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High rate performance carbon nano-cages with oxygen-containing functional groups as supercapacitor electrode materials
Carbon 111 (2017) 207e214
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Carbon journal homepage: www.elsevier.com/locate/carbon
High rate performance carbon nano-cages with oxygen-containing functional groups as supercapacitor electrode materials Liang Jiang a, *, Jing Wang b, Xuyan Mao a, Xiangyu Xu a, Bo Zhang c, Jie Yang a, Yunfei Wang d, Jun Zhu a, Shifeng Hou a, ** a
Bio-Nano & Medical Engineering Institute, Jining Medical University, 16 Hehua Road, Jining, 272067, China Physics and Information Engineering Department, Jining University, 1 Xingtan Road, Qufu, 273155, China Foundation Medical College, Jining Medical University, 16 Hehua Road, Jining, 272067, China d Gynecology Department, Affiliated Hospital of Jining Medical University, 89 Guhuai Road, Jining, 272000, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2016 Received in revised form 28 September 2016 Accepted 29 September 2016 Available online 30 September 2016
Graphitized carbon nano-cages can be prepared by carbon dioxide and magnesium metal as precursors. The carbon materials can produce a large amount of functional groups by the oxidation of nitric acid. Compared with amorphous carbon, the oxidation treatment can improve capacitive performance of carbon nano-cages, especially rate performance. At high scan rate, specific capacitance of carbon nanocages is much higher than that of the microporous activated carbon. So carbon nano-cages are promising high rate performance materials. A series of physical and chemical characterization exhibits that excellent capacitive performance is derived from carbon nano-cages structure themselves. It is due to higher electrical conductivity of graphite structure, lower charge transfer resistance of hierarchical porous structure and extra pseudo-capacitance of oxygen-containing functional groups. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction With increasing environmental pollution, the development of clean energy sources becomes an urgent task and is received more widespread attention. Supercapacitor is a new energy storage device utilizing electric double layer for storing charge. Compared with secondary batteries, supercapacitor has unique advantages including long cycle life, short charging time and high power density [1e4]. In general, carbon materials are the most common electrode materials for supercapacitors [5e8]. Activated carbons are most commonly used as electrode materials in carbon materials. But it also has its own shortcomings. Amorphous structure and rich microporous structure can lead to low electrical conductivity and high charge transfer resistance [9,10]. Capacitive performances of activated carbon significantly decay at high current density. While graphite has high conductivity, its low specific surface area is not suitable for supercapacitor electrode material. Highly graphitized porous carbon material has relatively higher
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Jiang), [email protected] (S. Hou). http://dx.doi.org/10.1016/j.carbon.2016.09.081 0008-6223/© 2016 Elsevier Ltd. All rights reserved.
specific surface area than graphite and higher electrical conductivity than amorphous carbon. Therefore, highly graphitized porous carbon materials are promising electrode materials for supercapacitors [11e13]. Compared to microporous structure of activated carbon, hierarchical structure of highly graphitized porous carbon has a synergistic effect of microporous, mesoporous and macroporous structures, and thus can show a better rate performance [14e17]. Meantime, some electrode surface of carbon materials can occur highly reversible redox reaction, which can generate another type of capacitor: pseudo-capacitance [18e20]. Generally, the surface of the carbon material has a large number of oxygen-containing functional groups, such as hydroxyl, carboxyl and carbonyl groups. Such functional groups on the carbon material surface can react with the electrolyte ion to produce pseudo-capacitance [21,22]. So the carbon materials with oxygen-containing functional groups as supercapacitor electrode materials can generate electric double layer capacitor and also generate pseudo-capacitor. The presence of pseudo-capacitance can greatly improve the specific capacitance value of carbon materials [23]. This kind of pseudo-capacitance is generated on the surface of the carbon material and it has higher electrical conductivity than that of metal oxide which can also generate pseudo-capacitor. Therefore, it
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exhibits a high capacitance value and a better rate capability. Carbon material can effectively generate oxygen-containing functional groups on the surface by the oxidation of nitric acid [24]. However, nitric acid with strong oxidizing can damage the pore structures of the amorphous carbon to a certain extent. Highly graphitized porous carbon materials have strong antioxidant properties and reduce the damage of pore structure. In this work, magnesium metal and carbon dioxide are used as precursors to prepare the multi-layer graphene nano-cages [25]. Further concentrated nitric acid is used to oxide graphene nano-cages, so that oxygen-containing functional groups are grafted onto the surface of the graphene nano-cages. It is expected to be a promising way to synthesize high-rate performance supercapacitor electrode materials.
