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Activated Porous Carbon Spheres with Customized Mesopores through Assembly of Diblock Copolymers for Electrochemical Capacitor Jing Tang, Jie Wang, Lok Kumar Shrestha, Md. Shahriar A. Hossain, Zeid Abdullah Alothman, Yusuke Yamauchi, and Katsuhiko Ariga ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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ACS Applied Materials & Interfaces

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Activated Porous Carbon Spheres with Customized

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Mesopores

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Copolymers for Electrochemical Capacitor

through

Assembly

of

Diblock

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Jing Tang1, Jie Wang2, Lok Kumar Shrestha,1 Md. Shahriar A. Hossain3, Zeid Abdullah Al-

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Othman3,4, Yusuke Yamauchi1,3,4*, Katsuhiko Ariga1,5*

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1

8 9

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

2

Key Laboratory of Materials and Technologies for Energy Conversion, College of

10

Materials Science & Engineering, Nanjing University of Aeronautics and Astronautics,

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Nanjing, 210016, P.R. China

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3

13 14

Australian Institute for Innovative Materials (AIIM), University of Wollongong, North Wollongong, NSW 2500, Australia

4

15

Advanced Materials Research Chair, Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

16

5

Graduate School of Frontier Science, The University of Tokyo, Kashiwa 277-0827, Japan

17

*

Corresponding Authors: [email protected]; [email protected]

ACS Paragon Plus Environment

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KEYWORDS: self-assembly, carbon sphere, controlled mesopore, chemical activation,

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electrochemical capacitor

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ABSTRACT: A series of porous carbon spheres with precisely adjustable mesopores (4~16

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nm), high specific surface area (SSA, ~2000 m2 g−1), and sub-micrometer particle size (~300 nm)

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have been synthesized through a facile co-assembly of diblock polymer micelles with a nontoxic

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dopamine source, and a common post-activation process. The mesopore size can be controlled

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by the diblock polymer, polystyrene-block-poly(ethylene oxide) (PS-b-PEO) templates and has

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an almost linear dependence on the square root of the degree of polymerization of the PS blocks.

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These advantageous structural properties make the product a promising electrode material for

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electrochemical capacitors. The electrochemical capacitive performance was studied carefully by

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using symmetrical cells in a typical organic electrolyte of 1 M tetraethylammonium

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tetrafluoroborate/acetonitrile (TEA BF4/AN) or in an ionic liquid electrolyte of 1-ethyl-3-

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methylimidazolium tetrafluoroborate (EMIMBF4), displaying a high specific capacitance of 111

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F g−1 and 170 F g−1 at 1 A g−1, respectively. The impacts of pore size distribution on the

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capacitance performance have been thoroughly investigated. It is revealed that large mesopores

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and a relatively low ratio of micropores are ideal for realizing high SSA-normalized capacitance.

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These results provide us with a simple and reliable way to screen future porous carbon materials

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for electrochemical capacitors and encourage researchers to design porous carbon with high

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specific surface area, large mesopores, and a moderate proportion of micropores.

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INTRODUCTION: Electrochemical capacitors (ECs) represent a fascinating electrical energy

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storage system that can fulfil the requirements of high power delivery/uptake and long cycle life

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in portable electronics, hybrid electric vehicles, and other equipment with high power

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demands.1,2 The electrochemical double layer capacitor (EDLC) is one kind of electrochemical

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capacitor, which stores energy using reversible ion adsorption onto the active materials, which

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generally possess large specific surface area (SSA).3,4 Up to now, porous carbon is still

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unbeatable as the electrode material for EDLCs due to its outstanding comprehensive properties,

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such as electrochemical stability, good electrical conductivity, adjustable porosity, and abundant

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sources.5,6

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In the initial research, SSA was considered as the key to achieving high capacitance by

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increasing the charging double layers in the porous carbon. Chemical activation by using KOH

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or CO2 is an effective approach to producing highly porous networks in bulk carbon materials.7,8

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Many activated carbon materials, such as carbon wires,9 graphene-based carbon,10 and ordered

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mesoporous carbons11 have shown much higher capacitance than their untreated counterparts.

