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Annual Reports on the Progress of Chemistry, Section A Vol 105
Inorganic Chemistry
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Hydrogen storage materials: present scenariosand future directions
Tapas K. Mandal and Duncan H. Gregory*
DOI: 10.1039/b818951j
This review describes the present state of contemporary solid state hydrogen
storage on the basis of research carried out during the last decade. The
article focuses on the key aspects of materials based on the physical and
chemical storage of hydrogen and emerging mechanisms for reversible
storage. Among chemical storage materials, we consider metal hydrides
(both light and complex), nitrides-imides-amides and other multi-
component systems and discuss the emergence of coordination polymers
(metal organic frameworks; MOFs) among solids exhibiting physicalstorage. Significant challenges remain if we are to meet the practical
demands required of a solid state storage system, namely high storage
density together with favourable sorption thermodynamics and kinetics and
prolonged cycleability and lifetime. This review emphasises both how our
understanding of the storage mechanism (as a process or phenomenon
during hydrogen cycling) is evolving and how this understanding impacts on
future materials design. The prospect of high capacity storage and fast
kinetics in nanostructured materials is highlighted as is the role of complex,
multi-component, composite systems in future hydrogen storage research.
1. Introduction
At the juncture of the 20th and 21st centuries it was realised that there was an urgent
requirement for an alternative and sustainable energy vector as reserves of fossil
fuels faded.1,2 Moreover, if the adverse effects of global warming and consequent
climate change is to be arrested then the utilisation of green and renewable energy
sources is imperative. As Grochala and Edwards and later van den Berg and Area n
note in their articles,3 Jules Verne first brought these issues to the attention of thepublic in a work of fiction over 100 years ago: . . . water will one day be employed as
a fuel, that hydrogen and oxygen will constitute it, used singly or together, will furnish
an inexhaustible source of heat and light and water will be the coal of the future
(The Mysterious Island, Jules Verne, 1874). Vernes work was to presage with
uncanny pertinence the role of hydrogen as the fuel of the future and predict the
coming of the so-called hydrogen economy.
An uninterrupted and secure energy supply for the developed nations and meeting
the increasing energy demands of the rapidly developing nations are essential. The
burgeoning need for energy coupled with the rapid depletion of fossil fuels pose
serious threats for sustainable development. Before the potential for hydrogen as a
West CHEM, Department of Chemistry, Joseph Black Building, University of Glasgow,Glasgow, UK. E-mail: d.gregory@chem.gla.ac.uk; Fax: +44 (0)141 330 8128;Tel: +44 (0)141 330 4888
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future energy carrier can be realised, fundamental research including components of
invention and discovery, subsequent implementation of new technology and socio-
economic acceptance must occur. These are the key steps to the hydrogen energy
transition.
The greater hydrogen energy picture centres around the production and storage of
hydrogen, the two most important steps that currently represent a bottle neck to
utilization of hydrogen more widely in fuel cell systems. It must be realized thatunlike coal or oil, hydrogen is not naturally available. Thus, stable large-scale
hydrogen production is necessary for the gradual switchover to the hydrogen
economy. However a safe, efficient and economic storage medium is likely to be a
crucial prerequisite before hydrogen would be globally acceptable as a fuel,
particularly in mobile (automotive) applications. It should be emphasised here that
while gaseous or liquid hydrogen is currently an option for prototype personal
vehicles (cars) or larger commercial transport, solid state storage of hydrogen is
potentially superior with regard to its storage capacity (both gravimetric and
volumetric), energy efficiency and safety.2 Nevertheless compact storage of hydrogen
in a solid medium is the most demanding and challenging part of realising thehydrogen economy as far as mobile applications are concerned (Fig. 1).2b
Hydrogen storage is thus a key research area where considerable international
effort is concentrated. Since this review is written from a materials perspective,
current chemical research in hydrogen storage materials will be highlighted captur-
ing the discoveries and developments over the past decade (19982009), the many
opportunities that could be seized and the future pathways that could be taken.
Although many of the materials classes meet the majority of the US Department of
Energy (US-DoE) criteria for vehicular applications (e.g. hydrogen storage capacity;
the amount of hydrogen stored per unit mass and per unit volume),4 other factors
such as non-reversibility, slow kinetics and sometimes thermodynamic barriers to
hydrogen uptake-release limit or prevent their practical use. Holistic and systematic
research towards understanding mechanism, structure and thermodynamics and
Fig. 1 Alternatives for storage of 4 kg hydrogen, with volume relative to the size of a car.
(Reprinted from ref. 2b, with permission from Macmillan Publishers Ltd.)
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their inter-relationships are crucial to innovation and materials development. This
review illustrates the extent of emerging materials discovery and design and
demonstrates how an improved chemical understanding of storage processes informs
an evolving materials design strategy.
2. Chemical storage
The chemical storage of hydrogen concerns materials where hydrogen is chemi-cally bound in a compound. Therefore, the loading and unloading of hydrogen
(hydrogenationdehydrogenation) in this class of compound requires a chemisorption
step prior to the absorption of hydrogen in the bulk and the formation of chemical
bonds to hydrogen. In contrast, physisorption (see section 3) is responsible in cases
where molecular hydrogen is only physically adsorbed on the surface of a solid or in
the internal volume of porous/framework structures. In the chemical storage scenario,
hydrogen is strongly bound while in physisorption it is so weakly held that often very
high pressures or low temperatures are required, making physisorbed materials much
less attractive for practical uses at ambient or elevated temperatures.
Among chemical storage materials, families of metal alloys and their hydrides
constitute a core grouping that has been widely investigated for over a century.3,5 In
general, the storage capacities of these materialswhich often contain f- and/or
d-block elementsare too low to meet the DOE targets for vehicular applications
but could find utility in stationary applications due to sometimes excellent reversi-
bility. For example, Mg2Ni (with a gravimetric capacity of 3.6 wt% hydrogen) and
LaNi5 (1.28 wt%) are two classic alloy systems that are noteworthy due to their
extraordinary cycleability.5 Light metal binary hydrides, however, are considered as
an important class with respect to high hydrogen storage density but are plagued by
high desorption temperatures, non-reversible thermodynamics and poor kinetics.6
MgH2 is one of the binary hydrides that still attracts huge attention owing to its light
weight and high gravimetric density even given its kinetic and thermodynamic
limitations for storage.7 On the contrary, complex chemical hydrides offer tremen-
dous opportunities over the binaries in many respects.6,810 Among them, the
borohydrides,810,11 aluminohydrides (alanates)6,810 and amides810,12,13 are close
to meeting US-DoE targets in terms of storage capacity, reversibility, cost and
toxicity. The complex hydrides divide rather neatly into 2 groups depending on their
composition and bonding: borohydrides and alanates, (for example LiBH4 and
LiAlH4) are hydridic in nature and contain hydrogen within complex anions
([BH4], [AlH4]) that can be represented as Hd. In contrast, the imides andamides, e.g. Li2NH, LiNH2 are protonic and contain hydrogen within complex
anions ([NH]2, [NH2]) that can be represented as Hd+.These two distinct and
discrete classes of complex hydride are covered separately in the sections below.
2.1 Light metal hydrides
Among the light metal binary hydrides, magnesium hydride (MgH2) is the most
attractive due to its low atomic weight, high hydrogen storage capacity and low
cost.7 In Table 1, the gravimetric and volumetric density along with hydrogen
desorption temperature and reversibility for selected light binary hydrides arecompared with that of gaseous and liquid hydrogen.3a,7,14 Although, MgH2 has a
high gravimetric capacity of 7.6 wt%, it suffers from slow kinetics and a high
decomposition temperature (B330 1C). A huge body of research has been devoted to
improving its performance.7,1527
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Particle size reduction by ball milling, alloying or doping are among the most
common approaches that are being pursued to speed up the kinetics in the Mg-Hsystem.15,16 Catalytically enhanced systems show great promise for improved
hydrogen storage characteristics and mechanical milling has been employed to
prepare catalysed MgH2 composites.1719 Various additives include carbon,15f,21
transition metals,15d,22 oxides23,24 and halides.25 Milled Ni-doped19 MgH2 shows a
6.5 wt% storage capacity at 150250 1C and a remarkable improvement in sorption
was reported for 1 mol% Nb2O5 doped milled MgH2 with high desorption kinetics
at 160 1C under a helium flow.20 Further, a recent investigation of the effect of ZrNi
activated alloys on milled MgH2 indicated significant improvements in desorption
kinetics.26 By comparing several compositions in this latter alloy system, it has been
argued that the alloys are playing a crucial catalytic role distinct from grain size
effects induced by milling.26 Although, the development of suitable catalysts has
drastically reduced the charge-discharge times, the thermodynamics of the process
remain almost unaltered.
