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 ) 1 2 5 7 – 1 2 6 6

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Dynamic synthesis of ternary Mg2FeH6

M. Polanski a, T. P1ocinski b, I. Kunce a, J. Bystrzycki a,*a Faculty of Advanced Technology and Chemistry, Military University of Technology, 2 Kaliskiego Str., 00-908 Warsaw, Polandb Faculty of Materials Science and Engineering, Warsaw University of Technology, 141 Wołoska Str., 02-507 Warsaw, Poland

a r t i c l e i n f o

Article history:

Received 10 June 2009

Received in revised form

5 September 2009

Accepted 5 September 2009

Available online 4 December 2009

Keywords:

Magnesium-iron ternary hydride

Mechanical (ball) milling

Dynamic synthesis

Decomposition

Structure

Hydriding/dehydriding properties

* Corresponding author. Tel.: þ48 (22) 683 71E-mail address: jbystrzycki@wat.edu.pl (J

0360-3199/$ – see front matter ª 2009 Profesdoi:10.1016/j.ijhydene.2009.09.010

a b s t r a c t

This work presents new results on the dynamic synthesis and decomposition of ternary

Mg2FeH6. A novel synthesis method was applied for the rapid and effective synthesis of

a ternary Mg–Fe hydride. This method consists of two processing routes. The first route

involves high-energy ball milling of the initial MgH2–Fe powder mixture, while the second

is composed of a unique pressurizing and heating cycle route to obtain a full phase

transformation within half an hour. The structural investigations carried out by X-ray

diffraction revealed that almost all of the initial powder mixture transforms into the

ternary hydride. Furthermore, the sample, which was synthesized, was also decomposed

and reloaded with hydrogen. The formation of Mg2FeH6 consists of two steps that involve

MgH2 as an intermediate compound. In contrast, the decomposition of Mg2FeH6 consists of

only one step and does not follow the inverse route. Some traces of iron were found in the

reaction products. TDP results show that a desorption peak occurs at 315 �C, and this is in

good agreement with DSC measurements showing only a single endothermic peak around

340 �C. Microstructural examinations revealed that the synthesized Mg2FeH6 powder

generally exhibits a duplex structure that consists of plate-like particles larger than 1 mm in

diameter and spherical particles smaller than 50 nm that show a tendency to agglomerate

and form larger particles exhibiting a sponge-like structure. The formation of Mg2FeH6

takes place at the phase boundary between Fe seeds and the growing hydride phase. In

contrast, the decomposition of the Mg2FeH6 phase takes place with the formation of the

separate nanosized Mg and Fe phases. The dehydrogenated powder sample shows oval Fe

precipitates of 10–100 nm in size that are embedded in the Mg-based matrix.

ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction Mg2FeH6 from the elements Mg and Fe, due to the absence of

The ternary hydride Mg2FeH6 is an attractive material for

hydrogen storage for fuel cell applications or for thermo-

chemical thermal energy storage near 500 �C. Mg2FeH6 has the

highest volumetric hydrogen density, 150 kg/m3, among all

the known complex hydrides, and it possesses a gravimetric

hydrogen density of 5.4 wt.% [27]. Furthermore, this hydride is

based on inexpensive metallic elements such as Mg and Fe;

therefore, its cost should be competitive with that of other

complex hydrides. However, it is difficult to synthesize

35; fax: þ48 (22) 683 9445. Bystrzycki).sor T. Nejat Veziroglu. Pu

an Mg2Fe intermetallic compound in the binary Mg–Fe system

(Mg and Fe are immiscible). Only in the ternary Mg–Fe–H

system does hydrogen act as a binding component to form

Mg2FeH6 with a cubic K2PtCl6-type structure in which the

octahedral [FeH6]�4 complexes of anions are surrounded by

Mg in an eight-fold cubic configuration [4].

