Rehydrogenation performance of an MgH2–Nb2O5 system modified by heptane and acetone

An MgH₂ + 1 mol% Nb₂O₅ system was modified by heptane and acetone through a high-energy ball milling process, and their rehydrogenation performances were investigated. XRD results indicated that except MgH₂ and Nb₂O₅ phases Mg and MgO phases existed after ball milling. The rehydrogenation results showed that after modification by heptane the capacity increased from 3.0 wt% and 4.2 wt% to 5.0 wt% and 5.5 wt% within 110 s at 523 K and 573 K, respectively. The hydriding rate increased from 0.08 wt%/s after 20 s to 0.22 wt%/s after 10 s at 523 K. However, after modification by acetone it only absorbed 1.8 wt% and 2.0 wt% of hydrogen even within 8000 s at 523 K and 573 K, respectively. Rietveld refinement results indicated that after modification by the heptane the content of MgO was reduced from 6.8 wt% to 4.2 wt%, while after the modification by the acetone the content of MgO was significantly increased from 6.8 wt% to 23.8 wt%. The difference in the rehydrogenation performance was believed to be attributed to the different contents of the MgO phase, which led to the difference in the contents of the MgH₂ phase. It implied that the heptane acted as a solvent without oxygen element in it to prevent the MgH₂ + Nb₂O₅ system from aggregation, crystallization and oxidation. It suggested heptane was suitable for the improvement of the rehydrogenation performance of MgH₂ system.     

Morphological effects of Nb2O5 on Mg–MgH2 system for thermal energy storage application

The solar thermal energy could be stored and reused at the desired locations and conditions. The requirement is to develop a suitable medium which could be able to store the solar thermal energy reversibly. Metal-metal hydride system may be one of the options. The high hydrogen storage capacity and fast hydrogenation-dehydrogenation kinetics are the desirable conditions for the practical applications. In this regard, magnesium – magnesium hydride (Mg-MgH₂) system has been considered as a potential candidate. The prerequisite is to fine tune the hydrogenation-dehydrogenation kinetics by a suitable additive. Niobium pentoxide (Nb₂O₅) has been recognized as a suitable additive to improve the hydrogenation-dehydrogenation kinetics. The Nb₂O₅ is found in various morphological forms, however; all of them are not suitable to enhance the hydrogenation-dehydrogenation kinetics of Mg-MgH₂ system. In the current study, it has been found that the mesoporous amorphous Nb₂O₅ has shown remarkable catalytic property on the Mg-MgH₂ system as compared to crystalline Nb₂O₅ under the similar experimental conditions.     

Nanostructured rapidly solidified LaMg11Ni alloy. II. In situ synchrotron X-ray diffraction studies of hydrogen absorption–desorption behaviours

Recently, the present authors [17] have reported dramatic improvements in the hydrogenation behaviours of nanostructured LaMg₁₁Ni prepared by Rapid Solidification, caused by modifications of the microstructure and crystal structure. The aim of the present work was to study the mechanism and kinetics of the hydrogen interaction with rapidly solidified LaMg₁₁Ni by employing in situ synchrotron X-Ray diffraction studies of hydrogen absorption–desorption processes in hydrogen gas or in vacuum.     

Synergetic effect of C (graphite) and Nb2O5 on the H2 sorption properties of the Mg–MgH2 system

Ternary Mg–Nb₂O₅ – graphitic C mixtures (molar ratio % = 97.5:0.5:2.0) were prepared by high-energy ball milling (BM) under Ar for different times (from 0.25 h to 4 h) and thoroughly characterized by manometric, calorimetric, X-ray powder diffraction, and scanning electron microscopy analyses. The aims of the work were: - to assess the effect of the simultaneous presence of the two dopants on the reactivity and the sorption properties of the Mg–MgH₂ system; - to study the influence of the milling time on the performance of the mixtures.     

Bulk nanocomposite MgH2/10 wt% (8 Nb2O5/2 Ni) solid-hydrogen storage system for fuel cell applications

Hydrogen, which holds tremendous promise as a new clean energy option is considered as an efficient source of primary energy. Unluckily, hydrogen storage presents the most crucial difficulty restricting utilization of hydrogen energy for real applications. However, Mg metal is the best known cheap solid-state hydrogen storage media with high hydrogen capacity and operational cost effectiveness; it shows high thermal stability and poor hydrogenation/dehydrogenation kinetics. In the present work we have succeeded to prepare nanocrystalline MgH₂ powders doped with a mixture of 8 wt% Nb₂O₅/2 wt% Ni nanocatalytic system. The synthesized nanocomposite powders possessed superior hydrogenation/dehydrogenation kinetics (2.6 min/3 min) at relatively low temperature (250 °C) with long cycle-life-time (400 h). The powders were consolidated into green-compacts, using cold pressing technique. The compacts were utilized as solid-state hydrogen source needed for charging a battery of a cell-phone device, using integrated Ti-tank/commercial proton-exchange membrane fuel cell system.     

