Hierarchical Fe3O4@Fe2O3 CoreâShell Nanorod Arrays as High

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Hierarchical Fe3O4@Fe2O3 Core-Shell Nanorod Arrays as High-performance Anode for Asymmetric Supercapacitors Xiao Tang, Ruyue Jia, Teng Zhai, and Hui Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09766 • Publication Date (Web): 23 Nov 2015 Downloaded from http://pubs.acs.org on November 28, 2015

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ACS Applied Materials & Interfaces

Hierarchical Fe3O4@Fe2O3 Core-Shell Nanorod Arrays as High-performance Anode for Asymmetric Supercapacitors

Xiao Tang 1,2, Ruyue Jia 1,2, Teng Zhai 1,2∗, and Hui Xia1,2∗

1

School of Materials Science and Engineering, Nanjing University of Science and Technology,

Nanjing 210094, China 2

Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology,

Nanjing 210094, China

ABSTRACT Anode materials with relatively low capacitance remain a great challenge for asymmetric supercapacitors (ASCs) to pursue high energy density. Hematite (α-Fe2O3) has attracted intensive attention as anode material for ASCs due to its suitable reversible redox reactions in negative potential window (0∼−1 V vs. Ag/AgCl), high theoretical capacitance, rich abundance, and non-toxic feature. Nevertheless, the Fe2O3 electrode cannot deliver large volumetric capacitance at high rate because of its poor electrical conductivity (∼10−14 S/cm), resulting in low power density and low energy density. In this work, a hierarchical heterostructure comprising Fe3O4@Fe2O3 core-shell nanorod arrays (NRAs) is presented and investigated as negative electrode for ASCs. Consequently, the Fe3O4@Fe2O3 electrode exhibits superior supercapacitive performance compared to the bare Fe2O3 and Fe3O4 NRAs electrodes,

ACS Paragon Plus Environment

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demonstrating large volumetric capacitance (up to 1206 F/cm3 with 1.25 mg/cm2 mass loading) as well as good rate capability and cycling stability. The hybrid electrode design is also adopted to prepare Fe3O4@MnO2 core-shell NRAs as positive electrode for ASCs. Significantly, the as assembled 2 V ASC device delivered a high energy density of 0.83 mWh/cm3 at a power density of 15.6 mW/cm3. This work constitutes the first demonstration of Fe3O4 as the conductive supports for Fe2O3 to address the concerns about its poor electronic and ionic transport. KEYWORDS: Fe2O3, Fe3O4, hierarchical, core-shell, anode, asymmetric supercapacitors.

INTRODUCTION High-efficient energy storage devices have stimulated tremendous attention from researchers due to the exhaustion of non-renewable resources and their increasing damage to environment.1–4 Supercapacitors (SCs), also known as electrochemical capacitors, are considered to represent a new class of energy storage devices because they are capable of storing more energy than conventional capacitors and providing higher power than batteries.1,5–11 However, in order to meet the increasing energy requirements for the next-generation electronic devices, the energy density of current SCs need to be further improved without sacrificing the power density and cycling life. According to the equation of energy density E =1/2CV2, two strategies aiming at extending the voltage window (V) and increasing capacitance (C) have been proposed to achieve this target.12 The first one can be achieved by the construction of asymmetric supercapacitors (ASCs), which possess an expanded voltage window up to ∼2.2 V in aqueous electrolytes.13–14 The second one is to develop nanostructured electrode materials with high capacitance and excellent rate capability.15–18 In recent years, cathode materials with excellent capacitive


