Efficient Ternary Synergism of Platinum/Tin Oxide/Nitrogen-Doped Carbon Leading to High-Performance Ethanol Oxidation

Article abstract of DOI:10.1021/acscatal.8b01573

Direct ethanol fuel cells are attractive alternative power sources due to the use of liquid fuels featuring high energy density, low toxicity, easy storage, and biomass-derived production. To date, most Pt-based electrocatalysts are still limited by low mass activity and high susceptibility to poisoning for the ethanol oxidation reaction (EOR) in acidic medium. Herein, we have constructed a ternary platinum/tin oxide/nitrogen-doped carbon electrocatalyst for the EOR by highly dispersing the hybridized platinum/tin oxide on nitrogen-doped carbon nanocages. CO electrooxidation from the stripping experiments is used as a sensitive indicator to evaluate the antipoisoning capability of the catalysts. By a comparison study on a series of designed catalysts, the correlation of the CO resistibility with the geometrical configuration has been well established for the catalysts. We demonstrate that the efficient ternary synergism of Pt/SnO_x/N-doped sp²-C via the heterointerfaces is the key to high CO resistibility, which could facilitate the oxidative removal of CO species at Pt sites by the adsorbed OH species generated at neighboring SnO_x sites and thereby the facile regeneration of Pt active sites. Accordingly, a synergistic catalyst has been optimized which shows high EOR performance in acidic medium with a mass activity of 1187 mA mg_Pt^-1 and high durability, in comparison with most reported catalysts to date. This study provides an approach of exploring advanced EOR electrocatalysts for potential applications.

Full text of DOI:10.1021/acscatal.8b01573

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Article
Efficient Ternary Synergism of Platinum/Tin Oxide/Nitrogen-
doped Carbon Leading to High-Performance Ethanol Oxidation
Zhiqi Zhang, Qiang Wu, Kun Mao, Yugang Chen, Lingyu Du,
Yongfeng Bu, Ou Zhuo, Lijun Yang, Xizhang Wang, and Zheng Hu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01573 • Publication Date (Web): 01 Aug 2018
Downloaded from http://pubs.acs.org on August 1, 2018
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Efficient Ternary Synergism of Platinum/Tin
Oxide/Nitrogenꢀdoped Carbon Leading to
HighꢀPerformance Ethanol Oxidation
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Zhiqi Zhang, Qiang Wu,* Kun Mao, Yugang Chen, Lingyu Du, Yongfeng Bu, Ou Zhuo, Lijun Yang,
Xizhang Wang, and Zheng Hu*
Key Laboratory of Mesoscopic Chemistry of MOE and Jiangsu Provincial Laboratory for
Nanotechnology, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023,
China.
ABSTRACT
Direct ethanol fuel cells are attractive alternative power sources due to the usage of liquid fuels featuring
high energy density, low toxicity, easy storage and biomassꢀderived production. To date, most Ptꢀbased
electrocatalysts are still limited by the low mass activity and high susceptibility to poisoning for ethanol
oxidation reaction (EOR) in acidic medium. Herein, we have constructed the ternary platinum/tin
oxide/nitrogenꢀdoped carbon electrocatalyst for EOR by highly dispersing the hybridized platinumꢀtin
oxide on nitrogenꢀdoped carbon nanocages. CO electrooxidation from the stripping experiments is used
as a sensitive indicator to evaluate the antiꢀpoisoning capability of the catalysts. By comparison study on
a series of designed catalysts, the correlation of the CO resistibility with the geometrical configuration has
been well established for the catalysts. We demonstrate that the efficient ternary synergism of
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Pt/SnO /Nꢀdoped sp ꢀC via the heterointerfaces is the key for high CO resistibility, which could facilitate
x
the oxidative removal of CO species at Pt sites by the adsorbed OH species generated at neighbouring
SnO
x
sites, thereby the facile regeneration of Pt active sites. Accordingly, the synergistic catalyst has been
optimized which shows the high EOR performance in acidic medium with a mass activity of 1187 mA
ꢀ
1
mgPt and high durability, in comparison with the most reported catalysts to date. This study provides an
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approach of exploring advanced EOR electrocatalysts for potential applications.
