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Mg2FeH6–LiBH4 and Mg2FeH6–LiNH2 composite materials for hydrogen storage
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Mg2FeH6eLiBH4 and Mg2FeH6eLiNH2 composite materials for hydrogen storage Henrietta W. Langmi a, G. Sean McGrady a,*, Rebecca Newhouse b, Ewa Ro¨nnebro c,1 a
Department of Chemistry, University of New Brunswick, PO Box 4400, Fredericton, NB E3B 5A3, Canada University of California, Santa Cruz, 1156 High St., Santa Cruz, CA, USA c Sandia National Laboratories, 7011 East Ave, Livermore, CA, USA b
article info
abstract
Article history:
Two composite hydrogen storage materials based on Mg2FeH6 were investigated for the
Received 12 September 2011
first time. The Mg2FeH6eLiBH4 composite of molar ratio 1:5 showed a hydrogen desorption
Received in revised form
capacity of 5.6 wt.% at 370 C, and could be rehydrogenated to 3.6 wt.% with the formation
3 January 2012
of MgH2, as the material was heated to 445
Accepted 6 January 2012
Mg2FeH6eLiNH2 composite of 3:10 molar ratio exhibited a hydrogen desorption capacity of
Available online 2 February 2012
4.3 wt.% and released hydrogen at 100 C lower then the Mg2FeH6eLiBH4 composite, but
Keywords:
show enhanced hydrogen storage properties in terms of desorption kinetics and capacity at
Hydrogen storage
these low temperatures. In particular, Mg2FeH6eLiNH2 exhibits a much lower desorption
C and held at this temperature. The
this mixture could not be rehydrogenated. Compared to neat Mg2FeH6, both composites
Composite hydride
temperature than neat Mg2FeH6, but only Mg2FeH6eLiBH4 re-absorbs hydrogen.
Transition metal hydride
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
According to the U.S. Department of Energy, ‘Hydrogen storage is a key enabling technology for the advancement of hydrogen and fuel cell power technologies in transportation, stationary, and portable applications’ [1]. Over the past two decades, a multitude of materials of various types have been investigated, in the quest for a hydrogen storage system with appropriate thermodynamics and rapid kinetics. In the field of metal hydrides, early work focused on interstitial-type transition metal hydrides [2], with subsequent activity concentrating on complex transition metal hydrides [3]. The complex hydrides of the lighter main group metals exhibit the highest hydrogen storage capacities, and after the discovery that Ticatalyzed NaAlH4 is reversible at moderate temperatures [4], research efforts have focused on alanates, borohydrides and
amides [5]. These materials operate at high temperatures above 150 C, and their kinetics are poor. By mixing borohydrides with binary hydrides, it has been shown that the operating temperature can be lowered significantly through formation of a so-called destabilized hydride or a reactive hydride composite [6e10]. However, mixtures of two or more complex hydrides to achieve this effect have not been reported. Here we describe the investigation of the composites formed between an Mg-based complex transition metal hydride and a borohydride or an amide, with the aim of enhancing the hydrogen storage properties of the individual components. The complex transition metal hydrides Mg2NiH4, Mg2CoH5 and Mg2FeH6 have been known for several decades [11e13]. These systems have theoretical hydrogen storage capacities of 3.6, 4.5, and 5.5 wt.% H, respectively. Amongst all complex
* Corresponding author. E-mail address: [email protected] (G.S. McGrady). 1 Current address: Pacific Northwest National Laboratory, 902 Battelle Blvd., Richland, WA 99352, USA. 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.01.020
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hydrides Mg2FeH6 has the highest known volumetric hydrogen density of 150 kg m3. It has also been reported that Mg2FeH6 has high volumetric and gravimetric thermal energy densities and excellent stability on cycling, among other attractive features, making it highly suitable for thermochemical thermal energy storage at about 500 C [14,15]. Synthesis of Mg2FeH6 is difficult, owing to the absence of an Mg2Fe intermetallic phase in the binary MgeFe system [16]. Didisheim et al. first prepared Mg2FeH6 by sintering mixtures of the elemental powders at 500 C and 20e120 bar H2 [13]. In addition to sintering, mechanical alloying and reactive mechanical milling have been used to synthesize Mg2FeH6 [17]. The yield of Mg2FeH6 is dependent on the starting materials and the conditions employed during synthesis. It is difficult to obtain a pure sample; unreacted Fe is typically present together with Mg2FeH6 [17,18], and MgH2, MgO and Mg also sometimes appear in the synthesized material [19]. Mg2FeH6 has been reported to reversibly store ca. 5.5 wt.% H at around 500 C over 1100 cycles [14,15]. Below 300 C, this system has also been shown to desorb 5.2 wt.% H in dynamic vacuum [20]. However, it re-absorbs only 3 wt.