Microstructural evolution during hydrogen sorption cycling of Mg–FeTi nanolayered composites

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Acta Materialia 59 (2011) 2083–2095 www.elsevier.com/locate/actamat

Microstructural evolution during hydrogen sorption cycling of Mg–FeTi nanolayered composites W.P. Kalisvaart ⇑, Alan Kubis, Mohsen Danaie, Babak Shalchi Amirkhiz, David Mitlin ⇑ Chemical and Materials Engineering, University of Alberta and National Research Council Canada, National Institute for Nanotechnology, Edmonton, AB, Canada T6G 2V4 Received 1 August 2010; received in revised form 2 December 2010; accepted 5 December 2010 Available online 10 January 2011

Abstract This paper describes the microstructural evolution of Mg–FeTi mutlilayered hydrogen storage materials during extended cycling. A 28 nm Mg–5 nm FeTi multilayer has comparable performance to a cosputtered material with an equivalent composition (Mg–10%Fe– 10%Ti), which is included as a baseline case. At 200 °C, the FeTi layers act as a barrier, preventing agglomeration of Mg particles. At 300 °C, the initial structure of the multilayer is preserved up to 35 cycles, followed by fracturing of the Mg layers in the in-plane direction and progressive delamination of the FeTi layers as observed by electron microscopy. Concurrently, an increase in the Mg grain size was observed from 32 to 76 nm between cycles 35 and 300. As a result, the absorption kinetics deteriorate with cycling, although 90% of the total capacity is still absorbed within 2 min after as many as 300 cycles. The desorption kinetics, on the other hand, remain rapid and stable, and complete desorption of 4.6 wt.% H is achieved in 1.5 min at ambient desorption pressure. In addition to showing good hydrogen storage performance, multilayers are an excellent model system for studying the relation between microstructure and hydrogen absorption/desorption kinetics. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen storage; Magnesium alloys; Nanocomposite; Multilayers

1. Introduction Because of its high gravimetric capacity of 7.6 wt.% H, MgH2 has attracted a lot of attention as a solid-state hydrogen storage medium. However, a key drawback is the hydride’s slow hydrogenation/dehydrogenation kinetics, due to, among other factors, the extremely low diffusivity of hydrogen in MgH2 [1–3] and the poor catalytic activity of Mg surface towards hydrogen dissociation [4]. Numerous studies have shown that the sorption kinetics can be significantly improved by ball-milling as it reduces the grain size [5,6] and introduces defects [7] providing rapid diffusion paths for hydrogen and reducing the diffu-

⇑ Corresponding authors. Tel.: +1 780 850 2678 (W.P. Kalisvaart), +1 780 492 1542 (D. Mitlin). E-mail addresses: [email protected] (W.P. Kalisvaart), [email protected] (D. Mitlin).

sion distances. Milling also enables intimate mixing of catalytic phases with the Mg [8–12] to facilitate rapid dissociation of hydrogen molecules on the Mg surface. However, in practical applications the kinetics not only have to be fast, but must also remain stable over hundreds of cycles. It was recently found that the addition of a mixture of single-wall carbon nanotubes, amorphous carbon and metal catalyst particles prevents coarsening of Mg particles and grains to a large extent [13]. Compared to the baseline case of milled MgH2 without additives, grain growth of Mg was slowed by over a factor three by the addition of the carbon mixture. The most important factor in stabilization of the kinetics was the prevention of sintering of the Mg particles, keeping the free surface area in contact with hydrogen gas sufficiently high for rapid cycling. Increase in absorption and desorption times during cycling was slowed down by a factor three or more for the MgH2-SWCNT mixture compared to the baseline. Analysis of the activation energies revealed that the

1359-6454/$36.00 Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2010.12.010

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catalytic activity of the metal particles was lost during cycling [13]. In view of the above, the challenge is to design the hydrogen storage material in such a way that: (1) grain growth of Mg is inhibited; (2) sintering of Mg particles is prevented; and (3) the activity of catalytic additives is preserved during cycling. By arranging the Mg and catalyst in a layered structure, the catalyst layers can act as a diffusion barrier against sintering of Mg while simultaneously catalyzing the dissociation of hydrogen. In order to try and preserve the layered structure during cycling, the catalyst phase must be nonreactive with Mg. Mg-based materials with layered morphologies can be produced by cold-rolling techniques and are being intensively studied as hydrogen storage media with superior properties compared to pure Mg [14–17]. As it is not very different from rolling processes used in the production of steel, this technique may be suitable for bulk production of ultra-fine microstructures, much more so than ball-milling [18]. The present paper presents an in-depth study of the hydrogen cycling behavior of sputtered Mg–FeTi multilayers as a model system for cold-rolled materials. This material combines all aforementioned properties; Mg–Fe and Mg–Ti are immiscible [19] and FeTi is a well-known catalyst for hydrogen dissociation [20,21]. The activation barrier for hydrogen dissociation on Mg is around 1 eV/H2 molecule (100 kJ mol1) [4], whereas for FeTi this value is much lower, around 0.2 eV or 20 kJ/H2 molecule [22]. The kinetics and capacity of the multilayers are studied as a function of temperature and the Mg and FeTi layer thicknesses, and the evolution of their microstructure is studied by electron microscopy and X-ray diffraction (XRD). As a baseline, cosputtered Mg–Fe–Ti layers with equivalent composition are tested under identical conditions. Cosputtered Mg–Fe–Ti films form a very finely dispersed Mg–catalyst composite with very rapid sorption kinetics upon cycling [21,23], which makes them a model system for ball-milled materials, the same way as sputtered multilayers are for cold-rolled composites. From a comparison between multilayers and cosputtered materials, a fundamental understanding of the mechanisms underlying the changes in hydrogen storage behavior and microstructure in the course of cycling is derived. 2. Experimental Mg-FeTi multilayers with periodicities of 100 and 33.3 nm were synthesized by sputter deposition. These multilayers will be denoted hereafter as 85/15 and 28/5 (Mg and FeTi layer thicknesses, respectively). We used Ar gas with a purity of 99.999% at a sputtering pressure of 5  103 mbar, with a maximum base pressure of 5  108 mbar. Deposition was performed using a DC magnetron co-sputtering

