University of Groningen
Stability of magnesium based nanoparticles for hydrogen storage
Krishnan, Gopi
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CHAPTER 1
Introduction
This chapter presents a brief introduction of hydrogen as an energy carrier, various
methods used for storing hydrogen, and associated problems. Furthermore, a detailed
discussion is provided why magnesium and its nanoparticles are interesting for hydrogen
storage. At the end, we introduce the basic concepts of the Kirkendall effect and its role
on the formation of hollow nanoparticles.
1.1. HYDROGEN AS A FUEL
Hydrogen is considered as the ideal candidate as an energy carrier for both mobile and
stationary applications. It offers an attractive alternative to fossil fuels due to its
abundance, easy synthesis, potential for implementation in a carbon- free emission cycle,
and high efficiency. Indeed, chemical energy is based on the energy of unpaired outer
electrons (valence electrons) eager to be stabilized by electrons from other atoms. The
hydrogen atom is most attractive because its electron (for charge neutrality) is
accompanied by only one proton. Hydrogen thus has the best ratio of valence electrons to
protons (and neutrons) of all the periodic table, and the energy gain per electron is very
high. The chemical energy per mass of hydrogen (142 MJ kg-1) is at least three times
larger than that of other chemical fuels (for example, the equivalent value for liquid
hydrocarbons is 47 MJ kg-1).
Therefore the use of hydrogen is advantageous in many aspects: (a) hydrogen has the
highest energy density per unit weight in comparison to any chemical, (b) it is non
polluting because the major by-product of its combustion is water, (c) it is in abundance
since it can be produced from water, and (d) the combination of hydrogen with fuel-cells
leads to efficient energy systems [1–4]. Whether hydrogen can be considered a clean
form of energy on a global scale depends on the primary energy that is used to split
water. Although it is advantageous there are two important reasons why hydrogen is not
Chapter 1
the major fuel of today’s energy consumption: First of all, hydrogen is just an energy
carrier. Although it is the most abundant element in the universe, it has to be produced,
since on earth it is basically only present in the form of water. This implies that there will
be a cost to pay for the production of the energy, which results in a difficult economic
task, because since the industrialization we are still consuming energy for free. The major
impediment to hydrogen use remains the problem with its storage.
In particular, with the sector of transportation, the use of hydrogen as a fuel would
significantly reduce the emission effect of greenhouse gases and thus hydrogen can create
new markets which will support for our economical growth [5–7]. In fact, it has been
recognized in early 1948 that hydrogen could substitute conventional gasoline in
combustion engines [8]. But the crucial aspect for both mobile and stationary applications
is a volumetric and gravimetric density of hydrogen in a storage system. Hydrogen can be
stored by six different methods and phenomena: (1) high pressure gas cylinders (up to
800 bar), (2) liquid hydrogen in cryogenic tanks (at 21 K), (3) adsorbed hydrogen on
materials with a large specific surface area (at T < 100 K), (4) absorbed within interstitial
sites in a host metal (at ambient pressure and temperature), (5) chemically bonded in
covalent and ionic compounds (at ambient pressure) and (6) oxidation of reactive metals
e.g. Li, Na, Mg, Al, Zn with water. Hydrocarbons can be burnt completely by oxidation
of carbon into CO2 and of hydrogen into H2O; some can also be considered as a liquid
storage medium for hydrogen if they can be hydrogenated and dehydrogenated; that is, if
their ratio of hydrogen to carbon atoms can be adapted reversibly. Cyclohexane (C6H12),
for example, reversibly desorbs six hydrogen atoms (7.1 mass%) and forms benzene
(C6H6). Stationary hydrogenation and dehydrogenation under steady-state conditions are
managed in numerous chemical plants, but the on-board process under variable
conditions is another matter [5-8]. Figure 1.1 shows how the size of the hydrogen tank
can be made smaller for a car, depending upon the method of hydrogen storage. It is
evident that storing hydrogen in the form of metal hydride gives us a good possibility to
reduce the size of the tank in the car.
2
Introduction
Mg2NiH4
LaNi5
H2 (liquid)
H2 (200 bar)
Figure 1.1 H-storage in tank systems. Volume of 4 kg of hydrogen compacted in different
ways, with size relative to the size of a car. Figure adopted from ref [17].
