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In situ hydrothermal preparation of mesoporous Fe3O4 film for high-performance negative electrodes of supercapacitors
Accepted Manuscript In situ hydrothermal preparation of mesoporous Fe3O4 film for high-performance negative electrodes of supercapacitors Ke Jiang, Baolong Sun, Mengqi Yao, Ni Wang, Wencheng Hu, Sridhar Komarneni PII:
S1387-1811(18)30080-5
DOI:
10.1016/j.micromeso.2018.02.015
Reference:
MICMAT 8778
To appear in:
Microporous and Mesoporous Materials
Received Date: 17 December 2017 Revised Date:
17 January 2018
Accepted Date: 13 February 2018
Please cite this article as: K. Jiang, B. Sun, M. Yao, N. Wang, W. Hu, S. Komarneni, In situ hydrothermal preparation of mesoporous Fe3O4 film for high-performance negative electrodes of supercapacitors, Microporous and Mesoporous Materials (2018), doi: 10.1016/j.micromeso.2018.02.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT In situ hydrothermal preparation of mesoporous Fe3O4 film for high-performance negative electrodes of supercapacitors Ke Jianga, Baolong Suna, Mengqi Yaoa, Ni Wang*a,b, Wencheng Hua, Sridhar Komarnenib Center for Applied Chemistry, University of Electronic Science and Technology of China,
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a
Chengdu, 610054, China b
Materials Research Institute and Department of Ecosystem Science and Management,
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Materials Research Laboratory, The Pennsylvania State University, University Park,
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Pennsylvania, 16802, USA.
*Corresponding Authors: N. Wang ([email protected]) Sridhar Komarneni, [email protected]
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Abstract
A mesoporous Fe3O4 film was prepared as binder-free electrode material for supercapacitors through a facile process that included the hydrothermal electroplating of an
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Fe/Zn alloy, in situ electrolytic dealloying to remove the Zn template, and oxidation in a water
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vapor environment. The Fe3O4 film showed a cubic structure and mesoporosity with a specific surface area of 247 m2 g−1. As a negative electrode material, the mesoporous Fe3O4 film delivered a high gravimetric capacity of 221 C g−1 at 1 A g−1, and the gravimetric capacity was maintained at 154 C g−1 even at a high current density of 50 A g−1. In addition, the mesoporous Fe3O4 electrode exhibited very high cycling stability (only 4.7% capacity loss after 10,000 galvanostatic charge–discharge cycles). Electrochemical impedance spectroscopy revealed that the mesoporous Fe3O4 film had excellent conductivity, implying its promising 1
ACCEPTED MANUSCRIPT application as a supercapacitor electrode. Keywords: mesoporous Fe3O4 film; supercapacitor; hydrothermal electroplating; high gravimetric capacity; cycling stability
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1. Introduction At present, considerable research attention is being paid for the sustainable development of various energy storage devices. Among these, supercapacitors are notable energy storage
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devices that can be charged or discharged quickly with very high-power density [1]. In the
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supercapacitors, energy is stored on the surface of the electrode material, which can maintain the long-term stability of its structure, leading to an ultra-long energy storage capability that is superior to that of lithium-ion batteries [1,2].
Studies on supercapacitors have received global attention. Depending on the energy storage
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mechanisms, electrode materials for supercapacitors are composed of three materials [3]: carbon-based materials (activated carbon, graphene, and carbon nanotubes [CNT]), pseudocapacitive materials (RuO2 and MnO2), and battery-type materials (NiO, CoO, and
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conducting polymer). Carbon-based materials with superior conductivity have been widely
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investigated as positive electrodes in acidic electrolytes [4], negative electrodes in alkaline electrolytes [5], and both positive and negative electrodes in neutral electrolytes [6]. As electric double-layer capacitors, carbon-based materials display an energy storage mechanism that mainly relies on the surface area to electrostatically accumulate charges at the material/electrolyte interface [7], leading to a relatively low capacitance compared with metal oxides. Low-cost metal oxides with high theoretical capacitance or capacity, such as manganese, nickel, and cobalt oxide, have been investigated as electrode materials for 2
ACCEPTED MANUSCRIPT supercapacitors [8,9]. These three oxides are positive electrode materials in alkaline and neutral electrolytes. For matched supercapacitors, manganese, nickel, and cobalt oxide electrodes must match with the relatively low capacitances of carbon-based electrodes (as
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negative electrodes) in order to assemble asymmetric or hybrid supercapacitors [10,11]. The development of low-cost negative electrode materials with remarkable capacitance or capacity has become more urgent than ever.
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Many materials have been utilized as negative electrodes in aqueous electrolytes [12]. One
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such material is iron oxide, which is inexpensive and environmentally friendly, making it an attractive alternative as a negative electrode material for supercapacitors [13]. Different methods have been used to prepare Fe2O3 as a negative electrode for supercapacitors [14–16]. Nonetheless, studies have demonstrated that key disadvantages, such as poor electrical
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conductivity, low “available” surface area, and binder addition, limited the iron oxide’s Faraday reaction for obtaining high specific capacity [17]. Therefore, iron oxides must be composited with graphene [18,19] and CNT [20] to improve their specific capacities.
