Trimetallic PtAuNi alloy nanoparticles as an efficient electrocatalyst for the methanol electrooxidation reaction

Article abstract of DOI:10.1039/c7dt02608k

Platinum is an excellent electrocatalyst. However, the disadvantages of Pt as an electrocatalyst lie in its poor earth abundance, high cost, and poor stability due to surface poisoning. Thus it remains a challenge to find suitable alternative electrocatalysts and/or reduce the use of Pt. Herein, we report the solvothermal synthesis of bimetallic (PtAu and PtNi) and trimetallic (PtAuNi) alloy nanoparticles (NPs) with controlled percentages of individual metals. With an optimized Ni content, the trimetallic (Pt₆₆Au₁₁Ni₂₃) alloy NPs show superior electrocatalytic activity (in terms of lower onset oxidation potential and higher mass activity) not only to bimetallic alloy NPs (PtAu and PtNi) but also to commercial Pt/C (20% Pt loading) for methanol electrooxidation (MEO). This enhanced electrocatalytic activity is due to the synergistic role of different metals in MEO catalysis. In particular, the catalytic activity is found to be controlled by the balance between the adsorption of methanol species on Pt and the removal of carbonaceous species from the catalyst surface by Au and Ni, as demonstrated here. The multimetallic alloy of optimal individual content thereby not only reduces the Pt content in the catalyst but also exhibits higher electrocatalytic activity than Pt/C for MEO that is desirable for fuel cell applications.

Full text of DOI:10.1039/c7dt02608k

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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Trimetallic PtAuNi Alloy Nanoparticles as an Efficient
Electrocatalyst for Methanol Electrooxidation Reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Kousik Bhunia, Santimoy Khilari, and Debabrata Pradhan*
DOI: 10.1039/x0xx00000x
Platinum is an excellent electrocatalyst. However, the disadvantages of Pt as an electrocatalyst lie in its poor earth
abundance, high cost, and poor stability due to surface poisoning. Thus it remains a challenge to find suitable alternate
electrocatalysts and/or reduce the use of Pt. Herein, we report solvothermal synthesis of bimetallic (PtAu and PtNi) and
trimetallic (PtAuNi) alloy nanoparticles (NPs) with controlled percentages of individual metals. With an optimized Ni
content, the trimetallic (Pt66Au11Ni23) alloy NPs shows superior electrocatalytic activity (in terms of lower onset oxidation
potential and higher mass activity) not only to bimetallic alloy NPs (PtAu and PtNi) but also to commercial Pt/C (20% Pt
loading) for methanol electrooxidation (MEO). This enhanced electrocatalytic activity is due to the synergistic role of
different metals in the MEO catalysis. In particular, the catalytic activity is found to be controlled by the balance between
adsorption of methanol species on Pt and the removal of carbonaceous species from the catalyst surface by Au and Ni as
demonstrated here. The multimetallic alloy of optimal individual content thereby not only reduces the Pt content in the
catalyst but also exhibited higher electrocatalytic activity than Pt/C for MEO that is desirable for fuel cell applications.
www.rsc.org/
1. Introduction
by forming alloy with another inexpensive and suitable metal
becomes a promising route to design new efficient catalyst for
The present world consumes much of its power from coalꢀ
based resources which are not only limited but also produce
green house gases. Thus it becomes a compelling issue to
explore sustainable energy resources. Among several energy
resources available, methanol can be used as fuel in the direct
methanol fuel cells (DMFC), one of the potential
environmentally clean power sources for the portable electronic
devices. If methanol is derived from biomass, then the resource
becomes fully sustainable and renewable. DMFC is thus an
attractive proposition for green energy generation because of its
easy storage and transport, high volumetric energy density, and
nonꢀtoxic byproducts.1,2 However, broad application of DMFC
is limited due to its high cost, mostly contributed from
expensive Pt catalyst used for the oxidation of methanol.3 In
addition, Pt exhibits instability during long term application
leading to high operational cost.4 Therefore, it is essential to
develop efficient and durable catalyst to replace or reduce the
Pt content in the DMFC. Several nonꢀPt catalysts were
investigated by researchers in last few years.5,6 However,
activity of those nonꢀPt catalysts remains far below the
expected level.5,6 Thus Ptꢀbased bimetallic alloy (BMA)
catalysts gained recent momentum to improve the methanol
electrooxidation (MEO) performance.4 Dilution of Pt content
MEO. The primary reasons for introducing another metal into
Pt are to reduce the cost and surface poising.7 Interestingly,