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The contribution of micropores to the capacitive storage is highlighted by recent studies, in

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which carbide derived carbons with exclusively narrow unimodal micropores (can be tuned in

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the range of 0.6-1.1 nm) were used.12 These studies proposed that the ions confined in

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micropores of the same size would display extremely high capacitance.13

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Later research proved that there is no linear relationship between the specific surface area and

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the electrochemical double layer capacitance.14 Other parameters, such as pore size distribution,

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interconnected porous structures, morphology and size of the carbons, wettability, and doped

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heteroatoms, are non-ignorable.15,16 For instance, only the pore sizes that are large enough to

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accommodate the ions/solvated ions can contribute to the double layer capacitance, and thus, the


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requirements for the pore sizes differ from the aqueous, to the organic, to the ionic liquid

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electrolytes.17 Thus, the effective double layer surfaces, which are markedly influenced by the

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pore size distribution rather than the total SSA, are decisive for the capacitance; and hierarchical

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porous textures (e.g. pores, including micro-, meso-, and macro-pores.) that are open enough for

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ionic mass transport will be helpful for maximizing the capacitance.18,19 Thus, ideal carbon

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materials for EDLCs would not only consist of abundant micropores for charge accumulation,

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but also possess enough accessible mesoporous pathways.

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Among the various nanostructured carbons from zero to three dimensions, porous carbon

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spheres with sub-micrometer sizes are attractive due to their quite short diffusion length for ion

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transportation in their interior nanopores, as well as the resulting low diffusion resistance.20,21

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Various porous carbon spheres have been synthesized for EDLCs by using the templating

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method or taking advantage of spherical self-polymerization of some specific carbon precursors.

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For example, mesoporous carbon nanospheres with a diameter of ~65 nm and mesopores 2.7 nm

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in size have been prepared by using mesoporous silica nanospheres as the template and ferrocene

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as the carbon precursor.22 N-doped hollow carbon microspheres were prepared by direct

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pyrolysis of polymerized melamine–formaldehyde resin spheres.23 Recently, our group proposed

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a facile route for the preparation of N-doped mesoporous carbon spheres with large mesopores

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by using a diblock copolymer, polystyrene-block-poly(ethylene oxide) (PS-b-PEO), as the

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removable pore forming agent and N-containing dopamine as the carbon precursor.24 It is

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noteworthy that the dopamine has many unique advantages, such as self-polymerization into

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spherical polydopamine under mild alkaline solution at room temperature, in-situ doping of high-

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content nitrogen by carbonization, and effective interaction with PS-b-PEO as pore directing


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agent.24-26 Our initial study showed that large mesopores were quite important in electrochemical

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catalysis by shortening and smoothing the mass transportation pathways.

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Inspired by this project and the above-introduced required properties for an electrode material

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in EDLCs, we have attempted to synthesize a fascinating carbon material with sub-micrometer

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sized spherical morphology, high porosity (increased by chemical activation), abundant

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mesopores (templated from diblock copolymer micelles), and even heteroatom doping (inherited

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from the N-containing carbon precursor of dopamine). Furthermore, according to our initial

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study, the size of mesopores can be controlled by the pore forming agent, diblock copolymer PS-

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b-PEO with different molecular weights. Thus there was a good opportunity for us to study the

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effects of the pore size distribution of carbon materials on the electrochemical double layer

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capacitance by preparing a series of carbon materials using the same carbon precursor and

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synthetic procedures, but adopting different pore forming agents. The electrochemical properties

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of our series of porous carbon spheres have been investigated in a typical organic electrolyte and

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an ionic liquid electrolyte.


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EXPERIMENTAL SECTION

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Chemicals. Polystyrene-block-poly(ethylene oxide) (PS-b-PEO) was purchased from Polymer

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Source Inc. Dopamine hydrochloride was purchased from Sigma-Aldrich. Tetrahydrofuran

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(THF), ethanol, ammonia aqueous solution (NH4OH, 25 wt%), potassium hydroxide, and

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sulfuric acid (98 wt%) were purchased from Nacalai Tesque, Inc. All of the chemicals were

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analytical grade and used as received.

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Preparation of N-doped mesoporous carbon spheres (NMCS). In a typical synthesis, 30 mg

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of diblock copolymer (PS-b-PEO) was first dissolved in 4 mL of THF under sonication.

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Meanwhile, 300 mg of dopamine hydrochloride was dissolved in 12 mL of a mixed solution of 4

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mL ethanol and 8 mL deionized water. Then, the above two kinds of transparent solutions were

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merged under mild stirring and became semi-transparent. After one hour, 500 µL of ammonia

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aqueous solution was injected into the solution to induce the self-polymerization of dopamine.

