Crystallite growth characteristics of Mg during hydrogen desorption of MgH2

Caiqin Zhou,Chaodong Hu,Yongtao Li,Qingan Zhang

School of Materials Science and Engineering,Anhui University of Technology,Maanshan,Anhui,243002,China

ABSTRACT MgH2 is one of promising hydrogen storage materials,but Mg crystallites grow up very fast during hydrogen desorption,leading to the degradation of hydrogen storage properties.Therefore the growth behavior and mechanism of Mg crystallites during hydrogen desorption of nanocrystalline MgH2 were investigated in this work.It was found that the transformation from MgH2 to Mg occurred by the surface-controlled‘nucleation and growth’mechanism.After the instantaneous nucleation of Mg at free surfaces of MgH2 particles,Mg crystallites grew through three stages,namely one-dimensional,then two-dimensional and finally one-dimensional growths.In the second stage,Mg crystallites grew quickly as compared with other stages.After complete hydrogen desorption,the average Mg crystallite size in MgH2−10 wt%Pr3Al11 sample was smaller than that in pure MgH2 sample due to the presence of Pr3Al11.

Keywords:Magnesium hydride Crystallite growth Hydrogen desorption

1.Introduction

Hydrogen energy has attracted more attention in recent years due to its environmental friendship and high efficiency.As one of promising hydrogen storage materials,MgH2is of great interest to scientists because of its high hydrogen capacity [1].However,the high hydrogen sorption temperature,slow desorption rate and poor cycle stability of MgH2limit its practical application [2].Hence,great efforts have been made to improve hydrogen storage properties of MgH2up to now [3].Among these efforts,nanosizing by ball milling is a simple and effective method [4].In spite of this,the crystallites of Mg grow up very fast during the hydrogen desorption of nanocrystalline MgH2,leading to the degradation of hydrogen storage properties during subsequent cycles[5].Therefore it is necessary to understand the growth behavior of Mg crystallites during the hydrogen desorption of MgH2,which is critical to approaching solutions for the improvement of cycle durability.

Until now,several models,such as ‘shrinking core’ mechanism,‘nucleation and growth’ mechanism and ‘multiple step’ mechanism,have been proposed for the desorption of nanocrystalline MgH2[6,7].In particular,the ‘shrinking core’ and ‘nucleation and growth’ models brought forward two quite different viewpoints.In the former it is believed that Mg skin is first formed surrounding MgH2and then the transition continues by shrinkage of MgH2core region during hydrogen desorption process[6,8],while the latter holds that the nucleation and growth of Mg randomly proceeds within MgH2[6,9].The reason for this argument is that both models can explain many experimental results.Recently Nogita et al.suggested that hydrogen desorption of bulk MgH2is based on the ‘nucleation and growth’ mechanism while dehydriding of nanostructured MgH2occurs by the ‘shrinking core’ mechanism [10].In spite of this argument,the dehydrogenation of MgH2is generally considered to be the nucleation and growth process controlled by interface reaction [3].

Based on the ‘nucleation and growth’ mechanism,the hydrogen desorption kinetics of MgH2can be described using Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation [7,11],namely

Where,f is hydrogen desorption fraction,k is hydrogen desorption kinetic parameter,n is Avrami exponent related to nucleation rate and growth dimensionality and t is time.From the JMAK equation,it is clear that the growth mechanism of Mg crystallites during hydrogen desorption of nanocrystalline MgH2can be revealed by the exponent n which may be expressed as nn+ng[11].The nnvalue is related to nucleation rate,namely,nn=0,0 1 stand for nucleation site saturation (instantaneous nucleation),decreasing nucleation rate,constant nucleation rate and increasing nucleation rate,respectively.The ngvalue is the growth dimensionality(1,2 or 3)when the growth process of Mg crystallites is controlled by interface reaction.Hence,the in-depth investigation of Avrami exponent n has great significance for clarifying growth mechanism of Mg crystallites during the hydrogen desorption of MgH2.

