Trans. Nonferrous Met. Soc. China 26(2016) 791-798

Effects of Ti-based additives on Mg₂FeH₆ dehydrogenation properties

Chen-chen XU, Xue-zhang XIAO, Jie SHAO, Lang-xia LIU, Teng QIN, Li-xin CHEN

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

Received 6 January 2015; accepted 4 April 2015

Abstract: Mg₂FeH₆ doped with and without Ti and its alloys (TiMn₂, TiAl) were prepared combing ball milling and heat treatment. The effects of these additives on the dehydrogenation performance of Mg₂FeH₆ were studied systematically. The results show that all additives have favor influence on improving the hydrogen desorption property of Mg₂FeH₆. Especially, TiMn₂ exhibits prominent effect on enhancing the dehydrogenation kinetics of Mg₂FeH₆. Moreover, the activation energy of TiMn₂-doped Mg₂FeH₆ calculated by Kissinger equation is 94.87 kJ/mol, which is 28 kJ/mol lower than that of the undoped Mg₂FeH₆. The cycling tests suggest that the improved dehydrogenation kinetics of Mg₂FeH₆ doped by TiMn₂ can maintain in the second cycle.

Key words: Mg₂FeH₆; Ti-based additives; dehydrogenation properties; kinetics

1 Introduction

Hydrogen is a promising energy carrier by virtue of its abundance, high energy density as well as its environmentally friendly property [1-4]. Yet the development of safe and efficient hydrogen storage materials is of vital importance to realize the so-called hydrogen economy [2]. Complex transitional metal hydrides, especially the Mg-based 3D-transitional metal hydrides are considered to be potential candidates as hydrogen storage materials, thus attracting much interest [5-9]. Among these, Mg₂FeH₆ possesses a volumetric hydrogen density of 150 kg/m³, which is the highest one so far. Additionally, Mg₂FeH₆ has an advantage of a larger gravimetric hydrogen density (5.47%, mass fraction) over others belonging to the same group, like Mg₂NiH₄ (3.6%), Mg₂CoH₅ (4.5%). However, the difficulties to prepare pure Mg₂FeH₆ resulting from the absence of the stable intermetallic compound between Mg and Fe hampered the deep study of Mg₂FeH₆, not to speak of the technique used to optimize its hydrogen storage properties.

In 1984, DIDISHEIM et al [10] firstly synthesized Mg₂FeH₆ through sintering under high pressure. Afterwards, a series of investigations relating to its synthesis were conducted [11-24], and the result turns out to be encouraging. While the reports regarding to the hydrogen storage properties indicated that further efforts are needed to improve the high sorption temperature and the sluggish kinetics. Strategies [25-28] including nanosizing, catalyzing, alloying and so forth seem to be effective in improving the properties of Mg-based hydrogen storage materials. To enhance the kinetics, additives usage has been widely applied. The effective additives reported mainly include transition metals, transition metal oxides and intermetallic compounds. Among the transition metals, Ti and its oxide exhibited superior catalytic effects on desorption and absorption properties [29]. ZHOU et al [30] studied the effects of Ti intermetallic catalysts on the hydrogen storage properties of MgH₂ and the results showed significant progress on the properties of MgH₂. In this work, we investigated the effect of Ti-based additives including TiMn₂, TiAl and Ti on the hydrogen desorption properties of Mg₂FeH₆. The in-situ adjunction of 5% (mole fraction) additives was adopted in the doping process. The hydrogen desorption properties were characterized by differential scanning calorimetry (DSC) and Sieverts apparatus. The activation energy was calculated via Kissinger equation. The consequence shows that TiMn₂ exhibits better catalytic effects over the others. The dehydrogenation reaction mechanism was also investigated.

2 Experimental

Mg (purity, 98%) and Ti powders (purity, 99%) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Fe (purity, 98%) was purchased from Shanghai Jinshan Smelter, China. TiMn₂ and TiAl alloys were prepared by suspension induction melting firstly, and then the as-prepared alloys were ball-milled for 4 h under 4 MPa H₂ atmosphere.