system (AJA International). The substrate temperature was maintained near ambient. Deposition was done in a sputter-up configuration with continuous substrate rotation. Total thickness of the stack was always 1.5 lm. To protect the stacks from oxidation and to catalyze the dissociation of hydrogen on the surface, a Ta/Pd catalyst [24] was deposited on the top and bottom. The Si substrates were first coated with a layer of photoresist to enable lift-off of the films in acetone after deposition [23]. Absorption and desorption kinetic measurements were performed in an automated Sieverts-type apparatus (HyEnergy Scientific Instruments PCTPro2000). Typical sample mass was 15–20 mg. Cycling tests were performed at 200 and 300 °C. The pressure in the absorption reservoir (volume: 11.9 ml) was set to 3 and 4.5 bar, respectively. For desorption, the reservoir (1025 ml) was initially put under primary vacuum at 200 °C and 1 bar at 300 °C. For the cycling tests at higher temperatures, the samples were first activated at 200 °C until the kinetics were stable before increasing the temperature to 300 °C. The absorption and desorption steps were terminated when an average rate lower than 0.005 wt.% min1 was measured over a period of 4 min. Scanning electron microscopy (SEM) observations were performed on a Zeiss NV vision 400 equipped with a gallium liquid metal ion source. Transmission electron microscopy (TEM) samples of as-deposited materials were prepared using focussed ion beam (FIB) lift-out. The FIB was operated at 30 kV and down to a lowest probe current of 80 pA to polish the surfaces of the TEM membranes. TEM observations were performed on a JEOL 2010 and a JEOL 2200FS at 200 kV accelerating voltage. TEM powder samples were prepared by directly dispersing the powders on amorphous carbon-coated copper grids. Conventional bright-field and dark-field imaging and energy-dispersive X-ray (EDX) spectrometry elemental mapping in scanning/transmission mode on cycled samples were performed on the JEOL 2200 FS. Imaging and selected-area diffraction (SAD) on as-deposited samples prepared with the FIB were done on the JEOL 2010. XRD experiments were performed on a Bruker AXS diffractometer (Bruker Discover 8) using a Cu Ka radiation ˚ ) that was monochromatized using a source (k = 1.5406 A single Go¨bel mirror. The diffractometer is equipped with a HiStar general area two-dimensional detection system (GADDs) with a sample–detector distance of 15 cm. Integral breadth analysis (IBA) was used to determine the crystallite size and microstrain for Mg in the cycled samples. The following relation was used [25]: ðd2hÞ

2

ðtanh0 Þ

2

¼

Kk d2h þ 16e2 D sinh0 tanh0

ð1Þ

where d2h is the integral breadth of the peak in radians and h0 is the position of the peak maximum. The values for the integral breadth were corrected for instrumental effects using a LaB6 standard obtained from NIST [26]. By

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plotting (d2h)2/(tanh0)2 vs. (d2h)/(sinh0tanh0), values for the crystallite size D and microstrain e can be determined from the slope and ordinate intercept, respectively. 3. Results 3.1. Mg 85/FeTi 15 nm multilayer Fig. 1A and B show a dark-field TEM micrograph of the as-deposited 85/15 multilayer and its electron diffraction pattern, respectively. In the as-deposited state, the individual Mg and FeTi layers can be clearly observed. The FeTi layers are, despite being only 15 nm thick, completely continuous. The layer thicknesses can be seen to be very close to the nominal ones. The electron diffraction pattern is consistent with a hexagonal close-packed pattern for Mg with [2  1 1 0] zone axis. Upon cycling, the material undergoes a remarkable transformation as can be seen from Fig. 1C. The individual FeTi layers are still intact and can be easily distinguished in the SEM micrograph. The Mg layers, on the other hand, have completely broken up into small particles that have retained contact with the FeTi layers. The 85/15 multilayer has relatively slow kinetics during the first few cycles (see Fig. 2). After 5–10 cycles, the first 3–3.5 wt.% is absorbed in a matter of seconds, which reflects

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the microstructural development seen in Fig. 1. In the first cycle, the only catalytically active sites are the Pd layers on the top and bottom, and hydrogen has to diffuse through each individual FeTi and Mg(H2) layer. When the Mg layers start breaking up into particles, hydrogen molecules can also be dissociated on the free FeTi surfaces between the Mg particles, leading to increased absorption rates. However, the hydrogen sorption rates are not stable at this initial high level and start to degrade quickly. As can be seen in Fig. 2, after 25 cycles the capacity has decreased from 4.5 to 4.0 wt.% and the final stage of absorption is markedly slower compared to, for example, the 5th cycle. The same trend is observed for desorption: after 25 cycles the capacity and kinetics have markedly degraded. It must be kept in mind here that an absorption or desorption cycle is stopped when the rate falls below 0.005 wt.%/min and that a decrease in capacity is, in principle, equivalent to degradation of the kinetics. 3.2. Mg 28 nm/FeTi 5 nm multilayer vs. cosputtered Mg– 10%Fe–10%Ti In order to try and improve upon the properties of the 85/15 multilayer, the layer thickness of both Mg and FeTi were reduced by a factor 3. For the 28/5 multilayer (see Fig. 3A and B), the picture is virtually the same as for

[0004]

[000 2] [0111]

[0111] [0110]

[0000] [0110]

[0111] [0111] [0002]

[0004]

(A)

(B)

[2110]

1 μm

(C) Fig. 1. (A): Dark-field TEM image of as-deposited Mg 85/FeTi 15 nm multilayer. (B) Electron diffraction pattern of as-deposited multilayer. (C) SEM image of cycled Mg 85/FeTi 15 nm multilayer. Note how the FeTi layers remain completely intact during cycling and how the Mg layers form small particles between the FeTi layers.

