Porous Carbon Prepared from Coal Tar Pitch was Doped with N/S to Prepare Supercapacitors

Abstract

Coal tar pitch (CTP), a hazardous coal-chemical byproduct, requires high-value recycling for sustainable energy applications, yet precise structural and electrochemical regulation of pitch-derived carbon materials remains challenging. Herein, a facile postcarbonization N/S codoping strategy via thiourea-assisted hydrothermal treatment is developed to optimize porous carbon for supercapacitors. Traditional one-step inorganic doping severely damages carbon frameworks and restricts pore development, while hydrothermal-first preparation also delivers inferior porous structures. To overcome these drawbacks, we adopt a sequential strategy: preconstructing hierarchical porous carbon skeleton followed by thiourea-mediated N/S doping. This method well preserves carbon integrity and porous features, overcoming the inherent deficiencies of conventional doping routes. Combined SEM, XRD, and Raman results verify that this strategy enhances graphitization by eliminating amorphous carbon while maintaining intact porous architecture. The optimized PC–NS–2 sample possesses an ultrahigh micropore proportion of 97% and a large specific surface area of 1426.73 m²/g. It achieves a specific capacitance of 307 F g^–1 at 0.5 A/g and maintains 93.98% capacitance retention after 10,000 cycles at 5 A/g, showing superior electrochemical kinetics and cycling stability. The assembled symmetric supercapacitor delivers an energy density of 27.8 Wh/kg at 0.5 A/g. Such excellent performance benefits from well-preserved hierarchical pores, optimized graphitization and improved conductivity brought by N/S codoping. This work provides an efficient structure-stable doping strategy for CTP high-value utilization and offers a feasible reference for advanced carbon electrode design.

1. Introduction

Due to the increasing shortage of petroleum resources, the development of energy storage devices utilizing green energy or solid waste resources has become a new research focus. Supercapacitors have emerged as a promising choice due to their high capacity, high power density, long cycle life, and efficient charging–discharging performance. Based on the energy storage mechanisms, supercapacitors can be classified into two categories: electric double-layer capacitors and Faradaic pseudocapacitors. Faradaic pseudocapacitors primarily store and convert energy through reversible redox reactions occurring on and near the surface of active electrode materials. Electric double-layer capacitors mainly store energy via the formation of an electric double layer at the electrode–electrolyte interface. This storage mechanism is highly reversible in the absence of chemical reactions. Given this mechanism, the selection of electrode materials is critical for achieving capacitors with high energy density. Carbon-based materials, owing to their high tunability, structural diversity, and ease of processing, have become the most widely used materials in energy storage applications.

Coal tar pitch, a heavy fraction of coal tar, accounts for up to 50 wt % of its composition. The polycyclic aromatic hydrocarbon structures in coal tar pitch make it an excellent precursor for the preparation of porous carbon. Current methods for producing porous carbon include physical activation, chemical activation template-assisted methods, chemical vapor deposition, and molten salt synthesis. Among these, template-assisted methods are often expensive and involve complex removal processes. Porous carbons prepared by physical activation often struggle to achieve a satisfactorily high specific surface area. Although molten salt synthesis shows promising potential in carbon material preparation, it has not yet reached the same level of maturity as chemical activation. Therefore, this study employs chemical activation, a well-established method capable of yielding porous carbon with a high specific surface area.

In the preparation of porous carbon materials, various strategies are employed to enhance their electrochemical performance. For example, transition metal oxides are doped to utilize the redox properties of metal ions for improved capacitance, or heteroatoms are introduced to modify surface functional groups and enhance energy storage behavior. Li· et al. prepared nitrogen-doped porous carbon, which achieved a specific capacity of 137 mAh/g at a current density of 18 A/g. Yang and his team fabricated an N/P codoped carbon nanocage that exhibited a high specific capacitance of 435 F g^–1 at 0.05 A/g. Qu et al.· used H₂O₂ and HNO₃ for preoxidation, followed by KOH activation to obtain porous carbon, which demonstrated a specific capacitance of 318.1 F g^–1 at 0.5 A/g. Discussed how our postdoping method achieves higher microporosity and better cycling stability than many reported predoping or one-step approaches.

