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Cross-linked carbon networks constructed from N-doped nanosheets with enhanced performance for supercapacitors
Accepted Manuscript Title: Cross-linked carbon networks constructed from N-doped nanosheets with enhanced performance for supercapacitors Author: Qingqing Liu Jialiang Zhong Zhipeng Sun Hongyu Mi PII: DOI: Reference:
S0169-4332(16)32597-1 http://dx.doi.org/doi:10.1016/j.apsusc.2016.11.156 APSUSC 34461
To appear in:
APSUSC
Received date: Revised date: Accepted date:
12-8-2016 24-10-2016 21-11-2016
Please cite this article as: Qingqing Liu, Jialiang Zhong, Zhipeng Sun, Hongyu Mi, Cross-linked carbon networks constructed from N-doped nanosheets with enhanced performance for supercapacitors, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.11.156 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Cross-linked carbon networks constructed from N-doped nanosheets with enhanced performance for supercapacitors Qingqing Liua, Jialiang Zhonga, Zhipeng Sunb, Hongyu Mia,* a
Xinjiang Uygur Autonomous Region Key Laboratory of Coal Clean Conversion and Chemical
Engineering Process, School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China b
Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, China
Corresponding Authors.
Fax: +86 991 8582809, Tel: +86 991 8582809, Email: [email protected].
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Graphical Abstract
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Highlights ► PEG and melamine co-carbonized N-rich cross-linked carbon networks were prepared. ► Systematic study on microstructure dependency and performance was achieved. ► The optimized carbons offered superior electrochemical performance.
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Abstract Hierarchically porous carbons offer great benefits for constructing advanced electrodes for energy-related applications. Herein, we reported facile synthesis of cross-linked carbon networks (HPCNs) made from N-doped nanosheets. By using MgO as self-sacrificial templates, the polyethylene glycol and melamine precursors were first uniformly coated on the template, and then annealed at the elevated temperature in inert atmosphere before removing the templates by mild acid etching. Interestingly, the capacitance performance of HPCNs could be easily modulated by adjusting the mass ratio of the precursors and templates, as well as the carbonization temperature. The optimized HPCNs showed specific capacitances of 192.6 F g–1 at 1.0 A g–1 and 156.2 F g–1 even at 20 A g–1 in 6.0 M KOH solution, and long-term cyclability with 85.5% capacitance retention at high current load of 10 A g–1 after 8000 successive cycles, which were attributed to structural merits of these continuous networks including high surface area of 370.8 m2 g–1, high pore volume of 1.65 cm3 g–1, as well as high nitrogen content of 9.920 wt. %. Owing to simplicity of the synthesis method and superior performance, such HPCNs may promise great potential in energy storage fields. Keywords:
Cross-linked
carbon
nanosheets;
Supercapacitors
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N-doped
structures;
Cycling
stability;
1. Introduction Recently, there has been a growing interest in the development of sustainable energy to address the issues caused by fast depletion of fossil fuels. Advanced energy storage technologies, such as supercapacitors, play a pivotal role in effectively utilizing these renewable resources. Porous carbons represent the most widely studied materials for supercapacitors due to their high conductivity, excellent stability, and wide availability [1–3]. Many types of porous carbons have been carefully investigated for applications as supercapacitor electrodes with remarkable progress achieved. However, the performance of carbon electrodes, especially the capacitance, requires further improvement to fulfill the increasing energy demand in the near future [4]. Typically, the capacitance of carbon materials can be enhanced by enlarging specific surface area, constructing a hierarchically porous system, creating highly efficient conductive paths with strong structural stability, doping with heteroatoms, and building hybrid carbon architectures [5– 25]. For instance, the reported three-dimensional (3D) hierarchical porous carbon derived from polypyrrole microsheets, exhibited the capacitances up to 318.2 and 189.0 F g−1 at 0.5 and 50 A g–1, as well as superior rate performance [20]. A capacitance as high as 225 F g–1 at 2 A g–1 was offered by heavily nitrogen doped porous carbons from yogurt [23]. A well-combined hybrids of hollow carbon spheres located on carbon nanotube surfaces could achieve a capacitance of 201.5 F g–1 at 0.5 A g–1 and good rate performance of 69% capacitance retention at 20 A g–1, with excellent stability with 90% retention over 5000 cycles [24]. Very recently, Wang and Qiu et al. reported B/N co-doped carbon nanosheets from biomass, which displayed excellent rate performance with 70% retention in a large current range 0.2–100 A g–1, as well as outstanding electrochemical stability [26].
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N-doped graphene nanosheets as excellent candidates are actively studied in many electrochemical areas, which are usually prepared by a two-step procedure of chemical exfoliation of graphite and the addition of N species. Nevertheless, both the complexity of the method and relatively high cost make these materials less suitable for mass-production [26,27]. Exploring a facile method to inexpensive substitutes of N-doped graphene nanosheets may be a favorable solution. In this research, we report cross-linked carbon networks constructed from Ndoped nanosheets. To date, the design that utilizes the linking of porous carbons from polyethylene glycol (PEG) and melamine towards N-doped continuous nanosheet/film networks is not presented. Besides, the comprehensive and extensive research on so-formed carbon networks is hardly implemented. In a typical process, MgO has been employed as self-sacrificial templates for the controllable synthesis of porous carbon architectures. This conventional MgOtemplated method is very popular because of the advantages of MgO template such as the structural stability, chemical and thermal stabilities, low cost and easy removal [9,28]. The carefully selected precursors (polyethylene glycol and melamine) can contribute to N-doped interconnected nanosheets which will be transformed into 3D network structures after annealing in inert atmosphere. Hierarchically porous N-doped carbon nanosheet networks (HPCNs) can thus be effectively produced by further removing the MgO template through mild acid etching. The structures and properties of HPCNs rely on the mass ratio of the precursors and templates, as well as the carbonization temperature, thus allowing efficient modulating their electrochemical performance. In particular, the optimized HPCNs (PEG-melamine-MgO ratio: 10:3:7; carbonization temperature: 700 oC) deliver a specific capacitance of 192.6 F g–1 at 1.0 A g–1, and long-term cyclability with a capacitance decay of 14.5% at a high current load of 10 A g–1 after
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8000 cycles when evaluated by a three-electrode system with 6.0 M KOH as the electrolyte solution. 2. Experimental 2.1. Preparation of HPCNs Magnesium oxide (MgO), polyethylene glycol (PEG, MW = 6000), and melamine were uniformly mixed in various mass ratios, and tableted under 30 MPa pressure, as the precursors. These tablets were treated in a tube furnace held at 900 oC for 3 h under N2 atmosphere (heating rate: 5 oC min–1). Subsequently, the annealed products were soaked in 3 M HCl solution, and then washed with distilled water and ethanol to remove MgO. Finally, the powder was dried at 80 oC in vacuum for 24 h to get a series of porous carbons (HPCNs). The products from various PEG-melamine-MgO mass ratios of 10:10:0, 10:7:3, 10:5:5, 10:3:7 and 10:1:9 were termed as W0, W1, W2, W3 (also denoted as W3-900) and W4, respectively. The mixtures with a mass ratio of 10:3:7 were heated at different temperatures (600 oC, 700 oC, 800 oC and 900 oC) under N2 flow to yield W3-600, W3-700, W3-800 and W3-900 samples. 2.2. Structure characterization and performance evaluation Scanning electron microscopy (SEM) images of the products were taken with a Zeiss 55VP microscope. Low- and high-resolution transmission electron microscopy (TEM and HRTEM) images were taken with HITACHI-600 and JEM-2100F microscopes. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance using Cu kα radiation. Raman spectra were collected using a Bruker Senterra spectrometer. Elemental analysis (EA) was performed using a Vario EL III elemental analyzer. The Brunauer-Emmett-Teller (BET) area was obtained through N2 adsorption/desorption isotherms on a Biaode SSA-4200 system, and pore size distribution (PSD) was evaluated by the Barrett-Joyner-Halenda (BJH) method.
