Facile Synthesis of Cobalt Nanoparticles Entirely Encapsulated in Slim

Research Article pubs.acs.org/journal/ascecg

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*,†,‡ †

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

‡

S Supporting Information *

ABSTRACT: Owing to the scarcity of platinum resources and their visible flaw with poor durability and severe anode methanol crossover, Pt-based catalysts for catalyzing ORR have been experiencing some problems lately. Hence, nonprecious 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-doped carbon nanotubes after process optimization, and the cobalt content even reached 20.9 wt %. In the catalytic aspect, the optimized Co@NSCNTs show a half wave potential equivalent to Pt/C in alkaline medium. Then, better durability (after 60,000 s chronoamperometry, the current retention still remains 95%) and commendable resistance to methanol crossover performed than the literature confirmed. All these characteristics superior to Pt/C should be ascribed to large amounts of entire encapsulated structures 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, and then improving oxygen reduction reaction (ORR) catalysis. KEYWORDS: Nonprecious metal catalyst, Encapsulated structure, Oxygen reduction reaction, Durability

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conspicuous.7 Therefore, looking for 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 nonprecious 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 materials with good stability and better tolerance for methanol crossover were synthesized.10 These were cladding-structure materials whose

INTRODUCTION

Car fuel cell technology development, as well as underwater unmanned vehicles being developed, herald huge opportunities for the development of fuel cells,1 which also indicates that the industrialization of fuel cells is just around the corner. But this still faces a number of practical problems2−4 like the slow kinetics of the cathodic oxygen reduction reaction (ORR).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 restricting factors for fuel cells, although in recent years the membrane electrode of platinum loaded from 10 to 0.02 mg cm−2. At the same time, another two issues, poor stability and severe anode crossover, are becoming more and more © 2017 American Chemical Society

Received: December 27, 2016 Revised: March 20, 2017 Published: March 22, 2017 3973

DOI: 10.1021/acssuschemeng.6b03173 ACS Sustainable Chem. Eng. 2017, 5, 3973−3981

Research Article

ACS Sustainable Chemistry & Engineering

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 on the catalyst compared to conventional nitrogendoping 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 oxygen reduction road to a bright future.

above characteristics are ascribed to their unique cladding structure with a firmly graphitized carbon layer, protecting inner particles from being eroded.11−13 It then appears that transition metal nanoparticles encapsulated in heteroatomdoped carbon nanotubes demonstrated tremendous potential as a catalyst for ORR.14,15 Years ago, there was a report using sodium azide and ferrocene to successfully synthesize iron nanoparticles encapsulated within pod-like carbon nanotubes with superior catalytic activities and excellent stability after 220 h of accelerated durability testing (ADT) for catalyzing ORR.16 Furthermore, they also had density functional theory calculations (DFT) to demonstrate the interaction between metal particles and carbon nanotubes that 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 the high temperature and high pressure demands, and security still needs to be considered. Subsequently, transitionmetal-catalyzed in situ catalysis was used to synthesize transition metal-coated catalysts, and dicyandiamide, ferric chloride, and carbon black were used as precursors to synthesize an 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, the adhesion of the carbon layer, so the catalytic mechanism, is difficult to determine. Similar to bamboo-like carbon nanotube/Fe3C nanoparticle hybrids as ORR catalysts,18 cobalt encapsulated in carbonitride nanotubes as HER catalysts, and Fenton-like catalysts,19,20 this material is widely used in the field of catalysis. But this material in the synthesis of simplification, structure of controllability, and catalytic performance is still facing challenges. Therefore, we used a facile pyrolysis method to carry out a cladding-structure catalyst with excellent microstructure and outstanding catalytic performance. Here, melamine and D(+)-glucosamine hydrochloride worked both as a nitrogen source and carbon source, and 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 °C, the nitrogen and carbon sources experienced intermolecular polymerization, and nitrogen doped into a carbon skeleton, forming a nitrogen-doped carbon layer. Meanwhile, cobalt nitrates decomposed to cobaltcontaining particles and dispersed to form nanoparticles between carbon layers; both the cobalt-containing particles and the nitrogen-doped carbon layer are disordered. Then, the transient temperature was maintained at 700 °C in order to firmly secure the formation of a lamellar structure to get the nitrogen content and the encapsulated cobalt amount doubled high within the encapsulated structure catalyst. Figures S1 and S2 show the TEM and XRD images at 700 °C. Similar to the floating catalyst chemical vapor deposition (CCVD) method,21,22 the 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 to catalyze melamine into the tube, we also add another appropriately quantified nitrogenous carbon source, D(+)-glucosamine hydrochloride to not only increase the nitrogen content but also make the tube more orderly and slender.