1.1.1. Compressed Hydrogen Gas
Classical high-pressure tanks made of fairly cheap steel are tested up to 300 bar and
regularly filled up to 200 bar in most countries. To store our 4 kg hydrogen still requires
an internal volume of 225 litres (about 60 gallons) or 5 tanks of 45 litres each. Novel
high-pressure tanks made of carbon-fibre-reinforced composite materials are being
developed; these are tested up to 600 bar and filled up to 450 bar for regular use. But they
need a special inert inner coating to prevent the high-pressure hydrogen reacting with the
polymer. Consequently, another approach is to use hydrogen-inert aluminium tanks and
to strengthen them with external carbon-fibre coatings. Spherical containers slightly
smaller than 60 cm in diameter would be able to carry our 4 kg, but for practical
fabrication a cylindrical shape is preferred.These high-pressure containers, when full,
would contain about 4% hydrogen by mass, but with significant disadvantages: the fuel
would be available at a pressure dropping from 450 bar to zero overpressure, so
additional pressure control would be essential. High-pressure vessels present a
considerable risk — the compression itself is the most dangerous and complicated part
[9-11].
3
Chapter 1
1.1.2. Liquid Hydrogen
Condensation into liquid or even solid hydrogen is, of course, particularly attractive from
the point of view of increasing the mass per container volume. Liquid hydrogen (LH2)
tanks can, in principle, store more hydrogen in a given volume than compressed gas
tanks, since the volumetric capacity of liquid hydrogen is 0.070 kg/L (compared to 0.039
kg/L at 700 bar) [10]. Key issues with LH2 tanks are hydrogen boil-off, the energy
required for hydrogen liquefaction, as well as tank cost. However, the driving range for
vehicles using liquid hydrogen, excluding the effects of boil-off, can be longer than that
for compressed hydrogen. The density of liquid hydrogen is 70.8 kg m-3 (70.6 kg m-3 for
solid hydrogen). But the condensation temperature of hydrogen at 1 bar is -252 °C and
the vaporization enthalpy at the boiling point amounts to 452 kJ kg-1. As the critical
temperature of hydrogen is -241 °C (above this temperature hydrogen is gaseous), liquid
hydrogen containers are open systems to prevent strong overpressure. Therefore, heat
transfer through the container leads directly to the loss of hydrogen. Larger containers
have a smaller surface to volume ratio than small containers, so the loss of hydrogen is
smaller. The continuously evaporated hydrogen may be catalytically burnt with air in the
overpressure safety system of the container or collected again in a metal hydride. (Solid
hydrogen is a molecular insulating solid; under high pressure it transforms into metallic,
possibly even superconducting hydrogen with Tc of 200–300 oC [9-13]. Table 1.1 shows
the important storage parameter targets that was set to achieve the goals in 2015 for
practical application by DOE (Department of energy USA).
For on-board energy storage, vehicles need a compact, light, safe and affordable
containment. A modern, commercially available car optimized for mobility and not
prestige with a range of 400 km burns about 24 kg of petrol in a combustion engine; to
cover the same range, 8 kg hydrogen are needed for the combustion engine version or 4
kg hydrogen for an electric car with a fuel cell. Hydrogen is a molecular gas. At room
temperature and atmospheric pressure, 4 kg of hydrogen occupies a volume of 45 m3.
This corresponds to a balloon of 5 m diameter — hardly a practical solution for a vehicle.
4
Introduction
Storage Parameter
2015 (Target)
System Gravimetric Capacity
1.8 kWh/Kg
(5.5 Wt %)
System Volumetric Capacity
1.3 kWh/L
(0.040 Kg/L)
Storage System Cost
$2-6 /kWh
Table 1.1 shows the subset of DOE hydrogen storage system targets for 2015.
However, safety concerns and the relatively low volumetric density for compressed
hydrogen gas along with the big cost of liquefaction of liquid storage in storage tanks
demand a better storage system. Solid state hydrides that include metal/intermetallics and
complex (chemical) hydrides provide a good alternative for hydrogen storage due to the
possibility of high volumetric and mass densities and long term stability, and they do not
suffer draw backs as those experienced by compressed and liquid hydrogen [9-11].
