Hierarchically Interconnected N-Doped Carbon Aerogels Derived from

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C: Energy Conversion and Storage; Energy and Charge Transport

Hierarchically Interconnected N-Doped Carbon Aerogels Derived from Cellulose Nanofibrils as High Performance and Stable Electrodes for Supercapacitor Zhen Zhang, Lei Li, Yan Qing, Xihong Lu, Yiqiang Wu, Ning Yan, and Wen Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06550 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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The Journal of Physical Chemistry

Hierarchically Interconnected N-doped Carbon Aerogels Derived from Cellulose Nanofibrils as High Performance and Stable Electrodes for Supercapacitor Zhen Zhang1, Lei Li1, Yan Qing1*, Xihong Lu2, Yiqiang Wu1*, Ning Yan1, 3, Wen Yang1 1

School of Materials Science and Engineering, Hunan Provincial Collaborative Innovation

Center for High-efficiency Utilization of Wood and Bamboo Resources, Central South University of Forestry and Technology. 2

MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of

Environment and Energy Chemistry, School of Chemistry, Sun Yat-Sen University. 3

Faculty of Forestry, University of Toronto, Toronto, Canada.

*Corresponding Authors Yan Qing Affiliation: School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha, Hunan 410004, China Email address: [email protected] Phone number: +86 731 85623301 Fax: +86 731 85623301 Yiqiang Wu Affiliation: School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha, Hunan 410004, China Email address: [email protected] Phone number: +86 731 85623989 Fax: +86 731 85623989

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Abstract: Nitrogen (N) is an important heteroatom that improves the capacitance of carbon materials used in supercapacitors. Here, N-doped carbon aerogels are prepared using cellulose nanofibril aerogel as carbon source and template. Two types of N-doped carbon aerogels are prepared by N-doping at two stages of the carbon aerogel preparation process: N-doping before and after carbonization. Irrespective of the type of process, the N-doped carbon materials maintain their interconnected hierarchical porous architectures and large specific surface areas. However, the amounts of N incorporated and the specific capacitance are highly dependent on the N-doping process. Compared with the carbon aerogel precursor, the N-doped carbon aerogels exhibit a larger volume, improved wettability, and excellent conductivity. In particular, the as-obtained N-doped carbon aerogel exhibit a high specific capacitance (152% higher than carbon aerogel) and good cycling stability (~94.5% capacitance retention after 10,000 cycles) as consequences of its mesoporosity and the positive effect of the incorporated N. Given their excellent electrochemical performance coupled with their simple and environmentally friendly synthesis method, N-doped carbon aerogels have strong application potential in supercapacitors.

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1. Introduction Supercapacitors (SCs) has drawn considerable interests as a storage device to address the worldwide energy crisis because of their high power density, high energy density, and long-term stability1,2. Carbon materials, for example, porous activated carbon3, carbon black4, graphene, and carbon aerogels (CAs) have been widely studied as electrode materials for SCs. Among them, CAs are considered as an ideal SC electrode carbon materials due to their high porosity, large specific surface area (SSA), good conductivity, and wide working voltage range5-7. CAs are usually derived from organic or inorganic aerogels such as resorcinol–formaldehyde (RF)8, cresol–formaldehyde9, carbon nanotube10, and graphene aerogels11. However, the harmful, high-price precursors8 and complex synthesis method9 of CAs seriously hinder their application in SCs12. Therefore, a simple, low-cost, and environmentally friendly CA preparation method is needed. Cellulose nanofibrils (CNFs), which are the most important component of wood fibers, have garnered interests as a CA precursor because of their renewability and biodegradability and because they can easily self-assemble into a three-dimensional (3D) network structure. Such a structure can provide a unique and efficient path for the migration of ions and large numbers of active sites for electrons, enabling electrolyte ions to transmit to the surface of the carbon continuously and stably13. Although the preparation of low-cost CAs through CNFs is straightforward, the low specific capacitance of the resultant CAs impedes their large-scale application. Bacterial cellulose CAs show a specific capacitance of only 73.2 F g−1 at a current density of 0.5 A g−1.14 Zu15 reported lignocellulosic CAs with a specific capacitance of