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performance were reported intensively.19–21 Yet, current anode materials still suffer from low capacitance to match large capacitance cathodes. Therefore, it remains a great challenge for ASCs to pursue high energy density at high power density. In order to make progress in the energy-density limit of ASCs, a new class of negative electrode materials combining high capacitance and conductivity is indispensable.27 In this regard, a series of negative electrode materials such as MoO3–x,26 V2O5,27 VN,28 and Fe2O329–30 have been developed for ASCs. Among these materials, Fe2O3 is considered as a promising electrode material in negative potential window (0∼−1 V vs. Ag/AgCl) owing to its outstanding features such as appropriate reversible redox reactions (Fe2+/Fe3+), large theoretical capacitance, rich abundance, and non-toxicity.31 Nevertheless, the capacitance of Fe2O3 electrode at higher current densities is still fairly low. This is mainly due to its poor electrical conductivity (∼10−14 S/cm), resulting in low power density and corresponding low energy density at high rate.32 Up to now, intensive researches focusing on overcoming this limitation were reported. Significantly, several approaches including the construction of nanostructured Fe2O3 electrodes, Fe2O3 composite

electrodes,

and

oxygen

vacancy

induced

Fe2O3

electrodes

have

been

developed.15,25,32,34,36 Among these approaches, the construction of Fe2O3 composite electrodes has attracted the most of efforts on improving the electrochemical properties. Various conductive supports such as graphene,15,23,24 doped V2O5,34 mesoporous carbon,32,37,22 PANI,25 SnO2,33 etc., were adopted to combine with the Fe2O3. Though enhanced electrochemical performance has been achieved, the limited electron and ion transport are still the crucial concerns for Fe2O3 composites as high performance electrodes in ASCs. In this work, we designed a hierarchical heterostructure consisting of Fe3O4@Fe2O3 coreshell nanorod arrays (NRAs) with enhanced electrochemical performance through the adoption


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of highly conductive Fe3O4 supports and the junction formed at the interface between Fe3O4 and Fe2O3. It has been shown that the junction formation at the Fe3O4/Fe2O3 interface can significantly improve the charge transport of the Fe3O4@Fe2O3 composites and further boost their photocatalytic activities.38 Moreover, the Fe3O4 has attracted tremendous attention due to its superior electrical conductivity (∼200 S/cm).39 We expect that the Fe3O4@Fe2O3 combination will substantially enhance the electron transport as well as the capacitive performance for the hybrid electrode. Herein, a porous Fe2O3 shell was coated on the surface of Fe3O4 NRs via a facile electrodeposition process followed by annealing. Significantly, a large volumetric capacitance (up to 1206 F/cm3 with an active mass loading of 1.25 mg/cm2) as well as good rate capability and cycle performance were achieved by the hierarchical Fe3O4@Fe2O3 core-shell NRAs. Moreover, a 2 V ASC device based on Fe3O4@Fe2O3 anode and Fe3O4@MnO2 cathode was constructed and delivered a high energy density of 0.83 mWh/cm3 at a power density of 15.6 mW/cm3.

EXPERIMENTAL SECTION Synthesis of Fe3O4 and Fe2O3 NRAs In a typical experiment, two aqueous solutions containing 0.946 g of FeCl3·6H2O and 0.479 g of Na2SO4, respectively, were mixed with vigorous stirring. Distilled water was then added to obtain a final 70 mL solution. After thoroughly magnetic stirring for 10 min, the mixture was transferred into a Teflon-lined stainless-steel autoclave with a clean Ti substrate and hydrothermally treated at 120 °C for 6 h. After cooling down to room temperature, the arraycovered substrate was rinsed several times with distilled water and then dried at 60 °C. Finally,