The primary novelty of this work lies in the adoption of a postcarbonization N/S doping strategy using thiourea-assisted hydrothermal treatment on preactivated porous carbon derived from coal tar pitch. Unlike conventional predoping methods that damage the carbon framework during activation, this sequential approach effectively preserves the hierarchical porous structure while introducing N/S heteroatoms, resulting in exceptionally high microporosity (∼97%) and optimized electrochemical performance. This strategy provides a new pathway for high-value utilization of coal tar pitch waste.

2. Experimental Section

2.1. Materials

Coal tar pitch was obtained from Tianyuan Chemical Co., Ltd. Potassium hydroxide (KOH), thiourea, ethanol, and hydrochloric acid (HCl) were supplied by Tianjin Comio Chemical Reagent Co., Ltd., Tianjin Xinbote Chemical Co., Ltd., Tianjin Hengxing Chemical Reagent Manufacturing Co., Ltd., and Tianjin Comio Chemical Reagent Co., Ltd., respectively. Acetylene black was purchased from Shanghai Macklin Biochemical Technology Co., Ltd. Polytetrafluoroethylene (PTFE) binder was provided by Tianjin Aweixin Chemical Technology Co., Ltd. Nickel foam was acquired from Suzhou Zhengtairong New Material Co., Ltd.

2.2. Material Synthesis

Given the low ash content (0.45%) of coal tar pitch (CTP), the raw CTP was directly sieved through a 120-mesh screen and ground into fine powder. Then, 2 g of CTP was mixed with 6 g of KOH and ground thoroughly. The mixture was sieved through a 60-mesh screen and placed in a tube furnace. Under a N₂ atmosphere, the temperature was raised to 700 °C at a heating rate of 5 °C/min and held for 2 h for activation. The resulting product was washed with 1 M HCl to remove residual KOH, rinsed with distilled water until neutral pH, and dried at 80 °C for 12 h. The obtained porous carbon was denoted as PC. Subsequently, 1 g of PC and n g (n = 1, 2, 3) of thiourea were dispersed in a mixed solvent of 20 mL ethanol and 30 mL deionized water. After stirring for 5 min, the mixture was transferred into a hydrothermal autoclave and maintained at 160 °C for 12 h. After cooling, the product was washed with deionized water under stirring for 30 min until neutrality was achieved, collected via suction filtration, and dried at 80 °C for 12 h. The final products were labeled as PC–NS–n (as illustrated in Figure ).

1.

Illustration of the preparation process of PC–NS–n.

CTP was first sieved through a 120-mesh screen. Then, 2 g of the sieved CTP and 2n g (n = 1, 2, 3) of thiourea were placed in a beaker, to which a mixed solvent of 20 mL ethanol and 30 mL deionized water was added. After stirring for 5 min, the mixture was subjected to hydrothermal treatment at 160 °C for 12 h. The resulting product was stirred in deionized water for 30 min until neutral, collected by suction filtration, and dried at 80 °C for 12 h. This intermediate was denoted as NS-CTP. Next, three portions of 1 g each of NS-CTP were taken and mixed thoroughly with 3 g of KOH by grinding. The mixture was sieved through a 60-mesh screen, lightly ground again, and activated in a tube furnace under N₂ atmosphere at 700 °C for 2 h with a heating rate of 5 °C/min. The activated material was washed with 1 M HCl, rinsed with distilled water to neutrality, and dried at 80 °C for 12 h. The final products were labeled as NS-n.

2.3. Material Characterization

Raman spectroscopy was performed on a Renishaw inVia system (UK) to evaluate the graphitization degree of the samples. The pore structural characteristics of the porous carbon were analyzed using a Micromeritics ASAP 2460 instrument (USA). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Scientific K-Alpha spectrometer (USA) to determine the surface functional groups. Morphological observations were conducted using a Sigma 300 scanning electron microscope (SEM, Germany). The crystal structure was examined by X-ray diffraction (XRD) on a Bruker D8 Advance diffractometer (Germany).