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Fe3O4 is a mixed valence compound containing +2 and +3 iron ions. The Verwey transition
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effect facilitates electron conduction between +2 and +3 iron ions, leading to considerably higher conductivity than Fe2O3 [21]. In the present work, we adopted a novel strategy for hydrothermally electrodepositing porous Fe films, which were in-situ oxidized to synthesize binder-free negative electrodes for a supercapacitor. The porous Fe3O4 film possessed a high specific surface area of 247 m2 g−1 with a mesoporous structure. It delivered a high specific capacity of 221 C g−1 at 1 A g−1, indicating its potential application as a negative electrode in supercapacitors. 3
ACCEPTED MANUSCRIPT 2. Experimental procedure 2.1. Synthesis of Fe films The porous Fe films were directly prepared on commercial pure copper plates (1 cm × 4 cm)
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with a thickness of 50 µm. The copper plates were rinsed in 20% HNO3 and deionized water and then placed as a cathode electrode of the modified hydrothermal system for metal electroplating as shown in Fig.1a, where the anode constituted of iron and zinc plates with an
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area ratio of 1:1, Ag-coated copper wires were used as the conducting wires. The Fe/Zn
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plating solution, which contained 300 g L−1 ZnCl2, 300 g L−1 FeCl2, 4 g L−1 MnCl2, 3 g L−1 saccharin, 20 g L−1 borax, and 0.1 g L−1 SDS, was poured into a PTFE vessel with a filling volume of 80%. The plating solution was poured into the Teflon vessel until half of the parallel-positioned copper plate and iron/zinc plate were immersed in solution (Fig. 1a) in the
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PTFE vessel followed by its assembly into the autoclave. The hydrothermal system in this work was an inverted assembly hydrothermal reactor, as illustrated in Fig.1. Fig. 1b presents the assembled system for electrodeposition. The connectors were made from pure copper
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coated with an Au/Ni layer, and a high temperature-enameled wire (Ag-coated copper wire)
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was used as the conducting wire. Fig. 1c shows a stainless steel supporting plate on the PTFE vessel to assist the PTFE vessel at high pressure. Fig. 1d displays a three-electrode system for the electrodeposition, and Fig. 1e depicts a two-electrode system for the electrochemical measurement of the plating bath. The hydrothermal system was placed in a high-temperature oven, and the temperature was increased to 120 °C. Hydrothermal electroplating of the Fe/Zn alloy was conducted at a current density of 300 mA cm−2 for 30 min. Subsequently, the cathode and anode were reversed from each other, and the dealloying process was operated at 4
ACCEPTED MANUSCRIPT an electrolytic voltage of 1 V to remove the Zn template. The rinsed samples were oxidized in a tube furnace at 280 °C under an atmospheric and water vapor environment for 3 h, thereby converting the zero-valent iron into Fe3O4.
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2.2. Materials characterization A D-Max-γ type A X-ray diffractometer (XRD, Rigaku Co., Japan) was employed to characterize the crystallinity of the porous Fe3O4 films. The surface morphologies were
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observed using an Inspect F field-emission scanning electron microscope (FESEM, FEI Co.,
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US) and Libra 200FE high-resolution transmission electron microscope (HRTEM). N2 adsorption/desorption isotherms were analyzed using a JW–BK112 Surface Analyzer (Beijing JWGB Co., China). BET equation and BJH approach were used to evaluate the specific surface area and pore size distributions, respectively.
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2.3 Electrochemical measurements
Standard three-electrode measurements were used to investigate the electrochemical performance of porous Fe3O4 films, the latter were defined as working electrodes without
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binders. The porous Fe3O4 films and copper plates acted as the electro-active material and
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current collector, respectively. An Hg/HgO electrode was assigned as the reference electrode, and a Pt wire was set as the counter electrode. A CHI660D electrochemical workstation was used to measure cyclic voltammetry (CV), chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS) in 2 mol L−1 KOH solution. A long galvanostatic charge– discharge (GCD) cycle experiment was conducted in a battery testing system (Lanhe, China). 3. Results and discussion 3.1 Structural characteristics 5
ACCEPTED MANUSCRIPT Fig. 2 presents a typical glancing-angle XRD pattern of a porous Fe3O4 film. The sample showed six clear peaks located at 30.26°, 35.57°, 43.26°, 53.75°, 57. 26°, 62.91°and 73.95°, which correspond with the (220), (311), (400), (422), (511), (440) and (533) reflections of
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Fe3O4 (JCPDS card no. 19-0629) [22]. The sample displayed a polycrystalline structure of Fe3O4 and no secondary phases could be observed (Fig. 2). The sharp peaks with strong intensities indicated the cubic Fe3O4 phase of high crystallinity. The particle size of cubic
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Fe3O4 was calculated to be 46.6 nm based on the Scherrer equation [23].
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The FESEM images of the samples at different steps illustrate the formation of porous Fe3O4 films. In Fig. 3a, the electroplated Fe/Zn film exhibited a dense, crack-free, and relatively smooth surface because of accurate current control under the hydrothermal condition. After the sample was set as an anode to electrolyze for dealloying, the gradual
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dissolution of the zinc atoms caused the film to be uneven (Fig. 3b). As shown in Fig. 3b, after dealloying for 30 min, the surface initially presented a porous structure. As the electrolysis process was continued for 60 min, the porous structure became clearer as the
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porous particles were connected together, forming a complete porous Fe film (Fig. 3c). When
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the iron element was oxidized to form Fe3O4, the porous particles became closer to one another, and the particle size increased (Fig. 3d). Apparently as a result of the introduction of oxygen causing a volume increase [24]. The porous Fe3O4 film peeled from the electrode was tested by HRTEM, and the TEM
images are shown in Fig. 4. The connected structure was clearly observed and the particle size was in the range of 190–400 nm, which matched the FESEM observations. At higher magnification by high resolution TEM (Fig. 4b), it can be seen that the particles were 6
ACCEPTED MANUSCRIPT composed of many fine grains and some of them show highly ordered continuous fringes, which can be indexed to the (311) lattice planes of the cubic structure of Fe3O4. The porous structure was also observed as lighter areas between the fine grains.
