Effect of size and oxidation state of platinum nanoparticles on the electrocatalytic performance of graphene-nanoparticle composites

Article abstract of DOI:10.1039/c5ra17087g

A surfactant and stabilizer free graphene-based composite with Pt nanoparticles is considered to be a promising electrocatalyst with greatly improved performance. Here we show that both the size and oxidation state of Pt nanoparticles can significantly influence the electrocatalytic performance of the nanocomposite. We have synthesized Pt-graphene nanocomposites with varied sizes and oxidation states of Pt nanoparticles and test their catalytic activity towards methanol electro-oxidation. We found that the size <1.5 nm with mixed oxidation offers methanol oxidation at lower onset potential and with better tolerance to CO poisoning. However, these benefits are lost due to catalyst durability and thus catalytic current decays rapidly with time. As the nanoparticle size increases in the range of 2-5 nm this onset potential increases, CO tolerance decreases but the catalytic current becomes more stable with time. Thus an optimum nanoparticle size of 2.2 nm shows the best catalytic activity and durability. The oxygenic platinum with variable oxidation states offers stable grafting with the graphene surface, prevents active Pt (0) sites and assists for better CO tolerance. This result would be useful in the design and development of electrocatalysts with better performances.

Full text of DOI:10.1039/c5ra17087g

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Effect of size and oxidation state of platinum
nanoparticles on the electrocatalytic performance
Cite this: RSC Adv., 2015, 5, 85196
of graphene-nanoparticle composites†
*
Avijit Mondal and Nikhil R. Jana
A surfactant and stabilizer free graphene-based composite with Pt nanoparticles is considered to be
a promising electrocatalyst with greatly improved performance. Here we show that both the size and
oxidation state of Pt nanoparticles can significantly influence the electrocatalytic performance of the
nanocomposite. We have synthesized Pt–graphene nanocomposites with varied sizes and oxidation
states of Pt nanoparticles and test their catalytic activity towards methanol electro-oxidation. We found
that the size <1.5 nm with mixed oxidation offers methanol oxidation at lower onset potential and with
better tolerance to CO poisoning. However, these benefits are lost due to catalyst durability and thus
catalytic current decays rapidly with time. As the nanoparticle size increases in the range of 2–5 nm this
onset potential increases, CO tolerance decreases but the catalytic current becomes more stable with
time. Thus an optimum nanoparticle size of 2.2 nm shows the best catalytic activity and durability.
The oxygenic platinum with variable oxidation states offers stable grafting with the graphene surface,
prevents active Pt (0) sites and assists for better CO tolerance. This result would be useful in the design
and development of electrocatalysts with better performances.
Received 24th August 2015
Accepted 25th September 2015
DOI: 10.1039/c5ra17087g
www.rsc.org/advances
performance by lowering the accessibility of surface metal
atoms.13
1. Introduction
One signicant advantage of graphene as a solid support is
that it can stabilize ultra-small nanoparticles as well as provides
accessibility of nanoparticle surface to reactants.12,14–17 However,
commonly used synthesis approach involves physical or
chemical linking between graphene and nanoparticle via
hydrophobic interaction, electrostatic interaction or covalent
bonding and in most cases additional linker molecule/
surfactant/polymer are used to stabilize the nano-
composite.7,18–20 We have recently shown that surfactant free
metal–graphene nanocomposite can be prepared by reacting
partially reduced colloidal graphene oxide with the respective
colloidal metal oxide/hydroxide and they can be used as high
performance electrocatalyst.21 This approach produces Pt–gra-
phene nanocomposite which is composed of highly dispersed
Pt–PtII–PtIV based nanoparticle of 2.2 nm size and produces
high and stable catalytic current for ethanol and formic acid
oxidation. This size of Pt nanoparticle is smaller than conven-
tionally used colloid-chemical roots12,18,22 along with mixed
oxidation states and thus produced better performance.
