FIELD OF THE INVENTION
The present invention generally relates to deposition technique of CNT/Pt inks
made using microwave treated CNTs for deposition of CNT/Pt on large area
electrodes. More particularly the present invention relates to a method to
prepare a stable suspension of CNT/Pt catalyst in the form of a catalyst ink for
making electrodes for proton exchange membrane fuel cell (PEMFCs).
BACKGROUND OF THE INVENTION
For sustainable existence of society which is critically dependent on energy,
environment friendly methods for production, storage and conversion of energy
are being continuously explored and implemented. Fuel cells offer one such
prospect of supplying the society with clean and sustainable energy.
A fuel cell is an electrical cell, which unlike storage cells can be continuously fed
with a fuel so that the electrical power output is sustained indefinitely. Through
electrochemical reaction, they convert hydrogen or hydrogen containing fuel and
oxygen directly into electrical energy and water. Fuel cells have the advantage
over conventional batteries, in that they produce several times as much energy
per equivalent unit of weight.

Different kinds of materials have been used for making electrodes and electrolyte
for fuel cells. Compared to others, the alkaline variety of fuel cell using alkaline
electrolyte offer the advantage of high power to weight ratio due to intrinsically
faster kinetics for oxygen reduction to the hydroxyl anion in an alkaline
environment [1]. Therefore, alkaline fuel cells were ideal for space applications.
However, these cells have disadvantage of carbon dioxide poisoning of the
electrolyte for terrestrial use due to carbon dioxide present in environment and
reformate gas.
The above problems have largely been mitigated by the advent of proton
exchange membrane fuel cells (PEMFCs). The efficiency of PEMFC directly
depends upon the catalysts used for electrochemical reaction. It is generally
required that these catalysts have high durability, low cost and higher activities
in oxygen reduction and/or fuel oxidation reaction. Currently, the most widely
used catalysts in the PEMFC are metal nanoparticles, mainly Pt and/or Pt based
alloys. Due to high surface area and suitable Fermi levels for redox reactions,
these metal nanoparticles have high activities in oxygen reduction and/or fuel
oxidation reaction. Individual metal nanoparticles are usually unstable and prone
Io loss of their catalytic activity due to their irreversible aggregation during the
electrochemical processes. Since these metal nanoparticles form the core of
PEMFC, their synthesis and distribution over the surface of electrodes largely
determines the efficiency of fuel cells. Different techniques for synthesis of metal
nanoparticles-loaded electro catalysts have been adopted [2].

Beard et al [3], described the method to make loaded catalyst with bimetallic
compositions by electroless deposition (ED) for fuel cell electrodes. ED is catalytic
or auto-catalytic process whereby a chemical reducing agent reduces a metallic
salt on to specific sites of a catalytic surface which can either be an active
substrate or an inert substrate seeded with a catalytically active metal. Carbon
support treated with nitric acid was seeded with ruthenium (Rh) particles by the
wet impregnation. Further, Pt was electroless deposited on Rh seed sites.
Sputter-deposition technique was used by Hirano et al [4] to prepare a low-Pt-
loading catalyst layer on an uncatalyzed carbon supported electrode. The
sputtered electrode showed slightly lower cathode potential and exchange
current density at a low current density region than that of the base electrode.
However, higher oxygen reduction reaction (ORR) potential compared to the
base electrode was achieved at a high current density region.
In 2004, a sonochemical process was developed by Xing et al [5] to treat carbon
nanotubes (CNTs) in nitric and sulfuric acids to create functional groups for metal
nanoparticle deposition. The Pt salt precursor, K2PtCl4, was then reduced in
ethylene glycol and water solution with CNTs to deposit Pt nanoparticles on
CNTs.
Boennemann et al [6] developed organo aluminum-stabilized metal colloids with
a particle size smaller than 2 nm at room temperature. In their method, PtCl4
was dissolved in ethylene glycol under vigorous and Na0H was added to adjust
pH of solution containing carbon black.

