Facile Synthesis of Cobalt Nanoparticles Entirely Encapsulated in Slim

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Facile synthesis of cobalt nanoparticles entirely encapsulated in slim nitrogen-doped carbon nanotubes as oxygen reduction catalyst Ailing Song, Wang Yang, Wu Yang, Gang Sun, Xucai Yin, Lijun Gao, Yazhou Wang, Xiujuan Qin, and Guangjie Shao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b03173 • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017

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ACS Sustainable Chemistry & Engineering

Facile synthesis of cobalt nanoparticles entirely encapsulated in slim nitrogen-doped carbon nanotubes as oxygen reduction catalyst Ailing Song,†,‡ Wang Yang,† Wu Yang,† Gang Sun,† Xucai Yin,† Lijun Gao,† Yazhou Wang,† Xiujuan Qin,*,†,‡ Guangjie Shao∗,†,‡ †State key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China ‡Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China

ABSTRACT: Owing to the scarcity of platinum resources and its visible flaw with poor durability and severe anode methanol crossover, Pt-based catalysts for catalyzing ORR has been experiencing some problems lately. Hence, non-precious metal catalysts which are expected to replace platinum-based catalysts have become popular green sustainable development topics. We chose a facile pyrolysis method for the synthesis of this material, making the resulting coating material present a state of effectively encapsulating all cobalt nanoparticles into slim nitrogen∗

Corresponding author. Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao, 066004, China. E-mail address: [email protected] (X.Qin), [email protected] (G. Shao).

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doped carbon nanotubes after process optimization, the cobalt content even reached 20.9 wt. %. In the catalytic aspect, the optimized Co@NSCNTs shows a half wave potential equivalent to Pt/C in alkaline medium. Then, better durability (after 60,000s chronoamperometry, the current retention still remains 95%), commendable resistance to methanol crossover performed than literature confirmed. All these characteristics superior to Pt/C should be ascribed to large amounts of entire encapsulated structure and synergism between encapsulated Co nanocrystal and graphitic N-doped carbon nanotubes. The presence of abundant metal particles allows more surface carbon and nitrogen atoms to be activated, in favor of the adsorption and dissociation of O2, then improving oxygen reduction reaction (ORR) catalysis.

KEYWORDS: Non-precious metal catalyst, Encapsulated structure, Oxygen reduction reaction, Durability INTRODUCTION By the wind of new energy, fuel cells nova to stay up. Car fuel cell technology development, as well as underwater unmanned vehicle being developed, these all herald a huge space for the development of fuel cells,1 which also indicates that the industrialization of fuel cells just around the corner. But this still faces a number of practical problems.2-4 Like the slow kinetics of the


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cathodic oxygen reduction reaction.5 At present, the most used ORR catalysts are still dominated by precious metal Pt-based catalysts.6 The more usage of cathode Pt-based catalysts raises the cost, which is one of the key restricted factors for fuel cells, although in recent years the membrane electrode of platinum load from 10 mg cm-2 to 0.02 mg cm-2. At the same time, another two issues, poor stability and severe anode crossover are becoming more and more conspicuous.7 Therefore, looking for the substitutes of precious metal catalysts is more important and urgent. Over the past few decades, in order to solve the above obstacles, a series of metal-free and carbon-based non-precious metal electrocatalysts got in-depth research and development due to their low-cost and high-activity for oxygen reduction reaction.1, 8-9 It is particularly gratifying that in the course of the development of these materials, catalyst material with good stability and better tolerance for methanol crossover was synthesized.10 That was cladding-structure materials whose above characteristics are ascribed to their unique cladding structure by firmly graphitized carbon layer, protecting inner particles from being eroded.11-13 It thence appears that transition metal nanoparticles encapsulated in heteroatom-doped carbon nanotubes demonstrated tremendous potential as a catalyst for ORR.14-15 Years ago, there was reported using sodium azide and ferrocene successfully synthesize iron nanoparticles encapsulated within pod-like carbon nanotubes with superior catalytic activities and excellent stability after 220h accelerated durability testing (ADT) for catalyzing ORR.16 Furthermore, they also made Density Functional Theory calculations (DFT) to demonstrate the interaction between metal particles and carbon nanotubes will reduce the local work function of the surface of carbon nanotubes and improve the chemical activity of functional areas. The only problem is that stringent experimental conditions make the industrial production of the former catalyst more difficult to achieve due to


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the high temperature and high pressure demand, and the security still need to be considered. Subsequently, transition-metal-catalyzed in-situ catalysis was used to synthesize transition metalcoated catalysts, and dicyandiamide, ferric chloride and carbon black were used as precursors to synthesize N-doped carbon nanotube coated iron carbide catalyst,17 although the prepared catalyst has better performance under acidic conditions，but its structure is complicated, in addition to the carbon nanotube coating structure, also the adhesion of carbon layer, so the catalytic mechanism is difficult to figure out. Then, bamboo-like carbon nanotube/Fe3C nanoparticle hybrids as ORR catalyst,18 cobalt encapsulated in carbonitride nanotubes as HER catalyst ,also Fenton-like catalyst,19-20 etc. this material is widely used in the field of catalysis. But this material in the synthesis of simplification, the structure of the controllability and catalytic performance is still facing challenges. Therefore, we used a facile pyrolysis method to carry out cladding-structure catalyst with excellent microstructure and outstanding catalytic performance. Here, melamine and D-(+)Glucosamine hydrochloride worked both as nitrogen source and carbon source, cobalt nitrate not only acted as a cobalt source, but also as a template to catalyze the formation of graphitized nitrogen-doped carbon nanotubes. When the temperature is maintained at 600℃, the nitrogen and carbon source experienced intermolecular polymerization, nitrogen doped into carbon skeleton, forming nitrogen-doped carbon layer. Meanwhile, the cobalt nitrates decomposed to cobalt-containing particles, and dispersed to form nanoparticles between carbon layers, both the cobalt-containing particles and the nitrogen-doped carbon layer are disordered. Then, transient temperature maintained at 700 ℃ was in order to firmly secure the formation of lamellar structure, to get the nitrogen content and the encapsulated cobalt amount double high encapsulated structure catalyst (Figure S1. and Figure S2. show the TEM and XRD images at