In the present work,the isothermal dehydrogenation properties of pure MgH2sample and MgH2-Pr3Al11composite have been studied at first.Then the growth characteristics of Mg crystallites during the hydrogen desorption processes have been investigated.Finally,the growth mechanism of Mg crystallites has also been discussed based on the JMAK analysis.

2.Materials and methods

2.1.Materials

Commercial MgH2(purity 98%,325 mesh),PrH2(99 wt%,325 mesh) and Al (99 wt%,325 mesh) powders were purchased from Alfa Aesar Chemical Co.Ltd.(Shanghai,China).The nano-sized Pr3Al11was prepared with the PrH2and Al powders as described in our previous work [12,13].

2.2.Preparation of nanocrystalline MgH2

For comparative investigations,MgH2and MgH2−10 wt% Pr3Al11were selected in this work.In order to obtain nanocrystalline MgH2,both samples were ball milled at 300 rpm for 100 h using a QM-1SP2 planetary mill (Nanjing Chi Shun Technology Development Co.Ltd.,China).The ball milling was performed under argon atmosphere and with a ball-to-powder weight ratio of 40:1.After ball milling,the average powder size was about 500 nm but the average crystallite sizes of MgH2in ball milled MgH2and MgH2−10 wt%Pr3Al11samples were 13 and 8 nm,respectively.

2.3.Hydrogen desorption experiments

Isothermal dehydrogenation experiments for ball milled MgH2and MgH2−10 wt% Pr3Al11samples were conducted using a Sieverts-type apparatus (Suzuki Shokan Co.Ltd.,Japan) under vacuum at 350 °C.Before the measurements,the samples were not subjected to any activation in order to avoid crystallite growth.Based on the measured isothermal dehydrogenation curves,different hydrogen desorption durations were selected for partial dehydrogenations of ball milled samples in order to obtain partially dehydrogenated samples for characterizations.

2.4.Characterizations of samples

Powder X-ray diffraction (XRD) measurements were carried out using a Rigaku D/Max 2500VL/PC diffractometer (Japan) with Cu Kα radiation operated at 50 kV and 200 mA.To determine the crystallite sizes and lattice strains of Mg and MgH2,Rietveld refinements of XRD data were conducted using the program RIETAN-2000 [14].The detailed methods of Rietveld refinements were previously reported by Nakamura et al.[15].In addition,the transmission electron microscopy(TEM) images were acquired using a FEI Tecnai F20 transmission electron microscope(USA).For TEM sample preparation,powders were dispersed in tetrahydrofuran with assistance of ultrasonic vibration and the suspensions were then deposited onto copper grids in a glove box.

3.Results and discussion

3.1.Isothermal hydrogen desorption performances

Fig.1 shows the isothermal dehydrogenation curves of ball milled MgH2and MgH2−10 wt% Pr3Al11samples at 350 °C.Obviously the MgH2−10 wt% Pr3Al11composite had better hydrogen desorption kinetics than pure MgH2sample though the desorbed hydrogen amounts were less than their corresponding theoretical values due to the oxidation of magnesium.The enhancement of hydrogen desorption kinetics may be caused by the decrease of activation energy for dehydrogenation due to the catalytic effect of Pr3Al11[16].However,both isothermal dehydrogenation curves followed the same trend of hydrogen desorption.In the initial stage of hydrogen desorption (within 180 s for MgH2and 60 s for MgH2−10 wt% Pr3Al11),the hydrogen desorption rates were very slow.Subsequently,the dehydrogenations of both samples proceeded more quickly.Nevertheless,their hydrogen desorption rates became slower and slower in the final stage.This hydrogen desorption performance is related to the growth behavior of Mg crystallites during the transition process from MgH2to Mg.Thus it is necessary to comparatively investigate the growth characteristics of Mg crystallites in the two samples with different hydrogen desorption kinetic properties.For this purpose,the partial dehydrogenations were performed with various cut-off hydrogen desorption durations,as marked in Fig.1.