However, effect of further reduction of size of Pt nanoparticle
and effect of nanoparticle size is not studied. Earlier reports
demonstrate the particle size effect and shows that atomic layer
or subnanometer Pt clusters or smaller Pt nanoparticles can
produce even better catalytic current but the catalytic current is
shown to decay rapidly with time.23,24
Graphene based composites with metal nanoparticles have
been widely used in fuel cell catalysis, photocatalysis and
catalytic organic transformation.1–3 A variety of noble metals
such as Pt, Pd, Au and their alloys have been used as electro-
catalysts for the oxidation of methanol, formic acid, ethanol
and oxygen reduction reaction.4–8 It is reported that graphene
offers an ideal catalyst support due to the outstanding
mechanical strength, chemical and thermal stability, high
surface area and high conductivity.9 The electrocatalytic
performance of these graphene-based nanocomposites depends
on the nature of the metal, size of the nanoparticles, loading of
nanoparticles and stability of the nanocomposite. It is reported
that smaller size of nanoparticle can greatly improve the cata-
lytic performance.8,10,11 However, catalytic current decreases
with time due to stability issues of composite and nanoparticles
either aggregate or detach from composite.12 In addition pres-
ence of surfactant and polymer stabilizer (that are used for
nanocomposite synthesis) can severely limit the catalytic
Centre for Advanced Materials, Indian Association for the Cultivation of Science,
Jadavpur, Kolkata-700032, India. E-mail: camnrj@iacs.res.in; Fax: +91-33-
24732805; Tel: +91-33-24734971
† Electronic supplementary information (ESI) available: Details of
characterization of nanocomposites, electrocatalytic oxidation of methanol by
nanocomposites with Pt nanoparticle size of 0.95 nm, 1.5 nm and 2.0 nm and
electrochemically active surface area measurement for three nanocomposites.
See DOI: 10.1039/c5ra17087g
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Here we have studied both the effect of Pt nanoparticle size prepared basic platinum salt solution was mixed with 1.2 mL of
as well as oxidation state on the electrochemical performance of colloidal solution of partially reduced graphene oxide. The
Pt–graphene nanocomposite. We have synthesized different mixture was kept under constant stirring for 12–24 h to prepare
Pt–graphene nanocomposites with varied Pt nanoparticle size different PtGN nanocomposites. The PtGN-1 was prepared with
from 1–5 nm and test their catalytic activity towards methanol 18 h incubation. Other two PtGN nanocomposites were
electro-oxidation under acidic condition. Methanol electro- prepared with 12 h and 24 h incubation. If the basic Pt salt
oxidation is selected because the direct methanol fuel cells is solution is reacted with reduced graphene oxide for 12 h then
considered as most attractive among various fuel cell technol- PtGN is formed with the Pt size of 0.95 nm. If reacted for 18 h
ogies due to high energy density, high efficiency, low cost and then PtGN-1 with Pt nanoparticle of 1.5 nm is formed. If reacted
eco-friendly nature.25–27 It is found that the Pt is the most throughout 24 h then PtGN with Pt nanoparticle of 2 nm is
effective one and playing important role to achieve faster formed. Resultant PtGN-1 was separated by centrifuge and then
kinetics of anodic reaction of methanol electro oxidation.28 We dispersed in fresh water. This separation–redispersion was
found that Pt nanoparticle of <1.5 nm size with mixed oxidation repeated for 3–4 times and nally PtGN-1 was dispersed in fresh
state offers the advantage of lower onset potential of methanol water for further experiments.
oxidation and better tolerance to CO poisoning but these
benets are lost with repeated electrochemical cycle due to poor
durability of catalyst. As the nanoparticle size increases to
2.2 nm the onset potential of methanol oxidation is increased
and CO tolerance is decreased but offers higher catalytic current
and better current stability. Further increase of particle size
lowers the catalytic performance due to higher onset oxidation
potential, lower CO tolerance and poor current stability. The
oxygenic platinum with variable oxidation state offers stable
graing with graphene surface, prevents active Pt (0) sites
during repeated cycles and facilitate conversion of CO to CO2.
This study shows the role of Pt nanoparticle size and oxidation
state in the electrochemical performance of nanocomposites
and would help in designing electrocatalyst with better
performance.
2.4 Preparation of PtGN-2
In this case 1.5 mL of H2PtCl6 solution (10 mM) was mixed with
30–50 mL NaOH solution (1 M) and kept undisturbed for 1–2
days at room temperature. This allows the formation of
colloidal platinum oxide nanocrystals.30,31 Next, 400 mL of this
colloidal platinum oxide solution was added to 1.2 mL of
colloidal solution of partially reduced graphene oxide and kept
under stirring for 7–8 h. Resultant PtGN-2 was separated by
centrifuge and then dispersed in fresh water. This separation–
redispersion was repeated for 3–4 times and nally PtGN-2 was
dispersed in fresh water for further experiments.
