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Phase, microstructure and hydrogen storage properties of Mg–Ni materials synthesized from metal nanoparticles
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 135704 (http://iopscience.iop.org/0957-4484/25/13/135704) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 169.230.243.252 This content was downloaded on 07/12/2014 at 08:45
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 135704 (6pp)
doi:10.1088/0957-4484/25/13/135704
Phase, microstructure and hydrogen storage properties of Mg–Ni materials synthesized from metal nanoparticles Huaiyu Shao1 , Chunguang Chen2 , Tong Liu2 and Xingguo Li3 1
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan 2 Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, People’s Republic of China 3 Beijing National Laboratory for Molecular Sciences (BNLMS), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China E-mail: [email protected], [email protected] and [email protected] Received 9 December 2013, revised 14 January 2014 Accepted for publication 16 January 2014 Published 28 February 2014
Abstract
After Mg and Ni nanoparticles were fabricated by hydrogen plasma metal reaction, Mg-rich Mgx Ni100−x (75 < x < 90) materials were synthesized from these metal nanoparticles to study the synergistic effects for hydrogen storage in these samples to show both good kinetics and high capacity. These Mgx Ni100−x materials may absorb hydrogen with a capacity of around 3.3–5.1 wt% in 1 min at 573 K. The Mg90 Ni10 sample shows a hydrogen capacity of 6.1 wt%. The significant kinetic enhancement is thought to be due to the unique nanostructure from the special synthesis route, the catalytic effect of the Mg2 Ni nano phase, and the synergistic effects between the Mg2 Ni and Mg phases in the materials. An interesting phenomenon which has never been reported before was observed during pressure composition isotherm (PCT) measurements. One steep step in the absorption process and two obviously separated steps in the desorption process during PCT measurements of Mg80 Ni20 and Mg90 Ni10 samples were observed and a possible reason from the kinetic performance of the Mg2 Ni and Mg phases in absorption and desorption processes was explained. These Mgx Ni100−x materials synthesized from Mg and Ni nanoparticles show high capacity and good kinetics, which makes these materials very promising candidates for thermal storage or energy storage and utilization for renewable power. Keywords: magnesium alloys, hydrogen storage, nanoparticle, kinetics S Online supplementary data available from stacks.iop.org/Nano/25/135704/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
renewable power and provide stable energy supply afterwards [6, 7]. For on-board hydrogen storage as in fuel cell vehicles, the working temperature should be below 423 K. In the latter two cases, the working temperature may be in the range of 500–700 K. The problems in Mg-based hydrogen storage materials are poor sorption kinetics (for all possible applications) and stable thermodynamics for the desorption process (for on-board applications). For Mg-based hydrogen
Mg-based materials have attracted much attention for hydrogen storage study due to their advantages such as high capacity (7.7 wt% for MgH2 ) and low cost (around 3 dollars kg−1 for Mg) [1, 2]. Their potential applications include on-board hydrogen storage [3], thermal storage [4, 5] and stationary energy storage combined with fuel cells to store fluctuating 0957-4484/14/135704+06$33.00
1
c 2014 IOP Publishing Ltd
Printed in the UK
Nanotechnology 25 (2014) 135704
H Shao et al
3. Measurement and characterization
storage materials with particle sizes above the micrometer scale, a temperature over 600 K is needed for absorption and desorption reactions even after an activation process. Common approaches to enhancing the kinetics in Mg-based hydrogen storage materials include structure modification [8], catalyst addition [9], mixing with other storage materials [10, 11], etc. To tailor the thermodynamics in Mg-based hydrogen storage materials and to lower the desorption enthalpy, researchers have tried the formation of new compounds [12, 13] and downsizing [14, 15]. However, controversial results are being reported in the literature [6, 15, 16] about whether downsizing could affect and reduce the desorption enthalpy. The ball milling technique to introduce nanostructure and defects into materials is the most widely used method for kinetic enhancement [17, 18] in hydrogen storage materials. Ball milling may decrease the crystallite size of materials to nanometer scale, but cannot reduce the particles to nanometer scale. Usually, after some cycles, the crystallite size will go back to the micrometer scale due to agglomeration during hydrogen absorption and desorption cycles [19]. After ball milling, usually a few per cent of Fe and other steel elements can be detected in the final products [20]; this is another disadvantage of the ball milling technique for the synthesis of hydrogen storage materials. The authors previously reported Mg nanoparticles with an average size of around 300 nm and Mg2 Ni nanoparticles with a size of around 40 nm for hydrogen storage study [21–23]. A Mg2 Ni nanoparticle sample with good kinetics but limited hydrogen capacity can absorb hydrogen at room temperature with a capacity of around 1.7 wt% in 30 min [23]. Mg nanoparticles can absorb hydrogen with almost the theoretic value of around 7.6 wt% at 573 K [21]. In this work, from Mg and Ni nanoparticles, we synthesized Mgx Ni100−x hydrogen storage materials to try to clarify the relationship between phase/microstructure and hydrogen storage performance to study the synergistic effects in these samples, and to develop materials with both good kinetics and high hydrogen capacity for energy storage applications.
