Significantly improved dehydrogenation of LiAlH4 catalysed with TiO2 nanopowder

The effects of TiO₂ nanopowder addition on the dehydrogenation behaviour of LiAlH₄ have been studied. The 5 wt.% TiO₂-added LiAlH₄ sample showed a significant improvement in dehydrogenation rate compared to that of undoped LiAlH₄, with the dehydrogenation temperature reduced from 150 °C to 60 °C. Kinetic desorption results show that the added LiAlH₄ released about 5.2 wt% hydrogen within 30 min at 100 °C, while the as-received LiAlH₄ just released below 0.2 wt.% hydrogen within same time at 120 °C. From the Arrhenius plot of the hydrogen desorption kinetics, the apparent activation energy is 114 kJ/mol for pure LiAlH₄ and 49 kJ/mol for the 5 wt.% TiO₂ added LiAlH₄, indicating that TiO₂ nanopowder adding significantly decreased the activation energy for hydrogen desorption of LiAlH₄. X-ray diffraction and Fourier transform infrared analysis show that there is no phase change in the cell volume or on the Al-H bonds of the LiAlH₄ due to admixture of TiO₂ after milling. X-ray photoelectron spectroscopy results show no changes in the Ti 2p spectra for TiO₂ after milling and after dehydrogenation. The improved dehydrogenation behaviour of LiAlH₄ in the presence of TiO₂ is believed to be due to the high defect density introduced at the surfaces of the TiO₂ particles during the milling process.     

Enhancing the dehydrogenation properties of LiAlH4 using K2NiF6 as additive

Lithium alanate (LiAlH₄) is a material that can be potentially used for solid-state hydrogen storage due to its high hydrogen content (10.5 wt%). Nevertheless, a high desorption temperature, slow desorption kinetic, and irreversibility have restricted the application of LiAlH₄ as a solid-state hydrogen storage material. Hence, to lower the decomposition temperature and to boost the dehydrogenation kinetic, in this study, we applied K₂NiF₆ as an additive to LiAlH₄. The addition of K₂NiF₆ showed an excellent improvement of the LiAlH₄ dehydrogenation properties. After adding 10 wt% K₂NiF₆, the initial decomposition temperature of LiAlH₄ within the first two dehydrogenation steps was lowered to 90 °C and 156 °C, respectively, that is 50 °C and 27 °C lower than that of the аs-milled LiAlH₄. In terms of dehydrogenation kinetics, the dehydrogenation rate of K₂NiF₆-doped LiAlH₄ sample was significantly higher as compared to аs-milled LiAlH₄. The K₂NiF₆-doped LiAlH₄ sample can release 3.07 wt% hydrogen within 90 min, while the milled LiAlH₄ merely release 0.19 wt% hydrogen during the same period. According to the Arrhenius plot, the apparent activation energies for the desorption process of K₂NiF₆-doped LiAlH₄ are 75.0 kJ/mol for the first stage and 88.0 kJ/mol for the second stage. These activation energies are lower compared to the undoped LiAlH₄. The morphology study showed that the LiAlH₄ particles become smaller and less agglomerated when K₂NiF₆ is added. The in situ formation of new phases of AlNi and LiF during the dehydrogenation process, as well as a reduction in particle size, is believed to be essential contributors in improving the LiAlH₄ dehydrogenation characteristics.     

LiAlH4–ZrCl4 mixtures for hydrogen release at near room temperature

The dehydrogenation temperature of LiAlH₄ was significantly reduced by the production of mixtures with ZrCl₄. Stoichiometric 4:1, and 5 mol % mixtures of LiAlH₄ and ZrCl₄ were produced by ball milling at room temperature and ?196 °C, and tested for dehydrogenation at low temperature. Cryogenic ball-milling resulted in an effective way to produce reactive mixtures for hydrogen release; because of achieving small aggregates size (5–20 μm) in 10 min of cryomilling while preventing substantial decomposition during preparation. Dehydrogenation reaction in the mixtures LiAlH₄/ZrCl₄ started around 31–47 °C under different heating rates. Partial dehydrogenation was proved at 70 °C: 4.4 wt % for the 5 mol% ZrCl₄–LiAlH₄ mixture, and 3.4 wt % for the best 4:1 stoichiometric mixture. Complete dehydrogenation up to 250 °C released 6.4 wt% and 4.1 wt%, respectively. Dehydrogenation reactions are exothermic, and the LiAlH₄/ZrCl₄ mixtures are unstable and difficult to handle. The activation energy of the exothermic reactions was estimated as 113.5 ± 9.8 kJ/mol and 40.6 ± 6.6 kJ/mol for 4LiAlH₄+ZrCl₄ and 5%mol ZrCl₄+LiAlH₄ samples milled in cryogenic conditions, respectively. The dehydrogenation pathway was changed in the LiAlH₄/ZrCl₄ mixtures as compared to pure LiAlH₄. Dehydrogenation reaction is proposed to form Al, LiCl, Zr, and H₂ as main products. Modification of the dehydrogenation reaction of LiAlH₄ was achieved at the cost of reducing the total hydrogen release capacity.     

Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4

Study on the catalytic roles of MgFe₂O₄ on the dehydrogenation performance of LiAlH₄ was carried out for the first time. Notable improvement on the dehydrogenation of LiAlH₄–MgFe₂O₄ compound was observed. The initial decomposition temperatures for the catalyzed LiAlH₄ were decreased to 95 °C and 145 °C for the first and second stage reactions, which were 48 °C and 28 °C lower than the milled LiAlH₄. As for the desorption kinetics performance, the MgFe₂O₄ doped-LiAlH₄ sample was able to desorb faster with a value of 3.5 wt% of hydrogen in 30 min (90 °C) while the undoped LiAlH₄ was only able to desorb 0.1 wt% of hydrogen. The activation energy determined from the Kissinger analysis for the first two desorption reactions were 73 kJ/mol and 97 kJ/mol; which were 31 and 17 kJ/mol lower as compared to the milled LiAlH₄. The X-ray diffraction result suggested that the MgFe₂O₄ had promoted significant improvements by reducing the LiAlH₄ decomposition temperature and faster desorption kinetics through the formation of active species of Fe, LiFeO₂ and MgO that were formed during the heating process.     

Desorption properties of LiAlH4 doped with LaFeO3 catalyst

The addition of a catalyst and ball milling process was found to be one of the efficient method to reduce the decomposition temperature and improve the desorption kinetics of lithium aluminium hydride (LiAlH₄). In this paper, a transition metal oxide, LaFeO₃ was used as a catalyst. Decomposition temperature of the 10 wt% of LaFeO₃-doped LiAlH₄ system was found to be lowered from 143 °C to 103 °C (first step) and from 175 °C to 153 °C (second step), respectively. In isothermal desorption kinetics, the amount of hydrogen released of the doped sample was improved to 3.9 wt% in 2.5 h at 90 °C. Meanwhile, the undoped sample had released less than 1.0 wt% of hydrogen under the same condition. The activation energy of the LaFeO₃-doped LiAlH₄ sample was measured to be 73 kJ/mol and 90 kJ/mol for the first two dehydrogenation reactions compared to 107 kJ/mol and 119 kJ/mol for the undoped sample. The improvements of desorption properties were the results from the formation of LiFeO₂, Fe and La or La-containing phase during the heating process.     

The effects of the micrometric and nanometric iron (Fe) additives on the mechanical and thermal dehydrogenation of lithium alanate (LiAlH4), its self-discharge at low temperatures and rehydrogenation

LiAlH₄ containing 5 wt.% of nanometric Fe (n-Fe) shows a profound mechanical dehydrogenation by continuously desorbing hydrogen (H₂) during high energy ball milling reaching ∼3.5 wt.% H₂ after 5 h of milling. In contrast, no H₂ desorption is observed during low energy milling of LiAlH₄ containing n-Fe. Similarly, no H₂ desorption occurs during high energy ball milling for LiAlH₄ containing micrometric Fe (μ-Fe) and, for comparison, both the micrometric and nanometric Ni (μ-Ni and n-Ni) additive. X-ray diffraction studies show that ball milling results in a varying degree of the lattice expansion of LiAlH₄ for both the Fe and Ni additives. A volumetric lattice expansion larger than 1% results in the profound destabilization of LiAlH₄ accompanied by continuous H₂ desorption during milling according to reaction: LiAlH₄ (solid) → 1/3Li₃AlH₆ + 2/3Al + H₂. It is hypothesized that the Fe ions are able to dissolve in the lattice of LiAlH₄ by the action of mechanical energy, replacing the Al ions and forming a substitutional solid solution. The quantity of dissolved metal ions depends primarily on the total energy of milling per unit mass of powder generated within a prescribed milling time, the type of additive ion e.g. Fe vs. Ni and on the particle size (micrometric vs. nanometric) of metal additive. For thermal dehydrogenation the average apparent activation energy of Stage I (LiAlH₄ (solid) → 1/3Li₃AlH₆ + 2/3Al + H₂) is reduced from the range 76 to 96 kJ/mol for the μ-Fe additive to about 60 kJ/mol for the n-Fe additive. For Stage II dehydrogenation (1/3Li₃AlH₆ → LiH+1/3Al + 0.5H₂) the average apparent activation energy is within the range 77–93 kJ/mol, regardless of the particle size of the Fe additive (μ-Fe vs. n-Fe). The n-Fe and n-Ni additives, the latter used for comparison, provide nearly identical enhancement of dehydrogenation rate during isothermal dehydrogenation at 100 °C. Ball milled (LiAlH₄ + 5 wt.% n-Fe) slowly self-discharges up to ∼5 wt.% H₂ during storage at room temperature (RT), 40 and 80 °C. Fully dehydrogenated (LiAlH₄ + 5 wt.% n-Fe) has been partially rehydrogenated up to 0.5 wt.% H₂ under 100 bar/160°C/24 h. However, the rehydrogenation parameters are not optimized yet.     