Hydrogen-storage properties of gravity cast and melt spun Mg–Ni–Nb2O5 alloys

95%(gravity cast Mg–23.5Ni)–-5%Nb₂O₅ alloy was prepared by horizontal ball milling in n-hexane of gravity cast Mg–23.5wt%Ni with Spex milled Nb₂O₅. Melt spun Mg–23.5wt%Ni after heat treatment at 523 K for 1 h was also ground by planetary ball milling with finer Nb₂O₅ prepared by milling with NaCl. The activated 90%(melt spun Mg–23.5Ni)–10%Nb₂O₅ alloy shows higher hydriding and dehydriding rates than the activated 95%(gravity cast Mg–23.5Ni)–5%Nb₂O₅ alloy, thanks to the homogeneous distribution of fine Mg₂Ni phase in melt spun Mg–23.5Ni and the finer Nb₂O₅ addition to melt spun Mg–23.5Ni, which leads to the effective diminution of the Mg particle size. The activated 90%(ms Mg–23.5Ni)–10%Nb₂O₅ alloy absorbs 4.70 wt%H at 573 K under 12 bar H₂ for 10 min, and desorbs 4.75 wt%H at 573 K under 1.0 bar H₂ for 25 min.     

Effective nanoconfinement of 2LiBH4–MgH2 via simply MgH2 premilling for reversible hydrogen storages

To improve nanoconfinement of LiBH₄ and MgH₂ in carbon aerogel scaffold (CAS), particle size reduction of MgH₂ by premilling technique before melt infiltration is proposed. MgH₂ is premilled for 5 h prior to milling with LiBH₄ and nanoconfinement in CAS to obtained nanoconfined 2LiBH₄–premilled MgH₂. Significant confinement of both LiBH₄ and MgH₂ in CAS, confirmed by SEM–EDS–mapping results, is achieved due to MgH₂ premilling. Due to effective nanoconfinement, enhancement of CAS:hydride composite weight ratio to 1:1, resulting in increase of hydrogen storage capacity, is possible. Nanoconfined 2LiBH₄–premilled MgH₂ reveals a single–step dehydrogenation at 345 °C with no B₂H₆ release, while dehydrogenation of nanoconfined sample without MgH₂ premilling performs in multiple steps at elevated temperatures (up to 430 °C) together with considerable amount of B₂H₆ release. Activation energy (E_A) for the main dehydrogenation of nanoconfined 2LiBH₄–premilled MgH₂ is considerably lower than those of LiBH₄ and MgH₂ of bulk 2LiBH₄–MgH₂ (ΔE_A = 31.9 and 55.8 kJ/mol with respect to LiBH₄ and MgH₂, respectively). Approximately twice faster dehydrogenation rate are accomplished after MgH₂ premilling. Three hydrogen release (T = 320 °C, P(H₂) = 3–4 bar) and uptake (T = 320–325 °C, P(H₂) = 84 bar) cycles of nanoconfined 2LiBH₄–premilled MgH₂ reveal up to 4.96 wt. % H₂ (10 wt. % H₂ with respect to hydride composite content), while the 1st desorption of nanoconfined sample without MgH₂ premilling gives 4.30 wt. % of combined B₂H₆ and H₂ gases. It should be remarked that not only kinetic improvement and B₂H₆ suppression are obtained by MgH₂ premilling, but also the lowest dehydrogenation temperature (T = 320 °C) among other modified 2LiBH₄–MgH₂ systems is acquired.     

An efficient ZnS-UV photocatalysts generated in situ from ZnS(en)0.5 hybrid during the H2 production in methanol–water solution

Highly active ZnS-UV was obtained in situ from ZnS(en)_0.5 hybrid during the hydrogen formation using a methanol–water solution under UV irradiation. X-ray diffraction patterns and UV spectroscopy for both ZnS-UV and ZnS-400 obtained from the calcination of the ZnS(en)_0.5 hybrid showed similar structural and photophysical properties; however, the efficiency of the ZnS-UV semiconductor was 7 times higher (4825 μmol h⁻¹ g⁻¹) compared to the ZnS-400. The highest H₂ production was obtained using a UV lamp of very low intensity (2.2 mW cm⁻¹) and it is attributed to a quantum size effect caused by the slow elimination of ethylenediamine (en) in the structural ZnS layer during the UV irradiation.     