2.4. Electrochemical Measurements

The working electrode was prepared by uniformly mixing PC–NS–n or NS-2 with acetylene black and polytetrafluoroethylene (PTFE) binder at a mass ratio of 8:1:1 in ethanol. The resulting slurry was coated onto a piece of nickel foam (1 cm × 1 cm) with an active material loading of approximately 3 mg. Electrochemical measurements were carried out in a three-electrode system using 6 M KOH as the electrolyte, with a platinum plate as the counter electrode and a Hg/HgO electrode as the reference electrode. For the assembly of the coin-type cell (CR2032), two nickel foam electrodes of the same size and active material loading were used as the positive and negative electrodes, separated by a glass fiber membrane (Whatman GF/D) and impregnated with 6 M KOH electrolyte. The electrochemical performance was evaluated using an electrochemical workstation through cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS).

3. Results and Discussion

3.1. Material Characterization Analysis

As shown in Figure a–c, the SEM images of PC–NS–n were compared with those of PC (Figure g–i). It can be observed that the surface of the carbon framework in PC–NS–n exhibited uneven protrusions around the pores, in contrast to the relatively smooth carbon framework of PC. The porous structure of PC is attributed to the activation by KOH and CTP at high temperature, during which gases such as CO and CO₂ are generated and contribute to pore formation The irregular protrusions on the framework of PC–NS–n likely resulted from the subsequent hydrothermal treatment with thiourea after the initial carbonization. In comparison with PC (Figure g–i), the NS-n samples (Figure d–f) showed severe etching on the carbon framework structure. This distortion may be caused by the etching effect of thiourea during the activation process when doping was performed prior to carbonization. Different doping sequences clearly lead to distinct morphological changes: the postdoping method largely preserved the original porous structure of PC, while the predoping process significantly damaged the carbon framework and reduced the porosity in NS-n. Energy-dispersive spectroscopy (EDS) mapping of PC–NS–2 (Figure j–l) confirmed the homogeneous distribution of C, N, and S elements. It has been reported that N- and S-containing functional groups can enhance the electrochemical performance of carbon-based electrodes.

2.

Scanning electron microscopy (SEM) images of (a–c) PC–NS–n, (d–f) NS-n, and (g–i) PC. (j–l) Energy-dispersive X-ray spectroscopy (EDS) elemental mappings of PC–NS–2.

The crystal structure and graphitization degree of PC–NS–n were analyzed by XRD and Raman spectroscopy. As shown in Figure a, the XRD pattern of PC–NS–n exhibited two characteristic microcrystalline diffraction peaks at approximately 23° and 43°, corresponding to the (002) and (100) planes of graphitic carbon, which are typical for porous carbon materials. Compared with the XRD patterns of PC, PC–NS–2, and NS-2 provided in Figure S1a, the intensity of the C (002) peak increased after thiourea doping, though the extent of this increase varied between NS-2 and PC–NS–2 due to differences in doping methods. The higher C (002) peak intensity in NS-2 is likely attributed to the formation of sulfur-containing radicals (e.g., H₂S) at temperatures above 500 °C, which cleaved unstable C–C bonds and promoted the development of more stable sp²-conjugated graphitic crystallites. These radicals also etched amorphous carbon, leading to a more stable carbon skeleton. These interpretations were further corroborated by Raman results. In Figure a, the intensity of the C(002) peak in PC–NS–n gradually increased with higher thiourea content. The stronger diffraction intensity at low angles for PC–NS–2 suggests a higher abundance of micropores. ,

3.

(a)­XRD patterns of PC–NS–n, (b) Raman spectra of PC–NS–n.

Raman spectra (Figures b and S1b) showed two prominent bands at around 1350 cm^–1 (D band) and 1580 cm^–1 (G band), which are associated with structural defects and the in-plane vibration of sp²-hybridized carbon atoms, respectively. The intensity ratio ID/IG is commonly used to evaluate the graphitization degree of carbon materials. Higher thiourea content promotes graphitization via etching of amorphous regions and radical-induced reorganization into more ordered sp² domains. , While dopants introduce defects, the net effect here is increased graphitic order due to selective removal of disordered carbon, as evidenced by decreasing ID/IG (1.01 → 0.96) with higher doping.