several Ptꢀbased bimetallic alloys (BMA) with other metals
such as Pd, Au, Fe, Ni, Cu, Zn, Ru, Ag, and Ir have offered a
new dimension in catalysis with enhance activity than that of
Pt.4,8 This is due to the bifunctional nature of BMAs and/or the
varied electronic effect.9,10,11,12,13,14 The bifunctional nature
involves adsorption and dissociation of methanol on one metal
(primarily Pt) and oxidation of adsorbed residues at a lower
potential by other (oxophilic) metal.3 However, durability of
Ptꢀbased BMA still remains a major challenge for sustainable
MEO catalysis.4,15
The improved electrocatalytic properties by BMAs lead to
further research on trimetallic alloys (TMA) involving Pt in
order to achieve multifunctionality and further improve the
MEO activity. Those trimetallic Ptꢀbased metal alloys include
PtRuNi,16 PdPtCu,17 AuPtPd,18
AuPtCu,19 FePtM
(M=Cu,Ni),20 FePtAu,15 FePtPd,21 and PtPdTe.22 These
multimetallic alloys show promising physicochemical activities
due to the synergy of individual metal components which offers
tunable electronic and lattice structure.17,18,20 The rational
design of multimetallic alloy nanostructures with narrow size
distribution is desirable to enhance the active sites and thereby
catalytic activity. To explore trimetallic Ptꢀbased metal alloys,
we chose PtAuNi TMA on the basis of several reports on PtAu
BMA with different compositions and recently published
research articles on the electrocatalytic activity of PtNi for
MEO.23,24,25,26,27,28 It is to be noted that there is no report so far
a.Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur-
721302, West Bengal, India
E-mail: deb@matsc.iitkgp.ernet.in
Electronic Supplementary Information (ESI) available: [EDX Spectra, Histograms on
the particle size, TEM image, CV plots]. See DOI: 10.1039/x0xx00000x
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on PtAuNi for MEO to the best of our knowledge. Ni is 200 °C for 72 h in an air oven. After a dwell time of 72 h, the
considered to promote oxidation of CO species but suffers from autoclave was cooled to room temperature naturally. The black
structural dissolution in an acidic medium like other suspension formed inside Teflonꢀlined container was
bimetallics.29 On the other hand, Au is known to be a stable centrifuged at 8000 rpm and the product was collected. The
electrocatalyst for oxidation of hydrazine, glucose, and ascorbic product was finally washed with water and acetone mixture for
acid oxidation.30,31,32 Although Au shows catalytic activity several times and dried at room temperature under vacuum for
towards MEO alone, its alloy with Pt is known to improve the overnight. The BMA and TMA NPs with different metal
MEO substantially.33,34,35 It is important to be noted that the compositions were synthesized using same protocol only by
size of Au particle plays an important role in its catalytic varying their initial precursor quantity. The initial concentration
activity toward MEO with an extremely small size Au NP of different metal precursors to synthesize BMA and TMA NPs
shows catalytic activity.36 There are several reports on the is presented in Table 1.
improved electrocatalytic properties of AuPt alloy because Au
Table 1. Initial precursor concentrations to synthesize BMA
modifies the Pt electronic structure and lower the dꢀband
and TMA NPs.
center.37,38 Thus incorporation of Au as third component into
Sl. No.
H2PtCl6
(mmol)
HAuCl4ꢁ
xHCl
(mmol)
0.03
0.03
0.03
NiCl2ꢁ
6H2O
(mmol)
ꢀ
0.05
0.1
Alloy
composition
PtNi BMA could not only reduces the instability but also
improves the electrocatalytic activity as demonstrated here.39
Different synthetic approaches such as seedꢀmediated growth,40
galvanic replacement,41 template method,42 and sacrificing
agentsꢀassisted synthesis18 have been considered by various
groups in recent year to develop efficient Ptꢀbased BMA and
TMA electrocatalysts. Despite extensive efforts, oneꢀpot
synthesis of wellꢀdispersed multimetallic catalysts is highly
desirable.
i
ii
iii
iv
v
0.1
0.1
0.1
0.1
0.1
Pt80Au20
Pt76Au10Ni14
Pt66Au11Ni23
Pt56Au9Ni35
Pt76Ni24
0.03
ꢀ
0.15
0.1
2.3 Characterization
In the present work, we demonstrate a simple oneꢀpot and
oneꢀstep solvothermal synthesis of PtAuNi nanoparticles (NPs)
with narrow diameter distribution, in the range of 3‒7 nm, for
the first time and demonstrate their superior MEO activity. As
the alloy composition also plays significant role on the
electrocatalytic properties, the composition of Ni in AuPt was
varied to find an optimum Ni content for maximum oxidation
current/mass activity for MEO. In addition to the
electrocatalytic activity, the durability of the asꢀsynthesized
TMA NPs towards MEO is tested over 1000 cycles and
compared with their bimetallic counterparts (Pt76Ni24 and
Pt80Au20) as well as commercial Pt/C.
