Phase structure and hydrogen storage properties of REMg8.35Ni2.18Al0.21 （RE=La, Ce, Pr, and Nd） hydrogen storage alloys

REMg 8.35Ni2.18Al0.21 (RE=La, Ce, Pr, and Nd) alloys were prepared by induction melting and following annealing. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results showed that the alloys were composed of Mg2Ni, (La, Pr, Nd)Mg2Ni, (La, Ce)2Mg17 , (Ce, Pr, Nd)Mg12 and Ce2Ni7 phases. The above phases were disproportioned into Mg2NiH4 , MgH2 and REH x (x=2.51 or 3) phases in hydriding. CeH2.51 phase transformed into CeH2.29 phase in dehydriding, whereas LaH3 , PrH3 and NdH3 phases remained unchanged. The PrMg8.41Ni2.14Al0.20 alloy had the fastest hydriding kinetics and the highest dehydriding plateau pressure while the CeMg8.35Ni2.18Al0.21 alloy presented the best hydriding/dehydriding reversibility. The onset hydrogen desorption temperature of the CeMg8.35Ni2.18Al0.21 hydride decreased remarkably owing to the phase transformation between the CeH2.51 and the CeH2.29 .     

Effect of lanthanum hydride on microstructures and hydrogen storage performances of 2LiNHe-MgH2 system

Hydrogen storage properties of 2LiNH2-MgH2 system were improved by adding lanthanum hydride （LaH3）, and the role of LaH3 in hydrogen sorption process of Li-Mg-N-H system was investigated. Temperature programmed sorption results showed that the addition of lanthanum hydride reduced the dehydriding/hydriding onset temperature of 2LiNH2-MgH2 system by at least 15 K. Moreover, A 0.053 wt.%/min average rate was determined for the hydrogen desorption of 2LiNH2-MgH2-0.05LaH3 composite, while it was only 0.035 wt.%/min for 2LiNH2-MgH2 system. Hydrogen absorption capacity increased from 1.62 wt.% to 2.12 wt.% within 200 min by adding LaH3 into 2LiNH2-MgH2 system at 383 K. In the dehydrogenation of 2LiNH2-MgH2-0.05LaH3 composite, LaH2 transferred to LaN phase, which reversed to LaH2 in the following hydrogen adsorption process. The reversible reaction of LaH2 ef- fectively promoted the hydrogen sorption of Li-Mg-N-H system. Moreover, the homogenous distribution of fine La hydride was fa- vorable to improving effect of lanthanum hydride.     

Hydriding/dehydriding behaviors of La2Mg17-10 wt.%Ni composite prepared by mechanical milling

The structure and hydriding/dehydriding behaviors of La2Mg17-10 wt.%Ni composite prepared by mechanical milling were investigated. Compared with the un-milled sample, the as-milled alloys were ready to be activated and the kinetics of hydrogen absorption was relatively fast even at environmental temperature. The composite milled for 10 h absorbed 3.16 wt.% hydrogen within 100 s at 290 K. The kinetic mechanisms of hydriding/dehydriding reactions were analyzed by using a new model. The results showed that hydrogenation processes for all composites were controlled by hydrogen diffusion and the minimum activation energy was 15.3 kJ/mol H2 for the composite milled for 10 h. Mechanical milling changed the dehydriding reaction rate-controlling step from surface penetration to diffusion and reduced the activation energy from 204.6 to 87.4 kJ/mol H2. The optimum milled duration was 5 h for desorption in our trials.     

Hydrogen storage properties of mechanically milled La2Mg17-x wt.% Ni （x=0, 50, 100, 150 and 200） composites

