4.06

Hydrogen Storage: Liquid and Chemical

P Chen, Dalian Institute of Chemical Physics, Dalian, China
© 2012 Elsevier Ltd. All rights reserved.

4.06.1
4.06.2
4.06.3
4.06.4
4.06.4.1
4.06.4.1.1
4.06.4.1.2
4.06.4.2
4.06.4.2.1
4.06.4.2.2
4.06.5
4.06.5.1
4.06.5.2
4.06.6
References

Introduction
Physical Hydrogen Storage
Metal Hydrides
Chemical Hydrides
Ammonia Borane
Homogeneous catalytic dehydrogenation of AB
Solid-state dehydrogenation of AB
Amidoboranes and Derivatives

Alkali and alkaline earth amidoboranes
Derivatives
Complex Hydrides
Borohydrides
Amide–Hydride Systems
Pending Issues

157
157
159
159
160
162
163
164
164
165
167
167
169
173
174

4.06.1 Introduction
Hydrogen-based energy systems offer a potential solution to the ever-increasing demand for sustainable energy systems. Although
over the long term, the ultimate technological challenge is large-scale hydrogen production from renewable sources, a critical
practical issue is how to store hydrogen efficiently and safely, particularly for onboard hydrogen fuel cell vehicles [1]. Tremendous
efforts have been devoted to the research and development of systems that can hold sufficient hydrogen in terms of gravimetric and
volumetric densities to allow fuel cell vehicles to achieve a satisfactory driving range and in the meantime exhibit acceptable
charging/discharging kinetics, safety, and cost (see Table 1) [2, 3].

Hydrogen can be stored by either physical or chemical means. For physical storage, the conventional options are compressed
hydrogen gas and cryogenic adsorption, that is, liquid hydrogen. For chemical storage, the hydrogen molecule can be dissociated either
homolytically or heterolytically and bonds with other elements to form hydrides. The hydrogen uptake and release can be either
reversible or irreversible depending on the thermodynamic parameters of the corresponding starting and final materials. During the
last decade of exploration and study, the scope of candidate hydrogen storage materials has expanded considerably, from conventional
metal hydrides, such as LaNi5 and MgH2, to complex and chemical hydrides [3–7] and from activated carbon to carbon nanotubes and
to metal organic frameworks (MOFs) [8–18]. The employment of advanced synthetic routes has also allowed the physical state of the
storage materials to change from being bulk crystalline to amorphous and to nano structures [16, 19, 20]. Advanced theoretical
simulations also have an increasing impact not only on the description of the physical properties of known materials but also on the
prediction of novel structures and reaction paths [21–23]. A variety of promising storage systems are under intensive investigations.
Systems of high hydrogen content are the focus of ongoing studies because they allow more space for material modification and
optimization. A comprehensive survey by Thomas, Sandrock, and Bowman, as shown in Figure 1, lists over 40 material candidates
that are being actively investigated. Among them, high surface area porous materials and nitrogen- and boron-containing hydrides are
the most studied systems [24, 25]. In this chapter, a short survey on existing hydrogen storage techniques will be presented. Emphasis
will be given to chemical and complex hydrides that have been under intensive research since 2005.

4.06.2 Physical Hydrogen Storage
Physical storage of hydrogen is normally achieved either under high pressure or at cryogenic temperatures. To store high-pressure
hydrogen, a compressed hydrogen gas tank made of aluminum or composite materials wrapped with carbon or glass fiber to ensure
light weight and high strength is used. The hydrogen energy stored in the compressed gas tank increases with an increase in pressure,
but not in a directly proportional manner. Compressed hydrogen at 350–700 bar has been well developed and adopted in
prototype fuel cell vehicles. The compression of H2 will consume ∼10–15% of the energy stored, and the size of the tank holding
∼4 kg of H2 is still too large to directly compare with a gasoline tank. There have been considerable efforts in the development of
lightweight tanks to store hydrogen up to 1000 bar in recent years. Even with considerable progress, the tanks are somewhat too
expensive (∼$15 kWh−1) to be practically viable [26]. A recent demonstration on storing compressed hydrogen at 77 K suggested an
alternative method with improved characteristics especially in gravimetric and volumetric storage densities. It is, however, of
practical importance to take a step forward in reducing the system cost and increasing the energy efficiency (Table 2).

Comprehensive Renewable Energy, Volume 4


doi:10.1016/B978-0-08-087872-0.00414-5

157


158

Hydrogen Storage: Liquid and Chemical

The vehicle performance targets

Table 1

Storage parameter

Units

2010

2015

Ultimate

System gravimetric capacity

kWh kg−1
(kg H2 kg−1)
kWh l−1
(kg H2 l−1)
$ kWh−1

($ kg H2−1)
$ gge−1 at pump

1.5
(0.045)
0.9
(0.028)
4
(133)
2–3

1.8
(0.055)
1.3
(0.040)
2
(67)
2–3

2.5
(0.075)
2.3
(0.070)
TBD

°C
°C
cycles
% of mean at % confidence
atm

atm

−30/50
−30/80
1000
90/90
4 FC/35 ICE
100

−40/60
−40/85
1500
99/90
4 FC/35 ICE
100

−40/60
−40/85
1500
99/90
2 FC/35 ICE
100

min
kg H2 min−1
(g s−1) kW −1
s
s
%H2


4.2
3.3
(1.2)
(1.5)
0.02
0.02
5
5
0.75
0.75
99.99 (dry basis)

scc h−1

Meets or exceeds applicable standards

(g h−1) kg H2−1

0.1

System volumetric capacity
Storage system cost
Fuel cost
Durability/operability
Operating ambient temperature
Min/max delivery temperature
Cycle life (1/4 tank to full)
Cycle life variation
Min H2 delivery pressure
Max delivery pressure

Charging/discharging rates
System fill time (for 5 kg H2)
Minimum full-flow rate
Start time to full flow
Transient response (10% to 90%)
Fuel purity
Environmental health and safety
Permeation and leakage
Toxicity
Safety
Loss of usable H2

2–3

2.5
(2.0)
0.02
5
0.75

0.05

0.05

Source: />
16
Open symbols denote
new mat’ls for FY2009
14


Material capacity
must exceed
system targets

chemical hydrides
CH Regen.
Required

12
Observed H2 capacity (wt.%)

solid AB (NH3BH3)

New DOE system
targets

AB/cat.
MD C-foam
Ultimate

sorbents

6

IRMOF-177

4

PCN-12
C aerogel

carbide-derived C

2

B/C
bridged cat./IRMOF-8
MOF-74
MD C-foam
bridged cat./AX21

0
−200

−100

0

H2 sorption temperature (�C)

Mg(BH4)2(NH3)2
Mg(BH4)2

Mg(BH4)2(NH3)2
AB ionic liq.
AIB4H11

Li−AB

10


8

metal hydrides

AIH3

Ca(BH4)2

LiBH4/MgH2

LiBH4/CA
MgH2
Li3AIH6/LiNH2
LiMgN
1,6 naphthyridine
Li3AIH6/Mg(NH2)2
2015
Ca(BH4)2/2LiBH4
Mg−Li−B−N−H
liq. AB/cat.
LiNH2/MgH2
NaAIH4
LiMn(BH4)3
Mg(BH4)(AIH4)
NaMn(BH4)4
PANI
Na2Zr(BH4)6
Ti-MOF-16
PANI
M-doped CA

PANI
100

M−B−N−H

200

300

Temperature for observed H 2 release (�C)

Figure 1 A survey of hydrogen storage materials. />
400


Hydrogen Storage: Liquid and Chemical

Table 2

159

Parameters of compressed and liquid H2 storage
Gravimetric
energy content
(MJ kg−1)

Volumetric
energy content
(MJ m−3)


Storage
techniques

Storing energy
(kJ kg−1)

Spent energy/
stored energy

Compressed
H2 (350 bar)
Compressed
H2 (700 bar)
Liquid H2

12 264

0.10

8.04

2492

14 883

0.12

7.20

3599


42 600

0.36

16.81

3999

With reference to />
Liquid hydrogen is another option to store hydrogen onboard. Comparatively, the energy density is nearly 2 times higher than
that of the 700 bar compressed H2; however, the energy cost to liquefy hydrogen reaches 30% or more of the actual hydrogen energy
stored. There is also continual hydrogen loss when stored onboard (namely boil-off) due to thermal conduction, convection, and
thermal radiation. One of the other drawbacks is the use of expensive multilayered vacuum superinsulated vessels. Another
important practical issue is the ‘cooling-down’ losses during refilling of liquid hydrogen at gas stations. The entire transfer line
between the liquid H2 source and the vehicle tank system has to be cooled down to about −253.8 °C, and therefore, additional H2
evaporation occurs. Clearly, these losses cannot be neglected and remain significant. There have been a few comprehensive reviews
on compressed and liquid hydrogen published recently [26, 27], and the commercial employment of these techniques is still an
open issue. However, currently, while most of the chemical storage systems to be discussed later remain at a research stage,
compressed and liquid H2 storage remain as choices for demonstration and evaluation purposes.
Porous materials are prone to adsorb hydrogen physically; however, due to the weak interaction of H2 and sorbents, cryogenic
conditions typically have to be applied. Materials with large surface areas and proper pore size of 2–3 nm are capable of adsorbing up to
7 wt.% of H2 [12–14, 28, 29]. Representative sorbent materials include carbon materials [8–11, 30–32], MOFs [12, 16, 17, 33–35], and
conjugated polymers [14]. A special note on hydrogen storage on MOFs was triggered by breakthroughs in material design and synthesis
[12]. The interesting bonding nature of the metal and organic link creates a variety of pore structures and active centers, and the nature of
the interaction of H2 and the active centers has been a hot topic of study. It is obviously of scientific interest to further the research in this
area; however, cryogenic adsorption in general is an energy-consuming process and has, in general, relatively low volumetric hydrogen
storage density; hence, the remainder of this chapter will focus on alternative solid-state forms of H2 storage.

4.06.3 Metal Hydrides

The homolytic dissociation of H2 into atomic H followed by diffusion of the H in the lattice of metals, especially transition metals and
alloys, leads to the formation of metal hydrides [2, 36]. Table 3 shows a list of conventional metal hydrides that have been extensively
studied in the past. For example, the H content in terms of the volumetric hydrogen density of LaNi5H6 is 115 kg m−3, which is higher than
that of compressed hydrogen and liquid hydrogen. However, transition metals and their alloys have relatively low gravimetric hydrogen
densities (normally < 3 wt.%). A recent work by Matsunaga et al. [37] on hybridizing metal hydrides with a high-pressure tank gives a
certain level of promise for the improvement of gravimetric density. In addition to the search for new multicomponent metal alloys, current
research also focuses on the reduction in size of Mg-based materials to enhance kinetics in dehydrogenation [16, 20].

