Nanosizing of TiFe hydrogen storage alloy is conducted to facilitate its activation. Here, pure intermetallic TiFe nanoparticles (45 nm) were prepared using chemical reduction of oxide precursors at 600 °C, which is the lowest temperature ever used in chemical synthesis. This was achieved using a strong reducing agent (CaH2) in a molten LiCl. When used for hydrogen absorption, the obtained nanoparticles surprisingly exhibited almost no hydrogen absorption. The results demonstrated that TiFe nanoparticles are more difficult to activate than the bulk powder because the oxidized surface layers of the nanoparticles become stabilized, which prevents the morphological change necessary for their activation.

We propose using hydrogen as a heat transfer medium to supply waste heat from hydrogen-driven devices to hydrogen storage tanks. In our model, MgH2 is used in the form of porous sheets, set in parallel in the tank, and heat is supplied via hot hydrogen flowed through the interspaces between the porous sheets. Feasibility of the hydrogen desorption reaction in this process was verified numerically. Hydrogen efficiently carried heat to the stack of porous MgH2 sheets via convective heat transfer and then carried heat into the porous MgH2 sheets via conductive heat transfer through the pores owing to its high thermal conductivity. We found that the hydrogen desorption is also fast enough to allow the supplied heat to be used efficiently to drive the endothermic hydrogen desorption reaction. It was understood that the thickness of the MgH2 sheet and hot hydrogen flow speed affected hydrogen desorption. These factors can be evaluated by using the dimensionless number of τs/τh which is the ratio of the space time to the time constant for heat transfer in the MgH2 sheet. Under τs/τh > 0.01 range, both the reaction and heat transfer are fast enough, the hydrogen desorption is limited by heat supply, and hydrogen desorption amount is proportional to the heat supplied to the reactor. The tank structure and operating conditions can be designed by using the dimensionless number of τs/τh.

TiFe nanostructures where prepared at temperatures as low as 600 °C with a Ti–Fe precursor and a CaH₂ reducing agent in molten LiCl. For the first time an intermetallic compound with a unique layered morphology was found which could have originated from the FeTiO₃ precursor.

In order to investigate a material transformation of alumina (Al2O3) and an influence on leakage current by application of high DC voltage at high temperatures, the leakage current of two alumina samples were measured for 100 hours and the elemental analyses were carried out. The leakage current tended to decrease, and the electrical conductivity decreased with increasing applied voltage. The activation energies of electrical conduction of two samples were nearly equal. Thus, the reproducibility of these results was confirmed. After the experiments, deposits of impurities contained in alumina were observed in the surrounding area of the positive electrode. Furthermore, Na, which was contained in Pt paste used for preparing the electrodes, can be a charge career of alumina because it was detected from only negative electrodes. Therefore, the reason why the leakage current decreased is the decrease in the amount of charge careers because of the deposits of impurities.

In this study, the dc voltage insulating properties in a hydrogen atmosphere at high temperatures (600 degrees C to 850 degrees C) were evaluated for alumina (Al2O3), magnesia (MgO), silicon nitride (Si3N4), and mica (KMg3(Si3Al)O-10(OH)(2)) to comprehend the difference in the insulating properties of oxide, nitride, and minerals. The activation energies of the electrical conductivity of alumina and magnesia in hydrogen were larger than those in air. On the other hand, the electrical conduction values for silicon nitride and mica in hydrogen were the same as those in air. Therefore a low oxygen partial pressure would have some influence on the electrical conduction of oxides. Increasing the temperature did not result in a large change in the electrical conduction mechanism in any of the materials in either atmosphere. The maximum partial discharge (PD) in hydrogen tended to increase compared to that in air at high voltage. The applied voltage at which the maximum amount of PD started to increase rapidly became lower with increasing temperature in all materials and in both atmospheres. The total amount of PD tended to decrease with increasing temperature in all materials and in both atmospheres. However, above a certain temperature, the total amount of PD either increased or showed a slight decrease.

LaNi5-based AB(5) type alloy has high tolerance to CO2 poisoning for hydrogen purification and storage from 20 to 25% CO2 mixed gas. To elucidate the CO2 poisoning factors of AB(5) type alloys, which are LaNi5, CaNi5, LaCo5, and MmNi(4.025)Co(0.4)Mn(0.275)Al(0.3) (Mm-Ni), the dependence of the constituent elements has been investigated on hydrogenation degradation by CO2 poisoning. The tendency of CO2 poisoning magnitude is CaNi5 < LaNi5 << Mm-Ni < LaCo5, which was evaluated by the hydrogenation rate and capacity under CO2 partial pressure and after CO2 exposure. The Ni element of B site in CaNi5 and LaNi5 is an important role to maintain higher tolerance of CO2 poisoning compared to Co element in LaCo5. Moreover, the element of A site effects on CO2 poisoning magnitude in AB(5) type alloy. The experimental tendency of CO2 poisoning magnitude is consistent with the theoretical CO2 adsorption energy on the (1010) surface plane of -1.39, -2.05, and -2.68 eV for CaNi5, LaNi5, and LaCo5, respectively. CO2 adsorbs on B site with electron charge transfer from AB(5) alloys to carbon. Not only Ni element in B site but also Ca element in A site decreases the energy of CO2 adsorption on B site in AB(5) type alloys. (C) 2015 Elsevier B.V. All rights reserved.

