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Review Article

Electrochemical hydrogen storage: Opportunities

for fuel storage, batteries, fuel cells, and
supercapacitors

Ali Eftekhari a,b,*, Baizeng Fang c

a
The Engineering Research Institute, Ulster University, Newtownabbey BT37 OQB, United Kingdom
b
School of Chemistry and Chemical Engineering, Queen's University Belfast, Stranmillis Road, Belfast BT9 5AG,
United Kingdom
c
Department of Chemical & Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC,
V6T 1Z3, Canada

article info abstract

Article history: Solid-state storage of hydrogen is a possible breakthrough to realise the unique futures of
Received 19 April 2017 hydrogen as a green fuel. Among possible methods, electrochemical hydrogen storage is
Received in revised form very promising, as can be conducted at low temperature and pressure with a simple device
15 August 2017 reversibly. However, it has been overshadowed by the physical hydrogen storage in the
Accepted 16 August 2017 literature, and thus, research efforts are not adequately connected to lead us in the right
Available online 8 September 2017 direction. On the other hand, electrochemical hydrogen storage is the basis of some other
electrochemical power sources such as batteries, fuel cells, and supercapacitors. For
Keywords: instance, available hydrogen storage materials can build supercapacitors with exception-
Hydrogen storage ally high specific capacitance in order of 4000 F g
1. In general, electrochemical hydrogen
Electrochemical energy storage storage plays a substantial role in the future of not only hydrogen storage but also elec-
Fuel cells trochemical power sources. There are some vague points which have obscured our un-
Batteries derstanding of the corresponding system to be developed practically. This review aims to
Supercapacitors portray the entire field and detect those ambiguous points which are indeed the key ob-
stacles. It is clarified that different materials have somehow similar mechanisms for
electrochemical hydrogen storage, which is initiated by hydrogen dissociation, surface
adsorption and probably diffusing deep within the bulk material. This mechanism is
different from the insertion/extraction of alkali metals, though battery materials look
similar. Based on the available reports, it seems that the most promising material design
for the future of electrochemical hydrogen storage is a class of subtly designed nano-
composites of Mg-based alloys and mesoporous carbons.
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. The Engineering Research Institute, Ulster University, Newtownabbey BT37 OQB, United Kingdom.
E-mail address: eftekhari@elchem.org (A. Eftekhari).
http://dx.doi.org/10.1016/j.ijhydene.2017.08.103
0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
25144 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25144
An outline of possible applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25145
Hydrogen as a fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25145
Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25145
Fuel cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25145
Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25145
Materials and mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25146
Hydrogen adsorption on platinum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25146
Diffusion of hydrogen into palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25147
Metal hydrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25148
Carbon nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25156
Metal chalcogenides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25156
Metal oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25157
Hydrogen vs. alkali metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25158
Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25158
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25158

utilised as good charge careers. However, there are not many

Introduction profound works devoted to the mechanisms of electro-
chemical hydrogen storage to strategically plan for the future
Needless to repeat that hydrogen is an excellent fuel but its materials. Most papers are limited to typical electrochemical
applicability has been limited by the difficulty of storage. hydrogen storage capability of new nanomaterials.
Available methods for storing hydrogen in tanks in the form of While there is no specific limitation for increasing the
liquid or compressed hydrogen is not suitable for everyday pressure to increase the hydrogen storage capacity in phys-
applications. Two main reasons are low energy density and isorption, electrochemical adsorption is limited to a specific
serious safety issues. It seems that the emerging solid-state range of energy dictated by the potential region of the corre-
storage is the ultimate approach to realise the benefits of sponding underpotential reaction. The unique feature of
hydrogen [1e13]. There are several solid-state methods for electrochemical hydrogen storage is its reversibility, while the
storing hydrogen; despite the growing interest, all of them are higher capacity of physical approaches is highlighted as an
still far behind the practical expectations (though, they are advantage. Fig. 1 typically depicts where the electrochemical
occasionally utilised). To build a roadmap, the US Department approach stands.
of Energy has launched a hydrogen program and set tangible
targets to materialise solid-state hydrogen. However, slow
progress causes the need for revising those targets, as they are
still out of reach.
One of the most promising methods for solid-state
hydrogen storage is electrochemical hydrogen storage.
Despite numerous works, this area is still underdeveloped due
to the lack of an appropriate strategy of research. One possible
reason is that this method has been lost between the fields of
electrochemical power sources and hydrogen storage. Phys-
isorption of hydrogen molecules is significantly different from
the adsorption of hydrogen atoms at the electrode/electrolyte
interface, though similar materials are used in both systems.
Confusion by the similarity of names, “hydrogen storage” has
made the understanding of this topic difficult. While the
former physically stores hydrogen molecules over the large
area of a solid material, the electrochemical method stores
hydrogen ions as mobile charges. This is indeed an analogue Fig. 1 e Comparison of the maximum gaseous phase
to the charge storage in batteries and supercapacitors. As will hydrogen storage capacity, the reversible gaseous phase
be discussed in the present review, this feature makes the hydrogen storage capacity, and the full electrochemical
applicability of the electrochemical method far beyond discharge capacity. Reproduced with permission from
hydrogen storage as a fuel, since the hydrogen ions can be Ref. [164]. Copyright 2012, Elsevier.
 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5 25145

