Addition of transition metals to lithium intercalated fullerides enhances hydrogen storage properties

We report an innovative synthetic strategy based on the solid state reaction of fullerene C₆₀ with lithium-transition metals alloys (platinum and palladium), which provides transition metal-decorated lithium intercalated fullerides, with improved hydrogen storage properties. Compounds with Li₆Pt_0.11C₆₀ and Li₆Pd_0.07C₆₀ stoichiometry were obtained and investigated with manometric/calorimetric techniques which showed an 18% increase of the final H₂ absorbed amount with respect to pure Li₆C₆₀ (5.9 wt% H₂) and an improved absorption process kinetic. The absorption mechanism was investigated with X-rays diffraction which allowed to identify the formation of the hydrofullerides. Scanning Electron Microscopy was applied to gain information on transition metal distribution and detected the presence of platinum and palladium aggregates which are shown to perform a surface catalytic activity towards hydrogen molecule dissociation process.     

Synthesis and hydrogen storage capacity of exfoliated turbostratic carbon nanofibers

Turbostratic carbon nanofibers (CNFs) with a rough surface, open pore walls, and a defect structure were continuously produced by the thermal decomposition of alcohol in the presence of an iron catalyst and a sulfur promoter at 1100 °C under a nitrogen atmosphere in a vertical chemical vapor deposition reactor. A graphite exfoliation technique using intercalation and thermal shock was employed to expand the graphene layers of the as-produced turbostratic CNFs. The hydrogen storage capacity of the turbostratic CNF samples was measured using the volumetric method with a pressure of up to 1 MPa at 77 K. The hydrogen storage capacities of the as-produced and exfoliated turbostratic CNFs were 1.5 and 5 wt%, respectively. The defects on the surface and expandable graphitic structure are considered important keys to increasing the hydrogen uptake in turbostratic CNFs.     

Improving hydrogen storage in Ni-doped carbon nanospheres

The effect of nickel distribution and content in Ni-doped carbon nanospheres on hydrogen storage capacity under conditions of moderate temperature and pressure was studied. It was found that the nickel distribution, obtained by using different doping techniques and conditions, has a noticeable influence on hydrogen storage capacity. The samples with the most homogeneous nickel distribution, obtained by pre-oxidising the carbon nanospheres, displayed the highest storage capacity. In addition, storage capacity is influenced by the amount of nickel. It was found a higher storage capacity in samples containing 5 wt.% of Ni. This is due to the greater interactions between the nickel and the support that produce a higher activation of the solid through a spillover effect.     

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.     

Mg-based composites for enhanced hydrogen storage performance

Hydrogen storage in solids of hydrides is advantageous in comparison to gaseous or liquid storage. Magnesium based materials are being studies for solid-state hydrogen storage due to their advantages of high volumetric and gravimetric hydrogen storage capacity. However, unfavorable thermodynamic and kinetic barriers hinder its practical application. In this work, we presented that kinetics of Mg-based composites were significantly improved during high energy ball milling in presence of various types of carbon, including plasma carbon produced by plasma-reforming of hydrocarbons, activated carbon, and carbon nanotubes. The improvement of the kinetics and de-/re-hydrogenation performance of MgH₂ and TiC-catalysed MgH₂ by introduction of carbon are strongly dependent on the milling time, amount of carbon and carbon structure. The lowest dehydrogenation temperature was observed at 180 °C by the plasma carbon–modified MgH₂/TiC. We found that nanoconfinement of carbon structures stabilised Mg-based nanocomposites and hinders the nanoparticles growth and agglomeration. Plasma carbon was found to show better effects than the other two carbon structures because the plasma carbon contained both few layer graphene sheets that served as an active dispersion matrix and amorphous activated carbons that promoted the spill-over effect of TiC catalysed MgH₂. The strategy in enhancing the kinetics and thermodynamics of Mg-based composites is leading to a better design of metal hydride composites for hydrogen storage.     

Magnesium hydrazinidoborane: Synthesis,characterization and features for solid-state hydrogen storage

After various attempts, we present a new alkaline-earth derivative of hydrazine borane (N₂H₄BH_3, HB), magnesium hydrazinidoborane (Mg(N₂H₃BH₃)₂, Mg(HB)₂, 10.5 wt % H), that was undoubtedly identified by FTIR and ¹¹B MAS NMR spectroscopy. Mg(HB)₂ was obtained by an alternative synthesis route, which is the reaction between HB and di-n-butylmagnesium in THF. The dehydrogenation properties of this compound were evaluated by two different approaches: an “open” system by thermogravimetric analysis and differential scanning calorimetry, and in a closed system, by heating the compound under isothermal conditions. Different results were obtained depending on the approach. Unlike other boron- and nitrogen-based compounds, it is likely that when Mg(HB)₂ is heated in a closed system, the dehydrogenation is limited and it occurs mainly due to the homopolar interaction between the protic hydrogen atoms of the molecule. Also, Mg(HB)₂ presents a contrasting thermal behavior in comparison with previous HB derivatives. In addition, a characterization by X-ray photoelectron spectroscopy was performed, and we detected instability of Mg(HB)₂ when it was irradiated with the X-ray beam. All of these results are presented and discussed in the context of materials for hydrogen storage in the solid-state.     