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140 F g−1; however, the specific capacitance decreased to 93% of the original capacitance after only 4,000 charge/discharge cycles. Hence, a better approach is needed to improve the electrochemical performance of cellulose nanofibrils CAs. Introducing heteroatoms into carbon networks is a usual strategy for improving CA performance16-19. In particular, nitrogen (N) can be easily doped into carbon networks than other elements because the atomic radius and the number of valence electrons of a nitrogen atom are similar to those of a carbon atom20. N-doping has been reported to change SSA21, surface wettability22, and electronic conductivity23 of carbon-based materials and to result in superb cyclability of carbon-based electrodes in SCs24. The N content in most N-doped carbonaceous materials such as N-doped RF CAs (3.2%)25 and N-doped graphene aerogels (2%)26, is too low to provide sufficient SC charge storage capacity. This shortcoming can be solved through the use of polymers with high N contents, including polypyrrole and polyaniline27-29. However, these precursors are toxic and the doping process is complex. Thus, relatively easily produced, low-cost N-doped carbonaceous materials with a high N content and good electrochemical performance are strongly desired. In this work, a facile and environmentally friendly route to prepare N-doped CAs with high SSA and 3D network structure is presented. CAs are obtained by pyrolysis of aerogels at high temperature that are prepared from cellulose nanofibril through freeze drying. Then the aerogels and CAs were placed in a relatively sealed container with urea respectively, two types of N-doped CAs were obtained after a simple gas-phase cycle reaction at high temperature, and the highest content of the N dopant can reach 6.89 at%. The N dopant imparts N-doped CAs with excellent wettability (the contact angle with an electrolyte is 65.9°)

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and good electrical conductivity (the resistance is 0.58 Ω), which in turn results in a large specific capacitance (253.7 F g−1). In particular, the N-doped CA exhibits good long-term cycling stability (94.5% capacitance retention after 10,000 cycles). The obtained N-doped CA is incorporated as an electrode material into a symmetrical supercapacitor (SSC) device that exhibited good energy density and power density. Accordingly, the developed N-doped CA has strong application potential as an electrode material in SCs. 2. Experimental 2.1 Materials Eucalyptus cellulose powder was purchased from Shizuoka University; potassium hydroxide (KOH), sulfuric acid (H2SO4), polyvinylidene fluoride (PVDF), and acetylene black were purchased from Sinopharm Chemical Reagent Corp. (China). 2.2 Preparation of CNF hydrogel Cellulose powder and H2SO4 solution (48 wt%) were placed in a flask at a mass ratio of 1:20 and stirred for 2 h at 45°C to obtain a CNF suspension. The CNF suspension was then washed with deionized water and centrifuged (3H24RI, Hunanherexi Instrument & Equipment Co., Ltd.) until the pH of the supernatant reached 7. To obtain the CNF hydrogel, the suspension was treated in a high-pressure homogenizer machine (M-110EH-30, American Microfluidics International Corp.) ten times with a 200-µm chamber and ten times with an 87-µm chamber. 2.3 Preparation of CNF aerogels, CAs, and N-doped CAs Freeze drying the CNF hydrogel in a freeze dryer (Biosafer-18A, NingBo Scientz Biotechnology Co., Ltd.) to obtain the CNF aerogels. For preparation of the CA precursor, the aerogels were placed in a tubular furnace (YQL1100-80, Shang Hai Yongqing Experimental