The mixtures with n(Mg):n(Fe):n(TMs) (TMs= TiMn₂, TiAl, Ti) of 2.2:1:0.05 were ball-milled for 20 h in a planetary mill under 4 MPa H₂. Hereafter, heat treatment was conducted at 500 °C for 40 h under 9 MPa H₂ for the ball-milled mixtures. Finally, all the samples were ball-milled for 5 h under 2 MPa H₂ atomosphere. All the materials handling was carried out in the glovebox filled with argon to prevent the pollution from H₂O and O₂.

The structural analysis of the sample was conducted by X-ray diffraction (XRD). The differential scanning calorimetry (DSC) was executed on a Netzsch STA 449F3 instrument heating from 30 to 450 °C under the continuous protective gas of argon with heating rates of 2, 5, 10 °C/min, respectively. Hydrogen desorption properties of the samples were examined combining temperature-programmed desorption (TPD) and isothermal desorption tests via a Sievert apparatus. To compare the hydrogen capacity of Mg₂FeH₆ directly, the TMs content was not taken into account in the determination of hydrogen desorption process.

3 Results and discussion

3.1 Characterization of samples

Figure 1 shows the XRD patterns of as-prepared Mg₂FeH₆ doped with and without TMs. The inset shows that TiMn₂ and TiAl were synthesized. According to the results, no distinct differences exist among the samples. All samples exhibit strong diffraction peaks of Mg₂FeH₆ with some residual Fe phase, which may be a result of the decomposition during the final 5 h ball-milling. Meanwhile, the existing MgO phase could result from the contamination during the XRD detection. Moreover, no TMs additives phases exist in the XRD pattern. It could result from the relatively less quantities added [22].

Fig. 1 XRD patterns of as-prepared Mg₂FeH₆ (a) and Mg₂FeH₆ doped with TiMn₂ (b), TiAl (c) and Ti (d)

Fig. 2 TPD curves of Mg₂FeH₆ (a), and Mg₂FeH₆ doped with TiMn₂ (b), TiAl (c) and Ti (d) conducted at 5 °C /min to 450 °C under 100 Pa H₂

3.2 Hydrogen desorption properties of samples

Figure 2 displays the hydrogen desorption properties of as-prepared Mg₂FeH₆ doped with and

without TMs measured by TPD conducted at a heating rate of 5 °C/min. It is obviously found that both the onset and offset hydrogen desorption temperatures of Mg₂FeH₆ doped with TMs decrease compared with those of undoped Mg₂FeH₆. The hydrogen desorption of TMs doped samples starts from 200 °C, nearly 20 °C lower than that of undoped Mg₂FeH₆, suggesting improved dehydrogenation properties. Moreover, TiMn₂ exhibits best performance in improving the dehydrogenation of Mg₂FeH₆, followed by Ti and TiAl. Given the similar crystalline size in Fig. 1 and similar particle size after ball-milling shown in Fig. 3, it is believed that the differences among dehydrogenation properties for the TMs doped samples derive from the differences of the catalytic component. The theoretical hydrogen released for the Mg₂FeH₆, Mg₂FeH₆-TiMn₂, Mg₂FeH₆-TiAl, Mg₂FeH₆-Ti are 5.47%, 5.08%, 5.3% and 5.34% (mass fraction), respectively, and the actual hydrogen amount released is over 95% of the total hydrogen storage capacity, which are 5.20%, 4.83%, 5.04% and 5.07% (mass fraction), respectively. This manifests the high purity of Mg₂FeH₆ which agrees well with the results of XRD patterns.

Fig. 3 SEM images of Mg₂FeH₆ (a), and Mg₂FeH₆ doped with TiMn₂ (b), TiAl (c) and Ti (d)

In addition, the isothermal hydrogen desorption tests of Mg₂FeH₆ doped with and without TMs were carried out at 260 °C under 100 Pa H₂, as shown in Fig. 4. All samples show favor hydrogen desorption kinetics. At a temperature as low as 260 °C, all samples could release 75% H₂ in 30 min. As expected, the Mg₂FeH₆ doped with TiMn₂ shows the best dehydrogenation performance among the samples, whose dehydrogenation rate is nearly two times faster than that of the undoped Mg₂FeH₆.

