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Nitrogen-doped bi-modal porous carbon nanostructure derived from glycine for supercapacitors
G Model JIEC 3866 No. of Pages 5
Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Nitrogen-doped bi-modal porous carbon nanostructure derived from glycine for supercapacitors In-Ae Choi1, Da-Hee Kwak1, Sang-Beom Han, Kyung-Won Park* Department of Chemical Engineering, Soongsil University, Seoul 156743, Republic of Korea
A R T I C L E I N F O
Article history: Received 6 January 2018 Received in revised form 1 February 2018 Accepted 1 February 2018 Available online xxx Keywords: Supercapacitor Nitrogen doping Bi-modal pore Glycine Rate performance
A B S T R A C T
We prepared a nitrogen-doped bi-modal porous carbon nanostructure (G-500/20) using a template method with 500 and 20 nm SiO2 beads and glycine. The G500/20 has a surface area of 403 m2 g 1 with meso/macroporous structure and N-doping content of 5.9 at%. In the supercapacitor performance, G-500/ 20 exhibits superior specific capacitances of 19.5 and 5.3 F g 1 at 200 mV s 1 and 20 A g 1 in 6 M NaOH, compared to a commercial activated carbon. In particular, the superior capacitances of G500/20 at high scan rates and current densities were achieved due to the bi-modal porous structure and nitrogen doping effect. © 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Introduction Supercapacitors, used as energy storage devices, have attracted attention due to their high power density, excellent reversibility, and long cycle life [1–4]. In general, according to the charge storage mechanism, supercapacitors are classified into either electrochemical double layer capacitors (EDLCs) or pseudo-capacitors [2]. In the case of EDLCs, the electrical energies can be stored by the electrostatic charge at an interfacial region between the aqueous or non-aqueous electrolytes and the electrode [2,3]. The electrode materials require a highly effective surface area and pore structure that can accumulate mobile ions in the electrolytes. In particular, carbon materials have been used for supercapacitors due to their electrochemical stability, excellent conductivity, and well-defined porosity [5,6]. Porous carbon nanostructured materials, such as activated carbons, carbon nanotubes, and, carbon nanofibers, have usually been utilized as electrode materials in high-performance EDLCs [2,7–9]. In the porous structure, the micropores (<2 nm) provide a highly effective surface area for increased double layer capacitance and the mesopores (2–50 nm) or macropores (>50 nm) can act as ion-buffering reservoirs and ion transport pathways, resulting in a decreased diffusion distance [8,10,11].
* Corresponding author. E-mail address: [email protected] (K.-W. Park). These authors equally contributed to this work.
1
Recently, mesoporous carbon electrode materials for supercapacitors with high capacitance and good rate capability have been intensively studied. In particular, carbon nanostructures doped with heteroatoms (N, B, P, and S) exhibited enhanced supercapacitor performance comparable to undoped carbons [12– 14]. The heteroatom doping containing electron-donating or electron-withdrawing properties can provide carbon materials with improved electrochemical properties in acid/base electrolytes. Moreover, the doped carbon electrodes can improve the capacitance because of the pseudocapacitive effect due to the Faradaic interaction between the ions of electrolyte and surface functional groups. Among the doped electrodes, nitrogen doping can simply control the local electronic properties and thus enhance the electrochemical performance [15]. Nitrogen-doped carbons can increase the electrical conductivity and improve the wettability of the electrode materials with an aqueous electrolyte in supercapacitors [16]. The nitrogen-doped carbons have been prepared using post heat treatment of porous carbons under a nitrogen atmosphere and in situ doping using nitrogen-containing precursors [17–19]. Herein, we prepared a nitrogen doped bimodal porous carbon material (G-500/20) using a template method with 500 and 20 nm SiO2 beads and glycine as a dopant and carbon source. The G-500/20 has a surface area of 403 m2 g 1 with a meso/macroporous structure and N-doping content of 5.9 at %. In terms of the supercapacitor performance, G-500/20 exhibits superior specific capacitances of 19.5 and 5.3 F g 1 at 200 mV s 1 and 20 A g 1 in 6 M NaOH electrolyte, compared to a commercial activated carbon.
