Dichalcogenides and difulfides nanostructures for hydrogen storage
Flavio Bento de Oliveira Andreia Luisa da Rosa




andreialuisa@ufg.br
Institute of Physica, Federal University of Goiás, Campus Samambaia, 74690-900, Goiânia, Goiás, Brazil.



 
Large-surface-area, two-dimensional (2D) layered materials are expected to have an advantage in hydrogen storage applications. Among a large number of 2D materials, monolayer dichacogenides have emerged as promising
candidate for hydrogen clean energy.  In this work we investigate MoS_2, porous MoS_2 and XS_2 as alternative materials for hydrogen storage. In this work we have investigated porous MoS_2 and XS_2 (X = Pd, Ni and
Pt) as alternative materials for hydrogen storage. Nickel selenide
appears as a viable candidate for hydrogen storage, boasting a
gravimetrics of 6.87%. We also show that MoS_2 nanopores could enhance
the gravimetry for hydrogen storage. This enhancement occurs through
the adsorption of H_2 molecules within the pore. Finally, AIMD calculations suggested that the structures identified in this work could be
stable even at room temperature.
  

Dichalcogenides and difulfides nanostructures for hydrogen storage
    Andréia Luisa da Rosa
    April 29, 2024
==================================================================





§ INTRODUCTION


The demand for energy development has continually increased, and
therefore energy consumption is constantly increasing, causing global
warming through the emission of pollutants into the atmosphere,
leading to a probable future shortage of fossil fuels. With this there
is a growing demand for renewable energy, with a growth rate per year
of 14.7% <cit.>. Hydrogen is a potential energy carrier
since its origin may come from clean sources. Nevertheless, storing
hydrogen at densities greater than its natural density poses a
challenge. There are four main methods for hydrogen storage:
compression, liquefaction, metal hydrides and physisorption. The
primary challenge lies in developing lightweight hydrogen storage
materials at reasonable costs. Hydrogen boasts three times more energy
value compared to fossil fuels <cit.>. Nevertheless, storing 1 kg of hydrogen requires
approximately 11 m^3 at ambient temperature, necessitating the
storage of hydrogen fuel in cylinders with pressures ranging from 20
MPa to 80 MPa <cit.>. This method is referred to as
compression. Another approach is liquefaction, which involves cooling
hydrogen down to 33.2 K. While hydrogen can be
stored in open systems using this method, the evaporation and
consumption of hydrogen are high and limited. The fuel is consumed in
a very short time, limiting its application to specific cases, such as
aircrafts <cit.>. The currently
used hydrogen storage in the industry is either metallic or in form of
complex hydrides. However these hydrides cannot resolve the high
pressure condition, and contain a large volume 
to be stored. Besides storage, another problem when using
these hydrides is the fact that after a few cycles the metals used end
up degrading or becoming brittle. Another problem often associated is
the thermodynamics of metal hydrides reactions, the large amount of
heat generated during refuelling, thus making it difficult to store
hydrogen.

It is stimates that storing hydrogen requires a weight (wt) between 4
wt% to 7 wt% of hydrogen <cit.>. This
has lead to the search for new technologies. In the search for new
materials, two-dimensional nanostructures have gained interest
following the discovery of graphene, whose properties differed
significantly from those of its three-dimensional counterpart,
graphite <cit.>. These materials exhibit electrical
properties highly appealing for technological applications and hold
promise for reducing the dimensions of currently produced electronic
devices. This has led to a growing interest in
exploration of other two-dimensional materials, in particular for
hydrogen storage <cit.>. It is expected
that large-surface-area, two-dimensional (2D) layered materials have
an advantage in hydrogen storage applications. Therefore, finding
materials that can effectively contain hydrogen is crucial for the
application of hydrogen energy.


