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dehydrogenation kinetics of hydrogen-storage material MgH2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
Available online at www.sciencedirect.com
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Influence of micro-amount O2 or N2 on the hydrogenation/dehydrogenation kinetics of hydrogen-storage material MgH2 En Xu a, Hui Li a,b, Xinmin You a, Chao Bu a, Longfei Zhang a, Qi Wang a, Zhigang Zhao a,b,* a
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing, China b College of Renewable Energy & Advanced Materials, Nanjing Tech University Pujiang Institute, Nanjing, China
article info
abstract
Article history:
Mg-based hydrogen-storage materials are regarded as promising hydrogen-storage alloys.
Received 3 November 2016
However, in practical applications, Mg-based hydrogen-storage materials may be exposed
Received in revised form
to air or to a low-purity-gas environment with particularly oxygen as impurity. In this
17 December 2016
paper, the influence of a micro-amount of oxygen or nitrogen on the performance of
Accepted 20 December 2016
adsorption/desorption of MgH2 hydrogen-storage material has been studied by placing
Available online xxx
samples in an argon-filled glove box (at atmospheric pressure) with 1000 ppm oxygen concentration. The hydrogen capacity and adsorption/desorption kinetics were measured
Keywords:
by using a high-pressure and high-temperature gas-adsorption analyzer. The samples
Mg-based hydrogen-storage mate-
were characterized by X-ray diffraction. The activation energy for hydrogenation has been
rials
determined by means of the Arrhenius equation. It is found that the adsorption/desorption
Magnesium hydride
of MgH2 improves significantly by a micro-amount of oxygen, especially at high
Oxygen influence
temperature.
Reaction kinetics
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Currently, the world is facing the problem of fossil fuel depletion [1e4]. Hydrogen, as an alternative energy, is an ideal, clean and easy for transport, renewable energy. Finding proper ways of hydrogen-storage is a key issue for the application of hydrogen energy. Solid hydrogen-storage materials belong currently to the most attractive ways of hydrogenstorage, especially for applications in zero-emission vehicles [5,6].
Hydrogen can be stored as (i) cryogenic liquid, (ii) pressurized gas, (iii) solid as chemical or physical combination of hydrogen with materials, such as carbon materials, metal hydrides and complex hydrides, or (iv) by on-board reformingmethanol technology [7,8]. Each of these options possesses attractive attributes for hydrogen-storage. For the applications in zero-emission vehicles (solid fuel), it is required that the hydrogen-storage materials have high hydrogen gravimetric and volumetric density [9].
* Corresponding author. State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing, China. E-mail address: [email protected] (Z. Zhao). http://dx.doi.org/10.1016/j.ijhydene.2016.12.102 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu E, et al., Influence of micro-amount O2 or N2 on the hydrogenation/dehydrogenation kinetics of hydrogen-storage material MgH2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.102
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
Metal hydrides, as hydrogen-storage materials, have been focus of intensive research. Magnesium hydride combines both high theoretical hydrogen capacity and low-cost advantages [6,10e12]. However, the main obstacles for practical application of magnesium hydride remain the poor hydrogen kinetics at low temperature (below 300 C) and high thermodynamic stability [13e15]. Recently, the properties of hydrogen-storage materials have been investigated with the purpose to improve the hydrating/dehydrating kinetics without reducing the high hydrogen capacity. The constituents of the hydrogen-storage materials are Mg or MgH2 and a micro-amount of transition metal such as Ti, V, Mn, Fe, Co, Ni, Cu or Pd or transition-metal oxide [16e18]. Oelerich et al. have studied the influence of metal oxides such as Sc2O3 and Cr2O3 on the hydrogen sorption and the results show that the metal oxides have enormous catalytic acceleration effect to hydrogen sorption [19]. Aeppatah et al. have studied the influence of multiple additions, Cr2O3 and Nb2O5, on the hydrogen sorption/desorption kinetics of MgH2. It was found that the kinetics of MgH2 is more improved by multiple oxide addition than by single addition [20]. The main disadvantages of MgH2 as hydrogen-storage material are the high hydrating/dehydrating temperature of 300e400 C, the sluggish kinetic properties and the high reactivity with air/oxygen. These disadvantages impede its practical application [21e23]. In the present study, in order to understand the influence of air/oxygen on the hydrogenation/ dehydrogenation kinetics of hydrogen-storage material MgH2, we have simulated the practical working environment of Mgbased hydrogen-storage materials; a micro-amount of oxygen was added to the high-purity Ar-gas atmosphere. The obtained results may be taken as a reference for those who design hydrogen-storage systems that may be applied for portable and stationary hydrogen fuel cells.
