Effect of admixing different carbon structural variants on the decomposition and hydrogen sorption...

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 4 1 3 1 – 4 1 3 7

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Effect of admixing different carbon structural variants on thedecomposition and hydrogen sorption kinetics ofmagnesium hydride

Rajesh Kumar Singh, Himanshu Raghubanshi, Sunil Kumar Pandey, O.N. Srivastava*

Hydrogen Energy Centre and Unit on Nano Science and Technology, Physics Department, Banaras Hindu University,

Varanasi 221005, U.P., India

a r t i c l e i n f o

Article history:

Received 4 October 2009

Received in revised form

1 February 2010

Accepted 1 February 2010

Available online 12 March 2010

Keywords:

Hydrogen storage materials

Thermal desorption

Kinetics

* Corresponding author. Tel.: þ91 542 368468E-mail address: [email protected] (O.N

0360-3199/$ – see front matter ª 2010 Profesdoi:10.1016/j.ijhydene.2010.02.015

a b s t r a c t

The effect of admixing catalysts comprised of carbon nanostructures, specifically planar,

helical and twisted carbon nanofibers, spherical carbon particles and multi-walled carbon

nanotubes, on the hydrogen storage properties of magnesium hydride has been investi-

gated. Optimum results were achieved with the mixture containing twisted carbon

nanofibers (TCNF) synthesized by Ni catalyst derived by oxidative dissociation of catalyst

precursor LaNi5. The desorption temperature of 2 wt.% TCNF admixed MgH2 is w65 K lower

than that of pristine MgH2 milled for the same duration. The enhancement in hydrogen

absorption capacity of MgH2 admixed with 2 wt.% TCNF has been found to be two-fold in

the first 10 minutes at 573 K and under a hydrogen pressure of 2 MPa, i.e. 4.8wt% as

compared to 2.5 wt% for MgH2 alone. The increase in capacity by a factor of about two

within the first 10 minutes as a result of the catalytic activity of TCNF is one of the exciting

results obtained for hydrogen absorption in catalyzed MgH2.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction and to increase sorption kinetics, ball-milling to reduce

Hydrogen is the most abundant element on the earth and is

a promising medium for both energy transmission and

storage. Hydrogen has to be conveniently and economically

stored before it can be used as a clean fuel in IC engines or fuel

cells. For hydrogen storage, solid state (metallic as well as

complex hydrides) has potential advantages over compressed

and liquid hydrogen storage [1–3]. The gravimetric hydrogen

densities for AB5 [4] AB2, AB and A2B type metal hydrides are

usually less than 3 wt% except for MgH2 and Mg2NiH4. Mg is

cheap and found in abundance in the earth’s crust. Magne-

sium hydride has high gravimetric (7.6 wt %) as well as volu-

metric (w110 kg H2/m3) capacity. However, MgH2 is too stable

and its absorption and desorption requires high temperature

(>573 K) [5]. In order to decrease decomposition temperature

; fax: þ91 542 2369889.. Srivastava).sor T. Nejat Veziroglu. Pu

particle size [5–7] and admixing with catalysts have been

extensively studied [8–37]. Several catalysts such as transition

metals [8–12], metal oxides [13–17], halides [18–20], interme-

tallic compounds that absorb hydrogen [21–23] and different

forms of carbon [24–33] have been used to improve sorption

kinetics and to lower the desorption temperature.

Graphite, activated carbon, carbon nanoforms (single/

multi walled carbon nano tubes (S/MWCNT), carbon nano-

fibres (CNF), fullerenes, etc.) have been used as catalysts for

MgH2, hydrides of intermetallics (e.g. LaNi5) and complex

hydrides (e.g.NaAlH4). Several studies have been carried out to

explore the catalytic effect of different carbon forms on

magnesium hydride. In these investigations S/MWCNT has

been found to show quite good catalytic property [30,32].

However, few studies have been done to evaluate the catalytic

blished by Elsevier Ltd. All rights reserved.

