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Easy one-step synthesis of N-doped graphene for supercapacitors
Author’s Accepted Manuscript Easy one-step synthesis of N-doped graphene for supercapacitors Yan-Zhen Liu, Yong-Feng Li, Fang-Yuan Su, LiJing Xie, Qing-Qiang Kong, Xiao-Ming Li, JianGuo Gao, Cheng-Meng Chen www.elsevier.com/locate/ensm
PII: DOI: Reference:
S2405-8297(15)30029-5 http://dx.doi.org/10.1016/j.ensm.2015.09.006 ENSM15
To appear in: Energy Storage Materials Received date: 13 July 2015 Revised date: 25 September 2015 Accepted date: 25 September 2015 Cite this article as: Yan-Zhen Liu, Yong-Feng Li, Fang-Yuan Su, Li-Jing Xie, Qing-Qiang Kong, Xiao-Ming Li, Jian-Guo Gao and Cheng-Meng Chen, Easy one-step synthesis of N-doped graphene for supercapacitors, Energy Storage Materials, http://dx.doi.org/10.1016/j.ensm.2015.09.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Easy one-step synthesis of N-doped graphene for supercapacitors
Easy one-step synthesis of N-doped graphene for supercapacitors
Yan-Zhen Liua, Yong-Feng Lib,*, Fang-Yuan Sua, Li-Jing Xiea, Qing-Qiang Konga, Xiao-Ming Lia, Jian-Guo Gaoa, Cheng-Meng Chena,*
a
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China b College of Mechanics, Taiyuan University of Technology, Taiyuan 030024, China
Corresponding authors. E-mail address: [email protected] (C. M. Chen) E-mail address: [email protected] (Y. F. Li)
Abstract: A N-doped graphene (NG) was synthesized in hexamethylenetetramine flame from graphite oxide (GO) powder within a few seconds. Results indicate that hexamethylenetetramine acts not only as the fuel of flame, but also as the nitrogen source and reduction agent. And the exfoliation, reduction and nitrogen doping are achieved simultaneously in the nitrogen-rich flame plasma. The as-prepared NG has a large surface area of up to 595 m2/g, and a nitrogen content of 2.95 at.% with pyridinic, pyrrolic, quaternary and amino types configurations. The NG was evaluated as electrode material for supercapacitor and exhibits a high specific capacitance of 1
205.3 F/g in 6 M KOH aqueous electrolyte with low internal resistance and good stability, which is attributed to the combination of high specific surface area, abundant mesopores, and reversible redox nitrogen sites. The method features high efficiency, cost-effective and facile operation, and shed a light for the synthesis of nitrogen doped carbons in gram-scale towards energy storage applications.
Keywords: N-doped graphene; Hexamethylenetetramine; Flame; Rapid synthesis; Supercapacitor
1. Introduction Supercapacitors, a unique class of energy storage devices have attracted extensive attention in recent years owing to their high power capability, low maintenance and long cycle life [1-3]. Electrode materials are usually considered to play a key role in supercapacitors. Among various electrode materials, carbonaceous materials such as carbon nanotubes, porous carbons and graphene, are most widely used owing to their high surface areas and electrical conductivities [4,5]. Graphene, the two-dimensional nanosheets composed of sp2-hybridized carbon atoms, is the mother of all graphitic materials such as fullerenes, carbon nanotubes and graphite [6,7]. With an extremely high theoretical surface area (2630 m2/g) and electrical conductivity, graphene-based materials have attracted tremendous attention as electrode materials for the next-generation supercapacitors [8]. However, the intrinsic capacitance of as-prepared highly reduced graphene oxide is no more than 100 F/g, which is far below the theoretical value of 550 F/g as expected [9]. This is due to the inadequate exfoliation of graphite oxide in practice, and also relying on the amount of residue oxygen which contribute extra pseudo-capacitance. Thus, numerous works have been accomplished to enhance capacitance of graphene-based 2
materials, which include hybridization with transition oxides (RuO2 [10], Mn3O4 [11] and NiO [12]), and modifications via curvation [13], activation [14], vertical orientation [15], solvation [16] as well as heteroatom-doping with N, B and O atoms. It has been found that N-doping to the 2D crystal is effective in improving the electrochemical capacitive performance due to the electronic modification by the N dopant [3, 17-19]. Up to now, N-doped graphenes (NGs) were synthesized by chemical vapor deposition, arc-discharge, NH3 annealing, nitrogen plasma treatment and thermal annealing of graphite oxide (GO) with nitrogen-containing compounds and so on [20-22]. For instance, Shi et al. synthesized N-doped DTG (NDTG) by CVD method using a gas mixture of CH4/NH3/Ar as carbon and nitrogen sources at 950 oC for 10 min, which shows significantly improved reactivity on oxygen reduction reaction [21]. Lin et al. presented controllable processes to fabricate nitrogen-doped graphene using a tunable hybrid plasma source [22]. Sheng et al. proposed a catalyst-free thermal annealing approach (800 oC for 1 h) for synthesis of NG using melamine as the nitrogen source [23]. Rigorous conditions (e.g. high temperature), special instruments or a very long time are often required for the synthesis of NG, which increase its cost and limit its practical applications. Thus, a simple, low-cost, large-scale production of NG remains to be a challenge. Hexamethylenetetramine (HMTA), a heterocyclic organic compound with the formula (CH2)6N4, is a green and versatile reagent in organic synthesis. Currently, HMTA was used to prepare C or/and N-doped titania and N-containing carbon nano-materials. Shen et al. [24] have reported that the environmental friendly HMTA can be used as an effective reducing agent to produce highly stable aqueous graphene dispersions. Herein, we report a simple, fast and high yield method for NG synthesis using HMTA flame under a thin layer of GO powder, where HMTA was used as the combustion fuel, nitrogen source and reducing agent. The present approach avoids sophisticated equipment, strict operation conditions as well as complicated and time-consuming process. The as-prepared NG shows very high quality such as large surface area (595 m2/g) and moderate nitrogen doping level (2.95 at.%). Meanwhile, 3