The powder Xꢀray diffraction (PXRD) patterns of the asꢀ
synthesized BMA and TMA NPs were collected to investigate
crystal structure using a PANalytical High Resolution XRD
(PW 3040/60) operated at 40 kV and 30 mA with Cu Kα Xꢀ
rays (1.54 Å) in the 2θ angle 35−85°. The morphology and
microstructure of BMA and TMA NPs were investigated by a
FEI Tecnai G2 transmission electron microscope (TEM)
operated at an accelerating voltage of 200 kV. The elemental
composition was measured with a CarlꢀZeiss SUPRA 40
FESEM attached with an Oxford EDS detector (Fig S1, ESI)
and the asꢀsynthesized alloy samples are assigned with their
individual atomic % as subscript of respective element as noted
in Table 1 and discussion thereafter. The Xꢀray photoelectron
2. Materials and methods
2.1 Chemicals
spectroscopy (XPS) measurement was performed with
a
PHI5000 Versa Probe II Scanning XPS Microprobe with a
monochromatic Al Kα source (1486.6 eV) to investigate the
chemical states and compositions of the asꢀsynthesized alloy
Dimethylformamide (DMF) (Merck, India), chloroplatinic acid
hexahydrate (H2PtCl6
(HAuCl4 xH2O, Loba Chemie), nickel chloride hexahydrate
(Ni(Cl)2 6H2O, Merck India), polyvinylpyrrolidone (PVP)
⋅xH2O, Sigma Aldrich), chloroauric acid
NPs
.
⋅
⋅
2.4 Electrochemical measurements
(W=55000, Loba Chemie) and all other chemicals were
purchased from Merck India and used without further
purification.
The electrochemical measurements were carried out at room
temperature with a CHI 760D (CH Instruments, Inc., USA)
electrochemical workstation with a conventional three electrode
system. To prepare the working electrode, a slurry was first
prepared by ultrasonically dispersing an appropriate amount of
asꢀsynthesized alloy NPs and Vulcan XC as support material,
1.9 mL ethanol, and 100 ꢂL Nafion solution (Aldrich, 5 wt%
Nafion) for approximately 30 min. Then 20 ꢂL of slurry was
drop casted on a 3 mm diameter preꢀcleaned glassy carbon
(GC) working electrode. The NPs coated GC electrode was
finally dried under ambient for overnight. The Pt wire and
saturated calomel electrode (SCE) were served as counter and
reference electrode, respectively. The cyclic voltammograms
2.2 Synthesis of bimetallic and trimetallic alloy NPs
The BMA and TMA NPs were synthesized by a simple oneꢀ
step solvothermal process. In a typical synthesis process of
Pt66Au11Ni23 TMA NPs, 250 mg of PVP was added to 30 mL
DMF and stirred at room temperature for 15 min. Subsequently,
0.1 mmol of H2PtCl6, 0.1 mmol of NiCl2⋅6H2O and 0.03 mmol
of HAuCl4 were added one by one and stirred for another 15
min. The whole transparent reaction mixture was transferred to
a 50 mL Teflonꢀlined stainlessꢀsteel autoclave and heated at
2 | J. Name., 2012, 00, 1-3
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(CVs) were recorded over a potential window of −0.2 to 1 V at confirmed by TEM (discussed later). In addition, the diffraction
50 mV/s in 0.5 M H2SO4 and 0.5 M CH3OH to study the features of TMA NPs were found to be slightly shifted toward
methanol oxidation. The chronoamperometry (CA) analysis higher 2 value which is more pronounced with a higher Ni
θ
was carried out at 0.7 V in the same electrolyte for 3500 s. The content. This suggests the replacement of larger size Pt by
electrolyte solution was purged with highꢀpurity nitrogen for at smaller Ni atom in the lattice and thereby decrease in the lattice
least 15 min prior to every electrochemical measurement.
spacing.26 Since the lattice mismatch between Au (a=4.08 Å)
and Pt (a=3.92 Å) is only 4%, the change in lattice parameter of
TMA NPs is believed to be due to the Ni (a=3.52 Å) as it
possesses higher lattice mismatch of 10% with respect to Pt,
thus introducing more structural strain in TMA NPs which is
3. Results and discussion
3.1 Structural and microstructural properties
The crystallographic phases and structure of the asꢀsynthesized expected to introduce bifunctional characteristic in the alloy
NPs were analyzed by PXRD. Fig 1 shows the PXRD patterns structure for MEO as explain later.44 The correlation between
of the asꢀsynthesized BMA and TMA NPs with different lattice constants of pure Ni and Pt, and BMAs/TMAs with
compositions (Table 1). The wellꢀdefined diffraction peaks different Pt% is presented in Fig 2. The calculated lattice
confirm polycrystalline nature of these BMA and TMA NPs parameters with different Pt% in the TMAs show almost linear
with a face centered cubic crystal structure.43 The diffraction relationship in accordance with the Vegard’s law.45
planes of Pt (the main constituent of the alloys) are only
assigned in the XRD pattern. The diffraction features of BMA
3.9
3.8
3.7
(v) Pt76Ni24
(iv) Pt56Au9Ni35
3.6
(iii) Pt66Au11Ni23
3.5
(ii) Pt76Au10Ni14
0
20 40 60 80 100
Pt %
Ni (Card No. 01-1258)
Fig 2. The lattice parameters of BMA and TMA NPs with
varying Pt contents as estimated from the respective XRD
patterns.