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the samples were annealed in Ar gas at 450 °C for 3h to obtain the Fe2O3 NRAs or annealed in H2 gas at 350 °C for 3 h to attain the Fe3O4 NRAs. Synthesis of Fe3O4@Fe2O3 core-shell NRAs The Fe3O4 NRAs grown on the Ti substrate was used as the scaffold for the growth of Fe2O3 porous shells via a simple electrodeposition method. The electrodeposition was performed in a standard three-electrode cell, using a platinum foil counter electrode, an Ag/AgCl reference electrode, and the Fe3O4 NRA working electrode. Initially, FeOOH layers with various mass loadings were deposited by the potential static method at 1.5 V in a 20 mL of 20 mM FeCl2 solution at 75oC for 5, 15, 30, and 45min, respectively. After electrodeposition, the samples were collected, rinsed with distilled water and ethanol for several times, and dried at a vacuum oven. Finally, annealing in Ar gas at 450 °C was carried out on the samples for 3 h to attain the Fe3O4@Fe2O3 core-shell NRAs. Synthesis of Fe3O4@MnO2 core-shell NRAs To fabricate the Fe3O4@MnO2 core-shell NRAs, the carbon-coated Fe3O4 was first obtained via immersing the Fe3O4 NRAs into a 0.04 M aqueous glucose solution for 24 h, followed by carbonization at 450 °C in Ar gas for 2 h. After carbon coating, the sample was put into a Teflonlined stainless steel autoclave containing a 0.03 M KMnO4 solution, which was subsequently heated at 160 °C for 1∼5 h. Finally, the samples were collected, washed with distilled water, and dried at 60 °C to obtain the Fe3O4@MnO2 core-shell NRAs. Materials Characterization


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The crystallographic information and phase purity of the products were investigated by X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The XRD patterns were performed by a Shimadzu XRD-6000 X-ray diffractometer with Cu Kα radiation between 10 and 80°. Raman spectra of different samples were acquired using a Renishaw in Via Reflex Raman microprobe with a 532 nm wavelength incident laser. The morphology and microstructure of the samples were investigated by field emission scanning electron microscopy (FESEM, Hitachi S4300), transmission electron microscopy (TEM, FEI-Philips CM300 UT/FEG), and high-resolution transmission electron microscopy (HRTEM). Electrochemical Measurements The mass loadings for both Fe3O4@Fe2O3 and Fe3O4@MnO2 films on Ti substrates are in the range of 1∼2 mg/cm2, which were measured by a microbalance with a resolution of 0.01 mg. The electrochemical measurements of individual electrodes (Fe3O4, Fe2O3, Fe3O4@Fe2O3, and Fe3O4@MnO2) were performed using three-electrode cells with a platinum foil as counter electrode and an Ag/AgCl reference electrode. ASCs with Fe3O4@Fe2O3 anode and Fe3O4@MnO2 cathode were assembled in Swagelok cells using porous nonwoven fabric as the separator. For both three-electrode cells and two-electrode cells, 1 M Na2SO4 was used as the electrolyte. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements of different electrodes were carried out on a CHI660D electrochemical workstation. Galvanostatic charge/discharge measurements were measured using a battery testing system LAND CT2001A at different current densities. The volumetric capacitance (C) of single electrode can be calculated based on CV curves and galvanostatic charge/discharge curves according to equation (1) and (2), respectively:


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CV =

Q S = V∆U 2vV∆U

(1)

CV =

Q ∆t =I V∆U V∆U

(2)

Where Cv (F/cm3) is the volumetric capacitance; Q (C) is the average charge during the charging and discharging process; V (cm3) is the volume of the active materials; ∆U (V) is the potential window; S (A V) is the integrated area of the CV curve; v (V/s) is the scan rate; I (A) is the constant discharging current; ∆t (s) is the discharging time.

For gravimetric specific (Cs)

capacitance, the V (cm3) in the calculations should be replaced by m (g). The mass density of Fe3O4@Fe2O3 is 1.25 mg/cm2. The thin film volume V (cm3) was estimated from the cross-section FESEM images shown in Figure S1 (Supporting Information). The average thickness of the Fe3O4@Fe2O3 core-shell NRAs is measured to be about 1.2 µm, and the whole volume of the active material film per cm2 is calculated to be about 0.00024 cm3 (considering the film on both sides of the substrate). The volumetric energy density (Ev) and average volumetric power density (Pv) of the assembled ASC device were calculated by using the following equation (3) and (4): 1 EV = CV U 2 2

(3)

EV ∆t

(4)

PV =

Where the Cv (F/cm3) is the volumetric capacitance of the whole device; U is the voltage window of the as assembled ASC device.