4.06.4 Chemical Hydrides
There are a vast variety of natural and manmade chemical hydrides including H2O, NH3, alcohol, boranes, and hydrocarbons. The
H–X (where X refers to O, N, B, C, etc.) bond is significantly stronger than that of a typical H–M bond of most of the interstitial
metal hydrides [1–3]. Chemical hydrides that have been investigated for the purpose of hydrogen storage are mainly those
Table 3

Structure and hydrogen storage properties of typical metal hydrides

Type of metal
hydrides

Metal

Hydrides

Structure

Mass% of
hydrogen

Peq, T


AB5
AB3
AB2
AB
A2B

LaNi5
CaNi3
ZrV2
TiFe
Mg2Ni

LaNi5H6
CaNi3H4.4
ZrV2H5.5
TiFeH1.8
Mg2NiH4

Hexagonal
Hexagonal
Hexagonal
Cubic
Cubic

1.4
1.8
3.0
1.9
3.6


2 bar, 298 K
0.5 bar, 298 K
10−8 bar, 323 K
5 bar, 303 K
1 bar, 555 K


160

Hydrogen Storage: Liquid and Chemical

H

H

H

H
H

H
N

N

B

B

H


H
H

H

H
Li

Figure 2 Molecular structures of AB and lithium amidoborane.

Dihydrogen bonding
~ 1.8 Å

Figure 3 Packing of AB molecules in the crystal. Yellow, blue, and white balls are N, B, and H, respectively.


Table 4

Summary of dehydrogenation of AB, alkali and alkaline earth amidoboranes, and their derivatives


Reactions

Conditions a

Temperature
( °C)

H

(mass %)

nNH3BH3 → (NH2BH2)n + nH2 → (NHBH)n + 2nH2
NH3BH3 → NBH6−x + xH2

Solid
SBA-15
Ir-based catalyst/THF
Ni-NHC catalyst/THF
Solid
In THF
Solid
Solid
Solid
Solid
Solid
Solid

70–200
50–100
Ambient temperature
60
75–95
40–55
80–90
90–245
80–200
50–250
75–300
50–300


12.9
6.7
6.7
16.5
10.9

LiNH2BH3 → LiNBH + 2H2
NaNH2BH3 → NaH + BN + 2H2
Ca(NH2BH3)2 → Ca(NBH)2 + 4H2
Sr(NH2BH3)2 → Sr(NBH)2 + 4H2
LiNH2BH3NH3BH3 → LiN2B2H + 5H2
Ca(NH2BH3)2·2NH3 → Ca(BN2H)2 + 6H2
Mg(NH2BH3)2·NH3 → MgB2N3H + 6H2
a

7.5
8.0
6.8
14.7
8.9
11.8

For solid-state dehydrogenation, most of the materials are under molten or semi-molten state.

containing H–B, H–N, and H–O bonds [4, 38, 39]. A distinctive feature of the most investigated chemical hydrides is the coexistence
of both protonic and hydridic H atoms. Representative entities are NH3BH3 (ammonia borane, AB) [7, 24, 40–45], metal
amidoboranes (MAB) [46–52], and H2O–borohydride systems. Figure 2 shows the molecular structures of AB and lithium
amidoborane. H bonded with N has positive charge, which is opposite to H bonded with B. When a crystal is formed, the shortest
distance between these two H atoms is found to be less than twice the van der Waals radius of H (see Figure 3). The exceptionally

high chemical potentials for the combination of H− and H+ to molecular H2 and the formation of strong B–N or B–O bond are likely
to be the driving forces for the dehydrogenation. For most of the chemical hydrides, dehydrogenation is an exothermic process. The
increase in stability of the dehydrogenated product is the result of the formation of strong B–N or B–O covalent bond. In this
section, the research activities on the development of AB and amidoboranes for hydrogen storage will be reviewed. Table 4 presents
the conditions applied in the dehydrogenation of AB, alkali and alkaline earth amidoboranes, and their derivatives.

4.06.4.1

Ammonia Borane

� mm and
AB, first synthesized in 1955 [53], is a plastic crystalline solid adopting a tetragonal crystal structure with space group I4
lattice parameters of a = b = 5.240 Å and c = 5.028 Å at room temperature [54, 55]. As shown in Figure 2, this molecular crystal is


Hydrogen Storage: Liquid and Chemical

161

stabilized by dihydrogen bonding between H(B) and H(N). The crystal melts at ∼100 °C and decomposes to hydrogen (1 equiv.)
between 70 and 112 °C to yield polyaminoborane (PAB, [NH2BH2]n) according to eqn [1]. Subsequently, [NH2BH2]n decomposes
with an additional 1 equiv. hydrogen loss, over a broad temperature range around 150 °C, forming amorphous polyiminoborane
(PIB, [NHBH]n) and a small fraction of borazine ([N3B3H6]), according to eqns [2] and [3], respectively. The decomposition of
[NHBH] to boron nitride (BN) occurs at temperatures in excess of 500 °C. This final step is not considered practical for hydrogen
storage due to high temperatures needed for hydrogen release. Thermodynamic analyses and theoretical calculations show that
hydrogen release from either AB or PAB or PIB is an exothermic process, revealing the irreversibility of hydrogen desorption from
these materials.
nNH3 BH3 ðsÞ →½NH2 BH2 Š n ðsÞ þ nH2 ðgÞ

½1Š


½NH2 BH2 Š n ðsÞ →½NHBH Š n ðsÞ þ nH2 ðgÞ

½2Š

½NH2 BH2 Š n ðsÞ →½N3 B3 H6 Š n = 3 ðlÞ þ nH2 ðgÞ

½3Š

Figure 4 presents some of the likely forms of products from releasing the first and second equivalent molecules of H2 from AB.
The structure and composition of the product vary with the conditions applied during dehydrogenation. As an example, on catalytic
dehydrogenation of AB by iridium (Ir) catalyst in tetrahydrofuran (THF), crystalline PAB is formed [43]. A recent report from He
et al. [68] demonstrated the formation of crystalline linear PAB in the FeB-catalyzed solid-state dehydrogenation of AB at ∼60 °C.
However, in most cases, the solid product is essentially amorphous and is a mixture of linear, cyclic, branched, and cross-linked B–N
structures.
Although it has an exceptionally high hydrogen content, AB has to overcome a few drawbacks to be practically viable. The first two
challenges are the mass production of AB and energy-efficient regeneration of the used fuel [56, 57], while the kinetic-borne dehydrogena­
tion of AB and the coproduction of unwanted gaseous products (such as borazine and NH3) are to be alleviated. Moreover, severe material
foaming in the dehydrogenation is also problematic. Tremendous efforts have been devoted to these issues since the first report on the
dehydrogenation of AB for hydrogen storage by Wolf et al. [40], among which investigations on catalytic modification of AB or dispersing
AB into porous materials attract significant attention [43–45, 58–71]. Dehydrogenation of AB in ionic liquids shows improved kinetics in
comparison with neat AB [42]. Moreover, a number of intermediates and products have been identified by in situ nuclear magnetic
resonance (NMR) and the density functional theory–gauge including/invariant atomic orbital (DFT–GIAO) calculations. As shown in
Figure 5, different states of hybridization (sp2 or sp3) and bonding environments (with H or N) have chemical shifts ranging from −26.9 to
+39.3 ppm. There are a few comprehensive reviews in this area to which the readers may like to further refer [24, 25, 56, 57].

Figure 4 Molecular structures of possible dehydrogenation products [56].


162


Hydrogen Storage: Liquid and Chemical

(a)
H H

H

H

H

B

N

B
H

H

H

B

N
H

N


B

H H

H

H

H

H

N

N

B

H
H

H

H

H

H

H


−9.5 ppm −11.4 ppm
−23.0 ppm
−11.8 ppm
(b)

H H H

H

H

N

B

N

B

H

B

H

H

N
H


H H

B

N
H

H

−13.8 ppm
H

H

B

−5.9 ppm −22.4 ppm

H

N

H

(c)

H
H


B

H

N
H

H

H H

H

H

−10.4 ppm

N
H

−26.9 ppm
H

H

B

H

B


N
H

N

H

H

B
H

H

−8.8 ppm

+8.7 ppm

+33.2 ppm

+30.4 ppm

+39.3 ppm

Figure 5 DFT–GIAO calculated 11B NMR chemical shift for possible structures arising from the dehydropolymerization of AB [42].

4.06.4.1.1

Homogeneous catalytic dehydrogenation of AB


As summarized by Hamilton et al. [57] and Smythe and Gordon [56], a few homogeneous catalysts including Ru-, Ir-, and Ni-based
complexes and Lewis acid (B(C6F5)3) have been developed, which are effective in removing 1–2.5 equiv. H2 from AB under mild
conditions. Figure 6 shows the time dependence of H2 evolution from a AB/THF solution with different concentrations of
(POCOPf)Ir(H)2. With 1 mol.% of catalyst, ∼1 equiv. H2 can be released within 5 min. A quantitative yield of crystalline PAB
was observed [43].
The dehydrogenation of AB by the Ni-N heterocyclic carbene (Ni-NHC) complex in a molar ratio of 10:1 shows unprecedented
evolution of 2.5 equiv. H2 at 60 °C [45]. The formation of Ni-NHC is by the reaction of biscyclooctadiene nickel (Ni(cod)2) with
Enders’ NHC. Theoretical investigation suggests that the formation of the first transition state (highest energy barrier) is through
transferring H(N) of AB to ligand carbene, which is different from the β-H elimination of AB in some other cases.
One way to activate AB by Lewis or Brønsted acid is by attracting a hydridic H(B) from AB to form the initiative cation
[H2BNH3]+ [63]. As shown in Figure 7, the overall process after the formation of the initiative cation resembles cationic
polymerization and dehydrogeneration. One equivalent H2 can be removed from AB at ambient temperature.

Equivalents of H2

1
O PtBu2
0.5

catalyst =

Ir

H
H

O PtBu2
1
0

0

10

20
Time (min)

Figure 6 Amount of H2 evolved per mole of AB using 0.25 mol.% (●), 0.5 mol.% (▲), and 1.0 mol.% (■) Ir catalyst [43].


Hydrogen Storage: Liquid and Chemical

H3N BH3

A

H2B NH3

1

+ H3NBH3

+

H2B NH3
H BH2

2

163


NH3

3

H2
+
N
H2B
BHNH3
H

H2

‡

H
+ N2
H2B
H
H
H

4

H

B

NH3


Figure 7 Reaction of NH3BH3 with Lewis or Brønsted acid (A) results in the formation of borenium cation 2. Subsequent reaction with another equivalent
of NH3BH3 results in the formation of 3 with subsequent expulsion of H2 and concomitant formation of 4 [63].

As shown above, the chemistry involved in the catalytic dehydrogenation of AB in solvent is considerably rich and worthy of
detailed experimental and theoretical investigations. The use of the solvent allows sufficient mobility of both reactant and catalyst
but will decrease the energy density of the system. It is a subject of system engineering to minimize the side effect of the solvent but
retain the efficiency of homogeneous catalysis.

