Hydrogen storage studies in Pd/Ti/Mg films

This paper describes investigations (a) on the efficacy of Ti layer as a barrier against the intermixing of Pd and Mg in Pd/Ti/Mg films and (b) the hydrogen storage characteristics of the tri-layered films and the related bulk composites. The Mg film was prepared by resistive evaporation while the Pd and Ti films were deposited by e-beam evaporation. The analysis by Rutherford backscattering spectrometry (RBS) and glancing-incidence X-ray diffraction (GI-XRD) of the Pd/Ti/Mg/Si(substrate) films annealed in vacuum in 348–573 K temperature range revealed that Ti effectively prevents the intermixing of Pd and Mg up to ～523 K. However, mixing across Pd/Ti, Ti/Mg and Mg/Si interfaces commences around 523 K that progresses with the temperature of annealing though PdMg phases are not formed even at 573 K. The as-deposited Pd/Ti/Mg films are hydrogenated to ～7 wt % (62 at%) at 323–423 K at 0.15 MPa hydrogen pressure and dehydrogenated completely at ～ 473 K. The extent of (de)hydrogenation of the films was determined non-destructively by the ¹H(¹⁹F,αγ)¹⁶O nuclear reaction. The powder composites derived from the films, on the other hand, reversibly stored ～2.2 wt% hydrogen up to 18 cycles in 323–473 K temperature range. The superior cyclic stability is attributed to the inhibition of mixing between Pd and Mg and, as a result, the formation of PdMg inter-metallics by titanium.     

Activated carbons doped with Pd nanoparticles for hydrogen storage

Three activated carbons (ACs) having apparent surface areas ranging from 2450 to 3200 m²/g were doped with Pd nanoparticles at different levels within the range 1.3–10.0 wt.%. Excess hydrogen storage capacities were measured at 77 and 298 K at pressures up to 8 MPa. We show that hydrogen storage at 298 K depends on Pd content at pressures up to 2–3 MPa, below which the stored amount is low (<0.2 wt.%). At higher pressures, the micropore volume controls H₂ storage capacity. At 77 K, Pd doping has a negative effect on hydrogen storage whatever the pressure considered. From N₂ adsorption at 77 K, TPR, XRD, TEM, and H₂ chemisorption studies, we concluded that: (i) Pd particles remained mainly decorating the outer surface of the ACs; (ii) increasing Pd content produced an increase of the metal particle size; (iii) ACs with higher surface area produced smaller metallic nanoparticles at a given Pd content.     

Hydrogenation-induced microstructure evolution in as cast and severely deformed Mg-10 wt.% Ni alloy

We determined the kinetics of hydrogen absorption of the hypoeutectic Mg-10 wt.% Ni alloy in the as-cast state and after processing by four passes of equal channel angular pressing (ECAP). While during the first hydrogenation cycle the ECAP-modified alloy exhibited faster absorption than its as-cast counterpart, this advantage was lost after the second hydrogenation cycle; parity was regained after six cycles. We attributed these differences in the hydrogen absorption kinetics to the formation of large (tens of micrometers) faceted Mg crystals observed during the first hydrogenation cycle. These crystals were significantly larger in the ECAP-modified alloy than in its as-cast counterpart. We discussed the growth of large Mg crystals during hydrogenation in terms of self-diffusion of Mg atoms driven by the metal-hydride transformation stress. The larger size of these crystals in the ECAP-processed alloy was attributed to the acceleration of diffusion by ECAP. Our metallographic studies revealed a number of microstructural changes in the alloys upon hydrogenation, such as cracking, accumulation of plastic strain in large Mg crystals, and re-distribution of the dispersed particles of Mg₂Ni phase in the partly hydrogenated alloys.     

Hydrogen storage properties of Mg-10 wt% Ni alloy co-catalysed with niobium and multi-walled carbon nanotubes

Mg-10 wt% Ni alloys containing up to 1 wt% Nb were fabricated by a casting technique, followed by ball-milling with 5 wt% multi-walled carbon nanotubes. Further mechanical alloying with 1.5, 3, and 5 at % Nb was applied to a cast Mg-10 wt% Ni-370 ppm Nb alloy to investigate the catalytic role of Nb in hydrogen dissociation. The microstructure and distribution of Nb and Mg₂Ni in the alloys were characterised by SEM. The absorption and desorption kinetics of the samples were measured by Sieverts’ apparatus at various temperatures. The results show that addition of Nb during casting accelerates the hydrogen diffusion compared to the cast binary Mg-10 wt% Ni alloy. Moreover, ball-milling of the alloy with metallic niobium leads to the formation of BCC phase of Mg-Nb solid solution, which significantly improves the hydrogenation properties of the alloy. DSC results show that mechanical alloying of Mg-10 wt%Ni-370 ppm Nb with Nb in excess of 1.5 wt% decreases the desorption temperature by approximately 100 °C compared to the ball-milled cast alloy.     