3.2.Growth characteristics of Mg crystallites during hydrogen desorption process

Figs.2 and 3 show the XRD patterns for pure MgH2and MgH2−10 wt% Pr3Al11samples,respectively,after dehydrogenations at 350 °C for various periods of time.From these XRD patterns,the following features can be seen:(1)the relative intensities of diffraction peaks for Mg gradually increased with dehydrogenation time,indicating that the phase transition from MgH2to Mg occurred in this process.(2) In the MgH2−10 wt% Pr3Al11sample,minor Pr3Al11remained unchanged during the hydrogen desorption process.(3) The diffraction peaks of Mg gradually narrowed with the prolongation of dehydrogenation time,implying that crystallite size of Mg increased or/and lattice strain decreased.In order to evaluate average crystallite sizes and lattice strains,the XRD patterns were refined by the Rietveld method (see Figs.2 and 3).The refined parameters were used to calculate the crystallite sizes and lattice strains of Mg and MgH2.

Fig.4 shows the crystallite sizes and lattice strains of Mg and MgH2in pure MgH2and MgH2−10 wt% Pr3Al11samples during isothermal dehydrogenation at 350°C for various durations.It can be seen that the lattice strains of MgH2in ball milled MgH2and MgH2−10 wt%Pr3Al11samples were 1.1% and 1.8%,respectively.After an initial period of dehydrogenation for 210 s,the lattice strains of MgH2decreased greatly,and those of nascent Mg also maintained low values.This is attributed to that the lattice strains originated in ball milling process can be relaxed within initial several minutes of isothermal treatment[17].It should be noted that the crystallite size of Mg in pure MgH2sample increased from 39 nm to 175 nm when dehydrogenation time ranges from 210 s to 1260 s.Similarly,Mg crystallites in MgH2−10 wt% Pr3Al11sample grew up from 53 nm to 165 nm as the dehydrogenation time increased from 220 s to 1200 s.However,the crystallite growth rates of both samples become slower in the final stage of hydrogen desorption.Unfortunately,the sizes of Mg crystallites in the initial stage of hydrogen desorption have not been determined because the phase abundance of Mg was very low.In spite of this,it is certain that Mg crystallites became large after hydrogen desorption of nanocrystalline MgH2(13 nm for ball milled MgH2sample and 8 nm for ball milled MgH2−10 wt% Pr3Al11sample before hydrogen desorption).

3.3.Growth mechanism of Mg crystallites during hydrogen desorption process

Fig.5a and b show the TEM image of pure MgH2sample dehydrogenated partially at 350°C for 600 s(as indicated by“M2”in Fig.1)and corresponding selected area electron diffraction (SAED) pattern from outer area of a particle.The SAED pattern indicates that several Mg crystallites existed together with MgH2crystallites in this region.After complete dehydrogenation at 350 °C for 3600 s (as indicated by“M6”in Fig.1),the SAED pattern showed two sets of diffraction spots of metallic Mg(see Fig.5c and d),indicating that only two Mg crystallites existed in the diffraction region of electron beam.This implies that MgH2entirely disappeared and Mg crystallites grew up after complete hydrogen desorption.These results support the‘nucleation and growth’mechanism for desorption of nanocrystalline MgH2.

Many evidences reported previously indicates that the nucleation of Mg occurs heterogeneously at the free surfaces of MgH2particles in the initial hydrogen desorption stage [18-21].Even if the JMAK model is based on the assumption of homogeneous nucleation through a bulk sample,the kinetic curves of isothermal hydrogen desorption of MgH2can be well fitted by the JMAK equation due to the limitation of kinetic model and the complexity of transition mechanism [21].Fig.6 shows the plots of ln[−ln(1−f)]versus lnt for pure MgH2and MgH2−10 wt%Pr3Al11samples during isothermal dehydrogenation process at 350 °C,where the f values were derived from Fig.1.Based on the JMAK equation (see Eq.(1)),the curves were well fitted with different n values in three stages of hydrogen desorption.Consequently,the Avrami exponents of dehydrogenation reactions for pure MgH2and MgH2−10 wt% Pr3Al11samples were determined to be n=1.0 and n=0.73 in the first stage,n=2.3 and n=2.0 in the second stage,and n=1.4 and n=1.1 in the final stage,respectively.It is generally suggested that the instantaneous nucleation of metallic Mg occurs followed by its growth during hydrogen desorption of MgH2,namely nn=0[22-25].Hence,the growth dimensionalities of metallic Mg for both samples are ng≈1 in the first stage,ng≈2 in the second stage and ng≈1 in the final stage of hydrogen desorption.

