University of Groningen

Stability of magnesium based nanoparticles for hydrogen storageKrishnan, 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

2

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.

Introduction

3

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].

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].

Chapter 1

4

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.

Introduction

5

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

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

Chapter 1

6

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

Introduction

7

related to the changes of enthalpy ∆H and entropy ∆S by the Van’t Hoff equation:[16-

18]. 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

RS

TRH

pp

H

eq00

2

ln ∆+

⋅∆

−=⎟⎟⎠

⎞⎜⎜⎝

⎛ (1.1)

Chapter 1

8

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

Introduction

9

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 so-

called 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 copper-

zinc couple. At the original interface between the two pure metals, fine marker wires

Chapter 1

10

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,

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

Pure Metal B Wire

Pure Metal A Interface or weld

(A)

x

Original interface

Wire

(B)

Penetration curve (Per cent A)

100 Per Cent A

0 Per Cent A

Introduction

11

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].

Chapter 1

12

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 faster-

moving 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 Fe-

Fe3O4 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.

Introduction

13

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].

c)

Chapter 1

14

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Chapter 1

18