Facile synthesis of highly porous N-doped CNTs/Fe₃C and its electrochemical properties

Abstract

Highly porous N-doped CNTs/Fe₃C was synthesized by a facile and one-pot method. In this method, melamine was used as the carbon precursor; iron chloride was employed as a catalyst of graphic carbon and iron carbon source, and zinc chloride as a chemical activator to obtain highly porous structure. Fe₃C nanorods were in situ formed and embedded into the inner structure of CNTs. The pore structure analysis shows that N-doped CNTs/Fe₃C possesses the large specific surface area up to 1021.26 m² g⁻¹. The N-doped CNTs/Fe₃C electrode exhibits a high specific capacitance of 181 F g⁻¹ at the current density of 0.1 A g⁻¹, and excellent capacitance rate and cycling stability.

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1 Introduction

Carbon nanotubes (CNTs) have attracted intensive attention due to their unique structure and properties, such as large pore size, high electric conductivity and chemical stability.^1–3 These features allow CNTs to be widely used as ideal electrode materials for supercapacitors,^4–7 lithium ion batteries,^8–10 and fuel cells.^7,11,12 However, CNTs generally possess a low specific surface area (below 200 m² g⁻¹), which leads to the low specific capacitance (20–80 F g⁻¹) for electric double-layer capacitors (EDLCs) electrode materials.^3,13

In order to solve this problem, KOH activation is regarded as an effective method to create lots of micropores by the strong chemical etching carbon of CNTs, dramatically increasing their surface area, and therefore its specific capacitance.¹⁴ The specific surface area of CNTs via KOH activation are about 1000 m² g⁻¹, and their specific capacitances are in the range of 80–100 F g⁻¹ in an aqueous electrolyte.^13,15–18 It is much lower than those of activated carbons for EDLCs electrode materials.

The heteroatoms doping has been shown to improve the specific capacitance of CNTs by introducing the additional pseudocapacitance.^7,19–21 For example, Denisa et al. reported that N enriched nonporous carbon materials exhibited a specific capacitance of 198 F g⁻¹ in aqueous electrolyte.²² Yun et al. reported that the specific capacitance of N-doped CNTs was 190.8 F g⁻¹.²¹ The surface modification is generally used to incorporate electroactive heteroatoms on CNTs.^12,23 Unfortunately, this technology is complicated and costly.

Recently, melamine has been employed to effectively prepare N-doped CNTs,^7,24 such as S. Chen et al.⁷ reported that Fe–N co-doped CNTs/graphene were prepared by calcining the mixture of melamine, FeSO₄, and graphene at 700–1000 °C, and it showed a superior electrocatalytic activity. L. Chen et al.²⁴ prepared N-doped CNTs with bamboo-like structure by the pyrolysis of melamine and FeCl₃ at 700 °C, possessing the enhanced hydrogen storage capacity. Wang et al.²⁵ reported that the morphologies of N-doped CNTs can be tuned by the molar ratio of melamine and FeCl₃ and the calcined temperature. When the molar ratio of melamine and FeCl₃ was 6, the specific surface area of as-prepared N-doped CNTs reaches 294.4 m² g⁻¹. However, the specific capacitance is only 67 F g⁻¹, which is not sufficient for the EDLCs electrode materials.

Iron carbide (Fe₃C) is attractive in the field of catalysis due to its favorable magnetic properties and good air stability.^26,27 Recently, C/Fe₃C was reported as an anode material for lithium ion batteries and a counter electrode for dye-sensitized solar cell.^28–31 However, to our knowledge, there are no reports of N-doped CNTs/Fe₃C for EDLCs electrode materials. In this study, based on our previous reported results,²⁵ highly porous N-doped CNTs/Fe₃C were one-pot prepared by the pyrolysis of melamine with FeCl₃ and ZnCl₂ additions. In this method, FeCl₃ was used as a catalyst, and ZnCl₂ as a chemical activator. The results show that the specific surface area of N-doped CNTs/Fe₃C reaches 1021.26 m² g⁻¹, and the specific capacitance was 181 F g⁻¹ in 6 M KOH.