Calcium hydrazinidoborane: Synthesis,characterization, and promises for hydrogen storage

Viewing calcium hydrazinidoborane Ca(N₂H₃BH₃)₂ (9.3 wt% H) as a potential hydrogen storage material, we long sought to synthesize it by solid-solid reaction of calcium hydride CaH₂ and hydrazine borane N₂H₄BH₃. However, it was elusive because of unsuitable experimental conditions. In situ synchrotron thermodiffraction helped us to identify the key role played by the temperature in the formation of the new phase. From 45 °C, new diffraction peaks appear, and the DSC analysis shows an exothermic signal. Thermal activation is thus required to make solid-state CaH₂ react with melted (liquid-state) N₂H₄BH₃. The XRD pattern can be indexed using a mixture of two phases: (i) unreacted CaH₂ as a minor phase (29 wt%) and (ii) the hitherto elusive Ca(N₂H₃BH₃)₂ (71 wt%). The as-formed Ca(N₂H₃BH₃)₂ crystallizes in a monoclinic Ic (No. 9) unit cell where the intermolecular interactions form chains (layers) along the a axis, resulting in intra-chain and inter-chain Ca⋅⋅⋅Ca distances as short as 4.39 and 7.04 Å respectively. Beyond 90 °C, Ca(N₂H₃BH₃)₂ decomposes, as evidenced by the diffraction peaks fading, an exothermic signal revealed by DSC, a weight loss (5.3 wt% at 200 °C) observed by TGA, and a gas release (H₂, and some N₂, NH₃, N₂H₄) monitored by MS. The as-formed thermolytic residue is amorphous and of complex polymeric composition. These results and the next challenges, are discussed herein.     

Synthesis and characterisation of a mesoporous carbon/calcium borohydride nanocomposite for hydrogen storage

Borohydrides with high hydrogen content are being extensively studied as potential hydrogen storage systems placing particular emphasis on upturning their unfavourable kinetic and thermodynamic properties which give rise to significantly high dehydrogenation temperatures and slow hydrogen release far away from the desired application window. In this work the encapsulation of Ca(BH₄)₂ particles in the pores of a CMK-3 type ordered mesoporous carbon scaffold by wet chemistry routes, also employing the use of TiCl₃ as a catalyst, is shown to have a beneficial effect on the hydrogen desorption profile of the bulk hydride by shifting its decomposition to noticeable lower temperatures.     

Synthesis and hydrogen plasma interaction of model mixed materials for fusion

Samples of mixed materials relevant to fusion (W/C/Mg, where Mg is a chemical stand-in for Be) have been synthesized by planetary ball milling. We explore here W-rich compounds, and investigate their structural and physico-chemical properties, both before and after hydrogen plasma exposure. After presenting the synthesis method experimental results are detailed: clear differences between incorporation of magnesium and/or carbon into a tungsten matrix are observed, as reflected in the specific surface area of the samples and their lattice parameters. Indeed, specific surface area decreases with increasing Mg content but increases with C content. The same trend is seen for the lattice parameter. A particular effect seen after hydrogen plasma exposure is attributed to the presence of magnesium which may also occur in the presence of beryllium.     

Light metal functionalized two-dimensional siligene for high capacity hydrogen storage: DFT study

In this work, the hydrogen storage capacities of two-dimensional siligene (2D-SiGe) functionalized with alkali metal (AM) and alkali-earth metal (AEM) atoms were studied using density functional theory calculations. One AM (Li, Na, K) or AEM (Be, Mg, Ca) atom was placed on the 2D-SiGe surface, and several H₂ molecules were placed in the vicinity of the adatom. The results demonstrate that the most favorable siligene site for the adsorption of Li, Na, K and Be atoms is the hollow site, while for the Mg and Ca atoms is the down site. The AM atoms are the only ones with considerable binding energies on the SiGe nanosheets. Pristine 2D-SiGe slightly adsorbs one H₂ molecule per hollow site and, therefore, it is not suitable for hydrogen storage. In some of the AM- and AEM-decorated 2D-SiGe, several hydrogen molecules can be physisorbed. In particular, the Na-, K- and Ca-functionalized 2D-SiGe can adsorb six hydrogen molecules, whereas Li and Mg atoms adsorbed three hydrogen molecules, and the Be adatom only adsorbed one hydrogen molecule. The complexes formed by hydrogen molecules adsorbed on the analyzed metal decorated 2D-SiGe are energetically stable, indicating that functionalized 2D-SiGe could be an efficient molecular hydrogen storage media.     

