Ynghun Zhng ,Xin Wei ,Wei Zhng ,Zeming Yun ,Jinling Go ,Huiping Ren

a Key Laboratory of Integrated Exploitation of Baiyun Obo Multi-Metal Resources,Inner Mongolia University of Science and Technology,No.7 Aerding Road,Kun District,Baotou 014010,China

b Department of Functional Material Research,Central Iron and Steel Research Institute,No.76 Xueyuannan Road,Haidian District,Beijing 100081,China

c Weishan Cisri-Rare Earth Materials Co.,Ltd.,Jining 277600,China

Abstract In this investigation,mechanical grinding was applied to fabricating the Mg-based alloys La7Sm3Mg80Ni10+5wt.% M(M=None,TiO2,La2O3)(named La7Sm3Mg80Ni10–5M(M=None,TiO2,La2O3)).The result reveals that the structures of as-milled alloys consist of amorphous and nanocrystalline.The particle sizes of the added M(M=TiO2,La2O3)alloys obviously diminish in comparison with the M=None specimen,suggesting that the catalysts TiO2 and La2O3 can enhance the grinding efficiency.What’s more,the additives TiO2 and La2O3 observably improve the activation performance and reaction kinetics of the composite.The time required by releasing 3wt.% hydrogen at 553,573 and 593K is 988,553 and 419s for the M=None sample,and 578,352 and 286s for the M=TiO2 composite,and 594,366,301s for the La2O3 containing alloy,respectively.The absolute value of hydrogenation enthalpy change |?H| of the M(M=None,TiO2,La2O3)alloys is 77.13,74.28 and 75.28 kJ/mol.Furthermore,the addition of catalysts reduces the hydrogen desorption activation energy().

Keywords: Mg-based alloy;Ball milling;Catalysts;Hydrogen storage kinetics.

1.Introduction

Hydrogen is considered the most promising fuel applied in fuel-cell vehicle because of the wide applicability,high application efficiency,safety,environmentally friendly and inexhaustible reserves characteristics of it[1–3].An accepted view considers that the widespread utilization of clean renewable energy vehicles is an effective measure to deal with environmental pollution and excessive consumption of the limited fossil fuels because transportation consumes about a quarter of global energy while there is about 23% carbon dioxide emission originating from the combustion of fossil fuels in the world [4–6].However,the chemical energy of unit volume hydrogen is low,which limits the commercialization of it.Therefore,a lot of investigations pay attention to dealing with this problem and finding the safe,suitable hydrogen storing materials that can meet the needs of using hydrogen in the field of motor vehicle and electronic products[7–11].The ideal hydrogen storage methods should have the following characteristics,large capacity,low-temperature fast adsorption rate and high cycle survivability [12].In present,hydrogen storage methods include high-pressure storage,liquid storage,physical adsorption storage and hydride storage.Generally,the gas storage needs 70 MPa pressure for reaching only 4.8wt.% H2capacity [13].The condition of liquid storage is low-temperature(20 K)that brings about huge energy consumption(about 30% for filling)and short boiling storage duration [14].Solid hydrogen storage is reckoned as the best one among the various methods [15,16].

Among the metal hydrides,MgH2with high hydrogen storage capacity(7.6 wt.%),low cost,non-toxicity,abundant reserves and superior reversibility characteristics is the most promising material for hydrogen storage [17,18].Nonetheless,the attempt of the widespread application of commercial MgH2is retarded by its some disadvantages,such as high thermodynamic stability(i.e.the high strength of the Mg-H bonds),the complicated activation procedure,the high dissociation temperature and the sluggish hydrogenation/dehydrogenation kinetics [19,20].To copy with the dilemma,many studies have been conducted to reduce the decomposition temperature,accelerate the adsorption kinetics and change the reaction thermodynamics by the method of grain refinement [11,21],mixing catalysts [22–24],ballmilling [25,26],alloying [27,28],surface modification [29,30]and the other [31].The mechanical grinding and meltspinning [32,33]are very efficient means to reduce the particle or grain size.Especially,the mechanical milling can realize the refinement of grains and particles simultaneously[38].In addition,mechanical milling brings on the formation of crystal defects and modifies the superficial characteristics of alloy particles [1].Hence,mechanical grinding is reckoned as the best way to improve the hydrogen storing performance of magnesium based materials [34].Besides,refining the particle to very small size will introduce the capillarity effect that can ameliorate the hydrogen absorption and desorption thermodynamics in theory.The calculation results indicate that a particle radius on the order of 5 nm will reduce the dehydrogenation enthalpy of pure Mg by about 10%[35].As well known,there is considerable energy used for the hydrogen dissociation on the magnesium surface and this segment is regarded as a rate controlling factor in the procedure of hydrogen absorption [36].Transition metals either in their pure form(e.g.:Ni,Ti,Nb,Fe,Co,Al etc.)[37,38]or as oxides(e.g.:Nb2O5,Fe2O3,TiO2etc.)[39–42],hydrides(e.g.:TiH2,ZrFe2Hx,etc.)[43,44],fluoride(e.g.:FeF3,TiF3,NiF2,NbF5etc.)[45,46]or intermetallics [47,48],can act as the catalysts because they can weaken this dissociation energy.As studied by Du et al.[49]and Pozzo and Alfe [50],the hydrogen dissociation energy on magnesium surface is 1.15 eV but it can be decreased to 0.03,0.06,0.56,0.39 eV by the addition of Co,Ni,Cu,Pd,severally.Daryani et al.[51]researched that adding 6 mol% TiO2could improve the hydrogen absorbing kinetics and reduce the decomposition temperature of as-milled Magnesium hydride by 100 K.According to the research of Shahi et al.[52],the composite MgH2?5 wt.%Ni absorbs 5.0 wt.%H2hydrogen at the temperature of 443 K in 15 min and at the temperature 613 K it starts to decompose.As investigated by Hou et al.[53],Mg2NiH4with the catalyst composed of MWCNTs and TiF3has the 503 K(516.6 K for pure Mg2NiH4)hydrogen releasing temperature(TD)and the activation energy(Ea)of it is 53.24 kJ/mol(90.13 kJ/mol for pure Mg2NiH4).In particular,adding appropriate rare earths or their oxides can obviously make the Mg-based hydrides unstable and accelerate the rate of dehydrogenation reaction [54,56].Lass [56]found that the Mg85Ni15-xMx(M=La,x=0 or 5)alloys possess a lower enthalpy change in the reaction of producing MgH2and Mg2NiH4.On the basis of the investigation of Luo et al.[55],the element Y is beneficial to improve the thermodynamics property of magnesium based materials and the composite Mg90In5Y5has a lower?H(about 62.9 kJ/(mol H2))in comparison with the Mg95In5binary alloy(about 67.9 kJ/(mol H2))and pure Mg(about 74.9 kJ/(mol H2)).Sadhasivam et al.[57]found that the original desorption temperature of the composite MgH2?5 wt.% Mm-oxide was reduced by 76 K from 654 to 578 K.Kalinichenka et al.[58]researched the improved reaction kinetics of Mg90Ni8RE2(RE=Y,Nd,Gd)and found that the activated Mg90Ni8RE2could reversible absorb and release 5.5 wt.% H2within 20 min.