4.06.4.1.2

Solid-state dehydrogenation of AB

As mentioned in Section 4.06.4.1, the thermal decomposition of solid-state AB is a stepwise process having considerable kinetic
barriers at each step. Efforts in alleviating the barrier in solid-state dehydrogenation are through dispersing AB into porous
substrates [7] and introducing a catalyst in the material (solid form) [68].
An introductory work by Gutowska et al. demonstrated that, when dispersing AB onto porous SBA-15 nanoscaffold, hydrogen
started to release at temperatures just above 50 °C and peaked at ∼100 °C, which is considerably lower than that of neat AB. Moreover,
the formation of borazine was largely depressed [7]. Further isothermal testing showed that the dispersed AB presented a significantly
shortened induction period and reduced kinetic barrier. As shown in Figure 8, 1 equiv. H2 can be released at 50 °C within 150 min.
However, for pristine AB, it has to go through a ∼100-min induction period and another 400–500 min to remove the same amount of
H2. A few successful attempts in using carbon cryogel, lithium catalysis and mesoporous carbon (Li-CMK-3), nano-BN, poly(methyl
acrylate) (PMA), and so on, to improve the dehydrogenation properties of solid AB have been reported in the past 5 years [70, 72, 73].
Another approach in improving dehydrogenation of AB is via solid-state catalysis through the use of transition metals or alloys.
He et al. reported that, upon the introduction of nano-sized Co- or Ni- or Fe-based catalyst to solid AB via the so-called
coprecipitation method, ∼1.0 equiv. or 6 wt.% of H2 can be released at 59 °C (shown in Figure 9) [68]. It was observed that the
presence of the nano-sized catalyst largely depressed the sample foaming and the coproduction of borazine. In the meantime,
crystalline rather than amorphous PAB was formed (Figure 10), which should be derived from the catalyst-oriented growth of
aminoborane and is significantly different from the ion-initiated dehydrogenation.


1.0

0.8

0.8
AB:SBA-15

50 �C

0.6

0.6
neat AB
80 �C

0.4

0.4

0.2

0.0

Extent of reaction

Relative heat released (arb. units)

1.0

0.2


0

100

200

300

400

500

600

0.0
700

t (min)
Figure 8 Scaled exotherms (solid lines) from isothermal differential scanning calorimetry (DSC) experiments that show the time-dependent release of H2
from AB and AB:SBA-15 (1:1 w/w). The area under the curve for neat AB corresponds to ΔHrxn = −21 kJ mol−1, and the area under the curve for AB:SBA-15
corresponds to ΔHrxn = −1 kJ mol−1. The release of hydrogen from AB proceeds at a more rapid rate and at lower temperatures in SBA-15. The dashed line
(-) is the integrated signal intensity; (•) is the point at which the reaction is 50% complete [7].


164

Hydrogen Storage: Liquid and Chemical

1.0


Equiv. H2

0.8

b

0.6

c

0.4
0.2
a

0.0
0

5

10

15

20

25

30


Time (h)
Figure 9 Volumetric hydrogen release measurements at 59 °C on the pristine (a), 2 mol.% Co-doped (b), and 2 mol.% Ni-doped (c) AB samples [68].

Intensity (a.u.)

PAB

PABFeB

6

5

4

3

2

d (Å)
Figure 10 XRD patterns of the postdehydrogenated neat AB (80 °C) and 2.0 mol.% Fe-doped AB samples (60 °C). ▼, crystalline linear PAB.

4.06.4.2
4.06.4.2.1

Amidoboranes and Derivatives
Alkali and alkaline earth amidoboranes

As shown in the previous section, various methods have been employed to lower the decomposition temperature of AB through the
use of additives and catalysts. A different approach has been applied recently in the manipulation of the thermodynamic properties of

compounds through chemical alteration to AB, that is, through substituting one H in the NH3 group in BH3NH3 with a more
electron-donating element [46, 49]. The rationale behind this approach is to alter the polarity and intermolecular interactions
(specifically the dihydrogen bonding) of AB to produce a substantially improved dehydrogenation profile. Lithium amidoborane
(LiNH2BH3) [47, 49, 50, 52, 74, 75], sodium amidoborane (NaNH2BH3) [48, 49, 76], calcium amidoborane [46, 50], and strontium
amidoborane [77] have been synthesized, which show substantially different dehydrogenation characteristics with respect to AB itself.
These alkali and alkaline earth amidoboranes (MABs) were synthesized mainly through the interactions of alkali or alkaline
earth metal hydrides (LiH, NaH, CaH2, and SrH2) with AB (see eqn [4]), which lead to the replacement of hydrogen atom of AB by
alkali or alkaline earth metals.
½4Š
MHx þ xNH3 BH3 →MðNH2 BH3 Þ x þ xH2
where x = 1 when M is an alkali metal and x = 2 when M is an alkaline earth metal.
The replacement of the H of the NH3 group in AB by alkali or alkaline earth element results in the alteration of the crystal
structure and dehydrogenation property. As shown in Figure 11, LiNH2BH3 crystallizes in the orthorhombic space group Pbca with
the lattice constants a = 7.112 74(6) Å, b = 13.948 77(14) Å, c = 5.150 18(6) Å, and V = 510.970(15) Å3. The Li–N bond is essentially
ionic and the B–H bond length is slightly longer than that in neat NH3BH3. NaNH2BH3 is of identical structure to LiNH2BH3 [49,
50]. Ca(NH2BH3)2, on the other hand, is a monoclinic structure with a = 9.100(2) Å, b = 4.371(1) Å, c = 6.441(2) Å, and
β = 93.19°(see Figure 12) [50]. Li or Na is coordinated with four NH2BH3 groups. Ca, on the other hand, sits in the center of an
octahedron made of NH2BH3 groups. The THF adduct of Ca(NH2BH3)2 was also determined [46]. It is interesting to note that,
unlike Ca and Sr, no report on the formation of Mg(NH2BH3)2 has appeared in the literature so far.
The experimental results show that more than 10 and 7 wt.% of hydrogen desorbs exothermically from LiNH2BH3 and
NaNH2BH3, respectively, at around 91 °C (Figure 13) [49]. Ca(NH2BH3)2, on the other hand, releases ∼8 wt.% H2 in the temperature
range of 100–300 °C. In all the cases, borazine production is beyond the detection limit of mass spectrometry (MS). The induction
period that is associated with the dehydrogenation of pristine NH3BH3 is absent from these amidoboranes, indicating that a different
dehydrogenation mechanism is occurring. There are a few theoretical and experimental investigations on the dehydrogenation
mechanism of these amidoboranes, especially of LiNH2BH3 [52, 75]. Kim et al. reported that the dehydrogenation of LiNH2BH3 is
via abstracting H from the BH3 group by Li [52]. An isotopic investigation also evidenced the bimolecular dehydrogenation.


Hydrogen Storage: Liquid and Chemical


165

Figure 11 Crystal structure of LiNH2BH3. Li, B, N, and H are represented by red, orange, green, and white spheres, respectively [49].

c
a
b

2.87 Å

3Å

3.0

2.466 Å
Figure 12 Crystal structure of Ca(NH2BH3)2. Ca, B, N, and H atoms are represented by orange, green, blue, and white spheres, respectively [50].

4.06.4.2.2

Derivatives

A number of compounds and complexes derived from amidoboranes have been synthesized since 2009 [51, 78, 79]. NH3 and THF
are prone to adduct to amidoboranes. Amidoborane ammoniates can be synthesized either by exposing amidoboranes (such as
Ca(NH2BH3)2) to NH3 or by reacting amides or imides with AB [51]. In general, amidoborane ammoniates were found to release
H2 at mild temperatures. Bowden et al. reported that when NH3BH3 reacts with LiNH2, H2 rather than NH3 was released in the
temperature range ∼25–300 °C [79]. Chua et al. synthesized Ca(NH2BH3)2·2NH3 and Mg(NH2BH3)2·NH3 through the reaction of
NH3BH3 with Ca(NH2)2 or MgNH, respectively [51]. In both cases, NH3 adducts to metal cations and forms dihydrogen bonding


166


Hydrogen Storage: Liquid and Chemical

H content (wt.%)

12

LiNH2BH3

10
8

NaNH2BH3

6

BH3NH3

4
2
0
0

5

10
Time (h)

15


20

Figure 13 Hydrogen evolution from heating neat NH3BH3, LiNH2BH3, and NaNH2BH3 at 91 °C [49].

H1

(a)

N3

(b)

H3

H2

B1

Mg1

N1
N2

2.157
2.104 2.123
1.993
2.129
2.126

B2


H7
H6

H4

H13


H8

H5

H12

H11

H9

H10
c
a
Figure 14

b

Molecular packing and network of N–H⋯H–B dihydrogen bonding in MgAB·NH3 (a) and close contacts around the Mg2+ center (b).

(a)
Intensity (a.u.)


H2

NH3
Borazine

H2 content (wt.%)

12

6

8

4

4

2

No. of equiv. H2

(b)

TPD-MS

0

0
0


100

200

300

Temperature (�C)
Figure 15 Temperature-programmed desorption (TPD)-MS spectra (a) and volumetric release (b) measurements on Mg(NH2BH3)2·NH3 at the heating
rate of 2 (a) and 0.5 °C min−1 (b).

with nearby H(B). The shortest (N)H⋯H(B) distance in Mg(NH2BH3)2·NH3, for example, is around 1.92 Å (Figure 14). The
difficulty in forming Mg(NH2BH3)2 and the existence of its ammoniate indicate that the unstable crystal of Mg(NH2BH3)2
(probably due to small but dense charged cation (Mg2+) and big anion) can be stabilized by NH3 through the establishment of
coordination of a lone pair of N to Mg2+. The thermal decomposition of Mg(NH2BH3)2·NH3 performed in a closed vessel
demonstrated a stoichiometric conversion of NH3 and desorption of ∼11.2 wt.% H2 (shown in Figure 15).


Hydrogen Storage: Liquid and Chemical

167

As shown above, chemical hydrides are capable of releasing large amounts of H2 at relatively low temperatures. However, in
most cases, hydrogen desorption is exothermic in nature, showing that these chemicals are kinetically stable. Intensive sunshine,
impact, impurities in the material, and so on may trigger self-decomposition leading to auto-accelerated dehydrogenation, which
may need serious consideration when applied practically.

4.06.5 Complex Hydrides
Complex hydrides have attracted considerable attention since 1997 when Ti was successfully introduced to NaAlH4 system [6]. In
the past 13 years, alanates, borohydrides, and amides have been extensively and intensively investigated. The following section will

mainly report on the progress on borohydrides and amides. There have been significant amounts of review work on alanates in the
past 10 years [3, 80–83]. Table 5 summarizes the systems developed over this period.

