Coal-Based Hierarchical Porous Carbon Synthesized with a Soluble

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Coal-Based Hierarchical Porous Carbon Synthesized with a Soluble Salt Self-Assembly-Assisted Method for High Performance Supercapacitors and Li-Ion Batteries Shasha Gao, Yakun Tang, Lei Wang, Lang Liu, Zhipeng Sun, Shan Wang, Hongyang Zhao, Ling Bing Kong, and Dianzeng Jia ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03421 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 4, 2018

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ACS Sustainable Chemistry & Engineering

Coal-Based Hierarchical Porous Carbon Synthesized with a Soluble Salt Self-Assembly-Assisted Method for High Performance Supercapacitors and Li-Ion Batteries Shasha Gao,†,‡ Yakun Tang,†,‡ Lei Wang,† Lang Liu,*,†,‡ Zhipeng Sun,‡ Shan Wang,†,‡ Hongyang Zhao,‡ Lingbing Kong,§ and Dianzeng Jia‡ †

School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi, 830046

Xinjiang, P. R. China. ‡

Key Laboratory of Energy Materials Chemistry, Ministry of Education; Key Laboratory of

Advanced Functional Materials, Autonomous Region; Institute of Applied Chemistry, Xinjiang University, Urumqi, 830046 Xinjiang, P. R. China. §

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang

Avenue, 639798, Singapore. KEYWORDS: Coal, Salt-templates, Hierarchical porous carbon, Supercapacitors, Energy storage devices

ACS Paragon Plus Environment

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ABSTRACT: Coal-based multiscale porous carbon materials (CPCs) have been triumphally fabricated through a friendly method, with NaCl, Na2CO3 and Na2SiO3 as structural templates, instead of alkali activation at a relatively low temperature. The method is through a freeze-drying and the calcination of the salts/acid treated coal, and combined washing to remove salts. The optimal product (CPCs-20) obtained by the above procedure displayed a high surface area of 1100 m2 g-1. As electrode materials of supercapacitors, CPCs-20 presented a specific capacitance of 304 F g-1 at 1 A g-1 and superior stability over 10000 cycles at 4 A g-1, owing to its hierarchical porosity with higher surface area, promoted diffusion of electrolyte and increased conductivity. It is also worth mentioning that a symmetric device based on the CPCs-20 can light up a light-emitting diode (LED) for 30 min. Furthermore, the CPCs-20 as the anode for Li-ion batteries exhibited a high reversible capacity of 450 mA h g-1 at 0.2 A g-1, excellent rate capability and outstanding cycling performance. Therefore, there are bright prospects of our CPCs as high performance electrode materials for energy storage applications. INTRODUCTION Convenient electronic products are extremely important for modern life, which requires fast charging devices and high efficiency chemical power sources.1-3 At present, supercapacitors (SCs) and lithium ion batteries (LIBs) have been widely studied as two typical energy storage devices.4-6 Nonetheless, the former has high power density but low energy density, the latter has high energy density but insufficient rate and long-cycle performance.7-9 Carbon materials have excellent thermal and electrical conductivity, chemical inertness and controllable surface area. Besides, they can be made into a variety of forms, such as powder, wire, fiber and so on. They are acknowledged to be the most potential electrode materials of both SCs and LIBs, thus attracting worldwide interests.10-12


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Among numerous forms, porous carbons (PCs) are commonly used, due to their abundance, high surface area, well-developed porosity, unique electron conductivity and so on.13,14 Traditionally, PCs are obtained through high-temperature calcination combined with physical and/or chemical activation. Although PCs with high surface area are readily obtain, their electrochemical performance is unsatisfied, due to the low utilization of the surfaces and narrow pore distributions.15-17 Therefore, hierarchical porous carbons (HPCs) with micro-, meso- and macropores have attracted much attention. As everyone knows, the surface area of materials is mainly contributed by micropores and micropores can offer more active sites. Furthermore, micropores match the dimensions of desolvated electrolyte ions. Generally, PCs with pores to be less than 2 nm could have maximum surface area in terms of normalized double-layer capacitance, thus leading to highest capacitance.18,19 Mesopores relatively store more ions and are conducive to the spread and transmission of electrolyte, so that the equivalent series resistance given rise to the process of desolvation could be eliminated, thus resulting in high rate capabilities.20 Macropores can be used as ion-buffering reservoirs and are favorable for mass transport of ions. As a result, HPCs have been explored an ideal electrode materials for SCs and LIBs.21-23 Generally, hierarchical porous carbons can be fabricated by using hard or soft template methods. For instance, mesoporous SiO2 or zeolites are often used to synthesize structurally tuned mesoporous carbons, then physically or chemically activation introduce micropores, leading to hierarchical structures. More recently, several new templates, such as nano-ZnO,24-26 nano-CaCO3,27 nano-MgO28 and nano-Fe2O3,29 which are dissolvable in HCl solution, have been employed to get mesostructured materials or HPCs. Meanwhile, soft templates, including a variety of ordered polymers formed by amphiphilic molecules and self-organization of polymers


