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Efficient Ternary Synergism of Platinum/Tin Oxide/Nitrogendoped 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/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* 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 biomass-derived production. To date, most Pt-based 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/nitrogen-doped carbon electrocatalyst for 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 anti-poisoning 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/SnOx/N-doped sp2-C via the heterointerfaces is the key for high CO resistibility, which could facilitate the oxidative removal of CO species at Pt sites by the adsorbed OH species generated at neighbouring SnOx 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 mg-1Pt and high durability, in comparison with the most reported catalysts to date. This study provides an approach of exploring advanced EOR electrocatalysts for potential applications.

KEYWORDS ethanol oxidation reaction, ternary synergism, platinum/tin oxide/N-doped sp2-C, CO resistibility, mass activity

1. INTRODUCTION Direct alcohol fuel cells are promising power sources for movable electric devices owing to their high energy conversion efficiency and easy storage/transportation of fuels.1 Ethanol fuel is particularly attractive in virtue of its high theoretical energy density (8 kWh kg-1), low toxicity and availability from biomass production.2,3 Generally, ethanol oxidation reaction (EOR) can be realized electrocatalytically via a 4 or 12 electron process to produce acetic acid or CO2, respectively.3,4 The 12 electron process is preferred in terms of energy density and fuel utilization,4,5 but is harder to follow owing to the difficulty in breaking C-C bond.6,7 Density functional theory simulations indicated that breaking the C-C bond in CHCO intermediate has a low activation energy over Pt (111) surface,5,7 implying the great potential of Pt electrocatalyst in EOR. However, Pt is expensive and highly susceptible to poisoning by surface-adsorbed intermediates such as CO and CHx.7-9 Hence, increasing the mass activity and anti-poisoning capability of Pt in the EOR is of great significance. A conventional wisdom is to construct hybrid catalysts with synergistic effect. With cost-efficient concern, oxophilic species are usually introduced to increase the anti-poisoning capability of Pt, which could boost the water dissociation to form adsorbed OH species (OHad), and help the removal of adsorbed intermediates (COad, CHx,ad) on Pt sites for regeneration.10,11


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Teng et al. prepared Pt46-(SnO2)54 core-shell particles with a strong capability for C-C bond breaking of ethanol in acidic medium, but the mass activity was partially suppressed owing to the coverage of Pt active sites by the SnO2 shell.7 Li et al. reported Pt-Ni(OH)2/graphene ternary catalysts for methanol oxidation with exceptional activity (1236 mA mg-1Pt ) and durability resulting from the collective synergy of the three functional components, which, however, only worked in alkaline medium.10 Exploring high-active and durable Pt-based EOR electrocatalysts working in acidic medium is still a challenging topic. Herein, with pristine or nitrogen-doped 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 anti-poisoning capability of the catalysts. By comparison study on six designed catalysts, we reveal that the efficient ternary synergism of Pt/SnOx/N-doped sp2-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 benzene precursor, respectively.12-14 NCNC presents the 3D hierarchical morphology, featuring large specific surface area (814 m2 g-1), coexisting micro-meso-macropores, high N content (8.73 at.%) and bulk conductivity (238 S m-1) (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 owing to the good wettability and hierarchical geometry14 which also guarantees the uniform distribution of Pt and SnOx.


