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Construction of hierarchical FeCo2O4@MnO2 core-shell nanostructures on carbon fibers for high-performance asymmetric supercapacitor
Accepted Manuscript Construction of hierarchical FeCo2O4@MnO2 core-shell nanostructures on carbon fibers for high-performance asymmetric supercapacitor Fangfang Zhu, Yu Liu, Ming Yan, Weidong Shi PII: DOI: Reference:
S0021-9797(17)31124-4 https://doi.org/10.1016/j.jcis.2017.09.093 YJCIS 22845
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
5 June 2017 16 September 2017 24 September 2017
Please cite this article as: F. Zhu, Y. Liu, M. Yan, W. Shi, Construction of hierarchical FeCo2O4@MnO2 core-shell nanostructures on carbon fibers for high-performance asymmetric supercapacitor, Journal of Colloid and Interface Science (2017), doi: https://doi.org/10.1016/j.jcis.2017.09.093
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Construction of Hierarchical FeCo2O4@MnO2 Core-shell Nanostructures on Carbon Fibers for High-Performance Asymmetric Supercapacitor
Fangfang Zhua, Yu Liua, Ming Yan b, Weidong Shia,*
a
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang,
212013, P. R. China. b
School of Material Science and Engineering, Jiangsu University, Zhenjiang, 212013,
P. R. China. China.Tel: +86-511-88791800, * E-mail: [email protected]
1
Abstract： ： In this work, the novel hierarchical FeCo 2O4@MnO2 core-shell nanosheet arrays have been synthesized by a facile hydrothermal method, which are grown directly on a flexible carbon fiber (CF) as an integrated electrode for supercapacitors. Scanning electron
microscopy and
high-resolution
transmission
electron
microscopy
measurements illustrate that MnO2 nanoflakes uniformly wrap around the surface of two-dimensional FeCo 2O4 nanosheets. The electrode exhibits high areal capacitance of 4.8 F cm-2 at a current density of 1 mA cm-2. Moreover, an asymmetric FeCo2O4@MnO2//active carbon (AC) cell is successfully fabricated. The asymmetric supercapacitor (ASC) displays high energy density/power density (22.68 Wh kg-1 at 406.01 W kg-1 and 7.06 Wh kg-1 at 1802.5 W kg-1), as well as excellent cycling stability with 90.1% of the initial capacitance after 5000 continuous cycles. Moreover, two ASCs connected in series can light a LED. These performances demonstrate great potential of the designed ASC in the field of energy storage due to their remarkable electrochemical properties. Keywords: FeCo2O4@MnO2; Core-shell; Asymmetric supercapacitor; Energy density
2
1. Introduction With the constant consumption of fossil fuel and increasing number of environmental problems in the world, supercapacitors (SCs) have drawn significant research attention because of their irreplaceable properties, such as high power density, fast charge and discharge capability, long cycle life and low maintenance cost [1-5]. According to the energy storage mechanism, supercapacitors are usually classified into two major types, electrical double-layer capacitors (EDLCs) and pseudo-capacitors [6]. EDLCs store energy by electrostatic accumulation of charges at the surface of the electrode material and mainly use some carbonaceous materials with high surface area [7, 8]. Electrical conductivity, pore structures and specific surface area of the electrode materials generally decide the electrochemical performances of EDLC electrodes. The pseudo-capacitors compounds utilize certain fast redox reactions taking place at the surface or nearsurface regions of the electrode to store electric charges and are mainly based on transition metal oxides, metal hydroxides, conductive polymers and metal sulfides [9-13].
The overall
electrochemical properties of pseudo-capacitors are always affected by their theoretical capacitance, geometric configuration and electrical behavior. Recently, many efforts have been devoted to explore the integrated and binder-free supercapacitor electrode with unique nanostructure by directly growing active materials on conductive substrate with nanorod [14, 15], nanoneedle [16, 17], nanoflake [18, 19] and other three dimensional structures to achieve high 3
electrochemical, electrical, optical and mechanical properties. Carbon fiber (CF) substrate is considered as one of the most promising conductive substrate in the field of supercapacitor, due to the good mechanical strength, highly conductivity and excellent flexibility. Nevertheless, CFs known as a kind of double layer carbon material possess negligible electrochemical capacitance for its low specific surface and less internal pore structure [20-23]. To solve above mentioned problem, transition-metal oxides and hydroxides, metal sulfides and conducting polymers with high pseudo-capacitive properties are grown or deposited on carbon fiber. For instance, Xiao et al. successfully prepared NiCo 2S4 single crystalline nanotube arrays deposited by a series of electroactive metal oxide materials (CoxNi1-x (OH) 2, MnO2, and FeOOH) with a highest discharge areal capacitance of 2.86 F cm-2 at 4 mA cm-2 [24]. Wen et al. synthesized three dimensional Co9S8 nanorod@Ni(OH)2 nanosheet core-shell structure on carbon fiber by a simple low temperature hydrothermal method with a s pecific capacitance of 1620 F g-1 at a current density of 0.5 A g-1 [25]. Yu et al. designed ultrathin MnO2 nanosheet/carbon fiber cathodes by a facile hydrothermal treatment with a specific capacitance of 634.5 F g-1 at a current density of 2.5 A g-1 [26]. Manganese dioxide (MnO2), a promising pseudo-capacitors electrode materials which is natural abundance and low cost, has been extensively investigated owing to its excellent electrochemical performance, such as a high theoretical specific capacity of 1370 F g-1, a wide electrochemical potential window of approximately 0.9-1.0 V and the ability using mild aqueous electrolytes [27-29]. However, the poor electrical 4
conductivity (10-5-10-6 S cm-1) has severely limited its performance far from the theoretical value because of electron transfer barriers. An emerging efficient strategy is to design smart integration of multi-component MnO2 mixed oxides and then make full use of the properties from individual component based on the synergistic effects to achieve a maximum electrochemical performance [27]. Recently, FeCo2O4 ，a ternary metallic oxide, have been investigated as electrode materials for supercapacitors
since
it
possess
higher
electrical conductivity and
good
electrochemical activity [30-32]. Moreover, as we all know, two-dimensional nanosheets can provide high efficient electrochemical active sites, reduce the ions and electron diffusion path and improve the structural stability [33]. Therefore, the FeCo2O4 nanosheets should be an excellent backbone material for supercapacitors. Herein, we have synthesized hierarchical FeCo 2O4@MnO2 core-shell nanosheet arrays on CFs via a facile two-step hydrothermal method for supercapacitors. The pre-formed free-standing FeCo2O4 precursor nanosheets are used as a backbone on CFs for anchoring MnO2 nanosheets. The network structure composed of FeCo2O4 nanosheets arranging orderly in a crisscross pattern not only offer a large surface area, but also support and provide reliable electrical connections to the outer MnO2 nanosheets. The hybrid electrode not only exhibits a high energy density of 22.68 Wh kg-1 at 406.01 W kg-1 and an energy density 7.06 Wh kg-1 at a high power density of 1802.5 W kg-1, but also an excellent cycling performance with 90.1% retention of the initial specific capacitance after 5000 cycles in 3M KOH solution.
5
Fig. 1. Schematic illustration of the synthesis procedure of hierarchical nanostructured FeCo2O4@MnO2.
2. Experimental section The analytical grade reagents ferrous sulfate (FeSO4·7H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), urea (CO(NH2)2), ammonium fluoride (NH4F), hydrochloric acid (HCl), potassium hydroxide (KOH) and absolute ethanol were all purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used throughout all the experiments.
