Asymmetric Supercapacitor Based on Porous N-doped Carbon

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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, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06630 • Publication Date (Web): 19 Jul 2016 Downloaded from http://pubs.acs.org on July 20, 2016

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ACS Applied Materials & Interfaces

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

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

ABSTRACT: Three dimensional (3D) porous framework-like N-doped carbon (PFNC) with a high specific surface area is 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 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 power density of 7500 W kg-1. What’s important is that the device is able to supply 1 ACS Paragon Plus Environment


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two light-emitting diodes for 25 min, which demonstrates a greate potential for application.

KEYWORDS: pomelo peel, carbon, N-doped, NiO, arrays, asymmetric supercapacitor 1.

INTRODUCTION With explosive increase in the number of humans and dramatic improvement of

people’s requirement, there gradually appears many problems demanding prompt solution in the past years.1 Among all of the problems, the most concerned issue is energy crisis. In order 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 temperature in work, comparing with 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 electrode materials and electrolyte, while EDLCs 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 and excellent performance EDLCs material to adapt to the development of society.7 Highly 2 ACS Paragon Plus Environment

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porous carbonaceous materials derived from biomass energy8 stand out and are 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 aerogels16 and activated carbon,17 have displayed high EDLCs performances. Besides, heteroatoms doping such as nitrogen,18,19 boron,20 sulfur21,22 and phosphorus,23 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 6 M KOH electrolytes.27 Cao et al. recently announced the construction of hierarchical porous N-doped 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 co-doped carbon nanosheets through annealing boric acid and gelatin, which delivered a capacitance of 230 F g-1 at 0.1 A g-1 and energy density of 6000 Wh kg-1 in 1 M H2SO4 electrolyte.29 In comparison with traditional carbon source, it is the pursuit for low cost and environmental friendly precursors that promote 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 dustbin. In fact, peel of pomelo is a renewable material, which is suitable for mass production. There had been some researches on pomelo-derived cathode in lithium ion batteries (LIBs),30-32 nitrogen-doped interconnected carbon nanosheet 3 ACS Paragon Plus Environment


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electrode materials in supercapacitors33 and porous carbon in adsorbent materials.34 However, the above-mentioned carbon materials based on pomelo peel had a low specific surface and undeveloped pore nanostructures, which seriously affected performances of carbon materials in application, especially in supercapacitor. As for supercapacitor, a high specific surface and large amount of macro- and mesopores will be advantageous to contact between materials and electrolytes and improve diffusivity of ions greatly. In this paper, we carried out a series of experiments using pomelo peel as carbon source, ammonia as dopant and KOH as activating agent, and obtained porous framework-like N-doped carbon (PFNC) materials for supercapacitors. Porous framework-like carbon (PFC) would be obtained when there was no dopant. PFNC based supercapacitor presents a specific capacitance of 260 F g-1 at 1 A g-1 while only 230 F g-1 at 1 A g-1 for PFC in 2 M KOH electrolyte. There have been masses of studies on asymmetric supercapacitors device based on carbon materials, few works deal with the fabrication of asymmetric supercapacitor devices based on biomass carbon retain a high power density and energy density.35,36 Often, nickel oxide was used as positive electrode when fabricating asymmetric supercapacitor configuration. Here, a convenient, rapid and efficient method was performed to construct hierarchical porous flake-like NiO (PFN) arrays on surface of Ni foam by a dipping and annealing process. By 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 2 M KOH solution, which illustrates great potential for application. 4 ACS Paragon Plus Environment

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Scheme 1. Illustration of the fabricated processes of PFC and PFNC materials 2. EXPERIMENTAL SECTION 2.1. Preparation of PFC and PFNC. All used chemicals were analytical reagents. In a typical synthesis, the middle white layer in pomelo peel was stripped, shredded and then dried at 60 °C for 12 h in air. Then, 8 g pomelo peel was poured into a 90 mL Teflon-lined stainless steel autoclave and immersed in 60 mL deionized water. Afterwards, 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 pre-carbonization, color of product would be transformed from brown to black. Afterwards, 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 1 M HCl solution and then washed several times with deionized water. Then, PFC would be obtained after drying at 60 °C in an oven for 12 h. As for the synthesis of PFNC, all the procedures were kept the same besides substituting 10% ammonia solution for deionized water in hydrothermal process. 5 ACS Paragon Plus Environment


