- Email: [email protected]
α-MnO2 nanowires composite electrode towards supercapacitor applications
Accepted Manuscript Nitrogen-containing porous carbon/α-MnO2 nanowires composite electrode towards supercapacitor applications Lidong Sun, Nian Li, Shudong Zhang, Xinling Yu, Cui Liu, Yongqiang Zhou, Shuai Han, Wenbo Wang, Zhenyang Wang PII:
S0925-8388(19)30907-7
DOI:
https://doi.org/10.1016/j.jallcom.2019.03.100
Reference:
JALCOM 49864
To appear in:
Journal of Alloys and Compounds
Received Date: 30 November 2018 Revised Date:
3 February 2019
Accepted Date: 5 March 2019
Please cite this article as: L. Sun, N. Li, S. Zhang, X. Yu, C. Liu, Y. Zhou, S. Han, W. Wang, Z. Wang, Nitrogen-containing porous carbon/α-MnO2 nanowires composite electrode towards supercapacitor applications, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.03.100. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Nitrogen-Containing Porous Carbon/α-MnO2 Nanowires Composite Electrode
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towards Supercapacitor Applications
Lidong Sun a, b, #, Nian Li a, #, Shudong Zhang a,*, Xinling Yu a, b, Cui Liu a, Yongqiang
a
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Zhou a, b, Shuai Han a, b, Wenbo Wang a, b, Zhenyang Wang a, *
Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031,
b
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China.
Department of Chemistry, University of Science and Technology of China, Hefei,
Anhui 230026, China.
These authors contributed equally to this work.
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#
* Corresponding Authors:
Abstract
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Email: [email protected], [email protected]
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Porous carbon composite electrodes have attracted extensive attention due to their excellent electrochemical performance toward energy storage applications. In this work, hierarchical nitrogen-containing porous carbon (HNPC) and α-MnO2 nanowires composite electrode is prepared through a template method. The as-prepared composite electrode, characterized as one-dimensional nanowires inserted into three-dimensional interconnected porous structures, exhibits high specific capacitance of 204.6 F g-1 at 1 A g-1 and excellent cycling stability (97.2 % remains after 5000 cycles). Moreover, the
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fabricated asymmetric supercapacitor based on HNPC//HNPC/MnO2 achieves a wide working voltage window of 1.8 V and a high energy density up to 30.1 Wh kg-1 at a power density of 900.0 W kg-1. The results indicate that the HNPC/MnO2 composite
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electrodes with excellent electrochemical performance have promising perspective in supercapacitor applications.
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Keywords: Porous carbon; Nitrogen doping; MnO2 nanowires; Pseudocapacitance; Supercapacitor
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1. Introduction
Presently, supercapacitors have attracted wide attention in several fields of high technology, such as new energy resource, smart grid, aeronautics and astronautics, due to their large power density, rapid charge and discharge, and excellent stability [1]. The
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challenge is to find ideal electrode materials which meet the high requirements of each parameter. The porous carbon materials as one of the most promising electrode materials [2], generally obtained from different biomass resources (eg. cotton [3], bacterial
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cellulose [4], gelatin [5], or wood agricultural waste [6]), have been receiving increasing
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attention for their high surface area, short diffusion paths, and excellent interconnectivities [7]. Porous carbon based electrode materials exhibit excellent performance, such as high specific capacitance and excellent cycle stability. However, the energy density of carbon based electric double-layer capacitors (EDLCs) is still limited [8, 9]. To improve the electrochemical properties of carbon materials, most researchers generally introduce heteroatom, metal oxides or conducting polymers [10, 11]. For instance, carbonization of raw materials with high heteroatom content is a
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successful method for introducing heteroatom doping in porous carbon electrodes. The nitrogen content of porous carbon carbonized from gelatin remains 9.6 at%. Although nitrogen doping is beneficial to the conductivity and specific capacitance of the electrode
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materials [12, 13, 14], it is still needed to further improve the performance to meet the requirement of practical applications. Metal oxides and conductive polymers are capable of
greatly
increasing
the
specific
capacitance
and
energy
density through
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pseudocapacitance redox reactions. Therefore, the electrode material combining
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heteroatom-doped carbon with pseudocapacitance material is highly desirable. Transition metal oxides, compared with conductive polymers, usually have higher specific capacitance, energy density and power density [15]. Among various metal oxides, manganese dioxide (MnO2) has attracted considerable interest for supercapacitor electrode material due to its abundance, low cost, environmental friendliness, and high
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theoretic specific capacitance [16]. It is well known that the morphology of MnO2 has remarkable influence on electrochemical properties [16]. Han at el. synthesized ε-MnO2 hollow microspheres which possess high specific surface area, resulting in the
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improvement of cycle stability and rate capacity [17]. Huang et al. fabricated mesoporous
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MnO2 nanotubes using polycarbonate membrane as the template through hydrothermal method [18]. The MnO2 nanotubes can accommodate large volume changes occurred during charge-discharge cycles, thereby extending electrode life. Yin et al. prepared long α-MnO2 nanowires with a relatively wide tunneling size (0.46 nm), which facilitates ion diffusion and thus increases capacitance [19]. The nanowire structure also shortens the ion diffusion pathway and adds additional electrically active sites [20]. Nevertheless, the poor conductivity of MnO2 electrodes limits their capacitance, cycling life and rate
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performance [16]. In order to address this obstacle, the combination of MnO2 nanowires with carbon material is an effective method because porous carbon can improve the conductivity of composite and increase the specific surface. To the best of our
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knowledge, the widely used synthetic methods for MnO2/porous carbon composite mainly include electrodeposition [21], hydrothermal treatment [9], and chemical coprecipitation [22]. The morphologies of MnO2 introduced into carbon materials by
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these methods are mainly thin film [23], needle-like [9] and amorphous [24]. However, there has been little direct work about some other excellent structures, such as hollow
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spheres, nanotubes and long nanowires, probably due to the difficulty to combine the unique MnO2 structures with porous carbon materials during the synthesis process. Herein, we synthesize the composite of hierarchical nitrogen-containing porous carbon (HNPC) with α-MnO2 nanowires inserted in through a template method. The
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mixture of MnO2, SiO2 and gelatin is treated at high temperature and subsequently with alkaline solution to realize the carbonization of gelatin and etch of SiO2 to form HNPC/MnO2 composite. The MnO2 nanowires are firstly prepared and then introduced
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into the process of porous carbon synthesis. Compared with the traditional method of
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preparing porous carbon materials by template method and then been combined with MnO2 by hydrothermal method, our strategy greatly simplifies the preparation process. Simultaneously, during the carbonization process, the carbon-coating formation from gelatin and SiO2 can be used as a semi-protective layer that suppresses the MnO2 phase transformation on the scale of reactions [25]. Finally, after the etching of the template [26], a three-dimensional porous composite material with a large specific surface area and a well-ordered pore structure is obtained. The hierarchical porous structure not only
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provides a continuous electron pathway to ensure high electrical conductivity, but also facilitates ion transportation by shortening the diffusion pathway. Therefore, we successfully synthesize the composites containing porous carbon material and MnO2 with
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specific morphology. Under the combined action of porous carbon, nitrogen doping and MnO2 nanowires, the unique structure of HNPC/MnO2 composite electrodes possessing a considerably high specific capacitance (204.6 F g-1 at 1 A g-1), large specific surface area
2. Experiments 2.1 Synthesis of MnO2 nanowires
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promising perspective in supercapacitor applications.
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(533.60 m2 g-1), and excellent cycling durability (97.2 % remains after 5000 cycles) have
All chemical reagents were analytical grade and directly used without further
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purification. MnO2 nanowires were synthesized through a hydrothermal method [27]. In a typical procedure, 0.75 g KMnO4 (Sinopharm Chemical Reagent Co. ltd., Shanghai, P.R. of China) and 1.30 g K2S2O8 (Sinopharm Chemical Reagent Co. ltd., Shanghai, P.R. of
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China) was dispersed in 15.00 ml distilled-deionized water by stirring for 10 minutes.
