How-to, Tips and Articles: Coal Tar Three-dimensional Interconnected Sheet-like Porous Carbons for High-performance Supercapacitor

ABSTRACT

Three-dimensional (3D) porous carbon materials made of two-dimensional carbon sheets possess great potential as high-performance electrode materials for supercapacitors. In this work, 3D interconnected sheet-like porous carbons (ISPCs) are constructed from cheap coal tar for the ﬁrst time by a conﬁned ionic liquid soft-template strategy coupled with in-situ KOH activation. The as-made 3D ISPCs are composed of thin carbon nanosheets with abundant short pores, possessing a high speciﬁc surface area up to 1593 m2 g−1. Due to these synergistic structural characteristics, ISPCs as supercapacitor electrode measured in two-electrode conﬁguration exhibit an outstanding electrochemical performance with a high speciﬁc capacitance (314F g−1 at 0.05 A g−1), good rate capability (195F g−1 at 100 A g−1) and superior cycle stability (97.1% capacitance retention after 10,000 cycles at 5 A g−1) in KOH electrolyte. This work paves a novel way for the production of high-performance ISPCs from carbon-rich liquid industrial by-products for energy storage devices.

1. Introduction

Carbon-based supercapacitor as one kind of important energy storage devices has been widely applied in hybrid electric vehicles, industrial power devices and consumer electronics by virtue of its high power density, short charging time and outstanding cycle stability [1–5]. The aforementioned advantages mainly depend on the electrode materials of carbon-based supercapacitors. To date, activated carbons are the main electrode materials for commercial carbon-based supercapacitors because they are low cost, environmental friendliness and commercial availability [6]. However, some long and tortuous pores in commercial activated carbons are not accessible to the electrolyte ions, leading to poor rate capability and low capacitance [7,8]. To solve this problem, carbon materials with various structures, such as carbon nanotube [9], carbon ﬁber [10], graphene [11], carbon nanosheet [12] and carbon nanosphere [13], have been investigated to replace commercial activated carbons for supercapacitors. Among them, graphene and carbon nanosheet as the two-dimensional (2D) carbon materials are highly desired as the potential candidates due to their short pore length and high conductivity for fast ion/electron transfer, leading to enhanced rate performance [14,15]. However, 2D carbon materials are easy to restack owing to the strong Van der Waals force between adjacent carbon sheets, which causes the decrease of the ion-accessible surface area, further resulting in low capacitance thereafter [16,17]. Recently, three-dimensional (3D) porous carbon materials have been designed successfully as high-performance electrode materials for su- percapacitors because they possess 3D framework to avoid the self-restacking. For example, Zhao and co-workers developed an in-situ porous Cu template method to prepare 3D few-layer graphene-like carbons, presenting a high speciﬁc capacitance of 231 F g−1 at 1 A g−1 [18]. Shao and co-workers demonstrated that 3D hierarchical porous graphene ﬁlms can be fabricated by ﬁltration assembly of partially reduced graphene oxide and a subsequent freeze-casting process, exhibiting an extreme high speciﬁc capacitance of 284.2 F g−1 at 1 A g−1 [19]. Therefore, 3D porous carbon materials are the promising candidates to improve the overall performance of supercapacitors.

Coal tar is a very cheap and abundant by-product obtained in the production of metallurgical coke, possessing various kinds of polycyclic aromatic hydrocarbons with viscous and thermoplastic characteristics [20]. In the heat-treatment process, these aromatic hydrocarbons can be polymerized easily to form large polymer ﬁlms in the template-conﬁned nanospace [21], and be further transformed into 3D carbon materials made of 2D carbon sheets by tuning the polymerization direction of these aromatic hydrocarbons. In other word, the low-cost coal tar is a promising carbon precursor for the preparation of high value-added 3D carbon materials.

