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Amino acids derived nitrogen-doped carbon materials for electrochemical capacitive energy storage
Materials Letters 145 (2015) 273–278
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Amino acids derived nitrogen-doped carbon materials for electrochemical capacitive energy storage Hanbin Jeong a,1, Hwa Jung Kim a,b,1, Yang Jin Lee b, Jun Yeon Hwang b, Ok-Kyung Park b, Jae-Hyeung Wee b, Cheol-Min Yang b, Bon-Cheol Ku b, Jae Kwan Lee a,n a
Department of Carbon Materials, Chosun University, Gwangju 501-759, Republic of Korea Carbon Convergence Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Jeonbuk 565-905, Republic of Korea
b
art ic l e i nf o
a b s t r a c t
Article history: Received 27 November 2014 Accepted 18 January 2015 Available online 3 February 2015
Environmentally benign natural amino acids, especially histidine-derived nitrogen-doped carbon materials were readily synthesized from polycondensation reaction and subsequent carbonization via the stepwise thermolysis process with high yields of 40% even at a high temperature of 1000 1C and the nitrogen-atom contents of around 5 wt%. These materials possessed rolled planar structures as well as thick 2D-like planar structures with specific surface area of 455 m2/g, exhibiting a notable specific capacitance of 58 F/g at current densities of 0.1 A/g and superior stability without deterioration of performance values up to 2000 cycles. & 2015 Elsevier B.V. All rights reserved.
Keywords: Amino acid Carbon material Electrochemical capacitor Polycondensation Nitrogen-doping
1. Introduction Carbon materials have received a great deal of attention in scientific applications, for example, as electrode materials in the fields of energy conversion and storage owing to their large surface area, high electrical conductivity, and excellent chemical stability [1–4]. Over the past decades, considerable efforts have been made to develop porous carbon materials by using versatile activation methods or various templates from versatile carbon sources such as biomass or oil origins, which are carbohydrates, polysaccharides, and lignocellulose or pitches, phenolic resins, and organic polymers [5–13]. Biomass is one of the most abundant and low-cost renewable resource, and hence, it has been widely used to synthesis porous carbon materials via simply thermolysis or hydrothermal reaction [8–10]. In particular, nitrogen-containing biomass or biomass derivatives such as glucosamine, chitosan, and gelatin have been investigated as precursors for the porous nitrogen-doped carbon materials (NCMs) [11–13]. NCMs usually exhibited pyridinic, pyrrolic, and quaternary N bonding configurations in the carbon lattice. Nitrogen doping induces extraordinary properties with electronic surfaces or active catalytic sites due to the electronegativity difference between carbon and nitrogen, also alters the electronic properties, which have a great potential in design and fabrication of tunable device [14]. Nitrogen doping into the carbonaceous electrode n
Corresponding author. Tel.: þ 82 62 230 7319; fax: þ 82 62 230 8122. E-mail address: [email protected] (J.K. Lee). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.matlet.2015.01.067 0167-577X/& 2015 Elsevier B.V. All rights reserved.
materials of supercapacitors dramatically can lead to significant enhancement in the specific capacitance, which is closely related to pseudocapacitive behavior and enhanced wettability of the electrode materials to electrolyte [15]. However, most of biomass resources are composed of macromolecules such as lignin and cellulose. Unless the biomass resources contain metal catalysts or templates, they are often transformed into non-graphitizable carbon materials of low electrical conductivity during thermolysis at high temperature (above 2000 1C) [4]. Very recently, metal-free organic small molecules such as ionic liquids, which are defined as molten organic salts with a melting point below an arbitrary temperature such as 100 1C, have been reported as attractive alternatives to macromolecular carbon precursors for NCMs [16]. Since small molecules decompose completely into volatile fragments under thermolysis, metal catalysts should be required to synthesize the carbon materials [17]. Meanwhile, ionic liquids comprising of nitrogen-containing cations such as imidazolium and pyridinium facilitate the formation of NCMs by simple metal catalysts-free thermolysis in an inert gas environment. The most distinctive features of ionic liquids are their nonvolatility and decomposition below their boiling point. Considering such properties, thermolysis induces the polycondensation of ionic liquid precursors and subsequent aromatization with dehydrogenation, resulting in a graphitic microstructure [18]. However, ionic liquids are not good precursors because they are very expensive and have quite low yields in carbonisation. In this regard, amino acids might be attractive carbon precursors owing to their unique zwitterionic and their tendency to
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Fig. 1. (a) Schematic description for formation of carbon materials from amino acids via polycondensation and carbonization processes and TGA thermogram of representative natural amino acids.