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Apparatus Co., Ltd.), which was subsequently flushed with pure nitrogen at a flow rate of 110 mL min−1 to achieve an O2-free environment. Then increased the temperature to 800°C (heating rate is 5°C min−1) and maintained for 2 h. Finally, the tube furnace was naturally cooled to 25°C and the CA was removed. The resulting aerogel and CA were subjected to N-doping via a gas-phase cycle reaction. In the N-doping process, urea was placed under samples in a tubular furnace that was subsequently heated from 25°C to 800°C (5°C min−1) under the protection of flowing nitrogen (80 mL min−1) and maintained for 2 h. The system was cooled to 25°C to obtain N-doped CAs. The N-doped CAs obtained by aerogel and CA were denoted as N-CA, and CA-N, respectively. 2.4 Characterizations The structure and morphology of the CAs were characterized with transmission electron microscopy (TEM, JEM-2100F), energy-filtered TEM mapping (EFTEM), and scanning electron microscopy (SEM, Quanta 450). The skeleton density of the sample was calculated by ρ = M/V, where ρ, V, and M are the density, volume, and mass of the sample, respectively. The compositions of the CNF aerogel and CAs were analyzed by X-ray diffraction (XRD, D8 FOCUS), Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet IN10), and X-ray photoelectron spectroscopy (XPS, Escalab 250Xi). The wettability of CAs were tested at room temperature with a contact angle meter (SL150). The SSA and pore size distribution of the CNF aerogel and CAs were analyzed through the Brunauer–Emmett–Teller (BET) method (BELSORP-max2). 2.5 Electrochemical performances

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The electrochemical performances of the samples were measured in 6 M KOH electrolyte with an electrochemical workstation (CHI660E A15004, Shanghai CH Instruments Co., Ltd.) using a three-electrode system. The acetylene black, PVDF, and CAs samples were made into a slurry (mass ratio of 1:1:8), which was then coated onto foamed nickel (1 × 1 cm2) as the working electrode. The reference electrode and the counter electrode were a Ag/AgCl2 electrode and a carbon rod, respectively. Galvanostatic charge/discharge (GCD) was carried out at a current density of 1 to 20 A g−1. Cyclic voltammetry (CV) was performed at different scanning rates in the working voltage window from −1 to 0 V. From the GCD curves, the specific capacitance of the CAs was calculated by C = (I△t)/(m△V), where △t is the discharge time, I is the current, △V is the working voltage, and m is the loading amount of the CAs. A simple SC was assembled by two CA-N electrodes and a separator (a cellulose paper). The performance of SC was tested with two electrode system in an electrochemical workstation. 3. Results and Discussion 3.1 The morphology and structure of aerogels and CAs

Figure 1. Schematically illustrated of the synthesis of N-doped CAs.

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N-doped CAs were prepared by a environmentally friendly and simple method (Fig. 1). SEM images of CNF aerogel and CAs showed a interconnected 3D network structure (Fig. 2). It can be observed from the image that the CNF aerogels were composed of interconnected nanofibers with diameters ranging from 10 to 200 nm. However, after carbonization and N-doping, the diameter of the fibers decreased. The high-resolution TEM image showed that the CA-N consists of randomly orientated graphene layers (Fig. 2h).

Figure 2. (a-d) SEM micrographs of CNF aerogel, CA, N-CA, and CA-N. (e-f) SEM micrographs of CA and CA-N with large area. (g-h) TEM micrograph and high-resolution TEM micrographs of CA-N. The textural properties of the CNF aerogels and CAs were both investigated via N2 adsorption/desorption measurements. As shown in Fig. 3a, the CNF aerogel, CA, N-CA, and CA-N samples exhibited Langmuir type IV curves. Compared with the SSA of aerogel (85.12 m2 g−1), those of the CA, N-CA and CA-N were substantially higher (298.40, 538.81, and 831.03 m2 g−1, respectively). The trend clearly shows that N atoms can significantly increases the SSA of CAs. The pore size distribution, the mesopore and micropore volumes of the CNF aerogel and CAs were measured by Barrett–Joyner–Halenda (BJH) method. Compared with the CA and N-CA samples, the CA-N sample exhibited a wider size distribution in Fig. 3b, 8


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including micropores (< 2 nm), mesopores (2 to 50 nm) and macroporous (> 50 nm) with different ratios. In addition, the pore volume of aerogel, CA, N-CA, and CA-N samples are approximately 0.095, 0.185, 0.123 and 0.508 cm3 g−1, respectively, revealing N-doping increased the pore volume of the samples. These observations indicate that the CA-N sample possesses a large pore volume and high SSA. Fig. 3c shows the skeletal (true) densities of different samples. The skeletal densities of aerogel, CA, N-CA and CA-N samples are 2.589, 3.427, 10.848 and 24.671 mg cm−3, respectively. These results demonstrated that the nanofibers were denser, the diameter of the pores was smaller, and the number of mesopores was greater in the N-doped samples, in agreement with the volumes and the pore size distribution of the CAs in Fig. 3b.

