Zhu Li,
Kunkun Guo* and
Xuli Chen
College of Materials Science and Engineering, Hunan University, Changsha, 410082, People's Republic of China. E-mail: kunkunguo@hnu.edu.cn
First published on 14th June 2017
Nitrogen-doped mesoporous carbons (NMC) have been controllably synthesized through a solvent evaporation induced self-assembly (EISA) method, where a novel nitrogen-doped carbon precursor is used, followed by thermo-polymerization and pyrolysis of this precursor. In this precursor, dicyandiamide is covalently integrated into low-molecular weight and soluble phenolic resins. The configuration of nitrogen and the effects of nitrogen on the structure and porosity before and after KOH activation are deeply investigated for the obtained NMC. The nitrogen functional groups are found to facilitate the KOH activation process, especially when the aperture is suitable for molten KOH immersion, leading to the formation of much more micropores and mesopores on the surface of the carbons. As a result, a large specific surface area of 2828.8 m2 g−1 in activated NMC (A-NMC) is significantly greater than that of activated mesoporous carbon without nitrogen (1244.6 m2 g−1), and the mesoporous ratio in A-NMC is as high as 44.1%. Meanwhile, supercapacitor electrodes based on the obtained A-NMC exhibit a specific capacitance as high as 388 F g−1 at the current density of 0.5 A g−1, while only 213.6 F g−1 is achieved by the nitrogen-free activated mesoporous carbons based electrode. Moreover, the A-NMC shows a good rate capability (70% of the capacitance retained at a high current density up to 50 A g−1) and an excellent cycling stability (no capacitance loss over 5000 cycles). The method demonstrated in this work is hopeful to open up a new route of designing nitrogen-doped mesoporous carbons for supercapacitor applications with good rate capability and excellent cycling stability.
To further enhance energy density and power output, one strategy is proposed to introduce pseudocapacitance through doping functional groups/heteroatoms, such as N, O, S, B and P to carbon materials.6,7 Among various heteroatoms, nitrogen atom can greatly improve the specific capacitance of carbon materials by both improving the surface wettability8 and inducing redox reactions to incorporate pseudocapacitance.9 Some nitrogen-containing compounds, such as melamine,10,11 urea,12 dicyandiamide,13–16 have been used to prepare nitrogen-doped mesoporous carbons in the applications of supercapacitor electrodes. Dicyandiamide that is low price with the nitrogen content about 66.7%, possesses higher water solubility and reactivity, and thus is an ideal nitrogen source. NMC synthesized by simply mixing dicyandiamide, low molecular weight phenolic resins with F127, exhibits a specific surface area of 537 m2 g−1, and the specific capacitance of 244 F g−1 at a low current density of 0.5 A g−1, which is higher than that of MC without nitrogens.13 Additionally, the ordered NMC fabricated by soft template hydrothermal method, where D-fructose and dicyandiamide were used as carbon source and nitrogen source, respectively, give a specific surface area of 375–730 m2 g−1, and a specific capacitance of 212 F g−1 even at a low scan rate of 1 mV s−1.14
Although the incorporation of nitrogen can provide some pseudo capacitance, it is very limited to increase in both the capacitance and surface area of mesoporous carbons. It is well known that the specific surface area is crucial to the capacitance of EDLCs, which has a direct effect on the performance of mesoporous carbons. The ordered NMC prepared by EISA method with both TEOS and F127 as template, dicyandiamide and phenolic resins as nitrogen source and carbon source, respectively, followed by HF etching of SiO2, showed a specific surface area of 1374 m2 g−1 and a specific capacitance of 308 F g−1 at low current density of 0.2 A g−1, which was far higher than that of NMC without using silicon template (631 m2 g−1 and 234 F g−1). However, at high current density of 20 A g−1, the specific capacitance of NMC with TEOS was only 131 F g−1 less than that of NMC without silicon template (145 F g−1), as well as the capacitance retention rate of NMC with TEOS is only 42.5%.16 Another nitrogen-doped ordered mesoporous carbons prepared by mechanically mixing lysine and phenolic resins, showed a limited specific surface area of 1866 m2 g−1 even after further KOH activation and a specific capacitance of 186 F g−1 in ionic liquid electrolyte at the low current density of 0.25 A g−1.17 Besides, a series of nitrogen-doped porous carbons (NPC) were synthesized by pyrolysis of the polymerized ethylenediamine and carbon tetrachloride, followed by KOH activation. The specific surface area of the obtained NPC is 1913 m2 g−1, and the specific capacitance is 363 F g−1 at a current density of 0.1 A g−1. However, the capacitance retention rate was only 50% at the current density of 20 A g−1.18 The specific surface area and the specific capacitance of nitrogen-doped porous carbons can be effectively increased by the extra method of silica template19,20 and KOH activation.21–25 Likewise, it is found that large micropores are further formed in these porous carbons after KOH activation. Therefore, the obtained materials can exhibit a good specific capacitance at small current density, not at high current densities. This phenomenon is mainly ascribed to the fact that electrolyte ions cannot freely enter and leave small pores at high current density.26 Therefore, controllable synthesis of nitrogen-doped mesoporous carbons with large specific surface area and moderate pore size is possible to improve the electrochemical performance at high current density.