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Electrochemical tests were operated on a CHI660D electrochemical working station (Chenhua Instruments Co. Ltd., Shanghai) and a CT2001A battery testing system (LAND electronics Co. Ltd, Wuhan), where a three-electrode system (including the working electrode, platinum counter electrode and Hg/HgO reference electrode) was used in 6.0 M KOH aqueous electrolyte. The working electrode was made by pressing a disk (1 cm diameter) containing active material, acetylene black and poly-(tetrafluoroethylene) (the mass ratio of 8:1:1) on nickel foam. The loading of active materials on nickel foam is ~2.9 mg cm–2. Scan rates for cyclic voltammetry (CV) were between 2 and 50 mV s–1 in a potential window -1.0 to 0 V. Current densities for constant charge-discharge (CD) were from 1 to 20 A g–1 in the same window. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range 105–10–2 Hz at open circuit potential with an AC perturbation of 5 mV. 3. Results and discussion 3.1. Fabrication of HPCNs N-doped porous carbons are generally synthesized by direct carbonization of nitrogenous precursors or post-modification of carbon materials with other nitrogenous small-molecules [29]. The former is more facile and simpler as compared to the latter. So we propose a strategy of the direct co-pyrolysis of both carbonaceous and nitrogenous precursors in the presence of the MgO template, aiming to obtain ideal architectures, namely, hierarchically porous N-doped carbon nanosheet networks with interconnected structures. The synthesis process is schematically presented in Fig. 1. We select low-cost PEG, melamine and MgO as initial materials because of their significant functions in achieving the target structures. PEG acts simultaneously as carbon sources and the linkers between melamine and MgO, and melamine serves as nitrogen sources, while MgO as the self-sacrificial template. In detail, PEG was firstly ground with MgO in order
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to homogenously disperse PEG around or the surface of MgO. Melamine is then mixed with them, and compressed into several sheets, ensuring strong adhesion among these components. After the carbonization and subsequent MgO etching, N-doped cross-linked carbon networks are produced, showing good structural integrity. Notably, the morphology transformation from rigid thick sheet aggregates to flexible film strongly depends on the PEG-melamine-MgO mass ratio. The ratio also affects the aggregation degree of porous carbons. Under the optimized conditions (10:3:7 mass ratio and carbonization at 700 oC), the ultrathin nanosheet assembled porous carbon networks are formed. Because of meeting the above-mentioned structural demands for improving the electrode performance, such as relatively high BET area, high porosity and flexibility, heteroatom doping, and 3D stable interconnected structures, the designed HPCNs may be superior as the electrode of supercapacitors. 3.2. Influence of mass ratio of initial materials on morphology Fig. 2a-e show SEM images of various HPCNs (W0, W1, W2, W3 and W4) from different PEG-Melamine-MgO mass ratios, which reveal the effect of the ratio on the morphology evolution. The W0 sample with the 10:10:0 ratio (Fig. 2a) is porous aggregates composed of thick sheets due to serious stacking of porous carbons in the absence of MgO. As the ratio varies from 10:7:3 and 10:5:5 to 10:3:7, the shape of HPCNs converts from comparatively thick sheets (Fig. 2b and c) to flexible film-interconnected networks (Fig. 2d). Furthermore, these film networks (W3) show a plenty of randomly distributed open channels, which are nearly graphene nanosheet form. At the ratio up to 10:1:9 (Fig. 2e), continuous nanosheets are partially replaced by some broken nanoparticle-formed film agglomerates, as direct results of high MgO content in the mixture and strong van der Waals attraction between thin carbon layers. Film geometry with high porosity for W3 is confirmed by TEM and HRTEM images in Fig. 2f and g. Combined with
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the SEM image in Fig. 2d, it is observed that W3 has not only rich pores in surfaces but also 3D flexible channels from the surface extending to interior, suggesting fast transport for ions. In short, the microscopic analysis may conclude that: (i) the mass ratio of initial materials holds an important role in tailoring the morphology and suppressing serious agglomeration or restacking of porous carbons; (ii) the MgO content in the ternary system is of importance in forming welldeveloped interconnected structures; (iii) the optimal proportion of PEG-melamine-MgO for producing continuous nanosheet networks is 10:3:7 based on the consideration of microstructures. 3.3. Influence of mass ratio of initial materials on porous property Fig. 3 presents N2 adsorption/desorption isotherms and PSD curves of HPCNs (W0, W1, W2, W3 and W4). The isotherms of all samples are IV-typed in the IUPAC classification (Fig. 3a), which verifies their mesoporous characteristic [30]. The details of pore properties are summarized in Table 1. With varying the PEG-melamine-MgO ratio from 10:10:0 to 10:3:7, the BET area and total pore volume of the HPCNs increase, i.e., 23.1 m2 g–1 and 0.11 cm3 g–1 for W0 from the 10:10:0 ratio, 155.7 m2 g–1 and 0.64 cm3 g–1 for W2 from the 10:5:5 ratio, and 286.6 m2 g–1 and 1.05 cm3 g–1 with a dominating mesoporous volume (0.931 cm3 g–1) for W3 from the 10:3:7 ratio. However, the area declines to 241.9 m2 g–1 for W4 from 10:1:9. Area variation well matches with the morphology evolution. These data account for high control of the ratio of initial materials on pore property, and the appropriate ratio can lead to further development of pore structures. PSD curves of all HPCNs in Fig. 3b and the inset show a similar peak around ca. 3.7 nm at the mesopore range. Based on the analysis, it is suggested that all HPCNs obtained are mesoporous materials, and W3 has good pore properties regarding area and pore volume. In
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general, high mesoporosity for electrode materials may improve the electrolyte ion transport and decrease ion transport resistance during electrochemical process [31]. 3.4. Influence of carbonization temperature on morphology To elucidate the influence of carbonization temperature on morphology, SEM and TEM morphologies of HPCNs (W3-600, W3-700 and W3-800) prepared at 600 oC, 700 oC and 800 oC in PEG-melamine-MgO ratio of 10:3:7 are displayed in Fig. 4. SEM images (Fig. 4a-c and Fig. 2d) reveal the formation of 3D porous carbon networks built from continuous nanosheets. This morphology is nearly that of interconnected carbon nanosheets reported [32]. Moreover, the increase of carbonization temperature causes the decrease in thickness of sheets. This tendency is because the release of lots of gases induced by high-temperature carbonization promotes the separation of carbon layers and the formation of thinner nanosheets. TEM and HRTEM images (Fig. 4d-i) further identify their similar sheet structures with high porosity. Despite of ultrathin nanosheets that might possess high theoretical BET area, they may restack due to strong interaction, thus resulting in the decrease of the accessible area. 3.5. Influence of carbonization temperature on porous property Fig. 5 presents N2 adsorption/desorption isotherms and PSD curves of HPCNs (W3-600, W3700, W3-800 and W3-900). As shown in Fig. 5a, IV-typed isotherms with a hysteresis loop in the relative pressure (P/P0) between 0.4 and 1.0 indicate their mesoporous nature. Detailed porosity parameters are listed in Table 2. The HPCNs synthesized at relatively low temperature (600 oC or 700 oC) show larger BET specific surface areas (360.5 and 370.8 m2 g–1 for W3-600 and W3-700, respectively), which, however, is gradually reduced at higher temperature (800 oC or 900 oC, 355.8 and 286.6 m2 g–1 for W3-800 and W3-900, respectively). Besides, total pore volume also displays the same tendency, which decreases from 1.65 to 1.05 cm3 g–1 with the
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temperature increasing from 700 oC to 900 oC. The decline may be contributed to the partial collapse of pores and restacking of carbon nanosheets through π–π attraction [33,34]. By comparison, W3-700 has the highest values regarding BET area, pore volume and mesoporosity (ca. 96%), inferring that the suitable temperature producing ideal HPCNs is 700 oC. PSD curves in Fig. 5b are close to each other, with a peak centered at ca. 3.7 nm. 3.6. Influence of carbonization temperature on structure XRD patterns and Raman spectra are conducted to investigate the influences on structure of HPCNs obtained from various carbonization temperatures. All XRD patterns in Fig. 6a show low-intensity peaks at ~24.8o and ~43.3o ascribing to graphitic (002) and (100) reflections, suggesting relatively low graphitic crystallinity. It is also found that the (002) diffraction peak becomes sharper with the increase of carbonization temperature, indicating the improvement of crystallinity. Graphitization degree of HPCNs is further confirmed by Raman spectra in Fig. 6b. All carbon samples display G and D bands, which are associated with ordered and disordered structures [35], respectively. Intensity ratios of G to D bands (IG/ID) for W3-600, W3-700, W3800, and W3-900 are 0.964, 0.982, 1.018 and 1.021, respectively. It signifies that the increase of the temperature from 600 oC to 800 oC can promote the graphitization level of porous carbons but only little enhancement occurs between 800 oC and 900 oC, which is close to the report on spherical nitrogen-doped porous carbon shells [36]. In addition, N-doped structures in HPCNs are verified by EA analysis, which shows that N contents of W3-600, W3-700, W3-800 and W3900 are 8.493, 9.920, 7.619 and 3.378 wt.%, respectively. Accordingly, W3-700 has the highest N-doped level as compared to other HPCNs. 3.7. Electrochemical performance The electrochemical performance of HPCN electrodes is carefully evaluated by means of