Herein, for the ﬁrst time, we present a facile strategy for the synthesis of 3D interconnected sheet-like porous carbons (ISPCs) from coal tar using 1-butyl-3-methylimidazolium tetraﬂuoroborate (BMIMBF4) ionic liquid (IL) as conﬁned soft template coupled with in- situ KOH activation. Firstly, the aromatic hydrocarbons in coal tar are dispersed in the narrow spaces formed by BMIMBF4 IL. Secondly, these aromatic hydrocarbons are polymerized into 3D interconnected polymer ﬁlms in the conﬁned spaces in the heat-treatment process. Lastly, the polymer ﬁlms are activated and carbonized at elevated temperature, leading to 3D ISPCs. The as-prepared ISPCs feature a unique interconnected sheet-like architecture with abundant short pores and high speciﬁc surface area. Such a 3D interconnected framework avoids the restacking of carbon sheets and is helpful to speed up charge transfer. Moreover, the abundant short pores favor fast ion transport while the high speciﬁc surface area provides more active sites for ion adsorption. Beneﬁting from these synergistic structural characteristics, ISPCs as the electrode materials for supercapacitors exhibit high speciﬁc capacitance, good rate capability and superior cycle stability. As a result, the high-performance ISPCs have a bright prospect as the electrode materials of supercapacitors.

2. Experimental

2.1. Preparation of ISPCs

Coal tar was obtained from Maanshan Iron & Steel Co. Ltd. (China); BMIMBF4 IL was purchased from Dibai Co. Ltd. (Shanghai, China), and KOH was purchased from Sinopharm Co. Ltd. (China). In a typical case, 3.0 g coal tar and 3.0 g BMIMBF₄ IL were mixed by continuously stirring for 30 min, followed by being added 12.0 g KOH powder and stirred for 10 min. Then, the resulting mixture was heated in a horizontal tubular furnace. In 40 mL min−1 ﬂowing argon atmosphere, the mixture was ﬁrstly heated to 300 °C at 5 °C min−1 for 30 min, followed by being heated to 800 °C at 5 °C min−1 for 1 h. Next, the product was soaked in 2 M HCl for 12 h and subsequently washed by distilled water repeatedly to remove the impurities. Last, the sample was obtained after being dried at 110 °C for 12 h in air. The as-made sample is termed as ISPCn, where the subscript n represents the mass ratio of KOH to coal tar. For comparison, with other conditions being constant, the sample prepared from 3.0 g coal tar and 12.0 g KOH at 800 °C in argon atmosphere without the addition of BMIMBF₄ IL is named as PC₄.

2.2. Characterization

Field emission scanning electron microscopy (FESEM, NanoSEM 430, USA) and transmission electron microscopy (TEM, JEOL-2100, Japan) was employed to investigate the morphology of ISPCs. Nitrogen adsorption–desorption isotherms were collected at −196 °C on an Autosorb-IQ system (Quantachrome, USA). The speciﬁc surface area (SBET) was calculated by the Brunauer-Emmett-Teller (BET) method. The pore diameter was analyzed from the adsorption branches of the isotherms using the density functional theory (DFT) method. X-ray diﬀraction was tested on an X-ray diﬀractometer (XRD, Ultima IV, Japan) equipped with graphite-monochromatized Cu-Kα radiation (λ = 1.54178 Å). Raman spectra were obtained on a Raman spectro- scopy (JYLab-Raman HR800) with a laser wavelength of 532 nm. X-ray photoelectron spectra were recorded on an X-ray photoelectron spec- troscopy (XPS, Thermo ESCALAB250, USA).

2.3. Fabrication of ISPC electrodes

Firstly, ISPCs and polytetraﬂuoroethylene at mass ratio of 9:1 were mixed together in deionized water. Secondly, the mixture was rolled and then made into some circular ﬁlms with diameter of 12 mm (ca. 0.10 mm in thickness). Thirdly, these ﬁlms were dried at 110 °C for 2 h in vacuum oven, followed by being pressed onto nickel foams at 20 MPa for 30 s to obtain the electrodes. Before being assembled in supercapacitors, the electrodes were soaked in 6 M KOH electrolyte for 1 h under vacuum. Finally, the coin-shape supercapacitor (two-electrode conﬁguration) was assembled using two similar electrodes separated by a polypropylene membrane. The mass loading on ISPC₃, ISPC₄ and ISPC₅ electrode is 3.27, 3.01 and 2.26 mg cm−2, respectively, and the corresponding packing density is ca. 0.327, 0.301 and 0.226 g cm−3. Please see Supplementary Materials for the calculation instance of the corresponding packing density.

