Enhanced hydrogen storage performance for MgH2–NaAlH4 system—theeffects of stoichiometry and Nb2O5 nanoparticles on cycling behaviour{

Rafi-ud-din,*ac Qu Xuanhui,*a Li Ping,a Lin Zhang,a Mashkoor Ahmad,d M. Zubair Iqbal,b M. Yasir Rafiqueb

and M. Hassan Farooqa

Received 21st March 2012, Accepted 21st March 2012

DOI: 10.1039/c2ra20518a

Nowadays, the technological utilization of reactive hydride composites (RHC) as promising hydrogen

storage materials is hampered by their reaction kinetics. In the present work, effects of reactant

stoichiometry on ensuing hydrogen sorption properties and pathway of the MgH2–NaAlH4 (mole

ratios 1 : 2, 1 : 1 and 2 : 1) system, both undoped and doped with Nb2O5 nanoparticles, were

investigated. It was found that the as-prepared reactant stoichiometry of MgH2/NaAlH4 system had a

profound impact on its dehydrogenation kinetics and reaction mechanism. Variable temperature

dehydrogenation data revealed that undoped binary composites possessed enhanced hydrogen

desorption properties compared to that of pristine NaAlH4 and MgH2. The use of Nb2O5 displayed

superior catalytic effects in terms of enhancing dehydriding/rehydriding kinetics and reducing the

dehydrogenation temperature of MgH2–NaAlH4 system. Isothermal volumetric measurements at

300 uC revealed that enhancements arising upon adding Nb2O5 were almost double that of undoped

MgH2–NaAlH4 composites. The apparent activation energies for NaAlH4, Na3AlH6, MgH2, and

NaH relevant decompositions in doped composite were found to be much lower than that for the

undoped one. Moreover, Nb2O5 doping also markedly enhanced the reversible capacity of

MgH2–NaAlH4 composites under moderate conditions, persisting well during three de/

rehydrogenation cycles. XRD, XPS, and FESEM-EDS analyses demonstrated that reduction of

Nb2O5 during first desorption was coupled to the migration of reduced niobium oxide species from

the bulk to the surface of the material. It was suggested that these finely dispersed oxygen-deficient

niobium species might contribute to kinetic improvement by serving as the active sites to facilitate

hydrogen diffusion through the diffusion barriers both during dehydrogenation and rehydrogenation.

Introduction

The development of new practical hydrogen storage materials

with high volumetric and gravimetric hydrogen densities is a key

technical challenge to implement the fuel cell technology for

transportation applications. According to the U.S. DOE’s 2015

targets, a viable hydrogen storage system for fuel cell applica-

tions has necessitated a system gravimetric capacity of more than

5.5 wt% with fast desorption kinetics (1.5 wt% min21). Solid-

state hydrogen storage is attractive because it offers a volumetric

hydrogen density greater than that of either compressed gas or

liquid hydrogen storage, without high pressure containment or

cryogenic tanks.1–5 During the past decade, MgH26–8 and light-

element complex hydrides such as alanates,9–14 amides,15,16

borohydrides,17–19 and ammonia borane20,21 are extensively

investigated due to their higher gravimetric and volumetric

hydrogen densities. Among the solid-state hydrogen storage

materials, magnesium hydride is a good candidate possessing

large gravimetric density (7.6 wt% H2), abundant resources, low

cost, and superior reversibility compared to that of conventional

transition metal-based hydrides. However, its use is unfortu-

nately hampered by its high thermodynamic stability and its slow

kinetics. During the recent years, these disadvantages have been

overcome by preparing nanocrystalline MgH2 produced by high

energy milling,22,23 or doping with various catalysts such as

metals,24–26 metal oxides,27–29 and metal halides,30,31 etc.

Nonetheless, all these efforts have only ameliorated its kinetic

performances. The thermodynamic characteristics of the inter-

action between Mg and hydrogen are hardly modified. During

the past decade, another pioneer work in the hydrogen storage

field is the use of a mixture of single and complex hydrides with

amides based systems.32–34 Later this approach has been

extended to borohydrides systems,35–37 which are known as

aState Key Laboratory for Advanced Metals and Materials, School ofMaterials Science and Engineering, USTB, Beijing 100083, China.E-mail: quxh@ustb.edu.cn (Qu Xuanhui); rafi_682@yahoo.com(Rafi-ud-din); Fax: +86-10-62334311; Tel: +86-10-62332700bDepartment of Physics, School of Applied Science, University of Scienceand Technology Beijing, Beijing 100083, P. R. ChinacChemistry Division, PINSTECH, P. O. Office, Nilore, Islamabad,PakistandPhysics Division, PINSTECH, P. O. Office, Nilore, Islamabad, Pakistan{ Electronic Supplementary Information (ESI) available. See DOI:10.1039/c2ra20518a/

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Reactive Hydride Composites (RHCs). This approach is aimed

at modifying the thermodynamics and kinetics of the hydrogen

sorption reaction based on mutual interactions between different

hydrides by reducing the enthalpy of dehydrogenation and

rehydrogenation reactions. Recent studies have demonstrated

that it is possible to reduce the activation energy and

decomposition enthalpy of magnesium hydride by mixing it

with a second hydride specie, such as LiAlH4, indicating an

effective way to improve the hydrogen storage properties of

LiAlH4 and MgH2.38–40 Analogous to LiAlH4, NaAlH4 has also

been used to modify the de/rehydrogenation thermodynamics of

MgH2, albeit up to now there is limited investigation on sorption

reactions of this RHC system by experimental means. Recently,

Ismail et al.41 have investigated the kinetic sorption properties

and reaction pathway of a binary MgH2–NaAlH4 mixture with

4 : 1 molar stoichiometry prepared by ball milling. Unlike the

results obtained for the mixture LiAlH4/MgH2, where the

destabilization could be caused by the formation of either

Al12Mg17 or LixMgy alloys, only the intermetallic Al12Mg17 is

ascribed as a destabilizing agent in the system NaAlH4/MgH2.

The present work intends to contribute to the clarification of the

sorption properties, more precisely, indicating the most probable

dehydrogenation pathway for each of the various stoichiometries

of this system. It is important to properly understand the effect

of stoichiometry on such multicomponent system as well as the

effect of the dehydrogenation conditions on the reaction paths

and sorption properties of the system. Moreover, in our recent

studies on LiAlH4,10 we have found that Nb2O5 nanoparticles

are superior to its analogue Cr2O3 in enhancing the kinetic and

thermodynamic performance of LiAlH4. Previous studies29,42

have also indicated that the additive Nb2O5 is known to

substantially improve the decomposition thermodynamics and

kinetics of MgH2. Motivated by these experimental findings, we

have further extended the usage of Nb2O5 nanoparticles in the

MgH2–NaAlH4 system. The addition of Nb2O5 is found to result

in significantly improved de/rehydrogenation kinetics in the

MgH2–NaAlH4 system.

To clarify the impact of reactant stoichiometry on the properties

of the undoped and doped MgH2–NaAlH4 composites, herein we

report the synthesis, characterization, hydrogen storage character-

istics, and hydrogen desorption pathway for composites with

stoichiometric ratios of 1 : 2, 1 : 1, and 2 : 1. Thermogravimetry

(TG), differential scanning calorimetry (DSC), and isothermal

sorption measurements have been conducted to investigate its

thermodynamic and kinetic behavior. On the basis of XRD, XPS,

and FESEM-EDS analyses, we suggest a set of consecutive

reactions that is consistent with the observed phase evolution and

hydrogen desorption behavior. On the basis of these data, we

discuss factors that may explain differences between these three sets

of related compositions. Furthermore, we have performed a full

analysis of the surface and bulk regions in order to investigate the

evolution, oxidation state, and the local structure of the Nb species

during the different steps of milling, dehydrogenation, and

rehydrogenation.