decompose prior to melting. Moreover, they are cheap and abundant biomass, which can be continuously supplied in living organism. They are also environmentally benign as opposed to macromolecular biomass resources. Natural amino acids are decomposed completely into volatile fragments by thermolysis, which mechanism was elucidated by gas chromatography and mass spectrometry [19,20]. However, amino acids such as lysine, arginine, trypthopan, or histidine having N-terminal side chain produce char-like carbonanecous materials, which might have originated from polycondensed materials by inter- or intramolecular condensation through thermolysis at inert atmosphere (Fig. 1). Among amino acids, histidine was found to produce carbonaceous materials with high carbon yields. Therefore, we synthesized the histidine-derived carbon materials by simple stepwise thermolysis up to high temperature of 1000 1C. The obtained carbon materials contained small quantity of nitrogen atom and were produced as silvery-black solids with thick 2D-like planar structure. Thermolysis at 1500 1C yielded highly graphitic microstructure. Herein, we report the characteristics of histidinederived nitrogen-doped carbon materials (His-NCMs), and their electrochemical capacitive performance in energy storage application.
Electrochemical characterization of His-NCMs: Supercapacitor electrodes for electrochemical testing were composed of 80 wt% His-NCMs sample as the active material, 10 wt% poly(tetrafluoroethylene) as the binder, and 10 wt% acetylene black as the conducting agent. The weighed mixture was pressed into pellets of 15 mm diameter at a pressure of 5 MPa. The electrode weight was adjusted to 30 mg. The supercapacitor consisted of two electrodes, which were arranged face to face, with a separator (glass fiber) inserted between them. A Ni plate was used as the current collector. An aqueous electrolyte of 6 M KOH was used in this study. The charge voltage of the aqueous electrolyte system was limited to 1.0 V to ensure the stability of the solvent. The capacitance (C) of the electrodes was calculated on the basis of the following equation: C¼ (I Δt)/(W ΔV), where I is the discharge current, Δt is the discharge time, ΔV is the voltage variation of the initial voltage, and W is the weight of the active material of both the electrodes. All the specific capacitances were calculated from the following relationship: the capacitance in a three-electrode system equals four times the capacitance in a two-electrode system [21–23]. A conventional three-compartment cell was also used to confirm the decomposition voltage window.
3. Results and discussion 2. Materials and methods Materials: All reagents were purchased from Sigma-Aldrich, TCI, Acros Organic, or Mitsubishi Chemical.
Fig. 2 shows (a) and (b) X-ray photoelectron spectroscopy (XPS), (c) Raman, and (d) X-ray diffraction (XRD) spectra of His-NCM (1000) and His-NCM (1500) samples obtained after thermolysis to
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Fig. 2. (a, b) X-ray photoelectron spectroscopy (XPS), (c) Raman, and (d) XRD patterns of His-NCM (1000) and His-NCM (1500) obtained after thermolysis to temperatures of 1000 and 1500 1C, respectively.
