Chemistry–Sustainability–Energy–Materials

Accepted Article

Title: Borophene and boron fullerene materials in hydrogen storage:

Opportunities and challenges

Authors: Jithu Joseph, Vishnu Sankar Sivasankarapillai, Sohrab

Nikazar, Muhammad Salman Shanawaz, Abbas Rahdar, Han
Lin, and George Z. Kyzas

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To be cited as: ChemSusChem 10.1002/cssc.202000782

Link to VoR: https://doi.org/10.1002/cssc.202000782

01/2020
ChemSusChem 10.1002/cssc.202000782

Borophene and boron fullerene materials in hydrogen storage:

Opportunities and challenges

Jithu Joseph1, Vishnu Sankar Sivasankarapillai2, Sohrab Nikazar3,

Muhammad Salman Shanawaz4, Abbas Rahdar5,*, Han Lin6,*, George Z.
Kyzas7,*

Accepted Manuscript
1
Department of Applied Chemistry, Cochin University of Science and Technology,
Kerala, India
2
Department of Chemistry, NSS Hindu College, Changanaserry, Kerala-686102, India
3
Chemical Engineering Faculty, Engineering College, University of Tehran, Tehran,
P.O.Box:14155-6455, Iran
4
Department of Chemistry, Sree Narayana College, Punalur, Kollam-691305, India
5
Department of Physics, University of Zabol, Iran
6
State Key Lab of High-Performance Ceramics and Superfine Microstructure, Shanghai
Institute of Ceramics,Chinese Academy of Sciences, Shanghai, 200050, P. R. China
7
Deaprtmet of Chemistry, International Hellenic University, Kavala, Greece

*Corresponding authors: George Z. Kyzas (kyzas@chem.ihu.gr); Han Lin

(inhan_cust@163.com); Abbas Rahdar (a.rahdar@uoz.ac.ir)

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ChemSusChem 10.1002/cssc.202000782

Abstract

Two-dimensional materials created a leap in material science research, especially in the

section of energy conversion or/and storage. Borophene and its spherical counterpart

boron fullerene represent the emerging type of materials which gained great attention in

the whole area of advanced energy materials and technologies. Owing to their prominent

Accepted Manuscript
features such as electronic environment and geometry, borophene and boron fullerene

endowed versatile applications such as supercapacitors, superconductors, anode materials

for photochemical water splitting, and biosensors. Herein, one of the most promising

applications/areas as hydrogen storage is discussed. Boron fullerenes have been

considered and discussed for its hydrogen storage applications, and recently borophene is

also included in the list of materials with promising hydrogen storage properties. Studies

focus mainly on doped borophene systems over pristine borophene due to enhanced

features available on decoration with metal atoms. This review article introduces the very

recent progress and novel paradigms on the aspects of both borophene derivatives and

boron fullerene-based systems reported for hydrogen storage, focused on the synthesis,

physiochemical properties, hydrogen storage mechanism and practical applications.

Keywords: borophene; boron fullerene; hydrogen storage; two-dimensional materials;

water splitting

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ChemSusChem 10.1002/cssc.202000782

1. Introduction

The research community is paying significant attention to tackle the problems

associated with energy demand owing to the need for tremendously rising population

levels. An effective solution for addressing the energy requirement is to find out novel

Accepted Manuscript
and efficient fuel resources that can replace the traditional energy resources which are

constantly depleting. Hydrogen can be characterized as one of the most advantageous

fuel system for commercial applications due to its features such as availability from

abundant resources as photo splitting of water, renewability, lightweight, and

[1]
environmental friendliness of the hydrogen energy process . Some explanation of the

above is the fact that fuel economy of conventional gasoline internal combustion engine

vehicles 2.5-2.7 times smaller than that of hydrogen [2].

Even though hydrogen is the hope for addressing the fuel demand for the future
[3]
, there are significant obstacles which prevent the practical implementation from the

system. The main reason is due to the excellent energy density by weight of hydrogen,

which makes the traditional approach to convert hydrogen to store in the form of liquid
[4]
hydrogen very difficult . Until today, various storage systems were reported for

hydrogen storage which can be widely categorized into two types: (i) chemical storage

and (ii) physical storage systems. Even though chemical storage systems offer excellent

features as strong bonding with hydrogen and high storage densities, the regeneration of

the storage medium cannot be achieved, which makes the practical application difficult.

Physical storage systems provide reversible storage but weak binding properties. The

department of Energy (DOE), USA has given the essential criteria for physical hydrogen

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ChemSusChem 10.1002/cssc.202000782

storage systems such as binding energy of 0.20 to 0.40 eV/H2 and at least 5.5 wt% in

gravimetric density at mild temperature and modest pressure (P<15 MPa). Surface to

volume ratio also plays a key role in determining the features of hydrogen storage

systems [5]. One of the most important advancements in the hydrogen storage materials is

the doping of host materials with metals. This metal decoration can resolve the problem

Accepted Manuscript
of weak binding between hydrogen and host materials and thereby enhance the storage
[6] [6]
capacity . The most common metals include (i) alkali metals as Li, Na, and K (ii)

alkaline earth metals as Ca [4] (iii) light metals as Al [7] and (iv) Transition metals as Ti [8].
[9]
Until now, nanomaterials have great attention about different applications . Especially,
[10]
there are several nanosystems reported for hydrogen storage as graphene , carbon

nanotubes [11], carbon nitrides [12] and silicone [13].