pestle. Rubber clay state was formed by adding a small amount of ethanol. Then the sample was pressed to form a circular electrode membrane of 9 mm in diameter and then sandwiched between two nickel foams to form an electrode assembly, which nickel foam was used as current collectors. And nickel-metal band was used as a wire. Electrochemical measurements were carried out in threeelectrode cell. Prepared sample membrane, Pt plate and Ag/AgCl electrode were used as work electrode, counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) tests, electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge tests were carried out on an electrochemical workstation (Shanghai CH Instruments, CHI760E). 3. Results and discussion
2. Experimental 2.1. Material synthesis In the typical experiment, dry ice was placed in de-ionized water. At the same time, carbon dioxide gas appeared in the upper part of the beaker. Magnesium ribbon was ignited in the carbon dioxide atmosphere. Then the sample was leached in HCl solution to remove magnesium oxide and unreacted magnesium metal, and washed thoroughly with de-ionized water. Finally, the samples were dried at 60 C in an oven. The black carbon material was labeled as CM. The CM material was heated in concentrated nitric acid at 60 C for 3 h. The obtained sample was washed thoroughly with deionized water, which was labeled as CMN. Activated carbons have the characteristics of high specific surface area, low cost. Therefore, activated carbons become the most commonly used commercial supercapacitor electrode materials [7,26]. As a comparison, sucrose was used as raw material to prepare activated carbon materials. A certain amount of sucrose was placed in a tube of a furnace. The pyrolysis of sucrose was carried out at 500 C for 1 h in N2 atmosphere, then cooled to room temperature. The obtained sample and KOH powder were mixed. The mass ratio pyrolysed sucrose/KOH was 1:4 in the work. The mixture was treated at 800 C for 2 h in N2 atmosphere. The mixture after treatment was labeled as SC. Similarly, the SC material was heated in concentrated nitric acid at 60 C for 3 h. The obtained sample was washed thoroughly with de-ionized water, which was labeled as SCN. 2.2. Physicochemical characterization The compositions of the samples were charactered by elemental analysis with a Vario EL Cube (Elementar). X-ray photoelectron spectroscopy (XPS) was used to investigate the surface information of the carbon, XPS data was collected using an Escalab 250 Xi X-ray photoelectron spectrometer (Thermo Scientific). The porous texture of carbon nano-cages were characterized by nitrogen adsorption at 77 K with a Quantachrome NovaWin system (Quantachrome). Surface areas were calculated by BrunauereEmmeteTeller (BET), pore size distributions were analyzed by BarretteJoynereHalenda (BJH) method. X-ray diffraction (XRD) was performed on a XRD-6100 (Shimadzu). Morphologies of carbon nano-cages were investigated by a JEM-2100 at 200 kV and Zeiss ultra plus at 3 kV. 2.3. Electrochemical tests 85 wt% carbon, 10 wt% acetylene black and 5 wt% polytetrafluoroethylene (PTFE) were mixed and grounded with a mortar and
The morphologies of CM and CMN are observed by SEM and TEM images, as shown in Fig. 1 and Fig. 2. SEM image of CM (Fig. 1a) shows that the CM material is formed by the accumulation of a large number of tiny cubic particles with a size range of 100e200 nm. Further increasing the magnification (Fig. 1b), these particles exhibit a specific nano-cages structure and most of nanocages are open. An in-situ MgO template method leads to the open nano-cages structure. Fig. S1 shows XRD pattern of the material from the combustion of magnesium ribbon and carbon dioxide. The peaks at 36.9 , 42.9 , 62.3 , 74.6 and 78.6 are assigned to diffraction from (111) (200) (220) (311) and (222) planes of the cubic structure of magnesium oxidation. According to the previous papers and our study, in carbon dioxide atmosphere, magnesium is ignited and releases a lot of heat. In the outer zone of the reaction, carbon dioxide and magnesium react to produce magnesium oxide and CO. In the inner zone, CO is reduced by liquid metal magnesium, forming carbon and magnesium oxide [27,28]. The reaction produces magnesium oxide crystal of cubic structure and carbon material is deposited on the surface of magnesium oxide. Due to the limited concentration of CO produced by magnesium reducing CO2, the deposition rate of carbon on magnesium oxide is slow and the deposition method is performed in layer by layer way. Moreover, a lot of heat is released during the reaction and the reaction temperature can reach to 2500 C [29]. At high temperature, graphite carbon layers are formed. Because the structure of magnesium oxide is cubic, and thus the structure of carbon material wrapped on it is also cubic. After acid treatment, cubic structure of carbon nano-cages will be formed. The open nano-cages can be used as a reservoir of electrolyte ions, which can reduce the distance of electrolyte ions into micropores and mesopores [14]. In the common method of preparing mesoporous carbon with magnesium oxide as template, first it requires the preparation of magnesium oxide template, and then mixed with carbon source. And generally amorphous carbon is usually obtained by the template method [30,31]. High specific surface area mesoporous graphite carbon can be prepared only by high temperature carbonization or addition of graphitization catalyst [32e34]. Therefore, the method in this paper is simple and does not need to be processed by high temperature to obtain graphite carbon nanocages material. Fig. 2 shows the SEM and TEM of CMN, which is the material of CM oxidized by nitric acid. It can be observed that the morphology and size of carbon nano-cages material do not change significantly, suggesting that the morphology of the carbon nano-cages material is not influenced by strong oxidizing nitric acid. Further highresolution transmission electron microscopy (HRTEM) characterization (Fig. 2d) reveals the carbon nano-cages material has a crystal structure. It is difficult for nitric acid to oxide and destroy
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Fig. 1. SEM (a, b), TEM (c) and HRTEM images of CM (d).
Fig. 2. SEM (a,b), TEM (c) and HRTEM images of CMN (d).
highly graphitized structure. So it is still able to maintain such a nano-cages structure. Fig. 3 shows XRD patterns of CM and CMN. The peaks at 26 and 44.3 are assigned to diffraction from (002) and (100) planes of the hexagonal structure of graphite. The XRD results are indicative of good graphitization, which is consistent with the HRTEM results. XRD pattern of CMN is similar to that of CM, suggesting that the oxidation of nitric acid does not change the crystal structure. As shown in Table S1, elemental analysis results show that no significant changes of nitrogen and hydrogen in carbon materials happens after the nitric acid treatment, while the content of oxygen increases from 4.25 wt% to 9.02 wt%. It suggests that the nitric acid treatment can increase the oxygen content in carbon materials but has no effects on nitrogen content.
The pseudo-capacitance is produced by the highly reversible redox reaction of the oxygen containing functional groups on the surface of carbon materials. XPS is an effective method to analyze the oxygen functional groups on the surface of carbon materials. Therefore, the XPS test is a more meaningful method for the study of CMN as a supercapacitor electrode material. XPS spectra of CM and CMN are presented in Fig. S2. There are no peaks of nitrogen appearing in the spectra. It suggests that no nitrogen has been attached on the surface of the carbon before and after nitric acid treatment. As shown in Table 1, the content of oxygen in CM and CMN material is measured by XPS. The total of oxygen content is 7.74 at. % and 9.39 at. % for CM and CMN, respectively. It suggests that the content of oxygen-containing functional groups on the carbon material surface has increased
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Fig. 3. XRD patterns of CM and CMN. (A colour version of this figure can be viewed online.)
