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Fine platinum nanoparticles supported on polyindole-derived nitrogen-doped carbon nanotubes for efficiently catalyzing methanol electrooxidation
Applied Surface Science 501 (2020) 144260
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Full Length Article
Fine platinum nanoparticles supported on polyindole-derived nitrogendoped carbon nanotubes for efficiently catalyzing methanol electrooxidation
T
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Kexin Huanga,c,1, Jingping Zhonga,1, Jiongrong Huanga, Huaguo Tanga, Youjun Fana, , ⁎ ⁎ Muhammad Waqasa, Bo Yanga, Wei Chena, , Jun Yangb, a
Guangxi Key Laboratory of Low Carbon Energy Materials, College of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guilin 541004, China State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China c Guangxi Vocational & Technical Institute of Industry, Nanning 530001, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Multi-walled carbon nanotubes Nitrogen-doping Polyindole Pt nanoparticles Methanol electrooxidation
Carbon-based nanomaterials e.g. activated carbon powder, carbon nanotubes and graphene are often used as supports to sustain high performance of metal nanoparticles in electrochemical reactions. In principle, doping the carbon-based nanomaterials with hetero-atoms is an efficient approach to increase their catalytic aspects as catalyst supports. In this study, nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) are prepared for the first time by annealing the polyindole (PIn) coated acid treated MWCNTs, and then fine platinum nanoparticles (PtNPs) are deposited on them to design the Pt/N-MWCNTs catalyst for the anodic reaction of direct methanol fuel cells (DMFCs). The characterizations including Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray energy dispersive spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) confirm that zero valent Pt metal is formed on the surfaces of N-MWCNTs with highly dispersion and uniformly fine sizes (ca. 2.11 nm). The Pt/N-MWCNTs exhibits the enhanced electro-catalytic efficacy, improved electrochemical stability and anti-CO poisoning capability compared to Pt/AO-MWCNTs (the precursor used to prepare Pt/N-MWCNTs) and commercially available Pt/C catalysts for methanol oxidation reaction (MOR) due to strong electronic interaction between the fine PtNPs and the N-MWCNT supports, as verified by electrochemical cyclic voltammetry and chronoamperometry methods.
1. Introduction Recently, the quick progress in portable electronic devices and electric vehicles has generated large demands for safe, stable and efficient movable powers. Direct methanol fuel cells (DMFCs) would be an excellent candidate for their advantages in high energy density, convenient operation and environment amity [1–5]. The catalytic behavior of methanol oxidation reaction (MOR) is a key factor for DMFCs from candidate to large-scale commercial application. Nowadays, scarce noble platinum (Pt)-based catalyst still plays an important role in MOR [6–8]. However, poor stability, high fabrication cost, and low CO-tolerance of Pt-based catalyst require us to adopt sorts of methods to improve its utilization, stability, efficiency and economy. Many studies have proved that the supports can impact the characteristics of the catalysts through their structure, surface properties and the interaction with the same [9–11]. Hence, it is necessary to develop some advanced supporting materials to improve the electro-catalytic performance of
catalysts [12–14]. The potential materials used as supports should possess some key features, such as (i) excellent electrical thermal conductivity, (ii) high specific areas, chemical stability, (iii) superior mechanical properties, and (iV) strong interaction with the loading metals [15]. Carbon nanotubes (CNTs) are a fascinating option as support for Pt-based electro-catalysts owing to their distinct features including high specific surface area, high durability, excellent strength and electronic conductivity. However, their chemical inert also makes CNTs lack enough surface active sites for effective deposition and dispersion of catalytic nanoparticles. Nevertheless, CNTs can be modified and functionalized through introducing defects for binding active metal via strong interaction [16,17]. Usually, treatment with strong oxidants (e.g., H2SO4, KMnO4 or K2Cr2O7) is a common approach to create defects in CNTs. The oxidized CNTs have many hydrophilic oxygen-enriched functional groups (such as -OH, -COOH, etc.) on their surfaces, which could lead to effective deposition and dispersion of metal nanoparticles on the CNTs [18–20].
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Corresponding authors. E-mail addresses: [email protected] (Y. Fan), [email protected] (W. Chen), [email protected] (J. Yang). 1 These authors contributed equally to the work. https://doi.org/10.1016/j.apsusc.2019.144260 Received 25 August 2019; Received in revised form 25 September 2019; Accepted 30 September 2019 Available online 11 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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(PIn), followed by annealing at elevated temperature to generate nitrogen-doped MWCNTs (N-MWCNTs). Subsequently, a novel Pt/NMWCNTs catalyst is designed by depositing PtNPs on the surface of NMWCNTs using a typical solvothermal method and applied to catalyze the anodic reaction of DMFCs. For comparative analysis, the Pt supporting on acid-treated MWCNTs (without PIn functionalization and annealing) is also prepared. To the best of our acknowledgement, using PIn as N-precursor to dope carbon nanomaterials has not been reported, and the electrochemical evaluations reveal that the as-fabricated Pt/NMWCNTs shows excellent electrocatalytic activity, stability and anti-CO poisoning capability for methanol oxidation.
However, oxygen-containing functional groups also accelerate the oxidation of defective CNTs, resulting in corrosion of CNTs and deactivation of Pt during electrochemical reactions [21]. In this regard, recent studies found that the defected CNTs created by doping B [22], N [23], P [24] and S [25] can not only avoid the above-mentioned shortcomings, but also considerably improve the dispersity and electrocatalytic efficacy of Pt nanoparticles. Particularly, the atomic radius of N (70 pm) is very close to that of C (77 pm), meaning that the N atom can substitute the C atom to dope into the CNTs more easily. Simultaneously, owing to the strong N-C covalent interactions, the structure of N-doped carbon is more stable. In addition, while N dopes into CNTs, their π-π bond and conductivity enhance greatly due to the electronic rearrangement induced by external electrons provided from N atom [26,27]. Upon deposition on N-doped CNTs, the stability and dispersion as well as the electrocatalytic property of the Pt nanoparticles would strengthen apparently [28,29]. For instance, Liang et al. fabricated the N-doped CNTs under low-temperature, and used as the Pt supports for MOR [30]. The MOR activity of the Pt catalysts they prepared is 1.56fold greater than that of Pt supported on original CNTs. Song et al. synthesized cobalt nanoparticles on N-doped CNTs to catalyze ORR in alkaline medium [31], and they found that the performance of this nonnoble catalyst is superior to Pt/C, probably