1. Introduction
Black carbon (BC) is a solid particulate material produced by incomplete combustion of carbon-based fuels. It generally contains 80% mass or more of carbon atoms that are usually connected by sp
2 bonds. BC from marine diesel engines is estimated to account for as high as 1–2% of the world’s total BC generation [
1]. This is because the ship fuel is lower in quality than other fossil fuels used in combustion engines, and therefore more BC is emitted per unit of ship fuel consumed [
2].
While global warming is a growing concern worldwide, the Arctic climate is known to be warming up almost twice as fast as the rest of the world [
3,
4]. BC is responsible for half of the Arctic warming [
5], as it strongly absorbs incoming and reflected light irradiation [
6,
7] and accelerates melting of snow and ice when it is deposited on these surface [
8]. Further, as a result of the shrinking Arctic ice, the Arctic shipping route has begun to develop, and an increase in shipping through that region will lead to even more BC discharge/deposition, thus exacerbating the warming.
In 2005, the International Maritime Organization (IMO) adopted the International Convention for the Prevention of Marine Pollution by Maritime Pollution Prevention Commission (MARPOL 73/78) in order to limit the emission of various air pollutants (NOx, SOx, and volatile organic compounds (VOC)). The regulation requires that all ocean vessels around the world use fuels with a sulfur content below 0.5% by the year 2020 [
9]. To address such regulations, shipping companies have considered three approaches: (1) use of low-sulfur oil, (2) use of liquefied natural gas (LNG) fuel, and (3) installation of scrubbers. Option (1) is the most viable in the short term, but the operating cost is high in the long run. The adoption of LNG fuel entails a high financial investment to construct LNG facilities, since currently the LNG fuel is not convenient owing to insufficient supply facilities around the world [
10]. Moreover, LNG prices tend to fluctuate unpredictably, and the problem associated with methane emission remains. In contrast, usage of scrubbers involves a high cost of installation; however, they can be applied directly to existing ships. Thus, many shipping companies are using this approach because it allows the continued usage of high-sulfur oil [
11]. As the number of vessels equipped with these scrubbers increases, the amount of BC discharged from the vessel will be reduced.
Scrubbers are divided into two types depending on the operation method. The loop-type scrubber circulates the washing water used to remove air pollutants from the ship emission, while the open-type discharges it into the sea. At present, regulations related to the discharge of washing water are insufficient. Therefore, many ship owners prefer the open-type scrubber because they are relatively inexpensive and easy to operate, even though they may be restricted through future regulations. To ameliorate the considerable marine pollution caused by BC collected from ship scrubbers, in this study, we attempted to reuse BC as a valuable material in electrochemical applications.
First, we confirmed that the morphology of the collected BC particles is similar to that of carbon black. In general, carbon black is produced through incomplete combustion or pyrolysis of hydrocarbons such as petroleum or natural gas; it is characterized by chain aggregates at least 1 μm long formed by primary particles that are 10 to 30 nm in size [
12]. Owing to the nature of the starting material, carbon black generally contains more than 95% net carbon and a minimum amount of oxygen, hydrogen, and nitrogen. The particle size, agglomerate size, porosity, and surface chemical properties of carbon black can be controlled through various process parameters. Based on these unique properties, carbon black is often used as an additive to improve the physical, electrical, electrochemical and optical properties of materials. For example, it is used at a high volume fraction as a reinforcing and performance additive in rubber products. The excellent conductivity of carbon black also makes it an electrode material for energy storage media. In the printing industry, it is used as a viscosity control additive for pigments and optimum print quality. As an additive, it may improve the performance of other materials, including electrostatic charge control and ultraviolet shielding [
13,
14,
15,
16,
17,
18,
19,
20,
21].
Owing to the identical morphology, BC from ships could be used in the above applications. Among them, a very attractive application would be reusing BC, a pollutant generated from the power source that moves the ship (i.e., the combustion engine) as an energy material in batteries to move the ship. Importantly, crystallization through an additional heat treatment process can increase the conductivity of the waste BC and make it a better conductive material in the lithium ion battery (LIB). We confirmed that the heat-treated BC has significantly lower surface area compared to conventional commercial carbon black, and it could substantially improve the Coulombic efficiency of the anode material. In addition, BC is a waste material produced by ships, while commercial carbon black tends to consume hydrocarbon as feedstock. Therefore, BC can be very attractive in terms of cost.
3. Results
Figure 2a shows a TEM image of the soot collected from the economizer. The observed morphology confirms that the primary particles are spherical and approximately 30–50 nm in size, and that these particles are aggregated into chain structures. The high-level agglomerate structures are large and generally longer than 2 μm. Since such a structure can easily form an intergranular network over a wide scale, it is expected to provide excellent conduction between the active material particles in the battery electrode. The high-resolution (HR) TEM image of the untreated soot (
Figure 2b) shows an amorphous structure without crystalline domains. During the high-temperature heat treatment at 2700 °C, crystallization proceeds from the outside of the spherical particle to the interior part (
Figure 2c,d). Spherical primary particles have been converted into angular spheres, some of which contain hollow cores.
