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Oxygen-rich and hierarchical porous carbons prepared from coal based humic acid for supercapacitor electrodes
Fuel Processing Technology 142 (2016) 1–5
Contents lists available at ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Short communication
Oxygen-rich and hierarchical porous carbons prepared from coal based humic acid for supercapacitor electrodes Guangxu Huang ⁎, Weiwei Kang, Baolin Xing, Lunjian Chen, Chuanxiang Zhang Henan Polytechnic University, Henan, Jiaozuo 454000, China
a r t i c l e
i n f o
Article history: Received 6 August 2015 Received in revised form 9 September 2015 Accepted 19 September 2015 Available online xxxx Keywords: Humic acid KOH activation Hierarchical porous carbons Oxygen content Volumetric capacitance
a b s t r a c t Hierarchical porous carbons (HPCs) were prepared from coal-derived humic acid (HA) through a facile KOH activation method and used as electrode materials for supercapacitors. The obtained HPCs possess hierarchical porous structure consisting of micropores less than 1.8 nm, mesopores mainly falling in the narrow range of 3.5–4.5 nm and macropores, together with high oxygen content (N24.0 wt.%). And the HPC based electrodes deliver high volumetric capacitances with a maximum value of 201 F cm−3 at a current density of 5 A g−1, as well as high rate capabilities and excellent cycling stabilities. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Electrochemical double layer capacitors (EDLCs), also known as supercapacitors, have attracted significant attention due to their excellent properties [1]. Porous carbons (PCs) are regarded as the most promising supercapacitor electrode materials, because of their high surface area, electrical conductivity, chemical stability and low cost [2,3]. It is well known that the actual energy storage occurs predominately in the smaller micropores of the PCs, and narrow pore size distribution (PSD) can lead to an increase in capacitance and stored energy density [2]. The mesopores can decrease ion-transport resistance, and the macropores shorten the ion diffusion distance [4,5]. Moreover, introducing of oxygen functional groups to the PC surface is an effective strategy to improve the wettability of electrode to aqueous electrolyte, which can also induce additional pseudocapacitance to the overall capacitance [1,3]. Considering the facts described above, as electrode materials for supercapacitors, it is of great significance to design and produce PCs with a combination of hierarchical porosity, centralized PSD and high oxygen content. Up to now, template techniques have been widely employed to synthesize PCs with well-defined pore characteristics. However, these procedures are complicated, time-consuming and high cost, which restrict their application to mass-production [6,7]. Furthermore, the incorporation of oxygen groups into the carbon matrix is often achieved through oxidative modifications, most of which are unstable and may lead to
⁎ Corresponding author. E-mail address: [email protected] (G. Huang).
http://dx.doi.org/10.1016/j.fuproc.2015.09.025 0378-3820/© 2015 Elsevier B.V. All rights reserved.
poor electrochemical performance [8]. In addition, the volumetric capacitance is also a crucial technology parameter in supercapacitor applications, which is calculated by multiplying the gravimetric capacitance with the corresponding packing density. However, the well-developed porosity often results in low mass density, and how to balance the porosity and mass density of porous carbons still remains challenging [9,10]. The present work developed a simple and low cost method to prepare hierarchical porous carbons (HPCs) from coal-derived humic acid (HA) through mild KOH activation. The as-obtained HPCs possess moderate surface area, centralized pore size distribution and high oxygen content, delivering high volumetric capacitance, excellent rate capability and cycling performance as supercapacitor electrodes. 2. Experiment Coal based HA (Xinjiang Shuanglong Co., Ltd) was treated with a mixture of hydrochloric acid and hydrofluoric acid to reduce the ash content to below 1 wt.%. Other deashing methods that are based on cationic surfactant or NH4OH can also be conducted, which avoid the use of hydrofluoric acid. The elemental analysis of the resultant pure HA was listed in Table 1. Potassium hydroxide and pure HA were mixed with low KOH/HA mass ratios (0.50, 0.75 and 1), followed by adding some deionized water and stirring overnight. The compounds were activated in a tube furnace under a N2 atmosphere at 700 °C for 1 h with a heating rate of 5 °C min−1. After cooling naturally to room temperature, the activated materials were washed with diluted hydrochloric acid, rinsed with deionized water until pH = 7, and dried at 110 °C for 2 h. The obtained materials were referred as HPC1, HPC2
2
G. Huang et al. / Fuel Processing Technology 142 (2016) 1–5
Table 1 Elemental analysis of the pure HA. Sample
C (wt.%)
H (wt.%)
N (wt.%)
Oa (wt.%)
HA
66.3
3.3
1.4
29.0
a
Oxygen determined by difference.
