Hydriding behavior of Mg-50 wt% ZrCrFe composite Prepared by high energy ball milling

Magnesium hydride has a high theoretically storage capacity, which amounts to 7.6 wt%. It is therefore a promising candidate for hydrogen storage applications. However, its major drawback is its high desorption temperature of well over 300 °C, which is related to the high stability of the Mg–H bonds and expressed in the high enthalpy of hydride formation (77 kJ/mol). The preparation of Mg composites with other hydrogen storage compounds is an effective method to improve the hydrogen storage properties of Mg. Thus we prepared Mg-50 wt% ZrCrFe alloy composite by high energy ball milling under argon atmosphere. X-ray diffraction (XRD) studies on the composite before and after hydriding cycles suggest no intermetalic phase is formed between Mg and the elements of the alloy. The morphological studies carried on by Scanning Electron Microscope (SEM) technique suggest that the alloy particles are homogeneously distributed throughout the Mg surface. A particle reduction after hydrogenation is also visible. Hydriding/dehydriding properties of the composites are investigated by PCT measurements using a dynamic system. The maximum hydrogen capacity for this composite is found to be 4.5 wt%. The reaction kinetics have also been recorded in a temperature range from RT to 300 °C and the thermodynamic parameters calculated from Van’t Hoff plot. From the results it is found that the alloy reacts with hydrogen also when cooled to room temperature while at higher temperature it works as catalyst.     

Low temperature milling of the LiNH2 + LiH hydrogen storage system

Ball milling of the LiNH₂ + LiH storage system was performed at 20 °C, −40 °C, and −196 °C, and the resulting powders were analyzed using X-ray diffraction, scanning electron microscopy, nuclear magnetic resonance (NMR), specific surface area analysis, and kinetics cycling measurements. Ball milling at −40 °C showed no appreciable deviations from the 20 °C sample, but the −196 °C powder exhibited a significant increase in the hydrogen desorption kinetics. NMR analysis indicates that a possible explanation for the kinetics increase is the retention of internal defects generated during the milling process that are annealed at the collision site at higher milling temperatures.     

Effect of CO on hydrogen storage performance of 2LiNH2 + MgH2 system

(2LiNH₂ + MgH₂) system is one of the most promising hydrogen storage materials due to its suitable operation temperature and high reversible hydrogen storage capacity. In studies and applications, impurities such as CO, CO₂, O₂, N₂ and CH₄ are potential factors which may influence its performance. In the present work, hydrogen containing 1 mol% CO is employed as the hydrogenation gas source, and directly participates in the reaction to investigate the effect of CO on the hydrogen sorption properties of (2LiNH₂ + MgH₂) system. The results indicate that the hydrogen capacity of the (Mg(NH₂)₂ + 2LiH) system declines from 5 wt.% to 3.45 wt.% after 6 cycles of hydrogenation and dehydrogenation, and can not restore to its initial level when use purified hydrogen again. The hydrogen desorption kinetics decreases obviously and the dehydrogenation activation energy increases from 133.35 kJ/mol to 153.35 kJ/mol. The main reason for these is that two new products Li₂CN₂ and MgO appear after (2LiNH₂ + MgH₂) react with CO. They are formed on the surface of materials particles, which may not only cause a permanent loss of NH²⁻, but also prevent the substance transmission during the reaction process. After re-mechanically milling, both kinetics and dehydrogenation activation energy can be recovered to the initial level.     

Hydrogen storage properties of nanostructured MgH2/TiH2 composite prepared by ball milling under high hydrogen pressure

Nanostructured MgH₂/0.1TiH₂ composite was synthesized directly from Mg and Ti metal by ball milling under an initial hydrogen pressure of 30 MPa. The synthesized composite shows interesting hydrogen storage properties. The desorption temperature is more than 100 °C lower compared to commercial MgH₂ from TG-DSC measurements. After desorption, the composite sample absorbs hydrogen at 100 °C to a capacity of 4 mass% in 4 h and may even absorb hydrogen at 40 °C. The improved properties are due to the catalyst and nanostructure introduced during high pressure ball milling. From the PCI results at 269, 280, 289 and 301 °C, the enthalpy change and entropy change during the desorption can be determined according to the van’t Hoff equation. The values for the MgH₂/0.1TiH₂ nano-composite system are 77.4 kJ mol⁻¹ H₂ and 137.5 J K⁻¹ mol⁻¹ H₂, respectively. These values are in agreement with those obtained for a commercial MgH₂ system measured under the same conditions. Nanostructure and catalyst may greatly improve the kinetics, but do not change the thermodynamics of the materials.     

