Ultrathin and coiled carbon nanosheets as Pt carriers for high and stable electrocatalytic performance

Journal Pre-proof Ultrathin and Coiled Carbon Nanosheets as Pt Carriers for High and Stable Electrocatalytic Performance Zhen Tong, Min Wen, Chao Lv, Qiulin Zhang, Yanhong Yin, Xianbin Liu, Yesheng Li, Chunfa Liao, Ziping Wu, Dionysios D. Dionysiou

PII:

S0926-3373(20)30179-X

DOI:

https://doi.org/10.1016/j.apcatb.2020.118764

Reference:

APCATB 118764

To appear in:

Applied Catalysis B: Environmental

Received Date:

6 December 2019

Revised Date:

10 February 2020

Accepted Date:

14 February 2020

Please cite this article as: Tong Z, Wen M, Lv C, Zhang Q, Yin Y, Liu X, Li Y, Liao C, Wu Z, Dionysiou DD, Ultrathin and Coiled Carbon Nanosheets as Pt Carriers for High and Stable Electrocatalytic Performance, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118764

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Ultrathin and Coiled Carbon Nanosheets as Pt Carriers for High and Stable Electrocatalytic Performance

Zhen Tong a, Min Wen a, Chao Lv a, Qiulin Zhang b, Yanhong Yin a, b, c,*, Xianbin Liu a, Yesheng

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Li a, Chunfa Liao a, Ziping Wu a, Dionysios D. Dionysiou c,*

School of Materials Science and Engineering, Jiangxi University of Science and Technology,

Ganzhou 341000, China

Chongyi Zhangyuan Tungsten Co ., Ltd., Ganzhou 341000, China

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Environmental Engineering and Science Program, Department of Chemical and Environmental

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Graphical Abstract

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Engineering, University of Cincinnati, Cincinnati, OH 45221, United States

The tungsten carbide can be used as the template to produce the ultrathin and

coiled carbon nanosheets (UC-CNS) repeatedly. The UC-CNS loaded Pt NPs displays high electrochemical properties and excellent stability.

Highlights The UC-CNS could be stripped away from the WC substrate without obvious destruction.

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The UC-CNS possessed high degree of graphitization and high specific surface area.

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The UC-CNS/Pt displayed high electrochemical activity and excellent stability.

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Excellent activity was ascribed to strong interaction between UC-CNS and Pt.

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Abstract:

Highly graphitic carbon nanomaterials with large specific surface area and good

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dispersion are very favorable supports for Pt-based catalysts in fuel cells. Ultrathin and coiled carbon nanosheets (UC-CNS) were successfully synthesized via chemical

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vapor deposition method using tungsten carbide (WC) as substrate. After the WC@UC-CNS was immersed in HF/HNO3/H2O solution, UC-CNS was completely stripped away from the WC substrate without obvious destruction, and possessed coiled edges and more oxygen-containing functional groups for anchoring Pt nanoparticles. Another obtained product (WO3) could be used as a tungsten source to

produce UC-CNS repeatedly. The purified UC-CNS, with thicknesses of 3.45-5.61 nm, possessed high specific surface area (547 m2 g-1) and high degree of graphitization (IG/ID=1.43). Pt loaded on the UC-CNS exhibited high electrocatalytic activity and extremely high stability for methanol oxidation reaction even after 14,000 potential cycles. We believe UC-CNS can be widely used in fuel cells and other related green chemical applications.

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Keywords: Graphitic carbon; Ultrathin; Coiled edge; Pt Carrier; Electrocatalyst

1 Introduction

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Direct methanol fuel cell (DMFC) is an ideal power supply because of its high

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energy conversion efficiency and pollution-free quality [1-3]. However, the commercialization of DMFC technologies is hindered by utilization efficiency of

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carbon-supported Pt electrocatalysts [4]. As for the commercial Pt/vulcan XC-72

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(carbon black) catalyst, the corrosion behavior of carbon black results in the formation of amorphous carbon and the migration, agglomeration, and detachment of

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loaded Pt NPs [5-7]; moreover, their electrocatalytic performance would be deteriorated gradually, and this phenomenon can be mainly attributed to the electrochemical

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oxidation of carbon black. To improve the anticorrosion performance of the carbon supports, one solution is to improve their degree of graphitization and mediate their microscopic morphology [8-10]. The highly graphitized carbon support with structured porosity possesses excellent toleration to the oxidizing environment and provides the channel for the transport of reactants (e.g. H+) [11], which can effectively improve the

electrocatalytic activity and stability of Pt catalysts. Generally, to achieve high catalytic performance and a long lifespan, novel carbon materials with large specific surface area and high electrical conductivity are usually served as carriers for loading Pt NPs, such as carbon nanotubes, nanoporous carbon, or graphene [12-14]. Carbon nanotubes with large specific surface area tend to expose more active sites for loading Pt NPs. Nevertheless, their high length-width

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ratio and hydrophobic surface limit their homogeneous dispersion, which inhibits uniform loading of Pt NPs. Carbon nanotubes can be dispersed by using a

functionalized method [15], but their electrical conductivity may decrease to some

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extent. Nanoporous carbon with large specific surface area and huge pore volume can