% H at 10 bar and 300 C, with the formation of MgH2. Decomposition of Mg2FeH6 occurs in a single step, to produce elemental Mg, Fe, and H2 [17,18]. However, in the reverse reaction, MgH2 is generated first and this then reacts further with Fe and H2 to form Mg2FeH6 [21,22]. Hydrogen absorptionedesorption by ternary lanthanide iron borides such as Nd2Fe14B is exploited in the processing of powders used to prepare powerful lanthanide-based magnetic materials [23,24]. Recently, it was reported that a ternary magnesium nickel boride, MgNi2.5B2, can be hydrogenated reversibly in combination with LiH and MgH2, forming Mg2NiH4 and LiBH4 below 300 C, although the observed hydrogen capacity was only 1 wt.% [25]. In essence, Mg2NiH4eLiBH4 is a kinetically coupled destabilized system that forms a ternary boride during dehydrogenation [26]. Although Mg2NiH4 has been extensively studied as a hydrogen storage material, it suffers from sluggish absorption kinetics and a high temperature for hydrogen absorptionedesorption [27]. However, production of Mg2NiH4 in a nanocrystalline form or modification with platinum group metals enhances the hydrogen storage characteristics [28,29]. Effectively, Mg2NiH4 is MgH2 destabilized with Ni: Mg2Ni is more thermodynamically stable than Mg metal. Destabilization of the complex transition metal hydrides themselves appears to be a completely overlooked approach in the field of hydrogen storage. Drawing on the recent approaches demonstrated by Vajo, Jensen and others [6e10], we set out to explore the properties of the reactive hydride composites formed between Mg2FeH6 and the high hydrogen content materials LiBH4 and LiNH2, as described in Equations (1) and (2). The anticipated decomposition products from a Mg2FeH6/LiBH4 mixture at 300e500 C are the stable phases LiH and MgB2, and the ironeboron phase FeB; Equation (1). Although Fe2B can also be formed by these two elements, this outcome is unlikely at the temperatures under consideration [30]. As regards Equation (2), Mg3N2 is a well characterized material, and the ternary nitride, Li3FeN2, is a stable phase that can be synthesized from the binary nitrides, and which has been reported to store hydrogen reversibly [31]. Thus, mixing Mg2FeH6 with LiBH4 and LiNH2 in the proportions
6695
described in Equations (1) and (2) offers the prospect of stable dehydrogenation products for each of the elements N, B, Mg and Fe, and should result in activation of both the main group and the transition metal hydride components of the mixture, resulting in the release of substantial amounts of hydrogen at reduced temperatures. Mg2FeH6 þ 5LiBH4 / 2MgB2 þ FeB þ 5LiH þ 10.5H2 (9.6 wt.% H)
(1)
3Mg2FeH6 þ 10LiNH2 / 3Li3FeN2 þ 2Mg3N2 þ LiH þ 18.5H2 (6.7 wt.% H) (2) There is a range of potential complications with these composite systems, such as poor reversibility, slow kinetics, and the formation of metastable intermediates. However, as the approach was completely unexplored for Mg2FeH6, and offers the prospect of some novel and unique systems with high hydrogen storage capacities, we investigated in some detail the Mg2FeH6eLiBH4 and Mg2FeH6eLiNH2 systems described in Equations (1) and (2).
2.
Experimental
Mg2FeH6 was prepared at Sandia National Laboratory in ca. 80% yield by a combination of high-energy ball milling and high-pressure sintering, according to the following procedure. The starting materials MgH2 (Pfaltz & Bauer, Inc.; 95%) and Fe (Alfa Aesar, spherical powder < 10 microns; 99.5%) were ball milled for 10 h in a molar ratio of 2.5: 1 under Ar in a SPEX8000 high-energy ball mill using 2 WC- (Spex SamplePrep) of 9 g each for 4 g of sample powder in a WC mill pot (Spex SamplePrep). The Fe/MgH2 mixture became a fine black powder after milling. The milled powders were pressed into pellets of diameters of 13 mm and placed in stainless steel crucibles, further enclosed in an AE Closure (Snap-tite, Inc.), which is a self-sealing vessel of standard material A-286 and nominal capacity 100 mL that utilizes internal pressure to tighten the seal. The samples were sintered at 320e370 C under ca. 700 bar H2 over a period of 4 days. After high-pressure treatment, the material was olive green in colour. All sample handling was performed in an Ar-filled glove box with less than 1 ppm O2 and H2O. Mg2FeH6 was combined with either LiBH4 (Strem; 95%) in a 1:5 ratio, or with LiNH2 (Aldrich; 95%) in a 3:10 ratio according to Eqs. (1) and (2), and milled using a Retsch planetary mill PM100. The ball-to-powder ratio was 23:1. The samples were milled for 2 h at 100 rpm in a 50 mL WC-vessel containing 3 WC-balls of diameter 10 mm. Powder XRD patterns were collected using a Bruker D8 Advance diffractometer (CuKa radiation). The samples were mounted in a plastic holder and covered with parafilm to protect them from contact with air during the measurements. The parafilm resulted in additional diffraction peaks at ca. 21.6 and 24 . Thermal analysis was carried out by differential scanning calorimetry (DSC) using a TA Instruments Q20P DSC. About 9 mg of material was used for the measurements. The as-prepared materials Mg2FeH6eLiBH4 and Mg2FeH6eLiNH2 were heated at
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3.
Results and discussion
3.1.