Experimental section

NaAlH4 (hydrogen storage grade, ¢93% purity) and MgH2

(hydrogen storage grade) were purchased from Sigma–Aldrich

Co. The high purity nano-oxide Nb2O5 was provided by

SINONANO Co., Ltd (China). All the materials were used as

received without any further purification. All material handling

(including weighing and loading) was performed in a high purity

argon filled glove box, with low oxygen and water vapour

content. A hydride mixture was prepared by mixing the as-

received MgH2 and NaAlH4 together in the mole ratios of 1 : 2,

1 : 1, and 2 : 1. Subsequently, the mixtures were ball milled for

30 min by a high energy Spex mill. All the samples were loaded

into the hardened steel vial under an argon atmosphere in a glove

box. Steel balls (1 g and 3 g) were added with a ball to powder

weight ratio of 15 : 1. Air-cooling of the vial was employed to

prevent its heating during the ball-milling process. 2 mol% of

Nb2O5 nanopowders was also ball milled with 1MgH2–

2NaAlH4, 1MgH2–1NaAlH4, and 2MgH2–1NaAlH4 under the

same conditions to investigate their catalytic effects. MgH2 and

NaAlH4 doped with 2 mol% of Nb2O5were also prepared under

the same conditions for comparison purposes.

Non-isothermal dehydrogenation performances were investi-

gated by thermogravimetry (TG) and differential scanning

calorimetry (DSC). The DSC and TG analyses were conducted

using NETZSCH STA 449C. All measurements were carried out

under a flow (50 ml min21) of high purity argon (99.999%).

Sample mass was typically 5 mg. Heating runs were performed at

different rates (4, 7 and 10 uC min21) from 35 to 500 uC.

The isothermal de/re-hydrogenation kinetics were measured

using a pressure composition–temperature (PCT) apparatus. The

details of the apparatus are given in our previous reports.9,10,43

The apparatus can be operated up to a maximum pressure of

10 MPa and 600 uC. About 0.5 g of sample was loaded into the

sample vessel. The isothermal dehydrogenation measurements

for the undoped and doped samples were performed at 280 uCand 300 uC under a controlled vacuum atmosphere. Following

the first complete dehydrogenation, the samples were subjected

to rehydrogenation studies at 300 uC under 9.5 MPa for 1 h. The

pressure drop with time in the closed system testified the

rehydrogenation of the samples. Subsequently, the rehydroge-

nated samples were dehydrogenated at similar temperature.

The phase structure of the sample following the ball milling,

dehydrogenation, and rehydrogenation was determined by a

MXP21VAHF X-ray diffractometer (XRD with Cu-Ka radia-

tion) at room temperature. XRD was done at a tube voltage of

40 kV and a tube current of 200 mA. The samples were covered

with paraffin film to prevent the oxidation during the XRD test.

X-ray photoelectron spectroscopy (XPS) was performed with a

PHI-5300 XPS spectrometer.

The as-received, doped, dehydrogenated, and rehydrogenated

samples were examined by a field emission scanning electron

microscope (FESEM-6301F) coupled with energy dispersive

spectroscopy (EDS). Sample preparation for the FESEM

measurement was carried out inside the glove box and, more-

over, the samples were transferred to the SEM chamber by

means of a device maintaining an Ar overpressure.

Results and discussion

The TG profiles in Fig. 1 depict the non-isothermal dehydrogena-

tion performance of the milled MgH2–NaAlH4 composites with

different mole ratios (1 : 2, 1 : 1 and 2 : 1) undoped and doped

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with 2 mol% Nb2O5. The manometric desorption profiles

recorded on the unary components NaAlH4 and MgH2, with

and without 2 mol% Nb2O5, are also reported in Fig. 1 for

comparison. In the case of pristine NaAlH4, the hydrogen

desorption starts at 178 uC. After heating to 450 uC, this material

has released a total hydrogen capacity of 6.3 wt%. For the bare

MgH2, the hydrogen release starts at 345 uC, and the weight loss is

about 7.2 wt% after heating to 450 uC. It is evident that both the

undoped and doped 1MgH2–2NaAlH4 and 1MgH2–1NaAlH4

systems display three significant stages of dehydrogenation

occurring during the heating process. On the contrary, the

undoped 2MgH2–1NaAlH4 composite has exhibited four-stage

hydrogen release. The relative proportion of hydrogen evolved

during these three or four stages is of course dependent upon the

ratio of the as prepared samples. The first stage for all the

undoped and doped composite samples seems to be the two-step

decomposition of NaAlH4 as indicated in eqn (1) and (2), because

both the dehydrogenation temperatures and the hydrogen

liberation amounts correspond to the decomposition of NaAlH4.

3NaAlH4 A Na3AlH6 + 2Al + 3H2 (1)

Na3AlH6 A 3NaH + Al + 3/2H2 (2)

Although the first step dehydrogenation temperature (175 uCand 170 uC) in the MgH2–NaAlH4 (molar ratios, 1 : 2 and 1 : 1)

samples has not exhibited the significant reduction compared to

that of the pristine NaAlH4 sample, faster dehydrogenation

kinetics have been observed for both samples. The first stage of

the decomposition of 1MgH2–2NaAlH4 and 1MgH2–1NaAlH4

samples terminates at 255 and 248 uC, respectively, which are

45 uC and 52 uC lower than that for the pure NaAlH4 sample.

The 1MgH2–2NaAlH4 composite starts to decompose at 278 uCand terminates at 326 uC for the second stage, whereas the

1MgH2–1NaAlH4 sample starts the second decomposition at

275 uC and concludes at 335 uC. Obviously, The MgH2

decomposition temperature in 1MgH2–1NaAlH4 composite has

reduced by 70 uC compared to that of the pristine MgH2 sample.

Further heating of the 1MgH2–1NaAlH4 sample leads to the

third decomposition, starting at about 360 uC, corresponding to

the decomposition of NaH according to eqn (3), which also

occurs at 50 uC lower than that of the pure NaAlH4 sample.

NaH A Na + 1/2H2 (3)

For the undoped 2MgH2–1NaAlH4 system, the first and

second steps (Fig. 1(i)) occur in the temperature ranges of 152–

225 uC and 240–300 uC while the onset temperatures of the third

and fourth desorption steps are at 320 uC and 360 uC,

respectively. The lowered dehydrogenation temperatures of

2MgH2–1NaAlH4 system compared to that for 1MgH2–

2NaAlH4 and 1MgH2–1NaAlH4 systems suggest that higher

amount of MgH2 enhances the decomposition of NaAlH4. After

heating to 390 uC the total desorption capacity from the 2MgH2–

1NaAlH4 mixture is about 6.9 wt%, which is higher than that of

both MgH2 and NaAlH4 alone at the same temperature. These

results indicate that the mixing between NaAlH4 and MgH2 can

decrease the onset desorption temperature compared to that of

NaAlH4 (178 uC) and MgH2 (345 uC). Therefore, the dehy-

drogenation properties of the MgH2–NaAlH4 system are

improved compared to those of its unary components

(NaAlH4 and MgH2), suggesting that a mutual destabilization

has occurred in the binary MgH2–NaAlH4 system. Obviously, a

greater reduction in the decomposition temperatures has

occurred for the composites with higher concentrations of

MgH2. This kinetic enhancement in the undoped 2 : 1 system

can be partially ascribed to the more refinement of powders in

these composites compared to that in other mixtures (Fig. 14).

