Properties and Applications of Metal (M) dodecahydro-closo-dodecaborates (Mn=1,2B12H12) and Their Implications for Reversible Hydrogen Storage in the Borohydrides

Abstract

Hydrogen has long been proposed as a versatile energy carrier that could facilitate a sustainable energy future. For an energy economy centred around hydrogen to function, a storage method is required that is optimised for both portable and stationary applications and is compatible with existing hydrogen technologies. Storage by chemisorption in borohydride species emerges as a promising option because of the advantages of solid-state storage and the unmatched hydrogen energy densities that borohydrides attain. One of the most nuanced challenges limiting the feasibility of borohydride hydrogen storage is the irreversibility of their hydrogen storage reactions. This irreversibility has been partially attributed to the formation of stable dodecahydro-closo-dodecaborates (Mn=1,2B12H12) during the desorption of hydrogen. These dodecaborates have an interesting set of properties that are problematic in the context of borohydride decomposition but suggest a variety of useful applications when considered independently. In this review, dodecaborates are explored within the borohydride thermolysis system and beyond to present a holistic discussion of the most important roles of the dodecaborates in modern chemistry.

Full text

inorganics
Review
Properties and Applications of Metal (M)
dodecahydro-closo-dodecaborates (Mn=1,2B12H12) and
Their Implications for Reversible Hydrogen Storage
in the Borohydrides
Aiden Grahame and Kondo-François Aguey-Zinsou *
MERlin Group, School of Chemical Engineering, The University of New South Wales, Sydney 2052, Australia;
aidengrahame@gmail.com
*Correspondence: f.aguey@unsw.edu.au; Tel.: +61-293-857-970
Received: 3 September 2018; Accepted: 27 September 2018; Published: 1 October 2018
Abstract:
Hydrogen has long been proposed as a versatile energy carrier that could facilitate
a sustainable energy future. For an energy economy centred around hydrogen to function, a
storage method is required that is optimised for both portable and stationary applications and
is compatible with existing hydrogen technologies. Storage by chemisorption in borohydride
species emerges as a promising option because of the advantages of solid-state storage and
the unmatched hydrogen energy densities that borohydrides attain. One of the most nuanced
challenges limiting the feasibility of borohydride hydrogen storage is the irreversibility of their
hydrogen storage reactions. This irreversibility has been partially attributed to the formation
of stable dodecahydro-closo-dodecaborates (M
n=1,2B12 H12
) during the desorption of hydrogen.
These dodecaborates have an interesting set of properties that are problematic in the context
of borohydride decomposition but suggest a variety of useful applications when considered
independently. In this review, dodecaborates are explored within the borohydride thermolysis
system and beyond to present a holistic discussion of the most important roles of the dodecaborates
in modern chemistry.
Keywords:
borohydride; dodecaborate; closo-dodecaborate; hydrogen storage; solid-state electrolyte
1. Introduction
The international community is coming to terms with the need for significant changes to the
global energy economy, but even the most aggressive renewable energy expansion efforts are limited
by the status of the technology. The most difficult quality of petrochemicals to replicate has proven to
be their unmatched versatility. Fossil fuels are unique in that they can serve as both a primary source
of energy and as a vector for transportation. Given the dynamic nature of energy utilisation in the
modern era, an effective energy carrier is key to the overall functionality of an energy economy.
The identification of a single, versatile energy carrier is especially crucial to the feasibility of
an energy economy based on renewables. Energy storage solutions are vital to the facilitation of
large-scale and remote renewable energy production, as well the use of renewable energy for portable
applications. Additionally, the standardisation of all energy systems to use one energy vector enables
the integration of a diverse range of energy sources into the supply chain without any modifications to
the energy utilisation infrastructure.
Hydrogen emerges as a promising energy carrier for a number of reasons. First, hydrogen is
extremely abundant and a major component in what is perhaps the most well-studied chemical: water.
Hydrogen’s high energy density (120 MJ kg
−1
compared to 45 MJ kg
−1
for gasoline) and the simplicity
Inorganics 2018,6, 106; doi:10.3390/inorganics6040106 www.mdpi.com/journal/inorganics

Inorganics 2018,6, 106 2 of 38
of its combustion reaction are also significant [
1
,
2
]. Furthermore, a method of producing electricity
from hydrogen has been known for almost two centuries, as the first fuel cells were invented by
chemists Christian Friedrich Schonbein and Sir William Robert Grove in the late 1830s [
3
]. It is these
considerations that gave rise to the concept of a “hydrogen economy”, wherein all energy is stored,
transported and distributed in the form of hydrogen (Figure 1).
This theory was considered as early the 19th century, when the work of the Danish scientist,
Poul La Cour concerning the storage of wind energy in gaseous hydrogen gave rise to his idea of a
“hydrogen society” wherein hydrogen is used as the dominant energy vector [
4
]. However, the modern
terminology used to describe this concept was only coined at the dawn of the energy crisis of the 1970s,
when the global community was first confronted with the instability of an energy economy entirely
dependent on petrochemicals. In a paper published in 1972, just over a year before the peak of the 1973
world oil shortage, John O’.M. Bockris and A. John Appleby originated the term “hydrogen economy”
to encompass the “energetic, ecological and economic aspects” of an energy system centred around
hydrogen [2,5].
Figure 1. Schematic overview of a hydrogen based energy economy.
The lack of a suitable hydrogen storage method is currently the most significant limiting factor to
the realisation of a hydrogen economy. Conventional methods include storage as a pressurised gas at
35–70 MPa [
6
] and cryogenic liquid storage at temperatures in the range of
−
250
◦
C [
7
]. However, these
established methods of hydrogen storage have been found to have various prohibitive limitations that
preclude widespread hydrogen usage [
8
–
10
]. Any viable storage method must be able to reversibly
release and absorb hydrogen at reasonable conditions, while attaining high gravimetric capacities and
having good hydrogen cycling kinetics.
Chemisorption by a borohydride compound has been identified as a promising mechanism
of solid-state hydrogen storage that has the potential to meet all of those outlined requirements.
The fundamental appeal of the metal borohydrides is their ability to achieve gravimetric and volumetric
capacities that far exceed those possible using liquid or pressurised gas storage [
1
,
11
]. However,
hydrogen storage using a borohydride carrier is not currently feasible due to the extremely high
temperatures required for thermal dehydrogenation. Additionally, the irreversibility of the metal
borohydride hydrogen cycling reactions is a serious barrier to their practical application.
Significant research has been dedicated to moving borohydrides closer to industrial system
requirements by lowering the hydrogen release temperature and optimising their desorption kinetics
to allow for faster hydrogen release [
12
–
17
]. However, the irreversibility of the borohydride
dehydrogenation process is a question that remains largely un-answered within the field, likely because
borohydrides have yet to reach optimal desorption conditions for a single cycle. If borohydrides

Inorganics 2018,6, 106 3 of 38
are ever to find widespread practical application, the ability to store hydrogen reversibly will be
non-negotiable.
The focus of this review is on the factors that contribute to this irreversibility, with particular
emphasis on the formation of dodecahydro-closo-dodecaborates (M
n=1,2B12 H12
) during decomposition.
Commonly referred to as the “dodecaborates”, these compounds have a distinctive icosahedral
molecular structure that has been found to result in a number of remarkable properties, including
exceptional thermal stability. To clarify the role of the dodecaborates in the overall dehydrogenation
scheme, the mechanisms that govern thermolysis are reviewed for the light alkali and alkaline
earth borohydrides. This review also serves to identify possible routes of mitigating the impact
of dodecaborates, thereby aiding in the development a borohydride-based hydrogen storage material
that meets reversibility targets. An exploration of the chemical properties and proposed applications
of dodecaborates is then discussed to inform the behaviour that has been observed within the context
of borohydride systems.
2. Metal Borohydrides for Hydrogen Storage
The ability of a hydrogen carrier to reliably complete a dehydrogenation and rehydrogenation
cycle is fundamental to its potential as a storage material. Hydrogen release from metal borohydrides
can be invoked through hydrolysis or thermolysis. Hydrogen generation from
NaBH4
by hydrolysis
has been extensively investigated because of its spontaneous, low temperature reaction with water
and the high gravimetric hydrogen storage capacity of the
NaBH4– H2O
liquid fuel [
18
]. Hydrogen
release from NaBH4through hydrolysis proceeds according to the following ideal reaction:
NaBH4+2 H2O−−→ NaBO2+4 H2(1)
However, the viability of borohydride dehydrogenation by hydrolysis is fundamentally limited by
the irreversibility of the hydrolysis reaction [
19
–
21
]. In the
NaBH4
system, the hydrated sodium borate
(
NaBO2
) by-product can be regenerated to borohydride by annealing with magnesium hydride
MgH2
under high
H2
back-pressure (0.1–7 MPa) [
22
]. Although this method achieves very high maximum
yields (97–98%), it is not optimised for cost or energy efficiency. Furthermore, this method only
achieves high yields using dehydrated
NaBO2
, rather than the true hydrated complex formed during
the solution-phase hydrolysis reaction. Various improvements have been proposed and remain under
investigation, but none have satisfied all of the technical and economic requirements for industrial
implementation [
23
–
26
]. Hence, thermolytic dehydrogenation processes are the focus of most current
investigations and this review.
Thermolysis refers to the decomposition of a compound through the application of heat.
During thermolysis, the chemisorption of hydrogen is reversed through the cleavage of the chemical
bonds that bind it within the lattice of a storage material [
7
]. The metal borohydrides are comprised
of two main components: the metal M
n+
cation and the [
BH4
]
−
anion, composed of four hydrogen
atoms covalently bonded to a central boron. During themolysis, sufficient energy must be input into
the system to break the boron–hydrogen bonds within the borohydride anion and allow hydrogen to
reform diatomic gas molecules.
Both the internal bonding of the [
BH4
]
−
anion and the bonding between the metal cation
and the anion have been shown to have a strong dependence upon the properties of the metal
species [
27
]. This dependence includes the desorption temperature of a given metal hydride, as an
inverse correlation has been observed between cation Pauling electronegativity and borohydride
stability [
28
,
29
]. The [
BH4
]
−
anion is formed by covalently bonding four hydrogen atoms to a central
boron, which requires the donation of an additional electron by a cation species. When the donating
cation is very electronegative, the charge transfer between the cation and borohydride anion is
suppressed and the boron-hydrogen bonds are destabilised [30].