Mg2FeH6eLiBH4 composite material
According to its powder XRD pattern, as-prepared Mg2FeH6 contained unreacted iron (Fig. 1). After milling a mixture of Mg2FeH6 and LiBH4 (molar ratio 1:5), the XRD pattern of the composite material showed diffraction peaks characteristic of both Mg2FeH6 and LiBH4, thus indicating no chemical reaction. Upon dehydrogenation on the Sievert’s-type instrument no clear diffraction peaks were observed, indicating that the products may be glassy or amorphous. However, after rehydrogenation on the Sievert’s-type instrument by heating the dehydrogenated products from room temperature to 445 C (at an initial H2 pressure of 30 bar) and holding the sample at 445 C for 18 h, MgH2 was formed, as evidenced by the distinct diffraction peaks alongside two broad reflections at 42.1e45.5 and 61.4e64.4 , suggesting that the hydrogenated material is amorphous to some extent. Analysis of the as-milled composite material using DSC was carried out in a flowing stream of N2 (Fig. 2a). Several endothermic events were observed in the DSC plot for Mg2FeH6eLiBH4. The events occurring at ca. 112 and 286 C are assigned to the orthorhombic-to-hexagonal phase transition and melting of LiBH4, respectively [32]. This is in agreement with the existence of LiBH4 diffraction peaks in the XRD pattern of the as-milled material. The other endothermic peaks, at ca. 367 and 410 C, are assigned to the release of
Fig. 1 e Powder XRD patterns for (a) Mg2FeH6, (b) Mg2FeH6eLiBH4 before dehydrogenation, (c) Mg2FeH6eLiBH4 after dehydrogenation, and (d) Mg2FeH6eLiBH4 after rehydrogenation.
1.0
Heat Flow (W/g)
1 C min1 to 500 C in a flow of N2. Meanwhile, the dehydrogenated materials were subjected to an initial hydrogen pressure of 30 bar and heated at 5 C min1 to 500 C. Hydrogen sorption measurements were performed using a Sievert’s-type instrument (HyEnergy PCTPro-2000). In each experiment, ca. 0.5 g sample was heated at 5 C min1 and then held for 24 h at 370 C while the desorption curve was measured. For absorption experiments 30 bar initial pressure of high purity H2 (Air Liquide, 99.999%) was employed.
0.5 0.0 -0.5 -1.0
b
-1.5 0.8
Heat Flow (W/g)
6696
0.0 -0.8 -1.6
a
Exo 50
100
150
200
250
300
350
400
450
500
o
Temperature ( C) Fig. 2 e DSC plots of (a) hydrogen desorption from as-milled Mg2FeH6eLiBH4, and (b) hydrogen absorption by dehydrogenated Mg2FeH6eLiBH4.
hydrogen from Mg2FeH6 and LiBH4, respectively. Hence, hydrogen desorption from Mg2FeH6eLiBH4 appears to be a two-step process, analogous to the behaviour reported for the MgH2eLiBH4 system [33]. Equation (1) can therefore be rewritten in two parts: Mg2FeH6 þ 5LiBH4 / 2Mg þ Fe þ 5LiBH4 þ 3H2
(1a)
2Mg þ Fe þ 5LiBH4 þ 3H2 / 2MgB2 þ FeB þ 5LiH þ 10.5H2
(1b)
Powder XRD analysis revealed that the products from Equation (1b) exist in an amorphous state and the true composition of the products could therefore not be verified (Fig. 1). DSC experiments under H2 pressure were carried out on the composite material after desorption at 370 C (Fig. 2b). Endothermic peaks corresponding to the orthorhombic-tohexagonal phase transition and melting of LiBH4 were again observed. This indicates that residual LiBH4 was still present in the dehydrogenated material, although this was not evident in the XRD pattern; this unreacted LiBH4 partly accounts for the lower-than-theoretical hydrogen desorption capacity observed. An endothermic peak with a maximum at 434 C was also observed in the DSC plot. This peak is likely due to the decomposition of residual Mg2FeH6. The sharp peak at 458 C might be associated with the alloying of Mg (from decomposition of Mg2FeH6) with Al (from Al pan used in DSC) [34]. The thermal event occurring at 466 C is most likely associated with the decomposition of destabilized LiH e a product of reaction (1). Desorption experiments were carried out at an initial pressure of zero bar using the PCT apparatus (Fig. 3). An amount of gas corresponding to 5.6 wt.% H was desorbed after maintaining the sample at 370 C for 24 h, although the
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rehydrogenation of Mg2FeH6eLiBH4 proceeds with the formation of MgH2, as has been reported for neat Mg2FeH6 [21,22].
380
b
5
340 300
c
4
260 220
3
180
2
140
a
1
Temperature (ºC)
Hydrogen desorption (wt.%)
6
100 60
0
20 0
5
10
15
20
25
Time (h)
Fig. 3 e Hydrogen desorption plots for as-milled (a) Mg2FeH6, (b) Mg2FeH6eLiBH4, and (c) Mg2FeH6eLiNH2.
possibility that this contained a vestigial amount of B2H6 cannot be ignored [5]. The amount of hydrogen desorbed is less than the theoretical amount (9.6 wt.%), which can be partly accounted for by the presence of Fe in the Mg2FeH6 starting material, and partly by the presence of unreacted LiBH4 (Fig. 2b). Nevertheless, when compared with milled Mg2FeH6, the composite material released more hydrogen overall, and with a lower onset temperature. Hydrogen release by Mg2FeH6 began after 82 min at 370 C (Fig. 3a). A total of 1.6 wt.% H was desorbed after holding the sample at this temperature for 12 h. The hydrogen desorption properties of the composite Mg2FeH6eLiBH4 system are significantly better than this. Hydrogen uptake experiments on the dehydrogenated material were also carried out using the PCT apparatus. At 370 C and an initial pressure of 30 bar H2, no hydrogen absorption was observed for milled Mg2FeH6. The dehydrogenated Mg2FeH6eLiBH4 was heated from room temperature to 445 C and held at this temperature for 18 h. Hydrogen absorption reached 3.6 wt.% (Fig. 4), which is consistent with the formation of MgH2, as evidenced from both XRD and DSC analyses. Therefore, we conclude that 6
3.2.