In order to further clarify the mechanistic effects, three

undoped binary composites have been also heated at much lower

heating rate of 1 uC min21. The results are displayed in Fig. S1 of

the ESI.{ Evidently, all the decomposition steps still exist, albeit

the dehydrogenation temperatures have been further reduced.

Fig. 2 and 3 present the evolution of the XRD patterns of the

1MgH2–1NaAlH4 and 2MgH2–1NaAlH4 samples upon ball-

milling and heating to different temperatures, successively. The

as-milled samples are detected as the physical mixtures of

MgH2 and NaAlH4. Fig. 2(c) and 3(b) characterize the NaH, Al

and MgH2 phases in the XRD patterns of the 1MgH2–1NaAlH4

and 2MgH2–1NaAlH4 systems after dehydrogenation at

250 uC and 230 uC, respectively. It excludes the possibility of

Fig. 1 Comparison of the TG profiles of neat MgH2 and NaAlH4, and

MgH2–NaAlH4 composites (in mole ratios of 1 : 2, 1 : 1 and 2 : 1), (i)

without Nb2O5 nanoparticles and (ii) with Nb2O5 nanoparticles. The

ramping rate is 4 uC min21.

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MgH2-relevant reactions, demonstrating that only self-decom-

position of NaAlH4 dominates this stage. For the 1 : 1 composite,

after dehydrogenation at 350 uC, the Al phase has disappeared,

and the peaks pertaining to Al12Mg17 and Mg are observed clearly

from the XRD pattern in Fig. 2(d). These results indicate that the

hydrogen release in the temperature range of 250–350 uC is mainly

from the reaction of Al with MgH2 and the decomposition of

MgH2 in accordance with the following reactions:

12Al + 17MgH2 A Al12Mg17 + 17H2 (4)

MgH2 A Mg + H2 (5)

After further heating to 400 uC, the NaH phase has

disappeared with the evolution of Na phase, suggesting that

the hydrogen release in the temperature range of 350–400 uC is

due to the decomposition of NaH as indicated in eqn(3). These

results demonstrate that the three stages of dehydrogenation in

the 1MgH2–1NaAlH4 composite correspond to the sequential

decomposition of the NaAlH4, MgH2 and NaH phases (NaH

resulting from the decomposition of NaAlH4).

The XRD pattern of 2MgH2–1NaAlH4 sample, heated to

310 C, shows the formation of the NaMgH3 phase together with

Al12Mg17 in the temperature range of 240–300 uC (Fig.3c). The

presence of NaMgH3, in agreement with the TG analysis of

Fig. 1 (the third decomposition at about 320 uC), strongly

suggests that apart from the reaction between Al and MgH2, a

partial reaction between Mg/MgH2 and NaH has also taken

place in the temperature range of 240–300 uC. The formation of

NaMgH3 during the 2MgH2–1NaAlH4 decomposition reaction

demonstrates that the desorption reaction occurs via the

formation of an intermediate phase. On the contrary, the

1MgH2–1NaAlH4 sample has not exhibited the formation of

NaMgH3. This corroborates that NaMgH3 can be formed while

the ratio of MgH2/NaAH4 is relatively high. Otherwise, NaH

and Mg are formed preferentially without the formation of

NaMgH3. Ismail et al.41 have also identified the formation of

NaMgH3 for 4MgH2–1NaAlH4 composite. XRD measurements

of 2MgH2–1NaAlH4 composite at 360 uC indicate that the

decomposition of NaMgH3 has accomplished below that

temperature according to the following reaction.

NaMgH3 A NaH + Mg + H2 (6)

Moreover, the XRD pattern in Fig. 3(e) confirms that the

hydrogen release in the temperature range of 360–390 uC is

induced by the decomposition of NaH. These experimental

results, described above, point out the fact that various

stoichiometric ratios of the MgH2/NaAlH4 system can lead to

significant variations of the reaction mechanism and the sorption

properties, even if the same final decomposition products

(Al12Mg17, Mg and Na) are obtained. It can be surmised that

the more MgH2-rich compositions facilitate the favorable

formation of NaMgH3 phase. It has already been hypothesized

that the formation of a mixed compound such as NaMgH3 is

strongly associated with the microstructure of the starting

materials, which should contain interfaces between the reacting

phases.36 It is believed that higher concentrations of MgH2

favour the formation of NaMgH3 by creating large amounts of

such interfaces. Moreover, the binary NaAlH4–MgH2 systems

displayed superior hydrogen storage properties compared to that

of the unary components NaAlH4 and MgH2. This phenomenon

may be attributed to the mutual interactions among the two

hydrides.

The desorption curves in Fig.1(ii) clearly depict that the use of

nanometric Nb2O5 additions has rendered quite striking effects

not only on the dehydrogenation characteristics of the binary

composites but also on the unary components. In order to

comprehend the catalytic role of Nb2O5 on the binary system,

the samples of NaAlH4 and MgH2 doped with 2 mol% Nb2O5

nanopowders are prepared and investigated in comparison with

the undoped binary samples as well as the undoped and doped

unary components. It can be seen that nanosized Nb2O5 is

effective in ameliorating the dehydrogenation properties of the

Fig. 2 XRD patterns for the MgH2–NaAlH4 (1 : 1) composite at

different states: (a) before dehydrogenation; (b) after dehydrogenation at

225 uC; (c) after dehydrogenation at 250 uC; (d) after dehydrogenation

at 350 uC; (e) after dehydrogenation at 400 uC.

Fig. 3 XRD patterns for the MgH2–NaAlH4 (2 : 1) composite at

different states: (a) before dehydrogenation; (b) after dehydrogenation at

230 uC; (c) after dehydrogenation at 310 uC; (d) after dehydrogenation

at 360 uC; (e) after dehydrogenation at 390 uC.

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unary components NaAlH4 and MgH2 as well. In the case of the

Nb2O5-doped NaAlH4 and MgH2, the onset dehydrogenation

temperatures are decreased to 100 uC and 275 uC, which are

78 uC and 70 uC lower than that of pure NaAlH4 (178 uC) and

MgH2 (345 uC) samples. In addition, the dehydrogenation

temperatures of the NaAlH4 and MgH2 in all the MgH2–

NaAlH4 systems are also lower than that of the pure and doped

NaAlH4 and MgH2 samples. The onset temperatures of

hydrogen desorption for the milled 2 mol% Nb2O5 added

MgH2–NaAlH4 with molar ratios of 2 : 1, 1 : 1, and 1 : 2

composites appear around 70, 80, and 95 uC, respectively, which

are 82, 90, and 80 uC lower than the corresponding decomposi-

tion temperatures of the milled MgH2–NaAlH4 composite

without Nb2O5 addition. Further heating leads to the decom-

position of MgH2 near 213, 220 and 235 uC (Fig. 1(ii)), which are

also lower than the corresponding decomposition temperatures

at 240, 275 and 278 uC in Fig. 1(i). In particular for the doped

samples with molar ratios of 1 : 1 and 2 : 1, the dehydrogenation

temperature ranges have lowered to 80–355 uC and 70–350 uCdue to the additive Nb2O5. Obviously, the addition of Nb2O5

nanoparticles plays a key role in decreasing the decomposition

temperature of the MgH2–NaAlH4 system. Moreover, the

nanometric Nb2O5-doped MgH2–NaAlH4 samples have shown

faster desorption rates compared to undoped ones during the

heating process. For example, the hydrogen desorption capa-

cities of 6.6, 6.4 and 5.9 wt% are reached on heating the doped

MgH2–NaAlH4 (molar ratios of 2 : 1, 1 : 1 and 1 : 2) samples to

350 uC. In contrast, the MgH2–NaAlH4 composites (molar ratios

of 2 : 1, 1 : 1 and 1 : 2) without additive have exhibited

hydrogen release capacities of 5.7, 5.8 and 5.6 wt% hydrogen,

respectively, by 350 uC. In addition, the total hydrogen release

contents from 2MgH2–1NaAlH4 doped samples are around

6.6 wt%. It indicates that a significant reduction in the

decomposition temperature arising from the addition of a

Nb2O5 catalyst is achieved without much penalty in the practical

capacity of the materials. The above results clearly indicate that

the dehydrogenation properties of the NaAlH4–MgH2 composites

have been enhanced by the addition of Nb2O5 nanopowders.