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significantly increase the loading mass of Pt NPs. However, Pt NPs are easily buried in the holes of nanoporous carbon and lose their catalytic activity [16]. Owing to its

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unique electronic properties, high specific area, good electrical conductivity and

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stability, two-dimensional graphene is a highly ideal catalyst carrier [17]. However, the reported carrier function of graphene is not satisfactory, which can be ascribed to

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almost no functional groups in its plane except for the edge part, and this property weakens the interaction force of graphene with Pt NPs. Thereby, some studies have

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been reported in anchoring Pt NPs by chemical modification of carbon-based supports or morphology adjustment [18-20]. For example, modulation of surface functional groups or mediating the morphology of graphene via an acid treatment or filtering method, which produces graphene oxide, reduced graphene oxide, functional graphene, and three-dimensional graphene; this finding has been observed to activate

graphene for loading Pt NPs. The active sites in the modified surface structure have been proposed to show enhanced force strength towards Pt NPs. However, the preparation process is complicated, degree of graphitization is partly decreased, and electrical conductivity is reduced [21-23]. Based on the above results, the development of an effective catalyst carrier with a large specific surface area, high degree of graphitization, and good dispersion is

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desirable but challenging [24-26]. In this study, we developed ultrathin and coiled

carbon nanosheets coating on the surface of WC particles by introducing carbon sources into WO3 particles. The carbon sources diffused inside of WO3, and

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transformed WO3 to generate WC [27]. Based on the ability of WC to dissolve traces of

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carbon sources, the supersaturated carbon dissolved in WC precipitated and formed carbon nanosheets surrounding the surface of WC particles as temperature decreased.

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After been stripped from the WC surface by soaking in HF/HNO3/H2O solution, WC

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would be oxidized to form WO3•0.33H2O and dissolved in the acid solution, meanwhile, some oxygen-containing functional groups were introduced to the UC-

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CNS. The UC-CNS displayed high surface activity and a crumpled morphology, which provided the UC-CNS with improved dispersion and enhanced ability to

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interact for anchoring Pt NPs (Scheme 1). The UC-CNS loaded Pt NPs catalyst showed high electrocatalytic activity and extremely high stability, and was expected to be widely used in fuel cells and other related green chemical applications. 2 Experimental Section 2.1 Preparation and separation of UC-CNS from WC substrate

The tungsten precursor was prepared by using CNTs as template in our previous work [28]. The prepared precursor (0.5 g) was evenly spread in the crucible and then pushed to the middle of the furnace. Methanol/ethanol (volume ratio of 8:2) mixed solution was used as carbon source and injected into the furnace at a 0.5 mL min-1 of flow rate along with a high purity-nitrogen flow (flow rate: 300 mL min-1). The precursor was carbonized in furnace at 1000 oC for 2 h and the WC@UC-CNS was

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then obtained. At last, in order to separate the UC-CNS from WC substrate, the

WC@UC-CNS was dispersed in HNO3/HF/H2O solution with two different volume

ratios (1:10:10 and 1:1:2) for 6 h at 25 oC under air conditions; the obtained products

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were noted as UC-CNS-1 and UC-CNS, respectively.

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2.2 Pt deposition on the UC-CNS

In this procedure, 8.5 mg UC-CNS was added in 200 mL chloroplatinate acid

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(H2PtCl6), deionized water and ethylene glycol (EG) solution (containing 1.5 mg of

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Pt, 100 mL EG and 100 mL deionized water) while stirring and heating to 140 oC for 4 h. Afterwards, the product was cooled to room temperature, filtered with excess

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deionized water and ethanol successively, and then dried at 100 oC for 2 h; Pt loading mass on UC-CNS was controlled to be 15 wt %. The commercial Pt/C (20 wt% of Pt)

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was used as a comparative material. 2.3 Characterization of samples XRD spectra of the different samples were obtained by a Thermo ARL SCINTAG X’TRA X-ray diffractometer, using Cu Kα (0.1541 nm) radiation source. The detailed microstructure of the samples was characterized by using transmission

electron microscope (TEM), which incorporated a Tecnai G2 F30 S-Twin equipped with high-resolution digital camera and energy-dispersive full range X-ray (EDX) microanalysis system. N2 adsorption/desorption isotherms were obtained at the liquid nitrogen temperature using an automatic analyzer (BELSORP-miniⅡ, BEL Co., Japan). Raman spectroscopy was conducted to examine the quality of the carbon materials using a Horiba Jobin Yvon HR 800 UV with a 514.5 nm excitation wavelength laser.

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The chemical compositions on the surface of different products were qualitatively

analyzed by Fourier transform infrared spectroscopy (FTIR) using Bruker Equinox 55

containing DTGS detector with a resolution of 4 cm−1. The Pt loading on the UC-CNS

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was determined by an inductively coupled plasma-optical emission spectrometer

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(ICP-OES; iCAP 7000 ICP-OES, Thermo Scientific, USA). Chemical states were examined by X-ray photoelectron spectra (XPS) performed on a K-ALPHA

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instrument (Thermo Fisher Scientific, USA).

