Improvement of desorption performance of Mg(BH4)2 by two-dimensional Ti3C2 MXene addition

Magnesium borohydride (Mg(BH₄)₂) is an attractive materials for solid-state hydrogen storage due to its high hydrogen content (14.9 wt%). In the present work, the dehydrogenation performance of Mg(BH₄)₂ by adding different amounts (10, 20, 40, 60 wt%) of two-dimensional layered Ti₃C₂ MXene is studied. The Mg(BH₄)₂-40 wt% Ti₃C₂ composite releases 7.5 wt% hydrogen at 260 °C, whereas the pristine Mg(BH₄)₂ only releases 2.9 wt% hydrogen under identical conditions, and the onset desorption temperature decreases from 210 °C to a relative lower temperature of 82 °C. The special layered structure of Ti₃C₂ MXene and fluorine plays an important role in dehydrogenation process especially at temperatures below 200 °C. The main dehydrogenation reaction is divided into two steps, and activation energy of the Mg(BH₄)₂-40 wt% Ti₃C₂ composite is 151.3 kJ mol⁻¹ and 178.0 kJ mol⁻¹, respectively, which is much lower than that of pure Mg(BH₄)₂.     

Enhanced low temperature hydrogen desorption properties and mechanism of Mg(BH4)2 composited with 2D MXene

Mg(BH₄)₂ occupies a large hydrogen storage capacity of 14.7 wt%, and has been widely recognized to be one of the potential candidates for hydrogen storage. In this work, 2D MXene Ti₃C₂ was introduced into Mg(BH₄)₂ by a facile ball-milling method in order to improve its dehydrogenation properties. After milling with Ti₃C₂, Mg(BH₄)₂–Ti₃C₂ composites exhibit a novel “layered cake” structure. Mg(BH₄)₂ with greatly reduced particle sizes are found to disperse uniformly on Ti₃C₂ layered structure. The initial dehydrogenation temperature of Mg(BH₄)₂ has been decreased to 124.6 °C with Ti₃C₂ additive and the hydrogen liberation process can be fully accomplished below 400 °C. Besides, more than 10.8 wt% H₂ is able to be liberated from Mg(BH₄)₂–40Ti₃C₂ composite at 330 °C within 15 min, while pristine Mg(BH₄)₂ merely releases 5.3 wt% hydrogen. Moreover, the improved dehydrogenation kinetics can be retained during the subsequent second and third cycles. Detailed investigations reveal that not only Ti₃C₂ keeps Mg(BH₄)₂ particles from aggregation during de/rehydrogenation, but also the metallic Ti formed in-situ serves as the active sites to catalyze the decomposition of Mg(BH₄)₂ by destabilizing the B–H covalent bonds. This synergistic effect of size reduction and catalysis actually contributes to the greatly advanced hydrogen storage characteristics of Mg(BH₄)₂.     

Improving the hydrogen storage performance of lithium borohydride by Ti3C2 MXene

Two-dimensional layered material of Ti₃C₂ has been used to improve the hydrogen desorption properties of LiBH₄. The results of temperature-programmed dehydrogenation (TPD) and isotherm dehydrogenation (TD) demonstrate that adding the Ti₃C₂ contributes to the hydrogen storage performance of LiBH₄. The dehydrogenation temperature decreases and the dehydrogenation rate increases with increasing the adding amounts of Ti₃C₂. The onset dehydrogenation temperature of LiBH₄ + 40 wt% Ti₃C₂ composite is 120 °C and approximately 5.37 wt% hydrogen is liberated within 1 h at 350 °C. Furthermore, the activation energy of LiBH₄ + wt.% Ti₃C₂ is also greatly reduced to 70.3 kJ/mol, much lower than that of pure LiBH₄. The remarkable dehydrogenation property of the LiBH₄+ 40 wt% Ti₃C₂ may be due to the layered active Ti-containing Ti₃C₂ and the high surface area of MXene.     