X-ray diffraction (XRD) measurements of the Mgx Ni100−x samples were carried out to obtain the structure and phase information of the samples using monochromatic Cu Kα radiation. The size distribution and morphology of metal nanoparticles were observed by transmission electron microscopy (TEM). The hydrogen absorption rate measurements at various temperatures and the pressure composition isotherm (PCT) measurements were conducted on a home-made Sievert-type measurement system. A conventional pressure–volume–temperature technique was used to obtain the hydrogen absorption/desorption capacity change versus time by recording the change of gas pressure in a constant volume. 325 mesh Mg powder samples (purity > 99.9%) were used for comparison of the kinetics. A very thin oxide layer on the surface of the Mg nanoparticle samples may help to prevent the nanostructure samples from further oxidization. Unlike other nanostructure materials, which are very sensitive to air or moisture, in our case, all the sample transfer can be conducted in air without any atmosphere protection. 4. Results and discussion
Figure 1 shows the TEM images and XRD curves of the Mg and Ni nanoparticles. From the XRD pattern, both the Mg and the Ni are almost pure single phase. A small amount of MgO can be detected in the XRD pattern of the Mg sample. This MgO phase is on the particle surface of the Mg and was formed during the passivation process before the Mg nanoparticles were taken out of the collection chamber [21]. This thin MgO layer may protect the Mg nanoparticle samples from further oxidization. The Mg phase is hexagonal structure with a space group of P63 /mmc. The Ni nanoparticle sample has a face-centered cubic structure (space group: Fm3m). From the XRD pattern, we can see that the peak broadening in the Ni nanoparticle sample is more obvious than that in the Mg sample because the Mg nanoparticles have an average particle size of 300 nm and the Ni ones are around 30 nm based on the TEM images in figures 1(a) and (b). The Mg shows much larger particle size than the Ni particles due to the much faster generation rate of Mg by plasma synthesis methods [24]. Figure 2 shows the XRD patterns of the Mg75 Ni25 , Mg80 Ni20 , Mg85 Ni15 and Mg90 Ni10 samples. All of the four samples consist of mainly Mg and Mg2 Ni phases with nanostructure based on the peak broadening as well as previous study on the nanostructure in Mg–H and Mg2 Ni–H systems [23, 25]. The Mg2 Ni phase was synthesized by a hydrogenation process at 623 K under 3.5 MPa H2 atmosphere and later evacuation expressed by the following reactions (equations (1)–(3))
2. Experimental details 2.1. Synthesis
Mg and Ni nanoparticles were synthesized by a hydrogen plasma metal reaction method in a mixed atmosphere of 50% Ar and 50% H2 gases with a total pressure of one bar [23]. Mg (purity > 99.9%) and Ni (purity > 99.7%) were vaporized in a reaction chamber by hydrogen plasma and taken by circulating gas with a controlled rate into a collection chamber to deposit into metal nanoparticles. Before the reactive Mg nanoparticles were taken out of the collection chamber, air gas was slowly provided to the chamber as a passivation process. Mgx Ni100−x samples were synthesized from mixed pieces of Mg and Ni nanoparticles with a molar ratio of x:(100 − x) under 3.5 MPa hydrogen at 623 K for 2 h followed by evacuation.
Mgx Ni100−x + xH2 ⇒ (100 − x)Mg2 NiH4
2
+ (3x − 200)MgH2
(1)
Mg2 NiH4 ⇒ Mg2 Ni + 2H2
(2)
MgH2 ⇒ Mg + H2 .
(3)
Nanotechnology 25 (2014) 135704
H Shao et al
Figure 1. TEM images and XRD curves of Mg and Ni nanoparticles synthesized by the hydrogen plasma metal reaction.
Figure 3. TEM image of a synthesized Mg85 Ni15 nanoparticle sample. The inset shows the indexed electron diffraction pattern of the sample. The transparent large particles are Mg and the small dark particles are Mg2 Ni. The Mg particles have an average size of around a few hundreds of nm and the Mg2 Ni particles have a mean size of about 30–50 nm. Mg2 Ni particles are also dispersed on large Mg particles.
Figure 2. XRD curves of Mgx Ni100−x samples synthesized by
hydrogenation of Mg and Ni nanoparticles at 623 K under 3.5 MPa hydrogen and then evacuation. (a) Mg75 Ni25 , (b) Mg80 Ni20 , (c) Mg85 Ni15 and (d) Mg90 Ni10 .
sorption of the Mg phase [1], which makes these Mgx Ni100−x nanostructure samples potentially show superior hydrogen storage performance. From figures 3 and S1 (supplementary information available at stacks.iop.org/Nano/25/135704/mm edia), we can see that just as we reported and explained before [23], 300 nm Mg and 30 nm Ni nanoparticles form into Mg2 Ni nanoparticles with a mean size of around 30–50 nm (dark particles) and the Mg particles are still a few hundreds of nm. In figure 4(a), these Mgx Ni100−x samples with Mg and Mg2 Ni nanostructure phases show very fast hydrogen absorption kinetics. The Mg90 Ni10 sample shows a hydrogen capacity of 5.1 wt% in only 1 min after hydrogenation under 3.5 MPa at 573 K. The Mg75 Ni25 , Mg80 Ni20 , Mg85 Ni15 and
There is some unreacted Ni in the Mg75 Ni25 , Mg80 Ni20 and Mg85 Ni15 samples after the synthesis. There are also unidentified peaks (at around 29◦ , 33◦ and 36◦ ) in the XRD patterns. The peak heights shown in figure 2 have been adjusted to the same value to clearly indicate the relative ratio of Mg and Mg2 Ni phases. It is not surprising that with the increasing x, the remaining Mg phase in the Mgx Ni100−x sample is increasing. By ball milling of Mgx Ni100−x samples starting from Mg and Ni, mostly amorphous or body-centered cubic structure is observed [26, 27]. Formation of Mg2 Ni phase via ball milling starting from Mg and Ni has also been reported in some cases [28]. The existence of Mg2 Ni (and some residual Ni) may show a catalytic effect for hydrogen 3