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2.4 Electrochemical measurements

The electrochemical activities of the catalysts were characterized with cyclic

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voltammetry. Experiments were performed in a three-electrode cell using electrochemical station (CHI760E) at room temperature. The working electrode was a

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glassy carbon electrode with a diameter of 5 mm covered with a thin layer Nafionimpregnated catalyst. The thin-film electrode was prepared as follows: 2.8 mg of the catalyst was dispersed in 0.05 mL 5 wt% Nafion and 0.95 mL ethanol by the sonication for 30 min. Then, a suitable volume of catalyst dispersion was transferred on to the glassy carbon disk by using a pipette and dried in room temperature. The Pt

loading mass on the electrode was controlled to be 20 μg cm-2. A Pt foil and an Ag/AgCl (sat. KCl) were used as the counter electrode and reference electrode, respectively. The electrocatalytic activity was measured in a mixed N2 saturated 0.5 mol L-1 H2SO4 and 1 mol L-1 CH3OH solution at a scan rate of 100 mV s-1 for potentials against saturated calomel electrode (SCE)ranging from -0.3 V to 0.999 V. The CO stripping voltammetry test was performed to evaluate the anti-CO toxicity

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ability of the catalysts. 3 Results and Discussion

Before and after the tungsten precursor was carbonized with methanol/ethanol as

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carbon source in a nitrogen atmosphere, the microstructures of the samples were

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examined by using SEM (Fig. S1) and TEM (Fig. 1). As shown in the SEM image of Fig. S 1a, WC@UC-CNS precusor with diameter of approximately 50-200 nm had

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soomth surface before carbonization, which was similar to TEM image results (Fig.

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1a). After carbonization, WC particles with no significant change in particle diameter compared with the WC@UC-CNS precursor surrounded by near-transparent materials

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with shell structures (Fig. S1b). The morphologies of composite materials were further characterized by using TEM (Fig. 1b), the light-colored nanosheets were

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entangled with the deep-colored particles. Fig. 1c shows large amounts of particles with deep color surrounded by numerous thin and near-transparent nanosheets (noted by orange arrows in Fig. 1c). The nanosheets with open structure could be observed easily (Fig. S2a); however, the layers with low thickness could not be evaluated easily. As shown in Fig. 1d, we found the sharp diffraction peaks indexed into WC

with good crystalline quality, the diffraction characteristic peaks located at 31.5°, 35.7°, 48.4°, 64.1°, 73.3°, 77.3°, and 84.3° correspond to the (001), (100), (101), (110), (111), (102), and (201) plane for WC, respectively. Due to the relatively less quantity of carbon with amorphous nature than that of WC, no obvious carbon peaks can be found in the sample of WC@UC-CNS, only minor intensity of the (220) plane appeared at around 40o corresponds to carbon [29-30]. Combined with the EDS result in

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Fig. S2b (the selected area in Fig. S2a), a large amount of carbon shells were prepared depending on the WC substrate, the main elements were W and C, indicated that the deep-colored particles and light-colored nanosheets were WC and graphite carbon,

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respectively. The atomic ratio of W and C is less than 1: 1, indicating that the

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precursor was completely carbonized and carbon was approximately in 1.58 % excess, and the supersaturated carbon precipitated surrounding the surface of WC

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particles as temperature decreased. As seen from the magnification of the selected

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area in Fig. S2a (Fig. S2c and Fig. S2d), we could find that the edges of the nanosheets were coiled and possessed high degree of graphitization. The near

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transparent nanosheets extended from WC particles possessed an average diameter of only 50 nm (Fig. S3a), and the WC particles mainly grew along the (100) plane (Fig.

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S3b). We can conclude that numerous carbon nanosheets can easily grow on the WC substrate by supplying sustainable liquid carbon sources of methanol/ethanol solution. The WC substrate covered with ultrathin and coiled carbon nanosheets can be successfully prepared. The WC@UC-CNS shows black color and can be easily dispersed in ethanol (Fig. S4). To investigate the microstructure of CNS and other

physical and chemical properties, we attempted to separate the UC-CNS from the WC substrate without serious damage. Two different methods were applied: in the first method, WC@UC-CNS was dispersed in ethanol through ultrasonic treatment for 10 h, and then the UC-CNS was easily collected on the surface of the solution according to its smaller density than that of WC, the results were shown in Fig. S5. TEM images of Fig. S5a showed some deep-colored particles were still attached to the nanosheets;

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however, the morphology and microstructure of the UC-CNS (Fig. S5b and S5d) were unchanged according to the TEM images in Fig. 1. The EDS results were shown in

Fig. S5c (selected area of Fig. S5a), a certain content of W remained in the composite,

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indicating that the UC-CNS could not be separated completely from the WC substrate

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only by using ultrasonic effect. The second method was based on the purification of UC-CNS by soaking the WC@UC-CNS in two different volume ratios (1:10:10 and

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1:1:2) of HNO3/HF/H2O solution. The purified samples of UC-CNS-1 and UC-CNS

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were investigated by using infrared spectra, and results were shown in Fig. 2. Peak positioned at 1573, 564, 825, 916, 1347, 3410, and 3364 cm-1 corresponded to the

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functional groups of (C=C); (C-H); (C-H, C-O); (C-H, C-O-C, O-H, C-O); (C-H, OH); (O-H); and (O-H), respectively. Compared with the infrared spectra of the