temperatures of 1000 and 1500 1C, respectively. As shown in Fig. 2a, common peaks assigned in typical nitrogen-doped carbon materials appeared at 531, 400, and 284 eV corresponding to O 1s, N 1s, and C 1s-orbitals peaks, respectively, indicating that histidine could be effective carbon material precursor. From the intensity ratio between the nitrogen peaks, we roughly determined the nitrogen contents of 3.97 wt% and 1.64 wt% in His-NCM (1000) and His-NCM (1500), respectively. We also determined the atomic concentration of nitrogen in His-NCMs by elemental analysis (EA). The nitrogen doping content in His-NCM (1000) and His-NCM (1500) as determined by EA was ca. 5.7 wt% and 3.2 wt%, respectively, demonstrating slightly higher concentration compared to the results obtained by XPS (Table S1). These indicated that the nitrogen content in HisNCM was reduced by thermal treatment at 1500 1C, indicating removal of nitrogen atom followed by further carbonization to make more graphitic microstructure compared to that at 1000 1C. Fig. 2b shows the N 1s-orbital spectra of His-NCMs, which exhibits three individual peaks assigned to pyridinic N (398.0– 398.4 eV), pyrrolic N (399.8–400.5 eV), quaternary N (401.0– 401.5 eV), and oxidized N (403.2–403.4 eV) species, respectively. As shown in Fig. 2b, the His-NCMs exhibited the highest intensive
quaternary N peak with pyridinic N, pyrrolic N, and oxidized N species, but the peak intensities for pyridinic N, pyrrolic N, and oxidized N species are significantly decreased at higher temperature of 1500 1C. The quaternary N bonding configuration of HisNCM could be retained at thermal treatment of 1500 1C. The crystallinity of His-NCM (1000) and His-NCM (1500) is shown in Fig. 2(c) or (d). These His-NCMs exhibited the typical D and G bands between 1000–2000 cm 1, showing a decrease in the intensity ratio (ID/IG) of His-NCM (1500). And a distinct (002) peaks in the XRD pattern of His-NCM (1000) and His-NCM (1500) were observed with broad diffraction in the 2θ range of 24.1 and 25.31, respectively. The (002) peaks in His-NCM (1500) were better intensive than that of His-NCM (1000). These results indicated that thermal treatment at higher temperature of 1500 1C could induce more graphitic carbon lattices. Fig. 3(a) and (b) shows high resolution transmission electron microscopy (HRTEM) images of His-NCM (1000) and His-NCM (1500). The graphitic domains of His-NCM (1000) and His-NCM (1500) were placed sporadically in HRTEM images. Notably, the HisNCM (1500) exhibited significantly extended graphitic lattices, but the collapse of microstructures due to its graphitization. Fig. 3(c)
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Fig. 3. (a, b) HRTEM images of His-NCM (1000) and His-NCM (1500) and (c, d) SEM images of His-NCM (1000).
and (d) shows scanning electron microscopy (SEM) images of HisNCM (1000), which presented mostly the thickness of 200 nm and planar structures of large size. Interestingly, the rolled structures of these planes were also observed (Fig. 3d). His-NCM (1500) exhibited a similar morphology with His-NCM (1000). Fig. 4 shows the electrochemical capacitive performance of HisNCMs for supercapacitor application. An N2 adsorption technique was used at 77 K to confirm the change in specific surface area (SSA) of His-NCM (1000) and His-NCM (1500). The N2 adsorption/ desorption isotherms of the His-NCM (1000) showed typical type I isotherms with a steep N2 uptake at lower P/P0 and a long plateau at higher P/P0, indicating the presence of uniform microporosity. (Fig. 4a) Meanwhile, the His-NCM (1500) exhibited little N2 adsorption isotherms. These might be closely related with the collapse of microstructures at higher temperature of 1500 1C shown in Fig. 3. Although the pristine His-NCM (1000) had 2Dlike planar structure, that exhibited a notable SSA value of 455 m2/ g with micropore volumn of 0.13 mL/g and average micropore width of 0.63 nm. From these results, we measured cyclic voltammograms (CV) to explore the potential application of the His-NCM (1000) in supercapacitors.(Fig. 4b) The capacitive behavior of HisNCM (1000) showed the form of rectangular shape, which indicate the typical characteristics of electric double layer capacitor (EDLC)
[24], of the voltammetry characteristics. Fig. 4c shows the galvanostatic charge/discharge curves at different current densities. The triangle charge/discharge curves are linear and symmetrical, indicating that the His-NCM (1000) carbon electrode has excellent electrochemical reversibility, coulombic efficiency and ideal EDLC behavior [25,26]. The specific capacitances calculated from these discharge curves were 67, 58, 41, 28, 24 and 19 F/g at current densities of 0.05, 0.1, 0.3, 0.5, 0.7 and 1.0 A/g, respectively. However, the specific capacitance of His-NCM (1000), which was calculated and plotted with the current density, was decreased at higher current density specific capacitance due to its diffusionlimited charge/discharge process [27]. (see Fig. S4 of Supplementary information) Although the absolute value in specific capacitance of His-NCM (1000) seems to be low, its relative specific capacitance (58 F g 1/455 m2 g 1 E0.127 F/m2) on SSA value obtained from 2D-like planar structure without carbon activation could be comparable with that (85 F g 1/1500 m2 g 1 E0.057 F/m2) of commercially available YP-17D activated carbon from coconut shells [28]. The enhanced specific capacitance per unit of SSA could be understood by the nitrogen doping effect. The stability in supercapacitors is one of the important criteria in the selection of electrode materials. The cycle stability of the His-NCM (1000) electrode was investigated from chronopotentiometry measurements at the current
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Fig. 4. (a) N2 absorption/desorption isotherms of His-NCMs and (b) CV curves at different scan rate, (c) Galvanostatic charge/discharge curves at different current densities, and (d) cyclic stability of His-NCM (1000).