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The porous Fe3O4 film coated on the copper plate was directly used in the N2 adsorption– desorption experiments, and the typical isotherms are shown in Fig. 5a. Type IV isotherm shape with H2 hysteresis loop indicated the mesoporous nature of the film [25]. The pore size
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distribution of the sample illustrated in Fig. 5b revealed that the porous Fe3O4 film possessed
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a broad pore-size distribution ranging from 2 to 11 nm but centered at 2.5 nm, suggesting that it was a mesoporous material. The abundant mesopores are expected to be beneficial to the diffusion of the solution and the transmission of electrolyte ions [26]. BET calculation revealed that the specific surface area of the copper plate with film was 22.1 m2 g−1. However,
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the porous Fe3O4 film by itself was calculated and it showed a high specific surface area of 247 m2 g−1, which is superior to previously reported results for porous iron oxides [27–29]. 3.2 Electrochemical performance
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We first conducted the CV experiments in the potential window from −1.1 V to 0 V to
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estimate the electrochemical properties of the porous Fe3O4 film and the obtained CV curves at different scanning rates are shown in Fig. 6. A pair of peaks existed in all curves, suggesting that a redox reaction occurred between the +2 and +3 Fe ions. Broussea [30] suggested that mesoporous Fe3O4 is a typical battery–electrode; a non–linear relation exists between the current density and potential window in the CV curve, in contrast to the case in MnO2 and RuO2 electrodes. As a typical battery–electrode of a supercapacitor, the ability to store charges should be redefined as capacity (C or mA h) and considered a substitute for 7
ACCEPTED MANUSCRIPT capacitance (F). In addition, Fig. 6 shows that copper plate did not offer an additional capacity in the CV test. To accurately evaluate the electrochemical properties for the practical application of an
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electrode, GCD measurements were performed to investigate the specific capacity at different current densities. The GCD curves of mesoporous Fe3O4 are shown in Fig. 7a, and the corresponding specific capacities (Cs, C g-1) based on the calculation of the galvanostatic
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discharge curves (Cs=Im×td, where Im is the current density, and td is the discharge time [31])
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are summarized in Fig. 7b. The GCD curves revealed the battery–electrode behavior of the film, as evidenced by the non-linear dependence of the potential versus the charge/discharge time. Moreover, charge and discharge plateaus were observed at −1 and −0.8 V, respectively, which agreed with the potential values of the oxidation and reduction reactions in the CV
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curves. The Cs value of the mesoporous Fe3O4 film was calculated to be 221 C g−1 at 1 A g−1, which is higher than those of α–Fe2O3 nanotube arrays [32], synthesized iron oxide nanoparticles [33], and α–Fe2O3 hollow nanoshuttles [34]. This Cs value is comparable with
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that of conductive-enhanced iron oxides using carbon materials, such as graphene/iron oxide
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electrodes [20] and 3D α-Fe2O3/carbon nanotube sponges. In particular, as the GCD current density increased to 50 A g−1, the capacity was 154 C g−1, retaining up to 69.77% of the initial value and suggesting a good rate capability [35]. The excellent electrochemical performance of our sample can be attributed to three factors: high specific surface area with mesoporous structure, absence of a binder, and mixed-valence state of iron oxides. The long cycle life experiment was another crucial index for evaluating the electrochemical stability of electrode materials. The repeating GCD cycle operated in the potential range of 8
ACCEPTED MANUSCRIPT −1.1 V to 0 V at 5 A g−1 was conducted in a battery testing system, and the result for an interval of 200 cycles is depicted in Fig. 8. During the initial 1000 cycles, the capacity gradually increased with increasing cycle number because of the gradual activation of the
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electrode materials and further wetting of the surface [36]. The binder-free mesoporous Fe3O4 film retained 95.3% of the initial capacity after 10,000 GCD cycles, thereby demonstrating its satisfactory cyclic stability. Thus, the mesoporous Fe3O4 film with a stable cubic structure
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could guarantee excellent stability.
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EIS measurement was employed to investigate the fundamental behavior of the mesoporous Fe3O4 electrode in the frequency range of 100 kHz–0.01 Hz before and after long-term GCD cycles, and the Nyquist plots are shown in Fig.9. The straight lines in the low-frequency region are greater than 45°, implying that the mesoporous Fe3O4 electrode
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displayed good capacitive behavior with excellent electron transfer and diffusion property [37]. The bulk solution resistance (Rs) and the charge–transfer resistance (Rct) intercepted from the Z-axis were 0.39 and 1.83 Ω, respectively. The low Rs value was due to the unique
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porous structure for wetting between the material surface and electrolyte solution and the high
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and effective surface for the Faradaic reaction [38]. In addition, the binder-free electrode structure and electron transferability between Fe2+ and Fe3+ ions existing in Fe3O4 also contributed to the low Rct value [39]. After 10,000 GCD cycles, these values slightly increased to 0.56 and 1.92 Ω, and the surface morphology of the mesoporous Fe3O4 film observed from the SEM image (Fig.9b) only showed a small change compared to the sample before the long GCD measurement, which explained the 4.7% loss of the initial capacity after the long-term life test. 9
ACCEPTED MANUSCRIPT 4. Conclusion In summary, we suggested a novel hydrothermal electroplating approach for depositing Fe/Zn alloys on copper plates. After the Zn atoms were removed from the Fe/Zn alloy film,
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the sample was oxidized in a tube furnace under a water vapor environment to prepare the mesoporous Fe3O4 film. The high specific surface area with reasonable pore structure led to the rapid diffusion of electrolyte ions, and the binder-free electrode along with the
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mixed-valence state of iron oxide facilitated electron conduction. The mesoporous Fe3O4 film
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presented a high specific capacity of 221 at 1 A g−1 and 154 C g−1 at 50 A g−1, indicating excellent rate capability. The long-term cycle life of up to 10,000 GCD cycles further demonstrated that the mesoporous Fe3O4 may have valuable practical application for
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supercapacitors.
[1] Simon, Patrice, Y. Gogotsi, and B. Dunn, Science 343.6176 (2014) 1210-1211.
