Boosting low-temperature de/re-hydrogenation performances of MgH2 with Pd-Ni bimetallic nanoparticles supported by mesoporous carbon

Bimetallic Pd-Ni nano-particles supported by a mesoporous carbon material CMK-3 (denoted as Pd₃₀Ni₇₀/CMK-3) were synthesized through solution impregnation and hydrogen reduction methods. Among those hierarchical Ni-Pd nano-particles, majorly large ones (>10 nm) are dispersed over the surface of CMK-3, while a litter small ones (<10 nm) are embedded into the pores. It significantly improves the de/re-hydrogenation performances of MgH₂ at low temperature. The onset desorption temperature of MgH₂-Pd₃₀Ni₇₀/CMK-3 is lowered by 150 K from that of pristine MgH₂ (above 593 K). About 6 wt% hydrogen could be released during its decomposition below 561 K. Noticeably, MgH₂-Pd₃₀Ni₇₀/CMK-3 is capable of releasing 1.3 wt% H₂ even at 373 K. 4 wt% hydrogen can be absorbed at 343 K under a hydrogen pressure of 3 MPa within 18000 s. Activation energy values of both hydrogen decomposition (65.9 kJ mol⁻¹) and absorption (78.9 kJ mol⁻¹) for MgH₂-Pd₃₀Ni₇₀/CMK-3 are greatly improved from those of as-milled MgH₂. Thermal stability of the composite system is remarkably destabilized by 4.3 kJ mol H₂⁻¹ from pristine MgH₂ according to pressure-composition isotherm curves and van't Hoff plots. The enhanced performances can be ascribed to the synergistic effects of both destabilization and catalysis from nano-dispersed Pd and Ni particles, respectively.     

Influence of different amounts of FeCl3 on decomposition and hydrogen sorption kinetics of MgH2

In the present work, the hydrogen storage properties of MgH₂-X wt.% FeCl₃ (X = 5, 10, 15 and 20) are investigated experimentally. It is found that the MgH₂ + 10 wt.% FeCl₃ sample exhibits the best comprehensive hydrogen storage properties, in terms of the onset dehydrogenation temperature, the hydrogen amounts de/reabsorbed as well as the relative de/rehydrogenation rates. The onset dehydrogenation temperature of the 10 wt.% FeCl₃-doped MgH₂ sample is reduced by about 90 °C compared to the as-milled MgH₂, and the sorption kinetics measurements indicate that the FeCl₃-doped sample displays an average dehydrogenation rate 5–6 times faster than that of the undoped MgH₂ sample. Higher levels of doping introduce negative effects, such as lower capacity and slower absorption/desorption rates compared to samples with lower FeCl₃ doping levels. The apparent activation energy for hydrogen desorption is decreased from 166 kJ•mol⁻¹ for as-milled MgH₂ to 130 kJ•mol⁻¹ by the addition of 10 wt.% FeCl₃. It is believed that the improvement of the MgH₂ sorption properties in the MgH₂/FeCl₃ composite is due to the catalytic effects of the in-situ generated Fe species and MgCl₂ that are formed during the heating process.     

Synergistic catalytic effects of ZIF-67 and transition metals (Ni,Cu, Pd,and Nb) on hydrogen storage properties of magnesium

MgTM/ZIF-67 nanocomposites were prepared by the deposition-reduction method using ZIF-67, MgCl₂, and TMCl_x (TM = Ni, Cu, Pd, Nb) as raw materials. The dehydrogenation activation energies of MgTM/ZIF-67 (TM = Ni, Cu, Pd, Nb) nanocomposites were calculated to be 115.4 kJ mol⁻¹ H₂, 115.7 kJ mol⁻¹ H₂, 113.6 kJ mol⁻¹ H₂, and 75.8 kJ mol⁻¹ H₂, respectively; hence, the MgNb/ZIF-67 nanocomposite manifested the best comprehensive hydrogen storage performance. The hydrogen storage capacity of the MgNb/ZIF-67 nanocomposite was hardly attenuated after the 100th hydrogen absorption-desorption cycle. The dehydrogenated enthalpies of MgH₂ and CoMg₂H₅ in MgNb/ZIF-67 hydride were calculated to be 72.4 kJ mol⁻¹ H₂ and 81.0 kJ mol⁻¹ H₂, respectively, which were lower than those of additive-free MgH₂ and Mg/ZIF-67. The improved hydrogen storage properties of MgNb/ZIF-67 can be ascribed to the CoMg₂–Mg(Nb) core-shell structure and the catalytic effects of NbH and niobium oxide nanocrystals.     