According to our investigation on REMg11Ni(RE=Sm,Y)+5 wt.% M(M=MoS2,CeO2)composites,the additives MoS2and CeO2play a catalytic role in improving hydrogen storing performance [59,60]and 5 wt.% addition of catalysts is optimal as studied in this reference [33].It must be very interesting to compare the effects of TiO2and La2O3additives with high hardness on the hydrogen storing performance of ball milling magnesium based materials.Thereby,the alloys La7Sm3Mg80Ni10–5 M(M=None,TiO2,La2O3)were fabricated by mechanical milling.The thermodynamics and dynamics of the experimental alloys were investigated.A comprehensive comparison of the impacts of different catalysts on the structure and hydrogen storing performances of the alloys is conducted.

2.Experimental

The La7Sm3Mg80Ni10material was fabricated by inductive melting La,Sm,Mg,Ni(purity ≥99.9%)under 0.04 MPa He(purity ≥99.999%)to inhibit the volatilization of magnesium.To compensate the melting losses,additional magnesium(8 wt.%)and RE(RE=La,Sm)(5 wt.%)are required.The above-mentioned materials were all provided by CISRI Corporation.A Varian Liberty 100 inductively-coupled plasma(ICP)was applied to determining the chemical composition of experimental alloys.Then the ingot was mechanically crushed and ground to the 200–400 meshes powders.The obtained power with 5 wt.% TiO2or La2O3(purity ≥99.9%)catalyst was mechanically ground by a mill crusher at the speed of 350 rpm(the weight ratio of specimen and balls is 1:40).The milling duration is set at 20 h.Thus,the chemical compositions of the as-milled powder were La7Sm3Mg80Ni10–5 M(M=None,TiO2,La2O3).In order to heat dissipation and reduce the cold welding of powder in the process of milling,the working mechanism of ball mill is to stop half an hour every 3 h and the powder adhered to the milling chamber walls and grinding balls needs to be scrapped in due course of time all of these operations were operated under the protective atmosphere of Ar.

X-ray diffraction(XRD)(D/max/2400)determined the phase structures and compositions of the alloys.The experimental parameters were 40 kV,160 mA,and 2°/min with 2θchanging from 20° to 90°.The radiation was CuKα1filtered by graphite.The particles morphology observation was completed by a scanning electron microscope(SEM)(QUANTA 400).A high resolution transmission electron microscope(HRTEM)(JEM-2100F,operated at 200 kV)was utilized to the characterization of microstructure and crystalline state.

Fig.1.XRD profiles of the as-milled(20 h)La7Sm3Mg80Ni10?5M(M=none,TiO2,La2O3)alloys:(a)before hydrogen absorption,(b)after hydrogen absorption,(c)after hydrogen desorption.

Hydrogenation and dehydrogenation kinetics curves of the as-milled specimens were tested by automatic Sieverts apparatus.Prior to measuring,the sample need to be activated by six hydriding/dehydriding cycles(633 K and original hydrogen pressure of 3 MPa for hydrogen absorption,633 K and 1×10?4MPa original pressure for hydrogen desorption).The temperature of hydrogen absorption was set as 473,513,533,553,573,593,613 and 633 K,severally,while 553,573,593,613 and 633 K for hydrogen desorption.The setting of initial hydrogen pressure is the same as activation.The sample mass required for every determination was 300 mg.Nonisothermal hydrogen desorption property was researched by utilizing thermogravimetry(TGA)and differential scanning calorimetry(DSC)(SDTQ600)whose heating rates were 5,10,15 and 20 K/min.