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and biological macromolecules, have been used to create orderly mesoporous structure on the macroporous carbon matrix.30,31 But, hard template methods are usually complicated, because corrosive HF or volatile HCl must be used to remove the hard templates,32 resulting in environmental pollution. Soft template methods are limited by the low temperature treatment, which leads to low electrochemical performances. More importantly, it is quite costly and time consuming for the two methods, because the templates should be prepared ahead of time. Hence, it is still an enormous challenge to prepare HPCs with promising electrochemical performance by using an environmentally friendly and energy-efficient method. In the work, we displayed a one-step pyrolysis method, with water-soluble NaX salts as the structure-making templates instead of the alkali activation, to obtain coal-based hierarchical porous carbons with high surface area. In other words, coal can be converted into valuable porous carbon materials without activating agents (such as KOH, NaOH, H3PO4, etc). Because both the coal and NaX salts are very cheap, the preparation procedure is very friendly and high efficiency as well, our method is a promising approach to prepare hierarchical porous carbons. Moreover, the coal-based hierarchical porous carbons exhibited excellent rate capability and superior cycling performance in SCs and LIBs. EXPERIMENTAL Materials. Coal was produced in Kuche, Xinjiang, China. The proximate analysis of coal has been previously reported by our group.33 H2SO4 (98%), HNO3 (63%), HCl (37%), sodium hydroxide, sodium chloride, sodium carbonate and sodium silicate were analytical grade and used with no purification. Synthesis of CPCs. Coal was pretreated with an improved method previously reported.34 Firstly, coal was pulverized to a certain degree. Then, 10 g coal powder was carefully added into


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a flask containing 160 mL mixed acid (VHNO3/VH2SO4 = 1:3) in an ice bath, which was vigorously stirred overnight. The acid treated coal was carefully added into a certain amount deionized water and then filtrated, followed by washing with hydrochloric acid (6 M) and deionized water for several times. Finally, a dark brown powder was obtained after drying at 80oC for 12 h. To prepare CPCs, acid treated coal (2 g), NaCl (20 g), Na2CO3 (0.5 g) and Na2SiO3 (0.5 g) dissolved in 100 mL deionized water and neutralized with NaOH. After ultrasonication, it was frozen in liquid nitrogen and freeze-dried in vacuum, resulting in mixed powder, which was calcined at 650oC for 2 h in N2. After natural cooling, coal-based porous carbons (CPCs) were obtained after the calcined product washing with water. The sample was signed as CPCs-20. Similarly, CPCs10 [coal (2 g), NaCl (10 g), Na2CO3 (0.25 g) and Na2SiO3 (0.25 g)] and CPCs-40 [coal (2 g), NaCl (40 g), Na2CO3 (1 g) and Na2SiO3 (1 g)] were also synthesized for comparison. Moreover, bulk carbon (CPCs-0) was obtained without the presence of any salts. Besides, the CPCs obtained with single salt of NaCl, Na2CO3 and Na2SiO3 are denoted as CPCs-B, CPCs-M and CPCs-S, respectively (-B, -M, -S, representing big, medium and small). Characterization and Electrochemical Measurements are displayed in Supporting Information. RESULTS AND DISCUSSION Structure Characterization. Synthesis process of the CPCs is schematically exhibited in Fig. 1. In a specific procedure, the three kinds of salts were dissolved in water and acid-treated coal was added into the above solutions, which then was treated with freeze-drying. The salts were crystallized and coated with the acid-treated coal during the freeze-drying process. Afterwards, upon heating the above precursor at 650oC under N2, acid-treated coal was carbonized to generate carbon while crystal structures of the salts were unchanged. Finally, the calcined


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products were washed with water to remove the template of salts, resulting the formation of CPCs.

