A First Principles study on Boron-doped Graphene decorated by Ni-Ti-Mg atoms for Enhanced Hydrogen Storage Performance

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received: 17 July 2015

accepted: 19 October 2015

Published: 18 November 2015

# A First Principles study on Boron- doped Graphene decorated by Ni-Ti-Mg atoms for Enhanced Hydrogen Storage Performance

Santhanamoorthi Nachimuthu, Po-Jung Lai, Ermias Girma Leggesse & Jyh-Chiang Jiang

We proposed a new solid state material for hydrogen storage, which consists of a combination
of both transition and alkaline earth metal atoms decorating a boron-doped graphene surface.
Hydrogen adsorption and desorption on this material was investigated using density functional
theory calculations. We find that the diffusion barriers for H atom migration and desorption energies
are lower than for the previously designed mediums and the proposed medium can reach the
gravimetric capacity of ~6.5 wt % hydrogen, which is much higher than the DOE target for the year
2015. Molecular Dynamics simulations show that metal atoms are stably adsorbed on the B doped
graphene surface without clustering, which will enhance the hydrogen storage capacity.

Hydrogen is one of the promising alternative energy sources for future generations due to its applications
in hydrogen fuel-cell powered vehicles 1–3 . However, designing solid state material for hydrogen storage
with high gravimetric and volumetric densities near ambient thermodynamic conditions is a challenging task 4,5 . In recent years, many systems have been proposed to store hydrogen, but none of them has
achieved the US Department of Energy (DOE) target for 2015. Therefore, it is essential to explore new
materials with desired properties for efficient and economic ways of storing hydrogen.

Carbon-based materials such as graphene and carbon nanotubes have been designed for hydrogen
storage due to their large surface area, light weight and tunable properties 6–9 . However, the interaction of
carbon with hydrogen is only through weak van der Waals forces and hence their binding energies are
very low, resulted in decreasing the storage capacity 10,11 . In recent years, carbon-based nanostructures
decorated with alkaline earth metal atoms (AEM) 12–14 and transition metal atoms (TM) 15–23 are proposed
to increase the interaction between the hydrogen molecules and the host materials. The decoration by
TM atoms increases the binding ability of hydrogen, because of the hybridization of H2 σ or σ * orbitals
with _d_ -orbitals of TM atoms (the so-called the interaction typical of Kubas complexes) 24–26 . However,
because of the large cohesive energy, TM atoms form a cluster, which decrease the hydrogen storage
capacity 27,28 . The recent studies suggest that the doping of Boron (B) atom on graphene can prevent the
clustering of the isolated TM atoms and increase the hydrogen storage capacity 29–32 . Also, Louie _et al_ . 33
reported that the B-doping enhanced the binding energy of Ca atom and it suppresses the aggregation
of Ca atoms.

Most of the previous studies describe the hydrogen storage medium decorated by either AEM or TM
atoms on the graphene surface. However, in the former case, the adsorption energy of hydrogen is too
small because of weak interaction and hence the number of adsorbed hydrogen molecules is less, whereas
in the latter case, desorption of hydrogen is challenging because of its larger adsorption energy. Recently,
we decorated a B-doped graphene sheet by three different TM atoms (Ni-Pd-Co) and reported their
hydrogen storage efficiency 34 . But, those TM atoms are heavy and their wt.% is also comparatively low.
Hence, in this present study we propose a new strategy to enhance both hydrogen adsorption/desorption

Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan,
[R.O.C. Correspondence and requests for materials should be addressed to J.C.J (email: jcjiang@mail.ntust.edu.tw)](mailto:jcjiang@mail.ntust.edu.tw)

Figure 1. Side view of initial (IS), Transition (TS) and Final state (FS) geometries for the diffusion of
one H atom from Ni to Ti in (a) minimum and (b) maximum hydrogen coverage.

at ambient conditions and to reach the gravimetric goal of DOE. Here we considered combination of
both TM and AEM atoms to decorate the B-doped graphene (BDG) surface and investigate the adsorption and desorption of hydrogen using first-principles calculations. To the best of our knowledge, the
combination of TM and AEM atoms for hydrogen storage application has not yet been reported.

