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Oct 7, 2016 - M−N/C (M = Fe, Co, Ni, or Mn) catalysts also produces “NC- type” active sites that are ... That is to say, there are two kinds of ...
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Porous N‑Doped Carbon Prepared from Triazine-Based Polypyrrole Network: A Highly Efficient Metal-Free Catalyst for Oxygen Reduction Reaction in Alkaline Electrolytes Mei Yang, Yijiang Liu, Hongbiao Chen,* Duanguang Yang, and Huaming Li*

ACS Appl. Mater. Interfaces 2016.8:28615-28623. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/07/19. For personal use only.

College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province P. R. China S Supporting Information *

ABSTRACT: Metal-free N-doped carbon (NC) materials have been regarded as one of the most promising catalysts for the oxygen reduction reaction (ORR) in alkaline media because of their outstanding ORR catalytic activity, high stability, and good methanol tolerance. Up to now, only a small minority of such catalysts have been synthesized from triazine-based polymeric networks. Herein, we report the synthesis of such NC catalyst by directly pyrolyzing a nitrogenrich, triazine-based polypyrrole network (TPN). The TPN is fabricated by oxidative polymerization of 2,4,6-tripyrrol-1,3,5triazine monomer using TfOH as the protonating agent and benzoyl peroxide as the oxidizing agent. The obtained NC-900 (pyrolyzed at 900 °C) catalyst exhibits excellent ORR activity in alkaline media with a high ORR onset potential (0.972 V vs RHE), a large kinetic-limiting current density (15.66 mA cm−2 at 0.60 V), and good MeOH tolerance and durability. The assynthesized NC-900 material is a potential candidate as a highly active, stable, and low-cost ORR catalyst for alkaline fuel cells. KEYWORDS: polymeric networks, porous carbon, dope, netal-free catalysts, ORR, fuel cells

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cyanamide,26 urea,27 ethylenediamine,28 etc., have been adopted. The last approach involves direct pyrolysis of N-rich polymers such as polyacrylonitrile (PAN),29,30 polyaniline (PANI),29,31 polypyrrole (PPy),29 and melamine−formaldehyde (MF) resin.32 In most cases, however, the as-claimed metal-free NC catalysts contain metal impurities, for example, iron residues in CNTs prepared by chemical vapor deposition,33,34 manganese residues in graphene oxide prepared by Hummer’s method,35 and FeCl3 in PPy and PANI prepared by oxidative polymerization.36,37 Although the metal residues were later removed by mineral acids leaching, the complete removal of metal residues is impossible. For instance, Jurkschat et al. have confirmed that even if the washing time of CNTs in 2 M HNO3 was extended to 36 h (namely “superwashing”), iron impurities could not completely removed.38 The residual metals, especially Fe, even at trace levels that are undetectable by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) can significantly promote ORR activity.39,40 Meanwhile, the preparation of the so-called M−N/C (M = Fe, Co, Ni, or Mn) catalysts also produces “NCtype” active sites that are analogous to the metal-free NC catalysts. That is to say, there are two kinds of active sites in the M−N/C catalysts: M−Nx active sites and “NC-type” sites. However, it is difficult to evaluate their respective contributions

INTRODUCTION The exploitation of novel energy-storage technologies is of pivotal importance to alleviate the pressures brought about by the energy crisis.1 Fuel cells (FCs) are just in response to this need, came into being. Although FCs are currently perceived as one of the most attractive energy conversion devices,2−4 they require a cathode catalyst for the oxygen reduction reaction (ORR). Currently, platinum (Pt) and its alloys are recognized as the best catalysts toward ORR. Unfortunately, these precious-metal-based catalysts usually suffer from high cost, prone to MeOH/CO poisoning, sluggish ORR kinetics, and inadequate durability, which obstruct the extensive utilization of FCs.5−8 Aiming at these problems, the global research priorities have been committed to seeking other non-Pt catalysts that can promote the ORR. Among alternative catalysts, nitrogen-doped carbon (NC) materials appear to be the most promising metalfree catalysts for ORR in terms of high catalytic activity, low cost, and good durability.9−12 Current methods for the preparation of NC catalysts can ordinarily be divided into three categories, that is, direct growth, postsynthesis, and direct pyrolysis. The direct-growth method involves in situ N doping on carbon framework during the carbon growth process, which is commonly used for synthesizing N-doped carbon nanotubes (CNTs) and carbon nanofibers (CNFs).13−17 The postsynthesis approach attempts to heat-treat carbons (CNTs, graphenes, carbon blacks) with N-enriched precursors.18−22 A variety of N-rich small molecules such as ammonia,23 ammonium fluoride,24,25 melamine,25 © 2016 American Chemical Society