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effect of carbon nanofibres. The catalytic effect of different

forms of carbon admixed with MgH2 has been investigated by

Lillo Rodenas et al. [27]. The results show that a nanofibre with

a small amount of metal (especially Ni, Fe) acts as a catalyst to

improve hydrogen storage property. The effect of carbon

supported nickel catalyst on MgH2 has also been studied [31].

These studies have shown that the interaction between

carbon and nanometric nickel lead to a decrease in decom-

position temperature.

Here, we have synthesized different carbon nano-

structures (CNSs) and studied their catalytic effect on the

absorption kinetics and decomposition temperature of MgH2.

It has been found that a small amount of (2wt%) CNS admixed

with MgH2 exhibits improved hydrogen storage properties. In

particular MgH2 admixed with 2 wt% twisted CNF (MgH2-

2TCNF) exhibited the optimum hydrogenation/dehydrogena-

tion among all admixed samples tested.

2. Experimental details

2.1. Synthesis of different carbon structures

Synthesis of CNF was performed by employing the catalytic

chemical vapor deposition (CCVD) method involving thermal

decomposition of carbon-rich acetylene (C2H2) gas. For an

effective thermal cracking, fine particles of catalyst precursor

of LaNi5 and ZrFe2 were used. The silica tube was evacuated

up to 10�4 torr in order to avoid oxidation of catalyst at high

temperature. Hydrogen and acetylene gas in the ratio 1:4 was

admitted to the silica tube at a total pressure of w450 torr (90

torr for H2 and 360 torr for C2H2). Thermal cracking of the

gases was achieved by heating in the presence of the above-

mentioned catalyst powders at different temperatures for

2 hours in a resistance heated furnace. At this stage the

carbon gets deposited inside the tube where the catalyst

precursor was located. TEM investigations have confirmed

that carbon nanofibres get formed on nano nickel catalyst

formed by oxidative dissociation of LaNi5.

X-ray diffraction analysis of sample was carried out with

an Xpert Pro (Panalytical) diffractometer employing CuKa

radiation (l¼ 1.54 A). The microstructural features were

monitored by Scanning Electron Microscopy (SEM) (Philips,

XL-20) using secondary electron imaging. Transmission elec-

tron microscopy (TEM) observations were performed using

a Technai 20 G2 microscope.

Planar, helical and twisted CNFs were formed by using

LaNi5 alloy as catalyst precursor. At 550 �C, by the interaction

of acetylene gas, LaNi5 produces spherical particles of Ni

which were decisive for the growth of planar CNFs (PCNFs).

The length of these CNFs was in the range of 6–7 mm and

average diameter was 300 nm (Fig. 1(a)). The diffraction

pattern in the TEM image shown in the inset on the upper

right corner of Fig. 1(a) confirms that the structures formed

were CNFs having platelet morphology. However, at 650 �C, by

the interaction of acetylene gas, LaNi5 produces faceted

polygonal particles of Ni which were decisive for the growth of

Helical and Twisted CNFs (HCNFs and TCNFs). HCNFs were

synthesized by using LaNi5 as a catalyst precursor at 650 �C for

30 minutes in the flowing gas environment. Average length of

these CNFs was 15 mm and average diameter was 2 mm

(Fig. 1(b)). TCNFs were synthesized by employing Ni as catalyst

derived through oxidative dissociation of LaNi5 at 650 �C for

2 hours. The length of these CNFs was in the range of 6–7 mm

and average diameter was 200 nm (Fig. 1(c)). The diffraction

patterns shown in the insets in the upper right corners of

Fig. 1(b) and (c) shows that the CNFs have herringbone

morphology. Spherical carbon particles (SCPs) were synthe-

sized by using ZrFe2 as a catalyst at 650 �C for 30 minutes in

the flowing gas environment (Fig. 1(d)). The details of the

synthesis of various forms of CNF are being studied and will be

published elsewhere.

