Materials Transactions, Vol. 55, No. 8 (2014) pp. 1149 to 1155
Special Issue on Advanced Materials for Hydrogen Energy Applications II
© 2014 The Japan Institute of Metals and Materials
Catalytic Effect of Multi-Wall Carbon Nanotubes Supported Nickel
on Hydrogen Storage Properties of Mg99Ni Prepared
by Hydriding Combustion Synthesis
Lingjun Wei1,2, Zhengwei Cui2, Yunfeng Zhu1 and Liquan Li1,+
1
College of Materials Science and Engineering, Nanjing Tech University,
5 Xinmofan Road, Nanjing 210009, People’s Republic of China
2
School of Mechanical Engineering, Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology,
Jiangnan University, 1800 Lihu Road, Wuxi 214122, People’s Republic of China
Multi-wall carbon nanotubes supported nickel (Nano-nickel/MWCNT) is added to the hydriding combustion synthesis (HCS) product of
Mg99Ni by mechanical milling to yield a designed composite for improving the hydrogen storage properties of magnesium. It is revealed that
there is a synergistic effect of nano-Mg2NiH4 and MWCNT on the hydrogen storage properties of Mg99Ni, which improves the hydrogenation
and dehydrogenation performance when compared to adding either nano-Ni or MWCNT alone. The composite requires only 80 s to reach its
saturated hydrogen capacity of 6.79 mass% at 373 K and desorbs 97.2% hydrogen within 1800 s at 543 K. The dehydrogenation activation
energy of this system is 105 kJ mol¹1, which is much lower than that of as-received MgH2 (153 kJ mol¹1). In addition, the composite preserves
stable hydrogen storage capacity and kinetics in the hydrogenation/dehydrogenation cycles at 423 K. [doi:10.2320/matertrans.MG201416]
(Received January 20, 2014; Accepted May 7, 2014; Published July 25, 2014)
Keywords: magnesium-based material, hydrogen storage, hydriding combustion synthesis, mechanical milling, multi-wall carbon nanotubes
supported nickel, synergistic effect
1.
Introduction
Developing an efficient and reliable hydrogen storage
system has been crucial for hydrogen fuel cell powered
vehicles. Many kinds of hydrogen storage materials, such
as metal organic frameworks,1,2) complex hydrides and
imides,3­5) nanostructural carbon6­8) and metal hydrides,9­11)
are under investigation to satisfy the combined stringent
demands on high capacity, mild operation condition, safety
and low cost.12)
Among those materials, magnesium has promising potential for on-board application owing to its reversibility,
abundance, low cost, low density and high theoretical
hydrogen storage capacity of 7.6 mass%.13,14) Unfortunately,
the high thermodynamic stability of MgH2 results in
temperature higher than 573 K for hydrogen absorption and
desorption reactions. Moreover, the reactions are too slow
for practical use. To improve the performance of MgH2 for
hydrogen storage, different kinds of methods have been
developed. Metal oxides and halides such as TiF3, ZrF4,
Nb2O5. NbF5 and VCl3 have been used to catalyze
dehydrogenation of MgH2.15­17) Metals (V, In, Ni or Pd)
are incorporated into MgH2 to destabilize it.18­20) A series of
nonmetal materials like carbon or polymer are used to
produce nanostructural MgH2 by means of incorporating the
hydride within a nanoporous/microporous carbon host or
polymer matrix, which decreases diffusion path lengths for
hydrogen and promotes hydriding/dehydriding kinetics.21­23)
Mechanical milling (MM) has been developed to accelerate
the hydriding/dehydriding kinetics by preparing Mg-based
materials with nanostructures and large specific surface areas
favorable for hydrogen diffusion.24­26) Apart from MM,
hydriding combustion synthesis (HCS) has been regarded as
an innovative method to produce Mg-based hydrogen storage
+
Corresponding author, E-mail: [email protected]
alloys due to the advantages like short processing time, low
energy requirement and high activity of the products.27,28)
In our previous study, a novel nanostructured magnesiumbased material with improved hydriding properties was
prepared by combining HCS with MM. The HCS+MM
product of Mg95Ni5 reaches its saturated hydrogen capacity
of 5.56 mass% under the initial hydrogen pressure of 3.0 MPa
even in 100 s at 373 K.29) However, the saturated hydrogen
absorption capacity is still less than the theoretical hydrogen
capacity of the system. Moreover, the dehydriding properties
of these products are still far from practical requirements.