(i) Pt80Au20
Au (Card No. 01-1172)
The sizes and microstructure of the asꢀsynthesized NPs
were analyzed by TEM. Fig 3 presents the TEM images,
selected area electron diffraction (SAED) patterns, highꢀ
resolution lattice images, and histograms on the particle size for
the BMA and TMA NPs. Fig 3(a1) shows a representative
Pt (Card No. 01-1194)
40
50
60
70
80
Two Theta (degree)
Fig 1. XRD patterns of the asꢀsynthesized BMA and TMA NPs TEM image of Pt80Au20 BMA NPs. The atomic ratio Pt and Au
solvothermally synthesized at 200
JCPDS patterns of pure Pt, Au, and Ni.
°
C along with standard was measured by EDX (Fig S1, ESI). The Pt80Au20 BMA NPs
were found mostly in the shape of hexagon in the size range of
8−30 nm as shown in the inset histogram [Fig 3(a1)]. The
and TMA were found to be shifted from their pure counterpart SAED pattern [Fig 3(a2)] shows scattered bright spots forming
such as Pt, Au, and Ni, indicating alloying effect. For rings indicating the crystalline nature of the NPs with different
comparison, the JCPDS data of pure Pt, Au, and Ni are also orientations. The rings on the SAED pattern are assigned to the
shown in Fig 1. Among the diffraction patterns, the Pt80Au20 corresponding crystal planes of Pt. An HRTEM image [Fig
BMA shows sharp diffraction peaks [Fig 1(i)], which is due to 3(a3)] taken from the edge of a Pt80Au20 NP shows continuous
the larger size (~20 nm) of NPs as shown later. All other alloys lattice with a spacing of 2.2 Å corresponding to {111} plane,
show broad diffraction features [Fig 1(ii
half maximum of most intense peak in the 2
The most intense (111) peak of pure Pt is found to be shifted was found to be decreased as shown in Fig 3(b
toward the (111) peak of pure Au for the Pt80Au20 BMA NPs as shows TEM image of Pt76Au10Ni14 TMA NPs. The
shown in Fig 1(i).25 This suggests incorporation of Au in the Pt Pt76Au10Ni14 TMA NPs were found in the size range of 3
7 nm.
lattice and forming a BMA. With incorporation of Ni in PtAu The distinct ring shape SAED pattern [Fig 3(b2)] confirms the
or Pt, diffraction peaks were broadened as shown in Fig 1(ii v). polycrystalline nature of these NPs as a whole. The HRTEM
This is due to decrease in the particle size of the TMA NPs as image [Fig 3(b3)] of an individual NP confirms the crystalline
−
v)] with fullꢀwidth which was the most intense peak in the PXRD pattern [Fig
range of 38‒44 1(i)]. Upon incorporating Ni into the PtAu or Pt, the size of NPs
e). Fig 3(b1)
θ
°.
−
a
−
−
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Fig 3. (a1−e1) TEM images, (a2−e2) SAED patterns, and (a3−e3) HRTEM images of (a) Pt80Au20, (b) Pt76Au10Ni14, (c)
Pt66Au11Ni23, (d) Pt56Au9Ni35, (e) Pt76Ni24 NPs. The inset of a1 and b1 represent the histograms on the frequency of NPs size.
nature with {111} orientation. With further increasing the Ni% xHCONMe2 + yMn+ + zH2O
in the PtAuNi TMA, the size of NPs was found to be in the Me2NCOOH CO2 + Me2NH
same range, i.e., 3
→
yM0 + xMe2NCOOH + nH+ (1)
(2)
→
−
6 nm for Pt66Au11Ni23 [Fig 3(c1)] and As the metal NPs tend to agglomerate once formed, PVP was
Pt56Au9Ni35 [Fig 3(d1)] TMA NPs. The histograms on the size used as stabilizing agent.32 The protection effect of the PVP
of Pt66Au11Ni23 and Pt56Au9Ni35 NPs are presented in the ESI come from the lone pair of the nitrogen and oxygen atom which
(Figure S2). The SAED patterns [Fig 3(c2,d2)] and HRTEM form complex with the metal. Once the metal NPs is formed,
images [Fig 3(c3,d3)] of these TMAs with higher Ni% show PVP protects the NPs from agglomeration.50 This leads to the
similar crystalline nature to Pt76Au10Ni14 TMA NPs. Fig 3(e1) formation of wellꢀdispersed BMA and TMA NPs in the present
shows a representative TEM image of Pt76Ni24 BMA NPs of case with an average size of 3
−6 nm. However, in the absence
uniform spherical shape in the size range of 3 6 nm and of PVP, 3 5 nm diameter Pt66Au11Ni23 TMA NPs were found to
−
−
corresponding histogram on size is shown in Fig S2 (ESI). Fig be agglomerated to larger clusters of size >30 nm as shown in
3(e2) and 3(e3) show the corresponding SAED pattern and Fig S3 (ESI).