density of 0.1 A/g for 2000 cycles (Fig. 4d). With increasing cycle number from 200 to 2000, the specific capacitance exhibited superior long cycle life without a loss of performance values.
Acknowledgment This study was supported by research fund from Chosun University, 2014.
4. Conclusions Appendix A. Supporting information We demonstrated the characteristics of histidine-derived nitrogen-doped carbon materials (His-NCMs), and their electrochemical capacitive performance in energy storage application. In this study, small molecule natural amino acids could be candidate as environmentally benign carbon material precursor. The synthesized HisNCMs presented the rolled structures of planes as well as the thick and planar structures of large size. Even these 2D-like planar structures, His-NCM (1000) exhibited microporocity with a notable SSA value of 455 m2/g, exhibiting the specific capacitances of 58 F/g at current densities of 0.1 A/g and superior stability up to 2000 cycles without a loss of performance values. Further studies on these materials via KOH activation or template-based approaches to induce the high SSA are ongoing. We believe that the findings of this study introduce a new direction for the development of environmentally friendly carbon materials precursors.
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2015.01.067. References [1] Liang C, Li Z, Dai S. Mesoporous carbon materials: synthesis and modification. Angew Chem Int Ed 2008;47:3696–717. [2] Frackowiak E, Béguin F. Carbon materials for the electrochemical storage of energy in capacitors. Carbon 2001;39:937–50. [3] Vemula PK, John G. Crops: a green approach toward self-assembled soft materials. Acc Chem Res 2008;41:769–82. [4] Zhai Y, Dou Y, Zhao D, Fulvio PF, Mayes RT, Dai S. Carbon materials for chemical capacitive energy storage. Adv Mater 2011;23:4828–50. [5] Mohamed AR, Mohammadi M, Darzi GN. Preparation of carbon molecular sieve from lignocellulosic biomass: a review. Renew Sustain Energy Rev 2010;14:1591–9.
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[6] Wang HT, Yao JF. Use of poly(furfuryl alcohol) in the fabrication of nanostructured carbons and nanocomposites. Ind Eng Chem Res 2006;45:6393–404. [7] Saufi SM, Ismail AF. Fabrication of carbon membranes for gas separation-a review. Carbon 2004;42:241–59. [8] Lee J, Kim J, Hyeon T. Recent progress in the synthesis of porous carbon materials. Adv Mater 2006;18:2073–94. [9] Titirici MM, Thomas A, Yu SH, Müller JO, Antonietti M. A direct synthesis of mesoporous carbons with bicontinuous pore morphology from crude plant material by hydrothermal carbonization. Chem Mater 2007;19:4205–12. [10] Titirici MM, Thomas A, Antonietti M. Replication and coating of silica templates by hydrothermal carbonization. Adv Funct Mater 2007;17:1010–8. [11] Baccile N, Antonietti M, Titirich MM. One-step hydrothermal synthesis of nitrogen-doped nanocarbons: albumine directing the carbonization of glucose. ChemSusChem 2010;3:246–53. [12] Xu B, Hou S, Cao G, Wu F, Yang Y. Sustainable nitrogen-doped porous carbon with high surface areas prepared from gelatin for supercapacitor. J Mater Chem 2012;22:19088–93. [13] Zhao L, Baccile N, Gross S, Zhang Y, Wei W, Sun Y, et al. Sustainable nitrogendoped carbonaceous materials from biomass serivatives. Carbon 2010;48:3778–87. [14] Wang H, Maiyalagan T, Wang X. Review on recent progress in nitrogen-doped graphene: synthesis, characterization, and its potential applications. ACS Catal 2012;2:781–94. [15] Zhou M, Pu F, Wang Z, Guan S. Nitrogen-doped porous carbons through KOH activation with superior performance in supercapacitors. Carbon 2014;68:185–94. [16] Zhang S, Miran MS, Ikoma A, Dokko K, Watanabe M. Protic ionic liquids and salts as versatile carbon precursors. J Am Chem Soc 2014;136:1690–3. [17] Choi CH, Park SH, Woo SI. Heteroatom doped carbons prepared by the pyrolysis of bio-derived amino acids as highly active catalysts for oxygen electro-reduction reactions. Green Chem 2011;13:406–12.
