J. Cent. South Univ. (2016) 23: 2754-2762

DOI: 10.1007/s11771-016-3337-0

Effect of melt spinning on gaseous hydrogen storage characteristics of nanocrystalline and amorphous Nd-added Mg₂Ni-type alloys

ZHANG Yang-huan(张羊换)^{1, 2}, YUAN Ze-ming(袁泽明)², YANG Tai(杨泰)², QI Yan(祁焱)²,GUO Shi-hai(郭世海)², ZHAO Dong-liang(赵栋梁)²

1. Key Laboratory of Integrated Exploitation of Baiyun Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, Baotou 014010, China;

2. Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing 100081, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: Nanocrystalline and amorphous Mg-Nd-Ni-Cu quaternary alloys with a composition of (Mg₂₄Ni₁₀Cu₂)_100-xNd_x (x=0, 5, 10, 15, 20) were prepared by melt spinning technology and their structures as well as gaseous hydrogen storage characteristics were investigated. The XRD, TEM and SEM linked with EDS detections reveal that the as-spun Nd-free alloy holds an entire nanocrystalline structure but a nanocrystalline and amorphous structure for the as-spun Nd-added alloy, implying that the addition of Nd facilitates the glass forming in the Mg₂Ni-type alloy. Furthermore, the degree of amorphization of the as-spun Nd-added alloy and thermal stability of the amorphous structure clearly increase with the spinning rate rising. The melt spinning ameliorates the hydriding and dehydriding kinetics of the alloys dramatically. Specially, the rising of the spinning rate from 0 (the as-cast was defined as the spinning rate of 0 m/s) to 40 m/s brings on the hydrogen absorption saturation ratio (a ratio of the hydrogen absorption quantity in 5 min to the saturated hydrogen absorption capacity) increasing from 36.9% to 91.5% and the hydrogen desorption ratio (a ratio of the hydrogen desorption quantity in 10 min to the saturated hydrogen absorption capacity) rising from 16.4% to 47.7% for the (x=10) alloy, respectively.

Key words: Mg₂Ni-type alloy; Nd addition; melt spinning; nanocrystalline and amorphous alloy; hydrogen storage kinetics

1 Introduction

Hydrogen storage is one of the important components of hydrogen economy. Of the available ways of hydrogen storage, metal hydride systems are regarded to be more accurate, efficient and safe, representing the frontiers of technology [1]. Among these materials, Mg and Mg-based compounds are looked upon as one of the most promising hydrogen storage materials applied to hydrogen fuel cell because of the high hydrogen capacity of the hydrides, e.g. 7.6% (mass fraction) for MgH₂, 3.6% for Mg₂NiH₄, 4.5% for Mg₂CoH₅ and 5.4% for Mg₂FeH₆ compared to the well established low temperature metal hydrides such as LaNi₅ (1.4%) or TiFe and TiMn (1.8%) at a lower cost [2-3]. On the other hand, although the theoretical hydrogen capacities of other complex hydrides such as LiBH₄ and NaAlH₄ are higher than that of Mg₂NiH₄ [2, 4], the use of these complex hydrides for hydrogen storage is severely nullified because of kinetics and thermodynamics limitations [2]. In fact, the Mg₂Ni-type alloys have been viewed as one of the most promising negative electrodes in Ni-MH batteries [5] or hydrogen storage materials applied in hydrogen fuel cell vehicle because of their high theoretical electrochemical capacity (about 1000 mA·h/g) and gaseous hydrogen absorption capacity (3.6%) for Mg₂NiH₄ [6-7]. However, the practical application of the Mg-based alloys is frustrated by some inherent disadvantages such as relatively high dehydrogenation temperature and sluggish hydriding/ dehydriding kinetics as hydrogen storage materials of on-board use. In spite of facing enormous challenges, the researchers in this field still persist in the firm confidence to improve the properties of the alloys and have made breakthrough progress. It is universally agreed that alloying and microstructure modification are effective approaches for improving the hydriding properties [8]. Particularly, the partial substitution of some elements (Y, La, Zr) for Mg and (Cu, Fe, V, Cr, Co) for Ni in Mg₂Ni alloy makes the stability of the hydride decrease and the hydrogen desorption reaction easier [9-11]. It was documented that the hydriding and dehydriding kinetics of the Mg₂Ni-type alloys are very sensitive to their structures [12]. Especially, their hydrogen storage properties are strongly affected by nanometer scale structures because of the thermodynamic and kinetic aspects. Heretofore, the high energy ball milling (HEBM) and melt-spinning technique are considered to be very effective methods to obtain an amorphous and/or nanocrystalline Mg₂Ni alloy. However, some shortcomings of ball milling technology seem to be unavoidable: the milled materials are easily polluted by steel balls and air, even though in very good protection conditions. Also, the cycle stabilities of the milled Mg and Mg-based alloys are very poor owing to the vanishment of the metastable structures generated by ball milling during the multiple hydrogen absorbing and desorbing cycles [13]. On the contrary, the microstructure created by melt spinning displays much higher stability during the hydrogen absorbing and desorbing cycles compared with the microstructure generated by HEBM [14-15]. Meanwhile, the nanocrystalline and amorphous Mg-based alloys prepared by melt-spinning exhibit excellent hydriding characteristics, similar to the alloys produced by the HEBM process [16-17].