and HPC3 correspondingly. For comparison, PC0 was also prepared following the same procedure but without the addition of KOH. It should be noted that the KOH/HA mass ratios are lower compared to those (2–5) in conventional preparation methods for porous carbons as supercapacitor electrodes, resulting in lower consumption of diluted hydrochloric acid for washing the activated materials correspondingly. N2 adsorption/desorption isotherms were measured using a Quantachrome Autosorb-iQ-MP analyzer at 77 K. The specific surface areas were calculated by a Brunauer–Emmett–Teller (BET) method. The total pore volumes were estimated from the single point adsorption (P/P0 = 0.99) and pore size distributions (PSD) were derived from density functional theory (DFT). The morphology of the sample was observed by a scanning electron microscope (SEM, JSM-6390LV, JEOL) and transmission electron microscopy (TEM, JEM-2100, JEOL). X-ray photoelectron spectroscopy (XPS, Axis Ultra) was used to investigate the surface chemical composition. The supercapacitor electrodes were made with 85 wt.% HPCs, 5 wt.% polytetrafluoroethylene and 10 wt.% acetylene black. A two-electrode cell was used for electrochemical tests with a 3 M KOH solution as the electrolyte. Galvanostatic charge/discharge (GC) and cyclic voltammetry
(CV) were carried out by an electrochemical analyzer system (SCTS, Arbin). The gravimetric capacitances (F g−1) were calculated from the charge–discharge curves according to Cg = I⊿ t/m⊿ V, where I, ⊿t, m and ⊿ V are the discharge current (A), discharge time (s), the mass (g) of the active materials in the signal electrode and the voltage drop upon discharge, respectively. Then, the gravimetric capacitance was converted to volumetric capacitance (F cm−3) by Cv = ρCg, where ρ is the packing density of single electrode. 3. Results and discussion The structure of coal-derived HA is dominated by aromatic moieties containing a wide variety of oxygen-containing functional groups such as carboxylic, phenolic hydroxyl and carbonyl groups [11]. These functional groups are distributed uniformly in the HA, which facilitate the homogeneous KOH activation, and the formation of massive pores with similar diameter. As shown in Fig. 1a, many interconnected macropores with uniform pore size were formed on the surfaces of HPC1. The TEM image (Fig. 1b) reveals that there are lots of mesoporous and microporous texture in HPC1. Fig. 1c shows N2 adsorption/desorption isotherms of the HPC samples. All isotherms exhibit combined characteristics of type I/IV curve, steep uptakes below P/P0 = 0.01 suggest that the samples possess some micropores, the obvious hysteresis loops at the P/P0 from 0.4 to 0.9 indicate the existence of a large percentage of mesopores, and the almost vertical tails at a relative pressure near to 1.0 denote the presence of macroporosity [12,13]. When the HPCs were used as electrode materials for supercapacitors, these macropores serving as
Fig. 1. a. SEM image of HPC1 sample. b. TEM image of HPC1 sample. c. N2 adsorption–desorption isotherms of HPC samples. The inset is the PSD curves.
G. Huang et al. / Fuel Processing Technology 142 (2016) 1–5
3
Table 2 Composition and structure of the HPC samples. Sample
PC0 HPC1 HPC2 HPC3 a
Elemental analysis
XPS analysis
Texture analysis
C
H
N
Oa
N
O
O/C
SBET
Vt
Vmic
Vmes/Vt
(wt.%)
(wt.%)
(wt.%)
(wt.%)
(at.%)
(at.%)
(at./at.)
(m2 g−1)
(cm3 g−1)
(cm3 g−1)
(%)
86.4 71.9 70.2 69.4
2.1 2.5 2.5 2.4
1.7 0.9 1 1.5
9.7 24.8 26.3 26.8
– 2.7 2.5 0.6
– 12.5 13.3 15.2
– 0.15 0.16 0.18
– 662 668 649
– 0.516 0.588 0.415
– 0.309 0.306 0.332
– 40.1 48 20
Oxygen determined by difference.
ion-buffering reservoirs are able to minimize the diffusion distance to interior surfaces [14]. The PSD curves are shown in the inset of Fig. 1c. All the HPCs demonstrate hierarchical pore structure with centralized PSD, a multimodal distribution of micropores less than 1.8 nm along with a large amount of mesopores centralized at 4 nm. It is worth noting that the mesopore size in HPCs falls in a very narrow range of 3.5–4.5 nm, which is the narrowest mesopore size distribution reported previously as far as author's knowledge. These uniform pores with certain pore size may be generated from the reactions between KOH
and different active sites related with oxygen-containing groups of HA. The abundant and uniform mesopores play a key role not only in improving the performance of the supercapacitor at high charge– discharge rates, but also in the high capacitance retention [15]. The porosity properties of the samples are summarized in Table 2. The HPCs display moderate surface areas close to 660 m2 g−1 and total pore volumes of 0.4–0.6 cm3 g−1, while exhibiting large mesopore percentages (Vmes/Vt). In contrast to the HPC1 and HPC2 samples, HPC3 shows relatively lower SBET, Vt and Vmes/Vt but higher Vmic, which is
Fig. 2. a. CV profiles of supercapacitors based on HPCs at different scan rates. b. GC curves of supercapacitors based on HPCs at different current densities. c. Volumetric capacitances of supercapacitors based on HPCs at different discharge rates. The inset is the cycle performance of supercapacitor based on HPC1.