Therefore, much attention should be paid not only to increase
the saturated hydrogen absorption capacity but also to
accelerate the dehydriding kinetics at lower temperatures.
Nickel has been found to be effective to destabilize MgH2
due to its active role in the dissociation and recombination of
hydrogen.30) Xie et al. reported that 6.1 mass% hydrogen is
desorbed in 10 min at 523 K under an initial pressure of
0.01 bar of H2 when MgH2 nanoparticle was doped by
10 mass% Ni nanoparticle.30) Hanada et al. examined the
catalytic effect of nanoparticle of 3d-transition metals such
as Fenano, Conano, Ninano and Cunano on hydrogen desorption
properties of MgH2.31) The results show that the decomposition of MgH2 is significantly improved by the addition of
Ninano and ³6.5 mass% hydrogen is desorbed in the temperature range from 423 to 523 K at a heating rate of 5 K min¹1
under He gas flow with no partial pressure of hydrogen for
the 2 mol% Ninano-doped MgH2.31) Varin et al. compared
the catalytic effect of different forms of nickel, including
micro-Ni, submicro-Ni, and nano-Ni, showing that both
the hydrogen absorption and desorption steps are largely
improved with the addition of 2 mass% nano-Ni: ³6 mass%
hydrogen is already absorbed after ³15 h of controlled
reactive mechanical milling and the DSC onset and peak
desorption temperatures are reduced by ³30 and 50 K,
respectively.32) Recently, Wronski et al. studied the effect of
1150
L. Wei, Z. Cui, Y. Zhu and L. Li
the nano-sized Ni produced by chemical vapour deposition
on MgH2.33) They claimed that the nano-Ni exhibits strong
catalytic properties and lowers greatly the activation energy
(EA) by 50%.33) Therefore, the hydrogen desorption properties of MgH2 are largely improved by nanosized nickel.
Milling with carbon additives is another effective way to
enhance the kinetics of magnesium-based hydrogen storage
materials.34­37) This is probably due to both the physical
contributions of the carbon additives in facilitating mechanical milling and its chemical catalytic role in hydrogen
dissociation and diffusion.
Considering that Nano-nickel/MWCNT contains nanosized nickel uniformly distributed on MWCNT which may
have a better enhancing effect on the hydrogenation and
dehydrogenation of MgH2, and given that HCS+MM has an
obvious advantage of preparing nanostructured magnesiumbased material with high activity, we have studied the
HCS+MM-Mg99Ni+Nano-nickel/MWCNT system to investigate the role of joint milling of MWCNT and nanosized
nickel on hydrogen storage properties of the HCS product of
Mg99Ni. Here we present a synergistic effect on enhancing
hydrogenation and dehydrogenation of Mg99Ni by adding
Nano-nickel/MWCNT. The mechanism for this synergistic
effect has been discussed based on the microstructures.
2.
Experimental Procedure
Nano-nickel/MWCNT catalyst was prepared via a simple
“chemical reduction” route according to previous studies38)
with slight changes. Prior to impregnation with Ni precursor,
MWCNT (97 mass% in purity, Shenzhen Nanotech Port Co.,
Ltd., 2 g) was oxidized in the concentrated HNO3 solution
(100 mL), which was then stirred and refluxed at 413 K for
6 h. The as-treated MWCNT was then washed in distilled
water and dried overnight in air at 343 K. Afterward, the
oxidized carbon material (1 g) and Ni(NO3)2·6H2O (7.4 g)
were homogenized by ultrasonic vibration in acetone (50 mL)
for 3600 s. After being completely dried in air at 326 K, the
mixture was put into an oven, treated at 673 K for 4 h under
an Ar flow and then reduced at 723 K for 4 h under a H2 flow.
