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hollow carbon nanorods composites as electrode materials for supercapacitor
Journal of the Taiwan Institute of Chemical Engineers 101 (2019) 244–250
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Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Hollow carbon spheres/hollow carbon nanorods composites as electrode materials for supercapacitor Xinyu Fu a, Lei Liu a,∗, Yifeng Yu a, Haijun Lv a, Yue Zhang a, Senlin Hou b,∗, Aibing Chen a,∗ a b
College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China The Second Hospital of Hebei Medical University, 215 Heping Road, Shijiazhuang 050000, China
a r t i c l e
i n f o
Article history: Received 19 December 2018 Revised 11 March 2019 Accepted 25 April 2019 Available online 4 June 2019 Keywords: Carbon-based materials Supercapacitor Dissolution–reassembly Composites materials Hollow carbon spheres/hollow carbon nanorods
a b s t r a c t Carbon-based materials with different morphologies have special properties suitable for application in adsorption, catalysis, energy storage and so on. Carbon composites consisting of different morphologies are proved to improve the performance due to combination of favorable structural features. In this work, hollow carbon spheres/hollow carbon nanorods (HCS/HCR) composites are prepared by “dissolution– reassembly” combined with hard template method. Taking advantage of compositional inhomogeneity of 3-aminophenol/formaldehyde (3-AF) resin sphere, 3-AF oligomers are obtained by dissolution of resin sphere with acetone and then used to reassemble with silica oligomer on MnO2 nanorods template under the function of CTAB to form HCS/HCR composites after carbonization and removing template. The obtained HCS/HCR composites with combined characteristics of hollow sphere and hollow nanorod exhibit high surface area (1590 m2 g−1 ), large pore volumes (2.4 cm3 g−1 ), and uniform pore size distribution (9.3 nm). When used as electrode material, the obtained HCS/HCR composites show good specific capacitance of 250 F g−1 at a current density of 1 A g−1 in 6 M KOH aqueous electrolyte solution, as well as good cycling stability (91.3% capacity retention after 50 0 0 cycles), suggesting that the HCS/HCR composites electrode materials have potential applications in high-performance supercapacitor. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors, also known as electrochemical capacitors, have been widely applied as promising energy storage devices owing to their high power density, high rate capacity and long service lifetimes [1–3]. Considering the great global demand for energy and development of new energy techniques, supercapacitors have predictable huge market potential in electrical vehicles, standalone low-power electronics, digital device pulsing techniques, regenerative braking systems, etc. [4,5]. Electrode material is the key component in supercapacitors, which largely determine the performance of devices. So much effort has been focused on the design and synthesis of high-performance electrode materials. Carbon-based materials have been widely used as electrodes due to their high surface area, good electronic conductivity and chemical stability [5,6]. Many carbon-based materials with different morphologies including spheres, tubes, wires, sheets, and monolith have been explored to obtain high-performance electrodes materials. Among them, typical 0-dimensional hollow car-
∗
Corresponding authors. E-mail addresses: [email protected] (L. Liu), [email protected] (S. Hou), [email protected] (A. Chen).
bon spheres with low density, regular morphology, large cavity and adjustable diameter have been intensively investigated to increase the charge storage capability [6–8]. The template method is one of the most effective methods to prepare hollow carbon spheres, and also realizes the controllable adjustment of wall thickness and surface properties [8,9]. Except for the hollow carbon sphere, typical 1-dimensional carbon, nanotubes or nanofibers are also widely used as electrode materials [9–11]. The formation of continuous conductive network and large surface area can enable the accommodation of charges in large quantity, which can provide efficient transport for both electrons and ions along its longitudinal axis when use as electrodes [12,13]. Considering the advantages of combined multi-dimensional structure such as spheres, sheets, and nanotube, the preparation of composite materials with hollow spheres/tube or other carbon composites with multi-dimensional structure have attracted wide attention in recent years, which can further improve the supercapacitor performance of materials [14]. For example, different types of spheres combined with 2-dimensional sheets, including sphere/sheet [15,16], hollow sphere/sheet [17], and mesoporous sphere/sheet [18] have been successfully prepared. In addition, other types of composites such as carbon tube/sheet [19] and carbon nanotube/sphere [20] have been synthesized. All of the composites can inherit the original characteristics from the sphere,
https://doi.org/10.1016/j.jtice.2019.04.043 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
X. Fu, L. Liu and Y. Yu et al. / Journal of the Taiwan Institute of Chemical Engineers 101 (2019) 244–250
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and hollow carbon nanorods can avoid particles aggregation and improve the electrochemical properties. Herein, hollow carbon sphere/hollow carbon nanorods (HCS/ HCR) composites were prepared via “dissolution–reassembly” strategy combined template method by using 3-aminophenol/ formaldehyde (3-AF) resin sphere as carbon precursor and MnO2 nanorods as hard templates. Acetone was added to selectively remove oligomer component of 3-AF resin sphere to form hollow structure. The hollow 3-AF resin sphere, MnO2 nanorods templates, 3-AF oligomer and tetraethyl orthosilicate (TEOS) were coassembled under assisting of cationic surfactant cetyltrimethylammonium bromide (CTAB). After removing the MnO2 template/silica, the HCS/HCR composites were obtained which combined the morphology and advantages of hollow sphere and nanorod structure, showing good performance when used as supercapacitor electrode. Fig. 1. Schematic illustration of the procedure to prepare HCS/HCR composites.
