Final Report (Graphene supported platinum nanoparticles) (1)

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Platinum-Graphene Nanocomposites
as electrocatalysts in PEM Fuel Cells
Submitted by
T.V. Sridharan
(USN No. 1RV12ME108)
Under the guidance of
Professor Manoj Neergat
Department of Energy Sciences and Engineering
Indian Institute of Technology, Bombay

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CERTIFICATE
Certified that the summer internship project report “Platinum-Graphene
nanocomposite as electrocatalysts in PEM Fuel Cells” is the bonafide work of
T.V. Sridharan, USN No:1RV12ME108, 2rd
year B.Tech in Mechanical
Engineering of R.V. College of Engineering, Bangalore carried out under my
supervision during 2.06.2014 to 28.07.2014.
Place: IITB, Powai Signature of the Supervisor
Date: 28-7-2014 Name of Supervisor-
Prof. Manoj Neergat

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Acknowledgement
With great pleasure, I would like to express my gratitude to my guide, Professor
Manoj Neergat for his ideas, suggestions and encouragement during the course of
the project. I would like to thank his PhD scholars Ramesh Singh, Naresh Nalajala,
Rahul R., Wasim Feroze, Tathaghat Kar, Devi Ruttala Varaprasad, Bapi Bera and
Arup Chakraborty for the academic and technical support they provided during the
project.

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CONTENTS
Title Page 1
Certificate by the Supervisor 2
Acknowledgement 3
CONTENTS
1. Abstract 5
2. Introduction 6
3. Literature Review 8
4. Experimental
4.1 Materials 11
4.2 Synthesis of Pt/rGO 11
4.3 Synthesis of Pt/C 12
4.4 Physical and Electrochemical Characterization 12
4.5 Electrode preparation 12
5. Results and Discussion
5.1 Physical Characterization
5.1.1 TEM Analysis 13
5.1.2 XRD Analysis 15
5.2. Electrochemical Characterization
5.2.1 Cyclic Voltammetry 17
5.2.2 Oxygen Reduction Reaction 20
6. Conclusion 22
7. References 23

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1. Abstract
The use of graphene as a support material for a dispersion of platinum nanoparticles was
explored as an alternative to the conventionally used carbon black. The Pt/rGO nanocomposite
was synthesized using a one pot modified polyol method. Platinum nanoparticles were deposited
onto graphene sheets by means of borohydride reduction of H2PtCl6 in a graphene oxide (GO)
suspension. Electrochemical experiments suggested a considerably higher effective catalytic
surface area for the Pt/rGO composite compared to Pt/C of similar metal loading. The activity of
the oxygen reduction reaction (ORR) however did not show any significant improvements. The
results indicated the need for future research efforts for graphene to vie for replacement of
carbon black as an effective and economical catalyst support.

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2. Introduction
Polymer Electrolyte Membrane fuel cells (PEMFCs) are regarded as promising energy sources
for mobile electronic applications due to their high efficiencies and low operating temperatures.
Their performance and cost are essentially governed by the nature of the electrocatalysts used. It
has long been acknowledged that platinum nanoparticles show superior performance in the
catalysis of the oxygen reduction reaction [1]. However, the commercial success of PEMFCs has
been greatly hindered by high cost of Pt and its ineffective utilization.
One of the prime objectives of PEMFC research is the reduction of precious metal loading on the
electrode without compromising the efficiency of the fuel cell. A great deal of work has focused
on supported metal catalysts which show higher activity and stability compared to unsupported
bulk metal catalysts [2]. A dispersion of the catalyst on support material increases the catalytic
surface area, thereby increasing the utilization efficiency of precious metal. Support materials are
characterized by their surface area, porosity, electrical conductivity, electrochemical stability and
have a strong influence on the performance and durability of catalysts.
Carbon black, because of its high surface area and low cost, has been extensively used as a
support material in PEMFCs [3]. However, carbon blacks are impaired by problems such as, (i)
the presence of organic impurities (ii) entrapment of catalyst nanoparticles in the deep
micropores making them inaccessible to reactants thus leading to reduced catalytic activity [2].
Furthermore, carbon supports and catalytic metals have been reported to degrade under
prevailing conditions of temperature, humidity and potential at the anode. Corrosion of the
support exacerbates the agglomeration and detachment of the catalyst from support materials [4].
Recently, various carbon based nanostructured support materials have come under investigation.
Graphene, a monolayer of graphite composed of carbon atoms in a honeycomb arrangement has
attracted the attention of a multitude of researchers [5]. This two-dimensional (2-D) material has
a theoretical surface area (2630 m2
g-1
) and high conductivity (103–104 S m-1
) that its use as a
catalyst support [5][6].