2.4. Electrochemical measurement

The cyclic voltammetry (CV) test was carried out on a CHI 760C electrochemical workstation (Shanghai Chenhua, China). The galvanostatic charge-discharge (GCD) measurement was performed on a supercapacitance test system (SCTs, Arbin Instruments, USA) and the electrochemical impedance spectroscopy (EIS) test was carried out on a Solartron impedance analyzer (Solartron Analytical, SI 1260, UK). The gravimetric capacitance of the single ISPC electrode (Cg, F g−1) was obtained from the galvanostatic discharge proﬁle according to Cg = 4I/ [m(ΔV/Δt)], where I (A) is the discharge current, m (g) is the ISPC mass in two electrodes, and ΔV/Δt (V s−1) is the average slope of discharge plot after IR drop. The volumetric capacitance of the single ISPC electrode (Cv, F cm−3) was obtained based on Cv = Cgρ, where ρ is the packing density of ISPC electrode. The energy density (E, Wh kg−1) and average power density (P, W kg−1) of the supercapacitors (based on the ISPC mass in two electrodes) were obtained from E = CgV2/28.8 and P = E/Δt, where V (V) refers to the discharge voltage after IR drop, and Δt (h) stands for the discharge time.

Fig. 1. Schematic illustration for the synthesis process of ISPCs.

3. Results and discussion

The synthesis process of ISPCs is illustrated in Fig. 1. Firstly, coal tar is dispersed in BMIMBF₄ IL by stirring. Then, KOH particles are added into the mixture to obtain reactants. Secondly, the reactants are heated in Ar atmosphere. At the relatively low temperature, the aromatic molecules in coal tar begin to polymerize to form interconnected sheet-like polymers. Subsequently, the BMIMBF₄ IL begins to decompose at ca. 350 °C and decomposes completely at ca. 500 °C, please see the weight retention of BMIMBF₄ IL versus temperature in Fig. S1 (Supplementary Materials). In addition, at elevated temperature, KOH starts to react with the polymers to make pores. Finally, the ISPCs are obtained after being carbonized at 800 °C and the removal of impurities.

The morphology of ISPCs was investigated by FESEM and TEM techniques. The FESEM images indicate that ISPC₄ possesses a homogeneous sheet-like structure (Fig. 2a) and the carbon sheets are interconnected (Fig. 2b). Moreover, the FESEM images of ISPC₃ and ISPC₅ also present the interconnected sheet-like characteristic (Fig. S2a and b, Supplementary Materials). The TEM image of ISPC₄ reﬂects the thin sheet-like characteristic (Fig. 2c). The high-resolution TEM (HRTEM) image of ISPC₄ presents that the thickness of carbon sheet in ISPC₄ is ca. 5.5 nm (Fig. 2d). Similarly, the TEM images also demonstrate that ISPC₃ and ISPC₅ are made of thin carbon sheets (Fig. S3a–d, Supplementary Materials). Fig. 2e and f is the FESEM image and TEM image of PC₄, respectively. It is clearly seen that PC₄ is relatively thick bulky porous carbons. Compared the morphology of PC₄ with that of ISPCs, the difference between them demonstrates that BMIMBF₄ IL functions as a conﬁned soft template to promote the formation of the as-made ISPCs. To further have an insight into the architectural features of ISPCs,

