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Nb2O5) addition on the sorption kinetics of MgH2
international journal of hydrogen energy 34 (2009) 3032–3037
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Influence of multiple oxide (Cr2O3/Nb2O5) addition on the sorption kinetics of MgH2 Aep Pataha,*, Akito Takasakia, Janusz S. Szmydb a
Division of Regional Environment Systems, Graduate School of Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan b AGH – University of Science and Technology, Al. Mickiewicza 30, Krakow 30-059, Poland
article info
abstract
Article history:
The influence of multiple additions of two oxides, Cr2O3 and Nb2O5, as additives on the
Received 4 August 2008
hydrogen sorption kinetics of MgH2 after milling was investigated. We found that the
Received in revised form
desorption kinetics of MgH2 were improved more by multiple oxide addition than by single
10 December 2008
addition. Even for the milled MgH2 micrometric size powders, the high hydrogen capacity
Accepted 30 January 2009
with fast kinetics were achieved for the powders after addition of 0.2 mol% Cr2O3 þ 1 mol%
Available online 25 February 2009
Nb2O5. For this composition, the hydride desorbed about 5 wt.% hydrogen within 20 min and absorbed about 6 wt.% in 5 min at 300 C. Furthermore, the desorption temperature
Keywords:
was decreased by 100 C, compared to MgH2 without any oxide addition, and the activation
Hydrogen sorption
energy for the hydrogen desorption was estimated to be about 185 kJ mol1, while that for
Magnesium hydride
MgH2 without oxide was about 206 kJ mol1.
Mechanical milling
ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Kinetics Metal oxides
1.
Introduction
Magnesium hydride (MgH2) is one of the most promising candidates for hydrogen storage materials, due to its high storage capacity with a theoretical value of 7.6 wt.%. However, the practical use of MgH2 for hydrogen storage is limited, due to its high thermodynamic stability (DH ¼ 74.5 kJ mol1) and slow desorption hydrogen kinetics [1–4]. Recently, many efforts such as the manufacture of nanocrystalline powder [5], addition of metal oxides as catalysts [6,7], or synthesis of composite metal hydrides [8] have been attempted to improve the sorption properties. Of these, the addition of transition metal oxides to MgH2 by mechanical milling seems to be one of the most promising methods for improvement of the hydrogen sorption properties. With respect to transition metal oxides, Nb2O5 and Cr2O3 have recently attracted considerable
interest [9–14]. Friedrichs et al. [13] reported that milling of MgH2 with 10 wt.% (ca. 1.09 mol%) of nanosized Nb2O5 powder resulted in absorption and desorption of hydrogen within a few minutes, through the intimate contact between MgH2 and Nb2O5. Hanada et al. [14] reported that 1 mol% mesoporous Nb2O5-catalyzed MgH2 exhibited the most superior hydrogen desorption properties. Addition of nanosized Cr2O3 is reported to enhance the thermal stability of the MgH2 microstructure, as well as impart stability for prolonged cycling [10]. It was also reported that addition of 5 wt.% Cr2O3 improved the absorption/desorption reaction rate of MgH2; addition of nanosized Cr2O3 increased the diffusion coefficient of hydrogen by one order of magnitude than that for addition of microsized Cr2O3 [9]. Furthermore Yao et al. [15] used an analytical model for hydrogen diffusion and showed that the practicality of using Mg could be enhanced by reducing the
* Corresponding author. Tel.: þ81 03 5859 8059; fax: þ81 03 5859 8001. E-mail addresses: [email protected] (A. Patah), [email protected] (A. Takasaki), [email protected] (J.S. Szmyd). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.01.086
international journal of hydrogen energy 34 (2009) 3032–3037
grain size to nanoscale. However, not all transition metal oxides could be adapted as additives and the effect of each oxide, whose physical or chemical roles may also be different, also depended on the mechanical milling conditions [11]. Most of the research regarding the effect of oxide addition has dealt with one oxide and optimization of the amount of oxide. Our previous report [16] was the first to suggest that the addition of two oxides (Cr2O3/Nb2O5 and ZnO/Nb2O5) could improve the hydrogen absorption/desorption ability of MgH2 more than the addition of a single oxide. This implies that a synergetic effect between those oxides and MgH2 may occur. In the present study, the influence of multiple additions of two oxides, Cr2O3 and Nb2O5, on the hydrogen sorption kinetics of MgH2 was investigated in more detail. In particular, the influence of the total amount of concurrent Cr2O3 and Nb2O5 addition was examined. The sorption kinetics, activation energy for hydrogen desorption, and the morphology changes of the hydride with and without addition of multiple oxides were compared.
2.
Experimental
The mechanical milling was performed using a Fritsch P7 planetary ball mill with a stainless steel vial and 18 stainless steel balls (8 mm diameter). The ball-to-powder mass ratio was 10:1. MgH2 powder (98% purity, balance Mg) with a mean particle size of approximately 70 mm was purchased from Alfa Aesar, Johnson Matthey Company. Cr2O3 and Nb2O5, with a mean particle size of 47 mm, were purchased from Furuuchi Chemical Company and used as milling additives. MgH2, with and without oxides, was milled for a total of 20 h at a rotational speed of 400 rpm under a high-purity (99.999%) argon atmosphere. Hydrogen storage properties of the samples were examined using an automatic Sievert-type apparatus manufactured by Japan Metals & Chemicals Company, in which pressure and temperature data were simultaneously collected by a computer. The mass of the sample for each measurement was approximately 200 mg. The measurements were performed at 300 C with a hydrogen pressure of 1 MPa for absorption and 1 kPa for desorption. The hydride samples were initially dehydrided for 20 h at 300 C in a vacuum prior to the measurements. Crystallographic characterization was performed using X-ray diffraction (XRD; Jeol JDX-8030) with Cu ˚ ). The morphology of the samples Ka radiation (l ¼ 1.5418 A was observed with a scanning electron microscope (SEM; Jeol JSM-7400F). The dehydrogenation performance of the milled MgH2 was examined by differential scanning calorimetry (DSC; Shimazu DSC-60) under a high-purity Ar gas flow (50 mL min1) at several heating rates.