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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, and obtaining the mixture catalyst after acid treatment. The specific process is as follows: 2.0 g of melamine (Kemi Chemical Reagent Co., Ltd.), 0.1 g of Co(NO3)2·6H2O (Tianjin Damao Chemical Reagent Factory), and 0.05 g of D(+)-glucosamine hydrochloride (Sinopharm Chemical Reagent Co., Ltd.) were mixed by grinding. The well-proportioned hybrids were then put into the tube furnace and heated at 600 °C for 1 h by flowing N2 at 100 mL min−1. Then, 10 min maintained at 700 °C was in order to firmly secure the formation of a lamellar structure to get the nitrogen content and the encapsulated amount doubled high cladding structure catalyst. After that, the temperature of the tube furnace was further increased to 800 °C for 1 h. The temperature rose at a rate of 5 °C min−1. Subsequently, the samples were cooled to room temperature under N2 ambient air, and the product was collected from the crucible. Finally, in order to remove the maintained metallic Co species on the outer wall of the carbon nanotubes, the annealed products were ultrasonically soaked into 0.5 M H2SO4 for 48 h, then filtered and washed with distilled water several times and dried at 80 °C overnight. The final product was denoted as Co@NSCNTs for cobalt nanopartiales encapsulated into slim nitrogen-doped carbon nanotubes. The acid treatment process also took some oxygencontaining functional groups on carbon layers, adding material hydrophilicity. Preparation of Co@NSCNTs-B. We took an appropriate amount of Co@NSCNTs and ball-milled it with a planetary ball-milling instrument for 4 h to destroy the protective carbon shells around the metal nanoparticles. Then, we collected the product, filtered and washed it with distilled water several times, and dried it at 80 °C overnight; we named it Co@NSCNT-B. Preparation of Co@NSCNTs-L. We took an appropriate amount of Co@NSCNTs and removed the inner cobalt particles after hot acid reflux. Then, we collected the product, filtered and washed it with distilled water several times, and dried it at 80 °C overnight; we named it Co@NSCNT-L. 20% commercial Pt/C was also selected as a catalyst for comparative experiments. Characterization of Materials. Using 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 microscopy (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 Kα radiation 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 a themogravimetric 3974


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ACS Sustainable Chemistry & Engineering analyzer (TGA, Pyris Diamond, PerkinElmer Thermal Analysis) under atmosphere at a heating rate of 10 °C min−1 from ambient temperature to 900 °C. The content of cobalt was further analyzed by inductively coupled plasma-optical emission spectroscopy (ICPOES, 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 a wavenumber 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. Two milligrams 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 disk electrode 5 mm in diameter (PINE Instruments), giving a loading of 200 μg cm−2. In 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 for at least 1 h. 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 a rotating speed from 400 to 2000 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) measurements of Co@NSCNTs and Pt/C were obtained at −0.3 V (vs Ag/AgCl) for 60,000 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 following equations:

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

(HRTEM). Figure 1a clearly shows the structure of Co nanoparticles integrally encapsulated in slim carbon nanotubes. The folded part of the material is like a graphitic layer composite structure, making the catalyst prepared with the combined characteristics of one-dimensional CNTs with twodimensional 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 the oxygen reduction process and improving the catalytic properties of the materials. Furthermore, no cobalt nanoparticles observed in the range of the electron microscope were unencapsulated by the carbon nanotubes after acid treatment. A low magnification transmission diagram is shown in Figure S3; the formation of the tube is very orderly and slender. According to the morphology of the nanotubes, under a low temperature condition, N atoms from the decomposition of melamine and D-(+)-glucosamine hydrochloride were directly introduced into the carbon framework to form nitrogen-doped carbon layers. The cobaltcontaining nanoparticles were dispersed in the C−N layer (Figure S1), like the top growth mechanism of CCVD, but a string of metal particles (Figure S3a) catalyzed the formation of the same carbon nanotube. During this process, under catalyzing of the cobalt-containing particles, the amorphous C−N 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 graphitization of the carbon nanotubes was further improved during the temperature-maintaining process (calcination at 800 °C for 1 h). During the whole process, the cobalt-containing nanoparticles reduced to metal cobalt particles with a 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 corresponded to the (111) crystal plane of the metallic cobalt with a cubic structure (lattice parameters, a = b = c). Also, a graphitic wall of carbon nanotubes ranges from about 5 to 8 layers with an

E(vsRHE) = E(vsAg/AgCl) + EAg/AgCl (reference) + 0.0591V × pH (1) (EAg/AgCl (reference) = 0.197 V vs NHE at 25 °C) 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)

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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, the cobalt particles are encapsulated in the nanotubes, and the surface of the nanotubes is folded. The 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 microscopy (TEM) and high-resolution transmission electron microscopy 3975


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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.

interlamination distance of 3.40 Å, which coincides with the (002) crystal plane of graphitic carbon. Meanwhile, the crystallinity of the graphite layer is not perfect from the HRTEM image, which may be ascribed to the local stress induced by the introduction of N dopant together with synergetic catalytic particles of cobalt. The diffraction pattern of cobalt nanoparticles of the Co@NSCNTs along with the diffraction of the Co (111), (200), (220), and (311) crystal planes 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 (Figures S5 and S6). From Figure S5a, we can see the signs of sample reunion after ball-milling, and then Figure S5b clearly shows the length of the nanotubes become shorter and the well marked fracture of the nanotubes due to ball-milling. Fracture of the nanotubes destroyed the characteristics of one-dimensional CNTs. They decreased the electrically conductivity and catalytic ability of the materials. Figure S6a is Co@NSCNTsL. The tubular structure was well maintained after acid treatment, and EDS of SEM did not show the presence of cobalt, proving 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. Then, from Figure 2a, we can further corroborate the presence of crystalline cobalt and graphic carbon by powder Xray diffraction (XRD). An apparent diffraction peak at 26.1° was attributed to the (002) plane of the graphitic-carbon lattice, and a weak reflection at 42.1° (not marked in figure) was ascribed to the (100) reflection of graphic carbon to further prove this. The diffraction peaks at 44.2°, 51.5°, and 75.9°

could be assigned to the (111), (200), and (220) planes of the cubic Co nanocrystal (PDF#15-0806), respectively, corresponding with the diffraction pattern of the cobalt nanoparticle. Then, Raman spectroscopy accurately showed 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, this means the surface order and defects are in a relatively balanced appropriate state, which proved that 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 (2Dband), which is secondary evidence to prove the graphitization degree is at a relatively good level. A pertinent degree of graphitization is expected to benefit oxygen reduction, accelerating the electron transfer rate. Moreover, the content of the Co element in the Co@NSCNTs was ascertained by thermogravimetric analysis (TGA) (Figure 2c, recorded under atmosphere), and there was 33.87 wt % remaining at above 900 °C, which could be attributed to the formation of Co3O4 during heating, corresponding to 20.9 wt % of the cobalt content. In order to determine the content of Co in Co@NSCNTs, we also analyzed with inductively coupled plasma mass spectrometry (ICP) that the result was 200,589 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 that the addition of D-(+)-glucosamine hydrochloride plays a fixed role with cobalt particles. Encapsulated with a high content of cobalt particles will enhance the synergies with doped nitrogen and 3976


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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.