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G. Huang et al. / Fuel Processing Technology 142 (2016) 1–5
attributed to the enhanced KOH activation with higher KOH/HA mass rate, thereby increasing the packing densities of the corresponding electrodes. The packing densities of HPC1, HPC2 and HPC3 electrodes were calculated as 0.762, 0.751 and 0.865 g cm− 3, respectively, which are much higher than that of conventional porous carbons (usually less than 0.5 g cm−3) [10]. The results of elemental and XPS analysis are shown in Table 2. The oxygen content in HPCs increased with the increase of KOH/HA mass ratio from 0.5 to 1. More importantly, all the HPCs have oxygen content higher than 24.0 wt.%, while only 9.7 wt.% oxygen was retained in PC0. A large quantity of oxygen can be retained with the aid of KOH activation, which will be analyzed in forthcoming research. The deconvoluted XPS spectra of the C1s show the existence of various oxygen functionalities on the HPC surface including carboxylic, hydroxyl and carbonyl groups. The abundant oxygen-containing groups not only improve the surface wettability, but also give rise to pseudocapacitance in carbon electrodes. From the SEM, N2 adsorption– desorption and XPS results, it can be concluded that the HPCs demonstrate hierarchical porous structure, centralized PSD and high oxygen content, which are promising as electrode materials for advanced supercapacitors. Fig. 2a shows cyclic voltammograms of the HPC electrodes at various scan rates from 5 to 40 mV s−1. All CV profiles exhibit approximately rectangular shape even at the high scan rate of 40 mV s−1, showing a characteristic of capacitive behavior and good rate capability [16,17].The charge/discharge curves at the current densities of 0.05, 0.25, 0.5 A g−1 exhibit almost symmetrical triangle and have not obvious voltage drop (Fig. 2b), suggesting nearly perfect capacitive behavior and stable electrochemical properties [18]. The corresponding gravimetric capacitances are calculated to be 244, 265 and 276 F g−1 at the current density of 0.05 A g−1 for HPC1, HPC2 and HPC3 electrodes, respectively. The HPC3 has the highest gravimetric capacitance among the three HPCs, which is consistent to its highest oxygen content as well as micropore volume. More importantly, the HPCs deliver considerable volumetric capacitances of 186–239 F cm− 3, and fairly large areal capacitances of about 37–43 μF cm−2 that demonstrates the easy accessibility of electrolyte ions to the interior carbon surfaces [9]. Rate capability is an important factor to evaluate the power applications of supercapacitors [9]. Fig. 2c shows the Cv retention as a function of the charge/discharge current density. As the current density increases to 5 A g−1, the HPCs remain high Cv in the range of 152–201 F cm−3 (over 80% of their initial values at 0.05 A g−1), owing to the abundant meso/macropores and surface oxygen groups that favor the transport of ions. For HPC3 electrode, its volumetric capacitance reaches up to 201 F cm−3 even at a high current density of 5 A g−1. The high volumetric capacitance of HPC3 electrode is competitive to those of some porous carbon materials reported previously, such as carbon hollow submicron spheres (b 200 F cm−3 at 5 A g− 1) [19], highly nanoporous carbons (b150 F cm−3 at 0.5 A g−1) [20], flexible carbon nanofiber/graphene composite paper (112 F cm−3 at 0.5 A g−1) [21], ordered mesoporous carbon nanospheres (107–125 F cm−3 at 1 mA cm−2) [9] and boron and oxygen co-doped carbon nanofiber films (179 F cm−3 at 1 A g−1) [10]. Maxsorb (Kansai, Japan) with a SBET of 1973 m2 g−1, the popularly applied activated carbon, exhibits similar cycle performance and rate capability to HPCs but a lower volumetric capacitance of 100 F cm−3 at 5 A g−1. Cycle life is also a crucial parameter for the supercapacitor application. The HPC1 shows excellent cyclability with little capacity decay at a scan rate of 2.5 A g− 1 after 1000 charge/discharge cycles (the inset of Fig. 2c). The excellent electrochemical performances of HPCs could be attributed to the following characteristics. Firstly, developed porosity with hierarchical structure and centralized PSD favors the ion diffusion and the formation of EDL. Secondly, the presence of oxygen in carbon network increases the surface of the pores accessible by the electrolyte ions, and may also afford stable pseudo-capacitance. Thirdly, the large packing densities as well as gravimetric capacitances of HPC electrodes lead