To increase the Pt utilization in PEMFC, Antoine et al [7] explored electrode
position method and fabricated carbon supported cathodes for PEMFC, Antoine
et al [7] explored electrode position method and fabricated carbon supported
cathodes for PEMFC. The carbon support for electrode was impregnated with
H2PtCl6 and nafion was coated over it. Pt was then electrodeposited over the
Nafion coated carbon supports so that only finer Pt nanoparticles diffuse to
carbon supports through increasing Pt utilization.
Yoshitake et al [8] reported the colloid method for the synthesis of Pt/CNT
catalysts for PEMFC cathode. In the preparation, Pt oxide colloid was formed by
addition of NaHSO3 and H202 into H2PtCl6 CNT was added to Pt oxide and sample
was dried and reduced with hydrogen gas.
Gamma irradiation technique was used by Delcourt et al [9] to prepare platinum
nanoparticles on carbon support, K2PtCL6 was reduced by gamma irradiation in
presence of CO-saturated water/ isopropanol, Pt nanoparticles thus formed were
impregnated on the carbon support.
Boutonnet et al [10] used micro emulsion method to prepare monodisperse Pt,
Pd, Rh and Ir nanoparticles by reducing water soluble metal salts with hydrogen
or hydrazine. The dispersed metal nanoparticles were then transferred to carbon
support without agglomeration.
Paschos et al [11] prepare platinum nanoparticles by chemical spray pyrolsis
method. The mist of H2PtCal6 was formed by 2.4 MHz ultrasonic nebulizer which
was then carried to preheated Si/Si02 CNT substrate by carrier gas (Argon).

H2PtCl6) decomposed on the surface of substrate forming Pt nanoparticles
supported on CNT.
Supercritical deposition technique was implemented by Bayrakceken et al [12]
to prepare Pt-based electrocatalysts for PEFMC. Dimethyl (cyclooctadiene)
platinum (II) (PtMe2COD) was used as the Pt precursor. Precursor was dissolved
in supercritical carbon dioxide with critical temperature of 31oC and a critical
pressure of 7.38 MPa was impregnated on pre heated carbon support at 150oC.
These carbon supported Pt nanoparticles were then used for PEMFC electrodes.
Last decade has seen a rise in the popularity of microwave assisted chemical
syntheses . Microwaves are particularly helpful in the reactions which require
harsh conditions to complete as they provide sufficient instantaneous energy for
the intermediates to cross the activation energy barrier. The advantages of
microwave assisted synthesis include lower bulk temperature, lower reaction
time, easy controllability, high reproducibility and scalability.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method to prepare a stable
suspension of CNT/Pt catalyst in the form of a catalyst ink for making electrodes
for proton exchange membrane fuel cell (PEMFCs).

Another object of the invention is to propose a method to prepare a stable
suspension of CNT/Pt catalyst in the form of a catalyst ink for making electrodes
for proton exchange membrane fuel cell (PEMFCs), in which the size of the
Platinum nano particles is controlled using microwave irradiation.
A further object of the invention is to propose a method to fabricate electrodes
for PEMFC by depositing the CNT/Pt ink in which the size of the Platinum nano
particles is controlled using microwave irradiation and depositing is made
adapting ultrasonic nozzle free spraying process.
SUMMARY OF THE INVENTION
The present invention discloses a method to functionalize CNTs so that metal
nano particles are anchored onto them. It also discloses the method to deposit
metal nano particles onto CNTs by reducing platinum salts using microwave
irradiation. Also, it discloses a method to form stable suspension of the CNT/Pt to
form catalyst ink that can be deposited onto suitable support like carbon paper
using ultrasonic nozzle free spraying to form electrodes for PEMFC application.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
. Figure 1 shows the SEM image of the platinum deposition on MWCNTs
without the use of stabiliser. As can be seen, the particle size of Pt is in the
range of 100-200 nm and there is no sufficient anchoring of the particles to
the nanotubes

. Figure 2 shows the STEM image of the platinum deposition on MWCNTs with
the use of stabiliser. Platinum particles have been uniformly deposited
throughout the nanotube surface. Also, the size of the particles is below
10nm.
. Figure 3 shows the Cyclic Voltagram for VulcanX/Pt that is commercially
purchased. Voltage range: -0.2 to 1.2 V Vs Ag/AgCl electrode. Scan rate:
10mV/s.
. Figure 4 shows The Cyclic Voltagram for CNT/Pt developed in this project.
Voltage range: -0.2 to 1.2 V Vs Ag/AgCl electrode. Scan rate: 10mV/s.
. Figure 5 shows the image of an electrode made of CNT/Pt printed using the
ultrasonic spray technique.
DETAILED DESCRIPTION OF THE INVENTION
A representative embodiment of the method is to treat the CNTs in harsh
oxidizing conditions so that -COOH group is added on the CNTs at as many
sites as possible by using microwave heating technique. The chemical
inertness of the pristine CNTs is the reason for the requirement of harsh
oxidizing conditions to functionalize the CNTs. The oxidizing agents can be
strong acids such as sulfuric acid, nitric acid, hydrochloric acid, hydrogen
peroxide or a combination thereof.