3.Results and discussion

3.1.Microstructure characteristics

Fig.1 gives the X-ray diffraction of the as-milled La7Sm3Mg80Ni10–5 M(M=none,TiO2,La2O3)composites before and after hydrogen absorption and desorption under the condition of 633 K,3 MPa and 633 K,1×10?4MPa,severally.ICDD(International Centre for Diffraction Data)identification of X-ray diffraction patterns shows that the as-milledM=none specimen consists of the major phase La2Mg17and secondary phases Mg2Ni and La2Ni3.The addition of catalysts TiO2and La2O3do not introduce any new phase,indicating that these additives are not involved in the reaction with alloy.Moreover,it is visible to observe the broadened diffraction peaks representing the typical nanocrystalline and amorphous structures of as-milled specimens in comparison with that of the as-cast one(XRD patterns are not show here).After hydrogen absorption,the diffraction peaks get narrow and sharp.Meanwhile there are four hydrides become visible and emerge in the specimens,including MgH2,Mg2NiH4,LaH3and Sm3H7.The reaction relationship between the elements is as follows:

La2Mg17+H2→LaH3+MgH2

Mg2Ni+H2→Mg2NiH4

La2Ni3+La2Mg17+H2→LaH3+Mg2NiH4+MgH2

Sm+H2→Sm3H7

After dehydrogenated,the four phases Mg,Mg2Ni,LaH3and Sm3H7can be found.Evidently,the LaH3and Sm3H7phases are not decomposed because of the high thermal steadiness of them.Hence,the hydrogen desorption reactions are summarized as the following two equations:

MgH2→Mg+H2

Mg2NiH4→Mg2Ni+H2

According to the above inference,we can see that in the process of hydrogenation and dehydrogenation,the reversible reactions of activated composites include

Mg+H2?MgH2

Mg2Ni+H2?Mg2NiH4

Fig.2.SEM morphologies of the as-milled(20 h)La7Sm3Mg80Ni10?5M(M=none,TiO2,La2O3)alloys:(a) M=none,(b) M=TiO2,(c) M=La2O3.

Through a careful observation,we find that the width of the XRD peak narrows down after dehydrogenation compared with that after hydrogen absorption,which was owing to the cell volume reduction and stress relief rendered by hydrogen desorption.As found by Montone et al.[61],the volume of a metallic Mg atom is about 33% smaller than that of Mg atom in MgH2.It has been reported in the literature [62,63]that lattice distortion along with expansion and contraction of cell volume are inevitable in hydrogen storage materials during hydrogen absorption and desorption,which will cause many lattice defects such as vacancy and dislocation.The formation of the defects will have a beneficial effect on the hydrogen absorption and desorption property of the alloy.Mechanical milling of Mg-based alloy with TiO2and La2O3catalysts creates the defects on the surface and inside the magnesium matrix,which generate reactive clean surfaces and shrink the particle size of Mg.The creation of defects facilitates nucleation,the production of the reactive clean surface enhances the superficial reactivity,and the diminution of particle decreases the diffusion distances of hydrogen atoms.These effects ameliorate the hydriding and dehydriding kinetics of magnesium based alloy significantly.

The morphologies of the as-milled La7Sm3Mg80Ni10–5 M(M=none,TiO2,La2O3)alloy powder are observed by SEM and presented in Fig.2.As can be observed,the alloy particles have the typical morphology of ball-milling powder and the size of them is in a range of 1–10 μm.After careful observation,it is failed to find the catalysts TiO2and La2O3particles,which indicates that the TiO2and La2O3particles are not appear on the superficial part of alloy,but is wrapped in them.Evidently,the agglomeration tendency of the as-milled particles of theM=TiO2andM=La2O3alloys was decreased(Fig.2b and c)with smaller size than that ofM=none alloy.It means that adding a certain amount of TiO2and La2O3can significantly improve the efficiency of ball milling.After comparing the particles with different catalysts,we found no obvious difference in particle size,suggesting that two catalysts have similar effect on the efficiency of ball milling.As considered by Floriano et al.[64],some catalysts with high hardness,e.g.La2O3,CeO2,TiO2Nd2O5,etc.can act as lubricants,dispersants and/or cracking agents in the procedure of milling and are helpful to further reduce refine the particles of as-milled alloy.A very similar result also appears in the investigation of Daryani et al.[51]and Aguey-Zinsou et al.[65].

The HRTEM micrographs and SAED(Selected Area Electron Diffraction)patterns of the as-milled La7Sm3Mg80Ni10?5 M(M=none,TiO2,La2O3)materials are presented in Fig.3.We have noticed the nanocrystalline and amorphous structures of the as-milled alloys and the emergence of crystal defects.The SAED patterns also prove the existence of La2Mg17,Mg2Ni and La2Ni3phases,and there is no any new phase caused by adding TiO2and La2O3.After hydrogen absorption,the structures of as-milled alloy still are amorphous and nanocrystalline(Fig.3b,e and h),but there is an observably decrease in amorphous phase,meaning that the dehydrogenation promotes the crystallization reaction.Four hydrides MgH2,Mg2NiH4LaH3and Sm3H7also can be identified after hydrogen desorption by SAED patterns.According to Fig.3(c),(f)and(i),we can see that the alloys after hydrogen desorption exhibit an entirely crystal structure,and the size of grain evidently increase,and Pukazhselvan et al.[66]also had the similar report.The SAED rings of dehydrogenated alloys reflect the existence of Mg,Mg2Ni,LaH3and Sm3H7.Apparently,it is consistent with the result of XRD,the LaH3and Sm3H7phases still exist after dehydrogenation.In addition,it is found from Fig.2 that the LaH3,Sm3H7,TiO2and La2O3nanoparticals distribute in Mg matrix dispersedly and uniformly,which is considered to be the preferred nucleation sites for hydride formation/decomposition.The phase interfaces of LaH3(or Sm3H7,TiO2and La2O3)/Mg(or MgH2)provide channels for the diffusion of hydrogen atoms.Therefore,the additives can be regarded as catalysts to improve the hydrogen storage performance of magnesium and Mg-based alloy [67].

