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Nitrogen-doped graphene for supercapacitor with long-term electrochemical stability
Energy xxx (2014) 1e6
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Nitrogen-doped graphene for supercapacitor with long-term electrochemical stability Kai Wang a, *, Liwei Li a, Tiezhu Zhang b, Zaifei Liu c a
School of Automation Engineering, Qingdao University, Qingdao 266071, China Vehicle Electronic Technology Research Institute, Qingdao University, Qingdao 266071, China c State Grid Heze Power Supply Company, Heze 274000, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 August 2013 Received in revised form 10 April 2014 Accepted 14 April 2014 Available online xxx
Nitrogen-doped graphene is prepared by a solid microwave method with EDA (ethylenediamine) as the nitrogen source. The experimental results reveal that nitrogen atoms from the grafted EDA molecules on the surface of graphene are successfully doped into the lattices. The NGS (nitrogen-doped graphene nanosheets) sample exhibits outstanding specific capacitances of 197 and 151 F g1 at the current densities of 0.5 and 5 A g1 in 6.0 mol L1 KOH aqueous electrolyte, respectively. Furthermore, the sample also displays more superior rate capacity, which can possess high specific capacitance retention of 77% and 70% at the high current densities of 5 and 40 A g1, respectively. In addition, a capacity fading lower than 2% after 5000 cycles of charging and discharging is obtained, indicating its long-term electrochemical stability. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Ethylenediamine Functionalization Microwave Supercapacitor
1. Introduction Following the astonishing discoveries of fullerene and CNT (carbon nanotube) in earlier decades, the emergence of graphene has recently opened up an exciting new field in the science and technology of two-dimensional nanomaterial with continuously growing academic and technological impetus [1e3]. Owing to its outstanding intrinsic physical properties, such as extraordinarily high electrical and thermal conductivity, large surface area, and transcendental chemical stability, graphene exhibits great potential for application in supercapacitors [4,5]. Furthermore, the doping in graphene with nitrogen atoms has drawn much attention because conjugation between the nitrogen long-pair electrons and the graphene p-system can change its chemical and physical properties, like modulating electrical conductivity, accelerating the growth of nanoparticles, enhancing the generated materials activity [6,7]. Nitrogen-containing functional groups can be introduced through either in situ doping method or post-synthesis method [8e 10]. In situ doping method, nitrogen source is added in chemical vapor deposition process to assure nitrogen atoms are doped in situ in lattices of graphene as it grows [11e13]. By contrast, in post-
synthesis method, graphene is treated with nitrogenous molecules at high temperature to conduct nitrogen doping. In this method, GO (graphite oxide) has been most widely used and confirmed to be the most successful raw material for mass production of graphene [14e16], due to the outstanding properties such as high efficient, low-cost operation and environmental friendliness compared with in situ doping method. As a result, post-synthesis method has a much more promising future [17]. NH3 has been by far the most frequently used reagent to introduce nitrogen atoms into graphene [18]. However, because of the serious toxicity and corrosion, NH3 sets a great demand on instruments, which is not appropriate for mass production [19,20]. In this study, we develop a rapid and effective solid microwave method with EDA (ethylenediamine) as nitrogen source to synthesize nitrogen-doped graphene. In brief, we utilize the ringopening reaction between EDA and epoxy groups of GO to fabricate FGS (functionalized graphene nanosheets), and then treat the dried FGS with microwave irradiation to carry out deep reduction and nitrogen doping at the same time. 2. Experimental 2.1. Synthesis of GO
* Corresponding author. Tel.: þ86 158 6306 0145. E-mail address: [email protected] (K. Wang).
