G. ZHU et al.: EFFECT OF Sc ON THE HYDROGEN-STORAGE PERFORMANCE OF Mg2Ni ...
95–101
EFFECT OF Sc ON THE HYDROGEN-STORAGE PERFORMANCE
OF Mg
2
Ni: A FIRST-PRINCIPLES STUDY
VPLIV Sc NA U^INKOVITOST ZLITINE MG
2
Ni ZA
SHRANJEVANJA VODIKA: [TUDIJA TEMELJE^A NA PRVEM
NA^ELU
Guosong Zhu, Xiaoming Du
*
,FuLi
School of Materials Science and Engineering, Shenyang Ligong University, Shenyang 110159, China
Prejem rokopisa – received: 2023-09-28; sprejem za objavo – accepted for publication: 2023-12-21
doi:10.17222/mit.2023.980
The effect of metal scandium (Sc) on the hydrogen-storage properties of the magnesium-nickel (Mg2Ni) alloy has been explored
using the ultrasoft pseudopotential approach, rooted in the principles of Density Functional Theory (DFT). The binding energy,
lattice constant, enthalpy of formation, standard enthalpy of reaction, charge density, density of states and bond order for the
Mg2-xScxNi (x = 0, 0.25, 0.5, 1) alloys and their hydrides were calculated. Furthermore, the analysis of the atomic bonding and
the structural stability of Mg2-xScxNi and hydrides were also facilitated. The results show that the preference site of the Sc atom
in Mg2-xScxNi (x = 0, 0.25, 0.5, 1) alloys is Mg (6i) under the condition of a Sc doping concentration of 0.25. This causes a de-
crease in the stability of the Mg1.75Sc0.25Ni alloy. Moreover, the addition of Sc to Mg2-xScxNiH4 weakens the interaction of H-Ni
and H-Mg, thereby facilitating the hydrogen-release reaction and effectively enhancing the hydrogen-release capability of
Mg2-xScxNiH4.
Keywords: Mg2Ni alloy, first principles, electronic structure, hydrogen storage
Avtorji so {tudirali vpliv dodatka skandija (Sc) Mg2Ni zlitini na njeno u~inkovitost oziroma sposobnost za shranjevanje vodika.
Uporabili so »ultramehki« psevdo-potencialni pristop, temelje~ na osnovah funkcionalne teorije gostote (DFT; angl.: Density
Functional Theory). Izra~unali so vezavno energijo, mre`no konstanto, tvorbeno entalpijo, standardno entalpijo reakcije, gostoto
naboja, gostoto stanja in stanje (red) vzave za zlitine vrste Mg2-xScxNi (x = 0, 0,25, 0,5, 1) in njene hidrides. Nadalje v ~lanku
opisujejo analizo atomske vezave in strukturno stabilnost Mg2-xScxNi in njenih hidridov. Rezultati analiz in izra~unov so
pokazali da so prednostni polo`aji atomov Sc v zlitinah Mg2-xScxNi (x = 0, 0.25, 0.5, 1) pri polo`ajih Mg (6i) in koncentraciji Sc
je 0,25. To povzro~a pomembno zmanj{anje stabilnosti zlitine Mg1.75Sc0.25Ni alloy. Nadalje dodatek Sc k hidridu vrste
Mg2-xScxNiH4 slabi interakcijo med H-Ni in H-Mg vezmi, kar omogo~a sprostitev vodika in tako u~inkovito pospe{uje
sposobnost za spro{~anje vodika pri povi{ani temperaturi iz Mg2-xScxNiH4.
Klju~ne besede: Mg2Ni zlitina, {tudija prvega na~ela, elektronska struktura, shranjevanje vodika
1 INTRODUCTION
Due to the high theoretical hydrogen-storage capac-
ity, large electrochemical discharge capacity, and the
ability to release hydrogen at 253 °C and 0.1 MPa,
Mg
2
Ni was considered one of the ideal candidates for the
negative electrode materials in hydrogen-storage batter-
ies. Among the A
2
B-type alloys, Mg
2
Ni has garnered ex-
tensive recognition as a preeminent contender among hy-
drogen-storage materials due to its considerable
potential.
1
However, as a medium-temperature hydro-
gen-storage alloy, Mg
2
Ni still faces several challenges,
such as poor kinetic performance at room temperature
and relatively high hydrogen-absorption/desorption tem-
peratures, which somewhat restrict its practical applica-
tion.
