0957-4484_20_20_204016

7/31/2019 0957-4484_20_20_204016

1/13

IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 20 (2009) 204016 (13pp) doi:10.1088/0957-4484/20/20/204016

The influence of SWCNTmetallicnanoparticle mixtures on the desorptionproperties of milled MgH2 powders

Babak Shalchi Amirkhiz1,2, Mohsen Danaie1,2 and David Mitlin1,2,3

1 Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB,

Canada2 National Institute for Nanotechnology, NRC, Edmonton, AB, Canada3 CME Building University of Alberta, Edmonton, T6G-2G6, Canada

E-mail: [email protected]

Received 4 August 2008, in final form 26 November 2008

Published 24 April 2009

Online at stacks.iop.org/Nano/20/204016

Abstract

We have examined the effect of single-walled carbon nanotube (SWCNT)metallic nanoparticle

additions on the hydrogen desorption behavior of MgH2 after high-energy co-milling. The

metallic nanoparticles were the catalysts used for the SWCNT growth. The co-milling consisted

of high-energy planetary milling in an inert argon environment of the hydride powder mixed

with the SWCNTs. Identically milled pure MgH2 powders were used as a baseline. The

composites were tested using a combined differential scanning calorimeter and

thermogravimetric analyzer, while the microstructures were examined using a variety of

techniques including x-ray diffraction and transmission electron microscopy (TEM). We foundthat the SWCNTnanoparticle additions do have an influence on the desorption kinetics.

However, the degree to which they are effective depends on the composites final state. The

optimum microstructure for sorption, obtained after 1 h of co-milling, consists of highly

defective SWCNTs in intimate contact with metallic nanoparticles and with the hydride. This

microstructure is optimum, presumably because of the dense and uniform coverage of the

defective SWCNTs on the MgH2 surface. Prolonged co-milling of 7 h destroys the SWCNT

structure and reduces the enhancement. Even after 72 h of co-milling, when the SWCNTs are

completely destroyed, the metallic nanoparticles remain dispersed on the hydride surfaces. This

indicates that the metallic nanoparticles alone are not responsible for the enhanced sorption and

that there is indeed something catalytically unique about a defective SWCNTmetal

combination. Cryo-stage TEM analysis of the hydride powders revealed that they are

nanocrystalline and in some cases multiply twinned. To our knowledge this is the first study

where the structure of milled -MgH2 has been directly imaged. Since defects are an integralcomponent of hydride-to-metal phase transformations, such analysis sheds new insight

regarding the fundamental microstructural origins of the sorption enhancement due to

mechanical milling.

1. Introduction

Rather than acting as a storage media, single-walled (SW) and

multi-walled (MW) carbon nanotubes (CNTs) are increasingly

being utilized to improve the elevated temperature hydrogen

sorption kinetics of magnesium and other hydride forming

materials. The first study on the subject was by Chen et al [1].The authors mixed metallic magnesium with 5 or 20 weight

percent large diameter (>50 nm) MWCNTs and with 0.5 wt%

Zr and 3 wt% Ni powders. They then reactively ball milled

the composites under a hydrogen atmosphere. The authors

found that the 20 wt% MWCNT composite had unacceptably

low gravimetric capacity while the 5 wt% one was similar

to the normal capacity of magnesium powders. The authors

observed an accelerated sorption rate with the 5 wt% MWCNTcomposite and prolonged cycling stability.

0957-4484/09/204016+13$30.00 2009 IOP Publishing Ltd Printed in the UK1
http://dx.doi.org/10.1088/0957-4484/20/20/204016mailto:[email protected]://stacks.iop.org/Nano/20/204016http://stacks.iop.org/Nano/20/204016mailto:[email protected]://dx.doi.org/10.1088/0957-4484/20/20/204016

7/31/2019 0957-4484_20_20_204016

2/13

Nanotechnology 20 (2009) 204016 B S Amirkhiz et al

Dehouche et al [2] examined the effect of additions

of single-walled carbon nanotubes (SWCNTs), graphite

and activated carbon on the sorption kinetics of Ti/Zr

doped NaAlH4 hydride. The nanotubes were purified and

mechanically co-milled with the alanates. The authors found

that the addition of SWCNTs promoted the formation of

Na3AlH6, as compared to the same alanate mixture milledwithout them. They found that doping the alanate with the

SWCNTs yielded improved adsorption and desorption kinetics

relative to the graphite and activated carbon-doped composites.

The sorption enhancement was attributed to the high hydrogen

diffusivity through the nanotube matrix and enhanced thermal

energy transfer. Another group looking at the effect of adding

various carbon structures on the sorption performance of Ti-

doped sodium alanates reached a similar conclusion regarding

the SWCNTs being the optimum catalytic structure [3].

The authors provided a different explanation for this effect,

attributing it to enhanced hydrogen spillover mechanism driven

by the unique structure of the SWCNTs. No microstructural

characterization was performed in that study.

Wu et al [4] examined the effect of using SWCNT on

the adsorption and desorption behavior of MgH2. The authors

added either as-prepared 5 wt% (SWCNT40 wt% Fe/Co/Ni

particle) mixtures, also containing amorphous carbon and other

carbon structures, or purified SWCNTs to MgH2. They co-

milled the composites and tested their hydrogen sorption.

Since milling was performed in stainless steel vials, some Fe

contamination (0.50.8 wt%) was also present in the samples.

