PtSn Nanoalloy Thin Films as Anode Catalysts in Methanol Fuel Cells

Article abstract of DOI:10.1021/acs.inorgchem.0c01147

Reactions of SnX2 (X = Cl, Br) with [PtMe2(bipy)], 1, (bipy = 2,2′-bipyridine), followed by NaBH4 reduction at the toluene/water interface in the presence or absence of graphene oxide support rendered PtSn nanoalloy thin films. They were characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The electrocatalytical activity of the PtSn thin films was investigated in the methanol oxidation reaction. Our studies showed that the PtSn/reduced-graphene oxide (RGO) thin film gave better catalytic results for MOR in comparison to bare PtSn or Pt thin films. A maximum jf/jb ratio (jf and jb are the maximum current densities in the forward and backward scans, respectively) of 6.77 was obtained for the PtSn/RGO thin film deriving from the 1 + SnBr2 + NaBH4 sequence. ?

Full text of DOI:10.1021/acs.inorgchem.0c01147

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PtSn Nanoalloy Thin Films as Anode Catalysts in Methanol Fuel Cells
Nahal Aramesh, S. Jafar Hoseini,* Hamid R. Shahsavari, S. Masoud Nabavizadeh, Mehrangiz Bahrami,
Mohammad Reza Halvagar, Elvira De Giglio, Mario Latronico, and Piero Mastrorilli*
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sı Supporting Information
ABSTRACT: Reactions of SnX (X = Cl, Br) with [PtMe (bipy)], 1,
2
2
(
bipy = 2,2′-bipyridine), followed by NaBH reduction at the toluene/
4
water interface in the presence or absence of graphene oxide support
rendered PtSn nanoalloy thin films. They were characterized by powder
X-ray diffraction, X-ray photoelectron spectroscopy, and transmission
electron microscopy. The electrocatalytical activity of the PtSn thin films
was investigated in the methanol oxidation reaction. Our studies showed
that the PtSn/reduced-graphene oxide (RGO) thin film gave better
catalytic results for MOR in comparison to bare PtSn or Pt thin films. A
maximum j /j ratio (j and j are the maximum current densities in the
f
b
f
b
forward and backward scans, respectively) of 6.77 was obtained for the
PtSn/RGO thin film deriving from the 1 + SnBr + NaBH sequence.
2
4
1
. INTRODUCTION
borohydride reduction and subsequent hydrothermal treat-
11
ment. Furthermore, the addition of Sn metal in Pt−Sn/
graphene electrocatalyst prepared by the solution-phase
reduction promotes the methanol oxidation process by
geometric effects and expanding Pt’s lattice spacing, causing
Direct methanol fuel cells (DMFCs) are promising devices
that make use of platinum-based electrocatalysts. Because Pt
1
metal has high cost and low CO poisoning tolerance, the
alloying of this metal with a cheap non-noble metal (such as
Sn, Co, Cu, or Ni) is a way to reduce the cost of the catalyst
and improve its catalytic activity. This process depends on the
chemical nature of the alloying metal. However, platinum-
based electrocatalysts have been largely investigated for the
methanol oxidation process. Supported and binary or ternary
alloyed nanoparticles (NPs) are two groups of commonly used
electrocatalysts. The effect of structure and oxidation state of
Pt NPs/γ-Al O in the methanol oxidation process was
reported, which was elucidated from a time-consuming method
of micelle encapsulation. Pt/CNdoped electrocatalysts were
synthesized at high temperature of 130 °C and exhibited the
maximum I /I (I and I are the maximum currents in the
forward and backward scans, respectively) of 1.1. Synthesis of
Pt/graphene oxide-polyvinylpyrrolidone (GO-PVP) nanoelec-
trocatalyst with the maximum current density of 70 mA cm
was reported. Pt/reduced-graphene oxide (RGO)-SnO2
nanocomposite was synthesized as a durable and effective
electrocatalyst using laser ablation and photoassisted in situ
12
a synergistic effect between Pt and Sn NPs. Many similar
13−29
investigations are reported,
but finding an electrocatalyst
with a simple synthetic route at room temperature, high
efficiency and durability, low metal loading, and low cost has
been a challenge. Organometallic species are commonly used
30
as precursors for the synthesis of electroactive nanomaterials.
The choice of appropriate organometallic complexes capable of
reduction to metal particles plays a key role for the
construction of metal thin films. The self-assembly at the
liquid/liquid interface (LLI) has received much attention in
recent years as an inexpensive, simple, and powerful route for
2
3
2
3
1
f
b
f
b
nanomaterial fabrication. This defect-free focal plane
provides a platform for nanostructure assembly by manipu-
lation of surface chemistry. In this strategy, the thin film forms
at the interface between two immiscible liquids via injecting
the reducing reagent and leads to the self-assembly of particles
at a contact angle of 90° with the LLI. This method can be
applied for the synthesis of supported nanostructures. Carbon-
based supports have been used to achieve excellent dispersity,
3
−
2
4
5
reduction methods with I /I of 1.14. PtNi alloy was
f
b
6
synthesized as an electrocatalyst with different morphologies
such as core−shell nanotubes with the maximum current
−
2
7
Received: April 19, 2020
density of 1.5 mA cm . Also, PtNi/C electrocatalysts with a
maximum current density of 110 mA cm− were synthesized.
