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Review Article
High-entropy alloys for solid hydrogen storage:
a review
Long Luo
a,b,c,d,*
, Liangpan Chen
d
, Lirong Li
e
, Suxia Liu
d
, Yiming Li
d
,
Chuanfei Li
e
, Linfeng Li
e
, Junjie Cui
e
, Yongzhi Li
b,e,**
a
Analytical and Testing Center, Inner Mongolia University of Science and Technology, Baotou 014010, China
b
Baotou Materials Research Institute of Shanghai Jiao Tong University, Baotou 014010, China
c
School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
d
School of Materials and Metallurgy, Inner Mongolia University of Science and Technology, Baotou 014010, China
e
School of Science, Inner Mongolia University of Science and Technology, Baotou 014010, China
highlights
The research status of high-entropy hydrogen storage alloys is reviewed.
The advantages of high-entropy alloys in hydrogen storage performance are shown.
The shortcomings of high-entropy hydrogen storage alloys are pointed out.
The development direction in the future is outlined.
article info
Article history:
Received 24 May 2023
Received in revised form
4 July 2023
Accepted 15 July 2023
Available online 08 August 2023
Keywords:
Metal hydrides
Hydrogen storage
HEAs
Multiprincipal-element alloys
Microstructure
abstract
The development of materials has coincided with the development of human civilization.
In recent years, high-entropy alloys (HEAs) have been extensively applied to structural and
functional materials owing to their unique physical and chemical properties. Therefore,
HEAs have emerged as a promising materials. This review summarizes recent research
progress on HEAs for hydrogen storage. First, the history and basic concepts of HEAs are
systematically introduced. Furthermore, recent developments in the field of HEA-based
hydrogen storage are reviewed and discussed. The preparation process, design methods,
microstructures, and hydrogen-storage performance of HEAs are systematically compared
and summarized. Other hydrogen-related applications are also presented. Finally, the
shortcomings of the HEAs currently used in hydrogen-storage applications are highlighted,
and future development directions are outlined.
©2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
*Corresponding author.
** Corresponding author.
E-mail addresses: luolong@imust.edu.cn (L. Luo), liyzh1983@imust.edu.cn (Y. Li).
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier.com/locate/he
international journal of hydrogen energy 50 (2024) 406e430
https://doi.org/10.1016/j.ijhydene.2023.07.146
0360-3199/©2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407
2. Definition of HEAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409
3. Four HEA “core effects”.................................................................................410
3.1. The high-entropy effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410
3.2. The lattice-distortion effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410
3.3. The sluggish-diffusion effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411
3.4. The cocktail effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411
4. Design and preparation of hydrogen storage alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411
4.1. Main parameters for predicting HEA phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412
4.2. Element selection and preparation method of hydrogen-storage HEAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414
5. Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417
6. Hydrogen storage properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420
6.1. Hydrogenation kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420
6.2. Hydrogenation thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421
6.3. Hydrogen capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422
6.4. Cycling properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422
7. Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423
8. Other hydrogen-related applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423
8.1. Catalyst for hydrogen storage materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
8.2. Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
Summary and outlook of future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
Declaration of competing interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425
1. Introduction
As stated on the International Energy Agency website,
hydrogen is a versatile energy carrier that can help tackle
various critical energy challenges [1]. Owing to its high energy
density (120e142 kJ/kg, which is 2.7 times that of gasoline) [2]
and the absence of CO
2
emissions when burned, hydrogen is
considered an increasingly important piece of the strategy to
achieve net-zero emissions by 2050. However, the energy
density per unit volume of hydrogenis only 0.01 MJ/L at 288.15 K
and 0.101 MPa. In comparison, the volumetric energy densities
of methane and gasoline are0.04 MJ/L and 32 MJ/L, respectively.
High-energy-density hydrogen-storage technology is essential
to bridge the gap between hydrogen production and its energy-
storage applications. At the same time, hydrogen is a flam-
mable and explosive gas: when the concentration of hydrogen
in air is 4.1e75 vol% [3], it will explode in case of fire. Therefore,
safety must be consideredwhen evaluating the advantages and
disadvantages of hydrogen-storage technologies. In addition,
the technology must consider factors such as economy, energy
consumption, and life cycle. To develop a hydrogen-storage
technology that considers factors such as density, safety,
cost, and service life, researchers from various countries have
conducted multiple studies.
At present, physical and chemical hydrogen storage are the
principal hydrogen-storage methods. Physical methods
mainly include compressed hydrogen, liquefied hydrogen,
cryo-compressed hydrogen, and physically adsorbed
hydrogen [4,5], while chemical methods mainly include metal
hydrides [6e8], complex hydrides [9], and liquid organic hy-
drides [10]. Table 1 compares the two approaches and reveals
that solid-state hydrogen storage is one of the most promising
methods. Among them, alloys have become leading hydrogen-
storage materials owing to their favorable cost, safety, oper-
ating conditions, particularly their high energy density by
volume. For example, the most commonly used commercial
hydrogen-storage alloy in nickelemetal hydride batteries is
the AB
5
alloy with a CaCu
5
crystal structure. However, con-
ventional alloys also face many problems in hydrogen storage.
Each alloy has its own advantages and disadvantages, and
their overall performance is still far from the targets of U.S.
Department of Energy (DOE) [11]. The technical targets of DOE
for on board hydrogen based light duty vehicles are given in
Table 2. However, a new class of alloy materials called high-
entropy alloys (HEAs) may offer hope.
HEAs were discovered as early as the late 18th century,
when German scientist and metallurgist Franz Karl Achard
and his colleagues conducted an innovative study that
involved the preparation of a series of multicomponent
equimass alloys containing 5e7 elements [15]. He was prob-
ably the first scientist to study multiprincipal-element alloys.
However, metallurgists worldwide mostly ignored this
remarkable work, and it was not until 1963 that Professor Cyril
Stanley Smith recognized it [16]. Owing to the neglect of
Achard's work, the development of HEAs fell silent for a period
international journal of hydrogen energy 50 (2024) 406e430 407
of time. It was not until the 1990s of the 20th century that
HEAs gained new opportunities for development. In 1993,
scientists at the University of Cambridge in the United
Kingdom proposed the famous "chaos principle,”which states
that the higher the entropy of alloy materials, the easier it is
for them to form amorphous structures. Simultaneously,
Chinese scholar J.W. Yeh et al. proposed a novel alloy-design
concept: a mixed, multicomponent alloy with a high en-
tropy, i.e., “high-entropy alloy.”However, relevant research
had not yet been published.
Finally, Yeh [17] and Cantor [18] independently reported
HEAs and equiatomic multicomponent alloys in 2004. Since
then, as an emerging class of alloys, HEAs have attracted more
and more attention in the materials community owing to their
unique physical, chemical, and mechanical properties, as
shown in Fig. 1a. Over the past few years, the concept of HEAs
has been extended to high-entropy ceramics, films, steels,
superalloys, lightweight aluminumemagnesium HEAs, high-
entropy cemented carbides, and high-entropy functional
materials. Thus far, most studies on HEAs have focused on
their mechanical properties. In addition to being used as
structural materials, HEAs also have great prospects as func-
tional materials for applications such as hydrogen storage.
Following the first publication on CoFeMnTi
x
V
y
Zr
z
HEAs for
hydrogen storage in 2010 [19], very few papers on hydrogen-
Table 1 eComparison of the main hydrogen-storage technologies.
Category Storage method Conditions Gravimetric
density (wt%)
Volumetric
density (g/L)
Volumetric energy
density (MJ/L)
Ref.
Physical compressed 0.1 MPa, RT 100 0.0814 0.01 [2]
compressed 35 MPa, RT 100 24.5 2.94 [2]
compressed 70 MPa, RT 100 41.4 4.97 [2]
liquefied 0.1 MPa, 253 C 100 70.8 8.5 [2]
cryo-compression 35 MPa, 253 C 100 80 9.6 [5]
activated carbon 196 C, 3 MPa 5.0 38.5 2.4 [4]
Metal-organic frameworks 196 C, 8 MPa 7.9 25.8 3.1 [4]
Chemical LaNi
5
5 MPa, RT 1.4 104 12.4 [6]
MgH
2
287 C, 0.1 MPa 7.6 110 13.2 [7]
FeTiH
2
RT, 1 MPa 1.89 114 13.7 [8]
Mg
2
NiH
4
1.4 MPa, 200 C 3.6 97 11.6 [9]
ZrCr
2
H
3.8
RT, 1 MPa 2.0 111 13.3 [10]
VH
2
RT, 5 MPa 3.9 160 19.2 [12]
NaAlH
4
100 C, 0.1 MPa 7.4 80 9.6 [13]
Methylcyclohexane/toluene Catalyzed @RT a, 0.1 MPa 6.2 47.3 5.68 [14]
Table 2 eDOE's technical targets for on board hydrogen
systems.
Storage
System
targets
Gravimetric
Density
(wt%)
Volumetric
density
(g/L)
Hydrogen
release
temperature
2020 4.5 30 20/100 C
Ultimate 6.6 50
Note: (1) To be able to meet the system targets, that any storage
material must contain greater than the system density; (2) As-
sumes a vehicle storage capacity of 5.6 kg of useable hydrogen, it
will cover an area of around 300e350 mile.
Fig. 1 e(a) Number of articles published from 2004 to December 2022 containing the terms “HEA,”“multiprincipal element
alloy,”“multicomponent alloy,”or “complex concentrated alloy.”(b) Number of articles published from 2010 to December
2022 with “HEA,”“multiprincipal element alloy,”“multi-component alloy,”or “complex concentrated alloy”and “hydrogen
storage”in the title, keywords, and abstract. Source: Web of Science.
international journal of hydrogen energy 50 (2024) 406e430408
storage alloys were published over the next eight years.
