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Mohamad Samsul Anrokhi, Pipit Fitriani, Oktaviardi Bityasmawan Abdillah, Fitri Aulia Permatasari, [Yoshiyuki Yamashita](https://orcid.org/0000-0003-0994-8095), Fatimah Arofiati Noor, Ferry Iskandar

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This version of the article has been accepted for publication, after peer review (when applicable) and is subject to Springer Nature’s AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.1007/s10853-025-11395-3.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Carbonization and Structural Evolution of Bituminous Coal-Derived Carbons Material Toward Supercapacitor Applications](https://mdr.nims.go.jp/datasets/e1a2e9ce-39a0-4869-98f6-4355815f7320)

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Carbonization and Structural Evolution of Bituminous Coal-1 Derived Carbons Material Toward Supercapacitor 2 Applications 3  4 Mohamad Samsul Anrokhi1, Pipit Fitriani1, Oktaviardi Bityasmawan Abdillah2, Fitri Aulia 5 Permatasari1, Yoshiyuki Yamashita3,4, Fatimah Arofiati Noor1, and Ferry Iskandar1,5* 6 *ferry@itb.ac.id 7  8 1Department of Physics, Faculty of Mathematics and Natural Sciences, Institut Teknologi 9 Bandung, Indonesia 40132 10 2Chemical Engineering Program, Department of Advanced Science and Engineering, Graduate 11 School of Advanced Science and Engineering, Hiroshima University 12 3Nano Electronics Device Materials Group, Research Center for Electronic and Optical 13 Materials, National Institute for Materials Science, 305-0044 1-1 Namiki Tsukuba Ibaraki, 14 Japan 15 4Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 16 Motooka 744, Fukuoka 819-0395, Japan 17 5Research Center for Nanoscience and Nanotechnology, Institut Teknologi Bandung, Indonesia 18 40132 19  20  21  22  23  24  25 mailto:*ferry@itb.ac.idAbstract   1 Coal-derived carbon materials have garnered significant interest for energy storage applications 2 due to their abundance, cost-effectiveness, and straightforward processing methods. This study 3 investigates the direct carbonization of bituminous coal at various temperatures (500, 700, and 900 4 °C) and analysed the evolution of carbon structure, nitrogen functionalities. In addition, the effects 5 on supercapacitor performance were studied. Structural transformations and nitrogen group 6 changes were observed with increasing temperature. At 500°C, pyrrolic-N was the predominant, 7 and it gradually converted to pyridinic-N at 700°C, significantly enhancing charge storage 8 capacity. Further increasing the temperature to 900°C transformed the pyridinic-N into Graphitic- 9 N, improving conductivity. Electrochemical analysis revealed that the sample carbonized at 700°C 10 exhibited the highest specific capacitance of 73.6 F/g, attributed to the high content of pyridinic-11 N, which contributes to pseudo capacitance via redox reactions. These findings highlight the 12 potential of coal-derived carbon via carbonization as cost-effective and efficient supercapacitor 13 electrode materials. This work demonstrates the importance of optimizing carbonization 14 conditions to balance aromatic carbon structure and nitrogen functionalities for enhanced 15 electrochemical performance.  16  17  18 Keyword: carbonization, coal, functional group, supercapacitor 19  20  21 1. Introduction  22 Rapid advancement in energy storage technologies has established supercapacitors as 23 crucial components in future energy systems due to their exceptional power density, rapid charge-24 discharge cycles, and long cycle life [1]. Their capability to bridge the gap between high power 25 output and moderate energy storage makes them essential for applications in electric vehicles, 26 renewable energy systems, and smart power grids [1–3]. However, fully realizing the potential of 27 supercapacitors relies significantly on the development of advanced and cost-effective electrode 28 materials [2]. Various emergent materials such as graphene, MXene, and metal-organic 29 frameworks (MOFs) exhibit remarkable potential, but their high costs and complex synthesis 30 processes limit their widespread applications [4–7].  31 In response to these challenges, there is increasing interest in more sustainable and cost-1 effective alternatives, such as coal-derived carbon material. This interest is driven by coal's 2 abundant availability, low cost, and advantageous properties as a precursor for carbon production. 3 Recent progress in coal-derived carbon material toward the supercapacitor application has focused 4 on optimizing various factors such as carbonization temperature, activation methods, and 5 heteroatom doping to enhance electrochemical properties[8–10]. Chemical activation with 6 potassium hydroxide (KOH) or zinc chloride (ZnCl₂) and template-assisted approaches have been 7 explored to create porous structures in activated carbon [8, 11, 12]. These methods improve material 8 properties but are often complex and resource-intensive [8, 13]. While activation techniques 9 enhance pseudo-capacitance by introducing acidic surface functionalities, acid activation is 10 suboptimal for carbon-derived materials as it degrades microporosity and diminishes 11 electrochemical performance [14]. 