Introduction
Significant efforts are underway to find suitable renewable and
environmentally safe energy sources to address the energy crisis arising
from the depletion of fossil fuel resources and their detrimental
effects on the environment [1]. Hydrogen has gained major attention
as a clean, renewable source of energy that holds promise in addressing
the elevated emissions resulting from fossil fuels [2,3]. As
hydrogen has a better specific heat capacity as well as lesser toxicity
compared to other conventional fuels such as gasoline and methane, it is
considered a safer and more efficient fuel [4,5] compared to fossil
fuels. While large-scale production of hydrogen can be achieved via
methods like water electrolysis, gasification, steam reforming etc
[6,7], its utilization is severely limited by the absence of
efficient storage technologies to transport or contain it until usage.
For industrial usage, hydrogen has been primarily stored in
high-pressure cylinders, or as a liquid in cryogenic vessels. However,
these methods are not highly scalable owing to the safety concerns
stemming from the use of high-pressure containers as well as the high
price of storing hydrogen at cryogenic temperatures [8]. Previous
methods of storing hydrogen such as storing it in glass microspheres
were discontinued after experiencing high losses in the form of escaped
H2 from the containers [9]. In recent times, a new
approach to chemically store hydrogen in solid state materials using
adsorption techniques has gained major attention. Substrates with high
specific area, low cost, and chemical stability are thus being
extensively studied in this regard [10].
To filter out feasible materials that can be used for storing hydrogen,
the United States Department of Energy (DOE) has set a target of 6.5
wt.% for the system gravimetric density for storing hydrogen via
adsorption onto solid materials [11]. Materials like metallic
hydrides, mixed metal oxides, zeolites, carbon and boron nanotubes, and
metal organic frameworks have been previously shown to provide
convincing results as adsorbents due to their high surface area to
volume ratio and porosity. [12-19]. In systems such as metal
hydrides hydrogen is stored via chemisorption which involves the
hydrogen molecules chemically bonding to the adsorbents. This leads to
high hydrogen desorption energies and hinders the efficiency of the
system. For the adsorption process to be practically efficient, the
hydrogen molecules must be adsorbed onto the material with binding
energies that are sufficiently low to allow for easy desorption.
Materials that adsorb hydrogen based on physisorption are thus being
seen as a viable choice to store hydrogen.
Recent studies have found two-dimensional (2D) materials or Xenes to be
competitive adsorbents for storing hydrogen. Materials such as graphene
and phosphorene have been previously used in the batteries and
supercapacitors industry mainly due to their band gap and tuneable
electronic properties [20-26]. Defect engineering and metal
functionalization have been shown to significantly increase the
adsorption capabilities of graphene with boron and sulphur doped
graphene being able to adsorb unwanted gases in the atmosphere such as
NO2 and SO2 [27]. High-defect
density systems of graphene were reported to exhibit gravimetric
densities up to 5.81 wt.% H2 [28]. Ambrusi et al.
investigated hydrogen adsorption onto rhodium decorated graphene in
various configurations and reported hydrogen binding energies of up to
-1.08 eV/H2 [29]. The high binding energies were a
result of the strong binding between the d-orbital of rhodium and the
s-orbital of hydrogen due to Kubas interaction. Modifications using
other transition metals such as nickel and iron have been carried out on
phosphorene to obtain high hydrogen adsorption energies as well [30,
31]. However, the large mass of the transitional metal adatoms used
for substrate decoration leads to low gravimetric densities in many of
these systems. Transition metals can be replaced with lighter alkali
metals such as lithium and sodium which can bind hydrogen to the
decorated substrate with viable adsorption energies while simultaneously
increasing the gravimetric density of the systems [32]. For
instance, blue phosphorene nanosheets decorated with Li were shown to
have hydrogen binding energies of -0.25 eV/H2 as
reported by John et. al [33]. Liu at el. also found lithium
functionalized B2S to adsorb hydrogen with up to 9.1%
wt./H2 and average binding energies of -0.14
eV/H2 showing the effectiveness of lithium as a metal
adatom for physisorption based hydrogen storage substrates [34].
Many of these alkali metal-based systems however suffer from low binding
energies between the alkali metal adatom and the substrate, especially
in the case of graphene, which can lead to agglomeration of the metal
adatoms on top of the substrate thus reducing its efficiency and
stability.
