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Supramolecular Assembly Templated Nitrogen-Doped Hollow Carbon Tubes as Highly Active and Durable Catalytic Support for Methanol Electrooxidation Lei Zhao, Xu-Lei Sui, Qing-Yan Zhou, Xiao-Fei Gong, Jia-Jun Cai, Xifei Li, and Zhen-Bo Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00768 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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ACS Applied Energy Materials

Supramolecular Assembly Templated Nitrogen-Doped Hollow Carbon Tubes as Highly Active and Durable Catalytic Support for Methanol Electrooxidation Lei Zhao, 1 Xu-Lei Sui, 1 Qing-Yan Zhou, 1 Xiao-Fei Gong, 1 Jia-Jun Cai, 1 Xifei Li, 2 Zhen-Bo Wang 1, * 1

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and

Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92 West-Da Zhi Street, Harbin, 150001 China 2

Institute of Advanced Electrochemical Energy, Xi'an University of Technology, NO.5 South Jinhua Road, Xi'an 710048, China * Corresponding author. Tel.: +86-451-86417853; Fax: +86-451-86418616. Email: [email protected] (Z.B. Wang)

Abstract: A supramolecular polymerization-assisted approach is proposed for the preparation of 1D hollow nitrogen doped carbon tubes (h-NCTs) as an advanced support material to immobilize metal nanoparticles (NPs) towards methanol electrooxidation. Supramolecular assembly (melamine cyanurate) is adopted as sacrificial structure-directing template and nitrogen source, rendering the as-prepared Pt/h-NCTs catalyst 1D hollow architecture, highly accessible surface area, efficient in-situ nitrogen doping and homogeneous dispersion of ultrafine Pt NPs. Owing to these favorable features, the resultant Pt/h-NCTs catalyst delivers an outstanding catalytic activity and improved stability for efficient methanol electrooxidation reaction in comparison with commercial Pt/C reference catalyst. We believe our work may contribute to the development of 1D structural heteroatoms doped carbon-based materials for advanced energy storage and conversion. Keywords: Superamoleculer assembly; Sacrificial template; Hollow tubular structure; 1

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Pt-based electro-catalyst; Methanol oxidation reaction 1. Introduction Direct methanol fuel cells (DMFCs) have been deemed as one of the most promising energy conversion devices for electronic equipment and small electric vehicles owing to their appealing features including high energy density, low-temperature operation, ease of handling with a liquid fuel and low pollutant emission.1-5 In the past decades, prominent progress in both theory and application has been gained, however, in an attempt to realize a genuinely practical commercialization which can be mass-produced and cost-effective, significant further improvements are still needed. Their commercial success mainly hinges on two aspects: significant cut of the manufacturing cost regarding to noble metals and remarkable increase in the operation durability.6,7 Methanol electrooxidation reaction (MOR) as the vital reaction process suffers from sluggish kinetics. Pt-based materials are generally recognized as the most efficient catalysts which can decrease the activation energy and reduce the over-potential.8,9 However, large-scale applications of Pt-based catalysts are usually precluded by their scarcity and high price.10-12 Nanosized Pt particles are commonly designed to deposit on appropriate supports with high surface area to realize a high dispersion to maximum the utilization and reduce the dosage of Pt.13-15 Carbon black is most frequently utilized as catalyst support for DMFCs owing to its high conductivity, relatively high surface area, various pore structure and low cost.16,17 Nevertheless, the insufficient resistance to corrosion especially in operation environment and weak interaction between Pt and support materials could lead to detrimental effect on the activity and stability of catalysts.18,19 The enthusiasm for the development of alternative 2


