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N-Enriched Hollow Porous Carbon Nanospheres with Tailored Morphology and Microstructure for All-Solid-State Symmetric Supercapacitors Ziyang Song, Dazhang Zhu, Danfeng Xue, Jingjing Yan, Xiaolan Chai, Wei Xiong, Zhiwei Wang, Yaokang Lv, Tongcheng Cao, Mingxian Liu, and Lihua Gan ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00928 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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ACS Applied Energy Materials

N-Enriched Hollow Porous Carbon Nanospheres with Tailored Morphology and Microstructure for All-Solid-State Symmetric Supercapacitors Ziyang Song,† Dazhang Zhu,† Danfeng Xue,† Jingjing Yan,† Xiaolan Chai,† Wei Xiong,‡ Zhiwei Wang,§, ‖ Yaokang Lv,⊥ Tongcheng Cao, †,# Mingxian Liu*, †,‖, and Lihua Gan*, † †

Biomedical Multidisciplinary Innovation Research Institute, Shanghai East Hospital, Shanghai

Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, P. R. China. ‡

School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan

430073, P. R. China. §

State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental

Science and Engineering, Tongji University, Shanghai 200092, P. R. China. ‖

Shanghai Institute of Pollution Control and Ecological Security, Tongji University, Shanghai

200092, P. R. China. ⊥

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, P. R.

China. #

Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji University,

Shanghai 201804, P. R. China.

ACS Paragon Plus Environment

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ABSTRACT: We develop a straightforward and highly efficient approach to design N-enriched hollow porous carbon nanospheres (N-HPCNs) based on the self-polymerization of dopamine in mixed water/ethanol solvents. The control of the morphology, pore parameters and nitrogen content of N-HPCNs can be effectively achieved by simple tuning of the volume ratios of water/ethanol and the amounts of the solvents. The representative N-HPCNs exhibit the characteristics of spherical and hollow geometry with a uniform diameter (~400 nm), high surface area (1789 m2 g−1), abundant ultramicropores with reasonable supermicro- and mesopores, and high-level nitrogen content (up to 6.86 wt.%). N-HPCNs as supercapacitor electrodes in 6 M KOH electrolyte show outstanding electrochemical performances including a high specific capacitance (353 F g−1 at 0.5 A g−1), superb rate capability (214 F g−1 at 20 A g−1) and excellent cycling stability (91.9% capacitance retention at 1.0 A g−1 after 10,000 cycles). Furthermore, the assembled symmetric all-solid-state supercapacitor based on N-HPCN electrodes and polyvinyl alcohol/KOH gel electrolyte exhibits high integrated energy-power density of 10.42 W h kg−1 at 250 W kg−1. This study provides promising prospects to design Ndoped

carbons

with

tailored

morphology

and

microstructure

for

high-performance

supercapacitors.

KEYWORDS: Hollow porous carbon nanosphere; High-level nitrogen-doping; Polydopamine; Morphology and microstructure modulation; Supercapacitor


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1. INTRODUCTION Nowadays, the energy consumption and exploitation of fossil fuels are increasing sharply, causing climate change and environmental destruction.1-6 In order to solve the series of problems, it is imminent to develop new types of energy conversion and storage systems which are environmentally friendly, highly efficient and sustainable.7-10 Due to their advantages of highrate capability, high power density, superb cycle stability and product safety,11-15 supercapacitors have been widely explored as highly efficient energy storage systems for hybrid electric vehicles, portable consumer electronics and backup power supply devices, etc.16-22 According to different energy storage mechanisms, supercapacitors can be divided into pseudocapacitors and electric double layer capacitors (EDLCs).23 The energy storage in pseudocapacitors is achieved from reversible redox reactions on the surface of electroactive materials,24-25 which generally suffers the demerits of unsatisfied cycle stability. While EDLCs stores energy through the accumulation of electrostatical charges at the interface of active materials/electrolyte.26-27 With the features of abundant source, low cost, good conductivity and stability, and outstanding service life,28 carbon materials presently are the most used electroactive materials for EDLCs which represent more than 80% of the commercially available supercapacitors.29 High surface area of carbon materials is a key concern to achieve large capacitance for EDLCs.30-31 However, one major demerit existed in carbon electrodes is that the electrochemical capacitance is still unsatisfied even if at an ultrahigh surface area above 3000 m2 g−1 because of low utilization resulted from irregular or unreasonable pore structure,32 which causes much difficulty to their widespread applications.33 Therefore, there are considerable efforts to achieve efficient charge storage and ion diffusion kinetics in porous carbons through controlling over their pore structure, morphology and functionality.34-36 Porous carbons with suitable micro- and