Received: August 5, 2016 Accepted: October 7, 2016 Published: October 7, 2016 28615

DOI: 10.1021/acsami.6b09811 ACS Appl. Mater. Interfaces 2016, 8, 28615−28623

Research Article

ACS Applied Materials & Interfaces

The NC catalysts were synthesized by directly pyrolyzing the TPN (200 mg) at 800−1000 °C for 2 h under flowing argon atmosphere. The heating rate was fixed at 4 °C min−1, and the products were labeled as NC-800, NC-900, and NC-1000. To highlight the importance of oxidative polymerization, the TPT monomer was directly pyrolyzed at 900 °C (the optimal pyrolysis temperature for the TPN) for 2 h, and the product was labeled as TPT-900. The electrochemical results confirmed that the ORR activity of the TPT900 sample was much lower than that of the NC-900 catalyst (Figure S1, see Supporting Information). Moreover, the yield of carbon in the monomer pyrolysis (0.7 wt %) was significantly lower than that in the TPN pyrolysis (25.8 wt %) because the monomer is easy to sublimate. The elemental composition and electrochemical performance of the NC-900 catalyst are listed in Table 1.

toward ORR exactly. In this sense, it is of great importance to prepare truly metal-free NC catalysts without applying any metal-containing raw materials throughout the entire synthesis procedure. Recently, the development of porous organic networks (PONs) offers an excellent chance for the controllable construction of the NC catalysts. The advantages of PONs include large specific surface areas and abundant pore distributions, which are favorable to increase the catalytic active sites. Moreover, the N atoms can be controllably introduced into the networks by using designed N-containing building blocks. The fixation of the N atoms within the rigid polymeric networks together with the high skeletal strength further offers an attractive route toward creating the selfsupported NC materials uniformly doped with N element. Up to now, however, only minorities of the N-enriched PONs were used to prepare such NC catalysts.41−44 Therefore, it is really a challenge to prepare novel N-enriched PONs for the production of the NC catalysts. On the basis of the above analysis, we herein developed a facile approach to fabricate metal-free NC catalyst by directly pyrolyzing an N-rich, triazine-based polypyrrole network (TPN). We chose N-rich 2,4,6-tripyrrol-1,3,5-triazine (TPT) monomer to prepare TPN. The TPT was oxidative polymerized in the presence of TfOH using benzoyl peroxide (BPO) as the oxidizing agent, in which TfOH was used to protonate TPT. Conversely, the common strong organic oxyacids such as p-toluenesulfonic acid cannot induce the polymerization. The advantages of using BPO and TfOH are that the polymerization can be completed within 1 h and that can completely dispense with the use of any metal precursors. In this manner, truly metal-free NC catalyst can be fabricated by directly pyrolyzing the obtained TPN. Our synthesis strategy offers a chance to investigate the nature of catalytic sites for ORR in a “clean” trinary system, with only carbon, oxygen, and nitrogen elements. Compared with the commercial Pt/C catalyst, the as-synthesized NC-900 (pyrolyzed at 900 °C) catalyst displays a slightly higher ORR activity in alkaline media with the onset potential of 0.972 V (vs RHE), half-wave potential of 0.855 V (vs RHE), and limiting current density of 5.05 mA cm−2. Considering that TPT is easily synthesized in multigram quantities at a price less than U.S. $1.2 per gram (Table S1, see Supporting Information), the as-prepared NC900 catalyst (U.S. $7.5 per gram) is much cheaper than the commercial 20 wt % Pt/C catalyst (Johnson Matthey, U.S. $97.3 per gram). Such low cost and remarkable electrochemical performance make this NC material as a promising candidate for feasible ORR catalyst.

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Table 1. ORR Performance of the NC-900 and Pt/C Catalysts in O2-Saturated 0.1 M KOH Solution

a

samples

E0 (V)

E1/2 (V)

JL (mA cm−2)

Jka (mA cm−2)

na

Pt/C NC-900

0.965 0.972

0.845 0.855

5.26 5.05

17.54 15.66

3.99 3.98

Estimated by the K−L plots at 0.60 V (vs RHE).