2
8
Pt Ni/RGO was prepared via a solvothermal method at high
3
9
temperature, which exhibited a very low current density.
PtNi concave octahedral structure was synthesized with an I /
3
f
1
0
Ib of 1.49. PtSn/C electrocatalyst was synthesized using
©
XXXX American Chemical Society
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A
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Scheme 1. Synthesis of PtSn Nanoalloy Thin Films
1
Figure 1. H NMR spectrum of 2 (dmso-d , 298 K). Peaks with asterisks are due to [Pt(Me) (Cl)(bipy)] as a side product.
6
3
high activity and stability, and suitable surface area for the
supported particles. Among them, GO is an excellent candidate
due to its unique properties, including high conductivity, large
surface area, and high mechanical, chemical, and thermal
(XPS), which confirmed the copresence of tin and platinum
in the materials.
1
The H NMR spectrum (Figure 1) of the solid obtained
after SnCl addition to 1 showed, as the main component, a
2
3
2
stability. Because of oxygen containing surface groups, GO-
species characterized in the high-field region by a singlet at δ
1
95
2
supported multimetallic NPs are used as efficient catalysts that
1.16 flanked by Pt satellites ( J
= 58 Hz), attributable to
H,Pt
33
can increase the interaction with metals, and they have also
shown higher catalytic performance compared to their
the equivalent methyl groups of 2. The bipy protons were
found at δ 7.82, δ 8.31, and δ 8.74 in a 1:1:2 ratio.
34
1
195
1
119
monometallic counterparts.
Here, we report on the synthesis of PtSn nanoalloys
The H− Pt and H− Sn heteronuclear multiple-
quantum correlation spectra (HMQC) revealed that the
methyl protons at δ 1.16 are coupled both with Pt and
Sn. Perusal of the H− Pt HMQC spectrum (Figure 2)
35
195
obtained by reaction of [PtMe (bipy)], 1, complex with
2
1
19
1
195
tin(II) halides followed by NaBH reduction, and on their
4
electrocatalytic activity toward the methanol oxidation reaction
(
MOR) in the presence or absence of GO support. In spite of
the high cost of platinum, the Pt loading is very low in PtSn
thin films, and most of the atoms are present on the surface of
the electrocatalyst. Therefore, a lower amount of thin film
catalyst is needed in comparison to some other types of
catalysts with non-noble metals in their structures. Hence, the
cost becomes lower in comparison to electrocatalysts even with
low cost metals and bulk structures or some other morphology.
2
. RESULTS AND DISCUSSION
.1. Synthesis and Characterization of the PtSn
2
Nanoalloy Thin Films. The PtSn nanoalloy thin films were
prepared in two steps. First, addition of SnX (X = Cl, Br) to
2
an acetone solution of the complex [PtMe (bipy)], 1, (1:1
2
molar ratio, room temperature) resulted in the precipitation of
1
195
Figure 2. H− Pt HMQC spectrum of 2 (dmso-d , 298 K). The
6
a solid containing the adduct [(X)(bipy)Me Pt(μ-SnX )-
2
2
1
195
red-circled cross peak is due to the H C−Pt−Sn− Pt correlation in
the isotopologue having only one Pt atom in the molecule.
3
PtMe (bipy)(X)], (2, X = Cl, yellow-orange; 3, X = Br,
195
2
orange). A diluted toluene solution of this solid was then
layered over water, and a fresh aqueous solution of NaBH was
4
added, which caused formation of a PtSn nanoalloy thin film at
the LLI (Scheme 1). The identities of 2 and 3 are proposed
based on NMR analysis of the reaction solution obtained after
showed that the molecule contains two chemically equivalent
1
95
Pt atoms, falling at δPt −2657 ppm, and that both methyl
1
95
117/119
protons and Pt nuclei are coupled to
Sn “passive”
1
95
SnX addition to 1. The characteristics of the thin films
nuclei. The presence of two chemically equivalent Pt atoms
can be inferred by the appearance of a central cross peak
(circled in red in Figure 2) that is due to methyl protons
2
produced by NaBH reduction were studied by powder X-ray
4
diffraction (XRD) and X-ray photoelectron spectroscopy
B
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bonded to non NMR active Pt atoms in an isotopologue
containing a chemically equivalent 1 Pt atom.
95
36
The methyl protons of 2 are characterized by a geminal
1
195
1
119
H− Pt coupling constant of 58 Hz and by a H− Sn
1
17/119
coupling constant of about 8 Hz (no clear
Sn satellites
1
are discernible in the H NMR spectrum, due to the broadness
195
119/117
of the methyl signal). Moreover, an averaged Pt−
Sn
coupling constant of about 14400 Hz is extractable from the
1
195
1
119
H− Pt HMQC spectrum (Figure 2). The H− Sn HMQC
spectrum of the solid obtained after SnCl addition to 1 is
2
illustrated in Figure 3, which confirmed the values of JH,Sn and
Figure 5. 195Pt{ H} NMR spectrum of 2 (dmso-d , 298 K).
1
6
a
Table 1. NMR Features of Pt Sn Complexes 2−5
2
b
c
d
d
2
3
4
5
CH3
Pt
Sn
2JH,Pt
1JH,Sn
1JPt,119Sn
1.16
−2657
−408
58
1.35
1.36
1.35
1
119
Figure 3. H− Sn HMQC spectrum of 2 (dmso-d , 298 K).