GO was synthesized from flaky graphite powder by a modified Hummers method. In brief, 5 g of graphite powder (180
http://dx.doi.org/10.1016/j.energy.2014.04.034 0360-5442/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang K, et al., Nitrogen-doped graphene for supercapacitor with long-term electrochemical stability, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.034
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mesh, Sinopharm Chemical Reagent Co. Ltd.) and 130 mL concentrated sulfuric acid (98%, Sinopharm Chemical Reagent Co. Ltd.) were put into a 1000 mL graduated beaker and stirred continuously in ice bath for 2 h. Then 15 g of potassium permanganate (Analytic grade, Sinopharm Chemical Reagent Co. Ltd.) was slowly added and the solution was stirred in ice bath for another 2 h. After that, the solution was stirred in water bath at 35 C for 1 h, further promoting the oxidation of graphite, and this is the mesothermal reaction stage. 230 mL deionized water was added and the suspension was heated up to 98 C for 30 min, which is hyperthermal reaction stage. At the end of hyperthermal reaction stage, 400 mL deionized water was added, and the solution was centrifuged and washed until neutral to obtain GO suspension. 2.2. Preparation of nitrogen-doped graphene The GO suspension was diluted to 1 mg mL1, and then 120 mL of the solution and 1.5 mL EDA (Sinopharm Chemical Reagent Co. Ltd.) were mixed in a 250 mL flask, and refluxed for 6 h at 95 C. After the reaction, the obtained precipitate was freeze-dried to prepare solid FGS. Nitrogen-doped graphene was obtained by treating the solid FGS in an automated focused microwave system under argon flow at full power for 1 min. 2.3. Characterization SEM (scanning electron microscopy) images of samples were performed on a JEOL S-4800 FESEM (field emission SEM), while the TEM (transmission electron microscope) image of NGS (nitrogendoped graphene nanosheets) was obtained on a Philips Tecnai G20 TEM. Surface functional groups were measured using a Bruker Equinox 55 FTIR (Fourier transform infrared spectrometer). XPS (Xray photoelectron spectra) were obtained on a VG ESCALAB MK II Xray photoelectron spectrometer. The XRD (X-ray diffraction) patterns were recorded on a Rigaku D/Max2400 diffractometer. Raman spectra were recorded using Renishaw Raman Spectrometer, Germany.
2.4. Electrochemical measurements In order to evaluate the capacitive performances of the NGS sample in electrochemical capacitors, a mixture of 80 wt.% the NGS sample, 15 wt.% acetylene black and 5 wt.% PTFE (polytetrafluoroethylene) binder was fabricated using ethanol as a solvent. Slurry of the above mixture was subsequently pressed onto nickel foam under a pressure of 20 MPa, serving as the current collector. The prepared electrode was placed in a vacuum drying oven at 120 C for 24 h. A three electrode experimental setup taking a 6.0 mol L1 KOH aqueous solution as electrolyte was used in cyclic voltammetry and galvanostatic chargeedischarge measurements on an electrochemical working station (CHI660D, ChenHua Instruments Co. Ltd., Shanghai). The prepared electrode, platinum foil (6 cm2) and SCE (saturated calomel electrode) were used as the working, counter and reference electrodes, respectively. The whole electrochemical measurements were carried out at w25 C. 3. Results and discussion 3.1. SEM, TEM and EDS of FGS and NGS As shown in Fig. 1(a), the GO material was in packing state due to strong interlayer vdW (van der Waals) forces. Functionalization and microwave irradiation effectively reduced the packing state degree of graphene sheets as shown in Fig. 1(c). Furthermore, the exfoliated sheets were distributed randomly and there were numerous winkles on the surface, indicating that graphene could be successfully synthesized through functionalization and microwave irradiation. Fig. 1(d) shows the typical TEM image of NGS. Continuous, transparent and crumpled graphene sheets were stacked together forming the multilayered structure, which was probably caused by the nitrogen atoms doped into the graphene lattices [21]. This kind of structure is conductive to make electrode materials be thoroughly exposed to electrolyte, providing space for forming EDLC (electronic double layer capacitor). The chemical states of elements in FGS and NGS were analyzed by EDS (Energy Dispersive Spectrometer), as shown in Fig. 1(e), and
Fig. 1. SEM and TEM images of GO, FGS and NGS as well as EDS analysis of FGS and NGS.