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.     

Effect of Ni/tubular g-C3N4 on hydrogen storage properties of MgH2

Additive doping is one of the effective methods to overcome the shortcomings of MgH₂ on the aspect of relatively high operating temperatures and slow desorption kinetics. In this paper, hollow g-C₃N₄ (TCN) tubes with a diameter of 2 μm are synthesized through the hydrothermal and high-temperature pyrolysis methods, and then nickel is chemically reduced onto TCN to form Ni/TCN composite at 278 K. Ni/TCN is then introduced into the MgH₂/Mg system by means of hydriding combustion and ball milling. The MgH₂–Ni/TCN composite starts to release hydrogen at 535 K, which is 116 K lower than the as-milled MgH₂ (651 K). The MgH₂–Ni/TCN composite absorbs 5.24 wt% H₂ within 3500 s at 423 K, and takes up 3.56 wt% H₂ within 3500 s, even at a temperature as low as 373 K. The apparent activation energy (E_a) of the MgH₂ decreases from 161.1 to 82.6 kJ/mol by the addition of Ni/TCN. Moreover, the MgH₂–Ni/TCN sample shows excellent cycle stability, with a dehydrogenation capacity retention rate of 98.0% after 10 cycles. The carbon material enhances sorption kinetics by dispersing and stabilizating MgH₂. Otherwise, the phase transformation between Mg₂NiH₄ and Mg₂NiH_0.3 accelerates the re/dehydrogenation reaction of the composite.     

Effect of ball milling on structural and hydrogen storage properties of Mg - x wt% FeTi (x=2 & 5) solid solutions

The effects of milling time on structural and hydrogen storage properties of Mg-xwt% FeTi (x = 2 & 5) solid solutions prepared by ball milling have been investigated. The solid solutions were analyzed by X-ray Diffraction (XRD) and Scanning Electron Microscope (SEM). It is shown that the application of mechanical (ball) milling is a simple technique to produce nanocrystalline powder. A clear reduction of crystallite size with an increase of microstrain is observed with increasing milling time. Hydrogen absorption isotherms have been measured at pressures up to 9 bar and in a temperature range 553 K to 583 K. After 5 h milling results show maximum hydrogen storage 5.32 wt% for Mg-2wt% FeTi while 5.21 wt% for Mg-5wt% FeTi solid solutions and also found that the hydrogen absorption pressure of the ball-milled sample is lower than that of the as-prepared (0 h milling) sample.     

Effects of TiF3 addition on the hydrogen storage properties of 4MgH2 + Cd composite

In this paper, the best performance of the MgH₂ destabilized system with different ratios of Cd (1:1, 2:1, 3:1 and 4:1) have been studied for the first time. Remarkable enhancements on the onset dehydrogenation temperature, as well as the isothermal de/rehydrogenation kinetics were shown by the 4MgH₂ + Cd composite. In order to improve the hydrogen storage properties of the 4MgH₂ + Cd, TiF₃ was added and its catalytic effects were investigated. Temperature programmed dehydrogenation result had revealed that the onset dehydrogenation temperature was improved once the 10 wt% TiF₃ was incorporated into the 4MgH₂ + Cd system. The absorption and desorption kinetics were also improved compared to the un-doped 4MgH₂ + Cd composite system. The scanning electron microscope result had displayed that the 4MgH₂ + Cd + 10 wt% TiF₃ had the smallest particle size compared to the pure and the ball-milled MgH₂, as well as the 4MgH₂ + Cd composite system. The X-ray diffraction results had demonstrated the formation of an intermediate compound, Mg₃Cd, which was formed during the heating process. For the TiF₃-doped sample, it is reasonable to conclude that the in-situ formed TiH₂ and F-containing species play a synergetic role to encourage interactions between the MgH₂ and the Cd and thus further ameliorate the performances of the hydrogen storage of 4MgH₂ + Cd composite system.     