Table 1 Total oxygen contents and percentages of different species in the samples. Sample
CM
CMN
Total oxygen content (%) Percentage (%)oxygen in carbonylic Oxygen in phenolic and lactone groups Oxygen in H2O
7.74 4.13 2.45 1.16
9.39 4.98 4.11 0.30
significantly. The high resolution O1s peaks of carbon nano-cages CM and CMN are fitted into three components originating from oxygen in carbonyl groups (531.2e531.6 eV), oxygen in phenolic and lactone groups (532.7e533.0eV), and oxygen in H2O (535.5e536.3 eV) as shown in Fig. 4 [21,22]. Oxygen configurations of each sample are summarized in Table 1. The content of oxygen in phenolic and lactone groups increase from 2.45 at. % to 4.11 at. % after the oxidation of nitric acid. The two kinds of oxygen species can produce CO by desorption, in favor of the formation of an electric double layer. Therefore, oxygen functional group structure on the surface of the carbon material can be changed by nitric acid to a certain extent to improve the performance of surpercapacitor. Fig. 5a shows the N2 adsorption-desorption curves of SC and SCN. Both SC and SCN curves are similar, which belong to the isotherms of type I (based on IUPAC classifidcation). It indicates the existence of microporous structure in SC and SCN, due to the activation of KOH. From pore distribution curves of SC and SCN (Fig. 5b), it can be observed that there is no significant pore size
distribution in the mesoporous range (>2 nm). This result further illustrates that both of them are microporous materials. Before and after the oxidation of nitric acid, the specific surface area of SC and SCN is 3368 and 2666 m2 g1, respectively. And pore volume of SC and SCN is 1.7 and 1.36 cm3 g1, respectively. There is a significant reduction in the surface area and pore volume after the nitric acid treatment. It is due to oxidation corrosion of carbon material own structure, so that its pore structure is destroyed. As shown in Fig. 5c, the N2 adsorptionedesorption curves of the carbon nano-cages materials CM and CMN all belong to the isotherms of type IV (based on IUPAC classification). Conversely, before and after the oxidation of nitric acid, the curves of CM and CMN materials are not significantly different. It is because that CM material is highly graphitized, with a more completely crystalline structure and not easy to damage the structure. The slope of the curve increases rapidly at relative pressure above 0.4. Moreover, it can be found the existence of a hysteresis loop, which indicates the existence of mesopores. Pore distribution curves of the CM and CMN materials are shown in Fig. 5d by the BJH method. The poresize distribution is mainly distributed in a narrow pore size range of 6e7 nm. In addition to the mesoporous distribution, it exists pore size distribution above 50 nm. It exhibits that the CM and CMN carbon nano-cages materials are hierarchical porous carbon of mesopores and macropores coexisting. The magnesium oxide is removed by acid washing and followed by the formation of nanocages structure, resulting in the existence of macropores. Before and after the oxidation of nitric acid, the specific surface area of CM and CMN is 806 and 866 m2 g1, respectively. And pore volume of CM and CMN is 1.51 and 1.55 cm3 g1, respectively. The specific surface area and pore volume has a certain increase after nitric acid oxidation and the integrity of the pore structure of the carbon material remains unchanged. CV measurements are used to investigate the capacitive performance of the nano-cages materials in 6.0 mol L1 KOH aqueous electrolyte and in the scan potential range from 1 to 0 V. As shown in Fig. 6, The voltammograms exhibit nearly symmetrical rectangular shapes, indicating that the capacitive response mainly comes from the electrochemical double layer capacitance. The carbon nano-cages material has high graphitization to improve good electrical conductivity and unique cage structure can be used as a reservoir of electrolyte ions to reduce transfer resistance of electrolyte ions. So it shows good capacitive performance. Compared with CM, the cyclic voltammetry curves of CMN have greater response current, even appear redox peaks. Apparent redox peaks can be observed at the potential of 0.4 V. From the XPS results, the surface of the carbon nano-cages contains a large
Fig. 4. The O1s XPS spectra of CM (a) and CMN (b). (A colour version of this figure can be viewed online.)
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Fig. 5. Nitrogen adsorptionedesorption isotherm (a) and pore size distribution (b) of SC and SCN; Nitrogen adsorptionedesorption isotherm (c) and pore size distribution (d) of CM and CMN. (A colour version of this figure can be viewed online.)
Fig. 6. Cyclic voltammograms of the electrodes at different scan rates in 6 mol L1 KOH aqueous solution of (a) CM, (b) CMN. (A colour version of this figure can be viewed online.)
amount of carbonyl functional groups, indicating that the presence of surface oxygen-containing functional groups leads to the occurrence of highly reversible redox reaction in the chargedischarge process, thereby the appearance of pseudo-capacitance. Moreover, the other increased electrical double layer capacitance is caused by the strengthened surface charge density from oxygen atoms with more negative electronic affinity, which enhances the space charge layer capacitance in the carbon side. The specific capacitance is calculated according to the following equation:
C¼
Q 2DVm
Where Q is the charge integrated from the whole voltage range, DV is the whole voltage difference, and m is the mass of the active material in a single electrode. The value of CM and CMN at 2 mV s1 is 88 and 110 F g1, respectively. With increasing the scan rate, the values of response current are continuously improved. At the same time, the cyclic voltammetry curve remains rectangular shape. It shows at higher scan rate, the specific capacitance of CMN has a
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more significant improvement than that of CM. Oxygen-containing functional groups on the carbon material surface can generate pseudo-capacitance through redox reaction. Compared with the electrical double layers capacitor, pseudocapacitance is generally low to respond at the high scan rate. So pseudo-capacitance electrode materials generally have relatively poorer rate performance. However, the cyclic voltammetry curve of CMN still remains nearly rectangular configuration and there is no apparent oblique angle at the higher scan rate of 1000 mV s1 (Fig. 7a). The results show that the CMN electrode material has a small equivalent series resistance ESR and excellent capacitive performance. Although the surface functional groups of the CMN carbon material occur the redox reaction, it is noteworthy that these oxygen-containing functional groups are attached to the high conductivity graphite surface and electrons can transfer smoothly in redox reaction. Thus the CMN pseudo-capacitive electrode material still keeps a high conductivity. As a comparison, the curves of the specific capacitance of SC and SCN are shown in Fig. 7b. The specific capacitance of SC reaches 304 F g1 at the scan rate of 2 mV s1. However, the specific capacitance of SCN only has 174 F g1 at the scan rate of 2 mV s1 because of the damage of structure. With increasing scan rate, the specific capacitance has a sharp decline. This is because that surface functional groups can adsorb water molecules to form large molecules and further the microporous of SC and SCN decreases, resulting in a large charge transfer resistance. At the same time, the amorphous structure of activated carbon itself has led to a large electronic resistance. Compared with amorphous carbon, graphitized carbons CM and CMN exhibit better rate performance. At 1000 mV s1, the specific capacitance of CMN reaches 96 F g1, meanwhile its specific capacitance is only 59 F g1 and 50 F g1 for SC and SCN. CM and CMN materials are mainly made up of mesoporous and macroporous structure. Water molecules adsorbed on the functional groups do not affect the electrolyte ion transport. Moreover, oxygen-containing functional groups can also participate in a redox reaction to improve the rate performance of the supercapacitor. Although CMN has lower capacitance value at low current density, its rate performance is very excellent. Therefore, it is more suitable for use at high current density. The galvanostatic charge/discharge curves are shown in Fig. 8 at different current densities. The curves exhibit symmetrical triangle and voltage changes linearly with time, indicating that CMN material has a typical capacitive behaviour. The specific capacitance is calculated according to the equation:
C¼
I Dt Um
Where I is the discharge current (A), Dt is the discharge time (s), U is the potential change window (V) excluding IR drop in the discharge process, and m is the mass of the active material in a single electrode (g). The specific capacitance of CMN material is 104 F g1 at the current density of 0.2 A g1. To further increase the current density to 10 A g1, the value of the specific capacitance of CMN material is 93 F g1. No obvious potential drop appears at the current density of 10 A g1 for CMN, exhibiting a better rate performance and a smaller resistance. The excellent performance comes from high conductivity of the graphite structure and low transport resistance of the electrolyte ion of the synergistic effect of rich mesopores and macropores [35]. Electrochemical impedance spectroscopy (EIS) is also used to estimate the supercapacitive performance. The intersection of the high-frequency region and the real axis is very small (Fig. 9). It is due to its low resistance of the graphite structure. The EIS curves of CM and CMN samples are both perpendicular to the real axis at low frequency region, closed to the characteristic of the ideal capacitive behaviors. No apparent semicircle can be observed at higher frequency region, suggesting that it occurs fast ion diffusion in the electrode porous material due to the existence of mesopores. To further investigate the electrolyte ion transport in porous electrode inside, EIS data is processed to get the imaginary part of the capacitance. The following formula is provided to calculate the capacitance: 00
C ¼
Z0 00 2pfm jZ 0 j2 þ jZ j2
In which, C00 is the imaginary part of the capacitance, Z0 is the real part of the electrode resistance, Z00 is the imaginary part of the electrode resistance, f is the operating frequency and m is the mass of the active material. The imaginary part of the capacitor reaches the maximum at the frequency of f0, which is defined as demarcation point of capacitance and resistance properties for electrode material [36e38]. The time constant t0 is described as a characteristic relaxation time of the whole system (the minimum time to discharge all of the energy from a device with an efficiency of more than 50%). The time constant t0 is equal to 1/f0. It is shown that CMN material exhibits fast frequency response capability. The time constant (t0) calculated is 0.15 s, closed to 0.12 s of CM material. It is suggesting that oxygen-containing functional
Fig. 7. Cyclic voltammograms for CM and CMN at the scan rate of 1000 mV s1 (a); Specific capacitance of carbons at different scan rates (b). (A colour version of this figure can be viewed online.)
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Fig. 8. Galvanostatic charge/discharge profiles of CMN at different current densities. (A colour version of this figure can be viewed online.)
Fig. 9. Nyquist plot for CM and CMN electrodes (a); the changes of imaginary parts of the specific capacitances with frequency for the CM and CMN electrodes (b). (A colour version of this figure can be viewed online.)
groups on the surface of CMN after the oxidation of nitric acid do not prevent the transport of the electrolyte ion in the porous carbon material. And CMN material contains a lot of mesoporous and macroporous structures, which further reduces the electrolyte ion transport resistance. Moreover, the carbon nano-cages structure of CMN contains graphene sheet layers and can reduce electron resistance. According to the characteristics of CMN above, it exhibits excellent rate performance. 4. Conclusions Magnesium and carbon dioxide are used as feedstock to prepare graphitized hierarchical porous carbon nano-cages material CM. However, the carbon nano-cages material CM exhibits lower specific capacitance value. To further improve the performance of capacitance, CM is oxidized by nitric acid to obtain oxygencontaining functional groups on its surface. The structure of graphitized hierarchical porous carbon nano-cages material after nitric acid oxidation process can be better maintained, compared with the amorphous microporous activated carbon material. A large number of oxygen-containing functional groups attached to the high conductivity graphite surface cause CMN material to be a novel electric double layer capacitor and pseudo-capacitance coexisting supercapacitor material. High conductivity, mesoporous and macroporous hierarchical distribution and oxygencontaining functional groups on the surface of the graphite layer make the carbon nano-cages material CMN exhibit excellent rate properties. The capacitance value reaches 96 F g1 at the scan rate of 1000 mV s1. Capacitive performance of CMN is far more than
that of amorphous microporous activated carbon. The carbon nanocages material after oxidation treatment will be a promising supercapacitor electrode material. Acknowledgements We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grants 21175059), Natural Science Foundation of Shandong Province (Grants 2014BSB14028) and Staring Foundation of Jining Medical University (Grants JY2015BS02 and JY14QD04). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2016.09.081. References [1] E. Faggioli, P. Rena, V. Danel, X. Andrieu, R. Mallant, H. Kahlen, Supercapacitors for the energy management of electric vehicles, J. Power Sources 84 (2) (1999) 261e269. [2] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publisher, New York, 1999. [3] A. Burke, Ultracapacitors: why, how, and where is the technology, J. Power Sources 91 (1) (2000) 37e50. [4] G. Gutmann, Hybrid electric vehicles and electrochemical storage systems - a technology push-pull couple, J. Power Sources 84 (2) (1999) 275e279. [5] H. Shi, Activated carbons and double layer capacitance, Electrochim. Acta 41 (10) (1996) 1633e1639. [6] A.B. Fuertes, G. Lota, T.A. Centeno, E. Frackowiak, Templated mesoporous