sheet or tube, which have shown improved physical and chemical properties, and enhanced electrochemical properties [21]. As we know, hollow carbon sphere is conducive to increase the effective surface area, decrease the mass diffusion and transport resistance [22,23], nanorod skeleton will improve the dispersity and the stability of the hollow spheres, which can produce more direct and rapid electron transfer in electrode materials [24]. Therefore, it is desirable to combine the unique properties of hollow carbon spheres and nanorods to obtain new electrode composites with enhanced supercapacitor performance. Charge transfer in the material structure is a complex process, it is necessary to consider the transmission resistance and diffusion distance. Ion-buffering reservoirs can be formed in the cavity of hollow carbon spheres to minimize the diffusion distances to the interior surfaces, and the hollow nanorod structure provides low-resistant pathways for the ions. Meanwhile, the composite structure of hollow carbon spheres
2. Experimental The detailed preparing process of HCS/HCR composites and measurements are shown in Supporting Information. 3. Results and discussion The synthesis of HCS/HCR composites by “dissolution– reassembly” combined template method is illustrated in Fig. 1. In brief, 3-AP and formaldehyde polymerized to form 3-AF resin spheres rapidly under the catalysis of ammonia. But the component of 3-AF resin spheres is inhomogeneous. The outer part of the sphere is highly polymerized resin while the inner part consists of soluble oligomers which could be dissolved to form large cavity by acetone with a large number of dissolved oligomers. Then, the CTAB functionalized MnO2 nanorods and TEOS are added to the reaction system as hard template and assistant reagent respectively. Subsequently, the oligomer, and TEOS co-assemble onto the surface of the hollow 3-AF resin spheres and MnO2 nanorods via
Fig. 2. SEM (a, b) and TEM (c, d) images of the HCS/HCR composites. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Fig. 3. (a) Nitrogen adsorption–desorption isotherms and (b) pore size distribution of HCS/HCR composites.
Fig. 4. (a) XPS spectrum; (b) C1s spectrum; (c) O1s spectrum; (d) N1s spectrum of HCS/HCR composites.
electrostatic interaction forming hollow resin spheres/nanorods composites. After annealing and etching the templates and silica, the HCS/HCR composites are obtained. Fig. 2 shows the SEM and TEM images of HCS/HCR composites. As can be seen in SEM image (Fig. 2(a)), a large amount of spheres distribute on the surface of nanotubes. High-magnification SEM (Fig. 2(b)) shows some holes on the spheres (the red sign), which proves that the spheres are hollow structure. The TEM images (Fig. 2(c) and (d)) further illustrate the HCS/HCR composites consisting of a lot of hollow spheres with uniform size distributed on the hollow nanorods. The hollow nanorods are woven together to form a network structure that facilitates charge transport and the hollow carbon spheres provide space for storage of charge. It is observed from the high-resolution TEM image (Fig. 2(d)) that outer diameter and inner diameter of the nanorods are ca. 60 and 30 nm, respectively, and the cavity and diameter size of hollow
spheres are ca. 260 and 295 nm, respectively. It indicates that the acetone can selectively remove the oligomer inside the 3-AF resin spheres to form the hollow sphere structure and MnO2 nanorods are effective template to prepare hollow nanorod structure. Hollow spheres and nanorods can be obtained by assembly of TEOS and dissolved oligomer resins under the action of CTAB. Nitrogen isothermal adsorption–desorption measurements are performed to analyze the textural properties of the HCS/HCR composites. As depicted in Fig. 3, a type of IV adsorption-desorption isotherm and a type of H3 hysteresis loop are observed. A slow increase trend at low relative pressure (P/P0 < 0.1) is observed, revealing the presence of some micropores in the composites [25]. The obvious hysteresis loop at a high relative pressure in 0.4 < P/P0 < 0.9 is indicative of mesoporous structure [26]. The adsorption isotherm with a sharp capillary condensation step at high relative pressures (P/P0 = 0.9–1.0) indicate large cavity in the HCS/HCR
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Fig. 5. Electrochemical capacitive behavior of HCS/HCR measured in a three-electrode system: (a) cyclic voltammetry curves of the HCS/HCR composites at 5–200 mV s− 1 ; (b) galvanostatic charge/discharge curves of the HCS/HCR composites at 1–10 A g− 1 ; (c) the specific capacitance of the HCS/HCR composites electrode with different current densities and the comparison with reported various morphology carbon electrodes; (d) Nyquist plot measured with frequency range of 10−2 –105 Hz; (e) Cycling stability of HCS/HCR composites upon charging-discharging at a current density of 5 A g− 1 and the insert shows the GCD curves for the 1st and 50 0 0th cycles.