MX_2 (M = Mo,W and X=S,Se,Te) have several polytypes the 2H (2H-MoS_2) has semiconductor behavior and 1T(1T-MoS_2) or 1T'(1T'-MoS_2) both metallic phase.   It has been suggested that the the performance of the layered material has a dependence with the structural phase <cit.>. For example, MoS_2 nanotubes showed 1.2 % wt hydrogen storage capacity. The authors attributed these results to the high specific surface area of MoS_2 nanotubes <cit.>. Another usual approach for improving hydrogen storage capacity is doping. Hydrogen storage performances of pure MoS_2 monolayer and the one decorated with 3d and 4d atoms were studied via the first-principles DFT calculations<cit.>.  Storage capacity of around 6 wt%. <cit.> has been achieved. Although doping is a possible route to enhance adsorption energies on MoS_2, doping has proven to be difficult in low-dimensional materials <cit.>. Previous investigations  on hydrogen  intercalation in MoS_2 bilayers <cit.> has suggested that the insertion energies of hydrogen molecules as a function of hydrogen concentration in MoS_2 can be accommodated in MoS_2 bilayers without a major change in the interlayer spacing.

Previous work on hydrogen intercalation in MoS_2
bilayers<cit.>.  The insertion energies of hydrogen
molecules as a function of hydrogen concentration in MoS_2 have
shown that at a resonable number of H_2 molecules can be
accommodated in MoS_2 bilayers without a major change in the
interlayer spacing.  Hydrogen can be liquified at ambient pressure at
a temperature of -253C, which increases its density to about 70
kg /m3, facilitating transportation of smaller volumes. Recently,
bulk and exfoliated MoS_2 were used to understand  hydrogen storage
potential of the bulk and exfoliated MoS_2 was addressed under 1-9
bar hydrogen pressure. After  the absorption process. The bulk and
exfoliated MoS_2 stored 220 and 450 sccm hydrogen,
respectively. Besides, th e weight percent hydrogen storage was
determined to be 1.2 and 2.4 wt%,
respectively<cit.>.

On the other hand similar layered structures of formed by Pd, Ni and
Pt TMDs remained largely unexplored with few publications focusing on
their electronic, mechanical, optical and thermoelectric properties of
monolayer PdS_2 and PdSe_2
 <cit.>. These structures show a different
reactivity than their MoS_2 counterparts and therefore could lead to other  weight percent hydrogen.

In this work we investigate MoS_2, porous MoS_2 and XS_2 as alternative materials for hydrogen storage. In this work we have investigated porous MoS_2 and XS_2 (X = Pd, Ni and
Pt) as alternative materials for hydrogen storage. Nickel selenide
appears as a viable candidate for hydrogen storage, boasting a
gravimetrics of 6.87%. We also show that MoS_2 nanopores could enhance
the gravimetry for hydrogen storage. This enhancement occurs through
the adsorption of H_2 molecules within the pore. Our AIMD calculations suggests that the structures identified in this work could be
stable even at room temperature.



§ COMPUTATIONAL DETAILS


In this work, we compute electronic and atomic properties using VASP
(Vienna Ab-initio Simulation Package) computational
code <cit.>. The calculations presented here have
400 eV. We use the PAW<cit.> method together with the
exchange-correlation functional in the PBE to describe the electronic
interactions <cit.>.  The criteria used for the convergence
of energy was 10^-4eV. AIMD calculations have been
performed using the Andersen thermostat at 300K. Time steps of for a
total simulation time of 5 ps. The NEB method is used to determine
the path which a atom on the crystal energy surface would take when
connected by two states, which are minimum energy states of the
system. NEB will seek for the lowest energy cost path between two
states. The path through the potential energy surface show us the
energy barrier which the atom needs to overcome to go to a minimum
energy state to another minimum energy state.



§ MONOLAYERS AND NANOPORES OF MOX_2


The most stable structure among the MoS_2 polytypes is 2H at ambient conditions.  Meanwhile, other polytypes of MX_2, including 1T-MoS_2 and 1T'-MoS_2 have been found <cit.>. These structures are metallic materials, while the 2H-MoS_2 structure exhibits semiconductor properties. 

