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Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors
JEAC-113673; No of Pages 7 Journal of Electroanalytical Chemistry xxx (xxxx) xxx
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Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors Lvye Yang a, Jianhao Qiu a, Yaquan Wang a, Shihang Guo a, Yi Feng a, Dehua Dong b, Jianfeng Yao a,⁎ a College of Chemical Engineering, Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Jiangsu Key Lab for the Chemistry & Utilization of Agricultural and Forest Biomass, Nanjing Forestry University, Nanjing, Jiangsu 210037, China b School of Material Science and Engineering, University of Jinan, Jinan 250022, China
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Article history: Received 28 September 2019 Received in revised form 25 October 2019 Accepted 19 November 2019 Available online xxxx Keywords: Supercapacitor Hierarchical porous carbon Molten salt Wood sawdust
a b s t r a c t Molten salt synthesis is applied in the preparation of porous carbon under air atmosphere. Wood sawdust is used as the carbon source, and the molten salt containing KCl and KOH acts as structure-directing agent to tailor microstructure of porous carbon, making the resulting carbon have improved electrochemical performance. The function of molten salt is controlled by the cooperation of KCl and KOH in different compounding ratios. Benefiting from the open pore structure, the optimized carbon (CS-H3) shows the highest specific capacitance of 286 F g−1 in 1 A g−1, and the energy density reaches 5.63 W h kg−1 at a power density of 9213 W kg−1 in 6 M KOH. Furthermore, CS-H3 also displays an excellent cycle life of high capacitance retention of 99.8% after 10,000 cycles. This work provides a high-efficiency route to prepare hierarchical porous carbon from biomass, showing a promise in the industrial production. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Electrochemical energy storage technologies change the society in replying the human demand of efficient and sustainable future life [1,2]. Supercapacitors, as a class electrochemical device with ultra-fast energy storage ability, play an attractive role in some fields such as transportation industry, smart grid and new energy [3,4]. Electrode materials are seen as one of the determinant materials in assembly of supercapacitor. Porous carbon (activated carbon) was initially studied in supercapacitors, and has been widely used till now [5,6]. Recently, preparing porous carbons from biomass resources has attracted considerable attention [7]. Biomass resources are naturally abundant, low-cost and sustainable, and thus can be taken into account for scalable manufacturing [8]. In view of the applications of carbon materials in supercapacitors, the high specific surface area is considered to be important to obtain sufficient sites for ion-storage [9,10]. In addition, the microstructure, such as pore distribution, also affects the mass transfer and the efficiency of ion-storage [11–13]. Multi-scale pores with a hierarchical structure are promising to improve the ion accessibility and thus the capacitive performance [5,14,15]. Macropores act as ion-
⁎ Corresponding author. E-mail address: [email protected] (J. Yao).
buffering reservoirs, and mesopores work as ion-transporting channels while small mesopores and micropores provide more active sites. In many cases, hydrothermal and physical/chemical activation and their combination are the most common methods used in the preparation of hierarchical porous carbon from biomass, especially the crude biomass [3,14,16–22]. However, these techniques are complicated to form hierarchical pores and ensure activation efficiency. Moreover, the fabrication of porous carbon from biomass is normally controlled under inert gas (e.g. N2 and Ar), which will increase the resource consumption. Molten salt (MS) chemistry has attracted great attention in energy, environment and resource sustainability [23,24]. Currently, MSs are reported as reaction media for carbonizing/pyrolyzing biomass to produce various microstructures. In addition to the hierarchical porous carbon [25,26], carbon sheets could be obtained via using KCl-LiCl in the carbonization of glucose [27–31], and even 1D carbon nanobelt could be synthesized by employing ZnCl2 as a “stripping and cutting” agent of tofu [32]. The electrochemistry properties of carbon materials were affected by MSs functioning as structuredirecting agents [31,33–35]. Additionally, inert MSs could be a protection media, preventing carbon from burning [36]. With an air atmosphere, Wang et al. sequentially synthesized the carbon sheets using clover stems in KCl and cornstalk in KCl-NaCl [37,38]. Such strategies brought a scalable route to yield porous carbon with desirable capacitor performance.
https://doi.org/10.1016/j.jelechem.2019.113673 1572-6657/© 2018 Elsevier B.V. All rights reserved.
