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Nitrogen doped carbon nanotubes with encapsulated ferric carbide as excellent electrocatalyst for oxygen reduction reaction in acid and alkaline media
Journal of Power Sources 286 (2015) 495e503
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Nitrogen doped carbon nanotubes with encapsulated ferric carbide as excellent electrocatalyst for oxygen reduction reaction in acid and alkaline media Guoyu Zhong, Hongjuan Wang*, Hao Yu, Feng Peng* School of Chemistry and Chemical Engineering, Key Laboratory of Fuel Cell Technology of Guangdong Province, South China University of Technology, Guangzhou 510640, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
N-doped carbon nanotube with encapsulated Fe3C(Fe3C@NCNT) is prepared by pyrolysis. Fe3C@NCNT shows good O2 reduction reaction (ORR) nature in acid and alkali media. Fe3C is encapsulated in the interior and N is distributed on the outside of CNTs. The inner Fe3C with outside C form synergetic active sites to enhance ORR activity. A direct four electron ORR path on Fe3C@NCNT is proved in acid and alkali media.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2015 Received in revised form 6 March 2015 Accepted 4 April 2015 Available online 6 April 2015
Nitrogen doped carbon nanotubes (NCNTs) with encapsulated Fe3C nanoparticles (Fe3C@NCNTs) are synthesized by a simple direct pyrolysis of melamine and ferric chloride. The characterization results reveal that Fe3C is mainly encapsulated in the interior of NCNTs and N species is mainly distributed on the outside surface of NCNTs. Iron and iron carbide catalyze the growth of NCNTs and are wrapped by carbon to form Fe3C@NCNTs. The as-prepared Fe3C@NCNTs catalyst exhibits superior oxygen reduction reaction (ORR) activity, excellent methanol tolerance and long-term stability in both acid and alkaline media. It is proven that the doped N is the main active site for ORR and the inner Fe3C with outside carbon form the synergetic active site to enhance ORR activity. The ORR mechanism of direct four electron transfer pathway is proved in acid and alkaline media. © 2015 Elsevier B.V. All rights reserved.
Keywords: Fuel cells Oxygen reduction reaction Carbon nanotubes Ferric carbide Active sites Catalytic mechanism
1. Introduction
* Corresponding authors. E-mail address: [email protected] (F. Peng). http://dx.doi.org/10.1016/j.jpowsour.2015.04.021 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Proton exchange membrane fuel cell (PEMFC) that directly converts chemical energy into electrical energy is a clean and highefficiency energy conversion system. However, the high price and
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the shortage resource of Pt hinder the commercialization of PEMFC [1]. Furthermore, Pt-based catalysts are subject to dissolution, aggregation, and poisoning under the reaction conditions, thus reducing both the active surface area and the catalytic efficiency, which leads to an increase in over potentials for fuelecell reactions, especially for the critical but sluggish oxygen reduction reaction (ORR) [2,3]. Therefore, many efforts have been devoted to developing the non-precious metals catalysts, such as, carbon-supported iron/nitrogen compounds (FeNx/C) and even non-metal N-doped carbon catalysts (NxC) [4e7]. FeNx/C catalysts, synthesized by pyrolyzing the mixture of carbon, nitrogen precursors and iron salts under shielding gas or NH3 [4], exhibit comparable ORR activity to that of commercial Pt/C. However, the stability of FeNx/C catalysts is poor in acid, due to the dissolution of FeeN complexes [8,9]. NxC materials can be considered as another alternative to Pt due to their good activity and high stability in alkaline solution [5,10,11]. But their activity is still very low in acid media, which is not competent for PEMFC application. Thus, it is highly challenging but extremely desirable to develop carbon-based catalyst with high ORR activity and stability in acidic medium. Recently, a new type of pod-like carbon nanotubes (CNTs) with encapsulated iron has been prepared and used as ORR catalyst in acid media by Bao [12]. They found that the electron transfer from Fe particles to the CNTs decreased local work function of the carbon surface. As a result, the catalyst showed a significantly enhanced ORR activity than CNTs without encapsulated Fe. Li et al. [13] also found that graphite carbon with encased Fe3C showed good ORR activity and high stability in acid and alkaline media. To the author's knowledge, there are two indistinct problems for this kind of carbon catalysts with encapsulated iron species (Fe@C). First, the effect of the surface metallic functionalities on ORR has not been determined. Second, the mechanism of catalytic reaction is unclear. Hence, it is necessary and worthwhile to take effort on research of Fe@C for ORR. Herein, a novel N-doped carbon nanotubes with encapsulated Fe3C nanoparticles (Fe3C@NCNTs) was synthesized as ORR catalysts. Their morphologies, composition and growth mechanism were systematically investigated. Compared with Pt/C catalyst, the as-synthesized Fe3C@NCNTs catalyst shows the excellent ORR activity, better methanol resistance and higher stability in both acid and alkaline media. The ORR active sites and catalytic mechanism were discussed in detail.
which was denoted as Fe3C@NCNTs-BM-HCl. The NCNTs were synthesized by a chemical vapor deposition [14]. In a typical experiment, 0.1 g FeMo/Al2O3 was put in the quartz tube. Then 10 mL aniline was injected in the quartz tube after the temperature reached to 800 C with 500 mL min1 of NH3 flow. After injection, the quartz tube was cooled down to room temperature and the resulting sample was collected from quartz boat. The as-prepared NCNTs were treated with 6 M HCl for 2 h to remove the metal impurities.
2. Experiment
2.3. Characterizations
2.1. Catalysts preparation
The surface structure and morphology of the catalysts were characterized by scanning electron microscope (SEM, 1530VP, LEO Co.) at 5 kV and transmission electron microscope (TEM, JEOL, JEM2010) at 200 kV. X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (D/max-IIIA, Japan) using Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG Scientific ESCALAB MK II analyzer using Al Ka radiation (1486.71 eV), and the C1s peak at 284.6 eV was taken as standard. The weight percentage of metals and carbon were measured by energy dispersive spectroscope (EDS, Oxford instruments) attached to a scanning electron microscope. The thermogravimetric analysis (TGA) was carried out on a NETZSCH TGASTA 409 PC analyzer.