Mg2FeH6eLiNH2 composite material
As expected, the XRD patterns for the milled Mg2FeH6eLiNH2 (molar ratio 3:10) showed diffraction peaks corresponding to both Mg2FeH6 and LiNH2, in addition to a feature corresponding to elemental Fe (Fig. 5). After Mg2FeH6eLiNH2 was dehydrogenated on the PCT instrument, LiNH2 and Fe reflections were clearly visible, indicating that hydrogen was released only from the Mg2FeH6 component, with the formation of Fe and probably Mg [17,18], although diffraction peaks from the latter were not observed. Therefore, the dehydrogenation of this composite material does not proceed as described in Equation (2). The XRD pattern of the sample after the hydrogenation procedure was very similar to that of the dehydrogenated material, indicating that little or no hydrogen uptake had occurred. The DSC trace for the Mg2FeH6eLiNH2 sample in a flow of N2 showed a very broad and weak endothermic event between ca. 245e345 C (Fig. 6a). This event is assigned to the release of hydrogen, followed by a sharp endothermic peak at 370 C, which is associated with the melting of LiNH2 [35,36]. The release of hydrogen from this system occurred at a lower temperature than for Mg2FeH6eLiBH4. DSC experiments carried out on dehydrogenated Mg2FeH6eLiNH2 under an initial H2 pressure of 30 bar showed a single sharp endotherm at 360 C, which is assigned to the melting of LiNH2 (Fig. 6b). Although hydrogen desorption was recorded in the PCT experiments on Mg2FeH6eLiNH2, the DSC plot showed no thermal events that can be associated with hydrogen absorption. Accordingly, we conclude that the Mg2FeH6eLiNH2 system desorbs hydrogen but the dehydrogenated material does not re-absorb hydrogen. The mechanism of this process is unclear, but it may involve interactions between the doubly-charged magnesium cations of Mg2FeH6
460 420
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2
140
Temperature (ºC)
Hydrogen absorption (wt.%)
6697
100
1
60 0
20 0
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Fig. 4 e Hydrogen absorption plot for dehydrogenated Mg2FeH6eLiBH4.
Fig. 5 e Powder XRD patterns for (a) Mg2FeH6, (b) Mg2FeH6eLiNH2 before dehydrogenation, (c) Mg2FeH6eLiNH2 after dehydrogenation, and (d) Mg2FeH6eLiNH2 after rehydrogenation.
Heat Flow (W/g)
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Mg2FeH6eLiNH2 displayed better desorption properties than Mg2FeH6 at the relatively low temperature of 370 C, and the former system partially re-absorbed hydrogen as it was heated to 445 C and held at this temperature. This represents the first destabilization study of a complex transition metal hydride, Mg2FeH6, with a borohydride or an amide.
0.8 0.4 0.0
b
Heat Flow (W/g)
-0.4 0.2
Acknowledgements
0.0 -0.2 -0.4
a
Exo 50
100
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o
Temperature ( C) Fig. 6 e DSC plots of (a) hydrogen desorption from as-milled Mg2FeH6eLiNH2, and (b) hydrogen absorption by dehydrogenated Mg2FeH6eLiNH2.
and the basic amide anions of LiNH2, which are strong enough to activate release of hydrogen from Mg2FeH6, but which do not lead to the formation of an amide or nitride phase of magnesium or iron. According to desorption experiments performed on the PCT instrument, hydrogen release began at 270 C for Mg2FeH6eLiNH2, and reached 4.3 wt.% after holding the sample at 370 C for 24 h (Fig. 3c). It has been reported that small amounts of NH3 are released along with hydrogen in amideehydride systems [37e39]. Therefore, we cannot rule out the presence of small amounts of NH3 in the desorbed H2 on the basis of these experiments. Further analysis of the desorbed gas by mass spectrometry will provide insight into the evolved species. As was the case with neat Mg2FeH6, no hydrogen uptake was observed when the dehydrogenated Mg2FeH6eLiNH2 composite was exposed to a pressure of 30 bar H2 and heated to 370 C, a result consistent with the DSC experiments on this material. However, the Mg2FeH6eLiNH2 composite showed better hydrogen storage properties than neat Mg2FeH6 in terms of its desorption kinetics, release temperature and desorption capacity.
4.
Conclusions
The composite hydrogen storage materials Mg2FeH6eLiBH4 (1:5 molar ratio) and Mg2FeH6eLiNH2 (3:10 molar ratio) were investigated in this study. A hydrogen desorption capacity of 5.6 wt.% was achieved for Mg2FeH6eLiBH4 after holding the sample at 370 C for 24 h. Both Mg2FeH6 and LiBH4 were shown to participate in the hydrogen desorption process, although the products obtained could not be characterized by powder XRD. Mg2FeH6eLiBH4 partially re-absorbed hydrogen (3.6 wt.%), with the formation of MgH2. The onset of hydrogen desorption for Mg2FeH6eLiNH2 occurred at 100 C lower than for Mg2FeH6eLiBH4. The Mg2FeH6eLiNH2 system did not demonstrate reversibility. Both Mg2FeH6eLiBH4 and
We are grateful to the Natural Sciences and Engineering Council of Canada, the Canadian Foundation for Innovation, HSM Systems Inc., and the Office of Hydrogen Fuel Cells and Infrastructure Technology of the U.S. Department of Energy for support of this work. Dennis Morrison is acknowledged for skilful technical assistance related to the high-pressure experiments at SNL.