Thus far, several fabrication methods have been developed

to produce Mg2FeH6, which usually involve either sintering at

elevated temperatures (w500 �C) under high hydrogen pres-

sure (20–120 bar) over several days or simply mechanical

.

blished by Elsevier Ltd. All rights reserved.

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 ) 1 2 5 7 – 1 2 6 61258

alloying/milling (MA/M) in an inert atmosphere or under

a hydrogen atmosphere (reactive mechanical alloying – RMA)

combined (or not) with sintering techniques [1], [3], [4–6], [7,8],

Konstanchuk et al., 1987 [9], [11,12], [15–22], [24–26] . The yield

of Mg2FeH6 fabricated by sintering, MA/M or RMA depends on

both the initial material properties and the processing

conditions. A recent comprehensive review of synthesis

methods of Mg2FeH6 and of other ternary transition metal

complex hydrides is given by Varin et al. [27]. Progress

regarding the yield has been made by using optimized sin-

tering and/or RMA conditions and, more recently, by hydrid-

ing combustion synthesis [10] or sintering of metal

nanoparticles produced by a hydrogen plasma metal reaction

[23]. However, one main drawback of the processing methods

with yields over 90% is the very long processing time involved,

which is usually 10 h. Additionally, the retained Fe is usually

present in the synthesized material; for this reason, a purifi-

cation process is necessary.

The difficulties related to the synthesis of Mg2FeH6 are

probably responsible for the limited knowledge regarding

synthesis and decomposition mechanisms of this ternary

hydride. Bogdanovic et al. [2] carried out exhaustive thermo-

dynamic and microstructural investigations of Mg2FeH6. They

studied the initial formation and the subsequent de- and

rehydrogenation process of Mg2FeH6 on a micro- and nano-

scale level by using combined HRTEM-EDX (HRTEM - High-

Resolution Transmission Electron Microscopy; EDX - Energy

Dispersive X-ray) investigations. Zhou et al. [28] studied the

energy and electronic structure of Mg2FeH6 by using the first-

principles plane-wave pseudopotential method to calculate

heats of formation and the following formation mechanism.

More research has been devoted to the study of hydriding/

dehydriding kinetics and thermodynamic properties [15–17],

[10], [2]

In this work, we present new results on the dynamic

synthesis and decomposition of ternary Mg2FeH6. Most

recently, our research group successively synthesized

Mg2FeH6 by a novel processing route using combined high-

energy mechanical ball milling of the starting 2MgH2–Fe

powder mixture under argon and subsequent sintering under

high hydrogen pressure (85–100 bar) at elevated temperatures

(up to 500 �C). The Mg2FeH6 yield of this processing route is

greater than 90%, leaving some Fe as an unreacted phase.

Moreover, it must be emphasized that the total time of the

applied processing route (including milling and dynamic sin-

tering) is less than 2 h. To the best of our knowledge, the results

presented here are the first example in which the rapid and

effective synthesis of the ternary Mg2FeH6 is shown. Addi-

tionally, the obtained experimental results are supported by

the microstructural and hydriding/dehydriding investigations.

2. Experimental procedure

The binary MgH2 powder (ABCR, 98.0%) and the elemental Fe

powder (ABCR, 99.9%) were mechanically milled at a molar

ratio of 2:1 in a planetary ball mill (Fritsch P6) in an 80-ml

stainless steel vessel with 30 stainless steel balls (10 mm in

diameter) for 1 h under argon. An inert hydrocarbon working

as a lubricant was added in the course of the milling process.

The total mass of each powder mixture was 5 g. The rotation

rate of the milling container was set to 650 rpm. All handling

of the powders was conducted in a Labmaster Glovebox

Workstation (MBraun) under a continuously purified argon

atmosphere. The amounts of oxygen and water were below

0.1 ppm.