Synthesis of low-cost biomass charcoal-based Ni nanocatalyst and evaluation of their kinetic enhancement of MgH2

In this study, a low-cost biomass charcoal (BC)-based nickel catalyst (Ni/BC) was introduced into the MgH₂ system by ball-milling. The study demonstrated that the Ni/BC catalyst significantly improved the hydrogen desorption and absorption kinetics of MgH₂. The MgH₂ + 10 wt% Ni/BC-3 composite starts to release hydrogen at 187.8 °C, which is 162.2 °C lower than the initial dehydrogenation temperature of pure MgH₂. Besides, 6.04 wt% dehydrogenation can be achieved within 3.5 min at 300 °C. After the dehydrogenation is completed, MgH₂ + 10 wt% Ni/BC-3 can start to absorb hydrogen even at 30 °C, which achieved the absorption of 5 wt% H₂ in 60 min under the condition of 3 MPa hydrogen pressure and 125 °C. The apparent activation energies of dehydrogenation and hydrogen absorption of MgH₂ + 10 wt% Ni/BC-3 composites were 82.49 kJ/mol and 23.87 kJ/mol lower than those of pure MgH₂, respectively, which indicated that the carbon layer wrapped around MgH₂ effectively improved the cycle stability of hydrogen storage materials. Moreover, MgH₂ + 10 wt% Ni/BC-3 can still maintain 99% hydrogen storage capacity after 20 cycles. XRD, EDS, SEM and TEM revealed that the Ni/BC catalyst evenly distributed around MgH₂ formed Mg₂Ni/Mg₂NiH₄ in situ, which act as a “hydrogen pump” to boost the diffusion of hydrogen along with the Mg/MgH₂ interface. Meanwhile, the carbon layer with fantastic conductivity enormously accelerated the electron transfer. Consequently, there is no denying that the synergistic effect extremely facilitated the hydrogen absorption and desorption kinetic performance of MgH₂.     

Enhancing hydrogen storage properties of the Mg/MgH2 system by the addition of bis(tricyclohexylphosphine)nickel(II) dichloride

This is a first report on the use of the bis(tricyclohexylphosphine)nickel (II) dichloride complex (abbreviated as NiPCy3) into MgH₂ based hydrogen storage systems. Different composites were prepared by planetary ball-milling by doping MgH₂ with (i) free tricyclohexylphosphine (PCy3) without or with nickel nanoparticles, (ii) different NiPCy3 contents (5–20 wt%) and (iii) nickel and iron nanoparticles with/without NiPCy3. The microstructural characterization of these composites before/after dehydrogenation was performed by TGA, XRD, NMR and SEM-EDX. Their hydrogen absorption/desorption kinetics were measured by TPD, DSC and PCT. All MgH₂ composites showed much better dehydrogenation properties than the pure ball-milled MgH₂. The hydrogen absorption/release kinetics of the Mg/MgH₂ system were significantly enhanced by doping with only 5 wt% of NiPCy3 (0.42 wt% Ni); the mixture desorbed H₂ starting at 220 °C and absorbed 6.2 wt% of H₂ in 5 min at 200 °C under 30 bars of hydrogen. This remarkable storage performance was not preserved upon cycling due to the complex decomposition during the dehydrogenation process. The hydrogen storage properties of NiPCy3-MgH₂ were improved and stabilized by the addition of Ni and Fe nanoparticles. The formed system released hydrogen at temperatures below 200 °C, absorbed 4 wt% of H₂ in less than 5 min at 100 °C, and presented good reversible hydriding/dehydriding cycles. A study of the different storage systems leads to the conclusion that the NiPCy3 complex acts by restricting the crystal size growth of Mg/MgH₂, catalyzing the H₂ release, and homogeneously dispersing nickel over the Mg/MgH₂ surface.     

Two-dimensional C@TiO2/Ti3C2 composite with superior catalytic performance for NaAlH4

Carbon coated titanium dioxide supported on two-dimensional titanium carbide (C@TiO₂/Ti₃C₂) is synthesized by simple annealing under a flowing acetylene (C₂H₂) atmosphere, and applied to improve the hydriding/dehydriding behavior of sodium alanate (NaAlH₄). The results indicate that as-prepared C@TiO₂/Ti₃C₂ composite exhibits excellent catalytic activity. The initial temperature for hydrogen desorption is reduced by 70 °C compared with the pristine sample. About 4.0 wt% hydrogen is released in 13 min at 140 °C. The apparent activation energies (E_a) of 10 wt% C@TiO₂/Ti₃C₂ catalyzing NaAlH₄ for the first two-steps dehydrogenation are 72.41 and 64.27 kJ mol⁻¹ respectively. The structural analyses reveal that C@TiO₂/Ti₃C₂ interacts with NaAlH₄ by using ball milling and decomposes to form Ti-species which works in combination with carbon to improve the dehydrogenation performance of NaAlH₄. This result provides an important progress in the hydrogen storage of NaAlH₄ catalyzed by MXene.     