3.
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Fig. 1 – The amount of the hydrogen desorbed as a function of reaction time at 300 8C under 1 kPa H2.
respectively. It was obvious that the addition of Nb2O5 improved the hydrogen desorption kinetics of MgH2 more than Cr2O3. Furthermore, the hydrogen desorption kinetics of MgH2 are much more improved by the concurrent addition of Nb2O5 with Cr2O3. A small addition of Cr2O3 (0.2 mol%) could improve the hydrogen desorption kinetics up to 5.1 wt.% in 20 min, as shown for the MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5 composition. Increasing the amount of Cr2O3 up to 1 mol% decreased the hydrogen capacity as well as the hydrogen desorption kinetics; the MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5 composition desorbed 4.3 wt.% of hydrogen in 20 min. This indicates that the hydrogen desorption capacity is not dependent only on the type of oxides, but also the total amount of oxides added. However, for the combined addition of Nb2O5 and Cr2O3, the hydrogen desorption kinetics were significantly improved. Although the slopes at the beginning of the absorption curves were almost the same (Fig. 2), which is related to the rate limiting step, the hydrogen absorbed for a single oxide addition was less than that for addition of two oxides. MgH2 and MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5 absorb 5.3 wt.%
Results and Discussion
Fig. 1 shows the desorption kinetics of MgH2 at 300 C with and without metal oxides. The addition of Nb2O5 increases the desorption kinetics of MgH2 as shown for the MgH2 þ 1 mol% Cr2O3 and MgH2 þ 1 mol% Nb2O5 compositions, which desorbed 1.5 wt.% and 3.6 wt.% of hydrogen in 20 min,
Fig. 2 – The amount of the hydrogen absorbed as a function of reaction time at 300 8C under 1 MPa H2.
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international journal of hydrogen energy 34 (2009) 3032–3037
hydrogen in 5 min, and MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5 showed the highest (6.0 wt.%) and fastest (5 min) hydrogen adsorption capacity, indicating that the addition of two oxides plays an important role in improving the hydrogen absorption. It also suggests that the hydrogen capacities of the samples are related to the total amount and type of oxides added. Fig. 3 shows the theoretical hydrogen capacity, hydrogen absorption in 5 min and the hydrogen desorption in 20 min for each of the samples. The adsorption/desorption times were selected to reflect the onset of curve to plateau. MgH2 þ 1 mol% Cr2O3, XRD patterns of MgH2, MgH2 þ 1 mol% Nb2O5, MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5, and MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5 after milling and those after subsequent dehydrogenation at 300 C are shown in Figs. 4 and 5, respectively. The XRD patterns of all samples after milling had broad diffraction lines, indicating grain refinement due to high energy ball milling, which usually occurs when crystals are refined by mechanical milling processes. In all cases, the most intense diffraction line was identified as tetragonal b-MgH2, and a metastable orthorhombic phase g-MgH2 was also observed. On the other hand, the most intense diffraction line for the dehydrogenated sample was Mg, as shown in Fig. 5, which transforms again into b-MgH2 after hydrogenation. Small amounts of MgO were also identified after milling, as well as after dehydrogenation, which is probably due to oxygen contamination during sample preparation. For the MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5 sample shown in Fig. 5, the intense diffraction line of Nb2O5 almost disappeared after dehydrogenation, even though it was still observed after milling (Fig. 4). In contrast, the diffraction line for Cr2O3 was still identified even after dehydrogenation, suggesting that no reaction occurred between Cr2O3 and MgH2 during dehydrogenation, which was also reported by Oelerich et al. [6]. Closer examination of the MgO diffraction line in the inset of Fig. 5 reveals the formation of a shoulder on the left side of the MgO peak, which is probably due to a ternary magnesium–
Fig. 3 – Comparison of theoretical H2 capacity and experimental results for absorption/desorption of MgH2 with and without additives. Samples named are: (A) MgH2; (B) MgH2 D 1 mol% Cr2O3; (C) MgH2 D 1 mol% Nb2O5; (D) MgH2 D 0.2 mol% Cr2O3 D 1 mol% Nb2O5 and (E) MgH2 D 1 mol% Cr2O3 D 1 mol% Nb2O5.
Fig. 4 – The XRD patterns of: (a) MgH2, (b) MgH2 D 1 mol% Cr2O3, (c) MgH2 D 1 mol% Nb2O5, (d) MgH2 D 0.2 mol% Cr2O3 D 1 mol% Nb2O5, (e) MgH2 D 1 mol% Cr2O3 D 1 mol% Nb2O5 after milling for a total of 20 h.
niobium-oxide (Mg–Nb-oxide). It is assumed that Nb2O5 particles are embedded in the MgH2 particle surfaces after milling, and some Nb2O5 is dissolved in the Mg matrix during the dehydriding process. Dehydrogenation changes MgH2 into Mg and some Mg may react with oxygen to form MgO, which is required as a precursor for the formation of the Mg–Nb-oxide. Oxygen could be produced from reduction of Nb2O5 or trapped-oxygen gas to complete the reaction and produce the ternary oxide. These reactions are spontaneous reactions that are related to the standard Gibbs free energy (DG0) for both oxides. The standard Gibbs free energy for MgO formation at 300 C is more negative than that for Nb2O5 [17]. The intense diffraction line for the ternary Mg–Nb-oxide shown in Fig. 5 is quite similar to the MgNb2O3.67 oxide (JCPDS database, Powder Diffraction File #25–052), and the same result was reported by Friedrichs et al. [18]. This ternary oxide possibly has a spinel or perovskite structure.