RESULTS AND DISCUSSION


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The strategy to prepare hierarchical Fe3O4@Fe2O3 core-shell NRAs is illustrated in Scheme 1. The FeOOH NRAs were first grown on a Ti substrate via a facile hydrothermal process (details in Experimental section). Then Fe3O4 was obtained through annealing the FeOOH in H2 atmosphere. A porous Fe2O3 layer consisting of ultrafine nanorods (NRs) were further electrodeposited on the as-prepared Fe3O4 NRAs. For better comparison, the pure Fe2O3 NRAs were also synthesized via simple annealing the FeOOH NRAs in Ar atmosphere (details in Experimental section). Figure 1a and the inset show that a uniform yellow film consisting of vertically aligned NRAs of around 30∼50 nm in diameter and 0.8∼1.2 µm in length. After annealing the FeOOH in H2 atmosphere, no obvious change in morphology can be observed for the Fe3O4 NRAs (Figure 1b), indicating its good thermal structural stability. As shown in Figure 1c, a uniform black film of Fe3O4@Fe2O3 core-shell NRAs with diameters of around 50∼100 nm and length of around 1 µm (Figure S1, Supporting Information) was successfully fabricated on the Ti substrate. The space between Fe3O4@Fe2O3 NRs could be utilized efficiently in such unique hierarchical structure, which will allow easier access of the electrolyte ions to the active materials. XRD analysis was also performed to identify the phase transformation. Figure 1d shows the XRD patterns of the as-prepared Fe3O4 and Fe3O4@Fe2O3 NRAs. Peaks located at 30.1°, 35.4°, 43.1° can be ascribed to the (220), (311), (400) of Fe3O4 (magnetite), confirming the successful transformation from FeOOH to Fe3O4. Due to the poor crystallinity of Fe2O3 (hematite), only one peak (located at 35.7°) can be observed from the XRD pattern of the Fe3O4@Fe2O3 NRAs, which can be ascribed to the α-Fe2O3 (JCPDS No. 33-0664). To further confirm the phase and composition of the as electrodeposited Fe2O3, Raman spectrum was shown in Figure 2a. Since hematite belongs to the D63d crystal space group with two A1g modes and five Eg modes, two peaks situated at 218 and 483 cm-1 are assigned to the A1g


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modes and three peaks situated at 282, 392, and 587 cm-1 can be assigned to Eg modes, respectively.40 In addition to the two peaks from Fe3O4, the peaks at 218, 282, 392, 483, and 587 cm-1 are also detected, demonstrating the successful electrodeposition of hematite. To study the chemical composition and oxidation state of the Fe3O4@Fe2O3 core-shell NR-As, we also performed XPS measurements on the sample. Figure 2b shows the Fe 2p core level XPS spectrum of the Fe3O4@Fe2O3 NRAs. Two main peaks located at 710.9 and 724.9 eV correspond to Fe 2p3/2 and Fe 2p1/2, respectively. For the Fe 2p3/2, it can be deconvoluted into two peaks, at 710.7 and 711.1 eV. In consistent with Fe 2p3/2, the Fe 2p1/2 can also be best fitted into two peaks located at 724.4 and 724.9 eV. Peaks at 710.7 eV and 724.4 eV can be ascribed to the Fe3+ from α-Fe2O3 and Fe3O4, while another two peaks at 711.1 eV and 724.9 eV come from Fe2+ in Fe3O4.41,42 Notably, shakeup satellite peaks situated at 719.3 eV and 732.2 eV can be ascribed to the fingerprint of the electronic structures of Fe2O3,43 again confirming the existence of Fe2O3 in the Fe3O4@Fe2O3 core-shell NRAs. Figure 2c shows the O 1s spectrum that can be deconvoluted into three peaks at 529.9, 531.1, and 533.3 eV. Specially, the peak at 529.9 eV is a typical state of O2- species corresponding to Fe3O4/Fe2O3. All the peaks observed in the XPS analysis demonstrate the hybrid phases of Fe3O4 and Fe2O3 for the composite. The results of XRD, Raman, and XPS above convincingly confirm the successful synthesis of Fe3O4@Fe2O3 coreshell NRAs without any trace of impurity phase. The nanostructures and crystal structures were further investigated by using TEM. As shown in Figure 3a, the porous Fe2O3 shell is constructed by numerous Fe2O3 nanosticks with average size of 10 to 20 nm in length on the Fe3O4 NRs. The inset of Figure 3a is the selected area electron diffraction (SAED) pattern of a single Fe3O4@Fe2O3 core-shell NR, showing that the two diffraction rings correspond well to the (104) and (113) planes of Fe2O3. Moreover, another