Thus, Nano-nickel/MWCNT catalyst having 60 mass% Ni
content was obtained.
Mg (³37 µm in size and 99.7 mass% in purity, Tangshang
Weihao Magnesium Powder Co., Ltd.) and nano-Ni powders
(³50 nm in size and 99.9 mass% in purity, Kunshan Miyou
Nanometer Material Technology Co., Ltd.) in 99 : 1 molar
ratio was milled in a 100 mL stainless steel vial with stainless
steel balls (a ball to powder ratio of 20 : 1, at 400 rpm) for 5 h
under 0.1 MPa argon and the as-milled product was used for
HCS. More details about HCS are described in our previous
study.29) The above HCS product was denoted as HCSMg99Ni. Afterwards, the HCS-Mg99Ni with 2 mass% Nanonickel/MWCNT catalyst was mechanically milled for 10 h
under 0.6 MPa hydrogen atmosphere (a ball to powder ratio
of 30 : 1, at 400 rpm). The above MM product was denoted
as HCS+MM-Mg99Ni+Nano-nickel/MWCNT. For comparison, the following mixtures, i.e., Mg99Ni, Mg99Ni+1.2
mass% nano-nickel, Mg99Ni+0.8 mass% MWCNT were
also prepared under identical conditions. The as-prepared
products were denoted as HCS+MM-Mg99Ni, HCS+MM-
Mg99Ni+nano-nickel, and HCS+MM-Mg99Ni+MWCNT,
respectively. Mechanical milling was conducted on a
planetary milling apparatus (QM-3SP2, Nanjing Nanda
Instrument Plant).
Structural characteristics of the HCS and HCS+MM
products were identified by an X-ray diffractometer (XRD,
ARL X’TRA) with Cu K¡ radiation (operating at 35 kV and
40 mA). The particle sizes and crystallite sizes of the
HCS+MM products and the distribution of Nano-nickel/
MWCNT were observed by using high-resolution transmission electron microscope (HRTEM, JEM-2010 UHR).
TEM samples were prepared by ultrasonic shaking of the
powder in ethanol and drying on a copper grid with a holey
carbon foil.
The hydrogen storage properties of the HCS+MM
products were evaluated by using a Sieverts type apparatus
(GRC, Advanced Materials Co.). About 0.5 g of the powders
was loaded into a stainless steel sample chamber in the glove
box filled with argon. Since the HCS+MM products were
hydrides, they were dehydrogenated completely by evacuating upon being heated to 603 K at a heating rate of 5 K min¹1
prior to the hydriding kinetics measurement. In addition, the
hydriding kinetics at 373 and 423 K were measured under the
initial hydrogen pressure of 3.0 MPa and the dehydriding
kinetics at 523, 533 and 543 K were measured under the
initial hydrogen pressure of 0.005 MPa. The cyclic measurements of hydrogenation were carried out according to the
following procedures: dehydrogenation was performed by
evacuating upon being heated to 603 K, while hydrogenation
was performed at 423 K for 800 s under a hydrogen pressure
of 3.0 MPa. The weights of the additives which did not
absorb or desorb hydrogen were excluded when calculating
the hydrogen absorption/desorption capacities.
3.
Results and Discussions
3.1 Hydrogenation properties
The hydriding kinetics curves of the HCS+MM products
of Mg99Ni with different additives at 373 K under 3.0 MPa
hydrogen pressure are shown in Fig. 1. It is demonstrated
that the pristine HCS+MM-Mg99Ni absorbed only 5.71
mass% H2 within 100 s. The comparison in the hydriding
kinetics and capacity of the four samples reveals several
phenomena as follows: (1) Nickel significantly enhances both
the hydriding kinetics and the capacity. (2) MWCNT largely
increases the capacity but has little impact on the kinetics.
