Preliminary Study of Carbonized Bituminous Raw Coal for Supercapacitor Electrode

Bituminous grade coal is a promising and cost-effective raw material for supercapacitor electrode application. Carbonization plays a crucial role in transforming the organic components present in raw coal into a carbon-rich structure with an enhanced capacity for charge storage. In this research, we study carbonized bituminous coal (CBC) as a preliminary evaluation of its electrochemical properties. The carbonized bituminous coal was synthesized through a simple heat treatment at temperatures of 700 and 900°C under a continuous flow of argon gas for 2 hours. The structural behavior of carbonized bituminous coal was analyzed using several characterizations, including x-ray diffraction (XRD), Fourier transform infrared (FTIR), and Raman spectroscopy. Furthermore, the electrochemical properties were evaluated using cyclic voltammetry (CV) and galvanostatic charge-discharge in a 3 M electrolyte solution of potassium hydroxide (KOH) with a three-electrodes configuration. The specific capacitances of 49.75 F g−1 and 20.00 F g−1 at a current density of 0.5 A g−1 were achieved for the sample produced at the carbonization temperature of 700 and 900°C, respectively. These values are considerably higher than the specific capacitance of raw bituminous coal (0.687 F g−1) at 0.5 A g−1. This result may offer valuable insight for the further development of coal-based supercapacitor electrodes.


Introduction
Coal is the most accessible and widely used energy source in the world [1].The primary uses of raw coal are in the production of power plants and for the iron and steel industry [2] [3].Coal is categorized into four ranks, which are anthracite, bituminous, sub-bituminous, and lignite.These ranks exhibit differing degrees of coalification, resulting in varied chemical compositions, with a particular emphasis on their carbon content.Anthracite contains the highest carbon content (>92%), bituminous (75-90%), sub-bituminous (70-75%), and lignite (60-70%) [3].Due to its high carbon content, coal has great potential to serve as a precursor of carbon-based materials.However, coal cannot be directly converted into carbon-based material because it contains minerals and other impurities.Several pretreatment techniques are available to enhance the carbon content and eliminate impurities from coal, such as demineralization [4] and carbonization [5].Carbonization is the process of converting precursors into carbon-rich materials through heat treatment in an inert atmosphere [6].These precursors can be found in natural renewable resources, such as coal, wood, and coconut shells as well as manufactured precursors like polymers [7].Carbonization offers notable benefits by operating in an oxygen-free environment without the need for solvents, thereby it significantly minimizes air pollutants and makes it environmentally sustainable [8].In the carbonization process, volatile gases are emitted from high-viscosity organic materials resulting in the generation of bubbles, which in turn leads to foaming [9].This aspect of carbonization has a significant impact on the manufacture of activated carbon and porous carbon, with activators such as KOH and ZnCl being employed to facilitate the process [10] [11].These activators are used to accelerate the creation of porous structures by reacting with minerals or metals in coal to form soluble metal salts that can be dissolved by acid washing, resulting in porosity.The structural transition into a porous state results in an enhanced surface area, making it appropriate for supercapacitor electrode applications as studied by Kierzek et al. [12] and Lv et al. [13].
The utilization of carbonized coal as of now is mainly used for sodium-ion battery anodes [14].Despite being used in the aforementioned applications, carbonized coal also has the potential to be applied as a supercapacitor electrode due to the molecular structure becoming more organized as aromatic carbon structure increases and aliphatic carbon structure decreases [15] [16].However, the study of the electrochemical properties of carbonized coal as a supercapacitor electrode has not been reported previously.
Carbonized coal is currently being employed in sodium-ion battery applications [14].However, it possesses the potential for broader utilization as a supercapacitor electrode due to structural changes, marked by increased aromatic carbon content and decreased aliphatic carbon content, as these modifications occur [15] [16].Despite these intriguing structural alterations, there has been a notable absence of prior research investigating the electrochemical characteristics of carbonized coal when employed specifically as a supercapacitor electrode.This highlights the importance of studying and using carbonized coal to fully understand and harness the potential of carbonized coal in this application.
Therefore, this study focuses on the carbonization of bituminous coal to conduct an initial evaluation of its electrochemical characteristics when utilized as supercapacitor electrodes.The rationale for choosing high-volatile bituminous raw coal as our carbon source is grounded in its distinct structural features and chemical composition, making it a compelling candidate for such applications.The carbonization temperature used in this study varied from 700 to 900°C, as highlighted in Hashimoto et al.'s research examining ash content changes in bituminous coal under different temperatures and treatment conditions [17].