The critically interesting point is that the advancement of key step in this direction is to find the appropriate anode and
electrochemical hydrogen storage is not limited to storing cathode materials to accommodate the charge carriers. For
hydrogen as a fuel, as this can be the basis for a variety of instance, the trivalent Al charge career is quite interesting in
electrochemical power sources. Historically, electrochemical terms of energy storage capability, but high charge density
hydrogen storage was the basis of commercially popular makes the intercalation of Al3þ difficult [17]. In the same
metal hydride (MH) batteries, where the purpose was storing fashion, potential electrode materials for accommodating
energy rather than hydrogen as a fuel. hydrogen are not as many as the available ones for Liþ, how-
In any case, understanding the electrochemical hydrogen ever, there is no need to highlight the application of electro-
storage is of vital importance for the future of energy storage chemical hydrogen storage in batteries, as MH batteries are
whether electrochemically or by hydrogen fuel. A crucial step commonly available in the market [18]. However, develop-
in this direction is to properly classify our current knowledge ment of new hydrogen storing materials paves the path for
about electrochemical hydrogen storage, as there is no review designing new types of batteries, as H has some advantages as
on this topic. Almost all reviews on hydrogen storage ignore a charge carrier. There is indeed a growing interest in novel
electrochemical possibility or mention it briefly. The present batteries whose electrodes (both anode and cathode) are
review attempts to summarise available reports to reach based on a conversion mechanism instead of intercalation
critical open questions, which should be addressed by the [19]. However, the conversion mechanism of an MH electrode
future research in this important field. is much more straightforward than other conversion-based
Electrochemical hydrogen storage is normally based on electrodes due to the small size of H atoms. As a result, the
two general mechanisms: adsorption on the surface or metallic electrode does not undergo the common massive
insertion within the bulk of the electroactive material. The volume changes, which is a severe issue of the conventional
former is more important for the present candidates of elec- conversion-based electrodes (e.g., novel anode materials for
trochemical hydrogen storage, but it will be revealed that the lithium-ion batteries) [20]. On the other hand, metal hydrides
practical candidates of the future are probably based on a mix are a promising anode material candidate for lithium-ion
of both mechanisms. Different potential materials storing batteries [21,22]. Although the latter is not a direct hydrogen
hydrogen by these two mechanisms are explained, and then, storage, it is still the same system, and similar considerations
possible applications of various systems in electrochemical are required to develop such anode materials in the practical
power sources are described. Owing to the novelty and cells.
undermined potentials of the electrochemical hydrogen sys-
tems, the present review basically aims to introduce the op- Fuel cells
portunities without deep discussion of each system to keep
the manuscript readable by a broad range of audience. Electrochemical hydrogen storage is (or can be) the basis
of various types of fuel cells. Hydrogen storing materials
can be used as anodes of alkaline fuel cells. As a matter
An outline of possible applications of fact, MHs are commonly used for this purpose, and
there is a subclass named metal hydride fuel cells
Electrochemical hydrogen storage is indeed one of the po- [23e25]. The capability of storing hydrogen in the metal
tential applications of the underlying electrochemical mech- hydride fuel cells is somehow similar to direct borohy-
anism, but the applications of hydrogen as a charge career in dride fuel cells, which are another electrochemical sys-
electrochemical systems is not limited to the storage of tem utilising hydrogen fuel. Of course, in this case,
hydrogen as a fuel. In fact, the electrochemical systems of hydrogen is not electrochemically stored, but the release
hydrogen can be utilised for other types of energy storage and of the hydrogen from the borohydride salt is conducted
conversion, as will be briefly summarised here. electrochemically.

Hydrogen as a fuel Supercapacitors

A key aim of electrochemical hydrogen storage is to condense Adsorption of hydrogen has a pseudocapacitive nature and
hydrogen as a fuel because the density of the liquid or com- can be used for building supercapacitors. Many electro-
pressed hydrogen is much lower than practical requirements chemical hydrogen storage systems are indeed excellent
[14,15]. This is similar to physisorption of hydrogen under supercapacitors, but due to a gap between these two areas of
pressure. Although the specific capacity of electrochemical research, these opportunities are simply ignored. For high
hydrogen storage is limited by the stable potential window, capacity materials in which hydrogen is stored within the
electrode/electrolyte interfacial issues, charge transfer, etc.; it solid, the electrochemical response is more like batteries
has unique advantages over physical methods. A key feature with flat plateaus in charge/discharge profiles, because the
of electrochemical hydrogen storage is that it is easy to control system is controlled by a redox system. However, some
the hydrogen release. cases have been reported that the solid-state diffusion fol-
lows a pseudocapacitive behaviour. For instance, hydrogen
Batteries storage in Mg89Y11 exhibits an ideal pseudocapacitive
behaviour with a specific capacity around 1000 mAh g
1
Because of the natural scarcity of Li, new charge carriers such (equivalent to Mg89Y11H1.17) over 0.9 V (Fig. 2). This is indeed
as Na and K are considered as potential alternatives [16]. The a supercapacitor of reaching an exceptionally superior
25146 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5

