Bifunctional Nitrogen-Doped Microporous Carbon Microspheres

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Bifunctional Nitrogen-Doped Microporous Carbon Microspheres Derived from Poly(o‑methylaniline) for Oxygen Reduction and Supercapacitors Yanzhen He,† Xijiang Han,*,† Yunchen Du,† Bo Song,† Ping Xu,*,†,‡ and Bin Zhang*,†,‡ †

Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China

‡

S Supporting Information *

ABSTRACT: Heteroatom-doped carbon materials have attracted significant attention because of their applications in oxygen reduction reaction (ORR) and supercapacitors. Here we demonstrate a facile poly(o-methylaniline)-derived fabrication of bifunctional microporous nitrogen-doped carbon microspheres (NCMSs) with high electrocatalytic activity and stability for ORR and energy storage in supercapacitors. At a pyrolysis temperature of 900 °C, the highly dispersed NCMSs present a high surface area (727.1 m2 g−1), proper total content of doping N, and high concentration of quaternary N, which exhibit superior electrocatalytic activities for ORR to the commercial Pt/C catalysts, high specific capacitance (414 F g−1), and excellent durability, making them very promising for advanced energy conversion and storage. The presented conducting polymer-derived strategy may provide a new way for the fabrication of heteroatom-doped carbon materials for energy device applications. KEYWORDS: nitrogen-doped carbon, conducting polymer, microporous, oxygen reduction reaction, supercapacitor

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INTRODUCTION In recent years, heteroatom-doped carbon materials including nanotubes,1 nanocages,2 nanosheets,3 and graphene4,5 have attracted significant attention because of their various applications, especially as oxygen reduction reaction (ORR) electrocatalysts and supercapacitor materials.6−10 The presence of heteroatom in the graphitic layer can exhibit a faster chargetransfer rate at the electrode/electrolyte interface, favorable to proton and electron transfer in the ORR.11,12 In addition, the capacity, surface wettability and electronic conductivity of the carbon materials can be improved at the same time by introducing heteroatoms into the carbon framework.13,14 Notably, heteroatom-doped carbon materials usually focused on postdoping of N, S, P, B, and their dual or multiple dopants to traditional carbon nanomaterials by some harsh methods, such as chemical vapor deposition, thermal annealing with NH3, nitrogen plasma treatment and arc-discharge method, which not only require special instruments or rigorous conditions, but also result in a lower content of nitrogen.15−21 Therefore, heteroatom sources are inevitably required for conventional carbon materials. However, these drawbacks can be overcome by employing N-containing materials as precursors. In view of this, there is still considerable space in improving the fabrication technique of heteroatom-doped carbon materials. © XXXX American Chemical Society

The development of conducting polymers (CPs) containing N or S heteroatoms, such as polypyrrole (PPy), polyaniline (PANI), and polythiophene (PT), provides a great opportunity for the fabrication of metal-free heteroatom-doped carbon materials because these polymers contain only nonmetallic elements and some key heteroatoms in their backbones.22−26 The heteroatoms homogeneously introduced into the carbon framework can not only be preserved at a relatively high content by adjusting the carbonization temperature, but stay stable under a harsh working condition by a simple procedure of direct carbonizing the CPs.27,28 In view of this, PANI-derived N- and O-doped, and N-, O-, and S-tridoped mesoporous carbon materials have been synthesized and these materials present good conversions and high selectivity for ORR and alcohol oxidation reaction (AOR).29,30 Dai et al. reported that three-dimensional N and P codoped mesoporous nanocarbon foams with bifunctional catalytic activities toward ORR and oxygen evolution reaction (OER) can be obtained via direct pyrolysis of PANI aerogels.31 In addition, other heteroatomSpecial Issue: Applied Materials and Interfaces in China Received: August 24, 2015 Accepted: October 13, 2015