1.2. METAL HYDRIDES
Hydrogen adsorbs at solid surfaces depending on the applied pressure and the
temperature. The variation of attractive surface forces as a function of distance from the
surface decides whether van der Waals-type weak physisorption of molecular hydrogen
occurs, or whether dissociation and chemisorption of atomic hydrogen takes place.
Owing to the attractive forces, the most stable position for an adsorbed molecule is with
its centre at about 1 molecular radius from the surface, and the attractive field rapidly
diminishes at greater distances. Once a monolayer of adsorbate molecules or atoms has
formed, the gaseous species interacts with the liquid or solid adsorbate. Therefore, the
binding energy of the second layer of adsorbates is similar to the latent heat of
sublimation or vaporization of the adsorbate. Consequently, adsorption at a temperature
at or above the boiling point of the adsorbate at a given pressure leads to the adsorption
5
Chapter 1
of a single monolayer. For storage purposes, the adsorption of hydrogen has been studied
on carbon species and Metal Organic Framework (MOF’S) [11-15].
Figure 1.2 Schematic illustrations showing the α-phase (solid solution) and β-hydride
phase during the hydrogen absorption of the metal. This figure is adopted from Ref [11].
Many metals and alloys are capable of reversibly absorbing large amounts of hydrogen.
Charging can be done using molecular hydrogen gas or hydrogen atoms from an
electrolyte. Molecular hydrogen is dissociated at the surface before absorption; two H
atoms recombine to H2 in the desorption process. Fig 1.2 shows the interaction of
hydrogen with a host metal and the formation of the α and β phases. The thermodynamic
aspects of hydride formation from gaseous hydrogen are described by pressure–
composition isotherms (Fig. 1.3). The host metal initially dissolves some hydrogen as a
solid solution (α-phase). As the hydrogen pressure together with the concentration of H
in the metal is increased, interactions between hydrogen atoms become locally important,
and we start to see nucleation and growth of the hydride (β) phase. While the two phases
coexist, the isotherms show a plateau of constant pressure, the length of which
determines how much H2 can be stored reversibly with small pressure variations. In the
pure β-phase, the hydroge concentration rises steeply with the H2 pressure. At higher H2
pressures, further plateaus and further hydride phases may be formed. The two-phase
region ends in a critical point TC, above which the transition from α to β phase is
continuous. The plateau or equilibrium pressure depends strongly on temperature and is
6
Introduction
related to the changes of enthalpy ∆H and entropy ∆S by the Van’t Hoff equation:[1618]. As the entropy change corresponds mostly to the change from molecular hydrogen
gas to dissolved hydrogen, it is roughly 130 J K-1 mol-1 for all metal–hydrogen systems
under consideration. The enthalpy term characterizes the stability of the metal–hydrogen
bond. To reach an equilibrium pressure of 1 bar at 300 K, ∆H should amount to 19.6 kJ
molH-1 [16-18]. The operating temperature of a metal hydride system is fixed by the
plateau pressure in thermodynamic equilibrium and by the overall reaction kinetics.
Figure 1.3 (a) Pressure composition isotherm (PCI) plot of hydrogen-metal systems. The solid
solution (α-phase), the hydride phase (β-phase) and the region of the coexistence of the two
phases. Van’t Hoff plot is shown on right hand side. The slope of the line is equal to the enthalpy
of formation divided by the gas constant and the interception is equal to the entropy of formation
divided by the gas constant. This figure is adopted from Ref [14].
Below mentioned is the Van’t Hoff Equation
⎛ p
ln ⎜ eq
⎜ pH
2
⎝
0
0
⎞
⎟ = − ∆H + ∆S
⎟
R ⋅T
R
⎠
(1.1)
7
Chapter 1
Where, R is a gas constant and T is a temperature. Light metals such as Li, Be, Na, Mg, B
and Al, form a large variety of metal-hydrogen compounds. They are interesting
especially due to their light weight and the number of hydrogen atoms per metal atom,
which in many cases is of the order of H/M=2.