The Fe3C@NCNTs catalyst was synthesized by simply pyrolyzing a mixture of ferric chloride and melamine without high pressure [12,13], followed by acid leaching, as illustrated in Fig. 1a. In a typical synthesis, 1.5 g FeCl3$6H2O and 1.5 g melamine were dissolved or dispersed uniformly in 100 mL ethanol by ultrasonication for 0.5 h. Then the mixture was dried at 110 C for 10 h. The dry samples were respectively heated to a preset temperature from 600 to 900 C at a rate of 10 C min1 in a flow of Ar (160 mL min1) and maintained at this temperature for 2 h. The resulting samples were washed in 6 M HCl for 12 h at room temperature to remove the exposed iron species, followed by washing with distilled water and ethanol, and dried under vacuum at 70 C for 10 h. These obtained samples were denoted as Fe3C@NCNTs-X (X indicates the pyrolyzing temperature, 600e900 C). Fe3C@NCNTs synthesized at 800 C was washed with hydrochloric acid (6 M) under different times, which was denoted as Fe3C@NCNTs-HCl-Y (Y indicated the leaching times, 0e4 h). Fe3C@NCNTs synthesized at 800 C was also treated by first ball-milling and then leaching in 6 M HCl for 12 h,
2.2. Electrochemical measurements Electrochemical measurements were performed at room temperature in a three-electrode cell connected to an electrochemical analyzer (CH Instruments 760D) and a rotating ring-disk electrode (pine Instrument Co.). Platinum wire and Ag/AgCl electrode with saturated KCl were served as counter electrode and reference electrode, respectively. The catalyst inks were prepared by ultrasonically dispersing 0.5e4 mg catalyst in a mixture solution containing 100 mL acetone, 385 mL deionized water and l5 mL 5% Nafion solution (DuPont). 15 mL catalyst ink was cast on the GC electrode surface and dried at room temperature in the air for 1 h. Rotating disk electrode (RDE) or rotating ring-disk electrode (RRDE) with the GC disk diameter of 5 mm were used to carry out cyclic voltammogram (CV) or linear scan voltammogram (LSV) in the oxygen saturated 0.1 M KOH solution from 1 V to 0.2 V or 1 M HClO4 from 0.2 V to 0.8 V at a scan rate of 10 mV s1. All the potentials used in this work are referenced to the Ag/AgCl reference electrode. In the methanol-tolerant experiment, the working electrode was immersed in 1 M HClO4 with 1 M CH3OH. Stability of the catalysts was evaluated based on the oxygen reduction peak current densities of CV (4 mm GC disk) at various cycle numbers in oxygen saturated 0.1 M KOH and 1 M HClO4 solution, respectively. Catalyst loading without special instruction was 152.9 mg cm2 for all catalysts in KOH solution, 611.5 mg cm2 for Fe3C@NCNTs and NCNTs in 1 M HClO4, and 152.9 mg cm2 for Pt/C in 1 M HClO4. The poisoning experiments were carried out by LSV at RDE in the oxygen saturated 1 M HClO4 solution with 5 mM of F, Cl, Br, and SCN ions, respectively, at a scan rate of 10 mV s1 and a rotation speed of 1600 rpm. Calculation methods of the transferred electron number (n) and H2O2 yield during ORR are described in supporting information.
3. Results and discussion 3.1. Structure characterization and growth mechanism The morphology of the typical catalyst Fe3C@NCNTs-800 was investigated by SEM (Fig. 1f and Fig. S1a, b) and TEM (Fig. 2b and
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Fig. 1. Schematic of (a) the synthesis route and (b) growth mechanism of Fe3C@NCNTs. SEM images of the samples prepared at (c) 600 C, (d) 680 C, (e) 700 C and (f) 800 C.
Fig. 2. TEM images of (a) Fe3C@NCNTs-700 and (b) Fe3C@NCNTs-800. (c) HRTEM image of Fe3C@NCNTs-800. (d) Elemental-mapping images of Fe3C@NCNTs-800.
Figs. S1c, d). Uniform nanotubes with diameters of 100e200 nm and with filled nanoparticles as the pea-pod structure reported by Bao [12] were observed. Compared with the pea-pod CNTs, the asprepared Fe3C@NCNTs show larger diameters, larger filled particles, abundance of microscopic defects, a lot of folds and no
compartment. The filled nanoparticles were further characterized by HRTEM analysis and elemental-mapping in Fig. 2c and d. The spacings of crystalline lattices in two directions are 0.195 nm and 0.186 nm with a characteristic angle of 76.9 , corresponding to the (112) and (221) planes of the Fe3C phase. XRD results further
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confirm that the nanoparticles encapsulated in the interior of nanotubes are dominantly Fe3C when the preparation temperature is above 680 C (Fig. 3a). All the samples may contain traces of metallic iron due to the same characteristic peak as that of Fe3C at 2q of 44.6 . It is also observed from HRTEM that the Fe3C nanoparticles were coated by not only the tube walls but also several clingy graphitic layers. The dual protection of clingy graphitic layers and tube walls stabilizes the Fe3C nanoparticles in acid solutions. Therefore, it is difficult to remove the Fe species encapsulated in carbon with the common way, such as ball-milling and leaching in hot acid [13]. As shown in SEM images (Figs. S1e, f), the CNTs were cut short and partially destroyed by ball-milling and leaching in HCl solution. The white nanoparticles in the CNTs confirm that Fe species has not been removed completely. In addition, the EDS result (Table S1) demonstrates that Fe content still remains 14 wt%. The bulk phase composition of the Fe3C@NCNTs catalyst was analyzed with EDS, as shown in Table S1. The results show that the content of nitrogen decreases with the pyrolysis temperature increasing, while the content of iron increases firstly with the pyrolysis temperature increasing until 650 C and then decreases. For Fe3C@NCNTs-800 sample, the contents of nitrogen and iron are 0.45 wt% and 22.47 wt%, respectively. TGA (Fig. S2a) reveals an iron content of 20.63 wt% in Fe3C@NCNTs-800, in good agreement with the EDS result, which is similar to the iron content of pea-pod CNTs [12] (18.1wt%) and iron carbide encased in carbon [13] (15.0 wt%). The surface composition of Fe3C@NCNTs was examined by XPS and the results are shown in Fig. 4 and Table S2. Contrary to the EDS results, the XPS results show that the surface contents of nitrogen and iron for Fe3C@NCNTs-800 are 6.16 wt% and 1.79 wt%, respectively. Incorporating the higher Fe content and lower N content by EDS and the less surface Fe content and more surface N content by XPS, it is reasonably concluded that the Fe in the form of Fe3C is mainly wrapped in the interior of CNTs and N species is mainly distributed on the outside surface of CNTs. Thus it is impossible to have large amounts of Fe-Nx species in the sample [15]. For the surface N in the samples, four different N-species can be assigned based on the XPS results (Fig. 4). Pyridinic-N and graphitic-N are the dominant N species for Fe3C@NCNTs-800 and Fe3C@NCNTs900. While for Fe3C@NCNTs-700 with the highest surface nitrogen content, the pyrrolic nitrogen content is much higher than that of the other two samples due to the low synthesis temperature [16]. In order to investigate the formation mechanism of the Fe3C@NCNTs, the samples prepared at different pyrolyzing temperatures were analyzed carefully. At 600 C, as shown in Fig. 1c and Table S1, the obtained sample is amorphous thick carbon sheets containing iron (40.26 wt%) and nitrogen (19.53 wt%), which is the graphite-like carbon nitride from melamine decomposition [17].
Increasing temperature to 650 C, iron species turns into iron oxide, Fe and Fe3C (Fig. 3a). Part of carbon separates out the basal surface with Fe3C nanoparticles. When the temperature reaches to 680 C, carbon further separates out, leading to carbon nanotubes encapsulated Fe3C nanoparticles (Fig. S3 and Fig. 1d). At 700 C, a large number of carbon nanotubes with encapsulated Fe3C are produced (Figs. 1e, 2a and 3a). High-quality carbon nanotubes with high graphitic degree and encapsulated Fe3C nanoparticles evenly dispersing in the interior of CNTs are formed at 800 C (Figs. 1f, 2b and 3a). When further increase the temperature to 900 C, the CNTs become no longer uniform in diameter and rougher on the surface with little Fe filling (Figs. S1g, h). The result demonstrates that the appropriate temperature for preparing Fe3C@NCNTs is 800 C. Based on the aforementioned discussion and general carbon nanotube growth process [18e21], the growth mechanism of Fe3C@NCNTs can be schematically represented in Fig. 1b, and the involved chemical reactions are displayed in supporting information. Prior to 600 C, melamine and ferric chloride are decomposed to form the thick graphite-like carbon nitride with encapsulated and supported amorphous Fe species [21]. From 600 to 650 C, amorphous Fe species is converted into iron oxide. Part of the iron oxide is firstly reduced into elemental Fe and further reacts with carbon nitride species to form Fe3C [21]. Amorphous carbon dissolves in metal phase and graphitic layers are formed by the carbon-through-metal diffusion [20]. Above 680 C, Fe2O3 is completely transformed into Fe and Fe3C, and graphitic layers begin to grow into carbon nanotubes [18,19]. Meanwhile, the hot liquidlike cementite particles axial move in graphitic layers due to the buildup of compressive stress during thickness growth of the graphitic tube wall, and due to increasing surface tensional forces between the core material and the tube [20]. Thereby, the carbon nanotubes with encapsulated Fe3C are formed. The hot liquid-like cementite particles possess an enormous capacity of carbon uptake which largely separates out when the particles are cooled down [20,22]. This leads to several clingy graphitic layers on Fe3C particles, as mentioned above. The formation mechanism of asprepared Fe3C@NCNTs is obviously different from pea-pod CNTs prepared by explosion method [12] and Fe-filled CNTs prepared by CVD [23]. The preparation method represented in this work is more simple, convenient and scalable for the practical application. 3.2. ORR activity and stability The ORR activities of Fe3C@NCNTs catalyst were evaluated and compared with those of the NCNTs and a commercial Pt/C catalyst (Pt: 20 wt%) by LSV and KouteckyeLevich (KeL) plots in acidic and alkaline solutions. In 0.1 M KOH (Fig. 5a), Fe3C@NCNTs-800 exhibits
Fig. 3. XRD patterns of Fe3C@NCNTs prepared at different temperatures (a) and treated by HCl with different times (b).