Borophene and boron fullerenes are the two most important systems which are

gaining interest as efficient hydrogen storage materials. In this review, we are discussing

the application of borophene and boron fullerenes for hydrogen storage applications by

focusing on the advancements reported in the literature. Various synthetic strategies for

borophene and Boron fullerenes were discussed, along with the common systems

reported as the hydrogen storage medium. Further, we also discuss the properties of

borophene and boron fullerenes, which make them interesting candidates among

researchers. Results of both pristine and doped systems were discussed in detail regarding

hydrogen storage.

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ChemSusChem 10.1002/cssc.202000782

Accepted Manuscript
Figure 1. Boron-based materials in hydrogen storage.

2. Preparation of borophene and boron fullerenes

2.1 Borophene

Generally, the preparation methods can be classified into: (i) chemical vapour

deposition (CVD), (ii) molecular beam epitaxy (MBE), and (iii) liquid phase exfoliation

(LPE). During the 1990s, physicists predicted the structure of two-dimensional (2D)

monolayer of boron atoms employing theoretical simulation models. Nowadays, there are

important articles that gather all appropriate data about the properties of boron-borophene
[14]
materials . The first experimental synthetic growth of borophene was reported by
[15]
Mannix et al. in 2015 . Under ultrahigh vacuum condition, atomic-scale borophene

sheets were realized on clean Ag(111) substrate which acts as an inert surface for

borophene growth. STM images show the planar structure of the borophene also shows

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ChemSusChem 10.1002/cssc.202000782

metallic and highly anisotropic characteristics which agree with theoretical simulations
[15]
. By chemical vapor deposition method CVD, Tai-et al. synthesized 2Dγ boron films

on copper foil by the usage of pure boron and boron oxide powders as the precursors [16].

γ-B28 film was identified as a semiconductor with a direct band gap of ~2.25 eV

by Kou et al. [17] which features a strong photoluminescence emission at 626 nm. It must

Accepted Manuscript
be noted that monolayer and bilayer γ-B28 films are intrinsically metallic, while the

thicker films possess intriguing electronic states that exhibit moderate to large bandgaps

in all the interior layers, but are nearly gapless at the surface. These surface electronic

states are tunable by strain, allowing the outermost layer to transition between a

semimetal and a narrow-gap semiconductor. Moreover, these surface states almost

exclusively occupy a wide energy range around the Fermi level, thus dominating the

electronic transport in γ-B28 films. The dispersions of the surface electronic bands are

direction sensitive, and with hole injection producing anisotropic and very high carrier

mobility up to 104 cm2 V−1 s−1. So, surface passivation can open a sizable bandgap, which

offers an additional avenue for effective band engineering and explains the experimental

observation of a large bandgap in the synthesized film. The same team identified

borophane to be ferroelastic with a stress-driven 90° lattice rotation in the boron layer,

accompanied by a remarkable orientation switch of the anisotropic Dirac transport

[18]
channels . These outstanding strain-engineered properties make borophane a highly

versatile and promising 2D material for innovative applications in

microelectromechanical and nanoelectronic devices [18].

Tsai et al. applied a combined boron ion implantation with plasma-assisted

technique to deposit a multilayer β-borophene on an insulating SiNx film on Si-substrate

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ChemSusChem 10.1002/cssc.202000782

[19]
. Li et al synthesized a few layers of boron sheets using the sonication assisted liquid-

phase exfoliation method. These sheets exhibit good diffraction with a d-spacing of 0.504
[20]
nm, agree with the presence of (104) plane of the β-rhombohedral boron crystal .

Kambe et al. synthesized borophene oxide layers (BoLs) by the oxidation of KBH4 in the

air dissolved in organic solvents [21].

Accepted Manuscript
The stability of the hexagonal network contains boron and bridging oxygen

atoms, which was found to be in accord with theoretical predictions. Feng (2016) et al.

using the molecular beam epitaxy(MBE) method synthesized 2D boron sheets β12 and χ3
[22]
sheets on an Ag(111) surface under ultrahigh vacuum (UHV) condition . These sheets

featuring triangular lattice but various arrangements of periodic holes are observed by

scanning tunneling microscopy (STM). Meanwhile, DFT calculations agree with the

experimental observations. Zhong et al. successfully synthesized borophene nanoribbons

[23]
(BNRs) by the process of self-assembly of boron on the Ag(110) surface . STM

images reveal the presence of boron nanoribbons. Self-assembly of BNRs also indicates

the strong template effect of the substrate on the growth of the boron. Kiraly et al.
[24]
synthesized borophene on Au(111) substrate . Boron diffuses into Au at elevated

temperatures and segregated to the surface to form borophene islands as the substrate

cools. The initial-growth modifications are agreed with theoretical simulations. Wenbin

Li (2018) et al. prepared graphene-as pure honeycomb borophene by Molecular beam

epitaxy (MBE) method using Al(111) surface as the substrate associated with the ultra-
[25]
high vacuum . STM images reveal the perfect monolayer borophene film with

honeycomb lattice. Theoretical simulations point out the fact that the honeycomb

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ChemSusChem 10.1002/cssc.202000782

borophene is stable on the Al(111) surface, due to the charge transfer between the

substrate and borophene film.

The synthesis of large-area high-quality borophene sheet on thin Cu(111) film on

sapphire realized by Wu et al recently [26]. They used the MBE method to synthesize the

borophene sheets of nano-scale thickness under UVH condition, and the images were

Accepted Manuscript
well-characterized by real-time feedback from low energy electron microscopy LEEM.

The free-standing borophene sheets (β12, χ3, and intermediate phases) were synthesized

by sonochemical exfoliation method by using acetone solvent as well as the reduction of

[27]
boron oxide BO, was first reported by Ranjan et al. . Electron microscopic techniques

confirm the presence of β12, χ3, and intermediate phases of borophene. XPS, STM

integrates with DFT band structure simulations agree with the phase purity and metallic

nature. TGA analysis points out that the borophene sheet is oxidized above 673 K in air,

so it is limited to systems working at ambient conditions.