Melting method was used to obtain La2Mg17 alloy,and then Ni powder was added by mechanical alloying method.The kinetics of hydriding process and electrochemical properties of La2Mg17-x wt.%Ni(x=0,50,100,150,200) composites were investigated.X-ray diffraction(XRD) and scanning electron microscopy(SEM) analyses showed that the crystal structure of composite alloy gradually transformed into amorphous phase by the effect of ball milling and Ni powders.The research of hydrogen absorption properties found that La2Mg 17-50 wt.%Ni reached the highest hydrogen absorption than other alloys with more addition of Ni content,reached to 5.796 wt.% at 3 MPa,and up to 5.229 wt.% merely in 2 min,which revealed that the amorphous phase reduced the H occupation of the lattice clearance,resulting in the decline of hydrogen absorption capacity.The electrochemical tests indicated that the maximum discharge capacity increased to 353.1 mAh/g at 30 oC,however,the cycle stability decreased considerably.A series of kinetic measurements demonstrated that the controlling steps of electrochemical process of La 2 Mg 17-x wt.%Ni alloys transferred from hydrogen diffusion on alloy bulk(x=50,100) to hydrogen diffusion on both alloy bulk and surface(x=150,200).     

The formation and dissociation of magnesium alloy hydrides and their use for fuel storage in the hydrogen car

The formation of hydrides by the reaction of high-pressure hydrogen (300 to 800 psi) with Mg-10Al and Mg-25Ni was studied at 400 and 450°C. Although the reaction kinetics for both alloys can be described by the Johnson-Mehl relationship, the morphology and nature of the reaction products were very different for the two alloys. The temperature dependence of the rates suggests that an interfacial reaction, possibly the transfer of hydrogen from the metal to the hydride, may be the rate-controlling step. One hydride, MgH₂, formed internally in the Mg-10Al alloy as spherical particles of nearly constant size at any time, indicating that nucleation was rapid. The reaction involving Mg-25Ni resulted in two hydrides, MgH₂ and Mg₂NiH₄, the former being more stable and forming first by consuming the primary magnesium phase of the eutectic structure. The hydride advanced into the alloy as a “front”, after which the eutectic plates of Mg₂Ni reacted to form the ternary hydride. Finally, the larger primary plates of Mg₂Ni reacted. Dehydriding of the Mg-10Al alloy hydride also followed the Johnson-Mehl relationship and was found to be complete in 90 min at 300°C, these conditions being favorable for the use of this hydride in vehicles combusting hydrogen. On the other hand, dehydriding of the hydrided Mg-25Ni alloy occurred by a twostep process in which some of the Mg₂NiH₄ dissociated followed by complete dissociation of this hydride and of the MgH₂. Only a small fraction, 10 pct of the available hydrogen could be recovered in several hours at 300°C. The Mg-10Al alloy exhibited a much higher resistance to fragmentation during hydriding than did the Mg-25Ni alloy. An analysis of numerous factors required of hydrides for use in vehicular applications showed that Mg-10Al was much better than Mg-25Ni, although the charging time is much too long.     

Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics

In this work, a Mg-based composite material with in-situ formed LaH₃, Mg₂NiH₄-LiBH₄ + 20 wt% LaH₃, was prepared by ball milling LiBH₄ and hydrogenated LaMg₂Ni and Mg₂Ni powder mixture, followed by heat treatment at 573 K. The onset dehydrogenation temperature of the composite is reduced by 50 K compared with that of Mg₂NiH₄-LiBH₄. The LaH₃-doped composite shows faster kinetics, absorbing 1.43 wt% hydrogen within 100 s at 423 K, which is 6.5 times faster than Mg₂NiH₄-LiBH₄. Moreover, the composite releases 1.24 wt% hydrogen within 500 s at 573 K, 0.69 wt% higher than Mg₂NiH₄-LiBH₄. The activation energy of the composite is reduced by 8.2 and 80 kJ/mol compared with that of Mg₂NiH₄-LiBH₄ and commercial MgH₂, respectively. The improvement in hydrogen storage properties is attributed to the fact that LaH₃ promotes the generation of nano-sized spongy Mg structure, which has good catalytic activity during the subsequent hydrogenation/dehydrogenation process.     