2.5 Preparation of PtGN-3
In this case 400 mL H2PtCl6 (10 mM) was mixed with 1.2 mL of
partially reduced graphene oxide solution. Next, 200 mL sodium
borohydride solution (5 mg mLÀ1) was added and stirred for
one hour. Resultant PtGN-3 was centrifuged and washed several
times with pure water. The nal products were dispersed in 400
mL water via ultrasonication.
2. Experimental section
2.1 Materials and reagents
Graphite powder (<20 mm), hydrazine monohydrate (98%),
hexachloroplatinic acid hexahydrate (H2PtCl6$6H2O) and Pt on
graphitized carbon (Pt/C, 20 wt%) were purchased from Sigma-
Aldrich and used as received. Methanol and sulphuric acid
(98%) were purchased from Merck, India. All other reagents
were of analytical grade and used without further purication.
All the solutions were prepared by using double-distilled water.
2.6 Materials characterization
Transmission electron microscopic (TEM) samples were
prepared by putting a drop of particle dispersion on carbon
coated copper grid and observed under FEI Tecnai G2 F20
microscope. Raman spectra were recorded using Agiltron R3000
Raman spectrometer with 785 nm excitation laser and X-ray
photoelectron spectroscopy was performed using Omicron
(Serial No. 0571) X-ray photoelectron spectrometer. Amount of
metal present in the composite materials was measured by
Optima 2100 DV (Perkin Elmer) inductively coupled plasma
atomic emission spectroscopy (ICP-AES).
2.2 Synthesis of partially reduced graphene oxide
Graphene oxide was prepared by modied Hummer's method29
and stock solution was made with concentration of ꢀ1.5 mg
mLÀ1. Next, ꢀ20 mL of hydrazine was added to 1.2 mL of gra-
phene oxide solution and heated at 80–90 ꢁC with constant
stirring for 45 minutes.21 Resultant partially reduced graphene
oxide was puried by adding 100 mL of NaCl solution
(160 mg mLÀ1) followed by centrifugation and repeated washing
of precipitate with pure water to remove any free reagents. The
residue was redispersed in 1.2 mL distilled water by
ultrasonication.
2.7 Electrochemical measurements
A glassy carbon electrode of 3 mm in diameter (surface area of
0.07 cm2) was carefully polished with 1, 0.3, and 0.05 mm
alumina powder, sequentially, until a mirror nish was ob-
tained. Next, the electrode was ultrasonically cleaned with
ethanol and deionized water and dried in air at room temper-
2.3 Preparation of PtGN-1
In a vial 1.5 mL of H2PtCl6 solution (10 mM) was mixed with ature. Then the electrode was immersed in 0.5 M H2SO4 and was
30–50 mL NaOH solution (1 M). Next, 400 mL of this freshly voltammetrically scanned from À0.4 to 1.2 V (vs. Ag/AgCl) at
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a rate of 100 mV sÀ1 to clean the surface. Finally, the composite
dispersion was dropped onto the GCE surface, dried in air at
room temperature for 2 h and used for electrochemical
measurements. The loading of Pt in PtGN-1/2/3 modied GCE
were 2.2 mg, 5.2 mg and 7.5 mg, respectively. Electrochemical
measurements were performed with
a CHI633D Electro-
chemical Analyzer. A conventional three-electrode system was
used for all electrochemical experiments, which consisted of
a platinum wire as auxiliary electrode, an Ag/AgCl/saturated KCl
as reference electrode and modied glassy carbon as working
electrode. All experiments were conducted at room tempera-
ture. For the electro-oxidation of methanol, the cyclic voltam-
mograms were recorded at a sweep rate of 50 mV sÀ1 in
a mixture of H2SO4 (0.5 M) and methanol (1 M). All the current
were normalized with the loading amount of catalyst in the
electrode.