Chemical destabilisation27 of MgH2 and many other complex hydrides in a multi-
component mixture with designed reaction paths is a fairly new development that will
be discussed separately below under multinary systems (section 2.4). However, it must
be pointed out here that chemical destabilisation of MgH2 by alloying with Cu dates
back to the work of Reilly and Wiswall in 1967.28 They reversibly hydrogenated Mg2Cu
to MgH2 and MgCu2 at 2401C (which is
B
401C lower than that of pure MgH2) withan equilibrium pressure of 1 bar.28 Similarly, in a recent study, alloying with Si was
shown to destabilise MgH2 significantly forming Mg2Si upon dehydrogenation.29 But
the MgH2/Si system suffers from slow kinetics at 150 1C, which is the required
temperature for hydrogenation at 100 bar.29 Another very recent emerging trend in
the MgH2 based materials is to chemically nanostructure hydride materials10,30 and
this approach is highlighted in section 4 under New materials for hydrogen storage.
These sections in particular emphasise the many opportunities that exist to improve
reversibility and thermodynamics in light-weight metal hydrides.
2.2 Complex hydridic hydrides, borohydrides and alanates ([BH4] and [AlH4]
)
Hydrides containing complex anionic units with light elements like Al, B and
achieving charge neutrality through ionic or partially covalent bonding with a more
electropositive cation, such as, Li, Na, K, Mg and Ca, have been classified as
Table 1 Hydrogen content of simple binary hydrides in comparison with gaseous and liquid
hydrogen.3a,7,14 Tdec and kinetic reversibility of the hydrides are also given.
Compound
Wt%
H2
Volumetric density NH(atoms H/ml 1022)
Tdec(1C)
Kinetic
reversibility
H2, liquid 100 4.2
H2, gas
(100 atm)
100 0.49
LiH 12.6 5.3 720 Poor
NaH 4.2 2.3 425 Good
CaH2 4.8 5.1 600 Good
MgH2 7.6 6.7 330 Very poor
AlH3 10.0 8.84 150 Irreversible
TiH2 4.0 9.1 380 a
a No literature data available.
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so-called chemical hydrides. In these complex hydrides, the hydrogen within the
anionic unit resides at the vertices of a tetrahedron with B or Al at the centre to form
tetrahydroborates e.g. Li(BH4), Mg(BH4)2 or tetrahydroaluminates (also known as
alanates), e.g. Li(AlH4), Na(AlH4). Although these tetrahydrometallates constitute
an interesting storage class, the very large gravimetric and volumetric hydrogen
densities (Table 2) that they exhibit is countered by their thermal stability and high
desorption temperature which often makes them unsuitable for application. Thedesorption temperature and gravimetric hydrogen densities for some selected
complex hydrides are shown in Table 2.
Of these, the lightest borohydride LiBH4 has attracted enormous interest as a
candidate hydrogen store due to its extremely high gravimetric hydrogen density
(18.4 wt%),8,3234 although as yet it has not been possible to overcome the profound
thermodynamic, kinetic and reversibility challenges that would enable it to be
employed as a practical storage material.
The first thermal desorption of hydrogen from LiBH4 was reported in 1964 by
Fedneva et al.35 and four years later Stasinevich and Egorenko36 showed that LiBH4
releases hydrogen at temperatures greater than 470 1C. According to Fedneva et al.35
the differential thermal analysis profile of LiBH4 consists of three endothermic
events. While the first reversible endothermic effect at 108112 1C is attributed to a
polymorphic transformation of LiBH4, the second peak at 268286 1C corresponds
to melting with the onset of gradual decomposition of LiBH4.35 The main hydrogen
release starts at 380 1C liberating 80% of the hydrogen in LiBH4. Moreover, another
thermal event at around 490 1C was unexplained. However, in their study,
Stasinevich and Egorenko conducted the thermal analysis under hydrogen pressures
of 10 bar and according to them, the dehydrogenation reaction of LiBH4 releases
only 3 of the 4 hydrogen atoms above 268 1C.36 This process can be represented as:
LiBH4- LiH + B + 3/2 H2 (1)
Analogous decomposition pathways are observed for other alkali metal boro-hydrides (NaBH4 and KBH4) but with an increase in the decomposition temperaturewith increasing atomic number of the alkali metal. In a later study, Zu ttel et al.demonstrated that three distinctive desorption peaks exist during the dehydro-genation of LiBH4 when performed at a low heating rate.
37 This suggests a morecomplicated mechanism and the presence of other intermediate phases during the
Table 2 Desorption temperature and gravimetric storage capacities of complex chemical
hydrides3a,8
Compound Tdes (1C)
Storage
capacity (wt%)
LiBH4 380 18.4
NaBH4 400 10.6
KBH4 500 7.4
Mg(BH4)2 260280 14.8
Ca(BH4)2 320 11.5
Al(BH4)3 20 16.8
LiAlH4 125 9.5
NaAlH4 210 7.4KAlH4 270 5.7
Mg(AlH4)2 140200 9.3
Ca(AlH4)2 250a 7.8
a For complete desorption from neat Ca(AlH4)2.31
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decomposition. Only recently, has the involvement of a borohydride cluster,Li2B12H12, been elucidated as an intermediate in the thermal desorption ofLiBH4.
38,39 A series of systematic theoretical and experimental studies has beencarried out to attempt to rationalise the thermodynamic stabilities of metalborohydrides, M(BH4)n (M = Li, Na, K, Mg, Cu, Zn, Sc, Zr and Hf).
40 A goodcorrelation has been observed between desorption temperature (Td), the enthalpy ofdesorption (DHdes) and the Pauling electronegativity (wP). Similar studies have been
performed for Ca(BH4)2 and it was established that the ionic bonding exists betweenCa and the covalent [BH4]
unit and that there is a linear relationship between theheat of formation and wP of the cation.
41 The same study also predicted a possibledecomposition pathway as:
Ca(BH4)2- 2/3 CaH2 + 1/3 CaB6 + 10/3 H2 (2)
with a calculated gravimetric capacity of 9.6 wt% of hydrogen. Thermogravimetricanalysis leant support to the proposed reaction, yielding an experimental weight lossof 9.2 wt% and suggested that an intermediate species could be involved in thedecomposition.41 Although the CaBH system has a high storage capacity,
subsequent studies indicated that the dehydrogenation starts only at 640 K andthe kinetics of dehydrogenation is very slow.42,43
Various approaches have been taken to address the thermodynamics and kinetics
of hydrogenationdehydrogenation cycling in LiBH4 and the other complex
borohydrides. These approaches were very much triggered by the chemical activa-
tion of NaAlH4 by Bogdanovic and Schwickardi in their seminal paper of 1997.44 In
their pioneering study they showed that 2% Ti-catalyzed NaAlH4 (sodium alanate)
undergoes reversible dehydriding-rehydriding reactions in the solid state under
moderate conditions.44 The hydrogen desorption takes place in two steps.
3 NaAlH42 Na3AlH6 + 2 Al + 3 H2 (3)
Na3AlH62 3 NaH + Al + 3/2 H2 (4)
While the first step (3) amounts to 3.7 wt% hydrogen (with DH= 37 kJ mol1 H2),
the second step (4) releases 1.8 wt% (with DH= 47 kJ mol1 H2) corresponding to a
total of 5.5 wt% of hydrogen. The enthalpy values correspond to dehydrogenation
temperatures of about 35 and 100 1C, respectively.45 The theoretical storage capacity
of 7.4 wt% for the NaAlH4 system can be achieved if the remaining hydrogen stored
in NaH is released. This, however, requires temperatures greater than 500 1C due to
the very high stability of NaH. In fact, the two step decomposition mechanism of
NaAlH4 to NaH, Al and H2 at 210220 and 2501
C respectively was first demon-strated over 40 years ago.46 Subsequent studies revealed that similar two step
decomposition reactions also take place for LiAlH4 and KAlH4.4752 Although,
there have been reports of reversibility in these systems,51 the kinetics are extremely
slow. Further, relatively harsh conditions are required (200400 1C and 1040 MPa)
for cycling and the reversibility is essentially only partial.