However, recently (since 2018), interest in this research area
has been revived, resulting in a marked increase in the
number of articles published each year since (see Fig. 1b).
Therefore, a comprehensive review of high-entropy hydrogen
storage alloys is very necessary. In 2021, F. Marques et al. [20]
conducted a comprehensive review of high-entropy alloys for
hydrogen storage. In 2022, F.S. Yang et al. [21] made an
extensive review from other focus. The two articles are very
important work, they outline the research status and future
development direction of HEAs for hydrogen storage. How-
ever, both review papers were accepted in 2021. According to
the survey, from 2022 to now, nearly 40 articles have been
published on high-entropy hydrogen storage alloys. By care-
fully reviewed and concluded that these studies provide a lot
of interesting information, such as high-entropy alloys as
catalysts for hydrogen storage materials. Therefore, it would
be meaningful to combine the latest research findings with
previous studies for a comprehensive review. In this paper,
the research progress in hydrogen-storage HEAs is summa-
rized, and the theory and current status are evaluated. The
preparation methods, theoretical predictions, alloy types, and
hydrogen-storage performance are reviewed. This article
provides insights into the latest advances in hydrogen-storage
HEAs, identifies gaps in the current knowledge, and projects
the future direction of research in this rapidly developing field.
2. Definition of HEAs
Currently, there is no definition for multiprincipal alloys [18],
whereas more than one definition exists for HEAs. The mul-
tiple definitions of HEAs have led to confusion, exacerbating
the controversy over whether some alloys can actually be
called HEAs. Commonly used definitions and disputes are
briefly introduced and discussed.
As the name implies, HEAs have high degrees of configu-
rational entropy. Yeh et al. [17] generalized these alloys to
"HEA" based on two known thermodynamic facts: (I) the
configuration entropy of binary alloys
(DS
conf
¼eR(C
A
lnC
A
þC
B
lnC
B
), where Ris the gas constant and
C
A
and C
B
are the fractions of species A and B, respectively, is
at a maximum when the elements are in equiatomic pro-
portions (as shown in Fig. 2a); and (II) the maximum config-
uration entropy (DS
conf, max
¼RlnN) in any system increases
with the number of elements (N) in the system (as shown in
Fig. 2b). An HEA is defined by the magnitude of DS
conf
(the
mixing entropy is expressed in the literature), where DS
conf
of
an ideal solid-solution is calculated from the Boltzmann
equation [23,24]:
DSconf ¼RX
n
i¼1
ciln ci(1)
where Ris the gas constant (8.314 J/kmol) and c
i
is the atomic
fraction of the i
th
element in a solid solution composed of n
elements. Thus, alloys can be classified as low-, medium-, and
high-entropy alloys. Based on this classification, Fig. 3 shows a
schematic of alloy classification. It should be noted that the
entropy-based definition assumes that alloy atoms randomly
occupy lattice positions at high temperatures or in a liquid
state [25]. However, early research indicated that atoms in
metal solutions do not always occupy random positions, and
binary metal liquids are often not ideal at their melting tem-
peratures [26]. These factors suggest that a definition based on
entropy is imperfect.
Another definition of HEAs is based on alloy composition.
Yeh's earliest paper defined HEAs as being composed of five or
Fig. 2 eRelationship between configuration entropy and alloy composition. Redrawn from Ref. [22].
Fig. 3 eSchematic diagram of alloy classification based on
configuration entropy.
international journal of hydrogen energy 50 (2024) 406e430 409
more principal elements in equimolar ratios [17]. The equi-
molar concentration requirement has been strictly main-
tained, but the definition has been further expanded since this
publication. Currently, HEAs are generally described as alloys
containing five or more elements in atomic fractions of
5e35 at% [22,25]. For this reason, the number of HEAs has
increased significantly in recent years; that is, HEAs are not
necessarily equimolar. In addition, minor elements can be
added to HEAs to improve their properties, thereby further
increasing their number. This definition specifies only the
elemental concentration; the magnitude of the entropy is not
restricted, and a single-phase solid solution is not required.
There are other definitions of HEAs, but they are not dominant
[26e28]. The two definitions introduced above are neither
correct nor incorrect; the appropriate definition depends on
the intention of the work performed.
3. Four HEA “core effects”
The microstructure and properties of HEAs obviously differ
from those of conventional alloys mainly owing to four core
effects proposed by J.W. Yeh in 2006 [25]. As shown in Fig. 4,
these effects include the sluggish diffusion effect, high en-
tropy effect, lattice distortion effect, and “cocktail”effect. The
root cause of these effects is the mismatch of the atomic size,
modulus, etc., caused by the large number of different ele-
ments in the alloy. The first three effects are real and have
been proven, whereas the final (cocktail) effect is not hypo-
thetical and does not need to be verified; it is a concept based
on the unpredictable synergy of the other three effects.
3.1. The high-entropy effect
One distinguishing feature of HEAs is their high-entropy ef-
fect. The mixing entropy is comparatively high in HEAs
because they contain several component elements. The con-
ventional Gibbs phase-law is as follows:
P¼Cþ1eF (2)
where F is the thermodynamic degrees of freedom, C is the
number of group elements, and P is the number of phases.
According to numerous experiments, HEAs typically form
simple solid-solution phases, i.e., face-centered cubic (FCC),
body-centered cubic (BCC), and hexagonal close-packed (HCP),
rather than the anticipated multiphase intermetallic com-
pounds, which are predicted to form when more group ele-
ments are present in the mixture. The Gibbs free-energy
expression as:
DGmix ¼DHmix TDSmix (3)
where DGmix is the Gibbs free energy, DHmix is the enthalpy of
mixing, Tis temperature, and DSmix is the mixing entropy. At
high temperatures, the higher the mixing entropy, the lower is
the free energy of the entire alloy system, simplifying the
formation of stable solid-solution phases [29e31]. According
to this, the formation of concentrated disordered solid solu-
tions would be favored, and the presence of secondary inter-
metallic phases limited.
3.2. The lattice-distortion effect
Large variations in the atomic radii and electron distributions
of various alloying elements lead to lattice distortions in HEAs
[32]. The energy in the system, and consequently, the char-
acteristics of the alloy, are affected when the atoms depart
from their equilibrium locations [33,34]. Fig. 5 illustrates how
HEAs exhibit more severe lattice distortions than typical al-
loys. There are a few studies that believe that lattice distortion
leads to an increase in the number of active sites, which fa-
cilitates the hydrogenation of alloys [35,36]. For example,
Sahlberg et al. [36] noted in their study of the hydrogenation
process of TiVZrNbHf alloy that the HEA can be hydrogenated
to an H/M ratio of 2.5, which they believe is directly related to
lattice distortion that makes the tetrahedral and octahedral
interstitial site easier to fill by hydrogen atoms. We believe
that there is no strong evidence that lattice distortion is more
beneficial to the hydrogen storage performance of alloys, and
further research is needed.
Fig. 4 eFour core effects of HEAs proposed by Yeh.
international journal of hydrogen energy 50 (2024) 406e430410
3.3. The sluggish-diffusion effect
In HEAs, atomic diffusion is considered to be sluggish. This is
because nanocrystals and amorphous phases are formed
during alloy solidification [17,25,38], which has been
confirmed by direct probing in some studies [39,40]. Tsai et al.
[39] provide a reasonable explanation for the root cause of the
sluggish diffusion. They believe that, owing to the large fluc-
tuation in lattice potential energy between lattice sites, HEAs
undergo slow atomic diffusion with a high activation energy.
In HEAs, many principal elements exist in the solid-solution
phase. Since each site in the lattice is surrounded by
different atoms, each site has a different bond configuration,
and thus a different lattice potential energy [41]. As shown in
Fig. 6, the mean difference represents the change in average
lattice potential energy during atomic migration. The atoms in
HEAs experience significantly larger lattice potential energy
fluctuations during diffusion than those in conventional al-
loys. The greater the lattice potential energy fluctuations
experienced by the atoms, the more difficult their diffusion.
Since atoms tend to minimize their energy, sites with lower
lattice potential energies become traps for atoms, increasing
the migration energy barrier and diffusion activation energy.
In addition, the saddle-point energy is more widely distributed
in HEAs than in conventional alloys, and a location with high
saddle-point energies becomes an obstacle to diffusion. This
further reduces atomic diffusion efficiency. As mentioned
earlier, the sluggish-diffusion effect can promote grain
refinement to form high-density grain boundaries, which are
favorable channels for hydrogen diffusion, so multicompo-
nent alloys generally have excellent hydrogenation kinetics
[12].
3.4. The cocktail effect
The cocktail effect, in which the properties of multicompo-
nent materials interact with each other to offer a distinctive
characteristic, was initially proposed by Indian scholar S.
Ranganathan [42]. This effect is comparable to a cocktail
beverage, in which many types of alcohol impart varied af-
tertastes. From the compound effect at the atomic level to that
at the microstructural level, the cocktail effect has been
explicitly demonstrated in HEAs [43]. Taking advantage of this
feature, specific elements can be added to optimize the per-
formance of hydrogen-storage HEAs. For example, with the
addition of 10% Ta, the HEA Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
Ta
0.10
demonstrated a significantly improved hydrogen capacity,
desorption performance, and cycling stability compared with
the quaternary alloy Ti
0.325
V
0.275
Zr
0.125
Nb
0.275
[44].