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WC@UC-CNS (curve a in Fig. 2), the functional groups increased considerably after acid treatment, and this phenomenon may be attributed to oxidation during acid treatment. The peak intensity of UC-CNS-1 (curve b in Fig. 2) was lower than that of UC-CNS (curve c in Fig. 2), indicating that UC-CNS-1 was obtained by using HNO3/HF/H2O (volume of 1:10:10) and slightly damaged. Additionally, this result

may be because HNO3 could provide oxygen functional groups and bring increased damage to UC-CNS, which made the UC-CNS possess high surface energy. Raman spectroscopy of the sample provided structural information for investigating the intrinsic quality of carbon materials [31-32]. Fig. S6 shows the Raman spectra of UC-CNS and UC-CNS-1. Three distinct peaks at 1310, 1575, and 2590 cm1

correspond to the Raman characteristic peaks of graphene of the D-band (the sp3

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carbons from defects), G-band (attributed to the sp2 carbons), and G'-band (the second order mode of the D band, also noted 2D band), respectively. The IG/ID value of the UC-CNS-1 (1.80) lower than UC-CNS (1.43) indicates that HNO3 with a higher

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volume ratio is more destructive to carbon nanosheets. Therefore, the influence of

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HNO3 on UC-CNS etching in HNO3/HF/H2O system should be controlled within the range of appropriate content. We listed the IG/ID values of previously reported

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graphitic carbon materials (Table S1) and their functionalization products (Table S2)

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compared with that of UC-CNS. We can find the IG/ID value of UC-CNS in this work is higher than that of some reported functionalized carbon materials (Table S2).

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Compared with the Raman spectra of WC@UC-CNS (Fig. S7b) and commercial WC (Fig. S7c), too much volume ratio of HNO3 may damage the UC-CNS severely and

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not completely remove the WC substrate (Fig. S7a), while too little of volume ratio of HNO3 is not prone to introduce enough oxygen-containing functional group onto the surface of the UC-CNS. The UC-CNS obtained from soaking WC@UC-CNS in HF/HNO3/H2O (1:1:2) solution possesses high degree of graphitization (Table S1 and S2) and oxygen-containing functional groups (Fig. 2). After stripping the UC-CNS

from the WC@UC-CNS in HF/HNO3/H2O (1:1:2) solution, another obtained product was proved to be WO3 0.33H2O (Fig. S8), which could be used as tungsten sources after dehydration to prepare WC. The above results show WC can be used as the template to produce the UC-CNS repeatedly (Fig. S9). The SEM image in Fig. 3a shows the hollow shell with the open structure of UCCNS, which was obtained after being stripped away from the WC substrate. The TEM

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image in Fig. 3b illustrates that the UC-CNS was completely separated from the WC substrate. It can be seen from XRD spectrum of the UC-CNS (Fig. S10), the

characteristic peak at 2θ of 26.38° corresponded to the (002) plane of graphite carbon.

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As seen from the magnification of Fig. 3b, we observed clearly that the edges of the

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purified UC-CNS were ultrathin and coiled, and the lattice stripes remained clearly. AFM images indicated that the thickness of UC-CNS was in the range from 3.45 nm

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to 5.61 nm (Fig. S11). We inferred that coiled carbon nanosheets with approximately

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10-16 layers (one carbon layer is about 0.34 nm, an inset in Fig. 3c) grew easily along the surface of WC substrate during the carbonization process. The morphology and

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microstructure of UC-CNS were not obviously changed, which were consistent with the TEM and HRTEM images in Fig. S2c and Fig. S2d, indicating that the UC-CNS

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was not destroyed by the HNO3/HF/H2O (1:1:2) solution. Fig. 3d shows the isothermal adsorption/desorption curves and pore size distribution of the UC-CNS measured at a reaction temperature of 77 K. The isothermal adsorption curve was calculated and analyzed by using the Brunauer-Emmett-Teller (BET) method, and the corresponding pore size distribution was calculated from a desorption branch of N2

isotherms by using the Barrett-Joyner-Halenda (BJH) method. The specific surface area of the obtained UC-CNS reached 547 m2 g-1, and the BJH desorption curve showed that the UC-CNS possessed mesoporous structure, and the pore size distribution mainly concentrated at 2-10 nm. Therefore, the fast transmission of reactants and extended reaction interface area of the triple-phase may be promoted by abundant mesopores within the UC-CNS with high specific area, thereby resulting in enhanced and stable

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electrochemical performance. Combined with Fig. 1, the ultrathin, coiled, and neartransparent material that covered the WC surface is graphite carbon. Therefore, the

morphology and high degree of graphitization.