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The mixed solution was then transferred into a Teflon-lined stainless autoclave, and reacted at 140 °C for 24 h. After cooling down naturally to room temperature, the products were collected and then washed with deionized water and absolute ethylalcohol several times before they were dried in a vacuum oven at 60 °C overnight. 2.2 Synthesis of HNPC/MnO2 composite electrodes
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The HNPC/MnO2 was prepared using gelatin, SiO2 and MnO2 nanowires as the carbon precursor, template and pseudocapacitance material, respectively. Specifically, 2.00 g gelatin (Xilong Chemical, Guangdong of China), 2.00 g SiO2 (particle size of 7-40
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nm from Aladdin Industrial Corporation, Shanghai, China) and 0.10 g MnO2 nanowires were dispersed in 50.00 ml deionized water by sonication for 30 minutes. After that, the mixed solution was stirred in a water bath at 343 K until all the solvent were evaporated,
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and freeze dried at -40 °C for 12 h. The carbonization of the obtained mixture was firstly performed at 573 K for 2 h with a heating rate of 2 K min-1 and then carried out at 1073
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K for 1 h with a heating rate of 2 K min-1 under nitrogen atmosphere. Finally, the SiO2 templates were removed by etching the products in 2 M KOH solution at 333 K for 24h to generate HNPC/MnO2 composite. The mass ratio of MnO2 is 25 %, so the composite is labeled as HNPC/MnO2-25 %. In addition, three control experiments were carried out.
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The mass of MnO2 was set to be 0 g, 0.05 g and 0.30 g, while all other parameters remained the same. The obtained products were named as HNPC, HNPC/MnO2-14 % and HNPC/MnO2-50 %, respectively.
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2.3 Material characterizations
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The morphology of products was characterized by field-emission scanning electron
microscope (FE-SEM, Sirion200) and transmission electron microscopy (TEM, JEM2100F, Japan). The structure of products was analyzed by X-ray diffraction (XRD) Philips X’pert diffractometer (X’Pert Pro MPD) with Cu Kα radiation (1.5418 Å). Fourier-transform infrared (FTIR) spectra were collected on a FT-IR spectrometer (MagnaIR 750, Nicolet Instrument, Inc., USA). X-ray photoelectron spectroscopy (XPS) analysis of the product was measured by an ESCALAB 250 (Thermo-VG Scientific).
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Raman measurements were tested via a WiTec alpha300R Raman microscope with 2.5 mW of 532 nm laser light using a 55×/0.5 NA objective. Surface areas of the product
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were performed on a Tristar II 3020M through Brunauer-Emmet-Teller (BET). 2.4 Material electrochemical properties
The electrochemical performances of HNPC, MnO2 and HNPC/MnO2 were
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measured on a CHI 660E (CH instruments, Inc., Shanghai, China) electrochemical workstation at room temperature. It was performed in 2M Ca (NO3)2 aqueous solution by
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a three-electrode system where a platinum plate and saturated calomel electrode were used as the counter electrode and the reference electrode, respectively. The product (80 wt%), polytetrafluoroethylene binder (10 wt%), and acetylene black (10 wt%) were mixed evenly and then coated on a nickel foam with the size of 10 mm × 10 mm × 1 mm,
about 2 mg.
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and finally dried at 80 °C for 1 h in an oven. The total mass loading of each electrode was
The specific capacitance of a single electrode is calculated based on CV (Ccv, F g-1)
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and GCD (Cgcd, F g-1) by the following equations [23]:
Ccv =
∫ IdU vm∆V
Cgcd =
I × ∆t ∆V × m
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(1)
(2)
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Where, I (A) is response current, U (V) is potential voltage, v (V s-1) is scanning rate, m (g) is the mass of the active material in the electrode, ∆V (V) is the potential drop during
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discharge, and ∆t (s) is the discharge time, respectively. In an asymmetrical SC system, the mass ratio of electrode material load in two-
R=
m+ C− × ∆V− = m− C+ × ∆V+
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electrode configuration is calculated by the formula below [22]:
(3)
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Where, m+ and m- are loading mass of anode and cathode, C+ and C- are the specific capacitance of anode and cathode, and ∆V+ and ∆V- are the voltage windows of anode and cathode.
The energy density (E) and power density (P) of supercapacitors are calculated from
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charge-discharge curves according to following equations [28]:
E 1 I = × × ∆U t 2 m
1 2 E = × C × ( ∆U ) 2
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P=
(4)
(5)
Where, I is the constant discharge current, m is the total mass of active materials, ∆U is the voltage of the supercapacitor, and C is the specific capacitance of the supercapacitor. 3. Results and discussion 3.1 Characterization of HNPC/MnO2
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An overview of the synthesis process of HNPC/MnO2 nanowires is provided in Fig. 1. As can be seen, gelatin, SiO2, and MnO2 nanowires are mixed and sonicated in deionized water. The mixed solution is continuously stirred and heated until all the
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solvent is evaporated to obtain uniformly dispersive solid mixture. After the treatment of freeze drying and carbonization, the mixture is further etched with KOH solution to remove SiO2 nanoparticles. It is worth noting that SiO2 nanoparticles play an important
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role in the entire preparation process. For one thing, the etch of SiO2 nanoparticles template will create a large number of pore structures. These porous structures not only
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increase the specific surface area, but also can serve as a buffer reservoir for electrolyte ions to ensure the rapid diffusion of electrolyte ions and effectively shorten the diffusion paths during the rapid charge/discharge process. For another, in the preparation process, SiO2 is added in order to disperse and immobilize MnO2 nanowires. SiO2 can be evenly
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dispersed in the solution under stirring because of their small diameters (7-40 nm), resulting in viscous solutions. Therefore, the MnO2 nanowires added can be easily dispersed in the solution under stirring and ultrasonication. Finally, during the
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carbonization process, the carbon-coating formed from gelatin and SiO2 serves as a semiprotective layer which suppresses the phase transformation of MnO2 on the scale of the
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reaction [26]. The hierarchical porous structure of HNPC can effectively reduce the iontransport resistance and shorten the ion-diffusion pathway to promote the electrochemical performance [2]. Eventually, the purpose of well combining MnO2 with porous carbon material is achieved.
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Fig. 1. The schematic diagram of the preparation process of HNPC/MnO2 nanowires.
The morphology and microstructures of the product can be studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 2a shows the SEM images of HNPC, which possesses a hierarchical porous structure with high specific surface area (722.38 m2 g-1), micropores around 1.4 nm, and wide mesoporous
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distribution around 8-40 nm (Fig. S1a, e). Hierarchical porous structure with micropores and mesopores which are favorable for diffusion of active ions [42]. It can be seen from
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the morphology of HNPC/MnO2 in Fig. 2c that MnO2 nanowires are uniformly dispersed in HNPC. The composite materials also show high specific surface area of HNPC/MnO2-
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14 % (664.01 m2 g-1), HNPC/MnO2-25 % (533.60 m2 g-1), HNPC/MnO2-50 % (377.47 m2 g-1), respectively (Fig. S1b, c, d). Their microporous distribution is also about 1.4 nm, mesoporous distribution around 8-40nm (Fig. S1f, g, h). As reported, the existence of large mesopores facilitates ion diffusion, while small mesopores and micropores increase the ion-accessible surface area and thus contribute greatly to the specific capacitance [2]. The morphology of HNPC is further validated by TEM. The continuous 3D interconnected hierarchical structure consisting of highly interconnected carbon
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nanosheets is observed in HNPC (Fig. 2d). The overlap between mesopores generates the interconnected hierarchical structure. Fig. 2e and Fig. 2f display the structure and morphology of MnO2 nanowires and HNPC/MnO2. It can be clearly seen that the
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diameter of MnO2 is about 16-23 nm and they are composited by HNPC. The crystal structure of MnO2 nanowires is further studied by HRTEM. Fig. S2a shows the lattice fringes of MnO2 nanowires is 0.19 nm and 0.24 nm, corresponding to (510) and (211)
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crystal planes of α-MnO2. Besides, Fig. S2b shows the MnO2 nanowires lattice fringes of HNPC/MnO2 is 0.31 nm and 0.24 nm corresponding to (310) and (211) crystal planes of
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α-MnO2 [29].
Fig. 2. SEM images of (a) HNPC, (b) MnO2 nanowires and (c) HNPC/MnO2-25 %; TEM images of (d) HNPC, (e) MnO2 nanowires, (f) HNPC/MnO2-25 %.