4. Design and preparation of hydrogen
storage alloys
Based on the traditional Hume-Rothery criteria, we can fore-
cast the composition of alloy phases for ordinary solid solu-
tions. However, because HEA do not have a solvent or solute,
and because there will be interactions between the group
components, it is now a research priority to identify and
forecast the phase development. Additionally, HEA prepara-
tion is essential, and by treating the HEA to strengthen its
Fig. 5 eSchematic diagram of the BCC crystal structure: (a) crystal structure of only one element; (b) lattice distortion owing
to differences in atomic radii and electron distribution, etc., between alloys in a multicomponent solid-solution [37].
Fig. 6 eSchematic diagram of the variation of lattice
potential energy and mean difference between P and P′
point energies during the atomic migration in diff erent
alloys (metal). The mean difference for pure metals is zero,
whereas that for HEA is the largest.
international journal of hydrogen energy 50 (2024) 406e430 411
microstructure, its hydrogen-storage capabilities may be
enhanced.
4.1. Main parameters for predicting HEA phases
Researchers have progressively provided various empirical
parameters based on the HumeeRothery criteria. By assessing
the microstructural properties of HEAs, Zhang et al. [37] pre-
sented two parameters that affect phase formation: the
atomic size difference (d) and the chemical compatibility be-
tween the components, known as the mixing enthalpy (DHmix).
Based on the Gibbs free-energy equation, the impact of the
mixing entropy (DSmix) on phase formation was also exam-
ined. A high mixing entropy can significantly lower the free
energy while simplifying the production of stable phases. The
experimental analysis revealed that it is easier to form solid
solution phases when d6.5%, 15 kJ/mol <DH<5 kJ/mol,
and 12 J/K$mol <DS<17.5 J/K$mol. The parameters d,DHmix,
and DSmix are defined as follows:
d¼100 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
X
n
i¼1
ci1ri
r2
s(4)
where nis the group fraction, ciis the atomic percentage of the
ith component, riis the atomic radius of the ith element, and r
is the average atomic radius (r¼Pn
i¼1ciri).
DHmix ¼X
n
i¼1;isj
Uijcicj(5)
where ciand ridenote the atomic percentages of the ith and jth
components, respectively; and Uij ¼4DHmix
AB and DHmix
AB is the
mixing enthalpy of the binary liquid alloy.
DSmix ¼RX
n
i¼1
ciln ci(6)
where Ris the gas constant (8.314 J/kmol) and ciis the molar
percentage of the component.
Yang and Zhang [45] also proposed another parameter, U,
for estimating the solid-solution formation ability in a multi-
component alloy system, using U1.1 and d6.6% as criteria
for the formation of stable solid-solution phases in order to
further capture the balance between mixing entropy and
mixing enthalpy.
U¼TmDSmix
jDHmixj(7)
where Tm¼Pn
i¼1ciðTmÞiand ðTmÞiis the melting point of the ith
group element.
Valence electron concentration (VEC) is a very important
parameter, which can be expressed:
VEC ¼X
n
i¼1
ciVECi(8)
where VECiis the valence electron concentration of the ith
element, and ciis the atomic percentage of the ith element.
The VEC is critical in predicting the structure of HEAs. Guo
et al. [46] reported that the VEC controls the stability of the FCC
or BCC solid-solution phase. When VEC <6.87, the HEA is more
likely to form the BCC structure, and when VEC >8, the HEA is
more likely to form the FCC structure. In addition, VEC pa-
rameters can also be used to predict the hydrogen storage
performance of HEAs. Nyga
˚rd et al. [47] state that when the
VEC >4.75, the dihydride is destabilized, and the dehydroge-
nation occurs at room temperature if the total VEC in a HEA is
set to 6.4. In addition, their study also showed that when
VEC>5, the maximum hydrogen storage capacity is reduced.
They also noted that the volume expansion per metal atom
increase linearly with the VEC of the HEA from the bcc alloy to
the corresponding hydride. Edalati et al. [48] designed a
TiZrCrMnFeNi alloy based on multiple parameters including
VEC. The VEC value of the alloy is set according to Nygard's
report [47]. The TiZrCrMnFeNi alloy showed the presence of
95 wt% of C14 Laves phase and absorbed and desorbed 1.7 wt%
of hydrogen reversibly at room temperature with a fast ki-
netics. In a recent report by Zlotea et al. [49], the authors
systematically investigated the effect of VEC changes on
hydrogen storage properties in a series of HEAs: Ti
0.30
V
0.25-
Zr
0.10
Nb
0.25
M
0.10
,M¼Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ta.
They suggested the maximum hydrogen storage capacity at
room temperature strongly depends on the VEC of the HEAs:
the capacity is high (1.5e2.0H/M) for low values of VEC (<4.9)
whereas, a drastic fading is observed for VEC 4.9 which is the
case for alloys with Mbeing a late 3 d transition metal.
Moreover, the authors also pointed out the onset temperature
of desorption increases almost linearly with VEC for this
composition series. Their findings suggest that alloys with low
VEC are more likely to become promising candidates for
hydrogen storage. Cheng et al. [50] found that the dehydro-
genation activation energy of TieVeNbeCr HEAs hydrides has
been proved to decrease with the decreasing of VEC. The
above studies show that possible combinations in the multi-
dimensional space of HEAs are extremely broad, and the use
of VEC to predict the influence of chemical composition
changes is essential to guide future research on high-entropy
hydrogen storage alloys.
In typical alloys, the solubility is expected to increase with
decreasing electronegativity difference (Dc). In HEAs, Dc pre-
dicts the stability of the topologically dense row phase (TCP),
which is stable when Dc >0.133, except for some HEAs con-
taining large amounts of Al [51].
Dc¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
X
n
i¼1
ciðcicÞ2
s(9)
where c¼Pn
i¼1ciciis the average electronegativity, and ciis
the Bowling electronegativity of the ith element.
Ye et al. [52] predicted from the hard-sphere model that the
phases formed in HEAs depend only on two dimensionless
thermodynamic parameters: one is related to the entropic
deviation of the alloy from the ideal solution, and the other is
related to the average heat of mixing.
In practice, the value of the mixing entropy deviates from
the ideal value. Therefore, the total mixing entropy is ST¼SCþ
SE, where SCis the mixing configuration entropy and SEis the
excess mixing entropy. SEis also related to the atomic stacking
percentage xiand atomic diameter di. The following re-
quirements have been suggested for the creation of a single-
phase solid solution when combined with the Gibbs free-
energy expression:
international journal of hydrogen energy 50 (2024) 406e430412
jSEj
SC
≪1jHmj
TSC
(10)
The results indicate that the lower the value of jSEj
SCversus
jHmj
TSC, the easier it is to form a single-phase solid solution.
Instead, a conclusion in line with the authors’ results is that
metallic glasses can more easily obtain a greater ratio. Fig. 7
shows a generalized thermodynamic phase diagram of the
HEA equilibrium phase.
Ye et al. [53] also proposed the thermodynamic parameter
Ffor the design of HEAs based on a simple design route that
maximizes the entropy of their hybrid configurations.
Parameter Fis defined as:
F¼SCSH
jSEj(11)
where SH¼jHaj=Tmis defined as the complementary entropy
formed with the enthalpy jHaj. From Eq. (11), it is evident that
to maximize Fand produce a high-entropy effect, it is
necessary to increase SC, reduce jSEj, decrease jHaj, and in-
crease Tm.
The traditional atomic size effect does not apply to HEAs,
so Wang et al. [54] proposed a new parameter, g, to account for
the atomic size difference. This parameter is more useful than
dfor distinguishing solid solutions from intermetallic com-
pounds because it more accurately captures the multicom-
ponent alloys'overall trend of phase selection.
g¼uS
uL
¼ 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðrSþrÞ2r2
ðrSþrÞ2
s!, 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðrLþrÞ2r2
ðrLþrÞ2
s!(12)
where rLis the maximum atomic radius, rSis the minimum
atomic radius, ris the average atomic radius, uLdenotes the
maximum atomic filling angle, and uSrepresents the mini-
mum atomic filling angle. From Fig. 8, it can be seen that the
HEA is more likely to form a solid solution when g<1.175 (see
Fig. 9).
To better understand the formation of disordered solid
solutions, Singh et al. [55] developed a purely geometric
parameter, L, that further revealed the nature and volume
fraction of the formed compound within the required sub-
system. Parameter Lis defined as follows:
L¼DSmix
d2(13)
Compared with the aforementioned parameters d,DHmix,U,
VEC, and Dc,Lcan better predict the phase of the HEA. When
L>0.96, a single-phase solid solution is formed; when
0.24 <L<0.96, a two-phase mixture is formed; and when
L<0.24, a mixture of compounds is formed. Fig. 9 shows the
main parameters affecting the formation of single-phase
HEAs.
With the continuous development of technology, in addi-
tion to empirical parameters, other methods have been
devised to predict HEA phases. Currently, a combination of
thermodynamic calculations based on the CALPHAD phase
diagram and the first natural principle have been successful in
accurately predicting the phase composition of various HEA
Fig. 7 eGeneralized thermodynamic phase diagram of the
equilibrium phase in HEA [46].
Fig. 8 eApplication of the atomic packing parameter on the
solid solubility of superalloys [54].
