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N-doped Porous Carbon Nanofibers/Porous Silver Network Hybrid for High Rate Supercapacitor Electrode Qingshi Meng, Kaiqiang Qin, Liying Ma, Chunnian He, Enzuo Liu, Fang He, Chunsheng Shi, Qunying Li, Jiajun Li, and Naiqin Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08610 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 24, 2017

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ACS Applied Materials & Interfaces

N-doped Porous Carbon Nanofibers/Porous Silver Network Hybrid for High Rate Supercapacitor Electrode Qingshi Menga , Kaiqiang Qina, Liying Maa,*, Chunnian Hea,b,c, Enzuo Liua,b, Fang Hea, Chunsheng Shia, Qunying Lia, Jiajun Lia and Naiqin Zhaoa,b,c,*

a

School of Materials Science and Engineering and Tianjin Key Laboratory of Composites and Functional Materials, Tianjin University, Tianjin 300350, China

b

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300350, China c

Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, Tianjin, 300350, China

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. * E-mail: [email protected].

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Keywords: N-doped, porous carbon nanofibers, porous silver network, silver mirror reaction, chemical vapor deposition, high rate, supercapacitor

ABSTRACT: A 3D cross-linked porous silver network (PSN) is fabricated by silver mirror reaction using polymer foam as the template. The N-doped porous carbon nanofibers (N-PCNFs) is further prepared on PSN by chemical vapor deposition and treated by ammonia gas subsequently. The PSN substrate serving as the inner current collector will improve the electron transport efficiency significantly. The ammonia gas can not only introduce nitrogen doping into PCNFs, but also increase the specific surface area of PCNFs at the same time. Because of its large surface area (801 m2/g), high electrical conductivity (211 S/cm) and robust structure, the as-constructed N-PCNFs/PSN demonstrates a specific capacitance of 222 F/g at the current density of 100 A/g with a superior rate capability of 90.8% of its initial capacitance ranging from 1 A/g to 100 A/g while applied as the supercapacitor electrode. The symmetric supercapacitor device based on N-PCNFs/PSN displays an energy density of 8.5 W h/kg with power density of 250 W/kg, and excellent cycling stability which attains 103% capacitance retention after 10, 000 charge-discharge cycles at a high current density of 20 A/g, which indicates that N-PCNFs/PSN is a promising candidate for supercapacitor electrode materials.

1. INTRODUCTION In the last decade, considerable research has been dedicated to carbon nanofibers (CNFs) which can be applied as the promising candidate for supercapacitor electrode materials because 2 ACS Paragon Plus Environment

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of

its

low

cost,

light

weight,

high

superior

electrochemical

stability,

and

fast

charging/discharging kinetics as well1-6. However, CNFs-based electric double layer capacitors (EDLCs) usually suffer from the low specific capacitance on account of their intrinsic doublelayer charge storage mechanism which makes use of electrostatic attraction between the electrolyte ions and charges presenting at the electrode surface7-8. In order to strengthen the capacitive performance of carbon nanomaterials, synthesizing porous carbon nanofibers (PCNFs)2,

9-11

or introducing heteroatom (N, B and P etc.) into carbon

frameworks12-17 is an effective strategy. In numerous studies, it has been demonstrated that PCNFs possess a high specific surface area (SSA) which can enhance the double layer charging/discharging capability at the electrode/electrolyte interface18-21. And the nitrogen doping (N-doping) is conducive to increasing the active sites for pseudocapacitance due to the additional surface redox reactions22-25. Moreover, N-doping can dramatically improve the surface wettability of carbon nanomaterials26. Nevertheless, the PCNFs powder needs extra binders or conductive additives when it is assembled into electrodes. The relatively long ion diffusion length and the large charge transfer resistance between the active materials and the current collector inhibit the fast electron transporting and collecting, which leads to an inferior rate capability27-29. In addition, the enhanced interlayer binding energy between the neighboring carbon layers resulted from Ndoping make the nanocarbon framework incline to agglomerate30-32, which can largely reduce the electrochemical performance of the N-doped CNFs. In order to cut down the drawback, many works33-37 so far have focused on combining CNFs with an excellent conductive material, such as graphene, carbon nanosheets, or metal. Silver nanowire, as one-dimensional (1D) metal material with good conductivity and fast charge 3 ACS Paragon Plus Environment