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cyclic voltammetry (CV), galvanostatic charge-discharge (CD) and electrochemical impedance spectroscopy (EIS) in three-electrode system in 6.0 M KOH aqueous solution. Fig. 7a presents CV curves of the electrodes at the scan rate 10 mV s–1. A nearly rectangular shape with a slight deviation is found in all CV curves. It is proposed that complex charge storage mechanisms exist in all HPCNs, which include typical electric double layer capacitive behavior from porous carbons, and pseudocapacitance behavior from nitrogen species. Additionally, the order in CV area is W3-700 > W3-600 > W3-800 > W3-900, implying the highest capacitance in W3-700. CV curves with a wide scan rate range 2–50 mV s–1 for W3-700 are shown in Fig. 7b. The shape of CV curves can be well maintained at all tested scanning rates, confirming that W3-700 has fast ion transport behavior. Constant charge-discharge (CD) tests on HPCN electrodes are used to evaluate the specific capacitance and rate performance. CD curves of various HPCNs at a current density of 1 A g–1 are presented in Fig. 8a. Nearly symmetrical shape in all curves verifies their ideal capacitance behavior and high reversibility. Besides, a slight deviation to linearity is detected, which is caused mainly by redox reactions of N species. The specific capacitance (Cm) is calculated from the discharge curve based on the equation: Cm = IΔt/mΔV (I for the charge-discharge current density, Δt for discharge time, m for the mass of active materials, and ΔV for the potential during the discharge process). Cm values for W3-600, W3-700, W3-800 and W3-900 are calculated to be 171.8, 192.6, 141.3 and 70.9 F g–1 at 1 A g–1, respectively. Obviously, W3-700 achieves the maximum capacitance, which is attributed to short diffusion pathways available for ions due to ideal pore properties and rich active sites from N-doping structures [37]. The Cm values for W3700 at 1, 2, 4, 6, 8, 10, 15 and 20 A g–1 can be obtained from discharge curves in Fig. 8b, which are around 192.6, 186.3, 179.9, 175.7, 170.7, 167.3, 163.1 and 156.2 F g–1, respectively. Besides,
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the capacitance values of HPCNs at various currents are listed in Table 3, which show that all samples have a slight decrease in specific capacitance as the current increases. It may be ascribed to the reduced utilization of active materials during rapid charge/discharge processes [17]. In this case, W3-700 still remains a capacitance up to 156.2 F g–1 even at 20 A g–1 as compared to other samples (130.2 F g–1 for W3-600, 112.8 F g–1 for W3-800, 47.5 F g–1 for W3-900), with 81.1% retention, conforming its superior rate performance. Galvanostatic cycling tests on W3-700 electrode are performed at a high load of 10 A g–1, as shown in Fig. 9. After cycled up to 8000 cycles, W3-700 still keeps 85.5% of initial capacitance. CD curves of W3-700 for the initial 10 cycles and the last 10 cycles are also interpolated into Fig. 9. No pronounced difference in shape is observed in both insets except a slight change in discharge time, suggesting good cycling stability. It is related to high porosity as well as interconnected nanosheet network structures of W3-700. Nyquist impedance plots are usually regarded as efficient evidence of capacitive behavior of electrode materials. The plots of W3-700 are displayed in Fig. 10 and its inset. Internal resistance Rs and charge-transfer resistance Rct are usually estimated by high frequency intercept on the real impedance axis and the diameter of semicircles in middle frequency, respectively. Rs and Rct values are 0.4 and 1.7 Ω. At low frequency, the deep-rising straight line nearly parallel to the imaginary axis. Both low resistances and ideal capacitive behavior is thus demonstrated, which may be contributed to synergistic effect of 3D interconnected networks, high BET area and high N-doped structures of W3-700. 4. Conclusions Porous carbon networks organized from ultrathin and N-doped carbon nanosheets are easily prepared through the MgO-templated method with both PEG and melamine as the precursors.
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The control over the morphology, and pore properties can be realized by simply altering mass ratio of PEG-melamine-MgO and carbonization temperature. Good electrochemical results including high capacitance, good rate capability and long cycle life are achieved with the optimized HPCNs, probably thanks to 3D sheet-interconnected network structures with high BET area and high porosity, high N content, and continuous conductive paths. This first systematic study on the microstructure dependency and electrochemical properties of melamine and PEG co-pyrolysis N-doped carbon nanosheet networks offers the general and valuable guidelines for designed formation of high performing 3D heteroatom-doped cross-linked porous carbons for use in many fields. Acknowledgements The authors are grateful to the financial support from the National Natural Science Foundation of China (Nos. 21363023, 21563029, U1403293).
References [1] M.H. Sun, S.Z. Huang, L.H. Chen, Y. Li, X.Y. Yang, Z.Y. Yuan, B.L. Su, Applications of hierarchically structured porous materials from energy storage and conversion, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine, Chem. Soc. Rev. 45 (12) (2016) 3479–3563. [2] M.G. Hahm, A.L.M. Reddy, D.P. Cole, M. Rivera, J.A. Vento, J. Nam, H.Y. Jung, Y.L. Kim, N.T. Narayanan, D.P. Hashim, C. Galande, Y.J. Jung, M. Bundy, S. Karna, P.M. Ajayan, R. Vajtai, Carbon nanotube–nanocup hybrid structures for high power supercapacitor applications, Nano Lett. 12 (11) (2012) 5616–5621.
15
[3] A. Yu, Z. Chen, R. Maric, Z. Zhang, J. Zhang, J. Yan, Electrochemical supercapacitors for energy storage and delivery: advanced materials, technologies and applications, Appl. Energy 153 (2015) 1–2. [4] S. Nardecchia, D. Carriazo, M. Luisa Ferrer, M. C. Gutiérrez, F. Del Monte, Three dimensional macroporous architectures and aerogels built of carbon nanotubes and/or graphene: synthesis and applications, Chem. Soc. Rev. 42 (2) (2013) 794–830. [5] M. Sevilla, A.B. Fuertes, Direct synthesis of highly porous interconnected carbon nanosheets and their application as high-performance supercapacitors, ACS Nano 8 (5) (2014) 5069–5078. [6] D. Hulicova-Jurcakova, M. Kodama, S. Shiraishi, H. Hatori, Z.H. Zhu, G.Q. Lu, Nitrogenenriched nonporous carbon electrodes with extraordinary supercapacitance, Adv. Funct. Mater. 19 (11) (2009) 1800–1809. [7] Y. Gao, L. Li, Y. Jin, Y. Wang, C. Yuan, Y. Wei, G. Chen, J. Ge, H. Lu, Porous carbon made from rice husk as electrode material for electrochemical double layer capacitor, Appl. Energy 153 (2015) 41-47. [8] G. Zu, J. Shen, L. Zou, F. Wang, X. Wang, Y. Zhang, X. Yao, Nanocellulose-derived highly porous carbon aerogels for supercapacitors, Carbon 99 (2016) 203–211. [9] T. Morishita, T. Tsumura, M. Toyoda, J. Przepiórski, A.W. Morawski, H. Konno, M. Inagaki, A review of the control of pore structure in MgO-templated nanoporous carbons, Carbon 48 (10) (2010) 2690–2707. [10] Y. Deng, Y. Xie, K. Zou, X. Ji, Review on recent advances in nitrogen-doped carbons: preparations and applications in supercapacitors, J. Mater. Chem. A 4 (2016) 1144–1173.
16
[11] P. Hao, Z. Zhao, J. Tian, H. Li, Y. Sang, G. Yu, H. Cai, H. Liu, C.P. Wong, A. Umar, Hierarchical porous carbon aerogel derived from bagasse for high performance supercapacitor electrode, Nanoscale 6 (20) (2014) 12120–12129. [12] V.B. Kumar, A. Borenstein, B. Markovsky, D. Aurbach, A. Gedanken, M. Talianker, Z. Porat, Activated carbon modified with carbon nanodots as novel electrode material for supercapacitors, J. Phys. Chem. C 120 (25) (2016) 13406−13413. [13] C. Zheng, X. Zhou, H. Cao, G. Wang, Z. Liu, Synthesis of porous graphene/activated carbon composite with high packing density and large specific surface area for supercapacitor electrode material, J. Power Sources 258 (2014) 290–296. [14] F. Markoulidis, C. Lei, C. Lekakou, D. Duff, S. Khalil, B. Martorana, I. Cannavaro, A method to increase the energy density of supercapacitor cells by the addition of multiwall carbon nanotubes into activated carbon electrodes, Carbon 68 (2013) 58–66. [15] X. Cui, R. Lv, R.U.R. Sagar, C. Liu, Z. Zhang, Reduced graphene oxide/carbon nanotube hybrid film as high performance negative electrode for supercapacitor, Electrochim. Acta 169 (2015) 342–350. [16] A. Bello, F. Barzegar, D. Momodu, J. Dangbegnon, F. Taghizadeh, N. Manyala, Symmetric supercapacitors based on porous 3D interconnected carbon framework, Electrochim. Acta 151 (2015) 386–392. [17] E.C. Vermisoglou, T. Giannakopoulou, G.E. Romanos, N. Boukos, M. Giannouri, C. Lei, C. Lekakou, C. Trapalis, Non-activated high surface area expanded graphite oxide for supercapacitors, Appl. Surf. Sci. 358 (2015) 110–121.