Effects of REF3 (RE = Y,La, Ce) additives on dehydrogenation properties of LiAlH4

The catalytic effects of rare earth fluoride REF₃ (RE = Y, La, Ce) additives on the dehydrogenation properties of LiAlH₄ were carefully investigated in the present work. The results showed that the dehydrogenation behaviors of LiAlH₄ were significantly altered by the addition of 5 mol% REF₃ through ball milling. The destabilization ability of these catalysts on LiAlH₄ has the order: CeF₃>LaF₃>YF₃. For instance, the temperature programmed desorption (TPD) analyses showed that the onset dehydrogenation temperature of CeF₃ doped LiAlH₄ was sharply reduced by 90 °C compared to that of pristine LiAlH₄. Based on differential scanning calorimetry (DSC) analyses, the dehydriding activation energies of the CeF₃ doped LiAlH₄ sample were 40.9 kJ/mol H₂ and 77.2 kJ/mol H₂ for the first and second dehydrogenation stages, respectively, which decreased about 40.0 kJ/mol H₂ and 60.3 kJ/mol H₂ compared with those of pure LiAlH₄. In addition, the sample doped with CeF₃ showed the fastest dehydrogenation rate among the REF₃ doped LiAlH₄ samples at both 125 °C and 150 °C during the isothermal desorption. The phase changes in REF₃ doped LiAlH₄ samples during ball milling and dehydrogenation were examined using X-ray diffraction and the mechanisms related to the catalytic effects of REF₃ were proposed.     

Improved hydrogen desorption in lithium alanate by addition of SWCNT-metallic catalyst composite

A LiAlH₄/single walled carbon nanotube (SWCNT) composite system was prepared by mechanical milling and its hydrogen storage properties investigated. The SWCNT - metallic particle addition resulted in both a decreased decomposition temperature and enhanced desorption kinetics compared to pure LiAlH₄. The decomposition temperature of the 5 wt.% SWCNT-added LiAlH₄ sample was reduced to 80 °C and 130 °C for the first and second stage, respectively, compared with 150 °C and 180 °C for as-received LiAlH₄. In terms of the desorption kinetics, the 5 wt.% SWCNT-added LiAlH₄ sample released about 4.0 wt.% hydrogen at 90 °C after 40 min dehydrogenation, while the as-milled LiAlH₄ sample released less than 0.3 wt.% hydrogen for the same temperature and time. Differential scanning calorimetry measurements indicate that enthalpies of decomposition in LiAlH₄ decrease with added SWCNTs. The apparent activation energy for hydrogen desorption was decreased from 116 kJ/mol for as-received LiAlH₄ to 61 kJ/mol by the addition of 5 wt.% SWCNTs. It is believed that the significant improvement in dehydrogenation behaviour of SWCNT-added LiAlH₄ is due to the combined influence of the SWCNT structure itself and the catalytic role of the metallic particles contained in the SWCNTs. In addition, the different effects of the SWCNTs and the metallic catalysts contained in the SWCNTs were also investigated, and the possible mechanism is discussed.     