The 350 mg ball milled powder samples were used to

perform the dynamic synthesis using an automated Sievert’s

apparatus (HTP1-S, Hiden Isochema). During the loading

procedure, the samples were not exposed to air. The direct

synthesis of each hydride began at 85 bar of hydrogen pres-

sure, which increased to almost 100 bar when the system

reached 500 �C. This processing route was carried out to

hinder MgH2 from decomposing to elemental magnesium at

a high temperature. Afterwards, the temperature was set to

500 �C with a linear heating ramp rate of 20 �C/min.

The X-ray diffraction profiles for all of the investigated

powder samples were recorded with a Seifert 3003 diffrac-

tometer using Co Ka radiation (l¼ 1.79 A) with operating

parameters of 30 mA and 50 kV and a step size of 0.02�/5 s.

The morphology and microstructure of the synthesized

and decomposed samples were examined with a high-reso-

lution field emission scanning electron microscope (HITACHI

S5500), equipped with a backscattered electron detector, an

energy dispersive X-ray spectrometer (EDS) and a duo-STEM

bright/dark field (BF/DF) detector. A transmission electron

microscope (JEOL JEM 1200EX) with an accelerating voltage of

120 kV was also used to investigate diffraction patterns for

phase identification. Since the investigated samples included

both nano- and microcrystalline structures, thin foils for

electron microscopy were prepared by the focused ion beam

(FIB) technique using the FB-2100 Hitachi system. A liquid ion

metal source was used as the source of the gallium ion beam.

The accelerating voltage used was 40 kV. Tungsten was used

as a protective layer. Fig. 1a presents a sample SEM micro-

graph showing a thin Mg2FeH6 sample cut from the largest

particle visible in Fig. 1b using the FIB technique.

To determine the dehydrogenation properties of the

synthesized samples, temperature programmed desorption

(TPD) tests were carried out using the same HTP1-S analyzer

coupled with a quadrupole mass spectrometer. The

measurements were performed under a high purity (99.999%)

helium flow, and heating rates of 1, 2 and 5 �C/min were

applied in order to determine the activation energy of the

hydride decomposition. Additionally, DSC measurements

were performed on samples of about 5 mg with a Setaram

Labsys apparatus. The experiments were performed under an

argon flow in an Al2O3 crucible in the range of 20–500 �C at

heating rates of 1, 2 and 5 �C/min.

3. Results and discussion

3.1. Synthesis of Mg2FeH6

As mentioned above, our dynamic synthesis method of

Mg2FeH6 generally consists of two processing routes:

mechanical (ball) milling and subsequent sintering under high

hydrogen pressure at elevated temperatures. Initially, our

preparation method of Mg2FeH6 appears to be very similar to

Fig. 1 – SEM micrograph showing a thin Mg2FeH6 sample (a)

cut by FIB from the largest particle visible in micrograph (b).

Fig. 2 – Hydrogen absorption kinetics measured during the

dynamic synthesis of Mg2FeH6 from the initial ball milled

MgH2–Fe composite powder. The synthesis was conducted

at 85 bar of hydrogen pressure at a constant heating rate of

5 8C/min.

Fig. 3 – X-ray diffraction patterns of MgH2–Fe powder

mixture after: (a) ball milling, (b) ball milling and

subsequent dynamic synthesis, (c) decomposition of

Mg2FeH6 and (d) beginning of Mg2FeH6 formation from Mg

and Fe powder mixture obtained after desorption.

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others applied so far [7], [19], [15] b). However, there are some

important differences that have an essential influence on the

synthesis mechanism of this ternary hydride and, as

a consequence, on the reaction rate and the final yield.

The first important point is associated with the total energy

of mechanical milling, which should only be suitable for

achieving MgH2–Fe composite powders in a heavy cold work

state. For this reason, the milling time in our experiment was

rather short (1 h) in order to avoid MA and the formation of

a small amount of Mg2FeH6. The second important point is

related to the sintering hydrogen pressure, which should be

sufficiently high (>80 bar of H2) at the beginning so that the

decomposition of MgH2 into Mg and H2 can be hindered.