https://doi.org/10.1016/j.jiec.2018.02.006 1226-086X/© 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: I.-A. Choi, et al., Nitrogen-doped bi-modal porous carbon nanostructure derived from glycine for supercapacitors, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.02.006
G Model JIEC 3866 No. of Pages 5
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Experimental section Synthesis of nitrogen-doped porous carbon nanostructures To synthesize doped porous carbon nanostructures, glycine (0.4 g, Aldrich) as both the carbon and nitrogen sources and SiO2 beads were mixed in 50 mL de-ionized (DI) water for 5 h with continuous stirring and sonicating 500 nm (0.4 g, Alfa Aesar) and
20 nm (0.13 g, 40% in water, Alfa Aesar) SiO2 beads were used as porous templates. The weight ratio of the 500 nm–20 nm SiO2 beads were 3:1. The samples prepared with 500 nm beads were denoted as G-500 and both the 500 and 20 nm beads were denoted as G-500/20. The completely dispersed mixture solutions were transferred to a glass petri dish and thoroughly dried in an oven at 50 C for 12 h. The dried sample was loaded on a glass boat and then heated at 900 C in a tube furnace under an N2 atmosphere for 3 h. Followed by the pyrolysis, the sample was washed with HF solution (10 vol.%, J.T Baker) for 2 h and then with DI water several times to remove SiO2 template and impurities. The resulting black powder was obtained by drying in a 50 C oven for 12 h [17–19]. Structural characterization of nitrogen-doped porous carbon nanostructures
Fig. 1. XRD patterns of the as-prepared samples.
The morphology and structure of the as-prepared samples were characterized using field emission-scanning electron microscopy (FE-SEM, JSM-7800F, JEOL) with an accelerating voltage of 15 kV. The crystal structure of the samples was analyzed using an X-ray diffraction (XRD, D2 Phase System, BRUKER) system with a Cu Ka radiation source (l = 1.54056 Å) and a Ni filter. The tube current and voltage were 10 mA and 30 kV, respectively. X-ray photoelectron spectroscopy (XPS, Thermo Scientific) with an Al Ka X-ray source of 1468.8 eV and power of 200 W was performed under a chamber pressure of 7.8 10 9 Torr in order to characterize the chemical composition of the samples. To characterize the specific surface area and pore structure of the samples, nitrogen
Fig. 2. SEM images of (a,b) G-500 and (c,d) G-500/20.
Please cite this article in press as: I.-A. Choi, et al., Nitrogen-doped bi-modal porous carbon nanostructure derived from glycine for supercapacitors, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.02.006
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adsorption/desorption isotherms were obtained using an ASAP 2020 adsoprtion analyzer (Micromeritics). The Brunauer–Emmett–Teller (BET) specific surface area was calculated using a nitrogen adsorption/desorption isotherm and the pore size was determined using the Barret–Joyner–Halenda (BJH) method. Electrochemical characterization of nitrogen-doped porous carbon nanostructures The electrochemical properties of the as-prepared samples were characterized at room temperature using a potentiostat (PGSTAT302N, AUTOLAB. The slurry was prepared by homogeneously mixing the as-prepared sample powder (0.1 g) with DI water (266.6 mL), isopropanol (2 mL, Aldrich), and 5 wt% Nafion1 solution (386 mL, Aldrich). For comparison, a commercial activated carbon (AC, Power Carbon Technology) was also used as an active material. The slurry was ultrasonically sprayed on a Ni foam (area 1.5 3 cm2) as a current collector and dried at room temperature for 12 h and the active area of the electrode is 1 1 cm2. In a 2electrode system, the samples on the Ni foams were used as both working and reference/counter electrodes. Cyclic voltammograms (CVs) were obtained between 0 and 1 V with different scan rates in Ar-saturated 6 M NaOH. Galvanostatic charge/discharge analysis was carried out between 0 and 1 V with different current rates in Ar-saturated 6 M NaOH. Results and discussion Fig. 1 shows XRD patterns of G-500, G-500/20, and commercial activated carbon (AC). In the XRD data, all samples exhibited broad peaks corresponding to (002) and (101) at 26 and 42 , respectively, representing the low crystallinity of carbon structure. The as-prepared G-500 showed a porous