KEYWORDS
2
ethanol oxidation reaction, ternary synergism, platinum/tin oxide/Nꢀdoped sp ꢀC, CO resistibility, mass
activity
1
. INTRODUCTION
Direct alcohol fuel cells are promising power sources for movable electric devices owing to their high
1
energy conversion efficiency and easy storage/transportation of fuels. Ethanol fuel is particularly
ꢀ1
attractive in virtue of its high theoretical energy density (8 kWh kg ), low toxicity and availability from
2,3
biomass production. Generally, ethanol oxidation reaction (EOR) can be realized electrocatalytically
3
,4
via a 4 or 12 electron process to produce acetic acid or CO , respectively. The 12 electron process is
2
4,5
preferred in terms of energy density and fuel utilization, but is harder to follow owing to the difficulty
6
,7
in breaking CꢀC bond. Density functional theory simulations indicated that breaking the CꢀC bond in
5,7
CHCO intermediate has a low activation energy over Pt (111) surface, implying the great potential of Pt
electrocatalyst in EOR. However, Pt is expensive and highly susceptible to poisoning by surfaceꢀadsorbed
7ꢀ9
intermediates such as CO and CHx. Hence, increasing the mass activity and antiꢀpoisoning capability of
Pt in the EOR is of great significance. A conventional wisdom is to construct hybrid catalysts with
synergistic effect. With costꢀefficient concern, oxophilic species are usually introduced to increase the
antiꢀpoisoning capability of Pt, which could boost the water dissociation to form adsorbed OH species
1
0,11
(OHad), and help the removal of adsorbed intermediates (COad, CHx,ad) on Pt sites for regeneration.
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Teng et al. prepared Pt ꢀ(SnO ) coreꢀshell particles with a strong capability for CꢀC bond breaking of
46
2 54
ethanol in acidic medium, but the mass activity was partially suppressed owing to the coverage of Pt
7
active sites by the SnO
2
shell. Li et al. reported PtꢀNi(OH)
2
/graphene ternary catalysts for methanol
ꢀ
1
oxidation with exceptional activity (1236 mA mgPt ) and durability resulting from the collective synergy
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of the three functional components, which, however, only worked in alkaline medium. Exploring
highꢀactive and durable Ptꢀbased EOR electrocatalysts working in acidic medium is still a challenging
topic.
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Herein, with pristine or nitrogenꢀdoped carbon nanocages (CNC or NCNC) as supports, we have
constructed the ternary platinum/tin oxide/NCNC (or CNC) catalysts for EOR in acidic medium. CO
electrooxidation from the stripping experiments is used as a sensitive indicator to evaluate the
antiꢀpoisoning capability of the catalysts. By comparison study on six designed catalysts, we reveal that
2
the efficient ternary synergism of Pt/SnO
x
/Nꢀdoped sp ꢀC via the heterointerfaces is the key for high CO
resistibility. Accordingly, the optimized catalyst presents a high durability and superb mass activity,
superior to the most EOR catalysts in acidic medium to date, which provides a promising approach for
exploring advanced EOR electrocatalysts.
2. EXPERIMENTAL SECTION
2.1 Synthesis
NCNC and CNC supports were prepared by in situ MgO template method at 800 °C using pyridine and
12ꢀ14
benzene precursor, respectively.
NCNC presents the 3D hierarchical morphology, featuring large
2
ꢀ1
specific surface area (814 m g ), coexisting microꢀmesoꢀmacropores, high N content (8.73 at.%) and
ꢀ1
bulk conductivity (238 S m ) (Figure S1 in the Supporting Information). As the support, NCNC not only
enables fast electron transfer due to the high conductivity, but also contributes to electrolyte accessibility
14
owing to the good wettability and hierarchical geometry which also guarantees the uniform distribution
x
of Pt and SnO .
