Organic Crosslinked Polymer-Derived N/O-Doped Porous Carbons for High-Performance Supercapacitor

Abstract

Supercapacitors, as a new type of green electrical energy storage device, are a potential solution to environmental problems created by economic development and the excessive use of fossil energy resources. In this work, nitrogen/oxygen (N/O)-doped porous carbon materials for high-performance supercapacitors are fabricated by calcining and activating an organic crosslinked polymer prepared using polyethylene glycol, hydroxypropyl methylcellulose, and 4,4-diphenylmethane diisocyanate. The porous carbon exhibits a large specific surface area (1589 m²·g⁻¹) and high electrochemical performance, thanks to the network structure and rich N/O content in the organic crosslinked polymer. The optimized porous carbon material (C_OCLP-4.5), obtained by adjusting the raw material ratio of the organic crosslinked polymer, exhibits a high specific capacitance (522 F·g⁻¹ at 0.5 A·g⁻¹), good rate capability (319 F·g⁻¹ at 20 A·g⁻¹), and outstanding stability (83% retention after 5000 cycles) in a three-electrode system. Furthermore, an energy density of 18.04 Wh·kg⁻¹ is obtained at a power density of 200.0 W·kg⁻¹ in a two-electrode system. This study demonstrates that organic crosslinked polymer-derived porous carbon electrode materials have good energy storage potential.

1. Introduction

Solutions to environmental problems, owing to economic development and the excessive use of fossil energy resources, are urgently being sought [1]. Supercapacitors, as a new type of green electrical energy storage device, have drawn increasing attention, owing to their high power density, fast charging/discharging, excellent reversibility, long life cycle, and environmental friendliness [2,3,4].

Theoretical research on and practical applications of supercapacitors have significantly progressed; however, insufficient energy density and high cost are still challenges requiring resolution [5,6,7]. Electrode materials, which can be divided into carbon materials [8,9], metal oxides [10,11], and conductive polymers [12,13], play an important role as core components in supercapacitors and are a key step in solving the existing problems. Among them, carbon materials are the most widely used electrode materials because of their high specific surface area, and good electrical conductivity and chemical stability [14,15,16]. Studies have shown that doping heteroatoms in a carbon-based framework increases the specific capacitance of carbon materials. On the one hand, it can improve the infiltration area between the electrode material and the electrolyte; on the other hand, the heterogeneous atoms can introduce pseudocapacitance during the charging/discharging process, further enhancing the electrochemical performance [17].

Nitrogen doping has been demonstrated to be an effective way to improve the wettability and conductivity of carbon materials and can also provide additional pseudocapacitance for supercapacitors. Generally, nitrogen-doped carbon materials can be prepared using two synthetic strategies, namely by the pyrolysis of nitrogen-containing precursors, such as biomass [18], synthetic polymers [19], small molecules [20], and ionic liquids [21], or by the chemical or thermal modification of premade carbon materials with reagents/gases containing nitrogen atoms [22]. Zhang et al. [23] used urea as a nitrogen-containing precursor and KOH as the activator to prepare a carbon material with an appropriate amount of N doping, which yielded a nitrogen-doped carbon material with a porous structure and large specific surface area. They also found that the capacitance of the carbon material reached up to 446.0 F·g⁻¹ at 0.5 A·g⁻¹ in a three-electrode system. The symmetrical supercapacitor device assembled with this nitrogen-doped carbon also displayed good performance, with an energy density of 16.3 Wh·kg⁻¹ at a power density of 348.3 W·kg⁻¹.

Organic crosslinked polymers are mainly composed of elements, such as carbon, nitrogen, oxygen, and hydrogen, which have the characteristics of a network structure. Porous carbon materials prepared using such polymers had a high heteroatom content, specific surface area, and outstanding electrochemical properties [24]. In particular, the structure of organic crosslinked polymers can be adjusted by changing the ratio of raw materials during the synthesis process. Zou et al. [25] prepared a new type of heteroatom-doped porous carbon material with a high specific surface area by carbonizing and activating polyphosphazenes, which exhibited a specific capacitance of 438 F·g⁻¹ at a current density of 0.5 A·g⁻¹ in a three-electrode system. Chen et al. [26] prepared a porous carbon material by calcining hypercrosslinked polymer (poly (vinylbenzyl chloride-co-divinylbenzene)), which exhibited a specific capacitance of 455 F·g⁻¹ at a current density of 0.5 A·g⁻¹.