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2.2. Preparation of self-assembled 3D hierarchical flake-like NiO arrays. Ni foam was immersed in 0.25 M Ni(NO3)2 solution containing 3 mM hexadecyl trimethyl ammonium 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 three-dimension (3D) porous flake-like NiO (PFN) arrays would be obtained. 3D interconnected flake-like NiO (IFN) arrays would be achieved when using 0.5 M Ni(NO3)2 solution with other conditions all the same. 2.3. Materials Characterizations. The morphological images of PFC, PFNC, PFN and IFN were performed through field emission scanning electron microscopy (FESEM; Hitachi S-4800). Energy-dispersive spectroscopy (EDS) measurement was characterized by a field emission scanning electron microscope (FESEM; FEI SIRION). Crystallographic information on the as-prepared samples was recorded using Powder X-ray diffraction patterns (XRD; PANalytical Empyrean) loaded with Cu Kα radiation (a wavelength of 1.5418 Å). Bright-field (BF) images and selected area electron diffraction (SAED) patterns were performed inside the transmission electron microscope (TEM; JEOL JEM-2010). High resolution 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 performed by a (Renishaw Invia Raman microscope) mirco-Raman system loaded with a wavelength of 514.5 nm laser under ambient conditions. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method and the pore-size 6 ACS Paragon Plus Environment

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distribution

data

was

evaluated

using

the

Barrett-Joyner-Halenda

(BJH)

(Autosorb-iQASIQ). X-ray photoelectron spectroscopy (XPS) was performed by VG Multilab 2000 with 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) were carried out through a Parstat 4000 electrochemical workstation in 2 M KOH electrolyte with 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 iso-propyl 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, they were dried in a vacuum electric oven at 60 °C for 12 h. All of tests were carried out at room temperature. PFN and PFNC electrodes acted as positive and negative electrodes respectively when fabricating asymmetric supercapacitor device. A piece of commercial supercapacitor separator (MPF30AC-100) was employed as the supercapacitor membrane. According to 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. 3. RESULTS AND DISCUSSION 3.1. Properties of PFC and PFNC Pomelo peel mainly consists of highly cross-linked polysaccharose and 7 ACS Paragon Plus Environment


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polysaccharose polyphenolic polymer, which are ideal precursors for carbon materials.38 Generally, KOH is a common activation reagent to generate porous network in carbons. It is well known 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 compound to form metallic K.39 As a result, 3D porous framework-like carbon materials will be developed. The schematic illustration process of PFC and PFNC is as shown in Scheme 1. In a typical synthesis, pomelo peel was firstly treated by a hydrothermal process to realize crosslink. Then, pre-carbonation was carried out to further crosslink and remove part of hydroxyl. Finally, graphitization and KOH activation were required in 800 °C to yield the porous framework-like carbon materials. To incorporate nitrogen heteroatoms, ammonia solution was required in the hydrothermal process. SEM images in Figure 1 exhibit the microstructures of 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 hydrothermal process and KOH activation process play vital roles in fabricating porous framework-like nanostructures. The unique porous nanostructures are of benefit to accessibility of electrolyte and transportation of ions. Consequently, electrochemical performance will be improved greatly. The TEM and HR-TEM images in Figure S1 clearly shows the porous N-doped carbon frameworks, which is consistent with the SEM results. 8 ACS Paragon Plus Environment

