Transcript IIT-M

Nitrogen containing carbon nanotubes as
supports for Pt – alternate anodes for
Fuel cell applications.
T.Maiyalagan and
Prof. B. Viswanathan
Department of Chemistry,
Indian Institute of Technology, Madras
Chennai 600 036, India
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FUEL CELLS
Direct Energy Conversion Vs Indirect Technology
Thermal Energy
ICE
Mechanical Energy
Fuel Cell
Chemical Energy
Electrical Energy
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BATTERIES/ICE /FUEL CELLS
• Batteries
– Needs recharging
– Dangerous chemicals
• Internal combustion engines
- Carnot limitations
- Moving parts and hence friction
- Noisy
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C. K. Dyer, J. Power. Sources, 106 (2002) 245
FUEL CELLS – ADVANTAGES
 EFFICIENCY
 RELIABILITY
 CLEANLINESS
 UNIQUE OPERATING CHARACTERISTICS
 PLANNING FLEXIBILITY
 FUTURE DEVELOPMENT POTENTIAL
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VARIOUS TYPES OF FUEL CELLS
dadf
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HOW DOES PEMFC WORK ?
O2 + 4H+ + 4e-2H20
2H2  4H+ + 4e-
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2H2 + O2  2H2O
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Cathode catalyst
Anode catalyst
H2
O2
Stack of several hundred
Electrolyte frame
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Bipolar plate
ADVANTAGES OF LIQUID FUELS
• Higher volumetric and gravimetric densities
• Easier to transport
• Storage and handling
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CHEMICAL AND ELECTROCHEMICAL DATA
ON VARIOUS FUELS
G0,
kcal/mol
E0theor (V)
E0max (V)
Energy density
(kWh/kg)
Hydrogen
-56.69
1.23
1.15
32.67
Methanol
-166.80
1.21
0.98
6.13
Ammonia
-80.80
1.17
0.62
5.52
Hydrazine
-143.90
1.56
1.28
5.22
Formaldehyde
-124.70
1.35
1.15
4.82
Carbon
monoxide
-61.60
1.33
1.22
2.04
Formic acid
-68-20
1.48
1.14
1.72
Methane
-195.50
1.06
0.58
-
Propane
-503.20
1.08
0.65
-
FUEL
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WHY METHANOL ?
High specific energy density
Clean liquid fuel
Larger availability at low cost
Easy to handle and distribute
Made from Natural gas and renewable sources
Possible direct methanol operation fuel cell
Economically viable option
Heinzel et al, J. Power Sources 105 (2002) 250
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Direct Methanol Fuel Cell (DMFC)
Overall Reaction
CH3OH + 3/2O2 +H2O  CO2 + 3H2O Ecell = 1.18 V
Anode
CH3OH + H2O  CO2 + 6H+ + 6eEo = 0.046 V
(electro-oxidation of methanol)
Driven Load
e-
Cathode
e-
3/2O2 + 6H+ + 6e-  3H2O
Eo = 1.23 V
H+
Oxygen
Carbon Dioxide
H+
Methanol + Water
Anode
Diffusion
Media
Water
H+
Anode
Acidic Electrolyte
Catalyst Solid Polymer
Layer
Electrolyte: PEM
(Proton Exchange
Membrane)
Nafion 117
Cathode Diffusion
Media
Cathode
Catalyst
Layer
Acidic electrolytes are
usually more advantageous
to aid CO2 rejection since
insoluble carbonates form
in alkaline electrolytes
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Advantages of DMFC Technology
• Longer membrane lifetime due to operating in
aqueous environment
• Reactant humidification is not required
Compared to H2 Systems with Methanol Reformer
• Low operating temperature of DMFC results in low
thermal signature
• DMFC system has faster start-up and load following
• DMFC system is simpler and has lower weight and
volume
• Can use existing infrastructure for gasoline
G.G. Park et al., Int.J. Hydrogen Energy 28 (2003) 645
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Status of DMFC Technology
• Large number of companies working on
DMFC technology for consumer applications
• Commercialization of DMFCs for cell phones
and laptops expected within 2-3 years
• Cost of DMFCs is coming down, and
becoming competitive with Li batteries
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DIFFICULTIES IN DMFC
POOR ANODE KINETICS
FUEL CROSSOVER
ELECTROCATALYSTS
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Challenges for DMFC Commercialization
 COST Cost of stacks
DECREASE OF NOBLE METAL LOADINGS
Overall objective:
 Reduce catalyst cost for direct methanol fuel cells
Present objective
Utilization 
Stability 
Template synthesised CNT as the
support for Pt, Pt-Ru, Pt-MoO3
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CNT: Concentric shells of graphite rolled into a cylinder
Why Supported Catalyst?
High Temperature
What is the support?
How to choose better
Support ?
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THE PROMISE OF NANOTUBES SUPPORT
● Single walled nanotubes are only
a few nanometers in diameter and
up to a millimeter long.
● High conductivity.
● High accessible surface area.
● High dispersion.
● Better stability.