−2844
−391
62
6
−436
59
−319
59
JPt,Sn reported above and showed that the 119Sn chemical shift
for 2 is δSn −408 ppm. The Sn NMR spectrum (Figure 4)
showed a 1:8:18:8:1 quintet, confirming that the Sn nucleus
is bonded to two chemically equivalent
Pt{ H} NMR spectrum (Figure 5) indicated the singlet at
δ −2657 ppm flanked by Sn ( J
119
8
11
119
14 701
13 141
15 317
12 947
195
a
b
c
Pt atoms. The
Chemical shifts are in ppm, J are in Hz. In THF-d . In DMSO-d .
8 6
d
1
95
1
In CDCl , from ref 37.
3
119
1
= 14 701 Hz) and
Pt
17
Pt,119Sn
1
1
Sn ( J
= 14 050 Hz) satellites.
formation of a small amount of [Pt(bipy)X ] (X = Cl, Br) can
Pt,117Sn
2
The NMR features described so far are compatible with the
structure proposed in Scheme 1 and are comparable to those
arise from reaction of 1 with HX, which forms when SnX2
reacts with adventitious water in the reaction medium. The
reported by Puddephatt et al. for the complexes [(X)(Bu
side products formed together with 2 were [Pt(Me) (Cl)-
(bipy)] and 6.
A similar behavior was observed when carrying out the
2
3
3
7
39
40
bipy)Me Pt(μ-SnXY)PtMe (Bu bipy)(X)], 4, X = Y = Cl;
2
2
2
37
and 5, X = Cl, Y = Ph (Table 1). We were not able to
selectively obtain 2 from reaction of 1 with SnCl , nor to
crystallize it devoid of other metal species formed from
concomitant side reactions. Our attempts to crystallize 2 ended
reaction of 1 with SnBr . The main product was the trimetallic
2
2
complex 3 (Scheme 1) contaminated by [Pt(Me) (Br)-
3
41
42
(bipy)] and [PtBr (bipy)], 7. Also, in the case of 3, our
2
up with the isolation of little crystals of [PtCl (bipy)], 6,
whose X-ray crystallography is reported in Figure S1. The
crystallization endeavors gave small crystals of 7, whose X-ray
crystallography is reported in Figure S2. Crystal data, data
2
38
43
Figure 4. 119Sn{ H} NMR spectrum of 2 (dmso-d , 298 K).
1
6
C
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Figure 6. XPS spectra and relevant curve fittings of Pt 4d5/2 (a and c) and Sn 3d5/2(b and d) regions. Spectra were recorded on PtSn resulting from
chloro species 2 (a and b) and PtSn resulting from bromo species 3 (c and d).
Table 2. Surface Atomic Percentages, Sn/Pt Ratios, and Pt and Sn Speciation on PtSn Samples
sample
C 1s
O 1s
Sn 3d5/2
Pt 4d5/2
Cl 2p
0.3
Br 3p3/2
[Sn]/[Pt]
[Pt(0)]:[PtO )]
[SnO ]:[SnO]:[Sn(Pt)]
x
2
PtSn(Cl)
PtSn(Br)
39.6
34.6
43.8
46.7
12.1
14.9
4.2
3.5
2.9
4.3
87:13
86:14
26.5:60.0:13.5
20.4:65.5:15.1
0.3
collection, and structure refinement details are listed in Table
S1. The NMR features of 3 are summarized in Table 1, while
NMR spectra for this complex are shown in Figures S3−S6.
Room temperature chemical reduction of 2 and 3 at the LLI
and Sn peak fittings (Figure 6) are summarized in Table 2. The
high carbon atomic percentages were due to ubiquitous
hydrocarbon contamination. Chlorine and bromine contribu-
tions were negligible, as expected. Pt signals showed on both
samples two components: a main component relevant to Pt(0)
with BE(Pt 4d5/2) = 314.4 ± 0.1 eV and a small contribution
attributable to platinum oxides with BE(Pt 4d5/2) = 317.8 ±
of toluene and water with NaBH as reducing agent was used
4
for the synthesis of PtSn and PtSn/RGO bimetallic thin films
in the presence or absence of GO as support. The latter was
used to increase interaction with the metals due to the
presence of oxygen containing surface groups (Figure S7).
Thin film formation at the LLI is accompanied by a color
change to black (Figure S8). In this process, the effect of the
type of used complexes as precursor and the addition of GO as
a stabilizer on the size of structures and their electrocatalytical
activity have been investigated. The thickness of the PtSn films
can be controlled by controlling the reaction temperature,
48
0.1 eV, in agreement with the literature. As far as tin signals
are concerned, the Sn 3d5/2 spectra can be deconvoluted into
three components on both samples. Sn(II) and Sn(IV) oxides
fall at 486.3 ± 0.1 eV and 487.0 ± 0.1 eV, respectively.
Moreover, a contribution ascribable to Sn in a Pt environment
(Sn(Pt)) was centered at 485.5 ± 0.1 eV, in agreement with
49
previously reported data. Because XPS probes only the
surface of a solid sample (at most 10 nm), a different
composition of the underlying sample layers cannot be
excluded.
metallic precursor concentration, NaBH concentration, rate of
4
the addition of NaBH , time, mechanical vibrations, beaker
4
44
diameter, and viscosity of the medium.
