Carboxyl-Assisted Synthesis of Nitrogen-Doped Graphene Sheets for Supercapacitor Applications
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The high ratio of pyridinic and pyridone N-doped graphene sheets have been synthesized by functionalizing graphene oxide (GO) with different oxygen groups on its surface. The typical N-doped graphene was determined to be ~3–5 layers by transmission electron microscopy (TEM) and atomic force microscopy (AFM), and the nitrogen content was measured as 6.8–8 at. % by X-ray photoelectron spectroscopy (XPS). The structure of the N-doped graphene with different surface functional groups was characterized by Raman spectroscopy. The research result indicates that the carboxylation of GO is the key factor to obtain pyridinic and pyridone N types during the N atom doping process. Compared to general N-doped graphene, the electrochemical test shows that specific capacitance of the GO-OOH-N sample reaches up to 217 F/g at a discharge current density 1 A/g and stable cycling performance (keep 88.8 % specific capacitance after 500 cycles at the same discharge current density) when applied to the supercapacitor electrode materials.
KeywordsN-doped graphene Carboxylation Electrochemical Supercapacitor
atomic force microscopy
electrochemical double-layer capacitors
electrochemical impedance spectroscopy
N-doped graphene based on GO
N-doped graphene based on GO at the react temperature of 180 °C
N-doped graphene based on GO-OOH
Transmission electron microscopy
X-ray photoelectron spectroscopy
Graphene is an allotrope of a new-type carbon with two-dimensional carbon atoms tightly packed into honeycomb lattice . The special intrinsic structure exhibits excellent mechanical, electrical, and chemical properties [2, 3, 4], which have shown promising wide applications in various fields [5, 6, 7, 8, 9, 10]. However, chemically inert and hydrophobic feature of graphene surface makes it weakly interactive with other media and poorly soluble in water and common organic solvents. In addition, graphene is a semimetal with zero bandgap which is the major obstacle limiting its utilization for many device applications that need variable electronic bandgap .
By doping nitrogen into graphene, the bandgap can be opened, which makes it possible to adjust the conductivity type and the electronic structure of graphene; thus, the physical and surface performance of graphene can be modulated [1, 11, 12]. Additionally, by introducing nitrogen atoms into the carbon grid of graphene, the active sites for adsorbing metal ions and redox reactions can be increased, which could enhance the interaction between metal particles and graphene, also the ability to trigger Faraday reaction . However, the N content of graphene cannot be infinitely improved. On the one hand, high doping content of N can lead to the formation of carbon nitride films which has extremely low electrical conductivity; on the other hand, the increasing defects caused by nitrogen doping inevitably lead to the decline of the intrinsic features of graphene [14, 15].
In general, the graphene and nitrogen atoms could be combined together in four ways [16, 17, 18, 19, 20, 21]. Considering the sp 2-hybridized carbons will hinder the N atoms doping into graphene lattice, the quaternary-type N is difficult to form, and often with lower N content or even the failure of doping . Oxidized pyridinic nitrogen is also hard to achieve a high doping content (few further discussions on its acting mechanism were referred in the paper). Besides, the pyridinic nitrogen and pyridone nitrogen (or pyrrolic nitrogen) are easily formed on the graphene surface and also proven to be valuable electrochemical active sites. Thus, the effective regulation of pyridinic, pyrrolic, or pyridone N in graphene-like network together with a well-defined pore structure is very important for accelerating application of carbon-based materials in energy storage fields such as lithium-ion batteries , fuel cells [24, 25], and supercapacitors .
Supercapacitors are new energy storage device, and they have many advantages attracting tremendous attentions, such as high power density, excellent cycling stability, and fast charge/discharge capability . Graphene is commonly used as electrodes in electrochemical double-layer capacitors (EDLC) because of its high surface area and conductivity. In order to improve the electrochemical properties of EDLC, decorating the graphene with N is a feasible way .