due to the increased synergetic effect between Co nanocrystal and N-doped CNTs. Sun et al. proved that the N-doped CNTs are ideal supports for the growth of ZrO2-protected ALD Pt catalysts [32]. The prepared ALD ZrO2-Pt/NCNTs catalysts show better stability towards ORR since the defective surface of N-CNTs favors the ALD process. As noted above, N-doped CNTs are excellent supports for binding metal electrocatalysts through strong interaction. Then, developing an appropriate method to prepare N-doped CNTs is of particular importance. Generally, N-doped CNTs are prepared by three methods: (i) chemical vapor deposition (CVD), (ii) pyrolysis of nitrogen-containing polymers, and (iii) post-treatment method. Although N-doped CNTs prepared by CVD method have high purity, the product with low degree of graphitization tends to be bent and deformed (looks like bamboo joints) [33]. The method of pyrolysis of nitrogen-containing polymers needs a nanotube-like template to form polymer nanotubes [34], and this method has two disadvantages, i.e. complexity and uncontrollable nitrogen content and purity at the same time. Compared with those two methods, post-treatment method is rather simple, and the nitrogen doping mainly occurs on CNT surface. More importantly, the nitrogen content and purity can be controlled easily; even the structure of carbon nanotubes could be well maintained. Especially, post-treatment with conductive polymer does not damage the structure of CNTs. Instead, it can maintain and improve the excellent properties of CNTs. For example, Liu et al. employed polyaniline as the hetero-atom precursors and prepared N-doped CNTs with enhanced ORR activity in KOH solution [35]. In our previous study, sulfur-doped multi-walled CNTs (SMWCNTs) were prepared by heat treatment of poly (3,4-ethylenedioxythiophene) (PEDOT) functionalized MWCNTS, and then Pt/SMWCNTs catalyst was obtained by depositing PtNPs on the S-MWCNTs, which displays favorable electrocatalytic performance for MOR [36]. Unlike some conductive polymers widely used as nitrogen precursors, i.e. polyaniline, polypyrrole and polydopamine, which have only a six-membered benzene ring or a five-membered nitrogen-containing pyrrole ring, polyindole (PIn), a polymer form of indole with a bicyclic structure of benzene and pyrrole rings, has attracted considerable interest for its potential applications in sensor, supercapacitor and electrocatalyst due to its high redox activity, good thermal stability and well corrosion inhibition [37–39]. According to these attributes, PIn might be a potential candidate as a nitrogen precursor for doping of MWCNTs. Therefore, in this study, we report an effective strategy for synthesizing N-doped CNTs and investigate their performance as Pt nanoparticle supports for catalyzing MOR. As illustrated by Scheme 1, in this strategy, multi-walled carbon nanotubes (MWCNTs) are firstly treated by concentrated HNO3, and then functionalized by polyindole
2. Experimental 2.1. Materials Nafion (5 wt%) and indole (C8H7N, ≥99%) were obtained from Sigma-Aldrich, sodium dodecyl sulfate (SDS, analytical grade) was bought form Aladdin, nitric acid (HNO3, 68%) and (NH4)2S2O8, (analytical grade) were gained from Shantou Xilong Scientific Co., Ltd., H2PtCl6⋅6H2O (analytical grade), C2H5OH (analytical grade) and CH3OH (analytical grade) were supplied by Sinopharm Chemical Reagent Co., Ltd. All the reagents are used as received. 2.2. Synthesis of Pt/N-MWCNTs In a typical experimental procedure, 100 mg of MWCNTs and 100 mL of HNO3 (68%, 8 mol L−1) were successively added to a 250-mL beaker, and then the mixture was put in an oil bath fixed at 60 °C and stirred for 4 h. After that, the mixture was cooled down to room temperature, centrifuged at 9000 rpm and washed by distilled water repeatedly to achieve neutral state (pH = 7). Finally, the acid-treated MWCNTs, labeled as AO-MWCNTs, were obtained by drying in a vacuum oven at 60 °C for 12 h. Subsequently, 2.307 g of SDS and 20 mL of distilled water were added in a 100-mL beaker and stirred by a magnetic stirrer until colorless. Then, 20 mg of the as-prepared AO-MWCNTs was added, followed by ultrasonic oscillation for 2 h and addition of 13 mg of indole in turn. In the next step, stirring was continued for 2 h and 20 mL of aqueous (NH4)2S2O8 (0.4 g) solution was added drop-wise under stirring conditions. After stirring for an extra 24 h the mixture was allowed to stand for 3 h, and the prepared slurry was centrifuged, followed by washing with tipple distilled water and ethanol to achieve homogeneously colorless condition. Finally, the surplus solid was dried in a vacuum oven at 60 °C for 12 h to obtain PIn modified MWCNTs, labeled as PIn/MWCNTs. Part of PIn/MWCNTs was then placed in a tube furnace and annealed at 800 °C for 3 h under the protection of argon to obtain nitrogen-doped MWCNTs (N-MWCNTs). For the fabrication of Pt/N-MWCNTs, the typical solvothermal strategy was employed. Firstly, 20 mg of N-MWCNTs, 20 mL of ethylene glycol and 1.334 mL of aqueous H2PtCl6 solution (0.0193 mol L−1) were sequentially added in a 50-mL beaker. Secondly, the mixture was ultrasonicated for 2 h and then transferred into a Teflon-autoclave (25 mL) for heating at 90 °C for 24 h. Finally, the as-obtained mixture was centrifuged, washed with distilled water and dried in a vacuum oven at 60 °C for 12 h to obtain the Pt/N-MWCNTs. For comparative analysis, an AO-MWCNTs supported Pt catalyst (Pt/AO-MWCNTs) was also synthesized likewise just without PIn modification. 2.3. Characterization The confocal microscopic Raman spectrometer (InVia, Renishaw, UK) was employed to detect the Raman spectra of supports. The X-ray diffraction (XRD) patterns were recorded by a X-ray diffractometer (D/ MAX-2500V/PC, Rigaku, Japan). The morphology of the samples were investigated by high-resolution transmission electron microscope 2
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Scheme 1. Schematic illustration showing the preparation of Pt/N-MWCNTs.
defect degree in carbon materials [41]. In addition, the ID/IG of the three samples by increasing order is PIn/MWCNTs (0.52), AO-MWCNTs (0.64) and N-MWCNTs (0.82). The ID/IG of PIn/MWCNTs is slighter than that of AO-MWCNTs, suggesting that the defect sites on the AOMWCNTs surface might be covered by the Pin, which causes decrease of surface disorder [36]. Notably, a distinctive contrast in ID/IG ratios between PIn/MWCNTs and N-MWCNTs indicates that the N-MWCNTs derived from further annealing of PIn/MWCNTs are more chaotic, also meaning that the doped N atoms create defects in the graphite lattice of MWCNTs and increase the structural disorder [41], which in turn support the successful doping of N atoms in the MWCNTs. Furthermore, the G band of the N-MWCNTs presents a slight blue-shift (3.6 cm−1) as compared to PIn/MWCNTs and AO-MWCNTs, which could be interpreted by the increase of the vibrational frequency due to the injection of N atoms into the carbon cloud of MWCNTs. The phase structures of the catalysts were detected via X-ray diffraction (XRD). Typically, the XRD patterns of Pt/N-MWCNTs (a) and Pt/AO-MWCNTs (b) are shown in Fig. 1B. The diffraction peak appeared at 26.2° is belonged to the diffraction of C (0 0 2), while the diffraction peaks at 39.8°, 46.4°, 67.6° and 81.6° are attributed to the diffractions of (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of