Asymmetric Supercapacitor Based on Porous N ... - ACS Publications

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Asymmetric Supercapacitor Based on Porous N‑doped Carbon Derived from Pomelo Peel and NiO Arrays Gan Qu,† Shuangfeng Jia,† Hai Wang,‡ Fan Cao,† Lei Li,† Chen Qing,‡ Daming Sun,‡ Bixiao Wang,‡ Yiwen Tang,*,‡ and Jianbo Wang*,† †

ACS Appl. Mater. Interfaces 2016.8:20822-20830. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/24/18. For personal use only.

School of Physics and Technology, Center for Electron Microscopy and MOE Key Laboratory of Artificial Micro- and Nano-structures, and Institute for Advanced Studies, Wuhan University, Wuhan, Hubei Province 430072, China ‡ Institute of Nano-Science & Technology, Department of Physics and Technology, Central China Normal University, Wuhan, Hubei Province 430079, China S Supporting Information *

ABSTRACT: A three dimensional (3D) porous framework-like N-doped carbon (PFNC) with a high specific surface area was successfully fabricated through ammonia doping and graphitization based on pomelo peel. The obtained PFNC exhibits an enhanced specific capacitance (260 F g−1 at 1 A g−1) and superior cycling performance (capacitance retention of 84.2% after 10000 cycles at 10 A g−1) on account of numerous voids and pores which supply sufficient pathways for ion diffusion during cycling. Furthermore, a fabricated asymmetric PFNC//PFN device based on PFNC and porous flake-like NiO (PFN) arrays achieves a specific capacitance of 88.8 F g−1 at 0.4 A g−1 and an energy density of 27.75 Wh kg−1 at a power density of 300 W kg−1 and still retains 44 F g−1 at 10 A g−1 and 13.75 Wh kg−1 at power density of 7500 W kg−1. It is important that the device is able to supply two light-emitting diodes for 25 min, which demonstrates great application potential. KEYWORDS: pomelo peel, carbon, N-doped, NiO, arrays, asymmetric supercapacitor becoming a research hotspot in EDLCs.9 Moreover, biomass carbonaceous materials, including carbon flakes,10 mesoporous carbon,11 fibers,12,13 carbon nanosheets,14,15 sponge carbonaceous aerogels,16 and activated carbon,17 have displayed high EDLC performance. Further, heteroatom doping such as nitrogen,18,19 boron,20 sulfur21,22 and phosphorus23 can enhance capacities by increasing wettability and conductivity of carbonaceous materials and supplying pseudocapacitive behavior.24−26 For instance, nitrogen-doped porous carbon prepared via gelatin graphitization displayed a capacitance of 235 F g−1 at 50 A g−1 in a 6 M KOH electrolyte.27 Cao et al. recently announced the construction of hierarchical porous Ndoped nanosheets by natural silk. The carbonized materials exhibited a capacitance of 242 F g−1 at 0.1 A g−1 and maintained an energy density of 102 Wh kg−1 in ionic liquid electrolytes.28 Qiu et al. announced B/N codoped carbon nanosheets through annealing boric acid and gelatin, which delivered a capacitance of 230 F g−1 at 0.1 A g−1 and an energy density of 6000 Wh kg−1 in a 1 M H2SO4 electrolyte.29 In comparison with traditional carbon sources, it is the pursuit for low cost and environmentally friendly precursors that promotes