the current NG exhibits outstanding performance for supercapacitor applications. The energy chemistry of the NG and proposed doping mechanism are discussed in this work. 2. Experimental 2.1 Synthesis Graphite oxide powder (GO) was prepared from natural flake graphite by the modified Hummer's method. All reagents were used as received without further purifications. NG was synthesized as follows. In a typical procedure, 3 g GO was spread evenly on the top of 15 g HMTA powder (a spreading area of 6 cm ×12 cm) placed in a flat combustion boat, then the HMTA powder was ignited using an ordinary lighter. The spreading area and mass of HMTA powder were greater than those of GO powder. The HMTA was burned and the light black powder left off within 2-3 s was the NG, where exfoliation, reduction and N-doping occurred simultaneously. 2.2 Characterization Samples were examined using X-ray diffraction (XRD, MiniFlex II, Japan) with Cu Kα radiation (λ = 0.15406 nm) as the X-ray source. C, H, N and O elemental contents of samples were measured by a X-ray photoelectron spectroscope (XPS, AXIS ULTRA DLD, UK) and an elemental analyzer (EA, vario EL CUBE, Germany). The XPS data were obtained by charge correction using a standard peak position of C1s (binding energy 284.8 eV). The data of binding energy and intensity were imported using a software named as XPS Peak4.1. Then background was clicked, followed by selecting Shirley type and clicking Optimise to set the baseline. For fitting procedure, peaks were added according to peak type in the standard spectra and positions were fixed on the basis of literatures, followed by clicking Optimise till good coincidence degree of the summery peak and parent peak. TGA-MS was performed on an apparatus (Evolution 16/18+ TENSOR27) under nitrogen flow with a heating rate of 5 °C/min. The N2 adsorption-desorption isotherms of the samples were performed at 77 K using a Quotation Max (ST) physical adsorption instrument 4
(Japan), and the specific surface area was calculated from the BET plot from the N2 adsorption isotherm. Infrared spectra were recorded on a VERTEX 70 FT-IR spectrometer (Bruker) in the frequency range 4000-500 cm-1, and Raman spectra were collected on a Raman spectrometer with a laser wavelength of 514.5 nm at room temperature. A Raman spectrum was drawn using Origin software and the Raman baseline was deducted in order to obtain precise Raman intensity. Then, two tangent lines were drawn along with the position of the connection between D-peak and G-peak, followed by drawing two vertical lines down the highest point of these two peaks to the tangent lines. Finally, ID/IG ratio was calculated from the height value of D-peak vertical line divided by one of G-peak. The microstructure and morphology of the samples were further characterized by SEM (JSM-7001F, Japan) and TEM (JEOL JEM2010). The electrical conductivity of the NG sample was measured by the four-point probe method. 2.3 Electrochemical measurements Cyclic
voltammetry
(CV),
galvanostatic
charge/discharge
(GCD)
and
electrochemical impedance spectroscopy (EIS) were performed on a CHI760D electrochemical workstation at room temperature. A supercapacitor was made as the following. The NG was mixed with polytetrafluoroethylene (PTFE) binder in a mass ratio of 90:10, and dispersed in absolute ethanol. The resulting mixture was homogenized by ultrasonication and punched to circular sheets of 1.0 cm2, which were dried at 80 oC for 24 h. Each electrode contained 2.8 mg active materials. Then the circular sheets were pressed on to Ni foam under pressure. The two electrodes were assembled into a 2032 coin cell separated by a thin polymer membrane (Celgard 2400). A two-electrode configuration was assembled and a 6 M KOH aqueous solution was used as the electrolyte. The specific capacitance (Cs, F/g), energy density (Et, Wh/kg), and power density (Pt, W/kg) were calculated with the equations 1, 2 and 3 [2, 25], respectively. The capacitance of Ni foam was neglected. 4𝐼∆𝑡
Cs = 𝑚∆𝑣
(1)
5
1
Et = 8 𝐶𝑠(∆𝑣)2 Pt =
𝐸𝑡 𝑡
(2) (3)
where I (A) is the constant current, m (g) is the total mass of active materials in both electrodes, Δt (s) is the discharge time, and the Δv(V) is the voltage change during discharging. 3. Results and discussion The synthesis strategy for the NG is schematically illustrated in Scheme 1. In brief, GO powder fabricated by strong oxidation of graphite is simultaneously exfoliated, deoxidized and doped to form NG powder with a help of HMTA flame, which acts as combustion fuel, nitrogen source and reduction agent. This novel approach to produce NG in gram scale is carried out in solid-phase, avoiding aggregation of graphene during dying fin liquid-phase method. It is well known that fire is a form of energy released from separation, collision and combination of plasmas and free radicals when substance molecular cracks into the low energy state. The combustion enthalpy of HMTA is 239.7 kJ/mol, and the fire temperatures from bottom to up are detected to be from around 210 to 280 oC. The positions of flame bottom, middle and up are located around at 0 cm, 5 cm, 11 cm over the surface of GO powder. The plasmas and free radicals generated from the HMTA flame move at a ultrahigh speed, resulting in an incorporating of nitrogen atoms into the graphene structure. Previous researches have reported that GO can be exfoliated to few layers, accompanying by a removal of oxygen at around 200 oC [19, 26]. In this work, the GO powder can be converted into graphene in a rapid expansion manner, resulting in vacuum at the bottom of the fire, by which the light powder, named as NG powder, floats away from the flame.
6
Scheme 1. A schematic illustration of the NG synthesis method in HMTA flame.
The morphology and microstructure of as-prepared NG powder were observed by SEM and TEM. The GO powder with spherical shape generated by the modified Hummers’ method and a spray drying process explodes to form NG with expanded sphere or flower-like shape at low magnification (Fig. 1a), resulting from the exfoliation of GO by the flame energy during the combustion process. As seen from Figs. 1b and 1c, the nanosheets exhibit a curved/wrinkled morphology with mesopores, which is quite different from that of NG prepared by a plasma-assisted downstream microwave technique [27]. This wrinkled sheet structure is probably generated by the fast exfoliation of the GO powder and removal of oxygen functional groups and nitrogen doping during the flame treatment. The energy dispersive X-ray spectra are shown in Fig. S1 confirm the presence of C, O and N elements in the NG sample. The N atoms distribute uniformly not only in the plane but also at the edges of the graphene sheet. These results further demonstrate that N atoms have been successfully doped into graphene by this flame synthesis method. The TEM images in Figs. 1d-f also show that the NG has a wrinkled structure and is composed of one and few atom layers, which is similar to the structure in the previous works [7, 24]. Combined with the surface area of 2630 m2/g for the single-layer graphene and statistical analysis of abundant HRTEM images, the average graphene number of NG powder was estimated to be about 3 to 5 layers.