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three diffraction rings corresponding to the (220), (422), and (511) planes of Fe3O4 can also be observed in the SAED pattern. The HRTEM (Figure 3c) image of the selected area (marked by the box) in Figure 3b reveals two interplanar lattice fringes of 0.48 and 0.27 nm, which can be attributed to the (111) plane of Fe3O4 and the (104) plane of Fe2O3, respectively. Figure 3d shows a HRTEM image taken from the nanostick region (marked by the circle) of the Fe3O4@Fe2O3 NR in Figure 3b. An interplanar spacing about 0.45 nm can be observed, which corresponds to the (0003) planes of the trigonal Fe2O3,44 further confirming the existence of Fe2O3 shell. To evaluate the electrochemical performance of the as-prepared Fe3O4@Fe2O3 core-shell NRAs with hierarchical heterostructure, electrochemical studies were conducted in a three electrode system with an Ag/AgCl reference electrode and a Pt counter electrode. 1 M Na2SO4 was selected as the electrolyte. Figure 4a compares the CV curves of the Fe3O4, Fe2O3, and Fe3O4@Fe2O3 electrodes at a scan rate of 100 mV/s. As expected, the Fe3O4@Fe2O3 electrode exhibits much higher current densities than those of the Fe3O4 and Fe2O3 electrodes with similar volume level. Moreover, the CV curves of Fe3O4@Fe2O3 electrode (Figure S2a, Supporting Information) remain good rectangular shape as the scan rate varies from 5 to 200 mV/s, indicating its outstanding capacitive behaviour and rate capability. Figure 4b presents the volumetric capacitance of the Fe3O4, Fe2O3, and Fe3O4@Fe2O3 electrodes as a function of scan rate ranging from 5∼200 mV/s. Significantly, the Fe3O4@Fe2O3 electrode delivers the highest volumetric capacitance at varied scan rates. For example, it yields a volumetric capacitance of 1206 F/cm3 (231.9 F/g based on the total mass of Fe3O4@Fe2O3) at scan rate of 5 mV/s, which is substantially higher than that of the Fe3O4 electrode (315 F/cm3) and Fe2O3 electrode (595 F/cm3). Meanwhile, the Fe3O4@Fe2O3 electrode also exhibits greatly improved rate capability


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with capacitance retention of about 46% as the scan rate increases from 5 to 200 mV/s, which is substantially higher than ∼25% for the pure Fe2O3 electrode. Moreover, the rate capability of the Fe3O4@Fe2O3 electrode is also much higher than or comparable with recently reported various Fe2O3-based composites, such as Fe2O3@PANI (45% from 5 to 100 mV/s),25 Fe2O3 nanotubes/reduced graphene oxide (41% from 2.5 to 100 mV/s),45 Fe2O3/MnO2 core-shell nanowire (11% from 2 to 100 mV/s),48 etc. The