With the aim of structurally elucidating the catalytic mechan-

ism of Nb2O5 nanoparticles, XRD measurements of the doped

composites, before and after being subjected to the dehydro-

genation at different temperatures, are displayed in Fig. 4. It is

clear that the Nb2O5 phase can be detected in the XRD pattern

of the as milled materials. It suggests that the Nb2O5

nanocrystalline particles remain stable with the composite matrix

during ball-milling under the high-energy impact mode.

Although, the dehydrogenation products of the doped 1 : 1

composite are analogous to that observed for undoped one,

nevertheless, some by-products due to the decomposition/

reaction of the dopant can be revealed in the discharged mixture

of the doped sample. Fig. 4a clearly depicts that the doped 1 : 1

sample, at the beginning, consists mainly of Nb2O5 and changes

significantly during the heating and desorption process. It is

evident that the heating of the sample up to 200 uC has induced

the disappearance of the crystalline Nb2O5, coupled to a parallel

growth of the newly reduced niobium species with different

oxidation states similar to NbO2 (+4) and NbO/NbH (+2, +1).

During further heating, the Mg liberated from the hydrogen

induces a fast reduction of the niobium oxide due to its very low

redox potential, leading to the evolution of new peaks

corresponding to pure MgO. Obviously, the reaction between

the reduced niobium oxide species and the Mg/MgH2 has also

yielded the formation of a ternary oxide phase MgNb2O3.67. It

has been previously documented42 that NbO and MgO exhibit

similar crystal structure. Moreover, both have similar lattice

parameters with comparable bond lengths between metal and

oxygen atoms. Therefore, a reaction between the niobium oxide

and the Mg/MgH2 with a formation of a ternary oxide

MgxNbyO is possible. Hence, the evolution of the additive to

the real active species has taken place in the form of mixed

MgNb2O3.67 phase. In the literature, the formation of ternary

MgxNbyO has already been reported for the MgH2 doped with

Nb2O5. It has been documented that these ternary Mg–Nb

oxides facilitate the hydrogen diffusion by acting as pathways by

the formation of metastable niobium hydrides.42 When the

desorption process is finished, a steady state is reached consisting

mainly of phases with the oxidation states rich in +1(NbH) and

+2(NbO).

Fig. 4e presents the evolution of the XRD pattern of the

Nb2O5-doped 2MgH2–1NaAlH4 composite upon heating to

300 uC. Interestingly, no diffraction peaks corresponding to the

NaMgH3 phase have been detected for the 2 : 1 doped mixtures

(in the sensitivity limits of the XRD), suggesting that the

presence of Nb2O5 alters the desorption pathway by inhibiting

the formation of NaMgH3. These results coincide well with those

obtained in Fig. 1, indicating that the four-stage decomposition

for the undoped 2MgH2–1NaAlH4 sample has transformed to

three- stage owing to the additive Nb2O5. Moreover, these results

are also in contrast with that reported by Milanese et al.,44

pointing to the fact that the dopants hamper the formation of

NaMgH3 phase.

The thermal decomposition behavior of the undoped and

doped MgH2–NaAlH4 (molar ratios of 2 : 1, 1 : 1 and 1 : 2)

composites, as well as of undoped unary components NaAlH4

and MgH2, is further investigated by DSC, as presented in Fig. 5,

Fig. 4 XRD patterns for the Nb2O5-doped MgH2–NaAlH4 (1 : 1)

composite at different states: (a) before dehydrogenation; (b) after

dehydrogenation at 200 uC; (c) after dehydrogenation at 300 uC; (d) after

dehydrogenation at 350 uC; (e) XRD pattern of the Nb2O5-doped

2MgH2–1NaAlH4 composite upon heating to 300 uC.

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6, and 7. Fig. 5 illustrates the DSC results of the as-received

MgH2 and alanate samples at various heating rates (4 uC min21,

7 uC min21 and 10 uC min21, respectively). the DSC results of

Nb2O5-doped NaAlH4 at various heating rates (4 uC min21, 7 uCmin21 and 10 uC min21, respectively) have been reported in Fig.

S2 of the ESI.{ The calorimetric profiles, recorded on the binary

1 : 1, 1 : 2 and 2 : 1 mixtures, are reported in Fig. 6. Five

endothermic events in the 1 : 1 and 1 : 2 mixtures are assigned to

the melting of NaAlH4, the decomposition of molten NaAlH4 to

Na3AlH6, the decomposition of Na3AlH6 to NaH and Al, the

decomposition of MgH2, and the decomposition of NaH,

respectively, which is comparable with the three-step dehydro-

genation in the TG results. Concomitant with the aforemen-

tioned XRD and TG results, the first four endothermic processes

in 2 : 1 sample are similar to that in other composites, while the

fifth and sixth events are attributed to the decompositions of

NaMgH3 and NaH, respectively. The resulting peak tempera-

tures, measured in Fig. 6(i), are small compared to that of pure

NaAlH4 and MgH2 samples. For instance, the peak tempera-

tures for 2 : 1 composite for the first and second dehydrogena-

tion steps of NaAlH4 are 191 uC and 213 uC, respectively, which

are 49 uC and 58 uC lower than those of the bare alanate sample.

It is also evident that the peak temperature for pure MgH2 is

406 uC, which is 97 uC higher than that for the MgH2-relevant

decomposition in 1MgH2–1NaAlH4 sample. These results

further testify the mutual destabilization between NaAlH4 and

MgH2. In contrast with the calorimetric profiles of the undoped

MgH2/NaAlH4 composites, the features of all the doped samples

are strikingly different, displaying only four endothermic peaks

[Fig. 7]. The results indicate that NaAlH4 decomposes at a much

lower temperatures without melting with Nb2O5 catalysis.

Obviously, the addition of Nb2O5 nanopowders has rendered a

significant reduction in the decomposition temperatures asso-

ciated with the MgH2–NaAlH4 system.

To acquire detailed information about the kinetics of the

reactions, the apparent activation energy related to the NaAlH4,

Na3AlH4, MgH2, and NaH decompositions in both the undoped

and doped 1MgH2–1NaAlH4 composites, are calculated by

using the nonisothermal Kissinger method45 which is based on

the shift in peak temperatures with the heating rates of 4, 7, and

10 uC min21. The activation energies of pristine NaAlH4 and

MgH2, and Nb2O5-doped NaAlH4, for hydrogen desorption, are

also reported. The derived values of the activation energies are

Fig. 5 The DSC profiles at various heating rates (4 uC min21,

7 uC min21 and 10 uC min21) for pristine (i) MgH2 and (ii) NaAlH4.