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The conductive support facilitates efficient collection and transfer of electrons to the electrode
surface. The large specific surface area coupled with its excellent thermal, electronic and
mechanical properties, make graphene a potential alternate substrate for the deposition of
inorganic nanoparticles to produce highly dispersed composites.
In this study, the effective surface area and activity of Pt/C was compared with Pt/G composite
of the same loading of platinum. Further, the catalytic performance of Pt/G nanocomposites with
different weight compositions of platinum and graphene was studied by means of their cyclic
voltammograms and oxygen reduction reaction (ORR) polarization curves. Pt/rGO catalyst was
prepared by simultaneous reduction of H2PtCl6.6H2O and graphene oxide using a modified
polyol method [7]. TEM and XRD were employed to characterize morphology and surface
composition of the samples. The role of graphene as an effective catalyst support was
investigated.

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3. Literature Review
The unique structural, physical and chemical properties of graphene draw attention to possibility
of the preparation of novel composite materials with superior catalytic properties. Although
single layer graphene catalytic supports have not been reported, promising results from few
layered graphene stacks are encouraging increased research efforts in this direction. Thus it is
important to discuss the properties of graphene which are of relevance to its application as a
catalyst support and its influence on the activity oxygen reduction reaction in PEMFCs.
Graphene has a hexagonal arrangement of carbon atoms in a 2 D plane forming a honeycomb
lattice (Figure 1). The planar sheet structure provides a very high surface area for the attaching
catalyst nanoparticles. The large surface area is an advantage lost when graphene sheets
irreversibly agglomerate. The extraordinary properties of graphene are associated with individual
layers.
Figure 1. Graphene monolayer with a honeycomb structure [20]
The carbon atoms in graphene are said to be sp2 hybridized. The  bonds provide a strong
hexagonal backbone, and the out-of plane ∏ bonds are responsible for interaction between
different graphene layers. The lone pairs of ∏ electrons are delocalized and facilitate conduction
of charge through the plane normal to the c-axis of graphite (electron conduction along c-axis is
much lower).
Graphene is considered a zero-band gap semiconductor since the valence and conduction bands
overlap. The electronic properties of graphene vary with the thickness, or the number of stacked
layers. At one atom thickness, graphene is transparent suggesting its application in photocatalytic
reactions.