Electrochemical Measurements. A CHI760D electrochemical workstation (Shanghai Chenhua Co., China) with a three-electrode system was used to carry out the electrochemical tests. A graphite electrode was used as the counter electrode, and an Ag/AgCl (KClsaturated) electrode was used as the reference electrode. All measured potentials were converted to the reversible hydrogen electrode (RHE) scale via the Nernst equation45 (see Supporting Information). The glassy carbon electrode (area = 0.196 cm2) was used as the working electrode. For the NC-900 catalyst, the mass of catalyst loaded onto the glassy carbon electrode is 357 μg cm−2. For the commercial 20 wt % Pt/C catalyst (Johnson Matthey, hereafter denoted as “Pt/C”), the mass of Pt/C catalyst loaded onto the electrode is 100 μg cm−2, corresponding to the metal Pt loading of 20 μg Pt cm−2. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) techniques on rotating disk electrode (RDE, Pine Research Instrumentation) as well as LSV technique on rotating ring disk electrode (RRDE, Pine Research Instrumentation) were carried out in O2 (or N2)-saturated 0.1 M KOH aqueous solution at 25 °C.

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RESULTS AND DISCUSSION Preparation and Characterization of Metal-Free NC Catalysts. In our previous article,46 we reported the preparation of self-supported, Fe/N-co-doped carbon (Fe− N/C) as the efficient ORR catalyst through pyrolysis of TPTbased PON and Fe(II) acetate mixture. The PON was fabricated by the FeCl3-catalyzed Friedel−Crafts polymerization of TPT with dimethoxymethane as the cross-linking agent. The obtained Fe−N/C catalyst showed a promising catalytic activity toward ORR in alkaline electrolytes, in which the Fe−Nx species was found to serve as the predominant active centers for ORR. In an attempt to evaluate the contribution of iron residues to the ORR performance, we try to remove the Fe residues completely. The as-synthesized Fe−N/C electrocatalyst was thus washed in aqueous solution of H2SO4 (0.5 M) at 80 °C for up to 1 week. It was found that the Fe content was first declined but then leveled off, indicating that sulfuric acid leaching cannot completely remove all the Fe residues from carbons, peculiarly Fe embedded in graphitic layers. Considering that the preparation of the M−N/C catalysts also produces nitrogenized groups as in the case of NC catalysts, it is difficult to assess their respective contributions toward ORR because some nitrogenized groups are also ORR active. From this point of view, it is extremely

EXPERIMENTAL SECTION

Synthesis of TPN and Metal-Free NC Catalysts. TPT was prepared according to the procedure reported previously by our group.22 For the synthesis of TPN, TPT (2.76 g, 10.0 mmol) and TfOH (9.00 g, 60.0 mmol) were dissolved in dichloroethane (100 mL). The reaction mixture was heated to 80 °C followed by the addition of BPO (8.72 g, 36.0 mmol). The polymerization was conducted under stirring for 1 h at this temperature. After cooling to room temperature, the polymer (TPN salt) was filtered, washed with water (3 × 200 mL), and acetone (3 × 100 mL). The TPN was further purified by dispersing the TPN salt (4.0 g) in NaOH aqueous solution (1.0 M, 200 mL). After stirring at ambient temperature for 12 h, the base TPN was filtered, washed with water until pH = 7, and finally dried under vacuum. 28616

DOI: 10.1021/acsami.6b09811 ACS Appl. Mater. Interfaces 2016, 8, 28615−28623

Research Article

ACS Applied Materials & Interfaces

Figure 1. SEM images of the TPN precursor (a) and the NC-900 catalyst (b) (the inset depicts a single spherical particle). TEM (c) and HR-TEM (d) pictures of the NC-900 catalyst.

Figure 2. Nitrogen sorption isotherms at 77 K (a) and DFT pore size distribution curve (b) of the NC-900 catalyst. Raman (c) and high-resolution N 1s XPS (d) spectra of the NC-900 catalyst.