[18] Paraknowitsch PJ, Thomas A, Antonietti M. A detailed view on the polycondensation of ionic liquid monomers towards nitrogen doped carbon materials. J Mater Chem 2010;20:6746–58. [19] Ratcliff MA, Medley EE, Simmonds PG. Pyrolysis of amino acids. mechanistic considerations. J Org Chem 1974;39:1481–90. [20] Chiavari G, Galletti GC. Pyrolysis-gas chromatography/mass spectrometry of amino acids. J Anal Appl Pyrolysis 1992;24:123–37. [21] Qu D, Shi H. Studies of activated carbons used in double-layer capacitors. J Power Source 1998;74:99–107. [22] Yang CM, Kim YJ, Endo M, Kanoh H, Yudasaka M, Iijima S, et al. Nanowindowregulated specific capacitance of supercapacitor electrodes of single-wall carbon nanohorns. J Am Chem Soc 2007;129:20–1. [23] Zhu Y, Murali S, Stoller MD, Ganesh KJ, Cai W, Ferreira PJ, et al. Carbon-based supercapacitors produced by activation of graphene. Science 2011;332:1537–41. [24] Wang R, Wang P, Yan X, Lang J, Peng C, Xue Q. Promising porous carbon derived from celtuce leaves with outstanding supercapacitance and CO2 capture performance. ACS Appl Mater Interfaces 2012;4:5800–6. [25] Li Y, Roy S, Ben T, Xu S, Qiu S. Micropore engineering of carbonized porous aromatic framework (PAF-1) for supercapacitors application. Phys Chem Chem Phys 2012;12:12909–17. [26] Sevilla M, Fuertes AB. Direct synthesis of highly porous interconnected carbon nanosheets and their application as high-performance supercapacitors. ACS Nano 2014;8:5069–78. [27] Wang T, Peng A, Wang Y, Tang J, Zheng G. MnO nanoparticle@mesoporous carbon composites grown on conducting substrates featuring highperformance lithium-ion battery, supercapacitor and sensor. Sci Rep 2013;3:2693. [28] Wei L, Sevilla M, Fuertes AB, Mokaya R, Yushin G. Hydrothermal carbonization of abundant renewable natural organic chemicals for high-performance supercapacitor electrodes. Adv Energy Mater 2011;1:356–61.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Amino acids derived nitrogen-doped carbon materials for electrochemical capacitive energy storage Hanbin Jeong a,1, Hwa Jung Kim a,b,1, Yang Jin Lee b, Jun Yeon Hwang b, Ok-Kyung Park b, Jae-Hyeung Wee b, Cheol-Min Yang b, Bon-Cheol Ku b, Jae Kwan Lee a,n a
Department of Carbon Materials, Chosun University, Gwangju 501-759, Republic of Korea Carbon Convergence Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Jeonbuk 565-905, Republic of Korea
b
art ic l e i nf o
a b s t r a c t
Article history: Received 27 November 2014 Accepted 18 January 2015 Available online 3 February 2015
Environmentally benign natural amino acids, especially histidine-derived nitrogen-doped carbon materials were readily synthesized from polycondensation reaction and subsequent carbonization via the stepwise thermolysis process with high yields of 40% even at a high temperature of 1000 1C and the nitrogen-atom contents of around 5 wt%. These materials possessed rolled planar structures as well as thick 2D-like planar structures with specific surface area of 455 m2/g, exhibiting a notable specific capacitance of 58 F/g at current densities of 0.1 A/g and superior stability without deterioration of performance values up to 2000 cycles. & 2015 Elsevier B.V. All rights reserved.
