Cooperatively enhanced catalytic properties of Ti@Al(100) near-surface alloy for aluminum hydrogenation

We have carried out detailed first-principles studies of the catalytic properties of Ti@Al(100) near-surface alloy. The single Ti atom, (0,2) Ti–Ti pair, and [0,2] Ti doping domain have better catalytic performances. These species doped in the top surface can develop back-bonding interaction with H₂ to catalyze the splitting, which however on the other hand hinder the dissociated H atoms to diffuse. Doped in the subsurface, they can also enhance hydrogen interaction on aluminum to catalyze H₂ splitting. The activation energies are 0.80, 0.68, and 0.48 eV for Ti atom, (0,2) pair, and [0,2] doping domain, respectively. Without Ti–H bond, the dissociated H atom could diffuse away with small energy cost. The structural expansion induced by titanium doping, the lower electronegativity of Ti, and the more valence electrons of Ti may cooperatively facilitate the charge transfer from the above Al atoms to H₂ molecule, accounting for the enhanced splitting properties.     

Synthesis of catalytically active rock salt structured MgxNb1−xO nanoparticles for MgH2 system

This communication deals with the ex-situ synthesis of rock salt type Mg_xNb_1−xO whose structural characteristics are closely related with MgO. XRD examination of 30 h ball milled MgH₂ + Nb₂O₅ confirms the formation of a rock salt product Mg_xNb_1−xO_, which is comparable to the recently reported active catalyst Mg_xNb_1−xO formed in-situ in MgH₂ milled with 8 mol.% Nb₂O₅. It is shown that MgH₂ catalyzed with the pre-made 2 wt.% Mg_xNb_1−xO desorbs hydrogen at least 50 °C lower than the in-situ 2 wt.% Nb₂O₅ catalyzed MgH₂ with improved reversible absorption. This result highlights that the proposed pathway mechanism on the basis of Nb₂O₅ catalyst may need further verification and that the addition of the Mg_xNb_1−xO catalyst in a pre-reduced state can offer distinct performance advantages over its in-situ preparation.     

Improved hydrogen storage properties of MgH2 with Ni-based compounds

The nanoscaled Ni-based compounds (Ni₃C, Ni₃N, NiO and Ni₂P) are synthesized by chemical methods. The MgH₂-X (X = Ni₃C, Ni₃N, NiO and Ni₂P) composites are prepared by mechanical ball-milling. The dehydrogenation properties of Mg-based composites are systematically studied using isothermal dehydrogenation apparatus, temperature-programmed desorption system and differential scanning calorimetry. It is experimentally confirmed that the dehydrogenation performance of the Mg-based materials ranks as following: MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P. The onset dehydrogenation temperatures of MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P are 160 °C, 180 °C, 205 °C and 248 °C, respectively. The four Mg-based composites respectively release 6.2, 4.9, 4.1 and 3.5 wt% H₂ within 20 min at 300 °C. The activation energies of MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P are 97.8, 100.0, 119.7 and 132.5 kJ mol^?1, respectively. It' found that the MgH₂Ni₃C composites exhibit the best hydrogen storage properties. Moreover, the catalytic mechanism of the Ni-based compounds is also discussed. It is found that Ni binding with low electron-negativity element is favorable for the dehydrogenation of the Mg-based composites.     

Enhanced dehydrogenation/hydrogenation kinetics of the Mg(NH2)2-2LiH system with NaOH additive

The effects of NaOH addition on hydrogen absorption/desorption properties of the Mg(NH₂)₂-2LiH system were investigated systematically by means of dehydrogenation/hydrogenation measurements and structural analyses. It is found that the NaOH-added Mg(NH₂)₂-2LiH samples exhibit an enhanced dehydrogenation/hydrogenation kinetics. In particular, a ∼36 °C reduction in the peak temperature for dehydrogenation is achieved for the Mg(NH₂)₂-2LiH-0.5NaOH sample with respect to the pristine sample. Structural examinations reveal that NaOH reacts with Mg(NH₂)₂ and LiH to convert to NaH, LiNH₂ and MgO during ball milling. Then, their co-catalytic effects result in a significant improvement in the dehydrogenation/hydrogenation kinetics of the Mg(NH₂)₂-2LiH system. This finding will help in designing and optimizing the novel high-performance catalysts to further improve hydrogen storage in the amide-hydride combined systems.     

Role of S-S interlayer spacing on the hydrogen storage mechanism of MoS2

Hydrogen storage mechanism and hydrogen storage capacity are the great challenges for the development of hydrogen energy technology. Besides the better catalytic properties, it is crucial to search for suitable material that provides enough space to store H₂ molecule. Similar to graphene, MoS₂ with S-S layered structure opens up a new way to improve the hydrogen storage capacity. By using the first-principles calculations, in this work, we investigate the hydrogen diffusion mechanism, hydrogenation process and hydrogen storage capacity of MoS₂ with S-S interlayer. We find that hydrogen prefers to diffuse into S-S interlayer along the interstitial site (path: IT-IT). H₂ molecule is a stable in S-S interlayer because the charge interaction of H-H atoms is stronger than that of H-S atoms. Finally, we predict that MoS₂ with S-S layered-by-layered stacking can effectively improve the hydrogen storage capacity.     