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PtꢀSnO /NCNC or PtꢀSnO /CNC catalysts were synthesized by a convenient microwaveꢀassisted
x
x
ethylene glycol (EG) reduction. Typically, 20 mg NCNC or CNC was dispersed ultrasonically into 50 mL
EG, and then an appropriate amount of H PtCl ·6H O and/or SnCl ·2H O was added to the suspension
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and stirred for 3 h. The pH value of the mixture was adjusted to 8 by using NaOH/EG solution. After
irradiating in microwave oven (700 W) for 120 s, the catalysts were obtained after filtrating, washing with
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ethanol and water, drying at 80 °C. By adjusting the amount of SnCl
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·2H
2
O, the catalysts with tunable
/CNC. To understand the
Sn/Pt ratio (n) could be synthesized, designated as PtꢀnSnO /NCNC or PtꢀnSnO
x
x
role of ternary interaction on CO resistibility, we prepared six catalysts for comparison study, with the
difference in support (NCNC or CNC), or n value (0 or 0.5), or preparation procedure. Specifically, for
n=0, the binary Pt/CNC and Pt/NCNC catalysts could give us the clue on contribution of nitrogen doping.
For n=0.5, in addition to Ptꢀ0.5SnO
Pt and SnO were also loaded on NCNC by the twoꢀstep process. Pt+0.5SnO
firstly loading Pt on NCNC followed by loading SnO on the asꢀprepared Pt/NCNC, and reversely for
.5SnO +Pt/NCNC. These four catalysts could give us the clue on the contribution of the ternary
synergism. A control catalyst of SnO /NCNC (16.3 wt %) is prepared in the similar way.
x
/NCNC and Ptꢀ0.5SnO
x
/CNC prepared by the coꢀdeposition process,
x
x
/NCNC was prepared by
x
0
x
x
2
.2 Characterization
The structure and morphology of the catalysts were characterized by Xꢀray diffraction (XRD, Bruker
Xꢀray diffractometer, D8 Advance A25, Co Kα radiation, λKα1=0.178897 nm) and transmission electron
microscopy (TEM, JEMꢀ2100). The Sn/Pt atomic ratio was measured by Xꢀray photoelectron
spectroscopy (XPS, VG ESCALAB MKII), and the binding energies refer to C1s at 284.6 eV. The mass
loading of Pt in the catalysts was analysed by inductively coupled plasmaꢀmass spectroscopy (ICPꢀMS,
2
Optima 5300DV) (Table S1 in the Supporting Information). N adsorption/desorption isotherm was
measured on a Thermo Fisher Scientific Surfer Gas Adsorption Porosimeter at 77 K, and the specific
surface area was calculated using the BrunauerꢀEmmettꢀTeller method based on the adsorption data.
2.3 Electrochemical measurements
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Electrochemical tests were conducted on Bioꢀlogic VMP3 electrochemical workstation using a standard
three electrode system at 25 °C. A Pt wire served as the counter electrode, a saturated calomel electrode
(SCE) as the reference electrode, and a glassy carbon (GC) disk (3 mm in diameter) modified by the
catalysts as the working electrode. The working electrode was fabricated as follows: 2 mg catalyst
powder was dispersed in a mixed solution of 800 ꢁL water, 200 ꢁL ethanol and 40 ꢁL Nafion (DuPont, 5
wt %). After 1 h ultrasonic treatment, a homogeneous black suspension was formed. Then, 10 ꢁL
suspension was dropped onto a GC electrode surface, and dried at room temperature for 10 h. The
electrochemical surface areas (ECSAs) of the catalysts were measured by the charge involved in the
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hydrogen adsorption/desorption region in 0.5 mol L H
2 4
SO solution in cyclic voltammograms (CV). The
ꢀ1
electrocatalytic activity for EOR was examined by collecting CV in a N ꢀpurged 0.5 mol L H SO and 1
2
2
4
ꢀ1
ꢀ1
mol L ethanol solution at a scan rate of 50 mV s . Several activation scans were performed until the
stable CV curves were obtained. In the CO stripping measurements, a monolayer of CO was adsorbed on
ꢀ1
catalyst by flowing CO in 0.5 mol L H
2
SO
4
for 20 min while holding the electrode potential at ꢀ0.15 V.
for 20 min. Stripping measurements
Nonꢀadsorbed CO was removed by bubbling the electrolyte with N
2
ꢀ1
were initiated from ꢀ0.15 V to 0.9 V in the forward scan at 50 mV s for at least two consecutive scans.
ꢀ
1
The stabilities of the catalysts were evaluated by chronoamperometric (CP) tests performed in 0.5 mol L
ꢀ1
2 4
H SO and 1 mol L ethanol at 0.6 V for a period of 5000 s. The response to intentional CO poisoning
was recorded by bubbling CO to the electrolyte at the middle of CP tests.