Results and Discussion
As reported in our earlier study and previous studies 34,35, the decoration of graphene by metal atoms
improves the hydrogen storage capacity. In order to increase the wt.% from our previously considered
TM atoms, here we considered Ni and Ti of TM atoms in one end to adsorb the hydrogen and Mg
of AEM atom for another end to desorb. We have carefully chosen these metals because of its strong,
medium and weak hydrogen adsorption energies. The calculated hydrogen adsorption energies for TM
atoms (Ni and Ti) are − 0.92 and − 0.52 eV, and for Mg it is nearly zero (− 0.01 eV) 34 . More importantly
the molecular weight of these atoms is much smaller than the previously considered atoms. These three
metal atoms (Ni-Ti-Mg) are used to decorate the BDG surface and we investigate the hydrogen storage
capacity by the spillover mechanism. Previous studies reported that the hydrogen storage capacity can be
improved by means of the spillover mechanism 36–40 . We expect that the relatively low hydrogen adsorption energy of Mg will contribute a low hydrogen desorption energy and hence, hydrogen can desorb at
room temperature, which is essential for the practical applications.

In order to find the possible binding sites of these metals on the BDG surface, we considered three
different sites such as 1) hollow site, (the center of the ring) 2) bridge site (in between the two atoms)
and 3) top site (top of the atoms) for the adsorption of a single metal on BDG surface. The binding
energy ( _E_ _b_ ) of these metals with surface is calculated using _E_ _b_ = ( _ETotal_ - _E_ _Metal_ - _EGraphene_ ) , where
_ETotal_, _E Metal_ and _EGraphene_ are the energies of total system, Metal atoms and B-doped graphene surface,
respectively. These calculations indicate that these three metal atoms Ni, Ti and Mg, are most stably
adsorbed on the surface by hollow sites with binding energies of − 3.53, − 2.10 and − 1.49 eV, respectively.
It is found that the calculated binding energies for Ni and Ti atoms on BDG surface are larger than the
pristine graphene layer 41, which indicate an increase in the stability of the materials after B-doping. As a
first step, we considered trimers of each of these three metal atoms adsorbed on the BDG surface via
three different sites such as, top of the boron atom (Site A), top of the carbon atom which is surrounded
by three boron atoms (Site B) and top of the carbon atom which is surrounded by two boron atoms (Site
C). Here, we determined the trimer of each metal atom that would increase the wt.%. of hydrogen storage
and also the distance between the two different types of metal trimer in surface that would be most
favorable for hydrogen diffusion. The top views of the three possible binding sites (Sites A, B and C) for
Ni metal trimer adsorbed on BDG surface are shown in Figure 1S of supplementary material. The calculated binding energies for different sites are given in Table 1. As can be seen from this table, Site A and
Site B have strong binding energies for both the TM atoms and AEM atom, respectively. It was reported
that strong binding energies of the metal atoms with the surface can prevent clustering, because of strong
covalent bonds between the metal atoms and the surface 42 . Therefore, we considered the geometries with
strong metal surface binding energies for the further calculations.

Metal

Binding
Site Eb _d_ M-G _d_ M-M

Site A - 4.99 1.54 2.40

Ni Site B - 4.57 1.55 2.43

Site C - 4.69 1.54 2.41

Site A - 8.43 1.61 2.53

Ti Site B - 8.26 1.76 2.52

Site C - 8.37 1.71 2.55

Site A - 3.62 1.79 3.03

Mg Site B - 3.85 1.91 3.08

Site C - 3.82 1.90 3.12

Table 1. The calculated binding energies (Eb in eV) of metal trimer on graphene layer with three
possible binding sites and nearest distances of metal to graphene surface ( _d_ M-G in Å) and metal to metal
( _d_ M-M in Å).