Please cite this article as: L. Yang, J. Qiu, Y. Wang, et al., Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors, Journal of Electroanalytical Chemistry, https://doi.org/10.1016/j.jelechem.2019.113673
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Wood is the most extensive lignocellulosic biomass in the world, making the study of preparing materials from wood meaningful and good for controlling the cost [39,40]. To prepare a porous carbon in this work, wood sawdust (waste derived from wood) was selected as the carbon source, and the MS containing KCl and KOH was deliberately designed for the carbonization. Inert KCl salt serves as a thermal media to prevent carbon product from burning, allowing the carbonization processing in air. KOH is necessarily introduced to boost the tailoring function of MS system, which cooperates with KCl to create the open pore structures. The content of KOH plays a critical impact on the microstructure and the specific surface area, thus affecting the performance of the capacitor. As a result, the optimized carbon CS-H3 showed the highest specific capacitance of 286 F g−1 in 1 A g−1 and the energy density could reach 5.63 W h kg−1 at a power density of 9213 W kg−1 in 6 M KOH. Besides, CS-H3 displayed the high capacitance retention of 99.8% after 10,000 cycles, indicating an excellent cycle life. 2. Experimental section 2.1. Molten salt synthesis of porous carbon Here, 3 g of dry wood sawdust were mixed with 15 g of KCl and 3 g of KOH. The resulting mixture was dried at 100 °C to constant weight, and subsequently heated in a muffle furnace at 800 °C for 3 h with a heating rate of 10 °C min−1 in air. After the muffle furnace was naturally cooled down, the resulting products were taken out and washed with deionized water to remove the salt and residual alkali. The resultant carbon was collected and dried, and denoted as CS-H3. Additionally, different amounts of KOH (0, 1, 2, and 5 g) were used to instead 3 g of KOH to prepare other carbon products with the same procedure as above, and the resulting products were named as CS, CS-H1, CS-H2 and CS-H5, respectively. 2.2. Characterization The microstructure and morphology of carbon materials were examined by scanning electron microscope (SEM) of JSM-7600F. The N2 adsorption-desorption isotherms were tested using micromeritics ASAP 2020. The crystal and physical structure were characterized by X-ray diffraction (XRD) of Rigaku MiniFlex II. The electrochemical performances were measured by electrochemical working station of CHI 760e (Chenhua, Shanghai). The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were conducted. Detailed electrode preparation and
the calculation of electrochemical performance were described in supporting information. 3. Results and discussion In the synthesis of porous carbon, the needs for complex ventilation equipment and inert gases were avoided due to that fact the carbonization process was in open air. Fig. 1 shows SEM images of the carbon materials. The obtained porous carbon CS with only KCl used in carbonization shows a smooth surface without any visible pore (Fig. 1a). After the addition of KOH, visible macropores appear in the resulting porous carbons, and the pores throughout the carbon are gradually formed with the increase of KOH amount (Fig. 1b–d). CS-H3 shows an open pore structure with interconnected thinwalls (Fig. 1d and e). Interestingly, when the amount of KOH was continuously increased, carbon sheets could be formed, as shown in CS-H5 (Fig. 1f). CS-H5 even possesses the thin layer in only a few tens of nanometers (More SEM images are available in supporting information, Fig. S1). The KOH-added MS has a tailoring effect on carbon morphology, and this effect becomes stronger with the increase of KOH. The surface areas of CS and CS-H1~5 were calculated by BrunauerEmmett-Teller (BET) model according to the N2 adsorption-desorption measurement (Fig. S2). CS-H1, CS-H2, CS-H3 and CS-H5 show the higher BET surface areas of 1667, 1998, 1869 and 1699 m2 g−1 than that of CS (719 m2 g−1), which benefited from the activation of KOH and plenty of micropores and mesopores introduced. Fig. 2 shows XRD patterns of CS and CS-H1~5. Two broad peaks corresponding to (002) and (100) can be observed, which suggests the amorphous carbon structure. The (100) peaks of the carbon are at around 43.8°. It is worth noting that the (002) peaks shift to lower angle from 23.6° in CS to 22.4° in CS-H5, implying that the interlayer distance increases. In addition to defects caused by KOH-activation, the larger interlayer distance can be mainly attributed to the thin-walled/ layered structure [20]. The sharp (101) peak at 26.6° observed in CS is related to the existence of silica derived from the crude wood [41,42]. In the KOH-added MS system, the silica would react with KOH and then be removed by the washing process [36,43]. As the result, the (101) peaks are weaker or even disappeared in the final carbons (CSH1~5). During the carbonization, the mixture of organic and inorganic phases gradually becomes uniform with the temperature going up and salt liquefying. Meanwhile, the wood sawdust would have a transition from sp3 C\\X bonds (X: e.g., C, O, H) to the aromatic sp2 C\\C bonds. Continuous cleavage of bonded carbons and the prevention of sp2
Fig. 1. SEM images of CS (a), CS-H1 (b), CS-H2 (c), CS-H3 (d and e) and CS-H5 (f).
Please cite this article as: L. Yang, J. Qiu, Y. Wang, et al., Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors, Journal of Electroanalytical Chemistry, https://doi.org/10.1016/j.jelechem.2019.113673
L. Yang et al. / Journal of Electroanalytical Chemistry xxx (xxxx) xxx
Fig. 2. XRD spectra of CS, CS-H1, CS-H2, CS-H3 and CS-H5.
carbon stacking are the keys to obtain the porous structure (thermodynamic instability at high temperature) [27]. The KOH with an oxidizing/ activating action attacks the outside of carbon, allowing the flowable MS to diffuse into the carbon. With the temperature going down, the MS acts as a solid template to prevent carbon from stacking to form porous structure. Further mechanism of carbon structure formation can be explained by the illustration of Fig. 3. At a lower KOH content in MS (CSH1), the limited oxidizing action of KOH limits the compatibility of MS and carbon, and thus reduces the template action of MS to form pores. The MS deeply penetrates the carbon phase with more amount of KOH used (CS-H3 and CS-H5), making the formation of open pore structure and even sheets. KCl acting as the main body of MS is indispensable for the flowability and the template role. KCl plays as a sealing agent to prevent organic matter from being lost by air, even in the solid state and such sealing action becomes stronger in liquid state [38]. With only chloride salts used, the thin-layer carbon could be obtained from glucose or the biomass such as clover and egg white [25,27,29,37,38]. However, a strong oxidant (KOH) is required in this work, probably due to the presence of a large amount of lignin with stable and dense aromatic structure in the wood, which forbids the diffusion of salts into the interior [8,40]. The electrochemistry performances of CS and CS-H1~5 were evaluated in KOH aqueous solution by using three-electrode system. As the CV curves, CS displays a depressed rectangle whereas CS-H1~5 present