Materials and mechanisms

Hydrogen adsorption on platinum

The simplest form of hydrogen adsorption in electrochemical

systems (at least because of our profound knowledge) is on
noble metals. This comes from numerous studies of the
electrochemical behaviour of noble metals in aqueous solu-
tion during the 1960s and 1970s. Besides the practical appli-
cations for hydrogen evolution reaction (HER) and oxygen
evolution reaction (OER), which are of considerable impor-
tance in industrial electrochemistry [28e36], an important aim
was to understand the electrochemistry of noble metals as
inert electrodes for the studies of electrochemical reactions
(with growing demand for replacing the dropping mercury,
i.e., the heart of polarography) [37e48].
Adsorption of hydrogen onto a platinum surface is a classic
system, which has been extensively studied, but its exact
mechanism is not still fully understood. In acidic media, the
anion is also adsorbed along with hydrogen [49e52]. Although
this process significantly changes the electrochemical
behaviour of the system depending on the anion involved, the
hydrogen adsorption remains intact. This is somehow
confusing, as it is believed that adsorbed anions affect the
hydrogen adsorption process; but, the type of anion has no
noticeable effect on the hydrogen adsorption. It is difficult to
understand how all anions play the same role. On the other
hand, the role of anion on the hydrogen adsorption cannot be
Fig. 2 e Pseudocapacitive behaviour appeared from neglected. In basic media, the cation is also a counter player
electrochemical hydrogen storage in (a) MgTi film and (b) [53e55]. Again, the intensity of hydrogen adsorption is not
NiFex/Carbon nanocomposites. The curves introduced in affected, but the rates of adsorption and desorption strongly
the inset belongs to the increasing specific capacities, depend on the cation.
respectively. (a) Reproduced with permission from During the underpotential deposition of hydrogen, H
Ref. [88]. Copyright 2013, Elsevier. (b) Reproduced with atoms should be captured from the electrolyte to be adsorbed
permission from Ref. [210]. Copyright 2013, Royal Society on the electrode surface. In the overpotential regions, the H
of Chemistry. atoms have enough energy to join and form H2 and leave the
electrode surface in gaseous form in the course of HER.
Underpotential adsorption of Hþ is almost independent of the
electrolyte (and its pH), and the shapes of voltammograms are
specific capacitance of almost 4000 F g
1. This value is far approximately identical in all cases, but the rate of the reac-
beyond those of the available supercapacitors. Obviously, tion significantly depends on the hydrogen adsorption
the cyclability is not comparable with the common super- mechanism. The reaction rate is approximately one order of
capacitors, but this is just the task of future research. Similar magnitude higher in acidic media as compared with basic
capacity has been obtained for other Mg-based metal hy- media [45]. In the former environment, H atoms are captured
drides such as Mg2NiH4 [26] but over a narrower potential from hydrated Hþ or H3O ions; but in the latter case, H atoms
window. should be captured from H2O molecules where OH
is also
The mechanism of hydrogen storage controls whether produced (Fig. 3, top), which can also be adsorbed. This is of
the electrochemical behaviour resembles a battery or a the vital importance, as almost all electrochemical hydrogen
supercapacitor. There are several hydrogen storage mate- storage systems are based on alkaline solutions.
rials delivering ideal pseudocapacitive behaviour, but they In general, adsorption occurs in two steps: (i) the reacting
have not been seriously considered as promising candi- species are electrocatalysed to form adatoms, and (ii) then the
dates for supercapacitors. It has been recently discussed adatoms reach active adsorption sites through surface diffu-
that the pseudocapacitive behaviour is the basis of elec- sion. The rate-determining step for excellent catalysts such as
trocatalytic reactions in the water splitting system [27]. This Pt is the second step, which is directly controlled by the sur-
suggests that the adsorption of hydrogen on various elec- face structure (Fig. 3, bottom).
troactive materials is directly connected with the pseudo- The key parameter controlling the hydrogen adsorption is
capacitive behaviour utilised for the fabrication of the atomic arrangement on the electrode surface. For
supercapacitors. instance, hydrogen absorptions on Pt(100), Pt(110), and Pt(111)
 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5 25147

Fig. 3 e A schematic of the hydrogen adsorption on Pt. (top) Simple scenarios for capturing H adatoms from H2O in alkaline
and H3O in acidic media. (bottom) Surface diffusion of H adatoms over different crystal facets.

are significantly different (Figs. 3 and 4). This is of particular Theoretically, Pt has an excellent capacity for storing
importance because this proves that the mechanism of hydrogen or even directly electrical energy (as a super-
hydrogen adsorption is not as simple as reaching a surface capacitor). The charge of full hydrogen coverage for clean
atom by hydrogen. Instead, there is a dynamic mechanism for polycrystalline Pt is typically 210 mC cm
2 [56]. Owing to the
a planar phenomenon over the 2D surface. possibility of preparing high surface area Pt samples (in order
Although electrodes with different surface structures have of 800 m2 g
1), hydrogen storage capacity of Pt can be in order
distinct double layers and charge transfer at the electrode/ of 280 mAh g
1 (equivalent to PtH2). Besides the fact that
electrolyte interface [37], the first step, in which charge hydrogen adsorption on porous Pt is too complicated, Pt is too
transfer at the point of interaction of the Pt atoms with the expensive to be used for this purpose.
electroactive species across the double layer results in
hydrogen dissociation, can be considered to be similar. Then, Diffusion of hydrogen into palladium
H adatoms move along the surface from various active sites. It
is controversial if the H atoms constantly move on the surface, Adsorption of hydrogen on Pd is similar to that of Pt but un-
or they are fixed; however, it is evident that dissociated H dertakes an additional step in which hydrogen diffuses deep
atoms should reach the most stable adsorption site. The within the bulk. Thus, the capacity of Pd for storing hydrogen
pathway to reach these adsorption sites depends on the sur- is not limited to its surface only, and Pd can easily reach the
face structure (Fig. 3). composition of PdH0.8 via diffusion of adsorbed hydrogen
In the low index single crystals, the energy required for within the bulk. Solid-state diffusion of hydrogen within the
reaching these adsorption sites (i.e., associated with specific Pd lattice structure is indeed the rate-determining step, and
pathways) defines the shape of cyclic voltammograms (Fig. 4). the capacity strongly depends on the Pd thickness and time.
In 110 crystal structure, moving on a step with a height of half Another interesting feature is that solid-state diffusion occurs
of the Pt atom is represented by a sharp peak, as a specific at a given potential similar to those of intercalation materials
energy is required. In high-index crystal structures, the (e.g., redox systems of battery materials). Contrary to the
shapes of cyclic voltammograms are dictated by the compe- surface adsorption where the adsorption energies were
tition between adsorption/surface diffusion overstepped or different for various sites, the energy required (i.e., applied
terraced atoms. In a sense, high-index crystal structures are potential) is the same for solid-state diffusion throughout the
mixtures of 110 steps and terrace structures of 100 or 111. Pd lattice. As a result, the pseudocapacitive behaviour of
25148 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5

Fig. 4 e Cyclic voltammograms of various Pt single crystals

in a solution of 0.5 M H2SO4 or 0.1 M NaOH. Scan rate Fig. 5 e Cyclic voltammograms of various Pd single crystals
20 mV s¡1. The experimental data can be found in Refs. [46] in a solution of 0.5 M H2SO4 or 0.1 M HClO4 saturated with
and [56]. Ar. Scan rate 20 mV s¡1. The experimental data can be
found in Ref. [163].