(3) The nanosized-nickel and MWCNT have synergistic
effects on the hydrogenation. (4) The Nano-nickel/MWCNT
is more efficient in improving the hydriding properties than
the addition of either MWCNT or nanosized-nickel. (5) The
HCS+MM-Mg99Ni+Nano-nickel/MWCNT system displays
superior hydrogen absorption property in both kinetics and
capacity, which reaches its saturated hydrogen capacity of
6.79 mass% in only 80 s.
3.2 Dehydrogenation properties
Figure 2 gives the dehydriding kinetics of the HCS+MM
products of Mg99Ni with different additives at 523 K under
initial hydrogen pressure of 0.005 MPa. It is apparent that the
HCS+MM-Mg99Ni+Nano-nickel/MWCNT system exhibits
7
6
6
5
4
3
2 mass% Nano-Ni/MWCNT
0.8 mass% MWCNT
1.2 mass% Nano-Ni
None
2
1
0
0
20
40
60
80
Time, t / s
100
120
90
80
70
60
4
50
3
40
2
140
30
523 K
533 K
543 K
1
0
200
400
600
20
10
0
800 1000 1200 1400 1600 1800
Time, t / s
Fig. 3 Dehydriding kinetics of the HCS+MM-Mg99Ni+Nano-nickel/
MWCNT under 0.005 MPa hydrogen pressure at 523, 533 and 543 K,
respectively.
1
2 mass% Nano-Ni/MWCNT
0.8 mass% MWCNT
1.2 mass% Nano-Ni
none
-1
543K 533K
523K
n=1.06, ln(k)=-6.56
2
R =0.99419
0
ln[-ln(1-a(t))]
Hydrogen content (mass%)
100
5
0
Fig. 1 Hydriding kinetics of the HCS+MM products of Mg99Ni with
different additives under 3.0 MPa hydrogen pressure at 373 K.
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1151
Desorbed tranformed fraction (%)
7
Hydrogen content (mass%)
Hydrogen content (mass %)
Catalytic Effect of Multi-Wall Carbon Nanotubes Supported Nickel on Hydrogen Storage Properties of Mg99Ni
n=1.25, ln(k)=-6.94
2
R =0.99903
-2
n=1.36, ln(k)=-7.45
2
R =0.99405
-3
-4
Linear fit
-5
ln[-ln(1- a(t))]=nln(t)+nln(k)
0
-6
200 400 600 800 1000 1200 1400 1600 1800
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
ln(t) (s)
Time, t / s
Fig. 2 Dehydriding kinetics of the HCS+MM products of Mg99Ni with
different additives under initial hydrogen pressure of 0.005 MPa at 523 K.
the best desorption kinetics, desorbing 4.33 mass% hydrogen
within 1800 s. Compared with the systems adding either
Nano-Ni or MWCNT, it is noted that there is a synergistic
effect on dehydrogenation in the Mg99Ni+Nano-nickel/
MWCNT system.
Figure 3 shows the isothermal dehydrogenation curves of
the HCS+MM-Mg99Ni+Nano-nickel/MWCNT at 523, 533
and 543 K under initial hydrogen pressure of 0.005 MPa. As
expected, both the dehydriding kinetics and the capacity are
improved when the temperature is increased from 523 to
543 K. It can be seen that around 6.87 mass% hydrogen can
be released at 543 K in 1800 s.
To further study the catalytic effect of Nano-nickel/
MWCNT on the dehydrogenation kinetics, the activation
energy (EA) for dehydrogenation is determined by the
Johnson-Mehl-Avrami (JMA) model and Arrhenius equation
after fitting the experimental dehydriding kinetics in Fig. 3.
The JMA model is the most popular model for the analysis
of Mg-based hydrogen storage materials,39,40) which is
expressed by the following general formulation:
Fig. 4 JMA plots of ln[¹ln(1 ¹ ¡)] vs ln(t) for the dehydrogenation of the
HCS+MM-Mg99Ni+Nano-nickel/MWCNT at different temperatures.