3. RESULTS AND DISCUSSION
The CO resistibility of the six catalysts was evaluated by CO electrooxidation from the stripping
experiments as shown in Figure 1. For the binary catalysts of commercial Pt/C, Pt/CNC and Pt/NCNC, a
main peak appears in the first forward scan with the peak potential of 0.60~0.61 V (Figure 1aꢀc), which
15
stems from the CO electrooxidation at Pt nanoparticles. Without nitrogen participation, the peak appears
in the ‘lanky’ one (Figure 1a,b). In contrast, with nitrogen participation, the peak appears in the ‘dumpy’
one with a shoulder peak at the left side and a small peak at 0.28 V (Figure 1c). This difference clearly
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indicates the positive contribution of PtꢀN interaction to the CO electrooxidation. Specifically, the PtꢀN
interaction induced the electron transfer from N dopants to Pt species (Figure S2 in the Supporting
Information). This electronic effect could weaken the adsorption strength of CO and facilitate the
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oxidation of CO at a lower potential, which could be responsible for the small peak at 0.28 V. In
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addition, the N doping favors the high dispersion of Pt nanoparticles.
Hence the sizes of partial Pt
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nanoparticles in Pt/NCNC are somewhat smaller than the cases in Pt/CNC, which may lead to the
shoulder peak at the left side. For the ternary catalysts, the main peak negatively shifts to ca. 0.54 V for
x x
Ptꢀ0.5SnO /NCNC and Ptꢀ0.5SnO /CNC, accompanied by an extended broad peak towards the low
potential side (Figure 1d,e). Such a change indicates the ternary synergism via the intimate contact of Pt,
SnO and NCNC (or CNC) further favors the CO electrooxidation. Similar to the difference in the binary
x
catalysts (Figure 1b,c), Nꢀparticipation makes the peak extend towards the left and change from ‘lanky’ to
‘dumpy’ in comparison with the case without Nꢀparticipation (Figure 1d,e). Therefore, both
Nꢀparticipation and ternary synergism is beneficial to the CO electrooxidation. Furthermore, the influence
x
of the contact situation of Pt, SnO and NCNC (or CNC) on the synergism thereof the CO
electrooxidation was examined, as demonstrated in the comparison of CV curves for Ptꢀ0.5SnO /NCNC,
x
Pt+0.5SnO /NCNC, and 0.5SnO +Pt/NCNC (Figure 1d,f,g). Ptꢀ0.5SnO /NCNC prepared by the
x
x
x
coꢀdeposition process possesses much more heterointerfaces of the closely contacted ternary species than
the Pt+0.5SnO /NCNC and 0.5SnO +Pt/NCNC by the twoꢀstep deposition process, especially for the
heterointerfaces between Pt and SnO . For Pt+0.5SnO /NCNC, there are two typical configurations, i.e.,
x
x
x
x
the unsatisfactory ternary contact parts and the isolated SnO /NCNC parts, leading to the slight peak shift
x
to ca. 0.57 V while without the contribution from SnO /NCNC (Figure 1f). In contrast, for
x
0.5SnO
x
+Pt/NCNC, there are two typical configurations, i.e., the unsatisfactory ternary contact parts and
the isolated Pt/NCNC parts. Since most N sites were covered by SnO
x
during the first step process, the
unsatisfactory ternary contact parts is similar to the case in Ptꢀ0.5SnO
x
/CNC (Figure 1e), leading to the
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shoulder peak at ca. 0.56 V extending to 0.22 V (Figure 1g); and the isolated Pt/NCNC parts is similar to
the cases in Pt/C and Pt/CNC (Figure 1a,b), leading to the peak at ca. 0.60 V.