1. INTRODUCTION With an explosive increase in the number of humans and a dramatic increase in people’s requirements, many problems have gradually appeared over the past few years that have demanded prompt solutions.1 Among all of these problems, the most concerning issue is the energy crisis. To tackle the challenge, there are more and more researchers devoting themselves to overcoming energy shortage. Supercapacitors, which are also addressed as electrochemical capacitors or ultracapacitors, are becoming a hotspot in energy field for their high power density, outstanding rate performance, excellent cyclic stability, and a wide range of work temperatures compared to that of lithium batteries.2−4 Supercapacitors could be classified as electrochemical double-layer capacitors (EDLCs) and pseudocapacitors based on their charge storage mechanisms.5 Pseudocapacitors store and supply energy through reversible redox reaction in the interface between the electrode materials and electrolyte, while EDLCs store and supply energy through ion adsorption and desorption.6 Generally, EDLCs deliver a higher power density and are more environmental-friendly and much safer compared with pseudocapacitors. Nowadays, many researchers are engaged in seeking a low cost, sustainable EDLC material with excellent performance to adapt to the development of society.7 Highly porous carbonaceous materials derived from biomass energy8 stand out and are © 2016 American Chemical Society

Received: June 2, 2016 Accepted: July 19, 2016 Published: July 19, 2016 20822

DOI: 10.1021/acsami.6b06630 ACS Appl. Mater. Interfaces 2016, 8, 20822−20830

Research Article

ACS Applied Materials & Interfaces Scheme 1. Illustration of the Fabricated Processes of PFC and PFNC Materials

1 M HCl solution and then washed several times with deionized water. Then, PFC was obtained after drying the samples at 60 °C in an oven for 12 h. For the synthesis of PFNC, all procedures were kept the same besides substituting a 10% ammonia solution for deionized water in the hydrothermal process. 2.2. Preparation of Self-Assembled Three-Dimensional (3D) Hierarchical Flake-like NiO Arrays. Ni foam was immersed in a 0.25 M Ni(NO3)2 solution containing 3 mM hexadecyl trimethylammonium bromide (CTAB) for 12 h. Next, the dipped Ni foam was heated to 400 °C (ramp rate: 4 °C per minute) and maintained for 3 h. As a result, self-assembled 3D porous flake-like NiO (PFN) arrays were obtained. 3D interconnected flake-like NiO (IFN) arrays were achieved when a 0.5 M Ni(NO3)2 solution was used with other conditions remaining the same. 2.3. Material Characterizations. The morphological images of PFC, PFNC, PFN, and IFN were performed through field emission scanning electron microscopy (FESEM; Hitachi S-4800). Energydispersive spectroscopy (EDS) measurements were characterized by an FESEM (FEI SIRION). Crystallographic information on the asprepared samples was recorded using powder X-ray diffraction patterns (XRD; PANalytical Empyrean) loaded with Cu Kα radiation (wavelength of 1.5418 Å). Bright-field (BF) images and selected area electron diffraction (SAED) patterns were performed inside a transmission electron microscope (TEM; JEOL JEM-2010). A highresolution image was carried through the transmission electron microscope (HR-TEM; JEOL JEM-2010FEF). The samples of PFN and IFN were scraped from Ni foam and dispersed in ethanol before analysis. Raman spectra were obtained by a mirco-Raman system (Renishaw Invia Raman microscope) loaded with a 514.5 nm wavelength laser under ambient conditions. The specific surface area was calculated using the Brunauer−Emmett−Teller (BET) method, and the pore-size distribution data was evaluated using the Barrett− Joyner−Halenda (BJH, Autosorb-iQASIQ) method. X-ray photoelectron spectroscopy (XPS) was performed using a VG Multilab 2000 instrument and ascribing the peak of C 1s to 284.6 eV. 2.4. Electrochemical Measurements. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) tests, and galvanostatic charge and discharge (GCD) tests were carried out through a Parstat 4000 electrochemical workstation in a 2 M KOH electrolyte with a mercuric oxide electrode (Hg/HgO) as the reference electrode, a platinum foil as the counter electrode, and samples as the working electrode. The working electrode was prepared through mixing the as-prepared materials, acetylene black, and poly(tetrafluoro ethylene) in a mass ratio of 8:1:1 in isopropyl alcohol. The mixture was ground adequately to form a slurry and then coated on one side of the pretreated nickel foam at 10 MPa pressure. Finally, the electrodes were dried in a vacuum electric oven at 60 °C for 12 h. All tests were carried out at room temperature. PFN and PFNC electrodes acted as positive and negative electrodes, respectively, when fabricating the asymmetric supercapacitor device. A piece of commercial supercapacitor separator (MPF30AC-100) was employed as the supercapacitor membrane. According to the charge balance theory37 and their respective specific capacitances in 2 M KOH electrolyte, the mass ratio of PFN to PFNC was fixed to 0.36:1.