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(A)

5

5th

2nd

10th

25th

4

wt% H

1st 3 2 1 0 0

5

10

15

20

25

30

Time (min)

(B)

0

wt% H

-1 -2

5th & 10th

-3

25th -4

1st

2nd -5 0

5

10

15

20

25

30

Time (min) Fig. 2. Hydrogen absorption and desorption data of the Mg 85/FeTi 15 nm multilayer capped with Ta and Pd. (A) Absorption cycle 1, 5, 10 and 25. (B) Desorption cycle 1, 5, 10 and 25. After 25 cycles, both capacity and kinetics are clearly degraded with respect to their maximum values from the second cycle.

the 85/15 multilayer. The FeTi and Mg layers are fully continuous in the as-deposited state. The electron diffraction pattern is essentially identical to that of the 85/15 multilayer; [2 1 1 0] zone axis and no additional diffraction spots that can be attributed to crystalline FeTi are visible in the SAD pattern. In the post-cycled state, the Mg does not seem to form spherical particles between the FeTi layers. Instead, only plate-like features are found and intact FeTi layers are also visible in the image. The hydrogen cycling behavior of the 28/5 multilayer is compared with that of the chosen baseline case, namely a cosputtered material with the same Mg:FeTi molar ratio (Mg–10%Fe–10%Ti) in Fig. 4A–C. Selected absorption and desorption cycles for the multilayer and cosputtered material are shown in Fig. 4A and B. Fig. 4C depicts the time to 90% absorption (left-hand y-axis) and the time to 90% desorption (right-hand y-axis). All the example curves in Fig. 4A and B look very similar, except that the multilayer exhibits a slightly longer activation period; the 5th absorption cycle of the multilayer is still markedly slower than subsequent cycles. For desorption, there is virtually no difference in kinetics between the multilayer and the cosputtered alloy. However, the absorption/desorption time plots in Fig. 4C reveal that the cycling stability of the cosputtered material is slightly better: both the absorption and desorption times of the multilayer show a slightly upward trend. Compared to the data in Fig. 2, however, the decrease in the layer thicknesses by a factor of three is shown to vastly improve the hydrogen cycling properties in terms of kinetic stability.

[0004] [000 2]

[0111]

50 nm

[0111]

[0000]

[0111] [0111] [0002] [0004]

(A)

(B)

[2110] Zone Axis

2 μm

(C)

Fig. 3. SEM micrographs of as-deposited (A) and cycled (B) Mg 28 nm/FeTi 5 nm multilayer.

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wt% H

(A)

5

5th cycle Mg-10%Fe-10%Ti 5th cycle Mg 28/FeTi 5nm 50th cycle Mg-10%Fe-10%Ti 50th cycle Mg 28/FeTi 5nm

4

100th cycle Mg-10%Fe-10%Ti 100th cycle Mg 28/FeTi 5nm

3

2 0

1

2

3

4

Time (min)

(B)

0

5th cycle Mg-10%Fe-10%Ti 5th cycle Mg 28/FeTi 5nm 50th cycle Mg-10%Fe-10%Ti 50th cycle Mg 28/FeTi 5nm

wt% H

-1 -2

100th cycle Mg-10%Fe-10%Ti 100th cycle Mg 28/FeTi 5nm

-3 -4 -5 0

5

10

15

20

25

30

Time (min) 20

50

40 15 30

desorption Mg 28/FeTi 5nm desorption Mg-10%Fe-10%Ti

10

absorption Mg 28/FeTi 5nm

20

absorption Mg-10%Fe-10%Ti

5 10

0 0

30

60

90

Time to desorb 90% (min)

Time to absorb 90% (s)

(C)

0 120

cycle number Fig. 4. Comparison between cosputtered Mg–10%Fe–10%Ti and Mg 28 nm/FeTi 5 nm multilayer at 200 °C. (A) Selected absorption cycles (B) Selected desorption cycles. (C) Time to absorb (left y-axis) 90% of the maximum capacity in seconds and time to desorb (right y-axis) 90% of the maximum capacity in minutes.