In this work, nitrogen/oxygen(N/O)-doped carbon-based porous materials were fabricated by carbonizing and activating an organic crosslinked polymer with a network structure. The organic crosslinked polymer was synthesized using polyethylene glycol (PEG 6000), hydroxypropyl methylcellulose (HPMC), and 4,4-diphenylmethane diisocyanate (MDI). The carbon material obtained by optimizing the ratio of the raw materials had a large specific surface area (1589 m²·g⁻¹) and a high specific capacitance of 522 F·g⁻¹ at a current density of 0.5 A·g⁻¹. Furthermore, its energy density reached 18.04 Wh·kg⁻¹ at a power density of 200.0 W·kg⁻¹ in a two-electrode system using 1 M Na₂SO₄ as the electrolyte. Mechanistic studies showed that the high electrochemical performance of the obtained carbon was attributed to the network structure and rich N/O content of the crosslinked polymer. Hence, the preparation method for porous carbon materials proposed in this study provides a new approach for the research and development of electrode materials.

2. Materials and Methods

2.1. Materials

Polyethylene glycol (PEG, Mw = 6000), 4,4-diphenylmethane diisocyanate (MDI, analytical grade), hydroxypropyl methylcellulose (HPMC, Mw = 10,000), polytetrafluoroethylene (PTFE), and N, N-dimethylformamide (DMF) were purchased from Aladdin. Analytical-grade potassium hydroxide (KOH) and acetylene black were obtained from Xilong Science Co., Ltd. (Shantou, China). None of the purchased reagents were purified before use. All aqueous solutions were prepared using ultrapure water (deionized water, resistance 18 MΩ cm⁻¹).

2.2. Synthesis of Organic Crosslinked Polymers

The organic crosslinked polymers were prepared by a one-pot method, which is a minor modification based on our previous report [27]. Briefly, PEG 6000 (12.0 g), MDI (1.0 g), and a certain amount of HPMC were stirred in a three-neck flask containing DMF (80 mL) under argon gas and an oil bath with a constant temperature of 75 °C. The organic crosslinked polymer obtained after 30 h of condensation reflux is referred to as OCLP. The mass of HPMC was 3.5, 4.5, and 5.0 g; therefore, the corresponding organic crosslinked polymers were named as OCLP_3.5, OCLP_4.5 and OCLP_5.0, respectively. Figure 1 presents a flowchart of the one-pot method for the preparation of organic crosslinked polymers.

2.3. Preparation of Porous Carbon Materials

The prepared OCLPs were directly carbonized by heating them in a tube furnace at 500 °C for 2 h under a N₂ atmosphere at a heating rate of 5 °C/min. The resulting carbon precursors were homogeneously ground with KOH in a mass ratio of 1.0:3.0, then calcined in a tube furnace at 600 °C under a N₂ atmosphere for 2 h. The calcined products were stirred with a 1 M hydrochloric acid solution for 2 h, followed by washing with distilled water and anhydrous ethanol sequentially until the filtrate was neutral. The obtained residues were dried in a blast oven at 80 °C for 24 h to obtain porous carbon materials, which were named as C_OCLP-3.5, C_OCLP-4.5, and C_OCLP-5.0, respectively.

2.4. Characterization

Fourier transform infrared (FTIR) spectroscopy was performed on the samples using a Thermo Fisher (Waltham, MA, USA) Nicolet 6700 spectrometer with KBr pellets. A powder X-ray diffractometer (XRD; D8 Advance Bruker, Billerica, MA, USA) operating at 40 kV and 40 mA with Cu Kα radiation (λ = 0.15406 nm) in the 2θ range of 5–90° with 0.01° step increments was used to analyze the microstructure of the materials. The chemical structure and graphitization of the samples were further characterized using Raman spectroscopy (Horiba JY, Palaiseau, France) at an excitation wavelength of 532 nm. The surface micromorphology of the samples was characterized using scanning electron microscopy (SEM; SU8010, HITACHI, Tokyo, Japan) and transmission electron microscopy (TEM; Tecnai G2 F20, FEI Company, Hillsboro, OR, USA), and elemental analysis was performed using energy-dispersive X-ray spectroscopy (EDS). The specific surface area and pore structure characteristics of the samples were characterized using a nitrogen adsorption–desorption analyzer (ASIQM0002-4, Quantachrome, Boynton Beach, Florida, USA) at −196 °C. Surface element analysis was performed using X-ray photoelectron spectroscopy (XPS; Thermo Scientific Escalab 250Xi, Waltham, MA, USA).