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Among the several exciting properties of graphene, the one that has attracted considerable
attention is its adsorption. Transition metals, and specifically platinum and palladium have been
shown to have remarkable catalytic properties. The adsorption mechanism and interaction
between metal atoms and the graphene support has become vital to fabricate graphene-based
composite catalyst materials. Several research groups have reported the performance of
Pt/Graphene composite as effective electrocatalysts.
Soin et al. synthesized Pt/Graphene nanoflakes electrode for the Methanol Oxidation Reaction.
Microwave plasma assisted chemical vapour deposition technique was used to grow the
vertically aligned graphene nanoflakes. Raman spectroscopy confirmed the characteristics of
highly crystallized few layered graphene. Pt nanoparticles were sputtered onto the graphene
nanoflakes. Cyclic Votammetry curves demonstrated fast electron transfer (ET) kinetics for the
Pt/Graphene electrodes. The rapid electron transfer kinetics was attributed to the highly
graphitized edge structure of the nanoflakes [8].
Ali Grinou et a.l used an aniline stabilized Pt/rGO composite for electrochemical studies. The
nanocomposite was reduced by ethylene glycol solution and aniline stabilized the Pt
nanoparticles, without altering the reduced graphene oxide structure. A marked enhancement of
the electrical conductivity of the composite prepared using the aniline stabilizer was reported and
was attributed to the morphological structure, small particle size, uniform dispersion in large
quantities of Pt NPs and good interfacial interaction between the Pt NPs and rGO hybrid. [9]
Jafri et a.l synthesized Pt nanoparticles supported on nitrogen doped graphene by thermal
exfoliation of graphite oxide. This was performed by dispersing the Pt nanoparticles on the
support using the sodium borohydride reduction process. Electrochemical characterization using
Pt/N-G and Pt/G as the ORR catalyst showed a maximum power density of 440mWcm−2
and
390mWcm−2
, respectively. The improved performance of Pt/N-G was attributed to the
incorporation of nitrogen in the C-backbone leading to increase in the conductivity of
neighbouring C atoms [10].

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The fast electron transport mechanism ascribed to the presence of graphene facilitates and speeds
up the Oxygen Reduction Reaction in fuel cells. Min Ho Seo et al explored the use of graphene
supported electrocatalysts for the oxygen reduction reaction (ORR) in alkaline medium. Both Pd
and Pt nanoparticles with a mean diameter of 1.8 nm were dispersed on graphene sheets (GNSs)
through chemical synthesis at a metal loading of 60 wt%. The ORR activity of these catalysts
was investigated in a 0.1 M NaOH solution and was reported to show significantly high activity
for ORR [11].
Several synthesis procedures have been adopted to prepare Pt-Graphene composites. Sequential
reduction involves the separate reduction of graphene from graphene oxide and platinum from its
precursor salt and the subsequent preparation of the composite. An alternate reduction method
involves the simultaneous reduction of both metal nanoparticles and the graphene oxide
[12][13][14]. In the microwave assisted synthesis methods, irradiation helps heating of the
reaction mixture uniformly and rapidly, allowing large-scale and efficient production of
graphene–metal composites [15].
Other techniques for metal nanoparticle decoration on the graphitic nanostructure include
electro-deposition [16], photochemical [17], and solventless bulk synthesis [18]. Although these
methods present some advantages over solution-based techniques, they are expensive and energy
consuming.
In this study, a single step modified polyol method has been employed to disperse Pt
nanoparticles on reduced graphene oxide using H2PtCl6 as a Pt precursor, and ethylene glycol
and NaBH4 as a reducing agent for Pt precursor and graphene oxide, respectively. This simpler
one-pot synthesis approach has been adopted to obtain a uniform dispersion of platinum
nanoparticles on the graphene support [7].

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4. Experimental Section
4.1 Materials. Graphite, H2PtCl6, NaBH4 and were obtained from Alfa-Aeser while KMnO4,
NaNO3, C2H6O2, H2O2 and were obtained from Merck. Nafion and N2H4-H2O was obtained from
Sigma Aldrich.
Graphite oxide (GO) was prepared by a modified Hummer’s method (Figure 2) [19]. In brief,
this method involves vigorous stirring of a mixture of graphite powder and sodium nitrate with
concentrated sulfuric acid followed by oxidation by potassium chlorate. The resulting solution is
then washed with deionized water, subject to several cycles of centrifugation and dried to obtain
graphite oxide flakes. This is stored and dispersed in solvents as needed.
Figure 2. Schematic diagram of Hummer’s method [21]
4.2 Synthesis of Pt/graphene nanocomposite.50mg of graphite oxide obtained from the
modified Hummer’s method was dispersed in ethylene glycol (1mg/ml). It was then
ultrasonicated for one hour to exfoliate the graphite oxide to graphene oxide. A homogeneous
graphene oxide slurry was obtained. Subsequently, 120mg of H2PtCl6.6H2O was dissolved in the
graphene oxide slurry and sonicated. The pH of the resultant solution was adjusted to 10 by the
addition 100mg of NaOH and stirring vigorously. The solution was transferred to a round bottom
flask and the temperature was increased up to 120˚C. When the reaction temperature reached
120˚C, 2ml of NaBH4 (20mg/ml) was added dropwise and refluxed at 120˚C for 1 hour. After
complete reduction of H2PtCl6.6H2O to platinum, the solution is cooled and neutralized using 0.1
M HCl (aq) so as to obtain a pH of 7, washed and centrifuged 3 times with water.