The Fig. 3a illustrates the XRD patterns of HNPC, MnO2 nanowires and
HNPC/MnO2 composites. It is obvious that the diffraction peaks at around 12.90°, 18.21°, 25.76°, 28.84°, 36.76°, 37.72°, 42.15°, 50.04°, 56.20°, 60.20°, 65.50° and 69.43° correspond to (110), (200), (220), (310), (400), (211), (301), (411), (600), (521), (002)
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and (541) corresponding crystal plane of lattice fringes of α-MnO2, respectively (JCPDS, card no: 44-0141). This proves that pure tetragonal α-MnO2 is obtained [29]. There are two broad diffraction peaks located at about 25.13° and 44.12° in the XRD pattern of
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HNPC, which can be ascribed to the typical (002) and (101) crystal planes of graphitic carbon, respectively [41]. The carbon-coating formation from gelation and SiO2 serves as a protective layer that suppresses partial phase transformation reactions. The diffraction
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peaks of the HNPC/MnO2 at 12.78°, 18.03°, 28.78°, 37.65°, 41.91° and 49.76° can be corresponded to (110), (200), (310), (400), (211) and (301) crystal planes, respectively. It
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can be observed that the XRD patterns of MnO2 are essentially the same in MnO2 nanowires and HNPC/MnO2 composites, which shows that the crystal form and structure of MnO2 do not change before and after composite with HNPC. Furthermore, the HNPC, MnO2 and HNPC/MnO2 are also investigated by Raman spectra (Fig. 3b). The peaks of
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HNPC and HNPC/MnO2 located at 1373 and 1588 cm-1, are characteristic D and G bands of carbon materials, respectively [12]. The peaks at about 573 and 640 cm-1 in MnO2 and HNPC/MnO2 are due to symmetric stretching vibration (Mn-O) of the MnO6 groups.
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FTIR spectra of HNPC, MnO2 and HNPC/MnO2 are shown in Fig. 3c. The bands appeared around 3320-3650 and 1636 cm-1 can be ascribed to the bending vibration of
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intercalated water molecules and the skeletal vibration of aromatic C=C and the O-H stretching vibration of water molecules, respectively. The weak peaks around 428-590 cm-1 in the spectra of HNPC/MnO2 and MnO2 can be attributed to the Mn-O-Mn and MnO vibrations [23].
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(b)300 Intensity (a.u.)
200
100
80
500
1000
1500
2000
Raman shift (cm-1)
4000
458 521
40 60 2θ (degree)
MnO2
3435
0 20
HNPC HNPC/MnO2
HNPC/MnO2
1636
Intensity (a.u.)
HNPC/MnO2
(c)
HNPC MnO2
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JCPDS 044-0141 HNPC MnO2
Intensity (a.u.)
(a)
3000
2000 1000 Wavenumber (cm-1)
Fig. 3. Composition characterization of HNPC, HNPC/MnO2 and MnO2: (a) XRD patterns, (b) Raman
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spectra, (c) FTIR spectra.
In the analysis of XPS measurements, the characteristic peaks of C, O and Mn peaks
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can be observed in HNPC/MnO2-25 % in Fig. 4a, suggesting the presence of MnO2. The curve fitting of C 1s is shown in Figure S4a, the main sub-pecks of 284.6, 285.2, 286.2, 289.2 and 291.2 eV are respectively ascribed to C-C, C-N, C-O, C=O and O-C=O bonds. The nitrogen content of HNPC/MnO2-25 % composite is 8.6 at%. The curve fitting of
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N1s shows four types of N-containing groups (Fig. 4d), which are respectively ascribed to pyridinic-N (398.3 eV), pyrrolic-N (400.5 eV), quaternary-N (401.4 eV), and N-oxide (404.0 eV) [45, 46]. The pyrrolic-N and pyridinic-N can generally provide the
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pseudocapacitance [12]. The quaternary-N can improve the electrical conductivity of carbon, which is benefit for electron transfer [12]. There are two peaks in the Mn 2p
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spectrum (Fig. 4b) with the binding energy values of 642.3 and 653.9 eV, which can be attributed to the spin orbit doublets of Mn 2p3/2 and Mn 2p1/2 peaks in α-MnO2, respectively. In Fig. 4c, the ∆E of Mn 3s peaks is 4.8 eV in the HNPC/MnO2-25 %. Binding energy widths of 5.79, 5.50, 5.41 and 4.8 eV are acquired from genuine samples of MnO, Mn3O4, Mn2O3 and MnO2, respectively [43]. The result shows that the average valence state of Mn element is 4, indicating that the most existence form of manganese is Mn4+ [22]. As shown in Fig. S3b, the XPS spectrum of O1s peak basically indicates three
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components centered at 529.9, 531.6 and 533.5 eV. The O1s peak at 529.9 eV is assigned to Mn-O bond in α-MnO2 crystal lattice. The peaks of 531.6 eV and 533.5 eV are ascribed the hydrated trivalent oxide (Mn-O-H) and the residual water (H-O-H),
the oxidized states.
(a)
(b)
Mn2P3/2
O Mn 2p
Mn 3s 0
200
400
600
O
800
1000
660
Energy binding (eV)
(c)
4.8eV
Mn2P1/2
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N
Mn2p
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Intensity (a.u.)
Intensity (a.u.)
C
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respectively [22]. Obviously, all the manganese elements in the composite material are in
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(d)
Mn3s
650
645
640
635
Binding energy (eV)
pyrrolic-N
N1s
94
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Intensity (a.u.)
Intensity (a.u.)
pyridinic-N
92
90 88 86 84 82 Binding energy (eV)
80
quaternary-N
N-oxide
408
404 400 396 Binding energy (eV)
392
Fig. 4. XPS patterns of the HNPC/MnO2-25 %: (a) XPS survey spectrum; High spectra of (b) Mn 2p,
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(c) Mn 3s, (d) N 1s.
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3.2 Electrochemical characterization Because of the large surface area of mesoporous structure with a short ion diffusion
path, high electrical conductivity of the HNPC and the pseudocapacitor property of MnO2, the composite has excellent electrochemical performance. The electrochemical performances of HNPC, HNPC/MnO2-14 %, HNPC/MnO2-25 % and HNPC/MnO2-50 % are evaluated using the technologies of cyclic voltammograms (CV), the galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) in 2 mol L-1
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Ca (NO3)2 aqueous solutions. The solution of Ca (NO3)2 is selected as the electrolyte concerning the fact that the specific capacitance value of MnO2 can be greatly improved
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using a divalent ion such as Ca2+ as electrolyte ion instead of a univalent cation [28, 44]. Fig. 5a displays the CV curves of HNPC, MnO2, and HNPC/MnO2-25 % electrodes at 100 mV s-1 scan rates within a voltage range from 0.0 to 1.0 V. The shape of CV
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curves of HNPC is nearly symmetric rectangle, indicating an ideal double-layer capacitor behavior. From the GCD curves in Fig. 5b, the discharge time of HNPC/MnO2 is
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obviously longer than those of pure HNPC and pure MnO2 at 1 A g-1. Due to the pseudocapacitance properties of MnO2 and the large specific surface area of HNPC, the HNPC/MnO2 capacitance is largely improved compared with pure HNPC and MnO2. It shows that there is a synergistic effect in the composite, fully exerting the large specific surface area and conductive properties of HNPC, and the pseudocapacitance properties of
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MnO2 for high special capacitance.
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20
HNPC/MnO2-25%
10
Potential (V)
0 -10 -20
1.0
HNPC/MnO2-25 %
0.8 0.6 0.4
0.0
0.0
(c) 30
0.2
0.4 0.6 0.8 Potential (V)
25
HNPC/MnO2-14 %
20
HNPC/MnO2-25 %
15
HNPC/MnO2-50 %
1.0
0
100
(d)
10
Potential (V)
Current density (A g-1)
HNPC MnO2 nanowires
0.2
-30
5 0 -5 -10
0.8
HNPC/MnO2-50 %
0.6 0.4
0.4
0.6
0.8
Potential (V)
(e) 40
5 mv s-1 20 mv s-1
20
0
1.0
100 200 300 400 500 600 700 Time (s)
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0.2
(f)
1.0
Potential (V)
0.8
0
-20
100 mv s-1 200 mv s-1
-40
400
HNPC/MnO2-14 % HNPC/MnO2-25 %
0.0 0.0
200 300 Time (s)
1.0
0.2
-15 -20
Current density (A g-1)
1.2
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Current density (A g-1)
(b)
HNPC MnO2 nanowires
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(a) 30
0.6
0.5 A g-1 1 A g-1 5 A g-1 10A g-1
0.4 0.2 0.0
0.2
0.4 0.6 0.8 Potential (V)
1.0
0
250
500 Time (s)
750
1000
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0.0
Fig. 5. (a) CV curves of HNPC, MnO2 nanowires and HNPC/MnO2; (b) GCD curves of HNPC, MnO2 nanowires and HNPC/MnO2; (c) CV curves of HNPC/MnO2 at different MnO2 content; (d) GCD curves of HNPC/MnO2 at different MnO2 content; (e) CV curves of HNPC/MnO2 at different scan
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rates; (f) GCD curves of HNPC/MnO2 at various current densities.