2.5. Electrochemical Measurements

The electrochemical performance of the samples, including galvanostatic charge–discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance (EIS), was measured using a CHI 660E instrument in a three-electrode system. A slurry mixture of carbon material (C_OCLP), acetylene black, and PTFE in a weight ratio of 8:1:1 was applied to nickel foam (2 cm × 2 cm) as the working electrode; platinum and Hg/HgO electrodes were used as the counter and reference electrodes, respectively, in the three-electrode system. The voltage was set to −1–0 V and the electrolyte was 6 M KOH. A symmetric supercapacitor was built for a two-electrode system using the C_OCLP, a 1 M Na₂SO₄ electrolyte, and a voltage range of 0–1.6 V.

For the three-electrode and two-electrode systems, the weight-specific capacitances (F·g⁻¹) of the electrode material were calculated based on the GCD curves using Equations (1) and (2), respectively.

$$C_{\mathrm{g}}=\frac{I\Delta t}{m\Delta V}$$

(1)

$$C_{\mathrm{g}}=\frac{2I\Delta t}{m\Delta V}$$

(2)

where I (A), Δt (s), ΔV (mV), and m (g) represent the discharge current, discharge time, discharge voltage range, and mass of the active material of a single electrode, respectively.

The energy density (E_cell) and power density (P_cell) of the symmetrical supercapacitor were calculated using Equations (3) and (4), respectively.

$$E_{cell}=\frac{C_{\mathrm{g}} \mathsf{\Delta }{V}^2}{8\times 3.6}$$

(3)

$$P_{cell}=\frac{3600 E_{cell}}{\mathsf{\Delta }t}$$

(4)

where C_g is obtained from Equation (2), ΔV is the working voltage of the discharge, and Δt is the discharge time.

3. Results and Discussion

3.1. Structural and Morphological Characterization

Figure 2 shows the FTIR spectra of the samples, which indicates that the characteristic absorption peaks for the OCLPs (OCLP_3.5, OCLP_4.5, and OCLP_5.0) are similar. The peaks around 3438, 1639, 1526, and 1106 cm⁻¹ correspond to the stretching vibration absorption peaks of the –OH, C=O, C–N, and C–O groups, respectively, which is consistent with the organic crosslinked polymer [27]. The above results illustrate that the OCLPs are a type of organic crosslinked polymer.

The C_OCLPs obtained from the OCLPs were characterized using XRD and Raman spectroscopy. Figure 3a summarizes the XRD spectra of the C_OCLP-3.5, C_OCLP-4.5, and C_OCLP-5.0, showing that all the C_OCLPs exhibit obvious diffraction peaks at 43°, corresponding to the (100) crystal planes of the graphite structure. The results indicate that C_OCLP-3.5, C_OCLP-4.5, and C_OCLP-5.0 have amorphous graphite structures [28,29]. The diffraction peak intensity of the (100) lattice plane for C_OCLP-4.5 is the weakest, demonstrating that C_OCLP-4.5 has the highest structural disorder [30]. Figure 3b shows that there are two characteristic peaks at 1343 and 1594 cm⁻¹, corresponding to the D and G peaks of graphite, respectively. The ratio of the areas of the D peak to the G peak (A_D/A_G) reflects the order degree of the C_OCLP structure [31]. The calculated ratios for C_OCLP-3.5, C_OCLP-4.5, and C_OCLP-5.0 are 1.15: 1, 1.18: 1, and 1.13: 1, respectively. This result also illustrates that C_OCLP-4.5 has more defects because the D peak represents a defect peak caused by the low symmetry or irregularity of the carbon material [32].