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Fig. 4. (a) XPS survey and high-resolution Fe 2p spectra (inset) of Fe3C@NCNTs samples, High-resolution N 1s spectra of Fe3C@NCNTs prepared at (b) 700, (c) 800 and (d) 900 C.
high onset potential of ca. 0.098 V (vs Ag/AgCl, the same below), half-wave potential of ca. 0.147 V and large limiting current of ca. 3.1 mA cm2, which is significantly superior to the NCNTs and even slightly better than the commercial Pt/C catalyst. The electron transfer number n of Fe3C@NCNTs-800 at different electrode potentials in 0.1 M KOH was calculated on the basis of K-L equation and plots (Figs. S4a, b), shown as the inset of Fig. 5a. The n value is larger than 3.8, indicating a dominated four-electron oxygen reduction process. In 1 M HClO4 (Fig. 5b), the Fe3C@NCNTs-800 also displays wonderful ORR activity with the onset potential of ca. 0.719 V, the half-wave potential of ca. 0.518 V and the limiting
current of ca. 3.6 mA cm2, which is comparable with the commercial Pt/C and the best MNx/C catalysts reported (the detailed comparison in Table S3). The electron transfer number n of Fe3C@NCNTs-800 at 0.05 to 0.2 V in 1 M HClO4 was calculated to be 4 (Figs. S5a, b), which is in good agreement with that of Pt/C (Figs. S6a, b). Besides the high ORR activity, Fe3C@NCNTs catalyst also shows excellent methanol tolerance (Fig. S7a). To evaluate the stability of the Fe3C@NCNTs, continuous potential cycling in an aqueous solution of O2-saturated 0.1 M KOH or 1 M HClO4 with an oxygen flow rate of 20 mL min1 was conducted. After 10,000 potential cycles, the peak currents for Fe3C@NCNTs-
Fig. 5. Linear sweep voltammograms (LSV) curves and electron transfer value n (inset) of Pt/C, NCNTs and Fe3C@NCNTs-800 in (a) alkaline and (b) acid media. The LSV results were obtained in O2-saturated 1 M HClO4 or 0.1 M KOH at a rotation speed of 1600 rpm and a scan rate of 10 mV s1.
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800 in KOH and HClO4 remain approximate 85.4% and 95.5%, respectively (Fig. S7b). In contrast, the Pt/C exhibited a obviously decrease with the current loss of 51.7%, which was investigated in our prior work [24,25] under the same conditions (1 M HClO4). Contrary to the unsatisfactory stability of FeNx/C [4,8], the high electrocatalytic stability of Fe3C@NCNTs catalyst in ORR can be ascribed to (1) N-doped carbon materials are stable in acid and alkaline condition [5]; (2) Fe3C is doubly protected by clingy graphite layers and carbon nanotube as discussed above. 3.3. Discussion on the ORR active sites Based on the references [4,5,12], so far, three possible catalytic active sites have been presented in FeNx/C and NxC catalysts, as shown in Fig. 6aec. For NxC catalysts (Fig. 6a), pyridinic- or graphitic-type nitrogen were considered as the active sites for ORR, which is due to the change of the charge density in carbon materials [5]. For FeNx/C catalysts (Fig. 6b), Fe coordination compounds with two-coordinate (FeN2), four-coordinate (FeN4), and six-coordinate (FeN6) were reported to be the real active sites [26e28]. The electrocatalytic activity of FeNx/C catalysts for ORR increases with the pyridinic- or pyrrolic-type nitrogen functionalities owing to the formation of Fe coordination compounds. Another new type of Fe@C catalysts (Fig. 6c) indicated that the Fe species inside carbon materials also plays a synergetic role with outside carbon in the ORR [12,13]. For the Fe3C@NCNTs in this work, the three types of active sites mentioned above may be responsible for the excellent ORR activity: (1) the abundant doped N on the outside surface of CNTs forming pyridinic- or quaternarytype nitrogen active sites, (2) the rich Fe3C in the interior of the carbon nanotube with outside carbon as synergetic active sites, and (3) the trace amount of Fe on the outside surface in the form
of Fe3C or FeeN coordination compounds as active sites. To determine the main influencing factors on the ORR activity of Fe3C@NCNTs, a series of experiments were carried out and discussed mainly in acid medium due to the importance for PEMFC application. The active nitrogen species in FeNx/C and NxC catalysts were believed to be formed during the thermal treatment process, with the result that nitrogen species were strongly dependent on the treatment temperature [16,29]. So far, the general viewpoint is that the pyrrolic nitrogen functionalities transform into pyridinic nitrogen and follow by a transition to graphitic nitrogen with the increase of thermal treatment temperature [16]. Based on this, the Fe3C@NCNTs samples synthesized under different temperatures (700, 800, and 900 C) were studied in detail by XPS, EDS, RDE and RRDE. From the XPS spectra (Fig. 4bed), it is clear that the changes of nitrogen species in the samples under different synthesis temperatures agree well with the viewpoint mentioned above. According to the XPS and EDS results shown in Fig. 4a, Tables S1 and S2, the surface and bulk contents of nitrogen and iron decrease with the synthesis temperatures increasing from 700 to 900 C. The ORR activities and the electron transfer numbers of these catalysts are in the order of Fe3C@NCNTs-900 < Fe3C@NCNTs-700 < Fe3C@NCNTs800, as shown in Fig. 6d and Fig. S4. Although Fe3C@NCNTs-700 has the highest contents of iron and nitrogen in the bulk and at the surface, its catalytic activity is lower than that of Fe3C@NCNTs-800. While, Fe3C@NCNTs-800 shows the highest ORR activity due to the highest contents of pyridinic and graphitic nitrogen, meaning that pyridinic and graphitic nitrogen are the main ORR active sites in Fe3C@NCNTs catalyst, which is consistent with the NxC catalysts [16,30]. Fe3C@NCNTs-900 shows the minimum ORR activity, not only because of the lowest surface nitrogen content, but also possibly because of the lowest Fe3C content inside the CNTs.
Fig. 6. Possible catalytic active sites in carbon-based ORR catalysts: (a) N-doped carbon, (b) Fe coordination compounds, and (c) N doped carbon with encapsulated Fe species, (d) LSV curves and electron transfer value n (inset) of Fe3C@NCNTs-700, 800 and 900 C. The LSV results were obtained in O2-saturated 1 M HClO4 at a rotation speed of 1600 rpm and a scan rate of 10 mV s1.