Inorganics 2018,6, 106 4 of 38
Light alkali and alkaline earth metal cations form gravimetrically optimised borohydrides, but
their low electronegativies also contribute to exceptional thermodynamic stability [
31
]. This stability
is a significant barrier to their utilisation as hydrogen carriers because of the resulting increase in
energy input required to free the hydrogen atoms from the solid lattice. As summarised in Table 1,
all alkali and alkaline earth borohydrides are found to have decomposition temperatures that are not
energetically or practically feasible.
Table 1.
Key thermodynamic properties for common
M(BH4)n
(M = Li, Na, K, Mg, Ca) compounds [
32
].
Enthalpy of decomposition (
∆
H
dec
) values are given as ranges to reflect differing results from the
variety of experimental and theoretical methods that have been utilised to specify this parameter.
Because of the impact of hydrogen back-pressure on the temperature at which hydrogen release begins,
the decomposition temperature (T
dec
) is reported for a hydrogen equilibrium pressure of P = 0.1 MPa
to allow for easy comparison between different species.
Borohydride ∆Hdec (kJ mol−1H2) Tdec at 0.1 MPa H2(◦C) Reference
LiBH456–75 370 [33,34]
NaBH489.6–108 539 [35,36]
KBH4113.9 826 [36]
Mg(BH4)239.3–57 157 [37,38]
Ca(BH4)240.6–87 278 [39,40]
Regenerating the borohydride species from decomposition products after thermal
dehydrogenation has also proven challenging. While rehydrogenation has been accomplished for
some of the borohydrides, the temperature/pressure conditions are not viable and capacity losses are
always registered [
41
–
43
]. For example, only 8.3 mass %
H2
of
LiBH4
total 18.5 mass %
H2
capacity is
regenerated after treatment at 600
◦
C under 15.5 MPa of
H2
[
44
], with an increase to 35 MPa of
H2
at
600
◦
C required to achieve nearly complete rehydrogenation [
45
]. All of these obstacles stem from
the problematic kinetics, associated mass transfer and thermodynamics of the borohydride thermal
dehydrogenation scheme.
2.1. Possible H2Desorption Pathways
As a chemisorption process, thermal dehydrogenation is known to proceed via a multi-step
mechanism. Each step in the scheme is governed by different kinetics and thermodynamics, and will
be impacted differently by the manipulation of reaction conditions such as temperature and pressure,
or the use of a particular destabilising agent [
46
,
47
]. Furthermore, the properties of the intermediate
products formed during these steps will also impact the performance of the overall hydrogen storage
system. Given these considerations, it is unsurprising that the thermal dehydrogenation of the
borohydrides has been found to be exceedingly complex. Although an appreciable amount of study
has been dedicated to describing the specifics of this mechanism, it remains controversial and poorly
understood. This is illustrated most clearly through a discussion of the multiple, often conflicting,
decomposition schemes that have been proposed for each borohydride species.
2.1.1. Alkali Borohydrides
The dehydrogenation processes of many of the alkali borohydrides have been studied in detail,
and dehydriding pathways have been proposed. However, the greatest volume of research exists for
the lightest alkali borohydrides of
LiBH4and NaBH4
because they are commonly available and have
advantageous hydrogen storage properties. Moreover, correlation between the electronegativity of the
metal species and the thermodynamic stability of its borohydride results in a preference for the most
electronegative alkali metals (Li and Na) that are predicted to dehydrogenate more readily [28,29].

Inorganics 2018,6, 106 5 of 38
Table 2.
Known experimental structures of the common alkali borohydrides
MBH4
(M = Li, Na, K) [
32
].
Metal Species Polymorph Crystal System Space Group Reference
Li o-LiBH4Orthorhombic Pnma [48]
h–LiBH4Hexagonal P63mc [48]
hp1–LiBH4Orthorhombic Ama2 [50]
hp2–LiBH4Cubic Fm3m [50]
Na α–NaBH4Cubic Fm3m [51]
β–NaBH4Tetragonal P421cor P42/nmc [31,52]
γ–NaBH4Orthorhombic Pnma [53]
Kα–KBH4Cubic Fm3m [51]
β’–KBH4Tetragonal P42/nmc or P421c[54]
γ–KBH4Orthorhombic Pnma [55]
LiBH4:
Because of its high gravimetric hydrogen capacity (18.5 mass %) and thermodynamic instability
relative to the other light alkali borohydrides, significant efforts have focused on developing
LiBH4
as a hydrogen storage material. At ambient conditions,
LiBH4
exists as a single polymorph:
o
-
LiBH4
,
which has an orthorhombic (Pnma) structure [
48
]. While the polymorphism of
LiBH4
was reported as
early as the 1970s [
49
], a complete specification of its crystal structure and phase transitions did not
occur until much later (Table 2).
One of the most comprehensive studies was carried out using a combination of synchrotron
and Raman spectroscopy to specify the crystal structure of
o
-
LiBH4
at ambient conditions and with
increasing temperature [
48
]. The ambient
o
-
LiBH4
was observed to undergo a phase transition
at approximately 108
◦
C, resulting in the formation of hexagonal h–
LiBH4
. Further investigation
found that this phase transition is endothermic, with an enthalpy of 4.18 kJ mol
−1
at 118
◦
C [
56
].
High-pressure polymorphs have been described using in-situ measurements up to 20 GPa [
50
].
The transition of ambient
o
-
LiBH4
at 1.2 GPa results in the formation of a secondary orthrohomic
phase with differing space group symmetry. Furthermore, a secondary transition is observed at 10
GPa, indicated by a volume drop of 2.9%. This third phase was found to have a cubic crystal system,
with a Fm3m arrangement of Li cations and BH4anions.
Most generally, LiBH4has been observed to decompose according to the following reaction [33]:
LiBH4−−→ LiH +B+3
2H2(2)
One of the first studies that considered
LiBH4
as a potential hydrogen storage material was
conducted by Züttel et al. They found that mixing
LiBH4
with
SiO2
powder successfully facilitated the
desorption reaction, exhibiting a similar thermal desorption spectra to pure
LiBH4
, with the hydrogen
release steps shifted to lower temperatures [
56
]. Later investigation of this system clarified that the
observed temperature shift was caused by a reaction between the two components rather than a
catalytic influence [
57
]. However, in the study by Züttel et al., they found that the dehydrogenation
profiles of both the pristine
LiBH4
and
LiBH4
/
SiO2
mixtures showed the same three hydrogen
desorption features, which were analysed to infer the following preliminary mechanism:
LiBH4−−→ LiBH4−x+1
2(x)H2T=108 ◦C (3a)
LiBH4−x−−→ “LiBH2”+1
2(1−x)H2T=200 ◦C (3b)
“LiBH2”−−→ LiH +B+1
2H2T=453 ◦C (3c)

Inorganics 2018,6, 106 6 of 38
Notably, the intermediate “LiBH
2
” is given in quotes, as the composition was estimated from the
amount of desorbed hydrogen measured, and had yet to be confirmed through structural analysis.
While further study was required to clarify specifics, this work provided compelling evidence that the
thermolysis reaction was a multi-step process in which intermediates play an important role.
Early first-principles study of the thermal decomposition of
LiBH4
applied DFT-based
methodology to predict the stability of potential reaction intermediates [
34
]. From theoretical
analysis, it was determined that the most energetically preferable pathway proceeded through a
dodecaborate intermediate,
Li2B12H12
. A proposed reaction scheme for the overall decomposition of
LiBH4, including the energetically optimised Li2B12H12 phase, is given below [34]:
LiBH4−−→ 1
12Li2B12H12 +5
6LiH +13
12H2−−→ LiH +B3
2H2(4)
This mechanism was also supported by first-principles studies conducted using a combination of
the prototype electrostatic ground state (PEGS) search method and a DFT-based linear programming
approach [58].
However, experimental research conducted by Friedrichs et al. found that diborane (
B2H6
)
plays a more significant role in both the formation and decomposition of
LiBH4
than previously
reported [
59
]. Diborane evolution during borohydride dehydrogenation is considered to be an
unfavourable by-product because it compromises the purity of the released hydrogen, represents a
safety concern and reduces the storage capacity of the system with each cycle because of the loss of
boron to the gas phase. However, Friedrichs et al. proposed that diborane evolution also contributes
to the formation of
Li2B12H12
as a reaction by-product (rather than an intermediate), resulting from
the in-situ reaction of LiBH4and B2H6[59].
2 LiBH4+5 B2H6−−→ Li2B12H12 +13 H2(5)
They also suggested an overall reaction mechanism, wherein
LiBH4
decomposition proceeds
through a
LiH
intermediate, accompanied by diborane evolution. Because of the thermal instability
of diborane and the high temperatures required for
LiBH4
thermolysis, diborane then spontaneously
decomposes to B and H2[59].
LiBH4−−→ LiH +1
2B2H6(6a)
B2H6−−→ B+3
2H2(6b)
A later first-principles study came to a similar conclusion, finding that
Li2B12H12
would be a
reaction product rather than an intermediate due to its highly negative enthalpy of formation [
60
].
In this work, the enthalpy and Gibbs free energy are reported for a range of possible decomposition
pathways of
LiBH4
. By modelling the mechanism as a decomposition into a combination of all
proposed products, several
Li2B12H12
formation mechanisms were identified in agreement with
those proposed previously in literature [
34
]. It was also proposed that diborane evolution can occur
concurrently to Li2B12H12 production, as shown in Reactions (7a) and (7b) [60].
14 LiBH4−−→ Li2B12H12 +12 Li +B2H6+19 H2(7a)
15 LiBH4−−→ Li2B12H12 +12 Li +LiB +B2H6+21 H2(7b)
These reactions were found to be energetically favourable as compared to those not including
Li2B12H12
formation. When considering the overall decomposition, it was then determined that the
pathway with the lowest enthalpy of reaction at T = 0 K and the lowest free energy of reaction per

Inorganics 2018,6, 106 7 of 38
mole of
LiBH4
proceeded via the formation of a ternary phase with the analytical formula LiBH
2.5
(Reaction (8)) [60].
4 LiBH4−−→ LiBH2.5 +3 H2(8)
This echoes the early experimental findings of Züttel et al., who referred to a ternary intermediate
“LiBH
2
” whose composition was estimated from the amount of desorbed hydrogen [
56
]. Hence,
through the manipulation of the ternary intermediate, some pathways could potentially result in
products that can be re-hydrogenated. This may be further tailored through adjustments to the
decomposition pressure and temperature.
NaBH4:
Because of the higher stability of
NaBH4
and the resulting increase in hydrogen desorption
temperature, much of the preceding research has focused on dehydrogenation by hydrolysis rather
than thermolysis [
61
]. The use of aqueous
NaBH4
as a liquid fuel continues to garner interest, despite
the technical challenges of regenerating
NaBH4
from hydrolysis products [
62
]. Like
LiBH4
,
NaBH4
also
exists as a single polymorph under ambient conditions:
α
–
NaBH4
, which has a cubic (Fm3m) structure
[
51
] (Table 2). However, unlike
LiBH4
,
NaBH4
also has a low temperature polymorph, transitioning to
β–NaBH4(tetragonal P421c) below temperatures of approximately −83 ◦C [52].
That same tetragonal polymorph is also observed to form in high-pressure conditions, when
α
–
NaBH4
transitions back to
β
–
NaBH4
at 6.3 GPa [
63
]. While
β
–NaBH4 was initially interpreting as
having the P42
1
cspace group structure, first-principles study proposed an alternate P4
2
/nmc symmetry
[
31
]. However, it is not possible to differentiate between these two symmetries based on diffraction
data so the P42
1
cstructure is most commonly accepted. Beyond 6.3 GPa, another transition to an
orthorhombic phase is observed at 8.9 GPa. This high-pressure
γ
–
NaBH4
phase was found to follow
Pnma symmetry, similar to the structure of BaSO4and the ambient polymorph of LiBH4[53].
The thermal dehydrogenation of
NaBH4
is extremely energy intensive, with decomposition under
0.1 MPa of
H2
occurring only at temperatures above approximately 534
◦
C [
35
]. The decomposition
directly to the metal elements is cited as the overall reaction because of the extreme conditions required
for NaBH4desorption.
NaBH4−−→ Na +B+2 H2(9)
This reaction mechanism was proposed based on dynamic pressure, composition and temperature
(PCT) measurements taken during
NaBH4
desorption under constant hydrogen flow [
35
]. The resulting
pressure-composition isotherms (Figure 2) reveal that hydrogen desorption occurs in a single step,
indicated by a single isotherm plateau.
Figure 2.
Pressure, composition, and temperature (PCT) isotherms measured for the thermal
decomposition of
NaBH4
under a constant hydrogen flow of 2, 1, and 0.5 cm
3
(STP) min
−1
. Reprinted
from [35].