Number
of H2

Ni Ti

Eads Eads/H2 Eads Eads/H2

1 - 1.90 - 1.90 - 2.02 - 2.02

2 - 2.51 - 1.25 - 3.11 - 1.56

3 - 3.00 - 1.00 - 3.13 - 1.04

4 - 3.71 - 0.93 - 3.12 - 0.78

5 - 3.72 - 0.74 - 3.62 - 0.72

6 - 3.74 - 0.62 - 3.67 - 0.61

7 - 3.77 - 0.54 - 3.67 - 0.52

8 - 3.81 - 0.48 - 3.66 - 0.46

9 - 3.83 - 0.43 - 3.69 - 0.41

Table 2. The calculated total hydrogen adsorption energies (Eads in eV) and adsorption energies per H2
molecule (Eads/H2 in eV) for Ni and Ti trimer decorated BDG surface.

To investigate the hydrogen storage capacity, we gradually increased the number of adsorbed H2
molecules on the Ni and Ti atom decorated BDG surface and calculated the adsorption energy values
using the formula, _E_ _ads_ = ( _ETotal_ - _E_ _surface_ - _nE_ _H_ 2) . The calculated total adsorption energy and average
adsorption energy per H2 molecule are given in Table 2. We found that the maximum of nine H2 molecules can be adsorbed on both the Ni and Ti decorated BDG surface with an average binding energy of

- 0.43 and - 0.41 eV per H2 molecule, respectively. It has been observed that beyond this maximum
value, the calculated binding energy values decreases abruptly, which may lead to decrease the stability
of the system. Our calculation indicates that among the nine H2 molecules, the first four are adsorbed
via chemisorption, i.e., dissociated into 8 hydrogen atoms on the both Ni and Ti decorated BDG surface
and further adsorption follows as a physisorption (i.e., molecular form).

It is well known that the hydrogen spillover is the diffusion of dissociated hydrogen atoms adsorbed
on one metal site to the nearest metal of a different type on the decorated surface. Here, hydrogen diffusion must occur according to the following steps: (1) The Ni-decorated BDG surface adsorbs H2 and
diffuses to nearest metal Ti; and (2) the Ti-decorated BDG surface diffuses hydrogen to the nearest Mg
metal. Here, we considered the atomic diffusion of H from one metal to another metal (i.e., from Ni to
Ti and Ti to Mg) and also we considered the effect of hydrogen coverage in each case ( i.e, minimum
and maximum hydrogen adsorption). In order to accommodate two kinds of metals, the BDG surface
is extended to 6 × 3 unit cell from 3 × 3 and we optimize the surface to ensure different kinds of metal
atoms will not attract each other to form clusters.

First, we considered one hydrogen molecule adsorbed (via chemisorption) on Ni trimer decorated
on the BDG surface and the diffusion of one hydrogen atom to the nearest Ti trimer. Figure 1 shows the
corresponding initial state (IS), transition state (TS) and final state (FS) for the atomic H diffusion from
Ni to Ti for both minimum and maximum hydrogen coverages for the chemisorption case. The diffusion
barriers and the minimum energy path (MEP) for all the cases considered in this study are calculated
using NEB method 43,44 and the values are given in the Table 1S of supplementary material. We found that

Figure 2. Side view of initial (IS), Transition (TS) and Final state (FS) geometries for the diffusion of
one H atom from Ti to Mg in (a) minimum and (b) maximum hydrogen coverage.