N2 adsorption/desorption, XRD, Raman and XPS techniques were employed, the corresponding results are shown in Fig. 3. The three I-type isotherms in Fig. 3a indicate dominated micropores in ISPCs. In addition, small hysteresis loops in all the isotherms demonstrate the existence of a few mesopores in ISPCs. Fig. 3b is the pore diameter distributions of ISPCs. The mesopore diameter of ISPC₃ and ISPC₄ mainly ranges from ca. 2.0–3.5 nm while that of ISPC5 ranges from ca. 2.0–4.4 nm. For ISPC₅, the enlarged mesopores are ascribed to extra activation of increased KOH dosage to some small mesopores (ca. 2.0–3.5 nm). Correspondingly, increased KOH dosage also breaks the as-formed macropores in ISPC₄ and ISPC₅, leading to decreased amount of macropores in ISPC₄ and ISPC₅ compared with ISPC₃ (Fig. S4, Supplementary Materials). The pore structure parameters and yields of ISPCs are listed in Table 1. The SBET and Vt of ISPCs increase accordingly in the order of ISPC₃ (1215 m2 g−1, 0.65 cm3 g−1) < ISPC₄ (1593 m2 g−1, 0.85 cm3 g−1) < ISPC₅ (1667 m2 g−1, 0.93 cm3 g−1) due to the increase of KOH dosage. Based on the above analysis results, it can be found that the pore structure of ISPCs can be tuned by changing the dosage of KOH. The XRD results of ISPCs are presented in Fig. 3c. The broad peaks at ca. 25° and 43° are indicative of amorphous characteristic of ISPCs [22]. Meanwhile, no other peaks are observed in the three samples, proving the complete removal of potassium-containing compounds in ISPCs. The amorphous property of ISPCs is also analyzed by Raman spectra, as shown in Fig. 3d. The D band of the three spectra at ca. 1352 cm−1 represents the defective graphitic structures and disordered carbons, while the G band at ca. 1585 cm−1 refers to the bond stretching of sp2-hybridized carbons [23]. The integration area ratio of ISPCs (AD/AG) is 2.95, 2.97 and 3.02 for ISPC₃, ISPC₄ and ISPC₅, respectively, meaning the low graphitization degree of ISPCs [24]. Moreover, the AD/AG ratio of ISPC₅ is the highest among the three samples because of that the highest activation degree of KOH results in the maximum lattice defects in ISPC₅. The surface chemical nature of ISPCs was detected by XPS technique. Three kinds of elements (carbon, nitrogen and oxygen) exist on the surface of the samples (Fig. 3e). The detailed element contents are listed in Table S1 (Supplementary Materials). A small amount of nitrogen element in ISPCs (less than 2%) is derived from coal tar, please see the elemental analysis results of coal tar in Table S2 and that of ISPCs in Table S3 (Supplementary Materials). The oxygen-containing functional group for ISPC₄ is analyzed (Fig. 3f). Clearly, two peaks at 532.2 eV and 533.2 eV correspond to C==O and C==O, respectively [25]. The O1s spectra of the other samples are presented in Fig. S5 (Supplementary Materials). These oxygen-containing functional groups can not only enhance the surface wettability and accessible electroactive surface area, but also provide capacitance. [26,27].

Fig. 2. (a, b) FESEM images of ISPC₄; (c, d) TEM images of ISPC₄; (e) FESEM image of PC₄; (f) TEM image of PC₄.

Thanks to the unique morphology and structure, the as-synthesized ISPCs are expected to exhibit excellent electrochemical performances as electrode materials for supercapacitors. Thus, the capacitive behaviors of ISPCs were investigated in two-electrode supercapacitors using 6 M KOH aqueous solution as electrolyte. For comparison, the capacitive behavior of PC₄ was also studied under the same conditions. Fig. 4a presents the CV curves of ISPC supercapacitors at the scan rate of 100 mV s−1. It can be clearly observed that all the three curves exhibit a symmetrical rectangle-like shape, implying the ideal electrical double- layer behaviors of ISPC supercapacitors [28]. As the increase of the scan rate from 10 to 500 mV s−1, the CV curve of ISPC₄ supercapacitor shown in Fig. 4b still remains a rectangular shape while that of PC₄ supercapacitors becomes a fusiform shape (Fig. S6a, Supplementary Materials), demonstrating that ISPC₄ supercapacitor has an excellent rate capability in comparison to PC₄ supercapacitor. The speciﬁc capacitances of the ISPC electrode at diﬀerent scan rates are listed in Table S4 (Supplementary Materials). Obviously, ISPC₄ electrode possesses the biggest capacitance among the three samples at all the scan rates. The electrochemical performances of ISPC and PC₄ supercapacitors were also evaluated by GCD tests. The three symmetrical