All samples exhibit typical Type I/IV hybrid isotherms with an H4-type hysteresis loop observed in the relative pressure range of 0.7 < P/P₀ < 1, indicating the presence of micro- and mesopores­(Figure ). The more pronounced hysteresis loop of PC–NS–2 suggests a higher proportion of mesopores alongside dominant micropores compared to PC and NS-2, which is corroborated by the pore structure parameters summarized in Table . As shown in Table , thiourea doping effectively increased the specific surface area (SSA) compared to PC. This enhancement can be attributed to the etching of amorphous carbon by sulfur- and nitrogen-containing species (such as H₂S or NH₃) released from thiourea during high-temperature treatment, facilitating additional pore formation. Etching not only contributes to porosity development but also helps stabilize the carbon skeleton by removing less ordered carbon phases. Furthermore, different doping methods lead to distinct modifications in the carbon structure. The N₂ adsorption–desorption isotherms of the PC–NS–n series are provided in Figure S2a. As seen in Figure S2b and Table , all PC–NS–n samples show similar pore size distributions and specific surface areas. This similarity arises because the precursor PC, having already undergone high-temperature activation, contains only a limited amount of etchable amorphous and sp³ carbon. Consequently, subsequent hydrothermal treatment with thiourea resulted in comparable textual properties across the PC–NS–n samples.

4.

(a) N2 adsorption–desorption isotherms and (b) pore size distribution of PC, PC–NS–2, NS-2.

1. Shows the N₂ Adsorption–Desorption Isotherms of NS-2, PC, and PC–NS–N.

sample	SSABET (m2 g–1)	SSAmic (m2 g–1)	PV(cm3 g–1)		D ap (nm)
			total	micro	(D ap: average pore size.)
PC–NS–1	1416.75	1353.67	0.67	0.53	1.88
PC–NS–2	1426.73	1384.25	0.66	0.54	1.85
PC–NS–3	1410.37	1361.92	0.65	0.53	1.86
NS-2	1285.29	793.12	0.63	0.33	1.95
PC	1045.54	641.59	0.53	0.31	2.05

2. Proportion of Elements in PC and PC–NS–N Samples.

sample	C (%)	O (%)	N (%)	S (%)
PC	80.87	18.39	0.72	0.02
PC–NS–1	58.44	39.14	1.93	0.50
PC–NS–2	64.11	33.39	1.88	0.61
PC–NS–3	59.42	37.86	2.11	0.61

The X-ray photoelectron spectroscopy (XPS) measurement results are shown in the figures. As indicated in Table , the atomic percentage of carbon gradually decreased with increasing thiourea doping, while the contents of nitrogen and sulfur increased, confirming the successful incorporation of N and S elements into the carbon skeleton through thiourea-assisted modification. As shown in Figure S3a, the high-resolution C 1s spectrum could be deconvoluted into four peaks located at 284.8, 285.4, 286.6, and 288.6 eV, corresponding to C–C, C–N, C–S, and CO bonds, respectively. The N 1s spectrum (Figures a and ) was fitted with three components assigned to pyrrolic N (400.3 eV), graphitic N (401.1 eV), and pyridinic N-oxide (403.2 eV). In comparison with Figure S3d, it was observed that with increasing thiourea doping, the contents of pyrrolic N and pyridinic N decreased, while those of pyridinic N-oxide and graphitic N increased. This change in N-configuration is believed to enhance electron transfer capability between the electrode and electrolyte. The high-resolution S 2p spectrum (Figure b) exhibited peaks at 163.9 and 165.0 eV, attributed to the S 2p₃/₂ and S 2p₁/₂ of thiophene-type sulfur (C–S–C) and sulfate-type sulfur (C–S), respectively, along with an oxidized sulfur species (C–SO _x , x = 2–4) at 168.6 eV. The incorporation of sulfur-containing functional groups can provide additional pseudocapacitance, thereby improving the electrochemical performance of the material. Based on the XPS results, a schematic diagram illustrating the modification mechanism of PC–NS–2 was proposed.