The first examples of the use of titanium to catalyse dehydrogenation reactions
took place in solution and include, for example, the Ti-catalysed dehydrogenation of
LiAlH4 in ether by Wieberg et al. in 1951.53 However, the above discovery of
Ti-catalysis for reversible hydrogen storage in the solid state under favorable thermo-
dynamic conditions, as first represented by the alanates,44 has created a vast amountof fresh research activity in main group complex hydrides. Many other catalytic
dopants have since been studied, but no other metal-containing phases appear to
outperform those containing Ti. Use of different additive compositions as a source for
Ti impregnation/dispersion in NaAlH4 have produced results with some variance in
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performance and catalytic enhancement and the doping methods themselves are also
found to exert an influence. For example, Jensen and Gross have shown that the onset
of dehydriding reaction differs appreciably (Fig. 2) for solution doped and mechani-
cally doped NaAlH4 materials.54 A nanocrystalline NaAlH4, synthesised directly by
ball milling of NaH/Al with TiF3 catalyst under 1525 bar of hydrogen, demonstrated
fast kinetics with a high reversible capacity of 4.7 wt%.55
Moreover, recently, Fang et al.56 reported a chemical activation of LiBH4 by
mechanically milling 3 equivalents of LiBH4 with 1 equivalent of TiF3 that can
rapidly release 5.7 wt% hydrogen at moderate temperatures (7090 1C) without any
other gas impurities. However, the isothermal desorption profiles indicated that it
takes roughly an hour to fully release the hydrogen. Recently, theoretical57,58 and
experimental5961 investigations have been performed on the Group-II metal
(Mg and Ca), Group-III metal (Al) and mixed LiMgCa borohydrides owing to
their potentially high storage capacity (Table 2). While the dehydriding reaction of
pure Mg(BH4)2 starts at nearly 500 K, the addition of TiCl3 decreased the desorptiontemperature to 361 K,59 whereas the ball-milling had no effect on the dehydriding
temperature.59a Also, reversible storage in TiCl3-catalysed Ca(BH4)2 has been
demonstrated with a capacity of 3.8 wt%, when the Ca(BH4)2 was rehydrogenated
at 623 K under 90 bar for 24 h.60 This improved rehydrogenation condition in
Ca(BH4)2 suggested further study to exploit its full potential. New reversible
hydrogen storage reactions have been predicted in the mixed LiMgCa boro-
hydride systems.57b There remain ample opportunities for further performance
optimisation and certainly there is a need to understand much more regarding the
catalytic processes involved in Ti addition to alanates and borohydrides.
2.3 Complex protonic hydrides; nitrides, imides and amides
Nitrides and their conjugate hydrogenated compounds imides and amides form the
second class of complex hydrides although strictly these materials are not hydrides.
The discovery of reversible hydrogen storage in Li3N by Chen et al.62 in 2002
Fig. 2 Temperature programmed desorption of hydrogen from NaAlH4 doped with 2 mol%
of Ti(OBun)4. (Reprinted from ref. 54 with permission from Springer)
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spurred intense activity in investigating the LiNH system and subsequently
quaternary LiANH (where A = Group I and Group II metals) imide-amide-based
systems.63101
The key chemical processes that occur during the hydrogenation and dehydro-
genation of Li3N are represented by eqn (5).
Li3N + 2 H22 Li2NH + LiH + H22 LiNH2 + 2 LiH (5)
Although the theoretical gravimetric hydrogen storage capacity of this system is11.5 wt%, experimentally, the amount of stored hydrogen is less by B2 wt%,effectively storing between 9.310 wt% of hydrogen (Fig. 3).
In spite of the fact that the above chemical process offers potentially very high
storage capacity, the system suffers from various drawbacks in terms of high
operational temperature (especially in desorption), reversibility (only the imide-
amide step is reversible at practical working temperatures) and poor kinetics.
Whereas the second step in eqn (5) involving the interconversion of imide and
amide is readily reversible, the reverse dehydrogenation step to revert to the parent
lithium nitride and hydrogen occurs at high temperature, under low pressure(4320 1C at 105 mbar). Immediately following the report of the (de)hydrogenation
reactions in the LiNH system, a major research effort focused on reducing
the desorption temperature for LiNH2/LiH, optimising performance and latterly
attempting to understand the storage mechanism.6370
Pressure-composition (P-C) measurements performed on lithium imide show that
B6.5 wt% of hydrogen can be reversibly stored at 255 1C according to the following
reaction.
Li2NH + H22 LiNH2 + LiH (6)
The enthalpy change for this reaction is calculated to be 44.5 kJ mol1
H2 leadingto favourable and reversible reaction thermodynamics.70 However, the temperature
Fig. 3 Gravimetric hydrogen absorption (Abs) and desorption (Des) curves in the
lithium nitride-imide-amide system (Reprinted from ref. 62, with permission from Macmillan
Publishers Ltd.).
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of hydrogen cycling remains high for practical on-board storage. Therefore,
chemical destabilisation of LiNH2 itself was an early target in developing new
materials with low desorption temperatures (Tdes). The substitution of more
electronegative elements than Li in LiNH2 was suggested66,68 to weaken the bonding
between Li+ and [NH2] thereby reducing Tdes. Synthesis of Mg-substituted LiNH2
from LiMg alloys and investigation of dehydrogenation reactions showed a drastic
decrease of desorption temperature fromB
2501
C in pure LiNH2 to an onsettemperature ofB100 1C in (Li,Mg)NH2.71
However, a theoretical study of the modified amides, (Li,Mg)NH2, showed that
the NH bonding is weakened in (Li,Mg)NH2 compared to LiNH2.72 In a later
study, Zhang and Alavi pointed out that the relative strength of the metalN
bonding was the key factor in explaining the experimental results of reduced
desorption temperature in Mg-substituted LiNH2.73
In an effort to gain mechanistic understanding of the hydrogen storage process in
the reaction between LiNH2 and LiH (eqn (6)), the work of Hu and Ruckenstein64,65
and Ichikawa et al.67 has led to the proposition of an ammonia mediated mechanism
(eqn (7) and (8)).
2 LiNH2- Li2NH + NH3 (7)
NH3 + LiH- LiNH2 + H2 (8)
It is worth pointing out that the above elementary two step pathway was confirmed
recently by in situ 1H NMR spectroscopy.70c However, others63,74,75 believed the
dehydrogenation reaction to be redox or acidbase type where protonic hydrogen
(Hd+) in LiNH2 and hydridic hydrogen (Hd) in LiH combine to give molecular
hydrogen (H2). The exothermic combination of H
and H
+
probably drives thehydrogen desorption reaction,3a which also successfully explains the chemical
destabilisation in amide-hydride systems.
H+ + H- H2 (DH = 17.37 eV) (9)
This, in practice, was a major driving force in the future materials design of complex
hydride systems, most notably in multi-component systems where chemical tuning of
amides through doping and use of hydrides and borohydrides as secondary
components in composite mixtures have met with some success. (These are discussed
in section 2.4 in more detail).Among other approaches, the use of catalysts and modification of the micro-
structure via mechanical milling have been adopted to improve the performance of
nitrogenous systems. While Ichikawa et al.70 studied the effect of ball milling and
catalysts on the desorption of hydrogen, Hu and Ruckenstein76 investigated the
storage performance of amides based on preparative routes. Ichikawa et al.67,70 have
shown that upon ball milling the hydrogen desorption temperature range decreases
to 180400 1C. Moreover, when catalysts were used during ball milling, the
desorption temperature was reduced further and evidence of ammonia evolution
could be almost negligible. The best performance was obtained when 1% of TiCl 3
was added as catalyst resulting in 5.5 wt% of hydrogen being released between150250 1C with no ammonia formation.67 Importantly, however, the process by
which these catalyst additives work and the mechanism by which they influence
reaction pathways is not yet known. In fact, the connections between processes at the
solid surface and those that occur in the bulk are vital ones to be made if these and
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other storage materials are to be understood and a step change in performance
effected.
Among several imide-amide systems, the role of structural modification on
hydrogen storage kinetics in simple Li2NH itself is not well understood. Recently,
Yang et al. have studied the crystal structure and phase transitions of a-, b- and
g-Li2NH.90 It was mentioned that the phase transitions in Li2NH is dependent on the
synthetic methods. Moreover, among all the modifications, the a-phase showedhigher reversible hydrogen storage capacity (6.8 wt%) and faster kinetics.76
Fig. 4 Hydrogen absorption at 230 1C and 7 atm initial hydrogen pressure (A) a-Li2NH,
(B) b-Li2NH and (C) g-Li2NH. In (B): a, b and c corresponds to Li2NH synthesised by LiNH2decomposition under vacuum at 230, 280 and 350 1C, respectively (Reprinted from ref. 76b with
permission from American Chemical Society).