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The colour of the solution changed from semi-transparent to brown in 10 minutes. Polydopamine

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(PDA)/PS-b-PEO composite spheres could be collected by centrifugation after 20 hours of

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reaction and were then washed several times with ethanol and deionized water. All synthetic

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experiments were carried out at room temperature (~22 ○C). In order to obtain NMCS, the

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PDA/PS-b-PEO composite spheres were calcined under N2 atmosphere with a heating rate of 1

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○

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The mesopore sizes of NMCS can be precisely adjusted by utilizing diblock copolymers (PS-b-

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PEO) with different degrees of polymerization in the PS block. In order to distinguish each

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diblock copolymer, we have labelled them as PSm-b-PEOn, where the subscript “m” represents

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the degree of the PS block (i.e. total molecular weight of the PS block divided by the molecular

C·min−1, and kept at 350 ○C and 900 ○C for 2 hours each.


6

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weight of the PS monomer), and the subscript “n” represents the degree of the PEO block (i.e.

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the total molecular weight of the PEO block divided by the molecular weight of the PEO

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monomer). In this work, PS37-b-PEO114, PS48-b-PEO50, PS91-b-PEO114, and PS173-b-PEO170 were

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used for preparing NMCS with different mesopore sizes, and the products are labelled as NMCS-

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1, NMCS-2, NMCS-3, and NMCS-4, respectively.

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Preparation of activated N-doped mesoporous carbon spheres (NMCS-A). For post-

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synthesis activation, the obtained NMCS was mixed with potassium hydroxide (KOH) in a

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weight ratio of 1:4.27 Then, the mixture was dispersed in deionized water under sonication for 30

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minutes. The slurry was dried at 100 °C, followed by thermal treatment at 800 °C for 2 hours

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under flowing N2, with a heating rate of 5 °C min−1. After cooling down to room temperature, the

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resulting powder was washed with deionized water at least five times until the pH value of the

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centrifugate was around 7. Finally, the activated NMCS was dried in an oven at 80 °C overnight

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and labelled as NMCS-A. For the specific NMCS-A, the products are labelled as NMCS-1-A,

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NMCS-2-A, NMCS-3-A, and NMCS-4-A by using NMCS-1, NMCS-2, NMCS-3, and NMCS-4

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as the raw materials.

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Characterizations. The morphology of the samples was observed by using a Hitachi SU-8000

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field-emission scanning electron microscope (SEM) at 5 kV. Transmission electron microscopy

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(TEM) and energy-dispersive X-ray analysis was conducted on a JEM-2100 operated at 200 kV.

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Nitrogen adsorption-desorption isotherms were obtained by using a BELSORP-mini (BEL,

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Japan) at 77 K. The specific surface area (SSA) was evaluated by the multipoint Brunauer-

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Emmett-Teller (BET) method at a relative pressure from 0.05 to 0.3 based on the adsorption

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data. The total pore volume and pore size distribution were estimated by using the Barrett-

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Joyner-Halenda (BJH) model based on the adsorption branch of the isotherm. Powder X–ray


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diffraction (XRD) patterns were acquired on a Rigaku Rint 2000 X-ray 27 diffractometer with

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monochromated Cu Kα radiation at a scanning rate of 2° min−1. Raman spectra were collected on

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a Horiba-Jovin Yvon T64000 using an excitation laser with a wavelength of 514.5 nm. X-ray

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photoelectronic spectroscopy (XPS) spectra were characterized on a JPS-9010TR instrument

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equipped with an Mg Kα X-ray source, and the binding energies were calibrated by referencing

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them to the C 1s (284.5 eV) binding energy.

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Electrochemical measurements: The working electrode was prepared by mixing our prepared

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carbon materials (85 wt%) with acetylene black (10 wt%) and poly(tetrafluoroethylene) (5 wt%).

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The mixture was rolled into a thin film and dried at 100 °C for 12 hours. The film was cut and

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pressed onto aluminium foil as working electrodes for symmetric supercapacitors. In this study,

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two kinds of electrolyte were used. The two working electrodes separated by a glass fiber film

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were soaked in 1 M tetraethylammonium tetrafluoroborate (TEA BF4)/acetonitrile (AN) organic

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electrolyte or 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) ionic liquid, and they

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then were assembled into coin-type cells. The electrochemical measurements were performed on

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a CHI 660D electrochemical workstation. Cyclic voltammograms (CVs) and galvanostatic

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charge–discharge curves (CDC) were collected to investigate the electrochemical properties of

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the electrodes. For the two-electrode cell, the gravimetric capacitance (Cg) of one electrode was

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calculated from the galvanostatic discharge curve according to the formula =

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I is the current (A), ∆t is the discharge time, m is the total mass of active material, and ∆V is the

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practical discharging voltage range from the end of the IR drop (ohmic potential drop) to the end

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of the discharging.