Meanwhile, in the HR-TEM image, the commercial carbon black (
Figure S1) shows a considerably different shape than that of waste soot (
Figure 2d). Based on electron microscopy studies, a number of models have been proposed for the structure of common commercial carbon black particles. Most of them describe the carbon particles as spherical arrays of quasi-microcrystalline domains, which are more disordered at the center as the size becomes smaller.
X-ray diffraction studies reveal that the commercial carbon black has a large
d-spacing (3.5–3.6 Å) compared to graphite carbon (3.354 Å). The quasi-graphite layers are oriented parallel to each other, but they are turbostratic. These microcrystalline domains have been reported to have only a few layers and a width of approximately 20–30 Å. TEM images of the actual conductive carbon black (
Figure S1) also support this interpretation. Thus, commercial carbon black can be said to have many microcrystalline domains that aggregate into the primary particles, which then form lumps. In contrast, waste soot particles show a smooth spherical shape without the fine domains. After heat treatment, their structure turns into an onion-like structure with a smoother surface, and such morphology changes the material properties significantly. In particular, in commercial carbon black, micropores exist between the microcrystalline domains, owing to which and it has a very large specific surface area, while the heat-treated waste soot is expected to contain no micropores owing to the long and continuous crystallization.
The difference in the specific surface area was confirmed by the BET analysis.
Figure 3 shows the N
2 adsorption isotherms of waste soot according to the heat treatment temperature along with that of SuperP (a commercial carbon black).
Table 4 shows the specific surface area calculated from the isotherms. The specific surface area of SuperP is high, above 50 m
2/g, and that of untreated waste soot is also relatively high, owing to its amorphous structure, as predicted by the TEM results. However, as the heat treatment progresses, the particle structure changes to a crystalline onion structure, and the final specific surface area is very low (less than 10 m
2/g). Such a low value indicates the absence of micropores on the surface, thus supporting the TEM observation.
To the best of our knowledge, materials with morphology identical to that of carbon black but with a specific surface area below 10 m2/g have not been reported to date. Therefore, this heat-treated soot was expected to possess unique properties. While the low specific surface area makes it unsuitable as a catalyst support, as it improves the Coulombic efficiency, the heat-treated soot is more advantageous compared to commercial conductive materials for use in LIBs. Therefore, we hypothesized that the heat-treated soot can be used as a conductive material in a LIB to increase the Coulombic efficiency of the entire electrode.
In the electrodes, various impurities in the waste soot may affect the electrochemical performance. Therefore, CHNS elemental analysis (
Table 5) and TGA thermogravimetric analysis (
Figure 4,
Table 6) were used to quantify these impurities.
The CHNS results confirmed that considerable amounts of sulfur and hydrogen existed in the soot before heat treatment. Since the soot is a combustion product of high-sulfur oil, it is likely to contain a large amount of sulfur. However, as the heat treatment temperature increased, the amount of sulfur and hydrogen decreased sharply and completely disappeared at 2700 °C.
As can be seen from the CHNS results, the carbon composition ratio increases as the heat treatment temperature increases. Thus, soot heat-treated at very high temperatures consists of only carbon. Because carbon is generally gasified by combining with oxygen of ~500–700 °C, it is oxidized during TGA analysis and disappears. Therefore, for soot heat-treated at 2700 °C, typically, no residue is observed during TGA analysis. On the other hand, the main component of the residue is a metal oxide [
2,
22,
23,
24]. The low-quality heavy fuel oil is used as a fuel oil for ships, which contains a significant amount of organometallic compounds (particularly, vanadium), and sometimes metal oxide catalysts are used to crack the crude oil. Meanwhile, these compounds do not decompose at low temperatures, but only at very high temperatures; therefore, the residues are considerably reduced in soot heat-treated at 2700 °C.
A galvanostatic charge/discharge experiment was performed to evaluate the electrochemical performance of the heat-treated soot as a conductive material in the anode.
Figure 5a shows the reversible capacity data according to the cycling of the electrode.
The soot annealed at 1400 °C showed the lowest reversible capacity, while that annealed at 2300 °C showed a capacity comparable to that of SuperP. Moreover, the first-cycle Coulombic efficiency shows that the soot treated at 2300 °C exceeds SuperP in efficiency (
Figure 5b). Since heat-treated soot has a much smaller specific surface area than SuperP, a lower amount of the solid electrolyte interphase (SEI) layer is formed on its surface, and consequently, the Coulombic efficiency of the entire electrode material is increased. Therefore, in order for the soot to surpass commercial carbon black in performance as a conductive material, the crystallization should be carried out at a temperature above 2000 °C.