Fig. 9 eParameters affecting the formation of single-phase
HEAs [59].
international journal of hydrogen energy 50 (2024) 406e430 413
systems [56e58]. Floriano et al. [56] studied the crystal struc-
ture and hydrogen-storage properties of a new isoatomic
TiZrNbCrFe HEA and used the CALPHAD method to predict the
phases formed. The formation of the C15-type Laves phase
was predicted under equilibrium conditions, and the forma-
tion of the C14-type Laves and BCC phases was predicted
under non-equilibrium conditions caused by arc melting. The
experimental results agreed with the CALPHAD calculations.
Hu et al. [58] investigated the hydrogen-storage properties of
TiZrVMoNb using the first-principle study and showed that
the BCC/FCC phase transition occurs during hydrogen ab-
sorption and that hydrogen atoms are more likely to enter
tetrahedral and octahedral interstitial sites.
In conclusion, thermodynamic simulations, first-principles
calculations, and empirical parametric methods can assist
with the composition design of HEAs to some extent and are
more efficient than traditional experimental trial-and-error or
empirical methods. These methods are mainly used to predict
the phase structure of HEAs and rarely involve direct optimi-
zation of specific property targets. Moreover, these methods do
not address the difficulty of obtaining the desired target alloy
composition, and limitations of these methods for HEA prop-
erty optimization and composition design still exist owing to
the lack of multiprincipal thermodynamic databases and
phase-formation issues. Therefore, more effective methods
and approaches are required to solve the challenges of HEA
composition design to meet the performance demands.
Recently, attention has been drawn to two other methods
for predicting the hydrogen storage properties of HEAs, which
can also improve the selection of composition. One is a
thermodynamic-based model proposed by Zepon et al. [60,61].
The thermodynamic parameters in this model need to be
obtained by combining experimental data and DFT. The
thermodynamic properties of alloys depend on the composi-
tion. By definition, HEAs have a wide compositional space,
which leads to a wide variety of alloys properties, i.e. different
platform pressures and hydrogen storage capacities. This
means that alloys screening through experimental testing is
time-consuming and uneconomical. Therefore, in order to be
able to effectively explore the hydrogen storage properties of a
large number of HEAs, machine/statistical learning will come
into play. Witman et al. [62] reported calculating the thermo-
dynamic properties of HEAs by machine learning (ML). The ML
model quickly screens hydride stability within a large HEA
space and permits down selection for laboratory validation
based on targeted thermodynamic properties. Zlotea et al. [63]
predicted the enthalpy of hydride formation of the Al
0.10-
Ti
0.30
V
0.30
Nb
0.30
alloy by ML to be 49 kJ/mol H
2
, which is very
consistent with the experimental value of 51 kJ/mol H
2
.In
addition, the ML model well predicted the trend of hydride
instability caused by Al addition, which confirmed the
robustness of the metal hydride ML method proposed by
Witman [62]. Lu et al. [64] predicted the hydrogen storage ca-
pacity of VTiCrFe alloy via ensemble ML, and the results
showed that VEC plays a key role in prediction. In addition to
the study of high-entropy hydrogen storage alloys, machine
learning is also widely used in the prediction of other
hydrogen storage materials [65,66], such as AB
2
[67], MOFs [68].
Here, Rao's work [69] must be mentioned that although this
work is not specific to the field of hydrogen storage materials,
it has very good guiding significance for the design of HEAs for
hydrogen storage. The authors believe thermodynamic alloy
design rules alone often fail in high-dimensional composition
spaces. They propose an active learning strategy that is a
closed loop. Integrated ML with DFT, thermodynamic calcu-
lations, and experiments to accelerate the design of HEAs in
an almost unlimited compositional space based on very
sparse data. From millions of possible compositions, they
identified two high-entropy steel alloys with extremely low
coefficients of thermal expansion. This is a suitable pathway
for the fast and automated discovery of HEAs with optimal
properties. Obviously, the combination of machine/statistical
learning methods with other computational methods may be
the most attractive way to quickly explore the hydrogen
storage properties of a large number of high-entropy alloys.
ML methods are powerful tools for calculating material
properties and establishing relationships between them in
various compositions. However, they need a reliable and
sufficiently extensive database to work successfully.
4.2. Element selection and preparation method of
hydrogen-storage HEAs
The emergence of new hydrogenated materials relies on
intuition and constant experimentation, but the elements
selected for conventional hydrogen-storage alloys has guided
the selection process for HEAs. The first component of con-
ventional hydrogen-storage alloys is a hydride-forming
element A, which readily interacts with hydrogen to produce
a stable hydride that generates a large amount of heat during
the process (DH<0). This component regulates the quantity of
hydrogen stored in the alloy. A non-hydride-forming element
B is the other component. This element does not typically
react with hydrogen and has minimal affinity for it, but when
hydrogen is dissolved in this metal, it absorbs heat (DH>0)
and regulates the reversibility of hydrogen absorption and
release [20,70]. The enthalpy of hydride formation can be
tuned via mixing elements with different enthalpies of hy-
dride formation. Details can be obtained from Fig. 10.
Since most of the HEAs reported so far are BCC structures,
the element selection of BCC structure solid solution hydrogen
storage alloysis taken as an example. According topreliminary
statistics, as of December 2022, a total of 20 elements, in
addition to rare earth element dopants, have been used in
studies of multicomponent and BCC-structured solid-solution
hydrogen-storage alloys, as shown in Fig. 10. The elements Ti,
V, Cr, Mn, Fe, Zr, Ni, Nb, Hf, Co, and Mo all appeared more than
20 times, and four elements (V, Ti, Cr, and Mn) appeared more
than 100 times. It can be seen that transition metals are more
likely to be selected. However, considering that to store
hydrogen with high gravimetric density in HEAs, light ele-
ments such as Mg and Al should be paid attention to, and the
corresponding preparation method should also be appropriate.
The HEA synthesis techniques included in this study can
be broadly divided into three states: solid, liquid, and gas.
Zhang et al. [71] discussed the electrochemical preparation
techniques. Mechanical alloying and spark plasma sintering
(SPS) for compaction are the main methods of solid-state
synthesis [72]; arc melting, resistance melting, induction
melting, laser melting, laser cladding, and laser-engineered
international journal of hydrogen energy 50 (2024) 406e430414
net shaping (LENS) are liquid-state synthesis techniques
[73,74]; and atomic layer deposition (ALD), molecular beam
epitaxy (MBE), and vapor-phase deposition are gas-phase
methods. HEA preparation techniques are discussed in
greater detail by Wang et al. [75]; therefore, we will not further
discuss them here.
Arc melting are the most frequently employed synthesis
techniques for hydrogen-storage HEAs [56,57,76e82], followed
by mechanical alloying [83e88] and LENS. Arc melting, which
involves heating metals to their melting points under the
protection of argon gas, is best suited for small production
runs or experimental applications. The organization of the
processed alloys is characterized by close-to-equilibrium
phases with minimal segregation [69]. To ensure that the
chemical composition of the sample is homogeneous, it is
typically melted repeatedly [89]. In addition to arc melting,
mechanical alloying is a popular method of HEA synthesis.
Fig. 11a illustrates this process. It produces a more homoge-
nous sample combination than the arc-melting approach and
does not require high temperatures. This synthesis procedure
entails numerous iterations of extrusion, cold welding, frac-
ture, and re-extrusion of the mixture, in addition to the high-
velocity collision of the alloy powder with the grinding balls in
a high-energy ball mill [90,91]. Numerous studies have been
conducted on the mechanical alloying process for creating
hydrogen-storage HEAs, particularly HEAs containing light
elements (Mg and Al) that are unsuitable for arc melting owing
to their high equilibrium vapor pressures and low melting
Fig. 10 e(a) Relatively subdivided periodic table of elements according to the enthalpies of formation of the binary metal
hydrides. (A-type: hydride-forming elements with DH<0; B-type, non-hydride-forming elements with DH>0). Adapted
from Ref. [20]. (b) Frequency of element use in ternary and higher BCC solid-solution hydrogen-storage alloys (statistical
data source: Web of Science).
international journal of hydrogen energy 50 (2024) 406e430 415
points, which are known to lead to elemental volatilization
losses at high temperatures. For instance, Montero et al. [85]
prepared Mg
0.10
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
by high-energy ball mill-
ing, concluding that adding light metals such as Mg to re-
fractory HEAs is advantageous for increasing the reversibility
of the hydrogen absorption/desorption cycle. In three studies,
Kunce et al. [92e94] used LENS to synthesize hydrogen-
storage HEAs; an MR-7 LENS device was used, as depicted in
Fig. 11c. This device enables the precise regulation of oper-
ating parameters such as the powder flow rate, laser intensity,
table feed rate, and heat-transfer rate [94]. In addition to these
examples, Borkar et al. [95] prepared Al
x
CrCuFeNi
2
(0 x1.5)
by employing a combination of high-purity elemental
aluminum, chromium, copper, iron, and nickel powders.
In addition to the synthesis techniques mentioned above,
there are other less-popular approaches for improving the effi-
ciency of hydrogen-storage HEAs. MgTiVCrFe was originally
synthesized by de Marco et al. [96]using high-energyball milling
before being subjected to high-pressure torsion (HPT) to
improve activation. Edalati et al. [97] prepared TiZrCrMnFeNi by
arc melting and placed the sample through five turns of HPT at
6 GPa prior to hydrogenation experiments, allowing the hydride
phase to be trapped for a longer time and facilitating the
determination of the hydride crystalstructure. To preparehigh-
purity, homogenous hydrogen-storage HEAs after arc melting,
Zadorozhnyy et al. [98]andSaracetal.[99]melt-spunamolten
alloy underan argon environment. Zhang et al. [100,101]created
a TiZrNbTaHEA on a copper mold using arcmelting and suction
casting,which not only produceda more uniform distribution of
the elements but also decreased the porosity and increased the
density of the HEA, improving its ability to store hydrogen.