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mesopore architecture have been developed to improve the capacitive properties because that micropores contribute to high electrochemical capacitance and energy density due to the high capability for interfacial charge accommodation,37-38 and mesopores provide high transport channels for electrolyte ions, especially at a fast charge–discharge process.39-40 On the other hand, hollow porous carbon nanospheres (HPCNs) with diameter-controllable spherical morphology and adjustable hollow architecture are expected as innovative and promising electrode materials for advanced EDLCs.41 Exterior package porosity among HPCNs as well as the interior hollow cavity can serve as dual ion buffer reservoirs which shorten the ion diffusion paths for fast charge transportation into the internal pore channels.42 Besides the regulation of the pore structure and morphology, heteroatom (e.g., N, O, P and B) doping is another efficient means to enhance the electrochemical behaviors,43-46 which not only can significantly improve electrical conductivity and the wettability of porous carbons,47 but also offer additional Faraday capacitance.48-49 Generally, N doping could be achieved via direct carbonization of nitrogen-containing precursors or post-treatment of carbon materials under a specific chemical atmosphere.23 In-situ self-doping strategy with the advantages of controlled nitrogen dopant content, homogenous heteroatom distribution and stable porosity in the final carbons has promisingly substituted the tedious post-treatment mean which may give rise to geometry and structure defects such as pore collapse or blockage in the carbons.50 Considering the above-introduced requirements for ideal EDLC electrodes, the introduction of self-doped and high-level nitrogen species into high-surface-area, and micro- and mesoporous HPCNs is highly attractive, but remains a challenge. Dopamine has unique features such as selfpolymerization into spherical polymer under mild condition, in-situ doping of high content nitrogen heteroatoms after carbonization, and effective tailoring of pore architecture through


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pore directing agent.51-52 Herein, we establish a novel strategy to fabricate N-enriched HPCNs (N-HPCNs) from dopamine for high-performance symmetric all-solid-state supercapacitors. The morphology, pore structure parameters and nitrogen contents of N-HPCNs can be effectively modulated via the simple control of the ratios of water/ethanol and the solvent amounts during the self-polymerization of dopamine. The resultant N-HPCNs combine with the merits of uniform and hollow hydrangea-like geometry, ultrahigh surface area together with micro- and mesopores, and high nitrogen dopant. Owing to these superiorities, N-HPCN electrode exhibits a high gravimetric capacitance, superb rate capability and cycling stability in KOH electrolyte. Besides, an assembled symmetric all-solid-state supercapacitor has a superior energy density of 10.42 W h kg−1 at a power density of 250 W kg−1. The present study offers a straightforward and efficient approach to manipulate the geometry and microstructure of carbon-based materials for energy storages.

2. Experimental Section 2.1. Fabrication of N-HPCNs Typically, 0.6 mL ammonia aqueous solution (25 wt.%) was added into 120 mL water-ethanol solution (v/v, 3:1). Then, 2 mL tetraethyl orthosilicate (TEOS) was added into the mixture under stirring for 15 min. Into this mixture, 0.5 g of dopamine hydrochloride dissolved in 10 mL of water was added under stirring for 24 h. The obtained SiO2@polydopamine was collected by filtration, washed and dried, followed by carbonization at 800 °C for 3 h under a nitrogen flow (2 °C min−1) to prepare SiO2@C (denoted as SiO2@C-x-y where hereinafter x and y refer to the volume of water and ethanol, respectively). After the etching of silica using 3 M KOH solution, the obtained carbons were mixed with KOH in a weight ratio of 2:1, followed by activation under 700 °C (1 h, N2 flow) for fabricating N-HPCNs (referred to N-HPCN-x-y).


