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Atomistic Modelling of Materials for Clean Energy

Applications

hydrogen generation, hydrogen storage, and Li-ion battery

Zhao Qian

Doctoral Thesis in Materials Science

Stockholm, Sweden 2013

KTH Royal Institute of Technology

School of Industrial Engineering and Management

Department of Materials Science and Engineering

SE-100 44 Stockholm, Sweden

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Abstract

In this thesis, a number of clean-energy materials for hydrogen generation, hydrogen

storage, and Li-ion battery energy storage applications have been investigated through

state-of-the-art density functional theory.

As an alternative fuel, hydrogen has been regarded as one of the promising clean

energies with the advantage of abundance (generated through water splitting) and

pollution-free emission if used in fuel cell systems. However, some key problems

such as finding efficient ways to produce and store hydrogen have been hindering the

realization of the hydrogen economy. Here from the scientific perspective, various

materials including the nanostructures and the bulk hydrides have been examined in

terms of their crystal and electronic structures, energetics, and different properties for

hydrogen generation or hydrogen storage applications. In the study of chemisorbed

graphene-based nanostructures, the N, O-N and N-N decorated ones are designed to

work as promising electron mediators in Z-scheme photocatalytic hydrogen

production. Graphene nanofibres (especially the helical type) are found to be good

catalysts for hydrogen desorption from NaAlH4. The milestone nanomaterial, C60, is

found to be able to significantly improve the hydrogen release from the (LiH+NH3)

mixture. In addition, the energetics analysis of hydrazine borane and its derivative

solid have revealed the underlying reasons for their excellent hydrogen storage

properties.

As the other technical trend of replacing fossil fuels in electrical vehicles, the Li-ion

battery technology for energy storage depends greatly on the development of

electrode materials. In this thesis, the pure NiTiH and its various metal-doped

hydrides have been studied as Li-ion battery anode materials. The Li-doped NiTiH is

found to be the best candidate and the Fe, Mn, or Cr-doped material follows.

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Keywords: Renewable energy, Materials science, Hydrogen production, Hydrogen

storage, Li-ion battery, Density functional theory

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To my parents & grandparents

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List of journal papers included in the Thesis:

Ⅰ. Z. Qian, B. Pathak, J. Nisar, and R. Ahuja. Oxygen- and nitrogen-chemisorbed

carbon nanostructures for Z-scheme photocatalysis applications. Journal of

anoparticle Research 2012, 14: 895.

Ⅱ. Z. Qian, M. S. L. Hudson, H. Raghubanshi, R. H. Scheicher, B. Pathak, C. M.

Araújo, A. Blomqvist, B. Johansson, O. N. Srivastava, and R. Ahuja. Excellent

catalytic effects of graphene nanofibers on hydrogen release of sodium alanate. The

Journal of Physical Chemistry C 2012, 116: 10861.

Ⅲ. Z. Qian, S. Li, B. Pathak, C. M. Araújo, R. Ahuja, and P. Jena. C60-mediated

hydrogen desorption in Li-N-H systems. anotechnology 2012, 23: 485406.

Ⅳ. Z. Qian, B. Pathak, and R. Ahuja. Energetic and structural analysis of N2H4BH3

inorganic solid and its modified material for hydrogen storage. International Journal

of Hydrogen Energy 2013, 38: 6718.

Ⅴ. Z. Qian, A. De Sarkar, T. A. Maark, X. Jiang, M. D. Deshpande, M. Bououdina,

and R. Ahuja. Pure and Li-doped NiTiH: potential anode materials for Li-ion

rechargeable batteries. Applied Physics Letters 2013, 103: 033902.

Ⅵ. Z. Qian, X. Jiang, A. De Sarkar, T. A. Maark, M. D. Deshpande, M. Bououdina,

B. Johansson, and R. Ahuja. Screening study of light-metal and

transition-metal-doped NiTiH hydrides as Li-ion battery anode materials. Submitted

2013.

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Contribution statement:

The author has performed the calculations and conducted the analysis of the results &

the writing of the journal papers above.

List of journal papers not in the Thesis:

Ⅶ. Z. Qian, M. S. L. Hudson, H. Raghubanshi, R. H. Scheicher, O. N. Srivastava,

and R. Ahuja. Graphene nanofibers as potential catalyst of hydrogen release in

hydrazine borane. In manuscript.

Ⅷ. M. Bououdina, Y. Oumellal, L. Dupont, L. Aymard, H. Al-Gharni, A. Al-Hajry,

T.A. Maark, A. De Sarkar, R. Ahuja, M.D. Deshpande, Z. Qian, and A.B. Rahane.

Lithium storage in amorphous TiNi hydride: electrode for rechargeable lithium-ion

batteries. Materials Chemistry and Physics 2013, 141: 348.

Ⅸ. Y. Li, B. Pathak, J. Nisar, Z. Qian, and R. Ahuja. Metal-decorated graphene oxide

for ammonia adsorption. Europhysics Letters 2013, 103: 28007.

International conference contributions:

Ⅰ. Z. Qian, R. H. Scheicher, C. M. Araújo, A. Blomqvist, B. Pathak, B. Johansson,

and R. Ahuja. Effects of carbon nanostructures on metal hydrides for renewable

energy applications. International Symposium on Clusters and anostructures

(Energy, Environment, and Health), November 7-10, 2011, Richmond, VA, U.S.A.

Ⅱ. Z. Qian, B. Pathak, R. H. Scheicher, and R. Ahuja. Hydrazine borane and its

derivative material for hydrogen storage: Understanding from ab initio calculations.

International Conference on Materials for Hydrogen Storage, June 14-18, 2012, by

Institutt for energiteknikk, Norway.

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Ⅲ. Z. Qian, R. Ahuja, C. M. Araújo, A. Blomqvist, B. Pathak, and R. H. Scheicher.

Catalytic effect of carbon-nanomaterials on light metal hydride systems. American

Physical Society (APS) March Meeting 2011, March 21-25, 2011, Dallas, TX, U.S.A.

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Contents

1 Introduction.………………………………………………………….…………….11

2 Methodology..………………………………………………………….…………15

2.1 Density functional theory.…..…..…………………………………………….15

2.1.1 The many-body problem…………………………………………………….15

2.1.2 Hohenberg-Kohn theorems………………………………………………….17

2.1.3 Kohn-Sham ansatz ..………………………………………………………....17

2.1.4 The exchange-correlation functionals……………………………………….18

2.1.5 The Bloch electrons………………………………………………………….19

2.1.6 The projector augmented wave method……………………………………20

2.2 Other tools………………………………………………………………………21

2.2.1 Work function………………………………………………………………21

2.2.2 Ab initio molecular dynamics……………………..........................................22

3 Materials for hydrogen energy…………………………..........................................25

3.1 Hydrogen generation…………………………....................................................26

3.1.1 Brief introduction……………………………………………………………26

3.1.2 Graphene-based nanostructures and Z-scheme photocatalytic hydrogen

production…………………………………………………………………...27

3.2 Hydrogen storage……………………………………………………………….32

3.2.1 Hydrogen storage methods and materials…………………………………...32

3.2.2 Complex hydrides…………………………………………………………...34

3.2.3 Graphene nanofibers and NaAlH4…………………………………………...34

3.2.4 C60 and Li-N-H systems……………………………………………………41

3.2.5 Chemical hydrides…………………………………………………………...45

3.2.6 Hydrazine borane solid for hydrogen storage……………………………….46

4 Li-ion battery (energy storage) material……………………………………………53

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4.1 Introduction……………………………………………………………………53

4.2 Metal hydrides as anode materials……………………………………………...54

4.2.1 Pure and Li-doped NiTiH……………………………………………………55

4.2.2 Screening study of light-metals and transition-metals doping………………59

5 Concluding remarks and outlook…………………………………………………65

5.1 Brief summary of the appended papers…………………………………………65

5.2 Conclusion and outlook…………………………………………………………67

6 List of Figures and Tables.…...…………………………………………………….69

7 Acknowledgements………………………………………………………………...73

8 Bibliography………………………………………………………………………75

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1 Introduction

Fossil fuels still dominate the energy consumption in the current world. Although

there have been some arguments on the reserve of worldwide fossil fuel resources, the

detrimental effects (air pollution, CO2 emission, etc.) of fossil fuels on environment

are facts and have become more and more challenging especially in developing

countries. Development of renewable energies or sustainable energies as alternatives

has received increasing focus in order to reduce the usage of fossil fuels. Take the

automobile industries for example, they can reach up to 75% of our current oil

consumption; so the realization of clean-energy vehicles would make a great impact

on the global society. The technical trends can be the long-term fuel cell vehicles or

the short-term electrical vehicles (EVs) powered by Li-ion batteries, which would

lead to the transformation of energy consumption to the hydrogen fuel or the

electricity.

For the successful applications of hydrogen energy, several issues have to be tackled.

One issue is the low-cost and mass generation of the hydrogen gas. Theoretically

there are several traditional strategies such as fossil fuel reforming, bio-inspired

chemistry, etc. to produce hydrogen, but none of these approaches can meet the above

large-scale demand and more importantly, some are just “converted” ways of fossil

fuel consumption. The most promising strategy can be hydrogen production through

solar water splitting, which is totally clean and cost-effective because the ultimate

energy comes from the sun. The other issue or bottleneck is finding the suitable

materials or methods to storage hydrogen at ambient conditions. There are several

requirements for hydrogen storage materials [1,2]. The Department of Energy (DOE,

USA) has set the objective of hydrogen storage system capacity by 2017 to be: 1.8

kWh/kg system (5.5 wt.% hydrogen) and 1.3 kWh/L system (0.040 kg hydrogen/L) at

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a cost of $12/ kWh ($400/kg H2 stored) for automobile applications. In addition, good

hydrogen storage materials should be favorable for hydrogen sorption both in

thermodynamics and kinetics. The operation temperature of hydrogen sorption should

be as low as possible with respect to the ambient conditions and the hydrogen sorption

rates should be as fast as possible. Besides, issues such as reversibility, cost, toxicity

and safety should also be considered. If hydrogen storage, the biggest bottleneck to

realize the hydrogen economy can be solved in the long run, the hydrogen fuel can be

utilized in the fuel-cell systems to achieve the “hydrogen energy cycle”.

Figure 1-1. Cycle of hydrogen fuel as renewable energy. Courtesy of Crabtree et al. [3]

and the the Materials Research Society (MRS).

For the short-term electrical vehicles and many other industrial systems, the research

and development of better Li-ion batteries is crucial for more efficient energy storage,

which represents another technical trend to reduce fossil fuels usage through the

application of electricity. As an electrochemical approach to making energy

conversions between chemical energy and electricity, the Li-ion battery technology

has also been facing many challenges, among which developing better materials for

electrodes is one of the key problems.

Computational materials science has been being developed as an important approach

in the field of modern materials design, especially with the rapid updates of high

performance computers (HPC). As a synergic research method with experiment, the

computational materials science can expose the very basic electronic structures of

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materials and predict various properties of them at much lower costs and can also

shorten the R&D period if applied in real industries.

In this thesis, the background theory and methods used in the studies have been

introduced briefly. Followingly, the chapter 3 will give a broad description of our

studies on materials for hydrogen energy. Both hydrogen generation materials and

hydrogen storage materials will be discussed. The purpose of this chapter is to design,

predict or explain the excellent properties of various functional materials for hydrogen

energy applications through controlling their compositions or the structures. The

involved materials include carbon nanostructures (graphene-based, C60, etc.), complex

hydrides (NaAlH4, Li-N-H systems, etc.) and chemical hydrides (N2H4BH3 based

material, etc.). The chapter 4 will describe our studies on using hydrides (NiTiH based,

etc.) as Li-ion battery anode materials for energy storage. Both pure and various metal

doped materials will be presented. The brief conclusions of the studies are given at the

end.

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2 Methodology

2.1 Density functional theory

Density functional theory (DFT) is a kind of quantum-mechanical method to

investigate the electronic structures of many-body systems. Since 1960s, the theory

has been developed and applied in many research fields such as materials science,

physics, chemistry, energy, etc. Especially in the field of computational materials

science, the DFT method has become a key approach in the design of novel materials

and prediction or understanding of materials properties.

2.1.1 The many-body problem

The solid materials are composed of ions and electrons. In the framework of a solid,

electrons can be regarded as the cohesive “glue” for ions to construct a crystal

structure. In this kind of many-body system, the Schrödinger equation is the

foundation of the electronic structure of a material. The properties of a material can be

estimated by solving the general Schrödinger equation:

H EΨ = Ψ , (2.1)

where the wave function Ψ is the function of all spatial coordinates of all electrons

and ions in the system [4], H is the Hamiltonian that describes all the interactions of

particles and E stands for the total energy of the many-body or many-particle

system. The Hamiltonian operator, H , can be written as follows:

2 22 2 22 2

,2 2 22I J I

i I

i I i j I J i Ie I I J i Ii j

Z Z e Z eeH

m M ≠ ≠

= − ∇ − ∇ + + −− −−

∑ ∑ ∑ ∑ ∑R R r Rr r

� �, (2.2)

where em and ir stand for the mass and the spatial coordinates of an electron;

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IM , IR and IZ mean the mass, the position, and the nuclear charge of an ion. In (2.2),

the first term and the second term respectively represent the kinetic energy of all

electrons and the kinetic energy of all ions. The third one and the fourth one

respectively mean the electron-electron interaction and the ion-ion interaction. The

last term of (2.2) represents the attractive interaction between ions and electrons.