2.1 Synthesis of FeCo 2O4@MnO2 core-shell arrays Prior to the synthesis, a piece of carbon fiber (1 cm×2 cm) was ultrasonicated by acetone, ethanol, and distilled water every ten minutes in sequence. In a typical procedure, 1 mmol ferrous sulfate (FeSO4·7H2O), 2 mmol cobalt nitrate hexahydrate 6
(Co(NO3)2·6H2O), 5 mmol urea (CO(NH2)2), and 2 mmol ammonium fluoride (NH4F) were dissolved in deionized water (30 mL) to form a red solution under vigorous magnetic stirring. After stirring for 30 min, the color of the solution changed from red to orange. Then the as-obtained orange solution was transferred into a 50 mL polytetrafluoroethylene (PTFE) Teflon-lined autoclave and the cleaned carbon cloth was immersed into the above solution, following by heating the autoclave at 140 ℃ for 7 h in an electric oven. After the autoclave was cooled to room temperature naturally, the carbon cloth coated golden brown precursor were carefully washed with deionized water and absolute ethanol to remove surface ions and molecules and then dried at 60 ℃ in a vacuum oven overnight. A piece of CF substrate with grown FeCo2O4 nanosheets was put into a 50 mL Teflon-lined autoclave including 30 mL KMnO4 solution (0.05 M) for MnO2 nanofilm growth. The autoclave was sealed and hydrothermally treated at 140 ℃ for 6 h. After the reaction, the solution was cooled down to room temperature naturally. Then the as-obtained sample was washed with deionized water and absolute ethanol, and dried at 60 ℃ in a vacuum oven. Finally, the precursor was annealed at 400 ℃ for 2 h, and then black core-shell arrays on the CF substrate were obtained. The total weight of FeCo 2O4@MnO2 core-shell nanosheet arrays on the CF substrate was 8 mg cm-2, and the mass loading of the FeCo2O4 nanosheets was 2 mg cm-2.
2.2 Materials characterization The morphology, compositions and size of the samples was investigated using the Hitachi S-4800 field emission SEM (FESEM, Hitachi, Japan). The Powder X-ray 7
diffraction (XRD) patterns were recorded on a D/MAX-2500 diffract meter (Rigaku, Japan) with a nickel-filtered Cu Kα radiation source (λ= 1.54056 Å).The X-ray photoelectron spectroscopy (XPS) was obtained by a Thermo ESCALAB 250X (America) electron spectrometer using 150WAl Ka X-ray sources. Transmission electron microscopy (TEM) images and high resolution transmission electron microscopy (HRTEM) images were taken on a F20 S-TWIN electron microscope (Tecnai G2, FEI Co.) with an acceleration voltage of 200 kV.
2.3 Electrochemical measurements The electrochemical measurements were carried out by an electrochemical workstation (CHI 760E, CH instrument Inc, China) in a 3.0 M aqueous KOH electrolyte by a typical three-electrode configuration. Ag/AgCl electrode and platinum electrode were used as the reference electrode and counter electrode, respectively. The FeCo2O4 nanosheets and the FeCo2O4@MnO2 core-shell naosheet arrays were directly used as the working electrodes without any binder and conductive actives. The working electrodes immersed in the electrolyte was 1 cm2 in nominal area. The electrochemical properties were investigated by cyclic voltammetry (CV) technique with varying the scan rate of 2-100 mV s-1 at potential between 0 and 0.5 V. Galvanostatic charge-discharge (GCD) experiments were performed with current densities ranged from 1 to 16 mA cm-2 at a potential of 0 to 0.4 V. Moreover, electrochemical impedance spectroscopy (EIS) testing was conducted with frequencies ranging from 0.01 to 10000 Hz. The areal capacitance (C ) and specific capacitance (C ) was estimated from the following Equation (1) and (2): 8
∆
C∝ =
∆
C =
∆
(1)
∆
(2)
where Ca (F cm-2) is the areal capacitance, and Cs (F g-1) is the specific capacitance, I (A) is the discharge current, Δt is the discharge time, and ΔV is the discharging potential range, S (cm-2) is the geometrical area of the electrode, m (g) is the mass of the active materials of the electrode. For ASC, the FeCo 2O4 nanosheets and the active carbon (AC) were used as the positive and negative electrode in a two-electrode system, respectively. 3 M KOH solution was used as the electrolyte. The negative electrode was fabricated by 80 wt% of AC material, 10 wt% of acetylene black and 10 wt% of polyvinylidene (PVDF) binder, then use right amount of N-Methyl-2-pyrrolidne to dissolve the above powder. According to the equation:
×∆
= ×∆
(3)
The mass ration obtained between the positive and negative is 0.8. Where and are the capacitances of the FeCo2O4@MnO2 and AC electrodes, respectively. and are the discharging potential range of the FeCo2O4@MnO2 and AC electrodes, respectively. The energy density and power density of the asymmetric cell are calculated from Equations (4) and (5): E= P= here
.×
(4)
. ×
(5)
∆
is the energy density (Wh Kg-1) , ! is power density (W Kg-1) and C is the
capacitance of the asymmetric cell (F g-1) and the capacitance C is based on the total mass of two electrodes, Δt is the total discharge time (s). 9
3. Results and discussion 3.1 Structure and morphology The fabrication procedure is illustrated in Fig. 1. Firstly, the self-assembled FeCo2O4 precursor nanosheets are grown uniformly on the CFs. Subsequently, the MnO2 nanosheets with different thickness which has been adjusted by controlling the KMnO4 solution reaction time in this experiment are successfully anchored onto the surfaces of the above nanosheets. Finally, the hybrid FeCo2O4@MnO2 core-shell nanosheets are obtained through a thermal annealing process. The XRD pattern of the FeCo2O4@MnO2 sample is shown in Fig. 2. The peaks at 12˚ and 37˚ can be readily indexed as birnessite-type δ-MnO2 (JPCDS No.86-0666), while the remaining peaks can be ascribed to the FeCo2O4 spinel phase (Fd3m space group). These results definitely confirm that the hybrid nanosheets is integrated by FeCo2O4 and MnO2.
Fig. 2. X-ray diffraction pattern (XRD) of the as-synthesized FeCo2O4 nanosheets and hybrid 10
FeCo2O4@MnO2 nanosheets on CFs.
Fig 3. XPS spectra of the Fe 2p region (a), Co 2p region (b), Mn 2p region (c) and O 1s region (d) of the hybrid electrode.
To further illustrate the surface composition and electronic states of the synthesized sample, the X-ray photoelectron spectroscopy (XPS) measurement is performed in Fig. 3. The Fe 2p core level spectrum shows prominent peaks at 710.5 eV for Fe 2p3/2, 724 eV for Fe 2p 1/2, and a “shoulder” satellite peak at 714.4 eV, indicating the presence of Fe (III) oxidation [34]. In addition, the energy separation between the spin orbit components (binding energy (BE), △BE = 13.5 eV) is in agreement with the Fe (III) in an oxide environment [35-40]. The Co 2p region is shown in the Fig. 3b, there are two obvious peaks corresponding to Co 2p3/2 and Co 2p 1/2, and they are divided into four peaks after fitting. The fitting peaks at 780.3 and 11
795.3 eV, along with the satellite peak of 788.8 eV are related with Co3+, while the fitting peaks at binding energies of 781.9, 796.2 eV and the satellite peak of 805.1 eV are indexed to Co2+. Therefore, the Co3+ and Co2+ coexist in the sample. The Mn 2p spectrum is displayed in the Fig. 3c, the peaks located at 642.5 and 654.2 eV, corresponding to the Mn 2p 3/2 and Mn 2p 1/2, respectively. The Fig. 3d exhibits the O 1s spectrum, the peak at a binding energy of 529.8 eV corresponds to a typical bond between metal and oxygen, and the peak at a binding energy of 531.5 eV corresponds to the OH- groups oxygen, which reveal the hydroxylation of the material surface.