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a more regular rectangle in a scan rate of 50 mV s−1 (Fig. 4a). The porous structure in CS-H1~5 facilitates the penetration of electrolyte into the interior of carbon and acts reservoirs to relieve the ion-transport pressure during the electrochemical process [18]. CS-H1~5 also show the larger rectangular areas compared with CS, indicating the higher specific capacitances. The GCD curves show the symmetrical triangular shapes in a specific current of 1 A g−1, suggesting the electrical double layer capacitance (Fig. 4b). CS-H1~5 have the longer charge/discharge times than CS, which are caused by the higher specific capacitance (The complete CV and GCD curves are given in Fig. S3 and Fig. S4). High specific surface area of carbon provides sufficient active sites in supercapacitor energy storage, and the microstructure affects the efficiency of ion-storage [20]. With the effect of KOH-added molten salt, CS-H1~5 possess the higher surface areas and the richer pore structures, and therefore perform the much higher capacitance values than CS with only KCl. Fig. 4c summarizes the specific capacitance by calculating the GCD results. CS-H3 shows the capacitance of 286 F g−1 in 1 A g−1 and 242 F g−1 in 20 A g−1, which are higher than those of CS (164 F g−1, 131 F g−1), CS-H1 (217 F g−1, 185 F g−1), CS-H2 (270 F g−1, 228 F g−1) and CS-H5 (256 F g−1, 219 F g−1). For CS-H1~5, CS-H3 shows a slightly higher capacitance than that of CS-H2 although the specific surface area of CS-H3 (1869 m2 g−1) is slightly lower than CS-H2 (1998 m2 g−1). Such result is attributed to the more open pore structure of CS-H3 than CS-H2. The importance of open structure for capacitance also can be demonstrated by comparing CS-H1 and CS-H5. CS-H5 (1699 m2 g−1) shows a similar surface area to CS-H1 (1667 m2 g−1) but a much higher capacitance, which is caused by the highly open sheet structure. Thus, in the balance of specific surface and open structure, the proper addition of KOH (CS-H3) is the key to achieve the maximum specific capacitance. CS-H3 is comparable to other biomass-derived carbons prepared by hydrothermal and activation [3,21,22,26] (the detailed comparison in Table S1). Benefiting from the open structure, CS-H3 shows the superior specific capacitance at a high current density. The Nyquist plots were conducted at the frequency range of 100 kHz to 0.1 Hz, as shown in Fig. 4d. The diameter of semicircle in high frequencies is relevant to the penetration of electrolyte into the electrode material and corresponds the charge transfer resistance [44,45]. The length of 45o inclined line in low frequencies represents the Warburg resistance [46]. With the smaller semicircular diameter and inclined-line length, both the charge transfer resistance and Warburg resistance decrease from CS to CS-H5, suggesting the better mass transfer ability. Aside from KOH, the effect of KCl amount to the preparation of porous carbon and their
Fig. 3. Schematic illustration of carbon formation with different amounts of KOH.
Please cite this article as: L. Yang, J. Qiu, Y. Wang, et al., Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors, Journal of Electroanalytical Chemistry, https://doi.org/10.1016/j.jelechem.2019.113673
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Fig. 4. Electrochemical performances of CS, CS-H1, CS-H2, CS-H3 and CS-H5: CV curves at 50 mV s−1 (a), GCD curves at 1 A g−1 (b), specific capacitance at different specific currents (c), and Nyquist plots (d).
Fig. 5. The comparisons of CS-H3 and CS0-H3: SEM images of CS-H3 (a) and CS0-H3 (b), XRD spectra (c), CV curves at 50 mV s−1 (d), GCD curves at 1 A g−1 (e).
Please cite this article as: L. Yang, J. Qiu, Y. Wang, et al., Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors, Journal of Electroanalytical Chemistry, https://doi.org/10.1016/j.jelechem.2019.113673
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capacitor performances were also explored (details in supporting information). Based on the XRD results in Fig. 2, silica is present. According to the previous studies of carbonizing biomass, the presence of silica would act as an inner template, and the pores could be generated after the template removed [14,24,47]. Therefore, in this study, we also investigated the effects of silica on the carbon synthesis. The original sawdust (~10 g) was mixed with HF aqueous solution (200 ml, ~4 wt%) and allowed to stand overnight. After an exhaustive wash, the obtained sawdust was experienced by the same process as CS and the resulting carbon was donated as CS0. The XRD of CS0 presents the vanished (101) peak of silica (details in Fig. S7). The HF-treatment sawdust was used to prepare a carbon named CS0-H3 in the same method as CS-H3. For CS0-H3 and CS-H3, they show different pore structure when observed by SEM (Fig. 5a and b) and a similar XRD pattern (Fig. 5c). The open pore is difficult to see in CS0-H3, suggesting that the formation of open pore structure is affected by the silica existing during the carbonization. Silica acting as the template in the interior part of the biomass so as building block in creating some potential pore spaces, and the silica is easily corroded by KOH, thus would be replaced by the corrosive MS. Or in another language: the existence of silica makes KOH-added MS permeate easily into the carbon and good for creating a richer pore structure. CS0-H3 also shows a smaller rectangular area along the CV curve and a shorter charge/discharge time of GCD test than that of CS-H3 (Fig. 5d and e), implying a poor capacitance value. Therefore, the existence of silica is beneficial to synthesize the porous carbon with a high modified capacitive performance in KOH-added MS (The complete CV and GCD results of CS0-H3 are available in Fig. S8).
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Two-electrode system in 6 M KOH was used to further explore the supercapacitor performance of CS-H3. The CV and GCD curves both be controlled in a voltage window from 0 to 1 V, as shown in Fig. 6a and b. The CV curves present the nearly perfect rectangle, and it even was held in a high scan rate of 200 mV s−1, indicating the ideal electrical double layer capacitor. From the GCD curves, the specific capacitance was calculated to be 60 F g−1 in the specific current of 0.5 A g−1 and retains 51 F g−1 in the high specific current of 20 A g−1. As shown in Ragone plot of Fig. 6c, the energy density reaches 8.55 W h kg−1 at a power density of 226 W kg−1 and 5.63 W h kg−1 at 9213 W kg−1, which is comparable to the biomass-derived porous carbon from hydrothermal and activation [1,10,17–19,22,48,49]. The cycle test was conducted at the specific current of 10 A g−1, after 10,000 cycles, the capacitance can maintain the 99.8% of the initial value (Fig. 6d). Such high capacitance retention demonstrates the ion accessibility in the pores does not change upon an extremely long working hours with a high-power behavior, suggesting an excellent electrochemical stability and reversibility of the CS-H3 electrode. These results further demonstrate the potential of MS using in preparing porous carbon from biomass. 4. Conclusion In summary, the hierarchical porous carbons were synthesized by using wood sawdust as the carbon source after calcining in molten salt. The mixture of KCl and KOH can function as protective media and structure-directing agent, and the carbonization is allowed to proceed in air. The microstructure of carbon is adjusted by the content of KOH used, and such carbon structure greatly affects the capacitor performance.