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Mg2FeH6eLiBH4 and Mg2FeH6eLiNH2 composite materials for hydrogen storage Henrietta W. Langmi a, G. Sean McGrady a,*, Rebecca Newhouse b, Ewa Ro¨nnebro c,1 a
Department of Chemistry, University of New Brunswick, PO Box 4400, Fredericton, NB E3B 5A3, Canada University of California, Santa Cruz, 1156 High St., Santa Cruz, CA, USA c Sandia National Laboratories, 7011 East Ave, Livermore, CA, USA b
article info
abstract
Article history:
Two composite hydrogen storage materials based on Mg2FeH6 were investigated for the
Received 12 September 2011
first time. The Mg2FeH6eLiBH4 composite of molar ratio 1:5 showed a hydrogen desorption
Received in revised form
capacity of 5.6 wt.% at 370 C, and could be rehydrogenated to 3.6 wt.% with the formation
3 January 2012
of MgH2, as the material was heated to 445
Accepted 6 January 2012
Mg2FeH6eLiNH2 composite of 3:10 molar ratio exhibited a hydrogen desorption capacity of
Available online 2 February 2012
4.3 wt.% and released hydrogen at 100 C lower then the Mg2FeH6eLiBH4 composite, but
Keywords:
show enhanced hydrogen storage properties in terms of desorption kinetics and capacity at
Hydrogen storage
these low temperatures. In particular, Mg2FeH6eLiNH2 exhibits a much lower desorption
C and held at this temperature. The
this mixture could not be rehydrogenated. Compared to neat Mg2FeH6, both composites
Composite hydride
temperature than neat Mg2FeH6, but only Mg2FeH6eLiBH4 re-absorbs hydrogen.
Transition metal hydride
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
According to the U.S. Department of Energy, ‘Hydrogen storage is a key enabling technology for the advancement of hydrogen and fuel cell power technologies in transportation, stationary, and portable applications’ [1]. Over the past two decades, a multitude of materials of various types have been investigated, in the quest for a hydrogen storage system with appropriate thermodynamics and rapid kinetics. In the field of metal hydrides, early work focused on interstitial-type transition metal hydrides [2], with subsequent activity concentrating on complex transition metal hydrides [3]. The complex hydrides of the lighter main group metals exhibit the highest hydrogen storage capacities, and after the discovery that Ticatalyzed NaAlH4 is reversible at moderate temperatures [4], research efforts have focused on alanates, borohydrides and
amides [5]. These materials operate at high temperatures above 150 C, and their kinetics are poor. By mixing borohydrides with binary hydrides, it has been shown that the operating temperature can be lowered significantly through formation of a so-called destabilized hydride or a reactive hydride composite [6e10]. However, mixtures of two or more complex hydrides to achieve this effect have not been reported. Here we describe the investigation of the composites formed between an Mg-based complex transition metal hydride and a borohydride or an amide, with the aim of enhancing the hydrogen storage properties of the individual components. The complex transition metal hydrides Mg2NiH4, Mg2CoH5 and Mg2FeH6 have been known for several decades [11e13]. These systems have theoretical hydrogen storage capacities of 3.6, 4.5, and 5.5 wt.% H, respectively. Amongst all complex
* Corresponding author. E-mail address: [email protected] (G.S. McGrady). 1 Current address: Pacific Northwest National Laboratory, 902 Battelle Blvd., Richland, WA 99352, USA. 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.01.020
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hydrides Mg2FeH6 has the highest known volumetric hydrogen density of 150 kg m3. It has also been reported that Mg2FeH6 has high volumetric and gravimetric thermal energy densities and excellent stability on cycling, among other attractive features, making it highly suitable for thermochemical thermal energy storage at about 500 C [14,15]. Synthesis of Mg2FeH6 is difficult, owing to the absence of an Mg2Fe intermetallic phase in the binary MgeFe system [16]. Didisheim et al. first prepared Mg2FeH6 by sintering mixtures of the elemental powders at 500 C and 20e120 bar H2 [13]. In addition to sintering, mechanical alloying and reactive mechanical milling have been used to synthesize Mg2FeH6 [17]. The yield of Mg2FeH6 is dependent on the starting materials and the conditions employed during synthesis. It is difficult to obtain a pure sample; unreacted Fe is typically present together with Mg2FeH6 [17,18], and MgH2, MgO and Mg also sometimes appear in the synthesized material [19]. Mg2FeH6 has been reported to reversibly store ca. 5.5 wt.% H at around 500 C over 1100 cycles [14,15]. Below 300 C, this system has also been shown to desorb 5.2 wt.% H in dynamic vacuum [20]. However, it re-absorbs only 3 wt.