7
Fig. 1. (a,b,c) SEM, (d,e) TEM and (f) HR-TEM images of as-prepared NG powder at various magnifications.
The porosity of the NG was investigated by nitrogen-adsorption. As shown in Fig. 2a, the N2 adsorption-desorption isotherms belong to a type IV with a distinct hysteresis loop in the p/p0 region of 0.32-0.99, indicating a presence of mesopores. The BET specific surface area was calculated to be 595 m2/g, which is higher than that of G-CBP-a (363 m2/g) [28], N-doped rGO (431 m2/g) [29] and N-CNSs-800 (549.5 m2/g) [30], and comparable to NDCN-22 (589 m2/g) by a templating method [31].
The
pore
size
distribution
curve
(inset
of
Fig.
2a)
from
the
Barrett-Joyner-Halenda (BJH) model shows a strong peak at around 4 nm and a wide peak at about 23.4-31.3 nm. The total pore volume reaches 5.7 cm3/g. The mesopore structure is expected to shorten the ion diffusion path in supercapacitors and ensure a full utilization of surface area. Fig. 2b shows the powder XRD patterns of the GO and NG. The GO exhibits a sharp diffraction peak at 10.9o, corresponding to a d-spacing of 0.81 nm, which might be caused by an interlayer widening due to the accommodation of various oxygen-containing groups and water molecules within the layers [3]. Subsequently, this peak completely disappears after the HMTA flame treatment and a broad peak is 8
formed at around 25o in NG, corresponding to an average d-spacing of 0.35 nm. These results demonstrate that the flame treatment restores partially the graphitic structure due to the reduction effect by the HMTA flame at moderate temperature, exfoliation and nitrogen doping. Obviously, this treatment is expected to increase the electrochemical performance. As shown in Fig. 3a, the Raman spectra show two remarkable peaks located at 1345 and 1594 cm-1 for the GO, 1358 and 1594 cm-1 for the NG, which can be attributed to D band associated with structural defects and G band for E 2g vibration mode of sp2 carbon domains, respectively [10]. The ID/IG ratio slightly increases from 0.81 for the GO to 0.88 for the NG due to the reduction and nitrogen doping. The slightly increased ID/IG value in present study is consistent with that in literatures as a result of nitrogen doping [5, 9]. TGA was performed to investigate the thermal degradation behavior of the GO and NG. As revealed in Fig. 3b, GO powder has a substantial mass loss of 22% at around 205 oC at a heating rate of 5 oC/min under N2 atmospheres. This might be caused by a loss of numerous oxygen functional groups and residues during the rapid expansion of the GO, and was observed in a previous study [32]. For the NG, the TG curve shows no abrupt or obvious mass loss at 200 oC, and only exhibits a mass loss of 15.2% at 1000 oC, indicating that the GO was effectively reduced [26]. It has been found in a literature that the specific capacitance of reduced graphene oxide annealed at 200 oC was the highest and decreases with increasing annealing temperature [33]. Moreover, the NG preparation method in the present study avoids a high temperature treatment to dope nitrogen in graphene compared with the previous method (800 oC, melamine) [23].
9
Fig. 2. (a) Nitrogen adsorption-desorption isotherm and BJH desorption pore-size distribution (inset of (a)) of the NG and (b) X-ray diffraction patterns of the NG and GO.
Fig. 3. (a) Raman spectra and (b) TGA curves of the NG and GO.
XPS was used to further evaluate the chemical composition of the GO and NG (Fig. 4). The NG contains 2.95 at.% of nitrogen atoms and 8.21 at.% of oxygen. The nitrogen content of the NG is comparable to that of SWNT/rGO fiber produced at 220 o
C (2.9 at.%, nitrogen source: ethylenediamine) [34], NDTG grew at 950 oC (3.02
at.%, nitrogen source: NH3) [21] and N-doped graphene synthesized at 600 oC (3.06 at.%, nitrogen source: NH3) [35]. The surface and bulk element contents were also tested by the EDS (Fig. S1) and elemental analysis (Table S1). These results suggest that an efficient doping can be attained by combustion of HMTA in the presence of GO powder. The as-prepared NG with expected nitrogen content is likely due to the 10
stable doping process for GO at the constant combustion temperature. Moreover, the C/O ratio for the N-doped graphene is 10.82, which is much higher than that of GO (1.56) as shown in Fig. 4a, indicating a substantial elimination of oxygen containing groups. As exhibited in Fig. 4a, the XPS spectrum of the GO shows only carbon (284.8 eV) and oxygen (532.1 eV) peaks, whereas additionally a peak at ~400 eV, corresponding to the nitrogen atom, can be clearly seen for the NG, confirming the existence of nitrogen atoms in graphene [17,19,23]. In the C1s spectrum of the NG, a sharp peak centered at 284.8 eV is found, which corresponds to C-C/C=C groups. This confirms that most of carbon atoms are in the conjugated honeycomb lattice. The peaks at 286.6, 288.3, 289 eV are associated with different C-O bonding configurations, which decrease considerably after the flame treatment. A new peak at 285.9 eV for the NG is associated with nitrogen atoms (C-N/C=N) (Figs. 4b and 4c) [8, 10]. These results suggest that most of oxygen-containing functional groups have been effectively removed and the nitrogen atoms have been doped in graphene by the flame treatment. These are also confirmed by the FTIR results (Fig. S2). To understand the nature of nitrogen species, the deconvolution of the N1s spectrum was performed and shown in Fig. 4d. The four peaks are ascribed to pyrridinc N at 398.6 eV, amino N at 399.5 eV, pyrrolic N at 400.3 eV and graphitic N at 401.2 eV [3,9,18]. The nitrogen chemical states were depicted in Fig. 4e. Unlike the production of NG by annealing with NH3 or annealing GO in the presence of melamine at high temperatures (500-1000 oC) [23], the treatment of GO in HMTA flame here is an energy-saving and cost-effective method to prepare NG. Not only HMTA acts as fuel to provide flame, but also the decomposed species, such as N-ion plasma and carbon free radicals, can react with the oxygen-containing groups of GO to generate amino N, and can also attack structure defects or the active sites generated from the removal of oxygen species to form pyrridinc N, pyrrolic N and graphitic N in graphene layer.