[28]
Another important work of Xu et al. showed that borophene can be stabilized

by full surface hydrogenation (borophane). It was found that boropheane has direction-

dependent Dirac cones, which are mainly caused by the in-plane px and py orbitals of

boron atoms. The Dirac fermions possess an ultrahigh Fermi velocity of up to 3.5×106

m/s under the HSE06 level, which is 4 times higher than that of graphene. The Young's

moduli are calculated to be 190 and 120 GPa nm along two different directions, which

are comparable to those of steel.

2.2. Boron fullerenes

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ChemSusChem 10.1002/cssc.202000782

The major preparation strategies reported for boron fullerenes are: (i) laser

ablation, (ii) electron beam irradiation, and (iii) chemical methods. Placa et al. prepared

boron clusters (Bn, n=2-52) produced by laser ablation of boron nitrides with a 532 nm
[29]
from a frequency-doubled Nd: YAG laser using moving boron nitride disc . The

created species are ionized with a 193 nm laser and analyzed with (TOF) mass

Accepted Manuscript
spectrometer detected by an electron multiplier, the output of which is processed.

Goldberg et al. successfully synthesized BN fullerenes with a small number of

layers (≤3) by electron beam irradiation method [30]. Electron irradiation and observations

were carried out in a field emission type JEM-3000F electron microscope. Hexagonal BN

flakes prepared by heating a fused mixture of boric acid and urea. HRTEM images

support the presence of octahedral BN fullerene. Chemical composition studied using

electron energy loss spectroscopy (EELS).

[31]
Oku et al., prepared BN nanocapsules by two essential methods . The first

method by using precursor materials as boric acid, urea, silver nitrate completely

dissolved in deionized water. It is then dried in a rotating Vacuum drier and was reduced

at 300 and 700 °C in hydrogen gas for 7 h. After the annealing, the samples obtained. In

the second procedure, arc-melting was preferred with a vacuum Arc-melting furnace. A

white or gray powder was obtained around the pellets. HREM images captured and

analyzing the samples.

Narita et al., synthesized boron nitride (BN) fullerene nanomaterials by Arc-

melting of Al/b, TiB2, VB2, Ga/B, YB6, YB6, NbB2 powder in N2/Ar mixture gas, which
[32]
are confirmed by HREM and EDAX . It is found that the majority of nanotubes BN

nanotubes are formed when rare earth metals (Y, Zr, Nb, Hf, Ta, and La) are used as

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ChemSusChem 10.1002/cssc.202000782

catalytic metals. Samples are analyzed by HREM, and EDAX. Both analyses confirm the

formation of BN fullerene nanomaterials.

[33]
Wang et al. successfully synthesized BN hollow spheres via chemical route .

Sodium azide NaN3, BBr3 are used as the starting materials as hydrothermal method

conditions. XRD patterns, XPS spectral data, Energy dispersed spectrum, TEM, HRTEM

Accepted Manuscript
data supports the sample characteristics.

Shi et al. synthesized nanocrystalline boron nitride (BN) with the needle-as and

hollow spherical morphology nitriding of MgBr2 with NH4Cl, NH4Cl-NaN3. It is found

that NaN3may play a vital role in controlling the hollow spherical morphology of

nanocrystalline hexagonal boron nitride (hBN). XRD patterns observed for the samples

very close to the reported for (hBN) [34].

Recently Zhai et al. reported an experiment carried out using a magnetic bottle
[35]
PES apparatus coupled with a laser vaporization cluster source . In this work the

authors produced the B40− clusters using a 10B-enriched boron disc target using helium as

carrier gas seeded with 5% Ar and were mass selected using a time-of-flight mass

analyzer. This work gives the possibility of developing efficient synthetic strategies for

Boron fullerene-based clusters and needs further exploration.

2.3. Stability and properties

The stability of the different forms of boron- and borophene-based systems is a

controversial topic in the scientific community [36]. An experimental study based on first-

principles calculations investigated the mechanical, dynamic, and thermodynamic

stabilities of striped β12 and χ3 borophene (Figure 2).

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ChemSusChem 10.1002/cssc.202000782

Accepted Manuscript
(a) (b)

Figure 2. Crystal structures of (a) β12 borophene, (b) χ3 borophene.

The results showed that β12 and χ3 borophene have good mechanical, dynamic, and

thermodynamic stabilities; on the other hand, striped borophene is not equally stable. The

origin of the high stiffness and high instability observed in striped borophene, along the

“a” direction, can be attributed both to strong directional bonding and to high stiffness
[37]
along the “a” direction . Another study conducted by Gao et al. also confirmed the

thermodynamic stability of β12 borophene and χ3 borophene [38]. On the contrary, Penev et

al. showed that both β12 borophene and χ3 borophene could show naïve phonon-mediated
[39]
superconductivity . Furthermore, interesting results obtained by first-principles

calculations and STM image simulation indicated that β12 and χ3 sheets are
[40]
thermodynamically unstable . Due to the structural instability of borophene, some

researchers have theoretically hypothesized a fully hydrogenated structure of borophene

[41]
defined as “borophene”, a structure that showed excellent stability in silico . Finally,

the good optoelectronic properties and ability to be a superconductor allow borophene-

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ChemSusChem 10.1002/cssc.202000782

based nanostructures to be strategic for the production of nano-devices that are applicable

in different medical fields [42]. As is can be see, the stability of borophene-based systems

is a limiting factor that hampers the synthesis of these materials. Therefore urgent

attention has to be given in enhancing the stability of these materials. This might be

possible by incorporating stabilizing agents or combining with other materials that will

Accepted Manuscript
enhance the stability of the boron based systems.