the development of biomass-based supercapacitors in practical applications.10 Pomelo, a popular fruit, spreads in almost every corner of the world, while its peels are often discarded. In fact, the peel of pomelo is a renewable material which is suitable for mass production. There has been some research on pomelo-derived cathodes for lithium-ion batteries (LIBs),30−32 nitrogen-doped interconnected carbon nanosheet electrode materials in supercapacitors,33 and porous carbon in adsorbent materials.34 However, the above-mentioned carbon materials based on pomelo peel had a low specific surface area and undeveloped pore nanostructures, which seriously affected the performance of carbon materials in applications, especially in supercapacitors. As for supercapacitors, a high specific surface area and a large amount of macro- and mesopores will be advantageous to contact between materials and electrolytes and will greatly improve diffusivity of ions. In this paper, we carried out a series of experiments using pomelo peel as a carbon source, ammonia as a dopant, and KOH as an activating agent, and obtained porous framework-like N-doped carbon (PFNC) materials for supercapacitors. Porous framework-like carbon (PFC) is obtained when there is no dopant. The PFNC-based supercapacitor presents a specific capacitance of 260 F g−1 at 1 A g−1, while PFC presents only 230 F g−1 at 1 A g−1 in 2 M KOH electrolyte. There have been many studies on asymmetric supercapacitor devices based on carbon materials, but few works deal with the fabrication of asymmetric supercapacitor devices based on biomass carbon that retains a high power and energy density.35,36 Often, nickel oxide is used as the positive electrode when fabricating asymmetric supercapacitor configurations. Here, a convenient, rapid, and efficient method was performed to construct hierarchical porous flakelike NiO (PFN) arrays on the surface of Ni foam by a dipping and annealing process. Through the assembly of positive PFN and negative PFNC materials, the established asymmetric PFN//PFNC device displays a high specific capacitance and superior cycling performance at a high charge−discharge rate in a 2 M KOH solution, which illustrates great application potential.

2. EXPERIMENTAL SECTION 2.1. Preparation of PFC and PFNC. All used chemicals were analytical reagents. In a typical synthesis, the middle white layer of the pomelo peel was stripped, shredded, and then dried at 60 °C for 12 h in air. Then, 8 g of pomelo peel was poured into a 90 mL Teflon-lined stainless steel autoclave and immersed in 60 mL of deionized water. Afterward, the autoclave was heated to 160 °C and kept for 12 h in a box electric oven. Then, the obtained brown gel was dried in a box electric oven at 60 °C for 12 h. Subsequently, the brown powder was put into a corundum crucible and heated to 300 °C (ramp rate: 5 °C per minute) and maintained for 1.5 h in a Muffle furnace in air. After precarbonization, color of the product transformed from brown to black. Afterward, the mixture of black powder and KOH at a weight ratio of 1:3 were put into a graphite crucible and heated to 800 °C (ramp rate: 4 °C per minute) and maintained for 2 h under N2 flow in a tube electric furnace. Finally, the activated samples were soaked in a

3. RESULTS AND DISCUSSION 3.1. Properties of PFC and PFNC. Pomelo peel mainly consists of highly cross-linked polysaccharose and polysaccharose polyphenolic polymer, which are ideal precursors for carbon materials.38 Generally, KOH is a common activation 20823


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Figure 1. SEM images of PFC (a and b) and PFNC (c and d) at different magnifications.