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due to the strong alloying tendency between Mg and Pd. More recently, it was also found that for Mg–Ti multilayers, which do not alloy and where the Ti layers hydrogenate before Mg, destabilization occurs for an Mg layer thickness below 10 nm [30]. In view of the above, it is worthwhile to check the equilibrium plateau pressures of the multilayers. Fig. 5 shows the desorption pressure–composition isotherms at 200, 250 and 300 °C for the 28/5 multilayer after 20 activation cycles at 200 °C. The plateau pressures are 0.05, 0.33 and 1.6 bar, respectively, giving a value for the heat of hydride formation of 79 kJ/H2 molecule, which is the same as found previously for cosputtered Mg–Fe–Ti films [21] and in good agreement with values for the hydrogenation enthalpy of Mg reported widely the literature [31]. Fig. 4 showed that the performance of the 28/5 multilayer is comparable to that of a cosputtered film with the same Mg/FeTi ratio at 200 °C. However, the plateau pressure at this temperature is only 50 mbar, whereas a practical hydrogen storage system would operate at nearatmospheric pressures. It was shown in Fig. 5 that in order to reach more than 1 bar equilibrium pressure, temperatures of around 300 °C are needed. The hydrogen cycling behavior at this higher temperature of the 28/5 multilayer is reported in the next section and compared with cosputtered material of equivalent composition. 3.4. Multilayers vs. cosputtered Mg–10%Fe–10%Ti at 300 °C 3.4.1. Microstructure SEM images of the 28/5 multilayer after 35, 150 and 300 cycles at 300 °C are presented in Fig. 6. After 35 cycles, all the material consisted, to the naked eye, of large flakes that were up to several millimeters wide (Fig. 6A). In the area indicated by the arrow, the FIB was used to make a cut perpendicular to the carbon support to reveal the internal structure of the flakes. A frontal view of the indicated area is depicted in Fig. 6B. Originally, the stack consists of 45 Mg/FeTi bilayers that have now partially delaminated and bent out of shape due to the repeated expansion/

1.0

573 K

3.3. Thermodynamic properties Theoretical works suggest that reducing the particle size of Mg(H2) to extremely small sizes can alter the thermodynamic properties [27], specifically for particles with fewer than 10 Mg atoms. The dimensions of the Mg in the present study are far from this theoretical limit, which would correspond to a layer thickness of approximately 2 nm. However, a study by Baldi et al. showed destabilization of a Mg layer confined between Pd and TiH2 [28,29], which was attributed to clamping effects at the Mg/Pd interface

ln P eq (bar)

0.0 -1.0

523 K -2.0 -3.0

473 K -4.0

ΔH = -79 kJ/mol H2 ΔS = -140 J/mol/K

-5.0 0

1

2

3

4

5

wt% H Fig. 5. Desorption pressure–composition isotherms at 200, 250 and 300 °C of a Mg 28 nm/FeTi 5 nm multilayer.

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(A)

(C)

(E)

50 μm

100 μm

1 μm

(B)

(D)

(F)

1 μm

1 μm

1 μm

Fig. 6. SEM micrographs of Mg–10%Fe–10%Ti and the 28/5 multilayer after cycling at 300 °C. (A) Macroscopic view after 35 cycles. The material consists of millimeter-sized flakes. The arrow indicates the spot where a FIB cut was made. (B) Frontal view of area indicated in (A). (C) Macroscopic view after 150 cycles. (D) Higher magnification after 150 cycles. (E) High-magnification image after 300 cycles. Note the delamination of FeTi from the Mg layers. (F) Backscattered electron image of same area as in (E).

contraction of the Mg layers during hydrogen absorption/ desorption cycling. In the lateral direction, these filaments are still intact over distances of several microns. After 150 cycles, this picture has radically changed. A large aggregate of smaller Mg filaments and a higher magnification image of the same are shown in Fig. 6C and D, respectively. Planar regions several microns long are still present, but the features visible within these regions strongly suggest that the FeTi layers are delaminating from their underlying Mg layers. This is even clearer after 300 cycles, as can be seen in Fig. 6E and the backscattered electron image of the same area in Fig. 6F. Some large intact sheets of material are visible on the left and in the middle of the image with a large number of very small particles on their edges. In the BSE image, the small particles appear very bright, which means they probably consist of FeTi. A dark-field TEM image and elemental mappings for the 28/5 multilayer after 150 cycles at 300 °C are shown

in Fig. 7. As in Fig. 6C and D, clear evidence of delamination of FeTi layers from the Mg layers is observed. EDX analysis showed one area of pure Mg, marked accordingly in Fig. 7, and on the edges of the particle what seems to be the remains of an FeTi layer that has snapped and peeled back from the underlying Mg layer. A bright-field (top left) and high-angle annular darkfield (HAADF) image (top right) of the multilayer after 300 cycles are shown in Fig. 8 along with elemental mappings. Comparing these images with those obtained after 150 cycles in Fig. 7, it is clear that the multilayer stack is in a more advanced state of delamination, similar to what was observed in Fig. 6C–F. From the HAADF image and the elemental mappings, the filament-like features can clearly be seen to be FeTi layers that have peeled off from the Mg layers during cycling. Fig. 9 shows a dark-field image of a representative area for cosputtered Mg–10%Fe–10%Ti after 250 cycles at

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very finely dispersed through the Mg matrix, although the encircled area in the bright-field image is almost pure Mg over an area more than 100 nm wide. The Ta from the catalyst bilayer is visible as small particles indicated by the arrows. Pd is found throughout the entire area in the image, but matches most strongly with Mg, indicating it has formed a Mg–Pd alloy. Fig. 10A shows the XRD patterns of the 28/5 multilayer after 35, 150 and 300 cycles at 300 °C. Besides Mg, only the strongest reflection, (822), of Mg6Pd is observed in the diffraction pattern. Crystalline FeTi, for which the strongest reflection, (1 1 0), is expected around 42.9° is not observed at any stage of cycling, which means it remains amorphous or nanocrystalline. The onset of crystallization for sputterdeposited FeTi has been found to occur at a temperature above 350 °C [32,33]. Since all hydrogen cycling experiments were performed at 300 °C and lower, crystallization of the FeTi is not expected here. For cosputtered Mg–10%Fe–10%Ti, diffraction patterns of the as-deposited state and the desorbed and absorbed state after 250 cycles at 300 °C are shown as patterns (a), (b) and (c), respectively, in Fig. 10B. In the as-deposited state, the Mg (0 0 2) peak is located around 35°, which indicates that the Fe and Ti are dissolved in Mg. After cycling, the strongest reflection of Mg6Pd is detected around 37.9°. As with the multilayer, Mg6Pd is the only crystalline phase that is detected besides Mg. After cycling, the Mg and MgH2 reflections are at their theoretical positions, which means the dissolved Fe and Ti have segregated from the Mg. The FeTi, again, remains amorphous.