Keywords: Amino acid Carbon material Electrochemical capacitor Polycondensation Nitrogen-doping
1. Introduction Carbon materials have received a great deal of attention in scientific applications, for example, as electrode materials in the fields of energy conversion and storage owing to their large surface area, high electrical conductivity, and excellent chemical stability [1–4]. Over the past decades, considerable efforts have been made to develop porous carbon materials by using versatile activation methods or various templates from versatile carbon sources such as biomass or oil origins, which are carbohydrates, polysaccharides, and lignocellulose or pitches, phenolic resins, and organic polymers [5–13]. Biomass is one of the most abundant and low-cost renewable resource, and hence, it has been widely used to synthesis porous carbon materials via simply thermolysis or hydrothermal reaction [8–10]. In particular, nitrogen-containing biomass or biomass derivatives such as glucosamine, chitosan, and gelatin have been investigated as precursors for the porous nitrogen-doped carbon materials (NCMs) [11–13]. NCMs usually exhibited pyridinic, pyrrolic, and quaternary N bonding configurations in the carbon lattice. Nitrogen doping induces extraordinary properties with electronic surfaces or active catalytic sites due to the electronegativity difference between carbon and nitrogen, also alters the electronic properties, which have a great potential in design and fabrication of tunable device [14]. Nitrogen doping into the carbonaceous electrode n
Corresponding author. Tel.: þ 82 62 230 7319; fax: þ 82 62 230 8122. E-mail address: [email protected] (J.K. Lee). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.matlet.2015.01.067 0167-577X/& 2015 Elsevier B.V. All rights reserved.
materials of supercapacitors dramatically can lead to significant enhancement in the specific capacitance, which is closely related to pseudocapacitive behavior and enhanced wettability of the electrode materials to electrolyte [15]. However, most of biomass resources are composed of macromolecules such as lignin and cellulose. Unless the biomass resources contain metal catalysts or templates, they are often transformed into non-graphitizable carbon materials of low electrical conductivity during thermolysis at high temperature (above 2000 1C) [4]. Very recently, metal-free organic small molecules such as ionic liquids, which are defined as molten organic salts with a melting point below an arbitrary temperature such as 100 1C, have been reported as attractive alternatives to macromolecular carbon precursors for NCMs [16]. Since small molecules decompose completely into volatile fragments under thermolysis, metal catalysts should be required to synthesize the carbon materials [17]. Meanwhile, ionic liquids comprising of nitrogen-containing cations such as imidazolium and pyridinium facilitate the formation of NCMs by simple metal catalysts-free thermolysis in an inert gas environment. The most distinctive features of ionic liquids are their nonvolatility and decomposition below their boiling point. Considering such properties, thermolysis induces the polycondensation of ionic liquid precursors and subsequent aromatization with dehydrogenation, resulting in a graphitic microstructure [18]. However, ionic liquids are not good precursors because they are very expensive and have quite low yields in carbonisation. In this regard, amino acids might be attractive carbon precursors owing to their unique zwitterionic and their tendency to
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Fig. 1. (a) Schematic description for formation of carbon materials from amino acids via polycondensation and carbonization processes and TGA thermogram of representative natural amino acids.
decompose prior to melting. Moreover, they are cheap and abundant biomass, which can be continuously supplied in living organism. They are also environmentally benign as opposed to macromolecular biomass resources. Natural amino acids are decomposed completely into volatile fragments by thermolysis, which mechanism was elucidated by gas chromatography and mass spectrometry [19,20]. However, amino acids such as lysine, arginine, trypthopan, or histidine having N-terminal side chain produce char-like carbonanecous materials, which might have originated from polycondensed materials by inter- or intramolecular condensation through thermolysis at inert atmosphere (Fig. 1). Among amino acids, histidine was found to produce carbonaceous materials with high carbon yields. Therefore, we synthesized the histidine-derived carbon materials by simple stepwise thermolysis up to high temperature of 1000 1C. The obtained carbon materials contained small quantity of nitrogen atom and were produced as silvery-black solids with thick 2D-like planar structure. Thermolysis at 1500 1C yielded highly graphitic microstructure. Herein, we report the characteristics of histidinederived nitrogen-doped carbon materials (His-NCMs), and their electrochemical capacitive performance in energy storage application.