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supports for fuel cell catalysts has never diminished. 1 D structured carbon nanotubes (CNTs) are considered as the appealing support due to their high surface area, good chemical and thermal stability as well as nice corrosion resistance.20,21 It is understood that low dimensional 1 D and 2 D nanostructures also possess high conductivity. However, pre-treatment is often necessary to ensure the attachment of Pt nanoparticles (NPs) uniformly distributed on CNTs due to the inert surface of CNTs.22,23 Intensive theoretical and experimental research has revealed that nitrogen species in carbon materials are conducive to the immobilization of precious metal NPs on support materials and promoting the dispersion. Moreover, Pt NPs are firmly anchored on the nitrogen-doped carbons through the interaction between active metal species and support, thus hindering the migration of noble metal NPs.24,25 In generally, nitrogen-doped CNTs are accomplished by various methods such as pyrolysis of CNTs in a nitrogen-containing atmosphere, thermal chemical vapor deposition associate with nitrogen-containing organic molecules or carbonization of nitrogen-containing tubular polymer.26-28 However, the current strategies are technically complicated, time-consuming, and environmentally hazardous. In addition, CNTs are usually the indispensable precursor and the obtained N-doped CNTs have a poor porous structure. Therefore, it is highly desired to design and fabricate N-doped carbon tubes with well-defined multi-scale pore structure by an efficient, facile and environmentally friendly method. Recently, we have designed a robust 1D hollow nitrogen doped carbon tubes (h-NCTs) through a supramolecular polymerization-assisted method for oxygen reduction reaction in KOH aqueous solution.29 h-NCTs are successfully fabricated through a combined hydrothermal carbonization and thermal treatment process with glucose as the carbon source, 3



and melamine cyanurate (MC) supramolecular assembly as the structure-guiding template and nitrogen source. This method is much simple, effective and environmental friendly. The obtained sample exhibits a hierarchically porous structure with proliferative N-doped active sites, which are able to accommodate ultrafine metal nanoparticles. However, this 1D nitrogen-doped tubular material has not yet been utilized as catalyst support for methanol electrooxidation reaction. Herein, 1D h-NCTs material and Pt-decorated h-NCTs as anode catalyst towards methanol electrooxidation were successfully prepared and studied in detail. The resulting Pt/h-NCTs composite possesses a 1D architecture, highly accessible surface area and hierarchically porous structure which is favorable for the smooth transportation of reaction-related species to electrocatalytic active sites, and most of all, uniformly dispersed Pt NPs. As a consequence, Pt/h-NCTs could substantially boost the electrocatalytic activity and stability when used as a catalyst for methanol electrooxidation. 2. Experimental Section 2.1 Preparation of MC supramolecular assembly Melamine cyanurate (MC) complex was prepared by a typical supramolecular self-assembling between melamine and cyanuric acid by multiple hydrogen bonds. An equimolar ratio of melamine and cyanuric acid were dissolved in 100 mL deionized water under strong agitation at 80 oC to initiate self-assembly. The obtained white precipitate was collected via filtration, repetitively washed, and dried at 60 °C to get the melamine cyanurate supramolecular aggregate. 2.2 Preparation of h-NCTs In a typical synthesis, 500 mg MC precursor was added into an aqueous solution of 4


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glucose (1 g glucose in 30 mL water) under sonication and stirring process. Then the resultant suspension was subjected to a 50 mL Teflon autoclave, and kept at 160 °C for 10 h in hydrothermal condition. A brown product collected through centrifugation, washed with ultrapure water, and dried overnight was labeled as MC@Glu. The final product h-NCTs were obtained by calcination of MC@Glu at 900 oC under Ar atmosphere. 2.3 Synthesis of Pt/h-NCTs Pt/h-NCTs catalyst with the Pt content of 20 wt% was fabricated through a microwave-assisted polyol process (MAPP). Briefly, 20 mg prepared h-NCTs was added into 60 mL ethylene glycol (EG) under ultrasonication for 1 h. Subsequently, the calculated amounts of H2PtCl6/EG solution was joined into the dispersion with agitation for 3 h. The pH value of the solution was adjusted to 12.0 by adding 1 mol L-1 NaOH/EG solution, followed by microwave heating treatment for dozens of seconds. Final products were obtained after washing repeatedly with ultrapure water followed by vacuum drying at 60 oC. Pt loading in Pt/h-NCTs catalyst is determined to be 18.3 wt% by inductively coupled plasma analysis, which is close to the theoretical value of 20 wt%. 2.4 Characterizations The morphologies of the samples were determined by a scanning electron microscope (SEM, Hitachi S4800), field emission transmission electron microscope (TEM) and high resolution TEM (HRTEM, FEI Tecnai G2 F20). X-ray diffraction (XRD) analysis was conducted on the D/max-RB diffractometer. X-Ray photoelectron spectroscopy (XPS) analysis was performed using a physical electronics PHI model 5700 instrument. Raman spectra of samples were collected on a Renishaw1000 Raman microscope. Pt content in the 5