Reinforce the dehydrogenation process of LiAlH4 by accumulating porous activated carbon

Herein, it is reported that activated carbon (AC) alters the hydrogen storage behavior of lithium alanate (LiAlH₄) prepared by the ball milling technique. Notable improvements in onset decomposition temperature and desorption kinetics are attained for LiAlH₄ added 10 wt.% of AC composite compared to as-received and as-milled LiAlH₄. The onset decomposition temperature of LiAlH₄-10 wt.% AC dropped to 100 °C and 160 °C for the first and second steps. The composite also released 3.4 wt.% of hydrogen after 90 min compared to as-received and as-milled which is less than 0.2 wt.% of hydrogen within the same period. The XRD result discovered an additional peak of the Li₃AlH₆ and Al compounds appeared after the milling process, concluding that LiAlH₄ becomes unstable after the addition of AC. FTIR measurement has verified the presence of the Li₃AlH₆ and carbon bonding in the LiAlH₄-10 wt.% AC composite. The composite's activation energy (E_a) for the first and second steps is 70 kJ/mol and 85 kJ/mol, respectively. These values decrease from as-milled LiAlH₄ for both steps, demonstrating the catalytic effect of AC in this system. FESEM images illustrate that after ball milling, the particle size of LiAlH₄-10 wt.% AC composite decreases. The considerable improvement in the hydrogen storage characteristic of the LiAlH₄-10 wt.% AC composite is thought to be the collaborative role of amorphous carbon.     

LiBH4–VCl3 and LiAlH4–VCl3 mixtures prepared in soft milling conditions for hydrogen release at low temperature

Mixtures of LiBH₄/VCl₃ and LiAlH₄/VCl₃ in 5:1, 3:1, and 5% mol stoichiometries were prepared and tested for hydrogen release. The mixtures were prepared in 10 min of ball milling at room temperature or with cryogenic (N₂-liquid) cooling. The mixtures demonstrated diverse hydrogen release levels, but all of them started releasing hydrogen at low temperatures (33–66 °C) with a change in the reaction pathway as compared to pure LiBH₄ or LiAlH₄. The driving force for that is the formation of the stable salt LiCl. The best material was the 5% mol VCl₃ + LiAlH₄ cryogenic mixture because of the low-temperature dehydrogenation onset of 34 °C; and the dehydrogenation level of 5.1 wt.%, and 6.4 wt.% that was achieved upon heating at 100 °C and 250 °C, respectively.     

Enhanced hydrogen properties of MgH2 by Fe nanoparticles loaded hollow carbon spheres

In the present investigation, we have reported the synergistic effect of Fe nanoparticles and hollow carbon spheres composite on the hydrogen storage properties of MgH₂. The onset desorption temperature for MgH₂ catalyzed with hollow carbon spheres and Fe nanoparticle (MgH₂-Fe-HCS) system has been observed to be 225.9 °C with a hydrogen storage capacity of 5.60 wt %. It could be able to absorb 5.60 wt % hydrogen within 55 s and desorb 5.50 wt % hydrogen within 12 min under 20 atm H₂ pressure at 300 °C. The desorption activation energy of MgH₂-Fe-HCS has been found to be 84.9 kJ/mol, whereas the desorption activation energies for as received MgH₂, Hollow carbon sphere catalyzed MgH₂ and Fe catalyzed MgH₂ are found to be 130 kJ/mol, 103 kJ/mol, and 94.2 kJ/mol respectively. MgH₂-Fe-HCS composite lowered the change in enthalpy of hydrogen desorption from MgH₂ by 20.02 kJ/mol as compared to pristine MgH₂. MgH₂-Fe-HCS shows better cyclability up to 24 cycles of hydrogenation and dehydrogenation of MgH₂. The mechanism for the better catalytic action of Fe and HCS on MgH₂ has also been discussed.     

Kinetic improvement of H2 absorption and desorption properties in Mg/MgH2 by using niobium ethoxide as additive

The hydrogen absorption and desorption properties of a MgH₂ – 1 mol.% Nb(V) ethoxide mixture are reported. The material was prepared by hand mixing the additive with previously ball-milled MgH₂. Nb ethoxide reacts with MgH₂ during heating, releasing C₂H₆ and H₂, and producing MgO and Nb or Nb hydride. Hydriding and dehydriding are greatly enhanced by the use of the alkoxide. At 250 °C the material with Nb takes up 1.8 wt% in 30 s compared with 0.1 wt% of pure Mg, and releases 4.2 wt% in 30 min, whereas MgH₂ without Nb does not appreciably desorb hydrogen. The absorption and desorption activation energies are reduced from 153 kJ/mol H₂ to 94 kJ/mol H₂, and from 176 kJ/mol H₂ to 75 kJ/mol H₂, respectively. The hydrogen sorption properties remain stable after 10 cycles at 300 °C. The kinetic improvement is attributed to the fine distribution of amorphous/nanometric NbH_x achieved by the dispersion of the liquid additive.     