The variation of the niobium (Nb) oxide catalyst during hydrogen absorption and desorption reactions was investigated by in-situ X-ray absorption spectroscopy (XAS). Results indicated that H-2 was easily dissociated on the catalyst because the hydrogen absorption kinetics was significantly improved, and then the hydrogen atoms were diffused through inside of the catalyst. Thus, the catalytic mechanism of Nb oxides is different from that of conventional metal catalysts, in which the dissociated H is moved on the surface of catalyst.

Capacity fading mechanism of pristine and FePO4-coated LiNi1/3Co1/3Mn1/3O2 has been studied by electrochemical and magnetic methods. Along with cycles, significant increase of cell polarization and charger transfer resistance (R-ct), which mainly cause the capacity fading at the initial charge/discharge cycles, have been observed according to the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis. In comparison, the existence of FePO4 coating layer can effectively suppress the deterioration of surface polarization and increase of Rct at the interface of electrode/electrolyte. The coated sample suppress the decrease of Li+ diffusion coefficient with prolonged cycles according to the galvanostatic intermittent titration technique (GITT) tests. It also shows larger Weiss constants (fitting above 130 K) and lower blocking temperatures, which indicate the surface coating can suppress the deterioration of Li/Ni disorder upon repeated cycles. This study may give the researchers some light on understanding the capacity fading mechanism of LiNi1/3Co1/3Mn1/3O2 and the effects of surface coating layer, and thus, give help to the future designing of superior coating layers for layered cathode materials in Li-ion batteries. (C) 2014 Elsevier Ltd. All rights reserved.

Energy storage will be critically important for stabilizing grids with renewable energy power sources; therefore various kinds of high performance devices such as batteries and capacitors are being developed at present. In particular, Li-ion batteries exhibit high energy density, and Li-ion capacitors have high input/output power ability with almost zero self-discharge. The authors have proposed a BAttery/Capacitor Hybrid Energy Storage system (BACHES) which delivers wide-range charge/discharge power with a large capacity by Li-ion capacitors and batteries. In this paper, the authors show the conceptual details and the features of this system. Also, experimental equipment was manufactured as specifications of maximum power of 6kW for connection to a 380V bus. The fundamental performances of the system components of Li-ion capacitors, Li-ion batteries, and converters were evaluated in test runs. This study concluded that the system operates itself by sharing roles between the Li-ion capacitors and the Li-ion batteries according to the characteristics of each, and then can control the bus voltage within 1.2% and 4.4% drops under sudden 0.47kW and 1.8kW power losses, respectively.

A novel tunnel Na0.61Ti0.48Mn0.52O2 material is explored as a cathode for sodium-ion batteries for the first time. It can deliver a reversible discharge capacity of 86 mA h g(-1) with an average voltage of 2.9 V at 0.2 C rate in a sodium half cell, exhibiting good rate capability and capacity retention at a cut-off voltage of 1.5-4 V. These results indicate that tunnel Na0.61Ti0.48Mn0.52O2 has a great potential application in large scale energy storage.

In this study, the insulating properties on DC voltage in hydrogen atmosphere at high temperatures (600-850°C) were evaluated for alumina (Al₂O₃), magnesia (MgO), silicon nitride (Si₃N₄) and mica (KMg₃(Si₃Al)O₁₀(OH)₂) to comprehend the difference in insulating properties of oxide, nitride and mineral. The activation energies of electrical conductivity of alumina and magnesia in hydrogen were larger than those in air. On the other hand, at silicon nitride and mica electrical conduction in hydrogen were same as those in air. Therefore low oxygen partial pressure would have some influence on electrical conduction of oxide. And increasing temperature didnt bring a huge change of electrical conduction mechanism in all materials and both of the atmospheres. The maximum amounts of partial discharge in hydrogen tended to increase compared to those in air at high voltage. Applied voltage, in which the maximum amounts of partial discharge started to increase rapidly, became lower with increasing temperature in all materials and both of the atmospheres. The total amounts of partial discharge tended to decrease with increasing temperature in all materials and both of the atmospheres. However, at exceeding certain temperatures, the total amounts of partial discharge increased or slightly decreased.

The CO Selective Adsorbent and Metal Hydride Intermediate Buffer (COA-MIB) method has been proposed as a hydrogen production/supply system with reformer. First, this method thoroughly eliminates carbon monoxide using an adsorption material that strongly selects carbon monoxide, and second it introduces a mixed gas to a metal hydride that purifies and stores hydrogen. A 100 NL/h laboratory scale apparatus was operated in daily start and stop operations for 1000 cycles for a total of 1500 h with quite good efficiency. The metal hydride used in this apparatus was an AB5-type whose hydrogen equilibrium pressure was adjusted to be 0.1 MPa at 25 C. As a result of the basic performance evaluation, we have experimentally verified that pure hydrogen can be produced from methanol reforming gas for 1000 cycles on a daily start/stop operation basis and a high hydrogen recovery rate of over 89% can be achieved. 3 N m3/h Bench scale apparatus was also operated with same mode for 100 cycles for a total of 150 h and shows almost the same performance. The new method enables us to minimize emissions of carbon dioxide by using compact and highly efficient fuel cells as a part of the smart community technologies. © 2013 Published by Elsevier B.V.