composites [27,28]. A narrow pore size distribution with a mean value of 9.3 nm (Fig. 3(b)) is calculated from the adsorption branch by BJH method. Notably, the obtained HCS/HCR composites have a high BET surface area of 1590 m2 g−1 and high BJH pore volume of 2.4 cm3 g−1 . The 3-AF not only played the role of carbon precursor, but also acted as an excellent nitrogen precursor, leading to in-situ nitrogen doping for HCS/HCR composites. XPS was used to provide
qualitative information about the surface elemental composition and its chemical environments, as shown in Fig. 4. The XPS of HCS/HCR composites clearly revealed three typical peaks of C1s (284.6 eV), O1s (532.5 eV) and N1s (400.4 eV) at Fig. 4(a). Elemental analysis of XPS revealed that the carbon, oxygen and nitrogen content in the HCS/HCR composites were 89.9, 5.6 and 4.5 wt%, respectively. These results further confirmed that the heteroatom of nitrogen had been successfully doped into the carbon skeleton
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Fig. 6. Electrochemical capacitive behavior of HCS/HCR composites measured in a two-electrode system: (a) cyclic voltammetry curves at 5–200 mV s− 1 ; (b) galvanostatic charge/discharge curves at 1–10 A g− 1 ; (c) the specific capacitance at different current densities; (d) energy density vs power density curves.
of HCS/HCR composites. High-resolution XPS spectra for each element were performed and fitted, and the corresponding forms of each element were analyzed. As shown in Fig. 4(b), the spectrum of C1s of HCS/HCR composites could be deconvoluted into three type peaks, corresponding to C–C at 284.6 eV, C–N at 285.4 eV, C=O at 288.8 eV, which further indicated the existence of elements N, O in the surface of HCS/HCR composites. The spectrum of O1s (Fig. 4(c)) could be deconvoluted into four type peaks with binding energies of 530.0, 531.1, 532.9 and 536.4 eV corresponding to oxygen in carboxyl groups, C=O and chemically adsorbed oxygen, respectively. The high resolution XPS spectrum of N1s (Fig. 4(d)) could be further deconvoluted into three peaks at 398.3, 400.9, and 403.8 eV corresponding to the contribution of pyridinic nitrogen (at 398.3 eV), quaternary nitrogen (at 400.9 eV) and pyridine N-oxide (at 403.8 eV). The pyridinic nitrogen provides the active sites for electrode materials, and the quaternary nitrogen not only is the most stable nitrogen species during the process of pyrolysis but also can improve the electrical conductivity of the carbon materials. The HCS/HCR composites consisting of hollow spheres and nanorods structure have high surface area, large pore volume and uniform pore size distribution, which would endow the composite with outstanding capacitive performance as electrode material. The supercapacitive performances of HCS/HCR composites were evaluated by CV in three-electrode systems with 6 M KOH aqueous as electrolyte. Fig. 5(a) shows the CV curves of the HCS/HCR composites electrode at different scan rates with potential windows
ranging from −1 to 0 V. The shapes of these curves are quasirectangular, indicating the ideal electrical double-layer capacitance behavior and fast charging/discharging process [28]. Rate capability is one of the important factors for evaluating the power applications of supercapacitors. The GCD curves of the HCS/HCR composites at different current densities are shown in Fig. 5(b). The nearly isosceles triangle shape of all charge–discharge curves further confirms its excellent reversibility, high Coulombic efficiency, and good charge transport [29]. The charge–discharge curves show that HCS/HCR composites exhibit the largest capacitance of 250 F g−1 at the current density of 1 A g−1 . It is also worthy to point out that a high areal capacitance of 207 F g−1 is maintained at the current density reached 10 A g−1 , which manifests a remarkable capacitance retention of 80.5% (Fig. 5(c)). The HCS/HCR composites have superior performance when compared with other carbon materials (details presented in Table S1 [22,24,29–38]) reported recently. The higher capacitance of HCS/HCR indicates that the hollow sphere and hollow nanorod structure allow easy electrolyte penetration to the interface between electrode and electrolyte and high diffusion and transport of electrolyte ions. The electrochemical impedance spectroscopy (EIS) plot is used to analyze the electrical double layer capacitor behavior. An EIS test is carried out in a frequency range from 10−2 –105 Hz at open circuit voltage of 5 mV to evaluate the electrochemical behaviors of the supercapacitor (Fig. 5(d)). The HCS/HCR composites sample shows a vertical line at the low-frequency region, indicating an ideal capacitive behavior of EDLC. There is no obvious semi-