3. Results and discussion
3.1 Synthesis and characterization of platinum–graphene
Fig. 1 TEM image at different magnifications and corresponding size
distribution histograms of three nanocomposites. Average sizes of
nanoparticles are 1.5 nm, 2.2 nm and 5.1 nm for PtGN-1, PrGN-2 and
PrGN-3, respectively.
nanocomposite (PtGN)
We have focused on three different Pt–graphene nano-
composites designated as PtGN-1, PtGN-2 and PtGN-3 as shown
in Scheme 1. The synthetic procedure for three composites and
details of their characterization are shown in Scheme 1, Fig. 1
and 2, Table 1 and ESI, Fig. S1–S7.† Colloidal graphene oxide is
synthesized by Hummer's method and then transformed into
partially reduced graphene oxide via hydrazine reduction.21,29
Reaction of partially reduced graphene oxide with the basic
solution of platinum salt produces PtGN-1. Platinum salt is
transformed into colloidal platinum oxide by incubating in
basic solution for 1–2 days and then reacted with partially
reduced graphene oxide in preparing PtGN-2. Platinum salt is
reduced by NaBH4 in presence of partially reduced graphene
oxide that produces PtGN-3. All the nanocomposites are iso-
lated as solid by centrifugation and redispersed in fresh water
via sonication. The colloidal form of nanocomposites is used as
stock solution for their deposition on electrode surface.
We have extensively investigated the size of platinum-based
nanoparticles present on each nanocomposite. Fig. 1 and ESI,
Fig. S1–S5† shows the TEM image of nanocomposites. Gra-
phene akes are clearly observed with uniformly distributed
platinum nanoparticles, particularly for PtGN-1 and PtGN-2.
However, nanoparticles present in PtGN-3 are observed with
sever aggregation and distribution of nanoparticle are relatively
less uniform. High resolution TEM image shows that there is
a clear difference in nanoparticle size in three nanocomposites.
The average sizes of nanoparticles are 1.5 nm, 2.2 nm and 5.1
nm for PtGN-1, PtGN-2 and PtGN-3, respectively. Other two
PtGN with Pt nanoparticle of 0.95 nm and 2.0 nm has also been
synthesized using the condition similar to PtGN-1.
X-ray photoelectron spectra (XPS) of PtGN-1 have been
measured to investigate the chemical signature and oxidation
state of Pt and carbon (Fig. 2 and ESI, Fig. S6 and S7†). Two
oxidation states of Pt are observed in the deconvoluted Pt 4f
Fig. 2 (a) XPS characterization of PtGN-1 showing that platinum
nanoparticles consist of PtII and PtIV. Deconvoluted Pt 4f spectrum
Scheme 1 Schematic representation of synthesis steps for three displays four fitted signals at 72.2 eV (Pt 4f7/2) and 75.5 eV (Pt 4f5/2
)
different platinum nanoparticle–graphene nanocomposites. Partially corresponding to PtII and 73.4 eV (Pt 4f7/2) and 77.7 (Pt 4f5/2) attributed
reduced graphene oxide is reacted with platinum salt for preparation to PtIV, (b) XPS characterization of C 1s of PtGN-1 showing signature of
of PtGN-1, partially reduced graphene oxide is reacted with colloidal different types of carbon and (c) Raman spectra of PtGN-1, PtGN-2,
platinum oxide for preparation of PtGN-2 and platinum salt is reduced PtGN-3 and partially reduced graphene oxide showing the defective
by NaBH4 in presence of partially reduced graphene oxide for the bands at 1300 cmÀ1 and graphitic bands at 1600 cmÀ1 and the
preparation of PtGN-3.
respective intensity ratio (ID/IG value).
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Table 1 Property of three different nanocomposites used in this study (If/Ib represents ratio of forward to backward current intensity)
Pt loading
(wt%)
Pt oxidation
states
Electrochemical surface
Forward peak
position
If/Ib at
Samples
area (m2 gÀ1
)
15th and 105th cycle
PtGN-1
PtGN-2
PtGN-3
10.9
22.5
25
PtII (63%), PtIV (37%)
Pt0 (62%), PtII (22%), PtIV (16%)
Pt0 (100%)
22
64
32
0.56 V
0.69 V
0.64 V
1.87, 1.6
0.87, 0.79
0.75, 0.81
spectra. The most intense doublet peaks at 72.2 eV (Pt 4f7/2) and
75.5 eV (Pt 4f5/2) are attributed to the PtII and other lower
intense doublet peaks at 73.8 eV (Pt 4f7/2) and 77.7 eV (Pt 4f5/2
)
are attributed to the PtIV species.32 The relative percentage of PtII
and PtIV are calculated from the peak areas which are 63% and
37%, respectively. The deconvoluted C 1s spectra of PtGN-1
shows signature corresponding to C]C/C–C, C–O, C–O–C/C]
O and O–C]O groups with respective peaks at 284.2 eV,
285.2 eV, 286.1 eV and 287.6 eV, respectively.33 Our earlier XPS
study shows that PtGN-2 contain mixture of Pt (0), Pt (II), Pt (IV)
and PtGN-3 contain Pt (0) only.21 The oxidation states of all the
composites are summarized in Table 1.