The dehydrogenation kinetics and reversibility improvements of Mg(BH4)2 doped with Ti nano-particles under mild conditions

Magnesium borohydride, Mg(BH₄)₂, is ball-milled with Ti nano-particles. Such catalyzed Mg(BH₄)₂ releases more hydrogen than pristine Mg(BH₄)₂ does during isothermal dehydrogenation at 270, 280, and 290 °C. The catalyzed Mg(BH₄)₂ also exhibits better dehydrogenation kinetics than the pristine Mg(BH₄)₂. Based on kinetics model fitting, the activation energy (E_a) of the catalyzed Mg(BH₄)₂ is calculated to be lower than pristine Mg(BH₄)₂. During partial dehydrogenation, the catalyzed Mg(BH₄)₂ releases 4.23 wt % (wt%) H₂ for the second dehydrogenation at 270 °C, comparing to 4.05, and 3.75 wt% H₂ at 280, and 290 °C. The reversibility of 4.23 wt% capacity is also one of the highest for Mg(BH₄)₂ dehydrogenation under mild conditions such as 270 °C as reported. 4 cycles of Mg(BH₄)₂ dehydrogenation are conducted at 270 °C. The capacities degrade during 4 cycles and tend to be stable at about 3.0 wt% for the last two cycles. By analyzing the hydrogen de/absorption products of the catalyzed sample, Mg(BH₄)₂ is found to be regenerated after rehydrogenation according to Fourier Transform Infrared (FTIR) spectroscopy. Ti nano-particles can react with Mg(BH₄)₂ during ball-milling and de/rehydrogenation. The products include TiH_1.924, TiB, and TiB₂, which can improve the dehydrogenation properties of Mg(BH₄)₂ from a multiple aspect.     

Improved hydrogen storage properties of Mg/MgH2 thanks to the addition of nickel hydride complex precursors

In order to improve the hydrogenation/dehydrogenation properties of the Mg/MgH₂ system, the nickel hydride complex NiHCl(P(C₆H₁₁)₃)₂ has been added in different amounts to MgH₂ by planetary ball milling. The hydrogen storage properties of the formed composites were studied by different thermal analyses methods (temperature programmed desorption, calorimetric and pressure-composition-temperature analyses). The optimal amount of the nickel complex precursor was found to be of 20 wt%. It allows to homogeneously disperse 1.8 wt% of nickel active species at the surface of the Mg/MgH₂ particles. After the decomposition of the complex during MgH₂ dehydrogenation, the formed composite is stable upon cycling at low temperature. It can release hydrogen at 200 °C and absorb 6.3 wt% of H₂ at 100 °C in less than 1 h. The significantly enhanced H₂ storage properties are due to the impact of the highly dispersed nickel on both the kinetics and thermodynamics of the Mg/MgH₂ system. The hydrogenation and dehydrogenation enthalpies were found to be of −65 and 63 kJ/mol H₂ respectively (±75 kJ/mol H₂ for pure Mg/MgH₂) and the calculated apparent activation energies of the hydrogen uptake and release processes are of 22 and 127 kJ/mol H₂ respectively (88 and 176 kJ/mol H₂ for pure Mg/MgH₂). The change in the thermodynamics observed in the formed composite is likely to be due to the formation of a Mg_0.992Ni_0.008 phase during dehydrogenation/hydrogenation cycling. The impact of another hydride nickel precursor in which chloride has been replaced by a borohydride ligand, namely NiH(BH₄)(P(C₆H₁₁)₃)₂, is also reported.     

Dual-cation K2TaF7 catalyst improves high-capacity hydrogen storage behavior of MgH2

