Metal organic framework supported niobium pentoxide nanoparticles with exceptional catalytic effect on hydrogen storage behavior of MgH_(2)

Nb_(2)O_(5)nanoparticles with an average particle size of 10 nm supported on a rhombic dodecahedral metal organic framework(MOF)were successfully synthesized by a facile one-pot hydrothermal reaction and subsequent calcination process.Experimental results demonstrated that the prepared catalyst drastically improved the hydrogen storage behavior of MgH_(2).7 wt%Nb_(2)O_(5)@MOF doped MgH_(2)started to desorb hydrogen at 181.9℃and 6.2 wt%hydrogen could be released within 2.6 min and 6.3 min at 275℃and 250℃,respectively.The fully dehydrogenated composite also displayed excellent hydrogenation by decreasing the onset absorption temperature to 25℃and taking up4.9 wt%and 6.5 wt%hydrogen within 6 min at 1750C and 1500C,respectively.Moreover,the corresponding activation energy was calculated to be 75.57±4.16 kJ mol^(-1)for desorption reaction and 51.38±1.09 kJ mol^(-1)for absorption reaction.After 20 cycles,0.5 wt%hydrogen capacity was lost for the MgH_(2)+7 wt%Nb_(2)O_(5)@MOF composite,much lower than 1.5 wt%of the MgH_(2)+7 wt%Nb_(2)O_(5)composite.However,the addition of Nb_(2)O_(5)@MOF had limited effect on reducing the dehydrogenation enthalpy of MgH_(2).Microstructure analysis revealed that Nb_(2)O_(5)particles were uniformly distributed on surface of the MgH_(2)matrix and synergistically improved the hydrogen storage property of MgH_(2)with MOF.

Bullet-like vanadium-based MOFs as a highly active catalyst for promoting the hydrogen storage property in MgH_(2)

The practical application of magnesium hydride(MgH_(2))was seriously limited by its high desorption temperature and slow desorp-tion kinetics.In this study,a bullet-like catalyst based on vanadium related MOFs(MOFs-V)was successfully synthesized and doped with MgH_(2) by ball milling to improve its hydrogen storage performance.Microstructure analysis demonstrated that the as-synthesized MOFs was consisted of V_(2)O_(3) with a bullet-like structure.After adding 7wt%MOFs-V,the initial desorption temperature of MgH_(2) was reduced from 340.0 to 190.6℃.Besides,the MgH_(2)+7wt%MOFs-V composite released 6.4wt%H_(2) within 5 min at 300℃.Hydrogen uptake was started at 60℃under 3200 kPa hydrogen pressure for the 7wt%MOFs-V containing sample.The desorption and absorption apparent activity energies of the MgH_(2)+7wt%MOFs-V composite were calculated to be(98.4±2.9)and(30.3±2.1)kJ·mol^(-1),much lower than(157.5±3.3)and(78.2±3.4)kJ·mol^(−1) for the as-prepared MgH_(2).The MgH_(2)+7wt%MOFs-V composite exhibited superior cyclic property.During the 20 cycles isothermal dehydrogenation and hydrogenation experiments,the hydrogen storage capacity stayed almost unchanged.X-ray diffraction(XRD)and X-ray photoelectron spectrometer(XPS)measurements confirmed the presence of metallic vanadium in the MgH_(2)+7wt%MOFs-V composite,which served as catalytic unit to markedly improve the hydrogen storage properties of Mg/MgH_(2) system.

Constructing graphene nanosheet-supported FeOOH nanodots for hydrogen storage of MgH2

Novel graphene-supported FeOOH nanodots(FeOOH NDs@G)were successfully prepared by a facile hydrothermal method and doped into MgH_(2)through mechanical ball-milling.MgH_(2)with 10wt%FeOOH NDs@G began to release hydrogen at 229.8℃,which is106.8℃ lower than that of pure MgH_(2).The MgH_(2)-10wt%FeOOH NDs@G composite could reversibly absorb 6.0wt%hydrogen at 200℃ under a 3.2 MPa hydrogen pressure within 60 min.With the addition of FeOOH NDs@G,the dehydrogenation and hydrogenation activation energy of MgH_(2)was decreased to 125.03 and 58.20 kJ·mol^(-1)(156.05 and 82.80 kJ·mol^(-1)for pure MgH_(2)),respectively.Furthermore,the hydrogen capacity of the FeOOH NDs@G composite retained 98.5%of the initial capacity after 20 cycles,showing good cyclic stability.The catalytic action of FeOOH NDs@G towards MgH_(2)could be attributed to the synergistic effect between graphene nanosheets and in-situ formed Fe,which prevented the aggregation of Mg/MgH2 particles and accelerated the hydrogen diffusion during cycling,thus enabling the Mg H_(2)-10wt%FeOOH NDs@G composite to exhibit excellent hydrogen storage performance.