Improved hydrogen storage performance of the LiNH2–MgH2–LiBH4 system by addition of ZrCo hydride

Significant improvements in the hydrogen absorption/desorption properties of the 2LiNH₂–1.1MgH₂–0.1LiBH₄ composite have been achieved by adding 3wt% ZrCo hydride. The composite can absorb 5.3wt% hydrogen under 7.0 MPa hydrogen pressure in 10 min and desorb 3.75wt% hydrogen under 0.1 MPa H₂ pressure in 60 min at 150 °C, compared with 2.75wt% and 1.67wt% hydrogen under the same hydrogenation/dehydrogenation conditions without the ZrCo hydride addition, respectively. TPD measurements showed that the dehydrogenation temperature of the ZrCo hydride-doped sample was decreased about 10 °C compared to that of the pristine sample. It is concluded that both the homogeneous distribution of ZrCo particles in the matrix observed by SEM and EDS and the destabilized N–H bonds detected by IR spectrum are the main reasons for the improvement of H-cycling kinetics of the 2LiNH₂–1.1MgH₂–0.1LiBH₄ system.     

Improved hydrogen storage performance of MgH2–NaAlH4 composite by addition of TiF3

In a previous paper, it was demonstrated that a MgH₂–NaAlH₄ composite system had improved dehydrogenation performance compared with as-milled pure NaAlH₄ and pure MgH₂ alone. The purpose of the present study was to investigate the hydrogen storage properties of the MgH₂–NaAlH₄ composite in the presence of TiF₃. 10 wt.% TiF₃ was added to the MgH₂–NaAlH₄ mixture, and its catalytic effects were investigated. The reaction mechanism and the hydrogen storage properties were studied by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry (DSC), temperature-programmed-desorption and isothermal sorption measurements. The DSC results show that MgH₂–NaAlH₄ composite milled with 10 wt.% TiF₃ had lower dehydrogenation temperatures, by 100, 73, 30, and 25 °C, respectively, for each step in the four-step dehydrogenation process compared to the neat MgH₂–NaAlH₄ composite. Kinetic desorption results show that the MgH₂–NaAlH₄–TiF₃ composite released about 2.4 wt.% hydrogen within 10 min at 300 °C, while the neat MgH₂–NaAlH₄ sample only released less than 1.0 wt.% hydrogen under the same conditions. From the Kissinger plot, the apparent activation energy, E_A, for the decomposition of MgH₂, NaMgH₃, and NaH in the MgH₂–NaAlH₄–TiF₃ composite was reduced to 71, 104, and 124 kJ/mol, respectively, compared with 148, 142, and 138 kJ/mol in the neat MgH₂–NaAlH₄ composite. The high catalytic activity of TiF₃ is associated with in situ formation of a microcrystalline intermetallic Ti–Al phase from TiF₃ and NaAlH₄ during ball milling or the dehydrogenation process. Once formed, the Ti–Al phase acts as a real catalyst in the MgH₂–NaAlH₄–TiF₃ composite system.     

Raman spectroscopic observation of dehydrogenation in ball-milled LiNH2–LiBH4–MgH2 nanoparticles

In situ Raman spectroscopy was used to monitor the dehydrogenation of ball-milled mixtures of LiNH₂–LiBH₄–MgH₂ nanoparticles. The as-milled powders were found to contain a mixture of Li₄BN₃H₁₀ and Mg(NH₂)₂, with no evidence of residual LiNH₂ or LiBH₄. It was observed that the dehydrogenation of both of Li₄BN₃H₁₀ and Mg(NH₂)₂ begins at 353 K. The Mg(NH₂)₂ was completely consumed by 415 K, while Li₄BN₃H₁₀ persisted and continued to release hydrogen up to 453 K. At higher temperatures Li₄BN₃H₁₀ melts and reacts with MgH₂ to form Li₂Mg(NH)₂ and hydrogen gas. Cycling studies of the ball-milled mixture at 423 K and 8 MPa (80 bar) found that during rehydrogenation of Li₄BN₃H₁₀ Raman spectral modes reappear, indicating partial reversal of the Li₄BN₃H₁₀ to Li₂Mg(NH)₂ transformation.     