Assuming that the reaction pathway in our synthesis

follows equation (1), then the hydrogen pressure should drop

visibly due to hydrogen absorption during the ternary hydride

formation. Taking into account the theoretical and measured

amount of absorbed hydrogen, the yield of Mg2FeH6 during

synthesis can be easily estimated.

Fig. 4 – SEM micrographs of synthesized Mg2FeH6 powder

showing the morphology of the particles: (a) larger

particles with a plate-like structure and (b) smaller

particles with a spherical and/or sponge-like structure.

Vermicular-like particles are shown by arrows.

Fig. 5 – Cross-section of a synthesized Mg2FeH6 particle

showing: (a) bright field image of sample prepared by FIB

and (b) image obtained from BSE/SE detectors working in

mixed mode, i.e., 80% of the signal is from the BSE detector.

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2MgH2þ FeþH2 / Mg2FeH6 (1)

Fig. 2 shows the hydrogen absorption kinetics measured

during the dynamic synthesis of Mg2FeH6 from the milled

MgH2–Fe composite powder under 85 bar of hydrogen pres-

sure at a constant heating rate of 5 �C/min. It can be clearly

seen that a phase transformation of the milled powder into

the ternary hydride starts just below 400 �C. The reaction runs

very quickly until the temperature reaches 500 �C, at which

point, the reaction stops; most likely, this is not due to

a completion of the reaction, but rather a realization of the

plateau pressure of MgH2. Up to 1.47 wt.% of hydrogen was

absorbed, while the theoretical value determined for two

hydrogen atoms in Mg2FeH6 is 1.81 wt.%. By adding the

absorbed hydrogen (1.47 wt.%) to the amount of hydrogen

bound in MgH2 (3.62 wt.%), it can be determined that about

5.09 wt.% of the hydrogen is absorbed by the synthesized

material. This indicates that the yield of our sintering tech-

nique is about 94%, assuming a theoretical value for Mg2FeH6

of 5.43 wt.%. This obtained value may be slightly lower

because the initial MgH2 powder contained some amount of

Mg (max. 2%). Nevertheless, the yield is about 90% of Mg2FeH6,

and this result will be compared later with the result calcu-

lated by TPD. The entire sintering process usually takes about

25 min (while heating at 20 �C/min), although the phase

transformation of the initial components into Mg2FeH6 takes

no longer than 10 min, which is a great improvement as

compared to other methods [7], [19], [15], [23].

Figs. 3a and b presents the XRD patterns of the MgH2–Fe

powder obtained after ball milling and subsequent dynamic

synthesis. After milling for 1 h, the XRD pattern shows only

the presence of MgH2 and Fe. The broadening of all of the

diffraction peaks can be attributed to a reduction in crystallite

size and/or plastic cold deformation. The product of dynamic

Fig. 6 – BSE micrograph (a) and EDS elemental maps showing the distribution of Mg (b) and Fe (c) in a cross-section of

a synthesized Mg2FeH6 particle.

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synthesis was identified as the Mg2FeH6 phase with a small

amount of unreacted iron. There are no traces of the magne-

sium hydride or magnesium. The sample did not exhibit any

contaminations in the form of oxides or hydroxides. The

obtained XRD results suggest that Mg2FeH6 can form directly

from the milled MgH2–Fe composite. More recently, this

hypothesis has been confirmed by in-situ synchrotron X-ray

diffraction measurements [14]., in preparation.

The morphology of the synthesized Mg2FeH6 powder is

shown in Fig. 4. The SEM observations revealed that there are

two kinds of particles, with regard to size and shape. There are

plate-like particles larger than 1 mm in diameter (Fig. 4)a. The

surfaces of these particles are terraced. There are also

spherical particles smaller than 50 nm. The spherical particles

tend to agglomerate and form larger particles with a sponge-

like structure (Fig. 4)b.