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Pt-SnOx/NCNC or Pt-SnOx/CNC catalysts were synthesized by a convenient microwave-assisted ethylene glycol (EG) reduction. Typically, 20 mg NCNC or CNC was dispersed ultrasonically into 50 mL EG, and then an appropriate amount of H2PtCl6·6H2O and/or SnCl2·2H2O was added to the suspension 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 ethanol and water, drying at 80 °C. By adjusting the amount of SnCl2·2H2O, the catalysts with tunable Sn/Pt ratio (n) could be synthesized, designated as Pt-nSnOx/NCNC or Pt-nSnOx/CNC. To understand the 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 Pt-0.5SnOx/NCNC and Pt-0.5SnOx/CNC prepared by the co-deposition process, Pt and SnOx were also loaded on NCNC by the two-step process. Pt+0.5SnOx/NCNC was prepared by firstly loading Pt on NCNC followed by loading SnOx on the as-prepared Pt/NCNC, and reversely for 0.5SnOx+Pt/NCNC. These four catalysts could give us the clue on the contribution of the ternary synergism. A control catalyst of SnOx/NCNC (16.3 wt %) is prepared in the similar way. 2.2 Characterization The structure and morphology of the catalysts were characterized by X-ray diffraction (XRD, Bruker X-ray diffractometer, D8 Advance A25, Co Kα radiation, λKα1=0.178897 nm) and transmission electron microscopy (TEM, JEM-2100). The Sn/Pt atomic ratio was measured by X-ray 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 plasma-mass spectroscopy (ICP-MS, Optima 5300DV) (Table S1 in the Supporting Information). N2 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 Brunauer-Emmett-Teller method based on the adsorption data. 2.3 Electrochemical measurements


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Electrochemical tests were conducted on Bio-logic 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 µL water, 200 µL ethanol and 40 µL Nafion (DuPont, 5 wt %). After 1 h ultrasonic treatment, a homogeneous black suspension was formed. Then, 10 µL 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 hydrogen adsorption/desorption region in 0.5 mol L-1 H2SO4 solution in cyclic voltammograms (CV). The electrocatalytic activity for EOR was examined by collecting CV in a N2-purged 0.5 mol L-1 H2SO4 and 1 mol L-1 ethanol solution at a scan rate of 50 mV s-1. Several activation scans were performed until the stable CV curves were obtained. In the CO stripping measurements, a monolayer of CO was adsorbed on catalyst by flowing CO in 0.5 mol L-1 H2SO4 for 20 min while holding the electrode potential at -0.15 V. Non-adsorbed CO was removed by bubbling the electrolyte with N2 for 20 min. Stripping measurements were initiated from -0.15 V to 0.9 V in the forward scan at 50 mV s-1 for at least two consecutive scans. The stabilities of the catalysts were evaluated by chronoamperometric (CP) tests performed in 0.5 mol L-1 H2SO4 and 1 mol L-1 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 1a-c), which stems from the CO electrooxidation at Pt nanoparticles.15 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 Pt-N interaction to the CO electrooxidation. Specifically, the Pt-N 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 oxidation of CO at a lower potential,9 which could be responsible for the small peak at 0.28 V. In addition, the N doping favors the high dispersion of Pt nanoparticles.16-19 Hence the sizes of partial Pt nanoparticles in Pt/NCNC are somewhat smaller than the cases in Pt/CNC,20 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 Pt-0.5SnOx/NCNC and Pt-0.5SnOx/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, SnOx and NCNC (or CNC) further favors the CO electrooxidation. Similar to the difference in the binary catalysts (Figure 1b,c), N-participation makes the peak extend towards the left and change from ‘lanky’ to ‘dumpy’ in comparison with the case without N-participation (Figure 1d,e). Therefore, both N-participation and ternary synergism is beneficial to the CO electrooxidation. Furthermore, the influence of the contact situation of Pt, SnOx and NCNC (or CNC) on the synergism thereof the CO electrooxidation was examined, as demonstrated in the comparison of CV curves for Pt-0.5SnOx/NCNC, Pt+0.5SnOx/NCNC, and 0.5SnOx+Pt/NCNC (Figure 1d,f,g). Pt-0.5SnOx/NCNC prepared by the co-deposition process possesses much more heterointerfaces of the closely contacted ternary species than the Pt+0.5SnOx/NCNC and 0.5SnOx+Pt/NCNC by the two-step deposition process, especially for the heterointerfaces between Pt and SnOx. For Pt+0.5SnOx/NCNC, there are two typical configurations, i.e., the unsatisfactory ternary contact parts and the isolated SnOx/NCNC parts, leading to the slight peak shift to ca. 0.57 V while without the contribution from SnOx/NCNC (Figure 1f). In contrast, for 0.5SnOx+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 SnOx during the first step process, the unsatisfactory ternary contact parts is similar to the case in Pt-0.5SnOx/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 synergism to CO resistibility can be deduced. As known, the SnOx species could boost the OHad formation by water dissociation,6 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 COad with the OHad assistance at respective Pt and SnOx sites. Accordingly, the synergism of Pt and SnOx at the heterointerfaces with intimate interaction is the best situation, which leads to the facile regeneration of Pt active sites. Hence, the Pt-0.5SnOx/NCNC catalyst presents the best CO-resistant capability, which is confirmed by the durability evaluations of CP measurements (Figure S3 in the Supporting Information). 8 (a)