Fig. 1 Prepared procedure of the CPCs. Fig. S1 shows XRD patterns of the CPCs-20 before and after the calcination. It can be seen that they are very similar. All the peaks were attributed to NaCl (JCPDS No. 77-2064), which indicated crystal structure of the salts was unchanged during the calcination. Fig. 2 shows SEM and elemental mappings of calcined sample. As seen in Fig. 2a, some salt particles with different sizes were coated with the carbon layer, while others are homogeneously dispersed on surface of the carbon layers. Such configuration not only ensured dense stacking of the salt particles that promoted the filling of the carbon precursor, but also led to the formation of a crosslinked porous structure that would be beneficial to electrochemical performance of the final products. Furthermore, elemental mapping (Fig. 2b) shows strong signals of C, Cl, O, Na, and Si in the nanocomposites, thus proving the presence of the salts before and after calcination. The result was in a good agreement with the XRD.


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Fig. 2 (a) SEM image of the calcined sample before washing with water. (b) Elemental mappings illustrating distribution profiles of Cl, Na, C, O and Si. Due to their water solubility, NaX could be readily removed through simple water washing. Fig. 3a shows XRD patterns of CPCs. It can be seen that XRD patterns of CPCs are similar, with all the obvious peaks of NaCl to be absent. The diffused diffraction peaks at 22.5o and 43.5o are ascribed to (002) and (100) planes of graphitic carbon. The weak peaks suggested that CPCs possessed poor crystallinity and low degree of graphitization.35 Meanwhile, with increasing content of salts, the (002) plane of the graphitic carbon slightly shifted to high angles, implying that the layer distance was gradually decreased.36 From Fig. 3b, Raman spectrum displays two distinct peaks including D and G band at 1350 and 1580 cm-1, corresponding to the sp3 defects of disordered ones in the hexagonal graphitic layers and vibration of sp2-bonded carbon atoms, respectively.36 The ID/IG of CPCs-0, CPCs-10, CPCs-20 and CPCs-40 are 0.98, 0.95, 0.94 and 0.93, demonstrating the degree of graphitization was increased with increase of salts.


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Fig. 3 (a) XRD patterns and (b) Raman spectra of CPCs. Fig. 4 demonstrates representative SEM and TEM images of CPCs. As seen in Fig. 4a, only bulk carbon was obtained without the use of salts. The carbon layer is very thick and densely compact. With the addition of salts, porous network was formed and the carbon layer was much thinner than that in CPCs-0 (Fig. 4b). The sample CPCs-20 exhibited a typical loose and porous network structure, consisting of interconnected carbon nanosheets. Magnified SEM demonstrated that the carbon nanosheets had a thickness of ~20 nm, pores were formed due to the cross-linking of the carbon nanosheets, as illustrated in Fig. S2a. The presence of the ultrathin carbon layers, as well as the porous network, was further confirmed by the TEM image of CPCs-20 in Fig. 4c. The HRTEM image (Fig. S2b) indicated that the carbon nanosheets had abundant and regular micropores with an average size of ~5 nm. Furthermore, the more salts were used, the more porous network would be formed (Fig. 4d). It is believed that the porous structures of the samples CPCs-20 and CPCs-40 would offer large accessible surfaces and boost the adsorption and diffusion of the electrolyte ions, so as to achieve high electrical conductivity and improve the electrochemical performance. Compared to the morphology of the sample in Fig. 2, it is concluded that crosslinked porous network structure has been achieved after the salts were removed through simple water washing. Therefore, salts actually acted as templates to produce pores.