12 Given the limitations of conventional processing techniques, enhancing electrode materials 13 through heteroatom doping has become another popular approach. This technique improves the 14 performance of carbon materials by incorporating elements such as Nitrogen, Sulphur, or Boron 15 into the carbon lattice, leading to significant enhancements in physicochemical properties [15–18]. 16 Nitrogen doping effectively enhances the electronic conductivity, charge storage capacity, and 17 surface wettability of coal-derived carbon materials. This is achieved through the formation of π-18 conjugated systems with carbon atoms, which modify local charge density, improve electron 19 transport, and increase hydrophilicity, thereby boosting overall electrochemical performance [19–20 22]. The type and concentration of nitrogen dopants (e.g., pyridinic-N, pyrrolic-N, or graphitic-N) 21 are crucial for optimizing electronic performance. Previous studies reported that the introduction 22 of small amount of nitrogen atoms (< 2 wt.%) into coal-derived carbon material can significantly 23 enhance performance of supercapacitor electrodes [23, 24]. However, in these reports, the nitrogen-24 containing precursors were intentionally added into carbon materials for introducing nitrogen 25 doping, which complicates the experimental procedure. Meanwhile, coal in its bare form also 26 offers a distinct advantage by inherently possessing approximately 1-2 wt.% nitrogen within their 27 structure, primarily in the form of heterocyclic rings [22]. This inherent nitrogen content offers a 28 distinct advantage by enabling the enhancement of material properties for supercapacitor electrode 29 applications without requiring additional doping steps.  30 Moreover, it simplifies the preparation process while complementing the carbon structure, 1 which consists of aromatic and aliphatic components, each playing a distinct role in defining the 2 physicochemical properties of coal-derived carbon materials. Aromatic carbons, with their planar 3 ring structures and delocalized π-electrons, contribute significantly to stability and conductivity. 4 Conversely, aliphatic carbons, characterized by their chain-like structures, enhance flexibility and 5 surface area [25]. To effectively enhance the performance of supercapacitor electrode, controlling 6 the aromaticity and nitrogen configuration in coal is essential to achieve a balance between 7 excellent electron transport, increased wettability, and additional charge storage capability [1][3]. 8 Therefore, understanding the chemistry of this relationship could be crucial for achieving effective 9 outcomes.  10 The pyrolysis of coal, commonly referred to as carbonization, serves as a fundamental 11 process for this purpose. Conducted under oxygen-free conditions and without the need for 12 chemical additives, it enables the detailed study of the evolution of carbon bonding, the behaviour 13 of organic atoms such as nitrogen and oxygen, and the transformation of mineral phases. These 14 insights are crucial for elucidating the relationship between nitrogen configurations, carbon 15 structures, and their impact on the electrochemical performance of supercapacitor materials [26–16 28]. Previous studies have been conducted to investigate structural changes of coal under various 17 carbonization temperatures. Li et al. [29] observe the alterations to the stacking structure that 18 happened when low-rank coals were heated to 900°C. The observation showed that the increase in 19 temperature affects the change in crystal parameters of aggregate structure of low-rank coal. 20 Meanwhile, Xu et al. [30] develop and use micro-Raman spectroscopy to examine alterations in 21 the char structure produced during the pyrolysis of Zhundong coal. According to the results, the 22 char becomes more crystalline and thermally stable at high temperatures because they improve the 23 order of carbon structures. On the other hand, study by Meng et al. [31] about pyrolysis-induced 24 modifications to coal's carbon structure and functional groups shows that at temperatures below 25 1000°C, the coal particles' surface and core experiences distinct changes in the aromatic rings' 26 structure. In the meanwhile, comparable changes often occur in the functional groups. At 1000°C, 27 the functional groups especially the aliphatic C-H in the centre are slightly altered and nearly all 28 of them break down on the surface, while the big and tiny aromatic ring structures significantly 29 increase. Alongside examining the development of carbon structure and its functional groups, 30 carbonized coal has been utilized for application as an anode in lithium and sodium ion batteries 31 which can provide the specific capacity of 384 mAhg-1 at 0.1C and 270.1 mAhg-1 at 0.1C, 1 respectively [32]. In addition, the research conducted by Yang et al. focused on the transformation 2 of nitrogen functional groups in Chlorella vulgaris-derived biochar under different pyrolysis 3 temperatures, demonstrating that specific nitrogen functionalities can serve as active sites for the 4 oxygen reduction reaction (ORR) application [33]. While their work contributed valuable insights 5 into nitrogen chemistry in biochar, our study extends this scope by exploring the concurrent 6 evolution of both inherent nitrogen functionalities and carbon structure and understanding how 7 these structural changes influence the electrochemical performance of the resulting carbon material 8 when applied as a supercapacitor electrode. 