FeTi

Mg

FeTi

100 nm

Ti

Mg

Fe Fig. 7. HAADF TEM image and elemental mappings of 28/5 multilayer after 150 cycles at 300 °C. The area enclosed by the rectangle was scanned with EDX to map the distribution of Mg, Ti and Fe. Region where only Mg is detected is indicated by arrow marked “Mg”. Remnants of an FeTi layer are visible on the edges.

300 °C. Elemental mappings of Mg, Fe, Ti, Pd and Ta were made of the entire imaged area. Ti and Fe can be seen to be

100 nm

100 nm

Mg

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Fe

Ti

Fig. 8. Bright-field TEM image (top left) and HAADF image of the same area (top right) and elemental mappings of 28/5 multilayer after 300 cycles at 300 °C. The entire area in the images was scanned with EDX to map the distribution of Mg, Ti and Fe.

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200 nm

200 nm

Mg

Fe

Pd

Ti

Ta

Fig. 9. Bright-field (top left) and HAADF (top right) TEM image and elemental mappings of cosputtered Mg–10%Fe–10%Ti after 250 cycles at 300 °C. The entire area in the image was scanned with EDX to map the distribution of Mg, Ti, Fe, Pd and Ta. Ta particles are indicated by the arrows.

Fig. 11A shows the results of IBA to determine the Mg grain size in the course of cycling for the multilayered material. For the 28/5 multilayer, the Mg grain size derived from the IBA analysis steadily increases from 32 nm after 35 cycles to 53 and 76 nm after 150 and 300 cycles, respectively. The value for the microstrain is almost constant at 0.10–0.13% for the desorbed multilayers. For the cosputtered material, the results of IBA analysis are shown in Fig. 11B. Going from 60 to 250 cycles at 300 °C, the crystallite size increases from 19 to 32 nm, which is a relatively small increase compared to the multilayers (36% per 100 cycles vs. 52% for the multilayer). The value obtained for the microstrain after 60 cycles is 0.11% and decreases to zero after 250 cycles. 3.4.2. Kinetics and cycling stability Selected absorption and desorption cycles at 300 °C for the 28/5 multilayer and cosputtered Mg–10%Fe–10%Ti are compared in Figs. 12 and 13. The performance of cosputtered and mutlilayered material is quite similar for the first

100 cycles, the same as was found at 200 °C. Beyond that point, however, the absorption behavior of the multilayer starts to change as can be seen in Fig. 12A and C. The absorption time to 90% of the maximum capacity increases from 35 s to over 2 min between cycle 100 and 200, a 4-fold increase. The cosputtered material hardly shows any degredation in the absorption time—from approximately 30 s in the first cycle to 1 min after 250 cycles. Selected desorption cycles at 300 °C for the 28/5 multilayer are shown in Fig. 13A. Contrary to absorption, the desorption kinetics are very stable for long cycling periods. Even after 300 cycles, it takes 1 min at 1 bar desorption pressure to fully desorb all the hydrogen from the material, thus demonstrating rapid and stable kinetics at practical operating conditions. For the cosputtered material, the desorption curves are shown in Fig. 13B. As with the multilayered material, the desorption time hardly changes during cycling and it takes aproximately 1.5 min to fully desorb. The total desorption capacity is approximately 4.5 wt.% for both the multilayer and the cosputtered material.

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(A)

(A)

(101)

Intensity (A.U.)

(100) (002)

Multilayer

2.8E-04 35 cycles D = 32 nm, e = 0.10%

(102)

(822)

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2.1E-04 300 cycles

150 cycles D = 53 nm, e = 0.13%

1.4E-04 150 cycles

300 cycles D = 76 nm, e = 0.12%

7.0E-05 35 cycles

30

35

40

45

0.0E+00

50

0

0.02

0.04

0.06

0.08

Intensity (A.U.)

(B) (B)

Cosputtered

8.0E-04

60 cycles D = 19 nm, e = 0.11%

6.4E-04

(c)

4.8E-04 Mg (002) 34.98o

(b)

Mg MgH2 Mg6Pd

3.2E-04

(a)

20

1.6E-04

30

40

50

60

0.0E+00

2θ (degrees)

0

Fig. 10. XRD patterns of (A) cycled 28/5 multilayer in desorbed state after 35, 150 and 300 cycles at 300 °C; (C, B) Mg–10%Fe–10%Ti in (a) asdeposited state, (b) desorbed state after 250 cycles and (c) absorbed state after 250 cycles at 300 °C.