Our previous investigations have found that the substitution of La for Mg and M (M=Cu, Co, Mn) for Ni could improve the electrochemical and gaseous hydrogen storage performances of the Mg₂Ni-type alloys dramatically [18-20]. Therefore, we expect that the joint addition of Cu and Nd combining with a proper melt spinning technique may ameliorate the hydrogen storage characteristics of the Mg₂Ni-type alloy more markedly. To validate this, a systematical investigation about the effects of spinning rate on the structures and hydrogen storage performances of the (Mg₂₄Ni₁₀Cu₂)_100-xNd_x (x= 0-20) electrode alloys has been performed, and some experimental results were provided.

2 Experimental

The compositions of the experimental alloys were (Mg₂₄Ni₁₀Cu₂)_100-xNd_x (x=0, 5, 10, 15, 20). For convenience, the alloys were denoted with Nd content as Nd₀, Nd₅, Nd₁₀, Nd₁₅ and Nd₂₀, respectively. The alloy ingots were prepared by using a vacuum induction furnace in a helium atmosphere at a pressure of 0.04 MPa to prevent Mg from volatilizing. A part of the as-cast alloys was re-melted and spun by melt spinning with a rotating copper roller cooled by water. The spinning rates used in the experiment were 10, 20, 30 and 40 m/s, respectively, which were approximately expressed by the linear velocity of the copper roller.

The phase structures of the as-cast and spun alloys were determined by X-ray diffraction (XRD) (D/max/ 2400). The diffraction, with the experimental parameters of 160 mA, 40 kV and 10 (°)/min, respectively, was performed with CuK_α1 radiation filtered by graphite.

The thin film samples of the as-spun alloys prepared by using ion etching technology were observed by high resolution transmission electron microscope (HRTEM) (JEM-2100F, operated at 200 kV) and their crystalline states were ascertained by electron diffraction (ED).

A Philips SEM (QUANTA 400) linked with an energy dispersive spectrometer (EDS) was used for morphological characterization and chemical composition analysis of the as-cast alloys.

Thermal stability and crystallization of the as-spun alloys were studied by means of differential scanning calorimetry (DSC). Also, the DSC curve was measured in argon flow, and the samples were placed in an alumina crucible, heating temperature from 30 to 550 °C with the heating rate of 10 °C/min.

The hydrogen absorption and desorption kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The hydrogen absorption was conducted at 2 MPa hydrogen pressure (in fact, this pressure is the initial pressure of hydriding process) and 200 °C, and the hydrogen desorption at the pressure of 1×10^-4 MPa and 250 °C.