The Raman spectra of composite displays G band at
1600 cmÀ1 corresponding to the sp2 hybridized carbon atoms
and the D band at 1310 cmÀ1 corresponding to disruption of the
sp2 hybridized carbon atoms. (Fig. 2) The intensity ratios of
D band to G band (ID/IG) for different composites lies between
1.98–2.3, which are little larger than 1.9 corresponding to
partially reduced graphene oxide. This suggests the increased
defects in graphene structure aer incorporation of metal
nanoparticle.34,35
Fig. 4 (a–c) Electrocatalytic oxidation of methanol by PtGN-1, PtGN-
2 and PtGN-3 nanocomposites, respectively. In each case glassy
carbon electrode is modified with nanocomposites and then oxidation
of 1 M methanol in 0.5 M H2SO4 is performed at a scan rate of 50 mV
s
À1. The current density shown in the CVs is normalized by the
respective mass of Pt loading. (d) Amperometric i–t study for
comparing the stability of three catalysts in 1 M methanol and 0.5 M
H2SO4 at 0.5 V.
carbonyl that involves removal of four electrons. The backward
scan shows a peak in the range of 0.3–0.4 V which corresponds
to the oxidation of adsorbed CO to CO2 that makes the catalyst
free from CO poisoning. This result indicates that catalytic
methanol oxidation by all nanocomposites follow CO poisoning
pathway and electrochemical reactions can be summarized in
following two equations (Scheme 2).36,37
3.2 Size and oxidation state effect of Pt on electrochemical
performance
Electrochemical oxidation of methanol in H2SO4 media has
been used to study the electrocatalytic performance of three
different composites. Cyclic voltammetry (CV) has been used to
systematically study the catalytic activity and onset potential of
methanol oxidation. (Fig. 3 and 4 and ESI Fig. S8 and S9†) Two
typical oxidation peaks appear on the CV for all the catalyst,
arising due to the oxidation of methanol and its intermediates.
The forward scan shows a peak in the range of 0.5–0.7 V which
corresponds to oxidation of adsorbed methanol to metal
Anodic reaction:
CH3OH / Pt(CH3OH)ads / 4H+ + 4eÀ + Pt–CO
(1)
(2)
Cathodic reaction:
Pt–CO + H2O / Pt + CO2 + 2H+ + 2eÀ
Successive CV for rst 10 cycles has been shown for three
composites. (Fig. 3) In the case of PtGN-1 and PtGN-2, the
forward and backward peak gradually becomes intense with
increasing cycles. These results indicate the active catalyst in
the form of Pt (0) is generated during electrochemical cycles and
becomes stabilized. However, in the case of PtGN-3 those peaks
Fig. 3 First 10 electrochemical potential cycles towards 1 M methanol
oxidation by three different nanocomposites. In each case glassy are intense from the rst cycle, suggesting that catalytically active
carbon electrode is modified with nanocomposites and then oxidation
Pt (0) is present from the rst cycle. This observation is corrob-
orated with oxidation state of Pt nanoparticle in three compos-
ites. (Table 1) Electrochemical surface area has been measured
of 1 M methanol in 0.5 M H2SO4 is performed at a scan rate of 50 mV
s
À1. The current density shown in the CVs are normalized by the
respective mass of Pt loading.
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PtGN-2 offers high and stable current with best catalyst
durability.
The differential electrocatalytic performance by three cata-
lysts can be explained based on the size and oxidation states of
platinum nanoparticle. The well-known mechanism of meth-
anol electro-oxidation in acidic media involves CO intermediate
as shown in eqn (1) and (2). Platinum nanoparticles present in
PtGN-1 and PtGN-2 are composed of mixed valence states and
active Pt (0) states are surrounded with oxygen of Pt (II) and Pt
(IV). (Scheme 2) These oxygenic platinum offers stable graing
with graphene surface, prevents destruction of active Pt (0) sites
and facilitate conversion of CO to CO2.38 Lower onset potential
and lower CO poisoning by PtGN-1 is linked to the smallest size
of nanoparticle. However, as the reaction progress, these
advantages are lost due to gradual decrease of active Pt (0) sites
formed during rst 10 cycles by the formation of irreversible
oxide layer which prevent the oxidation of methanol and
increase of nanoparticle size.39 Poorest catalytic activity of PtGN-
3 can be explained due to largest size of nanoparticles, severe
aggregation between nanoparticles and presence of exclusive Pt
(0) which is highly prone to attack by CO species. All these
results can explain the 2.2 nm Pt with mixed oxidation state as
the optimum size for best electrocatalytic performance. This
size offers moderate onset potential of methanol oxidation with
lower CO tolerance and the active Pt (0) is best stabilized.