× × ∆

× ∆

, where

22


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RESULTS AND DISCUSSION

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Scheme 1. Schematic illustration of the synthetic process for the chemically activated N-doped

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mesoporous carbon nanospheres (NMCS-A).

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The synthetic procedure is illustrated in Scheme 1. Nitrogen-containing dopamine was used as

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the carbon precursor. Diblock copolymer PS-b-PEO was used as the soft template for the

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mesopores, thanks to its micellization property in the specific mixed solvents and its resulting

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spherical micelles with PS cores and PEO coronas.28 After mixing dopamine with the PS-b-PEO

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micelles in alkaline solutions, the self-polymerization reaction of the dopamine molecules

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proceeded, and the products formed assemblies with the micelles through the hydrogen bonding

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between the catechols/quinone groups in polydopamine (PDA) and -OH groups in the PEO

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corona.26,

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shown in Scheme 1) after a continuous 20 hours of reaction. In order to obtain N-doped

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mesoporous carbon spheres (NMCS), the PDA/PS-b-PEO composites were calcined under inert

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atmosphere. During this process, the PS-b-PEO micelles were unstable and were initially

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removed at the relatively low temperature of 350 °C, acting as sacrificial pore-forming agents.

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PDA was gradually converted to N-doped carbon at the relatively high temperature of 900 °C.

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The porosity of the NMCS can be distinctly increased by common KOH chemical activation,27

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which would make the activated NMCS (NMCS-A) more suitable as electrode in EDLCs.

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As a result, dispersed PDA/PS-b-PEO composite nanospheres were formed (as


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The morphology of the products obtained in each synthetic stage was monitered by scanning

2

electron microscopy (SEM) and transmission electron microscopy (TEM) images. Figure 1(a-d)

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shows the morphology of PDA/PS-b-PEO composite spheres based on the four kinds of PS-b-

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PEO with different degrees of polymerization of PS and PEO chains, namely, PS37-b-PEO114,

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PS48-b-PEO50, PS91-b-PEO114, and PS173-b-PEO170. As shown in low-magnification SEM images

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and particle size distribution histograms (Figure S1 in the Supporting Information), the average

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diameters are 185 nm, 257 nm, 242 nm, and 220 nm for the PDA/PS37-b-PEO114, PDA/PS48-b-

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PEO50, PDA/PS91-b-PEO114, and PDA/PS173-b-PEO170 composite spheres, respectively. All the

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composite spheres show raspberry-like shapes, whereas the round bumps are much clearer on

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PDA/PS173-b-PEO170 (Figure 1d) than on PDA/PS37-b-PEO114 (Figure 1a) due to the larger

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micelles formed from the diblock copolymer PS-b-PEO with a higher degree of polymerization

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in its PS chains.24 The average micelle sizes calculated from the SEM images (Figure 1a-d) are

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11.6 nm, 15.0 nm, 20.0 nm, and 28.0 nm in PDA/PS37-b-PEO114, PDA/PS48-b-PEO50,

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PDA/PS91-b-PEO114, and PDA/PS173-b-PEO170, respectively. Inspired by our previous research

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and some recent reports,24,30 a linear fit with the equation Y= 2.23X-1.28 (R2 = 0.99) was

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obtained when plotting the average size of micelles for the PS-b-PEO series against the square

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root of the degree of polymerization of the PS blocks (Figure S2).


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Figure 1. SEM images of (a) PDA/PS37-b-PEO114 composite spheres, (b) PS48-b-PEO50

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composite spheres, (c) PS91-b-PEO114 composite spheres, and (d) PDA/PS173-b-PEO170

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composite spheres. (e-h) SEM and (i-l) TEM images of the corresponding N-doped mesoporous

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carbon spheres (NMCS) carbonized at 900 °C for (e, i) NMCS-1, (f, j) NMCS-2, (g, k) NMCS-3,

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and (h, l) NMCS-4.