Significantly improved hydrogen storage behaviors in MgH_(2) with Nb nanocatalyst

The study explores the excellent modification effect of Nb nanocatalyst prepared via surfactant assisted ball milling technique(SABM)on the hydrogen storage properties of MgH_(2).Optimal catalyst doping concentration was determined by comparing onset decomposition temperature,released hydrogen capacity,and reaction rate for different MgH_(2)-ywt%Nb(y=0,3,5,7,9)composites.The MgH_(2)-5wt%Nb composite started releasing hydrogen at 186.7℃ and a total of 7.0wt%hydrogen was released in the dehydrogenation process.In addition,5wt%Nb doped MgH_(2) also managed to release 4.2wt%H_(2) within 14 min at 250℃ and had the ability to absorb 4.0wt%hydrogen in 30 min at 100℃.Cycling tests revealed that MgH_(2)-5wt%Nb could retain 6.3wt%H_(2) storage capacity(89.2%)after 20 cycles.Dehydrogenation and hydrogenation activation energy values were decreased from 140.51±4.74 and 70.67±2.07 kJ·mol^(−1) to 90.04±2.83 and 53.46±3.33 kJ·mol^(−1) after doping MgH_(2) with Nb,respectively.Microstructure analysis proved that homogeneously distributed NbH acted as active catalytic unit for improving the hydrogen storage performance of MgH_(2).These results indicate SABM can be considered as an option to develop other nanocatalysts for energy related areas.

Boosting the hydrogen storage performance of magnesium hydride with metal organic framework-derived Cobalt@Nickel oxide bimetallic catalyst

In this study,a MOF-derived bimetallic Co@NiO catalyst was synthesized and doped into MgH_(2)to improve the hydrogen desorption and resorption kinetics.The Co@NiO catalyst decreased the onset dehydrogenation temperature of MgH_(2)by 160℃,compared with un-doped MgH_(2).The MgH^(2+)9%(mass)Co@NiO composite released 6.6%(mass)hydrogen in 350 s at 315℃and uptook 5.4%(mass)hydrogen in500 s at 165℃,showing greatly accelerated de/rehydrogenation rates.Besides,the desorption activation energy of MgH^(2+)9%(mass)Co@NiO was decreased to(93.8±8.4)kJ·mol^(-1).Noteworthy,symbiotic Mg_(2)NiH_(4)/Mg_(2)CoH_(5)clusters were in-situ formed from bimetallic precursors and inlaid on MgH_(2)surface,which are considered as"multi-step hydrogen pumps",and provides surface pathways for hydrogen absorption.Meanwhile,the introduced Mg_(2)NiH_(4)/Mg_(2)CoH_(5)interfaces could provide numerous low energy barrier H diffusion channels,therefore accelerating the hydrogen release and uptake.This research proposes new insights to design high-efficiency bimetallic catalyst for MgH_(2)hydrogen storage.

Extreme high reversible capacity with over 8.0 wt% and excellent hydrogen storage properties of MgH2 combined with LiBH4 and Li3AlH6

Magnesium hydride has attracted great attention because of its high theoretical capacity and outstanding reversibility, nevertheless, its practical applications have been restricted by the disadvantages of the sluggish kinetics and high thermodynamic stability. In this work, an unexpected high reversible hydrogen capacity over 8.0 wt% has been achieved from MgH2 metal hydride composited with small amounts of LiBH4 and Li3AlH6 complex hydrides, which begins to release hydrogen at 276 ℃ and then completely dehydrogenates at 360 ℃. The dehydrogenated MgH2+LiBH4/Li3AlH6 composite can fully reabsorb hydrogen below 300 ℃ with an excellent cycling stability. The composite exhibits a significant reduction of dehydrogenation activation energy from 279.7 kJ/mol(primitive MgH2) to 139.3 kJ/mol(MgH2+LiBH4/Li3AlH6),as well as a remarkable reduction of dehydrogenation enthalpy change from 75.1 k J/mol H2(primitive MgH2) to 62.8 kJ/mol H2(MgH2+LiBH4/Li3AlH6). The additives of LiBH4 and Li3AlH6 not only enhance the cycling hydrogen capacity, but also simultaneously improve the reversible de/rehydrogenation kinetics, as well as the dehydrogenation thermodynamics. This notable improvement on the hydrogen absorption/desorption behaviors of the MgH2+LiBH4/Li3AlH6 composite could be attributed to the dehydrogenated products including Li3Mg7, Mg17Al12 and MgAlB4, which play a key role on reducing the dehydrogenation activation energy and increasing diffusion rate of hydrogen. Meanwhile, the LiBH4 and Li3AlH6 effectively destabilize MgH2 with a remarkable reduction on dehydrogenation enthalpy change in terms of thermodynamics. In particular, the Li3Mg7, Mg17Al12 and MgAlB4 phases can reversibly transform into MgH2, Li3AlH6 and LiBH4 after rehydrogenation, which contribute to maintain a high cycling capacity.This constructing strategy can further promote the development of high reversible capacity Mg-based materials with suitable de/rehydrogenation properties.