In order to solve the above complex equation, firstly the level of Born-Oppenheimer

approximation has been proposed. The core concept of Born-Oppenheimer

approximation is to “freeze” the ions, that is, the ions are regarded as an external

potential that is applied to the electron cloud. The basic consideration of this

approximation is that the ion is much heavier thus much slower than the electron,

which makes the kenetic energy of the ions neglectable. For the electrons, their

Hamiltonian can thus be given by:

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2 i

i e

Hm

= − ∇∑�

+ 2

2i j i j

e

≠ −∑

r r

2

,

I

i I i I

Z e−

−∑

r R, (2.3)

among which the second term, i.e., the electron-electron interaction is still difficult to

be solved. There have been several approaches developed such as the Hartree-Fock

method, the Density Functional Theory (DFT), etc. to simplify the problem. Here in

this thesis, the DFT method has been employed in investigations.

Figure 2-1. The density functional theory uses the concept of electron density.

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Courtesy of Lusk et al. [4] and the Materials Research Society (MRS).

2.1.2 Hohenberg-Kohn theorems

The density functional theory is based on the two Hohenberg-Kohn theorems [5]. The

basic idea of theorem 1 is: for any system of interacting electrons in an external

potential ( )extV r , this potential ( )extV r is determined uniquely by the ground state

electron density ( )ρ r ; all ground state properties of the system are also determined

by the ground state electron density. The brief content of theorem 2 is: a universal

functional ( )F ρ , which consists of the kinetic and interaction energy of all electrons

in the system, can be defined and is valid for external potential and any number of

electrons. Thus the total energy functional ( )E ρ can be written as follows:

( ) ( ) ( ) ( )extE F V dρ ρ ρ= + ∫ r r r . (2.4)

The exact ground state electron density would be the density that can minimize the

above functional. But the question still exists, that is, how to give the exact form of

the universal functional ( )F ρ .

2.1.3 Kohn-Sham ansatz

The Kohn-Sham ansatz [6] has helped to solve the problem. They have replaced the

complex many-body problem with the more simple N (number of electrons in the

system) one-particle non-interacting problems. These non-interacting single particles

are subject to an effective potential. According to the Kohn-Sham ansatz, the ground

state total energy functional of the whole system can be described in the following

way:

1 ( ) ( )[ ( )] [ ( )] ( ) ( ) ' [ ( )]

2 'ext XC IIE T V d d d E E

ρ ρρ ρ ρ ρ= + + + +

−∫ ∫ ∫'

r rr r r r r r r r

r r, (2.5)

in which the first term is the kinetic energy of the non-interacting electrons; the

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second term means the external potential (electrostatic potential caused by the

interaction between the nuclei and the electrons) that the electronic system is subject

to; the third term (i.e., the Hartree energy) is the classical electron-electron Coulomb

interaction; the fourth one is the most special term, which is the exchange-correlation

functional that considers all non-classical many-body effects between electrons. The

last term just stands for the nuclei-nuclei interaction. As discussed before, the solution

of the non-interacting one-particle system can be regarded as the ground state

minimization of the Kohn-Sham energy functional. The following Kohn-Sham

equation:

22( ) ( ) ( )

2 eff i i i

e

Vm

ψ ε ψ− ∇ + =r r�

(2.6)

is finally led to after using variational principles. In the above equation, ( )iψ r stands

for the Kohn-Sham orbitals (but it is just a mathematical expression instead of the real

wave function of electrons); effV is called the effective potential,

[ ( )]( )

( )XC

eff ext

EV V d

δ ρρ

δρ= + +

−∫'

' rrr

r r' r. (2.7)

It means that the effective potential includes the external potential, the Coulomb

interaction and the exchange-correlation contribution. If we can obtain this effective

potential, the equation (2.6) will be solved. effV depends on the electron density,

while the electron density of the system can be expressed as:

2( ) ( )i

i

ρ ψ=∑r r . (2.8)

Thus, it becomes a iterative problem which can be solved through a self-consistent

way on condition that the exchange-correlation functional can be got. Following the

approximations for this will be briefly discussed.

2.1.4 The exchange-correlation functionals

Generally two kinds of approximations are most widely used, that is, the LDA (local

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density approximation) and the GGA (generalized gradient approximation). LDA is

established based on the homogenous electron gas and can be defined as:

hom( ) ( ) [ ( )]LDA

XC XCE dρ ρ ε ρ= ∫ r r r , (2.9)

in which the [ ( )]XCε ρ r is the exchange-correlation energy density that only depends

on the electron density. The LDA generally works well for some near free-electron

metals with a slowly varying density. For the some systems with quickly varying

electron densities, although it is not expected that the LDA will be good, but some

facts and experiences have shown that it also works surprisingly well and has also

been employed extensively. Nevertheless, the LDA has some drawbacks such as the

overestimation of cohesive energy of system, bond dissociation energy and adsorption

energy, etc. as compared with the experimental results. The GGA is one attempt to

improve these. By considering the gradient of electron density, the GGA can be

defined as:

( ) ( ) [ ( ), ( )]GGA GGA

XC XCE dρ ρ ε ρ ρ= ∇∫ r r r r . (2.10)

In this definition, the information of gradient of electron density is included in the

exchange-correlation energy density. Through this, it can give a better description of

binding energy, adsorption energy, bond length, etc. compared with the LDA results.

The commonly used GGAs are GGA-PW 91[7] by Perdew and Wang, GGA-PBE [8]

by Perdew, Burke and Ernzerhof, etc. Another kind of exchange-correlation functional

is hybrid functional. The hybrid functional is a proportional combination of the

Hartree-Fock exact exchange energy and the DFT exchange-correlation

approximation. Examples are such as B3LYP [9], PBE0 [10], HSE06 [11], etc.

2.1.5 The Bloch electrons

From (2.6), we have converted the complex many-body problem to the single-particle

problem. In a solid state system, the electrons are subject to a periodic external

potential. According to the Bloch theorem [12], the Kohn-Sham orbitals of the

equation (2.6) can be expressed as follows:

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( ) ( )ie uψ ⋅= k r

k kr r , (2.11)

in which the parameter k is a vector in the first Brillouin-zone; the function ( )uk

r

has the same periodicity with the crystal lattice; ie ⋅k r stands for the plane wave. The

function ( )uk

r can be written in a form of plane wave basis set:

( )uk

r = ic e ⋅∑ r G

k

G

, (2.12)

where the new vector G is defined, which represents the reciprocal lattice vector. If

the equation (2.12) is put into the (2.11), finally the following expression can be got:

( )ψ =k

r ( )ic e ⋅ +∑ r k G

k,G

G

. (2.13)

The basis set is k -dependent. In order to solve the equation (2.6) using plane wave

basis set, generally a large number of plane waves are needed to describe the wave

functions well, which makes the computations demanding. In real calculations,

different electronic cut-off energies are usually used, whose values depend on specific

systems and the accuracy needed.

2.1.6 The projector augmented wave method

In this thesis, the computational method of the projector augmented wave (PAW) [13]

that is implemented in the Vienna ab initio simulation package [14,15] is mainly

employed. As a good method for electronic structure calculations, the PAW method

has combined the features of the ultrasoft pseudopotential (USPP) method [16] and

the linear augmented-plane-wave (LAPW) method [17]. The introduction of the

pseudopotential is to replace the rapidly oscillating wave function in the core region.

As is known, the wave functions of the electrons are very much different, which is

dependent on the positions of electrons (i.e., whether they are close to the nuclei or far

away in the bonding region). In the core region, it is computationally hard to describe

the wave function using plane waves. Thus, the PAW method uses a partial wave

expansion to describe the electrons in the augmented region. While, the electrons are

described by plane waves or other convenient basis set outside the augmented region,

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because the wave functions variation in this region is smooth.

Using a transformation operator, τ , the all-electron wave function ψ is transferred

to a smooth pseudo wave function ψ� in the PAW method:

ψ τ ψ= � . (2.14)

For electrons in the augmented regions, the pseudo wave function ψ� can be

expanded in the form of pseudo partial waves,

i i

i

cψ φ=∑ �� . (2.15)

Also, the all-electron wave function ψ can be written as:

i i

i

cψ φ=∑ . (2.16)

What should be pointed out is that the all-electron partial waves can be got by solving

the radial Schrödinger equation for the isolated atom. The pseudo partial waves iφ�

and the all-electron partial waves iφ are equivalent outside the augmented region.

The pseudo partial waves state iφ� should be dual to a projector function ip� , thus

the following condition should be satisfied:

,i j i jp φ δ⟩ =�� . (2.17)

In the above equation, i and j belong to the same augmentation sphere. Finally,

the transformation operator τ can be given as:

1̂ ( )i i i

i

pτ φ φ= + −∑ � � . (2.18)

2.2 Other tools

2.2.1 Work function

The work function means the minimum energy that is needed to remove an electron

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from a solid surface to the infinite vacuum outside the solid. The calculation method

used in this thesis is based on the density functional theory and is shown as follows:

φ = V(∞)﹣EF. (2.19)

In this equation, φ is work function; V(∞) and EF respectively stand for the

electrostatic potential in a vacuum region far from the neutral solid surface and the

Fermi energy of the neutral surface system [18]. In paperⅠ,the work functions of

different chemisorbed graphene-nanostructures have been calculated. The vacuum

space of 15 Å has been used in the calculations of electrostatic potentials. For

photocatalytic hydrogen generation applications, the estimation of work functions of

materials are of significance for the alignment of electronic band edges. The band

levels of photocatalytic materials can be benchmarked with respect to the redox levels

to choose the most suitable candidates for water splitting. In this thesis, the standard

reduction level (H+/H2, -4.44 eV) and oxidation level (O2/H2O, -5.67 eV) have been

considered with respect to the vacuum level.

2.2.2 Ab initio molecular dynamics

Molecular dynamics (MD) simulations make it possible to consider the effects of

temperature and other dynamical parameters. The motions of atoms in materials obey

the second law of Newton which is well known. Different from the classical

molecular dynamics in which the forces on the atoms are obtained from the generated

atomic potentials, the ab initio molecular dynamics (AIMD) uses the forces that are

directly got from density functional theory calculations. Compared with the classical

molecular dynamics, the strength of ab initio MD technique is the relatively high

accuracy and the main weakness is its computational cost. Thus, the application of ab

initio MD is currently limited to a few hundreds of atoms. While for not very big and

complex material systems, this technique is feasible to get a good balance of accuracy

and computational time.

In this thesis, the method used to implement the ab initio MD is Born-Oppenheimer

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molecular dynamics. In Born-Oppenheimer molecular dynamics, the calculation of

the forces acting on the atoms (from the solution of Kohn-Sham equations) and the

motion of the nuclei are treated separately, which is different from the Car-Parrinello

MD. In Born-Oppenheimer MD, the time step is determined by the dynamics of

nuclei instead of those of electrons. At every step, the forces have to be calculated by

solving the quantum electronic equations (while the atoms are fixed at this time); thus

the Born-Oppenheimer molecular dynamics is more time-consuming. What should

also be pointed out is that typically in the beginning of MD simulations, there will be

oscillations of terms such as total energy, the distribution of atomic velocities and the

non-physical forces. After the MD simulations get equilibrated, these oscillations will

subside and the simulations will come to the really physical period.

The ab initio MD is a powerful tool to study some properties of materials. For

instance, in this thesis, the estimation of mean square displacement (MSD) has helped

to investigate the diffusion properties of Li-ion battery materials. The mean square

displacement of the individual atom in the material system can be expressed in the

following way:

MSD = 2[ ( ) (0)]

i i

i

t −∑ r r . (2.20)

In the above expression, stands for the number of investigated atoms in the

simulation box.

The diffusion coefficient of atoms can be calculated using the MSD data. Based on

the Einstein equation, in the linear region of the MSD curve the diffusion coefficient

D can be got through the fitting of the following equation to the data:

2[ ( ) (0)]

i i

i

t −∑ r r = 6 Dt + C. (2.21)

In this equation, the constant D is the calculated Einstein diffusion coefficient,

through which the diffusion properties of some key atoms in the solid system can be

explored.

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In addition to the methods described above, some other tools such as the nudged

elastic band method (NEB) [19-22], the Bader charge analysis [23], etc. have also

been employed to investigate the properties of different materials for clean energy

applications.

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3 Materials for hydrogen energy

Finite supply of fossil fuel and its related issue of environmental polution have made

alternative clean fuels studied and developed extensively all over the world. H2 is one

typical case among them. With the advantages of high enegy density, abundant

resources (H2O), possible clean generation (via solar power, etc.) and zero emission

(if used in fuel cell), H2 is becoming a promising and sustainable energy carrier.

