N/O Co-doped Porous Carbons Derived from Coal Tar Pitch for Ultra-high Specific Capacitance Supercapacitors.

In this paper, a series of N/O co-doped porous carbons (PCs) were designed and used to prepare coal tar pitch-based supercapacitors (SCs). The introduction of N/O species under the intervention of urea effectively improves the pseudocapacitance of PCs. The results show that the specific surface area of synthesized N3PC4-700 is 1914 m2 g-1, while the N and O contents are 1.3 and 7.2%, respectively. The unique interconnected pore structure and proper organic N/O co-doping, especially the introduction of pyridine-N and pyrrole-N, are beneficial for improving the electrochemical performance of PCs. In the three-electrode system, the specific capacitance and rate capability of N3PC4-700 are 532.5 F g-1 and 72.5% at the current densities of 0.5 and 20 A g-1, respectively. In addition, the specific capacitance of N3PC4-700 in a coin-type symmetric device is 315.5 F g-1 at 0.5 A g-1. The N3PC4-700 electrode provides an energy density of 43.8 W h kg-1 with a power density of 0.5 kW kg-1 and still maintains a value of 29.7 at 10 kW kg-1. After 10,000 charge/discharge cycles, the retention rate was as high as 96.7%. In order to obtain high-performance carbon-based SCs, the effective identification and regulation of organic N/O species is necessary.

Introduction

Carbon-based supercapacitors (SCs) with high specific capacitance, good cycling stability, and high-rate capability are promising energy storage materials.[1] Porous carbon (PC) is one of the most commonly used electrode materials for SCs.[2] However, conventional ash-containing raw materials need to undergo multiple post-treatments before they can be made into PCs with good properties.[3] Coupled with the shortage of traditional fossil fuels and the rapid increase of greenhouse gases, the development of green, efficient, and low-cost carbon-based SCs has become a trend.[4]

Coal tar pitch (CTP) is a heavy fraction after distillation of coal tars, accounting for more than 55 wt %. This raw material has the advantages of high carbon content, low ash content, and good thermoplasticity, which are beneficial to the preparation of PCs.[5] Liu et al.[6] used the eutectic salt template strategy to prepare CTP-derived hierarchical PCs, which has a specific capacitance of 320 F g–1 at a current density of 0.1 A g–1. Although CTP has been successfully used to synthesize PCs for SCs, due to the relatively low content of heteroatoms, the specific capacitance of electrode materials depends entirely on the behavior of electric double-layer capacitance, which makes the electrochemical performance of such conventional materials difficult to meet actual requirements.[7,8]

Heteroatom doping, especially N-doping, is one of the effective strategies to increase the specific capacitance of PCs.[9,10] The introduction of organic nitrogen species improves the hydrophilicity of the material surface and increases the electrical conductivity.[11] Obviously, the type and distribution of organic nitrogen species directly affect the performance of SCs. Pyrrole-N, pyridine-N, and graphite-N play different roles in enhancing electron-transfer activity,[12] providing electron pairs and pseudocapacitance,[13] and improving conductivity,[14] which are all conducive to realizing the effective electrochemical behavior. Zhong et al.[15] prepared CTP-derived N-doped PCs with melamine as the N-donor. The specific capacitance was 228 F g–1 at 1 A g–1, and the capacitance retention was 94.2% after 1000 cycles. The relatively poor electrochemical performance may be related to the lower contents of pyrrole-N and pyridine-N. In contrast, the introduction of urea in the activation stage can not only provide sufficient pyridine-N, pyrrole-N, and oxynitrides but also further promote the synergistic pore-enlarging effect of KOH in the carbonization stage.[16] Therefore, the regulation of N-donors and N-doping behavior in the activation process is particularly important. In practical applications, the specific capacitance is not linearly proportional to specific surface area (SSA) but is closely related to the pore structure.[17] Charge storage, fast ion transport, and short ion diffusion require reasonably distributed pore structures (including micropores, mesopores, and macropores) to provide a relatively high SSA, low-resistance channels, and buffer reservoirs.[18,19] It can be seen that the regulation of surface active sites and pore size distribution is a breakthrough for obtaining electrode materials with high electrochemical performance.