to high volumetric capacitances, which is highly desirable for practical applications. 4. Conclusions In summary, HPC samples were prepared by a mild KOH activation and used as high-performance supercapacitor electrode materials. With a hierarchical porous structure, centralized pore size distribution, high oxygen content and a high mass density, the HPC samples show high volumetric capacitances, as well as good rate performances and cycling stabilities. Thus the approach should pave a simple and low cost way to fabricate HPC materials for supercapacitors requiring enhanced volumetric capacitance. Conflict of interest The authors have no conflict of interest for the content of this paper. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (U1361119, 51404098, 51174077) and the Foundation of Henan Polytechnic University for Ph. D (B2014-008). References [1] J. Yan, Q. Wang, T. Wei, Z. Fan, Recent advances in design and fabrication of electrochemical supercapacitors with high energy densities, Adv. Energy Mater. 4 (2014) 1–43. [2] Y. Zhai, Y. Dou, D. Zhao, P.F. Fulvio, R.T. Mayes, S. Dai, Carbon materials for chemical capacitive energy storage, Adv. Mater. 23 (2011) 4828–4850. [3] L. Fan, S. Qiao, W. Song, M. Wu, X. He, X. Qu, Effects of the functional groups on the electrochemical properties of ordered porous carbon for supercapacitors, Electrochim. Acta 105 (2013) 299–304. [4] Y. Lv, L. Gan, M. Liu, W. Xiong, Z. Xu, D. Zhu, D.S. Wright, A self-template synthesis of hierarchical porous carbon foams based on banana peel for supercapacitor electrodes, J. Power Sources 209 (2012) 152–157. [5] Y. Han, X. Dong, C. Zhang, S. Liu, Easy synthesis of honeycomb hierarchical porous carbon and its capacitive performance, J. Power Sources 227 (2013) 118–122. [6] H. Fan, F. Ran, X. Zhang, H. Song, W. Jing, K. Shen, L. Kong, L. Kang, Easy fabrication and high electrochemical capacitive performance of hierarchical porous carbon by a method combining liquid–liquid phase separation and pyrolysis process, Electrochim. Acta 138 (2014) 367–375. [7] Y. Gao, W. Zhang, Q. Yue, B. Gao, Y. Sun, J. Kong, P. Zhao, Simple synthesis of hierarchical porous carbon from Enteromorpha prolifera by a self-template method for supercapacitor electrodes, J. Power Sources 270 (2014) 403–410. [8] C. Zhang, D. Long, B. Xing, W. Qiao, R. Zhang, L. Zhan, X. Liang, L. Ling, The superior electrochemical performance of oxygen-rich activated carbons prepared from bituminous coal, Electrochem. Commun. 10 (2008) 1809–1811. [9] X. Yu, J. Wang, Z. Huang, W. Shen, F. Kang, Ordered mesoporous carbon nanospheres as electrode materials for high-performance supercapacitors, Electrochem. Commun. 36 (2013) 66–70. [10] Z. Yu, L. Chen, L. Song, Y. Zhu, H. Ji, S. Yu, Free-standing boron and oxygen co-doped carbon nanofiber films for large volumetric capacitance and high rate capability supercapacitors, Nano Energy 15 (2015) 235–243. [11] G. Hu, B. Tang, X. Min, Synthesis and characterization of alternated (humic acid/Fe3 +)n multilayer film on alumina fiber, Surf. Coat. Technol. 206 (2012) 3586–3594. [12] J. Zhang, L. Jin, J. Cheng, H. Hu, Hierarchical porous carbons prepared from direct coal liquefaction residue and coal for supercapacitor electrodes, Carbon 55 (2013) 221–232. [13] R. Liu, Y. Liu, X. Zhou, Z. Zhang, J. Zhang, F. Dang, Biomass-derived highly porous functional carbon fabricated by using a free-standing template for efficient removal of methylene blue, Bioresour. Technol. 154 (2014) 138–147. [14] J. Jiang, M. Zhou, H. Chen, Z. Wang, L. Bao, S. Zhao, S. Guan, J. Chen, Hierarchically porous carbon derived from an aqueous curable composition for supercapacitors, Electrochim. Acta 168 (2015) 300–307. [15] X. He, R. Li, J. Qiu, K. Xie, P. Ling, M. Yu, X. Zhang, M. Zheng, Synthesis of mesoporous carbons for supercapacitors from coal tar pitch by coupling microwave-assisted KOH activation with a MgO template, Carbon 50 (2012) 4911–4921. [16] W. Tsai, P. Gao, B. Daffos, P. Taberna, C.R. Perez, Y. Gogotsi, F. Favier, P. Simon, Ordered mesoporous silicon carbide-derived carbon for high-power supercapacitors, Electrochem. Commun. 34 (2013) 109–112. [17] Q. Zhao, X. Wang, H. Xia, J. Liu, H. Wang, J. Gao, Y. Zhang, J. Liu, H. Zhou, X. Li, S. Zhang, X. Wang, Design, preparation and performance of novel three-dimensional hierarchically porous carbon for supercapacitors, Electrochim. Acta 173 (2015) 566–574.