Inorganics 2018,6, 106 8 of 38
From X-ray Diffraction (XRD) analysis of the solid residue remaining after desorption, it
was determined that the composite phases were elemental sodium, some boron-rich binary Na–B
compound and traces of NaH. While Na was the dominant phase, the traces of NaH present suggested
that decomposition proceeds at least partially through NaH. Using these findings to constrain
first-principles modeling, the energetic favourability of the two most commonly reported mechanisms
of alkali borohydride decomposition was compared (Reactions (10a) and (10b)).
NaBH4−−→ Na +B+2 H2∆HT=0K
reaction =245.5 kJ mol−1(10a)
2 NaBH4−−→ 2 NaH +2 B +3 H2∆HT=0K
reaction =199.9 kJ mol−1(10b)
2 NaBH4−−→ 2 NaH +2 B +3 H2−−*
)−− 2 Na +2 B +4 H2(10c)
While these paths are competitive from a thermodynamic perspective, it can also be noted that
the conditions required for complete
NaBH4
dehydrogenation exceed the desorption temperature of
NaH into its elements. Therefore, the equilibrium shown in Reaction
(10c)
favours elemental sodium
irrespective of the underlying mechanism.
Further DFT calculations have been used to characterise the underlying mechanism of mass
transport and diffusion in
NaBH4
by analysing the properties of its lattice defects [
64
]. These findings
emphasise the ionic character of
NaBH4
, proposing a mechanism in which hydrogen diffuses through
the lattice structure as the ion unit [
BH4
]
−
and decomposes to H
−
ions and
BH3
molecules on the
surface. While the H
−
ions convert
NaBH4
to NaH within the lattice, the
BH3
molecules may escape
to the gas phase. This could result in the production of diborane, which in turn might result in in-situ
formation of
Na2B12H12
through the reaction of diborane and
NaBH4
, in a similar mechanism as
proposed for LiBH4[65].
Despite these theoretical findings, experimental consensus does not exist to support the evolution
of diborane during
NaBH4
decomposition [
66
]. In contrast, multiple studies have confirmed the
presence of
Na2B12H12
in the decomposition products of various
NaBH4
thermolysis systems [
67
–
69
].
While this is compelling evidence for the significance of
Na2B12H12
to the thermal decomposition
of
NaBH4
, no fundamental mechanism for its formation has been proposed and its role in the
dehydrogenation reaction remains unclear.
KBH4:
Because of its higher decomposition temperature and lower gravimetric hydrogen capacity,
little research has focused on the thermal dehydrogenation of
KBH4
[
11
]. During early study of the
structure and properties of
NaBH4
and
KBH4
, it was found that
KBH4
shares the same cubic (Fm3m)
structure as
NaBH4
at room temperature [
51
] (Table 2). In addition, following the behaviour of
NaBH4
,
KBH4
was found to have a low temperature polymorph. At temperatures below
−
203
◦
C,
KBH4
transitions to a tetragonal crystal system [
54
]. At this low temperature, the [
BH4
]
−
complexes follow a
P42/nmc structure that is much more ordered than the Fm3m room temperature structure.
A study of
KBH4
under compression found that this tetragonal
β
–
KBH4
phase is also formed
at 3.8 GPa, following a similar P42
1
csymmetry [
55
]. A final high-pressure polymorph
γ
–
KBH4
(orthorhombic Pnma) is observed at pressures
>
6.8 GPa. In general, it can be noted that the overall
phase transition scheme for
KBH4
is remarkably similar to that of
NaBH4
. Likewise, it is assumed that
its overall decomposition is similar to
NaBH4
, with the reaction products being elemental K and B [
47
].
KBH4−−→ K+B+2 H2(11)
Preliminary first-principles calculations have also predicted the formation of
K2B12H12
intermediate compounds, as noted for other borohydrides [70].

Inorganics 2018,6, 106 9 of 38
2.2. Alkaline Earth Borohydrides
Of the alkaline earth metals,
Mg(BH4)2
and
Ca(BH4)2
have garnered the most attention.
In particular,
Mg(BH4)2
is especially well suited to dehydrogenation by thermolysis because of
the aforementioned inverse correlation observed between cation Pauling electronegativity and
borohydride stability. As the Pauling electronegativity of Mg (1.33) is greater than those of Na,
Li, and Ca (0.93, 0.98 and 1.00, respectively), it follows that
Mg(BH4)2
is the most unstable and will
decompose most readily (Table 1) [29].
Table 3.
Known experimental structures of the common alkaline earth borohydrides
M(BH4)2
(M = Mg, Ca) [
32
].
Metal Species Polymorph Crystal System Space Group Reference
Mg
α–Mg(BH4)2Hexagonal P6122 [71]
β–Mg(BH4)2Orthorhombic Fddd [72]
γ–Mg(BH4)2Cubic Ia3d [73]
δ–Mg(BH4)2Tetragonal P42nm [73]
ζ–Mg(BH4)2Hexagonal P3112 [73]
Ca
α–Ca(BH4)2Orthorhombic F2dd [74]
α’–Ca(BH4)2Tetragonal I42d [74]
β–Ca(BH4)2Tetragonal P4 or P42/m[74]
γ–Ca(BH4)2Orthorhombic Pbca [75]
Mg(BH4)2:
Because of the advantageous thermodynamic properties of
Mg(BH4)2
and its high
theoretical gravimetric hydrogen capacity (14.9 mass %), its dominance over the sphere of borohydride
research has been rivalled only by
LiBH4
. Evidenced by its large number of polymorphs,
Mg(BH4)2
is the most extreme example of structural complexity observed within the group of borohydrides
considered in this review. Experimentally,
Mg(BH4)2
has been shown to have as many as five different
polymorphs (Table 3). However, theoretical predictions indicate that
Mg(BH4)2
has many other
polymorphs that have yet to be observed [32].
The
α
–
Mg(BH4)2
(hexagonal P6
1
22) phase was specified through the investigation of well
crystallised
Mg(BH4)2
at room temperature [
71
]. Heat treatment of
α
–
Mg(BH4)2
yields a transition to
orthorhombic
β
–
Mg(BH4)2
(Fddd) at 180
◦
C [
72
]. However, the most notable
Mg(BH4)2
polymorph
is the
γ
–
Mg(BH4)2
phase, specified through the investigation of a novel synthesis method for
Mg(BH4)2
[
73
]. The structure of
γ
–
Mg(BH4)2
is unique, as it is essentially composed of a 3D matrix
of pores similar to those seen in zeolites and other microporous materials. Hence,
γ
–
Mg(BH4)2
is
the first hydride observed to have large, permanent porosity: with empty volume accounting for
approximately 33% of the unit cell.
Investigation of the thermal dehydrogenation of
Mg(BH4)2
has sparked an active debate and
numerous mechanisms have been proposed in literature without the emergence of a consensus
opinion. Matsunaga et al. investigated the synthesis and thermal dehydrogenation of
Mg(BH4)2
through dynamic PCT measurements and XRD analysis of the desorption products [
37
]. The PCT
measurements were carried out under a hydrogen back-pressure greater than 0.1 MPa, at three
temperatures (290, 320 and 350
◦
C). The resulting PCT showed two distinct isotherm plateaus for the
desorption at 350
◦
C, and single plateau at lower temperatures (Figure 3). The presence of a second
plateau in the high temperature isotherm indicates that hydrogenated products remain after the first
thermal decomposition steps.

Inorganics 2018,6, 106 10 of 38
Figure 3.
Absorption/desorption isotherms for
Mg(BH4)2
as synthesised from
LiBH4
and
MgCl2
through heat treatment at 320
◦
C: (
a
) desorption at 350
◦
C; (
b
) desorption at 320
◦
C; and (
c
) desorption
at 290 ◦C. Reprinted from [37].
XRD analysis performed on the residue resulting from each of the three desorption reactions
supported the conclusions drawn from the PCT.
MgH2
was observed in the products at low
decomposition temperature, while only Mg was observed in the products at 350
◦
C. Hence, a two-step
desorption process was proposed to explain the H2release from Mg(BH4)2.
Mg(BH4)2−−→ MgH2+2 B +3 H2T<350 ◦C (12a)
MgH2−−→ Mg +H2T>350 ◦C (12b)
However, a later study by Li et al. conducted using mass spectrometry and thermogravimetric
analysis indicated that further intermediate steps were probable [
38
]. A derivative of the
thermogravimetric curve was found to show four distinct peaks (Figure 4), suggesting that the
dehydrogenation of
Mg(BH4)2
occurs in four endothermic stages. This was further validated by
the presence of at least three overlapped peaks in the mass spectrum measured during thermal
dehydrogenation, indicating at least three desorption steps.
Figure 4.
TG-DTA and mass spectrum curves of the thermal desorption of
Mg(BH4)2
:
(
a
) the thermogravimetry curve (black), its derivative (red) and the differential thermal analysis
(green) curves of
Mg(BH4)2
; and (
b
) associated mass spectrum of the as-synthesised
Mg(BH4)2
(blue).
Reprinted from [38].