Experiment procedures Sample preparation The samples have been prepared by the ball-milling method. The mechanical milling was performed using a LB60GQ planetary ball mill with a stainless-steel vial and 20 stainlesssteel balls (10 mm diameter, 4 g) and 10 stainless-steel balls (6 mm diameter, 1 g). The ball-to-powder mass ratio was 30:1. The starting material MgH2 powder (J&K, 98%) was milled for 10 h at a speed of 400 rpm in a high-purity (99.999%) Ar atmosphere. By means of SEM the particle size was determined to be about 700 nm.
The oxygen-control system Fig. 1 presents a schematic illustration of the oxygen-control system; a method was devised to ensure that each sample was prepared in the glove box at the same oxygen concentration. In the experiments, air is taken as oxygen resource and brought into the glove box through the oxygen-control system where the sample is exposed for 15 min. Because the volume of the glove box is much larger than the volume of the
Fig. 1 e Schematic illustration of the oxygen-control system.
hydrogen to be absorbed by the sample, the oxygen concentration in the glove box remains unchanged during the time that the sample is exposed to the Ar gas atmosphere with 1000 ppm oxygen. In all experiments, high-purity Ar gas (~99.9999%) was used.
Adsorption/desorption measurements The hydrogen adsorption/desorption properties of the samples were determined by using a high-pressure and -temperature gas-adsorption analyzer. First, the initial samples were desorbed at 823 K for about 100 min. After the desorption, the samples were adsorbed at five different temperatures (470, 500, 530, 560, 590 K) at 3.0 MPa pressure. X-ray diffraction (XRD) measurements were performed at room temperature to investigate the crystal structures of the possible phases in the sample. A TM9KW X-ray diffractometer was used with Cu-Ka radiation (l ¼ 1.54 A) and the 2q scanning range is from 20 to 75 . As air is used in this experiment, nitrogen has been filled in the glove box together with oxygen. In order to understand the influence of nitrogen on the sorption properties of MgH2, a sample was exposed in the pure nitrogen for 15 min and an adsorption curve was measured.
Results and discussion Influence of a micro-amount oxygen on the hydrogenation/ dehydrogenation kinetics of MgH2 Fig. 2 shows the hydrogenation characteristics of MgH2 at five different temperatures ranging from 470 K to 590 K. In general, the results show that the hydrogen-adsorption kinetics is improved significantly after 15 min treatment by Ar gas with 1000 ppm O2. For example, comparing Fig. 2a and b, at 530 K, the sample in Fig. 2b absorbs 5.41 wt% H2 in the first 20 min, whereas the sample in Fig. 2a absorbs only 2.18 wt% H2 in the first 20 min. This indicates that addition of a micro-amount of oxygen enhances the hydrogen-adsorption capacity of MgH2 and the adsorption kinetics of MgH2 has been improved also due to the micro-amount of oxygen added. The desorption behavior of MgH2( treated at pure-Ar atmosphere) and at pure-Ar with 1000 ppm O2 impurities were
Please cite this article in press as: Xu E, et al., Influence of micro-amount O2 or N2 on the hydrogenation/dehydrogenation kinetics of hydrogen-storage material MgH2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.102
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
Fig. 2 e Isothermal hydrogenation profiles at five different temperatures for (a) MgH2; (b) MgH2, 1000 ppm O2.
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dehydrogenation characteristics of the two types of samples are very similar. However, at high temperature the hydrogen desorption of MgH2 is quite different: 3.25 wt% for the sample treated in Ar gas with 1000 ppm O2, but only 0.85 wt% for the sample that was placed in pure Ar. Above results indicate that addition of a micro-amount of oxygen increases the hydrogen-desorption capacity which means that the hydrogen-desorption kinetics of MgH2 has been improved. In order to understand the correlation between the phase compositions and the performance of the hydrogen adsorption/desorption compounds, XRD measurements have been performed. Fig. 4 shows the XRD results of MgH2 before and after ball milling. It can be seen that after ball milling a microamount of Mg is present which may be due to the dehydrogenation process during the ball milling. At the same time, no reflection of MgO can be seen. Fig. 5 shows the XRD pattern of a MgH2 sample placed in the glove box in an Ar-gas environment with 1000 ppm O2. It can be seen that the main constituting phase is MgH2. The intensities of the MgH2 reflections at 27.95 and 35.75 increase with increasing hydrogen-adsorption temperature. We also see that the crystallization of the samples is getting better due to the micro-amount of oxygen presented. The XRD results also indicate that MgO has formed due to the added micro-amount of oxygen. From the XRD results, we can conclude that the influence of the micro-amount of oxygen on the hydrogen adsorption/ desorption kinetics of MgH2 compound is associated with an improvement of the crystallization and the existence of micro-amount of MgO in the sample. From the XRD results in Fig. 5, the weight percentage of MgO in the sample can be estimated to be about 6.3%. This remains unchanged for the different temperatures of hydrogenation which means that the MgO observed has nothing with the hydrogenation process at the different temperatures. From the pattern in Fig. 5, it can be derived that the particle size is about 700 nm which is larger than many other reported values [24e26].