face-centered cubic (fcc) Pt (JCPDS, No. 65-2868), respectively. In addition, the broader peaks for Pt in Pt/N-MWCNTs imply smaller size of PtNPs, which can be verified by the following calculation and the TEM observations. Since non-overlap with other peaks, the diffraction peak of Pt (2 2 0) is applied to calculate and analyze the size of PtNPs through Scherrer formula [42]. Calculation reveals that the mean sizes of PtNPs in Pt/N-MWCNTs and Pt/AO-MWCNTs are 2.15 nm and 3.32 nm, respectively. Subsequently, TEM was employed to analyze the morphologies of the catalysts. Fig. 2A and B are the TEM image of Pt/N-MWCNTs and histogram showing the distribution of particle sizes (random statistics of 200 nanoparticles), respectively. According to the TEM image, the PtNPs are uniformly dispersed on N-MWCNTs, and the measured distance between two neighboring planes is 0.23 nm, corresponding to the lattice distance of Pt (1 1 1) [13]. The histogram in Fig. 2B shows that the average size of the PtNPs is ca. 2.11 nm, which is well consistent with XRD calculations. Furthermore, Fig. 2C for the EDX spectrum indicates that the as-designed Pt/N-MWCNTs only contains C, O, N and Pt elements, while no impurity peak was observed. Fig. 2D–G are the element distributions in Pt/N-MWCNTs obtained under HAADF-STEM mode. Obviously, the N and Pt elements are uniformly distributed on
(HRTEM, JEM-2100, JEOL, Japan) adopting 200 kV as the accelerating voltage. XPS spectra were collected by a X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, USA). Inductively coupled plasma mass spectrometer (ICP-MS, FLexar-NexION300X, Perkin Elmer, USA) was used to determine the Pt content. The results show that the Pt proportion in Pt/N-MWCNTs and Pt/AO-MWCNTs are 19.2%, and 13.3%, respectively. 2.4. Electrochemical measurements An electrochemical workstation (CHI 660D, huakutian, China) containing classic three-electrode electrolytic cell system was used to carry out all the electrochemical measurements. Typically, the working electrode is a glassy carbon electrode (GCE, φ = 5.0 mm) modified with Pt/N-MWCNTs and Pt/AO-MWCNTs, while the platinum sheet (1 cm2) and saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. Before use, the glassy carbon electrodes were successively polished by 5.0 μm, 1.0 μm and 0.3 μm of Al2O3 powder (the electrode surface was cleaned by distilled water under ultrasonic after each polish). Finally, the glassy carbon electrodes were ultrasonically cleaned with distilled water, anhydrous ethanol and distilled water for 2–3 min in turn, and then dried naturally at room temperature. Further, 3 mg of the as-synthesized catalysts was added into 3 mL of distilled water, followed by addition of 50 μL of Nafion solution. The mixture was ultrasonically dispersed for 30 min, and then 15 μL of this slurry was meticulously introduced onto the pre-polished GCE and then dried at room temperature. The electrocatalytic tests were performed in 0.5 M CH3OH and 0.5 M H2SO4 solution, while the cyclic voltammetry scan rate and chronoamperometry potential were set at 50 mV s−1 and 0.6 V, respectively. 3. Results and discussion Raman spectroscopy may provide evidence to show the defect degree in carbon. Fig. 1A is the Raman spectrum of N-MWCNTs (a), PIn/ MWCNTs (b) and AO-MWCNTs (c). All three curves show D and G characteristic peaks at nearly 1347 cm−1 and 1578 cm−1 [40]. Generally, the D peak of disordered band is attributed to the defect degree of the CNTs, while the G characteristic peak is ascribed to the sp2 bond vibration between the carbon atoms in graphite layer. In particular, the ratio of ID/IG denotes the defect degree of CNT surface, as reported previously. The higher ID/IG value indicates the presence of higher 3
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shows a signal peak of N 1s at 399.4 eV, indicating that N element has been validly doped in MWCNTs [44]. Fig. 4B shows the high resolution spectrum of N 1s for Pt/N-MWCNTs. The characteristic peaks appeared at 398.21 eV, 399.57 eV and 400.7 eV represent pyridine N, pyrrolic N and graphite N, respectively [45,46], and the nitrogen content is 0.63 wt% in N-MWCNTs. Fig. 4C and D are the high resolution spectra of Pt 4f in Pt/N-MWCNTs and Pt/AO-MWCNTs, respectively. The PtNPs display two valence states, i.e. zero valence (Pt0) and divalence (Pt2+) in both Pt/N-MWCNTs and Pt/AO-MWCNTs. In detail, for the Pt in Pt/ N-MWCNTs, the XPS peaks of Pt0 are at 71.69 eV (Pt 4f7/2) and 75.07 eV (Pt 4f5/2), and the peaks of Pt2+ emerge at 72.68 eV (Pt 4f7/2) and 76.44 eV (Pt 4f5/2), while for the Pt in Pt/AO-MWCNTs, the XPS peaks of Pt0 appear at 71.39 eV (Pt 4f7/2) and 74.77 eV (Pt 4f5/2), and the peaks of Pt2+ are at 72.07 eV (Pt 4f7/2) and 76.82 eV (Pt 4f5/2), respectively [27]. By contrast, a red-shift of ca. 0.38 eV is observed for the 4f binding energies of Pt in Pt/N-MWCNTs as compared to the Pt in Pt/AO-MWCNTs, and this slight shift might be an indication to show the electronic rearrangement in MWCNTs induced by doping of N atoms, which also vary the electronic feature of Pt, suggesting that there is strong electronic interaction between the doped N atoms and
the surface of MWCNTs, meaning that the N atoms are efficiently doped into MWCNTs and then the PtNPs can be uniformly loaded on the same. As contrasts, Fig. 3A and B show the TEM image of Pt/AO-MWCNTs and its corresponding size distribution histogram, respectively. As observed, because of without N-dopant, PtNPs are unevenly dispersed and obviously agglomerated on AO-MWCNTs. The PtNPs display a larger average particle size of ca. 3.12 nm, very close to the calculation from the XRD peak. Therefore, the N-doping has significant control on the size and its distribution of PtNPs on MWCNT supports. The doped N atoms may provide more homogeneous active sites to anchor PtNPs through strong electronic interaction [43], and the uniform distribution of doped N atoms in MWCNTs leads to extreme dispersion of Pt with finer sizes. XPS has advantages in determining the composition and electronic structure of substances, and the XPS is therefore employed to analyze the catalysts in this study. Fig. 4A is the wide XPS spectra of Pt/NMWCNTs (a) and Pt/AO-MWCNTs (b). As seen from the graphs, characteristic peaks of C 1s, O 1s, Pt 4d and Pt 4f are found in both aforesaid catalysts, witnessing that PtNPs have been successfully deposited on NMWCNTs and AO-MWCNTs. In addition, curve a (redline in Fig. 4A) 35
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g−1), where QH represents the charge for hydrogen desorption [13], The ECSA of Pt/N-MWCNTs is calculated to be 94.61 m2 g−1, higher than that of Pt/AO-MWCNTs (78.88 m2 g−1) and commercial Pt/C (54.50 m2 g−1). The larger ECSA of PtNPs in Pt/N-MWCNTs can be attributed to their smaller sizes and the strong electron interaction between PtNPs and N-MWCNT support. Fig. 5B is the cyclic voltammograms of Pt/N-MWCNTs (a), Pt/AO-MWCNTs (b) and commercial Pt/C (c) in 0.5 M CH3OH and 0.5 M H2SO4 solutions. All these three catalysts present a pair of irreversible current characteristic peaks near 0.87 V and 0.66 V for methanol oxidation in acidic media, respectively. This pair of current peaks corresponds to the oxidation of methanol in the forward scan and the oxidative removal of carbonaceous intermediate species generated in the backward scan, respectively.