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Fig. 4 SEM (left) and TEM (right) images of the calcined samples: (a) CPCs-0, (b) CPCs-10, (c) CPCs-20 and (d) CPCs-40. Fig. 5 shows N2 adsorption/desorption isotherms and pore size distributions (PSD) of CPCs. As observed in Fig. 5a, the sample CPCs-0 shows the lowest adsorbed volume of N2, indicating its low porosity without the use of salts. As illustrated in Fig. 5b-d, the volume of adsorbed N2 was increased steeply at the relatively low pressures, suggesting the presence of lots of micropores. Simultaneously, with increasing content of salts, the volume of adsorbed N2 was increased, due to the increase in porosity and the existence of a small number of mesopores. Besides, the small tails at P/P0 of about 1.0 (> 0.9) implied existence of macropores. Therefore,


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the samples CPCs-10, CPCs-20 and CPCs-40 had hierarchical porous structures, which were also confirmed by their PSD curves. They exhibited much more micropores and mesopores than CPCs-0. It is well known that macropores avail for ion-buffering, mesopores for ion transport and micropores for improvement of charge storage.25 Therefore, it is expected that CPCs-20 and CPCs-40 should possess promising electrochemical performance comparatively. BET surface area (SBET) and pore volumes of CPCs are summarized in Table S1. SBET values of CPCs-0, CPCs-10, CPCs-20 and CPCs-40 are 238, 761, 1100 and 1073 m2 g-1, respectively. Obviously, the SBET value was increased with increasing content of salts up to CPCs-20. Micropores contributed to 61% of SBET of CPCs-20. It is well known that micropores are specifically responsible for high capacity.18,19 As for CPCs-40, the contribution of micropores to SBET was reduced to 52%, which might be attributed to the fact that more macropores were produced due to the too high contents of salts. The effects of type of the salts on formation of pores have also been studied and the preparation parameters of different samples are listed in Table S2. N2 adsorption/desorption isotherms and PSD profiles of the CPCs obtained with single salts (NaCl or Na2CO3 or Na2SiO3) are displayed in Fig. S3. As shown in Fig. S3a-c, isotherms of the three samples belong to type II, IV and I isotherms, corresponding to structures dominant with macropores, mesopores and micropores, respectively. In other words, porous networks with different pore sizes could be readily generated by simply using salts with different particle sizes, i.e., NaCl (1-2 µm), Na2CO3 (50-100 nm) and Na2SiO3 (5-10 nm).37,38 This is the reason why hierarchical porous structures can be developed by using multi-component salts. SBET and pore volumes of the samples are listed in Table S1.


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Fig. 5 N2 adsorption/desorption isotherms and pore size distribution profiles (insets) of (a) CPCs0, (b) CPCs-10, (c) CPCs-20 and (d) CPCs-40. With high SBET and abundant pore structure, CPCs-20 was characterized by X-ray photoelectron spectroscopy (XPS), in order to further study surface chemical bonding state of the materials. High-resolution C 1s spectrum (Fig. 6a) contains C–C, C–N, C–O and C=O bond at 284.8, 285.9, 286.5 and 289.8 eV.39 The N 1s spectra (Fig. 6b) can be fitted by three contributors, including oxidized pyridine N (403.3 eV), pyrrolic/pyridone N (400.5 eV) and pyridinic N (398.5 eV). As we all know, pyrrolic/pyridone N and pyridinic N contribute pseudocapacitance and oxidized pyridine N can enhance the conductivity of carbon materials.40,41 As illustrated in Fig. 6c, peaks located at 531.6, 533.4 and 536.6 eV attributed to


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C=O, C–O/C–OH and –COOH, which can improve the wettability and contribute to pseudocapacitance.41 Additionally, the S 2p spectrum (Fig. 6d) can be deconvoluted with two components, 2p3/2 and 2p1/2, which intensity ratio is 2:1 and an energy difference is 1.2 eV. The 2p3/2/2p1/2 peaks at 163.9 eV/165.1 eV and 168.3 eV/169.5 eV are assigned to C–S and S–O groups, respectively.42 The N, O and S might be derived from the raw coal when it was pretreated with the mixed acid of HNO3 and H2SO4.

Fig. 6 High resolution pattern of (a) C 1s, (b) N 1s, (c) O 1s and (d) S 2p XPS peaks of CPCs-20. Electrochemical Performances. Electrochemical performances of CPCs-0, CPCs-10, CPCs20, CPCs-40 and CPCs-B, CPCs-M, CPCs-S were evaluated with a three-electrode system in 6 M KOH electrolyte. At 1 A g-1, CPCs-B, CPCs-M, CPCs-S had specific capacitance of 184, 290