Inorganics 2018,6, 106 11 of 38
Raman spectroscopic analysis performed to elucidate the intermediates formed during desorption
indicated that
MgB12H12
was likely involved in one or more of the steps. Based on these
conclusions, the following three-stage reaction scheme was proposed for the overall dehydrogenation
of Mg(BH4)2[38]:
Mg(BH4)2−−→ 1
6MgB12H12 +5
6MgH2+13
6H2−−→ MgH2+2 B +3 H2−−→ MgB2+4 H2(13)
Another study by Soloveichik et al. also confirmed at least four decomposition steps [
76
].
However, they proposed that the reaction proceeds through multiple amorphous intermediates,
including MgB12H12 (Figure 5).
Figure 5.
Decomposition pathways of
Mg(BH4)2
, reproduced from Soloveichik et al. (Path C),
where D1–D4 indicate hydrogen evolution events measured by temperature programmed desorption
(TPD) [
76
]. Path A [
37
,
77
] proposed by Matsunaga et al. and Path B [
38
] proposed by Li et al. refer
to mechanisms described in previous publications. Amorphous phases are denoted by an asterisk,
observed hydrogen evolution steps are marked by dashed red lines. Reproduced from [76].
Beyond the four endothermic decomposition steps, an exothermic event was observed at 350
◦
C
(X1 in Figure 5) and this was attributed to the crystallisation of
MgH2
. However, the X1 event could
also be the result of an exothermic decomposition of the intermediate “
MgB2
H
2.5
” phase to give
MgB12H12
and
MgH2
. As shown in Path C of Figure 5, the desorption of
Mg(BH4)2
proceeds through
at least three polyborane species, one of which was conclusively identified as
MgB12H12
, and results
in MgB2[38] as the final desorption product as opposed to Mg and B [37].
Beyond this mechanism, it can also be noted that the desorption process of
Mg(BH4)2
has
also been found to be extremely pressure dependent [
78
]. However, the findings of Soloveichik
et al. emphasised a fundamental dependence of the reaction pathway on the behaviour of various
polyborane intermediates (including MgB12H12 ) [76]. Not currently considered is the unprecedented
level of polymorphism of
Mg(BH4)2
(Table 3), which introduces another level to an already complicated
desorption process [
32
]. For example, the cubic polymorph
γ
–
Mg(BH4)2
was found to desorb via
an eight step decomposition mechanism that included two polymorphic transitions and several
unidentified ternary Mg-B-H phases [79].
Ca(BH4)2:
Although
Ca(BH4)2
has a slightly lower theoretical gravimetric hydrogen capacity (11.6
mass%) than
Mg(BH4)2
, it has still been well studied as a potential hydrogen storage material.
The commonly utilised wet chemistry synthesis of
Ca(BH4)2
in tetrahydrofuran (THF) solvent
yields a mixture of two room temperature polymorphs:
α
–
Ca(BH4)2
and
β
–
Ca(BH4)2
[
74
] (Table
3). Additionally, a phase transition at around 223
◦
C is also noted, resulting in a high-temperature
tetragonal polymorph, α’–Ca(BH4)2.

Inorganics 2018,6, 106 12 of 38
A fourth polymorph has also been specified, originating from a mechanochemical synthesis
procedure using
MgB2
and
CaH2
as the starting materials [
75
]. This orthorhombic
γ
–
Ca(BH4)2
structure was found in room temperature samples of the as-synthesised
Ca(BH4)2
, and after heating to
>
127
◦
C. A high-temperature variant of
β
–
Ca(BH4)2
with P4
2
/msymmetry has also been identified,
resulting from a phase transition of α–Ca(BH4)2above 127 ◦C.
A preliminary first-principles study of the thermal properties of
Ca(BH4)2
predicted that it would
desorb according to the following reaction [80]:
Ca(BH4)2−−→ 2
3CaH2+1
3CaB6+10
3H2(14)
A reaction pathway involving a dodecaborate product has also been theorised (Reaction
(15)
) and
predicted to be more energetically favourable than Reaction
(14)
[
58
]. However, the reaction enthalpies
calculated at T = 0 K for the proposed mechanisms (35.2 kJ mol
−1H2
for Reaction (14) and 34.2 kJ
mol
−1H2
for Reaction (15)) only differ by 1 kJ mol
−1H2
, which is not sufficient to conclusively exclude
one or the other on the basis of energetic preference.
6 Ca(BH4)2−−→ CaB12H12 +5 CaH2+13 H2(15)
The framework of the desorption mechanism was described experimentally by Kim et al. through
a comprehensive investigation of the thermal decomposition of
Ca(BH4)2
[
81
]. The adduct-free
Ca(BH4)2
used in the experiments was prepared from a
Ca(BH4)2– 2 THF
precursor and all heating
procedures were conducted under vacuum. From this characterisation, it was concluded that
Ca(BH4)2
decomposition begins with a polymorphic transformation (at 167
◦
C) and proceeds via an unknown
ternary Ca-B-H intermediate compounds (347–387
◦
C). However, in the XRD spectra of the final
desorption products, the only crystalline component detected was
CaH2
and further characterisation
of the amorphous products was not attempted.
Review of the research that has been amassed concerning the thermal dehydrogenation of
Ca(BH4)2
suggests that the desorption proceeds via more than one different mechanism and is strongly
influenced by the reaction conditions. In general, the findings of experimental investigations coalesce
around two competing reaction pathways [82]:
Ca(BH4)2−−→ amorphous intermediates (16a)
amorphous intermediates −−→ CaB2Hx+ (4−x
2)H2(16b)
CaB2Hx+ (4−x
2)H2−−→ 1
3CaB6+2
3CaH2+ ( x
2−2
3)H2(16c)
and
Ca(BH4)2−−→ amorphous intermediates (17a)
amorphous intermediates −−→ 1
6CaB12H12 +5
6CaH2+13
6H2(17b)
A
11
B MAS-NMR study of the decomposition process emphasised temperature dependence,
concluding that desorption under vacuum conducted within the temperature range of 320–350
◦
C
results in
CaB6
as the major boron phase in the products, whereas higher temperatures from 400–450
◦
C
result in amorphous elemental boron [
83
]. These findings suggest that
Ca(BH4)2
may also desorb
according to a third mechanism, involving the formation of amorphous boron:
Ca(BH4)2−−→ CaH2+2 B +3 H2(18)

Inorganics 2018,6, 106 13 of 38
The low-temperature mechanism discussed in this study aligns with Reactions (16a)–(16c), as
they identified
CaB2H6
(a
CaB2
H
x
phase) as the reaction intermediate in the 320–350
◦
C range and did
not detect
CaB12H12
in the desorption products. Alternatively, the
CaB2
H
x
intermediate has also been
assigned the
CaB2H2
stoichiometry [
84
], and other investigations have also reported potential crystal
structures [85].
Decomposition to
CaB12H12
has been supported by multiple theoretical studies using
first-principles calculations [
58
,
86
]. Notably, significant
CaB12H12
formation has been observed
experimentally in systems desorbed under
H2
back-pressure. An investigation of
Ca(BH4)2
desorption
was conducted under 0.1 MPa of
H2
using in-situ analysis by
11
B MAS-NMR and structural analysis
of desorbed products by XRD [
87
]. Under these conditions, uncharacterised amorphous intermediates
were found to decompose to both CaB6and CaB12H12 starting at approximately 340 ◦C.
A later investigation considered a wider range of hydrogen back-pressures (p(
H2
) = 0.1, 0.5, 1
and 1 MPa), finding that formation of the commonly reported
CaB2
H
x
intermediate was suppressed
with increasing back-pressure [
88
]. Also noted was a decrease in
CaB12H12
and
CaB6
formation under
high
H2
pressure, accompanied by an increase in amorphous boron (possibly indicating desorption
according to Reaction
(18)
). Despite these observations, the fundamental cause of the temperature and
pressure dependence of the
Ca(BH4)2
desorption mechanism is still uncertain and further clarification
is required to effectively control the reaction pathway.
3. Dodecaborates in the Borohydride System
In all of the potential mechanisms that have been discussed, thermolysis proceeds via a
multi-step process with multiple reaction intermediates. One interesting commonality shared
between the dehydrogenation pathways of many borohydrides is the presence of dodecaborate
compounds (M
n=1,2B12 H12
) formed either as by-products or intermediates. Dodecaborates have
garnered significant attention, as their thermodynamic stability has been cited as a potential cause for
difficulties experienced in re-hydrogenating borohydrides. While their importance is acknowledged,
the role of dodecaborates in borohydride dehydrogenation remains debated and largely unclear.
3.1. Chemical Structure and Properties
Dodecahydro-closo-dodecaborates are a unique borohydride species that exists as the dianion
[
B12H12
]
2−
and most commonly reacts with metal cations to form salts. The “closo” in their name refers
to the closed, icosahedral structure of the molecule (Figure 6). Because of this distinctive structure,
the closo-dodocaborates can be classified as cage compounds, in the same vein as closed carbon
nano-structures such as nanospheres [
89
]. Many of the most consequential properties of dodecaborates
can be attributed to the symmetry and regularity of their molecular structure.
Figure 6.
Schematic view of the icosahedral
[B12H12 ]2−
. Pink spheres denote boron atoms and white
spheres denote hydrogen atoms
Of most consequence to their role in the dehydrogenation of borohydrides is the exceptionally high
thermal stability of monometallic dodecaborate salts like those formed during the desorption process.
For example,
Cs2B12H12
(one of the more extensively studied alkali dodecaborate species because of

Inorganics 2018,6, 106 14 of 38
its relatively larger cation radius) can be heated up to 810
◦
C under vacuum without undergoing any
decomposition [
90
]. In contrast, hydrogen release from the smaller alkali dodecaborates has been
observed at much lower temperatures. For example,
Na2B12H12
was observed to release hydrogen
starting at around 450
◦
C under a helium flow [
91
]. Similar results have been reported for
K2B12H12
and Li2B12H12 , showing hydrogen release events beginning at 350 ◦C and 250 ◦C, respectively [92].
However, these low temperature hydrogen release steps have not been found to result in the
decomposition of the dodecaborate species to smaller polyboranes or metal hydrides. Investigation of
the decomposition mechanism of
Li2B12H12
carried out by XRD (Figure 7) revealed an increase in the
amorphous character of the dodecaborate with increasing temperatures [92].
Figure 7.
XRD data from thermally decomposed anhydrous crystalline
Li2B12H12
before and after the
main H2evolution peak at ca. 440 ◦C. Reprinted from [92].
Furthermore, no evidence was identified for the formation of any other lithium containing phases,
including LiH. From these observations, it was proposed that
Li2B12H12
decomposes via continuous
hydrogen release, resulting in the formation of an amorphous hydrogen deficient dodecaborate species,
Li2B12
H
12−x
[
92
,
93
]. After the formation of this hydrogen deficient phase, shifts of the major resonance
peaks of
[B12H12 ]2−
indicate that the icosahedral B
12
skeleton of the anion may be polymerising,
resulting in the formation of (
Li2B12
H
z
)
n
polymers [
94
]. This polymerised phase was only observed
to fully decompose to amorphous elemental boron when the temperature exceeded 650
◦
C [
92
].
This decomposition pathway has also been observed for
NaB12H12
, culminating in the formation of
(Na2B12Hz)npolymers at around 700 ◦C [94].
Similar behaviour has been observed for the alkaline earth dodecaborates,
MgB12H12
and
CaB12H12
. The decomposition of
MgB12H12
is observed to begin at approximately 190
◦
C, losing
around 77% its theoretical hydrogen content between 190–800
◦
C [
95
]. Structural analysis by
11
B
MAS-NMR (Figure 8) showed the formation of an amorphous hydrogen deficient dodecaborate species
MgB12H12−x, analogous to Li2B12 H12−x, upon thermal decomposition.