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[2] T. Zhai, L. Wan, S. Sun, Q. Chen, J. Sun, Q. Xia, H. Xia, Adv. Mater. 29 (2017) 1604167. [3] N. Wang, M. Yao, P. Zhao, W. Hu, S. Komarneni, J. Mater. Chem. A 5 (2017) 5838–5845
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[4] P. Díaz, Z. González, R. Santamaría, M. Granda, R. Menéndez, C. Blanco, Electrochim. Acta 212 (2016) 848–855 [5] X. Chen, K. Chen, H. Wang, D. Xue, Electrochim. Acta 147 (2017) 216-224 [6] L. Sheng, L. Jiang, T. Wei, Z. Liu, Z. Fan, Adv. Eng. Mater. 7 (2017) 1700668 [7] K. Chen, F. Liu, X. Liang, D. Xue, Surf. Rev. Lett. 24 (2017) 1730005 [8] L. Hu, Y. Deng, K. Liang, X. Liu, W. Hu, J. Solid State Electr. 19 (2015) 629–637 [9] R. Jia, F. Zhu, S. Sun, T. Zhai, H. Xia, J. Power Sources 341 (2017) 427-434 [10] S.C. Lin, Y.T. Lu, Y.A. Chen, J.A. Wang, T.H. You, Y.S. Wang, C.W. Lin, Ma, Cn. C. M. Ma, C. C. Hu, J. Power Sources 362 (2017) 258–269 10
ACCEPTED MANUSCRIPT [11] J.L. Yin, J.Y. Park JY, J. Solid State Electr. 19 (2015) 2391–2398 [12] Q. Qu, S. Yang, X. Feng, Adv. Mater. 23 (2011) 5574–5580 [13] K.A. Owusu, L. Qu, J. Li, Z. Wang, K. Zhao, C. Yang, K.M. Hercule, C. Lin, C. Shi, Q. Wei, L. Zhou, L. Mai, Nat. Commun. 8 (2017) 14264
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[14] P. Yang, Y. Ding, Z. Lin, Z. Chen, Y. Li, P. Qiang, M. Ebrahimi, W. Mai, C.P. Wong, Z.L. Wang, Nano Lett. 14 (2014) 731–736
[15] Y. Li, J. Xu, T. Feng, Q. Yao, J. Xie, H. Xia, Adv. Funct. Mater. 27 (2017) 1606728
[16] L.L. Tian, M. J. Zhang, C. Wu, Y. Wei, J.X. Zheng, L.P Lin, J. Lu, K. Amine, Q.C.
SC
Zhuang, F. Pan, ACS Appl. Mater. Interfaces 7 (2015) 26284–26290
[17] H.D. Liu, J.L. Zhang, D.D. Xu, L.H Huang, S.Z. Tan, W.J. Mai, J. Solid State Electr. 19
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(2015) 135–144
[18] Z. Song, W. Liu, W. Wei, C. Quan, N. Sun, Q. Zhou, G. Liu, X. Wen, J. Alloy Compd. 685 (2016) 355–363
[19] J. Min, K, Kierzek, X, Chen, P.K. Chu, X. Zhao, R.J. Kaleńczuk, T. Tang, E. Mijowska, New J. Chem. (2017), https:// DOI: 10.1039/C7NJ03416D
20927–20934
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[20] X. Cheng, X. Gui, Z. Lin, Y. Zheng, M. Liu, R. Zhan, Y. Zhu, J. Mater. Chem. A 3 (2015)
[21] X. Jiang, B. Xiao, H. Yang, X. Gu, H. Sheng, X. Zhang, Appl. Phys. Lett. 109 (2016)
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203110
[22] S.C. Hu, F. Shi, J.X. Liu, L. Yu, S.H. Liu, J. Porous. Mat. 23 (2016) 655–661
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[23] K.M. Butt, M.A. Farrukh, I. Muneer, J. Mater. Sci. Mater El. 27 (2016) 8493–8498 [24] E. Hourdakis, A.G. Nassiopoulou, Thin. Solid Films 621 (2017) 36–41 [25] N. Wang, P. Zhao, K. Liang, M. Yao, Y. Yang, W. Hu, Chem. Eng. J 307 (2017) 105–112 [26] N. Wang, P. Zhao, K. Liang, M. Yao, Y. Yang, W. Hu, J. Solid State Electr. 20 (2016) 1429–1434
[27] G. Binitha, M.S. Soumya, A.A. Madhavan, P. Praveen, A. Balakrishnan, K.R.V. Subramanian, M.V. Reddy, S.V. Nair, A.S. Nair, N. Sivakumar, J. Mater. Chem. A 1 (2013) 11698–11704 [28] S. Shivakumara, T.R. Penki, N. Munichandraiah, J. Solid State Electr. 18 (2014) 1057– 1066 11
ACCEPTED MANUSCRIPT [29] Y. Zeng, M. Yu, Y. Meng, P. Fang, X. Lu, Y. Tong, Adv. Eng. Mater. 6 (2016) 1601053 [30] T. Broussea, D. Bélanger, J.W. Long, J. Electr. Soc. 162 (2015) A5185–A5189 [31] N. Wang, P. Zhao, Q. Zhang, M. Yao, W. Hu, Compos. Part B.Eng. 113 (2017) 144–151 [32] K. Xie, J. Li, Y. Lai, W. Lu, Z. Zhang, Y. Liu, L. Zhou, H. Huang, Electr. Commun 13
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(2011) 657–660 [33] E. Mitchell, F.D. Souza, R.K. Gupta, P.K. Kahol, D. Kumar, L. Dong, B.K. Gupta, Powder Technol. 272 (2015) 295-299
[34] X. Zheng, X. Yan, Y. Sun, Y. Yu, G. Zhang, Y. Shen, Q. Liang, Q. Liao, Y. Zhang, J.