Recently, Zadorozhnyy et al. [83] prepared TiVZrNbTa by
combining pendant-drop melt extraction (PDME) and electron
beam melting (EBM), which overcame the shortcomings of each
method while enhancing the effectiveness and performance of
HEAs for hydrogen storage [102,103]. Table 3 shows a compari-
son of the advantages and disadvantages of common methods.
The current preparation methods for hydrogen-storage
HEAs are mainly solid and gaseous approaches, but a few
other methods exist. However, with the advancement of
technology and the growth in scale, it is anticipated that the
material cost of hydrogen-storage HEAs will continuously
decline. For instance, more effective and affordable prepara-
tion methods, such as plasma infiltration and chemical vapor
deposition, can be employed, or HEAs can be combined with
other materials to enhance the material quality and cut costs.
The use of these techniques will encourage the production of
HEAs for hydrogen storage at lower material costs. The prep-
aration of hydrogen storage high-entropy alloys has been
extensively studied, but the kinetics and thermodynamic
properties of these alloys need to be further improved to
expand their practical application range. L.Z. Ouyang and his
co-workers [104,105] developed a new material synthesis
method, dielectric barrier discharge plasma assisted milling, to
achieve simultaneously tune the thermodynamic and kinetic
properties of hydrogen storage alloys. This new material
Fig. 11 e(a) Schematic diagram of arc-melting method [106]. (b) Schematic of the high-energy ball-milling synthesis
mechanism for HEA powders [107]. (c) Scheme of the LENS system [94]. (d) Schematic of HPT [108].
international journal of hydrogen energy 50 (2024) 406e430416
synthesis method may be helpful for improving the properties
of HEAs, such as increasing platform pressure. More detailed
information about dual-tuning the thermodynamic and kinetic
properties can be obtained in the literature [104,105].
5. Microstructure
High group entropy, that is, the random occupation of lattice
sites by component atoms, leads to the formation of simple
solid solutions in HEAs. For example, FCC solid-solution alloys
(e.g., CoCrFeMnNi alloys) were discovered by Yeh et al. [112],
and BCC solid-solution alloys (e.g., AlCoCrFeNi alloys) were
discovered by Zhang et al. [113]. Miracle and Senkov [28]
summarized in detail the microstructure of HEAs, including
amorphous, nanocrystalline, single-phase, and multiphase,
and the observed phases are shown in Fig. 12a. Since the
atomic packing fractions of the BCC space structure (0.68)
were lower than those of the FCC (0.74) and HCP (0.74) struc-
tures, there were more gaps in the crystal lattice to
accommodate the hydrogen atoms, and the hydrogen atoms
diffused at a faster rate in the BCC-type structure. Thus, BCC is
the predominant structure in current hydrogen-storage HEAs.
For instance, TiZrHfMoNb HEAs with good thermal stability
and single-phase reversibility of BCC 4FCC in hydrogen ab-
sorption and discharge cycles were examined by Shen et al.
[76]. Silva et al. [114] prepared three BCC-structured hydrogen-
storage HEAs, namely, (TiVNb)
85
Cr
15
, (TiVNb)
95.3
Co
4.7
, and
(TiVNb)
96.2
Ni
3.8
. In addition, the BCC-structured HEA
HfNbTiVZr was developed by Karlsson et al. [115]. Additional
BCC HEAs will be described in Section 6, Table 4.
In addition to the previously mentioned primary BCC
structures, Fig. 12b depicts another hydrogen-storage HEA
structure: C14 Laves [19,81,98,99,116,117] (see Table 4). C14
Laves were explored by Chen et al. [117]. Cr
u
FeMnTiVZr,
CrFe
v
MnTiVZr, CrFeMn
w
TiVZr, CrFeMnTi
x
VZr, CrFeMnTiV
y
Zr,
and CrFeMnTiVZr
z
alloys (0 u,v,w,x,y,z2) were
researched for their hydrogen-storage behavior, and the lat-
tice structure and lattice parameters before and after
hydrogen absorption were evaluated using XRD. These
Table 3 ePreparation method.
Preparation method Advantage Disadvantage Ref.
Arc melting Relatively simple process, easy to control and
operate
Requires high temperature and high energy
input, which tends to make unstable phases
appear in the alloy
[106]
Mechanical alloying Capable of preparing hydrogen-storage HEAs under
relatively mild conditions and improving the crystal
structure
Impurities are easily produced [109]
LENS The proportion of alloying elements can be
controlled in the alloy particles, and the preparation
process is highly controllable
High cost of equipment [94]
HPT Optimizes the microstructure and phase
composition of materials
Refinement of material grains may lead to slower
hydrogenation reaction rates
[110]
Melt spinning The prepared HEA has a high specific surface area May cause some elements to volatilize,
decompose, and become inactive
[111]
Suction The gas inside the casting can be discharged quickly,
reducing gas inclusions, holes and other defects
Need to strictly control the alloy composition and
temperature, and the operation is difficult
[111]
Fig. 12 e(a) Number of occurrences of each phase: if a phase occurred more than once in the same alloy, then that phase was
counted each time [28]. (b) C14 schematic of Laves structure [118].
international journal of hydrogen energy 50 (2024) 406e430 417
Table 4 eSummary of hydrogen storage properties of different HEAs.
HEA Synthesis method Alloy phase Hydride phase Hydrogen storage
capacity (wt%)
H/M Temperature/K Ref.
ZrTiVCrFeNi LENS C14 Laves(major) a-Ti solid solution(minor) e1.81 ee [93]
TiZrNbMoV LENS(300 W) BCC(major) aZr-rich(minor) fcc dTiH
x
bcc NbH
~0.4
aZr-rich 2.3 ee [92]
TiZrNbMoV LENS(1000 W) BCC BCC 0.61 ee [92]
LaNiFeVMn LENS aþLa(Ni,Mn)
5
FCC þLa(Ni,Mn)
5
0.83 e308 [94]
TiVZrNbHf AM BCC FCC/BCT 2.7 2.5 572 [36,115]
TiZrNbHfTa AM BCC BCT/FCC 1.7 e573 [122]
Ti
0.325
V
0.275
Zr
0.125
Nb
0.275
BM BCC BCT 2.5 1.7 523 [125]
Ti
0.325
V
0.275
Zr
0.125
Nb
0.275
AM BCC BCT 2.5 1.75 298 [125]
Ti
0.325
V
0.275
Zr
0.125
Nb
0.275
RBM BCC BCT 2.0 1.25 298 [125]
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
Ta
0.10
AM BCC BCT/FCC 2.5 2 298 [44]
Mg
0.10
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
BM BCC FCC 2.7 1.5 298 [85]
Al
0.10
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
AM BCC BCT 2.6 1.6 298 [128]
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
Mo
0.10
AM BCC FCC 2.8 2.0 298 [128]
Ti
0.30
V
0.25
Cr
0.10
Zr
0.10
Nb
0.25
AM BCC FCC 3.0 2.0 298 [82]
Al
0.10
Ti
0.30
V
0.30
Nb
0.30
AM BCC FCC e1.6 298 [71]
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
Mn
0.10
AM BCC FCC e2.0 298 [132]
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
Fe
0.10
BM/RBM BCC FCC e1.16 298 [132]
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
Co
0.10
BM BCC FCC e1.23 298 [132]
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
Ni
0.10
BM/RBM BCC FCC e1.1 298 [132]
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
Cu
0.10
BM/RBM BCC FCC e1.35 298 [132]
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
Zn
0.10
BM/RBM BCC FCC e1.18 298 [132]
TiZrHfMoNb AM BCC/FCC FCC 1.94 2 e[120]
TiZrHfScMo AM BCC FCC 2.14 ee [133]
Ti
0.20
Zr
0.20
Hf
0.20
Nb
0.40
AM BCC FCC 1.12 ee [134]
Ti
0.20
Zr
0.20
Hf
0.20
Mo
0.10
Nb
0.3
AM BCC FCC 1.54 ee [134]
Ti
0.20
Zr
0.20
Hf
0.20
Mo
0.20
Nb
0.2
AM BCC FCC 1.18 ee [134]
Ti
0.20
Zr
0.20
Hf
0.20
Mo
0.30
Nb
0.1
AM BCC BCT 1.40 ee [134]
Ti
0.20
Zr
0.20
Hf
0.20
Mo
0.40
AM BCC BCT 0.92 ee [134]
TiZrVMoNb eBCC FCC 2.65 2.05 e[58]
Ti
0.20