$${({\mathrm{N}\mathrm{H}}_2)}_2{\mathrm{C}\mathrm{S}+2\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{O}}_2{+}_2{\mathrm{N}\mathrm{H}}_3{+\mathrm{H}}_2\mathrm{S}$$ 1

$${\mathrm{A}\mathrm{r}-\mathrm{H}+\mathrm{H}}_2\mathrm{S}\to {\mathrm{A}\mathrm{r}-\mathrm{S}-\mathrm{A}\mathrm{r}+2\mathrm{H}}_2$$

$$(\mathrm{A}\mathrm{r}-\mathrm{H}\mathrm{S}\mathrm{i}\mathrm{s}\mathrm{a}\mathrm{n}\mathrm{a}\mathrm{r}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{a}\mathrm{l}\mathrm{t}\mathrm{a}\mathrm{r}\mathrm{p}\mathrm{i}\mathrm{t}\mathrm{c}\mathrm{h})(\mathrm{s}\mathrm{u}\mathrm{l}\mathrm{f}\mathrm{u}\mathrm{r}\mathrm{d}\mathrm{o}\mathrm{p}\mathrm{e}\mathrm{d})$$ 2

$${\mathrm{A}\mathrm{r}-\mathrm{H}+\mathrm{H}}_2\mathrm{S}\to {\mathrm{A}\mathrm{r}-\text{SO}}_3\mathrm{H}$$ 3

$${\mathrm{A}\mathrm{r}-\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+\mathrm{N}\mathrm{H}}_3\to {\mathrm{A}\mathrm{r}-\mathrm{C}(\mathrm{O})\mathrm{N}\mathrm{H}}_2{+\mathrm{H}}_2(\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{d}\mathrm{o}\mathrm{p}\mathrm{i}\mathrm{n}\mathrm{g})$$ 4

$${\mathrm{A}\mathrm{r}-\mathrm{H}+\mathrm{N}\mathrm{H}}_3\to {\mathrm{A}\mathrm{r}-\mathrm{N}\mathrm{H}}_2{+\mathrm{H}}_2$$ 5

5.

(a) S 2p spectra of PC–NS–n (b) N 1s spectra of PC–NS–n.

6.

Experimental mechanism diagram of PC–NS–n.

Hydrothermal thiourea hydrolysis generates CO₂, NH₃ and H₂S. H₂S attacks aromatic carbon skeletons to form thiophene C–S–C bonds and oxidized sulfonate groups. NH₃ reacts with surface carboxyl groups and aromatic carbon to introduce pyridinic, pyrrolic and graphitic N species.

3.2. Performance Characterization Analysis

The electrochemical performance of the PC–NS–n series was evaluated, as shown in Figure a–d. In Figure a, PC–NS–2 exhibits the largest integrated area under the cyclic voltammetry (CV) curve. Combined with the longest charge/discharge duration observed in the galvanostatic charge–discharge (GCD) profiles (Figure b), PC–NS–2 demonstrates superior charge storage capability. The specific capacitance values, calculated using eq , are summarized in Figure d. Among the PC–NS–n samples, PC–NS–2 delivers the highest specific capacitance of 307 F g^–1 at 0.5 A/g, outperforming PC–NS–1 (261.5 F g^–1) and PC–NS–3 (176.5 F g^–1). The electrochemical analysis of the NS-n series is presented in Figure S5a–c. As depicted in Figure S5a, NS-2 shows a larger CV integrated area compared to NS-1 and NS-3. Furthermore, in Figure S5b, NS-2 displays the longest GCD time at 0.5 A/g. The electrochemical impedance spectroscopy (EIS) results (Figure S5c) reveal that NS-2 has the smallest semicircle radius in the high-frequency region, indicating the lowest internal resistance among the NS-n samples. Therefore, NS-2 exhibits the best overall electrochemical performance within the NS-n group.

$$C_{\mathrm{s}\mathrm{p}}=\frac{I_{\mathrm{\Delta }}t}{m_{\mathrm{\Delta }}V}$$ 6

I(A) is the discharge current, m(g). The average mass of the active material in the electrode, potential change of ΔV,ΔT is the discharge time.