The workers observed that while both types of additions

accelerated the sorption kinetics, the as-prepared SWCNTs

worked better, presumably due to the presence of the metallic

particles used for the SWCNT growth. Interestingly, scanningelectron microscopy (SEM) and limited transmission electron

microscopy (TEM) analysis of the post-milled composites

failed to detect any presence of the SWCNTs. In another

manuscript, the same group also looked at a variety of

additions to MgH2, including purified SWCNTs (metal content

7/31/2019 0957-4484_20_20_204016

3/13


A

200 nm

B

200 m

20 nm

C

200 m

D

Figure 1. (A) SEM micrograph of the as-received MgH2 powder; (B) SEM micrograph of the as-received SWCNTamorphouscarbonmetallic nanoparticle mixtures; (C) HRTEM micrograph of the SWCNTamorphous carbonmetallic nanoparticle mixture.(D) Z-contrast HADF micrograph of metallic nanoparticles in the as-received SWNT mixture.

The SWCNTs were co-milled with the MgH2 powder

for either the entire duration or only for a certain number of

minutes right at the end of the milling cycle. For the baselineanalysis, the as-received MgH2 powder, without the SWCNTs,

was milled at identical conditions. The labeling scheme for the

samples was the following: M stands for milled MgH2 powder;

following M is the amount of time of the milling; C means

that carbon nanotubes were also co-milled with the hydride;

following C is the amount of time that the carbon nanotubes

were co-milled. For example, M 1 h means the MgH2 powder,

without the SWCNTs, was milled for 1 h. M 1 h C 1 h indicates

that the hydride was co-milled with the SWCNTs for 1 h, while

M 1 h C 30 m indicates that the hydride was milled for 1 h

with the SWCNTs added in the last 30 min. The powders were

milled at various durations ranging from 30 min to 72 h, withthe majority of analysis being performed on the 30 min20 h

samples.

We used a simultaneous differential scanning calorimeter

(DSC) and thermogravimetric analyzer (TGA) (SDT Q600,

TA Instruments) to monitor the desorption behavior of the

powder samples. During thermal analysis, the powders were

held in alumina cups and heated to 600 C with 10 C min1

heating rate. Additional isothermal desorption tests were

performed at 300 C. The analysis was performed under

constant flow of high-purity (99.998% pure) argon to prevent

oxidation.

Kissinger [13] and Ozawa [14] methods were employed

to measure the activation energy of desorption. The methoddeveloped by Kissinger allows for obtaining the activation

energy for a chemical reaction of any order by considering

the variation of the temperature at the maximum reaction rate.

In the case of endothermic hydride to metal transformation,the maximum reaction rate is straightforward to obtain from

the constant heating rate DSC data and corresponds to the

minimum of the desorption DSC peak. The Kissinger analysis

can be expressed in the following form:

d(ln(/T2m))

d(1/Tm)+

E

R= const.

where is heating rate, Tm is the absolute temperature at the

maximum reaction rate, E is the activation energy and R is the

gas constant. The activation energy is obtained from the slope

of ln(/T2m) versus 1/Tm.

According to the Ozawa method an Arrhenius chemicalreaction can be expressed in the following simplified form:

log + 0.4567E

RT= const.

where T is the absolute temperature at a certain constant

amount of conversion. Therefore the activation energy for the

process may be obtained by plotting log versus 1/T. Here

we used the constant heating rate TGA analysis to obtain the

temperature at 80% MgH2 to Mg conversion by weight. For

both types of analyses the heating rates used were 10, 20, 25

and 30 min1.

For a qualitative analysis of the as-received powders we

used scanning electron microscopy (SEM). We used Hitachi S-4800 SEM operating at 7 kV accelerating voltage, and imaging

3

7/31/2019 0957-4484_20_20_204016

4/13


Table 1. Crystallographic data used for generating electron diffraction simulations.

Unit cell (nm)

Phase Space group a b c Wyckoff positions

-MgH2 P42/mn m (136) 0.451 76 0.45176 0.30206 Mg (2a): 0, 0, 0H (4f): 0.304, 0.304, 0

-MgH2 Pbcn (60) 0.45246 0.54442 0.49285 Mg (4c): 0, 0.3313, 0.25

H (8d): 0.2727, 0.1089, 0.0794Mg P63/mm c (194) 0.320 95 0.32095 0.52107 Mg (c2): 0, 0, 0

MgO Fm 3m (225) 0.422 7 0.422 7 0.422 7 Mg (4a): 0, 0, 0O (4b): 0.5, 0.5, 0.5

using secondary electron signal. The powder samples where

mounted with conductive carbon tape onto SEM stubs.

We used a Malvern 2000SM Mastersizer laser scattering

particle size analysis system to obtain quantitative MgH2particle size distributions. The manufacturer specified

resolution range of the system was sub-m to 2 mm. An

anhydrous alcohol reagent (anhydrous ethyl alcohol 90% 1%

v/v; methyl alcohol approx. 5% v/v; 2-propanol approx. 5%v/v as dispersant) was used as suspension media. All samples

were ultrasonicated for equal durations prior to measurement.

All measurements were performed in same stirring speed and

obscuration level.

We used a Bruker-AXS, D8 discover diffractometer

system with an area detector and Cu K radiation for x-ray

diffraction (XRD) experiments. X-ray diffraction was used for

general phase identification. XRD spectra were also analyzed

for grain and lattice strain measurement. Integral breadth

analysis (IBA) was utilized for de-convoluting the broadening

effects of grain size and lattice strain for the powders. Using

the following relation [15]

(2)2/ tan2 0 = K/D(2/ tan 0 sin 0) + 16e2 (3)

and plotting (2)2/ tan2 0 versus (2/ tan 0 sin 0), where

2 is the integral breadth of the peaks, and 0 is the

position of the peak maximum. K is a constant taken as

0.9 and is the x-ray wavelength (1.54 A). The grain size

(D) and microstrain (root mean squared strain) (e) can be

calculated from the fitted line slope and ordinate intercept,

respectively. The strain peak broadening is due to the presence

of dislocations in the material. We used the first three

-MgH2 peaks for the analysis. These peaks were well

resolved from the peaks of other potentially present phases,e.g. magnesium and magnesium oxide. Peak fitting was

performed on EVA commercial software. Instrumentation

broadening was automatically accounted for in the analysis.