Fig. 4. SEM images of primary FeCo2O4 nanosheet arrays (a) and FeCo2O4@MnO2 core-shell nanosheet arrays obtained at different time on CFs: (a) 0 h; (b) 2 h; (c) 4 h; (d) 6 h. The insert shows the enlarged image.
Fig. 4a shows the morphology of as-synthesized FeCo2O4 nanosheet arrays. It 12
can be seen that the obtained CFs are uniformly covered by the FeCo 2O4 nanosheets. The high-magnification SEM image in the inset of Fig. 4a reveals that the thickness of individual nanoflake is only 30 ~ 40 nm. FeCo2O4 nanosheets grown on the CFs can not only enhance the electroactive surface site for growing MnO2 but also provide effective electrical connection to MnO2 and overcome the problem of poor conductivity of MnO2. Additionally, the SEM images of FeCo2O4@MnO2 core-shell nanosheets which have been dipped into the permanganate solution for different time are displayed in Fig. 4b-d. The reaction for forming FeCo2O4 is performed according to the equation (6) and (7): xFe + 2xCo( + 6xOH = Fe, Co(, (OH) ,
(6)
2Fe,Co(, (OH) , + O( = 2xFeCo( O/ + 6xH( O
(7)
MnO2 is a product of the decomposition reaction of Potassium Permanganate. Compared with the Fig. 4a (0 h), there are small particles can be seen from the edge or surface of the FeCo 2O4 nanosheets (2 h, Fig. 4b) and the thickness of each nanosheet is broadened. It is noteworthy that when the hydrothermal reaction increases from 2 h to 6 h, the thickness of the shell is rapidly increased from few tens nanometers to one micron. Moreover, the MnO2 nanosheets connecting with each other are 8-10 nm in thickness. And when the relatively small MnO2 nanosheets aggregate to grow, there is space left among sheets, so the electrolyte ions can still penetrate. The core-shell structure disappears and the three-dimensional structure is completely wrapped in a thick MnO2 nanoflakes shell after 8 h of reaction (Fig. S1). It is undoubted that the 8 h sample is unsuitable for the diffusion of the electrolytes 13
ions.
Fig. 5. TEM and HRTEM images of hierarchical FeCo2O4@MnO2 core-shell nanostructure scratched from CFs.
The morphology and structure of the as-obtained FeCo2O4@MnO2 core-shell nanosheet arrays (3h) are further investigated by TEM. The image of the above FeCo2O4@MnO2 sample scratched from CFs clearly reveals that the surface of thin FeCo2O4 nanosheet is coated by numerous MnO2 nanosheets, which is consistent with the SEM images of the FeCo 2O4@MnO2 core-shell nanostructure and different from the SEM image of the pure MnO2 nanosheets which are a few nanometers in size on CFs (Fig. S1 8h). The HRTEM image marked by the red solid circle is shown in the Fig. 5b and its interplanar spacing of 0.67 nm can be indexed as the (001) plane of birnessite-type MnO2. From the TEM image, the thickness of MnO2 nanosheets is about 10 nm. In addition, the lattice spacing of 0.24 nm in the Fig. 5c indicated by the red dashed circle corresponds to the (311) planes of the FeCo 2O4 cubic phase, which is the alignment with the XRD results. These results confirm the formation of 14
FeCo2O4@MnO2 core-shell hybrid composite.
3.2 Electrochemical properties of FeCo2O 4@MnO2 Next, we directly applied the hierarchical FeCo 2O4@MnO2 core-shell heterostructured nanosheet arrays as electrode for SCs. Fig. 6a shows the CV curves of pristine FeCo 2O4 and FeCo 2O4@MnO2 core–shell hybrid electrodes with a potential window of 0-0.5 V at a scan rate of 5 mV s-1. Clearly, a pair of redox peaks is evident when the reaction time is lower than 2 h, the pseudo-capacitive behavior observed could be largely attributed to the reversible faradaic reactions relating to the following equations [41]: FeCo( O/ + H( O + OH ↔ FeOOH + 2CoOOH + e
(8)
CoOOH + OH ↔ CoO( + H( O + e
(9)
FeOOH + H( O ↔ Fe(OH) ↔ FeO/ ( + 3e
(10)
The CV curves of the FeCo2O4 are shown in the Fig. S2.
15
Fig. 6. (a) CV curves of the FeCo2O4 @MnO2 core-shell NSs arrays obtained at different time on CFs at a scan rate of 5 mv s-1; (b) Galvanostatic charge-discharge (GCD) curves of the FeCo2O4@MnO2 core-shell NSs arrays at a current density of 1 mA cm-2; (c) CV curves of the FeCo2O4@MnO2 core-shell NSs arrays (6 h) at different scan rates; (d) GCD curves of the FeCo2O4@MnO2 core-shell NSs arrays (6 h) at different current densities; (e) specific capacitance of the electrodes; (f) EIS spectra of the pristine FeCo2O4 and FeCo2O4 @MnO2 core-shell hybrid electrodes. 16
When the reaction time continues to increase, this pair of redox peaks becomes unobvious due to the thick MnO2 shell. Obviously, the area under CV curve of the FeCo2O4@ MnO2 hybrid electrode (6 h) is larger than that of pristine FeCo 2O4 (0 h) electrode and other FeCo2O4@ MnO2 hybrid electrodes (2, 4 and 8 h). The results indicate that the thickness of MnO2 nanosheets has a great influence on electrochemical properties, but it is difficult for ions to penetrate from a too thick shell into the inner microstructures of the electrode. As is known to us, the III oxidation state in the MnO2 will be oxidized to IV oxidation state of Mn during charging, then IV oxidation state of Mn will be reduced to III oxidation state of Mn during discharging, accompanying the adsorption of cations at the surface of MnO2. The following reactions may be involved in the transition: (11) Mn(IV)O( ∙ nH( O + δe + δ(1 − f)H O + δfM ↔ (H O):(;<)M:< =Mn (III): Mn (IV);: >O( ∙ nH( O
(11)
M+ are Na+, K+ and Li+. To illustrate the influence and importance of FeCo2O4 backbone, the CV curve of FeCo2O4@ MnO2 hybrid electrode (6 h) and bare MnO2 electrode (6 h) are shown in Fig. S3, showing that the FeCo2O4 backbone is vital for the increased capacity. As expected in Fig. 6b, FeCo2O4@ MnO2 hybrid electrode (6 h) exhibits the longest discharge time at the same current density of 1 mA cm-2. These could be attributed to the additional pseudo-capacitance contributed by the MnO2 shell and the FeCo 2O4 nanosheets can further provide fast pathways and abundant electroactive surface sites for δ-MnO2 shell to harvest a maximum capacitance. Moreover, the layered 17
network-like highly conductive FeCo2O4 nanosheets as a support can provide reliable electrical connections to the outer MnO2 shell which have the poor electronic/ionic conductivity. Because of the hierarchical structure, the electrolyte ions can quickly contact the electrode material from all directions and then rapidly transfer to the inner core part with high conduction and this is the reason why the voltage profiles show sudden increases in the beginning of charge. When the reaction time is Fig. 6c displays the CV curves of optimized FeCo2O4@ MnO2 hybrid electrode (6 h) collected at different scan rates from 2 to 100 mV s-1 within the potential window of 0-0.5 V. Fig. 6d shows the GCD curves of FeCo2O4@ MnO2 electrode (6 h) at different current densities ranging from 1 to 16 mA cm-2 and the nearly symmetrical GCD curves reveals good electrochemical capacitive characteristics and a superior reversibility. The areal capacitances of the FeCo2O4@ MnO2 electrode (6 h) can be calculated from the above-mentioned discharge curves. The corresponding specific area capacitances (Ca) are 4.8, 3.4, 2.5, 1.2, 0.19 F cm-2 at 1, 2 4, 6, 8, 16 mA cm-2, respectively, which are larger than that of the pristine FeCo 2O4 electrode (42 mF cm-2). The areal capacitances of other FeCo2O4@ MnO2 hybrid electrodes are shown in the Fig. 6e. Fig. 6f shows the impedance Nyquist plots of the pristine FeCo2O4, the bare MnO2 electrode (6 h) and FeCo2O4@ MnO2 hybrid electrode (6 h) in the frequency range of 0.01 to 100 kHz under open circuit potential. The Nyquist plots are mainly composed of a semicircle at high frequency region and a straight line at low frequency region. The inset in Fig. 6f is the equivalent circuit model. In the high frequency region, the value of the charge-transfer impedance (Rct) of the FeCo2O4@ MnO2 18
hybrid electrode, pure FeCo2O4 nanosheets on CFs and the pure MnO2 nanosheets on CFs are 5.06 Ω, 3.17 Ω, 14.45 Ω, respectively. It can be seen clearly that the introduction of the backbone FeCo 2O4 nanosheets to a large extent solved the problem of extremely poor conductivity of MnO2. Moreover, the bulk resistance (Rs) value of pure FeCo 2O4 nanosheets on CFs, FeCo 2O4@ MnO2 hybrid electrode (6 h) and the bare MnO2 electrode is 1.26 Ω, 1.74 Ω, 1.41 Ω, respectively. In the low frequency area, the slope of the curves of the pristine FeCo2O4 electrode and the FeCo 2O4@ MnO2 hybrid electrode are almost the same. All these results suggests that the FeCo2O4 nanosheet arrays have better conductivity than the MnO2 nanosheets and can make the FeCo2O4@ MnO2 hybrid electrode faster reaction kinetics.
