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A facile method for preparing nitrogen-doped graphene and its application in supercapacitors
Journal of Power Sources 273 (2015) 1156e1162
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
A facile method for preparing nitrogen-doped graphene and its application in supercapacitors Hong Jin a, Xiaomin Wang a, Zhengrong Gu a, *, Qihua Fan b, Bing Luo c a
Agricultural and Biosystems Engineering Department, South Dakota State University, P.O. Box 2120, 1400 North Campus Drive, AgE Building, SAE 221, Brookings, SD 57007, USA b Electrical Engineering and Computer Science Department, South Dakota State University, Office: SECS 229, USA c Characterization Facility, University of Minnesota, 100 Union Street SE, Minneapolis, MN 55455, USA
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
Nanostructured carbon with high capacitance >300 F g1 from DDGS. Nanostructured carbon containing graphene nanosheets. Nitrogen-doped graphene from DDGS source is firstly reported for producing supercapacitors. The as-prepared N-doped graphene presented extremely low internal resistance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2014 Received in revised form 17 September 2014 Accepted 2 October 2014 Available online 13 October 2014
Nitrogen-doped graphene (N-doped graphene) produced from dried distillers grains with solubles (DDGS) is an extremely attractive material for energy storage due to the high specific capacitance and low cost. The as-prepared samples are derived from DDGS, an inexpensive byproduct from fermentation processed optimized for first generation bio-fuel production. The scanning electron microscopy, transmission electron microscopy results demonstrate the 700 C treated sample (N-700) displays crumpled nanosheets and few-layer graphene nanostructure. XRD and Raman spectroscopy indicate N-700 presents the distinctive graphene diffraction peaks. The XPS of N-700 shows the N 1s can be split into pyrrolic-N and pyridinic-N. The cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy were measured based on N-700. A high specific capacitance of 324 F g1, and a relatively low inner resistance of 0.1 U were obtained. © 2014 Elsevier B.V. All rights reserved.
Keywords: N-doped graphene Surface area Pore structure Supercapacitors Biochar
1. Introduction Efficient energy conversion and storage is one of the great challenges society faces today [1]. It is crucial that highly efficient, low-cost and environmentally friendly systems can be developed for energy conversion and storage; therefore, developing energy
* Corresponding author. Tel.: þ1 605 688 5372. E-mail address: [email protected] (Z. Gu). http://dx.doi.org/10.1016/j.jpowsour.2014.10.010 0378-7753/© 2014 Elsevier B.V. All rights reserved.
storage materials have been become one of the quickest growing research fields [2e7]. Supercapacitors are particularly becoming more attractive since they can possess a large power density, long cycling life, modest energy density [8e12]. The performance of supercapacitors is mainly based on the electrode materials and because of the high surface area, high conductivity, and chemical stability, carbon materials are thought as the ideal candidate in the field of high performance supercapacitors [13,14]. Carbon aerogel materials have been studied in terms of the binderless electrode; however, the limitation is that the original
H. Jin et al. / Journal of Power Sources 273 (2015) 1156e1162
materials (organic xerogels) are not cost effective and not environmental friendly [15]. Thus, carbon aerogel is not the best choice for developing green electrode materials. Due to the low cost of its preparation and high surface area, activated carbon has been used commercially; however, it is not a good choice either for high quality supercapacitor [16]. The reason is that activated carbon has a long diffusion distance and a high ion-transfer resistance, which obstruct the application of supercapacitors in high current density. Carbon nanotubes have also been studied as the electrode for supercapacitors based on the excellent conductivity and high current density performance; nevertheless, the low specific capacitance and high cost as the critical drawbacks impede the application as good electrode for supercapacitors [17]. Graphene is considered as the attractive material in the area of nanocomposites [18], transparent conduction films [19], sensors [20], nanoelectronics [21], and energy storage devices [22,23] since it's been discovered in 2004. The theoretical capacitance of single layer graphene is 550 F g1, which is much higher than other carbon materials; therefore, it becomes more attractive than other carbon candidates for researchers working on the supercapacitors [24,25]. Several methods, such as mechanical exfoliation [26], chemical exfoliation [27], chemical synthesis [28], chemical vapor deposition [29] and electrochemical synthesis [30], etc. have been explored to synthesis high quality graphene. However, these synthesis methods are somehow not easy to scale up and the re-stacking of graphene becomes difficult to avoid and result in decreased specific capacitance. Chemical modification can improve the performance of graphene in the application of supercapacitors, specifically, nitrogen doped graphene has been considered as the good choice [31,32]. Various methods were used to synthesis the N-doped graphene. The electron beam deposition [33], arc discharge [34], nitrogen plasma [35,36] were used to produce N-dope graphene, with the assistance of ammonia, hydrazine or pyridine as nitrogen source. However, the high requirement of reaction condition, the low products yields, the high energy consumption and environmental issue due to the precursor of nitrogen sources are prompting researchers to find a facile way for getting high quality N-doped graphene. Our approach for N-doped graphene is innovative and cost effective. The protein rich (25%e35% protein) distillers dried grains with solubles (DDGS) was selected as the N-doped graphene precursor. Before dried, DDGS was immersed by Ni(NO3)2$6H2O solution. During this process, Nickel can be dispersed onto the surface of DDGS. After dried at 400 C, the mixture was immersed into KOH solution. The N-doped graphene was obtained by graphitizated activation at 700 C. The most important thing is that the asprepared N-doped graphene presented excellent electrochemical properties (high specific capacitance, low inner resistance and extremely good rectangular shape of cyclic voltammetry) as the electrode for supercapacitors. 