Inorganics 2018,6, 106 15 of 38
Figure 8. 11 B MAS-NMR spectra of anhydrous MgB12H12 heated to respective temperatures between
190 and 800 ◦C. Reprinted from [95].
As the temperature increases, shifts of the major resonance peak of
[B12H12 ]2−
are also observed,
indicating the polymerisation of the icosahedral B
12
framework and the formation of (MgB
y
H
z
)
n
polymers. These intermediate products eventually decompose to amorphous elemental boron when the
temperature exceeds 800
◦
C. These results are in agreement with an earlier study of
MgB12H12
/carbon
nanocomposites, with discrepancies in the temperatures of the intermediate transitions, possibly
caused by an influence of the carbon on the decomposition process [96].
CaB12H12
has also been observed to undergo the same series of transformations during heating as
MgB12H12
, culminating in the formation of (CaB
y
H
z
)
n
polymers [
95
]. Unlike
MgB12H12
, amorphous
elemental boron was not detected during the thermal treatment, even at temperatures greater than
750 ◦C. When considered collectively, these results are a strong indicator of the fundamental stability
of the icosahedral molecular geometry of the dodecaborates, demonstrating that this structure persists
even after intense thermal treatment.
The crystal structure and symmetry of the dodecaborates have been characterised for their ambient
temperature polymorphs, summarised in Table 4. The high-temperature polymorphs of the alkali
dodecaborates have also been found to display good ionic conduction and thus are potential candidates
for solid-state electrolytes [
97
,
98
]. The thermodynamic stability of some of the dodecaborates has also
been demonstrated theoretically by first-principles studies of their structure (Table 4). The extremely
exothermic enthalpies of formation that have been calculated indicate that the dodecaborate species
are very low energy state compounds and are therefore unlikely to react or decompose if formed.
Table 4.
Structural and thermodynamic data reported for the common alkali and alkaline earth
dodecaborates. The theoretically predicted enthalpy of formation (
∆
H
T=0K
f orma tion
) is given when values
are available in literature. All given structures refer to the ambient temperature polymorph.
Species ∆HT=0K
f orma tion (kJ mol−1) Crystal System Space Group
Li2B12H12 −945.95 [60] Cubic [99]Pa3 [99]
- Monoclinic [34]P21/n[34]
Na2B12H12 −1086.196 [91] Cubic [91]Pa3 [91]
−1086.381 [91] Monoclinic [100]P21/n[100]
K2B12H12 - Cubic [101]Fm3 [101]
CaB12H12 - Monoclinic [102]C2/c [102]
MgB12H12 - Monoclinic [58]C2/m [58]

Inorganics 2018,6, 106 16 of 38
3.2. Synthesis Methods
Dodecaborates were first synthesised in 1960 by Pitochelli and Hawthorne as a by-product of
the reaction of 2-iododecaborane and triethylamine [
103
]. Since then, numerous synthetic methods
have been developed specifically to produce dodecaborates in high yields [
89
,
104
]. Of particular
significance to borohydride chemistry is the synthetic method first reported by Miller et al. in 1964.
They achieved sodium dodecaborate yields of greater than 80% through the reaction of diborane and
sodium borohydride in diethylamine at 180 ◦C [105].
2 NaBH4+5 B2H6−−→ Na2B12H12 +13 H2(19)
Based on their investigation, they proposed that polyborane species such as dodecaborates could
be produced through a sequential addition of boron and hydrogen to an existing boron lattice, similar
to a polymerisation reaction, as shown in the following scheme [105]:
aH−+bBxHy−−→ [BbxHby+a−z]a−+ ( z
2)H2(20)
This scheme is equivalent to the known boron addition Reactions (21a) and (21b) [105,106].
B2H6+NaH −−→ 2 NaBH4(21a)
NaBH4+B2H6−−→ NaB3H8+H2(21b)
Furthermore, through investigation of Reaction
(19)
in different solvents, Miller et al. found
that the [
B11H14
]
−
anion was preferentially formed over [
B12H12
]
2−
under certain reaction conditions.
For example, when Reaction
(19)
is conducted in a dioxane solvent at temperatures between 90–120
◦
C,
[
B11H14
]
−
is the sole polyborane product. Conversely, they found that when the borohydride species
was in excess and temperatures exceeded 130
◦
C,
Na2B12H12
was the sole product of Reaction
(19)
,
irrespective of solvent. Beyond these steps proposed by Miller et al., the mechanism of boron addition in
a metal-boron-hydrogen system has not been conclusively determined, likely because of the emergence
of more advantageous dodecaborate synthetic methods [89].
For example, a contemporary study by Adams et al. found that
NaBH4
reacts with
B10H14
in
diglyme at 160
◦
C to give dodecaborate yields of greater than 90% [
107
]. Because decaborane (
B10H14
)
is stable under ambient conditions, it is preferred over diborane as a precursor in dodecaborate
synthesis processes [
89
]. While it is possible that
B10H14
is an intermediate compound in the reaction
scheme proposed by Miller et al., its high stability makes it more likely to be an alternative terminal
product. More recently, dodecaborates have been synthesised directly from gas–solid reactions between
B2H6
and a metal borohydride [
65
,
96
]. As discussed in Section 2.1.1, these successful synthesis
procedures have also been cited as evidence of the relevance of
B2H6
to dodecaborate formation during
borohydride decomposition [59].
3.3. Other BxHyCompounds
The formation of other complex [B
x
H
y
]
n−
anions during borohydride decomposition has been
investigated through experimental and theoretical means. Three species that are commonly discussed
are the
[B10H10 ]2−
dianion and the
[B11H14 ]−
and
[B3H8]−
anions.
[B10H10 ]2−
and
[B11H14 ]−
compounds have also been considered as potential hydrogen storage materials in their own right
because of their ability to release hydrogen through transition metal-catalysed hydrolysis [
108
]. All
three anions have been theoretically predicted as potential intermediate compounds in the desorption
schemes of the most commonly investigated borohydrides (MBH4,M=Li[34], Mg and Na [58]).
[B3H8]−
intermediate phases have also been observed experimentally during the thermal
desorption of
Y(BH4)3
[
109
] and
Mg(BH4)2
. When
Mg(BH4)2
is desorbed at 200
◦
C,
Mg(B3H8)2
was
identified as a reversible intermediate with a cycling capacity of 2.5 mass % [
110
]. Coordination of

Inorganics 2018,6, 106 17 of 38
Mg(BH4)2
with a THF adduct has also been found to result in the preferential formation of
MgB10H10
as a product of desorption at 180
◦
C [
111
]. However, these observations for the
Mg(BH4)2
–THF system
are anomalous, as the [
B12H12
]
2−
dianion has been recognised as the most energetically favourable
and thermodynamically stable of the proposed polyborane intermediates. Therefore, it is considered
to be both the most likely, and most problematic [B
x
H
y
]
n−
participant in the borohydride desorption
scheme [34,58,112].
This is illustrated by the thermodynamic properties of [B
x
H
y
]
n−
, summarised in Table 5.
As
NaB3H8
has been observed to decompose at temperatures as low as 100
◦
C [
113
], its formation
during
NaBH4
thermolysis would not present the same challenges as the significantly more stable
Na2B12H12
. Furthermore, although the energy state of the ionic salt varies with the metal species,
the extremely exothermic gas phase enthalpy of formation of the
[B12H12 ]2−
dianion suggests that its
formation is a more energetically favourable pathway when compared to the other polyborane species
proposed as participants in borohydride decomposition.
Table 5.
Key structural and thermodynamic properties tabulated for a range of polyborane anions that
have been investigated as intermediate phases during borohydride decomposition. The theoretically
predicted enthalpy of formation (
∆
H
formation
) at T = 0 K is given for the gas phase formation of each
anion. An approximate decomposition temperature (T
dec
) is given for the sodium salt (
Nan
(B
x
H
y
)
n−
)
of the given anion to allow for easy comparison between different species.
Anion ∆HT=0K
f orm ation (kJ mol−1)Tdec of Nan(BxHy)n−(◦C) Anion Geometry [112]
[B12H12 ]2−−328.4 [114] 612 (He flow) [115]
[B10H10 ]2−−26.8 [114] 577 (He flow) [116]
[B11H14 ]−−200.4 [114] 127 (He flow) [117]
[B3H8]−−72.8 [114] 100 (Ar flow) [113,118]
Beyond their relevance to borohydride desorption, extensive independent investigation of the
polyborane (B
x
H
y
/[B
x
H
y
]
n−
) cluster compounds has been conducted to clarify the details of their
unusual structural chemistry and properties. A significant volume of both neutral and anionic boron
hydride cluster compounds have been identified. Their chemistry is relatively well understood and
has been collected in numerous textbooks and reviews [
90
,
104
,
119
]. In general, the neutral polyborane
clusters have been found to be highly reactive and prone to explosive oxidation by oxygen or water.
They exhibit a unique form of electron-deficient bonding that can be described using polyhedral

Inorganics 2018,6, 106 18 of 38
skeletal electron pair theory, also known as Wade’s rules [
120
,
121
]. A wide range of applications have
been proposed for polyborane cluster compounds, including forms of optoelectronics, novel chemical
synthesis methods and various medical technologies [122].
3.4. Role of Mn=1,2 B12H12 in Borohydride Dehydrogenation
As discussed previously, the mechanism of dodecaborate formation during borohydride
thermolysis has not been conclusively determined and is widely disputed within the field. Table 6
summarises the proposed mechanisms leading to the formation of dodecaborates, including enthalpies
of reaction where available.
Table 6. Summary of the metal borohydride decomposition mechanisms that result in dodecaborate formation.
Metal Species Reaction ∆HT=0K
reaction (kJ mol−1) Reference
Li LiBH4−−→ 1
12 Li2B12H12 +5
6LiH + 13
12 H256 [34]
2 LiBH4+ 5 B2H6−−→ Li2B12 H12 + 13 H2Unspecified [59]
12 LiBH4−−→ Li2B12 H12 + 10 LiH + 13 H256.139 [60]
12 LiBH4−−→ Li2B12 H12 + 10 Li + 18 H2122.50 [60]
12 LiBH4−−→ Li2B12 H12 + 10 LiH + 13 H240.9 [58]
Na 2 NaBH4+ 5 B2H6−−→ Na2B12 H12 + 13 H2Unspecified [64]
12 NaBH4−−→ 10 Na + Na2B12H12 + 18 H2167.313 [91]
12 NaBH4−−→ 10 NaH + Na2B12H12 + 13 H2129.386 [91]
KKBH4−−→ 1
12 K2B12H12 +5
6KH + 13
12 H2117.5 [70]
Mg Mg(BH4)2−−→ MgH2+1
12 MgB12H12 + MgB4Unspecified [76]
6 Mg(BH4)2−−→ MgB12 H12 + 5 MgH2+ 13 H225 [58]
Ca 6 Ca(BH4)2−−→ CaB12 H12 + 5 CaH2+ 13 H234.2 [58]
Ca(BH4)2−−→ 2
3CaH2+1
3CaB6+10
3H235.2 [58]
Based on what is known of the thermolysis pathway, it is clear that by-products and intermediates,
such as dodecaborate compounds, play an important role. Despite this, many investigations of
borohydride decomposition fail to consider dodecaborates because of the sole use of XRD to identify
reaction products. Furthermore, because of the unusual properties of dodecaborates, there is
considerable controversy in the literature over their classification as a reaction intermediate versus
a process by-product. However, from a practical perspective, this distinction has less relevance, as
the formation of dodecaborates during dehydrogenation will present challenges either way. If any
dodecaborates remain in the system after a dehydrogenation process, they can have a significant impact
on the storage material’s cycling efficiency and the overall reversibility of the dehydrogenation process
(represented in Figure 9).
Dodecaborates, and other commonly reported by-products such as diborane, act as boron sinks,
meaning that their formation will lead to a slow degradation of the capacity of a storage material
over many dehydrogenation cycles. In the case of gaseous diborane, this is because boron is lost
with the release of hydrogen, but the stability of dodecaborates means that boron is trapped in
an unreactive decomposition product. This is illustrated by the hydrogen capacity degradation
observed after multiple hydrogenation cycles of the reactive hydride composites
LiBH4
–
MgH2
–Al and
LiBH4
–Al [
123
,
124
]. In both systems,
Li2B12H12
was observed to form during each decomposition step
and accumulate with consecutive hydrogen release and uptake processes. In this way, the reversibility
and storage capacity of a storage material is severely hindered by the formation of dodecaborates
during thermolysis.