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circle in correlation with the charge-transfer resistance, suggesting that the HCS/HCR composites have low impedance at the electrode/electrolyte interface. In the equivalent circuit, Rs corresponds to the electrolyte resistance, Rct is charge transfer resistance at the electrode/electrolyte interface, CPEdl and Cf correspond to double layer capacitance and faradic capacitance, and Zw corresponds to Warburg impedance [39,40]. The HCS/HCR composites also show excellent cycling stability (Fig. 5(e)) in three-electrode supercapacitor. After 50 0 0 cycles, this device shows 91.3% capacitance retention of capacitance compared to the initial value. Additionally, the 50 0 0th galvanostatic charge– discharge curve (inset in Fig. 5(e)) still displays a symmetrical triangular shape, indicating that the supercapacitor possesses good charge propagation and long term electrochemical stability. The electrochemical properties of electrode materials are also tested by two-electrode system which can reflect their performance in practical application [41]. CV and GCD are tested in two-electrode system to determine the capacitive characters of HCS/HCR composites. Fig. 6(a) shows the CV curves of HCS/HCR composites electrode at different scan rates in a 6 M KOH electrolyte. All the CV curves show quasi-rectangular shape with inconspicuous distortion as the scan rate increased, demonstrating the ideal capacitive behavior of HCS/HCR composites as electrode. The GCD curves at different current densities are presented in Fig. 6(b). All curves show isosceles triangular shapes demonstrating an almost ideal capacitor behavior, even at current densities as high as 10 A g−1 . A retain in capacitance as large as 64% was observed for the HCS/HCR composites when the current density increased from 1 A g−1 to 10 A g−1 (Fig. 6(c)). A high capacitance 175 F g−1 at a current density of 1 A g−1 is observed for HCS/HCR composites and the capacitance remains 112 F g−1 when current density reaches up to 10 A g−1 . The HCS/HCR composites in symmetric supercapacitor exhibit a maximum energy density of 24.3 Wh kg−1 (with a power density of 20 0 0 W kg−1 ) (Fig. 6(d)). Evidently, these encouraging results further reveal the potential of HCS/HCR composites as the electrodes in supercapacitor. 4. Conclusions In summary, the HCS/HCR composites are prepared by “dissolve–reassembly” strategy combined hard template method. This method is based on the internal/external polymerization heterogeneity of the 3-AF resin sphere, which can create hollow structure in 3-AF resin sphere and provide 3-AF resin oligomer by the dissolution of acetone. 3-AF resin oligomer further assembles with hard template MnO2 nanorods and TEOS together in the action of CTAB. The obtained HCS/HCR composites combined with 0-dimensional hollow sphere and 1-dimensional hollow rod structure show high surface area, large pore volumes, and uniform mesoporous. When used as electrode material, the HCS/HCR composites exhibit high specific capacitance of 250 F g−1 at a current density of 1 A g−1 , and good cycling stability of 91.3% capacitance retention ratio after 50 0 0 cycles at 5 A g−1 as revealed by electrochemical measurements. This work provides a new direction for preparation of multi-dimensional carbon-based composite material for supercapacitors catalysis and adsorption applications. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21676070), the Excellent Going Abroad Experts’ Training Program in Hebei Province, Hebei One Hundred-Excellent Innovative Talent Program (III) (SLRC2017034), Hebei Science and Technology Project (17214304D), Special Project for Synthesis and Application of Graphene in Hebei University of
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Science and Technology (2015PT65), and Beijing National Laboratory for Molecular Sciences.