Fig.3.HRTEM micrographs and SAD patterns of the as-milled La7Sm3Mg80Ni10?5M(M=none,TiO2,La2O3)alloys:(a),(b),(c) M=none alloy before hydrogen absorption,after hydrogen absorption,after hydrogen desorption;(d),(e),(f) M=TiO2 alloy before hydrogen absorption,after hydrogen absorption,after hydrogen desorption;(g),(h),(i) M=La2O3 alloy before hydrogen absorption,after hydrogen absorption,after hydrogen desorption.

3.2.Activation behaviors and hydrogen absorption kinetics

In this investigation,it was found that the alloy powder prepared by traditional mechanical milling can hardly absorb hydrogen because the alloy powder exposed to air easily forms an oxide film on the particle surface that blocks the contact between H2molecules and alloy surface and prevents H2from dissociating into H atoms.As well known,this dissociation process is the basic step in the phase transformation from metallic Mg to MgH2and it is necessary for the incorporation of H atoms into the Mg lattice.Fortunately,it is found that when the alloy sample is kept under proper temperature and hydrogen pressure for a long time,the oxide film formed can be broken gradually,which results in exposing the fresh alloy surface and restoring the hydrogen absorption capability of the alloys.This process is called as activation.The activation performance of specimens is greater if it needs less cycle numbers.Fig.4 demonstrates the isothermal hydrogen absorption and desorption curves of the activated La7Sm3Mg80Ni10–5 M(M=none,TiO2,La2O3)materials.It is found from Fig.4(a),(b)and(c),the alloys are almost fully activated after the first cycle since the activation curves of the next five cycles are almost identical.It is noted that the time spent on the first hydrogen absorption to a saturated state is long.It takes 21716 s for theM=none alloy,14568 s for theM=TiO2alloy and 13340 s for theM=La2O3alloy to achieve the saturated capacity of 5.15 wt.%,5.052 wt.% and 4.916 wt.%,severally,suggesting that the activation property of the alloy is considerably improved by adding TiO2and La2O3.For a given hydrogen absorption capacity of 4 wt.%,by which the time required is 4820,4624 and 4758 s corresponding to the as-milledM=none,M=TiO2andM=La2O3alloys,respectively.It indicates that the hydrogenation rate is in the orderM=TiO2>M=La2O3>M=none.The improved activation ability by adding TiO2and La2O3was attributed to the modified particles surface state and the increased defect density of the crystals resulted from adding catalysts TiO2and La2O3.The catalyst nanoparticles distributing on the particles surface significantly increase the dissociation rate(limiting factor of hydrogen absorption rate)of hydrogen molecules.

Generally speaking,the first activation reaction is a long procedure [68]and in this process,the H atoms penetrate the formed oxide layer to form mental hydrides.What’s more,the attendant mechanical stress and lattice distortion are unfavorable to the absorption of hydrogen [69].As well known,the nucleation of MgH2on the superficial sites of alloy is retarded by the thin oxide layers [70].Although the operation is conducted at inert gas atmospheres the oxide layers with 3–4 nm thickness still can easily form [69].The sluggish dissociation of H2on the alloy surface is another reason to explain the slow hydrogen absorption rate [71].The dissociation on pure Mg surface needs high energy [48].Besides,the diffusion of H atoms in the metal hydrides is difficult[56,57].The growth rate of MgH2is decided by the hydrogen pressure due to the fact that higher pressure provides the greater the thermodynamics driving force for the hydrogen absorption.Nevertheless,if the original hydrogenation process is fast enough,a superficial layer of magnesium hydride will form to retard the hydrogen permeation [71].Because hydrogen diffuses along the interfaces but not along the Mg hydride layer[72],the MgH2hydride grows up in the form of slow Mg/Mg hydride interface movement.When the thickness reaches a certain value(30–50 μm),the hydrogen absorption reaction stops [73],indicating that powdered magnesium used for hydrogenation changes into massive magnesium.So the hydrogen absorption rate is affected by the powder size [74].The hydrogenation kinetics is markedly enhanced after the first hydrogen absorption and desorption cycle.With the increase in the cycle number,the hydrogen absorption kinetics curves have little change,which means the great activation performance of experimental alloys.Noticeably,the hydrogen absorbing capacity of all the specimens after first cycle first is no more than 4.8 wt.%,which represents a visible decline in the capacity.The formation of stable hydrides LaH3and Sm3H7is most likely responsible for the 0.25 wt.% capacity loss.

Fig.4.The isothermal hydrogenation/dehydrogenation curves of the as-milled La7Sm3Mg80Ni10–5M(M=none,TiO2,La2O3)alloys in the cycle activation:(a),(b),(c) M=none, M=TiO2 and M=La2O3 hydrogen absorption;(d),(e),(f) M=none, M=TiO2 and M=La2O3 hydrogen desorption.

