Facile preparation of β-/γ-MgH2 nanocomposites under mild conditions and pathways to rapid dehydrogenation

A magnesium hydride composite with enhanced hydrogen desorption kinetics can be synthesized via a simple wet chemical route by ball milling MgH2 with LiCl as an additive at room temperature followed by tetrahydrofuran (THF) treatment under an Ar atmosphere. The as-synthesized composite comprises ca. 18 mass% orthorhombic γ-MgH2 and 80 mass% tetragonal β-MgH2 as submicron-sized particles. The β-/γ-MgH2 nanocomposite exhibits a dehydrogenation capacity of 6.6 wt.% and starts to release hydrogen at ~260 °C; ca. 140 °C lower than that of commercial MgH2. The apparent activation energy for dehydrogenation is 115±3 kJ mol-1, which is ca. 46 % lower than that of commercial MgH2. Analysis suggests that the meta-stable γ-MgH2 component either directly dehydrogenates exothermically or first transforms into stable β-MgH2 very close to the dehydrogenation onset. The improved hydrogen release performance can be attributed both to the existence of the MgH2 nanostructure and to the presence of γ-MgH2

Facile synthesis of Co/Pd supported by few-walled carbon nanotubes as an efficient bidirectional catalyst for improving the low temperature hydrogen storage properties of magnesium hydride

Catalytic doping is important for enhancing the hydrogen storage performance of metal hydrides, but it is challenging to develop a single catalyst to enhance both hydrogen desorption and absorption to a certain degree. Herein, a bidirectional Co/Pd catalyst, homogeneously loaded on bamboo-shaped carbon nanotubes (Co/Pd@B-CNTs), showed superior catalytic effects, improving both the hydrogen desorption and absorption properties of MgH 2 at relatively low temperatures. The MgH 2 -Co/Pd@B-CNTs composite starts to release hydrogen at 198.9 °C, which is 132.4 °C lower than as-milled MgH 2 . The hydrogen desorption activation energy for MgH 2 is reduced from 178.00 to 76.66 kJ mol -1 by the catalytic effects of Co/Pd@B-CNTs. The MgH 2 -Co/Pd@B-CNTs composite shows dramatically improved absorption kinetics; it rapidly uptakes 6.68 wt% H 2 within 10 s at 250 °C, and quickly absorbs 1.91 wt% H 2 within 100 s, even at a temperature as low as 50 °C. More importantly, a special mechanism for the bidirectional catalyst Co/Pd is proposed for the first time and discussed in detail. During the hydrogenation process, elemental Pd plays a dominant role in accelerating the preferential diffusion of hydrogen atoms at the Pd/Mg interface, while during dehydrogenation, phase transformation between Mg 2 Co and Mg 2 CoH 5 as well as a Mg-Pd alloy becomes the crucial factor, facilitating the release of hydrogen atoms by decreasing the diffusion barrier. Moreover, novel structures of bamboo-shaped carbon nanotubes with a large diameter (\u3e100 nm) and high specific surface area (146.8 m 2 g -1 ) allow the homogenous dispersion of Co/Pd NPs and enhance the direct contact with MgH 2 particles

Facile preparation of β-/γ-MgH₂ nanocomposites under mild conditions and pathways to rapid dehydrogenation

A magnesium hydride composite with enhanced hydrogen desorption kinetics can be synthesized via a simple wet chemical route by ball milling MgH2 with LiCl as an additive at room temperature followed by tetrahydrofuran (THF) treatment under an Ar atmosphere. The as-synthesized composite comprises ca. 18 mass% orthorhombic γ-MgH2 and 80 mass% tetragonal β-MgH2 as submicron-sized particles. The β-/γ-MgH2 nanocomposite exhibits a dehydrogenation capacity of 6.6 wt.% and starts to release hydrogen at ~260 °C; ca. 140 °C lower than that of commercial MgH2. The apparent activation energy for dehydrogenation is 115±3 kJ mol-1, which is ca. 46 % lower than that of commercial MgH2. Analysis suggests that the meta-stable γ-MgH2 component either directly dehydrogenates exothermically or first transforms into stable β-MgH2 very close to the dehydrogenation onset. The improved hydrogen release performance can be attributed both to the existence of the MgH2 nanostructure and to the presence of γ-MgH2

Insights into 2D graphene-like TiO2 (B) nanosheets as highly efficient catalyst for improved low-temperature hydrogen storage properties of MgH2