Figure 3-1 shows the concept of “hydrogen economy” which consists of chains of

hydrogen production, hydrogen storage and usage. It links the renewable energy

resources and electrical energy form with the making and utilization of the hydrogen

fuel through storage medium.

Figure 3-1. The concept of “hydrogen economy”. Courtesy of Crabtree et al. [24] and

the American Institute of Physics.

In this chapter, the hydrogen generation from solar water splitting and various

light-weight materials for hydrogen storage will be involved. Using state-of-the-art

density functional theory and closely connecting with experimental research, some

promising hydrogen energy materials including both nanostructured and bulk

materials have been investigated to design and/or elaborate the excellent properties of

them in hydrogen generation or hydrogen storage applications.

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3.1 Hydrogen generation

3.1.1 Brief introduction

As an energy carrier instead of a source of energy, H2 has to be produced from H-host

matters occurring mostly as H2O and some as liquid or gaseous hydrocarbons [25].

Production from fossil fuels such as steam reforming of methane doesn't reduce the

consumption of traditional fuels and the emission of greenhouse gas CO2. One of the

clean methods is to produce hydrogen by splitting water through photocatalysis.

Hydrogen generation through photocatalytic water splitting means the conversion of

solar energy to chemical hydrogen energy, which is attractive due to the abundance of

solar energy and water resource [26-30]. Many materials including both common

metal oxides and non-oxides have been studied as direct photocatalysts to improve the

photocatalytic process of water splitting [31-35]. During the general photocatalytic

process, the necessity of separation of evolved H2 from O2 is disadvantageous. This

problem could be overcome by using a Z-scheme photocatalysis system. As a kind of

two-photon system, the Z-scheme photocatalytic system generally consists of a

H2-evolving photocatalyst, an O2-evolving photocatalyst and an electron mediator and

occurs in natural photosynthesis of green plants [36,37], which is different from the

general single-photon photocatalytic system. The two-step photoexcitation system can

mimic the natural photosynthesis and has greater potential to work under sunlight to

split water. In Z-scheme photocatalytic system, the electron transfer or shuttle

between the H2-evolving photocatalyst and the O2-evolving photocatalyst is the

rate-determining process [38]. In 2011, Iwase et al. demonstrated that the reduced

graphene oxide could be used as an electron mediator for water splitting in Z-scheme

photocatalysis system, which paved a novel way of utilizing carbon materials for

Z-scheme photocatalysis applications. Figure 3-2 shows that a reduced graphene

27

oxide can be used to conduct electrons efficiently from an O2-evolving photocatalyst

to a H2-evolving photocatalyst. Different from previous studies on graphene-based

materials with proper band-gaps and band positions that can be directly used as

semiconductor photocatalysts [39,40], in Iwase et al.’s work the reduced graphene

oxide performs as electron mediator in Z-scheme photocatalysis system.

Figure 3-2. Reduced graphene oxide is used to be electron mediator for water splitting.

Courtesy of Iwase et al. [41] and the American Chemical Society.

In this section, using density functional theory we have studied different

graphene-based nanostructures chemisorbed by various types and amounts of species

such as O, N, OH, their electronic structures and work functions. It is supposed to

help to design better carbon-based nanomaterials for more efficient Z-scheme

photocatalytic hydrogen generation.

3.1.2 Graphene-based nanostructures and Z-scheme photocatalytic hydrogen

production

Firstly the favorable bonding sites of single oxygen or nitrogen atom on graphene

sheet are studied. Figure 3-3 shows the schematic configurations of various possible

bonding sites of the single oxygen (nitrogen also) atom before geometry optimization.

Based on this, the absorption energy (Eabs) has been calculated to determine the most

possible bonding site according to the following formula:

( )pure nX X pureabs

E nE EE

n

+ − −= (3.1)

where Epure+nX, Epure and EX mean the ground state total energies of the chemisorbed

28

nanostructures, the pure graphene sheet with 32 atoms in unit cell, and the oxygen or

nitrogen atoms, respectively. The term n is the number of oxygen or nitrogen atoms.

Figure 3-3. Considered bonding sites of single oxygen (nitrogen also) atom before

geometry optimization (all are unit cells for calculations). Carbon atoms are in gold

and the red color means oxygen or nitrogen atom in this figure.

After comparison of absorption energies of diferent sites, for both oxygen case and

nitrogen case, the configuration (a) is found to be the most favorable one. The value

of the corresponding absorption energy is -2.4 eV for O-chemisorption and -0.7 eV

for N-chemisorption, respectively. Based on this configuration, the density of states

has been investigated. Figure 3-4 shows the calculated DOS for pure, O, N

chemisorbed nanostructures respectively. After chemisorption of single oxygen atom,

the Fermi level is shifted from -2.25 eV of the pure case to -2.43 eV. Nitrogen has

made the Fermi level shifted to -2.96 eV because of the introduction of N 2p states.

The band gap is zero. Many states around the Fermi level will benefit the potential use

for electron mediator applications in Z-scheme photocatalysis.

29

Figure 3-4. Density of states of pure, O-, N-chemisorbed nanostructures.

The doubled concentrations of oxygen and nitrogen chemisorption have also been

considered. The structural model of graphite oxide [42] has been referred. Figure 3-5

(left) shows the relaxed structures. The bond lengths of C-O, C-N are 1.45-1.47 Å.

Figure 3-5 (right) exposes their density of states. Compared with the O-chemisorbed

case in Figure 3-4, the O-O chemisorption has further decreased the Fermi level to

-2.54 eV; the N-N chemisorption has lowered the Fermi energy level to -3.12 eV. For

the combinational chemisorption of O and N, the Fermi level is -3.1 eV. And it has

high density of states near to the Fermi energy. The hydroxyl chemisorption on the

graphene sheet has also been studied, because many reported structures of GO contain

hydroxyl and some natural photocatalysts such as chlorophyll also contain hydroxyls

[43-46]. Figure 3-6 (left) shows the configurations of OH- and OH-OH-chemisorbed

graphene nanostructures after geometry optimization. The corresponding density of

states of them is also plotted. The chemisoption of one hydroxyl leads to the shift of

Fermi energy to -2.48 eV and a small band gap of about 0.3 eV is opened. But this

small gap disappears when the doubled hydroxyl chemisoption is done. There exists a

large number of states around the Fermi level for the OH-chemisorbed case.

30

Figure 3-5. Left: relaxed configurations of (a) O-O, (b) O-N, (c) N-N chemisorbed

graphene nanostructures (all are unit cells). Gold color: carbon; red: O; purple: N; the

“filled” white color shows the “lifted” bonded carbon atoms because of chemisorption.

Right: density of states.

Figure 3-6. Left: relaxed configurations of (a) OH-, (b) OH-OH-chemisorbed

structures. Right: the calculated density of states.

Figure 3-7 shows the work functions evaluation for various chemisorbed structures

with respect to the reduction level (H+/H2) and oxidation level (O2/H2O) of

photocatalytic water splitting. The standard reduction level and oxidation level of

photocatalytic water splitting have been considered to be -4.44 eV and -5.67 eV (with

respect to vacuum level) respectively [47]. The results have indicated that the N, O-N

31

and N-N chemisorbed nanostructures with deeper potential levels (-4.86 eV, -5.1 eV

and -5.12 eV respectively) have great prospect to be used to shuttle photogenerated

electrons from O2-evolving photocatalysts to H2-evolving photocatalysts in Z-scheme

photocatalytic water splitting, which will help to improve the efficiency of the

Z-scheme photocatalysis system. For the O- or OH-chemisorbed cases, they can be

utilized as electron conductors between the conduction band of H2-evolving

photocatalyst and the reduction level (H+/H2) and can help hydrogen generation due to

their similar potential levels to the water splitting reduction level. If synthesized with

low losts in the future, these carbon nanostructures even have prospect to replace

some expensive noble metals in Z-scheme photocatalytic hydrogen production.

-6

-5.5

-5

-4.5

-4

-3.5

reduction level

oxidation level

V (

eV)

pure

O

O-O

O-NN

N-N

OH

OH-OH

Figure 3-7. Work functions for various chemisorbed structures.

32

3.2 Hydrogen storage

Among the techniques required to realize the hydrogen economy, hydrogen storage is

one major challenging barrier to introduce hydrogen in the transportation area [48-51].

It is essential to find some methods or materials with efficient and sustainable

hydrogen storage. One of the key parameter is the hydrogen storage capacity. DOE

(Department of Energy, USA) has set the goal of storage density of 1.8 kWh/kg

system (5.5 wt.% hydrogen) and 1.3 kWh/L system (0.04 kg hydrogen/L) at a cost of

$12/ kWh ($400/kg H2 stored) for automotive applications by 2017 [52]. In addition,

other important properties such as low desorption temperature, long cycle life and fast

refuelling rate should also be addressed.

3.2.1 Hydrogen storage methods and materials

Generally there are three approaches to store hydrogen, i.e., compressed hydrogen gas,

liquefied hydrogen, and hydrogen storage in solid state materials. For the gaseous

storage, the biggest problem is the low volumetric storage density, which means that

the storage tank must be very large and solid to avoid the leakage of hydrogen. In

addition, the safety issue is another key problem due to the high pressure (several

hundred bar) used to store hydrogen in the tank. For the liquid storage, the key

problem is too low temperature needed for liquefied hydrogen (the boiling point of

hydrogen is only about 20 K at 1 bar), which requires the use of cryogenic storage

tanks and a large amount of energy to cool down H2 to reach the liquid phase and thus

makes this storage method too expensive to be economically used in potential market.

Compared with the gaseous and liquid hydrogen storage, storing hydrogen in solid

state materials is a more promising path. Figure 3-8 illustrates the volumes of 4 kg of

hydrogen stored in various ways with respect to the size of a car. Obviously, hydrogen

storage in solid state materials has the best volumetric density.

33

Figure 3-8. Comparison of volumetric density of different ways to store hydrogen (4

kg of hydrogen). Courtesy of L. Schlapbach et al. [25] and the Nature Publishing

Group.

Solid state hydrogen storage can also be divided into two types according to different

mechanisms to store hydrogen in materials: physisorption and chemisorption. The

physisorption mechanism is based on Van der Waals interaction or weak dispersive

interaction between hydrogen molecules and host materials. This interaction is often

very weak with the strength of less than 10 kJ/mol, which gives physisorption the

advantage of fast kinetics of hydrogen adsorption/release but causes the requirement

of cryogenic temperature or high pressure to hold hydrogen molecules within the host

materials. The typical hydrogen storage materials having physisorption mechanism

are high surface-area materials such as carbon based materials, metal organic

frameworks (MOFs), covalent organic frameworks (COFs), zeolite, clathrate hydrates,

etc. The biggest obstacle to use them in real hydrogen storage is to obtain higher

hydrogen uptake at ambient temperatures and pressures instead of in too extreme

conditions. Chemisorption usually involves the chemical reaction between hydrogen

and condensed matters (most are metals) which results in the formation of a hydride

phase. Hereby, hydrogen is not held by weak dispersive interaction but by strong

chemical bonding with the atoms in host materials. The bonding energy is generally

more than 50 kJ/mol, which makes high temperature usually required to desorb or

34

release hydrogen from the hydrides. Hydrides can be classified into several catagories

according to the nature of chemical bonding between hydrogen and the matter host

[53]. Ionic hydrides are those formed by alkali or alkaline earth metals and hydrogen,

such as LiH, MgH2, etc. Covalent hydrides are those formed by non-metallic

elements (B, C, N, Si, etc.) and hydrogen. Metallic hydrides possess metallic bonding

between a transition metal (or a rare earth metal) and hydrogen, which have the

examples of LaNi5H6, FeTiH2, and so on. Another important category is complex

hydrides. This will be discussed in detail followingly.

3.2.2 Complex hydrides

In complex hydrides, group Ⅰ or group Ⅱ metals form stable salts with complexes

such as [AlH4]-,[BH4]

-,[NH2]- via ionic bonds; while within the complexes, the

chemical bonding is usually covalent [54-57]. Typical complex hydrides include

alanates, borohydrides, and the amide-hydride systems. They ususally have high

hydrogen gravimetric densities, which make them favorable for potential on-board

vehicles applications. However, the application of complex hydrides is hindered by

the relatively slow kinetics of dehydrogenation and/or rehydrogenation. Catalysts can

be used to enhance their kinetics under moderate conditions in solid states.