7.

(a–d) is the CV, GCD, EIS spectra and specific capacitance of PC–NS–n at different current densities. (e–h) Are CV, GCD, EIS spectra and specific capacitances at different current densities of PC, NS-2 and PC–NS–2. (i) PC–NS–2 is CV at different scan rates, (j) PC–NS–2 GCD at different current densities, (k) PC–NS–2 surface capacitance ratio at 150 mV/s. (l) The proportion of surface capacitance control of PC–NS–2 at different scan rates. (m) CV of PC–NS–2 at low scan rate. (n) The surface capacitance ratio of PC–NS–2 at low scan rate. (o) Porter plot of PC–NS–2. (p) PC–NS–2 has cycle stability and Coulombic efficiency after 10000 cycles.

Figure e–h presents a comparative evaluation among PC–NS–2, NS-2, and PC to demonstrate the enhancement in electrochemical performance resulting from N/S doping and the influence of doping methods. Comparative analysis reveals that the CV curve area of PC at 50 mV/s (Figure e) is significantly smaller than those of the N/S-doped samples. Furthermore, as shown in Figure f, the charge–discharge duration of the N/S-doped materials considerably exceeds that of PC. Using eq , the specific capacitances are calculated and summarized in Figure d. The specific capacitances of PC–NS–2 and NS-2 at 0.5 A/g are 307 F g^–1 and 248.5 F g^–1, respectively, which are markedly higher than that of PC (177.84 F g^–1). These results confirm the effectiveness of thiourea doping in improving electrochemical performance. Further electrochemical analysis indicates that PC–NS–2 exhibits superior electrochemical properties compared to NS-2, suggesting that the doping method also influences the performance. XRD, BET, and Raman characterization reveal that although NS-2 possesses a higher degree of graphitization than PC–NS–2, its specific surface area (1285.29 m²/g) is lower than that of PC–NS–2 (1426.73 m²/g). Moreover, the microporosity fraction in PC–NS–2 (97%) is significantly greater than that in NS-2 (62%). The microporous structure provides more active sites for charge storage and facilitates charge transfer. Therefore, further electrochemical analysis is conducted on the PC–NS–n series.

Figure e shows that the CV curves of PC–NS–n exhibit a quasi-rectangular shape. Combined with their GCD profiles, which display nearly symmetric triangular shapes, the slight deviations from ideal capacitive behavior can be attributed to pseudocapacitive contributions originating from N/S doping. As shown in Figure i, the CV curves maintain a similar shape with increasing scan rates, indicating the electrochemical reversibility of PC–NS–2 and demonstrating that its charge storage mechanism is dominated by electric double-layer capacitance (EDLC). Figure j presents the GCD curves of PC–NS–2 at different current densities. The symmetric triangular shape is well retained even at higher current densities, demonstrating excellent rate capability and further confirming the EDLC-dominated capacitive behavior. Together with Figure S5a,b, these results reveal that the energy storage mechanism of PC–NS–n is primarily based on EDLC, accompanied by outstanding electron transfer kinetics. The charge storage behavior and electrochemical reaction kinetics of PC–NS–n were further quantitatively analyzed using eqs and . Therefore, the charge/discharge kinetics is qualitatively evaluated according to the eqs and ().

$$i=a{v}^b$$ 7

$$\log (\mathbb{i})=\log (a)+b\log (v)$$ 8

where a and b are constants, I is the current density, and v s the scan rate. In general, a b -value approaching 0.5 indicates that the electrochemical process is dominated by diffusion-controlled pseudocapacitance with slower reaction kinetics, where as a b-value close to 1 suggests a capacitive-controlled process with fast reaction kinetics. As shown in Figure S7c, the b-value of PC–NS–2 is 0.85, which is higher than those of PC–NS–1 (0.75) and PC–NS–3 (0.78), indicating a more pronounced capacitive-controlled behavior and faster kinetics. To further quantify the capacitance contributions from surface capacitive effects (i.e., EDLC) and diffusion-controlled processes (i.e., pseudo capacitance), Dunn’s method was applied to the PC–NS–n series using eqs and .