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700 ℃ ). Similar to the floating catalyst chemical vapor deposition (CCVD) method,21-22 transition metal catalyzes the decomposition of the carbon-containing gas or the organic liquid, or the direct thermal decomposition of the transition metal carbide to prepare the carbon nanotube, where we use cobalt nitrate as a metal source catalyzed melamine into the tube, we also add another appropriately quantified nitrogenous carbon source, D-(+)-Glucosamine hydrochloride, not only increase the nitrogen content, but also make the tube more orderly, and slender. This process and method make more metal nanoparticles encapsulated in carbon nanotubes, so that more of the coated carbon layer in contact with cobalt is activated, making the active sites more effective in the process of catalysis. The half wave potential of this catalyst reached the level of commercial Pt/C, corresponding to the above statement. Furthermore, the synergistic effect of cobalt and nitrogen-doped graphitized carbon of the catalyst compared to conventional nitrogen-doping carbon materials results in more carbon atoms being activated and generating more active sites.23 All these herald the catalyst we synthesized on the catalytic of oxygen reduction road to a bright future. EXPERIMENTAL SECTION Preparation of Co@NSCNTs After in-depth exploration, we have selected an optimal experimental program. In existing melamine and metal nitrates, glucosamine salts were joined, grinding the mixture and calcining in the tube furnace, got mixture catalyst after acid treatment. Specific process is as follows. 2.0 g melamine (Kemi Chemical Reagent Co., Ltd), 0.1 g Co(NO3)2·6H2O (Tianjin Damao Chemical Reagent Factory) and 0.05 g D-(+)-Glucosamine hydrochloride (Sinopharm Chemical Reagent Co., Ltd) were mixed by grinding. The well-proportioned hybrids was then put into the tube


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furnace and heated at 600℃ for one hour by flowing N2 at 100 mL min−1. Then, 10 minutes maintained at 700℃ was in order to firmly secure the formation of lamellar structure, to get the nitrogen content and the encapsulated amount double high cladding structure catalyst. After that, the temperature of the tube furnace was further increased to 800℃ for 1 h. All temperature rose at a rate of 5 ℃ min−1. Subsequently, the samples were cooled to room temperature under N2 ambient, the product were collected from the crucible. Finally, in order to remove the maintained metallic Co species on the outer wall of carbon nanotubes, the annealed products were ultrasonically soaked into 0.5 M H2SO4 for 48 h, then filtered and washed with distilled water for several times, dried at 80℃ overnight. The final product was denoted as Co@NSCNTs for cobalt nanopartiales encapsulated into slim nitrogen-doped carbon nanotubes. Acid treatment process also took some oxygen-containing functional groups on carbon layer, adding material hydrophilicity. Preparation of Co@NSCNTs-B Took appropriate amount of Co@NSCNTs, ball-milled with planetary ball-milling instrument for 4 hours to destroy the protective carbon shells around the metal nanoparticles. Then, collected the product, filtered and washed with distilled water for several times, dried at 80℃ overnight, as Co@NSCNT-B. Preparation of Co@NSCNTs-L Took appropriate amount of Co@NSCNTs, removed the inner cobalt particles after hot acid reflux. Then, collected the product, filtered and washed with distilled water for several times, dried at 80℃ overnight, as Co@NSCNT-L. 20% commercial Pt/C was also selected as a catalyst for comparative experiments. Characterization of materials


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Field-emission scanning electron microscopy (FE-SEM, Hitachi Modle S-4800, KV), the elements on the surface of sample were identified by energy dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM, JEM2010), and High magnification transmission electron microscope （ HRTEM, H-7500 ） were employed to observe the morphology and microstructure of the materials. Selected Area Electron Diffraction (SAED) was further used to identify the crystal plane of Co nanoparticles of the as-prepared Co@NSCNTs sample. The crystal types of the sample were characterized by X-ray diffraction (XRD) on a Rigakud/MAX-2500/pc X-ray diffractometer operated at 40 kV using Cu Karadiation at a scan rate of 2° min-1. Raman spectra were recorded by a Horiva (LabRam HR-800) spectrometer (532 nm, 50 mW excitation laser). The content of cobalt encapsulated into the carbon layer after acid treatment was analyzed by themogravimetric analyzer (TGA, Pyris Diamond, PerkinElmer Thermal Analysis) under atmosphere at a heating rate of 10 ℃ min-1 from ambient temperature to 900℃. The content of cobalt was further analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Thermo Scientific iCAP6300, U.S.A). Fourier transform infrared spectroscopy (FT-IR) was used to analyze the surface functional groups using a Nicolet IS Fourier transformation infrared spectrometer with the wave number range of 400-4000 cm-1. Elemental quantitative analysis was gained using X-ray photoelectron spectroscopy (XPS) with a Kratos XSAM-800 spectrometer. Electrochemical measurements All electrochemical measurements were conducted using PARSTAT4000 Princeton Applied Research. 2 mg of catalyst and 20 µL of Nafion solution (DuPont, 5 wt. %) were dispersed in 800 µL of ethanol by sonication for at least 30 min at 150 W to form a homogeneous ink. Then 16 µL of the ink (containing 0.04 mg of catalyst) was loaded on a glassy carbon (GC) rotating


