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Preparation, scale-up and testing of nanoscale, doped amidesystems for hydrogen storage

Ulrich Ulmer a,*, Jianjiang Hu a, Matthias Franzreb b, Maximilian Fichtner a

aKarlsruhe Institute of Technology (KIT), Institute of Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, GermanybKarlsruhe Institute of Technology (KIT), Institute of Functional Interfaces, P.O. Box 3640, D-76021 Karlsruhe, Germany

a r t i c l e i n f o

Article history:

Received 8 August 2012

Received in revised form

8 October 2012

Accepted 31 October 2012

Available online 2 December 2012

Keywords:

Hydrogen storage

Amide system

Scale-up

* Corresponding author. Tel.: þ49 (0) 721 608E-mail addresses: Ulrich.Ulmer@kit.edu

imilian.Fichtner@kit.edu (M. Fichtner).0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2012.10.1

a b s t r a c t

A comparative study of the LiNH2eMgH2 hydrogen storage system has been made, and

several additives (LiBH4, KH and ZrCoH3) have been tested as single catalysts and in various

combinations in order to study potential synergistic effects.

It was verified that LiBH4 and KH significantly improve the de-/rehydrogenation

kinetics. However, the addition of KH results in irreversible reactions.

Testing of catalyst combinations showed that a system composed of 2 LiNH2e1.1 MgH2

e0.1 LiBH4e3 mass% ZrCoH3 shows superior absorption/desorption kinetics, with

a reversible capacity of 4.2 mass% H at 180 �C.

An optimal milling time of 5 h was found by variation of ball-milling conditions,

whereas too long milling durations resulted in a lower capacity and reduced reversibility.

Based on optimized conditions, the system was scaled up on semi-industrial milling

equipment and kilogramme quantities of material were produced for testing in a tank

coupled to a fuel cell.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction binding forces, the capacity of the system under practical

Hydrogen is one of the most promising candidates as

a potential energy carrier in the establishment of an economy

based on renewable energy sources. However, safe and

efficient hydrogen storage is still a key challenge to be over-

come [1].

Amide systems have been extensively investigated as

materials for solid state hydrogen storage due to their high

theoretical storage capacities and suitable thermodynamics.

In 2002 Chen et al. [2] identified a reaction system based on

Li amide and Li hydride, which theoretically absorbs 11.5mass

% of hydrogen. For thermodynamic reasons (equilibrium

pressure at 195 �C < 0.02 bar) and strong NeH and LieH

22680; fax: þ49 (0) 721 60(U. Ulmer), Jianjiang.Hu

2012, Hydrogen Energy P15

conditions is limited to 6.3 mass% at temperatures below

200 �C. In order to lower the reaction enthalpy of the sorption

and thereby lower the sorption temperature, MgH2 was added

to the system instead of LiH. With the substitution of LiH by

MgH2 the LieMgeNeH-system becomes a hydrogen storage

system with a high capacity (5.5 mass%) and suitable ther-

modynamics [3]. Although the thermodynamic driving force

and the rate of hydrogen release was improved by the

substitution of LiH by MgH2, the LieMgeNeH-system requires

a longer time to reach equilibrium as compared to the

LieNeH-system [4]. At 220 �C and 100 bar an initial metathesis

reaction 2 LiNH2 þ MgH2 / 2 LiH þ Mg(NH2)2 takes place [5].

The following reversible hydrogen storage reaction has

8 26368.@kit.edu (J. Hu), Matthias.Franzreb@kit.edu (M. Franzreb), Max-

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Table 1 e Composition of the samples prepared duringvariation of the stoichiometry.

System Dopant Theoreticalcapacity

2 LiNH2e1 MgH2 e 5.5 mass%

2 LiNH2e1 MgH2 0.1 LiBH4 5.4 mass%

2 LiNH2e1 MgH2 0.1 KH 5.3 mass%

2 LiNH2e1 MgH2 3 mass% ZrCoH3 5.4 mass%

2 LiNH2e1.1 MgH2 e 5.4 mass%

2 LiNH2e1.1 MgH2 0.05 LiBH4 5.2 mass%

2 LiNH2e1.1 MgH2 0.1 LiBH4 5.2 mass%

2 LiNH2e1.1 MgH2 0.2 LiBH 5.1 mass%

2 LiNH2e1.1 MgH2 0.05 KH 5.2 mass%

2 LiNH2e1.1 MgH2 0.1 KH 5.1 mass%

2 LiNH2e1.1 MgH2 3 mass% ZrCoH3 5.2 mass%

2 LiNH2e1.1 MgH2 0.05 LiBH4 þ 3 mass% ZrCoH3 5.1 mass%

2 LiNH2e1.1 MgH2 0.1 LiBH4 þ 3 mass% ZrCoH3 5.1 mass%

2 LiNH2e1.1 MgH2 0.2 LiBH4 þ 3 mass% ZrCoH3 4.9 mass%

2 LiNH2e1.1 MgH2 0.1 KH þ 0.1 LiBH4 4.9 mass%

2 LiNH2e1.1 MgH2 0.1 KH þ 3 mass% ZrCoH3 4.9 mass%

2 LiNH2e1.1 MgH2 0.1 KH þ 0.1 LiBH4 þ 3 mass%

ZrCoH3

4.8 mass%

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a theoretical capacity of 5.5 mass% and proceeds according to

the following equation:

2LiNH2 þMgH2/Li2MgðNHÞ2 þ 2H25MgðNH2Þ2 þ 2LiH (1)

Various groups have determined the enthalpy of reaction

between 39 kJ/mol H2 and 41.1 kJ/mol H2 [3,6].

Capacity and kinetics considerably depend on the prepa-

ration conditions. Liu et al. found that during ball-milling for

12 h LiH and Mg(NH2)2 formed in a conversion reaction

whereas ball-milling for 36 h led to the formation of MgNH,

resulting in a decrease in capacity and a higher DH value in

a different reaction pathway [7].

There exists a severe kinetic barrier, hindering sorption

reactions to proceed atmoderate temperatures and pressures.

Catalysts have been investigated, promising to decrease the

time to reach equilibrium and lower the kinetic barriers.