galvanostatic

charge/discharge

measurements

also

demonstrate

the

superior

electrochemical performance of the Fe3O4@Fe2O3 electrode (Figure S2b, Supporting Information). As shown in Figure S3 (Supporting Information), the charge/discharge curves of the three electrodes collected at a current density of 25 mA/cm2 show that the Fe3O4@Fe2O3 electrode presents the most prolonged charge/discharge curves, again confirming its outstanding capacitive performance. Furthermore, the (IR) drop of 0.093 V for the Fe3O4@Fe2O3 electrode is much smaller than 0.167 V of the pure Fe2O3 electrode, indicating the enhanced electrical conductivity of Fe3O4@Fe2O3 electrode. The smallest IR drop of Fe3O4 electrode illustrates its highly conductive feature. EIS measurement is an efficient way to demonstrate the electrochemical properties of the electrodes (Figure 4c). The equivalent series resistances (ESR) of the Fe3O4, Fe2O3, and Fe3O4@Fe2O3 electrodes are 2.3, 3.3, and 2.9 Ω, respectively. In addition to the smaller ESR compared with the pure Fe2O3 electrode, the Fe3O4@Fe2O3 electrode also possesses a charge transfer resistance (Rct) of 2 Ω, which is substantially smaller than that of pure Fe2O3 electrode (∼7 Ω), indicating the enhanced faradic reactions via the adoption of highly conductive Fe3O4 electrode. Furthermore, the long-term cycling stability of the Fe3O4@Fe2O3 electrode was collected at a scan rate of 1.25 mA/cm2 for 5000 charge/discharge cycles (Figure 4d). A slight decrease of around 6 % of the initial capacitance can be observed for the


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Fe3O4@Fe2O3 electrode after 5000 cycles, which is substantially smaller than the reported values of the Fe2O3-based composite electrodes, such as Fe2O3 nanotube,46 V2O5-doped Fe2O3,47 Fe2O3/mesoporous carbon,48 and comparable with the values of the recently reported oxygendeficient Fe2O3 NRs36 and [email protected] These above results convincingly indicate that the adoption of highly conductive Fe3O4 core can efficiently enhance the electron transport and further boost the capacitive performance of the Fe3O4@Fe2O3 core-shell NRAs electrode. To better illustrate the superiority of the highly conductive Fe3O4 NRAs with high surface area as good support, the Fe2O3 with different mass loadings were electrodeposited. The volumetric capacitances of the Fe3O4@Fe2O3 electrodes with different electrodeposition times (5∼45 min) on the Fe3O4 NRAs (Fe3O4@Fe2O3-n; n = 5∼45 min) were plotted as a function of the scan rate (Figure S4, Supporting Information). The volumetric capacitance of the Fe3O4@Fe2O3 electrode increases as the electrodeposition time increases to 30 min, indicating the conductive Fe3O4 NRAs is effective to improve the capacitive performance of the hybrid electrode with high Fe2O3 mass loadings. As an effective method to boost the energy density of SCs, the enlargement of SCs voltage window can be achieved via the construction of ASCs. As inspired by the rational electrode design of the Fe3O4@Fe2O3 core-shell NRAs, the Fe3O4 NRAs were employed for the cathode design and a porous MnO2 layer was coated on the Fe3O4 NRAs to fabricate the Fe3O4@MnO2 core-shell NRAs with match-able capacitive performance in the positive potential window of 0∼1 V vs. Ag/AgCl (Figure S5-S7, Supporting Information). Thus, we fabricated an ASC device based on the Fe3O4@Fe2O3 anode and the Fe3O4@MnO2 cathode with voltage window of 2 V (Figure S8a, Supporting Information) (1 M Na2SO4 was used as the electrolyte). Figure 5a shows the CV curves of the Fe3O4@Fe2O3//Fe3O4@MnO2 ASC device collected at varied scan rates