11
Fig. 4. (a) XPS spectra of the GO and NG; high resolution C1s spectra of (b) the GO and (c) the NG; (d) N1s spectrum of the NG and (e) a schematic representation of nitrogen chemical states.
As an example of the possible applications of the NG, a coin-type supercapacitor cell was assembled using the NG as active material and an aqueous KOH solution (6 M)) as the electrolyte, and its electrochemical performance was evaluated. Fig. 5a 12
exhibits the CV curves of the supercapacitor at different scan rates within a potential window from 0 to 0.9 V. It can be found that the CV curves show a rectangular shape at low scan rates, typical of electric double layer capacitive behavior [36] and a small deviation from the rectangular shape at 100 mV/s. This is in accordance with the results reported by previous works [36, 37] using graphene as the electrode materials of supercapacitors and might be caused by a kinetic limitation for the electrolyte ions to enter the small pores. Fig. 5b shows the galvanostatic charge-discharge profiles of the supercapacitor at different current densities from 0.1 to 10 A/g. All the charge-discharge curves are quasi-triangular and nearly symmetrical, which is attributed to a fast and efficient charge transfer and superior electrical conductivity owing to the presence of active nitrogen atoms [17, 25]. The specific capacitance of the NG is 205.3 F/g at a current density of 0.1 A/g (Fig. 5c). This value is comparable with those of nitrogen-doped porous carbon [36], reduced graphene oxide (201.3 F/g) [36], and higher than that of the reduced graphene oxide prepared by our flame-induced method [39] as listed in Table S2. Additionally, our research group has reported that the amine modified graphene prepared at 200 oC exhibits a higher capacitance than those at temperatures higher than 200 oC [9]. Thus, the high specific capacitance of the NG synthesized in the HMTA flame may be attributed to the mild condition, the enhanced surface area, and the high electronegativity of nitrogen that may create dipoles on the surface of graphene. The dipoles may attract charged species within the surface [8, 18]. The mechanism of N and O atoms in NG for their contribution to the capacitance has been proposed by the inductive effects of the σ-bonded structure from N and O heteroatoms, which cause a redistribution of the electrons and polarization of some bonds, leading to reversible Faradic redox reactions [9]. The cycling performance of the NG electrode in 6 M KOH electrolyte at a current density of 1 A/g is presented in Fig. 5c. The specific capacitance of the supercapacitor decreases by around 6% for the first 1000 charge–discharge cycles and remains 92.5% of its initial value after 3500 cycles, indicating a good electrochemical stability. Furthermore, the electrical conductivity of 13
the NG was measured to be 357 S/m, which is of great importance for electrode materials.
Fig. 5. (a) CV curves for the NG electrode at different scan rates in 6 M KOH electrolyte; (b) charge-discharge profiles at various current densities; (c) gravimetric specific capacitance at various current densities and cycling stability at a current density of 1 A/g; (d) EIS of the NG electrode at a frequency range from 100 mHz to 100 kHz, the inset is a magnification of the high frequency region and a Randles equivalent circuit is used for fitting.
To further evaluate the device performance, the frequency response of the NG-based supercapacitor was analyzed using electrical impedance spectroscopy (EIS). As shown in Fig. 5d, the high-frequency region shows a good electrode contact [5]. 14
The solution resistance (Rs) and the charge-transfer resistance (Rct) NG were estimated to be 1.07 and 1.9 Ω, respectively, by a fitting of experimental data with the Randles equivalent circuit model. The Warburg-type line (the slope of the 45o region of the plots) is short, implying that NG has short ion diffusion pathways, which is consistent with the previous work [40, 41]. The nearly 60° slope in the plot at the low-frequency region indicates a good capacitor behavior generated by Faradic redox reactions and double-layer behaviors. These results are attributed to the presence of the nitrogen atoms, which are consistent with the previous finding that N-doping simultaneously increases the electrical conductivity and electrochemical activity of graphene in the electrochemical process. The short Warburg-type line is ascribed to the mesopores that act as ion-buffering reservoirs and shorten the ion diffusion distance from the electrolyte bulk to the interior surfaces where charge separation takes place [18,19,41]. The NG-based symmetric supercapacitor exhibits an energy density of 5.5 Wh/kg at a power density of 450 W/kg, and still retains 2.8 Wh/kg at a high power density of 4500 W/kg. The energy density and power density for the NG is higher than those for the reduced graphene oxide without N-doping prepared in the alcohol flame [39], indicating that HMTA flame is more suitable for preparation of electrode materials. Thus, the NG synthesized rapidly in HMTA flame is very promising as an electrode material for high performance supercapacitors.
4. Conclusions We have developed a novel, rapid and flame method to prepare NG powder at large scale, using HMTA as fuel of the flame below GO powder. The HMTA not only acts as fuel, but also as nitrogen source and reduction agent of the NG. The NG powder has a high specific surface area of 595 m2/g and a nitrogen content of 2.95 at.%. This method avoids using a complicated experimental setup and a huge amount of energy consumption. Moreover, the NG was found to deliver a high specific capacitance (205.3 F/g) than those of the reduced graphene oxide (170 F/g) synthesized in liquid alcohol flames. The presence of the oxygen-containing groups 15
and nitrogen atoms increase the electrochemical activity of graphene by facilitating the interaction of the NG with the electrolyte. The abundant mesopores with a large surface area serves as ion-buffering reservoirs that can shorten the ion diffusion distance from the bulk electrolyte to the interior surfaces. This novel route is simple and energy-saving for a large-scale production of NG powder that it is expected as electrode materials of high performance supercapacitors.
Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (51302281), Natural Science Foundation of Shanxi Province (2013011012-7), and Shanxi Coal Transportation and Sales Group Co. Ltd (2013WT103).
Appendix A. Supplementary material Supplementary data associated with this article can be found…
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PII: DOI: Reference:
S2405-8297(15)30029-5 http://dx.doi.org/10.1016/j.ensm.2015.09.006 ENSM15
To appear in: Energy Storage Materials Received date: 13 July 2015 Revised date: 25 September 2015 Accepted date: 25 September 2015 Cite this article as: Yan-Zhen Liu, Yong-Feng Li, Fang-Yuan Su, Li-Jing Xie, Qing-Qiang Kong, Xiao-Ming Li, Jian-Guo Gao and Cheng-Meng Chen, Easy one-step synthesis of N-doped graphene for supercapacitors, Energy Storage Materials, http://dx.doi.org/10.1016/j.ensm.2015.09.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Easy one-step synthesis of N-doped graphene for supercapacitors
Easy one-step synthesis of N-doped graphene for supercapacitors
Yan-Zhen Liua, Yong-Feng Lib,*, Fang-Yuan Sua, Li-Jing Xiea, Qing-Qiang Konga, Xiao-Ming Lia, Jian-Guo Gaoa, Cheng-Meng Chena,*
a
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China b College of Mechanics, Taiyuan University of Technology, Taiyuan 030024, China
Corresponding authors. E-mail address: [email protected] (C. M. Chen) E-mail address: [email protected] (Y. F. Li)
Abstract: A N-doped graphene (NG) was synthesized in hexamethylenetetramine flame from graphite oxide (GO) powder within a few seconds. Results indicate that hexamethylenetetramine acts not only as the fuel of flame, but also as the nitrogen source and reduction agent. And the exfoliation, reduction and nitrogen doping are achieved simultaneously in the nitrogen-rich flame plasma. The as-prepared NG has a large surface area of up to 595 m2/g, and a nitrogen content of 2.95 at.% with pyridinic, pyrrolic, quaternary and amino types configurations. The NG was evaluated as electrode material for supercapacitor and exhibits a high specific capacitance of 1
205.3 F/g in 6 M KOH aqueous electrolyte with low internal resistance and good stability, which is attributed to the combination of high specific surface area, abundant mesopores, and reversible redox nitrogen sites. The method features high efficiency, cost-effective and facile operation, and shed a light for the synthesis of nitrogen doped carbons in gram-scale towards energy storage applications.
Keywords: N-doped graphene; Hexamethylenetetramine; Flame; Rapid synthesis; Supercapacitor
1. Introduction Supercapacitors, a unique class of energy storage devices have attracted extensive attention in recent years owing to their high power capability, low maintenance and long cycle life [1-3]. Electrode materials are usually considered to play a key role in supercapacitors. Among various electrode materials, carbonaceous materials such as carbon nanotubes, porous carbons and graphene, are most widely used owing to their high surface areas and electrical conductivities [4,5]. Graphene, the two-dimensional nanosheets composed of sp2-hybridized carbon atoms, is the mother of all graphitic materials such as fullerenes, carbon nanotubes and graphite [6,7]. With an extremely high theoretical surface area (2630 m2/g) and electrical conductivity, graphene-based materials have attracted tremendous attention as electrode materials for the next-generation supercapacitors [8]. However, the intrinsic capacitance of as-prepared highly reduced graphene oxide is no more than 100 F/g, which is far below the theoretical value of 550 F/g as expected [9]. This is due to the inadequate exfoliation of graphite oxide in practice, and also relying on the amount of residue oxygen which contribute extra pseudo-capacitance. Thus, numerous works have been accomplished to enhance capacitance of graphene-based 2
materials, which include hybridization with transition oxides (RuO2 [10], Mn3O4 [11] and NiO [12]), and modifications via curvation [13], activation [14], vertical orientation [15], solvation [16] as well as heteroatom-doping with N, B and O atoms. It has been found that N-doping to the 2D crystal is effective in improving the electrochemical capacitive performance due to the electronic modification by the N dopant [3, 17-19]. Up to now, N-doped graphenes (NGs) were synthesized by chemical vapor deposition, arc-discharge, NH3 annealing, nitrogen plasma treatment and thermal annealing of graphite oxide (GO) with nitrogen-containing compounds and so on [20-22]. For instance, Shi et al. synthesized N-doped DTG (NDTG) by CVD method using a gas mixture of CH4/NH3/Ar as carbon and nitrogen sources at 950 oC for 10 min, which shows significantly improved reactivity on oxygen reduction reaction [21]. Lin et al. presented controllable processes to fabricate nitrogen-doped graphene using a tunable hybrid plasma source [22]. Sheng et al. proposed a catalyst-free thermal annealing approach (800 oC for 1 h) for synthesis of NG using melamine as the nitrogen source [23]. Rigorous conditions (e.g. high temperature), special instruments or a very long time are often required for the synthesis of NG, which increase its cost and limit its practical applications. Thus, a simple, low-cost, large-scale production of NG remains to be a challenge. Hexamethylenetetramine (HMTA), a heterocyclic organic compound with the formula (CH2)6N4, is a green and versatile reagent in organic synthesis. Currently, HMTA was used to prepare C or/and N-doped titania and N-containing carbon nano-materials. Shen et al. [24] have reported that the environmental friendly HMTA can be used as an effective reducing agent to produce highly stable aqueous graphene dispersions. Herein, we report a simple, fast and high yield method for NG synthesis using HMTA flame under a thin layer of GO powder, where HMTA was used as the combustion fuel, nitrogen source and reducing agent. The present approach avoids sophisticated equipment, strict operation conditions as well as complicated and time-consuming process. The as-prepared NG shows very high quality such as large surface area (595 m2/g) and moderate nitrogen doping level (2.95 at.%). Meanwhile, 3