Enhanced hydrogen storage properties of 4MgH2 + LiAlH4 composite system by doping with Fe2O3 nanopowder

In this paper, the hydrogen storage properties and reaction mechanism of the 4MgH₂ + LiAlH₄ composite system with the addition of Fe₂O₃ nanopowder were investigated. Temperature-programmed-desorption results show that the addition of 5 wt.% Fe₂O₃ to the 4MgH₂ + LiAlH₄ composite system improves the onset desorption temperature to 95 °C and 270 °C for the first two dehydrogenation stage, which is lower 40 °C and 10 °C than the undoped composite. The dehydrogenation and rehydrogenation kinetics of 5 wt.% Fe₂O₃-doped 4MgH₂ + LiAlH₄ composite were also improved significantly as compared to the undoped composite. Differential scanning calorimetry measurements indicate that the enthalpy change in the 4MgH₂–LiAlH₄ composite system was unaffected by the addition of Fe₂O₃ nanopowder. The Kissinger analysis demonstrated that the apparent activation energy of the 4MgH₂ + LiAlH₄ composite (125.6 kJ/mol) was reduced to 117.1 kJ/mol after doping with 5 wt.% Fe₂O₃. Meanwhile, the X-ray diffraction analysis shows the formation of a new phase of Li₂Fe₃O₄ in the doped composite after the dehydrogenation and rehydrogenation process. It is believed that Li₂Fe₃O₄ acts as an actual catalyst in the 4MgH₂ + LiAlH₄ + 5 wt.% Fe₂O₃ composite which may promote the interaction of MgH₂ and LiAlH₄ and thus accelerate the hydrogen sorption performance of the MgH₂ + LiAlH₄ composite system.     

CoFe2O4 synthesized via a solvothermal method for improved dehydrogenation of NaAlH4

The effect of cobalt ferrite (CoFe₂O₄) nanopowder synthesised through a solvothermal method on the dehydrogenation properties of sodium alanate (NaAlH₄) was studied for the first time. The onset decomposition temperature of NaAlH₄ is significantly reduced after milling with CoFe₂O₄, in which the 10 wt% CoFe₂O₄-doped NaAlH₄ sample starts to decompose at ～100 °C. In contrast, the as-milled NaAlH₄ begins to decompose at ～200 °C, ～100 °C higher than the doped sample. With respect to desorption kinetic at constant temperature of 150 °C, the 10 wt% CoFe₂O₄-doped NaAlH₄ sample desorbed ～2.2 wt% hydrogen within 30 min, whereas the as-milled NaAlH₄ only desorbed below 0.2 wt% hydrogen. The Kissinger plot exhibited that the apparent activation energy (E_A) for hydrogen release from NaAlH₄ is significantly reduced after adding with 10 wt% CoFe₂O₄-doped NaAlH₄. The E_A values for the first and second stage dehydrogenation of the 10 wt% CoFe₂O₄-doped NaAlH₄ composite are calculated as 80.3 and 88.2 kJ/mol, respectively, and these values are reduced at approximately 34.3 and 30.5 kJ/mol compared with the as-milled NaAlH₄ (114.6 and 118.7 kJ/mol, respectively). Based on the X-ray diffraction result, the enhancement of desorption properties of NaAlH₄ with the presence of CoFe₂O₄ is presumably due to the synergistic catalytic effect played by new active species (Co₃O₄ and Fe) that in situ formed during the desorption process.     

Hydro-catalytic treatment of organoamine boranes for efficient thermal dehydrogenation for hydrogen production