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The residue was suspended in 20ml of ethanol and left to dry in the oven at 80˚C for 12 hours.
After drying, the catalyst was ground to a fine powder and used for characterization.
4.3 Synthesis of Pt/carbon nanocomposite. The Pt/C nanocomposite was synthesized through
the sulfito complex route followed by reduction using sodium borohydride. 156.10mg of the
platinum precursor, Na6Pt(SO3)4.6 H2O was dissolved in 25ml of 0.5M H2SO4. The platinum
sulfito complex was added to 200ml of water and the temperature of the mixture was raised to
80˚C. Separately, 40mg of carbon black was dispersed in 20ml of H2O and sonicated for 10
minutes. This was further added to the above platinum solution and stirred continuously. Next,
25ml of H2O2 (30%) was added dropwise for 1½ hours. 25ml of NaBH4 (1mg/ml) was then
dropped into the solution over half an hour to reduce the platinum. The resultant solution was
cooled, washed and centrifuged thrice. The residue was dispersed in 20ml of ethanol and dried in
the oven at 80°C for 12 hours. After drying, the catalyst was ground to a fine powder and used
for characterization.
4.4 Physical and Electrochemical Characterization. The XRD was conducted on a Rigaku
SmartLab® X-ray diffractometer using Cu K radiation (= 0.15406 nm). The HRTEM images
were recorded with JEOL JEM 2100 Field emission electron microscope.
Electrochemical measurements were performed in a three-electrode electrochemical cell using
WaveDriver 20 Bipotentiostat/Galvanostat system from Pine Research Instrumentation, USA.
Platinum served as the counter electrode while Ag/AgCl system was used as the reference
electrode.
4.5 Electrode Preparation: Catalyst ink was prepared by dispersing catalyst powder (5.0 mg)
ultrasonically in 5.0 mL distilled water to form a homogeneous black suspension. Then, 7μL of
Nafion was dropped into the dispersion followed by 10ml of isopropyl alcohol and sonicated for
half an hour. A volume of 56μL of this solution was drop cast onto a clean glassy carbon
electrode (0.196mm2
area) and used as the working electrode. For cyclic voltammetry
measurements, the working electrode was immersed in 0.1 M H2SO4 saturated with highly
purified argon and scanned between -200mV to 800mV. Before the conduction of oxygen
reduction experiment, the solution was purged with 99.9995% O2 for about 10 min. The ORR
was carried out in the same potential range as the cyclic voltammogram at the rate of 20mVs-1
.

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5. Results and Discussions
5.1 Physical Characterization
5.1.1 TEM Analysis
The surface morphology and dispersion of the platinum were determined from TEM analysis.
For the analysis, a drop of colloidal sample was dispersed on a lacey-carbon grid and dried in air.
The TEM images revealed the formation of platinum nanoparticles. Figure 3(a) displays the
reduced graphene oxide sheets on the lacey grid supporting a dispersion of platinum particles.
The folds depict an overlap of few graphene layers at the boundary.

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Figure 3. (a) TEM image of reduced graphene oxide and platinum nanoparticles. (b) Uniform
dispersion of platinum nanoparticles (c) HRTEM of higher magnification (d) selected area
electron diffraction (SAED) pattern of the Pt/rGO composite.
Figure 3(b) shows a uniform, well ordered distribution of platinum nanoparticles on the surface
of the graphene sheets. The high percentage of metal loading is clearly observed. TEM images of
relatively higher magnification from figure 3(c) enabled a rough estimation of the particle size.
The average size of the particles was expected to be ~4-7 nm. The selected area electron
diffraction (SAED) pattern (Figure 3(d)) shows bright rings due to the presence of platinum.
The sharp hexagonal spot patterns correspond to the presence of the graphene sheets.