Nanotechnology 25 (2014) 135704
H Shao et al
Mg90 Ni10 samples show hydrogen capacities of 3.6, 4.0, 4.7 and 5.5 wt% at 573 K in less than half an hour. Figure 4(b) presents the absorption rate expressed as (H/Mg) in comparison to 45 µm Mg powder, 300 nm Mg and 40 nm Mg2 Ni nanoparticle samples under hydrogen pressures of 1 MPa, 4 MPa and 4 MPa, respectively [21, 23]. Compared to the kinetic difference resulting from the nanostructure and measurement temperature, the pressure difference contributes little to the hydrogen absorption kinetics and hydrogen capacity as long as the provided hydrogen is well above the equilibrium pressure of the absorption reaction [29]. From figure 4(b), we may see that the micrometer scale Mg powder sample cannot show any hydrogen capacity at 573 K. The 300 nm Mg nanoparticles synthesized by the hydrogen plasma metal reaction technique show an absorption content of 1.97 H/M (7.5 wt%), which is very close to the theoretical value of 2.0 H/M. The nanostructured Mg2 Ni sample and Mgx Ni100−x samples absorb hydrogen with smaller H/M values but with a faster rate than the Mg nanoparticle sample. It takes about 20 min for the hydrogen absorption of a Mg nanoparticle to come to saturation. However, it only takes 1 min for the nanostructured Mg2 Ni sample and Mgx Ni100−x samples to absorb about 90% of the hydrogen content. With the existence of the nanostructured Mg2 Ni phase, which can act as a catalyst to the absorption reaction of the Mg phase [1], the Mg90 Ni10 and Mg85 Ni15 samples even absorb hydrogen faster than the Mg2 Ni nanoparticle sample (figure 4(b)). The Mg85 Ni15 sample also shows good absorption kinetics at lower temperatures (figure 5). After 1 h of absorption under 3.5 MPa hydrogen, it absorbs hydrogen with a capacity of 1.8 wt% at 373 K and 4.4 wt% at 523 K. We think that the following factors contribute to the superior kinetic performance in these Mgx Ni100−x samples.
Figure 4. Hydrogen absorption curves of (a) Mgx Ni100−x samples
at 573 K and (b) the comparison results with the Mg2 Ni nanostructured sample and different sized Mg samples (the pressures for the 45 µm Mg powder, 300 nm Mg and 40 nm Mg2 Ni nanoparticle samples are 1 MPa, 4 MPa and 4 MPa, respectively).
(a) The nanostructure in these materials compared to micrometer scale samples means that there is a much larger surface area for contact with hydrogen and a much shorter hydrogen diffusion distance during hydrogen absorption and desorption reactions. (b) The existence of nanostructured Mg2 Ni phase shows a catalytic effect for sorption reactions in the Mg phase. (c) The synergistic effect of nanostructured Mg2 Ni and Mg phases in these samples. The nanostructured Mg2 Ni phase synthesized from Mg and Ni nanoparticles is around 30– 50 nm and the Mg nanoparticle phase is around 300 nm. The mixture of two phases with different particle size may result in strain and defects along the phase boundaries. After one hydrogen cycle, the Mg nanoparticles show cracked surfaces and inner diffusion tunnels for hydrogen transfer [21]. Combination of the strain, defects, cracked surfaces and new hydrogen diffusion tunnels led to the synergistic effects for hydrogen storage in these materials.
Figure 5. Hydrogen absorption curves of the Mg85 Ni15 sample at
373, 523, 573 and 623 K.
two plateaus (steps) during both the absorption and desorption processes for these two Mg-rich Mg–Ni based samples. Also, the hydrogen content in the lower plateaus (due to the absorption and desorption of the Mg phase) and higher plateaus (due to the Mg2 Ni phase) should be in a ratio of 7:2 for the Mg90 Ni10 sample and 4:4 for the Mg80 Ni20 sample. According to earlier van’t Hoff equations and equilibrium pressure results in Mg and Mg2 Ni nanoparticle samples, theoretical PCT curves based on Mg90 Ni10 and Mg80 Ni20 compositions were
Figure 6 presents PCT curves of Mg90 Ni10 and Mg80 Ni20 samples at 573 K. Based on the XRD results, without consideration of impurity phases, these two samples consist of (7Mg + Mg2 Ni) and (4Mg + 2Mg2 Ni) phases, respectively. Theoretically and also experimentally [1], there should be 4
Nanotechnology 25 (2014) 135704
H Shao et al
Mg and Ni nanoparticles show quite unique characteristics compared to materials from other synthesis techniques: fast kinetics due to nanoscale particle size (not just nanoscale crystallite size); high hydrogen capacity; good durability in air compared to other nanomaterials. We believe that these Mgx Ni100−x materials are perfect candidates for applications like thermal storage or stationary hydrogen energy storage for utilization of renewable power. 5. Conclusions
The main conclusions of this work are as follows. (1) Mg75 Ni25 , Mg80 Ni20 , Mg85 Ni15 and Mg90 Ni10 samples with nanostructure were synthesized from Mg and Ni nanoparticles. (2) These Mgx Ni100−x samples show fast kinetics and high hydrogen capacity. The Mg90 Ni10 sample shows a hydrogen capacity of 6.1 wt% at 573 K. These Mgx Ni100−x materials may absorb hydrogen with a capacity of around 3.3–5.1 wt% in 1 min at 573 K. The mechanism for significant kinetic enhancement in these materials was clarified. (3) An interesting phenomenon that has never been reported before was observed during PCT measurements of the Mg90 Ni10 and Mg80 Ni20 samples. One steep step in the absorption process and two obviously separated steps in the desorption process were observed and the possible mechanism was explained.
