Enhancing hydrogen storage performance via optimizing Y and Ni element in magnesium alloy

Xu Png ,Lei Rn ,Yu’n Chen,b,c,* ,Yuxio Luo ,Fusheng Pn,b

a College of materials Science and engineering,Chongqing University,Chongqing 400044,China

b National Engineering Research for magnesium Alloys,Chongqing University,Chongqing 400044,China

c Guangdong Guoyan Science and Technology Research Center Co.,Ltd,Guangdong 518000,China

Abstract Magnesium-based hydrogen storage materials are considered as one of the most promising candidates for solid state hydrogen storage due to their advantages of high hydrogen capacity,excellent reversibility and low cost.In this paper,Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys were prepared by melting and ball milling.Their microstructures and phases were characterized by X-ray diffraction,scanning electron microscope and transmission electron microscope,and hydrogen absorbing and desorbing properties were tested by the high pressure gas adsorption apparatus and differential scanning calorimetry (DSC).In order to estimate the activation energy and growth mechanism of alloy hydride,the JMAK,Arrhenius and Kissinger methods were applied for calculation.The hydrogen absorption content of Mg92.8Ni2.4Y4.8 alloy reaches 3.84 wt.% within 5 min under 350 °C,3 MPa,and the maximum hydrogen capacity of the alloy is 4.89 wt.% in same condition.However,the hydrogen absorption of Mg91.4Ni7Y1.6 alloy reaches 5.78 wt.% within 5 min,and the maximum hydrogen absorption of the alloy is 6.44 wt.% at 350 °C and 3 MPa.The hydrogenation activation energy of Mg91.4Ni7Y1.6 alloy is 25.4 kJ/mol H2,and the enthalpy and entropy of hydrogen absorption are -60.6 kJ/mol H2 and 105.5 J/K/mol H2,separately.The alloy begins to dehydrogenate at 210 °C,with the dehydrogenation activation energy of 87.7 kJ/mol H2.By altering the addition amount of Ni and Y elements,the 14H-LPSO phase with smaller size and ternary eutectic areas with high volume fraction are obtained,which provides more phase boundaries and catalysts with better dispersion,and there are a lot of fin particles in the alloy,these structures are beneficia to enhance the hydrogen storage performance of the alloys.

Keywords: Hydrogen storage materials;LPSO phase;Catalytic effect;Hydrogen storage performance.

1.Introduction

As energy crisis and environmental pollution are two major enemies for mankind,developing new energy sources and realizing carbon neutrality have become an urgent task in the 21st century.Hydrogen is considered to be the ultimate form of energy in the future because of its advantages such as high calorifi value,pollution-free,and wide sources[1,2],but how to store and use it safely has always troubled everyone.

Solid hydrogen storage,which is a promising hydrogen storage technology,owns the advantages of high density of hydrogen storage in weight and volume,moderate operating pressure and high energy efficien y [3,4],compared to highpressure gas hydrogen storage with low hydrogen storage efficien y,poor safety and to low-temperature liquid hydrogen storage with high cost and large energy consumption [5].In recent decades,many solid hydrogen storage materials have been discovered and studied,which can be divided into chemical hydrogen storage and physical hydrogen storage according to different hydrogen storage methods.Metal hydrides belong to chemical hydrogen storage,which can store hydrogen in the form of hydrogen atoms in solid hydrogen storage materials,and exhibit excellent safety and cycle performance.Magnesium based hydrogen storage alloys have been extensively studied in the fiel of hydrogen storage [2,6,7]due to their high hydrogen storage capacity (7.6 wt.% for MgH2),low cost and abundant resources.Nevertheless,the poor thermodynamic and kinetic barrier of Mg-based hydrogen storage alloys lead to high working temperature and slow kinetics,which limits its practical application in the fiel of hydrogen energy [8,9].The modificatio methods,such as alloying [10],nano-crystallization [11,12] and catalyst addition [13,14],have been used to improve the performance of Mg-based hydrogen storage materials.Noteworthy,alloying can effectively lower the thermodynamic and kinetic barriers.Among various alloying elements,transition metal elements(TM) promote the dissociation of H2molecules and play a synergistic catalytic role in improving the hydrogen absorption and desorption performance of magnesium-based hydrogen storage alloys[15-18].Rare earth elements(RE)can also promote the hydrogen absorption and desorption properties of the alloys [19-22].