Formation and hydrogen storage properties of in situ prepared Mg–Cu alloy nanoparticles by arc discharge

Mg–Cu alloy nanoparticles were in situ prepared by a physical vapor condensation method (arc discharge) in a mixture of argon and hydrogen. Four crystalline phases, Mg, Mg₂Cu, MgCu₂ and MgO, were formed simultaneously during the arc-discharge evaporation. Detailed experiments revealed that nanostructured hydrogen-active phases of Mg₂Cu and Mg exhibit enhanced hydrogen absorption kinetics possibly due to the small grain size and surface defects. The maximal hydrogen storage contents of Mg–Cu alloy nanoparticles can reach 2.05 ± 0.10 wt% at 623 K.     

Catalytically enhanced dehydrogenation of MgH2 by activated carbon supported Pd–VOx (x=2.38) nanocatalyst

In this paper, we reported that a new multi-component catalyst of activated carbon supported nanosized Pd and VO_x (Pd–VO_x/AC, x = 2.38) prepared by wet impregnation method exhibited significant catalytic effect on hydrogen desorption of MgH₂. It is demonstrated that the nanocomposites of MgH₂ + Pd–VO_x/AC prepared by ball milling could reduce the desorption temperature, e.g. differential scanning calorimetry (DSC) measurement indicated that the peak desorption temperature decreased ∼40 °C, and also improve the desorption kinetics of MgH₂, e.g. desorbed 6.5 wt% hydrogen within 30 min at 300 °C under an initial pressure of 1 kPa. A significant decrease of activation energy (E_a) indicated that Pd–VO_x/AC catalyst is highly efficient for MgH₂ dehydrogenation, which may be ascribed to the synergistic effect of nanometric bimetals (metal oxides) and nanocarbon.     

Sorption behavior of the MgH2–Mg2FeH6 hydride storage system synthesized by mechanical milling followed by sintering

The hydrogen sorption behavior of the Mg₂FeH₆–MgH₂ hydride system is investigated via in-situ synchrotron and laboratory powder X-ray diffraction (SR-PXD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), particle size distribution (PSD) and volumetric techniques. The Mg₂FeH₆–MgH₂ hydride system is obtained by mechanical milling in argon atmosphere followed by sintering at high temperature and hydrogen pressure. In-situ SR-PXD results show that upon hydriding MgH₂ is a precursor for Mg₂FeH₆ formation and remained as hydrided phase in the obtained material. Diffusion constraints preclude the further formation of Mg₂FeH₆. Upon dehydriding, our results suggest that MgH₂ and Mg₂FeH₆ decompose independently in a narrow temperature range between 275 and 300 °C. Moreover, the decomposition behavior of both hydrides in the Mg₂FeH₆–MgH₂ hydride mixture is influenced by each other via dual synergetic-destabilizing effects. The final hydriding/dehydriding products and therefore the kinetic behavior of the Mg₂FeH₆–MgH₂ hydride system exhibits a strong dependence on the temperature and pressure conditions.     

B2O3-free SiO2–Al2O3–SrO–La2O3–ZnO–TiO2 glass sealants for intermediate temperature solid oxide fuel cell applications

In this study, the chemical and thermal stabilities of eleven B₂O₃-free SiO₂–Al₂O₃–SrO–La₂O₃–ZnO–TiO₂ glasses were investigated, and the adhesion and sealing properties of the glasses with respect to Gd_0.2Ce_0.8O_2−δ (GDC) electrolyte and SUS 430 stainless steel (SUS430) were evaluated for use in intermediate temperature solid oxide fuel cells (ITSOFCs). It was found that the major crystallites formed in the glasses were initiated during the joining process at 950 °C, and only slight changes were observed in the intensity of crystallite peaks for the glasses subsequently soaked at 700 °C for 200 h. Experiments on the glass sandwiched with GDC electrolyte and SUS430 indicated that the SALSTi11 and SALSZT10 glasses provided good adhesion along the interfaces after heat treatment. According to the results of leakage test, the seals with the SALSTi11 and SALSZT10 glasses at 700 °C for a duration of 500 h showed good thermal stability with low leak rates of 0.007 and 0.003 sccm/cm at 0.5 psi, respectively. This property indicates a highly promising long-term thermal stability qualifying the sealing materials for ITSOFC applications.     