The reacted fraction of 0.1 < ¡ < 0.7 was used.
ð1Þ
generally reflects the nucleation and growth morphology and
dimensionality, as well as the rate-limiting step. Figure 4
presents the JMA plots of ln[¹ln(1 ¹ ¡)] vs ln(t) for the
dehydrogenation of the HCS+MM-Mg99Ni+Nano-nickel/
MWCNT at different temperatures. The plots show good
linearity (R2 > 0.99) in the range of ¡ < 0.7. In addition, the
value of n is decreased from 1.36 at 523 K to 1.06 at 543 K,
which suggests that at lower temperatures, the hydrogen
diffusion along the MgH2/Mg interfaces cannot be neglected
because a value of n = 1.5 in the JMA model indicates a
three dimensional-diffusion controlled growth.39) However,
at higher temperatures (543 K), the value of n is close to 1,
which is more likely that the growth of the Mg nuclei is
limited by hydrogen reassociation on the surface as proposed
by Mitlin et al.40) The hydrogen desorption mechanism will
be further studied in Section 3.4. Furthermore, the apparent
activation energy for desorption is calculated according to the
Arrhenius formulation as
k
EA ¼ RT ln
ð2Þ
k0
where ¡ is the fraction desorbed for the hydride at time t, k
is a nucleation and growth rate constant and the Avrami
exponent n is also constant with respect to time, which
where EA is the activation energy, k is a temperaturedependent reaction rate constant determined by JMA
equation, R is the gas constant and T is the absolute
¡ ¼ 1 expðktn Þ
1152
L. Wei, Z. Cui, Y. Zhu and L. Li
-6.4
(a)
y=-12.70333x+16.8534
EA=105.62 kJ/mol H2
-6.6
2
R =0.99004
ln(k)
-6.8
-7.0
-7.2
-7.4
-7.6
1.84
1.85
1.86
1.87
1.88
1.89
1.90
1.91
1.92
-1
1000/T (K )
Fig. 5 Arrhenius plots for the dehydriding kinetics of the HCS+MMMg99Ni+Nano-nickel/MWCNT using JMA model.
(b)
7.0
6.0
Hydrogen content (mass%)
Hydrogen content (mass%)
6.5
5.5
5.0
4.5
4.0
3.5
3.0
7.5
7.0
6.5
6.0
5.5
1st
2nd
10th
5.0
4.5
4.00
2
2.5
0
100
200
4
6
8
Cycle number
300
400
10
500
600
700
800
Time, t / s
Fig. 6 Hydriding kinetics of the HCS+MM-Mg99Ni+Nano-nickel/
MWCNT in the 1st, 2nd and 10th cycles at 423 K under 3.0 MPa
hydrogen pressure. Inset shows the cyclic hydrogen absorption capacities
for 10 cycles.
temperature. As shown in Fig. 5, the activation energy (EA)
for the dehydrogenation in the HCS+MM-Mg99Ni+Nanonickel/MWCNT is calculated as 105 kJ mol¹1 H2, which is
much lower than that of the as-received commercial MgH2
(153 kJ mol¹1 H2).41) This indicates that Nano-nickel/
MWCNT decreased the mass transfer barrier for dehydriding,
and had an obvious catalytic effect on the dehydrogenation
reaction of MgH2.
3.3 Rehydrogenation properties
The reversibility of the HCS+MM-Mg99Ni+Nano-nickel/
MWCNT system was characterized by 10 cycles of hydrogenation at 423 K. As shown in Fig. 6, the present system
exhibits an excellent cyclic stability of high hydriding rate
upon the cycle tests. It is obvious that the hydriding kinetics
remains intact from the 1st to the 10th cycle, and it still
reaches its saturated hydrogen capacity within 80 s in the 10th
cycle. Moreover, the hydrogen capacity of this system only
degrades slightly following 10 cycles as shown in the inset of
Fig. 6. The saturated hydrogen capacity is 6.60 mass% in the
10th hydriding cycle, only a loss of 0.20 mass% compared
with 6.80 mass% in the 1st cycle. Thus, in this respect, the
Fig. 7 (a) HRTEM image of HCS+MM-Mg99Ni+Nano-nickel/MWCNT,
(b) the corresponding selected area electron diffraction pattern of the
region circled.