In recent years,Mg-Ni-Y hydrogen storage alloys have been widely studied.The transition metal element Ni in the alloys can reduce the activation energy of H2decomposition during the process of hydrogen absorption [23],and Mg2Ni generated by reaction with Mg can be used as a stable catalyst during hydrogen ab-and desorption reaction [24-26].YH2produced by rare earth element Y and H2can also catalyze the reaction of hydrogenation [27-29].Remarkably,long period ordered stacking phase (LPSO phase) can be formed in Mg-Ni-Y hydrogen storage alloys [30].After hydrogen absorption,the LPSO phase decomposes and forms nano-sized Mg2NiHx,MgH2and YHX,which can reduce the hydrogen diffusion path and promote the activation and the kinetics of hydrogen ab-and desorption in Mg-Ni-Y hydrogen storage alloys [31-33].Whereas,the influenc of size,distribution and type of LPSO phase on hydrogen storage performance remains not very clear.

In this work,Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys were prepared by melting and ball milling.The microstructure and hydrogen ab-and desorption properties of the alloys were analyzed.The effect of the size,distribution of LPSO phase and ternary eutectic microstructure in the alloys with different contents of Ni and Y on hydrogen uptake and release properties were studied in detail.The hydrogen absorption mechanism of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys with LPSO structure is also discussed in this work.It is found that the catalytic effect is better when the catalytic phases are more evenly dispersed in the magnesium matrix.The enhancement of in-situ decomposition of coarse LPSO phases may be lower than that of fin Mg-Mg2Ni eutectic structure.So the size and distribution of phases and the number of phase boundaries may be more conducive to improve the hydrogen storage performance of the alloy.It is believed that this paper can provide us with an idea that the improving effect of LPSO phases is limited,reducing the size of LPSO phases and eutectic structure can be beneficia to further enhance the hydrogen storage performance of Mg-Ni-Y alloy.

2.Experimental

2.1.Alloys preparation

The character of raw materials is shown in Table 1.The Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8casting ingots are prepared by melting in induction furnace in SF6atmosphere and cooling in a salt solution.The casting ingots were then broken into powder,and further refine in the ball mill (pulverisette5 planetary ball mill,Feichi company,Germany).The rotation speed of the ball mill is 250 rpm (positive and negative rotation cycle),the mass ratio of grinding ball to material is 20:1,and the ball grinding time lasts 7 h.

Table 1 Character of raw materials.

2.2.Characterization of microstructure

The phase structures of the as-cast,ball milled,hydrogenated and dehydrogenated alloys were determined by an X-ray diffractometer (XRD,RIGAKUD/max250pc) with Cu Kαradiation,at a diffraction angle (2θ) from 10° to 90° at 4° min-1.Scanning electron microscopy and corresponding elemental analysis were performed on a fiel emission scanning electron microscope (JEOL JSM-7800F) equipped with an energy dispersive X-ray spectrometer (EDS),which perform microanalysis of specifi areas.LPSO phases and hydrides were identifie via transmission electron microscopy(TEM,Talos F200s,Czech).

2.3.Hydrogen absorption and desorption measurements

Non-isothermal dehydrogenation behaviors of hydrides were performed using differential scanning calorimetric(DSC,STA449F3,NETZSCH,Germany) at the heating rate of 5 °C/min,10 °C/min and 15 °C/min ranging from room temperature to 500 °C.The hydrogen absorption kinetics curves and PCT curves at different temperatures were measured by high pressure gas adsorption instrument (Beth company 3h-2000 pH) at the initial pressure 3 MPa.

3.Results

3.1.Phase structures of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8

Fig.1.XRD patterns of the two alloys in as-cast,ball grinding,hydrogenation and dehydrogenation:(A) Mg91.4Ni7Y1.6 and (b) Mg92.8Ni2.4Y4.8.

Fig.2.SEM images of the two alloys with different magnification (a) Mg91.4Ni7Y1.6;(b) Mg92.8Ni2.4Y4.8.

Figure 1 shows the XRD patterns of as-cast,ball milled,hydrogenated and dehydrogenated Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys.The as-cast alloys consist ofα-Mg,Mg2Ni,LPSO [27,32,34] according to Fig.1.Due to the increasing Y content in Mg92.8Ni2.4Y4.8alloys,Mg24Y5phase is formed while Mg2Ni disappears compared with Mg91.4Ni7Y1.6alloys.After ball milling,the diffraction peaks of various phases are broadened,which indicates that the plastic deformation of the alloy results in lattice distortion and grain size refinement The refinemen of grain size is beneficia to enhance the hydrogen absorption and desorption properties.In addition,the types of phases in the alloy do not transform,and the LPSO phases still exist after ball milling.After hydrogen absorption,fi e kinds of hydrogen absorbing phases can be found,namely MgH2,Mg2NiH4,YH3,YH2and Mg2NiH0.3.After releasing hydrogen,the XRD analysis shows that the dehydrogenated alloys are mainly composed of Mg,Mg2Ni and YH2.The LPSO phases and Mg24Y5phases do not form any more,indicating that the decomposition of LPSO phase and Mg24Y5phases are irreversible.