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Colloid Interf. Sci. 466 (2016) 291–296
[35] X. Zhao, N. Wang, Y. Tan, Y. Liu, L. Kong, L. Kang, F. Ran, J. Alloy Compd. 710 (2017)
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72–79
[36] H. Wang, C.M.B. Holt, Z. Li, X. Tan, B.S. Amirkhiz, Z. Xu, B.C. Olsen, T. Stephenson, D. Mitlin, Nano Res. 5 (2012) 605–617
[37] T. Zhao, W. Yang, X. Ji, W. Jin, J. Hu, T. Li, Appl. Surf. Sci. (2017) https://doi.org/10.1016/j.apsusc.2017.09.046
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[38] J.M. Luo, G. Bo, X.G. Zhang, Mater Res. Bull. 43 (2008) 1119–1125
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[39] Z. Li, F. Wang, X. Wang, Small 13 (2017) 1603076
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Figure captions
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Fig.1. Modified hydrothermal system for metal electroplating: (a) schematic of the set-up, (b) inverted hydrothermal reactor, (c) Modified stainless steel supporting plate for 2 electrodes on the PTFE vessel (d) Teflon cover with 3 electrodes and (e) Teflon cover with 2 electrodes Fig.2. XRD pattern of typical porous Fe3O4 film and reference standard data for Fe3O4 of the JCPDS NO. 19-0629.
Fig.3. SEM images of (a) Fe-Zn film, (b) Fe-Zn film corroded electrochemically for 30min, (c)
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Fe-Zn film corroded electrochemically for 60min, (d) porous Fe3O4 film after oxidation of Fe.
Fig.4. HRTEM image of typical porous Fe3O4 film.
film.
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Fig.5. (a) N2 adsorption–desorption isotherms and (b) pore size distributions of porous Fe3O4
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Fig.6. CV curves of porous Fe3O4 film measured in 2M KOH electrolyte.
Fig.7. (a) GCD curves at different current densities and (b) plot of specific capacity as a function of current density of porous Fe3O4 film. Fig.8. Cycling stability of porous Fe3O4 electrode at a current density of 10A g−1.
Fig. 9. Nyquist plots obtained from the porous Fe3O4 electrode before and after the long cycle measurements, and SEM image after 10000 GCD cycles.
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ACCEPTED MANUSCRIPT Highlights • Novel hydrothermal electroplating was developed to prepare porous Fe3O4 film • The Fe3O4 film with a mesoporous structure showed a high specific surface area
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• The porous film delivered a high specific capacity of 221 C g−1 at 1 A g−1
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• The sample was very stable and only lost 4.7% capacity after 10,000 GCD cycles
S1387-1811(18)30080-5
DOI:
10.1016/j.micromeso.2018.02.015
Reference:
MICMAT 8778
To appear in:
Microporous and Mesoporous Materials
Received Date: 17 December 2017 Revised Date:
17 January 2018
Accepted Date: 13 February 2018
Please cite this article as: K. Jiang, B. Sun, M. Yao, N. Wang, W. Hu, S. Komarneni, In situ hydrothermal preparation of mesoporous Fe3O4 film for high-performance negative electrodes of supercapacitors, Microporous and Mesoporous Materials (2018), doi: 10.1016/j.micromeso.2018.02.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT In situ hydrothermal preparation of mesoporous Fe3O4 film for high-performance negative electrodes of supercapacitors Ke Jianga, Baolong Suna, Mengqi Yaoa, Ni Wang*a,b, Wencheng Hua, Sridhar Komarnenib Center for Applied Chemistry, University of Electronic Science and Technology of China,
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a
Chengdu, 610054, China b
Materials Research Institute and Department of Ecosystem Science and Management,
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Materials Research Laboratory, The Pennsylvania State University, University Park,
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Pennsylvania, 16802, USA.
*Corresponding Authors: N. Wang ([email protected]) Sridhar Komarneni, [email protected]
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Abstract
A mesoporous Fe3O4 film was prepared as binder-free electrode material for supercapacitors through a facile process that included the hydrothermal electroplating of an
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Fe/Zn alloy, in situ electrolytic dealloying to remove the Zn template, and oxidation in a water
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vapor environment. The Fe3O4 film showed a cubic structure and mesoporosity with a specific surface area of 247 m2 g−1. As a negative electrode material, the mesoporous Fe3O4 film delivered a high gravimetric capacity of 221 C g−1 at 1 A g−1, and the gravimetric capacity was maintained at 154 C g−1 even at a high current density of 50 A g−1. In addition, the mesoporous Fe3O4 electrode exhibited very high cycling stability (only 4.7% capacity loss after 10,000 galvanostatic charge–discharge cycles). Electrochemical impedance spectroscopy revealed that the mesoporous Fe3O4 film had excellent conductivity, implying its promising 1
ACCEPTED MANUSCRIPT application as a supercapacitor electrode. Keywords: mesoporous Fe3O4 film; supercapacitor; hydrothermal electroplating; high gravimetric capacity; cycling stability
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1. Introduction At present, considerable research attention is being paid for the sustainable development of various energy storage devices. Among these, supercapacitors are notable energy storage
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devices that can be charged or discharged quickly with very high-power density [1]. In the
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supercapacitors, energy is stored on the surface of the electrode material, which can maintain the long-term stability of its structure, leading to an ultra-long energy storage capability that is superior to that of lithium-ion batteries [1,2].
Studies on supercapacitors have received global attention. Depending on the energy storage
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mechanisms, electrode materials for supercapacitors are composed of three materials [3]: carbon-based materials (activated carbon, graphene, and carbon nanotubes [CNT]), pseudocapacitive materials (RuO2 and MnO2), and battery-type materials (NiO, CoO, and
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conducting polymer). Carbon-based materials with superior conductivity have been widely
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investigated as positive electrodes in acidic electrolytes [4], negative electrodes in alkaline electrolytes [5], and both positive and negative electrodes in neutral electrolytes [6]. As electric double-layer capacitors, carbon-based materials display an energy storage mechanism that mainly relies on the surface area to electrostatically accumulate charges at the material/electrolyte interface [7], leading to a relatively low capacitance compared with metal oxides. Low-cost metal oxides with high theoretical capacitance or capacity, such as manganese, nickel, and cobalt oxide, have been investigated as electrode materials for 2
ACCEPTED MANUSCRIPT supercapacitors [8,9]. These three oxides are positive electrode materials in alkaline and neutral electrolytes. For matched supercapacitors, manganese, nickel, and cobalt oxide electrodes must match with the relatively low capacitances of carbon-based electrodes (as
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negative electrodes) in order to assemble asymmetric or hybrid supercapacitors [10,11]. The development of low-cost negative electrode materials with remarkable capacitance or capacity has become more urgent than ever.