Inorganics 2018,6, 106 19 of 38
Figure 9.
Schematic representation of the role of dodecaborates in the borohydride hydrogenation cycle.
4. Approaches to the Mitigation of Mn=1,2 B12H12 Formation
Although most efforts in the field have focused on confirming the presence of dodecaborates
in the dehydrogenation reaction scheme and clarifying their impact, some initial attempts have
been made to improve hydrogen storage properties by specifically targeting dodecaborate formation.
In general, the consequences of dodecaborate formation can be countered through one or more of the
following mechanisms: M
n=1,2B12 H12
destabilisation, improvement of M
n=1,2B12 H12
rehydrogenation
properties and alteration of decomposition pathways to inhibit the occurence of M
n=1,2B12 H12
compounds altogether.
Of those three options, pathway alteration is the most holistic, and therefore most preferable
approach. If decomposition can be forced through a kinetically and thermodynamically favourable
mechanism that does not include M
n=1,2B12 H12
formation, the entire hydrogenation cycle could be
optimised while also addressing the problems posed specifically by dodecaborates. However, tuning
of this pathway has proven challenging due to the sheer number of impactful variables that must be
accounted for and controlled. This uncertainty must be considered when interpreting the results of
investigations that report successful pathway alteration.
4.1. Catalysis of Mn=1,2 B12H12 Dehydrogenation and Rehydrogenation
Multiple studies attempting to mitigate the impact of dodecaborates on borohydride thermolysis
have focused on the use of some additive to destabilise and catalyse the dehydrogenation and
rehydrogenation reactions of the dodecaborates. One of the first experimental studies that utilised
this method was carried out on
CaB12H12
synthesised through a wet chemistry procedure using a
Cs2B12H12
precursor [
102
]. Although the focus of this study was on elucidating the crystal structure of
CaB12H12
, milling of
CaB12H12
with
CaH2
was also investigated as a possible method of improving
the hydrogen cycling properties of the dodecaborate species.
Attempts were made to rehydrogenate the ball-milled material at 397
◦
C under 100 MPa of
hydrogen pressure, but no
Ca(BH4)2
formation was detected. However, thermal pretreatment of the
ball-milled material at 597
◦
C under vacuum before following the same rehydrogenation procedure
resulted in the production of small amounts of crystalline
Ca(BH4)2
. It was proposed that this
Ca(BH4)2
formation might be the result of a reaction between
CaH2
and
CaB6
that could have been generated
during the thermal pretreatment step. This aligns with a theoretical prediction of the following reaction
between CaH2and CaB12H12 to produce CaB6(∆HT=0K
reaction = 38.6 kJ mol−1) [58].
CaB12H12 +CaH2−−→ 2 CaB6+7 H2(22)

Inorganics 2018,6, 106 20 of 38
This finding has great significance, as the regeneration of
Ca(BH4)2
from
CaB6
has been achieved
up to around 60% by using various catalysts (including
MgH2
[
40
], and numerous transition metal
compounds [
125
–
127
] and metal halides [
128
,
129
]) and shows much more potential for optimisation
than the direct rehydrogenation of CaB12H12 [43,87]. Therefore, the identification of a mechanism for
the conversion of
CaB12H12
to the much more reactive
CaB6
is a potential step towards improved
reversibility. In addition to that reaction during thermal treatment, ball-milling with
CaH2
also had
a pronounced impact on the direct dehydrogenation of
CaB12H12
. When heated up to 597
◦
C, pure
CaB12H12
experienced a total mass loss of less than 1.5% while the ball-milled mixture lost 6 mass%
under the same treatment (Figure 10).
Figure 10.
TGA curves for the desorption of
CaB12H12
(green curve) and ball-milled
CaB12H12
:
CaH2
(1:1) (blue curve). Reprinted from [102].
A similar study was later carried out for the alkali borohydrides (M = K, Na, Li) and their
analogous dodecaborate compounds (
K2B12H12
,
Na2B12H12
, and
Li2B12H12
) [
130
]. The first method
explored was the ball milling of the dodecaborate species with its corresponding metal hydride with
the aim of facilitating the rehydrogenation of the system to give a metal borohydride (Reactions
(23)–(25)).
Li2B12H12 +10 LiH +13 H2−−→ 12 LiBH4(23)
Na2B12H12 +10 NaH +13 H2−−→ 12 NaBH4(24)
K2B12H12 +10 KH +13 H2−−→ 12 KBH4(25)
Each of the rehydrogenation reactions was conducted at 500
◦
C under 100 MPa of
H2
pressure.
XRD characterisation before and after rehydrogenation showed that rehydrogenated samples were
primarily composed of crystalline
MBH4
(M = K, Na, Li), with small quantities of remaining
M2B12H12
and
MH
. Following from these results, a recent study attempted to clarify the mechanism of hydrogen
uptake in the
M2B12H12
–
MH
(M = Na, Li) system [
131
]. Under the milder conditions utilised in
this investigation (400
◦
C under 54.7 MPa and 97 MPa of
H2
), no reactions were observed in the
Li2B12H12 -10LiH system.
During treatment of the
Na2B12H12
-10
NaH
mixture at 400
◦
C under 54.7 MPa
H2
, a pressure
decrease was registered corresponding to a hydrogen sorption of approximately 1.5 mass%
H2
.
However, the phases produced during this hydrogenation process could not be conclusively identified
and no
NaBH4
formation was observed. The same unidentifiable intermediates were detected in
Na2B12H12
-10
NaH
samples treated under higher hydrogen pressure (400
◦
C under 97 MPa
H2
).
Although further investigation is required to clarify the composition of these intermediates,
11
B
MAS NMR characterisation revealed structural similarities between the intermediates and [
B12H12
]
2−
,

Inorganics 2018,6, 106 21 of 38
suggesting that they could be some form of closo-polyborate anion or polymerised icosahedral B
12
skeleton, as has been previously observed in studies of the thermal decomposition of NaB12H12 [94].
In another approach, the destabilisation of
Li2B12H12
using
MgH2
was investigated under the
assumption that this system would form a reactive hydride composite in a similar manner as has been
observed for
LiBH4
and
MgH2
[
132
]. Following this approach,
Li2B12H12
was ball-milled with
MgH2
,
with the aim of forming the more reactive binary compounds
MgB2
and LiH after
H2
desorption [
130
].
Li2B12H12 +6 MgH2−−→ 6 MgB2+2 LiH +11 H2(26)
The theoretical decomposition temperature of Reaction
(26)
is reported as 215
◦
C [
58
], but
desorption of the as-synthesised
Li2B12H12
–
MgH2
composite did not begin until the temperature
exceeded 380
◦
C and the rate of
H2
desorption did not peak until around 600
◦
C. Furthermore, the
cumulative hydrogen release during this desorption only reached 5.9 mass %, significantly less than
the 7.7 mass % theoretical capacity of the Li2B12H12–MgH2mixture.
The use of nanocrystalline cobalt boride (Co
1.34
B) has also been proposed to catalyse the
rehydrogenation of
Li2B12H12
[
133
], based on prior evidence of its catalytic properties in other
borohydride systems such as the hydrolysis of
NaBH4
and the desorption of the
LiNH2
–
LiBH4
composite material [
134
,
135
]. Hydrogen was desorbed from a ball-milled
LiBH4
-Co
1.34
B composite
and the desorption products were then rehydrogenated at 400
◦
C under 10 MPa of
H2
. The composite
released 5.1 mass %
H2
after the first hydrogenation cycle and 3.6 mass % after a second. Hence the
system achieved 68% reversibility at much more reasonable conditions than have been observed for
the rehydrogenation of un-catalysed
LiBH4
(76% reversibility after one cycle when rehydrogenated at
600 ◦C under 15.5 MPa of H2[33]).
4.2. Reactive Hydride Composites
While metal hydrides catalysts could potentially solve the problem of dodecaborate boron sinks
by allowing them to participate in borohydride regeneration reactions during rehydrogenation, other
approaches aim to fully inhibit the formation of dodecaborates through an alteration of the desorption
pathway. One such method is the combination of a borohydride species with another chemical hydride
to form a eutectic mixture, resulting in the formation of a reactive composite with a lowered reaction
enthalpy [
136
]. This enthalpy reduction is attributed to the exothermic formation of boron-containing
intermediate phases, that decreases the cumulative reaction enthalpy of the endothermic desorption
process [137].
While the conventional application of reactive hydride composites has been in the destabilisation
of borohydrides to promote more thermodynamically favourable desorption [
138
], a similar concept
has been proposed to prevent the formation of boron sinks such as dodecaborates [
139
]. In the
context of dodecaborate inhibition, the secondary hydride species essentially acts as a reversible boron
carrier (Figure 11). During the thermolysis of the composite material, this hydride reacts with the
decomposing borohydride to form a boron-containing compound. During rehydrogenation, the newly
formed boron-containing compound (e.g., MgNi
2.5B2
in Figure 11) should be more prone to give up its
boron than other stable boron phases such as Mn=1,2B12H12 that would occur otherwise.
Figure 11.
Schematic representation of the function of the
Mg2NiH4
boron carrier in the metal
borohydride (MBH4) hydrogenation cycle, shown without consideration of reaction stoichiometry.