Study of the different ZrxNiy phases of Zr-based AB2 materials

Hydride-forming alloys are used as components of the negative electrode of nickel-metal hydride (NiMH) batteries. In previous works, the study of Zr-based AB₂-type alloys indicated that the material without heat treatment (annealing) had better electrochemical characteristics than the annealed one. The effect was attributed to the presence of secondary phases Zr_xNi_y formed during the solidification of the alloy button obtained by arc melting, and to the fact that these phases diminished their concentration or disappeared upon annealing. The main secondary phases formed by microsegregation are Zr₇Ni₁₀, Zr₉Ni₁₁ and Zr₈Ni₂₁.     

Improved reversible dehydrogenation properties of MgH2 by the synergetic effects of graphene oxide-based porous carbon and TiCl3

In this study, we used a combination of graphene oxide-based porous carbon (GC) and titanium chloride (TiCl₃) to improve the reversible dehydrogenation properties of magnesium hydride (MgH₂). Examining the effects of GC and TiCl₃ on the hydrogen storage properties of MgH₂, the study found GC was a useful additive as confinement medium for promoting the reversible dehydrogenation of MgH₂. And TiCl₃ was an efficient catalytic dopant. A series of controlled experiments were carried out to optimize the sample preparation method and the addition amount of GC and TiCl₃. In comparison with the neat MgH₂ system, the MgH₂/GC-TiCl₃ composite prepared under optimized conditions exhibited enhanced dehydrogenation kinetics and lower dehydrogenation temperature. A combination of phase/microstructure/chemical state analyses has been conducted to gain insight into the promoting effects of GC and TiCl₃ on the reversible dehydrogenation of MgH₂. Our study found that GC was a useful scaffold material for tailoring the nanophase structure of MgH₂. And TiCl₃ played an efficient catalytic effect. Therefore, the remarkably improved dehydrogenation properties of MgH₂ should be attributed to the synergetic effects of nanoconfinement and catalysis.     

Recoverable Ni2Al3 nanoparticles and their catalytic effects on Mg-based nanocomposite during hydrogen absorption and desorption cycling

The Mg-3.9 wt% Ni₂Al₃ nanocomposite is produced by hydrogen plasma-metal reaction method. The particle size of Mg is in range of 40–160 nm with an average size of 90 nm. The Ni₂Al₃ nanoparticles (NPs) of about 9 nm uniformly disperse on the surface of Mg NPs and in situ transform to Mg₂NiH_0.3 and Al after hydrogen absorption process. Surprisingly, the Mg₂NiH_0.3 and Al can recover to the initial state of Ni₂Al₃ after hydrogenation/dehydrogenation cycle. The Mg-Ni₂Al₃ nanocomposite shows enhanced hydrogen sorption rate and storage capacity. It can quickly uptake 6.4 wt% H₂ within only 10 min at 573 K, and release 6.1 wt% H₂ within 10 min at 623 K. The apparent activation energies for hydrogenation and dehydrogenation are calculated to be 55.4 and 115.7 kJ mol^?1 H₂. The enhanced hydrogen storage performances of the Mg-Ni₂Al₃ nanocomposite are attributed to both the nanostructure of Mg and the catalytic effects of Ni₂Al₃ NPs.     

Improvement of hydrogen storage properties in MgH2 catalysed by K2NbF7

The catalytic effects of K₂NbF₇ on the hydrogen storage properties of MgH₂ have been studied for the first time. MgH₂ + 5 wt% K₂NbF₇ has reduced the onset dehydrogenation temperature to 255 °C, which is 75 °C lower than the as-milled MgH₂. For the rehydrogenation kinetic, at 150 °C, MgH₂ + 5 wt% K₂NbF₇ absorbs 4.7 wt% of hydrogen in 30 min whereas the as-milled MgH₂ only absorbs 0.7 wt% of hydrogen under similar condition. For the dehydrogenation kinetic, at 320 °C, the MgH₂ + 5 wt% K₂NbF₇ is able to release 5.2 wt% of hydrogen in 5.6 min as compared to 0.3 wt% by the as-milled MgH₂ under similar condition. Comparatively, the E_a value of MgH₂ + 5 wt% K₂NbF₇ is 96.3 kJ/mol, which is 39 kJ/mol lower compared to the as-milled MgH₂. The MgF₂, the KH and the Nb that are found after the heating process are believed to be the active species that have improved the system properties. It is concluded that the K₂NbF₇ is a good catalyst to improve the hydrogen storage properties of MgH₂.     