2 Experimental

2.1 Preparation of porous N-doped CNTs/Fe₃C

All chemicals were in analytical grade and used without further purification. 3 g melamine, 3 g ZnCl₂ and different FeCl₃·6H₂O contents were added into 30 mL ethanol under magnetic stirring (the molar ratio of melamine and FeCl₃·6H₂O is 6 : 1, 4 : 1, and 2 : 1, denoted as CNT/Fe₃C-6, CNT/Fe₃C-4, and CNT/Fe₃C-2 in the following). Then the mixture was dried at 80 °C for 10 h. Subsequently, the obtained powders were heated to 573 K and 773 K (5 K min⁻¹) for 2 h, respectively, and then heated to 973 K (10 K min⁻¹) for 2 h in the flow of Ar gas. Finally, the collected product was treated with 1 M hydrochloric acid for 24 h to remove the catalyst residues, washed with deionized water several times, and dried at 100 °C for 10 h.

2.2 Structural characterization

The microstructures of the obtained products were investigated using a field emission scanning electron microscope (FE-SEM, JSM-7001F). Transmission electron microscopy (TEM) images were recorded on a JEOL JEM 2100F field emission transmission electron microscope. X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Advance diffractometer with Cu Kα radiation operating at 40 kV and 20 mA. Raman spectra were collected with a confocal microprobe Raman system (Thermo Nicolet Almega XR Raman Microscope). The thermal analysis was carried out by using the thermogravimetric-differential scanning calorimetry analysis (TG-DSC). Nitrogen adsorption–desorption isotherm measurements were performed on a Micromeritics ASAP 2020 volumetric adsorption analyzer at 77 K. The specific surface area of the materials was calculated by the Brunauer–Emmett–Teller (BET) theory. The micropore size distribution was calculated according to the Horvath–Kawazoe (HK) method.

2.3 Electrochemical measurements

The working electrode was prepared by loading the mixture of 95 wt% of active material and 5 wt% of polytetrafluoroethylene (PTFE) on the nickel foam. The assembled electrodes were dried at 110 °C for 12 h. The electrochemical measurements were conducted in a three-electrode system in 6 M KOH under ambient conditions. A platinum foil and saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. The capacitive performance was investigated by the cyclic voltammeter (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques using an electrochemical workstation (Bio-logic SP 200). EIS was performed in a frequency range from 0.01 Hz to 100 kHz at the open circuit potential of 5 mV amplitude. The working voltage windows of CV and GCD were between −0.8 and 0 V. The specific capacitance was calculated using the following eqn (1):

C = IΔt(mΔV) (1)

where C is the specific capacitance (F g⁻¹), I is the discharge current (A), Δt is the discharge time (s), m is the mass of the active material in the electrode (g), and ΔV is the potential change in discharge (V).

3 Results and discussion

The XRD pattern of the prepared sample without HCl treatment is shown in Fig. s1.† It indicates that many phases are observed for the sample without a HCl treatment. Besides the diffraction peaks of C, Fe₃C and Fe, the diffraction peaks of Zn-based compounds can be identified as ZnO, ZnFe₂O₄ and Zn₃FeC_0.5. After the treatment with 1 M HCl for 24 h, the diffraction peaks of Fe and Zn-based compounds disappear (Fig. 1a), implying the Fe and Zn-based compounds had been completely removed. The strong diffraction peaks can be indexed as the orthorhombic phase of Fe₃C (JCPDS 35-772) and graphitic carbon. A diffraction peak centered at about 26° indicates the existence of graphitic carbon. This graphitization of carbon has been further confirmed by a Raman spectroscopy measurement. A weak D peak and a strong G peak in the Raman spectrum of N-doped CNTs/Fe₃C are shown in Fig. 1b. Two characteristic peaks around 1373 cm⁻¹ and 1602 cm⁻¹ could be ascribed to D and G band, respectively. The I_G/I_D values increase with increasing FeCl₃ contents because FeCl₃ is a good catalyst for the graphitization, and their values of all samples are greater than 1, implying a crystalline structure of the obtained N-doped CNTs/Fe₃C.

Fig. 1 (a) XRD patterns, and (b) Raman spectroscopy of N-doped CNTs/Fe₃C.