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transportation38-42, has attracted more and more attention to improve the electrical conductivity of active materials. However, the addition of binders into electrodes will weaken the effective role of silver nanowire inevitably43-46. Compared to 1D silver nanowire, the three-dimension (3D) porous silver network47-48 (PSN) can form further joints at the crossing points of silver wire. The cross-linked freestanding structure of porous silver network provides a highly conductive path for electrons without extra binders or conductive additives. In this context, we present a facile method to construct a 3D PSN with strong cross-linked structure using polymer foam as the template via silver mirror reaction and annealing process. Furthermore, the N-doped porous carbon nanofibers/porous silver network hybrid film (NPCNFs/PSN) is prepared by chemical vapor deposition (CVD) and treated by ammonia gas subsequently. The 3D cross-linked PSN is capable of forming a freestanding film with N-doped porous carbon nanofibers without any binder. In addition, PSN serving as an inner current collector will afford more rapidly transporting channels for electron, which can greatly reduce the ion diffusion length and prevent the carbon nanofibers from agglomerating effectively. Moreover, on the one hand the ammonia gas treatment introduces nitrogen doping into PCNFs, on the other hand it increases the specific surface area of PCNFs and optimizes the distribution of pore size. As the supercapacitor (SC) electrode material, the N-PCNFs/PSN presents an enhanced capacitive property (245 F/g at 1 A/g) and superior rate capability of 90.8% with the current density increasing from 1 A/g to 100 A/g in a three electrode system. In addition, the twoelectrode symmetric SC device demonstrates a high energy density of 8.5 W h/kg with a power density of 250 W/kg and a remarkable cycling stability (103% capacitance retention after 10000 cycles at 20 A/g), suggesting its potential application in energy storage. 4 ACS Paragon Plus Environment

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2. EXPERIMENTAL SECTION 2.1. Synthesis of porous silver network. The polymer foam purchased from BASF Co. Ltd is usually employed as a household cleaning eraser, which can be directly used in our experiment without any treatment. The synthesis of PSN can be referred to our previous report49 and improved on the base of that. In order to prepare silver ammonia solution, an ammonia solution (25 wt%) was dropwise added to 10 mL silver nitrate solution (20 g/L) until the precipitate just dissolved. Then, 2.5 mL glucose solution (100 g/L) was added to silver ammonia solution and mixed uniformly. A piece of polymer foam with the size of 40 × 20 × 4 mm3 was fully immersed into the above mentioned mixed solution immediately for ten hours in ice water bath. The silver coated polymer foam sample was rinsed in deionized water and dried on the electric heating table at 80 °C for two hours subsequently. Moreover, it was placed in the furnace and annealed at 500 °C for 120 minutes to burn away the polymer template and the PSN was synthesized finally. 2.2. Synthesis of N-doped CNFs/porous silver network hybrid. The PCNFs/PSN was synthesized by CVD using a Ni catalyst. PSN was immersed into alcoholic solution of nickel nitrate (0.01 mol/L) for five minutes and dried on the electric heating table at 80 °C for ten minutes. After that, it was pressed to a thickness of ~110 µm. The as treated PSN was placed in the horizontal tube furnace and heated to 600 °C under Ar (400 sccm) flow. The CNFs growth was in progress with a mixture of Ar (400 sccm), H2 (100 sccm) and C2H2 (10 sccm) for 20 minutes. Subsequently, the furnace was heated directly to 800 °C and the temperature was held for 30 minutes with a mixture of Ar (250 sccm) and NH3 (35 sccm). Finally, the furnace was cooled to room temperature under Ar protection (250 sccm) and the N-PCNFs/PSN hybrid was synthesized as a result. 5 ACS Paragon Plus Environment