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and 171 F g-1, respectively. Comparatively, CPCs-20 exhibited greater performance, as shown in Fig. 7 and Fig. S4-5. CV curves collected at 5 mV s-1 and GCD behavior of CPCs at 1 A g-1 are presented in Fig. 7a, b. As seen in Fig. 7a, all the CPCs-based electrodes displayed a quasirectangular profile with a mirror image feature, which indicates that electrical double-layer capacitors (EDLC) were formed at the electrode-electrolyte interface during the charge-discharge process. Combined with CV, all the GCD curves were linear and symmetrical, revealing typical capacitive behavior of the porous carbon structures (Fig 7b). Besides, there are broad redox humps over the voltage range from -0.8 to -0.3 V, while a mild deviation from the isosceles triangle was observed at the end of discharge, implying contribution of Faradaic pseudocapacitance. The Faradaic pseudocapacitance is related to various doped heteroatom functionalities, such as pyridinic and pyrollic N species.40,41 Among all the samples, CPCs-20 exhibited the largest rectangular area and longest discharge time, delivering most superior capacitive performance due to the hierarchical porous network structure having large accessible surface area, allowing quick ions adsorption and diffusion. Fig. 7c shows gravimetric capacitances of CPCs as a function of current density. It is found that the capacitance of CPCs20 can reach up to 339 F g-1 at 0.5 A g-1. It is worth emphasizing that this value is well above those carbon electrode materials which were activated with alkali.1,17,43-47 (Table 1) Generally, with increasing current density, the capacitance drops due to the inadequate surface reaction and hindered ion diffusion into the inner pores. However, for CPCs-20, rather high specific capacitance was still maintained at high current densities, e.g., 223 F g-1 at 20 A g-1, further confirming the high-rate ion transfer in CPCs-20. Meanwhile, CPCs-40 also showed a promising rate performance, relating to presence of the mesopores and high conductivity.


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However, specific capacitance was not further enhanced as the content of salts was further increased. The high specific capacitance and distinguished rate performance of CPCs-20 can be ascribed to the unique porous network structures. Firstly, the interconnected porous network ensured its effective and efficient contact with the electrolyte. What is more, the hierarchical pores have a synergistic effect on the electrochemical performance. Micropores enlarged the surface area for charge accommodation, while mesopores served as a reservoir for electrolyte, thus conduciving to quick spread and transmission of ions. Nyquist plots of CPCs-based electrodes are shown in Fig. 7d. All the samples demonstrate analogical Nyquist plots. Fig. S6 revealed the Nyquist plots fitted by an electric equivalent circuit model. At high frequency parts, the equivalent series resistance (Re) for CPCs-0, CPCs-10, CPCs-20 and CPCs-40 are 0.56, 0.50, 0.53 and 0.42 Ω, involving intrinsic resistance, electrolyte and contact resistance between the interface of active material and current collector. The CPCs-based electrodes also demonstrated low charge transfer resistance (Rct), the values of Rct for CPCs-0, CPCs-10, CPCs-20 and CPCs-40 are 0.16, 0.08, 0.03 and 0.06 Ω. Double layer capacitor and Weber resistance can be expressed as Cd and Zw.45 Both CPCs-20 and CPCs-40 had a relatively small high-frequency semicircle, revealing their relatively low charge-transfer resistance, which was beneficial to decreasing the overall internal resistance. Besides, they possessed a steep slope at low frequencies, confirming a low masstransfer resistance controlled by the diffusion.


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Fig. 7 Electrochemical performances of CPCs: (a) CV curves at 5 mV s-1, (b) GCD profiles at 1 A g-1, (c) Specific capacitances as a function of current density, (d) Nyquist plots.


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Table 1 Structural properties and electrochemical performances of carbon-based materials for supercapacitors. Precursor

SBET (m2 g-1)

Current density (A g-1)

C (F g-1)

Electrolyte (KOH)

Ref.