HRTEM image of Pt76Ni24 BMA NPs suggesting the crystalline
nature of the NPs, respectively.
3.3 Composition and chemical states
The elemental composition of the asꢀsynthesized BMA and
TMA NPs was analyzed by EDX (Fig S1, ESI). It is to be noted
3.2 Formation mechanism of alloy NPs
The solvent play a crucial role in the solvothermal synthesis of that in all the EDX spectra, Xꢀray peaks from oxygen and
nano and microstructured materials.46,47 The reducing ability of aluminum were additionally appeared (not marked) and their
DMF make it a unique solvent for metal NPs synthesis.48 Thus, contribution was neglected for the estimation of elemental % of
in the present study, DMF was chosen as solvent as well as alloys. The oxygen and aluminum in the EDX spectra were
reducing agent to synthesize BMA and TMA NPs at 200 °C by contributed from the aluminum substrate on which alloy NPs
solvothermal method. The oxidation of DMF produces CO2 and were coated to study the EDX. It is important to be noted that
H2.49 On that basis, following reaction mechanism is proposed these peaks were not found in the XPS measurements as
for the generation of the TMAs from the respective metal salts presented below.
in DMF.
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For the XPS study, the asꢀsynthesized alloy NPs were lower BE. Fig 4c shows the Au 4f XPS region spectra of
mixed with Vulcan XC carbon black (as support) with a wt% Pt80Au20 and Pt66Au11Ni23 NPs. The Au 4f7/2 and 4f5/2 XPS
ratio of alloy NPs to Vulcan XC at 20:80. Fig 4a shows the peaks were respectively found at BE of 84.16 eV and 87.91 eV
survey XPS spectra of BMA NPs such as Pt80Au20 and Pt76Ni24, for Pt80Au20 and shifted to lower BE for Pt66Au11Ni23 NPs due
and a representative TMA NPs, i.e., Pt66Au11Ni23. All the XPS to the reason explained earlier. In both the cases, Au 4f peaks
spectra were calibrated taking C 1s peak at 284.5 eV for the were found at a lower BE value than that of metallic Au
carbon support.51 The alloys (Pt76Ni24 and Pt66Au11Ni23) with indicating the formation of alloy.57 Figure 4d shows the
Ni as one component show O 1s photoelectron peak due to the deconvoluted Ni 2p region XPS spectra of Pt76Ni24 and
formation of oxide/hydroxide on the Ni surface. Fig 4b shows Pt66Au11Ni23 NPs. In addition to metallic Ni 2p3/2 peak at 853.0
the deconvoluted Pt 4f XPS region spectra of Pt80Au20, Pt76Ni24, eV, peaks at 853.7 eV, 854.9 eV, and 855.8 eV are respectively
and Pt66Au11Ni23 NPs. The Pt 4f7/2 and 4f5/2 XPS peaks of assigned to Ni(+2) states of NiO, Ni(OH)2, and NiOOH,
Pt80Au20 and Pt76Ni24 were found at binding energy (BE) of respectively.55 A broad feature at ~862.4 eV in the Ni alloys is
71.4 eV and 74.7 eV, respectively, with a fixed spitꢀorbit due to the multi electron excitation satellite shakeꢀup peak.58
splitting of 3.3 eV, which can be assigned to the metallic Pt.22 The presence of NiO and Ni(OH)2 not only enhance the
The Pt 4f peaks were found to at a higher BE than that of bulk electrocatalytic activity towards MEO but are also capable of
Pt (70.61−
71.3 eV for 4f7/2) suggesting alloy formation.27,52,53,54 oxidizing methanol in acid and basic medium.59,60
These two peaks were found to be further shifted by 0.23 eV to