3.3 Electrochemical properties of FeCo2O4@MnO2//AC asymmetric supercapacitor To further investigate the electrochemical performance for an asymmetric capacitor, we use FeCo2O4@MnO2 core-shell nanosheets as the positive electrode and AC as the negative electrode (Fig. 7a). The electrochemical performance of AC is shown in the Fig. S4. The CV curves of AC which are operated in the voltage window from -1.0 to 0 V indicate good electric double-layer capacitance property. In the actual experiment, the quality of AC is about 10 mg. Fig. 7b displays the CV curves of the as-assembled asymmetric supercapacitor recorded at different voltage windows. Note that the high potential part of the CV curves of 0-1.8V and 0-2.0V has obvious upturned, indicating that the oxygen evolution reaction, so the operating potential of the ASC device is determined to 1.6 V. Then, the CV curves of the ASC device in a 19
voltage window of 0 to 1.6 V at different scan rates of 2-100 mV s-1 are recorded in the Fig. 7c. These CV curves display no obvious redox peaks, showing a good rate capability for power storage. Fig. 7d exhibits the GCD curves of the ASC device at various current densities. The GCD curves are nearly symmetric, which suggests the excellent capacitive performance of the ASC device.
Fig. 7 Electrochemical performance of the FeCo2O4@MnO2 (6 h)//AC asymmetric supercapacitor. (a) Schematic illustration of the assembled ASC configuration; (b) CV curves of the FeCo2O4@MnO2 (6 h)//AC ASC cell measured at different potential windows; (c) CV curves of the ASC device collected at different scam rates; (d) CV curves at different current densities.
20
Fig. 8 (a) Cycling performance of the ASC at current density of 25 mA cm-2, the inset shows 1 st and 5000th cycles of the charge–discharge tests; (b) Ragone plot of the ASC and some other devices from previous literature for comparison. (c) A picture obviously shows the red, blue, white LEDs can be lighted up, when the two assembled ASC devices connected in series be charged in the left picture.
Furthermore, the cycling performance of the capacitor based on the GCD cycling curves is shown in Fig. 8a. It is clearly that the capacitance increases during the first 2500 cycles. With the further infiltration of electrolyte ions, more and more hybrid nanostructures become activated after the intercalation and deintercalation process and contribute to the increase of capacitance. Moreover, the capacitance can be maintained at 90.1% of the initial capacitance after 5000 cycles, suggesting a good electrochemical stability. The Ragone plots of the FeCo2O4@ MnO2//AC device in a voltage window of 0-1.6 V are displayed in Fig. 8b. The device shows a maximum 21
energy density calculated from the GCD curves is 22.68 W h kg-1 at a power density of 406.01 W kg-1 and the corresponding current density is 10 mA cm-2. Even at a high current density of 40 mA cm-2, the energy density stills retains 7.06 W h kg-1 at power density of 1802.5 W kg-1. The excellent values are higher than those of the MnO2-component asymmetric supercapacitors, such as graphene/MnO2 ASC ( 6.8 Wh kg-1 at 62 W kg-1) [42], MnO2/GHCS ASC ( 22.1 Wh kg-1 at 100 W kg-1) [43], MnO2/CNT ASC ( 15.3 Wh kg-1 at 100 W kg-1) [44], NiCo 2O4/MnO2 ASC ( 9.4 Wh kg-1 at 175 W kg-1) [45], MnO2-modified diatomites ASC ( 3.75 Wh kg-1 at 250 W kg-1) [46], MnO2/CNTs ASC ( 13.3 Wh kg-1 at 600 W kg-1) [47] and PEDOT/MnO2 ASC ( 9.8 Wh kg-1 at 800 W kg-1) [48]. Thus, in order to further demonstrate the real practical applications of our products, we assembled two ASCs in series to light a LED. When the device in the left side of the Fig. 8c was charged for 30 s at a current density of 0.01 mA cm-2, our devices can power a LED brightly (red, blue and white). Especially, the red LED can be kept for 3 min, and the brightness images of the red LED is shown in the Fig. S5.
4. Conclusions In summary, we have designed a facile and cost-effective strategy to fabricate hierarchical FeCo 2O4@MnO2 core-shell nanosheet arrays with great electrochemical performance for supercapacitor applications. In this architecture, the FeCo 2O4 nanosheets not only act as backbone to enhance the electroactive surface sites but also provide effective electrical connection to MnO2. The as-obtained hybrid electrode exhibits a high areal capacitance of 4.8 F cm-2 at the current density of 1 mA cm-2. 22
Moreover, the ASC device based on the FeCo 2O4@MnO2 core-shell electrode has been successfully assembled and shows a high energy density of 22.68 Wh kg-1 at a power density of 406.01 W kg-1 and 7.06 W h kg-1 at a high power density of 1802.5 W kg-1. Even after 5000 cycles of charging-discharging process, the specific capacitance still remained 90.1% of the initial specific capacitance. Furthermore, two ASCs in series can light a LED. These results convince us that our work have great potential applications in hybrid electrode materials for energy storage application.
Acknowledgements It is pleasure to acknowledge the financial support provided by the National Natural Science Foundation of China (21477050, 21522603 and 51602131), the Chinese German Cooperation Research Project (GZ1091), the Excellent Youth Foundation of Jiangsu Scientific Committee (BK20140011 and BK20160526), the Program for New Century Excellent Talents in University (NCET-13-0835), the Henry Fok Education Foundation (141068), Six Talents Peak Project in Jiangsu Province (XCL-025), Jiangsu Province Postdoctoral Science Foundation (1601059C) and China Postdoctoral Science Foundation (2016M600368). Conflicts of interest: none.