Conventional hydrogenation catalysts such as Ti, Fe, Co, Ni,

Pd, Pt and/or their chlorides do not enhance sorption kinetics

significantly, probably because these compounds are poorly

soluble in amides and imides. It is believed that they cannot be

effectively involved in the interfacial reactions and/or mass

transport owing to the poor solubility in amide, imide or

hydride [8].

Consequently, focus of research has been directed to

intermediate-forming additives, which can lower the kinetic

barriers, too. A potassium-modified Mg(NH2)2/2LiH-system

showed a lowering of dehydrogenation onset temperature

from 186 �C to 108 �C. This was explained by KH diffusing into

the amide/imide phases and interacting with nitrogen, and

thus a weakening of the NeH bonds was assumed [8].

LiBH4 was investigated as a potential hydrogen storage

material by Zuttel et al., with a theoretical storage capacity of

18.4 mass%. However, it is thermodynamically too stable for

practical applications as a single component [9]. Yang et al.

found that the addition of LiBH4 to a system composed of

LiNH2 and MgH2 enhances sorption kinetics by lowering the

onset temperature [10]. Desorption of a LieMgeNeBeH-

system proceeds in two steps at different temperatures. The

first step begins at 130 �C and is catalysed by the addition of

LiBH4. In the second step at 260 �C, LiBH4 reacts with

Li2Mg(NH)2, resulting in a maximum capacity of 9.1 mass% at

350 �C [10,12].

Adding ZrCoH3 to a 2 LiNH2e1.1 MgH2e0.1 LiBH4-system

further improves the sorption characteristics of the system.

By comparing the IR spectra with and without addition of

ZrCo, it was shown that ZrCo weakens the NeH bonds and

thereby catalyses the sorption reactions [13].

The LieMgeNeH-systems doped with either LiBH4, KH or

ZrCoH3 have so far been tested in various labs. In this work,

a comparison of the different dopants was made by preparing

and testing doped hydride samples under identical condi-

tions. Furthermore, it was investigated whether synergetic

effects exist among the dopants.

Therefore, amide systems composed of LiNH2 and MgH2

were prepared in different stoichiometries and doped with

catalytic amounts of KH, LiBH4 and ZrCoH3 both as single

dopants and in combinations as co-catalysts. The aim of this

study was to find a modified composition based on the

LieMgeNeH-system, which has good kinetics, cyclic stability

and capacity. Nanocomposites were prepared using similar

procedures and tested under practical conditions with respect

to a potential application of an amide-based tank with an HT-

PEM fuel cell (Tmax ¼ 180 �C; pback ¼ 1 bar H2).

After identifying the optimum stoichiometric system, ball-

milling parameters were optimized. The optimized material

was finally scaled up on semi-industrial milling equipment in

order to produce a kilogramme quantity of material for testing

in a metal hydride tank coupled to a fuel cell. While tank tests

with NaAlH4 have already been performed by several groups

[14e16], a similar work based on a LieMgeNeH-system has

not been reported yet.

2. Experimental

2.1. Sample preparation

MgH2 (Alfa Aesar, 98%), LiNH2 (Merck, �95%) and LiBH4 (Alfa

Aesar, �95%) were used as received. KH powder was obtained

from a suspension in a protective oil (Alfa Aesar, 35%) by

filtering followed bywashingwith dry hexane and dryingwith

a Buchi rotation evaporator. ZrCoH3 was prepared from ZrCo

ingot (Saes Getters S.p.A., Italy) by exposing to 10 bar H2 at

ambient temperature.

It is assumed that systems prepared with an excess of

MgH2 show less ammonia generation than systems than

stoichiometric systems [3,5]. Therefore, systems composed of

LiNH2 and MgH2 were prepared in two molar ratios, in the

stoichiometric ratio of LiNH2 toMgH2 as 2:1 andwith an excess

of MgH2, in a ratio of 2:1.1. Compositions of the prepared

samples are listed in Table 1.

Lab-scale samples of 2 g were prepared using a Retsch PM-

400 planetary ball-mill with 250 mL milling jar made of

stainless steel. Three balls with a diameter of 15 mm and 15

balls with a diameter of 5 mm were used. The Ball-to-Powder

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 en e r g y 3 8 ( 2 0 1 3 ) 1 4 3 9e1 4 4 9 1441

Ratio (BPR) was about 80:1. Ball-milling time was 20 h at

300 rpm. To avoid possible overheating caused by mechanical

friction, a 1 min pause was made after every 5 min of ball-

milling, then the milling direction was reversed. Samples

were also prepared at different ball-milling parameters, while

the composition was kept constant. The material was pro-

cessed for 5, 10, 20, 30 and 60 h at a BPR of 80:1, in the same

milling jar and balls as described above.

The scale-up was performed on an eccentric-vibrational

high energy ball-mill by Siebtechnik GmbH, which is inte-

grated into a glove-box and works under argon atmosphere.

2000 steel balls with a diameter of 20 mm were used. 200 g of

material were prepared per batch with a BPR of 390:1 at

1200 rpm. The total milling time was 5 h, samples (approxi-

mately 2 g) were taken for characterization after 90, 180 and

300 min, respectively.

2.2. Characterization

Hydrogen sorption properties were characterized on a home-

built Sieverts apparatus. The measured variables were pres-

sure, temperature and sample weight. The temperature of the

reference volumes was passively controlled by a water bath,

its temperature was measured using a thermal element PT-

100 with a standard accuracy of 0.2%. Temperatures of the

piping and of the reactor were measured using thermal

elements manufactured by Watson, with a standard accuracy

of 0.75%. The pressure measurements were conducted using

transducers from Keller, Type X 33, with a standard accuracy

of 0.05% (full scale). Desorption temperature was set to 180 �C,1 bar of hydrogen backpressure was provided in the appa-

ratus. The heating rate from room temperature to 180 �C was

5 �C/min. Subsequent to desorption with 1 bar of hydrogen

backpressure, the system was evacuated and desorbed under

vacuum at 180 �C. Absorption was performed at 100 bar

hydrogen pressure and 150 �C. Heating rate was 12.5 �C/min

(10 min from room temperature to 150 �C). For the phase

identification samples of 60e70 mg were measured on

a Bruker D8 Advance powder X-ray diffractometer with Cu-Ka

radiation (wave length 1.54�A). The sampleswere loaded in the

glove-box and sealed hermetically. Measurements by Fourier

Transform Infrared Spectroscopy (FTIR) were conducted on

a Perkin Elmer Spectrum FX unit. 1e3 mg sample was mixed

with KBr in the glove-box and pressed to pellets. The material

was measured in transmission.