Mg

Number of H2

−Eads −Eads/H2

1 0.52 0.52

2 0.77 0.38

3 1.06 0.35

4 1.06 0.27

5 1.08 0.22

6 1.10 0.18

Table 3. The calculated desorption energies ( −Eads in eV) and desorption energies per H2 molecule
(−Eads/H2 in eV) for the hydrogen molecules adsorbed on Mg trimer decorated BDG surface.

the distance between the two H atoms is 2.42 Å in the initial state and the calculated activation energy
for the one H atom diffusion from Ni trimer to Ti is 0.65 eV. The calculated reaction energy is exothermic by − 0.67 eV. We included zero-point energy corrections in all of our calculations. In the maximum
coverage, (i.e,4 H2 adsorbed on Ni via chemisorption) among 8 H atoms, we considered diffusion of the
H atom that had the shortest distance to the nearest Ti trimer and the calculated activation barrier is
0.41 eV and the reaction energy is exothermic by − 1.53 eV. Second we considered the diffusion of one
H atom from Ti to Mg both in minimum and maximum coverages. The optimized geometries of IS,
TS, and FS for the diffusion path of the H atom from Ti to Mg are illustrated in Fig. 2. The activation
barrier for the migration of one H atom from Ti to Mg in the case of minimum coverage is found to be
0.67 eV and the calculated reaction energy is endothermic by 0.63 eV. The calculated diffusion barrier
and the reaction energy for the migration of one H atom in the case of maximum hydrogen coverage
(i.e.,4 H2 adsorbed on Ti via Chemisorption) is 0.32 and 0.23 eV, respectively. From Table 1S, we found
that the diffusion barrier decreases while increasing the hydrogen coverage which is in agreement with
our previous study 34 . Further, the calculated diffusion barriers for the migration of the H atom in this
combination of both TM and AEM atoms is comparatively lower than our recent report on a BDG surface decorated by TM atoms alone 34 and also other studies 32,40 . It has been observed that the replacing
the previously reported metal atoms (i.e., Pd by Ti and Co by Mg) significantly reduces the diffusion
barriers both in the minimum and maximum hydrogen coverages. Further, we expect molecular (H2)
diffusion from one metal to another one in the case of physisorption. We found in our previous study 34
that the diffusion barriers for H2 migration on metal atoms decorated BDG are much lower than those
for atomic diffusion. Similarly, here also we believe that the diffusion barriers for H2 migration should
be less than atomic diffusion.

Table 3 shows the calculated desorption energies (negative of the adsorption energy) of the H2 molecules from Mg decorated on the BDG surface. The desorption energy for the minimum coverage, i.e.,

Figure 3. Fluctuations of total energy as a function of simulation time for different metal atoms
decorated B-doped Graphene surface in molecular dynamics simulations at different temperatures (200
and 390 K).

only one H2 molecule, is found to be 0.52 eV and it decreases with increasing the hydrogen coverages.
We found that a maximum of six H2 molecules can adsorb on the Mg trimer. In the maximum coverage,
the desorption energy per H2 molecule is only 0.18 eV, which is close to the required energy for reversible
storage. Considered AEM atom Mg is lighter than the previously used TM atom Co and also the calculated desorption energies are very low compared to our previous study 34 and hence it is easy to desorb
the diffused H from the terminal part.

Further, in order to confirm that the proposed hydrogen storage medium is thermodynamically stable, we performed molecular dynamics (MD) simulations using the constant particle number, volume,
temperature (NVT) ensemble on these metal decorated B-doped graphene surface in the case of maximum hydrogen adsorption. The suggested temperature for H2 delivery by the DOE is in the range of
233− 393 K. Hence, we performed MD simulations for maximum number of H2 molecules adsorbed
on Ni, Ti and Mg decorated boron-doped graphene surfaces at three different temperatures (200, 300,
and 390 K). The equilibrated geometries of the different metal atoms decorated BDG surfaces at higher
temperature are shown in Figure 2S of supplementary information. The fluctuations of total energy as a
function of simulation time at different temperatures (200 and 390 K) are shown in Fig. 3 and for 300 K
given in Figure 3S of Supplementary information. As can be clearly seen from those figures, there is no
structural deformation of the proposed medium even at higher temperatures, which clearly confirms that
the considered metal atoms dispersed uniformly and stably adsorbed on the B doped graphene surface
without clustering. Our MD results show that H2 molecules are stably adsorbed on the metal atoms at

low temperature and all adsorbed H2 molecules start to desorb from the metal atoms with increasing the
temperature and are completely desorbed at higher temperatures.