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N-doped mesoporous carbon spheres were obtained after carbonizing the PDA/PS-b-PEO

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composite spheres at 900 °C in N2 atmosphere. Four types of NMCS, namely NMCS-1, NMCS-


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2, NMCS-3, and NMCS-4 were fabricated from PDA/PS37-b-PEO114, PDA/PS48-b-PEO50,

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PDA/PS91-b-PEO114, and PDA/PS173-b-PEO170 composite spheres, respectively. As shown in

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the low-magnification SEM images and particle size distribution histograms (Figure S3), the

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average sizes are 176 nm, 246 nm, 242 nm, and 180 nm for NMCS-1, NMCS-2, NMCS-3,

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NMCS-4, respectively, which are slightly smaller compared with the original PDA/PS-b-PEO

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composite spheres (Figure S1). The distinct mesopores are well distributed on the sphere surface

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(Figure 1e-h) and were roughly estimated to be 5.0 nm, 6.5 nm, 10.2 nm, and 15.9 nm for

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NMCS-1, NMCS-2, NMCS-3, and NMCS-4. These sizes were smaller than those of the

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corresponding pore-directing micelles, as shown in Figure 1a-d. The TEM images (Figure 1i-l)

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show that the mesopores not only distribute on the surface but also distribute throughout the

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carbon spheres.

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Figure 2. (a) Wide-angle XRD pattern, (b) Raman spectrum, (c) high-resolution N 1s XPS

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spectrum of the representative NMCS-3 sample.


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Wide-angle X-ray diffraction (XRD) and Raman spectroscopy was used to investigate the

2

carbon state in NMCS. As shown in Figure 2a, the XRD pattern of the representative sample

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NMCS-3 displays two broad diffraction peaks located at 23.0° and 44.0°, which can be

4

respectively ascribed to the (002) and (101) diffraction planes of disordered carbon (amorphous

5

carbon).31 The Raman spectrum of NMCS-3 shows two distinct vibration bands in Figure 2b.

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The D band located at 1360 cm–1 originates from the vibrations of disordered carbon or the

7

defects in the plane terminations,32 whereas the G band at 1590 cm–1 is related to the vibrations

8

of ordered sp2-bonded graphitic carbon sheets.33 The ratio of D band to G band (ID/IG) is

9

calculated by using the peak intensity of D band and G band after subtracting the background.

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The ratio of ID/IG is 1.03 in NMCS-3 sample, implying the coexistence of disordered carbon and

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ordered graphitic carbon.High-resolution X-ray photoelectron spectroscopy (XPS) was

12

conducted to investigate the electronic states and content of N element in NMCS. As shown in

13

Figure 2c, the N 1s spectrum of NMCS-3 can be deconvoluted into two binding energies

14

centered at 398.4 eV

15

respectively. The ratio of the pyridinic-N to the graphitic-N can be calculated by using the areas

16

of each fitting peaks and the ratio is 28.6%. The content of N in the representative NMCS-3

17

sample is around 2.7 atomic%, as detected by XPS. The percentage of nitrogen in NMCS-3

18

sample roughly estimated by energy-dispersive X-ray analysis (Figure S4) is 3.0 atomic%,

19

which is similar to the XPS result (2.7 atomic%).

34

and 400.7 eV,35 which are associated with pyridinic-N and graphitic-N,


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Figure 3. SEM images of (a) NMCS-1-A, (b) NMCS-2-A, (c) NMCS-3-A, and (d) NMCS-4-A.

3

As we described in the introdcution, high SSA in a porous carbon electrode usually leads to a

4

high double layer capacitance. Thus, we further increased the SSA of NMCS by chemical post

5

activation, and the products are labelled as NMCS-A. As revealed by SEM iamges (Figure 3),

6

NMCS-A keep the spherical morphology and distinct mesopores from NMCS (Figure 1e-h),

7

implying that the chemical activation will not destroy the basic shape of raw carbon materials.

8

The porosities of NMCS and NMCS-A were carefully measured by N2 adsorption-desorption

9

isotherms. The structural parameters of NMCS and NMCS-A have been calculated and are

10

summarized in Table 1. NMCS show type IV isotherms with hysteresis loops (Figure 4a). The

11

steep N2 adsorption at a low relative pressure (P/P0 < 0.1) is typically associated with the

12

adsoption of N2 in the micropores. The hysteresis loops that appear at intermediate relative

13

pressure are caused by the capillary condensation of N2 into the randomly distributed mesopores

14

in NMCS. The pore size distribution (PSD) curves of NMCS shown in Figure 4b reveal that all

15

the NMCS samples possess a micropore peak centered at 1.8 nm (inset in Figure 4b), whereas