As mentioned above, there are many related studies on CTP-based electrode materials for the preparation of supercapacitors.[15] However, related scholars have ignored the effect of different N-containing species on the electrochemical performance. In addition, there are few reports of a one-step preparation method that provides abundant oxynitrides and hierarchical pore structures. Noteworthily, the specific capacitance of CTP-based electrode materials is mostly limited to 300–400 F g–1.[20] In view of this, CTP-derived N-doped PCs were directly prepared by one-pot carbonization using urea as the N-donor. The focus was on the distribution regulation of pyrrole-N and pyridine-N as well as the acquisition of hierarchical pore structures. In addition, the electrochemical performance of synthesized PCs and the related regulation mechanisms were also investigated.

Experimental Section

Materials

Low-temperature CTP (softening point 176 °C) was provided by Yulin Coal Chemical Industry Upgrade Technology R&D Center. CTP was pulverized to pass through a 200-mesh sieve (particle size ≤74 μm), followed by vacuum desiccation at 80 °C for 24 h before use. Polytetrafluoroethylene (PTFE) was purchased from Aladdin Chemical Co., Ltd., China. KOH (85 wt %) and HCl solution (37 wt %) were purchased from Shanghai Chemical Reagent Co., Ltd., China. Ethanol and urea are both analytically pure and purchased from Tianjin Kemiou Chemical Reagent Co., Ltd.

Synthesis of PCs

In a typical procedure, 3 g of CTP and a certain amount of KOH were ground and mixed uniformly. The mass ratio of KOH to CTP was X (X = 3, 4, or 5). The mixture was fed into a quartz tube and heated to the set temperature (Y = 600, 700, or 800 °C) at 3 °C min–1 under N2 and kept for 3 h. After cooling to room temperature, the powder was placed in a 2 M HCl solution, stirred for 24 h, and washed repeatedly with deionized water to neutrality. Finally, the sample was vacuum-dried at 60 °C for 24 h before storing and named PC.

Urea was added in the above grinding step, and the mass ratio of CTP to urea was Z (Z = 2, 3, or 4). The activation and post-treatment processes were consistent with the preparation of PC. The resulting sample was named NPC.

Characterization

The pore structure of PCs was characterized by N2 physisorption (Micromeritics ASAP 2460) at −196 °C. Brunauer–Emmett–Teller (BET) and density functional theory (DFT) methods were used to determine the SSA and pore size distribution, respectively. Scanning electron microscopy (SEM, ZEISS Sigma 300) was used to obtain the microstructure and surface morphology. The internal structure was determined by transmission electron microscopy (TEM, Talos F200X G2, superX). The X-ray diffraction (XRD) measurement was carried out with a Bruker Advance D8 XRD. A HORIBA Scientific Lab RAM HR Evolution laser confocal Raman spectrometer was used to record Raman spectra in the range of 4000–400 cm–1. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Fisher Scientific Kα 1063 spectrophotometer. A JC2000C1 contact angle measuring instrument was used to explore the surface wettability of PCs.

Electrochemical Measurements

A PC, PTFE, and acetylene black were mixed at a mass ratio of 8:1:1 in 10 mL of ethanol and dried to obtain a working electrode slurry. A certain amount of mixed slurry was evenly coated on the pretreated Ni foam and vacuum-dried at 60 °C for 12 h. Before the test, the electrode material was soaked in a 6 M KOH solution for 12 h. Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) were performed on the CHI660E electrochemical workstation. In addition, the cyclic stability was tested on a LANDdt V7 test system (CT3002A).

In the three-electrode system, the Pt and Hg/HgO electrodes were used as the counter and reference electrodes, respectively. The CV and GCD tests were performed on the electrode sheet under the potential window of −1–0 V. The EIS measurements were carried out at an amplitude of 10 mV and a frequency of 10–2–105 Hz.

The specific capacitance (C) of a single electrode was calculated by the following equation (eq )where m (g) is the mass of the active material in the electrode, I (A) is the discharging current, and ΔV (V) is the potential change within the discharge time Δt (s).

