrogen (H2 or D2) and investigated by high-resolution mass spectrometry. In addition to pure helium and hydrogen cluster ions, hydrogen-fullerene complexes are observed upon electron ionization. The composition of the main ion series is (H2)(n)HC(m)(+) where m = 60 or 70. Another series of even-numbered ions, (H2)(n)C(m)(+), is slightly weaker in stark contrast to pure hydrogen cluster ions for which the even-numbered series (H2)(n)(+) is barely detectable. The ion series (H2)(n)HC(m)(+) and (H2)(n)C(m)(+) exhibit abrupt drops in ion abundance at n = 32 for C60 and 37 for C70, indicating formation of an energetically favorable commensurate phase, with each face of the fullerene ion being covered by one adsorbate molecule. However, the first solvation layer is not complete until a total of 49 H2 are adsorbed on C60(+); the corresponding value for C70(+) is 51. Surprisingly, these values do not exhibit a hydrogen-deuterium isotope effect even though the isotope effect for H2/D2 adsorbates on graphite exceeds 6%. We also observe doubly charged fullerene-deuterium clusters; they, too, exhibit abrupt drops in ion abundance at n = 32 and 37 for C60 and C70, respectively. The findings imply that the charge is localized on the fullerene, stabilizing the system against charge separation. Density functional calculations for C60-hydrogen complexes with up to five hydrogen atoms provide insight into the experimental findings and the structure of the ions. The binding energy of physisorbed H2 is 57 meV for H2C60(+) and (H2)2C60(+), and slightly above 70 meV for H2HC60(+) and (H2)2HC60(+). The lone hydrogen in the odd-numbered complexes is covalently bound atop a carbon atom but a large barrier of 1.69 eV impedes chemisorption of the H2 molecules. Calculations for neutral and doubly charged complexes are presented as well.

C60-mediated hydrogen desorption in Li-N-H systems

Hydrogen desorption from a LiH + NH(3) mixture is very difficult due to the formation of the stable LiNH(4) compound. Using cluster models and first-principles theory, we demonstrate that the C(60) molecule can in fact significantly improve the thermodynamics of ammonia-mediated hydrogen desorption from LiH due to the stabilization of the intermediate state, LiNH(4). The hydrogen desorption following the path of LiNH(4)-C(60) → LiNH(3)-C(60) + 1/2H(2) is exothermic. Molecular dynamic simulations show that this reaction can take place even at room temperature (300 K). In contrast, the stable LiNH(4) compound cannot desorb hydrogen at room temperature in the absence of C(60). The introduction of C(60) also helps to restrain the NH(3) gas which is poisonous in proton exchange membrane fuel cell applications.

Reversible hydrogen storage by NaAlH4 confined within a titanium-functionalized MOF-74(Mg) nanoreactor

We demonstrate that NaAlH(4) confined within the nanopores of a titanium-functionalized metal-organic framework (MOF) template MOF-74(Mg) can reversibly store hydrogen with minimal loss of capacity. Hydride-infiltrated samples were synthesized by melt infiltration, achieving loadings up to 21 wt %. MOF-74(Mg) possesses one-dimensional, 12 Å channels lined with Mg atoms having open coordination sites, which can serve as sites for Ti catalyst stabilization. MOF-74(Mg) is stable under repeated hydrogen desorption and hydride regeneration cycles, allowing it to serve as a "nanoreactor". Confining NaAlH(4) within these pores alters the decomposition pathway by eliminating the stable intermediate Na(3)AlH(6) phase observed during bulk decomposition and proceeding directly to NaH, Al, and H(2), in agreement with theory. The onset of hydrogen desorption for both Ti-doped and undoped nano-NaAlH(4)@MOF-74(Mg) is ∼50 °C, nearly 100 °C lower than bulk NaAlH(4). However, the presence of titanium is not necessary for this increase in desorption kinetics but enables rehydriding to be almost fully reversible. Isothermal kinetic studies indicate that the activation energy for H(2) desorption is reduced from 79.5 kJ mol(-1) in bulk Ti-doped NaAlH(4) to 57.4 kJ mol(-1) for nanoconfined NaAlH(4). The structural properties of nano-NaAlH(4)@MOF-74(Mg) were probed using (23)Na and (27)Al solid-state MAS NMR, which indicates that the hydride is not decomposed during infiltration and that Al is present as tetrahedral AlH(4)(-) anions prior to desorption and as Al metal after desorption. Because of the highly ordered MOF structure and monodisperse pore dimensions, our results allow key template features to be identified to ensure reversible, low-temperature hydrogen storage.

In Situ NMR Study on the Interaction between LiBH4-Ca(BH4)2 and Mesoporous Scaffolds

We discuss the use of nuclear magnetic resonance (NMR) spectroscopy to investigate the physical state of the eutectic composition of LiBH4-Ca(BH4)2 (LC) infiltrated into mesoporous scaffolds and the interface effect of various scaffolds. Eutectic melting and the melt infiltration of mixed borohydrides were observed through in situ NMR. In situ and ex situ NMR results for LC mixed with mesoporous scaffolds indicate that LiBH4 and Ca(BH4)2 exist as an amorphous mixture inside of the pores after infiltration. Surprisingly, the confinement of the eutectic LC mixture within the mesopores is initiated below the melting temperature, which indicates a certain interaction between the borohydrides and the mesoporous scaffolds. The confined borohydrides remain inside of the pores after cooling. These phenomena were not observed in microporous or nonporous materials, and this observation highlights the importance of the pore structure of the scaffolds. Such surface interactions may be associated with a faster dehydrogenation of the nanoconfined borohydrides.