While this type of analysis is useful for predicting general

trends during milling, the exact grain size values should be

interpreted cautiously. This is because the presence of twins

and stacking faults in the microstructure results in a reduced

measured grain size D, relative to what one would directly

observe using TEM [16].

TEM analysis was performed using the JEOL 2010

and JEOL 2200FS microscopes, both operating at 200 kV

accelerating voltage. The TEM analysis of -MgH2 was

performed using a cryogenic holder held near 90 K. This wasdone to significantly increase the stability time of the hydride

powder under the electron beam. At cryogenic temperature,

sample degradation occurred relatively slowly. Even in the

milled sample, only slight weakening of the hydride reflections

occurred after several minutes of beam exposure of one area.

This was long enough to do the analysis. TEM samples

were prepared by dry dispersing the powders onto amorphous

carbon sample grids. We used the commercial software,

Desktop Microscopist, for simulating experimental electrondiffraction patterns. The crystallographic data reported in [17]

as well as data from the XRD database on EVA software

were used for generating the simulations (data tabulated in

table 1). In the simulation, hydrogen superlattice reflections

were excluded since their intensity is significantly less than that

of the magnesium atoms within the hydride.

HRTEM analysis was used to investigate the presence

and the morphology of SWCNTs in the co-milled powders.

We also used Raman spectroscopy to track the structural

changes of the SWCNTs versus ball milling time. Raman

spectra of the MgH2SWCNT composites were obtained with

a HORIBA iHR550 using 770.5 nm at 6.63 W, collected in thebackscattering geometry at room temperature.

3. Results

Figure 2 shows the constant heating rate differential scanning

calorimetry (DSC) and thermal gravimetric analysis (TGA)

results for the MgH2SWCNT composites, as well as the

baseline as-received and milled MgH2. The heating rate used

for all specimens was 10 C min1. The DSC endothermic

peaks agree with the TGA results in terms of detecting the

onset of hydrogen desorption. Integrating the endothermic

peaks yields values in the range of 75 kJ mol

1

, associated withthe decomposition of-MgH2 to Mg and H2 [18]. Figure 2(A)

shows the 1 h milled data. Judging from the DSC results,

the as-received MgH2 powder begins hydrogen desorption at

over 400 C. The milled MgH2 powder begins to desorb near

350 C. The best performance is achieved in the 1 h co-milled

powders, where hydrogen evolution begins to occur below

320 C. Samples milled for a total of 1 h, but with the SWCNTs

added at the last 30 min, display an intermediate performance

between the two.

Figure 2(B) demonstrates that the 7 h co-milled powder

performs better than 7 h milled baseline MgH2, which

performs better than the as-received MgH2. However, optimum

properties are obtained in the samples where just the MgH2was milled for 6.5 h followed by co-milling during the last

4

7/31/2019 0957-4484_20_20_204016

5/13


Figure 2. Constant heating rate DSC and TGA results for milled MgH2 powder by itself and co-milled with 5 wt% SWCNT mixtures: (A) 1 hmilling; (B) 7 h milling; The SWCNT mixtures were added either at the beginning of each milling cycle (M 7 h C 7 h, etc), or in the last

30 min (M 7 h C 30 m, etc); (C) 20 h milling. (D) Best performance TGA data for each milling time. (E) and (F) Isothermal TGA desorptionresults for milled MgH2 powder by itself and co-milled with 5 wt% SWCNT mixtures.

half an hour. The difference in the performance of the 20 h

(figure 2(C)) co-milled batches and that of the milled MgH2baseline is fairly small. Both desorb better than the as-

received powder. Adding the SWCNTs in the last 30 min of

milling provides for the best overall desorption enhancement.

Figure 2(D) shows the best kinetics of each milling time. The

fastest constant heating rate desorption is achieved for the 1 h

co-milled powders. Within the times tested, no additional

benefits are observed as a result of additional milling. In the

case of the 7 and 20 h batches, adding the SWCNTs during the

last 30 min provides for significantly enhanced desorption over

the co-milled powders.

Figures 2(E) and (F) show isothermal 300 C TGA results

for the milled powders. Figure 2(E) demonstrates that for

the milling duration of 1 h, co-milling with the SWCNTs is

optimum. The additions of SWCNTs in the last 5 and 30 min

of milling are not as effective. Figure 2(F) shows the best-

of-the-lot isothermal results, again confirming that the 1 h co-

milled powders are optimum. Milling for longer times, with or

without the SWCNTs, actually degrades the performance.

Finally, it is important to indicate that the results obtained

here may not necessarily represent the most optimized process

conditions. Other variations on co-milling, such as millingfor 1 h followed by co-milling for 1 h, may yield even lower

desorption temperatures. Variations in the respective weight

ratios of the two powders may also affect the kinetics. What

we observe here though are some systematic trends that we

will now correlate to the microstructure of the composites.

Figures 3(A)(C) show the particle size distributions of the

milled powders, as a function of milling time. The SWCNTs

were too small to be detected by this measurement technique

and do not show up in the distribution. Some hydride particles

are also finer than what is resolvable by the instrument, since

they are thin enough to be electron transparent for TEM

analysis. However, these particles are in the minority. The

results are fairly consistent: milling reduces the particle size

while introducing a multi-modal particle size distribution into

the powders. Co-milling for the entire duration does not

produce any obvious trends in the size distributions, still

resulting in multi-modal curves. Adding the SWCNTs in the

30 min has the effect of de-agglomerating the powders.

Figures 3(A)(C) indicate that the batches where the

SWCNTs were added in the last 30 min have a monomodal size

distribution, whereas the baseline and the co-milled powders

are multi-modal. The SWCNTs effectively act as a lubricant

for the milling process, de-agglomerating powders that would

be otherwise mechanically stuck together. The SWNT bundlesare sheared during the milling process, acting as a lubricant

5

7/31/2019 0957-4484_20_20_204016

6/13


Figure 2. (Continued.)

similar to the action of graphite. Once the SWNT structures

are destroyed, and only the amorphous carbon remains, the de-

agglomeration effect is lost.