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For comparison, the N-doped porous carbon nanofibers/carbon foam (N-PCNFs/CF) which has similar structure to N-PCNFs/PSN was synthesized simultaneously. N-PCNFs/CF was synthesized by the growth of CNFs on polymer foam matrix via CVD process. The polymer foam would be carbonized and formed carbon foam at 600 °C with a mixture of Ar, H2 and C2H2. The synthesis process and parameter of the N-PCNFs/CF was consistent with NPCNFs/PSN hybrid. 2.3. Characterization. Scanning electron microscopy (SEM, Hitachi S4800) was used to characterize the morphology and structure of samples. The transmission electron microscopy (TEM) and scanning TEM (STEM) were performed on a FEI Tacnai G2 F20. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max diffractometer with Cu Kα radiation. Thermogravimetric analysis (TGA) was performed on a TA instruments with the heating rate of 5 °C/min. The Nitrogen adsorption isotherms of samples were measured at 77 K using an autosorb instrument (Quantachrome U.S.). The total surface area was calculated by BET method and the pore size distribution data was calculated based on the adsorption and desorption data using density functional theory (DFT) method. The elemental composition was evaluated by X-ray photoelectron spectroscopy (XPS, PHI5000 VersaProbe). Raman spectra measurement was carried on a LabRAM HR Raman spectrometer using a 514.5 nm argon ion laser source. The electrical conductivity of samples was tested on a RTS-8 four-point probe tester. 2.4. Electrochemical measurements. The electrochemical measurements consist of the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS, recorded in a frequency range from 0.01 Hz to 100 kHz at the open circuit voltage with a potential amplitude of 5 mV). The electrolyte is 6 M KOH solution. All the electrochemical measurements were performed on a CHI 660D (Chenhua China) electrochemical 6 ACS Paragon Plus Environment

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workstation at room temperature. For a three electrode system, a piece of N-PCNFs/PSN, PCNFs/PSN and N-PCNFs/CF with an area of 0.5 cm2 were used as the working electrode directly. The active material mass loading of N-PCNFs/PSN, CNFs/PSN and N-PCNFs/CF was 1.44 mg/cm2, 1.34 mg/cm2 and 1.55 mg/cm2 respectively. The counter electrode and reference electrode was a platinum plate and Hg/HgO electrode respectively. The specific capacitance of N-PCNFs/PSN,

CNFs/PSN

and

N-PCNFs/CF

were

estimated

from

galvanostatic

charge/discharge curves using the equation: Cs =I∆t/m∆V (1) where Cs, I, ∆t, m, and ∆V represent the specific capacitance (F/g), the discharge current (A), the discharge time (s), the mass of the active material (g) and the potential change (V) within the discharge time respectively. For a two-electrode symmetric SC device, two pieces of N-PCNFs/PSN with same area were used as the working electrodes (both anode and cathode). The energy density and power density of device were calculated from the following equations: Cm =I∆t/M∆V (2) E =(1/7.2)Cm (∆V)2 (3) P=E/∆t (4) where Cm, I, ∆t, M, ∆V, E and P represent the measured device capacitance (F/g), the discharge current (A), the discharge time (s), the total mass of the active material on the two electrodes (g), 7 ACS Paragon Plus Environment


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the potential change (V) within the discharge time, the energy density (W h/kg) and the average power density (W/kg), respectively38.

3. RESULTS AND DISCUSSIONS

Figure 1. The fabrication process schematic of N-PCNFs/PSN.