Further, the functionalized CNTs are added into a reaction mixture where
the reduction of platinum salt is under way so that the reduced platinum
particles anchor to the CNTs to form CNT/Pt (CNTs having Platinum nano
particles anchored onto their surface). The platinum salts include but are not
limited to Platinum acetyl acetonate, platinic chloride, hydrochloric platinic
acid etc. The reducing agents of the platinum salts can include but are not
limited to sodium borohydride, glacial acetic acid or any other sodium salt
capable of reducing the platinum salts to form platinum nano particles.
process is usually carried out in a polyol such as ethylene glycol. All the
processes are closed vessel reactions in a microwave digestion system under
controlled temperature and pressure and under constant mechanical stirring.
The microwave heating brings in the advantages of reducing the reaction
times and the reaction conditions under microwave heating can be optimized
for required platinum nano particle sizes. Microwave reactions are also
reliable and repeatable. The size of platinum particles are also controlled
using stabilizers which limit the growth of platinum particles. Stabilizers are
including but not limited to glycerols, aldehydes, ketones or any other organic
stabilizer. The microwave heating under constant mechanical stirring will also
ensure uniform deposition of the platinum particles onto the CNTs.
The CNT/Pt are dispersed in a suitable solvent usually any alcohol such as
ethanol or isopropanol along with a polymer binder, usually nafion, using
ultrasonic mixing technique. The concentrations of CNT/Pt and polymer
binder can vary depending upon the Platinum loading required for the anodic
and the cathodic electrodes for the PEMFC. Depending upon the source of

CNTs and the size of the platinum particle deposition, DI water is also added
during the ink formation. For dispersion, the techniques include but are not
limited to mechanical mixing, ultrasonic bath, impeller mixture or a
combination of these. The catalyst ink is treated with mixing until a uniform
suspension of the CNT/Pt resulting in the catalyst ink formation.
The next step is the catalyst deposition onto the carbon paper. Ultrasonic
nozzle free spraying technology is most suitable for deposition of the catalyst
ink, given that the catalyst ink is a suspension. This is because when
conventional mixing devices and pumps are used for dispensing
nanoparticles, the particles tend to agglomerate and separate from the liquid
suspension. But here, the energy used for breaking the agglomerations of the
suspended nano particles is the same as used for the atomization.
Ultrasonic nozzles operate by converting a high frequency electrical signal
into mechanical expansion and contraction of the transducers. This causes
vibrations at the nozzle's atomizing tip. Liquid traveling down the center of
the nozzle forms capillary waves as a result of this vibrational energy. As the
liquid emerges onto the atomizing surface, it reaches critical wave amplitude
and is broken into a spray of tiny drops by the ultrasonic energy concentrated
there. This ultrasonic nozzle design provides an easily controllable atomized
spray that does not clog because of the large liquid feed orifice and the self-

cleaning ultrasonic vibration. In an ultrasonically produced spray, drop size is
governed by the frequency at which the nozzle vibrates, and by the surface
tension and density of the liquid being atomized. The higher the frequency,
the smaller the median drop size.
The main objective of using this ultrasonic technique is to avail of the
platinum catalyst property to the maximum extent by making sure of the
maximum surface area exposure of platinum, hence making largest platinum
availability for the flow of gas. The process also ensures maximum
repeatability as there will be better control of the process variables i.e.
droplet size, droplet energy, flow rate and the coating speed. The deposited
electrodes are dried under vacuum under optimized conditions to form
electrodes for PEMFC application.
Example
Acid Functionalization: MWCNTs are highly chemically inert. Hence, the
probability of platinum particles anchoring on the carbon nanotubes is very
small. Defect sites and dangling functional groups serve as anchor sites for
platinum particles. To create these, MWCNTs are treated with high
concentrated acid (H2SO4 and 70%-HNO3in the ratio of 1:3) solution in a
reflux system attached to a microwave digestion system at 60 C for 3 hours.