the deposited PtNPs. In addition, through deconvolution of the XPS peaks, the zero valent content of Pt in Pt/N-MWCNTs calculated by its XPS peak area is 79.55%, 6.17% higher than that of Pt in Pt/AOMWCNTs (73.38%), manifesting that the metallic content of Pt in Pt/NMWCNTs is more dominant. The higher Pt (0) content in Pt/NMWCNTs would also be benefit for its improved activity in electrochemical reactions. Fig. 5A is the cyclic voltammograms of Pt/N-MWCNTs (a), Pt/AOMWCNTs (b) and commercial Pt/C (c) in 0.5 M H2SO4 solutions. All the catalysts have obvious hydrogen adsorption and desorption characteristic peaks between 0.01 V and 0.34 V. As an important indicator of electrochemical performance, the electrochemically active area (ECSA) is calculated according to the formula: ECSA = QH/(2.1 × mPt) (m2
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Fig. 4. (A) XPS full spectra of Pt/N-MWCNTs (a) and Pt/AO-MWCNTs (b); (B) High resolution N 1s spectrum of Pt/N-MWCNTs; High resolution Pt 4f spectra of Pt/NMWCNTs (C) and Pt/AO-MWCNTs (D). 5
Applied Surface Science 501 (2020) 144260
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Fig. 5. Cyclic voltammograms of Pt/N-MWCNTs (a), Pt/AO-MWCNTs (b) and commercial Pt/C (c) in 0.5 mol L−1 H2SO4 (A) and 0.5 mol L−1 CH3OH + 0.5 mol L−1 H2SO4 (B) solution at 50 mV s−1. (C) Current-time curves of methanol oxidation on Pt/N-MWCNTs (a), Pt/AO-MWCNTs (b) and commercial Pt/C (c) in 0.5 mol L−1 CH3OH + 0.5 mol L−1 H2SO4 solution, measured at 0.6 V. (D) CO stripping voltammograms of Pt/N-MWCNTs (a), Pt/AO-MWCNTs (b) and commercial Pt/C (c) in 0.5 mol L−1 H2SO4 at 50 mV s−1. Table 1 Performance comparison of methanol oxidation on the Pt/N-MWCNTs and other published Pt-based carbon nanocomposite catalysts. Catalyst
Size (nm)
ECSA (m2 g−1)
If (mA mg−1
Pt/h-NCTs Pt/NCQDs-MWCNT 10 wt%Pt/NOMC-2D PtFe/RGO HNSs Pt/NG180 + MWNT Pt/NGA Pt/S-rGO Pt/rGO G-P-G Pt/N-MWCNTs
2.0 – 1.4 – 3.3 2.0 3.8 1.8 2.16 2.1
73.6 46.5 79.7 43.46 36 60.6 39.4 101.3 73.9 94.6
505 420 360.0 461.5 318.0 507.5 465 333.3 462.8 539.4
Pt)
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Reference
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0.5 0.5 0.5 0.5 1.0 0.5 1.0 0.5 0.5 0.5
[18] [47] [48] [49] [50] [51] [52] [53] [54] This study
0.5 M CH3OH and 0.5 M H2SO4 solutions. The current-time (i-t) curves display same tendency with sharp dropping at the first 200 s and gently decrease then. In particular, the current density of Pt/N-MWCNTs is the highest all over the time. The current drops sharply at the beginning mainly because the free active sites on catalyst surface are wrapped and poisoned by CO-like intermediate species rapidly [55]. After that, each step of MOR tends to be stable, responding for smooth liberation and adsorption of the electrocatalytic active sites [2], so the current-time (it) curve turns to be gently. It is noteworthy that at the end, Pt/NMWCNTs (a) still maintains a higher current density (73.03 mA mg−1 Pt ), which is about 1.4 times and 1.3 times as high as Pt/AO-MWCNTs (b) −1 (53.23 mA mg−1 Pt ) and commercial Pt/C (c) (56.18 mA mgPt ), respectively, illustrating the superior electrocatalytic activity and durability of Pt/N-MWCNTs. Fig. 5D is the CO stripping voltammograms of Pt/NMWCNTs (a), Pt/AO-MWCNTs (b) and commercial Pt/C (c) in 0.5 M H2SO4 solutions. The curves reveal that the hydrogen absorption and desorption current on each catalyst surface are completely suppressed
Therefore, the MOR activity for different catalysts can be compared through the peak current density in the forward scan. The peak current density of Pt/N-MWCNTs (a) in forward scan is 539.4 mA mg−1 Pt , which is significantly higher than that of Pt/AO-MWCNTs (450.3 mA mg−1 Pt ) and Pt/C (375.8 mA mg−1 Pt ). Moreover, the onset potential of methanol oxidation on the Pt/N-MWCNTs (0.44 V) is lower than that of the Pt/ AO-MWCNTs (0.46 V) and commercial Pt/C catalysts (0.46 V). The results reveal that the as-prepared Pt/N-MWCNTs has the highest MOR electrocatalytic activity compared to other two counterparts. Because of their largest ECSA, the Pt/N-MWCNTs exposes the most Pt active sites along with strong electronic interaction between Pt and supports, which also promotes the methanol oxidation, leading to its highest electrocatalytic activity. Importantly, in comparison with other PtNPs on carbon-based supports reported in some recent literatures [18,47–54], as listed in Table 1, the Pt/N-MWCNTs prepared in this work also has higher mass activity for MOR. Fig. 5C is the chronoamperometry curve of Pt/N-MWCNTs (a), Pt/AO-MWCNTs (b) and commercial Pt/C (c) in 6
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(N-MWCNTs) were synthesized by annealing the PIn-modified MWCNTs, and then a novel Pt/N-MWCNT catalyst was synthesized by depositing PtNPs on this N-MWCNT support. The TEM, XRD and XPS results reveal that the PtNPs supported on the N-MWCNTs have smaller particle size with more uniform dispersion, which is closely related to the active sites created by doped N atoms and the strong electron interaction between PtNPs and N dopants. Moreover, the electrocatalytic measurements show that the prepared Pt/N-MWCNTs presents higher electrocatalytic activity, electrochemical stability and CO-tolerance capability for methanol oxidation reaction.
(B)
(A)
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Declaration of Competing Interest
(C)
(D)
The authors declare no competing financial interest. Acknowledgements
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20 nm (E)
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This work was supported by the National Natural Science Foundation of China (21463007, 21573240), Natural Science Foundation of Guangxi Province (2017GXNSFDA198031, 2016GXNSFAA380199), the BAGUI Scholar Program (2014A001), the Project of Talents Highland of Guangxi Province and Innovation Project of Guangxi Graduate Education (XYCBZ2019004).
(F)
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Fig. 6. TEM images of Pt/N-MWCNTs (A,B), Pt/AO-MWCNTs (C,D) and commercial Pt/C (E,F) before (A,C,E) and after 7200 s chronoamperometric tests (B,D,F) in 0.5 mol L−1 CH3OH + 0.5 mol L−1 H2SO4 solution. Test potential: 0.6 V.
because of the saturation adsorption of CO in the first scan, but the current returns to normal after CO is oxidized and removed in the second scan. Further, the onset potential of CO oxidation in Pt/NMWCNTs is 0.67 V, less than that of Pt/AO-MWCNTs (0.74 V) and commercial Pt/C (0.76 V), indicating that the Pt/N-MWCNTs has better CO anti-poisoning capability than the other two catalysts. Strong electron transfer in Pt/N-MWCNTs may accelerate the oxidation of CO-like species, responsible for their lowest onset potential and best CO antipoisoning capability. Further, to appraise the long-term electrochemical stability of the catalyst, the TEM images of the catalysts taken prior and after electrochemical tests were compared. Fig. 6 is the TEM images of Pt/NMWCNTs (A and B), Pt/AO-MWCNTs (C and D) and commercial Pt/C (E and F) before and after chronoamperometric tests in 0.5 M CH3OH + 0.5 M H2SO4. As revealed by these images, the morphology and dispersity of PtNPs in Pt/N-MWCNTs have no distinct change after 7200 s test. However, the PtNPs in the other two catalysts become significantly larger and show serious agglomeration after electrochemical tests, indicating that the doping of N in MWCNTs can apparently enhance the electrochemical stability of deposited PtNPs.
