Study of FePt deposited reduced graphene oxide's utility as a catalyst towards oxygen reduction and methanol oxidation reactions

Article abstract of DOI:10.1039/c5ra05418d

Hydrogen and methanol fuelled polymer electrolyte fuel cell's (PEFC) penetration in the commercial market is slowed by the use of expensive Pt and PtRu as electrocatalysts. Transition metal based Pt alloy catalysts have historically struggled for durability in acidic environments. Reduced graphene oxide (RGO) supported Pt alloy catalysts have gained significant interest recently due to improvements in catalyst-support interaction that lead to better durability and performance. In this report we investigate the performance and durability aspects of FePt supported on RGO towards oxygen reduction and methanol oxidation reactions. PXRD and TEM results show that the FePt nanoparticle size is in the range of 4-7 nm and TGA measurements show that the metal loading of the catalyst is ～55%. Electrochemical measurements towards ORR reveal a significant improvement in activity and durability for FePtGO over commercial PtC and FePtC. The utilization of RGO as a support certainly increases the lifetime of transition metal-Pt alloys that are generally susceptible to durability issues under acidic environments in fuel cells.

Full text of DOI:10.1039/c5ra05418d

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Study of FePt deposited reduced graphene oxide's
utility as a catalyst towards oxygen reduction and
methanol oxidation reactions†
a
Cite this: RSC Adv., 2015, 5, 36993
R. Kannan, A. A. Silva,a F. M. Cardoso,a G. Gupta,a Z. Aslam,b S. Sharmaa
*
and R. Steinberger-Wilckensa
Hydrogen and methanol fuelled polymer electrolyte fuel cell's (PEFC) penetration in the commercial market
is slowed by the use of expensive Pt and PtRu as electrocatalysts. Transition metal based Pt alloy catalysts
have historically struggled for durability in acidic environments. Reduced graphene oxide (RGO) supported
Pt alloy catalysts have gained significant interest recently due to improvements in catalyst–support
interaction that lead to better durability and performance. In this report we investigate the performance
and durability aspects of FePt supported on RGO towards oxygen reduction and methanol oxidation
reactions. PXRD and TEM results show that the FePt nanoparticle size is in the range of 4–7 nm and TGA
measurements show that the metal loading of the catalyst is ꢀ55%. Electrochemical measurements
towards ORR reveal a significant improvement in activity and durability for FePtGO over commercial PtC
and FePtC. The utilization of RGO as a support certainly increases the lifetime of transition metal–Pt
alloys that are generally susceptible to durability issues under acidic environments in fuel cells.
Received 26th March 2015
Accepted 14th April 2015
DOI: 10.1039/c5ra05418d
www.rsc.org/advances
toward oxygen reduction.16 However, in perchloric acid medium
Pt (111) has shown better catalytic activity than Pt (100).
Introduction
Pt–M alloy catalysts have shown promise to increase the
catalytic activity of Pt towards ORR especially when M is Fe, Co
or Ni.3,5,17 Volcano plots describing specic activity as a function
of Pt's d-band value suggests Co as the best alloying transition
metal while Fe is not far behind with superior abundance than
Co.18 However, when transition metals such as Fe, Co or Ni are
used for alloying with carbon as the support, they are etched
and dissolved in the highly acidic PEFC operating conditions
leading to performance loss. Further, Fe and Co ions generated
during operation are known to damage the polymer electrolyte
membranes by producing hydrogen peroxide through Fenton's
reaction.19,20 Structural modications in the FePt alloy from a
face centred cubic (FCC) structure to face centred tetragonal
(FCT) structure has resulted in improved durability under acidic
conditions while issues associated with scaling up and sup-
porting on a carbon support remain unsolved.21 However, the
discovery of graphene and its evolution as a better support for
catalysts have rejuvenated this section of research and recent
reports suggest an increase in durability for transition metal
based Pt alloy catalysts under previously harsh fuel cell oper-
ating conditions.17,22–27 For example, Guo et al. have self-
assembled FePt nanoparticles, which were grown separately,
on a graphene support and observed improved catalytic activity
and durability.17 However, the catalysts prepared in these
studies are normally bigger than commercial catalysts and
results in a loss of electrochemical surface area. Hence, it is
imperative to study the catalytic activity of differently sized FePt
Among the energy conversion technologies, polymer electrolyte
membrane fuel cells (PEFCs) fuelled by hydrogen and methanol