The Coulombic efficiency (η) was calculated by using eq where td (s) and tc (s) are the discharge and charge times, respectively.

In the two-electrode system, a 6 M KOH solution was used as an electrolyte and two electrode sheets with the same mass were used as positive and negative electrodes to form a coin-shaped symmetrical device. The electrochemical tests were carried out on the electrochemical workstation and the LANDdt V7 test system. The specific capacitance of a single electrode was calculated from the GCD values according to the following equation (eq )where I (A) is the discharging current, m (g) is the average mass of the active material in the two electrodes, and ΔV (V) is the potential change within the discharge time Δt (s).

The energy density (E) and power density (P) were defined using eqs and 5where P (W kg–1) is the specific power density, E (W h kg–1) is the specific energy density, C (F g–1) is the specific capacitance, and ΔV (V) is the potential change within the discharge time Δt (s).

Results and Discussion

Materials Characterization

As shown in Figure S1, all adsorption and desorption curves obey the Langmuir model. Obviously, micropores dominate the as-synthesized PCs, while the pore size is mostly concentrated within 2 nm. Such micropores provide a large number of attached electrolyte ion points. As further listed in Table , properly raising the temperature and increasing the ratio of KOH to CTP are beneficial to increase the SSA and pore volume (PV) of PCs. During the carbonization, KOH and dehydrated K2O reacted with C and CO2 produced by the rapid thermal condensation of CTP to release K and various reducing gases, such as CO and H2, which are conducive to the formation of hierarchical pores.[21] As a result, PC4-700 exhibits the largest SSA and PV, while the proportion of micropore volume is 87.4%. These are the important factors for PC4-700 to provide a relatively large specific capacitance (309.7 F g–1 at 0.5 A g–1).

Table 1

Textural Parameters of the Samplesa

			PV (m3 g–1)	Dap (nm)
sample	SSABET (m2 g–1)	SSAmic (m2 g–1)	total	micro	specific capacitance (F g–1)
PC4-600	1960	1811	0.95	0.80	1.93	293.5
PC4-700	2789	2628	1.35	1.18	1.93	309.7
PC4-800	2258	2164	1.08	0.98	1.92	219.5
PC3-700	1885	1822	0.84	0.77	1.78	295.5
PC5-700	2569	2361	1.13	1.11	2.05	265.5
N2PC4-700	1222	1181	0.56	0.48	1.83	380.5
N3PC4-700	1914	1859	0.85	0.78	1.79	532.5
N4PC4-700	1286	1258	0.57	0.51	1.77	405.5

Dap: average pore size.

As displayed in Figure a, NZPC4-700 exhibit type I/IV isotherms, indicating that micropores are also dominant in N-doped PCs. The clear hysteresis loops in the relative pressure range of 0.5–1 mean that there is still a small proportion of mesopores in PCs. Apparently, the organic nitrogen species formed by urea during the activation process played the role of ammoniation and pore expansion. As further demonstrated in Figure b, similar to PC4-700, the pore size of NZPC4-700 is also mainly distributed in the range of 1–5 nm. Interestingly, the pore proportion of PC4-700 in the range of 1–3 nm is larger than that of NZPC4-700, while the opposite is true around 4 nm. These results further confirmed that the introduction of urea enriched the pore type of PCs.

Figure 1

(a) N2 adsorption/desorption isotherms and (b) pore size distribution curves of the samples.