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surface chemistry of activated carbons, Carbon 37 (9) (1999) 1379e1389. [23] H. Zhu, X.L. Wang, F. Yang, X.R. Yang, Promising carbons for supercapacitors derived from fungi, Adv. Mater. 23 (24) (2011) 2745e2748. [24] D.W. Wang, F. Li, M. Liu, H.M. Cheng, Improved capacitance of SBA-15 templated mesoporous carbons after modification with nitric acid oxidation, New Carbon Mater. 22 (4) (2007) 307e314. [25] A. Chakrabarti, J. Lu, J. Skrabutenas, T. Xu, Z. Xiao, J. Maguire, et al., Conversion of carbon dioxide to few-layer graphene, J. Mater. Chem. 21 (26) (2011) 9491e9493. [26] L. Mao, Y. Zhang, Y.T. Hu, K.H. Ho, Q.Q. Ke, H.J. Liu, et al., Activation of sucrosederived carbon spheres for high-performance supercapacitor electrodes, RSC Adv. 5 (12) (2015) 9307e9313. [27] M.K. King, A simplified two-reaction zone model of magnesium combustion in carbon dioxide, P Combust. Inst. 29 (2) (2002) 2931e2938. [28] M. Motiei, J. Calderon-Moreno, A. Gedanken, The formation of carbon-coated MgO cubes and carbon cubes, Adv. Mater. 14 (16) (2002) 1169e1172. [29] S. Dyjak, W. Kicinski, M. Norek, A. Huczko, O. Labedz, S. Budner, et al., Hierarchical, nanoporous graphenic carbon materials through an instant, selfsustaining magnesiothermic reduction, Carbon 96 (2016) 937e946. [30] T. Morishita, T. Tsumura, M. Toyoda, J. Przepiorski, A.W. Morawski, H. Konno, et al., A review of the control of pore structure in MgO-templated nanoporous carbons, Carbon 48 (10) (2010) 2690e2707. [31] T. Morishita, Y. Soneda, T. Tsumura, M. Inagaki, Preparation of porous carbons from thermoplastic precursors and their performance for electric double layer capacitors, Carbon 44 (12) (2006) 2360e2367. [32] M.S.A. Rahaman, A.F. Ismail, A. Mustafa, A review of heat treatment on polyacrylonitrile fiber, Polym. Degrad. Stab. 92 (8) (2007) 1421e1432. [33] E.P. Sajitha, V. Prasad, S.V. Subramanyam, S. Eto, K. Takai, T. Enoki, Synthesis and characteristics of iron nanoparticles in a carbon matrix along with the catalytic graphitization of amorphous carbon, Carbon 42 (14) (2004) 2815e2820. [34] M. Sevilla, A.B. Fuertes, Catalytic graphitization of templated mesoporous carbons, Carbon 44 (3) (2006) 468e474. [35] W. Xing, S.Z. Qiao, R.G. Ding, F. Li, G.Q. Lu, Z.F. Yan, et al., Superior electric double layer capacitors using ordered mesoporous carbons, Carbon 44 (2) (2006) 216e224. [36] Y.R. Nian, H.S. Teng, Influence of surface oxides on the impedance behavior of carbonbased electrochemical capacitors, J. Electroanal. Chem. 540 (2003) 119e127. [37] P.L. Taberna, P. Simon, J.F. Fauvarque, Electrochemical characteristics and impedance spectroscopy studies of carbon-carbon supercapacitors, J. Electrochem. Soc. 150 (3) (2003) A292eA300. [38] X. Geng, F. Li, D.W. Wang, H.M. Cheng, The electrochemical performance of a multi-wall carbon nanotube/activated carbon mixture as the electrode of electric double layer capacitors analyzed by electrochemical impedance, New Carbon Mater 26 (4) (2011) 307e312.
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Carbon journal homepage: www.elsevier.com/locate/carbon
High rate performance carbon nano-cages with oxygen-containing functional groups as supercapacitor electrode materials Liang Jiang a, *, Jing Wang b, Xuyan Mao a, Xiangyu Xu a, Bo Zhang c, Jie Yang a, Yunfei Wang d, Jun Zhu a, Shifeng Hou a, ** a
Bio-Nano & Medical Engineering Institute, Jining Medical University, 16 Hehua Road, Jining, 272067, China Physics and Information Engineering Department, Jining University, 1 Xingtan Road, Qufu, 273155, China Foundation Medical College, Jining Medical University, 16 Hehua Road, Jining, 272067, China d Gynecology Department, Affiliated Hospital of Jining Medical University, 89 Guhuai Road, Jining, 272000, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2016 Received in revised form 28 September 2016 Accepted 29 September 2016 Available online 30 September 2016
Graphitized carbon nano-cages can be prepared by carbon dioxide and magnesium metal as precursors. The carbon materials can produce a large amount of functional groups by the oxidation of nitric acid. Compared with amorphous carbon, the oxidation treatment can improve capacitive performance of carbon nano-cages, especially rate performance. At high scan rate, specific capacitance of carbon nanocages is much higher than that of the microporous activated carbon. So carbon nano-cages are promising high rate performance materials. A series of physical and chemical characterization exhibits that excellent capacitive performance is derived from carbon nano-cages structure themselves. It is due to higher electrical conductivity of graphite structure, lower charge transfer resistance of hierarchical porous structure and extra pseudo-capacitance of oxygen-containing functional groups. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction With increasing environmental pollution, the development of clean energy sources becomes an urgent task and is received more widespread attention. Supercapacitor is a new energy storage device utilizing electric double layer for storing charge. Compared with secondary batteries, supercapacitor has unique advantages including long cycle life, short charging time and high power density [1e4]. In general, carbon materials are the most common electrode materials for supercapacitors [5e8]. Activated carbons are most commonly used as electrode materials in carbon materials. But it also has its own shortcomings. Amorphous structure and rich microporous structure can lead to low electrical conductivity and high charge transfer resistance [9,10]. Capacitive performances of activated carbon significantly decay at high current density. While graphite has high conductivity, its low specific surface area is not suitable for supercapacitor electrode material. Highly graphitized porous carbon material has relatively higher
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Jiang), [email protected] (S. Hou). http://dx.doi.org/10.1016/j.carbon.2016.09.081 0008-6223/© 2016 Elsevier Ltd. All rights reserved.
specific surface area than graphite and higher electrical conductivity than amorphous carbon. Therefore, highly graphitized porous carbon materials are promising electrode materials for supercapacitors [11e13]. Compared to microporous structure of activated carbon, hierarchical structure of highly graphitized porous carbon has a synergistic effect of microporous, mesoporous and macroporous structures, and thus can show a better rate performance [14e17]. Meantime, some electrode surface of carbon materials can occur highly reversible redox reaction, which can generate another type of capacitor: pseudo-capacitance [18e20]. Generally, the surface of the carbon material has a large number of oxygen-containing functional groups, such as hydroxyl, carboxyl and carbonyl groups. Such functional groups on the carbon material surface can react with the electrolyte ion to produce pseudo-capacitance [21,22]. So the carbon materials with oxygen-containing functional groups as supercapacitor electrode materials can generate electric double layer capacitor and also generate pseudo-capacitor. The presence of pseudo-capacitance can greatly improve the specific capacitance value of carbon materials [23]. This kind of pseudo-capacitance is generated on the surface of the carbon material and it has higher electrical conductivity than that of metal oxide which can also generate pseudo-capacitor. Therefore, it
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exhibits a high capacitance value and a better rate capability. Carbon material can effectively generate oxygen-containing functional groups on the surface by the oxidation of nitric acid [24]. However, nitric acid with strong oxidizing can damage the pore structures of the amorphous carbon to a certain extent. Highly graphitized porous carbon materials have strong antioxidant properties and reduce the damage of pore structure. In this work, magnesium metal and carbon dioxide are used as precursors to prepare the multi-layer graphene nano-cages [25]. Further concentrated nitric acid is used to oxide graphene nano-cages, so that oxygen-containing functional groups are grafted onto the surface of the graphene nano-cages. It is expected to be a promising way to synthesize high-rate performance supercapacitor electrode materials.