The surface morphologies of C_OCLP-3.5, C_OCLP-4.5, and C_OCLP-5.0 were characterized using SEM, as shown in Figure 4. Figure 4 indicates that the three C_OCLPs are all porous and present a three-dimensional network structure. The number of pores in the C_OCLP increases with an increase in the amount of HPMC; however, when the HPMC content is increased to 5.0 g, the pore structure is only partially formed, and the number of pores decreases. The result demonstrates that the pore structure of C_OCLP-4.5 was excellent. Generally, an abundant number of pores can significantly increase the specific surface area of C_OCLPs, thereby providing more storage sites and transport channels for electrolyte ions. This is beneficial for improving the electrochemical performance [33].

Additionally, Figure 5a further demonstrates that C_OCLP-4.5 is a porous C_OCLP. When C_OCLP-4.5 is used as the electrode material, these disordered microporous structures can provide sufficient active sites for charge storage [34]. Figure 5b–e are element distribution diagrams obtained from the EDS analysis of C_OCLP-4.5, showing that carbon, nitrogen, and oxygen were uniformly distributed in the carbon framework. Abundant nitrogen and oxygen can introduce pseudocapacitance and enhance the capacitance performance of the electrode material.

The C_OCLPs were subjected to N₂ adsorption–desorption measurements to explore the pore characteristics. Figure 6 shows that all the C_OCLPs exhibit obvious type I isotherm characteristics, indicating that these samples are rich in micropores [35].

Table 1. Channel structure parameters of the C_OCLPs.

Samples	Specific Surface Area (m2·g−1)			Pore Volume (cm3·g−1)
	Total	Microporous	Mesoporous	Total	Microporous	Mesoporous
COCLP-3.5	942	894	48	0.399	0.353	0.046
COCLP-4.5	1589	1509	80	0.657	0.592	0.065
COCLP-5.0	1102	1040	62	0.482	0.407	0.075

summarizes the pore structure characteristics of the C_OCLPs, showing that the specific surface area and pore volume of these samples are mainly provided by the micropores and mesopores. Among the three samples, C_OCLP-4.5 has the largest specific surface area (1589 m²·g⁻¹) and the highest pore volume (0.657 cm³·g⁻¹), which further confirm that C_OCLP-4.5 has the best pore structure. Numerous studies have demonstrated that the large specific surface area and rich pore structure of porous carbon material can greatly promote the storage and rapid migration of ions, resulting in the excellent specific capacitance performance of supercapacitors [36,37]. The aqueous electrolytes currently used in supercapacitors are mainly sulfuric acid (H₂SO₄, acidic), KOH (alkaline), and sodium sulfate (Na₂SO₄, neutral). The electrolyte ions in these electrolytes mainly exist as hydrated ions (H⁺, K⁺, OH⁻, Na⁺, and ${\mathrm{SO}}_4^{2-}$). Based on Table 1, it can be found that the C_OCLPs obtained can meet the fast migration requirements of these electrolyte ions, thereby significantly improving the conductivity of carbon-based electrodes and enhancing their electrochemical performance.

Further analysis of the surface electronic states and elemental compositions of the C_OCLPs samples was performed using XPS. Figure 7a shows that there are three peaks in the spectra of all the samples. The binding energies of the three peaks are 285, 400, and 532 eV, corresponding to C 1s, N 1s, and O 1s, respectively. The results also prove that carbon, nitrogen, and oxygen are present in the three samples.

Table 2. Surface element content of the C_OCLPs.

Samples	Element Content
	Carbon (%)	Nitrogen (%)	Oxygen (%)
COCLP-3.5	92.83	1.98	5.19
COCLP-4.5	85.75	1.68	12.75
COCLP-5.0	84.51	2.65	12.84