Figure 2. XRD patterns (a) and Raman spectra (b) of PFC and PFNC.

reagent used to generate porous networks in carbons. It is wellknown that carbon reacts with KOH through solid−solid and solid−liquid reactions, which results in oxidation of carbon to carbon oxide and carbonate and reduction of potassium compounds to form metallic K.39 As a result, 3D porous framework-like carbon materials are developed. The schematic illustration process of PFC and PFNC is as shown in Scheme 1. In a typical synthesis, pomelo peel was first treated by a hydrothermal process to realize the cross-links. Then, precarbonation was carried out to further cross-link and remove part of the hydroxyl. Finally, graphitization and KOH activation were required in 800 °C to yield the porous framework-like carbon materials. To incorporate nitrogen heteroatoms, an ammonia solution was required in the hydrothermal process. SEM images in Figure 1 exhibit the microstructures of the obtained PFC and PFNC. Although there is no clear difference in the graphical morphologies, it is obvious that both PFC and PFNC possess 3D porous framework-like microstructures with dense and independent pores throughout all the bulk materials. It is reasonable that both the hydrothermal and KOH activation processes play vital roles in fabricating porous framework-like nanostructures. The unique porous nanostructures are of benefit to the accessibility of electrolytes and transportation of ions. Consequently,

electrochemical performance is greatly improved. The TEM and HR-TEM images in Figure S1 clearly show the porous Ndoped carbon frameworks, which are consistent with the SEM results. XRD patterns and Raman spectra were carried out to reveal the purity and nanostructures of PFC and PFNC, as indicated in Figures 2a and b. It is indicated that the XRD patterns of PFC and PFNC are similar. There are two weak peaks at 22.5° and 43.5° which are ascribed to {002} and {100} planes of graphitic carbon, respectively. Formation of carbon was verified by Raman shift of PFC and PFNC at 1350 and 1580 cm−1. The peak at 1350 cm−1 is ascribed to disordered carbon which is induced by sp3-bond carbon atoms, while the peak at 1580 cm−1 is ascribed to graphite in-plane vibrations40 induced by sp2-bond carbon atoms in a two-dimensional hexagonal graphitic layer. The ID/IG ratios for PFC and PFNC are 1.05 and 1.06, respectively, indicating both of the two products are composed of a partially graphitized structure and disordered carbon.41 Figure 3 exhibits the surface composition of PFNC, which was performed through XPS. Figure 3a confirmed that C, N, and O exist in the PFNC skeleton. The incomplete carbonization could lead to the presence of oxygen. As for the N 1s spectrum in Figure 3b, pyridinic, pyrrolic, and 20824


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Figure 3. XPS survey scan of PFNC (a), high-resolution N 1s (b), C 1s (c), and elemental mapping images of PFNC. Inset shows atomic percentage of C, O, and N for PFC and PFNC (d).

Figure 4. (a and d) CV curves of PFC and PFNC over a potential range from −0.8 to 0.2 V at different sweep rates. (b and e) GCD curves of PFC and PFNC in a three-electrode system. (c and f) Specific capacitances and cyclic stability of PFC and PFNC at 10 A g−1 for 10000 cycles.

could introduce pseudocapacitor behavior and improve electrochemical performance.44 As shown in Figures 4a and d, both PFC and PFNC exhibit rectangular-shaped CV curves from −0.8 to 0.2 V at various sweep rates, which are typical and ideal capacitive behaviors. Compared with those of PFC, it is apparent that the CV plots of PFNC exhibit a small peak at about −0.2 V, which is assigned to redox reactions of doped N atoms, and display a current density response higher than that of PFC. Further, GCD was used to further investigate the electrochemical performance in Figures 4b and e. The specific capacitance was calculated by GCD from 1 to 10 A g−1. For

quaternary nitrogen reside at 399.2, 400.2, and 401.4 eV, respectively.42 Concerning the high resolution C 1s spectrum in Figure 3c, the two peaks at 284.6 and 285.3 eV are ascribed to sp2-hybridized graphite-like carbon and sp3-hybridized diamond-like carbon, respectively, overlapping with the sp2 carbon bound to nitrogen. The peaks at 286.1, 287.4 and 288.7 eV are attributed to surface oxygen groups (designated as C−O, C O, and −COO, respectively).43 Additionally, it is clear that carbon, nitrogen, and oxygen exist in the PFNC skeleton uniformly based on the elemental mapping in Figure 3d. Usually, it is believed that heteroatoms in carbon materials 20825