Please cite this article in press as: Wang K, et al., Nitrogen-doped graphene for supercapacitor with long-term electrochemical stability, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.034
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Fig. 2. FTIR spectra of GO, FGS and NGS.
the mass and mol ratio were shown in Fig. 1(f). The XPS spectrum of FGS and NGS exhibited three peaks at 0.27, 0.39 and 0.52 keV, and they were assigned to C1s, N1s and O1s peaks, respectively. In Fig. 1(e), N1s and O1s peaks of NGS decreased significantly, because polar oxygen-containing functional groups decomposed to CO, CO2 and H2O during microwave irradiation. At the same time, grafted EDA molecules decomposed and separated from the surface, and then they were doped in lattices at the high temperature caused by microwave irradiation. On the basis of quantitative analysis, C/O atom mass rate changed from 3.41 to 5.08 after microwave irradiation and the content of nitrogen element was about 7.34%. 3.2. FTIR of GO, FGS and NGS Characteristic FTIR spectra of GO, FGS and NGS are shown in Fig. 2. GO possessed the peaks of CeO at 1050 cm1 and C]O at 1732 cm1 because of their stretching vibration, as well as the absorption peak at 1614 cm1 which was ascribed to the skeletal vibration of aromatic ring. After functionalization, the stretching vibration of CeO weakened significantly and two peaks of NeH appeared at 1471 and 1534 cm1, indicating that EDA molecules were grafted onto the surface of graphene successfully. As for NGS, peaks of oxygen-containing functional groups observably decreased after microwave irradiation. Combing the EDS analysis, we can conclude that microwave irradiation can remove oxygencontaining groups on the surface of GO effectively. 3.3. XPS of FGS and NGS In order to further investigate how nitrogen atoms exist, XPS spectroscopy was performed, as shown in Fig. 3. The spectra of FGS
3
and NGS exhibited three peaks, namely C1s peak at 284.6 eV, N1s peak at 399 eV and O1s peak 531.8 eV. However, compared with the N1s and O1s peaks intensities of FGS, those of NGS decreased obviously, which is consistent with the EDS analysis. Because of the outstanding chemical stability of carbon materials, only at high temperature can they be doped. As a result, the high-resolution XPS N1s core-level spectrum of FGS exhibited no evidence of doped nitrogen but of amidogen-containing functional groups, which consistently matched the previous reports [22,23]. During microwave irradiation, at the extremely high temperature caused by the intense interaction between microwave and graphene, most of the functional groups separated from the surface and then a part of the molecules and free radicals generated by amidogen thermal degradation were doped into lattices of graphene [24]. As shown in Fig. 3(c), three types of nitrogen doping including pyridinic N, pyrrolic N, and graphitic N can occur. The corresponding specific binding energies in the XPS spectra were 398.2, 399.7, and 401.3 eV for the pyridinic, pyrrolic, and graphitic Ns, respectively, which indicated the successful nitrogen doping. 3.4. XRD of GO, FGS and NGS Fig. 4 shows the XRD patterns of GO, FGS and NGS. The diffraction pattern of GO showed a clear (001) peak centered at 11.48 , indicating that flake graphite was intercalated completely. It was helpful to reduce the interaction of graphite layers for the next exfoliation. After functionalization, the (001) peak totally disappeared in the diffraction of FGS, indicating that the grafted EDA molecules effectively weakened interlayer vdW (van der Waals) forces and prevented the graphite structure from restoring. Furthermore, FGS present a wide (002) peak between 20 and 30 , demonstrating that layers of FGS were randomly distributed. After microwave irradiation, the peak between 20 and 30 of NGS increased compared with that of FGS, and its central position shifted to the right, deviating from the (002) peak of graphite. It was due to the reason that the wrinkles generated on the surface hindered graphene layers from restacking. 3.5. Raman of GO, FGS and NGS Fig. 5 shows the Raman spectra of GO, FGS and NGS with different exposure time. All of GO, FGS and NGS possessed two remarkable characteristic peaks at 1344 cm1 and 1590 cm1 corresponding to the well-defined D band and G band, respectively. After intense oxidation-intercalation, the ID/IG of GO increased obviously to 0.97 from 0 compared with that of flake graphite [25e 27]. In addition, the ID/IG of FGS further increased from 0.97 to 1.25 owing to the more carbon sp3 atoms brought about by functionalization. However, the D band intensity of NGS present a decrease and the ID/IG fell to 1.16, which was due to the reason that the
Fig. 3. (a) XPS spectra of FGS and NGS, N 1s spectra of FGS (b) and NGS (c).