The MWCNTs used in the present investigation were

synthesized by chemical vapor deposition technique using

ferrocene dissolved in benzene precursor. Details of the

synthesis process of CNTs have already been described by our

group on carbon nanomaterials [34,35]. The optimum flow

rate and concentration of ferrocene in benzene were found to

be w2 ml/min and 50 mg/ml. The SEM micrographs of as-

grown CNTs are shown in Fig. 2(a). The as-grown CNTs are

multi-walled as seen from the TEM image (Fig. 2(b)). The inset

in Fig. 2(b) shows HRTEM image of MWCNTs clearly showing

the multi-walled structure.

2.2. Ball-milling and hydrogen sorption measurement

The commercial MgH2 (Alfa Aesar 98%) was used as received.

Admixing was done by mechanical milling using a locally

fabricated miller. This consisted of a chrome steel vial with

a volume of 110 cm3. This contained two steel balls of

12.5 mm diameter and one steel ball with 4 mm diameter.

The miller was kept in an argon-filled glove box. All material

handling (including weighing and loading) was performed in

an argon (high purity)-filled glove box. Through several trial

runs employing varying concentration of CNS, it was found

that optimum admixing occurred for 1gm of as-obtained

MgH2 and 2wt% of CNS admixed through milling at 500 rpm

for 10 minutes. For comparison MgH2 was also milled alone

for 10 minutes under similar conditions and then used for

analysis. Henceforth this milled MgH2 will be referred to

simply as MgH2.

Hydrogenation/dehydrogenation properties were measured

through P–C–T system (Advanced Materials Corporation,

Pittsburgh, PA, USA) attached to a Furnace controlled to within

an accuracy of �2 K. A 300-mg sample was loaded in a cylin-

drical SS reactor for each measurement. The hydrogen

desorption behaviour was monitored under a primary vacuum

of 10�3 torr. Thermal desorption of the ball-milled samples was

performed using a computer-programmed furnace under

dynamic heating condition (5 K/min). After loading the sample,

the system was first evacuated by a rotary pump. The absorp-

tion was carried out under 2 MPa pressure of high purity

hydrogen gas at an operating temperature of 573 K.

3. Results and discussions

All CNSs were synthesized and admixed without purification

since. as stated earlier, carbon and metal particle interaction

also plays a positive catalytic effect in MgH2.

Fig. 1 – SEM images of different carbon structures (a) planar CNF (b) helical CNF (c)twisted CNF and (d) spherical carbon

particles. Inset of (a), (b) and (c) shows the diffraction pattern obtained by transmission electron microscope.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 4 1 3 1 – 4 1 3 7 4133

3.1. SEM analysis results

The morphologies of the as-received MgH2 sample and

10-minute ball-milled TCNF admixed MgH2 as revealed by

SEM are presented in Fig. 2(c) and (d). Particle size of as-

Fig. 2 – (a). SEM images of MWCNT (b) TEM image of MWCNT (Ins

as-received MgH2 and ball-milled MgH2.

received MgH2 observed from Fig. 2(c) seems to be in the

range of w 50-100 mm. After milling of MgH2 and CNS admixed

MgH2 the particle size of MgH2 is reduced to w5-10 mm and it

seems to be flattened. The microstructure of CNS is not

discernible since it is present in very small quantity (2% of the

et shows HRTEM image of MWCNT) (c) and (d) SEM image of


weight of MgH2). Analysis of SEM micrographs of ball-milled

MgH2 and different CNS admixed samples show no signifi-

cant difference in the particle morphology and hence are not

presented here.

Fig. 4 – Hydrogen absorption curve of 2-wt% CNS admixed

Mg at 573 K and an applied pressure of 2 MPa.