structure with an average diameter of 500 nm, implying the good transfer of 500 nm SiO2 beads to carbon structure formed using glycine as a carbon source (Fig. 2(a) and (b)). On the other hand, the as-prepared G-500/20 contained bi-modal pores with average diameters of 500 and 20 nm, representing a well-defined porous carbon structure formed using 500 and 20 nm SiO2 beads with glycine as a carbon source (Fig. 2(c) and (d)). Fig. 3 shows the nitrogen adsorption–desorption isotherms and pore size distributions of the samples. The specific active surface areas of G-500, G-500/20, and AC were 210, 403, and 1777 m2 g 1, respectively, representing the relatively high value of AC due to a predominant portion of micropore with a diameter of <2 nm. In particular, the surface area of G-500/20 was 1.9-fold higher than that of G-500, demonstrating an improvement of surface area by the addition of 20 nm sized silica bead in the template. However, compared to the AC, G-500 and G-500/20, which were prepared
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using 500 and 500/20 nm beads, respectively, exhibited a typical IV-type isotherm curve with meso-pores. Thus, the well-defined porous structure in G-500 and G-500/20 was completely transferred by the replica method using silica beads. To characterize the composition of AC, G-500, and G-500/20, XPS spectra were obtained (Fig. 4). The amount of nitrogen as a dopant in G-500 and G-500/20 was 5.6 at% and 5.9 at%, respectively, whereas AC showed no detection of nitrogen. According to the literature, nitrogen-doped carbon nanostructures have improved electrochemical performance in alkaline and acid media [10,13,20,21]. The doped N states could form unshared electron pairs in the carbon edges and so enhance the improved electron sharing [22]. The carbon-neighboring N state with Lewis basicity might improve the adsorption of molecules, thus enhancing electrochemical properties [23–25]. Fig. 5(a) and (c) shows the CVs of G-500, G-500/20, and AC, respectively, measured between 0 and 1 V with different scan rates in Ar-saturated 6 M NaOH. In the CVs measured at 5 mV s 1, AC showed excellent capacitor performance (78.6 F g 1) compared to those of G-500 and G-500/20 (25.5 and 33.2 F g 1, respectively) (Fig. 5(d)). However, with increasing scan rates, G-500 and G-500/ 20 maintained rectangular shaped CV curves, compared to the CVs of AC, representing the main double layer capacitor nature of the charge/discharge process. Also, the relatively fast response of the carbon nanostructure samples in the electrochemical reaction can promote ion diffusion due to the mesoporous structure and Ndoping effect. In particular, the higher specific current value of G500/20 than that of G-500 can result from the increased surface area due to a mixture of 500 and 20 nm silica beads. Fig. 6(a) and (c) shows galvanostatic charge/discharge curves of G-500, G-500/20, and AC, respectively, measured at different current densities at 25 C. From the curves at 1 A g 1, the
Fig. 4. XPS spectra of AC, G-500, and G-500/20.
Fig. 3. Nitrogen adsorption/desorption characteristic curves of (a) G-500, (b) G-500/20, and (c) AC. The insets indicate the pore size distributions of the samples.
Please cite this article in press as: I.-A. Choi, et al., Nitrogen-doped bi-modal porous carbon nanostructure derived from glycine for supercapacitors, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.02.006
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Fig. 5. CVs of (a) G-500, (b) G-500/20, and (c) AC measured at different scan rates between 0 and 1 V in Ar-saturated 6 M NaOH at 25 C. (d) Comparison of CVs of G-500, G500/20, and AC measured at 5 mV s 1.
Fig. 6. Galvanostatic charge/discharge curves of (a) G-500, (b) G-500/20, and (c) AC measured at different current densities in Ar-saturated 6 M NaOH at 25 C. (d) Comparison of CVs of G-500, G-500/20, and AC measured at 1 A g 1.
capacitances of AC, G-500, and G-500/20 were determined to be 68.2, 19.0 and 35.3 F g 1, respectively (Fig. 6(d)). However, with different current densities, G-500 and G-500/20 maintained a triangular shape in the charge/discharge curves, whereas AC exhibited the charge/discharge curves with a relatively large IR drop of 0.12 V due to a relatively low electrical conductivity of AC.