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

100

2000 1500 1000 500

400

1580

2500

480 Intensity/a.u.

002

3500 3000

(b) 560

PFC PFNC

1350

(a) 4000

Intensity/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PFC PFNC

320 240 160 80

0 20

30

40 50 60 2θ/degree

70

0

80

1000

1500 2000 Raman shift/cm-1

2500

Figure 2. XRD patterns (a) and Raman spectra (b) of PFC and PFNC. XRD patterns and Raman spectra were carried out to reveal the purity and nanostructures of PFC and PFNC, as indicated in Figure 2a and b. It is indicated that XRD patterns of PFC and PFNC are similar. There are two weak peaks at 22.5° and 43.5°, 9 ACS Paragon Plus Environment


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 cm-1 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 which is induced by sp2-bond carbon atoms in a two-dimension hexagonal graphitic layer. The ID/IG ratio for PFC and PFNC are 1.05 and 1.06 respectively, indicating both of the two products are composed of partial graphitized structure and disordered carbon.41 (a) 7000

6000

C 1s

O 1s

Element C (At%) O (At%) N (At%)

PFN PFNC 87.6 88.2 12.4 7.9 0 3.9

4000 3000 N 1s

2000

(b)

690 Intensity/a.u.

Intensity/a.u.

5000

660

400.2 eV

N 1s Piridinic-N Pyrrolic-N Quaternary-N

399.2 eV 401.4 eV

630

1000 0

0

200

400

600

800

600

1000 1200

398

Binding Energy/eV

404

C 1s

1400

C-C sp3

1200 284.6 eV 1000 800 600

285.3 eV 286.1 eV 287.4 eV

400 200 0

400 402 Binding Energy/eV

(d)

(c) 1600

Intensity/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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C-C sp2 C-O C=O -COO

C

288.7 eV

N 284

O

288 292 Binding Energy/eV

Figure 3. XPS survey scan of PFNC (a), high resolution N 1s (b), O 1s (c) and element mapping images of PFNC, inset shows atomic percentage of C, O and N for PFN and PFNC (d). Figure 3 exhibits the surface composition of PFNC, which was performed through 10 ACS Paragon Plus Environment

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XPS. Figure 3a confirmed that there exist C, N, and O elements in PFNC skeleton. The incomplete carbonization could lead to the presence of oxygen. As for N 1s spectrum in Figure 3b, pyridinic, pyrrolic and quaternary nitrogen reside in 399.2 eV, 400.2 eV and 401.4 eV respectively.42 Concerning the high resolution C 1s spectrum in Figure 3c, the two peaks at 284.6 eV and 285.3 eV are ascribed to sp2-hybridized graphite-like carbon and sp3-hybridized diamond-like carbon respectively, overlapping with sp2 carbon bound to nitrogen. The peaks at 286.1 eV, 287.4 eV 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 PFNC skeleton uniformly based on the element mapping in Figure 3d. Usually, it is believed that heteroatom in carbon materials could introduce pseudocapacitor behavior and improve electrochemical performance.44 As shown in Figure 4a and d, both PFC and PFNC exhibit rectangular shaped CV curves from -0.8 V to 0.2 V at various sweep rates, which are typical and ideal capacitive behaviors. Comparing with PFC, it is apparent that 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 higher current density response than PFC. Besides, GCD was used to further investigate the electrochemical performance in Figure 4b and e. The specific capacitance is calculated by GCD from 1 to 10 A g-1. For PFC and PFNC, the specific capacitances were 230 F g-1 and 260 F g-1 respectively at 1 A g-1. The specific capacitance will decrease slightly when increasing GCD current density. It is normal for supercapacitors that there is no enough time for electrolyte ion into pores. But PFNC electrode material still has a 11 ACS Paragon Plus Environment


specific capacitance of 232 F g-1 even at 10 A g-1. The cyclic performance was evaluated at 10 A g-1 in Figure 4c and f, which demonstrated PFNC had a better cyclic stability and reversibility.

The

specific

capacitance

comparison

with

other

carbon

based

supercapacitors is shown in Table S1. What’s more, PFNC still maintains a specific capacitance of 200 F g-1 after 10000 cycles.