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Fig. 4. XRD patterns of GO, FGS and NGS.
functional groups on the surface were removed effectively at the high temperature caused by microwave irradiation. 3.6. Electrochemical measurements Fig. 6 showed the CV (cyclic voltammetric) curves of the NGS material at different scan rates ranging from 5 to 20 mV s1. As shown in Fig. 6, rectangular shapes were observed. These results demonstrated that the NGS material has a high charge storage capacity with fast ion and charge transfer. Fig. 7 exhibited the comparative galvanostatic charge/discharge curves under different current densities. In the case of the NGS material, the specific capacitance calculated from the galvanostatic discharge curve was up to 197 F g1 at a current density of 0.5 A g1, which is much better than those reported by Wang et al. (205 F g1 at 100 mA g1) [28]. When the current density increased to 5 A g1, the specific capacitance still remained at 151 F g1 with w77% capacitance retention. These results indicated a good rate capability of the NGS supercapacitor. Variations in specific capacitance values with increase in current density for the supercapacitor were shown in Fig. 8. The figure
Fig. 5. Raman spectra of GO, FGS and GS.
Fig. 6. Cyclic voltammograms of NGS at various scan rates.
indicated that NGS showed good rate performance; where over 77% and 70% of the initial capacitance was retained at high current density of 5 A g1 and 40 A g1, respectively. The result is superior to that reported by Lin et al. (74% of the initial capacitance was retained at a current density of 5 A g1) or by Zhao et al. (68% of the initial capacitance was retained at a high current density of 20 A g1) [29,30]. In general, at current densities lower than 5 A g1, the specific capacitance decreased with the increase in discharge current density and above that the specific capacitance tended to be stable. The cycle life, a key evaluation factor for practical applications of supercapacitors, was measured by galvanostatic charge-discharge technique between 0 and 1 V at a current density of 0.5 A g1 for 5000 cycles, as shown in Fig. 9. NGS based supercapacitor exhibited excellent capacitance retention of w98% after 5000 cycles. Those good capacity retentions suggested the NGS sample held good stability and high degree of reversibility in the repetitive chargedischarge cycling. The EIS (Electrochemical Impedance Spectroscopy) data in the form of Nyquist plots in Fig. 10 showed that NGS displayed inconspicuous arcs in the high frequency region and straight lines in the low frequency region. The high frequency loop was related to
Fig. 7. Galvanostatic chargeedischarge curves measured at various current densities.
Please cite this article in press as: Wang K, et al., Nitrogen-doped graphene for supercapacitor with long-term electrochemical stability, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.034
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electronic resistance and NGS showed very small electronic resistance (w0.18 U). The magnitude of ESR (equivalent series resistance) was obtained from the x-intercept and it was w0.32 U, indicating excellent power density of the NGS based supercapacitors. The vertical shape at lower frequencies indicated a pure capacitive behavior and represents ion diffusion in the structure of electrode. 4. Conclusions
Fig. 8. Specific capacitances at various current densities.
In conclusion, a solid microwave method has been developed to prepare nitrogen-doped graphene, using EDA as functionalizing agent and nitrogen source. In contrary to traditional methods, this approach has several advantages for the synthesis of the nitrogendoped graphene: (1) EDA can serve as not only functionalizing agent but also nitrogen source, and the existence of nitrogen incorporated into graphene can benefit for the increase of capacitive performance for supercapacitors. (2) Highly toxic and corrosive chemicals such as NH3 are avoided, which is environmentally friendly. (3) The NGS sample exhibits excellent long-term electrochemical stability with 5000 cycles. The nitrogen-doped graphene represents an alternative electrode material for supercapacitors, which can potentially be applied for mass production. Acknowledgments The work was supported by the Shandong province Natural Science Foundation of China (No. Y2008F23), Shandong province Science and Technology Development Plan (No. 2011GGB01123) and 863 programme (No. 2012AA110407). References
Fig. 9. Cycle stability of the carbon samples.