The dehydrogenation curves of the alloys are provided in Fig.4(d),(e)and(f).It is visible that the dehydrogenation rate is fast and the dehydrogenation kinetics of the alloy was markedly improved by adding TiO2and La2O3.In particular,the first hydrogen desorption took less time.For a given hydrogen desorption capacity of 3 wt.%,by which the time required is 193,162 and 175 s corresponding to the as-milledM=none,M=TiO2andM=La2O3alloys,respectively.Evidently,the dehydrogenation rate is in the orderM=TiO2>M=La2O3>M=none.The improved activation performance is deemed to be related to the decreased particle size,the surface modification and the weakening effect of Mg-H bond strength resulted from the additive TiO2or La2O3.The improvement of thermodynamics performance is directly associated with the weakening of Mg-H bond strength.The co doping of multi elements,especially the transition element(or their compounds)and rare element(or their compounds)is beneficial to the reduction of thermal stability of MgH2[75,76].The reduction of particle observably enhances the decomposition rate of H2on the particle surface and is beneficial to the H atoms diffusion thus enhance the activation performance [77].Particularly,because TiO2and La2O3are high hardness particles,they are likely to cut into the alloy particles and form a new interface under the action of high impact stress in the ball-milling process,which may become the nucleation sites of hydride,acting as rapid paths for atoms diffusion [78].So the addition of TiO2and La2O3not only enhance the efficiency of mechanical milling,make the particle size decrease but also modify the surface of alloy particles,make the nucleation of hydrides more easy.

After the activation treatment,the hydrogen absorbing and desorbing properties of experimental composites were improved significantly.It is necessary to explore the change of structures in the process of activation.With the help of SEM,the morphological variations of the experimental composites before and after activation process are provided in Fig.5.Clearly,the particles show irregular morphologies with the very rough surface.After six hydriding and dehydriding cycles,the particle morphologies of the alloy have a dramatically change.It is very evident that many cracks appear on powder surface due to the lattice stress forming in the process of hydrogen absorption.When the lattice stress exceeds the fracture strength of the material,the pulverization of the alloy is inevitable and results in the improved properties.Through the above structural analysis,we believe that the activation is significant to the formation and decomposition of hydrides,the oxide film on the surface of alloy particles breaks,and along with the cracking of alloy particles,the specific surface area of the alloy is increased,thus improving the hydrogen absorbing and desorbing properties.

To explore the influence of the different catalysts,the hydrogen absorption curves of the as-milled La7Sm3Mg80Ni10–5 M(M=none,TiO2,La2O3)composites were tested at different temperatures from 423 to 633 K and 3 MPa,as shown in Fig.6.It is noted that at the initial stage corresponding to the rapid formation of hydride layer near the surface,the rate is very fast and hydrogen absorption capacity can reach more than 85% of saturated capacity in less than 200 s,while in the following stage it takes long time to achieve the saturated state due to the hindrance of formed hydride layer acting on the hydrogen diffusion.Freidlmeier et al.[79]considered that when the thickness of hydride layer reached to a certain value(100 nm),the rate of hydrogen absorption tended to 0.Aiming at investigate the hydrogen absorption kinetics more deeply,the time required to absorb 4 wt.% hydrogen was calculated and compared.As obtained from Fig.6,the spent time is 108,67 and 56 s at 473,513 and 533 K for theM=none specimen,96,62 and 48 s for theM=TiO2composite,and 103,64 and 53 s for theM=La2O3material,respectively.Apparently,the hydrogenation rate of the composites is in order ofM=TiO2>M=La2O3>M=none,which suggests that the added TiO2and La2O3notably ameliorate the hydrogen absorption kinetics,but this favorable effect decreases rapidly with hydrogen absorption temperature rising.Noticeably,the alloys almost display the same hydrogen absorption kinetics when the temperature exceeds 513 K,indicating the predominant role of temperature among many factors acting on the rate of hydrogen absorption.As we all know,there are three steps happening in the hydrogenation procedure of magnesium [80],namely a)the dissociation of superficial H2molecules,b)the diffusing of H atoms through grain boundaries,c)the combination of H atoms and Mg atoms to form MgH2on the Mg/catalyst interfaces.Because the hydrogen dissociation needs quite high energy,it is reckoned as the rate-controlling step[81].As confirmed by Sakintuna et al.[35],the additives transition metals or their oxides in magnesium can act as the catalysts to decrease the dissociation energy.Liu et al.[82]considered that theoretically,the substitution atoms weakened the stability of Mg-H bond owing to the interaction between the valence electrons of H and the unsaturated d/f electron shell of the transition metals or oxides,improving the hydrogen absorption performance.Agarwal et al.[47]reported that it is difficult to refine the grains of Mg by mechanical milling due to the inevitable agglomeration of particles.The additive of brittle oxides or intermetallics provides convenience for reducing the particle size of Mg.The refined particles ameliorate the hydrogen absorbing and releasing properties due to the decreased diffusion length and the larger reactive surfaces of H2caused by particle refinement [83].Compared with the experimental alloy,TiO2and La2O3have higher hardness.Therefore,the existence of TiO2or La2O3nanoparticles increases the brittleness of alloy and eventually makes the equilibrium between fragmentation and agglomeration change to a reduced particle size,as stated by Rafi-ud-din et al.[84].The shorter diffusion channels for H atoms and larger specific surface for H2dissociation caused by particle refinement facilitate to enhance hydrogen absorption kinetics [85].