catalyst is measured by an inductively coupled plasma analysis (ICP). The specific surface area of sample was explored by N2 adsorption-desorption with Brunauer-Emmett-Teller (BET) methods using a QUADRASORB SI analyzer. Electrochemical measures were performed on a CHI 650E electrochemical analysis instrument using a standard three-electrode cell at an ambient temperature, with a glassy carbon disk electrode modified with prepared catalyst, Hg/Hg2SO4 (0.68 V relative to reversible hydrogen electrode, RHE) and a platinum wire, as working, reference, and counter electrodes, respectively. In this study, all potentials are presented with respect to RHE. More detailed information concerning material synthesis, electrode preparation, physical characterization and electrochemical test is provided in the supporting information. 3 Results and discussions 3.1 Synthetic procedure for Pt/h-NCTs The employed synthetic strategy for Pt/h-NCTs hybrids applied in this study is illustrated in Figure 1. The preparation procedure of h-NCTs has been announced in our previous report. 29

Typically, melamine cyanurate (MC) supramolecular assembly is produced by mixing

together of melamine and cyanuric acid in equal mole ratio in hot deionized water under strong stirring. Then MC assembly and an amount of glucose are hydrothermally treated in a 50 mL autoclave at 160 oC for 10 h. During this process, the surface of MC assembly appears a coating of carbonized glucose (denoted as MC@Glu) through the dehydration and cross-linking processes. Subsequently, the resultant samples are vacuum dried and subjected to calcination at 900 oC for 2 h under an Ar atmosphere. Finally, Pt NPs are deposited on the surfaces of the obtained nitrogen-doped carbon tubes via an efficient microwave-assisted 6


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polyol process (MAPP) strategy, generating Pt/h-NCTs catalyst. 3.2 Physical characterization of Pt/h-NCTs The morphology and structural evolution of the as-prepared products were garnered from a scanning electron microscope (SEM) and transmission electron microscope (TEM). As presented in Figure 2b and S1. The prepared MC assembly exhibits a rod-like morphology with diameters ranging from submicrometer to micrometers and few micrometers in length. The rod-like structure is developed due to the self-assembly process induced by strong hydrogen bonds and π-π interactions.30 The successful formation of MC assembly is proved by X-ray diffraction (XRD) technology, in which the characteristic diffraction peaks in Figure 2a prove to be a good match with the standard card for MC (JCPDS file no. 00-005-0217).31 After hydrothermal process, the obtained MC@Glu presents the similar morphology to the MC assembly (Figure 2c). However, the surface of samples becomes rough indicating the MC assembly is coated with amorphous carbon layers stemmed from the carbonization of glucose generated from hydrothermal process. Finally, MC@Glu is subjected to annealing treatment in Ar, resulting in the complete decomposition of MC assembly and further carbonization of the outer glucose layers. Typical TEM and SEM images (Figure 2d and S1) further disclose that the prepared samples (h-NCTs) appear a tubular structure and similar in appearance to an inverted replica of MC assembly. This result suggests that the MC assembly plays a critical role in formation and evolution of the tubular structure by acting as a sacrificial structuredirecting template. Close inspection reveals that h-NCTs exhibit a hollow cavity structure, attesting to the total conversion of MC assembly during the annealing step. This is also confirmed by XRD data for h-NCTs (Figure S2), which contains only a C (002) diffraction 7



