Applied Science and Convergence Technology 2024; 33(1): 13-17
Published online January 30, 2024
Copyright © The Korean Vacuum Society.
aFaculty of Applied Energy System, Jeju National University, Jeju, 63243, Republic of Korea
bElectric Energy Research Center, Jeju National University, Jeju 63243, Republic of Korea
cDepartment of Nuclear and Energy Engineering, Jeju National University, Jeju 63243, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, distribution and reproduction in any medium without alteration, provided that the original work is properly cited.
Methane pyrolysis using plasma has recently attracted considerable attention. This method is eco-friendly and produces hydrogen and solid carbon black without producing carbon dioxide. However, it requires high energy to generate the plasma, making it economically ineffective. However, its economic feasibility can be secured using the by-products generated from methane pyrolysis as high-value materials. In this study, methane pyrolysis via thermal plasma and annealing was performed at 500−1,000 ∘C to improve the properties of the by-products. The synthesized carbon black was nano-sized with an amorphous structure and its performance as a conductive material in batteries was analyzed. As a result, the specific surface area and electrical conductivity were improved to 45.87 m2/g and 6.5 S/cm, respectively, when carbon black was annealed at 1,000 ∘C. Moreover, when used as a battery conductive material, the capacity was measured to be 320 mAh/g, which is comparable to that of Super-P, a commercial material. Finally, the annealing of carbon black by methane pyrolysis confirmed that its properties are similar to those of commercial carbon black and it could be applied as a conductive material for batteries.
Keywords: Thermal plasma, Methane, Carbon black, Annealing, Lithium-ion battery
With the increasing global energy demand and continued use of fossil fuels, substantial amounts of greenhouse gases are emitted, which increases global warming. Methane, a potent greenhouse gas that exacerbates global warming, has a global warming potential 21 times greater than that of carbon dioxide and is the second largest contributor to climate change after carbon dioxide [1,2]. Furthermore, methane has a short-term greenhouse effect that is approximately 80 times more potent than carbon dioxide, earning it the label of the ‘worst greenhouse gas’. Accordingly, methane emission regulations continue to be strengthened, and there exists a need to establish methods to utilize methane rather than simply suppressing its production and emission .
A prominent approach to utilize methane involves its conversion into necessary gases via methane reforming . Methane reforming can be categorized into steam and dry methods. Steam methane reforming produces hydrogen through the reaction of hydrocarbons with steam (H2O). This method primarily uses catalysts, such as transition metals with support, and operates at a reaction temperature ranging from 700−1,000 °C. Steam methane reforming offers the advantages of high hydrogen concentration and production yield; however, this process relies on fossil fuels and emits carbon dioxide [5–7]. On the other hand, dry methane reforming presents the advantage of utilizing carbon dioxide, which makes it environmentally friendly. Dry methane reforming involves the conversion of methane and carbon dioxide into hydrogen and carbon monoxide, an industrial raw material, through a chemical reaction. However, this method suffers from a lower efficiency than steam methane reforming. Accordingly, intensive studies have recently been conducted to overcome this drawback using plasma [8–10].
Plasma is often used in methane reforming at low or high temperatures . For low-temperature plasma, methane is easily decomposed with high efficiency; however, the amount of byproducts obtained is very small [12,13]. Conversely, high-temperature plasma, i.e., thermal plasma, has low efficiency but generates a large amount of hydrogen and carbon black. Carbon black by-products can be obtained via an environmentally friendly method without emitting carbon dioxide. However, despite these advantages, energy costs increase when thermal plasma is used; therefore, much research is being conducted to lower the energy costs [14–16].
A promising approach to compensate for these energy costs is to use carbon black obtained by methane pyrolysis as a commercial material [17,18]. Carbon black is commonly used as a reinforcing material in automobile tires and various rubber products, and it is also utilized as a conductive material in lithium-ion batteries . Currently, four prominent methods are used for producing carbon black: lamp black, channel/gas black, acetylene black, and furnace black [20,21]. However, 98 % of global carbon black is produced using the furnace black method, wherein carbon black is formed by combusting a carbon precursor, such as oil or gas, in a chamber. During this process, various harmful substances such as carbon dioxide are emitted . Consequently, if carbon black derived from plasma-based methane pyrolysis, which is simpler than the furnace black method, is utilized as a high -value-added material, it can lead to substantial energy cost savings .