Fig. 5 – The XRD patterns of: (a) MgH2 D 1 mol% Cr2O3, (b) MgH2 D 1 mol% Nb2O5, (c) MgH2 D 0.2 mol% Cr2O3 D 1 mol% Nb2O5, (d) MgH2 D 1 mol% Cr2O3 D 1 mol% Nb2O5 after milling for a total of 20 h and subsequently dehydrogenated (1 kPa, 300 8C).
international journal of hydrogen energy 34 (2009) 3032–3037
The presence of Cr2O3 assists in the formation of the ternary oxide, as shown in the inset of Fig. 5. The addition of Cr2O3 together with Nb2O5 increases formation of the ternary oxide, thereby decreasing the formation of MgO. Cr2O3 particles are also be embedded on the MgH2 surfaces during milling and can be identified even after dehydrogenation (Fig. 5(d)), indicating that Cr2O3 is not dissolved or reduced on Mg surfaces during dehydrogenation. The standard Gibbs free energy for Cr2O3 at 300 C is more positive than that for Nb2O5 [17], which is probably one reason that Cr2O3 was not reduced. This suggests that Cr2O3 facilitates the reaction of MgO and Nb2O5 to form the Mg–Nb-oxide. Furthermore, it was concluded that the addition of multiple oxides results in a synergetic effect for formation of the ternary oxide (MgNb2O3.67), which is considered to improve the kinetics and hydrogen absorption/desorption capabilities of MgH2. Fig. 6(a)–(d) is representative SEM micrographs showing the particle size and morphology of the as-received MgH2, MgH2 and those samples with metal oxides present after milling for 20 h. The particle size of MgH2 decreased after milling for 20 h, resulting in pure MgH2 particles in the range from 150 nm to 3 mm. The bright specks on the surface of MgH2 in Fig. 6(c) and (d) correspond to particles of Nb2O5 and Cr2O3. The oxide particles were embedded in the MgH2 matrix and distributed heterogeneously (Fig. 6(c)), which confirmed that substantial surface modifications occurred as a result of milling (Fig. 6(d)). Nb2O5 and Cr2O3 are hard materials, with Mohs hardness of approximately 5–6 and 8.5, respectively. It should be noted that the initial particle sizes for MgH2 and the oxides are of micrometer size. Addition of Nb2O5 and/or Cr2O3 might lead to
3035
substantial refinement of the powder particles during milling. Nb2O5 is reported to act as a lubricant, cracking agent and prevents the agglomeration and cold welding of MgH2 particles, and therefore facilitates the refinement of MgH2 particles during milling [11,19]. It is generally thought that the fast absorption/desorption kinetics of MgH2 or MgH2-oxide systems are due to the high defect surfaces of MgH2 powder [5,9,19]. Therefore, further improvement of the sorption kinetics of MgH2 is expected with refined particles. Fig. 7 shows a compilation of DSC curves for the investigated powders, compared to the DSC curve for MgH2. The DSC curves of all powders were obtained after milling and without any activation treatment. The DSC curve of MgH2 displayed a strong endothermic peak at approximately 466 C and a small endotherm at approximately 380 C. The appearance of double peaks in the DSC curve could be attributed to inhomogeneity of particle size, as finer particles are supposed to exhibit a lower temperature endotherm than coarser particles. One other possibility is that the small peak may correspond to the decomposition of a high-pressure phase of MgH2 (g-MgH2). This phase usually appears after mechanical milling processes, especially high energy ball milling. Compared to the b-MgH2 phase, this phase is metastable and therefore easily transforms to the b-MgH2 phase after annealing treatment. Because the g-MgH2 is metastable, the hydrogen desorption temperature for this phase is lower than b-MgH2. Considering to this phase, the appearance of double peaks in the DSC curve cannot be solely explained by the presence of finer and coarser fraction particles. However, it might be due to a combination of particle size inhomogeneity,
Fig. 6 – The SEM micrographs, showing the morphology and the particle size of the samples: (a) as-received MgH2; (b) milled MgH2 for 20 h; (c) milled MgH2 D 1 mol% Cr2O3 D 1 mol% Nb2O5 for 20 h; (d) milled MgH2 D 0.2 mol% Cr2O3 D 1 mol% Nb2O5 for 20 h.
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international journal of hydrogen energy 34 (2009) 3032–3037
i h ln ß=T2p ¼ EA =RTp þ ln½k0 R=EA
Fig. 7 – The DSC hydrogen desorption curves for MgH2, MgH2 D 1 mol% Cr2O3, MgH2 D 1 mol% Nb2O5, MgH2 D 0.2 mol% Cr2O3 D 1 mol% Nb2O5 and MgH2 D 1 mol% Cr2O3 D 1 mol% Nb2O5 after milling for a total of 20 h. The heating rate is 5 8C/min.