hydrogen adsorption turns into a sharp peak in cyclic vol-

tammetry (Fig. 5). inserted Pd is sometimes used as a reference electrode in
A key point, which is usually neglected, is indeed the third electrochemistry. In any case, palladium is also too expensive
step in the process of H adsorption. Thus, the surface for large-scale hydrogen storage. In practice, the maximum
adsorption of Hþ significantly controls the bulk capacity of Pd. hydrogen uptake of palladium is PdH0.8, i.e., equal to an elec-
Fig. 5 shows the cyclic voltammograms of Pd/H systems for trochemical specific capacity of 201 mAh g
1 or 0.75 wt%.
various single crystals. It is evident that the solid state diffu- It has been argued that Pd is not a good catalyst for
sion and storage of H within the Pd bulk is dependent on the hydrogen spillover in hydrogen storage systems [59]. The Pde
surface atomic arrangement because H adatoms are actually H bond is very strong, and hydrogen is not mobile to move
diffusing toward the bulk. The preferred crystal face for solid- along. However, it should be taken into account that mecha-
state diffusion is 100 and 111 in which Hþ can diffuse nism of most hydrogen storage systems are not based on
perpendicularly. In rich-step crystal facets (110), the tendency classical hydrogen spillover, and thus, Pd can significantly
is for catalytic formation of H2, and the Hþ solid-state diffu- improve the hydrogen storage capacity (Fig. 6) [60]. Never-
sion peak disappeares. Again, the competition between step- theless, Honarpazhouh et al. reported that Pd nanoparticles
ped and terraced atoms at high-index crystal structures serve as active sites for hydrogen adsorption in nano-
reveals the phenomenon. composites of porous silicon or porous silicon/graphene oxide
Although hydrogen ions are fairly small to diffuse within [61].
the palladium lattice, they cause a significant change in the
lattice volume [57]. This process is also accompanied by severe Metal hydrides
plastic deformation, which is more damaging on the electrode
surface [58], and then, changes the hydrogen adsorption at the Another possibility for chemically storing hydrogen is metal
primary stage of hydrogen storage within the bulk. hydrides, which normally can store hydrogen up to an atomic
Despite poor cyclability of palladium (more specifically not ratio of 1:1 (hydrogen to metal); though there are cases with
acceptable for the practical applications), it is a well-defined the theoretical possibility to reach the stoichiometric ratio of
system for the electrochemical hydrogen storage. Hydrogen- MH2 and beyond, this usually does not happen in practice.
 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5 25149

Fig. 6 e Discharge curves of (A) MgxCo100ex (x ¼ 40, 45, 50, 55, 60, 63) alloys and (B) MgxCo100ex 5 at.% Pd (x ¼ 40, 45, 50, 55,
60, 63) alloys at the first cycle. Reproduced with permission from Ref. [93]. Copyright 2016, Elsevier.

Several Mg-based alloys can experimentally reach the ratio of fuel. However, metal hydrides have the richest history for
MH2.25. There is an exceptional possibility of increasing the hydrogen storage in the realm of electrochemistry.
ratio by introducing alkali metals in a ternary metal hydride A common problem is poor cyclability of metal hydrides
structure such as K2ReH9, which normally have unusual (some data can be found in Table 1), as the transition from
hydrogen bonds [62], but there is no report for reversible metal to metal hydride is accompanied by structural changes,
hydrogen storage in this class of materials. which are usually irreversible. Therefore, having a rigid and
The popularity of metal hydrides in electrochemical sys- stable lattice structure is necessary to attain an acceptable
tems comes from the industrial transition from Ni/Cd to Ni/ cyclability. It has been claimed that mischmetal superlattices
MH batteries [63e65]. Metal hydrides have several advantages can stabilise the host lattice structure to deliver reversible
over the classic Cd anode. In the Ni/MH batteries, the main cycling over 6000 cycles [68].
cost and capacity limit are related to the MH component. A more general approach for improving hydrogen uptake is
Thus, most of the research efforts in the realm of Ni/MH to use alloys in lieu of a single metal. Using a mixture of metals
batteries aim to find better MH candidates. Another possibility alters the closely packed structure to some extent, as the size
is to replace NiO cathode to form MH/air battery [66,67]. In of metal atoms are different. This increases the lattice vol-
general, the main attention to metal hydrides is to store ume, and thus, larger empty spaces between the metal atoms,
charge in battery systems rather than storing hydrogen as a which are indeed the pathways for hydrogen diffusion. Mixed-
25150 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5

Table 1 e An overview of various materials used for electrochemical hydrogen storage represented by the charge/discharge
experiments. The electrolyte solution was 6 M KOH and reference electrode Hg/HgO unless otherwise noted.
Material Structure E1/V E2/V Reference Rate/mA g
1 Capacity/mAh g
1 Ref.
electrode
Co microparticles
0.9
0.6 Hg/HgO 500 360 [165]
Co
0.95
0.5 50 184 [166]
CoB
0.95
0.6 60 970 [167]
CoNi 1.4 1 Cell 75 325 [168]
CoNi 1.4 1 1875 178 [168]
CoB 100 350 [169]
CoB 1.3 0 770 [60]
Ni 2000 nm
0.95
0.6 30 290 [160]
Ni 100 nm
0.95
0.6 30 340 [160]
Ni 40 nm
0.95
0.6 30 350 [160]
NaNi5
0.9
0.6 60 287 [170]
NaNi5 Coated by CuCl
0.9
0.6 60 308 [170]
TiNi
0.95
0.6 60 210 [171]
ZrCr0.7Ni1.3 60 255 [172]
Ti1.4V0.6Ni 200 [173]
Ti1.4V0.6Ni 60 248 [174]
LaNi5 60 255 [175]
LaNi4Co 320 [175]
LaNi4Co 235 [175]
LaNi4Fe
0.95
0.6 330 310 [176]
LaNi3.2FeMn0.8 250 [176]
LaNi4.4Co0.3Mn0.3 1.4 1 220 [177]
LaNi4.1Co0.3Mn0.3Al0.3 305 [177]
La0.75Mg0.25Ni3.5Co0.2
0.95 0.5 60 404 [178]
LaMg12/Ni
0.95
0.6 1020 [179]
LaMg12eNi
0.95
0.6 900 [179]
LaMg12eNi 1 wt% rutile
0.95
0.6 1085 [179]
LaMg12eNi 1 wt% anatase
0.95
0.6 1090 [179]
LaMg12eNi 5 wt% anatase
0.95
0.6 1230 [179]
ReNi2.6Co0.9 80 210 [180]
LaeNieAl
0.95
0.6 60 255 [106]
ReNi2.15Mn0.45Co0.9
0.95
0.6 60 355 [106]
Zr8Ni21
0.95
0.6 50 83 [164]
Ti2Ni
0.95 0.6 335 [181]
Ti2Ni
0.95 0.6 60 178 [182]
Ti2Ni Large particles
0.95
0.6 60 145 [183]
Ti1.7Mg0.3Ni
0.95
0.6 60 180 [183]
Ti2Ni
0.95
0.6 60 270 [183]
Ti1.7Mg0.3Ni
0.95
0.6 60 250 [183]
Ti55V10Ni35
0.95
0.6 60 216 [184]
TiV30Cr15Mn15 10% MWCNT
1.1
0.6 10 1200 [185]
Ce5Mg41 þ200 wt% Ni
0.95
0.6 1050 [186]
Ml(NiMnAl)4.2Co0.3Fe0.5
0.9
0.6 310 [187]
MgPd
0.95 0 160 [188]
MgTi
0.95 0 360 [188]
Mg89Y11
0.85 0 810 [100]
Mg78Y22
0.85 0 1590 [100]
Mg28Y72
0.85 0 560 [100]
Mg48Ni52/Pd
0.85 0 340 [102]
Mg84Ni16/Pd
0.85 0 210 [102]
Mg94Ni6/Pd
0.85 0 480 [102]
Mg1.5Al0.5Ni
1
0.6 60 245 [189]
Mg2NiH4
0.95
0.6 30 578 [90]
Mg2NiH4 þ Ni nanoparticles
0.95 896 [90]
TiMgNi
0.95 148 [190]
MgNi
0.95 495 [191]
Mg0.8Pd0.2Ni
0.95 190 [191]
Mg50Co50
0.95
0.6 30 375 [94]
MgCoPd5
0.95
0.6 460 [94]
Mg2NiH4 þ Ni nanoparticles
0.95
0.6 30 990 [26]
MgNi
0.95
0.6 495 [96]
Mg0.9Ti0.1Ni
0.95
0.6 500 [96]
 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5 25151

