Hierarchical α-Fe2O3@MnO2 core-shell nanotubes as electrode materials for high-performance supercapacitors

Accepted Manuscript Title: Hierarchical ␣-Fe2 O3 @MnO2 core-shell nanotubes as electrode materials for high-performance supercapacitors Authors: Guangdi Nie, Xiaofeng Lu, Maoqiang Chi, Yun Zhu, Zezhou Yang, Na Song, Ce Wang PII: DOI: Reference:

S0013-4686(17)30301-8 http://dx.doi.org/doi:10.1016/j.electacta.2017.02.037 EA 28902

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

22-12-2016 20-1-2017 7-2-2017

Please cite this article as: Guangdi Nie, Xiaofeng Lu, Maoqiang Chi, Yun Zhu, Zezhou Yang, Na Song, Ce Wang, Hierarchical ␣-Fe2O3@MnO2 core-shell nanotubes as electrode materials for high-performance supercapacitors, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.02.037 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.

Graphical Abstract Hierarchical α-Fe2O3 nanotube@MnO2 nanosheet networks with tunable mass ratio between MnO2 and α-Fe2O3 are prepared via a simple two-step method for high-performance supercapacitor electrodes.

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Highlights  Hierarchical α-Fe2O3 nanotube@MnO2 nanosheet core-shell networks were prepared.  The mass ratio of MnO2 to α-Fe2O3 can be easily controlled.  The α-Fe2O3@MnO2 heterostructures exhibited improved supercapacitive property.

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Hierarchical α-Fe2O3@MnO2 core-shell nanotubes as electrode materials for high-performance supercapacitors Guangdi Nie, Xiaofeng Lu,* Maoqiang Chi, Yun Zhu, Zezhou Yang, Na Song, Ce Wang* Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun, 130012, P. R. China. *

Corresponding author:

Fax & Tel.: +86-431-85168292; E-mail: [email protected]; [email protected]

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Abstract The incompetency of conventional single-phase electrode materials remains a stumbling

block

for

making

further

breakthroughs

in

high-performance

supercapacitors. In this work, α-Fe2O3 nanotube@MnO2 nanosheet hierarchical networks with tunable mass ratio of MnO2 to α-Fe2O3 are prepared via a simple two-step method for supercapacitor electrodes. The α-Fe2O3@MnO2 core-shell heterostructures, especially the FM10020 containing 60.1 wt% of MnO2, exhibit a larger specific capacitance of 289.9 F g-1 at 1.0 A g-1, a better rate capability of 40.8% at 5.0 A g-1 and a higher cycling stability of 85.3% after 1200 cycles than the pure MnO2, highlighting the advantages of such unique configuration accompanied by the synergistic effect. It is believed that these intriguing results will provide an alternative way for the construction of metal oxide-based composite nanostructures with improved electrochemical performance.

Keywords: α-Fe2O3 nanotubes; MnO2 nanosheets; core-shell heterostructures; electrospinning; supercapacitors

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1. Introduction The growing concerns for fossil fuel consumption and environmental pollution scream for the development of clean, efficient and alternative power sources [1]. Supercapacitors (SCs), as an indispensable energy storage device, can bridge the gap between rechargeable batteries and traditional dielectric capacitors due to their high power/energy density, fast charge-discharge rate and long cycling lifetime [2-4]. As is well known, transition metal oxides are typical pseudocapacitive materials that store charges by reversible faradaic redox reactions occurring at the electrode surface [5,6]. Thereinto, manganese dioxide (MnO2) is considered as the promising candidate on account of its attractive theoretical specific capacitance of 1370 F g-1, wide potential window of approximately 0.9-1.0 V, relatively low cost, environmental compatibility and abundant natural resource [7,8]. However, for densely packed MnO2 electrodes, their poor intrinsic conductivity (10-5-10-6 S cm-1), low ionic diffusion constant (~10-13 cm2 V-1 s-1), electrochemical dissolution issue and lack of structural stability result in the unsatisfactory specific capacitance that is far below the expected value [9,10]. It is still an urgent issue to construct high-performance supercapacitors based on novel MnO2 nanostructures. The incompetency of common single-phase electrode materials remains the obstacle for making further breakthroughs in high-performance SCs [11]. To date, numerous attempts have been dedicated to rationally constructing bicomponent or multicomponent

nanostructures

with

tunable

functions,

among

which

one-dimensional (1D) core-shell heterostructures not only possess large active surface 5

area and good electronic/ionic conductivity, but also exhibit potential synergistic effects, leading to the improved electrochemical property, particularly the working stability because of their unique protection mechanism to the scaffold and hybrid components

[12-14].