4.06.5.1

Borohydrides

Borohydrides, having a common formula of M(BH4)n, where M refers to the metal element and n the valence of M, have been
extensively studied in the past decade. Representative systems are LiBH4, Mg(BH4)2, and Ca(BH4)2.
Figure 16 shows the structure of LiBH4, Mg(BH4)2, and Ca(BH4)2, respectively. LiBH4 crystalizes in an orthorhombic structure
(Pnma) at ambient temperature and transfers to a hexagonal structure (P63mc) at 135 °C [84–86]. Mg(BH4)2, on the other hand,
transfers from a hexagonal structure to an orthorhombic structure at ∼180 °C [87–89]. Three different polymorphs of Ca(BH4)2 have
been reported up to date [90–92]. The structures of low-temperature α-Ca(BH4)2 (orthorhombic, space group Fddd) and
high-temperature β-Ca(BH4)2 (tetragonal, P42/m) phases have been resolved [90, 91], and the third phase γ-Ca(BH4)2, of ortho­
rhombic structure Pbca, was also reported [92]. Ca(BH4)2 undergoes phase transformation prior to its thermal decomposition.
Considerable attention has been paid to LiBH4 due to its high hydrogen content (∼18.4 mass%) [39, 85, 86]. However, hydrogen
desorption from this chemical is highly endothermic (∼67 kJ mol−1) and, thus, requires temperatures higher than 300 °C. A few
approaches have been introduced recently to destabilize LiBH4. Comparatively, reacting LiBH4 with chemicals, such as LiNH2
[93–96], MgH2 [22, 38, 97, 98], and CaH2 [99–101], can considerably change the overall dehydrogenation thermodynamics due to
the formation of more stable products. As examples, hydrogen desorption from the LiBH4–2LiNH2 mixture is an exothermic
reaction [93, 96]; combination of LiBH4 and MgH2 in a molar ratio of 2:1 leads to ∼25 kJ mol−1 H2 decrease in enthalpy change
compared with the pristine LiBH4 [38]. As shown in Figure 17, hydrogen desorption from the composite starts at ∼300 °C and
rehydrogenation occurs at ∼250 °C. The formation of the stable product MgB2 alters the thermodynamics of the dehydrogenation.
Table 5

Dehydrogenation of borohydrides and amide–hydride combinations

Reactions

Mass% of

H2

Temperature
(°C) a

Borohydrides
2LiBH4 → 2LiH + 2B + 3H2
2LiBH4 + MgH2 = 2LiH + MgB2 + 4H2
Mg(BH4)2 → MgB2 + 4H2
3Mg(BH4)2·2(NH3) → Mg3B2N4 + 2BN + 2B + 21H2
Ca(BH4)2 → CaH2 + 2B + 3H2
Zn(BH4)2 → Zn + B2H6 + H2

13.6
11.5
14.8
15.9
8.6
2.1

200–550
270–440
290–500
100–400
300–500
90–140

Amide/hydride
LiNH2 + 2LiH = Li2NH + LiH + H2 = Li3N + 2H2
CaNH + CaH2 = Ca2NH + H2

Mg(NH2)2 + 2LiH = Li2Mg(NH)2 + 2H2
3Mg(NH2)2 + 8LiH = 4Li2NH + Mg3N2 + 8H2
Mg(NH2)2 + 4LiH = Li3N + LiMgN + 4H2
2LiNH2 + LiBH4 → ‘Li3BN2H8’ → Li3BN2 + 4H2
Mg(NH2)2 + 2MgH2 → Mg3N2 + 4H2
2LiNH2 + LiAlH4 → LiNH2 + 2LiH + AlN + 2H2 = Li3AlN2 + 4H2
3Mg(NH2)2 + 3LiAlH4 → Mg3N2 + Li3AlN2 + 2AlN+12H2
Mg(NH2)2 + CaH2 → MgCa(NH)2 + 2H2
NaNH2 + LiAlH4 → NaH + LiAl0.33NH + 0.67Al + 2H2
2LiNH2 + CaH2 = Li2Ca(NH)2 + 2H2
4LiNH2 + 2Li3AlH6 → Li3AlN2 + Al + 2Li2NH + 3LiH + 15/2H2
2Li4BN3H10 + 3MgH2 → 2Li3BN2 + Mg3N2 + 2LiH + 12H2

10.5
2.1
5.6
6.9
9.1
11.9
7.4
5.0
8.5
4.1
5.2
4.5
7.5
9.2

150–450
350–650

100–250
150–300
150–300
150–350
20b
20b–500
20b–350
20b–500
20b
100–330
100–500
100–400

a
b

Experimental observation.
Under ball milling condition.


168

Hydrogen Storage: Liquid and Chemical

c
c
a

b


a

b

c
b

a

Figure 16 Crystal structures of LiBH4, Mg(BH4)2, and Ca(BH4)2, respectively. Green, orange, and blue spheres represent Li, Mg, and Ca.

(a)

350
(d)

8

300

(c)

250

6

200
(b)

150


4
(a)

100

Temperature (�C)

Hydrogen uptake (wt.%)

10

2
50
0
0

2

4

6

8


0

Time (hr)



(c)

400

8
(b)

300

6

200

4
(a)

100

2
0

Temperature (�C)

Desorbed hydrogen (wt.%)

(b) 10

0


1

2

3

4

5

0

Time (hr)
Figure 17 Hydrogenation and dehydrogenation of milled LiH + ½MgB2 with 3 mol.% TiCl3. (a) Hydrogen uptake during heating in 100 bar of hydrogen.
Curve (a) shows the temperature profile. Curve (b) shows the initial uptake of hydrogen. Curves (c) and (d) show uptake during the second and third
cycles, respectively. (b) Desorption following hydrogenation into a closed evacuated volume. Curve (a) shows the temperature profile. Curves (b) and (c),
respectively, show the wt.% of desorbed hydrogen following the initial and second hydrogenation cycles that are shown in part (a) [38].


Hydrogen Storage: Liquid and Chemical

(a)

169

0

TG (mass%)

−3


−9
−12

as-synthesized
2 h milling
5 h milling

−15

(Exo.

DTA (a.u.)

)

(b)

−6

300

400
500
600
Temperature (K)

700

800


Figure 18 (a) Thermogravimetry and (b) differential thermal analysis curves of Mg(BH4)2 for as-synthesized and after 2 and 5 h milling [102].

There has been a discussion on whether the dehydrogenation is via a stepwise manner, that is, if the first step is via
self-decomposition of LiBH4 to LiH, B, and H2 followed by MgH2 + B to MgB2 and H2.
Having a hydrogen content of ∼15 mass%, Mg(BH4)2 is another attractive hydrogen storage candidate [102–105]. Mg(BH4)2 can
be synthesized via a metathesis of LiBH4 and MgCl2 [104–106]. At temperatures above 340 °C, it decomposes to hydrogen (see
Figure 18) [103]; the first step in dehydrogenation has an enthalpy change of 39.3 kJ mol−1 H2 and an entropy change of
91.3 J mol−1 H2 [106]. An easy calculation from eqn [5] will give a temperature of ∼150 °C to desorb 1.0 bar hydrogen.
Obviously, it is ∼70 °C higher than the operation temperature of the proton exchange membrane (PEM) fuel cell.
RTlnðPÞ ¼ ΔH −ΔS Â T

½5Š

Ca(BH4)2 has a hydrogen capacity of 11.6 wt.% and a lower hydrogen desorption (thermal decomposition) temperature compared
with that of LiBH4 as predicted by thermodynamic analysis based on an ab initio calculation [107]. A previous study showed that
Ca(BH4)2 desorbs 9.0 wt.% hydrogen at a temperature as high as 770 K, and CaH2 is the only crystalline phase in the solid residue
[108, 109]. Doping it with Ti or Nb species did not show obvious catalytic effect on decreasing the decomposition temperature [110].
Other borohydrides, such as Zn(BH4)2 [72], also decompose to hydrogen; however, these either tend to have poor thermo­
dynamics or create unwanted side products (i.e., B2H6).
Nakamori et al. demonstrated an interesting relationship between the heat of formation of metal borohydrides and the
electronegativities of metal in M(BH4)n (M = Li, Na, Ca, Mg, Zn, etc.). Further investigation shows that the dehydriding temperature
of M(BH4)n decreases with the increase of the electronegativity of M [111]. It is worth noting that the dehydriding temperature
obtained by the temperature-programmed desorption (TPD) technique [103] is not only a measure of the thermodynamic stability
of a reactant but also reflects the kinetic barrier associated with the decomposition process. It is also worth highlighting that the
chemical state of B in dehydrogenated products, which can bond with metals (such as MgB2) or be in elemental or in amorphous
B–Hx states, will considerably change the dehydrogenation thermodynamic parameters [38, 112].
An amine complex of Mg(BH4)2, that is, Mg(BH4)2·2NH3, was identified [113], which releases hydrogen endothermically at
temperatures above 150 °C (see Figures 19 and 20). Although with side gaseous product(s) such as NH3, this new chemical shows
certain advantages over AB.


4.06.5.2

Amide–Hydride Systems

Studies on hydrogen storage over amide–hydride systems were initiated when researchers accidentally noticed that a mixture of metallic
lithium and carbon nanotubes pretreated in a purified N2 atmosphere absorbed large amounts of hydrogen at elevated temperatures (see
Figure 21) [4]. Through X-ray diffraction characterizations, the hydrogenated solid-state sample was found to contain LiNH2, LiH, and
the unreacted carbon nanotubes. Further investigations revealed that the N2-treated Li–C mixture converted to Li3N and carbon
nanotubes. The reversible hydrogen storage over Li3N follows reaction [6], and ∼10.5 mass% of hydrogen can be stored [4, 114].
Li3 N þ 2H2 ¼ Li2 NH þ LiH þ H2 ¼ LiNH2 þ 2LiH

½6Š

Thermodynamic analyses showed that hydrogen desorption from LiNH2–2LiH and LiNH2–LiH is endothermic with the heat of
desorption of 80 and 66 kJ mol−1 H2, respectively [4]. The operation temperature at 1.0 bar equilibrium H2 pressure is above 250 °C
for the LiNH2–LiH (1:1 molar ratio) system, which is too high for practical applications.


170

Hydrogen Storage: Liquid and Chemical

H5c

H5a

B5

H4d


H4c

2.209 Å

H5d

1.999 Å
2.400 Å

H5b

B4

Mg1
H2c

2.149 Å

2.156 Å

2.030 Å

H3b

H4a

N3

N2

H3a
H2a

H4b

2.145 Å

2.887 Å

H3c

H2b

Figure 19 Molecular structure of Mg(BH4)22NH3 [113].

14
12

wt.% H2

10
Mg(BH4)2 2NH3

8
6

Mg(BH4)2

4
2

0
0

50

100

150 200 250 300
Temperature (�C)

350

400

Figure 20 Gas evolution from Mg(BH4)2·2NH3 (magenta line) and Mg(BH4)2 (blue line). The vertical axis calibration is wt.% H2 and is based on the
assumption that all the evolved gas is hydrogen [113].

10

Abs

Wt.% H2

8

6
4

Des
2

0
50

100 150 200 250 300 350 400 450
Temperature (�C)

Figure 21 Weight variations during hydrogen absorption and desorption processes over Li3N samples [4].

Li3N has been regarded as a superior Li ion conductor. The structure of Li3N is illustrated in Figure 22, which consists of Li and
Li2N layers, in which Li migrates along the Li2N layer having a relatively low barrier. When hydrogen is pumped in, one-third of the
Li will be removed from the Li3N structure, which combines with H to form LiH. H replaces Li and bonds to N to form Li2NH.
Further hydrogenation results in the additional exchange between Li in Li2NH and H in H2 and the formation of LiNH2 and LiH.


Hydrogen Storage: Liquid and Chemical

171

Figure 22 Crystal structure of Li3N. N is in blue and Li in purple.