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(5∼800 mV/s) in a voltage range between 0 and 2 V. The rectangular shape of CV curve is still well retained even at a high scan rate of 800 mV/s, presenting the good capacitive behavior and fast charge/discharge capability of the as-assembled ASC device in the voltage window of 0∼2 V. The symmetric and prolonged charge/discharge curves at varied current densities (Figure S8b, Supporting Information) confirm the good capacitive behavior of ASC device. The volumetric capacitance

calculated

for

the

Fe3O4@Fe2O3//Fe3O4@MnO2 device

based

on

the

charge/discharge curves as a function of current densities is shown in Figure 5b. The Fe3O4@Fe2O3//Fe3O4@MnO2 device delivered a volumetric capacitance of 1.49 F/cm3 based on the whole volume of the device at 1.25 mA/cm2, which is considerably higher or comparable than the recently reported values for ASC devices.6,50–53 Significantly, an excellent capacitance retention of 58% of the initial capacitance was achieved as the current densities increased from 1.25 mA/cm2 to 40 mA/cm2. Despite the good rate capability, the Fe3O4@Fe2O3//Fe3O4@MnO2 ASC device also exhibited excellent cycling stability, retaining 92% of the initial capacitance in the 2 V voltage windows at 5 mA/cm2 after 5000 cycles (Figure 5c). Energy density and average power density are two important parameters for the evaluation of the as-assembled ASC device. Noticeably, the Fe3O4@Fe2O3//Fe3O4@MnO2 device delivered the highest energy density of 0.83 mWh/cm3 (26.6 Wh/kg) at a power density of 15.6 mW/cm3 (500 W/kg) (Figure 5d). Even at a high power density of 500 mW/cm3 (16 kW/kg), the Fe3O4@Fe2O3//Fe3O4@MnO2 device still can achieve an energy density of about 0.45 mWh/cm3 (14.5 Wh/kg), which is superior to the previously reported supercapacitor systems, including graphene-based SCs (0.06 mWh/cm3, 135 mW/cm3),50 TiN-based SCs (0.05 mWh/cm3, 120 mW/cm3),6 TiO2@C-based SCs (0.01 mWh/cm3, 19 mW/cm3),51 ZnO@MnO2//rGO-based SCs (0.234 mWh/cm3, 5 mW/cm3),52 and even comparable to the values of α-Fe2O3@PANI//PANI-


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based SCs (0.35 mWh/cm3, 120.51 mW/cm3),25 MnO2//Fe2O3-based SCs (0.4 mWh/cm3, 60 mW/cm3),36 VOx//VN-based SCs (0.6 mWh/cm3, 80 mW/cm3),28 TiO2@MnO2//TiO2@C-based SCs (0.3 mWh/cm3, 190 mW/cm3).53

CONCLUSIONS In summary, for the first time, we have successfully designed the Fe3O4@Fe2O3 core-shell NRAs directly on a Ti substrate for ASCs. In the rational hybrid electrode design, the Fe3O4 core affords fast electron transport while the porous Fe2O3 shell brings in large surface area for charge storage, synergizing to boost the supercapacitive performance. As a consequence, the Fe3O4@Fe2O3 core-shell NRAs electrode delivers a maximum volumetric capacitance of approximately 1206 F/cm3, which is much larger than those of the Fe3O4 electrode and Fe2O3 electrode. In addition, the Fe3O4@Fe2O3 core-shell NRAs electrode also achieves excellent rate capability due to the synergetic effect of the highly conductive Fe3O4 and porous Fe2O3. A 2 V ASC device was further fabricated using Fe3O4@Fe2O3 as anode and Fe3O4@MnO2 as cathode in 1 M Na2SO4 electrolyte. An energy density of 0.83 mWh/cm3 (26.6 Wh/kg) at a power density of 15.6 mW/cm3 (500 W/kg) and an energy density of 0.45 mWh/cm3 (14.5 Wh/kg) at a power density of 500 mW/cm3 (16 kW/kg) can be achieved by the Fe3O4@Fe2O3//Fe3O4@MnO2 device, again confirming the outstanding capacitive behavior of the Fe3O4@Fe2O3 anode. Moreover, the Fe3O4@Fe2O3//Fe3O4@MnO2 ASC device also exhibits superior long-term cycling stability, with more than 92% capacitance retention after 5000 cycles. This work constitutes the first demonstration of Fe3O4 as the conductive supports for Fe2O3 to address the concerns about its poorly electronic and ionic transport.