3.2.Microstructure of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8

Figure 2 shows the SEM images of as-cast Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys with different magnification In the Mg91.4Ni7.1Y1.6alloy,the black elliptical pre-precipitated phase and lamellar eutectic structure can be clearly observed.When the atomic ratio of Ni to Y is greater than 0.5,there are three phases ofα-Mg,LPSO and Mg2Ni in Mg-Ni-Y alloy,and eutectic reaction occurs during solidification:→α-Mg+LPSO+Mg2Ni [29].So occurs the eutectic reaction in Mg91.4Ni7Y1.6alloys with Ni/Y atomic ratio of 4.4.In addition,based on the XRD data and the SEM microstructure in Fig.2(a),the Mg91.4Ni7Y1.6alloys consist of black Mg matrix and a large number of gray eutectic structures,which is composed of fin lamellar LPSO phases,Mg2Ni andα-Mg alternating layers.The TEM patterns of LPSO phase regions in Mg91.4Ni7Y1.6alloy is observed in Fig.3.The results of selected area electron diffraction and HRTEM micrograph analysis show that the lamellar LPSO phase in Mg91.4Ni7Y1.6alloys is 14H-type LPSO structure.The microstructure of the Mg92.8Ni2.4Y4.8alloy observed by Scanning Electron Microscopy is shown in Fig.2(b).The Mg92.8Ni2.4Y4.8alloy is made up of black Mg matrix,a large number of bulk LPSO phases and gray eutectic structure,and the LPSO phases,Mg24Y5andα-Mg are distributed alternatively in the small eutectic structure.The TEM patterns of LPSO phase regions in Mg92.8Ni2.4Y4.8alloy is observed in Fig.4.The results of selected area electron diffraction and HRTEM micrograph analysis confirm the bulk LPSO phase in Mg92.8Ni2.4Y4.8alloys belong to 18R-type LPSO structure.In Mg92.8Ni2.4Y4.8there are less eutectic phase and more bulk 18R-LPSO,whose width is more about 10 μm or even wider.Whereas the lamellar 14H-LPSO phase is more and fine,its width is about 200-500 nm,and the content of ternary eutectic areas is more in Mg91.4Ni7Y1.6alloy.

Fig.3.The fin structure of LPSO phase in the Mg91.4Ni7Y1.6 alloys:(a) the BF TEM image;(b) the DF TEM image;(c) the corresponding SAED pattern of 14H phase;and (d) HRTEM micrograph of 14H phase.

Fig.4.The fin structure of LPSO phase in the Mg92.8Ni2.4Y4.8 alloys:(a)(b) TEM image with different magnification (c) the corresponding SAED pattern of 18R phase;and (d) HRTEM micrograph of 18R phase.

In summary,there will be more grain boundaries and phase boundary where it is easier to diffuse for hydrogen atoms.Therefore,kinetic properties of hydrogen absorption and desorption of Mg91.4Ni7Y1.6alloys will be more excellent.

Fig.5 shows the SEM pictures of the particle size and surface morphology of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys after ball milling under the same conditions.According to Fig.5,the particles of Mg91.4Ni7Y1.6alloy are fine and there are more cracks and other defects on the surface compared with the ones of Mg92.8Ni2.4Y4.8alloy,which can provide surface where H2can be adhered and increase the diffusion rate of hydrogen atoms.

Fig.5.SEM images of ball grinding of the two alloys under different magnification (a) (c) Mg91.4Ni7Y1.6;(b) (d) Mg92.8Ni2.4Y4.8.