The effect of different Co phase structure(FCC/HCP)on the catalytic action towards the hydrogen storage performance of MgH_(2)

High hydrogen desorption temperature and sluggish reaction kinetics are the major limitations for the practical application of MgH_(2).In this study,Co particles with a face centered cubic(FCC)structure and a hexagonal close packed(HCP)structure were prepared facilely and proved to be good catalysts for magnesium hydride.Co particles with FCC structure presented better catalytic effect on MgH_(2)than that with HCP structure.Both 7%(mass)Co FCC and HCP particle modified MgH_(2)decreased the initial dehydrogenation temperature from 301.3℃ to approximately 195.0℃,but 7%(mass)Co with FCC structure modified MgH_(2)has a faster desorption rate,and around 6.5%(mass)H_(2)was desorbed in 10 min at325℃.Hydrogen uptake was detected at 70℃ under 3.25 MPa hydrogen pressure and 6.0%(mass)H_(2)was recharged in 40 min at 150℃.The hydrogen desorption and absorption activation energy for 7%(mass)FCC Co modified MgH_(2)was significantly decreased to(76.6±8.3)kJ·mol^(-1) and(68.3±6.0)kJ·mol^(-1),respectively.Thermodynamic property was also studied,the plateau pressures of MgH_(2)+7%(mass)FCC Co were determined to be 0.14,0.28,0.53 and 0.98 MPa for 300℃,325℃,350℃ and375℃.The decomposition enthalpy of hydrogen(ΔH)for MgH_(2)+7%(mass)FCC Co was(80.6±0.1)kJ·mol^(-1),5.8 kJ·mol^(-1)lower than that of as-prepared MgH_(2).Moreover,cycling performance for the first20 cycles revealed that the reaction kinetics and capacity of MgH_(2)-FCC Co composite remained almost unchanged.The result of density functional theory calculation demonstrated that cobalt could extract the Mg AH bond and reduced the decompose energy of magnesium hydride.Our paper can be presented as a reference for searching highly effective catalysts for hydrogen storage and other energy-related research fields.

Enhanced hydrogen storage properties of Mg by the synergistic effect of grain refinement and NiTiO_(3)nanoparticles

As a promising hydrogen storage material,the practical application of magnesium is obstructed by the stable thermodynamics and sluggish kinetics.In this paper,three kinds of NiTiO3catalysts with different mole ratio of Ni to Ti were successfully synthesized and doped into nanocrystalline Mg to improve its hydrogen storage properties.Experimental results indicated that all the Mg-NiTiO3composites showed prominent hydrogen storage performance.Especially,the Mg-NiTiO3/TiO2composite could take up hydrogen at room temperature and the apparent activation energy for hydrogen absorption was dramatically decreased from 69.8±1.2(nanocrystalline Mg)k J/mol to 34.2±0.2 k J/mol.In addition,the hydrogenated sample began to release hydrogen at about 193.2℃and eventually desorbed 6.6 wt%H2.The desorption enthalpy of the hydrogenated Mg-NiTiO3-C was estimated to be 78.6±0.8 k J/mol,5.3 k J/mol lower compared to 83.9±0.7 k J/mol of nanocrystalline Mg.Besides,the sample revealed splendid cyclic stability during 20 cycles.No obvious recession occurred in the absorption and desorption kinetics and only 0.3 wt%hydrogen capacity degradation was observed.Further structural analysis demonstrates that nanosizing and catalyst doping led to a synergistic effect on the enhanced hydrogen storage performance of Mg-NiTiO3-C composite,which might serve as a reference for future design of highly effective hydrogen storage materials.