Direct Synthesis of Intermetallic Platinum-Alloy Nanoparticles Highly Loaded on Carbon Supports for Efficient Electrocatalysis

Article abstract of DOI:10.1021/jacs.0c05140

Compared to nanostructured platinum (Pt) catalysts, ordered Pt-based intermetallic nanoparticles supported on a carbon substrate exhibit much enhanced catalytic performance, especially in fuel cell electrocatalysis. However, direct synthesis of homogeneous intermetallic alloy nanocatalysts on carbonaceous supports with high loading is still challenging. Herein, we report a novel synthetic strategy to directly produce highly dispersed MPt alloy nanoparticles (M = Fe, Co, or Ni) on various carbon supports with high catalyst loading. Importantly, a unique bimetallic compound, composed of [M(bpy)3]2+ cation (bpy = 2,2′-bipyridine) and [PtCl6]2- anion, evenly decomposes on carbon surface and forms uniformly sized intermetallic nanoparticles with a nitrogen-doped carbon protection layer. The excellent oxygen reduction reaction (ORR) activity and stability of the representative reduced graphene oxide (rGO)-supported L10-FePt catalyst (37 wt %-FePt/rGO), exhibiting 18.8 times higher specific activity than commercial Pt/C catalyst without degradation over 20a ?000 cycles, well demonstrate the effectiveness of our synthetic approach toward uniformly alloyed nanoparticles with high homogeneity.

Full text of DOI:10.1021/jacs.0c05140

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Article
Direct Synthesis of Intermetallic Platinum-Alloy Nanoparticles
Highly-Loaded on Carbon Supports for Efficient Electrocatalysis
Tae Yong Yoo, Ji Mun Yoo, Arun Kumar Sinha, Megalamane S Bootharaju, Euiyeon Jung, Hyeon
Seok Lee, Byoung-Hoon Lee, Jiheon Kim, Wytse Hooch Antink, Yong Min Kim, Jongmin Lee, Eungjun
Lee, Dong Wook Lee, Sung-Pyo Cho, Sung Jong Yoo, Yung-Eun Sung, and Taeghwan Hyeon
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c05140 • Publication Date (Web): 27 Jul 2020
Downloaded from pubs.acs.org on July 27, 2020
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Direct Synthesis of Intermetallic Platinum-Alloy Nanoparticles Highly-
Loaded on Carbon Supports for Efficient Electrocatalysis
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Tae Yong Yoo,†,‡,§ Ji Mun Yoo,†,‡,§ Arun Kumar Sinha,†,‡,§ Megalamane S. Bootharaju,†,‡
Euiyeon Jung,†,‡ Hyeon Seok Lee,†,‡ Byoung-Hoon Lee,†,‡ Jiheon Kim,†,‡ Wytse Hooch
Antink,†,‡ Yong Min Kim,†,‡ Jongmin Lee,†,‡ Eungjun Lee,‡,┴ Dong Wook Lee,┴,∇ Sung-Pyo
Cho,# Sung Jong Yoo,┴,∆,ǁ Yung-Eun Sung,*,†,‡ and Taeghwan Hyeon*,†,‡
† Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of
Korea.
‡ School of Chemical and Biological Engineering, and Institute of Chemical Processes, Seoul
National University (SNU), Seoul 08826, Republic of Korea.
┴ Center for Hydrogen and Fuel Cell Research, Korea Institute of Science and Technology
(KIST), Seoul 02792, Republic of Korea.
# National Center for Inter-University Research Facilities, Seoul National University, Seoul
08826, Republic of Korea.
∆ Division of Energy & Environment Technology, KIST School, Korea University of Science and
Technology (UST), Seoul 02792, Republic of Korea.
ǁ KHU-KIST Department of Converging Science and Technology, Kyung Hee University, Seoul
02447, Republic of Korea.
∇
Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Republic
of Korea.
§These authors contributed equally to this work.
*Corresponding Authors: ysung@snu.ac.kr; thyeon@snu.ac.kr.
