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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: [email protected]; Fax: +44 (0)141 330 8128;Tel: +44 (0)141 330 4888

Annu. Rep. Prog. Chem., Sect. A, 2009, 105, 2154 | 21

This journal is c The Royal Society of Chemistry 2009

REVIEW www.rsc.org/annrepa | Annual Reports A

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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.)

22 | Annu. Rep. Prog. Chem., Sect. A, 2009, 105 , 2154



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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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mandal_h2 storage materials

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