The catalytic effect of an inert additive (SrTiO3) on the hydrogen storage properties of 4MgH2Na3AlH6

Investigations on the catalytic effects of a non-reactive and stable additive, SrTiO₃, on the hydrogen storage properties of the 4MgH₂Na₃AlH₆ destabilized system were carried out for the first time. The Na₃AlH₆ compound and the destabilized systems used in the investigations are prepared using ball milling method. The doped system, 4MgH₂Na₃AlH₆SrTiO₃, had an initial dehydrogenation temperature of 145 °C, which 25 °C lower as compared to the un-doped system. The isothermal absorption and desorption capacity at 320 °C has increased by 1.2 wt% and 1.6 wt% with the addition of SrTiO₃ as compared to the 4MgH₂Na₃AlH₆ destabilized system. The decomposition activation energy of the doped system is estimated to be 117.1 kJ/mol. As for the XRD analyses at different decomposition stages, SrTiO₃ is found to be stable and inert. In addition to SrTiO₃, similar phases are found in the doped and the un-doped system during the decomposition and dehydrogenation processes. Therefore, the catalytic effect of the SrTiO₃ is speculated owing to its ability to modify the physical structure of the 4MgH₂Na₃AlH₆ particles through pulverization effect.     

Processing of LiBH4 from its elements by ball milling method

Investigations of alternative renewable energy resources continue, with many studies concentrating on hydrogen storage. However, there are a few problems such as storage, transportation, delivery to the user and usage safely, to be addressed to facilitate commercialization and wide usage of the hydrogen. The absorbed form within the metal hydrides seems to be the best solution of this problem. Since Li is the lightest metal, it has the advantage as the stored amount of hydrogen mass ratio. LiBH₄ production process was investigated using elemental Li, B and H₂. Spex type ball milling with tungsten carbide, stainless steel and zirconia type vessels, was used to mix the different amount of Li and B under argon atmosphere. X-ray diffraction pattern demonstrated that the LiB was obtained. A system was designed to provide a hydrogen atmosphere of 60 bars to force hydrogen into the LiB structure. FTIR analysis strongly indicated the LiBH₄ compound when the mol ratio of B/Li is 0.214. Thermal decomposition and heat flow experiments performed simultaneously with DSC and TGA techniques also indicate hydrogen-rich structure showing greater mass loss. One gram of lithium borohydride sample released 1423 ml of hydrogen with Ni catalyst while NiO caused 1972.94 ml of hydrogen gas desorption, equaling to 90% of the theoretical yield of commercial LiBH₄. Indicating that, hydrogen of water can be obtained by either Ni or NiO catalysts.     

Development of a hydrogen catalytic heater for heating metal hydride hydrogen storage systems

This paper describes the design, fabrication and performance evaluation of a high efficiency, compact heater that uses the catalytic oxidation of hydrogen to provide heat to a hydrogen storage system. The heater was designed to transfer up to 30 kW of heat from the catalytic reaction to the hydrogen storage system via a recirculating heat transfer fluid.     

Improvement in the hydrogenation-dehydrogenation performance of a Mg–Al alloy by graphene supported Ni

Mg-based materials are very promising candidates for hydrogen storage. In this paper, the graphene supported Ni was introduced to the Mg₉₀Al₁₀ system by hydrogenation synthesis (HS) and mechanical milling (MM). The 80 wt%Ni@Gn catalyst was synthesized by a facile chemical reduction method. The microstructures of the catalyst and composite show that Ni nanoparticles are well supported on the surface of graphene and they are dispersed uniformly on the surface of MgH₂ particles. After heating to 450 °C and holding at 340 °C for 2 h subsequently under 2.0 MPa hydrogen pressure, all the samples are almost completely hydrogenated. According to the temperature programmed desorption test, the Mg₉₀Al₁₀-8(80 wt%Ni@Gn) composite could desorb 5.85 wt% H₂ which comes up to 96% of the theoretical hydrogen storage capacity. Moreover, it shows the optimal hydriding/dehydriding performance, absorbing 5.11 wt% hydrogen within 400 s at 523 K, and desorbing 5.81 wt% hydrogen within 1800 s at 573 K.     

Enhanced hydrogen desorption from the Co-catalyzed LiBH4–Mg(BH4)2 eutectic composite

Co-based catalyst can significantly improve the dehydrogenation kinetics of the eutectic composite of LiBH₄–Mg(BH₄)₂ (1/1 M ratio). The onset hydrogen desorption temperature of the composite is at about 155 °C, which is ca. 245, 110 or 27 °C lower than that of LiBH₄, Mg(BH₄)₂ or pristine LiBH₄–Mg(BH₄)₂, respectively. Upon holding the samples at 270 °C, the Co catalyzed composite can release hydrogen at a rate 1.6 times faster than that of the pristine one. Electron Paramagnetic Resonance (EPR) characterization evidenced that Co was in a reduced state of Co⁺ which may serve as the functional species in catalyzing the dehydrogenation of the composite.