MgH₂ has been extensively regarded as a low-cost hydrogen storage material with high gravimetric hydrogen capacity of approximately 7.6 wt%. However, the hydrogen release and absorption kinetics in MgH₂ still needs further improving. For the first time, the catalytic impacts of a new dual-cation metal fluoride K₂TaF₇ upon the hydrogen storage characteristics of MgH₂ have been investigated in this work. With only 1 wt% K₂TaF₇ dopant, the initial dehydrogenation temperature of MgH₂ was lowered by about 130 °C, releasing more than 7.3 wt% hydrogen totally. The desorption activation energy of MgH₂ + 1 wt% K₂TaF₇ composite was decreased to 107.2 ± 1.2 kJ mol^?1. Besides, at 190 °C, the dehydrogenated MgH₂ + 1 wt% K₂TaF₇ sample could absorb 6.56 wt% H₂, while pristine MgH₂ re-absorbed only 3.45 wt% H₂. Further studies revealed that K₂TaF₇ could react with MgH₂ during dehydrogenation and produce symbiotic hydrides KMgH₃ and TaH_0.8, which could play the role of hydrogen pumps during hydrogen release and uptake. The cooperative catalysis between the hydrogen pump effect and the active interface in the multi-hydride area significantly enhanced the reversible hydrogen storage in the MgH₂+1 wt% K₂TaF₇ composite. This study provides new thinking for novel catalysts to elevate the hydrogen storage performance of MgH₂.     

CNTs decorated with CoFeB as a dopant to remarkably improve the dehydrogenation/rehydrogenation performance and cyclic stability of MgH2

The chain-like carbon nanotubes (CNTs) decorated with CoFeB (CoFeB/CNTs) prepared by oxidation-reduction method is introduced into MgH₂ to facilitate its hydrogen storage performance. The addition of CoFeB/CNTs enables MgH₂ to start desorbing hydrogen at only 177 °C. Whereas pure MgH₂ starts hydrogen desorption at 310 °C. The dehydrogenation apparent activation energy of MgH₂ in CoFeB/CNTs doped-MgH₂ composite is only 83.2 kJ/mol, and this is about 59.5 kJ/mol lower than that of pure MgH₂. In addition, the completely dehydrogenated MgH₂−10 wt% CoFeB/CNTs sample can start to absorb hydrogen at only 30 °C. At 150 °C and 5 MPa H₂, the MgH₂ in CoFeB/CNTs doped-MgH₂ composite can absorb 6.2 wt% H₂ in 10 min. The cycling kinetics can remain rather stable up to 20 cycles, and the hydrogen storage capacity retention rate is 98.5%. The in situ formation of Co₃MgC, Fe, CoFe and B caused by the introduction of CoFeB/CNTs can provide active and nucleation sites for the dehydrogenation/rehydrogenation reactions of MgH₂. Moreover, CNTs can provide hydrogen diffusion pathways while also enhancing the thermal conductivity of the sample. All of these can facilitate the dehydrogenation/rehydrogenation performance and cyclic stability of MgH₂.     

In-situ synthesis of amorphous Mg(BH4)2 and chloride composite modified by NbF5 for superior reversible hydrogen storage properties

The solvent-free amorphous Mg(BH₄)₂ composite was in-situ synthesized by ball milling LiBH₄ and MgCl₂. It is found that the onset dehydrogenation temperature of the as-synthesized composite is 126.9 °C, which is roughly 156 °C lower than that of pristine Mg(BH₄)₂. The activation energy of the amorphous Mg(BH₄)₂ and pristine Mg(BH₄)₂ for the first dehydrogenation step was calculated as 120.01 kJ/mol and 487.99 kJ/mol, respectively. Hence the kinetics improvement is certified by the lower E_a value of the dehydrogenation process. When adding NbF₅ into the composite, the catalyzed composite exhibits better hydrogen storage properties compared to pristine and amorphous Mg(BH₄)₂. The catalyzed composite starts to release hydrogen at proximately 120 °C with a total capacity of 10.04 wt%. The reversibility of the catalyzed composite is also improved. The capacity of the catalyzed composite at the second cycle is 5.5 wt%. For the third and fourth cycles the catalyzed composite can still liberate 4 wt% H₂. Besides, the onset hydrogen desorption temperature during four cycles are extremely lower than those of pristine and amorphous Mg(BH₄)₂. The peaks of the intermediate MgB₁₂H₁₂ is detected by FTIR as the regenerated hydrogenation product in the catalyzed composite. It can be speculated from the detailed analysis that there are mainly three reasons for the improved properties. Firstly, the additive NbF₅ is favorable to enhance the hydrogen storage properties by modifying the dehydrogenation path and producing MgF₂ and NbB₂ as new products. Secondly, the in-situ formation of amorphous Mg(BH₄)₂ is likely to improve the dehydrogenation properties of the samples due to its different reactivity comparing to crystal ones. Finally, LiCl can serve as buffer in the composite and thus improve the dehydrogenation properties.     