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disk electrode of 5 mm in diameter (PINE Instruments), giving a loading of 200 µg cm-2. The same way, 8 µL of Pt/C ink was slowly dropped onto the disk electrode (Pt loading, 20 µg cm-2). Then, a typical three-electrode system was used in all the following electrochemical tests, Ag/AgCl (saturated KCl) as the reference electrode (calibration before use), platinum wire as the counter electrode and the sample modified glassy carbon electrode as the working electrode. Electrolyte (0.1 M KOH) was saturated with oxygen by bubbling O2 prior to the measurements system at least one hour. A flow of O2 was maintained over the electrolyte at room temperature during the recording of CVs. The working electrode was cycled 30 times in order to reach the steady-state of the system before data were recorded at a scan rate of 50 mV s–1. In control experiments, CV measurements were performed in an N2-saturated electrolyte. For the rotating disk electrode (RDE) measurements, the working electrode was scanned cathodically at a rate of 5 mV s-1 with varying rotating speed from 400 rpm to 2,000 rpm. Koutecky–Levich plots (J-1 vs ω-1/2) were analyzed at various electrode potentials. The slopes of the linear fit lines were used to calculate the number of electrons transferred (n) on the basis of the Koutecky–Levich equation. The detailed calculation process showed in the supporting information. Current–time (I–T) measurement of Co@NSCNTs and Pt/C was obtained at -0.3 V (vs. Ag/AgCl) for 60,000 s and 40,000 s respectively in 0.1 M KOH solution with continuous oxygen bubbling. All work results were presented against the reversible hydrogen electrode (RHE). The conversion between potentials versus Ag/AgCl and versus RHE is performed using the equations: E (vsRHE) = E (vsAg/AgCl) + EAg/AgCl (reference) + 0.0591 V × pH

(1)

(EAg/AgCl (reference) = 0.197 V vs NHE at 25℃)


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In this work, we used 0.1 M KOH, then pH=13, subsequently, the formula result is E (vsRHE) = E (vsAg/AgCl) +0.9653

(2)

RESULTS AND DISCUSSION Material Characterization Almost immediately, we proceed from the characterization process of the sample to verify our inference of the mechanism of the structure formation. From Figure 1a and b, the Scanning Electron Microscopy (SEM) characterization of Co@NSCNTs, we can clearly see that the samples have uniform distribution and morphology, and the cobalt particles are encapsulated in the nanotubes, the surface of the nanotubes is folded. Figure 1b arrow refers to the nanotubes as a hollow structure. As illustrated in Figure 1c and d, the morphology and microstructures of the obtained Co@NSCNTs catalyst was further characterized by Transmission Electron Microscope (TEM) and high-resolution transmission electron microscopy (HRTEM). Figure 1a clearly shows the structure of Co nanoparticles integrally encapsulated in slim carbon nanotubes. Fold part of the material as like graphitic layer composite structures makes the catalyst prepared combined the characteristics of one-dimensional CNTs with two-dimensional graphene, endowing the material with a larger surface area, good electrical conductivity, more exposed active sites and better chemical stability, further promoting the transfer of electrons in oxygen reduction process and improving the catalytic properties of the materials. Furthermore, no cobalt nanoparticles observed in the range of the electron microscope was unencapsulated by the carbon nanotubes after acid treatment. Low magnification transmission diagram can be seen in Figure S3, the formation of the tube is very orderly and slender. According to the morphology of the nanotubes, firstly, under low temperature condition, N atoms from the decomposition of melamine and D(+)-Glucosamine hydrochloride were directly introduced into the carbon framework to form


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nitrogen-doped carbon layers. The cobalt-containing nanoparticles dispersed in the C-N layer (Figure S1), like top growth mechanism of CCVD, but a string of metal particles (Figure S3a) catalysed the formation of the same carbon nanotube. During this process, under the catalyzing of the cobalt-containing particles, the amorphous CN layer through the metal particles became orderly and the degree of graphitization was improved, which was advantageous to electron transfer. Finally, the particles when the end of the tube is catalyzed are encapsulated in the port position to complete the tube forming process. The degree of the graphitization of the carbon nanotubes was further improved during the temperature maintaining process (calcination at 800 ℃ for 1 h). During the whole process, the cobalt-containing nanoparticles reduced to metal cobalt particles with cubic structure by carbon layers. Almost immediately, the HRTEM images in Figure 1b, further confirm the microstructure of the Co@NSCNTs, where the lattice distance of 0.205 Å is corresponded to the (111) crystal plane of the metallic cobalt with cubic structure (lattice parameters, a=b=c). Also, graphitic wall of carbon nanotubes range from about 5 to 8 layers with interlamination distance of 3.40 Å, which is coincide with the (002) crystal plane of graphitic carbon. Meanwhile, the crystallinity of graphite layer is not perfect from the HRTEM image, which may be ascribed to the local stress induced by the introduction of N dopant with together synergetic catalytic particles of cobalt. The diffraction pattern of cobalt nanoparticles of the Co@NSCNTs along with the diffraction of Co (111) (200) (220) and (311) crystal plane shows a polycrystalline pattern (inset Figure 1b). The morphology and structure of Co@NSCNTs-B and Co@NSCNTs-L are also characterized by SEM and TEM (Figure S5 & S6). From Figure S5a, we can see the signs of sample reunion after ball-milling, then Figure S5b clearly shows the length of nanotubes become shorter and the well marked fracture of the nanotubes due to ball-milling. Fracture of the nanotubes destroyed the characteristics of one-


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dimensional CNTs, there were decline in the electrically conductive, and also catalytic ability of the materials. Figure S6a is Co@NSCNTs-L, the tubular structure was well maintained after acid treatment, EDS of SEM did not show the presence of cobalt, proved that cobalt was basically removed after the treatment of hot acid. Subsequently, we conducted a series of accurate element content analysis by other characterization methods as follows.