9 In this study, direct carbonization of bituminous coal was performed at 500, 700, and 900°C 10 to investigate structural changes in carbonized coal. The alterations in carbon structure are 11 anticipated to improve the electrochemical characteristics as a supercapacitor electrode. By fine-12 tuning carbonization conditions, we aim to achieve a favorable balance of high aromatic carbon 13 content, suitable aliphatic carbon levels, and enhanced nitrogen functionalities, without additional 14 doping and activation steps. Unlike previous studies by Cheng et al. [34], which focused solely on 15 carbon structure, and Niu et al. [35], which examined only the thermal behaviour of functional 16 groups, our work explores the interplay between these factors. This study demonstrates how these 17 elements can improve supercapacitor performance and offer a cost-effective solution for advanced 18 energy storage applications.  19  20 2. Experimental section  21 2.1 Preparation and Carbonization of Bituminous Coal 22 The bituminous raw coal used in this study was sourced from PT. Bukit Asam, Indonesia 23 and classified as high-volatile bituminous-B based on proximate and ultimate analyses. It exhibited 24 39.57 wt.% volatile matter and 1.35 wt.% nitrogen, as detailed in a previous study [33]. These 25 properties are important since the high volatile matter enhances the yield and porosity of the 26 carbonized material while the inherent nitrogen facilitates nitrogen doping without additional 27 processing. Prior to the carbonization process, the raw coal was ground and sieved to achieve a 28 particle size of approximately 200 μm. The raw coal powder was then dried in an oven at 110°C 29 for 2 hours to remove moisture content. Following drying, the coal powder was subjected to 30 carbonization in a tubular furnace at temperatures ranging from 500 to 900°C for 2 hours in a 0.4 31 l/m flow rate of Ar atmosphere. The carbonized coal produced at varying temperatures (500, 700 1 and 900°C) were labeled as C-500, C-700, and C-900, respectively, while the uncarbonized raw 2 coal was labeled as RC. 3  4 2.2 Characterization  5 The infrared spectroscopy analysis was conducted to identify functional groups using IR 6 Prestige-21 Shimadzu covering the wavenumber range from 400-4500 cm-1 using the KBr pellet 7 technique. Raman spectroscopy was employed through a MacroRam benchtop using a 532 nm 8 laser to identify and quantify defects within the structure. X-ray Diffraction (XRD) used to identify 9 the crystalline phase was performed using a Bruker D8 Advance diffractometer, utilizing a Cu K-10 α radiation source (λ=0.15418 nm). In addition, carbon surface chemistry measurements were 11 performed using a X-ray photoelectron spectroscopy (XPS) HI Quantes (ULVAC-PHI) instrument 12 using a photon energy of 1486.6 eV Al Kα to determine the surface chemistry of samples with the 13 energy resolution of 0.51 eV.  14  15 2.3 Electrochemical measurement  16 The working electrodes were prepared by mixing 80 wt.% carbonized coal, 10 wt.% carbon 17 black, and 10 wt.% polyvinylidene fluoride (PVDF) in 1-methyl-2-pyrrolidinone (NMP) to form 18 a slurry. This slurry was then coated onto a nickel foam current collector (1 cm × 1 cm). The coated 19 electrodes were dried at 100°C for 12 hours in a vacuum oven. Electrochemical properties were 20 assessed using a three-electrode setup under ambient conditions, with a platinum mesh and an 21 Ag/AgCl electrode serving as the counter and reference electrodes, respectively. Cyclic 22 voltammetry (CV), and galvanostatic charge-discharge (GCD) were performed using a Parstat 23 3000A electrochemical workstation in a 3 M KOH electrolyte at room temperature. The specific 24 capacitance (𝑆𝐶) value obtained from galvanostatic charge-discharge (GCD) was determined using 25 the equation below [34]: 26  27 𝑆𝐶 =𝐼∆𝑡𝑚∆𝑉                         (1)                                                             28  29  30 3. Results and Discussions  1 Materials Characterization 2 Fig. 1 shows the X-ray diffraction (XRD) patterns of carbonized coal samples subjected to 3 various carbonization temperatures. The patterns reveal a pronounced background intensity, 4 indicating a significant amount of amorphous carbon and considerable structural disorder within 5 the carbonized coal [35]. Diffraction peaks identified at 2θ value of 20.9°, 26.7°and 50.1° are 6 assigned to the crystal