Figure 6. PCT curves of Mg90 Ni10 and Mg80 Ni20 samples at
573 K.
simulated (see figure S2 and table S1 in the supplementary information available at stacks.iop.org/Nano/25/135704/mme dia). However, in figure 6, a very interesting phenomenon was observed. The desorption curves for these two samples consist of two flat steps (more obviously for the Mg90 Ni10 sample) due to the reactions of equations (2) and (3); this is in agreement with the theoretical expectation. However, the absorption curves of both of these two samples show most likely only one steep step and we cannot easily find the separation boundary for the two absorption reactions of the Mg and Mg2 Ni phases in the two samples, (100 − x)Mg2 Ni + (3x − 200)Mg + xH2 ⇒ (100 − x)Mg2 NiH4 + (3x − 200)MgH2 .
(4)
Acknowledgments
We believe that the reason for this phenomenon is that when one tries to get equilibrium plateaus from PCT measurements for hydrogen storage materials, actually the plateau results for the PCT curves depend greatly on the kinetic performance and the equilibrium parameters set for the PCT measurements. In our case, the Mg90 Ni10 and Mg80 Ni20 samples both have two phases—the Mg phase with lower absorption and desorption plateaus and relatively poor kinetics, and the Mg2 Ni phase with higher plateaus and better kinetics. During the absorption process, the Mg with poorer kinetics should absorb hydrogen first because of the lower equilibrium pressure, but this absorption part of the Mg phase merges with that from the Mg2 Ni phase with better kinetics (although with higher hydrogen pressures) during the hydrogenation reaction of the Mg phase reaching its equilibrium state, and forms into only one steep absorption step (equation (4)). During the desorption process, the Mg2 NiH4 phase with fast kinetics and higher equilibrium pressure desorbs first completely during decrease of the hydrogen pressure and then the MgH2 phase desorbs. This is why these two steps can be clearly separated in the desorption process. To the best of our knowledge, this is the first on the kinetic performance difference in the two phases affecting the thermodynamic performance (reaction steps in the PCT curves). According to the above discussion, we may say that the nanostructured Mgx Ni100−x materials synthesized from
HS acknowledges support from the International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), established by the World Premier International Research Center Initiative (WPI), MEXT, Japan. XL acknowledges support from the Ministry of Science and Technology of China (No. 2012CBA01207). TL acknowledges support from the China Program of Magnetic Confinement Fusion under grant number 2012GB102006. References [1] Reilly J J and Wiswall R H 1968 Reaction hydrogen with alloys magnesium and nickel and formation of Mg2 NiH4 Inorg. Chem. 7 2254–6 [2] www.infomine.com [3] Yang J, Sudik A, Wolverton C and Siegel D J 2010 High capacity hydrogen storage materials: attributes for automotive applications and techniques for materials discovery Chem. Soc. Rev. 39 656–75 [4] Bogdanovi´c B, Ritter A and Spliethoff B 1990 Active MgH2 –Mg systems for reversible chemical energy-storage Angew. Chem. Int. Edn 29 223–34 [5] Gross A F, Ahn C C, Van Atta S L, Liu P and Vajo J J 2009 Fabrication and hydrogen sorption behaviour of nanoparticulate MgH2 incorporated in a porous carbon host Nanotechnology 20 204005 5
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[6] Shao H, Xin G, Zheng J, Li X and Akiba E 2012 Nanotechnology in Mg-based materials for hydrogen storage Nano Energy 1 590–601 [7] Sekhar B S, Muthukumar P and Saikia R 2012 Tests on a metal hydride based thermal energy storage system Int. J. Hydorg. Energy 37 3818–24 [8] Edalati K, Yamamoto A, Horita Z and Ishihara T 2011 High-pressure torsion of pure magnesium: evolution of mechanical properties, microstructures and hydrogen storage capacity with equivalent strain Scr. Mater. 64 880–3 [9] Barkhordarian G, Klassen T and Bormann R 2003 Fast hydrogen sorption kinetics of nanocrystalline Mg using Nb2 O5 as catalyst Scr. Mater. 49 213–7 [10] Felderhoff M and Bogdanovic B 2009 High temperature metal hydrides as heat storage materials for solar and related applications Int. J. Mol. Sci. 10 325–44 [11] Chaise A, de Rango P, Marty P, Fruchart D, Miraglia S, Olives R and Garrier S 2009 Enhancement of hydrogen sorption in magnesium hydride using expanded natural graphite Int. J. Hydorg. Energy 34 8589–96 [12] Anastasopol A, Pfeiffer T V, Middelkoop J, Lafont U, Canales-Perez R J, Schmidt-Ott A, Mulder F M and Eijt S W H 2013 Reduced enthalpy of metal hydride formation for Mg–Ti nanocomposites produced by spark discharge generation J. Am. Chem. Soc. 135 7891–900 [13] Zhong H C, Wang H, Liu J W, Sun D L and Zhu M 2011 Altered desorption enthalpy of MgH2 by the reversible formation of Mg(In) solid solution Scr. Mater. 65 285–7 [14] Wagemans R W P, van Lenthe J H, de Jongh P E, van Dillen A J and de Jong K P 2005 Hydrogen storage in magnesium clusters: quantum chemical study J. Am. Chem. Soc. 127 16675–80 [15] Li W Y, Li C S, Ma H and Chen J 2007 Magnesium nanowires: enhanced kinetics for hydrogen absorption and desorption J. Am. Chem. Soc. 129 6710 [16] Lu J, Choi Y J, Fang Z Z, Sohn H Y and Ronnebro E 2009 Hydrogen storage properties of nanosized MgH2 –0.1TiH2 prepared by ultrahigh-energy-high-pressure milling J. Am. Chem. Soc. 131 15843–52 [17] Froes F H, Suryanarayana C, Russell K and Li C G 1995 Synthesis of intermetallics by mechanical alloying Mater. Sci. Eng. A 192 612–23 [18] Baba T, Taketoshi N and Yagi T 2012 Development of pulsed light heating thermoreflectance methods under