3.2.3 Graphene nanofibers and :aAlH4

NaAlH4 has been regarded to be one of the most promising complex hydrides for

hydrogen storage. But proper catalysts are needed to improve the hydrogen sorption

kinetics and lower the desorption temperature. The pioneering work of catalyzing

NaAlH4 with titanium done by Bogdanovic and Schwickardi [58] has promoted many

studies on transition metal catalysts to enhance the hydrogen sorption properties of

light metal hydrides [59-65]. While, some metal catalysts have the disadvantages of

detrimental environmental effects, high costs, etc. Carbon-based nanomaterials have

been becoming increasingly recognized for their applications in catalyzing hydrogen

35

storage [66]. In this part, the catalytic effects of graphene nanofibers which are

composed of layered graphene stacks on NaAlH4 have been investigated

Figure 3-9 shows the morphologies of the HGNF (helical graphene nanofibre), the

PGNF (planar graphene nanofibre), the HGNF admixed NaAlH4 and the SAED

(selected area electron diffraction) pattern (using 10-micron selected area aperture) of

2 wt.% HGNF admixed NaAlH4. The SAED patterns of HGNF and PGNF

(respectively shown in the insets of (a) & (b)) have suggested their different

configurations.

Figure 3-9. TEM images of (a) HGNF, (b) PGNF, (c) HGNF admixed NaAlH4, and (d)

SAED pattern of 2 wt.% HGNF admixed NaAlH4 used in the study.

The kinetic properties of hydrogen desorption have been investigated. Firstly Figure

3-10 (left) illustrates the TPD curves of NaAlH4 admixed with 2 wt.% various

carbonaceous nanomaterials. The heating rate is 2ºC/min. It can be seen that when

admixed with 2 wt.% GNF, the desorption behavior of NaAlH4 gets improved

significantly. Especially for the HGNF admixed case, the temperature corresponding

to the desorption of 3.5 wt.% H2 has been lowered from 246°C (pristine NaAlH4) to

182°C. Figure 3-10 (right) shows the desorption kinetic curves of 2 wt.% carbon

36

nanovariants admixed NaAlH4. It is found that the dehydrogenation from 2 wt.% GNF

admixed NaAlH4 is faster than that of 2 wt.% SWCNT admixed NaAlH4. Especially

for the HGNF admixed one, the catalytic effect is excellent.

Figure 3-10. Left: temperature programmed desorptions of NaAlH4 admixed with 2

wt.% carbon nanovariants. Right: desorption kinetic curves.

The basic constituents of GNFs are graphene layers, repeatedly stacked on top of one

another, with the characteristic variation in their long-range stacking arrangement

resulting in either HGNFs or PGNFs. Because the experimental FTIR spectra of

GNFs does not reveal any signature of C-H, C-O or C=O, here we have studied the

interaction of NaAlH4 with various sites on the edges of one pure single-layer

graphene patch from first-principles theory to understand the mechanism behind the

catalytic effects of GNFs. The molecular approach is employed as we successfully

used before to mimic the hydrogen desorption properties of hydrides [56,67]. Figure

3-11 has shown the equilibrium configurations of NaAlH4 clusters facing various sites

of zigzag or armchair edges of the graphene patch and that of the pristine NaAlH4

cluster, which are visualized through VESTA (Visualization for Electronic and

STructural Analysis) [68]. A, B and C correspond to the cases in which NaAlH4 has

been placed near various sites at the zigzag edge. E and F are for the armchair edge

sites. D is the pristine NaAlH4 cluster.

37

Figure 3-11. Ground state equilibrium configurations of NaAlH4 with Na facing

various edge sites and of isolated NaAlH4 cluster. Na in yellow, Al in blue, H in pink,

and C in green. The spheres in “filled” light green color represent the carbon atoms

that are fixed in our calculations to simulate those atoms bonding to other carbon

atoms deeper within the single graphene layer.

Based on the configurations shown above, the hydrogen removal energies of NaAlH4

in all cases have been calculated. Both the hydrogen removal from the longer Al-H

bond and the removal from the shorter Al-H bond of NaAlH4 cluster have been

considered. From Figure 3-12, it can be seen that the hydrogen removal energies of

NaAlH4 placed near a carbon edge (independent of whether it is a zigzag or an

armchair edge) are greatly decreased compared with that of the pristine NaAlH4. The

hydrogen removal energy can be lowered to 1.56 eV at site A, which is much lower

than the 3.8 eV of pristine NaAlH4 (decreased by 58%). B and C also show evident

effects on the hydrogen removal energy with the decrease of 54% and 43%

respectively. The armchair edges (E and F) show similarly good performance. Thus,

38

both the zigzag edge and the armchair edge are able to weaken the Al-H bonds greatly

and decrease the hydrogen removal energy of sodium alanate a lot. This is very useful

for improving the hydrogen desorption kinetics of this material for hydrogen storage.

Another point is that the decreased hydrogen removal energies are lower than those of

NaAlH4 catalyzed by C60 or CNTs (with the values of about 3 eV [56]), which can

help to understand why graphene nanofibres (GNFs) show better catalytic

performance for the hydrogen desorption of NaAlH4 than SWCNTs.

Figure 3-12. Hydrogen removal energies of NaAlH4 placed at various graphene patch

edge sites.

Bader charge analysis [69-71] has been carried out to investigate the charge states

before and after the hydrogen removal from NaAlH4 respectively, based on the above

various configurations. Figure 3-13 (upper) shows the charge transfer from Na to

various graphene patch edge sites before the hydrogen removal from sodium alanate.

At the zigzag edge sites A and B, the highest charge transfer can be found, which is

followed in magnitude by the armchair edge sites E and F. The charge transfer is

lowest at the zigzag edge site C. This trend of charge transfer is consistent with that of

39

the hydrogen removal energy shown in Figure 3-12. It is because here, the graphene

patch edges have “grabbed” some charge of Na and have affected the normal charge

donation of Na to the [AlH4]- complex, which results in the destabilization of covalent

bonds between Al and H atoms. The zigzag edge can attract more charge from

NaAlH4 than the armchair edge, thus the zigzag edge can destabilize sodium alanate

more than the armchair edge. Figure 3-13 (lower) shows the charge states of the AlH3

moiety after one hydrogen atom is removed from the reactant NaAlH4. At both the

zigzag edge sites and the armchair edge sites, after hydrogen removal from NaAlH4,

the single valence electron of Na is transferred to the carbon edges instead of to the

AlH3 moiety, which makes the AlH3 moiety nearly neutral (the most stable charge

state for this cluster). Thus the hydrogen removal or dissociation product has

evidently been stabilized in this study. The graphene patch edges can make the

functions of both destabilizing the reactant NaAlH4 and stabilizing the removal

product, which can account for the large decrease of hydrogen removal energies in

Figure 3-12. More importantly, the graphene nanofibers consist of large amounts of

graphene patch layers (which would supply a large quantity of exposed graphene

edges) and thus could work as excellent nanocatalysts to improve hydrogen sorption

of NaAlH4. HGNF possesses a larger surface area than PGNF and could therefore

work better when used as a catalyst.

40

Figure 3-13. Upper: Amount of charge transfer from Na atom to various carbon edge

sites before hydrogen removal. Lower: the charge states of the AlH3 moiety after one

hydrogen atom has been removed from NaAlH4.

41

3.2.4 C60 and Li-:-H systems

Amide-hydride system, which contains metal and nitrogen-based complex hydrides, is

one of the promising hydrogen storage candidates. The system of (LiNH2 + LiH)

[72,73] is one typical case, which has the hydrogen storage capacity of 10.5 wt.%.

Their desorption reaction mechanism is commonly regarded as stepwise. The first

step of them is proposed to actually occur also stepwisely [74-76]:

2LiNH2 → Li2NH + NH3 (3.2)

NH3 + LiH → LiNH2 + H2. (3.3)

Firstly LiNH2 decomposes to ammonia and lithium imide (endothermically) and then

ammonia reacts with LiH to produce H2 molecules (exothermically). While, NH3 is

poisonous for the polyelectrolyte membrane of the PEM fuel cell; thus we have to

promote the reaction (3.3) to prevent the release of NH3 [77,78]. For the reaction (3.3),

it will take place also stepwisely through the formation of a stable [79] intermediate

phase LiNH4:

NH3 + LiH → LiNH4 (3.4)

LiNH4 → LiNH2 + H2. (3.5)

The reaction (3.5) shows a hydrogen storage capacity of about 8 wt%, while its

kinetics is not favorable. In this section, the study of using fullerene to improve the

hydrogen release of this Li-N-H system has been performed motivated by our former

effort to use carbon nanomaterials to catalyze the hydrogen storage reactions of

complex hydride.

Firstly the comparison of total energies of various possible LiH binding sites has

determined that the LiH molecule is most stable on top of the pentagon sites of C60,

which is shown in Figure 3-14(d). The binding energy of this configuration has been

calculated to be -0.63 eV. This means that LiH interacts with C60 more strongly than

purely dispersive interaction, which can also be further verified by change of the Li-H

bond length from 1.60 Å to 1.68 Å. The effect of C60 on decomposition of pure LiH

42

has been evaluated first. From Table 3-1, it can be seen that the desorption of LiH is

endothermic with the reaction energy of 0.20 eV/f.u. (DMol3); while the reaction

becomes exothermic (-1.39 eV/f.u.) when LiH is supported on a C60 molecule. Ab

initio molecular dynamics (MD) simulations at 300K have been performed and it has

been found that the LiH molecule moves away from C60 instead of decomposing to Li

and H2. All of these suggest the existence of an energy barrier for the decomposition

of LiH to Li and H2.

Figure 3-14. Equilibrium configurations of (a) LiH, (b) NH3, (c) LiH+NH3, (d)

LiH-C60, (e) NH3-C60, and (f) (LiH+NH3)-C60 after geometry optimization at 0 K.

43

Table 3-1. The calculated reaction energies with and without C60 as catalyst using

DMol3. (The values in parentheses have been calculated at the B3LYP level of theory

using Gaussian 09.)

Reactions without C60 ∆E (eV/f.u.) Reactions with C60 ∆E (eV/f.u.)

LiH → Li+1/2H2 0.20 (0.14) LiH-C60 → Li-C60 +1/2 H2 -1.39 (-1.01)

LiH+NH3 → LiNH4 -0.91 (-0.92) LiH-C60+NH3 → LiNH4-C60 -2.15 (-1.99)

LiNH4→ LiNH2+H2 0.39 (0.44) LiNH4-C60 → LiNH2-C60+H2 0.79 (1.42)

LiNH4 → LiNH3+1/2H2 1.02 (0.35) LiNH4-C60→LiNH3-C60+1/2

H2 -0.37 (-0.16)

The interaction between NH3 and C60 has also been investigated. The binding energy

is calculated to be -0.06 eV. This means that their interaction is through van der Waals

force (dispersive interaction). MD simulation results show that the NH3 molecule

moves away from C60 in the early stage, which is very consistent with the 0 K

optimization. Thus the N-H bonds in NH3 can not be weakened by C60.

Followingly, the effect of C60 on the mixture of (LiH + NH3) has been investigated.

First the reaction energies of (3.4) and (3.5) are calculated to be -0.91 eV/f.u. and 0.39

eV/f.u. respectively. This confirms the formation of the stable intermediate state

LiNH4. Then besides reaction (3.5), the following desorption reaction has also been

considered:

LiNH4→ LiNH3 + 1/2H2. (3.6)

The reaction energy of (3.6) is calculated to be 1.02 eV/f.u. (DMol3). The hydrogen

desorption of pure LiNH4 seems to more favorably follow the path of (3.5). The

nudged elastic band (NEB) calculations have been performed to find that the

desorption barrier for the reaction (3.5) is about 0.98 eV (Figure 3-15).

44

Figure 3-15. The calculated reaction energy barrier of LiNH4 → LiNH2 + H2 through

NEB method.

When the fullerene C60 is introduced into the following reaction:

LiH-C60 + NH3 → LiNH4-C60, (3.7)

the reaction energy is calculated to be much more exothermic (-2.15 eV/f.u.) than the

formation of pure LiNH4. Thus we can say that the fullerene stabilizes the formation

of the intermediate state. Based on this, the two desorption paths have also been

investigated with the support of C60:

LiNH4-C60 → LiNH2-C60 + H2 (3.8)

LiNH4-C60 → LiNH3-C60 + 1/2H2. (3.9)

The reaction energies are calculated through DMol3 to be 0.79 eV/f.u. and -0.37

eV/f.u., respectively. So the fullerene will make the reaction (3.9) more

thermodynamically favorable. Two LiNH4 molecules will be needed to desorb one H2

molecule in reaction (3.9), thus the hydrogen capacity that can be released reduces to

about 4 wt.%. The MD simulations indicate that the configuration in Figure 3-14(f) is

actually metastable when considering the temperature (dynamics) effect. The

LiNH4-C60 would decompose to LiNH3-C60 and 1/2 H2 molecule at 300 K. From

45

Figure 3-16, because only one LiNH4 molecule is used in our MD simulations, only

one extra H atom attached to the fullerene is found in the study. Considering the

overall reactions of (3.2) and (3.3), C60 will also help to stimulate the consumption of

poisonous NH3, which is good for PEM fuel cell applications.