Figures 4(A) and (B) show the raw XRD data for the

milled powders, with and without the SWCNTs respectively.

The vast majority of the powder is composed of the

thermodynamically stable -MgH2. The as-received powder

also contains a small amount of Mg. With prolonged milling

the -MgH2 peaks broaden. Milling for 1 h and longer

promotes the formation of the metastable -MgH2 phase.

Judging from the relative peak intensities, the amount of this

phase is fairly minor and is quite similar for the baseline and

the SWCNT containing powders. Overall it can be concluded

that the addition of SWCNTs does not affect the diffraction

patterns in any appreciable way. This means that the SWCNTs

do not promote a formation of any new phases nor do they

change the width or the relative intensity of the magnesium

hydride diffraction peaks. As will be shown in the next set

of figures, the grain size and the microstrain in the MgH2are also independent of the SWNCT presence. This will

clearly indicate that the SWCNTs do not affect the bulk MgH2microstructure.

Figure 5(A) shows the IBA grain diameter, obtained from

the XRD curves, as a function of milling time. The as-

received MgH2 powder starts out nanocrystalline. Milling

further reduces the grain size, with no discernable difference

between the baseline MgH2 or the MgH2SWCNT mixtures.Figure 5(B) indicates that milling does introduce significant

6

7/31/2019 0957-4484_20_20_204016

7/13


Figure 3. Particle size distributions for the baseline hydride, the hydride co-milled with the SWCNTs for the entire duration and the hydridewhere the SWCNTs were added during the last 30 min. (A) 1 h milling; (B) 7 h milling; (C) 20 h milling.

(This figure is in colour only in the electronic version)

rms lattice strain into the microstructure. However, neither

prolonged milling nor the presence of SWCNTs affects the

amount of strain past the initial 10 min of milling.

Figures 6 and 7 illustrate the two predominant microstruc-

tural features observed in -MgH2 via TEM. First, most of the

observed hydride powder particles were polycrystalline, with

nanoscale grain sizes. Second, the larger hydride grains were

multiply twinned. To our knowledge, this second observation

has never been reported in literature. These two features were

identically present in the baseline and the co-milled powders.

Since the hydride powder was very unstable at room temper-

ature under the electron beam, it is only through the use of a

cryogenic sample stage that we were able to make these obser-

vations.Figures 6(A) and (B) shows bright field and dark field

TEM micrographs of the 1 h SWCNT co-milled MgH2 powder.

The large -MgH2 grain imaged in figure 6(B) was obtained

using the g = [101] reflection, shown in figure 6(C).

Occasionally, it was possible to image the entire particle in

dark field using a single reflection. However, more often

the particles were polycrystalline with grain sizes roughly

on the same order of magnitude as determined by XRD

analysis. The grain in figure 6(B) shows mottled (non-uniform,

blocky) contrast, previously reported for highly deformed ionic

minerals such as phyllosilicates [19, 20]. It is tempting to

indicate that this is the first direct observation of the high

dislocation content in the magnesium hydride induced by themilling process. This observation would also confirm much

of what has been surmised from indirect XRD-based evidence

regarding the effects of milling on the hydride microstructures.

However, electron beam damage may also be responsible for

such contrast and more work is needed to separate the two

effects.

Figures 7(A)(C) provide an example of a multiply

twinned structure observed in the larger hydride crystallites.

The density of twins varied from grain to grain, with the

smaller crystallites normally not containing any at all. The

twins were virtually undetectable in bright field mode but

showed up clearly in dark field. The dark field micrograph

in figure 7(B) was obtained using the g = 120 type

reflection. Since the grain was far from a symmetric zone

axis, the relative spot spacing was used to identify the labeled gvectors. Researchers generally assume that mechanical milling

of magnesium hydride increases its dislocation content and

reduces its grain size. This may be the first evidence to show

that mechanical milling also induces deformation twins. If

mechanical milling did introduce twinning into the -MgH2microstructure, it would in a fundamental way change the

understanding of sorption kinetics in this important material.

In fact, we believe that the twin interfaces, in addition to the

dislocation cores, act as the fast paths for hydrogen diffusion

out of the system. The twinned interface may also be the

preferred initiation site for the -MgH2 to Mg transformation.

However a more systematic study is needed to conclusively

confirm both hypotheses. It can now be stated that all the -MgH2 microstructural features of the two materials, including

7

7/31/2019 0957-4484_20_20_204016

8/13


A

B

Figure 4. Raw XRD results for the milled hydride (A) and thehydride co-milled with the SWCNTs for the entire duration (B).

the grain size, strain and twin content, are unaffected by the

presence of SWCNTs.

Finally, we should point out that figures 6 and 7

clearly show the magnesium hydride microstructure rather

than desorbed magnesium or MgO. While upon prolonged

beam exposure the hydride did transform, the process was

straightforward to track: single grains of -MgH2 that

were tens or even hundreds on nanometers in diameter

would transform to a dense distribution of randomly oriented

crystallites that were on the scale of single nanometers.

Figure 8(A) shows a HRTEM micrograph of several

SWCNTs (arrowed) and a metallic nanoparticle (arrowed)attached to a hydride particle, obtained from a batch that

was co-milled for 30 min. After 30 min of co-milling, the

SWCNTs, with long sections still intact, covered the MgH2grains. After 1 h of milling (figure 8(B)), SWCNTs were

still detectable covering the hydride particles. However, their

sections were now much shorter and the bundles appeared

broken (arrowed). Extensive search for SWCNTs in the

7 h co-milled powders did not yield any evidence of their

existence. Only amorphous carbon and metallic nanoparticles

were detected. Both were lining the surfaces of the hydride

particles.