Electrochemical characterization of His-NCMs: Supercapacitor electrodes for electrochemical testing were composed of 80 wt% His-NCMs sample as the active material, 10 wt% poly(tetrafluoroethylene) as the binder, and 10 wt% acetylene black as the conducting agent. The weighed mixture was pressed into pellets of 15 mm diameter at a pressure of 5 MPa. The electrode weight was adjusted to 30 mg. The supercapacitor consisted of two electrodes, which were arranged face to face, with a separator (glass fiber) inserted between them. A Ni plate was used as the current collector. An aqueous electrolyte of 6 M KOH was used in this study. The charge voltage of the aqueous electrolyte system was limited to 1.0 V to ensure the stability of the solvent. The capacitance (C) of the electrodes was calculated on the basis of the following equation: C¼ (I Δt)/(W ΔV), where I is the discharge current, Δt is the discharge time, ΔV is the voltage variation of the initial voltage, and W is the weight of the active material of both the electrodes. All the specific capacitances were calculated from the following relationship: the capacitance in a three-electrode system equals four times the capacitance in a two-electrode system [21–23]. A conventional three-compartment cell was also used to confirm the decomposition voltage window.
3. Results and discussion 2. Materials and methods Materials: All reagents were purchased from Sigma-Aldrich, TCI, Acros Organic, or Mitsubishi Chemical.
Fig. 2 shows (a) and (b) X-ray photoelectron spectroscopy (XPS), (c) Raman, and (d) X-ray diffraction (XRD) spectra of His-NCM (1000) and His-NCM (1500) samples obtained after thermolysis to
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Fig. 2. (a, b) X-ray photoelectron spectroscopy (XPS), (c) Raman, and (d) XRD patterns of His-NCM (1000) and His-NCM (1500) obtained after thermolysis to temperatures of 1000 and 1500 1C, respectively.
temperatures of 1000 and 1500 1C, respectively. As shown in Fig. 2a, common peaks assigned in typical nitrogen-doped carbon materials appeared at 531, 400, and 284 eV corresponding to O 1s, N 1s, and C 1s-orbitals peaks, respectively, indicating that histidine could be effective carbon material precursor. From the intensity ratio between the nitrogen peaks, we roughly determined the nitrogen contents of 3.97 wt% and 1.64 wt% in His-NCM (1000) and His-NCM (1500), respectively. We also determined the atomic concentration of nitrogen in His-NCMs by elemental analysis (EA). The nitrogen doping content in His-NCM (1000) and His-NCM (1500) as determined by EA was ca. 5.7 wt% and 3.2 wt%, respectively, demonstrating slightly higher concentration compared to the results obtained by XPS (Table S1). These indicated that the nitrogen content in HisNCM was reduced by thermal treatment at 1500 1C, indicating removal of nitrogen atom followed by further carbonization to make more graphitic microstructure compared to that at 1000 1C. Fig. 2b shows the N 1s-orbital spectra of His-NCMs, which exhibits three individual peaks assigned to pyridinic N (398.0– 398.4 eV), pyrrolic N (399.8–400.5 eV), quaternary N (401.0– 401.5 eV), and oxidized N (403.2–403.4 eV) species, respectively. As shown in Fig. 2b, the His-NCMs exhibited the highest intensive
quaternary N peak with pyridinic N, pyrrolic N, and oxidized N species, but the peak intensities for pyridinic N, pyrrolic N, and oxidized N species are significantly decreased at higher temperature of 1500 1C. The quaternary N bonding configuration of HisNCM could be retained at thermal treatment of 1500 1C. The crystallinity of His-NCM (1000) and His-NCM (1500) is shown in Fig. 2(c) or (d). These His-NCMs exhibited the typical D and G bands between 1000–2000 cm 1, showing a decrease in the intensity ratio (ID/IG) of His-NCM (1500). And a distinct (002) peaks in the XRD pattern of His-NCM (1000) and His-NCM (1500) were observed with broad diffraction in the 2θ range of 24.1 and 25.31, respectively. The (002) peaks in His-NCM (1500) were better intensive than that of His-NCM (1000). These results indicated that thermal treatment at higher temperature of 1500 1C could induce more graphitic carbon lattices. Fig. 3(a) and (b) shows high resolution transmission electron microscopy (HRTEM) images of His-NCM (1000) and His-NCM (1500). The graphitic domains of His-NCM (1000) and His-NCM (1500) were placed sporadically in HRTEM images. Notably, the HisNCM (1500) exhibited significantly extended graphitic lattices, but the collapse of microstructures due to its graphitization. Fig. 3(c)
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Fig. 3. (a, b) HRTEM images of His-NCM (1000) and His-NCM (1500) and (c, d) SEM images of His-NCM (1000).