% H at 10 bar and 300 C, with the formation of MgH2. Decomposition of Mg2FeH6 occurs in a single step, to produce elemental Mg, Fe, and H2 [17,18]. However, in the reverse reaction, MgH2 is generated first and this then reacts further with Fe and H2 to form Mg2FeH6 [21,22]. Hydrogen absorptionedesorption by ternary lanthanide iron borides such as Nd2Fe14B is exploited in the processing of powders used to prepare powerful lanthanide-based magnetic materials [23,24]. Recently, it was reported that a ternary magnesium nickel boride, MgNi2.5B2, can be hydrogenated reversibly in combination with LiH and MgH2, forming Mg2NiH4 and LiBH4 below 300 C, although the observed hydrogen capacity was only 1 wt.% [25]. In essence, Mg2NiH4eLiBH4 is a kinetically coupled destabilized system that forms a ternary boride during dehydrogenation [26]. Although Mg2NiH4 has been extensively studied as a hydrogen storage material, it suffers from sluggish absorption kinetics and a high temperature for hydrogen absorptionedesorption [27]. However, production of Mg2NiH4 in a nanocrystalline form or modification with platinum group metals enhances the hydrogen storage characteristics [28,29]. Effectively, Mg2NiH4 is MgH2 destabilized with Ni: Mg2Ni is more thermodynamically stable than Mg metal. Destabilization of the complex transition metal hydrides themselves appears to be a completely overlooked approach in the field of hydrogen storage. Drawing on the recent approaches demonstrated by Vajo, Jensen and others [6e10], we set out to explore the properties of the reactive hydride composites formed between Mg2FeH6 and the high hydrogen content materials LiBH4 and LiNH2, as described in Equations (1) and (2). The anticipated decomposition products from a Mg2FeH6/LiBH4 mixture at 300e500 C are the stable phases LiH and MgB2, and the ironeboron phase FeB; Equation (1). Although Fe2B can also be formed by these two elements, this outcome is unlikely at the temperatures under consideration [30]. As regards Equation (2), Mg3N2 is a well characterized material, and the ternary nitride, Li3FeN2, is a stable phase that can be synthesized from the binary nitrides, and which has been reported to store hydrogen reversibly [31]. Thus, mixing Mg2FeH6 with LiBH4 and LiNH2 in the proportions
6695
described in Equations (1) and (2) offers the prospect of stable dehydrogenation products for each of the elements N, B, Mg and Fe, and should result in activation of both the main group and the transition metal hydride components of the mixture, resulting in the release of substantial amounts of hydrogen at reduced temperatures. Mg2FeH6 þ 5LiBH4 / 2MgB2 þ FeB þ 5LiH þ 10.5H2 (9.6 wt.% H)
(1)
3Mg2FeH6 þ 10LiNH2 / 3Li3FeN2 þ 2Mg3N2 þ LiH þ 18.5H2 (6.7 wt.% H) (2) There is a range of potential complications with these composite systems, such as poor reversibility, slow kinetics, and the formation of metastable intermediates. However, as the approach was completely unexplored for Mg2FeH6, and offers the prospect of some novel and unique systems with high hydrogen storage capacities, we investigated in some detail the Mg2FeH6eLiBH4 and Mg2FeH6eLiNH2 systems described in Equations (1) and (2).
2.
Experimental
Mg2FeH6 was prepared at Sandia National Laboratory in ca. 80% yield by a combination of high-energy ball milling and high-pressure sintering, according to the following procedure. The starting materials MgH2 (Pfaltz & Bauer, Inc.; 95%) and Fe (Alfa Aesar, spherical powder < 10 microns; 99.5%) were ball milled for 10 h in a molar ratio of 2.5: 1 under Ar in a SPEX8000 high-energy ball mill using 2 WC- (Spex SamplePrep) of 9 g each for 4 g of sample powder in a WC mill pot (Spex SamplePrep). The Fe/MgH2 mixture became a fine black powder after milling. The milled powders were pressed into pellets of diameters of 13 mm and placed in stainless steel crucibles, further enclosed in an AE Closure (Snap-tite, Inc.), which is a self-sealing vessel of standard material A-286 and nominal capacity 100 mL that utilizes internal pressure to tighten the seal. The samples were sintered at 320e370 C under ca. 700 bar H2 over a period of 4 days. After high-pressure treatment, the material was olive green in colour. All sample handling was performed in an Ar-filled glove box with less than 1 ppm O2 and H2O. Mg2FeH6 was combined with either LiBH4 (Strem; 95%) in a 1:5 ratio, or with LiNH2 (Aldrich; 95%) in a 3:10 ratio according to Eqs. (1) and (2), and milled using a Retsch planetary mill PM100. The ball-to-powder ratio was 23:1. The samples were milled for 2 h at 100 rpm in a 50 mL WC-vessel containing 3 WC-balls of diameter 10 mm. Powder XRD patterns were collected using a Bruker D8 Advance diffractometer (CuKa radiation). The samples were mounted in a plastic holder and covered with parafilm to protect them from contact with air during the measurements. The parafilm resulted in additional diffraction peaks at ca. 21.6 and 24 . Thermal analysis was carried out by differential scanning calorimetry (DSC) using a TA Instruments Q20P DSC. About 9 mg of material was used for the measurements. The as-prepared materials Mg2FeH6eLiBH4 and Mg2FeH6eLiNH2 were heated at
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3.
Results and discussion
3.1.