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■ ABSTRACT
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Compared to nanostructured platinum (Pt) catalysts, ordered Pt-based intermetallic
nanoparticles supported on a carbon substrate exhibit much enhanced catalytic performance,
especially in fuel cell electrocatalysis. However, direct synthesis of homogeneous intermetallic
alloy nanocatalysts on carbonaceous supports with high loading is still challenging. Herein, we
report a novel synthetic strategy to directly produce highly dispersed MPt alloy nanoparticles (M
= Fe, Co or Ni) on various carbon supports with high catalyst loading. Importantly, a unique
bimetallic compound, composed of [M(bpy)3]2+ cation (bpy = 2,2′-bipyridine) and [PtCl6]2- anion,
evenly decomposes on carbon surface and forms uniformly-sized intermetallic nanoparticles with
a nitrogen-doped carbon protection layer. The excellent oxygen reduction reaction (ORR)
activity and stability of the representative reduced graphene oxide (rGO)-supported L10-FePt
catalyst (37wt%-FePt/rGO), exhibiting 18.8 times higher specific activity than commercial Pt/C
catalyst without degradation over 20,000 cycles, well demonstrate the effectiveness of our
synthetic approach towards uniformly alloyed nanoparticles with high homogeneity.
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■ INTRODUCTION
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Bimetallic alloy nanomaterials exhibit novel properties that are distinctly different from
those of constituent monometallic counterparts derived from their unique electronic and
geometric structures.1-7 Especially, carbon-supported alloy nanoparticles composed of platinum
group metals and transition metals have been intensively investigated as fuel cell electrocatalysts
because they show much enhanced activity and stability with less amount of Pt on electrode.8-18
However, synthetic methodologies need to be rationally designed to deliver both optimal alloy
crystal structures and catalyst morphologies that the catalytic performance hinges on.1-3, 8-13
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Although typical synthetic methods, including impregnation, co-precipitation, and
simultaneous chemical reduction of two different metal precursors directly on a support, are
simple, they lack control over the homogeneity of size and composition of the alloy
nanoparticles.3, 19-21 On the other hand, colloidal synthesis has enabled homogeneous formation
of nanostructured alloys, but they still require surfactants and reductants that are often toxic and
expensive.22-24 Furthermore, additional steps such as loading of the alloy nanoparticles on a
support, annealing at high temperature to get active phase, and removal of protecting ligands are
required to obtain the active form of catalysts.25 Therefore, achieving homogeneous formation of
alloy nanoparticles directly on support, with high catalyst loading, promises scalable and facile
production of supported alloy nanocatalysts. Nevertheless, precise control of the nucleation and
alloying of two different metals on supports has only been limited to a monolayer-level on oxide
supports through electrostatic anchoring of metal complexes.19, 26
Recently, Pt-alloys with atomically ordered crystal structures, including L12 and L10
structures, have been found to largely boost both electrocatalytic activity and stability through
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increased d-d orbital electronic interaction between Pt and transition metal.8, 12, 20, 27-32 In
particular, L10-structured Pt-alloys have very strong electronic interaction between the two
atomic elements along c-axis of the crystal structure, as revealed by hard-magnetic properties,
making them one of the most promising candidates for fuel cell electrocatalysis.29-32 Therefore, it
is very important to synthesize Pt-alloy nanoparticles with 1:1 atomic ratio and ordered L10
structure.33 However, confining the stoichiometry of every single alloy nanoparticle into 1:1
composition has been a demanding job in colloidal chemistry, which is even more challenging
for the direct synthesis on carbon supports. Only a few approaches through colloidal synthesis
have achieved homogeneous L10-structured Pt-alloys, typically via thermal annealing of surface-
layer-protected random alloys followed by the removal of the protective layers.12, 29, 34 On the
other hand, several attempts for the direct synthesis of Pt-alloy nanoparticles on carbon supports
have been far behind the level of precision of colloidal synthesis in terms of the uniformity in
size, composition and degree of atomic alloying.35-38 Therefore, rational synthetic strategies are
highly desirable to directly create ordered Pt-alloy nanoparticles on carbon supports with
required properties, including well-defined L10-structure with 1:1 atomic ratio, uniform particle
size, and high degree of atomic alloying.