Figure 3. (a-b) N2 adsorption/desorption isotherms and pore size distribution of CNF aerogel, CA, N-CA, and CA-N. (c) Density of CNF aerogel, CA, N-CA, and CA-N. 3.2 XRD and FTIR analyses of the aerogels and carbon aerogels XRD measurements were performed to analyze the crystalline nature of the samples (Fig. 4a). The XRD pattern of the original CNF aerogel showed two peaks centered at 22.6°and 16.7°, which are corresponding to the (200) and (110) crystallographic planes30,31. These results confirm that a highly crystalline CNF suspension was obtained32,33. These peaks disappeared after carbonization, which indicats that the crystallinity of the CNF aerogels was damaged, 9



and the strength of the peaks at 23.4° and 43.5° corresponding to the (002) and (100) graphite planes were enhanced. The peak at 43.5° in the XRD pattern of the CA-N clearly indicated that N-doping positively affected the graphitization of carbon. CNF aerogels and CAs was also researched by FTIR (Fig. 4b). The bands at 2897 and 3354 cm−1 are belong to the C–H and O–H vibrations of the CNF aerogel, respectively. The C–O–C vibration bands appear at 1025, 1059, and 1197 cm−1. The peak at 1370 cm−1 is attributed to a C–H vibration. After carbonization and N-doping, the strength of peaks such as C–O, C–H, and O–H became smaller. The C=C vibration appeared near 1600 cm−1, indicating that the deoxidation and hydroxylation reaction dominated the process of carbonation34.

Figure 4. (a) XRD pattern of CNF aerogel, CA, N-CA, and CA-N. (b) FTIR spectroscopy of CNF aerogel, CA, N-CA, and CA-N. (c) XPS spectrum of CA, N-CA, and CA-N. (d) High-resolution N 1s spectrum of CA-N. (e) EFTEM elemental mapping of CA-N. 10


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The content and chemical state of N atoms after N-doping were characterized through XPS measurements (Fig. 4c). After N-doping, approximately 3.97 at% and 6.89 at% of N was detected for N-CA and CA-N samples, respectively. The difference in the amount of N between N-CA and CA-N was explained as follows. Compared with aerogels, CAs exhibited a weakly bonded structure that was easier in incorporating N via an N-doping reaction. The N1s peak in the spectrum of CA-N was deconvoluted into three different peaks at binding energies of 401.5, 400.1, and 397.1 eV, which corresponding to quaternary N, pyrrolic N, and pyridinic N, respectively (Fig. 4d).35 EFTEM elemental mapping of CA-N demonstrated that the N atom were well-distributed in the nanofibers (Fig. 4e). 3.3 Electrochemical performances of the CAs

Figure 5. (a) CV curves of N-CA exposed with different urea mass amounts. (b) CV curves of CA-N exposed with different urea mass amounts. (c) The effect of different urea mass amounts on specific capacitance and N content of samples (the number in the figure is the N content of the sample). The electrochemical performances of the CAs were investigated by a electrochemical workstation. In this paper, the aerogels and CAs were doped with 1, 3, 5, or 7 g urea; the resultant aerogel products are denoted as N-CA-1, N-CA-3, N-CA-5, and N-CA-7, and the resultant CAs are denoted as CA-N-1, CA-N-3, CA-N-5, and CA-N-7, respectively. The effect of urea mass and the N content on the specific capacitance of the samples was studied 11



