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Hydrogen storage thermodynamics and dynamics of Mg–Y–Ni–Cu based alloys synthesized by melt spinning
Journal of Physics and Chemistry of Solids xxx (xxxx) xxx
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Hydrogen storage thermodynamics and dynamics of Mg–Y–Ni–Cu based alloys synthesized by melt spinning Yanghuan Zhang a, b, *, Xin Wei b, Zeming Yuan a, Zhonghui Hou a, Yan Qi b, Shihai Guo b a b
Key Laboratory of Integrated Exploitation of Baiyun Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, Baotou, 014010, China Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing, 100081, China
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Mg–Ni-Based alloy C. Activation energy C. Melt spinning C. Thermodynamics D. Hydrogen storage kinetics
Excessive heat stability, poor kinetics of hydrogen absorption and desorption reaction are viewed as the major setbacks for the Mg-based hydrogen storing materials application. The microstructures of samples Mg25-xYxNi9Cu (x ¼ 0, 1, 3, 5, 7) alloys were observed by XRD, SEM, and TEM. Besides, aiming at studying the characteristics of hydrogenation/dehydrogenation, the Sievert instrument, DSC and TGA were applied in the experiments. Substituting Mg with Y can promote the formation of the second phase YNi3 but does not transform the Mg2Ni major phase. This substitution greatly prompts the amorphization. The increase of Y dosage triggers a mild decrease in the absolute values of ΔH and ΔS and remarkably boosts the improvement of dehydrogenation ki netics. Moreover, the increase in Y amount brings on a decrease in hydrogen absorption capability. The hydrogen desorption activation energy markedly lowers with increasing Y content (the Ede a values of as-spun Y0 and Y3
samples are 71.38 and 66.13 kJ/mol, severally, and Ede k values of them are 69.03 and 65.53 kJ/mol, respec tively), which brings about the promotion of dehydrogenation kinetics of the experimental samples.
1. Introduction Hydrogen is considered to be the non-substitutable bunkers in the 21st century with pollution-free characteristics as a fuel and the most abundant distribution as an element in the universe [1–3]. There are various applications about hydrogen including hydrogen power gener ation, energy for the vehicle, jet planes and combustion energy in our daily lives. It is worth noting that there are many projects dedicated to the application of hydrogen in the field of vehicle fuel [4]. Compared with traditional fossil fuel, the hydrogen fuel will not trigger the issues caused by combustion like global warming or air pollution [5–7]. A critical step to utilize hydrogen energy in automobiles is to exploit the vehicle-mounted hydrogen energy systems [8–11]. Among the hydrogen storing techniques, metal hydride is identified as a promising way which can meet the needs of mobile application [12]. Wang et al. [13] pre pared the ZrCo alloy decorated with Pd particles and found that the hydrogenation kinetic property of the alloys after decoration treatment was greater than the initial specimen. In addition, on the basis of in vestigations, some hydrogen storage materials have achieved the application aims. Unfortunately, there is no one can reach to the
standard presented by the U.S. Department of Energy for the vehicle industry field [14,15]. According to major advantages of Mg–Ni-based metallic hydrides, Mg2NiH4, holding 3.6 wt% theoretical gaseous hydrogen absorption capacity and 1000 mAh/g electrochemical capac ity [16–19] is viewed as a prospective material which can be applied in hydrogen energy automobiles or in batteries [20]. Nevertheless, as for Mg-based alloys, the absence of moderate dehydrogenation tempera ture, fast kinetics characteristic, and good electrochemical cycle stability is the serious impediment that restricts the practical application of them [21,22]. Aiming at ameliorating the hydrogen storing characteristics, the efficient approaches: alloying and microstructure modification were utilized [23–25]. Especially, partial substitution of Mg in Mg2Ni-type alloy with rare earth elements results in the decline of hydrides steadi ness and the promotion of hydrogen desorption [26,27]. Yong et al. [28] successfully prepared the Mg90Ce5Y5 alloy which had better kinetics than pure MgH2 specimen owing to the formation of rare earth hydride phase. Furthermore, according to the previous investigations, the dy namics is closely related to the microstructures [29]. As researched by Pang et al. [30], the complicated hydride with nanoscale structure was fabricated by a mechanical-force-driven physical vapour deposition
* Corresponding author. Department of Functional Material Research, Central Iron and Steel Research Institute, No. 76 Xueyuannan Road, Haidian District, 100081, Beijing, China. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.jpcs.2019.109252 Received 1 July 2019; Received in revised form 24 October 2019; Accepted 30 October 2019 Available online 1 November 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Yanghuan Zhang, Journal of Physics and Chemistry of Solids, https://doi.org/10.1016/j.jpcs.2019.109252
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method. Siarhei et al. [31] refined the grain of Mg-based alloys to nanometer level and their hydrogenation/dehydrogenation properties could be significantly ameliorated. Some methods like ball milling [32] and melt spinning [33] have been favorably applied to obtaining non-crystal or nanocrystalline structures that have high homogeneous element distribution. Just like investigated by Huang et al. [34] that by using the preparation technology of strip casting, the (Mg60Ni25)90Nd10 alloy with non crystal and nanocrystalline structures can be gained and has 580 mAh/g discharge capacity. In addition, Spassov et al. [35] proposed that partly substituting Mm (Mm ¼ Ce, La-rich mischmetal) for Mg heightens the hydrogen absorbing capability of the as-spun Mg75Ni20Mm5 specimens. In present work, so as to prepare nanocrystalline and no crystal structure in Mg25-xYxNi9Cu (x ¼ 0–7) specimens, the preparation method of melt spinning technique and substituting Y for Mg partially in Mg–Ni-based alloy were applied. Subsequently, the influences of Y content and melt spinning on thermodynamic and dynamic perfor mances of hydrogen storage alloys were studied minutely.