Mg2FeH6eLiBH4 composite material
According to its powder XRD pattern, as-prepared Mg2FeH6 contained unreacted iron (Fig. 1). After milling a mixture of Mg2FeH6 and LiBH4 (molar ratio 1:5), the XRD pattern of the composite material showed diffraction peaks characteristic of both Mg2FeH6 and LiBH4, thus indicating no chemical reaction. Upon dehydrogenation on the Sievert’s-type instrument no clear diffraction peaks were observed, indicating that the products may be glassy or amorphous. However, after rehydrogenation on the Sievert’s-type instrument by heating the dehydrogenated products from room temperature to 445 C (at an initial H2 pressure of 30 bar) and holding the sample at 445 C for 18 h, MgH2 was formed, as evidenced by the distinct diffraction peaks alongside two broad reflections at 42.1e45.5 and 61.4e64.4 , suggesting that the hydrogenated material is amorphous to some extent. Analysis of the as-milled composite material using DSC was carried out in a flowing stream of N2 (Fig. 2a). Several endothermic events were observed in the DSC plot for Mg2FeH6eLiBH4. The events occurring at ca. 112 and 286 C are assigned to the orthorhombic-to-hexagonal phase transition and melting of LiBH4, respectively [32]. This is in agreement with the existence of LiBH4 diffraction peaks in the XRD pattern of the as-milled material. The other endothermic peaks, at ca. 367 and 410 C, are assigned to the release of
Fig. 1 e Powder XRD patterns for (a) Mg2FeH6, (b) Mg2FeH6eLiBH4 before dehydrogenation, (c) Mg2FeH6eLiBH4 after dehydrogenation, and (d) Mg2FeH6eLiBH4 after rehydrogenation.
1.0
Heat Flow (W/g)
1 C min1 to 500 C in a flow of N2. Meanwhile, the dehydrogenated materials were subjected to an initial hydrogen pressure of 30 bar and heated at 5 C min1 to 500 C. Hydrogen sorption measurements were performed using a Sievert’s-type instrument (HyEnergy PCTPro-2000). In each experiment, ca. 0.5 g sample was heated at 5 C min1 and then held for 24 h at 370 C while the desorption curve was measured. For absorption experiments 30 bar initial pressure of high purity H2 (Air Liquide, 99.999%) was employed.
0.5 0.0 -0.5 -1.0
b
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Heat Flow (W/g)
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0.0 -0.8 -1.6
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150
200
250
300
350
400
450
500
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Temperature ( C) Fig. 2 e DSC plots of (a) hydrogen desorption from as-milled Mg2FeH6eLiBH4, and (b) hydrogen absorption by dehydrogenated Mg2FeH6eLiBH4.
hydrogen from Mg2FeH6 and LiBH4, respectively. Hence, hydrogen desorption from Mg2FeH6eLiBH4 appears to be a two-step process, analogous to the behaviour reported for the MgH2eLiBH4 system [33]. Equation (1) can therefore be rewritten in two parts: Mg2FeH6 þ 5LiBH4 / 2Mg þ Fe þ 5LiBH4 þ 3H2
(1a)
2Mg þ Fe þ 5LiBH4 þ 3H2 / 2MgB2 þ FeB þ 5LiH þ 10.5H2
(1b)
Powder XRD analysis revealed that the products from Equation (1b) exist in an amorphous state and the true composition of the products could therefore not be verified (Fig. 1). DSC experiments under H2 pressure were carried out on the composite material after desorption at 370 C (Fig. 2b). Endothermic peaks corresponding to the orthorhombic-tohexagonal phase transition and melting of LiBH4 were again observed. This indicates that residual LiBH4 was still present in the dehydrogenated material, although this was not evident in the XRD pattern; this unreacted LiBH4 partly accounts for the lower-than-theoretical hydrogen desorption capacity observed. An endothermic peak with a maximum at 434 C was also observed in the DSC plot. This peak is likely due to the decomposition of residual Mg2FeH6. The sharp peak at 458 C might be associated with the alloying of Mg (from decomposition of Mg2FeH6) with Al (from Al pan used in DSC) [34]. The thermal event occurring at 466 C is most likely associated with the decomposition of destabilized LiH e a product of reaction (1). Desorption experiments were carried out at an initial pressure of zero bar using the PCT apparatus (Fig. 3). An amount of gas corresponding to 5.6 wt.% H was desorbed after maintaining the sample at 370 C for 24 h, although the
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rehydrogenation of Mg2FeH6eLiBH4 proceeds with the formation of MgH2, as has been reported for neat Mg2FeH6 [21,22].
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b
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4
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Temperature (ºC)
Hydrogen desorption (wt.%)
6
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0
20 0
5
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Fig. 3 e Hydrogen desorption plots for as-milled (a) Mg2FeH6, (b) Mg2FeH6eLiBH4, and (c) Mg2FeH6eLiNH2.
possibility that this contained a vestigial amount of B2H6 cannot be ignored [5]. The amount of hydrogen desorbed is less than the theoretical amount (9.6 wt.%), which can be partly accounted for by the presence of Fe in the Mg2FeH6 starting material, and partly by the presence of unreacted LiBH4 (Fig. 2b). Nevertheless, when compared with milled Mg2FeH6, the composite material released more hydrogen overall, and with a lower onset temperature. Hydrogen release by Mg2FeH6 began after 82 min at 370 C (Fig. 3a). A total of 1.6 wt.% H was desorbed after holding the sample at this temperature for 12 h. The hydrogen desorption properties of the composite Mg2FeH6eLiBH4 system are significantly better than this. Hydrogen uptake experiments on the dehydrogenated material were also carried out using the PCT apparatus. At 370 C and an initial pressure of 30 bar H2, no hydrogen absorption was observed for milled Mg2FeH6. The dehydrogenated Mg2FeH6eLiBH4 was heated from room temperature to 445 C and held at this temperature for 18 h. Hydrogen absorption reached 3.6 wt.% (Fig. 4), which is consistent with the formation of MgH2, as evidenced from both XRD and DSC analyses. Therefore, we conclude that 6
3.2.