Borophene as a new 2D material is crystalline two-dimensional boron sheets and

can be considered as a “cousin” of graphene. It seems to be polymorphic and presents

many states close to the ground-state energy line. The latter can be attributed to the
[43]
hollow hexagons (HHs) network having as basis the triangular lattice . Due to a

reduction in dimensionality, it exhibits different properties compared to the bulk form of

boron. For example, its mechanical properties are completely different and hence of high

interest. Firstly, because of the lower weight of boron, borophene has the lowest mass

density among 2D materials. Secondly, it has flexibility against off-plane deformation.

Boron fullerenes with different cage configurations have (more or less) higher stability
[44]
which can be resembled to that of carbon fullerenes . They are in the form of Bn,
[43]
where n can vary from 3 to 100 . Initially, larger boron clusters, lile B80, were found

(predictions) to create fullerene-like structure. The balance between 3-center- and 2-

center-bonds can explain the aforementioned “strange” stability [45].

3. Mechanism of hydrogen storage and available materials

Hydrogen is a highly abundant molecule which is tasteless, odorless, and

colorless. It is non-toxic and very famous for clean burning which means a clean

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ChemSusChem 10.1002/cssc.202000782

combustion without any kind of pollutants and greenhouse gases. Moreover, its high

chemical energy per mass (142 MJ), high energy content by weight (3 times more than

that of gasoline), low energy content by volume (4 times less than that of gasoline), and

fast burning make it a proper candidate to solve the global energy and environmental

concerns. Nevertheless, there is still a major barrier in the development of the hydrogen

Accepted Manuscript
related technologies (e.g. fuel cells) which is mainly about storage. It is of high

importance to find a safe, environmentally-friendly, and cost-effective method to store

hydrogen [46].

To date, several methods have been proposed and studied in order to store

hydrogen . Except compressing and liquifying hydrogen which are simple but inefficient

methods, hydrogen can be also stored in a variety of materials under different operating

conditions using two main mechanisms: (i) chemisorption, (ii) physisorption.

In the chemical storage method, the hydrogen is usually generated through a

chemical reaction. A reaction between atomic hydrogen and solids forms hybrids, such as

complex, chemical, and metal hybrids. Complex hybrids have shown high energy

densities, but the hydrogenation and dehydrogenation reactions are complex and

reversible which limits the complex hybrids applications. Chemical hybrids can provide

higher energy density because they are able to release hydrogen at a moderate operating

condition. An example of chemical complexes is using hydrolysis reaction which can

provide high theoretical hydrogen yield, but it has some drawbacks, such as irreversible

dehydrogenation reaction and too much heat generation. Hence, metal hybrids seem to be

the most feasible way to store hydrogen chemically due to its their effectiveness.

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ChemSusChem 10.1002/cssc.202000782

In physisorption, hydrogen adsorption occurs by weak physical forces at the

surface of the material. Physisorption is an effective method because neither high initial

pressure nor a large amount of energy input is required. The hydrogen bonding by

chemical and/or physical forces is advantageous from the safety point of view and

generally more beneficial than storing compressed and liquid hydrogen. Physisorption

Accepted Manuscript
leads to higher energy efficiency compared to chemisorption. Furthermore,

adsorption/desorption process occurs reversibly, at a lower temperature, and faster in

physisorption in contrast to chemisorption. However, the amount of absorbed gas is

larger in chemisorption. Although chemisorption is an effective way to store hydrogen,

there are some limitations that hamper its application, such as high temperature

adsorption/desorption process and high energy requirement for releasing the stored

hydrogen [47].

3.1 Available materials in hydrogen storage

Several materials are able to store hydrogen by chemisorption such as ammonia,

metal hybrids, Metal nitrides, amides, and imides. Ammonia (NH3), as one of the most

produced chemicals, can provide high hydrogen storage when it is stored in the liquid

form. It can produce hydrogen by reforming or mixing with the common fuels with no

CO2 emission (Figure 3).

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ChemSusChem 10.1002/cssc.202000782

Accepted Manuscript
Figure 3. The five steps in the process for producing energy from H2 from stored NH3,

and the commercial availability of the technologies. Reprinted with permission taken by

Elsevier [48].

Although, it is a toxic gas with a potent odor that cannot be completely removed from the
[49]
generated hydrogen, the process is not energy efficient . Metal hybrids (e.g. NaAlH4,

AlH3, LiBH4, Mg(BH4)2, NH3BH3, Li3NH, and MgH2) is another group of materials with

a superior ability of absorbing hydrogen and releasing at the temperature range of 120-

200 °C. They are good choices for the applications with the need of high safety and high
[5, 50]
hydrogen storage capacities . Furthermore, low-pressure equipment and little energy

requirements are the other advantages of this technology. Metal nitrides, amides, and

imides also exhibit promising hydrogen storage capacity, especially at low temperatures.

Nevertheless, the hydrogenation and dehydrogenation reactions are somehow complex

which is hampering their potential applications.

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ChemSusChem 10.1002/cssc.202000782

A group of materials which has been extensively studied and attracted attention in

hydrogen storage using physisorption is porous materials, such as carbon materials,

zeolites, metal organic frameworks, covalent organic frameworks, and covalent organic

frameworks. Carbon materials normally possess unique structures with high surface area.

Van der Waals bonding plays a key role in adsorbing hydrogen on the carbon surface. As

Accepted Manuscript
the surface area increases, the storage capacity of these materials increases which shows

the linear proportion of storage capacity of the carbonaceous material to their surface area
[51]
. Based on an estimation, approximately a surface area of 3300 m2/g will be required

to meet the hydrogen storage target of UFF US Department of Energy (US DOE) (7.5

wt%) [52].