Hydrogen absorption and desorption properties of Mg/MgH2 with nanometric dispersion of small amounts of Nb(V) ethoxide

A study to determine the optimal content of Nb(V) ethoxide required to efficiently catalyze the H₂ sorption kinetics in the Mg/MgH₂ system is reported. The materials were synthesized by hand mixing different amounts of additive (from 0.10 to 1 mol%) to pre-milled MgH₂. Considering kinetics and capacity the best performance corresponds to a 0.25 mol% of Nb ethoxide concentration. With this material, a remarkable kinetic behavior with excellent reversibility is obtained: 5.3 wt% and 5.1 wt% of hydrogen are absorbed and desorbed respectively at 300 °C in 3 min. At 250 °C the material absorbs 5.2 wt% of hydrogen and releases 3.7 wt% in 10 min. Thermal desorption starts at 247 °C and peaks at 268 °C. The H₂ sorption properties of all the materials remain unchanged after 10 cycles of absorption and desorption at 300 °C, and the best material reversibly takes in and releases 5.3 wt% of H₂ during a 10 min combined cycle. The kinetic improvement of the hydrogen desorption and absorption properties is attributed to an enhancement of the kinetic processes that occur on the surface of the material, due to the excellent spreading of the liquid additive at nanometric level, as revealed by SEM/EDS and TEM/EELS.     

Facile synthesized Fe nanosheets as superior active catalyst for hydrogen storage in MgH2

Reversible hydrogen storage in MgH₂ under mild conditions is a promising way for the realization of “Hydrogen Economy”, in which the development of cheap and highly efficient catalysts is the major challenge. Herein, A two-dimensional layered Fe is prepared via a facile wet-chemical ball milling method and has been confirmed to greatly enhance the hydrogen storage performance of MgH₂. Minor addition of 5 wt% Fe nanosheets to MgH₂ decreases the onset desorption temperature to 182.1 °C and enables a quick release of 5.44 wt% H₂ within 10 min at 300 °C. Besides, the dehydrogenated sample takes up 6 wt% H₂ in 10 min under a hydrogen pressure of 3.2 MPa at 200 °C. With the doping of Fe nanosheets, the apparent activation energy of the dehydrogenation reaction for MgH₂ is reduced to 40.7 ± 1.0 kJ mol⁻¹. Further ab initio calculations reveal that the presence of Fe extends the Mg–H bond length and reduces its bond strength. We believe that this work would shed light on designing plain metal for catalysis in the area of hydrogen storage and other energy-related issues.     

Influence of multiple oxide (Cr2O3/Nb2O5) addition on the sorption kinetics of MgH2

The influence of multiple additions of two oxides, Cr₂O₃ and Nb₂O₅, as additives on the hydrogen sorption kinetics of MgH₂ after milling was investigated. We found that the desorption kinetics of MgH₂ were improved more by multiple oxide addition than by single addition. Even for the milled MgH₂ micrometric size powders, the high hydrogen capacity with fast kinetics were achieved for the powders after addition of 0.2 mol% Cr₂O₃ + 1 mol% Nb₂O₅. For this composition, the hydride desorbed about 5 wt.% hydrogen within 20 min and absorbed about 6 wt.% in 5 min at 300 °C. Furthermore, the desorption temperature was decreased by 100 °C, compared to MgH₂ without any oxide addition, and the activation energy for the hydrogen desorption was estimated to be about 185 kJ mol⁻¹, while that for MgH₂ without oxide was about 206 kJ mol⁻¹.     

Effect of in-situ formed Mg2Ni/Mg2NiH4 compounds on hydrogen storage performance of MgH2

Magnesium-based hydrogen storage materials (MgH₂) are promising hydrogen carrier due to the high gravimetric hydrogen density; however, the undesirable thermodynamic stability and slow kinetics restrict its utilization. In this work, we assist the de/hydrogenation of MgH₂ via in situ formed additives from the conversion of an MgNi₂ alloy upon de/hydrogenation. The MgH₂–16.7 wt%MgNi₂ composite was synthesized by ball milling of Mg powder and MgNi₂ alloy followed by a hydrogen combustion synthesis method, where most of the Mg converted to MgH₂, and the others reacted with the MgNi₂ generating Mg₂NiH₄, which produced in situ Mg₂Ni during dehydrogenation. Results showed that the Mg₂Ni and Mg₂NiH₄ could induce hydrogen absorption and desorption of the MgH₂, that it absorbed 2.5 wt% H₂ at 473 K, much higher than that of pure Mg, and the dehydrogenation capacity increased by 2.6 wt% at 573 K. Besides, the initial dehydrogenation temperature of the composite under the promotion of Mg₂NiH₄ decreased greatly by 100 K, whereas it is 623 K for MgH₂. Furthermore, benefiting from the catalyst effect of Mg₂NiH₄ during dehydrogenation, the apparent activation energy of the composite reduced to 73.2 kJ mol⁻¹ H₂ from 129.5 kJ mol⁻¹ H₂.     