reduction and are more conducive to the transfer of electrons than the three-dimensional sp3 hybridized quaternary-N. The content in Co@NSCNTs is 58.8%, even more than the planarN 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 nanotube.21,26 Besides, the deconvolution of the C 1s spectrum displayed five types of C species (Figure 3c), among them 59% CC and 12% C−N occupied the main bonding of the carbon atoms to prove that in addition to graphitized carbon, carbon atoms are mainly in conjunction with doped nitrogen atoms for the catalytic active sites. Of course, there was also some combination of C−O bonds and some of the oxygencontaining 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 and around 793.3 eV corresponding to a Co 2p3/2 binding energy and Co 2p1/2 states, respectively, which are consistent with metallic cobalt. The presence of the two peaks at 780.3 and 795.3 eV hint at reciprocal action between cobalt 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 concrete analysis about morphology and specific elements of materials, and from these results, the catalysis materials we compounded by facile synthesis are expected to contribute to the preparation of the catalytic oxygen fuel cell cathode of the restore process.

provide more active sites for the ORR catalysis process. Afterward, an FT-IR spectroscopy test was also done to make a cursory view of the chemical bonding situations of the Co@ NSCNTs in Figure 2d. 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 the XPS signal is 87.51, 2.02, 10.08, and 0.39 at. %, respectively. Apparently, the metallic Co signal is very weak, and 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, or 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 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 the 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. % (398.5 eV for pyridinic-N, 400.7 eV for pyrrolic-N, and 401.5 eV 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 are considered to be the most dominant for ORR. In the article of Wei,25 it was also proved that planar-N types in sp2 hybridization, i.e., pyridine-N and pyrrole-N, play a dominant role in the catalytic oxygen 3977


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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: LSVs of Co@NSCNTs, Co@NSCNTs-B, and Co@NSCNTs-L). (c, 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 potentials (inset: dependence of electron transfer number).

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5 mV s−1 at 1600 rpm to reveal the electrocatalyst kinetics of the ORR. The LSV curve is shown in Figure 4b. From the figure, we can clearly see that the electrocatalytic performance of Co@NSCNTs matches the 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 the Pt/C catalyst is only a 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 a planetary ball-milling instrument for 4 h for Co@NSCNT-B to destroy the protective carbon shells around the metal nanoparticles and another was to remove 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 in the

MATERIAL ELECTROCATALYSIS Figure 4a first shows a cyclic voltammetry (CV) technique test saturated with N2 or O2 in a 0.1 M KOH aqueous solution at a scan rate of 50 mV s−1 at room temperature with the electrocatalytic activities. Samples of the commercial Pt/C for the control experiment were investigated under the same conditions. It is shown 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 to the cathodic currents density to commercial Pt/C saturated with O2. To gain further insight on the electrochemical performance of the Co@NSCNTs catalyst, a rotating disk electrode (RDE) test was performed to produce liner sweep voltammetry (LSV) in a 0.1 M KOH solution saturated with O2 at a scanning rate of 3978


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ACS Sustainable Chemistry & Engineering Table 1. Comparison of Catalytic Properties of Different Catalysts catalyst

L (μg/cm)

Eonset (V)

E1/2 (V)

L-Pt (μg/cm)

Eonset (V)

E1/2 (V)

Lratio (cat/Pt, %)

ΔEo (mV)

E1/2 (mV)

Fe3C/NCNTs/OBP-900 Fe3C/C-800 PMF-800 Co-600-800 Co@NCNT-700 Ni@NCNT-700 FeNC-800 Co@NSCNTs

400 600 1200 88.3 100 100 900 200

0.928 1.05 − − 0.89 0.84 1.10 0.90

0.785 0.83 − − − − 0.88 0.80

40 50 25 17.66 20 20 30 20

0.903 1.05 − − 0.95 0.95 1.05 0.91

− 0.83 − − − − 0.83 0.80

10 12 48 5 5 5 30 10

25 0 − 12.3 60 110 50 10

− 0 49 34 − − 50 0

elec sol 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

M M M M M M M M

HClO4 KOH KOH KOH KOH KOH KOH KOH

ref 17 13 18 27 28 28 29 this work

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 a 0.1 M O2-saturated KOH solution. (b) Chronamperometry test of 60,000 and 40,000 s at −0.3 V (vs Ag/AgCl) with a rotation rate of 1600 rpm for Co@NSCNTs and Pt/C, respectively.