It can be seen in Fig. 12 that, after 150 cycles, the hydrogenation response of the multilayer starts to show a distinct sigmoidal shape that is typical of nucleation-and-growth processes that can be described by the Johnson–Mehl– Avrami (JMA) equation [34]: f ¼ 1  expðKtn Þ

250 cycles D = 32 nm, e = 0

70

ð2Þ

where f is the transformed fraction and K is a constant associated with the nucleation and growth rates. The value of the exponent n can, depending on the type of nucleation (e.g. constant nucleation rate or site-saturation), dimensionality of the growth and the rate-limiting step during growth (interface or diffusion-controlled), range between 0.5 and 4 [35]. A plot of ln(ln(1/1  f)) vs. lnt then gives a straight line with slope n and intercept lnK. The 250th absorption cycle for the 28/5 multilayer is shown in Fig. 14. A plot of ln(ln(1/1  f)) vs. lnt is shown in the inset on the bottom right. Two stages are clearly observed with different slopes, giving different values for the exponent n. The first stage ranges from a transformed fraction near zero to aproximately 0.35 and n has a value near 2 (2.27) here. The second stage ranges from f = 0.5 to 1 and a value for n of 1.23 is obtained. The data depicted in Fig. 14 are representative of the materials’ behavior in ”stage III” (see Fig. 12C). Between cycle 200 and 300, values obtained for n range between 1.95 and 2.27 for f 6 0.35 and between 1.14 and 1.23 for f P 0.5. The difference

0.02

0.04

0.06

0.08

0.1

Fig. 11. Integral breadth analysis for the cycled multilayers and the cosputtered material in the desorbed state. Values for the crystallite size and microstrain are indicated.

between the exponents is always approximately equal to 1 (0.85–1.05). For desorption, the entire curve can be fitted with a single value for the exponent n and rate constant K. Fig. 14B shows the 250th desorption cycle for the 28/5 mutlilayer, together with the fitted values for n and lnK and the resulting fit to the desorption curve. The values obtained for n and lnK are 1.09 and 3.29, respectively. Between cycle 200 and 300, the value of n varies between 1.09 and 1.25. 4. Discussion For both the 85/15 and 28/5 multilayers, intact FeTi layers could still be discerned in SEM images (see Figs. 1 and 3). At low temperatures and sufficient thickness, the mechanical strength of the FeTi layers is apparently sufficient to function as a barrier between Mg layers, preventing sintering of the Mg particles in the direction perpendicular to the layers. For the 28/5 multilayer, the hydrogen cycling performance is comparable to that of the cosputtered material. It can therefore be concluded that at 200 °C, the idea to use the FeTi in the dual role of catalyst and agglomeration barrier works perfectly when the bilayer thickness is sufficiently low. In the case of the 85/15 multilayer, the absorption kinetics decayed rapidly. In particular, the final stage of absorption from 3.5 to 4 wt.% H absorbed becomes very slow (see

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(A)

Absorption at 300oC

5

(A)

Mg 28/FeTi 5nm multilayer -1

3 2

Cycle 1

Cycle 200

Cycle 50

Cycle 250

Cycle 100

Cycle 300

wt% H

4

wt% H

Desorption at 300oC

0

-2 -3

Cycle 150 -4

1

Mg 28/FeTi 5nm multilayer

0 0

2

4

6

-5

wt% H

Cycle 1 Cycle 50

3

Cycle 150 2

Cycle 200

1

Cycle 250

3

4

5

6

0

Cosputtered Mg-10%Fe-10%Ti

0

2

4

6

-3

200th, 250th

1st, 50th, 150th -5 0

1

2

8

Time (min)

3

4

5

6

Time (min) Fig. 13. Desorption kinetics at 300 °C over the course of cycling for (A) 28/5 multilayer and (B) cosputtered Mg–10%Fe–10%Ti.

150 125

-2

-4

Cosputtered Mg-10%Fe-10%Ti

0

Multilayer

(A)

Cosputtered

25

II

I

III

0 0

50

100

150

200

250

300

cycle number Fig. 12. Absorption kinetics at 300 °C over the course of cycling for (A) 28/5 multilayer, (B) cosputtered Mg–10%Fe–10%Ti and (C) time to 90% absorption of the total capacity as a function of cycle number for 28/5 multilayer and cosputtered Mg–10%Fe–10%Ti.

Fig. 2). From the SEM image in Fig. 1, it is clear that most of the particles are now several hundreds of nanometers across, much larger than the original layer thickness. Assuming that hydrogen dissociation primarily takes place on exposed FeTi surface, the hydride will nucleate at the FeTi/Mg particle interface. From that moment on, hydrogen diffusion into the Mg particles will be progressively dominated by bulk diffusion through a product layer of MgH2. Hao and Sholl performed density functional theory calculations on hydrogen diffusion in MgH2 and calculated a value for the self-diffusion coefficient of H interstitials at 473 K (200 °C) of 1020 m2 s1 [36]. Assuming the effective diffusion coefficient in a chemical potential gradient is the same, it would take approximately 1 h for a MgH2

I

0.8

II

I: f ≤ 0.35

2

ln[ln(1/1-f)]

75 50

1

0.6 0.4

II: f ≥ 0.5

n = 1.23 lnK= -4.99

0 -2 -4

n = 2.27 lnK= -8.55

-6

0.2

-8 0

1

2

3

4

5

lnt

0 0

100

200

300

Time (s)

(B)

1 0.8 2

ln[ln(1/1-f)]

100

Transformed fraction f

Time to 90% (s)

2

-1

Transformed fraction f

wt% H

(B)

1

5 4

(C)

0

8

Time (min)

(B)

50th -300 th every 50 cycles 1st

0.6 0.4 0.2

0 -2

n = 1.09 lnK= -3.29

-4 0

1

2

3

4

5

lnt

0 0

40

80

120

160

Time (s) Fig. 14. (A) 250th absorption cycle for the 28/5 multilayer plotted as transformed fraction f vs. time in seconds. The inset in the bottom right shows the evaluation of the data using Eq. (2). Two stages with different slopes n are clearly observed. Theoretical kinetic responses using the values for n and K obtained from the fit are also shown in the graph (solid lines). (B) Same as in (A) but for the 250th desorption cycle.