Recently,

Co3O4@MnO2,

ZnO@MnO2,

CuO@MnO2,

SnO2@MnO2 and α-Fe2O3@MnO2 core-shell arrays were demonstrated to achieve both high capacitance and long-lasting durability by combining desired natures of individual constituents [15-19]. Unfortunately, these MnO2-based architectures might be electrically isolated stemming from the weak interaction and loose contact between them [20]. Thus, it is still an enormous challenge to design continuous core-shell building blocks with accelerated mass transport, ion penetration and electron transfer. Herein, we present a novel route to fabricate α-Fe2O3 nanotube@MnO2 nanosheet hierarchical networks, which have the following advantages as supercapacitor electrodes. First, the α-Fe2O3 core, as a robust mechanical support and electron delivery channel (10-4-10-5 S cm-1) [19], can significantly prevent the aggregation and volume expansion of MnO2 layer during the redox reactions. Second, in an ideal case, both the core and shell materials will participate in the energy storage process, contributing to the superior electrochemical performance of the composite nanostructures. Third, the α-Fe2O3 nanotubes are prepared via electrospinning technique and thermal treatment, which guarantees the high crystal quality, large aspect ratio and enhanced accessible surface area of the core in comparison with conventional hydrothermal method [21-24]. Finally, the thin MnO2 shell coating on the lapped α-Fe2O3 nanotubes provides a short ion diffusion pathway and effective 6

electrical contact between the continuous networks. In addition, such a unique architecture can to some extent reduce the “dead volume” in electrodes [25]. Therefore, the proposed α-Fe2O3@MnO2 core-shell heterostructures are found to show higher specific capacitance, better rate capability and longer cycling lifespan than pure MnO2. 2. Experimental 2.1 Materials and methods All the chemical reagents were of analytical grade and used as received without any further purification. Ultrapure water was involved in the preparation of aqueous solutions. The α-Fe2O3 nanotubes were initially synthesized as described in our previous work [26]. In detail, 1.8 g of polyvinylpyrrolidone (PVP, Mw = 1300000, Sigma-Aldrich) and 1.8 g of Fe(NO3)3∙9H2O (Tianjin East China Reagent Factory) were completely dissolved in 20 mL of the mixed solvent containing N,N-dimethylformamide (DMF) and isopropyl alcohol (IPA) (Tianjin Tiantai Refine Chemicals Co., Ltd., volume ratio = 1:1) under vigorous stirring at room temperature. Then, the precursor solution was transferred to a single glass capillary for electrospinning using a high-voltage power supply (Gamma High Voltage Research, Inc., US) maintained at 18 kV with a collection distance of about 20 cm. The electrospun fibrous membranes were lastly calcined in a muffle furnace at 450 °C for 3 h in air to obtain the brick-red powder of α-Fe2O3 nanotubes. The heating rate was controlled at 2 °C min-1 before 250 °C, 1 °C min-1 between 250 and 300 °C, and 7