The relatively fast response (i.e. rate performance) and small voltage drop of G-500 and G-500/20 in the electrochemical reaction can be attributed to the N-doped porous structure prepared using glycine as a carbon and doping source. Moreover, compared to G-500 formed using a mixture of 500 nm silica beads as a template the higher specific current value of G-500/20 can
Please cite this article in press as: I.-A. Choi, et al., Nitrogen-doped bi-modal porous carbon nanostructure derived from glycine for supercapacitors, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.02.006
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Fig. 7. Plots of specific capacitances of the samples from (a) CVs and (b) galvanostatic charge/discharge curves.
result from an increased surface area of the carbon nanostructure formed using a mixture of 500 and 20 nm silica beads as a template. Fig. 7(a) and (b) shows plots of the specific capacitances of the samples calculated from the CVs measured at different scan rates and galvanostatic charge/discharge curves measured at different current densities, respectively. In particular, G-500 and G-500/20 exhibited near constant capacitances at varying scan rates and current densities, representing a fast charge-discharge performance. On the other hand, AC exhibited higher specific capacitances at low can rates up to 50 mV s 1 and low current densities up to 5 A g 1 than those of G-500 and G-500/20. However, at high scan rates and high current densities, G-500/20 showed high specific capacitances, compared to AC, due to the sufficient ionic diffusion into the bi-modal porous electrode structure. Therefore, the enhanced rapid electrochemical reaction (i.e. rate capability) of G-500/20 can be attributed to the N-doped porous nanostructure and the increased surface area due to a mixture of 500 and 20 nm silica beads.
References
Conclusions
[17]
In summary, we prepared a nitrogen-doped bi-modal porous carbon material for a high rate performance supercapacitor. The nitrogen doped bi-modal porous carbon material (G-500/20) was synthesized using a template method with 500 and 20 nm SiO2 beads and glycine as a dopant and carbon source. The bi-modal doping process increases the surface area with the meso/macroporous structure, compared to G-500, and provides N-doping content in the carbon structure. The superior capacitances of G-500/20, especially, at high scan rates and current densities, resulted from the bi-modal porous structure and nitrogen doping effect.
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Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1A2B2016033).
Please cite this article in press as: I.-A. Choi, et al., Nitrogen-doped bi-modal porous carbon nanostructure derived from glycine for supercapacitors, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.02.006
Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Nitrogen-doped bi-modal porous carbon nanostructure derived from glycine for supercapacitors In-Ae Choi1, Da-Hee Kwak1, Sang-Beom Han, Kyung-Won Park* Department of Chemical Engineering, Soongsil University, Seoul 156743, Republic of Korea
A R T I C L E I N F O
Article history: Received 6 January 2018 Received in revised form 1 February 2018 Accepted 1 February 2018 Available online xxx Keywords: Supercapacitor Nitrogen doping Bi-modal pore Glycine Rate performance
A B S T R A C T
We prepared a nitrogen-doped bi-modal porous carbon nanostructure (G-500/20) using a template method with 500 and 20 nm SiO2 beads and glycine. The G500/20 has a surface area of 403 m2 g 1 with meso/macroporous structure and N-doping content of 5.9 at%. In the supercapacitor performance, G-500/ 20 exhibits superior specific capacitances of 19.5 and 5.3 F g 1 at 200 mV s 1 and 20 A g 1 in 6 M NaOH, compared to a commercial activated carbon. In particular, the superior capacitances of G500/20 at high scan rates and current densities were achieved due to the bi-modal porous structure and nitrogen doping effect. © 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Introduction Supercapacitors, used as energy storage devices, have attracted attention due to their high power density, excellent reversibility, and long cycle life [1–4]. In general, according to the charge storage mechanism, supercapacitors are classified into either electrochemical double layer capacitors (EDLCs) or pseudo-capacitors [2]. In the case of EDLCs, the electrical energies can be stored by the electrostatic charge at an interfacial region between the aqueous or non-aqueous electrolytes and the electrode [2,3]. The electrode materials require a highly effective surface area and pore structure that can accumulate mobile ions in the electrolytes. In particular, carbon materials have been used for supercapacitors due to their electrochemical stability, excellent conductivity, and well-defined porosity [5,6]. Porous carbon nanostructured materials, such as activated carbons, carbon nanotubes, and, carbon nanofibers, have usually been utilized as electrode materials in high-performance EDLCs [2,7–9]. In the porous structure, the micropores (<2 nm) provide a highly effective surface area for increased double layer capacitance and the mesopores (2–50 nm) or macropores (>50 nm) can act as ion-buffering reservoirs and ion transport pathways, resulting in a decreased diffusion distance [8,10,11].