It is noteworthy that tin has a negative standard redox
2
+
The characterization of the PtSn and PtSn/RGO thin films
was made using XRD, XPS, and transmission electron
microscopy (TEM) techniques. In the XRD patterns of PtSn
thin films obtained from 2 and 3 (Figures S9a and S9c), the
main diffraction peak at 2θ value of 40.1° can be assigned to
potential (E(Sn /Sn) = −0.14 V) and is linked between two
platinum in 2 or 3; therefore, the reduction of Sn(II) to Sn(0)
is more difficult than that of Pt. In addition, Sn(0) is converted
to tin oxide compounds after formation in the alkaline reaction
medium and in the presence of air. Coordination of the metal
in the complex varies the redox potential, implying that the
metals can be oxidized or reduced more easily during the redox
45
the (111) index of face-centered cubic (fcc) Pt. As for the
XRD patterns of PtSn/RGO thin films (Figures S9b and S9d),
in addition to the main diffraction peak of Pt, a broad and weak
diffraction peak at 2θ = 24.8° is attributable to RGO that can
50,51
reactions.
TEM images (Figures S10 and S11) of the PtSn thin films
obtained from 2 and 3 exhibited spherical structures with the
average diameter of 8 and 6 nm, respectively (Figures S10c and
S11c). In addition, TEM images of PtSn/RGO thin films
obtained from 2 and 3 are shown in Figures 7 and 8,
respectively. In both cases, the films are characterized by
spherical structures well-dispersed on the support. The mean
diameters of the NPs obtained from chloro species 2 (Figure
46
be assigned to the (002) plane. Relative to the same
reflections for Pt(0), the diffraction peaks of these thin films
are shifted to higher 2θ values, revealing a decreased lattice
47
parameter and a high level of alloying.
Figure 6 shows the XPS spectra recorded on PtSn thin films.
The surface atomic composition of the two analyzed samples
as well as the Sn/Pt ratios and the different contributions in Pt
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52
Cl− that could prevent aggregation of NPs. This observation
confirms that halide ions can modify the surface energy and
53−55
can lead to an anisotropic growth of the NPs.
The
importance of anisotropic metal NPs is due to the dependence
of the physical and chemical properties to their shape and size.
Based on the reported experimental and theoretical inves-
56−58
tigations,
it can be suggested that halide anions (chloride,
bromide, or iodide) considerably have effective influence on
the morphology of NPs. This is the result of their strong
tendency for adsorbing on the surface of metallic NPs, which
leads to rational control of size and shape during the synthesis
process. In other words, halide ions have an important role as
surface active species that can devise and control the successful
growth and properties of NPs, alloys, and bimetallic
nanostructures. NP growth kinetics is affected by halide ions
through several ways, i.e. binding selectively on crystal facets,
complex formation with the precursors, and oxidative
59
etching.
PtSn dispersity and growth on the GO support is tuned by
controlling the metallic precursor concentration, NaBH4
Figure 7. (a−c) TEM images of PtSn/RGO thin film obtained from
chloro species 2 and (d) histogram of particle size distribution.
concentration, and the rate of the addition of NaBH . For
4
the synthesis of PtSn/RGO thin film, metallic precursor is
dissolved in toluene phase and GO with carbonyl, epoxy,
hydroxyl and carboxyl functional groups is dispersed in
aqueous phase (Scheme 2a). By the addition of NaBH4,
reduction simultaneously occurs in both organic and aqueous
phases. The rate of this step has a key role for controlling the
nucleation and growth speed, which are affected by the
addition of very small drops of reducing agent using a syringe.
In addition to reduction of platinum-based complexes, GO is
also gradually converted to RGO by the addition of reducing
agent. Thus, they are moved toward the interface, which is the
result of the reduction of their polar functional group and
increasing hydrophobicity. Polar functional groups of GO play
an important role in trapping the formed NPs (Scheme 2b).
Additional polar groups of GO are reduced by keeping on the
addition of the reducing agent. The produced nuclei of Pt and
Sn NPs were grown and self-assembled on the surface of RGO.
RGO acts as a stabilizer and limits the particle size. PtSn/RGO
thin films were completely formed after 24 h (Scheme 2c).
2.2. Electrocatalytic Activity of the Thin Films for
MOR. Preliminarily, the electrochemistry of PtSn and PtSn/
RGO thin films was studied by cyclic voltammetric measure-
ments performed in a 0.50 M H SO solution at a scan rate of
Figure 8. (a−c) TEM images of a PtSn/RGO thin film obtained from
bromo species 3 and (d) histogram of particles size distribution.
2
4
−
1
50 mV s at room temperature (Figures 9a and S12a, c, e).
7
3
d) and bromo species 3 (Figure 8d) are approximately 7 and
nm, respectively.