17
[18] Y. Zhou, S. L. Candelaria, Q. Liu, E. Uchaker, G. Cao, Porous carbon with high capacitance and graphitization through controlled addition and removal of sulfur-containing compounds, Nano Energy 12 (2015) 567–577. [19] D.M. Anjos, J.K. McDonough, E. Perre, G.M. Brown, S.H. Overbury, Y. Gogotsi, V. Presser, Pseudocapacitance and performance stability of quinone-coated carbon onions, Nano Energy 2 (5) (2013) 702–712. [20] L. Qie, W. Chen, H. Xu, X. Xiong, Y. Jiang, F. Zou, X. Hu, Y. Xin, Z. Zhang, Y. Huang, Synthesis of functionalized 3D hierarchical porous carbon for high-performance supercapacitors, Energy Environ. Sci. 6 (8) (2013) 2497–2504. [21] S. Isikli, M. Lecea, M. Ribagorda, M.C. Carreño, R. Díaz, Influence of quinone grafting via Friedel–Crafts reaction on carbon porous structure and supercapacitor performance, Carbon 66 (2014) 654–661. [22] W. Wang, S. Guo, M. Penchev, I. Ruiz, K.N. Bozhilov, D. Yan, M. Ozkan, C.S. Ozkan, Three dimensional few layer graphene and carbon nanotube foam architectures for high fidelity supercapacitors, Nano Energy 2 (2) (2013) 294–303. [23] M. Wahid, G. Parte, D. Phase, S. Ogale, Yogurt: a novel precursor for heavily nitrogen doped supercapacitor carbon, J. Mater. Chem. A 3 (3) (2015) 1208–1215. [24] Q. Wang, J. Yan, Y. Wang, G. Ning, Z. Fan, T. Wei, J. Cheng, M. Zhang, X. Jing, Template synthesis of hollow carbon spheres anchored on carbon nanotubes for high rate performance supercapacitors, Carbon 52 (2013) 209–218. [25] Q. Xie, S. Zhou, A. Zheng, C. Xie, C. Yin, S. Wu, Sandwich-like nitrogen-enriched porous carbon/graphene composites as electrodes for aqueous symmetric supercapacitors with high energy density, Electrochim. Acta 189 (2016) 22–31.
18
[26] Z. Ling, Z. Wang, M. Zhang, C. Yu, G. Wang, Y. Dong, S. Liu, Y. Wang, J. Qiu, Sustainable synthesis and assembly of biomass-derived B/N co-doped carbon nanosheets with ultrahigh aspect ratio for high-performance supercapacitors, Adv. Funct. Mater. 26 (1) (2016) 111–119. [27] V. León, M. Quintan, M.A. Herrero, J.L.G. Fierro, A.D.L. Hoz, M. Prato, E. Vázquez, Fewlayer graphenes from ball-milling of graphite with melamine, Chem. Commun. 47 (39) (2011) 10936–10938. [28] W. Geng, F. Ma, G. Wu, S. Song, J. Wan, D. Ma, MgO-templated hierarchical porous carbon sheets derived from coal tar pitch for supercapacitors, Electrochim. Acta 191 (2016) 854–863. [29] N.P. Wickramaratne, J. Xu, M. Wang, L. Zhu, L. Dai, M. Jaroniec, Jaroniec M, Nitrogen enriched porous carbon spheres: attractive materials for supercapacitor electrodes and CO2 adsorption, Chem. Mater. 26 (9) (2014) 2820–2828. [30] L. You, Y. Zhang, S. Xu, J. Guo, C. Wang, Movable magnetic porous cores enclosed within carbon
microcapsules:
structure-controlled
synthesis
and
promoted
carbon-based
applications, ACS Appl. Mater. Interfaces 6 (17) (2014) 15179–15187. [31] Y. Qiang, J. Jiang, Y. Xiong, H. Chen, J. Chen, S. Guan, J. Chen, Facile synthesis of N/P co-doped carbons with tailored hierarchically porous structures for supercapacitor applications, RSC Adv. 6 (2016) 9772–9778. [32] H. Peng, G. Ma, K. Sun, Z. Zhang, Q. Yang, Z. Lei, Nitrogen-doped interconnected carbon nanosheets from pomelo mesocarps for high performance supercapacitors, Electrochim. Acta 190 (2016) 862–871.
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[33] X. Liu, L. Zhou, Y. Zhao, L. Bian, X. Feng, Q. Pu, Hollow, spherical nitrogen-rich porous carbon shells obtained from a porous organic framework for the supercapacitor, ACS Appl. Mater. Interfaces 5 (20) (2013) 10280–10287. [34] D. Liu, Z. Jia, D. Wang, Preparation of hierarchically porous carbon nanosheet composites with graphene conductive scaffolds for supercapacitors: an electrostatic-assistant fabrication strategy, Carbon 100 (2016) 664–677. [35] M. Wahid, D. Puthusseri, D. Phase, S. Ogale, Enhanced capacitance retention in a supercapacitor made of carbon from sugarcane bagasse by hydrothermal pretreatment, Energy Fuels 28 (6) (2014) 4233–4240. [36] B. Friedel, S. Greulich-Weber, Preparation of monodisperse, submicrometer carbon spheres by pyrolysis of melamine–formaldehyde resin, Small 2 (7) (2006) 859–863. [37] Q. Zhao, X. Wang, H. Xia, J. Liu, H. Wang, J. Gao, Y. Zhang, J. Liu, H. Zhou, X. Li, S. Zhang, X. Wang, Design, preparation and performance of novel three-dimensional hierarchically porous carbon for supercapacitors, Electrochim. Acta 173 (2015) 566–574.
20
Figure Captions
Fig. 1. Brief description for the fabrication of HPCNs.
21
Fig. 2. SEM images of HPCNs prepared with different mass ratios of PEG- melamine-MgO: W0 (10:10:0), (b) W1 (10:7:3), (c) W2 (10:5:5), (d) W3 (10:3:7), (e) W4 (10:1:9). (f) TEM and (g) HRTEM images of W3.
22
Fig. 3. (a) N2 adsorption/adsorption isotherms at 77 K and (b) PSD curves of W0, W1, W2, W3 and W4.
23
Fig. 4. SEM, TEM and HRTEM images of HPCNs prepared under different carbonization temperature in PEG-melamine-MgO ratio of 10:3:7: (a,d,g) 600 oC (W3-600), (b,e,h) 700 oC (W3-700), (c,f,i) 800 oC (W3-800).
24
Fig. 5. (a) N2 adsorption-desorption isotherms and (b) PSD curves of W3-600, W3-700, W3-800 and W3-900.
25
Fig. 6. (a) XRD patterns and (b) Raman spectra of W3-600, W3-700, W3-800 and W3-900.
26
Fig. 7. (a) CV curves of various HPCN electrodes at a scan rate of 10 mV s–1 in 6.0 M KOH solution. (b) CV curves of W-700 electrode at scan rates ranging from 2 to 50 mV s–1.
27
Fig. 8. (a) Constant CD curves of various HPCN electrodes at a current density of 1 A g–1. (b) Constant CV curves of W-700 electrode at current densities ranging from 1 to 20 A g–1.
28
Fig. 9. Cycling performance of W3-700 electrode over 8000 charge-discharge cycles at a current density of 10 A g–1. The inset is CD curves for the first 10 cycles and the last 10 cycles.
29
Fig. 10. Nyquist impedance plot of W3-700 electrode. The inset is the enlarged plots in high frequency region.
30
Table 1 Textural properties of various HPCNs Sample
Vmicro (cm3 g–1)
SBET
Vtotal
(m2 g–1)
(cm3 g–1)
W0 (10:10:0)
23.1
0.11
0.009
0.101
W1 (10:7:3)
78.0
0.32
0.033
0.287
W2 (10:5:5)
155.7
0.64
0.064
0.576
W3 (10:3:7)
286.6
1.05
0.119
0.931
W4 (10:1:9)
241.9
1.06
0.100
0.96
(the
mass ratio)
Vmeso (cm3 g–1)
Table 2 Textural properties of various HPCNs Sample
SBET
Vtotal
Vmicro
Vmeso
(m2 g–1)
(cm3 g–1)
(cm3 g–1)
(cm3 g–1)
W3-600
360.5
1.02
0.158
0.962
W3-700
370.8
1.65
0.165
1.585
W3-800
355.8
1.32
0.162
1.158
W3-900
286.6
1.05
0.119
0.931
Table 3 Specific capacitances of various HPCNs at different current densities
Sample
Specific capacitance (F g–1) –1
1Ag
–1
4Ag
–1
8Ag
–1
–1
–1
10 A g
15 A g
20 A g
Capacitance retention (%)
W3-600
171.8
156.9
150.1
144.1
136.4
130.2
75.8%
W3-700
192.6
179.9
170.7
167.3
163.1
156.2
81.1%
31
W3-800
141.3
130.4
123.8
121.4
116.1
112.8
79.8%
W3-900
70.9
59.8
54.6
53.0
50.3
47.5
67.0%
32
S0169-4332(16)32597-1 http://dx.doi.org/doi:10.1016/j.apsusc.2016.11.156 APSUSC 34461
To appear in:
APSUSC
Received date: Revised date: Accepted date:
12-8-2016 24-10-2016 21-11-2016
Please cite this article as: Qingqing Liu, Jialiang Zhong, Zhipeng Sun, Hongyu Mi, Cross-linked carbon networks constructed from N-doped nanosheets with enhanced performance for supercapacitors, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.11.156 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Cross-linked carbon networks constructed from N-doped nanosheets with enhanced performance for supercapacitors Qingqing Liua, Jialiang Zhonga, Zhipeng Sunb, Hongyu Mia,* a
Xinjiang Uygur Autonomous Region Key Laboratory of Coal Clean Conversion and Chemical
Engineering Process, School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China b
Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, China
Corresponding Authors.