* Corresponding author. E-mail address: [email protected] (K.-W. Park). These authors equally contributed to this work.
1
Recently, mesoporous carbon electrode materials for supercapacitors with high capacitance and good rate capability have been intensively studied. In particular, carbon nanostructures doped with heteroatoms (N, B, P, and S) exhibited enhanced supercapacitor performance comparable to undoped carbons [12– 14]. The heteroatom doping containing electron-donating or electron-withdrawing properties can provide carbon materials with improved electrochemical properties in acid/base electrolytes. Moreover, the doped carbon electrodes can improve the capacitance because of the pseudocapacitive effect due to the Faradaic interaction between the ions of electrolyte and surface functional groups. Among the doped electrodes, nitrogen doping can simply control the local electronic properties and thus enhance the electrochemical performance [15]. Nitrogen-doped carbons can increase the electrical conductivity and improve the wettability of the electrode materials with an aqueous electrolyte in supercapacitors [16]. The nitrogen-doped carbons have been prepared using post heat treatment of porous carbons under a nitrogen atmosphere and in situ doping using nitrogen-containing precursors [17–19]. Herein, we prepared a nitrogen doped bimodal porous carbon material (G-500/20) using a template method with 500 and 20 nm SiO2 beads and glycine as a dopant and carbon source. The G-500/20 has a surface area of 403 m2 g 1 with a meso/macroporous structure and N-doping content of 5.9 at %. In terms of the supercapacitor performance, G-500/20 exhibits superior specific capacitances of 19.5 and 5.3 F g 1 at 200 mV s 1 and 20 A g 1 in 6 M NaOH electrolyte, compared to a commercial activated carbon.
https://doi.org/10.1016/j.jiec.2018.02.006 1226-086X/© 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: I.-A. Choi, et al., Nitrogen-doped bi-modal porous carbon nanostructure derived from glycine for supercapacitors, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.02.006
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Experimental section Synthesis of nitrogen-doped porous carbon nanostructures To synthesize doped porous carbon nanostructures, glycine (0.4 g, Aldrich) as both the carbon and nitrogen sources and SiO2 beads were mixed in 50 mL de-ionized (DI) water for 5 h with continuous stirring and sonicating 500 nm (0.4 g, Alfa Aesar) and
20 nm (0.13 g, 40% in water, Alfa Aesar) SiO2 beads were used as porous templates. The weight ratio of the 500 nm–20 nm SiO2 beads were 3:1. The samples prepared with 500 nm beads were denoted as G-500 and both the 500 and 20 nm beads were denoted as G-500/20. The completely dispersed mixture solutions were transferred to a glass petri dish and thoroughly dried in an oven at 50 C for 12 h. The dried sample was loaded on a glass boat and then heated at 900 C in a tube furnace under an N2 atmosphere for 3 h. Followed by the pyrolysis, the sample was washed with HF solution (10 vol.%, J.T Baker) for 2 h and then with DI water several times to remove SiO2 template and impurities. The resulting black powder was obtained by drying in a 50 C oven for 12 h [17–19]. Structural characterization of nitrogen-doped porous carbon nanostructures
Fig. 1. XRD patterns of the as-prepared samples.
The morphology and structure of the as-prepared samples were characterized using field emission-scanning electron microscopy (FE-SEM, JSM-7800F, JEOL) with an accelerating voltage of 15 kV. The crystal structure of the samples was analyzed using an X-ray diffraction (XRD, D2 Phase System, BRUKER) system with a Cu Ka radiation source (l = 1.54056 Å) and a Ni filter. The tube current and voltage were 10 mA and 30 kV, respectively. X-ray photoelectron spectroscopy (XPS, Thermo Scientific) with an Al Ka X-ray source of 1468.8 eV and power of 200 W was performed under a chamber pressure of 7.8 10 9 Torr in order to characterize the chemical composition of the samples. To characterize the specific surface area and pore structure of the samples, nitrogen
Fig. 2. SEM images of (a,b) G-500 and (c,d) G-500/20.