GCD curves measured at 1.2 A g−1 exhibit equicrural triangle shape (Fig. 4c), indicating the ideal capacitive behavior and reversible electrochemical behavior of ISPCs [29,30], which is consistent with the analysis result of the CV curves in Fig. 4a. Accordingly, the IR drop of ISPC₃, ISPC₄ and ISPC₅ supercapacitors is only 0.0055 V, 0.0037 V and 0.0041 V, respectively, revealing the very small internal resistance of ISPCs. The GCD curves of ISPC supercapacitors (Fig. 4d) show that ISPC₄ possesses the smallest IR drop among the three samples at the current density of 100 A g−1. In addition, the IR drops of ISPCs at different current density are listed in Table S5 (Supplementary Materials). It can be found that the IR drops of ISPC₄ are very small, indicating the ISPC₄ possesses very low internal resistance. The gravimetric speciﬁc capacitances of ISPC electrodes at the current densities from 0.05 to 100 A g−1 calculated by GCD curves are presented in Fig. 4e. As shown, the gravimetric speciﬁc capacitance of ISPC₃, ISPC₄ and ISPC₅ electrode is 204 F g−1, 314 F g−1, and 285 F g−1 at 0.05 A g−1, respectively. With the increase of the current density up to 100 A g−1, the corresponding gravimetric speciﬁc capacitance only drops to 119 F g−1, 195 F g−11 and 158 F g−1. It is worth noting that the gravimetric capacitance of ISPC₄ electrode exceeds those in literature, as shown in Table 2 [16,30–42].

Fig. 3. (a) Nitrogen adsorption-desorption isotherms, (b) pore diameter distributions, (c) XRD patterns, (d) Raman spectra and (e) XPS survey spectra of ISPCs; (f) O1s spectrum of ISPC₄.

Table 1 The pore structure parameters and yields of ISPCs.

The volumetric speciﬁc capacitance of ISPC₃, ISPC₄ and ISPC₅ electrode is 67 F cm−3, 95 F cm−3 and 71 F cm−3 at 0.05 A g−1, and 39 F cm−3, 59 F cm−3 and 44 F cm−3 at 100 A g−1, respectively, as shown in Fig. S7a (Supplementary Materials). Remarkably, the speciﬁc capacitances of ISPCs at low current densities decrease fast. In this work, ISPCs possess abundant micropores. At lower current densities, micropores can be fully utilized for charge storage. With the increase of the current density, the utilization of micropores will be greatly reduced [18]. Thus, the speciﬁc capacitances of ISPCs still decrease very fast at low current densities. In addition, although the speciﬁc surface area of ISPC₄ (1593 m2 g−1) is slightly lower than that of ISPC₅ (1667 m2 g−1), the capacitance of ISPC₄ is higher than that of ISPC₅ . This is because that ISPC₄ has the largest amount of micropores below 1 nm among the three samples (please see Fig. 3b), which are conducive to improve the capacitance [43].

Fig. 4. (a) CV curves of ISPC electrodes at the scan rate of 100 mV s−1; (b) CV curves of ISPC₄ electrodes at diﬀerent scan rates from 10 to 500 mV s−1; (c) GCD curves of ISPC electrodes at the current density of 1.2 A g−1; (d) GCD curves of ISPC electrodes at the current density of 100 A g−1; (e) gravimetric capacitances of ISPC electrode at diﬀerent current densities from 0.05 to 100 A g−1; (f) cycle stability of ISPC₄ electrodes at 5 A g−1, inset is the ﬁrst and last 5 cycles.

Moreover, the IR drops of ISPC₄ at diﬀerent current densities are almost the smallest among the three samples. Thus, the capacitance of ISPC₄ electrode is bigger than that of ISPC₅. The gravimetric speciﬁc capacitance of PC₄ electrode is 255 F g−1 at 0.05 A g−1 and 84 F g−1 at 100 A g−1, as shown in Fig. S6b. Obviously, PC₄ electrode reveals a poor rate capability in comparison to ISPC₄, which is well consistent with the results of CV curves in Fig. S6a and Fig. 4b, respectively. As known, at a very small current density, the electrolyte ions have enough time to migrate into micropores [44]. Consequently, more inner pores in porous carbon are utilized for ion adsorption/desorption, leading to high capacitance. However, at a big current density, some micropores are jammed as a consequence of the retardation movement of electrolyte ions in micropores, resulting in the decay of capacitance [45]. In this work, ISPC₄ possesses an interconnected thin sheet-like structure. It is well-known that thin carbon sheets with abundant short pores provide lots of short ion transport channels [14]. Thus, at a relatively high current density, the electrolyte ions in ISPC₄ still quickly pass into micropores without seriously clogging micropores. On the contrary, PC₄ is thickly granular porous carbons, which does not possess the same merits as ISPC₄.

Table 2 The Cg of diﬀerent carbon-based electrode materials in two-electrode supercapacitor.