the current NG exhibits outstanding performance for supercapacitor applications. The energy chemistry of the NG and proposed doping mechanism are discussed in this work. 2. Experimental 2.1 Synthesis Graphite oxide powder (GO) was prepared from natural flake graphite by the modified Hummer's method. All reagents were used as received without further purifications. NG was synthesized as follows. In a typical procedure, 3 g GO was spread evenly on the top of 15 g HMTA powder (a spreading area of 6 cm ×12 cm) placed in a flat combustion boat, then the HMTA powder was ignited using an ordinary lighter. The spreading area and mass of HMTA powder were greater than those of GO powder. The HMTA was burned and the light black powder left off within 2-3 s was the NG, where exfoliation, reduction and N-doping occurred simultaneously. 2.2 Characterization Samples were examined using X-ray diffraction (XRD, MiniFlex II, Japan) with Cu Kα radiation (λ = 0.15406 nm) as the X-ray source. C, H, N and O elemental contents of samples were measured by a X-ray photoelectron spectroscope (XPS, AXIS ULTRA DLD, UK) and an elemental analyzer (EA, vario EL CUBE, Germany). The XPS data were obtained by charge correction using a standard peak position of C1s (binding energy 284.8 eV). The data of binding energy and intensity were imported using a software named as XPS Peak4.1. Then background was clicked, followed by selecting Shirley type and clicking Optimise to set the baseline. For fitting procedure, peaks were added according to peak type in the standard spectra and positions were fixed on the basis of literatures, followed by clicking Optimise till good coincidence degree of the summery peak and parent peak. TGA-MS was performed on an apparatus (Evolution 16/18+ TENSOR27) under nitrogen flow with a heating rate of 5 °C/min. The N2 adsorption-desorption isotherms of the samples were performed at 77 K using a Quotation Max (ST) physical adsorption instrument 4
(Japan), and the specific surface area was calculated from the BET plot from the N2 adsorption isotherm. Infrared spectra were recorded on a VERTEX 70 FT-IR spectrometer (Bruker) in the frequency range 4000-500 cm-1, and Raman spectra were collected on a Raman spectrometer with a laser wavelength of 514.5 nm at room temperature. A Raman spectrum was drawn using Origin software and the Raman baseline was deducted in order to obtain precise Raman intensity. Then, two tangent lines were drawn along with the position of the connection between D-peak and G-peak, followed by drawing two vertical lines down the highest point of these two peaks to the tangent lines. Finally, ID/IG ratio was calculated from the height value of D-peak vertical line divided by one of G-peak. The microstructure and morphology of the samples were further characterized by SEM (JSM-7001F, Japan) and TEM (JEOL JEM2010). The electrical conductivity of the NG sample was measured by the four-point probe method. 2.3 Electrochemical measurements Cyclic
voltammetry
(CV),
galvanostatic
charge/discharge
(GCD)
and
electrochemical impedance spectroscopy (EIS) were performed on a CHI760D electrochemical workstation at room temperature. A supercapacitor was made as the following. The NG was mixed with polytetrafluoroethylene (PTFE) binder in a mass ratio of 90:10, and dispersed in absolute ethanol. The resulting mixture was homogenized by ultrasonication and punched to circular sheets of 1.0 cm2, which were dried at 80 oC for 24 h. Each electrode contained 2.8 mg active materials. Then the circular sheets were pressed on to Ni foam under pressure. The two electrodes were assembled into a 2032 coin cell separated by a thin polymer membrane (Celgard 2400). A two-electrode configuration was assembled and a 6 M KOH aqueous solution was used as the electrolyte. The specific capacitance (Cs, F/g), energy density (Et, Wh/kg), and power density (Pt, W/kg) were calculated with the equations 1, 2 and 3 [2, 25], respectively. The capacitance of Ni foam was neglected. 4𝐼∆𝑡
Cs = 𝑚∆𝑣
(1)
5
1
Et = 8 𝐶𝑠(∆𝑣)2 Pt =
𝐸𝑡 𝑡
(2) (3)
where I (A) is the constant current, m (g) is the total mass of active materials in both electrodes, Δt (s) is the discharge time, and the Δv(V) is the voltage change during discharging. 3. Results and discussion The synthesis strategy for the NG is schematically illustrated in Scheme 1. In brief, GO powder fabricated by strong oxidation of graphite is simultaneously exfoliated, deoxidized and doped to form NG powder with a help of HMTA flame, which acts as combustion fuel, nitrogen source and reduction agent. This novel approach to produce NG in gram scale is carried out in solid-phase, avoiding aggregation of graphene during dying fin liquid-phase method. It is well known that fire is a form of energy released from separation, collision and combination of plasmas and free radicals when substance molecular cracks into the low energy state. The combustion enthalpy of HMTA is 239.7 kJ/mol, and the fire temperatures from bottom to up are detected to be from around 210 to 280 oC. The positions of flame bottom, middle and up are located around at 0 cm, 5 cm, 11 cm over the surface of GO powder. The plasmas and free radicals generated from the HMTA flame move at a ultrahigh speed, resulting in an incorporating of nitrogen atoms into the graphene structure. Previous researches have reported that GO can be exfoliated to few layers, accompanying by a removal of oxygen at around 200 oC [19, 26]. In this work, the GO powder can be converted into graphene in a rapid expansion manner, resulting in vacuum at the bottom of the fire, by which the light powder, named as NG powder, floats away from the flame.
6
Scheme 1. A schematic illustration of the NG synthesis method in HMTA flame.
The morphology and microstructure of as-prepared NG powder were observed by SEM and TEM. The GO powder with spherical shape generated by the modified Hummers’ method and a spray drying process explodes to form NG with expanded sphere or flower-like shape at low magnification (Fig. 1a), resulting from the exfoliation of GO by the flame energy during the combustion process. As seen from Figs. 1b and 1c, the nanosheets exhibit a curved/wrinkled morphology with mesopores, which is quite different from that of NG prepared by a plasma-assisted downstream microwave technique [27]. This wrinkled sheet structure is probably generated by the fast exfoliation of the GO powder and removal of oxygen functional groups and nitrogen doping during the flame treatment. The energy dispersive X-ray spectra are shown in Fig. S1 confirm the presence of C, O and N elements in the NG sample. The N atoms distribute uniformly not only in the plane but also at the edges of the graphene sheet. These results further demonstrate that N atoms have been successfully doped into graphene by this flame synthesis method. The TEM images in Figs. 1d-f also show that the NG has a wrinkled structure and is composed of one and few atom layers, which is similar to the structure in the previous works [7, 24]. Combined with the surface area of 2630 m2/g for the single-layer graphene and statistical analysis of abundant HRTEM images, the average graphene number of NG powder was estimated to be about 3 to 5 layers.