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Volumetric hydrogen desorption studies show that while a-Li2NH absorbs
5.4 wt% in 10 min, the b- and g-polymorphs display much slower kinetics with less
than 2 wt% and 4 wt% reversible hydrogen capacity, respectively (Fig. 4).76
Thehigher storage capacity and faster kinetics have been attributed to the particle size
that varies with thermal treatments under the synthesis condition.76 Nevertheless, it
was also found that doping with Mg or Ca can reduce the dehydrogenation
temperature of hydrogenated Li2NH.77,95,99
The structural and compositional changes in the LiNH system have been
investigated by ex situ synchrotron X-ray diffraction to elucidate the processes that
occur during hydrogen uptake-release cycling in the solid state.93 The hydrogenation
dehydrogenation of the imide-amide system occurs through non-stoichiometric
intermediates, Li2xNH1+x (0 r x r 1), based on a cubic anti-fluorite structure
(Fig. 5). More importantly, the process seems to generate Frenkel-type defects,which in turn creates a mechanism for Li+ and H+ ion mobility. Subsequent
structural studies by in situ neutron diffraction during deuterationdedeuteration
of Li3N indicated cell volume changes of Li2NH that was attributed to defect
formation in cubic Li2NH, implying non-stoichiometric compounds.94
A recent study98 by synchrotron X-ray and neutron diffraction of the LiMgNH
system with the amide-hydride mixtures identified a mixed imide a-Li2Mg(NH)2phase that has a superstructure based on the high temperature cubic anti-fluorite
Li2NH.102,103 The replacement of 2 Li atoms by one Mg atom creates ordered cation
vacancies and results in a Li2NH supercell. The imide protons are also ordered,
orienting the NH bonds between two vacant cation sites.98 At higher temperatures
this a-Li2Mg(NH)2 phase undergoes a structural transition to b- and g-Li2Mg(NH)2(similar to the high temperature Li2NH phase)
104 where the Li, Mg and vacancies are
randomly distributed over the tetrahedral sites. The structural transitions are
believed to be driven by progressive disordering of the cations and vacancies as
the temperature increases.98
A dramatic particle size-dependent catalytic enhancement of hydrogen absorption
desorption has recently been reported in the LiMgNH system.101 A three-
dimensional diffusion-controlled kinetic mechanism has been identified for the
dehydrogenation process. The hydrogenation and isothermal dehydrogenationcurves (Fig. 6) reveal that the onset of hydrogen absorption was, remarkably,
lowered to 80 1C for a sample ball-milled for 36 h and consequently the dehydro-
genation kinetics at 160 1C were notably improved compared to 3 h milled or hand
milled samples.101
Fig. 5 Structural transformation of lithium imide, Li2NH, to lithium amide, LiNH2, via
non-stoichiometric intermediates (Reprinted from ref. 12b, with permission from The Japan
Chemical Journal Forum and Wiley Periodicals).
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From the close similarity of the mechanisms in the LiNH and LiMgNHsystems, it is apparent that the vacancy distribution, defect concentration and the
ionic mobility of Li+ and H+ play an important role in the storage processes.
Recently, the crystal structure of a ternary imide, Li2Ca(ND)2, was determined by
NPD.100a The structure of Li2Ca(ND)2 resembled a combined imide structure
with an ordered arrangement of Ca[NH]6 octahedra and Li[NH]4 tetrahedra
alternating along the c-axis, distinctly different from any of the polymorphs of
Li2Mg(NH)2.100a The Li vacancy was attributed to defect reactions at elevated
temperatures and the dehydrogenation mechanism was proposed to be mediated by
small mobile ion migration in both amide and hydride.27b,100 Similar factors may
well be fundamental to other complex hydride systems and could eventually lead tothe improved understanding and better design of materials. Further work is required
with respect to understanding and exploiting the defect chemistry, complex
phase behaviour and the involvement of chemical intermediates in these and similar
systems.
Fig. 6 (A) Gravimetric hydrogen uptake versus temperature for Li2MgN2H2 as a function of
milling time and (B) Isothermal dehydrogenation curves (at 160 1C) of hydrogenated
Li2MgN2H2 samples (Reprinted from ref. 101, with permission from American Chemical
Society).
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2.4 Other multi-component systems: amide-hydride-borohydrides
The underlying strategy behind employing more than one component is to alter the
overall thermodynamics of hydrogen release, lowering Tdes, in addition to improving
the kinetics of sorption. Indeed, it has been observed that in multinary systems the
hydrogen uptake-release temperatures are lower and the kinetics often better than
for the individual component systems. Performance is highly dependent on the
reaction pathway of hydrogenation and dehydrogenation, together with the stabilityof the product formed in the dehydrogenated state.
The report of reversible hydrogen storage by Chen et al.62,63 in a mixture of LiH
and LiNH2 initiated a flurry of research activity, which later opened up a number of
diverse routes for complex hydride destabilization in the form of binary, ternary or
multinary reactive mixtures involving amides, hydrides, borohydrides and alanates
with substantial improvements in (de)hydrogenation thermodynamics.105137 The
various multinary combinations studied can be classified mainly as binary
(two component) and ternary (three component) systems. Moreover, the binary
systems can be considered as simple when the combination involves a single metal
hydride and an imide/amide, as compared to complex binaries where a hydride
containing a complex anion, such as a borohydride or aluminohydride, is involved.
As the number of components increases, the system becomes more and more
complex often due to the increased number of intermediates (and/or reaction steps)
involved in the hydrogenationdehydrogenation cycles and the non-trivial interplay
between structures and reactivities.
Simple binary component systems: Amides-hydrides. The lithium amidelithium
hydride [LiNH2 + LiH] combination, containing both protonic and hydridic
components was perhaps the first step towards a reversible and light-weight multi-component hydrogen store.62,63 The reactions with varying amide:hydride ratios and
their mechanisms have been widely investigated. With the realisation that the
LiNH2LiH system was restricted in its ability to release hydrogen under favourable
and practical conditions, several alternative binary combinations of amides and
hydrides have been investigated.74,105133 In addition to reacting Group-II hydrides
such as MgH2 or CaH2 with LiNH2, another option was to react binary hydrides
(such as LiH) with Mg(NH2)2 with the reasoning that the latter amide has a lower
decomposition temperature than the lithium counterpart, LiNH2. The initial studies
by Chen and co-workers reacted LiNH2 with MgH2 or CaH2 with the aim of
destabilising the ternary imides.74 Their results showed a remarkable drop in thehydrogen uptake and release temperatures as revealed by temperature programmed
absorption and desorption studies (Fig. 7).
Based on the starting composition, the chemical formula of the compound
produced in the LiNH2:MgH2 2:1 reaction was proposed to be Li2MgN2H2.
Interestingly, after the rehydrogenation step, the initial components (LiNH2 and
MgH2) were not regenerated and instead Mg(NH2)2 and LiH were the products.74
Subsequent studies of the LiMgNH system with varying ratios of Mg(NH2)2:LiH
(or LiNH2:MgH2)105110 demonstrated that an onset of hydrogen desorption as low
as 150 1C was achievable while maintaining up to B5 wt% of hydrogen in these
systems.74,106108
Recently, Alapati et al.111 predicted a 1:1 LiNH2-MgH2 hydrogen storage system
with 8.19 wt% capacity by density functional theory (DFT) calculations:
LiNH2 + MgH22 LiMgN + 2 H2 (10)
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First-principles calculations of total energies and vibrational free energies for thesame binary composition have suggested the following sequential reactions:112
LiNH2 + MgH2- 1/2 Mg(NH2)2 + LiH + 1/2 MgH2
- 1/4 Mg(NH2)2 + 1/4 Mg3N2 + LiH + H2
- 1/4 Li2MgN2H2 + 1/4 Mg3N2 + 1/2 LiH + 3/2 H2
- LiMgN + 2 H2 (11)
However, experimentally a new intermediate compound with compositionLi2Mg2(NH)3 was identified113 on dehydrogenation of the initial mixture at
210 1C although this mixed imide phase was known long ago.114
LiNH2 + MgH2- 1/3 Li2Mg2(NH)3 + 1/3 MgH2 + 1/3 LiH + H2 (12)
A recent XRD and IR investigation showed that the 1:1 LiNH2MgH2 systemreleased 6.1 wt% H2 first during ball milling and then by subsequent heating thatfollowed a four step reaction.115
LiNH2 + MgH2- 1/2 Mg(NH2)2 + LiH + 1/2 MgH2 (12 h ball milled)
-1/2 MgNH + 1/4 Mg(NH2)2 + LiH + 1/4 MgH2 + 1/2 H2 (36 h ball milled)
-1/2 Li2MgN2H2 + 1/2 MgNH + 1/2 LiH + 1/4 MgH2 + H2 (at 220 1C)
-1/4 Mg3N2 + 1/4 Li2MgN2H2 + 1/2 LiH + 3/2 H2 (at 390 1C) (13)
Fig. 7 Temperature-programmed absorption (a) and desorption (b) curves for (I) 2LiNH2 +
CaH2, (II) 2LiNH2 + MgH2 and (III) Li2NH samples (Reprinted from ref. 74, with permission
from Wiley-VCH).