Recently, scholars have endeavored to improve the
hydrogen-storage capacity of the Mg
2
Ni alloy by modifi-
cation using doping techniques.
2
Cui et al. implemented
a comprehensive inquiry into the intricate hydrogen-dif-
fusion mechanism entrenched within electrodes forged
from hydrogen-storage alloys of the Mg
2
Ni type.
3
Through a concerted amalgamation of the potentiostatic
polarization method based on the spherical diffusion
model, electrochemical impedance spectroscopy (EIS)
and an array of advanced methodologies, they ascer-
tained that the deliberate substitution of Magnesium
(Mg) and Nickel (Ni) within the Mg
2
Ni matrix with va-
nadium (V) and aluminum (Al) engendered a notable
augmentation in the alloy’s capacity to facilitate hydro-
gen diffusion. This heightened modification thereby cul-
minates in a substantial amplification of the electrode’s
discharge capacity, further corroborating the proposition
that the introduction of dopant elements can improve the
hydrogen absorption and desorption properties of Mg
2
Ni.
Gao et al. also conducted an extensive exploration of the
myriad modes of hydrogen absorption and desorption
transpiring along the crystalline plane at the interface of
Mg
2
Ni doped with rare-earth elements, most notably yt-
trium, cerium, lanthanum, and scandium, along the des-
ignated orientation (010).
4
An insightful revelation un-
derscores that the incorporation of the rare-earth element
Materiali in tehnologije / Materials and technology 58 (2024) 1, 95–101 95
UDK 669.721.5:543.272.2 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(1)95(2024)
*Corresponding author's e-mail:
du511@163.com. (Xiaoming Du)

scandium exerts a marked reduction in the cohesive
strength at the interface between hydrogen and the
Mg
2
Ni lattice. This profound transformation, in turn,
contributes to the amelioration of the intrinsic energy
barriers governing hydrogen diffusion and desorption,
thereby substantially enhancing the efficiency of the hy-
drogen storage in Mg
2
Ni. Nonetheless, there remains a
dearth of comprehensive research elucidating the intri-
cate mechanisms through which the rare-earth element
Sc influences the hydrogen absorption and dehydro-
genation properties of Mg
2
Ni alloys. Therefore, there is a
compelling need for more profound and extensive inqui-
ries, specifically focused on delineating the optimal dop-
ing concentration and unraveling the underlying influen-
tial mechanisms. Such efforts are imperative to not only
enhance the hydrogen-absorption and dehydrogenation
properties, but also to pave the way for its seamless inte-
gration into diverse realms of practical production and
daily existence.
The purpose of this study is to explore the effect of
Sc doping on both the crystal structure and electronic
configuration of the Mg
2
Ni alloys. Calculations are exe-
cuted for a range of Mg
2-x
Sc
x
Ni (x = 0, 0.25, 0.5, 1) al-
loys and their corresponding hydrides. The comprehen-
sive evaluation encompasses critical factors such as
binding energy, lattice constant, formation enthalpy, hy-
drogen-absorption enthalpy, density of states, charge
density, bond order, as well as an in-depth analysis of the
atomic bonding and structural stability. A detailed inves-
tigation is carried out to reveal the consequences of Sc
replacement for the structural characteristics and hydro-
gen-storage capabilities of both the Mg
2
Ni alloy and its
affiliated hydrides. These findings unveil intricate traits
that often prove elusive in experimental measurements.
By illuminating the effect of Sc on strengthening the sta-
bility of the hydride structure, along with its cascading
influence on the hydrogen-storage capabilities of Mg
2
Ni,
this effort is expected to offer theoretical insights that
contributes to the refinement of the hydrogen-storage al-
loy’s design and application strategies.
2 COMPUTATIONAL DETAILS
Utilizing the Quantum-ESPRESSO package,
5
which
is firmly grounded in Density Functional Theory (DFT),
the computations for this study were conducted.
6
The
electronic exchange correlation potential energy was ac-
curately determined using the Generalized Gradient Ap-
proximation (GGA) of the Perde/Wang (PW91) version.
7
The pseudo potentials used in this study were classified
as ultrasoft type, characterized by their reciprocal space
properties. The designated kinetic cutoff energy for
plane-wave calculations was established at 410 eV.