Silicene is another 2-D material that shows a strong structural
resemblance to graphene due to its hexagonal repetition of silicon atoms
[35]. Unlike graphene however, the atoms in silicene arrange
themselves in a buckled fashion [36]. Silicene has found
ground-breaking applications in enhancing the performance of lithium-ion
batteries due to its high capacity with regards to lithium adsorption
[37-39]. Silicene is also being studied for possible applications in
spintronics, batteries and superconductors [40, 41]. Owing to its
structural and surface properties, applications of silicene have also
extended towards hydrogen storage in recent times. Metal functionalized
hydrogenated silicene or silicane was studied by Hussain et al. to
understand the effectiveness of hydrogen adsorption on its surface
[42]. Wang et. al adsorbed hydrogen on metal decorated silicene
systems to conclude that hydrogen adsorption in the case of Li and Na
was via physisorption whereas Sc and Ti bound to H2 in a
chemisorbed fashion [43]. Different structures of silicene such as
penta-silicene have also shown promising hydrogen binding energies of
-0.35eV/H2 with gravimetric densities of around 6.42
wt.% when decorated with Li [44]. Silicene also shows metal adatom
binding energies higher than the metal-metal cohesive energies in most
cases thus preventing metal adatom agglomeration. One approach to
further increase the hydrogen storage capacity of the substrate has been
to increase the number of viable sites via creation of defects onto the
surface. These defects, such as vacancies not only lead to an increase
in the metal-substrate decoration energies but also increase the
gravimetric density of the substrate. While the sole effect of defects
and metal decorations on hydrogen storage has been heavily discussed in
the case of other 2-D materials such as graphene, such literature is
sparse in the case of silicene. The combined effect of both metal
decoration as well as defects as a means of improving hydrogen
adsorption in silicene has also not been discussed in the literature.
This study aims to use Density Functional Theory (DFT) calculations to
understand the effect of lithium decoration and defects on the hydrogen
storage capabilities of silicene based systems. The single vacancy
defect and Stone-Wales defect were chosen due to their low formation
energies and were created by removing a Si atom and rotating a Si-Si
bond in the silicene monolayer respectively. The presence of defects not
only increases the metal adatom binding energies preventing clustering
and is more practical as defects are an inherent part of the material
synthesis process. Hydrogen adsorption was subsequently studied onto the
lithium decorated defective silicene substrates. It was observed that
lithium decoration greatly enhances the hydrogen binding energies in the
defective silicene systems. Double side Li decoration of the substrates
was further performed to investigate the total number of
H2 molecules that can be effectively stored onto these
defective substrates. An analysis of the projected density of states
(pDOS) plots of the Li decorated systems showed combined interactions of
the Li(s) and Li(p) orbitals with the H(s) orbitals which lead to
enhanced hydrogen adsorption energies. Charge density diagrams were also
plotted to conclude that there is a significant electronic interaction
between Li and H2 which leads to an improvement in the
hydrogen adsorption energies. Ab-initio molecular dynamics (AIMD)
simulations were performed at 300 K and verified that Li decoration and
subsequently H2 adsorption is stable on the defective
substrates. All the lithium decorated systems were found to show high
reversibility with the hydrogen binding energies being favourable even
for higher number of H2 molecules showing a maximum
gravimetric density of 5.97 wt.% H2.
2. Computational
Details
QUANTUM ESPRESSO was used to carry out spin-polarized Density Functional
Theory (DFT) calculations [45]. In order to approximate the exchange
correlation energy, the Generalized Gradient Approximation using Perdew,
Burke and Ernzerhof - PBE-GGA [46] was used. 45 Ry was chosen as the
cut-off energy for the plane wave expansion and the Projector Augmented
Wave (PAW) was used to describe the interactions between ions and
electrons. The Monkhorst-Pack scheme of 9x9x1 and 18x18x1 k-points were
used for relaxation and the electronic calculations respectively
[47]. To account for the van der Waals interaction between the
H2 molecules and the other atomic species, the DFT-D3
approach was used [48]. The structure geometries were relaxed using
the Broyden-Fletcher-Goldfarb-Shanno (BFGS) method [49]. Pristine
silicene was modelled using a monolayer of 32 atoms arranged in a 4x4
supercell with a vacuum spacing of 20 Å in the z- direction to avoid the
repeating unit cell interactions that occur due to periodic boundary
conditions. The unit cell parameter was found to be a = 3.83 Å which
matches with previous studies.
The formation energy for the pristine monolayer of silicene was assumed
to be 0 eV and the defect formation energy was then calculated using -
Ef = Edef -
NpEp (1)
where Ef is the defect formation energy,
Edef is the energy of the defective silicene system and
Np is the number of silicon atoms in the defective
system which was taken to be 31 and 32 for the single vacancy and
Stone-Wales defective system respectively. Ep is the
energy per silicon atom in the pristine system.
The lithium decoration energy was calculated as:
ELi ads = ELi+system –
(Esystem+ELi) (2)
Where ELi ads is the binding energy of a single lithium
atom onto the monolayer system surface, E Li+system is
the energy of the lithium decorated monolayer and
Esystem is the energy of the defective silicene
substrate. ELi is taken to be the energy of a single
lithium atom.
The hydrogen adsorption energies (EH2 ads) for multiple
pairs of H2 molecules on the lithium decorated systems
were calculated using:
EH2 ads = EH2+Li+system –
ELi+system + (NH2*EH2))/
NH2 (3)
Here, EH2+Li+system and EH2 refers to
the total energy of the hydrogen adsorbed lithium decorated structures
and the energy of a single H2 molecule respectively.
NH2 is the total number of hydrogen molecules adsorbed
onto the substrate at any given instance.
Gravimetric densities were also calculated for the substrates using –
Gravimetric Density = (n1WH2)/
(n1WH2 +
n2WSi +2WLi) (4)
Where WH2, WC/Si and WLiare the mass of the H2 molecule, silicon atom and
lithium atom respectively. The number of H2 molecules
and number of atoms in the silicene substrate are represented using
n1 and n2 respectively.