desirable to prepare truly metal-free NC catalysts dispensing with the need for any metallic precursors as mentioned above. In this paper, the TPN was fabricated by oxidative polymerization of TPT monomer using TfOH as the protonating agent and BPO as the oxidizing agent (Scheme S1, see Supporting Information), which would dispense with the use of any metal precursors. The most probable polymerization mechanism involves protonation of TPT by TfOH, followed by oxidation of protonated pyrrole rings in the TPT with benzoyloxy radicals, leading to the formation of

pyrrole radical cations. Such radical cations undergo dimerization followed by deprotonation to form pyrrole dimer. The bipyrrole is then protonated and oxidized in a similar manner and combines with another oxidized species. In the next growth step, protonation, reoxidation, combination, and deprotonation go on to produce TPN eventually (Scheme S2).47−49 The metal-free NC catalysts were then synthesized by the pyrolysis of the TPN from 800 to 1000 °C under flowing Ar; the products are labeled as NC-800, NC-900, and NC-1000. The electrochemical results showed that the NC-900 catalyst 28617

DOI: 10.1021/acsami.6b09811 ACS Appl. Mater. Interfaces 2016, 8, 28615−28623

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a) CVs of the NC-900 and Pt/C catalysts in N2- and O2-saturated 0.1 M KOH solution with a scan rate of 50 mV s−1. (b, c) LSV curves of the NC-900 catalyst in N2- and O2-saturated 0.1 M KOH solution with various rotation speeds at a sweep rate of 10 mV s−1 (b) and LSVs of the NC-900 and Pt/C catalysts in O2-saturated 0.1 M KOH solution with the rotation speed and sweep rate of 1600 rpm and 10 mV s−1, respectively (c; the inset of (c) shows the K−L plots of the NC-900 and Pt/C catalyst at 0.60 V). (d) The Jk and n involved in the ORR of the NC-900 and Pt/C catalysts at 0.65, 0.60, and 0.55 V (vs RHE).

NC-900 catalyst possesses an ID/IG value of 0.98, indicating that the extent of graphitization is very low. The wide-scan XPS spectrum of the NC-900 catalyst (Figure S5) demonstrates the existence of three kinds of elements: carbon (90.5%), nitrogen (3.02%), and oxygen (6.48%). Notably, the abnormal high O content is unlikely from adsorption as exposed to air. But it could be understood considering the radical coupling reaction between benzoyloxy radicals and the propagating PPy radical cations. At the final stage of the polymerization, the propagating PPy radical cations become insoluble or on the verge of insolubility in solvent. Combination of propagating PPy radical cations via their radical ends becomes progressively more difficult. In contrast, the cross-coupling reaction between benzoyloxy radicals and the growing PPy radical cations becomes predominant owing to the high concentration and easy movement of the benzoyloxy radicals. Such reaction can generate ester group in the TPN polymer, which is evident by the appearance of ester CO absorption band at 1728 cm−1 in the FTIR spectrum of TPN (Figure S6). Supporting evidence comes from elemental analysis of the TPN polymer, which shows an atomic composition of 57.33% C, 4.53% H, 19.24% N, and 18.9% O (Table S3). The deconvoluted C 1s spectrum (Figure S7) shows four peaks at 284.7 ± 0.2 eV (CC−C), 285.6 ± 0.2 eV (C−N/ C−O), 288.1 ± 0.2 eV (CO), and 291.1 ± 0.2 eV (π−π*). Similarly, the deconvoluted O 1s spectrum (Figure S8) also shows four peaks at 531.1 ± 0.1 eV (CO), 532.4 ± 0.1 eV (C−O), 533.5 ± 0.1 eV (C−OH), and 536.1 ± 0.1 eV (adsorbed H2O or O2).31 The deconvoluted N 1s XPS spectrum in Figure 2d again displays four peaks at 398.6 ± 0.1 eV (pyridinic-N), 400.3 ± 0.1 eV (pyrrolic-N), 401.2 ± 0.1 eV (graphitic-N), and 403.7 ± 0.1 eV (pyridinic-N+−O−),54,55