3 Results and discussion

3.1 Microstructure characteristics

Figure 1 shows the XRD patterns of the as-cast and spun Nd₀ and Nd₁₀ alloys, from which it is found that the as-cast and spun Nd₀ alloy displays a typical crystalline structure. Also, we note that the melt spinning renders the diffraction peaks of the Nd₀ alloy evidently broaden, to be put down to the significant refinement of the grain. Differing from Nd₀ alloy, the merged and broadened diffraction peaks of the as-cast Nd₁₀ alloy after melt spun (10 m/s) indicate that the crystalline structure has transformed into an amorphous and nanocrystalline structure. With the spinning rate increasing, the diffraction peaks of the Nd₁₀ alloy become broader and flatter, suggesting that the amount of amorphous phase is growing. It means that the addition of Nd facilitates the glass forming in the Mg₂Ni-type alloy. Amorphous forming ability mainly related to the elements and atomic radius of alloy. The more elements and the lager atomic size difference there are, the stronger amorphous forming ability will be. The presence of Nd facilitates the formation of amorphous phase because the atomic size of Nd (0.264 nm) is lager than that of Mg (0.172 nm) and the atomic size difference is indeed the main factor for forming amorphous alloys [21]. This conclusion is also evidenced by TEM detections, as demonstrated in Fig. 2, from which there is an obvious evidence that the as-spun Nd₀ alloy is strongly disordered and nanostructured, meanwhile, some crystal defects such as subgrains and grain boundaries can be seen clearly, and in its electron diffraction (ED) patterns sharp multi-haloes appear, corresponding to a nanocrystalline structure. Nevertheless, the as-spun Nd₁₀ alloy differing from Nd₀ alloy exhibits a clear feature of the nanocrystalline embedded in the amorphous matrix, and its electron diffraction patterns consist of broad and dull halo, indicating the existence of an amorphous structure, which corresponds with the XRD inspection as depicted in Fig. 1. Meanwhile, the phase components of the as-cast Nd₀ and Nd₁₀ alloys are analyzed by SEM, as illustrated in Fig. 3. Apparently, the addition of Nd results in the morphologies of the as-cast alloys changing obviously, namely the typical dendritic structure of the as-cast Nd₀ alloy completely disappearing and some secondary phases emerging, which are Mg₆Ni, Nd₅Mg₄₁ and NdNi determined by EDS patterns.

Fig. 1 XRD patterns of as-cast and spun alloys:

Fig. 2 HRTEM micrographs and ED patterns of as-spun (Mg₂₄Ni₁₀Cu₂)_100-xNd_x (x=0-20) alloys:

Fig. 3 SEM images (a, b, c, d) together with typical EDS spectra (e, f, g) of as-cast alloys:

3.2 Thermal stability and crystallization

To examine the thermal stability and the crystallization of the as-spun amorphous and nanocrystalline/amorphous alloys, DSC analysis was conducted, as demonstrated in Fig. 4. Evidently, some sharp exothermic DSC peaks emerge at about 460 °C, corresponding to a crystallization reaction (ordering) of the amorphous into nanocrystalline. Furthermore, we note that increasing the spinning rate from 0 to 40 m/s gives rise to the crystallization temperature rising from 452 to 483 °C, suggesting that rising spinning rate facilitates the stability of the amorphous structure.

3.3 Hydrogen storage properties

3.3.1 Hydrogen storage capacity and kinetics

Figure 5 describes the relationship between the hydrogen absorption quantities and the reaction time of the as-cast and spun Nd₀ and Nd₁₀ alloys at 200 °C. Evidently, all the as-cast and spun alloys display fast hydrogen absorption rates in the initial stage after that the hydrogen contents are almost saturated at the next quite a long hydrogenation time. For all the experimental alloys, the hydrogen absorption capacities in 100 min are more than 95% of their saturated hydrogen absorption capacities. Thereby, it is considered to be reasonable taking value as hydrogen absorption capacity of the alloy. Evidently, the as-spun alloys exhibit much higher value than the as-cast ones, indicating that the melt spinning makes an obviously positive contribution to the hydrogen absorption capacity of the alloy. It is derived that enhancing the spinning rate from 0 to 40 m/s renders the value growing from 2.95% to 3.58% for the Nd₀ alloy and from 2.82% to 3.16% (30 m/s) and then to 3.01% for the Nd₁₀ alloy.