hydride phase duality and external influence like oxide addition. The hydrogen desorption temperature for MgH2 with added oxides decreases by approximately 50 C compared to that for pure MgH2. The DSC curves for the MgH2 þ 1 mol% MgH2 þ 1 mol% Nb2O5 and MgH2 þ 1 mol% Cr2O3, Cr2O3 þ 1 mol% Nb2O5 samples have strong endothermic peaks at ca. 410, 415, and 401 C, respectively. This lowering of the main endothermic peak by the addition of oxide could be due to the catalytic effect of the oxides and/or the role of the oxides as a lubricating agent for the formation of finer MgH2 particles with a high density of interfaces. MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5 had the lowest hydrogen desorption temperature compared with the other samples examined in this study. The fast kinetics observed for this composition are believed to be not only due to particle size, but also the catalytic effect of the oxides, which contributes to a significant lowering of the hydrogen desorption temperature of MgH2. Mechanical milling and the addition of oxides have obviously reduced the hydrogen desorption temperature of MgH2. Varin et al. [20] reported that refinement of the MgH2 particle size (sub-micrometer range, ca. 700–300 nm) caused a dramatic decrease in the hydrogen desorption temperature. Because we have ignored the effect of reducing the initial particle size down to sub-micrometer size, it is concluded that lowering of the DSC desorption temperature is a result of the presence of oxides. Addition of two oxides (Cr2O3/Nb2O5) decreases the hydrogen desorption temperature of MgH2 much more in this case; this combination of oxides reduced the main desorption temperature by 65 C for MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5 and by 100 C for MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5. The activation energies (EA) for dehydrogenation of MgH2, MgH2 þ 1 mol% Cr2O3, MgH2 þ 1 mol% Nb2O5, MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5 and MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5 can be estimated using the Kissinger method [21], according to the following equation:
(1)
where b is the heating rate (K min1), Tp indicates the peak temperatures (K) in the DSC curves, R is the gas constant (J K1 mol1) and k0 is the frequency factor. In this work, Tp was obtained for several heating rates (1, 5, 10, and 20 K min1). Kissinger plots, i.e., ln[b/T2p] as a function of the inverse of Tp, are shown in Fig. 8. The activation energies were estimated from the slope of the straight line to be 206 kJ mol1 for MgH2, 190 kJ mol1 for MgH2 þ 1 mol% Cr2O3, 197 kJ mol1 for MgH2 þ 1 mol% Nb2O5, 185 kJ mol1 for MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5, and 136 kJ mol1 for MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5. The Cr2O3 and activation energies for MgH2 þ 1 mol% MgH2 þ 1 mol% Nb2O5 are almost the same (ca. 190 kJ mol1), although these samples gave different results for the hydrogen desorption kinetics [Fig. 1]. Comparison of the diffraction lines for these compositions after dehydrogenation [Fig. 5(a)–(b)] demonstrates a clear difference. The appearance of Mg–Nboxide could be one reason that MgH2 þ 1 mol% Nb2O5 provides fast desorption kinetics compared with MgH2 þ 1 mol% Cr2O3. The concurrent addition of Nb2O5 and Cr2O3 to MgH2 decreased the activation energy for the hydrogen desorption process. Addition of 1 mol% Cr2O3 in MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5 composition is an excessive amount of additive and caused the hydrogen capacity to drop, as shown in Figs. 1 and 2. However, increasing the amount of Cr2O3 to 1 mol% resulted in further lowering of the activation energy. This indicates that the combination of two oxides is more effective for reduction of the activation energy. It is thought that the activation energy is not directly related to the hydrogen desorption kinetics; the activation energy is a barrier that must be overcome to start the release of hydrogen, but the speed of hydrogen release also depends on other factors, such as catalytic effects, surface area, particle size, distribution of oxides, and also the nature of the oxide added.
Fig. 8 – The Kissinger plot for: (a) MgH2; (b) MgH2 D 1 mol% Cr2O3; (c) MgH2 D 1 mol% Nb2O5; (d) MgH2 D 0.2 mol% Cr2O3 D 1 mol% Nb2O5 and (e) MgH2 D 1 mol% Cr2O3 D 1 mol% Nb2O5. The activation energies for dehydrogenation are 206 kJ molL1, 190 kJ molL1, 197 kJ molL1, 185 kJ molL1 and 136 kJ molL1, respectively.
international journal of hydrogen energy 34 (2009) 3032–3037
The formation of Mg–Nb-oxide (MgNb2O3.67) during the desorption process may be important for decreasing the activation energy and may facilitate improvement of the absorption/desorption kinetics. Recently, the role of Mg–Nboxide (MgNb2O3.67) as a reversible uptake of molecular hydrogen has been reported by Dolci et al. [22]. The Mg–Nboxide (MgNb2O3.67) is, in fact, active in absorption and desorption of molecular hydrogen. Furthermore, refinement of the MgH2 powder to nanometer size combined with the addition of oxides and the generation of surface defects are expected to improve the kinetics as well as hydrogen sorption capability. There are also other factors, such as the nature and local electronic structure of the added oxides, which would affect the introduction of defects on the surface of MgH2 particles. Oxide addition could also result in the generation of diffusion paths that would affect the absorption/desorption kinetics. Further investigation (e.g., TEM analyses) of these factors is required to clarify the role of oxide addition on the increased absorption/desorption kinetics of MgH2.
4.
Conclusion
The hydrogen desorption kinetics and the absorption/ desorption capability of MgH2 were further improved by the addition of multiple oxides (Cr2O3 and Nb2O5) over that by single addition of oxide, even if the size of the milled MgH2 particles is in the micrometer range. The activation energy for hydrogen desorption was also decreased from 206 kJ mol1 (MgH2 without oxide) to 136 kJ mol1 by the addition of multiple oxides. Subsequent dehydrogenation processes led to the formation of Mg–Nb-oxide (MgNb2O3.67), and the addition of Cr2O3 supported the formation of the Mg–Nb-oxide (MgNb2O3.67), which is strongly related to improvement of the hydrogen desorption kinetics and hydrogen capability of MgH2.
