Facile and Scalable Fabrication of Nitrogen-Doped Porous Carbon

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Facile and Scalable Fabrication of Nitrogen-Doped Porous Carbon Nanosheets for Capacitive Energy Storage with Ultrahigh Energy Density Yingbo Xiao, Jun Huang, Yazhou Xu, Kai Yuan, and Yiwang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04393 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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ACS Applied Materials & Interfaces

Facile and Scalable Fabrication of Nitrogen-Doped Porous Carbon Nanosheets for Capacitive Energy Storage with Ultrahigh Energy Density

Yingbo Xiao, Jun Huang, Yazhou Xu, Kai Yuan,* and Yiwang Chen* College of Chemistry/Institute of Polymers and Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China.

ABSTRACT: Porous carbon materials are the most commonly used electrode materials for supercapacitors due to their abundant structures, excellent conductivities and chemical stability. However, the manufacture of carbon materials possesses sizable pores and remarkable wettability with the electrolyte remains challenging. Herein, we developed a facile and industrially scalable method for the production of nitrogen-doped porous carbon nanosheets (PNDC-4) with excellent pore size distribution, large specific surface area (>1200 m2 g-1), high conductivity (>700 S m-1), and superb wettability either in aqueous or organic electrolyte. Particularly, PNDC-4 shows high capacitance of 387 F g-1 (1 A g-1) in threeelectrode system with 3 M KOH and 80 F g-1 (1 A g-1) in symmetric two-electrode system with EMIMBF4. The device exhibits an ultrahigh energy density of 81 Wh kg-1 at a power density of 1.3 kW kg-1, which can still maintain 60.8 Wh kg-1 when the power density increased to 266.6 kW kg-1. Moreover, the devices show superb stability that 94% of its initial capacitance is still maintained after 100,000 cycles at 20 A g-1.

KEYWORDS: carbon nanosheets; hierarchical porous structure; industrially scalable; high energy density; supercapacitors

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1. INTRODUCTION Supercapacitors have attracted worldwide attentions for their higher power density, faster charge/discharge rate, and longer cycle life compared to batteries.1 Supercapacitors can be divided into two types according to different charge storage mechanisms: electric double layer capacitors (EDLC) and pseudocapacitors.2,3 A perfect EDLC electrode material should meet the following conditions: large specific surface area (SSA), well-proportioned pore size distribution, excellent electrical conductivity, and good wettability with electrolyte.4-7 For the sake of the above requirements, porous carbon materials possess excellent conductivities, abundant structure morphologies, controllable size of the pores, and easily modified electronic structures which worked as supercapacitor electrodes widely.8-11 The structural design of porous carbon materials is generally based on the manufacture of different dimensions nanometer-sized porous carbon.12 Specially, 2D porous carbon nanosheets (PCN), represented by graphene and graphene derivatives, have caused a lot of attention owing to their open structures with high SSA.13-15 The unique open structures can shorten the transmission distance of electrolyte ions and achieve more effective charge storage.16-19 In addition to the open structures, the wettability of carbon materials with the electrolyte is also an important influence factor in charge storage. However, the pure carbon materials are generally hydrophobic with less active sites for charge storage in electrolyte.20 Heteroatom doping is an efficient strategy to solve this problem. It is beneficial to electrolyte transfer and boost the wettability of the carbon materials with electrolyte.21-24 For example, the electron distribution of the carbon materials could be changed by nitrogen doping, and provide the adequate space for charge storage, which is beneficial for improving the performance of supercapacitors.25 Another difficulty is the creation of cost-effective and scalable process to prepare large quantities of PCN. The preparation of PCN can generally be carried out in two ways: “top2 ACS Paragon Plus Environment