structure of SiO2 (quartz) based on JCPDS no 46-1045 [36] These 7 diffraction peaks correspond to the inorganic crystalline mineral phases inherently present in the 8 raw coal. Additionally, two main broad diffractions are observed around the 2θ = ~23°and 43°, 9 which corresponds to the (002) and (100) plane of carbon. The first peak centered in the 23°, 10 corresponds to the reflection of the microcrystalline (002) plane and arises from the stacking of 11 aromatic layers. The second broad peak, located at approximately 43°, is attributed to the (100) 12 plane and denotes the presence of graphite-like atomic arrangements within the individual layers 13 [35, 37]. Furthermore, the Gaussian fitting method was employed to assess the asymmetry of the 14 (002) peak, enabling precise deconvolution of overlapping signals to reveal the coexistence of 15 ordered and disordered carbon structures, as well as provide quantitative insights into stacking, 16 crystallite size, and phase distribution [38]. The representative Gaussian fits for the bands in the 17 coal samples are illustrated in Fig. 1(b-d). This analysis revealed that the presence of an additional 18 band around 15°, attributed to the γ band, which arises from saturated aliphatic sidechains attached 19 to the edges of coal crystallites [38, 39]. The interlayer spacing (d002) calculated using Bragg's 20 equation (Eq. 2) reveals the degree of graphitization. Meanwhile, the peak positions and the full 21 width at half maximum (FWHM) values obtained from Gaussian fitting of the 100 and 002 peaks 22 from the XRD patterns were used to calculate the lateral size (La) and stacking height (Lc) based 23 on Scherrer equation (Eq.3) and (Eq.4), respectively [29]. Additionally, aromaticity is determined 24 from the area under the (002) and γ bands as calculated using Eq. 5, representing the ratio of carbon 25 atoms in aromatic rings to those in aliphatic chains [27]. 26 𝑑002 =𝜆2𝑠𝑖𝑛 𝜃002                                   (2)  27 𝐿𝑎 =1.84𝜆𝛽100 cos 𝜃100                                  (3)  28 𝐿𝐶 =0.9𝜆𝛽002 cos 𝜃002                                  (4) 29 𝑓𝑎 =𝐶𝑎𝑟𝐶𝑎𝑙=𝐴002𝐴002+𝐴𝛾                  (5)          1 where λ is the wavelength of the X-ray (1.5406 Å), and 𝜃002 and 𝜃100 are the peak positions of 2 002 and 100 bands, respectively. 𝐴002 and 𝐴𝛾 are the integrated areas of 002 and γ bands, 3 respectively. The peak positions and the calculated results of Eqs. (2), (3), (4), and (5) are 4 summarized in Table 1.  5  6 Fig. 1 (a) XRD patterns of coal subjected at different carbonization temperatures; the curve fitting 7 of the XRD spectra of (b) CBC-500, (c) CBC-700, and (c) CBC-900  8 As the carbonization temperature increases, the (002) and (100) peaks in the XRD patterns 9 exhibit a slight shift to higher angles, indicating changes in the interlayer spacing. While these 10 shifts suggest some degree of structural reorganization towards a more ordered graphite-like 11 structure, the broad nature of the (002) and (100) peaks across all samples highlights the significant 12 presence of turbostratic carbon. Additionally, mineral peaks are observed in Figure 1, with quartz 1 (SiO₂) being the most prominent mineral detected in all samples. The observed increase in 𝐿𝑎 with 2 higher temperatures reflects the expansion of aromatic domains, as the thermal energy facilitates 3 the growth of aromatic rings, contributing to improved electron transport and structural stability. 4 However, a decrease in 𝐿𝑐 at 700°C compared to 500 and 900°C suggests a transitional phase in 5 the carbonization process, where the decomposition of aliphatic groups and volatile components 6 temporarily disrupts vertical stacking, but lateral ordering progresses undisturbed resulting 7 imbalance at temperature 700°C represents an intermediate stage in the carbonization process. 8 Furthermore, the increase in aromaticity (𝑓𝑎) with temperature highlights the progressive 9 conversion of aliphatic carbon to aromatic structures, enhancing conductivity and stability [32].  10  11 Table 1. The peak positions, 𝑑002, 𝐿𝑐 , 𝐿𝑎 , 𝑓𝑎  and Unit cell of CBC-500, CBC-700 and CBC-900  12 Sample 2𝜃002(°) 2𝜃100(°) 𝑑002 (Å) 𝐿𝑎  (Å) 𝐿𝑐  (Å) 𝑓𝑎(%) Unit Cell (ų) CBC-500 23.50 43.07 3.78 19.49 9.32 70.68 38.52 CBC-700 23.31 43.52 3.81 25.75 8.47 79.90 38.00 CBC-900 23.74 43.84 3.74 33.41 9.38 80.67 36.83  13 To elucidate the molecular structure and functional groups of coal before and after the 14 carbonization process, the FTIR spectra of the coal samples were analyzed and categorized into 15 four functional group regions: hydroxyl groups (3600 - 3000 cm⁻¹), aliphatic groups (3000 - 2700 16 cm⁻¹), oxygen-containing functional groups (1800 - 1000 cm⁻¹), and aromatic groups (900 - 700 17 cm⁻¹) [40]. As shown in Fig. 2, the carbonization process leads to a decrease in the intensity of the 18 peaks corresponding to all four functional group regions, while the peak positions remain largely 19 unchanged. In the hydroxyl region (3600 - 3000 cm⁻¹), all samples exhibit a broad, intense band, 20 which diminishes in intensity with increasing