Fig. 10. Nyquist plots of NGS.
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Nitrogen-doped graphene for supercapacitor with long-term electrochemical stability Kai Wang a, *, Liwei Li a, Tiezhu Zhang b, Zaifei Liu c a
School of Automation Engineering, Qingdao University, Qingdao 266071, China Vehicle Electronic Technology Research Institute, Qingdao University, Qingdao 266071, China c State Grid Heze Power Supply Company, Heze 274000, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 August 2013 Received in revised form 10 April 2014 Accepted 14 April 2014 Available online xxx
Nitrogen-doped graphene is prepared by a solid microwave method with EDA (ethylenediamine) as the nitrogen source. The experimental results reveal that nitrogen atoms from the grafted EDA molecules on the surface of graphene are successfully doped into the lattices. The NGS (nitrogen-doped graphene nanosheets) sample exhibits outstanding specific capacitances of 197 and 151 F g1 at the current densities of 0.5 and 5 A g1 in 6.0 mol L1 KOH aqueous electrolyte, respectively. Furthermore, the sample also displays more superior rate capacity, which can possess high specific capacitance retention of 77% and 70% at the high current densities of 5 and 40 A g1, respectively. In addition, a capacity fading lower than 2% after 5000 cycles of charging and discharging is obtained, indicating its long-term electrochemical stability. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Ethylenediamine Functionalization Microwave Supercapacitor
1. Introduction Following the astonishing discoveries of fullerene and CNT (carbon nanotube) in earlier decades, the emergence of graphene has recently opened up an exciting new field in the science and technology of two-dimensional nanomaterial with continuously growing academic and technological impetus [1e3]. Owing to its outstanding intrinsic physical properties, such as extraordinarily high electrical and thermal conductivity, large surface area, and transcendental chemical stability, graphene exhibits great potential for application in supercapacitors [4,5]. Furthermore, the doping in graphene with nitrogen atoms has drawn much attention because conjugation between the nitrogen long-pair electrons and the graphene p-system can change its chemical and physical properties, like modulating electrical conductivity, accelerating the growth of nanoparticles, enhancing the generated materials activity [6,7]. Nitrogen-containing functional groups can be introduced through either in situ doping method or post-synthesis method [8e 10]. In situ doping method, nitrogen source is added in chemical vapor deposition process to assure nitrogen atoms are doped in situ in lattices of graphene as it grows [11e13]. By contrast, in post-
synthesis method, graphene is treated with nitrogenous molecules at high temperature to conduct nitrogen doping. In this method, GO (graphite oxide) has been most widely used and confirmed to be the most successful raw material for mass production of graphene [14e16], due to the outstanding properties such as high efficient, low-cost operation and environmental friendliness compared with in situ doping method. As a result, post-synthesis method has a much more promising future [17]. NH3 has been by far the most frequently used reagent to introduce nitrogen atoms into graphene [18]. However, because of the serious toxicity and corrosion, NH3 sets a great demand on instruments, which is not appropriate for mass production [19,20]. In this study, we develop a rapid and effective solid microwave method with EDA (ethylenediamine) as nitrogen source to synthesize nitrogen-doped graphene. In brief, we utilize the ringopening reaction between EDA and epoxy groups of GO to fabricate FGS (functionalized graphene nanosheets), and then treat the dried FGS with microwave irradiation to carry out deep reduction and nitrogen doping at the same time. 2. Experimental 2.1. Synthesis of GO
* Corresponding author. Tel.: þ86 158 6306 0145. E-mail address: [email protected] (K. Wang).