Table 1 e (continued )
Material Structure E1/V E2/V Reference Rate/mA g
1 Capacity/mAh g
1 Ref.
electrode
Mg0.9Zr0.1Ni
0.95
0.6 505 [96]
Mg0.9Al0.1Ni
0.95
0.6 405 [96]
Mg0.9B0.1Ni
0.95
0.6 500 [96]
La0.70Mg0.36Ni0.245Co0.75Al0.30
0.95
0.6 320 [75]
Mg1.5Ti0.3Zr0.1Pd0.1Ni
0.95
0.6 100 485 [192]
Mg1.5Ti0.3Zr0.1Fe0.1Ni
0.95
0.6 405 [192]
Mg1.5Ti0.3Zr0.1C0.1Ni
0.95
0.6 408 [192]
Mg1.5Ti0.3Zr0.1B0.1Ni
0.95
0.6 480 [192]
Mg1.5Ti0.3Zr0.1Ni0.1Ni
0.95
0.6 320 [192]
La0.67Mg0.33Ni2.75Co0.25
0.95
0.6 340 [193]
La0.67Mg0.18Ca0.15Ni2.75Co0.25
0.95
0.6 150 [193]
La0.65Cd0.2Mg0.15Ni3.1Co0.3Al0.1
0.95
0.6 300 356 [194]
(Mg24Ni10Cu2)85Nd15
0.95
0.5 415 [195]
Mg64Pd3Co33
0.95
0.6 625 [196]
Mg78Y22
0.9 0 1590 [196]
Mg80Sc20
0.9 0 1790 [89]
Mg80Ti20
0.9 0 1750 [89]
Mg80V20
0.9 0 1700 [89]
Mg80Cr20
0.9 0 1270 [89]
Mg1.8La0.2Ni/Ni 1.45 1 720 [197]
CNT
0.95
0.4 800 277 [110]
CNF
0.95
0.4 70 [110]
Hollow core/mesoporous shell carbon
1.05
0.55 SCE 25 586 [198]
Graphene
0.9
0.1 100 10 [199]
Graphene D 0.355 nm
0.9
0.1 110 [199]
Graphene D 0.365 nm
0.9
0.1 147 [199]
OMC Mesoporous
1.15
0.55 SCE 25 527 [200]
MCNF Nanofibers 1.1
0.6 SCE 3000 585 [201]
MWCNT Purified
0.9 0 410 [158]
SWCN
0.95
0.4 25 503 [202]
MWCNT
0.95
0.4 102 [203]
MWCNT/TiO2
0.95
0.4 602 [203]
G/TiO2
0.95
0.4 374 [204]
G/TiO2 Microspheres
0.95
0.4 205 [204]
Ni/Graphene
1.2 0 164 [205]
Co@C60 1.4 0 910 [161]
Co/G/Co 836 [206]
CNT
1.3
0.4 SCE 1000 46 [207]
CNT/Ni2B 131 [207]
SWCNT/Ni 0.95 0.4 1404 [208]
Co/G
1
0.4 800 133 [209]
Co/G [209]
Ni/Carbon
1.2 0 350 [210]
NiFe/Carbon 410 [210]
NiFe2/Carbon 420 [210]
Co/G
1.3 0 30 242 [211]
Se
0.9 0 50 270 [212]
BN
0.9
0.4 1000 56 [213]
MoS2
0.95
0.3 50 375 [139]
MoS2 50 260 [140]
Bi2S3 Hollow spheres
0.9
0.1 50 165 [214]
Bi2S3 Solid spheres 148 [214]
Bi2S3 Nanorod
1.1 0 SCE 50 77 [142]
Bi2S3 Nanoflower 123 [142]
Bi2S3 Flower-like 0.9 0.1 50 148 [143]
Bi2S3/Ni 165 [143]
Bi2S3@C/Ni 162 [143]
Bi2S3@C/Ni Nanowire 76 [143]
Bi2S3 Nanocrystals
0.8
0.1 102 [144]
Bi2S3 Nanoflowers 84 [144]
Bi2S3
1.05
0.65 50 143 [145]
Co9S8/RGO 1.5 0 2580 [215]
Co9S8 1.2 0 Cell 30 480 [216]
(continued on next page)
25152 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5