Sai Raman et al. [19] observed sponge-like particles for

synthesized Mg2FeH6 powder prepared by MA. The agglom-

eration and granular morphology of the surface were also

found in synthesized hydride powder obtained by RMA [5,17].

Some additional similarities in the morphology of the

synthesized powder can be found as compared to the initial

formation morphology of Mg2FeH6 observed by Bogdanovic

et al. [2] during TEM–EDX investigations. They observed

a morphology of Mg2FeH6 in the form of characteristic

Table 1 – Quantitative EDS analysis of the synthesizedMg2FeH6 sample taken in different regions, as shown inFig. 7a.

Spot number Mg–K (at. %) Fe–K (at. %)

1 62.7� 0.6 28.7� 0.6

2 61.6� 0.7 30.6� 0.7

3 45.8� 0.6 48.1� 0.6

4 13.6� 0.5 81.4� 0.7

5 9.8� 0.3 90.2� 0.8

6 63.8� 0.6 29.1� 0.6

vermicular particles of 2–3 mm in size. This morphology of the

particles was maintained over the course of 500 hydrogena-

tion/dehydrogenation cycles. In Figs. 4a and b, the arrows

show similar vermicular-like particles of 100–300 nm in

length. As will be shown later, these were also observed in the

powder after desorption (Fig. 10)b.

The plate-like structure of the larger Mg2FeH6 particles has

not been previously observed, probably due to the fact that the

processing pathway in our dynamic synthesis differs from

others with regard to the synthesis mechanism, the reaction

rate and the yield. Moreover, thus far, only a few FE SEM

studies of the morphology of synthesized Mg2FeH6 particles

have been carried out.

The microstructural investigation conducted with a STEM

detector on the cross-section of a thin sample prepared from

the largest synthesized particle obtained by FIB showed

a crystalline morphology with a layer porosity (white phase)

forming along the interphase boundary between the gray and

Fig. 7 – TEM bright field image showing the microstructure

of synthesized Mg2FeH6 powder and the corresponding

electron diffraction pattern (EDP). The TEM bright field

image shown in Figure 7 corresponds to the same particle

cross-section reported in Figs. 5 and 6.

Fig. 8 – Thermal desorption spectrum of hydrogen for

synthesized Mg2FeH6 sample obtained at a constant

heating rate of 5 8C/min.

Fig. 10 – SEM micrographs of Mg2FeH6 sample after

desorption showing morphology of powders: (a)

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black phases shown by the arrows in Fig. 5a. There were also

two well visible phases - black and gray – observed by the

bright field detector. The observations carried out in BSE/SE

detectors working in the mixed mode (Fig. 5)b combined with

EDS analysis (Fig. 6; Table 1) revealed that there are regions of

unreacted iron (black phase in Fig. 5a; light specks in Figs. 5b

and 6a) present at the grain boundaries and within the Mg–Fe

matrix of the particle (gray phase in Figs. 5b and 6a).

Fig. 9 – Differential scanning calorimetry of synthesized

Mg2FeH6 sample obtained at a constant heating rate of 5 8C/

min.

micrograph at low magnification, (b) agglomerated

spherical particles with a sponge-like structure.

Vermicular-like particles are shown by arrows.

Bogdanovic et al. [2] also observed that the formation of

Mg2FeH6 occurs at the phase boundary between Fe seeds and

the growing hydride phase, by the insertion of newly formed

Mg2FeH6 layers between the two phases. In the course of the

growing process, the Mg2FeH6 phase moves away from the

phase boundary, forming the characteristic vermicular

appearance.

The elemental maps of Mg and Fe after synthesis show that

both these elements are distributed rather uniformly in the

gray matrix and that much more Fe is observed in the light

specks visible in Fig. 6. A homogeneous distribution of Mg and

Fe in the Mg2FeH6 phase was also found in papers by Li et al.