Fig. 6. Electrochemical performances of CS-H3 in two-electrode system: CV curves at different scan rates (a), GCD curves at different specific currents (b), Ragone plot (c) and cycling stability at 10 A g−1 (d).
Please cite this article as: L. Yang, J. Qiu, Y. Wang, et al., Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors, Journal of Electroanalytical Chemistry, https://doi.org/10.1016/j.jelechem.2019.113673
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The resulting porous carbon CS-H3 shows the highest specific capacitance of 286 F g−1 in 1 A g−1. This work further demonstrates the advantages of molten salt synthesis in controlling carbon morphology. In future work, the synthesis of carbon from biomass using molten salt while focusing on the molten salts recycling and the material dimension controlling will be very meaningful. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors are grateful for the financial support of National Key Research and Development Program of China (2017YFD0601006). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jelechem.2019.113673. References [1] Y. Huang, L. Peng, Y. Liu, G. Zhao, J.Y. Chen, G. Yu, Biobased nano porous active carbon fibers for high-performance supercapacitors, ACS Appl. Mater. Inter. 8 (2016) 15205–15215. [2] H.-S. Huang, K.-H. Chang, N. Suzuki, Y. Yamauchi, C.-C. Hu, K.C.W. Wu, Evaporationinduced coating of hydrous ruthenium oxide on mesoporous silica nanoparticles to develop high-performance supercapacitors, Small 9 (2013) 2520–2526. [3] H. Feng, H. Hu, H. Dong, Y. Xiao, Y. Cai, B. Lei, Y. Liu, M. Zheng, Hierarchical structured carbon derived from bagasse wastes: a simple and efficient synthesis route and its improved electrochemical properties for high-performance supercapacitors, J. Power Sources 302 (2016) 164–173. [4] B.P. Bastakoti, H.-S. Huang, L.-C. Chen, K.C.W. Wu, Y. Yamauchi, Block copolymer assisted synthesis of porous alpha-Ni(OH)2 microflowers with high surface areas as electrochemical pseudocapacitor materials, Chem. Commun. 48 (2012) 9150–9152. [5] T. Liu, F. Zhang, Y. Song, Y. Li, Revitalizing carbon supercapacitor electrodes with hierarchical porous structures, J. Mater. Chem. A 5 (2017) 17705–17733. [6] S. Makino, Y. Yamauchi, W. Sugimoto, Synthesis of electro-deposited ordered mesoporous RuOx using lyotropic liquid crystal and application toward microsupercapacitors, J. Power Sources 227 (2013) 153–160. [7] J. Deng, M. Li, Y. Wang, Biomass-derived carbon: synthesis and applications in energy storage and conversion, Green Chem. 18 (2016) 4824–4854. [8] J. Deng, T. Xiong, H. Wang, A. Zheng, Y. Wang, Effects of cellulose, hemicellulose, and lignin on the structure and morphology of porous carbons, ACS Sustain. Chem. Eng. 4 (2016) 3750–3756. [9] Y. Lu, S. Zhang, J. Yin, C. Bai, J. Zhang, Y. Li, Y. Yang, Z. Ge, M. Zhang, L. Wei, M. Ma, Y. Ma, Y. Chen, Mesoporous activated carbon materials with ultrahigh mesopore volume and effective specific surface area for high performance supercapacitors, Carbon 124 (2017) 64–71. [10] E.Y.L. Teo, L. Muniandy, E.-P. Ng, F. Adam, A.R. Mohamed, R. Jose, K.F. Chong, High surface area activated carbon from rice husk as a high performance supercapacitor electrode, Electrochim. Acta 192 (2016) 110–119. [11] L. Qie, W. Chen, H. Xu, X. Xiong, Y. Jiang, F. Zou, X. Hu, Y. Xin, Z. Zhang, Y. Huang, Synthesis of functionalized 3D hierarchical porous carbon for high-performance supercapacitors, Energy Environ. Sci. 6 (2013) 2497–2504. [12] L. Yang, Y. Feng, D. Yu, J. Qiu, X.-F. Zhang, D. Dong, J. Yao, Design of ZIF-based CNTs wrapped porous carbon with hierarchical pores as electrode materials for supercapacitors, J. Phys. Chem. Solids 125 (2019) 57–63. [13] C. Young, R.R. Salunkhe, J. Tang, C.-C. Hu, M. Shahabuddin, E. Yanmaz, M.S.A. Hossain, J.H. Kim, Y. Yamauchi, Zeolitic imidazolate framework (ZIF-8) derived nanoporous carbon: the effect of carbonization temperature on the supercapacitor performance in an aqueous electrolyte, Phys. Chem. Chem. Phys. 18 (2016) 29308–29315. [14] Z. Chen, H. Zhuo, Y. Hu, L. Zhong, X. Peng, S. Jing, Q. Liu, X. Zhang, C. Liu, R. Sun, Selfbiotemplate preparation of hierarchical porous carbon with rational mesopore ratio and high oxygen content for an ultrahigh energy-density supercapacitor, ACS Sustain. Chem. Eng. 6 (2018) 7138–7150. [15] Y. Feng, M. Cao, L. Yang, X.-F. Zhang, Y. Wang, D. Yu, X. Gu, J. Yao, Bilayer N-doped carbon derived from furfuryl alcohol-wrapped melamine sponge as highperformance supercapacitor, J. Electroanal. Chem. 823 (2018) 633–637. [16] L. Zhu, F. Shen, R.L. Smith Jr., L. Yan, L. Li, X. Qi, Black liquor-derived porous carbons from rice straw for high-performance supercapacitors, Chem. Eng. J. 316 (2017) 770–777.