Material, synthesis, and characterizations
The coal used in this study was bituminous raw coal (RC) collected from PT. Bukit Asam Tbk. in Lampung, Sumatera.The composition of RC was characterized by proximate and ultimate analysis as can be seen in Table 1.Before carbonization, bituminous coal was grounded and sieved to obtain an average particle size of about 212 μm.RC powder was further subjected to heat treatment at temperatures of 700 and 900°C for 2 hours under an argon gas stream.The resulting samples were labeled as CBC-700 and CBC-900 for carbonization at temperatures of 700, and 900°C, respectively.
To analyze the crystalline structure and chemical properties, all samples were thoroughly characterized.The carbonized products were measured using X-ray diffraction (XRD) (Bruker D8 Advance) to determine the ash components formed from minerals within coal and carbon phase change in the samples.In addition, Fourier transforms infrared (FTIR) (Shimadzu, Irspirit) was employed to analyze molecular vibrations and functional groups.Raman Spectroscopy (MacroRam benchtop, λ=785 nm) supplemented existing approaches by providing detailed information on crystallinity and defects within the samples.

Electrochemical measurement
The fabrication of working electrodes was prepared by mixing RC or CBC powder 80 wt.%, carbon black (CB) 10 wt.%, polyvinylidinefloride (PVDF) 10 wt.%, and 1-methyl 2-pyrrolidinone (NMP, Pure Analysis) 10 wt.% as solvent (CB, PVDF, and NMP were purchased from MTI Corp).The formulated slurry was then layered onto nickel foam (1× 3 cm) and dried for twelve hours in a vacuum oven at 100°C.To acquire a better understanding of the electrochemical properties of the samples, the electrochemical characteristics were measured using a three-electrode setup in an electrochemical working station (Parstat 3000A).The counter and reference electrode were made of Pt wire and Ag/AgCl, respectively.A solution of potassium hydroxide (KOH) with a concentration of 3 M was utilized as an electrolyte.Cyclic voltammetry (CV) tests were conducted with a scan rate of 5 mV s -1 within the voltage range -0.2 and -1.0 V. From the CV curve, the specific capacitance (SC) of the samples was calculated using the equation [18]: where m represents the active mass put on the nickel foam substrate, s represents the scan rates mV s -1 , V represents the applied voltage window, and ∫   2  1 represents the absolute area under the CV curve Meanwhile, the SC value from galvanostatic charge-discharge (GCD) was determined using the equation below [18] : where I, m, ∆ dan ∆ are current, active mass, discharging time, and potential window, respectively.