[10] and Bogdanovic et al. [2]. The quantitative EDS analysis

carried out in different regions of the particle shown in Fig. 6a

shows that the amount of Mg and Fe in the gray phase ranges

from 62 to 64 and 29 to 31 at. %, respectively (Table 1). The

remaining concentration may belong to the hydrogen in the

ternary hydride Mg2FeH6 and some amount of oxygen,

because the thin sample was transferred in air from the FIB to

the FE SEM and TEM. Although it is well known that the EDS

signal can overlap the phases underneath the surface (not

Fig. 11 – SEM micrographs of Mg2FeH6 sample after

desorption showing cross-sections of desorbed particles:

(a) image from BSE/SE detectors working in mixed mode,

(b) Z-contrast and (c) bright field.

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visible), the chemical composition determined in the gray

phase in Fig. 6a (spots 1, 2 and 6) indicates that it is rather the

ternary Mg2FeH6 phase. Similar results from extensive

chemical compositional analysis were found by Sai Raman

et al. [19].

Fig. 7 shows a TEM bright field image showing the micro-

structure of the synthesized Mg2FeH6 powder and the corre-

sponding electron diffraction pattern (EDP). The obtained

electron diffraction rings shown in Fig. 7 indicate the presence

of small well-crystallized grains. The EDP analysis confirmed

the earlier EDS and XRD results showing the presence of the

Mg2FeH6 phase and unreacted iron in the synthesized sample.

3.2. Decomposition of Mg2FeH6

Figure 3c shows the XRD pattern measured during the

decomposition of Mg2FeH6 under 1 bar of argon pressure. It

can be seen that Mg2FeH6 decomposes directly to elemental

Mg and Fe according to the following reaction (2):

Mg2FeH6 / 2Mgþ Feþ 3H2 (2)

There is no formation of MgH2 as an intermediate phase. A

similar result was found by Konstanchuck et al. (1986), Gen-

nari et al. [5] and Bogdanovic et al. [2].

TPD measurements carried out on the synthesized Mg2FeH6

sample heated at 5 �C/min under a helium flow revealed that

there is a single endothermic peak at w315 �C due to the

decomposition of Mg2FeH6 (Fig. 8). The desorption of Mg2FeH6

starts at 250 �C, and the total amount of desorbed hydrogen is

5.24 wt.%, which is very close to the theoretical capacity of

5.43 wt.% for Mg2FeH6. As can be seen, the hydrogen desorp-

tion is completed within a relatively narrow range of temper-

atures. Varin et al. [25]a) pointed out that a narrow hydrogen

TPD peak indicates the presence of small well-crystallized

hydride particles. Therefore, such an explanation can be

consistent with the microstructural results shown in Figs. 4–7.

The non-symmetrical shape of the TPD peak may be associated

with the presence of unreacted Fe working as a catalyst [6,5].

DSC investigations carried out for the synthesized Mg2FeH6

sample heated at 5 �C/min confirm that there is an endo-

thermic peak around 340 �C, starting at about 300 �C (Fig. 9).

This peak corresponds to the decomposition of Mg2FeH6,

which is in agreement with the previous TPD results shown in

Fig. 8. Similar results have been previously observed by others

[6,5,15], [1]. Puszkiel et al. [15]a) showed that the desorption

peak temperature is strongly dependent on the thermal

history of the sample. The chemical homogenization and

microstructural modifications associated with cycling or sin-

tering can influence the desorption temperature.

The calculated activation energy of hydrogen desorption

for the synthesized Mg2FeH6 was calculated to be about 93 kJ/

mol. This value is closed to the activation energy of the cata-

lyzed ball milled MgH2 [13].