$$\mathbb{i}=k_{1}v+k_2{\nu }^{1/2}$$ 9

$$\mathbb{i}/{\nu }^{1/2}=k_1{\nu }^{1/2}+k_2$$ 10

k ₁ v and k ₂ν^1/2 represent the surface capacitive and diffusion-controlled current contributions. As shown in Figure m,n, the capacitive current contributionscategorized into surface-controlled and diffusion-controlled processeswere quantified for PC–NS–2 based on low scan rate CV measurements. In conjunction with the comparative analysis provided in Figure S5e–f across the PC–NS–n series, it is evident that PC–NS–2 exhibits a surface-controlled capacitive contribution of up to 91% at 5 mV/s, the highest among all PC–NS–n materials. This result highlights its superior rate capability. Further analysis was conducted at higher scan rates, as summarized in Figure i. Even at a scan rate as high as 150 mV/s, the surface-controlled contribution remains notably high at 86.24%, as illustrated in Figure k,l, consistent with its excellent rate performance.

Figure o displays the Bode plots derived from electrochemical impedance spectroscopy (EIS) measurements of the PC–NS–n series, from which the relaxation time constant τ was calculated (where τ = 1/f ₀, and f ₀ represents the characteristic frequency at a phase angle of −45°). The calculated τ values are 1.35 s for PC–NS–1, 1.18 s for PC–NS–2, and 2.38 s for PC–NS–3. The smallest τ value of PC–NS–2 indicates rapid ion diffusion within the electrode and demonstrates its capability for high-rate charging and discharging. As shown in Figure p, the cycling stability and Coulombic efficiency were evaluated over 10,000 cycles at a current density of 5 A/g. The specific capacitance retention remains at 93.98% after 10,000 cycles, confirming outstanding electrochemical stability. The inserted GCD curves (comparing the first and 10,000th cycles) further support the excellent electrochemical durability.

A 6 mol/L KOH aqueous solution was employed as the electrolyte, and coin-type symmetric supercapacitors were assembled using PC–NS–n as both the positive and negative electrodes, with an operating voltage window of 0–1 V. Figure a presents the CV curves measured at a scan rate of 50 mV s^–1. The CV curve of PC–NS–2 exhibits a nearly rectangular shape with the largest enclosed area. As further supported by Figure c, the CV profiles of PC–NS–2 show negligible distortion with increasing scan rates, indicating excellent rate capability and electrochemical reversibility. In addition, the GCD curves in Figure b reveal that PC–NS–2 displays the longest charge–discharge duration, confirming its superior electrochemical performance over the other two materials when configured in a symmetric supercapacitor. Figure e shows the GCD curves of PC–NS–2 at various current densities. The profiles retain a symmetric triangular shape even at elevated current densities, further corroborating its outstanding electrochemical reversibility.

$$C=\frac{2I_{\mathrm{\Delta }}t}{m_{\mathrm{\Delta }}V}$$ 11

$$E=\frac{Ct\mathrm{\Delta }{v}^2}{2\times 3.6}$$ 12

$$P=\frac{3100E}{\mathrm{\Delta }t}$$ 13

I (A) is the discharge current, m (g) is the average mass of the active material in the two electrodes, the potential change in ΔV, Δt is the discharge time, and the energy density (E) and power density (P).

8.

(a) is the CV of PC–NS–n, (b) is the GCD of PC–NS–n, (c) is the CV of PC–NS–2 at different scan rates, (d) is the GCD of PC–NS–2 at different current densities, (e) the specific capacitance of PC–NS–2 at different current densities. (f) Ragone plots in comparison with other works.

The specific capacitance under various current densities, as calculated using eq , is summarized in Figure e. The device exhibits a specific capacitance of 210 F g^–1 at 0.5 A/g. The decrease in capacitance compared to the three-electrode measurement is likely attributed to prolonged ion diffusion paths and resulting concentration polarization in the two-electrode configuration. Energy and power densities, calculated via eqs and for the coin-cell supercapacitor, are presented in Figure f. At 0.5 A/g, the device achieves an energy density of 27.8 Wh/kg with a corresponding power density of 502 W kg^–1. Even at a high current density of 10 A/g, it maintains an energy density of 15.2 Wh/kg and a power density of 10,000 W kg^–1. These values are superior to those reported for many other carbon-based symmetric supercapacitors, − demonstrating the outstanding performance of the PC–NS–2 material.