Fig. 3 e Temperature dependence of dehydrogenation for (a) MgH2, (b) MgH2, 1000 ppm O2. investigated through temperature-programmed desorption experiments. Fig. 3 shows the temperature dependence of desorption of the two types of MgH2 samples ranging from 470 K to 590 K. In the beginning of the desorption process, the
Fig. 4 e XRD patterns of MgH2 (a) before ball milling and (b) after ball milling for 10 h.
Please cite this article in press as: Xu E, et al., Influence of micro-amount O2 or N2 on the hydrogenation/dehydrogenation kinetics of hydrogen-storage material MgH2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.102
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
activation energy of the hydrogenation reaction. The activation energy for hydrogenation was obtained by means of the Arrhenius equation [27]: Ea ¼ RT lnðk=k0 Þ
(1)
where Ea is the activation energy, k the temperaturedependent reaction-rate constant, k0 is a pre-exponential factor, R the gas constant and T the absolute temperature. The constant k can be obtained by applying the Avrami equation [28]: f ¼ 1 expð ktn Þ
Fig. 5 e XRD patterns of MgH2 after hydrogenation at five temperatures. The samples have been exposed for 15 min to Ar with 1000 ppm O2 before the hydrogenation measurements.
Influence of nitrogen on the hydrogen adsorption/desorption of MgH2 In order to demonstrate the influence of nitrogen on the hydrogenation properties of MgH2, Fig. 6 shows the hydrogenadsorption profiles of MgH2 at 470 K and 590 K. The samples were treated with and without nitrogen as mentioned in Section “Adsorption/desorption measurements”. It can be seen that, at each temperature, the data for MgH2 with and without nitrogen treatment overlap completely. This proves that nitrogen does not affect the hydrogen-storage performance of MgH2.
Evaluation of the activation energy for the hydrogenation process The improved hydrogenation kinetics of MgH2 due to the micro-amount of oxygen can be characterized by the
(2)
where f is the reaction function (here it is the weight percentage of hydrogenation of the sample), n the order of reaction and t the reaction time. For solid-state to solid-state phase transitions, the value of n normally falls between 1 and 4. From Eq. (2) we get the following equation: ln ½ ln ð1 fÞ ¼ nln t þ ln k
(3)
In order to analyze the absorption behavior, the value of the Avrami exponent n has to be determined. In this study, n was derived from plots based on Eq. (3) and the data were taken at the initial time region of Fig. 2. Fig. 7 shows the results of the plot for (a) MgH2 and (b) MgH2 with 1000 ppm O2. Table 1 lists the results for n and ln k based on Eq. (3) and Fig. 2. It can be seen that the value of n is between 1 and 2 for temperatures below or equal to 500 K. n is smaller than 1 when the temperature is above or equal to 530 K. The obtained n value (1 < n < 2) for temperatures below or equal to 500 K corresponds to one- or two-dimensional grain growth with constant nucleation rate. At higher temperatures, n is smaller than 1. The mechanism of the phase transform diffusion is attributed to lamellar precipitation which may be due to the larger particle size of the sample (about 700 nm). Using the values derived from isothermal hydrogenation data at different temperatures, presented in Fig. 2, the activation energy (Ea) for the hydrogenation reaction can be calculated, as shown in Fig. 7. The activation energy of the H2-adsorption processes for MgH2 treated in pure Ar gas and for MgH2 treated in Ar gas with 1000 ppm O2 are evaluated to be 207 kJ/mol and 182 kJ/ mol, respectively, showing that the activation energy for the samples that were exposed to oxygen is lower than for those exposed to pure Ar gas only. In comparison with other reported values, 86 kJ/mol by Kojima et al. [29], and 84 kJ/mol by Singh et al. [30], the present results are much higher. This is very likely due to the larger particle size of the present samples. The XRD and SEM results indicate that the average particle size is 700 nm, almost 10 times larger than that reported in Ref. [17e21] (see Fig. 8).