16

the mesopore peak gradually shifts from 4.2 nm in NMCS-1, to 5.4 nm in NMCS-2, 7.6 nm in


14

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NMCS-3, and 15.9 nm in NMCS-4, which is slightly smaller than the average mesopore size

2

measured from the SEM images (Figure 1e-h). We also obtained a linear fit with the function

3

Y= 1.63X-6.29 (R2 = 0.93) after plotting the peak values of mesopores in NMCS against the

4

square root of the degree of polymerization of the PS blocks (Figure S2). As listed in Table 1,

5

the SSA of NMCS is ~ 400 m2 g−1. After chemical activation by KOH, NMCS-A shows much

6

higher N2 uptake at a low relative pressure (P/P0 < 0.1, Figure 4c), indicating the tremendous

7

amount of micropores. The SSA of NMCS-A is obviously increased and exceeds 1800 m2 g−1

8

(Table 1). The peak value for micropores in the PSD curves is reduced slightly to 1.7 nm (inset

9

in Figure 4d), which is likely due to the abudant small micropores produced during activation.

10

Although the peaks of mesopores in the PSD curves of NMCS-A are not as clear as those for

11

NMCS (Figure 4d), the mesopores in NMCS-A were basically retained, as demonstrated by the

12

SEM images (Figure 3). The activated NMCS-2-A, NMCS-3-A, and NMCS-4-A samples show

13

slightly expanded mesopores, as can be concluded from Table 1.


15


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Page 16 of 28

1 2

Figure 4 (a, c) N2 adsorption–desorption isotherms and (b, d) pore size distribution (PSD) curves

3

of all the samples. Insets in (b,d) are the magnified PSD of micropores. For clarity, the isotherms

4

for NMCS-2, NMCS-3, and NMCS-4 are offset by 20, 80, and 120 cm3 g−1, respectively. The

5

PSD curves for NMCS-2, NMCS-3, and NMCS-4 are offset vertically by 0.02, 0.04, and 0.06

6

cm3 nm−1 g−1, respectively. The isotherms for NMCS-4-A are offset by 250 cm3 g−1. The PSD

7

curves for NMCS-2-A, NMCS-3-A, and NMCS-4-A are offset vertically by 0.1, 0.2, and 0.3 cm3

8

nm−1 g−1, respectively.

9

Table 1. The physicochemical properties of all the samples. Peak values SSA

Smicro

Smicro/SSA

Vpore

Vmicro

(%)

3

3

Vmicro/Vpore

Sample

in PSD 2

−1

2

−1

(m g ) (m g )

−1

(cm g )

−1

(cm g )

(%) (nm)


16

Page 17 of 28

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


NMCS-1

405

331

81.7%

0.289

0.173

59.9%

1.8, 4.2

NMCS-2

412

320

77.6%

0.391

0.184

47.1%

1.8, 5.4

NMCS-3

400

276

69.0%

0.496

0.151

30.4%

1.8, 7.6

NMCS-4

397

260

65.5%

0.621

0.142

22.9%

1.8, 15.9

NMCS-1-A

1883

1301

69.1%

1.052

0.572

54.4%

1.7, 4.2

NMCS-2-A

2107

1177

55.9%

1.410

0.551

39.1%

1.7, 5.8

NMCS-3-A

2320

1412

60.8%

1.245

0.645

51.8%

1.7, 9.2

NMCS-4-A

2103

1573

74.8%

1.235

0.674

54.6%

1.7, 16.0

1 2

Due to the unique structure of the NMCS-A samples, including small-sized spherical

3

morphology (~300 nm), interconnected hierarchical porous structure (micro- and mesopores),

4

and especially the high porosity (~2000 m2 g−1) via chemical activation, NMCS-A is expected to

5

be a promising electrode material for application in supercapacitors. In order to demonstrate the

6

superiority of NMCS-A compared with NMCS without chemical activation, we quickly

7

investigated the electrochemical performance of NMCS-1 and NMCS-1-A in a three-electrode

8

system by using 1M H2SO4 as electrolyte. As shown in Figure S5 in supporting information,

9

NMCS-1 without chemical activation displays a relative low gravimetric capacitance of 190 F

10

g−1, whereas NMCS-1-A shows a much higher gravimetric capacitance of 339 F g−1 mainly due

11

to the increased SSA from 405 m2 g−1 to 1883 m2 g−1. As is well known, the cell potential

12

window for operation in EDLCs is limited by the decomposition potential of the electrolyte, and

13

for this reason, organic electrolyte and ionic liquid electrolyte have attracted much attention due

14

to their 2-3 times higher cell voltage than with the aqueous electrolyte, resulting in increased

15

energy density.5,36,37 In this work, we have investigated the electrochemical properties of NMCS-


17


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Page 18 of 28

1

A electrodes in an organic electrolyte, 1 M TEA BF4/AN, and a common ionic liquid electrolyte,

2

EMIMBF4, respectively, by using symmetrical electrochemical cells.