displayed the maximum catalytic activity toward ORR (Figures S2 and S3); we therefore focus on the NC-900 catalyst in the following research. Figure 1a shows the SEM images of the TPN precursor. As can be seen, the TPN polymer exhibits an irregularly spherical architecture with diameter ranging from 130 to 750 nm. In addition, aggregated particles can be clearly seen. After pyrolysis, the spherical morphology is still maintained, but the surface becomes rough and porous (inset of Figure 1b). Figure 1c depicts the TEM images of the as-prepared NC-900 catalyst. In agreement with the SEM observation, agglomerated spherical particles are observed again. The high-magnification TEM image in Figure 1d reveals that the NC-900 catalyst is basically amorphous carbon. The porosity of the NC-900 catalyst was investigated by N2 sorption analysis. The BET surface area of TPN precursor is 100 m2 g−1 (Figure S4a). After pyrolysis, the BET surface area of the NC-900 catalyst increases greatly and reaches up to 779 m2 g−1 (Figure 2a). According to the IUPAC classification, the NC-900 sample exhibits a type-IV isotherm with a type-H4 hysteresis loop at p/p0 = 0.45. The appearance of type-H4 hysteresis loop is indicative of the presence of mesopores.50,51 Figure 2b depicts the pore size distribution (PSD) for the NC900 catalyst calculated by density functional theory (DFT) method. It is found that the porosity of the NC-900 catalyst is consisted of a large proportion of mesopores and a very small proportion of macropores. The Raman spectrum of the NC-900 catalyst shown in Figure 2c exhibits the ordered graphitic carbon band at 1589 cm−1 (G band) and defect band at 1336 cm−1 (D band).52,53 The ratio of intensity of D/G bands (ID/IG) can give an quantitative estimation of the degree of graphitization. The 28618

DOI: 10.1021/acsami.6b09811 ACS Appl. Mater. Interfaces 2016, 8, 28615−28623

Research Article

ACS Applied Materials & Interfaces

Figure 4. RRDE voltammograms of the NC-900 (a) and Pt/C (b) catalysts at 900 rpm in O2-saturated 0.1 M KOH with a scan rate of 10 mV s−1. Peroxide yield (black) and n (red) of the NC-900 (c) and Pt/C (d) catalysts derived from RRDE measurement.

NC-900 catalyst, their ORR current densities are also compared in terms of mass activity (Table S7). The Tafel slope of the NC-900 catalyst in the potential range of 0.95−0.85 V (vs RHE) is ca. 97 mV decade−1, which is also comparable to the Pt/C catalyst (ca. 95 mV decade−1, Figure S10). This demonstrates that the ORR kinetics on the NC-900 catalyst resembles that on the Pt/C catalyst. With respect to the E0, E1/2, and JL values, the ORR activity of the NC-900 catalyst is considerably higher than most recently reported NC catalysts but is still inferior to some M-N/C catalysts reported to date (Table S9). LSVs for the NC-900 catalyst with varying catalyst loading were also performed at 1600 rpm in O2-saturated alkaline media to investigate the influence of catalyst loading on its activity toward ORR. As depicted in Figure S11, the E0, E1/2, and JL values measured with the NC-900 at a loading of 257 μg cm−2 decreased obviously compared to that of 357 μg cm−2 loading. When the loading of NC-900 catalyst increased to 457 μg cm−2, the E0 value remained almost unchanged although the JL value increased slightly, indicating that the ORR catalytic activity relied on the mass loading of catalyst but had an optimal catalyst loading. To understand the catalytic mechanism of ORR on the NC900 catalyst, LSV measurements on a RDE were further carried out in O2-saturated alkaline solution with different rotation rates. The Koutecky−Levich (K−L)56 profiles (J−1 vs ω−1/2) of the NC-900 and Pt/C catalysts were compared at 0.6 V (vs RHE) and other potentials (inset of Figure 3c; Figure S13). All K−L plots are linear regardless of the potentials. Furthermore, the value of kinetic current density (JK) can be estimated from the intercepts of the linear K−L plots in O2-saturated alkaline electrolyte. As shown in Figure 3d, the corresponding JK value of the NC-900 catalyst is 15.66 mA cm−2 at 0.60 V (vs RHE), close to that of the Pt/C catalyst (17.54 mA cm−2). This may be due to the high specific surface area and abundant mesoporous structure of the NC-900 catalyst, which led to a