-10 -20

-0.2 -0.4 -0.6

-30 -0.8

-0.6

-0.4 -0.2 Potential/V

0.0

(d) 40 30

10 mv/s 30 mv/s 50 mv/s 80 mv/s

20

-0.8

0.2

0 -10

100

200 300 Time/s

400

1 A/g 2 A/g 5 A/g 10 A/g

0.2 0.0 -0.2 -0.4

-20 -0.6

-30 -40

-0.8

-0.6

-0.4 -0.2 Potential/V

0.0

0.2

-0.8

0

100

200

300 Time/s

400

500

200

100

180

90

160

80

140 70

120 100

500

(e) 0.4

Potential/V

10

0

110 220

0

2000

4000 6000 Cycle Number

8000

60 10000

(f) 240

110 100

210 90 180 80 150 70 120 0

2000

4000 6000 Cycle Number

8000

60 10000

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

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Capacity retention/%

0

0.0

(c)

Capacity retention/%

10

1 A/g 2 A/g 5 A/g 10 A/g

0.2

Specific capacitance/(F/g)

20

(b) 0.4


10 mv/s 30 mv/s 50 mv/s 80 mv/s

30

Potential/V

Current density/(A/g)

(a)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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PFC

600 500

Mesopore: 40.3%

0.4

400 300 200 100

0.3 0.2 0.1 0.0 1.0

0

0.0

1.5

2.0 2.5 Pore width/nm

3.0

0.2 0.4 0.6 0.8 Relative pressure/(P/P0)

(c) 14

3.5

0.8

10

0.6

8

0.4

4

0.2

0.4

0.6

0.8

1.0

Mesopore: 92.2%

0.30

300 200 100

0.25 0.20 0.15 0.10 0.05 0.00

-0.05

0

0

2

4

10

1.0

1.2

Primary Fitting

1.0

1.2

8

0.2 0.4 0.6 0.8 Relative pressure/(P/P0)

12

0.8

10

0.6

8

0.4 0.2

6

0.0 0.0

4

2

0.2

0.4

0.6

0.8

1.0

1.2

2 Wo

0

6

Pore width/nm

(d) 14

Primary Fitting

0.0 0.0

0.35

400

0.0

0.2

6

500

-Z''/ohm

12

PFNC

1.0

1.2 1.0

(b) 600

Pore volume/ (cm3/g)

700

Pore volume/ (cm 3/g)

Volume adsorbed/(cm3 g-1 STP)

(a)

-Z''/ohm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Volume adsorbed/(cm3 g-1 STP)

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0

2

4

6 8 Z'/ohm

10

Wo

12

0

14

0

2

4

6

8 Z'/ohm

10

12

14

Figure 5. Nitrogen adsorption desorption isotherms of PFC (a) and PFNC (b), inset exhibits pore size distribution and meso-pore proportion; Nyquist plots of PFC (c) and PFNC (d), inset shows equivalent circuit diagram and magnified high frequency regions. To analyze pore structure of PFC and PFNC, N2 adsorption desorption measurement was carried out. It is 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 0.8925 cm3 g-1 and 1648.6 m2 g-1 for PFNC, as indicated in Figure 5a and b. Insets of Figure 5a and b show pore diameters of PFC and PFNC are mainly distributed in about 1.3 nm and 2.6 nm which are classified as micropores and mesopores respectively. The above evaluates confirm that both PFC and PFNC have a hierarchical porous nanostructure. Meanwhile, it is clear that PFC has a higher specific surface area and smaller pore size than PFNC. However, as to micropores, 13 ACS Paragon Plus Environment