In contrast, the SSA and PV of PCs decreased significantly after N-doping (Table ), which may be attributed to the partial conversion of micropores and mesopores to macropores under the effect of ammoniation.[22] It can also be clearly observed in Figure S2 that the pore size increases after N-doping, especially for N3PC4-700. In addition, proper introduction of N-containing species still maintains the higher SSA and PV of PCs. The hierarchical pore structure could increase ion diffusion channels, reduce diffusion resistance, and improve the utilization of micropores, which are beneficial to improve the electrochemical performance of PCs.[23,24] It can be seen that urea has the following advantages in the synthesis of N-doped PCs: (1) a higher N-doping efficiency, (2) better wettability, and (3) the evolution of abundant organic N-containing species, which are also related to the strong hydrogen bonds formed between O-containing species in CTP and urea.[25,26]

As shown in Figure a–c, NZPC4-700, especially N3PC4-700, have a large number of uniformly distributed 3D pore structures, while the different pore sizes mean that the as-synthesized PCs have hierarchical pore characteristics. Compared with PC, the pore structure of N3PC4-700 is more three-dimensional, with larger and more abundant pores (Figure S2). These changes are related to the synergistic activation of KOH and urea, making the porous network more regular and interconnected. TEM images (Figure d–f) further show that NPC4-700 are composed of less carbon layers and have abundant edge defects. In addition, abundant interconnected channels and obviously rough surfaces are typical characteristics of amorphous PCs caused by high-temperature KOH activation. As further described in Figure g–i, the abundant non-graphitized micropores provide more active sites for the ion storage and transfer on electrode materials.

Figure 2

SEM images of (a) N2PC4-700, (b) N3PC4-700, and (c) N4PC4-700. TEM images of (d,g) N2PC4-700, (e,h) N3PC4-700, and (f,i) N4PC4-700. (j–l) Elemental mapping of N3PC4-700.

As shown in Figure a, all samples exhibited two typical diffraction peaks around 23° (002) and 43° (100). The relatively weak intensity of the (100) diffraction peak indicates that PCs are dominated by amorphous carbon and structurally disordered.[27,28] The intensity difference of (002) diffraction peaks among PCs implies the correlation of the non-graphitized structure with urea content. All samples have higher diffraction peaks at low angles, indicating that there are a large number of micropores in PCs. The order of low-angle diffraction peak intensity is N3PC4-700 > N4PC4-700 > N2PC4-700, which is consistent with the BET analysis. As further exhibited in Figure b, peaks D and G represent the lattice defects of PCs and the in-plane stretching vibration of SP2 hybrid C species, respectively.[29] The ID/IG is usually used to indicate the degree of lattice defects. The larger the ratio, the more lattice defects of PCs. The calculated ID/IG value of the sample is N3PC4-700 (1.01) > N4PC4-700 (0.98) > N2PC4-700 (0.94). It can be seen that when the carbon to nitrogen ratio is 3:1, the sample has the most defects, which is consistent with the XRD analysis.

Figure 3

(a) XRD patterns and (b) Raman spectra of the samples.

As demonstrated in Figure S3 and Table S1, the N element was detected in all samples. As a N-donor, the introduction of urea effectively doped N species while significantly reducing the oxygen content in PCs. As shown in Figure a, the binding energies of pyridine-N (N1), pyrrole-N (N2), graphite-N (N3), and N-oxides (N4) are around 398, 400, 401, and 402.5 eV, respectively. The total proportion of N1 and N2 in all samples exceeds 55% (Table S2). Appropriate N-doping can effectively regulate the proportion of N1–N4 species. Obviously, the surface of N3PC4-700 has the highest proportion of N1 and N2 species. Figure b shows that the binding energies around 531, 532.5, and 534 eV in the O 1s spectrum of N3PC4-700 are attributed to > C=O, >C–O–, and −COOH functional groups, respectively.[30,31] As further listed in Table S2, the ratio of >C–O– is much higher than other O-containing moieties.

Figure 4

High resolution of samples (a) N 1s and (b) O 1s.