3 4

Figure 5. Electrochemical performance of NMCS-A (NMCS-1-A, NMCS-2-A, NMCS-3-A, and

5

NMCS-4-A) in 1 M TEA BF4/AN. (a) CV curves of NMCS-A at a scan rate of 10 mV s−1; (b)

6

galvanostatic charge–discharge curves of NMCS-A electrodes at a current density of 1 A g−1; (c)

7

comparison of the specific capacitances of NMCS-A electrodes at different current densities; (d)

8

plot of capacitance at 1 A g−1 normalized by the SSA (CSSA) against the peak mesopore size of

9

each NMCS-A sample.

10

As shown in Figure 5a, all the samples display rectangular CV curves at 10 mV s−1 between 0

11

and 2.0 V, revealing the typical electrical double layer capacitive properties of NMCS-A in

12

organic electrolyte. The discharging plots are generally symmetric to their corresponding


18

Page 19 of 28

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1

charging counterparts, suggesting high capacitive reversibility in NMCS-A (Figure 5b). The

2

specific capacitance for each electrode has been calculated based on the discharge curves at

3

various current densities from 1−10 A g−1. As shown in Figure 5c, NMCS-3-A displays the

4

highest specific capacitance of 111 F g−1 at a current density of 1 A g−1, which is much higher

5

than for NMCS-1-A (62 F g−1), NMCS-2-A (88 F g−1), and NMCS-4-A (89 F g−1). When the

6

current density increased to 10 A g−1, the specific capacitance of NMCS-3-A still retained the

7

highest value of 90 A g−1. On comparing their texture parameters listed in Table 1, the excellent

8

capacitance of NMCS-3-A is probably contributed synergistically by its highest SSA (2320 m2

9

g−1), accessible mesopores, and a moderate proportion of micropores (60.8%).

10

Considering that all of the samples were prepared by using the similar synthetic procedures

11

and reactants, the most obvious differences between them are the peak of mesopore size, which

12

is determined by the different degrees of polymerization of the diblock copolymer PS-b-PEO.

13

Thus, we tried to determine the relationship between the electrochemical capacitance and the

14

pore size in the series of mesoporous carbon spheres. In order to eliminate the influence of

15

specific surface area (SSA) in different samples, we normalized the capacitance by the SSA

16

(CSSA, gravimetric capacitance at 1 A g−1 divided by SSA) and plotted the CSSA as a function of

17

the peak mesopore size in Figure 5d. The plot shows that the CSSA obviously became higher as

18

the mesopore size grew larger, revealing the increased utilization ratio of the surface area in the

19

samples that possessed large mesopores. The CSSA of NMCS-4-A (16 nm mesopores) is slightly

20

decreased compared with NMCS-3-A (9.2 nm mesopores), but this can be explained by the

21

relative proportions of micropores. As listed in Table 1, NMCS-4-A features a much higher

22

proportion of micropores (74.8%) than NMCS-3-A (60.8%), which probably influences the

23

utilization ratio of the surface area.


19


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Page 20 of 28

1 2

Figure 6. Electrochemical performance of NMCS-A (NMCS-1-A, NMCS-2-A, NMCS-3-A, and

3

NMCS-4-A) in EMIMBF4. (a) CV curves of NMCS-A at a scan rate of 10 mV s−1; (b)

4

galvanostatic charge–discharge curves of NMCS-A at a current density of 1 A g−1; (c)

5

comparison of the specific capacitances of the NMCS-A samples at different current densities;

6

(d) plot of capacitance at 1 A g−1 normalized by the SSA (CSSA) against the peak mesopore size

7

of each NMCS-A sample.

8

In addition, we studied the electrochemical performance of NMCS-A in EMIMBF4 electrolyte,

9

in which the potential window for operation can be further extended to 3 V. As shown in Figure

10

6a,b, all the samples display rectangular CV curves at 10 mV s−1 between 0 and 3.0 V, and

11

exhibit almost symmetric charge–discharge curves, revealing the electrical double layer

12

capacitive properties of NMCS-A in ionic liquid. The specific capacitances for each electrode

13

have been calculated based on the discharge curves at various current densities from 1−10 A g−1.