Further lowering of size can offer methanol oxidation at low
over-potential and with high CO tolerance but the catalytic
activity and current stability is compromised. Poor performance
by larger size Pt is due to higher over-potential of methanol
oxidation, lower CO tolerance, low accessibility of active sites
and poor current stability.
Scheme
nanocomposite.
2 Mechanism of methanol oxidation by Pt–graphene
for three catalysts and is highest for PtGN-2 (ESI, Fig. S9†) This
result indicates that PtGN-2 provides most accessible active sites.
This has been manifested from CV with the current densities
normalized against loading of Pt. (Fig. 4a–c) The current density
is highest for PtGN-2 and relatively stable aer repeated cycles as
compared to other two catalysts. The CO poisoning tolerance,
measured by the ratio of forward to backward peak current, is
signicantly higher for PtGN-1 as compared to other two cata-
lysts. For example, the ratio of forward to backward peak current
is 1.87 for PtGN-1 but the values are 0.87 and 0.75 for PtGN-2 and
PtGN-3, respectively. This tolerance of CO poisoning decreases
with repeated electrochemical cycles and the change of values
from 15th cycle to 105th cycle for each nanocomposites shows that
the effect is most prominent for PtGN-1. (Table 1) Amperometric
i–t study has been performed for three composites for the eval-
uation of catalyst stability. (Fig. 4d) The current density shows
most rapid decay for PtGN-1 during the initial period but for
other two composites the decay is slow. This result indicates that
deactivation of PtGN-1 is faster than other two composites.
Thus electrocatalytic methanol oxidation shows ve signi-
cant differences between three nanocomposites which are due
to the effect of Pt nanoparticle size and oxidation state. First,
catalytic current increases with increasing electrochemical cycle
for PtGN-1 and PtGN-2, but for PtGN-3 catalytic current is high
from beginning. This is because the active catalyst in the form
of Pt (0) is generated during electrochemical cycles in the case of
PtGN-1 and PtGN-2, but for PtGN-3 the active Pt (0) is already
present. Second, the forward oxidation peak of methanol is
lowest for PtGN-1 (0.56 V) but for other two composites the
values are very similar. (0.67 V and 0.64 V) which suggests that
anodic methanol oxidation is easier at PtGN-1 surface
compared to other composites. Third, CO poisoning tolerance
is signicantly higher for PtGN-1 as compared to other two
catalysts but this tolerance decreases with repeated electro-
chemical cycles and the effect is most prominent for PtGN-1.
This suggests that the advantage of high CO poisoning toler-
ance by PtGN-1 is lost rapidly during electro-catalysis. Fourth,
electrochemical surface area and normalized catalytic current
are highest for PtGN-2 as compared to PtGN-1 and PtGN-3. This
suggests that PtGN-2 offers most accessible active sites with best
overall catalytic performance. Fih, among all the catalysts the
4. Conclusion
In summary, we have synthesized graphene-based composite
with platinum nanoparticles where the size of nanoparticle
varies from 1 to 5 nm with different oxidation states of metal
and test their catalytic activity towards methanol electro-
oxidation. We found that 2.2 nm platinum nanoparticle with
mixed oxidation state offers best overall electro-catalytic
performance. This optimum size requirement can be
explained by effect of platinum nanoparticle size on methanol
oxidation over-potential, CO poisoning tolerance and catalyst
stability under repeated electrochemical cycle. Although the
smaller size offers low over-potential methanol oxidation and
high CO tolerance, the catalytic current rapidly decays with
repeated cycle. Poor performance by larger size Pt is due to
increased over-potential of methanol oxidation, lower CO
tolerance, low accessibility of active sites and poor current
stability. In the optimum size of 2.2 nm the advantages of
smaller size is partially compromised but accessibility of active
sites and catalyst stability are compensate them. Mixed
valence oxygenic Pt offers better CO tolerance and long term
catalyst stability. This understanding of nanoparticle size
effect would offer in the design of electrocatalyst with better
performance.
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