Energy Storage in Azobenzene and
Ice confined in Carbon Nanotubes
Pramod Verma
CCMT Group, Department of Physics
IISc, Bangalore
QCMJC
15th November, 2012
Questions:
Solar Thermal Fuels:
1. What is so special about Azobenzene compared to other solar thermal fules?
2. Why confinement is required?
3. What are theoretical and experimental challenges to store energy in efficient way?
Hydrogen storage in ice
1. How ice is formed inside the CNTs and what is role of CNT diameter on the structure
of ice ?
2. How to load H2 in ice?
3. What is the use of this hydrogen storage in ice in our real life?
OUTLINE
1. Solar Energy Fuels
2. Present energy storage systems
3. Azobenzene as solar thermal fuel
4. Azobenzene confined in the carbon naotubes
5. Ice formation inside the carbon nanotubes.
6. Storage of Hydrogen in ice.
7. Conclusions
8. Future directions
9. Refferences
Renewable energy category
hydroelectricity
Biomass energy
Geothermal energy
Wind energy
Solar energy
What is Solar Energy?


Originates with the
thermonuclear fusion
reactions occurring in the
sun.
Represents the entire
electromagnetic radiation
(visible light, infrared,
ultraviolet, x-rays, and
radio waves).
Some facts about solar energy
. The surface recieves about 47% of the total solar energy
that reaches the earth. Only this amount is usable.
Solar thermal fuels
. Store energy from the sun in the chemical bonds of photoactive
Molecules.
. Upon absorption of light with energy hν, a photoactive “fuel” molecule
undergoes a Conformational change to a higher energy metastable
state, thus storing energy ΔH.
Previous studied solar thermal fuels
The concept of storing solar energy in the chemical bonds of molecules is not a
new one.
In 1970s and 1980s.
. Norbordiene <--> quadricyclane
. Anthracene <--> diaanthracene
Their limitations
. Easily degradable after few cycles of storage.
. Less efficient in storage.
. Cost effective.
Recently found new solar thermal fuel
Tetracarbonyl-diruthenium fulvalene
Advantage:
It can cycle solar energy numerous times without degradation. This has renewed
Interest in the possibility of practical solar thermal fuels.
Disadvantages:
. Ru-fulvalene is not a practical condidate for large scale use due to the requirement
of Ru, a rare and expensive element, and to date efforts to replace the Ru with
cheaper and more abandunt transition metals (e.g., Fe) have faced the challenge of
poor thermal stability and/or low energy density.
. The gravimetric energy density of the Ru-fulvalene fuel is comparable to that of current
Li-ion batteries, its volumetric energy density in solution is several orders of magnitude
smaller, making portability unfeasible as well as increasing storage costs.
To increase the energy storage
capacity via substitution of
functional groups in several isomers
have lead only to small (~10-20%)
increase in ΔH, and furthermore
often result in an undesired
decrease of Ea.
Is there any new material that can
transform solar energy into a
comercially viable energy
technology?
AZOBENZENE --> SOLAR THERMAL FUEL
Cis and Trans isomers
Azobenzene can store solar energy
Why confinement in CNT?
. The potential advantages of azo/CNT hybrid
nanostructures as solar thermal fuesl comes
from the close-packed, highly ordered array of
adsorbed photomolecules imposed by the CNT
substrate.
Two key effects because of confinement
> The number of photoactive molecules per volume (i.e., photoisomer
concentration) significantly increases w.r.t a solution of free
photomolecules. Leading to an increased volumetric density of 5-7
orders of magnitude.
> Both the proximity and the ordered arrangement of the adsorbed
molecules enable systematic manipulation of the inter and
intramolecular interactions, providing a highly effective set of tuning
parameters for maximizing both the energy storage capacity and the
storage life time of the solar thermal fuel.
Additional tuning parameters:
> Molecular packing density, the diameter of CNT,
and the orientation of the adsorbed molecules
and any functional groups on the adsorbed
molecules w.r.t the CNT.
Azobenzene confined inside the carbon naotubes
2,2',5'-trihydroxy diazobenzene
Molecules covalently attached
to a CNT undergoes a
photoinduced trans-to-cis
isomerization, storing Δh = 1.55
eV per azobenzene.
A thermal barrier, Ea , prevents
the back reaction from occuring
under storage conditions.
White--> H
Gray --> C
Blue --> N
Red --> O
Nanotubes carbons are a
lighter gray for clarity.
Density Functional Theory
To investigate several new solar thermal fuel candidates based on the azo/CNT
System.
Pps--> Ultrasoft
Exchange and correlations: Perdew-Berkey-Ernzerhof generalized gradient
Approximation (PBE-GGA).
Code: The Quantum Espresso
Supercells: 1.5nm of vaccum separating periodic copies in the y- and z- directions.
Nanotube axis: x-direction with a periodicity of 0.424 nm.
K-mesh: (8 1 1)
Computation of ΔH
A significant increase in ΔH can be seen
Compared to an isolated azobenzene
Molecule.
The packing interactions due to unsubstituted
Azobenzene alone lead to a net increase of
0.2 eV per molecule in the magnitude of ΔH
for a spacing of 0.424 nm.