2.5 °C min-1 from 300 to 450 °C. The calcination process was held respectively at 100 and 300 °C for 1 h to remove the absorbed moisture and stabilize the continuous 1D morphology. For the preparation of α-Fe2O3@MnO2 core-shell nanostructures, the H+ ions are enriched in the vicinity of α-Fe2O3 walls, where MnO2 nanosheets tend to be deposited through the self-limiting decomposition of KMnO4 in the acidic medium [27]. In a typical procedure, 10 mg of the above α-Fe2O3 nanotubes were dispersed in 30 mL of 2 M H2SO4 (Beijing Chemical Works) aqueous solution by ultrasonic treatment, which was heated to 90 °C and subsequently added with 100 mg of KMnO4 (Sinopharm Chemical Reagent Co., Ltd.) under magnetic stirring for another 20 min. The brown product designated as FM10020 was separated by centrifugation, rinsed thoroughly with deionized water and dried in a vacuum freeze drier for further characterization and use. As a contrast, other α-Fe2O3@MnO2 composites and individual MnO2 were also fabricated by varying reaction time and precursor amount as illustrated in Table 1. 2.2 Characterization A field-emission scanning electron microscopy (FESEM, FEI Nova NanoSEM) and a transmission electron microscopy (TEM, JEOL JEM-2000EX) operated at 15 and 100 kV, respectively, were applied to observe the morphologies of samples. The high-resolution TEM (HRTEM) images, fast Fourier transform (FFT) version, selected area electron diffraction (SAED) pattern and energy dispersive X-ray (EDX) spectrum of FM10020 were acquired from a FEI Tecnai G2 F20 electron microscope 8

at an acceleration voltage of 200 kV, which was equipped with a high-angle annular dark-field (HAADF) detector in the scanning TEM (STEM) system. The crystallographic natures were confirmed by X-ray diffraction (XRD, PANalytical B.V. Empyrean) with Cu Kα radiation. Raman characterizations were conducted on a Horiba LabRAM HR Evolution apparatus using a 633 nm laser as the excitation source. The chemical composition of FM10020 was investigated by an X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB250). The contents of MnO2 in the nanocomposites were determined with an inductively coupled plasma emission spectrometer (ICP, Agilent 725). 2.3 Electrochemical measurements Electrochemical measurements were mainly implemented at room temperature on a CHI 660E electrochemical workstation (Shanghai Chenhua instrument Co., Ltd.) and a Land Battery workstation (Wuhan Land Instrument Company, China) using 3 M of KOH (Beijing Chemical Works) aqueous solution as electrolyte. A three-electrode cell configuration was adopted to examine the electrochemical performance of the obtained samples, which was assembled with a working electrode, a saturated calomel electrode (SCE, reference electrode) and a platinum foil counter electrode. The working electrode was fabricated as reported elsewhere [28]. First, 8 mg of FM10020 (active material) was blended with acetylene black (commercially available) and polytetrafluoroethylene (PTFE, Aladdin) at a weight ratio of 8:1:1 in a few drops of ethanol (Beijing Chemical Works). Next, this slurry was coated onto the nickel foam current collector (~ 1×1 cm2) and dried at 40 °C overnight. Cyclic voltammetry (CV) 9

experiments were performed in a potential range of -0.4-0.5 V at various scan rates from 3 to 100 mV s-1. As for galvanostatic charge-discharge (GCD) curves at different current densities of 1.0, 2.0, 3.0, 4.0 and 5.0 A g-1, a potential window of -0.4-0.4 V was selected unless otherwise stated. The electrochemical impedance spectroscopy (EIS) was carried out over the frequency range of 105-0.01 Hz with an amplitude of 5 mV. 3. Results and discussion The typical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of individual α-Fe2O3 and MnO2 nanostructures are displayed in Fig. 1. It is clearly observed that the electrospun α-Fe2O3 nanofibers with obvious tubular shape are randomly oriented (Fig. 1A and B). Most of the fiber diameters fall within the range of 90-340 nm. Interestingly, there are some α-Fe2O3 nanoparticles inside the hollow nanostructure, which is in agreement with Chen’s result due to the phase separation caused by the volatilization of the two solvents during the electrospinning process [29]. As shown in Fig. 1C and D, the sphere-like MnO2 assembled with the interconnected nanosheets are prepared in the absence of α-Fe2O3 nanotubes and suffers a significant self-aggregation, hindering the access of electrolyte ions into its interior. The hierarchical α-Fe2O3@MnO2 core-shell nanotubes with different contents of MnO2 have been constructed by tuning the reaction conditions (Table 1), which play an important role in the uniformity of the sample morphology. It is evident that the MnO2 layer becomes denser and thicker along with the increasing of either KMnO4 10