7
Fig. 1. (a,b,c) SEM, (d,e) TEM and (f) HR-TEM images of as-prepared NG powder at various magnifications.
The porosity of the NG was investigated by nitrogen-adsorption. As shown in Fig. 2a, the N2 adsorption-desorption isotherms belong to a type IV with a distinct hysteresis loop in the p/p0 region of 0.32-0.99, indicating a presence of mesopores. The BET specific surface area was calculated to be 595 m2/g, which is higher than that of G-CBP-a (363 m2/g) [28], N-doped rGO (431 m2/g) [29] and N-CNSs-800 (549.5 m2/g) [30], and comparable to NDCN-22 (589 m2/g) by a templating method [31].
The
pore
size
distribution
curve
(inset
of
Fig.
2a)
from
the
Barrett-Joyner-Halenda (BJH) model shows a strong peak at around 4 nm and a wide peak at about 23.4-31.3 nm. The total pore volume reaches 5.7 cm3/g. The mesopore structure is expected to shorten the ion diffusion path in supercapacitors and ensure a full utilization of surface area. Fig. 2b shows the powder XRD patterns of the GO and NG. The GO exhibits a sharp diffraction peak at 10.9o, corresponding to a d-spacing of 0.81 nm, which might be caused by an interlayer widening due to the accommodation of various oxygen-containing groups and water molecules within the layers [3]. Subsequently, this peak completely disappears after the HMTA flame treatment and a broad peak is 8
formed at around 25o in NG, corresponding to an average d-spacing of 0.35 nm. These results demonstrate that the flame treatment restores partially the graphitic structure due to the reduction effect by the HMTA flame at moderate temperature, exfoliation and nitrogen doping. Obviously, this treatment is expected to increase the electrochemical performance. As shown in Fig. 3a, the Raman spectra show two remarkable peaks located at 1345 and 1594 cm-1 for the GO, 1358 and 1594 cm-1 for the NG, which can be attributed to D band associated with structural defects and G band for E 2g vibration mode of sp2 carbon domains, respectively [10]. The ID/IG ratio slightly increases from 0.81 for the GO to 0.88 for the NG due to the reduction and nitrogen doping. The slightly increased ID/IG value in present study is consistent with that in literatures as a result of nitrogen doping [5, 9]. TGA was performed to investigate the thermal degradation behavior of the GO and NG. As revealed in Fig. 3b, GO powder has a substantial mass loss of 22% at around 205 oC at a heating rate of 5 oC/min under N2 atmospheres. This might be caused by a loss of numerous oxygen functional groups and residues during the rapid expansion of the GO, and was observed in a previous study [32]. For the NG, the TG curve shows no abrupt or obvious mass loss at 200 oC, and only exhibits a mass loss of 15.2% at 1000 oC, indicating that the GO was effectively reduced [26]. It has been found in a literature that the specific capacitance of reduced graphene oxide annealed at 200 oC was the highest and decreases with increasing annealing temperature [33]. Moreover, the NG preparation method in the present study avoids a high temperature treatment to dope nitrogen in graphene compared with the previous method (800 oC, melamine) [23].
9
Fig. 2. (a) Nitrogen adsorption-desorption isotherm and BJH desorption pore-size distribution (inset of (a)) of the NG and (b) X-ray diffraction patterns of the NG and GO.
Fig. 3. (a) Raman spectra and (b) TGA curves of the NG and GO.
XPS was used to further evaluate the chemical composition of the GO and NG (Fig. 4). The NG contains 2.95 at.% of nitrogen atoms and 8.21 at.% of oxygen. The nitrogen content of the NG is comparable to that of SWNT/rGO fiber produced at 220 o
C (2.9 at.%, nitrogen source: ethylenediamine) [34], NDTG grew at 950 oC (3.02
at.%, nitrogen source: NH3) [21] and N-doped graphene synthesized at 600 oC (3.06 at.%, nitrogen source: NH3) [35]. The surface and bulk element contents were also tested by the EDS (Fig. S1) and elemental analysis (Table S1). These results suggest that an efficient doping can be attained by combustion of HMTA in the presence of GO powder. The as-prepared NG with expected nitrogen content is likely due to the 10
stable doping process for GO at the constant combustion temperature. Moreover, the C/O ratio for the N-doped graphene is 10.82, which is much higher than that of GO (1.56) as shown in Fig. 4a, indicating a substantial elimination of oxygen containing groups. As exhibited in Fig. 4a, the XPS spectrum of the GO shows only carbon (284.8 eV) and oxygen (532.1 eV) peaks, whereas additionally a peak at ~400 eV, corresponding to the nitrogen atom, can be clearly seen for the NG, confirming the existence of nitrogen atoms in graphene [17,19,23]. In the C1s spectrum of the NG, a sharp peak centered at 284.8 eV is found, which corresponds to C-C/C=C groups. This confirms that most of carbon atoms are in the conjugated honeycomb lattice. The peaks at 286.6, 288.3, 289 eV are associated with different C-O bonding configurations, which decrease considerably after the flame treatment. A new peak at 285.9 eV for the NG is associated with nitrogen atoms (C-N/C=N) (Figs. 4b and 4c) [8, 10]. These results suggest that most of oxygen-containing functional groups have been effectively removed and the nitrogen atoms have been doped in graphene by the flame treatment. These are also confirmed by the FTIR results (Fig. S2). To understand the nature of nitrogen species, the deconvolution of the N1s spectrum was performed and shown in Fig. 4d. The four peaks are ascribed to pyrridinc N at 398.6 eV, amino N at 399.5 eV, pyrrolic N at 400.3 eV and graphitic N at 401.2 eV [3,9,18]. The nitrogen chemical states were depicted in Fig. 4e. Unlike the production of NG by annealing with NH3 or annealing GO in the presence of melamine at high temperatures (500-1000 oC) [23], the treatment of GO in HMTA flame here is an energy-saving and cost-effective method to prepare NG. Not only HMTA acts as fuel to provide flame, but also the decomposed species, such as N-ion plasma and carbon free radicals, can react with the oxygen-containing groups of GO to generate amino N, and can also attack structure defects or the active sites generated from the removal of oxygen species to form pyrridinc N, pyrrolic N and graphitic N in graphene layer.