Enhanced hydrogen desorption/absorption properties of magnesium hydride with CeF3@Gn

In order to improve the hydrogen storage performance of MgH₂, graphene and CeF₃ co-catalyzed MgH₂ (hereafter denoted as MgH₂+CeF₃@Gn) were prepared by wet method ball milling and hydriding, which is a simple and time-saving method. The effect of CeF₃@Gn on the hydrogen storage behavior of MgH₂ was investigated. The experimental results showed that co-addition of CeF₃@Gn greatly decreased the hydrogen desorption/absorption temperature of MgH₂, and remarkably improved the dehydriding/hydriding kinetics of MgH₂. The onset hydrogen desorption temperature of Mg + CeF₃@Gn is 232 °C，which is 86 °C lower than that of as-milled undoped MgH₂, and its hydrogen desorption capacity reaches 6.77 wt%, which is 99% of its theoretical capacity (6.84 wt%). At 300 °C and 200 °C the maximum hydrogen desorption rates are 79.5 and 118 times faster than that of the as-milled undoped MgH₂. Even at low temperature of 150 °C, the dedydrided sample (Mg + CeF₃@Gn) also showed excellent hydrogen absorption kinetics, it can absorb 5.71 wt% hydrogen within 50 s, and its maximum hydrogen absorption rate reached 15.0 wt% H₂/min, which is 1765 times faster than that of the undoped Mg. Moreover, no eminent degradation of hydrogen storage capacity occurred after 15 hydrogen desorption/absorption cycles. Mg + CeF₃@Gn showed excellent hydrogen de/absorption kinetics because of the MgF₂ and CeH_2-3 that are formed in situ, and the synergic catalytic effect of these by-products and unique structure of Gn.     

Facile synthesis of nickel-vanadium bimetallic oxide and its catalytic effects on the hydrogen storage properties of magnesium hydride

To improve the dehydrogenation/hydrogenation performance of magnesium hydride (MgH₂), a nickel-vanadium bimetallic oxide (NiV₂O₆) was prepared by a simple hydrothermal method using ammonium metavanadate and nickel nitrate as raw materials. This oxide was used to improve the hydrogen storage performance of MgH₂. NiV₂O₆ reacted with Mg to form Mg₂Ni and V₂O₅; Mg₂Ni and V₂O₅ played an important role in improving the hydrogen storage properties of MgH₂. The NiV₂O₆-doped MgH₂ had an excellent hydrogen absorption and desorption kinetics performance, and it could absorb 5.59 wt% of hydrogen within 50 min at 150 °C and release about 5.3 wt% of hydrogen within 12 min. The apparent activation energies for the dehydrogenation and hydrogenation of MgH₂-NiV₂O₆ were 92.9 kJ mol^?1 and 24.9 kJ mol^?1, respectively. These were 21.7% and 66.3% lower than those of MgH_2, respectively. The mechanism analysis demonstrated that the improved kinetic properties of MgH₂ resulted from the heterogeneous catalysis of vanadium and nickel.     

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.     

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.     

Study on catalytic effect and mechanism of MOF (MOF = ZIF-8, ZIF-67, MOF-74) on hydrogen storage properties of magnesium

Using a deposition-reduction method, Mg/MOF nanocomposites were prepared as composites of Mg and metal-organic framework materials (MOFs = ZIF-8, ZIF-67 and MOF-74). The addition of MOFs can enhance the hydrogen storage properties of Mg. For example, within 5000 s, 0.6 wt%, 1.2 wt%, 2.7 wt%, 3.7 wt% of hydrogen were released from Mg, Mg/MOF-74, Mg/ZIF-8, Mg/ZIF-67, respectively. Activation energy values of 198.9 kJ mol⁻¹ H₂, 161.7 kJ mol⁻¹ H₂, 192.1 kJ mol⁻¹ H₂ were determined for the Mg/ZIF-8, Mg/ZIF-67, Mg/MOF-74 hydrides, which are 6 kJ mol⁻¹ H₂, 43.2 kJ mol⁻¹ H₂, and 12.8 kJ mol⁻¹ H₂ lower than that of Mg hydride, respectively. Moreover, the cyclic stability characterizing Mg hydride was significantly improved when adding ZIF-67. The hydrogen storage capacity of the Mg/ZIF-67 nanocomposite remained unchanged, even after 100 cycles of hydrogenation/dehydrogenation. This excellent cyclic stability may have resulted from the core-shell structure of the Mg/ZIF-67 nanocomposite.     