Reversible Hydriding of LaNi5 − xAlx ― Pd Composites in the Presence of Carbon Monoxide

Carbon monoxide is one of the most dangerous impurities in gas mixtures involved in the hydriding of intermetallic compounds, and up to the present time the concentration limit 0.03 vol.% CO has not been surmounted. The objective of the present work was to investigate the possibility of reversibly hydriding LaNi_{5 − x} Al _x ― Pd composites in gas mixtures containing up to 5 vol.% CO. The experimental specimens were mixtures of palladium black and LaNi₅, LaNi_4.7Al_0.3, LaNi_4.5Al_0.5, LaNi_4.2Al_0.8, and LaNi₄Al powders cold pressed at 300 MPa. The intermetallics were preliminarily dispersed by hydriding and mechanical grinding. The concentration of palladium black in the powder mixtures was varied from 0 to 1.5 mass%. It was shown that it is possible to reversibly hydride LaNi_{5 − x} Al _x ― Pd composites in mixtures of hydrogen and carbon monoxide containing up to 5 vol.% CO at the temperatures 423 and 473 K. It was established that alloying LaNi₅ with aluminum in amounts which raised the stoichiometric concentration above 0.5 stabilized the absorption properties of LaNi_{5 − x} Al _x ― Pd in the presence of CO. The composition LaNi₄Al was optimal. Palladium additions to LaNi_{5 − x} Al _x ― Pd composites also increased the stability of hydrogen absorption by the intermetallic. It is noted that the effect of palladium was more positive than that of aluminum.

     

Hydrogen Storage Properties of Graphite-Modified Mg-Ni-Ce Composites Prepared by Mechanical Milling Followed by Microwave Sintering

The Mg₁₇Ni_1.5Ce_0.5 hydrogen storage composites with different contents of graphite were prepared by a new method of mechanical milling and subsequent microwave sintering. The small particle size (~25 ??m) and the low echo ratio of power indicate that graphite plays an important role not only as a lubricant during mechanical milling but also as a supplementary heating material during microwave sintering. As a catalyst in the hydriding/dehydriding (H/D) reaction, graphite also improved the hydrogen storage properties of the composites. The hydrogen absorption and desorption capacities of Mg₁₇Ni_1.5Ce_0.5 with 5 wt pct graphite were 5.34 and 5.30 wt pct H₂ at 573 K (300 °C), its onset temperature of dehydriding reaction was 511 K (238 °C), and its activation energies of H/D reaction were 40.9 and 54.5 kJ/mol H₂, respectively. The kinetic mechanisms of the H/D reaction are also discussed.     

Reversible Hydriding of LaNi5 − xAlx ― Pd Composites in the Presence of Carbon Monoxide

Carbon monoxide is one of the most dangerous impurities in gas mixtures involved in the hydriding of intermetallic compounds, and up to the present time the concentration limit 0.03 vol.% CO has not been surmounted. The objective of the present work was to investigate the possibility of reversibly hydriding LaNi_{5 ? x} Al _x ― Pd composites in gas mixtures containing up to 5 vol.% CO. The experimental specimens were mixtures of palladium black and LaNi₅, LaNi_4.7Al_0.3, LaNi_4.5Al_0.5, LaNi_4.2Al_0.8, and LaNi₄Al powders cold pressed at 300 MPa. The intermetallics were preliminarily dispersed by hydriding and mechanical grinding. The concentration of palladium black in the powder mixtures was varied from 0 to 1.5 mass%. It was shown that it is possible to reversibly hydride LaNi_{5 ? x} Al _x ― Pd composites in mixtures of hydrogen and carbon monoxide containing up to 5 vol.% CO at the temperatures 423 and 473 K. It was established that alloying LaNi₅ with aluminum in amounts which raised the stoichiometric concentration above 0.5 stabilized the absorption properties of LaNi_{5 ? x} Al _x ― Pd in the presence of CO. The composition LaNi₄Al was optimal. Palladium additions to LaNi_{5 ? x} Al _x ― Pd composites also increased the stability of hydrogen absorption by the intermetallic. It is noted that the effect of palladium was more positive than that of aluminum.