are one of the front runners to replace the incumbent
combustion engines mainly in transportation applications.1
This is primarily due to their low temperature operation that
allows quicker start-ups and ease of handling. However, the cost
and durability of the technology still remains an issue despite
signicant effort for the past two decades, chiey due to the use
of expensive Pt as electrocatalyst for the sluggish oxygen
reduction reaction (ORR). Further, in direct methanol fuel cells
(DMFCs), catalyst poisoning by methanol crossover to the
cathode is another major issue for ORR catalysts. Efforts to
reduce the Pt use in the electrode involves: (1) reducing the Pt
particle sizes and controlling their shape, (2) alloying with
transition metal ions to modify the electronic structure of the
Pt, (3) core–shell structures with cheap metals as core and Pt as
shell and (4) alternative non-precious catalysts that replace Pt
altogether in the catalyst layer.2–15 For example, Pt nanocubes
have shown 4 times higher catalytic activity than Pt polyhedron
towards ORR in sulphuric acid medium as the preferential
attachment of sulphite ions on Pt (111) hinders its activity
aCentre for Fuel Cell and Hydrogen Research, School of Chemical Engineering,
University of Birmingham, Birmingham, UK. E-mail: chemkannan@yahoo.com
bLENNF, University of Leeds, Leeds, UK
† Electronic supplementary information (ESI) available: Detailed synthesis of
graphene oxide, CV data and XPS results. See DOI: 10.1039/c5ra05418d
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nanoparticles supported in RGO to investigate RGO's role as electrode and Pt mesh as counter electrode. Methanol oxidation
support for varying particle sizes. studies were carried out in 0.5 M CH3OH in 0.5 M H2SO4 elec-
Here we present the results of a new preparation method and trolyte. A Ag/AgCl electrode was used as the reference electrode
its effectiveness for the preparation of FePt/RGO catalysts. We and Pt mesh as the counter electrode. Pt mesh was heated to red
decided a step-wise addition of iron and platinum in order to hot condition before each fresh measurement to ensure it was
use Fe nanoparticle as nucleating sites for the Pt nanoparticle free of contaminants. The commercial PtC used in this work for
formation. The purpose of this procedure is to study the effect comparison is TEC10E50E (supplied by TKK and referred from
of a strong reducing agent in reducing the particle size and here as PtC-TKK). Cyclic voltammetry, linear sweep voltammetry
improving the performance. We have used cyclic voltammetry experiments were performed using an Autolab PGSTAT12/30/
and linear sweep voltammetry under various rotation rates for 302. A pine rotation controller was used for the rotating disc
the catalysts. Further, the catalytic activity of FePtGO towards electrode measurements.
methanol oxidation was also studied to analyse its suitability as
ORR catalyst in direct methanol fuel cells.
Results and discussion
PXRD measurements on FePtGO and FePtC revealed the
formation of polycrystalline Pt nanoparticles with (111) as the
Experimental
Synthesis of FePtGO and FePtC
major phase (Fig. 1a). This is the dominant phase under both
Iron(II) acetate Fe(OAc)2, K2PtCl6, NaBH4, were all procured
supports (GO and graphitic carbon) and it is clear that the
from Sigma Aldrich UK. Solvents and other chemicals used in
the experiments were of analytical grade purchased from Sigma
Aldrich. 5% Naon was procured from Electrochem Inc. Gra-
support material does not play a role in phase formation here.
Intensities (I111/I110) are in similar range for both catalysts
making it suitable for comparing their catalytic activity. The
phene oxide (GO) was prepared in-house by a modied
Hummers method.28 Detailed procedures can be found in the
ESI.† 100 mg of GO was dispersed in 50 mL of ethanol in a
round bottom ask (RBF). 50 mg of 1,10-phenanthroline (Sigma
Aldrich UK) dispersed in 50 mL of ethanol was added to the GO
solution. 78 mg of iron(II) acetate dispersed in 50 mL ethanol
was slowly added to the solution via a burette with continuous
stirring and a deep red colour indicating the formation of
Fe–phenanthroline complex was observed. An excess of NaBH4
dissolved in DI water was added slowly to this mixture and
stirring was continued for another two hours. The mixture was
ltered, washed with plenty of DI water, dried in a vacuum oven
at 40 ꢁC for 2 days and stored for further experiments. The
sample was labelled as FeGO. For FePtGO preparation, 60 mg of
FeGO was dispersed in DI water and 100 mg of K2PtCl6 was
added into this mixture. To this mixture an excess of NaBH4 was
added slowly and stirring continued for another 2 hours. The
resulting solution was ltered, washed and dried in a vacuum
oven at 40 ꢁC for 2 days and the sample was labelled as FePtGO.