(Fig. 5). The highest N contents of the N-treated aerogels and CAs are approximately 4 at% and 6.9 at%, respectively. The specific capacitance increased with increasing N content, indicating that the N content is an important factor affecting the capacitance of the samples. Moreover, the specific capacitances of N-CA (147.1 F g−1) and CA-N (253.7 F g−1) were the highest when the urea mass was 3 g. Therefore, the mass of urea used in subsequent experiments was 3 g. Fig. 6a and 6b illustrate the CV curves and GCD curves of the different CA electrodes, respectively. The CV curves of the CA sample presented a square-like shape, demonstrating that they exhibit good charge transfer ability36. The CV area of CA and N-CA was much smaller than those of CA-N, indicating that CA-N exhibited a larger specific capacitance than the other two samples. The GCD curves of the CA sample exhibited a triangular-like shape, which show that the electrochemical reaction of the materials had excellent reversibility37. The specific capacitance of the CA, N-CA, and CA-N samples measured from the GCD curves are 100.8, 147.1, and 253.7 F g−1, respectively. The high capacitance of the CA-N was attributed to its outstanding wettability, interconnected nanostructure, and excellent conductivity. The wetting surface area can increase the penetrability of ions toward the inner surface, thereby reducing the diffusion resistance of electrolyte ions in the pores and improving the specific capacitance of the electrode. The surface contact angle measurements (Fig. 6c) confirm the aforementioned results. Noticeably, the contact angle of the the CA (139.2°) and N-CA (99.4°) samples is larger than that of CA-N sample (65.9°), clearly demonstrating that N-doping enhanced the surface wettability. This enhanced wettability can increase the surface charge exchange capacity of carbon materials and their surface polarity.

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A lower resistivity also helps to increase the specific capacitance of carbon materials. The electrochemical impedance spectroscopy results for the CA, N-CA, and CA-N electrodes are shown in Fig. 6d. The series resistance (Rs) was calculated from the Nyquist plot (inset of Fig. 6d).38-40 The Rs of the CA, N-CA, and CA-N samples were calculated to be 1.77, 0.86, and 0.58 Ω, respectively, indicating N-doped CAs have good ion migration performance.

Figure 6. (a) CV tests of the CA, N-CA, and CA-N samples collected at 100 mV s−1. (b) GCD curves collected for CA, N-CA, and CA-N samples at 1 A g−1. (c) Contact angle of an electrolyte droplet on the surface of CA, N-CA, and CA-N. (d) Nyquist plots of the CA, N-CA, and CA-N. The aforementioned experiments exhibited that the electrochemical performance of CA-N was even better than that of N-CA. This improved performance was attributed to the multilevel pores, good wettability, high SSA. At the same time, the interconnected hierarchical porous architectures can provide CA-N with large SSA for N doping to increase N amount and thus improve capacitance. The CV tests of the CA-N sample at different scan 13



rates were given in Fig. 7a. The CA-N sample exhibited capacitances of 113.32 and 357.23 F g−1 at a scan rate of 400 and 5 mV s−1, respectively. The phenomenon of the capacitance is inversely proportional to the scanning rate is due to the resistances of charge transfer hindering the capacitive of the sample and ion transport in the interconnected structure falling behind the potential variation at higher scan rates41. The GCD tests of the CA-N sample at different current densities was also tested (Fig. 7b). The specific capacitances of the CA, N-CA, and CA-N samples at different current densities were presented in Fig. 7c.

Figure 7. (a) CV tests of CA-N sample collected at different scan rates. (b) GCD tests collected for CA-N sample at a current density of 1 to 20 A g−1. (c) The capacitance of CA, N-CA, and CA-N samples at a current density of 1 to 20 A g−1. (d) Cycling performance of CA and CA-N samples for 10,000 cycles and the last 10 cycles GCD curves of CA-N sample. (e) Comparison of the specific capacitances and cyclic stability (the number in the figure is the number of life test cycles). (f) The SEM diagram of CA and CA-N samples before and after the cycle durability test. Cycle life is also an important concern for SC electrode materials. Fig. 7d presents the cycle life performance of the CA and CA-N samples. Interestingly, CA-N exhibited a very stable 14