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Many materials have been utilized as negative electrodes in aqueous electrolytes [12]. One
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such material is iron oxide, which is inexpensive and environmentally friendly, making it an attractive alternative as a negative electrode material for supercapacitors [13]. Different methods have been used to prepare Fe2O3 as a negative electrode for supercapacitors [14–16]. Nonetheless, studies have demonstrated that key disadvantages, such as poor electrical
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conductivity, low “available” surface area, and binder addition, limited the iron oxide’s Faraday reaction for obtaining high specific capacity [17]. Therefore, iron oxides must be composited with graphene [18,19] and CNT [20] to improve their specific capacities.
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Fe3O4 is a mixed valence compound containing +2 and +3 iron ions. The Verwey transition
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effect facilitates electron conduction between +2 and +3 iron ions, leading to considerably higher conductivity than Fe2O3 [21]. In the present work, we adopted a novel strategy for hydrothermally electrodepositing porous Fe films, which were in-situ oxidized to synthesize binder-free negative electrodes for a supercapacitor. The porous Fe3O4 film possessed a high specific surface area of 247 m2 g−1 with a mesoporous structure. It delivered a high specific capacity of 221 C g−1 at 1 A g−1, indicating its potential application as a negative electrode in supercapacitors. 3
ACCEPTED MANUSCRIPT 2. Experimental procedure 2.1. Synthesis of Fe films The porous Fe films were directly prepared on commercial pure copper plates (1 cm × 4 cm)
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with a thickness of 50 µm. The copper plates were rinsed in 20% HNO3 and deionized water and then placed as a cathode electrode of the modified hydrothermal system for metal electroplating as shown in Fig.1a, where the anode constituted of iron and zinc plates with an
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area ratio of 1:1, Ag-coated copper wires were used as the conducting wires. The Fe/Zn
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plating solution, which contained 300 g L−1 ZnCl2, 300 g L−1 FeCl2, 4 g L−1 MnCl2, 3 g L−1 saccharin, 20 g L−1 borax, and 0.1 g L−1 SDS, was poured into a PTFE vessel with a filling volume of 80%. The plating solution was poured into the Teflon vessel until half of the parallel-positioned copper plate and iron/zinc plate were immersed in solution (Fig. 1a) in the
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PTFE vessel followed by its assembly into the autoclave. The hydrothermal system in this work was an inverted assembly hydrothermal reactor, as illustrated in Fig.1. Fig. 1b presents the assembled system for electrodeposition. The connectors were made from pure copper
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coated with an Au/Ni layer, and a high temperature-enameled wire (Ag-coated copper wire)
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was used as the conducting wire. Fig. 1c shows a stainless steel supporting plate on the PTFE vessel to assist the PTFE vessel at high pressure. Fig. 1d displays a three-electrode system for the electrodeposition, and Fig. 1e depicts a two-electrode system for the electrochemical measurement of the plating bath. The hydrothermal system was placed in a high-temperature oven, and the temperature was increased to 120 °C. Hydrothermal electroplating of the Fe/Zn alloy was conducted at a current density of 300 mA cm−2 for 30 min. Subsequently, the cathode and anode were reversed from each other, and the dealloying process was operated at 4
ACCEPTED MANUSCRIPT an electrolytic voltage of 1 V to remove the Zn template. The rinsed samples were oxidized in a tube furnace at 280 °C under an atmospheric and water vapor environment for 3 h, thereby converting the zero-valent iron into Fe3O4.
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2.2. Materials characterization A D-Max-γ type A X-ray diffractometer (XRD, Rigaku Co., Japan) was employed to characterize the crystallinity of the porous Fe3O4 films. The surface morphologies were
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observed using an Inspect F field-emission scanning electron microscope (FESEM, FEI Co.,
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US) and Libra 200FE high-resolution transmission electron microscope (HRTEM). N2 adsorption/desorption isotherms were analyzed using a JW–BK112 Surface Analyzer (Beijing JWGB Co., China). BET equation and BJH approach were used to evaluate the specific surface area and pore size distributions, respectively.
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2.3 Electrochemical measurements
Standard three-electrode measurements were used to investigate the electrochemical performance of porous Fe3O4 films, the latter were defined as working electrodes without
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binders. The porous Fe3O4 films and copper plates acted as the electro-active material and
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current collector, respectively. An Hg/HgO electrode was assigned as the reference electrode, and a Pt wire was set as the counter electrode. A CHI660D electrochemical workstation was used to measure cyclic voltammetry (CV), chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS) in 2 mol L−1 KOH solution. A long galvanostatic charge– discharge (GCD) cycle experiment was conducted in a battery testing system (Lanhe, China). 3. Results and discussion 3.1 Structural characteristics 5
ACCEPTED MANUSCRIPT Fig. 2 presents a typical glancing-angle XRD pattern of a porous Fe3O4 film. The sample showed six clear peaks located at 30.26°, 35.57°, 43.26°, 53.75°, 57. 26°, 62.91°and 73.95°, which correspond with the (220), (311), (400), (422), (511), (440) and (533) reflections of
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Fe3O4 (JCPDS card no. 19-0629) [22]. The sample displayed a polycrystalline structure of Fe3O4 and no secondary phases could be observed (Fig. 2). The sharp peaks with strong intensities indicated the cubic Fe3O4 phase of high crystallinity. The particle size of cubic
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Fe3O4 was calculated to be 46.6 nm based on the Scherrer equation [23].