Hydrogen was introduced in commercial-purity (99%) aluminum by electrochemical charging to study the existing state of hydrogen and its effects on the mechanical properties of aluminum. Electrochemical charging was conducted in an aqueous H2SO4 solution with 0.1% NH4SCN as a hydrogen recombination poison. The potential and during the charging were determined from the immune. passive, and corrosive regions in the Pourbaix diagram to determine the optimum conditions or the charging. The maximum amount of hydrogen absorbed was obtained in the immune region. The amount of hydrogen and its existing state were examined using hydrogen desorption curves, which were obtained by thermal desorption spectroscopy. The curves showed distinctive peaks corresponding to trapping sites of hydrogen in the material. One of the peaks was observed at approximately 100 degrees C, and it corresponds to vacancies and dislocations in the material; another peak was observed at approximately 400 degrees C and it corresponds to molecular hydrogen in blisters. It was presumed that charged hydrogen diffuses into the bulk of the material to form hydrogen-vacancy pairs, and then these pairs cluster to form blisters. The fracture strain of charged aluminum in the immune region decreased with decreasing strain rate, showing an inverse dependence on the fracture strain of the uncharged material. This phenomenon was considered to be caused by hydrogen transport by dislocations through the interaction between hydrogen and dislocations. The phenomenon was further confirmed by the observation of hydrogen release during tensile deformation, where the amount of hydrogen was high in the strain rate range where the interaction between dislocations and hydrogen was prominent. [doi: 10.2320/matertrans.M2011035]

In this study, we analyzed the effects of deformation on hydrogen absorption and desorption properties of titanium to improve such properties. Hydrogen was introduced into commercially pure (99.5%) titanium by the electrochemical method. The amount and existing state of hydrogen were examined using hydrogen desorption curves obtained by thermal desorption spectroscopy. Hydrogen absorption was promoted by applying tensile deformation prior to charging, which leads to hydride formation within a short charging time. The amount of hydrogen absorbed decreased when the volume fraction of deformation twins exceeded about 0.2. It was considered that hydrogen was mainly trapped by dislocations forming hydride while a large fraction of deformation twins hindered dislocation motion, thus reducing dislocation density leading to a decrease in the amount of absorbed hydrogen. Almost half the charged hydrogen was released when in-plane compressive stress was applied to a charged plate specimen at room temperature due to hydride decomposition under compressive stress. (C) 2010 Elsevier B.V. All rights reserved.

Liquid ammonia was electrolyzed based on a new concept by using metal amide as the supporting electrolyte for generation of hydrogen. Different metal amides including LiNH(2), NaNH(2) and KNH(2) were tested, and the influences of the solubility and the concentration of different electrolytes upon the electrolysis current were investigated. Electrolysis efficiency was evaluated on the basis of chronopotentiometry tests at different current densities. Platinized platinum electrodes with fine structure were introduced in order to improve the electrolysis current and reduce the electrolysis potential. (C) 2010 Elsevier B.V. All rights reserved.

The electrochemical properties of Mg + 2LiH and Al + 3LiH are investigated by applying a Li-ion insertion and extraction system to form magnesium and aluminum hydrides. For MgH(2) formation, the voltage-composition (VC) curve for Mg + 2LiH during charging exhibits a plateau voltage at 0.58 V, then the final composition is obtained with 1.05 mol Li extraction at 3.0 V. After the charging, the MgH(2) phase is observed by XRD measurement. Therefore, MgH(2) is produced from Mg and LiH by electrochemical charging. With respect to AlH(3) formation, Al + 3LiH is charged at a plateau voltage of 0.81 V, which corresponds to the reaction of Al with hydrogen in LiH to form AlH(3). And the final composition at 3.0 V is 0.6 mol Li. In the XRD profile after charging, the AlH(3) phase is not detected. (C) 2010 Elsevier B.V. All rights reserved.

Hydrogen is introduced in commercial (99% pure) aluminum by electrochemical charging to study the existing state of hydrogen and its effect on the mechanical properties of aluminum. Electrochemical charging is conducted in an aqueous solution of H2SO4 with 0.1 mass% NIH4SCN as a hydrogen recombination poison. The potential and pH during the charging are chosen from the immune, passive, and corrosive regions on Pourbaix diagram to determine the optimum conditions for the charging. The maximum amount of hydrogen absorbed is obtained in the immune region. The amount of hydrogen and its existing state are examined using hydrogen desorption curves, which are obtained by thermal desorption spectroscopy. The curves show distinctive peaks that correspond to trapping sites of hydrogen in the material. One of the peaks is observed at approximately 100 degrees C and it corresponds to vacancies and dislocations in the material; another peak is observed at approximately 400 degrees C and it corresponds to molecular hydrogen in blisters. It is presumed that charged hydrogen diffuses into the bulk of the material to form hydrogen vacancy pairs, and then these pairs cluster to form blisters. The fracture strain of charged aluminum in the immune region decreased with a slower strain rate, showing an inverse dependence on the fracture strain of the uncharged material. This phenomenon is considered to be caused by the transport of hydrogen by dislocations through the interaction between hydrogen and the dislocations. The phenomenon is further confirmed by the observation of hydrogen release during tensile deformation, where the amount of hydrogen is higher in the strain rate region where the interaction between the dislocations and hydrogen is more prominent.