amount or growth time. For the obtained FM5020 (Fig. 2A and D), fewer MnO2 nanosheets are coated on the α-Fe2O3 nanotubes, leaving partial exposed surface, while with regard to FM10040 (Fig. 2C and F), some individual MnO2 nanospheres are attached to the composites with compact shells blocking both ends of the nanotubes. Only FM10020 (Fig. B and E) exhibits well-defined hollow heterostructures, which are intercrossed with one another, forming an intricate transportation network. It can be seen from the TEM image of FM10020 (Fig. 3A) that MnO2 nanosheet clusters are arranged homogeneously on the α-Fe2O3 nanotube substrate. The interplanar spacings shown in the high-resolution TEM (HRTEM) images (Fig. 3B and C) are measured to be 0.27 and 0.48 nm, corresponding to the values for d104 of hexagonal hematite and d101 of hexagonal birnessite, respectively. The inset in Fig. 3B is the relevant fast Fourier transform (FFT) version, indicating the high crystallinity of α-Fe2O3. The obscure rings observed in the selected area electron diffraction (SAED) pattern (Fig. 3D) can be assigned to the (006) and (119) planes of MnO2 with weak polycrystalline character. The energy dispersive X-ray (EDX) spectrum (Fig. 3E) has detected the signals of C, Mn, O, Fe, Cu, Si, and K elements, among which C, Cu, and Si atoms should be attributed to the carbon-coated copper grid and the instrument substrate, confirming the formation of the α-Fe2O3@MnO2 hybrid. Fig. 3F shows the scanning TEM (STEM) micrograph and EDX mapping analysis of FM10020. The K species distributed throughout the entire nanotube may originate from the surface adsorption and the inserted interlayer ions in birnessite-type MnO2 [30]. 11

X-ray diffraction (XRD) patterns are illustrated in Fig. 4A. By comparison, most of the sharp peaks for the resultant α-Fe2O3@MnO2 heterostructures can be readily indexed to hexagonal α-Fe2O3 (JCPDS card no. 33-0664). Besides, some visible peaks belonging to hexagonal MnO2 (JCPDS card no. 18-0802) are also probed without any other impurity phases. The Raman spectra are provided in Fig. 4B to validate the crystal phase and the microstructure of samples. According to group theory, there are seven Raman-active phonon modes (ΓRaman = 2A1g + 5Eg) for α-Fe2O3 [31]. The A1g vibrations appear at 226 and 497 cm-1, the Eg modes at 246, 293, 412, and 612 cm-1. It is noted that the broad band at 293 cm-1 can be fitted to double Eg modes at 287 and 298 cm-1. The unusual peak at 1318 cm-1 is identified as a two-magnon scattering, that is, a collective spin transition [32]. The forbidden peak at around 659 cm-1 is induced by the surface and grain boundary disorder or other defects [33,34]. For the α-Fe2O3@MnO2 core-shell nanotubes, two additional bands at 566 and 643 cm-1 are related to the Mn-O stretching vibration in the basal plane and symmetric stretching mode of the MnO6 group, revealing the lamellar structure of birnessite-type MnO2 [35]. Further information on the surface electronic state of FM10020 has been acquired from X-ray photoelectron spectroscopy (XPS). The wide-scan XPS spectrum (Fig. 5A) demonstrates the existence of Mn, Fe, and O elements in the sample. The signal of C species is mainly caused by the carbonaceous contaminant. As depicted in Fig. 5B, the dual binding peaks of Fe 2p3/2 and Fe 2p1/2 with splitting width of 13.6 eV are accompanied by two small satellite lines at about 718.9 and 732.9 eV, which are 12

associated with Fe3+ in α-Fe2O3 [36]. In the Mn 2p region (Fig. 5C), a single spin-orbit doublet of Mn 2p3/2 and Mn 2p1/2 are observed at 642.3 and 654.0 eV, respectively. The high-resolution O 1s spectrum (Fig. 5D) can be deconvoluted into three peaks of O-Fe/Mn-O-Mn (529.9 eV), Fe-O-H/Mn-O-H/O=C (531.4 eV) and H-O-H (surface-adsorbed water, 532.6 eV) [29,37]. Fig. 5E exhibits the K 2p core level, a superposition of K 2p3/2 and K 2p1/2 peaks with weak intensity due to the trace amount of K-containing species. Cyclic

voltammetry

(CV)

and

galvanostatic

charge-discharge

(GCD)