Here, a kind of soluble resins (dicyandiamide formaldehyde resin, DPF) covalently inter-connected among dicyandiamide, phenol and formaldehyde via a basic catalyzed process, possesses high nitrogen content and stable structure. As illustrated in Scheme 1, using triblock copolymer F127 as a soft template, and dicyandiamide phenolic resins as the precursor, nitrogen-doped mesoporous carbons are controllably synthesized through EISA method followed by thermo-polymerization and carbonization. After KOH activation, large micropores and mesoporous are formed in the obtained nitrogen-doped mesoporous carbons, giving A-NMC with a large specific surface area and a high ratio of mesoporous. The as-made carbons based supercapacitor electrodes greatly improve the electrochemical capacitive performance compared with MC without nitrogen, especially showing good rate capability and excellent cycling stability.
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Fig. 3 Small-angle XRD patterns of NMC-0.0, NMC-0.5 and NMC-1.0, where the short curves of NMC-0.0 and NMC-0.5 are the magnification with 20 times. |
Fig. 4 shows the nitrogen sorption results of NMC before and after KOH activation. A sharp uptake at P/P0 below 0.1 are displayed in all isotherms of Fig. 4(a) and (c), indicative of large micropores found in both NMC-x and A-NMC-x.28 Likewise, all nitrogen sorption isotherms exhibit the combination of type I and IV curves. However, NMC-x before KOH activation contain the typical type-IV curves with steep H4 hysteresis loops in the pressure of 0.5–1.0, which evidently indicate the existence of both mesoporous and micropores.29 Likewise, the pore size distributions decided by density functional theory (DFT)15 are presented in Fig. 4(b) and (d), also giving the existence of both micropores and mesoporous in both NMC-x and A-NMC-x. As shown in Fig. 4(b), when x is above 1.0, the mesoporous (pore width > 2.0 nm) possess much wide distribution, indicating that the pores formed by the cylindrical micelles (F127) are not uniform any more. Along with the continuous increase of nitrogen atoms, such interactions between nitrogen functional groups and the carbon surface would be prone to cause the expansion of the mesoporous in certain directions, and finally leading to the distortion and collapses of the original ordering structure.13 As such, when x is equal to 0.5, the width of mesoporous is slightly larger than those of NMC-0.0. With further increase of x, the distortions and collapses of the ordered mesostructures appear evidently, giving that the distribution of mesoporous becomes much wider. These results are well consistent with those obtained from both TEM and SAXS.
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Fig. 4 Nitrogen adsorption–desorption isotherms and DFT pore size distributions of NMC-x (a) and (b) and A-NMC-x (c) and (d). |
Textural properties deduced from these isotherms are listed in Table 1. It is found that in the presence of nitrogen atoms, the volumes of both micropores and mesoporous increase sharply after KOH activation. It is also deduced from Table 1 that when x ≤ 1.0, the mesoporous volumes in NMC-x and A-NMC-x are about half of their total pore volumes, that is to say, these mesoporous ratios in both NMC-x and A-NMC-x are close to 50%. Likewise, the specific surface area after KOH activation is found to increase along with the increase of nitrogen. When x is increased up to 1.0, the specific surface area and pore volume of A-NMC-1.0 increases up to 2828.8 m2 g−1 and 1.27 cm3 g−1, which are almost twice of A-NMC-0.0. With the further increase of x, for example, x = 1.2, the specific surface, the mesoporous ratio and the pore volume start to decrease conversely. The evident increase in the surface area of carbons with nitrogen-doping after KOH activation have also been observed in previous result that the specific surface area of N-doped activated carbon sheets fabricated by using glucose as carbon source and melamine as nitrogen source after KOH activation, is as high as 1997.5 m2 g−1, much higher than both this carbon sheets before activation (125.8 m2 g−1) and this carbon sheets without nitrogen (1120.9 m2 g−1).30 These results might be ascribed that during activation process, the molten KOH is feasible to immerse into the mesoporous channels, as well as the presence of nitrogen atoms would seriously promote the etching of the carbon framework, the gasification of carbon and the expansion of carbon lattices, thus leading to the further formation of much more micropores and mesoporous on the surface of carbons. Additionally, the excess of nitrogen would seriously distort and collapse the pre-formed mesoporous channels in NMC-x, leading to the decrease of the mesoporous ratio of both NMC-1.2 and A-NMC-1.2. Likewise, during KOH activation, only the nitrogen atoms on the surface of the effective pores would contribute to the increase of both the specific surface area and the pore volume. In addition, the pore sizes of NMC-0.0, NMC-0.5 and NMC-1.0 shown in Table 1 are found to agree well with the results of TEM.