Ball-milling preparation of titanium/graphene composites and its enhanced hydrogen storage ability

Mass pure Ti/graphene (GE) composites have been synthesized by direct ball-milling Ti powders and GE nanosheets. Through adjusting the reaction conditions, Ti–C bonding in Ti/GE composites was constructed on the interface between Ti nanoparticles and GE nanosheets, which was also confirmed by the following Raman and XPS measurements. Pressure-composition-isotherm (PCT) curves showed that the Ti/GE composites exhibited a higher hydrogen storage capacity (4.3 wt% for H₂ at 300 °C, 0.08 bar) and lower equilibrium pressure compared with that of simply physically mixed sample. In addition, the hydrogen adsorption process and the mechanism for the Ti/GE composites have also been investigated, in which a Kubas-enhanced adsorption process was found in the composites. In this Kubas-type adsorption process, hydrogen was adsorbed firstly by Ti atoms with a formation of Ti–H bonding and then the bonded hydrogen was spilled over to graphene, forming a C–H interaction.     

Inorganic nanotube composites based on polyaniline: Potential room-temperature hydrogen storage materials

Inorganic nanotubes as a support material for polyaniline were used for hydrogen storage. To this end, a solid-state preparation method has been developed for controlling the hydrogen storage capacity of these nanocomposites. The hydrogen storage capacities have measured at room temperature and at a low pressure of 0.5 MPa using the precise quadrupole quartz crystal microbalance technique in a chamber free of moisture. The optimum nanocomposite shows an enhanced hydrogen storage capacity of 0.78 wt.% with excellent reversibility when compared to less than 0.019 wt.% capacity of the pristine nanotubes and 0.05 wt.% of polyaniline. This large increase in the hydrogen capacity can be attributed to the chemisorption hydrogen uptake, which was enhanced by the sorption sites created through the milling process of polyaniline with the nanotubes. This is in addition to the hydrogen adsorption contribution by a controlled lumen size that is suitable for a maximum hydrogen adsorption through inserting polyaniline chains into the nanotubes.     

Effect of Al and Mo substitution on the structural and hydrogen storage properties of CaNi5

A simple mechanical milling and annealing process has been used to synthesize CaNi₅-based hydrogen storage alloys. Heat treatment at 800 °C under vacuum results in the formation of a crystalline CaNi₅ phase. Secondary phases, including Ca₂Ni₇ and Mo–Ni, are formed when substituting Mo for Ni. Replacement of Ni by Al or Mo leads to an increase in the unit cell volume of the CaNi₅ phase. The hydrogen storage capacity of all substituted alloys is reduced and the plateau pressures are lower than those of pure CaNi₅. Fairly flat plateau regions are retained for all compositions except the CaNi_4.8Mo_0.2 composition where a Ca₂Ni₇ phase is dominant. The incorporation of Mo also causes slow sorption kinetics for the CaNi_4.9Mo_0.1 alloy. CaNi_4.9Al_0.1 maintains its initial hydrogen absorption capacity for 20 cycles performed at 85 °C but the other substituted alloys lose their capacity rapidly, especially the CaNi_4.8Mo_0.2 composition.     

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.     

Pellets of MgH2-based composites as practical material for solid state hydrogen storage

Hydrogenation properties and mechanical stability of pellets made starting from compressed ball-milled MgH₂ powders mixed with catalysts (Nb₂O₅ and graphite) and a binding agent (aluminium powder) have been investigated. Structural characterization with X-ray diffraction and gas–solid reaction kinetic and thermodynamic tests with a Sievert's apparatus have been done on the samples up to 50 hydrogen absorption/desorption (a/d) cycles. The best cycling behaviour and mechanical strength stability have been observed for pellets of catalysed MgH₂ powders added with 5 wt% aluminium annealed in vacuum at 450 °C before starting the a/d cycles. This mechanical stability to cycles has been attributed to the formation of a solid solution of aluminium in magnesium.     

Investigation of hydrogen storage capacity of multi-walled carbon nanotubes deposited with Pd or V