Fig. 10a presents representative SEM micrographs of the

Mg2FeH6 powder after desorption, taken at a lower magnifi-

cation. The particles show irregular shapes resembling

a sponge-like structure. A detailed view of the particles is

shown in Fig. 10b, where the agglomerated spherical particles

are well visible. In contrast to the synthesized Mg2FeH6

powder, the plate-like structure in the desorbed powder was

not observed. Only the characteristic vermicular-like particles

of 100–300 nm in length were observed in the dehydrogenated

samples as shown by the arrows in Fig. 10b, similar to the

observations reported by Bogdanovic et al. [2].

Fig. 12 – BSE micrograph (a) and EDS elemental maps showing the distribution of Mg (b) and Fe (c) in a cross-section of the

Mg2FeH6 sample after desorption.

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Fig. 11 shows cross-sections of the desorbed particles

examined by STEM using different detectors. The micrograph

taken from the BSE/SE detectors working in the mixed mode

illustrates that there are two chemically distinguishable

regions (Fig. 11)a. Light oval precipitates are embedded in the

gray matrix. An examination of the precipitates carried out

with the DF and BF detectors, revealed that the oval precipi-

tates have a nanoscale dimension, smaller than roughly

100 nm in diameter (Figs. 11b, c). The smallest particles, with

a size below 20 nm, have a rather spherical shape. They most

likely coalesce while growing to form the more globular

structures exhibiting a ‘‘popcorn ball’’ appearance (Fig. 11)c.

Fig. 12 presents elemental maps of Mg and Fe obtained for

a sample after desorption. As can be seen, there are nanosized

regions (<100 nm) in which Mg or Fe dominates distinctly.

There also exists a Mg–Fe matrix in which both Mg and Fe are

present together.

The microstructural results obtained for the desorbed

sample are in good agreement with the data reported by

Bogdanovic et al. [2], who carried out exhaustive microstruc-

tural investigations of Mg2FeH6 after decomposition on

a micro- and nanoscale level using combined HRTEM-EDX

techniques. They found that the dehydrogenated sample

showed spherical or dark oval Fe precipitates of 10–100 nm in

size. The EDX analysis showed that the precipitates were Fe,

embedded in the Mg matrix. Transparent regions of pure Mg

were also found. The authors concluded that the desorption of

Mg2FeH6 takes place with the formation of separate nanosized

Mg and Fe phases.

Fig. 13 – Hydrogen absorption kinetics measured during

the dynamic synthesis of Mg2FeH6 from the Mg-Fe powder

mixture obtained after desorption. The synthesis was

conducted at 90-97 bar of hydrogen pressure at a constant

heating rate of 5 8C/min.

3.3. Formation of Mg2FeH6 from elemental Mg and Fepowders

Fig. 13 shows the hydrogen absorption curve of the Mgþ Fe

powder mixture obtained after the full desorption of Mg2FeH6.

A hydrogen pressure of 87 bar was applied at the beginning of

the process, and the total amount of absorbed hydrogen was

calculated on the base of the pressure change measured

during the heating of the sample from 30 to 500 �C at

a constant rate of 5 �C/min. The absorption starts at levels

close to 0.5 wt.%, due to the fact that, after dosing the

hydrogen pressure at RT, before the heating begins, the

sample rapidly absorbed hydrogen to that level in less than

10 min, and it would absorb even more if more time were

allowed. This behavior is in good agreement with the XRD

observations, in which the MgH2 peaks are visible from the

beginning of Mg2FeH6 formation (Fig. 3)d. The character of the

curve in Fig. 13 suggests that the sample absorbs hydrogen

during heating, even at temperatures close to RT, reaching the

first plateau near 300 �C. This indicates the transformation of

pure magnesium into magnesium hydride. Near 400 �C, the

phase transformation of the formed magnesium hydride and

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iron into Mg2FeH6 occurs, which can easily be observed by an

increase in the slope of the absorption curve. The total

amount of absorbed hydrogen is 4.55 wt.%. (the theoretical

capacity of Mg2FeH6 is 5.43 wt.%). No magnesium hydride

residues were found in the synthesized mixture. This suggests

that the reaction yield is about 84%. However, taking into

account the fact that the obtained curve is not saturated at the

end of the synthesis, the yield can probably be increased to

values close to that obtained during the first absorption

(starting from the milled MgH2–Fe composite powder; Fig. 2)

by using a longer annealing time at 500 �C.