Activated carbon (AC) is a high porosity form of processed carbon containing

graphite crystallites and amorphous carbon. It normally consists of pores with the

diameter of<1 nm and exhibits an average surface area of 3000 m2/g. An average

measured hydrogen storage of AC has been reported in the range of 0.5 and 5 wt% in the

literature [53]. Uner reported hydrogen storage of 12.87 mg/g in Arundo donax derived AC

under 805 mmHg and 77 K. Arundo donax was a precursor to produce AC with the

surface area of 1784 m2/g [54]

. Hirscher managed to reach 4.5% wt. hydrogen storage by

AC having surface area of 2560 m2/g at 77.4 K [51a]

. Nevertheless, some methods have

been examined in order to improve the AC capacity of hydrogen storage, including

palladium doping [55], platinum doping [56], grinding [57], and microwave treatment [58].

Graphite, sp2-hybridized having sheet as structure, was also studied as a hydrogen

storage material. Generally, pure graphite has low capacity of hydrogen storage due to its

low surface area and small interlayer distance (4.06 Å) [59]. It is reported that alkali metals

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ChemSusChem 10.1002/cssc.202000782

or alkali bonded to organic ligand introduction can increase the graphene sheets up to

8.7-12.4 Å [60].

Fullerene is aC60moleculewith pentagonal or hexagonal rings creating spherical

curvature. In its structure, each carbon is located into 12 pentagonal faces and more than

one hexagonal face. A metal atom supported fullerene attracts electron and leaves the

Accepted Manuscript
metal atom in cationic form. Therefore, the metal ions can trap molecular hydrogen.

Based on density functional theory, Li2C60 can store up to 120 hydrogen atoms.

Scandium and titanium coating is also studied in this area. Yildirim et al have studied

titanium and scandium coated fullerenes and found out that hydrogen storage capacity of

8 wt% is theoretically possible with binding energy range of 0.3-0.5 Ev [61].

Zeolites are another group of material with a remarkable hydrogen storage

capacity. They are microporous hydrated aluminosilicates with open and infinite 3D

structure. They have attracted massive attention due to their adsorption characteristics

and hence gas storage. Hydrogen moves into the open spaces present in the various pore

structure of the molecular sieve. This normally happens under elevated temperatures and

pressure. After cooling the zeolites down, the hydrogen molecule gets trapped inside the

cavities and by increasing the system temperature, H2 releasing process begins. The same

conclusion also exists for metal organic frameworks (MOFs). MOFs are a group of

porous materials composed of metal ions/clusters and organic linkers. They are promising

materials for H2 storage due to possessing high surface area of up to 6000 m2/g, as well as

some other significant properties as adjustable pore sizes and thermal stability.

Significant hydrogen storage capacity has been observed in MOF-5, MOF-177, MIL-53,

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ChemSusChem 10.1002/cssc.202000782

[62]
and MIL-101 . Despite having good H2 storage capacity at cryogenic temperatures,

operating at ambient temperatures is still challenging as well as acceptable pressures.

4. Hydrogen storage applications of borophene and boron fullerene

Borophene has been recently successfully discussed for its extraordinary

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hydrogen storage efficiency by the Department of Energy (USA). This value is very

much promising than other conventional materials used for the same applications and

thus made Borophene as a hotspot of research in developing as an efficient material for

Hydrogen storage applications. Studies are moving as theoretical route due to the

feasibility of the calculation process and the prediction of various borophene systems

having enhanced storage capacities. Researchers observed that doping borophene sheets

with alkali metals and other dopants as nitrogen can improve the level of the efficiency

regarding the H2 storage due to the redistribution of electronic environment/media and

conductivity compared to pristine borophene sheets.

Li et al. investigated that Li-decorated Borophene is exhibiting enhanced

hydrogen storage properties using three important functionals (vDW, LDA, and PBE)
[63]
based on DFT calculations . The higher binding energy and adsorption energy of Li-

decorated borophene demands this material is the right candidate for hydrogen storage

compared to undoped -borophene systems. The adsorption maximum was found to be 3-4

hydrogen molecules per Li atoms (Figure 4). It is found that one adsorbed Li atom can

uptake 3 molecules of H2 (this is the maximum level). Also, Li adsorbed atoms existed on

both sides of borophene may similarly bind/adsorb up to 4 H2 molecules. One of the

significant observations is that the storage capacity of hydrogen can reach up to 13.7

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ChemSusChem 10.1002/cssc.202000782

wt%; the latter can be happened presenting average adsorption energy equal to 0.142 and

0.176 eV/H2 for vDW and LDA functional. Li-borophene increases the H2 adsorption

with two “mechanisms”; either by charge transfer or/and orbital hybridization.

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Figure 4. The adsorption sites and the distances of B-Li, Li-H and H-H of H2 molecules

adsorbed on two Li-doped borophene. (a-d) H2 molecules on the 2Li/borophene with

n=1-4 per Li, respectively. Reprinted with permission taken by Elsevier [63].

Yujin Ji (2017) et al. observed that Li-decorated β12 and χ3 borophene are

potential hydrogen storage materials. Based on DFT calculations, Li-decorated χ3

borophene shows strong binding effects than that of Li-decorated β12 borophene systems.

The above methods show adsorption maximum of five hydrogen molecules per Li-atoms.