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[20] X.Y. Chen, Y.Y. He, H. Song, Z.J. Zhang, Structure and electrochemical performance of highly nanoporous carbons from benzoate–metal complexes by a template carbonization method for supercapacitor application, Carbon 72 (2014) 410–420. [21] C. Ma, X. Wang, Y. Ma, J. Sheng, Y. Li, S. Li, J. Shi, Carbon nanofiber/graphene composite paper for flexible supercapacitors with high volumetric capacitance, Mater. Lett. 145 (2015) 197–200.
Contents lists available at ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Short communication
Oxygen-rich and hierarchical porous carbons prepared from coal based humic acid for supercapacitor electrodes Guangxu Huang ⁎, Weiwei Kang, Baolin Xing, Lunjian Chen, Chuanxiang Zhang Henan Polytechnic University, Henan, Jiaozuo 454000, China
a r t i c l e
i n f o
Article history: Received 6 August 2015 Received in revised form 9 September 2015 Accepted 19 September 2015 Available online xxxx Keywords: Humic acid KOH activation Hierarchical porous carbons Oxygen content Volumetric capacitance
a b s t r a c t Hierarchical porous carbons (HPCs) were prepared from coal-derived humic acid (HA) through a facile KOH activation method and used as electrode materials for supercapacitors. The obtained HPCs possess hierarchical porous structure consisting of micropores less than 1.8 nm, mesopores mainly falling in the narrow range of 3.5–4.5 nm and macropores, together with high oxygen content (N24.0 wt.%). And the HPC based electrodes deliver high volumetric capacitances with a maximum value of 201 F cm−3 at a current density of 5 A g−1, as well as high rate capabilities and excellent cycling stabilities. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Electrochemical double layer capacitors (EDLCs), also known as supercapacitors, have attracted significant attention due to their excellent properties [1]. Porous carbons (PCs) are regarded as the most promising supercapacitor electrode materials, because of their high surface area, electrical conductivity, chemical stability and low cost [2,3]. It is well known that the actual energy storage occurs predominately in the smaller micropores of the PCs, and narrow pore size distribution (PSD) can lead to an increase in capacitance and stored energy density [2]. The mesopores can decrease ion-transport resistance, and the macropores shorten the ion diffusion distance [4,5]. Moreover, introducing of oxygen functional groups to the PC surface is an effective strategy to improve the wettability of electrode to aqueous electrolyte, which can also induce additional pseudocapacitance to the overall capacitance [1,3]. Considering the facts described above, as electrode materials for supercapacitors, it is of great significance to design and produce PCs with a combination of hierarchical porosity, centralized PSD and high oxygen content. Up to now, template techniques have been widely employed to synthesize PCs with well-defined pore characteristics. However, these procedures are complicated, time-consuming and high cost, which restrict their application to mass-production [6,7]. Furthermore, the incorporation of oxygen groups into the carbon matrix is often achieved through oxidative modifications, most of which are unstable and may lead to
⁎ Corresponding author. E-mail address: [email protected] (G. Huang).
http://dx.doi.org/10.1016/j.fuproc.2015.09.025 0378-3820/© 2015 Elsevier B.V. All rights reserved.
poor electrochemical performance [8]. In addition, the volumetric capacitance is also a crucial technology parameter in supercapacitor applications, which is calculated by multiplying the gravimetric capacitance with the corresponding packing density. However, the well-developed porosity often results in low mass density, and how to balance the porosity and mass density of porous carbons still remains challenging [9,10]. The present work developed a simple and low cost method to prepare hierarchical porous carbons (HPCs) from coal-derived humic acid (HA) through mild KOH activation. The as-obtained HPCs possess moderate surface area, centralized pore size distribution and high oxygen content, delivering high volumetric capacitance, excellent rate capability and cycling performance as supercapacitor electrodes. 2. Experiment Coal based HA (Xinjiang Shuanglong Co., Ltd) was treated with a mixture of hydrochloric acid and hydrofluoric acid to reduce the ash content to below 1 wt.%. Other deashing methods that are based on cationic surfactant or NH4OH can also be conducted, which avoid the use of hydrofluoric acid. The elemental analysis of the resultant pure HA was listed in Table 1. Potassium hydroxide and pure HA were mixed with low KOH/HA mass ratios (0.50, 0.75 and 1), followed by adding some deionized water and stirring overnight. The compounds were activated in a tube furnace under a N2 atmosphere at 700 °C for 1 h with a heating rate of 5 °C min−1. After cooling naturally to room temperature, the activated materials were washed with diluted hydrochloric acid, rinsed with deionized water until pH = 7, and dried at 110 °C for 2 h. The obtained materials were referred as HPC1, HPC2
2
G. Huang et al. / Fuel Processing Technology 142 (2016) 1–5
Table 1 Elemental analysis of the pure HA. Sample
C (wt.%)
H (wt.%)
N (wt.%)
Oa (wt.%)
HA
66.3
3.3
1.4
29.0
a
Oxygen determined by difference.