Similarly FeC and FePtC were also prepared by using graphitic
carbon support. Commercial PtC (46 wt%) obtained from
Tanaka (PtC-TKK) was used for comparison. Prepared catalysts
were characterized by PXRD, XPS (VG Escalab 25), TEM (Philips
CM200 FEGTEM operated at 200 kV) and TGA (TG 209 F1 Libra,
NETZSCH). PXRD measurements were carried out in D2 phase
analyser with a cobalt source.
Electrochemical experiments
The catalyst ink was prepared by dispersing 3.4 mg of the
catalyst, 16.66 mL of 5% Naon solution, 3.153 mL of DI water
and 1 mL of IPA through probe sonication for 10 minutes on a
5 s on/off cycle.29 10 mL of this ink was dispensed on the clean
glassy carbon electrode (5 mm diameter) and dried in an oven at
50 C. ORR experiments were carried out in 0.1 M HClO4 elec-
Fig. 1 (a) PXRD patterns obtained for commercial PtC-TKK, in house
prepared FePtGO and FePtC catalysts. (b) TGA plots obtained for
ꢁ
trolyte with a reversible hydrogen electrode (RHE) as reference FePtGO, FePtC, FeGO and FeC.
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particle size calculated by the Scherer equation is in the range of particle size observed from TEM analysis is in accordance with
4–6 nm for FePtGO while it is in the range of 5–7 nm for FePtC. the PXRD results. Further, no FePt particle is detected on the
Commercial PtC-TKK has particles in the range of 2–3 nm. TEM grids outside RGO suggesting a better interaction between
An impurity peak is observed with FePtC at 2 theta value of ꢀ65ꢁ the nanoparticle and its support. The particle sizes observed for
despite the cleaning procedures undertaken while no such our catalysts or slightly bigger than the commercial PtC catalyst
impurity peak was observed in FePtGO. TGA measurement (2–3 nm) while FePt particles reported in the literature vary from
shown in Fig. 1b revealed the overall metal content of the 2–8 nm.6,21,33,34
prepared catalysts were around 52% in FePtC and 60% in
Electrocatalytic properties of the prepared catalysts were
FePtGO. The Fe content in the FeGO and FeC is ꢀ25% indi- analysed through cyclic voltammograms (CVs) which are shown
cating a net Pt content of 27 and 35% in the nal FePtGO and in Fig. S1a.† The normalized CVs show a higher double layer
FePtC catalysts, respectively. TGA results further reveal that region for FePtGO indicating the higher surface area of RGO
despite the use of NaBH4 as the reducing agent, a signicant along with displaying the characteristic peaks of Pt nano-
amount of functional groups remained in the GO aer reduc- particles. Linear sweep voltammograms (LSV) obtained at a
tion. About 25% and 10% weight loss are observed below 200 ꢁC rotation rate of 2000 rpm normalized with the electrochemical
for FeGO and FePtGO respectively that could be attributed to surface area (ECSA) are shown in Fig. 3a. FePtGO showed
water molecules and hydroxyl groups attached to GO surface enhanced performance to that of PtC-TKK and FePtC. At 0.9 V,
functional groups.30–32
the current density observed with FePtGO is 5 and 3.5 times
Bright eld transmission electron microscopic (TEM) images higher than that of currents obtained with PtC-TKK and FePtC
of both catalysts were obtained to study their nanoscale respectively. Further, at a current density of 0.5 mA cmÀ2, about
morphologies. Representative images obtained for FePtGO and 77 mV less overpotential is observed for FePtGO in comparison
FePtC are given in Fig. 2. FePt nanoparticles show a narrow to that of FePtC. The catalytic activity per gram of Pt calculated
distribution in size in the range of 4–7 nm on both supports. for FePtGO is comparable to that of the commercial catalyst
However, due to the high metal content (55–60%) agglomera- despite its larger size (ꢀ5 nm against ꢀ2.5 nm) which clearly
tion and clustering of the nanoparticles is found in few places indicates the superior catalytic activity of FePtGO (Fig. S1b†).
and could not be avoided in the solution reduction method with This could further assist in improving the durability of the
a strong reducing agent such as NaBH4. Nevertheless, the
Fig. 3 (a) Linear sweep voltammograms obtained for FePtGO, FePtC
Fig. 2 TEM images of the as prepared (a) FePtGO and (b) FePtC. Insets and commercial PtC-TKK at a rotation rate of 2000 rpm (b) KL plots
show the particles in the size range of 4–6 nm.
obtained for FePtGO catalyst at various potentials.