> The formation of orderede close-packed
Array Increases the energy storage capacity
per Molecule by 30% compared to gas phase
Azobenzene.
DFT-computed values of ΔH and Ea for a zo/CNT systems
The increase of ∆H with
respect to isolated azobenzene
Arises from the presence of the
CNT substrate which
(i) imposes a close-packed
Crystalline-like state,
(ii) breakes molecular symmetry
(iii) enables design of specific
Ordered, fixed interactions
between functional groups on
Neighboring molecules.
Volumetric energy density as a function of CNT diameter
For several nanotube
packing densities.
Volumetric energy
densities of azobenze
and Ru-fulvalene
solutions < 0.1 Wh/l.
Challenges to overcome before comercializing such a technology:
Theoretical point of view:
Need to develope a fundamental understanding of the relationship between
the geometry of the hybrid nanostructure-based fuels and their solubility in
Water and other solvents, and then design and optimize highly soluble
structures that still have a high energy density, are thermally stable and are
good at cycling.
Experimental point of view:
First thing need to be done is to adapt synthesis methods so that we can
attach a large number of photo-switch molecules per CNT, or other template
molecules that yield the desired properties.
And Finally, we must find ways to integrate thermal fuels with existing
technologies or design novel and inexpensive new devices.
Flow of water through carbon
nanotubes, formation of ice and
hydrogen storage
Water
Water is a liquid at standard temperature and
pressure. It is tasteless and odorless. The
intrinsic colour of water and ice is a very slight
blue hue, although both appear colorless in
small quantities. Water vapour is essentially
invisible as a gas
Water Models
.TIP3P
.TIP4P
.TIP5P
Using water to create nano scale
power cells
.Several years ago, scientists found that they could create an electric current by
pushing water through a singlewalled carbon nanotube (SWCNT)—the direction
of the electric potential along the tube could even be flipped by changing the
course of the water flow. Last year, Chinese scientists led by Lianfeng Sun
managed to make hydroelectric power converters based on this phenomenon,
which led them to suggest that "SWNTs can be exploited as unique, tunable
molecular channels for water and might find potential application in nanoscale
energy conversion."
. Water molecules move through the nanotube in a perfect single file formation.
They form ordered hydrogen bonds with one another, and
the ability to hydrogen bond was responsible for the formation of this singlefile
Chain.
Dipole moment of water molecule
Each water molecule in that chain has a dipole moment and is polar, as the
oxygen atom is more electronegative than the hydrogen atoms. Thus, when
hydrogen bonded in a single file fashion, all the water molecules contribute to
give the collective chain a dipole moment as well.
The dipole moment of the chain creates a polarity difference through the
SWCNT, resulting in a charge of 0.134e at one end of the tube and a charge of
0.005e at the other end. Yuan and Zhao calculated that the voltage difference
between the two ends was 17.2 mV, the electric current was 1.72A, and the
electric field of the tube was 10 V/m.
Molecular dynamics studies of water
flow through CNTs
Entropy and the driving force for the
filling of carbon nanotubes with water
What is so interestng abou ice Ih?
. Hexagonal Crystal structure.
. Stable down to -200oC.
. Can exist upto pressures 0.2GPa.
. Density less than liquid water, of
0.917 g/cm³ due to the extremely
low density of its crystal lattice.
Formation of ordered ice nanotubes
inside carbon nanotubes
Axial pressure = 50 MPa
Phase transition
(16,16) CNT SWNT Chemical potential of liquid water (filled
circles
P = 50MPa
And dashed line) and the hexagonal ice
nanotube (solid line) againts temperature
(14,14) SWCN
Hydrogen storage in ice
Average loading of H2 molecules
In ice Ih predicted from GCMC
simulations
H2 Loading
A 1 nm deep ice Ih is first loaded
With H2 at 100 bar and 150K using
GCMC procedure.
MD Results
Coclusions
1. Volumetric energy density of azobenzene is 10,000 times of
any solar energy fuels.
2. The energy storage capacity and the thermal stability of azobenzene
increases with confinement in carbon nanotubes.
3. Inspite of being hydrophobic nature of carbon nanotube, water can pass
Through it.
4. Water becomes ice for some specific diameters of nanotubes.
5. Flow of water through nanotubes produces electricity.
6. Hydrogen can be stored inside the ice and the efficiency of storage
depends on the temperature, pressure and the diameter of nanotubes.
7. Hexagonal ice is the best ice to store the hydrogen.
Future directions
1. A search for new materials which can store solar
energy very efficiently.
2. Bringing these solar thermal fuesls and hydrogen
storage in real life and Solving the future energy
problem.
Refferences:
[1] Alexie M. Kolpak and Jeffrey C. Grossman, Nano Lett. 2011, 11, 3156-3162.
[2] Tod A. Pascal, Christopher Boxe, and William A Goddard,
J. Phys. Chem. Lett 2011, 2, 1417-1420.
[3] Kenichoro Koga, G. T. Gao, Hidekl Tanaka & X. C. Zeng,
NATURE|VOL 412|23 AUGUST 2001.
[4] G. Hummer, J.C. Rasaiah & J. P. Noworyta, Nature|VOL 414| 8 NOVEMBER
2001.
[5] Tod A. Pascal, William A. Goddard, and Yousung Jung, PNAS, 2011.