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In order to understand the possible effect of Fe-Nx in ORR, the poisoning experiment with halide that can poison Fe-Nx active sites by forming a strong ligand [31,32] was carried out, as shown in Fig. 7a and b. It can be seen that halide ions have no influence on the ORR activity of the Fe3C@NCNTs, which is different from the FeNx/C catalysts containing Fe-Nx active sites [31]. Because the trace amount of FeeN4 loading is enough to obtain a definite effect on the ORR activity of catalysts [33], the poisoning experiment with halide indicates that the Fe-Nx species is negligible in Fe3C@NCNTs catalyst. Therefore, based on the XRD and EDS results, the small amount of Fe on the outside of CNTs should be Fe3C. To probe the influence of outside Fe3C on ORR, Fe3C@NCNTs samples prepared at 800 C were treated by hydrochloric acid leaching with different times. According to the results of SEM (Fig. S8), XRD (Fig. 3b) and EDS (Table S1), it is clear that the acid leaching decreases the content of Fe3C without a destruction of the structure of CNTs. Compared with the bulk composition (Table S1) of Fe3C@NCNTs-HCl-4h and Fe3C@NCNTs-800 (12 h), it is found that the content of Fe keeps constant (ca. 23%) after the acid leaching time of 4 h owing to the stability of Fe3C inside the CNTs. From the surface composition (Table S2) of Fe3C@NCNTs-HCl-1h and Fe3C@NCNTs-800 (12 h), it reveals that the surface N content keeps constant and the surface Fe content decreases with the leaching time increasing. Fig. 7c shows that the ORR activity of Fe3C@NCNTs increases with acid leaching increasing from 0 to 4 h, indicating that the lower content of Fe3C on the surface results in a higher ORR activity. Thus it can be concluded that the Fe3C on the outside surface is inactive for ORR. To explore the role of Fe3C inside the CNTs in ORR, the poisoning experiment with SCN was designed. Fig. 7d shows the LSV of Fe3C@NCNTs for ORR in 1 M HClO4 with or without SCN. It can be seen that SCN ions caused ca. 200 mV negative shift of the onset
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and half-wave potentials, and reduced diffusion-limited current for the ORR. The result of the elemental mappings (Inset in Fig. 7d) demonstrates S element is mainly detected on the sites where Fe particles are observed. This indicates that the Fe3C encapsulated in CNTs was poisoned by sulfur cyanide complexation, resulting in the decreased ORR activity. Therefore, it can be concluded that the Fe3C encapsulated in CNTs, though not directly contacts the O2, plays a key role in activating outside carbon as synergetic active sites to enhance ORR activity, as discussed by Bao [12]. 3.4. Discussion on the ORR mechanism In spite of a similar N content to NCNTs, the as-prepared Fe3C@NCNTs catalyst displays a higher ORR activity and an apparent electron transfer number of 4 in acid medium, quite different from NCNT with n value of 3.5 in this work or 2.88 in literature [24]. It implies that the Fe3C encapsulated in NCNTs changes the ORR mechanism. Fe3C@NCNTs-800 displays a Tafel slope of 69 mV per decade at low over-potentials in Fig. 8b, close to 68 mV per decade for the Pt/C catalyst, revealing that the first electron transfer is the rate determining step under Temkin condition of intermediate adsorption [34]. As shown in Fig. 8a, there are three possible 4e ORR paths: 1) the direct 4e pathway (O2/H2O); 2) the peroxide pathway (O2/H2O2/H2O); 3) the electrochemical-chemical (EC) pathway (O2/H2O2, H2O2 / H2O þ 1/2O2) [36]. The catalyst loading and rotation rate have different effects on the measured H2O2 amount during ORR in three ORR paths [37]. For a direct 4e pathway, the generated H2O2 amount should be insensitive to the catalyst loading and rotation rate because of almost no H2O2 forming in the ORR. While for peroxide pathway and EC pathway, H2O2 amount would increase with the increased rotation rate or the decreased catalyst
Fig. 7. Effects of (a) Cl, (b) Br, (c) acid leaching time and (d) SCN on ORR activity of Fe3C@NCNTs. Inset in (d) shows TEM and corresponding elemental-mapping images of Fe3C@NCNTs-800 after poisoning by SCN. The LSV results were obtained in O2-saturated 1 M HClO4 at a rotation speed of 1600 rpm and a scan rate of 10 mV s1.
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Fig. 8. (a) Possible 4e-ORR paths. (b) Tafel plots obtained from the RDE measurements of Fe3C@NCNTs and Pt/C at 1600 rpm. (c) RRDE curves and (d) H2O2 amount of Fe3C@NCNTs with different catalyst loadings in O2-saturated 1 M HClO4 at a rotation speed of 1600 rpm and a scan rate of 10 mV s1. (e) H2O2 amount of Fe3C@NCNTs at different rotation speeds in 1 M HClO4. (f) H2O2 electro-reduction on Fe3C@NCNTs in N2-saturated 0.1 M HClO4 with 1.3 mM H2O2 at different rotation speeds. The insets in (d) and (e) show electron transfer value n.
loading, since less amount of H2O2 might either diffuse into the electrocatalyst or take part in the disproportionation reaction [35,37]. Fig. 8cee shows the influences of catalyst loading and rotation rate on ORR activity and H2O2 yield during ORR in 1 M HClO4. The H2O2 amount shows a little increase from ca. 5 to ca. 12% with the catalyst loading largely decreasing from 611.5 to 76.4 mg cm2, and meanwhile the electron transfer number n keeps at ca. 3.9 to ca. 3.7, indicating that the loading of Fe3C@NCNTs catalyst has a slight effect on the H2O2 yield. Fig. 8e shows that the H2O2 amount increases slightly with the increase of rotation rate from 200 to 3000 rpm, indicating that the influence of rotation rate is also very slight. In 0.1 M NaOH alkaline medium, the same experimental results are also displayed (Fig. S9). However, for NCNTs, the result (Fig. S10) shows that electron transfer number n decreases from ca. 3.9 to ca. 2.8 with
the catalyst loading decreasing from 611.5 to 152.9 mg cm2, which is quite different from that of Fe3C@NCNTs. Furthermore, for the 4e peroxide pathway, H2O2 electroreduction (HPER) is necessary, and the reaction should be fast enough to completely reduce the H2O2 to H2O [35]. HPER of Fe3C@NCNTs was tested by RDE in a N2-saturated 0.1 M HClO4 solution containing 1.3 mM H2O2 (the maximum H2O2 produced during ORR in RDE [35]) at the same potential range as ORR. As shown in Fig. 8f, the HPER current curve observed over Fe3C@NCNTs has no platform at low potential, indicating that HPER on Fe3C@NCNTs is practically under kinetic control during ORR and is not fast enough [35], further implying that the 4e peroxide pathway is not suitable to explain apparent four electron transfer process. All these results confirm that the ORR mechanism of Fe3C@NCNTs catalyst in acid and alkaline media is mainly a direct 4e reduction.