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Herein, we design a facile and generalizable approach to directly synthesize platinum alloy
nanoparticles on various kinds of carbon supports especially with high loading. We prepared
bimetallic compounds, [M(bpy)3][PtCl6] (M = Fe, Co or Ni; bpy = 2,2′-bipyridine) composed of
[M(bpy)3]2+ cation and [PtCl6]2- anion, which can be thermally decomposed into homogeneously
alloyed MPt nanoparticles on carbon substrates. Indeed, the sub-micrometer-sized bimetallic
compound grains were found to evenly decompose on carbon surfaces to deliver uniform-sized
Pt-alloy nanoparticles on various carbon supports (MPt/C, C = reduced graphene oxide (rGO),
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Ketjen Black, Vulcan XC, and carbon nanotube). We herein focused on the formation of highly
loaded intermetallic L10-FePt nanoparticles on rGO (37wt%-FePt/rGO), which exhibits high
electrocatalytic performance towards oxygen reduction reaction (ORR) with excellent stability
originating from its highly ordered L10 structure and robust nitrogen-doped carbon protection.
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■ RESULTS AND DISCUSSION
Preparation of Bimetallic FePt Compound. For the preparation of [Fe(bpy)3][PtCl6]
compound (denoted as FePt compound), the [Fe(bpy)3]2+ complex was first prepared by adding
2,2′-bipyridine into an aqueous solution of FeSO4 (Scheme S1). An instantaneous color change
from light yellow to dark-red (Figure S1a,b) indicates the formation of the [Fe(bpy)3]2+ complex,
confirmed by its characteristic ultraviolet-visible (UV-vis) absorption features (Figure S2a).39
The following addition of ethanolic H2PtCl6 solution (Figure S1c) into the [Fe(bpy)3]SO4
solution leads to the rapid precipitation of the FePt compound (Figure S1d,e) with sub-
micrometer-sized grains (Figure S3a). The atomic ratio between Fe and Pt is measured at five
different sites and it is found to be close to 1:1 (Table S1), as revealed by energy-dispersive X-
ray spectroscopy (EDS). The elemental EDS mapping (Figure S3b-d) shows uniform
distribution of Fe and Pt elements. The powder X-ray diffraction (XRD) pattern strongly
suggests that the FePt compound is highly crystalline with a completely different crystal
structure from [Fe(bpy)3]SO4 salt (Figure S4).40,
Intact absorption features from both
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complexes confirm the existence of the Fe(bpy)3 and PtCl6 complexes (Figure S2a) in the
FePt compound, while a significant red-shift of 524 nm peak to 538 nm indicates the electrostatic
attraction between them (black line, Figure S2a). Conversely, the sequential dilution of FePt
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compound results in a peak shift from 538 nm back to 524 nm, showing the weakening of the
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electrostatic attraction between Fe and Pt complexes (Figure S2b).
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Synthesis and Characterization of FePt/rGO. The as-prepared FePt compound was
adsorbed onto the graphene oxide (GO) surface and heated up to 700 °C, during which a
complete wetting of FePt compound over GO occurs, followed by its decomposition into
bimetallic alloy nanoparticles (Figure 1a-d). The intermediate obtained at 500 °C has a smooth
surface without any crystalline feature of FePt compound in the XRD pattern (Figure 1c,e).
Fourier-transform infrared spectroscopy (FTIR) provides additional surface structural
information of the intermediate, the decomposition of bipyridine ligands with the evolution of C-
O and C=C functional groups (Figure 1f).42 EDS mapping in Figure 1g clearly demonstrates the
homogeneous decomposition of FePt compound on rGO. The morphological and structural
evolution of the FePt/rGO composite with temperature (up to 700 °C) was tracked by ex-situ
transmission electron microscopy (TEM) and XRD measurements (Figures S5 and S6,
respectively), revealing that the FePt nanoparticles start forming at 600 °C. We observed
uniform-sized alloy nanoparticles in FePt/rGO by TEM and scanning electron microscopy (SEM)
images (Figures S7 and S8). Interestingly, thin N-doped carbon layer (< 1.5 nm) on the surface
of nanoparticles, derived from bipyridine ligands, serves as a protective shell that contributes to
the confined growth of alloy nanoparticles during the heat treatment at 700 °C, as identified by
high-resolution scanning transmission electron microscopy (STEM) and TEM (Figure S9).
Furthermore, the STEM-EDS mapping suggests that the entire surface of FePt/rGO is covered
with N-doped carbon layer (Figure S10).
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The synthesized FePt/rGO with high catalyst loading was comprehensively characterized.