Figure 3-16. Ab initio molecular dynamics simulations of the LiNH4-C60 system at

300 K. (a) initial structure, and (b) structure after 1000 fs.

3.2.5 Chemical hydrides

In these years chemical hydrides, represented by ammonia borane, have absorbed

increasing interest. Several B-N-based chemical hydrides with both protic and

hydridic hydrogen atoms and their metal-modified derivatives have been studied

intensively [80-83]. Typical materials are NH3BH3, NH4BH4, Ca(NH2BH3)2,

LiNH2BH3·NH3BH3, Ca(NH2BH3)2·2NH3, etc. Although the thermodynamics of this

type of material is not very favorable, they possess definitely high hydrogen

capacities. Especially for those metal-free N-B based chemcial hydrides, the hydrogen

46

capacity can be easily more than 10 wt.%. Taking NH3BH3 for example, its hydrogen

storage capacity can be as high as 19.6 wt.%. But for ammonia borane (AB), the

hydrogen release kinetics is not good enough and the volatile dehydrogenation

byproduct, borazine [84,85], can poison the polymer electrolyte membrane (PEM)

fuel cells. Thus, developing better light-weight chemical hydrides (N-B-based) has

been one of the important branches of hydrogen storage materials field. Recently, the

N2H4BH3 (hydrazine borane, HB) solid material with 15.4 wt% hydrogen storage

capacity, which can desorb 5.8 wt% H2 at 140℃ in 12 min and can release 11 wt% H2

at 150℃ in an hour after mixing with LiH [86], has been investigated experimentally

as a novel N-B-based material for hydrogen storage through different methods such as

thermal decomposition, hydrolysis, etc. [87-90]. For the thermal dehydrogenation of

HB, although there still occur some problems about the regeneration of spent material,

etc., the hydrogen release rate is encouraging compared with many traditional

hydrogen storage materials. While, in order to meet the more demanding requirements

of practical applications, better hydrogen desorption kinetics is expected. To help

understanding of HB-relevant solid materials and help experimentalists and engineers

to develop better candidate materials for hydrogen storage, hereby in this section, we

have investigated the hydrogen removal energetics and electronic structure of HB

relevant materials, which would be very meaningful to improve the potential

hydrogen release properties of HB.

3.2.6 Hydrazine borane solid for hydrogen storage

Figure 3-17 shows the crystal structure of hydrazine borane solid after geometry

optimization. It can be seen that the unit cell of HB solid contains 8 formula units

(N2H4BH3). Within every formula unit, we define the two nitrogen atoms as N1 (the

middle nitrogen atom) and N2 (the end or the terminal nitrogen atom) respectively.

The orthorhombic structure belongs to the space group Pbcn at ambient conditions

and the calculated lattice parameters are 9.66 Å, 5.26 Å and 12.34 Å.

47

Figure 3-17. Equilibrium crystal structure (unit cell) of HB solid determined from ab

initio calculations. B: dark green small spheres; N: light blue; H: pink.

The total and partial density of states (DOS) for HB solid is shown in Figure 3-18.

There exist three evident regions: lower energy region of valence band (below -2 eV),

top of the valence band (from -2 eV to 0 eV) and conduction band (above 5.6 eV).

The calculated band gap is 5.6 eV (might be underestimated due to the well-known

problem of GGA), which means HB is a wide band-gap insulator. The strong sp

hybridization (energy overlapping) between B and H (B) and between N and H (N) in

valence band leads to the strong bonding between B and H (B) atoms and between N

and H (N) atoms. The electron localization function (ELF) [91,92] analysis (Figure

3-19) also confirms that NH2 and BH3 are covalently bonded, seen from the connected

ELF isosurface. The purpose of knowing these bonding is to try to weaken them for

hydrogen storage applications.

48

Figure 3-18. Total and partial DOS for the hydrazine borane solid. The Fermi energy

is set at zero; s-orbital and p-orbital contributions are distinguished with black color

and red color, respectively.

49

Figure 3-19. Calculated electron localization function for HB solid plotted as

green-colored transparent isosurfaces at a level of 0.5.

Followingly, the hydrogen removal energies have been investigated. The 1×2×1

supercell has been used and one hydrogen atom bonded either with nitrogen (N1 or

N2) or with boron has been taken away in all calculations. The ground state total

energy of single hydrogen atom (not hydrogen molecule) was used. The calculated

average hydrogen removal energy from N1 site is 4.16 eV; the value is 5.14 eV for N2

site and 4.83 eV for B site. Thus N1 site is the weakest one. To modify the pristine

HB solid and weaken the H-host bonds, ab initio calculations have been performed to

design other structural candidates for hydrogen storage materials. Based on former

experimental studies, here the modification with light-weight element Li has been

studied. After full optimizations of all possible geometries, one of the Li-modified HB

structure found to be most stable without much structural distortion is shown in

Figure 3-20. Figure 3-21 shows the comparison of calculated hydrogen removal

energies of HB and Li-modified HB. All three hydrogen removal sites have been

considered. Evidently, the hydrogen removal energies of Li-modified HB are

evidently lower than those of HB regardless of the removal sites. Especially for the

N1-H bond, the hydrogen removal energy of Li-modified HB (2.10 eV) has been

decreased by as large as 50% compared with that of HB (4.16 eV). The hydrogen

removal energy is also lowered by 50% for the N2-H bond (from pristine 5.14 eV to

2.61 eV). Therefore, Li-modified HB is proposed to have favorable hydrogen release

properties than pristine HB from the energetic perspective.

50

Figure 3-20. Unit cell of one structure of Li-modified HB material obtained from ab

initio calculations.

Figure 3-21. Calculated hydrogen removal energies of the pristine HB solid (blue) and

the Li-modified HB (red). A, B and C are different hydrogen removal sites. A: N1 site;

B: N2 site; C: boron site.

Further investigation of bond-weakening mechanism has been explored. Table 3-2

shows the bond length changes. The B-H bond length has elongated to 1.25 Å in

Li-modified HB. But there are no evident changes in terms of N-H bond lengths.

51

Followingly the Bader atomic charge analysis has been done to compare the two

materials. Some changes have been found. Table 3-3 lists the differences in average

charge states of H atoms that are bonded with N before any hydrogen removal. It is

found that the charge states of H (N) atoms are lowered in Li-modified HB (i.e., H (N)

atoms in Li-modified HB are more electronegative.), which would destabilize the N-H

bonds compared with the pristine HB. Table 3-4 shows the changes of atomic charge

states (in units of e) of the residual N atom (N1 or N2) after one H atom is removed

from N1-H bond or from N2-H bond. The results have confirmed the stabilization of

residual N-H bond after hydrogen removal. Thus it is suggested the combination of

destabilization of N-H bonds before hydrogen removal and stabilization of residual

N-H bond after hydrogen removal accounts for the N-H bonds weakening in

Li-modified HB, which would be good for the improvement of hydrogen release

kinetics. Figure 3-22 shows the local structures that can reflect the dihydrogen

bondings in HB and Li-modified HB materials respectively. They will benefit the

hydrogen release, i.e., every two dihydrogen-bonded atoms would detach from the

system and combine to one H2 molecule during dehydrogenation of the materials.

Table 3-2. Calculated average bond-lengths of H-host bonds in pure HB and

Li-modified one.

52

Table 3-3. Charge states (in units of e) of H atoms bonded with N in HB and

Li-modified HB before hydrogen removal.

Table 3-4. Charge states (in units of e) of residual N atom (N1 or N2) after one H

atom is removed from HB or from Li-modified HB.

Figure 3-22. Dihydrogen bondings (Å) in HB solid and Li-modified HB solid (local

structures).

53

4 Li-ion battery (energy storage) material

A Li-on battery is a kind of rechargeable (secondary) battery for energy storage. Its

working principle is based on the phenomena/process that the Li-ions move from the

anode to the cathode during discharging and back during charging. The basic

configuration of a Li-ion battery consists of the negative electrode (anode), the

positive electrode (cathode), the electrolyte, and the separator. The cathode materials

are generally some Li-containing metal oxides. While the anode materials can be

classified into the type of insertion electrodes (titanate, carbon, etc.), the type of

conversion electrodes (nickel oxides, iron oxides, etc.) and the alloying-type

electrodes (Si, Ge, etc.). As a good ion-conductor and electron-insulator, the

electrolyte functions as a good conducting medium for Li-ions.

Since commercialized in early 1990s, the Li-ion battery has been widely used in the

portable electronics industry with the advantages of high energy density, high voltage,

long lifetime, and environmentally-friendly capability compared with conventional

batteries. For more demanding applications, the energy density still has to be

increased, which depends on the electrochemical capacity and the output voltage.

These parameters can be enhanced by improving the electrochemical properties of the

electrode materials [93].

4.1 Introduction

In this chapter the anode materials are mainly discussed. For the commercialized

Li-ion batteries, carbon materials are commonly used. The average potential

difference of carbon electrode such as graphite is 0.1-0.2 V. There are several types of

carbon anode materials, among which graphite and the hard carbon are two typical

54

ones. The main advantages of graphite as anode materials are their stable specific

capacity and good cycling properties. While, the hard carbon has a high capacity and

good power density; but its electrical conductivity is poor [94]. Although showing the

above strengths, there are still some weaknesses for carbon materials concerning the

electrolyte solvent co-insertion problems. Also, their storage capacity is not high

enough. Other materials such as Si, Ge, Sn, etc. have also been investigated as anodes

in the form of elements or alloys in these years. For example, the Si anode can have

the high specific capacity of 4212 mAh/g and Ge shows the value of more than 1600

mAh/g. But for this kind of materials, the main drawback is the severe problem of

large volume expansion during discharging and charging, which limits the application

of these materials in practical Li-ion batteries.

4.2 Metal hydrides as anode materials

With the sustainable trend of transformation of traditional fuels economy to wider

utilization of electricity, the electrical storage is becoming more and more important

[95-99]. This issue has to be addressed especially as the usage of electricity has been

moving from consumer electronics to large-scale electric vehicles and industrial

systems [100-102]. As a promising solution to tackle the above energy storage

problems, the Li-ion battery technology still faces challenges in its intrinsic limitation

concerning the kinetics of the insertion or de-insertion of Li-ions during charging and

discharging [103-106]. Different from many earlier studies, Y. Oumellal et al. have

proposed a new concept to use metal hydrides as conversion anode materials for

Li-ion rechargeable batteries applications [107,108]. In their pioneering work, the

metal hydrides such as MgH2 and TiH2 have been experimentally examined and have

shown good properties as anode materials. Here in the following sections, materials

based on another hydride NiTiH have been investigated by us to explore their

properties and novel applications of them in Li-ion batteries while making use of the

relative maturity and availability of this hydride in industry.

55

4.2.1 Pure and Li-doped :iTiH

The equilibrium “quasi-layered” crystal structure of pure NiTiH hydride obtained

from density functional theory is shown in Figure 4-1, from which it can be seen that

the unit cell contains 48 atoms including 16 H atoms, 16 Ni atoms and 16 Ti atoms.

16 Ti atoms are classified into five different sites, i.e., 2a, 2b, 4c, 4d, and 4e. All the

Ni atoms are equivalent. During charging of a Li-ion battery, the Li-ions would

migrate from electrolyte into the NiTiH anode and diffuse inside. Before the study of

diffusion, it is important to know the preferential site for the insertion of the migrated

Li ion into the NiTiH lattice. Here in our study, one additional Li atom has been

introduced into the pure NiTiH lattice and been put at different possible sites. These

different possible sites have been systematically weighed from the thermodynamic

perspective. The results have been shown in Table 4-1. It is found that the migration

of a Li-ion to the 2b (Ti) site induced by pushing aside the corresponding Ti atom

from this site is most energetically favorable (although it might be kinetically limited

by an activation energy barrier).

56

Figure 4-1. The crystal structure of NiTiH hydride (48 atoms per unit cell, space

group I4/mmm) determined from first-principle calculations. Ti: green color, with

different labels showing different sites; Ni: blue; H: yellow.

57

Table 4-1. The energetic comparison when a migrated Li-ion stays at different sites in

pure NiTiH lattice. E: total energy of each relaxed configuration that contains 49

atoms (Ni16Ti16H16 and one extra migrated Li atom) in a unit cell.

Considered migration sites E (eV)

16m site -284.22

2a site -284.55

2b site -284.73

4c site -284.20

4d site -284.14

4e site -284.28

Interstitial site -284.64

Based on the selected 49-atom system, the mobility of a Li-ion through the pure

NiTiH lattice has been studied via ab initio molecular dynamics. It can be shown in

Figure 4-2. At room temperature, there is very small diffusion. At 600 K, the

corresponding diffusion constant of the Li-ion in pure NiTiH has been calculated

based on the Einstein equation. The calculated diffusion constant of the migrated

Li-ion in pure NiTiH is about 2.3×10-10 m2s-1 at 600 K.