Zr
0.20
Hf
0.20
Nb
0.40
eBCC FCC e1.5 573 [135]
TiZrHfMoNbPt
0.0025
AM BCC FCC 1.749 e293 [136]
TiZrHfMoNbPd
0.0025
AM BCC FCC 1.764 e293 [136]
TiZrNbTa AM þsuction casting BCC fct ε-ZrH2, fct ε-TiH2, b-(Nb,Ta)H 1.67 e293 [100]
TiZrNbTa AM þsuction casting BCC e1.25 e493 [100]
Ti
0.2
Zr
0.2
Nb
0.2
V
0.2
Cr
0.17
Fe
0.03
AM BCC þFCC FCC-Monohydride 1.73 e298 [137]
CoFeMnTiVZr AM þmelt spinning C14 Laves C14_1
C14_2
1.7 ee [99]
Ti
20
Zr
20
V
20
Nb
20
Ta
20
AM BCC e1.6 ee [138]
Ti
25
Zr
25
Nb
15
V
15
Ta
20
AM BCC e1.7 ee [138]
Ti
20
Zr
20
V
15
Nb
15
Ta
15
Hf
15
AM BCC e1.5 ee [138]
ZrTiVFe AM C14 þHCP C14 Laves 1.67 e473 [79]
ZrTiVAl AM C14 þHCP C14 Laves 1.4 e473 [79]
international journal of hydrogen energy 50 (2024) 406e430418
(ZrTiVFe)
80
Al
20
AM C14 þHCP þtetragonal C14 þHCP 1.4 e473 [35]
(ZrTiVFe)
0.95
Cu
0.05
AM C14 (major), a-Zr (minor), a-Ti C14 þHCP 1.504 e473 [139]
(ZrTiVFe)
0.90
Cu
0.10
AM C14 (major), a-Ti, a-Zr (minor) C14 þHCP 1.357 e473 [139]
(ZrTiVFe)
0.80
Cu
0.20
AM C14 þZrTiCu
2
þCu
8
Zr
3
C14 þHCP 1.112 e473 [139]
MgZrTiFe
0.5
Co
0.5
Ni
0.5
BM BCC FCC 1.2 e623 [140]
Mg
12
Al
11
Ti
33
Mn
11
Nb
33
BM BCC e1.75 1.05 548 [141]
MgVAlCrNi BM BCC e0.3 0.1 e[88]
MgTiNbCr
0.5
Mn
0.5
Ni
0.5
BM/RBM BCC FCC 1.6 ee [142]
Mg
0.68
TiNbNi
0.55
BM/RBM BCC FCC 1.6 ee [142]
TiZrCrMnFeNi AM C14 (major) þcubic phase (minor) C14 Laves 1.7 1.0 303 [97]
TiZrNbFeNi AM C14 Laves C14 Laves 1.64 1.17 305 [81]
Ti
20
Zr
20
Nb
5
Fe
40
Ni
15
AM C14 Laves (major) þBCC (minor) C14 Laves 1.38 0.95 305 [81]
(TiVNb)
85
Cr
15
AM BCC FCC 3.18 2.0 298 [114]
(TiVNb)
95.3
Co
4.7
AM BCC FCC 3.11 2.0 298 [114]
(TiVNb)
96.2
Ni
3.8
AM BCC FCC 3.17 1.9 298 [114]
(TiVNb)
85
Cr
15
AM BCC (major) þFCC (minor) CaF
2
-type (major)þFCC (minor) 3.0 1.9 297 [57]
(TiVNb)
75
Cr
25
AM BCC (major) þFCC (minor) CaF
2
-type (major)þFCC (minor) 2.7 1.7 297 [57]
(TiVNb)
65
Cr
35
AM BCC (major) þFCC (minor) BCC (major) þTi-rich FCC (minor) 2.8 1.7 297 [57]
TiZrNbCrFe AM C14 þBCC C14 þFCC 1.95 1.35 303 [56]
TiZrNbCrFe AM C14 þBCC C14 þFCC 1.9 1.32 473 [56]
MgAlTiFeNi BM BCC FCC 1 e598 [84]
MgAlTiFeNi RBM BCC FCC 0.94 ee [84]
Ti
31
V
26
Nb
26
Zr
12
Fe
5
AM BCC þC14 FCC þC14 2.92 1.9 298 [143]
Ti
31
V
26
Nb
26
Zr
12
Co
5
AM BCC þC14 FCC þC14 2.89 1.9 298 [143]
Ti
31
V
26
Nb
26
Zr
12
Ni
5
AM BCC þC14 FCC þC14 2.93 1.9 298 [143]
TiZrNbCrFeNi AM C14 Laves C14 Laves 1.5 0.9 303 [143]
Mg
35
Al
15
Ti
25
V
10
Zn
15
BM BCC þMg FCC þMgH
2
þBCT þMgZn
2
2.5 e648 [144]
Mg
35
Al
15
Ti
25
V
10
Zn
15
RBM BCC þMg FCC þMgH
2
2.75 ee [144]
Ti
0.24
V
0.17
Zr
0.17
Mn
0.17
Co
0.17
Fe
0.08
RF induction melting C14 Laves C14 Laves 0.53 e698 [145]
CoFeMnTi
2
VZr AM C14 Laves C14 Laves 1.82 e298 [19]
CoFeMnTiV
2.6
Zr AM C14 Laves C14 Laves 1.6 e298 [19]
CoFeMnTiVZr
2.3
AM C14 Laves C14 Laves 1.80 e298 [19]
CrMnTiVZr AM C14 Laves C14 Laves 2.23 e278 [118]
CrFeMnTiVZr
2
AM C14 Laves C14 Laves 2.17 e278 [118]
CrFe
1.5
MnTiVZr AM C14 Laves C14 Laves 1.14 e353 [118]
CrFeMnTiVZr
0.5
AM C14 Laves C14 Laves 1.14 e278 [118]
MgTiVCrFe BM þHPT BCC BCCþb-MgH
2
0.3 e303 [86]
V
0.3
Ti
0.3
Cr
0.25
Mn
0.1
Nb
0.05
AM BCC FCC 3.45 e298 [77]
ZrTiNbMoCr AM C14 Laves C14 Laves eee[146]
ZrTiVNiCrFe AM þmelt spinning C14 Laves C14 Laves 1.6 e298 [98]
AlCrFeMnNiW BM BCC þFCC BCC þFCC 0.615 e298 [87]
Ti
20
Zr
20
V
20
Cr
20
Ni
20
AM C14 Laves C14 Laves 1.52 e305 [147]
TiZrFeMnCrV AM C14 Laves C14 Laves 1.80 e273 [130]
TiVZrNbTa EBM-PDME BCC FCC 1.6 1.5 673 [83]
AlMg
2
TiZn BM BCC FCC 1.35e1.4 e583~643 [148]
Zr
0.2
Ti
0.2
Ni
0.2
Cr
0.2
Mn
0.2
AM C14 þB2 e1.66 e323 [123]
(continued on next page)
international journal of hydrogen energy 50 (2024) 406e430 419
findings demonstrated that the investigated alloys and their
hydrides had a common C14 Laves structure. Owing to the
increased chemical bond stability and improved chemical
inertness between the elements in the C14 Laves structure, it
is feasible to construct hydrogen-storage HEAs with extended
cycle lives. In addition, although few HEAs with FCC struc-
tures exist, they are still used in hydrogen storage. Zhao et al.
[119] investigated the hydrogen absorption and plastic defor-
mation behaviors of two FCC HEAs (CoCrFeNi and CoCr-
FeMnNi) and discovered that grain size can influence these
two alloys. Hu et al. [120] created BCC and FCC TiZrHfMoNb
alloys to examine and compare the preference of hydrogen
atoms to occupy interstitial sites.
According to the findings of current research, BCC is one of
the most promising hydrogen-storage HEA architectures.
Generally, as hydrogen atoms occupy voids during hydrogen
absorption, the lattice constants of hydrogen-storage HEAs
with BCC structures gradually increase. This structural phase
change causes the HEA with a BCC structure to change into an
FCC structure when the number of adsorbed hydrogen atoms
reaches a certain threshold. The performance of HEAs,
including their hydrogen storage capacity, hydrogen-
absorption properties, and discharge kinetics, are also
affected by changes in their lattice parameters, crystal struc-
ture, and atomic spacing after hydrogen absorption. There-
fore, in the research of hydrogen-storage HEAs, in-depth
studies on their adsorption and diffusion processes, structural
phase changes, and hydrogen absorption and discharge
properties are needed to realize the development of efficient
and stable hydrogen-storage materials.
6. Hydrogen storage properties
6.1. Hydrogenation kinetics
The hydrogen-storage kinetics of HEAs are affected by
various factors, including temperature, pressure, chemical
composition, and the microstructure of the hydrogen-storage
alloys. Temperature and pressure are the main factors
affecting the kinetics. Cheng et al. [121] tested the hydrogen
uptake and release rates of three TiVNbCr alloys with
different Cr contents after activation, all of which had t
0.9
(the time required for hydrogen uptake and release to reach
90% of the saturation capacity) values close to 60 s, as shown
in Fig. 13a. These rapid kinetics are mainly due to the severe
lattice distortion in the HEA. During hydrogen release,
compared with hydrogen absorption, the release volume
decreases significantly, and the required hydrogen-release
temperature surpasses the hydrogen absorption tempera-
ture. Therefore, the authors used three typical hydride ki-
netic models to fit the hydrogen desorption process (the
JMAK equation, Jander diffusion model, and the
GinstlingeBrounshtein model). Their results showed that the
GinstlingeBrounshtein model was the best fit. Reactive ball
milling was used by Cardoso et al. [84] to create MgAlTiFeNi
alloy hydrides, and they. Analyzed two sequential hydrogen-
release curves at 325 C. Both the release profiles exhibited
high desorption kinetics, particularly the second, as
hydrogen was entirely desorbed in <100 s. By combining
Table 4 e(continued )
HEA Synthesis method Alloy phase Hydride phase Hydrogen storage
capacity (wt%)
H/M Temperature/K Ref.
Ti
4
V
3
NbCr
2
AM BCC FCC 3.7 2.01 300 [149]
Ti
27.5
V
27.5
Nb
20
Cr
12.5
Mn
12.7
AM BCC FCC 3.38 2.1 298 [127]
Ti
35
V
30
Nb
10
Cr
25
AM BCC FCC 3.72 e300 [121]
Note: AM, BM, and RBM refer to arc melting, ball milling, and reactive ball milling, respectively.
international journal of hydrogen energy 50 (2024) 406e430420
thermogravimetry and quadrupole mass-spectrometry mea-
surements, the authors predicted that hydride phases may
be present during dehydrogenation and Mg
2
FeH
6
breakdown.