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4. Conclusions In summary, NCNTs with encapsulated Fe3C nanoparticles (Fe3C@NCNTs) were successfully prepared through a simple and scalable pyrolysis process. The characterization results show that the Fe in the form of Fe3C is mainly encapsulated in the interior of NCNTs and N mostly exists on the surface of NCNTs. The asprepared Fe3C@NCNTs catalyst exhibits excellent ORR performances in both acidic and alkaline media. Excluding the possible effect of Fe-Nx and Fe3C on the surface of NCNTs, it is proposed that the doped N is the main active site and the inner Fe3C with outside carbon form the synergetic active site to enhance ORR activity. An apparent four electron transfer pathway was determined in acid and alkaline media for ORR, and a direct four electron path was further proved. Acknowledgments We acknowledge the financial support from the National Natural Science Foundation of China (No. 21373091, 21133010), the Guangdong Provincial Natural Science Foundation (No. S20120011275, 2014A030312007) and Program for New Century Excellent Talents in Universities of China (NCET-12-0190). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.04.021. References [1] [2] [3] [4] [5] [6]
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Nitrogen doped carbon nanotubes with encapsulated ferric carbide as excellent electrocatalyst for oxygen reduction reaction in acid and alkaline media Guoyu Zhong, Hongjuan Wang*, Hao Yu, Feng Peng* School of Chemistry and Chemical Engineering, Key Laboratory of Fuel Cell Technology of Guangdong Province, South China University of Technology, Guangzhou 510640, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
N-doped carbon nanotube with encapsulated Fe3C(Fe3C@NCNT) is prepared by pyrolysis. Fe3C@NCNT shows good O2 reduction reaction (ORR) nature in acid and alkali media. Fe3C is encapsulated in the interior and N is distributed on the outside of CNTs. The inner Fe3C with outside C form synergetic active sites to enhance ORR activity. A direct four electron ORR path on Fe3C@NCNT is proved in acid and alkali media.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2015 Received in revised form 6 March 2015 Accepted 4 April 2015 Available online 6 April 2015
Nitrogen doped carbon nanotubes (NCNTs) with encapsulated Fe3C nanoparticles (Fe3C@NCNTs) are synthesized by a simple direct pyrolysis of melamine and ferric chloride. The characterization results reveal that Fe3C is mainly encapsulated in the interior of NCNTs and N species is mainly distributed on the outside surface of NCNTs. Iron and iron carbide catalyze the growth of NCNTs and are wrapped by carbon to form Fe3C@NCNTs. The as-prepared Fe3C@NCNTs catalyst exhibits superior oxygen reduction reaction (ORR) activity, excellent methanol tolerance and long-term stability in both acid and alkaline media. It is proven that the doped N is the main active site for ORR and the inner Fe3C with outside carbon form the synergetic active site to enhance ORR activity. The ORR mechanism of direct four electron transfer pathway is proved in acid and alkaline media. © 2015 Elsevier B.V. All rights reserved.
Keywords: Fuel cells Oxygen reduction reaction Carbon nanotubes Ferric carbide Active sites Catalytic mechanism
1. Introduction
* Corresponding authors. E-mail address: [email protected] (F. Peng). http://dx.doi.org/10.1016/j.jpowsour.2015.04.021 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Proton exchange membrane fuel cell (PEMFC) that directly converts chemical energy into electrical energy is a clean and highefficiency energy conversion system. However, the high price and
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the shortage resource of Pt hinder the commercialization of PEMFC [1]. Furthermore, Pt-based catalysts are subject to dissolution, aggregation, and poisoning under the reaction conditions, thus reducing both the active surface area and the catalytic efficiency, which leads to an increase in over potentials for fuelecell reactions, especially for the critical but sluggish oxygen reduction reaction (ORR) [2,3]. Therefore, many efforts have been devoted to developing the non-precious metals catalysts, such as, carbon-supported iron/nitrogen compounds (FeNx/C) and even non-metal N-doped carbon catalysts (NxC) [4e7]. FeNx/C catalysts, synthesized by pyrolyzing the mixture of carbon, nitrogen precursors and iron salts under shielding gas or NH3 [4], exhibit comparable ORR activity to that of commercial Pt/C. However, the stability of FeNx/C catalysts is poor in acid, due to the dissolution of FeeN complexes [8,9]. NxC materials can be considered as another alternative to Pt due to their good activity and high stability in alkaline solution [5,10,11]. But their activity is still very low in acid media, which is not competent for PEMFC application. Thus, it is highly challenging but extremely desirable to develop carbon-based catalyst with high ORR activity and stability in acidic medium. Recently, a new type of pod-like carbon nanotubes (CNTs) with encapsulated iron has been prepared and used as ORR catalyst in acid media by Bao [12]. They found that the electron transfer from Fe particles to the CNTs decreased local work function of the carbon surface. As a result, the catalyst showed a significantly enhanced ORR activity than CNTs without encapsulated Fe. Li et al. [13] also found that graphite carbon with encased Fe3C showed good ORR activity and high stability in acid and alkaline media. To the author's knowledge, there are two indistinct problems for this kind of carbon catalysts with encapsulated iron species (Fe@C). First, the effect of the surface metallic functionalities on ORR has not been determined. Second, the mechanism of catalytic reaction is unclear. Hence, it is necessary and worthwhile to take effort on research of Fe@C for ORR. Herein, a novel N-doped carbon nanotubes with encapsulated Fe3C nanoparticles (Fe3C@NCNTs) was synthesized as ORR catalysts. Their morphologies, composition and growth mechanism were systematically investigated. Compared with Pt/C catalyst, the as-synthesized Fe3C@NCNTs catalyst shows the excellent ORR activity, better methanol resistance and higher stability in both acid and alkaline media. The ORR active sites and catalytic mechanism were discussed in detail.
which was denoted as Fe3C@NCNTs-BM-HCl. The NCNTs were synthesized by a chemical vapor deposition [14]. In a typical experiment, 0.1 g FeMo/Al2O3 was put in the quartz tube. Then 10 mL aniline was injected in the quartz tube after the temperature reached to 800 C with 500 mL min1 of NH3 flow. After injection, the quartz tube was cooled down to room temperature and the resulting sample was collected from quartz boat. The as-prepared NCNTs were treated with 6 M HCl for 2 h to remove the metal impurities.
2. Experiment
2.3. Characterizations
2.1. Catalysts preparation
The surface structure and morphology of the catalysts were characterized by scanning electron microscope (SEM, 1530VP, LEO Co.) at 5 kV and transmission electron microscope (TEM, JEOL, JEM2010) at 200 kV. X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (D/max-IIIA, Japan) using Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG Scientific ESCALAB MK II analyzer using Al Ka radiation (1486.71 eV), and the C1s peak at 284.6 eV was taken as standard. The weight percentage of metals and carbon were measured by energy dispersive spectroscope (EDS, Oxford instruments) attached to a scanning electron microscope. The thermogravimetric analysis (TGA) was carried out on a NETZSCH TGASTA 409 PC analyzer.
The Fe3C@NCNTs catalyst was synthesized by simply pyrolyzing a mixture of ferric chloride and melamine without high pressure [12,13], followed by acid leaching, as illustrated in Fig. 1a. In a typical synthesis, 1.5 g FeCl3$6H2O and 1.5 g melamine were dissolved or dispersed uniformly in 100 mL ethanol by ultrasonication for 0.5 h. Then the mixture was dried at 110 C for 10 h. The dry samples were respectively heated to a preset temperature from 600 to 900 C at a rate of 10 C min1 in a flow of Ar (160 mL min1) and maintained at this temperature for 2 h. The resulting samples were washed in 6 M HCl for 12 h at room temperature to remove the exposed iron species, followed by washing with distilled water and ethanol, and dried under vacuum at 70 C for 10 h. These obtained samples were denoted as Fe3C@NCNTs-X (X indicates the pyrolyzing temperature, 600e900 C). Fe3C@NCNTs synthesized at 800 C was washed with hydrochloric acid (6 M) under different times, which was denoted as Fe3C@NCNTs-HCl-Y (Y indicated the leaching times, 0e4 h). Fe3C@NCNTs synthesized at 800 C was also treated by first ball-milling and then leaching in 6 M HCl for 12 h,
2.2. Electrochemical measurements Electrochemical measurements were performed at room temperature in a three-electrode cell connected to an electrochemical analyzer (CH Instruments 760D) and a rotating ring-disk electrode (pine Instrument Co.). Platinum wire and Ag/AgCl electrode with saturated KCl were served as counter electrode and reference electrode, respectively. The catalyst inks were prepared by ultrasonically dispersing 0.5e4 mg catalyst in a mixture solution containing 100 mL acetone, 385 mL deionized water and l5 mL 5% Nafion solution (DuPont). 15 mL catalyst ink was cast on the GC electrode surface and dried at room temperature in the air for 1 h. Rotating disk electrode (RDE) or rotating ring-disk electrode (RRDE) with the GC disk diameter of 5 mm were used to carry out cyclic voltammogram (CV) or linear scan voltammogram (LSV) in the oxygen saturated 0.1 M KOH solution from 1 V to 0.2 V or 1 M HClO4 from 0.2 V to 0.8 V at a scan rate of 10 mV s1. All the potentials used in this work are referenced to the Ag/AgCl reference electrode. In the methanol-tolerant experiment, the working electrode was immersed in 1 M HClO4 with 1 M CH3OH. Stability of the catalysts was evaluated based on the oxygen reduction peak current densities of CV (4 mm GC disk) at various cycle numbers in oxygen saturated 0.1 M KOH and 1 M HClO4 solution, respectively. Catalyst loading without special instruction was 152.9 mg cm2 for all catalysts in KOH solution, 611.5 mg cm2 for Fe3C@NCNTs and NCNTs in 1 M HClO4, and 152.9 mg cm2 for Pt/C in 1 M HClO4. The poisoning experiments were carried out by LSV at RDE in the oxygen saturated 1 M HClO4 solution with 5 mM of F, Cl, Br, and SCN ions, respectively, at a scan rate of 10 mV s1 and a rotation speed of 1600 rpm. Calculation methods of the transferred electron number (n) and H2O2 yield during ORR are described in supporting information.