[figure]style=plain,subcapbesideposition=top











Gravimetric analysis is a quantitative analytical technique that enables the determination of the amount of a substance in a given mixture, thus
the gravimetric hydrogen storage capacity for 2H-MoS_2 reached 6.97wt%, it was seen that the hydrogen molecules are inert on the 2H-MX_2 layer, being more energetically favorable the molecule perpendicular to the structures with type 2H-MX_2, molybdenum disulfide (MoS_2) demonstrates a structure with higher storage capacity in relation to other transition metal dichalcogenides. Adsorption energy of around 200 meV as shown in Fig. <ref>, for the 2H lies in the physisorption range. For the 1T' we see formation of bonds between hydrogen and the surface atoms.




In order to find a new route for hydrogen storage, defective 2H-MoS_2 was investigated. In our previous work we have investigated  several point defects <cit.>. The most common defects in MoS_2 are S and Mo vacancies. Beyond spontaneous defects, ion beam irradiation can also be used to promote larger defects <cit.> so we calculated the 2H-MoS_2 in vacancy defects in order to understand the behavior of hydrogen molecules in the presence of these defects, as shown in the Fig. <ref>. 




In order to estimate the possibility of additional H_2 molecules within the nanopores,  the reaction of H_2 molecules was investigated. We have performed  NEB <cit.> calculations to estimate the potential barrier for the molecule to diffuse through the pore.  The NEB was performed along a path perpendicumar to  the MoS_2 plane. The intermediate states are shown in Fig. <ref>. Five images were chosen along the diffusion path. Along the chosen path a barrier of 0.35 eV is found, as shown in Fig. <ref>. This implies that additional molecules could reside within the pore, thus enhancing the storage capacity of 2H-MoS_2. 



The gravimetric storage of H_2 adsorbed on the defective layers containing one vacancy results in 7.09 wt%. Defect engineering could be a path for increasing hydrogen storage capacity.



§ INTERCALATION OF H_2 IN BILAYERS OF MOS_2


Effective approaches to fabricate bandgap engineered bilayer MoS_2 structures, an important step to fabricate homo/hetero-structures, and thus advanced electronic devices have been recently developed <cit.>. This is specially interesting for energy storage applications, since molecules can be trapped between the layers. We now discuss hydrogen intercalation in MX_2 bilayers. The relaxed configurations are shown in Fig. <ref>. We calculated the gravimetric hydrogen storage capacity for the 2H-MoS_2 and 1T'-MoS_2 bilayers respectively. The gravimetric hydrogen storage varied from 3.39 wt% and 2.17wt%, as shown in Tab. <ref>. We analyzed the adsorption energy of the 2H-bilayer and 1T'-bilayer as a function of the H_2 numbers.  The results are shown in Fig. <ref>











§ MOLECULAR DYNAMICS OF H_2 ADSORBED ON MOS_2


In order to investigate temperature effects of adsorption of  H_2 on both polytypes, 1T'-MoS_2 and 2H-MoS_2 MD simulations were performed at a temperature of 300 K with the Andersen thermostat bath in contact with the system. The RDF revealed that for the 2H phase, the equidistances equivalent to the first and second neighbors of a referential Mo, are illustrated in Ref. <ref>. Additionally, the structure of 2H-MoS_2 did not shown structural distortion.





On the other hand, for 1T'-MoS_2, owned to the asymetry between the S-Mo-S bonds, additional peaks were seen, as depicted in Fig. <ref>(b).