Please cite this article in press as: I.-A. Choi, et al., Nitrogen-doped bi-modal porous carbon nanostructure derived from glycine for supercapacitors, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.02.006
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adsorption/desorption isotherms were obtained using an ASAP 2020 adsoprtion analyzer (Micromeritics). The Brunauer–Emmett–Teller (BET) specific surface area was calculated using a nitrogen adsorption/desorption isotherm and the pore size was determined using the Barret–Joyner–Halenda (BJH) method. Electrochemical characterization of nitrogen-doped porous carbon nanostructures The electrochemical properties of the as-prepared samples were characterized at room temperature using a potentiostat (PGSTAT302N, AUTOLAB. The slurry was prepared by homogeneously mixing the as-prepared sample powder (0.1 g) with DI water (266.6 mL), isopropanol (2 mL, Aldrich), and 5 wt% Nafion1 solution (386 mL, Aldrich). For comparison, a commercial activated carbon (AC, Power Carbon Technology) was also used as an active material. The slurry was ultrasonically sprayed on a Ni foam (area 1.5 3 cm2) as a current collector and dried at room temperature for 12 h and the active area of the electrode is 1 1 cm2. In a 2electrode system, the samples on the Ni foams were used as both working and reference/counter electrodes. Cyclic voltammograms (CVs) were obtained between 0 and 1 V with different scan rates in Ar-saturated 6 M NaOH. Galvanostatic charge/discharge analysis was carried out between 0 and 1 V with different current rates in Ar-saturated 6 M NaOH. Results and discussion Fig. 1 shows XRD patterns of G-500, G-500/20, and commercial activated carbon (AC). In the XRD data, all samples exhibited broad peaks corresponding to (002) and (101) at 26 and 42 , respectively, representing the low crystallinity of carbon structure. The as-prepared G-500 showed a porous structure with an average diameter of 500 nm, implying the good transfer of 500 nm SiO2 beads to carbon structure formed using glycine as a carbon source (Fig. 2(a) and (b)). On the other hand, the as-prepared G-500/20 contained bi-modal pores with average diameters of 500 and 20 nm, representing a well-defined porous carbon structure formed using 500 and 20 nm SiO2 beads with glycine as a carbon source (Fig. 2(c) and (d)). Fig. 3 shows the nitrogen adsorption–desorption isotherms and pore size distributions of the samples. The specific active surface areas of G-500, G-500/20, and AC were 210, 403, and 1777 m2 g 1, respectively, representing the relatively high value of AC due to a predominant portion of micropore with a diameter of <2 nm. In particular, the surface area of G-500/20 was 1.9-fold higher than that of G-500, demonstrating an improvement of surface area by the addition of 20 nm sized silica bead in the template. However, compared to the AC, G-500 and G-500/20, which were prepared
3
using 500 and 500/20 nm beads, respectively, exhibited a typical IV-type isotherm curve with meso-pores. Thus, the well-defined porous structure in G-500 and G-500/20 was completely transferred by the replica method using silica beads. To characterize the composition of AC, G-500, and G-500/20, XPS spectra were obtained (Fig. 4). The amount of nitrogen as a dopant in G-500 and G-500/20 was 5.6 at% and 5.9 at%, respectively, whereas AC showed no detection of nitrogen. According to the literature, nitrogen-doped carbon nanostructures have improved electrochemical performance in alkaline and acid media [10,13,20,21]. The doped N states could form unshared electron pairs in the carbon edges and so enhance the improved electron sharing [22]. The carbon-neighboring N state with Lewis basicity might improve the adsorption of molecules, thus enhancing electrochemical properties [23–25]. Fig. 5(a) and (c) shows the CVs of G-500, G-500/20, and AC, respectively, measured between 0 and 1 V with different scan rates in Ar-saturated 6 M NaOH. In the CVs measured at 5 mV s 1, AC showed excellent capacitor performance (78.6 F g 1) compared to those of G-500 and G-500/20 (25.5 and 33.2 F g 1, respectively) (Fig. 5(d)). However, with increasing scan rates, G-500 and G-500/ 20 maintained rectangular shaped CV curves, compared to the CVs of AC, representing the main double layer capacitor nature of the charge/discharge process. Also, the relatively fast response of the carbon nanostructure samples in the electrochemical reaction can promote ion diffusion due to the mesoporous structure and Ndoping effect. In particular, the higher specific current value of G500/20 than that of G-500 can result from the increased surface area due to a mixture of 500 and 20 nm silica beads. Fig. 6(a) and (c) shows galvanostatic charge/discharge curves of G-500, G-500/20, and AC, respectively, measured at different current densities at 25 C. From the curves at 1 A g 1, the
Fig. 4. XPS spectra of AC, G-500, and G-500/20.