Figure 1. a) and b) SEM image of Co@NSCNTs. c) TEM image of Co@NSCNTs and HRTEM image d) with inset corresponding diffraction pattern of cobalt nanoparticle. Then, from Figure 2a, we can further corroborate the presence of crystalline cobaltand graphic carbon by powder X-ray diffraction (XRD). An apparent diffraction peak at 26.1° was attributed to (002) plane of graphitic-carbon lattice, and a weak reflection at 42.1° (Not marked in figure)


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ascribed to (100) reflection of graphic carbon was further to prove this. The diffraction peaks at 44.2°, 51.5° and 75.9° could be assigned to (111), (200) and (220) planes of cubic Co nanocrystal (PDF#15-0806) respectively, corresponding with the diffraction pattern of cobalt nanoparticle. Then, Raman spectroscopy accurately shows the degree of graphitization of the catalyst we synthesized. Figure 2b records two well-defined peaks at about 1350 and 1580 cm−1, tending to typical D- and G-bands of carbon, respectively. While the value of ID/IG is 0.91, an appropriate value to ensure a certain degree of graphitization and quantity of the active sites, means the surface order and defects in a relatively balanced appropriate state, which proved the annealing temperature as well as the amount of nitrogen that we select is appropriate. In addition to this, another peak appears at 2681 cm−1 (2D-band), the secondary evidence to prove the graphitization degree at a relatively good level. Pertinent degree of graphitization is expected to benefiting oxygen reduction, accelerating the electron transfer rate. Moreover, the content of Co element in the Co@NSCNTs was ascertained by thermogravimetric analysis (TGA) (Figure 2c, recorded under atmosphere), there were 33.87 wt. % of weight remained at above 900℃, which could be attributable to the formation of Co3O4 during heating, corresponding to 20.9 wt. % of cobalt content. In order to determine the content of Co in Co@NSCNTs, we also analyzed with inductively coupled plasmamass spectrometry (ICP), the result was 200589 mg kg-1, meaning 20.0 wt. %. The result was consistent with the TGA test result. This amount of coating compared with reported literature is relatively high.19-20, 24 One possible explanation is the addition of D(+)-Glucosamine hydrochloride plays a fixed role of cobalt particles. Encapsulated with high content of cobalt particles will enhance the synergies with doped nitrogen, and provide more active sites for ORR catalysis process. Afterwards, an FT-IR spectroscopy test is also done to make a cursory view of chemical bonding situations of the Co@NSCNTs in Figure 2d.


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Figure 2. a) XRD pattern of Co@NSCNTs. b) Raman and c) TGA thermograms in air, d) FT-IR spectroscopy of the Co@NSCNTs.


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Figure 3. a) XPS survey spectra, and high-resolution scans of b) N 1s c) C 1s and d) Co 2p of Co@NSCNTs. On the basis of the above characterizations, the chemical elements state of Co@NSCNTs was further analyzed by XPS. From Figure 3a, four elements were detected as C, N, O and Co. The content for each element after calculated on the basis of XPS signal is 87.51, 2.02, 10.08 and 0.39 at. %. Apparently, the metallic Co signal is very weak, the content is just 0.39 at. %. Since XPS is the study of surface characteristics of Materials Characterization methods, such a low content of Co nanoparticles on the surface of carbon nanotubes inferred that most even all metallic Co nanoparticles were encapsulated in the carbon nanotubes. In other words, no other metal compounds outside the CNTs contribute to ORR catalysis except for our synthesized


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structure. The results are in agreement with those observed by the transmission electron microscope. In addition, in order to investigate the bonding state of nitrogen, we did a more detailed analysis of N 1s signal, as shown in Figure 3b, the content of pyridinic-N, pyrrolic-N and graphitic-N are 26.3, 32.5 and 26.6 at. % (398.5eV for pyridinic-N, 400.7eV for pyrrolic-N, and 401.5eV for graphitic-N respectively, occupying more than 80% of the nitrogen content, this will greatly improve the catalytic activity for oxygen reduction, because these three types of nitrogen is considered to be the most dominant for ORR. In the article of Wei,25 it was also proved that planar-N in sp2 hybridization, i.e. pyridine-N and pyrrole-N play a dominant role in the catalytic oxygen reduction, and are more conducive to the transfer of electrons than the threedimensional sp3 hybridized quaternary-N. The content in Co@NSCNTs is 58.8%, even more than the planar-N content of total nitrogen in the material prepared by Wei. From any point of view, the nitrogen incorporated in the Co@NSCNTs catalyst is in a more favorable state for catalysis. In addition, according to the literature, nitrogen functional groups, especially the introduction of pyrrole nitrogen is the reason why the fold presents in the synthetic carbon tube21, 26

. Besides, the deconvolution of C1s spectrum displayed five types of C species (Figure 3c),

among them 59% C=C and 12% C-N occupied the main bonding way of carbon atoms to prove that in addition to graphitized carbon, carbon atoms mainly in conjunction with doped nitrogen atoms for the catalytic active sites. Of course there was also some combination of C-O bonds, and into some of the oxygen-containing functional groups, enhancing the hydrophilicity of the catalyst materials. The Co 2p3/2 and 2p1/2 high-resolution spectra (Figure 3d) were also briefly analyzed. The peaks were found at 778.0 eV and around 793.3 eV corresponding to Co 2p3/2 binding energy and Co 2p1/2 states, respectively, which are consistent with metallic cobalt. The presence of two peaks at 780.3 and 795.3 eV hint the reciprocal action between cobalt


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nanoparticles and N-doped CNTs, which accordingly leads to a lower electron density at the metallic Co sites.19 All of the above characterization tools have done concrete analysis about morphology and specific elements of materials, from these results, the catalysis material we compounded by one-step synthesis are expected to contribute to the preparation of catalytic oxygen fuel cell cathode of the restore process. Material Electrocatalysis Figure 4a firstly shows cyclic voltammetry (CV) technique text saturated with N2 or O2 in 0.1 M KOH aqueous solution at a scan rate of 50 mV s-1 at room temperature about the electrocatalytic activities. Sample of the commercial Pt/C for control experiment were investigated under the same conditions. It can be seen from the figure that the cyclic voltammograms of the two catalysts show no apparent reduction peak under nitrogen conditions. The contrast is obvious that Co@NSCNTs showed a more conspicuous positive reduction peak potential (0.77 V relative to the reversible hydrogen electrode (RHE)), comparable cathodic currents density to commercial Pt/C saturated with O2.