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down” and “bottom-up”, such as mechanical peeling and solution-chemical synthesis.20,26,27 However, these approaches also are complex and expensive. Therefore, if this kind of carbon material fabricated in a simple and effective process, will have a positive effect on academic and industrial circles. We report herein a facile and industrially scalable approach using ball milling process and self-sacrificing template method to prepare nitrogen doping porous carbon nanosheets (PNDC-4). The prepared carbon nanosheets with suitable pore size distribution, high SSA (>1200 m2 g-1), high conductivity (>700 S m-1), and superb wettability in both KOH and EMIMBF4 electrolyte. In particular, PNDC-4 delivers a high capacitance of 387 F g-1 (1 A g1)

in three-electrode system with 3 M KOH. In symmetric two-electrode system, the PNDC-4

also shows a high capacitance of 80 F g-1 (1 A g-1) with EMIMBF4. The device exhibits an ultrahigh energy density of 81 Wh kg-1 at a power density of 1.3 kW kg-1, which can still maintain 60.8 Wh kg-1 when the power density increased to as high as 266.6 kW kg-1. Moreover, the devices show superb stability that 94% of its initial capacitance is still maintained after 100,000 cycles at 20 A g-1. 2. EXPERIMENTAL SECTION 2.1. Chemicals. Sodium alginate (SA), sodium citrate (SC), melamine, potassium hydroxide, polyvinylidene fluoride (PVDF), acetylene black and EMIMBF4. 2.2. Preparation of SSM-x. Typically, 0.6 g of SA, 0.6 g of SC, and 0.8 g of melamine were dispersed in 40 mL deionized water, then mixed by ball milling at 400 rpm for 4 h. The type of ball mill machine is MSK-SFM-1. The mass ratio of balls and materials is 10:1~20:1. After ball milling, the obtained dispersion was freeze-dried to remove water. The resultant precursor is denoted as SSM-4. To synthesize SSM-3 and SSM-2, the same procedure was followed except the addition of melamine and SC, respectively. To synthesize SSM-1, only SA was used. 3 ACS Paragon Plus Environment


2.3. Preparation of NDC-x. The SMM-x was calcined under nitrogen atmosphere at 600 ℃ for 2 hours, the rate of heating is 1 ℃ min-1 and the flow rate of nitrogen is 100 sccm. Then, the obtained samples were washed with deionized water. At last, these samples were dried under vacuum 60 ℃ for 3 hours. The samples were expressed as NDC-x (x = 1, 2, 3, 4) and correspond with SMM-x one-to-one. 2.4. Preparation of PNDC-x. In order to prepare porous carbon nanosheets, the NDC-x was mixed with KOH, the mass ratio is 1:1. And then activated under nitrogen atmosphere at 800 ℃ for 2 hours, the rate of heating is 2 ℃ min-1 and the flow rate of nitrogen is 100 sccm. Then, the obtained samples were washed with deionized water. At last, these samples were dried under vacuum 60 ℃ for 3 hours. The samples were expressed as PNDC-x (x = 1, 2, 3, 4) and correspond with NDC-x one-to-one. 2.5. Fabrication of soft pack Supercapacitors Based on PNDC-4. The PNDC-4 was mixed with acetylene black and PVDF by ball milling, the mass ratio is 8:1:1. A few drops of NMP was added into the homogeneous black powder and then uniform coated on nickel foil (length: 9 cm, width: 7 cm). Finally, the electrodes were dried under vacuum 80 ℃ for 12 hours. The mass of active substance on nickel foil was 40.95 mg, the areal density was 0.65 mg cm-2. The soft pack supercapacitors were constructed by assembling two nickel foil electrode with polypropylene (PP) as a separator, laminated aluminum film as package and KOH or EMIIMBF4 as electrolyte. 2.6. Materials Characterization. Scanning electron microscopy (SEM, JEOL JSM6380LV), transmission electron microscopy (TEM, JEOL JEM-2012), Raman (HORIBA PLUS, 532.4 nm laser), N2 adsorption/desorption (QUADRASORB SI/MP, BrunauerEmmett-Teller (BET) method), X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD), atomic force microscope (AFM, Bruker-Multi Mode 8), X-ray diffraction (XRD, Bruker D8 Advance, Cu Kα), and conductivity meter (Keithley 2400, four-wire method). 4 ACS Paragon Plus Environment

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2.7. Dynamic Contact Angle Measurements. PNDC-x was mixed with acetylene black and PVDF in NMP then uniform coated on nickel foil and dried at 80 ℃ overnight. The DSA100 KRUSS was used to record the contact angle evolution of water and EMIIMBF4. 2.8. Electrochemical Characterizations. The electrochemical performance tests of PNDC-x were performed by using a CHI 760E instrument (Shanghai Chenhua). The cycle stability was calculated according to chronopotentiometry charge/discharge (GCD). Specific capacitance (C, F g-1) of PNDC-x and devices were calculated from GCD discharge curves:

C 

I m   V / t 

(1)

C 

I M   V / t 

(2)

where I represents the constant current in GCD,  t represents discharge time, m represents the mass of active materials in there-electrode system, M represents the weight of electrode in two-electrode system (device measurements) and  V represents the voltage window during the GCD discharge process (except IR drop). The energy density (E, Wh kg-1) and power density (P, W kg-1) of the device can be calculated by the following formulas:

C  V 2 E= 2  3.6

(3)

E  3600 t

(4)

P=



3. RESULTS AND DISCUSSION Schematic illustration of the preparation of PNDC-4 is summarized in Figure 1a, including three standard industry steps: (1) ball milling sodium alginate (SA), sodium citrate (SC), and melamine to produce precursor SSM-4, (2) carbonization of SSM-4 to obtain nitrogen-doped carbon NDC-4, and (3) chemical activation to achieve nitrogen-doped porous carbon PNDC-4 with desired porous structure, large SSA and high conductivity. As comparison, PNDC-1, PNDC-2, PNDC-3 are also prepared with the same process from pure SA, SA and melamine, SA and SC, respectively (Figure S1). The morphology of SSM-4 was first investigated by SEM, as shown in Figure 1b and Figure S2. SSM-4 displays a hierarchical flake-on-flake structure, that is, small flakes are vertically distributed on a large flake. This phenomenon is caused by the interaction between carboxyl groups from SA and amino groups from melamine during ball milling.28,29 This result is confirmed by IR spectroscopy (Figure S5 and Table S1) and a series of similar cases (Figure S6-S10). This connection method is beneficial to the doping of nitrogen and manufacturing pores during pyrolysis under high temperatures, as evidenced by numerous pores on the surface of large flakes after carbonization (Figure 1c, S3). Finally, PNDC-4 is composed of graphene-like nanosheets after chemical activation (Figure 1d, S4). TEM and HRTEM further reveal that the PNDC-4 is consist of highly porous nanosheets with a 2D perspective (Figure 1e, f). The thickness of the flakes is about 5~10 nm, which calculated from AFM images (Figure S11). SAED showing the amorphous structure of PNDC-4 (Figure 1g). The presence of C, N, and O elements in the PNDC-4 nanosheets are confirmed by EDX elemental mapping images (Figure 1h). We use nitrogen adsorption and desorption measurements to characterize the porous structure of NDC-x and PNDC-x, as shown in Figure S12 and Figure 2a, respectively. PNDC4 exhibits the largest SSA of 1259 m2 g-1, greater than those of PNDC-3 (888 m2 g-1), PNDC6 ACS Paragon Plus Environment

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2 (596 m2 g-1), and PNDC-1 (552 m2 g-1), as summarized in Table S2. Specifically, PNDC-4 possesses a hierarchical porous structure containing multiscale pores. The sharp absorption below P/P0 = 0.01 in the nitrogen adsorption-desorption isotherms indicates the presence of lots of micropores (Figure 2a). The hysteresis loop above P/P0 = 0.5, revealing the coexistence of macropores and mesopores, which can be confirmed through pore size distribution profiles (Figure 2b). The specific surface area of PNDC-4 is greatly improved due to the decomposition of SC and the activation of KOH. The unique hierarchical porous structure provides abundant active sites and shorten electron transfer distance during electrochemical processes. The PNDC-x are amorphous carbon, which can be confirmed by the two broad peaks at 25.5° (002) and 43° (100) in XRD patterns (Figure S13).30 The Raman spectra of PNDC-x are presented in Figure S14, which show two obvious peaks corresponding to D-band (~1350 cm1)

and G-band (~1580 cm-1).31 The PNDC-4 possesses the highest D band intensity with ID/IG