3.2. Hydrogen desorption/absorption on CNS-catalyzedMgH2

Fig. 3 shows the temperature programmed desorption curve of

MgH2 and different catalytic CNS-MgH2 admixtures at a heat-

ing rate of 5 K/min in a closed chamber. To avoid the effect of

increased pressure on desorption temperature, equal weights

of each sample were taken for comparison. As seen from the

figure the starting H -desorption temperature is 695 K for

MgH2, 653 K for SCP and MWCNT, 623 K for HCNF, 613 K for

PCNF and 603 K for TCNF admixed MgH2 respectively. It can

also be seen from the figure that there is a temperature shift in

the starting H-desorption temperature of about 90 K for 2wt%

TCNF admixed sample as compared to MgH2. The hydrogen

desorption temperature gets lowered by w65 K in the case of

optimum catalyst.

Fig. 4 presents a comparative study of hydrogen absorption

behaviour of desorbed MgH2 and different CNS admixed des-

orbed MgH2 samples at 300 �C and an applied pressure of

2 MPa. It was observed that the absorption kinetics of MgH2

are considerably improved by admixing different CNSs. It can

also be seen from the figure that MgH2-2TCNF shows 4.8 wt%

absorption as compared to 2.5 wt% for MgH2 alone in the first

10 minutes. The absorption rate for the first 10 minutes for

MgH2 -2TCNF is 0.48 wt%/min which is w200% higher

compared to some of the recently reported rates for catalyzed

MgH2, for example Nb2O5-catalyzed MgH2 ball-milled for

40 hours by V.V. Bhat et al. [36]. In the work of C.Z. Wu et al.

[30,32] absorption kinetics were higher than in our case, which

may be due to longer ball-milling time, which has an effect on

absorption rate. However, the decomposition temperature in

these studies is still higher than that in the present investi-

gation. Some other workers have also reported absorption

Fig. 3 – Thermal desorption of hydrogen from 2-wt% CNS

admixed MgH2 heating @ 5 K/min.

rates better than our results but these are for samples ball-

milled for more than 5 hours.

The X-ray diffractograms of as-received MgH2, 10-minute

milled MgH2 with 2 wt% TCNF and MgH2 after 5th cycle of

hydrogenation/dehydrogenation are shown in Fig. 5(a)–(c)

respectively. After milling the main phases of the sample

remained b MgH2. A change in intensity and peak width of the

samples after the 5th cycle was observed which is due to an

increase in crystallite size of MgH2 due to crystallization.

A small amount of unhydrided Mg is also present; this leads to

a decrease in storage capacity. This unhydrided Mg may be

present due to high energy impact of balls during milling. It is

possible to hydrogenate unhydrided Mg by applying pressure

higher than 5 MPa at 300 �C for 2 hours.

Fig. 5 – XRD patterns of (a) as-received MgH2 (b) 10-minute

ball-milled MgH2 (c) MgH2 after five dehydriding/hydriding

cycles.

Fig. 7 – Plot of ln[Lln(1 L a)] vs ln(t) of Mg-2TCNF at

553 K,573 K and 593 K. Inset shows ln k vs 1/T plot of Mg

and Mg-2TCNF.


Fig. 6 shows the XRD of as-synthesized TCNF without

purification. The presence of a graphitic carbon peak arising

from CNF, and La2O3 and Ni peaks from chemical decompo-

sition of native catalyst particles (LaNis) [37] can be clearly

seen. Ni particles are known to have better catalytic activity

for dissociation of hydrogen molecules to atoms [9,11] and

carbon facilitates the diffusion of H in MgH2 [30,32]. Several

workers found that the interaction of carbon and Ni enhances

hydrogen storage behaviour [38,39] and act as a better catalyst

also [27,31]. Therefore, enhancement in the hydrogen storage

properties of MgH2-TCNF can be taken to be the combined

interactive effect of Ni and carbon. In order to confirm this we

have admixed 2 wt% of purified TCNF and 2 wt% of Ni sepa-

rately as catalyst in MgH2. In both cases hydrogen storage

properties were enhanced as compared to milled MgH2 but

less than that of MgH2 catalyzed through TCNF with Ni cata-

lyst which stands as impurity in the CNF matrix. HCNF and

PCNF have also been synthesized mainly by Ni but the catalyst

used is in the micrometer range which, on synthesis, breaks

into smaller size particles w >250 nm.