Figire 6 and Fig.7 show the SEM images of Mg91.4Ni7Y1.6alloys and Mg92.8Ni2.4Y4.8alloys after hydrogenation and dehydrogenation,respectively.It is found in Fig.6 and Fig.7 that the particle size of Mg91.4Ni7Y1.6alloy is significantly smaller than that of Mg92.8Ni2.4Y4.8alloy.Moreover,the particle surfaces of Mg92.8Ni2.4Y4.8alloys after hydrogen absorption is relatively dense,and there are fewer fin particles on the alloy surfaces after hydrogenation and dehydrogenation.However,lots of flocculatin particle clusters appear on the surfaces of Mg91.4Ni7Y1.6alloys after hydrogen adsorption.These nano-sized particles can improve the hydrogen absorption and desorption dynamics of the alloy because it shortens the diffusion distance of H atoms.The TEM,energy spectrum,high resolution electron microscope and selected area electron diffraction pattern of a hydrogenated particle can be observed in Fig.8 and Fig.9.The EDS results show that Mg,Ni and Y elements are slightly evenly distributed in the alloy particles after hydrogen absorption.The calibration results of electron diffraction pattern and high resolution electron microscope data confir also that MgH2,Mg2NiH4,Mg2NiH0.3and YH3phases are uniformly distributed in the alloy particles after hydrogen absorption.After hydrogen desorption,Nano-sized compounds,such as Mg2Ni and YH2,are evenly dispersed on the surface of the alloy particles.According to Nobuko,Hanada et al.[35],transition metal and Rare earth metal nano-sized compounds uniformly dispersed on the surface of MgH2can modify the surface condition of the particle and greatly reduce the activation energy of hydrogen desorption on the alloy surface.Therefore,Nano-sized hydride on the surface of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys can not only absorb hydrogen,but also play the important role of surface modification leading to better kinetic properties of hydrogenation and dehydrogenation of the alloys.

Fig.6.SEM images of hydrogen absorption of the two alloys at different multiples:(a) (b) Mg91.4Ni7Y1.6;(c) (d) Mg92.8Ni2.4Y4.8.

Fig.7.SEM images of dehydrogenation of the two alloys at different multiples:(a) (c) Mg91.4Ni7Y1.6;(b) (d) Mg92.8Ni2.4Y4.8.

Fig.8.The microstructure of hydrogenated sample in the Mg91.4Ni7Y1.6 alloys:(a) the HAADF image;(b-d) the EDS of three elements.

Fig.9.The high resolution electron microscope and selected electron diffraction of Mg91.4Ni7.1Y1.6 alloy after hydrogen absorption:(a) high-resolution;(b-e)high-resolution partial enlarged view;(f) selected area electron diffraction spot.

Fig.10.The DSC curves of alloys at different heating rates:(a) Mg91.4Ni7Y1.6;(a) Mg92.8Ni2.4Y4.8.

3.3.Thermodynamics investigation of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8

The DSC curves of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8during hydrogen desorption at various heating rates are presented in Fig.10.As one can see,three endothermic peaks of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys can be found.The hydrogen desorption temperature of Mg2NiH4is lower than that of MgH2and YH3[36-38].In the hydrogen desorption process,Mg2NiH4firs desorbs hydrogen to Mg2NiH0.3and undergoes a significan volume contraction,causing a contraction strain on MgH2around it,facilitating hydrogen desorption of MgH2.Hence,the desorption processes of primary MgH2and Mg2NiH4are not isolated,the synergistic reaction of them displays the lowest desorption peak temperature:Mg2NiH4?Mg2NiH0.3+2H2,MgH2?Mg+H2.The dehydrogenation of Mg2NiH0.3into Mg2Ni when the decomposition of MgH2is complete,so the dehydrogenation process of Mg2NiH0.3will occur at a higher temperature:Mg2NiH0.3?Mg2Ni+H2.The dehydrogenation temperature of YH2and YH3is generally around 790 °C and 400 °C,respectively [36,38].Therefore,DSC curve shows no heat absorption peak of YH2and the highest desorption peak temperatures is YH3?YH2+H2.When the heating rate is 5°C/min,the onset and peak decomposition temperatures of Mg91.4Ni7Y1.6hydride are only 210.3 °C and 237.7 °C,both of which are lower than that of Mg92.8Ni2.4Y4.8hydride:227.9 °C and 248.2 °C,which indicates that more Ni doped in the alloys would contribute to enhancing thermodynamic property.