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Graphical abstract
28
S0021-9797(17)31124-4 https://doi.org/10.1016/j.jcis.2017.09.093 YJCIS 22845
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
5 June 2017 16 September 2017 24 September 2017
Please cite this article as: F. Zhu, Y. Liu, M. Yan, W. Shi, Construction of hierarchical FeCo2O4@MnO2 core-shell nanostructures on carbon fibers for high-performance asymmetric supercapacitor, Journal of Colloid and Interface Science (2017), doi: https://doi.org/10.1016/j.jcis.2017.09.093
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.
Construction of Hierarchical FeCo2O4@MnO2 Core-shell Nanostructures on Carbon Fibers for High-Performance Asymmetric Supercapacitor
Fangfang Zhua, Yu Liua, Ming Yan b, Weidong Shia,*
a
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang,
212013, P. R. China. b
School of Material Science and Engineering, Jiangsu University, Zhenjiang, 212013,
P. R. China. China.Tel: +86-511-88791800, * E-mail: [email protected]
1
Abstract： ： In this work, the novel hierarchical FeCo 2O4@MnO2 core-shell nanosheet arrays have been synthesized by a facile hydrothermal method, which are grown directly on a flexible carbon fiber (CF) as an integrated electrode for supercapacitors. Scanning electron
microscopy and
high-resolution
transmission
electron
microscopy
measurements illustrate that MnO2 nanoflakes uniformly wrap around the surface of two-dimensional FeCo 2O4 nanosheets. The electrode exhibits high areal capacitance of 4.8 F cm-2 at a current density of 1 mA cm-2. Moreover, an asymmetric FeCo2O4@MnO2//active carbon (AC) cell is successfully fabricated. The asymmetric supercapacitor (ASC) displays high energy density/power density (22.68 Wh kg-1 at 406.01 W kg-1 and 7.06 Wh kg-1 at 1802.5 W kg-1), as well as excellent cycling stability with 90.1% of the initial capacitance after 5000 continuous cycles. Moreover, two ASCs connected in series can light a LED. These performances demonstrate great potential of the designed ASC in the field of energy storage due to their remarkable electrochemical properties. Keywords: FeCo2O4@MnO2; Core-shell; Asymmetric supercapacitor; Energy density
2
1. Introduction With the constant consumption of fossil fuel and increasing number of environmental problems in the world, supercapacitors (SCs) have drawn significant research attention because of their irreplaceable properties, such as high power density, fast charge and discharge capability, long cycle life and low maintenance cost [1-5]. According to the energy storage mechanism, supercapacitors are usually classified into two major types, electrical double-layer capacitors (EDLCs) and pseudo-capacitors [6]. EDLCs store energy by electrostatic accumulation of charges at the surface of the electrode material and mainly use some carbonaceous materials with high surface area [7, 8]. Electrical conductivity, pore structures and specific surface area of the electrode materials generally decide the electrochemical performances of EDLC electrodes. The pseudo-capacitors compounds utilize certain fast redox reactions taking place at the surface or nearsurface regions of the electrode to store electric charges and are mainly based on transition metal oxides, metal hydroxides, conductive polymers and metal sulfides [9-13].
The overall
electrochemical properties of pseudo-capacitors are always affected by their theoretical capacitance, geometric configuration and electrical behavior. Recently, many efforts have been devoted to explore the integrated and binder-free supercapacitor electrode with unique nanostructure by directly growing active materials on conductive substrate with nanorod [14, 15], nanoneedle [16, 17], nanoflake [18, 19] and other three dimensional structures to achieve high 3
electrochemical, electrical, optical and mechanical properties. Carbon fiber (CF) substrate is considered as one of the most promising conductive substrate in the field of supercapacitor, due to the good mechanical strength, highly conductivity and excellent flexibility. Nevertheless, CFs known as a kind of double layer carbon material possess negligible electrochemical capacitance for its low specific surface and less internal pore structure [20-23]. To solve above mentioned problem, transition-metal oxides and hydroxides, metal sulfides and conducting polymers with high pseudo-capacitive properties are grown or deposited on carbon fiber. For instance, Xiao et al. successfully prepared NiCo 2S4 single crystalline nanotube arrays deposited by a series of electroactive metal oxide materials (CoxNi1-x (OH) 2, MnO2, and FeOOH) with a highest discharge areal capacitance of 2.86 F cm-2 at 4 mA cm-2 [24]. Wen et al. synthesized three dimensional Co9S8 nanorod@Ni(OH)2 nanosheet core-shell structure on carbon fiber by a simple low temperature hydrothermal method with a s pecific capacitance of 1620 F g-1 at a current density of 0.5 A g-1 [25]. Yu et al. designed ultrathin MnO2 nanosheet/carbon fiber cathodes by a facile hydrothermal treatment with a specific capacitance of 634.5 F g-1 at a current density of 2.5 A g-1 [26]. Manganese dioxide (MnO2), a promising pseudo-capacitors electrode materials which is natural abundance and low cost, has been extensively investigated owing to its excellent electrochemical performance, such as a high theoretical specific capacity of 1370 F g-1, a wide electrochemical potential window of approximately 0.9-1.0 V and the ability using mild aqueous electrolytes [27-29]. However, the poor electrical 4
conductivity (10-5-10-6 S cm-1) has severely limited its performance far from the theoretical value because of electron transfer barriers. An emerging efficient strategy is to design smart integration of multi-component MnO2 mixed oxides and then make full use of the properties from individual component based on the synergistic effects to achieve a maximum electrochemical performance [27]. Recently, FeCo2O4 ，a ternary metallic oxide, have been investigated as electrode materials for supercapacitors
since
it
possess
higher
electrical conductivity and
good
electrochemical activity [30-32]. Moreover, as we all know, two-dimensional nanosheets can provide high efficient electrochemical active sites, reduce the ions and electron diffusion path and improve the structural stability [33]. Therefore, the FeCo2O4 nanosheets should be an excellent backbone material for supercapacitors. Herein, we have synthesized hierarchical FeCo 2O4@MnO2 core-shell nanosheet arrays on CFs via a facile two-step hydrothermal method for supercapacitors. The pre-formed free-standing FeCo2O4 precursor nanosheets are used as a backbone on CFs for anchoring MnO2 nanosheets. The network structure composed of FeCo2O4 nanosheets arranging orderly in a crisscross pattern not only offer a large surface area, but also support and provide reliable electrical connections to the outer MnO2 nanosheets. The hybrid electrode not only exhibits a high energy density of 22.68 Wh kg-1 at 406.01 W kg-1 and an energy density 7.06 Wh kg-1 at a high power density of 1802.5 W kg-1, but also an excellent cycling performance with 90.1% retention of the initial specific capacitance after 5000 cycles in 3M KOH solution.
5
Fig. 1. Schematic illustration of the synthesis procedure of hierarchical nanostructured FeCo2O4@MnO2.
2. Experimental section The analytical grade reagents ferrous sulfate (FeSO4·7H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), urea (CO(NH2)2), ammonium fluoride (NH4F), hydrochloric acid (HCl), potassium hydroxide (KOH) and absolute ethanol were all purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used throughout all the experiments.