Improvement of dehydrogenation performance by adding CeO2 to α-AlH3

The α-AlH₃ + nCeO₂ (n = 0.5, 1 and 2, mol%) composite materials were prepared by ball milling, and the phase composition and dehydrogenation performance of the composites were investigated. The results revealed that α-AlH₃ doped with CeO₂ could effectively decrease the dehydrogenation temperature. The dehydrogenation onset temperature of α-AlH₃ + 2 mol% CeO₂ dropped to 106 °C, which was significantly reduced by 24.3% compared with pure α-AlH₃. At 100 °C, the α-AlH₃ + 2 mol% CeO₂ composite released 3.8 wt% hydrogen within 100 min, while the pure α-AlH₃ released only 0.12 wt% hydrogen in the same duration. Kissinger analysis indicated that apparent activation energy for hydrogen desorption of α-AlH₃ was significantly decreased with CeO₂ doping. The improvement of dehydrogenation performance was due to the finely dispersed CeH_2+x, which was formed by the reaction between CeO₂ and α-AlH₃.     

Enhanced hydrogen-storage properties of MgH2 by Fe–Ni catalyst modified three-dimensional graphene

High dehydrogenation temperature and slow dehydrogenation kinetics impede the practical application of magnesium hydride (MgH₂) serving as a potential hydrogen storage medium. In this paper, Fe–Ni catalyst modified three-dimensional graphene was added to MgH₂ by ball milling to optimize the hydrogen storage performance, the impacts and mechanisms of which are systematically investigated based on the thermodynamic and kinetic analysis. The MgH₂+10 wt%Fe–Ni@3DG composite system can absorb 6.35 wt% within 100 s (300 °C, 50 atm H₂ pressure) and release 5.13 wt% within 500 s (300 °C, 0.5 atm H₂ pressure). In addition, it can absorb 6.5 wt% and release 5.7 wt% within 10 min during 7 cycles, exhibiting excellent cycle stability without degradation. The absorption-desorption mechanism of MgH₂ can be changed by the synergistic effects of the two catalyst materials, which significantly promotes the improvement of kinetic performance of dehydrogenation process and reduces the hydrogen desorption temperature.     

Effect of Ti-based nanosized additives on the hydrogen storage properties of MgH2

MgH₂-based nanocomposites were synthesized by high-energy reactive ball milling (RBM) of Mg powder with 0.5–5 mol% of various catalytic additives (nano-Ti, nano-TiO₂, and Ti₄Fe₂O_x suboxide powders) in hydrogen. The additives were shown to facilitate hydrogenation of magnesium during RBM and substantially improve its hydrogen absorption-desorption kinetics. X-ray diffraction analysis showed the formation of nanocrystalline MgH₂ and hydrogenation of nano-Ti and Ti₄Fe₂O_x. The possible reduction of TiO₂ during RBM in hydrogen was not observed, which is in agreement with lower hydrogenation capacity of the corresponding composite, 5.7 wt% for Mg + 5 mol% nano-TiO₂ compared to 6.5 wt% for Mg + 5 mol% nano-Ti. Hydrogen desorption from the as-prepared composites was studied by Thermal Desorption Spectroscopy (TDS) in vacuum. A significant lowering of the hydrogen desorption temperature of MgH₂ by 30–90 °C in the presence of the additives is associated with lowering activation energy from 146 kJ/mol for nanosized MgH₂ down to 74 and 67 kJ/mol for MgH₂ modified with nano-TiO₂ and Ti₄Fe₂O_0.3 additives, respectively. After hydrogen desorption at 300–350 °C, these materials are able to absorb hydrogen even at room temperature. It is shown that nano-structuring and addition of Ti-based catalysts do not decrease thermodynamic stability of MgH₂. The thermodynamic parameters, obtained from hydrogen desorption isotherms for the Mg–Ti₄Fe₂O_0.3 nanocomposite, ΔH_des = 76 kJ/mol H₂ and ΔS_des = 138 J/K·mol H₂, correspond to the reported literature values for pure polycrystalline MgH₂. Hydrogen absorption-desorption characteristics of the composites with nano-Ti remain stable during at least 25 cycles, while a gradual decay of the reversible hydrogen capacity occurred in the case of TiO₂ and Ti₄Fe₂O_x additives. Cycling stability of Mg/Ti₄Fe₂O_x was substantially improved by introduction of 3 wt% graphite into the composite.     