Effect of NaH/MgB2 ratio on the hydrogen absorption kinetics of the system NaH + MgB2

In this work the effect of the ratio of starting reactants on the hydrogen absorption reaction of the system xNaH + MgB₂ is investigated. At a constant hydrogen pressure of 50 bar, depending on the amount of NaH present in the system NaH + MgB₂, different hydrogen absorption behaviors are observed. For two system compositions: NaH + MgB₂ and 0.5NaH + MgB₂, the formation of NaBH₄ and MgH₂ as only crystalline hydrogenation products is achieved. The relation between the ratio of the starting reactants and the obtained hydrogenation products is discussed in detail.     

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 MWCNTs on improving the hydrogen storage performance of the Li3N system

In this paper, the influence of multi-walled carbon nanotube (MWCNT)-doping on the hydrogen storage properties of the Li₃N system was systematically investigated. Compared with the pure Li₃N sample, the MWCNT-doped Li₃N sample shows faster hydrogen absorption and desorption kinetics and a drastically improved cycling stability. Along with increasing MWCNT content, the hydrogen storage improvement becomes more apparent. When the MWCNT doping level reaches 10 mol%, the enhancement effect is significant. The improved hydrogen storage properties of the Li₃N system by MWCNT-doping can be reduced to the physical effect of ball milling, the increased specific surface area and large pores as well as the good thermal conductivity of MWCNTs.     

On the reversibility of hydrogen storage in Ti- and Nb-catalyzed Ca(BH4)2

The hydrogen sorption properties of calcium borohydride (Ca(BH₄)₂) catalyzed with a small amount of TiF₃, TiCl₃, NbF₅ or NbCl₅ are investigated using thermal analyses and X-ray diffraction. NbF₅ exhibits the best performance among all the catalysts; it causes a decrease in the hydrogen desorption temperature which leads to hydrogen absorption at practical temperature and pressure conditions. The hydrogen content of Ca(BH₄)₂ with NbF₅ reaches about 5.0 wt.% after hydrogen absorption at 693 K for 24 h under 90 bar of hydrogen. The main dehydrogenation product of Ca(BH₄)₂ with NbF₅ is a CaH_2−xF_x solid solution with a CaF₂ (C1) structure, while pure Ca(BH₄)₂ produces CaH₂ after hydrogen desorption.     

Convenient storage of concentrated hydrogen peroxide as a CaO2·2H2O2(s)/H2O2(aq) slurry for energy storage applications

Prior investigations have proposed, and successfully implemented, a stand-alone supply of aqueous hydrogen peroxide for use in fuel cells. An apparent obstacle for considering the use of aqueous hydrogen peroxide as an energy storage compound is the corrosive nature of the nominally required 50 wt.% maximum concentration. Here we propose storage of concentrated hydrogen peroxide in a high weight percent solid slurry, namely the equilibrium system of CaO₂·2H₂O₂(s)/H₂O₂(aq), that mitigates much of the risk associated with the storage of such high concentrations. We have prepared and studied surrogate slurries of calcium hydroxide/water that are assumed to resemble the peroxo compound slurries. These slurries have the consistency of a paste rather than a distinct two-phase (liquid plus solid) system. This paste-like property of the prepared surrogates enable them to be contained within a 200 lines-per-inch. (LPI) nickel mesh screen (33.6% open area) with no solids leakage, and only liquid transport driven by an adsorbent material is placed in physical contact on the exterior of the screen. This hydrogen peroxide slurry approach suggests a convenient and safe mechanism of storing hydrogen peroxide for use in, say, vehicle applications. This is because fuel cell design requires only aqueous hydrogen peroxide use, that can be achieved using the separation approach utilizing the screen material here. This proposed method of storage should mitigate hazards associated with unintentional spills and leakage issues arising from aqueous solution use.     