Table 1 e Avrami exponent n and ln k for MgH2, at different temperatures. T(K)
Fig. 6 e Isothermal hydrogenation profiles of MgH2 with and without nitrogen at 470 K and 590 K.
n (without O2) ln k (without O2) n (1000 ppm O2) ln k (1000 ppm O2)
470
500
530
560
590
1.33 15.5 4.18 12.9
1.47 13.9 1.06 9.9
0.51 7.52 0.72 7.4
0.79 8.3 0.39 5.3
0.22 4.7 0.093 3.6
Please cite this article in press as: Xu E, et al., Influence of micro-amount O2 or N2 on the hydrogenation/dehydrogenation kinetics of hydrogen-storage material MgH2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.102
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
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hydrogen-storage materials. Both adsorption and desorption kinetics as well as the hydrogen capacity are enhanced due to the improvement of crystallization and due to an appropriate amount of MgO in the MgH2 samples, especially when adsorption and desorption at high temperature. Nitrogen does not influence the adsorption and desorption of MgH2 hydrogen-storage materials. The larger particle size probably leads to higher activation energies that reported in literature.
Acknowledgements The authors thank Prof. Dr. F.R. de Boer for providing valuable discussions and suggestions. This work was supported by the discovery project of State Key Laboratory of MaterialsOriented Chemical Engineering, Nanjing Tech University (38901159); The Hydrogen Energy Key Materials R&D Center of Nanjing City (51201311); and the collaboration program between Nanjing Tech University and Delft University.
references
Fig. 7 e Plots of ln [¡ln (1¡f)] vs ln t for (a) MgH2 and (b) MgH2 with 1000 ppm O2.
Fig. 8 e Arrhenius plot of the composites MgH2.
Conclusions A micro-amount of oxygen, present during the material preparation, has large influence on the performance of MgH2
[1] Sanborn Scott D. Fossil sources: “Running Out” is not the problem. Int J Hydrogen Energy 2005;30:1e7. [2] Oleander U. Lunar mineral extraction to correct for potential loss of Earth mass caused by fossil fuel consumption. Sci Total Environ 2016;911:557e8. [3] Rather SU, Taimoor AA, Muhammad A, Alhamed YA, Zaman SF, Ali AM. Kinetics of hydrogen adsorption on MgH2/CNT composite. Mater Res Bull 2016;77:23e8. [4] Zhang Y, Liu X, Qin B, Shi K, Deng J, Zhou Y. Aquatic vegetation in response to increased eutrophication and degraded light climate in Eastern Lake Taihu: implications for lake ecological restoration. Sci Rep 2016;6:23867. [5] Schlapbach L, Zu¨ttel A. Hydrogen-storage materials for mobile applications. Nature 2001;414:353e8. [6] Wagemans RW, van Lenthe JH, de Jongh PE, Van Dillen AJ, de Jong KP. Hydrogen storage in magnesium clusters: quantum chemical study. J Am Chem Soc 2005;127:16675e80. [7] Ogden JM. Developing an infrastructure for hydrogen vehicles: a Southern California case study. Int J Hydrogen Energy 1999;24:709e30. [8] Mori D, Hirose K. Recent challenges of hydrogen storage technologies for fuel cell vehicles. Int J Hydrogen Energy 2009;34:4569e74. [9] Dogan B. Hydrogen storage tank systems and materials selection for transport applications. Am Soc Mech Eng 2006:571e8. € m-Olsen J. Nano- crystalline metal [10] Zaluski L, Zaluska A, Stro hydrides. J Alloys Compd 1997;253:70e9. [11] Jain IP, Lal C, Jain A. Hydrogen storage in Mg: a most promising material. Int J Hydrogen Energy 2010;35:5133e44. [12] Selvam P, Viswanathan B, Swamy CS, Srinivasan V. Magnesium and magnesium alloy hydrides. Int J Hydrogen Energy 1986;11:169e92. [13] Bououdina M, Grant D, Walker G. Review on hydrogen absorbing materials-structure, microstructure, and thermodynamic properties. Int J Hydrogen Energy 2006;31:177e82. [14] Guo ZX, Shang C, Aguey-Zinsou KF. Materials challenges for hydrogen storage. J Eur Ceram Soc 2008;28:1467e73.