graphite filtering CuKα1 radiation. The parameters were 10� /min, 40 kV and 160 mA in this experiment. The observation of morphology was conducted with the help of Philips scanning electron microscope Philips SEM (QUANTA 400) equipped with an energy dispersive spectrometer (EDS). By means of the utilization of a JEM-2100 F high resolution trans mission electron microscope (HRTEM) made by JEOL Ltd, the as-spun specimens were observed. The crystallized states of specimens were determined by electron diffraction (ED). An automatic control Sieverts instrument with a stove controlling temperature in the range of �2 K was utilized to measure the P-C-T curves and dynamics of the sample. The temperatures of hydrogen ab sorption were 553–593 K at 3 MPa pressure with 300 mg alloy in a tubbiness reactor for each measure. When it comes to the performances of hydrogen desorption, the differential scanning calorimetry (DSC) with 5, 10, 15 and 20 K/min heated rates was used for testing the samples. 3. Results and discussion
2. Experimental
3.1. Microstructure characteristics
Aiming at inhibiting the volatilization of the Mg element, we used the vacuum induction furnace with a protective gas of 0.04 MPa He (Helium with purity of 99.999% is provided by CISRI Corporation) to fabricate the Mg25-xYxNi9Cu (x ¼ 0, 1, 3, 5, 7) alloys. For ease of pre sentation, corresponding Y contents in alloys are represented by Y0, Y1 and so on. The purities of the metallic materials of Mg, Ni, Cu, and Nd are at least 99.99% provided by CISRI Corporation. The molten alloy was poured into a copper mold for cooling down and then a cast ingot was prepared. Some specimens were re-melted and treated by melt spinning. In the melt spinning experiment, the spinning rate was approximately represented by the linear velocity of the copper roller due to the difficulty in measuring the precise spinning rate. The rate of 20 m/ s was set as the spinning rates in the experiment. The manufacturer of automatically controlled Sieverts apparatus (PCT-4SDWIN) is Suzuki Shokan Co., Ltd. The weight of sample put into the reactor was 0.5 g, and after six cycles activation treatment under the temperature of 573 K and the H2 pressure of 3Mpa, the specimens could be entirely activated. X-ray diffraction (XRD) (D/max/2400) produced by Rigaku is a general apparatus to determine the phase structures of specimens with
Fig. 1 gives the XRD images, from which the information of structure, and phase constituent of as-cast and as-spun specimens can be obtained. Fig. 1 (a) shows that substituting Y for Mg gives rise to the appearance of the secondary phase YNi3, and the structural evolution of the as-cast specimens is owing to the increase of Y element and the decline in the Mg element, the amount of phase Mg2Ni decreases, conversely, the quantity of phase YNi3 increases. The diffraction peaks of as-spun (20 m/ s) sample vary with Y content as displayed in Fig. 1 (b). By observing the as-spun Y5 and Y7 specimens, the existence of typical amorphous structure can be found, which suggests that the increase of Y content has a stimulative effect on the glass formation of Mg–Ni-based alloys. SEM is utilized to observe the morphologies, as demonstrated in Fig. 2, in which the morphology varies with Y content greatly. The EDS identification reveals that the Y0 alloy has a biphasic structure, con taining Mg2Ni and Mg. Substituting Y for Mg brings about the formation of the secondary phase YNi3. Besides, the phase YNi3 increases in quantity with the growth of Y dosage, which is the same as the XRD analysis. The observation of morphologies was achieved by the usage of
Fig. 1. XRD profiles of as-cast and as-spun Mg25-xYxNi9Cu (x ¼ 0–7) alloys: (a) as-cast alloys, (b) as-spun alloys (20 m/s). 2
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Fig. 2. SEM images together with typical EDS spectra of the as-cast alloys: (a) Y0 alloy, (b) Y3 alloy, (c) Y7 alloy.
HRTEM and ED for confirming the crystal states, as presented in Fig. 3. Clearly, Y0, Y1 and Y3 specimens display a mass of nanocrystalline structures, disordered areas and crystal defects. Besides, the ED patterns of Y0, Y1 as well as Y3 specimens have evident multi-haloes, which is in line with the feature of nanocrystalline. However, the as-spun Y7 sample exhibits a clear characteristic of nanocrystalline inserting into amor phous matrix, indicating that the glass formation of Mg–Ni-based alloys is facilitated owing to the Y additive.
(Where the abbreviated form PH2 expresses the equilibrium hydrogen gas pressure, the sample temperature and the letter R expresses the gas constant) By means of the logarithmic conversion of equation (1), the Van’t Hoff graphs of lnPH2 =P0 versus 1/T for Y0 and Y3 samples were plotted, as presented in Fig. 5. There is a linear relation of lnPH2 =P0 and 1/T for Y0 and Y3 alloys. Therefore, the thermodynamic parameters which are generally referred to enthalpy change (ΔH) and entropy change (ΔS) were figured up effortlessly based on equation (1). Table 1 displays the enthalpy and entropy changes, suggesting that replacing Mg by Y brings on a mild reduction in ΔH and ΔS of as-spun specimens. The as-cast and as-spun (20 m/s) Mg25-xYxNi9Cu (x ¼ 0–7) alloys at saturated hydrogen absorption state were placed in a reactor with 5 K/min heating rate so as to obtain the desorption curve, as shown in Fig. 6. Samples taken at each time were the same weight, which aims at avoiding the effect caused by increased pressure on desorption temperature. Based on the graph, the temperature of hydrogen desorption clearly declines with adding Y element, indicating the hydride stability reduction caused by this substitution. The decreased starting hydrogen desorption tempera ture caused by melt spinning can be ascribed to the formation of the nanocrystalline and amorphous structures on basis of the fact that the refined microstructure plays a dominant role in decreasing the dehy drogenation temperature [39,40]. The as-cast alloys’ starting hydrogen desorption temperature cuts down from 532.8 to 499.5 K and from 513.2 to 491.3 K for the as-spun alloys with changing Y content. Ac cording to Fig. 6 (a) and (b), when Y content is fixed, the as-spun alloys have lower starting hydrogen desorption temperature than the as-cast alloys, signifying the decline of hydride steadiness induced by melt spinning.
3.2. P-C-T curves and hydrogen storage thermodynamics In Fig. 4, the P-C-T curves of as-cast and spun samples at 573 K are presented. Obviously, the existence of two pressure plateaus in every curve can be found. The shorter and lower plateau expresses MgH2 while the longer and higher one represents the hydride Mg2NiH4 [36,37]. Moreover, an obvious gradient and a big hysteresis (Hf ¼ ln (Pa/Pd)) are exhibited in the hydrogenation/dehydrogenation pressure platforms. Melt spinning brings on the obvious ascent of hydro genation/dehydrogenation plateaus pressure owing to the fact that melt spinning results in the increase of internal strain. It is worth noting that the samples’ hydrogen storage capacity changes with the variety of Y dosage. Fig. 4 shows the relation between Y content and hydrogen storage capacity. The rise of Y brings about the hydrogen storage ca pacity of as-cast alloy decreasing from 4.12 to 2.82 wt% and 4.24 to 2.58 wt% for that of the as-spun (20 m/s) sample. The temperatures of measuring the as-spun (20 m/s) Y0 and Y3 samples were 553–593 K so as to make sense of the influence of substitution on thermodynamics, as described in Fig. 5. The following is the Van’t Hoff equation [38], � � P H2 ΔH ΔS In (1) ¼ RT R P0 3
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Fig. 3. HRTEM micrographs and ED patterns the as-spun (20 m/s) alloys: (a) Y0 alloy, (b) Y1 alloy, (c) Y3 alloy, (d) Y7 alloy.
Fig. 4. P-C-T curves of the as-cast and as-spun Mg25-xYxNi9Cu (x ¼ 0–7) alloys at 573 K: (a) as-cast alloys, (b) as-spun alloys (20 m/s).