Figure 9 shows a bright field and a Z-contrast high angle

annular dark field (HAADF) STEM micrograph of the 72 hco-milled powders. The powder composite was purposely

Figure 5. XRD IBA analysis grain size (A), and root mean square(rms) strain (B), for the baseline hydride and hydride co-milled withthe SWCNTs for the entire duration.

overmilled to examine the evolution of the size/morphology of

the metallic nanoparticles. These samples show no trace of any

graphitic bonds in the carbon, but clearly show the presence of

metallic nanoparticles. Three such remaining nanoparticles are

arrowed in figure 9(B). The composition of the largest of the

three particles was determined by EDXS to be MnFe based.

A comparison of the sizes and morphologies of the metallic

particles in the as-received SWNTs (figure 1(D)) with theparticles shown here yields no obvious differences. Hence, we

can conclude that while milling drastically affects the structure

of the SWCNTs, the metallic nanoparticle phase appears fairly

unaffected. This may be explained by the fact that the particles

are largely encapsulated by carbon that prevents nanoparticle

agglomeration or significant alloying with the hydride.

Figure 10(A) shows the Raman spectroscopy results for

the MgH2SWCNT composites as a function of milling

time. The spectra clearly shows a peak around 1580 cm1

corresponding to the graphite CC bands (G-band) that are

characteristic of the structure in SWCNTs [21]. The

increase in intensity of the D-band, around 1330 cm1

, isan indication of the formation of defects in the structure of

SWCNTs [2224]. The spectrum after co-milling for 7 h

was similar to that of amorphous carbon. Figure 10(B)

shows a plot of the intensity ratio of the G-versus the D-

band, also as a function of the milling time. The ratios in

figure 10(B) were calculated from the data in figure 10(A),

using the maximum intensities of the two bands. Both figures

confirm the conclusion that prolonged milling damages and

then entirely destroys the SWCNT structure.

4. Discussion

We now have enough information regarding the co-milled

microstructures to discuss the role of the SWCNTmetallic

8

7/31/2019 0957-4484_20_20_204016

9/13


A B

250 nm

C

[010] -10-1

-101

g = [101]

Figure 6. (A) Bright field micrograph of several MgH2 particles from the batch that was co-milled with SWCNTs for 1 h. (B) and (C) darkfield and corresponding SAD of an individual hydride crystallite within the largest particle.

C

120

011

250 nm

A B

g =

Figure 7. (A) Bright field micrograph of several MgH2 particles from the batch that was co-milled with SWCNTs for 1 h. (B) and (C) darkfield micrograph and corresponding SAD of the twins within the hydride grains (arrowed).

20 nm

A

20 nm

B

Figure 8. HRTEM images of SWCNTs attached to MgH2 particles. (A) MgH2 + SWCNT composite co-milled for 30 min.(B) MgH2 + SWCNT composite co-milled for 60 min. Arrows in the image point to the SWCNTs, except for the top arrow in (A) whichmarks a metallic nanoparticle.

nanoparticle additions on the desorption behavior of MgH2.

The dissociation of MgH2 is known to be sluggish even

when appreciable thermodynamic driving force exists. The

origin of the kinetic sluggishness is still somewhat debatable,

being attributed to several rate limiting mechanisms: (1) the

slowness of the MgMgH2 phase boundary movement dueto the slow diffusivity of the hydrogen ions both through the

hydride and through the metal [2529]; (2) the diffusion of

hydrogen along a metalMgH2 interface [30, 31]; (3) surface

passivation/poisoning due to adsorbed oxygen or sulfur

containing species [32]; (4) the high activation energy of

hydrogen dissociation/re-association on MgH2 surface [33].

This debate may be to some extent artificial since the degreeto which mechanism is the rate limiting one fundamentally

9

7/31/2019 0957-4484_20_20_204016

10/13


100 nm

A B

Figure 9. Bright field and Z-contrast HAADF STEM micrographs of metallic nanoparticles (arrowed) on the hydride powder surface after72 h of co-milling. EDXS confirmed that the lower arrowed particle is MnFe based.

Figure 10. (A) Raman spectra for hydrideSWCNT mixturesco-milled for various durations; (B) G-band to D-band ratio as afunction of milling time.

depends on the surface preparation and on the bulk

microstructure, which vary from experiment to experiment.

Figures 11(A) and (B) show desorption activation energy

plots obtained from the Ozawa and the Kissinger analysis,

respectively. Though there is an absolute value difference

between the two forms of analysis, the trends are fairlyconsistent. The activation energies may be roughly separated

into three classes: the baseline hydride powders and the seven-

hour milled hydride powders have by far the highest activationenergies for desorption. The one-hour co-milled powders

and the powders that were milled for seven hours with the

SWCNTs added during the last 30 min possess the lowest

activation energies. The one-hour milled magnesium hydride

and the seven-hour co-milled powders have intermediate

values of activation energies. This analysis confirms our

previous conclusions that optimum catalytic function is

achieved when the SWCNTs are well dispersed throughout

the microstructure and are defective. However even in the

overmilled state (M 7 h C 7 h) some catalytic enhancement

is still achieved.

We have shown that the addition of SWCNTs does notinfluence the amount of grain boundary area, or the dislocation

content of the hydride. This can be seen from figure 5.

There is also no discernable difference in the twin density

with or without the SWCNTs. It is unlikely that the SWCNTs

influence the rate of grain boundary or bulk hydrogen diffusion.