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Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Hollow carbon spheres/hollow carbon nanorods composites as electrode materials for supercapacitor Xinyu Fu a, Lei Liu a,∗, Yifeng Yu a, Haijun Lv a, Yue Zhang a, Senlin Hou b,∗, Aibing Chen a,∗ a b
College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China The Second Hospital of Hebei Medical University, 215 Heping Road, Shijiazhuang 050000, China
a r t i c l e
i n f o
Article history: Received 19 December 2018 Revised 11 March 2019 Accepted 25 April 2019 Available online 4 June 2019 Keywords: Carbon-based materials Supercapacitor Dissolution–reassembly Composites materials Hollow carbon spheres/hollow carbon nanorods
a b s t r a c t Carbon-based materials with different morphologies have special properties suitable for application in adsorption, catalysis, energy storage and so on. Carbon composites consisting of different morphologies are proved to improve the performance due to combination of favorable structural features. In this work, hollow carbon spheres/hollow carbon nanorods (HCS/HCR) composites are prepared by “dissolution– reassembly” combined with hard template method. Taking advantage of compositional inhomogeneity of 3-aminophenol/formaldehyde (3-AF) resin sphere, 3-AF oligomers are obtained by dissolution of resin sphere with acetone and then used to reassemble with silica oligomer on MnO2 nanorods template under the function of CTAB to form HCS/HCR composites after carbonization and removing template. The obtained HCS/HCR composites with combined characteristics of hollow sphere and hollow nanorod exhibit high surface area (1590 m2 g−1 ), large pore volumes (2.4 cm3 g−1 ), and uniform pore size distribution (9.3 nm). When used as electrode material, the obtained HCS/HCR composites show good specific capacitance of 250 F g−1 at a current density of 1 A g−1 in 6 M KOH aqueous electrolyte solution, as well as good cycling stability (91.3% capacity retention after 50 0 0 cycles), suggesting that the HCS/HCR composites electrode materials have potential applications in high-performance supercapacitor. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors, also known as electrochemical capacitors, have been widely applied as promising energy storage devices owing to their high power density, high rate capacity and long service lifetimes [1–3]. Considering the great global demand for energy and development of new energy techniques, supercapacitors have predictable huge market potential in electrical vehicles, standalone low-power electronics, digital device pulsing techniques, regenerative braking systems, etc. [4,5]. Electrode material is the key component in supercapacitors, which largely determine the performance of devices. So much effort has been focused on the design and synthesis of high-performance electrode materials. Carbon-based materials have been widely used as electrodes due to their high surface area, good electronic conductivity and chemical stability [5,6]. Many carbon-based materials with different morphologies including spheres, tubes, wires, sheets, and monolith have been explored to obtain high-performance electrodes materials. Among them, typical 0-dimensional hollow car-
∗
Corresponding authors. E-mail addresses: [email protected] (L. Liu), [email protected] (S. Hou), [email protected] (A. Chen).
bon spheres with low density, regular morphology, large cavity and adjustable diameter have been intensively investigated to increase the charge storage capability [6–8]. The template method is one of the most effective methods to prepare hollow carbon spheres, and also realizes the controllable adjustment of wall thickness and surface properties [8,9]. Except for the hollow carbon sphere, typical 1-dimensional carbon, nanotubes or nanofibers are also widely used as electrode materials [9–11]. The formation of continuous conductive network and large surface area can enable the accommodation of charges in large quantity, which can provide efficient transport for both electrons and ions along its longitudinal axis when use as electrodes [12,13]. Considering the advantages of combined multi-dimensional structure such as spheres, sheets, and nanotube, the preparation of composite materials with hollow spheres/tube or other carbon composites with multi-dimensional structure have attracted wide attention in recent years, which can further improve the supercapacitor performance of materials [14]. For example, different types of spheres combined with 2-dimensional sheets, including sphere/sheet [15,16], hollow sphere/sheet [17], and mesoporous sphere/sheet [18] have been successfully prepared. In addition, other types of composites such as carbon tube/sheet [19] and carbon nanotube/sphere [20] have been synthesized. All of the composites can inherit the original characteristics from the sphere,
https://doi.org/10.1016/j.jtice.2019.04.043 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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and hollow carbon nanorods can avoid particles aggregation and improve the electrochemical properties. Herein, hollow carbon sphere/hollow carbon nanorods (HCS/ HCR) composites were prepared via “dissolution–reassembly” strategy combined template method by using 3-aminophenol/ formaldehyde (3-AF) resin sphere as carbon precursor and MnO2 nanorods as hard templates. Acetone was added to selectively remove oligomer component of 3-AF resin sphere to form hollow structure. The hollow 3-AF resin sphere, MnO2 nanorods templates, 3-AF oligomer and tetraethyl orthosilicate (TEOS) were coassembled under assisting of cationic surfactant cetyltrimethylammonium bromide (CTAB). After removing the MnO2 template/silica, the HCS/HCR composites were obtained which combined the morphology and advantages of hollow sphere and nanorod structure, showing good performance when used as supercapacitor electrode. Fig. 1. Schematic illustration of the procedure to prepare HCS/HCR composites.