Figure 1 displays the typical fabrication process of N-PCNFs/PSN. Firstly, a freestanding PSN film was fabricated by silver mirror reaction using polymer foam as the substrate. The SEM images in Figure 2a and Figure S1b indicate that the skeleton of polymer foam (Figure S1a) was coated by silver uniformly after silver mirror reaction. Then the cross-linked porous silver network was obtained after removing the polymer foam template by annealing at 500 °C. SEM 8 ACS Paragon Plus Environment

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image shows that PSN consists of silver wire roughly 4.5 µm in diameters (Figure 2b). The crystal phase of PSN was further confirmed by X-ray diffraction. The diffraction pattern of PSN (Figure S2) has five sharp peaks at 38.1°, 44.3°, 64.4°, 77.5° and 81.5°, corresponding to (111), (200), (220), (311) and (222) of silver respectively. The TGA curve (Figure S3) displays that the polymer foam can be burned away completely at 500 °C for 120 minutes in the air. Subsequently, N-PCNFs/PSN hybrid film was prepared by CVD and further treated by NH3. SEM images exhibit the N-CNFs were seamlessly grown on silver wire and the macropores between the silver wires were filled with N-CNFs (Figure 2c and d). TEM image (Figure 2e) reveals that the asprepared N-CNFs exhibit mesoporous structure and the diameter of N-CNFs ranges from 30 to 50 nm. Furthermore, STEM and elemental mapping (Figure 2f) were performed to determine the distribution of carbon and nitrogen, revealing the nitrogen atom doped in carbon nanofibers matrix homogeneously. For comparison, the polymer foam was carbonized to carbon foam and N-PCNFs was directly grown onto the surface of carbon foam (CF) using CVD approach and further treated by NH3. As shown in figure S1c, the N-PCNFs/CF has a similar structure to NPCNFs/PSN. The N-CNFs were grown on carbonized skeleton of polymer foam uniformly. There are some obvious macropores in N-PCNFs/CF. The thickness of N-PCNFs/CF (1245 µm) is much larger than that of PCNFs/PSN and N-PCNFs/PSN (Figure S1d, e). Figure S1f shows the optical image of the as-prepared freestanding PSN and N-PCNFs/PSN hybrid film, which demonstrates their highly flexibility and plasticity.



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Figure 2. SEM images of (a) the polymer foam coated with silver, (b) PSN and (c), (d) NPCNFs/PSN. (e) TEM image of N-CNFs. (f) STEM image and elemental mappings exhibiting the distribution of C, N.

To examine the porous nature of N-PCNFs/CF, PCNFs/PSN and N-PCNFs/PSN, nitrogen adsorption-desorption isotherms curves (Figure 3a) were measured and identified as type IV, which has a hysteresis loop at a relative pressure ranging from 0.4 to 0.97. It suggests a narrow 10 ACS Paragon Plus Environment

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mesopore size distribution in Figure 3b. Density functional theory (DFT) analyses show that the mesopore size of N-PCNFs/CF and N-PCNFs/PSN is about 3 - 6 nm, which is larger than that of PCNFs/PSN (3 - 4 nm). The values of specific surface area, total pore volume, micropore surface area and t-method external surface area are listed in Table S1. N-PCNFs/PSN exhibits a micropore surface area of 516.125 m2/g and an external surface area of 285.215 m2/g, which are larger than those of PCNFs/PSN, indicating that NH3 treatment not only increased the number of micropores but also enlarged the diameter of mesoporous. X-ray photoelectron spectroscopy (XPS) was carried out to characterize the elemental composition and clarify the types of nitrogen introduced to the CNFs surface. As shown in Figure 3c, four typical peaks corresponding to the binding energies of C 1s, Ag 3d5, N 1s and O 1s can be observed in the full survey XPS spectra of N-PCNFs/PSN. From the XPS survey spectrum, the nitrogen content in N-PCNFs/CF and N-PCNFs/PSN was calculated to be around 4.08% and 3.27% respectively. The N 1s spectrum can be separated into three peaks which located at 398.2 eV, 399.6eV and 401.2 eV, corresponding to the pyridinic-N, pyrrolic-N and quaternary-N25,