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The FESEM images of the samples at different steps illustrate the formation of porous Fe3O4 films. In Fig. 3a, the electroplated Fe/Zn film exhibited a dense, crack-free, and relatively smooth surface because of accurate current control under the hydrothermal condition. After the sample was set as an anode to electrolyze for dealloying, the gradual
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dissolution of the zinc atoms caused the film to be uneven (Fig. 3b). As shown in Fig. 3b, after dealloying for 30 min, the surface initially presented a porous structure. As the electrolysis process was continued for 60 min, the porous structure became clearer as the
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porous particles were connected together, forming a complete porous Fe film (Fig. 3c). When
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the iron element was oxidized to form Fe3O4, the porous particles became closer to one another, and the particle size increased (Fig. 3d). Apparently as a result of the introduction of oxygen causing a volume increase [24]. The porous Fe3O4 film peeled from the electrode was tested by HRTEM, and the TEM
images are shown in Fig. 4. The connected structure was clearly observed and the particle size was in the range of 190–400 nm, which matched the FESEM observations. At higher magnification by high resolution TEM (Fig. 4b), it can be seen that the particles were 6
ACCEPTED MANUSCRIPT composed of many fine grains and some of them show highly ordered continuous fringes, which can be indexed to the (311) lattice planes of the cubic structure of Fe3O4. The porous structure was also observed as lighter areas between the fine grains.
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The porous Fe3O4 film coated on the copper plate was directly used in the N2 adsorption– desorption experiments, and the typical isotherms are shown in Fig. 5a. Type IV isotherm shape with H2 hysteresis loop indicated the mesoporous nature of the film [25]. The pore size
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distribution of the sample illustrated in Fig. 5b revealed that the porous Fe3O4 film possessed
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a broad pore-size distribution ranging from 2 to 11 nm but centered at 2.5 nm, suggesting that it was a mesoporous material. The abundant mesopores are expected to be beneficial to the diffusion of the solution and the transmission of electrolyte ions [26]. BET calculation revealed that the specific surface area of the copper plate with film was 22.1 m2 g−1. However,
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the porous Fe3O4 film by itself was calculated and it showed a high specific surface area of 247 m2 g−1, which is superior to previously reported results for porous iron oxides [27–29]. 3.2 Electrochemical performance
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We first conducted the CV experiments in the potential window from −1.1 V to 0 V to
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estimate the electrochemical properties of the porous Fe3O4 film and the obtained CV curves at different scanning rates are shown in Fig. 6. A pair of peaks existed in all curves, suggesting that a redox reaction occurred between the +2 and +3 Fe ions. Broussea [30] suggested that mesoporous Fe3O4 is a typical battery–electrode; a non–linear relation exists between the current density and potential window in the CV curve, in contrast to the case in MnO2 and RuO2 electrodes. As a typical battery–electrode of a supercapacitor, the ability to store charges should be redefined as capacity (C or mA h) and considered a substitute for 7
ACCEPTED MANUSCRIPT capacitance (F). In addition, Fig. 6 shows that copper plate did not offer an additional capacity in the CV test. To accurately evaluate the electrochemical properties for the practical application of an
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electrode, GCD measurements were performed to investigate the specific capacity at different current densities. The GCD curves of mesoporous Fe3O4 are shown in Fig. 7a, and the corresponding specific capacities (Cs, C g-1) based on the calculation of the galvanostatic
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discharge curves (Cs=Im×td, where Im is the current density, and td is the discharge time [31])
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are summarized in Fig. 7b. The GCD curves revealed the battery–electrode behavior of the film, as evidenced by the non-linear dependence of the potential versus the charge/discharge time. Moreover, charge and discharge plateaus were observed at −1 and −0.8 V, respectively, which agreed with the potential values of the oxidation and reduction reactions in the CV
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curves. The Cs value of the mesoporous Fe3O4 film was calculated to be 221 C g−1 at 1 A g−1, which is higher than those of α–Fe2O3 nanotube arrays [32], synthesized iron oxide nanoparticles [33], and α–Fe2O3 hollow nanoshuttles [34]. This Cs value is comparable with
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that of conductive-enhanced iron oxides using carbon materials, such as graphene/iron oxide
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electrodes [20] and 3D α-Fe2O3/carbon nanotube sponges. In particular, as the GCD current density increased to 50 A g−1, the capacity was 154 C g−1, retaining up to 69.77% of the initial value and suggesting a good rate capability [35]. The excellent electrochemical performance of our sample can be attributed to three factors: high specific surface area with mesoporous structure, absence of a binder, and mixed-valence state of iron oxides. The long cycle life experiment was another crucial index for evaluating the electrochemical stability of electrode materials. The repeating GCD cycle operated in the potential range of 8
ACCEPTED MANUSCRIPT −1.1 V to 0 V at 5 A g−1 was conducted in a battery testing system, and the result for an interval of 200 cycles is depicted in Fig. 8. During the initial 1000 cycles, the capacity gradually increased with increasing cycle number because of the gradual activation of the
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electrode materials and further wetting of the surface [36]. The binder-free mesoporous Fe3O4 film retained 95.3% of the initial capacity after 10,000 GCD cycles, thereby demonstrating its satisfactory cyclic stability. Thus, the mesoporous Fe3O4 film with a stable cubic structure
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could guarantee excellent stability.