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5.1.2 XRD Analysis
Figure 4 depicts the XRD patterns obtained from the characterization of reduced graphene oxide
(rGO), 50 wt% Pt/C and 50 wt% Pt/rGO composite catalyst. A sharp peak is observed at
26.6°from the XRD pattern of the rGO. With the reduction of graphite oxide, this peak is
expected to be close to that of graphite structure as seen. Both Pt/C and Pt/rGO displayed
diffraction peaks at 2 theta angles of 39.8°, 46.3°, 67.6° and 81.4° can be indexed to the (1 1 1),
(2 0 0), (2 2 0) and (3 1 1) planes of the face-centered cubic (FCC) Pt crystal (JCPDS card NO.
04-0802.)
Figure 4. X-ray diffraction pattern of rGO (blue), Pt/rGO (red), Pt/C

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From the XRD, particle size of the platinum nanoparticles dispersed on the graphene support was
estimated using Scherrer’s equation (Equation1).
Equation1.  = k/ cos
  is the particle Size
 k is a dimensionless shape factor , typical taken as 0.94, but varies with the actual shape
of the crystallite;
 λ is the wavelength of the X-ray = 0.154 nm for Cu k radiation;
 β is the line broadening at half the maximum intensity (FWHM), after.
 θ is the Bragg angle.
The plane (2 2 0) was used for calculation at a 2θ angle of = 67.23°. The FWHM, β , was found
to be 2.30576°. Upon calculation using equation1,  = 4.32 nm. The size of Pt particles indicated
a good dispersion on the graphene support.

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5.2 Electrochemical Characterization
5.2.1 Cyclic Voltammetry
To investigate the utilization and electrochemical activity of the Pt/rGO composite catalyst in
comparison to the Pt/C catalyst, a cyclic voltammogram was performed in 0.1 M H2SO4
saturated with highly purified argon at a sweep rate of 20mVs-1
. As the potential was increased
in the forward scan, hydrogen desorption peaks were observed in the potential window -0.2 to
0.05V (Figure 5). The potential range 0.05 to 0.5V corresponds to the charge of the double
layer by the oxygenated groups on the carbon/graphene support surface. The oxide formation
region is between 0.55-0.8V to form platinum oxides. In the reverse scan, oxygen evolution from
the platinum surface results in a reduction peak at 0.51V as Pt-O reduces to platinum metal. As
lower potentials from 0.05 to -0.2V on the reverse scan, peaks corresponding to the
adsorption of hydrogen on the surface of platinum are observed. The potential range for
hydrogen adsorption/ desorption processes comprises the hydrogen underpotential deposition
(HUPD) region.
Figure 5. Cyclic voltammograms of Pt/rGO (black) and Pt/C recorded at room temperature in an
argon saturated solution of 0.1M H2SO4

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A clear distinction was observed in the voltammograms of Pt/C and Pt/rGO. The double layer
current in the potential regime 0.05-0.5V in the case of the Pt/rGO composite is significantly
higher as when compared with conventional Pt/C (with equal Pt loading). Graphene is known to
display a high interfacial capacitance partly due to its large specific surface area. The enhanced
double layer consequently renders a higher HUPD current.
Electrochemically active Surface Area: The electrochemically active surface area (ECSA) is
an important parameter that provides information about the number of available active sites. It
accounts not only for the catalytic surface area available for charge transfer but also the access of
a conductive path for electron transfer between the catalyst and the electrode surface. Hydrogen
adsorption/desorption region in an electrochemical system is commonly used to evaluate the
ECSA. The area under the curve is a measure of the hydrogen desorbed, which provides an
estimate of the ECSA.
Equation2 below is commonly employed to calculate the effective surface area.
Equation2. ECSA [cm2
Pt/g Pt] = charge [Qh μC/cm2
]/ (210 [ μC/cm2
]*electrode loading
[gPt/cm2
])
 QH- average charge integrated from the voltammogram of the adsorbtion/desorbtion
hydrogen process on the CV curve (mC)
 constant 210 shows the charge in theoretical calculation to oxidize a single hydrogen
layer adsorbed on bright platinum (mC)
 mPt is the platinum loading on the surface sample (g cm−2
)
The ESCA of the Pt/rGO composite was calculated to be 61.67 m2
/g Pt while that of Pt/C was
35.51 m2
/g Pt. This result indicates a smaller particle size and a far better utilization of Pt in the
Pt/rGO nanocomposites which is essential for improving the practical performance of the
PEMFCs.

