Presentation

Materials for Hydrogen Storage:
From nanostructures to complex hydrides
Puru Jena
Virginia Commonwealth University, Richmond, VA.
Talk given at University of Tennessee, Knoxville, October 1 , 2009

Outline
‣Basic facts and need for a hydrogen
economy
‣Requirements for Hydrogen Storage
‣Nanostructures as hydrogen storage
materials and as catalysts in
complex light metal hydrides -
Sodium Alanates

Acknowledgement
‣Nanostructures for H storage
Q. Sun, Q. Wang, S. Li, A. Kandalam, K. Boggavarapu,
VCU, Y. Kawazoe, Tohoku University
‣ Carbon Nanostructures as catalysts in
Sodium Alanates
A. Blomqvist, C. M. Araújo, R. H. Scheicher, R. Ahuja,
University of Uppsala
Polly A. Berseth, Andrew Harter, Ragaiy Zidan – Savannah
River National Laboratory
Work supported by: DOE, NSF

Basic Facts
‣ Growing demand for energy
‣ 80% of the current energy needs is supplied by oil, gas, and
coal and much of it goes to meet the needs of mobile industry
‣ Resources are limited and lead to climate change
Hydroelectric (4%) Biomass (3%) Other renewables (1%)
Nuclear (8%)
Coal (22%)
Gas (23%)
Oil (39%)

Energy Crisis?
It is not a question of “if”, but
“when”?

Solution
‣ Life Style Changes
‣ New Energy Source that are
abundant, secure, renewable,
clean, and affordable

“Yes my friends, I believe that water will one
day be employed as fuel, that hydrogen and
oxygen which constitute it, used singly or
together, will furnish an inexhaustible source of
heat and light, of an intensity of which coal is
not capable.... When the deposits of coal are
exhausted we shall heat and warm ourselves
with water. Water will be the coal of the
future.”
Jules Verne, The Mysterious Island (1874-5)

Hydrogen: A National Initiative
“Tonight I'm proposing $1.2 billion in research
funding so that America can lead the world in
developing clean, hydrogen-powered
automobiles… With a new national commitment,
our scientists and engineers will overcome
obstacles to taking these cars from laboratory to
showroom, so that the first car driven by a child
born today could be powered by hydrogen, and
pollution-free.”
President Bush, State-of the-Union Address,
January 28, 2003

Advantages in a Hydrogen Economy
‣ Hydrogen is the third most abundant element on
earth
‣ It is renewable and clean
‣ Packs the highest energy per unit mass of any
element

Problems with a Hydrogen Economy
‣ Hydrogen is not an energy source, but an energy
carrier. It does not occur freely in nature.
‣ Hydrogen is gaseous at ambient temperature and
pressure
‣ Difficulties with its production, transportation,
storage, and use
‣ Problems with cost and safety

The Hydrogen Economy
solar
automotive
H 2 O wind
fuel cells
hydro
nuclear/solar
gas or
thermochemical
hydride
cycles H 2 storage H 2
consumer
electronics
Bio- and
bioinspired
fossil fuel
reforming
production
9M tons/yr
storage
4.4 MJ/L (Gas, 10,000 psi)
8.4 MJ/L (LH2)
stationary
electricity/heat
generation
use
in fuel cells
$3000/kW
40M tons/yr
(Transportation only)
9.72 MJ/L
(2015 FreedomCARTarget)
$35/kW
(Internal Combustion Engine)

For more information, see website http://www.sc.doe.gov/bes/hydrogen.pdf

Hydrogen Storage Requirements
Transportation Applications
‣ High gravimetric (9 wt %) and volumetric density
(70 g/L)
‣ Fast kinetics
‣ Favorable thermodynamics
‣ Effective heat transfer
‣ Long cycle lifetime for hydrogen
absorption/desorption
‣ Safety, durability, and cost effectiveness
75% of U.S. oil consumption is used to meet
transportation energy needs

Hydrogen Storage Media
‣ Gaseous storage – High Pressure
Energy content = 4.4 MJ/L (at 10,000 psi)
compared to that of 31.6 MJ/L for fossil fuel.
Costs associated with compression, leakage and safety
are issues of concern.
‣ Liquid Storage - Cryogenic Temperatures
Density of liquid of H 2 at 20 K= 70 g/L
Energy content = 8.4 MJ/L