lists the surface element contents of the three samples. These samples are mainly a carbon-based framework with oxygen and nitrogen. Fine analyses of the C 1s, N 1s, and O 1s spectra of C_OCLP-4.5 are performed using the peak differentiation fitting method, as shown in Figure 7b–d. The C 1s spectrum (Figure 7b) can be matched by four peaks at 284.8, 285.7, 286.8, and 289.0 eV, corresponding to the C–C, C–N, C–O, and COOR groups, respectively [38]. The N 1s spectrum, shown in Figure 7c, is deconvoluted into four peaks of 398.8, 400.3, 400.8, and 402.4 eV, corresponding to pyridinic-N (N-6) (11.70%), pyrrolic-N (N-5) (52.13%), quaternary-N (N–Q) (29.79%), and oxidized N (N–X) (6.38%), respectively. In particular, the pyridinic-N and pyrrolic-N contents reach 63.83%. A high content of N-6 and N-5 is beneficial for introducing pseudo-capacitance and providing electrochemically active sites and quaternary nitrogen (N–Q) can effectively improve the conductivity of C_OCLPs and promote electron transfer in the carbon matrix [35,39]. The deconvoluted O 1s peak displayed four peaks at 531.2, 532.3, 533.3, and 534.2 eV, representing the oxygen atoms in the C=O, C–O/C–OH, COOR, and N–O groups, respectively (shown in Figure 7d) [35,39]. According to a previous report [40], the oxygen groups are evenly distributed in the carbon framework, which can improve the interfacial tension between the carbon-based porous material and electrolyte to reduce the interfacial resistance.

3.2. Electrochemistry Measurements

The electrochemical performances of the electrode materials were evaluated using a three-electrode system. Figure 8a shows the CV plots of the different C_OCLPs (C_OCLP-3.5, C_OCLP-4.5, and C_OCLP-5.0) at a sweep rate of 5 mV·s⁻¹. All the samples display a typical rectangular shape, indicating that the capacitive behavior of these materials is mainly electric double-layer capacitance. Concurrently, these curves have a broad peak in the voltage window of −0.8 to −0.3 V, which is caused by the oxidation–reduction reaction of nitrogen and oxygen atoms contained in these samples during the charge and discharge process. Moreover, the pseudo-capacitance introduced by the redox reaction can significantly increase the specific capacitance of carbon electrodes. The C_OCLP-4.5 sample exhibits the largest encircled area of the CV curve among the three samples, which also illustrates that C_OCLP-4.5 has the highest specific capacitance. Figure 8b shows the constant GCD curves for the C_OCLPs at a current density of 1 A·g⁻¹. The GCD curves for the three samples are all quasi-isosceles triangle shapes, indicating that the capacitance is mainly electric double-layer capacitance (EDLC), and the slight deformation is attributed to the existence of pseudo-capacitance. According to Equation (1), the specific capacitances of C_OCLP-3.5, C_OCLP-4.5, and C_OCLP-5.0 at a current density of 1 A·g⁻¹ are 302, 503, and 330 F·g⁻¹, respectively. This result shows that the specific capacitance of C_OCLP-4.5 is the largest, owing to its large specific surface area (1589 m²·g⁻¹) and pore volume (0.657 cm³·g⁻¹). Figure 8c presents the CV curves for C_OCLP-4.5 at different scanning rates. It reveals that the C_OCLP-4.5 still maintains a quasi-rectangular shape at scan rates of 5–50 mV·s⁻¹, indicating that the good pore structure of C_OCLP-4.5 enables the rapid migration of electrolyte ions to result in its good rate capability. Figure 8d presents the GCD curves for C_OCLP-4.5 at current densities of 0.5–20 A g⁻¹, showing that the GCD curve does not exhibit a significant IR drop at a high current density of 20 A·g⁻¹. Therefore, it demonstrates that the C_OCLP-4.5 has a high conductivity, good rate capability, and electrochemical reversibility. The specific capacitances are calculated as 522, 503, 432, 396, 363, and 319 F·g⁻¹ at current densities of 0.5, 1, 2, 5, 10, and 20 A·g⁻¹, respectively. Comparing the electrochemical performance of C_OCLP-4.5 with that of the references, the result is listed in

Table 3. Comparison of the specific capacitances of the C_OCLP-4.5 electroactive material to recently reported carbonaceous materials.

Material	Electrolyte	Current Density (A·g−1)	Capacitance (F·g−1)	Reference
Grape marc	6 M KOH	0.5	446	[23]
Polyphosphazene	6 M KOH	0.5	438	[25]
Polypyrrole/Polythiophene	KOH	0.5	455	[41]
Cotton stalk	1 M H2SO4	0.2	338	[42]
L-tyrosine	KOH	0.3	512	[43]
Coal tar pitch	6 M KOH	0.5	298	[44]
CNTs@Gr-CNF	6 M KOH	0.25	521	[45]
CTAB	6 M KOH	1.0	241	[46]
3-aminophenol-formaldehyde resin	6 M KOH	0.5	381	[47]
Organic crosslinked polymer	6 M KOH	0.5	522	This work

. According to Table 3, the electrochemical performance of C_OCLP-4.5 is better than that of other electroactive materials reported in the literature. This is attributed to the unique network structure and rich N/O content of the crosslinked polymer fabricated in this study.