Figure 11 show the fitte Kissinger curves of the Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys.The DSC curves of the synergistic catalytic de-/hydrogenation reaction between MgH2and Mg2NiH4at different heating rates were analyzed.The endothermic peak temperatures of Mg91.4Ni7Y1.6alloy are 230.7°C,248.3 °C and 262 °C,and that of Mg92.8Ni2.4Y4.8alloy are 248.2 °C,266.6 °C and 273.1 °C,respectively.The activation energies of hydrogen desorption of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys were calculated by fittin the straight line ln(β/Tm2)vs.1/Tmthrough Kissinger equation [39].As shown in Fig.10,the hydrogen desorption activation energies of Mg91.4Ni7Y1.6alloy and Mg92.8Ni2.4Y4.8alloy are 87.7 and 112.4 kJ/mol H2,respectively,which are lower than that of [40] ball-milled pure MgH2,250 kJ/mol H2.Combined with SEM and TEM analysis,the synergistic catalysis of uniformly distributed YH2,YH3,Mg2NiH0.3and Mg2NiH4,which are generated by hydrogen absorption of cluster-parallel LPSO phases,gives rise to in-situ element catalysis and nano-scale effect [41].Thus,these kinds of effect would greatly reduce the activation energy of hydrogen release and improve dehydrogenation performance.

Fig.11.The fitte Kissinger curves of alloys:(a) Mg91.4Ni7Y1.6;(b) Mg92.8Ni2.4Y4.8.

Fig.12.Hydrogenation absorption/desorption PCT curves of the two alloys at 330 °C,350 °C and 380 °C:(a) Mg91.4Ni7Y1.6;(b) Mg92.8Ni2.4Y4.8.

The experimental results are consistent with the analysis results of microstructure characteristics observed by SEM,that is,the more eutectic phase,the fine and more dispersed LPSO phase,leading to better in-situ catalytic effect after hydrogen absorption.

The de-/hydrogenation process of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys includes three chemical reactions:

The respective de-/hydrogenation plateau pressures of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys at different temperatures can be obtained from the PCT curves in Fig.12(a) and(b).The low-plateau pressures corresponds to the transformation of Mg/MgH2while the high-plateau pressures relates to the transformation of Mg2Ni/Mg2NiHx.Due to the low content of Ni element in Mg92.8Ni2.4Y4.8alloy,the synergistic catalytic effect is weak,resulting part of the hydrogen is still retained in Mg92.8Ni2.4Y4.8alloy at 330 °C,indicating that its dehydrogenation temperature under 3 MPa pressure is higher than 330 °C.Moreover,there is only one de-/hydrogenation platform,and the higher Mg2Ni platform was not obvious.In addition,we found that the reversible hydrogen storage capacity of the two alloys at 380 °C is slightly less than the hydrogen storage capacity at 350 °C and 330 °C.Combined with the analysis of the hydrogen absorption kinetic mechanism on page 15 of the article,with the increase of absorption temperature,the hydride nucleation rate decreases.There are more hydride grains with large sizes and a thicker hydride layer [42] during the hydrogenation of the Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys at 380°C,which are not conducive to the diffusion of hydrogen atoms,resulting in a lower hydrogen storage capacity.Wenjie Song [41] also reported the similar situation.

Fig.13.The Van’t Hoff plots for Mg/MgH2 transformation in two alloys:(a) Mg91.4Ni7Y1.6;(b) Mg92.8Ni2.4Y4.8.

The corresponding van’t Hoff plots for both hydrogen absorption and desorption are shown in Fig.13(a) and (b).According to the fittin result,the formation enthalpy changes(ΔH) of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8hydride are calculated to be -60.6 and -68.6 kJ/mol H2,and the formation entropy changes (ΔS) are 105.5 and 116.2 J/K/mol H2,respectively.According to Fig.12,The Mg92.8Ni2.4Y4.8alloy release hydrogen incompletely at 330 °C,and the complete dehydrogenation PCT curve was not obtained.It is difficul to distinguish the hydrogen release platform.Therefore,it is difficul to calculate the enthalpy change and entropy change of dehydrogenation through van’t Hoff plots.In addition,the dehydrogenation capacity of the Mg91.4Ni7Y1.6alloy is more significant so the dehydrogenation enthalpy change and entropy change of the Mg92.8Ni2.4Y4.8alloy can be omitted.The decomposition enthalpy and entropy changes of Mg91.4Ni7Y1.6alloys are 56.9 kJ/mol H2and 97.9 J/K/mol H2,separately.

These values are all lower than that of MgH2theoretical values [43],which indicates that the addition of Y and Ni reduce the thermodynamic stability of MgH2and show better enhancement effects than just adding Ni or Y.Because of the large atomic radius of Ni and Y elements,the Mg-H bond distance is increased,which reduces the binding energy,thus reducing the stability.In addition,the activation energy and enthalpy of the hydrogen storage alloy will be reduced due to the nanostructure,which will greatly improve the hydrogen storage performance of the alloy [44].In Mg91.4Ni7Y1.6alloy,the LPSO phase is decomposed to form the fine and more dispersed nanoscale catalytic phases,and the value of hydrogen absorption enthalpy decreases more than that of Mg92.8Ni2.4Y4.8alloy.This indicated that the Mg91.4Ni7Y1.6alloy presents better hydrogen absorption performance and lower de-/hydrogenation temperature.