2. Results and discussion 2.1. Physiochemical characterization of the samples The N-600, N-700, N-800, N-900 and N-1000 were named based on the reaction temperature. The chemical compositions of the as-prepared samples were characterized by the X-ray photoelectron spectroscopy (XPS). The results are shown in Fig. S1. The N600 and N-700 samples show the N 1s peak, while it is not obvious for the N-800 and there is no N 1s peak for N-900 and N-1000. These results indicate that the N-doped graphene structures were firstly formed at the lower temperature, and with the temperature increase, the N-doped carbon would thermodynamically
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decompose, specifically, when the temperature higher than 900 C, there is no nitrogen left in the carbon materials. Two components centered at 398.8 eV and 400.8 eV are detected in the N 1s peak of N-700 as shown in Fig. 1a, which are corresponded to the nitrogen forms of pyridinic-N and pyrrolic-N, respectively and the schematic structure of N-700 is shown in Fig. 1c. The pyrrolic-N is the dominant nitrogen form in N-600, N700 and N-800. The elemental contents were characterized by XPS as well, as shown in Table S1. The pyrrolic-N ratios of these three samples are 70.2%, 87.3% and 100% separately. It indicates that the decomposition of nitrogen was firstly performed in pyridinic-N, which is not like the simple pyrolysis process. Because the melting metal ions are easier to be adsorped by pyridinic-N, since pyridinic-N is more electron negative than pyrrolic-N. The total nitrogen ratios of these three samples are 4.77%, 2.64% and 0.74%. Besides, for the samples of N-900, N-1000, there are no nitrogen elements left. This result demonstrates that all of the nitrogen parts are decomposed when the temperature goes up to 900 C. The C 1s peaks of N-700 are shown in Fig. 1b. The main peak is located at 284.8 eV, which is the same as the graphite-like sp2 carbon. While the N-sp2 carbon and N-sp3 carbon are reflected by the peaks at 285.8 eV and 287.3 eV. In addition, the peak at 290.2 eV is because of the carboneoxygen bond. As shown in Fig. 1b, most of the carbons are in the C-sp2 form, while there are some connected with nitrogen forming carbonenitrogen bond. The intensity of the peak at 285.8 eV is stronger than the peak at 287.3 eV, which indicates nitrogen is mainly formed as N-sp2 carbon, while there is minor N-sp3 carbon existing in N-700. The morphology and microstructure of the samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and BrunauereEmmetteTeller (BET) surface area analysis. The distinctive microstructures and the complex three-dimensional porous structure of N-700 are shown in Fig. 2a. The wrinkled nanostructures, nanoporous structures and the few-layer graphene structures are shown in Fig. 2(bef). As shown in Fig. 2b & c, it is clear that there are wrinkled layers in the sample N-700. And the more detailed structures are characterized as shown in Fig. 2d, e and f. Most of the sample is composed with graphene layers less than 10-layer and the length of the graphene structures are in the nanoscale. Very few parts of graphene structure are about 10e30 layers. These nanostructures were further proved by nitrogen adsorptionedesorption isotherm curve and the pore size distribution curve. As Fig. 3a shows, the small hysteresis loop at the relative pressure P/P0 between 0.45 and 1.0 indicates that N-700 contains a small amount of mesopores. And the pore size distribution curve is further used to analyze N-700, while the data are shown in Fig. 3b and Table S2. The result shows the total pore volume is 0.85 cm3 g1, and 0.45 cm3 g1 pores are less than 2 nm, while 0.29 cm3 g1 pores are ranging from 2 to 10 nm and 0.11 cm3 g1 pores are above 10 nm, which means N-700 is presenting a hierarchical nanopore structure. The crystalline property of N-700 was also characterized by Raman spectroscopy as shown in Fig. 4a. The Raman spectrum shows N-700 is a typical partly disordered graphene system. The intensity ratio of G band versus D band (IG/ID) is 1.71, which means that N-700 contains more graphitized structures than disordered carbon. The compressed 2D band is due to the defective structure of graphene, namely it is caused by the nitrogen doping affection. The X-ray diffraction (XRD) pattern of N-700 is shown in Fig. 4b. It is clear that N-700 presents the peak 100, while the 002 peak is not obvious. This result is in accordance with the TEM characterization, which means that N-700 is composed by more single layer or few layer N-doped graphene sheets but not graphite chunk. XRD results comparison of these five samples is shown in Fig. S2. As is obvious the peak 002 of N-900 and N-1000 is sharper than the others,
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Fig. 1. a) XPS spectrum detailed N 1s, b) XPS spectrum detailed C 1s, c) schematic structure of N-700.