3. Results and discussion 3.1. Structure characterization and growth mechanism The morphology of the typical catalyst Fe3C@NCNTs-800 was investigated by SEM (Fig. 1f and Fig. S1a, b) and TEM (Fig. 2b and
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Fig. 1. Schematic of (a) the synthesis route and (b) growth mechanism of Fe3C@NCNTs. SEM images of the samples prepared at (c) 600 C, (d) 680 C, (e) 700 C and (f) 800 C.
Fig. 2. TEM images of (a) Fe3C@NCNTs-700 and (b) Fe3C@NCNTs-800. (c) HRTEM image of Fe3C@NCNTs-800. (d) Elemental-mapping images of Fe3C@NCNTs-800.
Figs. S1c, d). Uniform nanotubes with diameters of 100e200 nm and with filled nanoparticles as the pea-pod structure reported by Bao [12] were observed. Compared with the pea-pod CNTs, the asprepared Fe3C@NCNTs show larger diameters, larger filled particles, abundance of microscopic defects, a lot of folds and no
compartment. The filled nanoparticles were further characterized by HRTEM analysis and elemental-mapping in Fig. 2c and d. The spacings of crystalline lattices in two directions are 0.195 nm and 0.186 nm with a characteristic angle of 76.9 , corresponding to the (112) and (221) planes of the Fe3C phase. XRD results further
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confirm that the nanoparticles encapsulated in the interior of nanotubes are dominantly Fe3C when the preparation temperature is above 680 C (Fig. 3a). All the samples may contain traces of metallic iron due to the same characteristic peak as that of Fe3C at 2q of 44.6 . It is also observed from HRTEM that the Fe3C nanoparticles were coated by not only the tube walls but also several clingy graphitic layers. The dual protection of clingy graphitic layers and tube walls stabilizes the Fe3C nanoparticles in acid solutions. Therefore, it is difficult to remove the Fe species encapsulated in carbon with the common way, such as ball-milling and leaching in hot acid [13]. As shown in SEM images (Figs. S1e, f), the CNTs were cut short and partially destroyed by ball-milling and leaching in HCl solution. The white nanoparticles in the CNTs confirm that Fe species has not been removed completely. In addition, the EDS result (Table S1) demonstrates that Fe content still remains 14 wt%. The bulk phase composition of the Fe3C@NCNTs catalyst was analyzed with EDS, as shown in Table S1. The results show that the content of nitrogen decreases with the pyrolysis temperature increasing, while the content of iron increases firstly with the pyrolysis temperature increasing until 650 C and then decreases. For Fe3C@NCNTs-800 sample, the contents of nitrogen and iron are 0.45 wt% and 22.47 wt%, respectively. TGA (Fig. S2a) reveals an iron content of 20.63 wt% in Fe3C@NCNTs-800, in good agreement with the EDS result, which is similar to the iron content of pea-pod CNTs [12] (18.1wt%) and iron carbide encased in carbon [13] (15.0 wt%). The surface composition of Fe3C@NCNTs was examined by XPS and the results are shown in Fig. 4 and Table S2. Contrary to the EDS results, the XPS results show that the surface contents of nitrogen and iron for Fe3C@NCNTs-800 are 6.16 wt% and 1.79 wt%, respectively. Incorporating the higher Fe content and lower N content by EDS and the less surface Fe content and more surface N content by XPS, it is reasonably concluded that the Fe in the form of Fe3C is mainly wrapped in the interior of CNTs and N species is mainly distributed on the outside surface of CNTs. Thus it is impossible to have large amounts of Fe-Nx species in the sample [15]. For the surface N in the samples, four different N-species can be assigned based on the XPS results (Fig. 4). Pyridinic-N and graphitic-N are the dominant N species for Fe3C@NCNTs-800 and Fe3C@NCNTs900. While for Fe3C@NCNTs-700 with the highest surface nitrogen content, the pyrrolic nitrogen content is much higher than that of the other two samples due to the low synthesis temperature [16]. In order to investigate the formation mechanism of the Fe3C@NCNTs, the samples prepared at different pyrolyzing temperatures were analyzed carefully. At 600 C, as shown in Fig. 1c and Table S1, the obtained sample is amorphous thick carbon sheets containing iron (40.26 wt%) and nitrogen (19.53 wt%), which is the graphite-like carbon nitride from melamine decomposition [17].
Increasing temperature to 650 C, iron species turns into iron oxide, Fe and Fe3C (Fig. 3a). Part of carbon separates out the basal surface with Fe3C nanoparticles. When the temperature reaches to 680 C, carbon further separates out, leading to carbon nanotubes encapsulated Fe3C nanoparticles (Fig. S3 and Fig. 1d). At 700 C, a large number of carbon nanotubes with encapsulated Fe3C are produced (Figs. 1e, 2a and 3a). High-quality carbon nanotubes with high graphitic degree and encapsulated Fe3C nanoparticles evenly dispersing in the interior of CNTs are formed at 800 C (Figs. 1f, 2b and 3a). When further increase the temperature to 900 C, the CNTs become no longer uniform in diameter and rougher on the surface with little Fe filling (Figs. S1g, h). The result demonstrates that the appropriate temperature for preparing Fe3C@NCNTs is 800 C. Based on the aforementioned discussion and general carbon nanotube growth process [18e21], the growth mechanism of Fe3C@NCNTs can be schematically represented in Fig. 1b, and the involved chemical reactions are displayed in supporting information. Prior to 600 C, melamine and ferric chloride are decomposed to form the thick graphite-like carbon nitride with encapsulated and supported amorphous Fe species [21]. From 600 to 650 C, amorphous Fe species is converted into iron oxide. Part of the iron oxide is firstly reduced into elemental Fe and further reacts with carbon nitride species to form Fe3C [21]. Amorphous carbon dissolves in metal phase and graphitic layers are formed by the carbon-through-metal diffusion [20]. Above 680 C, Fe2O3 is completely transformed into Fe and Fe3C, and graphitic layers begin to grow into carbon nanotubes [18,19]. Meanwhile, the hot liquidlike cementite particles axial move in graphitic layers due to the buildup of compressive stress during thickness growth of the graphitic tube wall, and due to increasing surface tensional forces between the core material and the tube [20]. Thereby, the carbon nanotubes with encapsulated Fe3C are formed. The hot liquid-like cementite particles possess an enormous capacity of carbon uptake which largely separates out when the particles are cooled down [20,22]. This leads to several clingy graphitic layers on Fe3C particles, as mentioned above. The formation mechanism of asprepared Fe3C@NCNTs is obviously different from pea-pod CNTs prepared by explosion method [12] and Fe-filled CNTs prepared by CVD [23]. The preparation method represented in this work is more simple, convenient and scalable for the practical application. 3.2. ORR activity and stability The ORR activities of Fe3C@NCNTs catalyst were evaluated and compared with those of the NCNTs and a commercial Pt/C catalyst (Pt: 20 wt%) by LSV and KouteckyeLevich (KeL) plots in acidic and alkaline solutions. In 0.1 M KOH (Fig. 5a), Fe3C@NCNTs-800 exhibits
Fig. 3. XRD patterns of Fe3C@NCNTs prepared at different temperatures (a) and treated by HCl with different times (b).