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DOI: 10.1021/acsami.5b07865 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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temperature at 5 °C/min (700, 800, 900, or 1000 °C), and kept for 1 h. After cooling down, the N-doped carbon microspheres were obtained. The final samples were labeled as NCMSs-T, where T (= 700, 800, 900, or 1000 °C) represents the second pyrolysis temperature. Characterization. Scanning electron microscopic (SEM) images were recorded on an S-4800 (Hitachi) electron microscope. Transmission electron microscopic (TEM) images were acquired with Topcon 002B transmission electron microscope. The Raman spectra were determined on a Renishaw in Via micro Raman spectroscopy system, using the TE air-cooled 576 × 400 CCD array in a confocal Raman system (wavelength: 532 nm). The incident laser power was kept at 0.1 mW, and total accumulation times of 5 s were employed. The X-ray diffraction (XRD) patterns were conducted on a Rigaku/Max-3A X-ray diffractometer with Cu Kα radiation (λ= 1.54178 Å), where the operation voltage and current were maintained at 40 kV and 40 mA. X-ray photoelectron spectra were obtained with PHI 5700 ESCA system equipped with an A1 Kα radiation as a source (hυ = 1486.6 eV). Nitrogen adsorption/desorption isotherms were measured on a QUADRASORB SI-KR/MP (Quantachrome, USA) after heating the materials under vacuum at 120 °C. Electrochemical Performances. Electrochemical measurements were performed using a rotating disk electrode (RDE) in a conventional three-electrode cell with 0.1 M KOH solution as electrolyte on a CHI electrochemical station (Model 660D) at room temperature. A platinum wire and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. For preparing the catalyst ink, appropriate amount of NCMSs-T materials or commercial Pt/C (20 wt %, Alfa Aesar) was ultrasonically dispersed in an alcoholic solution containing 5 wt % Nafion for at least 1 h. The ink was then dropped on a glassy-carbon disk of RDE with a geometric area of 0.19625 cm2. The catalyst loading was controlled at 0.2 mg cm−2. The working electrode was first cycled between −0.8 and 0.2 V vs SCE for 50 cycles at 50 mV s−1 in an O2-saturated 0.1 M KOH solution to produce a clean electrode surface. Then cyclic voltammogram (CV) tests were performed at a scan rate of 50 mV·s−1 in either O2- or N2-saturated 0.1 M KOH solution. The linear sweep voltammograms (LSVs) were obtained on a RDE at a scan rate of 5 mV s−1 in O2-saturated 0.1 M KOH solution with varying rotating speeds ranging from 225 to 1225 rpm. Methanol crossover test was performed by adding 3 M methanol into the electrolyte solution, and then the cathodic ORR current density was measured using the LSV curves. The electron transfer number (n) per oxygen molecule was determined by the Koutechy-Levich (K-L) equation31

doped carbon materials derived from conducting polymers or their derivatives have also been reported in recent years,32,33 such as 2D sandwich-like conjugated microporous polymers,34 N-doped porous carbon derived from poly(o-phenylenediamine),35 and N-doped hierarchically porous carbons.36 Therefore, direct carbonization of heteroatom-containing precursors, especially conducting polymers, has been proven to be a facile strategy for fabricating functional carbon materials.37 However, most researches have focused on conventional conducting polymers (PANI and PPy), where conducting polymer derivatives (such as poly(o-methylaniline), POT) have been paid less attention. As reported, the presence of methyl group can improve the properties of POT, such as the solubility, crystallinity, and electrical conductivity, which may be beneficial for the ultimate nitrogen-doped carbon materials.38 So far, there is no report on the fabrication of spherical carbon materials derived from POT spheres for the application as ORR catalysts and supercapacitor materials. It has been proved that the carbon submicrometer spheres with monodispersity and maintained size are of great practical application because of their being extraordinary building blocks for multifunctional hybrid materials.39−42 With this point of view, it is challenging but desirable to explore new nitrogendoped carbon spheres with excellent thermal stability and high monodispersity derived from the derivatives of CPs. Herein, we demonstrate the synthesis of metal-free microporous nitrogen-doped carbon microspheres (NCMSs) via a direct carbonization of a PANI derivative, poly(o-methylaniline) (POT), which acted as the C and N precursors. Especially, the NCMSs prepared by pyrolysis of POT at 900 °C (NCMSs900) possess a very high specific surface area of 727.1 m2 g−1 and an optimized surface functionality with the desired nitrogen doping. These unique structural features enable NCMSs-900 with a superior ORR activity to commercial Pt/C catalyst and a specific capacitance of 414 F g−1 in 6 M KOH solution. The NCMSs materials might be a promising alternative as a kind of bifunctional carbon materials for advanced energy devices.