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Nevertheless, the predicted product lithium magnesium nitride, LiMgN, was notrealized in this study and only 1 equivalent of hydrogen was rechargeable under highhydrogen pressure.115
Given that in the initial studies of the reactions of LiNH2 with MgH2 an initial
exchange reaction occurred to produce Mg(NH2)2 on dehydrogenation and subsequent
rehydrogenation, further explorations employed various ratios of Mg(NH2)2 and
LiH, as starting materials. According to Nakamori et al.,116 the following overall
reaction occurs for a 1:4 Mg(NH2)2LiH ratio:
Mg(NH2)2 + 4 LiH2 1/3 Mg3N2 + 4/3 Li3N + 4 H2 (14)
Although a calculated value of 9.1 wt% of hydrogen was expected from the above
reaction, the experimental weight loss at 527 1C was only B7 wt%. However,
importantly, the major weight loss occurred below 230 1C which is lower than the
equivalent desorption in the LiNH2 + 2 LiH system. Amide:hydride ratios of 1:2
and 3:8 for the Mg(NH2)2 LiH system were also investigated, viz:74,107,109,117121
Mg(NH2)2 + 2 LiH2 Li2Mg(NH)2 + 2 H2 (15)
3 Mg(NH2)2 + 8 LiH2 4 Li2NH + Mg3N2 + 8H2 (16)
Reactions (15) and (16) were expected to release 5.6 and 6.9 wt% of hydrogen. The
above combinations were systematically investigated by thermogravimetry, PCT and
XRD studies.120,121 While the pressure-composition isotherms were similar at
250 1C, the quantity of released hydrogen was more with higher LiH content.
(5.4, 4.5 and 5.1 wt% for reaction 14, 15 and 16, respectively).120 Moreover, the mass
spectrometry profiles atB190 1C indicated an increase of ammonia production for
compositions with smaller equivalents of LiH.
According to the report of Aoki et al. all the reactions, Mg(NH2
)2
+ 2 LiH,
3 Mg(NH2)2 + 8 LiH and Mg(NH2)2 + 4 LiH, showed similar desorption PC
isotherms with the formation of Li2Mg(NH)2 as the final product at 250 1C and the
equilibrium dehydriding reactions had no serious effect on the quantity of LiH.121
The structure of the intermediate imide Li2Mg(NH)2 was previously described in
section 2.3. One fascinating outcome is the identification of a Li-rich and H-poor
imide, Li2+xMgN2H2x, in the 3 Mg(NH2)2 + 8 LiH system studied by Nakamura
and co-workers.119b The intermediate phase was synthesized and characterized as
Li2.6MgN2D1.4 (using LiND2 and D2 gas)119b by combined synchrotron X-ray and
neutron diffraction experiments. The structure of Li2.6MgN2D1.4 has the same
symmetry as the previously described Li2Mg(NH)2 but with a smaller cell dimension(Fig. 8). In addition, in the non-stoichiometric imide structure 30% of the D-sites are
vacant [as compared to fully occupied sites in Li2Mg(NH)2] and the adjacent cation
Fig. 8 Structure of LiMgND intermediate (left) and formation scheme for
Li2+xMgN2H2x (right) (Reprinted from ref. 119b, with permission from Elsevier B. V.).
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sites are 20% occupied (by contrast, vacant in Li2Mg(NH)2). Based on this, the
following scheme has been proposed for the formation of the intermediate defect
imide phase mediated by interfacial diffusion of hydrogen (H from LiH and H+
from LiMgNH phase) (Fig. 8).
More in situ studies by synchrotron and neutron diffraction would enhance the
understanding of the hydrogen uptake-release processes that occur in specific
systems and the role of intermediates. A deeper understanding of intermediatestructures and their formation mechanisms are key elements in the future design of
materials.
Complex binary component systems: Amides-complex hydrides. Similar to the
simple binary mixtures, complex binary composite systems are considered here as
reactive mixtures of complex hydrides, such as borohydrides or aluminohydrides
with amides or binary hydrides. Thermodynamic destabilisation with combined
chemical systems occurs either through chemical destabilisation of the hydrogenated
state or by formation of stable products on dehydrogenation.3b An illustration of the
chemical destabilisation of LiBH4 was provided by Vajo and co-workers with areduced decomposition temperature achieved in a 2:1 mixture of LiBH4 and MgH2as compared to pure LiBH4.
122 The formation of stable MgB2 (reaction (17)) in
the dehydrogenated state seems to drive the reaction with an enthalpy gain of
25 kJ mol1 of H2.122
2 LiBH4 + MgH22 2 LiH + MgB2 + 4 H2 (17)
A reversible storage of more than 9 wt% hydrogen (theoretical capacity 11.9 wt%)
has been demonstrated for reaction (17) when catalysed with 23 mole% of TiCl3.122
It is worth noting here that a very small amount of LiBH4 mixed with MgH2 alsoenhanced the dehydrogenation/hydrogenation kinetics of MgH2.
123 The requirement
of a reduced temperature (168 1C) for an equilibrium hydrogen pressure of 1 bar in
the 2LiBH4MgH2 system indicates that both LiBH4 and MgH2 are destabilised.27
Subsequently, similar destabilisation strategies have been employed for many
other complex hydride combinations.124 With the successful demonstration of the
hydrogenation reactions (18) and (19), it has been suggested that MgB2 facilitates
production of metal borohydrides by promoting the formation of the [BH4]
complex anion.124b
2 NaH + MgB2 + 4 H2- 2 NaBH4 + MgH2 (18)
CaH2 + MgB2 + 4 H2- Ca(BH4)2 + MgH2 (19)
Very recently, another new coupled complex hydride system, LiBH4/CaH2, was
reported with a theoretical capacity of 11.7 wt% via the following reaction:124f
6 LiBH4 + CaH22 6 LiH + CaB6 + 10 H2 (20)
Interestingly, the above reaction, with TiCl3 added, reproducibly stores 9.1 wt%
hydrogen (95% of the theoretical total accounting for the added wt. of catalyst in the
calculation) and cycles between LiBH4CaH2 and LiHCaB6 in the hydrogenatedand dehydrogenated state respectively.124f Despite many advantages, this system is
highly unlikely to be suitable for onboard storage due to the high equilibrium
temperature at 1 bar H2 (418 1C) predicted from thermodynamic calculations based
on experimental data under differing conditions.124f
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Another destabilised complex binary mixture, which has been investigated
simultaneously by Aoki et al. and Pinkerton et al., is that of LiBH4 and
LiNH2.125,126 A dehydrogenation enthalpy of 23 kJ mol1 of H2 was predicted for
a (2:1) LiNH2 + LiBH4 mixture, which is lower than either of the individual LiNH2and LiBH4 components, according to the following dehydrogenation reaction (21)
by first-principle calculations.125a
LiBH4 + 2 LiNH2- Li3BN2 + 4 H2 (21)
Experimental investigations of this mixture were carried out by ball milling followed by
XRD analysis and by p-c isothermal measurements. A new phase was identified upon
ball milling and was indexed in a cubic cell (10.67 A ).125a In a separate study Pinkerton
et al. reported formation of a quaternary hydride, Li3BN2H8, that formed by either
ball-milling or heating (above 95 1C) the same binary mixture.126a They also noticed
melting of Li3BN2H8 at B190 1C followed by Z10 wt% hydrogen release above
B250 1C.126a Soon after, the compound was identified as a lithium borohydride amide,
Li4(NH2)3BH4, with a bcc structure (lattice parameter 10.66 A ).127 This phase, however,
did not match with the starting stoichiometry indicating a non-stoichiometric reactionof LiBH4 with 2 LiNH2. The ideal reaction stoichiometry was determined to be a 3:1
ratio of LiNH2 and LiBH4 to produce the LiNBH quaternary phase. Although
Li4(NH2)3BH4 had a high hydrogen content of 11.9 wt% and the major decomposition
product of this phase above 260 1C was hydrogen, it is non-reversible.127 Recently,
many new destabilised systems with enhanced hydrogen capacity and thermodynamics
have been recognised by DFT based computational studies.111,128 However, it is often
observed that the introduction of a non-hydride second species diminishes the overall
gravimetric capacity of the mixed system.