0.60 V

(a’)

4 0 -4 8 (b)

Pt/C 0.61 V

(b’)

4 0 Pt/CNC

-4 8 (c)

0.60 V

4

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-2

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Pt/NCNC

-4 8 (d) 4

0.09 V 0.19 V

(d’)

0.54 V

0 Pt-0.5SnOx/NCNC

-4 8 (e) 4

(e’) 0.54 V

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0.56 V

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0.0

0.2 0.4 0.6 E (V vs. SCE)

0.8

1.0

C

N

Pt-1


Pt-2

SnOx

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Figure 1. CO stripping experiments in 0.5 mol L-1 H2SO4. (a) 20 wt % Pt/C. (b) Pt/CNC. (c) Pt/NCNC. (d) Pt-0.5SnOx/NCNC. (e) Pt-0.5SnOx/CNC. (f) Pt+0.5SnOx/NCNC. (g) 0.5SnOx+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 (a-g). Commercial Pt/C catalyst in (a) is used as the benchmark. Pt-1: with neighbouring N or/and SnOx; Pt-2: without neighbouring N or SnOx. Note: For Pt+0.5SnOx/NCNC and 0.5SnOx+Pt/NCNC prepared by the two-step deposition process, most of the Pt-N moieties for the former or N sites for the latter were actually covered during the deposition of the SnOx species. Hence, the contribution from Pt-N interaction is limited. The unsatisfactory ternary contact parts present the similar contribution in CV curves to that of Pt-0.5SnOx/CNC (Figure 1e-g).

We have further prepared a series of Pt-nSnOx/NCNC catalysts (n=0, 0.3, 0.5, 0.8, 1.0 and 1.7, respectively)

by

co-deposition

process

to

optimize

EOR

performance

and

examine

the

performance-structure relationship. Figure 2 depicts the XPS spectra of Pt 4f and Sn 3d of the catalysts. The Pt species exist in Pt(0) and Pt(II),21,22 while the Sn species mainly in Sn(IV) with a small fraction of Sn(II).23,24 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 Pt-1.7SnOx/NCNC, which should be associated with the feedstock of SnCl2·2H2O. In catalyst preparation, the presence of slight SnCl2·2H2O could boost the EG reduction of PtCl62-,25 thus increase the Pt(0) species. But the high SnCl2·2H2O 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 Pt-nSnOx/NCNC catalysts. Note: The binding energies for the marked species 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 Pt-nSnOx/NCNC present the diffraction peaks of metal Pt and SnO2 (Figure S6 in the Supporting Information). The half-peak 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 SnO2 (PDF# 41-1445), 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 Pt-nSnOx/NCNC catalysts. Pt nanoparticles are uniformly dispersed on the NCNC supports owing to the nitrogen participation (Figure 3a-d).16-19 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. High-resolution TEM (HRTEM) images display the characteristic lattice fringes of Pt (111) with the interplanar distance of 0.227 nm for all the catalysts (Figure 3e-h). In the ternary cases, the lattice fringes with the distance of 0.335 nm were observed, assigned to the (110) planes of SnO2 (Figure 3f-h). The SnO2 species present as the nanosheet-like morphology as reflected by the image contrast. All the Pt nanoparticles overlap with