GO was synthesized from flaky graphite powder by a modified Hummers method. In brief, 5 g of graphite powder (180
http://dx.doi.org/10.1016/j.energy.2014.04.034 0360-5442/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang K, et al., Nitrogen-doped graphene for supercapacitor with long-term electrochemical stability, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.034
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mesh, Sinopharm Chemical Reagent Co. Ltd.) and 130 mL concentrated sulfuric acid (98%, Sinopharm Chemical Reagent Co. Ltd.) were put into a 1000 mL graduated beaker and stirred continuously in ice bath for 2 h. Then 15 g of potassium permanganate (Analytic grade, Sinopharm Chemical Reagent Co. Ltd.) was slowly added and the solution was stirred in ice bath for another 2 h. After that, the solution was stirred in water bath at 35 C for 1 h, further promoting the oxidation of graphite, and this is the mesothermal reaction stage. 230 mL deionized water was added and the suspension was heated up to 98 C for 30 min, which is hyperthermal reaction stage. At the end of hyperthermal reaction stage, 400 mL deionized water was added, and the solution was centrifuged and washed until neutral to obtain GO suspension. 2.2. Preparation of nitrogen-doped graphene The GO suspension was diluted to 1 mg mL1, and then 120 mL of the solution and 1.5 mL EDA (Sinopharm Chemical Reagent Co. Ltd.) were mixed in a 250 mL flask, and refluxed for 6 h at 95 C. After the reaction, the obtained precipitate was freeze-dried to prepare solid FGS. Nitrogen-doped graphene was obtained by treating the solid FGS in an automated focused microwave system under argon flow at full power for 1 min. 2.3. Characterization SEM (scanning electron microscopy) images of samples were performed on a JEOL S-4800 FESEM (field emission SEM), while the TEM (transmission electron microscope) image of NGS (nitrogendoped graphene nanosheets) was obtained on a Philips Tecnai G20 TEM. Surface functional groups were measured using a Bruker Equinox 55 FTIR (Fourier transform infrared spectrometer). XPS (Xray photoelectron spectra) were obtained on a VG ESCALAB MK II Xray photoelectron spectrometer. The XRD (X-ray diffraction) patterns were recorded on a Rigaku D/Max2400 diffractometer. Raman spectra were recorded using Renishaw Raman Spectrometer, Germany.
2.4. Electrochemical measurements In order to evaluate the capacitive performances of the NGS sample in electrochemical capacitors, a mixture of 80 wt.% the NGS sample, 15 wt.% acetylene black and 5 wt.% PTFE (polytetrafluoroethylene) binder was fabricated using ethanol as a solvent. Slurry of the above mixture was subsequently pressed onto nickel foam under a pressure of 20 MPa, serving as the current collector. The prepared electrode was placed in a vacuum drying oven at 120 C for 24 h. A three electrode experimental setup taking a 6.0 mol L1 KOH aqueous solution as electrolyte was used in cyclic voltammetry and galvanostatic chargeedischarge measurements on an electrochemical working station (CHI660D, ChenHua Instruments Co. Ltd., Shanghai). The prepared electrode, platinum foil (6 cm2) and SCE (saturated calomel electrode) were used as the working, counter and reference electrodes, respectively. The whole electrochemical measurements were carried out at w25 C. 3. Results and discussion 3.1. SEM, TEM and EDS of FGS and NGS As shown in Fig. 1(a), the GO material was in packing state due to strong interlayer vdW (van der Waals) forces. Functionalization and microwave irradiation effectively reduced the packing state degree of graphene sheets as shown in Fig. 1(c). Furthermore, the exfoliated sheets were distributed randomly and there were numerous winkles on the surface, indicating that graphene could be successfully synthesized through functionalization and microwave irradiation. Fig. 1(d) shows the typical TEM image of NGS. Continuous, transparent and crumpled graphene sheets were stacked together forming the multilayered structure, which was probably caused by the nitrogen atoms doped into the graphene lattices [21]. This kind of structure is conductive to make electrode materials be thoroughly exposed to electrolyte, providing space for forming EDLC (electronic double layer capacitor). The chemical states of elements in FGS and NGS were analyzed by EDS (Energy Dispersive Spectrometer), as shown in Fig. 1(e), and
Fig. 1. SEM and TEM images of GO, FGS and NGS as well as EDS analysis of FGS and NGS.