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configurations of rear heating/front detection and front heating/front detection THERMINIC 2012: 18th International Workshop on Thermal Investigations of ICs and Systems pp 1–5 Shao H, Felderhoff M and Schuth F 2011 Hydrogen storage properties of nanostructured MgH2 /TiH2 composite prepared by ball milling under high hydrogen pressure Int. J. Hydrog. Energy 36 10828–33 Fernandes B B, Rodrigues G, Coelho G C and Ramos A S 2005 On iron contamination in mechanically alloyed Cr–Si powders Mater. Sci. Eng. A 405 135–9 Shao H Y, Wang Y T, Xu H R and Li X G 2004 Hydrogen storage properties of magnesium ultrafine particles prepared by hydrogen plasma–metal reaction Mater. Sci. Eng. B 110 221–6 Shao H Y, Liu T and Li X X 2003 Preparation of the Mg2 Ni compound from ultrafine particles and its hydrogen storage properties Nanotechnology 14 L1–3 Shao H Y, Xu H R, Wang Y T and Li X G 2004 Preparation and hydrogen storage properties of Mg2 Ni intermetallic nanoparticles Nanotechnology 15 269–74 Ohno S and Uda M 1984 Generation rate of ultrafine metal particles in ’hydrogen plasma–metal’ reaction Nippon Kinzoku Gakkaishi 48 640–6 Shao H Y, Liu T, Wang Y T, Xu H R and Li X G 2008 Preparation of Mg-based hydrogen storage materials from metal nanoparticles J. Alloys Compounds 465 527–33 Shao H Y, Asano K, Enoki H and Akiba E 2009 Preparation and hydrogen storage properties of nanostructured Mg–Ni BCC alloys J. Alloys Compounds 477 301–6 Mizutani U, Tanaka K, Matsuda T, Suzuki N, Fukunaga T, Ozaki Y, Yamada Y and Nakayama K 1993 Electronic-structure and electron-transport properties of amorphous Mg–Ni–La and Mg–Cu0 –Y alloys J. Non-Cryst. Solids 156 297–301 Janot R, Aymard L, Rougier A, Nazri G A and Tarascon J M 2003 Enhanced hydrogen sorption capacities and kinetics of Mg2 Ni alloys by ball-milling with carbon and Pd coating J. Mater. Res. 18 1749–52 Luo Q, An X-H, Pan Y-B, Zhang X, Zhang J-Y and Li Q 2010 The hydriding kinetics of Mg–Ni based hydrogen storage alloys: a comparative study on Chou model and Jander model Int. J. Hydrog. Energy 35 7842–9
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Phase, microstructure and hydrogen storage properties of Mg–Ni materials synthesized from metal nanoparticles
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 135704 (http://iopscience.iop.org/0957-4484/25/13/135704) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 169.230.243.252 This content was downloaded on 07/12/2014 at 08:45
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 135704 (6pp)
doi:10.1088/0957-4484/25/13/135704
Phase, microstructure and hydrogen storage properties of Mg–Ni materials synthesized from metal nanoparticles Huaiyu Shao1 , Chunguang Chen2 , Tong Liu2 and Xingguo Li3 1
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan 2 Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, People’s Republic of China 3 Beijing National Laboratory for Molecular Sciences (BNLMS), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China E-mail: [email protected], [email protected] and [email protected] Received 9 December 2013, revised 14 January 2014 Accepted for publication 16 January 2014 Published 28 February 2014
Abstract
After Mg and Ni nanoparticles were fabricated by hydrogen plasma metal reaction, Mg-rich Mgx Ni100−x (75 < x < 90) materials were synthesized from these metal nanoparticles to study the synergistic effects for hydrogen storage in these samples to show both good kinetics and high capacity. These Mgx Ni100−x materials may absorb hydrogen with a capacity of around 3.3–5.1 wt% in 1 min at 573 K. The Mg90 Ni10 sample shows a hydrogen capacity of 6.1 wt%. The significant kinetic enhancement is thought to be due to the unique nanostructure from the special synthesis route, the catalytic effect of the Mg2 Ni nano phase, and the synergistic effects between the Mg2 Ni and Mg phases in the materials. An interesting phenomenon which has never been reported before was observed during pressure composition isotherm (PCT) measurements. One steep step in the absorption process and two obviously separated steps in the desorption process during PCT measurements of Mg80 Ni20 and Mg90 Ni10 samples were observed and a possible reason from the kinetic performance of the Mg2 Ni and Mg phases in absorption and desorption processes was explained. These Mgx Ni100−x materials synthesized from Mg and Ni nanoparticles show high capacity and good kinetics, which makes these materials very promising candidates for thermal storage or energy storage and utilization for renewable power. Keywords: magnesium alloys, hydrogen storage, nanoparticle, kinetics S Online supplementary data available from stacks.iop.org/Nano/25/135704/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
renewable power and provide stable energy supply afterwards [6, 7]. For on-board hydrogen storage as in fuel cell vehicles, the working temperature should be below 423 K. In the latter two cases, the working temperature may be in the range of 500–700 K. The problems in Mg-based hydrogen storage materials are poor sorption kinetics (for all possible applications) and stable thermodynamics for the desorption process (for on-board applications). For Mg-based hydrogen
Mg-based materials have attracted much attention for hydrogen storage study due to their advantages such as high capacity (7.7 wt% for MgH2 ) and low cost (around 3 dollars kg−1 for Mg) [1, 2]. Their potential applications include on-board hydrogen storage [3], thermal storage [4, 5] and stationary energy storage combined with fuel cells to store fluctuating 0957-4484/14/135704+06$33.00
1
c 2014 IOP Publishing Ltd
Printed in the UK
Nanotechnology 25 (2014) 135704
H Shao et al