     

Investigations on hydrogen storage properties of LaMg8.52Ni2.23M0.15 （M=Ni, Cu, Cr） alloys

LaMg8.52Ni2.23M0.15 (M=Ni, Cu, Cr) alloys were prepared by induction melting. X-ray diffraction showed that all the three alloys had a multiphase structure, consisting of La2Mg17, LaMg2Ni and Mg2Ni phases. Energy dispersive X-ray spectrometer results revealed that most of Cu and Cr distributed in Mg2Ni phase. La2Mg17 and LaMg2Ni phases decomposed into MgH2, Mg2NiH4 and LaH3 phases during the hydrogenation process. Hydriding/dehydriding measurements indicated that the reversible hydrogen storage capacities of Mg2Ni phase in LaMg8.52Ni2.23M0.15 (M=Cu, Cr) alloys increased to 1.05 wt.% and 0.97 wt.% from 0.79 wt.% of Mg2Ni phase in LaMg8.52Ni2.38 alloy at 523 K. Partial substitution of Cu and Cr for Ni decreased the onset dehydrogenation temperature of the alloy hydrides and the temperature lowered by 18.20 and 5.50 K, respectively. The improvement in the dehydrogenation property of the alloys was attributed to that Cu and Cr decreased the stability of Mg2NiH4 phase.     

Gaseous hydrogen storage properties of Mg-Y-Ni-Cu alloys prepared by melt spinning

For purpose of promoting the hydrogen absorption and desorption thermodynamics and kinetics properties of Mg-Ni-based alloys, partially substituting Y and Cu for Mg and Ni respectively and melt spinning technique were applied for getting Mg_(25-x)Y_xNi_9 Cu(χ = 0-7) alloys. Their microstructures and phases were characterized with the help of X-ray diffraction and transmission electron microscopy. Their hydrogen absorbing and desorbing properties were tested by a Sievert apparatus, DSC, and TGA, which were connected with a H_2 detector. In order to estimate the dehydrogenation activation energy of alloy hydride, both Arrhenius and Kissinger methods were applied for calculation. It is found that their hydriding kinetics notably declines, however, their hydrogen desorption kinetics conspicuously improves, with spinning rate and Y content increasing. Their hydrogen desorption activation energy markedly decreases under the same constraint, and it is found that melt spinning and Y substituting Mg improve the real driving force for dehydrogenation. As for the tendency of hydrogen absorption capacity,it presents an elevation firstly and soon after a decline with the rising of spinning rate, however, it always lowers with Y content growing. With Y content and spinning rate increasing, their thermodynamic parameters(△H and △S absolute values) visibly decrease, and the starting hydrogen desorption temperatures of alloy hydrides obviously lower.     

Hydriding and Dehydriding Characteristics of Mechanically Alloyed LaMg_(17) Ni Composite Material

Magnesiumandmagnesiumalloyshavebeenin vestigatedashydrogenstoragematerialsforseveralde cadesbecausefarmorehydrogenbyweightcanbestoredinthemthaninmostoftheothercurrentlyknownhydrogenstoragealloys .Moreover ,thehighnaturalabundanceofMg ,itslightmassandenviron mentalcompatibilitypotentiallymakemagnesiumoneofthemostprospectivecandidatesforfuturehydrogenstoragematerials .Unfortunately ,thepracticalappli cationofMganditsalloyshasbeenlimitedonlytocertainstoragedevicebecauseoftheirpoorhydriding dehydr…     

Factors controlling hydrogen-induced amorphization of C15 Laves compounds

The radii of tetrahedral holes occupied by hydrogen atoms in the amorphous hydrides a-RFe₂H_x (R = a rare earth metal) are larger than the empirical minimum hole size, 0.040 nm, of the general stable hydrides A_xB_1−xH_y. However, the radii of the holes in the corresponding crystalline hydrides c-RFe₂H_x are smaller than this value. Thus, the higher stabilities of the amorphous hydrides which are considered to be the origin for the driving force of hydrogen-induced amorphization (HIA) in the C15 Laves compounds are interpreted on the basis of the differences in the hole sizes. Furthermore, the occurrence of HIA in C15 AB₂ Laves compounds has been analyzed in terms of the tetrahedral hole size, the stability of the compounds, the atomic size ratio, the valence electron concentration and the contraction or the expansion of the constituent elements and the holes. The atomic size ratio is the single most important factor controlling the occurrence of HIA in these compounds, and the compounds with the ratio above 1.37 are amorphized by hydrogenation.     