Samples | SBET (m2 g−1) | Vtotal (cm3 g−1) | Vmirco (cm3 g−1) | Pore width (nm) |
---|---|---|---|---|
a SBET: BET surface area, Vtotal: total pore volume calculated by density functional theory (DFT) method, Vmicro: pore volume of micropores calculated using the t-plot method. | ||||
NMC-0.0 | 453.5 | 0.27 | 0.15 | 1.52, 2.83, 3.82 |
NMC-0.5 | 448.8 | 0.26 | 0.14 | 1.52, 3.96 |
NMC-0.8 | 385.6 | 0.20 | 0.13 | 1.54, 3.95 |
NMC-1.0 | 421.9 | 0.29 | 0.14 | 1.52, 2.83, 3.82 |
NMC-1.2 | 384.5 | 0.25 | 0.12 | 1.52, 2.83, 3.82 |
A-NMC-0.0 | 1244.6 | 0.79 | 0.43 | 1.23, 1.52, 1.87, 3.38 |
A-NMC-0.5 | 1890.8 | 1.15 | 0.64 | 1.23, 1.53, 1.88, 3.83 |
A-NMC-0.8 | 2218.4 | 1.13 | 0.55 | 1.23, 1.53, 1.88, 3.83 |
A-NMC-1.0 | 2828.8 | 1.27 | 0.71 | 1.23, 1.53, 1.87, 2.21, 3.83 |
A-NMC-1.2 | 1841.9 | 1.00 | 0.68 | 1.23, 1.53, 1.87, 2.29, 2.67, 3.41 |
X-ray photoelectron spectroscopy (XPS) has been employed to clarify the relative surface concentrations and chemical environments in the carbon structures. One typical spectrum of NMC-1.0, including survey spectrum, C 1s, N 1s and O 1s spectra, is given in Fig. 5. The fractions of carbon atoms, four types of nitrogen atoms and three types of oxygen atoms in each sample (NMC-x) calculated from XPS analysis are summarized in Table 2. As listed in Table 2, the surface concentration of nitrogen increases as a function of the molar ratio of dicyandiamide to phenol, x. The nitrogen yields of NMC-x deduced by the surface concentration of nitrogen and the carbon yield, show that NMC-0.5 exhibits the highest nitrogen yield of 13.04%, and other NMC-x contain the nitrogen yield only close to 7.5%. In addition, the nitrogen yield of NMC-x fabricated by the DPF-x precursors in the present study is far higher than the previous report only by mechanically mixing the phenolic resins and dicyandiamide as the precursor.13 As shown in Fig. 5(b), three distinct peaks (284.3 eV, 285.0 eV and 287.8 eV) in the C 1s spectrum of NMC-1.0 correspond to the form of sp2 graphitic carbon, C–N and CO, respectively. Four distinct peaks in the N 1s spectrum shown in Fig. 5(c) indicate pyridinic-N (N-6, 398.6 eV), pyrrolic-N (N-5, 400.0 eV), quaternary-N (N-Q, 400.9 eV) and pyridine-N-oxide (N-X, 403 eV). Pyridinic-N (N-6, 398.6 eV) which is sp2 bonded to two carbon atoms, donates one p electron to the aromatic system. Pyrrolic-N (N-5, 400.0 eV) associates with phenolic or carbonyl groups on the neighboring carbon atoms of the five membrane ring, while quaternary-N (N-Q, 400.9 eV) is bonded to three carbon atoms in the central or valley positions of the graphene layer. As shown in Fig. 5(d), oxygen atoms are assigned to C
O quinine-type oxygen (O-1, 531 eV), C–OH phenol groups or C–O–C ether groups (O-2, 532 eV) and COOH chemisorbed oxygen or water (O-3, 533 eV).16 As reported previously,14 N-5, N-6 and N-X are responsible for the pseudo-capacitance, where N-Q can affect electron transfer through the carbon frameworks. Among oxygenous groups, only quinine-type oxygen (O-1) is electrochemically active in an acidic electrolyte.16 Scheme 3 presents the reaction mechanisms of N-5, N-6, N-X and O-1.