Magnesium hydride, MgH2, has the highest energy density (9MJ/Kg Mg) of all
reversible hydrides applicable for hydrogen storage. MgH2 can store up to 7.7 wt % of
hydrogen gravimetrically and ~110 KgH2m-3 in volume with the benefits of low cost and
abundance of Mg in the earth [19,20]. Furthermore, the use of magnesium is relatively
safe in comparison to alkaline metals, which react violently under oxidizing conditions
[21]. The main disadvantages of MgH2 for hydrogen storage is the high temperature of
hydrogen discharge, slow hydriding / dehydriding kinetics, low storage efficiency due to
the high enthalpy of formation, and thermal management during the hydriding reaction
and a high reactivity towards air and oxygen [22,23]. The thermodynamic properties of
the magnesium hydride system have been investigated. The results showed high
operating temperature not suitable for practical on-board applications [24]. The high
thermodynamic stability of MgH2 results in a relatively high desorption enthalpy, which
corresponds to an unfavorable desorption temperature of 300 oC at 1bar H2 [25,26],
remaining an important obstacles for the use of Mg in hydrogen storage. To overcome
these issues, use of catalyst and alloying of magnesium has been carried out showing the
effective result that the kinetics is improved, but the thermodynamics is unaffected.
Another option is nanoengineering, offering new ways of tackling these issues by taking
the advantage of the distinctive chemical and physical properties observed in
nanostructures in order to improve the thermodynamics of Mg-hydride.
1.3. MAGNESIUM NANOSTRUCTURES.
In order to reduce the particle size, various methods starting from bottom up and top
down approaches have been employed. Nanoparticles and nanowires of magnesium with
particles size above 50 nm do not show any significant improvement, which clearly
proves that the thermodynamics improvements should not be expected for larger particle
size [27,28]. Simultaneously the nanoconfinement of Mg within scaffold material, i.e.
porous carbon obtained by melt infiltration and impregnation with appropriate metallic
8
Introduction
precursor, only shows a good enhancement in kinetics, controlling of aggregation and
sintering of the particles, but no change in thermodynamics has been observed [29-31].
The main motivation for the reduction of size started from a theoretical calculation which
predicts that a reduction in size below 2 nm will reduce the enthalpy of MgH2 formation.
[32,33]. which means that the strength of the magnesium and hydrogen bond will be
reduced and comparatively a lower temperature will be required for the desorption of the
hydrogen from Mg. The thermodynamics properties of nanoparticles are known to
depend on their size and shape [34,35]. The classical example is Au nanoparticles
[36,37]. Investigations with Pd nanoparticles show that the hydrogen-storage properties
of metals, i.e. kinetics and thermodynamics, can be controlled with size effects [38,39],
but for Mg (a promising candidate for actual hydrogen storage) detailed studies are still
missing.
Recently the interest in Mg nanoparticle production with sizes below 5 nm has
attracted lot of attention (mentioned in last paragraph). Nevertheless, for a successful
application of Mg a clear understanding of the structure of Mg nanoparticles and a
fundamental knowledge about their behavior in hydrogen atmosphere during annealing is
very important. Also an understanding of Mg structure and their shape is necessary
during hydrogen absorption and desorption which could play a role in improving the
cyclability of Mg. In this respect, we made an effort in identifying the hydriding behavior
of gas phase synthesized Mg nanoparticles, which are completely characterized with high
resolution transmission electron microscopy (HRTEM). The fundamental physical
properties of magnesium nanoparticles should also be addressed because it may be
different from its bulk, and will be very essential for its storage applications. Nonetheless,
during annealing of Mg nanoparticles it became evident that processes related to the socalled Kirkendall effect [40,41] (associated with Mg oxidation and evaporation) were
playing very important roles. For this reason we will explain the basics of this effect in
the following paragraphs.
1.4. KIRKENDALL EFFECT AND KIRKENDALL VOID
The experiment by Smigelskas and Kirkendall [40,41] studied the diffusion of a copperzinc couple. At the original interface between the two pure metals, fine marker wires
9
Chapter 1
were incorporated. After annealing, the concentration profiles were determined across the
interface. The interesting result of their study was that the marker wires moved during the
diffusion process. This is shown schematically in Fig. 1.4,
Pure Metal B
Pure Metal A
Wire
Interface or weld
(A)
x
100 Per Cent A
Original interface
Wire
Penetration curve
(Per cent A)
(B)
0 Per Cent A
Figure 1.4 Marker movement in a Kirkendall diffusion couple; figure adapted from reference
[42].