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EXPERIMENTAL SECTION

Materials. o-Methylaniline (≥98.0%, Acros) was distilled and stored at low temperature under nitrogen protection. Hydrogen peroxide (H2O2, 30 v/v % aqueous solution), ferric chloride hexahydrate (FeCl3·6H2O), and phosphoric acid (H3PO4) were all AR grade (National medicine group chemical reagent co., LTD) and used as received without further purification. Synthesis of Poly(o-methylaniline) (POT) Microspheres. The synthesis of POT microspheres was completed through a modified hydrothermal route as reported in a previous literature.43 In a typical experimental procedure, 1 mL of o-methylaniline was dissolved in 400 mL of 0.4 M H3PO4 at ambient condition under vigorous stirring for 10 min to form a uniform solution. Then 1.2 mL of H2O2 and 0.2 mL of FeCl3 aqueous solution were added into the above solution under magnetic stirring. The solution was later transferred into a Teflonlined stainless steel autoclave. The autoclave was sealed quickly and maintained at 140 °C for 6 h in a digital temperature-controlled oven. Then, the autoclave was cooled to room temperature spontaneously. The precipitate was filtered and washed adequately with deionized water and ethanol until the filtrate became colorless. At last, the precipitate was dried in a vacuum drier at 80 °C for overnight. Synthesis of Nitrogen-Doped Carbon Microspheres (NCMSs). N-doped carbon microspheres (NCMSs) were obtained by a controlled carbonization process under N2 protection from the POT microspheres. The as-prepared POT microspheres were first heated from room temperature to 400 °C at 5 °C/min and kept at this temperature for 1.5 h. Then, the temperature was raised to a higher

1/j = 1/jk + 1/Bω1/2

(1)

where jk and ω are the kinetic current and electrode rotating rate, respectively. B can be determined from the slope of the K-L plots according to Levich equation

B = 0.2nFCo(Do)2/3 v−1/6

(2) −1

where F is the Faraday constant (96485 C mol ), C0 is the saturated O2 concentration in the electrolyte, D0 is the diffusion coefficient of O2 in the electrolyte, and v is the kinetic viscosity of the solution. Here, the values of constant are C0 = 1.2 × 10−6 mol cm−3, D0 = 1.9 × 10−5 cm2 s−1, and v = 0.01 cm2 s−1. To test the electrochemical activities of NCMSs-T as supercapacitor materials, we obtained CV curves at scan rates from 2 to 500 mV s−1, galvanostatic charge and discharge profiles measured at varying current densities of 0.5 to 5 A g−1 using a CHI 660D electrochemical workstation. The capacitance was calculated from the discharge CV data according to the following equation9

C=

1 υ(Vf − Vi )

∫V

i

Vf

I(V )dV

(3)

where C is the capacitance from the electrodes, υ is the scan rate (V/ s), Vf and Vi are the integration potential limits of the voltammetric curve, and I(V) is the discharge current (A). The mass capacitance Cm (F/g) was calculated based on the mass of samples. B


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RESULTS AND DISCUSSION The strategy for the fabrication of POT microspheres and related NMCSs materials is presented in Scheme 1. The POT