The effect of N doping types of graphene on supercapacitor performances has been previously investigated [5, 26, 29, 30]. The research found that pyridone nitrogen atoms can be considered as pyridine nitrogen atoms with one adjacent carbon atom that adsorbs a hydroxy group, and the conversion between pyridone and pyridine nitrogen atoms through Faraday processes is reasonable [28, 31]. Through the redox reaction between them, the pseudocapacitance is increased, which contributes to the increase of the capacity of the supercapacitors. Recently, pyridinic-like holes in situ formed during the reduction and nitrogen doping processes of GO have been reported in many papers [18, 31, 32]. These micropore/mesopore textures can enhance the charge accommodations and finally endow the electrode material with a high energy density . In addition, quaternary-type nitrogen will help to improve the wettability between the electrolyte and the electrode, thereby enhancing the pseudocapacitance effect. Moreover, quaternary-type nitrogen and oxidized pyridinic nitrogen are able to maintain a stable capacity during fast charging and discharging .
In this paper, we obtained the N-doped graphene with different N types and content by adjusting the oxygenic functional groups on the graphene surface. As a result, the graphene with a high percentage of both pyridinic and pyridone N was obtained from carboxylic graphene oxide. The graphene with designed N types exhibits superior capacitance and cycling performance in the electrochemical tests, suggesting that special oxygen functionalization of graphene surface is a promising method to synthesize N-doped graphene with optimized N-doped types for high-performance supercapacitors.
Materials and Equipments
Materials used were as follows: natural flake graphite (graphite, purity 99.8 %, 600 meshes, Qingdao), KMnO4 (AR, Sinopharm Chemical Reagent Co., Ltd.), N2H4 ·H2O (80 %, AR, Tianjin Tianli Chemical Reagent Co., Ltd.), H2SO4 (98 %, AR, Xinyang Chemical Reagent Factory), H3PO4 (85 %, AR, Sinopharm Chemical Reagent Co., Ltd.), HCl (37 %, Sinopharm Chemical Reagent Co., Ltd.), H2O2 (30 %, Sinopharm Chemical Reagent Co., Ltd.), C2H5OH (Sinopharm Chemical Reagent Co., Ltd.), ClCH2CO2H (Shanghai Taishan Chemical Plant), NH3 ·H2O (30 %), and home-made secondary deionized water. All medicines were not treated with further purification.
All the graphene and N-doped graphene sheets were characterized with the JOL2010 transmission electron microscope (TEM; accelerating voltage of 200 KV), PHI-5400 X-ray photoelectron spectroscopy (XPS), Rxn1-785 Raman spectrometer (λ = 690 nm), and Nanoscope IIIa controller atomic force microscope (AFM).
Synthesis of Graphene Oxide (GO)
Three grams of natural flake graphite was added into 400 ml of ice-mixed acid (V H2SO4:V H3PO4 = 9:1). Then 18 g of KMnO4 was slowly added. The temperature was reached to 50 °C and kept at the temperature for 60 h accompanied by magnetic stirring. Finally, the solution was stewed and cooled down to room temperature. After that, deionized water of 300 ml was slowly added for dilution, and 10 ml of H2O2 and 100 ml of HCl were added to remove impurities. Then a large amount of distilled water was used to make washing and centrifugation, which were repeated for several times until the pH of the solution was neutral. The brown precipitate was frozen and dried for preservation.
Synthesis of Carboxylic Graphene (GO-OOH)
The as-prepared GO (0.5 g) was dispersed in 250 ml of distilled water using ultrasonic equipment. Yellow transparent colloidal solution was regained. Then NaOH (10 g) and ClCH2CO2H (7.5 g) were added, and the solution was stirred for 3 h. After the reaction was complete, the solution was washed repeatedly to remove impurity ions until the neutral state was reached. Vacuum drying was conducted at 60 °C to harvest GO-OOH.
Synthesis of N-Doped Graphene Based on GO (GO-N)
Twenty-five milliliters of GO solution (6 mg/ml) was taken, and its pH was adjusted to 10 by NH3·H2O. Then 20 ml of N2H4·H2O solution was added followed by stirring for 30 min. The solution was then transferred into the reaction vessel and kept for 5 h at 130 °C, and black product was obtained. Centrifugal washing was performed for several times with distilled water and ethanol. N-RGO was finally obtained by vacuum drying at 60 °C.