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Fig. 5 XPS spectra of NMC-1.0. (a) Survey spectrum, (b) C 1s spectrum, (c) N 1s spectrum, (d) O 1s spectrum. |
Samples | Carbon content (at%) | Nitrogen content (at%) | Oxygen content (at%) | Composition of nitrogen (%) | Composition of oxygen (%) | Carbon yield (%) | Nitrogen yield (%) | |||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
N-6 | N-5 | N-Q | N-X | O-1 | O-2 | O-3 | ||||||
NMC-0.0 | — | — | — | — | — | — | — | — | — | — | 51.2 | — |
NMC-0.5 | 87.49 | 3.93 | 8.58 | 29.26 | 49.67 | 21.07 | — | 8.30 | 45.10 | 46.60 | 52.4 | 13.04 |
NMC-0.8 | 85.75 | 3.96 | 10.29 | 38.13 | 23.17 | 36.71 | 1.99 | 22.20 | 42.29 | 35.51 | 48.2 | 8.11 |
NMC-1.0 | 86.24 | 4.09 | 9.67 | 36.84 | 22.59 | 26.08 | 14.49 | 17.84 | 40.97 | 41.19 | 46.4 | 6.78 |
NMC-1.2 | 86.98 | 5.50 | 7.52 | 29.23 | 35.84 | 31.65 | 3.82 | 20.53 | 44.07 | 35.40 | 44.4 | 7.65 |
Fig. 6(b) shows the CV curves of A-NMC-x at a scan rate of 5 mV s−1. In the presence of nitrogen, the CV curves exhibit humps as well as rectangular shapes. The incorporated heteroatoms, N and O, would attribute to several redox reactions overlapping one to another, thus leading to a couple of broad current peaks.16 As such, electrical double layer formation and redox reactions are contributed to the capacitances of A-NMC-x. The capacitances as reported previously31 are divided into EDLCs and pseudocapacitance and are shown in Fig. 6(d). It is found that the EDLCs of A-NMC-0.0 is low to 166.8 F g−1, as well as the EDLCs of A-NMC-x is found to enhance along with the increase x. When x increases up to 1.0, A-NMC-1.0 achieves the EDLCs up to the maximum of 257.7 F g−1. This result is mainly due to the fact that A-NMC-1.0 possesses the largest specific surface area, moderate pore size, and inter-connected bridge structures. Similar change trends are observed for the dependence of the pseudocapacitance on x, but the pseudocapacitances of all A-NMC-x are less than 60 F g−1. Thus, the contributions of EDLCs, not the pseudocapacitance play important role in the specific capacitances of A-NMC-x. Likewise, the specific surface area of A-NMC-x contributed to the EDLCs, increases sharply as a function of nitrogen. Whereas, the N-5, N-6, N-X and O-1 functional groups which can provide pseudocapacitance through the faradic redox reactions, possess less contents in all NMC-x samples as deduced from the above XPS results. Therefore, carbons with large specific surface area could exhibit the outstanding electrochemical performance as supercapacitance electrodes.
Fig. 6(c) shows the Nyquist plots of A-NMC-x ranged from 10 mHz to 100 kHz. The impedance spectra of all curves are almost similar in the form of an arc at a higher frequency region and a spike at a lower frequency region. The measured impedance spectra are analyzed using the complex nonlinear least-squares (CNLS) fitting method32 in the light of the equivalent circuit shown in the middle of Fig. 6. At very high frequencies, the intercept at real part (Z′) indicates a resistance at the active material/current collector interface (Re). This value (Re) for all samples is close to 2.8 Ω due to these electrodes located at the same electrolyte. The semicircle in the high-frequency range corresponds to both the charge transfer resistance (Rct) caused by the faradic reactions and the double-layer capacitance (Cdl) on the grain surface.33 It is well known that the higher the charge-transfer resistance of the electrode, the lower the specific capacitance is. Rct of A-NMC-1.0 is approximately 0.46 Ω with the least value among those samples, whereas A-NMC-1.0 have the maximum values of both the double-layer capacitance (Cdl) and the specific surface area. In addition, the Warburg resistance (Zw) can obtained from the slope of the 45 degree portion of the curve. A-NMC-0.0 in the absence of nitrogen attains the least value of Zw, exhibiting typically double-layer capacitance.