demonstrates that this spherical structure of POT is thermally stable (Figure 1b−e). This differs greatly from previous studies that morphology of carbon materials derived from pyrolysis of conducting polymers usually varies from that of the pristine polymer.44,45 The molecular rearrangement and sterical hindrance during the doping process could be affected by the presence of substituents along the aromatic rings, which affect both the chemical structure and the final physical and chemical properties of the synthesized materials.46,47 In other words, it is the presence of methyl group that results in the thermally stable spherical structures. TEM images inset in Figure 1b−e also confirmed that the spherical morphology and size feature could be maintained. To verify the successful conversion of POT to carbon materials and the graphitization degree of the NCMSsT samples, we compared their Raman spectra. As shown in Figure 1f, one can see two distinct D and G peaks at 1360 and 1580 cm−1, but no Raman fingerprints related to the pristine POT polymer (Figure S1), a strong evidence of the transformation from conducting polymer to carbon.48 The D and G bands correspond to the disordered carbon/structural defects and graphitic layers (sp2 bonded carbon atoms) of carbon materials, respectively.49 As the G band is related to tangential vibrations of sp2 carbon atoms, its presence in the spectra indicates the existence of graphitic carbon structure in NCMSs-T. The D band, which corresponds to the defect band, should be due to the presence of dopant atoms and concomitant absence of some graphitic carbons in the NCMSs-T materials.30 It is well-known that the relative intensity ratio of the D band to G band (ID/IG) is a pointer to confirm the degree of graphitization or defect density in carbon materials, and increased ID/IG value represents more disordered structure in graphitic carbon or increased graphitization degree of amorphous carbon.50−52 In the case of NCMSs-T materials, ID/IG ratio increases from 0.89 to 0.99

Scheme 1. Schematic Illustration of the Preparation of Poly(o-methylaniline) (POT) by a Hydrothermal Technique and N-Doped Carbon Microspheres (NCMSs) from Pyrolysis of POT

microspheres were first synthesized by the polymerization of omethylaniline with the assistance of H2O2 and FeCl3·6H2O through a hydrothermal technique. Followed carbonization of the resulting POT microspheres leads to carbon materials containing different nitrogen bonding configurations. Monodispersed POT microspheres with a diameter of about 1 μm were obtained by the hydrothermal method, as shown in Figure 1a. From the TEM image inset in Figure 1a, one can see the as-prepared POT microspheres are dense inside. Notably, the resultant carbon materials after high-temperature pyrolysis appeared to preserve the structural integrity, monodispersity, and spherical morphology with a negligible size change, which

Figure 1. SEM images of pristine (a) POT and (b) NCMSs-700, (c) NCMSs-800, (d) NCMSs-900, and (e) NCMSs-1000; Insets of a−e are their corresponding TEM images of a single particle. Scale bars in SEM and TEM images are 1 μm and 200 nm, respectively. (f) Raman spectra of the NCMSs materials obtained at different temperatures. C