Fig. 6 The DSC profiles for (i) MgH2–NaAlH4 composites (in mole

ratio of 1 : 2, 1 : 1 and 2 : 1) at heating rate of 4 uC min21 and (ii)

MgH2–NaAlH4 composite (in mole ratio of 1 : 1) at various heating rates

(4 uC min21, 7 uC min21 and 10 uC min21).

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listed in Tables 1 and 2. For comparison, the activation energies

of NaAlH4 and MgH2, undoped and doped with other additives,

are also recapitulated in Tables 1 and 2.6,25–27,46–49,50–55 The

calculated apparent activation energies for the first, second, and

third decomposition steps for the pure alanate are 116 kJ mol21,

149 kJ mol21, and 180 kJ mol21, respectively. It is evident in

Table 1 that the activation energy values for the first and second

stages obtained in our work, agrees very well with that reported

by Fan et al.47 The apparent activation energy calculated is

about 153 kJ mol21 for pure MgH2, which is consistent with

the values of 150, 156, 158, and 160 kJ mol21 H2 for un-milled

MgH2 in Table 2. With the mixing of two hydrides,

the activation energy has lowered to 109, 124, 120, and

164 kJ mol21 for NaAlH4, Na3AlH6, MgH2, and NaH relevant

decompositions, respectively, which exhibits an enhancement in

kinetics. This result clearly indicates that the mixing of two

hydrides reduces the energy barriers for all the four stages but is

more efficient for the last three stages, i.e., the dehydrogenation

of Na3AlH6, MgH2, and NaH, respectively. For the uncatalyzed

MgH2–NaAlH4 composite, the reduction of activation energy

may be attributed to the particle size reduction and the

occurrence of mutual interactions between two species. The

derived apparent activation energies corresponding to NaAlH4,

Na3AlH6, MgH2, and NaH decompositions for the MgH2–

NaAlH4 composite, catalyzed with Nb2O5 nanoparticles, are 68,

81, 75, and 130 KJ mol21, respectively. It implies that the

activation energies of four stages are reduced by around 41, 43,

44, and 34 kJ mol21, compared with the activation energies

associated with the undoped 1MgH2–1NaAlH4 sample, respec-

tively. Table 1 and 2 clearly demonstrate that these values are

slightly higher than those reported for TiO2-doped NaAlH4 and

MgH2 doped with TiH2, but smaller than that reported for

NaAlH4 and MgH2 doped with all other additives. Thereby, it is

surmised that the presence of the Nb2O5 nanoparticles has

essentially rendered the diminution of activation energy barrier

for the dehydrogenation of the MgH2–NaAlH4 samples.

Fig. 7 The DSC profiles for (i) Nb2O5-doped MgH2–NaAlH4 compo-

sites (in mole ratios of 1 : 2, 1 : 1 and 2 : 1) at heating rate of 4 uC min21

and (ii) Nb2O5-doped MgH2–NaAlH4 composite (in mole ratio of 1 : 1)

at various heating rates (4 uC min21, 7 uC min21 and 10 uC min21).

Table 1 Comparison of Activation energies (KJ mole21) of pure and doped NaAlH4

Sample Determination method NaAlH4 NaAlH3 References

Pristine NaAlH4 Kissinger Method 114.2,132 156.8,268 47,46Pristine NaAlH4 Isothermal 118.1 120.7 48Pristine NaAlH4 Kissinger Method 116 149 Present workNaAlH4 + MgH2 Kissinger Method 109 124 Present workNaAlH4 + MgH2 + Nb2O5 Kissinger Method 68 81 Present workNb2O5-doped Kissinger Method 65 86 Present workCeCl3-doped Kissinger Method 80.8 97.2 47CeAl4-doped Kissinger Method 80.9 98.9 47TiCl3-doped Isothermal 80 97.5 48Ti-doped Kissinger Method 77 49TiO2-doped Kissinger Method 67 49Ti/Til3N6THF-doped Kissinger Method 96.2 197.2 50Ti/TiCl3-doped Kissinger Method 139.5 50CeAl-doped Kissinger Method 72.3 98.9 51LaCl3-doped Kissinger Method 86.4 96.1 52La3Al11-doped Kissinger Method 92.9 99.2 52SmCl3-doped Kissinger Method 89 96.7 52SmAl3-doped Kissinger Method 91.9 98.9 52TiF3-doped Kissinger Method 98 130 46SiO2-doped Kissinger Method 127 138 46TiF3 + SiO2-doped Kissinger Method 99 122 46

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The pronounced effect of Nb2O5 nanopowders in promoting

dehydrogenation reactions of the Na–Mg–Al–H system is

further demonstrated in the isothermal volumetric measure-

ments. Fig. 8 presents the dehydriding kinetics at 280 uC and

300 uC for the samples with and without the Nb2O5 additive,

respectively. For comparison, the dehydrogenation kinetics of

the pure MgH2, decomposed at 280 uC and 300 uC, are also

included. The desorption rate is much slower for the pristine

MgH2 both at 280 uC and 300 uC. The desorption kinetics exhibit

an enhancement after mixing with NaAlH4. However, the

enhancements arising upon the addition of Nb2O5 to the

MgH2/NaAlH4 sample are much more significant. Within

60 min, the pure MgH2 has only desorbed 0.7 wt% and

1.1 wt% hydrogen at 280 uC and 300 uC, respectively. The

1MgH2–1NaAlH4 composite can release 4.5 wt% and 5.2 wt%

hydrogen in 60 min at 280 uC and 300 uC, respectively. Under the

same conditions, the doped sample has yielded 5.9 wt% and

6.2 wt% of hydrogen. Moreover, the MgH2–NaAlH4–Nb2O5

sample has desorbed about 5.2 wt% hydrogen after 25 min at

300 uC, which is higher than for the MgH2–NaAlH4 (3.2 wt%)

and much higher than for the pure MgH2 (0.5 wt%) at same

temperature. In contrast, 60 min is required for the undoped

MgH2–NaAlH4 sample to release 5.2 wt% hydrogen at 300 uC.

These results indicate that the dehydrogenation kinetics of MgH2

is significantly improved by combining with NaAlH4, and also

further enhanced by the addition of Nb2O5. In conclusion, the

Nb2O5-doped MgH2/NaAlH4 mixture exhibits superior dehy-

drogenation properties than that of the pure MgH2 as well as the

uncatalyzed MgH2/NaAlH4 samples.

In order to further analyze the catalytic efficiency of Nb2O5

nanoparticles for the rehydrogenation/dehydrogenation cycles,

the reversibility and cyclic properties of the Nb2O5-doped binary

composite system have been investigated. The rehydrogenation

of the samples has been conducted under 9.5 MPa of H2 at

300 uC after complete dehydrogenation. The undoped binary

system has also been examined for comparison. Fig. 9 displays

the isothermal rehydrogenation kinetics for the first three

hydrogenation cycles of the Nb2O5-doped 1 : 1 system and the

first rehydrogenation cycle of the undoped 1 : 1 sample. It is

evident that the rehydriding capacity for the undoped sample

(3.3 wt%) is smaller than that of the Nb2O5-doped sample

(4.4 wt%), indicating that the reversibility is facilitated by the

Nb2O5 dopant. Moreover, the recycled MgH2–NaAlH4–Nb2O5

sample exhibits well maintained kinetics and some capacity loss

compared to the performance in the first cycle. It decreases from

4.4 wt% (first rehydrogenation) to 4.2 wt% (second rehydro-

genation) and 4 wt% (third rehydrogenation). The XRD profiles

of both the undoped and doped samples following the

rehydrogenation, shown in Fig. 10b and c, indicate the

reformation of MgH2 and NaH. This suggests that reaction (3)

is reversible. However, no Al3Mg2 alloy is found in the

rehydrogenated samples, indicating that full recovery of MgH2

from Al12Mg17 alloy has been acquired according to the

following equations.