Fig. 5. P-C-T curves of as-spun (20 m/s) Y0 and Y3 samples at 553 K, 573 K and 593 K: (a) Y0 alloys, (b) Y3 alloys.
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heightened, the Ra5 of as-spun sample declines from 87.5% to 79.6%, but that of as-cast sample rises from 85.8% to 91.5%. Taking the positive influence of substitution on as-cast alloy into consideration, the enlargement of cell volume and the formation of induced phase YNi3 can be utilized to explain this phenomenon. Besides, the phase boundaries can offer convenient access for the diffusion of hydrogen and the descent of atoms diffusion activation energy caused by increasing cell volume also has a beneficial influence on samples’ hydrogen absorption dy namics [41]. As for the as-spun alloy, the negative action of substituting Y for Mg on the hydrogenation kinetics is due to the glass formation, which is based on the principle that the hydrogen atoms have better diffusing ability in nanocrystalline structure than in amorphous structure. Analogously, the hydrogen desorption ratio (Rdt ) (the ratio of hydrogen desorbing capability at specified time to saturated hydrogen desorbing capability) is also a significant parameter which can reflect samples’ dehydrogenation dynamic property. The definition of the
Table 1 ΔH and ΔS values of the as-spun (20 m/s) Y0 and Y3 alloys. Samples
Absorption ΔH (kJ/mol)
Y0 Y3
64.40 60.90
Desorption ΔS (J/mol/K) 120.92 115.22
ΔH (kJ/mol)
ΔS (J/mol/K)
68.24 64.29
123.02 116.84
3.3. Hydrogen absorption and desorption kinetics Under the hydrogenation condition of 573 K and 3 MPa, the varia tions in experimental specimens’ hydrogen absorption capacity are described in Fig. 7. The as-cast and spun compounds in the beginning stage demonstrate quick hydrogen absorption velocity, later the almost hydrogen saturated state lasts for a long time. With regard to hydrogen absorption capacity, it is a crucial factor to enable hydrogen storage materials to be applied successfully in vehicle-mounted hydrogen stor age system, and that also cannot ignore the factor: hydrogen reaction rate. In this experiment, the Rat (the ratio of hydrogen absorbing capa bility at specified time to saturated hydrogen absorbing capability) is used to express the hydrogen absorption kinetics with the definition of Rat ¼ Cat =Ca100 � 100%. The abbreviated form Cat represents the hydrogen absorption capacity at t min. Because all the samples used in this experiment obtained the hydrogen absorption capacity that reaches 98% of the saturated state in 100 min, the Ca100 value is utilized to ex press the saturated absorption capacity. In order to compare conve niently, Ra5 (t ¼ 5) was set as a standard criterion to establish the relation with Y dosage, as given in Fig. 7. Obviously, increasing Y content makes the Ra5 value of as-spun samples decline. As for the as-cast samples, the situation is exactly opposite. To be exact, when the Y dosage is
hydrogen desorption ratio is Rdt ¼ Cdt =Ca100 � 100%, where Cdt represents the hydrogen desorption capacity in t min and the meaning of Ca100 is the same as the previous definition. As described in Fig. 8, in order to establish a relation of hydrogen desorption ratio and Y content, it is necessary to institute 10 min hydrogen absorption time as a criterion. The augmentation of dehydriding rates with increasing Y content is obvious. Specifically, for the as-cast sample, adding Y from 0 to 7 en ables Rd10 value to rise from 50.4% to 70.5% and 57.7% to 71.6% for as-
spun sample. The as-spun sample emerges much higher Rd10 value compared with the as-cast one in the case of fixed Y content, manifesting that melt spinning can ameliorate the hydrogen desorption kinetics. The positive effect on dehydrogenation kinetics triggered by melt spinning is considered to be caused by the increase of internal stress and the grain
Fig. 6. Temperature programmed desorption curve of the as-cast and as-spun samples after hydrogen absorption at a heating rate of 5 K/min: (a) as-cast alloys, (b) as-spun alloys (20 m/s).
Fig. 7. Hydrogen absorption kinetic curves of the as-cast and spun Mg25-xYxNi9Cu (x ¼ 0–7) alloys at 573 K: (a) as-cast alloys, (b) as-spun alloys (20 m/s). 5
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the dehydrogenation reaction of samples [44]. According to the slope and intercept of fitting lines, the η and ηlnk values at 593, 573 and 553 K were obtained and the rate constant k was also easy to calculate. In the light of the Arrhenius equation as follows, the dehydrogenation activa tion energy (Ede a ) can be gotten [45]: Using A to stand for temperature coefficient, using R to represent the ideal gas constant, in addition, using T to express the absolute temper ature of samples. The k is the same as the previous definition. The Arrhenius images are presented in Fig. 10 to describe the hydrogen desorption dynamic of samples. According to the slopes of plots, the activation energy Ede a was obtained. When it comes to the as-spun Y0 and
Y3 alloys, the Ede a values of them are 71.38 and 66.13 kJ/mol, severally, which manifests that replacing Mg by Y makes the activation energy of dehydrogenation decrease. In order to compare with the estimates of the Arrhenius method, the dehydrogenation activation energy was determined by using the Kis singer method. The Kissinger equation is [46]: �� � d In β=T 2P Ede k ¼ (4) dð1=TP Þ R
Fig. 8. Evolution of the values of the as-cast and as-spun Mg25-xYxNi9Cu (x ¼ 0–7) alloys with Y content.
refinement, because when the size of grains is reduced below micro meter level, the Mg-based alloys’ hydrogen desorption characteristics can get great enhancement [42].