The presence of both halides from 2 or 3 and GO support,
The humps on these diagrams are associated with atomic
hydrogen desorption and adsorption (I and IV regions,
respectively) and also metal oxide formation and reduction
those have an effective impact on the size of the NPs is
apparent from comparison of the TEM images (Figures 7, 8,
S10, and S11). In fact, the addition of GO to the PtSn alloy
thin films resulted in a decrease of the NP size, an effect that
was registered also passing from 2 to 3 (Table 3). The latter
(
regions II and III). To study the electrocatalytic properties of
PtSn and PtSn/RGO thin films in the MOR, we performed
cyclic voltammetric measurements in a solution of 0.5 M
H SO and 0.50 M CH OH at a scan rate of 50 mV s at
room temperature (Figures 9b and S12b, d, f). These
voltammograms show the appearance of an oxidation current
in the positive potential region. The forward anodic peaks can
be attributed to methanol oxidation, and the oxidation peaks in
the backward scan can be ascribed to the oxidations of
adsorbed CO or CO-like species. It is known that the ratio of
the forward anodic peak current (j ) to the backward anodic
peak current (j ), j /j , is an index of the catalyst tolerance to
incompletely oxidized species aggregated on the surface of the
electrode. A larger ratio represents more complete methanol
−1
2
4
3
−
effect can be ascribed to the larger size of Br with respect to
Table 3. NP Size of PtSn Thin Films
entry
thin film
PtSn(Cl)
PtSn(Br)
PtSn(Cl)/RGO
PtSn(Br)/RGO
morphology
size (nm)
1
2
3
4
spherical NPs
spherical NPs
spherical NPs
spherical NPs
8
6
7
3
f
b
f
b
E
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Scheme 2. Schematic Illustration of Formation of PtSn Nanoalloy Dispersed on RGO
Figure 9. Cyclic voltammograms of PtSn thin films obtained from chloro species 2 (a) in the presence of H SO (0.5 M) and (b) in H SO
4
2
4
2
−
1
electrolyte (0.5 M) containing CH OH (0.5 M) with a scan rate of 50 mV s .
3
oxidation and more efficient elimination of the poisoning
stands for the charge due to the hydrogen adsorption/
desorption in the hydrogen region of the cyclic voltammo-
grams (mC). The highest ECSA value, considerable mass
activity, j /j ratio, and current density in the case of PtSn(Br)/
60
species on the catalyst surface.
As shown in Figures 9b and S12b, d, f, the j /j ratios for
f
b
PtSn/RGO and for PtSn nanoalloy thin films obtained from
bromo species 3 are 6.77 and 1.83, respectively, to be
compared with the values of 2.05 and 0.92 for PtSn/RGO and
PtSn thin films obtained from chloro species 2, respectively.
f
b
RGO thin film make this catalyst unique. Furthermore, PtSn
NP thin films were produced and then placed on GO support.
According to Table 4 entries 41 and 42, these electrocatalysts
exhibit electrocatalytic activity lower than that of PtSn/RGO
thin films with the same metal loadings. Lower current density,
j /j ratio, ECSA, and mass activity confirm our claim (Table 4
61
The j /j ratio reported for Pt NPs thin film is 1.28 while
f
b
other Pt-based electrocatalysts reported in the literature
showed j /jb ratio ranging from 0.60 to 1.88 (Table 4).
f
f
b
Therefore, PtSn/RGO nanoalloy thin film electrode resulting
from precursor containing Br can lead to more complete
methanol oxidation and less accumulation of CO or CO-like
species due to its large j /j ratio. These results demonstrate
entries 39−42 and Figure S13). The lower activity of PtSn thin
film placed on GO is ascribed to less interaction of PtSn NPs
with RGO support. Also, using Nafion to transfer the
electrocatalysts to the surface of glassy carbon electrode
leads to a decrease of the current density, caused by the
f
b
that the PtSn/RGO nanoalloy thin film obtained from bromo
species 3 not only enhances electroactive surface area of thin
film but also increases the performance of MOR in comparison
with other thin films. Here, electrocatalytic performance of
various catalysts in MOR is summarized in Table 4.
65
insulator nature of Nafion. Hence, these facts hamper the
activity of PtSn NPs placed on GO in comparison with PtSn/
RGO thin films.
According to TEM images and Table 3, the size of PtSn/
RGO nanoalloy thin film obtained from the precursor
containing Br is smaller than other thin films. Therefore, this
thin film shows the best electrocatalytic activity (Table 4, entry
Mass activity and electrochemical active surface area
ECSA) are calculated for the as-synthesized thin films
(
according to eqs 1 and 2 and compared with other reported
data (Table 4). Mass activity term is calculated using eq 1:
40). Also, dispersion of NPs on the GO as a support can
i0.9
W
increase the surface area. The PtSn/RGO nanohybrid thin
films obtained from precursors containing Cl and Br exhibit
better catalytic activity compared to other thin films in the
absence of GO support, as shown in Table 4. So, these thin
films can lead to more complete methanol oxidation and less
accumulation of CO or CO-like species. Furthermore, due to
the smaller particle size of PtSn/RGO nanohybrid thin film
obtained from precursor containing Br, it shows a better
electrocatalytic activity than PtSn/RGO nanohybrid thin film
obtained from precursor containing Cl due to its higher surface
to volume ratio.