Scanning electron microscopy (SEM) was performed with

a LEO 1530 instrument at an acceleration voltage of 15 keV

using carbon tape as substrate.

Fig. 1 e Hydrogen desorption curves measured under 1 bar

H2 for the 2 LiNH2e1 MgH2-system with and without

addition of LiBH4, ZrCoH3 or KH.

3. Results and discussion

For a comparison of the hydrogen storage properties of the

two systems (stoichiometric and with MgH2-excess) as well as

the dopants and their combinations, the materials were

prepared following the same procedure.

It was proposed by Luo et al. that samples preparedwith an

excess of 10% MgH2 showed less ammonia generation than

the samples prepared in a stoichiometric ratio [3]. However, it

is not clear whether an excess of MgH2 does also affect the

sorption kinetics and overall capacity of the systems. In

Sections 3.1 and 3.2 the two systems are tested and compared

to each other with respect to capacity, overall kinetics and

onset temperature.

3.1. Stoichiometric system: 2 LiNH2e1 MgH2

3.1.1. Hydrogen sorption propertiesHydrogen desorption curves of the stoichiometric systemwith

LiNH2 and MgH2 at the ratio of 2:1 without or with single

dopants are shown in Fig. 1.

The curves present a sigmoidal shape, which is typical for

solid state chemical reactions of metal hydrides. However,

they differ in their onset temperature and the slope of the

respective region. The pristine system shows an onset

temperature of 161 �C. Doping the system with 3 mass%

ZrCoH3, 0.1 LiBH4 or 0.1 KH lowered the onset temperature to

140 �C, 125 �C or 113 �C, respectively. Yang et al. found a 2

LiNH2e1 LiBH4e1 MgH2-system to desorb hydrogen from

150 �C under 1 bar H2 backpressure with a temperature ramp

of 5 �C/min [10].Wang et al.measured an onset temperature of

a 0.1 KH-doped system of 107 �C while desorbing against

vacuum at a heating rate of 2 �C/min. The higher onset

temperatures obtained for the KH-doped systems in the

present work can be explained by the faster heating rate and

desorption under 1 bar H2 backpressure. A higher heating rate

shifts the onset temperature to higher values. Hydrogen

backpressure lowers the thermodynamic driving force for

desorption. Therefore, a higher temperature is necessary in

order to compensate for this lower driving force.

The addition of LiBH4 and KH considerably enhanced

desorption kinetics, while the ZrCoH3-doped system showed

unchanged kinetics as compared to the undoped system.

The ZrCoH3-doped system showed the lowest overall

capacity of 4.0 mass% after desorption at 180 �C under

vacuum, which is a reproducible effect of yet unknown origin.

The system doped with LiBH4 has the highest capacity of 4.6

mass%. 4.5 mass% were measured with the undoped system.

All capacities listed above were measured after desorption at

180 �C against 1 bar hydrogen backpressure and subsequent

vacuum desorption at 180 �C.

Fig. 2 e Hydrogen desorption curves under 1 bar for the 2

LiNH2e1.1 MgH2-system with and without addition of

LiBH4, ZrCoH3 or KH in varying amounts.

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Subsequent to desorption at 1 bar hydrogen backpressure

the systems were evacuated and desorbed at 180 �C. The total

capacities from the two desorption steps are listed in Table 2.

3.2. Excess of MgH2: 2 LiNH2e1.1 MgH2

3.2.1. Hydrogen desorption propertiesHydrogen desorption kinetic curves of the systems with an

excess of MgH2 are shown in Fig. 2.

The onset temperature of the pristine system was 148 �C,which is 13 �C lower than the onset of the 2:1 stoichiometric

system presented in Fig. 1. The onset temperature of the

systems doped with LiBH4 decreased with an increase of the

fraction of LiBH4 (130 �C, 128 �C and 126 �C with molar frac-

tions of 0.05, 0.1 and 0.2 LiBH4, respectively). One can see that

desorption kinetics show a strong dependence on the chem-

ical nature and the amount of dopant added to the system.

Desorption process of the 0.05 and 0.1 KH-doped systems

began at 115 �C or 110 �C, respectively. The addition of ZrCoH3

lowered the onset temperature by only 2 �C as compared to

the pristine sample. The KH- as well as the ZrCoH3-modified

samples desorbed a smaller amount of hydrogen in 80 min

when compared to the pristine system. The samples desorbed

only 2.7 mass%, 3.1 mass% with addition of 0.05 or 0.1 KH,

respectively, 3.3 mass% with addition of 3 mass% ZrCoH3,

whereas 3.6 mass% H was measured in 80 min for the sample

without dopant despite the positive effect of dopants on the

decrease of the onset temperatures. Hydrogen desorption

kinetics are enhanced by the addition of LiBH4 and KH,

however, the addition of ZrCoH3 as single dopant does not

have an obvious effect on desorption kinetics as compared to

the pristine system. Furthermore, the addition of ZrCoH3

lowered the effective hydrogen storage capacity of the system

by 0.2 mass%, due to the addition of inert material and

possibly also because the addition of ZrCoH3 alone interferes

with hydrogen sorption steps. The highest capacities of 4.6

and 4.7 mass% were measured with the addition of LiBH4.

The capacities are listed in Table 3, as determined after

desorption at 180 �C and 1 bar H2 backpressure and subse-

quent vacuum desorption at 180 �C.Compared to the systems with a stoichiometric amount of

MgH2, the amount of hydrogen desorbed in a period of 80 min

was higher in all the systems with an excess of MgH2. In both

the stoichiometric systems as well as the systems with an

excess of MgH2, the doped systems behaved similarly with

respect to onset temperature. Furthermore, it was proposed

by Luo that an excess of MgH2 added to the system caused less

ammonia generation than systemswith a stoichiometric ratio

[3]. Therefore, we chose to further investigate systemswith an

Table 2 e Capacities of the 2 LiNH2e1 MgH2-system withand without addition of LiBH4, ZrCoH3 or KH.