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it will be difficult for electrolyte entering channels of pores, especially for solvated ions. As a result, these mircopores will be 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 Figure 5c and d, Nyquist plots were fitted by equivalent circuit models. In high frequency region, Nyquist plots of PFC and PFNC present a semicircle, while an inclined line in low frequency region. The point of intersection between semicircle and real axis signifies equivalent series resistance, which is 0.67 Ω for PFC and 0.64 Ω for PFNC respectively. Charge transfer resistance between electrolyte and electrode is calculated by diameter of circle in high frequency region, which is 0.53 Ω for PFC and 0.45 Ω for PFNC respectively. Obviously, PFNC has a lower equivalent series resistance and charge transfer resistance than PFC. What’s more, the 45° region in plot of PFNC is short, indicating a typical Warburg impedance. However, 45° region in plot of PFC is a little longer.45 Besides, Bode plots in Figure S2 show that phase angle for PFNC reaches -86.8° in low frequency region which approaches an ideal capacitor while phase angel is only -85.1° for PFC. The above results manifest that PFNC is more fit for supercapacitor electrode materials than PFC. 3.2. Properties of various morphological NiO arrays Nickel oxide was often used as positive electrode when fabricating asymmetric supercapacitor configuration. Here, a convenient, rapid and efficient method was performed to construct hierarchical PFN and IFN nanostructures on surface of Ni foam 14 ACS Paragon Plus Environment

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by a dipping and annealing process, as shown in Scheme 2. Firstly, CTAB mixed with Ni2+ and NO3- and adhered on the surface of Ni foam. Then, Ni(NO3)2 would discompose into NiO and NO2 in annealing process. Hierarchical PFN and IFN nanostructures arrays would be obtained through controlling concentration of Ni(NO3)2 solution. The morphological features of PFN and IFN were investigated using SEM and TEM techniques. Figure 6a and b shows that PFN are aligned regularly on surface of Ni foam and each flake is composed of numerous nanosized particles and pores. In Figure 6c and d, it is displayed that hierarchical IFN is constructed. TEM images in Figure S3 further prove hierarchical nanostructures of PFN and IFN on Ni foam. Insets of Figure S3 are SAED patterns of PFN and IFN respectively. Both of SAED patterns are concentric rings, which are consistent well with simulated polycrystalline diffraction patterns of NiO. Moreover, Figure S4a shows XRD patterns of as-treated Ni foam, which confirms formation of NiO nanostructures on surface of Ni foam. It is clearly noted that 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, which are well consistent with standard diffraction data of NiO (ICSD # 47-1049) and similar to the reported literature.46 Apart from 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 higher specific capacitance than IFN, which may be ascribed to the reason that each flake comprises many pores and NiO nanoparitcles, and thus the unique nanostructure is convenient for electrolyte contact and ion diffuison. 15 ACS Paragon Plus Environment


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Scheme 2. Illustration of fabrication processes of PFN and IFN arrays

Figure 6. SEM images of PFN (a, b) and IFN (c,d) nanostructures at different magnifications. 16 ACS Paragon Plus Environment

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40 20

(b) 60

5 mv/s 10 mv/s 30 mv/s 50 mv/s



(a) 60

0 -20 -40

PFNC PFN

40 20 0 -20 -40

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5 mv/s 10 mv/s 30 mv/s 50 mv/s

0.4 A/g 0.6 A/g 1 A/g 2 A/g 5 A/g 10 A/g

1.4 1.2 Potential/V


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 -5

1.0 0.8 0.6 0.4

-10

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-15 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Potential/V

0.0

0

100 200 300 400 500 600 700 800 Time/s

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 2 M KOH electrolyte (c) and GCD at 0.4, 0.6, 1, 2, 5, 10 A g-1 (d). Hence, electrochemical behaviors of PFN naostructures were further characterized by CV in a three electrode system. It is exhibited a typical pseudocapacitor behavior at different sweep rates from 0 V to 0.7 V in Figure 7a. A pair of redox peaks in CV plots are described as redox reaction of Ni2+ ↔ Ni3+, which referres to NiO + OH- ↔ NiOOH + e-.47 Although reduction and oxidation peak currents increase and locations of redox peaks shift when increasing sweep rates, shapes of CV curves display no significant changes, instructing excellent electrochemical performances of PFN naostructures. The electrochemical behaviors of PFNC and PFN were measured by CV, as shown in Figure 17 ACS Paragon Plus Environment