Fig. 4 Isothermal hydrogen desorption curves at 260 °C under 100 Pa H₂

To further understand the improved kinetics by calculating the activation energy, the DSC experiment was conducted by heating up to 450 °C with the heating rates of 2, 5, 10 °C/min, respectively. Figure 5 shows the DSC profiles of Mg₂FeH₆ doped with and without TMs. Each curve presents one endothermic peak indicating the one-step dehydrogenation of Mg₂FeH₆, as manifested in the previous reports [21,31]. Table 1 presents the peak temperatures relevant to the DSC consequences. For Mg₂FeH₆ doped with 5% TiMn₂, the dehydrogenation peak temperature is 266.9 °C, nearly 25 °C lower than that of the as-prepared Mg₂FeH₆, thus, the result is in good agreement with the TPD results.

The activation energy of the hydrogen desorption process can be estimated according to the Kissinger equation [32]:

                            (1)

where T_p is the peak temperature corresponding to the heating rate β, E_a is the activation energy and R is the gas

constant. By plotting 1000/T_p against , E_a can be obtained, as shown in Fig. 6. The activation energies of as-prepared Mg₂FeH₆ doped with and without TMs calculated according to Eq. (1) are list in Table 1. For the as-prepared Mg₂FeH₆, the activation energy is estimated as 123.48 kJ/mol, while the activation energies are 94.87, 107.66 and 97.93 kJ/mol for the Mg₂FeH₆ doped with TiMn₂, TiAl and Ti, respectively. The result indicates that the addition of TMs reduces the activation energy, thus improving the dehydrogenation kinetics, among which TiMn₂ is the most efficient catalyst.

Fig. 5 DSC profiles of Mg₂FeH₆ (a), and Mg₂FeH₆ doped with TiMn₂ (b), TiAl (c) and Ti (d)

Table 1 Peak temperatures of DSC profiles and calculated activation energy

Fig. 6 Kissinger plots of Mg₂FeH₆, and Mg₂FeH₆ doped with TiMn₂ , TiAl and Ti

To investigate the kinetic mechanism of the hydrogen desorption process, isothermal dehydroge- nation kinetic analysis was executed in the range of 270-300 °C, as shown in Fig. 7. As TiMn₂ exhibited the best performance among the samples, this sample was used to select a most fitting model. Jone’s method [33] is a widely used one in the rapid model selection. (t/t_0.5)_theo is constant for a certain model when t is defined, where t is the time and t_0.5 is the time when the fraction α is 0.5. The theoretical value (t/t_0.5)_theo closest to the experimental value (t/t_0.5)_exp is the most reliable model. Therefore, we plot the (t/t_0.5)_exp against the (t/t_0.5)_theo and the fitting linear slope closest to 1 is the reliable model. Figure 8 shows the relationship between the (t/t_0.5)_exp and the (t/t_0.5)_theo of the nine kinetic mechanisms, which is also listed in Table 2. From the curves, we found out that the model best fitting the hydrogen desorption is the F1 model, implying that the hydrogen desorption was controlled by the concentration of reactant. To further verify the model, the time dependence of F1 model was used at different temperatures with the fraction α ranging from 0.1 to 0.7 (Fig. 9) and other three samples were tested using this model. It is clearly seen that the linear coefficient R² for all samples are larger than 0.99, signifying the same reaction mechanism.

Fig. 7 Isothermal dehydrogenation curves of Mg₂FeH₆ (a), and Mg₂FeH₆ doped with TiMn₂ (b), TiAl (c) and Ti (d)