58

Figure 4-2. The mean square displacement of the Li-ion as a function of time at both

300 K and 600 K.

In order to improve the electrochemical capacity of the anode, the Li-doped

NiTiH has also been investigated beyond the pure hydride. One of the Ti atoms or Ni

atoms in the 48-atom unit cell has been substituted by a Li atom (2.08 at.% doping

concentration). All the possible substitutional doping sites have been considered to

calculate the respective formation energies. Table 4-2 shows the results. The 2b site

has been found to be most favorable for Li substitutional doping. Based on this new

doped material, the average voltages have been firstly calculated. The following two

reactions have been used:

Ni16Ti16H16 + 16Li+ + 16e- ＝ Ni16Ti16 + 16LiH (4.1)

Ni16Ti15LiH16 + 16Li+ + 16e- ＝ Ni16Ti15Li + 16LiH. (4.2)

The value of average voltage is estimated by dividing the reaction energies by the

Faraday constant. Table 4-3 has shown the calculated results, in which there is a slight

increase in voltage from 0.40 V to 0.42 V when the electrode changes from pure

NiTiH to Li-doped NiTiH hydride. About the unit-cell volume, no much expansion is

found. The electrochemical capacity has also been calculated based on Faraday’s law.

As one of the key properties for energy storage, the calculated electrochemical

capacity of Li-doped NiTiH is 255.25 mAh/g, which is higher than that of pure NiTiH

anode.

59

Table 4-2. Calculated formation energies for different substitutionally Li-doped NiTiH

hydrides as anode materials.

Sites for substitutional doping Eform (eV/f.u.)

Ti site (2a) 0.067

Ti site (2b) 0.028

Ti site (4c) 0.044

Ti site (4d) 0.042

Ti site (4e) 0.048

Ni site 0.074

Table 4-3. Properties of pure NiTiH and Li-doped NiTiH for Li-ion battery anodes

applications: unit-cell volume, average voltage and electrochemical capacity.

Anode material Volume (Å3) Voltage (Volts) Electrochemical capacity

(mAh/g)

Pure NiTiH 462.45 0.40 249.18

Li-doped NiTiH 464.12 0.42 255.25

4.2.2 Screening study of light-metals and transition-metals doping

In this section, the effects of various light-metals (Al, Mg) and transition-metals (V,

Cr, Mn, Fe, Co, Cu, Zn) dopants on the electrochemical properties of pristine NiTiH

hydride have been screened for Li-ion battery anode materials applications. The basic

structure of pristine NiTiH is just as the one in Figure 4-1. Based on the structure, one

Ni or Ti atom in a unit cell has been substituted by one light-metal or transition-metal

atom. Five different Ti sites have been considered. Table 4-4 lists the total energies

60

(eV per unit cell) of various light-metal or transition-metal substitutionally doped

NiTiH hydrides. Through comparison, the 4d Ti site has been chosen among 5

different Ti sites for the calculation of formation energy in terms of Mg dopant. For Al,

V, Cr, Mn, Fe, Co, Cu and Zn, the most stable Ti substitutional doping site is 2a, 2a,

4d, 2a, 2a, 4c, 2a and 2a site respectively.

Table 4-4. Calculated total energies (eV per unit cell) of various light-metals or

transition-metals substitutionally doped NiTiH materials.

Then the formation energies have been calculated to choose the preferential doping

site (either Ni site or the most stable Ti site) for every metal dopant respectively. From

Table 4-5, it is finally determined that the 4d Ti site (Mg-doped NiTiH), 2a Ti site

(Al-doped NiTiH), 2a Ti site (V-doped NiTiH), Ni site (Cr-doped NiTiH), Ni site

(Mn-doped NiTiH), Ni site (Fe-doped NiTiH), Ni site (Co-doped NiTiH), Ni site

(Cu-doped NiTiH) and the 2a Ti site (Zn-doped NiTiH) are the most

thermodynamically stable sites respectively.

61

Table 4-5. Formation energies (eV/f.u.) for various substitutionally metal-doped

NiTiH materials.

Based on the above results, the systematic studies of structural volume, average

voltage and specific electrochemical capacity have been done to find the most

promising metal dopants to enhance the properties of pristine NiTiH for anode

materials. Figure 4-3 shows the comparison of unit-cell volumes of pure and various

light-metal or transition-metal-doped NiTiH materials. The purpose of examining this

is to find the candidates that can meet the requirement of structural stability for Li-ion

battery anodes applications. Mg-doped, Cr-doped, Mn-doped, Fe-doped and

Co-doped NiTiH are found to have few volume changes compared with the pure

NiTiH hydride. Figure 4-4 illustrates the changes in terms of average voltage, which

is one of the key properties of Li-ion battery anodes. In the calculations, the following

reactions are employed:

Ni16Ti16H16 + 16Li+ + 16e- ＝ Ni16Ti16 + 16LiH (4.3)

Ni16-xTi16-yMH16 + 16Li+ + 16e- ＝ Ni16-xTi16-yM + 16LiH. (4.4)

In (4.4), x + y = 1 and x, y = 0 or 1, which stands for different doping sites for

respective dopant metals (M). From the results, the Mg-doped, V-doped, and

Zn-doped NiTiH with too high voltages have been excluded.

62

Figure 4-3. Calculated volumes (unit-cell) of intrinsic and various light-metal or

transition-metal-doped NiTiH hydrides.

Figure 4-4. Comparison of average voltages of various light-metal or

transition-metal-doped NiTiH for anodes.

The specific electrochemical capacities of various metal-doped NiTiH anodes have

63

also been calculated. Figure 4-5 shows the changes. Compared with the 249.18

mAh/g of pure NiTiH electrode, Mg-doped, Al-doped, Cr-doped, Mn-doped, and

Fe-doped NiTiH hydrides can respectively increase the specific capacity to 252.64

mAh/g, 252.24 mAh/g, 250.15 mAh/g, 249.72 mAh/g and 249.59 mAh/g.

Figure 4-5. Specific capacities of pure and various metal-doped NiTiH hydrides as

Li-ion battery anodes.

64

65

5 Concluding remarks and outlook

5.1 Brief summary of the appended papers

Firstly in paper Ⅰ,the various graphene-based nanostructures chemisorbed by

various types of species such as O, N, OH and their electronic structures have been

investigated for electron mediator or conductor applications in Z-scheme

photocatalytic hydrogen generation. It is found that the N, O-N and N-N chemisorbed

nanostructures can be utilized as good electron mediators in Z-scheme photocatalysis;

their work functions (absolute values) are in the range of 4.8–5.2 eV. While, the O or

OH chemisorbed ones have best prospect to function as electron conductors between

the reduction level (H+/H2) of water splitting and the H2-evolving photocatalyst.

In paper Ⅱ, we have investigated the graphene nanofibers (GNFs) as catalytic

materials for hydrogen release of sodium alanate. It is found that the GNFs, especially

the helical GNF, can function as excellent catalysts by enhancing the hydrogen

desorption kinetics greatly. Using the PAW method and the GGA functional, the ab

initio materials modelling has helped to understand the mechanisms. The graphene

sheet edges (whether the zigzag edge or the armchair edges) are found to be able to

weaken the Al-H bonds of NaAlH4 and drastically decrease the hydrogen removal

energies. Further exploration has revealed that it is the result of a combination of

hydrogen desorption reactant destabilization and dissociation product stabilization.

The study implies the potential of using graphene-based nanomaterials as catalysts for

complex light metal hydrides.

In paper Ⅲ, the potential effect of C60 on hydrogen desorption of Li-N-H systems

66

has been studied. The results have shown that C60 can stabilize the formation of

LiNH4, i.e., the intermediate state of hydrogen release reaction from the (LiH + NH3)

mixture. With the presence of C60, the hydrogen desorption from LiH+NH3 mixture

can take place following the overall path: LiH-C60 +NH3 → LiNH3-C60+1/2H2. Also,

this reaction can take place at 300K. While for the LiNH4 alone, it can not desorb

hydrogen at room temperature. In addition, C60 can also help restrain the NH3 gas that

is poisonous in PEM fuel cells.

In paper Ⅳ,the electronic structure, chemical bonding of the N2H4BH3 inorganic

material for hydrogen storage applications have been investigated and its hydrogen

removal energetics has also been considered. The hydrogen removal energies of pure

hydrazine borane have been compared with those of a Li-modified HB structure

obtained through ab initio calculations and it is found that the hydrogen removal

energies of Li-modified HB can be dramatically decreased by as much as 50%

compared with those of pristine hydrazine borane. The underlying mechanisms of the

H-host bonds (both the B-H bonds and the N-H bonds in HB) weakening have been

given.

In paper Ⅴ,we have studied the electrochemical properties of pure and Li-doped

NiTiH hydrides for Li-ion battery (energy storage) anode material applications and

have also investigated the diffusion of Li-ion through the pure NiTiH lattice.

Equilibrium crystal structure of NiTiH has been obtained and the ab initio molecular

dynamics simulations have shown a great increase in diffusion of Li ions at 600 K

with respect to the room temperature case. The determined most stable Li-doped

NiTiH hydride has been examined and it has better electrochemical capacity and a

marginal increase in voltage and structural volume compared with those of the pure

NiTiH.

In paper Ⅵ,a more screening study of the effects of light-metals (Mg, Al) and

transition-metals (V, Cr, Mn, Fe, Co, Cu, Zn, etc.) on the electrochemical properties of

67

NiTiH hydride has been performed. The most thermodynamically stable doping sites

of various dopant-metals have been determined, base on which a systematic study in

terms of the unit-cell volume, the average voltage and the specific electrochemical

capacity has been done to screen the most promising metal dopants to enhance the

pristine NiTiH anode material. After comprehensive analysis, it is finally found that

the transition metals Fe, Mn, and Cr are the most effective metal dopants for the

pristine NiTiH. This theoretical study is proposed to guide the design and

development of better metal-hydrides materials for Li-ion battery anode applications.

5.2 Conclusion and outlook

As a fundamental pillar of technical progresses, the materials science is playing a

more and more important role in the energy transformation from the traditional fossil

fuels to the future clean energy systems. Closely in collaboration with the

experimental studies, the approach of computational materials design has become

increasingly recognized with the development of more efficient methods and

installations of faster high performance computers. For the wide applications of

hydrogen energy, the main bottlenecks will be eliminated if the technologies of mass

hydrogen generation from solar water-splitting and of more efficient hydrogen storage

can be achieved. For this part we have mainly considered the chemisorption and the

usage of carbon-based nanomaterials and various practical light-element complex or

chemical hydrides. For the real applications the more ambient temperatures and other

conditions should be considered in further studies. Possible combination of ab initio

simulation approach with other methods such as multi-scale modelling, computational

thermodynamics, etc. can be conducted in the future. For the part of Li-ion battery

materials for energy storage, the more detailed study on diffusion of Li-ions in the

electrode materials can be further explored. Also, combining with experiments, the

larger scale study coupled with some classical or empirical molecular dynamics

methods can be done to know more about the electrochemical processes inside the

68

materials or the devices in the near future.

69

6 List of Figures and Tables

Figure 1-1. Cycle of hydrogen fuel as renewable energy. Materials Research Society.

Figure 2-1. The density functional theory uses the concept of electron density.

Materials Research Society.

Figure 3-1. The concept of “hydrogen economy”. American Institute of Physics.

Figure 3-2. Reduced graphene oxide is used to be electron mediator for water splitting.

American Chemical Society.

Figure 3-3. Considered bonding sites of single oxygen (nitrogen also) atom before

geometry optimization (all are unit cells for calculations). Carbon atoms are in gold

and the red color means oxygen or nitrogen atom in this figure. Springer.

Figure 3-4. Density of states of pure, O-, N-chemisorbed nanostructures. Springer.

Figure 3-5. Left: relaxed configurations of (a) O-O, (b) O-N, (c) N-N chemisorbed

graphene nanostructures (all are unit cells). Gold color: carbon; red: O; purple: N; the

“filled” white color shows the “lifted” bonded carbon atoms because of chemisorption.

Right: density of states. Springer.

Figure 3-6. Left: relaxed configurations of (a) OH-, (b) OH-OH-chemisorbed

structures. Right: the calculated density of states. Springer.

Figure 3-7. Work functions for various chemisorbed structures. Springer.

Figure 3-8. Comparison of volumetric density of different ways to store hydrogen (4

kg of hydrogen). ature Publishing Group.

Figure 3-9. TEM images of (a) HGNF, (b) PGNF, (c) HGNF admixed NaAlH4, and (d)

SAED pattern of 2 wt.% HGNF admixed NaAlH4 used in the study. American

Chemical Society.

Figure 3-10. Left: temperature programmed desorptions of NaAlH4 admixed with 2

wt.% carbon nanovariants. Right: desorption kinetic curves. American Chemical

Society.

70

Figure 3-11. Ground state equilibrium configurations of NaAlH4 with Na facing

various edge sites and of isolated NaAlH4 cluster. Na in yellow, Al in blue, H in pink,

and C in green. The “filled” light green color stands for the carbon atoms that are

fixed in modelling to simulate those bonding to other carbon atoms within the single

graphene layer. American Chemical Society.

Figure 3-12. Hydrogen removal energies of NaAlH4 placed at various graphene patch

edge sites. American Chemical Society.

Figure 3-13. Upper: Amount of charge transfer from Na atom to various carbon edge

sites before hydrogen removal. Lower: the charge states of the AlH3 moiety after one

hydrogen atom has been removed from NaAlH4. American Chemical Society.