Using Ede k to stand for activation energy, using a Greek alphabet β to express the heating rate, using TP to represent the absolute temperature at which there is a maximum desorption rate existing in DSC plots, in addition, using a capital letter R to express the ideal gas constant. The measurement of hydrogen desorption reactions in the Mg25-xYxNi9Cu (x ¼ 0–7) alloys was taken by using DSC whose heating rates were set between 5 and 20 K/min. What needs to pay attention to is that the tested samples were saturated state at 3 MPa, and 573 K. To illustrate the measurement results, the as-spun samples’ dehydrogenation DSC curves are presented in Fig. 11. All curves have a clear and alike endothermic peak in the dehydrogenation process, manifesting that the reaction of each alloy contains similar process. In addition, the phenomenon that the as-spun samples’ endothermic peak has the tendency to transfer to low temperature indicates that melt spinning has a commendably beneficial function to promote the hydrogen desorbing reaction. The relation of Inβ=T 2P vs. 1/TP was established by making use of the loga rithmic transform of equation (4) based on the experimental datum, as displayed in Fig. 11. Taking linearization of the Kissinger plots into consideration, it is effortless to figure out the activation energy Ede k on the basis of the Kissinger plots’ slope. The Ede k values of as-spun Y0 and Y3 alloy are 69.03 and 65.53 kJ/mol, respectively. According to the results of activation energy figured out by utilizing the two methods mentioned above, the relation of the alloys’ Ede (Ede ¼ Ede a is calculated on the basis
3.4. Hydrogen desorption activation energy In general, activation energy is not only the barrier that needs to be overcome in reaction but also an extremely important parameter in the kinetics. In dehydrogenation reaction, there is a total energy barrier existing, so the activation energy generally reflects the driving force for dehydrogenation reaction. In order to expose the function mechanism of replacing Mg by Y on dehydrogenation dynamic, the Arrhenius and Kissinger methods were utilized to obtain the calculation results. On basis of the applicable premises of the Arrhenius method, the dehy drogenation dynamic curves of as-spun Mg25-xYxNi9Cu (x ¼ 0–7) speci mens were tested between 553 and 593 K. Fig. 9 presents the dehydrogenation kinetics curves of as-spun specimens. In general, the Johnson-Mehl-Avrami (JMA) model is used for simulating the gas-solid reactions such as dehydrogenation reaction of metal hydrides. The JMA equation is [43]: ln½
lnð1
αÞ � ¼ ηlnk þ ηlnt
(2)
where phase fraction at t time expressed by an ancient Greek alphabet α and identified by standardized hydrogen wt.%, using a Greek alphabet η to represent the Avrami coefficient, as well as the dynamics coefficient, is expressed by lowercase letter k. The Johnson-Mehl-Avrami graphs ln [-ln (1-α)] vs. lnt were established by making use of the logarithmic transformation of equation (2) at 593, 573 and 553 K, as presented in Fig. 9. The trend of Johnson-Mehl-Avrami plots is linear in the process of
of the Arrhenius method; Ede ¼ Ede k is calculated by Kissinger method) and Y content is presented in Fig. 12. Clearly, the as-cast and as-spun specimens’ Ede values decline with the increase of Y dosage.
Fig. 9. Hydrogen desorption kinetic curves of the as-spun (20 m/s) Y0 and Y7 alloys at 553, 573 and 593 K and Avrami plots of ln [-ln (1-α)] vs. lnt: (a) Y0 alloys, (b) Y3 alloys. 6
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Fig. 10. Arrhenius plots of the as-spun (20 m/s) Mg25-xYxNi9Cu (x ¼ 0–7) alloys: (a) Y0 alloy, (b) Y3 alloy. � de � Ea K ¼ A exp RT
(3)
Fig. 11. Kissinger plots and DSC curves of as-spun (20 m/s) Mg25-xYxNi9Cu (x ¼ 0–7) alloys at various heating rates: (a) Y0 alloys, (b) Y3 alloys.
Furthermore, the Ede of as-spun alloy is much lower in comparison with that of as-cast alloy when the Y content is fixed. Due to the lessening of the dehydrogenation activation energy triggered by this substitution and melt spinning, the treatments mentioned above have a positive influence on the hydrogen desorption kinetics. 4. Conclusions The hydrogen storing thermodynamics and dynamics of Mg25-xYx Ni9Cu (x ¼ 0–7) alloys were systematically researched in this paper. The following are summarized conclusions: 1. Replacing Mg by Y leads to the decline in thermodynamic parameters (ΔH and ΔS) and brings about the decrease of hydrogen desorption activation energy, the descent of this activation energy evidently improves the dehydrogenation kinetics property of experimental specimens. 2. Substituting Y for Mg enables the hydride steadiness to decline. By means of increasing Y dosage from 0 to 7, the as-cast alloys’ initial hydrogen desorption temperature reduces from 532.8 to 499.5 K and for as-spun (20 m/s) alloys it changes from 513.2 K to 491.3 K. 3. Arrhenius and Kissinger methods were used for calculating the dehydrogenation activation energy. According to the calculation results, increasing Y content leads to the descent of activation en ergy, which has an active effect on the hydrogen desorption dynamics.
Fig. 12. Relations of the Ede values of as-cast and as-spun (20 m/s) Mg25-xYx Ni9Cu (x ¼ 0–7) alloys and Y content.
Declaration of competing interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
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Acknowledgments
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It is sincere thanks to the National Natural Science Foundation of China, China (51901105, 51761032, and 51871125) and Natural Sci ence Foundation of Inner Mongolia, China (2019BS05005) for this financial support of the work. References [1] I.P. Jain, Hydrogen the fuel for 21st century, Int. J. Hydrogen Energy 34 (2009) 7368–7378. [2] S. Banger, V. Nayak, U.P. Verma, Hydrogen storage in lithium hydride: atheoretical approach, J. Phys. Chem. Solids 115 (2018) 6–17. [3] R.F. Li, J. Wan, F. Wang, C.P. Ding, R.H. Yu, Effect of non-stoichiometry on microstructure and electrochemical performance of La0.8GdxMg0.2Ni3.15Co0.25 Al0.1 (x ¼ 0-0.4) hydrogen storage alloys, J. Power Sources 301 (2016) 229–236. [4] Q. Luo, J.D. Li, B. Li, B. Liu, H.Y. Shao, Q. Li, Kinetics in Mg-based hydrogen materials: enhancement and mechanism, J. Magnes. Alloys 7 (2019) 58–71. [5] C.J. Webb, A review of catalyst-enhanced magnesium hydride as a hydrogen storage material, J. Phys. Chem. Solids 84 (2015) 96–106. [6] D. Mori, K. Hirose, Recent challenges of hydrogen storage technologies for fuel cell vehicles, Int. J. Hydrogen Energy 34 (2009) 4569–4574. [7] H.K. Liu, An overview-functional nanomaterials for lithium rechargeable batteries, supercapacitors, hydrogen storage, and fuel cells, J.Mater. Res. Bull. 48 (2013) 4968–4973. [8] T. Umegaki, J.-M. Yan, X.B. Zhang, S. Hiroshi, H. Shioyama, Q. Xu, Boron-and nitrogen-based chemical hydrogen storage materials, Int. J. Hydrogen Energy 34 (2009) 2303–2311. [9] L. Schlapbach, A. Züttel, Hydrogen-storage materials for mobile applications, Nature 414 (2001) 353–358. [10] L. Zubizarreta, J.A. Men�endez, J.J. Pis, A. Arenillas, Improving hydrogen storage in Ni-doped carbon nanospheres, Int. J. Hydrogen Energy 34 (2009) 3070–3076. [11] B. Liao, Y.Q. Lei, G.L. Lu, L.X. Chen, H.G. Pan, Q.D. Wang, The electrochemical properties of LaxMg3-xNi9 (x ¼ 1.0-2.0), J. Alloy. Comp. 356 (2003) 746–749. [12] M. Anik, F. Karanfil, N. Küçükdeveci, Development of the high performance magnesium based hydrogen storage alloy, Int. J. Hydrogen Energy 37 (2012) 299–308. [13] F. Wang, R.F. Li, C.P. Ding, W.K. Tang, Y.B. Wang, S.M. Xu, R.H. Yu, Enhanced hydrogen storage properties of ZrCo alloy decorated with flower-like Pd particles, Energy 139 (2017) 8–17. [14] L.E. Klebanoff, J.O. Keller, 5 Years of hydrogen storage research in the U.S. DOE metal hydride center of excellence (MHCoE), Int. J. Hydrogen Energy 38 (2013) 4533–4576. [15] L. Shaw, J. Pratt, L. Klebanoff, T. Johnson, M. Arienti, M. Moreno, Analysis of H2 storage needs for early market “man-portable” fuel cell applications, Int. J. Hydrogen Energy 38 (2013) 2810–2823. [16] D. Chandra, A. Sharma, R. Chellappa, W.N. Cathey, F.E. Lynch, R.C. Bowman, J. R. Wermer, S.N. Paglieri, Hydriding and structural characteristics of thermally cycled and cold-worked V-0.5 at.