Acknowledgements This work was partially supported by the European Commission (project Dev-BIOSOFC, FP6-042436, MTKD-CT-2006042436).
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Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Influence of multiple oxide (Cr2O3/Nb2O5) addition on the sorption kinetics of MgH2 Aep Pataha,*, Akito Takasakia, Janusz S. Szmydb a
Division of Regional Environment Systems, Graduate School of Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan b AGH – University of Science and Technology, Al. Mickiewicza 30, Krakow 30-059, Poland
article info
abstract
Article history:
The influence of multiple additions of two oxides, Cr2O3 and Nb2O5, as additives on the
Received 4 August 2008
hydrogen sorption kinetics of MgH2 after milling was investigated. We found that the
Received in revised form
desorption kinetics of MgH2 were improved more by multiple oxide addition than by single
10 December 2008
addition. Even for the milled MgH2 micrometric size powders, the high hydrogen capacity
Accepted 30 January 2009
with fast kinetics were achieved for the powders after addition of 0.2 mol% Cr2O3 þ 1 mol%
Available online 25 February 2009
Nb2O5. For this composition, the hydride desorbed about 5 wt.% hydrogen within 20 min and absorbed about 6 wt.% in 5 min at 300 C. Furthermore, the desorption temperature
Keywords:
was decreased by 100 C, compared to MgH2 without any oxide addition, and the activation
Hydrogen sorption
energy for the hydrogen desorption was estimated to be about 185 kJ mol1, while that for
Magnesium hydride
MgH2 without oxide was about 206 kJ mol1.
Mechanical milling
ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Kinetics Metal oxides
1.
Introduction
Magnesium hydride (MgH2) is one of the most promising candidates for hydrogen storage materials, due to its high storage capacity with a theoretical value of 7.6 wt.%. However, the practical use of MgH2 for hydrogen storage is limited, due to its high thermodynamic stability (DH ¼ 74.5 kJ mol1) and slow desorption hydrogen kinetics [1–4]. Recently, many efforts such as the manufacture of nanocrystalline powder [5], addition of metal oxides as catalysts [6,7], or synthesis of composite metal hydrides [8] have been attempted to improve the sorption properties. Of these, the addition of transition metal oxides to MgH2 by mechanical milling seems to be one of the most promising methods for improvement of the hydrogen sorption properties. With respect to transition metal oxides, Nb2O5 and Cr2O3 have recently attracted considerable
interest [9–14]. Friedrichs et al. [13] reported that milling of MgH2 with 10 wt.% (ca. 1.09 mol%) of nanosized Nb2O5 powder resulted in absorption and desorption of hydrogen within a few minutes, through the intimate contact between MgH2 and Nb2O5. Hanada et al. [14] reported that 1 mol% mesoporous Nb2O5-catalyzed MgH2 exhibited the most superior hydrogen desorption properties. Addition of nanosized Cr2O3 is reported to enhance the thermal stability of the MgH2 microstructure, as well as impart stability for prolonged cycling [10]. It was also reported that addition of 5 wt.% Cr2O3 improved the absorption/desorption reaction rate of MgH2; addition of nanosized Cr2O3 increased the diffusion coefficient of hydrogen by one order of magnitude than that for addition of microsized Cr2O3 [9]. Furthermore Yao et al. [15] used an analytical model for hydrogen diffusion and showed that the practicality of using Mg could be enhanced by reducing the
* Corresponding author. Tel.: þ81 03 5859 8059; fax: þ81 03 5859 8001. E-mail addresses: [email protected] (A. Patah), [email protected] (A. Takasaki), [email protected] (J.S. Szmyd). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.01.086
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grain size to nanoscale. However, not all transition metal oxides could be adapted as additives and the effect of each oxide, whose physical or chemical roles may also be different, also depended on the mechanical milling conditions [11]. Most of the research regarding the effect of oxide addition has dealt with one oxide and optimization of the amount of oxide. Our previous report [16] was the first to suggest that the addition of two oxides (Cr2O3/Nb2O5 and ZnO/Nb2O5) could improve the hydrogen absorption/desorption ability of MgH2 more than the addition of a single oxide. This implies that a synergetic effect between those oxides and MgH2 may occur. In the present study, the influence of multiple additions of two oxides, Cr2O3 and Nb2O5, on the hydrogen sorption kinetics of MgH2 was investigated in more detail. In particular, the influence of the total amount of concurrent Cr2O3 and Nb2O5 addition was examined. The sorption kinetics, activation energy for hydrogen desorption, and the morphology changes of the hydride with and without addition of multiple oxides were compared.
2.
Experimental
The mechanical milling was performed using a Fritsch P7 planetary ball mill with a stainless steel vial and 18 stainless steel balls (8 mm diameter). The ball-to-powder mass ratio was 10:1. MgH2 powder (98% purity, balance Mg) with a mean particle size of approximately 70 mm was purchased from Alfa Aesar, Johnson Matthey Company. Cr2O3 and Nb2O5, with a mean particle size of 47 mm, were purchased from Furuuchi Chemical Company and used as milling additives. MgH2, with and without oxides, was milled for a total of 20 h at a rotational speed of 400 rpm under a high-purity (99.999%) argon atmosphere. Hydrogen storage properties of the samples were examined using an automatic Sievert-type apparatus manufactured by Japan Metals & Chemicals Company, in which pressure and temperature data were simultaneously collected by a computer. The mass of the sample for each measurement was approximately 200 mg. The measurements were performed at 300 C with a hydrogen pressure of 1 MPa for absorption and 1 kPa for desorption. The hydride samples were initially dehydrided for 20 h at 300 C in a vacuum prior to the measurements. Crystallographic characterization was performed using X-ray diffraction (XRD; Jeol JDX-8030) with Cu ˚ ). The morphology of the samples Ka radiation (l ¼ 1.5418 A was observed with a scanning electron microscope (SEM; Jeol JSM-7400F). The dehydrogenation performance of the milled MgH2 was examined by differential scanning calorimetry (DSC; Shimazu DSC-60) under a high-purity Ar gas flow (50 mL min1) at several heating rates.