The obtained results can indicate that, after the desorption

of Mg2FeH6, rehydrogenation occurred according to the

following reactions (3,4):

MgþH2 / MgH2 (3)

2MgH2þ FeþH2 / Mg2FeH6 (4)

As can be seen from our studies, the reconstruction of the

Mg2FeH6 phase from the nanosized Mg and Fe proceeds

according to the same mechanism shown in reactions (1) and

(4), where the MgH2 phase acts as a precursor to the phase

transformation of the MgH2–Fe mixture into the ternary

Mg2FeH6 phase.

Our results are in good agreement with those reported

recently by Shao et al. [23] regarding the synthesis of the

Mg2FeH6 phase obtained while sintering elemental Mg and Fe

nanopowders under a hydrogen atmosphere. The earlier

suggestions of Bogdanovic et al. [2] related to the synthesis and

decomposition mechanisms are also well within in the frame

of our findings. In contrast, the hypothetical model previously

given by Gennari et al. [5] and Puszkiel et al. [15,16], critically

discussed by Varin et al. [25], is not confirmed by our structural

studies showing that free Mg is not found in the formation of

ternary Mg2FeH6. The MgH2 phase is a precursor to the phase

transformation of the MgH2–Fe mixture into Mg2FeH6.

4. Conclusions

We have applied a novel synthesis method for the rapid and

effective synthesis of the ternary Mg–Fe hydride. This method

consists of two processing routes. The first route involves

high-energy ball milling of the initial MgH2–Fe powder

mixture, while the second consists of a unique pressurizing

and heating cycle route to obtain a full phase transformation

within half an hour.

Moreover, in the present work, we have reported the

mechanisms of Mg2FeH6 synthesis and decomposition. The

formation of Mg2FeH6 consists of two steps that involve MgH2

as an intermediate compound. Mg does not participate in the

formation of ternary Mg2FeH6. The MgH2 phase is a precursor

to the phase transformation of the MgH2–Fe mixture into

Mg2FeH6. In contrast, the decomposition of Mg2FeH6 consists

of only one step and does not follow the inverse route. Traces

of iron were found in the reaction products.

The TDP results showed that the desorption onset

temperature is as low as 250 �C. The desorption peak

temperature is 315 �C. This indicates that the hydrogen

desorption is completed within a relatively narrow range of

temperatures and that it can be related to the presence of

small well-crystallized hydride particles. The DSC curve

shows only a single endothermic peak around 340 �C, and it is

consistent with the obtained TPD results.

The microstructural investigations carried out on the

Mg2FeH6 phase revealed that the synthesized powder gener-

ally exhibits a duplex structure that consists of plate-like

particles larger than 1 mm and spherical particles smaller than

50 nm with a tendency to agglomerate and form larger parti-

cles exhibiting a sponge-like structure. The formation of

Mg2FeH6 takes place at the phase boundary between Fe seeds

and the growing hydride phase. In contrast, the decomposi-

tion of the Mg2FeH6 phase takes place with the formation of

the separate nanosized Mg and Fe phases. The dehydro-

genated powder sample shows oval Fe precipitates of 10–

100 nm in size that are embedded in the Mg-based matrix.

Vermicular-like particles of 100–300 nm in length were

observed in both the synthesized and decomposed samples.

Acknowledgments

This work was supported by the Polish Ministry of Science and

Higher Education (Grant No PBZ-KBN-117/T08/01). The TPD

and DSC investigations in this work have been done in 2009

year under financial support the Polish Ministry of Sciences

and Higher Education, Key Project POIG.01.03.01-14-016/08.

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