ratio of 1.11 due to the introduction of defects sites by doping of nitrogen atoms and activation. Such abundant defects lead to amorphous carbon and are beneficial to supercapacitor.32,33 The present peak for nitrogen (400.02 eV) in the XPS spectra (Figure 2c), confirmed that PNDC-4 and PNDC-2 are successfully doped with nitrogen. The result is in line with the EDX mapping images (Figure 1h). The atomic ratio of N in PNDC-4 and PNDC2 were 4.71 and 5.65 at%, respectively. The N 1s peaks of PNDC-4 at 398.6, 399.8, and 400.9 eV, represent pyridinic-N (N-6), pyrrolic-N (N-5) and graphitic-N (N-Q), respectively (Figure 2d).34,35 PNDC-4 possesses the highest proportion of N-6 (45.9 %). The C 1s peaks of PNDC4 at 284.7, 285.8, 287.5, and 288.5 eV, represent sp2-C, sp3-C, C=O/C-N, and O-C=O/N-C=O, respectively (Figure 2e).36 The O 1s peaks of PNDC-4 at 531.3, 533.2, 535.2 eV, represent C=O, O-C=O and N-C=O, respectively (Figure 2f).37,38 To explore the relationship between nitrogen doping and wettability, we performed contact angle tests for PNDC-x with water and EMIMBF4. It is well known that pure carbon materials 7 ACS Paragon Plus Environment


are generally very hydrophobic, for example, the contact angle of pure carbon is almost constant, ≈ 126°.39,40 Specifically, PNDC-4 shows the best hydrophilic. At the beginning, the contact angle of PNDC-4 is 98.5°, and then the KOH solution is completely assimilated in 90 s (Figure 3a). The wettability of PNDC-x in EMIMBF4 represent similar regularity with in water. PNDC-4 possesses smaller initial contact angle of 43.8° in EMIMBF4 than in water and complete wetting of ≈ 60 s (Figure 3b). These results clearly indicate the much better wettability of PNDC-4 than other smples due to the nitrogen doping and remarkable surface property, which facilitates to explore advanced supercapacitor electrode materials. 39,41 The electrochemical performances of PNDC-x are first analyzed in three-electrode system. The cyclic voltammetry (CV) curves are performed at different scan rates (Figure S15). At 50 mV s-1, the CV of PNDC-x show quasi-rectangular shapes without any peaks (Figure 4a), indicating the ideal EDLC behavior. PNDC-4 possesses the largest integral area capacitance, and the capacitance decreases from 329 F g-1 for PNDC-4 to 278 F g-1 for PNDC-3, 252 F g-1 for PNDC-2, and 240 F g-1 for PNDC-1. As the scan rate increased to 1000 mV s-1, PNDC-1, PNDC-2, and PNDC-3 show fairly good capacitive behavior and excellent rate capability, but lower than those for PNDC-4 (Figure 4b). To further investigate their electrochemical performances, GCD tests were carried out at different current densities (Figure S16). At 20 A g-1, the GCD of PNDC-x show very limited voltage (IR) drop, symmetrical and linear graphic, similar to that of ideal EDLC (Figure 4c). The capacitance increases from 270 F g-1 for PNDC-1 to 282 F g-1 for PNDC-2, 296 F g-1 for PNDC-3 and 344 F g-1 for PNDC-4. The IR drops for PNDC-1,2,3,4 decrease from 0.143 to 0.116, 0.047, and 0.028 V, respectively. With the current density grows up to 200 A g-1, PNDC-4 possesses a high capacitance of 325 F g-1, which is approximately 86% of its initial capacitance (Figure 4d). The superior performance are assigned to the excellent wettablity, high conductivity and short electron transfer distance of PNDC-4.42,43