© 2020 Elsevier Ltd The development of low temperature hydrogen storage system is crucial for the wide application of renewable energy. A key obstacle for this system is the lack of efficient catalysts, in which two-dimensional nanosheets have attracted considerable attention because of their unique properties. Herein, we synthesized graphene-like TiO2 (B), and applied them as a highly efficient catalyst to dramatically enhance the low temperature hydrogen storage performances of MgH2. The MgH2s–TiO2 (B) quickly absorbs 5.32 and 5.50 wt% H2 at low temperatures of 50 and 60 °C, respectively, which is superior to most of the previously catalyzed MgH2 composites. Moreover, MgH2–TiO2 (B) begins to desorb hydrogen at ~200 °C and release ~6.29 wt% H2 below 288 °C. Careful microstructure analyses reveal that TiO2 (B) nanosheets are reduced to metallic Ti nanoparticles and wrinkled Ti2O3 upon ball milling and (de)hydriding processes, which creates lots of boundary interfaces between MgH2 and Ti-based catalysts, thus facilitating the hydrogen diffusion. Besides, the in-situ formed Ti has the intermediate electronegativity between Mg and H, which could weaken the Mg–H bonds and decrease the dehydrogenation kinetic barriers. While in rehydrogenation, Ti nanoparticles act as effective heterogeneous nucleation agents of MgH2 nuclei, further promoting hydrogen absorption properties of MgH2–TiO2 (B). The present investigation provides clear evidence for remarkable catalytic effect of graphene-like TiO2 (B) on hydrogen absorption/desorption properties of MgH2, which is not only important for deeply understanding the mechanism, but also sheds lights for catalysis design towards practical low temperature hydrogen storage

Reversible hydrogen storage behaviors and microstructure of TiC-doped sodium aluminum hydride

http://deepblue.lib.umich.edu/bitstream/2027.42/174876/2/s10853-009-3726-y.pdfPublished versionDescription of s10853-009-3726-y.pdf : Published versio

Significantly enhanced hydrogen desorption properties of Mg(AlH4)2 nanoparticles synthesized using solvent free strategy

Mg(AlH4)2 nanoparticles with a particle size less than 10 nm have been successfully synthesized by mechanochemical method using LiAlH4 and MgCl2 as raw materials together with LiCl buffering additive. In comparison to Mg(AlH4)2 microparticles, Mg(AlH4)2 nanoparticles exhibit a faster hydrogen desorption kinetics and lower desorption temperature. The hydrogen desorption temperatures of the first and second dehydrogenation steps are 80 and 220 °C for the Mg(AlH4)2 nanoparticles, which are about 65 and 60 °C, respectively, lower than those of Mg(AlH4)2 microparticles. The decomposition activation energy is reduced from 135 kJ/mol for Mg(AlH4)2 microparticles to 105.3 kJ/mol for Mg(AlH4)2 nanoparticles. It is proposed that the shortened diffusion distance and enhanced diffusivity of Mg(AlH4)2/MgH2 nanoparticles provide an energy destabilization for lowering the dehydrogenation temperature, and thus being the key factor for promoting the hydrogen desorption kinetics. More importantly, it is demonstrated that the dehydrided nano MgH2 hydride with a particle size below 10 nm can be formed after rehydrogenation process, resulting in the good cycling hydrogen desorption performance of nano MgH2

Superior Reversible Hydrogen Storage Properties and Mechanism of LiBH₄–MgH₂–Al Doped with NbF₅ Additive

LiBH₄ is one of the most potential candidates for hydrogen storage materials among several sorts of complex borohydrides. Utilizing reactive hydride composites on LiBH₄ could destabilize the thermodynamics and improve dehydrogenation behaviors, such as the excellent reversibility of LiBH₄–MgH₂ and the fast dehydrogenation of LiBH₄–Al. The strategy of combining both outstanding effects of MgH₂ and Al to form LiBH₄–MgH₂–Al system has been proposed. However, reduction of hydrogen capacity during cycles has not been solved for the LiBH₄–MgH₂–Al system, which is considered as the principal problem. In this work, we investigated the reversible hydrogen storage performance and reaction mechanism of LiBH₄–MgH₂–Al doped with/without NbF₅ additive. It can be found that the dehydrogenation of 4LiBH₄–MgH₂–Al can release about 9.0 wt % H₂ quickly without incubation period, compared with 2LiBH₄–MgH₂. Moreover, it is the first time to achieve completely reversible hydrogen desorption property of LiBH₄–MgH₂–Al by doping with NbF₅ and dehydrogenating under hydrogen back pressure in experiment. Microstructure analysis shows that the formation of Mg–Al alloys could result in the formation of Li₂B₁₂H₁₂ and subsequently lead to the capacity degradation. With the additive NbF₅, it shows a totally different pathway and a significant inhibition effect on the alloying between Mg and Al, leading to an improved de/rehydrogenation behavior without the byproduct Li₂B₁₂H₁₂. Meanwhile, NbF₅ could be hydrogenated into NbH₂ and react with element B to form NbB₂, promoting the reaction between Mg/Al metals and B element to form MgAlB₄. On the other hand, those niobium compounds could facilitate the products MgAlB₄ and LiH to be fully rehydrogenated into LiBH₄, MgH₂, and Al, which contributes to the complete reversibility of LiBH₄–MgH₂–Al. A better understanding of the capacity fade mechanism of LiBH₄–MgH₂–Al system and the effects of additives might promote further development of high-capacity hydrogen storage materials