The TiZrCrMnFeNi HEA created by Edalati et al. [97] exhibited
quick kinetics at room temperature, did not undergo any
activation processing, and could be handled and stored
directly in air without experiencing deactivation. Zlotea et al.
[122] reported that TiZrNbHfTa can begin hydrogen sorption
at very low pressures and undergo two-phase transitions
during the sorption process. The activation energy for
desorption by the Kissinger method was 80.2 kJ/mol, which is
comparable to catalytic hydrogen sorption from nano-
crystalline MgH
2
.
6.2. Hydrogenation thermodynamics
The thermodynamic characteristics of HEAs directly affect
their hydrogen-storage efficiency and capacity, such as their
thermodynamic stability, heat of hydrogen adsorption, en-
tropy, free energy, and heat of reaction. The hydrogen-
storage capabilities of the ZreTieNieCreMn HEA were
reviewed by Fukagawa et al. [123], and its PCI (Pressur-
eecomposition isotherms) curves were measured at 323 K.
In their hydride enthalpy of formation (DH) and entropy (DS)
calculations, the authors discovered that eDHdecreases
with decreasing lattice constant and came to the conclusion
that eDSis the degree of freedom generated during
hydrogen desorption and adsorption, which has a constant
value of 130 J$mol$K
1
. All the samples displayed good
reversibility. In addition, as the Ni content increased,
hydrogen absorption decreased, and the hysteresis
improved. Moore et al. [124] studied the HEA TiZrNbHfTa
and through calculations found that the vibrational entropy
combined with the conformational entropy accurately pre-
dicted HEA hydride decomposition. In the PCI curves of
(TiVNb)
1-x
Cr
x
alloys, Strozi et al. [57] discovered that the
maximal hydrogen absorption did not significantly decrease
as the concentration increased and that hydride formation
was reversible, albeit with more severe hysteresis. There-
fore, with the increase in non-hydride-forming elements (Cr)
in the HEA, no substantial capacity loss occurred. By
substituting Fe with Mn in the alloy V
0.3
Ti
0.3
Cr
0.25
Mn
0.1
Nb
0.05
,
which was reported in earlier research, Liu et al. [77]
developed a novel alloy with a maximum hydrogen storage
capacity of 3.45 wt%. They measured the PCI curves for
hydrogen desorption at 298, 323, and 343 K. The alloy des-
orbed in two phases, as evidenced by the tilt of the pressure
plateau and incomplete desorption. Calculating the enthalpy
of desorption using the Van't Hoff equation yielded values of
31.1 kJ/mol and 101.8 J/K/mol, indicating that the hydride is
comparatively stable. Montero et al. [125] prepared TiVZrNi
alloys by arc melting and ball milling, and metal hydrides
were produced by reactive grinding in a hydrogen environ-
ment. PCI were used to test the ability of the alloys to adsorb
hydrogen. At normal temperatures, TiVZrNi alloys produced
by electric arc melting exhibit superior absorption charac-
teristics. The desorption characteristics of the alloys were
examined using TDS after full hydrogen absorption, and
they displayed similar curve trends. This was in contrast to
the metal hydrides prepared via reactive grinding, in which
case the desorption temperatures were lower, and the
desorption kinetics were better. Silva et al. [114] used the
CALPHAD method to design three BCC HEAs with the same
VEC (4.87), that is, (TiVNb)
85
Cr
15
, (TiVNb)
95.3
Co
4.7
, and
(TiVNb)
96.2
Ni
3.8
. The three alloys exhibited a two-step hy-
drogenation sequence and eventually reached a hydrogen
uptake of 1.6e2.0H/M. The entropy and enthalpy values of
the three alloys were calculated from the Van't Hoff plots in
Fig. 13b and are comparable to those of the Mg
2
Ni and
TiVZrNbHf hydrides, but are higher than those of LaNi
5
-
related intermetallic compounds. To better understand the
cyclic effect, the thermal desorption behaviors of the three
alloys were investigated using TDS. The results showed that
the desorption behavior was influenced not only by the VEC
but also by Cr, Co, and Ni substitution. Recently, Andrade
et al. [126] investigated a new isoatomic HEA, TiZrNbCrFeNi,
and measured the PCI curves for two cycles at 303, 353, 373,
and 473 K without activation. At 473 K, the alloy reversibly
absorbed and desorbed 1.1 wt% hydrogen, exhibiting very
good performance in terms of reversibility. Furthermore, the
Fig. 13 e(a) Hydrogen-absorption kinetics of the TiVNbCr alloys at 300 K [121]. (b) Van't Hoff plots of the (TiVNb)
85
Cr
15
,
(TiVNb)
95.3
Co
4.7
and (TiVNb)
96.2
Ni
3.8
alloys [114]. (c) Activation curves and maximum hydrogen-absorption capacities of the
as-cast alloys. Measurements under 2 MPa H
2
at 25 C[127].
international journal of hydrogen energy 50 (2024) 406e430 421
measured PCT curves lacked a clearly defined pressure
plateau, which the authors hypothesized may be because
hydrogen absorption in the C14 structure occurs only via an
interstitial solid-solution mechanism.
6.3. Hydrogen capacity
The composition, crystal structure, atomic size and shape,
pore structure, and surface topography of HEAs impact their
ability to store hydrogen. Moreover, hydrogen-storage condi-
tions such as hydrogen pressure, temperature, and cycle
period affect their hydrogen-storage capacity. When
designing hydrogen-storage HEAs, the above factors must be
considered, and a reasonably optimized design is required to
achieve a higher hydrogen-storage capacity. In 2016, Sahlberg
et al. [34] discovered that the capability of the TiVZrNbHf HEA
to store hydrogen can approach 2.5H/M (2.7 wt%). Karlsson
et al. [115] investigated the mechanism of hydrogen absorp-
tion in the HEA TiVZrNbHf, which underwent a structural
phase transition from BCC to BCT during hydrogenation. As
revealed by neutron diffraction, the hydrogen atoms occupied
the tetrahedral and octahedral gaps, which also contributed to
the high hydrogen-storage capacity of this alloy. Montero et al.
[82] found that the addition of the lightweight metal Mg not
only changed the cyclic properties but also increased the
hydrogen-storage capacity of HEAs. Recently, Serrano et al.
[127] designed three HEAs, namely, Ti
35
V
35
Nb
20
Cr
5
Mn
5
,Ti
32-
V
32
Nb
18
Cr
9
Mn
9
, and Ti
27.5
V
27.5
Nb
20
Cr
12.5
Mn
12.5
, using thermo-
dynamic calculations and tested their hydrogen-absorption
kinetics at room temperature. As shown in Fig. 13c, the ki-
netics of Ti
27.5
V
27.5
Nb
20
Cr
12.5
Mn
12.5
suddenly accelerated after
450 min, and the saturation hydrogen absorption reached
3.38 wt%, making it the HEA with the highest hydrogen-
storage capacity at the present time. However, the
maximum hydrogen-storage capacity and kinetic properties
of this alloy declined significantly after hydrogen absorption
and discharge cycles.
6.4. Cycling properties
The cyclic properties of HEAs during hydrogen absorption and
desorption cycles are an important performance index for
their use as hydrogen-storage materials, which mainly
include cycle stability and cycle life. According to Montero
et al. [44], the hydrogen-storage capacity of Ti
0.30
V
0.25
Zr
0.10-
Nb
0.25
Ta
0.10
gradually decreases after the first cycle until
reaching a steady state in the eighth cycle, when it is
approximately 86% of its initial capacity (1.71H/M; 2.19 wt%).
The maximum hydrogen absorption at room temperature
was 2.0H/M (2.5 wt%). The structure of the alloy and its hy-
drides did not experience considerable phase segregation and
oxidation despite a slight reduction in hydrogen-storage ca-
pacity. The authors also reported that the addition of Mg
improved the cycling performance of the TieVeZreNb alloy
[85], as shown in Fig. 14a. It can be seen that the five-member
alloy Mg
0.10
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
had a higher maximum
hydrogen-storage capacity than the quaternary alloy Ti
0.325-
V
0.275
Zr
0.125
Nb
0.275
, and its hydrogen storage capacity was
stabilized at 2.4 wt% after the second cycle. Therefore, the
addition of Mg to refractory HEAs improves the reversibility of
the hydrogen absorption/desorption cycle. A similar study
was performed on Al
0.10
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
HEAs [128]. Ac-
cording to Bouzidi et al. [129], the inclusion of 10% Mo not only
enhanced the hydrogen-absorption kinetics during cycling of
the Ti
0.325
V
0.275
Zr
0.125
Nb
0.275
alloy but also optimized its cyclic
characteristics. A single C14 Laves structure of the HEA
TiZrFeMnCrV was prepared by Chen et al. [130] via arc melting
and mechanical ball milling, achieving a maximum
hydrogen-storage capacity of 1.80 wt%. The hydrogen-storage
capacity was essentially maintained in the range of
1.73e1.76 wt% after the first cycle, indicating a strong
reversible hydrogen-storage performance, although t
0.9
was
reduced after 50 cycles of hydrogen absorption and discharge
testing. However, t
0.9
did not surpass 60 s in the range of cy-
cles studied. To adjust the hydrogen-storage temperature and
pressure of a hydrogen-storage HEA, Mohammadi et al. [131]
used the concept of binding energy. They created and syn-
thesized Ti
x
Zr
2-x
CrMnFeNi (x¼0.4e1.6) and discovered
through PCT as well as kinetic tests on this alloy series that
the performance of Ti
0.4
Zr
1.6
CrMnFeNi is excellent. The
hydrogen-storage capacity of this alloy remained almost un-
changed throughout the cycle, and the PCT curves did not
vary noticeably, as illustrated in Fig. 14b and c, which were
examined after the 4th, 30th, 100th, and, 1000th cycles.