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The following cyclic voltammogram, (Figure 6.) depicts the cyclic voltammogram of Pt/rGO
nanocomposite with different Pt loading. It was observed that graphene largely masks the Pt
features for loadings less than 50%. This is due to the high double layer capacitance of the
graphene support.
Figure6. Cyclic voltammograms of Pt/rGO with platinum loading of 20% (red) and 50% (blue)
recorded at room temperature in an argon saturated solution of 0.1M H2SO4

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5.2.2 Oxygen Reduction Reaction
The oxygen reduction reaction was carried out in the potential regime -200 to 800mV in an
oxygen purged solution of 0.1M H2SO4 to compare the activity of Pt/C and Pt/rGO catalyst.
From the ORR depicted in Figure 7, it is observed that the curves are comparable. In the case of
the Pt/rGO composite however, current decays more rapidly as the potential is increased between
-200 and 600mV. This can be ascribed to the presence of unreduced functional groups on the
graphene sheets that hinder the diffusion of oxygen to the surface of the electrocatalyst.
Figure7. ORR polarization curves of Pt/rGO and Pt/C catalyst recorded at room temperature
with a sweep rate of 20mVs-1
in O2-saturated 0.1 M H2SO4 solution.
At higher potentials in the oxide formation region, it is observed that the Pt/rGO current is
marginally higher.

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The effect of varying the platinum loading on the catalytic performance was also studied. It is
seen from Figure 8 that an increase in the metal loading from 20% to 50% resulted in an
increase in the limiting current and half wave potential.
Figure8. ORR polarization curves of Pt/rGO catalyst at metal loading of 20% (black) and 50%
(blue) recorded at room temperature at a rate of 20mVs-1
in O2-saturated 0.1 M H2SO4 solution.

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6. Conclusions
1. A single step modified polyol method was adopted to prepare Pt/rGO composite. This
strategy allowed for efficient synthesis of highly loaded Pt catalyst with small
nanoparticle size and uniform particle dispersion.
2. The platinum features were masked at metal loadings less than 50% on graphene
supports. This is due to the high double layer capacitance of graphene. The requirement
of high metal loadings is a potential area for future research efforts to realize the
application of Pt/rGO composite electrocatalysts.
3. Comparison of the effective surface area of Pt/rGO to that of Pt/C with similar metal
loading revealed the presence of increased number of active sites and higher utilization of
the platinum supported on graphene.
4. Comparison of the oxygen reduction reaction however did not show any significant
improvement in activity of the Pt/rGO composite. This was attributed to the presence of
oxygen moieties on the surface of partially reduced graphene hampering mass transport
of oxygen to the electrode.
The effort to utilize graphene as an alternative support material for platinum catalysts in
fuel cells showed both promise as well as the challenges involved in leveraging the
theoretical properties of high specific surface area, thermochemical stability and
conductivity of graphene that suggest its use as an excellent catalytic support. Further
work is necessary to develop strategies to improve the sluggish oxygen reduction kinetics
and reduce the precious metal loading for to make it an economical and viable alternative
to the presently used carbon supports.

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7. References
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& K.P. Loh, Nat. Commun, 2012, 3, 1
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