A valence state and a local structure of transition metals (Nb, V, and Ti) in MgH2 doped with metal oxides (Nb2O5, V2O5, and TiO2nano) by ball milling were examined by X-ray absorption spectroscopy (XAS). The main edge regions of the Nb, V, and Ti K-edges in the X-ray absorption near edge structure (XANES) profiles are located between 0 and +5 in the oxidation states. Since these spectra coincide with those of NbO, VO, and Ti2O3, respectively, the additives are reduced by MgH2 to the metal oxides, which have lower oxidation states than those of the starting materials. Furthermore, in order to examine the local structures around the transition metal atoms, the extended X-ray absorption fine structure (EXAFS) spectra were analyzed. In the Fourier transformation curves of the EXAFS spectra, all samples doped with the metal oxides show two peaks corresponding to metal-oxygen and metal-metal bonds, being the same as the references of NbO, VO, and Ti2O3. The local structure formed after ball milling or dehydrogenation is close to that of each of the reduced metal oxides (NbO, VO, and Ti2O3) but in a more disarrangement state.

Enthalpy change (Delta H) due to hydrogen desorption (H-desorption) for the lithium amide/imide system was evaluated by differential scanning calorimetry (DSC) measurement. In order to obtain the accurate and precise value of Delta H, we have paid special attention to following two points for correcting raw experimental data. One is to determine a cell constant of DSC equipment, which was evaluated by using the TiO2-doped MgH2 compound as a reference because of its quite similar hydrogen desorption properties to that of the lithium amide/imide system. The other is to estimate the sample amount corresponding to the H-desorption reaction from weight loss in the thermogravimetric (TG) analysis. By performing both the corrections, the Delta H value due to the H-desorption reaction from LiNH2 + LiH to Li2NH + H-2 was evaluated to be 67 kJ/mol H-2. (c) 2007 Elsevier B.V. All rights reserved.

The microstructures of MgH2 catalyzed with Ni nano-particle or Nb2O5 mesoporous powders are examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. For MgH2 catalyzed with Ni, the Ni particles with the diameter smaller than 1 mu m were detected on the MgH2 particles with the diameter smaller than 5 mu m by the back scattering electron (BSE) microscopy. In details, the TEM micrograph indicates that the Ni particles distribute similar to 20 nm in diameter on MgH2 uniformly, which was the same size as the additive doped in MgH2 before milling. On the other hand, for MgH2 catalyzed with Nb2O5, the additive particles could not be found anywhere in the BSE image. Even in the TEM micrograph by much larger magnification than the SEM micrograph, the particles corresponding to the additive cannot be observed at all. Furthermore, an energy dispersive X-ray (EDX) analysis in spots with a diameter of 20 nm indicated that the existing ratio of Mg to Nb was evaluated to 98:2, being the same as the starting ratio before milling. Therefore, the metal oxide Nb2O5 becomes extremely small particle that could not be observed by the present work after milling compared to metal Ni-nano. (C) 2006 Elsevier B.V. All rights reserved.

The thermal decomposition steps of Mg(BH(4))(2) were investigated under He flow and various hydrogen pressures up to 50 bar. In a He flow, the main decomposition of Mg(BH(4))(2) occurs between 250 and 410 degrees C until 12.2 mass% is lost, with three main peaks of hydrogen desorption. In the first decomposition step the crystalline phase of Mg(BH(4))(2) disappears while a small amount of Mg is detected in the XRD profile. However, the major part of the sample is in an amorphous state. After the second step, crystalline MgH(2) is observed together with the Mg phase. The third step of hydrogen desorption corresponds to the decomposition of MgH(2) and Mg remains the only crystalline phase observed by XRD measurement after heating to 410 degrees C. Further hydrogen evolution of 1.4 mass% is observed from 410 degrees C to 580 degrees C. Only after this hydrogen desorption, MgB(2) appears in the XRD spectra of the sample. These results indicate that amorphous, hydrogen containing boron compounds take part as intermediates in the reaction. Under hydrogen pressure, the decomposition events of Mg(BH(4))(2) shift to higher temperatures in the DSC (differential scanning calorimetry) profiles: while there is only a small shift for steps 1 and 2 there is a clear separation of the succeeding reactions under a background pressure of hydrogen. These data show that the decomposition proceeds via several well defined steps. The final stable decomposition compound of Mg(BH(4))(2) is MgB(2) under both inert and hydrogen gas atmosphere.