Graphene-anchored Ni6MnO8 nanoparticles with steady catalytic action to accelerate the hydrogen storage kinetics of MgH2

Transition metal-based oxides have been proven to have a substantial catalytic influence on boosting the hydrogen sorption performance of MgH₂. Herein, the catalytic action of Ni₆MnO₈@rGO nanocomposite in accelerating the hydrogen sorption properties of MgH₂ was investigated. The MgH₂ + 5 wt% Ni₆MnO₈@rGO composites began delivering H₂ at 218 °C, with about 2.7 wt%, 5.4 wt%, and 6.6 wt% H₂ released within 10 min at 265 °C, 275 °C, and 300 °C, respectively. For isothermal hydrogenation at 75 °C and 100 °C, the dehydrogenated MgH₂ + 5 wt% Ni₆MnO₈@rGO sample could absorb 1.0 wt% and 3.3 wt% H₂ in 30 min, respectively. Moreover, as compared to addition-free MgH₂, the de/rehydrogenation activation energies for doped MgH₂ composites were lowered to 115 ± 11 kJ/mol and 38 ± 7 kJ/mol, and remarkable cyclic stability was reported after 20 cycles. Microstructure analysis revealed that the in-situ formed Mg₂Ni/Mg₂NiH₄, Mn, MnO₂, and reduced graphene oxide synergically enhanced the hydrogen de/absorption properties of the Mg/MgH₂ system.     

Air exposure improving hydrogen desorption behavior of Mg–Ni-based hydrides

It is well established that H₂O and O₂ have an inauspicious influence on hydrogen reactivity of hydrogen storage alloys. In this work, an unexpected improvement of the desorption behavior was discovered by just exposing the magnesium rich Mg–Ni hydrides into the air for a certain period. Upon an exposure duration of 4 months, the dehydrogenation peak and onset temperature were sharply lowered by 150 °C and 130 °C. Furthermore, the air-exposed sample could quickly absorb 3.08 wt% H₂ and desorb 2.81 wt% H₂ within 400 s at 300 °C. Besides the refinement of the powders due to the spontaneous hydrolysis reaction, the in-situ formed magnesium hydroxide layer and Ni are thought to be responsible for the remarkable improvement. This work gives interesting insights that the self-generating surface passivation is not necessarily harmful in the solid-state hydrogen storage area, especially for the cases where active sites of catalysis are present.     

Achievement of excellent hydrogen sorption through swift hydrogen transport in 1:2 Mg(NH2)2–LiH catalyzed by Li4BH4(NH2)3and carbon nanostructures

The present studies deal with the catalytic character of carbon nanostructure (Graphene (Gr) and single-wall carbon nanotubes (SWNTs), and their composite versions) on the hydrogen sorption behavior of 1:2 Mg(NH₂)₂–LiH/Li₄BH₄(NH₂)₃. The inclusion of an optimal quantity of 2 wt% SWNTs in Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ resulted in superior hydrogen sorption over 2 wt% Gr and 2 wt% of (Gr and SWNT) composite. The onset desorption temperature for SWNTs catalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ is 108 °C which is 32 °C, 44 °C lower compared to Gr catalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ and uncatalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ respectively. The de/re-hydrogenation kinetics of the SWNT catalyzed sample has been found to be 4.02 wt% and 4.63 wt% within 15min at 170 °C and 7 MPa H₂ pressure, correspondingly. The activation energy for SWNT catalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ has been found to be 69.75 kJ/mol. The SWNT catalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ shows good cyclic stability (almost no degradation) up to 10 cycles. The better hydrogen sorption for SWNTs is attributed to the ballistic transport of hydrogen atoms within and across the amide/hydride matrix. In contrast, Gr sheets agglomerate, which adversely affects hydrogen sorption from Gr and Gr+SWNT composites. A hydrogen sorption mechanism has been proposed based on structural, microstructural, Fourier-transform infrared spectroscopy, and Raman characterization results.     