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Figure 4. a) Cyclic voltammetry (CV) in 0.1 M KOH aqueous solution saturated with N2 or O2 at a sweep rate of 50 mV s–1. b) Linear sweep voltammograms (LSVs) of Co@NSCNTs and Pt/C at 1600 rpm in O2-saturated 0.1 M KOH (inset: the LSVs of Co@NSCNTs, Co@NSCNTs-B and Co@NSCNTs-L). c) and d) Linear sweep voltammetry of Co@NSCNTs and Pt/C in O2saturated 0.1 M KOH with a sweep rate of 5 mV s–1 at different rotation rates indicated. e) Koutecky–Levich (K–L) plots of Co@NSCNTs, and Pt/C catalysts at 0.3 V (vs RHE). f) K–L


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plots of Co@NSCNTs at different potential (inset: the dependence of the electron transfer number). To gain further insight on the electrochemical performance of the Co@NSCNTs catalyst, rotating disk electrode (RDE) test was performed to proceed liner sweep voltammetry (LSV) in 0.1M KOH solution saturated with O2 at a scanning rate of 5 mV s–1 at 1600 rpm to reveal the electrocatalyst kinetics of the ORR. LSV curve as shown in Figure 4b, from the figure we can clearly see that the electrocatalytic performance of Co@NSCNTs matches up to commercial Pt/C. After careful analysis, the onset potential of Co@NSCNTs is 0.90 V, and half-wave potential of it is 0.80 V, the former with Pt/C catalyst only 10 mV gap and the latter is equivalent to commercial Pt/C (Eonset= 0.91 V, E1/2=0.80 V). The commendable catalytic activity of the catalyst is due to the high-speed electron transfer from the synergism between its encapsulated cobalt particles and the dopant carbon network. In order to prove this synergistic effect, we also made a set of comparative experiments, one was ball-milled with planetary ball-milling instrument for 4 hours for Co@NSCNT-B to destroy the protective carbon shells around the metal nanoparticles and another was removed the inner cobalt particles after hot acid reflux for Co@NSCNT-L to test their performance compared with the original sample without the aforementioned treatment to prove this synergistic effect. The LSV curve is shown inset Figure 4b). It can be seen from the figure that after ball milling, the graphitized nitrogen-doped carbon network as an electron transport channel is destroyed, so that the electron transport is blocked, so that both the half-wave potential and the limiting current performance are deteriorated. Similarly, after the hot acid treatment, the cobalt metal particles were removed, and the graphitized nitrogen-doped carbon layer was no further activated because of the presence of cobalt, so that the LSV curve showed poor catalytic performance. In summary, the synergistic effect of cobalt


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particles and graphitized nitrogen-doped carbon layer is very important throughout the catalytic process, and the lack of one will degrade the catalytic performance of Co@NSCNT. We also present a table summarizing the study of this material over the last two to three years (Table 1). The onset potential, half-wave potential, and Pt/C catalyst characterization of the oxygen reduction catalytic performance are listed in the table. The ratio of catalyst loading to Pt/C loading is listed for the first time, making the comparison of catalyst performance under different literature test conditions more meaningful. Table 1. Comparison of the Catalytic Properties of Different Catalysts. Eonset E1/2 (V) (V)

L-Pt (µg/ cm)

Eonset (V)

E1/2 (V)

Lratio (Cat/ Pt, %)

Fe3C/NCNT 400 s/OBP-900

0.92 8

0.78 5

40

0.903

-

Fe3C/C-800

600

1.05

0.83

50

1.05

PMF-800

1200

-

-

25

Co-600-800

88.3

-

-

Co@NCNT -700

100

0.89

Ni@NCNT100 700 FeNC-800 Co@NSCN Ts

Cat

∆Eo

E1/2

(mV)

(mV)

10

25

-

0.1M HClO4

0.83

12

0

0

0.1M KOH

-

-

48

-

49

0.1M KOH

17.6 6

-

-

5

12.3

34

0.1M KOH

-

20

0.95

-

5

60

-

0.1M KOH

0.84

-

20

0.95

-

5

110

-

0.1M KOH

900

1.10

0.88

30

1.05

0.83

30

50

50

0.1M KOH

200

0.90

0.80

20

0.91

0.80

10

10

0

0.1M KOH

L (µg/cm)


Elec Sol

Cit

17

13

18

27

28

28

29

This wok

19


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We then calculated the number of electrons transferred by the catalyst based on the KouteckyLevich equation (the exact calculation method is described in the experimental section). Typical linear sweep voltammetric curve using a rotating disk electrode in O2-saturated 0.1 M KOH at different rotating speeds (from 400 to 2000 rpm) were recorded (Figure 4c and d). The corresponding K-L plots of the catalyst electrode show a good linearity (Figure 4e), which indicates a first-order reaction kinetics in relation to the concentration of dissolved O2. The calculated electron transfer number of the Co@NSCNTs is 3.94, which is very close to four, proves that the electron transfer process is very biased towards the four-electron process, this is consistent with the previous XPS analysis. Then we also made a different electrode potential of the K-L diagram, the linearity is also very good, and the gap of the slope is very small, which proved the process of electron transfer to maintain a relatively stable state, which can also be seen from Figure 4f embedded illustrations.

Figure 5. a) Current−time (i−t) Chronoamperometric response for ORR at Co@NSCNTs and Pt/C electrodes on addition of 1 M methanol after about 180 s in 0.1 M O2-saturated KOH solution. b) Chronamperometry test of 60,000 s and 40,000 s at -0.3V (vs Ag/AgCl) with rotation rate of 1600 rpm for Co@NSCNTs and Pt/C, respectively.


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Then, as if the actual cathodic catalysts for direct methanol fuel cell, we made a 600 s chronoamperometry test in Figure 5a to show the effect of the methanol crossover. When time got to about 180 s, we joined 1 M methanol rapidly to the electrolyte solution parallel to the electrode side. We can clearly see the current change of Pt/C is very obvious and almost completely inactivate after reversion. While the curve of Co@NSCNTs after adding methanol almost has no obvious fluctuations, so the performance of Co@NSCNTs for methanol crossover effect is very good. Finally, as Fuel Cell catalysts, the durability of Co@NSCNTs is also an important indicator to determine its characteristics. We made chronamperometry test of 60,000 s and 40,000 s respectively at -0.3V (vs Ag/AgCl) with rotation rate of 1600 rpm for Co@NSCNTs and Pt/C to prove this (shown in Figure 5b). In the whole testing process, the catalytic activity of Pt/C continuously decays over time, when time reached 40,000 seconds, the remain keeps 76 %, while the attenuation of Co@NSCNTs in the whole process is very slow, even reach 40,000 s, it to maintain 95%, and almost reaches a stable state, no longer decay, even if after 60,000 s chronoamperometry, Co@NSCNTs remains still at 95%, no longer attenuated. Then,

observing the

changes

in

the morphology of the catalysts

after 60,000s

chronoamperometry (Figure S5), obviously, no significant structural changes were observed in the graphs. This shows excellent stability of Co@NSCNTs. We also made a table summarizing the study of the long-term stability of this material over the last two years. Co@NSCNTs also shows excellent properties as follows.