HCS+MM-Mg99Ni+Nano-nickel/MWCNT is superior to
those reported in previous works.42,43)
3.4 Discussion
According to the microstructural analysis, we propose a
mechanism about the synergetic effect of nanosized nickel
and MWCNT on the hydrogenation and dehydrogenation. Figure 7(a) is a TEM image of the HCS+MMMg99Ni+Nano-nickel/MWCNT. It can be seen that the
tubular MWCNT are aggregated along the particle surface
and the grain boundaries inside the Mg particles. There are
black particles with nanosize homogeneously embedded in
the matrix of MgH2. The selected area electron diffraction
pattern (SAED) of the region circled is given in Fig. 7(b).
The index of the diffraction pattern reveals the presence of
MgH2 and Mg2NiH4 phases. Thus, it indicates that these
black particles in Fig. 7(a) are actually nanosized Mg2NiH4.
Figure 8 gives XRD patterns of the HCS-Mg99Ni and
the HCS+MM-Mg99Ni+Nano-nickel/MWCNT. The HCSMg99Ni consists of MgH2 and un-reacted Mg. After milling
with Nano-nickel/MWCNT, the diffraction peaks of MgH2
are broadened due to crystal grain refinement and lattice
stress. Moreover, the diffraction peaks of un-reacted Mg
disappear. However, diffraction peaks corresponding to
Mg2NiH4 are not found in the XRD patterns, which may
be attributed to the nanostructure and the relatively small
amount of Mg2NiH4 in the sample.
Catalytic Effect of Multi-Wall Carbon Nanotubes Supported Nickel on Hydrogen Storage Properties of Mg99Ni
Mg
HCS+MM-Mg99Ni+nano-nickel/MWCNT
Intensity (arb. unit)
MgH 2
HCS-Mg99Ni
20°
30°
40°
50°
60°
70°
2θ
Fig. 8 XRD patterns of HCS-Mg99Ni and HCS+MM-Mg99Ni+Nanonickel/MWCNT.
could nucleate and grow around the uniformly-dispersed
active Mg2Ni site. Therefore, the hydrogen absorption
property is improved. Moreover, the MWCNT aggregated
along the surface can inhibit the formation of a new oxide
layer, favorable for increasing the saturated hydrogen
absorption capacity.
The dehydrogenation mechanism is the reverse process
to hydrogenation. Mg2NiH4 nanoparticles desorb hydrogen
firstly, and then MgH2 near the Mg2Ni particle starts to
decompose. MWCNT helps transfer H-atom to the Mg2Ni
particle. Afterwards, the Mg2Ni particle recombines H-atom
into hydrogen molecules. Therefore, according to the JMA
fitting results (Fig. 4), Nano-nickel/MWCNT could not only
enhance greatly hydrogen diffusion, but also promote the
recombination of hydrogen which improves the dehydriding
kinetics and capacity of MgH2.
Further, to help understand the excellent cyclic stability
of the HCS+MM-Mg99Ni+Nano-nickel/MWCNT, HRTEM
images of the composite after 10 hydriding cycles (in
hydrogenated state) are shown in Fig. 10. It shows that
the particle size of the composite is almost unchanged
(Fig. 10(a)) and Mg2NiH4 nanoparticles are still distributed
uniformly in the MgH2 matrix (Fig. 10(b)). Moreover, the
tubular MWCNT are still aggregated along the particle
surface and the grain boundaries inside the Mg particles
(Fig. 10(c)). Therefore, the Nano-nickel/MWCNT may also
act as a grain growth inhibitor that prevent the sintering
and agglomeration of Mg or MgH2 particles during the
dehydrogenation-hydrogenation cycling. This result is similar
to a previous report by Lu et al.,24) which showed that TiH2
additive hinders the MgH2 from grain growth during the
cycles at 573 K.