Synthesis of N-Doped Graphene Based on GO (GO-N-180)
The process for preparation is the same as GO-N, except that the react temperature is 180 °C.
Synthesis of N-Doped Graphene Based on GO-OOH (GO-OOH-N)
The process for preparation of GO-OOH-N is the same as GO-N, except that the precursor solution GO is replaced by GO-OOH.
Electrochemical Measurements in a Three-Electrode System
A mixture containing 80 wt % active materials (3 mg), 10 wt % acetylene black, and 10 wt % poly-tetrafluoroethylene (PTFE) was well mixed in N,N-dimethylformamide(DMF) until they formed a slurry with the proper viscosity, and then the slurry was uniformly laid on a piece of Ni foam about 1 cm2 that was used as a current collector and then dried at 80 °C for 2 h. The Ni foam coated with the composite was pressed for 1 min under 1.0 MPa and dried at 120 °C for another 12 h. A Pt electrode and Ag/AgCl electrode filled with saturated KCl aqueous solution were used as the counter electrode and reference electrode, respectively. Cyclic voltammetry (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) were measured on a CHI760E electrochemical workstation in a three-electrode system. The specific capacitance is calculated according to equation C = It/V, where I is the charge/discharge current density (A/g), t is the discharge time (t), and V is the voltage (1 V). The reported specific capacitance and energy density are all normalized to the weight of sample.
Results and Discussion
Elemental composition and distribution of type of carbon-containing groups on the surface of GO and GO-OOH
C (at. % )
O (at. % )
C Distribution (%)
Four N1s sub-peaks could be fitted  in Fig. 4: pyridinic nitrogen, pyrrolic or pyridone nitrogen, oxidized pyridinic nitrogen, and adsorbed nitrogen. The peak with the bonding energy at 400–400.6 eV can be attributed to five-membered ring of pyrrole or pyridone structure (pyridone). If the peak is attributed to pyridone, it can be mutually converted with pyridinic nitrogen in specific condition. This is because both oxidized pyridinic nitrogen and pyridone nitrogen are in oxidized states, while pyridinic nitrogen is in reducing state .
Elemental composition and distribution of the type of nitrogen-containing groups on the surface of GO-N and GO-OOH-N samples
C (at. % )
N (at. % )
N distribution (at. % )
In order to know the relationship between the other oxygen groups and the content and types of N-doped graphene, we also made hydroxy group on graphene surface and the related synthesis method and characterization were in the supporting information. The XPS spectra of G-OH and G-OH-N show that the reduced graphene obtained with 3.9 at. % N doping, in which the content of pyridinic nitrogen, pyridone nitrogen, and oxidized pyridinic nitrogen was 1.2, 2.1, and 0.4 at. %, respectively.(Additional file 1: Figure S2, Additional file 1: Table S2).
The specific capacitance of the electrode is obtained from the equation: Csp = It/△Em where Csp, I, t, △E, and m are specific capacitance (F/g), constant current (A), discharge time (s), potential window (V), and mass of the active material (g), respectively. As shown in Fig. 6c, the Csp of GO-N, GO-N-180, and GO-OOH-N at 1 A/g is 205, 162, and 217 F/g, respectively, which is in accord with the CV analysis. To probe the potential causes of this increase in capacitance value, we made an investigation on the samples from the point of both chemical composition and physical structure. The negatively charged nitrogen-containing groups (N-6/N-5) can facilitate the electrostatic interaction between K+ and N atoms, giving rise to abundant electrolyte ions occupied on the electrode surface . Clearly, the similar curve of GO-N and GO-OOH-N has been observed in BET results (Additional file 1: Figure S4 and Additional file 1: Table S3). The specific surface areas of GO-N and GO-OOH-N are 262 and 223 m2/g, respectively. The pore size and distribution are close for both samples (especially for the mesopore), indicating that this markedly enhanced capacitance ability of GO-OOH-N ought to be attributed to the high N content and favored N structures benefiting from an artificial carboxylation on graphene oxide precursor instead of pore structure difference. Unexpectedly, the Csp of GO-OOH-N undergoes a visible drop from 1 to 2 A/g and reaches to a lower capacitance level compared to that of GO-N; this observation may be elucidated by the instability of the pseudocapacitive redox pairs at high current density , which can also be proven by the absence of clear mutation in the discharge curve segment of GO-OOH-N at a current density of more than 2 A/g in Fig. 6d. In contrast, GO-N can still exhibit a superior capacitance retention even at high current densities, herein it can be ascribed to the important role of the positively charged groups (N-X) which helps in an electron transfer especially at fast charge/discharge process [18, 28].