FePt mass loading of 37 wt% was obtained by inductively coupled plasma atomic emission
spectroscopy (ICP-AES). The XRD peaks of FePt/rGO match well with the intermetallic L10-
FePt phase, which is clearly distinguished from other possible phases, such as Fe, Pt, L12-Fe3Pt
and L12-FePt3 (Figure 2a).43 Furthermore, the characteristic ferromagnetism of the L10-FePt
phase is identified with a high coercivity of 22.5 kOe at room temperature (Figure 2b).44 Figure
2c is a representative atomic-resolution microscopic image obtained by high-angle annular dark-
field STEM (HAADF-STEM) imaging. A layer-by-layer atomic arrangement along the (001)
plane in a L10-FePt nanoparticle is clearly distinguished by the large difference in the Z-contrast
of Fe and Pt atoms (Figure 2d,e). The fast Fourier-transform (FFT) pattern of the atomic image
further verifies the face-centered tetragonal crystal structure of the L10-FePt phase (inset of
Figure 2c). The homogeneous distribution of Fe and Pt atoms in the FePt nanoparticles is
established by EDS single-particle mapping (Figure 2f). Furthermore, a highly magnified EDS
mapping reveals a layer-by-layer distribution of Fe and Pt atoms in the intermetallic L10-FePt
nanoparticle (Figure 2g). Thus, the homogeneous 1:1 Fe to Pt atomic ratio of the FePt
compound is shown to be maintained in an atomically ordered way after the formation of alloy
nanoparticles on rGO, as corroborated by XRD, magnetic property measurements, HAADF-
STEM imaging, and EDS mapping.
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Control experiments in the absence of either bipyridine ligands or a carbon support show
uncontrolled generation of bimetallic alloy nanoparticles (Figures S11 and S12). Without
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bipyridine ligands, the separate Fe2+ cations and PtCl6 anions could not be anchored on
negatively-charged GO in a controlled manner, resulting in non-homogeneous metallic species
on rGO. Carbon support provides high surface area, where a large amount of FePt compound can
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be attached and decomposed to cover the entire surface. The uniform wetting and the
decomposition of the FePt compound can also be extended to the facile synthesis of highly
uniform and alloyed FePt nanoparticles on other carbon supports such as Ketjen Black, Vulcan
XC and carbon nanotube (Figure S13).
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Electrochemical Study of FePt/rGO. Electrocatalytic properties of two FePt/rGO
samples with controlled FePt loading amounts, 24wt%- and 37wt%-FePt/rGO (Figure 3a,b),
were investigated using the rotating-disk electrode (RDE) technique. Commercial Pt/C was used
for both qualitative and quantitative comparisons. Because thick N-doped carbon shell is
reported to seriously mitigate ORR activity by blocking active sites and hindering oxygen mass
transfer,12 we checked electrochemical response of FePt/rGO and found the surface of as-
prepared FePt/rGO to be completely covered by N-doped carbon shell (Figure S14). To
effectively expose the L10-FePt surface, a facile thermal-etching strategy was applied to
FePt/rGO samples (see experimental section for details).45 After the etching procedure, the
overall morphology and ordered L10-FePt phase of 37wt%-FePt/rGO are well-preserved without
particle agglomeration (Figure S15). The efficacy of the etching procedure is corroborated by
clear signs of exposed FePt alloy surface in both cyclic voltammogram (CV) and CO stripping
experiments (Figure S16).
With well-exposed FePt surface, the FePt/rGO samples show superior ORR activities as
shown in Figure 3c. Kinetic current densities calculated from Koutecky-Levich equation are
compared in Tafel plots (Figure 3d). In all potential regions, L10-FePt alloy nanoparticles in
FePt/rGO exhibit higher kinetic current densities than Pt/C. For the quantitative comparison,
electrochemical surface area (ECSA) values were obtained based on both CO oxidation charge in
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CO stripping (ECSACO) and Hupd charge in CV (Figure 3e).46 ECSACO values of 24wt%- and
37wt%-FePt/rGO are 32 and 41% lower than Pt/C, respectively, due to the bigger particle size.