3.
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Fig. 1 – The amount of the hydrogen desorbed as a function of reaction time at 300 8C under 1 kPa H2.
respectively. It was obvious that the addition of Nb2O5 improved the hydrogen desorption kinetics of MgH2 more than Cr2O3. Furthermore, the hydrogen desorption kinetics of MgH2 are much more improved by the concurrent addition of Nb2O5 with Cr2O3. A small addition of Cr2O3 (0.2 mol%) could improve the hydrogen desorption kinetics up to 5.1 wt.% in 20 min, as shown for the MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5 composition. Increasing the amount of Cr2O3 up to 1 mol% decreased the hydrogen capacity as well as the hydrogen desorption kinetics; the MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5 composition desorbed 4.3 wt.% of hydrogen in 20 min. This indicates that the hydrogen desorption capacity is not dependent only on the type of oxides, but also the total amount of oxides added. However, for the combined addition of Nb2O5 and Cr2O3, the hydrogen desorption kinetics were significantly improved. Although the slopes at the beginning of the absorption curves were almost the same (Fig. 2), which is related to the rate limiting step, the hydrogen absorbed for a single oxide addition was less than that for addition of two oxides. MgH2 and MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5 absorb 5.3 wt.%
Results and Discussion
Fig. 1 shows the desorption kinetics of MgH2 at 300 C with and without metal oxides. The addition of Nb2O5 increases the desorption kinetics of MgH2 as shown for the MgH2 þ 1 mol% Cr2O3 and MgH2 þ 1 mol% Nb2O5 compositions, which desorbed 1.5 wt.% and 3.6 wt.% of hydrogen in 20 min,
Fig. 2 – The amount of the hydrogen absorbed as a function of reaction time at 300 8C under 1 MPa H2.
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hydrogen in 5 min, and MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5 showed the highest (6.0 wt.%) and fastest (5 min) hydrogen adsorption capacity, indicating that the addition of two oxides plays an important role in improving the hydrogen absorption. It also suggests that the hydrogen capacities of the samples are related to the total amount and type of oxides added. Fig. 3 shows the theoretical hydrogen capacity, hydrogen absorption in 5 min and the hydrogen desorption in 20 min for each of the samples. The adsorption/desorption times were selected to reflect the onset of curve to plateau. MgH2 þ 1 mol% Cr2O3, XRD patterns of MgH2, MgH2 þ 1 mol% Nb2O5, MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5, and MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5 after milling and those after subsequent dehydrogenation at 300 C are shown in Figs. 4 and 5, respectively. The XRD patterns of all samples after milling had broad diffraction lines, indicating grain refinement due to high energy ball milling, which usually occurs when crystals are refined by mechanical milling processes. In all cases, the most intense diffraction line was identified as tetragonal b-MgH2, and a metastable orthorhombic phase g-MgH2 was also observed. On the other hand, the most intense diffraction line for the dehydrogenated sample was Mg, as shown in Fig. 5, which transforms again into b-MgH2 after hydrogenation. Small amounts of MgO were also identified after milling, as well as after dehydrogenation, which is probably due to oxygen contamination during sample preparation. For the MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5 sample shown in Fig. 5, the intense diffraction line of Nb2O5 almost disappeared after dehydrogenation, even though it was still observed after milling (Fig. 4). In contrast, the diffraction line for Cr2O3 was still identified even after dehydrogenation, suggesting that no reaction occurred between Cr2O3 and MgH2 during dehydrogenation, which was also reported by Oelerich et al. [6]. Closer examination of the MgO diffraction line in the inset of Fig. 5 reveals the formation of a shoulder on the left side of the MgO peak, which is probably due to a ternary magnesium–
Fig. 3 – Comparison of theoretical H2 capacity and experimental results for absorption/desorption of MgH2 with and without additives. Samples named are: (A) MgH2; (B) MgH2 D 1 mol% Cr2O3; (C) MgH2 D 1 mol% Nb2O5; (D) MgH2 D 0.2 mol% Cr2O3 D 1 mol% Nb2O5 and (E) MgH2 D 1 mol% Cr2O3 D 1 mol% Nb2O5.
Fig. 4 – The XRD patterns of: (a) MgH2, (b) MgH2 D 1 mol% Cr2O3, (c) MgH2 D 1 mol% Nb2O5, (d) MgH2 D 0.2 mol% Cr2O3 D 1 mol% Nb2O5, (e) MgH2 D 1 mol% Cr2O3 D 1 mol% Nb2O5 after milling for a total of 20 h.
niobium-oxide (Mg–Nb-oxide). It is assumed that Nb2O5 particles are embedded in the MgH2 particle surfaces after milling, and some Nb2O5 is dissolved in the Mg matrix during the dehydriding process. Dehydrogenation changes MgH2 into Mg and some Mg may react with oxygen to form MgO, which is required as a precursor for the formation of the Mg–Nb-oxide. Oxygen could be produced from reduction of Nb2O5 or trapped-oxygen gas to complete the reaction and produce the ternary oxide. These reactions are spontaneous reactions that are related to the standard Gibbs free energy (DG0) for both oxides. The standard Gibbs free energy for MgO formation at 300 C is more negative than that for Nb2O5 [17]. The intense diffraction line for the ternary Mg–Nb-oxide shown in Fig. 5 is quite similar to the MgNb2O3.67 oxide (JCPDS database, Powder Diffraction File #25–052), and the same result was reported by Friedrichs et al. [18]. This ternary oxide possibly has a spinel or perovskite structure.