Fig. 8 (t/t_0.5)_theo vs (t/t_0.5)_exp for TiMn₂-doped Mg₂FeH₆ at 290 °C

Table 2 Kinetic models examined in isothermal desorption curves

Considering that the TiMn₂ exhibits the best performance among the TMs additives in improving the dehydrogenation of Mg₂FeH₆, we investigate the reversibility of the undoped and TiMn₂-doped Mg₂FeH₆. The isothermal dehydrogenation was performed at 300 °C under 100 Pa H₂. Here, the dehydrogenation performance was employed to replace the hydrogen absorption property given that little change in temperature could bring about huge variation in pressure under high pressure [34]. Figure 10 shows the dehydrogenation performance of the undoped and TiMn₂-doped Mg₂FeH₆. Obviously, both composites can release over 90% H₂ which comes from the decomposition of Mg₂FeH₆ and the residual MgH₂. Remarkably, TiMn₂-doped Mg₂FeH₆ releases 80% H₂ in 5 min, whereas the undoped one needs 10 min. The results imply that TiMn₂ can help to sustain the cycle stability of dehydrogenation kinetics of Mg₂FeH₆.

Fig. 9 Time dependence of F1 model at different temperatures for Mg₂FeH₆ (a), and Mg₂FeH₆ doped with TiMn₂ (b), TiAl (c) and Ti (d)

Fig. 10 Hydrogen desorption curves of Mg₂FeH₆ (a) and Mg₂FeH₆ doped with TiMn₂ (b) at 300 °C after rehydrogenation

4 Conclusions

1) This research indicates that the TMs additives (TiMn₂, TiAl and Ti) show favor catalytic effects on improving the dehydrogenation properties of Mg₂FeH₆.

2) Particularly, the Mg₂FeH₆ doped with TiMn₂ exhibits the best desorption performance. The starting dehydrogenation temperature is reduced by 20 °C and the activation energy is decreased by 28 kJ/mol for the TiMn₂ doped Mg₂FeH₆ compared with that for the undoped Mg₂FeH₆.

3) The dehydrogenation mechanism was investigated as the first-order reaction model, implying that the hydrogen desorption was controlled by the concentration of reactant.

4) Moreover, the TiMn₂-doped Mg₂FeH₆ shows the best dehydrogenation kinetics stability during dehydrogenation cycle.

References

[1] EDWARDS P, Kuznetsov V, David W. Hydrogen energy [J]. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2007, 365(1853): 1043-1056.

[2] Schlapbach L, Züttel A. Hydrogen-storage materials for mobile applications [J]. Nature, 2001, 414 (6861): 353-358.

[3] van den Berg A W, Areán C O. Materials for hydrogen storage: Current research trends and perspectives [J]. Chemical Communications, 2008, 6(6): 668-681.

[4] Yang J, Sudik A, Wolverton C, Siegel D J. High capacity hydrogen storage materials: Attributes for automotive applications and techniques for materials discovery [J]. Chemical Society Reviews, 2010, 39(2): 656-675.

[5] Bobet J, Akiba E, Nakamura Y, Darriet B. Study of Mg-M (M=Co, Ni and Fe) mixture elaborated by reactive mechanical alloying—Hydrogen sorption properties [J]. International Journal of Hydrogen Energy, 2000, 25(10): 987-996.

[6] Dornheim M, Doppiu S, Barkhordarian G, Boesenberg U, Klassen T, Gutfleisch O, Bormann R. Hydrogen storage in magnesium-based hydrides and hydride composites [J]. Scripta Materialia, 2007, 56(10): 841-846.

[7] Huot J, Pelletier J, Liang G, Sutton M, Schulz R. Structure of nanocomposite metal hydrides [J]. Journal of Alloys and Compounds, 2002, 330: 727-731.

[8] Sakintuna B, Lamari-Darkrim F, Hirscher M. Metal hydride materials for solid hydrogen storage: A review [J]. International Journal of Hydrogen Energy, 2007, 32(9): 1121-1140.

[9] Orimo S, Fujii H. Materials science of Mg-Ni-based new hydrides [J]. Applied Physics A, 2001, 72(2): 167-186.

[10] Didisheim J, Zolliker P, Yvon K, Fischer P, Schefer J, Gubelmann M, Williams A. Dimagnesium iron (II) hydride, Mg₂FeH₆, containing octahedral FeH₆^4- anions [J]. Inorganic Chemistry, 1984, 23(13): 1953-1957.

[11] Gennari F, Castro F, Andrade Gamboa J. Synthesis of Mg₂FeH₆ by reactive mechanical alloying: Formation and decomposition properties [J]. Journal of Alloys and Compounds, 2002, 339(1): 261-267.