and HPC3 correspondingly. For comparison, PC0 was also prepared following the same procedure but without the addition of KOH. It should be noted that the KOH/HA mass ratios are lower compared to those (2–5) in conventional preparation methods for porous carbons as supercapacitor electrodes, resulting in lower consumption of diluted hydrochloric acid for washing the activated materials correspondingly. N2 adsorption/desorption isotherms were measured using a Quantachrome Autosorb-iQ-MP analyzer at 77 K. The specific surface areas were calculated by a Brunauer–Emmett–Teller (BET) method. The total pore volumes were estimated from the single point adsorption (P/P0 = 0.99) and pore size distributions (PSD) were derived from density functional theory (DFT). The morphology of the sample was observed by a scanning electron microscope (SEM, JSM-6390LV, JEOL) and transmission electron microscopy (TEM, JEM-2100, JEOL). X-ray photoelectron spectroscopy (XPS, Axis Ultra) was used to investigate the surface chemical composition. The supercapacitor electrodes were made with 85 wt.% HPCs, 5 wt.% polytetrafluoroethylene and 10 wt.% acetylene black. A two-electrode cell was used for electrochemical tests with a 3 M KOH solution as the electrolyte. Galvanostatic charge/discharge (GC) and cyclic voltammetry
(CV) were carried out by an electrochemical analyzer system (SCTS, Arbin). The gravimetric capacitances (F g−1) were calculated from the charge–discharge curves according to Cg = I⊿ t/m⊿ V, where I, ⊿t, m and ⊿ V are the discharge current (A), discharge time (s), the mass (g) of the active materials in the signal electrode and the voltage drop upon discharge, respectively. Then, the gravimetric capacitance was converted to volumetric capacitance (F cm−3) by Cv = ρCg, where ρ is the packing density of single electrode. 3. Results and discussion The structure of coal-derived HA is dominated by aromatic moieties containing a wide variety of oxygen-containing functional groups such as carboxylic, phenolic hydroxyl and carbonyl groups [11]. These functional groups are distributed uniformly in the HA, which facilitate the homogeneous KOH activation, and the formation of massive pores with similar diameter. As shown in Fig. 1a, many interconnected macropores with uniform pore size were formed on the surfaces of HPC1. The TEM image (Fig. 1b) reveals that there are lots of mesoporous and microporous texture in HPC1. Fig. 1c shows N2 adsorption/desorption isotherms of the HPC samples. All isotherms exhibit combined characteristics of type I/IV curve, steep uptakes below P/P0 = 0.01 suggest that the samples possess some micropores, the obvious hysteresis loops at the P/P0 from 0.4 to 0.9 indicate the existence of a large percentage of mesopores, and the almost vertical tails at a relative pressure near to 1.0 denote the presence of macroporosity [12,13]. When the HPCs were used as electrode materials for supercapacitors, these macropores serving as
Fig. 1. a. SEM image of HPC1 sample. b. TEM image of HPC1 sample. c. N2 adsorption–desorption isotherms of HPC samples. The inset is the PSD curves.
G. Huang et al. / Fuel Processing Technology 142 (2016) 1–5
3
Table 2 Composition and structure of the HPC samples. Sample
PC0 HPC1 HPC2 HPC3 a
Elemental analysis
XPS analysis
Texture analysis
C
H
N
Oa
N
O
O/C
SBET
Vt
Vmic
Vmes/Vt
(wt.%)
(wt.%)
(wt.%)
(wt.%)
(at.%)
(at.%)
(at./at.)
(m2 g−1)
(cm3 g−1)
(cm3 g−1)
(%)
86.4 71.9 70.2 69.4
2.1 2.5 2.5 2.4
1.7 0.9 1 1.5
9.7 24.8 26.3 26.8
– 2.7 2.5 0.6
– 12.5 13.3 15.2
– 0.15 0.16 0.18
– 662 668 649
– 0.516 0.588 0.415
– 0.309 0.306 0.332
– 40.1 48 20
Oxygen determined by difference.
ion-buffering reservoirs are able to minimize the diffusion distance to interior surfaces [14]. The PSD curves are shown in the inset of Fig. 1c. All the HPCs demonstrate hierarchical pore structure with centralized PSD, a multimodal distribution of micropores less than 1.8 nm along with a large amount of mesopores centralized at 4 nm. It is worth noting that the mesopore size in HPCs falls in a very narrow range of 3.5–4.5 nm, which is the narrowest mesopore size distribution reported previously as far as author's knowledge. These uniform pores with certain pore size may be generated from the reactions between KOH
and different active sites related with oxygen-containing groups of HA. The abundant and uniform mesopores play a key role not only in improving the performance of the supercapacitor at high charge– discharge rates, but also in the high capacitance retention [15]. The porosity properties of the samples are summarized in Table 2. The HPCs display moderate surface areas close to 660 m2 g−1 and total pore volumes of 0.4–0.6 cm3 g−1, while exhibiting large mesopore percentages (Vmes/Vt). In contrast to the HPC1 and HPC2 samples, HPC3 shows relatively lower SBET, Vt and Vmes/Vt but higher Vmic, which is
Fig. 2. a. CV profiles of supercapacitors based on HPCs at different scan rates. b. GC curves of supercapacitors based on HPCs at different current densities. c. Volumetric capacitances of supercapacitors based on HPCs at different discharge rates. The inset is the cycle performance of supercapacitor based on HPC1.