Mg17 Al12 + (17 2 2y)H2 AyMg2Al3 + (17 2 2y)MgH2 + (12 2 3y)Al (7)

Table 2 Comparison of Activation energies (KJ mole21) of pure and doped MgH2

Sample Determination method MgH2 References

Pristine MgH2 Kissinger Method 120,135,150,191,191.3 53,6,27,26,54Pristine MgH2 Isothermal 158.5,156,160 25Pristine MgH2 Kissinger Method 153.5 Present workNaAlH4 + MgH2 Kissinger Method 120 Present workNaAlH4 + MgH2 + Nb2O5 Kissinger Method 75 Present workTiC-doped Kissinger Method 144.6 54NbF5-doped Kissinger Method 90,88 53Cr2O3-doped Kissinger Method 84 27Fe2O3-doped Kissinger Method 124 27Ni-doped Kissinger Method 81 26TiO2-doped Kissinger Method 94 27Fe3O4-doped Kissinger Method 115 27In2O3-doped Kissinger Method 122 27ZnO-doped Kissinger Method 147 27TiH2-doped Kissinger Method 58.4 6Nb2O5-doped Kissinger Method 71 55CoCl2-doped Kissinger Method 121.3 25NiCl2-doped Kissinger Method 102.6 25

Fig. 8 Comparison of the isothermal dehydriding curves for neat MgH2

at (a) 280 uC and (b) at 300 uC, undoped MgH2–NaAlH4 composite (in

mole ratio of 1 : 1) at (c) 280 uC and (d) at 300 uC, and Nb2O5-doped

MgH2–NaAlH4 composite (in mole ratio of 1 : 1) at (e) 280 uC and (f) at

300 uC.

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Mg2Al3 + 2H2 A 2MgH2 + 3Al (8)

In addition, the Na3AlH6 phase has been detected for the

rehydrogenated doped sample (Fig. 10c), which is not found for

the rehydrogenated undoped sample (Fig. 10b). The formation

of Na3AlH6 may be accounted for the enhanced rehydriding

capacity of the doped sample engendered by the Nb2O5 dopant,

indicating that the Nb2O5 component in the MgH2–NaAlH4–

Nb2O5 sample plays a catalytic role through the formation of

Nb-containing catalytic species.

To further analyze the milling and cycling process, the

chemical characterization of the Nb-based additive has been

also investigated by XPS in the surface and sub surface regions

of the milled 1MgH2–1NaAlH4 + 2 mol% Nb2O5 sample before

and after hydrogen cycling. Fig. 11 depicts the Nb 3d

photoelectron peak region for the as prepared sample as

compared to the fully desorbed and fully reabsorbed samples.

According to Fig. 11a for the as prepared sample, no Nb catalyst

has been detected at the surface. During desorption, the reduced

Nb species migrate from the bulk to the surface.56 This is

consistent with the results obtained from XRD. It is hard to

specify whether the inception of the reduction of Nb2O5 takes

place during milling or not. XRD results for the milled sample

have not revealed the existence of any reduced Nb species.

During the first hydrogen desorption, the reduction of Nb2O5

commences and the originating products disperse in the sample

and emerge to the surface. This fact is further strengthened by

the XPS spectra from the Mg 2p regions of the doped sample

(Fig. 12b), exhibiting the occurrence of MgO after first

dehydrogenation. The Na3AlH6 photoelectron peak in

Fig. 13(a–d) for the rehydrogenated samples, located at

64.92 eV,57 further proves the reformation of Na3AlH6 during

the rehydrogenation process. These observations are in accor-

dance with the results of X-ray diffraction, showing that the

enhanced rehydriding capacity of the doped sample is induced by

the reformation of Na3AlH6.

The FESEM qualitative and quantitative microstructural

investigations have been employed to examine the particle size

and morphology of the as-milled and cycled samples. Fig. 14a

and b indicates that the pure MgH2 and NaAlH4 particles are

irregularly shaped with a mean particle size of more than 30 mm.

Fig. 14c,d brings out the microstructural features of the as-milled

undoped 1MgH2–1NaAlH4 and 2MgH2–1NaAlH4 powders.

Remarkably, the stoichiometry has exhibited an evident influ-

ence on the particle size distribution reached after the milling

process. This noticeable difference may be ascribed to the higher

brittleness of the MgH2 powders which, indeed, improves the

efficiency of the ball milling procedure. The FESEM analysis

carried out on the cross-section of 1MgH2–1NaAlH4 + 2 mol%

Nb2O5 composite has revealed the substantial refinement

induced by the Nb2O5 nanoparticles (Fig. 14e). The size of most

of the particles is less than 5 mm for the sample with Nb2O5

dopant, indicating the significant diminution of particle size

Fig. 9 Comparison of the rehydriding curves for the undoped MgH2–

NaAlH4 composite (in mole ratio of 1 : 1) and Nb2O5-doped MgH2–

NaAlH4 composite (in mole ratio of 1 : 1) at 300 uC under 9.5 MPa.

Fig. 10 XRD spectra for (a) undoped MgH2–NaAlH4 (1 : 1) composite

after complete dehydrogenation, (b) undoped MgH2–NaAlH4 (1 : 1)

composite after rehydrogenation, and (c) Nb2O5-doped MgH2–NaAlH4

(1 : 1) composite after rehydrogenation.

Fig. 11 Narrow scan XPS Nb 3d spectra for Nb2O5-doped MgH2–

NaAlH4 (1 : 1) composite before dehydrogenation, after dehydrogena-

tion, and after 3rd rehydrogenation.

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Fig. 12 Narrow scan XPS Mg 2p spectra for (a) pristine MgH2 and (b)

Nb2O5-doped MgH2–NaAlH4 (1 : 1) composite before dehydrogenation,

after dehydrogenation, and after 3rd rehydrogenation.

Fig. 13 Narrow scan XPS Na 2s spectra for pristine NaAlH4 and

Nb2O5-doped MgH2–NaAlH4 (1 : 1) composite before dehydrogenation,

after dehydrogenation, and after 3rd rehydrogenation.

Fig. 14 Field emission scanning electron microscopy (FESEM) images

of (a) as-received MgH2, (b) as-received NaAlH4, (c) undoped MgH2–

NaAlH4 (1 : 1) composite, (d) undoped MgH2–NaAlH4 (2 : 1) compo-

site, (e) Nb2O5-doped MgH2–NaAlH4 (1 : 1) composite after ball milling,

and (f) Nb2O5-doped MgH2–NaAlH4 (1 : 1) composite after third

rehydrogenation.

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compared to that of undoped unary as well as binary samples. It

is vividly discernible in the inset of Fig. 14e that the oxide

particles are embedded heterogeneously in the composite matrix,

resulting in the substantial surface modifications exhibiting

many deformed and disordered surface regions. Moreover, the

hardness of Nb2O5 is much higher than that of MgH2 and

NaAlH4. Therefore, the fraction of embedded Nb2O5 nanopar-

ticles will enhance the hardness and brittleness of hydride

particles that will ultimately shift the balance between fracturing

and agglomeration to smaller particle sizes. Moreover, Nb2O5 is

well known for its good lubricant and dispersive properties that

prevent the agglomeration and cold welding of hydride particles

and thereby facilitates the refinement of matrix particles during

milling. Fig. 14f brings out the microstructural features of the

doped materials after third rehydrogenation. The small pores are

observed at the matrix surface are imputable to the repeated

volume shrinkage and expansion during the sorption cycles.