Please cite this article in press as: Wang K, et al., Nitrogen-doped graphene for supercapacitor with long-term electrochemical stability, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.034
K. Wang et al. / Energy xxx (2014) 1e6
Fig. 2. FTIR spectra of GO, FGS and NGS.
the mass and mol ratio were shown in Fig. 1(f). The XPS spectrum of FGS and NGS exhibited three peaks at 0.27, 0.39 and 0.52 keV, and they were assigned to C1s, N1s and O1s peaks, respectively. In Fig. 1(e), N1s and O1s peaks of NGS decreased significantly, because polar oxygen-containing functional groups decomposed to CO, CO2 and H2O during microwave irradiation. At the same time, grafted EDA molecules decomposed and separated from the surface, and then they were doped in lattices at the high temperature caused by microwave irradiation. On the basis of quantitative analysis, C/O atom mass rate changed from 3.41 to 5.08 after microwave irradiation and the content of nitrogen element was about 7.34%. 3.2. FTIR of GO, FGS and NGS Characteristic FTIR spectra of GO, FGS and NGS are shown in Fig. 2. GO possessed the peaks of CeO at 1050 cm1 and C]O at 1732 cm1 because of their stretching vibration, as well as the absorption peak at 1614 cm1 which was ascribed to the skeletal vibration of aromatic ring. After functionalization, the stretching vibration of CeO weakened significantly and two peaks of NeH appeared at 1471 and 1534 cm1, indicating that EDA molecules were grafted onto the surface of graphene successfully. As for NGS, peaks of oxygen-containing functional groups observably decreased after microwave irradiation. Combing the EDS analysis, we can conclude that microwave irradiation can remove oxygencontaining groups on the surface of GO effectively. 3.3. XPS of FGS and NGS In order to further investigate how nitrogen atoms exist, XPS spectroscopy was performed, as shown in Fig. 3. The spectra of FGS
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and NGS exhibited three peaks, namely C1s peak at 284.6 eV, N1s peak at 399 eV and O1s peak 531.8 eV. However, compared with the N1s and O1s peaks intensities of FGS, those of NGS decreased obviously, which is consistent with the EDS analysis. Because of the outstanding chemical stability of carbon materials, only at high temperature can they be doped. As a result, the high-resolution XPS N1s core-level spectrum of FGS exhibited no evidence of doped nitrogen but of amidogen-containing functional groups, which consistently matched the previous reports [22,23]. During microwave irradiation, at the extremely high temperature caused by the intense interaction between microwave and graphene, most of the functional groups separated from the surface and then a part of the molecules and free radicals generated by amidogen thermal degradation were doped into lattices of graphene [24]. As shown in Fig. 3(c), three types of nitrogen doping including pyridinic N, pyrrolic N, and graphitic N can occur. The corresponding specific binding energies in the XPS spectra were 398.2, 399.7, and 401.3 eV for the pyridinic, pyrrolic, and graphitic Ns, respectively, which indicated the successful nitrogen doping. 3.4. XRD of GO, FGS and NGS Fig. 4 shows the XRD patterns of GO, FGS and NGS. The diffraction pattern of GO showed a clear (001) peak centered at 11.48 , indicating that flake graphite was intercalated completely. It was helpful to reduce the interaction of graphite layers for the next exfoliation. After functionalization, the (001) peak totally disappeared in the diffraction of FGS, indicating that the grafted EDA molecules effectively weakened interlayer vdW (van der Waals) forces and prevented the graphite structure from restoring. Furthermore, FGS present a wide (002) peak between 20 and 30 , demonstrating that layers of FGS were randomly distributed. After microwave irradiation, the peak between 20 and 30 of NGS increased compared with that of FGS, and its central position shifted to the right, deviating from the (002) peak of graphite. It was due to the reason that the wrinkles generated on the surface hindered graphene layers from restacking. 3.5. Raman of GO, FGS and NGS Fig. 5 shows the Raman spectra of GO, FGS and NGS with different exposure time. All of GO, FGS and NGS possessed two remarkable characteristic peaks at 1344 cm1 and 1590 cm1 corresponding to the well-defined D band and G band, respectively. After intense oxidation-intercalation, the ID/IG of GO increased obviously to 0.97 from 0 compared with that of flake graphite [25e 27]. In addition, the ID/IG of FGS further increased from 0.97 to 1.25 owing to the more carbon sp3 atoms brought about by functionalization. However, the D band intensity of NGS present a decrease and the ID/IG fell to 1.16, which was due to the reason that the
Fig. 3. (a) XPS spectra of FGS and NGS, N 1s spectra of FGS (b) and NGS (c).