Table 1 e (continued )
Material Structure E1/V E2/V Reference Rate/mA g
1 Capacity/mAh g
1 Ref.
electrode
CoS2 420 [216]
In4SnS8 1.4 1 130 [217]
Sb2S3
1 0 50 105 [218]
Ni3S2 Sponge-like
1.5
0.4 30 385 [219]
Ni3S2 Flakes 308 [219]
Ni3S2 Flake-like porous 236 [219]
Ni3S2 Spheres 96 [219]
Fe3S4 1.4 1 Cell 220 [202]
Bi2Se3 50 185 [148]
Bi2Se3 860 [150]
Bi2Se3-xSx 935 [150]
Sb2Se3 Nanobelts
0.95
0.3 50 230 [149]
Sb2Se3 Urchin-like 205 [149]
Sb2Se3 Nanoplates
1.1
0.6 50 240 [151]
Sb2Se3 Nanostructure
0.9
0.4 50 250 [151]
Sb2Se3 Microspheres 195 [151]
Ni0.67Co0.33Se 0.5 0 1000 310 [152]
Cu2Se
1 0 140 [153]
ZnV2O4 0.5 0 1000 50 [220]
Co3O4 Nanoflake 1.5 1.1 Cell 1000 62 [221]
LaFeO3 230 [222]
LaFeO3 Carbon-coated 355 [222]
LaFeO3 Polyaniline-coated 350 [222]
Cu(OH)2
0.8 0 10 180 [223]

metal (alloy) hydrides are common materials for electro- complicated. It has been reported that hydrogen is adsorbed in
chemical hydrogen storage as the main component of metal the form of the anion (H
) instead of cation (Hþ) in molten salt
hydride batteries, which have been a promising commercial electrolytes [70].
candidate for portable rechargeable batteries for over two Electrochemical hydrogen storage in metal hydrides is
decades. normally conducted through two different steps (or regions):
Comparing with common rechargeable batteries such as surface adsorption and solid-state diffusion in bulk.
lithium-ion, metal hydrides have poor cyclability, as inser- These two mechanisms can be distinguished in the electro-
tion/extraction of hydrogen is accompanied by severe struc- chemical experiments, though they are usually ignored in
tural change rather than simply having a fixed position in a the literature due to the narrow potential window (cutoffs)
rigid lattice structure (i.e., the case for ceramic electrode ma- utilised. If the potential window is wide enough, which is in
terials). This structural change can be easily observed in sig- some systems, two plateaus are distinguishably observable
nificant changes in morphology after a few cycles (Fig. 7). (Fig. 8).
In general, metal hydrides cannot deliver a good cyclability Contrary to common intercalation systems, solid-state
beyond a hundred cycles. Different factors are involved. The diffusion is not simply the rate-determining step here.
most important one is the structural change of the lattice Although solid-state diffusion is slow, the interfacial resis-
upon cycling because hydrogen atoms are a voluminous part tance can hinder the surface adsorption and consequently
of the lattice (at least for high capacity materials). In most slow down the hydrogen storage process. Thus, the specific
cases, these structural changes block the diffusion pathways, capacity may decrease or increase by the particle size [69].
but sometimes, they open the channels for the Hþ diffusion Investigating the influence of temperature can reveal the
too. Some materials need a few cycles to reach the maximum competition between solid-state diffusion and interfacial
capacity of hydrogen storage, because the as-synthesized barriers since the temperature has more effects on solid-state
material does not have well-ordered diffusion pathways, diffusion rather than surface adsorption (Fig. 9). It should al-
and initial diffusion of H atoms paves the way for the subse- ways be taken into account that interfacial resistance be-
quent diffusion (Fig. 7). However, after reaching the maximum tween MH particles plays a crucial role in the specific capacity.
capacity, the capacity starts to fade as a result of further Therefore, the particle size and morphology of each MH
structural changes. should be optimised to achieve the highest hydrogen uptake,
A common drawback of electrochemical hydrogen storage i.e., the best battery performance.
in MHs is the common instability of metals in aqueous solu- Nevertheless, the overall system is much more compli-
tions resulting in the formation of metal oxides/hydroxides, cated. Similar to the Pd/H system described above, the solid-
which can prevent further hydrogen adsorption. This is the state diffusion is initiated by the H adatoms adsorbed on the
reason that smaller particles have lower capacity in spite of electrode surface. Thus, the surface structure controlling the
shorter solid-state diffusion paths because the role of inter- H adsorption is also controlling the solid-state diffusion. This
facial resistance is more significant [69]. A possible solution is can be judged by inspecting the same materials with different
to use nonaqueous media, but they are more expensive and surface structures (Fig. 10).
 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5 25153

Fig. 7 e SEM images of La(Ni0.8Fe0.2)5 alloy (a) before and (b) after cycling. Reproduced with permission from Ref. [162].
Copyright 2007, Elsevier.

The common type of MHs for the commercial Ni/MH bat- in the system as a whole. In general, the hydrogen storage
teries is AB5-type alloys [71]. Although these alloys are made capability can be significantly adjusted by changing the
of mixed metals, more additives are included to improve the chemical composition of these alloys, as this is indeed an
electrochemical performance, particularly cycle life. Co is a active area of research in the literature [75e82].
common additive to improve the cyclability, but is expensive The lattice structure of such alloys is not straightforward
and decreases the specific capacity [72]. Al is another choice, and can be subject to noticeable changes by thermal treat-
but Al interferes with the diffusion of H adtoms towards the ment. This can be a controllable factor to tune the hydrogen
bulk [73,74]. In addition to the structural changes (more storage capacity of these alloys, as it has been reported that a
importantly, diffusion pathways) made by dopants, the roles thermal treatment is required to achieve a good cyclability,
and contributions of possible additives should be investigated though it is vital to find the optimum temperature as higher
25154 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5

Fig. 8 e Charge/discharge curves of (a) a C60/Co nanocomposite (1:1) in 6 M KOH at 30 mA/g, and (b) CoB alloy (1:2). (a)
Reproduced with permission from Ref. [161]. Copyright 2014, American Chemical Society. (b) Reproduced with permission
from Ref. [6] Copyright 2012, Elsevier.