and (d) shows scanning electron microscopy (SEM) images of HisNCM (1000), which presented mostly the thickness of 200 nm and planar structures of large size. Interestingly, the rolled structures of these planes were also observed (Fig. 3d). His-NCM (1500) exhibited a similar morphology with His-NCM (1000). Fig. 4 shows the electrochemical capacitive performance of HisNCMs for supercapacitor application. An N2 adsorption technique was used at 77 K to confirm the change in specific surface area (SSA) of His-NCM (1000) and His-NCM (1500). The N2 adsorption/ desorption isotherms of the His-NCM (1000) showed typical type I isotherms with a steep N2 uptake at lower P/P0 and a long plateau at higher P/P0, indicating the presence of uniform microporosity. (Fig. 4a) Meanwhile, the His-NCM (1500) exhibited little N2 adsorption isotherms. These might be closely related with the collapse of microstructures at higher temperature of 1500 1C shown in Fig. 3. Although the pristine His-NCM (1000) had 2Dlike planar structure, that exhibited a notable SSA value of 455 m2/ g with micropore volumn of 0.13 mL/g and average micropore width of 0.63 nm. From these results, we measured cyclic voltammograms (CV) to explore the potential application of the His-NCM (1000) in supercapacitors.(Fig. 4b) The capacitive behavior of HisNCM (1000) showed the form of rectangular shape, which indicate the typical characteristics of electric double layer capacitor (EDLC)
[24], of the voltammetry characteristics. Fig. 4c shows the galvanostatic charge/discharge curves at different current densities. The triangle charge/discharge curves are linear and symmetrical, indicating that the His-NCM (1000) carbon electrode has excellent electrochemical reversibility, coulombic efficiency and ideal EDLC behavior [25,26]. The specific capacitances calculated from these discharge curves were 67, 58, 41, 28, 24 and 19 F/g at current densities of 0.05, 0.1, 0.3, 0.5, 0.7 and 1.0 A/g, respectively. However, the specific capacitance of His-NCM (1000), which was calculated and plotted with the current density, was decreased at higher current density specific capacitance due to its diffusionlimited charge/discharge process [27]. (see Fig. S4 of Supplementary information) Although the absolute value in specific capacitance of His-NCM (1000) seems to be low, its relative specific capacitance (58 F g 1/455 m2 g 1 E0.127 F/m2) on SSA value obtained from 2D-like planar structure without carbon activation could be comparable with that (85 F g 1/1500 m2 g 1 E0.057 F/m2) of commercially available YP-17D activated carbon from coconut shells [28]. The enhanced specific capacitance per unit of SSA could be understood by the nitrogen doping effect. The stability in supercapacitors is one of the important criteria in the selection of electrode materials. The cycle stability of the His-NCM (1000) electrode was investigated from chronopotentiometry measurements at the current
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Fig. 4. (a) N2 absorption/desorption isotherms of His-NCMs and (b) CV curves at different scan rate, (c) Galvanostatic charge/discharge curves at different current densities, and (d) cyclic stability of His-NCM (1000).
density of 0.1 A/g for 2000 cycles (Fig. 4d). With increasing cycle number from 200 to 2000, the specific capacitance exhibited superior long cycle life without a loss of performance values.
Acknowledgment This study was supported by research fund from Chosun University, 2014.
4. Conclusions Appendix A. Supporting information We demonstrated the characteristics of histidine-derived nitrogen-doped carbon materials (His-NCMs), and their electrochemical capacitive performance in energy storage application. In this study, small molecule natural amino acids could be candidate as environmentally benign carbon material precursor. The synthesized HisNCMs presented the rolled structures of planes as well as the thick and planar structures of large size. Even these 2D-like planar structures, His-NCM (1000) exhibited microporocity with a notable SSA value of 455 m2/g, exhibiting the specific capacitances of 58 F/g at current densities of 0.1 A/g and superior stability up to 2000 cycles without a loss of performance values. Further studies on these materials via KOH activation or template-based approaches to induce the high SSA are ongoing. We believe that the findings of this study introduce a new direction for the development of environmentally friendly carbon materials precursors.
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