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2.2. Characterization The morphologies were observed by a scanning electron microscopy (SEM, Hitachi S-4800) and a transmission electron microscopy (TEM, JEM-2100). Nitrogen sorption were conducted using an ASAP 2460 analyzer (Micromeritics) at liquid nitrogen temperature (−196 °C). The surface areas were obtained using the Brunauer–Emmett–Teller method within P/P0 = 0.05−0.25. Nonlocal density functional theory equilibrium model was utilized to estimate the pore size distribution. Powder X-ray diffraction (XRD) analysis was done on Focus D8 Advance diffractometer (Bruker) with Cu Kα radiation (λ = 0.154 nm). Raman spectra (Renishaw Invia) were measured using 514 nm laser excitation. The chemical element composition and nitrogen valence were obtained using an AXIS Ultra DLD X-ray photoelectron spectrometer (XPS). 2.3. Electrochemical Measurement The electrochemical characterizations were first conducted in a three-electrode system (CHI660D electrochemical workstation). The working electrode was prepared by mixing carbon materials, graphite and polytetrafluoroethylene (8:1:1, w/w) dispersion in ethanol. The dried mixture was pressed onto nickel foam (1 cm2) and then a 0.25 mm thick circle electrode (with a diameter of 0.5 cm) was dried overnight at 100 ºC (the active material mass on electrode was about 4 mg). Pt was employed as a counter electrode and Hg/Hg2Cl2 electrode as a reference electrode. Cyclic voltammetry (CV), gravimetric charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) were analyzed in 6 M KOH as electrolyte. The electrochemical capacitances were calculated using the discharge curves based on the formula:

Cm =

I × ∆t m × ∆V

(1)


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where Cm is the capacitance (F g−1), I is the discharge current (A), ∆t represents the discharge time (s), ∆V is the range of charge-discharge voltage (V) and m is the total mass of active material (g). The electrochemical behaviors of N-HPCN electrodes were further investigated by a twoelectrode configuration with a gel electrolyte. Typically, 2.0 g polyvinyl alcohol (PVA) powder was dissolved in 20 mL water under stirring at 85 ºC until a transparent PVA solution was formed and then cool to room temperature. Into the mixture, an excessive 6 M KOH aqueous solution was added dropwise. Finally, two N-HPCN electrodes with the same mass were immersed in PVA/KOH gel for 24 h, and they were packaged into an all-solid-state device. CV and GCD tests were measured in a voltage window between 0–1 V. The capacitances were calculated using the discharge curves according to the equation: Cs = 4Ccell =

4 ×I × ∆t (2) m × ∆V

where Cs is the single electrode capacitance (F g–1), Ccell is the gravimetric capacitance of the symmetric supercapacitor device (F g–1), I, ∆t and ∆V show the same denotation with Equ. (1), and m is the total mass of active materials (g). The energy density (E, W h kg–1) and power density (P, W kg–1) of the device were calculated using the equations:

E =

1 Ccell ∆V 2 (3) 7.2

P =

E × 3600 (4) ∆t

3. RESULTS AND DISCUSSION Figure 1 exhibits SEM images for SiO2@C and N-HPCNs. Using water as a solvent, the obtained SiO2@C-120-0 exhibits regular spherical morphology with a uniform diameter of ~360


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nm (Figure 1a). With a substitute of ethanol for equal volume water (i.e., remaining unchanged total volumes of water/ethanol), the diameters of SiO2@C nanospheres increase to 400 (SiO2@C-90-30) and 420 nm (SiO2@C-60-60), accompanied with decreased homogeneity (Figure 1b, c). Further increase the ethanol volume brings cohesion and less dispersion and regularity of SiO2@C-30-90 nanospheres (Figure 1d). These SiO2@C nanospheres show definite core-shell structures with increased cores of SiO2 particles (from about 300 to 430 nm) and decreased carbon shells (from about 27 to 17 nm) when the ethanol volume increase from 0 to 90 mL (Figure 2a−d). After the etching of SiO2 template, N-HPCNs prepared in case of pure or majority of water as the solvent (N-HPCN-120-0 and N-HPCN-90-30) still exhibit spherical geometry, but with highly wrinkled surfaces like hydrangeas (Figure 1f and g). TEM images (Figure 2f and g) indicate these hydrangea-like carbons have hollow structures. While N-HPCN60-60 and N-HPCN-30-90 have very thin carbon shells (20 and 17 nm) which could not enough support the framework during the template removal, leading to the collapse of the spherical architecture (Figure 1h, i and Figure 2h, i). In absence of water (ethanol as a sole solvent), SiO2@C-0-120 spheres show highly mutual adhesion with a large difference in particle size from nanometer to micrometer scale (Figure 1e). N-HPCN-0-120 inherits the spherical geometry with inhomogeneity of SiO2@C-0-120 in size (Figure 1j, Figure 2j) because small hollow cavities resulted from SiO2 cores and very thick carbon shells withstand the contraction. However, it is difficult to observe the core-shell structure of SiO2@C-0-120 from Figure 2e due to the small cores and thick carbon layers.