carbonization temperature. This reduction is 21 attributed to the loss of hydroxyl (-OH) and amino (-NH) groups [41]. The aliphatic region (3000 22 - 2700 cm⁻¹), which primarily consists of methyl (-CH₃), methylene (-CH₂), and methine (-CH-) 23 groups, also shows a marked decrease in peak intensity. These aliphatic groups are less 24 thermostable due to their lower binding energies, making them more prone to degradation at higher 25 temperatures [42]. Finally, the peaks in the 956 - 1050 cm⁻¹ range correspond to C-O-C functional 26 groups [43]. The CBC-700 sample exhibits a notably sharper peak in this region compared to the 27 CBC-500 and CBC-900 samples, indicating structural reorganization of oxygen-containing 1 functional groups. The overall trends in the FTIR spectra demonstrate that increasing the 2 carbonization temperature significantly transforms the molecular structure of coal, primarily 3 through the removal of aliphatic and hydroxyl functional groups and the rearrangement of aromatic 4 structures. These transformations yield a more thermally stable and structurally ordered carbon 5 material, which is essential for enhancing the performance of carbonized coal in various 6 applications. 7  8  9 Fig. 2 FTIR spectrum of carbonized-coal samples at different temperatures 10 Furthermore, Raman spectroscopy was used to assess the degree of structural disorder and 11 graphitization in the carbonized coal samples. Fig. 3(a) presents the Raman spectra of coal samples 12 subjected to carbonization at various temperatures. The spectra exhibit two prominent broad and 13 overlapping peaks centered at 1358 and 1585 cm⁻¹, corresponding to the D and G bands of 14 disordered graphite, respectively. The D band at 1358 cm⁻¹ is indicative of defects and disorder 15 within the graphite structure, primarily arising from the breathing modes of sp² atoms in aromatic 16 rings. In contrast, the G band at 1585 cm⁻¹ is associated with the in-plane vibrations of sp²-bonded 17 carbon atoms in graphite-like materials [44]. These observations confirm the presence of 18 significant structural disorder in the carbonized coal samples, consistent with the typical Raman 19 spectral characteristics of disordered and amorphous carbon materials [44]. The Raman results 20 complement the FTIR findings, highlighting the structural transformations observed in both 21 analyses. The reduction in aliphatic and oxygen-containing functional groups in the FTIR spectra 22 corresponds to a decrease in the Raman spectral intensity with increasing temperature. Together, 1 these results indicate the removal of structural defects and oxygenated components, along with the 2 progressive formation of more ordered aromatic carbon structures during carbonization. In 3 addition, the relatively broad D and G bands across all carbonized coal samples suggest a small 4 carbon crystallite size [39]. To gain deeper insights into the structure of carbonized coal, Raman 5 spectrum deconvolution was employed. The overlap of the D and G bands tends to obscure critical 6 structural details [45]. Among the available deconvolution techniques, the method combining 7 Lorentzian and Gaussian functions, as proposed by Sadezky et al. [46] has been identified as the 8 most suitable for these samples. The results of this deconvolution are presented in Fig. 3(b-d), 9 with the five deconvoluted bands and their assignments succinctly summarized in Table 2. 10  11 Fig. 3 (a) Raman spectrum of carbonized coal; deconvolution of (b) CBC-500, (c) CBC-700, and 12 (d) CBC-900 13  1 Table 2. The five bands resulting from the fitting deconvolution in the Raman spectra [44, 46–49] 2 Band name Band position (cm-1) Description G 1565 (CBC-500) 1565 (CBC-700) 1567 (CBC-900) Ideal graphitic lattice (E2g symmetry) D1 1374 (CBC-500) 1355 (CBC-700) 1361 (CBC-900) Disordered graphitic lattice, (graphene layer edges, A1g symmetry) D2 1601 (CBC-500) 1600 (CBC-700) 1599 (CBC-900) Disordered graphitic lattice, (surface graphene layer edges, E2g symmetry) D3 1492 (CBC-500) 1500 (CBC-700) 1491 (CBC-900) Amorphous carbon D4 1289 (CBC-500) 1269 (CBC-700) 1292 (CBC-900) Disordered graphitic lattice, (A1g symmetry)  3 Based on the data presented in Table 2 and the amorphization trajectory discussed in the 4 previous study [44], the position of the G peak (1600-1510 cm-1) suggests that the CBC samples 5 exhibit structural characteristics intermediate between nanocrystalline graphite and amorphous 6 carbon. This observation indicates that the samples comprise a combination of highly disordered 7 carbon and partially ordered graphitic domains. As the carbonization temperature increases, the G 8 band wavenumber is relatively stable, with only minimal shift observed at 900°C while the D1 9 band tends to shift toward lower wavenumbers. This shift indicates a gradual transition from 10 amorphous to more ordered graphitic structures. If the carbonization temperature continues to 11 increase towards the graphitization temperature (1700-2900°C), it is expected that the G band will 12 approach a band position of 1580 cm⁻¹ and the D1 band will reach approximately 1300 cm⁻¹, which 13 are characteristic of disordered graphitized carbon [50]. These trends underscore the influence of 14 temperature on the structural evolution of carbonized coal, highlighting the potential to tailor 15 material properties by controlling the carbonization process.  