This work presents the synthesis and characterization of multi-walled carbon nanotubes (multi-walled CNTs) deposited with Pd or V and their hydrogen storage capacity measured by Sievert's volumetric apparatus. The CNTs were grown by the CVD method using LPG and LaNi₅ as the carbon source and catalyst, respectively. Pd was impregnated on the CNTs by the reflux method with hydrogen gas as a reducing agent, while V was embedded on the CNTs by the vapor deposition method. The average metal particle size deposited on the CNTs was around 5.8 nm for Pd and 3.6 nm for V. Hydrogen adsorption experiments were performed at room temperature and at −196 °C under a hydrogen pressure of 65 bar. At −196 °C, the treated CNTs had a maximum hydrogen uptake of 1.21 wt%, while the CNTs deposited with Pd (Pd-CNTs) and CNTs deposited with V (V-CNTs) possessed lower surface areas, inducing lower hydrogen adsorption capacities of 0.37 and 0.4 wt%, respectively. For hydrogen sorption at room temperature, the CNTs decorated with the metal nanoparticles had a higher hydrogen uptake compared to the treated CNTs. Hydrogen adsorption capacity was 0.125 and 0.1 wt% for the Pd-CNTs and V-CNTs, respectively, while the hydrogen uptake of the treated CNTs was <0.01 wt%. For the second cycle, only half of the first hydrogen uptake was obtained, and this was attributed to the re-crystallization of the defect sites on the carbon substrate after the first hydrogen desorption.     

Simulation of hydrogen trapping at defects in Pd

The interaction of hydrogen with defects in palladium was studied using qualitative electronic structure calculations in the framework of the Atom Superposition and Electron Delocalization Molecular Orbital (ASED-MO) theory. Interatomic distances, energies and electronic structure for hydrogen at a dislocation and at a vacancy were determined and compared with that for an octahedral site in the Pd fcc lattice. We found that a repulsive H–H interaction is developed if these atoms occupy interstitial sites in a regular lattice. However, when the H atoms are close to a dislocation, the accumulation becomes possible. It was found that the dislocation allows hydrogen association at interatomic distances close to molecular hydrogen. The Density of States (DOS) and Crystal Orbital Overlap Population (COOP) curves were used to shed more light on the interstitial-Pd-defect interactions. Hydrogen produce changes in the local and global electronic structure of the host metal and induce changes in the cohesive forces between atoms in the host matrix. In all cases, it was found that strong bonds between Pd and H atoms are formed while metal–metal bonds are weakened. The metal–hydrogen bonds were characterized by charge transfer from metal atoms to hydrogen.     

In situ deposition of Pd nanoparticles with controllable diameters in hollow carbon spheres for hydrogen storage

A novel in situ synthesis of Pd nanoparticles supported in hollow carbon spheres (HCS) is reported. The size of the nanoparticles can be tuned via application of different Pd precursors. The hydrogen storage properties of Pd supported in HCS under room temperature were examined at partial pressures. We observed significant difference between the storage capacities of two samples containing Pd nanoparticles with different diameter distributions. The results showed that the sample with suitable diameters of Pd nanoparticles was more favorable for the H₂ storage, even lower mass of Pd was used. The maximum hydrogen storage of 0.36 wt % exhibited the sample with Pd nanoparticles with the diameter of 11 nm (measured at 298 K and 24 bar) and it was enhanced by the factor of two in respect to the pristine HCS. The enhanced storage capacity is due to cumulative hydrogen adsorption by HCS and Pd nanoparticles. We also propose the mechanism of hydrogen storage in our material.     

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.     

Reduction and unusual recovery in the reversible hydrogen storage capacity of V1−xTix during hydrogen cycling

The effect of the vanadium content on the cyclic stability of V–Ti binary alloys was investigated. V_1−xTi_x, x = 0.2 and 0.5 samples were hydrogenated and dehydrogenated at 410 K and 553 K respectively, for more than 100 times. During hydrogen cycling, reduction in the reversible hydrogen storage capacity was clearly observed from both samples. No prominent V-effect was found. In fact, the reduction rates of two samples were similar; both samples showed a ∼25% reduction in the reversible hydrogen storage capacity after 100 cycles. In addition, the shape of the pressure–composition-isotherm (PCT) curves was significantly altered over the testing cycle period; the absorption and desorption plateaus got markedly inclined and the hysteresis became evidently smaller. We found that even after the hydrogen storage capacity of V_1−xTi_x was significantly reduced, at low enough temperature V_1−xTi_x was able to absorb hydrogen as much as it did at the first cycle. Furthermore, the reversible hydrogen storage capacity of V_0.8Ti_0.2 at 410 K was recovered to a certain degree after hydrogenating the sample at low temperatures.     

Reversible hydrogen storage in electrospun polyaniline fibers

Due to its many physisorption sites as well as chemisorption sites, polyaniline (PANI) has been investigated for hydrogen storage purposes. The PANI was produced in house via traditional chemical synthesis methods and then electrospun to produce fibers. These PANI fibers were investigated and compared with standard bulk PANI and found to be stable up to 150 °C. When investigating PANI fibers, using PCT measurements, it was found that a reversible hydrogen storage capacity of ∼3–10 wt.% could be obtained at different temperatures. Hydrogen kinetic sorption measurements in prolonged cycles (up to 66 cycles) reveal an uptake and release of >6–10 wt.% on these PANI materials. The importance of the type of measurement is discussed as to its effect on the morphology and structure of the PANI nanofibers. The surface morphologies before and after hydrogen sorption on these PANI fibers encompass significant changes in the microstructure (nanofibrallar swelling effect). Detailed chemical and physical characterization of the PANI fibers is reported as part of this work.     