In this study, hydrogen and carbon black were synthesized by methane pyrolysis using thermal plasma . Following the annealing of by products, their shapes, sizes, and electrical properties were analyzed using various techniques [25–27]. Furthermore, we used the synthesized carbon black as a conductive material to fabricate lithiumion batteries and evaluated their performance . Consequently, we confirmed the usefulness of the by-products by comparison with those of Super-P, a commercially available conductive material.
Figure 1(a) shows a schematic diagram of methane pyrolysis using thermal plasma. The system contains three plasma torches (DCTP30K, Plasnix), a power supply (DW-TP30K, Dawonsys) for each torch, a reactor chamber, a graphite liner, and a gas chromatograph (YL6500, Young In). The system combines three thermal plasma jets generated from three direct current (DC) thermal plasma torches at the center, enabling more effective thermal decomposition of methane through a longer, high-temperature flame than that generated from a conventional single DC thermal plasma torch. The internal graphite liner extends the high-temperature area, enabling methane to remain in the high-temperature region for a longer period .
The experimental conditions are summarized in Table I. All experiments were performed at atmospheric pressure, and each torch had a fixed input current of 100 A, resulting in a total output of 30 kW. To generate the plasma, 15 L/min of N2 was injected for each torch. The synthesized carbon black was annealed in a vertical furnace to enhance the characterization of the by-products. A schematic of the vertical furnace used for annealing is shown in Fig. 1(b). To prevent carbon black from oxidizing and reacting with other gases at high temperatures, Ar gas was introduced into the chamber at 1 L/min to establish an Ar atmosphere. The annealing conditions are summarized in Table II, with a fixed annealing time of 4 h at temperatures of 500 and 1,000 °C .
To examine the crystal structure of the synthesized carbon black, an X-ray diffractometer (XRD) (Empyrean, Malvern Panalytical) equipped with CuKα radiation (λ = 1.5406 Å) was employed, covering a range of 10−90∘. Additionally, a field-emission transmission electron microscope (FE-TEM) (Talos F200X G2) operating at an acceleration voltage of 200 kV was utilized to examine the nanostructure of the synthesized carbon black. To analyze the size distribution of the carbon black particles, the average size was determined by counting 100 particles in the FE-TEM images. A LamRam RH Evolution Raman spectrometer (Raman, Horiba Jobin-Yvon) with a laser wavelength of 514 nm was used to measure the Raman spectrum of the synthesized carbon black. Furthermore, the electrical conductivity and specific surface area of the synthesized carbon black were measured using a powder resistivity measurement system (Hantech, HPRM-FA2) and a Brunauer-Emmett-Teller (BET) specific surface area analyzer (Tristar-2 plus 3030), respectively.
Figure 2 outlines the battery manufacturing process using synthesized carbon black as the conductive material. The electrodes were created using an active material (graphite) and the binder carboxymethyl cellulose (CMC) in combination with carbon black as the conductive material, following the specified conditions. A slurry was prepared by mixing the active material, conductive material, and binder with distilled water at mass ratios of 60, 20, and 20 %, respectively. The resulting slurry was uniformly spread to a thickness of 25 µm on copper foil using a doctor blade and subsequently dried in a vacuum oven at 110 °C for 10 h. Subsequently, the dried electrode was cut into a circle with a diameter of 15 mm, and a CR2032 type coin cell was assembled in an Ar atmosphere glove box. The assembly process involved the use of a lithium chip and 1M LiPF6 (ethylene carbonate:diethyl carbonate:dimethyl carbonate = 1:1:1 in volume) as the electrolyte.
A battery testing system (BTS-4000, Newware) was employed to measure the capacity change in the assembled coin cell as a function of the number of charge and discharge cycles. This system evaluated the voltage range from 0.01−2.50 V at an input current of 50 mA/g.