sheet or tube, which have shown improved physical and chemical properties, and enhanced electrochemical properties [21]. As we know, hollow carbon sphere is conducive to increase the effective surface area, decrease the mass diffusion and transport resistance [22,23], nanorod skeleton will improve the dispersity and the stability of the hollow spheres, which can produce more direct and rapid electron transfer in electrode materials [24]. Therefore, it is desirable to combine the unique properties of hollow carbon spheres and nanorods to obtain new electrode composites with enhanced supercapacitor performance. Charge transfer in the material structure is a complex process, it is necessary to consider the transmission resistance and diffusion distance. Ion-buffering reservoirs can be formed in the cavity of hollow carbon spheres to minimize the diffusion distances to the interior surfaces, and the hollow nanorod structure provides low-resistant pathways for the ions. Meanwhile, the composite structure of hollow carbon spheres
2. Experimental The detailed preparing process of HCS/HCR composites and measurements are shown in Supporting Information. 3. Results and discussion The synthesis of HCS/HCR composites by “dissolution– reassembly” combined template method is illustrated in Fig. 1. In brief, 3-AP and formaldehyde polymerized to form 3-AF resin spheres rapidly under the catalysis of ammonia. But the component of 3-AF resin spheres is inhomogeneous. The outer part of the sphere is highly polymerized resin while the inner part consists of soluble oligomers which could be dissolved to form large cavity by acetone with a large number of dissolved oligomers. Then, the CTAB functionalized MnO2 nanorods and TEOS are added to the reaction system as hard template and assistant reagent respectively. Subsequently, the oligomer, and TEOS co-assemble onto the surface of the hollow 3-AF resin spheres and MnO2 nanorods via
Fig. 2. SEM (a, b) and TEM (c, d) images of the HCS/HCR composites. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Fig. 3. (a) Nitrogen adsorption–desorption isotherms and (b) pore size distribution of HCS/HCR composites.
Fig. 4. (a) XPS spectrum; (b) C1s spectrum; (c) O1s spectrum; (d) N1s spectrum of HCS/HCR composites.
electrostatic interaction forming hollow resin spheres/nanorods composites. After annealing and etching the templates and silica, the HCS/HCR composites are obtained. Fig. 2 shows the SEM and TEM images of HCS/HCR composites. As can be seen in SEM image (Fig. 2(a)), a large amount of spheres distribute on the surface of nanotubes. High-magnification SEM (Fig. 2(b)) shows some holes on the spheres (the red sign), which proves that the spheres are hollow structure. The TEM images (Fig. 2(c) and (d)) further illustrate the HCS/HCR composites consisting of a lot of hollow spheres with uniform size distributed on the hollow nanorods. The hollow nanorods are woven together to form a network structure that facilitates charge transport and the hollow carbon spheres provide space for storage of charge. It is observed from the high-resolution TEM image (Fig. 2(d)) that outer diameter and inner diameter of the nanorods are ca. 60 and 30 nm, respectively, and the cavity and diameter size of hollow
spheres are ca. 260 and 295 nm, respectively. It indicates that the acetone can selectively remove the oligomer inside the 3-AF resin spheres to form the hollow sphere structure and MnO2 nanorods are effective template to prepare hollow nanorod structure. Hollow spheres and nanorods can be obtained by assembly of TEOS and dissolved oligomer resins under the action of CTAB. Nitrogen isothermal adsorption–desorption measurements are performed to analyze the textural properties of the HCS/HCR composites. As depicted in Fig. 3, a type of IV adsorption-desorption isotherm and a type of H3 hysteresis loop are observed. A slow increase trend at low relative pressure (P/P0 < 0.1) is observed, revealing the presence of some micropores in the composites [25]. The obvious hysteresis loop at a high relative pressure in 0.4 < P/P0 < 0.9 is indicative of mesoporous structure [26]. The adsorption isotherm with a sharp capillary condensation step at high relative pressures (P/P0 = 0.9–1.0) indicate large cavity in the HCS/HCR
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Fig. 5. Electrochemical capacitive behavior of HCS/HCR measured in a three-electrode system: (a) cyclic voltammetry curves of the HCS/HCR composites at 5–200 mV s− 1 ; (b) galvanostatic charge/discharge curves of the HCS/HCR composites at 1–10 A g− 1 ; (c) the specific capacitance of the HCS/HCR composites electrode with different current densities and the comparison with reported various morphology carbon electrodes; (d) Nyquist plot measured with frequency range of 10−2 –105 Hz; (e) Cycling stability of HCS/HCR composites upon charging-discharging at a current density of 5 A g− 1 and the insert shows the GCD curves for the 1st and 50 0 0th cycles.