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Please cite this article as: L. Yang, J. Qiu, Y. Wang, et al., Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors, Journal of Electroanalytical Chemistry, https://doi.org/10.1016/j.jelechem.2019.113673
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Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors Lvye Yang a, Jianhao Qiu a, Yaquan Wang a, Shihang Guo a, Yi Feng a, Dehua Dong b, Jianfeng Yao a,⁎ a College of Chemical Engineering, Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Jiangsu Key Lab for the Chemistry & Utilization of Agricultural and Forest Biomass, Nanjing Forestry University, Nanjing, Jiangsu 210037, China b School of Material Science and Engineering, University of Jinan, Jinan 250022, China
a r t i c l e
i n f o
Article history: Received 28 September 2019 Received in revised form 25 October 2019 Accepted 19 November 2019 Available online xxxx Keywords: Supercapacitor Hierarchical porous carbon Molten salt Wood sawdust
a b s t r a c t Molten salt synthesis is applied in the preparation of porous carbon under air atmosphere. Wood sawdust is used as the carbon source, and the molten salt containing KCl and KOH acts as structure-directing agent to tailor microstructure of porous carbon, making the resulting carbon have improved electrochemical performance. The function of molten salt is controlled by the cooperation of KCl and KOH in different compounding ratios. Benefiting from the open pore structure, the optimized carbon (CS-H3) shows the highest specific capacitance of 286 F g−1 in 1 A g−1, and the energy density reaches 5.63 W h kg−1 at a power density of 9213 W kg−1 in 6 M KOH. Furthermore, CS-H3 also displays an excellent cycle life of high capacitance retention of 99.8% after 10,000 cycles. This work provides a high-efficiency route to prepare hierarchical porous carbon from biomass, showing a promise in the industrial production. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Electrochemical energy storage technologies change the society in replying the human demand of efficient and sustainable future life [1,2]. Supercapacitors, as a class electrochemical device with ultra-fast energy storage ability, play an attractive role in some fields such as transportation industry, smart grid and new energy [3,4]. Electrode materials are seen as one of the determinant materials in assembly of supercapacitor. Porous carbon (activated carbon) was initially studied in supercapacitors, and has been widely used till now [5,6]. Recently, preparing porous carbons from biomass resources has attracted considerable attention [7]. Biomass resources are naturally abundant, low-cost and sustainable, and thus can be taken into account for scalable manufacturing [8]. In view of the applications of carbon materials in supercapacitors, the high specific surface area is considered to be important to obtain sufficient sites for ion-storage [9,10]. In addition, the microstructure, such as pore distribution, also affects the mass transfer and the efficiency of ion-storage [11–13]. Multi-scale pores with a hierarchical structure are promising to improve the ion accessibility and thus the capacitive performance [5,14,15]. Macropores act as ion-
⁎ Corresponding author. E-mail address: [email protected] (J. Yao).
buffering reservoirs, and mesopores work as ion-transporting channels while small mesopores and micropores provide more active sites. In many cases, hydrothermal and physical/chemical activation and their combination are the most common methods used in the preparation of hierarchical porous carbon from biomass, especially the crude biomass [3,14,16–22]. However, these techniques are complicated to form hierarchical pores and ensure activation efficiency. Moreover, the fabrication of porous carbon from biomass is normally controlled under inert gas (e.g. N2 and Ar), which will increase the resource consumption. Molten salt (MS) chemistry has attracted great attention in energy, environment and resource sustainability [23,24]. Currently, MSs are reported as reaction media for carbonizing/pyrolyzing biomass to produce various microstructures. In addition to the hierarchical porous carbon [25,26], carbon sheets could be obtained via using KCl-LiCl in the carbonization of glucose [27–31], and even 1D carbon nanobelt could be synthesized by employing ZnCl2 as a “stripping and cutting” agent of tofu [32]. The electrochemistry properties of carbon materials were affected by MSs functioning as structuredirecting agents [31,33–35]. Additionally, inert MSs could be a protection media, preventing carbon from burning [36]. With an air atmosphere, Wang et al. sequentially synthesized the carbon sheets using clover stems in KCl and cornstalk in KCl-NaCl [37,38]. Such strategies brought a scalable route to yield porous carbon with desirable capacitor performance.
https://doi.org/10.1016/j.jelechem.2019.113673 1572-6657/© 2018 Elsevier B.V. All rights reserved.