Hydriding/dehydriding properties of MgH2/5 wt.% Ni coated CNFs composite

In this paper, we reported that the prepared nickel coated carbon nanofibers (NiCNFs) by electroless plating method exhibited superior catalytic effect on hydrogen absorption/desorption of magnesium (Mg). It is demonstrated that the nanocomposites of MgH₂/5 wt.% NiCNFs prepared by ball milling could absorb hydrogen very fast at low temperatures, e.g. absorb ∼6.0 wt.% hydrogen in 5 min at 473 K and ∼5.0 wt.% hydrogen in 10 min even at a temperature as low as 423 K. More importantly, the desorption of hydrogen was also significantly improved with additives of NiCNFs. Diffraction scanning calorimetry (DSC) measurement indicated that the peak desorption temperature decreased 50 K and the on-set temperature for desorption decreased 123 K. The composites also desorbed hydrogen fast, e.g. desorb 5.5 wt.% hydrogen within 20 min at 573 K. It is suggested that the new phase of Mg₂Ni, and the nano-sized dispersed distribution of Ni and carbon contributed to this significant improvement. Johnson–Mehl–Avrami (JMA) analysis illustrated that hydrogen diffusion is the rate-limiting step for hydrogen absorption/desorption.     

Enhanced room-temperature hydrogen storage capacity in Pt-loaded graphene oxide/HKUST-1 composites

Series of Pt-loaded graphene oxide (GO)/HKUST-1 composites were synthesized by the reaction between Pt@GO and precursors of HKUST-1. The parent materials and composites have been characterized by powder X-ray diffraction (XRD), Infrared (IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and gas adsorption analyzer. The XRD and IR analysis showed that the incorporation of Pt@GO did not prevent the formation of HKUST-1 units. SEM, TEM and EDS results revealed that Pt nanoparticles were well-dispersive and anchored tightly into composites. Meanwhile, the percentage of Pt@GO has an obvious effect on morphologies, crystallinities and surface areas of composites. More importantly, the significant enhancement of hydrogen storage capacity at ambient temperature for the composite with low Pt@GO content can be ascribed to the hydrogen spillover mechanism in such system.     

Enhanced hydrogen storage capacity of Pt-loaded CNT@MOF-5 hybrid composites

We report on an easy synthesis method for the preparation of a hybrid composite of Pt-loaded MWCNTs@MOF-5 [Zn₄O(benzene-1,4-dicarboxylate)₃] that greatly enhanced hydrogen storage capacity at room temperature. To prepare the composite, we first prepared Pt-loaded MWCNTs, which were then incorporated in-situ into the MOF-5 crystals. The obtained composite was characterized by various techniques such as powder X-ray diffractometry, optical microscopy, porosimetry by nitrogen adsorption, and hydrogen adsorption. The analyses confirmed that the product has a highly crystalline structure with a Langmuir specific surface area of over 2000 m²/g. The hybrid composite was shown to have a hydrogen storage capacity of 1.25 wt% at room temperature and 100 bar, and 1.89 wt% at cryogenic temperature and 1 bar. These H₂ storage capacities represent significant increases over those of virgin MOF-5s and Pt-loaded MWCNTs.     

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.     

MgCNi3 prepared by powder metallurgy for improved hydrogen storage properties of MgH2

With the depletion of global energies resources, improvement of hydrogen storage properties of materials like MgH₂ is of great interest for future efficient renewable resources. In this study, a novel antiperovskite MgCNi₃ was synthesized by powder metallurgy then introduced into Mg to fabricate MgMgCNi₃ composite. The hydrogen storage properties of the obtained MgMgCNi₃ composite were evaluated. MgMgCNi₃ showed a high capacity of hydrogen storage and fast kinetics of hydrogen uptake/release at relatively low temperatures. About 4.42 wt% H₂ was absorbed within 20 min at 423 K, and 4.81 wt% H₂ was reversibly released within 20 min at 593 K. By comparison, milled MgH₂ absorbed only 0.99 wt% H₂ and hardly underwent any hydrogen evolution under the same conditions. In addition, MgMgCNi₃ composite showed outstanding cycling stability, with hydrogen absorption capacity retention rates reaching 98% after ten cycles at 623 K. The characterization analyses revealed that MgCNi₃ and Mg formed Mg₂NiH₄ hydride and carbonaceous material during hydrogenation, where Mg₂NiH₄ induced dehydrogenation of MgH₂ and carbon played a dispersive role during the composite reaction. Both features synergistically benefited the hydrogen storage properties of MgH₂.