Fig. 14 e(a) Comparison of the reversible hydrogen-storage capacities of quinary Mg
0.10
Ti
0.30
V
0.25
Zr
0.10
Nb
0.25
and
quaternary Ti
0.325
V
0.275
Zr
0.125
Nb
0.275
alloys [85]. (b) Hydrogenation test of 1000 cycles at room temperature. (c)
Corresponding PCT absorption/desorption isotherms for 4, 30, 100, and 1000 cycles for the HEA Ti
0.4
Zr
1.6
CrMnFeNi [131].
international journal of hydrogen energy 50 (2024) 406e430422
7. Cost
The price of HEAs for hydrogen storage varies according to
several variables, including the price of the raw materials,
preparation method, processing, testing, shipping, and man-
agement. The cost of the raw materials is a major consider-
ation. Vanadium, titanium, chromium, manganese, and other
elements are frequently employed in hydrogen storage HEAs,
as shown in Fig. 10.Fig. 15 presents the price and abundance of
elements for designing hydrogen storage materials. The data
in Fig. 15 is for reference only, as discrepancies arise when
multiple processes are involved in extracting the pure ele-
ments, and upon additional market variation and prices
change significantly over time [150]. HEA materials can be
prepared using various techniques, and the costs of these
techniques may vary depending on the equipment and tech-
nology used.
Here, we only discuss costs from the point of view of raw
materials. According to the study of Fu et al. [151], that the
median price of alloys increases as the number of constituents
rises (as shown in Fig. 16), by about an order of magnitude by
the time a six-component system is reached; mixing elements
raises the average price because there is a higher likelihood of
a combination including an element from the higher price
groups. This suggests that if alloy design is conducted without
considering price, the resulting HEAs are likely to be expensive
compared with even current specialty metals. They also noted
that the alloy complexity level rises, the spread of possible
alloy prices drops considerably, from almost five orders-of-
magnitude at N¼1 to about two by N¼6. Lai et al. [150]
suggested that the cost (COAxBy) of the raw hydrogen storage
alloys (A
x
B
y
) can be determined from:
COAxBy¼COAx
xþyNAþCOBy
xþyNB
x
xþyNAþy
xþyNB
(14)
where CO
A
,CO
B
correspond to the cost of the elements A and
B ($ per kg), x and y are the respective amount of the elements
A and B, and N
A
and N
B
their respective molar mass (g/mol). By
using the cost of single elements, the cost of binary to higher
quinary alloys with (non)-stoichiometric compositions, can be
determined from Eq. (14). For more information, the reader is
also encouraged to check Ref. [150,151].
8. Other hydrogen-related applications
This comprehensive overview of hydrogen-storage HEAs re-
veals that transition metal elements with high atomic weights
are often used as alloy components, reducing their hydrogen-
storage capacity to a relatively low level. For example, the
mass capacity of TiZrNbMoV is in the range of 1.78e2.30 wt%
[92]. Therefore, most HEAs are not considered promising
candidates for hydrogen storage. However, there are other
hydrogen-related applications for HEAs.
Fig. 15 ePrice and abundance of elements for designing hydrogen storage materials. Periodic table summarizing the
elements forming hydrides, their market price (AUD $ per kg), and abundance [150].
Fig. 16 eRange in alloy price (in $/mol) as number of
elements increases [151].
international journal of hydrogen energy 50 (2024) 406e430 423
8.1. Catalyst for hydrogen storage materials
HEAs with tunable lattice distortion, activation site, and
microstructure, are regarded as unique materials for various
catalytic reactions. Among them, it has been widely studied as
a catalyst for other hydrogen storage materials [152e155].
Magnesium hydride (MgH
2
) is a promising candidate for
hydrogen storage, but suffers from its sluggish de/hydroge-
nation kinetics. Catalytic addition is considered one of the
most effective methods to improve the kinetics of Mg-based
hydrides. Zhang et al. [152] studied equiatomic TiVNb-based
HEAs as catalysts for MgH
2
. The results show that C14 Laves
structured TiVNbZrFe alloy shows a superior catalytic effect in
improving the de/hydrogenation kinetics and cycling proper-
ties of MgH
2
. The MgH
2
eTiVNbZrFe starts to release hydrogen
at about 209 C, nearly 170 C lower than that of the pure MgH
2
.
Also, the apparent activation energy of dehydrogenating
MgH
2
eTiVNbZrFe can be reduced to 63.03 kJ mol
1
, which is
about 90 kJ mol
1
lower than that of pure MgH
2
. Wan et al. [153]
reported the FeCoNiCrMn HEA loaded MgH
2
and HEA's effect
on the hydrogen storage properties of Mg/MgH
2
. The FeCo-
NiCrMn alloy shows high catalytic activity toward hydrogen
dissociation and recombination reaction, and successfully
suppressed activation energy of dehydrogenation reaction
from 151.9 to 90.2 kJ mol
1
. Cermak et al. [154] prepared Mg-
10 wt% HEA composition by high-energy ball milling. It was
found that activation energy of hydrogen desorption for
composition was significantly decreased compared to pure Mg.
Wang et al. [155] also studied the use of HEAs as catalysts to
improve the hydrogen storage performance of MgH
2
. The re-
sults show that addition of Mn had a great impact on the per-
formance of HEAs catalysts. The MgH
2
eCrMnFeCoNi
composite could release 6.5 wt% H
2
in 10 min at 300 C and
started to absorb H
2
at 40 C. The above studies well show the
beneficial application of HEAs as catalysts in other hydrogen
storage materials. These excellent catalytic effects may be the
cocktail effect of HEAs exerted synergic action between
multicomponent to improve the overall catalytic efficiency.
8.2. Others
The HEAs can also be used as hydrogen compression mate-
rials, which is also very important for hydrogen applications.
For example, TieCreFe-based HEAs hydrogen compression
materials enable the final-stage compression units up to more
than 45 MPa for metal hydride hydrogen compressors
[156,157]. HEAs also can be used as an electrode material for
nickel-metal hydride batteries to enhance the high-rate
discharge properties [158,159].
The inclusion of hydrogen atoms in the metal lattice is an
indicator of the structural differences between HEAs and
conventional alloys [133]. Another crucial area of research in
the field of HEAs is the impact of hydrogen on the mechanical
characteristics of alloy systems [160e171]. According to Zhang
et al. [172], the HEA Ni
20
Fe
20
Mo
10
Co
35
Cr
15
can be used as the
hydrogen-precipitation electrocatalyst in acidic and alkaline
electrolytes. This dual functionality has brought hydrogen
storage and HEAs closer together. Li et al. [173]studiedtheef-
fects of an inhomogeneous electron density distribution on the
hydrogen distribution in TiZrTaNbAl HEA. The results shown
that the electron density has a significant effect on the
hydrogen distribution in TiZrTaNbAl MPEA. The work provided
new insight into the design of materials with high hydrogen
storage capacity and high hydrogen embrittlement resistance.
In addition, HEAs also have great potential in tritium storage,
and Zhang et al. [174] reported a comprehensive review of this.
Summary and outlook of future work
The problems associated with hydrogen energy storage and
transportation may be greatly improved by using HEAs, a new
type of hydrogen storage material with the benefits of high
hydrogen-storage capacity, high stability, and good cycling
performance. Several elements, such as alloy composition,
crystal structure, and preparation method, must be consid-
ered when designing and creating HEAs for hydrogen storage.
Mechanical alloying, vacuum arc melting, and other prepa-
ration techniques are commonly employed. To obtain a
crystal structure with high purity and good uniformity, the
advantages and disadvantages of various preparation tech-
niques must be weighed based on the particular scenario. An
essential metric to consider when assessing the utility of an
HEA for hydrogen storage is its ability to store hydrogen. The
results of this study indicate that HEAs are potential
hydrogen-storage materials; however, additional research
and improvements are required to enhance their hydrogen
storage/release rate and cycle stability. HEAs are anticipated
to become increasingly important in hydrogen energy storage
and transportation in the future. Further improvements in the
alloy composition, preparation process, and hydrogen storage
performance are required, and efficient and reasonable
preparation techniques and application technologies must be
developed to achieve commercial applicability. In addition,
the thermophysical properties (such as thermal conductivity,
expansion properties, etc.) of high-entropy alloys are poorly
studied, which is also very important for future applications of
alloys and needs to be studied. It should be pointed out that
alloying generally reduces the thermal conductivity of metals,
so the thermal conductivity of high-entropy alloys may not be
ideal. How to improve its thermal conductivity may also be a
key issue that needs to be solved. It is important to encourage
the practical application of HEA technology for hydrogen
storage to support the long-term growth of hydrogen energy.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by the Natural Science Foundation
of Inner Mongolia, China (grant nos. 2022MS05011,
2020LH01006 and 2022FX02), the National Natural Science
international journal of hydrogen energy 50 (2024) 406e430424
Foundation of China (grant numbers 52261041 and 51961032),
the Major Science and Technology Project of Inner Mongolia
(grant number 2021ZD0029) and the Fundamental Research
Funds for Inner Mongolia University of Science &Technology.
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international journal of hydrogen energy 50 (2024) 406e430430