Solid-state reactions of Ca(AIH(4))(2) and various additives were investigated experimentally for hydrogen desorption. The Ca(AIH(4))(2) + Si, Ca(AIH(4))(2) + 2MgH(2), Ca(AIH(4))(2) + 2LiH, and Ca(AIH(4))(2) + 2LiNH(2) systems were chosen among reactions proposed theoretically(1) to study their hydrogen storage capacity and an appropriate reaction enthalpy. For all systems investigated, the reversible reactions proposed should have more than 6.5 mass % hydrogen capacity and reaction enthalpies in the range of 30-55 kJ/mol H-2. However, most of the experimentally observed reactions do not conform to theoretical propositions because different final products were obtained in all cases but one. Two tendencies were observed in the experiments. One is that the reaction comprises several distinct steps at different temperatures as observed in the Ca(AIH(4))(2) + Si and Ca(AIH(4))(2) + 2 MgH2 systems. In this case, Ca(AIH(4))(2) decomposes first and is followed by a reaction of the remaining compounds. The same kinetic reaction barriers are encountered here as in pure Ca(AIH(4))(2). Second, LiH and LiNH2 additions yield exothermic reactions of Ca(AIH(4))(2) and the other reactant as early as in the ball milling or annealing steps, leading to final products not considered in the reactions calculated.

The hydrogen absorption kinetics of magnesium hydride (MgH2) composite doped with 1 mol% Nb2O5 prepared by ball milling was examined under various temperatures and pressures. The composite after dehydrogenation at 200 degrees C absorbs gaseous hydrogen of similar to 4.5 mass% within 15 s even at room temperature under 1.0 MPa hydrogen pressure or at 0 degrees C under 3.0 MPa, and finally their capacities reach up to 5 mass%. At 150 and 250 degrees C, a large amount of hydrogen gas of more than 5.0 mass% is absorbed within 30 s and their capacity reach up to 5.7 mass% under 1.0 MPa. Interestingly, the absorption kinetics of the catalyzed Mg shows two unusual behaviors in the initial reaction stage of the time scale within 30 s. One is that the kinetics decreases with increase in the temperature from 150 to 250 degrees C under any pressures (0.2, 1.0 and 3.0 MPa). The other is that the amount of hydrogen absorption drastically increases with increase in the initial pressure from 1.0 to 3.0 MPa at 0 degrees C or from 0.2 to 1.0 MPa at room temperature (similar to 20 degrees C). These behaviors may be explained by taking into account heat generation of Mg due to fast hydrogen uptake in such a short time. (C) 2006 Elsevier B.V. All rights reserved.

Thermal analyses of the mixture of MgH(2) and LiBH(4) doped with TiCl(3), and each element MgH(2) or LiBH(4) doped with TiCl(3) were performed under an inert gas flow and 0.5 MPa H(2)-gas conditions. It was indicated that the hydrogen desorption reaction of MgH(2) + 2LiBH(4) to MgB(2) + 2LiH + 4H(2) phases proceeded at temperature above 400 degrees C under 0.5 MPa hydrogen, whereas, under an inert gas atmosphere, the reaction of the same mixture was transformed into Mg + 2B + 2LiH + 4H(2) phases in a temperature range from 350 to 430 degrees C. Before these reactions, the dehydrogenation of MgH(2) and the melting of LiBH(4) took place under both hydrogen and the inert gas atmospheres with increasing temperature. In addition, the molten LiBH(4) did not decompose into LiH, B and H(2) below 450 degrees C under 1 MPa H(2), while the LiBH(4) decomposed even below 450 degrees C under an inert gas atmosphere. From these results, it is deduced that the reaction producing MgB(2) is a solid-liquid reaction between solid Mg and liquid LiBH(4) above 400 C without decomposition of molten LiBH(4) Under a hydrogen atmosphere, while MgH(2) dehydrogenate. A characteristic solid-liquid reaction is realized under a hydrogen atmosphere for proceeding of the MgB(2) producing reaction in this system. (C) 2007 Elsevier B.V. All rights reserved.

In this paper, we review our recent results on hydrogen storage properties in light metals(M)-nitrogen(N)-hydrogen(H) systems prepared by mechanochemical method. At first, the composite mixture of LiH and LiNH2 doped with TiCl3 as a catalyst was prepared by ball milling for 2 h under a H-2 gas pressure of 1 MPa. The TDS profile indicated that similar to 6 mass% H-2 was desorbed by the reaction LiH + LiNH2 &lt;-&gt; Li2NH + H-2 in the temperature range from 150 to 250 degrees C under a He gas flow at a heating rate of 5 degrees C min(-1), but the H-desorption equilibrium pressure P-H2 was similar to 0.1 MPa at 250 degrees C. This temperature is too high for onboard use, indicating that further improvement is necessary to destabilize the above Hstorage reaction. For that, we clarified the H-desorption mechanism by the isotopic exchange experiments, on the basis of which we designed a new Li-Mg-N-H composite system with the reaction 8LiH + 3Mg(NH2)(2)&lt;-&gt; 4Li(2)NH + Mg3N2 + 8H(2). This composite materials desorbed similar to 7 mass% H-2 in the range from 120 to 200 degrees C and the H-desorption equilibrium pressure P-H2 was higher than 5 MPa at 200 degrees C, indicating that this system has an excellent potential for onboard applications. (c) 2006 Elsevier B.V. All rights reserved.