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Fig. 5c and Fig. 5d show the CV and GCD curves of HNPC composite with different contents of MnO2. Obviously, the HNPC/MnO2-25 % exhibits high specific capacitance of 204.6 F g-1 at current densities of 1 A g-1, much higher than HNPC/MnO214 % and HNPC/MnO2-25 %, thus, the HNPC/MnO2-25 % is the best among the three composites. The suitable mass ratio of the HNPC and MnO2 can take full advantage of the pseudocapacitance properties of MnO2 and the large specific surface area of HNPC to improve the electrochemical performance of the composite.
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Furthermore, the discharge specific capacitance of the HNPC/MnO2-25 % (Fig. 5f) is calculated to be 216.0, 204.6, 162.3 and 147.5 F g-1 at current densities of 0.5, 1, 5 and 10 A g-1, respectively. Fig. S4 shows that the specific capacitance of HNPC/MnO2-14 %,
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HNPC/MnO2-25 % and HNPC/MnO2-50 % at different scan rates. Clearly, the specific capacitance of HNPC/MnO2-25 % and HNPC/MnO2-50 % are almost the same and are both higher than that of HNPC/MnO2-14 %. As the scan rate increases, the capacitance of
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HNPC/MnO2-25 % declines slowly compared with the capacitance of HNPC/MnO2-50 %. The capacitance of HNPC/MnO2-25 % at 200 mV s-1 is still 64.5 % of the values at 5
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mV s-1, and the capacitance of HNPC/MnO2-50 % at 200 mV s-1 is only 45.1 % of the values at 5 mV s-1. The result shows that the HNPC/MnO2-25 % also has an excellent
(a) 80
HNPC HNPC/MnO2
60
MnO2
40 30 20
7 6 5
Z''/ohm
Z'' / ohm
50
(b)100
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4 3 2 1
10
0
0
0
10
20
2
3 4 Z'/ohm
30 40 50 Z' / ohm
5
60
6
70
80 60 40 20
HNPC/MnO2-25%
7
80
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0
1
Capacitance retention (%)
rate performance.
0 0
1000 2000 3000 4000 5000 Cycling number
Fig. 6. (a) Nyquist plots of HNPC, MnO2 nanowires, and HNPC/MnO2; (b) the cycle stability of
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HNPC/MnO2-25 % at a current density of 10.0 A g-1 for 5000 cycles.
The Nyquist plot (Fig. 6a) can evaluate the electric resistance of the HNPC, α-MnO2
and HNPC/MnO2-25 %. The shape of Nyquist plots of the HNPC, α-MnO2 and HNPC/MnO2-25 % are all semicircular at high−frequency region, representing the charge transfer resistance (Rct). The Rct of HNPC/MnO2-25 % is smaller than the Rct of α-MnO2, caused by the reason that the specific hierarchically porous structure of HNPC/MnO2-25 % can increase the contact between the electrolyte and electrode. The linear slope of the 17
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Nyquist plots in the low-frequency zone stands for diffusion resistance [21]. Apparently, the slope of HNPC/MnO2-25 % is much bigger than that of the pure α-MnO2, which means that the HNPC/MnO2-25 % possesses lower diffusive resistance and faster ion
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transport than pure α-MnO2. Additionally, the cycle stability test (Fig. 6b) results show that the capacitance of HNPC/MnO2-25 % remains 97.2 % after cycling 5000 times at 10.0 A g-1, indicating excellent cycling stability without obvious performance
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degradation. The excellent cycling stability is determined by the structure of composite materials. This hierarchical porous structure provides effective diffusion pathways and
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more active sites for electrolyte ions. Therefore, the HNPC/MnO2-25 % composite exhibits high capacitance and excellent cycle stability performance due to its high
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specific surface area and the high pseudocapacitive behavior of MnO2.
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(b) 15
0 -10 -20
5
0
-5 -0.5 0.0 0.5 Potential (V) 10 mV s-1 50 mV s-1
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100 mV s-1 200 mV s-1
0.0
(d)2.0
0
0.4
1.0
0.5
0.0
1.6
(f) 100
1 A g-1 2 A g-1 3 A g-1
1.6 1.2
5 A g-1 10 A g-1
0.8 0.4 0.0
Capacitance retention (%)
(e) 2.0
0.8 1.2 Potential (V)
2.0
0-1 V 0-1.2 V 0-1.4 V 0-1.6 V 0-1.8 V
-10 0.0
1.0 1.5 Potential (V)
1.5
500 mV s-1
10
0.5
0
50
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30
1.0
Potential (V)
(c)
Potential (V)
0-1V 0-1.2 V 0-1.4 V 0-1.6 V 0-1.8 V
10
100
150 200 Time (s)
250
300
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-1 Current Density (A g )
10
-1.0
Current Density (A g-1)
Current Density (A g-1)
HNPC HNPC/MnO2-25%
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(a)
80 60 40 20
HNPC//HNPC/MnO2
0
0
50
100 150 200 250 300 Time (s)
0
500 1000 1500 2000 2500 3000 Cycling number
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Fig. 7. (a) CV curves of HNPC and HNPC/MnO2 electrodes at a scan of 100 mV s-1; (b) CV curves of the ASCs at different potential windows at a scan rate of 100 mV s-1; (c) CV curves of the ASCs at different scan rates; (d) GCD curves of the ASCs at a current density of 1.0 A g-1; (e) GCD curves of
cycles.
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ASCs at different density; (f) the cycle stability of ACSs at a current density of 5.0 A g-1 for 3000
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The asymmetric supercapacitors (ASCs) HNPC//HNPC/MnO2-25 % is assembled with HNPC as the anode and HNPC/MnO2-25 % as the cathode, respectively. In order to investigate
the
steady
voltage
window
and
electrochemical
properties
of
HNPC//HNPC/MnO2-25 %, the CV measurements of the composite is carried out in 2.0 M Ca(NO3)2 at 100 mV s-1 (Fig. 7a). The stable voltage window of the as-prepared HNPC//HNPC/MnO2 asymmetric SC can be extended to be 1.8 V. It also can be seen that HNPC and HNPC/MnO2 have good cyclic reversibility from Fig. 7d. Furthermore, the
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voltage window of ASCs can reach 1.8 V, which is greatly extended comparing with that of the SCs. And, the mass ratio between anode (HNPC) and cathode (HNPC/MnO2-25 %) in ASCs is 2:1, which is calculated by eq 3. The GCD curves of HNPC//HNPC/MnO2
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at different current densities shows that the ASCs have a high specific capacitance of 66.9 F g-1 at 1 A g-1. In addition, the capacitance of HNPC//HNPC/MnO2 remains 88.3% after cycling 3000 times at 5.0 A g-1 (Fig. 7f). It shows that ASCs have excellent cycle
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stability. Moreover, because of its wide voltage window and high specific capacitance, the prepared device has a high energy density of 30.1 Wh kg-1 at power density of 900.0
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W kg-1. The result is higher than that of some similar reported devices (Fig. 8a). As shown in Fig. 8b, the Light-emitting diodes (LED) is successfully lit up by two devices in series, demonstrating that the HNPC/MnO2 is a potential electrode material for energy
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storage devices.
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Fig. 8. (a) Ragone plot of the ASCs; (b) The ASCs illuminate the Light-emitting diodes (LED).
4. Conclusions
A new method for preparing porous carbon materials and MnO2 composite materials
is presented. The HNPC/MnO2 composite fabricated by the template method have a unique nanowire interpenetrating hierarchical porous structure, a large specific surface area and an ordered porous structure. HNPC/MnO2 composite have a high specific
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capacitance of 204.6 F g-1 at 1 A g-1, which remains 97.2 % after 5000 cycles. The high capacitance is mainly due to the hierarchical porous structure and the pseudocapacitive effect of nitrogen doping and MnO2 nanowires. The good cycle stability is benefit from
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the hierarchical porous carbon nanostructures which provides a continuous electron path to ensure good electrical contact and also shorten the diffusion path of ion transportation. The assembled HNPC//HNPC/MnO2 nanowires asymmetric supercapacitor exhibits a
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wide working voltage window of 1.8 V and achieve an energy density of 30.1 Wh kg-1 at
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a power density of 900.0 W kg-1. Acknowledgements
This study was financially supported by the National Natural Science Foundation of China (no. 61605222, U1432132), the Innovative Program of the Development
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Foundation of Hefei Center for Physical Science and Technology (2017FXCX006) and the Youth “spark” of Chinese Academy of China (YZJJ2018QN25).
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ACCEPTED MANUSCRIPT Highlights: 1. The composite of porous carbon and α-MnO2 nanowires with good electrochemical performance is successfully prepared.
increases ion-accessible surface area.