Fig. 4 DSC curves of as-spun Nd₁₀ alloy

Fig. 5 Evolution of hydrogen absorption quantities of (Mg₂₄Ni₁₀Cu₂)_100-xNd_x (x=0-20) alloys with time varying at 200 °C:

As is wellknown, in addition to decreasing hydrogenation and dehydrogenation temperatures, improving the hydriding and dehydriding kinetics is also extremely important for putting Mg₂Ni-type alloy into practice application. Therefore, it is very necessary to examine the influence of spinning rate on the hydriding and dehydriding kinetics of the alloy. Here, the hydriding kinetics of the alloy is characterized by its hydrogen absorption saturation ratio a ratio of the hydrogen absorption capacity at a fixed time to the saturated hydrogen absorption capacity of the alloy, which is defined as where and are hydrogen absorption capacities at t min and 100 min, respectively. Considering the better possibility of mutual comparison, we take the hydrogen absorption time of 5 min as a criterion and establish the relationship between the(t=5) values of the (Mg₂₄Ni₁₀Cu₂)_100-xNd_x (x=0-20) alloys and the spinning rate, just as provided in Fig. 6. Evidently, the growing of the spinning rate from 0 to 10 m/s results in the values of the alloys increasing dramatically, after that the growing rate of the values clearly declines with the spinning rate rising, meaning that the melt spinning plays a beneficial role on the hydriding kinetics of the alloy. Particularly, the value is enhanced from 32.3% to 88.4% for the Nd₀ alloy and from 36.9% to 91.5% for the Nd₁₀ alloy by increasing the spinning rate from 0 to 40 m/s. Also, it is found that the minimum difference of the value between the as-spun and as-cast alloy for the fixed Nd content is much larger than the maximum difference of the value generated by changing the Nd content for the fixed spinning rate. Hence, it can be surmised that the hydrogen absorption kinetics of the alloy is chiefly dominated by its structure.

Fig. 6 Evolution of values of (Mg₂₄Ni₁₀Cu₂)_100-xNd_x (x=0-20) alloys with spinning rate varying

Fig. 7 Evolution of hydrogen desorption quantities of (Mg₂₄Ni₁₀Cu₂)_100-xNd_x (x=0-20) alloys with time varying at 250 °C:

Presented in Fig. 7 is the relationship between the hydrogen desorption quantities and the reaction time of the as-cast and spun (Mg₂₄Ni₁₀Cu₂)_100-xNd_x (x=0-20) alloys at 250 °C. An important feature of alloys emerging in the dehydriding process exhibits fast initial hydrogen desorption, subsequently the dehydrogenation rate dwindles sharply. Apparently, the melt spinning considerably enhances the dehydrogenation capability of the alloys. From Fig. 7, it is derived that the increasing of the spinning rate from 0 to 40 m/s engenders the (H-desorbed capacity in 100 min) values growing from 0.30% to 1.98% for the Nd₀ alloy and from 0.53% to 2.20% for the Nd₁₀ alloy, respectively. Likewise, the hydrogen desorption kinetics of an alloy is symbolized by hydrogen desorption ratio a ratio of the H-desorbed capacity at a fixed time to the saturated hydrogen absorption capacity of the alloy, which is defined as where is the hydrogen absorption capacity at 100 min and is the hydrogen desorption capacity at the time of t min, respectively. In order to facilitate comparison, here, we take hydrogen absorption time of 10 min as the standard. Thus, the relationship between the (t=10) values and the spinning rate of the (Mg₂₄Ni₁₀Cu₂)_100-xNd_x (x= 0-20) alloys can be founded easily, as illustrated in Fig. 8. Evidently, the melt spinning makes a positive contribution to the dehydriding kinetics of the alloys. More specifically, enhancing the spinning rate from 0 to 40 m/s gives rise to the value rising from 8.2% to 38.9% for the Nd₀ alloy and from 16.4% to 47.7% for the Nd₁₀ alloy, respectively. What is more, it is noticeable that, whatever the spinning rate is, the Nd-added alloys show a much larger value than the as-cast one, suggesting that the addition of Nd facilitates the dehydriding rate of the alloy.