Characterizations of composite NaNH2-NaBH4 hydrogen storage materials synthesized via ball milling

Composite NaNH₂-NaBH₄ (molar ratio of 2/1) hydrogen storage materials are prepared by a ball milling method with various ball milling times. The compositions and hydrogen generation characteristics are investigated by means of X-ray diffraction (XRD) and thermo gravimetric-differential thermal analysis (TG-DTA). The structural characteristics imply that ball milling produces a new phase of Na₃(NH₂)₂BH₄, and mechanical energy accumulated in the ball milling process may be responsible for the phase change of Na₃(NH₂)₂BH₄. TG-DTA demonstrates that the phase change temperature of the composite NaNH₂-NaBH₄ (2/1) ball milled for 16 h is 141.8 °C, and the melting point is 197.3 °C; below 400 °C, composite hydrogen storage material is mainly decomposed to give hydrogen and Na₃BN₂; while above 400 °C, the previous by-product Na₃BN₂ continues to decompose so as to give metal Na gradually.     

The recent development on MgH2 system by 16 wt% nickel addition and particle size reduction through ball milling: A noticeable hydrogen capacity up to 5 wt% at low temperature and pressure

The addition of nickel as catalyst and particle size reduction through milling process is considered as the best approach for MgH₂ properties to be thermodynamically and kinetically more favorable. The recent development in MgH₂ is done by adding Ni (14–16 wt%) and particle size reduction through simple mechanical milling at a large sample of 100 g. The research sequences are easy to adopt for the implementation and sustainable research of MgH₂. Starting from the moisture test to the milling process, continue with particle size distribution (PSD) for the milled sample and final moisture test after PSD. Some critical finding from our research includes high capacity storage of Mg₈₄:Ni₁₆ above 5 wt% H₂ within 20 min at 573 K, the effect of moisture content on system performance and the different effect of carbon (C) in the system at specific temperature and pressure that may have.     

The effect of ball-milling gas environment on the sorption kinetics of MgH2 with/without additives for hydrogen storage

A study of the effect of the ball-milling gas environment on the kinetic enhancement of MgH₂ with different additives was conducted using argon and hydrogen. The as-sourced MgH₂ was milled for 20 h and then milled for a further 2 h after adding 1–2 mol% of one of the additives titanium isopropoxide, niobium oxide or carbon buckyballs, varying the gas environment for both ball-milling stages. The milling environment had little or no effect on the desorption kinetics in most cases. However, in some cases, the absorption uptake differed by up to 2 wt%, depending on the gas used_. This effect was not consistent among the composite samples surveyed, demonstrating the importance of reporting all information about the ball-milling processes used, including the gas environment.     

Mechanistic investigations on significantly improved hydrogen storage performance of the Ca(BH4)2-added 2LiNH2/MgH2 system

The hydrogen storage properties and mechanisms of the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system were systematically investigated. The results showed that the addition of Ca(BH₄)₂ pronouncedly improved hydrogen storage properties of the 2LiNH₂–MgH₂ system. The onset temperature for dehydrogenation of the 2LiNH₂–MgH₂–0.3Ca(BH₄)₂ sample is only 80 °C, a ca. 40 °C decline with respect to the pristine sample. Further hydrogenation examination indicated that the dehydrogenated 2LiNH₂–MgH₂–0.1Ca(BH₄)₂ sample could absorb ca. 4.7 wt% of hydrogen at 160 °C and 100 atm while only 0.8 wt% of hydrogen was recharged into the dehydrogenated pristine sample under the same conditions. Structural analyses revealed that during ball milling, a metathesis reaction between Ca(BH₄)₂ and LiNH₂ firstly occurred to convert to Ca(NH₂)₂ and LiBH₄, and then, the newly developed LiBH₄ reacted with LiNH₂ to form Li₄(BH₄)(NH₂)₃. Upon heating, the in situ formed Ca(NH₂)₂ and Li₄(BH₄)(NH₂)₃ work together to significantly decrease the operating temperatures for hydrogen storage in the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system.     