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Table 2. Comparison of the Durability Properties of Different Catalysts.

Catalyst

Chronoamperometry test time / s

Current holding ratio (%)

Citation

Co-600-800

10,000

90.4

27

Co@NCNT-700

10,000

95.4

28

FeNC-800

5,000

95

29

Co/N-CNTs

39,600

96.8

30

Fe-N-C@CNT

10,000

74.17

24

Co@NSCNTs

60,000

95

This work

In summary, the unique encapsulated structure play an important role in the protection of cobalt nanoparticles, improve the stability and durability of materials; graphitized carbon nanotube is an effective electron transport highway, the existence of pyridinic-N and pyrrole-N can accelerate the transfer of electrons; at the same time, the presence of cobalt changes the electronic structure of the surrounding carbon layer,31 further making the active site more efficient; interwoven three-dimensional network space is conducive to mass transfer, so the catalyst can be obtained with equivalent catalytic activity of Pt/C. CONCLUSIONS The results show that Co@NSCNTs has a more orderly and finer structure by adding glucosamine hydrochloride in proportion. By controlling the temperature, the graphitization degree and the number of the active sites are in a state more favorable for the catalytic activity to be exerted. The catalyst shows good catalytic activity, favorable four-electron selectivity, and great durability, excellent tolerance for methanol crossover, these features benefit from the “entire-encapsulated” structure of cobalt nanoparticles, the synergistic effect between


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encapsulated metal particle clusters and C-N meshwork consisted of nitrogen-doped carbon nanotubes, the changes of the electronic structure of the surrounding carbon layer by large amounts of cobalt metal particles, and particular slim structure of the cladding make these effect more effective. Briefly, the stability and durability of the encapsulated structure are remarkably superior to common metal cladding-free catalyst, namely, do suffer the catalytic leaching problem and poisoning issue on the surface of the catalysts in electrolyte. These characteristics will promote the development of the catalyst in all oxygen reduction reaction involved various types of fuel cells. It is also faced with further increase of metal encapsulated amount and incorporation of heteroatom under the premise of the stability in the future development. The purpose is to expose more active sites on the surface of carbon nanotubes and make the active sites more effective in the process of catalysis. All these mean a bright future on the road of the sustainable non-precious metal catalyst of oxygen reduction development prospects. ASSOCIATED CONTENT Supporting Information TEM images of the nanocomposites prepared at 700℃ and 725℃, and the not adding melamine comparison sample; XRD at 700℃; low-magnification TEM image; SEM (including EDS) and TEM images of the Co@NSCNTs-B and Co@NSCNTs-L; TEM of the Co@NSCNTs after 60,000s chronoamperometry test. AUTHOR INFORMATION Corresponding Authors * E-mail address: [email protected]. Tel.: 0086-335-8061569; fax: 0086-335-8059878.


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* E-mail address: [email protected].: 0086-335-8061569; fax: 0086-335-8059878. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This work was financially supported by the National Natural Science Foundation of China (No. 51674221). REFERENCES (1) Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R., Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chemical Reviews 2016, 116, 3594-3657. (2) 486, 43-51.

Debe, M. K., Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012,

(3) Zhang, C.; Grass, M. E.; Mcdaniel, A. H.; Decaluwe, S. C.; El, G. F.; Liu, Z.; Mccarty, K. F.; Farrow, R. L.; Linne, M. A.; Hussain, Z., Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ X-ray photoelectron spectroscopy. Nature Materials 2010, 9, 944-949. (4) Wang, L.; Li, Y.; Xia, M.; Li, Z.; Chen, Z.; Ma, Z.; Qin, X.; Shao, G., Ni nanoparticles supported on graphene layers: An excellent 3D electrode for hydrogen evolution reaction in alkaline solution. Journal of Power Sources 2017, 347, 220-228. (5)

Sealy, C., The problem with platinum. Materials Today 2015, 527, 65-68.

(6) Dai, L.; Xue, Y.; Qu, L.; Choi, H.-J.; Baek, J.-B., Metal-Free Catalysts for Oxygen Reduction Reaction. Chemical Reviews 2015, 115, 4823-4892. (7) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L., Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323, 760-764. (8) Daems, N.; Sheng, X.; Vankelecom, I. J.; Pescarmona, P., Metal-free doped carbon materials as electrocatalysts for the oxygen reduction reaction. Journal of Materials Chemistry A 2014, 2, 4085-4110. (9) Liu, B.; Yao, H.; Daniels, R. A.; Song, W.; Zheng, H.; Jin, L.; Suib, S. L.; He, J., A facile synthesis of Fe3C@mesoporous carbon nitride nanospheres with superior electrocatalytic activity. Nanoscale 2016, 8, 54415445. (10) Jiang, W.-J.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L.-J.; Wang, J.-Q.; Hu, J.-S.; Wei, Z.; Wan, L.-J., Understanding the High Activity of Fe–N–C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe–Nx. Journal of the American Chemical Society 2016, 138, 3570-3578.