4. Conclusion A creative method for preparation nitrogen-doped carbon nanotubes was developed. Nitrogen-doped multi-walled carbon nanotubes 7
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Full Length Article
Fine platinum nanoparticles supported on polyindole-derived nitrogendoped carbon nanotubes for efficiently catalyzing methanol electrooxidation
T
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Kexin Huanga,c,1, Jingping Zhonga,1, Jiongrong Huanga, Huaguo Tanga, Youjun Fana, , ⁎ ⁎ Muhammad Waqasa, Bo Yanga, Wei Chena, , Jun Yangb, a
Guangxi Key Laboratory of Low Carbon Energy Materials, College of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guilin 541004, China State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China c Guangxi Vocational & Technical Institute of Industry, Nanning 530001, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Multi-walled carbon nanotubes Nitrogen-doping Polyindole Pt nanoparticles Methanol electrooxidation
Carbon-based nanomaterials e.g. activated carbon powder, carbon nanotubes and graphene are often used as supports to sustain high performance of metal nanoparticles in electrochemical reactions. In principle, doping the carbon-based nanomaterials with hetero-atoms is an efficient approach to increase their catalytic aspects as catalyst supports. In this study, nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) are prepared for the first time by annealing the polyindole (PIn) coated acid treated MWCNTs, and then fine platinum nanoparticles (PtNPs) are deposited on them to design the Pt/N-MWCNTs catalyst for the anodic reaction of direct methanol fuel cells (DMFCs). The characterizations including Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray energy dispersive spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) confirm that zero valent Pt metal is formed on the surfaces of N-MWCNTs with highly dispersion and uniformly fine sizes (ca. 2.11 nm). The Pt/N-MWCNTs exhibits the enhanced electro-catalytic efficacy, improved electrochemical stability and anti-CO poisoning capability compared to Pt/AO-MWCNTs (the precursor used to prepare Pt/N-MWCNTs) and commercially available Pt/C catalysts for methanol oxidation reaction (MOR) due to strong electronic interaction between the fine PtNPs and the N-MWCNT supports, as verified by electrochemical cyclic voltammetry and chronoamperometry methods.
1. Introduction Recently, the quick progress in portable electronic devices and electric vehicles has generated large demands for safe, stable and efficient movable powers. Direct methanol fuel cells (DMFCs) would be an excellent candidate for their advantages in high energy density, convenient operation and environment amity [1–5]. The catalytic behavior of methanol oxidation reaction (MOR) is a key factor for DMFCs from candidate to large-scale commercial application. Nowadays, scarce noble platinum (Pt)-based catalyst still plays an important role in MOR [6–8]. However, poor stability, high fabrication cost, and low CO-tolerance of Pt-based catalyst require us to adopt sorts of methods to improve its utilization, stability, efficiency and economy. Many studies have proved that the supports can impact the characteristics of the catalysts through their structure, surface properties and the interaction with the same [9–11]. Hence, it is necessary to develop some advanced supporting materials to improve the electro-catalytic performance of
catalysts [12–14]. The potential materials used as supports should possess some key features, such as (i) excellent electrical thermal conductivity, (ii) high specific areas, chemical stability, (iii) superior mechanical properties, and (iV) strong interaction with the loading metals [15]. Carbon nanotubes (CNTs) are a fascinating option as support for Pt-based electro-catalysts owing to their distinct features including high specific surface area, high durability, excellent strength and electronic conductivity. However, their chemical inert also makes CNTs lack enough surface active sites for effective deposition and dispersion of catalytic nanoparticles. Nevertheless, CNTs can be modified and functionalized through introducing defects for binding active metal via strong interaction [16,17]. Usually, treatment with strong oxidants (e.g., H2SO4, KMnO4 or K2Cr2O7) is a common approach to create defects in CNTs. The oxidized CNTs have many hydrophilic oxygen-enriched functional groups (such as -OH, -COOH, etc.) on their surfaces, which could lead to effective deposition and dispersion of metal nanoparticles on the CNTs [18–20].
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Corresponding authors. E-mail addresses: [email protected] (Y. Fan), [email protected] (W. Chen), [email protected] (J. Yang). 1 These authors contributed equally to the work. https://doi.org/10.1016/j.apsusc.2019.144260 Received 25 August 2019; Received in revised form 25 September 2019; Accepted 30 September 2019 Available online 11 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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(PIn), followed by annealing at elevated temperature to generate nitrogen-doped MWCNTs (N-MWCNTs). Subsequently, a novel Pt/NMWCNTs catalyst is designed by depositing PtNPs on the surface of NMWCNTs using a typical solvothermal method and applied to catalyze the anodic reaction of DMFCs. For comparative analysis, the Pt supporting on acid-treated MWCNTs (without PIn functionalization and annealing) is also prepared. To the best of our acknowledgement, using PIn as N-precursor to dope carbon nanomaterials has not been reported, and the electrochemical evaluations reveal that the as-fabricated Pt/NMWCNTs shows excellent electrocatalytic activity, stability and anti-CO poisoning capability for methanol oxidation.
However, oxygen-containing functional groups also accelerate the oxidation of defective CNTs, resulting in corrosion of CNTs and deactivation of Pt during electrochemical reactions [21]. In this regard, recent studies found that the defected CNTs created by doping B [22], N [23], P [24] and S [25] can not only avoid the above-mentioned shortcomings, but also considerably improve the dispersity and electrocatalytic efficacy of Pt nanoparticles. Particularly, the atomic radius of N (70 pm) is very close to that of C (77 pm), meaning that the N atom can substitute the C atom to dope into the CNTs more easily. Simultaneously, owing to the strong N-C covalent interactions, the structure of N-doped carbon is more stable. In addition, while N dopes into CNTs, their π-π bond and conductivity enhance greatly due to the electronic rearrangement induced by external electrons provided from N atom [26,27]. Upon deposition on N-doped CNTs, the stability and dispersion as well as the electrocatalytic property of the Pt nanoparticles would strengthen apparently [28,29]. For instance, Liang et al. fabricated the N-doped CNTs under low-temperature, and used as the Pt supports for MOR [30]. The MOR activity of the Pt catalysts they prepared is 1.56fold greater than that of Pt supported on original CNTs. Song et al. synthesized cobalt nanoparticles on N-doped CNTs to catalyze ORR in alkaline medium [31], and they found that the performance of this nonnoble catalyst is superior to Pt/C, probably due to the increased synergetic effect between Co nanocrystal and