measurements were carried out using a standard three-electrode configuration in 3 M KOH aqueous electrolyte for a better understanding of the electrochemical properties of the sample-modified electrodes. It is apparent that the CV curves of both α-Fe2O3 and FM10020 electrodes show a pair of redox peaks, which is mainly derived from the reversible conversion reaction between Fe(II) and Fe(III), indicating their pseudocapacitive behaviors. The large increment of the CV integral area (Fig. 6A) furnishes incontrovertible evidence that the FM10020 electrode possesses a higher specific capacitance than the neat α-Fe2O3 and MnO2 electrodes at the same scan rate of 5 mV s-1. As expected, an analogous conclusion is drawn from the GCD profiles at a current density of 1.0 A g-1 (Fig. 6B). The capacitance value is determined on the basis of the following formula: C (𝐹 𝑔−1 ) =

I × ∆t m × ∆V

where, I (A) is the applied current, Δt (s) is the discharge time, m (g) is the total mass (~8 mg) of the active materials in a single electrode, and ΔV (V) is the potential 13

window excluding the initial potential drop. Impressively, compared with FM5020 (176.9 F g-1) and FM10040 electrodes (243.6 F g-1), a larger specific capacitance of 289.9 F g-1 is achieved for the as-prepared FM10020 electrode, which is more than twice that of MnO2 (133.5 F g-1) and five times that of α-Fe2O3 (57.3 F g-1) owing to the unique architecture of α-Fe2O3@MnO2 composite nanotubes. The obtained value is also higher than or comparable with those of the previously reported CuO@MnO2 arrays (298.7 F g-1 at 1.0 A g-1, 1 M Na2SO4) [17], MnO2@Fe2O3 nanospindles (159 F g-1 at 0.1 A g-1, 0.5 M K2SO4) [38], FeS2@Fe2O3 hybrids (255 F g-1 at 1.0 A g-1, 1 M Li2SO4) [39], CeO2/Fe2O3 nanospindles (142.6 F g-1 at 5 mV s-1, 6 M KOH) [40], Fe2O3@N-doped graphene (274 F g-1 at 1.0 A g-1, 2 M KOH) [41], MnO2/CNFs (228 F g-1 at 1.0 mA cm-2, 6 M KOH) [42], C-MnO2 (167 F g-1 at 0.05 A g-1, 3 M KOH) [43], and CNF/graphene/MnO2 (225 F g-1 at 1.0 mA cm-2, 6 M KOH) [44]. The enhanced specific capacitance of the α-Fe2O3@MnO2 core-shell nanostructures should be due to the thin porous MnO2 shells on the surface of α-Fe2O3 nanotubes, offering short ion diffusion and electron transport pathways during charge-discharge processes. Scan rate and current density are important factors to evaluate the rate capability of the supercapacitor electrodes. Taking the FM10020 electrode as an example, the shape of CV curves (Fig. 7A) remains constant even at a very high scan rate of 100 mV s-1, signifying its good rate capability. In addition, there is a minor shift in the peak position along with the increase of scan rate on account of the weak electrode polarization [45]. The current density-dependent GCD profiles are displayed in Fig. 7B with the plateaus matching well with the redox peaks in the CV curves. All 14

discharge branches are nearly symmetrical to their charge counterparts, testifying the ideal capacitive behavior of this electrode. It is calculated that the capacitance retention of FM10020 is 40.8% as the current density increases from 1.0 to 5.0 A g-1 (Fig. 7C), which is comparable with that of FM5020 (41.2%), but higher than those of FM10040 (37.4%) and MnO2 (28.5%), implying that the less content of MnO2, the better the rate capability. The electrochemical impedance spectra (EIS) are simulated using an equivalent circuit. As shown in Fig. 7D, the Nyquist plots of the MnO2-based electrodes are composed of a semicircle in the high-frequency area and a straight line at low frequency. It can be found that the tail segment of FM10020 is closer to the imaginary axis as that of α-Fe2O3, demonstrating that the diffusion resistances in such systems are smaller than those in FM5020, FM10040 and MnO2 electrodes. At high frequency, only the α-Fe2O3 electrode exhibits an invisible semicircle, indicating a very small charge-transfer resistance (Rct) [46]. By comparing the semicircle diameter of the MnO2-based electrodes, we infer that the obtained FM10020 possesses a much lower Rct value. It is believed that the faster ion diffusion and more effective electron transfer give rise to the improved specific capacitance of FM10020. Long-term cycling stability, another important aspect for SCs, is evaluated by GCD measurement at 1.0 A g-1 (Fig. 8). After 1200 cycles, approximately 85.3% of the initial capacitance value is retained for FM10020, much higher than that for MnO2 (62.6%), which should be further enhanced for practical applications. The decrease of specific capacitance during the cyclic process is partially due to the dissolution of MnO2 as a result of the mechanical stress introduced by the insertion and expulsion of ions [47]. 15