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(11) Zhong, G.; Wang, H.; Yu, H.; Peng, F., A Novel Carbon-Encapsulated Cobalt-Tungsten Carbide as Electrocatalyst for Oxygen Reduction Reaction in Alkaline Media. Fuel Cells 2013, 13, 387–391. (12) Xiao, M., Meso/macroporous nitrogen-doped carbon architectures with iron carbide encapsulated in graphitic layers as an efficient and robust catalyst for the oxygen reduction reaction in both acidic and alkaline solutions. Advanced Materials 2015, 27, 2521-2527. (13) Hu, Y.; Jensen, J. O.; Zhang, W.; Cleemann, L. N.; Xing, W.; Bjerrum, N. J.; Li, Q., Hollow spheres of iron carbide nanoparticles encased in graphitic layers as oxygen reduction catalysts. Angewandte Chemie 2014, 53, 3675-3679. (14) Wang, J.; Wang, G.; Miao, S.; Jiang, X.; Li, J.; Bao, X., Synthesis of Fe/Fe 3 C nanoparticles encapsulated in nitrogen-doped carbon with single-source molecular precursor for the oxygen reduction reaction. Carbon 2014, 75, 381-389. (15) Deng, J.; Yu, L.; Deng, D.; Chen, X.; Yang, F.; Bao, X., Highly active reduction of oxygen on a FeCo alloy catalyst encapsulated in pod-like carbon nanotubes with fewer walls. J.mater.chem.a 2013, 1, 14868-14873. (16) Deng, D.; Yu, L.; Chen, X.; Wang, G.; Jin, L.; Pan, X.; Deng, J.; Sun, G.; Bao, X., Iron encapsulated within pod-like carbon nanotubes for oxygen reduction reaction. Angewandte Chemie 2013, 52, 371-376. (17) Zhu, J.; Xiao, M.; Liu, C.; Ge, J.; Stpierre, J.; Xing, W., Growth mechanism and active site probing of Fe3C@N-doped carbon nanotubes/C catalysts: guidance for building highly efficient oxygen reduction electrocatalysts. Journal of Materials Chemistry A 2015, 3, 21451-21459. (18) Yang, W.; Liu, X.; Yue, X.; Jia, J.; Guo, S., Bamboo-like Carbon Nanotube/Fe3C Nanoparticle Hybrids and Their Highly Efficient Catalysis for Oxygen Reduction. Journal of the American Chemical Society 2015, 137, 1436-1439. (19) Dai, X.; Li, Z.; Ma, Y.; Liu, M.; Du, K.; Su, H.; Zhuo, H.; Yu, L.; Sun, H.; Zhang, X., Metallic Cobalt Encapsulated in Bamboo-Like and Nitrogen-Rich Carbonitride Nanotubes for Hydrogen Evolution Reaction. ACS Applied Materials & Interfaces 2016, 8, 6439-6448. (20) Yao, Y.; Chen, H.; Lian, C.; Wei, F.; Zhang, D.; Wu, G.; Chen, B.; Wang, S., Fe, Co, Ni nanocrystals encapsulated in nitrogen-doped carbon nanotubes as Fenton-like catalysts for organic pollutant removal. Journal of Hazardous Materials 2016, 314, 129-139. (21) Liu, H.; Zhang, Y.; Li, R.; Sun, X.; Désilets, S.; Abou-Rachid, H.; Jaidann, M.; Lussier, L. S., Structural and morphological control of aligned nitrogen-doped carbon nanotubes. Carbon 2010, 48, 1498-1507. (22) Tang, D. M.; Ren, C. L.; Lv, R.; Yu, W. J.; Hou, P. X.; Wang, M. S.; Wei, X.; Xu, Z.; Kawamoto, N.; Bando, Y., Amorphization and Directional Crystallization of Metals Confined in Carbon Nanotubes Investigated by in Situ Transmission Electron Microscopy. Nano Letters 2015, 15, 4922-4927. (23) Liu, X.; Wang, Y.; Dong, L.; Chen, X.; Xin, G.; Zhang, Y.; Zang, J., One-step synthesis of shell/core structural boron and nitrogen co-doped graphitic carbon/nanodiamond as efficient electrocatalyst for the oxygen reduction reaction in alkaline media. Electrochimica Acta 2016, 194, 161-167. (24) Zhang, Y.; Jiang, W. J.; Guo, L.; Zhang, X.; Hu, J. S.; Wei, Z.; Wan, L. J., Confining Iron Carbide Nanocrystals inside CNx@CNT toward an Efficient Electrocatalyst for Oxygen Reduction Reaction. Acs Applied Materials & Interfaces 2015, 7, 11508-11515. (25) Yao, N.; Xie, X.; Chen, S.; Wei, D.; Qi, X.; Yao, W.; Wang, J.; Wei, L.; Wei, Z.; Shao, M., Towards Effective Utilization of Nitrogen-Containing Active Sites: Nitrogen-doped Carbon Layers Wrapped CNTs


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Electrocatalysts for Superior Oxygen Reduction. Electrochimica Acta 2015, 187, 153-160. (26) Trasobares, S.; Stéphan, O.; Colliex, C.; Hsu, W. K.; Kroto, H. W.; Walton, D. R. M., Compartmentalized CNx nanotubes: Chemistry, morphology, and growth. Journal of Chemical Physics 2002, 116, 8966-8972. (27) Cao, T.; Wang, D.; Zhang, J.; Cao, C.; Li, Y., Bamboo-Like Nitrogen-Doped Carbon Nanotubes with Co Nanoparticles Encapsulated at the Tips: Uniform and Large-Scale Synthesis and High-Performance Electrocatalysts for Oxygen Reduction. Chemistry - A European Journal 2015, 21, 14022-14029. (28) Hao, Y.; Lu, Z.; Zhang, G.; Chang, Z.; Luo, L.; Sun, X., Cobalt‐Embedded Nitrogen Doped Carbon Nanotubes as High Performance Bifunctional Oxygen Catalysts. Energy Technology, [Online early access].DOI:10.1002/ente.201600559.PublishedOnline:Nov12,2016.http://onlinelibrary.wiley.com/doi/10.1002/en te.201600559/full.html (accessed Nov 12, 2016). (29) Zhao, P.; Xu, W.; Hua, X.; Luo, W.; Chen, S.; Cheng, G., Facile Synthesis of a N-Doped Fe3C@CNT/Porous Carbon Hybrid for an Advanced Oxygen Reduction and Water Oxidation Electrocatalyst. The Journal of Physical Chemistry C 2016, 120, 11006-11013. (30) Liu, Y.; Jiang, H.; Zhu, Y.; Yang, X.; Li, C., Transition metals (Fe, Co, and Ni) encapsulated in nitrogen-doped carbon nanotubes as bi-functional catalysts for oxygen electrode reactions. Journal of Materials Chemistry A 2016, 4, 1694-1701. (31) Wang, Y.; Nie, Y.; Ding, W.; Chen, S. G.; Xiong, K.; Qi, X. Q.; Zhang, Y.; Wang, J.; Wei, Z. D., Unification of catalytic oxygen reduction and hydrogen evolution reactions: highly dispersive Co nanoparticles encapsulated inside Co and nitrogen co-doped carbon. Chemical Communications 2015, 51, 8942-8945.