Remarkably improved hydrogen storage properties of carbon layers covered nanocrystalline Mg with certain air stability

Different nanocrystalline magnesium with carbon layers were successfully synthesized via a facile wet-chemical ball milling method for 20, 30 and 40 h, respectively. Based on Scherrer formula and X-ray diffraction results, the average crystallite size of all the three samples was below 30 nm. TEM observations showed that the hydrogenated Mg particles were covered with carbon layers. Moreover, the 40 h ball milled Mg sample showed outstanding hydrogen storage performance especially in the aspect of hydrogen absorption. The as-prepared sample started to take up hydrogen at nearly room temperature and eventually absorbed 6.8 wt% hydrogen at 200 °C. The apparent activation energy (E_a) of hydrogen absorption for the sample was decreased to 26.7 kJ/mol, much lower than that of other reported systems. For the dehydrogenation experiments, the hydrogenated sample could start to release hydrogen at about 275 °C and 6.5 wt% hydrogen was desorbed in 20 min at 325 °C. Interestingly, the prepared samples showed noteworthy air stability. Been placed in the air for 60 min, the dehydrogenation kinetics and hydrogen capacity of the three samples were basically unchanged, making it possible to be used in future commercial applications.     

Synthetical catalysis of nickel and graphene on enhanced hydrogen storage properties of magnesium

Reduced graphene-oxide-supported nickel (Ni@rGO) nanocomposite catalysts were synthesized, and incorporated into magnesium (Mg) hydrogen storage materials with the aim of improving the hydrogen storage properties of these materials. The experimental results revealed that the catalytic effect of the Ni@rGO nanocomposite on Mg was more effective than that of single nickel (Ni) nanoparticles or graphene. When heated at 100 °C, the Mg–Ni and Mg–Ni@rGO composites absorbed 4.70 wt% and 5.48 wt% of H₂, respectively, whereas the pure Mg and Mg@rGO composite absorbed almost no hydrogen. The addition of the Ni@rGO composite as a catalyst yielded significant improvement in the hydrogen storage property of the Mg hydrogen storage materials. The apparent activation energy of the pure Mg sample (i.e., 163.9 kJ mol⁻¹) decreased to 139.7 kJ mol⁻¹ and 123.4 kJ mol⁻¹, respectively, when the sample was modified with single rGO or Ni nanoparticles. Under the catalytic action of the Ni@rGO nanocomposites, the value decreased further to 103.5 kJ mol⁻¹. The excellent hydrogen storage properties of the Mg–Ni@rGO composite were attributed to the catalytic effects of the highly surface-active Ni nanoparticles and the unique structure of the composite nanosheets.     

New mechanism and improved kinetics of hydrogen absorption and desorption of Mg(In) solid solution alloy milling with CeF3

This paper presents improving the hydrogen absorption and desorption of Mg(In) solid solution alloy through doped with CeF₃. A nanocomposite of Mg_0.95In_0.05-5 wt% CeF₃ was prepared by mechanical ball milling. The microstructures were systematically investigated by X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy. And the hydrogen storage properties were evaluated by isothermal hydrogen absorption and desorption, and pressure-composition-isothermal measurements in a temperature range of 230 °C–320 °C. The mechanism of hydrogen absorption and desorption of Mg_0.95In_0.05 solid solution is changed by the addition of CeF₃. Mg_0.95In_0.05-5 wt% CeF₃ nanocomposite transforms to MgH₂, MgF₂ and intermetallic compounds of MgIn and CeIn₃ by hydrogenation. Upon dehydrogenation, MgH₂ reacts with the intermetallic compounds of MgIn and CeIn₃ forming a pseudo-ternary Mg(In, Ce) solid solution, which is a fully reversible reaction with a reversible hydrogen capacity～4.0 wt%. The symbiotic nanostructured CeIn₃ impedes the agglomeration of MgIn compound, thus improving the dispersibility of element In, and finally improving the reversibility of hydrogen absorption and desorption of Mg(In) solution alloy. For Mg_0.95In_0.05-5 wt% CeF₃ nanocomposite, the dehydriding enthalpy is reduced to about 66.1 ± 3.2 kJ⋅mol⁻¹⋅H₂, and the apparent activation energy of dehydrogenation is significantly lowered to 71.9 ± 10.0 kJ⋅mol⁻¹⋅H₂, a reduction of ～73 kJ⋅mol⁻¹⋅H₂ relative to that for Mg_0.95In_0.05 solid solution. As a result, Mg_0.95In_0.05-5 wt% CeF₃ nanocomposite can release ～57% H₂ in 10 min at 260 °C. The improvements of hydrogen absorption and desorption properties are mainly attributed to the reversible phase transition of Mg(In, Ce) solid solution combing with the multiphase nanostructure.