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Figure 1. SEM images of SiO2@C-120-0 (a), SiO2@C-90-30 (b), SiO2@C-60-60 (c), SiO2@C30-90 (d), SiO2@C-0-120 (e), N-HPCN-120-0 (f), N-HPCN-90-30 (g), N-HPCN-60-60 (h), NHPCN-30-90 (i), and N-HPCN-0-120 (j). The water/ethanol ratio has a crucial influence on the morphology of SiO2@C and the resultant N-HPCNs. The solubility parameter is an important factor for the interaction between the solvent and the polymer precursor.53-54 Water and ethanol have a solubilizability (δ) value of 23.4 and 12.9 (cal cm–3)1/2, respectively.55 The δ value of the mixed water/ethanol double-solvent system


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can be calculated based on the formula δ = δ1φ1 + δ1φ1, in which δ1 and δ2 are the solubility parameters of each component, and φ1 and φ2 are the corresponding volume fractions. Compared with ethanol, water shows stronger swelling and stabilizing ability for polydopamine. According to the basic theory of similar dissolve mutually,56 in the self-polymerization of dopamine, the solubility parameters of the mixed solution gradually decrease with the increase of ethanol/water ratio, which causes decreased stabilizing ability of dopamine and consequently increased inhomogeneity of SiO2@C spheres.

Figure 2. TEM images of SiO2@C-120-0 (a), SiO2@C-90-30 (b), SiO2@C-60-60 (c), SiO2@C30-90 (d), SiO2@C-0-120 (e), N-HPCN-120-0 (f), N-HPCN-90-30 (g), N-HPCN-60-60 (h), NHPCN-30-90 (i), and N-HPCN-0-120 (j).


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Besides the influence of water/ethanol ratios, the amounts of the mixed solvents also have an important influence on the morphologies of final nitrogen-doped carbons. Figure 3 shows typical SEM images of SiO2@C and N-HPCNs prepared by tuning the amount of the solvents (keeping the water/ethanol ratio of 3). With increased solvent amount, the obtained SiO2@C nanospheres show increased diameters from 370 (SiO2@C-60-20, Figure 3a), 400 (SiO2@C-90-30, Figure 1b) to 550 nm (SiO2@C-120-40, Figure 3b). The resultant N-HPCN-60-20 (and N-HPCN-90-30) basically keeps the spherical morphology with a hydrangea-like geography, as shown in Figure 3c. While N-HPCN-120-40 exhibits deflated spherical geometry (Figure 3d) like N-HPCN-60-60 and N-HPCN-30-90. These N-HPCNs also show hollow architecture, as shown in Figure S1. NHPCNs present two relatively broad diffraction peaks in the XRD patterns (Figure S2a), corresponding to amorphous carbons. Raman spectra of N-HPCNs (Figure S2b) exhibit a D peak (1350 cm−1) and a G peak (1580 cm−1), which reflects respectively the defects and the vibration of sp2 graphitic structures in carbons.57 The ID/IG values (0.81–0.91) indicate amorphous structures of N-HPCNs.58


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Figure 3. SEM images of SiO2@C-60-20 (a), SiO2@C-120-40 (b), N-HPCN-60-20 (c), and NHPCN-120-40 (d). With steep rise at P/P0 < 0.05 in adsorbed volumes, N-HPCNs mainly display type-I isotherms in the nitrogen sorption profiles (Figure 4a and c), which denotes plentiful micropores.37 NHPCNs exhibit micro- and mesopores centered at about 0.50, 0.80, 1.27 and 2.73 nm, respectively (Figure 4b and d). It is often useful to differentiate between the narrow micropore with small ultramicropore (< 0.7 nm) and large supermicropore according to the classification of International Union of Pure and Applied Chemistry (IUPAC).59 A good example is that there is an anomalous enhance of EDLC capacitance when the pore size is comparable to the dimensionality of the ionic species (below 1 nm).60-63 The regular ultramicropores (~0.5 nm) derive from the intrinsic framework cavities of polydopamine, and the supermicropores (~0.80 and 1.27 nm) result from the decomposition of polymer in carbonization/activation. The hydrolysis and condensation of TEOS not only generate big SiO2 particles as cores for polydopamine coating, but also form some small nanoparticles (Figure 2), and their removal leads to the generation of the mesopores of 2.73 nm.64 The pore structure parameters of NHPCNs are summarized in Table 1. N-HPCN-120-0 fabricated using water as the sole solvent has a surface area of 1305 m2 g−1. When increase the ethanol volume to 30 and 60 mL, the obtained N-HPCN-90-30 and N-HPCN-60-60 show enhanced surface areas of 1789 and 1412 m2