Am
=
(1)
A stands for the mass activity of the electrocatalyst, i stands
m
0.9
2
for the current density at 0.9 V (A/cm ), and W stands for the
loading of metal especially platinum (mg/cm ). To prevent the
2
inclusion of any concentration polarization, the potential value
(
0.9 V) is chosen. ECSA is calculated using eq 2:
QH
ECSA = 0
.21Mcatalyst
(2)
In this equation, Mcatalyst stands for the loading of the metal,
especially platinum, on the working electrode (mg), and QH
Cyclic voltammograms recorded for PtSn and PtSn/RGO
thin films in 0.5 M H SO and 0.5 M CH OH at different scan
2
4
3
F
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Table 4. Electrocatalytic Activity of Various Catalysts in Methanol Oxidation
current density (mA·
ECSA
mass activity
−1
−
2
2
−1
entry
electrode
j / j ratio
cm
)
E (V, NHE)
(m g Pt)
(A mg )
Pt
ref
f
b
1
2
3
4
5
6
7
8
9
Pt/C
PtRu/C
PtRuCo/C
PtRu/XC-72
Pt/Pd thin film
Pt-CeO2
0.605
0.629
0.868
1.050
1.190
1.200
1.280
1.320
1.500
1.880
1.100
1.590
1.140
1.010
1.564
1.490
0.697
0.647
0.347
0.647
0.540
0.544
0.730
0.494
0.520
0.377
0.484
0.400
0.397
0.544
0.543
0.467
0.380
0.300
0.344
66.67
56.50
233.33
1.780
3.370
15.350
0.167
0.558
62
62
62
63
46
64
61
64
46
63
3
4
5
7
9
10
11
11
12
13
13
13
13
13
13
14
16
18
19
21
22
25
30.00
317.00
203.10
40.00
61.81
45.80
Pt thin film
PtFe O /CeO
2
31.30
0.047
3.500
0.271
0.375
3
4
PtPd/RGO
PtRu/C(E-TEK)
Pt/CN‑doped
Pt/GO-PVP
Pt0.89/RGO-SnO2
PtNi core−shell
109.50
70.00
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
a
70.00
80.56
50.08
0.638
1.60
2.10
1.19
0.17
0.085
Pt Ni/RGO
3
PtNi /C
69.20
24.30
17.70
57.89
31.25
24.13
28.43
35.20
29.00
32.50
3.12
99.40
1.99
54.50
70.20
53.00
190.80
0.823
0.037
0.014
3
PtSn/C
Pt Sn/C
3
PtSn/graphene
1.341
Ag@Pt core-rods
Ag@Pt core-dendrites
Pt/Ccomm NPs
Au@Pt nanodendrites
Cu@PtRu nanowires
PdNi@Pt
0.310
0.220
0.080
0.450
0.450
0.965
1.070
0.447
b
NiCoSn NPs
140.00
60.00
0.90
c
Pd Sn nanorods
0.750
1.140
1.760
1.340
1.580
2
Pt Sn
0.397
0.430
0.444
0.494
−0.486
0.550
3
PtCo/C nanocrystals
Pt Cu nanowires
3.70
2.110
0.707
0.750
32
68
Pt Cu microwires
4.50
60
40
hollow PtNi nanoboxes
Au@Pd/RGO
Pt NPs/LDCNT-NG
28.50
0.330
0.871
0.949
27
27
27
29
this work
this work
this work
this work
this work
this work
d
132.40
24.60
9.59
163.10
120.29
162.81
248.67
25.13
e
Pt/NiFe-LDH-RGO
Pt Pd /C NPs
1.170
0.920
1.830
2.050
6.770
1.960
2.400
0.70
177.70
187.00
182.00
160.10
45.53
0.349
0.380
0.400
0.300
0.320
0.570
0.580
48
52
f
PtSn(Cl) thin film
PtSn(Br) thin film
PtSn(Cl)/RGO thin film
PtSn(Br)/RGO thin film
PtSn(Cl) NPs thin film placed on GO
PtSn(Br) NPs thin film placed on GO
0.625
1.730
2.000
5.240
0.191
0.190
g
h
i
j
k
68.12
97.90
a
b
−1
c
d
3
0 wt % Pt/nitrogen-doped ordered mesoporous carbon-3D. mg NiCoSn (in the absence of Pt). In the case of ethanol oxidation. LDCNT:
e
f
g
h
low defect carbon nanotube, NG: N-doped grapheme. Layered double hydroxide. Pt loading of 0.07 mg. Pt loading of 0.0579 mg. Pt loading of
0
i
j
k
.039 mg. Pt loading of 0.018 mg. Pt loading of 0.0403 mg. Pt loading of 0.021 mg.
−
1
+
−
rates (ranging from 20 to 100 mV s ) are shown in Figures 10
and S14. An increase in the current density with the scan rate is
observed. Also, Figures 10 and S14 show that peak current
densities are linearly proportional to the square root of the
scan rates, suggesting that the electrocatalytic oxidation of
methanol on these thin films is a diffusion-controlled process.