Due to a poorer electronegativity, H bonded with N is positively charged (Hδ+). On the contrary, H in hydrides, especially ionic
hydrides, is negatively charged (Hδ−). The abnormally high potential of the combination of Hδ+ and Hδ− to H2 together with the
strong electrostatic attraction between the N anion in amide and the metal cation in hydride will likely induce a direct reaction
between the amide and hydride and lead to the release of H2 [2, 115]. One can expect that such an interaction should exist in other
amide–hydride combination systems. A variety of metal–N–H systems with different hydrogen capacities and thermodynamic
parameters have been developed [23, 114, 116–155] (Table 5). As examples, the reaction between Mg(NH2)2 and MgH2 in a molar
ratio of ½ gives more than 7.4 mass% H2 and the solid product of Mg3N2 [134]; more than 5 mass% of hydrogen can be released
exothermically from a mixture of NaNH2/LiALH4 (1:1 molar ratio) upon energetic ball milling [147].
The reactions of amides and complex hydrides including LiBH4 [94, 96, 133, 154, 156–159], LiAlH4, and Li3AlH6 [136, 138,
142, 144, 149] brought considerable interesting features to the amide–hydride system. It was reported that more than 11 wt.% of

hydrogen can be desorbed from a mixture of 2LiNH2–LiBH3 exothermically in a temperature range of 250–350 °C (see Figure 23).
Li3BN2 is the final product [93]. The dehydrogenation feature is significantly different from the highly endothermic
self-decomposition of LiBH4 and LiNH2. Due to the exceptionally high hydrogen content, this system can be used for onboard
hydrogen production, provided that the operation temperature can be substantially reduced. Attempts in catalyzing the dehydro­
genation by introducing nano-sized Pd, Pt, Ni, and Co have successfully brought down the dehydrogenation temperature to
∼150 °C [156, 160, 161]. However, further reduction of temperature may depend not only on the catalytic modification but also on
the optimization of the physical state of the reactant. Experimental results show that the catalyzed dehydrogenation reaches the
maximum rate upon the melting of Li3BN2H8, indicating that the mobility of the reacting species (Li3BN2H8) is essential to the
effective contact of catalyst [161]. It is likely that an additive which can form an eutectic compound with Li3BN2H8 could further
enhance the reaction kinetics. Clearly, such an attempt has to be based on an in-depth structural understanding [94, 133, 159]. Yang
et al. introduced MgH2 to the LiBH4–LiNH2 system and observed a multiple-step reaction involving the formation of Li4BN3H10,
400

10%
300
10.24 wt.%

8%
Temperature

364 �C
200

6%
4%
2 LiNH2 + LiBH4
Ball milled 16 hours

2%


100

Desorption

0%

0

500

1000

0
1500

Time (min)
Figure 23 Volumetric measurement of thermal desorption from Li3BN2H8 heating at 0.5 °C min−1 to 364 °C [93].

Temperature (�C)

Hydrogen desorption (wt.%)

12%


172

Hydrogen Storage: Liquid and Chemical

Li2Mg(NH)2, Li3BN2, and Mg3N2 [150]. Each step has different thermodynamic and kinetic parameters. Investigations on the

interaction between LiNH2 and LiAlH4 revealed that the transition of [AlH4]− to [AlH6]3− is fairly easy in the presence of LiNH2
[149]. From a 2LiNH2–LiAlH4 sample, 4 equiv. H was desorbed during a ball milling treatment. NMR measurements of samples
collected at different intervals of ball milling showed that an Al–N bond was established upon the contact of these two chemicals,
revealing a direct interaction between –NH2 and [AlH4]−. Complete dehydrogenation at elevated temperatures results in the
formation of Li3AlN2, which can only be partially rehydrogenated to LiNH2, LiH, and AlN (Table 5) [149]. The LiNH2–Li3AlH6
combination was also investigated [136, 138]. Different results were observed by different groups. Kojima et al. detected the
formation of Li3AlN2, Li2NH, Al, and LiH after the dehydrogenation of Li3AlH6–2LiNH2 [136]. The Al–N bonding was not observed
by Lu et al. A fully reversible reaction between Li3AlH6 and LiNH2 (molar ratio 1:3) was reported [138]. Large amounts of hydrogen
desorption from Mg(NH2)2–LiAlH4 [142] and Mg(NH2)2–Li3AlH6 [144] were also observed, in which 6.2 wt.% of hydrogen can be
reversibly stored in a Mg(NH2)2–Li3AlH6 combination at 300 °C.
Comparatively, the Mg(NH2)2–LiH system has attracted more attention due to its reversible nature and suitable thermodynamic
parameters [5, 116, 117, 119, 122, 123, 128, 129, 139, 140, 145, 163–178]. A few Mg(NH2)2 and LiH combinations have been
investigated thus far, which gave different reaction paths and hydrogen capacity [119, 140, 145, 179]. However, Mg(NH2)2–2LiH
provides more ‘usable’ hydrogen at lower temperatures [117, 122]. The dehydrogenation of Mg(NH2)2 and 2LiH takes place in the
temperature range of 150–250 °C, which gives 5.6 mass% hydrogen and a solid product Li2Mg(NH)2, which is a new compound of
an orthorhombic structure (see Figure 24). Due to almost identical ion radii, Li+ and Mg2+ are indistinguishable in the lattice, thus
bringing particular interest to the crystallographic analyses [180]. Pressure–composition–temperature (PCT) measurements show
that dehydrogenation of Mg(NH2)2 + 2LiH exhibits a pressure plateau and a slope region [117, 122]. The heat of hydrogen
desorption within the pressure plateau is ∼38.9 kJ mol−1 H2. The results of a van’t Hoff plot (see Figure 25) indicate that the
temperature to desorb hydrogen at 1.0 bar equilibrium pressure is ∼80 °C [162], which is clearly in the range of typical operational
temperatures of PEM fuel cells. However, there is a severe kinetic barrier that probably originates from interface reactions and mass
transport through the product layer, which provides a hurdle to low-temperature dehydrogenation [174]. Catalytic modification to
the system is challenging partly due to the catalytic additives that have to be involved in the interface reactions and/or mass
transport. Hu et al. introduced LiBH4 to the system and lowered the energy barrier. It is observed that complete dehydrogenation
and hydrogenation can be achieved at 140 and 100 °C, respectively [174]. More recently, Wang et al. introduced ∼3 mol.% K in the
Mg(NH2)2–2LiH system and enabled a full dehydrogenation and rehydrogenation cycle at a temperature near 100 °C (see
Figure 26) [178]. Theoretical investigations indicated that K may bond with N and activate the N–H bond. It is rather interesting
to understand the way K functions in the dehydrogenation and hydrogenation as it may shine a light on catalyst design for complex
hydrides.
As mentioned earlier, the diverse combinations of amides and hydrides enable a series of novel chemical processes for

hydrogen storage and production. On top of that, the superior bonding capability of N with metal and hydrogen allows the
formation of a variety of new compounds, such as Li2Mg(NH)2 [122, 180], Li2Ca(NH)2 [146, 153], MgCa(NH)2 [132],
Li4BN3H10 [93, 133], and Li2BNH6 [159]. Although a number of promising materials have been identified, the further
development of amide–hydride systems for hydrogen storage largely relies on the proper match of N–H and N–M bonds in

Figure 24 The crystal structure of Li2Mg(NH)2. Li and Mg are in purple, N in blue, and hydrogen in white.


Pressure (bar)

Hydrogen Storage: Liquid and Chemical

173

ln(P) = −4683.65/T + 13.47
ΔH = 38.9 KJ mol−1 H2
ΔS = 112 J mol−1 K−1 H2
10

1
0.0020 0.0022 0.0024 0.0026 0.0028 0.0030
1/T (K−1)

Pressure (bar)

100

10

ln(Peq /P Θ )


Figure 25 van’t Hoff plot of Mg(NH2)2-2LiH system [162].

y = −5052.50x + 14.26
Δ H = 42.0 kJmol−1 H2

2.4
1.8
1.2
0.00225

0.00240

0.00255

1/T (K−1)

130 °C

107 °C

1

Pristine sample at 107 �C
0.1
0.0

0.5

1.0


1.5

2.0
2.5
H content

3.0

3.5

4.0

Figure 26 Pressure–composition–temperature (PCT) desorption isotherms of the K-modified samples at 107 (●) and 130 °C (○) and the post-milled
pristine sample at 107 °C (□). For the pristine sample, the dehydrogenation is too slow to reach equilibrium. H content refers to the equivalent H atoms
desorbed from the sorbent. The inset shows the van’t Hoff plot of the K-modified sample [178].

amides and M–H bond in hydrides. In this regard, theoretical simulation is essential in predicting the potential systems
[181]. In the meantime, in-depth understanding of the reaction mechanisms will also significantly benefit the kinetic
optimization.

4.06.6 Pending Issues
The comparative H content in complex and chemical hydrides is high. More than 10 attracting systems have been developed within
the past 10 years [3, 83, 182], and further promising systems are continually emerging. In the meantime, considerable new
chemistry of B- and N-containing compounds has been accumulated, which will greatly facilitate the ongoing research and
development. The broad range of materials and varieties of approaches in the material optimization and engineering enable
these systems to be the most promising candidates for hydrogen storage.
There are a few common challenges in the chemical and complex hydride systems, one of which is material stability. Most of the
chemical hydrides and some of the complex hydrides are kinetically stable and are reactive to moisture and oxygen. There are also
by-products such as NH3, borazine, and B2H6 in the dehydrogenation, which are toxic and may deteriorate the performance of, for

example, a PEM fuel cell.
The other challenge the complex and chemical hydride systems have to overcome is the slow kinetics of the hydrogenation and
dehydrogenation stages, which is an intrinsic property of solid-state reactions. The kinetic problem could be more prominent for a
system of multiple phases and/or undergoing a stepwise reaction, where a catalyst is demanded. The effective catalytic modification
depends strongly on the identification of the origin of the kinetic barrier(s); therefore, mechanistic understanding of the corre­
sponding reaction is essential.