2.1 Synthesis of FeCo 2O4@MnO2 core-shell arrays Prior to the synthesis, a piece of carbon fiber (1 cm×2 cm) was ultrasonicated by acetone, ethanol, and distilled water every ten minutes in sequence. In a typical procedure, 1 mmol ferrous sulfate (FeSO4·7H2O), 2 mmol cobalt nitrate hexahydrate 6
(Co(NO3)2·6H2O), 5 mmol urea (CO(NH2)2), and 2 mmol ammonium fluoride (NH4F) were dissolved in deionized water (30 mL) to form a red solution under vigorous magnetic stirring. After stirring for 30 min, the color of the solution changed from red to orange. Then the as-obtained orange solution was transferred into a 50 mL polytetrafluoroethylene (PTFE) Teflon-lined autoclave and the cleaned carbon cloth was immersed into the above solution, following by heating the autoclave at 140 ℃ for 7 h in an electric oven. After the autoclave was cooled to room temperature naturally, the carbon cloth coated golden brown precursor were carefully washed with deionized water and absolute ethanol to remove surface ions and molecules and then dried at 60 ℃ in a vacuum oven overnight. A piece of CF substrate with grown FeCo2O4 nanosheets was put into a 50 mL Teflon-lined autoclave including 30 mL KMnO4 solution (0.05 M) for MnO2 nanofilm growth. The autoclave was sealed and hydrothermally treated at 140 ℃ for 6 h. After the reaction, the solution was cooled down to room temperature naturally. Then the as-obtained sample was washed with deionized water and absolute ethanol, and dried at 60 ℃ in a vacuum oven. Finally, the precursor was annealed at 400 ℃ for 2 h, and then black core-shell arrays on the CF substrate were obtained. The total weight of FeCo 2O4@MnO2 core-shell nanosheet arrays on the CF substrate was 8 mg cm-2, and the mass loading of the FeCo2O4 nanosheets was 2 mg cm-2.
2.2 Materials characterization The morphology, compositions and size of the samples was investigated using the Hitachi S-4800 field emission SEM (FESEM, Hitachi, Japan). The Powder X-ray 7
diffraction (XRD) patterns were recorded on a D/MAX-2500 diffract meter (Rigaku, Japan) with a nickel-filtered Cu Kα radiation source (λ= 1.54056 Å).The X-ray photoelectron spectroscopy (XPS) was obtained by a Thermo ESCALAB 250X (America) electron spectrometer using 150WAl Ka X-ray sources. Transmission electron microscopy (TEM) images and high resolution transmission electron microscopy (HRTEM) images were taken on a F20 S-TWIN electron microscope (Tecnai G2, FEI Co.) with an acceleration voltage of 200 kV.
2.3 Electrochemical measurements The electrochemical measurements were carried out by an electrochemical workstation (CHI 760E, CH instrument Inc, China) in a 3.0 M aqueous KOH electrolyte by a typical three-electrode configuration. Ag/AgCl electrode and platinum electrode were used as the reference electrode and counter electrode, respectively. The FeCo2O4 nanosheets and the FeCo2O4@MnO2 core-shell naosheet arrays were directly used as the working electrodes without any binder and conductive actives. The working electrodes immersed in the electrolyte was 1 cm2 in nominal area. The electrochemical properties were investigated by cyclic voltammetry (CV) technique with varying the scan rate of 2-100 mV s-1 at potential between 0 and 0.5 V. Galvanostatic charge-discharge (GCD) experiments were performed with current densities ranged from 1 to 16 mA cm-2 at a potential of 0 to 0.4 V. Moreover, electrochemical impedance spectroscopy (EIS) testing was conducted with frequencies ranging from 0.01 to 10000 Hz. The areal capacitance (C ) and specific capacitance (C ) was estimated from the following Equation (1) and (2): 8
∆
C∝ =
∆
C =
∆
(1)
∆
(2)
where Ca (F cm-2) is the areal capacitance, and Cs (F g-1) is the specific capacitance, I (A) is the discharge current, Δt is the discharge time, and ΔV is the discharging potential range, S (cm-2) is the geometrical area of the electrode, m (g) is the mass of the active materials of the electrode. For ASC, the FeCo 2O4 nanosheets and the active carbon (AC) were used as the positive and negative electrode in a two-electrode system, respectively. 3 M KOH solution was used as the electrolyte. The negative electrode was fabricated by 80 wt% of AC material, 10 wt% of acetylene black and 10 wt% of polyvinylidene (PVDF) binder, then use right amount of N-Methyl-2-pyrrolidne to dissolve the above powder. According to the equation:
×∆
= ×∆
(3)
The mass ration obtained between the positive and negative is 0.8. Where and are the capacitances of the FeCo2O4@MnO2 and AC electrodes, respectively. and are the discharging potential range of the FeCo2O4@MnO2 and AC electrodes, respectively. The energy density and power density of the asymmetric cell are calculated from Equations (4) and (5): E= P= here
.×
(4)
. ×
(5)
∆
is the energy density (Wh Kg-1) , ! is power density (W Kg-1) and C is the
capacitance of the asymmetric cell (F g-1) and the capacitance C is based on the total mass of two electrodes, Δt is the total discharge time (s). 9
3. Results and discussion 3.1 Structure and morphology The fabrication procedure is illustrated in Fig. 1. Firstly, the self-assembled FeCo2O4 precursor nanosheets are grown uniformly on the CFs. Subsequently, the MnO2 nanosheets with different thickness which has been adjusted by controlling the KMnO4 solution reaction time in this experiment are successfully anchored onto the surfaces of the above nanosheets. Finally, the hybrid FeCo2O4@MnO2 core-shell nanosheets are obtained through a thermal annealing process. The XRD pattern of the FeCo2O4@MnO2 sample is shown in Fig. 2. The peaks at 12˚ and 37˚ can be readily indexed as birnessite-type δ-MnO2 (JPCDS No.86-0666), while the remaining peaks can be ascribed to the FeCo2O4 spinel phase (Fd3m space group). These results definitely confirm that the hybrid nanosheets is integrated by FeCo2O4 and MnO2.
Fig. 2. X-ray diffraction pattern (XRD) of the as-synthesized FeCo2O4 nanosheets and hybrid 10
FeCo2O4@MnO2 nanosheets on CFs.
Fig 3. XPS spectra of the Fe 2p region (a), Co 2p region (b), Mn 2p region (c) and O 1s region (d) of the hybrid electrode.
To further illustrate the surface composition and electronic states of the synthesized sample, the X-ray photoelectron spectroscopy (XPS) measurement is performed in Fig. 3. The Fe 2p core level spectrum shows prominent peaks at 710.5 eV for Fe 2p3/2, 724 eV for Fe 2p 1/2, and a “shoulder” satellite peak at 714.4 eV, indicating the presence of Fe (III) oxidation [34]. In addition, the energy separation between the spin orbit components (binding energy (BE), △BE = 13.5 eV) is in agreement with the Fe (III) in an oxide environment [35-40]. The Co 2p region is shown in the Fig. 3b, there are two obvious peaks corresponding to Co 2p3/2 and Co 2p 1/2, and they are divided into four peaks after fitting. The fitting peaks at 780.3 and 11
795.3 eV, along with the satellite peak of 788.8 eV are related with Co3+, while the fitting peaks at binding energies of 781.9, 796.2 eV and the satellite peak of 805.1 eV are indexed to Co2+. Therefore, the Co3+ and Co2+ coexist in the sample. The Mn 2p spectrum is displayed in the Fig. 3c, the peaks located at 642.5 and 654.2 eV, corresponding to the Mn 2p 3/2 and Mn 2p 1/2, respectively. The Fig. 3d exhibits the O 1s spectrum, the peak at a binding energy of 529.8 eV corresponds to a typical bond between metal and oxygen, and the peak at a binding energy of 531.5 eV corresponds to the OH- groups oxygen, which reveal the hydroxylation of the material surface.