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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 mv s-1 to 50 mv s-1 with a potential windiow of 1.5 V. As it is revealed that, CV curves of PFN//PFNC device exhibit a large current response and still retain primary shape even at a sweep rate of 50 mv s-1 without evolution of hydrogen and oxygen, which confirms an efficient charge sotrage behavior. To evaluate electrochemical performance further, GCD tests were conducted at various current density ranging from 0.4 A g-1 to 10 A g-1, as shown in Figure 7d. It is 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 stability is a key standard. Hence, the cycling stabiltiy of the PFN//PFNC device was investigated at 5 A g-1 in Figure 8a. It is obtained that the specific capacitance of PFN//PFNC device decreases slightly from 50 F g-1 to 40.1 F g-1 after 5000 cycles, with excellent capacitance retention of 80.2% of 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. Figure 8b and c dispaly the Nyquist plots of the PFN//PFNC device at open circuit voltage after 10 cycles and 5000 cycles respectively. Also, the inset illustrates the equivalent circuit diagram. In the interface process, intercept of semicircle with real axis in high frequcency 18 ACS Paragon Plus Environment

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is equivalent series resistance and diameter of the semicircle refers to charge transfer resistance. It is calculated that equivalent series resistance increases slightly from 1.5 Ω to 2.3 Ω and charge transfer resistance increases a little from 3.3 Ω to 6.2 Ω in the fitting circuit. The slope in lower frequcency describes a diffusive resistance of electrolyte in PFN//PFNC device. After 5000 cycles, the slope at lower frequcency nearly shows no obvious change and suggests fast electrolyte ion diffusioin, instructing excellent electrochemical performance of PFN//PFNC device. For supercapacitors, energy density and power density are two important paramaters in estimating peformances of supercapacitors. It is noted that the PFN//PFNC device delivers an energy density of 27.75 Wh kg-1 at power density of 300 W kg-1, and still remains an energy density of 13.75 Wh kg-1 at power density of 7500 W kg-1 according to total active material mass, in Figure 8d. This performance surpasses most of reported hierarchical NiO//carbon device, such as NiO//carbon,48 NiO//AC49 and N-PMNC//N-PNMC supercapacitors.33 What’s more, two PFN//PFNC devices in series were used to power two commercial red LEDs in parellel for more than 25 min, as shown in Figure S5. All the results verify excellent electrochemical performance and great potential application of the PFN//PFNC device.



(b)

45 90 40 35 80

30 25

(c)

0

1000

2000 3000 4000 Cycle number

0

Energy density/(Wh

40 30 20 10 0

10

20

30 40 Z'/ohm

50

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20 Z'/ohm

30

40

(d) 50 40

Primary Fitting

50

0

20 10

5000

70 60

30 -Z''/ohm

50

Primary Fitting

40 Capacitance retention/%

100

kg-1)


(a) 60 55

-Z''/ohm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30 This work

20

10

10

100 1000 Power density(W kg-1)

10000

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

4. CONCLUSIONS In conclusion, we have successfully fabricated hierarchical PFNC using pomelo peel through 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 20 ACS Paragon Plus Environment

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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. Besides, an asymmetric supercapacitor device is 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 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 power density of 7500 W kg-1, indicating outstanding energy storage performance. What’s more, this device is able to light up two light-emitting diode for 25 min, which is promsing for next generation energy storage devices. ASSOCIATED CONTENT Supporting Information. TEM and HR-TEM images of PFNC, Bodes plots of PFC and PFNC based on supercapacitor, TEM images and SAED patterns of PFN and IFN, XRD patterns of PFN and IFN and photographs of PFN//PFNC device powering two commercial red LEDs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (J. Wang) * E-mail: [email protected] (Y. Tang) 21 ACS Paragon Plus Environment


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the 973 Program (2011CB933300), the National Natural Science Foundation of China (51572102, 51271134, J1210061, 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 (2014T70734), the Self-determined Research Funds of CCNU from the Colleges’ basic Research and the Operation of MOE (CCNU15GF006). REFERENCES (1)

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Asymmetric Supercapacitor Based on Porous N-doped Carbon

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