As can be seen from Figs. 3 and 4 all the three types of

fibres, namely PCNF, TCNF and HCNF, were found to signifi-

cantly catalyze the sorption characteristics of Mg/MgH2. The

diameter of the CNF depends on the size of the catalyst

formed during the synthesis process. The typical SEM micro-

graphs of TCNF and PCNF as shown in Fig. 1(a) and (c) reveal

that the diameter of PCNF (>300 nm) is larger than that of

TCNF (<200 nm). Since the size of the catalyst particle decides

the diameter of the CNF, the nickel catalyst particle leading to

growth of PCNF can be taken to be>300 nm while that of TCNF

<200 nm. The larger size Ni particle is expected to have

a lower degree of carbon-nickel interaction than that of

smaller particles as in the case of TCNF. This will lead to

higher catalytic activity for TCNF. The interaction of catalyst

with carbon in the CNF is mainly at tip on which it grows. In

the case of HCNF the length of the fibre is much larger as

compared to PCNF and TCNF. Due to this, carbon-nickel

interaction is of lower degree. This is most probably the

Fig. 6 – XRD pattern of as-synthesized TCNF prepared by

employing LaNi5 as catalyst.

reason for the less effective catalytic behaviour of HGNF as

compared to PCNF and TCNF. The catalyst used in case of

MWCNT and SCP is Fe based and hence does not show

prominent effect.

We have also calculated the activation energy for hydrogen

absorption in MgH2 as well as the optimum catalyzed material

MgH2-2TCNF. The activation energies Ea, for the hydrogen

absorption of samples MgH2-2TCNF and MgH2 were estimated

from an Arrehenius plot, which can be derived from the

equation:

k ¼ Ae�Ea=RT

The absorption curves in Fig. 4 are not linear. Therefore the

rate constant k was obtained by the kinetic model for nucle-

ation and crystal growth in solids formulated by John-Mehl-

Avrami [40] outlined in the following equation:

½ � lnð1� aÞ�1=n¼ kt

where, a is the extent of the reaction which can be identified

with a normalized hydrogen wt% (range: from 0 to 1), t is the

time and k and n are constants (at constant temperature).

The logarithmic transform of the above equation has

been used to construct a graph of ln [�ln(1� a)] vs. ln(t) in

which isothermal experimental data are linear. The loga-

rithmic transforms of the equation as calculated from

hydrogenation curves at (a) 553 K, (b) 573 K and (c) 593 K for

Mg-2TCNF are shown in Fig. 7. The dependence of ln k on the

inverse temperature is shown in the inset of Fig. 7 for

hydrogenation of MgH2 and MgH2-2TCNF. From the slope of

the Arrhenius plot of ln k vs 1/T, the activation energy has

been calculated. The activation energy Ea, obtained from the

curve for absorption of hydrogen in Mg-2TCNF is 66 kJ/mol

H2 and 98 kJ/mol H2 for pure MgH2 (milled). This value is

lower than that for the formation of MgH2 calculated in

earlier studies.


4. Conclusions

The starting H-desorption temperature is 695 K for pure MgH2,

653 K for SCP and MWCNT, 623 K for HCNF, 613 K for PCNF and

the least 603 K (the lowest) for TCNF admixed MgH2 respec-

tively. For the first 10 minutes Mg-2TCNF shows 4.8 wt%

absorption as compared to 2.5 wt% for pure Mg H2. This shows

about a two-fold enhancement in the absorption rate. The

activation energy for the absorption comes out to be 66 kJ/mol

H2 for twisted CNF admixed MgH2 as compared to 98 kJ/mol H2

for pure MgH2.

Acknowledgements

The authors are grateful to Prof. T.N. Veziroglu (President, IAHE

Florida USA) and Prof. M. Groll (Stuttgart, Germany) for helpful

discussions. Financial assistance from CSIR (New Delhi, India),

UNANST (DST) and Ministry of New and Renewable Energy

(New Delhi, India) is gratefully acknowledged.

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