3. Measurement and characterization
storage materials with particle sizes above the micrometer scale, a temperature over 600 K is needed for absorption and desorption reactions even after an activation process. Common approaches to enhancing the kinetics in Mg-based hydrogen storage materials include structure modification [8], catalyst addition [9], mixing with other storage materials [10, 11], etc. To tailor the thermodynamics in Mg-based hydrogen storage materials and to lower the desorption enthalpy, researchers have tried the formation of new compounds [12, 13] and downsizing [14, 15]. However, controversial results are being reported in the literature [6, 15, 16] about whether downsizing could affect and reduce the desorption enthalpy. The ball milling technique to introduce nanostructure and defects into materials is the most widely used method for kinetic enhancement [17, 18] in hydrogen storage materials. Ball milling may decrease the crystallite size of materials to nanometer scale, but cannot reduce the particles to nanometer scale. Usually, after some cycles, the crystallite size will go back to the micrometer scale due to agglomeration during hydrogen absorption and desorption cycles [19]. After ball milling, usually a few per cent of Fe and other steel elements can be detected in the final products [20]; this is another disadvantage of the ball milling technique for the synthesis of hydrogen storage materials. The authors previously reported Mg nanoparticles with an average size of around 300 nm and Mg2 Ni nanoparticles with a size of around 40 nm for hydrogen storage study [21–23]. A Mg2 Ni nanoparticle sample with good kinetics but limited hydrogen capacity can absorb hydrogen at room temperature with a capacity of around 1.7 wt% in 30 min [23]. Mg nanoparticles can absorb hydrogen with almost the theoretic value of around 7.6 wt% at 573 K [21]. In this work, from Mg and Ni nanoparticles, we synthesized Mgx Ni100−x hydrogen storage materials to try to clarify the relationship between phase/microstructure and hydrogen storage performance to study the synergistic effects in these samples, and to develop materials with both good kinetics and high hydrogen capacity for energy storage applications.
X-ray diffraction (XRD) measurements of the Mgx Ni100−x samples were carried out to obtain the structure and phase information of the samples using monochromatic Cu Kα radiation. The size distribution and morphology of metal nanoparticles were observed by transmission electron microscopy (TEM). The hydrogen absorption rate measurements at various temperatures and the pressure composition isotherm (PCT) measurements were conducted on a home-made Sievert-type measurement system. A conventional pressure–volume–temperature technique was used to obtain the hydrogen absorption/desorption capacity change versus time by recording the change of gas pressure in a constant volume. 325 mesh Mg powder samples (purity > 99.9%) were used for comparison of the kinetics. A very thin oxide layer on the surface of the Mg nanoparticle samples may help to prevent the nanostructure samples from further oxidization. Unlike other nanostructure materials, which are very sensitive to air or moisture, in our case, all the sample transfer can be conducted in air without any atmosphere protection. 4. Results and discussion
Figure 1 shows the TEM images and XRD curves of the Mg and Ni nanoparticles. From the XRD pattern, both the Mg and the Ni are almost pure single phase. A small amount of MgO can be detected in the XRD pattern of the Mg sample. This MgO phase is on the particle surface of the Mg and was formed during the passivation process before the Mg nanoparticles were taken out of the collection chamber [21]. This thin MgO layer may protect the Mg nanoparticle samples from further oxidization. The Mg phase is hexagonal structure with a space group of P63 /mmc. The Ni nanoparticle sample has a face-centered cubic structure (space group: Fm3m). From the XRD pattern, we can see that the peak broadening in the Ni nanoparticle sample is more obvious than that in the Mg sample because the Mg nanoparticles have an average particle size of 300 nm and the Ni ones are around 30 nm based on the TEM images in figures 1(a) and (b). The Mg shows much larger particle size than the Ni particles due to the much faster generation rate of Mg by plasma synthesis methods [24]. Figure 2 shows the XRD patterns of the Mg75 Ni25 , Mg80 Ni20 , Mg85 Ni15 and Mg90 Ni10 samples. All of the four samples consist of mainly Mg and Mg2 Ni phases with nanostructure based on the peak broadening as well as previous study on the nanostructure in Mg–H and Mg2 Ni–H systems [23, 25]. The Mg2 Ni phase was synthesized by a hydrogenation process at 623 K under 3.5 MPa H2 atmosphere and later evacuation expressed by the following reactions (equations (1)–(3))
2. Experimental details 2.1. Synthesis
Mg and Ni nanoparticles were synthesized by a hydrogen plasma metal reaction method in a mixed atmosphere of 50% Ar and 50% H2 gases with a total pressure of one bar [23]. Mg (purity > 99.9%) and Ni (purity > 99.7%) were vaporized in a reaction chamber by hydrogen plasma and taken by circulating gas with a controlled rate into a collection chamber to deposit into metal nanoparticles. Before the reactive Mg nanoparticles were taken out of the collection chamber, air gas was slowly provided to the chamber as a passivation process. Mgx Ni100−x samples were synthesized from mixed pieces of Mg and Ni nanoparticles with a molar ratio of x:(100 − x) under 3.5 MPa hydrogen at 623 K for 2 h followed by evacuation.
Mgx Ni100−x + xH2 ⇒ (100 − x)Mg2 NiH4
2
+ (3x − 200)MgH2
(1)
Mg2 NiH4 ⇒ Mg2 Ni + 2H2
(2)
MgH2 ⇒ Mg + H2 .