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ChemSusChem 10.1002/cssc.202000782

Liu et al. investigated the comparison of hydrogen adsorption capacity of pristine

β12 borophene based on DFT simulations. The same was achieved also for the case of Li-
[64]
decorated β12 borophene systems . Li-decoration enhances the hydrogen storage

capacity to a large extent supported by DFT calculations on pure β12 borophene and Li-

decorated β12 borophene systems. One to two Li-decorated β12 Borophene can absorb 7

Accepted Manuscript
to 14 hydrogen molecules, respectively.

Wang et al. reported another important addition about the efficient H2 storage of

some alkali metals (lithium, sodium, potassium) doped borophene systems [65]. So, it was

found that the decorated borophene of potassium and lithium showed relatively strong

binding strengths with H2 than that of Na doped borophene systems. Compared to three

alkali metal-doped materials, Li-doped one shows higher amount of hydrogen storage

capacity than any other novel Li-decorated materials.

Chen and co-workers were performed a theoretical investigation of hydrogen

storage capacities of Ca-decorated borophene S1, S2, and S3 systems based on DFT
[4]
calculations . The 2D configured types of Ca-decorated borophene which were

synthesized with finely tuned temperature are abbreviated as S1, S2 and S3 (Figure 5). S2

and S3 borophene systems were found to be efficient (promising) materials for H2 storage

than that of the S1 system. On the other hand, gravimetric hydrogen density of 9.5 and

7.2% found for S2 and S3 Ca-decorated borophene, respectively make them suitable H2

storage materials. DFT simulations point out that Ca atoms have strong binding strength

with borophene having Eb of -1.54eV on S1,-1.52 eV on S2, and -1.06 eV even on S3.

The higher binding energies do not allow any easy creation of calcium clusters. The

polarization mechanism and Kubas interaction support the adsorption maximum of 2

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ChemSusChem 10.1002/cssc.202000782

hydrogen molecules by S1 and 6 H2 molecules by S2 and S3. Ca-doped borophene of S2

and S3 can be treated as the most promising materials for Hydrogen storage than that of

the S1 system.

Accepted Manuscript
Figure 5. Top and side views of the charge density difference plots of Ca-decorated on

borophene. Blue area represents the accumulation of electron and yellow represents the

electron depletion. (For interpretation of the references to colour in this figure legend, the

reader is referred to the web version of this article). Reprinted with permission taken by

Elsevier [4].

Shahsavar et al. reported that the H2 storage properties of metalized borophene

[66]
enhanced by N-doping . Depending on the metal and N-content, the H2 adsorption

energies vary in the range between -0.11 and -0.21 eV. Metalized substrates having

higher Li-content doped with N shows high performance in H2 adsorption. At a higher

level of the dopant decrease in adsorption energy is observed.

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ChemSusChem 10.1002/cssc.202000782

Nagpal et al. studied the effect of the H2 storage capacity of alkali-metals (Li, Na,
[67]
and K) decorated borophene systems compared to pristine borophene systems . The

dramatic increase in hydrogen adsorption is mainly due to electrostatic interaction

between Hydrogen and metal-decorated borophene and Coulomb repulsion among

adsorbed H2 molecules. Sandip (2018) et al. also reported similar study on the H2 storage

Accepted Manuscript
[68]
capacity of Li, Na, and Ca incorporated and defective borophene systems . DFT

calculations show that alkali and alkaline earth metals decoration enhances the H2storage

capacity of Borophene based systems. Borophene shows excellent mechanical properties

as young’s modulus 398 GPa nm and exhibits Dirac transport properties and

superconductivity. About Hydrogen gravimetric density, 7.5 wt% shows borophene

systems.

Studies have made any indication that Li is an ideal alkali metal dopant on

borophene for the enhancement of H2 storage. Lebon et al. investigated that the H2
[69]
storage capacity Li-decorated β-sheet and alpha sheet phase of borophene systems . β-

sheet phase of borophene is more stable than the alpha form, due to the weak binding

effects with the H2 β-sheet phase of borophene is not suitable for hydrogen storage

(Figure 6). DFT studies on the Li-decorated beta form of borophene shows higher

hydrogen storage capacity by the inclusion of van der Waals forces than that of the alpha

sheet phase of the borophene systems.

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ChemSusChem 10.1002/cssc.202000782

Accepted Manuscript
Figure 6. From top to bottom, the most stable configurations for Li-decorated β-

borophene with 1, 1.5 and 2 H2 molecules per Li atom and for the Ti-decorated ZGNR

saturated with 4 H2 molecules per Ti atom. Reprinted with permission taken by Elsevier
[69]
.

Li et al. investigated that charged borophene nanosheets are excellent H2 storage

[70]
materials compared with neutral borophene systems . Based on DFT calculations, H2

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ChemSusChem 10.1002/cssc.202000782

adsorption energies of charged borophene nanosheets significantly rise as -0.231 eV for

positively, and -0.236 eV for negatively charged striped borophene nanosheets and that of

the neutral system is only -0.057eV. The presence of extra charges makes these results.

Wang et al. (2019) reported that the Li-doped alpha sheet of borophene shows
[71]
enhance hydrogen storage capacity . After Li decoration, the adsorption energy of H2

Accepted Manuscript
molecules increases and shows strong binding effects with borophene systems. Li- doped

alpha sheet phase borophene is most promising material than those alkali-metal decorated

TMDs and silicones using for hydrogen storage.

Apart from Borophene, Boron fullerenes have also studied as candidates for

hydrogen storage applications. Similar to Borophene systems, Boron fullerenes were

also modified with several dopants, which showed enhanced hydrogen storage

applications compared to pristine systems. Gaurab (2019) et al. studied the role of
[72]
solvents in hydrogen storage of the (BN)24 fullerene systems . DFT and Ab initio

simulations show that nucleophilic solvents can prevent oligomerization of the (BN)24 by

strongly binding to the fullerene. DFT calculations pointed out the fact that THF would

be an ideal solvent where (BN)24 hydrogenated to the extent of 5.13% hydrogen storage

medium. The solute-solvent interactions play a vital role in the dehydrogenation of the

hydrogenated fullerene systems associated with electrophilic or nucleophilic solvents.