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.     

Electrochemical reactivity of magnesium hydride toward lithium: New synthesis route of nano-particles suitable for hydrogen storage

Electrochemical conversion reaction of MgH₂ with Li ion enables the production of in-situ nanometric Mg and MgH₂ particles so-called (nano-Mg) _INSITU and [(nano-MgH₂)_INSITU] showing interesting hydrogen sorption properties with hydrogen absorption at 100 °C under 10 bars of hydrogen pressure (PH₂) and desorption at 200 °C under primary vacuum, respectively. Differential Scanning Calorimetry (DSC) measurements of MgH₂ electrochemically prepared confirmed a decrease in the desorption temperature from 416 °C to 295 °C and in the heat of formation from −74 kJ mole mol⁻¹ (H₂)⁻¹ to −56 kJ mol⁻¹ (H₂)⁻¹ for commercial (particle size diameter: 10 μm–100 μm) and as prepared MgH₂ hydride (particle size: 10 nm–40 nm), respectively.     

Effects of trimesic acid-Ni based metal organic framework on the hydrogen sorption performances of MgH2

A metal-organic framework based on Ni (II) as metal ion and trimasic acid (TMA) as organic linker was synthesized and introduced into MgH₂ to prepare a Mg-(TMA-Ni MOF)-H composite through ball-milling. The microstructures, phase changes and hydrogen storage behaviors of the composite were systematically studied. It can be found that Ni ion in TMA-Ni MOF is attracted by Mg to form nano-sized Mg₂Ni/Mg₂NiH₄ after de/rehydrogenation. The hydriding and dehydriding enthalpies of the Mg-MOF-H composite are evaluated to be −74.3 and 78.7 kJ mol⁻¹ H₂, respectively, which means that the thermodynamics of Mg remains unchanged. The absorption kinetics of the Mg-MOF-H composite is improved by showing an activation energy of 51.2 kJ mol⁻¹ H₂. The onset desorption temperature of the composite is 167.8 K lower than that of the pure MgH₂ at the heating rate of 10 K/min. Such a significant enhancement on the sorption kinetic properties of the composite is attributed to the catalytic effects of the nanoscale Mg₂Ni/Mg₂NiH₄ derived from TMA-Ni MOF by providing gateways for hydrogen diffusion during re/dehydrogenation processes.     

Effects of ball milling time on the microstructure and hydrogen storage performances of Ti21.7Y0.3Fe16Mn3Cr alloy

TiFe alloy can store hydrogen at room temperature and low hydrogen pressure, and its theoretical hydrogen storage capacity is up to 1.8 wt%. However, TiFe alloy needs to be activated at high pressure (5 MPa hydrogen) and high temperature (673–723 K), which limits the practical application of TiFe alloy. The as-cast Ti_21.7Y_0.3Fe₁₆Mn₃Cr alloy was milled for 0, 0.5, 0.75, 1, and 3 h to study the effects of ball milling on phase structures and hydrogen storage performances. Emphasis was focused on the activation process of as-milled alloys at different temperatures, including the activation process at 483, 443, and 403 K. The results show that the alloys were consisted of TiFe phase, and [Fe, Cr] solid solution. The nanocrystalline boundary produced by milling and the phase boundary provided by the second phase provide a large number of channels for hydrogen diffusion and promote the improvement of hydrogen storage performances. The time required for activation process of as-milled alloys was significantly reduced, and the activation time of as-milled (0.75 h) was only 4 min, and its enthalpy variation for hydrogen absorption and desorption was 22.943 and 26.215 kJ mol⁻¹ H₂, respectively.     

The hydrogen storage properties and catalytic mechanism of the CuFe2O4-doped MgH2 composite system

The influence of CuFe₂O₄ addition on the sorption performances of MgH₂ prepared by ball milling was studied for the first time. The MgH₂ + 10 wt% CuFe₂O₄ sample exhibited an enhancement in hydrogen storage performance compared to that of as-milled MgH₂, with the onset decomposition temperature decreased from 340 °C to 250 °C. Dehydrogenation kinetic result revealed that CuFe₂O₄-added MgH₂ released around 5.3 wt% H₂ within 10 min at 320 °C, while the as-milled MgH₂ released below 1.0 wt% H₂ under the same condition. Furthermore, about 5.0 wt% H₂ was absorbed at 250 °C in 30 min for the 10 wt% CuFe₂O₄-doped MgH₂ sample. In contrast, the un-doped MgH₂ only absorbed 4.0 wt% H₂ at 250 °C in 30 min. From the Kissinger analysis, the apparent activation energy of as-milled MgH₂ was 166.0 kJ/mol and this value decreased to 113.0 kJ/mol for 10 wt% CuFe₂O₄-added MgH₂. The enhanced sorption performance of MgH₂ in the presence of CuFe₂O₄ is believed to be due to the role of in situ formed Fe, Mg-Cu alloy, and MgO phases as an active species to catalyse the hydrogen storage properties of MgH₂.     