Chemical destabilization of LiAlH4 by LiNH2 was originally shown by Lu and Fang
employing a thermogravimetric study of a 2LiAlH4 + LiNH2 mixture.129
The systemreleasedB8.1 wt% of hydrogen between 85 and 320 1C in three dehydrogenation steps
without any catalysts.129 The reaction involved a two step decomposition of LiAlH4similar to NaAlH4 (eqns (3) and (4)) followed by a third step as per the dehydrogenation
in eqn (6). The resulting overall reaction can be represented as in eqn (22).129
2 LiAlH4 + LiNH2- LiH + Li2NH + 2 Al + 4 H2 (22)
Ball-milling of other molar ratios, (1:1) and (1:2), of the LiAlH4LiNH2 system have
also been investigated. The (1:1) LiAlH4LiNH2 system has emerged as an inter-
esting case that releases 6.1 wt% hydrogen under ball-milling as reported by Xiong
et al. and the enthalpy effect of the H2 release reaction was very low.130 Afterwards,reversible storage was reported by other groups in the LiAlNH system.131,132 For
example, a Li3AlH63LiNH2 mixture with 4 wt% TiCl31/3AlCl3 releases 7.1 wt%
of hydrogen in two dehydrogenation steps between 150 and 300 1C.132 It was
demonstrated that in short-cycle experiments the system is 100% reversible with the
following overall reaction (23):132
Li3AlH6 + 3 LiNH22 Al + 3 Li2NH + 9/2 H2 (23)
Recently, Liu et al. have reported a new chemical system LiAlH4Mg(NH2)2 (1:1)with a hydrogen content of 8.5 wt% and releasing 6.2 wt% under ball-milling near
ambient temperature.133 The overall reaction was expressed as:
Mg(NH2)2 + LiAlH4- 1/3 Mg3N2 + 1/3 Li3AlN2 + 2/3 AlN + 4 H2 (24)
Further investigations are suggested to explore its potential for on-board storageconcentrating on the capacity and reversibility issues.
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Ternary composite systems: Amides-hydrides-borohydrides. In addition to simple
and complex binaries the strategy of hydride destabilisation was extended to ternary
mixtures.134137 Recently Yang et al. have reported a self-catalysing ternary
composite LiNH2/LiBH4/MgH2 (in a 2:1:1 molar ratio) that has resulted in
enhanced low temperature desorption kinetics with reduction in ammonia liberation
as compared to the state-of-the-art binary composites (Fig. 9).134 It is believed that
the self-catalysing effect is due to a set of coupled reactions,LiNH2 + LiBH4- Li4BN3H10 (25-1)
2 Li4BN3H10 + 3 MgH2- 3 Mg(NH2)2 + 2 LiBH4 + 6 LiH (25-2)
2 Li4BN3H10 + 3 MgH2- 3 Li2Mg(NH)2 + 2 LiBH4 + 6 H2 (25-3)
Mg(NH2)2 + 2 LiH2 Li2Mg(NH)2 + 2 H2 (25-4)
where the first step (251) occurs during the milling and the produced Li4BN3H10melts and reacts with MgH2 when the mixture is heated to 100 1C (reaction (252)).
The first hydrogen desorption begins at 150 1C by reaction (253). The total capacity
of 8.2 wt% can only be achieved at higher temperatures (up to 375 1C) with furtherreaction of Li2Mg(NH)2 and LiBH4 forming Mg3N2, Li3BN2, traces of LiH plus an
unknown phase.134c It is thought that pre-formation of Li2Mg(NH)2 in step (25-3)
provides nuclei for subsequent reaction (step (254)) in a Li4BN3H10 melt (acting as
Fig. 9 Hydrogen and ammonia desorption kinetics data as a function of temperature for 2:1:1
LiNH2/LiBH4/MgH2 ternary mixtures and comparison with other unary and binary systems
(Reprinted from ref. 134c, with permission from Wiley-VCH).
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an effective mass transfer medium) and is self-catalysed.134c Another point to note is
that step (254) is the only reversible one in the entire reaction sequence.
A subsequent study systematically investigated the impact of MgH2 on the ternary
system LiNH2LiBH4MgH2 with 2:1:x molar ratios and 0 r x r 1. This study
suggested that the addition of MgH2 facilitates milling-induced reaction and reduces
the ammonia release.135 Conversely, however, with the addition of MgH2 the total
theoretical capacity of the system decreased. This work confirmed the directinvolvement of MgH2 in the first desorption event as x is varied in the LiNH2
LiBH4MgH2 ternary mixture. It corroborated the previous study for the (2:1:1)
system (i.e., when x = 1) in demonstrating that this was the optimum composition
for low temperature hydrogen release (B4 wt%) and indicated future opportunities
for further kinetic enhancements.135
In a study of another ternary mixture, Mg(NH2)22LiHLiBH4, and with
comparison with the LiNH2LiBH4MgH2 ternary system above, the former
showed very similar hydrogen desorption, thermal effects and structural changes.136
However, Mg(NH2)22LiH0.67LiBH4 gave 9.1 wt% hydrogen at 300 1C, higher
than the total theoretical capacity of the 2 LiNH2MgH2LiBH4 composite.136
Theeffects of varying the reaction stoichiometry in the (LiNH2)X(LiBH4)Y(MgH2)Zternary system (ratio x:y:z) have been recently investigated by Sudik et al.137 Among
the 2:1:2, 1:1:1, 2:0.5:1 and 2:1:1 ratios tested (in ref. 134), only 2:1:1 and 2:0.5:1 were
reversible. The 2:0.5:1 stoichiometry had a maximum capacity of 8.6 wt% of which
only 3.5 wt% was reversible at low temperature (180 1C).
2.5 Ammonia-borane and amido-boranes
Ammonia-borane (NH3BH3) is considered to be an attractive candidate within the
chemical storage category largely due to its safety features and a high hydrogencontent of 19.6 wt% that exceeds that of gasoline.138141 NH3BH3 is a colourless
solid that is stable at ambient temperature and was first synthesised in 1955.142 Many
other preparative methods, both in solution and in the solid state, have since been
described.143146 In the context of amide-hydride destabilization combining the
protonic and hydridic hydrogens, NH3BH3 is a remarkable compound in that both
hydridic BH and protonic NH bonds exist within the same molecule.147 Moreover,
the strong BN bond prevents ammonia and borane emission prior to hydrogen
release under most thermolysis conditions. Although the detailed mechanism for the
decomposition of NH3BH3 is not clear yet, significant progress has been made
towards understanding its hydrogen release reactions.141,148156During thermal decomposition several intermediates have been reported and the
desorption products vary depending on the thermolysis conditions (temperature
range, ramping rate). Intermediate species such as polyaminoborane, [NH2BH2]n,
polyiminoborane, [NHBH]n, and even a small fraction of borazine, [N3B3H6], have
been identified as part of the dehydrogenation process. According to Wolf and
co-workers149,150 the stepwise solid state decomposition of NH3BH3 to release
12 wt% H2 can be represented as:
n NH3BH3 (s)- [NH2BH2]n (s) + n H2 (g) (26-1)
[NH2BH2]n (s)- [NHBH]n (s) + n H2 (g) (26-2)
[NH2BH2]n (s)- [N3B3H6]n/3 (l) + n H2 (g) (26-3)
NH3BH3 decomposition often requires a long induction period. Use of ionic solventshas been shown to reduce the induction period and speed up the kinetics of hydrogen
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release.152 Several approaches have been employed to reduce dehydrogenationtemperatures,139 increase the kinetics of H2 release
152,157168 and minimize borazineemission.152 Use of transition metals,157,158 base-metal catalysts,162 acid catalysis,163
nanoscaffolds,139 ionic liquids152 and carbon cryogels164 are reported to improvedehydrogenation properties.
Recently, a significant development in the hydrogen release temperature has been
reported by Xiong et al. in alkali-metal amidoboranes, LiNH2BH3 and NaNH2BH3,
releasing 10.9 and 7.5 wt% of hydrogen (Fig. 10), respectively, at B90 1C via:169
n LiNH2BH3 (s)- (LiNBH)n (s) + 2n H2 (g) 10.9 wt% (27)
or
n NaNH2BH3 (s)- (NaNBH)n (s) + 2n H2 (g) 7.5 wt% (28)
In addition, the compounds do not release unwanted borazine by-product and the
reactions are devoid of lengthy induction periods and foaming. Moreover, the
dehydrogenation process is less exothermic than NH3BH3 (the enthalpies of
dehydrogenation from Li- and Na-amidoborane are 3 and 5 kJ mol
1 vs. 5 kJ mol
1
for NH3BH3), which would facilitate its off-board materials regeneration.169,172
Another derivative of NH3BH3, solvent-containing calcium amidoborane,
Ca(NH2BH3)2 2THF,170,171 has also been reported with enhanced dehydrogenation
kinetics and suppressed borazine release. Contrary to previous reports, a recent
study indicated a significant amount of ammonia emission from the sodium
amidoborane, thereby decreasing the practical hydrogen content and suitability
for use in fuel cells.173
Despite the several advantages of ammonia-borane and amidoborane systems, the
most serious drawback is the lack of facile reversibility which makes them unsuitable
for on-board storage applications. The thermal decomposition of ammonia-borane
and amidoboranes is less exothermic in comparison to their hydrolysis, making them
nearly thermoneutral which would greatly facilitate their off-board regeneration
Fig. 10 Time evolution of hydrogen release from alkali amidoboranes and post milled
ammonia-borane at about 91 1C (Reprinted from ref. 169, with permission from Macmillan
Publishers Ltd.).