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[15] Sakintuna B, Lamaridarkrim F, Hirscher M. Metal hydride materials for solid hydrogen storage: a review. Int J Hydrogen Energy 2007;32:1121e40. [16] Hanada N, Ichikawa T, Fujii H. Catalytic effect of nanoparticle 3d-transition metals on hydrogen storage properties in magnesium hydride MgH2 prepared by mechanical milling. J Phys Chem B 2005;109:7188e94. [17] Chen M, Yang XB, Cui J, Tang JJ, Gan LY, Zhu M, et al. Stability of transition metals on Mg(0001) surfaces and their effects on hydrogen adsorption. Int J Hydrogen Energy 2012;37:309e17. [18] Shahi RR, Tiwari AP, Shaz MA, Srivastava ON. Studies on de/ rehydrogenation characteristics of nanocrystalline MgH2 cocatalyzed with Ti, Fe and Ni. Int J Hydrogen Energy 2013;38:2778e84. [19] Oelerich W, Klassen T, Bormann R. Metal oxides as catalysts for improved hydrogen sorption in nanocrystalline Mg-based materials. J Alloys Compd 2001;315:237e42. [20] Patah A, Takasaki A, Szmyd JS. Influence of multiple oxide (Cr2O3/Nb2O5) addition on the sorption kinetics of MgH2. Int J Hydrogen Energy 2009;34:3032e7. [21] Barkhordarian G, Klassen T, Bormann R. Effect of Nb2O5 content on hydrogen reaction kinetics of Mg. J Alloys Compd 2004;364:242e6. € m-Olsen J. Nano- crystalline [22] Zaluska A, Zaluski L, Stro magnesium for hydrogen storage. J Alloys Compd 1999;288:217e25.
[23] Rashidi AM, Nouralishahi A, Khodadadi AA, Mortazavi Y, Karimi A, Kashefi K. Modification of single wall carbon nanotubes (SWNT) for hydrogen storage. Int J Hydrogen Energy 2010;35:9489e95. [24] Yang L, Qian L, Chou KC. Dehydriding reaction kinetic mechanism of MgH2-Nb2O5 by Chou model. Trans Nonferrous Met Soc China 2008;18:s235e41. [25] Agueyzinsou K, Aresfernandez J, Klassen T, Bormann R. Effect of Nb2O5 on MgH2 properties during mechanical milling. Int J Hydrogen Energy 2007;32:2400e7. [26] Gosalawit-Utke R, Thiangviriya S, Javadian P, Laipple D, Pistidda C, Bergemann N, et al. Effective nanoconfinement of 2LiBH4eMgH2 via simply MgH2 premilling for reversible hydrogen storages. Int J Hydrogen Energy 2014;39:15614e26. ¨ ber die Reaktionsgeschwind- igkeit bei der [27] Arrhenius S. U € uren. Z Phys Chem (N F) Inversion von Rohrzucker durch Sa 1889;4:226e48. [28] Avrami M. Kinetics of phase change.ⅠGeneral theory. J Chem Phys 1939;7:1103e12. [29] Kojima Y, Kawai Y, Haga T. Magnesium-based nanocomposite materials for hydrogen storage. J Alloys Compd 2006;424:294e8. [30] Singh RK, Sadhasivam T, Sheeja GI, Singh P, Srivastava ON. Effect of different sized CeO2 nano particles on decomposition and hydrogen absorption kinetics of magnesium hydride. Int J Hydrogen Energy 2013;38:6221e5.
Please cite this article in press as: Xu E, et al., Influence of micro-amount O2 or N2 on the hydrogenation/dehydrogenation kinetics of hydrogen-storage material MgH2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.102
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Influence of micro-amount O2 or N2 on the hydrogenation/dehydrogenation kinetics of hydrogen-storage material MgH2 En Xu a, Hui Li a,b, Xinmin You a, Chao Bu a, Longfei Zhang a, Qi Wang a, Zhigang Zhao a,b,* a
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing, China b College of Renewable Energy & Advanced Materials, Nanjing Tech University Pujiang Institute, Nanjing, China
article info
abstract
Article history:
Mg-based hydrogen-storage materials are regarded as promising hydrogen-storage alloys.
Received 3 November 2016
However, in practical applications, Mg-based hydrogen-storage materials may be exposed
Received in revised form
to air or to a low-purity-gas environment with particularly oxygen as impurity. In this
17 December 2016
paper, the influence of a micro-amount of oxygen or nitrogen on the performance of
Accepted 20 December 2016
adsorption/desorption of MgH2 hydrogen-storage material has been studied by placing
Available online xxx
samples in an argon-filled glove box (at atmospheric pressure) with 1000 ppm oxygen concentration. The hydrogen capacity and adsorption/desorption kinetics were measured
Keywords:
by using a high-pressure and high-temperature gas-adsorption analyzer. The samples
Mg-based hydrogen-storage mate-
were characterized by X-ray diffraction. The activation energy for hydrogenation has been
rials
determined by means of the Arrhenius equation. It is found that the adsorption/desorption
Magnesium hydride
of MgH2 improves significantly by a micro-amount of oxygen, especially at high
Oxygen influence
temperature.