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Figure 5. Nitrogen adsorption−desorption isotherms of PFC (a) and PFNC (b). Insets exhibit the pore size distribution and meso-pore proportion. Nyquist plots of PFC (c) and PFNC (d). Insets show equivalent circuit diagram and magnified high-frequency regions.

Scheme 2. Illustration of the Fabrication Processes of PFN and IFN Arrays

To analyze pore structure of PFC and PFNC, N 2 adsorption−desorption measurements were carried out. It was verified that PFC has a specific pore volume of 1.0083 cm3 g−1 and a specific surface area of 1727.7 m2 g−1, whereas PFNC has a specific pore volume of 0.8925 cm3 g−1 and a specific surface area of 1648.6 m2 g−1, as indicated in Figures 5a and b. The insets of Figures 5a and b show that the pore diameters of PFC and PFNC are mainly distributed at about 1.3 and 2.6 nm, which are classified as micropores and mesopores, respectively. The above evaluation confirms that both PFC and PFNC have a hierarchical porous nanostructure. Meanwhile, it is clear that PFC has a specific surface area higher than and a pore size smaller than PFNC. However, for

PFC and PFNC, the specific capacitances were 230 and 260 F g−1, respectively, at 1 A g−1. The specific capacitance will decrease slightly when increasing the GCD current density. It is normal for supercapacitors to not have enough time for electrolyte ions to move into pores. However, the PFNC electrode material still has a specific capacitance of 232 F g−1 even at 10 A g−1. The cyclic performance was evaluated at 10 A g−1 in Figures 4c and f, which demonstrated that PFNC had a better cyclic stability and reversibility. The specific capacitance comparison with other carbon-based supercapacitors is shown in Table S1. Further, PFNC still maintains a specific capacitance of 200 F g−1 after 10000 cycles. 20826


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Figure 6. SEM images of PFN (a and b) and IFN (c and d) nanostructures at different magnifications.

Figure 7. CV curves of PFN at different sweep rates (a). PFNC and PFN at 30 mv s−1 (b). PFN//PFNC asymmetric supercapacitor device at different sweep rates in a 2 M KOH electrolyte (c). GCD at 0.4, 0.6, 1, 2, 5, and 10 A g−1 (d).

Ω for PFNC. The charge transfer resistance between the electrolyte and electrode is calculated by the diameter of the circle in the high-frequency region, which is 0.53 Ω for PFC and 0.45 Ω for PFNC. Obviously, PFNC has a lower equivalent series resistance and charge transfer resistance than PFC. Further, the 45° region in the plot of PFNC is short, indicating a typical Warburg impedance. However, the 45° region in the plot of PFC is a little longer.45 Additionally, the Bode plots in Figure S2 show that the phase angle for PFNC reaches −86.8° in the low-frequency region, which approaches an ideal capacitor, while phase angle is only −85.1° for PFC. The

micropores, it is difficult for electrolytes entering the pore channels, especially for solvated ions. As a result, these mircopores are not favorable for improvement of electrochemical performance because of restricting fast ion transfer. Besides, EIS was performed to illustrate divergence between PFC and PFNC in a three-electrode system. As shown in the insets of Figures 5c and d, Nyquist plots were fitted by equivalent circuit models. In the high-frequency region, Nyquist plots of PFC and PFNC present a semicircle, while an inclined line is presented in the low-frequency region. The point of intersection between the semicircle and real axis signifies equivalent series resistance, which is 0.67 Ω for PFC and 0.64 20827