Ternary transition metal alloy FeCoNi nanoparticles on graphene as new catalyst for hydrogen sorption in MgH2

The present investigation deals with the synthesis of ternary transition metal alloy nanoparticles of FeCoNi and graphene templated FeCoNi (FeCoNi@GS) by one-pot reflux method and there use as a catalyst for hydrogen sorption in MgH₂. It has been found that the MgH₂ catalyzed by FeCoNi@GS (MgH₂: FeCoNi@GS) has the onset desorption temperature of ~255 °C which is 25 °C and 100 °C lower than MgH₂ catalyzed by FeCoNi (MgH₂: FeCoNi) (onset desorption temperature 280 °C) and the ball-milled (B.M) MgH₂ (onset desorption temperature 355 °C) respectively. Also MgH₂: FeCoNi@GS shows enhanced kinetics by absorbing 6.01 wt% within just 1.65 min at 290 °C under 15 atm of hydrogen pressure. This is much-improved sorption as compared to MgH₂: FeCoNi and B.M MgH₂ for which hydrogen absorption is 4.41 wt% and 1.45 wt% respectively, under the similar condition of temperature, pressure and time. More importantly, the formation enthalpy of MgH₂: FeCoNi@GS is 58.86 kJ/mol which is 19.26 kJ/mol lower than B.M: MgH₂ (78.12 kJ/mol). Excellent cyclic stability has also been found for MgH₂: FeCoNi@GS even up to 24 cycles where it shows only negligible change from 6.26 wt% to 6.24 wt%. A feasible catalytic mechanism of FeCoNi@GS on MgH₂ has been put forward based on X-ray diffraction (XRD), Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Photoelectron Spectroscopy (XPS), and microstructural (electron microscopic) studies.     

Effects of SnO2 on hydrogen desorption of MgH2

The hydrogen desorption properties of Magnesium Hydride (MgH₂) ball milled with cassiterite (SnO₂) have been investigated by X-ray powder diffraction and thermal analysis. Milling of pure MgH₂ leads to a reduction of the desorption temperature (up to 60 K) and of the activation energy, but also to a reduction of the quantity of desorbed hydrogen, referred to the total MgH₂ present, from 7.8 down to 4.4 wt%. SnO₂ addition preserves the beneficial effects of grinding on the desorption kinetics and limits the decrease of desorbed hydrogen. Best tradeoff – activation energy lowered from 175 to 148 kJ/mol and desorbed hydrogen, referred to the total MgH₂ present, lowered from 7.8 to 6.8 wt% – was obtained by co-milling MgH₂ with 20 wt% SnO₂.     

Synthesis of BaFe12O19 by solid state method and its effect on hydrogen storage properties of MgH2

Previous studies have shown that ferrites give a positive effect in improving the hydrogen sorption properties of magnesium hydride (MgH₂). In this study, another ferrite, i.e., BaFe₁₂O₁₉, has been successfully synthesised via the solid state method, and it was milled with MgH₂ to enhance the sorption kinetics. The result showed that the MgH₂ + 10 wt% BaFe₁₂O₁₉ sample started to release hydrogen at about 270 °C which is about 70 °C lower than the as-milled MgH₂. The doped sample was able to absorb hydrogen for 4.3 wt% in 10 min at 150 °C, while as-milled MgH₂ only absorbed 3.5 wt% of hydrogen under similar conditions. The desorption kinetic results showed that the doped sample released about 3.5 wt% of hydrogen in 15 min at 320 °C, while the as-milled MgH₂ only released about 1.5 wt% of hydrogen. From the Kissinger plot, the apparent activation energy of the BaFe₁₂O₁₉-doped MgH₂ sample was 115 kJ/mol which was lower than the milled MgH₂ (141 kJ/mol)_. Further analyses demonstrated that MgO, Fe and Ba or Ba-containing contribute to the improvement by serving as active species, thus enhancing the MgH₂ for hydrogen storage.