Please cite this article as: L. Yang, J. Qiu, Y. Wang, et al., Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors, Journal of Electroanalytical Chemistry, https://doi.org/10.1016/j.jelechem.2019.113673
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Wood is the most extensive lignocellulosic biomass in the world, making the study of preparing materials from wood meaningful and good for controlling the cost [39,40]. To prepare a porous carbon in this work, wood sawdust (waste derived from wood) was selected as the carbon source, and the MS containing KCl and KOH was deliberately designed for the carbonization. Inert KCl salt serves as a thermal media to prevent carbon product from burning, allowing the carbonization processing in air. KOH is necessarily introduced to boost the tailoring function of MS system, which cooperates with KCl to create the open pore structures. The content of KOH plays a critical impact on the microstructure and the specific surface area, thus affecting the performance of the capacitor. As a result, the optimized carbon CS-H3 showed the highest specific capacitance of 286 F g−1 in 1 A g−1 and the energy density could reach 5.63 W h kg−1 at a power density of 9213 W kg−1 in 6 M KOH. Besides, CS-H3 displayed the high capacitance retention of 99.8% after 10,000 cycles, indicating an excellent cycle life. 2. Experimental section 2.1. Molten salt synthesis of porous carbon Here, 3 g of dry wood sawdust were mixed with 15 g of KCl and 3 g of KOH. The resulting mixture was dried at 100 °C to constant weight, and subsequently heated in a muffle furnace at 800 °C for 3 h with a heating rate of 10 °C min−1 in air. After the muffle furnace was naturally cooled down, the resulting products were taken out and washed with deionized water to remove the salt and residual alkali. The resultant carbon was collected and dried, and denoted as CS-H3. Additionally, different amounts of KOH (0, 1, 2, and 5 g) were used to instead 3 g of KOH to prepare other carbon products with the same procedure as above, and the resulting products were named as CS, CS-H1, CS-H2 and CS-H5, respectively. 2.2. Characterization The microstructure and morphology of carbon materials were examined by scanning electron microscope (SEM) of JSM-7600F. The N2 adsorption-desorption isotherms were tested using micromeritics ASAP 2020. The crystal and physical structure were characterized by X-ray diffraction (XRD) of Rigaku MiniFlex II. The electrochemical performances were measured by electrochemical working station of CHI 760e (Chenhua, Shanghai). The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were conducted. Detailed electrode preparation and
the calculation of electrochemical performance were described in supporting information. 3. Results and discussion In the synthesis of porous carbon, the needs for complex ventilation equipment and inert gases were avoided due to that fact the carbonization process was in open air. Fig. 1 shows SEM images of the carbon materials. The obtained porous carbon CS with only KCl used in carbonization shows a smooth surface without any visible pore (Fig. 1a). After the addition of KOH, visible macropores appear in the resulting porous carbons, and the pores throughout the carbon are gradually formed with the increase of KOH amount (Fig. 1b–d). CS-H3 shows an open pore structure with interconnected thinwalls (Fig. 1d and e). Interestingly, when the amount of KOH was continuously increased, carbon sheets could be formed, as shown in CS-H5 (Fig. 1f). CS-H5 even possesses the thin layer in only a few tens of nanometers (More SEM images are available in supporting information, Fig. S1). The KOH-added MS has a tailoring effect on carbon morphology, and this effect becomes stronger with the increase of KOH. The surface areas of CS and CS-H1~5 were calculated by BrunauerEmmett-Teller (BET) model according to the N2 adsorption-desorption measurement (Fig. S2). CS-H1, CS-H2, CS-H3 and CS-H5 show the higher BET surface areas of 1667, 1998, 1869 and 1699 m2 g−1 than that of CS (719 m2 g−1), which benefited from the activation of KOH and plenty of micropores and mesopores introduced. Fig. 2 shows XRD patterns of CS and CS-H1~5. Two broad peaks corresponding to (002) and (100) can be observed, which suggests the amorphous carbon structure. The (100) peaks of the carbon are at around 43.8°. It is worth noting that the (002) peaks shift to lower angle from 23.6° in CS to 22.4° in CS-H5, implying that the interlayer distance increases. In addition to defects caused by KOH-activation, the larger interlayer distance can be mainly attributed to the thin-walled/ layered structure [20]. The sharp (101) peak at 26.6° observed in CS is related to the existence of silica derived from the crude wood [41,42]. In the KOH-added MS system, the silica would react with KOH and then be removed by the washing process [36,43]. As the result, the (101) peaks are weaker or even disappeared in the final carbons (CSH1~5). During the carbonization, the mixture of organic and inorganic phases gradually becomes uniform with the temperature going up and salt liquefying. Meanwhile, the wood sawdust would have a transition from sp3 C\\X bonds (X: e.g., C, O, H) to the aromatic sp2 C\\C bonds. Continuous cleavage of bonded carbons and the prevention of sp2
Fig. 1. SEM images of CS (a), CS-H1 (b), CS-H2 (c), CS-H3 (d and e) and CS-H5 (f).
Please cite this article as: L. Yang, J. Qiu, Y. Wang, et al., Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors, Journal of Electroanalytical Chemistry, https://doi.org/10.1016/j.jelechem.2019.113673
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Fig. 2. XRD spectra of CS, CS-H1, CS-H2, CS-H3 and CS-H5.
carbon stacking are the keys to obtain the porous structure (thermodynamic instability at high temperature) [27]. The KOH with an oxidizing/ activating action attacks the outside of carbon, allowing the flowable MS to diffuse into the carbon. With the temperature going down, the MS acts as a solid template to prevent carbon from stacking to form porous structure. Further mechanism of carbon structure formation can be explained by the illustration of Fig. 3. At a lower KOH content in MS (CSH1), the limited oxidizing action of KOH limits the compatibility of MS and carbon, and thus reduces the template action of MS to form pores. The MS deeply penetrates the carbon phase with more amount of KOH used (CS-H3 and CS-H5), making the formation of open pore structure and even sheets. KCl acting as the main body of MS is indispensable for the flowability and the template role. KCl plays as a sealing agent to prevent organic matter from being lost by air, even in the solid state and such sealing action becomes stronger in liquid state [38]. With only chloride salts used, the thin-layer carbon could be obtained from glucose or the biomass such as clover and egg white [25,27,29,37,38]. However, a strong oxidant (KOH) is required in this work, probably due to the presence of a large amount of lignin with stable and dense aromatic structure in the wood, which forbids the diffusion of salts into the interior [8,40]. The electrochemistry performances of CS and CS-H1~5 were evaluated in KOH aqueous solution by using three-electrode system. As the CV curves, CS displays a depressed rectangle whereas CS-H1~5 present