transfer process is very biased toward 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 is also shown from the inset in Figure 4f . Then, for the actual cathodic catalysts for a direct methanol fuel cell, we conducted a 600 s chronoamperometry test (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 that 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, the performance of Co@NSCNTs for the 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 conducted a chronamperometry test of 60,000 and 40,000 s, respectively, at −0.3 V (vs Ag/AgCl) with a rotation rate of 1600 rpm for Co@ NSCNTs and Pt/C to prove this (Figure 5b). In the whole testing process, the catalytic activity of Pt/C continuously decays over time, and when time reached 40,000 s, it remained at 76%. While the attenuation of Co@NSCNTs in the whole process is very slow and even reaches 40,000 s, it to maintains 95% and almost reaches a stable state with no more decay. Even after a 60,000 s chronoamperometry, Co@NSCNTs remained at 95% and was no longer attenuated. Then, observing the changes in the morphology of the catalysts after 60,000 s chronoamperometry (Figure S7), obviously no significant

inset of Figure 4b. It is shown that after ball milling, the graphitized nitrogen-doped carbon network as an electron transport channel is destroyed, so that the electron transport is blocked and 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 longer activated because of the presence of cobalt, so that the LSV curve showed poor catalytic performance. In summary, the synergistic effect of cobalt particles and graphitized nitrogendoped 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. We then calculated the number of electrons transferred by the catalyst based on the Koutecky−Levich equation (the exact calculation method is described in the Experimental Section). Typical linear sweep voltammetric curves 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 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 4, and proves that the electron 3979



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structural changes were observed in the graphs. This shows excellent stability of Co@NSCNTs. Table 2 summarizes the study of the long-term stability of this material over the last two years. Co@NSCNTs also shows excellent properties.

chronoamperometry test time (s)

current holding ratio (%)

ref

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

10,000 10,000 5000 39,600 10,000 60,000

90.4 95.4 95 96.8 74.17 95

27 28 29 30 24 this work

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03173. TEM images of the nanocomposites prepared at 700 and 725 °C and the not adding melamine comparison sample, XRD at 700 °C, low-magnification TEM image, SEM (including EDS) and TEM images of Co@ NSCNTs-B and Co@NSCNTs-L, and TEM of the Co@NSCNTs after 60,000 s chronoamperometry test. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 0086-335-8061569. Fax: 0086335-8059878. (X. Qin) *E-mail: [email protected]. Tel.: 0086-335-8061569. Fax: 0086-335-8059878. (G. Shao)

In summary, the unique encapsulated structure plays an important role in the protection of cobalt nanoparticles, improving the stability and durability of materials. The graphitized carbon nanotube is an effective electron transport highway, and 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. An interwoven three-dimensional network space is conducive to mass transfer, so the catalyst can be obtained with equivalent catalytic activity of Pt/C.

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ASSOCIATED CONTENT

S Supporting Information *

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

Research Article

ORCID

Guangjie Shao: 0000-0001-6957-4828 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant 51674221).

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CONCLUSIONS

REFERENCES

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The results show that Co@NSCNTs have a more orderly and finer structure by adding glucosamine hydrochloride in proportion. By controlling the temperature, the graphitization degree and number of 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, great durability, and excellent tolerance for methanol crossover. These features benefit from the “entire encapsulated” structure of cobalt nanoparticles. The synergistic effect between the encapsulated metal particle clusters and the C−N meshwork consisting of nitrogen-doped carbon nanotubes, the changes in the electronic structure of the surrounding carbon layer by large amounts of cobalt metal particles, and the particular slim structure of the cladding make these features more effective. Briefly, the stability and durability of the encapsulated structure are remarkably superior to the common metal cladding-free catalyst; namely, they 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 reactions involving various types of fuel cells. They also face a further increase in the metal encapsulated amount and incorporation of heteroatoms under the premise of stability in 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 indicate a bright future for the sustainable nonprecious metal catalyst of oxygen reduction development prospects. 3980


Research Article


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Facile Synthesis of Cobalt Nanoparticles Entirely Encapsulated in Slim

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