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ACS Applied Materials & Interfaces when the pyrolysis temperature is raised from 700 to 1000 °C, an indication of increased degree of carbon graphitization.52 As known, graphitization of carbon can enhance the electronic conductivity and corrosion resistance during electrocatalysis, but the type and content of doping heteroatoms are also very crucial in creating catalytically active sites of carbon materials.53,54 The structural information on POT and NCMSs-T was further investigated by XRD measurements. As shown in Figure S2, the pristine POT presents one amorphous characteristic peak at 2θ ≈ 23°, indicating the unsuccessful formation of the graphite structure. However, the XRD patterns of all the NCMSs-T samples reveal two characteristic peaks located at 2θ ≈ 25 and 43°, corresponding to the (002) and (100)/(101) lattice planes of amorphous carbon materials.50 As the pyrolysis temperature increases, the (002) diffraction peak was slightly shifted toward a higher 2θ angle due to increased graphitization degree of the carbon materials and contraction of interplanar spacing (d002) because of the decreased content of incorporated heteroatoms at higher temperature,32 which agrees well with the Raman data. Above results demonstrate that though the asprepared NCMSs-T materials are mainly amorphous carbon in nature, higher pyrolysis temperature will lead to increased graphitization degree and reduced content of heteroatoms. Nitrogen adsorption/desorption measurements were conducted to investigate the BET surface areas and pore structures of the samples. As for the pristine POT, there is an unconspicuous I/IV-type adsorption−desorption isothermal curve and its BET surface area is only 3.2 m2/g (Figure S3). However, the N2 adsorption of NCMSs-T showed an I/IV-type adsorption−desorption isothermal curve (Figure 2a and Figure S3), and surprisingly, the surface areas of these samples were dramatically increased as compared to that of POT. The BET surface areas of NCMSs-700, NCMSs-800, NCMSs-900, and NCMSs-1000 are calculated to be 388.6, 480.7, 727.1, and 202.4 m2 g−1, respectively, which indicates the introduction of porous nature of NCMSs-T. Carbon materials with high specific surface area can offer an ample electrode/electrolyte interface for ion or charge accumulation.55 The pore size distribution evaluated by nonlocal density functional theory (DFT) method indicates that all the NCMSs-T samples derived from high-temperature pyrolysis of POT microspheres present microporous structure (inset in Figure 2a and Figure S3),55 which can well explain their high specific surface areas. Therefore, high-temperature pyrolysis can introduce micropores in NCMSs-T materials, though the surface morphology was not changed as compared to the pristine POT. As-prepared NCMSs-T materials with high specific areas are expected to be ideal as electrocatalysts for ORR and energy storage materials in supercapacitors. The electrocatalytic activity of NCMSs-T toward the ORR was first evaluated by CV and LSV in 0.1 M KOH electrolyte. For comparison, LSV curves of pristine POT as well as commercial 20 wt % Pt/C were also collected under the same conditions (Figure 3a). The pristine POT shows very limited response for ORR, while the electrocatalytic activities of NCMSs-T materials are dramatically improved and pyrolysis temperature-dependent. It can be seen that the activity is enhanced gradually when the pyrolysis temperature increases from 700 to 900 °C, but then gets decreased with a pyrolysis temperature of 1000 °C. Remarkably, distinctively high activity of NCMSs-900 for ORR can be seen from its large onset potential of 0.981 V vs RHE and half-wave potential (E1/2) of 0.915 V vs RHE at 900

Figure 2. (a) Nitrogen adsorption/desorption isotherm and pore size distribution (inset) of NCMSs-900, and (b) BET surface area of pristine POT and NCMSs-T materials.

rpm and a scan rate of 5 mV s−1, which is close to those of commercial Pt/C (1.01 V, 0.93 V vs RHE) and better than that of other reference samples (see Table S1).11,30,41,48 CV profiles for NCMSs-900 were carried out in 0.1 M KOH solution saturated with N2 or O2 at a scan rate of 50 mV s−1 (Figure 3b). One can see a strong cathodic peak at 0.882 V vs RHE in O2saturated solution, but featureless background in N2-saturated electrolyte, an indication of NCMSs-900’s electrocatalytic activity toward ORR. RDE measurements of NCMSs-900 (Figure 3c) and other samples (Figure S4) at various rotating speeds from 225 to 1225 rpm were carried out according to a previous report.56 The ORR kinetics of NCMSs-900 and other materials were studied by Koutecky−Levich (K-L) plots, where the excellent linearity indicates a feature of first order reaction.31 Using RDE current density as well as K-L plots, the electron transfer number per oxygen molecule (n) during the ORR process with NCMSs-900 is calculated to be 4.0, suggesting a four-electron process over a wide potential range (Figure 3d).30,54 However, with other NCMSs-T samples (T = 700, 800, 1000), n only falls in the range of 3.0−3.4, suggesting a mixed two-electron and four-electron process. The stability of a catalyst is a key feature in deciding its possibility for practical applications. To investigate the electrochemical stability of NCMSs-900, continuous CV tests were performed in O2-saturated 0.1 M KOH solution. As shown in Figure 3e, even after 10 000 cycles, the characteristic reduction current with a well-defined cathodic peak at 0.882 V vs RHE can still be well-maintained for the NCMSs-900 D


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NCMSs-900 also exhibits the largest CV area as compared to the pristine POT and the other NCMSs-T, indicating most charges stored as a capacitor (Figure 4a). The specific