50-52

respectively (Figure 3d). Furthermore, the Raman spectrum of N-

PCNFs/PSN presents two peaks at about 1344 cm-1 and 1595 cm-1 which correspond to the disorder-induced D band and in-plane vibrational G band of carbonaceous materials (Figure S4). Generally, the intensity radio of D peak to G peak (ID/IG) can be applied to evaluate the crystallization of CNFs. The ID/IG of N-PCNFs/PSN (1.12) is higher than that of PCNFs/PSN (0.91), which indicates that there are more defects in the CNFs matrix resulted from the successful introduction of nitrogen atoms.



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Figure 3. (a) Nitrogen adorption-desoption isotherms and (b) pore size distribution of NPCNFs/CF, CNFs/PSN and N-PCNFs/PSN. (c) XPS spectra of N-PCNFs/CF, CNFs/PSN and NPCNFs/PSN. (d) N1s spectra of N-PCNFs/PSN

In order to investigate the electrochemical performance of N-PCNFs/PSN, PCNFs/PSN and NPCNFs/CF, cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements were carried out in a three-electrode system with 6 M KOH as the electrolyte. In comparison with N-PCNFs/PSN, the area of CV curve of PSN is negligible, indicating that PSN has less contribution to the capacitance, as shown in Figure 4a. The CV curves of N-PCNFs/PSN (Figure 4b) retain quasi-rectangular shape under the scan rate from 5 mV/s to 1000 mV/s, inferring a 12 ACS Paragon Plus Environment

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superior conductivity of N-PCNFs/PSN. Even at high scan rate of 1000 mV/s, the quasirectangular shape of N-PCNFs/PSN has almost little change, suggesting the rapid ions diffusion in the electrode and the speedy current response to voltage variation. Moreover, the CV curves of N-PCNFs/PSN at various scan rates have much larger area than that of PCNFs/PSN (Figure S5a), indicating that N-PCNFs/PSN has a higher electrochemical capacitive property and nitrogen doping has great contribution to capacity enhancement. For N-PCNFs/CF, the CV curves deviate quasi-rectangular shape when the scan rate reaches up to 200 mV/s (Figure S5b), inferring a rapidly decay of capacitance. The GCD profiles (Figure 4c, S6) of N-PCNFs/PSN at different current densities show the approximate linear line and symmetrical charging curves with their discharging counterpart, demonstrating a good capacitive performance and high reversibility. The specific capacitance of N-PCNFs/PSN is calculated to be 245 F/g at 1 A/g (Figure 4d) which is 3.5 times larger than that of PCNFs/PSN (70 F/g). More importantly, the capacitance of NPCNFs/PSN remains 222 F/g even at a high current density of 100 A/g (90.8% of the initial capacitance at 1 A/g), while the retention capacitance of N-PCNFs/CF is only 13.6% under the current densities from 1 A/g to 50 A/g. As far as we know, such high capacitance and superior rate capability are outstanding with respect to similar CNFs based materials30, 34, 53-60 (Table S2). The electrochemical impedance spectra measurements were also carried out to attain deeper insights regarding internal resistance of the N-PCNFs/PSN. As shown in Figure S7, NPCNFs/PSN displays a semicircle with a diameter of 0.74 Ω at the high frequency region, which is similar to that of PCNFs/PSN (0.68 Ω) and further smaller than that of N-PCNFs/CF (3.91 Ω), suggesting a lower charge transfer resistance. Furthermore, the nearly vertical line at the low frequency region indicates the N-PCNFs/PSN has a superior ion diffusion behavior. In addition, the electrical conductivity of N-PCNFs/PSN measured by four-point probe tester is 211 S/cm, 13 ACS Paragon Plus Environment