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UC-CNS possesses a graphene-like three-dimensional structure according to the

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As is known to all, the dispersion uniformity of nano-sized Pt particles has great influence on the structure and morphology of the catalyst, and this further affects the

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electrooxidation activity of methanol [33-36]. Fig. 4 shows TEM and HRTEM images of

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the UC-CNS deposited Pt NPs. Some black rounded-shaped Pt NPs depositing on the surface of the UC-CNS are clearly observed and the ultrathin and coiled morphology

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of the UC-CNS are consistent with the images in Fig. 3 (Fig. 4a). It seems that Pt NPs deposited on UC-CNS are non-uniform, especially at the corner of the coiled edges of

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UC-CNS. In this work, after acid etching, UC-CNS was stripped from the WC particles and inherited from their morphologies. The obtained UC-CNS possessed hollow shell-like and open structure (Fig. 3(a)), which was different from the morphologies of the commercial carbon support. When Pt NPs located at the corner of the coiled edges of UC-CNS, which seemed non-uniformly distributed in Fig. 4(a)

because of the coiled edges. The image was further magnified (Fig. 4b) to provide a clear view of the morphology of the UC-CNS/Pt, numerous Pt NPs were uniformly distributed on the surface of the UC-CNS in the magnified TEM image and exhibited without obvious aggregation. The distributions of Pt NPs were obtained by calculating the selected particles, and centered at ca. 2.0 nm (inset image in Fig. 4b). As shown in Fig. S12, the aggregation of Pt nanoparticles with 3.5-4 nm in the commercial Pt/C

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was very obvious. Therefore, Pt NPs with smaller size (1.5-2.5 nm) and better

dispersion deposited on UC-CNS could provide more active sites for electrocatalytic reactions, thus showing higher electrochemical performance. EDS result (Fig. 4c)

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shows that the main elements in the sample (the selected area in Fig. 4a) are

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composed of C and Pt, there are also a very small amount of W, O and Cu element. The Pt loading mass in the sample is 14.48 wt%, reaching to 96.5 % of theoretical Pt

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loading (15 wt%), the value corresponds well with those obtained by ICP-OES (13.76

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wt%). From XRD spectrum of the UC-CNS/Pt catalyst (Fig. S13), the characteristic peaks at 2θ of 39.54, 45.98, and 67.06° correspond to (111), (200), and (220) planes

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of Pt with a face-centered cubic structure (JCPDS: 04-0802), respectively. The grains with lattice spacing of 0.23 nm showed dominant preferential orientation along the

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growth direction of (111) of Pt (Fig. 4d). Besides, the Pt peak maintained, which was consistent with the selected area electron diffraction (SAED) observation (insert image in Fig. 4d), indicating that Pt NPs were successfully deposited on the UC-CNS surface. We also found that the clear diffraction characteristic peak at 2θ of 26.38° (Fig. S13) corresponded to the (002) plane of graphite carbon, which is in agreement

with the XRD result of UC-CNS (Fig. S10). Hence, the UC-CNS existed in the catalysts and its original phase structure was unchanged after the Pt NPs were uniformly distributed on the UC-CNS. As seen from the High-Angle Annular DarkField Scanning Transmission Electron Microscope (HAADF-STEM) (Fig. 4e), C, Pt and minor W elements were homogeneously dispersed in the UC-CNS/Pt electrocatalyst. The results may be attributed to the partial chemical bonds and

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oxygen-containing functional groups introduced on UC-CNS after soaking in acid, which makes Pt NPs can be uniformly distributed on the supports without serious

agglomeration during the anchoring process. A very small amount of W element can

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still be observed after the WC substrate removing.

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The chemical environment and valence electron of the UC-CNS/Pt were characterized by X-ray photoelectron spectroscopy (XPS). The survey spectrum was

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shown in Fig. 5, the existence of C, Pt, and O elements was obviously identified, and

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minor W element was also identified. The C 1s spectrum of UC-CNS could be fitted into three peaks (Fig. 5b), i.e., C=C/C-C (284.8 eV), C-O (286.0 eV), and O=C-O

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(289.0 eV). The peaks at 71.7 eV and 75.1 eV were assigned to Pt (Pt 4f7/2, Pt 4f5/2) (Fig. 5c), which were known to be demonstrated as the active species toward MOR.

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To determine the different oxidation states of Pt, the complex Pt 4f spectrum of the UC-CNS/Pt could be fitted to three pairs of peaks. The binding energies of 71.7 eV and 75.1 eV, 72.6 eV and 76.0 eV, 72.7 eV and 78.0 eV correspond respectively to three different oxidation states of Pt (0), Pt (2+), and Pt (4+). Pt (0) was the major species in the Pt particle surface. The Pt 4f spectrum of commercial Pt/C is shown in

Fig. S14. It could be seen that the Pt 4f spectrum of UC-CNS/Pt is similar to that of the commercial Pt/C, which means Pt (o) was the major species on the surface of Pt NPs, and the Pt NPs were successfully deposited on UC-CNS. Moreover, the typical W4f XPS spectra were shown in Fig. 5d, the main peaks at high binding energy (35.9 eV and 38.0 eV) were attributed to WO3, resulting from the inevitable surface oxidation of WC substrate from WC@UC-CNS in HF/HNO3/H2O solution, which meant the

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WC substrate had been removed from the WC@UC-CNS, but minor oxidation

product of WC remained, which was consistent with the results of Fig. 4e, Fig. S8, and Fig. S9.