Reaction kinetics
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Currently, the world is facing the problem of fossil fuel depletion [1e4]. Hydrogen, as an alternative energy, is an ideal, clean and easy for transport, renewable energy. Finding proper ways of hydrogen-storage is a key issue for the application of hydrogen energy. Solid hydrogen-storage materials belong currently to the most attractive ways of hydrogenstorage, especially for applications in zero-emission vehicles [5,6].
Hydrogen can be stored as (i) cryogenic liquid, (ii) pressurized gas, (iii) solid as chemical or physical combination of hydrogen with materials, such as carbon materials, metal hydrides and complex hydrides, or (iv) by on-board reformingmethanol technology [7,8]. Each of these options possesses attractive attributes for hydrogen-storage. For the applications in zero-emission vehicles (solid fuel), it is required that the hydrogen-storage materials have high hydrogen gravimetric and volumetric density [9].
* Corresponding author. State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing, China. E-mail address: [email protected] (Z. Zhao). http://dx.doi.org/10.1016/j.ijhydene.2016.12.102 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu E, et al., Influence of micro-amount O2 or N2 on the hydrogenation/dehydrogenation kinetics of hydrogen-storage material MgH2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.102
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
Metal hydrides, as hydrogen-storage materials, have been focus of intensive research. Magnesium hydride combines both high theoretical hydrogen capacity and low-cost advantages [6,10e12]. However, the main obstacles for practical application of magnesium hydride remain the poor hydrogen kinetics at low temperature (below 300 C) and high thermodynamic stability [13e15]. Recently, the properties of hydrogen-storage materials have been investigated with the purpose to improve the hydrating/dehydrating kinetics without reducing the high hydrogen capacity. The constituents of the hydrogen-storage materials are Mg or MgH2 and a micro-amount of transition metal such as Ti, V, Mn, Fe, Co, Ni, Cu or Pd or transition-metal oxide [16e18]. Oelerich et al. have studied the influence of metal oxides such as Sc2O3 and Cr2O3 on the hydrogen sorption and the results show that the metal oxides have enormous catalytic acceleration effect to hydrogen sorption [19]. Aeppatah et al. have studied the influence of multiple additions, Cr2O3 and Nb2O5, on the hydrogen sorption/desorption kinetics of MgH2. It was found that the kinetics of MgH2 is more improved by multiple oxide addition than by single addition [20]. The main disadvantages of MgH2 as hydrogen-storage material are the high hydrating/dehydrating temperature of 300e400 C, the sluggish kinetic properties and the high reactivity with air/oxygen. These disadvantages impede its practical application [21e23]. In the present study, in order to understand the influence of air/oxygen on the hydrogenation/ dehydrogenation kinetics of hydrogen-storage material MgH2, we have simulated the practical working environment of Mgbased hydrogen-storage materials; a micro-amount of oxygen was added to the high-purity Ar-gas atmosphere. The obtained results may be taken as a reference for those who design hydrogen-storage systems that may be applied for portable and stationary hydrogen fuel cells.
Experiment procedures Sample preparation The samples have been prepared by the ball-milling method. The mechanical milling was performed using a LB60GQ planetary ball mill with a stainless-steel vial and 20 stainlesssteel balls (10 mm diameter, 4 g) and 10 stainless-steel balls (6 mm diameter, 1 g). The ball-to-powder mass ratio was 30:1. The starting material MgH2 powder (J&K, 98%) was milled for 10 h at a speed of 400 rpm in a high-purity (99.999%) Ar atmosphere. By means of SEM the particle size was determined to be about 700 nm.
The oxygen-control system Fig. 1 presents a schematic illustration of the oxygen-control system; a method was devised to ensure that each sample was prepared in the glove box at the same oxygen concentration. In the experiments, air is taken as oxygen resource and brought into the glove box through the oxygen-control system where the sample is exposed for 15 min. Because the volume of the glove box is much larger than the volume of the
Fig. 1 e Schematic illustration of the oxygen-control system.
hydrogen to be absorbed by the sample, the oxygen concentration in the glove box remains unchanged during the time that the sample is exposed to the Ar gas atmosphere with 1000 ppm oxygen. In all experiments, high-purity Ar gas (~99.9999%) was used.
Adsorption/desorption measurements The hydrogen adsorption/desorption properties of the samples were determined by using a high-pressure and -temperature gas-adsorption analyzer. First, the initial samples were desorbed at 823 K for about 100 min. After the desorption, the samples were adsorbed at five different temperatures (470, 500, 530, 560, 590 K) at 3.0 MPa pressure. X-ray diffraction (XRD) measurements were performed at room temperature to investigate the crystal structures of the possible phases in the sample. A TM9KW X-ray diffractometer was used with Cu-Ka radiation (l ¼ 1.54 A) and the 2q scanning range is from 20 to 75 . As air is used in this experiment, nitrogen has been filled in the glove box together with oxygen. In order to understand the influence of nitrogen on the sorption properties of MgH2, a sample was exposed in the pure nitrogen for 15 min and an adsorption curve was measured.