Willow catkin Bamboo byproduct Bagasse Pomelo mesocarps Pomelo peel Sodium dodecyl sulfonate Cotton Coal

1533

0.5

298

6M

17

1472

0.1

301

6M

43

2296

0.5

320

6M

44

974.6

0.5

245

2M

45

1648.6

1

260

2M

1

2154.0

1

259

6M

46

1563 1100

0.1 0.5

314 339

6M 6M

47 This work

Fig. 8 shows electrochemical performance of CPCs-20 in a two-electrode system in 6 M KOH. Similarly, it exhibited rectangular CV curves at different scan rates, as observed in Fig. 8a, confirming its ideal capacitive behavior and good reversibility. Fig. 8b displays galvanostatic discharge curves of the CPCs-20//CPCs-20 symmetric cell at different current densities, the specific capacitance is 199 F g-1 at 0.5 A g-1. The galvanostatic discharge curves still retained a linear shape at 20 A g-1, demonstrating its outstanding rate capability. Besides, IR drops were not obviously in the curves, suggesting it had a low internal resistance and high conductivity, which was further confirmed by the EIS measurement result. As shown in Fig. 8c, the semicircle of CPCs-20//CPCs-20 symmetric cell had a small diameter. Moreover, a nearly vertical profile at low-frequency region also suggested its low ionic diffusion resistance. Fig. S7 exhibited the fitted Nyquist plot. The values of Re and Rct are 0.59 and 0.1 Ω. To verify feasibility of the CPCs-20 capacitor for energy supply, a four-tandem cell was assembled to power a green LED. The device can power the LED for 30 min (Fig. 8d). As seen


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in Fig. 8e, the CPCs-20 based device had a pretty high gravimetric energy density (6.9 Wh kg-1 at 250 W kg-1). Fig. 8f showed cycling stability of the CPCs-20//CPCs-20 cell, where the specific capacitance was only slightly decreased after 10000 cycles, demonstrating its distinguished cycling stability.

Fig. 8 Electrochemical performances of the CPCs-20//CPCs-20 symmetric cell: (a) CV curves at various sweep rates, (b) GCD curves recorded at 0.5-25 A g-1, (c) Nyquist plots, (d) Photograph of a green LED lighted by the four tandem-capacitor cell, (e) Ragone plot and (f) Cycling stability measured at 4 A g-1 with inset showing GCD curves (1 A g-1) before and after 10,000 cycles. The electrochemical properties of CPCs-20 as anode material of LIBs were also characterized. The CV curves of CPCs-20 at 0.1 mV s-1 are depicted in Fig. 9a. Both cathodic and anodic peaks above 0.5 V were appeared in the first cycle and disappeared in subsequent cycles, which owned to the formation of SEI film that led to an irreversible process.7,22,48 Additionally, the intensity was hardly changed in the subsequent cycles, indicating its high cycling stability. Fig. 9b shows


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the charge-discharge curves of the 1st, 2nd, 3rd, 20th and 50th cycle for the CPCs-20 electrode at 2 A g-1. All curves exhibited similar Li insertion/extraction profiles without distinct potential plateaus. The first discharge and charge capacities were 795.7 and 333.9 mA h g-1, with the initial Coulombic efficiency of ~47.99%. It is pretty common for carbon electrodes having large irreversible capacity loss at first cycle due to SEI film formed on the surface of active materials. Reversible capacities were 626, 445, 360 and 270 mA h g-1 at 0.1, 0.5, 1 and 3 A g-1, respectively (Fig. 9c). Even at a relatively high current density (6 A g-1), a reversible capacity of as high as 200 mA h g-1 was still retained. Remarkably, after the charge-discharge process at high rates, specific capacity of the electrode could be readily rebound to the initial value at low current densities, indicating its good rate capability. Moreover, CPCs-20 showed high reversible capacity. As seen in Fig. 9d, the electrode exhibited a capacity of as high as 450 mA h g-1 after 100 cycles at 0.2 A g-1, while 305 and 205 mA h g-1 were achieved after 450 cycles at 2 and 5 A g-1 and Columbic efficiency over 99% under different current densities up to 100 or 450 cycles. It can be seen from Fig. S8, the CPCs-20 still consisted of carbon nanosheets. Though the carbon nanosheets became a certain degree of aggregation, the original structural integrity of the active material in the CPCs-20 electrode was well retained after 450 cycles at 5 A g-1, indicating a high structural stability of the synthesized porous carbons. Definitely, the hierarchical porous structure and high electrical conductivity of CPCs-20 ensured affluent pathways for quick move of both ions and electrons, resulting in a high Coulombic efficiency and high cycling performance.