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Fig. 6a shows the cyclic voltammetric (CV) curves of the UC-CNS/Pt and the

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commercial Pt/C in H2SO4 solution. In Fig. 6a, two distinct current peaks of the UCCNS/Pt were observed in the oxidation zone of hydrogen, corresponding to desorption

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peaks of hydrogen in the Pt (111) and Pt (100) crystal surfaces located at 0.625 V to

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0.725 V and 0.725 V to 1.075 V, respectively. In the absorbance/detachment areas of hydrogen, the current density of the UC-CNS/Pt catalyst is higher than that of the

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commercial Pt/C catalyst. The first desorption peak of hydrogen appears at the potential of 0.625 V to 0.725 V in the forward scan direction of abscissa in the range

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of potential scanning, and the current density of the desorption peaks of UC-CNS/Pt and commercial Pt/C are 34.1 and 23.5 mA cm-2, respectively. A wide second desorption peak of hydrogen appears at a potential of 0.725 V to 1.075 V. The current density of the desorption peaks of the UC-CNS/Pt and the commercial Pt/C located are 25.6 and 6.4 mA cm-2, respectively. The above results showed enhanced catalytic

activity on the Pt (111) surface and conferred the Pt catalyst with increased catalytic activity, which is due to the state density of the Pt (111) plane was higher than that of the (100) plane. The energy band shows gentle changes and small width near the Fermi energy level; thus, the corresponding density is relatively high [11]. Therefore, excellent catalytic performance of the UC-CNS/Pt catalyst was demonstrated. The detachment peak area of hydrogen represents the amount of electricity involved in the

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oxidation reaction, according to the formula [37] of effective electrochemical activity surface area (ECSA)=Q/mβ, where Q = S v-1, S is integral area of hydrogen for the

detached peak, v is the scanning speed (100 mV s-1), m is the quantity of the loading

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Pt on the work electrode (20 μg cm-2), and β is the charge corresponding to 1.3×1015

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hydrogen atoms per square centimeter of Pt surface (0.21 mC cm-2). The ECSA values of the UC-CNS/Pt and the commercial Pt/C catalysts were calculated to be 121.98 and

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68.62 m2 g-1, respectively. The electrochemical activity of the prepared UC-CNS/Pt

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catalyst was higher than that of the commercial Pt/C catalyst, and even that of the WC@UC-CNS/Pt and the reduction graphene oxide/Pt (rGO/Pt) (51.06 m2 g-1 and

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76.58 m2 g-1, Fig. S15a). The rGO is prepared according to the literature reported by Xianbin Liu et al [38], and the rGO/Pt catalyst was obtained by loading the same mass

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percentage of Pt on rGO in the same way as UC-CNS/Pt. The above results were described to the CNS with coiled edges and high specific surface area hindering the coagulation of Pt NPs and further improving the dispersion of Pt NPs，exposed more active centers of Pt NPs and improved the dehydrogenation efficiency and oxygen reduction.

Fig. 6b shows the CV curves of UC-CNS/Pt and the commercial Pt/C in the H2SO4/CH3OH electrolyte, ranking it among the better active MOR catalysts reported in recent literature (Table S3). The positive sweep peak at 1.525 V-1.725 V is the oxidation peak of methanol, and the negative sweep peak at 1.125 V-1.425V is the oxidation of residual carbon species (such as CO) produced during electrooxidation of methanol. The peak current density of methanol oxidation over the UC-CNS/Pt

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catalyst was 91.7 mA cm-2, which was higher than that of the commercial Pt/C

catalyst (42.2 mA cm-2) and even higher than that of the WC@UC-CNS/Pt and rGO

(64.5 and 24.6 mA cm-2, Fig. S15b). The methanol oxidation activity of UC-CNS/Pt

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catalyst was 2.16 times of that of the commercial Pt/C catalyst. The peak current

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density of the oxidation of residual carbon species over the UC-CNS/Pt catalyst (74.7 mA cm-2) was higher than that of the commercial Pt/C (42.7 mA cm-2), respectively,

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and even higher than that of the WC@UC-CNS/Pt and rGO (53.5 and 13.8 mA cm-2,

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Fig. S15b).

To further investigate the anti-CO poisoning ability [13, 39] of the UC-CNS/Pt

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catalyst in methanol acidic medium, CO-stripping voltammetry test was performed, and the results are shown in Fig. 6c. It can be seen that the onset potential of the COad

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oxidation reaction of the UC-CNS/Pt catalyst (Pt loading is 15 wt %) (1.385 V) is obviously lower than that (1.535 V) of the commercial Pt/C catalyst (Pt loading is 20 wt %). The order of the ECSA of CO is as follows: the UC-CNS/Pt catalyst (16.09 m2 g-1) < the commercial Pt/C catalyst (26.26 m2 g-1). These results suggested that the UC-CNS/Pt catalyst exhibited good electrocatalytic activity and enhanced CO toxicity

resistance in methanol electrooxidation, which was most likely because UC-CNS with excellent electrical conductivity and good stability hindered the coagulation, improved the dispersion and exposed more active sites Pt NPs. Fig. 6e shows the chronoamperometric current curves of UC-CNS/Pt and the commercial Pt/C catalyst at a polarization voltage of 1.605 V. The current density of the two samples decayed rapidly at the beginning, then decreased gradually, and

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finally stabilized. The rapid decay of the current density was attributed to the

intermediate products, which were produced continuously during the oxidation of

methanol and tended to adsorb on the surface of the catalysts and further reduce the

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performance of the catalysts [20]. The UC-CNS/Pt catalyst showed higher residual