8
For
Mg
2
Ni, Mg
2
NiH
4
, and Mg
2-x
Sc
x
NiH
4
, the k-point grid
numbers were 6×6×2, 2×4 ×4 and 2×4×4, respectively.
The wave functions of pseudopotentials for the elements
Sc, Mg, Ni and H were denoted as follows: H (1s
1
), Sc
(3s
2
3p
6
3d
1
4s
2
), Mg (2p
6
3s
2
), and Ni (3d
8
4s
2
).
9
To attain
the stable structures, the geometric optimization of the
crystal cells was executed through the utilization of the
BFGS (Broyden-Fletcher-Goldfarb-Shanno) methodol-
ogy.
10
The total energy of the alloy system is calculated
using self-consistent iteration techniques, where the con-
vergence threshold of the total energy is set to 1.0 × 10
–5
eV/atom, stress error value less than 0.05 GPa, atomic
force less than 0.03 eV/Å.
11
The calculation of the H
2
molecular energy adopts a simple cubic model with a
size of 1 nm.
The Mg
2
Ni cell model was shown in Figure 1a with
a space group of P6222 (No. 180). The model consists of
12 Mg atoms and 6 Ni atoms, with lattice constants a =
0.5205 nm and c = 1.3236 nm. To investigate the varia-
tion of Sc occupancy in Mg
2
Ni crystal cells, a Sc atom
was used to replace the Mg (6i) and Mg (6f) occupancies
in Mg
2
Ni crystal cells. The corresponding cell models
were shown in Figure 1b and 1c respectively. Through
geometric optimization calculation and analysis, it was
found that the optimal site for Sc in Mg
2
Ni crystal cells
was Mg (6i). Therefore, four Sc-doped Mg
2-x
Sc
x
Ni (x =
0, 0.25, 0.5, 1) models were constructed and the cell
models were displayed in Figures 1a, 1d, 1e and 1f.T o
explore the influence of scandium alloying on the hydro-
gen-desorption characteristics of the hydride Mg
2
NiH
4
,a
model of Mg
2-x
Sc
x
NiH
4
(where x = 0, 0.25, 0.5, 1) was
built for analysis, as shown in Figures 1g, 1h, 1i and 1j.
At standard temperature and pressure, Mg
2
NiH
4
has a
monoclinic crystalline structure, characterized by a C2/c
G. ZHU et al.: EFFECT OF Sc ON THE HYDROGEN-STORAGE PERFORMANCE OF Mg2Ni ...
96 Materiali in tehnologije / Materials and technology 58 (2024) 1, 95–101
Figure 1: Crystal structure of a) Mg
2
Ni, b) Mg
11
Sc
Mg(6i)
Ni
6
,c)Mg
12
Sc
Ni(3d)
Ni
5
,d)Mg
1.75
Sc
Mg(6i)0.25
Ni,e)Mg
1.5
Sc
Mg(6i)0.5
Ni, f) MgSc
Mg(6i)
Ni,
g) Mg
2
NiH
4
,h)Mg
1.75
Sc
0.25
NiH
4
,i)Mg
1.5
Sc
0.5
NiH
4
, j) MgScNiH
4

(No. 15) space group. The lattice constants are defined as
follows: a = 1.4363 nm, b = 0.64038 nm, c = 0.6483 nm,
and = 113.52°.
3 RESULTS AND DISCUSSION
3.1 Structural properties
To validate the reasonableness of the used variables
and circumstances, a comparison is made between the
lattice constant of the calculated and the measured value,
as shown in Table 1. The data presented in Table 1 re-
veals a remarkable proximity between the computed lat-
tice constant of the Mg
2
Ni alloy and the empirical mea-
surement, displaying a maximum deviation of 0.11 %.
This observation strongly suggests that the calculated pa-
rameters and conditions used in this study were indeed
reasonable and well-suited. Moreover, Table 1 also re-
veals that after Sc replaces a portion of the atoms in the
Mg
2
Ni alloy, significant changes occur in the lattice con-
stant of Mg
2
Ni. With the increase of Sc concentration,
the a, b lattice constants and the volume of the Mg
2
Ni
models gradually expand, resulting in a volume expan-
sion. This phenomenon can be attributed to the atomic
radii of Sc and Mg, which were 0.162 nm and 0.160 nm,
respectively. As Sc has a larger radius than Mg, its sub-
stitution in the Mg
2
Ni cell leads to expansion. This was
consistent with the principle that the substitution of small
atoms by larger ones causes the lattice to expand.