Mg2FeH6eLiNH2 composite material
As expected, the XRD patterns for the milled Mg2FeH6eLiNH2 (molar ratio 3:10) showed diffraction peaks corresponding to both Mg2FeH6 and LiNH2, in addition to a feature corresponding to elemental Fe (Fig. 5). After Mg2FeH6eLiNH2 was dehydrogenated on the PCT instrument, LiNH2 and Fe reflections were clearly visible, indicating that hydrogen was released only from the Mg2FeH6 component, with the formation of Fe and probably Mg [17,18], although diffraction peaks from the latter were not observed. Therefore, the dehydrogenation of this composite material does not proceed as described in Equation (2). The XRD pattern of the sample after the hydrogenation procedure was very similar to that of the dehydrogenated material, indicating that little or no hydrogen uptake had occurred. The DSC trace for the Mg2FeH6eLiNH2 sample in a flow of N2 showed a very broad and weak endothermic event between ca. 245e345 C (Fig. 6a). This event is assigned to the release of hydrogen, followed by a sharp endothermic peak at 370 C, which is associated with the melting of LiNH2 [35,36]. The release of hydrogen from this system occurred at a lower temperature than for Mg2FeH6eLiBH4. DSC experiments carried out on dehydrogenated Mg2FeH6eLiNH2 under an initial H2 pressure of 30 bar showed a single sharp endotherm at 360 C, which is assigned to the melting of LiNH2 (Fig. 6b). Although hydrogen desorption was recorded in the PCT experiments on Mg2FeH6eLiNH2, the DSC plot showed no thermal events that can be associated with hydrogen absorption. Accordingly, we conclude that the Mg2FeH6eLiNH2 system desorbs hydrogen but the dehydrogenated material does not re-absorb hydrogen. The mechanism of this process is unclear, but it may involve interactions between the doubly-charged magnesium cations of Mg2FeH6
460 420
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Temperature (ºC)
Hydrogen absorption (wt.%)
6697
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Fig. 4 e Hydrogen absorption plot for dehydrogenated Mg2FeH6eLiBH4.
Fig. 5 e Powder XRD patterns for (a) Mg2FeH6, (b) Mg2FeH6eLiNH2 before dehydrogenation, (c) Mg2FeH6eLiNH2 after dehydrogenation, and (d) Mg2FeH6eLiNH2 after rehydrogenation.
Heat Flow (W/g)
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Mg2FeH6eLiNH2 displayed better desorption properties than Mg2FeH6 at the relatively low temperature of 370 C, and the former system partially re-absorbed hydrogen as it was heated to 445 C and held at this temperature. This represents the first destabilization study of a complex transition metal hydride, Mg2FeH6, with a borohydride or an amide.
0.8 0.4 0.0
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Heat Flow (W/g)
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Acknowledgements
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Temperature ( C) Fig. 6 e DSC plots of (a) hydrogen desorption from as-milled Mg2FeH6eLiNH2, and (b) hydrogen absorption by dehydrogenated Mg2FeH6eLiNH2.
and the basic amide anions of LiNH2, which are strong enough to activate release of hydrogen from Mg2FeH6, but which do not lead to the formation of an amide or nitride phase of magnesium or iron. According to desorption experiments performed on the PCT instrument, hydrogen release began at 270 C for Mg2FeH6eLiNH2, and reached 4.3 wt.% after holding the sample at 370 C for 24 h (Fig. 3c). It has been reported that small amounts of NH3 are released along with hydrogen in amideehydride systems [37e39]. Therefore, we cannot rule out the presence of small amounts of NH3 in the desorbed H2 on the basis of these experiments. Further analysis of the desorbed gas by mass spectrometry will provide insight into the evolved species. As was the case with neat Mg2FeH6, no hydrogen uptake was observed when the dehydrogenated Mg2FeH6eLiNH2 composite was exposed to a pressure of 30 bar H2 and heated to 370 C, a result consistent with the DSC experiments on this material. However, the Mg2FeH6eLiNH2 composite showed better hydrogen storage properties than neat Mg2FeH6 in terms of its desorption kinetics, release temperature and desorption capacity.
4.
Conclusions
The composite hydrogen storage materials Mg2FeH6eLiBH4 (1:5 molar ratio) and Mg2FeH6eLiNH2 (3:10 molar ratio) were investigated in this study. A hydrogen desorption capacity of 5.6 wt.% was achieved for Mg2FeH6eLiBH4 after holding the sample at 370 C for 24 h. Both Mg2FeH6 and LiBH4 were shown to participate in the hydrogen desorption process, although the products obtained could not be characterized by powder XRD. Mg2FeH6eLiBH4 partially re-absorbed hydrogen (3.6 wt.%), with the formation of MgH2. The onset of hydrogen desorption for Mg2FeH6eLiNH2 occurred at 100 C lower than for Mg2FeH6eLiBH4. The Mg2FeH6eLiNH2 system did not demonstrate reversibility. Both Mg2FeH6eLiBH4 and
We are grateful to the Natural Sciences and Engineering Council of Canada, the Canadian Foundation for Innovation, HSM Systems Inc., and the Office of Hydrogen Fuel Cells and Infrastructure Technology of the U.S. Department of Energy for support of this work. Dennis Morrison is acknowledged for skilful technical assistance related to the high-pressure experiments at SNL.
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