The SWCNTs would have to be embedded in the hydride

particles and have to traverse at least half the thickness of the

structure. From our TEM results we see that in the optimum

processing condition (1 h milling) the SWCNTs are truly

nanoscale in length and cover the hydride surface. Even if it

was indeed possible to embed SWCNTs into the MgH2 matrix,

there are simply no carbon structures that are even remotelyon the same scale as the hydride particles. It is also quite

unlikely that it would be possible to create a continuous path

of broken SWCNTs throughout the bulk of the hydride. Thus,

we conclude that the addition of SWCNTs does not influence

(1) the rate of the MgMgH2 phase boundary movement or

(2) the diffusion of hydrogen along a metalMgH2 interface.

Experimental results show that the particle size differences

at most have a secondary influence on the temperature where

appreciable hydrogen desorption begins. There also does not

seem to be a clear trend in the shape of the DSC endothermic

peaks being related to width of the particle size distributions.

For example, in the case of the 1 h milled samples, the co-

milled powders show more than 30 lower onset of desorptionrelative to the milled baseline. Yet the particle size distribution

10

7/31/2019 0957-4484_20_20_204016

11/13


Figure 11. Constant heating rate analysis for activation energy of desorption (A) Ozawa TGA-based analysis using 80% reaction completionby weight. (B) Kissinger DSC-based analysis using maximum reaction rate.

of the 1 h co-milled and the baseline powders quite similar. Infact, the main difference between the two is that the co-milled

powder shows a secondary peak centered around 55 m.

On the side of the small particles, the two distributions are

almost identical. One could argue that in the one-hour milled

powders the wider size distribution leads to a wider DSC peak.

However, this trend is reversed in the 20 h milled powders.

Examining the DSC heat evolution curves for the 20 h milled

samples shows that the sharpest endothermic peak (the baseline

hydride) occurs for the powder batch with the widest sizedistribution (volume fraction peaks centered at 20, 40, 57 and

85 m). Conversely, the widest DSC peak (SWCNTs added

in the last 30 min) occurs for the batch with the narrowestdistribution (a single volume fraction peak centered at 45 m).

What is observed is that the optimum desorption

properties are achieved when the SWCNTs are well dispersed,

contain defects and are in contact with the metallic

nanoparticles. The desorption properties of both the baseline

and the co-milled powders do get worse with increasing milling

time, which may be in part due to increasing oxide formation.

However it is the desorption enhancement due to the SWCNTs

that is reduced when the hydrideSWCNT composites are

overmilled and the SWCNTs are destroyed. This could be seenby considering the maximum reaction rate as corresponding to

the trough of the endothermic peaks. For example, in the 7 hmilled samples the baseline hydride reaction peak is roughly

at 400 C. If the SWCNTs are added in the last 30 min, in

which case they survive the milling cycle but are dispersed and

damaged, the endothermic peak is at 360 C. If the SWCNTs

and the hydride are co-milled for 7 h, resulting the complete

destruction of the SWCNT structure, the peak is at 375 C.

Since the samples were not removed from the glove box at

any point of the milling, the total oxide in each batch should

be identical. The remaining catalytic enhancement in the 7 h(and the 20 h) co-milled samples relative to the pure hydride

baseline is due to the metallic nanoparticles which survive the

milling.We can now conclude that it is the SWCNTs presence,

in a defective state and coupled to a metal catalyst, which

promotes enhanced desorption kinetics. There may be several

scenarios by which the SWCNTmetal combinations may be

effective. One is that the defective SWCNTs in intimate

contact with the hydride particles may act as preferential

pathways for hydrogen past the surface hydroxide. This would

relate to decreasing the influence of mechanism (3) surface

passivation/poisoning due to adsorbed oxygen containing

species. Even in nominally inert environments, -MgH2 isalways covered by a layer of hydroxide several nanometers in

11

7/31/2019 0957-4484_20_20_204016

12/13


scale [34]. Individual and bundles of SWCNTs could penetrate

this thin hydroxide shell, acting as hydrogen pumps to move

the atoms to the surface. The optimum co-milling time of 1 h

may then be the duration required to firmly embed the SWNTs

into the hydride particles. SWCNTs containing defects would

have additional diffusion paths as well.

Additionally, the defective SWCNTmetal combinationmay be optimum catalytically. This would relate to reducing

mechanism (4) the high activation energy of hydrogen

dissociation/re-association on MgH2 surface. It is well known

that the activation energy for hydrogen recombination on

pure -MgH2 is quite high. The literature quoted values

include 144, 172, or even 323 kJ mol1 (see reference [33]

and the citations therein). This is an energy barrier at least

twice as high as the heat of formation of the hydride itself

(75 kJ mol1) [18]. The SWCNTmetal nanoparticle couples,

which cover the hydride surface, may act as preferential

low-energy sites for hydrogen re-association. The metallic

nanoparticles used for SWCNT growth are also known to be

catalytic for splitting hydrogen, and their contact with the

SWCNTs may be synergistic. Recent density functional theory

(DFT) calculations on hydrogen interaction with transition

metalSWCNT combinations support this scenario [6].

5. Conclusions

We examined the role of a combination of SWCNTs and

metallic nanoparticles on the hydrogen desorption properties

of MgH2 powders. A nanocomposite of these was created

by high-energy co-milling for various durations. SWCNT

metallic nanoparticle additions catalyze the desorption of

hydrogen, lowering the onset temperature by more than 30 Cover those of identically milled pure MgH2 powders. However,

co-milling for longer than one hour degrades this effect,

essentially yielding a similar sorption performance in the pure

and the composite powders.

Using Raman spectroscopy and HRTEM analysis, we

show that a significant portion of SWCNTs do survive up

to 1 h of milling, though becoming broken and defective

in the process. At longer milling times the SWCNTs are

completely destroyed and the enhancement is largely lost

despite the continued presence of the metallic particles. X-ray

diffraction analysis of the -MgH2 grain size and strain, and

laser scattering analysis of the -MgH2 particle size, indicatesthat the addition of the SWCNTmetallic nanoparticles does

not appreciably affect these parameters during milling. Cryo-

stage TEM analysis confirmed that the majority of the hydride

powder particles were polycrystalline, with nanoscale grain

sizes. TEM also revealed that the larger hydride crystallites

are multiply twinned.