3.2 Enthalpy of formation
The enthalpy of formation was a crucial parameter
for evaluating the stability of alloys. Typically, a negative
value of the enthalpy of formation indicates a more sta-
ble alloy. When investigating the stability of Sc-doped
Mg
2
Ni alloy structure, the enthalpies of formation for
both Mg
2
Ni and Mg
2-x
Sc
x
Ni (x = 0, 0.25, 0.5, 1) were
calculated according to the following expressions:
15
()
ΔH
xyz
Ex Ey Ez E
form tot solid
Mg
solid
Ni
solid
Sc
=
++
−−−
1
(1)
where E
tot
represents the total energy in equilibrium lat-
tice per unit; E
solid
Mg
, E
solid
Ni
and E
solid
Sc
are the total energy
of hcp Mg, fcc Ni and bcc Sc in their stable state per
unit cell, respectively; x, y and z refer to the numbers of
Mg, Ni and Sc atoms in the unit cell of Mg
2-x
Sc
x
Ni, re-
spectively. The calculation of single atom solid-state en-
ergy is implemented using the same method as the sum
of cell energy in intermetallic compounds. The total en-
ergy of elemental Mg, Ni, and Sc crystal cells was cal-
culated to be –974.5887 eV, –1356.74886 eV, and
–1277.89783 eV, respectively. By using Eq. (1), the
enthalpy of formation for each crystal cell was com-
puted, and the results are also listed in Table 1. The
enthalpy of formation of Mg
2
Ni was –15.47 kJ/mol, ex-
hibiting a notable approximation to the experimental
value of –13 kJ/mol.
6
However, it is important to note
that the calculations in this work were performed in an
ideal0Ken vironment. The difference between the cal-
culated and experimental values may be attributed to
temperature variations. It is clearly indicated from Ta-
ble 1 that the enthalpy of formation for Mg
12
Sc
Ni(3b)
Ni
5
and Mg
12
Sc
Ni(3d)
Ni
5
alloys is lower than that of Mg
2
Ni,
suggesting successful replacement of Ni (3b) and Ni
(3d) atoms in the Mg
2
Ni crystal lattice with Sc, thereby
enhancing the stability of the alloy system. Similarly,
the substitution of Mg (6f) and Mg (6i) sites in the
Mg
2
Ni crystal cells with Sc improves the stability of al-
loy structures. Especially, Sc preferentially occupies the
Mg (6i) site upon addition to the Mg
2
Ni alloy. A thor-
ough analysis and comparison of the enthalpy of forma-
tion for Mg
2
Ni, Mg
1.75
Sc
Mg(6i)0.25
Ni, Mg
1.5
Sc
Mg(6i)0.5
Ni,
and MgSc
Mg(6i)
Ni indicate a decreasing order of alloy
phase structure stability as follows : MgSc
Mg(6i)
Ni >
Mg
1.5
Sc
Mg(6i)0.5
Ni>Mg
1.75
Sc
Mg(6i)0.25
Ni>Mg
2
Ni. It can
be concluded that the optimal concentration of Sc-doped
Mg
2
Ni alloy system is x = 0.25.
G. ZHU et al.: EFFECT OF Sc ON THE HYDROGEN-STORAGE PERFORMANCE OF Mg2Ni ...
Materiali in tehnologije / Materials and technology 58 (2024) 1, 95–101 97
Table 1: The cell parameters, total energy and enthalpy of formation of Mg
2
Ni alloys and their hydrides.