Fax: +86 991 8582809, Tel: +86 991 8582809, Email: [email protected].
1
Graphical Abstract
2
Highlights ► PEG and melamine co-carbonized N-rich cross-linked carbon networks were prepared. ► Systematic study on microstructure dependency and performance was achieved. ► The optimized carbons offered superior electrochemical performance.
3
Abstract Hierarchically porous carbons offer great benefits for constructing advanced electrodes for energy-related applications. Herein, we reported facile synthesis of cross-linked carbon networks (HPCNs) made from N-doped nanosheets. By using MgO as self-sacrificial templates, the polyethylene glycol and melamine precursors were first uniformly coated on the template, and then annealed at the elevated temperature in inert atmosphere before removing the templates by mild acid etching. Interestingly, the capacitance performance of HPCNs could be easily modulated by adjusting the mass ratio of the precursors and templates, as well as the carbonization temperature. The optimized HPCNs showed specific capacitances of 192.6 F g–1 at 1.0 A g–1 and 156.2 F g–1 even at 20 A g–1 in 6.0 M KOH solution, and long-term cyclability with 85.5% capacitance retention at high current load of 10 A g–1 after 8000 successive cycles, which were attributed to structural merits of these continuous networks including high surface area of 370.8 m2 g–1, high pore volume of 1.65 cm3 g–1, as well as high nitrogen content of 9.920 wt. %. Owing to simplicity of the synthesis method and superior performance, such HPCNs may promise great potential in energy storage fields. Keywords:
Cross-linked
carbon
nanosheets;
Supercapacitors
4
N-doped
structures;
Cycling
stability;
1. Introduction Recently, there has been a growing interest in the development of sustainable energy to address the issues caused by fast depletion of fossil fuels. Advanced energy storage technologies, such as supercapacitors, play a pivotal role in effectively utilizing these renewable resources. Porous carbons represent the most widely studied materials for supercapacitors due to their high conductivity, excellent stability, and wide availability [1–3]. Many types of porous carbons have been carefully investigated for applications as supercapacitor electrodes with remarkable progress achieved. However, the performance of carbon electrodes, especially the capacitance, requires further improvement to fulfill the increasing energy demand in the near future [4]. Typically, the capacitance of carbon materials can be enhanced by enlarging specific surface area, constructing a hierarchically porous system, creating highly efficient conductive paths with strong structural stability, doping with heteroatoms, and building hybrid carbon architectures [5– 25]. For instance, the reported three-dimensional (3D) hierarchical porous carbon derived from polypyrrole microsheets, exhibited the capacitances up to 318.2 and 189.0 F g−1 at 0.5 and 50 A g–1, as well as superior rate performance [20]. A capacitance as high as 225 F g–1 at 2 A g–1 was offered by heavily nitrogen doped porous carbons from yogurt [23]. A well-combined hybrids of hollow carbon spheres located on carbon nanotube surfaces could achieve a capacitance of 201.5 F g–1 at 0.5 A g–1 and good rate performance of 69% capacitance retention at 20 A g–1, with excellent stability with 90% retention over 5000 cycles [24]. Very recently, Wang and Qiu et al. reported B/N co-doped carbon nanosheets from biomass, which displayed excellent rate performance with 70% retention in a large current range 0.2–100 A g–1, as well as outstanding electrochemical stability [26].
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N-doped graphene nanosheets as excellent candidates are actively studied in many electrochemical areas, which are usually prepared by a two-step procedure of chemical exfoliation of graphite and the addition of N species. Nevertheless, both the complexity of the method and relatively high cost make these materials less suitable for mass-production [26,27]. Exploring a facile method to inexpensive substitutes of N-doped graphene nanosheets may be a favorable solution. In this research, we report cross-linked carbon networks constructed from Ndoped nanosheets. To date, the design that utilizes the linking of porous carbons from polyethylene glycol (PEG) and melamine towards N-doped continuous nanosheet/film networks is not presented. Besides, the comprehensive and extensive research on so-formed carbon networks is hardly implemented. In a typical process, MgO has been employed as self-sacrificial templates for the controllable synthesis of porous carbon architectures. This conventional MgOtemplated method is very popular because of the advantages of MgO template such as the structural stability, chemical and thermal stabilities, low cost and easy removal [9,28]. The carefully selected precursors (polyethylene glycol and melamine) can contribute to N-doped interconnected nanosheets which will be transformed into 3D network structures after annealing in inert atmosphere. Hierarchically porous N-doped carbon nanosheet networks (HPCNs) can thus be effectively produced by further removing the MgO template through mild acid etching. The structures and properties of HPCNs rely on the mass ratio of the precursors and templates, as well as the carbonization temperature, thus allowing efficient modulating their electrochemical performance. In particular, the optimized HPCNs (PEG-melamine-MgO ratio: 10:3:7; carbonization temperature: 700 oC) deliver a specific capacitance of 192.6 F g–1 at 1.0 A g–1, and long-term cyclability with a capacitance decay of 14.5% at a high current load of 10 A g–1 after
6
8000 cycles when evaluated by a three-electrode system with 6.0 M KOH as the electrolyte solution. 2. Experimental 2.1. Preparation of HPCNs Magnesium oxide (MgO), polyethylene glycol (PEG, MW = 6000), and melamine were uniformly mixed in various mass ratios, and tableted under 30 MPa pressure, as the precursors. These tablets were treated in a tube furnace held at 900 oC for 3 h under N2 atmosphere (heating rate: 5 oC min–1). Subsequently, the annealed products were soaked in 3 M HCl solution, and then washed with distilled water and ethanol to remove MgO. Finally, the powder was dried at 80 oC in vacuum for 24 h to get a series of porous carbons (HPCNs). The products from various PEG-melamine-MgO mass ratios of 10:10:0, 10:7:3, 10:5:5, 10:3:7 and 10:1:9 were termed as W0, W1, W2, W3 (also denoted as W3-900) and W4, respectively. The mixtures with a mass ratio of 10:3:7 were heated at different temperatures (600 oC, 700 oC, 800 oC and 900 oC) under N2 flow to yield W3-600, W3-700, W3-800 and W3-900 samples. 2.2. Structure characterization and performance evaluation Scanning electron microscopy (SEM) images of the products were taken with a Zeiss 55VP microscope. Low- and high-resolution transmission electron microscopy (TEM and HRTEM) images were taken with HITACHI-600 and JEM-2100F microscopes. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance using Cu kα radiation. Raman spectra were collected using a Bruker Senterra spectrometer. Elemental analysis (EA) was performed using a Vario EL III elemental analyzer. The Brunauer-Emmett-Teller (BET) area was obtained through N2 adsorption/desorption isotherms on a Biaode SSA-4200 system, and pore size distribution (PSD) was evaluated by the Barrett-Joyner-Halenda (BJH) method.