Hence, PC₄ only exhibits high capacitance at low current densities while presents very low capacitance at high current densities because of the serious blocking of micropores by electrolyte ions. In a word, the rate capability of ISPC₄ is superior to that of PC₄ due to their diﬀerence in morphology. Furthermore, compared the rate capabilities of other ISPC samples (Fig. 4e) with that of PC₄ (Fig. S6b), it is also seen that the rate capabilities of ISPC₃ and ISPC₅ are better than that of PC₄, further demonstrating the superiority of ISPCs on rate capability. Besides, Fig. S7b presents the Ragone plots of ISPC electrodes. The energy density of ISPC₄ electrodes is 11 Wh kg−1 at 0.05 A g−1, which is also the highest among the three samples. In addition to the capacitance and rate cap- ability, the cycle stability also plays an important role on the evaluation of electrochemical performance for supercapacitor. The capacitance retention of ISPC₄ electrode calculated by GCD tests reaches 97.1% after 10,000 cycles at 5 A g−1 (Fig. 4f), revealing an outstanding cycle stability.

To further evaluate the electrochemical performances of ISPC supercapacitors, the EIS was obtained at the frequency ranging from 105 Hz to 10−2 Hz. Fig. 5a presents the Nyquist plots of ISPCs. It can be found that all the ISPC supercapacitors present a straight line paralleled to the Y axis at low-frequency part, indicative of the ideal capacitive behavior [15]. At high-frequency region, the small X-intercept and semicircle (inset in Fig. 5a) demonstrate that ISPC supercapacitors have very low intrinsic ohmic resistance (Rs) and charge transfer resistance (Rct), respectively [45]. Moreover, it is also seen that the Rs and Rct of ISPC₄ are the smallest among the three samples, demonstrating that ISPC₄ possesses excellent electrical conductivity and the fastest charge transfer speed among the three samples [24]. In addition, compared the Nyquist plot of PC₄ with that of ISPC₄ (Fig. S6c), it is obviously found that the Rs and Rct of PC₄ are higher than that of ISPC₄ . This demonstrates that the interconnected structure of ISPCs in our work helps to promote charge transfer [46]. Considering the relationship between the structure and the electrochemical properties of ISPCs, the schematic illustration of the transfer process for ion and electron in ISPCs is presented in Fig. 5b. As shown, on one hand, the interconnected sheets are helpful to improve the electron conduction, leading to enhanced capacitance. On the other hand, abundant short pores in ISPCs are beneﬁcial to the fast migration and adsorption of electrolyte ions, resulting in the excellent rate capability and high capacitance. Therefore, ISPCs are the promising electrode materials for high-performance supercapacitors.

4. Conclusions

In summary, ISPCs were synthesized by using low-cost coal tar as carbon precursor with BMIMBF₄ ionic liquid as conﬁned template coupled with in-situ KOH activation. The as-prepared ISPCs possess a 3D interconnected structure for good electron conduction and abundant short pores for ion adsorption and fast transport. Thus, ISPC electrode for supercapacitor exhibits high speciﬁc capacitance of 314 F g−1 at 0.05A g−1, good rate capability of 195 F g−1 at 100 A g−1 and excellent cycle stability with 97.1% initial capacitance after 10,000 charge-discharge cycles. This work opens up a feasible pathway for the synthesis of 3D sheet-like porous carbon materials for high-performance supercapacitors.

Fig. 5. (a) Nyquist plots of ISPC supercapacitors, inset is the magniﬁed plots at high-frequency range; (b) schematic illustration of the ion/electron transfer process in ISPCs.

Source: Xiaoyu Xiea, Xiaojun Hea,⁎, Hanfang Zhanga, Feng Weia, Nan Xiaob, Jieshan Qiuc,⁎
a School of Chemistry and Chemical Engineering, Anhui Key Lab of Coal Clean Conversion and Utilization Recycling, Ministry of Education, Anhui University of Technology
b Carbon Research Lab, State Key Lab of Fine Chemicals, Dalian University of Technology
c School of Chemical Engineering and Technology, Xi’an Jiaotong University

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Monday, March 9, 2020

Coal Tar Three-dimensional Interconnected Sheet-like Porous Carbons for High-performance Supercapacitor