Ozawa and Kissinger constant heating rate analysis

conclusively demonstrated that in the optimum processed

condition (1 h co-milled) the activation energy for hydrogen

desorption is the lowest. Through the process of elimination it

can be concluded that the catalytic effect of the SWCNTs is a

result of one or a combination of the following effects: during

mechanical milling the SWCNTs penetrate the thin surfacehydroxide shell and act as hydrogen pumps; or the surface

dispersed and damaged SWCNTs reduce the high activation

energy of hydrogen re-association on the MgH2 surface.

Acknowledgments

We would like to sincerely thank Benoit Simard at Steacie

Institute for Molecular Sciences Ottawa, ON, Canada forsupplying the SWCNTs and for excellent discussions. The

authors would also like to recognize the instrumentation

assistance from Steve Launspach and Daniel Salamon. This

research was supported by an NSERC Discovery Grant,

Alberta Ingenuity, and by NINT NRC.

References

[1] Chen D, Chen L, Ma C X, Chen D M and Wang L B 2004J. Alloys Compounds 372 231

[2] Dehouche Z, Lafi L, Grimard N, Goyette J and Chahine R 2005Nanotechnology 16 402

[3] Wang J, Ebner A D and Ritter J A 2006 J. Phys. Chem. B110 17353

[4] Wu C, Wang P, Yao X, Liu C, Chen D, Lu G Q andCheng H 2005 J. Phys. Chem. B 109 22217

[5] Wu C, Wang P, Yao X, Liu C, Chen D, Lu G Q andCheng H 2006 J. Alloys Compounds 414 259

[6] Yao X, Wu C, Du A, Lu G Q, Cheng H, Smith S C, Zou J andHe Y 2006 J. Phys. Chem. B 110 11697

[7] Wu C Z, Wang P, Yao X, Liu C, Chen D M, Lu G Q andCheng H M 2006 J. Alloys Compounds 420 278

[8] Yoo Y, Tuck M, Kondakindi R, Seo C-Y, Dehouche Z andBelkacemi K 2007 J. Alloys Compounds 446 84

[9] Huang Z G, Guo Z P, Calka A, Wexler D and Liu H K 2007J. Alloys Compounds 427 94

[10] Yao X, Wu C, Du A, Zou J, Zhu Z, Wang P, Cheng H,Smith S and Lu G 2007 J. Am. Chem. Soc. 129 15650

[11] Lillo-Rodenas M A, Guo Z X, Aguey-Zinsou K F,Cazorla-Amoros D and Linares-Solano A 2008 Carbon46 126

[12] Kim K S, Cota-Sanchez G, Kingston C T, Imris M,Simard B and Soucy G 2007 J. Phys. D: Appl. Phys.40 2375

[13] Kissinger H E 1957 Anal. Chem. 29 1702[14] Ozawa T 1965 Bull. Chem. Soc. Japan 38 1881

Ozawa T 1992 Thermochim. Acta 203 159[15] Klug H P and Alexander L E 1974 X-ray Diffraction

Procedures (New York: Wiley) p 643[16] Warren B E 1959 Prog. Metal. Phys. 8 147[17] Vajeeston P, Ravindran P, Hauback B C, Fjellvag H,

Kjekshus A, Furuseth S and Hanfland M 2006 Phys. Rev. B73 224102

[18] Grochala W and Edwards P P 2004 Chem. Rev. 104 1283[19] Bozhilov K N, Green H W II and Dobrzhinetskaya L 1999

Science 284 128[20] Noe D C and Veblen D R 1999 Am. Mineral. 84 1932[21] Dresselhaus M S, Dresselhaus G, Saitoc R and Joriod A 2005

Phys. Rep. 409 47[22] Pierard N, Fonseca A, Colomer J-F, Bossuot C, Benoit J-M,

Van Tendeloo G, Pirard J-P and Nagy J B 2004 Carbon42 1691

[23] Tao Z, Genga H, Yuc K, Yanga Z and Wang Y 2004 Mater.Lett. 58 3410

[24] Huang Z G, Guo Z P, Calka A, Wexler D and Liu H K 2007J. Alloys Compounds 427 94

[25] Stadler C M 1977 Int. Inorg. Nucl. Chem. 39 221

[26] Karty A, Grunzweig-Genossar J and Rudman P S 1979 J. Appl.Phys. 50 7200[27] Rudman P S 1979 J. Appl. Phys. 50 7195