16 The area ratios of the D1 band to the G band (ID1/IG) and the G band to the total area (IG/IAll) 17 are critical metrics for assessing the degree of order within the carbon structure. Specifically, the 18 ID1/IG ratio is inversely related to the plane size of the carbon crystals, where a decrease in ID1/IG 19 signifies an increase in structural order[51, 52]. Conversely, the G peak representing the vibrational 1 mode of aromatic compounds, becomes more pronounced with a higher IG/IAll ratio, indicating a 2 more ordered carbon structure [51, 52]. As illustrated in Fig. 4, an increase in the carbonization 3 temperature from 500 to 700°C results in the ID1/IG value rising from 1.69 to 2.76. However, when 4 the temperature is further elevated to 900°C, the ID1/IG value decreases slightly to 2.55. 5 Simultaneously, the IG/IAll value decreases from 0.221 to 0.150 as the temperature rises from 500°C 6 to 700°C, but this trend reverses slightly with an increase to 0.155 at 900°C. In the range 7 temperature of 500 and 700°C, dehydrogenation of hydroaromatic compounds and the formation 8 of aromatic rings increase the aromatic content. Despite this, the structure remains highly 9 disordered and differs significantly from graphite crystals. The polycondensation process breaks 10 downside chains and produces small molecules that deposit on the coal surface. This deposition 11 introduces defects and amorphous structures, disrupting carbon regularity. As the temperature 12 rises, these defects and amorphous regions gradually transform into more ordered structures. It 13 reflects a balance between disorder and the development of graphite-like domains at higher 14 temperatures. The change of (ID3+ID4)/IG is regarded as crosslinking density related to the 15 formation of alkyl-aryl C-C. There is a fast increase in (ID3+ID4)/IG in the 500 - 700°C temperature 16 range due to the release of functional groups as small molecular entities, which promotes the 17 formation of alkyl-aryl C-C bonds. Meanwhile, the rise slows between 700 and 900°C because the 18 release of functional groups as tiny molecular substances reduces, impacting the creation of 19 (ID3+ID4)/IG. 20  21  22 Fig. 4 Evolution of structural parameters obtained from Raman spectroscopy characterization in 23 carbonized coal  24  1  2 Fig. 5 High resolution XPS spectra of C1s and N1s peaks obtained from (a,d) CBC-500, (b,d) 1 CBC-700, and (c,f) CBC-900, respectively. N-Q, N-5, and N-6 correspond to quaternary, pyrrolic, 2 and pyridinic configuration of nitrogen atoms bonded with carbon atoms 3 Table 3. Quantitative analysis of chemical bonds obtained by deconvolution of C1s XPS spectra 4 Sample C=C/C-C (at. %) C-OH/C=N (at. %) C-O / C-N (at. %) CBC-500 47.75 44.97 7.28 CBC-700 56.50 37.21 6.28 CBC-900 70.94 23.77 5.29  5 Table 4. Quantitative analysis of chemical bonds obtained by deconvolution of N1s XPS spectra 6 Sample Pyridinic / N-6 (at. %) Pyrrolic / N-5 (at. %) Quaternary / N-Q (at. %) CBC-500 32.54 45.76 21.69 CBC-700 38.63 25.95 35.42 CBC-900 22.68 23.36 53.96  7 XPS analysis was utilized to further study the surface chemical bonds of the as-synthesized 8 CBC samples. Full scan XPS of all samples shown in Fig. S1 exhibit 7 peaks, attributed to the C 9 1s, C KLL, O1s, O KLL, N 1s, Si 2s, and Si 2p peaks. It is evident that in addition to the carbon 10 and oxygen constituents commonly found in coal-derived samples, trace quantities of nitrogen and 11 silicon elements are also present across all samples. The detection of silicon is consistent with the 12 X-ray diffraction data, which reveals the presence of SiO2 in every sample analyzed. Then, the 13 quantification of each element is depicted in Table S1. It reveals that increasing carbonization 14 temperature from 500 to 700°C escalates the carbon content in the CBC sample while the O 15 element decreases, and N element remain stable. The decrease in O element is caused by 16 decomposition of oxygen-containing functional groups in coal sample owing to elevated 17 temperature [53]. However, further increasing of temperature to 900 °C induces decomposition of 18 carbon species to COx gas ultimately decreasing of carbon composition in CBC-900 [54]. The SiO2 19 remains stable at 900°C [55,56], however as N and C decompose, the relative concentrations of Si 20 and O increase. Similar concepts of interaction-guided performance tuning have also been 21 explored in ionic hydrogel systems [55]. 