Michael chojecki (2018) et al. is done an investigational theoretical analysis of the

stability of endohedral and exohedral complexes of B40 fullerene with the H2, N2, H2O,
[73]
and CO2 . The binding and interaction energies obtained from symmetry adapted

perturbation theory. Supermolecular approaches reveal the important observation that

B40 fullerene endohedral complexes show a weak stable complexation with H2 molecule

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ChemSusChem 10.1002/cssc.202000782

and repulsion with other molecules. Zero-point vibration correction for hydrogenated B40

fullerene shows that endohedral complexes are thermodynamically unstable at 0 K.

Si et al. investigated that sodium decorated B28 boron fullerene exhibits excellent

hydrogen adsorption (16 H2 molecules) than other molecules (Li, K, Ca, Mg, Sc, Y, Ti)

with 7.99% hydrogen gravimetric density more substantial than the suitable for hydrogen

Accepted Manuscript
[74]
storage (5.5wt%) . It is observed that B28 boron fullerene, smallest boron cage, and

contains one hexagonal hole and two octagonal holes allow them suitable for storage

applications (Figure 7). DFT simulations show that average hydrogen adsorption energies

in the range 0.2-0.6 eV, such that Na atom can adsorb 16 H2 molecules compared with

(Li, K, Mg, Sc, Y, Ca) corresponding (12, 14, 12, 12, 18 and 15) hydrogen molecules.

The desorption temperature and molecular dynamics simulations pointing out the fact

that those hydrogenated systems can desorb hydrogen makes the B28 boron fullerene,

potential candidate for hydrogen storage applications. The B28 cages can be combined by

the B5 chain to form the stable hydrogen storage structure -[B28(M3-nH2)-B5-B28(M3-

nH2)]m. sodium decoration on B28 boron fullerene acts as a potential hydrogen storage

system.

This article is protected by copyright. All rights reserved.

ChemSusChem 10.1002/cssc.202000782

Accepted Manuscript
Figure 7. (a)∼(e) Five lowest energy structures of the B28 cluster; (F) The hexagonal ring

(H) in the most stable B28 cage; (g)The octagonal ring (O) in the most stable B28 cage.

(h)∼(l) Five different isomers of B28M (M=Li, Na, K, Mg, Ca, Ti, Sc, and Y). (h) The

site near to the hollow site of the hexagonal hole inside of the B28 cage (IH); (i) The site

near to the hollow site of the hexagonal hole outside of the B28 cage (OH); (j) the cage

center (C); (k) The site near to the hollow site of the octagonal hole inside of the B28 cage

(IO); (l) The site near to the hollow site of the octagonal hole outside of the B28 cage

(OO). Reprinted with permission taken by Elsevier [74].

Liu et al. investigations are based on first-principle density functional calculations

[75]
. Fe, Co, Ni atoms show strong binding to the hexagonal holes of the B38 fullerenes

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ChemSusChem 10.1002/cssc.202000782

without clustering. Fe4B38 adsorb 24 H2 molecules with an average adsorption energy of

0.175 eV, and corresponding Co4 B38shows 0.18eV, Ni4 B38 having 0.202 eV. Fe

decoration makes another fascinating property of higher gravimetric density around

7.34%, Co4 B38 having 7.21% and for Ni4 B38 is 7.22%. Theoretical observations show

that transition metal decoration as alkali metal decoration enhances the hydrogen

Accepted Manuscript
adsorption properties of the B38 fullerenes system.

Liu et al. reported that the theoretical investigational studies in B40 fullerene

systems show that alkali-metal adsorbed (AM) B40 fullerene can serve as a potential
[76]
hydrogen storage material . From surface characterization, it is found that the surface

of B40 fullerene contains 4 Heptagonal rings and 2 hexagonal rings, which promotes

metal atom adsorption of the B40 fullerene. The position of AM atoms on the B40 surface

plays a vital role in the H2 adsorption properties. The higher binding energies enable the

relaxation of the clustering problem with B40 fullerene and make it a suitable reversible

hydrogen storage medium. The observed hydrogen gravimetric density of (AM)6 B40 is

around 8%, exceeds the target of the US Department of Energy. H2 storage capacities of

AM)6 B40 expressed in terms of hydrogen gravimetric density as follows (Li)6 B40 having

7.8%, for (Na)6 B40around 8.4%, and (K)6 B40 it is about 8.8% respectively. The

computed absorption spectra show the hydrogen molecule adsorption induces a red and

blue shift for (AM)6 B40 fullerene systems.

Ganguly et al. reported that first principle DFT simulation studies on B24N24

fullerene show that it is a potentially reversible hydrogen storage material having

[77]
hydrogen gravimetric density up to 5.13% . Theoretical observations are well fitted

with experimentally synthesized B24N24 fullerene. Hydrogenation and dehydrogenation in

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ChemSusChem 10.1002/cssc.202000782

B24N24 fullerene always carried out in the presence of solvents to prevent aggregation. It

is found that the hydrogenation and dehydrogenation of the metal-free B24N24 fullerene

can be achieved at reasonable rates based on the DFT calculations.