crystallinity is not necessarily in favour of better performance hydrogen through Mg closely packed lattice. An obvious
(in terms of both specific capacity and capacity retention) [83]. approach to achieve a considerable specific capacity of pure
In general, the main problem of alloys for hydrogen storage Mg is to increase the specific surface area.
is significantly low specific capacity. The only possible solu- Adding another metal such as Ti [87e89], Ni [90e92], Co
tion is to shift towards lighter metals. Mg is a promising [93,94], Sc [89,95], V [89], Zr [96], Ce [97], Ca [98], Fe [99], Y [100],
candidate, as it can easily store hydrogen up to the ratio of Al [97,101], Cr [89], etc is a practical approach for improving
MgH2 [84,85]. However, Mg is not stable in the conventional the hydrogen storage in Mg. These additives have two roles: (i)
electrochemical environments; surface passivation increases catalysing the hydrogen dissociation to initiate the adsorp-
the charge transfer resistance resulting in a higher over- tion, and (ii) stabilising the Mg lattice structure with more
potential and lower capacity [86]. It is tough to achieve even a spacious diffusion channels. For the latter, it is necessary to
fraction of the extremely high specific capacity of Mg (i.e., in form intermetallic compounds [102,103]; otherwise, the sys-
order of 2200 mAh g
1), because of slow solid-state diffusion of tem reversibility will be poor for the practical performance
 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5 25155

Fig. 9 e Temperature-dependency of charge/discharge of three Ni samples with different particle sizes (T255: 2000 nm,
N100: 100 nm, and N40: 40 nm). The electrolyte solution is 6 M KOH, and charge/discharge rate 60 mA/g. Reproduced with
permission from Ref. [160]. Copyright 2016, Elsevier.

[84]. In multi-metal Mg-based alloys, mixed metal oxides are

formed instead of Mg(OH)2; for instance, MgTi2O4 in
Mg0.7Ti0.225Al0.075Ni [104] or a protective layer of (NiO)x(P-
dO)y(TiO2)z over Mg0.8Ti0.1Pd0.1Ni [69]. Another efficient
approach is to coat the Mg-based alloy with an H permeable
film, in the best case, Pd [105]. It has been claimed that Mg78Y22
can achieve an exceptionally high specific capacity of
1590 mAh g
1 (equivalent to Mg78Y22H2.28) [100]. LaeNieAl
hydrogen storage alloy has also been investigated for elec-
trochemical hydrogen storage [106]. It was reported that a Ni-
rich and Al-poor layer had formed after the fluorination
treatment, and the electroless plating enabled a homogeneous
precipitation of the nanosized NieP particles on the alloy
surface. Both the fluorination treatment and the electroless
plating contributed to enhanced rate capability of the alloy.
MHs suffer from several serious disadvantages, which
make them less promising for reversible hydrogen storage: (i)
specific capacity is low due to the ratio of heavy metal in the
compound, (ii) they are subject to severe structural change
Fig. 10 e AFM images of three Mg89Y11 films prepared at (a) during cycling, (iii) they are not stable in common aqueous
room temperature, (b) 60, and (c) 100  C; and corresponding media and oxide layers easily prevent future hydrogen stor-
discharge curves in a 6 M KOH solution. Reproduced with age. Nevertheless, MHs are still the main candidates for
permission from Ref. [159]. Copyright 2014, Elsevier. hydrogen storage.
25156 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5

Carbon nanomaterials Hydrogen spillover [125e129], which is a common mecha-

nism in catalytic organic synthesis, is a possible mechanism
In the 1990s, the popularity of carbon nanotubes caused the in hydrogen storage systems. In an electrochemical system,
discovery of new applications for carbon nanomaterials. the mechanism is not exactly the common spillover, but
Nutzenadel and coworkers showed an electrochemical pos- electrochemical hydrogen storage is somehow involved in a
sibility for storing hydrogen [107]. In comparison with MHs, sort of spillover. Note that the source of H adtoms in electro-
carbon has a low atomic mass, and thus, the theoretical spe- chemical systems is not H2. However, similar to the spillover
cific capacity is much higher. However, hydrogen storage on mechanism, H adtoms can spread across the electrode surface
carbon nanomaterials is based on surface adsorption, and or in some case deep within.
hydrogen does not diffuse deep within the solid, except Since the metallic impurities of carbon nanomaterials are
diffusion through graphitic layers along with the 002 d inter- electrochemically active, it is quite difficult to precisely mea-
layer. In any case, the specific capacity of carbon nano- sure the capacity of hydrogen storage [130,131]. Even, chang-
materials for hydrogen storage is much below their theoretical ing the acid treatment for carbon purification can
capacities [107e117], and actually in the level of metal hy- substantially affect the electrochemical hydrogen storage in
drides. Nevertheless, carbon nanomaterials have three carbon nanotubes (Fig. 11). This is indeed the subject of con-
noticeable advantages over metal hydrides: (i) lower cost of troversies and debates among researchers, and it is some-
material, and (ii) faster storing and releasing hydrogen, and times claimed that carbon nanotubes are not practically
(iii) significantly better cyclability. The latter ones are due to capable of storing hydrogen. In summary, normal sp2 carbons
the trivial role of solid-state diffusion (i.e., rate-determining are not capable of capturing hydrogen from the electrolyte
and causing structural changes). solution, and H adatoms are the result of catalytic activities of
Since hydrogen storage is based on adsorption, the specific metallic impurities, edge atoms, or altered sp2 carbon atoms.
capacity is somehow proportional to the specific surface area. Then, overall carbon nanostructure is capable of hosting H
It should be taken into account that the real surface area adtoms. It should be understood that carbon nanotubes are
measured by common methods is significantly different from not standalone materials in the future of electrochemical
the electrochemically accessible area. This is the reason that hydrogen storage.
the pore size of mesoporous carbon materials is much more However, the current attention towards carbon nano-
important than the overall specific surface area for electro- materials is because of a high surface area provided as a
chemical hydrogen storage as well as capacitive behaviour in nanocomposite matrix. Several electroactive materials have
supercapacitors [118,119]. Another important factor, which is been composited with carbon nanomaterials to improve the
usually ignored, is the reactivity of carbon atoms. Adsorption electrochemical hydrogen storage performance [61,132,133].
of hydrogen is mostly conducted through active sites. An ideal
flat sheet of graphene provides an incredibly high surface Metal chalcogenides
area, but sp2 carbon atoms in the hexagonal arrangement are
not reactive enough to facilitate hydrogen dissociation and Metal sulphides from the family of chalcogenides are common
subsequently adsorption; but altered sp2 carbon atoms, materials for electrochemical hydrogen storage. MoS2 has a
different types of defects, and similar reactive sites can. well-defined structure and electrochemical behaviour and has
Charge transfer over these active sites is significantly been widely examined in various electrochemical systems
different, and substantially change the pathway of electro- [134e138]. A noticeable advantage of metal sulphides is while
chemical reactions [120]. An alternative to carbon reactive having a rigid lattice; they can easily form nanostructures
sites is to use catalysts to initiate hydrogen adsorption on
carbon nanomaterials. To this aim, carbon is decorated with
metals such as Ni, Fe, etc.
Mechanism of hydrogen storage in carbon nanomaterial is
still controversial, and most works have focused on the
practical applications rather than fundamental studies.
Although chemisorption of hydrogen seems to be the
preferred mechanism, it is also believed the physisorption
might be the dominant mechanism in some cases. Raman
spectroelectrochemical studies revealed that electrochemical
hydrogen storage in SWCNTs is via physisorption, while
chemisorption is the dominant mechanism for MWCNTs
[121]. This means that chemisorption only occurs through
graphitic interlayers, but this is not an active process, as the
specific capacity of MWCNTs is much lower than that of
SWCNTs [116,117]. In fact, the experimental data are in
contradiction with the common assumption that hydrogen Fig. 11 e Discharge curves of the electrodes prepared from
atoms are stored between the graphene layers [122,123]. MWCNTs purified by (a) HCl, (b) HNO3, (c) H2SO4, and (d) HF
Graphitic samples with more spacious 002 interlayer display acid treatment. Reproduced with permission from
higher capacity for hydrogen storage [124]. Ref. [158]. Copyright 2008, Elsevier.
 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5 25157