3.4.The activation and hydrogen absorption kinetics of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8

Figure 14 gives the hydrogen absorption kinetics curves of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys.The amount of hydrogen absorbed and the rate of hydrogen absorption reaction increase with the rising temperature.The hydrogen absorption kinetic curve of the alloy is composed of two parts,which are fast hydrogen absorption and stable hydrogen absorption.The hydrogen absorption rate of the alloy rises steeply in the firs 5 min,which is the rapid hydrogen absorption.However,the hydrogen absorption of the alloy shows a slow upward trend in the stable hydrogen absorption stage.The reason is as follows:after the hydride layer is formed on the surface of the particles,further hydrogenation of the alloy requires H atoms to penetrate through the hydride layer and diffuse into the alloy particles.However,the diffusion rate of H atoms in the hydride layer is lower than that in metallic magnesium,which causes the hydrogen absorption rate of the alloy to continuously decrease until it can no longer continue to absorb hydrogen when the size of the hydride layer grows to a certain thickness.Therefore,the hydrogen storage capacity of the alloy is determined by the rapid hydrogen absorption stage,and the high hydrogen storage capacity in the rapid hydrogen absorption stage results in a high total hydrogen storage capacity of the alloy.Representative hydrogen absorption data for Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys are summarized in Table 2.The data shows that Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloy can absorb 5.78 wt.% and 3.84 wt.%hydrogen respectively under an initial hydrogen pressure of 3 MPa at 350 °C in 5 min,reaching 90% and 78% of the maximum hydrogen absorption capacity.Apparently,the theoretical hydrogen uptake of Mg91.4Ni7Y1.6alloy is lower than that of Mg92.8Ni2.4Y4.8alloy,but Mg91.4Ni7Y1.6has a higher maximum hydrogen uptake and a faster hydrogen absorption rate.

Table 2 Hydrogen absorption properties of Mg91.4Ni7Y1.6 alloy and Mg92.8Ni2.4Y4.8 alloy at different temperatures.

To explain the hydrogen absorption kinetic mechanism of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys,the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model are adopted to analyze the evolution of kinetics [45]:

whereαis the reaction fraction of the hydrogen storage material converted to hydride corresponding to time,k is the reaction rate constant,n (commonly between 0～3) is the Avrami exponent of reaction order.When the value of 0.5＜n＜1.5,the nucleation and growth mode of the alloy is three-dimensional growth.After nucleation,the core takes a spherical shape and grows in a three-dimensional manner.When the spherical hydrides grow up and collide with each other,a continuous hydride layer is formed.In contrast to the value ofn＜0.5,the nucleation and growth method of the alloy is one-dimensional growth,and the core-shell structure will be formed in Mg-MgH2transition,and the rate of hydrogenation is one to two orders of magnitude slower [31].

Fig.14.Hydrogen absorption kinetics of the two alloys 100 °C,150 °C,200 °C,300 °C and 350 °C:(a) Mg91.4Ni7Y1.6;(b)Mg92.8Ni2.4Y4.8.

The hydrogenation curves of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloy at 100 °C,200 °C and 300 °C under 3 MPa are analyzed by JMAK model,which is shown in Fig.15 (a-c) and Fig.16(a-c).The fitte curve is in good agreement with the experimental data (R2＞0.99).At 100°C,the hydride nucleation and growth of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys are all three-dimensional diffusioncontrolled methods.However,at 200°C,Mg91.4Ni7Y1.6alloy forms a cored shell structure while Mg92.8Ni2.4Y4.8alloy still grows at the three-dimensional manner,and the cored shell structure in Mg91.4Ni7Y1.6alloy is formed at a lower temperature,indicating the nucleation rate of the alloy is very fast at a lower temperature.The rapid nucleation promotes the alloy to absorb a large amount of hydrogen during the rapid hydrogen absorption stage,and the hydride core quickly grows to form a hydride layer.

Temperature greatly affects the hydrogen absorption reaction rate.When the temperature is low,the reaction rate is not fast enough to form cored shell structure.After rising to a certain temperature,nucleation and growth rate of alloy is accelerated,the core-shell structure is formed,and this gives rise to reducing the rate of reaction of materials.As grain boundaries and formed compounds act as pathways for hydrogen diffusion [46],interfacial diffusion can be maintained at higher temperatures.Therefore,although the Mg91.4Ni7Y1.6alloy has a high nucleation rate and is easy to form a core-shell structure,its powder particles are fine and a large number of fin LPSO phases and phase interfaces are contained in the alloy,which can provide diffusion path for H atoms and reduce the diffusion distance of H atoms,thereby improving the hydrogen storage performance of Mg91.4Ni7Y1.6alloy.