Figure 8e presents the EIS curves for C_OCLPs and the equivalent circuit model (the inset of Figure 8e), showing that C_OCLP-4.5 has the lowest R_ct (internal charge transfer resistance) and R_s (contact resistance with the electrolyte) among the three materials. That is, in the high-frequency region, the R_ct of C_OCLP-4.5 is 0.042 Ω, lower than those of C_OCLP-3.5 (0.152 Ω) and C_OCLP-5.0 (0.183 Ω). The low R_s demonstrates that the electrolyte ions are readily transferred to the surface of the C_OCLP-4.5 electrode [48]. Additionally, the linear curve of C_OCLP-4.5 is almost vertical in the low-frequency region. The EIS results illustrate that the structure of C_OCLP-4.5 is beneficial for charge transfer and the efficient diffusion of electrolyte ions. For supercapacitors, the cycling stability is a significant parameter to estimate their practical application. Figure 8f shows that C_OCLP-4.5 retains 83% of its initial specific capacitance value after 5000 cycles at a current density of 5 A g⁻¹. The surface morphology of C_OCLP-4.5 after cycling was characterized by SEM, as shown in Figure 9. Compared with the C_OCLP-4.5, before (Figure 4b) shows that the pore structure of C_OCLP-4.5 has some damage and collapses after 5000 cycles.

A symmetric supercapacitor was constructed using C_OCLP-4.5 to evaluate its practical application. Figure 10a shows the CV curves for the symmetric supercapacitor at different scan rates. The curves maintained a quasi-rectangular shape at a scan rate of 50 mV·s⁻¹. A slight deformation indicates that the electrochemical behavior of a symmetric supercapacitor is a combination of the EDLC and pseudocapacitance. Figure 10b shows that the GCD curves for the symmetric supercapacitor increased with an increasing current density from 1 to 20 A·g⁻¹. Based on Equation (2), the specific capacitance of C_OCLP-4.5 is 203 F·g⁻¹ at 1 A·g⁻¹ and its specific capacitance remains 150 F·g⁻¹ at 10 A·g⁻¹, demonstrating a good rate capability even at high current densities for the symmetric supercapacitor. Figure 10c shows the cycle stability curve at a current density of 10 A·g⁻¹. It displays that the capacitance retention of the device is 84.0% after 5000 cycles, reflecting good cycling stability. Figure 10d indicates that the symmetric capacitor obtains an energy density of 18.04 Wh·kg⁻¹ at a power density of 200.0 W·kg⁻¹ based on Equations (3) and (4), significantly higher than those reported in recent years (13. [25], 10.83 [49], 13.60 [50], 7.00 [51], 13.86 [52], 10.60 [53], and 15.50 Wh·kg⁻¹ [54]). Specifically, the symmetric supercapacitor device successfully powers up a light-emitting diode (the inset of Figure 10d). As shown in the video (see Supplementary Materials File S1), the light-emitting diode can last for a while. Obviously, the N/O-doped porous C_OCLPs are expected to be used in supercapacitors.

4. Conclusions

In this study, a network-structured organic crosslinked polymer was used as a carbon source to obtain N/O-doped porous C_OCLPs. The results indicated that the C_OCLP-4.5 obtained by optimizing the raw materials exhibited an excellent electrochemical performance. For instance, the specific capacitance of C_OCLP-4.5 was as high as 522 F·g⁻¹ at a current density of 0.5 A·g⁻¹, and still exhibited 309 F·g⁻¹ at 20 A·g⁻¹ in a three-electrode system. Furthermore, the symmetric capacitor achieved an energy density of 18.04 Wh·kg⁻¹ at a power density of 200.0 W·kg⁻¹. The C_OCLPs benefitted from the net structure of organic crosslinked polymers to form hierarchical porous carbon and the pseudocapacitance introduced by heteroatoms. Therefore, the method for fabricating carbon material proposed in this study provides a new strategy for the development of electrode materials with high electrochemical performance.