§ MOX_2 (X= PD, PD AND NI)






Next we investigate XY_2 nanostructures, namely (X = Ni,Pt,Pd) and
(Y = S, Se) in 1T and 2H polytypes. First we discuss the thermodynamic stability of pristine structures. We find that the dichalcogenides,
1T-NiS_2, 1T-PtS_2, and 1T-PdS_2 are higher in energy compared to their respective 2H polytypes (2H-NiS_2,
2H-PtS_2, and 2H-PdS_2).  The lattice parameters for 1T-NiS_2,
1T-PtS_2, and 1T-PdS_2 are 3.35 Å, 3.54 Å, and 3.56 Å,
respectively. In the 2H polytype, 2H-NiS_2 has a lattice parameter
of 3.54 Å, 2H-PtS_2 has 3.48 Å, and 2H-PdS_2 has 3.92
Å, as shown in Figs. <ref>(a)-(c). For diselenides,
a similar trend is found for NiSe_2, PdSe_2, and PtSe_2. The 1T
polytypes show higher thermodynamic stability with lattice parameters
of 3.53 Å, 3.74 Å, and 3.75 Å, respectively, compared to
the 2H polytypes with lattice parameters of 3.56 Å, 4.01 Å,
and 3.63 Å, as depicted in Figs. <ref>(d)-(e).
	






Adsoprtion energy for a single molecule on <ref>.  It
was found that due to the low concentration of H_2 in the network,
it caused the dissociation of H_2 on PdS_2 and PtS_2, forming
S-H bonds.  When compared to adsorption at high
concentrations of H_2, the adsorption energy shows a dependency on
the concentration. As the concentration of H_2 increases, the
available adsorption sites on the surface become unavailable. As all surface
sites become occupied, the addition of further H_2 molecules does not
lead to a significant decrease in adsorption energy.



The adsorption energy of the investigated structures reached the order of 100 meV. A close-up view of the region with high H_2 concentration reveals that as the H_2 concentration increases, the available adsorption sites on the surface  become saturated, forming layers of H_2 as shown in Fig. <ref>.

For 1T-NiS_2, it was seen that for a quantity of H_2 adsorption molecules higher than 28 molecules, the nickel disulfide (NiS_2) structure exhibits structural disorder caused by the absorption of H_2, forming S-H bonds as demonstrated in Fig. <ref>. A storage capacity of 10.8 wt% was obtained. Alternatively, considering the quantity of 28 H_2 molecules, the gravimetry is 8.83 wt%.  Calculations show that for 2H-NiS_2, the gravimetric value is 11.44 wt%, and there is no significant structural distortion.



For 1T-PdS_2, a similar behavior was obtained from 28 H_2 molecules. This result is independent on the molecule orientation. A storage capacity of 8.04 wt% is found with a small amount of H_2 dissociating on the surface sulphur atoms (0.3%). Due to larger weight of Pd compared to Ni, 2H-PdS_2 was found to have  a storage capacity of 8.53 wt%.



It was found that for the 1T-PtS_2 polytype, the H_2 adsorption behavior was similar to 1T-PdS_2. However, due to the fact that Pt is heavier than Pd, the storage capacity of 1T-PtS_2 was 5.45 wt%, whereas for 2H-PtS_2, it was found to be 5.79 wt%, as shown in Tab. <ref>.




Adsorption on  dichalcogenides resulted in the dissociation of H_2 and a modification of the structure, leading to structural changes. Nonetheless, when compared with the 1T-NiS_2 structures, an increase in storage capacity from 8.61 wt% to up to 11.44 wt% was found. Therefore, both NiS_2 polytypes and PdS_2 polytypes can be considered promising candidates for hydrogen storage.



§ CONCLUSIONS


In this work we have investigated porous MoS_2 and XS_2 (X = Pd, Ni and
Pt) as alternative materials for hydrogen storage. Nickel selenide
appears as a viable candidate for hydrogen storage, boasting a
gravimetry of 6.87% wt. We also show that MoS_2 nanopores could enhance
the gravimetry for hydrogen storage. This enhancement occurs through
the adsorption of H_2 molecules within the pore. 
AIMD suggested that the structures identified in this work could be
stable even at room temperature. 














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