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g−1, respectively, due to decreased carbon shells which benefit the KOH activation to generate more abundant micropores. However, compared with N-HPCN-90-30, thinner shells of NHPCN-60-60 lead to the collapse of the carbon architectures resulted in decreased surface area. This structure collapse becomes more obvious, makes the surface area further decrease to 1130 m2 g−1 for N-HPCN-30-90. In case of ethanol as the solvent, as-prepared N-HPCN-0-120 shows a lower surface area of 860 m2 g−1 since much thicker carbon shells hinder the full activation of KOH. The similar results are found when changing the amounts of the mixed solvents. With increasing the total volumes of water/ethanol from 80 to 160 mL, the surface areas of N-HPCNs gains a boost from 1217 to 1789 m2 g−1, and then decreased to 1430 m2 g−1.

Figure 4. N2 adsorption/desorption isotherms (a, c) and pore size distribution curves (b, d) of NHPCNs.


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Table 1. Pore Structure Parameters of N-HPCNsa.

a

SBET

Vtotal

Smicropore

Samples

(m2 g−1)

(cm3 g−1)

(m2 g−1)

(nm)

(nm)

(nm)

N-HPCN-120-0

1305

0.86

1052

0.50;

0.80; 1.27

2.73

N-HPCN-90-30

1789

1.18

1462

0.54

0.80; 1.27

2.73

N-HPCN-60-60

1412

0.92

1133

0.54

0.80; 1.27

2.73

N-HPCN-30-90

1130

0.75

897

0.50

0.80; 1.27

2.73

N-HPCN-0-120

860

0.73

567

0.50

0.80; 1.27

2.73

N-HPCN-60-20

1217

0.63

1157

0.54

0.86; 1.27

2.73

N-HPCN-120-40

1430

0.87

1207

0.54

0.80; 1.27

2.73

Pultramicropore Psupermicropore

Pmesopore

SBET, specific surface area; Vtotal, the total pore volume; Smicropore, micropore surface area;

Pultramicropore, Psupermicropore, Pmesopore, pore size of ultramicropore, supermicropore and mesopore. In wide-scan XPS spectra (Figure 5), N-HPCNs exhibit featured peaks for C 1s (285.1 eV), N 1s (400 eV) and O 1s (533.1 eV). The fitted high-resolution N 1s XPS spectra show pyridinic N (N-6), pyrrolic N (N-5), quaternary N (N-Q), pyridinic N-oxide (N-X), ammonia and chemisorbed nitrogen oxide N (N-Ox) species which are assigned to binding energies of 398.1, 399.5, 400.5, 401.7 and 403.8 eV, respectively (Figure S3a−e). The survey and fitted highresolution XPS spectra of N-HPCNs prepared with different solvent amounts were shown in Figure S4. Table 2 summarizes the contents of C, N, O of N-HPCNs and Table S1 demonstrates relative nitrogen species content of N 1s. N-HPCNs show nitrogen contents of 3.29–6.86 wt.% and oxygen contents of 6.53–10.26 wt.% dependent on the water/ethanol ratios and the dosages


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of the solvents. The self-polymerization of dopamine is rather complicated,65-66 and is influenced greatly by the synthetic parameters (as shown in Figure 1–3), leading to much difference in nitrogen and oxygen contents. Oxygen functional groups improve the hydrophilic properties of the electrode, and provide pseudocapacitance.67 Nitrogen dopants not only enhance the surface polarity, but also strengthen the surface wettability and thus reduce the ion diffusion resistance.68 Particularly, N-6 and N-5 play dominant roles in the enhancement of the pseudocapacitance due to their electroactive and electron donor inclination.32, 69 They also can create masses of active sites and defects to accelerate the ion diffusion and transfer in alkaline electrolyte.70 While N-Q increases the electronic conductivity of the electrode materials to improve the capacitance performance.71 However, the detailed electrochemical mechanisms of N-X and N-Ox are still not clear.72

Figure 5. XPS spectra of N-HPCNs.