As we all know,hydrogen storage alloys need to overcome a certain energy barrier in the hydrogenation process in order to absorb hydrogen.This energy barrier is called the activation energy,which can be obtained by the Arrhenius equation.According to the Arrhenius equation,the activation energy of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys can be calculated by lnk vs.1000/RT fitting their hydrogen absorption activation energy was reduced from 100 kJ/mol H2of pure Mg[47] to 25.4 kJ/mol H2and 28.6 kJ/mol H2,respectively,as shown in Fig.15(d) and Fig.16(d).Through the analysis of hydrogen absorption kinetics curve and activation energy,Mg91.4Ni7Y1.6alloy has a better hydrogen absorption kinetics performance.Adding different amount of Ni and Y element,Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys have different structure after smelting.Compared with the Mg92.8Ni2.4Y4.8,the Mg91.4Ni7Y1.6alloy has fine LPSO phases and more eutectic structures,and the hydrogenated YH2/YH3and Mg2NiHx phases are more dispersed,contributing to better synergistic catalytic effect and kinetics performance.

Fig.15.Plots of α vs.t for the hydrogenation of Mg91.4Ni7Y1.6 after activated at different temperature and Arrhenius plots of lnk vs.1000/RT for the hydrogenation of Mg91.4Ni7Y1.6:(a) 100 °C,(b) 200 °C,(c) 300 °C,(d) Arrhenius plots of lnk vs.1000/RT.

Fig.16.Plots of α vs.t for the hydrogenation of Mg92.8Ni2.4Y4.8 after activated at different temperature and Arrhenius plots of lnk vs.1000/RT for the hydrogenation of Mg92.8Ni2.4Y4.8 alloy:(a) 100 °C,(b) 200 °C,(c) 300 °C,(d) Arrhenius plots of lnk vs.1000/RT.

Fig.17.The schematic diagram of the hydrogen absorption process of Mg91.4Ni7Y1.6 and Mg92.8Ni2.4Y4.8 alloys:(a) Mg91.4Ni7Y1.6;(b) Mg92.8Ni2.4Y4.8.

4.Discussions

4.1.Mechanism of LPSO phase enhancing the performance of de/hydrogenation

Based on the analysis to XRD,SEM and TEM,it can be seen that there are a lot of LPSO phases in Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys,as shown in Figs.1-4.The LPSO phase is a long-period stacking ordered phase containing Ni and Y atoms.Moreover,the LPSO structure with the periodic distribution of Y and Ni has an orderly composition and an orderly stacking in the entire LPSO phase.Combined with Fig.1 and Fig.9,the alloys after hydrogen absorption consist of hydrides such as MgH2,Mg2NiH4,YH3,YH2and Mg2NiH0.3.This means that the hydrogen induced decomposition of the LPSO phase occurs upon initial hydrogen absorption,which can be expressed as:

where YH2and Mg2NiH0.3are further hydrogenated into YH3and Mg2NiH4,respectively:

After releasing hydrogen,the XRD analysis shows that the dehydrogenated alloys are mainly composed of Mg,Mg2Ni and YH2.Since the decomposition temperature of YH2is as high as 790°C [36],YH2cannot release hydrogen at the experimental temperature.The dehydrogenation reaction can be described as follow:

The LPSO phases do not form any more,indicating that the decomposition of LPSO phase is irreversible.

A large number of uniformly distributed nano-hydrides formed during hydrogen absorption have a significan catalytic effect on the hydrogen absorption and desorption performance of the alloy [25,48],which is attributed to the nanosizing and in-situ catalyzing effects.In the process of hydrogen absorption,Ni can promote the decomposition of H2molecules into H atoms,and the ability to form YHxhydrides is better than that of Mg2NiHxand MgH2[49,50].The formation of YHxwill causes significan lattice distortion and then promote the formation of Mg2NiHxand MgH2.In the hydrogen desorption process,Mg2NiH4firs desorbs hydrogen to Mg2NiH0.3and undergoes a significan volume contraction,causing a contraction strain on MgH2around it,facilitating hydrogen desorption of MgH2.In addition,the multi-phase nanostructure can prevent the growth of Mg grains during the hydrogen absorption and desorption cycle,which can shorten the diffusion distance of H atoms.