Figure 3(a) shows the XRD analysis graph of carbon black based on the annealing conditions. The same crystal structure as that of the commercially available carbon black Super-P was observed under all conditions. However, the synthesized carbon black exhibited a more amorphous pattern than Super-P, with no significant change in the crystal structure as the annealing temperature increased. This can be observed in the first peak of carbon black, where amorphous peaks are widely dispersed. Consequently, the synthesized carbon black was determined to be relatively more amorphous than Super-P, as evidenced by its broader distribution.
The Raman spectra for the different annealing conditions are illustrated in Fig. 3(b). The D (1,345 cm−1) and G peaks (1,575 cm−1), commonly found in various carbon materials, are evident in the nontreated carbon black. Upon annealing, noticeable changes were observed in the intensities of the D and G peaks. The samples under the non-treated, 500, and 1,000 °C conditions exhibited ID/IG ratios of 0.87, 0.92, and 0.97, respectively. The intensity of the D peak increased as the annealing temperature was increased, and the ratio between the D and G peaks (ID/IG) also increased, approaching that of Super-P. We believe that the increase in the ratio (ID/IG) was caused by the removal of hydrocarbon impurities attached to carbon black during annealing.
The FE-TEM images obtained based on the annealing conditions are shown in Figs. 4(a)–(d). When examining the particle shape of Super-P, an aggregation of spherical particles with typical characteristics of carbon black was observed. In contrast, the non-treated carbon black appeared to combine with each other. This formation was assumed to occur when hydrocarbons were not fully converted into carbon during methane pyrolysis. However, upon annealing at 1,000 °C, it was observed that the particles tended to separate from each other and gradually assumed individual shapes. Furthermore, it is suggested that the particles composed of pure carbon alone agglomerate during annealing, causing a slight increase in particle size.
The size distribution of carbon black based on the annealing process is depicted in Fig. 4(e). In this size analysis, the diameters of 100 particles were measured and averaged from the FE-TEM images. The size of the non-treated carbon black particles was 28 nm, which is similar to that of Super-P. As the annealing temperature increased, the diameter increased, approaching 39 nm. This increase in size can be attributed to the slight crystallization and agglomeration induced by the annealing process.
Figure 5(a) shows the specific surface area determined by BET analysis results across various annealing conditions. The specific surface area of non-treated carbon black was half that of Super-P; however, with annealing, the specific surface area increased to 45.87 m2/g at 1,000 °C. This is because the area with the impurities in carbon black evaporated. Figure 5(b) shows the total pore volume of carbon black based on the BET analysis results. As the annealing temperature increased, the total pore volume increased from 0.07 to 0.09 cm3/g; these results indicate that the specific surface area and pore volume tend to increase together. Moreover, these findings suggest that annealing carbon black has a positive effect on battery performance by enhancing the ion transport pathway within carbon black.
Figure 5(c) shows the electrical conductivity of the carbon black based on the annealing conditions, including Super-P. Super-P exhibited a rapid increase in electrical conductivity, which was dependent on the density, measuring up to 30 S/cm. However, in the case of the non-treated carbon black, the electrical conductivity was nearly zero. We believe that numerous impurities hinder the flow of electricity when carbon black is compressed. Annealing at 500 °C did not yield a significant increase in electrical conductivity, similar to the non-treated case. Hence, it is determined that 500 °C is insufficient for complete impurity removal. However, when annealing at 1,000 °C, a significant increase in electrical conductivity was observed, reaching 6–7 S/cm. These results indicate that electrical mobility experiences a substantial boost when annealing at 1,000 °C or higher, enabling it to serve as a conductive material. Figure 5(d) displays the electrical conductivity at a density of 0.7 g/cc among the results of Fig. 5(c) to facilitate easier reference and better understanding. Annealing of the synthesized carbon black at 1,000 °C increased its electrical conductivity from 0.03 to 6.50 S/cm. This can be attributed to the enhanced purity of carbon black, as impurities residing between the carbon black particles are gradually eliminated during annealing, as indicated by the various analysis results. Although the electrical conductivity increased with higher annealing temperatures, it remained at one-third of that exhibited by Super-P.