Superior ORR kinetics on lower ECSA value shows excellent specific activities of FePt/rGO,
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calculated at 0.9 V to be 2.21 and 4.1 mA cmPt , which are 10.2 and 18.8 times higher than that
of commercial Pt/C, respectively (Figure 3f). Recently, it was reported that improved ORR
activity on intermetallic Pt alloy surface results from increased electronic interaction between Pt
and transition metal,47 and ORR specific activity of FePt alloys is much more enhanced in fully-
ordered phase compared to the partially-ordered one.48 Therefore, the much higher specific
activity of 37wt%-FePt/rGO than 24wt%-FePt/rGO can be attributed to the increased Fe-Pt
electronic interaction from a higher degree of Fe-Pt atomic ordering, as confirmed by the larger
coercivity value (Figures 2b and S17). In line with the superior specific activity, mass activities
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of 24wt%- and 37wt%-FePt/rGO at 0.9 V are calculated to be 1.18 and 1.96 A mgPt , 6.9 and
11.5 times higher than Pt/C, respectively (Figure 3f). Superior ORR activities of the FePt/rGO
samples are also confirmed at 0.95 V (Figure S18). Both mass and specific activities of 37wt%-
FePt/rGO are higher than other intermetallic FePt alloys in the previous reports (Figure S19), as
confirmed to be one of the most active FePt alloy ORR electrocatalysts (Table S2).
To assess the long-term stability of FePt/rGO, accelerated degradation tests (ADT) were
performed for 20,000 cycles. After ADT, the FePt/rGO samples show little change in CV curves
while the Hupd region of Pt/C is largely decreased (Figure S20). The robust stability of FePt/rGO
samples compared to Pt/C is also confirmed by the barely changed ORR polarization curves and
the Tafel plots (Figure 4a,b). The ECSA of Pt/C decreases by 30% after ADT, whereas those of
FePt/rGO samples slightly increase by 4-7% (Figure 4c). In terms of mass activity and specific
activity, Pt/C shows severe degradation by 60% and 44% after ADT, respectively (Figure 4d).
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However, the mass activities of FePt/rGO samples slightly increase while specific activities
reveal little change after ADT (Figures 4d and S21), suggesting that high specific activities are
retained while ECSA values slightly increase during ADT. From TEM analysis and EDS
mapping on three different sites, we could further confirm the stability of the FePt nanoparticles
under the harsh ADT condition that usually causes particle-particle agglomeration and Fe-
dissolution (Figures S22 and S23). The excellent robustness of FePt/rGO indicates that N-doped
carbon protection, together with the high degree of atomic ordering in FePt, contributes to the
prevention of coalescence and dissolution of FePt nanoparticles.12, 48
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Our synthetic strategy is further extended to other platinum alloys by simply changing the
[Fe(bpy)3]2+ complex with [Ni(bpy)3]2+ or [Co(bpy)3]2+ (Figure S24). The obtained NiPt and
CoPt compounds thermally decompose on rGO into NiPt and CoPt nanoparticles, respectively, in
the same annealing condition as that for the FePt nanoparticles. From TEM and STEM-EDS
images of NiPt/rGO (Figure 5a,b) and CoPt/rGO (Figure 5c,d), highly uniform NiPt and CoPt
alloy nanoparticles are found to be homogeneously deposited on rGO, as in the case of FePt/rGO.
Their well-alloyed bimetallic structures, intermetallic L10-CoPt and randomly-alloyed NiPt49, 50
phases, are further confirmed by XRD patterns (Figure 5e), and the ORR activities of NiPt/rGO
(46 wt%) and CoPt/rGO (22 wt%) are measured to be excellent, with comparable mass activities
to FePt/rGO (Figures 5f and S25). Other similar kinds of FePt compounds can also be prepared
by the use of different organic ligands analogous to 2,2′-bipyridine, such as 1,10-phenanthroline
(phen) and 2,2′:6′,2′′-terpyridine (terpy). The [Fe(phen)3][PtCl6] and [Fe(terpy)2][PtCl6]
compounds decompose evenly on GO to produce FePt nanoparticles coated with N-doped carbon
shell as in the case of [Fe(bpy)3][PtCl6] compound (Figures S26 and S27).