composites [27,28]. A narrow pore size distribution with a mean value of 9.3 nm (Fig. 3(b)) is calculated from the adsorption branch by BJH method. Notably, the obtained HCS/HCR composites have a high BET surface area of 1590 m2 g−1 and high BJH pore volume of 2.4 cm3 g−1 . The 3-AF not only played the role of carbon precursor, but also acted as an excellent nitrogen precursor, leading to in-situ nitrogen doping for HCS/HCR composites. XPS was used to provide
qualitative information about the surface elemental composition and its chemical environments, as shown in Fig. 4. The XPS of HCS/HCR composites clearly revealed three typical peaks of C1s (284.6 eV), O1s (532.5 eV) and N1s (400.4 eV) at Fig. 4(a). Elemental analysis of XPS revealed that the carbon, oxygen and nitrogen content in the HCS/HCR composites were 89.9, 5.6 and 4.5 wt%, respectively. These results further confirmed that the heteroatom of nitrogen had been successfully doped into the carbon skeleton
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Fig. 6. Electrochemical capacitive behavior of HCS/HCR composites measured in a two-electrode system: (a) cyclic voltammetry curves at 5–200 mV s− 1 ; (b) galvanostatic charge/discharge curves at 1–10 A g− 1 ; (c) the specific capacitance at different current densities; (d) energy density vs power density curves.
of HCS/HCR composites. High-resolution XPS spectra for each element were performed and fitted, and the corresponding forms of each element were analyzed. As shown in Fig. 4(b), the spectrum of C1s of HCS/HCR composites could be deconvoluted into three type peaks, corresponding to C–C at 284.6 eV, C–N at 285.4 eV, C=O at 288.8 eV, which further indicated the existence of elements N, O in the surface of HCS/HCR composites. The spectrum of O1s (Fig. 4(c)) could be deconvoluted into four type peaks with binding energies of 530.0, 531.1, 532.9 and 536.4 eV corresponding to oxygen in carboxyl groups, C=O and chemically adsorbed oxygen, respectively. The high resolution XPS spectrum of N1s (Fig. 4(d)) could be further deconvoluted into three peaks at 398.3, 400.9, and 403.8 eV corresponding to the contribution of pyridinic nitrogen (at 398.3 eV), quaternary nitrogen (at 400.9 eV) and pyridine N-oxide (at 403.8 eV). The pyridinic nitrogen provides the active sites for electrode materials, and the quaternary nitrogen not only is the most stable nitrogen species during the process of pyrolysis but also can improve the electrical conductivity of the carbon materials. The HCS/HCR composites consisting of hollow spheres and nanorods structure have high surface area, large pore volume and uniform pore size distribution, which would endow the composite with outstanding capacitive performance as electrode material. The supercapacitive performances of HCS/HCR composites were evaluated by CV in three-electrode systems with 6 M KOH aqueous as electrolyte. Fig. 5(a) shows the CV curves of the HCS/HCR composites electrode at different scan rates with potential windows
ranging from −1 to 0 V. The shapes of these curves are quasirectangular, indicating the ideal electrical double-layer capacitance behavior and fast charging/discharging process [28]. Rate capability is one of the important factors for evaluating the power applications of supercapacitors. The GCD curves of the HCS/HCR composites at different current densities are shown in Fig. 5(b). The nearly isosceles triangle shape of all charge–discharge curves further confirms its excellent reversibility, high Coulombic efficiency, and good charge transport [29]. The charge–discharge curves show that HCS/HCR composites exhibit the largest capacitance of 250 F g−1 at the current density of 1 A g−1 . It is also worthy to point out that a high areal capacitance of 207 F g−1 is maintained at the current density reached 10 A g−1 , which manifests a remarkable capacitance retention of 80.5% (Fig. 5(c)). The HCS/HCR composites have superior performance when compared with other carbon materials (details presented in Table S1 [22,24,29–38]) reported recently. The higher capacitance of HCS/HCR indicates that the hollow sphere and hollow nanorod structure allow easy electrolyte penetration to the interface between electrode and electrolyte and high diffusion and transport of electrolyte ions. The electrochemical impedance spectroscopy (EIS) plot is used to analyze the electrical double layer capacitor behavior. An EIS test is carried out in a frequency range from 10−2 –105 Hz at open circuit voltage of 5 mV to evaluate the electrochemical behaviors of the supercapacitor (Fig. 5(d)). The HCS/HCR composites sample shows a vertical line at the low-frequency region, indicating an ideal capacitive behavior of EDLC. There is no obvious semi-