In summary, we have designed a new and efficient solid state material consisting of a combination of
both TM and AEM atoms which have potentially a good reversibility of hydrogen uptake and release.
These TM and AEM atoms decorated BDG surface can have the maximum hydrogen gravimetric capacity of 6.4% for double-sided adsorptions, which is much higher than the DOE new target of 5.5% for the
year 2015 45 . Molecular Dynamics simulations were also carried out to test the stability of the designed
medium at different temperatures. Results show that the clustering of metal atoms is suppressed in the
proposed medium and it is stable even at larger temperature. Hence, the designed medium will be a
good candidate for the efficient hydrogen storage at ambient conditions and we hope this theoretical
study can guide experimental works on the designing of solid-state media for hydrogen storage based
on carbon nanomaterials.

Methods
All the DFT calculations and first-principles molecular dynamics (MD) simulations were carried out
using the Vienna ab initio simulation package (VASP) 46 with a projector-augmented-wave (PAW)
method 47 . The exchange-correlation energy functional in the generalized gradient approximation (GGA)
of Perdew and Wang (PW91) 48 was used. The graphene used in our simulation consists of a 3 × 3 supercell of dimension 7.60 × 7.60 × 13.00 (Å) with a vacuum thickness of 13 Å to avoid interlayer interaction.
Further, the possible atomic arrangement for B-doped graphene (BC5) is based on previous experimental
results 49 . The sampling of the Brillouin zone is done for 3 × 3 and 6 × 3 super cells with 5 × 5 × 1 and
2 × 5 × 1 Monkhorst-Pack k-point grids, respectively. Spin− polarized calculations have been performed
in all the cases. The cutoff energy was set to 500 eV for a fully relaxed geometry optimization. All the
structures were optimized until the total energy converged to less than 10 −5 eV per atom and the maximum force converged to lower than 0.02 eVÅ −1 . The possible reaction pathway for hydrogen atom diffusion from one metal to the nearest metal has been considered and the nudged elastic band (NEB)
method 43,44 was used to estimate the transition structures and diffusion barriers. Zero-point energy
(ZPE) corrections have been included for all the diffusion barrier calculations, which is calculated as:

_hν_

_ZPE_ = ∑ _i_ 2 _i_

_hν_

_ZPE_ = ∑ _i_ 2 _i_, where h is Planck’s constant and _ν_ _i_ (Hz) is runs over all computed frequencies. In order to

reduce the computational cost, we considered only metal and hydrogen atoms for the frequency calculations. Further, in order to confirm the thermal stability of these metal decorated B-doped surface, we
performed ab initio molecular dynamics as implemented in VASP. The energy optimized structures were
chosen as an initial configurations where Γ points are used for Brillouin zone integration throughout the
simulations. The velocity Verlet algorithm 50 was used to integrate Newton’s equation of motion with the
simulation time step of 0.5 fs for 4 ps. The Nosé-Hoover thermostat 51 method was used to control the
different temperatures during the simulations.

= ∑

_hν_

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Acknowledgements
We acknowledge the financial support from the Ministry of Science and Technology, Taiwan (MOST
104-2113-M-011-002 & MOST 103-2219-M-011-002). We are also thankful to the National Center of
High-Performance Computing (NCHC) for donating computer time and facilities.

Author Contributions
J.C.J. designed and directed the Project. S.N. and P.J.L performed all calculations and analyzed the results.
All authors wrote the paper. All authors reviewed the manuscript.

Additional Information
Supplementary information [accompanies this paper at http://www.nature.com/srep](http://www.nature.com/srep)

Competing financial interests: The authors declare no competing financial interests.

How to cite this article : Nachimuthu, S. _et al._ A First Principles study on Boron-doped Graphene
decorated by Ni-Ti-Mg atoms for Enhanced Hydrogen Storage Performance. _Sci. Rep._ 5, 16797;
doi: 10.1038/srep16797 (2015).

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