Figure 4. (a) CV curves of all the samples at a scan rate of 50 mV s−1. (b) Specific capacitances of all the samples at various scan rates. (c) Galvanostatic charge/discharge curves of all the samples at a current density of 1 A g−1. (d) Cycle performance of NCMSs-900. Figure 3. (a) LSV curves of ORR over NCMSs-T materials on RDE rotating at 900 rpm; (b) CV curves of CNMSs-900 in O2 and N2saturated 0.1 M KOH solutions; (c) LSV curves of ORR over NCMSs900 on RDE rotating at different speeds, the inset is the K-L plots of J−1 versus ω−1/2 of NCMSs-900 at a potential from −0.2 to −0.45 V derived from the LSV curves; (d) electron transfer number in ORR on NCMSs-900 material; (e) electrochemical stability of NCMSs-900 as determined by continuous CV scanning in O2-saturated 0.1 M KOH at a scan rate of 50 mV s−1; (f) LSV curves of NCSMs-900 and commercial Pt/C catalyst (inset) in O2-saturated 0.1 M KOH solution with or without 3 M methanol.

capacitance of as-prepared materials as a function of scan rate by integrating the voltammetric charge in the CV profiles is shown in Figure 4b.60,61 As calculated, the specific capacitance of NCMSs-900 (414 F g−1) is always higher than those of the other samples (e.g., 41 F g−1 for pristine POT, 342 F g−1 for NCMSs-700, 413 F g−1 for NCMSs-800, and 330 F F g−1 for NCMSs-1000) at a same scan rate of 2 mV s−1, and the value of mass capacitance is 310 F g−1 even at a high scan rate of 500 mV s−1, and the capacitance of other NCMSs still maintain over 255 F g−1 at high scan rate of 500 mV s−1, which is superior to the values of reported carbon materials.57,58,62−64 To further estimate the performance of the as-prepared materials, we measured galvanostatic charge/discharge curves at a function of current densities from 0.5 to 5 A g−1. The characteristic galvanostatic charge/discharge curves of the as-prepared materials at a given current density of 1 A g−1 are presented in Figure 4c. One can see the linear voltage−time curves and symmetric charge/discharge characteristics without obvious iR drop response except the pristine POT, indicating the ideal capacitive characteristics with a rapid I−V response. And it was found that the NCMSs-900 took the longest discharging time and possessed the highest specific capacitance, which is superior to the reported nitrogen-doped porous carbon nanofibers.13 This finding is self-consistent with the results of CV curves. What’s more, we have also examined the cyclic stability of the NCMSs-900 by means of continuous CV cycling experiments. As shown in Figure 4d, the NCMSs-900 presents good capacitance retention, which was evaluated by CV measurement at a scan rate 100 mV s−1 in a 6 M KOH electrolyte. NCMSs-900 displays a high stability of 93% capacitance retention after 3000 cycles. Such good cycle performance is highly promising for practical applications as electrode materials in supercapacitors. To investigate why NCMSs-900 displays the best ORR catalysis and supercapacitor performance, we performed X-ray photoelectron spectroscopy (XPS) to analyze the elemental

catalyst. Crossover effect of the electrocatalysts is also very critical for their practical applications in fuel cells. Herein, the electrocatalytic selectivity of NCMSs-900 as a model material was performed for the electro-oxidation of methanol. As shown in Figure 3f, one can see a better response for ORR of NCMSs900 after the addition of 3 M methanol to an O2-saturated 0.1 M KOH solution as compared to the commercial Pt/C catalyst. This demonstrates that NCMSs-900 presents high selectivity toward the ORR and better ability to avoid crossover effects. Heteroatom-doped carbon materials are potential electrode materials for supercapacitors mainly owing to their high specific surface area and conductivity.57−59 Inspired by the use of the aforementioned heteroatom-doped or metal-free carbon materials in supercapacitors, we performed the CV measurements of as-fabricated pristine POT and NCMSs-T materials at a scan rate from 2 to 500 mV s−1, and the choice of voltage window is according to the reported works.21,22,44 As shown in Figure S5, one can see a gradual increase of the current density with increased scan rate, which confirms that the as-prepared materials have desirable fast charge/discharge capability for power devices.9 What’s more, the CV curves of all samples show nearly rectangular shapes without obvious redox peaks even at a high scan rate of 500 mV·s−1 (Figure S5), indicating a typical electrical double-layer capacitive behavior except for the pristine POT. Notably, besides the distinct ORR catalysis, E