Figure 3-14. Equilibrium configurations of (a) LiH, (b) NH3, (c) LiH+NH3, (d)

LiH-C60, (e) NH3-C60, and (f) (LiH+NH3)-C60 after geometry optimization at 0 K. IOP

(Institute of Physics, UK) Publishing.

Figure 3-15. The calculated reaction energy barrier of LiNH4 → LiNH2 + H2 through

NEB method. IOP Publishing.

Figure 3-16. Ab initio molecular dynamics simulations of the LiNH4-C60 system at

300 K. (a) initial structure, and (b) structure after 1000 fs. IOP Publishing.

Figure 3-17. Equilibrium crystal structure (unit cell) of HB solid determined from ab

initio calculations. B: dark green small spheres; N: light blue; H: pink. Elsevier.

Figure 3-18. Total and partial DOS for the hydrazine borane solid. The Fermi energy

is set at zero; s-orbital and p-orbital contributions are distinguished with black color

and red color, respectively. Elsevier.

Figure 3-19. Calculated electron localization function for HB solid plotted as

green-colored transparent isosurfaces at a level of 0.5. Elsevier.

Figure 3-20. Unit cell of one structure of Li-modified HB material obtained from ab

initio calculations. Elsevier.

Figure 3-21. Calculated hydrogen removal energies of the pristine HB solid (blue) and

the Li-modified HB (red). A, B and C are different hydrogen removal sites. A: N1 site;

B: N2 site; C: boron site. Elsevier.

Figure 3-22. Dihydrogen bondings (Å) in HB solid and Li-modified HB solid (local

71

structures). Elsevier.

Figure 4-1. The crystal structure of NiTiH hydride (48 atoms per unit cell, space

group I4/mmm) determined from first-principle calculations. Ti: green color, with

different labels showing different sites; Ni: blue; H: yellow. American Institute of

Physics.

Figure 4-2. The mean square displacement of the Li-ion as a function of time at both

300 K and 600 K. American Institute of Physics.

Figure 4-3. Calculated volumes (unit-cell) of intrinsic and various light-metal or

transition-metal-doped NiTiH hydrides.

Figure 4-4. Comparison of average voltages of various light-metal or

transition-metal-doped NiTiH for anodes.

Figure 4-5. Specific capacities of pure and various metal-doped NiTiH hydrides as

Li-ion battery anodes.

Table 3-1. The calculated reaction energies with and without C60 as catalyst using

DMol3. (The values in parentheses have been calculated at the B3LYP level of theory

using Gaussian 09.) IOP Publishing.

Table 3-2. Calculated average bond-lengths of H-host bonds in pure HB and

Li-modified one. Elsevier.

Table 3-3. Charge states (in units of e) of H atoms bonded with N in HB and

Li-modified HB before hydrogen removal. Elsevier.

Table 3-4. Charge states (in units of e) of residual N atom (N1 or N2) after one H

atom is removed from HB or from Li-modified HB. Elsevier.

Table 4-1. The energetic comparison when a migrated Li-ion stays at different sites in

pure NiTiH lattice. E: total energy of each relaxed configuration that contains 49

atoms (Ni16Ti16H16 and one extra migrated Li atom) in a unit cell. American Institute

of Physics.

Table 4-2. Calculated formation energies for different substitutionally Li-doped NiTiH

hydrides as anode materials. American Institute of Physics.

Table 4-3. Properties of pure NiTiH and Li-doped NiTiH for Li-ion battery anodes

applications: unit-cell volume, average voltage and electrochemical capacity.

72

American Institute of Physics.

Table 4-4. Calculated total energies (eV per unit cell) of various light-metals or

transition-metals substitutionally doped NiTiH materials.

Table 4-5. Formation energies (eV/f.u.) for various substitutionally metal-doped

NiTiH materials.

73

7 Acknowledgements

First of all, I would like to thank my supervisor, Prof. Rajeev Ahuja for your guidance

and introducing me to the 4 years’ PhD study of computational materials science after

I finished my master’s experimental research in 2009. I am thankful for your

open-minded help, encouragement, support and making a research atmosphere that

encourages free thoughts and ideas. These will be great merits for scientific research. I

am also grateful to Prof. Börje Johansson, my second supervisor for your help and

arrangement especially in KTH.

Very special thanks to Cissi. I will never forget in the very beginning of my PhD

(when I was in very bad feelings for my personal reasons) you helped me registration

and many procedures in Materialvetenskap and for your always being kind and

friendly to me both in KTH and Uppsala. Great thanks to Ralph and Moyses for your

cordial and responsible guides on hydrogen storage project especially in the beginning

of my PhD, which made me learn a lot. I would thank Biswarup for guiding me a lot

on the studies of various energy materials throughout my first three years. I would

also like to thank Andon, Jawad, Abir, Tuhina, etc. for your nice guidances about

different energy materials.

The collaborators outside Sweden are acknowledged. Great thanks to Sterlin,

Himanshu, Prof. O. N. Srivastava, Sa Li, Prof. Puru Jena, etc. I would also thank my

colleagues Henrik, Thanayut, Mohammad, Wei, Tanveer and the former members

Pornjuk, Peter, etc. in condensed matter theory group for sharing the unforgettable

study time in Uppsala. All the friends in materials theory group are sincerely thanked.

I would like to thank my Chinese friends in and out of Ångström lab, and both in

Uppsala and in Stockholm. Special thanks to uncle Niancheng, Li, and Zizhen. I feel

at home when together with you. Special thanks to Ying. I am very happy to meet

Yang and Yuan so good friends in Ångström. Many regards to Kelin and Na, wish you

74

and your coming baby a happy life. Special thanks to Xiangyang and Jiefang, I still

clearly remember the first day when I arrived at Uppsala. I also cherish the happy

times with uncle Wei Liu and his family, Shuyi, Yanling, Shuanglin, Xin, Yiming, Wei

Zhang, Dan, Zhoujie, Junhong, Rui, Qingguo, Baisheng, Yunguo, Xue, Wen Li,

Yanling Li, Yang Yang, Jie, Fei, Ellen, Rongzhen, Maofeng, Song, Qiang, Shen, etc.

during my PhD life both in Uppsala and in KTH.

Finally I would dedicate this thesis and my endless thanks to my parents and

grandparents. I always love you.

75

8 Bibliography

[1] Huot J., Ravnsbaek D. B., Zhang J., Cuevas F., Latroche M., Jensen T. R.

Mechanochemical synthesis of hydrogen storage materials. Progress in Materials

Science 58: 30-75 (2013).

[2] Dornheim M., Eigen N., Barkhordarian G., Klassen T., Bormann R. Tailoring

hydrogen storage materials towards application. Advanced Engineering Materials 8:

377-385 (2006).

[3] Crabtree G. W., Dresselhaus M. S. The hydrogen fuel alternative. MRS Bulletin 33:

421-428 (2008).

[4] Lusk M. T., Mattsson A. E. High-performance computing for materials design to

advance energy science. MRS Bulletin 36: 169-174 (2011).

[5] Hohenberg P., Kohn W. Inhomogeneous electron gas. Physical Review 136: B864

(1964).

[6] Kohn W., Sham L. J. Self-consistent equations including exchange and correlation

effects. Physical Review 140: A1133 (1965).

[7] Perdew J. P., Wang Y. Accurate and simple analytic representation of the

electron-gas correlation energy. Physical Review B 45: 13244-13249 (1992).

[8] Perdew J. P., Burke K., Ernzerhof M. Generalized gradient approximation made

simple. Physical Review Letters 77: 3865-3868 (1996).

[9] Lee C., Yang W., Parr R. G. Development of the Colle-Salvetti correlation-energy

formula into a functional of the electron density. Physical Review B 37: 785-789

(1988).

[10] Adamo C., Barone V. Toward reliable density functional methods without

adjustable parameters: The PBE0 model. Journal of Chemical Physics 110:

6158-6170 (1999).

[11] Paier J., Marsman M., Hummer K., et al. Screened hybrid density functionals

76

applied to solids. Journal of Chemical Physics 124: 154709-154713 (2006).

[12] Kittel C. Introduction to solid state physics. John Wiley and Sons, 1996.

[13] Blöchl P. E. Projector augmented-wave method. Physical Review B 50:

17953-17979 (1994).

[14] Kresse G., Hafner J. Ab initio molecular dynamics for open-shell transition

metals. Physical Review B 48: 13115-13118 (1993).

[15] Kresse G., Furthmüller J. Efficient iterative schemes for ab initio total-energy

calculations using a plane-wave basis set. Physical Review B 54: 11169-11186 (1996).

[16] Hamann D. R., Schlüter M., Chiang C. Physical Review Letters 43: 1494-1497

(1979).

[17] Andersen O. K. Physical Review B 12: 3060-3083 (1975).

[18] Kajita S., Nakayama T., Yamauchi J. Density functional calculation of work

function using charged slab systems. Journal of Physics: Conference Series 29:

120-123 (2006).

[19] Jónsson H., Mills G., Jacobsen K. W. Classical and Quantum Dynamics in

Condensed Phase Simulations. World Scientific, 1998.

[20] Henkelman G., Uberuaga B. P., Jónsson H. A. Journal of Chemical Physics 113:

9901 (2000).

[21] Sheppard D., Terrell R., Henkelman G. Journal of Chemical Physics 128:

134106 (2008).

[22] Sheppard D., Xiao P., Chemelewski W., Johnson D. D., Henkelman G. Journal

of Chemical Physics 136: 074103 (2012).

[23] Bader R. F. W. Atoms in molecules: a quantum theory, Internation series of

monographs on chemistry. Oxford: Clarendon, 1990.

[24] Crabtree G. W., Dresselhaus M. S., Buchanan M. V. The hydrogen economy.

Physics Today 57: 39-44 (2004).

[25] Schlapbach L.; Züttel A. Hydrogen-storage materials for mobile applications.

ature 414: 353-358 (2001).

[26] Fujishima A., Honda K. Electrochemical photolysis of water at a semiconductor

electrode. ature 238: 37-38 (1972).

77

[27] Kudo A., Miseki Y. Heterogeneous photocatalyst materials for water splitting.

Chemical Society Reviews 38: 253-278 (2009).

[28] Kudo A. Photocatalysis and solar hydrogen production. Pure and Applied

Chemistry 79: 1917-1927 (2007).

[29] Turner J. A. A realizable renewable energy future. Science 285: 687-689 (1999).

[30] Lewis N. S., Nocera D. G. Powering the planet : chemical challenges in solar

energy utilization. Proceedings of the ational Academy of Sciences (USA) 103:

15729-15735 (2006).

[31] Maeda K., Domen K. New non-oxide photocatalysts designed for overall water

splitting under visible light. Journal of Physical Chemistry C 111: 7851-7861 (2007).

[32] Suwanchawalit C., Wongnawa S. Triblock copolymer-templated synthesis of

porous TiO2 and its photocatalytic activity. Journal of anoparticle Research 12:

2895-2906 (2010).

[33] Grätzel M. Energy resources through photochemistry and catalysis. Academic

Press, New York, 1983.

[34] Hoffmann M. R., Martin S. T., Choi W., Bahnemann D.W. Environmental

applications of semiconductor photocatalysis. Chemical Reviews 95: 69-96 (1995).

[35] Chen X. B., Shen S. H., Guo L. J., Mao S. S. Semiconductor-based

photocatalytic hydrogen generation. Chemical Reviews 110: 6503-6570 (2010).

[36] Kudo A. Z-scheme photocatalyst systems for water splitting under visible light

irradiation. MRS Bulletin 36: 32-38 (2011).

[37] Fu N., Jin Z., Wu Y., Lu G., Li D. Z-scheme photocatalytic system utilizing

separate reaction centers by directional movement of electrons. Journal of Physical

Chemistry C 115: 8586-8593 (2011).

[38] Sasaki Y., Nemoto H., Saito K., Kudo A. Solar water splitting using powdered

photocatalysis driven by Z-schematic interparticle electron transfer with an electron

mediator. Journal of Physical Chemistry C 113: 17536-17542 (2009).

[39] Lightcap I. V., Kosel T. H., Kamat P. V. Anchoring semiconductor and metal

nanoparticles on a two-dimensional catalyst mat. storing and shuttling electrons with

reduced graphene oxide. ano Letters 10: 577-583 (2010).

78

[40] Xiang Q., Yu J., Jaroniec M. Graphene-based semiconductor photocatalysts.

Chemical Society Reviews 41: 782-796 (2012).

[41] Iwase A., Ng Y. H., Ishiguro Y., Kudo A., Amal R. Reduced graphene oxide as a

solid-state electron mediator in Z-scheme photocatalytic water splitting under visible

light. Journal of the American Chemical Society 133: 11054-11057 (2011).

[42] Gao W., Alemany L. B., Ci L., Ajayan P. M. New insights into the structure and

reduction of graphite oxide. ature Chemistry 1: 403-408 (2009).

[43] Wang L., Lee K., Sun Y. Y., Lucking M., Chen Z., Zhao J. J., Zhang S. Graphene

oxide as an ideal substrate for hydrogen storage. ACS ano 10: 2995-3000 (2009).