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which is close to that of PCNFs/PSN (189 S/cm) and higher than that of N-PCNFs/CF (0.08 S/cm). It indicates that PSN has an obvious strengthening effect on the electrical conductivity of N-PCNFs. The capacitive performance of N-PCNFs/PSN can be attributed to the combined effect in three main factors: (1) The cross-linked porous silver network provides rapid transmission channels for electrons and effectively prevents PCNFs from agglomerating after nitrogen doping. (2) The introduction of nitrogen atoms affords more active sites for pseudocapacitance. The redox reaction between the pyridine-N and the hydroxyl ions in the electrolyte61 is able to enhance the pseudocapacitance of PCNFs. (3) The ammonia gas treatment increases the specific area of N-PCNFs and further optimizes the distribution of pore size.


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Figure 4. Electrochemical performance of N-PCNFs/PSN, PCNFs/PSN and N-PCNFs/CF measured in a three-electrode system. (a) CV curves at 100 mV/s. (b) CV curves of NPCNFs/PSN with scan rate from 5 mV/s to 1000 mV/s. (c) Galvanostatic charge/discharge profiles of N-PCNFs/PSN at various current densities.

(d) Specific capacitance at various

current densities.

A symmetrical two-electrode device was also built to evaluate the electrochemical performance of N-PCNFs/PSN with 6 M KOH as electrolyte. Figure 5a shows the CV curves of the symmetrical device based on N-PCNFs/PSN at various scan rates. It can be found that the shape of CV curves remains rectangular even at high scan rate of 1000 mV/s, inferring the excellent conductivity and the fast ions diffusion in the N-PCNFs/PSN electrode. Attributed to the construction of cross-linked porous silver network framework which could enhance the electrical conductivity, GCD curves in Figure 5b exhibit approximate triangular shape, indicating the symmetric device has a fast I-V response. The specific capacitance of device is 58.4 F/g at 1A/g, which is comparable to those of previously reported CNFs-based two-electrode devices. Meanwhile, it achieves a remarkable capability retention rate of 75.6% when the current density increases from 1 to 20 A/g (Figure 5c). The capability of retaining high capacitance under ultrafast charging/discharging infers that the symmetrical two-electrode device of N-PCNFs/PSN is a promising candidate for high performance supercapacitor device. As shown in Figure S8, the EIS spectra of symmetrical device show a small semicircle diameter (1.9 Ω) in the high frequency region and a nearly vertical tail in the low frequency region, suggesting the low charge transfer resistance and the excellent ion diffusion behavior. The N-PCNFs/PSN performs high energy density of 8.5 W h/kg with a power density of 250 W/kg (Figure S9), which is higher 15 ACS Paragon Plus Environment


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than that of other carbon based aqueous supercapacitors such as freestanding porous carbon nanofibers (3.22 W h/kg at a power density of 600 W/kg)57, CNFs/CNTs/PANI ternary composites (5.1 W h/kg at a power density of 10.1 kW/kg)54 and cross-linked N-doped carbon nanofibers network (5.9 W h/kg at a power density of 1200 W/kg)55. Moreover, the cycle stability of device was measured at a current density of 20 A/g by a GCD measurement (Figure 5d). The N-PCNFs/PSN delivers 103% retention of the initial specific capacitance after 10 000 cycles, demonstrating the superb chemical stability of device.

Figure 5. Electrochemical performance of N-PCNFs/PSN measured in a symmetrical twoelectrode system. (a) CV curves at scan rate increasing from 100 mV/s to 1000 mV/s. (b) Galvanostatic charge/discharge curves at various current densities. (c) Specific capacitance at various current densities. (d) Long-cycle performance conducted at 20 A/g.