and these four kinds of N atoms are found to be 26.18%, 16.45%, 38.85%, and 18.52%, respectively. The high graphitic-N content (38.85%) endows the catalyst with enhanced electrical conductivity. To prove this, the electrical conductivity of the NC-900 catalyst was studied using a four probe measurement and was found to be 3.4 S cm−1. Furthermore, the NC-900 sample also showed small internal resistance (0.36 Ω) and charge transfer resistance (0.66 Ω) as determined by EIS measurement (Figure S9). Such low resistance as well as N and O codoping will play a very important role for its high ORR activity. ORR Activity. Cyclic voltammograms (CV) of the NC-900 catalyst in N2- or O2-saturated alkaline media (0.1 M KOH solution) were initially investigated to evaluate the ORR activity. In N2-saturated alkaline solution, no redox peaks can be observed for the NC-900 catalyst (Figure 3a). In contrast, a distinct ORR peak was observed for the NC-900 catalyst when O2 was introduced. The peak potential of the NC-900 catalyst is 0.809 ± 0.005 V (vs RHE), which is slightly positive compared to the Pt/C catalyst (0.805 ± 0.005 V vs RHE). In addition, the NC-900 catalyst has a higher ORR peak current density (3.00 ± 0.02 mA cm−2) than the Pt/C catalyst (2.40 ± 0.02 mA cm−2). Linear sweep voltammograms (LSVs) for the NC-900 catalyst in O2-saturated alkaline solution were then performed to further evaluate its ORR performance. All LSV curves present in the figure have been corrected by removing the background signals obtained in N2-saturated alkaline electrolyte. The ORR onset (E0) and half-wave (E1/2) potentials for the NC-900 catalyst shown in Figure 3c were found to be 0.972 and 0.855 V (vs RHE), respectively, which were comparable to the Pt/C catalyst (E0 = 0.965 V, E1/2 = 0.845 V vs RHE). Additionally, the limiting current density (JL) of the NC-900 catalyst is about 5.05 mA cm−2 in KOH solution, which is comparable to the Pt/C catalyst (5.26 mA cm−2). Considering that the loading for the Pt/C catalyst is less than that for the 28619

DOI: 10.1021/acsami.6b09811 ACS Appl. Mater. Interfaces 2016, 8, 28615−28623

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ACS Applied Materials & Interfaces

Figure 5. ORR polarization plots measured with the NC-900 (a) and Pt/C (b) catalysts during cycling durability tests in O2-saturated 0.1 M KOH solution; inset of (a) shows MeOH-poison effect evaluation on the current−time (i−t) chronoamperometric response, and inset of (b) shows durability evaluation on the current−time (i−t) chronoamperometric response for 10 000 s. For all tests, the rotation rate is 1600 rpm and the scan rate is 10 mV s−1.

and O/N atoms (C−O/C−N) are 11.45% and 23.00%, respectively, in which both CO and C−O groups should be ORR active as mentioned above. Furthermore, the high level of C−O/C−N groups indicates that the NC-900 catalyst contains plenty of C−OH groups, many of them are neighbored by the N-containing species because the N content is only ca. 3%. That is to say, pyridone groups must exist in the NC-900 catalyst and should be included in the peak of pyrrolicN in the high-resolution N 1s XPS spectrum (Figure 2d).63 Note that pyridone is also proposed as the ORR active site owing to the fact that it can undergo tautomerization with hydroxypyridine, which can stabilize singlet dioxygen through the formation of oxygen adduct, as demonstrated by Asefa and co-workers.30,64 Finally, the relatively high surface area (779 m2 g−1) and predominant mesostructure of the NC-900 catalyst have an almost direct impact on the ORR activity because they provide abundant ORR active sites and be beneficial to the mass transfer of O2. All in all, the excellent ORR activity of the NC-900 catalyst is virtually a collective property of all beneficial effects mentioned above and can hardly be separated. To investigate the MeOH crossover effect and durability of the NC-900 catalyst, we carried out chronoamperometric measurements in O2-saturated alkaline solution. It is found that the corresponding amperometric response for the NC-900 catalyst has stayed the same after adding MeOH (3 M) at 145 s (inset of Figure 5a), showing a remarkably good tolerance to crossover effects. In contrast, the Pt/C catalyst suffers a sharp decrease, demonstrating that the Pt/C catalyst is prone to MeOH poisoning. The durability evaluation on the i−t chronoamperometric response was further performed on the NC-900 and Pt/C catalysts for 10 000 s (inset of Figure 5b). It was found that the NC-900 catalyst exhibited better long-term stability than the Pt/C catalyst. The electrochemical stabilities of the NC-900 and Pt/C catalysts were then evaluated by endurance tests for 3000 cycles in O2-saturated alkaline solution with the rotation speed and scan rate of 1600 rpm and 50 mV s−1, respectively. After 3000 cycles, the E1/2 shifted negatively by 5.0 mV (vs RHE) for the NC-900 catalyst (Figure 5a). For the Pt/C catalyst (Figure 5b), however, the corresponding value was changed to 10 mV (vs RHE), implying that the NC-900 catalyst has a very fine application prospects in FCs.