% C alloy, J. Alloy. Comp. 452 (2008) 312–324. [17] J. Zhang, D.W. Zhou, L.P. He, P. Peng, J.S. Liu, First-principles investigation of Mg2Ni phase and high/low temperature Mg2NiH4 complex hydrides, J. Phys. Chem. Solids 70 (2009) 32–39. [18] X.J. Hou, R. Hu, Y.L. Yang, L. Feng, G.Q. Suo, A powerful strategy for superior thermodynamics and hysteresis of Mg-Ni hydrogen energy storage alloys, J. Alloy. Comp. 766 (2018) 112–122. [19] X.J. Hou, R. Hu, Y.L. Yang, L. Feng, Isothermal activation, thermodynamic and hysteresis of MgH2 hydrides catalytically modified by high-energy ball milling with MWCNTs and TiF3, Int. J. Hydrogen Energy 42 (2017) 22953–22964. [20] A. Ebrahimi-Purkani, S.F. Kashani-Bozorg, Nanocrystalline Mg2Ni-based powders produced by high-energy ball milling and subsequent annealing, J. Alloy. Comp. 456 (2008) 211–215. [21] I.P. Jain, C. Lal, A. Jain, Hydrogen storage in Mg: a most promising material, Int. J. Hydrogen Energy 35 (2010) 5133–5144. [22] Y. Wang, Y.J. Wang, Recent advances in additive-enhanced magnesium hydride for hydrogen storage, Prog. Mater. Sci. 27 (2017) 41–49.
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Journal of Physics and Chemistry of Solids journal homepage: http://www.elsevier.com/locate/jpcs
Hydrogen storage thermodynamics and dynamics of Mg–Y–Ni–Cu based alloys synthesized by melt spinning Yanghuan Zhang a, b, *, Xin Wei b, Zeming Yuan a, Zhonghui Hou a, Yan Qi b, Shihai Guo b a b
Key Laboratory of Integrated Exploitation of Baiyun Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, Baotou, 014010, China Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing, 100081, China
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Mg–Ni-Based alloy C. Activation energy C. Melt spinning C. Thermodynamics D. Hydrogen storage kinetics
Excessive heat stability, poor kinetics of hydrogen absorption and desorption reaction are viewed as the major setbacks for the Mg-based hydrogen storing materials application. The microstructures of samples Mg25-xYxNi9Cu (x ¼ 0, 1, 3, 5, 7) alloys were observed by XRD, SEM, and TEM. Besides, aiming at studying the characteristics of hydrogenation/dehydrogenation, the Sievert instrument, DSC and TGA were applied in the experiments. Substituting Mg with Y can promote the formation of the second phase YNi3 but does not transform the Mg2Ni major phase. This substitution greatly prompts the amorphization. The increase of Y dosage triggers a mild decrease in the absolute values of ΔH and ΔS and remarkably boosts the improvement of dehydrogenation ki netics. Moreover, the increase in Y amount brings on a decrease in hydrogen absorption capability. The hydrogen desorption activation energy markedly lowers with increasing Y content (the Ede a values of as-spun Y0 and Y3
samples are 71.38 and 66.13 kJ/mol, severally, and Ede k values of them are 69.03 and 65.53 kJ/mol, respec tively), which brings about the promotion of dehydrogenation kinetics of the experimental samples.
1. Introduction Hydrogen is considered to be the non-substitutable bunkers in the 21st century with pollution-free characteristics as a fuel and the most abundant distribution as an element in the universe [1–3]. There are various applications about hydrogen including hydrogen power gener ation, energy for the vehicle, jet planes and combustion energy in our daily lives. It is worth noting that there are many projects dedicated to the application of hydrogen in the field of vehicle fuel [4]. Compared with traditional fossil fuel, the hydrogen fuel will not trigger the issues caused by combustion like global warming or air pollution [5–7]. A critical step to utilize hydrogen energy in automobiles is to exploit the vehicle-mounted hydrogen energy systems [8–11]. Among the hydrogen storing techniques, metal hydride is identified as a promising way which can meet the needs of mobile application [12]. Wang et al. [13] pre pared the ZrCo alloy decorated with Pd particles and found that the hydrogenation kinetic property of the alloys after decoration treatment was greater than the initial specimen. In addition, on the basis of in vestigations, some hydrogen storage materials have achieved the application aims. Unfortunately, there is no one can reach to the
standard presented by the U.S. Department of Energy for the vehicle industry field [14,15]. According to major advantages of Mg–Ni-based metallic hydrides, Mg2NiH4, holding 3.6 wt% theoretical gaseous hydrogen absorption capacity and 1000 mAh/g electrochemical capac ity [16–19] is viewed as a prospective material which can be applied in hydrogen energy automobiles or in batteries [20]. Nevertheless, as for Mg-based alloys, the absence of moderate dehydrogenation tempera ture, fast kinetics characteristic, and good electrochemical cycle stability is the serious impediment that restricts the practical application of them [21,22]. Aiming at ameliorating the hydrogen storing characteristics, the efficient approaches: alloying and microstructure modification were utilized [23–25]. Especially, partial substitution of Mg in Mg2Ni-type alloy with rare earth elements results in the decline of hydrides steadi ness and the promotion of hydrogen desorption [26,27]. Yong et al. [28] successfully prepared the Mg90Ce5Y5 alloy which had better kinetics than pure MgH2 specimen owing to the formation of rare earth hydride phase. Furthermore, according to the previous investigations, the dy namics is closely related to the microstructures [29]. As researched by Pang et al. [30], the complicated hydride with nanoscale structure was fabricated by a mechanical-force-driven physical vapour deposition
* Corresponding author. Department of Functional Material Research, Central Iron and Steel Research Institute, No. 76 Xueyuannan Road, Haidian District, 100081, Beijing, China. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.jpcs.2019.109252 Received 1 July 2019; Received in revised form 24 October 2019; Accepted 30 October 2019 Available online 1 November 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Yanghuan Zhang, Journal of Physics and Chemistry of Solids, https://doi.org/10.1016/j.jpcs.2019.109252
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method. Siarhei et al. [31] refined the grain of Mg-based alloys to nanometer level and their hydrogenation/dehydrogenation properties could be significantly ameliorated. Some methods like ball milling [32] and melt spinning [33] have been favorably applied to obtaining non-crystal or nanocrystalline structures that have high homogeneous element distribution. Just like investigated by Huang et al. [34] that by using the preparation technology of strip casting, the (Mg60Ni25)90Nd10 alloy with non crystal and nanocrystalline structures can be gained and has 580 mAh/g discharge capacity. In addition, Spassov et al. [35] proposed that partly substituting Mm (Mm ¼ Ce, La-rich mischmetal) for Mg heightens the hydrogen absorbing capability of the as-spun Mg75Ni20Mm5 specimens. In present work, so as to prepare nanocrystalline and no crystal structure in Mg25-xYxNi9Cu (x ¼ 0–7) specimens, the preparation method of melt spinning technique and substituting Y for Mg partially in Mg–Ni-based alloy were applied. Subsequently, the influences of Y content and melt spinning on thermodynamic and dynamic perfor mances of hydrogen storage alloys were studied minutely.