Kinetics of hydrogen absorption and desorption reactions was investigated on the MgH2 composite doped with 1 mol% Nb2O5 as a catalyst by ballmilling. The composite after dehydrogenation at 200 degrees C absorbed gaseous hydrogen of similar to 4.5 mass% even at room temperature under lower pressure than I MPa within 15 s and finally its capacity reached more than 5 mass%. On the other hand, the catalyzed MgH2 after rehydrogenation desorbed similar to 6 mass% hydrogen at 160 degrees C under purified He flow, which followed the first order reaction. From the Kissinger plot, the activation energy for hydrogen desorption was estimated to be similar to 71 kJ/mol H-2, indicating the product was significantly activated due to the catalytic effect of Nb2O5. (c) 2005 Elsevier B.V. All rights reserved.

We examined a catalytic effect of niobium oxide (Nb2O5) on the hydrogen storage properties of MgH2 prepared by mechanical ball milling method. The MgH2 composite doped with I mol% Nb2O5 by ball milling for 20 h desorbed hydrogen up to similar to 6 mass% in the temperature range from 200 to 250 degrees C at the heating rate of 5 degrees C/min under a purified helium flow. After dehydrogenation at 200 degrees C, the product showed remarkable hydrogen absorption kinetics. A large amount of gaseous hydrogen up to similar to 4.5 mass% was absorbed even at room temperature under 1 MPa hydrogen pressure within 15 s and finally its capacity reached up to 5 mass%. Furthermore, the valence state of Nb2O5 doped in MgH2 was examined by X-ray absorption near edge structure (XANES) measurement. The results indicated that additive Nb2O5 was reduced by MgH2 during mechanical milling. This suggests that the Nb compound, in which the valence state of Nb atom is less than 5 +, acts as a catalyst for the hydrogen absorbing/desorbing kinetics. (c) 2006 Elsevier B.V. All rights reserved.

The synthesis and decomposition properties of some metal amides M(NH2)(x) such as LiNH2, NaNH2, Mg(NH2)(2) and Ca(NH2)(2) were investigated, which play important roles for designing a new family of metal-N-H hydrogen storage systems. Both the gas chromatographic examination and X-ray diffraction measurement indicated that the reaction between alkali or alkaline earth metal hydride MHx (such as LiH, NaH, MgH2 and CaH2) and gaseous NH3 could quickly proceed at room temperature by ball milling and the corresponding metal amides were easily synthesized in high quality. The kinetics of these kind of reactions is faster in the order of NaH &gt; LiH &gt; CaH2 &gt; MgH2, which is consistent with the inverse order of electronegativity of those metals, i.e. Na &lt; Li = Ca &lt; Mg. The thermal decomposition properties indicated that both Mg(NH2)(2) and Ca(NH2)(2) decomposed and emitted NH3 at lower temperature than LiNH2. (c) 2005 Elsevier B.V. All rights reserved.

The Li-N-H system expressed by LiNH2 + LiH &lt;-&gt; Li2NH + H-2 can be expected as a promising candidate for the hydrogen storage materials because of possessing a large amount of reversible hydrogen (6.5 wt.%), a satisfactorily fast kinetics and a relatively small enthalpy change. In this work, we investigated the hydrogen storage properties of the Li-N-H system from three different points of view. Firstly, we claim that the ball milled 1: 1 mixture of lithium amide (LiNH2) and lithium hydride (LiH) containing a small amount (I mol %) of titanium chloride (TiCl3) shows superior hydrogen storage properties; a large amount of H-2 gas desorbs in the temperature range from 150 to 250 degrees C at a heating rate of 5 degrees C/n-dn and it reveals an excellent reversibility. Secondly, we clarify that the above hydrogen desorption reaction is composed of two kinds of elementary reactions: The one is that 2LiNH(2) decomposes to Li2NH and emits ammonia (NH3). The other is that the emitted NH3 reacts with LiH and transforms into LiNH2 and H-2, indicating that NH3 plays an important role on this H-2 desorption reaction. Finally, we examined the reaction of LiH and LiOH to clarify the influence of exposing the product to air. This is because due to the fact that LiOH is easily produced by exposing LiH and LiNH2 to air. The reaction between LiH and LiOH indicated better kinetics but worse durability and an extra H2 desorption due to transforming into Li2O. (c) 2005 Elsevier B.V. All rights reserved.