Hydrogen storage properties of Ti2−xCrVMx (M = Fe,Co, Ni) alloys

In this work, quaternary alloys having compositions Ti_1.9CrVM_0.1 and Ti_1.8CrVM_0.2 (M = Fe, Co and Ni) have been studied in detail for their structural aspects and hydrogen absorption–desorption properties. All the alloys form bcc phase solid solutions and after hydrogen absorption the structures change to fcc. The pressure composition isotherms, hydrogen storage capacities and hydrogen absorption kinetics were studied using Sievert's type of volumetric setup. The Ti_1.9CrVFe_0.1, Ti_1.9CrVCo_0.1 and Ti_1.9CrVNi_0.1 alloys are found to absorb maximum 3.80, 3.68 and 3.91 wt.% of hydrogen respectively; whereas, Ti_1.8CrVCo_0.2 and Ti_1.8CrVNi_0.2 alloys show 3.52 and 3.67 wt.% of hydrogen at room temperature. All the alloys show fast hydrogen absorption kinetics at the room temperature. From differential scanning calorimetric measurements, it has been found that Fe, Ni and Co substitution in place of Ti decreased the hydrogen desorption temperature drastically compared to the parent alloy.     

Superior hydrogen storage properties of Mg95Ni5 + 10 wt.% nanosized Zr0.7Ti0.3Mn2 + 3 wt.% MWCNT prepared by hydriding combustion synthesis followed by mechanical milling (HCS + MM)

Significant improvement of the hydrogen storage property of the magnesium-based materials was achieved by the process of hydriding combustion synthesis (HCS) followed by mechanical milling (MM) and the addition of nanosized Zr_0.7Ti_0.3Mn₂ and MWCNT. Mg₉₅Ni₅ doped by 10 wt.% nanosized Zr_0.7Ti_0.3Mn₂ and 3 wt.% MWCNT prepared by the process of HCS + MM absorbed 6.07 wt.% hydrogen within 100 s at 373 K in the first hydriding cycle and desorbed 95.1% hydrogen within 1800 s at 523 K. The high hydriding rate remained well and the hydrogen capacity reached 5.58 wt.% within 100 s at 423 K in the 10th cycle. The dehydrogenation activation energy of this system was 83.7 kJ/mol, which was much lower than that of as-received MgH₂. A possible hydrogenation–dehydrogenation mechanism was proposed in terms of the structural features derived from the HCS + MM process and the synergistic catalytic effects of nanosized Zr_0.7Ti_0.3Mn₂ and MWCNT.     

Effect of ball milling time on the hydrogen storage properties of TiF3-doped LiAlH4

In the present work, the catalytic effect of TiF₃ on the dehydrogenation properties of LiAlH₄ has been investigated. Decomposition of LiAlH₄ occurs during ball milling in the presence of 4 mol% TiF₃. Different ball milling times have been used, from 0.5 h to 18 h. With ball milling time increasing, the crystallite sizes of LiAlH₄ get smaller (from 69 nm to 43 nm) and the dehydrogenation temperature becomes lower (from 80 °C to 60 °C). Half an hour ball milling makes the initial dehydrogenation temperature of doped LiAlH₄ reduce to 80 °C, which is 70 °C lower than as-received LiAlH₄. About 5.0 wt.% H₂ can be released from TiF₃-doped LiAlH₄ after 18 h ball milling in the range of 60 °C–145 °C (heating rate 2 °C min⁻¹). TiF₃ probably reacts with LiAlH₄ to form the catalyst, TiAl₃. The mechanochemical and thermochemical reactions have been clarified. However, the rehydrogenation of LiAlH₄/Li₃AlH₆ can not be realized under 95 bar H₂ in the presence of TiF₃ because of their thermodynamic properties.