Platinum deposition reaction: After dispersing the acid treated MWCNTs in
cold water, 3.75ml of chloroplatinic acid of 7.4mgPt/ml concentration was
added to the microwave vessel in which the reaction is to be carried out.
Further, 10ml of 2.5M NaOH is added dropwise to the solution under vigorous
stirring. The reaction temperature is set at 200 C and reaction was performed
for 7 minutes. After several experiments, it is observed that addition of a 2ml
of 40% formaldehyde solution will control the platinum particles to 2-10 nm
range. Hence, this has been used during the reaction
After the reaction, the CNT/Pt powder is extracted using a vacuum filter and
PTFE membrane filter paper. Also, the powder is washed with dilute HCl to
remove any excess NaOH that may be present. It is washed until the pH test
shows neutral. Then, the powder is washed with DI water and atmosphere
dried in a closed vessel for 4 hours.
Figure1 depicts the SEM image of the Platinum particles on CNT without the
addition of formaldehyde during the reaction. As can be seen in the picture,
the platinum particles are in 100-200 nm range. The dispersion of the
particles is also not uniform. But according to the popular literature, ideal
platinum particles size for good catalytic property and maximum utilization of
platinum, the size should be between 4-10 nm.

Figure 2 depicts the SEM image of the platinum particles on CNT with the
addition of formaldehyde to the reaction mixture. It can be clearly seen that
the particle size is in the range of 2-10nm.
The extracted CNT/Pt is then mixed with 5 wt% nafion solution, DI water and
isopropyl alcohol (with DI water and IPA in 3:1 ratio) and sonicated for 3
hours to form a stable printable ink. Cyclic Voltammetry measurements are
carried out to estimate the performance of the as prepared catalyst. It is
compared against the commercially available 40 wt% Vulcan-X/Pt. For the
cyclic voltammetry measurements, 1N H2SO4 was used as the electrolyte.
Ag/AgCl electrode is used as the reference and a platinum rod is used as the
counter electrode. A glassy carbon electrode on which 50 micro litres of each
of the stable inks has been deposited is used as a working electrode.
Nitrogen gas is purged in the electrolyte for 20 min prior to measurement to
remove any dissolved gas. Nitrogen is also passed during the experiment to
avoid the formation of bubbles on the working electrode. For the
measurement of electrochemically active surface area (ECSA), slow scan is
performed in the potential range of -0.2V to +1.2V at 10mV/s.
The voltagram for the catalyst powder commercially purchased (VulcanX/Pt)
is shown below in figure 3. Similarly, the figure 4 shows the cyclic voltagram
for the prepared catalyst CNT/Pt.

The ink was used to print large area electrodes of size 101X242 mm using the
ultrasonic nozzle free spraying method. The electrodes are used in the usual
MEA making process by hot pressing the electrodes at 130 C. Figure 5 shows
the image of the electrode made using CNT/Pt ink using the ultrasonic spray
technique.
Literature References
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methods of low Pt-loading electrocatalysts for proton exchange membrane
fuel cell systems” Energy 2010, 30:3941-3957.
2. Jing Pan, ShanfuLu, Yan Li AibinHuang.LinZhuang, Juntao Lu “High-
Performance Alkaline Polymer Electrolyte for Fuel Cell Applications”
Advanced Functional Materials 2010,20:2:312-19.
3. Beard KD Schaal MT, Van Zee HW, Monnier JR, “Preparation of highly
dispersed PEM fuel cell catalysts using electroless deposition methods”
Applied Catalysis B: Environmental 2007, 72:262-71.
4. Hirano S. Kim J. Srinivasan S “High performance proton exchange
membrane fuel cells with sputter-deposited Pt layer electrodes”
Electrochimica Acta 1977, 42:1587-93.