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■ CONCLUSION
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In conclusion, we have demonstrated an effective synthetic strategy that enables direct
formation of well-defined bimetallic alloy nanoparticles on various carbons with metal loadings
of ~40 wt%. The FePt compound, [Fe(bpy)3][PtCl6] (bpy = 2,2′-bipyridine), thermally
decomposes on carbon surfaces to generate intermetallic L10-FePt nanoparticles with 1:1 atomic
composition. The microscopic and magnetic studies of the intermetallic L10-FePt on rGO (37wt%-
FePt/rGO) show the homogeneous bimetallic alloying with ordered atomic arrangement. The
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excellent ORR mass activity (1.96 A mgPt ) and specific activity (4.1 mA cmPt ) of 37wt%-
FePt/rGO, which are 11.5 and 18.8 times higher than those of commercial Pt/C respectively, are
well-preserved after 20,000 potential cycling. Other 1:1 Pt-alloy nanoparticles including
NiPt/rGO and CoPt/rGO can also be prepared on carbon supports, exhibiting comparable ORR
activities to FePt/rGO. We believe that our proposed synthetic concept can be further generalized
and modified to prepare different supported alloy nanoparticles with high homogeneity.
■ EXPERIMENTAL SECTION
Materials. 2,2′-bipyridine (bpy, ≥99 %), 1,10-phenanthroline (phen, ≥99 %), 2,2′:6′,2′′-
terpyridine (terpy, 98 %), iron (II) sulfate heptahydrate (FeSO4·7H2O, ≥ 99.0 %), cobalt (II)
sulfate heptahydrate (CoSO4·7H2O, ≥99 %), nickel (II) sulfate hexahydrate (NiSO4·6H2O,
≥98 %) and chloroplatinic acid hexahydrate (H2PtCl6·6H2O, ≥37.50 % Pt basis) were purchased
from Sigma Aldrich. Graphene oxide (GO, 4 mg/mL in water) was purchased from Graphenea.
Ethanol (anhydrous, 99.9 %) and isopropanol (>99.99 %) were purchased from Samchun
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Chemical. Perchloric acid (HClO4, 70-72 %) was purchased from Merck. Ultrapure de-ionized
water (Milli-Q, resistivity of 18.2 MΩ, total organic carbon of 2 ppb) was used for the synthesis
of FePt/rGO and the preparation of electrolytes.
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Synthesis of [Fe(bpy)3][PtCl6] compound (FePt compound). 0.5 mmol of
FeSO4·7H2O was dissolved in 15 mL of de-ionized (DI) water (or 15 mL ethanol) and 1.5 mmol
of 2,2′-bipyridine was dissolved in 15 mL of ethanol (99.9 %). These two separate solutions were
then mixed to form dark-red [Fe(bpy)3]SO4, which was stirred overnight at room temperature.
Into the dark-red solution, 30 mL ethanolic solution of 0.5 mmol of H2PtCl6·6H2O was added
instantly. Brighter-red FePt compound was precipitated right after the addition of Pt solution
without any difference after 5 h of stirring at room temperature. The red precipitate can be easily
separated by short-time centrifugation (10 min) at 10,000 rpm. Even with several washing steps
on FePt compound with ethanol, the solid precipitate can be completely obtained after the
centrifugation. The precipitate was then dried in oven at 40 °C overnight. Fine powder can be
obtained by grinding the dried precipitate in a mortar with a pestle.
Synthesis of FePt/rGO. The freshly prepared FePt compound was dispersed in 80 mL of
DI water (36 and 144 mg of FePt compound for FePt loading of about 24 and 37 wt%,
respectively). The resulting mixture was sonicated for 10 min and stirred for 2 h, after which 10
mL of aqueous GO solution (4 mg/mL) was added dropwise. FePt compound is well attached on
GO after mixing them for 2 h followed by freeze-drying. The composite containing FePt
compound grains on GO foam was then thermally treated at 700 °C for 6 h in argon flow of 100
sccm, during which a complete wetting and decomposition of FePt compound occurs on the
surface of GO, forming atomically ordered FePt alloy nanoparticles on rGO denoted as FePt/rGO.
Synthesis of FePt nanoparticles on different carbon supports. The synthesis of
FePt nanoparticles on carbon nanotube (CNT, Nanostructured & Amorphous Materials. 7-8 wt%
in water) followed the same procedure as that for FePt/rGO except that aqueous CNT solution
was added dropwise instead of the GO solution.