174

Hydrogen Storage: Liquid and Chemical

References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]

[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]

[48]
[49]
[50]
[51]
[52]
[53]
[54]
[55]
[56]

Schlapbach L and Zuttel A (2001) Hydrogen-storage materials for mobile applications. Nature 414: 353.
Grochala W and Edwards PP (2004) Thermal decomposition of the non-interstitial hydrides for the storage and production of hydrogen. Chemical Reviews 104: 1283.
Orimo SI, Nakamori Y, Eliseo JR, et al. (2007) Complex hydrides for hydrogen storage. Chemical Reviews 107: 4111.
Chen P, Xiong ZT, Luo JZ, et al. (2002) Interaction of hydrogen with metal nitrides and imides. Nature 420: 302.
Chen P and Zhu M (2008) Recent progress in hydrogen storage. Materials Today 11: 36.
Bogdanovic B and Schwickardi M (1997) Ti-doped alkali metal aluminium hydrides as potential novel reversible hydrogen storage materials. Journal of Alloys and Compounds
253: 1.
Gutowska A, Li LY, Shin YS, et al. (2005) Nanoscaffold mediates hydrogen release and the reactivity of ammonia borane. Angewandte Chemie International Edition 44: 3578.
Dillon AC, Jones KM, Bekkedahl TA, et al. (1997) Storage of hydrogen in single-walled carbon nanotubes. Nature 386: 377.
Dresselhaus MS, Williams KA, and Eklund PC (1999) Hydrogen adsorption in carbon materials. MRS Bulletin 24: 45.
Dillon AC and Heben MJ (2001) Hydrogen storage using carbon adsorbents: Past, present and future. Applied Physics A: Materials Science & Processing 72: 133.
Darkrim FL, Malbrunot P, and Tartaglia GP (2002) Review of hydrogen storage by adsorption in carbon nanotubes. International Journal of Hydrogen Energy 27: 193.
Rosi NL, Eckert J, Eddaoudi M, et al. (2003) Hydrogen storage in microporous metal-organic frameworks. Science 300: 1127.
Dybtsev DN, Chun H, Yoon SH, et al. (2004) Microporous manganese formate: A simple metal-organic porous material with high framework stability and highly selective gas
sorption properties. Journal of the American Chemical Society 126: 32.
Kitagawa S, Kitaura R, and Noro S (2004) Functional porous coordination polymers. Angewandte Chemie International Edition 43: 2334.
Pan L, Sander MB, Huang XY, et al. (2004) Microporous metal organic materials: Promising candidates as sorbents for hydrogen storage. Journal of the American Chemical
Society 126: 1308.
Seayad AM and Antonelli DM (2004) Recent advances in hydrogen storage in metal-containing inorganic nanostructures and related materials. Advanced Materials 16: 765.
Rowsell JLC and Yaghi OM (2005) Strategies for hydrogen storage in metal-organic frameworks. Angewandte Chemie International Edition 44: 4670.

Elias DC, Nair RR, Mohiuddin TMG, et al. (2009) Control of Graphene’s properties by reversible hydrogenation: Evidence for Graphane. Science 323: 610.
Berube V, Radtke G, Dresselhaus M, et al. (2007) Size effects on the hydrogen storage properties of nanostructured metal hydrides: A review. International Journal of Energy
Research 31: 637.
Varin RA, Czujko T, and Wronski Z (2006) Particle size, grain size and gamma-MgH2 effects on the desorption properties of nanocrystalline commercial magnesium hydride
processed by controlled mechanical milling. Nanotechnology 17: 3856.
Vajeeston P, Ravindran P, and Fjellvag H (2008) Novel high pressure phases of beta-AlH3: A density-functional study. Chemistry of Materials 20: 5997.
Alapati SV, Johnson JK, and Sholl DS (2006) Identification of destabilized metal hydrides for hydrogen storage using first principles calculations. Journal of Physical Chemistry
B 110: 8769.
Wolverton C, Siegel DJ, Akbarzadeh AR, et al. (2008) Discovery of novel hydrogen storage materials: An atomic scale computational approach. Journal of Physics: Condensed
Matter 20: 064228.
Marder TB (2007) Will we soon be fueling our automobiles with ammonia-borane? Angewandte Chemie International Edition 46: 8116.
Stephens FH, Pons V, and Baker RT (2007) Ammonia – Borane: The hydrogen source par excellence? Dalton Transactions 36: 2613.
Ahluwalia RK, Huaa TQ, Peng JK, et al. (2010) Technical assessment of cryo-compressed hydrogen storage tank systems for automotive applications. International Journal of
Hydrogen Energy 35: 4171.
Hua TQ, Ahluwalia RK, Peng JK, et al. (2011) Technical assessment of compressed hydrogen storage tank systems for automotive applications. International Journal of
Hydrogen Energy 36: 3037.
Lee H, Lee JW, Kim DY, et al. (2005) Tuning clathrate hydrates for hydrogen storage. Nature 434: 743.
Morris RE and Wheatley PS (2008) Gas storage in nanoporous materials. Angewandte Chemie International Edition 47: 4966.
Chen P, Wu X, Lin J, et al. (1999) High H2 uptake by alkali-doped carbon nanotubes under ambient pressure and moderate temperatures. Science 285: 91.
Baughman RH, Zakhidov AA, and de Heer WA (2002) Carbon nanotubes – The route toward applications. Science 297: 787.
Zhao JJ, Buldum A, Han J, et al. (2002) Gas molecule adsorption in carbon nanotubes and nanotube bundles. Nanotechnology 13: 195.
Chen BL, Ockwig NW, Millward AR, et al. (2005) High H2 adsorption in a microporous metal-organic framework with open metal sites. Angewandte Chemie International Edition
44: 4745.
Mueller U, Schubert M, Teich F, et al. (2006) Metal-organic frameworks – Prospective industrial applications. Journal of Materials Chemistry 16: 626.
Sun DF, Ma SQ, Ke YX, et al. (2006) An interweaving MOF with high hydrogen uptake. Journal of the American Chemical Society 128: 3896.
Akiba E and Iba H (1998) Hydrogen absorption by Laves phase related BCC solid solution. Intermetallics 6: 461.
Matsunaga T, Kon M, Washio K, et al. (2009) TiCrVMo alloys with high dissociation pressure for high-pressure MH tank. International Journal of Hydrogen Energy 34: 1458.
Vajo JJ, Skeith SL, and Mertens F (2005) Reversible storage of hydrogen in destabilized LiBH4. Journal of Physical Chemistry B 109: 3719.
Zuttel A, Rentsch S, Fischer P, et al. (2003) Hydrogen storage properties of LiBH4. Journal of Alloys and Compounds 356: 515.
Wolf G, Baumann J, Baitalow F, et al. (2000) Calorimetric process monitoring of thermal decomposition of B-N-H compounds. Thermochimica Acta 343: 19.

Baitalow F, Baumann J, Wolf G, et al. (2002) Thermal decomposition of B-N-H compounds investigated by using combined thermoanalytical methods. Thermochimica Acta
391: 159.
Bluhm ME, Bradley MG, Butterick R, et al. (2006) Amineborane-based chemical hydrogen storage: Enhanced ammonia borane dehydrogenation in ionic liquids. Journal of the
American Chemical Society 128: 7748.
Denney MC, Pons V, Hebden TJ, et al. (2006) Efficient catalysis of ammonia borane dehydrogenation. Journal of the American Chemical Society 128: 12048.
Clark TJ, Whittell GR, and Manners I (2007) Highly efficient colloidal cobalt- and rhodium-catalyzed hydrolysis of H3N center dot BH3 in air. Inorganic Chemistry 46: 7522.
Keaton RJ, Blacquiere JM, and Baker RT (2007) Base metal catalyzed dehydrogenation of ammonia-borane for chemical hydrogen storage. Journal of the American Chemical
Society 129: 1844.
Diyabalanage HVK, Shrestha RP, Semelsberger TA, et al. (2007) Calcium amidotrihydroborate: A hydrogen storage material. Angewandte Chemie International Edition 46: 8995.
Xiong ZT, Chua YS, Wu GT, et al. (2008) Interaction of lithium hydride and ammonia borane in THF. Chemical Communications 44: 5595.
Xiong ZT, Wu GT, Chua YS, et al. (2008) Synthesis of sodium amidoborane (NaNH2BH3) for hydrogen production. Energy & Environmental Science 1: 360.
Xiong ZT, Yong CK, Wu GT, et al. (2008) High-capacity hydrogen storage in lithium and sodium amidoboranes. Nature Materials 7: 138.
Wu H, Zhou W, and Yildirim T (2008) Alkali and alkaline-earth metal amidoboranes: Structure, crystal chemistry, and hydrogen storage properties. Journal of the American
Chemical Society 130: 14834.
Chua YS, Wu GT, Xiong ZT, et al. (2009) Calcium amidoborane ammoniate-synthesis, structure, and hydrogen storage properties. Chemistry of Materials 21: 4899.
Kim DY, Singh NJ, Lee HM, et al. (2009) Hydrogen-release mechanisms in lithium amidoboranes. Chemistry: A European Journal 15: 5598.
Shore SG and Parry RW (1955) The crystalline compound ammonia borane, H3NBH3. Journal of the American Chemical Society 77: 6084.
Hughes EW (1956) The crystall structure of ammonia borane, H3NBH3. Journal of the American Chemical Society 78: 502.
Lippert EL and Lipscomb WN (1956) The structure of H3NBH3. Journal of the American Chemical Society 78: 503.
Smythe NC and Gordon JC (2010) Ammonia borane as a hydrogen carrier: Dehydrogenation and regeneration. European Journal of Inorganic Chemistry 2010: 509.


Hydrogen Storage: Liquid and Chemical

175

[57] Hamilton CW, Baker RT, Staubitz A, et al. (2009) B-N compounds for chemical hydrogen storage. Chemical Society Reviews 38: 279.
[58] Clark TJ, Lee K, and Manners I (2006) Transition-metal-catalyzed dehydrocoupling: A convenient route to bonds between main-group elements. Chemistry: A European Journal
12: 8634.
[59] Xu Q and Chandra M (2006) Catalytic activities of non-noble metals for hydrogen generation from aqueous ammonia-borane at room temperature. Journal of Power Sources

163: 364.
[60] Clark TJ and Manners I (2007) Transition metal-catalyzed dehydrocoupling of group 13-group 15 Lewis acid-base adducts. Journal of Organometallic Chemistry 692: 2849.
[61] Fulton JL, Linehan JC, Autrey T, et al. (2007) When is a nanoparticle a cluster? An operando EXAFS study of amine borane dehydrocoupling by Rh4-6 clusters. Journal of the
American Chemical Society 129: 11936.
[62] Paul A and Musgrave CB (2007) Catalyzed dehydrogenation of ammonia-borane by iridium dihydrogen pincer complex differs from ethane dehydrogenation. Angewandte

Chemie International Edition 46: 8153.

[63] Stephens FH, Baker RT, Matus MH, et al. (2007) Acid initiation of ammonia-borane dehydrogenation for hydrogen storage. Angewandte Chemie International Edition 46: 746.
[64] Blaquiere N, Diallo-Garcia S, Gorelsky SI, et al. (2008) Ruthenium-catalyzed dehydrogenation of ammonia boranes. Journal of the American Chemical Society 130: 14034.
[65] Dietrich BL, Goldberg KI, Heinekey DM, et al. (2008) Iridium-catalyzed dehydrogenation of substituted amine boranes: Kinetics, thermodynamics, and implications for hydrogen
storage. Inorganic Chemistry 47: 8583.
[66] Yan JM, Zhang XB, Han S, et al. (2008) Iron-nanoparticle-catalyzed hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage. Angewandte Chemie

International Edition 47: 2287.