Based on the preceding experimental results and analysis, the different characteristics of these catalysts
could be figured out as schematically shown in Figure 1a’ꢀg’, and the contribution of the ternary
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synergism to CO resistibility can be deduced. As known, the SnO species could boost the OHad
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formation by water dissociation, which in turn facilitates the electrooxidation removal of the COad at Pt
sites generated during EOR, thereby the facile regeneration of Pt active sites. The smooth proceeding of
this sequential process needs the quick removal of CO with the OH assistance at respective Pt and
ad
ad
SnO
x
sites. Accordingly, the synergism of Pt and SnO
x
at the heterointerfaces with intimate interaction is
/NCNC
the best situation, which leads to the facile regeneration of Pt active sites. Hence, the Ptꢀ0.5SnO
x
catalyst presents the best COꢀresistant capability, which is confirmed by the durability evaluations of CP
measurements (Figure S3 in the Supporting Information).
8
4
0
(a)
0.60 V
0.61 V
0.60 V
(
a’)
-
4
Pt/C
8
(b)
(c)
(
b’)
c’)
4
0
-4
8
Pt/CNC
(
4
0
0.28 V
-
-
-
4
Pt/NCNC
8
(d)
(e)
(f)
(d’)
0
.54 V
4
0
0.09 V
0.19 V
Pt-0.5SnO /NCNC
4
8
x
(e’)
0.54 V
4
0
0.14 V
0.16 V
Pt-0.5SnO /CNC
x
4
8
(f’)
0.57 V
4
0
Pt+0.5SnO /NCNC
-4
8
x
(
g)
0.56 V
(g’)
0
.60 V
0.22 V
4
0
0.5SnO +Pt/NCNC
-4
x
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
C
N
Pt-1
Pt-2
SnOx
E (V vs. SCE)
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ꢀ
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Figure 1. CO stripping experiments in 0.5 mol L H SO . (a) 20 wt % Pt/C. (b) Pt/CNC. (c) Pt/NCNC.
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(
x x x x
d) Ptꢀ0.5SnO /NCNC. (e) Ptꢀ0.5SnO /CNC. (f) Pt+0.5SnO /NCNC. (g) 0.5SnO +Pt/NCNC. The red and
black curves come from the first and second scan, respectively. (a’ꢀg’) The schematic structural
characteristics corresponding the catalysts in (aꢀg). Commercial Pt/C catalyst in (a) is used as the
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benchmark. Ptꢀ1: with neighbouring N or/and SnO ; Ptꢀ2: without neighbouring N or SnO .
x
x
Note: For Pt+0.5SnO /NCNC and 0.5SnO +Pt/NCNC prepared by the twoꢀstep deposition process, most
x
x
of the PtꢀN moieties for the former or N sites for the latter were actually covered during the deposition of
the SnO species. Hence, the contribution from PtꢀN interaction is limited. The unsatisfactory ternary
contact parts present the similar contribution in CV curves to that of Ptꢀ0.5SnO /CNC (Figure 1eꢀg).
x
x
We have further prepared a series of PtꢀnSnO /NCNC catalysts (n=0, 0.3, 0.5, 0.8, 1.0 and 1.7,
x
respectively) by coꢀdeposition process to optimize EOR performance and examine the
performanceꢀstructure relationship. Figure 2 depicts the XPS spectra of Pt 4f and Sn 3d of the catalysts.
2
1,22
The Pt species exist in Pt(0) and Pt(II),
while the Sn species mainly in Sn(IV) with a small fraction of
With increasing the Sn/Pt ratio, both Pt(II) and Sn(IV) species show the evolutions of first
decrease then increase (Figure 2, Figure S4 in the Supporting Information), presenting nearly pure SnO2
species for Ptꢀ1.7SnO /NCNC, which should be associated with the feedstock of SnCl ·2H O. In catalyst
preparation, the presence of slight SnCl
the Pt(0) species. But the high SnCl ·2H
2
3,24
Sn(II).
x
2
2
2ꢀ 25
2
2
·2H
2 6
O could boost the EG reduction of PtCl , thus increase
2
O dosage introduced too much coordinated water, leading to the
oxidation of metal species (Figure S5 in the Supporting Information). It is noted that the binding energies
for these species are almost unchanged, suggesting the negligible alloying of Pt and Sn.
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Figure 2. XPS spectra of PtꢀnSnO /NCNC catalysts. Note: The binding energies for the marked species
x
are about 71.07 eV [4f7/2 of Pt(0)], 72.24 eV [(4f7/2 of Pt(II)], 486.74 eV [3d5/2 of Sn(IV)] and 485.50 eV
[3d5/2 of Sn(II)].