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current after 3,500 s when the current density stabilized. This finding showed that the UC-CNS/Pt catalyst had higher electrochemical activity and higher stable current for

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a long time than that of the commercial Pt/C catalyst during methanol oxidation. To

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further investigate the high electrochemical properties and stability of the UC-CNS/Pt catalyst in methanol acidic medium, multiple-loop scanning was performed, and the

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results are shown in Fig. 6d. The peak current density of methanol oxidation increased from 91.7 to 102.8 mA cm-2 after 7,000 cycles, and the peak current density of the

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intermediate product increased from 74.1 to 101.8 mA cm-2, indicating more and more Pt active sites participates in the oxidation of methanol, while more and more intermediate products are emerged simultaneously. The above results showed that the open and coiled edges of CNS provide space for continuous Pt particles, and facilitate the transport of electrolyte ions and electron, which indicated that the intermediate

carbon products (CO, etc.) generated during the electrochemical reaction can gradually occupy the active position of Pt NPs. After 14,000 cycles, the peak current density of methanol oxidation decreased from 102.8 to 91.9 mA cm-2, and the peak current density of the intermediate products increased from 101.8 to 102.5 mA cm-2. The results showed that more CO was finally adsorbed on the surface of Pt NPs in the catalyst and covered the active sites of Pt NPs, thus indicating that large amounts of

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intermediate products emerged through the incomplete decomposition of methanol oxidation, and gradually increased as the reaction proceeded; therefore, the

electrocatalytic activity of the catalyst was influenced. The performance changes of

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the UC-CNS/Pt catalysts after 1 cycle, 7,000 cycles and 14,000 cycles were listed in

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Table 1.

Based on the above results, the high electroactivity and excellent stability of the

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UC-CNS/Pt catalyst may be attributed to the following reasons: (1) the UC-CNS with

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high degree of graphitization has good corrosion resistance and tends to restrict the aggregation and migration of Pt NPs during electrochemical process; (2) the UC-CNS

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with high specific surface area improves the dispersion of Pt NPs and the porous nature of the UC-CNS facilitates the transport of reactants; and (3) the oxygen-

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containing functional groups introduced on UC-CNS tend to anchor Pt NPs strongly to prevent migration and shedding during electrochemical process. To further investigate the morphology and microstructure of UC-CNS/Pt after 14,000 cycles of CVs in H2SO4/CH3OH electrolytes, TEM and HRTEM images were assessed and shown in Fig. 7. The morphology of the UC-CNS/Pt (Fig. 7 a-c) did not change

noticeably after 14,000 cycles; thus, UC-CNS/Pt showed excellent durability in acidic electrolyte. The particle size of the dispersed Pt NPs throughout the catalysts without evident aggregation, and centered at ca. 2.25 nm (inset image in Fig. 7c). The EDS result of Fig. 7d showed that the Pt content of the UC-CNS/Pt (selected area in Fig. 7b) was about 11.1 wt% and remained 74.0 % theoretical Pt loading (15 wt%). Results indicated the Pt NPs retained uniform distribution and good loading density.

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Conclusion

We successfully prepared UC-CNS with ultrathin and coiled edges via chemical vapor deposition with WC as substrate and methanol/ethanol as carbon source.

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Numerous carbon nanosheets can easily grow on the WC substrate through a

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sustainable system for successively supplying liquid methanol/ethanol sources. The UC-CNS (consist of 10-16 carbon layers) with ultrathin and coiled edges can be

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easily stripped away from the WC substrate and chemically modified. WC can be

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used as a template to produce UC-CNS repeatedly. The UC-CNS with high degree of graphitization (ID/IG of 1.43) and high specific surface area (547 m2 g-1) deposited Pt

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NPs displays high electrochemical properties and excellent stability, which can be used as an electrocatalyst for fuel cells and in other related green chemical fields.

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Acknowledgements

This work was supported by the China Scholarship Council [201908360233]; the

Postdoctoral Science Foundation of China [2018M632591]; and the Department of Science & Technology of Jiangxi Province [GJJ160604]. Z. P. Wu acknowledges support from the National Natural Science Foundation of China [51861009, 51202095

and 51264010]; and the Department of Science and Technology of Jiangxi Province [GJJ150617, GJJ160596, 20171ACB21043]. D. D. Dionysiou acknowledges support from the University of Cincinnati through a UNESCO co-Chair Professor Position on “Water Access and Sustainability” and the Herman Schneider Professorship in the College of Engineering and Applied Sciences. Appendix A. Supplementary data

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Supplementary materials related to this article can be found, in the online version, at XXX

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Declaration of Interests

The authors declare that they have no known competing financial interests or personal

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relationships that could have appeared to influence the work reported in this paper.

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Figures

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Scheme 1. Scheme illustration of WC@UC-CNS, UC-CNS and UC-CNS/Pt.

a

b

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Fig. 1 (a) TEM image of WC@UC-CNS precursor with soomth surface before carbonization; (b,

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c) TEM images and (d) XRD spectrum of WC@UC-CNS.