Fig. 15(a–b) is the surface EDS spectra collected at the positions

circled in Fig. 14e,f. The variation in the Nb peak positions

following the cycling (Fig. 15b) further corroborates that Nb2O5

has been reduced during cycling.

The above experimental results suggest that all the stoichio-

metries follow the same initial low-temperature reaction pathway

(decomposition of NaAlH4). Nevertheless, the differences in the

respective pathways begin to emerge at elevated temperatures

(the formation of NaMgH3), depending upon the proportion of

the potential reactant (MgH2). The improvement in dehydro-

genation kinetics of binary composite is believed to be due to the

interaction between MgH2 and alanate that changes the

thermodynamics of the reactions by either lowering the enthalpy

of the dehydrogenation reaction or leads the balance of the

reaction towards one direction. Moreover, the smaller powder

agglomerates are obtained for the 2 : 1 composites (Fig. 14),

which accounts for their enhanced dehydrogenation kinetics

compared to that of other samples. The results of the present

investigation also reveal that additive, i.e. Nb2O5 nanopowders

can lead to the generation of surface defects by inducing a

substantial refinement of powder particles during the high-

energy milling, which are expected to further ameliorate the

kinetics as well as the hydrogen sorption capability. It is

documented in our previous work9,10,43 that the hydrogen

desorption/absorption is closely associated with the surface

defects and the refinement, and the hydrogen sorption capability

increases with the increase in the defects in the nanostructures. In

addition, the lubricant, dispersive, and hardness properties of

Nb2O5 can facilitate a further reduction of hydride particles by

preventing the agglomeration of matrix particles during milling,

which has to be considered also an important factor for

enhancing the reaction kinetics. Nonetheless, other factors, such

as the nature and local electronic structure of the added oxide as

well as its reduction during heating also have to be taken into

account. Since the Nb2O5 is reduced during the heating, the

possible candidates for catalysis remain in the non-stoichiometric

magnesium–niobium oxide or even other possible niobium

phases with lower oxidation states formed during cycling.

Similarly, in previous studies by ourselves and others,10,42 it is

suggested that the partially reduced Nb species with a wide range

of valence might play a major role as a catalyst. Thereby, the

finely dispersed oxygen-deficient Nb species may contribute to

kinetic improvement by facilitating the diffusion of hydrogen

through the diffusion barriers both in dehydrogenation and

rehydrogenation processes.

Conclusions

In conclusion, we have examined the hydrogen storage proper-

ties and reaction pathways of three distinct stoichiometries

within the MgH2–NaAlH4 binary composite system both

undoped and doped with Nb2O5 nanopowders. The premilled

reactant stoichiometry of the MgH2/NaAlH4 system has a

profound impact on the reaction kinetics and the sorption

properties because of the reactant availability. It has also been

demonstrated that the dehydrogenation kinetics of MgH2 and

NaAlH4 can be improved by combining them with each other.

Apart from the existence of the mutual interactions between two

hydrides, the crystallite size of the undoped binary composites is

also found to be smaller than that of the pristine components,

which explains for the superior hydrogen performance of the

mixed samples. Significant improvements in the dehydrogena-

tion/rehydrogenation properties of the MgH2–NaAlH4 system

Fig. 15 Energy-dispersive spectroscopy (EDS) results of Nb2O5-doped

MgH2–NaAlH4 (1 : 1) composite (a) after ball milling (area circled in

Fig. 14e) and (b) after third rehydrogenation (area circled in Fig. 14f).

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have been achieved by adding a small amount of Nb2O5

nanoparticles. The onset as well as the peak temperatures of

hydrogen desorption shift to lower temperatures. The kinetics of

hydrogen desorption with Nb2O5 additions are found to be

2–6 times as fast as the undoped MgH2 and MgH2–NaAlH4

samples. The rehydrogenation properties of the MgH2–NaAlH4–

Nb2O5 composite are also improved significantly as compared to

the MgH2–NaAlH4 composite. The reversible capacity of

4.4 wt% is acquired for the MgH2–NaAlH4–Nb2O5 composite,

which is larger than that of MgH2–NaAlH4 (3.3 wt%).

Moreover, this catalytically enhanced rehydrogenation capacity

persists well during three de/rehydrogenation cycles. XRD, XPS,

and FESEM-EDS analyses suggest that hydrogen cycling has

induced the reduction of Nb2O5, which leads to the formation of

oxygen-deficient reduced niobium oxide species. Accordingly, it

is believed that the fine dispersion of these oxygen-deficient

niobium oxide nanoparticles facilitates the dehydrogenation

process by serving as the active sites for nucleation and growth of

the dehydrogenated product associated with the shortening of

the diffusion paths among the reaction ions and thus reducing

the kinetic barriers and rendering the amelioration of dehydro-

genation kinetics. Besides, the nanocrystalline reduced niobium

oxide phases present in the mixture can also provide the active

catalytic sites for the hydrogen dissociation and surface

adsorption, which improves the kinetics of the rehydrogenation

as well. Moreover, the lubricant, dispersive, and hardness

properties of Nb2O5 increase the surface defects and grain

boundaries by a large reduction in the particle size, creating a

larger surface area for hydrogen to interact, thereby decreasing

the temperature for decomposition.

Acknowledgements

This work is financially supported by the University of Science

and Technology Beijing (USTB). The authors also thank the

Higher Education Commission (HEC) of Pakistan for the

financial support to Rafi-ud-din.

References

1 R. A. Varin, T. Czujko and Z. S. Wronski, Nanomaterials for SolidState Hydrogen Storage; Springer Science + Business Media, NewYork, 2009.

2 J. Yang, A. Sudik, C. Wolverton and D. J. Siegel, Chem. Soc. Rev.,2010, 39, 656–675.

3 A. W. C. Berg and C. O. Arean, Chem. Commun., 2008, 668–681.4 S.-I. Orimo, Y. Nakamori, J. R. Eliseo, A. Zuttel and C. M. Jensen,

Chem. Rev., 2007, 107, 4111–4132.5 T. Umegaki, J. M. Yan, X. B. Zhang, H. Shioyama, N. Kuriyama

and Q. Xu, Int. J. Hydrogen Energy, 2009, 34, 2303.6 L. Jun, J. C. Young, Z. F. Zhigang, Y. S. Hong and R. J. Ewa, AM.

CHEM. SOC., 2009, 131, 15843–15852.7 J. F. Mao, Z. Wu, T. J. Chen, B. C. Weng, N. X. Xu and T. S.

Huang, J Phys Chem C, 2007, 111, 12495–12498.8 M. David, B. J. Daniel, S. Toyoto, N. Dag, K. Daisuke, S. Tetsuo, K.

Naoyuki and Y. Hitoshi, J. Mater. Chem., 2009, 19, 8150–8161.9 Rafi-ud-din, L. Zhang, L. Ping and Q. Xuanhui, J. Alloys Compds.,

2010, 508, 119–129.10 Rafi-ud-din, Q. Xuanhui, L. Ping, L. Zhang and M. Ahmad, J Phys

Chem C, 2011, 115, 13088–13099.11 L. Li, F. Qiu, Y. Wang, Y. Wang, G. Liu, C. Yan and C. An, J.