We examined the catalytic effect of Ni nano-particle and Nb oxide on hydrogen desorption (HD) properties in MgH2 prepared by mechanical ball milling under a hydrogen gas atmosphere of 1 MPa. The MgH2 composite with 2 mol% Ni nano-particle prepared by milling for a short time of 15 min at 200 rpm desorbed a large amount of hydrogen (similar to 6.5 wt.%) in the temperature range from 150 to 250 degrees C at heating rate of 5 degrees C/min. On the other hand, the MgH2 Composite with I mol% Nb-2 O-5 prepared by milling for a long time of 20 h at 400 rpm desorbed similar to 6.0 wt.% H-2 in the temperature range from 200 to 250 degrees C at heating rate of 5 degrees C/min. After second hydrogen absorption/desorption cycles at 200 degrees C, the HD properties of the Ni-catalyzed MgH2 composite became worse than those before cycling, while the Nb2O5-catalyzed composite showed better HD properties than those before cycling. (c) 2005 Elsevier B.V. All rights reserved.

The effect of some different type Ti additives on kinetics of the reaction, LiH + LiNH2 &lt;-&gt; Li2NH + H-2, was intensively investigated in this work. The mixture of LiH and LiNH2 powders with the 1: 1 molar ratio and Ti additives with different chemical form were mechanically ball milled under a hydrogen gas atmosphere of I MPa at 400 rpm for 2 It and the measurements of thermal hydrogen desorption spectrum (TDS), thermogravimetry (TG) and X-ray diffraction (XRD) were performed. Here, we used Ti (nano particle), Ti (micro particle), TiCl3, TiO2 (nano particle) and TiO2 (micro particle) as the additives. The results indicated that the Ti-nano, TiCl3 and TiO2nano doped composites revealed a superior catalytic effect on the TDS properties, while the Ti-micro and TiO2micro did not show so good catalytic effect being similar to the sample without any additives. In the XRD profiles, there are traces of Ti and TiO2 phases in the Ti-micro and TiO2micro doped composites, respectively, whereas no trace of Ti, TiCl3 and TiO2 was found in the Ti-nano TiCl3 and TiO2nano doped composites. These results indicate that the uniform distribution of nano particle Ti metal between LiH and LiNH2 plays an important role for catalytic effect. (c) 2005 Elsevier B.V. All rights reserved.

Three metal amides LiNH2, NaNH2 and Mg(NH2)(2) were synthesized by ball milling the metal hydrides under gaseous ammonia NH3 at room temperature. The decomposition behaviours from these metal amides were investigated by thermal desorption mass spectroscopy and thermogravimetry analysis methods. The results showed that LiNH2 decomposed at T &gt; 230 degrees C and was transformed into the imide Li2NH with emitting NH3, while Mg(NH2)(2), decomposed at T &gt; 180 degrees C and was transformed into MgNH and finally into Mg3N2 with emitting NH3 within 500 degrees C. Then, a new metal-N-H system composed of Mg(NH2)(2) and LiH with a molar ratio of 3:8 was designed by ball milling treatment and examined the hydrogen storage properties. The results showed that this system could reversibly absorb/desorb a large amount of hydrogen (similar to 7 wt.%) at a moderate temperature and pressure, which was better than the system of LiNH2 and LiH for hydrigen storage. (c) 2005 Elsevier B.V. All rights reserved.

We examined the catalytic effect of nanoparticle 3d-transition metals on hydrogen desorption (HD) properties of MgH2 prepared by mechanical ball milling method. All the MgH2 composites prepared by adding a small amount of nanoparticle Fe-nano, Co-nano, Ni-nano, and Cu-nano metals and by ball milling for 2 h showed much better HD properties than the pure ball-milled MgH2 itself. In particular, the 2 mol % Ni-nano-doped MgH2 composite prepared by soft milling for a short milling time of 15 min under a slow milling revolution speed of 200 rpm shows the most superior hydrogen storage properties: A large amount of hydrogen (similar to 6.5 wt %) is desorbed in the temperature range from 150 to 250 degrees C at a heating rate of 5 degrees C/min under He gas flow with no partial pressure of hydrogen. The EDX micrographs corresponding to Mg and Ni elemental profiles indicated that nanoparticle Ni metals as catalyst homogeneously dispersed on the surface of MgH2. In addition, it was confirmed that the product revealed good reversible hydriding/dehydri ding cycles even at 150 degrees C. The hydrogen desorption kinetics of catalyzed and noncatalyzed MgH2 could be understood by a modified first-order reaction model, in which the surface condition was taken into account.

We have investigated the hydrogen storage properties of a ball-milled mixture of 3Mg(NH2)(2) and 8LiH after first synthesizing Mg(NH2)(2) by ball milling MgH2 under an atmosphere of NH3 gas at room temperature. The thermal desorption mass spectra of the mixture without any catalysts indicated that a large amount of hydrogen (similar to7 wt %) was desorbed from 140 degreesC, and the desorption peaked at similar to190 degreesC under a heating rate of 5 degreesC/min with almost no ammonia emission. Moreover, the reversibility of the hydrogen absorption/desorption reactions was confirmed to be complete. The above results indicate that this system is one of the promising metal-N-H systems for hydrogen storage.