peak and no peaks for MC. Raman spectra in Figure S3 shows two intensive peaks at around 1350 and 1590 cm-1, which can correspond to D and G band, respectively, further demonstrating the graphitic structure of h-NCTs materials.32 High-resolution TEM (HRTEM) image of h-NCTs in Figure S4 shows that there are plenty of disordered defeats and short-range sp2 hybrid carbon structure in the carbon layers of h-NCTs. An amorphous and disordered carbon structure could expose more active sites for nitrogen doping, which is conducive to the immobilization of Pt precursors and to Pt nucleation. Meanwhile, the thermal decomposition of MC assembly would release large quantity of nitrogen containing gas for the subsequent in-situ nitrogen doping, leading to the formation of h-NCTs materials. It should be noticed that our synthetic strategy is apparently different and in fact superior to the previous ones. In the previous works, ionic liquid or polymers was applied as nitrogen source for N doping, leading to the formation of carbon-based composites caused by the nitrogen-containing carbon residues on the surface.33,34 On clear contrast, supramolecular MC assembly as a sacrificial structure-directing template and nitrogen source could completely decompose and produce a pure N doped carbon tube.29,35 Scanning transmission electron microscopy (STEM, Figure 3) with the corresponding elemental mapping images reveals that h-NCTs are composed of N, C, and O elements, which are homogenously distributed in carbon tubes. N2 physisorption plot in Figure S5a indicate that h-NCTs possess a type IV isotherm, revealing a domination of hierarchical porous structure of micropores and mesopores. This is further revealed by the pore size distribution curve (Figure S5b), in which the pore appears in a range of 0.7-22.0 nm in size. The hierarchical porous structure gives h-NCTs sample a high surface area. The surface area of 849.0 m2 g-1 is garnered for h-NCTs, 8


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calculating from the Brunauer–Emmett–Teller (BET) method, which is much higher than that of common used XC-72 carbon black (200.0 m2 g-1).36 Therefore, it is reasonable to deduce that h-NCTs can offer more exposed active sites for the capture of Pt precursors and for Pt nucleation. Pt NPs were grown on the surfaces of h-NCTs materials via the MAPP. In comparison with the h-NCTs, the tubular structure of Pt/h-NCTs is maintained after the loading of Pt NPs (Figure 4a), implying the good mechanical stability of Pt/h-NCTs. Close inspection suggests that ultrafine Pt NPs are uniformly distributed on the surfaces of the individual carbon tubes (Figures 4b-d). The average size of the Pt NPs is ~ 2 nm, and there is no obvious agglomeration happening. On clear contrast, as displayed in Figure S6, Pt NPs anchor on the pristine carbon black show a larger particle diameter and tend to form aggregates in this case, due to its relatively weak interactions. In this context, our prepared h-NCTs are more suitable to be applied as support materials for immobilization and dispersion of Pt NPs. This could be caused by the existence of proliferative nitrogen-doping induced active sites in h-NCTs, which could efficiently capture and anchor metal particles. Theoretical and experimental calculations have confirmed N-doped carbon materials are favorable for the immobilization and dispersion of Pt NPs than their undoped counterparts, which can sustainably reinforce the interaction between metal and support.37,38 A high-resolution transmission electron microscopy image clearly reveals a crystal plane distance of 0.227 nm, indexed to the (111) planes of a face-centered cubic (fcc) crystalline Pt (Figures 4f and S7).39 To further interrogate the elemental composition of the Pt/h-NCTs hybrids, X-ray photoelectron spectroscopy (XPS) technique and elemental analysis were conducted. As 9