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2. The hierarchical porous structure of composite facilitates ion diffusion and
3. The composite electrodes exhibit excellent cycling durability (97.2 % remains after
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5000 cycles).
S0925-8388(19)30907-7
DOI:
https://doi.org/10.1016/j.jallcom.2019.03.100
Reference:
JALCOM 49864
To appear in:
Journal of Alloys and Compounds
Received Date: 30 November 2018 Revised Date:
3 February 2019
Accepted Date: 5 March 2019
Please cite this article as: L. Sun, N. Li, S. Zhang, X. Yu, C. Liu, Y. Zhou, S. Han, W. Wang, Z. Wang, Nitrogen-containing porous carbon/α-MnO2 nanowires composite electrode towards supercapacitor applications, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.03.100. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Nitrogen-Containing Porous Carbon/α-MnO2 Nanowires Composite Electrode
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towards Supercapacitor Applications
Lidong Sun a, b, #, Nian Li a, #, Shudong Zhang a,*, Xinling Yu a, b, Cui Liu a, Yongqiang
a
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Zhou a, b, Shuai Han a, b, Wenbo Wang a, b, Zhenyang Wang a, *
Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031,
b
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China.
Department of Chemistry, University of Science and Technology of China, Hefei,
Anhui 230026, China.
These authors contributed equally to this work.
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#
* Corresponding Authors:
Abstract
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Email: [email protected], [email protected]
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Porous carbon composite electrodes have attracted extensive attention due to their excellent electrochemical performance toward energy storage applications. In this work, hierarchical nitrogen-containing porous carbon (HNPC) and α-MnO2 nanowires composite electrode is prepared through a template method. The as-prepared composite electrode, characterized as one-dimensional nanowires inserted into three-dimensional interconnected porous structures, exhibits high specific capacitance of 204.6 F g-1 at 1 A g-1 and excellent cycling stability (97.2 % remains after 5000 cycles). Moreover, the
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fabricated asymmetric supercapacitor based on HNPC//HNPC/MnO2 achieves a wide working voltage window of 1.8 V and a high energy density up to 30.1 Wh kg-1 at a power density of 900.0 W kg-1. The results indicate that the HNPC/MnO2 composite
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electrodes with excellent electrochemical performance have promising perspective in supercapacitor applications.
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Keywords: Porous carbon; Nitrogen doping; MnO2 nanowires; Pseudocapacitance; Supercapacitor
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1. Introduction
Presently, supercapacitors have attracted wide attention in several fields of high technology, such as new energy resource, smart grid, aeronautics and astronautics, due to their large power density, rapid charge and discharge, and excellent stability [1]. The
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challenge is to find ideal electrode materials which meet the high requirements of each parameter. The porous carbon materials as one of the most promising electrode materials [2], generally obtained from different biomass resources (eg. cotton [3], bacterial
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cellulose [4], gelatin [5], or wood agricultural waste [6]), have been receiving increasing
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attention for their high surface area, short diffusion paths, and excellent interconnectivities [7]. Porous carbon based electrode materials exhibit excellent performance, such as high specific capacitance and excellent cycle stability. However, the energy density of carbon based electric double-layer capacitors (EDLCs) is still limited [8, 9]. To improve the electrochemical properties of carbon materials, most researchers generally introduce heteroatom, metal oxides or conducting polymers [10, 11]. For instance, carbonization of raw materials with high heteroatom content is a
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successful method for introducing heteroatom doping in porous carbon electrodes. The nitrogen content of porous carbon carbonized from gelatin remains 9.6 at%. Although nitrogen doping is beneficial to the conductivity and specific capacitance of the electrode
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materials [12, 13, 14], it is still needed to further improve the performance to meet the requirement of practical applications. Metal oxides and conductive polymers are capable of
greatly
increasing
the
specific
capacitance
and
energy
density through
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pseudocapacitance redox reactions. Therefore, the electrode material combining
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heteroatom-doped carbon with pseudocapacitance material is highly desirable. Transition metal oxides, compared with conductive polymers, usually have higher specific capacitance, energy density and power density [15]. Among various metal oxides, manganese dioxide (MnO2) has attracted considerable interest for supercapacitor electrode material due to its abundance, low cost, environmental friendliness, and high
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theoretic specific capacitance [16]. It is well known that the morphology of MnO2 has remarkable influence on electrochemical properties [16]. Han at el. synthesized ε-MnO2 hollow microspheres which possess high specific surface area, resulting in the
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improvement of cycle stability and rate capacity [17]. Huang et al. fabricated mesoporous
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MnO2 nanotubes using polycarbonate membrane as the template through hydrothermal method [18]. The MnO2 nanotubes can accommodate large volume changes occurred during charge-discharge cycles, thereby extending electrode life. Yin et al. prepared long α-MnO2 nanowires with a relatively wide tunneling size (0.46 nm), which facilitates ion diffusion and thus increases capacitance [19]. The nanowire structure also shortens the ion diffusion pathway and adds additional electrically active sites [20]. Nevertheless, the poor conductivity of MnO2 electrodes limits their capacitance, cycling life and rate
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performance [16]. In order to address this obstacle, the combination of MnO2 nanowires with carbon material is an effective method because porous carbon can improve the conductivity of composite and increase the specific surface. To the best of our
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knowledge, the widely used synthetic methods for MnO2/porous carbon composite mainly include electrodeposition [21], hydrothermal treatment [9], and chemical coprecipitation [22]. The morphologies of MnO2 introduced into carbon materials by
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these methods are mainly thin film [23], needle-like [9] and amorphous [24]. However, there has been little direct work about some other excellent structures, such as hollow
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spheres, nanotubes and long nanowires, probably due to the difficulty to combine the unique MnO2 structures with porous carbon materials during the synthesis process. Herein, we synthesize the composite of hierarchical nitrogen-containing porous carbon (HNPC) with α-MnO2 nanowires inserted in through a template method. The
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mixture of MnO2, SiO2 and gelatin is treated at high temperature and subsequently with alkaline solution to realize the carbonization of gelatin and etch of SiO2 to form HNPC/MnO2 composite. The MnO2 nanowires are firstly prepared and then introduced
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into the process of porous carbon synthesis. Compared with the traditional method of
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preparing porous carbon materials by template method and then been combined with MnO2 by hydrothermal method, our strategy greatly simplifies the preparation process. Simultaneously, during the carbonization process, the carbon-coating formation from gelatin and SiO2 can be used as a semi-protective layer that suppresses the MnO2 phase transformation on the scale of reactions [25]. Finally, after the etching of the template [26], a three-dimensional porous composite material with a large specific surface area and a well-ordered pore structure is obtained. The hierarchical porous structure not only
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provides a continuous electron pathway to ensure high electrical conductivity, but also facilitates ion transportation by shortening the diffusion pathway. Therefore, we successfully synthesize the composites containing porous carbon material and MnO2 with
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specific morphology. Under the combined action of porous carbon, nitrogen doping and MnO2 nanowires, the unique structure of HNPC/MnO2 composite electrodes possessing a considerably high specific capacitance (204.6 F g-1 at 1 A g-1), large specific surface area
2. Experiments 2.1 Synthesis of MnO2 nanowires
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promising perspective in supercapacitor applications.
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(533.60 m2 g-1), and excellent cycling durability (97.2 % remains after 5000 cycles) have
All chemical reagents were analytical grade and directly used without further
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purification. MnO2 nanowires were synthesized through a hydrothermal method [27]. In a typical procedure, 0.75 g KMnO4 (Sinopharm Chemical Reagent Co. ltd., Shanghai, P.R. of China) and 1.30 g K2S2O8 (Sinopharm Chemical Reagent Co. ltd., Shanghai, P.R. of
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China) was dispersed in 15.00 ml distilled-deionized water by stirring for 10 minutes.
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The mixed solution was then transferred into a Teflon-lined stainless autoclave, and reacted at 140 °C for 24 h. After cooling down naturally to room temperature, the products were collected and then washed with deionized water and absolute ethylalcohol several times before they were dried in a vacuum oven at 60 °C overnight. 2.2 Synthesis of HNPC/MnO2 composite electrodes
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The HNPC/MnO2 was prepared using gelatin, SiO2 and MnO2 nanowires as the carbon precursor, template and pseudocapacitance material, respectively. Specifically, 2.00 g gelatin (Xilong Chemical, Guangdong of China), 2.00 g SiO2 (particle size of 7-40
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nm from Aladdin Industrial Corporation, Shanghai, China) and 0.10 g MnO2 nanowires were dispersed in 50.00 ml deionized water by sonication for 30 minutes. After that, the mixed solution was stirred in a water bath at 343 K until all the solvent were evaporated,
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and freeze dried at -40 °C for 12 h. The carbonization of the obtained mixture was firstly performed at 573 K for 2 h with a heating rate of 2 K min-1 and then carried out at 1073
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K for 1 h with a heating rate of 2 K min-1 under nitrogen atmosphere. Finally, the SiO2 templates were removed by etching the products in 2 M KOH solution at 333 K for 24h to generate HNPC/MnO2 composite. The mass ratio of MnO2 is 25 %, so the composite is labeled as HNPC/MnO2-25 %. In addition, three control experiments were carried out.