Fig. 4. SEM images of primary FeCo2O4 nanosheet arrays (a) and FeCo2O4@MnO2 core-shell nanosheet arrays obtained at different time on CFs: (a) 0 h; (b) 2 h; (c) 4 h; (d) 6 h. The insert shows the enlarged image.
Fig. 4a shows the morphology of as-synthesized FeCo2O4 nanosheet arrays. It 12
can be seen that the obtained CFs are uniformly covered by the FeCo 2O4 nanosheets. The high-magnification SEM image in the inset of Fig. 4a reveals that the thickness of individual nanoflake is only 30 ~ 40 nm. FeCo2O4 nanosheets grown on the CFs can not only enhance the electroactive surface site for growing MnO2 but also provide effective electrical connection to MnO2 and overcome the problem of poor conductivity of MnO2. Additionally, the SEM images of FeCo2O4@MnO2 core-shell nanosheets which have been dipped into the permanganate solution for different time are displayed in Fig. 4b-d. The reaction for forming FeCo2O4 is performed according to the equation (6) and (7): xFe + 2xCo( + 6xOH = Fe, Co(, (OH) ,
(6)
2Fe,Co(, (OH) , + O( = 2xFeCo( O/ + 6xH( O
(7)
MnO2 is a product of the decomposition reaction of Potassium Permanganate. Compared with the Fig. 4a (0 h), there are small particles can be seen from the edge or surface of the FeCo 2O4 nanosheets (2 h, Fig. 4b) and the thickness of each nanosheet is broadened. It is noteworthy that when the hydrothermal reaction increases from 2 h to 6 h, the thickness of the shell is rapidly increased from few tens nanometers to one micron. Moreover, the MnO2 nanosheets connecting with each other are 8-10 nm in thickness. And when the relatively small MnO2 nanosheets aggregate to grow, there is space left among sheets, so the electrolyte ions can still penetrate. The core-shell structure disappears and the three-dimensional structure is completely wrapped in a thick MnO2 nanoflakes shell after 8 h of reaction (Fig. S1). It is undoubted that the 8 h sample is unsuitable for the diffusion of the electrolytes 13
ions.
Fig. 5. TEM and HRTEM images of hierarchical FeCo2O4@MnO2 core-shell nanostructure scratched from CFs.
The morphology and structure of the as-obtained FeCo2O4@MnO2 core-shell nanosheet arrays (3h) are further investigated by TEM. The image of the above FeCo2O4@MnO2 sample scratched from CFs clearly reveals that the surface of thin FeCo2O4 nanosheet is coated by numerous MnO2 nanosheets, which is consistent with the SEM images of the FeCo 2O4@MnO2 core-shell nanostructure and different from the SEM image of the pure MnO2 nanosheets which are a few nanometers in size on CFs (Fig. S1 8h). The HRTEM image marked by the red solid circle is shown in the Fig. 5b and its interplanar spacing of 0.67 nm can be indexed as the (001) plane of birnessite-type MnO2. From the TEM image, the thickness of MnO2 nanosheets is about 10 nm. In addition, the lattice spacing of 0.24 nm in the Fig. 5c indicated by the red dashed circle corresponds to the (311) planes of the FeCo 2O4 cubic phase, which is the alignment with the XRD results. These results confirm the formation of 14
FeCo2O4@MnO2 core-shell hybrid composite.
3.2 Electrochemical properties of FeCo2O 4@MnO2 Next, we directly applied the hierarchical FeCo 2O4@MnO2 core-shell heterostructured nanosheet arrays as electrode for SCs. Fig. 6a shows the CV curves of pristine FeCo 2O4 and FeCo 2O4@MnO2 core–shell hybrid electrodes with a potential window of 0-0.5 V at a scan rate of 5 mV s-1. Clearly, a pair of redox peaks is evident when the reaction time is lower than 2 h, the pseudo-capacitive behavior observed could be largely attributed to the reversible faradaic reactions relating to the following equations [41]: FeCo( O/ + H( O + OH ↔ FeOOH + 2CoOOH + e
(8)
CoOOH + OH ↔ CoO( + H( O + e
(9)
FeOOH + H( O ↔ Fe(OH) ↔ FeO/ ( + 3e
(10)
The CV curves of the FeCo2O4 are shown in the Fig. S2.
15
Fig. 6. (a) CV curves of the FeCo2O4 @MnO2 core-shell NSs arrays obtained at different time on CFs at a scan rate of 5 mv s-1; (b) Galvanostatic charge-discharge (GCD) curves of the FeCo2O4@MnO2 core-shell NSs arrays at a current density of 1 mA cm-2; (c) CV curves of the FeCo2O4@MnO2 core-shell NSs arrays (6 h) at different scan rates; (d) GCD curves of the FeCo2O4@MnO2 core-shell NSs arrays (6 h) at different current densities; (e) specific capacitance of the electrodes; (f) EIS spectra of the pristine FeCo2O4 and FeCo2O4 @MnO2 core-shell hybrid electrodes. 16
When the reaction time continues to increase, this pair of redox peaks becomes unobvious due to the thick MnO2 shell. Obviously, the area under CV curve of the FeCo2O4@ MnO2 hybrid electrode (6 h) is larger than that of pristine FeCo 2O4 (0 h) electrode and other FeCo2O4@ MnO2 hybrid electrodes (2, 4 and 8 h). The results indicate that the thickness of MnO2 nanosheets has a great influence on electrochemical properties, but it is difficult for ions to penetrate from a too thick shell into the inner microstructures of the electrode. As is known to us, the III oxidation state in the MnO2 will be oxidized to IV oxidation state of Mn during charging, then IV oxidation state of Mn will be reduced to III oxidation state of Mn during discharging, accompanying the adsorption of cations at the surface of MnO2. The following reactions may be involved in the transition: (11) Mn(IV)O( ∙ nH( O + δe + δ(1 − f)H O + δfM ↔ (H O):(;<)M:< =Mn (III): Mn (IV);: >O( ∙ nH( O
(11)
M+ are Na+, K+ and Li+. To illustrate the influence and importance of FeCo2O4 backbone, the CV curve of FeCo2O4@ MnO2 hybrid electrode (6 h) and bare MnO2 electrode (6 h) are shown in Fig. S3, showing that the FeCo2O4 backbone is vital for the increased capacity. As expected in Fig. 6b, FeCo2O4@ MnO2 hybrid electrode (6 h) exhibits the longest discharge time at the same current density of 1 mA cm-2. These could be attributed to the additional pseudo-capacitance contributed by the MnO2 shell and the FeCo 2O4 nanosheets can further provide fast pathways and abundant electroactive surface sites for δ-MnO2 shell to harvest a maximum capacitance. Moreover, the layered 17
network-like highly conductive FeCo2O4 nanosheets as a support can provide reliable electrical connections to the outer MnO2 shell which have the poor electronic/ionic conductivity. Because of the hierarchical structure, the electrolyte ions can quickly contact the electrode material from all directions and then rapidly transfer to the inner core part with high conduction and this is the reason why the voltage profiles show sudden increases in the beginning of charge. When the reaction time is Fig. 6c displays the CV curves of optimized FeCo2O4@ MnO2 hybrid electrode (6 h) collected at different scan rates from 2 to 100 mV s-1 within the potential window of 0-0.5 V. Fig. 6d shows the GCD curves of FeCo2O4@ MnO2 electrode (6 h) at different current densities ranging from 1 to 16 mA cm-2 and the nearly symmetrical GCD curves reveals good electrochemical capacitive characteristics and a superior reversibility. The areal capacitances of the FeCo2O4@ MnO2 electrode (6 h) can be calculated from the above-mentioned discharge curves. The corresponding specific area capacitances (Ca) are 4.8, 3.4, 2.5, 1.2, 0.19 F cm-2 at 1, 2 4, 6, 8, 16 mA cm-2, respectively, which are larger than that of the pristine FeCo 2O4 electrode (42 mF cm-2). The areal capacitances of other FeCo2O4@ MnO2 hybrid electrodes are shown in the Fig. 6e. Fig. 6f shows the impedance Nyquist plots of the pristine FeCo2O4, the bare MnO2 electrode (6 h) and FeCo2O4@ MnO2 hybrid electrode (6 h) in the frequency range of 0.01 to 100 kHz under open circuit potential. The Nyquist plots are mainly composed of a semicircle at high frequency region and a straight line at low frequency region. The inset in Fig. 6f is the equivalent circuit model. In the high frequency region, the value of the charge-transfer impedance (Rct) of the FeCo2O4@ MnO2 18
hybrid electrode, pure FeCo2O4 nanosheets on CFs and the pure MnO2 nanosheets on CFs are 5.06 Ω, 3.17 Ω, 14.45 Ω, respectively. It can be seen clearly that the introduction of the backbone FeCo 2O4 nanosheets to a large extent solved the problem of extremely poor conductivity of MnO2. Moreover, the bulk resistance (Rs) value of pure FeCo 2O4 nanosheets on CFs, FeCo 2O4@ MnO2 hybrid electrode (6 h) and the bare MnO2 electrode is 1.26 Ω, 1.74 Ω, 1.41 Ω, respectively. In the low frequency area, the slope of the curves of the pristine FeCo2O4 electrode and the FeCo 2O4@ MnO2 hybrid electrode are almost the same. All these results suggests that the FeCo2O4 nanosheet arrays have better conductivity than the MnO2 nanosheets and can make the FeCo2O4@ MnO2 hybrid electrode faster reaction kinetics.