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catalyst as smaller particles are known to agglomerate during
fuel cell operation. A maximum current density of 0.425 A gÀ1 at
0.7 V was obtained with FePtGO which is comparatively higher
than the 0.25 A gÀ1 reported for the FePt nanoparticles sup-
ported on graphene by Guo et al. and others.17,21,35,36 However,
Chen et al. has reported higher mass activity values for FePt
nanoparticles prepared by a one pot synthesis that involves
ꢁ
heating up to 160 C although durability studies are available
only for 750 cycles.37 Koutcheky–Levich (KL) plots obtained for
FePtGO are presented in Fig. 3b. The ‘n’ values obtained from
these plots are close to 4 suggesting a direct conversion of
oxygen to water (eqn (1)). However, FePt supported on carbon
has shown an ‘n’ value close to 3 indicating a partial formation
of hydrogen peroxide as well (eqn (2)). The KL plots obtained for
all three catalysts at 0.78 V are shown in Fig. S1c† for compar-
ison. These results reveal the signicant role played by the
support in improving the 4 electron reduction of oxygen to
water.
O2 + 4H+ + 4eÀ / 2H2O
O2 + 2H+ + 2eÀ / H2O2
(1)
(2)
The durability of FePt catalysts–support system has been a
concern as the Fe particles corrode in the highly acidic oper-
ating conditions of PEFCs. We cycled the prepared catalysts
between 1.1–0.6 V for 20 000 cycles and the results presented in
Fig. 4a evidently reveal the enhanced durability of FePtGO over
FePtC and PtC-TKK. For example, aer 10 000 cycles FePtGO
lost only 20% of its initial catalytic activity whereas PtC-TKK and
FePtC lost 47% and 70% respectively. Even aer 20 000 cycles
FePtGO retained 67% activity against 45% of PtC-TKK. The
corresponding CVs obtained during stability tests are given in
Fig. S2.† While the CVs of PtC-TKK and FePtGO retain their Pt
peak features, the FePtC CV shows a signicant reduction in Pt
peaks suggesting Pt detachment from the carbon support along
with peak position shi. The oxygen defect sites in RGO could
have played a crucial role, as nucleating sites for Fe and in turn
to Pt, in strongly binding the catalyst particles to the support.
During electrochemical treatment, Pt is known to form a skin
on the FePt catalyst surface that could further be attributed to
the increased stability of FePtGO.38,39 However, no such
enhancement in durability was observed with the FePtC cata-
lyst. The reduced stability of the commercial catalyst could be
attributed to the agglomeration of catalyst as many reports
suggest. Polarization plots obtained from the electrochemical
measurements shown in Fig. 4b reveal that FePtGO and
PtC-TKK have better catalytic activity than FePtC. Further, the
activation drop is signicantly higher for FePtC than the other
two catalysts.
Fig. 4 (a) Durability of the studied catalysts measured by the loss of
ECSA against the cycle number. (b) Polarization data obtained for
FePtGO, FePtC and PtC-TKK through RDE measurements.
signicantly proves the higher retention of functional groups in
the RGO matrix along with TGA results where close to 15%
weight loss is observed due to hydroxyl functional groups. The O
1s spectrum further reveals the presence of metal oxides (M–O)
at 530.1 eV, carbonyl groups (C]O) at 531.9 eV and ether type
XPS results obtained for FePtGO are shown in Fig. 5a–d while
additional XPS plots of FePtC, FeGO and FeC are given in
Fig. S3–S5 and Table S1.† The C 1s spectrum of FePtGO reveals
the presence of C]C, C–O and C]O through peaks at 284.6,
286.7 and 288.4 eV respectively with ꢀ40% of carbon atoms
attached to oxygen by either a single or double bond. This
Fig. 5 XPS obtained for FePtGO (a) C 1s (b) O 1s (c) Pt 4f and (d) Fe 2p
revealing the presence of surface functional groups in RGO and Fe(II)
and Pt(II) along with Pt(0).