Inorganics 2018,6, 106 22 of 38
Beyond acting as a boron carrier, the dopant hydride should also be lightweight and preferably
contain hydrogen so that the overall gravimetric capacity of the storage material does not suffer.
One such compound that has been proposed is the ternary hydride
Mg2NiH4
. In an extension of
their previous work with
LiBH4
composites, Vajo et al. investigated the desorption reaction of a
LiBH4
–
Mg2NiH4
compound synthesised by ball-milling of the two compounds [
140
]. They found
that the desorption of this composite material begins at a much lower temperature than either of the
component hydrides (Figure 12).
Figure 12.
Dehydrogenation of
4 LiBH4
+
5 Mg2NiH4
,
Mg2NiH4
, and
LiBH4
conducted using a
2◦C min−1temperature ramp under 4 bar of H2. Reprinted from [140].
The first step of the hydrogen desorption is attributed to Reaction (27), which includes the
formation of the boron containing ternary compound
MgNi2.5B2
. While a discussion of dodecaborate
formation was not considered in this investigation, a small loss of hydrogen capacity was noted after
the first cycle which could be attributed to the formation of a boron sink. However, the material was
able to complete 10 hydrogenation cycles, which demonstrates the promise of borohydride–
Mg2NiH4
composites, and validates the postulated advantages of a ternary boron carrier [140].
4 LiBH4+5 Mg2NiH4−−→ MgNi2.5B2+4 LiH +8 MgH2+8 H2(27)
These findings were further supported through similar results obtained for a
NaBH4
–
Mg2NiH4
composite material [
141
]. However, the most compelling evidence for the potential of
borohydride–
Mg2NiH4
composite materials is a study on
Ca(BH4)2
–
Mg2NiH4
conducted with the goal
of inhibiting the formation of stable boron sinks [
139
]. The
Ca(BH4)2
–
Mg2NiH4
system was synthesised
by ball-milling and then desorbed by ramping at 5
◦
C min
−1
from ambient temperature to 450
◦
C
under 0.1 MPa
H2
. The composite was characterised using
11
B MAS-NMR before dehydrogenation,
after dehydrogenation and after an attempt to rehydrogenate desorption products under 39.5 MPa
H2
at a temperature of 400 ◦C.
In the NMR spectra, a peak at
−
141.5 ppm was attributed to the boron carrier
MgNi2.5B2
(also
observed in the
LiBH4
–
Mg2NiH4
system [
140
]) and a peak at
−
32.6 ppm to
Ca(BH4)2
. Beyond those
major peaks, another small resonance peak was tentatively attributed to some B-H binary compound.
Hence, it was concluded that
Mg2NiH4
was able to successfully act as a boron carrier during the
composite desorption reaction, as no other boron containing compounds were observed in the products
beyond
MgNi2.5B2
and the small peak at
−
14.7 ppm. Based on these findings and other characterisation
by in-situ and ex-situ XRD, they proposed that the
Ca(BH4)2
–
Mg2NiH4
composite desorbs according
to the following reaction [139]:
Ca(BH4)2+2.5 Mg2NiH4−−→ CaH2+MgNi2.5B2+4 Mg +8 H2(28)
This successful inhibition of dodecaborate formation is significant, but the overall system requires
further optimisation. During their attempts to rehydrogenate the desorption products, Bergemann et

Inorganics 2018,6, 106 23 of 38
al. estimated from NMR spectra that only around 1/3 of
Ca(BH4)2
was reformed. It is believed that
this low reversibility results from kinetic barriers that could be overcome using some form of catalysis
or optimisation of reaction conditions. However, even if complete rehydrogenation could be achieved,
the reduction of theoretical gravimetric hydrogen capacity between pure
Ca(BH4)2
and the composite
material is considerable.
Investigation of reactive hydride composite systems has also provided additional evidence that
the application of hydrogen back-pressure during desorption can have an impact on the formation
of dodecaborates. For a
LiBH4
–
MgH2
–Al composite, samples decomposed under a p(
H2
) = 0.5 MPa
back-pressure reversibly formed a larger proportion of
LiBH4
compared to those cycled under lower
hydrogen pressure [
123
]. This was attributed to a decrease in the formation of
Li2B12H12
, confirmed
by XRD and
11
B MAS NMR. Similarly, in-situ measurements of the desorption of a
LiBH4
–
MgH2
composite under p(
H2
) = 0.5 MPa showed the composite preferentially decomposing to LiH and
MgB2[142].
4.3. Nanoconfinement
Investigations of the catalysis of M
n=1,2B12 H12
dehydrogenation/rehydrogenation and the use
of boron carriers have often attributed issues to kinetic barriers that inhibit the progression of
thermodynamically favourable reactions. Kinetic inefficiency is common in reactions with phase
separations, especially gas–solid reactions wherein slow mass transport across the reacting solid can
prove to be a limiting factor [
7
]. One widely utilised method of improving reaction kinetics is the
nanosizing of solid materials, which has been observed to alter their properties significantly and cause
them to react through different pathways than their bulk analogues.
However, the use of nanosizing to improve the properties of borohydride storage materials is
challenging given the tendency of nanoparticles to agglomerate under heat treatment. One method of
overcoming this tendency is to confine nanosized materials in a microporous matrix or other hollow
nanostructures. These configurations have been described as “nano-reactors” wherein solid reactants
are kept contained at nanoscale and improved reaction kinetics can be maintained after thermal
cycling [143] (Figure 13).
Figure 13.
Reversible behaviour of a sodium borohydride (
NaBH4
) nanoparticle embedded in a
nanoscale structure. In this scenario, the confined elements remain in close vicinity during hydrogen
cycling, which should facilitate hydrogen reversibility.
For example, during an investigation of the dehydrogenation pathways of
NaBH4
, reversible
hydrogen storage in
NaBH4
confined in mesoporous carbon was attributed to the reduced diffusion
distance between
Na2B12H12
and Na resulting from nanoconfinement [
69
]. Zhao-Karger et al. utilised
a similar method, but considered the reactive hydride composite
LiBH4
–
Mg(BH4)2
instead of a single
borohydride species, to investigate the interplay between the impacts of nanosizing and the property
modifications observed in mixed hydrides [
144
]. In this system, diborane emission was found to
be inhibited in the nanoconfined sample compared to the bulk
LiBH4
–
Mg(BH4)2
. This finding is

Inorganics 2018,6, 106 24 of 38
significant, as diborane inhibition seems to be crucial to the viability and reversibility of a borohydride
storage reaction, primarily because diborane emission results in boron loss from the system and
presents a safety concern.
11
B MAS-NMR spectra were also produced for both the nanoconfined and bulk
LiBH4
–
Mg(BH4)2
at different temperatures within the desorption range to elucidate the reaction pathway and
intermediates. The spectra suggested that at temperatures T
>
280
◦
C, most of the borohydrides
in the infiltrated samples had been converted to an amorphous elemental boron phase in a single
reaction step. This is a significant deviation from the desorption mechanism observed for the bulk
LiBH4–Mg(BH4)2, which proceeds via several steps through the formation of other boron-containing
intermediates, including MgB2[145].
Despite this observed pathway alteration, the presence of some
[B12H12 ]2−
anions was also
detected in the desorbed nanoconfined samples. Zhao-Karger et al. proposed that the dodecaborate
formation resulted from variation in pore size within the carbon matrix, such that some pores were
large enough that the confined
LiBH4
–
Mg(BH4)2
displayed bulk behaviour. While these findings are
compelling evidence of the effectiveness of nano-confinement, the system has fundamental gravimetric
limitations. In the nanoconfined
LiBH4
–
Mg(BH4)2
samples investigated, the maximum gravimetric
hydrogen capacity is only around 4 mass %, as the active borohydride-hydride only comprises 27% of
the total mass of the carbon composite material.
4.4. Perspective
Because of the lack of specific research that has focused on dodecaborates within the borohydride
system, there are a number of knowledge gaps that must be addressed. The underlying challenge is a
fundamental lack of understanding of the borohydride decomposition mechanism, as evidenced by the
number of conflicting pathways that have been proposed (see Section 2.1). Pathway alteration through
nanosizing or the use of a catalyst/dopant could be the ultimate solution to the challenge of capacity
loss during borohydride cycling. The suppression of dodecaborate formation that has been achieved in
certain systems through the application of hydrogen back-pressure also raises the possibility of tuning
the decomposition pathway by exerting fine control over the desorption conditions [88,123,142].
However, current attempts at alteration are exerted on a decomposition process that is essentially a
black box, which is unlikely to yield comprehensive or reliable results. Therefore, it is unsurprising that
none of the methods of dodecaborate mitigation that have been attempted have achieved completely
satisfactory results. Of the techniques that focus on destabilising dodecaborates after they have
formed, both investigations using alkali/alkaline earth hydrides reported sluggish rehydrogenation
kinetics and capacity losses in the rehydrogenated materials [
102
,
130
]. Furthermore, both of these
investigations attempted to directly rehydrogenate M
n=1,2B12 H12
compounds and did not consider
how the catalysts they used impact the overall dehydrogenation process.
The use of a “boron carrier”, such as
Mg2NiH4
[
139
], shows some potential, as it successfully
forces the boron of the decomposing borohydride to react with the secondary species instead of forming
dodecaborates [
139
–
141
]. However, this dehydrogenation process is not completely reversible (in the
Ca(BH4)2
–
Mg2NiH4
system only 1/3 of the initial
Ca(BH4)2
was regenerated [
139
]) and the composite
materials also suffers from a severe degradation of gravimetric capacity compared to the un-doped
borohydride species.
Nanoconfinement in mesoporous carbon results in similar gravimetric limitations, but the findings
of Zhao-Karger show more potential as indicators of the underlying factors that impact the borohydride
decomposition that could be exploited in other ways [
144
]. In particular, their conclusion that
nanosizing successfully alters the borohydride thermolysis mechanism to prevent dodecaborate
formation merits further investigation in a system wherein nanosizing impacts can be isolated from
the interactions of the borohydride with the carbon host.

Inorganics 2018,6, 106 25 of 38
5. Applications of Dodecaborates and Their Derivatives
The identification of the relevance of M
n=1,2B12 H12
compounds to borohydride thermolysis has
sparked a renaissance in dodecaborate research that is currently ongoing. This research has the
potential to yield exciting new technologies, especially in the development of new solid electrolytes
and a variety of innovative medical applications.
5.1. Lithium-Ion Battery Technology
One of the most intriguing potential applications for the dodecaborates and their derivatives is as
a solid-state electrolyte compatible with a variety of different battery configurations, including the
dominant lithium-ion battery. Lithium-ion batteries are discharged through the migration of lithium
cations from the anode to the cathode, liberating an electron that can be diverted to an external circuit
to perform work [
146
] (Figure 14). Lithium ions are conducted within the battery cell by an electrolyte,
which also serves to provide physical separation between the electrodes. In commercial lithium-ion
batteries, this electrolyte is usually a liquid-phase solution of a lithium salt dissolved in some solvent.
Figure 14. Schematic representation of the working principle of a generic lithium-ion battery.
The most common solvents are organic liquid carbonates, which have a number of prohibitive
drawbacks that have prompted a search for alternatives [
147
]. Their most critical weakness is their
flammability, which presents a serious safety concern and has already resulted in numerous incidents
of fire and explosion [
148
]. The development of an inorganic, solid-state electrolyte has the potential to
increase the stability of a lithium-ion battery by eliminating the flammability risk and increasing the
mechanical robustness of the cell. Beyond solving the safety issues, an entirely solid-state battery cell
would also simplify the overall configuration and facilitate higher overall energy densities [149].
The application of the dodecaborates as solid-state electrolytes was first considered upon the
discovery of a high-temperature order–disorder phase transition undergone by a number of alkali
dodecaborates that was accompanied by a significant increase in ionic conductivity [
97
,
98
]. As shown
in Figure 15, the cation sites of the low temperature structure are fully occupied. In contrast, the
high-temperature cubic phase can accommodate a variety of off-centre cation positions, allowing the
ions to be much more delocalised [
150
]. This cation delocalisation is the mechanism of conduction in
most of the superior solid ionic conductors, including
RbAg4I5
, which has one of the highest room
temperature conductivities reported for a solid material (0.12 S cm−1at 22 ◦C) [151,152].