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from solid decomposition products (BNHx). In spite of major progress, significant
challenges remain for the development of efficient off-board regeneration of ammonia-
borane. With an efficient regeneration procedure, these systems could possibly serve
as an alternative to other complex chemical hydrides as a sustainable hydrogen
storage medium.
3. Physical storage
Unlike chemical storage where the hydrogen is held chemically to component atoms
within a solid (formation of chemical bonds), in physical storage systems the
hydrogen is attached/included to/in a host-a surface or porous solid-by much
weaker interactions/forces commonly defined as a process of physical adsorption
or physisorption. Coordination polymers, metal organic frameworks (MOFs),
polymers with intrinsic microporosity (PIMs), activated carbons and zeolites are
among the widely investigated examples of materials for hydrogen storage that
operate via physisorption.3b In physisorption the forces of attraction between the
hydrogen and the host originate mainly from weak van der Waals interactions,
thereby restricting any significant hydrogen adsorption to that at very low tempera-
tures and/or at very high pressures. Notable progress has been made during the past
few years with gas storage in MOFs using innovative materials design to improve
their performance in terms of increased storage capacity.
Metal-organic frameworks (MOFs)
MOFs (also known as coordination frameworks, coordination network materials or
coordination polymers) can broadly be thought of as synthetic analogs of naturally
occurring zeolitic materials, mostly with a three dimensional framework. These
frameworks are interconnected in such a fashion that ordered networks of channels
and pores are generated within the structure. Due to their porous structures, various
small organic molecules, solvent molecules and gas molecules can diffuse in and out
of the channel structures, sometimes selectively. The primary building blocks that
form the MOFs are multidentate organic ligands (linkers) coordinating to a metal
ion or a cluster of metal atoms. MOF materials often form with solvent molecules
trapped within the pores and following removal of these solvent guests, the host
materials are typically high in surface area and of low density.
The first report of a MOF as a potential hydrogen storage medium was published
by Yaghi and co-workers in 2003.
174
They demonstrated an uptake of 4.5 wt%of hydrogen at 78 K and 1 wt% at ambient temperature and a pressure of 20 bar
in a Zn-containing framework material (MOF-5) of composition Zn4O(BDC)3(BDC = 1,4-benzenedicarboxylate).174 They also reported up to 2 wt% of hydrogen
uptake in topologically similar (replacing BDC with naphthalene linkers) isoreticular-
MOF-8 (IRMOF-8) at room temperature and 10 bar.174 After these remarkable
reports, the hydrogen uptake in MOF-5 was established independently at about
4.55.2 wt% at 77 K and B50 bar.175
Realising their high potential as new materials for hydrogen storage, Rowsell and
Yaghi outlined several strategies to improve the storage capacity of MOFs under
more favourable working conditions.176 A variety of parameters, such as pore size,adsorption energy, impregnation, catenation and incorporation of open metal sites
and lighter metals in MOFs were proposed that can be tuned for performance
optimisation.176 Many of these parameters are interrelated so, for example, pore size
can be modified by changing organic linkers or by catenation that occurs with
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interpenetration or interweaving of networks. Moreover, impregnation of another
adsorbent within large-pore MOFs can lead to reduced pores. Pore size, in turn, has
a direct influence on the hydrogen adsorption enthalpy. A pore size comparable to
the diameter of molecular hydrogen should increase the van der Waals interaction
thereby increasing the enthalpy.176 Similarly, higher enthalpy would also result from
interaction of hydrogen with frameworks having coordinatively unsaturated metal
centres or open metal sites.Subsequently, a variety of framework materials have been synthesized and
investigated for hydrogen storage applications.175189 Hydrogen uptake studies on
a series of MOFs (IRMOF-1, -6, -11, -20, -74, MOF-177 and HKUST-1) reveal that
saturation uptake correlates with the surface area.175a,d At 77 K the saturation
uptake and the saturation pressure varied widely in the series reaching a strikingly
high capacity of 6.7 and 7.5 wt% in IRMOF-20 and MOF-177 at saturation
pressures of 70 and 80 bar, respectively.175a Many of the initial studies aimed at
highly porous frameworks with high surface areas. Another recent report by Lin
et al. showed high hydrogen uptake of 6.07 wt% at 20 bar (at 77 K) and
projected 7.01 wt% from Langmuir plots for MOFs with a BET surface area of2932 m2 g1.181 It has been demonstrated in MOFs that high surface area and pore
volume are approximately proportional to the amount of stored hydrogen.175a,181,190
Very high storage capacities have also been reported in a series of MIL (Mate riaux
de lInstitut Lavoisier) MOFs possessing extremely high surface areas and concen-
trations of unsaturated metal sites. It is noteworthy that MIL-101 (based on
Cr-carboxylates) adsorbs large amounts of hydrogen reaching nearly 6.1 wt% at
77 K.188 Its high heat of adsorption (10 kJ mol1) at low pressures was believed to be
related to small pore sizes.188
It must be pointed out that the maximum hydrogen uptake (up to the plateau of
isotherm at 77 K) in the MOFs varies almost linearly with the specific surface area.
However, a conflicting scenario appears with large pores. While a large pore volume is
required to enhance storage capacity, the interaction strength with hydrogen becomes
weak with wider pores. In a Ni-based nanoporous framework with flexible linkers, the
hydrogen was shown to confine within the pores where the entry to the pores was
controlled by dynamical opening of the pore windows.191 This implies that once hydrogen
is adsorbed at high pressure it can be stored at low pressure. Moreover, the metal centre
and the organic linker units were identified as the two major hydrogen binding sites in the
MOFs by inelastic neutron scattering experiments and theoretical studies.192,193
MOFs with unsaturated coordination positions or open metal sites have beenexplored and shown to be attractive for hydrogen storage.177,178,184,192194 For
example, HKUST-1 or Cu-BTC based on Cu(COO)4 unit and benzene-1,3,5-
tricarboxylate linkers possessing smaller pores (as compared to MOF-5) and Cu(II)
open metal sites show enhanced hydrogenadsorbent interactions and consequently
higher heats of adsorption.177,184,194196 Many possibilities for creating MOFs with
exposed metal sites have been reviewed recently.197 It is proposed that increasing the
hydrogen binding affinity and accordingly imparting a high adsorption enthalpy can
be achieved by systematic incorporation of unsaturated metal ions. Bhatia and
Myers calculated the optimal enthalpy of adsorption to be 15.1 kJ mol1 for storage
at 298 K with operating pressures between 1.5 and 30 bar.198 Limitations due todecreased H2 adsorption with increasing temperature have been improved in
some MOFs by increasing H2-adsorbent surface interactions.184,199201 Recently,
strong interaction between hydrogen and exposed metal sites within MOFs
have been demonstrated with very high heat of adsorption values of 12.3 and
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13.5 kJ mol1.202,203 The total stored amount was dependent on surface area and
pore volume which led to the conclusion that, a high surface density of strong
adsorption sites is required for efficient storage under workable conditions.203
Recently, many so-called covalent organic frameworks (COFs) have been syn-
thesised and structurally characterised.204208 By contrast to MOFs, within COFs
the organic structural building units are held together by strong covalent bonds
(for example, CC, CO, BO, SiC) as opposed to metals. The resulting solidspossess a very low density and an ultrahigh porosity. These properties of COFs make
them excellent candidates for hydrogen storage. Theoretical studies on prototypical
COFs predict that storage of up to 10 wt% H2 in COF-5 (at 80 bar) and COF-8
(at 100 bar) is possible.204,207 In COF-8 an excess hydrogen uptake of 10 wt% has
been found experimentally at 77 K and 100 bar (Fig. 11).208 These values are
significantly higher than the highest values reported in MOFs (7 wt% in MOF-177
and 7.1 wt% in MOF-5) although the pressure employed is quite high at 77 K. 175a,209
Also, the 3-D COFs show 2.5 to 3 times higher storage capacity than the 2-D COFs.
The associative hydrogen storage in COF-8 is the highest value reported for a
framework material.208
Another new development for increasing the hydrogen uptake of MOFs is the
hydrogen spillover technique.210 By employing this technique Li et al. reported an
eightfold enhancement in hydrogen uptake in MOF-5 and IRMOF-8.210 The
enhancement was attributed to the secondary spillover of hydrogen atoms from a
Pt/activated carbon (AC) catalyst to the surface of MOFs in a composite system.
Further, significant improvement in H2 uptake capacity at near ambient temperature
(30 1C) has been predicted in Li-doped MOF-C30 with a gravimetric uptake of
6 wt% at 100 bar and Fig. 12 shows how enhanced gravimetric capacity for an array
Fig. 11 Hydrogen adsorption isotherm of COF-108 in wt% at 77 K (Reprinted from ref. 208,
with permission from American Chemical Society).