11
Fig. 4. (a) XPS spectra of the GO and NG; high resolution C1s spectra of (b) the GO and (c) the NG; (d) N1s spectrum of the NG and (e) a schematic representation of nitrogen chemical states.
As an example of the possible applications of the NG, a coin-type supercapacitor cell was assembled using the NG as active material and an aqueous KOH solution (6 M)) as the electrolyte, and its electrochemical performance was evaluated. Fig. 5a 12
exhibits the CV curves of the supercapacitor at different scan rates within a potential window from 0 to 0.9 V. It can be found that the CV curves show a rectangular shape at low scan rates, typical of electric double layer capacitive behavior [36] and a small deviation from the rectangular shape at 100 mV/s. This is in accordance with the results reported by previous works [36, 37] using graphene as the electrode materials of supercapacitors and might be caused by a kinetic limitation for the electrolyte ions to enter the small pores. Fig. 5b shows the galvanostatic charge-discharge profiles of the supercapacitor at different current densities from 0.1 to 10 A/g. All the charge-discharge curves are quasi-triangular and nearly symmetrical, which is attributed to a fast and efficient charge transfer and superior electrical conductivity owing to the presence of active nitrogen atoms [17, 25]. The specific capacitance of the NG is 205.3 F/g at a current density of 0.1 A/g (Fig. 5c). This value is comparable with those of nitrogen-doped porous carbon [36], reduced graphene oxide (201.3 F/g) [36], and higher than that of the reduced graphene oxide prepared by our flame-induced method [39] as listed in Table S2. Additionally, our research group has reported that the amine modified graphene prepared at 200 oC exhibits a higher capacitance than those at temperatures higher than 200 oC [9]. Thus, the high specific capacitance of the NG synthesized in the HMTA flame may be attributed to the mild condition, the enhanced surface area, and the high electronegativity of nitrogen that may create dipoles on the surface of graphene. The dipoles may attract charged species within the surface [8, 18]. The mechanism of N and O atoms in NG for their contribution to the capacitance has been proposed by the inductive effects of the σ-bonded structure from N and O heteroatoms, which cause a redistribution of the electrons and polarization of some bonds, leading to reversible Faradic redox reactions [9]. The cycling performance of the NG electrode in 6 M KOH electrolyte at a current density of 1 A/g is presented in Fig. 5c. The specific capacitance of the supercapacitor decreases by around 6% for the first 1000 charge–discharge cycles and remains 92.5% of its initial value after 3500 cycles, indicating a good electrochemical stability. Furthermore, the electrical conductivity of 13
the NG was measured to be 357 S/m, which is of great importance for electrode materials.
Fig. 5. (a) CV curves for the NG electrode at different scan rates in 6 M KOH electrolyte; (b) charge-discharge profiles at various current densities; (c) gravimetric specific capacitance at various current densities and cycling stability at a current density of 1 A/g; (d) EIS of the NG electrode at a frequency range from 100 mHz to 100 kHz, the inset is a magnification of the high frequency region and a Randles equivalent circuit is used for fitting.
To further evaluate the device performance, the frequency response of the NG-based supercapacitor was analyzed using electrical impedance spectroscopy (EIS). As shown in Fig. 5d, the high-frequency region shows a good electrode contact [5]. 14
The solution resistance (Rs) and the charge-transfer resistance (Rct) NG were estimated to be 1.07 and 1.9 Ω, respectively, by a fitting of experimental data with the Randles equivalent circuit model. The Warburg-type line (the slope of the 45o region of the plots) is short, implying that NG has short ion diffusion pathways, which is consistent with the previous work [40, 41]. The nearly 60° slope in the plot at the low-frequency region indicates a good capacitor behavior generated by Faradic redox reactions and double-layer behaviors. These results are attributed to the presence of the nitrogen atoms, which are consistent with the previous finding that N-doping simultaneously increases the electrical conductivity and electrochemical activity of graphene in the electrochemical process. The short Warburg-type line is ascribed to the mesopores that act as ion-buffering reservoirs and shorten the ion diffusion distance from the electrolyte bulk to the interior surfaces where charge separation takes place [18,19,41]. The NG-based symmetric supercapacitor exhibits an energy density of 5.5 Wh/kg at a power density of 450 W/kg, and still retains 2.8 Wh/kg at a high power density of 4500 W/kg. The energy density and power density for the NG is higher than those for the reduced graphene oxide without N-doping prepared in the alcohol flame [39], indicating that HMTA flame is more suitable for preparation of electrode materials. Thus, the NG synthesized rapidly in HMTA flame is very promising as an electrode material for high performance supercapacitors.
4. Conclusions We have developed a novel, rapid and flame method to prepare NG powder at large scale, using HMTA as fuel of the flame below GO powder. The HMTA not only acts as fuel, but also as nitrogen source and reduction agent of the NG. The NG powder has a high specific surface area of 595 m2/g and a nitrogen content of 2.95 at.%. This method avoids using a complicated experimental setup and a huge amount of energy consumption. Moreover, the NG was found to deliver a high specific capacitance (205.3 F/g) than those of the reduced graphene oxide (170 F/g) synthesized in liquid alcohol flames. The presence of the oxygen-containing groups 15
and nitrogen atoms increase the electrochemical activity of graphene by facilitating the interaction of the NG with the electrolyte. The abundant mesopores with a large surface area serves as ion-buffering reservoirs that can shorten the ion diffusion distance from the bulk electrolyte to the interior surfaces. This novel route is simple and energy-saving for a large-scale production of NG powder that it is expected as electrode materials of high performance supercapacitors.
Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (51302281), Natural Science Foundation of Shanxi Province (2013011012-7), and Shanxi Coal Transportation and Sales Group Co. Ltd (2013WT103).
Appendix A. Supplementary material Supplementary data associated with this article can be found…
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