7
Electrochemical tests were operated on a CHI660D electrochemical working station (Chenhua Instruments Co. Ltd., Shanghai) and a CT2001A battery testing system (LAND electronics Co. Ltd, Wuhan), where a three-electrode system (including the working electrode, platinum counter electrode and Hg/HgO reference electrode) was used in 6.0 M KOH aqueous electrolyte. The working electrode was made by pressing a disk (1 cm diameter) containing active material, acetylene black and poly-(tetrafluoroethylene) (the mass ratio of 8:1:1) on nickel foam. The loading of active materials on nickel foam is ~2.9 mg cm–2. Scan rates for cyclic voltammetry (CV) were between 2 and 50 mV s–1 in a potential window -1.0 to 0 V. Current densities for constant charge-discharge (CD) were from 1 to 20 A g–1 in the same window. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range 105–10–2 Hz at open circuit potential with an AC perturbation of 5 mV. 3. Results and discussion 3.1. Fabrication of HPCNs N-doped porous carbons are generally synthesized by direct carbonization of nitrogenous precursors or post-modification of carbon materials with other nitrogenous small-molecules [29]. The former is more facile and simpler as compared to the latter. So we propose a strategy of the direct co-pyrolysis of both carbonaceous and nitrogenous precursors in the presence of the MgO template, aiming to obtain ideal architectures, namely, hierarchically porous N-doped carbon nanosheet networks with interconnected structures. The synthesis process is schematically presented in Fig. 1. We select low-cost PEG, melamine and MgO as initial materials because of their significant functions in achieving the target structures. PEG acts simultaneously as carbon sources and the linkers between melamine and MgO, and melamine serves as nitrogen sources, while MgO as the self-sacrificial template. In detail, PEG was firstly ground with MgO in order
8
to homogenously disperse PEG around or the surface of MgO. Melamine is then mixed with them, and compressed into several sheets, ensuring strong adhesion among these components. After the carbonization and subsequent MgO etching, N-doped cross-linked carbon networks are produced, showing good structural integrity. Notably, the morphology transformation from rigid thick sheet aggregates to flexible film strongly depends on the PEG-melamine-MgO mass ratio. The ratio also affects the aggregation degree of porous carbons. Under the optimized conditions (10:3:7 mass ratio and carbonization at 700 oC), the ultrathin nanosheet assembled porous carbon networks are formed. Because of meeting the above-mentioned structural demands for improving the electrode performance, such as relatively high BET area, high porosity and flexibility, heteroatom doping, and 3D stable interconnected structures, the designed HPCNs may be superior as the electrode of supercapacitors. 3.2. Influence of mass ratio of initial materials on morphology Fig. 2a-e show SEM images of various HPCNs (W0, W1, W2, W3 and W4) from different PEG-Melamine-MgO mass ratios, which reveal the effect of the ratio on the morphology evolution. The W0 sample with the 10:10:0 ratio (Fig. 2a) is porous aggregates composed of thick sheets due to serious stacking of porous carbons in the absence of MgO. As the ratio varies from 10:7:3 and 10:5:5 to 10:3:7, the shape of HPCNs converts from comparatively thick sheets (Fig. 2b and c) to flexible film-interconnected networks (Fig. 2d). Furthermore, these film networks (W3) show a plenty of randomly distributed open channels, which are nearly graphene nanosheet form. At the ratio up to 10:1:9 (Fig. 2e), continuous nanosheets are partially replaced by some broken nanoparticle-formed film agglomerates, as direct results of high MgO content in the mixture and strong van der Waals attraction between thin carbon layers. Film geometry with high porosity for W3 is confirmed by TEM and HRTEM images in Fig. 2f and g. Combined with
9
the SEM image in Fig. 2d, it is observed that W3 has not only rich pores in surfaces but also 3D flexible channels from the surface extending to interior, suggesting fast transport for ions. In short, the microscopic analysis may conclude that: (i) the mass ratio of initial materials holds an important role in tailoring the morphology and suppressing serious agglomeration or restacking of porous carbons; (ii) the MgO content in the ternary system is of importance in forming welldeveloped interconnected structures; (iii) the optimal proportion of PEG-melamine-MgO for producing continuous nanosheet networks is 10:3:7 based on the consideration of microstructures. 3.3. Influence of mass ratio of initial materials on porous property Fig. 3 presents N2 adsorption/desorption isotherms and PSD curves of HPCNs (W0, W1, W2, W3 and W4). The isotherms of all samples are IV-typed in the IUPAC classification (Fig. 3a), which verifies their mesoporous characteristic [30]. The details of pore properties are summarized in Table 1. With varying the PEG-melamine-MgO ratio from 10:10:0 to 10:3:7, the BET area and total pore volume of the HPCNs increase, i.e., 23.1 m2 g–1 and 0.11 cm3 g–1 for W0 from the 10:10:0 ratio, 155.7 m2 g–1 and 0.64 cm3 g–1 for W2 from the 10:5:5 ratio, and 286.6 m2 g–1 and 1.05 cm3 g–1 with a dominating mesoporous volume (0.931 cm3 g–1) for W3 from the 10:3:7 ratio. However, the area declines to 241.9 m2 g–1 for W4 from 10:1:9. Area variation well matches with the morphology evolution. These data account for high control of the ratio of initial materials on pore property, and the appropriate ratio can lead to further development of pore structures. PSD curves of all HPCNs in Fig. 3b and the inset show a similar peak around ca. 3.7 nm at the mesopore range. Based on the analysis, it is suggested that all HPCNs obtained are mesoporous materials, and W3 has good pore properties regarding area and pore volume. In
10
general, high mesoporosity for electrode materials may improve the electrolyte ion transport and decrease ion transport resistance during electrochemical process [31]. 3.4. Influence of carbonization temperature on morphology To elucidate the influence of carbonization temperature on morphology, SEM and TEM morphologies of HPCNs (W3-600, W3-700 and W3-800) prepared at 600 oC, 700 oC and 800 oC in PEG-melamine-MgO ratio of 10:3:7 are displayed in Fig. 4. SEM images (Fig. 4a-c and Fig. 2d) reveal the formation of 3D porous carbon networks built from continuous nanosheets. This morphology is nearly that of interconnected carbon nanosheets reported [32]. Moreover, the increase of carbonization temperature causes the decrease in thickness of sheets. This tendency is because the release of lots of gases induced by high-temperature carbonization promotes the separation of carbon layers and the formation of thinner nanosheets. TEM and HRTEM images (Fig. 4d-i) further identify their similar sheet structures with high porosity. Despite of ultrathin nanosheets that might possess high theoretical BET area, they may restack due to strong interaction, thus resulting in the decrease of the accessible area. 3.5. Influence of carbonization temperature on porous property Fig. 5 presents N2 adsorption/desorption isotherms and PSD curves of HPCNs (W3-600, W3700, W3-800 and W3-900). As shown in Fig. 5a, IV-typed isotherms with a hysteresis loop in the relative pressure (P/P0) between 0.4 and 1.0 indicate their mesoporous nature. Detailed porosity parameters are listed in Table 2. The HPCNs synthesized at relatively low temperature (600 oC or 700 oC) show larger BET specific surface areas (360.5 and 370.8 m2 g–1 for W3-600 and W3-700, respectively), which, however, is gradually reduced at higher temperature (800 oC or 900 oC, 355.8 and 286.6 m2 g–1 for W3-800 and W3-900, respectively). Besides, total pore volume also displays the same tendency, which decreases from 1.65 to 1.05 cm3 g–1 with the
11
temperature increasing from 700 oC to 900 oC. The decline may be contributed to the partial collapse of pores and restacking of carbon nanosheets through π–π attraction [33,34]. By comparison, W3-700 has the highest values regarding BET area, pore volume and mesoporosity (ca. 96%), inferring that the suitable temperature producing ideal HPCNs is 700 oC. PSD curves in Fig. 5b are close to each other, with a peak centered at ca. 3.7 nm. 3.6. Influence of carbonization temperature on structure XRD patterns and Raman spectra are conducted to investigate the influences on structure of HPCNs obtained from various carbonization temperatures. All XRD patterns in Fig. 6a show low-intensity peaks at ~24.8o and ~43.3o ascribing to graphitic (002) and (100) reflections, suggesting relatively low graphitic crystallinity. It is also found that the (002) diffraction peak becomes sharper with the increase of carbonization temperature, indicating the improvement of crystallinity. Graphitization degree of HPCNs is further confirmed by Raman spectra in Fig. 6b. All carbon samples display G and D bands, which are associated with ordered and disordered structures [35], respectively. Intensity ratios of G to D bands (IG/ID) for W3-600, W3-700, W3800, and W3-900 are 0.964, 0.982, 1.018 and 1.021, respectively. It signifies that the increase of the temperature from 600 oC to 800 oC can promote the graphitization level of porous carbons but only little enhancement occurs between 800 oC and 900 oC, which is close to the report on spherical nitrogen-doped porous carbon shells [36]. In addition, N-doped structures in HPCNs are verified by EA analysis, which shows that N contents of W3-600, W3-700, W3-800 and W3900 are 8.493, 9.920, 7.619 and 3.378 wt.%, respectively. Accordingly, W3-700 has the highest N-doped level as compared to other HPCNs. 3.7. Electrochemical performance The electrochemical performance of HPCN electrodes is carefully evaluated by means of