The synthesis of FePt nanoparticles on Ketjen Black (KB) and Vulcan XC (VC) was done
by the following modified procedure. After dispersing a certain amount of FePt compound in 80
mL of isopropanol (or ethanol), the solution was sonicated for 10 min and stirred for 2 h. Then,
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50 mg of KB or VC powder was added into the solution that contains FePt compound and the
resulting mixture was sonicated for 10 min. After stirring for 2 h, the composite containing FePt
compound grains on carbon support was obtained by rotary evaporation and dried in oven at
40 °C. The dried powder was ground in a mortar and then heat-treated at 700 °C under argon
atmosphere.
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Synthesis of CoPt and NiPt nanoparticles on rGO. [Co(bpy)3][PtCl6] and
[Ni(bpy)3][PtCl6] compounds were prepared following the same protocol for the synthesis of
FePt compound except that CoSO4·7H2O and NiSO4·6H2O were used in place of FeSO4·7H2O.
The subsequent synthetic processes for the preparation of CoPt/rGO and NiPt/rGO is the same as
that for the synthesis of FePt/rGO.
Preparation of [Fe(phen)3][PtCl6] and [Fe(terpy)2][PtCl6] compounds for the
synthesis of FePt/rGO. [Fe(phen)3][PtCl6] and [Fe(terpy)2][PtCl6] compounds were prepared
following the same protocol for the synthesis of [Fe(bpy)3][PtCl6] compound except that
stoichiometric amounts of 1,10-phenanthroline (phen) and 2,2′:6′,2′′-terpyridine (terpy) were
used in place of 2,2′-bipyridine. The subsequent synthetic processes for the preparation of
FePt/rGO samples were the same as that done with [Fe(bpy)3][PtCl6] compound.
Electrochemical measurements. Standard three-electrode configuration was used for
electrochemical measurement with rotating-disk electrode setup (Autolab, PGSTAT 302N).
Homemade FEP cell was used with salt bridge to prevent possible crossover of anion species
from reference electrode. Before the measurement, electrochemical cell was immersed in 0.5 M
H2SO4 and boiled in DI water multiple times to remove possible impurities. Glassy carbon
(geometric area of 0.196 cm2) was used as working electrode substrate. Ag/AgCl (BASi, 3M
KCl) and Pt foil were used as reference and counter electrodes, respectively. All potentials were
referenced to reversible hydrogen electrode (RHE) by measuring reduction/oxidation reaction on
polycrystalline Pt electrode in H2-saturated 0.1 M HClO4.
MPt/C catalysts were pre-treated with mild air-etching at 200-230 °C for 12 h before
electrochemical measurements to fully expose the surface. Catalyst ink was prepared by
dispersing 5 mg of electrocatalysts in 2 mL of isopropanol through ultrasonication, followed by
addition of 30 μL of Nafion ionomer (5 wt%, Sigma Aldrich). To prepare uniform thin-film
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electrode, the catalyst ink was pipetted and drop-casted on the glassy carbon substrate. Catalyst
loadings were set to 7 and 15 μgPt cm-2 for MPt/rGO and Pt/C (Johnson Matthey Co. 20 wt%),
respectively.
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All electrochemical measurements were conducted in 0.1 M HClO4 electrolyte (Merck,
70 %) under room temperature. To fully activate catalyst surface, the working electrode was
pretreated in Ar-saturated electrolyte by cycling between 0.05 and 1.1 V (vs. RHE) at a scan rate
of 200 mV s-1 until reaching stable voltammograms. The pretreated electrode was transferred to
fresh Ar-saturated electrolyte and polarized between 0.05 and 1.1 V (vs. RHE) at a scan rate of
20 mV s-1 to measure cyclic voltammograms.
For CO stripping, the pretreated electrode was immersed in Ar-saturated electrolyte and
quickly held at 0.05 V (vs. RHE). The electrolyte was flowed with CO gas for 3 min, and
subsequently flowed with Ar for 30 min to completely remove residual CO molecules in the
electrolyte. Polarization curve was measured between 0.05 and 1.1 V (vs. RHE) at a scan rate of
20 mV s-1.
For ORR measurement, the pretreated electrode was immersed in O2-saturated electrolyte
and polarization curves were recorded from 0.05 to 1.1 V at a scan rate of 20 mV s-1 while the
working electrode was rotated at 1600 rpm. iR-drop correction was applied during the
measurement with the solution resistance value obtained from the impedance measurement at
0.68 V. Accelerated degradation test (ADT) was performed by cycling the working electrode
between 0.6 and 1.0 V for 20,000 cycles under Ar-saturated 0.1 M HClO4.