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oxygen atoms at 533.3 eV indicating the large range of func-
tional groups present in the RGO.40,41 The Fe 2p XPS spectrum
reveals that most of the Fe is in the oxidized (Fe2+) state despite
the double reduction procedure. FePt alloys with varying sizes
and thin lms have been reported to form Fe2+ even under air
exposure through oxidation especially severe for small nano-
particles ca. 5 nm in size.42,43 Further, Pt(II) is also observed in
small quantities in comparison to Pt(0) that suggests some-
what partial reduction. Interestingly, the amount of Fe
observed through XPS analysis is signicantly lower in the
FePtGO in comparison to the TGA results. Since XPS is a
surface analysis technique with an X-ray penetration of not
more than 1 micro metre, it is not uncommon to observe
differences between metal loading values obtained from XPS
and TGA. TGA based values provide a more accurate analysis of
total metal loading in such samples while XPS indicate the
surface composition. However, to investigate this discrepancy
in Fe content between the measurements, we analysed the
energy dispersive spectroscopy (EDS) mapping of the samples
and the images obtained are given in Fig. S6 and S7.† EDS
mapping of FePtGO show a predominant coverage of Pt over Fe
with Fe observed at Pt particles edges suggesting Fe might be
buried under the Pt cover while FePtC show the presence of
both Fe and Pt in equal amounts adjacent to each other. This
could also explain the poor durability of FePtC as these exposed
Fe particles can dissolve easily in the acidic electrolyte.
Nevertheless, inductively coupled plasma-atomic emission
spectroscopy would give the most reliable data on composition
although we could not do it due to unavailability of the
instrument in our lab. The level of Pt wt% observed through
XPS is ꢀ25% which is lower than the weight percentage
calculated from TGA results.
While the FePtGO catalyst prepared in this study is slightly
bigger than the commercial PtC catalysts, it showed ꢀ160%
improvement in mass activity over PtC towards ORR. Similarly,
FePtGO's mass activity of 0.45 A gÀ1 is signicantly higher than
that of the ꢀ0.25 A gÀ1 obtained with G/FePts of 7 nm size which
further involves the synthesis of FePt in a separate step at 220 ꢁC
followed by their assembly over the graphene support.17 Simi-
larly, the one pot synthesis of FePt/RGO with ꢀ2 nm particle
size also involved reducing the catalyst at 160 ꢁC in an autoclave
whereas FePtGO catalysts reported in this study are all prepared
under room temperature conditions and are easy to scale up.37
We also studied the catalytic activity of FePtGO catalysts
towards methanol oxidation reaction (MOR) as the residual
hydroxyl group in rGO has been reported to improve the
methanol tolerance of Pt based catalysts.22,23 Cyclic voltammo-
grams obtained for these catalysts towards MOR are given in
Fig. 6. PtRuC shows maximum conversion of methanol followed
by PtC-TKK, FePtGO and FePtC. In general, the peak intensity
observed in the forward scan suggests the amount of methanol
oxidized while the reverse peak intensity indicates the oxidation
of partially oxidized species that are attached to the catalyst
surface (eqn (3) and (4)).
Fig. 6 Cyclic voltammograms obtained for FePtGO, FePtC, PtC-TKK
and PtRuC towards methanol oxidation at a scan rate of 100 mV sÀ1
.
Pt–COads + OHads / Pt–H + CO2
(4)
The ratio between forward and reverse peak intensities
normally indicates the tolerance of the catalyst towards CO
poisoning.28 This is 2–3 times higher for the FePtGO catalyst
than for PtRuC and PtC-TKK indicating a better tolerance
towards CO despite poor catalytic activity towards methanol
oxidation thus making it a suitable catalyst for ORR in DMFCs
as well.
Conclusions
FePt nanoparticles supported on RGO have been successfully
synthesized and studied for their catalytic activity towards ORR
and MOR. The catalytic activity per ECSA is signicantly higher
for the alloy catalysts than the commercial PtC-TKK catalyst.
RGO plays a crucial role in improving the catalytic activity of
FePt nanoparticles with a direct 4eÀ reduction mechanism
while FePt nanoparticles supported on carbon show a mixed
two and four electron mechanisms. However, there is no
signicant improvement in catalytic activity towards MOR
despite the presence of Fe and RGO. This is in contrast with
reported results. Further experiments are needed to conrm
this observation and also to study the role of RGO in improving
the catalytic activity of FePt particles of sizes smaller than 4 nm.
Acknowledgements
RK thanks the European Union Marie Skłodowska – Curie
actions for funding this work through an FP7 fellowship
(Project number 300983 – NECPEM). We also thank EPSRC and
University of Leeds (LENNF) for TEM and XPS (Dr Benjamin
Johnson) measurements.
Notes and references
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Pt + CH3OH / dCO2 + dPt–COads + H2O
(3)
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