In summary, this study demonstrates that thiourea doping significantly enhances the electrochemical performance of porous carbon materials. Furthermore, the doping method considerably influences the material properties. Specifically, the approach involving initial carbonization followed by N/S doping was found to be more effective in enhancing electrochemical performance than the conventional method of predoping N/S prior to carbonization. Postcarbonization doping notably modifies the pore structure of porous carbon, resulting in a microporosity of 97% for PC–NS–2, substantially higher than the 62% observed for NS-2. This highly microporous structure provides abundant active sites for charge storage, leading to a high specific capacitance of 307 F g^–1 for PC–NS–2, markedly superior to the 248.5 F g^–1 achieved by NS-2. Rate capability analysis revealed that the surface-controlled capacitive contribution of PC–NS–2 remains at 86.24% even at a high scan rate of 150 mV s^–1, consistent with its rapid electrochemical kinetics. When assembled into a coin-type symmetric supercapacitor, the device delivered an energy density of 27.8 Wh/kg at a power density of 502 W kg^–1(Table ).

3. Comparison of the Specific Capacitance of PC Electrode.

material	electrolyte (mol/L)	CD (A/g)	SC (F/g)	ED (Wh/kg)	PD (W/kg)	cycles	CR (%)	ref
PC–NS–2	6.0 KOH	0.5	307	27.8	502	10000	93.98	This
PC-CTP (N,O)	6.0 KOH	0.5	318	7.7	50	10000	90.9
NS-IPCN800	6.0 KOH	0.05	302	9.71	25.98	10000	94.2
NSPC	6.0 KOH	0.5	208.6	8.94	50	10000	95.22
Ni0.3CTP/FCCOA	1.0 Na2SO4	0.5	216	27.45	253	10000	88.9
FHNC	6.0 KOH	1	280.3	10.1	250	10000	96.3
PGCM-2	6.0 KOH	0.05	293	148.3	204.2	7000	97.4
MZS	6.0 KOH	0.1	320	10.6	50.1	10000	94

4. Conclusion

In summary, porous carbon derived from CTP and doped with N/S heteroatoms demonstrates significantly enhanced electrochemical performance compared to undoped PC. Postdoping preserves preformed porous architecture (high SSA, 97% microporosity), while predoping leads to excessive etching/damage during activation (lower micropore fraction ∼ 62%, distorted framework per SEM). This results in more active sites, better ion diffusion, and superior capacitance/stability for PC–NS–2 despite slightly lower graphitization than NS-2.

The optimal material, PC–NS–2, delivers a high specific capacitance of 307 F g^–1 at 0.5 A/g and exhibits excellent charge transfer kinetics, along with remarkable electrochemical stabilityretaining 93.98% of its initial capacitance after 10,000 cycles at 5 A/g. When assembled into a coin-type symmetric supercapacitor, PC–NS–2 achieves an energy density of 27.8 Wh/kg at 0.5 A/g.

The study is the demonstration of a milder, structure-preserving doping route that overcomes limitations of one-step methods (framework damage, underdeveloped pores), enabling superior micropore-dominated carbons from industrial waste for high-performance supercapacitors. This bridges gaps in sustainable carbon material design.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c05731.

• XRD and Raman spectra of PC, PC–NS–2 and NS-2 (Figure S1); N₂ adsorption–desorption isotherms and pore size distribution of PC–NS–n (n = 1, 2,3) (Figure S2); XPS full spectrum and high-resolution C 1s, O 1s, N 1s spectra (Figure S3); CV, GCD and EIS curves of NS-n samples (Figure S4); GCD curves under different current densities, CV curves at low scan rates, and the diffusion-controlled/surface-capacitive contribution ratio of PC–NS–1 and PC–NS–3 (Figure S5) (PDF)

The authors declare no competing financial interest.