Models
Lattice constants /nm
Cell volume /nm
–3
Total energy /eV  H
form
/ kJ·mol
–1
abc
Mg
2
Ni 0.5216 – 1.3227 311.7 –19839.4716 –15.47
Mg
2
Ni
Exp.12
0.5205 – 1.3236 313.6 – –13.00
13
Mg
2
Ni
Cal.14
0.5218 – 1.3212 – – –
Mg
11
Sc
Mg(6f)
Ni
6
0.5219 – 1.3220 313.7 –20143.3562 –16.83
Mg
11
Sc
Mg(6f)
Ni
6
0.5182 – 1.3411 313.1 –20143.2269 –16.73
Mg
12
Sc
Ni(3b)
Ni
5
0.5270 – 1.4700 336.5 –19759.2643 –16.33
Mg
12
Sc
Ni(3d)
Ni
5
0.4934 – 1.4916 332.7 –19759.4255 –16.33
Mg
1.75
Sc
Mg(6i)0.25
Ni 0.5220 – 1.3243 314.0 –20465.4659 –17.25
Mg
1.5
Sc
Mg(6i)0.5
Ni 0.5282 – 1.3120 318.8 –20750.6378 –17.62
MgSc
Mg(6i)
Ni 0.5368 – 1.3048 325.4 –21661.0790 –18.81
Mg
2
NiH
4
1.4422 0.6421 0.6513 554.1 –26971.8466 –64.02
Mg
1.75
Sc
0.25
NiH
4
1.4343 0.6404 0.6483 546.0 –27579.2132 –49.81
Mg
1.5
Sc
0.5
NiH
4
1.4343 0.6404 0.6483 546.0 –28186.7942 –64.01
MgScNiH
4
1.4343 0.6404 0.6483 546.0 –29402.2551 –64.12

The standard enthalpy of the reaction for the transfor-
mation of Mg
2
Ni to hydride Mg
2
NiH
4
, denoted as  H,
can be calculated using Equation (2),
ΔHE E E =−−
1
16
1
12
32 2
()() ( ) Mg Ni H Mg Ni H
16 8 12 6
(2)
Upon computation using Equation (2), the enthalpy
of hydrogen absorption for Mg
2
Ni is determined to be
–64.02 kJ/(mol · H
2
). Importantly, this value closely
agrees with the experimental measurement of
–65.5 kJ/(mol · H
2
),
16
indicating a slight discrepancy be-
tween theoretical calculations and the experimental mea-
surement. This provides a method for the evaluation of
the reaction enthalpy in the context of Mg
2-x
Sc
x
Ni (x=0,
0.25, 0.5, 1) hydrides. This computational endeavour
draws upon the energy associated with an individual
metal atom. The hydrogen absorption reaction can be
concisely expressed as follows:
1
12
1
16
2224
Mg Sc Ni H Mg Sc NiH
−−
+=
x x x x
(3)
The standard enthalpy of hydrogen absorption reac-
tion can be calculated using Equation (4),
ΔHE
EE
x x
x x
Sc
(Mg Sc NiH
(Mg Sc Ni) (H
=−
−−
−
−
1
16
1
12
24
22
)
)
(4)
The total energy of the H
2
molecule was calculated
by employing the Barth-Hedin exchange-correlation po-
tential function, resulting in a value of –31.565 eV. The
standard enthalpy of reaction for the hydrogen absorp-
tion of Mg
2-x
Sc
x
NiH
4
(x = 0, 0.25, 0.5, 1) calculated from
Equation (4) is presented in Table 1. The standard
enthalpy of reaction for hydrogen absorption in the four
hydrides follows a decreasing order: Mg
1.75
Sc
0.25
NiH
4
>
Mg
1.5
Sc
0.5
NiH
4
>Mg
2
NiH
4
> MgScNiH
4
. This result in-
dicates that during the hydrogen-release process, the
Mg
1.75
Sc
0.25
Ni alloy exhibits the lowest stability, suggest-
ing that under the same conditions, the hydrogen release
reaction in this cell is more favourable. This result pro-
vides further evidence, from an energy perspective, that
doping 25 % Sc significantly enhances the hydrogen-re-
lease capability of the Mg
2
Ni alloy.
3.3 Density of States
In order to attain an insight into the influence of Sc
on the structural stability of the Mg
2
Ni alloy and its hy-
dride from an electronic structure perspective, Figure 2
illustrates the total densities of states (TDOS) and partial
densities of states (PDOS) for Mg
2-x
Sc
x
Ni (x = 0, 0.25,
0.5, 1) alloys, employing the Fermi energy level E
F
as the
reference point for energy. Figure 2 reveals a visible
bonding peak in the energy range of –7.5 eV to 4 eV for
the total density of states of Mg
2
Ni, with the Ni (3d)
electronic orbitals significantly influencing the bonding
within Mg
2
Ni crystals. Nevertheless, the bonding effect
between Mg and Ni atoms appears relatively modest. To
probe the effect of Sc alloying, Figures 2b to 2d show
the total density of states and partial density of states for
Mg
1.75
Sc
0.25
Ni, Mg
1.5
Sc
0.5
Ni, and MgScNi, respectively. A
comparative analysis with Figure 2a reveals two notable
distinctions. (1) The height of bonding peak of Ni (3d) in
TDOS and PDOS of Mg
2-x
Sc
x
Ni decreases, showing the
beneficial influence of Sc alloying on the hydrogen re-
lease reaction of Mg
2
NiH
4
, leading to an enhancement in
its hydrogen release property. (2) To discern the intrica-
cies of the crystal cell interactions, the addition of Sc
promotes orbital hybridization between Sc (3s4s), Sc
(3p), and Sc (3d) electron orbitals with Mg (3s) and Ni
(3d) within the interval spanning from –4.5 eV to 0 eV.