Inorganics 2018,6, 106 26 of 38
Figure 15.
Schematic representation of the high-temperature order–disorder phase transition of
Li2B12H12
and its impact on the ionic conductivity in a lithium ion battery; molecular geometries
for the phases sourced from [115].
In these disordered phases, the
[B12H12 ]2−
anions were also found to undergo fast molecular
reorientations within the cubic unit cell [
150
,
153
]. In addition to the cation delocalisation, the high
reorientational mobility of the anions may also be contributing to the superior ionic conductivity of
the disordered phase [
154
]. When considering conduction in the solid-state, it has been proposed
that anions with high reorientational mobility move with the diffusing cation, thereby decreasing the
system’s resistance. In effect, these reorientations are thought to act as a “paddle wheel” that propels
the cation through the solid matrix [155].
Despite these findings, the high temperatures (Table 7) required to stimulate the order–disorder
phase transition in the pure alkali dodecaborates is an obstacle that must be overcome before these
materials can be considered for practical application. To exploit the favourable properties of the
disordered cubic phase, some method must be developed to stabilise it at moderate conditions and
prevent its conversion back to the ambient temperature phase. Alternatively, chemical modifications
could be considered to produce a dodecaborate derivative that achieves a lower transition temperature
without compromising conductivity.
Table 7.
Tabulated values for the order–disorder transition temperature and the ionic conductivity
reported at a given temperature for a selection of dodecaborates and dodecaborate derivatives.
Species Transition Temperature (◦C) Ionic Conductivity (S cm−1)
NaCB11H12 107 [156] 0.15 (130 ◦C) [156]
LiCB11H12 127 [156] 0.12 (110 ◦C) [156]
Ag2B12H12 200 [157] 0.035 (227 ◦C) [157]
LiNaB12H12 215 [158] 0.79 (277 ◦C) [158]
Na2B12H12 247 [97] 0.1 (267 ◦C) [154]
Li2B12H12 355 [98] 0.07 (277 ◦C) [158]
Na2(B12H12 )0.5(B10H10)0.5, N/A 0.0009 (20 ◦C) [159]
Ag(2+x)IxB12H12 N/A 0.002 (25 ◦C) [157]
Cs2B12H12 256 [97] -
Rb2B12H12 469 [97] -
K2B12H12 538 [97] -

Inorganics 2018,6, 106 27 of 38
Although this field of research has only recently attracted significant interest, several potential
modifications have already been proposed and investigated. Duchêne et al. ball-milled an equimolar
mixture of
Na2B12H12
and
Na2B12H10
, resulting in the composite
Na2(B12H12 )0.5(B10H10)0.5
material.
They found that this material did not undergo an order–disorder phase transition with heating, but
still achieved a reasonable 0.9 mS cm−1sodium ion conductivity at ambient conditions.
He et al. considered the impacts of a system that includes multiple cation species through
investigation of
LiNaB12H12
[
158
]. This composite material was produced by sintering of
LiBH4
,
NaBH4
, and
B10H14
, resulting in the bimetallic
LiNaB12H12
compound. This modification lowered the
phase transition temperature compared to the Li/Na analagoues (Table 7) and showed an extreme
peak in ionic conductivity to 0.79 S cm−1above 227 ◦C.
Beyond multi-cation systems, anion substitution has also been attempted as a method
of modifying the crystal structure of a dodecaborate species to improve room temperature
ion conductivity. The efficacy of this technique was demonstrated for
Ag2B12H12
, which
undergoes the characteristic polymorphic order–disorder transition and accompanying peak in ionic
conductivity at around 200
◦
C (Table 7). In comparison, an iodide substituted composite (with the
formula Ag
(2+x)
I
xB12H12
, where
x≈
1) displayed high ionic conductivity from room temperature
(Figure 16) [157].
Figure 16.
Ionic conductivities measured by Paskevicius et al. for silver dodecaborate and decaborate
Ag2B12H12
(red) and
Ag2B10H10
(blue) and the novel iodide substituted variations Ag
(2+x)
I
xB12H12
(green) and Ag
(2+x)
I
xB10H10
(purple), plotted with comparisons of other related materials [
157
].
Conductivities are plotted as a function of inverse temperature. Reprinted from [157].
When considering modifications to the dodecaborate cage itself, Tang et al. investigated the
substitution of a boron atom with a carbon atom to give the carborane anion
[CB11H12 ]−
[
156
].
While
[CB11H12 ]−
and
[B12H12 ]2−
share very similar icosahedral structures, the carbon substitution
results in a reduction in anionic charge from 2
−
to 1
−
and thereby halves number of alkali cation
required for salt neutrality. Both of the two carborane species considered,
NaCB11H12
and
LiCB11H12
,
were found to experience an order–disorder phase transition at temperatures significantly lower
than their pure dodecaborate analogues (Table 7). Most notably, it was also determined that the
ionic conductivities of
NaCB11H12
and
LiCB11H12
far exceeded the pure dodecaborates for the entire
range of temperatures probed. As shown in Figure 17, the carboranes show remarkably high ionic

Inorganics 2018,6, 106 28 of 38
conductivities, even at ambient temperature, and both peak in range of 0.1 S cm
−1
after their respective
phase transitions.
Figure 17.
Ionic conductivities measured by Tang et al. for
LiCB11H12
(blue) and
NaCB11H12
(red),
plotted with comparisons of other related materials. Conductivities are plotted as a function of inverse
temperature. Circles and squares denote the conductivities of the respective 1st and 2nd temperature
cycles. Closed and open symbols denote respective heating and cooling processes. Reprinted from [
156
].
Halogenation of the dodecaborate cage has also been considered, beginning as early as the
1980s with the investigation of perchlorinated lithium dodecaborate (
Li2B12Cl12
) as a solvated
electrolyte
[160,161]
. More recently, the thermal stability and conductive properties of the halogenated
variations of
Na2B12X12
(where X= Cl, Br, I) have been characterised [
162
]. While the order–disorder
transition temperatures for these compounds significantly exceed that of
Na2B12H12
, their exceptional
thermal stabilities indicate that they could be suitable for high temperature applications.
Although these preliminary observations are promising, all of the modifications discussed in this
section are in the very early stages of experimental investigation. For each of these systems, overall
battery chemistry and configuration must be considered, including the dodecaborate electrolyte’s
compatibility with commonly used electrode materials. While this specification will come with further
electrochemical testing, a greater fundamental understanding of the origins of the conductivity of
the dodecaborates is also required to aid in the optimisation of the properties of the dodecaborate
electrolytes [163].
It is also interesting to note that the borohydrides themselves have also been considered as
innovative alternatives to other components of battery chemistry [
15
].
LiBH4
has been proposed as a
conversion type anode material to replace the intercalation/insertion-type electrodes that are used in
current lithium-ion battery configurations [
164
]. Conversion type electrodes are advantageous because
of their high theoretical energy capacities (
LiBH4
has a theoretical capacity of 4992 mAh g
−1
, compared
to 372 mAh g
−1
for graphite, the most commonly commercialised anodic material [
165
]). Unfortunately,
this application of
LiBH4
has not been extensively investigated and preliminary studies have reported
low practical lithium capacities and poor reversibility of the electrochemical reactions [165,166].
5.2. Other Applications
Beyond their potential as solid-state electrolytes, the dodecaborates and their derivatives have
been investigated for application in a number of other fields. However, the largest volume of research
has been devoted to medical applications because of the low toxicity and resistance to hydrolysis of
the dodecaborates and their derivatives [89].

Inorganics 2018,6, 106 29 of 38
In medicine, the most notable use that has been proposed is as a boron source in boron neutron
capture therapy (BNCT), a novel cancer treatment that uses a boron-containing compound to “capture”
neutron radiation and selectively target cancerous cells [
167
]. Sodium borocaptate, a thiol derivative of
the
[B12H12 ]2−
anion with the chemical formula
Na2B12H11 SH
(often abbreviated as BSH, as shown
in Figure 18), is one of only two BNCT agents that have found extensive clinical application [
168
].
However, these clinical studies revealed a high degree of variability in the effectiveness of sodium
borocaptate as a boron delivery agent, partially attributed to uneven uptake of the drug by tumour
cells [169].
When considering dodecaborate derivatives, various materials have been proposed as carriers
to enhance the delivery of boron-containing compounds to tumour cells. Conjugation of sodium
borocaptate and other dodecaborate derivatives with organic polymers has been explored as a method
of improving transport properties [
170
,
171
]. A range of nanosized delivery vehicles has also been
investigated, including silicon nanowires [
172
] and boron cluster-containing redox nanoparticles [
173
].
Figure 18.
Schematic view of the
[B12H12 ]2−
anion vs. the
[B12H11 SH]2−
anion component of the
BNCT agent, sodium borocaptate (BSH,
Na2B12H11 SH
). Pink spheres denote boron atoms and white
spheres denote hydrogen atoms.
Liposomes, defined as an aqueous volume contained within lipid bilayer [
169
], are another notable
carrier compound that are useful for selectively transporting materials into tumour cells [
174
]. Sodium
borocaptate molecules can be contained in the aqueous volume and encapsulated by the liposome,
allowing the selectivity of the delivery system to be tailored through the design of the lipid bilayer.
For example, sodium borocaptate encapsulated in transferrin-PEG liposomes has shown effectiveness
at treating solid tumours in mice [
175
]. Additionally, sodium borocaptate can be incorporated into
the lipid bilayer, facilitating the synthesis of closo-dodecaborate lipid liposomes and maximising the
boron-content of the system [176,177].
6. Conclusions
In this review, the origins of the irreversibility of the borohydride hydrogen storage cycle are
investigated by analysing the mechanism of borohydride decomposition during thermolysis. The
formation of exceptionally stable dodecaborate compounds during hydrogen desorption is identified as
having serious implications for the reversibility of borohydride dehydrogenation. These dodecaborates
act as boron sinks that cannot be rehydrogenated and decrease the overall hydrogen capacity of the
material with each hydrogenation cycle. Review of the borohydride desorption mechanisms reported in
literature also reveals numerous instances of contradiction between different investigations, indicating
that the details of the decomposition process are not yet fully understood.
Furthermore, no mechanism for dodecaborate formation during borohydride dehydrogenation
has ever been conclusively determined, though analysis of dodecaborate synthesis procedures suggests
a possible route through polyborane intermediates. Adding another layer of complexity is the
noted dependence of both the borohydride decomposition pathway and the dodecaborate formation

Inorganics 2018,6, 106 30 of 38
mechanism on hydrogen back-pressure and desorption temperature. Based on these considerations, it
is the recommendation of this review that future research focus on establishing a consensus opinion
on the mechanism of hydrogen release from the borohydrides. Through an understanding of this
mechanism, stable by-products such as dodecaborates can be inhibited by exerting fine control over
the reactions and preventing decomposition via unfavourable pathways.
The unusual properties of dodecaborates also suggest a number of potential applications when
considered independently, most notably as a solid-state superionic conductor. The current universal
focus on identifying novel energy storage solutions could help to sustain the recent surge of research
interest and progress in the development of dodecaborates and their derivatives as solid-state
electrolytes. Further optimisation of BNCT technology also presents a dynamic research challenge with
a dodecaborate derivative at its center. When considering the dodecaborate derivatives, carboranes
in particular have been identified as a promising new iteration of closo-polyborate compounds that
could ultimately find utility. Overall, the dodecahydro-closo-dodecaborates are an important facet
of boron–hydride chemistry that will continue to have interdisciplinary relevance and intrigue into
the future.
Author Contributions: The manuscript was written by A.G. and K.-F.A.-Z.
Funding:
Financial support by the Office of Naval Research (Award No: ONRG-NICOP-N62909-16-1-2155) is
gratefully acknowledged.
Conflicts of Interest: The authors declare no conflict of interest.
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