The impedance analyses of all products are measured in the frequency range of 100 kHz–0.01 Hz at open circuit potential to evaluate the kinetic and fundamental behavior of electrode materials and are shown in Fig. 6e. The Nyquist plots of N-doped graphene samples are composed of a partial semicircle in high-frequency region and a nearly straight line in low-frequency regions. The semicircle diameter is commonly used for the measurement of the surface properties of the electrode materials and corresponds to the charge transfer resistance. The smaller diameter of GO-OOH-N indicates that it can offer faster charge transportation, which can be contributed to the relative low defect density (indicated in Raman spectrum in Fig. 2b) and the desired N structures as discussed above. The nearly vertical line in the mid- to low-frequency region for all electrode materials reflects that the double-layer structure can be established rapidly, indicating a fast ion transfer kinetics. The excellent dynamic behavior is believed to be closely linked to the good wettability of materials benefiting from the suitable oxygen contents (~10 at. % ) and the hydrophilic groups (i.e., carboxyl groups) incorporated in the carbon networks. In addition, the equivalent series resistance (ESR) obtained from the X intercept of the curves  appears to be small at ~0.25 Ω for GO-OOH-N, which is slightly smaller than GO-N and GO-N-180, suggesting that GO-OOH-N electrode has lower resistance with good current response.
Figure 6f illustrates the Columbic efficiency of the GO-N and GO-OOH-N electrodes recorded during 500 continuous cycles at a current density of 1 A/g. After 300 cycles, the specific capacitance of GO-N and GO-OOH-N in 6 M KOH both retain approximately 90 % of the initial value. Moreover, the retention of specific capacitance of GO-N and GO-OOH-N decreases to 82.1 and 88.8 %, respectively. The promotion of the cycling performance of GO-OOH-N electrode reveals that a redistribution of oxygenic groups on precursor surface is an effective way to improve the stability of N-doped graphene during the charge/discharge process, due to a beneficial regulation on its chemical groups.
In this article, the changes of N types in graphene were investigated by using different functionalized graphene oxide precursors under varied experimental conditions. The research result shows that low temperature was favorable for the generation of pyridone nitrogen, while high temperature was beneficial to the generation of pyridinic nitrogen. When GO-OOH was used as the precursor, the presence of carboxyl groups was favorable for the both generations of pyridinic nitrogen and pyridone nitrogen. The GO-OOH-N shows the highest capacitance, and an inferior of GO-OOH-N to GO-N in rate capability at high current density convinced the instability of pseudocapacitance in the fast charge/discharge process. Moreover, GO-OOH-N also showed an impressive specific capacitance and cyclic stability after 500 cycles at a discharge current density 1 A/g. These observations demonstrate that the regulation of nitrogen structures in graphene could be achieved by the well-controlled oxygen groups on graphene surface. We believe that the present way is beneficial for the supercapacitor applications or other fields.
This work was supported by the National Natural Science Foundation of China (Grant No. 41202022), the Fundamental Research Funds for National University, China University of Geosciences (Wuhan), the National Natural Science Foundation of China (Grant No. 51372063), and Anhui International Cooperation Project (Grant No. 1303063014).
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