22 The high resolution of C 1s and N 1s peaks from XPS spectra were demonstrated in Fig. 5 23 (a) and (b). In C 1s spectrum, the C=C peak (~ 284 eV) increases gradually as temperature and is 24 followed by decreasing amount of nitrogen and oxygen-containing functional groups (C-OH, C-25 O, C-N, and C=N) (Table 3). It suggests the reduction of functional groups during carbonization 1 process leading to an increasing amount of aromatic carbon. This phenomenon is in accordance 2 with XRD result that higher carbonization temperature causes high aromaticity of CBC sample. 3 Besides, the deconvolution of N 1s peaks also reveals that the evolution of nitrogen species 4 contained in carbonized coal has also occurred as increasing carbonization temperature.  5 The CBC-500 is dominated by N-5 (pyrrolic) followed by N-6 (pyridinic) and N-Q 6 (quaternary) configuration. When the carbonization temperature increased to 700 °C, pyrrolic 7 content is decreased, while the pyridinic and graphitic content increased. This evolution involves 8 dehydrogenation, aromatization and carbon restructuration as an increase of heating temperature 9 [56]. Further escalation of temperature to 900 °C caused pyridinic nitrogen to be reduced as does 10 the pyrrolic which continuously decreased. Meanwhile, the quaternary continues to increase due 11 to transformation of pyridinic to graphitic in high temperature[56, 57]. The XPS results reveal that 12 heating temperature can easily tune the amount and configuration of oxygen and nitrogen species 13 in the carbonized coal.  14  15 Surface area and pore distribution analysis.  16 The porosity characteristic of carbonized coal were measured via N2 adsorption-17 desorption isotherm. The detailed parameter information is presented in Table 5.  18  19 Table 5.  the pore structure parameters of obtained by N2 (77.3 K) 20 Sample SBet (m2g-1) Vtot (cm3g-1) Davr (nm) CBC-500 0.087 0.001 30.4 CBC-700 4.277 0.008 7.14 CBC-900 2.541 0.006 4.55  21 As shown in table 5, SBET increases rapidly from 0.087 to 4.277 m3g-1. This indicates 22 that the release of small molecules at around 700°C provides an advantage for pore structure 23 formation. Subsequently, SBET decreases at 900°C due to the collapse of pores[58]. The pore size 24 distribution of all samples by using BJH method are shown in Fig.6. the CBC-500 shows pore size 25 distribution dominate around 30.4 nm.  26 Although 500°C is a low temperature, the pore size and analysis of pore size distribution 1 shows the largest pore size, which is around 30.4 nm, with the pore size distribution only around 2 that pore size. At 500°C, it is the initial decomposition process that releases volatile gases and 3 creates a large pore structure. However, because the temperature is not sufficient for significant 4 aromatization and graphitization, the pores remain open and do not experience narrowing. 5 Conversely, at higher temperatures (700 and 900), there is an increase in the degree of 6 aromatization, which causes the pores to narrow.  7  8 Fig. 6 Pore size distribution (a) CBC-500, (b) CBC-700 (c) CBC-900 9  10 Electrochemical Characteristics 11 The electrochemical characteristics of CBC samples were evaluated in a 3M KOH aqueous 12 electrolyte at ambient temperature using a three-electrode supercapacitor cell. Fig. 6(a) shows the 13 cyclic voltammetry (CV) results of CBC samples within the voltage range -0.2 to -1.0 V at a scan 1 rate of 10 mV/s. The CV curves exhibit capacitive behavior typical of electrical double-layer 2 capacitors. The area inside CV curve of CBC-700 is notably more rectangular and wider compared 3 to those of CBC-500 and CBC-900. This suggests that CBC-700 exhibits superior capacitive 4 behavior and charge propagation at the electrode surface, indicative of enhanced electric double-5 layer capacitance.  6  7 Fig. 7 Electrochemical performance of CBC prepared under various carbonization temperatures 8 including (a) CV data from -1.0 to -0.2 V vs. Ag/AgCl at scan rate of 10 mV/s, (b) GCD data from 9 -1.0 to -0.2 V vs. Ag/AgCl at 0.5 A/g, and (c) rate capability obtained from GCD at various current 10 densities 11 Galvanostatic charge-discharge tests were carried out on each sample to further assess their 12 electrochemical capabilities within the voltage range -0.2 V to -1.0 V, employing a current density 13 of 0.5 A/g. GCD curves as shown in Fig. 7 (b) showed non-ideal triangular shape profiles. The 14 specific capacitance (SC) of all samples was calculated from GCD curves by using the equation 2. 15 The corresponding capacitance from discharge is 1.98, 11.4, 73.6, and 27.7 F/g of Raw coal, CBC-1 500, CBC-700, and CBC-900, respectively. The CBC-700 possesses the highest specific 2 capacitance value, attributed to the contribution of the nitrogen moieties as shown in XPS results. 3 The presence of the nitrogen moieties improves the wettability of the electrode due to an elevated 4 hydrophilic polar site [59]. It allows electrochemical reactions to occur more efficiently due to 5 enhance electrode-electrolyte contact. In addition, among the different types of N, the pyridinic-N 6 plays a role in the contribution of additional pseudo capacitance from redox reactions, especially 7 in basic media [60]. Meanwhile, graphitic-N enhances capacitive performance by altering the 8 electronic structure of carbon to be more conductive [61]. On the other hand, pyrrolic-N reduces 9 the normalized capacitance while pyridinic-N increases the normalized capacitance [62]. Under 10 negative voltage, pyridinic-N and graphitic-N can store charge better than pyrrolic-N. On the 11 contrary, pyrrolic-N does so at positive potential [63]. Given that aqueous-based electrolytes are 12 used in this study, the electrode's water absorption capacity becomes critical, as it influences 13 wettability, ion transport, and electrochemical properties.  