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ASSOCIATED CONTENT Supporting Information: The cross-section FESEM images of the Fe3O4@Fe2O3 core-shell NRAs, the CV curves and charge/discharge curves of the Fe3O4@Fe2O3 core-shell NRAs, the FESEM, TEM, XRD, Raman, and EDS results for the Fe3O4@MnO2 core-shell NRAs, the CV and charge/discharge curves for the Fe3O4@MnO2 core-shell NRAs and the Fe3O4@Fe2O3//Fe3O4@MnO2 ASC device. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding authors: *E-mail: [email protected]; [email protected]. Tel: (86) 25 84303408 , Fax: (86) 25 84303408

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (No. 51572129, U1407106), Natural Science Foundation of Jiangsu Province (No. BK20131349), QingLan Project of Jiangsu Province, A Project Funded by the Priority Academic Program Development


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of Jiangsu Higher Education Institutions (PAPD), the Fundamental Research Funds for the Central Universities (No. 30915011204).

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Figure Legends Scheme 1. Schematic illustration of the synthesis procedure for the hierarchical Fe3O4@Fe2O3 core-shell NRAs.

Figure 1. The FESEM images of the a) FeOOH, b) Fe3O4, and c) Fe3O4@Fe2O3 core-shell NRAs. d) XRD patterns of the Fe3O4 and Fe3O4@Fe2O3 NRAs.

Figure 2. a) Raman spectra of the Fe3O4@Fe2O3 NRAs. b) XPS Fe 2p and c) O 1s core level spectra of the Fe3O4@Fe2O3 NRAs.

Figure 3. a-b) TEM and c-d) HRTEM images of the Fe3O4@Fe2O3 core-shell NRs. The inset of a) shows the SAED pattern for the Fe3O4@Fe2O3 core-shell NR. The box and circle in b) correspond to the HRTEM images for the selected areas of c) and d), respectively.

Figure 4. a) CV curves collected for the Fe3O4, Fe2O3, and Fe3O4@Fe2O3 electrodes at a scan rate of 100 mV/s. b) Volumetric capacitance of the Fe3O4, Fe2O3, and Fe3O4@Fe2O3 electrodes as a function of scan rate ranging from 5∼200 mV/s. c) Nyquist plots for the Fe3O4, Fe2O3, and Fe3O4@Fe2O3 electrodes. The inset of c) is the magnified Nyquist plots in high frequency region. d) Cycling stability of the Fe3O4@Fe2O3 electrode collected at a current density of 1.25 mA/cm2 for 5000 charge/discharge cycles.

Figure 5. a) CV curves of the Fe3O4@Fe2O3//Fe3O4@MnO2 ASC device collected at varied scan rates in a voltage range between 0 and 2 V. b) Volumetric capacitance calculated for Fe3O4@Fe2O3//Fe3O4@MnO2 device based on the charge/discharge curves as a function of current densities. c) Cycle performance of as-assembled ASC device collected at 5 mA/cm2 for 5000 charge/discharge cycles. Inset shows the charge/discharge curve collected at a current


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density of 5 mA/cm2. d) Ragone plots of the Fe3O4@Fe2O3//Fe3O4@MnO2 ASC device and other ASCs reported in literature.6,25,28,36,50,51,52,53


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Scheme 1


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Table of Contents (TOC) graphic

Hierarchical Fe3O4@Fe2O3 Core-Shell Nanorod Arrays as High-performance Anode for Asymmetric Supercapacitors

Xiao Tang 1,2, Ruyue Jia 1,2, Teng Zhai 1,2∗, and Hui Xia1,2∗

1

School of Materials Science and Engineering, Nanjing University of Science and Technology,

Nanjing 210094, China 2

Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology,

Nanjing 210094, China Keyword： Fe2O3, Fe3O4, hierarchical, core-shell, anode, asymmetric supercapacitors.


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Hierarchical Fe3O4@Fe2O3 CoreâShell Nanorod Arrays as High

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