Fig. 8 Evolution of values of (Mg₂₄Ni₁₀Cu₂)_100-xNd_x (x=0-20) alloys with spinning rate varying

In the case of the improved hydriding and dehydriding kinetics by melt spinning, some elucidations can be provided. As known to all, the hydrogen absorption process of hydrogen storage alloys consists of the following steps: the rate of hydrogen molecular dissociation on the surface, the capability of hydrogen atoms penetrating from the oxide layer surface into the metal, the rate of hydrogen atoms diffusing into the bulk metal and through the hydride already formed, yet, the hydrogen absorption rate is predominated by the slowest step. The positive impact of the melt spinning on the hydriding kinetics is principally ascribed to the changed structure of the alloy by the melt spinning. The crystalline material, when melt spun, becomes at least partially disordered and its structure turns into nanocrystalline, generating a lot of new crystallites and grain boundaries (Fig. 2). Hence, some crystal defects such as dislocations, stacking faults and grain boundaries are introduced, as evidenced by our previous work [22], which may promote the diffusion of hydrogen in materials by providing numerous sites with low diffusion activation energy [23]. Moreover, it has been ascertained that the hydriding and dehydriding kinetics of the Mg₂Ni-type alloy is very sensitive to its structure. Especially, the hydrogen storage properties are considered to be strongly affected by their nanometer scale structures because of the thermodynamic and kinetic aspects. As stated by KUMAR et al [24], the changing of the structure of the Mg₂Ni alloy from polycrystalline to nanocrystalline results in a drop of about 100 K in absorbing/desorbing temperature, namely from 573 K to 473 K. It is evidenced by ZHAO et al [25] that hydrogen adheres to the surface of the nanocrystalline nickel more strongly than that of the polycrystalline nickel, facilitating the dissociation reaction of hydrogen molecular. With respect to the positive contribution of Nd adding to the dehydriding kinetics, it is considered to be related to the following two reasons [26]. Firstly, the addition of Nd facilitates the glass forming of Mg₂Ni-type alloy, and amorphous Mg₂Ni shows an excellent hydrogen desorption capability. Secondly, such addition decreases the stability of the hydride and makes the desorption reaction easier.

3.3.2 Hydriding and dehydriding cycle stability

Cycle stability, one of the major performances which are used to evaluate whether a kind of an alloy can be applied as a hydrogen storage material or not, is signified by the capacity retaining rate (S_n), defined as where is the saturated hydrogen capacity and C_n is the hydrogen absorption capacity at the nth hydriding and dehydriding cycle, respectively. Evidently, it means that the larger the capacity retaining rate (S_n) is, the better the cycle stability of the alloy will be. The variations of the S_n values of the as-spun (40 m/s) Nd₀ and Nd₁₀ alloys depending on the cycle numbers are provided in Fig. 9, from which the degradation process of the hydrogen capacity of the alloys can be seen clearly. In order to directly exhibit the degradation of the hydrogen capacity with the cycle number, the evolution of the values of the Nd₀ and Nd₁₀ alloys with cycle number is also inserted in Fig. 9. The slopes of the curves qualitatively reflect the degradation rate of the hydrogen capacity during the hydriding and dehydriding cycles, namely the smaller the slope of the curve is, the better the cycle stability of the alloy will be. It can be seen that the S_n values of the as-spun (40 m/s) Nd₀ and Nd₁₀ alloys exhibit small drop with cycle number increasing, reflecting excellent cycle stability. Stated concretely, after cycling for 100 times, the S_n values of the Nd₀ and Nd₁₀ alloys still keep at 88.3% and 96.8%, respectively. Noticeably, the slope of the as-spun Nd₁₀ alloy is less than that of the Nd₀ one, indicating that the addition of Nd plays a beneficial role on the cycle stability of the alloy.

Fig. 9 Evolution of capacity maintaining rate (S_n) together with hydrogen absorption capacity of as-spun (40 m/s) Nd₀ and Nd₁₀ alloys with cycle number