Characterization. Transmission electron microscopy and scanning transmission electron
microscopy measurements with energy-dispersive spectroscopy (EDS) were conducted on a
JEOL EM-2010 microscope operated at 200 kV. X-ray diffraction patterns were acquired on a
Rigaku SmartLab X-ray diffractometer using Cu Kα radiation. Ultraviolet-visible absorption
spectroscopy was performed on a Cary 5000 UV-Vis-NIR spectrophotometer (Agilent). Fourier-
transform infrared spectroscopy was conducted on a TENSOR27 model (Bruker, Germany) in
ATR mode with a spectral range from 4,000 to 400 cm-1. Scanning electron microscopy images
were acquired using JSM-7800F Prime model (JEOL Ltd, Japan). Inductively coupled plasma
atomic emission spectroscopy analyses were performed on ICPS-8100 model. Magnetic
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measurements were conducted on a PPMS-14 system (Quantum Design, USA) scanning with a
field up to 70 kOe at room temperature. High-angle annular dark-field scanning transmission
electron microscopy measurements (with EDS) were conducted using JEM-ARM200F (Cold
FEG, JEOL Ltd, Japan) model at the National Center for Inter-University Research Facilities
(Seoul National University). Simulations for the analyses of HAADF-STEM data were
performed with MacTempasX software (parameters were set based on the well-known crystal
structure information on L10-FePt phase12, 51 with a crystal thickness of 4 nm).
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■ ASSOCIATED CONTENT
Supporting Information
Scheme S1.
Tables S1, S2.
Figures S1-S27.
This material is available free of charge via the Internet at http://pubs.acs.org.
■ AUTHOR INFORMATION
Corresponding authors.
*ysung@snu.ac.kr
*thyeon@snu.ac.kr.
Author contributions.
§T. Y. Y., §J. M. Y., §A. K. S. equally contributed to this work.
Notes.
The authors declare no competing financial interests.
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■ ACKNOWLEDGMENTS
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T.H. acknowledges the financial support by IBS-R006-D1. Y.-E.S acknowledges the financial
support by IBS-R006-A2. S.J.Y. acknowledges the financial support by the NRF grant funded by
the Korea government (MIST) (2018M1A2A2061975, 2019R1A2B5B03004854).
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Figure 1. (a) Schematic illustration of the synthesis of rGO-supported ordered FePt alloy
nanoparticles with N-doped carbon protection. (b-d) TEM, (e) powder XRD, and (f) FTIR data
of [Fe(bpy)3][PtCl6]/GO, intermediate and FePt/rGO as described in (a). (g) STEM-EDS
mapping images showing uniform distribution of C, Pt, Fe atoms in the intermediate.
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Figure 2. (a) XRD pattern of 37 wt% FePt/rGO and reference peak positions of the candidate
phases. (b) Magnetic hysteresis loop of 37 wt% FePt/rGO. (c) Measured HAADF-STEM image
(inset: FFT pattern of the image), (d) its enlarged image of 37 wt% FePt/rGO, and (e) a
simulated image of an L10-FePt nanocrystal. STEM-EDS mappings of (f) a single FePt
nanoparticle and (g) a highly magnified region of 37 wt% FePt/rGO.
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Figure 3. (a,b) TEM images of 24wt%- and 37wt%-FePt/rGO (insets show size distributions of
FePt nanoparticles). (c) ORR polarization curves, (d) Tafel plots, (e) ECSA, and (f) mass
activities and specific activities of the prepared FePt/rGO catalysts compared with the
commercial Pt/C.
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Figure 4. (a) ORR polarization curves, (b) Tafel plots, (c) ECSA, and (d) mass activities and
specific activities before and after ADT (20,000 potential cycles) of the prepared FePt/rGO
catalysts compared with the commercial Pt/C.
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Figure 5. (a) TEM and (b) STEM-EDS data of NiPt/rGO (46 wt%), and (c) TEM and (d) STEM-
EDS data of CoPt/rGO (22 wt%). (e) Powder XRD patterns, and (f) ORR polarization curves of
NiPt/rGO and CoPt/rGO.
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