Based on the results and discussion above,the growth mechanism of Mg crystallites during hydrogen desorption of MgH2can be proposed(see Fig.7).Upon hydrogen desorption of nanocrystalline MgH2,metallic Mg nucleates instantaneously at free surfaces of particles.After the instantaneous nucleation of Mg,the interface-controlled growth of metallic Mg occurs one-dimensionally (1D) in the first stage of hydrogen desorption,which agrees with the previously reported result that the initial growth occurs nearly along 1D line defects [26,27].In the second stage,the growth of Mg crystallites can be described as being two-dimensional (2D).In other words,the interface-controlled thickening of linear Mg crystallites occurs predominately till neighbouring crystallites are interconnected with each other.Thus the hydrogen desorption rate is relatively quick in this stage.After this,the Mg crystallites grow only in the longitudinal direction(1D)towards the end of transition so that the hydrogen desorption rate becomes slower and slower.

3.4.Effect of Pr3Al11 on crystallite growth of Mg

It should be noted that the durations of the initial stage and the second stage for MgH2−10 wt% Pr3Al11sample are shorter than those of pure MgH2sample,respectively (see Fig.6).This is because that Pr3Al11has a catalytic effect on hydrogen desorption that accelerates hydrogen recombination on particle surfaces[28-34].Accordingly,the MgH2−10 wt% Pr3Al11sample exhibits a larger growth rate of Mg crystallites in the second stage than the pure MgH2sample(see Fig.4).After complete hydrogen desorption,however,the former has a smaller average crystallite size than the latter.As mentioned above,the nucleation of Mg occurs instantaneously at free surfaces of particles upon hydrogen desorption of MgH2[22-25].Thus the average crystallite size of Mg in the completely dehydrogenated sample is related to the number of Mg nuclei [19,29].However,the number of metallic Mg nuclei per particle is few[18,29],leading to larger Mg crystallites in the completely dehydrogenated pure MgH2sample.

To elucidate the effect of Pr3Al11on crystallite growth of Mg,HRTEM observations were performed in the entirely dehydrogenated MgH2−10 wt% Pr3Al11sample.As shown in Fig.8,metallic Mg formed on the Pr3Al11surface during hydrogen desorption.This demonstrates that the defects on the MgH2/Pr3Al11interface could act as nucleation sites for Mg,which is consistent with the findings of other catalyzed MgH2samples [21,33-37].Thus the number of Mg nuclei in MgH2−10 wt% Pr3Al11sample was more than that in pure MgH2sample.As a result,the MgH2−10 wt% Pr3Al11sample had finer Mg crystallites than pure MgH2sample after complete hydrogen desorption.

4.Conclusions

In summary,the rapid growth phenomenon of Mg crystallites has been observed during hydrogen desorption of nanocrystalline MgH2.The transformation from MgH2to Mg is identified as the surface-controlled ‘nucleation and growth’ mechanism.After the instantaneous nucleation of Mg at free surfaces of MgH2particles,the Mg crystallites grow in the sequence of one dimension,two dimensions and one dimension.The rapid growth of Mg crystallites occurs in the second stage of growth in two dimensions.The presence of Pr3Al11lead to the increase of growth rate of Mg crystallites in the second stage due to its catalytic effect but to the decrease of average Mg crystallite size after complete hydrogen desorption because the defects on the MgH2/Pr3Al11interface can act as nucleation sites for Mg.These findings provide an important guidance for the improvement of hydrogen storage properties in Mg-based materials.

Declaration of competing interest

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

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No.51871002).