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To reach an in-depth understanding of the ion transfer and interactions in the electrode materials, electrochemical impendance apectroscopy (EIS) measurements were carried out. In Figure 4e, the Nyquist plots are obtained from the frequency range of 100 to 0.01 kHz. As illustrated in Figure 4e, EIS data are fitted by the equivalent circuit model. Equivalent series resistance (Rs) and charge transfer resistance (Rct) can be calculated based on the EIS data. The specific values of Rs and Rct for PNDC-1,2,3,4 are 0.17 Ω and 0.58 Ω, 0.15 Ω and 0.51 Ω, 0.14 Ω and 0.42 Ω, and 0.12 Ω and 0.24 Ω, respectively. PNDC-4 has the smallest Rs with good ion response. The semicircle in the middle frequency indicates the smallest Rct between the electrode and electrolyte interface.44,45 As revealed by the Bode phase diagram (Figure 4f), the frequency (f0) of PNDC-4 at the -45° phase angle is 12.2 Hz, time constant (τ0 = 1/f0) is about 0.082 s. This value is the smallest one among recently reported materials: multiwalled carbon nanotubes (0.7 s),46 3D graphene (0.53 s),47 graphene film (0.17 s),48 highly conductive onion-like carbons (about 0.1 s)49 and activated carbon (10 s).50 The small τ0 represents the fast frequency response of PNDC-4 and is in consistent with its high electrical conductivity (742 S m−1, Figure S17) and excellent rate capability. The preceding results show that PNDC-4 with ideal capacitive, excellent rate performance and low mass transfer resistance in three-electrode system, which suffices the general requirements of a supercapacitor electrode material. Therefore, the electrochemical performances of PNDC-4 is further evaluated by the two-electrode soft pack supercapacitor either in KOH aqueous solution or in EMIMBF4 electrolyte. In the beginning, we build symmetric supercapacitor devices with 3 M KOH. The rectangle-like CV (Figure S18a) and the symmetrical triangles GCD (Figure S18b) demonstrate the excellent EDLC behavior of the PNDC-4. At 1 A g-1, the capacitance of the device approximately 88 F g-1. With the current density grows up to 200 A g-1, the capacitance is about 61.6 F g-1, retaining 70% of its initial capacitance (Figure S18c). The EIS was used to further explore the ion transfer and interactions of the device (Figure S18d). The Nyquist plot shows minor Rs (0.66 Ω) and Rct 9 ACS Paragon Plus Environment


(1.51 Ω) of the device, which indicates the low ionic resistance between electrodes and electrolyte along with the high conductivity of the electrodes. The frequency (f0) of PNDC-4 at the -45° phase angle is 1.33 Hz, time constant is 0.75 s, indicating the fast charge/discharge rate (Figure S18e). The device exhibits superb stability that 90% of its initial capacitance is still maintain after 50,000 cycles at 20 A g-1 (Figure S18f). In addition, this device shows energy density of 12.2 Wh kg-1 at power density of 0.5 kW kg-1, which can maintain 8.55 Wh kg-1 when the power density improved to 100 kW kg-1 (Figure S19). As we all known, the energy density of supercapacitor is proportional to the square of its voltage interval. The operating voltage window of the aqueous electrolyte limits the energy density of supercapacitor device. Thus, we analysis the electrochemical performance of PNDC-4 in EMIMBF4 electrolyte, the operating voltage window improved from 0-1 V to 0-2.7 V (Figure 5). As the scan rate increased to 1000 mV s-1, the rectangle CV curves indicate efficient charge and ion transfer within PNDC-4 (Figure 5a). At 1 A g-1, the capacitance of the device approximately 80 F g-1 (Figure 5b). The very close capacitance in EMIMBF4 and KOH is attributed to the better wettability of PNDC-4 in EMIMBF4 than in KOH, which countervails the negative effects of the high viscosity of EMIMBF4.51 As the current density improved to 200 A g-1, the capacitance is about 60 F g-1 (75% of the initial capacitance), higher than the device with KOH (Figure 5c). The Rs and Rct of device are 1.98 Ω and 0.98 Ω by calculating from Nyquist plot (Figure 5d), respectively. Significantly, the Rs, Rct and τ0 (0.98 s) of the device are much smaller than the EDLC using EMIMBF4 electrolyte in literatures (Figure 5e).52-56 This is also related to the excellent wettablity, high conductivity, and unique structure for PNDC-4. As expected, the symmetric device with EMIMBF4 possesses ultrahigh energy density of 81 Wh kg-1 at power density of 1.3 kW kg-1, which can still maintain 60.8 Wh kg-1 when the power density increased to as high as 266.6 kW kg-1 (Figure S20). Assuming that the weight 10 ACS Paragon Plus Environment