pestle. Rubber clay state was formed by adding a small amount of ethanol. Then the sample was pressed to form a circular electrode membrane of 9 mm in diameter and then sandwiched between two nickel foams to form an electrode assembly, which nickel foam was used as current collectors. And nickel-metal band was used as a wire. Electrochemical measurements were carried out in threeelectrode cell. Prepared sample membrane, Pt plate and Ag/AgCl electrode were used as work electrode, counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) tests, electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge tests were carried out on an electrochemical workstation (Shanghai CH Instruments, CHI760E). 3. Results and discussion
2. Experimental 2.1. Material synthesis In the typical experiment, dry ice was placed in de-ionized water. At the same time, carbon dioxide gas appeared in the upper part of the beaker. Magnesium ribbon was ignited in the carbon dioxide atmosphere. Then the sample was leached in HCl solution to remove magnesium oxide and unreacted magnesium metal, and washed thoroughly with de-ionized water. Finally, the samples were dried at 60 C in an oven. The black carbon material was labeled as CM. The CM material was heated in concentrated nitric acid at 60 C for 3 h. The obtained sample was washed thoroughly with deionized water, which was labeled as CMN. Activated carbons have the characteristics of high specific surface area, low cost. Therefore, activated carbons become the most commonly used commercial supercapacitor electrode materials [7,26]. As a comparison, sucrose was used as raw material to prepare activated carbon materials. A certain amount of sucrose was placed in a tube of a furnace. The pyrolysis of sucrose was carried out at 500 C for 1 h in N2 atmosphere, then cooled to room temperature. The obtained sample and KOH powder were mixed. The mass ratio pyrolysed sucrose/KOH was 1:4 in the work. The mixture was treated at 800 C for 2 h in N2 atmosphere. The mixture after treatment was labeled as SC. Similarly, the SC material was heated in concentrated nitric acid at 60 C for 3 h. The obtained sample was washed thoroughly with de-ionized water, which was labeled as SCN. 2.2. Physicochemical characterization The compositions of the samples were charactered by elemental analysis with a Vario EL Cube (Elementar). X-ray photoelectron spectroscopy (XPS) was used to investigate the surface information of the carbon, XPS data was collected using an Escalab 250 Xi X-ray photoelectron spectrometer (Thermo Scientific). The porous texture of carbon nano-cages were characterized by nitrogen adsorption at 77 K with a Quantachrome NovaWin system (Quantachrome). Surface areas were calculated by BrunauereEmmeteTeller (BET), pore size distributions were analyzed by BarretteJoynereHalenda (BJH) method. X-ray diffraction (XRD) was performed on a XRD-6100 (Shimadzu). Morphologies of carbon nano-cages were investigated by a JEM-2100 at 200 kV and Zeiss ultra plus at 3 kV. 2.3. Electrochemical tests 85 wt% carbon, 10 wt% acetylene black and 5 wt% polytetrafluoroethylene (PTFE) were mixed and grounded with a mortar and
The morphologies of CM and CMN are observed by SEM and TEM images, as shown in Fig. 1 and Fig. 2. SEM image of CM (Fig. 1a) shows that the CM material is formed by the accumulation of a large number of tiny cubic particles with a size range of 100e200 nm. Further increasing the magnification (Fig. 1b), these particles exhibit a specific nano-cages structure and most of nanocages are open. An in-situ MgO template method leads to the open nano-cages structure. Fig. S1 shows XRD pattern of the material from the combustion of magnesium ribbon and carbon dioxide. The peaks at 36.9 , 42.9 , 62.3 , 74.6 and 78.6 are assigned to diffraction from (111) (200) (220) (311) and (222) planes of the cubic structure of magnesium oxidation. According to the previous papers and our study, in carbon dioxide atmosphere, magnesium is ignited and releases a lot of heat. In the outer zone of the reaction, carbon dioxide and magnesium react to produce magnesium oxide and CO. In the inner zone, CO is reduced by liquid metal magnesium, forming carbon and magnesium oxide [27,28]. The reaction produces magnesium oxide crystal of cubic structure and carbon material is deposited on the surface of magnesium oxide. Due to the limited concentration of CO produced by magnesium reducing CO2, the deposition rate of carbon on magnesium oxide is slow and the deposition method is performed in layer by layer way. Moreover, a lot of heat is released during the reaction and the reaction temperature can reach to 2500 C [29]. At high temperature, graphite carbon layers are formed. Because the structure of magnesium oxide is cubic, and thus the structure of carbon material wrapped on it is also cubic. After acid treatment, cubic structure of carbon nano-cages will be formed. The open nano-cages can be used as a reservoir of electrolyte ions, which can reduce the distance of electrolyte ions into micropores and mesopores [14]. In the common method of preparing mesoporous carbon with magnesium oxide as template, first it requires the preparation of magnesium oxide template, and then mixed with carbon source. And generally amorphous carbon is usually obtained by the template method [30,31]. High specific surface area mesoporous graphite carbon can be prepared only by high temperature carbonization or addition of graphitization catalyst [32e34]. Therefore, the method in this paper is simple and does not need to be processed by high temperature to obtain graphite carbon nanocages material. Fig. 2 shows the SEM and TEM of CMN, which is the material of CM oxidized by nitric acid. It can be observed that the morphology and size of carbon nano-cages material do not change significantly, suggesting that the morphology of the carbon nano-cages material is not influenced by strong oxidizing nitric acid. Further highresolution transmission electron microscopy (HRTEM) characterization (Fig. 2d) reveals the carbon nano-cages material has a crystal structure. It is difficult for nitric acid to oxide and destroy
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Fig. 1. SEM (a, b), TEM (c) and HRTEM images of CM (d).