shown in Figure 5a, C, N, O, Pt elements simultaneously emerge in the hybrids. The high-resolution N1s spectra can be fitted into mainly four peaks, located at 398.3, 399.8, 401.2 and 402.6 eV, which sequentially belong to pyridinic, pyrrolic, graphitic, and oxidized N, respectively (Figure 5b).40 Meanwhile, high-resolution C 1s spectra in Figure S8 reveal that a C-N bond appears at 285.6 eV, further suggesting the successful incorporation of N atoms in carbon matrix.41 As evidenced in the literature, doped N species could cause strong interactions with platinum. In addition, from Figure 5c, there are two states of Pt in Pt/h-NCTs catalyst: the double binding energies at 71.5 and 74.9 eV assign to metallic Pt, while the other couple is bivalent PtO with the two binding energies at 72.8 and 76.2 eV.42 X-ray diffraction (XRD) pattern also confirms the face-centered cubic (fcc) crystalline structure of Pt in Pt/h-NCTs hybrids (Figure 5d). Whereas the intensities of the diffraction peaks are quite weak, this could arise from the ultrafine Pt particle size and is consistent with the HRTEM investigation. 3.3 Electrocatalytic activity for MOR DMFC with the liquid methanol as anode fuel has been regarded to be an appealing power source, in which methanol electrooxidation reaction (MOR) is the essential process. Pt/h-NCTs catalyst was tested to evaluate their MOR performance in acid medium. Before assessing the MOR activity, Pt/h-NCTs hybrids are examined by means of cyclic voltammetry in 0.5 mol L-1 H2SO4 solutions. To make a comparison, commercial Pt/C is also measured under the same condition as a benchmark. Remarkably, Figure 6a illustrates the representative CV curves for Pt-based catalysts, in which hydrogen adsorption/desorption peaks, Pt oxidation/reduction regions and non-featured double-layer capacitance regions in between can 10


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be observed. The obvious larger capacitive current background in Pt/h-NCTs than that in commercial Pt/C stems from the higher specific surface area of h-NCTs support.43 Integrating the coulombic charge for hydrogen desorption, the electrochemical active specific surface area (ECSA) of catalyst can be acquired. As calculated, Pt/h-NCTs gives an ECSA of 73.6 m2 g-1, greatly higher than 54.4 m2 g-1 for commercial Pt/C. The high ECSA implies Pt/h-NCTs can provide more accessible active sites for electrocatalytic reaction. MOR activities are garnered from CV curves in a solution containing 0.5 mol L-1 H2SO4 and 0.5 mol L-1 CH3OH. Insights into MOR activity can be acquired by calculating the forward current density in CV curve. As displayed in Figure 6b, Pt/h-NCTs achieves a forward current density of 505.0 mA mg-1, which is 1.4 times higher than that of 360.2 mA mg-1 reached by commercial Pt/C. This agrees well with the ECSA result as elaborated above. It should be noted that the MOR activity of Pt/h-NCTs is considered good among the previously reported catalysts with nitrogen doped carbon tubes as support materials (Table S1). Electrochemical impedance spectroscopy (EIS) is a powerful measurement to assess the charge transport property of electrochemical reaction, thus revealing the kinetic characteristic and activity for methanol electrooxidation.44 Figure 6c discloses the Nyquist plots of methanol electrooxidation on Pt/h-NCTs and commercial Pt/C electrodes in acidic methanol medium. Obviously, the charge transfer resistance for Pt/h-NCTs is lower than that for commercial Pt/C, implying Pt/h-NCTs possesses a faster charge-transfer capacity during MOR.45 Generally speaking, our prepared Pt/h-NCTs catalyst delivers a superior MOR activity compared with commercial Pt/C reference. Such a phenomenon can be attributed to the profound synergistic impact between Pt NPs and doped N atoms. In this regard, N-doping is beneficial to the homogeneous 11



distribution of Pt NPs, meanwhile, the doped N atoms would polarize plenty of neighboring carbon atoms and facilitate the generation of OHads by significantly expediting water dissociation, thus giving rise to the acceleration of oxidative removal of carbonaceous poisoning products .46,47 3.4 Electrocatalytic durability for MOR Besides the MOR activity, the poor durability is another issue plaguing the development of DMFC electrocatalysts. The stability of the as-prepared materials is first screened by chronoamperometric measurement at a constant potential of 0.65 V for 3600 s (Figure 6d). The response current densities of the two samples decrease promptly at the initial stage, probably owing to the poison of carbonaceous species generated during the MOR process.48 Clearly, Pt/h-NCTs displays the higher oxidation current in comparison with commercial Pt/C throughout the entire testing time. The current density on Pt/h-NCTs electrode after 1h operation retains 20.6 mA mg-1, which is 3.2 times higher than that on commercial Pt/C. This result is also borne out by the long-term stability deduced from the continuous CV tests in methanol acidic solution, as presented in Figure 7 and S9. It can be obtained that the peak current densities decline gradually along with the consecutive scan. Pt/h-NCTs losts only 25% of its initial forward peak current density after 1000 CV cycles, whereas commercial Pt/C suffers 40% loss under the same condition. Additionally, it is worth noting that the peak current density on Pt/h-NCTs is significantly higher compared with that on commercial Pt/C during the CV operation, revealing the outstanding long-term durability of Pt/h-NCTs. The structural evolution of the samples during the stability test can be reflected by TEM investigation. TEM images with Pt NPs size distributions of Pt/h-NCTs and commercial Pt/C, 12