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Fig. 9 (a) CV curves at 0.1 mV s-1, (b) Charge-discharge curves at 2 A g-1, (c) Rate capacity and (d) Cycle performance and Columbic efficiency in the voltage range of 0.01–3.0 V (versus Li+/Li) at different current densities of CPCs-20. CONCLUSIONS A facile approach has been established for large-scale synthesis of hierarchical porous carbons from coal, with water-soluble NaX salts as structural templates instead of the commonly used alkali activation techniques. Structure and electrochemical properties of the porous carbon materials could be well controlled by simply using salts with different particle sizes. Among all samples, the CPCs-20 based supercapacitor electrode exhibited promising charge storage capacity, with specific capacitances of 339 F g-1 and 199 F g-1 at 0.5 A g-1 in 6 M KOH electrolyte, for the three- and two- electrode systems, respectively. Furthermore, symmetric devices based on CPCs-20 were able to power a LED for 30 min. The high electrochemical


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performances of CPCs-20 could be readily attributed to its high accessible surfaces closely related to the hierarchical porous structure, high microporosity and heteroatom doping, thus enabling large storage and high-rate transport of ions. It also demonstrated superior rate capability and long cycling life as anode of LIBs. Noting the features of easy operation, low time and energy consumption, as well as abundance of coal, the approach developed in our current study could find broad applications in energy storage.

ASSOCIATED CONTENT Supporting Information Additional data including XRD patterns of the sample before and after calcination; Magnified SEM and HRTEM images of CPCs-20; BET surface areas and pore structure parameters of the samples; Compositions of the precursors for making materials with different pore sizes; N2 adsorption-desorption isotherm and specific capacitance of CPCs-B, CPCs-M and CPCs-S; Fitted Nyquist plots of the materials (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected] Author Contributions S. S. Gao and Y. K. Tang contributed equally to this work. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the Joint Funds of NSFC-Xinjiang of China (U1303391), the National Natural Science Foundation of China (21362037, 51672235), the Doctoral Innovation Program of Xinjiang University (XJUBSCX-2016009), the Graduate Research Innovation Project of Xinjiang (XJGRI2016002), the Opened Fund of the Key Laboratory of Xinjiang Uygur Autonomous Region of China (2015KL010), the Program for Changjiang Scholars and Innovative Research Team in the University of Ministry of Education of China (IRT1081). REFERENCES (1) Qu, G.; Jia, S. F.; Wang, H.; Cao, F.; Li, L.; Qing, C.; Sun, D. M.; Wang, B. X.; Tang, Y. W.; Wang, J. B. Asymmetric Supercapacitor Based on Porous N-Doped Carbon Derived from Pomelo Peel and NiO Arrays. ACS Appl. Mater. Inter. 2016, 8, 2082220830. (2) Pandolfo, A. G.; Hollenkamp, A. F. Carbon Properties and Their Role in Supercapacitors. J. Power Sources 2006, 157, 11-27. (3) Beguin, F.; Presser, V.; Balducci, A.; Frackowiak, E. Carbons and Electrolytes for Advanced Supercapacitors. Adv. Mater. 2014, 26, 2219-2251. (4) Miller, J. R.; Simon, P. Electrochemical Capacitors for Energy Management. Science 2008, 321, 651-652. (5) Ye, G.; Zhu, X.; Chen, S.; Li, D.; Yin, Y.; Lu, Y.; Komarneni, S.; Yang, D. Nanoscale Engineering of Nitrogen-Doped Carbon Nanofiber Aerogels for Enhanced Lithium Ion Storage. J. Mater. Chem. A 2017, 5, 8247-8254. (6) Ma, Q. T.; Wang, L. X.; Xia, W.; Jia, D. Z.; Zhao, Z. B. Nitrogen-Doped Hollow Amorphous Carbon Spheres@Graphitic Shells Derived from Pitch: New Structure Leads


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Table of Contents The hierarchical porous carbons with a high specific surface area have been successfully synthesized by using a one-step pyrolysis method, with water-soluble NaX (X: Cl-, CO32-, SiO32-) salts as the structure-making templates instead of the alkali activation from coal. Structure of the porous carbon materials could be well controlled by simply using salts with different particle sizes. As electrode materials for SCs and LIBs, the coal-based hierarchical porous carbons exhibited excellent rate capability and superior cycling performance. The coal and NaX salts are very cheap, commercially available and the preparation procedure is much simplified, time saving and easy to operate, which indicates a great prospect in industry.


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Coal-Based Hierarchical Porous Carbon Synthesized with a Soluble

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