Fig. 2. SEM (a,b), TEM (c) and HRTEM images of CMN (d).
highly graphitized structure. So it is still able to maintain such a nano-cages structure. Fig. 3 shows XRD patterns of CM and CMN. The peaks at 26 and 44.3 are assigned to diffraction from (002) and (100) planes of the hexagonal structure of graphite. The XRD results are indicative of good graphitization, which is consistent with the HRTEM results. XRD pattern of CMN is similar to that of CM, suggesting that the oxidation of nitric acid does not change the crystal structure. As shown in Table S1, elemental analysis results show that no significant changes of nitrogen and hydrogen in carbon materials happens after the nitric acid treatment, while the content of oxygen increases from 4.25 wt% to 9.02 wt%. It suggests that the nitric acid treatment can increase the oxygen content in carbon materials but has no effects on nitrogen content.
The pseudo-capacitance is produced by the highly reversible redox reaction of the oxygen containing functional groups on the surface of carbon materials. XPS is an effective method to analyze the oxygen functional groups on the surface of carbon materials. Therefore, the XPS test is a more meaningful method for the study of CMN as a supercapacitor electrode material. XPS spectra of CM and CMN are presented in Fig. S2. There are no peaks of nitrogen appearing in the spectra. It suggests that no nitrogen has been attached on the surface of the carbon before and after nitric acid treatment. As shown in Table 1, the content of oxygen in CM and CMN material is measured by XPS. The total of oxygen content is 7.74 at. % and 9.39 at. % for CM and CMN, respectively. It suggests that the content of oxygen-containing functional groups on the carbon material surface has increased
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Fig. 3. XRD patterns of CM and CMN. (A colour version of this figure can be viewed online.)
Table 1 Total oxygen contents and percentages of different species in the samples. Sample
CM
CMN
Total oxygen content (%) Percentage (%)oxygen in carbonylic Oxygen in phenolic and lactone groups Oxygen in H2O
7.74 4.13 2.45 1.16
9.39 4.98 4.11 0.30
significantly. The high resolution O1s peaks of carbon nano-cages CM and CMN are fitted into three components originating from oxygen in carbonyl groups (531.2e531.6 eV), oxygen in phenolic and lactone groups (532.7e533.0eV), and oxygen in H2O (535.5e536.3 eV) as shown in Fig. 4 [21,22]. Oxygen configurations of each sample are summarized in Table 1. The content of oxygen in phenolic and lactone groups increase from 2.45 at. % to 4.11 at. % after the oxidation of nitric acid. The two kinds of oxygen species can produce CO by desorption, in favor of the formation of an electric double layer. Therefore, oxygen functional group structure on the surface of the carbon material can be changed by nitric acid to a certain extent to improve the performance of surpercapacitor. Fig. 5a shows the N2 adsorption-desorption curves of SC and SCN. Both SC and SCN curves are similar, which belong to the isotherms of type I (based on IUPAC classifidcation). It indicates the existence of microporous structure in SC and SCN, due to the activation of KOH. From pore distribution curves of SC and SCN (Fig. 5b), it can be observed that there is no significant pore size
distribution in the mesoporous range (>2 nm). This result further illustrates that both of them are microporous materials. Before and after the oxidation of nitric acid, the specific surface area of SC and SCN is 3368 and 2666 m2 g1, respectively. And pore volume of SC and SCN is 1.7 and 1.36 cm3 g1, respectively. There is a significant reduction in the surface area and pore volume after the nitric acid treatment. It is due to oxidation corrosion of carbon material own structure, so that its pore structure is destroyed. As shown in Fig. 5c, the N2 adsorptionedesorption curves of the carbon nano-cages materials CM and CMN all belong to the isotherms of type IV (based on IUPAC classification). Conversely, before and after the oxidation of nitric acid, the curves of CM and CMN materials are not significantly different. It is because that CM material is highly graphitized, with a more completely crystalline structure and not easy to damage the structure. The slope of the curve increases rapidly at relative pressure above 0.4. Moreover, it can be found the existence of a hysteresis loop, which indicates the existence of mesopores. Pore distribution curves of the CM and CMN materials are shown in Fig. 5d by the BJH method. The poresize distribution is mainly distributed in a narrow pore size range of 6e7 nm. In addition to the mesoporous distribution, it exists pore size distribution above 50 nm. It exhibits that the CM and CMN carbon nano-cages materials are hierarchical porous carbon of mesopores and macropores coexisting. The magnesium oxide is removed by acid washing and followed by the formation of nanocages structure, resulting in the existence of macropores. Before and after the oxidation of nitric acid, the specific surface area of CM and CMN is 806 and 866 m2 g1, respectively. And pore volume of CM and CMN is 1.51 and 1.55 cm3 g1, respectively. The specific surface area and pore volume has a certain increase after nitric acid oxidation and the integrity of the pore structure of the carbon material remains unchanged. CV measurements are used to investigate the capacitive performance of the nano-cages materials in 6.0 mol L1 KOH aqueous electrolyte and in the scan potential range from 1 to 0 V. As shown in Fig. 6, The voltammograms exhibit nearly symmetrical rectangular shapes, indicating that the capacitive response mainly comes from the electrochemical double layer capacitance. The carbon nano-cages material has high graphitization to improve good electrical conductivity and unique cage structure can be used as a reservoir of electrolyte ions to reduce transfer resistance of electrolyte ions. So it shows good capacitive performance. Compared with CM, the cyclic voltammetry curves of CMN have greater response current, even appear redox peaks. Apparent redox peaks can be observed at the potential of 0.4 V. From the XPS results, the surface of the carbon nano-cages contains a large
Fig. 4. The O1s XPS spectra of CM (a) and CMN (b). (A colour version of this figure can be viewed online.)
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Fig. 5. Nitrogen adsorptionedesorption isotherm (a) and pore size distribution (b) of SC and SCN; Nitrogen adsorptionedesorption isotherm (c) and pore size distribution (d) of CM and CMN. (A colour version of this figure can be viewed online.)