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ratio of active materials in commercial supercapacitors is about 37%, our device exhibits ultrahigh energy density (29.97 Wh kg-1), larger than commerical supercapacitors and literature results (Table S3). This device could compensate for the blank gap between traditional capacitor and batteries owing to ultrahigh energy and power density (Figure 5f). Notably, single device possesses high operating voltage and large energy storage. For example, a single device can easily power a digital clock for more than 24 hours after charging at 20 A g-1 to 2.7 V (Figure S21). Moreover, the devices can be linked in parallel and series to improve the capacitance and voltage window. For example, the operating voltage of the CV and GCD curves increased from 2.7 V for a single cell to 5.4 V for two cells in series. Meanwhile, two cells in parallel shows double of capacitance compared with a single cell (Figure 5g, h). PNDC-4 with EMIMBF4 electrolyte also possess superb stability that 95% of its initial capacitance is still maintain after 100,000 cycles at 20 A g-1 (Figure 5i). The preceding discussion indicate that the device with PNDC-4 possesses excellent electrochemical performance in both KOH and EMIMBF4 electrolytes. The excellent supercapacitor performance mainly results from the porous structure of nitrogen doping carbon which combines the advantages of high SSA, suitable pore structure, good conductivity, and excellent wettability in both KOH and EMIMBF4 electrolytes. Hence, the PNDC-4 shows the best performance. This cost-effective and high performance symmetrical supercapacitor could be promising for energy storage. We are conducting pilot production (Figure S22), and the experimental results are worth to be anticipated. 4. CONCLUSION In conclusion, PNDC-4 with large SSA, suitable pore size distribution, superb conductivity, and excellent wettability is prepared by a cost-effective ball milling along with a subsequent carbonization and activation process. The facile synthesis method is highly scalable to meet 11 ACS Paragon Plus Environment


the needs of industrial manufacturing. Supercapacitors fabricated with PNDC-4 in either KOH or EMIMBF4 electrolyte show both high energy and power density, remarkable stability (94% of its initial capacitance is still maintained after 100,000 cycles at 20 A g-1). This work could open up exciting opportunities for porous carbon fabrication and high-power-density applications in a wide range. ASSOCIATED CONTENT Supporting Information. Additional SEM, XRD, Raman, BET and electrochemical performance results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is financially supported by the National Science Fund for Distinguished Young Scholars (51425304), the China Postdoctoral Science Foundation (2018M632599), the National Natural Science Foundation of China (21704038, 51763018), the National Postdoctoral Program for Innovative Talents (BX201700112), the Natural Science Foundation of Jiangxi Province (20171ACB21009, 2018ACB21021), and the NSFC-DFG Joint Research Project(51761135114).


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Figure 1. Fabrication and characterization of PNDC-4. (a) Schematic illustration of the synthesis of PNDC-4. SEM images of (b) SSM-4, (c) NDC-4, and (d) PNDC-4. (e) TEM, (f) HRTEM, (g) SAED, (h) TEM and corresponding EDX mapping images of PNDC-4.



Figure 2. (a) Nitrogen adsorption-desorption isotherms, (b) pore size distribution, and (c) XPS survey spectra of PNDC-x. High resolution XPS spectra of (d) N 1s, (e) C 1s, and (f) O 1s for PNDC-4.


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Figure 3. Dynamic contact angle measurements for PNDC-x: (a) for water, (b) for EMIMBF4.



Figure 4. Electrochemical performances of PNDC-x in the three-electrode system with 3 M KOH as electrolyte. (a) CV curves of PNDC-x at the scan rate of 50 mV s-1. (b) Specific capacitance at different scan rates. (c) GCD curves at the current density of 20 A g-1. (d) Specific capacitance at different current densities. (e) Nyquist plots and (f) the respective Bode plots. Inset in (e) is the magnification of high-frequency range and the electrical equivalent circuit used for fitting EIS spectra.


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Figure 5. Supercapacitor performance of PNDC-4 in the two-electrode system with EMIMBF4 as electrolyte. (a) CV curves. (b) GCD curves. (c) Specific capacitance at different current densities. (d) Nyquist plots and (e) the corresponding Bode plot of phase angle versus frequency. Inset in (d) is the magnification of high-frequency range and the electrical equivalent circuit used for fitting EIS spectra. (f) Ragone plots for the device in comparison with traditional capacitors, commercial ECs, and lithium-ion batteries. The data are based on all device mass (assuming a 37% weight ratio for active material). (g) CV curves at the scan rate of 50 mV s-1 and (h) GCD curves at the current density of 20 A g-1 for the two devices connected in series and in parallel. (i) Cycling stability and coulombic efficiency at 20 A g-1 for 100000 cycles.



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Facile and Scalable Fabrication of Nitrogen-Doped Porous Carbon

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