with high surface areas. Thus, they can provide high capac-
ities for hydrogen storage with high charge/discharge rates
[139,140]. Similar to the case of metal hydrides, having a
mixture of different metals can alter the lattice structure to
open space for more facile hydrogen insertion. For example, a
mixture of Fe and Mo in the form of FeMo4S6 provides better
performance for hydrogen storage in comparison with the
individual metal sulphides [141]. Bismuth sulphide also has a
considerable electrochemical behaviour for storing hydrogen
[142e145]. Similar to MHs, metal sulphides suffer from a se-
vere structural change in the course of cycling. The
morphology of metal sulphides plays a critical role in the
capability of hydrogen storage [146].
In comparison with metal sulphides, metal selenides have
recently attracted considerable attention due to a higher
electrical conductivity, which is indeed an essential require-
ment in the electrochemical systems [147]. Metal selenides
can show exceptionally high capacities for electrochemical
hydrogen storage [148e153]. However, poor cyclability is still Fig. 12 e Charge/discharge profiles of La0.6Sr0.4FeO3 at
the main drawback. In general, moving towards larger atoms different temperatures. Reproduced with permission from
in the transition metal chalcogenides provides a better Ref. [157]. Copyright 2009, Elsevier.
versatility for adjusting the material properties by altering the
2D structure [147,154].

Metal oxides

Although metal oxides are the most common electroactive

materials for the insertion/extraction of alkali metals in sec-
ondary batteries, they are not ideal choices for hydrogen
storage. Similar to the available cathode materials of lithium
batteries, metal oxides usually show good cyclability for the
insertion/extraction of H atoms. This is due to the rigid metal
oxide structure, which remains intact when a cation is added
or captured. This large lattice structure (in comparison with
the inserting cation) is accompanied by a severe disadvantage
namely low specific capacity. In general, metal oxides have
low specific surface areas, and thus, the hydrogen storage
occurs via solid-state diffusion rather than surface adsorp-
tion. This noticeably reduces the rate capability.
Similar to other cases mentioned above, a mixture is more
suitable for the electrochemical hydrogen storage rather than
a crystalline material. This also provides an opportunity for
tuning the hydrogen storage capacity. Gholami and Salavati-
Niasari investigated the effect of the Cu/Al ratio on the
hydrogen capacity of CuO/Al2O3 nanocomposite, as the spe-
cific capacity could be in the range of 20e240 mA h g
1 [155]. In
addition, the reaction potential was also changed by changing
the chemical composition.
Perovskite-type oxides have also been introduced as
promising candidates for electrochemical hydrogen storage
[156]. Like other types of metal oxides, they suffer from low
specific capacity due to the huge lattices, and poor rate
capability due to the low electrical conductivity and diffusion Fig. 13 e Charge/discharge profiles for (a) Li insertion/
coefficient. However, their rigid lattice structures can guar- extraction into Cu2Se as an anode material of lithium ion
antee good cyclability. Interestingly, it has been reported that battery, and (b) electrochemical hydrogen storage in the
the specific capacity can be substantially increased at elevated same electrode. A current of 50 mA g¡1 applied in
temperatures. For instance, La1exSrxFeO3 with negligible ca- electrolytes of 1 M LiPF6 in a mixture of ethylene carbonate
pacities at room temperature can deliver high specific capac- and diethyl carbonate, and 6 M KOH, respectively.
ities over 500 mAh g
1 (equivalent to La1exSrxFeO3H4.5) at 60  C Reproduced with permission from Ref. [153]. Copyright
(Fig. 12). This indicates that there are other possibilities for 2014, Royal Society of Chemistry.
25158 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 5 1 4 3 e2 5 1 6 5

hydrogen storage too; in this case, no practical attention has promising candidates for electrochemical hydrogen storage
been given to high-temperature electrochemical hydrogen might be subtly designed nanocomposite of Mg-based alloys
storage yet. with mesoporous carbon. Obviously, this is not a simple
nanocomposite made by physical mixing, as the scenario of
Hydrogen vs. alkali metals dissociation of H adatoms, spreading across abundant
adsorption sites and diffusion into high capacity electroactive
From the periodic table, H looks like alkali metals, and thus, material should be planned. Moreover, handling magnesium
electrochemical hydrogen storage in MHs or other materials hydride materials is not easy, and the electrode assembly is an
seems similar to the Li insertion/extraction in electrode ma- active area of research.
terials of lithium ion batteries. Despite many similarities,
these two processes are different. As explained for the case of
Pd, hydrogen storage always starts with surface adsorption, references
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