lower BE for Pt66Au11Ni23 NPs. This downward shift suggest 3.4 Electrochemical studies
the transfer of electrons from Au and Ni to Pt due to the 3.4.1 Electrochemical active surface area. Electrochemical
difference
in
their
electronegativity
values
(the active surface area (ESCA) of the asꢀprepared catalysts was
electronegativity values for Ni, Au, and Pt are 1.9, 2.54, and measured by CV analysis in acid medium (0.5 M H2SO4). Fig
2.28 respectively).55,56 Moreover, additional set of less intense 5a presents the CVs of BMAs, TMAs, and Pt/C after 100 cycles
peaks at 72.4 eV and 75.8 eV in the case Pt80Au20 and Pt76Ni24 recorded over the potential range of −0.2 to 1.0 V (vs. SCE) at a
can be assigned to Pt(+2) state of PtO and/or Pt(OH)2 species.42 scan rate of 50 mV/s. Initial 100 CV cycles were carried out to
These peaks were also found for Pt66Au11Ni23 NPs at slightly remove the surface impurities to record the ESCA of the
Pt 4f7/2
Pt 4f5/2
(a) Survey
Pt66Au11Ni23
(b) Pt 4f
Pt66Au11Ni23
Pt76Ni24
Pt76Ni24
Pt80Au20
Pt80Au20
78
76
74
72
70
1000
800
600
400
200
0
)
H
(c) Au 4f
(d) Ni 2p
O
(
i
N
i
N
Au 4f7/2
Pt66Au11Ni23
Pt66Au11Ni23
Au 4f5/2
Satellite Peak
Pt80Au20
Pt76Ni24
90
88
86
84
868 864
860
856
852
Binding Energy (eV)
Binding Energy (eV)
Fig 4. (a) Survey XPS spectra, Region XPS spectra (b) Pt 4f, (c) Au 4f, and (d) Ni 2p of different BMA and TMA NPs.
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electrocatalysts. The CVs recorded with present electrocatalysts commercial Pt/C.61 It is also known that the presence of nonꢀPt
are found to be similar in shape as previous reports.61 The CVs atoms in multimetallics significantly change the electronic
show two distinct potential regions which are (i) structure of Pt thereby change in their electrochemical
adsorption/desorption of hydrogen (Had) that occur in the region activity.55 However, nonꢀPt transition metal atoms are prone to
̶
of
−
0.2 to 0 V (H+ + e = Had) and (ii) the formation of adsorbed corrosion and dissolve in acid medium as reported for Ni in
hydroxyl (OHad) layer beyond 0.5 V (2H2O = OHad + H3O+ + PtNi alloy.63 Thus the stability test of the asꢀsynthesized
e−).62 The former potential region (
0.2 to 0 V) related to Had electrocatalysts was performed by repeat CV cycles in 0.5 M
adsorption/desorption process is commonly used to evaluate the H2SO4. The CVs of Pt80Au20, Pt76Ni24 and Pt66Au11Ni23 NPs at
ECSA of various Ptꢀbased
electrocatalysts.11 its 100th and 1000th cycle are presented in Fig S4 (ESI). It was
−
The ECSA of an electrocatalyst significantly control the found that after 900 cycles of CVs, Pt66Au11Ni23 retains almost
electrode kinetics for different redox reactions which take place 90% of its ECSA value whereas 20% and 19% loss in ESCA
at the electrode surface. The higher ECSA value of a catalyst with Pt76Ni24 and Pt80Au20 NPs, respectively, suggesting TMA
signifies a higher number of electrocatalytic active sites and NPs are superior electrocatalysts than that of BMA NPs. This
vice versa. Thus the MEO kinetics at different Ptꢀbased also ascertains the minimum structural dissolution of
electrodes is governed by the ECSA of the active electrode Pt66Au11Ni23 NPs than that of BMAs in the acidic medium.