Fig. 6. Cyclic voltammograms of the electrodes at different scan rates in 6 mol L1 KOH aqueous solution of (a) CM, (b) CMN. (A colour version of this figure can be viewed online.)
amount of carbonyl functional groups, indicating that the presence of surface oxygen-containing functional groups leads to the occurrence of highly reversible redox reaction in the chargedischarge process, thereby the appearance of pseudo-capacitance. Moreover, the other increased electrical double layer capacitance is caused by the strengthened surface charge density from oxygen atoms with more negative electronic affinity, which enhances the space charge layer capacitance in the carbon side. The specific capacitance is calculated according to the following equation:
C¼
Q 2DVm
Where Q is the charge integrated from the whole voltage range, DV is the whole voltage difference, and m is the mass of the active material in a single electrode. The value of CM and CMN at 2 mV s1 is 88 and 110 F g1, respectively. With increasing the scan rate, the values of response current are continuously improved. At the same time, the cyclic voltammetry curve remains rectangular shape. It shows at higher scan rate, the specific capacitance of CMN has a
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more significant improvement than that of CM. Oxygen-containing functional groups on the carbon material surface can generate pseudo-capacitance through redox reaction. Compared with the electrical double layers capacitor, pseudocapacitance is generally low to respond at the high scan rate. So pseudo-capacitance electrode materials generally have relatively poorer rate performance. However, the cyclic voltammetry curve of CMN still remains nearly rectangular configuration and there is no apparent oblique angle at the higher scan rate of 1000 mV s1 (Fig. 7a). The results show that the CMN electrode material has a small equivalent series resistance ESR and excellent capacitive performance. Although the surface functional groups of the CMN carbon material occur the redox reaction, it is noteworthy that these oxygen-containing functional groups are attached to the high conductivity graphite surface and electrons can transfer smoothly in redox reaction. Thus the CMN pseudo-capacitive electrode material still keeps a high conductivity. As a comparison, the curves of the specific capacitance of SC and SCN are shown in Fig. 7b. The specific capacitance of SC reaches 304 F g1 at the scan rate of 2 mV s1. However, the specific capacitance of SCN only has 174 F g1 at the scan rate of 2 mV s1 because of the damage of structure. With increasing scan rate, the specific capacitance has a sharp decline. This is because that surface functional groups can adsorb water molecules to form large molecules and further the microporous of SC and SCN decreases, resulting in a large charge transfer resistance. At the same time, the amorphous structure of activated carbon itself has led to a large electronic resistance. Compared with amorphous carbon, graphitized carbons CM and CMN exhibit better rate performance. At 1000 mV s1, the specific capacitance of CMN reaches 96 F g1, meanwhile its specific capacitance is only 59 F g1 and 50 F g1 for SC and SCN. CM and CMN materials are mainly made up of mesoporous and macroporous structure. Water molecules adsorbed on the functional groups do not affect the electrolyte ion transport. Moreover, oxygen-containing functional groups can also participate in a redox reaction to improve the rate performance of the supercapacitor. Although CMN has lower capacitance value at low current density, its rate performance is very excellent. Therefore, it is more suitable for use at high current density. The galvanostatic charge/discharge curves are shown in Fig. 8 at different current densities. The curves exhibit symmetrical triangle and voltage changes linearly with time, indicating that CMN material has a typical capacitive behaviour. The specific capacitance is calculated according to the equation:
C¼
I Dt Um
Where I is the discharge current (A), Dt is the discharge time (s), U is the potential change window (V) excluding IR drop in the discharge process, and m is the mass of the active material in a single electrode (g). The specific capacitance of CMN material is 104 F g1 at the current density of 0.2 A g1. To further increase the current density to 10 A g1, the value of the specific capacitance of CMN material is 93 F g1. No obvious potential drop appears at the current density of 10 A g1 for CMN, exhibiting a better rate performance and a smaller resistance. The excellent performance comes from high conductivity of the graphite structure and low transport resistance of the electrolyte ion of the synergistic effect of rich mesopores and macropores [35]. Electrochemical impedance spectroscopy (EIS) is also used to estimate the supercapacitive performance. The intersection of the high-frequency region and the real axis is very small (Fig. 9). It is due to its low resistance of the graphite structure. The EIS curves of CM and CMN samples are both perpendicular to the real axis at low frequency region, closed to the characteristic of the ideal capacitive behaviors. No apparent semicircle can be observed at higher frequency region, suggesting that it occurs fast ion diffusion in the electrode porous material due to the existence of mesopores. To further investigate the electrolyte ion transport in porous electrode inside, EIS data is processed to get the imaginary part of the capacitance. The following formula is provided to calculate the capacitance: 00
C ¼
Z0 00 2pfm jZ 0 j2 þ jZ j2
In which, C00 is the imaginary part of the capacitance, Z0 is the real part of the electrode resistance, Z00 is the imaginary part of the electrode resistance, f is the operating frequency and m is the mass of the active material. The imaginary part of the capacitor reaches the maximum at the frequency of f0, which is defined as demarcation point of capacitance and resistance properties for electrode material [36e38]. The time constant t0 is described as a characteristic relaxation time of the whole system (the minimum time to discharge all of the energy from a device with an efficiency of more than 50%). The time constant t0 is equal to 1/f0. It is shown that CMN material exhibits fast frequency response capability. The time constant (t0) calculated is 0.15 s, closed to 0.12 s of CM material. It is suggesting that oxygen-containing functional
Fig. 7. Cyclic voltammograms for CM and CMN at the scan rate of 1000 mV s1 (a); Specific capacitance of carbons at different scan rates (b). (A colour version of this figure can be viewed online.)
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Fig. 8. Galvanostatic charge/discharge profiles of CMN at different current densities. (A colour version of this figure can be viewed online.)
Fig. 9. Nyquist plot for CM and CMN electrodes (a); the changes of imaginary parts of the specific capacitances with frequency for the CM and CMN electrodes (b). (A colour version of this figure can be viewed online.)
groups on the surface of CMN after the oxidation of nitric acid do not prevent the transport of the electrolyte ion in the porous carbon material. And CMN material contains a lot of mesoporous and macroporous structures, which further reduces the electrolyte ion transport resistance. Moreover, the carbon nano-cages structure of CMN contains graphene sheet layers and can reduce electron resistance. According to the characteristics of CMN above, it exhibits excellent rate performance. 4. Conclusions Magnesium and carbon dioxide are used as feedstock to prepare graphitized hierarchical porous carbon nano-cages material CM. However, the carbon nano-cages material CM exhibits lower specific capacitance value. To further improve the performance of capacitance, CM is oxidized by nitric acid to obtain oxygencontaining functional groups on its surface. The structure of graphitized hierarchical porous carbon nano-cages material after nitric acid oxidation process can be better maintained, compared with the amorphous microporous activated carbon material. A large number of oxygen-containing functional groups attached to the high conductivity graphite surface cause CMN material to be a novel electric double layer capacitor and pseudo-capacitance coexisting supercapacitor material. High conductivity, mesoporous and macroporous hierarchical distribution and oxygencontaining functional groups on the surface of the graphite layer make the carbon nano-cages material CMN exhibit excellent rate properties. The capacitance value reaches 96 F g1 at the scan rate of 1000 mV s1. Capacitive performance of CMN is far more than
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