This study aims to present the hydro-catalytic treatment of organoamine boranes for efficient thermal dehydrogenation for hydrogen production. Organoamine boranes, methylamine borane (MeAB), and ethane 1,2 diamine borane (EDAB), known as ammonia borane (AB) carbon derivatives, are synthesized to be used as a solid-state hydrogen storage medium. Thermal dehydrogenation of MeAB and EDAB is performed at 80 °C, 100 °C, and 120 °C under different conditions (self, catalytic, and hydro-catalytic) for hydrogen production and compared with AB. For this purpose, a cobalt-doped activated carbon (Co-AC) catalyst is fabricated. The physicochemical properties of Co-AC catalyst is investigated by well-known techniques such as ATR/FT-IR, XRD, XPS, ICP-MS, BET, and TEM. The synthesized Co-AC catalyst obtained in nano CoOOH structure (20 nm, 12% Co wt) is formed and well-dispersed on the activated carbon support. It has indicated that Co-AC exhibits efficient catalytic activity towards organoamine boranes thermal dehydrogenation. Hydrogen release tests show that hydro-catalytic treatment improves the thermal dehydrogenation kinetics of neat MeAB, EDAB, and AB. Co-AC catalyzed hydro-treatment for thermal dehydrogenation of MeAB and EDAB acceleras the hydrogen release from 0.13 mL H₂/min to 46.12 mL H₂/min, from 0.16 mL H₂/min to 38.06 mL H₂/min, respectively at 80 °C. Moreover, hydro-catalytic treatment significantly lowers the H₂ release barrier of organoamine boranes thermal dehydrogenation, from 110 kJ/mol to 19 kJ/mol for MeAB and 130 kJ/mol to 21 kJ/mol for EDAB. In conclusion, hydro and catalytic treatment presents remarkable synergistic effect in thermal dehydrogenation and improves the hydrogen release kinetics.     

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.     

Microstructure characteristics,hydrogen storage thermodynamic and kinetic properties of Mg–Ni–Y based hydrogen storage alloys

In this paper, the Mg_95-X-Ni_x-Y₅ (x = 5, 10, 15) alloy were prepared by vacuum induction melting. The X-ray diffraction was used to analytical phase composition in different states, and the Scanning Electron Microscope and Transmission Electron Microscope were used to characterize the microstructure and crystalline state. Meanwhile, the kinetic properties of isothermal hydrogen adsorption and desorption at different temperatures also were tested by the Sievert isometric volume method. The results indicate that the hydrogenated Mg–Ni–Y samples is a nanocrystalline structure consists of MgH₂, Mg₂NiH₄, and YH₃ phases. And, the in-situ formed YH₃ phase not decompose in the process of dehydrogenation and evenly dispersed in the mother alloy, which plays a paly a positive the catalytic role for the reversible cyclic reaction of Mg and Mg₂Ni phases. In addition, the Ni elements are effectively to improve the thermodynamic properties of the Mg-based hydrogen storage alloy, the desorption enthalpy of the Ni₅, Ni₁₀, and Ni₁₅ samples successively decrease to 84.5, 69.1, and 63.5 kJ/mol H₂. The hydrogen absorption and desorption kinetics of the Mg–Ni–Y alloy are improved obviously with the increase of Ni content, especially for Mg₈₀Ni₁₅Y₅ alloy, which the optimal hydrogenated temperature is reduced to 200 °C, and the 90% of the maximum hydrogen storage capacity can be absorbed within 1 min, about 5.4 wt % H₂. Besides, the dehydrogenated activation energy of the Mg₈₀Ni₁₅Y₅ alloy also is reduced to 67.0 kJ/mol, and it can completely release hydrogen at 320 °C within 5 min, which is almost reached the hydrogen desorption capability of Ni₅ alloy at 360 °C. This means that Ni element is a very positive element to reduce 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.     

ZrO2@Nb2CTx composite as the efficient catalyst for Mg/MgH2 based reversible hydrogen storage material

While Mg/MgH₂ system has a high hydrogen storage capacity, its sluggish hydrogen desorption rate has hindered practical applications. Herein, we report that the hydrogen absorption and desorption kinetics of Mg/MgH₂ system can be significantly improved by using the synergetic effect between Nb₂CT_x MXene and ZrO₂. The catalyst of Nb₂CT_x MXene loading with ZrO₂ (ZrO₂@Nb₂CT_x) is successfully synthesized, and the dehydrogenation activation energy of MgH₂ becomes as low as 60.0 kJ/mol H₂ when ZrO₂@Nb₂CT_x is used as the catalyst, which is far smaller than the case of ZrO₂ (94.8 kJ/mol H₂) and Nb₂CT_x MXene (125.6 kJ/mol H₂). With the addition of ZrO₂@Nb₂CT_x catalyst, MgH₂ can release about 6.24 wt.% and 5.69 wt.% of hydrogen within 150 s at 300 °C and within 900 s even at 240 °C, respectively. Moreover, it realizes hydrogen absorption at room temperature, which can uptake 2.98 wt.% of hydrogen within 1800 s. The catalytic mechanism analysis demonstrates that the in-situ formed nanocomposites can weaken the Mg–H bonding and provide more hydrogen diffusion channels, enabling the dissociation and recombination of hydrogen under milder reaction conditions.     

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.