Dopant Capacity(180 �C, 1 bar H2)

Capacity(180 �C, vacuum)

e 3.1 mass% 4.5 mass%

0.1 LiBH4 3.8 mass% 4.6 mass%

3 mass% ZrCoH3 2.0 mass% 4.0 mass%

0.1 KH 3.4 mass% 4.5 mass%

excess of 10 mol% MgH2 and performed a second desorption

cycle for a few selected systems. The results are shown in

Fig. 3.

During the second cycle the onset temperature of the

ZrCoH3-doped system was determined to be 148 �C, a similar

value as compared to the first cycle and close to the onset

temperature of the undoped system. We thereby conclude

that ZrCoH3 as a single catalyst does not have a significant

effect in decreasing the onset desorption temperature of the

system. Desorption kinetics of the KH-doped system slowed

down during the second cycle. At first the curve ran steeply,

but it took a longer time until it converged at a specific value as

compared to the first cycle. During the third cycle, only 3.0

mass% of H weremeasured, and the capacity of the KH-doped

system gradually decreased during the subsequent cycles.

This suggests that the addition of KH leads to irreversible

reactions under the applied temperature and pressure

conditions.

Fig. 4 shows the desorption curves of the 2 LiNH2e1.1

MgH2-system with combinations of catalysts during the first

cycle. It was investigated whether there are synergetic effects

among the individual catalysts which may improve the

sorption kinetics and capacity of the systems.

There was no correlation between the amount of LiBH4

added to the system and the onset temperature for the 2

LiNH2e1.1 MgH2e(3 mass% ZrCoH3 þ x LiBH4) systems. The

systems desorbed hydrogen from 126 �C, 118 �C or 127 �C at

Table 3 e Capacities of the 2 LiNH2e1.1 MgH2-systemwith various dopants.

Dopant Capacity(180 �C, 1 bar H2)

Capacity(180 �C, vacuum)

e 3.6 mass% 4.3 mass%

0.1 LiBH4 4.1 mass% 4.6 mass%

3 mass% ZrCoH 3.4 mass% 4.1 mass%

0.1 KH 3.4 mass% 4.3 mass%

0.05 LiBH4 3.5 mass% 4.7 mass%

0.2 LiBH4 4.0 mass% 4.6 mass%

0.05 KH 2.7 mass% 3.8 mass%

Fig. 3 e Hydrogen desorption curves of the second

desorption cycle under 1 bar H2 for the 2 LiNH2e1.1 MgH2-

system with and without addition of 0.05, 0.1 and 0.2

LiBH4, 3 mass% ZrCoH3 or 0.1 KH.

Table 4 e Capacities of the 2 LiNH2e1.1 MgH2-systemwith combined catalysts during the first desorption cycle.

Co-dopants Capacity(180 �C, 1 bar H2)

Capacity(180 �C, vacuum)

0.05 LiBH4 þ 3

mass% ZrCoH3

3.0 mass% 3.9 mass%

0.1 LiBH4 þ 3

mass% ZrCoH3

3.2 mass% 3.8 mass%

0.2 LiBH4 þ 3

mass% ZrCoH3

3.3 mass% 3.9 mass%

0.1 LiBH4 þ 0.1 KH 2.7 mass% 3.5 mass%

0.1 LiBH4 þ 0.1 KH þ 3

mass% ZrCoH3

2.8 mass% 3.3 mass%

0.1 KH þ 3 mass%

ZrCoH3

2.9 mass% 3.4 mass%

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x ¼ 0.05, 0.1 or 0.2 LiBH4. The combination of KH and LiBH4

resulted in an onset temperature of 129 �C. Adding ZrCoH3 to

the same system increased the onset temperature to 136 �C.The KH- and ZrCoH3-doped system desorbed H2 from 115 �C.Table 4 summarizes the total capacities of various 2 LiNH2e1.1

MgH2-systems after desorbing against 1 bar hydrogen back-

pressure and subsequent vacuum desorption at 180 �C.The capacity of each 2 LiNH2e1.1 MgH2e(3 mass%

ZrCoH3 þ x LiBH4) system was at least 0.3 mass% higher than

the capacities of (KH þ LiBH4)-, (KH þ LiBH4 þ ZrCoH3)- or

(KH þ ZrCoH3)-doped systems. For this reason we primarily

focused on (3 mass% ZrCoH3 þ x LiBH4) doped systems and

performed a second desorption cycle. Desorption curves are

shown in Fig. 5.

During the second cycle, the onset temperature of the 2

LiNH2e1.1 MgH2e(3 mass% ZrCoH3 þ x LiBH4) systems were

determined as 125 �C, 122 �C or 119 �C with molar fractions of

0.05, 0.1 or 0.2 LiBH4, respectively. The LiBH4-, KH- and

ZrCoH3-co-catalysed system already converged at a very low

capacity as compared to the other systems, an indication for

a decreasing capacity during cycling and deteriorating

Fig. 4 e Hydrogen desorption curves under 1 bar for the 2

LiNH2e1.1 MgH2-system with combination catalysts.

kinetics. The capacities of the systems after desorption

against 1 bar backpressure and subsequent vacuum desorp-

tion at 180 �C are listed in Table 5.

Capacity and kinetics vary with the chemical nature and

the amount of dopant added to the system. The stoichiometric

system, composed of 2 LiNH2 and MgH2, desorbed 4.5 mass%.

The onset temperatures of the undoped systems were 161 �Cfor the 2:1 system and 148 �C for the 2:1.1 system, respectively.

Desorption curves of ZrCoH3-catalysed systems as a single

dopant show different characteristics. The system with an

excess of MgH2 desorbed much faster and showed a higher

capacity (4.1 mass%) than the 2:1 system (4.0 mass%). In case

of the KH-doped samples, the onset temperature was

decreased considerably for both the 2:1 and the 2:1.1 systems

from 161 �C or 148 �C to 113 �C or 110 �C, respectively. Thishigher onset temperature can be explained by the faster

heating rate applied in this work and the presence of

hydrogen backpressure as compared to Wang et al.’s work.