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TOC graphic

Facile synthesis of cobalt nanoparticles entirely encapsulated in slim nitrogen-doped carbon nanotubes as oxygen reduction catalyst

Ailing Song, Wang Yang, Wu Yang, Gang Sun, Xucai Yin, Lijun Gao, Yazhou Wang, Xiujuan Qin, Guangjie Shao

Co@NSCNTs are synthesized for oxygen reduction catalyst, cathode catalyst for green energy of fuel cell, high activity, sustainable utilization.


27


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Figure Captions Abstract map. Figure 1. SEM, TEM and HRTEM of Co@NSCNTs, prove the apparent structure and microstructure of the material. Figure 2. XRD, Raman, TGA, and FTIR of Co@NSCNTs, characterize the material crystal surface structure, graphitization degree, cobalt content and structure of the bond. Figure 3. a) XPS survey spectra, and high-resolution scans of b) N 1s c) C 1s and d) Co 2p of Co@NSCNTs. Figure 4. A series of electrochemical characterization, including cyclic voltammetry, linear sweep voltammetry, linear sweep voltammetry at different sweeps, and K-L plot, electron transfer number. Figure 5. a) Current−time (i−t) Chronoamperometric response for ORR at Co@NSCNTs and Pt/C electrodes on addition of 1 M methanol after about 180 s in 0.1 M O2-saturated KOH solution. b) Chronamperometry test of 60,000 s and 40,000 s at -0.3V (vs Ag/AgCl) with rotation rate of 1600 rpm for Co@NSCNTs and Pt/C, respectively. Table Captions Table 1. Comparison of the catalytic properties of different catalysts. Table 2. Comparison of the durability properties of different catalysts.


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Abstract map.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. a) and b) SEM image of Co@NSCNTs. c) TEM image of Co@NSCNTs and HRTEM image d) with inset corresponding diffraction pattern of cobalt nanoparticle.


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Figure 2. a) XRD pattern of Co@NSCNTs, b) Raman and c) TGA thermograms in air, d) FTIR spectroscopy of the Co@NSCNTs.



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Figure 3. a) XPS survey spectra, and high-resolution scans of b) N 1s c) C 1s and d) Co 2p of Co@NSCNTs.


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Figure 4. a) Cyclic voltammetry (CV) in 0.1 M KOH aqueous solution saturated with N2 or O2 at a sweep rate of 50 mV s–1. b) Linear sweep voltammograms (LSVs) of Co@NSCNTs and Pt/C at 1600 rpm in O2-saturated 0.1 M KOH (inset: the LSVs of Co@NSCNTs, Co@NSCNTs-B and Co@NSCNTs-L). c) and d) Linear sweep voltammetry of Co@NSCNTs and Pt/C in O2-saturated 0.1 M KOH with a sweep rate of 5 mV s–1 at different rotation rates indicated. e) Koutecky–Levich (K–L) plots of Co@NSCNTs, and Pt/C catalysts at 0.3 V (vs RHE). f) K–L plots of Co@NSCNTs at different potential (inset: the dependence of the electron transfer number).



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Figure 5. a) Current−time (i−t) Chronoamperometric response for ORR at Co@NSCNTs and Pt/C electrodes on addition of 1 M methanol after about 180 s in 0.1 M O2-saturated KOH solution. b) Chronamperometry test of 60,000 s and 40,000 s at -0.3V (vs Ag/AgCl) with rotation rate of 1600 rpm for Co@NSCNTs and Pt/C, respectively.


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Table 1. Comparison of the Catalytic Properties of Different Catalysts.

(μg/cm)

Eonset (V)

E1/2 (V)

L-Pt (μg/ cm)

Eonset (V)

E1/2 (V)

Lratio (Cat/ Pt, %)

Fe3C/NCNTs/ OBP-900

400

0.928

0.785

40

0.903

-

10

Fe3C/C-800

600

1.05

0.83

50

1.05

0.83

PMF-800

1200

-

-

25

-

Co-600-800

88.3

-

-

17.66

100

0.89

-

100

0.84

FeNC-800

900

Co@NSCNTs

200

Cat

Co@NCNT-70 0 Ni@NCNT-70 0

L

∆Eo

E1/2 (mV)

Elec Sol

Cit

25

-

0.1M HClO4

16

12

0

0

-

48

-

49

-

-

5

12.3

34

20

0.95

-

5

60

-

-

20

0.95

-

5

110

-

1.10

0.88

30

1.05

0.83

30

50

50

0.1M KOH

28

0.90

0.80

20

0.91

0.80

10

10

0

0.1M KOH

This wok


(mV)

0.1M KOH 0.1M KOH 0.1M KOH 0.1M KOH 0.1M KOH

12

17

26

27

27


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Page 36 of 36

Table 2.Comparison of the Durability Properties of Different Catalysts. Catalyst

Chronoamperometry test time / s

Current holding ratio (%)

Citation

Co-600-800 Co@NCNT-700 FeNC-800 Co/N-CNTs Fe-N-C@CNT Co@NSCNTs

10,000 10,000 5,000 39,600 10,000 60,000

90.4 95.4 95 96.8 74.17 95

26


27 28 29 23

This work

Facile Synthesis of Cobalt Nanoparticles Entirely Encapsulated in Slim

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