12
cyclic voltammetry (CV), galvanostatic charge-discharge (CD) and electrochemical impedance spectroscopy (EIS) in three-electrode system in 6.0 M KOH aqueous solution. Fig. 7a presents CV curves of the electrodes at the scan rate 10 mV s–1. A nearly rectangular shape with a slight deviation is found in all CV curves. It is proposed that complex charge storage mechanisms exist in all HPCNs, which include typical electric double layer capacitive behavior from porous carbons, and pseudocapacitance behavior from nitrogen species. Additionally, the order in CV area is W3-700 > W3-600 > W3-800 > W3-900, implying the highest capacitance in W3-700. CV curves with a wide scan rate range 2–50 mV s–1 for W3-700 are shown in Fig. 7b. The shape of CV curves can be well maintained at all tested scanning rates, confirming that W3-700 has fast ion transport behavior. Constant charge-discharge (CD) tests on HPCN electrodes are used to evaluate the specific capacitance and rate performance. CD curves of various HPCNs at a current density of 1 A g–1 are presented in Fig. 8a. Nearly symmetrical shape in all curves verifies their ideal capacitance behavior and high reversibility. Besides, a slight deviation to linearity is detected, which is caused mainly by redox reactions of N species. The specific capacitance (Cm) is calculated from the discharge curve based on the equation: Cm = IΔt/mΔV (I for the charge-discharge current density, Δt for discharge time, m for the mass of active materials, and ΔV for the potential during the discharge process). Cm values for W3-600, W3-700, W3-800 and W3-900 are calculated to be 171.8, 192.6, 141.3 and 70.9 F g–1 at 1 A g–1, respectively. Obviously, W3-700 achieves the maximum capacitance, which is attributed to short diffusion pathways available for ions due to ideal pore properties and rich active sites from N-doping structures [37]. The Cm values for W3700 at 1, 2, 4, 6, 8, 10, 15 and 20 A g–1 can be obtained from discharge curves in Fig. 8b, which are around 192.6, 186.3, 179.9, 175.7, 170.7, 167.3, 163.1 and 156.2 F g–1, respectively. Besides,
13
the capacitance values of HPCNs at various currents are listed in Table 3, which show that all samples have a slight decrease in specific capacitance as the current increases. It may be ascribed to the reduced utilization of active materials during rapid charge/discharge processes [17]. In this case, W3-700 still remains a capacitance up to 156.2 F g–1 even at 20 A g–1 as compared to other samples (130.2 F g–1 for W3-600, 112.8 F g–1 for W3-800, 47.5 F g–1 for W3-900), with 81.1% retention, conforming its superior rate performance. Galvanostatic cycling tests on W3-700 electrode are performed at a high load of 10 A g–1, as shown in Fig. 9. After cycled up to 8000 cycles, W3-700 still keeps 85.5% of initial capacitance. CD curves of W3-700 for the initial 10 cycles and the last 10 cycles are also interpolated into Fig. 9. No pronounced difference in shape is observed in both insets except a slight change in discharge time, suggesting good cycling stability. It is related to high porosity as well as interconnected nanosheet network structures of W3-700. Nyquist impedance plots are usually regarded as efficient evidence of capacitive behavior of electrode materials. The plots of W3-700 are displayed in Fig. 10 and its inset. Internal resistance Rs and charge-transfer resistance Rct are usually estimated by high frequency intercept on the real impedance axis and the diameter of semicircles in middle frequency, respectively. Rs and Rct values are 0.4 and 1.7 Ω. At low frequency, the deep-rising straight line nearly parallel to the imaginary axis. Both low resistances and ideal capacitive behavior is thus demonstrated, which may be contributed to synergistic effect of 3D interconnected networks, high BET area and high N-doped structures of W3-700. 4. Conclusions Porous carbon networks organized from ultrathin and N-doped carbon nanosheets are easily prepared through the MgO-templated method with both PEG and melamine as the precursors.
14
The control over the morphology, and pore properties can be realized by simply altering mass ratio of PEG-melamine-MgO and carbonization temperature. Good electrochemical results including high capacitance, good rate capability and long cycle life are achieved with the optimized HPCNs, probably thanks to 3D sheet-interconnected network structures with high BET area and high porosity, high N content, and continuous conductive paths. This first systematic study on the microstructure dependency and electrochemical properties of melamine and PEG co-pyrolysis N-doped carbon nanosheet networks offers the general and valuable guidelines for designed formation of high performing 3D heteroatom-doped cross-linked porous carbons for use in many fields. Acknowledgements The authors are grateful to the financial support from the National Natural Science Foundation of China (Nos. 21363023, 21563029, U1403293).
References [1] M.H. Sun, S.Z. Huang, L.H. Chen, Y. Li, X.Y. Yang, Z.Y. Yuan, B.L. Su, Applications of hierarchically structured porous materials from energy storage and conversion, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine, Chem. Soc. Rev. 45 (12) (2016) 3479–3563. [2] M.G. Hahm, A.L.M. Reddy, D.P. Cole, M. Rivera, J.A. Vento, J. Nam, H.Y. Jung, Y.L. Kim, N.T. Narayanan, D.P. Hashim, C. Galande, Y.J. Jung, M. Bundy, S. Karna, P.M. Ajayan, R. Vajtai, Carbon nanotube–nanocup hybrid structures for high power supercapacitor applications, Nano Lett. 12 (11) (2012) 5616–5621.
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[3] A. Yu, Z. Chen, R. Maric, Z. Zhang, J. Zhang, J. Yan, Electrochemical supercapacitors for energy storage and delivery: advanced materials, technologies and applications, Appl. Energy 153 (2015) 1–2. [4] S. Nardecchia, D. Carriazo, M. Luisa Ferrer, M. C. Gutiérrez, F. Del Monte, Three dimensional macroporous architectures and aerogels built of carbon nanotubes and/or graphene: synthesis and applications, Chem. Soc. Rev. 42 (2) (2013) 794–830. [5] M. Sevilla, A.B. Fuertes, Direct synthesis of highly porous interconnected carbon nanosheets and their application as high-performance supercapacitors, ACS Nano 8 (5) (2014) 5069–5078. [6] D. Hulicova-Jurcakova, M. Kodama, S. Shiraishi, H. Hatori, Z.H. Zhu, G.Q. Lu, Nitrogenenriched nonporous carbon electrodes with extraordinary supercapacitance, Adv. Funct. Mater. 19 (11) (2009) 1800–1809. [7] Y. Gao, L. Li, Y. Jin, Y. Wang, C. Yuan, Y. Wei, G. Chen, J. Ge, H. Lu, Porous carbon made from rice husk as electrode material for electrochemical double layer capacitor, Appl. Energy 153 (2015) 41-47. [8] G. Zu, J. Shen, L. Zou, F. Wang, X. Wang, Y. Zhang, X. Yao, Nanocellulose-derived highly porous carbon aerogels for supercapacitors, Carbon 99 (2016) 203–211. [9] T. Morishita, T. Tsumura, M. Toyoda, J. Przepiórski, A.W. Morawski, H. Konno, M. Inagaki, A review of the control of pore structure in MgO-templated nanoporous carbons, Carbon 48 (10) (2010) 2690–2707. [10] Y. Deng, Y. Xie, K. Zou, X. Ji, Review on recent advances in nitrogen-doped carbons: preparations and applications in supercapacitors, J. Mater. Chem. A 4 (2016) 1144–1173.
16
[11] P. Hao, Z. Zhao, J. Tian, H. Li, Y. Sang, G. Yu, H. Cai, H. Liu, C.P. Wong, A. Umar, Hierarchical porous carbon aerogel derived from bagasse for high performance supercapacitor electrode, Nanoscale 6 (20) (2014) 12120–12129. [12] V.B. Kumar, A. Borenstein, B. Markovsky, D. Aurbach, A. Gedanken, M. Talianker, Z. Porat, Activated carbon modified with carbon nanodots as novel electrode material for supercapacitors, J. Phys. Chem. C 120 (25) (2016) 13406−13413. [13] C. Zheng, X. Zhou, H. Cao, G. Wang, Z. Liu, Synthesis of porous graphene/activated carbon composite with high packing density and large specific surface area for supercapacitor electrode material, J. Power Sources 258 (2014) 290–296. [14] F. Markoulidis, C. Lei, C. Lekakou, D. Duff, S. Khalil, B. Martorana, I. Cannavaro, A method to increase the energy density of supercapacitor cells by the addition of multiwall carbon nanotubes into activated carbon electrodes, Carbon 68 (2013) 58–66. [15] X. Cui, R. Lv, R.U.R. Sagar, C. Liu, Z. Zhang, Reduced graphene oxide/carbon nanotube hybrid film as high performance negative electrode for supercapacitor, Electrochim. Acta 169 (2015) 342–350. [16] A. Bello, F. Barzegar, D. Momodu, J. Dangbegnon, F. Taghizadeh, N. Manyala, Symmetric supercapacitors based on porous 3D interconnected carbon framework, Electrochim. Acta 151 (2015) 386–392. [17] E.C. Vermisoglou, T. Giannakopoulou, G.E. Romanos, N. Boukos, M. Giannouri, C. Lei, C. Lekakou, C. Trapalis, Non-activated high surface area expanded graphite oxide for supercapacitors, Appl. Surf. Sci. 358 (2015) 110–121.