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Table 2. Elemental Composition of N-HPCNs. C

N

O

Samples

C

N

O

(wt.%)

(wt.%)

(wt.%)

Samples (wt.%)

(wt.%)

(wt.%)

N-HPCN-120-0

85.01

4.92

10.07

N-HPCN-0-120

86.96

3.29

9.75

N-HPCN-90-30

86.20

6.86

6.93

N-HPCN-60-20

87.12

5.37

7.51

N-HPCN-60-60

84.55

5.18

10.26

N-HPCN-120-40

88.81

4.66

6.53

N-HPCN-30-90

85.90

4.16

9.93

EIS of N-HPCN electrodes are given in Figure S5. Each Nyquist plots show a typical nearly vertical line in the low frequency region, indicating an ideal electrochemical capacitance behavior. A line with a slope of about 45º in the middle frequency region reveals Warburg impedance which reflects the characteristic of ion diffusion into porous electrodes. The diameters of semicircles in the magnified high-frequency region reflect the electron transfer process.63, 73 The intercept of EIS curves at the Z' axis reflects the equivalent series resistance (ESR) composed of the ohmic resistance of electrolyte, the contact resistance between active materials/current collector, and the intrinsic resistance of active materials.74 N-HPCN electrodes have low ESR (0.28−0.63 Ω), indicating excellent conductivity.


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Figure 6. CV profiles of N-HPCN electrodes at 10 mV s−1 (a) and N-HPCN-90-30 electrode under various scanning speeds (b). Electrochemical tests were initially conducted on a three-electrode system in 6 M KOH electrolyte. CV profiles of N-HPCN electrodes (Figure 6a) exhibit quasi-rectangular shapes at 10 mV s−1, indicating a typical EDLC nature of energy storage.75 High N dopants of N-HPCNs contribute Faradic capacitance, however, there are no redox peaks in CV curves, indicating a charge/discharge at a pseudo-constant frequency during whole voltage range.76 The electrode capacitance integrated from the CV profile declines as followings: N-HPCN-90-30 > N-HPCN60-60 > N-HPCN-120-0 > N-HPCN-30-90 > N-HPCN-0-120. CV curves of N-HPCN electrodes prepared with different dosages of solvents are shown in Figure S6a, which indicates N-HPCN90-30 electrode also has the largest specific capacitance. Figure 6b shows CV profiles of NHPCN-90-30 electrode at different scan rates. When the scanning rate increases from 5 to 100 mV s−1, the profiles maintain a rectangular shape with a slight distortion, indicating an increased resistance at larger scanning rate. GCD profiles of N-HPCN electrodes depicted in Figure 7 show almost isosceles triangles with high coulombic efficiency (~96%) and an excellent electrochemical reversibility. N-HPCN-90-30 electrode reveals a gravimetric capacitance of 316 F g−1 (Figure 7a), much higher than those of N-HPCN-60-60 (270 F g−1), N-HPCN-120-0 (242 F


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g−1), N-HPCN-30-90 (234 F g−1), and N-HPCN-0-120 (216 F g−1) at 1.0 A g−1, as well as NHPCN-60-20 (227 F g−1) and N-HPCN-120-40 electrodes (248 F g−1) shown in Figure S6b. Figure 7b gives GCD curves of N-HPCN-90-30 electrode at different loading current densities. The electrode has 353 F g−1 at 0.5 A g−1 and remains 214 F g−1 even at 20 A g−1, showing a good rate capability with strong endurance. Figure S7a gives a comparison of the electrochemical capacitances of N-HPCN electrodes. N-HPCN-90-30 electrode also has a higher capacitance than those of many other reported N-doped carbon electrodes including graphene, nanospheres, nanofibers, carbon grape skins, carbon nanosheets, nanocages, etc. (Table S2). In addition, NHPCN-90-30 electrode exhibits 91.9% capacitance retention of the initial value at 1.0 A g−1 for 10,000 cycles (Figure S7b), indicating an excellent long-term cycling stability.