This observation suggests a weakening of the interaction
among the Mg (3s), Mg (2p), and Ni (3d) electron
orbitals. Ultimately, it leads to a decrease in the stability
of the crystal cells. The primary contribution to the den-
sity of states in Mg
2
Ni crystal cells originates from the
Ni (3d) orbital electrons. Within the energy span extend-
G. ZHU et al.: EFFECT OF Sc ON THE HYDROGEN-STORAGE PERFORMANCE OF Mg2Ni ...
98 Materiali in tehnologije / Materials and technology 58 (2024) 1, 95–101
Figure 2: a) The DOS and PDOS of Mg
2
Ni and b), c), d) Mg
2-x
Sc
x
Ni (x = 0.25, 0.5, 1)

ing from –10 eV to 0 eV for the Mg
2-x
Sc
x
Ni unit cell, the
number of electrons participating in the bonding within
the Ni (3d) orbital is correspondingly quantified as 40.1
electrons (x = 0), 38.7 electrons (x = 0.25), 37.1 electrons
(x = 0.5), and 35.9 electrons (x = 1). The reduction in the
number of bonding electrons weakens the bonding effect
in the unit cell, consequently diminishing the stability of
the phase structure. These findings agree harmoniously
with the results of the enthalpy of formation.
17
To comprehensively analyse the impact of Sc alloy-
ing on the hydrogen-release ability of Mg
2
Ni hydrides,
TDOS and PDOS of the hydrides Mg
2-x
Sc
x
Ni
4
were
shown in Figure 3.InFigure 3a, the TDOS of Mg
2
NiH
4
exhibits the main bonding peak in the energy spectrum
from –8.5 eV to 0 eV, and exhibits strong orbital hybrid-
ization between Ni (3d) and H (1s). This hybridization
improves the structural stability of Mg
2
NiH
4
, and there-
fore make hydrogen-gas release difficult under ambient
temperature and pressure conditions. This characteristic
contributes to the enhanced hydrogen-absorption perfor-
mance of Mg
2
Ni alloys. Furthermore, it can be seen from
Figure 3a that Mg
2
NiH
4
possesses a band gap of 1.35 eV
between its valence and conduction bands, indicative of
its non-metallic attributes. In the energy range of the va-
lence band, a distinct convergence is present between the
3d orbitals of Ni atoms and the 1s orbitals of H atoms.
Conversely, the overlap between the 1s orbitals of H at-
oms and the 3s orbitals of Mg atoms is relatively mini-
mal. This observation implies that the interaction be-
tween Mg and H is substantially weaker in comparison
to that between Ni and H. Consequently, the interplay
among Ni and H atoms for Mg
2
NiH
4
assumes a pivotal
significance in upholding the stability of the hydrides.
As shown in Figure 3b, the density of states for
Mg
1.75
Sc
0.25
NiH
4
reveals that there is a reduction in
PDOS associated with Ni and H atoms in the region lo-
cated below the Fermi level. The reduction of orbital hy-
bridization between Ni and H atoms, a result of the Sc
doping, precipitates a reduction in the intensity of the
Ni-H bond. Consequently, the dehydrogenation reaction
of Mg
1.75
Sc
0.25
NiH
4
becomes convenient. It is indicated
that Sc alloying enhances the hydrogen release for
Mg
2
NiH
4
. Moreover, the orbital hybridization between
Ni and H atoms in Mg
2-x
Sc
x
NiH
4
hydrides is significantly
reduced. In Figures 3b to 3d, it is evident that the bond-
ing peak of TDOS shifts towards a lower energy region
due to the substitution of Sc, leading to a similar shift in
the PDOS of Ni, Mg, and H atoms towards lower energy
levels. The 1s orbitals of the H atom in the valence band
converge more extensively with the 3d orbitals of the Sc
atom than with the 3s orbitals of the Mg atom. This ob-
servation implies that the interaction of the Sc atom with
the H atom demonstrates greater potency compared to
that of the Mg atom with the H atom for Mg
2-x
Sc
x
NiH
4
.