ACS Catalysis

the neighbouring SnO2 nanosheets due to the co-deposition process. With increasing the Sn content, the overlapping degree gets more and more severe (Figure 3f-h), leading to the decreasing of exposed Pt

(b)

5 0 0 1 2 3 4 5 6 7 8 9 Particle size (nm)

20 nm

(c)

10 5 0 0 1 2 3 4 5 6 7 8 9 Particle size (nm)

20 nm

10 0 0 1 2 3 4 5 6 7 8 9 Particle size (nm)

SnO2 (110) 0.335 nm

(h)

2 nm

10 0 0 1 2 3 4 5 6 7 8 9 Particle size (nm)

Pt (111) 0.227 nm

SnO2 (110) 0.335 nm Pt (111) 0.227 nm

Pt (111) 0.227 nm

20

20 nm

(g) Pt (111) 0.227 nm

2 nm

(d)

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(f)

(e)

20

Percentage (%)

10

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Percentage (%)

sites. Percentage (%)

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2 nm

2 nm

SnO2 (110) 0.335 nm

Figure 3. Typical (HR)TEM images and corresponding Pt particle size distributions of the catalysts. (a,e) Pt/NCNC. (b,f) Pt-0.5SnOx/NCNC. (c,g) Pt-1.0SnOx/NCNC. (d,h) Pt-1.7SnOx/NCNC. Black arrows show the Pt nanoparticles wrapped by SnO2 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 anti-poisoning 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 extended-peak 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 extended-peak. Such a profile evolution of CV curves is in agreement with the morphological change (Figure 3), i.e., a suitable quantity of SnOx can form effective heterointerfaces for the ternary synergism while too much SnOx will lead to the decreasing of exposed Pt sites by


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over-wrapping. 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 Pt-nSnOx/NCNC presents a volcano-type change with increasing n (the Sn/Pt ratio), i.e., first increases then decreases with the optimal for n=0.5.

Figure 4. CO stripping experiments for Pt-nSnOx/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 Pt-nSnOx/NCNC catalysts are shown in Figure 5. Based on the CV curves tested in the N2-saturated 0.5 mol L-1 H2SO4 solution (Figure 5a), ECSAs are calculated to be 29.8, 44.4, 77.6, 63.0, 49.3 and 39.7 m2 g-1Pt for n=0, 0.3, 0.5, 0.8, 1.0 and 1.7, respectively. ECSAs first increase then decrease with the maximum of 77.6 m2 g-1Pt for n=0.5, which is larger than 47.3 m2 g-1Pt 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 volcano-type evolution of ECSAs. The CV curves of Pt-containing catalysts display two peaks derived from EOR while that for the control catalyst of SnOx/NCNC is featureless, which indicates the origin of EOR activity from Pt species (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 densities of 637, 957, 1187, 1023, 910 and 646 mA mg-1Pt , respectively. A volcano-type 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, SnOx and NCNC, but will be suppressed to some extent when the Pt is over-wrapped by SnOx. Therefore, both the ternary synergism and the suitable Pt exposure are crucial for improving the mass activity. Worthy to mention is that the mass activity of Pt-0.5SnOx/NCNC (1187 mA mg-1Pt ) is almost 4.0-fold higher than that of the 20 wt % commercial Pt/C (297 mg-1Pt ), and also obviously larger than the data of most reported catalysts to date (Table S2),26-32 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 the Sn/Pt ratio. After the stability tests, SnO2 species still exist adjacent to the Pt nanoparticles, indicating that the SnOx can survive in acidic medium for a long operation time (Figure S7 in the Supporting 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 anti-poisoning 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 Pt-nSnOx/NCNC catalysts (n=0, 0.3, 0.5, 0.8, 1.0 and 1.7). (a) CV curves in 0.5 mol L-1 H2SO4. (b) CV curves in 0.5 mol L-1 H2SO4 and 1 mol L-1 ethanol. (c) Evolutions of ECSAs and mass activities. (d) The comparison between the optimal Pt-0.5SnOx/NCNC (dashed curve) and the control catalysts. The curves for 20 wt % Pt/C and SnOx/NCNC are presented for comparison.