graphite filtering CuKα1 radiation. The parameters were 10� /min, 40 kV and 160 mA in this experiment. The observation of morphology was conducted with the help of Philips scanning electron microscope Philips SEM (QUANTA 400) equipped with an energy dispersive spectrometer (EDS). By means of the utilization of a JEM-2100 F high resolution trans mission electron microscope (HRTEM) made by JEOL Ltd, the as-spun specimens were observed. The crystallized states of specimens were determined by electron diffraction (ED). An automatic control Sieverts instrument with a stove controlling temperature in the range of �2 K was utilized to measure the P-C-T curves and dynamics of the sample. The temperatures of hydrogen ab sorption were 553–593 K at 3 MPa pressure with 300 mg alloy in a tubbiness reactor for each measure. When it comes to the performances of hydrogen desorption, the differential scanning calorimetry (DSC) with 5, 10, 15 and 20 K/min heated rates was used for testing the samples. 3. Results and discussion
2. Experimental
3.1. Microstructure characteristics
Aiming at inhibiting the volatilization of the Mg element, we used the vacuum induction furnace with a protective gas of 0.04 MPa He (Helium with purity of 99.999% is provided by CISRI Corporation) to fabricate the Mg25-xYxNi9Cu (x ¼ 0, 1, 3, 5, 7) alloys. For ease of pre sentation, corresponding Y contents in alloys are represented by Y0, Y1 and so on. The purities of the metallic materials of Mg, Ni, Cu, and Nd are at least 99.99% provided by CISRI Corporation. The molten alloy was poured into a copper mold for cooling down and then a cast ingot was prepared. Some specimens were re-melted and treated by melt spinning. In the melt spinning experiment, the spinning rate was approximately represented by the linear velocity of the copper roller due to the difficulty in measuring the precise spinning rate. The rate of 20 m/ s was set as the spinning rates in the experiment. The manufacturer of automatically controlled Sieverts apparatus (PCT-4SDWIN) is Suzuki Shokan Co., Ltd. The weight of sample put into the reactor was 0.5 g, and after six cycles activation treatment under the temperature of 573 K and the H2 pressure of 3Mpa, the specimens could be entirely activated. X-ray diffraction (XRD) (D/max/2400) produced by Rigaku is a general apparatus to determine the phase structures of specimens with
Fig. 1 gives the XRD images, from which the information of structure, and phase constituent of as-cast and as-spun specimens can be obtained. Fig. 1 (a) shows that substituting Y for Mg gives rise to the appearance of the secondary phase YNi3, and the structural evolution of the as-cast specimens is owing to the increase of Y element and the decline in the Mg element, the amount of phase Mg2Ni decreases, conversely, the quantity of phase YNi3 increases. The diffraction peaks of as-spun (20 m/ s) sample vary with Y content as displayed in Fig. 1 (b). By observing the as-spun Y5 and Y7 specimens, the existence of typical amorphous structure can be found, which suggests that the increase of Y content has a stimulative effect on the glass formation of Mg–Ni-based alloys. SEM is utilized to observe the morphologies, as demonstrated in Fig. 2, in which the morphology varies with Y content greatly. The EDS identification reveals that the Y0 alloy has a biphasic structure, con taining Mg2Ni and Mg. Substituting Y for Mg brings about the formation of the secondary phase YNi3. Besides, the phase YNi3 increases in quantity with the growth of Y dosage, which is the same as the XRD analysis. The observation of morphologies was achieved by the usage of
Fig. 1. XRD profiles of as-cast and as-spun Mg25-xYxNi9Cu (x ¼ 0–7) alloys: (a) as-cast alloys, (b) as-spun alloys (20 m/s). 2
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Fig. 2. SEM images together with typical EDS spectra of the as-cast alloys: (a) Y0 alloy, (b) Y3 alloy, (c) Y7 alloy.
HRTEM and ED for confirming the crystal states, as presented in Fig. 3. Clearly, Y0, Y1 and Y3 specimens display a mass of nanocrystalline structures, disordered areas and crystal defects. Besides, the ED patterns of Y0, Y1 as well as Y3 specimens have evident multi-haloes, which is in line with the feature of nanocrystalline. However, the as-spun Y7 sample exhibits a clear characteristic of nanocrystalline inserting into amor phous matrix, indicating that the glass formation of Mg–Ni-based alloys is facilitated owing to the Y additive.
(Where the abbreviated form PH2 expresses the equilibrium hydrogen gas pressure, the sample temperature and the letter R expresses the gas constant) By means of the logarithmic conversion of equation (1), the Van’t Hoff graphs of lnPH2 =P0 versus 1/T for Y0 and Y3 samples were plotted, as presented in Fig. 5. There is a linear relation of lnPH2 =P0 and 1/T for Y0 and Y3 alloys. Therefore, the thermodynamic parameters which are generally referred to enthalpy change (ΔH) and entropy change (ΔS) were figured up effortlessly based on equation (1). Table 1 displays the enthalpy and entropy changes, suggesting that replacing Mg by Y brings on a mild reduction in ΔH and ΔS of as-spun specimens. The as-cast and as-spun (20 m/s) Mg25-xYxNi9Cu (x ¼ 0–7) alloys at saturated hydrogen absorption state were placed in a reactor with 5 K/min heating rate so as to obtain the desorption curve, as shown in Fig. 6. Samples taken at each time were the same weight, which aims at avoiding the effect caused by increased pressure on desorption temperature. Based on the graph, the temperature of hydrogen desorption clearly declines with adding Y element, indicating the hydride stability reduction caused by this substitution. The decreased starting hydrogen desorption tempera ture caused by melt spinning can be ascribed to the formation of the nanocrystalline and amorphous structures on basis of the fact that the refined microstructure plays a dominant role in decreasing the dehy drogenation temperature [39,40]. The as-cast alloys’ starting hydrogen desorption temperature cuts down from 532.8 to 499.5 K and from 513.2 to 491.3 K for the as-spun alloys with changing Y content. Ac cording to Fig. 6 (a) and (b), when Y content is fixed, the as-spun alloys have lower starting hydrogen desorption temperature than the as-cast alloys, signifying the decline of hydride steadiness induced by melt spinning.