[67] Yao CF, Zhuang L, Cao YL, et al. (2008) Hydrogen release from hydrolysis of borazane on Pt- and Ni-based alloy catalysts. International Journal of Hydrogen Energy 33: 2462.
[68] He T, Xiong ZT, Wu GT, et al. (2009) Nanosized Co- and Ni-catalyzed ammonia borane for hydrogen storage. Chemistry of Materials 21: 2315.
[69] Kalidindi SB, Joseph J, and Jagirdar BR (2009) Cu2+-induced room temperature hydrogen release from ammonia borane. Energy & Environmental Science 2: 1274.
[70] Li L, Yao X, Sun CH, et al. (2009) Lithium-catalyzed dehydrogenation of ammonia borane within mesoporous carbon framework for chemical hydrogen storage. Advanced
Functional Materials 19: 265.
[71] Li YQ, Xie L, Li Y, et al. (2009) Metal-organic-framework-based catalyst for highly efficient H2 generation from aqueous NH3BH3 solution. Chemistry: A European Journal 15:
8951.
[72] Feaver A, Sepehri S, Shamberger P, et al. (2007) Coherent carbon cryogel-ammonia borane nanocomposites for H2 storage. Journal of Physical Chemistry B 111: 7469.
[73] Zhao JZ, Shi JF, Zhang XW, et al. (2010) A soft hydrogen storage material: Poly(methyl acrylate)-confined ammonia borane with controllable dehydrogenation. Advanced

Materials 22: 394.

[74] Kang XD, Fang ZZ, Kong LY, et al. (2008) Ammonia borane destabilized by lithium hydride: An advanced on-board hydrogen storage material. Advanced Materials 20: 2756.
[75] Lee TB and McKee ML (2009) Mechanistic study of LiNH2BH3 formation from (LiH)(4) + NH3BH3 and subsequent dehydrogenation. Inorganic Chemistry 48: 7564.

[76] Fijakowski KJ and Grochala W (2009) Substantial emission of NH3 during thermal decomposition of sodium amidoborane, NaNH2BH3. Journal of Materials Chemistry 19: 2043.
[77] Zhang QG, Tang CX, Fang CH, et al. (2010) Synthesis, crystal structure, and thermal decomposition of strontium amidoborane. Journal of Physical Chemistry C 114: 1709.
[78] Wu CZ, Wu GT, Xiong ZT, et al. (2010) LiNH2BH3 center dot NH3BH3: Structure and hydrogen storage properties. Chemistry of Materials 22: 3.
[79] Graham KR, Kemmitt T, and Bowden ME (2009) High capacity hydrogen storage in a hybrid ammonia borane-lithium amide material. Energy & Environmental Science 2: 706.
[80] Jensen CM and Gross KJ (2001) Development of catalytically enhanced sodium aluminum hydride as a hydrogen-storage material. Applied Physics A: Materials Science &
Processing 72: 213.
[81] Sandrock G, Gross K, and Thomas G (2002) Effect of Ti-catalyst content on the reversible hydrogen storage properties of the sodium alanates. Journal of Alloys and Compounds
339: 299.
[82] Schuth F, Bogdanovic B, and Felderhoff M (2004) Light metal hydrides and complex hydrides for hydrogen storage. Chemical Communications 40: 2249.
[83] Sakintuna B, Lamari-Darkrim F, and Hirscher M (2007) Metal hydride materials for solid hydrogen storage: A review. International Journal of Hydrogen Energy 32: 1121.
[84] Soulie JP, Renaudin G, Cerny R, et al. (2002) Lithium boro-hydride LiBH4. Journal of Alloys and Compounds 346: 200.
[85] Miwa K, Ohba N, Towata S, et al. (2004) First-principles study on lithium borohydride LiBH4. Physical Review B 69: 245120.
[86] Lodziana Z and Vegge T (2004) Structural stability of complex hydrides: LiBH4 revisited. Physical Review Letters 93: 145501.
[87] Ozolins V, Majzoub EH, and Wolverton C (2008) First-principles prediction of a ground state crystal structure of magnesium borohydride. Physical Review Letters 100: 135501.
[88] Her JH, Stephens PW, Gao Y, et al. (2007) Structure of unsolvated magnesium borohydride Mg(BH4)2. Acta Crystallographica Section B: Structural Science 63: 561.
[89] Filinchuk Y, Chernyshov D, and Dmitriev V (2008) Light metal borohydrides: Crystal structures and beyond. Zeitschrift Fur Kristallographie 223: 649.
[90] Miwa K, Aoki M, Noritake T, et al. (2006) Thermodynamical stability of calcium borohydride Ca(BH4)2. Physical Review B 74: 155122.
[91] Riktor MD, Sorby MH, Chlopek K, et al. (2007) In situ synchrotron diffraction studies of phase transitions and thermal decomposition of Mg(BH4)2 and Ca(BH4)2. Journal of
Materials Chemistry 17: 4939.
[92] Buchter F, Lodziana Z, Remhof A, et al. (2008) Structure of Ca(BD4)2 beta-phase from combined neutron and synchrotron X-ray powder diffraction data and density functional
calculations. Journal of Physical Chemistry B 112: 8042.
[93] Pinkerton FE, Meisner GP, Meyer MS, et al. (2005) Hydrogen desorption exceeding ten weight percent from the new quaternary hydride Li3BN2H8. Journal of Physical Chemistry
B 109: 6.
[94] Meisner GP, Scullin ML, Balogh MP, et al. (2006) Hydrogen release from mixtures of lithium borohydride and lithium amide: A phase diagram study. Journal of Physical
Chemistry B 110: 4186.
[95] Filinchuk YE, Yvon K, Meisner GP, et al. (2006) On the composition and crystal structure of the new quaternary hydride phase Li4BN3H10. Inorganic Chemistry 45: 1433.
[96] Aoki M, Miwa K, Noritake T, et al. (2005) Destabilization of LiBH4 by mixing with LiNH2. Applied Physics A: Materials Science & Processing 80: 1409.
[97] Vajo JJ, Salguero TT, Gross AE, et al. (2007) Thermodynamic destabilization and reaction kinetics in light metal hydride systems. Journal of Alloys and Compounds 446: 409.
[98] Bosenberg U, Doppiu S, Mosegaard L, et al. (2007) Hydrogen sorption properties of MgH2-LiBH4 composites. Acta Materialia 55: 3951.
[99] Pinkerton FE and Meyer MS (2008) Reversible hydrogen storage in the lithium borohydride-calcium hydride coupled system. Journal of Alloys and Compounds 464: L1.

[100] Jin SA, Lee YS, Shim JH, et al. (2008) Reversible hydrogen storage in LiBH4-MH2 (M = Ce, Ca) composites. Journal of Physical Chemistry C 112: 9520.
[101] Barkhordarian G, Jensen TR, Doppiu S, et al. (2008) Formation of Ca(BH4)2 from hydrogenation of CaH2+MgB2 composite. Journal of Physical Chemistry C 112: 2743.
[102] Li HW, Kikuchi K, Nakamori Y, et al. (2008) Dehydriding and rehydriding processes of well-crystallized Mg(BH4)2 accompanying with formation of intermediate compounds.
Acta Materialia 56: 1342.
[103] Li HW, Kikuchi K, Nakamori Y, et al. (2007) Effects of ball milling and additives on dehydriding behaviors of well-crystallized Mg(BH4)2. Scripta Materialia 57: 679.
[104] Chlopek K, Frommen C, Leon A, et al. (2007) Synthesis and properties of magnesium tetrahydroborate, Mg(BH4)2. Journal of Materials Chemistry 17: 3496.
[105] Nakamori Y, Li HW, Miwa K, et al. (2006) Syntheses and hydrogen desorption properties of metal-borohydrides M(BH44)n (M = Mg, Sc, Zr, Ti, and Zn; n=2-4) as advanced
hydrogen storage materials. Materials Transactions 47: 1898.
[106] Matsunaga T, Buchter F, Mauron P, et al. (2008) Hydrogen storage properties of Mg[BH4]2. Journal of Alloys and Compounds 459: 583.
[107] Ozolins V, Majzoub EH, and Wolverton C (2009) First-principles prediction of thermodynamically reversible hydrogen storage reactions in the Li-Mg-Ca-B-H system. Journal of
the American Chemical Society 131: 230.
[108] Kim JH, Jin SA, Shim JH, et al. (2008) Reversible hydrogen storage in calcium borohydride Ca(BH4)2. Scripta Materialia 58: 481.
[109] Kim JH, Jin SA, Shim JH, et al. (2008) Thermal decomposition behavior of calcium borohydride Ca(BH4)2. Journal of Alloys and Compounds 461: L20.
[110] Kim JH, Shim JH, and Cho YW (2008) On the reversibility of hydrogen storage in Ti- and Nb-catalyzed Ca(BH4)2. Journal of Power Sources 181: 140.


176

Hydrogen Storage: Liquid and Chemical

[111] Nakamori Y, Miwa K, Ninomiya A, et al. (2006) Correlation between thermodynamical stabilities of metal borohydrides and cation electronegativites: First-principles calculations
and experiments. Physical Review B 74: 045126.
[112] Hwang SJ, Bowman RC, Reiter JW, et al. (2008) NMR confirmation for formation of [B12H12]2− complexes during hydrogen desorption from metal borohydrides. Journal of
Physical Chemistry C 112: 3164.
[113] Soloveichik G, Her JH, Stephens PW, et al. (2008) Ammine magnesium borohydride complex as a new material for hydrogen storage: Structure and properties of Mg(BH4)2
center dot 2NH3. Inorganic Chemistry 47: 4290.
[114] Ichikawa T, Isobe S, Hanada N, et al. (2004) Lithium nitride for reversible hydrogen storage. Journal of Alloys and Compounds 365: 271.
[115] Xiong ZT, Chen P, Wu GT, et al. (2003) Investigations into the interaction between hydrogen and calcium nitride. Journal of Materials Chemistry 13: 1676.
[116] Leng HY, Ichikawa T, Hino S, et al. (2004) New metal-N-H system composed of Mg(NH2)2 and LiH for hydrogen storage. Journal of Physical Chemistry B 108: 8763, 12628.
[117] Luo WF (2004) (LiNH2-MgH2): A viable hydrogen storage system. Journal of Alloys and Compounds 381: 284.