The introduction of organic N species, especially N1 and N2, is beneficial for increasing the wettability of electrode materials, adjusting the behavior of electron donors, and thereby improving the electrochemical performance.[32,33] As further summarized in Figure a, the water contact angle of N3PC4-700 was almost zero, implying that the introduction of N species greatly improves the hydrophilicity of PCs and increases the diffusion rate of electrolyte ions on the surface and inside of carbon materials. These differences are related to the unique distribution of nitrogen species in N3PC4-700.[12−14] In addition, the abundant oxygen-containing functional groups on its surface can also provide abundant reactive sites for additional pseudocapacitance (Figure b).[34] The change of chemical structure also proved the improvement of the hydrophilicity of N3PC4-700. Among them, the >C–O– part showing good electrochemical redox activity is retained in the high-temperature carbonization stage, which is more conducive to improving the pseudocapacitance of electrode materials.[35] As displayed in Figure S4 and Table S2, the binding energies around 289.1, 286.3, 285.0, and 284.1 eV in C 1s spectra are attributed to > C=O, >C–O–, >C–N<, and >C=C< moieties, respectively.[36] It can be seen that the total proportion of >C=O and >C–O– functional moieties in N3PC4-700 is moderate, while that of >C–N< functional groups is the highest, which further proves that N3PC4-700 has a suitable graphitized structure and is rich in defects and active edge sites.[37]

Figure 5

(a) Water contact angle tests of PC4-700 and N3PC4-700 at different times and (b) schematic illustration for the possible chemical structure of N/O co-doped PCs.

Electrochemical Characterization

Three-Electrode System

Figure shows various electrochemical characterizations of NZPC4-700 in the three-electrode system. The rectangular shape of the CV curve shows that NZPC4-700 have excellent double-layer capacitor behavior (Figure a). In addition, the redox process of −0.8––0.3 V can be clearly observed on all CV curves, which should be attributed to the redox reaction of N/O functional groups.[38] The PC4-700 electrode has no obvious redox peak (Figure S5c), which proves that the pseudocapacitance of N3PC4-700 mainly comes from the N species introduced by urea doping. In addition, N3PC4-700 has the largest CV curve area (Figures a and S5a), indicating the highest specific capacitance. Figure b shows that the GCD curve shapes of NPC4-700 electrodes are slightly deviated from the isosceles triangle, indicating that the energy storage of PCs is controlled by the electric double layer and pseudocapacitance behavior.[39,40] The GCD curves of the NPC4-700 electrode have no obvious IR value, indicating that the internal resistance is relatively small. Furthermore, N3PC4-700 has the longest discharge time and the largest specific capacitance, indicating that the urea dosage and activation temperature of N3PC4-700 are the best conditions (Figures b and S5b).

Figure 6

Electrochemical tests of NZPC4-700 in a three-electrode system: (a,b) CV and GCD curves of NZPC4-700, (c,d) CV and GCD curves of N3PC4-700, (e) corresponding rate capabilities, (f) Nyquist plots, (g) Bode plots, and (h) cycle performance and Coulombic efficiency of N3PC4-700 under 20 A g–1.

The high specific capacitance of N3PC4-700 is caused by the excellent pore structure and suitable doping amount of nitrogen species. On one hand, the reasonable hierarchical pore structure can provide more channels, which is beneficial to the efficient transport of electrolyte ions.[19] In addition, for the three nitrogen-doped PCs, N3PC4-700 has the largest SSAmic and micropore volume, implying the highest proportion of micropores, which in turn provides more attachment sites.[16] These physical structure changes are beneficial to improve the electrochemical performance of PCs.[41] On the other hand, rational doping of urea also introduces abundant N-containing groups, especially N1 and N2. For the physical structure, the increase of edge defects and surface wrinkle/distortion are beneficial to the improvement of electron transport efficiency. For the chemical structure, rational doping of nitrogen species enhances the wettability of PCs, which in turn facilitates the penetration and absorption of electrolyte ions.