Moreover, the overlap between the 3d orbitals of Ni and
the 1s orbitals of H in Mg
2-x
Sc
x
NiH
4
is much smaller
than that between the 3d orbitals of Ni and the 1s orbitals
ofHinMg
2
NiH
4
.
18
This effect is particularly noticeable
when doped with 25 % Sc. The doping of Sc accelerates
the weakening of the interaction between Ni and H,
which helps to improve the hydrogen desorption effi-
ciency of the hydrides. This conclusion agrees with the
results obtained from the analysis of the standard
enthalpy of reaction of hydrogen absorption. It further
confirms that Sc alloying positively affects the hydrogen
desorption reaction of the Mg
2
Ni hydride system and im-
proves its hydrogen desorption property.
3.4 Differential Charge Density
As shown in Figure 4, a differential charge density
plot of Mg
2-x
Sc
x
Ni (x = 0, 0.25, 1) is provided to explain
the interactions between different atoms in Mg
2
NiH
4
,
Mg
1.75
Sc
0.25
NiH
4
, and MgScNiH
4
crystal cells. The inter-
action between H-Ni is stronger than that between H-Mg
in the Mg
2
NiH
4
(Figure 4a). It is indicated that the addi-
tion of Ni in Mg based alloys weakens the interaction be-
tween H and Mg atoms. The calculation results are iden-
tical to results based on density functional theory by
Paula et al.
19,20
In the Mg
2-x
Sc
x
NiH
4
crystal cell of Sc al-
loying (Figures 4b and 4c), it becomes evident that the
G. ZHU et al.: EFFECT OF Sc ON THE HYDROGEN-STORAGE PERFORMANCE OF Mg2Ni ...
Materiali in tehnologije / Materials and technology 58 (2024) 1, 95–101 99
Figure 3: The DOS and PDOS of Mg
2
NiH
4
and Mg
2-x
Sc
x
Ni (x = 0.25, 0.5, 1), a) Mg
2
NiH
4
, (b) Mg
1.75
Sc
0.25
NiH
4
,c )M g
1.5
Sc
0.5
NiH
4
,d )
MgScNiH
4

interaction between H-Sc is greater than that of H-Mg. It
coincides with the results of the DOS. Comparing the
differential charge density of Mg
2-x
Sc
x
NiH
4
cells with
those of Mg
2
NiH
4
cells (Figure 4a), it is found that the
Sc-H interaction generated by the addition of Sc weak-
ens the interaction between H-Mg and H-Ni, which is
particularly significant in the Mg
1.75
Sc
0.25
NiH
4
cells. It is
beneficial for the hydrogen-absorption and desorption
process of Mg
2
Ni alloy. This is because the addition of
Sc reduces the binding energy between H atoms and Mg
atoms, as well as between H atoms and Ni atoms, which
is beneficial for promoting the progress of hydrogen re-
lease reaction and hydrogen desorption reaction.
21
3.5 Charge Population Analysis
To quantitatively analyse the bonding characteristics
of each atom in Mg
2-x
Sc
x
NiH
4
, we examined the bond or-
der of hydrides. The strength of the interaction between
the atoms can be determined by the bond order value
(BO
S
), calculated as BO
S
= BO/BL, where BO represents
the bond length, and BL represents the charge population
between the atoms. A positive value of BO
S
with a larger
bond order indicates a strong covalent bond and a more
robust interaction between the two atoms. Conversely, a
negative value of BO
S
with a smaller bond order indi-
cates stronger repulsion between the atoms.
22
Table 2
presents the calculated values of BO, BL and BO
S
of
H-Mg, H-Ni, and H-Sc in Mg
2-x
Sc
x
NiH
4
hydride.