12
http://dx.doi.org/10.1016/j.jallcom.2003.08.104http://dx.doi.org/10.1016/j.jallcom.2003.08.104http://dx.doi.org/10.1088/0957-4484/16/4/012http://dx.doi.org/10.1088/0957-4484/16/4/012http://dx.doi.org/10.1021/jp062574hhttp://dx.doi.org/10.1021/jp062574hhttp://dx.doi.org/10.1021/jp0545041http://dx.doi.org/10.1021/jp0545041http://dx.doi.org/10.1016/j.jallcom.2005.07.021http://dx.doi.org/10.1016/j.jallcom.2005.07.021http://dx.doi.org/10.1021/jp057526whttp://dx.doi.org/10.1021/jp057526whttp://dx.doi.org/10.1016/j.jallcom.2005.10.028http://dx.doi.org/10.1016/j.jallcom.2005.10.028http://dx.doi.org/10.1016/j.jallcom.2006.12.054http://dx.doi.org/10.1016/j.jallcom.2006.12.054http://dx.doi.org/10.1016/j.jallcom.2006.03.069http://dx.doi.org/10.1016/j.jallcom.2006.03.069http://dx.doi.org/10.1021/ja0751431http://dx.doi.org/10.1021/ja0751431http://dx.doi.org/10.1016/j.carbon.2007.10.033http://dx.doi.org/10.1016/j.carbon.2007.10.033http://dx.doi.org/10.1088/0022-3727/40/8/S17http://dx.doi.org/10.1088/0022-3727/40/8/S17http://dx.doi.org/10.1021/ac60131a045http://dx.doi.org/10.1021/ac60131a045http://dx.doi.org/10.1246/bcsj.38.1881http://dx.doi.org/10.1246/bcsj.38.1881http://dx.doi.org/10.1016/0040-6031(92)85192-Xhttp://dx.doi.org/10.1016/0040-6031(92)85192-Xhttp://dx.doi.org/10.1016/0502-8205(59)90015-2http://dx.doi.org/10.1016/0502-8205(59)90015-2http://dx.doi.org/10.1103/PhysRevB.73.224102http://dx.doi.org/10.1103/PhysRevB.73.224102http://dx.doi.org/10.1021/cr030691shttp://dx.doi.org/10.1021/cr030691shttp://dx.doi.org/10.1126/science.284.5411.128http://dx.doi.org/10.1126/science.284.5411.128http://dx.doi.org/10.1016/j.physrep.2004.10.006http://dx.doi.org/10.1016/j.physrep.2004.10.006http://dx.doi.org/10.1016/j.carbon.2004.02.031http://dx.doi.org/10.1016/j.carbon.2004.02.031http://dx.doi.org/10.1016/j.matlet.2004.05.045http://dx.doi.org/10.1016/j.matlet.2004.05.045http://dx.doi.org/10.1016/j.jallcom.2006.03.069http://dx.doi.org/10.1016/j.jallcom.2006.03.069http://dx.doi.org/10.1016/0022-1902(77)80003-1http://dx.doi.org/10.1016/0022-1902(77)80003-1http://dx.doi.org/10.1063/1.325832http://dx.doi.org/10.1063/1.325832http://dx.doi.org/10.1063/1.325831http://dx.doi.org/10.1063/1.325831http://dx.doi.org/10.1063/1.325831http://dx.doi.org/10.1063/1.325832http://dx.doi.org/10.1016/0022-1902(77)80003-1http://dx.doi.org/10.1016/j.jallcom.2006.03.069http://dx.doi.org/10.1016/j.matlet.2004.05.045http://dx.doi.org/10.1016/j.carbon.2004.02.031http://dx.doi.org/10.1016/j.physrep.2004.10.006http://dx.doi.org/10.1126/science.284.5411.128http://dx.doi.org/10.1021/cr030691shttp://dx.doi.org/10.1103/PhysRevB.73.224102http://dx.doi.org/10.1016/0502-8205(59)90015-2http://dx.doi.org/10.1016/0040-6031(92)85192-Xhttp://dx.doi.org/10.1246/bcsj.38.1881http://dx.doi.org/10.1021/ac60131a045http://dx.doi.org/10.1088/0022-3727/40/8/S17http://dx.doi.org/10.1016/j.carbon.2007.10.033http://dx.doi.org/10.1021/ja0751431http://dx.doi.org/10.1016/j.jallcom.2006.03.069http://dx.doi.org/10.1016/j.jallcom.2006.12.054http://dx.doi.org/10.1016/j.jallcom.2005.10.028http://dx.doi.org/10.1021/jp057526whttp://dx.doi.org/10.1016/j.jallcom.2005.07.021http://dx.doi.org/10.1021/jp0545041http://dx.doi.org/10.1021/jp062574hhttp://dx.doi.org/10.1088/0957-4484/16/4/012http://dx.doi.org/10.1016/j.jallcom.2003.08.104

7/31/2019 0957-4484_20_20_204016

13/13


[28] Barkhordarian G, Klassen T and Bormann R 2006 J. AlloysCompounds 407 249

[29] Kelekar R, Griffard H, Kelly S T and Clemens B M 2007J. Appl. Phys. 101 114311

[30] Schober T 1981 Met. Trans. A 12 951[31] Zhdanov V P, Krozer A and Kasemo B 1993 Phys. Rev. B

47 11044

[32] Bloch J and Mintz M H 1997 J. Alloys Compounds253/254 529

[33] Du A J, Smith S C and Lu G Q 2007 J. Phys. Chem. C111 8360

[34] Friedrichs O, Sanchez-Lopez J C, Lopez-Cartes C,Dornheim M, Klassen T, Bormann R and Fernandez A 2006

Appl. Surf. Sci. 252 2334

13
http://dx.doi.org/10.1016/j.jallcom.2005.05.037http://dx.doi.org/10.1016/j.jallcom.2005.05.037http://dx.doi.org/10.1063/1.2736304http://dx.doi.org/10.1063/1.2736304http://dx.doi.org/10.1007/BF02643475http://dx.doi.org/10.1007/BF02643475http://dx.doi.org/10.1103/PhysRevB.47.11044http://dx.doi.org/10.1103/PhysRevB.47.11044http://dx.doi.org/10.1016/S0925-8388(96)03070-8http://dx.doi.org/10.1016/S0925-8388(96)03070-8http://dx.doi.org/10.1021/jp072191zhttp://dx.doi.org/10.1021/jp072191zhttp://dx.doi.org/10.1016/j.apsusc.2005.04.018http://dx.doi.org/10.1016/j.apsusc.2005.04.018http://dx.doi.org/10.1016/j.apsusc.2005.04.018http://dx.doi.org/10.1021/jp072191zhttp://dx.doi.org/10.1016/S0925-8388(96)03070-8http://dx.doi.org/10.1103/PhysRevB.47.11044http://dx.doi.org/10.1007/BF02643475http://dx.doi.org/10.1063/1.2736304http://dx.doi.org/10.1016/j.jallcom.2005.05.037

0957-4484_20_20_204016

Documents