Hydrogen storage
2kg of hydrogen
(200km of fuel cell car)
gas-H2
(300K, 0.1MPa)
~25,000l
liquid-H 2
(20K, 0.1MPa)
~ 35l
compress.-H 2
(300K, 35MPa)
~ 75l

Solid State Storage:
Materials Requirements
‣ For high gravimetric density (9 wt %) host
materials must consist of light elements: Li,
Be, B, C, N, Na, Mg, Al
‣ Difficulties: Bonding of hydrogen is strong
(covalent or ionic) and thermodynamics and
kinetics are not ideal
‣ Ideal Bonding: not too weak or not too strong
‣ Ways of altering the chemistry of hydrogen
bonding: nanostructuring or catalysis

Nature of Hydrogen Interaction
‣ Physisorption

3.0 Å
0.76 Å

Nature of Hydrogen Interaction
‣ Chemisorption

Hydrogen Storage Materials
‣ Materials where hydrogen is weakly bonded:
Metal-Organic Frameworks (MOFs),
Clathrates
‣Materials where hydrogen is strongly bonded:
Complex light metal hydrides and chemical
hydrides: NaAlH 4 , MgH 2 , LiBH 4 , NH 3 BH 3

Nature of Hydrogen Interaction
‣ Bonding intermediate between
physisorption and chemisorption
‣Molecular chemisorption

~2.5 Å
~2.5 Å

Lessons from early studies on
dihydrogen bonding
‣ Binding to a metal cation
o K. R. Atkins, Phys. Rev. 116, 1339 (1959)
o M. Manninen and P. Hautojarvi, Phys. Rev. B 17, 2129 (1978)
o
o
o
J. Niu, B. K. Rao, and P. Jena, “Binding of hydrogen molecules by a
transition-metal ion”, Phys. Rev. Letters 68, 2277 (1992)
B. K. Rao and P. Jena, “Hydrogen uptake by an alkali metal ion”,
Europhys. Lett. 20, 307 (1992)
J. Niu, B. K. Rao, P. Jena, and M. Manninen, “Interaction of H 2 and
He with metal atoms, clusters, and ions”, Phys. Rev. B 51, 4475 (1995)
‣ Binding to a neutral transition metal atom
o G. J. Kubas, Acc. Chem. Res, 21, 120(1988)
o L. Gagliardi and P. Pyykko, J. Am. Chem. Soc. 126, 15014 (2004).

Nanostructures for H storage
‣Organo-metallic systems
‣Metal coated carbon fullerenes and
nanotubes
‣Metal decorated polymers
‣Metal decorated Nano-porous carbon
‣Complex light metal hydrides

Computational Approach
‣ Density Functional Theory
‣ Generalized Gradient Approximation
Atomic Clusters
‣ Gaussian Basis sets, and Gaussian 98 code
Crystals
‣ Supercell Band Structure Methods and VASP code

0.832Å
1.96Å
0.82Å
1.75Å
1.91Å
1.75Å
1.98Å
2.40Å
1.76Å
1.76Å
1.76Å
(a)
(b)
(b′)
0.83Å
1.91Å
0.84Å
1.95Å
0.84Å
1.75Å
1.90Å
0.80Å
1.93Å
0.80Å
1.95Å 1.76Å
0.84Å
0.84Å
0.80Å
2.00Å 1.90Å
1.90Å
(c) (c′) (d)
Optimized geometries of Ti C 5 H 5 (H 2 ) n , (n=1-4) along with important bond lengths (Å).
The energy difference (∆E) between (b) and (b′) is 0.12 eV; (c) and (c′) is 0.02 eV
with (b) and (c) being lower in energy than (b′) and (c′), respectively.

Ti doped C 60
6.5 wt%
8 wt%
Zhao et al, Phys. Rev. Lett. 94, 155504 (2005); T. Yildirim and S. Ciraci, Phys. Rev. Lett. 94, 175501 (2005)
Sun et al. J. Am. Chem. Soc. 127, 14582 (2005)

Li Coated C 60 Fullerene
‣Q. Sun, P. Jena, Q. Wang, and M. Marquez,
“First Principles Study of Hydrogen Storage of
Li 12 C 60 ,”
J. Am. Chem. Soc, 128, 9741-9745, (2006)

E=0.0eV
E= + 2.20eV

Interaction energy: 4.50eV
Weight percent: 13%

Interaction between Li 12 C 60 clusters-I
E=0.4 eV

Interaction between Li 12 C 60 clusters-II
E=1.18 eV

Interaction of H 2 with Li 12 C 60 dimer
E H2 = 0.18 eV

Interaction of H 2 with Li 12 C 60 dimer
E= 0.18 eV/H 2

Why not Li 12 C 60 ?
‣Li atoms do not cluster, but binding of
hydrogen is too weak!