The upper figure 1.4 represents the situation before the heat treatment, while the lower
figure shows the position after diffusion had occurred. The position of the original
interface, where also the markers are positioned initially, is determined geometrically
where the two metals A and B are joined. It turns out that after diffusion markers can
move compared to this geometrically defined original interface. The distance of the
marker movement was found to vary with the square root of time the specimen was kept
at the diffusion temperature. The moving plane in which the markers are situated is called
the Kirkendall plane. This marker movement could only be explained by a different speed
of diffusion for the different types of atoms. In this way the effect confirms the vacancy
mechanism of diffusion, with different rates of jumps into a vacancy for both types of
atoms [40-44].
Due to the difference in diffusivity of the atoms in a binary solution, one of the
components in the diffusion couple will experience a loss of mass while the other
component will gain mass. As a result of the mass transfer, shrinkage and expansion will
10
Introduction
occur in parts of the system. In this way a state of stress is introduced in the diffusion
zone. The part which suffers a loss of mass is placed under a two dimensional tensile
stress (Zinc), while the side that gains mass will be placed under a compressive stress
(Cu).
Figure 1.5 The Kirkendall effect at the boundary between two solids diffusing into each other at
different rates, for example zinc and copper, their alloy (brass) grows in the direction of the
faster-moving species (zinc). Unfilled voids are left behind and coalesce into large pores: Figure
adopted from ref [45].
These stress fields may bring about plastic flow. Furthermore, if one of the components
in a binary diffusion couple diffuses faster than the second component, a vacancy flux
passes in the direction of the slowest component [42-44]. The vacancies are both created
and annihilated in the metal coupled at sources and sink such as dislocations or internal
interfaces. The combination of vacancy flow and vacancy condensation in combination
with a state of tensile stress makes it possible that voids are formed [43].
11
Chapter 1
The boundary between two metals, zinc and copper for example (Fig. 1.5), is formed by a
growing layer of alloy — brass, in this case it expands in the direction of the fastermoving species, zinc. The Kirkendall effect says that the atoms of the two solids do not
change places directly; rather diffusion occurs where voids open, making room for atoms
to move in. In the wake of the faster-moving material, large pores or cavities form as
unfilled voids coalesce [40-45].
1.5. NANOSCALE KIRKENDALL EFFECT
The Kirkendall effect, a vacancy flux and subsequent void formation resulting from
diffusivity differences at interfaces, was first reported for nanoparticles in 2004, when
solid metal nanoparticles were converted to hollow metal oxide, sulfide, and selenide
nanoparticles. Such conversions through the Kirkendall effect are known as the nanoscale
Kirkendall effect. Typically, when a metal nanoparticle is exposed to oxygen,
phosphorus, sulfur or nitrogen under elevated temperatures, it results in a diffusion
couple. When outward diffusion of the metal cations is much faster than the inward
diffusion of the anions, an inward flux of vacancies accompanies the outward metal
cation flux to balance the diffusivity difference. When the vacancies supersaturate, they
coalesce into a void (or in some cases, several small voids that usually merge into a larger
void) [46-50], the reaction products are hollow nanoparticles with binary compositions.
Hollow metal-oxide NPs have frequently been synthesized by reactions in solution or
oxidation in air at elevated temperatures [46, 47, 50-63]. These hollow nanoparticles are
attractive for their potential use in catalysis [64,65] energy storage [53,66,67], and
biomedical applications [68,69] due to their high surface area to volume ratio and internal
void morphology. Figure 1.6 shows an example of a nanoscale Kirkendall effect based on
oxidation. The hollow Fe3O4 nanoparticles have a thicker oxide shell than the initial FeFe3O4 nanoparticle. This example shows the reason behind the difference in the
nanoparticle size before and after oxidation at higher temperatures. Here in this work, we
will show how the Kirkendall effect can contribute to the void formation due to
oxidation, and evaporation of the Mg core.
12
Introduction
c)
Figure 1.6 Bright Field TEM image of (a) 13 nm Fe-Fe3O4 nanoparticle as prepared. (b) 16 nm
hollow Fe3O4 nanoparticle after oxidation (C) Synthesis of core-shell –void Fe-Fe3O4 and hollow
Fe3O4; Figures adapted from reference [70].
13
Chapter 1
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17
Chapter 1
18