which is due to, in the high temperature, the graphene layers were aggregated and formed more graphite structure. As Fig. S3 shows, there are more thick layers structures while the catalysis reaction was performed in 1000 C. 2.2. Electrochemical performances of activated carbons The supercapacitor properties of the N-700 were studied through a set of symmetrical two-electrode 2032 coin-type system in a 6 M potassium hydroxide electrolyte. The cyclic voltammetry curves (CVs) of N-700 supercapacitor at various scan rates are shown in Fig. 5a. The typical rectangular shape of all of the scan rates ranging from 5 to 200 mV s1 indicates pure electric double layer capacitive features of N-700. The specific capacitances at different scan rates are shown in Fig. 5b. The high capacitance of
293 F g1 can be obtained at the scan rate of 5 mV s1, while it is shown high capacitance of 236 F g1 at the high scan rate of 200 mV s1. Fig. S4 demonstrates that the cyclic voltammetry curve of N-700 is more rectangular than the other four samples, which means N-700 is the best among all of these five samples. And the specific capacitance is higher than others as well. The galvanostatic charge/discharge properties are shown in Fig. 6. As can be observed, the charging plots are mainly symmetric to the discharging plots at the current densities, especially at the current density of 0.1 and 1.0 A g1. This indicates N-700 is a high capacitive reversible material for the application as the electrode of supercapacitors. The specific capacitances are shown in Fig. 6b. The specific capacitance is as high as 324 F g1 at the current density of 0.1 A g1, which is higher than most of the reported materials. More importantly, at the high current density of 10.0 A g1, the specific
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Fig. 2. Scanning electron microscopy image and transmission electron microscopy images of N-700, a) SEM image with a scale bar of 10 mm, b) TEM image with a scale bar of 100 nm, c) TEM image with a scale bar of 50 nm, d) TEM image with a scale bar of 20 nm, e) TEM image with a scale bar of 10 nm, f) TEM image with a scale bar of 5 nm.
capacitance can still achieve 270 F g1. The stability of N-700 is shown in Fig. S6, as is obvious that at the current density of 0.5 and 1.0 A g1, there is no drop of the capacitance. And as shown in Fig. S7, the specific capacitance of N-700 is much higher than the other samples.
The electrochemical properties of the N-700 were further characterized by electrochemical impedance spectroscopy (EIS). As shown in Fig. 7, the Nyquist plot of N-700 electrode was performed in a frequency ranging from 20 kHz to 10 mHz. The inner resistance (equivalent series resistance) is around 0.1 U, which indicates the
Fig. 3. Nitrogen adsorption/desorption results of N-700. a) Isothermal curve, b) pore size distribution.
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Fig. 4. Raman spectrum and X-ray diffraction of N-700, a) Raman spectrum b) XRD curve.
Fig. 5. Cyclic voltammetry results of N-700, a) cyclic voltammetry curves, b) The relationship between specific capacitance and scan rate.
excellent conductivity of the sample N-700. And the vertical linear part in the lower frequencies indicates the as prepared supercapacitor is an ideal double-layer supercapacitor. The inset in Fig. 7 shows properties of the medium frequency region. As is shown here, there is a 45 phase shift after the semicircle in the high frequency, which is caused by the porous structure of the N-doped graphene. The transport of ions from bulk electrolyte solution through the porous structure of the electrode to the surface of the electrode formed a diffusion inner resistance. And the diffusion inner resistance is very small in the supercapacitor, which is because the ions transport is promoted by the hierarchical porous
structures. Fig. S7 shows EIS comparison of these five samples. As is clear that the semicircle of N-700 is smaller than other samples, and linear part is shorter than others as well, which demonstrate that N-700 is an excellent candidate for high performance supercapacitors. 3. Conclusion According to the SEM, TEM, XRD, Raman spectroscopy, BET, and XPS results, the N-700 sample is the crumpled nanoporous fewlayer N-doped graphene. The high performance N-doped
Fig. 6. Galvanostatic charge/discharge results of N-700, a) The galvanostatic charge/discharge curve, b) The relationship between specific capacitance and current density.
H. Jin et al. / Journal of Power Sources 273 (2015) 1156e1162
Fig. 7. The electrochemical impedance spectrum of N-700.
graphene has been developed by a facile method with the DDGS as the carbon and nitrogen precursor. The carbonization temperature is found as the key factor to the properties of the N-doped graphene. The most important observation is that the N-700 sample presented high specific capacitance (324 F g1) and extremely low inner resistance (0.1 U) while it is used as the electrode for supercapacitors. The synthesis method is extremely attractive, since the N-doped graphene can be produced economically and effectively. This method is easy to scale-up and the precursor is easy to get and extremely cheap; nevertheless, it will be an attractive way to synthesis N-doped graphene for the application of supercapacitors in industry.