W.P. Kalisvaart et al. / Acta Materialia 59 (2011) 2083–2095

layer to grow 15 nm when obeying parabolic growth. Cermak and Kral found that hydrogen diffusion through an interphase boundary is much faster, resulting in an effective diffusion coefficient several orders of magnitude higher than the bulk value [2]. As absorption progresses, ever fewer Mg/MgH2 boundaries will intersect with the Mg–FeTi interface, resulting in a progressive decrease in the effective diffusion coefficient. As the Mg particles grow larger during cycling, this decline becomes noticeable at a point when the particles are not fully hydrogenated, yet results in strongly deteriorated kinetics compared to the first five cycles. At 300 °C, the situation is completely different. From the SEM images in Fig. 6 it was clear that only in the initial stages of cycling is the structure of the as-deposited material preserved. Further cycling induces progressive disintegration. First the Mg–FeTi filaments fracture in the in-plane direction (see Fig. 6C and D) and the beginnings of delamination of the FeTi layers from Mg were observed after 150 cycles. After 300 cycles (Fig. 6E and F) delamination was in a much more advanced stage. The same trend could be seen in the TEM images, where after 150 cycles a planar Mg particle which was only partially covered by FeTi was observed and after 300 cycles the same “tangles” of delaminated FeTi layers that were observed with SEM were seen. Apparently, the elevated temperature degrades the mechanical properties of the FeTi layers to the extent that repeated hydrogenation/dehydrogenation and the accompanying 32% volume expansion causes them not Hydride nucleation at H2/FeTi/Mg triple-points

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only to fracture but to detach from the underlying Mg layer as well. This is depicted schematically in Fig. 15. The progressive delamination of the FeTi from the Mg layers that was observed with SEM and TEM increases the distance between catalytic sites for H2 dissociation/ recombination and thus potential nucleation sites for the hydride and metal phases also move further apart. This would enable individual crystallites to grow larger before they impinge on each other, which is reflected by the increasing Mg grain size found from IBA. At the same time, the absorption times start to increase, eventually by a factor of 4 compared to the initial stages of cycling. Given the extremely slow hydrogen diffusion in MgH2, absorption would be expected to be affected most by changes in the distribution of catalyst. Thus, the microstructural changes as observed with SEM, TEM and XRD and changes in the hydrogen absorption kinetics are in very good agreement with each other. The absorption curves of the 28/5 mutlilayer showed a clear two-stage behavior with different values for the JMA exponent n. The exponent n can be expressed as d/m + 1 when there is continuous nucleation and as d/m when all nuclei are pre-formed and simply grow during the transformation. Here, d is the number of dimensions in which the particles grow and m is the growth mode: m = 1 for interface-controlled growth and 2 for diffusioncontrolled growth [35]. From the SEM micrographs in Fig. 6, it is clear that almost all the FeTi catalyst particles are on an external surface in stage III, in direct contact

Large volume expansion; brittle fracture of FeTi and Mg layers

FeTi MgH2

MgH2

Mg

H | H Mg H | H

MgH2 FeTi

FeTi

FeTi

FeTi

FeTi

MgH2

FeTi FeTi Smaller sizes: isotropic expansion, further fracture of FeTilayers Fig. 15. Schematic representation of the microstructural development for 28/5 multilayer. Dashed lines indicate semi-infinite crystal; solid lines denote real boundaries of particles.

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with the gas phase. It is therefore likely that at some time during absorption, saturation of nucleation sites, which are expected at or near the FeTi particles, will occur. The fact that the values for n found for the two different stages during absorption differ by 1 strongly suggests that this does indeed happen between f = 0.35 and f = 0.5 and that there is a transition from nucleation-and-growth to growth-only. The values found for n in the later stage of absorption (f P 0.5) are close to 1, which matches with diffusionlimited growth in two dimensions (m = 2 and d = 2). Considering the slow diffusivity of hydrogen in MgH2 and the planar morphology of the Mg particles, this is the most plausible explanation. Hao and Sholl calculated an effective activation energy for diffusion of 1.33 eV (128 kJ mol1), which is close to the value found from nuclear magnetic resonance measurements of the H hopping rate; 140 kJ mol1 [3,36]. Based on these values, the hydrogen diffusion coefficient is expected to increase approximately by a factor 500 going from 200 to 300 °C. Based on the grain sizes from Fig. 11, hydrogenation is expected to complete in approximately 2 min, which is close to what was found from the absorption measurements in Fig. 12. For desorption, the entire curve can be fitted with single values for n and K; the value of n is close to 1. Hydrogen diffusion in Mg metal is many orders of magnitude faster than in MgH2 [2,37], yet the desorption time is only three times shorter than absorption and unchanged during cycling, despite the observed increase in grain size and changes in the catalyst distribution. It is therefore unlikely that “n = 1” reflects 2-D diffusion-limited growth in this case. It is more likely that growth of the Mg nuclei is limited by other factors such as hydrogen reassociation on the surface. For the cosputtered material, IBA showed that the Mg crystallite size increases much more slowly compared to the multilayer. Between the 60th and 250th cycle, the Mg grain size increases from 19 to 32 nm. This was also reflected by the trend in the absorption times (see Fig. 12C), where a much smaller increase was found for the cosputtered material compared to the multilayer. It is known that precipitate particles can act as pinning points for grain boundaries, preventing grain growth in the matrix material. However, if the precipitate particles coalesce above a certain critical size, grain growth in the matrix phase can proceed [38]. Considering the fact that every catalyst particle is a potential nucleation site during (de)hydrogenation, the Mg(hydride) grain size will be primarily determined by the distance between these catalytic particles, as was derived here from IBA and SEM micrographs on the multilayers. It is therefore reasonable to assume that coarsening of the catalyst phase is the dominant factor influencing the absorption/desorption kinetics and the Mg(H2) grain size. This is corroborated by a study by Friedrichs et al. on MgH2 ball-milled with Nb2O5 [39]. It was found that annealing the ball-milled mixture at 400 °C increased the