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a more regular rectangle in a scan rate of 50 mV s−1 (Fig. 4a). The porous structure in CS-H1~5 facilitates the penetration of electrolyte into the interior of carbon and acts reservoirs to relieve the ion-transport pressure during the electrochemical process [18]. CS-H1~5 also show the larger rectangular areas compared with CS, indicating the higher specific capacitances. The GCD curves show the symmetrical triangular shapes in a specific current of 1 A g−1, suggesting the electrical double layer capacitance (Fig. 4b). CS-H1~5 have the longer charge/discharge times than CS, which are caused by the higher specific capacitance (The complete CV and GCD curves are given in Fig. S3 and Fig. S4). High specific surface area of carbon provides sufficient active sites in supercapacitor energy storage, and the microstructure affects the efficiency of ion-storage [20]. With the effect of KOH-added molten salt, CS-H1~5 possess the higher surface areas and the richer pore structures, and therefore perform the much higher capacitance values than CS with only KCl. Fig. 4c summarizes the specific capacitance by calculating the GCD results. CS-H3 shows the capacitance of 286 F g−1 in 1 A g−1 and 242 F g−1 in 20 A g−1, which are higher than those of CS (164 F g−1, 131 F g−1), CS-H1 (217 F g−1, 185 F g−1), CS-H2 (270 F g−1, 228 F g−1) and CS-H5 (256 F g−1, 219 F g−1). For CS-H1~5, CS-H3 shows a slightly higher capacitance than that of CS-H2 although the specific surface area of CS-H3 (1869 m2 g−1) is slightly lower than CS-H2 (1998 m2 g−1). Such result is attributed to the more open pore structure of CS-H3 than CS-H2. The importance of open structure for capacitance also can be demonstrated by comparing CS-H1 and CS-H5. CS-H5 (1699 m2 g−1) shows a similar surface area to CS-H1 (1667 m2 g−1) but a much higher capacitance, which is caused by the highly open sheet structure. Thus, in the balance of specific surface and open structure, the proper addition of KOH (CS-H3) is the key to achieve the maximum specific capacitance. CS-H3 is comparable to other biomass-derived carbons prepared by hydrothermal and activation [3,21,22,26] (the detailed comparison in Table S1). Benefiting from the open structure, CS-H3 shows the superior specific capacitance at a high current density. The Nyquist plots were conducted at the frequency range of 100 kHz to 0.1 Hz, as shown in Fig. 4d. The diameter of semicircle in high frequencies is relevant to the penetration of electrolyte into the electrode material and corresponds the charge transfer resistance [44,45]. The length of 45o inclined line in low frequencies represents the Warburg resistance [46]. With the smaller semicircular diameter and inclined-line length, both the charge transfer resistance and Warburg resistance decrease from CS to CS-H5, suggesting the better mass transfer ability. Aside from KOH, the effect of KCl amount to the preparation of porous carbon and their
Fig. 3. Schematic illustration of carbon formation with different amounts of KOH.
Please cite this article as: L. Yang, J. Qiu, Y. Wang, et al., Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors, Journal of Electroanalytical Chemistry, https://doi.org/10.1016/j.jelechem.2019.113673
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Fig. 4. Electrochemical performances of CS, CS-H1, CS-H2, CS-H3 and CS-H5: CV curves at 50 mV s−1 (a), GCD curves at 1 A g−1 (b), specific capacitance at different specific currents (c), and Nyquist plots (d).
Fig. 5. The comparisons of CS-H3 and CS0-H3: SEM images of CS-H3 (a) and CS0-H3 (b), XRD spectra (c), CV curves at 50 mV s−1 (d), GCD curves at 1 A g−1 (e).
Please cite this article as: L. Yang, J. Qiu, Y. Wang, et al., Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors, Journal of Electroanalytical Chemistry, https://doi.org/10.1016/j.jelechem.2019.113673
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capacitor performances were also explored (details in supporting information). Based on the XRD results in Fig. 2, silica is present. According to the previous studies of carbonizing biomass, the presence of silica would act as an inner template, and the pores could be generated after the template removed [14,24,47]. Therefore, in this study, we also investigated the effects of silica on the carbon synthesis. The original sawdust (~10 g) was mixed with HF aqueous solution (200 ml, ~4 wt%) and allowed to stand overnight. After an exhaustive wash, the obtained sawdust was experienced by the same process as CS and the resulting carbon was donated as CS0. The XRD of CS0 presents the vanished (101) peak of silica (details in Fig. S7). The HF-treatment sawdust was used to prepare a carbon named CS0-H3 in the same method as CS-H3. For CS0-H3 and CS-H3, they show different pore structure when observed by SEM (Fig. 5a and b) and a similar XRD pattern (Fig. 5c). The open pore is difficult to see in CS0-H3, suggesting that the formation of open pore structure is affected by the silica existing during the carbonization. Silica acting as the template in the interior part of the biomass so as building block in creating some potential pore spaces, and the silica is easily corroded by KOH, thus would be replaced by the corrosive MS. Or in another language: the existence of silica makes KOH-added MS permeate easily into the carbon and good for creating a richer pore structure. CS0-H3 also shows a smaller rectangular area along the CV curve and a shorter charge/discharge time of GCD test than that of CS-H3 (Fig. 5d and e), implying a poor capacitance value. Therefore, the existence of silica is beneficial to synthesize the porous carbon with a high modified capacitive performance in KOH-added MS (The complete CV and GCD results of CS0-H3 are available in Fig. S8).
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Two-electrode system in 6 M KOH was used to further explore the supercapacitor performance of CS-H3. The CV and GCD curves both be controlled in a voltage window from 0 to 1 V, as shown in Fig. 6a and b. The CV curves present the nearly perfect rectangle, and it even was held in a high scan rate of 200 mV s−1, indicating the ideal electrical double layer capacitor. From the GCD curves, the specific capacitance was calculated to be 60 F g−1 in the specific current of 0.5 A g−1 and retains 51 F g−1 in the high specific current of 20 A g−1. As shown in Ragone plot of Fig. 6c, the energy density reaches 8.55 W h kg−1 at a power density of 226 W kg−1 and 5.63 W h kg−1 at 9213 W kg−1, which is comparable to the biomass-derived porous carbon from hydrothermal and activation [1,10,17–19,22,48,49]. The cycle test was conducted at the specific current of 10 A g−1, after 10,000 cycles, the capacitance can maintain the 99.8% of the initial value (Fig. 6d). Such high capacitance retention demonstrates the ion accessibility in the pores does not change upon an extremely long working hours with a high-power behavior, suggesting an excellent electrochemical stability and reversibility of the CS-H3 electrode. These results further demonstrate the potential of MS using in preparing porous carbon from biomass. 4. Conclusion In summary, the hierarchical porous carbons were synthesized by using wood sawdust as the carbon source after calcining in molten salt. The mixture of KCl and KOH can function as protective media and structure-directing agent, and the carbonization is allowed to proceed in air. The microstructure of carbon is adjusted by the content of KOH used, and such carbon structure greatly affects the capacitor performance.