4. Experimental Preparation of the samples: The protein rich DDGS was used as the feeding material for generating N-dope graphene. 1.45 g of Ni(NO3)2$6H2O was dissolved in 20 g of water, and then the solution was transferred into a crucible where 20 g of DDGS was loaded as well. The mixture was placed for 1 h at room temperature, and then put into muffle furnace. These soaked mixture was then dried at 400 C in muffle furnace (chamber is 15*15*22 cm) in N2 atmosphere (N2 flow is 500 ml min1) for 20 min to decompose Ni(NO3)2$6H2O. The as-prepared product was cooled down and soaked into 20 ml 7.5 mol L1 potassium hydroxide solution for 1 h. This mixture was then put into oven and dried at 105 C for 24 h. After that the graphitizated activation was carried out at 600, 700, 800, 900 and 1000 C separately for 1 h with the protection of 500 ml min1 N2 flow. The obtained samples were cooled down in muffle furnace in the same N2 atmosphere. N-dope graphene samples were further purified in the 1.0 mol L1 HCl at 150 C within the hydrothermal reactor, and then washed with deionized water to pH value at 7 and dried at 105 C overnight under vacuum. Characterization of the samples: Physisorption analyses with N2 were carried out at 77 K (liquid nitrogen bath), using Tristar 3000 Micropore analyzer. The specific surface areas were calculated using the BrunauereEmmetteTeller (BET) equation. The total pore volumes were obtained at the relative pressure P/ P0 ¼ 0.978. The micropore volume, mesopore volume and the pore size distribution were determined by the NLDFT analysis for carbon with slit pore model (Micromeritics Inc.) based on the N2 isotherm adsorption data. Spectrum (Horiba LABRam confocal Raman microscope) was used to characterize N-dope graphene,
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while the excitation wavelength was set at 532 nm from a diode pumped solid-state laser. The micro-morphology was detected by SEM (HITACHI S-3400N). The XPS measurements were performed on an SSX-100 system (Surface Science Laboratories, Inc.) equipped with a monochromated Al Ka X-ray source, a hemispherical sector analyzer (HSA) and a resistive anode detector. The base pressure of the XPS system was 6.0 1010 Torr. During the data collection, the pressure was ca. 1.0 108 Torr. JEM-2100F was used to characterize the crystal and porous structure of N-doped graphene structure. Electrode preparation and electrochemical measurements: The supercapacitors were assembled as the general. For all of the electrochemical characterization, the surface area of the electrode was 1 cm2. Meanwhile, the electrolytic medium was 6 mol L1 KOH. The electrodes were prepared by pressing the slurry of N-doped graphene, carbon black (conducting material from Fisher scientific) and PTFE (binder) with a mass ration of 8:1:1 on nickel foam. Twoelectrode sandwich-type cells were built using a stainless coin cell (2032) with a microporous PP separator celgard-3501 between the electrodes. Cyclic voltammetry was performed using a SP-150 multichannel potentiostategalvanostat-EIS (Biologic, France) and galvanostatic charge/discharge cycling was performed in Neware battery test systems. The electrochemical impedance spectroscopy was tested by a SP-150 multichannel potentiostategalvanostat-EIS (Biologic, France) with the frequency ranging from 0.1 Hz to 200,000 Hz and the potential amplitude of 10 mV). The specific capacitance of the electrode was calculated using Equation (1) [37]
C¼
2IDt mDV
(1)
here, I is the charge or discharge current density, Dt is the corresponding charge or discharge time, m is one of the symmetry electrodes mass and DV is the total corresponding potential change. Acknowledgments This research is funded by following projects: “Development of high value carbon based adsorbents from thermochemically produced biochar” 2011-67009-20030 USDA-NIFA Agriculture and Food Research Initiative Sustainable Bioenergy Program funded Micropore analyzer, electrochemical analyzer and instruments for preparing graphene materials. NSF EpsCor Dakota BioCon center supported Dr. Hong Jin for his PhD study and XPS analysis. The TEM test was performed by Dr. Masahiro Kawasaki in JEOL Inc. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2014.10.010. References [1] J.W. Lee, A.S. Hall, J.-D. Kim, T.E. Mallouk, Chem. Mater. 24 (2012) 1158e1164. ~ ero, F. Leroux, F. Be guin, Adv. Mater. 18 (2006) 1877e1882. [2] E. Raymundo-Pin [3] G. Wang, X. Sun, F. Lu, H. Sun, M. Yu, W. Jiang, C. Liu, J. Lian, Small 8 (2012) 452e459. [4] H. Jiang, J. Ma, C. Li, Adv. Mater. 24 (2012) 4197e4202. [5] G. Zheng, L. Hu, H. Wu, X. Xie, Y. Cui, Energy Environ. Sci. 4 (2011) 3368. [6] Y. Huang, J. Liang, Y. Chen, Small 8 (2012) 1805e1834. [7] H. Jiang, P.S. Lee, C. Li, Energy Environ. Sci. 6 (2013) 41. [8] J. Huang, B.G. Sumpter, V. Meunier, Angew. Chem. Int. Ed. Engl. 47 (2008) 520e524. [9] Z.S. Wu, A. Winter, L. Chen, Y. Sun, A. Turchanin, X. Feng, K. Mullen, Adv. Mater. 24 (2012) 5130e5135. [10] X. Lu, T. Zhai, X. Zhang, Y. Shen, L. Yuan, B. Hu, L. Gong, J. Chen, Y. Gao, J. Zhou, Y. Tong, Z.L. Wang, Adv. Mater. 24 (2012) 938e944. [11] L. Hao, X. Li, L. Zhi, Adv. Mater. 25 (2013) 3899e3904. [12] K. Wang, H. Wu, Y. Meng, Z. Wei, Small 10 (2014) 14e31.