Fig. 3. Nitrogen adsorption/desorption characteristic curves of (a) G-500, (b) G-500/20, and (c) AC. The insets indicate the pore size distributions of the samples.
Please cite this article in press as: I.-A. Choi, et al., Nitrogen-doped bi-modal porous carbon nanostructure derived from glycine for supercapacitors, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.02.006
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Fig. 5. CVs of (a) G-500, (b) G-500/20, and (c) AC measured at different scan rates between 0 and 1 V in Ar-saturated 6 M NaOH at 25 C. (d) Comparison of CVs of G-500, G500/20, and AC measured at 5 mV s 1.
Fig. 6. Galvanostatic charge/discharge curves of (a) G-500, (b) G-500/20, and (c) AC measured at different current densities in Ar-saturated 6 M NaOH at 25 C. (d) Comparison of CVs of G-500, G-500/20, and AC measured at 1 A g 1.
capacitances of AC, G-500, and G-500/20 were determined to be 68.2, 19.0 and 35.3 F g 1, respectively (Fig. 6(d)). However, with different current densities, G-500 and G-500/20 maintained a triangular shape in the charge/discharge curves, whereas AC exhibited the charge/discharge curves with a relatively large IR drop of 0.12 V due to a relatively low electrical conductivity of AC.
The relatively fast response (i.e. rate performance) and small voltage drop of G-500 and G-500/20 in the electrochemical reaction can be attributed to the N-doped porous structure prepared using glycine as a carbon and doping source. Moreover, compared to G-500 formed using a mixture of 500 nm silica beads as a template the higher specific current value of G-500/20 can
Please cite this article in press as: I.-A. Choi, et al., Nitrogen-doped bi-modal porous carbon nanostructure derived from glycine for supercapacitors, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.02.006
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Fig. 7. Plots of specific capacitances of the samples from (a) CVs and (b) galvanostatic charge/discharge curves.
result from an increased surface area of the carbon nanostructure formed using a mixture of 500 and 20 nm silica beads as a template. Fig. 7(a) and (b) shows plots of the specific capacitances of the samples calculated from the CVs measured at different scan rates and galvanostatic charge/discharge curves measured at different current densities, respectively. In particular, G-500 and G-500/20 exhibited near constant capacitances at varying scan rates and current densities, representing a fast charge-discharge performance. On the other hand, AC exhibited higher specific capacitances at low can rates up to 50 mV s 1 and low current densities up to 5 A g 1 than those of G-500 and G-500/20. However, at high scan rates and high current densities, G-500/20 showed high specific capacitances, compared to AC, due to the sufficient ionic diffusion into the bi-modal porous electrode structure. Therefore, the enhanced rapid electrochemical reaction (i.e. rate capability) of G-500/20 can be attributed to the N-doped porous nanostructure and the increased surface area due to a mixture of 500 and 20 nm silica beads.
References
Conclusions
[17]
In summary, we prepared a nitrogen-doped bi-modal porous carbon material for a high rate performance supercapacitor. The nitrogen doped bi-modal porous carbon material (G-500/20) was synthesized using a template method with 500 and 20 nm SiO2 beads and glycine as a dopant and carbon source. The bi-modal doping process increases the surface area with the meso/macroporous structure, compared to G-500, and provides N-doping content in the carbon structure. The superior capacitances of G-500/20, especially, at high scan rates and current densities, resulted from the bi-modal porous structure and nitrogen doping effect.
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Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1A2B2016033).
Please cite this article in press as: I.-A. Choi, et al., Nitrogen-doped bi-modal porous carbon nanostructure derived from glycine for supercapacitors, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.02.006