Results and discussion
The chemical analysis of RC was conducted by using the proximate and ultimate analysis presented in Table 1.The proximate analysis revealed that the RC has low ash (3.72%) and fixed carbon (44.98%).The overall moisture content of RC, including both intrinsic and surface moisture, was found to be 11.73%, which is relatively high.The ultimate analysis shows carbon content in the RC is quite high (79.10%)and low sulfur content (0.46%).These characteristics indicate that the coal utilized in this research is a high-volatile bituminous b-type coal [3].absence of a well-defined (002) Miller index [19].The turbostratic structure appears in CBC samples at 2θ of 23°, representing a hybrid structure incorporating graphite and amorphous characteristics.The sharpness of this structure increases with higher temperatures, yet it has not reached the graphite structure due to insufficient temperature.The results suggest that elevating the temperature will enhance the coal's graphitic phase in the carbon structure.Furthermore, the CBC samples reveal a new peak in the range of 40~50° associated to the stacking aromatic layer and identified by the (100) Miller index.It is shown that the majority peaks are attributed to the minerals present in the coal, notably quartz (SiO 2 ) observed at 2θ of 26.5°, 36.5°,39.5°, 50.0°, 67.84°, and 59.74°.To provide clarity concerning the quartz peaks in the RC and CBC samples, an inset image emphasizing the 2θ range of approximately 36° to 39° is presented.The absent of quartz peaks in RC are now indicated at both angles for CBC because of the rising temperatures during carbonization.
The FTIR spectrum of all samples is shown in Figure 2 (a).The RC showed a broad peak at 3400 cm -1 at a high-frequency region ascribed to the stretching state of the O-H bond which indicates the existence of a hydroxyl group in the RC.The peaks at around 2913 cm -1 and 2850 cm -1 can be designated as asymmetric stretching of the -CH bond.These peaks did not appear in the CBC-700 and CBC-900 samples because the CH bond will break during heat treatment.However, there is a new peak in CBC-700 and CBC-900 approximately 3000 cm -1 referring to the stretching of =C-H bond.In addition, at CBC-700, two new peaks appear at around 2150 and 1900 cm -1 whereas at CBC-900 these peaks are insignificant.These peaks can be attributed to C=C and C-H, respectively.The peak at 1605 cm -1 comes from the aromatic sp 2 -C=C bond.The peak at roughly 1400 cm-1 corresponds to the C-O stretching vibration of the alkoxy group.Meanwhile, the peaks at low-frequency areas are associated vibration of Si-O-Si in quartz and koalinite.The Raman spectra of RC and CBC are displayed in Figure 2 (b).There are two prominent peaks which can be stated as the D peak and G peak at about 1300 cm -1 and 1590 cm -1 , respectively.The D peak is attributed to a disordered carbon band, while the G peak shows stretching of the sp 2 carbon atom in two forms of carbon structure in coal chains and rings [14].The ID/IG ratio represents the intensity defect to G ratio.The ID/IG ratio decreases from 2.59 to 2.20 for CBC-700 and CBC-900, respectively, which indicates the creation of a more interconnected hexacyclic carbon structure and a decrease in carbon structure defects with increasing temperature [14].The presence of defects may originate from the minerals contained in the coal, as Lan et al found that minerals in the coal do not facilitate the graphitization process, instead, they hinder it and result in defects [20].The CV curve of all samples at a scanning speed of 5 mV s -1 and a voltage range of -0.2 to -1.0 V are depicted in Figure 3 (a).The CV curve of CBC-700 has a greater area than the CV curves of RC and CBC 900, indicating the highest specific capacitance of CBC-700.A similar conclusion was reached from the GCD result of CBC in Figure 3(b).The GCD measurements were conducted using a current density of 0.5 A g -1 and a potential window of 0.8 V.The discharge time of CBC-700 was 79 s, this time was the longest in our samples.Meanwhile, discharge times for CBC-900 and RC were 32 s and 1.1 s, respectively.From the result of the GCD measurements, the SC values of the samples can be determined using Equation 2. The SC of RC, CBC-700, and CBC-900 were 0.68, 20.00, and 49.75 F g -1 , respectively.

Conclusions
We report the electrochemical properties of RC and carbonized CBC at temperatures 700 and 900°C.Analysis of XRD and FTIR revealed distinctive characteristics of RC and CBC.Wherein on CBC

Table 1 .
Proximate and ultimate analysis result of raw coalFigure 1 depicts the XRD patterns of RC and CBC samples.The broad diffraction peak at 2θ of 23° for RC sample is indicative of amorphous carbon structures, as suggested by the