Fig. 5 – The XRD patterns of: (a) MgH2 D 1 mol% Cr2O3, (b) MgH2 D 1 mol% Nb2O5, (c) MgH2 D 0.2 mol% Cr2O3 D 1 mol% Nb2O5, (d) MgH2 D 1 mol% Cr2O3 D 1 mol% Nb2O5 after milling for a total of 20 h and subsequently dehydrogenated (1 kPa, 300 8C).
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The presence of Cr2O3 assists in the formation of the ternary oxide, as shown in the inset of Fig. 5. The addition of Cr2O3 together with Nb2O5 increases formation of the ternary oxide, thereby decreasing the formation of MgO. Cr2O3 particles are also be embedded on the MgH2 surfaces during milling and can be identified even after dehydrogenation (Fig. 5(d)), indicating that Cr2O3 is not dissolved or reduced on Mg surfaces during dehydrogenation. The standard Gibbs free energy for Cr2O3 at 300 C is more positive than that for Nb2O5 [17], which is probably one reason that Cr2O3 was not reduced. This suggests that Cr2O3 facilitates the reaction of MgO and Nb2O5 to form the Mg–Nb-oxide. Furthermore, it was concluded that the addition of multiple oxides results in a synergetic effect for formation of the ternary oxide (MgNb2O3.67), which is considered to improve the kinetics and hydrogen absorption/desorption capabilities of MgH2. Fig. 6(a)–(d) is representative SEM micrographs showing the particle size and morphology of the as-received MgH2, MgH2 and those samples with metal oxides present after milling for 20 h. The particle size of MgH2 decreased after milling for 20 h, resulting in pure MgH2 particles in the range from 150 nm to 3 mm. The bright specks on the surface of MgH2 in Fig. 6(c) and (d) correspond to particles of Nb2O5 and Cr2O3. The oxide particles were embedded in the MgH2 matrix and distributed heterogeneously (Fig. 6(c)), which confirmed that substantial surface modifications occurred as a result of milling (Fig. 6(d)). Nb2O5 and Cr2O3 are hard materials, with Mohs hardness of approximately 5–6 and 8.5, respectively. It should be noted that the initial particle sizes for MgH2 and the oxides are of micrometer size. Addition of Nb2O5 and/or Cr2O3 might lead to
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substantial refinement of the powder particles during milling. Nb2O5 is reported to act as a lubricant, cracking agent and prevents the agglomeration and cold welding of MgH2 particles, and therefore facilitates the refinement of MgH2 particles during milling [11,19]. It is generally thought that the fast absorption/desorption kinetics of MgH2 or MgH2-oxide systems are due to the high defect surfaces of MgH2 powder [5,9,19]. Therefore, further improvement of the sorption kinetics of MgH2 is expected with refined particles. Fig. 7 shows a compilation of DSC curves for the investigated powders, compared to the DSC curve for MgH2. The DSC curves of all powders were obtained after milling and without any activation treatment. The DSC curve of MgH2 displayed a strong endothermic peak at approximately 466 C and a small endotherm at approximately 380 C. The appearance of double peaks in the DSC curve could be attributed to inhomogeneity of particle size, as finer particles are supposed to exhibit a lower temperature endotherm than coarser particles. One other possibility is that the small peak may correspond to the decomposition of a high-pressure phase of MgH2 (g-MgH2). This phase usually appears after mechanical milling processes, especially high energy ball milling. Compared to the b-MgH2 phase, this phase is metastable and therefore easily transforms to the b-MgH2 phase after annealing treatment. Because the g-MgH2 is metastable, the hydrogen desorption temperature for this phase is lower than b-MgH2. Considering to this phase, the appearance of double peaks in the DSC curve cannot be solely explained by the presence of finer and coarser fraction particles. However, it might be due to a combination of particle size inhomogeneity,
Fig. 6 – The SEM micrographs, showing the morphology and the particle size of the samples: (a) as-received MgH2; (b) milled MgH2 for 20 h; (c) milled MgH2 D 1 mol% Cr2O3 D 1 mol% Nb2O5 for 20 h; (d) milled MgH2 D 0.2 mol% Cr2O3 D 1 mol% Nb2O5 for 20 h.
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i h ln ß=T2p ¼ EA =RTp þ ln½k0 R=EA
Fig. 7 – The DSC hydrogen desorption curves for MgH2, MgH2 D 1 mol% Cr2O3, MgH2 D 1 mol% Nb2O5, MgH2 D 0.2 mol% Cr2O3 D 1 mol% Nb2O5 and MgH2 D 1 mol% Cr2O3 D 1 mol% Nb2O5 after milling for a total of 20 h. The heating rate is 5 8C/min.