Fig.5.SEM morphologies of the as-milled(20 h)La7Sm3Mg80Ni10–5M(M=none,TiO2,La2O3)alloys:(a)as-milled M=TiO2 alloy,(b)After six hydriding and dehydriding cycles M=none alloy,(c)After six hydriding and dehydriding cycles M=TiO2 alloy,(d)After six hydriding and dehydriding cycles M=La2O3 alloy.

Fig.6.Isothermal hydrogen absorption kinetic curves of the as-milled(20 h)La7Sm3Mg80Ni10?5M(M=none,TiO2,La2O3)alloys at different temperatures:(a) M=none;(b) M=TiO2;(c) M=La2O3;(d)Rietveld refinement on the XRD patterns of the as-milled La7Sm3Mg80Ni10 alloy hydrogenated at 3MPa and 633K.

To investigate the hydrogenation degree of the alloys and the phase structure changes during hydrogenation,The Rietveld refinements of the XRD patterns of as-milled La7Sm3Mg80Ni10alloy hydrogenated at 3 MPa and 593 K are provided,as illustrated in Fig.6(d).The milled alloys cannot be fitted with the Rietveld method because their XRD detections are amorphous.After hydrogenation,the amorphous phase is completely crystallized.Thus,the Rietveld method can be used analyzed the evolution of the phases of the asmilled alloy after hydrogenation.The result reveals that the as-milled hydrogenated alloy is composed of four hydrides,viz.MgH2,Mg2NiH4,LaH3and Sm3H7and the relative content of each phase is 56.1,25.9,11.5 and 6.5%,respectively.It suggested that the alloy is in saturated hydrogenation state.

3.3.Hydrogen desorption kinetics and hydrogen desorption activation energy

In order to research the relationships between the catalysts TiO2,La2O3and hydride stability,the temperature programmed desorption and DSC of the as-milled La7Sm3Mg80Ni10–5 M(M=none,TiO2,La2O3)after complete hydrogen absorption were measured at the heating rate of 5 K/min,as presented in Fig.7.It is observed that the adding catalyst renders an obvious effect on the hydride stability.The onset dehydrogenation temperature of La7Sm3Mg80Ni10–5 M(M=none,TiO2,La2O3)composites after hydrogenation is 547.4,537.2 and 540.1 K,severally.The temperatures of endothermic peaks in DSC curves of the alloys are 553.2,546.4 and 548.9 K,respectively.The change of initial hydrogen desorption temperature can reflect the hydrides stability.Evidently,the stability of the hydrogenated La7Sm3Mg80Ni10–5 M(M=none,TiO2,La2O3)composites is the following orderM=none>M=La2O3>M=TiO2.The decreased stability is attributed to the decline in Mg-H bond energy.The additives transition metals [86]or rare-earth elements can reduce the Mg-H bond energy and act as the catalysts owing to the electronic exchange reaction between these catalysts and magnesium hydride [57].Pighin et al.[87]investigated the function of various catalysts on Mg-H bond energy and found that the addition of transition metals or their oxides effectively decreased this bond energy,the magnesium hydride stability and the dehydrogenation temperature.According to the research of Abdellaoui et al.[12],the existence of new bonds weakens the bond strength between Mg and H,which can help us to understand the system instability and the decreased hydrogen desorption temperature mentioned above.

Fig.7.Temperature programmed desorption curve of the as-milled(20 h)La7Sm3Mg80Ni10?5M(M=none,TiO2,La2O3)alloys after hydrogen absorption at a heating rate of 5K/min.

Fig.8.Isothermal hydrogen desorption kinetic curves of the as-milled(20 h)La7Sm3Mg80Ni10?5M(M=none,TiO2,La2O3)alloys at different temperatures:(a) M=none;(b) M=TiO2;(c) M=La2O3,(d)Rietveld refinement on the XRD patterns of the as-milled La7Sm3Mg80Ni10 alloy dehydrogenated at 1×10–4 MPa and 633 K.