Super-P, a commercial carbon black, was initially utilized to produce an anode material to compare with the synthesized carbon black. The anode material comprised an active material (graphite) and a binder, CMC, with the mass ratio of the active material, conductive material, and binder set at 60:20:20. The results of charge/discharge tests using Super-P are shown in Fig. 6(a). The current was held constant at 50 mA/g, and the capacity change with respect to the voltage was measured. The initial charging and discharging results were unstable and unreliable; therefore, the results are shown after 50−150 cycles. Consequently, a capacity of 340−350 mAh/g or higher, which is equivalent to the theoretical capacity of artificial graphite, was achieved. The charging and discharging results for each annealing condition are shown in Figs. 6(b)–(d). The Super-P and annealed carbon black exhibited similar voltage curves; however, lower annealing temperatures correspond to lower discharge-starting voltages. This phenomenon can be attributed to the instability within the battery, which was charged up to 2.5 V, but the voltage dropped rapidly during the resting period between charging and discharging, causing a voltage drop at the beginning of discharging. Based on these results, it is inferred that the internal stability of the battery increases with the annealing temperature.
A plot of the discharge capacity versus the cycle number is shown in Fig. 6(e). The initial capacities of the batteries using Super-P, nontreated carbon black, and carbon black treated at 500 and 1,000 °C were 343.0, 140.5, 155.0, and 320.1 mAh/g, respectively. It is evident that non-treated and treated carbon black at 500 °C exhibited similar capacity characteristics, with a capacity half of the theoretical capacity and increased from the initial level. Generally, the battery capacity decreases during charging and discharging cycles. However, when a ‘break-in’ effect occurs, the capacity increases. This is because the electrode structure creates an empty space in the initially inactive anode. Consequently, as lithium ions are inserted and desorbed during the charging and discharging processes, the electrolyte enters the electrode owing to volume changes, gradually increasing the capacity. The carbon black treated at 1,000 °C showed a capacity similar to that of Super-P. Most of the carbon black analysis results exhibited a trend similar to that of Super-P, supporting the conclusion that carbon black has a capacity akin to Super-P. However, a decrease in capacity was observed after 100 charge and discharge cycles. This decrease is believed to be caused by its relatively lower characteristics compared to Super-P and the influence of the remaining impurities. Nevertheless, the results show that the annealed carbon black can be applied as a conductive material.
In this study, carbon black was synthesized via methane pyrolysis using plasma. When using plasma, it is crucial to enhance the economic viability of the process by increasing the carbon black content. The synthesized carbon black was annealed at various temperatures and employed as a conductive material for lithium-ion batteries; its properties were subsequently compared with those of Super-P, a commercially available carbon black. Annealing was performed at 500 and 1,000 °C. As the annealing temperature of the carbon black increased, the ID/IG ratio also increased; this is because the impurities were removed and the structural defects increased. Although the characteristic aggregation of carbon black particles did not occur, it was observed that as the annealing temperature increased, the combined particles tended to separate from each other. Furthermore, as the annealing temperature increased, the specific surface area and total pore volume increased owing to the removal of impurities, thus enhancing ion mobility; this is likely to have a positive impact on the battery performance. The electrical conductivity increased when the synthesized carbon black underwent annealing, particularly when annealed at a temperature close to 1,000 °C. Carbon black annealed at 1,000 °C exhibited a capacity similar to that of Super-P, measuring 320.1 mAh/g. This increase can be attributed to the enhanced electrical conductivity and specific surface area. However, the capacity decreased after 100 charge and discharge cycles, leading to a deterioration in battery performance. This problem could be addressed by adjusting the annealing conditions to induce future improvements in carbon black properties, such as crystallinity, purity, and electrical conductivity.
This work was supported by the Technology Innovation Program (RS-2023-00265608, 1415188462, Development of production technology of high value-added chemical using by-product gas in Naphtha Cracker) funded By the Ministry of Trade Industry & Energy (MOTIE, Korea).
The authors declare no conflicts of interest.