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Fig. 4. (a) XPS survey and high-resolution Fe 2p spectra (inset) of Fe3C@NCNTs samples, High-resolution N 1s spectra of Fe3C@NCNTs prepared at (b) 700, (c) 800 and (d) 900 C.
high onset potential of ca. 0.098 V (vs Ag/AgCl, the same below), half-wave potential of ca. 0.147 V and large limiting current of ca. 3.1 mA cm2, which is significantly superior to the NCNTs and even slightly better than the commercial Pt/C catalyst. The electron transfer number n of Fe3C@NCNTs-800 at different electrode potentials in 0.1 M KOH was calculated on the basis of K-L equation and plots (Figs. S4a, b), shown as the inset of Fig. 5a. The n value is larger than 3.8, indicating a dominated four-electron oxygen reduction process. In 1 M HClO4 (Fig. 5b), the Fe3C@NCNTs-800 also displays wonderful ORR activity with the onset potential of ca. 0.719 V, the half-wave potential of ca. 0.518 V and the limiting
current of ca. 3.6 mA cm2, which is comparable with the commercial Pt/C and the best MNx/C catalysts reported (the detailed comparison in Table S3). The electron transfer number n of Fe3C@NCNTs-800 at 0.05 to 0.2 V in 1 M HClO4 was calculated to be 4 (Figs. S5a, b), which is in good agreement with that of Pt/C (Figs. S6a, b). Besides the high ORR activity, Fe3C@NCNTs catalyst also shows excellent methanol tolerance (Fig. S7a). To evaluate the stability of the Fe3C@NCNTs, continuous potential cycling in an aqueous solution of O2-saturated 0.1 M KOH or 1 M HClO4 with an oxygen flow rate of 20 mL min1 was conducted. After 10,000 potential cycles, the peak currents for Fe3C@NCNTs-
Fig. 5. Linear sweep voltammograms (LSV) curves and electron transfer value n (inset) of Pt/C, NCNTs and Fe3C@NCNTs-800 in (a) alkaline and (b) acid media. The LSV results were obtained in O2-saturated 1 M HClO4 or 0.1 M KOH at a rotation speed of 1600 rpm and a scan rate of 10 mV s1.
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800 in KOH and HClO4 remain approximate 85.4% and 95.5%, respectively (Fig. S7b). In contrast, the Pt/C exhibited a obviously decrease with the current loss of 51.7%, which was investigated in our prior work [24,25] under the same conditions (1 M HClO4). Contrary to the unsatisfactory stability of FeNx/C [4,8], the high electrocatalytic stability of Fe3C@NCNTs catalyst in ORR can be ascribed to (1) N-doped carbon materials are stable in acid and alkaline condition [5]; (2) Fe3C is doubly protected by clingy graphite layers and carbon nanotube as discussed above. 3.3. Discussion on the ORR active sites Based on the references [4,5,12], so far, three possible catalytic active sites have been presented in FeNx/C and NxC catalysts, as shown in Fig. 6aec. For NxC catalysts (Fig. 6a), pyridinic- or graphitic-type nitrogen were considered as the active sites for ORR, which is due to the change of the charge density in carbon materials [5]. For FeNx/C catalysts (Fig. 6b), Fe coordination compounds with two-coordinate (FeN2), four-coordinate (FeN4), and six-coordinate (FeN6) were reported to be the real active sites [26e28]. The electrocatalytic activity of FeNx/C catalysts for ORR increases with the pyridinic- or pyrrolic-type nitrogen functionalities owing to the formation of Fe coordination compounds. Another new type of Fe@C catalysts (Fig. 6c) indicated that the Fe species inside carbon materials also plays a synergetic role with outside carbon in the ORR [12,13]. For the Fe3C@NCNTs in this work, the three types of active sites mentioned above may be responsible for the excellent ORR activity: (1) the abundant doped N on the outside surface of CNTs forming pyridinic- or quaternarytype nitrogen active sites, (2) the rich Fe3C in the interior of the carbon nanotube with outside carbon as synergetic active sites, and (3) the trace amount of Fe on the outside surface in the form
of Fe3C or FeeN coordination compounds as active sites. To determine the main influencing factors on the ORR activity of Fe3C@NCNTs, a series of experiments were carried out and discussed mainly in acid medium due to the importance for PEMFC application. The active nitrogen species in FeNx/C and NxC catalysts were believed to be formed during the thermal treatment process, with the result that nitrogen species were strongly dependent on the treatment temperature [16,29]. So far, the general viewpoint is that the pyrrolic nitrogen functionalities transform into pyridinic nitrogen and follow by a transition to graphitic nitrogen with the increase of thermal treatment temperature [16]. Based on this, the Fe3C@NCNTs samples synthesized under different temperatures (700, 800, and 900 C) were studied in detail by XPS, EDS, RDE and RRDE. From the XPS spectra (Fig. 4bed), it is clear that the changes of nitrogen species in the samples under different synthesis temperatures agree well with the viewpoint mentioned above. According to the XPS and EDS results shown in Fig. 4a, Tables S1 and S2, the surface and bulk contents of nitrogen and iron decrease with the synthesis temperatures increasing from 700 to 900 C. The ORR activities and the electron transfer numbers of these catalysts are in the order of Fe3C@NCNTs-900 < Fe3C@NCNTs-700 < Fe3C@NCNTs800, as shown in Fig. 6d and Fig. S4. Although Fe3C@NCNTs-700 has the highest contents of iron and nitrogen in the bulk and at the surface, its catalytic activity is lower than that of Fe3C@NCNTs-800. While, Fe3C@NCNTs-800 shows the highest ORR activity due to the highest contents of pyridinic and graphitic nitrogen, meaning that pyridinic and graphitic nitrogen are the main ORR active sites in Fe3C@NCNTs catalyst, which is consistent with the NxC catalysts [16,30]. Fe3C@NCNTs-900 shows the minimum ORR activity, not only because of the lowest surface nitrogen content, but also possibly because of the lowest Fe3C content inside the CNTs.
Fig. 6. Possible catalytic active sites in carbon-based ORR catalysts: (a) N-doped carbon, (b) Fe coordination compounds, and (c) N doped carbon with encapsulated Fe species, (d) LSV curves and electron transfer value n (inset) of Fe3C@NCNTs-700, 800 and 900 C. The LSV results were obtained in O2-saturated 1 M HClO4 at a rotation speed of 1600 rpm and a scan rate of 10 mV s1.