As further shown in Figure S5d, the specific capacitance of N3PC4-700 at 0.5 A g–1 is 532.5 F g–1, which is much higher than that of PC4-700 (309.7 F g–1). The significant increase in specific capacitance may be due to the following three reasons: (1) N3PC4-700 has good hydrophilicity (Figure a), resulting in low diffusion resistance, which is beneficial to the storage and transportation of electrolyte ions; (2) the introduction of heteroatoms provides more attachment sites for electrolyte ions,[42] and the redox reaction of N/O functional groups provides additional pseudocapacitance; (3) the hierarchical pore structure formed by urea doping makes the NPC4-700 electrodes with short and efficient electron and ion transmission pathways, thereby ensuring high capacity and rate performance.[43] Due to the increase of transmission resistance, the specific capacitance of N3PC4-700 decreases with the increase of the current density (Figure d,e). As exhibited in Figure e, N3PC4-700 has a capacity retention rate of 72.5% at a current density of 20 A g–1, indicating its excellent rate performance. The high rate performance may be attributed to the abundant micropores for storing electrolyte ions, and N-doping improves the surface wettability of PCs and reduces the transport resistance of electrolyte ions.[44] In addition, the loading effect of active materials on the capacitance is also evident. As shown in Figure S6, the specific capacitance of N3PC4-700 decreases with the increase of active material loading, and its specific capacitance is 421 F g–1 when the mass is 5.4 mg. This is because when the mass of active materials increases, the ion migration pathway of the electrolyte becomes longer and slower, resulting in a decrease in specific capacitance.

Figure f shows the Nyquist plots of NZPC4-700, and the equivalent circuit diagram used the nonlinear Schrodinger equation to fit the EIS data (Figure f inset). The intrinsic resistance (Rc) values of NZPC4-700 are all less than 0.62, indicating that the internal resistance of PCs is small. In the high-frequency region, the small arc span indicates that the charge-transfer resistance (Rct) value is low. N3PC4-700 has the lowest Rct value, suggesting that its charge-transfer resistance is the lowest, which is conducive to the rapid storage and release of electrolyte ions to the electrode surface.[45] In addition, the slope of curves in the low-frequency region also has a similar trend, indicating that the mass-transfer rate between the electrode and electrolyte is accelerated. This may be due to the N-doping and layered pore structure enhancing the surface wettability and promoting the charge transfer of PCs.[46] Moreover, the relaxation time constant (τo = 1/f, f is the frequency at −45°) obtained from the Bode plot is 1.21 s for N3PC4-700 (Figure g). The low relaxation time constant of N3PC4-700 means that electrolyte ions can penetrate well into the electrode material, giving it good capacitance and fast charging capabilities. Figure h further shows that N3PC4-700 has good cycle stability (95.2% capacitance retention ratio after 10,000 cycle) and a good Coulombic efficiency (100% after 10,000 cycles).

As mentioned above, NPC4-700 is dominated by electric double-layer energy storage due to its large SSA, unique hierarchical pore structure, and pseudocapacitance induced by N-doping. Therefore, the charge/discharge kinetics is qualitatively evaluated according to eqs and 7(47)where a and b are constants, while i and v are the current density and scan rate, respectively. In general, when b is close to 0.5, the electrochemical behavior is dominated by pseudocapacitance and the reaction kinetics is slow. When b is close to 1, the electrochemical behavior is dominated by electric double-layer capacitance, and the reaction kinetics is fast. As shown in Figure a, the calculated b values of NPC4-700 ranged from 0.66 to 0.89. Furthermore, Dunn’s method was used to quantitatively analyze the capacitive contributions of the NPC4-700 surface capacitance-controlled and diffusion-controlled processes, as shown in eqs and 9.[47]where k1ν and k2ν1/2 represent the surface capacitance-controlled and diffusion-controlled current contributions, respectively. Figure b shows the surface capacitance-controlled and diffusion-controlled current contributions of N3PC4-700 at 20 mV s–1. The results show that the surface capacitance controls the contribution of most dynamic processes. Figure c shows that the N3PC4-700 surface-controlled capacitance increases to 92% when the scan rate is as high as 100 mV s–1, consistent with its high rate capability.

Figure 7

(a) Linear plot of log i vs log ν in both charge and discharge processes, (b) capacitive contribution at 20 mV s–1, and (c) diffusion-controlled and capacitive-controlled contribution ratios at different scan rates.