Figure 5c compares the CV curves of the first cycle for the annealed soot samples, confirming the presence of residual impurities in a voltage range of 0.005 to 3 V. For the soot annealed at 1400 °C, redox peaks, which are presumed to be due to impurities in the carbon rather than electrolyte decomposition, appeared at 1.8 V. However, for soot annealed at 2700 °C, these impurity peaks disappeared. The first three cycles of the CV curves for the soot annealed at 2700 °C (
Figure 5d) confirm the formation of a stable SEI layer. The first peak located at 1.1 V can be assigned to the irreversible reduction of the electrolyte additive FEC. The second broader cathode peak at 0.25–1.1 V corresponds to the decomposition of EC/DEC and the formation of an SEI layer. Finally, a sharp peak indicating the insertion of Li ions is observed at 0.25 V or lower. After the first cycle, the cathode reduction peaks disappear, and CV curves in different cycles almost overlap without any apparent change in the peak current or peak potential. This indicates the excellent reversibility of the Li insertion and extraction reactions and the stability of the soot-based conductive material.
Impedance measurements were performed to examine the degradation of the soot-based conductive material after numerous cycles.
Figure 5e,f show the experimental data after the first and 50th cycles. Even after 50 cycles, it maintained the same value of resistance at high frequency, and so the soot-based conductive material between the graphite particles shows stable performance. The parametric values analyzed from EIS are summarized in
Table 7.
The C-rate capability of the soot-based conductive material is shown in
Figure 5g. The reversible capacity decreased as the C-rate increased from 0.1, 0.2, 0.5, 1, 2, to 5 C. The reversible capacity reduction rate at 0.2 C versus 2.0 C was 76%, indicating excellent C-rate capability. Finally, after cycling at high C-rates, the capacity gradually recovered when the C-rate was decreased: when the current rate returned to 0.1 C in the 55th cycle, the reversible capacity was 367 mAh/g. These results indicate that the heat-treated soot performs reliably at various current densities.
These measurements show that the heat-treated waste soot displays high Coulombic efficiency owing to its low specific surface area, with better electrochemical performance than that of commercial carbon black. Therefore, the heat-treated waste soot can function as a unique conductive material to increase the Coulombic efficiency of secondary batteries. Moreover, we demonstrated that soot emitted from ship engines, which is usually considered a pollutant, can be utilized as a superior electrode material for energy storage after a simple heat treatment. While increasing the amount of global cargo shipping would lead to increased generation of soot in the coming years, we have found a great way to reuse it.
Finally, we consider the reasons why waste soot can develop a carbon black structure, and why it grows into smooth spherical particles rather than small microcrystalline domains. This observation can be deduced from the difference between the combustion process in the ship engine and the commercial process of carbon black production (
Figure 6).
First, in the engine’s combustion chamber, pyrolysis of the fuel produces odd carbons such as methyl group (CH
3), propargyl (C
3H
3), and benzene (C
6H
6) ring by C
2H
2. The benzene rings fuse together to form polycyclic aromatic hydrocarbons (PAHs). The addition of C
2H
2 leads to much larger PAH molecules, which coalesce to form primary particles several nanometers in size [
25,
26,
27,
28,
29,
30]. At this time, it is unclear how the spheres are generated from large nuclei of carbon atoms/radicals, but one of the most widely proposed hypothesis is their nucleation from a pentagonal carbon ring following spiral shell growth [
31,
32,
33,
34]. This can be considered to process in four steps: (a) pentagonal nucleation, (b) growth of quasi-cubic shells, (c) formation of helical shell carbon particles, and (d) growth in size. On the other hand, the primary spherical particles formed through the above process are further grown by the HACA (hydrogen abstraction acetylene addition) reaction in the flame, and coagulation of the primary particles results in high-level structures that flow in the flame. Particularly, in a ship diesel engine, particles generated in the combustion chamber at high-temperature and high-pressure pass through a very long exhaust pipe at ~300 °C, and the temperature gradient therein causes soot coagulation by thermophoresis. In other words, in the exhaust pipe, aggregates formed in the combustion chamber become smooth spherical particles in a continuous agglomeration process at the high temperature. Upon collecting the soot and subjecting it to a crystallization process through high-temperature heat treatment, the particle surface became perfectly smooth with an onion structure, and the particles grew into smooth spherical agglomerate structures without micropores.
In contrast, carbon black often undergoes rapid temperature changes during its production. Specifically, a significant amount of quenching water is directly sprayed several times into the combustion chamber to control the temperature. In addition, quenching water is sprayed several times during the wet palletization, separation, and collection of carbon black. Thus, carbon black often undergoes rapid temperature changes during the growth process, which make it impossible for it to achieve continuous surface growth with PAH. As a result, during the condensation under decreasing temperature, the nanocrystals agglomerate into a number of nanodomain states instead. These nanodomains are presumed to form a porous structure on the surface instead of a perfect spherical structure (
Figure 6c).