5. Xing Y. “Synthesis and electrochemical characterization of uniformly
dispersed high loading Pt nanoparticles on sonochemically-treated
carbonnanotubes” Journal of Physical Chemistry B 2004, 108:19255-9.
6. Boennemann H Binkmann R Britz P Endruschat U Moertel R Paulus UA et
al “New Mater” Electrochemical System 2000, 3:199.
7. Antoine 0 Durand R. “In situ electrochemical deposition of Pt nanoparticles
carbon and inside nafion “Electrochemical Solid State Letters 2001, 4:A55-
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8. Yoshitake T Shimakawa Y Kuroshima S Kimura H Ichihashi T Kudo Y
“Preparation of fine platinum catalyst supported on single-wall
carbonnano horns for fuel cell application” Physica B 2002, 323:124-6.
9. Le Gratiet B Remita H Picq G Delcourt MO “CO-stabilized supported Pt
catalysts for fuel cells: radiolytic synthesis” J Catalysis 1996, 164:36-43.
10. Boutonnet M Kizling J Stenius P Maire G “The preparation of mono
disperse colloidal metal particles from micro emulsions” Colloids and
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11. Paschos 0 Choi P Efstathiadis H Haldar P “Synthesis of platinum
nanoparticles by aerosol assisted deposition method: Thin Solid Films
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Patent No. Date Inventors Title of the Patent
WO 2011116169 17th March 2011 Jiefeng Lin et al Durable
platinum/multi-
A2
walled carbon
nanotube catalysts
US 20090220835 24th February 2007 Yushan Yan et al Platinum and
A1 Platinum Based
Alloy Nanotubes
as Electrocatalysts
for Fuel Cells
WO 2012131718 29th March 2012 Harshal Dilip An improved
Al CHAUDHARI et al process for the
preparation of
membrane
electrode
assemblies (meas)
US 20090050258 22nd August 2008 Branko N Popov et Development of
A1 al pem fuel cell
electrodes using
pulse electro
position

WE CLAIM :
1. A method to prepare a stable suspension of CNT/Pt catalyst in the form of
catalyst ink for making electrodes for proton exchange membrane fuel
cell (PEMFCs,) comprising :
- providing a CNT/Pt nanopowder, and
- synthesizing the CNT/Pt nanopowder using a microwave synthesis
wherein the platinum particle size is between 1-100 nm, wherein the
solvent used during the synthesis is D1 water, and wherein an organic
stabilizer is added to the solvent to control the platinum particle size.
2. The method as claimed in claim 1, wherein, an acid functionalization of
the nano tubes prior to platinum deposition is carried out in microwave.
3. The method as claimed in claim 1, wherein strong acids including but not
limited to concentrated sulfuric acid, concentrated nitric acids are used for
functionalization.
4. The method as claimed in claim 1, wherein the source of platinum
includes all platinum salts including but not limited to hexa chloro platinic
acid, platinum acetyl acetonate etc.

5. The method as claimed in claim 4, wherein the agents used for reducing
platinum salts include but are not limited to sodium borohydride, glacial
acetic acid, sodium hydroxide or any other reducing agent capable of
reducing the platinum salts to form platinum nano particles.
6. The method as claimed in any of the preceding claims, wherein reaction
time varies from 1 to 60 min depending upon the power of microwave,
nature and quantity of the reaction mixture and its pH, and wherein the
temperature of the reaction solution is maintained constant between
50oC to 2400C.
7. The method as claimed in claim 1, wherein the microwave radiation of
2.45GHz is used to deliver power ranging from 100W to 1000W depending
upon the nature of precursors, wherein the obtained powder can be
deposited on large area carbon paper to form electrodes for used in PEM
fuel cell, and wherein the processor of deposition include but not limited
to spin coating, spray coating, screen printing or ultra-nozzle spray
technique.
8. The method as claimed in claim 7, wherein the length and breath of a
printable electrode can vary from 5 mm to 500 mm.

9. The method as claimed in claim 1, wherein the obtained CNT/Pt catalyst
is mixed with a polymer and a combination of Dl water and IPA to form a
stable suspension of the catalyst for ultra-nozzle spray deposition, wherein
the polymer can be any proton conducting and non-electron conducting
material like nafion, wherein the proportion of IPA in the ink formulation
can vary between 0 to 100%, and wherein the ink is stable at least for
one week when maintained at room temperature without disturbing.
10. The method as claimed in claim 1, wherein CNT/Pt powders in amounts
varying from 0.1 mg to 1kg can be produced.