NPC4-700 Symmetric SCs

In order to further study the electrochemical performance of NPC4-700, coin-shaped symmetrical devices were assembled in 6 M KOH. As displayed in Figure a–d, the CV and GCD curves of NPC4-700//NPC4-700 symmetric SCs as well as those of N3PC4-700//N3PC4-700 symmetric SCs at different scan rates and current densities were tested. The specific electrochemical behavior analyses of NPC4-700//NPC4-700 symmetric SCs are similar to those of the three-electrode system. In addition, the specific capacitance of N3PC4-700 increases significantly (Figure S7), which is consistent with the analysis of the three-electrode system. As illustrated in Figure e, the N3PC4-700//N3PC4-700 symmetric SCs have good cycle stability (96.7% capacitance retention ratio after 10,000 cycle) and η (100% after 10,000 cycles). Noteworthily, the button battery with N3PC4-700 and zinc flakes as positive/negative materials lighted up the LED lamp, indicating that such SCs have a high practical value (Figure e inset). Moreover, the self-discharge performance of SCs is one of the main factors in their applications. After charging the N3PC4-700//N3PC4-700 symmetric SC to 1V and self-discharging for 24 h, its open-circuit voltage was 0.78 V, which maintained 78% of the initial voltage value (Figure S8).

Figure 8

Electrochemical performance of a symmetric SC with a 6 M KOH electrolyte: (a,b) CV and GCD curves of NPC4-700, (c,d) CV and GCD curves of N3PC4-700, (e) cycle performance and Coulombic efficiency of N3PC4-700 under 20 A g–1 (inset: an LED light, powered by an assembled N3PC4-700-based button battery), and (f) Ragone plot.

Energy and power densities are important indicators for measuring SCs. The assembled N3PC4-700//N3PC4-700 symmetrical SCs can provide a maximum energy density of 43.8 W h kg–1 at a power density of 0.5 kW kg–1. More significantly, the energy density can still be kept at 29.7 W h kg–1 even at a power density as high as 10 kW kg–1. The electrochemical performances of some coal-based and pitch-based carbon materials are compared and listed in Table . The various electrochemical properties of N-doped PCs reported in this work are more prominent.

Table 2

Electrochemical Performance of the Samples

material	current density (A g–1)	specific capacity (F g–1)	energy density (Wh kg–1)	power density (W kg–1)	cycling performance (%)	reference
CTP	0.50	532.5	43.8	500.0	96.7 (10,000 cycles)	This work
CTP	0.10	320.0	10.6	50.1	94.0 (10,000 cycles)	(6)
CTP	1.00	228.0			94.2 (1000 cycles)	(15)
petroleum pitch	0.05	293.0 (2E)	10.0	25.6	97.4 (7000 cycles)	(48)
raw coal	0.50	298.0	10.4	125.0	95.3 (10,000 cycles)	(49)
graphitized coal	0.50	225.0	79.4(EMIMBF4)		91.0 (10,000 cycles)	(50)
petroleum coke	0.05	342.8	8.2	280.0	71.8 (1000 cycles)	(51)
lignitea	0.05	390.0	11.0	22.5	94.1 (2000 cycles)	(52)

3 M KOH, the electrolyte of other samples is 6 M KOH.

Conclusions

The specific capacitance of as-synthesized PCs was greatly improved under the intervention of urea. Compared with other PCs, N3-PC4-700 has suitable pore size distribution and a suitable N/O co-doping amount. These properties not only facilitate the penetration and absorption of electrolyte ions but also guarantee the fast and efficient electron transfer on the N3-PC4-700 surface. In the three-electrode system, N3-PC4-700 exhibits the highest specific capacitance (532.5 F g–1 at 0.5 A g–1) and excellent rate capability (72.5% at 20 A g–1). In addition, after 10,000 charge–discharge cycles, the capacitance retention and η were as high as 96.7 and 100%, respectively. In a coin-type symmetric device, the specific capacitance of N3-PC4-700//N3-PC4-700 symmetric SCs was 315.5 F g–1 at 0.5 A g–1. The N3-PC4-700//N3-PC4-700 symmetric SCs have an energy density of 43.8 W h kg–1 and a power density of 0.5 kW kg–1 and still maintain a value of 29.7 W h kg–1 at 10 kW kg–1. In conclusion, this work confirms that the introduction of pyridine-N and pyrrole-N can effectively improve the electrochemical performance of PCs.