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
A facile method for preparing nitrogen-doped graphene and its application in supercapacitors Hong Jin a, Xiaomin Wang a, Zhengrong Gu a, *, Qihua Fan b, Bing Luo c a
Agricultural and Biosystems Engineering Department, South Dakota State University, P.O. Box 2120, 1400 North Campus Drive, AgE Building, SAE 221, Brookings, SD 57007, USA b Electrical Engineering and Computer Science Department, South Dakota State University, Office: SECS 229, USA c Characterization Facility, University of Minnesota, 100 Union Street SE, Minneapolis, MN 55455, USA
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
Nanostructured carbon with high capacitance >300 F g1 from DDGS. Nanostructured carbon containing graphene nanosheets. Nitrogen-doped graphene from DDGS source is firstly reported for producing supercapacitors. The as-prepared N-doped graphene presented extremely low internal resistance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2014 Received in revised form 17 September 2014 Accepted 2 October 2014 Available online 13 October 2014
Nitrogen-doped graphene (N-doped graphene) produced from dried distillers grains with solubles (DDGS) is an extremely attractive material for energy storage due to the high specific capacitance and low cost. The as-prepared samples are derived from DDGS, an inexpensive byproduct from fermentation processed optimized for first generation bio-fuel production. The scanning electron microscopy, transmission electron microscopy results demonstrate the 700 C treated sample (N-700) displays crumpled nanosheets and few-layer graphene nanostructure. XRD and Raman spectroscopy indicate N-700 presents the distinctive graphene diffraction peaks. The XPS of N-700 shows the N 1s can be split into pyrrolic-N and pyridinic-N. The cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy were measured based on N-700. A high specific capacitance of 324 F g1, and a relatively low inner resistance of 0.1 U were obtained. © 2014 Elsevier B.V. All rights reserved.
Keywords: N-doped graphene Surface area Pore structure Supercapacitors Biochar
1. Introduction Efficient energy conversion and storage is one of the great challenges society faces today [1]. It is crucial that highly efficient, low-cost and environmentally friendly systems can be developed for energy conversion and storage; therefore, developing energy
* Corresponding author. Tel.: þ1 605 688 5372. E-mail address: [email protected] (Z. Gu). http://dx.doi.org/10.1016/j.jpowsour.2014.10.010 0378-7753/© 2014 Elsevier B.V. All rights reserved.
storage materials have been become one of the quickest growing research fields [2e7]. Supercapacitors are particularly becoming more attractive since they can possess a large power density, long cycling life, modest energy density [8e12]. The performance of supercapacitors is mainly based on the electrode materials and because of the high surface area, high conductivity, and chemical stability, carbon materials are thought as the ideal candidate in the field of high performance supercapacitors [13,14]. Carbon aerogel materials have been studied in terms of the binderless electrode; however, the limitation is that the original
H. Jin et al. / Journal of Power Sources 273 (2015) 1156e1162
materials (organic xerogels) are not cost effective and not environmental friendly [15]. Thus, carbon aerogel is not the best choice for developing green electrode materials. Due to the low cost of its preparation and high surface area, activated carbon has been used commercially; however, it is not a good choice either for high quality supercapacitor [16]. The reason is that activated carbon has a long diffusion distance and a high ion-transfer resistance, which obstruct the application of supercapacitors in high current density. Carbon nanotubes have also been studied as the electrode for supercapacitors based on the excellent conductivity and high current density performance; nevertheless, the low specific capacitance and high cost as the critical drawbacks impede the application as good electrode for supercapacitors [17]. Graphene is considered as the attractive material in the area of nanocomposites [18], transparent conduction films [19], sensors [20], nanoelectronics [21], and energy storage devices [22,23] since it's been discovered in 2004. The theoretical capacitance of single layer graphene is 550 F g1, which is much higher than other carbon materials; therefore, it becomes more attractive than other carbon candidates for researchers working on the supercapacitors [24,25]. Several methods, such as mechanical exfoliation [26], chemical exfoliation [27], chemical synthesis [28], chemical vapor deposition [29] and electrochemical synthesis [30], etc. have been explored to synthesis high quality graphene. However, these synthesis methods are somehow not easy to scale up and the re-stacking of graphene becomes difficult to avoid and result in decreased specific capacitance. Chemical modification can improve the performance of graphene in the application of supercapacitors, specifically, nitrogen doped graphene has been considered as the good choice [31,32]. Various methods were used to synthesis the N-doped graphene. The electron beam deposition [33], arc discharge [34], nitrogen plasma [35,36] were used to produce N-dope graphene, with the assistance of ammonia, hydrazine or pyridine as nitrogen source. However, the high requirement of reaction condition, the low products yields, the high energy consumption and environmental issue due to the precursor of nitrogen sources are prompting researchers to find a facile way for getting high quality N-doped graphene. Our approach for N-doped graphene is innovative and cost effective. The protein rich (25%e35% protein) distillers dried grains with solubles (DDGS) was selected as the N-doped graphene precursor. Before dried, DDGS was immersed by Ni(NO3)2$6H2O solution. During this process, Nickel can be dispersed onto the surface of DDGS. After dried at 400 C, the mixture was immersed into KOH solution. The N-doped graphene was obtained by graphitizated activation at 700 C. The most important thing is that the asprepared N-doped graphene presented excellent electrochemical properties (high specific capacitance, low inner resistance and extremely good rectangular shape of cyclic voltammetry) as the electrode for supercapacitors. 