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The mass of MnO2 was set to be 0 g, 0.05 g and 0.30 g, while all other parameters remained the same. The obtained products were named as HNPC, HNPC/MnO2-14 % and HNPC/MnO2-50 %, respectively.
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2.3 Material characterizations
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The morphology of products was characterized by field-emission scanning electron
microscope (FE-SEM, Sirion200) and transmission electron microscopy (TEM, JEM2100F, Japan). The structure of products was analyzed by X-ray diffraction (XRD) Philips X’pert diffractometer (X’Pert Pro MPD) with Cu Kα radiation (1.5418 Å). Fourier-transform infrared (FTIR) spectra were collected on a FT-IR spectrometer (MagnaIR 750, Nicolet Instrument, Inc., USA). X-ray photoelectron spectroscopy (XPS) analysis of the product was measured by an ESCALAB 250 (Thermo-VG Scientific).
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Raman measurements were tested via a WiTec alpha300R Raman microscope with 2.5 mW of 532 nm laser light using a 55×/0.5 NA objective. Surface areas of the product
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were performed on a Tristar II 3020M through Brunauer-Emmet-Teller (BET). 2.4 Material electrochemical properties
The electrochemical performances of HNPC, MnO2 and HNPC/MnO2 were
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measured on a CHI 660E (CH instruments, Inc., Shanghai, China) electrochemical workstation at room temperature. It was performed in 2M Ca (NO3)2 aqueous solution by
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a three-electrode system where a platinum plate and saturated calomel electrode were used as the counter electrode and the reference electrode, respectively. The product (80 wt%), polytetrafluoroethylene binder (10 wt%), and acetylene black (10 wt%) were mixed evenly and then coated on a nickel foam with the size of 10 mm × 10 mm × 1 mm,
about 2 mg.
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and finally dried at 80 °C for 1 h in an oven. The total mass loading of each electrode was
The specific capacitance of a single electrode is calculated based on CV (Ccv, F g-1)
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and GCD (Cgcd, F g-1) by the following equations [23]:
Ccv =
∫ IdU vm∆V
Cgcd =
I × ∆t ∆V × m
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(1)
(2)
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Where, I (A) is response current, U (V) is potential voltage, v (V s-1) is scanning rate, m (g) is the mass of the active material in the electrode, ∆V (V) is the potential drop during
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discharge, and ∆t (s) is the discharge time, respectively. In an asymmetrical SC system, the mass ratio of electrode material load in two-
R=
m+ C− × ∆V− = m− C+ × ∆V+
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electrode configuration is calculated by the formula below [22]:
(3)
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Where, m+ and m- are loading mass of anode and cathode, C+ and C- are the specific capacitance of anode and cathode, and ∆V+ and ∆V- are the voltage windows of anode and cathode.
The energy density (E) and power density (P) of supercapacitors are calculated from
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charge-discharge curves according to following equations [28]:
E 1 I = × × ∆U t 2 m
1 2 E = × C × ( ∆U ) 2
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P=
(4)
(5)
Where, I is the constant discharge current, m is the total mass of active materials, ∆U is the voltage of the supercapacitor, and C is the specific capacitance of the supercapacitor. 3. Results and discussion 3.1 Characterization of HNPC/MnO2
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An overview of the synthesis process of HNPC/MnO2 nanowires is provided in Fig. 1. As can be seen, gelatin, SiO2, and MnO2 nanowires are mixed and sonicated in deionized water. The mixed solution is continuously stirred and heated until all the
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solvent is evaporated to obtain uniformly dispersive solid mixture. After the treatment of freeze drying and carbonization, the mixture is further etched with KOH solution to remove SiO2 nanoparticles. It is worth noting that SiO2 nanoparticles play an important
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role in the entire preparation process. For one thing, the etch of SiO2 nanoparticles template will create a large number of pore structures. These porous structures not only
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increase the specific surface area, but also can serve as a buffer reservoir for electrolyte ions to ensure the rapid diffusion of electrolyte ions and effectively shorten the diffusion paths during the rapid charge/discharge process. For another, in the preparation process, SiO2 is added in order to disperse and immobilize MnO2 nanowires. SiO2 can be evenly
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dispersed in the solution under stirring because of their small diameters (7-40 nm), resulting in viscous solutions. Therefore, the MnO2 nanowires added can be easily dispersed in the solution under stirring and ultrasonication. Finally, during the
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carbonization process, the carbon-coating formed from gelatin and SiO2 serves as a semiprotective layer which suppresses the phase transformation of MnO2 on the scale of the
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reaction [26]. The hierarchical porous structure of HNPC can effectively reduce the iontransport resistance and shorten the ion-diffusion pathway to promote the electrochemical performance [2]. Eventually, the purpose of well combining MnO2 with porous carbon material is achieved.
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Fig. 1. The schematic diagram of the preparation process of HNPC/MnO2 nanowires.
The morphology and microstructures of the product can be studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 2a shows the SEM images of HNPC, which possesses a hierarchical porous structure with high specific surface area (722.38 m2 g-1), micropores around 1.4 nm, and wide mesoporous
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distribution around 8-40 nm (Fig. S1a, e). Hierarchical porous structure with micropores and mesopores which are favorable for diffusion of active ions [42]. It can be seen from
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the morphology of HNPC/MnO2 in Fig. 2c that MnO2 nanowires are uniformly dispersed in HNPC. The composite materials also show high specific surface area of HNPC/MnO2-
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14 % (664.01 m2 g-1), HNPC/MnO2-25 % (533.60 m2 g-1), HNPC/MnO2-50 % (377.47 m2 g-1), respectively (Fig. S1b, c, d). Their microporous distribution is also about 1.4 nm, mesoporous distribution around 8-40nm (Fig. S1f, g, h). As reported, the existence of large mesopores facilitates ion diffusion, while small mesopores and micropores increase the ion-accessible surface area and thus contribute greatly to the specific capacitance [2]. The morphology of HNPC is further validated by TEM. The continuous 3D interconnected hierarchical structure consisting of highly interconnected carbon
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nanosheets is observed in HNPC (Fig. 2d). The overlap between mesopores generates the interconnected hierarchical structure. Fig. 2e and Fig. 2f display the structure and morphology of MnO2 nanowires and HNPC/MnO2. It can be clearly seen that the
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diameter of MnO2 is about 16-23 nm and they are composited by HNPC. The crystal structure of MnO2 nanowires is further studied by HRTEM. Fig. S2a shows the lattice fringes of MnO2 nanowires is 0.19 nm and 0.24 nm, corresponding to (510) and (211)
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crystal planes of α-MnO2. Besides, Fig. S2b shows the MnO2 nanowires lattice fringes of HNPC/MnO2 is 0.31 nm and 0.24 nm corresponding to (310) and (211) crystal planes of
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α-MnO2 [29].
Fig. 2. SEM images of (a) HNPC, (b) MnO2 nanowires and (c) HNPC/MnO2-25 %; TEM images of (d) HNPC, (e) MnO2 nanowires, (f) HNPC/MnO2-25 %.
The Fig. 3a illustrates the XRD patterns of HNPC, MnO2 nanowires and
HNPC/MnO2 composites. It is obvious that the diffraction peaks at around 12.90°, 18.21°, 25.76°, 28.84°, 36.76°, 37.72°, 42.15°, 50.04°, 56.20°, 60.20°, 65.50° and 69.43° correspond to (110), (200), (220), (310), (400), (211), (301), (411), (600), (521), (002)
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and (541) corresponding crystal plane of lattice fringes of α-MnO2, respectively (JCPDS, card no: 44-0141). This proves that pure tetragonal α-MnO2 is obtained [29]. There are two broad diffraction peaks located at about 25.13° and 44.12° in the XRD pattern of
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HNPC, which can be ascribed to the typical (002) and (101) crystal planes of graphitic carbon, respectively [41]. The carbon-coating formation from gelation and SiO2 serves as a protective layer that suppresses partial phase transformation reactions. The diffraction
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peaks of the HNPC/MnO2 at 12.78°, 18.03°, 28.78°, 37.65°, 41.91° and 49.76° can be corresponded to (110), (200), (310), (400), (211) and (301) crystal planes, respectively. It
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can be observed that the XRD patterns of MnO2 are essentially the same in MnO2 nanowires and HNPC/MnO2 composites, which shows that the crystal form and structure of MnO2 do not change before and after composite with HNPC. Furthermore, the HNPC, MnO2 and HNPC/MnO2 are also investigated by Raman spectra (Fig. 3b). The peaks of
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HNPC and HNPC/MnO2 located at 1373 and 1588 cm-1, are characteristic D and G bands of carbon materials, respectively [12]. The peaks at about 573 and 640 cm-1 in MnO2 and HNPC/MnO2 are due to symmetric stretching vibration (Mn-O) of the MnO6 groups.