[118] Nakamori Y, Kitahara G, and Orimo S (2004) Synthesis and dehydriding studies of Mg-N-H systems. Journal of Power Sources 138: 309.
[119] Nakamori Y and Orimo S (2004) Destabilization of Li-based complex hydrides. Journal of Alloys and Compounds 370: 271.
[120] Nakamori Y and Orimo S (2004) Li-N based hydrogen storage materials. Materials Science and Engineering B: Solid State Materials for Advanced Technology 108: 48.
[121] Orimo S, Nakamori Y, Kitahara G, et al. (2004) Destabilization and enhanced dehydriding reaction of LiNH2: An electronic structure viewpoint. Applied Physics A: Materials
Science & Processing 79: 1765.
[122] Xiong ZT, Wu GT, Hu HJ, et al. (2004) Ternary imides for hydrogen storage. Advanced Materials 16: 1522.
[123] Chen P, Xiong ZT, Wu GT, et al. (2005) Development of metal-N-H compounds for hydrogen storage. Abstracts of Papers of the American Chemical Society 230: U1642.
[124] Herbst JF and Hector LG (2005) Energetics of the Li amide/Li imide hydrogen storage reaction. Physical Review B 72: 125120.
[125] Hino S, Ichikawa T, Leng HY, et al. (2005) Hydrogen desorption properties of the Ca-N-H system. Journal of Alloys and Compounds 398: 62.
[126] Ichikawa T, Hanada N, Isobe S, et al. (2005) Hydrogen storage properties in Ti catalyzed Li-N-H system. Journal of Alloys and Compounds 404: 435.
[127] Isobe S, Ichikawa T, Hanada N, et al. (2005) Effect of Ti catalyst with different chemical form on Li-N-H hydrogen storage properties. Journal of Alloys and Compounds 404: 439.
[128] Nakamori Y, Kitahara G, Miwa K, et al. (2005) Hydrogen storage properties of Li-Mg-N-H systems. Journal of Alloys and Compounds 404: 396.
[129] Nakamori Y, Kitahara G, Miwa K, et al. (2005) Reversible hydrogen-storage functions for mixtures of Li3N and Mg3N2. Applied Physics A: Materials Science & Processing 80: 1.
[130] Nakamori Y, Kitahara G, Ninomiya A, et al. (2005) Guidelines for developing amide-based hydrogen storage materials. Materials Transactions 46: 2093.
[131] Pinkerton FE (2005) Decomposition kinetics of lithium amide for hydrogen storage materials. Journal of Alloys and Compounds 400: 76.
[132] Xiong ZT, Hu JJ, Wu GT, et al. (2005) Hydrogen absorption and desorption in Mg-Na-N-H system. Journal of Alloys and Compounds 395: 209.
[133] Chater PA, David WIF, Johnson SR, et al. (2006) Synthesis and crystal structure of Li4BH4(NH2)3. Chemical Communications 42: 2439.
[134] Hu JJ, Wu GT, Liu YF, et al. (2006) Hydrogen release from Mg(NH2)2-MgH2 through mechanochemical reaction. Journal of Physical Chemistry B 110: 14688.
[135] Hu JJ, Xiong ZT, Wu GT, et al. (2006) Hydrogen releasing reaction between Mg(NH2)2 and CaH2. Journal of Power Sources 159: 116.
[136] Kojima Y, Matsumoto M, Kawai Y, et al. (2006) Hydrogen absorption and desorption by the Li-Al-N-H system. Journal of Physical Chemistry B 110: 9632.
[137] Liu YF, Xiong ZT, Hu JJ, et al. (2006) Hydrogen absorption/desorption behaviors over a quaternary Mg-Ca-Li-N-H system. Journal of Power Sources 159: 135.
[138] Lu J, Fang ZZ, and Sohn HY (2006) A new Li-Al-N-H system for reversible hydrogen storage. Journal of Physical Chemistry B 110: 14236.
[139] Luo WF and Sickafoose S (2006) Thermodynamic and structural characterization of the Mg-Li-N-H hydrogen storage system. Journal of Alloys and Compounds 407: 274.
[140] Xiong ZT, Wu GT, Hu JJ, et al. (2006) Investigations on hydrogen storage over Li-Mg-N-H complex – The effect of compositional changes. Journal of Alloys and Compounds
417: 190.
[141] David WIF, Jones MO, Gregory DH, et al. (2007) A mechanism for non-stoichiometry in the lithium amide/lithium imide hydrogen storage reaction. Journal of the American
Chemical Society 129: 1594.
[142] Liu Y, Hu J, Wu G, et al. (2007) Large amount of hydrogen desorption from the mixture of Mg(NH2)2 and LiAlH4. Journal of Physical Chemistry C 111: 19161.
[143] Liu YF, Hu JJ, Xiong ZT, et al. (2007) Investigations on hydrogen desorption from the mixture of Mg(NH2)2 and CaH2. Journal of Alloys and Compounds 432: 298.
[144] Lu J, Fang ZZ, Sohn HY, et al. (2007) Potential and reaction mechanism of Li-Mg-Al-N-H system for reversible hydrogen storage. Journal of Physical Chemistry C 111: 16686.

[145] Osborn W, Markmaitree T, and Shaw LL (2007) Evaluation of the hydrogen storage behavior of a LiNH2+MgH2 system with 1 : 1 ratio. Journal of Power Sources 172: 376.
[146] Wu GT, Xiong ZT, Liu T, et al. (2007) Synthesis and characterization of a new ternary imide-Li2Ca(NH)2. Inorganic Chemistry 46: 517.
[147] Xiong ZT, Hu JJ, Wu GT, et al. (2007) Large amount of hydrogen desorption and stepwise phase transition in the chemical reaction of NaNH2 and LiAlH4. Catalysis Today 120:
287.
[148] Xiong ZT, Wu GT, Hu JJ, et al. (2007) Ca-Na-N-H system for reversible hydrogen storage. Journal of Alloys and Compounds 441: 152.
[149] Xiong ZT, Wu GT, Hu JJ, et al. (2007) Reversible hydrogen storage by a Li-Al-N-H complex. Advanced Functional Materials 17: 1137.
[150] Yang J, Sudik A, Siegel DJ, et al. (2007) Hydrogen storage properties of 2LiNH2+ LiBH4+MgH2. Journal of Alloys and Compounds 446: 345.
[151] Shaw LL, Osbom W, Markmaitree T, et al. (2008) The reaction pathway and rate-limiting step of dehydrogenation of the LiHN2+LiH mixture. Journal of Power Sources 177: 500.
[152] Sudik A, Yang J, Halliday D, et al. (2008) Hydrogen storage properties in (LiNH2)2-LiBH4-(MgH2)(X) mixtures (X=0.0-1.0). Journal of Physical Chemistry C 112: 4384.
[153] Wu H (2008) Structure of ternary imide Li2Ca(NH)2 and hydrogen storage mechanisms in amide-hydride system. Journal of the American Chemical Society 130: 6515.
[154] Yang J, Sudik A, Siegel DJ, et al. (2008) A self-catalyzing hydrogen-storage material. Angewandte Chemie International Edition 47: 882.
[155] Kojima Y and Kawai Y (2004) Hydrogen storage of metal nitride by a mechanochemical reaction. Chemical Communications 40: 2210.
[156] Pinkerton FE, Meyer MS, Meisner GP, et al. (2006) Improved hydrogen release from LiB0.33N0.67H2.67 with noble metal additions. Journal of Physical Chemistry B 110: 7967.
[157] Pinkerton FE, Meyer MS, Meisner GP, et al. (2007) Phase boundaries and reversibility of LiBH4/MgH2 hydrogen storage material. Journal of Physical Chemistry C 111: 12881.
[158] Chater PA, Anderson PA, Prendergast JW, et al. (2007) Synthesis and characterization of amide-borohydrides: New complex light hydrides for potential hydrogen storage.
Journal of Alloys and Compounds 446: 350.
[159] Chater PA, David WIF, and Anderson PA (2007) Synthesis and structure of the new complex hydride Li2BH4NH2. Chemical Communications 43: 4770.
[160] Pinkerton FE, Meyer MS, Meisner GP, et al. (2007) Improved hydrogen release from LiB0.33N0.67H2.67 with metal additives: Ni, Fe, and Zn. Journal of Alloys and Compounds
433: 282.
[161] Tang WS, Wu G, Liu T, et al. (2008) Cobalt-catalyzed hydrogen desorption from the LiNH2-LiBH4 system. Dalton Transactions 37: 2395.
[162] Xiong ZT, Hu JJ, Wu GT, et al. (2005) Thermodynamic and kinetic investigations of the hydrogen storage in the Li-Mg-N-H system. Journal of Alloys and Compounds 398: 235.
[163] Ichikawa T, Tokoyoda K, Leng HY, et al. (2005) Hydrogen absorption properties of Li-Mg-N-H system. Journal of Alloys and Compounds 400: 245.
[164] Ichikawa T, Leng HY, Isobe S, et al. (2006) Recent development on hydrogen storage properties in metal-N-H systems. Journal of Power Sources 159: 126.
[165] Leng H, Ichikawa T, and Fujii H (2006) Hydrogen storage properties of Li-Mg-N-H systems with different ratios of LiH/Mg(NH2)2. Journal of Physical Chemistry B 110: 12964.
[166] Aoki M, Noritake T, Kitahara G, et al. (2007) Dehydriding reaction of Mg(NH2)2-LiH system under hydrogen pressure. Journal of Alloys and Compounds 428: 307.
[167] Janot R, Eymery JB, and Tarascon JM (2007) Investigation of the processes for reversible hydrogen storage in the Li-Mg-N-H system. Journal of Power Sources 164: 496.
[168] Liu YF, Hu JJ, Xiong ZT, et al. (2007) Improvement of the hydrogen-storage performances of Li-Mg-N-H system. Journal of Materials Research 22: 1339.
[169] Nakagawa T, Ichikawa T, Iida R, et al. (2007) Observation of hydrogen absorption/desorption reaction processes in Li-Mg-N-H system by in-situ X-ray diffractmetry. Journal of
Alloys and Compounds 430: 217.
[170] Sorby MH, Nakamura Y, Brinks HW, et al. (2007) The crystal structure of LiND2 and Mg(ND2)2. Journal of Alloys and Compounds 428: 297.

[171] Tsumuraya T, Shishidou T, and Oguchi T (2007) First-principles study on lithium and magnesium nitrogen hydrides for hydrogen storage. Journal of Alloys and Compounds
446: 323.
[172] Wang Y and Chou MY (2007) First-principles study of cation and hydrogen arrangements in the Li-Mg-N-H hydrogen storage system. Physical Review B 76: 014116.


Hydrogen Storage: Liquid and Chemical

177

[173] Yang J, Sudik A, and Wolverton C (2007) Activation of hydrogen storage materials in the Li-Mg-N-H system: Effect on storage properties. Journal of Alloys and Compounds
430: 334.
[174] Hu JJ, Liu YF, Wu GT, et al. (2008) Improvement of hydrogen storage properties of the Li-Mg-N-H system by addition of LiBH4. Chemistry of Materials 20: 4398.
[175] Isobe S, Ichikawa T, Leng HY, et al. (2008) Hydrogen desorption processes in Li-Mg-N-H systems. Journal of Physics and Chemistry of Solids 69: 2234.
[176] Liu YF, Hu JJ, Wu GT, et al. (2008) Formation and equilibrium of ammonia in the Mg(NH2)2-2LiH hydrogen storage system. Journal of Physical Chemistry C 112: 1293.
[177] Ma LP, Wang P, Dai HB, et al. (2009) Catalytically enhanced dehydrogenation of Li-Mg-N-H hydrogen storage material by transition metal nitrides. Journal of Alloys and
Compounds 468: L21.
[178] Wang JH, Liu T, Wu GT, et al. (2009) Potassium-modified Mg(NH2)2/2 LiH system for hydrogen storage. Angewandte Chemie International Edition 48: 5828.
[179] Leng HY, Ichikawa T, Hino S, et al. (2004) New metal-N-H system composed of Mg(NH2)2 and LiH for hydrogen storage. Journal of Physical Chemistry B 108: 8763.
[180] Rijssenbeek J, Gao Y, Hanson J, et al. (2008) Crystal structure determination and reaction pathway of amide-hydride mixtures. Journal of Alloys and Compounds 454: 233.
[181] Alapati SV, Johnson JK, and Sholl DS (2007) Using first principles calculations to identify new destabilized metal hydride reactions for reversible hydrogen storage. Physical
Chemistry Chemical Physics 9: 1438.
[182] Chen P, Xiong ZT, Wu GT, et al. (2007) Metal-N-H systems for the hydrogen storage. Scripta Materialia 56: 817.