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attributed to the enhanced KOH activation with higher KOH/HA mass rate, thereby increasing the packing densities of the corresponding electrodes. The packing densities of HPC1, HPC2 and HPC3 electrodes were calculated as 0.762, 0.751 and 0.865 g cm− 3, respectively, which are much higher than that of conventional porous carbons (usually less than 0.5 g cm−3) [10]. The results of elemental and XPS analysis are shown in Table 2. The oxygen content in HPCs increased with the increase of KOH/HA mass ratio from 0.5 to 1. More importantly, all the HPCs have oxygen content higher than 24.0 wt.%, while only 9.7 wt.% oxygen was retained in PC0. A large quantity of oxygen can be retained with the aid of KOH activation, which will be analyzed in forthcoming research. The deconvoluted XPS spectra of the C1s show the existence of various oxygen functionalities on the HPC surface including carboxylic, hydroxyl and carbonyl groups. The abundant oxygen-containing groups not only improve the surface wettability, but also give rise to pseudocapacitance in carbon electrodes. From the SEM, N2 adsorption– desorption and XPS results, it can be concluded that the HPCs demonstrate hierarchical porous structure, centralized PSD and high oxygen content, which are promising as electrode materials for advanced supercapacitors. Fig. 2a shows cyclic voltammograms of the HPC electrodes at various scan rates from 5 to 40 mV s−1. All CV profiles exhibit approximately rectangular shape even at the high scan rate of 40 mV s−1, showing a characteristic of capacitive behavior and good rate capability [16,17].The charge/discharge curves at the current densities of 0.05, 0.25, 0.5 A g−1 exhibit almost symmetrical triangle and have not obvious voltage drop (Fig. 2b), suggesting nearly perfect capacitive behavior and stable electrochemical properties [18]. The corresponding gravimetric capacitances are calculated to be 244, 265 and 276 F g−1 at the current density of 0.05 A g−1 for HPC1, HPC2 and HPC3 electrodes, respectively. The HPC3 has the highest gravimetric capacitance among the three HPCs, which is consistent to its highest oxygen content as well as micropore volume. More importantly, the HPCs deliver considerable volumetric capacitances of 186–239 F cm− 3, and fairly large areal capacitances of about 37–43 μF cm−2 that demonstrates the easy accessibility of electrolyte ions to the interior carbon surfaces [9]. Rate capability is an important factor to evaluate the power applications of supercapacitors [9]. Fig. 2c shows the Cv retention as a function of the charge/discharge current density. As the current density increases to 5 A g−1, the HPCs remain high Cv in the range of 152–201 F cm−3 (over 80% of their initial values at 0.05 A g−1), owing to the abundant meso/macropores and surface oxygen groups that favor the transport of ions. For HPC3 electrode, its volumetric capacitance reaches up to 201 F cm−3 even at a high current density of 5 A g−1. The high volumetric capacitance of HPC3 electrode is competitive to those of some porous carbon materials reported previously, such as carbon hollow submicron spheres (b 200 F cm−3 at 5 A g− 1) [19], highly nanoporous carbons (b150 F cm−3 at 0.5 A g−1) [20], flexible carbon nanofiber/graphene composite paper (112 F cm−3 at 0.5 A g−1) [21], ordered mesoporous carbon nanospheres (107–125 F cm−3 at 1 mA cm−2) [9] and boron and oxygen co-doped carbon nanofiber films (179 F cm−3 at 1 A g−1) [10]. Maxsorb (Kansai, Japan) with a SBET of 1973 m2 g−1, the popularly applied activated carbon, exhibits similar cycle performance and rate capability to HPCs but a lower volumetric capacitance of 100 F cm−3 at 5 A g−1. Cycle life is also a crucial parameter for the supercapacitor application. The HPC1 shows excellent cyclability with little capacity decay at a scan rate of 2.5 A g− 1 after 1000 charge/discharge cycles (the inset of Fig. 2c). The excellent electrochemical performances of HPCs could be attributed to the following characteristics. Firstly, developed porosity with hierarchical structure and centralized PSD favors the ion diffusion and the formation of EDL. Secondly, the presence of oxygen in carbon network increases the surface of the pores accessible by the electrolyte ions, and may also afford stable pseudo-capacitance. Thirdly, the large packing densities as well as gravimetric capacitances of HPC electrodes lead