Fig. 7(a) shows the first charge/discharge curves of A-NMC-1.0 at the current density varying from 0.5 to 10 A g−1. The specific capacitance of A-NMC-1.0 is found to decrease from 368.2 ± 19.8 F g−1 at 0.5 A g−1 down to 279.9 ± 8.7 F g−1 at 10 A g−1. The average capacitances of A-NMC-x obtained from five measurements are presented in Fig. 7(b) as a function of the current density. The capacitance of all samples are found to decrease as a function of the current density, but when the current density increases up to 20 A g−1, the capacitance of each sample almost keeps unchanged along with the further increase of the current density. In addition, it is found that when the current density increases from 0.5 to 50 A g−1, the specific capacitance of all samples possess a retention rate close to 70%, which is much higher than the previously reported results of nitrogen-doped porous carbons.16,18 The high retention rate in the high current density might be ascribed to the high mesoporous ratios of A-NMC-x. Additionally, the incorporation of dicyandiamide into phenolic resins cannot completely change the interconnected micropores/mesoporous structures. However, nitrogen functional groups are favorable to improve both the specific surface area and the interface wettability between the electrode and the electrolyte through the hydrophilic polar sites, giving the higher specific capacitance along with the increase of nitrogen atoms.
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Fig. 7 (a) The first charge–discharge curves of A-NMC-1.0 at different current densities, (b) the average specific capacitances of A-NMC-x at the current density ranged from 0.5 A g−1 to 50 A g−1. |
Fig. 8 presents the CV curves of A-NMC-0.0 (a) and A-NMC-1.0 (b) as a function of scan rates. Both A-NMC-0.0 and A-NMC-1.0 exhibits the typical capacitive behavior with several humps and rectangular shaped voltammetry characteristic from −0.2 to 0.7 V over a wide range of scan rates. Evidently, A-NMC-1.0 shows better rate capability than that of A-NMC-0.0 as far as the current responses at different scan rates are concerned.
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Fig. 8 CV curves of A-NMC-0.0 (a) and A-NMC-1.0 (b) at different scan rates varying from 5 to 100 mV s−1. |
To investigate the cycle stability, 5000 consecutive cycles of charge–discharge are recorded for A-NMC-0.0 and A-NMC-1.0 at a current density of 10 A g−1, as shown in Fig. 9. In the inset of Fig. 9(a), the charge/discharge curves of A-NMC-1.0 remain better triangle shapes even up to 5000th cycle. After 5000 cycles, the capacitance retention of A-NMC-1.0 is close 100%, slightly higher than that of A-NMC-0.0 (about 95%). The impedance spectra of A-NMC-1.0 after 1st and 5000th cycle are shown in Fig. 9(b). It is found that the values of Re, Rct and Cdl all decreases slightly after 5000 cycles, and the Warburg resistance (Zw) attributed by the frequency dependence of ion diffusion/transport in the electrode surface decreases evidently from 0.35 to 0.038 Ω. Less Zw of the electrode indicates that due to the decrease of the diffusion and transfer of electrons/ions arisen from Faraday reactions, the electrode attains the typical double-layer capacitance. Therefore, the result indicates that after 5000 cycles, A-NMC-1.0 shows better double-layer capacitance, while the pseudocapacitance gradually disappears. Likewise, the presence of nitrogen would improve the wettability between electrode and aqueous electrolytes. This result also suggests that after 5000 cycles, the capacitance retention of A-NMC-1.0 could achieve close to 100%.
Fig. 10 presents the Ragone curves of A-NMC-x. The Ragone curves are obtained by E = C(ΔV)2/8 and P = E/Δt,34 where E is the energy density (W h kg−1), C is the specific capacitance (A h kg−1 V−1), ΔV is a potential range (V), the voltage drop (IR drop) is considered when the current density more than 2 A g−1. P is the power density (W kg−1), Δt is the discharge time (h). At the current density of 0.5 A g−1, the energy density of A-NMC-1.0 is up to 10.83 W h kg−1, and the corresponding power density is 225 W kg−1, while the energy density of A-NMC-0.0 is only 6.15 W h kg−1, and the power density is 225 W kg−1. When the current density increases up to 20 A g−1, the energy density of A-NMC-1.0 still remains 5.44 W h kg−1, and the power density is 7464 W kg−1. At this time, the energy density of A-NMC-0.0 decrease sharply to 2.82 W h kg−1, and the power density is 6775 W kg−1. The energy density of A-NMC-1.0 are larger than those reported previously in N-doped carbons.34–38 Therefore, the N-doped mesoporous carbons presented here has a very good application prospect as supercapacitor electrode materials.
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