23
The
data reveals that in Mg
2
NiH
4
, the average bond length of
Ni-H was 0.155546 nm, and the average bond order was
0.69 nm, showing a strong covalent bonds between Ni
and H, and there is a strong interaction between Ni and
H atoms. To release hydrogen from Mg
2
NiH
4
, it is neces-
sary to have conditions of high temperature or high pres-
sure, which explains the difficulty of Mg
2
NiH
4
releasing
hydrogen under low-temperature conditions. In
Mg
2-x
Sc
x
NiH
4
(x = 0, 0.25, 1) the interaction between
Mg-H and Sc-H was weaker than that of Ni-H. There-
fore, to improve the hydrogen-release performance of the
hydrides, it is necessary to reduce the interaction be-
tween Ni and H. A comparison of the H-Ni bond order
values in Table 2 demonstrates that the Ni-H interaction
is weakest in the Mg
1.75
Sc
0.25
NiH
4
hydride. The addition
of Sc can improve the hydrogen-release kinetic property
of Mg
2
Ni. This result is consistent with the previous con-
clusions on differential charge density, density of states,
and enthalpy of the hydrogen-absorption reaction, further
verifying that Sc alloying has a positive impact on the
hydrogen-release performance of Mg
2
Ni hydrides.
4 CONCLUSIONS
The crystal structure, thermodynamic stability, and
electronic structure of the Mg
2-x
Sc
x
Ni (x = 0,0.25,0.5,1)
alloy and its hydride Mg
2-x
Sc
x
NiH
4
(x = 0,0.25,0.5,1)
were studied using a density-functional-theory-based,
ultrasoft-pseudopotential method. The results indicate
that with the increase of Sc content, the structure stabil-
G. ZHU et al.: EFFECT OF Sc ON THE HYDROGEN-STORAGE PERFORMANCE OF Mg2Ni ...
100 Materiali in tehnologije / Materials and technology 58 (2024) 1, 95–101
Table 2: Bond length (BL, nm), average bond order (BO) and unit bond order per unit length (BO
S
, nm) between atoms in Mg
2-x
Sc
x
NiH
4
(x=0,
0.25, 1)
Hydrides
H-Mg H-Ni H-Sc
BO BL BO
S
BO BL BO
S
BO BL BO
S
Mg
2
NiH
4
–0.008 0.214840 –0.04 0.108 0.155546 0.69 – – –
Mg1.75Sc
0.25
NiH4 –0.023 0.210030 –0.11 0.054 0.158793 0.34 0.018 0.214279 0.08
MgScNiH4
–0.013 0.207878 –0.06 0.074 0.160386 0.46 0.014 0.218836 0.06
Figure 4: Differential Charge Density of: a) Mg
2
NiH
4
,b)Mg
1.75
Sc
0.25
NiH
4
and c) MgScNiH
4

ity of the Mg
2-x
Sc
x
Ni (x = 0, 0.25, 0.5, 1) alloy gradually
increases, and Sc occupies the Mg (6i) position. The cal-
culation results of electronic density of states indicate
that the addition of Sc leads to a decrease in the bonding
peak height of Ni (3d) electron orbitals in Mg
2-x
Sc
x
Ni al-
loys, greatly reducing the structural stability of
Mg
2-x
Sc
x
Ni alloys and facilitating the occurrence of hy-
drogen-release reactions. In Mg
2
NiH
4
, there is a strong
interaction between H and Ni, while the interaction be-
tween H and Mg and Niand Mg is weak. The charge den-
sity between H and Ni and H and Mg in Mg
2-x
Sc
x
NiH
4
decreases. This is due to the addition of Sc weakening
the interaction between H and Ni and H and Mg, result-
ing in a decrease in the binding energy between H atoms
and Ni atoms, as well as between H atoms and Mg at-
oms. This helps to promote the hydrogen-release reac-
tion and improve the hydrogen-release ability of
Mg
2-x
Sc
x
NiH
4
. The calculation results of differential
charge density, hydride bond order, and enthalpy of hy-
drogen absorption reaction show that the interaction be-
tween Ni-H in Mg
1.75
Sc
0.25
NiH
4
is the weakest and the
structural stability is the lowest when the Sc content is
x = 0.25, which is helpful to the occurrence of the hydro-
gen-release reaction. These results reveal the beneficial
effect of Sc alloying on the hydrogen-desorption ability
of Mg
2
Ni hydrides. They emphasize the enormous poten-
tial for significantly improving the practicality of these
materials in the field of hydrogen-storage technology.
Acknowledgment
This work was supported by Liaoning Provincial Ap-
plied Basic Research Project (2023JH2/101300233) in
2023; Basic Research Projects of Higher Education Insti-
tutions in Liaoning Province in 2023 (JYTZD20230004,
JYTMS20230193).
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Materiali in tehnologije / Materials and technology 58 (2024) 1, 95–101 101