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Fig. 4. XRD patterns of GO, FGS and NGS.
functional groups on the surface were removed effectively at the high temperature caused by microwave irradiation. 3.6. Electrochemical measurements Fig. 6 showed the CV (cyclic voltammetric) curves of the NGS material at different scan rates ranging from 5 to 20 mV s1. As shown in Fig. 6, rectangular shapes were observed. These results demonstrated that the NGS material has a high charge storage capacity with fast ion and charge transfer. Fig. 7 exhibited the comparative galvanostatic charge/discharge curves under different current densities. In the case of the NGS material, the specific capacitance calculated from the galvanostatic discharge curve was up to 197 F g1 at a current density of 0.5 A g1, which is much better than those reported by Wang et al. (205 F g1 at 100 mA g1) [28]. When the current density increased to 5 A g1, the specific capacitance still remained at 151 F g1 with w77% capacitance retention. These results indicated a good rate capability of the NGS supercapacitor. Variations in specific capacitance values with increase in current density for the supercapacitor were shown in Fig. 8. The figure
Fig. 5. Raman spectra of GO, FGS and GS.
Fig. 6. Cyclic voltammograms of NGS at various scan rates.
indicated that NGS showed good rate performance; where over 77% and 70% of the initial capacitance was retained at high current density of 5 A g1 and 40 A g1, respectively. The result is superior to that reported by Lin et al. (74% of the initial capacitance was retained at a current density of 5 A g1) or by Zhao et al. (68% of the initial capacitance was retained at a high current density of 20 A g1) [29,30]. In general, at current densities lower than 5 A g1, the specific capacitance decreased with the increase in discharge current density and above that the specific capacitance tended to be stable. The cycle life, a key evaluation factor for practical applications of supercapacitors, was measured by galvanostatic charge-discharge technique between 0 and 1 V at a current density of 0.5 A g1 for 5000 cycles, as shown in Fig. 9. NGS based supercapacitor exhibited excellent capacitance retention of w98% after 5000 cycles. Those good capacity retentions suggested the NGS sample held good stability and high degree of reversibility in the repetitive chargedischarge cycling. The EIS (Electrochemical Impedance Spectroscopy) data in the form of Nyquist plots in Fig. 10 showed that NGS displayed inconspicuous arcs in the high frequency region and straight lines in the low frequency region. The high frequency loop was related to
Fig. 7. Galvanostatic chargeedischarge curves measured at various current densities.
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electronic resistance and NGS showed very small electronic resistance (w0.18 U). The magnitude of ESR (equivalent series resistance) was obtained from the x-intercept and it was w0.32 U, indicating excellent power density of the NGS based supercapacitors. The vertical shape at lower frequencies indicated a pure capacitive behavior and represents ion diffusion in the structure of electrode. 4. Conclusions
Fig. 8. Specific capacitances at various current densities.
In conclusion, a solid microwave method has been developed to prepare nitrogen-doped graphene, using EDA as functionalizing agent and nitrogen source. In contrary to traditional methods, this approach has several advantages for the synthesis of the nitrogendoped graphene: (1) EDA can serve as not only functionalizing agent but also nitrogen source, and the existence of nitrogen incorporated into graphene can benefit for the increase of capacitive performance for supercapacitors. (2) Highly toxic and corrosive chemicals such as NH3 are avoided, which is environmentally friendly. (3) The NGS sample exhibits excellent long-term electrochemical stability with 5000 cycles. The nitrogen-doped graphene represents an alternative electrode material for supercapacitors, which can potentially be applied for mass production. Acknowledgments The work was supported by the Shandong province Natural Science Foundation of China (No. Y2008F23), Shandong province Science and Technology Development Plan (No. 2011GGB01123) and 863 programme (No. 2012AA110407). References
Fig. 9. Cycle stability of the carbon samples.
Fig. 10. Nyquist plots of NGS.
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Please cite this article in press as: Wang K, et al., Nitrogen-doped graphene for supercapacitor with long-term electrochemical stability, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.034