The microstructures of N-doped CNTs/Fe₃C were investigated by SEM, and shown in Fig. s2.† It can be seen that the nanotubular structure was observed for all the samples regardless of FeCl₃ contents. The nanotubes were tangled together into clusters with a length of more than 10 μm. A magnification SEM image of CNT/Fe₃C-6 in Fig. 2a demonstrates that the diameter of CNTs is about 100 nm, and the cracks were observed in the surface of CNTs. The internal structure and components of CNT/Fe₃C-6 were further investigated by TEM images. As shown in Fig. 2b and c, the wall thickness of CNTs is about 5–10 nm. Fe₃C nanorods were encapsulated by graphitic carbons, embedded into the inner of CNTs, leading to be difficult to remove them by HCl treatment. HRTEM images of CNT/Fe₃C-6 are shown in Fig. 2d. It can be seen that the well-crystallized structures with lattice fringes of about 0.34 and 0.21 nm, corresponding to an interplanar spacing of graphite (002) and Fe₃C (211) crystal plane, respectively.³⁰

Fig. 2 (a) SEM images, (b) and (c) TEM, and (d) HRTEM images of CNT/Fe₃C-6.

In order to define the quality of Fe₃C in CNT/Fe₃C-6, the thermal analysis was carried out by using TG-DSC. The strongest endothermal peak appears in the temperature range of 450–660 °C, and the weight of the sample decreased rapidly (Fig. 3a). It indicates that the oxidation of carbon and Fe₃C in this temperature range at the same time. Based on the chemical equations: C + O₂ = CO₂, 2Fe₃C + 13/2O₂ = 3Fe₂O₃ + 2CO₂,³¹ the weight percentage of Fe₃C in the composites is about 13.2 wt%.

Fig. 3 (a) TG-DSC thermograms for heating CNT/Fe₃C-6, (b) nitrogen adsorption–desorption isotherms of CNT/Fe₃C-6, and the inset shows the pore size distributions, which are calculated by HK method.

Nitrogen adsorption–desorption isotherms of N-doped CNTs/Fe₃C are shown in Fig. s3.† They exhibit a typical I isotherm, which is indicative of microporous structures in the sample. The specific surface area of samples increases with decreasing FeCl₃ contents. CNT/Fe₃C-6 exhibits the maximum specific surface area up to 1026.23 m² g⁻¹, and the pore volume is 0.57 cm³ g⁻¹. The pore size distributions calculated by HK method, as shown in inset in Fig. 3b, is indicative of a quite uniform pore size distribution centered in micropore ranges of about 0.68 nm and 0.78 nm. In our previous report,²⁵ the specific surface area of N-doped CNTs was only 294.4 m² g⁻¹ without a ZnCl₂ activator. It suggests that ZnCl₂ is an effective chemical activator to improve the high specific surface area of N-doped CNTs/Fe₃C, which is much higher than the specific surface area of CNTs activated by a KOH chemical activator.¹³ Therefore, it is expected that CNT/Fe₃C-6 with high specific surface area will possess superior capacitive performance.

The X-ray photoelectron spectroscopy (XPS) of CNT/Fe₃C-6 is shown in Fig. 4. The wide XPS spectrum (Fig. 4a) confirms that CNT/Fe₃C-6 contains nitrogen (398.2 eV), oxygen (531.1 eV), and Fe (711.9 eV). The surface N, O and Fe contents in the composite was 12.83 at%, 15.05 at% and 1.3 at%, respectively. The high-resolution N1s XPS spectrum of CNT/Fe₃C-6 (Fig. 4b) shows three peaks with binding energies at 398.3 eV, 399.8 eV, and 400.9 eV, assigned to pyridinic-, pyrrolic-, and graphitic-like nitrogen species, respectively.¹² The high N and O contents are considered to contribute to additional pseudocapacitance and improved wettability.³²

Fig. 4 (a) XPS spectrum of a wide survey scan, and (b) high-resolution N1s XPS spectra of CNT/Fe₃C-6.

The electrochemical performances of samples were evaluated by CV, GCD, and EIS. Fig. s4a† and 5a show the CV curves of all samples at a scan rate of 100 mV s⁻¹ and CNT/Fe₃C-6 at scan rates ranging from 5 to 300 mV s⁻¹, respectively. It shows that the curves maintain the quasi-rectangular shape with little distortion at the scan rate up to 300 mV s⁻¹, indicating the favorable accessibility of electrolyte ions into the pores and superior capacitive behavior.³² The GCD curves of N-doped CNT/Fe₃C electrode are presented in Fig. s4b† and 5b. The curves show symmetric triangular shapes between 0 and −0.8 V vs. SCE electrode, indicating the good capacitive properties. The curves still remain a typical triangle shape at a high current density of 20 A g⁻¹, implying excellent columbic efficiency. The specific capacitances of N-doped CNT/Fe₃C electrodes were calculated according to the eqn (1), and the relationship between the specific capacitance and current densities is shown in Fig. 5c. It exhibits that the specific capacitances of N-doped CNT/Fe₃C increase with decreasing FeCl₃ contents, which is in accordance with the specific surface area. CNT/Fe₃C-6 shows the maximum specific capacitance of 181 F g⁻¹ at a current density of 0.1 A g⁻¹. The specific capacitance retention of CNT/Fe₃C-6 electrode is about 51% for the current densities ranging from 0.5 to 20 A g⁻¹, which exhibits the good rate capability.