The LiBH4-catalysed systems started desorbing from

126 �Ce130 �C. KHeffected a lowering of the onset temperature

to around 110 �Ce115 �C. Catalysing the 2:1.1 system with

a combination of 0.1 KH and 0.1 LiBH4, hydrogen desorption

was found to begin at 129 �C. Therefore, the onset temperature

of a (KHþ LiBH4)-co-catalysed systemwas similar to that of the

Fig. 5 e Hydrogen desorption curves under 0.1 MPa for the

2 LiNH2e1.1 MgH2-system with combination catalysts

during the second cycle.

Table 5 e Capacities of the 2 LiNH2e1.1 MgH2-systemwith combined catalysts during the second desorptioncycle.

Dopants Capacity(180 �C, 1 bar H2)

Capacity(180 �C, vacuum)

0.05 LiBH4 þ 3

mass% ZrCoH3

3.0 mass% 3.8 mass%

0.1 LiBH4 þ 3

mass% ZrCoH3

3.3 mass% 3.9 mass%

0.2 LiBH4 þ 3

mass% ZrCoH3

3.4 mass% 3.8 mass%

0.1 LiBH4 þ 0.1 KH þ 3

mass% ZrCoH3

2.0 mass% 2.7 mass%

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LiBH4-catalysed system. Wang et al. proposed that KH was

diffusing into the amide/imide phase, interactedwith nitrogen

and consequently decreased the onset temperature. With the

presence of LiBH4, however, hydrogen desorption began at

a higher temperature, in the range of the systems which were

catalysed only by LiBH4. We assume that the system was not

catalysed in the same way as they would be without presence

of LiBH4, and thereby the onset temperature is not offset as

much as it is in the systems catalysed only by KH.

3.2.2. Hydrogen absorption propertiesThe pressure curves of absorption experiments are presented

in Fig. 6.

A rise of apparatus pressure was detected first as a conse-

quence of heating up the reactor. As soon as the temperature

reached a certain value, the kinetic barrier was overcome and

the absorption process began. This was characterized by

a slowed down pressure increase, which reverted to decrease

when the absorption accelerated at higher temperatures. As

soon as the equilibrium was reached the pressure stayed

constant.

It is obvious that the time until the equilibrium is reached

(the pressure in the apparatus stays constant) depends

strongly on the chemical nature and amount of dopant added

to the system. In case of the pristine system the pressure

dropped at a high rate at the beginning of the absorption

Fig. 6 e Pressure curves of the 2 LiNH2e1.1 MgH2-system

with various single catalysts and catalyst combinations.

process. After about 40 min the pressure decreased much

slower, and it took another 800 min until the chemical equi-

libriumwas reached and the pressure stayed constant. Adding

0.1 LiBH4 to the system improved the absorption kinetics

considerably. The 0.1 LiBH4-doped system reached an equi-

librium state after 300 min, indicating a much faster absorp-

tion rate as compared to the pristine sample. The addition of

0.1 KH further improved the absorption kinetics in this first

absorption cycle, reaching an equilibrium state after 110 min.

ZrCoH3 has the effect of an accelerated absorption only at the

beginning of the process. The pressure stayed constant 30min

after the beginning of the heating process. However, with the

combination of ZrCoH3 and LiBH4, the pressure decreased

rapidly after the initiation of absorption. The absorption

process was already completed before the final absorption

temperature had reached 150 �C. Even though the tempera-

ture was ramped at 12.5 �C/min for all experiments, some

differences in the temperature profiles within the reactor

cannot be avoided. In order to demonstrate this, the temper-

ature profile during the absorption of the ZrCoeLiBH4-doped

system is also included in the figure. It becomes apparent that

the temperature still increases after the absorption was

complete. This is why the apparatus pressure further

increased after the completion of the absorption. Further-

more, it was calculated from the Van’t Hoff equation as

determined by Hu et al. [11] of the 2 LiNH2e1 MgH2e0.1 LiBH4-

system that hydrogen desorption is thermodynamically

impossible at 100 bar and 150 �C.It is apparent that the system 2 LiNH2e1.1 MgH2e(0.1

LiBH4 þ 3 mass% ZrCoH3) shows the best absorption kinetics.

To further investigate the system, the absorption temperature

was varied. The results are shown in Fig. 7. The figure also

shows a good correlation of absorption temperature and the

absorption kinetics.

After reaching the pressuremaxima, pressure decreased to

equilibrium pressure at different rates. By a temperature

increase of 10 �C from 120 to 130 �C, the total absorption time

was shortened from 175 min to 36 min. At 150 �C the total

absorption timewas further lowered by 19min. The apparatus

pressure rose slightly after the completion of the absorption

process at 150 �C, because the temperature had not yet

reached 150 �C when the absorption was already completed.

Fig. 7 e Pressure curves of the absorptions of the 2

LiNH2e1.1 MgH2e(0.1 LiBH4 D 3 mass% ZrCoH3) system at

120, 130 and 150 �C.

Fig. 9 e First desorption of the 2 LiNH2e1.1 MgH2e(0.1

LiBH4 D 3 mass% ZrCoH3) system after ball-milling for 5,

10, 20, 30 and 60 h.

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 en e r g y 3 8 ( 2 0 1 3 ) 1 4 3 9e1 4 4 9 1445

Zhang et al. conducted isothermal absorption measure-

ments at 150 �C and 70 bar H2. There, the absorption was

complete after 10 min [13]. The absorption onset temperature

in this work was 121 �C, which was reached within 13 min

after the initiation of the temperature ramp. The pressure

remained constant after only 20min after the beginning of the

heating process. Consequently, the absorptionwas completed

within only 7 min. This lower absorption time can be

explained by the higher thermodynamic driving force put on

the system in this work, because the absorption pressure was

set to 100 bar, as compared to 70 bar in Zhang et al.’s work.