Hydriding/dehydriding properties of NdMgNi alloy with catalyst CeO2

Hydrogen storage composites Nd2Mg17-50 wt.%Ni-x wt.%CeO2（x=0, 0.5, 1.0, 1.5, 2.0） were obtained by induction-ball milling method. The catalytic effect of CeO₂ on hydriding kinetics of Nd₂Mg17-50 wt.%Ni composite was investigated. X-ray diffraction（XRD） and high resolution transmission electron microscopy（HRTEM）, selected area electron diffraction（SAED） analyses showed that Nd₂Mg17-50 wt.%Ni alloy had a multiphase structure, consisting of NdMg12, NdMg₂Ni, Mg₂Ni and Ni phases and the addition of catalyst CeO₂ prompted the composites to be partly transformed into amorphous strucutre. The CeO₂ improved the maximum hydrogen capacity of Nd₂Mg17-50 wt.%Ni alloy from 3.192 wt.% to 3.376 wt.%（x=1.0）. What’s more, the increment of diffusion coefficient D led to the faster hydriding kinetics, which was calculated by Avrami-Erofeev equation. The dehydrogenation temperature reduced from 515.54 to 504.72 K was mainly caused by the decrease of activation energy from 93.28 to 69.36 kJ /mol, which was proved by the Kissinger equation.     

CeO2-x modified Ru/γ-Al2O3 catalysts for ammonia decomposition reaction

Developing high-performance ammonia decomposition catalysts for preparing CO_x-free hydrogen shows great practical significance.Herein,CeO₂ is used as a promoter to modulate the metal-support interaction to enhance the catalytic performance of Ru/Al₂O₃ catalysts.A series of 1Ru/xCe-10AI(x=0.5,1,or 3)catalysts was prepared by a facile colloidal deposition method.We find that the optimized 1 Ru/1Ce-10Al catalyst exhibits excellent activity for the decompo...     

Highly ameliorated gaseous and electrochemical hydrogen storage dynamics of nanocrystalline and amorphous LaMg12-type alloys prepared by mechanical milling

Nanocrystalline and amorphous LaMg12-type alloy-Ni composites with a nominal composition of LaMg11 Ni+x wt.% Ni (x=100, 200) were synthesized via ball milling.The influences of ball mill-ing duration and Ni adding amount x on the gaseous and electrochemical hydrogen storage dynamics of the alloys were systematically studied.Gaseous hydrogen storage performances were studied by a differential scanning calorimeter and a Sievert apparatus.The dehydrogenation activation energy of the alloy hydrides was evaluated by Kissinger method.The electrochemical hydrogen storage dynam-ics of the alloys was investigated by an automatic galvanostatic system.The H atom diffusion and ap-parent activation enthalpy of the alloys were calculated.The results demonstrate that a variation in Ni content remarkably enhances the gaseous and electrochemical hydrogen storage dynamics perform-ance of the alloys.The gaseous hydriding rate and high-rate discharge (HRD) ability of the alloys ex-hibit maximum values with varying milling duration.However, the dehydriding kinetics of the alloys is always accelerated by prolonging milling duration.Specifically, rising milling time from 5 to 60 h makes the hydrogen desorption ratio (a ratio of the dehydrogenation amount in 20 min to the saturat-ed hydrogenation amount) increase from 57% to 66% for x=100 alloy and from 57% to 70% for x=200.Moreover, the improvement of gaseous hydrogen storage kinetics is attributed to the descending of dehydrogenation activation energy caused by the prolonging of milling duration and growing of Ni content.     