[12] Huot J, Hayakawa H, Akiba E. Preparation of the hydrides Mg₂FeH₆ and Mg₂CoH₅ by mechanical alloying followed by sintering [J]. Journal of Alloys and Compounds 1997, 248(1): 164-167.

[13] Leiva D R, de Souza Villela A C, Paiva-Santos C d O, Fruchart D, Miraglia S, Ishikawa T T, Botta W J. High-yield direct synthesis of Mg₂FeH₆ from the elements by reactive milling [J]. Solid State Phenomena, 2011, 170: 259-262.

[14] Niaz N, Ahmad I, Khalid N, Ahmed E, Abbas S, Jabeen N. Preparation of Mg₂FeH₆ nanoparticles for hydrogen storage properties [J]. Journal of Nanomaterials, 2013, 2013: 610642.

[15] Polanski M,  T, Kunce I, Bystrzycki J. Dynamic synthesis of ternary Mg₂FeH₆ [J]. International Journal of Hydrogen Energy, 2010, 35(3): 1257-1266.

[16] Retuerto M,  E, Serafini D, Alonso J. High-pressure synthesis of Mg₂FeH₆ complex hydride [J]. International Journal of Hydrogen Energy, 2010, 35(15): 7835-7841.

[17] Sai Raman S, Davidson D, Bobet J L, Srivastava O. Investigations on the synthesis, structural and microstructural characterizations of Mg-based K₂PtCl₆ type (Mg₂FeH₆) hydrogen storage material prepared by mechanical alloying [J]. Journal of Alloys and Compounds, 2002, 333(1): 282-290.

[18] Selvam P, Yvon K. Synthesis of Mg₂FeH₆, Mg₂CoH₅ and Mg₂NiH₄ by high-pressure sintering of the elements [J]. International Journal of Hydrogen Energy, 1991, 16(9): 615-617.

[19] Varin R, Li S, Calka A, Wexler D. Formation and environmental stability of nanocrystalline and amorphous hydrides in the 2Mg-Fe mixture processed by controlled reactive mechanical alloying (CRMA) [J]. Journal of Alloys and Compounds, 2004, 373(1): 270-286.

[20] Wang Yan, Cheng Fang-yi, Li Chun-sheng, Tao Zhan-liang, Chen Jun. Preparation and characterization of nanocrystalline Mg₂FeH₆ [J]. Journal of Alloys and Compounds, 2010, 508(2): 554-558.

[21] Zhang Xuan-zhou, Yang Rong, Qu Jiang-lan, Zhao Wei, Xie Lei, Tian Wen-huai, Li Xing-guo. The synthesis and hydrogen storage properties of pure nanostructured Mg₂FeH₆ [J]. Nanotechnology, 2010, 21(9): 095706.

[22] Li Song-lin, Tang Sheng-long, Liu Yi, Peng Shu-ke, Cui Jian-min. Synthesis of nanostructured Mg₂FeH₆ hydride and hydrogen sorption properties of complex [J]. Transactions of Nonferrous Metals Society of China, 2010, 20(12): 2281-2288.

[23] Li S L, Varin R A. Structural and hydrogen storage capacity evolution of Mg₂FeH₆ hydride synthesized by reactive mechanical alloying [J]. Transactions of Nonferrous Metals Society of China, 2004, 14(4): 649-653.

[24] MA Jian-li, WANG Yan, TAO Zhan-liang, CHEN Jun. Preparation and hydrogen storage properties of Mg₂FeH₆ nanocrystals [J]. Chemical Journal of Chinese Universities, 2012, 33(3): 536-540. (in Chinese)

[25] Jeon K J, Moon H R, Ruminski A M, Jiang B, Kisielowski C, Bardhan R, UrbanJ J. Air-stable magnesium nanocomposites provide rapid and high-capacity hydrogen storage without using heavy-metal catalysts [J]. Nature Materials, 2011, 10(4): 286-290.