N-doped CNTs. Sun et al. proved that the N-doped CNTs are ideal supports for the growth of ZrO2-protected ALD Pt catalysts [32]. The prepared ALD ZrO2-Pt/NCNTs catalysts show better stability towards ORR since the defective surface of N-CNTs favors the ALD process. As noted above, N-doped CNTs are excellent supports for binding metal electrocatalysts through strong interaction. Then, developing an appropriate method to prepare N-doped CNTs is of particular importance. Generally, N-doped CNTs are prepared by three methods: (i) chemical vapor deposition (CVD), (ii) pyrolysis of nitrogen-containing polymers, and (iii) post-treatment method. Although N-doped CNTs prepared by CVD method have high purity, the product with low degree of graphitization tends to be bent and deformed (looks like bamboo joints) [33]. The method of pyrolysis of nitrogen-containing polymers needs a nanotube-like template to form polymer nanotubes [34], and this method has two disadvantages, i.e. complexity and uncontrollable nitrogen content and purity at the same time. Compared with those two methods, post-treatment method is rather simple, and the nitrogen doping mainly occurs on CNT surface. More importantly, the nitrogen content and purity can be controlled easily; even the structure of carbon nanotubes could be well maintained. Especially, post-treatment with conductive polymer does not damage the structure of CNTs. Instead, it can maintain and improve the excellent properties of CNTs. For example, Liu et al. employed polyaniline as the hetero-atom precursors and prepared N-doped CNTs with enhanced ORR activity in KOH solution [35]. In our previous study, sulfur-doped multi-walled CNTs (SMWCNTs) were prepared by heat treatment of poly (3,4-ethylenedioxythiophene) (PEDOT) functionalized MWCNTS, and then Pt/SMWCNTs catalyst was obtained by depositing PtNPs on the S-MWCNTs, which displays favorable electrocatalytic performance for MOR [36]. Unlike some conductive polymers widely used as nitrogen precursors, i.e. polyaniline, polypyrrole and polydopamine, which have only a six-membered benzene ring or a five-membered nitrogen-containing pyrrole ring, polyindole (PIn), a polymer form of indole with a bicyclic structure of benzene and pyrrole rings, has attracted considerable interest for its potential applications in sensor, supercapacitor and electrocatalyst due to its high redox activity, good thermal stability and well corrosion inhibition [37–39]. According to these attributes, PIn might be a potential candidate as a nitrogen precursor for doping of MWCNTs. Therefore, in this study, we report an effective strategy for synthesizing N-doped CNTs and investigate their performance as Pt nanoparticle supports for catalyzing MOR. As illustrated by Scheme 1, in this strategy, multi-walled carbon nanotubes (MWCNTs) are firstly treated by concentrated HNO3, and then functionalized by polyindole
2. Experimental 2.1. Materials Nafion (5 wt%) and indole (C8H7N, ≥99%) were obtained from Sigma-Aldrich, sodium dodecyl sulfate (SDS, analytical grade) was bought form Aladdin, nitric acid (HNO3, 68%) and (NH4)2S2O8, (analytical grade) were gained from Shantou Xilong Scientific Co., Ltd., H2PtCl6⋅6H2O (analytical grade), C2H5OH (analytical grade) and CH3OH (analytical grade) were supplied by Sinopharm Chemical Reagent Co., Ltd. All the reagents are used as received. 2.2. Synthesis of Pt/N-MWCNTs In a typical experimental procedure, 100 mg of MWCNTs and 100 mL of HNO3 (68%, 8 mol L−1) were successively added to a 250-mL beaker, and then the mixture was put in an oil bath fixed at 60 °C and stirred for 4 h. After that, the mixture was cooled down to room temperature, centrifuged at 9000 rpm and washed by distilled water repeatedly to achieve neutral state (pH = 7). Finally, the acid-treated MWCNTs, labeled as AO-MWCNTs, were obtained by drying in a vacuum oven at 60 °C for 12 h. Subsequently, 2.307 g of SDS and 20 mL of distilled water were added in a 100-mL beaker and stirred by a magnetic stirrer until colorless. Then, 20 mg of the as-prepared AO-MWCNTs was added, followed by ultrasonic oscillation for 2 h and addition of 13 mg of indole in turn. In the next step, stirring was continued for 2 h and 20 mL of aqueous (NH4)2S2O8 (0.4 g) solution was added drop-wise under stirring conditions. After stirring for an extra 24 h the mixture was allowed to stand for 3 h, and the prepared slurry was centrifuged, followed by washing with tipple distilled water and ethanol to achieve homogeneously colorless condition. Finally, the surplus solid was dried in a vacuum oven at 60 °C for 12 h to obtain PIn modified MWCNTs, labeled as PIn/MWCNTs. Part of PIn/MWCNTs was then placed in a tube furnace and annealed at 800 °C for 3 h under the protection of argon to obtain nitrogen-doped MWCNTs (N-MWCNTs). For the fabrication of Pt/N-MWCNTs, the typical solvothermal strategy was employed. Firstly, 20 mg of N-MWCNTs, 20 mL of ethylene glycol and 1.334 mL of aqueous H2PtCl6 solution (0.0193 mol L−1) were sequentially added in a 50-mL beaker. Secondly, the mixture was ultrasonicated for 2 h and then transferred into a Teflon-autoclave (25 mL) for heating at 90 °C for 24 h. Finally, the as-obtained mixture was centrifuged, washed with distilled water and dried in a vacuum oven at 60 °C for 12 h to obtain the Pt/N-MWCNTs. For comparative analysis, an AO-MWCNTs supported Pt catalyst (Pt/AO-MWCNTs) was also synthesized likewise just without PIn modification. 2.3. Characterization The confocal microscopic Raman spectrometer (InVia, Renishaw, UK) was employed to detect the Raman spectra of supports. The X-ray diffraction (XRD) patterns were recorded by a X-ray diffractometer (D/ MAX-2500V/PC, Rigaku, Japan). The morphology of the samples were investigated by high-resolution transmission electron microscope 2
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Scheme 1. Schematic illustration showing the preparation of Pt/N-MWCNTs.
defect degree in carbon materials [41]. In addition, the ID/IG of the three samples by increasing order is PIn/MWCNTs (0.52), AO-MWCNTs (0.64) and N-MWCNTs (0.82). The ID/IG of PIn/MWCNTs is slighter than that of AO-MWCNTs, suggesting that the defect sites on the AOMWCNTs surface might be covered by the Pin, which causes decrease of surface disorder [36]. Notably, a distinctive contrast in ID/IG ratios between PIn/MWCNTs and N-MWCNTs indicates that the N-MWCNTs derived from further annealing of PIn/MWCNTs are more chaotic, also meaning that the doped N atoms create defects in the graphite lattice of MWCNTs and increase the structural disorder [41], which in turn support the successful doping of N atoms in the MWCNTs. Furthermore, the G band of the N-MWCNTs presents a slight blue-shift (3.6 cm−1) as compared to PIn/MWCNTs and AO-MWCNTs, which could be interpreted by the increase of the vibrational frequency due to the injection of N atoms into the carbon cloud of MWCNTs. The phase structures of the catalysts were detected via X-ray diffraction (XRD). Typically, the XRD patterns of Pt/N-MWCNTs (a) and Pt/AO-MWCNTs (b) are shown in Fig. 1B. The diffraction peak appeared at 26.2° is belonged to the diffraction of C (0 0 2), while the diffraction peaks at 39.8°, 46.4°, 67.6° and 81.6° are attributed to the diffractions of (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of face-centered cubic (fcc) Pt (JCPDS, No. 65-2868), respectively. In addition, the broader peaks for Pt in Pt/N-MWCNTs imply smaller size of PtNPs, which can be verified by the following calculation and the TEM observations. Since non-overlap with other peaks, the diffraction peak of Pt (2 2 0) is applied to calculate and analyze the size of PtNPs through Scherrer formula [42]. Calculation reveals that the mean sizes of PtNPs in Pt/N-MWCNTs and Pt/AO-MWCNTs are 2.15 nm and 3.32 nm, respectively. Subsequently, TEM was employed to analyze the morphologies of the catalysts. Fig. 2A and B are the TEM image of Pt/N-MWCNTs and histogram showing the distribution of particle sizes (random statistics of 200 nanoparticles), respectively. According to the TEM image, the PtNPs are uniformly dispersed on N-MWCNTs, and the measured distance between two neighboring planes is 0.23 nm, corresponding to the lattice distance of Pt (1 1 1) [13]. The histogram in Fig. 2B shows that the average size of the PtNPs is ca. 2.11 nm, which is well consistent with XRD calculations. Furthermore, Fig. 2C for the EDX spectrum indicates that the as-designed Pt/N-MWCNTs only contains C, O, N and Pt elements, while no impurity peak was observed. Fig. 2D–G are the element distributions in Pt/N-MWCNTs obtained under HAADF-STEM mode. Obviously, the N and Pt elements are uniformly distributed on
(HRTEM, JEM-2100, JEOL, Japan) adopting 200 kV as the accelerating voltage. XPS spectra were collected by a X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, USA). Inductively coupled plasma mass spectrometer (ICP-MS, FLexar-NexION300X, Perkin Elmer, USA) was used to determine the Pt content. The results show that the Pt proportion in Pt/N-MWCNTs and Pt/AO-MWCNTs are 19.2%, and 13.3%, respectively. 2.4. Electrochemical measurements An electrochemical workstation (CHI 660D, huakutian, China) containing classic three-electrode electrolytic cell system was used to carry out all the electrochemical measurements. Typically, the working electrode is a glassy carbon electrode (GCE, φ = 5.0 mm) modified with Pt/N-MWCNTs and Pt/AO-MWCNTs, while the platinum sheet (1 cm2) and saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. Before use, the glassy carbon electrodes were successively polished by 5.0 μm, 1.0 μm and 0.3 μm of Al2O3 powder (the electrode surface was cleaned by distilled water under ultrasonic after each polish). Finally, the glassy carbon electrodes were ultrasonically cleaned with distilled water, anhydrous ethanol and distilled water for 2–3 min in turn, and then dried naturally at room temperature. Further, 3 mg of the as-synthesized catalysts was added into 3 mL of distilled water, followed by addition of 50 μL of Nafion solution. The mixture was ultrasonically dispersed for 30 min, and then 15 μL of this slurry was meticulously introduced onto the pre-polished GCE and then dried at room temperature. The electrocatalytic tests were performed in 0.5 M CH3OH and 0.5 M H2SO4 solution, while the cyclic voltammetry scan rate and chronoamperometry potential were set at 50 mV s−1 and 0.6 V, respectively. 3. Results and discussion Raman spectroscopy may provide evidence to show the defect degree in carbon. Fig. 1A is the Raman spectrum of N-MWCNTs (a), PIn/ MWCNTs (b) and AO-MWCNTs (c). All three curves show D and G characteristic peaks at nearly 1347 cm−1 and 1578 cm−1 [40]. Generally, the D peak of disordered band is attributed to the defect degree of the CNTs, while the G characteristic peak is ascribed to the sp2 bond vibration between the carbon atoms in graphite layer. In particular, the ratio of ID/IG denotes the defect degree of CNT surface, as reported previously. The higher ID/IG value indicates the presence of higher 3
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shows a signal peak of N 1s at 399.4 eV, indicating that N element has been validly doped in MWCNTs [44]. Fig. 4B shows the high resolution spectrum of N 1s for Pt/N-MWCNTs. The characteristic peaks appeared at 398.21 eV, 399.57 eV and 400.7 eV represent pyridine N, pyrrolic N and graphite N, respectively [45,46], and the nitrogen content is 0.63 wt% in N-MWCNTs. Fig. 4C and D are the high resolution spectra of Pt 4f in Pt/N-MWCNTs and Pt/AO-MWCNTs, respectively. The PtNPs display two valence states, i.e. zero valence (Pt0) and divalence (Pt2+) in both Pt/N-MWCNTs and Pt/AO-MWCNTs. In detail, for the Pt in Pt/ N-MWCNTs, the XPS peaks of Pt0 are at 71.69 eV (Pt 4f7/2) and 75.07 eV (Pt 4f5/2), and the peaks of Pt2+ emerge at 72.68 eV (Pt 4f7/2) and 76.44 eV (Pt 4f5/2), while for the Pt in Pt/AO-MWCNTs, the XPS peaks of Pt0 appear at 71.39 eV (Pt 4f7/2) and 74.77 eV (Pt 4f5/2), and the peaks of Pt2+ are at 72.07 eV (Pt 4f7/2) and 76.82 eV (Pt 4f5/2), respectively [27]. By contrast, a red-shift of ca. 0.38 eV is observed for the 4f binding energies of Pt in Pt/N-MWCNTs as compared to the Pt in Pt/AO-MWCNTs, and this slight shift might be an indication to show the electronic rearrangement in MWCNTs induced by doping of N atoms, which also vary the electronic feature of Pt, suggesting that there is strong electronic interaction between the doped N atoms and
the surface of MWCNTs, meaning that the N atoms are efficiently doped into MWCNTs and then the PtNPs can be uniformly loaded on the same. As contrasts, Fig. 3A and B show the TEM image of Pt/AO-MWCNTs and its corresponding size distribution histogram, respectively. As observed, because of without N-dopant, PtNPs are unevenly dispersed and obviously agglomerated on AO-MWCNTs. The PtNPs display a larger average particle size of ca. 3.12 nm, very close to the calculation from the XRD peak. Therefore, the N-doping has significant control on the size and its distribution of PtNPs on MWCNT supports. The doped N atoms may provide more homogeneous active sites to anchor PtNPs through strong electronic interaction [43], and the uniform distribution of doped N atoms in MWCNTs leads to extreme dispersion of Pt with finer sizes. XPS has advantages in determining the composition and electronic structure of substances, and the XPS is therefore employed to analyze the catalysts in this study. Fig. 4A is the wide XPS spectra of Pt/NMWCNTs (a) and Pt/AO-MWCNTs (b). As seen from the graphs, characteristic peaks of C 1s, O 1s, Pt 4d and Pt 4f are found in both aforesaid catalysts, witnessing that PtNPs have been successfully deposited on NMWCNTs and AO-MWCNTs. In addition, curve a (redline in Fig. 4A) 35
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g−1), where QH represents the charge for hydrogen desorption [13], The ECSA of Pt/N-MWCNTs is calculated to be 94.61 m2 g−1, higher than that of Pt/AO-MWCNTs (78.88 m2 g−1) and commercial Pt/C (54.50 m2 g−1). The larger ECSA of PtNPs in Pt/N-MWCNTs can be attributed to their smaller sizes and the strong electron interaction between PtNPs and N-MWCNT support. Fig. 5B is the cyclic voltammograms of Pt/N-MWCNTs (a), Pt/AO-MWCNTs (b) and commercial Pt/C (c) in 0.5 M CH3OH and 0.5 M H2SO4 solutions. All these three catalysts present a pair of irreversible current characteristic peaks near 0.87 V and 0.66 V for methanol oxidation in acidic media, respectively. This pair of current peaks corresponds to the oxidation of methanol in the forward scan and the oxidative removal of carbonaceous intermediate species generated in the backward scan, respectively.