2. Results and discussion 2.1. Physiochemical characterization of the samples The N-600, N-700, N-800, N-900 and N-1000 were named based on the reaction temperature. The chemical compositions of the as-prepared samples were characterized by the X-ray photoelectron spectroscopy (XPS). The results are shown in Fig. S1. The N600 and N-700 samples show the N 1s peak, while it is not obvious for the N-800 and there is no N 1s peak for N-900 and N-1000. These results indicate that the N-doped graphene structures were firstly formed at the lower temperature, and with the temperature increase, the N-doped carbon would thermodynamically
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decompose, specifically, when the temperature higher than 900 C, there is no nitrogen left in the carbon materials. Two components centered at 398.8 eV and 400.8 eV are detected in the N 1s peak of N-700 as shown in Fig. 1a, which are corresponded to the nitrogen forms of pyridinic-N and pyrrolic-N, respectively and the schematic structure of N-700 is shown in Fig. 1c. The pyrrolic-N is the dominant nitrogen form in N-600, N700 and N-800. The elemental contents were characterized by XPS as well, as shown in Table S1. The pyrrolic-N ratios of these three samples are 70.2%, 87.3% and 100% separately. It indicates that the decomposition of nitrogen was firstly performed in pyridinic-N, which is not like the simple pyrolysis process. Because the melting metal ions are easier to be adsorped by pyridinic-N, since pyridinic-N is more electron negative than pyrrolic-N. The total nitrogen ratios of these three samples are 4.77%, 2.64% and 0.74%. Besides, for the samples of N-900, N-1000, there are no nitrogen elements left. This result demonstrates that all of the nitrogen parts are decomposed when the temperature goes up to 900 C. The C 1s peaks of N-700 are shown in Fig. 1b. The main peak is located at 284.8 eV, which is the same as the graphite-like sp2 carbon. While the N-sp2 carbon and N-sp3 carbon are reflected by the peaks at 285.8 eV and 287.3 eV. In addition, the peak at 290.2 eV is because of the carboneoxygen bond. As shown in Fig. 1b, most of the carbons are in the C-sp2 form, while there are some connected with nitrogen forming carbonenitrogen bond. The intensity of the peak at 285.8 eV is stronger than the peak at 287.3 eV, which indicates nitrogen is mainly formed as N-sp2 carbon, while there is minor N-sp3 carbon existing in N-700. The morphology and microstructure of the samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and BrunauereEmmetteTeller (BET) surface area analysis. The distinctive microstructures and the complex three-dimensional porous structure of N-700 are shown in Fig. 2a. The wrinkled nanostructures, nanoporous structures and the few-layer graphene structures are shown in Fig. 2(bef). As shown in Fig. 2b & c, it is clear that there are wrinkled layers in the sample N-700. And the more detailed structures are characterized as shown in Fig. 2d, e and f. Most of the sample is composed with graphene layers less than 10-layer and the length of the graphene structures are in the nanoscale. Very few parts of graphene structure are about 10e30 layers. These nanostructures were further proved by nitrogen adsorptionedesorption isotherm curve and the pore size distribution curve. As Fig. 3a shows, the small hysteresis loop at the relative pressure P/P0 between 0.45 and 1.0 indicates that N-700 contains a small amount of mesopores. And the pore size distribution curve is further used to analyze N-700, while the data are shown in Fig. 3b and Table S2. The result shows the total pore volume is 0.85 cm3 g1, and 0.45 cm3 g1 pores are less than 2 nm, while 0.29 cm3 g1 pores are ranging from 2 to 10 nm and 0.11 cm3 g1 pores are above 10 nm, which means N-700 is presenting a hierarchical nanopore structure. The crystalline property of N-700 was also characterized by Raman spectroscopy as shown in Fig. 4a. The Raman spectrum shows N-700 is a typical partly disordered graphene system. The intensity ratio of G band versus D band (IG/ID) is 1.71, which means that N-700 contains more graphitized structures than disordered carbon. The compressed 2D band is due to the defective structure of graphene, namely it is caused by the nitrogen doping affection. The X-ray diffraction (XRD) pattern of N-700 is shown in Fig. 4b. It is clear that N-700 presents the peak 100, while the 002 peak is not obvious. This result is in accordance with the TEM characterization, which means that N-700 is composed by more single layer or few layer N-doped graphene sheets but not graphite chunk. XRD results comparison of these five samples is shown in Fig. S2. As is obvious the peak 002 of N-900 and N-1000 is sharper than the others,
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Fig. 1. a) XPS spectrum detailed N 1s, b) XPS spectrum detailed C 1s, c) schematic structure of N-700.