Concepts for improving hydrogen storage in nanoporous materials

Hydrogen storage in nanoporous materials has been attracting a great deal of attention in recent years, as high gravimetric H₂ capacities, exceeding 10 wt% in some cases, can be achieved at 77 K using materials with particularly high surface areas. However, volumetric capacities at low temperatures, and both gravimetric and volumetric capacities at ambient temperature, need to be improved before such adsorbents become practically viable. This article therefore discusses approaches to increasing the gravimetric and volumetric hydrogen storage capacities of nanoporous materials, and maximizing the usable capacity of a material between the upper storage and delivery pressures. In addition, recent advances in machine learning and data science provide an opportunity to apply this technology to the search for new materials for hydrogen storage. The large number of possible component combinations and substitutions in various porous materials, including Metal-Organic Frameworks (MOFs), is ideally suited to a machine learning approach; so this is also discussed, together with some new material types that could prove useful in the future for hydrogen storage applications.     

Ab initio study of hydrogen storage on metal-decorated GeC monolayers

Bidimensional nanostructures have been proposed as hydrogen-storage systems owing to their large surface-to-volume ratios. Germanium carbide monolayers (GeC-MLs) can offer attractive opportunities for H₂ adsorption compared to graphene. However, this possibility has not been explored in detail. In this work, the adsorption of H₂ molecules on GeC-MLs decorated with alkali metal (AM) and alkaline earth metal (AEM) adatoms was investigated using the density functional theory. Results showed that the AM adatoms were chemisorbed on the GeC-ML, whereas AEM adatoms were physisorbed. The H₂ molecules presented negligible adsorption energies on the weakly adsorbed AEM adatoms. Conversely, the AM adatoms improved the H₂ adsorption, possibly due to a large charge transfer from the adatoms to the GeC-ML. The potassium-decorated GeC-ML exhibited the most optimal H₂ storage capacity, adsorbing up to six molecules and with a lower possibility of forming metal clusters than the other studied cases. These results may aid in the development of new efficient hydrogen-storage materials.     

A search for new Mg- and K-containing alanates for hydrogen storage

In this work Mg- and K-containing alanates have been investigated as possible hydrogen storage materials. Ball milling was carried out under argon or at moderate/high hydrogen pressure in order to obtain an improved driving force for the formation of potential new alanate phases. Powder X-ray diffraction and volumetric measurements were used in order to identify reaction mechanisms and phases forming in these systems. New unidentified peaks were detected for the mixtures 2MgH₂ + 3Al + KH and 2CaH₂ + Al + 2KH. However, they do not seem to belong to reversible hydride phases.     

Hydrogen storage materials for hydrogen and energy carriers

Hydrogen storage technology is essentially necessary to promote renewable energy. Many kinds of hydrogen storage materials, which are hydrogen storage alloys, inorganic chemical hydrides, carbon materials and liquid hydrides have been studied. In those materials, ammonia (NH₃) is easily liquefied by compression at 1 MPa and 298 K, and has a highest volumetric hydrogen density of 10.7 kg H₂/100 L. It also has a high gravimetric hydrogen density of 17.8 wt%. The theoretical hydrogen conversion efficiency is about 90%. NH₃ is burnable without emission of CO₂ and has advantages as hydrogen and energy carriers.     

Synthesis and characterization of porous HKUST-1 metal organic frameworks for hydrogen storage

Hydrogen storage capacity has been investigated on a copper-based metal organic framework named HKUST-1 with fine structural analyses. The crystalline structure of HKUST-1 MOF has been confirmed from the powder X-ray diffraction and the average particle diameter has been found about 15–20 μm identified by FE-SEM. Nitrogen adsorption isotherms show that HKUST-1 MOF has approximately type-I isotherm with a BET specific surface area of 1055 m²g⁻¹. Hydrogen adsorption study shows that this material can store 0.47 wt.% of H₂ at 303 K and 35 bar. The existence of Cu (II) in crystalline framework of HKUST-1 MOF has been confirmed by pre-edge XANES spectra. The sharp feature at 8985.8 eV in XANES spectra represents the dipole-allowed electron transition from 1s to 4p_xy. In addition, EXAFS spectra indicate that HKUST-1 MOF structure has the Cu–O bond distance of 1.95 Å with a coordination number of 4.2.     