14 The specific capacitance values of CBC-500, CBC-700, and CBC-900 measured under 15 various current densities are displayed in Fig. 7 (c). The specific capacitance of CBC samples is 16 reduced as the current density increased from 0.5 A g−1 to 4 A g−1 as shown in Fig. 6 (c), which 17 was connected to the limited movement of electrolyte ions on the electrode material surface during 18 fast charging [64]. After the charge-discharge measurement at 4 A g-1, the specific capacitance of 19 these electrodes remains 11.58, 53.65, and 36.11 F/g compared to the specific capacitance at 0.5 20 A g-1. The carbonization process at 500°C still obtains a low aromaticity of coal and low interlayer 21 spacing which is not favorable for electron transfer which causes a low-rate capability of electrode. 22 When the temperature of carbonization increases to 700°C, the better capability of electrode can 23 be obtained as the higher remaining specific capacitance is still observed. It is attributed to 24 combination of several effects, such as high aromaticity, large interlayer distance, and good 25 wettability of carbonized coal owing to dominant N-pyridinic content as resulted from XRD and 26 XPS measurement [65]. These effects facilitate faster electrolyte ions and electrons during the 27 charge-discharge process, thus when high current densities are applied, they can induce higher 28 specific capacitance. Meanwhile, although further increasing carbonization temperature to 900°C 29 can improve the aromaticity degree of coal, structural changes in carbon at that temperature affect 30 surface area, pore size, and functional groups, all of which affect capacitance values. As 31 demonstrated in Table 5, and Figure 6 depicts the pore size distribution. The specific surface area 1 and pore size of CBC-900 are smaller than those of CBC-700, as is the number of pyridinic-N 2 functional groups decreases which is less favorable for ionic penetration into active sites for charge 3 storage. It induces the imbalance electron and ionic transport which results in lower capability of 4 CBC-900 than CBC-700 [65]. These results reveal that different heating temperatures of coal can 5 affect the electrochemical properties of supercapacitor electrodes that are related to the changing 6 of structural and chemical bonds during carbonization process.  7  8  9 Conclusion  10 This study investigated the effects of carbonization temperature on nitrogen 11 transformation, structural properties, and the specific capacitance performance of coal-derived 12 carbon materials. The results demonstrated that carbonization temperature significantly influences 13 coal’s molecular structure, with higher temperatures increasing aromaticity and altering nitrogen 14 configurations. At 500°C, pyrrolic nitrogen was the dominant functional group and it transformed 15 into pyridinic nitrogen at 700°C, which contributed to the highest specific capacitance of 73.6 F/g. 16 At 900°C, graphitic nitrogen became more prevalent, enhancing conductivity but reducing charge 17 storage performance. The findings highlight CBC-700 as the optimal material for supercapacitor 18 electrodes, emphasizing the role of pyridinic nitrogen in achieving superior capacitance.  19  20 Acknowledgment 21 This work was fully supported by the Ministry of Finance, Republic of Indonesia, through 22 the Endowment Fund for Education (LPDP) for doctoral scholarship program.  23 CRediT Authorship contribution statement 24 Mohamad Samsul Anrokhi: Conceptualized the study, conducted the experiments, performed data 25 collection and analysis, and drafted the initial manuscript. 26 Pipit Fitriani: Contributed data analysis and interpretation and assisted in manuscript preparation. 27 Oktaviardi Bityasmawan Abdillah: Supported data analysis and participated in manuscript 28 drafting. 29 Fitri Aulia Permatasari: Reviewed the manuscript and provided revisions. 30 Yoshiyuki Yamashita: Performed material characterization and contributed to data interpretation. 31 Fatimah Arofiaty Noor: Assisted in supervision and manuscript review. 32 Ferry Iskandar: Supervised the research, guided the methodology, and oversaw the overall project 1 execution. 2 Conflicts of interest  3 The authors declare that they have no conflicts of interest. 4 Data and code availability  5 Research data are not shared. 6 Supplementary information  7 Supplementary information associated with this article is provided. 8 Ethical approval  9  Not applicable.   10  11 References  12 1.  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