rapid electron transfer. The average electron transfer numbers (n) of the NC-900 catalyst are calculated to be ca. 3.98 in alkaline solution, indicating that the ORR process on the NC900 catalyst is dominated by a 4e reduction pathway. Further evidence of the 4e reduction pathway comes from the RRDE tests, in which the n and the H2O2 generation rate of the NC-900 catalyst can be determined. The disk current (id) and the ring current (ir) response on the NC-900 and Pt/C catalysts at 900 rpm in 0.1 M KOH solution are depicted in Figure 4a,b. The corresponding H2O2 yield (%) and n are shown in Figure 4c,d. For the NC-900 catalyst, obviously, the H2O2 yield is less than 10%, and the average n value is 3.92, which are very close to the Pt/C catalyst. The average n value of the NC-900 catalyst determined by RRDE tests (3.92) is almost equal to that estimated by K−L plots (3.98), again demonstrating good selectivity toward the 4e pathway. Based on the above research results, the excellent ORR activity of the NC-900 catalyst is possibly due to the following reasons. First, the relative amount of N-based functional groups detected on the NC-900 catalyst by XPS is pyridinic-N (26.18%), pyrrolic-N (16.45%), graphitic-N (38.85%), and pyridinic-N+−O− (18.52%) (Table S5) as mentioned before, in which both pyridinic-N and graphitic-N have been suggested as the ORR active sites.57,58 Clearly, the total content of ORR active N (pyridinic-N + graphitic-N) can reach up to 65.03% in the NC-900 catalyst. The high proportion of graphitic-N can also favor 4e process in ORR as confirmed by previous studies.30,59 Second, the relatively high O content (6.48%) of the NC-900 catalyst is at least part of the reason causing its high ORR activity because O-containing groups, especially the quinone groups, can facilitate O2 adsorption and its subsequent reduction.60,61 As a result, the catalytic effects of the Ofunctionalities (such as carboxyl, carbonyl, and hydroxyl groups) on the CNFs surface toward ORR were recently confirmed by Niu and Zhang et al.,62 in which they found that the carboxylic acid groups have the highest catalytic activity, followed by the carbonyl groups and the hydroxyl groups. Interestingly, further N-functionalization of the O-modified CNFs can change the ORR pathway from a two-plateau 2e to a one-plateau 4e reduction due to the coexistence of O- and Ncontaining groups in CNFs.62 In our case, the synergistic influence of N- and O-containing functionalities was also investigated by analyzing the deconvoluted C 1s spectrum of the NC-900 catalyst (Figure S7 and Table S4). The relative contents of C atoms that directly bonded to O atoms (CO) 28620

DOI: 10.1021/acsami.6b09811 ACS Appl. Mater. Interfaces 2016, 8, 28615−28623

Research Article

ACS Applied Materials & Interfaces

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CONCLUSION In summary, we have successfully developed a facile and economic method for synthesis of metal-free NC catalyst toward ORR by simply pyrolyzing polymeric network (TPN) in Ar. The TPN was fabricated by oxidation polymerization of TPT monomer using TfOH as the protonating agent and BPO as the oxidizing agent. Our synthesis strategy offers a chance to investigate the nature of catalytic sites for ORR in a “clean” trinary system, with only carbon, oxygen, and nitrogen elements. The as-prepared NC-900 catalyst possessed a mesoporous structure and a relatively high nitrogen (3.02 at. %) and oxygen (6.48 at. %) contents. This novel metal-free catalyst exhibited excellent ORR activity as well as high selectivity toward a 4e reduction pathway in alkaline media, which places it among the best metal-free NC catalysts for ORR reported to date.

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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.6b09811. Schematic illustration for the preparation of TPN; possible mechanism for the TPT polymerization; N2 sorption isotherms and PSD for TPN; FT-IR spectrum of TPN; wide-scan XPS, and deconvoluted C 1s and O 1s spectra for NC-900 catalyst; LSV, CV, and Nyquist plots for NC-800, NC-900, and NC-1000 catalysts; LSVs of NC-900 catalyst with different mass loadings; Tafel plots for NC-900 and Pt/C catalysts; LSVs of Pt/C at different rotation speeds; K−L profiles of NC-900 and Pt/C catalysts at different potentials; cost estimation for NC-900 catalyst; ORR performances of various ORR catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel +86 731 58298572; Fax +86 731 58293264 (H.L.). *E-mail [email protected] (H.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Program for NSFC (51272219, 51674219), RFDP (20124301110006), and the Construct Program of the Key Discipline in Hunan Province is greatly acknowledged.

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REFERENCES

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