(3)
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Figure 1. TEM images and XRD curves of Mg and Ni nanoparticles synthesized by the hydrogen plasma metal reaction.
Figure 3. TEM image of a synthesized Mg85 Ni15 nanoparticle sample. The inset shows the indexed electron diffraction pattern of the sample. The transparent large particles are Mg and the small dark particles are Mg2 Ni. The Mg particles have an average size of around a few hundreds of nm and the Mg2 Ni particles have a mean size of about 30–50 nm. Mg2 Ni particles are also dispersed on large Mg particles.
Figure 2. XRD curves of Mgx Ni100−x samples synthesized by
hydrogenation of Mg and Ni nanoparticles at 623 K under 3.5 MPa hydrogen and then evacuation. (a) Mg75 Ni25 , (b) Mg80 Ni20 , (c) Mg85 Ni15 and (d) Mg90 Ni10 .
sorption of the Mg phase [1], which makes these Mgx Ni100−x nanostructure samples potentially show superior hydrogen storage performance. From figures 3 and S1 (supplementary information available at stacks.iop.org/Nano/25/135704/mm edia), we can see that just as we reported and explained before [23], 300 nm Mg and 30 nm Ni nanoparticles form into Mg2 Ni nanoparticles with a mean size of around 30–50 nm (dark particles) and the Mg particles are still a few hundreds of nm. In figure 4(a), these Mgx Ni100−x samples with Mg and Mg2 Ni nanostructure phases show very fast hydrogen absorption kinetics. The Mg90 Ni10 sample shows a hydrogen capacity of 5.1 wt% in only 1 min after hydrogenation under 3.5 MPa at 573 K. The Mg75 Ni25 , Mg80 Ni20 , Mg85 Ni15 and
There is some unreacted Ni in the Mg75 Ni25 , Mg80 Ni20 and Mg85 Ni15 samples after the synthesis. There are also unidentified peaks (at around 29◦ , 33◦ and 36◦ ) in the XRD patterns. The peak heights shown in figure 2 have been adjusted to the same value to clearly indicate the relative ratio of Mg and Mg2 Ni phases. It is not surprising that with the increasing x, the remaining Mg phase in the Mgx Ni100−x sample is increasing. By ball milling of Mgx Ni100−x samples starting from Mg and Ni, mostly amorphous or body-centered cubic structure is observed [26, 27]. Formation of Mg2 Ni phase via ball milling starting from Mg and Ni has also been reported in some cases [28]. The existence of Mg2 Ni (and some residual Ni) may show a catalytic effect for hydrogen 3
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Mg90 Ni10 samples show hydrogen capacities of 3.6, 4.0, 4.7 and 5.5 wt% at 573 K in less than half an hour. Figure 4(b) presents the absorption rate expressed as (H/Mg) in comparison to 45 µm Mg powder, 300 nm Mg and 40 nm Mg2 Ni nanoparticle samples under hydrogen pressures of 1 MPa, 4 MPa and 4 MPa, respectively [21, 23]. Compared to the kinetic difference resulting from the nanostructure and measurement temperature, the pressure difference contributes little to the hydrogen absorption kinetics and hydrogen capacity as long as the provided hydrogen is well above the equilibrium pressure of the absorption reaction [29]. From figure 4(b), we may see that the micrometer scale Mg powder sample cannot show any hydrogen capacity at 573 K. The 300 nm Mg nanoparticles synthesized by the hydrogen plasma metal reaction technique show an absorption content of 1.97 H/M (7.5 wt%), which is very close to the theoretical value of 2.0 H/M. The nanostructured Mg2 Ni sample and Mgx Ni100−x samples absorb hydrogen with smaller H/M values but with a faster rate than the Mg nanoparticle sample. It takes about 20 min for the hydrogen absorption of a Mg nanoparticle to come to saturation. However, it only takes 1 min for the nanostructured Mg2 Ni sample and Mgx Ni100−x samples to absorb about 90% of the hydrogen content. With the existence of the nanostructured Mg2 Ni phase, which can act as a catalyst to the absorption reaction of the Mg phase [1], the Mg90 Ni10 and Mg85 Ni15 samples even absorb hydrogen faster than the Mg2 Ni nanoparticle sample (figure 4(b)). The Mg85 Ni15 sample also shows good absorption kinetics at lower temperatures (figure 5). After 1 h of absorption under 3.5 MPa hydrogen, it absorbs hydrogen with a capacity of 1.8 wt% at 373 K and 4.4 wt% at 523 K. We think that the following factors contribute to the superior kinetic performance in these Mgx Ni100−x samples.
Figure 4. Hydrogen absorption curves of (a) Mgx Ni100−x samples
at 573 K and (b) the comparison results with the Mg2 Ni nanostructured sample and different sized Mg samples (the pressures for the 45 µm Mg powder, 300 nm Mg and 40 nm Mg2 Ni nanoparticle samples are 1 MPa, 4 MPa and 4 MPa, respectively).
(a) The nanostructure in these materials compared to micrometer scale samples means that there is a much larger surface area for contact with hydrogen and a much shorter hydrogen diffusion distance during hydrogen absorption and desorption reactions. (b) The existence of nanostructured Mg2 Ni phase shows a catalytic effect for sorption reactions in the Mg phase. (c) The synergistic effect of nanostructured Mg2 Ni and Mg phases in these samples. The nanostructured Mg2 Ni phase synthesized from Mg and Ni nanoparticles is around 30– 50 nm and the Mg nanoparticle phase is around 300 nm. The mixture of two phases with different particle size may result in strain and defects along the phase boundaries. After one hydrogen cycle, the Mg nanoparticles show cracked surfaces and inner diffusion tunnels for hydrogen transfer [21]. Combination of the strain, defects, cracked surfaces and new hydrogen diffusion tunnels led to the synergistic effects for hydrogen storage in these materials.