the deposited PtNPs. In addition, through deconvolution of the XPS peaks, the zero valent content of Pt in Pt/N-MWCNTs calculated by its XPS peak area is 79.55%, 6.17% higher than that of Pt in Pt/AOMWCNTs (73.38%), manifesting that the metallic content of Pt in Pt/NMWCNTs is more dominant. The higher Pt (0) content in Pt/NMWCNTs would also be benefit for its improved activity in electrochemical reactions. Fig. 5A is the cyclic voltammograms of Pt/N-MWCNTs (a), Pt/AOMWCNTs (b) and commercial Pt/C (c) in 0.5 M H2SO4 solutions. All the catalysts have obvious hydrogen adsorption and desorption characteristic peaks between 0.01 V and 0.34 V. As an important indicator of electrochemical performance, the electrochemically active area (ECSA) is calculated according to the formula: ECSA = QH/(2.1 × mPt) (m2
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Fig. 4. (A) XPS full spectra of Pt/N-MWCNTs (a) and Pt/AO-MWCNTs (b); (B) High resolution N 1s spectrum of Pt/N-MWCNTs; High resolution Pt 4f spectra of Pt/NMWCNTs (C) and Pt/AO-MWCNTs (D). 5
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Fig. 5. Cyclic voltammograms of Pt/N-MWCNTs (a), Pt/AO-MWCNTs (b) and commercial Pt/C (c) in 0.5 mol L−1 H2SO4 (A) and 0.5 mol L−1 CH3OH + 0.5 mol L−1 H2SO4 (B) solution at 50 mV s−1. (C) Current-time curves of methanol oxidation on Pt/N-MWCNTs (a), Pt/AO-MWCNTs (b) and commercial Pt/C (c) in 0.5 mol L−1 CH3OH + 0.5 mol L−1 H2SO4 solution, measured at 0.6 V. (D) CO stripping voltammograms of Pt/N-MWCNTs (a), Pt/AO-MWCNTs (b) and commercial Pt/C (c) in 0.5 mol L−1 H2SO4 at 50 mV s−1. Table 1 Performance comparison of methanol oxidation on the Pt/N-MWCNTs and other published Pt-based carbon nanocomposite catalysts. Catalyst
Size (nm)
ECSA (m2 g−1)
If (mA mg−1
Pt/h-NCTs Pt/NCQDs-MWCNT 10 wt%Pt/NOMC-2D PtFe/RGO HNSs Pt/NG180 + MWNT Pt/NGA Pt/S-rGO Pt/rGO G-P-G Pt/N-MWCNTs
2.0 – 1.4 – 3.3 2.0 3.8 1.8 2.16 2.1
73.6 46.5 79.7 43.46 36 60.6 39.4 101.3 73.9 94.6
505 420 360.0 461.5 318.0 507.5 465 333.3 462.8 539.4
Pt)
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[18] [47] [48] [49] [50] [51] [52] [53] [54] This study
0.5 M CH3OH and 0.5 M H2SO4 solutions. The current-time (i-t) curves display same tendency with sharp dropping at the first 200 s and gently decrease then. In particular, the current density of Pt/N-MWCNTs is the highest all over the time. The current drops sharply at the beginning mainly because the free active sites on catalyst surface are wrapped and poisoned by CO-like intermediate species rapidly [55]. After that, each step of MOR tends to be stable, responding for smooth liberation and adsorption of the electrocatalytic active sites [2], so the current-time (it) curve turns to be gently. It is noteworthy that at the end, Pt/NMWCNTs (a) still maintains a higher current density (73.03 mA mg−1 Pt ), which is about 1.4 times and 1.3 times as high as Pt/AO-MWCNTs (b) −1 (53.23 mA mg−1 Pt ) and commercial Pt/C (c) (56.18 mA mgPt ), respectively, illustrating the superior electrocatalytic activity and durability of Pt/N-MWCNTs. Fig. 5D is the CO stripping voltammograms of Pt/NMWCNTs (a), Pt/AO-MWCNTs (b) and commercial Pt/C (c) in 0.5 M H2SO4 solutions. The curves reveal that the hydrogen absorption and desorption current on each catalyst surface are completely suppressed
Therefore, the MOR activity for different catalysts can be compared through the peak current density in the forward scan. The peak current density of Pt/N-MWCNTs (a) in forward scan is 539.4 mA mg−1 Pt , which is significantly higher than that of Pt/AO-MWCNTs (450.3 mA mg−1 Pt ) and Pt/C (375.8 mA mg−1 Pt ). Moreover, the onset potential of methanol oxidation on the Pt/N-MWCNTs (0.44 V) is lower than that of the Pt/ AO-MWCNTs (0.46 V) and commercial Pt/C catalysts (0.46 V). The results reveal that the as-prepared Pt/N-MWCNTs has the highest MOR electrocatalytic activity compared to other two counterparts. Because of their largest ECSA, the Pt/N-MWCNTs exposes the most Pt active sites along with strong electronic interaction between Pt and supports, which also promotes the methanol oxidation, leading to its highest electrocatalytic activity. Importantly, in comparison with other PtNPs on carbon-based supports reported in some recent literatures [18,47–54], as listed in Table 1, the Pt/N-MWCNTs prepared in this work also has higher mass activity for MOR. Fig. 5C is the chronoamperometry curve of Pt/N-MWCNTs (a), Pt/AO-MWCNTs (b) and commercial Pt/C (c) in 6
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20 nm
(N-MWCNTs) were synthesized by annealing the PIn-modified MWCNTs, and then a novel Pt/N-MWCNT catalyst was synthesized by depositing PtNPs on this N-MWCNT support. The TEM, XRD and XPS results reveal that the PtNPs supported on the N-MWCNTs have smaller particle size with more uniform dispersion, which is closely related to the active sites created by doped N atoms and the strong electron interaction between PtNPs and N dopants. Moreover, the electrocatalytic measurements show that the prepared Pt/N-MWCNTs presents higher electrocatalytic activity, electrochemical stability and CO-tolerance capability for methanol oxidation reaction.
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Declaration of Competing Interest
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(D)
The authors declare no competing financial interest. Acknowledgements
20 nm
20 nm (E)
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This work was supported by the National Natural Science Foundation of China (21463007, 21573240), Natural Science Foundation of Guangxi Province (2017GXNSFDA198031, 2016GXNSFAA380199), the BAGUI Scholar Program (2014A001), the Project of Talents Highland of Guangxi Province and Innovation Project of Guangxi Graduate Education (XYCBZ2019004).
(F)
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20 nm
Fig. 6. TEM images of Pt/N-MWCNTs (A,B), Pt/AO-MWCNTs (C,D) and commercial Pt/C (E,F) before (A,C,E) and after 7200 s chronoamperometric tests (B,D,F) in 0.5 mol L−1 CH3OH + 0.5 mol L−1 H2SO4 solution. Test potential: 0.6 V.
because of the saturation adsorption of CO in the first scan, but the current returns to normal after CO is oxidized and removed in the second scan. Further, the onset potential of CO oxidation in Pt/NMWCNTs is 0.67 V, less than that of Pt/AO-MWCNTs (0.74 V) and commercial Pt/C (0.76 V), indicating that the Pt/N-MWCNTs has better CO anti-poisoning capability than the other two catalysts. Strong electron transfer in Pt/N-MWCNTs may accelerate the oxidation of CO-like species, responsible for their lowest onset potential and best CO antipoisoning capability. Further, to appraise the long-term electrochemical stability of the catalyst, the TEM images of the catalysts taken prior and after electrochemical tests were compared. Fig. 6 is the TEM images of Pt/NMWCNTs (A and B), Pt/AO-MWCNTs (C and D) and commercial Pt/C (E and F) before and after chronoamperometric tests in 0.5 M CH3OH + 0.5 M H2SO4. As revealed by these images, the morphology and dispersity of PtNPs in Pt/N-MWCNTs have no distinct change after 7200 s test. However, the PtNPs in the other two catalysts become significantly larger and show serious agglomeration after electrochemical tests, indicating that the doping of N in MWCNTs can apparently enhance the electrochemical stability of deposited PtNPs.
4. Conclusion A creative method for preparation nitrogen-doped carbon nanotubes was developed. Nitrogen-doped multi-walled carbon nanotubes 7
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