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In order to understand the possible effect of Fe-Nx in ORR, the poisoning experiment with halide that can poison Fe-Nx active sites by forming a strong ligand [31,32] was carried out, as shown in Fig. 7a and b. It can be seen that halide ions have no influence on the ORR activity of the Fe3C@NCNTs, which is different from the FeNx/C catalysts containing Fe-Nx active sites [31]. Because the trace amount of FeeN4 loading is enough to obtain a definite effect on the ORR activity of catalysts [33], the poisoning experiment with halide indicates that the Fe-Nx species is negligible in Fe3C@NCNTs catalyst. Therefore, based on the XRD and EDS results, the small amount of Fe on the outside of CNTs should be Fe3C. To probe the influence of outside Fe3C on ORR, Fe3C@NCNTs samples prepared at 800 C were treated by hydrochloric acid leaching with different times. According to the results of SEM (Fig. S8), XRD (Fig. 3b) and EDS (Table S1), it is clear that the acid leaching decreases the content of Fe3C without a destruction of the structure of CNTs. Compared with the bulk composition (Table S1) of Fe3C@NCNTs-HCl-4h and Fe3C@NCNTs-800 (12 h), it is found that the content of Fe keeps constant (ca. 23%) after the acid leaching time of 4 h owing to the stability of Fe3C inside the CNTs. From the surface composition (Table S2) of Fe3C@NCNTs-HCl-1h and Fe3C@NCNTs-800 (12 h), it reveals that the surface N content keeps constant and the surface Fe content decreases with the leaching time increasing. Fig. 7c shows that the ORR activity of Fe3C@NCNTs increases with acid leaching increasing from 0 to 4 h, indicating that the lower content of Fe3C on the surface results in a higher ORR activity. Thus it can be concluded that the Fe3C on the outside surface is inactive for ORR. To explore the role of Fe3C inside the CNTs in ORR, the poisoning experiment with SCN was designed. Fig. 7d shows the LSV of Fe3C@NCNTs for ORR in 1 M HClO4 with or without SCN. It can be seen that SCN ions caused ca. 200 mV negative shift of the onset
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and half-wave potentials, and reduced diffusion-limited current for the ORR. The result of the elemental mappings (Inset in Fig. 7d) demonstrates S element is mainly detected on the sites where Fe particles are observed. This indicates that the Fe3C encapsulated in CNTs was poisoned by sulfur cyanide complexation, resulting in the decreased ORR activity. Therefore, it can be concluded that the Fe3C encapsulated in CNTs, though not directly contacts the O2, plays a key role in activating outside carbon as synergetic active sites to enhance ORR activity, as discussed by Bao [12]. 3.4. Discussion on the ORR mechanism In spite of a similar N content to NCNTs, the as-prepared Fe3C@NCNTs catalyst displays a higher ORR activity and an apparent electron transfer number of 4 in acid medium, quite different from NCNT with n value of 3.5 in this work or 2.88 in literature [24]. It implies that the Fe3C encapsulated in NCNTs changes the ORR mechanism. Fe3C@NCNTs-800 displays a Tafel slope of 69 mV per decade at low over-potentials in Fig. 8b, close to 68 mV per decade for the Pt/C catalyst, revealing that the first electron transfer is the rate determining step under Temkin condition of intermediate adsorption [34]. As shown in Fig. 8a, there are three possible 4e ORR paths: 1) the direct 4e pathway (O2/H2O); 2) the peroxide pathway (O2/H2O2/H2O); 3) the electrochemical-chemical (EC) pathway (O2/H2O2, H2O2 / H2O þ 1/2O2) [36]. The catalyst loading and rotation rate have different effects on the measured H2O2 amount during ORR in three ORR paths [37]. For a direct 4e pathway, the generated H2O2 amount should be insensitive to the catalyst loading and rotation rate because of almost no H2O2 forming in the ORR. While for peroxide pathway and EC pathway, H2O2 amount would increase with the increased rotation rate or the decreased catalyst
Fig. 7. Effects of (a) Cl, (b) Br, (c) acid leaching time and (d) SCN on ORR activity of Fe3C@NCNTs. Inset in (d) shows TEM and corresponding elemental-mapping images of Fe3C@NCNTs-800 after poisoning by SCN. The LSV results were obtained in O2-saturated 1 M HClO4 at a rotation speed of 1600 rpm and a scan rate of 10 mV s1.
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Fig. 8. (a) Possible 4e-ORR paths. (b) Tafel plots obtained from the RDE measurements of Fe3C@NCNTs and Pt/C at 1600 rpm. (c) RRDE curves and (d) H2O2 amount of Fe3C@NCNTs with different catalyst loadings in O2-saturated 1 M HClO4 at a rotation speed of 1600 rpm and a scan rate of 10 mV s1. (e) H2O2 amount of Fe3C@NCNTs at different rotation speeds in 1 M HClO4. (f) H2O2 electro-reduction on Fe3C@NCNTs in N2-saturated 0.1 M HClO4 with 1.3 mM H2O2 at different rotation speeds. The insets in (d) and (e) show electron transfer value n.
loading, since less amount of H2O2 might either diffuse into the electrocatalyst or take part in the disproportionation reaction [35,37]. Fig. 8cee shows the influences of catalyst loading and rotation rate on ORR activity and H2O2 yield during ORR in 1 M HClO4. The H2O2 amount shows a little increase from ca. 5 to ca. 12% with the catalyst loading largely decreasing from 611.5 to 76.4 mg cm2, and meanwhile the electron transfer number n keeps at ca. 3.9 to ca. 3.7, indicating that the loading of Fe3C@NCNTs catalyst has a slight effect on the H2O2 yield. Fig. 8e shows that the H2O2 amount increases slightly with the increase of rotation rate from 200 to 3000 rpm, indicating that the influence of rotation rate is also very slight. In 0.1 M NaOH alkaline medium, the same experimental results are also displayed (Fig. S9). However, for NCNTs, the result (Fig. S10) shows that electron transfer number n decreases from ca. 3.9 to ca. 2.8 with
the catalyst loading decreasing from 611.5 to 152.9 mg cm2, which is quite different from that of Fe3C@NCNTs. Furthermore, for the 4e peroxide pathway, H2O2 electroreduction (HPER) is necessary, and the reaction should be fast enough to completely reduce the H2O2 to H2O [35]. HPER of Fe3C@NCNTs was tested by RDE in a N2-saturated 0.1 M HClO4 solution containing 1.3 mM H2O2 (the maximum H2O2 produced during ORR in RDE [35]) at the same potential range as ORR. As shown in Fig. 8f, the HPER current curve observed over Fe3C@NCNTs has no platform at low potential, indicating that HPER on Fe3C@NCNTs is practically under kinetic control during ORR and is not fast enough [35], further implying that the 4e peroxide pathway is not suitable to explain apparent four electron transfer process. All these results confirm that the ORR mechanism of Fe3C@NCNTs catalyst in acid and alkaline media is mainly a direct 4e reduction.
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4. Conclusions In summary, NCNTs with encapsulated Fe3C nanoparticles (Fe3C@NCNTs) were successfully prepared through a simple and scalable pyrolysis process. The characterization results show that the Fe in the form of Fe3C is mainly encapsulated in the interior of NCNTs and N mostly exists on the surface of NCNTs. The asprepared Fe3C@NCNTs catalyst exhibits excellent ORR performances in both acidic and alkaline media. Excluding the possible effect of Fe-Nx and Fe3C on the surface of NCNTs, it is proposed that the doped N is the main active site and the inner Fe3C with outside carbon form the synergetic active site to enhance ORR activity. An apparent four electron transfer pathway was determined in acid and alkaline media for ORR, and a direct four electron path was further proved. Acknowledgments We acknowledge the financial support from the National Natural Science Foundation of China (No. 21373091, 21133010), the Guangdong Provincial Natural Science Foundation (No. S20120011275, 2014A030312007) and Program for New Century Excellent Talents in Universities of China (NCET-12-0190). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.04.021. References [1] [2] [3] [4] [5] [6]
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