to high volumetric capacitances, which is highly desirable for practical applications. 4. Conclusions In summary, HPC samples were prepared by a mild KOH activation and used as high-performance supercapacitor electrode materials. With a hierarchical porous structure, centralized pore size distribution, high oxygen content and a high mass density, the HPC samples show high volumetric capacitances, as well as good rate performances and cycling stabilities. Thus the approach should pave a simple and low cost way to fabricate HPC materials for supercapacitors requiring enhanced volumetric capacitance. Conflict of interest The authors have no conflict of interest for the content of this paper. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (U1361119, 51404098, 51174077) and the Foundation of Henan Polytechnic University for Ph. D (B2014-008). References [1] J. Yan, Q. Wang, T. Wei, Z. Fan, Recent advances in design and fabrication of electrochemical supercapacitors with high energy densities, Adv. Energy Mater. 4 (2014) 1–43. [2] Y. Zhai, Y. Dou, D. Zhao, P.F. Fulvio, R.T. Mayes, S. Dai, Carbon materials for chemical capacitive energy storage, Adv. Mater. 23 (2011) 4828–4850. [3] L. Fan, S. Qiao, W. Song, M. Wu, X. He, X. Qu, Effects of the functional groups on the electrochemical properties of ordered porous carbon for supercapacitors, Electrochim. Acta 105 (2013) 299–304. [4] Y. Lv, L. Gan, M. Liu, W. Xiong, Z. Xu, D. Zhu, D.S. Wright, A self-template synthesis of hierarchical porous carbon foams based on banana peel for supercapacitor electrodes, J. Power Sources 209 (2012) 152–157. [5] Y. Han, X. Dong, C. Zhang, S. Liu, Easy synthesis of honeycomb hierarchical porous carbon and its capacitive performance, J. Power Sources 227 (2013) 118–122. [6] H. Fan, F. Ran, X. Zhang, H. Song, W. Jing, K. Shen, L. Kong, L. Kang, Easy fabrication and high electrochemical capacitive performance of hierarchical porous carbon by a method combining liquid–liquid phase separation and pyrolysis process, Electrochim. Acta 138 (2014) 367–375. [7] Y. Gao, W. Zhang, Q. Yue, B. Gao, Y. Sun, J. Kong, P. Zhao, Simple synthesis of hierarchical porous carbon from Enteromorpha prolifera by a self-template method for supercapacitor electrodes, J. Power Sources 270 (2014) 403–410. [8] C. Zhang, D. Long, B. Xing, W. Qiao, R. Zhang, L. Zhan, X. Liang, L. Ling, The superior electrochemical performance of oxygen-rich activated carbons prepared from bituminous coal, Electrochem. Commun. 10 (2008) 1809–1811. [9] X. Yu, J. Wang, Z. Huang, W. Shen, F. Kang, Ordered mesoporous carbon nanospheres as electrode materials for high-performance supercapacitors, Electrochem. Commun. 36 (2013) 66–70. [10] Z. Yu, L. Chen, L. Song, Y. Zhu, H. Ji, S. Yu, Free-standing boron and oxygen co-doped carbon nanofiber films for large volumetric capacitance and high rate capability supercapacitors, Nano Energy 15 (2015) 235–243. [11] G. Hu, B. Tang, X. Min, Synthesis and characterization of alternated (humic acid/Fe3 +)n multilayer film on alumina fiber, Surf. Coat. Technol. 206 (2012) 3586–3594. [12] J. Zhang, L. Jin, J. Cheng, H. Hu, Hierarchical porous carbons prepared from direct coal liquefaction residue and coal for supercapacitor electrodes, Carbon 55 (2013) 221–232. [13] R. Liu, Y. Liu, X. Zhou, Z. Zhang, J. Zhang, F. Dang, Biomass-derived highly porous functional carbon fabricated by using a free-standing template for efficient removal of methylene blue, Bioresour. Technol. 154 (2014) 138–147. [14] J. Jiang, M. Zhou, H. Chen, Z. Wang, L. Bao, S. Zhao, S. Guan, J. Chen, Hierarchically porous carbon derived from an aqueous curable composition for supercapacitors, Electrochim. Acta 168 (2015) 300–307. [15] X. He, R. Li, J. Qiu, K. Xie, P. Ling, M. Yu, X. Zhang, M. Zheng, Synthesis of mesoporous carbons for supercapacitors from coal tar pitch by coupling microwave-assisted KOH activation with a MgO template, Carbon 50 (2012) 4911–4921. [16] W. Tsai, P. Gao, B. Daffos, P. Taberna, C.R. Perez, Y. Gogotsi, F. Favier, P. Simon, Ordered mesoporous silicon carbide-derived carbon for high-power supercapacitors, Electrochem. Commun. 34 (2013) 109–112. [17] Q. Zhao, X. Wang, H. Xia, J. Liu, H. Wang, J. Gao, Y. Zhang, J. Liu, H. Zhou, X. Li, S. Zhang, X. Wang, Design, preparation and performance of novel three-dimensional hierarchically porous carbon for supercapacitors, Electrochim. Acta 173 (2015) 566–574.
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