Figure 7. GCD profiles of N-HPCN electrodes at 1.0 A g−1 (a) and N-HPCN-90-30 electrode under various current densities (b). N-HPCN-90-30 is an optimized sample considering from morphology, specific surface area, nitrogen content and consequent electrochemical performance. Based on this, we further carried out temperature-dependent activation to study the evolution of the pore structures, nitrogen contents and electrochemical properties of N-HPCNs (the notation N-HPCNX denotes the materials prepared by KOH activation at X °C). With the activation temperature increases from


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600 to 700 °C, the micro- and mesopores in N-HPCNs become developed (Figure S8), accompanying with enhanced surface areas from 1122 to 1789 m2 g−1. While high temperatures (800 and 900 °C) cause obvious framework shrinkage and reduce the surface areas to 1565 and 1281 m2 g−1 (Table S3). Correspondingly, the N contents reduce from 9.50 to 3.23 wt.% (Figure S9 and Table S4). N-HPCN-90-30 (i.e., activated at 700 °C) is optimum in view of pore parameters and N content to gain the highest electrochemical capacitance (Figure S10). N-HPCN-90-30 electrode was assembled into a symmetrical all-solid-state supercapacitor using PVA/KOH gel electrolyte. Compared with aqueous electrolytes, gel electrolytes are not easy to leakage, light-weight, friendly to the environment and flexible.8 CV curves of the symmetrical supercapacitor (Figure 8a) are quasi-rectangular, exhibiting an EDLC feature. GCD profiles (Figure 8b) depict triangular shapes and high symmetry with no obvious IR drop varying from 0.2 to 5 A g−1, suggesting effective ion diffusion and a low transport resistance. As shown in Figure S11a, the electrochemical capacitance of N-HPCN-90-30 electrode measured in a twoelectrode cell reaches 315 F g−1 at 0.2 A g−1 and 192 F g−1 even at 20 A g−1. Correspondingly, the supercapacitor has 79 and 48 F g−1 at 0.2 and 20 A g−1. Figure S11b presents EIS of the supercapacitor, which exhibits a small impedance (1.26 Ω) and thus a superior ion diffusion process in the gel electrolyte. The leakage current of N-HPCNs based solid-state supercapacitor was ~15 µA (Figure S11c). The stability of the assembled device and its capability for maintaining charges were further confirmed by a self-discharge measurement exhibited by the time course of the open-circuit voltage (Figure S11d). The device after being charged at 1.0 V exhibits a slow self-discharge feature, with an output voltage of ~0.64 V after 24 h. The energy densities of the device achieve 10.42 and 6.7 W h kg−1 corresponding to the power densities of 0.25 and 10 kW kg−1, respectively (Figure 8c). It shows superior energy density compared with


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many previously reported carbon-based symmetric supercapacitors (Table S5). The inset of Figure 8c exhibits a lighted blue LED lamp by two coin cells of the supercapacitor with a voltage window of 0–2 V. In addition, N-HPCN-90-30 electrode and the assembled device show a high cycling stability with 86% capacitance retention after 10,000 cycles (Figure 8d).

Figure 8. Electrochemical performance of the symmetric solid-state supercapacitor based on NHPCN-90-30 electrodes tested in the PVA/KOH gel electrolyte: CV curves at different scanning rates (a), GCD curves at different current densities (b), the relationship between the energy density and power density of the supercapacitor (c, the inset image shows a lighted blue LED lamp through two symmetric supercapacitors connected in series), and cycling stability of the device at 0.5 A g−1 (d).


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The excellent electrochemical performance of N-HPCN-90-30 electrode as well as the assembled symmetric all-solid-state supercapacitor can be ascribed to regular geometry, high surface area, micro- and mesoporous structure and high-level N content. To be specific, (i) NHPCN-90-30 possesses spherical geography and hollow architecture where external package porosity as well as the internal hollow cavity can act as dual ion buffer reservoirs which shorten the ion diffusion paths for fast charge transportation into the internal pore channels in all directions; (ii) high surface area resulted from abundant micropore, wrinkled carbon shell and the hollow structure (provide doubled surface area) provide plenty of ion-accessible adsorption sites for energy storage; (iii) uniform and well-developed ultramicropores (0.54 nm) contribute to high electrochemical capacitance, and supermicropores and mesopores provide lower resistance ion-transport channels for rapid kinetic process of ion diffusion into electrode interior; (iv) highlevel N content of N-HPCN-90-30 reduces the internal resistance due to the enhanced electric conductivity for carbon matrix and improved wettability for ion diffusion, and offer additional pseudocapacitance to achieve excellent electrochemical performance.