4.
Fig. 9 HRTEM image of Nano-nickel/MWCNT.
According to above phase identification by TEM and
XRD, MgH2 and Mg2NiH4 phases exist in the HCS+MMMg99Ni+Nano-nickel/MWCNT. As shown in Fig. 9, the
nickel particles formed in situ by chemical reduction are very
small, only about 30 nm in size, and they have very low
activation energies for alloying.44) These nanosized nickel
particles can react with Mg easily under hydrogen atmosphere during mechanical milling. Therefore, the nanosized Mg2NiH4 phase in the HCS+MM-Mg99Ni+Nanonickel/MWCNT is formed probably according to the reaction
shown below:
2Mg þ Ni þ 2H2 ! Mg2 NiH4
ð3Þ
The as-synthesized nanosized Mg2NiH4 can absorb/desorb
hydrogen more easily and faster than MgH2. As seen from
Fig. 7(a), Mg2NiH4 nanoparticles are embedded uniformly in
MgH2 matrix, thus they work as a hydrogen pump and
transport media during the hydrogenation/dehydrogenation
process. During hydrogenation, Mg2Ni nanoparticles provide
active catalytic sites for the dissociation of hydrogen
molecules.30) Then, MWCNT which is in an intimate contact
between Mg2Ni and Mg works as a pathway favoring
hydrogen diffusion to Mg grain.45) Finally, More MgH2 phase
1153
Conclusion
The effect of Nano-nickel/MWCNT on the hydrogen
storage properties of HCS-Mg99Ni was investigated. It has
been shown that the HCS+MM-Mg99Ni+Nanosized-nickel
composite exhibits much better hydriding/dehydriding
kinetics. Also the addition of MWCNT in the shape of
Nano-nickel/MWCNT can further improve the hydrogenation/dehydrogenation performance. About 6.79 mass% of
hydrogen can be absorbed within 80 s at 373 K and around
97.2% hydrogen can be desorbed within 1800 s at 543 K
under initial hydrogen pressure of 0.005 MPa from the
HCS+MM-Mg99Ni+Nano-nickel/MWCNT composite. This
composite also exhibits an excellent cyclic stability of high
hydriding rate at 423 K. There is a synergetic catalytic effect
of nanosized nickel and MWCNT on the HCS+MMMg99Ni+Nano-nickel/MWCNT system. It indicates that
in situ formed Mg2NiH4 nanoparticles from nano-nickel in
Nano-nickel/MWCNT play an important role in promoting
the hydriding/dehydriding properties of MgH2, and
MWCNT facilitates the hydrogen diffusion, benefits for the
hydrogen capacity increase and prevents the particles from
sintering and agglomerating during cycling, leading to the
enhanced cyclic stability. In addition, the dehydrogenation
activation energy of this system is reduced to 105 kJ mol¹1,
probably ascribing to the synergetic effect of nanosized
Mg2NiH4 and MWCNT.
1154
L. Wei, Z. Cui, Y. Zhu and L. Li
(a)
(b)
(c)
Fig. 10 HRTEM images of the HCS+MM-Mg99Ni+Nano-nickel/MWCNT after 10 hydriding cycles (in hydrogenated state): (a) Bright
field image, (b) dark field image, (c) MWCNT covered on or inlaid in the particle surface.
Acknowledgments
This work was supported by the NSFC (No. 51171079),
Undergraduate Training Programs for Innovation of Jiangnan
University, Project on the Integration of Industry, Education
and Research (SBY 201320100), Natural Science Foundation
(No. 13KJA430003), Innovation Foundation for Graduate
Students (No: CXZZ12_0408), Qing Lan Project and the
Priority Academic Program Development (PAPD) of Jiangsu
Higher Education Institutions.
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