Fig. 5 (a) CV curves at different scan rates, (b) GCD curves at various current densities of CNT/Fe₃C-6, (c) the relationship between the specific capacitances and current densities, and (d) Nyquist plots of N doped CNTs/Fe₃C.

Nyquist plots of N-doped CNT/Fe₃C electrodes are shown in Fig. 5d. The curves are composed of a semicircle in the high frequency region and a nearly vertical line in the low frequency region indicating a typical capacitive behavior of EDLCs electrodes. The semicircle shape represents the charge-transfer resistance (R_ct) at the electrode/electrolyte interface. The obtained values of R_ct are 0.43, 1.17, and 3.32 Ω for CNT/Fe₃C-2, CNT/Fe₃C-4 and CNT/Fe₃C-6, respectively. The R_ct of CNT/Fe₃C-6 is much higher than that of the previous reported CNTs,^13,25 which is assigned to the high microposity and Fe₃C nanorods of N-doped CNT/Fe₃C-6. However, the equivalent series resistances (ESR) are only 0.26 Ω calculated from x-intercepts with the real axis of the Nyquist plots in high frequency part due to the high graphitization, which is close to the values of CNTs activated by KOH activation.¹³ The lower ESR of N-doped CNT/Fe₃C electrodes is beneficial to enhance the electrochemical performance.

The cycling stability plays an important role for the practical application of supercapacitors. Fig. 6 shows that after 5000 successive charging–discharging cycles at 1 A g⁻¹, the specific capacitance of CNT/Fe₃C-6 remains 135.6 F g⁻¹, which is 93.6% of its initial value (144.8 F g⁻¹), indicating the high electrochemical stability. No significant electrochemical changes were observed after long-term process, as shown in the inset of Fig. 6. It further proves the excellent cycling stability of CNT/Fe₃C-6.

Fig. 6 The cycling performance of CNT/Fe₃C-6 at the current density of 1 A g⁻¹, and the inset shows the charge/discharge curves of the 10 cycles of the electrode.

4 Conclusions

In summary, a highly porous N-doped CNTs/Fe₃C was facilely synthesized through one-step pyrolysis of melamine with a FeCl₃ catalyst and ZnCl₂ activator. The as-prepared CNT/Fe₃C-6 possess the characteristics of large specific surface area (1021.26 m² g⁻¹), and Fe₃C nanorods were embedded into the inner of N-doped CNTs. As a result, CNT/Fe₃C-6 electrode exhibits the outstanding capacitive behavior (181 F g⁻¹ at 0.1 A g⁻¹), rate capacity and cycling stability in 6 M KOH aqueous electrolyte, which is promising for applications as EDLCs electrode materials.

Acknowledgements

The work was financially supported by the National Natural Science Foundation of China (No. 51102216 and 51572140), the Program for the Innovative Talents of Higher Learning Institutions of Shanxi, and Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi.