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circle in correlation with the charge-transfer resistance, suggesting that the HCS/HCR composites have low impedance at the electrode/electrolyte interface. In the equivalent circuit, Rs corresponds to the electrolyte resistance, Rct is charge transfer resistance at the electrode/electrolyte interface, CPEdl and Cf correspond to double layer capacitance and faradic capacitance, and Zw corresponds to Warburg impedance [39,40]. The HCS/HCR composites also show excellent cycling stability (Fig. 5(e)) in three-electrode supercapacitor. After 50 0 0 cycles, this device shows 91.3% capacitance retention of capacitance compared to the initial value. Additionally, the 50 0 0th galvanostatic charge– discharge curve (inset in Fig. 5(e)) still displays a symmetrical triangular shape, indicating that the supercapacitor possesses good charge propagation and long term electrochemical stability. The electrochemical properties of electrode materials are also tested by two-electrode system which can reflect their performance in practical application [41]. CV and GCD are tested in two-electrode system to determine the capacitive characters of HCS/HCR composites. Fig. 6(a) shows the CV curves of HCS/HCR composites electrode at different scan rates in a 6 M KOH electrolyte. All the CV curves show quasi-rectangular shape with inconspicuous distortion as the scan rate increased, demonstrating the ideal capacitive behavior of HCS/HCR composites as electrode. The GCD curves at different current densities are presented in Fig. 6(b). All curves show isosceles triangular shapes demonstrating an almost ideal capacitor behavior, even at current densities as high as 10 A g−1 . A retain in capacitance as large as 64% was observed for the HCS/HCR composites when the current density increased from 1 A g−1 to 10 A g−1 (Fig. 6(c)). A high capacitance 175 F g−1 at a current density of 1 A g−1 is observed for HCS/HCR composites and the capacitance remains 112 F g−1 when current density reaches up to 10 A g−1 . The HCS/HCR composites in symmetric supercapacitor exhibit a maximum energy density of 24.3 Wh kg−1 (with a power density of 20 0 0 W kg−1 ) (Fig. 6(d)). Evidently, these encouraging results further reveal the potential of HCS/HCR composites as the electrodes in supercapacitor. 4. Conclusions In summary, the HCS/HCR composites are prepared by “dissolve–reassembly” strategy combined hard template method. This method is based on the internal/external polymerization heterogeneity of the 3-AF resin sphere, which can create hollow structure in 3-AF resin sphere and provide 3-AF resin oligomer by the dissolution of acetone. 3-AF resin oligomer further assembles with hard template MnO2 nanorods and TEOS together in the action of CTAB. The obtained HCS/HCR composites combined with 0-dimensional hollow sphere and 1-dimensional hollow rod structure show high surface area, large pore volumes, and uniform mesoporous. When used as electrode material, the HCS/HCR composites exhibit high specific capacitance of 250 F g−1 at a current density of 1 A g−1 , and good cycling stability of 91.3% capacitance retention ratio after 50 0 0 cycles at 5 A g−1 as revealed by electrochemical measurements. This work provides a new direction for preparation of multi-dimensional carbon-based composite material for supercapacitors catalysis and adsorption applications. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21676070), the Excellent Going Abroad Experts’ Training Program in Hebei Province, Hebei One Hundred-Excellent Innovative Talent Program (III) (SLRC2017034), Hebei Science and Technology Project (17214304D), Special Project for Synthesis and Application of Graphene in Hebei University of
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Science and Technology (2015PT65), and Beijing National Laboratory for Molecular Sciences.
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