Synthesis of boron and nitrogen co-doped carbon nanotubes and their application in hydrogen storage

Boron and nitrogen codoped carbon nanotubes (B,N-CNTs) were synthesized by floating catalyst chemical vapor deposition (FCCVD) using ethanol, ferrocene, boric acid and imidazole as carbon source, catalyst, boron and nitrogen precursors, respectively. The samples were analyzed using transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis and X-ray photoemission spectroscopy. 1.5 at% B and 1.34 at% N could be doped in the resultant structure, which has higher length (few μm) with higher thermal stability (621 °C). At pressure 16 bar, hydrogen adsorption for B,N-CNTs was found to be 1.96 and 0.35 wt% at 77 K and 303 K, respectively. Hydrogen storage as function of time was also reported for both the cases. The adsorption process follow pseudo second order kinetics. The present study reveals that the codoping of CNTs aid in tuning properties of CNTs for hydrogen storage application.     

Recent advances in carbon nitride-based nanomaterials for hydrogen production and storage

There are different types of materials comprising of carbon and nitrogen elements. Typical materials are the cyanogen family, beta carbon nitride, graphitic carbon nitride, azafullerenes, and heterofullerenes, N-containing heterocycles. Except cyanogen (C₂N₂) is a gas, most others are solid. Among these solids, the graphitic carbon nitride, with the general chemical formula of C₃N₄, is widely studied in heterogeneous catalysis and energy storage. Such applications exploit the resiliency of the material in different environments due to its labile protons and Lewis acid functionalities, as well as its layered structure. The structure of graphitic C₃N₄ allows it to store a significant amount of hydrogen. Furthermore, it offers the space for dopants, which are used purposely for tuning the band gap and the electronic properties of C₃N₄ to make it suitable for water splitting using sun light, or many other applications in waste water treatment under radiation. We think that the material is important and it is not being exploited at its highest capability, especially in hydrogen production via water splitting technique. This review aims to summarize recent outcomes using the carbon nitride material in hydrogen production, and a brief about hydrogen storage. We also highlight future research directions which might worth being persuaded.     

Fabrication of Pt-doped carbon aerogels for hydrogen storage by radiation method

A radiation method was investigated to fabricate Pt-doped carbon aerogels (CAsPt). The physicochemical properties of the pristine CAs and CAsPt were systematically characterized by X-ray diffraction, scanning electron microscope, transmission electron microscopy, and nitrogen adsorption measurements. The results showed that not only a great number of Pt nanospheres but also many Pt nanoparticles presented in the network of CAs after radiation. The influence of Pt doping on the hydrogen uptake capacity of CAs was studied. In comparison with the pristine CAs, it was remarkable that the hydrogen uptake capacity of the CAsPt had been significantly enhanced, which was contributed by the hydrogen spillover of Pt.     

Macroscopic kinetics of hydrate formation of mixed hydrates of hydrogen/tetrahydrofuran for hydrogen storage

Hydrogen/Tetrahydrofuran mixed hydrate formation studies were conducted in a stirred tank reactor. Hydrate formation kinetics at driving forces of 2 MPa, 5 MPa and 7 MPa were studied. Tetrahydrofuran (THF) concentration was varied between 1 mol% and 5 mol% and its effect on hydrate formation kinetics was investigated. With an increase in the driving force there was an increase in gas uptake till super saturation. However, the increased driving force had little effect on the reduction of induction time even at high promoter (THF) concentration. 5 mol% THF solutions behave distinctly exhibiting an increased hydrate growth compared to low promoter concentrations due to the occurrence of multiple nucleation events (observed based on temperature spikes and gas uptake). Rate of hydrate formation increased with an increase in driving force for a given concentration of THF promoter. Water conversion to hydrates in the range of 2.8–10.8% was achieved for all the experiments. Addition of Sodium Dodecyl Sulphate (SDS) surfactant had no effect in improving the kinetics of mixed hydrogen/THF hydrates. This study highlights the kinetic challenges that need to be overcome in storing hydrogen as clathrate hydrates.     

Synthesis and hydrogen storage properties of ultrafine Mg-Zn particles

Mg-6.9 at.% Zn ultrafine particles (UFPs) were prepared by hydrogen plasma-metal reaction (HPMR) method. The electron microscopy study revealed that they were spherical in shape with particle size in the range 100-700 nm. Each fine particle was composed of single crystal structure of α-Mg(Zn) solid solution and amorphous structure of Mg-Zn alloy. After one absorption and desorption cycle, these UFPs transformed from the single crystal into the nanocrystalline structure and the mean particle size changed from 400 to 250 nm. It was found that the Mg-Zn UFPs could absorb 5.0 wt.% hydrogen in 20 min at 573 K and accomplish a high hydrogen storage capacity of 6.1 wt.% at 573 K. The fine particle size, nanocrystalline structure and the low oxide content of the obtained sample promoted the hydrogen sorption process with low hydrogen absorption activation energy of 56.3 kJ/mol. The enhanced hydrogen sorption properties of high absorbing rate and high storage capacity were due to the improved kinetics rather than the change in enthalpy.