Transmittance

a b c 564 3410 916

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Fig. 2 The infrared spectrogram of (a) WC@UC-CNS, (b) UC-CNS-1 and (c) UC-CNS purified

by WC@UC-CNS soaking in two different volume ratios (1:10:10 and 1:1:2) of HNO3/HF/H2O

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solution.

b

a

500 nm

100 nm

1000

0.08 0.07 0.06 0.05 0.04 0.03 0

10

500

0

20

30

40

Pore radius(nm)

-p

20 nm

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5 nm

1500

dVp/drp(cm3g-1nm-1)

Coiled edges

3 -1

2000

0.34nm

adsorption desorption

0.09

2500

Volume adsorbed(cm g STP)

d

c

0.2

0.4

60

0.6

70

Relative pressure(P/P0)

0.8

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0.0

50

1.0

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Fig. 3 The morphology and microstructure of the UC-CNS stripped away from the WC substrate. (a) SEM image; (b, c) TEM images, the insert in (c) is HRTEM image of coiled edges; (d)

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adsorption/desorption isothermal curve and pore size distribution.

b

a

mean size=2 nm

10 nm

20 nm

1.5

Element

Weight%

C Pt W O Cu

79.93 14.48 0.38 1.30 3.90

d

Atomic% 46.80 52.18 0.01 0.43 0.57

Pt(111)

Pt Cu

Pt OCu W W

Cu Pt

Pt

3.5

4.0

0.23 nm

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keV

15

3.0

-p

5 nm 0 5 10 Full Scale 408 cts Cursor:0.014(1171 cts)

2.5

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cC

2.0

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Fig. 4 The morphology and composition o5f the UC-CNS/Pt catalyst. (a, b) TEM images, (c) EDS result of the selected area of (a), and (d) HRTEM image, particle size distributions and SAED of

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Pt NPs is given as an insert in (b) and (d); (e) HAADF-STEM image and the corresponding EDX

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mapping images of Pt, C and W.

a

b

C 1s Pt 4f

W 4f

C-O

C=O-O

Intensity (a.u.)

c

500

400

300

200

Binding Energy (eV)

100

0

292

288

Pt 4f5/2

286

284

Binding Energy (eV) W 4f7/2

d

Pt 4f7/2

Pt0 Pt2+ Pt4+

290

W 4f5/2

282

WC WO3

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600

Intensity (a.u.)

700

C 1s

Intensity (a.u.)

Intensity (a.u.)

O 1s

C-C/C=C

81

78

75

72

Binding Energy (eV)

69

42

40

38

36

34

Binding Energy (eV)

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84

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W 4f5/2

32

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Fig. 5 (a) XPS survey spectrum of UC-CNS/Pt, and the corresponding high resolution of (b) C

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1s, (c) Pt 4f, (d) W 4f in UC-CNS/Pt.

0.04

a

0.09

Current density/A cm-2

0.02

b

Current density/A cm-2

UC-CNS/Pt Commercial Pt/C

UC-CNS/Pt Commercial Pt/C

0.06

0.00

0.03

-0.02

0.00

ECSAUC-CNS/Pt =121.98 m2g-1 ECSACommercial Pt/C =68.62 m2g-1

-0.04

-0.03

-0.06

-0.06 -0.08 0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.6

Potential (V.vs RHE) UC-CNS/Pt Commercial Pt/C

Current density/A cm-2

0.009

ECSAUC-CNS/Pt =16.09 m2g-1

0.03

0.003

0.46V

0.00

0.61V

0.000

-0.03

-p

-0.003

-0.06 -0.006 1.0

1.2

1.4

1.6

Potential (V.vs RHE)

1.8

1.4

1.6

1.8

2.0

2.0

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Potential (V.vs RHE)

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Current density/A cm-2

e

0.09

1.2

1th 7000th 14000th

0.06

ECSACommercial Pt/C =26.26 m2g-1

0.006

d

0.09

1.0

Potential (V.vs RHE)

ro of

c

Current density/A cm-2

0.012

0.8

UC-CNS/Pt Commercial Pt/C

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0.06

0.03

1000

2000

Time (s)

3000

ur

0

na

0.00

Fig. 6 CV curves of the UC-CNS/Pt and the commercial Pt/C catalyst (a) in 0.5M H2SO4 and (b)

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in 0.5M H2SO4 +1M CH3OH, and (c) CO stripping curves at a scanning rate of 100 mv s-1 at room temperature; (d) CV curves of the UC-CNS/Pt catalyst after 1st, 7,000 and 14,000 cycles at a scanning rate of 100 mv s-1; (e) chronoamperometric current curves at a polarization voltage of 0.68 V. Note: Pt loading on the working electrode was controlled to be 0.2 mg cm-2.

Table 1 Mass Activity and Specific Activity of UC-CNS/Pt catalysts after 1 cycle, 7,000 cycles and 14,000 cycles. Mass Activity (A mg-1 Pt)

Specific Activity (mA cm-2)

1

4.59

3.76

7,000

5.14

4.21

14,000

4.60

3.77

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Number (cycle)

a

b

mean size=2.25 nm

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Fig. 7 The morphology of the UC-CNS/Pt catalyst after 14,000 potential cycles at a scanning rate

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of 100 mv s-1 at room temperature. (a-c) TEM and HRTEM images, the insert in (a) is particle size

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distributions of Pt NPs, and (d) EDS result of the selected area of (b).