Li coated B doped C 60 – Li 12 B 12 C 48
‣ To increase hydrogen binding energy, make Li to
remain in a +1 charge state
‣ Doping B to C 60 will make the fullerene electron
deficient
‣ Electron affinity of B n C 60-n will increase
‣ Li atoms will more readily transfer charge
Q. Sun, Q. Wang, and P. Jena,
Appl. Phys. Lett. 94, 013111(2009)

(a) (b) (c)
(a) B doped C 60 , (b) HOMO, (c) LUMO

(a) 0.00 eV
(b) +7.942 eV (c) +8.718 eV (d) +7.355 eV
(a) Isolated Li atoms, (b, c, d) clustered Li atoms

(a) (b)
(c)
(d)
(a) HOMO, (b) LUMO of Li 12 B 12 C 48 ; (c) HOMO, (d) LUMO
of Li C

(a)
C 48 B 12 Li 12 -12H 2
0.172 eV / H2
(b)
C 48 B 12 Li 12 -24H 2
0.147eV / H2
(c)
C 48 B 12 Li 12 -36H 2
0.135eV / H2
(a) 1 H 2 , (b) 2 H 2 , (c) 3 H 2 decorated fullerenes

Number of attached H 2 , binding energy,
distances in Li 12 B 12 C 48 hetero-fullerene
x E b
R H2
R Li-H2
wt %
12 0.172 0.761 1.990 3
24 0.147 0.757 2.100 6
36 0.135 0.753 2.200 9

Ca coated C 60 fullerene: Ca 32 C 60
Q. Sun, Q. Wang, P. Jena, and Y. Kawazoe
J. Chem. Theo. Computation 5, 374 (2009)

R Ca-Ca =3.681
R Ca-C =2.752
R C56 =1.465
R C66 =1.446
C 60 Ca 32

Do Ca atoms Cluster?
Ca 4 C 60
E= 0.0 eV E= +0.7914 eV

Electronic structure of Ca 32 C 60
a: charge difference plot
b: HOMO
c: LUMO

2.79
E=0.05eV
Top-site
0.74
2.03
R H-Ca =2.05
R H-H =0.74
Bridge-site
Hollow-site
R H-Ca =2.30 E=0.735 eV/H

(a) Geometry and (b) charge difference of H 60 Ca 32 C 60

Metal-coated fullerene M 32 C 60
(M=Ca, Mg, Li)
a: initial geometry
b-d: optimized geometries of Ca 32 C 60 , Mg 32 C 60 and
Li 32 C 60

Hydrogen Storage in AlN
nanostructures
Q. Wang, Q. Sun, P. Jena, and Y. Kawazoe,
ACS Nano 3, 621 (2009).

Al 12 N 12 nano-cage
(a) Geometry, (b) charge density difference, (c) HOMO, and
(d) LUMO of Al 12 N 12 cage.

Binding of H 2 on AlN nanocage
(a) Single H 2 and (b) 12 H 2 molecules adsorbed on
Al 12 N 12 cage surface, (c) LUMO of 12H 2 -Al 12 N 12 cage.

(a) Geometry and (b) HOMO of AlN nanocone.

Adsorption of a single hydrogen molecule on
different Al sites in AlN nano cone.

AlN naotube

Hydrogen storage in AlN nanotube

Hydrogen storage in AlN nanowire

Idea:
How does a charged metal site adsorb H 2 molecules?
Charged metal
ion
+ +
+
++
+
local electric field
adsorption
Polarizing
H 2
Question: Can the same objective be achieved by
directly applying an external electric field to hydrogen
storage materials?

1. BN sheet : Experimental findings

Effect of electric field
A single H 2 molecule adsorbed on the BN sheet: (a) the top view where the red
lines show the supercell used in the simulations, (b) the side view, where
vertical E-field is applied in the -z direction, (c) the model demonstrating the
polarized H2 molecule and the polarized electrons at N site on the BN sheet.

Iso-surface of the electrostatic potential of one H 2 molecule
absorbed on BN sheet in E-field of 0.05 a.u.