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Figure 8. (a) Specific capacitance and cyclic stability of the PFN//PFNC asymmetric supercapacitor device at 5 A g−1 for 5000 cycles. Nyquist plots of the PFN//PFNC asymmetric supercapacitor device before (b) and after (c) 5000 cyclic measurements. (d) Ragone plot of power density vs energy density of the PFN//PFNC asymmetric supercapacitor device. Inset shows the equivalent circuit diagram of the PFN//PFNC asymmetric supercapacitor device.

above results show that PFNC is more fit to be used as a supercapacitor electrode material than PFC. 3.2. Properties of Various Morphological NiO Arrays. Nickel oxide was often used as positive electrode when fabricating asymmetric supercapacitor configurations. Here, a convenient, rapid, and efficient method was performed to construct hierarchical PFN and IFN nanostructures on the surface of Ni foam by a dipping and annealing process, as shown in Scheme 2. First, CTAB mixed with Ni2+ and NO3− adhered on the surface of Ni foam. Then, Ni(NO 3 ) 2 discomposes into NiO and NO2 in the annealing process. Hierarchical PFN and IFN nanostructure arrays were obtained through controlling the concentration of the Ni(NO3) 2 solution. The morphological features of PFN and IFN were investigated using SEM and TEM techniques. Figures 6a and b show that PFN are aligned regularly on the surface of Ni foam, and each flake is composed of numerous nanosized particles and pores. In Figures 6c and d, it can be seen that hierarchical IFN is constructed. TEM images in Figure S3 further prove hierarchical nanostructures of PFN and IFN on Ni foam. The insets of Figure S3 are SAED patterns of PFN and IFN. Both of the SAED patterns are concentric rings, which are consistent with the simulated polycrystalline diffraction patterns of NiO. Moreover, Figure S4a shows XRD patterns of as-treated Ni foam, which confirms the formation of NiO nanostructures on the surface of the Ni foam. It is clearly noted that the diffraction peaks locate at 37.2, 43.3, 62.9, and 75.4°, corresponding to {111}, {200}, {220}, and {311} crystal planes of face-centered cubic NiO phase, respectively, are consistent with standard diffraction data of NiO (ICSD #47-1049) and similar to the reported literature.46 Apart from the NiO peaks, the peaks of Ni are also observed at 44.8, 52.2, and 76.8°, which are assigned to Ni foam. Figure S4b shows that PFN displays a specific

capacitance higher than that of IFN, which may be ascribed to the reason that each flake comprises many pores and NiO nanoparticles, and thus the unique nanostructure is convenient for electrolyte contact and ion diffuison. Hence, electrochemical behaviors of PFN nanostructures were further characterized by CV in a three-electrode system. Typical pseudocapacitor behavior at different sweep rates from 0 to 0.7 V is exhibited in Figure 7a. A pair of redox peaks in the CV plots are described as the redox reaction of Ni2+ ↔ Ni3+, which refers to NiO + OH− ↔ NiOOH + e−.47 Although the reduction and oxidation peak currents increase and locations of redox peaks shift with increasing sweep rates, the shapes of the CV curves display no significant changes, indicating excellent electrochemical performance of the PFN nanostructures. The electrochemical behaviors of PFNC and PFN were measured by CV, as shown in Figure 7b. PFNC and PFN are representatives of EDLC and pseudocapacitor materials, respectively, both of which possess typical energy storage mechanisms. Therefore, a PFN//PFNC asymmetric supercapacitor device was fabricated by employing positive PFN and negative PFNC materials. Figure 7c illustrates CV plots of the assembled PFN//PFNC device at various sweep rates from 5 to 50 mv s−1 with a potential window of 1.5 V. It is revealed that CV curves of the PFN//PFNC device exhibit a large current response and still retain their primary shape even at a sweep rate of 50 mv s−1 without evolution of hydrogen and oxygen, which confirms an efficient charge storage behavior. To evaluate electrochemical performance further, GCD tests were conducted at various current densities ranging from 0.4 to 10 A g−1, as shown in Figure 7d. It can be noted that the PFN// PFNC device delivers a specific capacitance of 88.8 F g−1 at 0.4 A g−1 and still maintains 44 F g−1 at 10 A g−1, indicating outstanding rate performance. For supercapacitors, cyclic 20828