The results can be explained by the dependence of the

reaction rate on temperature. Reaction rates in solid state

reactions can be calculated by Arrhenius equations, and thus

show an exponential dependence on temperature. However, it

should be noted that absorption time cannot be increased

infinitely by further increasing absorption temperature. The

thermodynamic driving force for absorption decreases with

increasing temperature. Therefore, for each material there

exists an optimum absorption temperature, which is high

enough to ensure good reaction rates, but at the same time

allows for a high driving force.

The system composed of 2 LiNH2e1.1 MgH2e(0.1 LiBH4 þ 3

mass% ZrCoH3) was characterized by XRD. Fig. 8 shows XRD

patterns of the as-milled system, the re-absorbed system after

the second absorption and the desorbed system after the

second desorption at 180 �C and 1 bar hydrogen pressure and

subsequent vacuum desorption at 180 �C.The diffraction pattern of the system ball-milled for 20 h

showed broadened MgH2 and ZrCoH3 peaks with comparably

lowintensity.Allotherphaseswereamorphousandcouldnotbe

identified by XRD. However, it was shown by FTIR in Fig. 12 that

themetathesis reaction 2LiNH2 þMgH2/2LiHþMgðNH2Þ2 hastaken place. The remaining MgH2-reflexes can be explained by

the excess of MgH2 added to the system.

The re-absorbed system showed peaks of LiH, Mg(NH2)2and ZrCoH3. Hence, the system can reversibly absorb

hydrogen for at least the first two cycles.

Fig. 8 e XRD patterns of the 2 LiNH2e1.1 MgH2e(0.1

LiBH4 D 3 mass% ZrCoH3) system in (A) as-milled state

after 20 h ball-milling, (B) re-absorbed state after the

second absorption, (C) desorbed state after the second

desorption.

The phases identified after vacuum desorption could be

assigned to Li2Mg(NH)2 and ZrCoH3. Therefore, ZrCoH3 did not

participate at the reaction and acted as a catalyst.

4. Variation of ball-milling parameters

4.1. Hydrogen sorption properties

Based on the results obtained in the above sections, the ball-

milling conditions were varied for the optimum composition

2 LiNH2e1.1 MgH2e(0.1 LiBH4 þ 3 mass% ZrCoH3). The milling

periods were 5, 10, 20, 30 and 60 h. For investigating the cyclic

stability of the material, the systems ball-milled for 5 and 60 h

were cycled 5 times.

Fig. 9 shows the first desorption after various ball-milling

durations.

The systems ball-milled between 10 and 60 h showed

similar kinetics, with an onset temperature of 120 �C.Hydrogen desorption of the material ball-milled for 5 h began

at 124 �C. The reason for this elevated onset temperature

Fig. 10 e Second desorption of the 2 LiNH2e1.1 MgH2e(0.1

LiBH4 D 3 mass% ZrCoH3) system after ball-milling for 5,

10, 20, 30 and 60 h.

Fig. 11 e Capacities of the 2 LiNH2e1.1 MgH2e(0.1 LiBH4 D 3

mass% ZrCoH3) system ball-milled for 5 and for 60 h during

the first five cycles.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 3 9e1 4 4 91446

might be that the conversion reaction 2LiNH2 þMgH2/2LiHþMgðNH2Þ2 has not completely taken place, as verified also by

FTIR measurements shown in Fig. 12. It was also obvious that

the capacity during the first cycle largely depended on ball-

milling time. Extended ball-milling can result in the forma-

tion of Li2Mg(NH)2, as seen from FTIR measurement, and

thereby resulted in a decrease of capacity during the first

cycle. Especially after milling for 60 h, a considerable pressure

increase in themilling vial was detected during opening of the

milling jar in the glove-box. This was an indication that

hydrogen desorption already occurred during ball-milling.

The capacities during the first desorption cycle against

1 bar hydrogen backpressure and subsequent vacuum

desorption at 180 �C of the 2 LiNH2e1.1 MgH2e(0.1 LiBH4 þ 3

mass% ZrCoH3) system prepared with different milling times

are listed in Table 6.

In Fig. 10 the graphs of the second desorption are shown.

Fig. 12 e FTIR spectra of LiNH2 and the 2 LiNH2e1.1

MgH2e(0.1 LiBH4 D 3 mass% ZrCoH3) systems after ball-

milling for 5, 10, 20, 30 and 60 h.

Smaller differences in the desorbed amounts of hydrogen

were detectable during the second cycle. Also during the

second cycle thematerial ball-milled for 5 h exhibited a higher

onset temperature of 124 �C as compared to the other systems,

which desorbed from 120 �C.The capacities are listed in Table 7.

4.1.1. Cyclic stabilityFor investigating the cyclic stability of the systems, the

materials obtained after ball-milling for 5 h and for 60 h were

cycled five times. Results are presented in Fig. 11. The capac-

ities were measured after desorbing against 1 bar hydrogen

backpressure at 180 �C and subsequent vacuum desorption at

180 �C.The capacity of the material ball-milled for 5 h had a value

constantly between 3.9 and 4.2 mass% during the first five

cycles. The material ball-milled for 60 h desorbed only 3.3

mass% during the first cycle. As mentioned above, hydrogen

was desorbed already during ball-milling. Thereby less

hydrogen could be desorbed during the first cycle whereas 4.0

mass% H were desorbed during the second cycle. It was

observed that the capacity gradually decreases after the

second cycle. Zhang et al. reported that the capacity of the

material with the same composition, ball-milled for 10 h,

could be kept constant by prolonging the time of the vacuum

desorption [13]. For thematerial ball-milled for 60 h, this could

not be achieved, even by prolonging the time of the vacuum

desorption. This suggests a declining reversibility of the

material with longer ball-milling. To further clarify the

reasons for the declining reversibility, the as-milled samples

were subjected to FTIR measurements. The results are shown

in Fig. 12.

Pure LiNH2 showed characteristic absorption bands from

NeH stretching vibration at 3258 cm�1 and 3312 cm�1.