MgH2 grain size from 6.7 to 94 nm, without influencing the kinetics. After a single subsequent absorption/desorption cycle, the grain size had actually decreased back to 69.5 nm. For milled MgH2 without additives, the grain size simply kept increasing from 8.4 to 144 nm after annealing and to 167.5 nm after a single absorption/desorption cycle. From a practical viewpoint, it is important to realize that our cosputtered and multilayered materials represent idealized cases for ball-milled or melt-spun and rolled materials, respectively. Because a fold-and-roll or roll-and-stacking process can, in principle, be repeated indefinitely [18], achieving sufficient catalyst dispersion should be possible. However, it has to be kept in mind that in order to fully preserve the activity of the catalytic phase, incorporation of oxygen has to be kept to a minimum. It has been shown, for instance, that partially oxidized Nb hardly has any catalytic activity towards hydrogen at all [40]. FeTi is also notoriously difficult to activate after air exposure when prepared as a bulk alloy [41,42]. Therefore, laboratory-scale demonstration of a Mg–catalyst composite produced by rolling with sorption kinetics comparable to our sputtered multilayers will be very difficult as the number of required rolling cycles will be very high and the whole process needs to be carried out in an inert atmosphere. Apart from the effects of hydrogen sorption cycling, metallic multilayers are known to be subject to thermal instability due to grain boundary grooving [43,44] and so-called Asaro–Tiller–Grinfeld instabilities due to crystallographic mismatch and differences in elastic moduli between the thin layer (FeTi) and the matrix (Mg) [45]. Theoretically, both these processes would cause the FeTi layers to be more sensitive to fracture and delamination. However, low-melting metals such as Mg tend to have low grain boundary energy and all diffraction measurements suggested that the FeTi layers are amorphous. Therefore, the initial structure with straight, smooth interfaces between Mg and FeTi has high thermal stability. If the problems related to delamination due to mismatch in expansion between Mg and the catalyst could be solved, it is likely that the fast and stable kinetics from the initial 100 cycles would be preserved much longer. 5. Conclusions Microstructure–property relationships in Mg–FeTi hydrogen storage composites have been studied. For multilayers, a large periodicity of 100 nm leads to break-up of the Mg layers which form spherical particles between intact FeTi layers. The kinetics at 200 °C rapidly deteriorate as the Mg particle size is already too large to avoid diffusion limitations. Decreasing the layer thicknesses by a factor three greatly improves the kinetics and cycling stability. At 200 °C, a Mg–FeTi multilayer with a periodicity of 33 nm performs as well as a cosputtered material with the same overall composition. The FeTi layers remain intact during cycling and thus constrain grain growth of Mg in the direction perpendicular to the layers.

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During cycling at 300 °C, the original microstructure of the multilayers is gradually lost. SEM and TEM observations after 35, 150 and 300 cycles showed that the layers fracture in the in-plane direction down to ever smaller sizes. In the final stages of cycling, the FeTi layers delaminate from the Mg layers. This leads to an increase in the Mg grain size from 32 to 76 nm and a slight deterioration of the absorption kinetics; however, the FeTi particles remain effective as a catalyst. Even after 300 cycles, 90% of the total capacity is absorbed within 2 min, whereas desorption of 4.6 wt.% H is complete within 1.5 min. Cosputtered Mg–10%Fe–10%Ti has very rapid and stable kinetics at 300 °C as well. The absorption time to 90% of the total capacity is only 1 min after 250 cycles. Complete desorption takes place in 1.5 min. This shows that both a layered and randomly dispersed starting microstructure are effective for maintaining rapid absorption and desorption kinetics over hundreds of cycles, despite a gradual increase in the Mg grain size with cycling for both the multilayer and cosputtered alloy. The values obtained for the JMA exponent n for the multilayers during absorption are consistent with site-saturation when the hydrided fraction reaches 50% and 2-D diffusion-limited growth. This agrees very well with the SEM and TEM observations that all FeTi is in contact with the gas-phase and the shape of the Mg particles remains planar. This relatively straightforward interpretation of the kinetic data opens up possibilities of using multilayers as model systems for evaluating and comparing the performance of different catalysts. Considering the excellent performance of the 28/5 multilayer, it may be possible to reduce the FeTi layer thickness further, increasing the useful hydrogen storage capacity. Establishing a lower limit for the FeTi layer thickness for maintaining fast kinetics as well as evaluation of other transition metal-based catalysts in the same way as demonstrated here for FeTi will be the subject of further study. Acknowledgements This work was financially supported by NINT-NRC and NSERC Hydrogen Networks. The National Institute of Nanotechnology is acknowledged for providing FIB and TEM facilities. Erik Luber is acknowledged for many helpful discussions during the course of the study. References [1] Vermeulen P, Ledovskikh A, Danilov D, Notten P. Acta Mater 2009;57(17):4967–73. [2] Cermak J, Kral L. Acta Mater 2008;56(12):2677–86. [3] Conradi M, Mendenhall M, Ivancic T, Carl E, Browning C, Notten P, et al. J Alloy Compd 2007;446:499–503. [4] Banerjee S, Pilla C, Majumder C. J Chem Phys 2008;129:174703. [5] Paik B, Jones I, Walton A, Mann V, Book D, Harris I. J Alloy Compd 2010;492(1–2):515–20.

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