The mechanism of the hydrogen desorption (HD) reaction from the 1:1 mixture of lithium amide (LiNH2) and lithium hydride (LiH) to lithium imide (Li2NH) and hydrogen (H-2) has been proposed on the basis of our experimental results in this paper. The proposed model is constituted by 2 kinds of elementary reactions: the one is that 2LiNH(2) decomposes to Li2NH and ammonia (NH3) the other is that the emitted NH3 reacts with LiH and transforms into LiNH2 and H-2. Since the former and the latter reactions are, respectively, endothermic and exothermic, the HD reaction corresponding to the latter reaction occurs as soon as LiNH2 has decomposed into Li2NH and NH3. Therefore, the HD reaction can be understood by the following processes: at the first step, LiNH2 decomposes into Li2NH/(2) + NH3/2, and then the emitted NH3/2 quickly reacts with LiH/2, transforming into LiNH2/2 + H-2/2; at the second one, the produced LiNH2/2 decomposes to Li2NH/4 + NH3/4, and then NH3/4 + LiH/4 transform to LiNH2/4 + H-2/4, and such successive steps continue until LiNH2 and LiH completely transform into Li2NH and H-2, even at low temperatures, by the catalytic effect of TiCl3.(.)

We examined the catalytic effect of nano-particle 3d-transition metals on hydrogen desorption (HD) properties of MgH2 prepared by mechanical ball milling method. All the MgH2 composites prepared by adding a small amount of nano-particle Fenano, Conano, Ninano and Cunano metals and by ball milling for 2h showed much better HD properties than the pure ball-milled MgH2 itself. Especially, the 2 mol% Ninano-doped MgH2 composite prepared by soft milling for a short milling time of 15 min under a slow milling revolution speed of 200 rpm shows the most superior hydrogen storage properties: A large amount of hydrogen (~6.5 wt.%) is desorbed in the temperature range from 150 to 250 ºC at a heating rate of 5 ºC /min under He gas flow with no partial pressure of hydrogen. The EDX micrographs corresponding to Mg and Ni elemental profiles indicated that nano-particle Ni metals as catalyst homogeneously dispersed on the surface of MgH2. In addition, it was confirmed that the product revealed good reversible hydriding/dehydriding cycles even at 150 ºC. The hydrogen desorption kinetics of catalyzed and non-catalyzed MgH2 could be understood by a modified first order reaction model, in which the surface condition was taken into account.

In this work, we clarified the correlation between hydrogen storage and crystallographic properties in nanostructural magnesium hydride MgH2 prepared by mechanical milling under hydrogen gaseous atmosphere. At the early stage within 2h milling, the amount of desorbed hydrogen decreases similar to16% from 7.3 to 6.1 wt.% and the onset temperature of dehydrogenation decreases by 70 K from 670 K, while both the powder size and the crystallite size in powder decrease with increasing the milling time down to 1 mum and 15 nm, respectively, and the lattice strain of 0.3% is rapidly introduced. At the middle stage with longer milling time than 2 h, however, the crystallite size hardly change, but the lattice strain is once released at 2-5 h milling and again increases for longer milling time than 5 h. On the other hand, the amount of desorbed hydrogen suddenly increases from 2 to 5 h, and again decreases with a little increase of lattice strain during 5-80 h milling. At the final stage, the hydrogen capacity and desorption temperature reach to saturation of, respectively, 6.5 wt.% and 600 K, whereas the crystallite size and lattice strain reach to saturation of similar to7 nm and 0.2%, respectively. The results obtained indicate that the reduction of crystallite size as well as the introduction of lattice strain in MgH2 during milling gives rise to the decrease in hydrogen storage capacity. (C) 2003 Elsevier B.V. All rights reserved.

In this paper, we examined the basic properties in the 1:1 mixture of lithium amide LiNH2 and lithium hydride LiH as a candidate of reversible hydrogen storage materials. The thermal desorption mass spectra of the ball milled mixture without any catalysts indicated that hydrogen H-2 is released in the temperature range from 180 to 400degreesC while emitting a considerable amount of ammonia NH3. On the other hand, the ball milled mixture containing a small amount of TiCl3 as a catalyst showed the most superior hydrogen storage properties among the 1: 1 mixtures with a small amount of catalysts, Ni, Fe, Co metals and TiCl3 (I mol.%). That is, the product desorbs a large amount of hydrogen (similar to5.5 wt.%) in the temperature from 150 to 250degreesC under the condition of a heating rate of 5 degreesC/min, but it does not desorb ammonia at all within our experimental accuracy. In addition, we confirmed that the product shows an excellent cycle retention with an effective hydrogen capacity of more than 5 wt.% and a high reaction rate until at least 3 cycles. (C) 2003 Elsevier B.V. All rights reserved.

Ternary Laves phase structures with compositions MgYNi4, MgCaNi4 and CaYNi4 were prepared, and the relationship between the structures and hydriding properties was studied in detail. Only in MgYNi4 are Mg and Y found to be ordered and a plateau pressure is clearly observed in the P-C isotherm during the dehydriding process. In MgCaNi4, however, Mg and Ca are disordered, and hydrogen content of MgCaNi4 is similar to30% larger than that of MgYNi4. Control of their order/disorder in Laves phase structures may provide the hydriding properties with higher hydrogen concentrations and flatter plateau regions. (C) 2003 Elsevier B.V. All rights reserved.