XRD patterns of PtꢀnSnO
Supporting Information). The halfꢀpeak widths of Pt increase with increasing Sn content, implying the
downsizing of Pt nanoparticles. For the sample with n=1.7, the pattern could be assigned to SnO (PDF#
1ꢀ1445), in consistence with the XPS results. Worthy to mention is that the diffraction peaks for Pt show
a negligible shift with increasing the Sn content, which again suggests negligible alloying of Pt and Sn.
Figure 3 shows the typical TEM characterizations of the PtꢀnSnO /NCNC catalysts. Pt nanoparticles
are uniformly dispersed on the NCNC supports owing to the nitrogen participation (Figure 3aꢀd).
x 2
/NCNC present the diffraction peaks of metal Pt and SnO (Figure S6 in the
2
4
x
1
6ꢀ19
The
average size of Pt nanoparticles is 3.9, 3.4, 2.5 and 1.9 nm for n=0, 0.5, 1.0 and 1.7, respectively, i.e.,
decreases with increasing Sn/Pt ratio, in agreement with the XRD results. Highꢀresolution TEM
(
HRTEM) images display the characteristic lattice fringes of Pt (111) with the interplanar distance of
.227 nm for all the catalysts (Figure 3eꢀh). In the ternary cases, the lattice fringes with the distance of
0.335 nm were observed, assigned to the (110) planes of SnO (Figure 3fꢀh). The SnO species present as
0
2
2
the nanosheetꢀlike morphology as reflected by the image contrast. All the Pt nanoparticles overlap with
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the neighbouring SnO
2
nanosheets due to the coꢀdeposition process. With increasing the Sn content, the
overlapping degree gets more and more severe (Figure 3fꢀh), leading to the decreasing of exposed Pt
sites.
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0
(a)
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(b)
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(c)
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Particle size (nm)
Particle size (nm)
Particle size (nm)
Particle size (nm)
2
0 nm
20 nm
(f)
20 nm
20 nm
(h)
(
g)
Pt (111)
0.227 nm
(
e)
Pt (111)
.227 nm
SnO (110)
0
2
SnO (110)
2
0
.335 nm
0.335 nm
Pt (111)
.227 nm
Pt (111)
0.227 nm
0
SnO (110)
2
0
.335 nm
2
nm
2 nm
2 nm
2 nm
Figure 3. Typical (HR)TEM images and corresponding Pt particle size distributions of the catalysts. (a,e)
Pt/NCNC. (b,f) Ptꢀ0.5SnO /NCNC. (c,g) Ptꢀ1.0SnO /NCNC. (d,h) Ptꢀ1.7SnO /NCNC. Black arrows show
x
x
x
2
the Pt nanoparticles wrapped by SnO nanosheets. The particle size distribution was determined by
counting 300 nanoparticles.
The CO stripping experiments are also performed for this series of catalysts to examine the
antiꢀpoisoning capability, as shown in Figure 4. All the peak potentials for the ternary catalysts (n≠0) are
lower than 0.60 V for the binary catalyst (n=0), which results from the ternary synergism as expected.
With increasing n from 0 to 0.5, the peak potential gradually shifts from 0.60 V (n=0) to 0.59 V (n=0.3)
and 0.54 V (n=0.5), accompanied by an increasing extendedꢀpeak towards the low potential side. Further
increasing n leads to the shifting of the peak potential back to 0.57 V (n=0.8, 1.0, 1.7), accompanied by a
shrinking of the extendedꢀpeak. Such a profile evolution of CV curves is in agreement with the
morphological change (Figure 3), i.e., a suitable quantity of SnO
x
can form effective heterointerfaces for
the ternary synergism while too much SnO will lead to the decreasing of exposed Pt sites by
x
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overꢀwrapping. This changing behaviour is indeed what we expected according to the synergism picture
in Figure 1, which further supports the synergism mechanism. Hence, the CO resistibility of
x
PtꢀnSnO /NCNC presents a volcanoꢀtype change with increasing n (the Sn/Pt ratio), i.e., first increases
then decreases with the optimal for n=0.5.