Fig. 6. Electrochemical performances of CS-H3 in two-electrode system: CV curves at different scan rates (a), GCD curves at different specific currents (b), Ragone plot (c) and cycling stability at 10 A g−1 (d).
Please cite this article as: L. Yang, J. Qiu, Y. Wang, et al., Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors, Journal of Electroanalytical Chemistry, https://doi.org/10.1016/j.jelechem.2019.113673
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The resulting porous carbon CS-H3 shows the highest specific capacitance of 286 F g−1 in 1 A g−1. This work further demonstrates the advantages of molten salt synthesis in controlling carbon morphology. In future work, the synthesis of carbon from biomass using molten salt while focusing on the molten salts recycling and the material dimension controlling will be very meaningful. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors are grateful for the financial support of National Key Research and Development Program of China (2017YFD0601006). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jelechem.2019.113673. References [1] Y. Huang, L. Peng, Y. Liu, G. Zhao, J.Y. Chen, G. Yu, Biobased nano porous active carbon fibers for high-performance supercapacitors, ACS Appl. Mater. Inter. 8 (2016) 15205–15215. [2] H.-S. Huang, K.-H. Chang, N. Suzuki, Y. Yamauchi, C.-C. Hu, K.C.W. Wu, Evaporationinduced coating of hydrous ruthenium oxide on mesoporous silica nanoparticles to develop high-performance supercapacitors, Small 9 (2013) 2520–2526. [3] H. Feng, H. Hu, H. Dong, Y. Xiao, Y. Cai, B. Lei, Y. Liu, M. Zheng, Hierarchical structured carbon derived from bagasse wastes: a simple and efficient synthesis route and its improved electrochemical properties for high-performance supercapacitors, J. Power Sources 302 (2016) 164–173. [4] B.P. Bastakoti, H.-S. Huang, L.-C. Chen, K.C.W. Wu, Y. Yamauchi, Block copolymer assisted synthesis of porous alpha-Ni(OH)2 microflowers with high surface areas as electrochemical pseudocapacitor materials, Chem. Commun. 48 (2012) 9150–9152. [5] T. Liu, F. Zhang, Y. Song, Y. Li, Revitalizing carbon supercapacitor electrodes with hierarchical porous structures, J. Mater. Chem. A 5 (2017) 17705–17733. [6] S. Makino, Y. Yamauchi, W. Sugimoto, Synthesis of electro-deposited ordered mesoporous RuOx using lyotropic liquid crystal and application toward microsupercapacitors, J. Power Sources 227 (2013) 153–160. [7] J. Deng, M. Li, Y. Wang, Biomass-derived carbon: synthesis and applications in energy storage and conversion, Green Chem. 18 (2016) 4824–4854. [8] J. Deng, T. Xiong, H. Wang, A. Zheng, Y. Wang, Effects of cellulose, hemicellulose, and lignin on the structure and morphology of porous carbons, ACS Sustain. Chem. Eng. 4 (2016) 3750–3756. [9] Y. Lu, S. Zhang, J. Yin, C. Bai, J. Zhang, Y. Li, Y. Yang, Z. Ge, M. Zhang, L. Wei, M. Ma, Y. Ma, Y. Chen, Mesoporous activated carbon materials with ultrahigh mesopore volume and effective specific surface area for high performance supercapacitors, Carbon 124 (2017) 64–71. [10] E.Y.L. Teo, L. Muniandy, E.-P. Ng, F. Adam, A.R. Mohamed, R. Jose, K.F. Chong, High surface area activated carbon from rice husk as a high performance supercapacitor electrode, Electrochim. Acta 192 (2016) 110–119. [11] L. Qie, W. Chen, H. Xu, X. Xiong, Y. Jiang, F. Zou, X. Hu, Y. Xin, Z. Zhang, Y. Huang, Synthesis of functionalized 3D hierarchical porous carbon for high-performance supercapacitors, Energy Environ. Sci. 6 (2013) 2497–2504. [12] L. Yang, Y. Feng, D. Yu, J. Qiu, X.-F. Zhang, D. Dong, J. Yao, Design of ZIF-based CNTs wrapped porous carbon with hierarchical pores as electrode materials for supercapacitors, J. Phys. Chem. Solids 125 (2019) 57–63. [13] C. Young, R.R. Salunkhe, J. Tang, C.-C. Hu, M. Shahabuddin, E. Yanmaz, M.S.A. Hossain, J.H. Kim, Y. Yamauchi, Zeolitic imidazolate framework (ZIF-8) derived nanoporous carbon: the effect of carbonization temperature on the supercapacitor performance in an aqueous electrolyte, Phys. Chem. Chem. Phys. 18 (2016) 29308–29315. [14] Z. Chen, H. Zhuo, Y. Hu, L. Zhong, X. Peng, S. Jing, Q. Liu, X. Zhang, C. Liu, R. Sun, Selfbiotemplate preparation of hierarchical porous carbon with rational mesopore ratio and high oxygen content for an ultrahigh energy-density supercapacitor, ACS Sustain. Chem. Eng. 6 (2018) 7138–7150. [15] Y. Feng, M. Cao, L. Yang, X.-F. Zhang, Y. Wang, D. Yu, X. Gu, J. Yao, Bilayer N-doped carbon derived from furfuryl alcohol-wrapped melamine sponge as highperformance supercapacitor, J. Electroanal. Chem. 823 (2018) 633–637. [16] L. Zhu, F. Shen, R.L. Smith Jr., L. Yan, L. Li, X. Qi, Black liquor-derived porous carbons from rice straw for high-performance supercapacitors, Chem. Eng. J. 316 (2017) 770–777.
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Please cite this article as: L. Yang, J. Qiu, Y. Wang, et al., Molten salt synthesis of hierarchical porous carbon from wood sawdust for supercapacitors, Journal of Electroanalytical Chemistry, https://doi.org/10.1016/j.jelechem.2019.113673