During ball-milling, the conversion reaction 2

LiNH2 þ MgH2 / 2 LiH þ Mg(NH2)2 gradually took place. The

sample after 5 h of ball-milling showed the typical doublet

NeH vibration of LiNH2 at 3258/3312 cm�1, however, the

absorbance was already broadened towards higher wave-

numbers. This suggests that a small fraction of LiNH2 had

already been converted to Mg(NH2)2 during ball milling,

which has characteristic symmetric and asymmetric NeH

vibrations at 3272/3326 cm�1. For the sample after 10 h of

ball-milling the absorbance became much broader and could

be attributed to both the NeH vibrations in LiNH2 and

Mg(NH2)2. After 20 h of ball-milling the band shifted to

Table 6 e Capacities of the 2 LiNH2e1.1 MgH2e(0.1LiBH4 D 3 mass% ZrCoH3) system during the firstdesorption cycle after ball-milling for 5, 10, 20, 30 and60 h.

Millingduration

Capacity(180 �C, 1 bar H2)

Capacity(180 �C, vacuum)

5 h 3.5 mass% 4.1 mass%

10 h 3.6 mass% 4.2 mass%

20 h 3.3 mass% 3.8 mass%

30 h 3.3 mass% 3.8 mass%

60 h 2.8 mass% 3.3 mass%

Table 7 e Capacities of the 2 LiNH2e1.1 MgH2e(0.1LiBH4 D 3 mass% ZrCoH3) system during the seconddesorption cycle after ball-milling for 5, 10, 20, 30 and60 h.

Millingduration

Capacity(180 �C, 1 bar H2)

Capacity(180 �C, vacuum)

5 h 3.5 mass% 3.9 mass%

10 h 3.5 mass% 3.9 mass%

20 h 3.4 mass% 3.9 mass%

30 h 3.3 mass% 4.0 mass%

60 h 3.4 mass% 4.0 mass%

Fig. 13 e XRD patterns of the 2 LiNH2e1.1 MgH2e(0.1

LiBH4 D 3 mass% ZrCoH3) systems ball-milled for 5 and

60 h after the fifth absorption.

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 en e r g y 3 8 ( 2 0 1 3 ) 1 4 3 9e1 4 4 9 1447

3272 cm�1, the characteristic wavenumber of Mg(NH2)2. Also

after ball-milling for 30 and 60 h the band was mainly in this

range. Hu et al. recently showed for the TiCl3-doped

MgH2:LiNH2 in a stoichiometric ratio of 1:1 that the conver-

sion reaction is taking place after 20 h of ball-milling when

milling at 150 rpm [17]. The conversion reaction in this study

occurred at lower milling time, which can be explained by

the higher milling intensity (300 rpm) and the resulting

larger mechanical forces and higher temperatures affecting

the material, as well as the addition of LiBH4 as the presence

of LiBH4 significantly decreases the temperature of the

conversion reaction [18].

Furthermore, in case of the materials milled for 20, 30 and

60 h some absorbance appears at 3174 cm�1, which corre-

sponds to the characteristic absorbance band of imide NeH

bonds. This suggests the formation of Li2Mg(NH)2 or MgNH.

Thus, hydrogen has been desorbed during themilling process,

which was already observed by the pressure rise while

opening the milling jar.

Various authors proposed the formation of MgNH or

a Magnesium-imide-like structures during ball-milling for

long periods [7,19]. MgNH shows vibration absorbances at

3251 and 3199 cm�1. The absorbance at 3251 cm�1 was difficult

to distinguish in our work due to the weak and broad absor-

bance. However, a peak broadening of the characteristic

Mg(NH2)2 absorbance at 3272 cm�1 towards smaller wave-

numbers could be seen after ball-milling for more than 20 h,

suggesting the formation of imides. The formation of imide-

like structures might have possibly caused the declining

capacity and reduced reversibility of the material ball-milled

for 60 h.

Fig. 13 shows the XRD patterns of the rehydrogenated

samples ball-milled for 5 h and 60 h after the fifth cycle.

The XRD pattern of the material ball-milled for 5 h showed

sharp peaks of high intensity, the material was obviously well

crystalline. After five cycles the phases formed were LiH,

Mg(NH2)2 and ZrCoH3. The pattern was in good agreement

with the XRD pattern shown in Ref. [12] of the same system

after 10 cycles.

After ball-milling for 60 h the peak intensities were much

lower. This suggests a higher fraction of amorphous material.

The pattern of the re-absorbed system milled for 60 h still

showed Li2Mg(NH)2-peaks. To clarify that no more hydrogen

absorption occurred in the sample, the apparatus pressure

was kept at 100 bar at 150 �C, and only after the pressure was

constant for 60 min the system was cooled down to room

temperature. Pressure constancy suggests that no hydrogen is

absorbed, and no more Li2Mg(NH)2 will react to LiH and

Mg(NH2)2.

No significant change in particle size was observed after

ball-milling for 5 h. This is concluded from the SEM micro-

graphs, also presented in the Supporting information.

5. Scale-up

The material investigated in the lab-scale was scaled up on

semi-industrial milling equipment, which can produce

batches in the kg-scale. As the operation principle of the

vibrational ball-mill differs from the planetary ball-mill’s

operation principle used in the lab-scale the milling process

could therefore not be scaled up by applying a theoretical

model, and the optimum milling parameters had to be

determined empirically. 200 g of the 2 LiNH2e1.1 MgH2e(0.1

LiBH4 þ 3 mass% ZrCoH3) system were filled into the vibra-

tional ball-mill. Because milling intensities of the vibrational

ball-mill are generally larger than those of the planetary ball-

mills used in the lab-scale, the total milling timewas set as 5 h

and samples of about 2 g were removed after 90, 180 and

300 min and then characterized in order to determine the

optimum ball-milling time.

In Fig. 14 the first two desorption cycles of the samples

removed after 90 and 180 �C are presented.

Desorption graphs of the up-scaled materials ball-milled

for 90 and 180 min showed a similar sigmoidal shape as the

materials prepared in the lab-scale. This suggests that the

material shows similar characteristics when prepared in

a semi-industrial scale as compared to the material prepared

in the lab-scale.