material.60 The ECSA of Ptꢀbased electrocatalysts was
calculated by employing following equation.62
3.4.2 Electrocatalytic activity. The electrocatalytic activity of
the asꢀsynthesized BMA and TMA NPs toward MEO was
tested by CV analysis (Fig 5b) in 0.5 M H2SO4 and 0.5 M
ECSA [cm2/g of Pt] = charge [QH, µC/cm2] / [210 (µC/cm2)
×
catalyst loading (g of Pt/cm2)]
(3)
where QH represents the total charge for H+ ions desorption. CH3OH electrolyte. The CVs consist of two wellꢀdefined peaks
The numerical value of QH was calculated from the integral area in the potential range of 0.2−1.0 V as shown in Fig 5b. The first
under desorption peak of CVs over the potential range of 0 to peak with maximum current appeared at ~0.7 V during forward
−
0.2 V. The calculated ECSA for different catalysts (Table 2) scan (from negative to positive potential), which corresponds to
synthesized in the present work follow the order of the oxidation of freshly chemisorbed methanol on the catalyst
Pt66Au11Ni23 > Pt76Au10Ni14 > Pt76Ni24 > Pt56Au9Ni35 > Pt80Au20 surface, whereas second peak centered at ~0.5 V observed
> Pt/C. This suggests the important role of TMA in the during the reverse scan was responsible for the removal of
hydrogen adsorption/desorption. Similar higher ESCA values carbonaceous species such as CO, CHO etc. from the catalyst
have earlier been reported for multimetallic alloys than that of surface formed in the forward scan.2 The peak current density
30
20
10
0
(ii)
(iii)
Ip,f
(iii)
(ii)
(i) Pt80Au20, (ii) Pt76Au10Ni14
(iii) Pt66Au11Ni23, (iv) Pt56Au9Ni35
(v) Pt76Ni24, (vi) Pt/C
(b)
10 (a)
Ip,r
60
40
20
0
,
(iii)
(v)
(v)
(iv)
(i)
(ii)
(v)
(iv)
(vi)
(i)
5
0
(vi)
(iv)
0.1 0.2 0.3 0.4
Potential vs. SCE (V)
(i)
(vi)
-5
(i) Pt80Au20, (ii) Pt76Au10Ni14
,
(iii) Pt66Au11Ni23, (iv) Pt56Au9Ni35
,
-20
-10
(v) Pt76Ni24, (vi) Pt/C
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
Potential vs. SCE (V)
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
Potential vs. SCE (V)
300
250
200
150
100
50
40
30
20
10
0
(d)
(c)
(i) Pt80Au20, (ii) Pt76Au10Ni14
,
(iii) Pt66Au11Ni23, (iv) Pt56Au9Ni35
,
(v) Pt76Ni24, (vi) Pt/C
(iii)
(ii)
(v)
(iv)
(i)
(vi)
0
0
1000
2000
3000
Time (s)
Fig 5. Cyclic voltammograms with different electrocatalysts (a) in 0.5 M H2SO4 to measure the electrochemical active surface area
and (b) in 0.5 M H2SO4 and 0.5 M CH3OH, (c) mass activity of different electrocatalysts of MEO (d) Chronoamperometry
measurements in 0.5 M H2SO4 + 0.5 M CH3OH with different electrocatalysts.
6 | J. Name., 2012, 00, 1-3
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in the forward and reverse scan are assigned as Ip,f and Ip,r, peak current density obtained from CV divided by the mass of
respectively. The MEO peak current density with all alloy NPs the catalyst used. Fig 5c presents the mass activity of the asꢀ
was found to be higher than that of commercial Pt/C, signifying synthesized BMA and TMA NPs electrocatalyst, which follows
the role of Ptꢀbased alloys for the improved MEO kinetics. The the same order of Ip,f/Ip,r value. This suggests that an optimum
onset potential for MEO follows the order Pt66Au11Ni23
<
Ni content in TMA has the highest mass activity. In particular,
Pt76Au10Ni14 < Pt76Ni24 < Pt56Au9Ni35 < Pt80Au20 < Pt/C, as the mass activity of Pt66Au11Ni23 TMA NPs is found to be 2.47
shown in the inset of Fig 5b and also presented in Table 2. The times higher than that of the Pt/C. Moreover, presence of Au in
lowest onset potential observed for Pt66Au11Ni23 and the alloy is crucial for the improved MEO as demonstrated
Pt76Au10Ni14 indicates the enhanced electrocatalytic behavior of here. Furthermore, the stability of the asꢀsynthesized
TMAs compared to the BMAs and Pt/C. However, a slightly electrocatalysts
was
tested
by
chronoamperometry
higher onset potential for Pt56Au9Ni35 as compared to Pt76Ni24 measurements (Fig 5d) in 0.5 M H2SO4 + 0.5 M CH3OH
was due to smaller Pt content in the former.
electrolyte. The current decay mainly results from the inhibition
of surface active sites by the accumulation of CO species and
adsorption of the SO4 on the catalyst.65,66 The anodic current
density first rapidly decreases because of the formation of
intermediate species during MEO in each catalyst. Gradually a
pseudoꢀsteady state was achieved and current decays slowly.44
The amperometry current density with Pt66Au11Ni23 was found
to be much higher than that of commercial Pt/C after 3500 s
underlying the significance of TMA NPs with an optimum Ni
content.
2
−
Table 2. The electrochemical properties of asꢀsynthesized
electrocatalyst.
Onset
potential
vs. SCE
(V)
Peak
Mass
current
density
(mA/cm
activity
(mA/m
̶
1
gPt )
2
)
Pt80Au20
Pt76Au10Ni14
Pt66Au11Ni23
Pt56Au9Ni35
Pt76Ni24
20
37
44
26
28
1.07
1.39
1.12
1.27
1.11
0.28
0.11
0.1
0.14
0.13
38
50
65
42
48
134
174
228
148
167
The electrochemical MEO properties (lower onset potential
and high mass activity) of TMA (Pt66Au11Ni23) were found to
be higher than that of BMAs (Pt80Au20 and Pt76Ni24) and
commercial Pt/C. The only exception of lower MEO
performance with Pt56Au9Ni35 than Pt76Ni24 is attributed to
lower Pt content in the former, leading to a fewer number of Pt
sites available for methanol dehydrogenation and thereby
sluggish MEO. Similar observation was made by Wang et al.