Dong et al. reported that DFT calculations titanium decorated B40 fullerene can be
[78]
chosen as an efficient hydrogen storage material . Comparison with other transition

Accepted Manuscript
metals Ti exhibits excellent binding properties with a B40fullerene system. The titanium

coated (Ti)6 B40 fullerene can uptake 34 H2 molecules, which corresponds to a maximum

hydrogen gravimetric density of about 8.7 wt%. The energy needed to add one H2

molecule is about 0.2-0.4 eV/H2, which makes the reversible storage of the H2 molecule

under ambient conditions. B40 fullerene with titanium functioning as an excellent medium

for hydrogen storage applications.

Lu et al. investigated that alkali and alkaline earth metal decoration (A=Li, Na, K,

Mg, Ca) on B38 fullerene show excellent storage properties based on DFT calculations
[79]
. The presence of metal atom favor face capping on the top of the center of hexagonal

holes of the B38 fullerene system. The elimination of the clustering effect of alkali and

alkaline earth metals is due to the considerable binding energy associated with it, which

enables them good for H2 storage applications.DFT simulations pointed out that for

Ca4B38 fullerene, store up to 20 H2 molecules, correlating hydrogen gravimetric density

about 6.47wt %. The average binding energy of the system is observed to be 0.0075-

0.240 eV/H2. Ca-decorated B38 fullerene systems show eminent hydrogen adsorption

properties resulting from Charge induced dipole interaction within the doped system.
[80]
Tang et al. used first-principles calculations to demonstrate that calcium-

decorated β12 nanosheets are the ideal materials for hydrogen storage where Ca can be

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ChemSusChem 10.1002/cssc.202000782

uniformly captured in the naturally hexagonal center. This kind of substrate was very

stable and up to six H2 molecules can be adsorbed successively to each Ca atom. The

B30·4Ca complex as a hydrogen storage material can store all 24 H2 molecules

at adsorption energies of ∼ 0.2 eV/H2, suggesting a maximum gravimetric density of

8.92 wt%. A deeper insight into the interaction mechanism was revealed, suggesting that

Accepted Manuscript
both orbitals hybridization and polarization have effect on the binding of H2 molecule to

Ca-decorated β12 boron sheet.

Wu et al. studied the hydrogen storage properties of yttrium doped B80 fullerene
[81]
using the DFT method by using the ab-initio code (SIESTA) . It is found that there is

an energetic enhancement in hydrogen adsorption without any clustering effect mainly

due to the Functionalization of Y atoms at the pentagonal site fullerene system.B80Y12

bind up to six H2 molecules yields a gravimetric density up to 6.85 wt% with an average

binding energy of single yttrium atom decorated B80 fullerenes are of -0.55 eV/H2. B80

fullerenes with Yttrium atoms exhibits enhanced hydrogen storage properties mainly

arise from Dewar-Kubas interaction and the polarization effects.

Er et al. studied the hydrogen storage properties of calcium decorated C48B12

[82]
boron-carbon hetero fullerenes by DFT calculations . The dispersed Ca atoms are

found to be thermodynamically stable against clustering and make them efficient material

for hydrogen storage applications. There are six H2 molecules per Ca atom adsorbed with

an average binding energy of 0.24 eV/H2 (DFT-D2) and 0.15 eV/H2 (DW91) simulations.

The adsorption properties mainly arise from the strong ionic character between Ca and

C48B12bond. DFT analysis formulating the fact that calcium doped C48B12 fullerene shows

a hydrogen gravimetric density of about 7.1 wt%.

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ChemSusChem 10.1002/cssc.202000782

4. Conclusions and outlook

Advancements in energy research have revealed the possibility of replacing

conventional fuels with clean fuels having environmental friendly nature and abundance.

But one of the major limitations arises in developing successful and efficient storage

Accepted Manuscript
systems for these clean fuels as Hydrogen. The search for novel energy storage systems

are always a hotspot of research comprising interdisciplinary areas of material science.

Reversible storage and strong binding are two sides of same coin regarding storage of

hydrogen. Borophene and boron fullerenes are novel hydrogen storage systems which are

emerging candidates as efficient platforms for storing hydrogen fuel. In this review we

have discussed the research progress of these materials in employing as hydrogen storage

media. We have also briefly addressed the synthetic strategies and properties of these

materials which make them unique candidates for various applications including

hydrogen storage. Even though the literature is progressing there exist the need of urgent

attention to make these results implemented for practical applications or from ‘lab to

reality’. One of the major problems to be tackled is the necessity of novel and efficient

synthetic methods of borophene. Since most of the preparation includes sophisticated

protocols, more feasible synthetic strategies as biological methods have to be elucidated

to assist rapid and cost effective preparation methods. The stability of borophene-based

systems is a limiting factor that hampers the synthesis of these materials. Therefore

urgent attention has to be given in enhancing the stability of these materials. This might

be possible by incorporating stabilizing agents or combining with other materials that will

enhance the stability of the boron based systems. Another challenge involves the lack of

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ChemSusChem 10.1002/cssc.202000782

promising experimental works in literature regarding borophene and boron fullerene

systems. Most of the works relies on theoretical tools which investigate the hydrogen

binding efficiency of pristine and metal doped systems. Researchers should aware of the

rapid developing field of theory that contrasts with the slower development of

experimental studies. This will adversely affects the research output which in turn makes

Accepted Manuscript
the data confined to an academic framework which would not give any practical benefits

for hydrogen storage applications. One of the best and effective way to address this

problem is to develop a synergistic approach consisting of theoretical and experimental

protocols. The theoretical part should focus methods as High throughput screening (HTS)

which helps to select suitable material for hydrogen storage from a library of available

materials and experimental part should verify and confirm the efficiency of the selected

system. We hope that this review will give valuable insights to researchers working on

hydrogen energy research to tackle the challenges and gap of the field and develop

successful hydrogen storage systems that are able to resolve the energy demands of near

future.

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ChemSusChem 10.1002/cssc.202000782

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