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FTIR spectra of HNPC, MnO2 and HNPC/MnO2 are shown in Fig. 3c. The bands appeared around 3320-3650 and 1636 cm-1 can be ascribed to the bending vibration of
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intercalated water molecules and the skeletal vibration of aromatic C=C and the O-H stretching vibration of water molecules, respectively. The weak peaks around 428-590 cm-1 in the spectra of HNPC/MnO2 and MnO2 can be attributed to the Mn-O-Mn and MnO vibrations [23].
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(b)300 Intensity (a.u.)
200
100
80
500
1000
1500
2000
Raman shift (cm-1)
4000
458 521
40 60 2θ (degree)
MnO2
3435
0 20
HNPC HNPC/MnO2
HNPC/MnO2
1636
Intensity (a.u.)
HNPC/MnO2
(c)
HNPC MnO2
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JCPDS 044-0141 HNPC MnO2
Intensity (a.u.)
(a)
3000
2000 1000 Wavenumber (cm-1)
Fig. 3. Composition characterization of HNPC, HNPC/MnO2 and MnO2: (a) XRD patterns, (b) Raman
SC
spectra, (c) FTIR spectra.
In the analysis of XPS measurements, the characteristic peaks of C, O and Mn peaks
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can be observed in HNPC/MnO2-25 % in Fig. 4a, suggesting the presence of MnO2. The curve fitting of C 1s is shown in Figure S4a, the main sub-pecks of 284.6, 285.2, 286.2, 289.2 and 291.2 eV are respectively ascribed to C-C, C-N, C-O, C=O and O-C=O bonds. The nitrogen content of HNPC/MnO2-25 % composite is 8.6 at%. The curve fitting of
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N1s shows four types of N-containing groups (Fig. 4d), which are respectively ascribed to pyridinic-N (398.3 eV), pyrrolic-N (400.5 eV), quaternary-N (401.4 eV), and N-oxide (404.0 eV) [45, 46]. The pyrrolic-N and pyridinic-N can generally provide the
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pseudocapacitance [12]. The quaternary-N can improve the electrical conductivity of carbon, which is benefit for electron transfer [12]. There are two peaks in the Mn 2p
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spectrum (Fig. 4b) with the binding energy values of 642.3 and 653.9 eV, which can be attributed to the spin orbit doublets of Mn 2p3/2 and Mn 2p1/2 peaks in α-MnO2, respectively. In Fig. 4c, the ∆E of Mn 3s peaks is 4.8 eV in the HNPC/MnO2-25 %. Binding energy widths of 5.79, 5.50, 5.41 and 4.8 eV are acquired from genuine samples of MnO, Mn3O4, Mn2O3 and MnO2, respectively [43]. The result shows that the average valence state of Mn element is 4, indicating that the most existence form of manganese is Mn4+ [22]. As shown in Fig. S3b, the XPS spectrum of O1s peak basically indicates three
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components centered at 529.9, 531.6 and 533.5 eV. The O1s peak at 529.9 eV is assigned to Mn-O bond in α-MnO2 crystal lattice. The peaks of 531.6 eV and 533.5 eV are ascribed the hydrated trivalent oxide (Mn-O-H) and the residual water (H-O-H),
the oxidized states.
(a)
(b)
Mn2P3/2
O Mn 2p
Mn 3s 0
200
400
600
O
800
1000
660
Energy binding (eV)
(c)
4.8eV
Mn2P1/2
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N
Mn2p
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Intensity (a.u.)
Intensity (a.u.)
C
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respectively [22]. Obviously, all the manganese elements in the composite material are in
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(d)
Mn3s
650
645
640
635
Binding energy (eV)
pyrrolic-N
N1s
94
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Intensity (a.u.)
Intensity (a.u.)
pyridinic-N
92
90 88 86 84 82 Binding energy (eV)
80
quaternary-N
N-oxide
408
404 400 396 Binding energy (eV)
392
Fig. 4. XPS patterns of the HNPC/MnO2-25 %: (a) XPS survey spectrum; High spectra of (b) Mn 2p,
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(c) Mn 3s, (d) N 1s.
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3.2 Electrochemical characterization Because of the large surface area of mesoporous structure with a short ion diffusion
path, high electrical conductivity of the HNPC and the pseudocapacitor property of MnO2, the composite has excellent electrochemical performance. The electrochemical performances of HNPC, HNPC/MnO2-14 %, HNPC/MnO2-25 % and HNPC/MnO2-50 % are evaluated using the technologies of cyclic voltammograms (CV), the galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) in 2 mol L-1
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Ca (NO3)2 aqueous solutions. The solution of Ca (NO3)2 is selected as the electrolyte concerning the fact that the specific capacitance value of MnO2 can be greatly improved
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using a divalent ion such as Ca2+ as electrolyte ion instead of a univalent cation [28, 44]. Fig. 5a displays the CV curves of HNPC, MnO2, and HNPC/MnO2-25 % electrodes at 100 mV s-1 scan rates within a voltage range from 0.0 to 1.0 V. The shape of CV
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curves of HNPC is nearly symmetric rectangle, indicating an ideal double-layer capacitor behavior. From the GCD curves in Fig. 5b, the discharge time of HNPC/MnO2 is
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obviously longer than those of pure HNPC and pure MnO2 at 1 A g-1. Due to the pseudocapacitance properties of MnO2 and the large specific surface area of HNPC, the HNPC/MnO2 capacitance is largely improved compared with pure HNPC and MnO2. It shows that there is a synergistic effect in the composite, fully exerting the large specific surface area and conductive properties of HNPC, and the pseudocapacitance properties of
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MnO2 for high special capacitance.
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20
HNPC/MnO2-25%
10
Potential (V)
0 -10 -20
1.0
HNPC/MnO2-25 %
0.8 0.6 0.4
0.0
0.0
(c) 30
0.2
0.4 0.6 0.8 Potential (V)
25
HNPC/MnO2-14 %
20
HNPC/MnO2-25 %
15
HNPC/MnO2-50 %
1.0
0
100
(d)
10
Potential (V)
Current density (A g-1)
HNPC MnO2 nanowires
0.2
-30
5 0 -5 -10
0.8
HNPC/MnO2-50 %
0.6 0.4
0.4
0.6
0.8
Potential (V)
(e) 40
5 mv s-1 20 mv s-1
20
0
1.0
100 200 300 400 500 600 700 Time (s)
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0.2
(f)
1.0
Potential (V)
0.8
0
-20
100 mv s-1 200 mv s-1
-40
400
HNPC/MnO2-14 % HNPC/MnO2-25 %
0.0 0.0
200 300 Time (s)
1.0
0.2
-15 -20
Current density (A g-1)
1.2
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Current density (A g-1)
(b)
HNPC MnO2 nanowires
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(a) 30
0.6
0.5 A g-1 1 A g-1 5 A g-1 10A g-1
0.4 0.2 0.0
0.2
0.4 0.6 0.8 Potential (V)
1.0
0
250
500 Time (s)
750
1000
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0.0
Fig. 5. (a) CV curves of HNPC, MnO2 nanowires and HNPC/MnO2; (b) GCD curves of HNPC, MnO2 nanowires and HNPC/MnO2; (c) CV curves of HNPC/MnO2 at different MnO2 content; (d) GCD curves of HNPC/MnO2 at different MnO2 content; (e) CV curves of HNPC/MnO2 at different scan
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rates; (f) GCD curves of HNPC/MnO2 at various current densities.
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Fig. 5c and Fig. 5d show the CV and GCD curves of HNPC composite with different contents of MnO2. Obviously, the HNPC/MnO2-25 % exhibits high specific capacitance of 204.6 F g-1 at current densities of 1 A g-1, much higher than HNPC/MnO214 % and HNPC/MnO2-25 %, thus, the HNPC/MnO2-25 % is the best among the three composites. The suitable mass ratio of the HNPC and MnO2 can take full advantage of the pseudocapacitance properties of MnO2 and the large specific surface area of HNPC to improve the electrochemical performance of the composite.