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ACS Applied Materials & Interfaces stability is a key standard. Hence, the cycling stability of the PFN//PFNC device was investigated at 5 A g−1 in Figure 8a. The specific capacitance of the PFN//PFNC device decreases slightly from 50 to 40.1 F g−1 after 5000 cycles, with excellent capacitance retention of 80.2% of the initial value. The outstanding performance of the PFN//PFNC device may be attributed to hierarchical porous nanostructures of carbon and perfect flake-like nanostructures of PFN. Figures 8b and c display the Nyquist plots of the PFN//PFNC device at open circuit voltage after 10 and 5000 cycles, respectively. Also, the inset illustrates the equivalent circuit diagram. In the interface process, the intercept of the semicircle with the real axis in a high frequency is equivalent to the series resistance and diameter of the semicircle referring to charge transfer resistance. It was calculated that equivalent series resistance increases slightly from 1.5 to 2.3 Ω and charge transfer resistance increases slightly from 3.3 to 6.2 Ω in the fitting circuit. The slope in the lower frequency describes a diffusive resistance of electrolyte in the PFN//PFNC device. After 5000 cycles, the slope at the lower frequency shows almost no obvious change and suggests fast electrolyte ion diffusion, indicating excellent electrochemical performance of the PFN// PFNC device. For supercapacitors, energy and power density are two important parameters in estimating performance of supercapacitors. It can be noted that the PFN//PFNC device delivers an energy density of 27.75 Wh kg−1 at a power density of 300 W kg−1 and still remains at an energy density of 13.75 Wh kg−1 at power density of 7500 W kg−1 according to the total active material mass in Figure 8d. This performance surpasses most of the reported hierarchical NiO//carbon devices, such as NiO//carbon,48 NiO//AC,49 and N-PMNC// N-PNMC supercapacitors.33 Further, two PFN//PFNC devices in series were used to power two commercial red LEDs in parallel for more than 25 min, as shown in Figure S5. All of these results verify excellent electrochemical performance and great potential applications for the PFN//PFNC device.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Program (Grant 2011CB933300), the National Natural Science Foundation of China (Grants 51572102, 51271134, J1210061, and 51501132), the Fundamental Research Funds for the Central Universities, the CERS-1-26 (CERS-China Equipment and Education Resources System), the China Postdoctoral Science Foundation (Grant 2014T70734), the Self-Determined Research Funds of CCNU from the Colleges’ Basic Research and Operation of MOE (Grant CCNU15GF006).