Mater. Chem., 2012, 22, 3127–3132.12 J. Chen, N. Kuriyama, Q. Xu, H. T. Takeshita and T. Sakai, J. Phys.

Chem. B, 2001, 105, 11214–11220.

13 F. Schuth, B. Bogdanovic and M. Felderhoff, Chem. Commun., 2008,668–681.

14 J. F. Mao, X. B. Yu, Z. P. Guo, C. K. Poh, H. K. Liu, Z. Wu and J.Ni, J. Phys. Chem. C, 2009, 113, 10813–10818.

15 P. Chen, Z. T. Xiong, L. F. Yang, G. T. Wu and W. F. Luo, J PhysChem B, 2006, 110, 14221–14225.

16 H. Y. Leng, T. Ichikawa, S. Hino, N. Hanada, S. Isobe and H. Fujii,J Power Sources, 2006, 156, 166–170.

17 M. Au, A. R. Jurgensen, W. A. Spencer, D. L. Anton, F. E.Pinkerton and S. J. Hwang, J Phys Chem C, 2008, 112, 18661–18671.

18 X. B. Yu, G. L. Xia, Z. P. Guo and H. K. Liu, J Mater Res, 2009, 24,2720–2727.

19 Y. H. Guo, X. B. Yu, L. Gao, G. L. Xia, Z. P. Guo and H. K. Liu,Energy Environ. Sci., 2010, 3, 465–470.

20 X. D. Kang, Z. Z. Fang, L. Y. Kong, H. Cheng, X. D. Yao, G. Luand P. Wang, Adv. Mater., 2008, 20, 2756–2759.

21 J. Zhao, J. Shi, X. Zhang, F. Cheng, J. Liang and Z. Tao, Adv. Mater,2010, 22, 394–397.

22 B. Peng, J. Liang, Z. Tao and J. Chen, J. Mater. Chem., 2009, 19, 2877–288.23 F. Zeppelin, H. Reule and M. Hirscher, J. Alloys Compds., 2002, 330,

723–726.24 C. X. Shang, M. Bououdina1, Y. Song and Z. X. Guo, Int. J.

Hydrogen Energy, 2004, 29, 73–80.25 J. F. Mao, Z. Guo, X. Yu, H. Liu, Z. Wu and J. Ni, Int. J. Hydrogen

Energy, 2010, 35, 4569–4575.26 L. Xie, Y. Liu, X. Zhang, J. Qu, Y. Wang and X. Li, J. Alloys

Compds., 2009, 482, 388–392.27 M. Polanski and J. Bystrzycki, J. Alloys Compds., 2009, 486, 697–701.28 D. L. Croston, D. M. Grant and G. S. Walker, J. Alloys Compds.,

2010, 492, 251–258.29 O. Friedrichs, T. Klassen, J. C. Sanchez-Lopez, R. Bormann and A.

Fernandez, J. scripta mat., 2006, 54, 1293–1297.30 L. P. Ma, P. Wang and H. M. Cheng, Int. J. Hydrogen Energy, 2010,

35, 3046–3050.31 N. Recham, V. V. Bhat, M. Kandavel, L. Aymard, J. M. Tarascon

and A. Rougier, J. Alloys Compds., 2008, 464, 377–382.32 J. Lu and Z. G. Z. Fang, J. Phys. Chem., 2005, 109, 20830–20834.33 Z. T. Xiong, G. T. Wu, J. J. Hu and P. Chen, J. Power Sources, 2006,

159, 167–170.34 P. Wang, L. P. Ma, Z. Z. Fang, X. D. Kang and P. Wang, Energy

Environ. Sci., 2009, 2, 120–123.35 G. S. Walker, D. M. Grant, T. C. Price, X. Yu and V. Legrand, J.

Power Sources, 2009, 194, 1128–1134.36 D. Pottmaier, C. Pistidda, E. Groppo, S. Bordiga and G. Spoto, Int.

J. Hydrogen Energy, 2011, 36, 7891–7896.37 J. F. Mao, Z. Guo, H. Leng, Z. Wu, Y. Guo, X. Yu and H. Liu, J.

Phys. Chem. C, 2010, 114, 11643–11649.38 J. F. Mao, Z. Guo, X. Yu, M. Ismail and H. Liu, Int. J. Hydrogen

Energy, 2011, 36, 5369–5374.39 Y. Zhang, T. Qi-Feng, L. Shu-Sheng and S. Li-Xian, J. Power

Sources, 2008, 185, 1514–1518.40 M. Ismail, Y. Zhao, X. B. Yu and S. X. Dou, RSC Advances, 2011, 1,

408–415.41 M. Ismail, Y. Zhao, X. B. Yu, J. F. Mao and S. X. Dou, Int. J.

Hydrogen Energy, 2011, 36, 9045–9050.42 O. Friedrichs, F. Aguey-Zinsou, J. R. Ares Fernandez, J. C. Sanchez-

Lopez, A. Justo, T. Klassen, R. Bormann and A. Fernandez, Actamat., 2006, 54, 105–110.

43 A. Mashkoor, Rafi-ud-Din, P. Caofeng and Z. Jing, J. Phys. Chem.C, 2010, 114, 2560.

44 C. Milanese, S. Garroni, A. Girella, G. Mulas, V. Berbenni, G. Bruni,S. Surinach, M. D. Baro and A. Marini, J. Phys. Chem. C, 2011, 115,3151–3162.

45 H. E. Kissinger, Anal. Chem., 1958, 29, 1702.46 S. Zheng, Y. Li, F. Fang, G. Zhou, X. Yu, G. Chen, D. Sun, L.

Ouyang and M. Zhu, J. Mater. Res., 2010, 25, 2047–2053.47 X. Fan, X. Xiao, L. Chen, S. Li and Q. Wang, J. Alloys Compds.,

2011, 509, 386–389.48 G. Sandrock, K. Gross and G. Thomas, J. Alloys Compd., 2002, 339,

299–308.49 D. Pukazhselvan, M. Hudson, A. Sinha and O. Srivastava, Energy,

2010, 35, 5037–5042.50 O. Kircher and M. Fichtner, J. Alloys Compd., 2005, 404, 339–342.51 X. Fan, X. Xiao, L. Chen, S. Li, H. Ge and Q. Wang, J. Phys. Chem.

C, 2011, 115, 2537–2543.

RSC Adv. This journal is � The Royal Society of Chemistry 2012

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nloa

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pubs

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i:10.

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/C2R

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View Online

52 X. Fan, X. Xiao, L. Chen, L. Han, S. Li, H. Ge and Q.Wang, Int. J.Hydrogen Energy, 2011, 36, 10861–10869.

53 Y. Luo, P. Wang, L. Ma and H. Cheng, J. Alloys Compd., 2008, 453, 138–142.54 M. Fan, S. Liu, Y. Zhang, J. Zhang, L. Sun and F. Xu, Energy, 2010,

35, 3417–3421.

55 N. Hanada, T. Ichikawa, S. Hino and H. Fujii, J. Alloys Compd.,2006, 420, 46–49.

56 http://srdata.nist.gov/xpx/.57 C. Wan, X. Ju, Y. Qi, S. Wang, X. Liu and L. Jiang, J. Alloys

Compd., 2009, 486, 436–441.

This journal is � The Royal Society of Chemistry 2012 RSC Adv.

Dow

nloa

ded

on 2

6 A

pril

2012

Publ

ishe

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22

Mar

ch 2

012

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ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C2R

A20

518A

View Online