3.2. P-C-T curves and hydrogen storage thermodynamics In Fig. 4, the P-C-T curves of as-cast and spun samples at 573 K are presented. Obviously, the existence of two pressure plateaus in every curve can be found. The shorter and lower plateau expresses MgH2 while the longer and higher one represents the hydride Mg2NiH4 [36,37]. Moreover, an obvious gradient and a big hysteresis (Hf ¼ ln (Pa/Pd)) are exhibited in the hydrogenation/dehydrogenation pressure platforms. Melt spinning brings on the obvious ascent of hydro genation/dehydrogenation plateaus pressure owing to the fact that melt spinning results in the increase of internal strain. It is worth noting that the samples’ hydrogen storage capacity changes with the variety of Y dosage. Fig. 4 shows the relation between Y content and hydrogen storage capacity. The rise of Y brings about the hydrogen storage ca pacity of as-cast alloy decreasing from 4.12 to 2.82 wt% and 4.24 to 2.58 wt% for that of the as-spun (20 m/s) sample. The temperatures of measuring the as-spun (20 m/s) Y0 and Y3 samples were 553–593 K so as to make sense of the influence of substitution on thermodynamics, as described in Fig. 5. The following is the Van’t Hoff equation [38], � � P H2 ΔH ΔS In (1) ¼ RT R P0 3
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Fig. 3. HRTEM micrographs and ED patterns the as-spun (20 m/s) alloys: (a) Y0 alloy, (b) Y1 alloy, (c) Y3 alloy, (d) Y7 alloy.
Fig. 4. P-C-T curves of the as-cast and as-spun Mg25-xYxNi9Cu (x ¼ 0–7) alloys at 573 K: (a) as-cast alloys, (b) as-spun alloys (20 m/s).
Fig. 5. P-C-T curves of as-spun (20 m/s) Y0 and Y3 samples at 553 K, 573 K and 593 K: (a) Y0 alloys, (b) Y3 alloys.
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heightened, the Ra5 of as-spun sample declines from 87.5% to 79.6%, but that of as-cast sample rises from 85.8% to 91.5%. Taking the positive influence of substitution on as-cast alloy into consideration, the enlargement of cell volume and the formation of induced phase YNi3 can be utilized to explain this phenomenon. Besides, the phase boundaries can offer convenient access for the diffusion of hydrogen and the descent of atoms diffusion activation energy caused by increasing cell volume also has a beneficial influence on samples’ hydrogen absorption dy namics [41]. As for the as-spun alloy, the negative action of substituting Y for Mg on the hydrogenation kinetics is due to the glass formation, which is based on the principle that the hydrogen atoms have better diffusing ability in nanocrystalline structure than in amorphous structure. Analogously, the hydrogen desorption ratio (Rdt ) (the ratio of hydrogen desorbing capability at specified time to saturated hydrogen desorbing capability) is also a significant parameter which can reflect samples’ dehydrogenation dynamic property. The definition of the
Table 1 ΔH and ΔS values of the as-spun (20 m/s) Y0 and Y3 alloys. Samples
Absorption ΔH (kJ/mol)
Y0 Y3
64.40 60.90
Desorption ΔS (J/mol/K) 120.92 115.22
ΔH (kJ/mol)
ΔS (J/mol/K)
68.24 64.29
123.02 116.84
3.3. Hydrogen absorption and desorption kinetics Under the hydrogenation condition of 573 K and 3 MPa, the varia tions in experimental specimens’ hydrogen absorption capacity are described in Fig. 7. The as-cast and spun compounds in the beginning stage demonstrate quick hydrogen absorption velocity, later the almost hydrogen saturated state lasts for a long time. With regard to hydrogen absorption capacity, it is a crucial factor to enable hydrogen storage materials to be applied successfully in vehicle-mounted hydrogen stor age system, and that also cannot ignore the factor: hydrogen reaction rate. In this experiment, the Rat (the ratio of hydrogen absorbing capa bility at specified time to saturated hydrogen absorbing capability) is used to express the hydrogen absorption kinetics with the definition of Rat ¼ Cat =Ca100 � 100%. The abbreviated form Cat represents the hydrogen absorption capacity at t min. Because all the samples used in this experiment obtained the hydrogen absorption capacity that reaches 98% of the saturated state in 100 min, the Ca100 value is utilized to ex press the saturated absorption capacity. In order to compare conve niently, Ra5 (t ¼ 5) was set as a standard criterion to establish the relation with Y dosage, as given in Fig. 7. Obviously, increasing Y content makes the Ra5 value of as-spun samples decline. As for the as-cast samples, the situation is exactly opposite. To be exact, when the Y dosage is
hydrogen desorption ratio is Rdt ¼ Cdt =Ca100 � 100%, where Cdt represents the hydrogen desorption capacity in t min and the meaning of Ca100 is the same as the previous definition. As described in Fig. 8, in order to establish a relation of hydrogen desorption ratio and Y content, it is necessary to institute 10 min hydrogen absorption time as a criterion. The augmentation of dehydriding rates with increasing Y content is obvious. Specifically, for the as-cast sample, adding Y from 0 to 7 en ables Rd10 value to rise from 50.4% to 70.5% and 57.7% to 71.6% for as-
spun sample. The as-spun sample emerges much higher Rd10 value compared with the as-cast one in the case of fixed Y content, manifesting that melt spinning can ameliorate the hydrogen desorption kinetics. The positive effect on dehydrogenation kinetics triggered by melt spinning is considered to be caused by the increase of internal stress and the grain
Fig. 6. Temperature programmed desorption curve of the as-cast and as-spun samples after hydrogen absorption at a heating rate of 5 K/min: (a) as-cast alloys, (b) as-spun alloys (20 m/s).