Results and discussion Influence of a micro-amount oxygen on the hydrogenation/ dehydrogenation kinetics of MgH2 Fig. 2 shows the hydrogenation characteristics of MgH2 at five different temperatures ranging from 470 K to 590 K. In general, the results show that the hydrogen-adsorption kinetics is improved significantly after 15 min treatment by Ar gas with 1000 ppm O2. For example, comparing Fig. 2a and b, at 530 K, the sample in Fig. 2b absorbs 5.41 wt% H2 in the first 20 min, whereas the sample in Fig. 2a absorbs only 2.18 wt% H2 in the first 20 min. This indicates that addition of a micro-amount of oxygen enhances the hydrogen-adsorption capacity of MgH2 and the adsorption kinetics of MgH2 has been improved also due to the micro-amount of oxygen added. The desorption behavior of MgH2( treated at pure-Ar atmosphere) and at pure-Ar with 1000 ppm O2 impurities were
Please cite this article in press as: Xu E, et al., Influence of micro-amount O2 or N2 on the hydrogenation/dehydrogenation kinetics of hydrogen-storage material MgH2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.102
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Fig. 2 e Isothermal hydrogenation profiles at five different temperatures for (a) MgH2; (b) MgH2, 1000 ppm O2.
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dehydrogenation characteristics of the two types of samples are very similar. However, at high temperature the hydrogen desorption of MgH2 is quite different: 3.25 wt% for the sample treated in Ar gas with 1000 ppm O2, but only 0.85 wt% for the sample that was placed in pure Ar. Above results indicate that addition of a micro-amount of oxygen increases the hydrogen-desorption capacity which means that the hydrogen-desorption kinetics of MgH2 has been improved. In order to understand the correlation between the phase compositions and the performance of the hydrogen adsorption/desorption compounds, XRD measurements have been performed. Fig. 4 shows the XRD results of MgH2 before and after ball milling. It can be seen that after ball milling a microamount of Mg is present which may be due to the dehydrogenation process during the ball milling. At the same time, no reflection of MgO can be seen. Fig. 5 shows the XRD pattern of a MgH2 sample placed in the glove box in an Ar-gas environment with 1000 ppm O2. It can be seen that the main constituting phase is MgH2. The intensities of the MgH2 reflections at 27.95 and 35.75 increase with increasing hydrogen-adsorption temperature. We also see that the crystallization of the samples is getting better due to the micro-amount of oxygen presented. The XRD results also indicate that MgO has formed due to the added micro-amount of oxygen. From the XRD results, we can conclude that the influence of the micro-amount of oxygen on the hydrogen adsorption/ desorption kinetics of MgH2 compound is associated with an improvement of the crystallization and the existence of micro-amount of MgO in the sample. From the XRD results in Fig. 5, the weight percentage of MgO in the sample can be estimated to be about 6.3%. This remains unchanged for the different temperatures of hydrogenation which means that the MgO observed has nothing with the hydrogenation process at the different temperatures. From the pattern in Fig. 5, it can be derived that the particle size is about 700 nm which is larger than many other reported values [24e26].
Fig. 3 e Temperature dependence of dehydrogenation for (a) MgH2, (b) MgH2, 1000 ppm O2. investigated through temperature-programmed desorption experiments. Fig. 3 shows the temperature dependence of desorption of the two types of MgH2 samples ranging from 470 K to 590 K. In the beginning of the desorption process, the
Fig. 4 e XRD patterns of MgH2 (a) before ball milling and (b) after ball milling for 10 h.
Please cite this article in press as: Xu E, et al., Influence of micro-amount O2 or N2 on the hydrogenation/dehydrogenation kinetics of hydrogen-storage material MgH2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.102
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activation energy of the hydrogenation reaction. The activation energy for hydrogenation was obtained by means of the Arrhenius equation [27]: Ea ¼ RT lnðk=k0 Þ
(1)
where Ea is the activation energy, k the temperaturedependent reaction-rate constant, k0 is a pre-exponential factor, R the gas constant and T the absolute temperature. The constant k can be obtained by applying the Avrami equation [28]: f ¼ 1 expð ktn Þ
Fig. 5 e XRD patterns of MgH2 after hydrogenation at five temperatures. The samples have been exposed for 15 min to Ar with 1000 ppm O2 before the hydrogenation measurements.