Oxidation of methanol on the surface of Pt-based electro-
catalysts has two main steps: (i) a fast step that contains inner-
sphere electron transfer and known as dehydrogenation step,
and (ii) a rate-determining step that contains electrochemical
PtSn − CH O → PtSn − CH O + H + e
3
2
(4)
5)
+
−
PtSn − CH O → PtSn − CHO + H + e
(
2
+
−
PtSn − CHO → PtSn − CO + H + e
PtSn − OH2 → PtSn − OH + H + e−
(6)
+
(7)
(8)
(9)
PtSn − OH → PtSn − O + H + e−
PtSn − CO + PtSn − O → CO2
The overall reaction is
+
55
oxygen transfer reaction and known as oxidation step. The
oxidation mechanism is as follows (eqs 3−10):
+
−
+
−
PtSn + CH OH → PtSn − CH O + H + e
(3)
CH OH + H O → CO + 6H + 6e
(10)
3
3
3
2
2
G
https://dx.doi.org/10.1021/acs.inorgchem.0c01147
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Article
Figure 10. Cyclic voltammograms of the PtSn thin film obtained from chloro species 2 at different scan rates in 0.5 M H SO + 0.5 M CH OH and
2
4
3
(
b) dependence of the peak currents on the square root of the scan rates.
6
6
According to studies on Pt-based electrocatalysts as well as our
In this equation, J is the initial current density, easily
0
61,66,67
good experience in this field,
most of the poisoning
obtained by extrapolating the linear current decay, and (dJ/dt)
species are carbon monoxide. At the beginning of this field or
even when catalyst composition was new, researchers used gas
chromatography to detect the result of methanol oxidation or
ratio is the slope of the linear portion of the current decay
(
Table 5).
6
8,69
find a mechanism.
Now, the most aspects of this field on
Table 5. Comparing the Long-Term Poisoning Rates of
Different Electrocatalysts
Pt-based electrocatalysts are clear, and just finding a new
catalyst with low metal loading and effective catalytic activity in
addition to high CO tolerance is a challenge.
The stability and recyclability of the as-synthesized electro-
catalysts were examined using chronoamperometry, long-term
poisoning rate δ, and also inductively coupled plasma (ICP)
and TEM investigations. Figure 11 shows the chronoamper-
entry
electrocatalyst
δ (% s−1)
6
6
6
6
6
6
1
2
3
4
5
6
7
8
PtPdCu/RGO thin film
PtCo/RGO thin film
PtCu thin film
0.1200
0.0750
0.0870
Pt thin film
PtSn(Cl) thin film
PtSn(Br) thin film
PtSn(Cl)/RGO thin film
PtSn(Br)/RGO thin film
0.0469
0.0343
0.0420
0.0290
According to Table 5, the δ values exhibit considerable
tolerance of electrocatalysts’ active sites against carbon
monoxide poisoning in comparison to δ values reported for
other thin films. PtSn(Br)/RGO electrocatalyst has the lower δ
value. This means that this electrocatalyst exhibits the lower
poisoning rate or the higher stability against poisoning, as
shown in Table 5. In addition to the electrochemical
investigations, ICP analysis was used for the recycled
electrocatalysts, and the obtained results exhibit no consid-
erable metal leaching. Also, the TEM analysis was applied from
the used and recycled electrocatalysts to confirm the catalyst
stability and shows no considerable agglomeration (Figure
S15).
Figure 11. Chronoamperometric curve of (a) PtSn(Cl), (b)
PtSn(Cl)/RGO, (c) PtSn(Br), and (d) PtSn(Br)/RGO thin films
in 0.5 M H SO and CH OH at 25 °C.
2
4
3
3. CONCLUSION
PtSn thin films described in this paper revealed efficient
electrocatalysts in MOR. In particular, PtSn/RGO thin films
exhibited higher electrocatalytic activity than PtSn thin films
due to their large surface area and superior electrical
conductivity. Moreover, these thin films have higher current
density, higher ratios of forward peak current density to
backward peak current density, and lower oxidation potentials
with respect to Pt thin films and some other Pt-based
electrocatalysts. The major advantages of these catalysts are (i)
applying a facile synthetic procedure to prepare metal alloys,
ometry curves for (a) PtSn(Cl), (b) PtSn(Cl)/RGO, (c)
PtSn(Br), and (d) PtSn(Br)/RGO thin films at 25 °C. As is
obvious in the case of all the as-synthesized electrocatalysts, the
current is rapidly decreased, which is related to the carbon
monoxide formation during the process of methanol oxidation.
Long-term poisoning rate δ is applied to confirm the stability
of the as-synthesized electrocatalysts from the linear decay of
the current measurement for a period of more than 500 s using
chronoamperometry curves and eq 11:
(
ii) reducing costs associated with the use of a nonprecious
δ = 1 × j
00
idJ y
z
z
j
z
j
metal (Sn) for Pt-based alloy, and (iii) improving the
performance via using PtSn/RGO thin films. These results
J
dt
k
{
(11)
0
H
https://dx.doi.org/10.1021/acs.inorgchem.0c01147
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
clearly indicate that this method and these kinds of catalysts
are promising for fuel cells and sensors.
voltammograms for electrochemical investigation of the
studied materials (PDF)
Accession Codes
4
. EXPERIMENTAL SECTION
CCDC 1815430−1815431 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
The complex [PtMe (bipy)], 1, was prepared according to the
2
35
literature. GO was synthesized from graphite powder by a modified
70
Hummers−Offeman method. The NMR spectra of the mixtures
obtained upon addition of SnX (X = Cl, Br) to 1 were recorded using
2
a Bruker Advance 400 MHz spectrometer in dmso-d6 or thf-d8.