The material milled for 300 min did not release any

hydrogen during the first cycle. Evidently, the milling process

has stressed the material too much to make any hydrogen

desorption during the first cycle impossible. It should be noted

that the material could possibly absorb hydrogen and release

hydrogen after the second cycle. But because it was shown in

Fig. 14 e First and second desorption cycle of the samples

milled for 90 and 180 min.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 3 9e1 4 4 91448

the lab-scale that too long milling duration resulted in

a steady decrease of capacity during cycling, 300 min were

regarded as a too long ball-milling time.

The samples ball milled at 90 and 180 min showed onset

temperatures of 130 �C during the first two hydrogen release

cycles, an increase of 10 �C as compared to the same system

prepared in lab-scale. Hence there exists a higher kinetic

barrier before the first desorption process begins. Within the

first 100 min of the first desorption cycle, the two samples

released 3.8 and 3.7 mass%, respectively, whereas the sample

milled in the lab-scaleplanetaryballmill for 5hor 10h released

only 3.5mass% in the sametime.Also the total capacities of the

semi-industrial scale materials were higher than those of the

lab-scale materials. The samples desorbed 4.5 mass% and 4.2

mass% after milling for 90 min and 180 min, respectively, as

compared to 4.1 mass% for the lab-scale material.

During the second cycle the material milled for 90 and

180 min desorbed 3.5 mass% within the first 100 min, as

compared to 3.6 mass% in the lab-scale material. It becomes

apparent that the capacity is degrading during the first two

cycles.

For investigating the cyclic stability of the material, the

sample ball-milled for 90 min was cycled five times. The

capacities during the first five cycles are listed in Table 8.

The capacity decreased during the first two desorption

cycles from 4.6 mass% in the first cycle to 3.6 mass% in the

third cycle. The capacity stayed constant at around 3.5

mass% during the subsequent desorption cycles. An

explanation for the decrease of capacity during the first two

Table 8 e Capacities of the up-scaled material ball-milledfor 90 min during the first five cycles.

Cycle 1 2 3 4 5

Capacity

(180 �C, 1 bar H2)

4.1

mass%

3.7

mass%

3.3

mass%

3.2

mass%

3.2

mass%

Capacity

(180 �C, vacuum)

4.6

mass%

4.3

mass%

3.6

mass%

3.6

mass%

3.6

mass%

cycles might be the formation of ammonia, which was

released into the gaseous phase. For each molecule of

ammonia which is released, three hydrogen atoms less can

be absorbed by the material. The capacity stayed constant

at 3.5 mass%, which is an indicator that no more ammonia

was formed during the subsequent cycles. In case of

ammonia formation, the capacity would have declined

further. A possible explanation for the constancy of the

capacity after the second desorption cycle is that in order to

form the dehydrogenation product, Mg(NH2)2 needed LiH in

the proximity. In case no LiH was available, the otherwise

reacting NH3 group splits from the remaining Mg and

evaporates. Possibly, all NH3 groups, which had no LiH

reaction partner in their proximity evaporated during the

first two cycles. The NH3 groups, which had a reaction

partner, remained in the material, and thus the capacity

stayed constant thereafter.

Possibly, the growth of crystalline structures of the mate-

rials could be a factor for a slowed down kinetics as the

distance between the reactants can be increased. This may

explain the higher onset temperatures and slower sorption

rates of the up-scaled materials. However, this may not have

such a pronounced effect on the cyclic stability of the

material.

In Fig. 15 the XRD patterns are presented of the as-milled

materials ball-milled for 90 and 180 min and the re-absorbed

material ball-milled for 90 min after the fifth cycle.

The materials ball-milled for 90 and 180 min showed

LiNH2-, MgH2- and ZrCoH3-peaks. Due to the materials stress

induced by ball-milling the peak intensities of the material

ball-milled for 180 min declined as compared to the peak

intensities of the material ball-milled for 90 min. The re-

absorbed sample showed peaks of Mg(NH2)2-, LiH-, ZrCoH3-

and Li2Mg(NH)2-peaks. Few minor peaks are also visible,

which could not be attributed to any of the phases. During the

fifth absorption, the apparatus pressure was constant after

about 300 min, suggesting that no more Mg(NH2)2 or LiH was

formed.

Fig. 15 e XRD patterns of the 2 LiNH2e1.1 MgH2e(0.1

LiBH4 D 3 mass% ZrCoH3) systems as-milled for 90 min (A),

180 min (B) and the re-absorbed material after the fifth

cycle, ball-milled 90 min (C).

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 en e r g y 3 8 ( 2 0 1 3 ) 1 4 3 9e1 4 4 9 1449

6. Conclusion

A hydrogen storage system composed of the complex

hydrides LiNH2 and MgH2 was systematically doped with

catalytic additives. The composites were tested on their

hydrogen sorption properties. The undoped systems show

only poor sorption kinetics. Under the applied temperature-

and pressure conditions KH-doped systems show good initial

kinetics but both capacity and kinetics degrades during

cycling. The LiBH4-catalysed systems show already good

desorption kinetics, however, the addition of ZrCoH3 further

improves the absorption kinetics of the system. A mixture

with the optimized composition 2 LiNH2e1.1 MgH2e(0.1

LiBH4 þ 3 mass% ZrCoH3) was identified as a suitable material

for further investigation. The systemdesorbs 3.4mass% under

1 bar H2 at 180 �Cwithin 100min and can be rehydrogenated in

only 7 min.

For this composition the preparation procedure was opti-

mized by variation of ball-milling time from 5 to 60 h. For ball-

milling times between 5 and 30 h, the materials exhibited

a stable cyclability. Too long ball-milling time caused the

capacity to decline to 3.1mass% after the fifth cycle. After ball-

milling for 5 h the capacity was stable between 4.0 and 4.3

mass%. Finally, the material was scaled up on semi-industrial

vibrational ball-milling equipment, and a capacity of 4.5 mass

% was obtained during the first cycle. After two cycles the

capacity declined to 3.5mass%, but stayed constant during the

subsequent three cycles. It was shown that the material is

suitable for up-scaling and shows similar characteristics as

compared to the lab-scale samples.

Acknowledgements

Financial support by the EU project “SSH2S” (grant # 256653) is

gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.ijhydene.2012.10.115

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