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Furthermore, the discharge specific capacitance of the HNPC/MnO2-25 % (Fig. 5f) is calculated to be 216.0, 204.6, 162.3 and 147.5 F g-1 at current densities of 0.5, 1, 5 and 10 A g-1, respectively. Fig. S4 shows that the specific capacitance of HNPC/MnO2-14 %,
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HNPC/MnO2-25 % and HNPC/MnO2-50 % at different scan rates. Clearly, the specific capacitance of HNPC/MnO2-25 % and HNPC/MnO2-50 % are almost the same and are both higher than that of HNPC/MnO2-14 %. As the scan rate increases, the capacitance of
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HNPC/MnO2-25 % declines slowly compared with the capacitance of HNPC/MnO2-50 %. The capacitance of HNPC/MnO2-25 % at 200 mV s-1 is still 64.5 % of the values at 5
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mV s-1, and the capacitance of HNPC/MnO2-50 % at 200 mV s-1 is only 45.1 % of the values at 5 mV s-1. The result shows that the HNPC/MnO2-25 % also has an excellent
(a) 80
HNPC HNPC/MnO2
60
MnO2
40 30 20
7 6 5
Z''/ohm
Z'' / ohm
50
(b)100
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70
4 3 2 1
10
0
0
0
10
20
2
3 4 Z'/ohm
30 40 50 Z' / ohm
5
60
6
70
80 60 40 20
HNPC/MnO2-25%
7
80
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0
1
Capacitance retention (%)
rate performance.
0 0
1000 2000 3000 4000 5000 Cycling number
Fig. 6. (a) Nyquist plots of HNPC, MnO2 nanowires, and HNPC/MnO2; (b) the cycle stability of
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HNPC/MnO2-25 % at a current density of 10.0 A g-1 for 5000 cycles.
The Nyquist plot (Fig. 6a) can evaluate the electric resistance of the HNPC, α-MnO2
and HNPC/MnO2-25 %. The shape of Nyquist plots of the HNPC, α-MnO2 and HNPC/MnO2-25 % are all semicircular at high−frequency region, representing the charge transfer resistance (Rct). The Rct of HNPC/MnO2-25 % is smaller than the Rct of α-MnO2, caused by the reason that the specific hierarchically porous structure of HNPC/MnO2-25 % can increase the contact between the electrolyte and electrode. The linear slope of the 17
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Nyquist plots in the low-frequency zone stands for diffusion resistance [21]. Apparently, the slope of HNPC/MnO2-25 % is much bigger than that of the pure α-MnO2, which means that the HNPC/MnO2-25 % possesses lower diffusive resistance and faster ion
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transport than pure α-MnO2. Additionally, the cycle stability test (Fig. 6b) results show that the capacitance of HNPC/MnO2-25 % remains 97.2 % after cycling 5000 times at 10.0 A g-1, indicating excellent cycling stability without obvious performance
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degradation. The excellent cycling stability is determined by the structure of composite materials. This hierarchical porous structure provides effective diffusion pathways and
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more active sites for electrolyte ions. Therefore, the HNPC/MnO2-25 % composite exhibits high capacitance and excellent cycle stability performance due to its high
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specific surface area and the high pseudocapacitive behavior of MnO2.
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(b) 15
0 -10 -20
5
0
-5 -0.5 0.0 0.5 Potential (V) 10 mV s-1 50 mV s-1
20
100 mV s-1 200 mV s-1
0.0
(d)2.0
0
0.4
1.0
0.5
0.0
1.6
(f) 100
1 A g-1 2 A g-1 3 A g-1
1.6 1.2
5 A g-1 10 A g-1
0.8 0.4 0.0
Capacitance retention (%)
(e) 2.0
0.8 1.2 Potential (V)
2.0
0-1 V 0-1.2 V 0-1.4 V 0-1.6 V 0-1.8 V
-10 0.0
1.0 1.5 Potential (V)
1.5
500 mV s-1
10
0.5
0
50
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30
1.0
Potential (V)
(c)
Potential (V)
0-1V 0-1.2 V 0-1.4 V 0-1.6 V 0-1.8 V
10
100
150 200 Time (s)
250
300
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-1 Current Density (A g )
10
-1.0
Current Density (A g-1)
Current Density (A g-1)
HNPC HNPC/MnO2-25%
20
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(a)
80 60 40 20
HNPC//HNPC/MnO2
0
0
50
100 150 200 250 300 Time (s)
0
500 1000 1500 2000 2500 3000 Cycling number
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Fig. 7. (a) CV curves of HNPC and HNPC/MnO2 electrodes at a scan of 100 mV s-1; (b) CV curves of the ASCs at different potential windows at a scan rate of 100 mV s-1; (c) CV curves of the ASCs at different scan rates; (d) GCD curves of the ASCs at a current density of 1.0 A g-1; (e) GCD curves of
cycles.
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ASCs at different density; (f) the cycle stability of ACSs at a current density of 5.0 A g-1 for 3000
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The asymmetric supercapacitors (ASCs) HNPC//HNPC/MnO2-25 % is assembled with HNPC as the anode and HNPC/MnO2-25 % as the cathode, respectively. In order to investigate
the
steady
voltage
window
and
electrochemical
properties
of
HNPC//HNPC/MnO2-25 %, the CV measurements of the composite is carried out in 2.0 M Ca(NO3)2 at 100 mV s-1 (Fig. 7a). The stable voltage window of the as-prepared HNPC//HNPC/MnO2 asymmetric SC can be extended to be 1.8 V. It also can be seen that HNPC and HNPC/MnO2 have good cyclic reversibility from Fig. 7d. Furthermore, the
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voltage window of ASCs can reach 1.8 V, which is greatly extended comparing with that of the SCs. And, the mass ratio between anode (HNPC) and cathode (HNPC/MnO2-25 %) in ASCs is 2:1, which is calculated by eq 3. The GCD curves of HNPC//HNPC/MnO2
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at different current densities shows that the ASCs have a high specific capacitance of 66.9 F g-1 at 1 A g-1. In addition, the capacitance of HNPC//HNPC/MnO2 remains 88.3% after cycling 3000 times at 5.0 A g-1 (Fig. 7f). It shows that ASCs have excellent cycle
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stability. Moreover, because of its wide voltage window and high specific capacitance, the prepared device has a high energy density of 30.1 Wh kg-1 at power density of 900.0
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W kg-1. The result is higher than that of some similar reported devices (Fig. 8a). As shown in Fig. 8b, the Light-emitting diodes (LED) is successfully lit up by two devices in series, demonstrating that the HNPC/MnO2 is a potential electrode material for energy
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storage devices.
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Fig. 8. (a) Ragone plot of the ASCs; (b) The ASCs illuminate the Light-emitting diodes (LED).
4. Conclusions
A new method for preparing porous carbon materials and MnO2 composite materials
is presented. The HNPC/MnO2 composite fabricated by the template method have a unique nanowire interpenetrating hierarchical porous structure, a large specific surface area and an ordered porous structure. HNPC/MnO2 composite have a high specific
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capacitance of 204.6 F g-1 at 1 A g-1, which remains 97.2 % after 5000 cycles. The high capacitance is mainly due to the hierarchical porous structure and the pseudocapacitive effect of nitrogen doping and MnO2 nanowires. The good cycle stability is benefit from
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the hierarchical porous carbon nanostructures which provides a continuous electron path to ensure good electrical contact and also shorten the diffusion path of ion transportation. The assembled HNPC//HNPC/MnO2 nanowires asymmetric supercapacitor exhibits a
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wide working voltage window of 1.8 V and achieve an energy density of 30.1 Wh kg-1 at
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a power density of 900.0 W kg-1. Acknowledgements
This study was financially supported by the National Natural Science Foundation of China (no. 61605222, U1432132), the Innovative Program of the Development
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Foundation of Hefei Center for Physical Science and Technology (2017FXCX006) and the Youth “spark” of Chinese Academy of China (YZJJ2018QN25).
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References
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ACCEPTED MANUSCRIPT Highlights: 1. The composite of porous carbon and α-MnO2 nanowires with good electrochemical performance is successfully prepared.
increases ion-accessible surface area.
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2. The hierarchical porous structure of composite facilitates ion diffusion and
3. The composite electrodes exhibit excellent cycling durability (97.2 % remains after
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5000 cycles).