The contribution of the ternary synergism is also supported by comparing the performances of Pt-0.5SnOx/NCNC and several control catalysts. Generally, the binary Pt/NCNC catalyst exhibits much inferior EOR activity to the ternary catalysts, and SnOx/NCNC has little EOR activity (Figure 5b). The EOR peak potential of Pt-0.5SnOx/NCNC is ca. 30 mV lower than that of Pt-0.5SnOx/CNC, which may result from the electronic effect of Pt-N 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 Pt-0.5SnOx/CNC compared with Pt-0.5SnOx/NCNC (896 vs 1187 mA mg-1Pt ) (Figure 5d). These results indicate the ternary synergism of Pt, SnOx and NCNC is crucial to the high EOR activity of Pt-0.5SnOx/NCNC. Actually, the loading sequence influences the EOR activity a lot. The control catalysts of Pt+0.5SnOx/NCNC and 0.5SnOx+Pt/NCNC


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with unsatisfactory ternary contacts show the peak current densities of 799 and 758 mA mg -1Pt , respectively, only about two-thirds of that for Pt-0.5SnOx/NCNC (Figure 5d). This result further demonstrates the significance of the intimate interaction via the heterointerfaces of Pt, SnOx and NCNC by co-deposition process which favors their synergism thereby the high mass activity. Actually, our test for methanol oxidation in acidic medium exhibits ca. 3.7-fold higher mass activity for the Pt-0.5SnOx/NCNC than the benchmark Pt/C (1109 vs 298 mA mg-1Pt ), suggesting the potential of the synergistic catalysts in alcohol oxidation reactions (Figure S9 in the Supporting Information). 4. CONCLUSION In summary, we have successfully constructed a series of ternary Pt/SnOx/N-doped sp2-C electrocatalysts for EOR in acidic medium. By taking CO electrooxidation as a sensitive indicator for anti-poisoning capability, the correlation of the CO resistibility with the geometrical configuration has been well established for the catalysts. The efficient ternary synergism of Pt, SnOx and NCNC via heterointerfaces 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 SnOx sites, thereby the facile regeneration of Pt active sites. The optimized synergistic catalyst Pt-0.5SnOx/NCNC demonstrates the excellent EOR performance in acidic medium with a superb mass activity of 1187 mA mg-1Pt 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 anti-poisoning 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: 10.1021/xxxx. XPS, ICP-MS, XRD, durability and CO resistibility characterizations of the electrocatalysts, characterizations of NCNC, methanol oxidation activities, and typical performances of Pt-based EOR electrocatalysts in acidic medium. AUTHOR INFORMATION Corresponding Authors. *E-mail for Q.W.: [email protected]. *E-mail for Z.H.: [email protected]. 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). REFERENCES (1) Bianchini, C.; Shen, P. K. Palladium-Based Electrocatalysts for Alcohol Oxidation in Half Cells and in Direct Alcohol Fuel Cells. Chem. Rev. 2009, 109, 4183-4206. (2) Erini, N.; Loukrakpam, R.; Petkov, V.; Baranova, E. A.; Yang, R.; Teschner, D.; Huang, Y.; Brankovic, S. R.; Strasser, P. Ethanol Electro-Oxidation on Ternary Platinum-Rhodium-Tin Nanocatalysts: Insights in the Atomic 3D Structure of the Active Catalytic Phase. ACS Catal. 2014, 4, 1859-1867. (3) Antolini, E. Catalysts for Direct Ethanol Fuel Cells. J. Power Sources 2007, 170, 1-12. (4) Rao, L.; Jiang, Y. X.; Zhang, B. W.; Cai, Y. R.; Sun, S. G. High Activity of Cubic PtRh alloys Supported on Graphene towards Ethanol Electrooxidation. Phys. Chem. Chem. Phys. 2014, 16, 13662-13671.


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SYNOPSIS TOC 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*


Tin Oxide ... - ACS Publications

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