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0
x
Figure 4. CO stripping experiments for PtꢀnSnO /NCNC catalysts (n=0, 0.3, 0.5, 0.8, 1.0 and 1.7). The
curves in Figure 1c,d (n=0, 0.5) are replotted here for convenient comparison. The red and black curves
come from the first and second scan, respectively.
The electrochemical performances of the PtꢀnSnO /NCNC catalysts are shown in Figure 5. Based on
x
ꢀ1
the CV curves tested in the N
2 2 4
ꢀsaturated 0.5 mol L H SO solution (Figure 5a), ECSAs are calculated to
2
ꢀ1
be 29.8, 44.4, 77.6, 63.0, 49.3 and 39.7 m gPt for n=0, 0.3, 0.5, 0.8, 1.0 and 1.7, respectively. ECSAs
2
ꢀ1
2
ꢀ1
first increase then decrease with the maximum of 77.6 m gPt for n=0.5, which is larger than 47.3 m gPt
for the Pt/C benchmark catalyst. As revealed by HRTEM observation, with increasing n, Pt nanoparticles
gradually decrease in size, accompanied by the increasing overlapping of Pt nanoparticles and SnOx
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nanosheets (Figure 3). The compromise of these two factors results in the volcanoꢀtype evolution of
ECSAs.
The CV curves of Ptꢀcontaining catalysts display two peaks derived from EOR while that for the
control catalyst of SnO
Figure 5b). The mass activities were evaluated by comparing the peak current densities in the forward
scans. Specifically, with increasing n from 0 to 1.7, the six catalysts produce the maximum current
x
/NCNC is featureless, which indicates the origin of EOR activity from Pt species
(
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ꢀ
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densities of 637, 957, 1187, 1023, 910 and 646 mA mgPt , respectively. A volcanoꢀtype change is also
observed with the highest mass activity for n=0.5 (Figure 5c), in accordance with the evolution of
ECSAs. This result clearly indicates that the mass activity of Pt can be effectively enhanced by the
ternary synergism of Pt, SnO and NCNC, but will be suppressed to some extent when the Pt is
x
overꢀwrapped by SnO . Therefore, both the ternary synergism and the suitable Pt exposure are crucial for
x
x
improving the mass activity. Worthy to mention is that the mass activity of Ptꢀ0.5SnO /NCNC (1187 mA
ꢀ
1
ꢀ1
mgPt ) is almost 4.0ꢀfold higher than that of the 20 wt % commercial Pt/C (297 mgPt ), and also obviously
26ꢀ32
larger than the data of most reported catalysts to date (Table S2),
suggesting the great potential for
application. The stability of the series of catalysts shows a tendency of first increase (from n=0 to 0.5) and
then decrease (from n=0.5 to 1.7) after 5000 s test, similar to the changing trend of the EOR activity with
2
the Sn/Pt ratio. After the stability tests, SnO species still exist adjacent to the Pt nanoparticles, indicating
that the SnO can survive in acidic medium for a long operation time (Figure S7 in the Supporting
x
Information). It is noted that the evolutions of EOR performances, ECSAs and CO resistibility with
increasing n highly coincide with each other. Hence, in addition to acting as the sensitive indicator for
antiꢀpoisoning capability, CO electrooxidation can even be used to reflect the EOR performance for this
series of catalysts.
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Figure 5. Electrochemical performances of PtꢀnSnO /NCNC catalysts (n=0, 0.3, 0.5, 0.8, 1.0 and 1.7). (a)
ꢀ
1
ꢀ1
ꢀ1
CV curves in 0.5 mol L H
Evolutions of ECSAs and mass activities. (d) The comparison between the optimal Ptꢀ0.5SnO
dashed curve) and the control catalysts. The curves for 20 wt % Pt/C and SnO /NCNC are presented for
comparison.