[26] Liu Guang, Wang Yi-jing, Qiu Fang-yuan, Li Li, Jiao Li-fang, Yuan Hua-tang. Synthesis of porous Ni@ rGO nanocomposite and its synergetic effect on hydrogen sorption properties of MgH₂ [J]. Journal of Materials Chemistry, 2012, 22(42): 22542-22549.

[27] Malka I, Czujko T, Bystrzycki J. Catalytic effect of halide additives ball milled with magnesium hydride [J]. International Journal of Hydrogen Energy, 2010, 35(4): 1706-1712.

[28] Zhang J, Cuevas F,  W, Bonnet J P, Aymard L, Bobet J L, Latroche M. Highlighting of a single reaction path during reactive ball milling of Mg and TM by quantitative H₂ gas sorption analysis to form ternary complex hydrides (TM= Fe, Co, Ni) [J]. The Journal of Physical Chemistry C, 2011, 115(11): 4971-4979.

[29] PAN Yin-cheng, ZOU Jian-xin, ZENG Xiao-qin, DING Wen-jiang. Hydrogen storage properties of Mg-TiO₂ composite powder prepared by arc plasma method [J]. Transactions of Nonferrous Metals Society of China, 2014, 24(12): 2834-3839.

[30] Zhou Cheng-shang, Fang Zhi-gang, Ren Chai, Li Jing-zhu, Lu Jun. Effect of Ti intermetallic catalysts on hydrogen storage properties of magnesium hydride [J]. The Journal of Physical Chemistry C, 2013, 117(25): 12973-12980.

[31] Puszkiel J, Arneodo Larochette P, Gennari F. Thermodynamic–kinetic characterization of the synthesized Mg₂FeH₆-MgH₂ hydrides mixture [J]. International Journal of Hydrogen Energy, 2008, 33(13): 3555-3560.

[32] Kissinger H E. Reaction kinetics in differential thermal analysis [J]. Analytical Chemistry, 1957, 29(11): 1702-1706.

[33] Jones L, Dollimore D, Nicklin T. Comparison of experimental kinetic decomposition data with master data using a linear plot method [J]. Thermochimica Acta, 1975, 13(2): 240-245.

[34] FANG Z Z, KANG X D, YANG Z X, WALKER G S, WANG P. Combined effects of functional cation and anion on the reversible dehydrogenation of LiBH₄ [J]. The Journal of Physical Chemistry C, 2011, 115(23): 11839-11845.

Ti基添加剂对Mg₂FeH₆放氢性能的影响

徐晨晨，肖学章，邵 杰，刘朗夏，秦 腾，陈立新

浙江大学 材料科学与工程学院，硅材料国家重点实验室，浙江省电池新材料与应用技术研究重点实验室，杭州 310027

摘  要：结合球磨和热处理制备掺杂Ti，TiMn₂和TiAl的Mg₂FeH₆样品，并系统研究这些添加剂对Mg₂FeH₆样品放氢行为的影响。研究结果表明：所有添加剂都能够在一定程度上改善Mg₂FeH₆的放氢性能，特别是TiMn₂对Mg₂FeH₆的放氢动力学性能改善效果显著。根据Kissinger方程计算出掺杂TiMn₂的Mg₂FeH₆样品的放氢活化能为94.87 kJ/mol，与未掺杂的Mg₂FeH₆样品相比，降低了28 kJ/mol。另外，掺杂TiMn₂的Mg₂FeH₆样品在循环测试过程中仍具有良好的放氢性能改善效果。

关键词：Mg₂FeH₆；Ti基添加剂；放氢性能；动力学

(Edited by Mu-lan QIN)

Foundation item: Project (2010CB631300) supported by the National Basic Research Program of China; Project (2012AA051503) supported by the National High Technology Research & Development Program of China; Projects (51001090, 51171173) supported by the National Natural Science Foundation of China; Project (IRT13037) supported by the Program for Innovative Research Team in University of Ministry of Education of China

Corresponding author: Xue zhang XIAO; Tel: +86 571 87951876; Fax: +86 571 87951152; E mail: xzxiao@zju.edu.cn

DOI: 10.1016/S1003-6326(16)64169-9