The mechanism of MEO on the catalyst surface can be
summarized through following reactions.64
Pt + CH3OH → Pt
−
−
CH3OHads
CHOads / Pt−COads
(4)
(5)
(6)
(7)
for methanol oxidation in presence of Pt−Ni catalyst with an
Pt
Pt
Pt
−
−
−
CH3OHads → Pt
CHOads + Ni
COads + Ni
excess Ni loading.67 This suggests an optimum balance of each
component in the TMA NPs for the maximum activity. The
primary factors responsible for the improved MEO properties
of TMAs are (i) change in the lattice parameters leading to the
structural strain and thus electronic properties, (ii) improved
CO tolerance, and (iii) increase in the adsorbed OH−.68,55,69 The
change in the lattice parameters upon introducing foreign
metals (Au and Ni) on the Pt sites in the TMA and BMA NPs is
shown in Fig 2. Previous report suggests that deviation of the
alloy’s lattice parameter from the pure Pt is the key in
enhancing the catalytic activity.65 As Au catalyzes the CO
oxidation, it would reduce the CO accumulation and thereby
reduce poisoning of the catalyst surface. In addition, being a
noble metal and filled dꢀband, Au has weak chemisorption
properties.70,25 This suggest an important role of Au for
enhanced MEO activity in TMA NPs. The nonꢀPt metals such
as Ni provide oxygenated species such as OH− at the Pt site
which facilitates electrochemical oxidation of the adsorbed
carbonaceous species such as CO.53 XPS analysis here shows
̶
−
OHads → Pt + Ni + CO2 + 2H+ + 2e
̶
−
OHads → Pt + Ni + CO2 + H+ + e
As noted in the above reaction steps, surface poisoning by
carbonaceous species (CO and CHO) is a major limiting step in
the DMFC with Pt electrocatalyst. The CO tolerance of a
catalyst is explained by the ratio of forward and reverse peak
current density, i.e., Ip,f/Ip,r which describes accumulation of
CHOads and COads on Pt surface leading self poisoning.
Traditionally, high Ip,f/Ip,r indicates good methanol oxidation to
carbon dioxide in the forward scan and minimum accumulation
of carbonaceous species. The Ip,f/Ip,r value in the present work
follow the order of Pt76Au10Ni14 > Pt56Au9Ni35 > Pt66Au11Ni23
>
Pt76Ni24 > Pt80Au20 > Pt/C (Table 2). This suggests the
poisoning effect is lower for TMAs than that of BMAs and Pt/C
(Ip,f/Ip,r = 0.9). Moreover, this trend indicates that the lowest CO
poisoning of a catalyst is linked to an optimum Pt and Ni
percentage. The mass activity is another important factor to
determine the performance of an electrocatalyst. The mass
activity was estimated by measuring the maximum oxidation
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11 B. Y. Xia, H. Bin Wu, X. Wang and X. W. Lou, J. Am. Chem.
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of TMA favors the formation of surface OH− layer to a greater
extent and thereby assists in removing the carbonaceous
species. The removal of carbonaceous species is presented as
equations (6) and (7), which allow freeing up of both the Pt and
Ni sites for further methanol oxidation.64 The higher CO
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shown in Fig 5b. This concludes the requirement of an optimal
ratio of nonꢀPt metal contents, i.e., Ni and Au in the alloy for
the maximum MEO activity, which is found with Pt66Au11Ni23.
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In summary, synthesis of highly uniform PtAuNi TMA NPs
with narrow size distribution (3−6 nm) and controlled atomic
Soc., 2013, 135, 7130–7133.
percentage is demonstrated here using a simple oneꢀstep
solvothermal method. The PtAuNi TMA NPs with an optimum
content of Pt, Ni, and Au (Pt66Au11Ni23) exhibit superior MEO
activity as compared to their BMA counterparts and Pt/C. This
was attributed to the synergistic role of Pt, Ni, and Au in the
TMA NPs. While Ni facilitates adsorption of OHad species on
NPs to enhance the oxidation of carbonaceous species, Au
deters adsorption of carbonaceous species that poison the active
Pt sites of the catalyst. The design of the PtAuNi TMA NPs
with an enhanced MEO activity as presented here has a strong
bearing to the development of DMFC.
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Acknowledgements
KB thanks to Scientific & Industrial Research (CSIR), New
Delhi for a senior research fellowship. This work is supported
by CSIR [Grant No. 01(2724)/13/EMRꢀII], New Delhi, India.
Authors also acknowledge the DSTꢀFIST funded facility at the
Department of Physics for XPS measurement.
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