References

1. S. Iijima, Nature, 1991, 354, 56–58.
2. E. Frackowiak and F. Béguin, Carbon, 2002, 40, 1775–1787.
3. G. Lota, K. Fic and E. Frackowiak, Energy Environ. Sci., 2011, 4, 1592–1605.
4. H. Pan, J. Li and Y. Feng, Nanoscale Res. Lett., 2010, 5, 654–668.
5. R. A. Fisher, M. R. Watt and W. Jud Ready, ECS J. Solid State Sci. Technol., 2013, 2, M3170–M3177.
6. P. Bondavalli, D. Pribat, J. P. Schnell, C. Delfaure, L. Gorintin, P. Legagneux, L. Baraton and C. Galindo, Eur. Phys. J.: Appl. Phys., 2012, 60, 10401–10414.
7. S. Zhang, H. Zhang, Q. Liu and S. Chen, J. Mater. Chem. A, 2013, 1, 3302–3308.
8. H. Zhang, G. Cao and Y. Yang, Energy Environ. Sci., 2009, 2, 932–943.
9. J. Wang, L. Li, C. L. Wong and S. Madhavi, Nanotechnology, 2012, 23, 495401.
10. W. D. Zhang, B. Xu and L. C. Jiang, J. Mater. Chem., 2010, 20, 6383–6391.
11. X. Xie, M. Ye, L. Hu, N. Liu, J. R. McDonough, W. Chen, H. Alshareef, C. S. Criddle and Y. Cui, Energy Environ. Sci., 2012, 5, 5265–5270.
12. Z. Wang, R. Jia, J. Zheng, J. Zhao, L. Li, J. Song and Z. Zhu, ACS Nano, 2011, 5, 1677–1684.
13. B. Xu, F. Wu, Y. Su, G. Cao, S. Chen, Z. Zhou and Y. Yang, Electrochim. Acta, 2008, 53, 7730–7735.
14. J. Wang and S. Kaskel, J. Mater. Chem., 2012, 22, 23710–23725.
15. E. Frackowiak, S. Delpeux, K. Jurewicz, K. Szostak, D. Cazorla-Amoros and F. Béguin, Chem. Phys. Lett., 2002, 361, 35–41.
16. C. Emmenegger, P. Mauron, P. Sudan, P. Wenger, V. Hermann, R. Gallay and A. Züttel, J. Power Sources, 2003, 124, 321–329.
17. Q. Jiang, Y. Zhao, X. Y. Lu, X. T. Zhu, G. Q. Yang, L. J. Song, Y. D. Cai, X. M. Ren and L. Qian, Chem. Phys. Lett., 2005, 410, 307–311.
18. E. Raymundo-Piñero, P. Azaïs, T. Cacciaguerra, D. Cazorla-Amorós, A. Linares-Solano and F. Béguin, Carbon, 2005, 43, 786–795.
19. G. Wang, R. Liang, L. Liu and B. Zhong, Electrochim. Acta, 2014, 115, 183–188.
20. L. X. Li, J. Tao, X. Geng and B. G. An, Acta Phys.-Chim. Sin., 2013, 29, 111–116.
21. Y. S. Yun, H. H. Park and H. J. Jin, Materials, 2012, 5, 1258–1266.
22. D. Hulicova-Jurcakova, M. Kodama, S. Shiraishi, H. Hatori, Z. H. Zhu and G. Q. Lu, Adv. Funct. Mater., 2009, 19, 1800–1809.
23. P. Ayala, R. Arenal, M. Rümmeli, A. Rubio and T. Pichler, Carbon, 2010, 48, 575–586.
24. L. Chen, K. Xia, L. Huang, L. Li, L. Pei and S. Fei, Int. J. Hydrogen Energy, 2013, 38, 3297–3303.
25. Y. Wang, Y. Liu, W. Liu, H. Chen, G. Zhang and J. Wang, Mater. Lett., 2015, 154, 64–67.
26. A. Kraupner, A. Markus, R. Palkovits, K. Schlicht and C. Giordano, J. Mater. Chem., 2010, 20, 6019–6022.
27. Z. Wen, S. Ci, F. Zhang, X. Feng, S. Cui, S. Mao, S. Luo, Z. He and J. Chen, Adv. Mater., 2012, 24, 1399–1404.
28. L. Su, Z. Zhou and P. Shen, Electrochim. Acta, 2013, 87, 180–185.
29. Y. Tan, K. Zhu, D. Li, F. Bai, Y. Wei and P. Zhang, Chem. Eng. J., 2014, 258, 93–100.
30. Y. Liao, K. Pan, L. Wang, Q. Pan, W. Zhou, X. Miao, B. Jiang, C. Tian, G. Tian, G. Wang and H. Fu, ACS Appl. Mater. Interfaces, 2013, 5, 3663–3670.
31. X. Zhao, D. Xia, J. Yue and S. Liu, Electrochim. Acta, 2014, 116, 292–299.
32. Y. Zhao, M. Liu, X. Deng, L. Miao, P. K. Tripathi, X. Ma, D. Zhu, Z. Xu, Z. Hao and L. Gan, Electrochim. Acta, 2015, 153, 448–455.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07101e

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