17
[18] Y. Zhou, S. L. Candelaria, Q. Liu, E. Uchaker, G. Cao, Porous carbon with high capacitance and graphitization through controlled addition and removal of sulfur-containing compounds, Nano Energy 12 (2015) 567–577. [19] D.M. Anjos, J.K. McDonough, E. Perre, G.M. Brown, S.H. Overbury, Y. Gogotsi, V. Presser, Pseudocapacitance and performance stability of quinone-coated carbon onions, Nano Energy 2 (5) (2013) 702–712. [20] L. Qie, W. Chen, H. Xu, X. Xiong, Y. Jiang, F. Zou, X. Hu, Y. Xin, Z. Zhang, Y. Huang, Synthesis of functionalized 3D hierarchical porous carbon for high-performance supercapacitors, Energy Environ. Sci. 6 (8) (2013) 2497–2504. [21] S. Isikli, M. Lecea, M. Ribagorda, M.C. Carreño, R. Díaz, Influence of quinone grafting via Friedel–Crafts reaction on carbon porous structure and supercapacitor performance, Carbon 66 (2014) 654–661. [22] W. Wang, S. Guo, M. Penchev, I. Ruiz, K.N. Bozhilov, D. Yan, M. Ozkan, C.S. Ozkan, Three dimensional few layer graphene and carbon nanotube foam architectures for high fidelity supercapacitors, Nano Energy 2 (2) (2013) 294–303. [23] M. Wahid, G. Parte, D. Phase, S. Ogale, Yogurt: a novel precursor for heavily nitrogen doped supercapacitor carbon, J. Mater. Chem. A 3 (3) (2015) 1208–1215. [24] Q. Wang, J. Yan, Y. Wang, G. Ning, Z. Fan, T. Wei, J. Cheng, M. Zhang, X. Jing, Template synthesis of hollow carbon spheres anchored on carbon nanotubes for high rate performance supercapacitors, Carbon 52 (2013) 209–218. [25] Q. Xie, S. Zhou, A. Zheng, C. Xie, C. Yin, S. Wu, Sandwich-like nitrogen-enriched porous carbon/graphene composites as electrodes for aqueous symmetric supercapacitors with high energy density, Electrochim. Acta 189 (2016) 22–31.
18
[26] Z. Ling, Z. Wang, M. Zhang, C. Yu, G. Wang, Y. Dong, S. Liu, Y. Wang, J. Qiu, Sustainable synthesis and assembly of biomass-derived B/N co-doped carbon nanosheets with ultrahigh aspect ratio for high-performance supercapacitors, Adv. Funct. Mater. 26 (1) (2016) 111–119. [27] V. León, M. Quintan, M.A. Herrero, J.L.G. Fierro, A.D.L. Hoz, M. Prato, E. Vázquez, Fewlayer graphenes from ball-milling of graphite with melamine, Chem. Commun. 47 (39) (2011) 10936–10938. [28] W. Geng, F. Ma, G. Wu, S. Song, J. Wan, D. Ma, MgO-templated hierarchical porous carbon sheets derived from coal tar pitch for supercapacitors, Electrochim. Acta 191 (2016) 854–863. [29] N.P. Wickramaratne, J. Xu, M. Wang, L. Zhu, L. Dai, M. Jaroniec, Jaroniec M, Nitrogen enriched porous carbon spheres: attractive materials for supercapacitor electrodes and CO2 adsorption, Chem. Mater. 26 (9) (2014) 2820–2828. [30] L. You, Y. Zhang, S. Xu, J. Guo, C. Wang, Movable magnetic porous cores enclosed within carbon
microcapsules:
structure-controlled
synthesis
and
promoted
carbon-based
applications, ACS Appl. Mater. Interfaces 6 (17) (2014) 15179–15187. [31] Y. Qiang, J. Jiang, Y. Xiong, H. Chen, J. Chen, S. Guan, J. Chen, Facile synthesis of N/P co-doped carbons with tailored hierarchically porous structures for supercapacitor applications, RSC Adv. 6 (2016) 9772–9778. [32] H. Peng, G. Ma, K. Sun, Z. Zhang, Q. Yang, Z. Lei, Nitrogen-doped interconnected carbon nanosheets from pomelo mesocarps for high performance supercapacitors, Electrochim. Acta 190 (2016) 862–871.
19
[33] X. Liu, L. Zhou, Y. Zhao, L. Bian, X. Feng, Q. Pu, Hollow, spherical nitrogen-rich porous carbon shells obtained from a porous organic framework for the supercapacitor, ACS Appl. Mater. Interfaces 5 (20) (2013) 10280–10287. [34] D. Liu, Z. Jia, D. Wang, Preparation of hierarchically porous carbon nanosheet composites with graphene conductive scaffolds for supercapacitors: an electrostatic-assistant fabrication strategy, Carbon 100 (2016) 664–677. [35] M. Wahid, D. Puthusseri, D. Phase, S. Ogale, Enhanced capacitance retention in a supercapacitor made of carbon from sugarcane bagasse by hydrothermal pretreatment, Energy Fuels 28 (6) (2014) 4233–4240. [36] B. Friedel, S. Greulich-Weber, Preparation of monodisperse, submicrometer carbon spheres by pyrolysis of melamine–formaldehyde resin, Small 2 (7) (2006) 859–863. [37] Q. Zhao, X. Wang, H. Xia, J. Liu, H. Wang, J. Gao, Y. Zhang, J. Liu, H. Zhou, X. Li, S. Zhang, X. Wang, Design, preparation and performance of novel three-dimensional hierarchically porous carbon for supercapacitors, Electrochim. Acta 173 (2015) 566–574.
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Figure Captions
Fig. 1. Brief description for the fabrication of HPCNs.
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Fig. 2. SEM images of HPCNs prepared with different mass ratios of PEG- melamine-MgO: W0 (10:10:0), (b) W1 (10:7:3), (c) W2 (10:5:5), (d) W3 (10:3:7), (e) W4 (10:1:9). (f) TEM and (g) HRTEM images of W3.
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Fig. 3. (a) N2 adsorption/adsorption isotherms at 77 K and (b) PSD curves of W0, W1, W2, W3 and W4.
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Fig. 4. SEM, TEM and HRTEM images of HPCNs prepared under different carbonization temperature in PEG-melamine-MgO ratio of 10:3:7: (a,d,g) 600 oC (W3-600), (b,e,h) 700 oC (W3-700), (c,f,i) 800 oC (W3-800).
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Fig. 5. (a) N2 adsorption-desorption isotherms and (b) PSD curves of W3-600, W3-700, W3-800 and W3-900.
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Fig. 6. (a) XRD patterns and (b) Raman spectra of W3-600, W3-700, W3-800 and W3-900.
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Fig. 7. (a) CV curves of various HPCN electrodes at a scan rate of 10 mV s–1 in 6.0 M KOH solution. (b) CV curves of W-700 electrode at scan rates ranging from 2 to 50 mV s–1.
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Fig. 8. (a) Constant CD curves of various HPCN electrodes at a current density of 1 A g–1. (b) Constant CV curves of W-700 electrode at current densities ranging from 1 to 20 A g–1.
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Fig. 9. Cycling performance of W3-700 electrode over 8000 charge-discharge cycles at a current density of 10 A g–1. The inset is CD curves for the first 10 cycles and the last 10 cycles.
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Fig. 10. Nyquist impedance plot of W3-700 electrode. The inset is the enlarged plots in high frequency region.
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Table 1 Textural properties of various HPCNs Sample
Vmicro (cm3 g–1)
SBET
Vtotal
(m2 g–1)
(cm3 g–1)
W0 (10:10:0)
23.1
0.11
0.009
0.101
W1 (10:7:3)
78.0
0.32
0.033
0.287
W2 (10:5:5)
155.7
0.64
0.064
0.576
W3 (10:3:7)
286.6
1.05
0.119
0.931
W4 (10:1:9)
241.9
1.06
0.100
0.96
(the
mass ratio)
Vmeso (cm3 g–1)
Table 2 Textural properties of various HPCNs Sample
SBET
Vtotal
Vmicro
Vmeso
(m2 g–1)
(cm3 g–1)
(cm3 g–1)
(cm3 g–1)
W3-600
360.5
1.02
0.158
0.962
W3-700
370.8
1.65
0.165
1.585
W3-800
355.8
1.32
0.162
1.158
W3-900
286.6
1.05
0.119
0.931
Table 3 Specific capacitances of various HPCNs at different current densities
Sample
Specific capacitance (F g–1) –1
1Ag
–1
4Ag
–1
8Ag
–1
–1
–1
10 A g
15 A g
20 A g
Capacitance retention (%)
W3-600
171.8
156.9
150.1
144.1
136.4
130.2
75.8%
W3-700
192.6
179.9
170.7
167.3
163.1
156.2
81.1%
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W3-800
141.3
130.4
123.8
121.4
116.1
112.8
79.8%
W3-900
70.9
59.8
54.6
53.0
50.3
47.5
67.0%
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