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Why Nitrogen containing carbon nanotubes?
Good electronic conductivity.
Electronic structure and band gap can be tuned by varying the nitrogen content .
Addition of nitrogen increases the conductivity of the material by raising the
Fermi level towards the conduction band .
Catalytic properties of the surface are determined by the position of the Fermi
level of the catalyst. Consequently Fermi level acts as a regulator of the catalytic
activity of the catalyst.
The nitrogen functionality in the carbon nanotube support determines the the size
of Pt by bonding with lone pairs of electrons at the nitrogen site.
Pt bound strongly to nitrogen sites so sintering doesn’t takes place.
The increased electron donation from nitrogen bound carbon nanotubes to Pt
might be responsible for enhancement in kinetics of methanol oxidation.
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Synthesis Of Nitrogen containing carbon nanotubes
Present work
NITROGEN CONTAINING POLYMERS
PPP
N= 0%
PVP
PPY
PVI
N=12.9% N=21.2% N=33.0%
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Schematic Diagram
impregnation
Polymer
Polymer solution
ALUMINA MEMBRANE
carbonization
48 % HF
24 HRS
CNT
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SYNTHESIS OF PVP-CNT
PVP
In
DCM
Alumina membrane
Carbonization Ar atm
PVP/alumina
48% HF 24 hrs
CNTPVP
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Carbonization apparatus
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Thermogravimetric analysis
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ELEMENTAL ANALYSIS
EXPERIMENTAL at 9000C
CALCULATED
SAMPLE
%C
%N
%H
%C
%N
%H
PPP-CNT 93.0
0.00
4.9
92.3
0.00
1.8
PVP-CNT 64.82 12.62 8.17 86.98
6.63
0.81
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SEM PICTURE OF PVP -CNT
(a) The top view of the CNTs.
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SEM PICTURE OF PVP -CNT
(b) The lateral view of the well aligned CNTs ( Low magnification) .
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SEM PICTURE OF PVP -CNT
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(c) The lateral view of the well aligned CNTs ( High magnification) .
TEM PICTURES OF PVP -CNT
200nm
HR-TEM images of carbon nanaotubes obtained by the carbonisation of
polyvinyl pyrolidone (a-b) Carbonisation at 1173 K, 4hrs
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RAMAN SPECTRUM
Intensity (arbitrary unit)
0.0025
D-Band
G -Band
0.0020
0.0015
1650
1500
1350
1200
-1
Raman shift (cm )
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FT – IR SPECTRUM
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FT – IR SPECTRUM
C=C
O-H
C=N C-N
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XPS - SPECTRA
C1s
284.5
N1s
Intensity (arb.units)
Intensity (arb.units)
397.6
287.05
275
280
285
290
Binding Energy (eV)
295
392
396
400
Binding Energy (eV)
399.4
404
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Loading of catalyst inside nanotubes
73mM H2PtCl6
12 hrs
H2 823 K
3 hrs
48% HF 24 hrs
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TEM PICTURE OF Pt/CNT
EDX spectrum
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TEM PICTURE OF Pt/CNT
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ELECTROCHEMICAL STUDIES
Electrode Fabrication
10 mg CNT/ 100 l water
Ultrasonicated, 30 min
Dispersion (10 l) / Glassy Carbon (0.07 cm2)
Dried in air
5 l Nafion (binder)
Solvent evaporated
ELECTRODE
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METHANOL OXIDATION
Cyclic Voltammograms of (a) Pt in 1 M H2SO4/1 MCH3OH run at 50 mV/s
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Cyclic Voltammograms of (b) GC/ETek 20 % Pt/C Nafion in 1 M H2SO4/1 MCH3OH run at
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50 mV/s
Cyclic Voltammograms of (c)GC/CNTpvp-Pt--Nafion in 1 M H2SO4/1 MCH3OH run
at 50 mV/s
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Electrochemical activity of the electrodes based on carbon nanotubes in
comparison with commercial catalysts for methanol oxidation
Electrode
Activity
Ipa(mA/cm2)
Pt
GC/ETek20%Pt/C-Naf
GC/CNT-Pt-Naf
0.076
11.4
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Data evaluated from cyclic voltammogram run in 1M H2SO4/1M CH3OH at 50 mV/s
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Conclusions
1. The template aided synthesis of carbon nanotubes using polymer as
a carbon source yielded well aligned carbon nanotube with the
pore diameter matching with the template used.
2. The higher electrochemical surface area of the CNT and the highly
dispersed catalytic particles may be responsible for the better
utilization of the catalytic particles. The tubular morphology might
be the reason for the better dispersion.
3. The higher activity of the nitrogen containing carbon nanotube
catalyst suggest that the Nitrogen present in the carbon nanotube
(after carbonisation) plays an important role not only in the
dispersion, but also in increasing the hydrophilic nature of the
catalyst.
4. There is a correlation between the catalytic activity of the carbon
nanotube electrode material and the nitrogen concentration (at%).
Future work will be focused on ways to enrich the N content42on
the surface of CNT supports.
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