Aiming at studying the hydrogen desorption kinetics of the composites with additives TiO2and La2O3,the plots of capacity versus time were tested at 553,573,593,613 and 633 K and presented in Fig.8.As can be observed,the reaction temperature greatly affects the hydrogen desorption kinetics of experimental alloys.Under the high-temperature conditions,all the alloys have a very fast reaction rate.In addition,it is noted that the adding catalysts TiO2and La2O3generates a favorable impact on the isothermal dehydrogenation kinetics.For further making sense of the influence of adding TiO2and La2O3on the kinetics,the time inquired by releasing 3 wt.%hydrogen is regarded as a reference standard.As shown in Fig.8,the time required by releasing 3 wt.% hydrogen at the temperatures of 553,573,593,613 and 633 K is 988,553,419,227 and 152 s for theM=none alloy,and 578,352,286,188,and 112 s for theM=TiO2alloy,and 594,366,301,197 and 132 s for theM=La2O3specimen,respectively.Obviously,the dehydrogenation rate is in the orderM=TiO2>M=La2O3>M=none.Based on the above data,the relationship between the time needed by desorbing 3 wt.% H2and temperature can be constructed,as displayed in Fig.8(d).It indicates that the additives TiO2and La2O3markedly ameliorate the hydrogen absorption kinetics,but the positive contribution decreases rapidly with the increase of hydrogen desorption temperature,which suggests that among all the factors affecting the dehydrogenation kinetics of alloys,temperature is predominate.It has come to light that the hydrogen desorption of MgH2is completed through three stages:(a)magnesium phase nucleates and grows,(b)hydrogen diffuses from the magnesium hydride matrix to the surfaces,and(c)two adjacent hydrogen atoms combine to form hydrogen molecules[88].The improved hydrogen desorption kinetics of magnesium based alloys is most likely attributed to the decline in hydride stability,the diminution of the particles size and the increase of the defect density on the particle surface of the alloys caused by adding catalysts TiO2and La2O3.As mentioned above,the rare-earth elements and transition metals or their oxides can reduce the bond energy of Mg-H,thus,weaken the magnesium hydrides stability and accelerate the hydrides decomposition [47].High hardness catalyst particles are likely to be embedded into the interior of alloy particles under the action of repeated impact stress in the process of ball grinding,so the alloy particles broken and particle size greatly reduced,as considered by Jain et al.[89].Meanwhile,the additive high-hardness catalyst induces the surface defects and brings on the particle refinement of alloy in the procedure of mechanical milling [84].The induced defects are favorable to the nucleation and increase the surface reactivity.Besides,the decreased particle size makes the length of hydrogen diffusion channels shorten.These factors definitely accelerate the hydrogenation and dehydration of the Mg-based materials[90].Nevertheless,the improved kinetics because of adding TiO2and La2O3particles is not only due to the decline in particles size.Other factors such as the nature of the added oxides and the local electronic structure and the reduction of the oxide during heating should also be considered.Partially reduced oxides are expected to have different valence states and may act on ameliorating the hydrogen desorption property.Generally speaking,the main catalysis of transition metal-based catalysts is engendered by the transition metal ions and their ability to form hydrogen bonds.In this way,transition metal-based catalysts provide a faster route for H atoms diffusion.It has been reported that oxygen vacancies(also known as anoxic surfaces)on oxide surfaces also have catalytic activity [84].Hence,we believe that the hydrogen desorption results from thermodynamically induced surface vacancies.In summary,the improved kinetics may be due to the uniform dispersion of these anoxic oxide particles,which shorten the diffusion path between reaction ions.The oxygen vacancies act as the sites for nucleation and growth of dehydrogenation products,and promote the dehydrogenation process.

To investigate the dehydrogenation degree of the hydrides and the phase structure changes during dehydrogenation,The Rietveld refinements of the XRD patterns of as-milled saturated hydrides dehydrogenated at 1×10?4MPa and 633 K are given,as illustrated in Fig.8(d).It reveals that the phase Mg,Mg2Ni,LaH3and Sm3H7exist in the dehydrogenated alloy.It is very clear that rare earth hydrides LaH3and Sm3H7remain undecomposed at experimental temperatures and pressures.The relative content of each phase in the alloy is 55.7,26.1,11.6 and 6.6%,respectively.

Fig.9.JMA plots and Arrhenius plots for the dehydrogenation of the as-milled(20 h)La7Sm3Mg80Ni10–5M(M=none,TiO2,La2O3)alloys at different temperatures:(a) M=none;(b) M=TiO2;(c) M=La2O3,(d)The hydrogen desorption activation energy(E)of the experimental alloys.

Generally,the occurrence of gas-solid reaction needs to overcome a total energy barrier that can be reflected in terms of the apparent activation energy.Hence,when the activation energy reaches a certain requirement,the reaction can take place smoothly.The apparent activation energy of the hydrogenated La7Sm3Mg80Ni10–5 M(M=none,TiO2,La2O3)alloys in hydrogen desorption is evaluated by the Arrhenius and Kissinger methods.As well known,the nucleation and growth of dehydrogenation products are the crucial factors that control the hydrogen desorption reaction of magnesium based materials [91].In general,the Johnson-Mehl-Avrami-Kolmogorov(JMAK)model can simulate this solid-state reaction [92]:

a—the reaction fraction;

n—the avrami index;

k—the dehydrogenation rate constant;

t—the reaction time.

According to Fig.8,the fitting curves ln [-ln(1-α)]vs.lntat 573,593,613 and 633 K can be ploted,as provided in Fig.9.As can be observed,the JMAK sketches are almost linear,suggesting the dehydrogenation of the composite is composed of two steps,including the first stage instantaneous nucleation and the second stage 3D growth controlled by interface [93].Theηandηlnkvalues were obtained according to the slope and intercept in fitting curves at the corresponding temperature.Thus the value ofkcan be acquired.The apparent activation energy()of dehydrogenation reaction was estimated gained by using Arrhenius formula [57]:

A—a temperature independent coefficient;

R—the gas constant(8.3145 J/mol/K);

T—the absolute temperature of reaction;

k—the dehydrogenation rate constant.

The Arrhenius plots of lnkvs.1/Tof the alloys are sketched,as presented in Fig.9.The apparent activation energyof the as-milled alloys was acquired from the slopes of the Arrhenius plots.68.1,62.1 and 63.6 kJ/mol correspond to the La7Sm3Mg80Ni10–5 M(M=none,TiO2,La2O3)alloys,severally.Obviously,the apparent activation energy of the dehydrogenation of the alloys is in sequencedM=TiO2

Fig.10.DSC curves and Kissinger plots of the as-milled(20 h)La7Sm3Mg80Ni10?5M(M=none,TiO2,La2O3)alloys at different temperatures:(a)M=none;(b) M=TiO2;(c) M=La2O3,(d)The hydrogen desorption activation energy(E)of the experimental alloys.