Processing analysis of the ternary LiNH2–MgH2–LiBH4 system for hydrogen storage

In this article, we investigate the ternary LiNH₂–MgH₂–LiBH₄ hydrogen storage system by adopting various processing reaction pathways. The stoichiometric ratio of LiNH₂:MgH₂:LiBH₄ is kept constant with a 2:1:1 molar ratio. All samples are prepared using solid-state mechano-chemical synthesis with a constant rotational speed, but with varying milling duration. Furthermore, the order of addition of parent compounds as well as the crystallite size of MgH₂ are varied before milling. All samples are intimate mixtures of Li–B–N–H quaternary hydride phase with MgH₂, as evidenced by XRD and FTIR measurements. It is found that the samples with MgH₂ crystallite sizes of approximately 10 nm exhibit lower initial hydrogen release at a temperature of 150 °C. Furthermore, it is observed that the crystallite size of Li–B–N–H has a significant effect on the amount of hydrogen release with an optimum size of 28 nm. The as-synthesized hydrides exhibit two main hydrogen release temperatures, one around 160 °C and the other around 300 °C. The main hydrogen release temperature is reduced from 310 °C to 270 °C, while hydrogen is first reversibly released at temperatures as low as 150 °C with a total hydrogen capacity of ∼6 wt.%. Detailed thermal, capacity, structural and microstructural properties are discussed and correlated with the activation energies of these materials.     

Synergic effect of V2O5 and P2O5 on the sealing properties of barium–strontium–alumino-silicate glass/glass–ceramics

The effect of combined addition of P₂O₅ and V2O₅ on structural and sealing properties of glasses with nominal composition (mol.%) 27SiO₂–23SrO–32BaO–4Al₂O₃–10B₂O₃–(4-x)P₂O₅–xV₂O₅ is reported in the present study. ²⁹Si,³¹P,⁵¹V,²⁷Al and ¹¹B MAS-NMR have been used to characterize the local environment in the glasses and glass–ceramics. The main network-forming elements (Si, Al, B) are not significantly affected, however, P and V show different local environment when they are associated in the composition. Characteristic temperatures (T_g, T_soft, T_shrink) decrease when V₂O₅ is added to the glass composition, which enable to decrease the sealing temperature. However, the combined addition of P₂O₅ and V₂O₅ is favorable to avoid extensive crystallization during sealing operation. TEC of the glass and glass–ceramics samples is found compatible with those of metallic alloys and do not change after heat treatment. EPMA images show that the seals have good thermal and chemical stabilities when treated up to 1500 h at 800 °C. Sandwich seals tested under pressure are resistant to thermal cycling.     

Improved reversible dehydrogenation properties of 2LiBH4–MgH2 composite by milling with graphitic carbon nitride

The 2LiBH₄–MgH₂ reactive hydride composite is a promising hydrogen storage system due to the combined high hydrogen capacity and relatively moderate reaction enthalpy. However, the sluggish de/rehydrogenation kinetics severely impedes its practical applications. In this study, graphitic carbon nitride (C₃N₄) as a metal-free additive was added to the 2LiBH₄–MgH₂ composite and examined with respect to the promoting effect on the hydrogen storage properties of the composite. Our study found that mechanically milling with small amount of C₃N₄ additive can eliminate the incubation period between two dehydrogenation steps and thus markedly enhance the dehydrogenation kinetics of the LiBH₄–MgH₂ composite. Further cyclic study found that the composite with C₃N₄ additive exhibits improved cyclic dehydrogenation property although it also shows capacity loss upon cycling, particularly in the second cycle. Combined dehydrogenation property, phase analysis and a series of designed experiments suggested that the C₃N₄ additive could react with both LiBH₄ and MgH₂ in heating process, and the resulting products may improve the reversible dehydrogenation property of the composite system.     

Nanoconfined 2LiBH4–MgH2–TiCl3 in carbon aerogel scaffold for reversible hydrogen storage

Nanoconfinement of 2LiBH₄–MgH₂–TiCl₃ in resorcinol–formaldehyde carbon aerogel scaffold (RF–CAS) for reversible hydrogen storage applications is proposed. RF–CAS is encapsulated with approximately 1.6 wt. % TiCl₃ by solution impregnation technique, and it is further nanoconfined with bulk 2LiBH₄–MgH₂ via melt infiltration. Faster dehydrogenation kinetics is obtained after TiCl₃ impregnation, for example, nanoconfined 2LiBH₄–MgH₂–TiCl₃ requires ∼1 and 4.5 h, respectively, to release 95% of the total hydrogen content during the 1st and 2nd cycles, while nanoconfined 2LiBH₄–MgH₂ (∼2.5 and 7 h, respectively) and bulk material (∼23 and 22 h, respectively) take considerably longer. Moreover, 95–98.6% of the theoretical H₂ storage capacity (3.6–3.75 wt. % H₂) is reproduced after four hydrogen release and uptake cycles of the nanoconfined 2LiBH₄–MgH₂–TiCl₃. The reversibility of this hydrogen storage material is confirmed by the formation of LiBH₄ and MgH₂ after rehydrogenation using FTIR and SR-PXD techniques, respectively.