[44] Dreyer D. R., Park S., Bielawski C. W., Ruoff R. S. The chemistry of graphene

oxide. Chemical Society Reviews 39: 228-240 (2010).

[45] Gao W., Singh N., Song L., Liu Z., Reddy A. L. M., Ci L., Vajtai R., Zhang Q.,

Wei B., Ajayan P. M. Direct laser writing of micro-supercapacitors on hydrated

graphite oxide films. ature anotechnology 6: 496-500 (2011).

[46] Yeh T. F., Chan F. F., Hsieh C. T., Teng H. Graphite oxide with different

oxygenated levels for hydrogen and oxygen production from water under illumination:

the band positions of graphite oxide. Journal of Physical Chemistry C 115:

22587-22597 (2011).

[47] Chakrapani V., Angus J. C., Anderson A. B., Wolter S. D., Stoner B. R.,

Sumanasekera G. U. Charge transfer equilibria between diamond and an aqueous

oxygen electrochemical redox couple. Science 318: 1424-1430 (2007).

[48] Züttel A. Materials for hydrogen storage. Materials Today 6: 24-33 (2003).

[49] Weidenthaler C., Felderhoff M. Solid-state hydrogen storage for mobile

applications: Quo Vadis? Energy & Environmental Science 4: 2495-2502 (2011).

[50] Chen P., Zhu M. Recent progress in hydrogen storage. Materials Today 11:

36-43 (2008).

[51] Yang J., Sudik A., Wolverton C., Siegel D. J. High capacity hydrogen storage

materials: attributes for automotive applications and techniques for materials

discovery. Chemical Society Reviews 39: 656-675 (2010).

[52] http://www.eere.energy.gov/

79

[53] Berube V., Radtke G., Dresselhaus M., Chen G. Size effects on the hydrogen

storage properties of nanostructured metal hydrides: A review. International Journal

of Energy Research 31: 637-663 (2007).

[54] Orimo S., Nakamori Y., Eliseo J. R., Züttel A., Jensen C. M. Complex hydrides

for hydrogen storage. Chemical Reviews 107: 4111-4132 (2007).

[55] Kojima Y., Kawai Y. IR characterizations of lithium imide and amide. Journal of

Alloys and Compounds 395: 236-239 (2005).

[56] Berseth P. A., Harter A. G., Zidan R., Blomqvist A., Araújo C. M., Scheicher R.

H., et al. Carbon nanomaterials as catalysts for hydrogen uptake and release in

NaAlH4. ano Letters 9: 1501-1505 (2009).

[57] Liu C., Li F., Ma L. P., Cheng H. M. Advanced materials for energy storage. Adv

anced Materials 22: E28-62 (2010).

[58] Bogdanovic B., Schwickardi M. Ti-doped alkali metal aluminium hydrides as

potential novel reversible hydrogen storage materials. Journal of Alloys and

Compounds 253-254: 1-9 (1997).

[59] Zidan R. A., Takara S., Hee A. G., Jensen C. M. Hydrogen cycling behavior of

zirconium and titanium-zirconium-doped sodium aluminum hydride. Journal of

Alloys and Compounds 285: 119-122 (1999).

[60] Sandrock G., Gross K., Thomas G. Effect of Ti-catalyst content on the reversible

hydrogen storage properties of the sodium alanates. Journal of Alloys and Compounds

339: 299-308 (2002).

[61] Anton D. L. Hydrogen desorption kinetics in transition metal modified NaA1H4.

Journal of Alloys and Compounds 356-357: 400-404 (2003).

[62] Bogdanovic B., Felderhoff M., Pommerin A., Schüth F., Spielkamp N. Advanced

hydrogen storage materials based on Sc-, Ce-, and Pr-doped NaA1H4. Advanced

Materials 18: 1198-1201 (2006).

[63] Sakintuna B., Lamari-Darkrim F., Hirscher M. Metal hydride materials for solid

hydrogen storage: a review. International Journal of Hydrogen Energy 32: 1121-1140

(2007).

[64] Yu D., Nagelli E., Du F., Dai L. Metal-free carbon nanomaterials become more

80

active than metal catalysts and last longer. Journal of Physical Chemistry Letters 1:

2165-2173 (2010).

[65] Jeon K. J., Moon H. R., Ruminski A. M., Jiang B., Kisielowski C., Bardhan R.,

Urban J. J. Air-stable magnesium nanocomposites provide rapid and high-capacity

hydrogen storage without using heavy-metal catalysts. ature Materials 10: 286-290

(2011).

[66] Teprovich J. A., Wellons M. S., Lascola R., Hwang S. J., Ward P. A., Compton R.

N., Zidan R. Synthesis and characterization of a Lithium-doped fullerane for

reversible hydrogen storage. ano Letters 12: 582-589 (2012).

[67] Scheicher R. H., Li S., Araujo C. M., Blomqvist A., Ahuja R., Jena P. Theoretical

study of C60 as catalyst for dehydrogenation in LiBH4. anotechnology 22: 335401

(2011).

[68] Momma K., Izumi F. VESTA: a Three-Dimensional Visualization System for

Electronic and Structural Analysis. Journal of Applied Crystallography 41: 653-658

(2008).

[69] Henkelman G., Arnaldsson A., Jónsson H. Fast and stable algorithm for Bader

analysis of charge density grids. Computational Materials Science 36: 354-360

(2006).

[70] Sanville E., Kenny S. D., Smith R., Henkelman G. An improved grid-based

algorithm for Bader charge allocation. Journal of Computational Chemistry 28:

899-908 (2007).

[71] Tang W., Sanville E., Henkelman G. A grid-based Bader analysis algorithm

without lattice bias. Journal of Physics: Condensed Matter 21: 084204 (2009).

[72] Chen P., Xiong Z. T., Luo J. Z., Lin J. Y., Tan K. L. Interaction of hydrogen with

metal nitrides and imides. ature 420: 302-304 (2002).

[73] Chen P., Xiong Z. T., Luo J. Z., Lin J. Y., Tan K. L. Interaction between lithium

amide and lithium hydride. Journal of Physical Chemistry B 107: 10967-10970

(2003).

[74] Ichikawa T., Isobe S., Hanada N., Fujii H. Lithium nitride for reversible

hydrogen storage. Journal of Alloys and Compounds 365: 271-276 (2004).

81

[75] Ichikawa T., Hanada N., Isobe S., Leng H. Y., Fujii H. New Metal-N-H system

composed of Mg(NH2)2 and LiH for hydrogen storage. Journal of Physical Chemistry

B 108: 7887-7892 (2004).

[76] Pinkerton F. E. Decomposition kinetics of lithium amide for hydrogen storage

materials. Journal of Alloys and Compounds 400: 76-82 (2005).

[77] Hu Y. H., Ruckenstein, E. Ultrafast reaction between LiH and NH3 during H2

storage in Li3N. Journal of Physical Chemistry A 107: 9737-9739 (2003).

[78] Varin R. A., Czujko T., Wronski Z. S. Nanomaterials for Solid State Hydrogen

Storage. Springer, 2009.

[79] Kar T., Scheiner S., Li L. J. Theoretical investigation on the mechanism of

dehydriding reaction LiH + NH3 → LiNH2 + H2. Journal of Molecular Structure:

THEOCHEM 857: 111-114 (2008).

[80] Guo Y., Yu X., Sun W., Sun D., Yang W. The hydrogen-enriched Al-B-N

system as an advanced solid hydrogen-storage candidate. Angewandte Chemie

International Edition 50: 1087-1091 (2011).

[81] Kang X., Wu H., Luo J., Zhou W., Wang P. A simple and efficient approach to

synthesize amidoborane ammoniates: case study for Mg(NH2BH3)2(NH3)3 with

unusual coordination structure. Journal of Materials Chemistry 22: 13174-13179

(2012).

[82] Wang P. Solid-state thermolysis of ammonia borane and related materials for

high-capacity hydrogen storage. Dalton Transactions 41: 4296-4302 (2012).

[83] Tang Z., Tan Y., Chen X., Yu X. Regenerable hydrogen storage in lithium

amidoborane. Chemical Communications 48: 9296-9298 (2012).

[84] Nutt W. R., McKee M. L. Theoretical study of reaction pathways to borazine.

Inorganic Chemistry 46: 7633 (2007).

[85] Staubitz A., Robertson A. P. M., Manners I. Ammonia-borane and related

compounds as dihydrogen sources. Chemical Reviews 110: 4079-4124 (2010).

[86] Hügle T., Kühnel M. F., Lentz D. Hydrazine borane : a promising hydrogen

storage material. Journal of the American Chemical Society 131: 7444-7446 (2009).

[87] Hügle T., Hartl M., Lentz D. The route to a feasible hydrogen-storage material:

82

MOFs versus ammonia borane. Chemistry - A European Journal 17: 10184-10207

(2011).

[88] Hannauer J., Akdim O., Demirci U. B., Geantet C., Herrmann J. M., Miele P., et

al. High-extent dehydrogenation of hydrazine borane N2H4BH3 by hydrolysis of BH3

and decomposition of N2H4. Energy & Environmental Science 4: 3355-3358 (2011).

[89] Karahan S., Zahmakiran M., Özkar S. Catalytic hydrolysis of hydrazine borane

for chemical hydrogen storage: highly efficient and fast hydrogen generation system

at room temperature. International Journal of Hydrogen Energy 36: 4958-4966

(2011).

[90] Moury R., Moussa G., Demirci U. B., Hannauer J., Bernard S., Petit E., et al.

Hydrazine borane : synthesis, characterization, and application prospects in chemical

hydrogen storage. Physical Chemistry Chemical Physics 14: 1768-1777 (2012).

[91] Becke A. D., Edgecombe K. E. A simple measure of electron localization in

atomic and molecular systems. Journal of Chemical Physics 92: 5397-5403 (1990).

[92] Wagner F. R. http://www.cpfs.mpg.de/ELF/index.php?content=02topol.txt,2002.

[93] Van Schalkwijk W., Scrosati B., Eds. Advances in lithium-ion batteries. Springer,

Berlin, 2002.

[94] Kaskhedikar N. A., Maier J. Lithium storage in carbon nanostructures. Advanced

Materials 21: 2664-2680 (2009).

[95] Shi Z., Liu M., Naik D., Gole J. L. Electrochemical properties of Li-Mg alloy

electrodes for lithium batteries. Journal of Power Sources 92: 70-80 (2001).

[96] Taberna P. L., Mitra S., Poizot P., Simon P., Tarascon J. M. High rate capabilities

Fe3O4-based Cu nano-architectured electrodes for lithium-ion battery applications.

atural Materials 5: 567-573 (2006).

[97] Park M., Kim Y. U., Kim H., Sohn H. J. Enhancement of the rate capability and

cyclability of an Mg-C composite electrode for Li secondary batteries. Journal of

Power Sources 158: 1451-1455 (2006).

[98] Larcher D., Beattie S., Morcrette M., Edström K., Jumas J. C., Tarascon J. M.

Recent findings and prospects in the field of pure metals as negative electrodes for

Li-ion batteries. Journal of Materials Chemistry 17: 3759-3772 (2007).

83

[99] Bruce P. G., Scrosati B., Tarascon J. M. Nanomaterials for rechargeable lithium

batteries. Angewandte Chemie International Edition 47: 2930-2946 (2008).

[100] Goodenough J. B., Kim Y. Challenges for rechargeable Li batteries. Chemistry

of Materials 22: 587-603 (2010).

[101] Armstrong A. R., Lyness C., Panchmatia P. M., Islam M. S., Bruce P. G. The

lithium intercalation process in the low-voltage lithium battery anode Li1+xV1-xO2.

atural Materials 10: 223-229 (2011).

[102] Kim T. H., Park J. S., Chang S. K., Choi S., Ryu J. H., Song H. K. The current

move of lithium ion batteries towards the next phase. Advanced Energy Materials 2:

860-872 (2012).

[103] Tarascon J. M., Armand M. Issues and challenges facing rechargeable lithium

batteries. ature 414: 359-367 (2001).

[104] Tirado J. L. Inorganic materials for the negative electrode of lithium-ion

batteries: State-of-the-art and future prospects. Materials Science and Engineering: R:

Reports 40: 103-136 (2003).

[105] Obrovac M. N., Christensen L., Le D. B., Dahn J. R. Alloy design for

lithium-ion battery anodes. Journal of the Electrochemical Society 154: A849–A855

(2007).

[106] Armand M., Tarascon J.-M. Building better batteries. ature 451: 652-657

(2008).

[107] Oumellal Y., Rougier A., Nazri G. A., Tarascon J.-M., Aymard L. Metal hydrides

for lithium-ion batteries. atural Materials 7 : 916-921 (2008).

[108] Oumellal Y., Rougier A., Tarascon J.-M., Aymard L. 2LiH + M (M=Mg, Ti) :

New concept of negative electrode for rechargeable lithium-ion batteries. Journal of

Power Sources 192: 698-702 (2009).