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of Li-doped MOFs compares theoretically to the undoped materials.211 Hence
despite a linear relationship with surface area in both doped and undoped materials,
it is the lithium concentration that is believed to be the dominant factor for high
uptake near room temperature.211 The introduction of Li-atoms within the
framework has been proposed to functionalise the organic linkers211,212 and a
gravimetric uptake of 4.5 wt% at room temperature and 10 wt% at 77 K at
100 bar has been predicted in MOFs by following this Li-doping strategy.213
Among the various MOFs reported, after independent authentication of 7.5 wt%
hydrogen uptake at 77 K and 70 bar, MOF-177 is considered as an excellent material
to serve as a benchmark adsorber.214 A very recent study by Schro der and
co-workers showed promising uptake of 7.3 wt% in a Cu(II)-tetracarboxylate based
framework (NOTT-103) at 77 K and 60 bar.
215
This drop in pressure can beconsidered significant in comparison to most of the previous studies conducted at
pressures up to 100 bar.
In summary, the small enthalpy change (heat of adsorption) in physically
adsorbed MOFs offers a huge advantage for faster kinetics and reversibility over
the chemisorbed materials. Heats of adsorption vary with the type of metal,
framework topology, surface area and pore size and hence the extent of hydrogen
adsorption in MOF materials can be influenced by manipulating these parameters.216
Therefore, the exploration of new MOFs and developing an understanding of
adsorption mechanisms are very important steps towards optimising the hydrogen
storage properties of MOFs at near ambient conditions. Incorporation of moieties orfunctionalities that increase the interaction of the framework with hydrogen
may increase the temperatures at which large quantities of hydrogen can be adsorbed.
In addition, optimization of pore size without compromising the specific surface area
and total pore volume will play a key role in developing viable materials.
Fig. 12 Gravimetric uptake of hydrogen as a function of BET surface area for MOF and
Li-doped MOF at 300 K and 100 bar (Reprinted from ref. 211, with permission from American
Chemical Society).
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4. New materials for hydrogen storage
In this section, novel emerging materials for hydrogen storage will be considered.
This will mainly encompass nanosized and nanostructured materials spanning the
classes of solids described above. Both experimental studies and promising theo-
retical predictions of improved hydrogen storage will be highlighted. In parallel to
common concepts in catalysis, it has been shown that decreasing the particle size to
the nanoscale drastically improves the kinetics of hydrogenationdehydrogenationin metal hydrides and alloys. In this regard simple ball-milling can have a significant
effect in altering the grain size. Moreover, ball-milling can be also employed to
disperse catalysts into chemical hydrides thus generating nanosized catalysts.
As described in section 2.1, bulk MgH2 offers a high storage capacity of 7.6 wt%
but its use is limited by the poor reversibility originating from the high thermal
stability of the hydride and slow desorption kinetics.7 Although considerable effort
has been devoted to modify the kinetics and lower Tdes of the bulk hydride, these
approaches have afforded only relatively small improvements in the sorption kinetics
and exerted little influence over the desorption temperature (300 1C for bulk
MgH2).7
However, in a combined quantum chemical (HartreeFock) and DFT study,
Wagemans et al. have shown that both Mg and MgH2 become less stable with
decreasing cluster size (below 20 Mg atoms), thereby decreasing the hydrogen
desorption energy.217 The study showed significant decreases in desorption energy
below a crystal grain size ofB1.3 nm. Moreover, another interesting outcome was
the fact that the destabilization of MgH2 was stronger than Mg (Fig. 13).217 Similar
destabilisation effects have been predicted in the case of Mg and MgH2 nanowires as
well by DFT calculations.218 The calculations reveal the possibility of a room
temperature hydrogen storage in MgH2 nanowires of 0.85 nm diameter.218
Recently, Li et al. have successfully synthesized nanowires of Mg and experimen-
tally demonstrated the enhanced kinetics of hydrogen absorptiondesorption of
nanowires as compared to bulk MgH2 and, moreover, the thinner the nanowire the
lower the desorption energy (Fig. 14).219 Interestingly, despite the complete
Fig. 13 Calculated energies for Mg and MgH2 clusters as a function of cluster size (Reprinted
from ref. 217, with permission from American Chemical Society).
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disappearance of nanowires (and their collapse to nanoparticles) after only
10 cycles, there was no change in capacity even after 50 hydridingdehydriding
cycles.219 Also, Aguey-Zinsou and Ares-Ferna ndez have reported 7.6 wt% reversible
storage near room temperature for Mg nanoparticles of 5 nm diameter.220 The size
dependent performances even for a complex hydride, NaAlH4, have been reported
recently by Balde et al.221 Hydrogen desorption temperatures of below 70 1C were
achievable for 210 nm particles of NaAlH4 followed by reabsorption under lowhydrogen pressures (beginning at 20 bar).221 Furthermore, recent reports describe
the hydrogen desorption of hollow nanospheres of Mg(NH2)2 mixed with 2 moles of
MgH2 nanoparticles starting at 375 K under hydrogen with enhanced desorption
kinetics compared to bulk materials.222 Recently, the prospects of metal/metal
hydride nanostructures were reviewed by Be rube et al. considering both their
advantages and disadvantages.223 Peng et al. have described some recent develop-
ments in Mg-nanostructures.30 There are advantages of low temperatue hydrogen
cycling and fast kinetics but poor cycleability, degradation of sorption properties
and a reduction in thermal conductivity would seem to suggest practical challenges
that would need to be overcome.223
Therefore, the concept of hydride destabilization
Fig. 14 Hydrogen absorption (top) and desorption (bottom) curves of Mg nanowires at
different temperatures; triangle, 3050 nm; circle, 80100 nm and square, 150170 nm diameters
(Reprinted from ref. 219, with permission from American Chemical Society).
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via nanowire, nanotube or nanoparticle formation presents some immense
opportunities for further development. More research is now required towards the
development of synthesis methods for nanostructured materials together with much
needed insight into every aspect of structure. Further, it is also worth noting that
other structural factors may be utilised to modify and optimise sorption perfor-
mance. For example, different polymorphs of MgH2 (a-, b- and g-phase) appear to
have intrinsically different desorption temperatures.
7c,224
5. Future directions and challenges for materials design
Despite significant progress made in the development of solid state hydrogen storage
materials, there remains immense challenges ahead before we enter into a genuine
hydrogen energy era. Considering the two major storage types discussed, namely
chemical and physical, there are advantages and disadvantages for mobile applica-
tions in both categories. For example, many of the complex chemical hydrides have
high storage capacity with favourable kinetics or thermodynamics (to some extent),but mostly lack reversibility. Conversely, MOFs have very good reversibility, but the
higher storage capacity is realized only at low temperatures (77 K) which is far
beyond the operational temperature of PEM fuel cells.
Apart from those discussed above, there are many other classes of materials that
are also being investigated for potential hydrogen stores. These include PIMs,225227
carbon nanostructures,228,229 clathrates,230233 and zeolites.3b,234 Clathrates are
compounds that are formed by association of two or more molecular entities and
not by strong chemical bonds wherein one set of molecules encloses the other.
Hydrogen is stored by a mode of encapsulation in a clathrate and can be released by
changing the pressure or temperature.233 It is only recently that these materials have
been suggested for hydrogen storage after the discovery of hydrogen clathrates.230233
More future work is necessary with clathrates before their potential can be
judged.
PIMs are formed either by polymerisation of monomers with rigid units (having
restricted conformation or rotation) or by cross-linking of polymeric gels.225,226
They have been investigated for hydrogen storage due to their microporosity which
gives rise to a large surface area and low density material. Storage capacities up to
2.17 wt% at 77 K and 10 bar have been reported in PIMs.227 However, due to the
weak nature of interaction with hydrogen, similar to that of MOFs, their ambienttemperature storage capacities are significantly low. Strategies similar to those
adopted for MOFs can be applied to PIMs towards improving their performance.
Carbon materials including carbon nanotubes have been intensively invesitgated
for hydrogen storage applications.228 Yang et al. have recently reported a reversible
hydrogen uptake of 6.9 wt% at 77 K and 20 bar in an ordered porous carbon
material.229 This is very a encouraging result and requires further attention. Besides
carbon, hydrogen storage properties of inorganic nanowires and nanotubes of boron
nitride (BN), titanium disulfide (TiS2) and molybdenum disulfide (MoS2) have been
investigated.235 It seems apparent from the very limited studies of such systems that
those involving light elements are more attractive than heavier ones to maximise thegravimetric capacity. For example, in collapsed BN nanotubes hydrogen uptake up
to 4.2 wt% was reported at a pressure of 10 MPa.236 These diverse examples of very
different materials show that there is still plenty of room for further invention and
discovery in the bid to produce new solid state hydrogen storage solutions.
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