4.2.The influenc of microstructure size on hydrogen storage performance

Although plenty of LPSO phases are contained in Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys,the hydrogen storage performance of Mg91.4Ni7Y1.6is better than that of Mg92.8Ni2.4Y4.8alloy.The reason is that the fin eutectic structure in Mg91.4Ni7Y1.6alloy and many fin particles formed after activation are conducive to increasing the specifi surface area,providing many reaction nucleation and diffusion interfaces,and enhancing the diffusion ability of H atoms in the alloy.The schematic diagram of the hydrogen absorption process of Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys is shown in Fig.17.According to the results of SEM and TEM analysis,there are a large number of bulky 18RLPSO phases and a small amount of phase interfaces in the Mg92.8Ni2.4Y4.8alloy,which causes most of the hydrides to nucleate only on the surface of the alloy.The growth of hydride requires H atoms to penetrate through the hydride layer and diffuse into the alloy.The growth of the hydride layer stops when it grows to a certain thickness,which results in part of the alloy structure not participating in the hydrogenation reaction,thereby reducing the maximum hydrogen absorption capacity and slowing down the hydrogen absorption rate of the alloy.On the contrary,Mg91.4Ni7Y1.6alloy contains a large amount of Mg+Mg2Ni+14H-LPSO ternary eutectic,the size of LPSO phase is small,and there are many phase interfaces in the alloy.The hydride can not only nucleate on the surface of alloy particles,but also nucleate at the phase interface.In addition,the diffusion speed of H atoms in the LPSO phase is faster than that in the Mg phase,and the particles size of the Mg91.4Ni7Y1.6alloy is smaller than that of the Mg92.8Ni2.4Y4.8alloy,which makes the diffusion distance of the H atoms shorter.Therefore,the hydrogen absorption rate and total hydrogen absorption of Mg91.4Ni7Y1.6alloy are higher than that of Mg92.8Ni2.4Y4.8alloy under the same reaction conditions.

5.Conclusions

In this paper,Mg91.4Ni7Y1.6and Mg92.8Ni2.4Y4.8alloys were prepared by smelting and ball grinding.The microstructure and hydrogen storage properties are studied in detail.The catalytic mechanisms of alloying Ni and Y are elaborated.The results are as follows:

(a) There are three phases in Mg91.4Ni7Y1.6alloys,including Mg,Mg2Ni and abundant lamellar 14H-LPSO,while the massive bulk 18R-LPSO phases appear in the Mg92.8Ni2.4Y4.8alloy.With the increase of the relative content of Y and the decrease of Ni,Mg24Y5are formed in the Mg92.8Ni2.4Y4.8alloy,and Ni element is completely distributed in the LPSO phases,inducing Mg2Ni phases disappears.

(b) During the hydrogen absorption,LPSO phases decompose,causing hydride Mg2NiHxand YH2/YH3to generate in situ.These phases distributed more uniformly in the alloy can play a better role of in-situ catalysis.In addition to the better dispersion of these catalysts,Mg91.4Ni7Y1.6alloys also have more phase boundaries,so hydrogen atoms can diffuse better and improve the dynamic properties of the material.

(c) A large number of fin particles are contained in Mg91.4Ni7Y1.6alloys,which exposes more second-phase hydrides to the alloy surface and shortens the diffusion distance of H atoms.Not only can the maximum amount of hydrogen absorption of the material be increased,but also the activation energy of hydrogen absorption can be reduced,and the rate of hydrogen absorption of the material can be improved.

(d) Mg91.4Ni7Y1.6alloys have better kinetic and thermodynamic properties.Under the conditions of 300°C,350°C and 3 MPa,the hydrogen absorption contents of Mg91.4Ni7Y1.6alloys reach 4.64 wt.% and 5.78 wt.% in 5 min,respectively.The activation energy of hydrogen absorption was 25.4 kJ/mol H2,and the enthalpy and entropy of hydrogen absorption were -60.6 kJ/mol H2and 105.5 J/K/mol H2,separately.The alloy begins to dehydrogenate at 210°C,with the dehydrogenation activation energy of 87.7 kJ/mol H2,and theΔH andΔS of dehydrogenation are 56.9 kJ/mol H2and 97.9 J/K/mol H2,respectively.

Declaration of Competing Interest

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.

Acknowledgments

This work was financiall supported by Chongqing Special Key Project of Technology Innovation and Application Development,China (Grant No.cstc2019jscx-dxwtB0029)