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which experienced 1000 cycling tests, are shown in Figure 8. Both the two catalysts show the growth of Pt NPs after 1000 cycles, which is in consistent with their reduced current densities. Based on TEM analysis, the Pt NPs sizes on Pt/h-NCTs and commercial Pt/C grow to 2.74 and 3.61 nm, increasing by 38.4% and 47.8%, relative to those before stability test, respectively. In addition, Pt NPs on Pt/h-NCTs still remain highly dispersive and free of agglomeration after the potential cycling, in contrast, dramatic agglomeration of Pt NPs happens to commercial Pt/C catalyst. These findings prove that Pt/h-NCTs catalyst is quite stable during the durability operation process, explaining well the good stability of Pt/h-NCTs. Based on the above findings, it is reasonable to infer that h-NCTs are significantly favorable for its utility as an electrocatalyst support for MOR derived from their unique properties. First, h-NCTs with 1D architecture, highly accessible surface area and hierarchically porous structure can offer a great many accessible active sites and enable the smooth transportation of reaction-related species to electrocatalytic active center.49 Second, the incorporation of N atoms within the carbon matrix is beneficial for the reinforcement of the Pt NPs, through strengthening the interaction between metal and support, guaranteeing the highly dispersive state of Pt NPs.38,50 Third, as-prepared 1D structural catalyst could cross link together to form a 3D hybrid architecture, which facilitate rapid electron transport along the 1D pathways.51,52 Finally, the ultrafine Pt NPs with an even size distribution on the 1D h-NCTs support can promote the utilization of Pt and maximize the numbers of accessible catalytic active sites. Integrating these merits above, the newly designed Pt/h-NCTs with exceptional catalytic activity and stability hold great potential for the development of 13



DMFCs. 4 Conclusions In conclusion, we have brought forward a supramolecular polymerization-assisted method to 1D hollow nitrogen doped carbon tubes as high-efficient electrocatalyst support for methanol electrooxidation. Melamine cyanurate supramolecular assembly servers as both structure directing template and nitrogen source for in-situ N-doping. By means of the excellent structural superiorities including unique 1D tubular architecture, highly accessible surface area, sufficient nitrogen-doping and homogeneous distribution of ultrafine Pt NPs, the obtained Pt/h-NCTs delivers an excellent electrocatalytic performance in terms of good catalytic activity and high stability when employed as an anode catalyst toward methanol electrooxidation. It is believed that such a valid synthetic protocol has open a novel avenue for the more-general synthesis of 1D heteroatoms doped carbon-based architectures for catalysis, environment and energy resource areas. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Certain reagents, physical and electrochemical characterizations. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] Notes The authors declare no competing financial interest. 14