Fig. 7. Hydrogen absorption kinetic curves of the as-cast and spun Mg25-xYxNi9Cu (x ¼ 0–7) alloys at 573 K: (a) as-cast alloys, (b) as-spun alloys (20 m/s). 5
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the dehydrogenation reaction of samples [44]. According to the slope and intercept of fitting lines, the η and ηlnk values at 593, 573 and 553 K were obtained and the rate constant k was also easy to calculate. In the light of the Arrhenius equation as follows, the dehydrogenation activa tion energy (Ede a ) can be gotten [45]: Using A to stand for temperature coefficient, using R to represent the ideal gas constant, in addition, using T to express the absolute temper ature of samples. The k is the same as the previous definition. The Arrhenius images are presented in Fig. 10 to describe the hydrogen desorption dynamic of samples. According to the slopes of plots, the activation energy Ede a was obtained. When it comes to the as-spun Y0 and
Y3 alloys, the Ede a values of them are 71.38 and 66.13 kJ/mol, severally, which manifests that replacing Mg by Y makes the activation energy of dehydrogenation decrease. In order to compare with the estimates of the Arrhenius method, the dehydrogenation activation energy was determined by using the Kis singer method. The Kissinger equation is [46]: �� � d In β=T 2P Ede k ¼ (4) dð1=TP Þ R
Fig. 8. Evolution of the values of the as-cast and as-spun Mg25-xYxNi9Cu (x ¼ 0–7) alloys with Y content.
refinement, because when the size of grains is reduced below micro meter level, the Mg-based alloys’ hydrogen desorption characteristics can get great enhancement [42].
Using Ede k to stand for activation energy, using a Greek alphabet β to express the heating rate, using TP to represent the absolute temperature at which there is a maximum desorption rate existing in DSC plots, in addition, using a capital letter R to express the ideal gas constant. The measurement of hydrogen desorption reactions in the Mg25-xYxNi9Cu (x ¼ 0–7) alloys was taken by using DSC whose heating rates were set between 5 and 20 K/min. What needs to pay attention to is that the tested samples were saturated state at 3 MPa, and 573 K. To illustrate the measurement results, the as-spun samples’ dehydrogenation DSC curves are presented in Fig. 11. All curves have a clear and alike endothermic peak in the dehydrogenation process, manifesting that the reaction of each alloy contains similar process. In addition, the phenomenon that the as-spun samples’ endothermic peak has the tendency to transfer to low temperature indicates that melt spinning has a commendably beneficial function to promote the hydrogen desorbing reaction. The relation of Inβ=T 2P vs. 1/TP was established by making use of the loga rithmic transform of equation (4) based on the experimental datum, as displayed in Fig. 11. Taking linearization of the Kissinger plots into consideration, it is effortless to figure out the activation energy Ede k on the basis of the Kissinger plots’ slope. The Ede k values of as-spun Y0 and Y3 alloy are 69.03 and 65.53 kJ/mol, respectively. According to the results of activation energy figured out by utilizing the two methods mentioned above, the relation of the alloys’ Ede (Ede ¼ Ede a is calculated on the basis
3.4. Hydrogen desorption activation energy In general, activation energy is not only the barrier that needs to be overcome in reaction but also an extremely important parameter in the kinetics. In dehydrogenation reaction, there is a total energy barrier existing, so the activation energy generally reflects the driving force for dehydrogenation reaction. In order to expose the function mechanism of replacing Mg by Y on dehydrogenation dynamic, the Arrhenius and Kissinger methods were utilized to obtain the calculation results. On basis of the applicable premises of the Arrhenius method, the dehy drogenation dynamic curves of as-spun Mg25-xYxNi9Cu (x ¼ 0–7) speci mens were tested between 553 and 593 K. Fig. 9 presents the dehydrogenation kinetics curves of as-spun specimens. In general, the Johnson-Mehl-Avrami (JMA) model is used for simulating the gas-solid reactions such as dehydrogenation reaction of metal hydrides. The JMA equation is [43]: ln½
lnð1
αÞ � ¼ ηlnk þ ηlnt
(2)
where phase fraction at t time expressed by an ancient Greek alphabet α and identified by standardized hydrogen wt.%, using a Greek alphabet η to represent the Avrami coefficient, as well as the dynamics coefficient, is expressed by lowercase letter k. The Johnson-Mehl-Avrami graphs ln [-ln (1-α)] vs. lnt were established by making use of the logarithmic transformation of equation (2) at 593, 573 and 553 K, as presented in Fig. 9. The trend of Johnson-Mehl-Avrami plots is linear in the process of
of the Arrhenius method; Ede ¼ Ede k is calculated by Kissinger method) and Y content is presented in Fig. 12. Clearly, the as-cast and as-spun specimens’ Ede values decline with the increase of Y dosage.
Fig. 9. Hydrogen desorption kinetic curves of the as-spun (20 m/s) Y0 and Y7 alloys at 553, 573 and 593 K and Avrami plots of ln [-ln (1-α)] vs. lnt: (a) Y0 alloys, (b) Y3 alloys. 6
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Journal of Physics and Chemistry of Solids xxx (xxxx) xxx
Fig. 10. Arrhenius plots of the as-spun (20 m/s) Mg25-xYxNi9Cu (x ¼ 0–7) alloys: (a) Y0 alloy, (b) Y3 alloy. � de � Ea K ¼ A exp RT
(3)
Fig. 11. Kissinger plots and DSC curves of as-spun (20 m/s) Mg25-xYxNi9Cu (x ¼ 0–7) alloys at various heating rates: (a) Y0 alloys, (b) Y3 alloys.
Furthermore, the Ede of as-spun alloy is much lower in comparison with that of as-cast alloy when the Y content is fixed. Due to the lessening of the dehydrogenation activation energy triggered by this substitution and melt spinning, the treatments mentioned above have a positive influence on the hydrogen desorption kinetics. 4. Conclusions The hydrogen storing thermodynamics and dynamics of Mg25-xYx Ni9Cu (x ¼ 0–7) alloys were systematically researched in this paper. The following are summarized conclusions: 1. Replacing Mg by Y leads to the decline in thermodynamic parameters (ΔH and ΔS) and brings about the decrease of hydrogen desorption activation energy, the descent of this activation energy evidently improves the dehydrogenation kinetics property of experimental specimens. 2. Substituting Y for Mg enables the hydride steadiness to decline. By means of increasing Y dosage from 0 to 7, the as-cast alloys’ initial hydrogen desorption temperature reduces from 532.8 to 499.5 K and for as-spun (20 m/s) alloys it changes from 513.2 K to 491.3 K. 3. Arrhenius and Kissinger methods were used for calculating the dehydrogenation activation energy. According to the calculation results, increasing Y content leads to the descent of activation en ergy, which has an active effect on the hydrogen desorption dynamics.
Fig. 12. Relations of the Ede values of as-cast and as-spun (20 m/s) Mg25-xYx Ni9Cu (x ¼ 0–7) alloys and Y content.
Declaration of competing interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
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Acknowledgments
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