Figure 5. Hydrogen absorption curves of the Mg85 Ni15 sample at
373, 523, 573 and 623 K.
two plateaus (steps) during both the absorption and desorption processes for these two Mg-rich Mg–Ni based samples. Also, the hydrogen content in the lower plateaus (due to the absorption and desorption of the Mg phase) and higher plateaus (due to the Mg2 Ni phase) should be in a ratio of 7:2 for the Mg90 Ni10 sample and 4:4 for the Mg80 Ni20 sample. According to earlier van’t Hoff equations and equilibrium pressure results in Mg and Mg2 Ni nanoparticle samples, theoretical PCT curves based on Mg90 Ni10 and Mg80 Ni20 compositions were
Figure 6 presents PCT curves of Mg90 Ni10 and Mg80 Ni20 samples at 573 K. Based on the XRD results, without consideration of impurity phases, these two samples consist of (7Mg + Mg2 Ni) and (4Mg + 2Mg2 Ni) phases, respectively. Theoretically and also experimentally [1], there should be 4
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Mg and Ni nanoparticles show quite unique characteristics compared to materials from other synthesis techniques: fast kinetics due to nanoscale particle size (not just nanoscale crystallite size); high hydrogen capacity; good durability in air compared to other nanomaterials. We believe that these Mgx Ni100−x materials are perfect candidates for applications like thermal storage or stationary hydrogen energy storage for utilization of renewable power. 5. Conclusions
The main conclusions of this work are as follows. (1) Mg75 Ni25 , Mg80 Ni20 , Mg85 Ni15 and Mg90 Ni10 samples with nanostructure were synthesized from Mg and Ni nanoparticles. (2) These Mgx Ni100−x samples show fast kinetics and high hydrogen capacity. The Mg90 Ni10 sample shows a hydrogen capacity of 6.1 wt% at 573 K. These Mgx Ni100−x materials may absorb hydrogen with a capacity of around 3.3–5.1 wt% in 1 min at 573 K. The mechanism for significant kinetic enhancement in these materials was clarified. (3) An interesting phenomenon that has never been reported before was observed during PCT measurements of the Mg90 Ni10 and Mg80 Ni20 samples. One steep step in the absorption process and two obviously separated steps in the desorption process were observed and the possible mechanism was explained.
Figure 6. PCT curves of Mg90 Ni10 and Mg80 Ni20 samples at
573 K.
simulated (see figure S2 and table S1 in the supplementary information available at stacks.iop.org/Nano/25/135704/mme dia). However, in figure 6, a very interesting phenomenon was observed. The desorption curves for these two samples consist of two flat steps (more obviously for the Mg90 Ni10 sample) due to the reactions of equations (2) and (3); this is in agreement with the theoretical expectation. However, the absorption curves of both of these two samples show most likely only one steep step and we cannot easily find the separation boundary for the two absorption reactions of the Mg and Mg2 Ni phases in the two samples, (100 − x)Mg2 Ni + (3x − 200)Mg + xH2 ⇒ (100 − x)Mg2 NiH4 + (3x − 200)MgH2 .
(4)
Acknowledgments
We believe that the reason for this phenomenon is that when one tries to get equilibrium plateaus from PCT measurements for hydrogen storage materials, actually the plateau results for the PCT curves depend greatly on the kinetic performance and the equilibrium parameters set for the PCT measurements. In our case, the Mg90 Ni10 and Mg80 Ni20 samples both have two phases—the Mg phase with lower absorption and desorption plateaus and relatively poor kinetics, and the Mg2 Ni phase with higher plateaus and better kinetics. During the absorption process, the Mg with poorer kinetics should absorb hydrogen first because of the lower equilibrium pressure, but this absorption part of the Mg phase merges with that from the Mg2 Ni phase with better kinetics (although with higher hydrogen pressures) during the hydrogenation reaction of the Mg phase reaching its equilibrium state, and forms into only one steep absorption step (equation (4)). During the desorption process, the Mg2 NiH4 phase with fast kinetics and higher equilibrium pressure desorbs first completely during decrease of the hydrogen pressure and then the MgH2 phase desorbs. This is why these two steps can be clearly separated in the desorption process. To the best of our knowledge, this is the first on the kinetic performance difference in the two phases affecting the thermodynamic performance (reaction steps in the PCT curves). According to the above discussion, we may say that the nanostructured Mgx Ni100−x materials synthesized from
HS acknowledges support from the International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), established by the World Premier International Research Center Initiative (WPI), MEXT, Japan. XL acknowledges support from the Ministry of Science and Technology of China (No. 2012CBA01207). TL acknowledges support from the China Program of Magnetic Confinement Fusion under grant number 2012GB102006. References [1] Reilly J J and Wiswall R H 1968 Reaction hydrogen with alloys magnesium and nickel and formation of Mg2 NiH4 Inorg. Chem. 7 2254–6 [2] www.infomine.com [3] Yang J, Sudik A, Wolverton C and Siegel D J 2010 High capacity hydrogen storage materials: attributes for automotive applications and techniques for materials discovery Chem. Soc. Rev. 39 656–75 [4] Bogdanovi´c B, Ritter A and Spliethoff B 1990 Active MgH2 –Mg systems for reversible chemical energy-storage Angew. Chem. Int. Edn 29 223–34 [5] Gross A F, Ahn C C, Van Atta S L, Liu P and Vajo J J 2009 Fabrication and hydrogen sorption behaviour of nanoparticulate MgH2 incorporated in a porous carbon host Nanotechnology 20 204005 5
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