In order to compare with the JMAK model,the Kissinger method is employed to estimate the activation energy,as following equation [94]:

β—the heating rate;

TP—the absolute temperature corresponding to the maximal desorption rate,

R—the gas constant(8.3145 J/mol/K).

The as-milled La7Sm3Mg80Ni10–5 M(M=none,TiO2,La2O3)alloys need to absorb H2to the saturated state before the measure of DSC.Fig.10 demonstrates the non-isothermal dehydrogenation curves tested at the heating rates of 5,10,15 and 20 K/min,severally.As we can see,an endothermic peak exists in each DSC curve,suggesting the same reaction procedure of each specimen.According to Fig.10,the graphs of ln(β/)vs.1/TPcan be sketched,as presented in Fig.10.It is noted that ln(β/)vs.1/TPplot is almost linear,so from the slopes of it,the activation energywas obtained.According to the calculation,apparent activation energies of the La7Sm3Mg80Ni10–5 M(M=none,TiO2,La2O3)alloys are 65.5,60.2 and 61.7 kJ/mol and in order ofM=TiO2

3.4.P-C-T curves and hydrogen storage thermodynamics

Reducing the thermal stability of magnesium hydride is the main goal to improve its hydrogen storage performance and realize its practical application.To inspect the effect of adding TiO2and La2O3on the thermodynamics,the P-C-T curves of the as-milled specimens were tested at the temperature of 593,613 and 633 K and given in Fig.11.Obviously,the pressure platform is quite flat and the hysteresis coefficient(Hf=ln(Pa/Pd))is small.The catalyst TiO2or La2O3has no evidently change in the platform characteristic reflected in the P-C-T curves of alloys.As we can observe,two pressure plateaus emerge in every P-C-T curve and the higher and lower platform pressures stand for the formation/dissociation of the Mg2NiH4and MgH2hydrides,severally [96,97].According to the plateau pressures(PaandPd)in Fig.11,the thermodynamics parameters enthalpy change?Hand entropy change?Sare evaluated by Van’t Hoff equation [98]:

Fig.11.P-C-T curves and Van’t Hoff plots of the as-milled(20 h)La7Sm3Mg80Ni10?5M(M=none,TiO2,La2O3)alloys at different temperatures:(a)M=none;(b) M=TiO2;(c) M=La2O3,(d)The absolute value of hydriding and dehydriding enthalpy of the experimental alloys.

PH2—the equilibrium hydrogen gas pressure corresponding to MgH2;

P0—the standard atmospheric pressure;

R—the gas constant(8.3145J/mol/K);

T—the absolute temperature of reaction.

The Van’t Hoff graphs of lnPH2/P0vs.1/Tfor the as-milled La7Sm3Mg80Ni10–5 M(M=none,TiO2,La2O3)composites can be sketched.Hence,the?Hand?Scan be calculated according to the slopes and intercepts in Van’t Hoff diagrams and listed in Table 1.It uncovers that the addition of TiO2and La2O3has not notably impact on the improvement of the experimental materials’thermodynamics and the reduction of corresponding hydrides stability.The addition of catalysts TiO2or La2O3decreases the stability magnesium hydride.A similar result also emerged in the investigations of Anik et al.[99]and Bououdina et al.[39].Obviously,the absolute values of dehydrogenation enthalpy change?Hdeof the alloys are in following orderM=none>M=La2O3>M=TiO2.Based on the above results,we can find that both isothermal and non-isothermal analyses reveal that the catalysts TiO2and La2O3weaken the magnesium hydride stability and improve the hydrogen absorption and desorption kinetics.The positive contribution to the hydrogen storing thermodynamic and dynamics of the specimens caused by two catalysts is in following orderM=TiO2>M=La2O3>M=none.

Table 1.Enthalpy ?H and entropy ?S of the as-milled alloys.

4.Conclusion

(1)The addition of TiO2and La2O3has no change in the phase composition but reduces the agglomeration tendency of particles in the process of mechanical milling and make the particle size of the as-milled alloy markedly decreased.It is this modification of the microstructure that remarkably enhances the hydrogen absorption and desorption performances.

(2)The addition of TiO2and La2O3have obviously positive contribution to the hydrogenation and dehydrogenation kinetics of the experimental alloys,which is in orderM=TiO2>M=La2O3>M=none.It is ascribed to the decline in the size of grains and particles,the generation of the fresh surface and the creation of the various crystal defects derived from ball milling and adding catalysts.

(3)The addition of TiO2and La2O3catalysts has a slightly favorable influence on the improvement of the thermodynamics of alloy and the stability of the hydride,which is in sequenceM=TiO2>M=La2O3>M=none.

Declaration of Competing Interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work,there is no professional or other personal interest of any nature or kind in any product,service and/or company that could be construed as influencing the position presented in,or the review of the manuscript entitled.

Acknowledgments

This study was financially supported by the National Natural Science Foundation of China(Nos.51901105,51871125,and 51761032),Natural Science Foundation of Inner Mongolia,China(2019BS05005),Inner Mongolia University of Science and Technology Innovation Fund(2019QDL-B11)and Major Science and Technology Innovation Projects in Shandong Province(2019JZZY010320).