which is due to, in the high temperature, the graphene layers were aggregated and formed more graphite structure. As Fig. S3 shows, there are more thick layers structures while the catalysis reaction was performed in 1000 C. 2.2. Electrochemical performances of activated carbons The supercapacitor properties of the N-700 were studied through a set of symmetrical two-electrode 2032 coin-type system in a 6 M potassium hydroxide electrolyte. The cyclic voltammetry curves (CVs) of N-700 supercapacitor at various scan rates are shown in Fig. 5a. The typical rectangular shape of all of the scan rates ranging from 5 to 200 mV s1 indicates pure electric double layer capacitive features of N-700. The specific capacitances at different scan rates are shown in Fig. 5b. The high capacitance of
293 F g1 can be obtained at the scan rate of 5 mV s1, while it is shown high capacitance of 236 F g1 at the high scan rate of 200 mV s1. Fig. S4 demonstrates that the cyclic voltammetry curve of N-700 is more rectangular than the other four samples, which means N-700 is the best among all of these five samples. And the specific capacitance is higher than others as well. The galvanostatic charge/discharge properties are shown in Fig. 6. As can be observed, the charging plots are mainly symmetric to the discharging plots at the current densities, especially at the current density of 0.1 and 1.0 A g1. This indicates N-700 is a high capacitive reversible material for the application as the electrode of supercapacitors. The specific capacitances are shown in Fig. 6b. The specific capacitance is as high as 324 F g1 at the current density of 0.1 A g1, which is higher than most of the reported materials. More importantly, at the high current density of 10.0 A g1, the specific
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Fig. 2. Scanning electron microscopy image and transmission electron microscopy images of N-700, a) SEM image with a scale bar of 10 mm, b) TEM image with a scale bar of 100 nm, c) TEM image with a scale bar of 50 nm, d) TEM image with a scale bar of 20 nm, e) TEM image with a scale bar of 10 nm, f) TEM image with a scale bar of 5 nm.
capacitance can still achieve 270 F g1. The stability of N-700 is shown in Fig. S6, as is obvious that at the current density of 0.5 and 1.0 A g1, there is no drop of the capacitance. And as shown in Fig. S7, the specific capacitance of N-700 is much higher than the other samples.
The electrochemical properties of the N-700 were further characterized by electrochemical impedance spectroscopy (EIS). As shown in Fig. 7, the Nyquist plot of N-700 electrode was performed in a frequency ranging from 20 kHz to 10 mHz. The inner resistance (equivalent series resistance) is around 0.1 U, which indicates the
Fig. 3. Nitrogen adsorption/desorption results of N-700. a) Isothermal curve, b) pore size distribution.
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Fig. 4. Raman spectrum and X-ray diffraction of N-700, a) Raman spectrum b) XRD curve.
Fig. 5. Cyclic voltammetry results of N-700, a) cyclic voltammetry curves, b) The relationship between specific capacitance and scan rate.
excellent conductivity of the sample N-700. And the vertical linear part in the lower frequencies indicates the as prepared supercapacitor is an ideal double-layer supercapacitor. The inset in Fig. 7 shows properties of the medium frequency region. As is shown here, there is a 45 phase shift after the semicircle in the high frequency, which is caused by the porous structure of the N-doped graphene. The transport of ions from bulk electrolyte solution through the porous structure of the electrode to the surface of the electrode formed a diffusion inner resistance. And the diffusion inner resistance is very small in the supercapacitor, which is because the ions transport is promoted by the hierarchical porous
structures. Fig. S7 shows EIS comparison of these five samples. As is clear that the semicircle of N-700 is smaller than other samples, and linear part is shorter than others as well, which demonstrate that N-700 is an excellent candidate for high performance supercapacitors. 3. Conclusion According to the SEM, TEM, XRD, Raman spectroscopy, BET, and XPS results, the N-700 sample is the crumpled nanoporous fewlayer N-doped graphene. The high performance N-doped
Fig. 6. Galvanostatic charge/discharge results of N-700, a) The galvanostatic charge/discharge curve, b) The relationship between specific capacitance and current density.
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Fig. 7. The electrochemical impedance spectrum of N-700.
graphene has been developed by a facile method with the DDGS as the carbon and nitrogen precursor. The carbonization temperature is found as the key factor to the properties of the N-doped graphene. The most important observation is that the N-700 sample presented high specific capacitance (324 F g1) and extremely low inner resistance (0.1 U) while it is used as the electrode for supercapacitors. The synthesis method is extremely attractive, since the N-doped graphene can be produced economically and effectively. This method is easy to scale-up and the precursor is easy to get and extremely cheap; nevertheless, it will be an attractive way to synthesis N-doped graphene for the application of supercapacitors in industry.