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N content, and high concentration of quaternary N that prompts the superior electrochemical activities of NCMSs-900.

composition and nitrogen bonding configurations in NCMSs-T materials (Figure 5 and Figure S6). XPS spectra show the

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CONCLUSIONS In summary, we have prepared metal-free microporous nitrogen-doped carbon microspheres (NCMSs) by direct pyrolysis of poly(o-methylaniline) (POT) microspheres as carbon and nitrogen precursors. Microporous feature has been introduced into NCMSs materials during the high-temperature pyrolysis process, which greatly increases the specific surface areas. The NCMSs materials obtained at different pyrolysis temperatures are applied as metal-free carbon materials for oxygen reduction reaction (ORR) and supercapacitors. With highest surface area and quaternary N content, the sample obtained at 900 °C emerges not only as a superior ORR electrocatalyst with high selectivity and long stability, but also as an ideal supercapacitor material with high specific capacitance. This study not only provides a facile method to fabricate nitrogen-doped carbon materials based on the derivatives of conducting polymers, but also gives an insight for the application of carbon materials as ORR electrocatalyst in fuel cells and electrode material for supercapacitors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07865. Figures S1−S7 and Table S1 (PDF)

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Figure 5. (a) N 1s XPS spectrum of NCMSs-900; (b) evolution of N/ C atomic ratios and the content of different N species of NCMSs materials as a function of the pyrolysis temperature.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

presence of only carbon and nitrogen atoms, which confirmed the successful incorporation of N atoms into the as-prepared NCMSs-T materials by the transformation from POT. The bonding configurations of N atoms in NCMSs-T materials were characterized by high-resolution N 1s spectra, where six main peaks centered at 399.0 (N1), 400.3 (N2), 401.4 and 402.6 (N3), 403.8 and 405.3 eV (N4) can be attributed to pyridinic N, pyrrolic N, quaternary N and oxidized N (Figure 5a and S7).65,66 In order to assess the type and level of N-doping with the change of pyrolysis temperature, the content of each type of N species was calculated (Figure 5b). With the increase in pyrolysis temperature, the total content of N atoms decreases from 4.35% for NCMSs-700, 3.25% for NCMSs-800, 1.86% for NCMSs-900, to 1.08% for NCMSs-1000. And the N/C atomic ratio significantly decreases from 0.057 to 0.011 as the pyrolysis temperature increases from 700 to 1000 °C (see the magenta line in Figure 5b), indicating the loss of total content of N with higher pyrolysis temperature. Remarkably, with increase in the pyrolysis temperature, quaternary N and oxidized N became dominant, accompanied by the reduced content of pyridinic N and pyrrolic N because of their low thermal stability. As reported, pyridinic N and pyrrolic N may also be transformed into quaternary N and oxidized N during the high-temperature pyrolysis process.66 Furthermore, it has been demonstrated that quaternary N could not only greatly increase the limiting current density for ORR, but also enhance the electronic conductivity and initial active sites in pseudocapacitive interactions, while other N species such as pyrrolic N or oxidized N had little effect on the electrochemical performance of carbon materials.41,55,66 As a result, we believe that it is the synergistic effect between higher BET surface area, tuned total

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support from NSFC (21471039, 21203045), Fundamental Research Funds for the Central Universities (Grant HIT. NSRIF. 2010065 and 2011017, PIRS of HIT A201502 and HIT. BRETIII. 201223), China Postdoctoral Science Foundation (2014M560253), Postdoctoral Scientific Research Fund of Heilongjiang Province (LBHQ14062, LBH-Z14076), Natural Science Foundation of Heilongjiang Province (B2015001), Open Project Program of Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin Normal University, China (PEBM 201306), Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (ES201411), and Open Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201401).

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REFERENCES

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Bifunctional Nitrogen-Doped Microporous Carbon Microspheres

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