2
SO
4
. (b) CV curves in 0.5 mol L H
2
SO
4
and 1 mol L ethanol. (c)
x
/NCNC
(
x
The contribution of the ternary synergism is also supported by comparing the performances of
Ptꢀ0.5SnO /NCNC and several control catalysts. Generally, the binary Pt/NCNC catalyst exhibits much
inferior EOR activity to the ternary catalysts, and SnO /NCNC has little EOR activity (Figure 5b). The
EOR peak potential of Ptꢀ0.5SnO /NCNC is ca. 30 mV lower than that of Ptꢀ0.5SnO /CNC, which may
x
x
x
x
result from the electronic effect of PtꢀN interaction on the EOR, as supported by electrochemical
impedance spectroscopy characterization (Figures S2 and S8 in the Supporting Information). In addition,
replacing NCNC with CNC leads to ~25% decrease in mass activity for Ptꢀ0.5SnO
x
/CNC compared with
/NCNC (896 vs 1187 mA mgPt ) (Figure 5d). These results indicate the ternary synergism of Pt,
and NCNC is crucial to the high EOR activity of Ptꢀ0.5SnO /NCNC. Actually, the loading sequence
influences the EOR activity a lot. The control catalysts of Pt+0.5SnO /NCNC and 0.5SnO +Pt/NCNC
ꢀ
1
Ptꢀ0.5SnO
SnO
x
x
x
x
x
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with unsatisfactory ternary contacts show the peak current densities of 799 and 758 mA mgPt
,
respectively, only about twoꢀthirds of that for Ptꢀ0.5SnO /NCNC (Figure 5d). This result further
x
demonstrates the significance of the intimate interaction via the heterointerfaces of Pt, SnO
by coꢀdeposition process which favors their synergism thereby the high mass activity.
x
and NCNC
Actually, our test for methanol oxidation in acidic medium exhibits ca. 3.7ꢀfold higher mass activity for
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ꢀ1
the Ptꢀ0.5SnO /NCNC than the benchmark Pt/C (1109 vs 298 mA mgPt ), suggesting the potential of the
x
synergistic catalysts in alcohol oxidation reactions (Figure S9 in the Supporting Information).
4. CONCLUSION
2
In summary, we have successfully constructed a series of ternary Pt/SnO
x
/Nꢀdoped sp ꢀC electrocatalysts
for EOR in acidic medium. By taking CO electrooxidation as a sensitive indicator for antiꢀpoisoning
capability, the correlation of the CO resistibility with the geometrical configuration has been well
established for the catalysts. The efficient ternary synergism of Pt, SnO
is crucial to the high CO resistibility, which could facilitate the oxidative removal of CO species at Pt
sites by the adsorbed OH species generated at neighbouring SnO sites, thereby the facile regeneration of
Pt active sites. The optimized synergistic catalyst Ptꢀ0.5SnO /NCNC demonstrates the excellent EOR
x
and NCNC via heterointerfaces
x
x
ꢀ
1
performance in acidic medium with a superb mass activity of 1187 mA mgPt and high durability, superior
to the most reported catalysts to date. The consistence in the evolutions of EOR performances, ECSAs
and CO resistibility with increasing Sn/Pt ratio indicates that CO electrooxidation can not only act as the
sensitive indicator for antiꢀpoisoning capability, but also be used to predict the EOR performance for this
series of catalysts. This result suggests that the regeneration of Pt active sites should be the predominant
factor in EOR process with these catalysts, which is associated with the reaction mechanism and needs
further research. This study provides a new approach of exploring advanced EOR electrocatalysts by
efficient multicomponent synergism for promoting the applications of direct ethanol fuel cells.
ASSOCIATED CONTENT
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Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
0.1021/xxxx.
1
XPS, ICPꢀMS, XRD, durability and CO resistibility characterizations of the electrocatalysts,
1
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characterizations of NCNC, methanol oxidation activities, and typical performances of Ptꢀbased
EOR electrocatalysts in acidic medium.
AUTHOR INFORMATION
Corresponding Authors.
*
*
Eꢀmail for Q.W.: wqchem@nju.edu.cn.
Eꢀmail for Z.H.: zhenghu@nju.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was jointly supported by National Basic Research Program of China (2017YFA0206500),
NSFC (21773111, 51571110, 21473089, 21573107).
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SYNOPSIS TOC
Efficient Ternary Synergism of Platinum/Tin Oxide/NitrogenꢀDoped Carbon Leading to
HighꢀPerformance Ethanol Oxidation
Zhiqi Zhang, Qiang Wu,* Kun Mao, Yugang Chen, Lingyu Du, Yongfeng Bu, Ou Zhuo, Lijun Yang,
Xizhang Wang, and Zheng Hu*
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