4. Conclusion In conclusion, a novel and straightforward strategy to synthesize polydopamine-derived NHPCNs with controllable geometry, surface area, pore structure and nitrogen content, which can be simply realized by tuning the ratios of water/ethanol and the solvent amounts during the synthetic process. As-prepared N-HPCNs show combined advantages of uniform and hollow spherical architecture, ultrahigh surface area, abundant ultramicropores coupled with reasonable supermicro- and mesopores, and high-level nitrogen heteroatoms. Consequently, N-HPCN electrode delivers a high electrochemical capacitance, high-rate capability and superb cycle


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stability in three-electrode system using KOH aqueous electrolyte. More importantly, an assembled symmetric all-solid-state supercapacitor based on N-HPCN electrodes in a PVA/KOH gel electrolyte exhibits a superior energy density of 10.42 W h kg−1 at a power density of 250 W kg−1, and the device has an 86% capacitance retention after 10,000 cycles. This study highlights great potential to develop N-enriched carbons with well-designed architecture for advanced supercapacitor applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm……. TEM images of N-HPCNs; XRD patterns and Raman spectra of N-HPCNs; XPS spectra of NHPCNs; Relative contents of N species in N-HPCNs; EIS, CV and GCD curves of N-HPCN electrodes; Specific capacitances of N-HPCN electrodes and cycle stability of N-HPCN-90-30 electrode; Comparison of the specific capacitances of reported N-doped porous carbon electrodes; N2 adsorption/desorption analysis and XPS spectra of N-HPCNX; Elemental compositions and relative contents of nitrogen species in N-HPCNX; CV and GCD curves and EIS of N-HPCNX electrodes; Specific capacitances of N-HPCN-90-30 electrode in two-electrode cell and EIS of the symmetric all-solid-state supercapacitor; Comparison of the energy density and power density of reported porous carbon-based supercapacitors (PDF)

AUTHOR INFORMATION Corresponding Authors


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*(M.L.) E-mail: [email protected]. *(L.G.) E-mail: [email protected]. ORCID Zhiwei Wang: 0000-0001-6729-2237 Mingxian Liu: 0000-0002-9517-2985 Lihua Gan: 0000-0002-3652-8822 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21473122, 21501135, 51503152, 21703161 and 51772216), the Science and Technology of Shanghai Municipality, China (14DZ2261100), the Fundamental Research Funds for the Central Universities, and the Large Equipment Test Foundation of Tongji University.

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(73) Shelke, M. V.; Gullapalli, H.; Kalaga, K.; Rodrigues, M.-T. F.; Devarapalli, R. R.; Vajtai, R.; Ajayan, P. M. Facile Synthesis of 3D Anode Assembly with Si Nanoparticles Sealed in Highly Pure Few Layer Graphene Deposited on Porous Current Collector for Long Life Li-Ion Battery. Adv. Mater. Interfaces 2017, 4, 1601043‒1601049. (74) Navarro-Suárez, A. M.; Carretero-González, J.; Rojo, T.; Armand, M. Poly(quinoneamine)/Nanocarbon Composite Electrodes with Enhanced Proton Storage Capacity. J. Mater. Chem. A 2017, 5, 23292‒23298. (75) Zhang, G.; Zhang, J.; Qin, Q.; Cui, Y.; Luo, W.; Sun, Y.; Jin, C.; Zheng, W. Tensile ForceInduced Tearing and Collapse of Ultrathin Carbon Shells to Surface-Wrinkled Grape Skins for High Performance Supercapacitor Electrodes. J. Mater. Chem. A 2017, 5, 14190‒14197. (76) Liu, L.; Xu, S. D.; Yu, Q.; Wang, F. Y.; Zhu, H. L.; Zhang, R. L.; Liu, X. Nitrogen-Doped Hollow Carbon Spheres with a Wrinkled Surface: Their One-Pot Carbonization Synthesis and Supercapacitor Properties. Chem. Commun. 2016, 52, 11693−11696.


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N-Enriched Hollow Porous Carbon Nanospheres ... - ACS Publications

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