hydride phase duality and external influence like oxide addition. The hydrogen desorption temperature for MgH2 with added oxides decreases by approximately 50 C compared to that for pure MgH2. The DSC curves for the MgH2 þ 1 mol% MgH2 þ 1 mol% Nb2O5 and MgH2 þ 1 mol% Cr2O3, Cr2O3 þ 1 mol% Nb2O5 samples have strong endothermic peaks at ca. 410, 415, and 401 C, respectively. This lowering of the main endothermic peak by the addition of oxide could be due to the catalytic effect of the oxides and/or the role of the oxides as a lubricating agent for the formation of finer MgH2 particles with a high density of interfaces. MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5 had the lowest hydrogen desorption temperature compared with the other samples examined in this study. The fast kinetics observed for this composition are believed to be not only due to particle size, but also the catalytic effect of the oxides, which contributes to a significant lowering of the hydrogen desorption temperature of MgH2. Mechanical milling and the addition of oxides have obviously reduced the hydrogen desorption temperature of MgH2. Varin et al. [20] reported that refinement of the MgH2 particle size (sub-micrometer range, ca. 700–300 nm) caused a dramatic decrease in the hydrogen desorption temperature. Because we have ignored the effect of reducing the initial particle size down to sub-micrometer size, it is concluded that lowering of the DSC desorption temperature is a result of the presence of oxides. Addition of two oxides (Cr2O3/Nb2O5) decreases the hydrogen desorption temperature of MgH2 much more in this case; this combination of oxides reduced the main desorption temperature by 65 C for MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5 and by 100 C for MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5. The activation energies (EA) for dehydrogenation of MgH2, MgH2 þ 1 mol% Cr2O3, MgH2 þ 1 mol% Nb2O5, MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5 and MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5 can be estimated using the Kissinger method [21], according to the following equation:
(1)
where b is the heating rate (K min1), Tp indicates the peak temperatures (K) in the DSC curves, R is the gas constant (J K1 mol1) and k0 is the frequency factor. In this work, Tp was obtained for several heating rates (1, 5, 10, and 20 K min1). Kissinger plots, i.e., ln[b/T2p] as a function of the inverse of Tp, are shown in Fig. 8. The activation energies were estimated from the slope of the straight line to be 206 kJ mol1 for MgH2, 190 kJ mol1 for MgH2 þ 1 mol% Cr2O3, 197 kJ mol1 for MgH2 þ 1 mol% Nb2O5, 185 kJ mol1 for MgH2 þ 0.2 mol% Cr2O3 þ 1 mol% Nb2O5, and 136 kJ mol1 for MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5. The Cr2O3 and activation energies for MgH2 þ 1 mol% MgH2 þ 1 mol% Nb2O5 are almost the same (ca. 190 kJ mol1), although these samples gave different results for the hydrogen desorption kinetics [Fig. 1]. Comparison of the diffraction lines for these compositions after dehydrogenation [Fig. 5(a)–(b)] demonstrates a clear difference. The appearance of Mg–Nboxide could be one reason that MgH2 þ 1 mol% Nb2O5 provides fast desorption kinetics compared with MgH2 þ 1 mol% Cr2O3. The concurrent addition of Nb2O5 and Cr2O3 to MgH2 decreased the activation energy for the hydrogen desorption process. Addition of 1 mol% Cr2O3 in MgH2 þ 1 mol% Cr2O3 þ 1 mol% Nb2O5 composition is an excessive amount of additive and caused the hydrogen capacity to drop, as shown in Figs. 1 and 2. However, increasing the amount of Cr2O3 to 1 mol% resulted in further lowering of the activation energy. This indicates that the combination of two oxides is more effective for reduction of the activation energy. It is thought that the activation energy is not directly related to the hydrogen desorption kinetics; the activation energy is a barrier that must be overcome to start the release of hydrogen, but the speed of hydrogen release also depends on other factors, such as catalytic effects, surface area, particle size, distribution of oxides, and also the nature of the oxide added.
Fig. 8 – The Kissinger plot for: (a) MgH2; (b) MgH2 D 1 mol% Cr2O3; (c) MgH2 D 1 mol% Nb2O5; (d) MgH2 D 0.2 mol% Cr2O3 D 1 mol% Nb2O5 and (e) MgH2 D 1 mol% Cr2O3 D 1 mol% Nb2O5. The activation energies for dehydrogenation are 206 kJ molL1, 190 kJ molL1, 197 kJ molL1, 185 kJ molL1 and 136 kJ molL1, respectively.
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The formation of Mg–Nb-oxide (MgNb2O3.67) during the desorption process may be important for decreasing the activation energy and may facilitate improvement of the absorption/desorption kinetics. Recently, the role of Mg–Nboxide (MgNb2O3.67) as a reversible uptake of molecular hydrogen has been reported by Dolci et al. [22]. The Mg–Nboxide (MgNb2O3.67) is, in fact, active in absorption and desorption of molecular hydrogen. Furthermore, refinement of the MgH2 powder to nanometer size combined with the addition of oxides and the generation of surface defects are expected to improve the kinetics as well as hydrogen sorption capability. There are also other factors, such as the nature and local electronic structure of the added oxides, which would affect the introduction of defects on the surface of MgH2 particles. Oxide addition could also result in the generation of diffusion paths that would affect the absorption/desorption kinetics. Further investigation (e.g., TEM analyses) of these factors is required to clarify the role of oxide addition on the increased absorption/desorption kinetics of MgH2.
4.
Conclusion
The hydrogen desorption kinetics and the absorption/ desorption capability of MgH2 were further improved by the addition of multiple oxides (Cr2O3 and Nb2O5) over that by single addition of oxide, even if the size of the milled MgH2 particles is in the micrometer range. The activation energy for hydrogen desorption was also decreased from 206 kJ mol1 (MgH2 without oxide) to 136 kJ mol1 by the addition of multiple oxides. Subsequent dehydrogenation processes led to the formation of Mg–Nb-oxide (MgNb2O3.67), and the addition of Cr2O3 supported the formation of the Mg–Nb-oxide (MgNb2O3.67), which is strongly related to improvement of the hydrogen desorption kinetics and hydrogen capability of MgH2.
Acknowledgements This work was partially supported by the European Commission (project Dev-BIOSOFC, FP6-042436, MTKD-CT-2006042436).
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