3.3 Electrochemical properties of FeCo2O4@MnO2//AC asymmetric supercapacitor To further investigate the electrochemical performance for an asymmetric capacitor, we use FeCo2O4@MnO2 core-shell nanosheets as the positive electrode and AC as the negative electrode (Fig. 7a). The electrochemical performance of AC is shown in the Fig. S4. The CV curves of AC which are operated in the voltage window from -1.0 to 0 V indicate good electric double-layer capacitance property. In the actual experiment, the quality of AC is about 10 mg. Fig. 7b displays the CV curves of the as-assembled asymmetric supercapacitor recorded at different voltage windows. Note that the high potential part of the CV curves of 0-1.8V and 0-2.0V has obvious upturned, indicating that the oxygen evolution reaction, so the operating potential of the ASC device is determined to 1.6 V. Then, the CV curves of the ASC device in a 19
voltage window of 0 to 1.6 V at different scan rates of 2-100 mV s-1 are recorded in the Fig. 7c. These CV curves display no obvious redox peaks, showing a good rate capability for power storage. Fig. 7d exhibits the GCD curves of the ASC device at various current densities. The GCD curves are nearly symmetric, which suggests the excellent capacitive performance of the ASC device.
Fig. 7 Electrochemical performance of the FeCo2O4@MnO2 (6 h)//AC asymmetric supercapacitor. (a) Schematic illustration of the assembled ASC configuration; (b) CV curves of the FeCo2O4@MnO2 (6 h)//AC ASC cell measured at different potential windows; (c) CV curves of the ASC device collected at different scam rates; (d) CV curves at different current densities.
20
Fig. 8 (a) Cycling performance of the ASC at current density of 25 mA cm-2, the inset shows 1 st and 5000th cycles of the charge–discharge tests; (b) Ragone plot of the ASC and some other devices from previous literature for comparison. (c) A picture obviously shows the red, blue, white LEDs can be lighted up, when the two assembled ASC devices connected in series be charged in the left picture.
Furthermore, the cycling performance of the capacitor based on the GCD cycling curves is shown in Fig. 8a. It is clearly that the capacitance increases during the first 2500 cycles. With the further infiltration of electrolyte ions, more and more hybrid nanostructures become activated after the intercalation and deintercalation process and contribute to the increase of capacitance. Moreover, the capacitance can be maintained at 90.1% of the initial capacitance after 5000 cycles, suggesting a good electrochemical stability. The Ragone plots of the FeCo2O4@ MnO2//AC device in a voltage window of 0-1.6 V are displayed in Fig. 8b. The device shows a maximum 21
energy density calculated from the GCD curves is 22.68 W h kg-1 at a power density of 406.01 W kg-1 and the corresponding current density is 10 mA cm-2. Even at a high current density of 40 mA cm-2, the energy density stills retains 7.06 W h kg-1 at power density of 1802.5 W kg-1. The excellent values are higher than those of the MnO2-component asymmetric supercapacitors, such as graphene/MnO2 ASC ( 6.8 Wh kg-1 at 62 W kg-1) [42], MnO2/GHCS ASC ( 22.1 Wh kg-1 at 100 W kg-1) [43], MnO2/CNT ASC ( 15.3 Wh kg-1 at 100 W kg-1) [44], NiCo 2O4/MnO2 ASC ( 9.4 Wh kg-1 at 175 W kg-1) [45], MnO2-modified diatomites ASC ( 3.75 Wh kg-1 at 250 W kg-1) [46], MnO2/CNTs ASC ( 13.3 Wh kg-1 at 600 W kg-1) [47] and PEDOT/MnO2 ASC ( 9.8 Wh kg-1 at 800 W kg-1) [48]. Thus, in order to further demonstrate the real practical applications of our products, we assembled two ASCs in series to light a LED. When the device in the left side of the Fig. 8c was charged for 30 s at a current density of 0.01 mA cm-2, our devices can power a LED brightly (red, blue and white). Especially, the red LED can be kept for 3 min, and the brightness images of the red LED is shown in the Fig. S5.
4. Conclusions In summary, we have designed a facile and cost-effective strategy to fabricate hierarchical FeCo 2O4@MnO2 core-shell nanosheet arrays with great electrochemical performance for supercapacitor applications. In this architecture, the FeCo 2O4 nanosheets not only act as backbone to enhance the electroactive surface sites but also provide effective electrical connection to MnO2. The as-obtained hybrid electrode exhibits a high areal capacitance of 4.8 F cm-2 at the current density of 1 mA cm-2. 22
Moreover, the ASC device based on the FeCo 2O4@MnO2 core-shell electrode has been successfully assembled and shows a high energy density of 22.68 Wh kg-1 at a power density of 406.01 W kg-1 and 7.06 W h kg-1 at a high power density of 1802.5 W kg-1. Even after 5000 cycles of charging-discharging process, the specific capacitance still remained 90.1% of the initial specific capacitance. Furthermore, two ASCs in series can light a LED. These results convince us that our work have great potential applications in hybrid electrode materials for energy storage application.
Acknowledgements It is pleasure to acknowledge the financial support provided by the National Natural Science Foundation of China (21477050, 21522603 and 51602131), the Chinese German Cooperation Research Project (GZ1091), the Excellent Youth Foundation of Jiangsu Scientific Committee (BK20140011 and BK20160526), the Program for New Century Excellent Talents in University (NCET-13-0835), the Henry Fok Education Foundation (141068), Six Talents Peak Project in Jiangsu Province (XCL-025), Jiangsu Province Postdoctoral Science Foundation (1601059C) and China Postdoctoral Science Foundation (2016M600368). Conflicts of interest: none.
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