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Acknowledgment This research is financially supported by the National Natural Science Foundation of China (Grant No. 21273058 and 21673064), China postdoctoral science foundation (Grant No. 2018M631938 and 2017M621284), Research Fund of State Key Laboratory for Marine Corrosion and Protection of Luoyang Ship Material Research Institute (No. KF160410) and HIT Environment and Ecology Innovation Special Funds (No. HSCJ201620). References 1. Debe, M. K., Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43-51. 2. Zhang, K.; Yang, W.; Ma, C.; Wang, Y.; Sun, C.; Chen, Y.; Duchesne, P.; Zhou, J.; Wang, J.; Hu, Y.; Banis, M. N.; Zhang, P.; Li, F.; Li, J.; Chen, L., A Highly Active, Stable and Synergistic Pt Nanoparticles/Mo2C Nanotube Catalyst for Methanol Electro-Oxidation. NPG Asia Mater. 2015, 7, e153. 3. Yang, P.; Yuan, X.; Hu, H.; Liu, Y.; Zheng, H.; Yang, D.; Chen, L.; Cao, M.; Xu, Y.; Min, Y.; Li, Y.; Zhang, Q., Solvothermal Synthesis of Alloyed PtNi Colloidal Nanocrystal Clusters (CNCs) with Enhanced Catalytic Activity for Methanol Oxidation. Adv Funct Mater 2018, 28, 1704774. 4. Wang, J.; Zhang, X.-B.; Wang, Z.-L.; Wang, L.-M.; Xing, W.; Liu, X., One-Step and Rapid Synthesis of "Clean" and Monodisperse Dendritic Pt Nanoparticles and Their High Performance toward Methanol Oxidation and P-Nitrophenol Reduction. Nanoscale 2012, 4, 1549-1552. 15



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a Melamine

Self-assembly

Cyanuric acid Melamine cyanurate

b

Figure 1 Self-assembling of melamine and cyanuric acid leads to the formation of MC assembly (a). Schematic illustration of the synthetic procedure of Pt/h-NCTs catalyst (b). It includes 1) preparation of MC assembly by self-assembling between cyanuric acid and melamine, 2) MC@Glu was synthesized through the hydrothermal treatment of glucose in the presence of a MC template, 3) formation of 1D h-NCTs architectures by complete decomposition of MC assembly during calcination process, and 4) controllable growth of Pt NPs on the surfaces of h-NCTs via MAPP.

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b

a

1 µm c

d

1 µm

1 µm

Figure 2 XRD pattern of MC assembly (a); TEM images of MC assembly (b), MC@Glu (c) and h-NCTs (d).

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a

b

1 µm

c

C map

d

O map

N map

Figure 3 STEM image (a) of h-NCTs with EDS elemental mapping for C (b), N (c), and O (d).

a

d

b

c

f

e

Pt Pt Pt

0.227 nm Pt

Figure 4 TEM images of Pt/h-NCTs at different magnifications (a-d); the size histogram of particle diameters (e) and HRTEM image of Pt/h-NCTs (f).

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a

b

c

d

Figure 5 XPS survey (a); high resolution N 1s (b) and Pt 4f (c) XPS spectrum of Pt/h-NCTs; XRD pattern of Pt/h-NCTs (d).

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a

b

c

d

Figure 6 CV curves of Pt/h-NCTs and commercial Pt/C catalysts in 0.5 mol L−1 H2SO4 (a) and in a solution of 0.5 mol L−1 H2SO4 containing 0.5 mol L−1 CH3OH (b). Scanning rate: 50 mV s−1; test temperature: 25 oC. Nyquist plots of EIS for Pt/h-NCTs and commercial Pt/C catalysts in a solution of 0.5 mol L−1 H2SO4 containing 0.5 mol L−1 CH3OH solution (c). Chronoamperometric curves for Pt/h-NCTs and commercial Pt/C catalysts in a solution of 0.5 mol L−1 H2SO4 containing 0.5 mol L−1 CH3OH at a ﬁxed potential of 0.6 V vs. RHE (d).

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a

b

c

d

Figure 7 CV of commercial Pt/C (a) and Pt/h-NCTs (b) in a solution of 0.5 mol L−1 H2SO4 containing 0.5 mol L−1 CH3OH before and after stability test. Scanning rate: 50 mV s−1; test temperature: 25 oC. Mass activities of Pt/h-NCTs and commercial Pt/C catalysts with cycle numbers during stability test (c). The normalization of initial forward peak current density of Pt/h-NCTs and commercial Pt/C catalysts with cycle numbers during the stability test (d).

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a

e

c

b

g

f

d

h

Figure 8 TEM images with associated Pt NPs size distributions of commercial Pt/C (a-d) and Pt/h-NCTs (e-h) catalyst before and after stability test.

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Supramolecular Assembly Templated Nitrogen ... - ACS Publications

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