Powder XRD patterns were obtained using a Bruker AXS (D8
Advance) instrument employing the reflection Bragg−Brentano
geometry with Cu Kα radiation (λ = 1.54184 Å). TEM images
were taken with a Philips CM-10 microscope operated at 100 kV. XPS
analyses were carried out using a scanning microprobe PHI 5000
VersaProbe II (Physical Electronic) with monochromatized Al Kα X-
ray radiation source. Survey scans and high-resolution spectra were
acquired in Fixed Analyzer Transmission (FAT) mode, with pass
energy of 117.4 and 29.35 eV, respectively. The MultiPak software
AUTHOR INFORMATION
Corresponding Authors
■
S. Jafar Hoseini − Professor Rashidi Laboratory of
Organometallic Chemistry, Department of Chemistry, College of
Sciences, Shiraz University, Shiraz 71946-84795, Iran;
Department of Chemistry, Faculty of Sciences, Yasouj University,
Yasouj 75918-74831, Iran; orcid.org/0000-0003-4514-
1096; Email: sj.hoseini@shirazu.ac.ir, sjhoseini54@
yahoo.com
(
v.9.9.0) was exploited for data analysis.
.1. Preparation of PtSn Thin Films. Five milligrams of the
product of the reaction between 1 and SnCl or SnBr was dissolved
4
2
2
in toluene (25 mL) and placed in an ultrasonic bath for 10 min to
afford a yellow-orange (Cl) or orange (Br) solution. Then, this
solution was added to a beaker (100 mL) containing distilled water
Piero Mastrorilli − DICATECh, Politecnico di Bari, I-70125
Bari, Italy; orcid.org/0000-0001-8841-458X;
Email: p.mastrorilli@poliba.it
(
25 mL) and allowed to stand in contact with each other. In the next
step, when the two layers were stabilized, a fresh aqueous solution of
Authors
NaBH (5 mL, 0.10 M) was injected dropwise using a syringe. After 4
4
Nahal Aramesh − Department of Chemistry, Faculty of Sciences,
Yasouj University, Yasouj 75918-74831, Iran
h, the organic phase turned pale yellow, and thin film formation was
completed after 24 h.
Hamid R. Shahsavari − Department of Chemistry, Institute for
Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-
66731, Iran; orcid.org/0000-0002-2579-2185
S. Masoud Nabavizadeh − Professor Rashidi Laboratory of
Organometallic Chemistry, Department of Chemistry, College of
Sciences, Shiraz University, Shiraz 71946-84795, Iran;
orcid.org/0000-0003-3976-7869
Mehrangiz Bahrami − Professor Rashidi Laboratory of
Organometallic Chemistry, Department of Chemistry, College of
Sciences, Shiraz University, Shiraz 71946-84795, Iran
Mohammad Reza Halvagar − Department of Inorganic
Chemistry, Chemistry and Chemical Engineering Research
Center of Iran, Tehran 14968-13151, Iran
4
.2. Preparation of PtSn/RGO. First, GO (10 mg) in distilled
water (25 mL) was sonicated for 10 min to make a good dispersion.
Then, the product of the reaction between 1 and SnCl or SnBr (5
mg, 25 mL) in toluene was placed in a sonic bath for 10 min, leading
to a yellow-orange solution. Finally, this solution was added to the
GO solution in a beaker (100 mL), and they were allowed to stand in
contact with each other. Once the two layers were stabilized, a fresh
aqueous solution of NaBH (10 mL, 0.10 M) was injected into the
aqueous layer via a syringe. The onset of reduction was indicated by a
coloration of the toluene−water interface. After 24 h, PtSnO /RGO
thin film formation was completed.
2
2
4
2
4
.3. Electrode Preparation. Transferring of thin films from LLI
to the surface of carbon electrode was performed via lifting-up
method of the electrode from the liquid phase. The organic phase was
removed by a syringe; the thin film was transferred to a glassy lamella,
and then the surface of glassy carbon electrode was squeezed on the
lamella surface until dried.
Elvira De Giglio − Dipartimento di Chimica, Universita
studi di Bari “Aldo Moro”, I-70125 Bari, Italy
̀
degli
Mario Latronico − DICATECh, Politecnico di Bari, I-70125
Bari, Italy
4
.4. Electrochemical Measurements. Electrochemical measure-
ments were carried out by an Autolab Potentiostat/Galvanostat
PGSTAT12 (Eco Chemie, Switzerland) at room temperature in a
standard three-electrode cell. Ag/AgCl was applied as a reference
electrode, and a platinum wire was used as a counter electrode. A
modified glassy carbon electrode of 2 mm diameter with prepared
platinum-based thin films was used as a working electrode. All
potentials were converted to values with reference to a normal
hydrogen electrode (NHE). All data of cyclic voltammograms were
recorded under the same conditions.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01147
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Shiraz University Research Council, Yasouj
University Research Council, Iran National Science Founda-
tion (Grant 96010235), and Iran Science Elites Federation for
their support. This manuscript is dedicated to Prof. Richard J.
Puddephatt on his 77th birthday.
ASSOCIATED CONTENT
■
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01147.
REFERENCES
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Figures S1 and S2 show the structural figures of the
studied materials; Table S1 provides the crystal data,
data collection, and structure refinement details for the
studied materials; Figures S3−S15 exhibit some NMR
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