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4. CONCLUSION In summary, N-doped carbon nanofibers/porous silver network hybrid has been fabricated via silver mirror reaction and CVD process. The cross-linked freestanding porous silver network provides a fast conduction channel for electrons and greatly enhances the conductivity of PCNFs. Ammonia treatment plays an important role in introducing nitrogen doping, increasing specific surface area and optimizing pore size distribution as well. The combined effect of above mentioned factors makes N-PCNFs/PSN hybrid electrode perform a high rate capability. The capacitance retention of N-PCNFs/PSN can reach 90.8% with the current density increasing from 1 A/g to 100A/g, suggesting its great potential in supercapacitor electrode materials. This study demonstrates that the porous silver network is an ideal substrate for enhancing the conductivity of active materials. Furthermore, the performance of N-PCNFs/PSN can be further enhanced through decorating other active materials (such as conducting polymers or transition metal oxides) on N-CNFs to expand its application in energy storage field.

ASSOCIATED CONTENT Supporting Information Detailed preparation of samples, morphology characterizations for the samples (SEM), physical and chemical characteristics (the XRD, TGA, BET and RAMAN results), and additional electrochemical properties such as CV, GCD, EIS and ragone plot.



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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Science and Technology Support Programme of Tianjin (No. 16ZXCLGX00110), the National Natural Science Foundation of China (Grant Nos. 51472177, 11474216, and 51272173), and the State Key Program of National Natural Science of China (Grant No. 51531004). REFERENCES 1.

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55. Cheng, Y.; Huang, L.; Xiao, X.; Yao, B.; Yuan, L.; Li, T.; Hu, Z.; Wang, B.; Wan, J.; Zhou, J. Flexible and Cross-Linked N-Doped Carbon Nanofiber Network for High Performance Freestanding Supercapacitor Electrode. Nano Energy 2015, 15, 66-74. 56. Ma, L.; Liu, R.; Niu, H.; Xing, L.; Liu, L.; Huang, Y. Flexible and Freestanding Supercapacitor Electrodes Based on Nitrogen-Doped Carbon Networks/Graphene/Bacterial Cellulose with Ultrahigh Areal Capacitance. ACS Appl. Mater. Inter. 2016, 8, 33608-33618. 57. Liu, Y.; Zhou, J.; Chen, L.; Zhang, P.; Fu, W.; Zhao, H.; Ma, Y.; Pan, X.; Zhang, Z.; Han, W. Highly Flexible Freestanding Porous Carbon Nanofibers for Electrodes Materials of High-Performance All-Carbon Supercapacitors. ACS Appl. Mater. Inter. 2015, 7, 23515-23520. 58. Song, L.-T.; Wu, Z.-Y.; Liang, H.-W.; Zhou, F.; Yu, Z.-Y.; Xu, L.; Pan, Z.; Yu, S.-H. Macroscopic-Scale Synthesis of Nitrogen-Doped Carbon Nanofiber Aerogels by TemplateDirected Hydrothermal Carbonization of Nitrogen-Containing Carbohydrates. Nano Energy 2016, 19, 117-127. 59. Sheng, J.; Ma, C.; Ma, Y.; Zhang, H.; Wang, R.; Xie, Z.; Shi, J. Synthesis of Microporous Carbon Nanofibers with High Specific Surface using Tetraethyl Orthosilicate Template for Supercapacitors. Int. J. Hydrogen Energ. 2016, 41, 9383-9393. 60. Chen, L. F.; Zhang, X. D.; Liang, H. W.; Kong, M.; Guan, Q. F.; Chen, P.; Wu, Z. Y.; Yu, S. H. Synthesis of Nitrogen-Doped Porous Carbon Nanofibers as an Efficient Electrode Material for Supercapacitors. ACS nano 2012, 6, 7092-7102.


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61. Wang, D. W.; Li, F.; Yin, L. C.; Lu, X.; Chen, Z. G.; Gentle, I. R.; Lu, G. Q. M.; Cheng, H. M. Nitrogen-Doped Carbon Monolith for Alkaline Supercapacitors and Understanding Nitrogen-Induced Redox Transitions. Chem.- Eur. J. 2012, 18, 5345-5351.



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Porous Silver Network Hybrid for

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