Influence of nitrogen on the hydrogen adsorption/desorption of MgH2 In order to demonstrate the influence of nitrogen on the hydrogenation properties of MgH2, Fig. 6 shows the hydrogenadsorption profiles of MgH2 at 470 K and 590 K. The samples were treated with and without nitrogen as mentioned in Section “Adsorption/desorption measurements”. It can be seen that, at each temperature, the data for MgH2 with and without nitrogen treatment overlap completely. This proves that nitrogen does not affect the hydrogen-storage performance of MgH2.
Evaluation of the activation energy for the hydrogenation process The improved hydrogenation kinetics of MgH2 due to the micro-amount of oxygen can be characterized by the
(2)
where f is the reaction function (here it is the weight percentage of hydrogenation of the sample), n the order of reaction and t the reaction time. For solid-state to solid-state phase transitions, the value of n normally falls between 1 and 4. From Eq. (2) we get the following equation: ln ½ ln ð1 fÞ ¼ nln t þ ln k
(3)
In order to analyze the absorption behavior, the value of the Avrami exponent n has to be determined. In this study, n was derived from plots based on Eq. (3) and the data were taken at the initial time region of Fig. 2. Fig. 7 shows the results of the plot for (a) MgH2 and (b) MgH2 with 1000 ppm O2. Table 1 lists the results for n and ln k based on Eq. (3) and Fig. 2. It can be seen that the value of n is between 1 and 2 for temperatures below or equal to 500 K. n is smaller than 1 when the temperature is above or equal to 530 K. The obtained n value (1 < n < 2) for temperatures below or equal to 500 K corresponds to one- or two-dimensional grain growth with constant nucleation rate. At higher temperatures, n is smaller than 1. The mechanism of the phase transform diffusion is attributed to lamellar precipitation which may be due to the larger particle size of the sample (about 700 nm). Using the values derived from isothermal hydrogenation data at different temperatures, presented in Fig. 2, the activation energy (Ea) for the hydrogenation reaction can be calculated, as shown in Fig. 7. The activation energy of the H2-adsorption processes for MgH2 treated in pure Ar gas and for MgH2 treated in Ar gas with 1000 ppm O2 are evaluated to be 207 kJ/mol and 182 kJ/ mol, respectively, showing that the activation energy for the samples that were exposed to oxygen is lower than for those exposed to pure Ar gas only. In comparison with other reported values, 86 kJ/mol by Kojima et al. [29], and 84 kJ/mol by Singh et al. [30], the present results are much higher. This is very likely due to the larger particle size of the present samples. The XRD and SEM results indicate that the average particle size is 700 nm, almost 10 times larger than that reported in Ref. [17e21] (see Fig. 8).
Table 1 e Avrami exponent n and ln k for MgH2, at different temperatures. T(K)
Fig. 6 e Isothermal hydrogenation profiles of MgH2 with and without nitrogen at 470 K and 590 K.
n (without O2) ln k (without O2) n (1000 ppm O2) ln k (1000 ppm O2)
470
500
530
560
590
1.33 15.5 4.18 12.9
1.47 13.9 1.06 9.9
0.51 7.52 0.72 7.4
0.79 8.3 0.39 5.3
0.22 4.7 0.093 3.6
Please cite this article in press as: Xu E, et al., Influence of micro-amount O2 or N2 on the hydrogenation/dehydrogenation kinetics of hydrogen-storage material MgH2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.102
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hydrogen-storage materials. Both adsorption and desorption kinetics as well as the hydrogen capacity are enhanced due to the improvement of crystallization and due to an appropriate amount of MgO in the MgH2 samples, especially when adsorption and desorption at high temperature. Nitrogen does not influence the adsorption and desorption of MgH2 hydrogen-storage materials. The larger particle size probably leads to higher activation energies that reported in literature.
Acknowledgements The authors thank Prof. Dr. F.R. de Boer for providing valuable discussions and suggestions. This work was supported by the discovery project of State Key Laboratory of MaterialsOriented Chemical Engineering, Nanjing Tech University (38901159); The Hydrogen Energy Key Materials R&D Center of Nanjing City (51201311); and the collaboration program between Nanjing Tech University and Delft University.
references
Fig. 7 e Plots of ln [¡ln (1¡f)] vs ln t for (a) MgH2 and (b) MgH2 with 1000 ppm O2.
Fig. 8 e Arrhenius plot of the composites MgH2.
Conclusions A micro-amount of oxygen, present during the material preparation, has large influence on the performance of MgH2
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Please cite this article in press as: Xu E, et al., Influence of micro-amount O2 or N2 on the hydrogenation/dehydrogenation kinetics of hydrogen-storage material MgH2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.102