20

Page 21 of 28

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1

As shown in Figure 6c, NMCS-3-A displays the highest specific capacitance of 170 F g−1 at a

2

current density of 1 A g−1, which is much higher than for NMCS-1-A (89 F g−1), NMCS-2-A

3

(106 F g−1), and NMCS-4-A (118 F g−1). When the current density was increased to 10 A g−1, the

4

specific capacitances of NMCS-3-A still retained the highest value of 68 A g−1. We also

5

normalized the capacitance by the SSA (CSSA, gravimetric capacitance at 1 A g−1 divided by

6

SSA) and have plotted the CSSA against the peak mesopore size in Figure 6d. It is noteworthy

7

that there was a sharper increase in CSSA than in the organic electrolyte when the peak mesopore

8

size increased from 5.8 nm to 9.2 nm, indicating the greater impact of mesopore size on CSSA in

9

ionic liquid electrolyte compared to organic electrolyte (Figure 5d and Figure 6d). Furthermore,

10

the proportion of micropores also had more influence on CSSA in ionic liquid. As shown in

11

Figure 6d, CSSA was reduced quickly from NMCS-3-A (9.2 nm mesopores, 60.8% micropores)

12

to NMCS-4-A (16 nm mesopores, 74.8% micropores) even though the mesopore size was

13

expanded from 9.2 nm to 16 nm. Based on the elelctrochemical results in organic and ionic

14

liquid electrolytes, a larger mesopore size and lower proportion of micropores would increase the

15

utilization ratio for the surface area. Nevertheless, the specific surface area, as well as the related

16

specific capacitance, would be decreased if we only kept pursuing large mesopores and a low

17

ratio of micropores in porous carbons, leading to a low capacitance. Thus, it is quite important

18

for us to maintain a balance between all the structural parameters.

19

CONCLUSION

20

A series of porous carbon spheres with abundant micropores and adjustable mesopores have

21

been prepared by using a method based on soft-templating and post-activation. Dopamine was

22

used as the carbon sources due to its self-polymerization in alkaline solution and its ability to

23

form assemblies with diblock copolymer micelles that would act as a forming agent for


21


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Page 22 of 28

1

mesopores. After carbonization and chemical activation by KOH, small porous carbon spheres

2

(~300 nm) with high specific surface area (~2000 m2 g−1), and tunable mesopores (4−16 nm)

3

were successfully obtained. Due to the special structural properties of NMCS-A samples, we

4

attempted to employ them as electrode materials in electrical double layer capacitors and

5

investigated their electrochemical properties in symmetrical cells containing both organic and

6

ionic liquid electrolytes. The electrochemical results demonstrated that NMCS-3-A with a peak

7

mesopore size of ~ 9 nm exhibited the highest specific capacitance of 111 F g−1 and 170 F g−1 in

8

1 M TEA BF4/AN and EMIMBF4, respectively, at a current density of 1 A g−1, and showed the

9

best utilization ratio of surface area due to the large accessible mesopores and moderate

10

proportion of micropores.


22

Page 23 of 28

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1

ASSOCIATED CONTENT

2

Supporting Information. The following files are available free of charge. Complementary SEM

3

images, particle size distribution histograms, fitting plots, energy-dispersive X-ray analysis, and

4

cyclic voltammograms (PDF).

5

AUTHOR INFORMATION

6

Corresponding Authors

7

* Y. Yamauchi. E-mail address: [email protected].

8

* K. Ariga. E-mail address: [email protected]

9

Author Contributions

10

The manuscript was written through contributions of all authors. All authors have given approval

11

to the final version of the manuscript.

12

Notes

13

The authors declare no competing financial interest.

14

ACKNOWLEDGMENT

15

This work was supported by the University of Wollongong’s Australian Institute for Innovative

16

Materials (AIIM) for AIIM-MANA 2016 grant. This study was partially supported by JSPS

17

KAKENHI Grant Number JP16H06518 (Coordination Asymmetry) and CREST, JST. Y.Y and

18

Z.A.A. are grateful to the Deanship of Scientific Research, King Saud University for funding

19

through Vice Deanship of Scientific Research Chairs.

20


23


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Page 24 of 28

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TABLE OF CONTENT

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Activated Porous Carbon Spheres with ... - ACS Publications

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