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REFERENCES

(1) Wang, X. F.; Chen, Y.; Schmidt, O. G.; Yan, C. L. Engineered Nanomembranes for Smart Energy Storage Devices. Chem. Soc. Rev. 2016, 45, 1308−1330. (2) Zhi, M. J.; Xiang, C. C.; Li, J. T.; Li, M.; Wu, N. Q. Nanostructured Carbon-Metal Oxide Composite Electrodes for Supercapacitors: A Review. Nanoscale 2013, 5, 72−88. (3) Yan, C. L.; Xi, W.; Si, W. P.; Deng, J. W.; Schmidt, O. G. Highly Conductive and Strain-Released Hybrid Multilayer Ge/Ti Nanomembranes with Enhanced Lithium-Ion-Storage Capability. Adv. Mater. 2013, 25, 539−544. (4) Chen, Y.; Liu, L. F.; Xiong, J.; Yang, T. Z.; Qin, Y.; Yan, C. L. Porous Si Nanowires from Cheap Metallurgical Silicon Stabilized by a Surface Oxide Layer for Lithium Ion Batteries. Adv. Funct. Mater. 2015, 25, 6701−6709. (5) Wang, G. P.; Zhang, L.; Zhang, J. J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797−828. (6) Vijayakumar, S.; Nagamuthu, S.; Muralidharan, G. Supercapacitor Studies on NiO Nanoflakes Synthesized through a Microwave Route. ACS Appl. Mater. Interfaces 2013, 5, 2188−2196. (7) Sahu, V.; Shekhar, S.; Sharma, R. K.; Singh, G. Ultrahigh Performance Supercapacitor from Lacey Reduced Graphene Oxide Nanoribbons. ACS Appl. Mater. Interfaces 2015, 7, 3110−3116. (8) Yang, T. Z.; Qian, T.; Wang, M. F.; Shen, X. W.; Xu, N.; Sun, Z. Z.; Yan, C. L. A Sustainable Route from Biomass Byproduct Okara to High Content Nitrogen-Doped Carbon Sheets for Efficient Sodium Ion Batteries. Adv. Mater. 2016, 28, 539−545. (9) Yuan, A.; Zhang, Q. L. A Novel Hybrid Manganese Dioxide/ Activated Carbon Supercapacitor Using Lithium Hydroxide Electrolyte. Electrochem. Commun. 2006, 8, 1173−1178. (10) Qian, W. J.; Sun, F. X.; Xu, Y. H.; Qiu, L. H.; Liu, C. H.; Wang, S. D.; Yan, F. Human Hair-Derived Carbon Flakes for Electrochemical Supercapacitors. Energy Environ. Sci. 2014, 7, 379−386. (11) Kang, D. M.; Liu, Q. L.; Gu, J. J.; Su, Y. S.; Zhang, W.; Zhang, D. Egg-Box”-Assisted Fabrication of Porous Carbon with Small Mesopores for High-Rate Electric Double Layer Capacitors. ACS Nano 2015, 9, 11225−11233. (12) Gui, Z.; Zhu, H. L.; Gillette, E.; Han, X. G.; Rubloff, G. W.; Hu, L. B.; Lee, S. B. Natural Cellulose Fiber as Substrate for Supercapacitor. ACS Nano 2013, 7, 6037−6046. (13) Liu, Y.; Shi, Z. J.; Gao, Y. F.; An, W. D.; Cao, Z. Z.; Liu, J. R. Biomass-Swelling Assisted Synthesis of Hierarchical Porous Carbon

4. CONCLUSIONS In conclusion, we successfully fabricated a hierarchical PFNC using pomelo peel through a hydrothermal process in ammonia and activation in KOH. The obtained PFNC displays a specific pore volume of 0.8925 cm3 g−1 and a specific surface area of 1648.6 m2 g−1 due to numerous macro- and mesopores, which are convenient for ion diffusion. The obtained PFNC exhibits a specific capacitance of 260 F g−1 at 1 A g−1 and excellent cyclic performance at 10 A g−1. Further, an asymmetric supercapacitor device was optimized using positive PFN and negative PFNC materials. This PFN//PFNC device achieves a remarkable performance with a specific capacitance of 88.8 F g−1 at 0.4 A g−1 and an energy density of 27.75 Wh kg−1 at power density of 300 W kg−1 and still retains 44 F g−1 at 10 A g−1 and 13.75 Wh kg−1 at a power density of 7500 W kg−1, indicating outstanding energy storage performance. Additionally, this device is able to power two light-emitting diodes for 25 min, which is promising for next-generation energy storage devices.

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and SAED patterns of PFN and IFN, XRD patterns of PFN and IFN, and photographs of the PFN//PFNC device powering two commercial red LEDs (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06630. TEM and HR-TEM images of PFNC, Bodes plots of PFC and PFNC-based on supercapacitor, TEM images 20829


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Asymmetric Supercapacitor Based on Porous N ... - ACS Publications

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