掺杂对氢化燃烧合成镁基储氢合金性能的影响

借助XRD、SEM和自制放氢量的测试装置研究了掺杂对氢化燃烧合成镁镍储氢合金性能的影响。结果表明：三种掺杂中以掺富铈镧系金属产生的镁镍氢化物最多,掺铜产生镁镍氢化物的晶格畸变最为明显。晶胞分析显示Mg_2NiH_4的晶胞参数都有一定的变化。300℃、0．1 MPa下放氢速率的测量显示,掺杂降低了放氢温度,放氢速率一般为6～10 min。掺铜放氢量为2．68％,掺钛放氢量为2．35％,掺富铈镧系金属放氢量为3．10％,掺钛、掺富铈镧系金属活化可适当提高吸放氢量。     

Influence of partial substitution of cerium for lanthanum on magnetocaloric properties of La_(1–x)Ce_xFe_(11.44)Si_(1.56) and their hydrides

The structure and magnetocaloric properties of La1–xCexFe11.44Si1.56 and their hydrides La1–xCexFe11.44Si1.56Hy（x=0, 0.1, 0.2, 0.3, 0.4） were investigated.The samples crystallized mainly in the cubic Na Zn13-type structure with a small amount of α-Fe phase as impurity.The lattice constants and Curie temperature presented the same change tendency with increasing of Ce content.For the hydrides, the influence of Ce content on lattice constants was weakened and the values of H concentration y were approximate to be 1.56.The La1–xCexFe11.44Si1.56 compounds exhibited large values of isothermal entropy change –ΔSm around the Curie temperature TC under a low magnetic field change of 1.5 T.The value of –ΔSm increased and then decreased with increasing Ce content, reached the maximum, 26.07 J/kg·K for x=0.3.TC increased up to the vicinity of room temperature by hydrogen absorption for the Ce substituted compounds, but TC only slightly decreased with increasing Ce content.The first-order metamagnetic transition was still kept in the hydrides and the maximum values of –ΔSm were lower than those of the La1–xCexFe11.44Si1.56 compounds, but still remained large values, about 10.5 J/kg K under a magnetic field change of 1.5 T.The values of –ΔSm were nearly independent of the Ce content and did not increase with increasing x for the hydrides.The La1–xCexFe11.44Si1.56Hy（x=0–0.4） hydrides exhibited large magnetic entropy changes, small hysteresis loss and effective refrigerant capacity covered the room temperature range from 305 to 317 K.These hydrides are very useful for the magnetic refrigeration applications near room temperature under low magnetic field change.     

Microstructural Evolution,Powder Characteristics,Compaction Behavior and Sinterability of Al 6061–B<Subscript>4</Subscript>C Composites as a Function of Reinforcement Content and Milling Times

Al 6061_100–x–x wt % B₄C (x = 0, 5, 10, 20, 30 and 40) composites, prepared by mechanical alloying and compacted at room temperature, have been used for the present investigation. The effects of B₄C content and milling time on the powder morphology, powder particle size, and other powder characteristics such as the apparent density, tap density, flow rate, cohesiveness, and hausner ratio are systematically investigated. The steady state of milling process is determined by observing the correlation between apparent densities and milling time explained by the morphological evolution of the powder particles during the milling process. The Hausner ratio (HR), estimated to evaluate friction between the particles, decreases with an increase in milling duration and B₄C content due to the changes in morphology and hardness of the powders. The compressibility behavior of post-compacts as a function of compaction pressure and the B₄C content was analyzed by using several linear and non-linear powder compaction equations. The linear Panelli and Ambrozio Filho, and non-linear Van Der Zwan and Siskens equations give the highest regression coefficients. The results are explained in terms of the plastic deformation capacity and plastic deformation coefficient of the powders, which are influenced by the hardness and the morphology of the powder. After compaction, the supersolidus liquid phase sintering was performed at various temperatures (585, 610 and 630°C) under high purity nitrogen atmosphere. The results revealed that the sinterability was degraded by increasing the reinforcement content, particularly above 10 wt % B₄C. Neutron radiography measurements conducted on the rolled composite sheet have revealed the uniform distribution of B₄C particles in the composite.     

Low-temperature catalytic combustion of trichloroethylene over MnOx-CeO2 mixed oxide catalysts

The cerium-based catalysts were investigated for the catalytic co mbustion of trichlo roethylene(TCE) and exhibit a surprising catalytic activity.MnO_x was doped into CeO₂ by a citric acid(CA) sol-gel method,and the effect of Mn content on the physicochemical properties and catalytic activities of MnO_x-CeO₂ mixed oxides was investigated systemically.The introduction of MnO_x into CeO₂ can evidently improve the catalytic activity and...