Calculated (black points) and fitted curve (red) of (a) H-H bond
length R1 (Å), (b) N-H distance R2 (Å), (c) the induced dipole
moment of H 2 molecule, and (d) binding energy.

A layer of H 2 molecules adsorbed on the BN sheet: (a) the top view,
(b) the side view, (c) the changes of binding energy with E-field.

More polarizable substrate the less E-field
Graphene sheet
> 0.06 a.u.
BN sheet
0.045 a.u.
AlN sheet
0.03 a.u.
More polarizable
Less E-field

AlN sheet
B Al
Al is more metallic-like than B,
AlN is more polarizable than BN

3. graphene sheet
Covalent bonding less polarizable than BN and AlN sheets

We used graphene, BN and AlN sheets to demonstrate the
concept of using electric field as a new parameter to store
hydrogen; more plarizable the substrate, easier it is to use
electric field to store hydrogen for practical applications.
We have shown that the electron-rich N sites are favorable sites
for H 2 adsorption. However, this picture is not limited to just N
sites, any substrate with ionic bonding such as metal oxide or
metal sulfide may be suitable.

In applications, following points should be kept in mind:
(a) In order to have high weight percentage, the polarizable
substrate should consist of light elements.
(b) Since the polarized H 2 molecules are aligned along the
electric field, the polarizable substrate should have enough
space for H 2 molecules to adopt this parallel orientation, so
porous substrates with large surface area may be favorable.

The possible new solution to the challenge:
Design polarizable porous materials or polymers
Electric field
Hydrogen storage

Advantages:
1. Contrary to the local electric field produced by exposed metal
sites embedded in a substrate or a matrix, an external applied
electric field is more effective and provides better control for
reversible hydrogen storage. In particular, the energetics and
kinetics can be manipulated by tuning the field strength.
2. This process avoids complicated synthesis routes for embedding
metal ions and does not suffer from the possibility that these
ions may either cluster during repeated hydrogenation
/dehydrogenation process or be poisoned by other gases such as
oxygen. We don’t need to worry about the clustering problem of
metal atoms .

3. In contrast to the ionic hydrides and alanates where the release of
H2 might render the materials unstable or incapable of reabsorption
of H 2 if its chemical nature is altered, here the H 2
release can be very easily achieved by switching off the electric
field.
4. The reversibility, kinetics, and material stability can be
controlled by choosing appropriate electric field strength and
appropriate substrate.

Complex Light Metal Hydrides
and
Role of catalysts

Sodium Alanates

Structure of Alanates
For more information, see website http://www.sc.doe.gov/bes/hydrogen.pdf

Sodium Alanate – NaAlH 4
‣ Discovered: (Finholt & Schlesinger 1955)
‣ Direct Synthesis: THF, 140ºC, 150 bar H 2 (Ashby 1958, Clasen 1961)
‣ Principal Use: Chemical Reducing Agent
‣ Sensitive to air and water exposure reacting strongly with O 2 and OH
‣ NaAlH 4 melts at 182ºC
‣ PCT Desorption Measurements
‣ 2-step Decomposition
1 2
NaAlH ↔ Na AlH + Al + H
3 3
3
Na3AlH 6
↔ 3NaH + Al + H
2
2
4 3 6 2
1 2
NaAlH ↔ Na AlH + Al + H
3 3
3
Na3AlH 6
↔ 3NaH + Al + H
2
2
4 3 6 2
Dymova, T. N., Eliseeva, N. G., Bakum, S. I.,
And Dergachev, Y. M., Doklady Akademii
Nauk. SSSR, 215 (6) (1974) 1369

Desorption of hydrogen from NaAlH 4
Compare Second Desorption NaAlH 4 /Carbon
4.5
4.0
3.5
Wt % H2 (relative to NaAlH4)
C60
TiCl3
3.0
2.5
2.0
10-20 nm CNT
1.5
1.0
0.5
No catalyst
0.0
a b c d e f g h i j

NaAlH4 supported on Nano-Carbon

Energy gain in adding a H atom
The energy gain in adding a hydrogen atom to AlH n–1– , AlH n–1 , and
NaAlH n–1 clusters.

Time evolution of charge transfer from
NaAlH 4 to C 60

Conclusions
‣Nano-structures are useful
‣Carbon fullerenes and nanotubes can serve
as catalysts for hydrogen uptake and release
in complex metal hydrides
‣Curvature matters
‣Metal coated C 60 can store hydrogen with
favorable thermodynamics