4. Conclusions In summary, a simple and novel approach is proposed for the preparation of the hierarchical α-Fe2O3@MnO2 core-shell nanotube networks. When utilized as supercapacitor electrodes, the α-Fe2O3@MnO2 heterostructures, especially the FM10020 with well-defined morphology, show higher specific capacitance, better rate capability and longer cycling lifespan than the pure MnO2. Such excellent electrochemical performance is attributed to the unique configuration and the synergistic effect that can combine the desired functions of individual components. These intriguing results will provide an alternative way for the construction of metal oxide hybrids with increased capacitive properties. Acknowledgements This work was financially supported by the research grants from the National Natural Science Foundation of China (21474043, 51473065, 51273075, and 21274052), and the Graduate Innovation Fund of Jilin University (2015011). References [1] X.H. Cao, C.L. Tan, X. Zhang, W. Zhao, H. Zhang, Solution-processed two-dimensional metal dichalcogenide-based nanomaterials for energy storage and conversion, Adv. Mater. 28 (2016) 6167. [2] L.L.

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Figure Captions Table 1. Experimental parameters for the synthesis of α-Fe2O3@MnO2 and MnO2 nanostructures. Fig. 1. (A, C) SEM and (B, D) TEM images of the as-prepared (A, B) α-Fe2O3 nanotubes and (C, D) MnO2 nanoflowers. Fig. 2. (A, B, C) SEM and (D, E, F) TEM images of α-Fe2O3@MnO2 core-shell heterostructures: (A, D) FM5020, (B, E) FM10020 and (C, F) FM10040. Fig. 3. Morphological and structural analyses for FM10020: TEM images at (A) low and (B, C) high magnifications. Inset: the relevant FFT version. (D) SAED pattern, (E) EDX spectrum and (F) STEM micrograph and EDX mappings of Fe, O, Mn and K elements. Fig. 4. (A) XRD patterns and (B) Raman spectra of samples: (a) α-Fe2O3, (b) FM5020, (c) FM10020, (d) FM10040, and (e) MnO2 nanostructures. Fig. 5. XPS spectra of FM10020: (A) full survey spectrum, (B) Fe 2p, (C) Mn 2p, (D) O 1s, and (E) K 2p regions. Fig. 6. (A) CV curves measured at a scan rate of 5 mV s-1 and (B) GCD profiles at a current density of 1.0 A g-1 for α-Fe2O3, α-Fe2O3@MnO2 and MnO2 electrodes. Fig. 7. (A) CV curves at multiple scan rates and (B) GCD profiles at diverse current densities of FM10020 electrode. (C) Specific capacitance and capacitance retention as functions of discharge current density. (D) Nyquist plots recorded at an open-circuit voltage. Insets: the magnified plots at high frequency and the equivalent circuit diagram. 24

Fig. 8. (A) Cycling stabilities of α-Fe2O3, FM10020 and MnO2 electrodes. (B) The first and last 10 cycles of the GCD curves at 1.0 A g-1 for the FM10020 electrode.

Table 1. Sample Name α-Fe2O3 (mg) KMnO4 (mg) Time (min) MnO2 content (wt%) FM5020

10

50

20

46.2

FM10020

10

100

20

60.1

FM10040

10

100

40

65.2

MnO2

0

100

40

―

Fig. 1.

25

Fig. 2.

Fig. 3.

26

Fig. 4.

27

Fig. 5.

28

Fig. 6.

Fig. 7.

29

Fig. 8.

30