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capacitance (94.5% of the initial capacitance) after 10,000 cycles of GCD, indicating durable electrochemical stability. The specific capacitance and cycle life of the CA-N electrodes were compared with those of other reported N-doped carbon materials in Fig. 7e. Encouragingly, CA-N exhibited higher specific capacitances and greater cyclic stabilities. The overall performance of the CA-N was superior to that of some N-doped carbon and cellulose carbon materials, such as N-doped PF CAs42, N-doped porous carbon27, N-doped carbon nanotubes43, and N-doped bacterial-cellulose carbon networks44. The durable electrochemical stability of the sample was because of the interconnected hierarchically nanostructure of the CAs. After the CAs were prepared to electrode materials, these porous nanostructures can provide a stable transport for electrolyte ions transportation. The existence of micropores can enhance charge storage capacity, the mesopores and the macroporous can accelerate the diffusion of ions in electrodes to enhance conductivity. The introduction of N into the structure of carbon materials increased the number of defects in the carbon materials, thereby enhancing the activity and stability of their electrochemical reactions. After the cycle durability test in electrolyte solution, obvious pore structures were observed on the surface of the CA-N (Fig. 7f). These structures could function as ion-buffering reservoirs, providing a short diffusion distance and stable transport of electrolyte ions to the interior of the bulk material. The N atoms on the carbon skeleton could also provide lone-pair electrons, promote the transport of electrons in the carbon substrate, and attract electrolyte ions to improve the electrical double layer concentration, thereby increasing the material’s capacity and cyclic stability45. Finally, a SSC device was assembled with CA-N samples (denoted as CAN-SSC) and subsequently tested under conditions mimicking real usage. The CV tests of the CAN-SSC at

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different scan rates showed square-like shapes in Fig. 8a, indicating excellent capacitive behavior of the CAN-SSC. The GCD tests at different current densities indicating excellent electrochemical performance of the CAN-SSC (Fig. 8b). The calculated Rct value was 3.16 Ω, implying that the device exhibited excellent conductivity (Fig. 8c). The power and energy densities of the CAN-SSC were presented in Fig. 8d. The CAN-SSC possessed an energy density of 9.525 Wh kg−1 at a power density of 250 W kg−1; these power and energy densities were larger than those of other carbonaceous SCs reported by previous studies46-51.

Figure 8. (a) The CV tests of CAN-SSC at different scan rates (b) The GCD tests of CAN-SSC at different current densities (c) Nyquist plots and equivalent circuit of CAN-SSC (d) Power and energy densities of CAN-SSC. 4. Conclusions In summary, N-doped CAs with enhanced electrochemical performance were prepared from cellulose nanofibrils by the pyrolysis of CNF aerogel and by exposing CAs with urea in an oxygen-free atmosphere. Benefiting from the hierarchical porous, improved surface area as

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well as enhanced wettability, the as-obtained CA-N sample exhibited excellent capacitance performance when used as an SC electrode. The specific capacitances of the CA-N electrode reached 118.0 F g−1 and 253.7 F g−1 at current densities of 20 A g−1 and 1 A g−1, respectively. In addition, the specific capacitance retains 94.5% of its original value after 10,000 GCD cycles, implying good cycling stability. Moreover, the SSC possessed an energy density of 9.525 Wh kg−1 at a power density of 250 W kg−1. The low-cost, environmentally friendly porous CA-N presented in this work is a candidate electrode material for SCs. Acknowledgement This work was financially supported by National Natural Science Foundation of China (31530009, 31500476), National Key Research and Development Program of China (2017YFD0600804), Young Elite Scientists Sponsorship Program by CAST (2016QNRC001), Outstanding Innovative Youth Training Program of Changsha (KQ1707019), and Hunan Provincial Technical Innovation Platform and Talent Program in Science and Technology (2016RS2010, 2016TP1013). Reference [1] Chien, H. C.; Cheng, W. Y.; Wang, Y. H.; Lu, S. Y. Ultrahigh Specific Capacitances for Supercapacitors Achieved by Nickel Cobaltite/Carbon Aerogel Composites. Adv. Funct. Mater. 2012, 22, 5038-5043. [2] Zhu, Y.; Murali, S.; Stoller, M. D.; Ganesh, K. J.; Cai, W.; Ferreira, P. J.; Pirkle, A.; Wallace, R. M.; Cychosz, K. A.; Thommes, M.; et al. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science. 2011, 332, 1537. [3] Zhai, Y. P; Dou, Y. Q.; Zhao, D. Y.; Fulvio, P. F.; Mayes, R. T.; Dai, S. Carbon Materials for Chemical Capacitive Energy Storage. Adv. Mater. 2011, 23, 4828-4850.

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Hierarchically Interconnected N-Doped Carbon Aerogels Derived from

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