In an attempt to elucidate the possible reasons for the cycle performance loss of the as-spun alloys, we investigate the structure changes of the alloy generated by hydriding and dehydriding cycle. First, the morphologies of the Nd₀ and Nd₁₀ alloy particles before and after cycling are observed by SEM, as illustrated in Fig. 10. A lot of cracks can be easily seen on the surfaces of the alloy particles after 20 cycles, and with cycle number increasing, the pulverization degree of the particles becomes more serious. Noticeably, we find that there are much fewer cracks on the surface of the Nd₁₀ alloy particle after cycling than that of the Nd₀ one, reflecting that the addition of Nd can clearly enhance the anti-pulverization ability of the alloy. The faster capacity degradation of the as-spun (40 m/s) Nd₀ alloy is principally ascribed to the structure stability of Nd₀ alloy which is lower than Nd₁₀ alloy. The crystal defects of Nd₀ alloy prone to merge even disappear during the hydriding-dehydriding cycle, which results in the occupied site of atomic hydrogen losing and the capacity decreasing. The function of Nd adding is surmised to be ascribed to two aspects. On the one hand, adding the rare elements (La, Nd, Sm and Y) engenders the cell volume of the Mg₂Ni alloy enlarging clearly [18], decreasing the ratios of expansion/contraction in the process of hydrogen absorption/desorption, hence increasing the anti-pulverization capability. On the other hand, the glass formation facilitated by Nd adding is extremely important because an amorphous phase improves not only anti-pulverization ability but also anti-corrosion and anti-oxidation abilities of the alloy electrode in a corrosive electrolyte [19, 27]. It must be pointed out that the capacity degradation of the alloys can not be ascribed to the pulverization, which is supported by two aspects. Firstly, the cycle testing is carried out under almost absolute vacuum, which prevents the surfaces of alloy particles from being oxidized. Secondly, it is generally accepted that the reduction of the alloy particles’ size facilitates to enhance hydrogen storage capacity and kinetics of the alloys. What is more, we investigate the crystallization behavior of amorphous structure. As considered by LIANG et al [14], the crystallization of amorphous phase in the Mg-based alloy causes an obvious change of hydrogen storage performances. As a matter of fact, the result shown in Fig. 4 indicates that crystallization of the amorphous phase will not occur at the dehydrogenation temperature because the crystallization temperatures of the amorphous phase in the alloys are all above 452 °C, much higher than dehydrogenation temperature (250 °C). But in fact, the crystallization of the amorphous phase is inevitable during repeated cycling in spite of the crystallization temperature being much higher than the dehydrogenation one, which is also validated by XRD data, as depicted in Fig. 11. Obviously, the as-spun Nd₁₀ alloy still holds amorphous structure but only a little crystal Mg₂Ni phase appears after 20 cycles. After cycling for 50 times, the amorphous structure is found to turn into nanocrystalline completely. Based on the above-mentioned results, it can be concluded that the degradation of cycling properties of the as-spun alloy is principally ascribed to the varying structure, particularly crystallization of amorphous phase.

Fig. 10 SEM images of granular morphologies of as-spun (40 m/s) Nd₀ and Nd₁₀ alloys before and after hydriding and dehydriding cycles:

Fig. 11 XRD patterns of as-spun (40 m/s) Nd₁₀ alloy before and after hydriding and dehydriding cycle

4 Conclusions

The hydrogen storage characteristics of the as-cast and spun (Mg₂₄Ni₁₀Cu₂)_100-xNd_x (x=0-20) alloys and their structure evolution during cycling were investigated. The major conclusions are summarized as follows:

1) All the as-cast alloys hold a multiphase structure, consisting of the major phase Mg₂Ni and some secondary phases. The addition of Nd renders Nd₅Mg₄₁ and NdNi phases forming whose amounts markedly grow with the Nd content increasing. The addition of Nd facilitates the glass forming in the Mg₂Ni-type alloy and the degree of amorphization significantly increases with the Nd content rising.

2) The melt spinning exerts a significantly positive impact on the hydrogen absorption capacity together with hydriding and dehydriding kinetics of the alloys, which is principally attributed to the nanocrystalline and amorphous structure generated by melt spinning.

3) The as-spun alloys display excellent cycle stability, to be attributed to high stability of the nanocrystalline and amorphous structure generated by melt spinning. The repeated cycling for many times results in a gradual evolution of the nanocrystalline and amorphous structure, namely the crystallization of amorphous, which is responsible for the degradation of the cycle performances.

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(Edited by FANG Jing-hua)

Foundation item: Projects(51161015, 51371094) supported by the National Natural Science Foundation of China

Received date: 2015-10-16; Accepted date: 2015-12-04

Corresponding author: ZHANG Yang-huan, Professor, PhD; Tel: +86-10-62183115; E-mail: zhangyh59@sina.com