4. Experimental Preparation of the samples: The protein rich DDGS was used as the feeding material for generating N-dope graphene. 1.45 g of Ni(NO3)2$6H2O was dissolved in 20 g of water, and then the solution was transferred into a crucible where 20 g of DDGS was loaded as well. The mixture was placed for 1 h at room temperature, and then put into muffle furnace. These soaked mixture was then dried at 400 C in muffle furnace (chamber is 15*15*22 cm) in N2 atmosphere (N2 flow is 500 ml min1) for 20 min to decompose Ni(NO3)2$6H2O. The as-prepared product was cooled down and soaked into 20 ml 7.5 mol L1 potassium hydroxide solution for 1 h. This mixture was then put into oven and dried at 105 C for 24 h. After that the graphitizated activation was carried out at 600, 700, 800, 900 and 1000 C separately for 1 h with the protection of 500 ml min1 N2 flow. The obtained samples were cooled down in muffle furnace in the same N2 atmosphere. N-dope graphene samples were further purified in the 1.0 mol L1 HCl at 150 C within the hydrothermal reactor, and then washed with deionized water to pH value at 7 and dried at 105 C overnight under vacuum. Characterization of the samples: Physisorption analyses with N2 were carried out at 77 K (liquid nitrogen bath), using Tristar 3000 Micropore analyzer. The specific surface areas were calculated using the BrunauereEmmetteTeller (BET) equation. The total pore volumes were obtained at the relative pressure P/ P0 ¼ 0.978. The micropore volume, mesopore volume and the pore size distribution were determined by the NLDFT analysis for carbon with slit pore model (Micromeritics Inc.) based on the N2 isotherm adsorption data. Spectrum (Horiba LABRam confocal Raman microscope) was used to characterize N-dope graphene,
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while the excitation wavelength was set at 532 nm from a diode pumped solid-state laser. The micro-morphology was detected by SEM (HITACHI S-3400N). The XPS measurements were performed on an SSX-100 system (Surface Science Laboratories, Inc.) equipped with a monochromated Al Ka X-ray source, a hemispherical sector analyzer (HSA) and a resistive anode detector. The base pressure of the XPS system was 6.0 1010 Torr. During the data collection, the pressure was ca. 1.0 108 Torr. JEM-2100F was used to characterize the crystal and porous structure of N-doped graphene structure. Electrode preparation and electrochemical measurements: The supercapacitors were assembled as the general. For all of the electrochemical characterization, the surface area of the electrode was 1 cm2. Meanwhile, the electrolytic medium was 6 mol L1 KOH. The electrodes were prepared by pressing the slurry of N-doped graphene, carbon black (conducting material from Fisher scientific) and PTFE (binder) with a mass ration of 8:1:1 on nickel foam. Twoelectrode sandwich-type cells were built using a stainless coin cell (2032) with a microporous PP separator celgard-3501 between the electrodes. Cyclic voltammetry was performed using a SP-150 multichannel potentiostategalvanostat-EIS (Biologic, France) and galvanostatic charge/discharge cycling was performed in Neware battery test systems. The electrochemical impedance spectroscopy was tested by a SP-150 multichannel potentiostategalvanostat-EIS (Biologic, France) with the frequency ranging from 0.1 Hz to 200,000 Hz and the potential amplitude of 10 mV). The specific capacitance of the electrode was calculated using Equation (1) [37]
C¼
2IDt mDV
(1)
here, I is the charge or discharge current density, Dt is the corresponding charge or discharge time, m is one of the symmetry electrodes mass and DV is the total corresponding potential change. Acknowledgments This research is funded by following projects: “Development of high value carbon based adsorbents from thermochemically produced biochar” 2011-67009-20030 USDA-NIFA Agriculture and Food Research Initiative Sustainable Bioenergy Program funded Micropore analyzer, electrochemical analyzer and instruments for preparing graphene materials. NSF EpsCor Dakota BioCon center supported Dr. Hong Jin for his PhD study and XPS analysis. The TEM test was performed by Dr. Masahiro Kawasaki in JEOL Inc. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2014.10.010. References [1] J.W. Lee, A.S. Hall, J.-D. Kim, T.E. Mallouk, Chem. Mater. 24 (2012) 1158e1164. ~ ero, F. Leroux, F. Be guin, Adv. Mater. 18 (2006) 1877e1882. [2] E. Raymundo-Pin [3] G. Wang, X. Sun, F. Lu, H. Sun, M. Yu, W. Jiang, C. Liu, J. Lian, Small 8 (2012) 452e459. [4] H. Jiang, J. Ma, C. Li, Adv. Mater. 24 (2012) 4197e4202. [5] G. Zheng, L. Hu, H. Wu, X. Xie, Y. Cui, Energy Environ. Sci. 4 (2011) 3368. [6] Y. Huang, J. Liang, Y. Chen, Small 8 (2012) 1805e1834. [7] H. Jiang, P.S. Lee, C. Li, Energy Environ. Sci. 6 (2013) 41. [8] J. Huang, B.G. Sumpter, V. Meunier, Angew. Chem. Int. Ed. Engl. 47 (2008) 520e524. [9] Z.S. Wu, A. Winter, L. Chen, Y. Sun, A. Turchanin, X. Feng, K. Mullen, Adv. Mater. 24 (2012) 5130e5135. [10] X. Lu, T. Zhai, X. Zhang, Y. Shen, L. Yuan, B. Hu, L. Gong, J. Chen, Y. Gao, J. Zhou, Y. Tong, Z.L. Wang, Adv. Mater. 24 (2012) 938e944. [11] L. Hao, X. Li, L. Zhi, Adv. Mater. 25 (2013) 3899e3904. [12] K. Wang, H. Wu, Y. Meng, Z. Wei, Small 10 (2014) 14e31.
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