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EIS measurement was employed to investigate the fundamental behavior of the mesoporous Fe3O4 electrode in the frequency range of 100 kHz–0.01 Hz before and after long-term GCD cycles, and the Nyquist plots are shown in Fig.9. The straight lines in the low-frequency region are greater than 45°, implying that the mesoporous Fe3O4 electrode
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displayed good capacitive behavior with excellent electron transfer and diffusion property [37]. The bulk solution resistance (Rs) and the charge–transfer resistance (Rct) intercepted from the Z-axis were 0.39 and 1.83 Ω, respectively. The low Rs value was due to the unique
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porous structure for wetting between the material surface and electrolyte solution and the high
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and effective surface for the Faradaic reaction [38]. In addition, the binder-free electrode structure and electron transferability between Fe2+ and Fe3+ ions existing in Fe3O4 also contributed to the low Rct value [39]. After 10,000 GCD cycles, these values slightly increased to 0.56 and 1.92 Ω, and the surface morphology of the mesoporous Fe3O4 film observed from the SEM image (Fig.9b) only showed a small change compared to the sample before the long GCD measurement, which explained the 4.7% loss of the initial capacity after the long-term life test. 9
ACCEPTED MANUSCRIPT 4. Conclusion In summary, we suggested a novel hydrothermal electroplating approach for depositing Fe/Zn alloys on copper plates. After the Zn atoms were removed from the Fe/Zn alloy film,
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the sample was oxidized in a tube furnace under a water vapor environment to prepare the mesoporous Fe3O4 film. The high specific surface area with reasonable pore structure led to the rapid diffusion of electrolyte ions, and the binder-free electrode along with the
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mixed-valence state of iron oxide facilitated electron conduction. The mesoporous Fe3O4 film
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presented a high specific capacity of 221 at 1 A g−1 and 154 C g−1 at 50 A g−1, indicating excellent rate capability. The long-term cycle life of up to 10,000 GCD cycles further demonstrated that the mesoporous Fe3O4 may have valuable practical application for
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supercapacitors.
[1] Simon, Patrice, Y. Gogotsi, and B. Dunn, Science 343.6176 (2014) 1210-1211.
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[2] T. Zhai, L. Wan, S. Sun, Q. Chen, J. Sun, Q. Xia, H. Xia, Adv. Mater. 29 (2017) 1604167. [3] N. Wang, M. Yao, P. Zhao, W. Hu, S. Komarneni, J. Mater. Chem. A 5 (2017) 5838–5845
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[4] P. Díaz, Z. González, R. Santamaría, M. Granda, R. Menéndez, C. Blanco, Electrochim. Acta 212 (2016) 848–855 [5] X. Chen, K. Chen, H. Wang, D. Xue, Electrochim. Acta 147 (2017) 216-224 [6] L. Sheng, L. Jiang, T. Wei, Z. Liu, Z. Fan, Adv. Eng. Mater. 7 (2017) 1700668 [7] K. Chen, F. Liu, X. Liang, D. Xue, Surf. Rev. Lett. 24 (2017) 1730005 [8] L. Hu, Y. Deng, K. Liang, X. Liu, W. Hu, J. Solid State Electr. 19 (2015) 629–637 [9] R. Jia, F. Zhu, S. Sun, T. Zhai, H. Xia, J. Power Sources 341 (2017) 427-434 [10] S.C. Lin, Y.T. Lu, Y.A. Chen, J.A. Wang, T.H. You, Y.S. Wang, C.W. Lin, Ma, Cn. C. M. Ma, C. C. Hu, J. Power Sources 362 (2017) 258–269 10
ACCEPTED MANUSCRIPT [11] J.L. Yin, J.Y. Park JY, J. Solid State Electr. 19 (2015) 2391–2398 [12] Q. Qu, S. Yang, X. Feng, Adv. Mater. 23 (2011) 5574–5580 [13] K.A. Owusu, L. Qu, J. Li, Z. Wang, K. Zhao, C. Yang, K.M. Hercule, C. Lin, C. Shi, Q. Wei, L. Zhou, L. Mai, Nat. Commun. 8 (2017) 14264
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[14] P. Yang, Y. Ding, Z. Lin, Z. Chen, Y. Li, P. Qiang, M. Ebrahimi, W. Mai, C.P. Wong, Z.L. Wang, Nano Lett. 14 (2014) 731–736
[15] Y. Li, J. Xu, T. Feng, Q. Yao, J. Xie, H. Xia, Adv. Funct. Mater. 27 (2017) 1606728
[16] L.L. Tian, M. J. Zhang, C. Wu, Y. Wei, J.X. Zheng, L.P Lin, J. Lu, K. Amine, Q.C.
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Zhuang, F. Pan, ACS Appl. Mater. Interfaces 7 (2015) 26284–26290
[17] H.D. Liu, J.L. Zhang, D.D. Xu, L.H Huang, S.Z. Tan, W.J. Mai, J. Solid State Electr. 19
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(2015) 135–144
[18] Z. Song, W. Liu, W. Wei, C. Quan, N. Sun, Q. Zhou, G. Liu, X. Wen, J. Alloy Compd. 685 (2016) 355–363
[19] J. Min, K, Kierzek, X, Chen, P.K. Chu, X. Zhao, R.J. Kaleńczuk, T. Tang, E. Mijowska, New J. Chem. (2017), https:// DOI: 10.1039/C7NJ03416D
20927–20934
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[20] X. Cheng, X. Gui, Z. Lin, Y. Zheng, M. Liu, R. Zhan, Y. Zhu, J. Mater. Chem. A 3 (2015)
[21] X. Jiang, B. Xiao, H. Yang, X. Gu, H. Sheng, X. Zhang, Appl. Phys. Lett. 109 (2016)
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203110
[22] S.C. Hu, F. Shi, J.X. Liu, L. Yu, S.H. Liu, J. Porous. Mat. 23 (2016) 655–661
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[23] K.M. Butt, M.A. Farrukh, I. Muneer, J. Mater. Sci. Mater El. 27 (2016) 8493–8498 [24] E. Hourdakis, A.G. Nassiopoulou, Thin. Solid Films 621 (2017) 36–41 [25] N. Wang, P. Zhao, K. Liang, M. Yao, Y. Yang, W. Hu, Chem. Eng. J 307 (2017) 105–112 [26] N. Wang, P. Zhao, K. Liang, M. Yao, Y. Yang, W. Hu, J. Solid State Electr. 20 (2016) 1429–1434
[27] G. Binitha, M.S. Soumya, A.A. Madhavan, P. Praveen, A. Balakrishnan, K.R.V. Subramanian, M.V. Reddy, S.V. Nair, A.S. Nair, N. Sivakumar, J. Mater. Chem. A 1 (2013) 11698–11704 [28] S. Shivakumara, T.R. Penki, N. Munichandraiah, J. Solid State Electr. 18 (2014) 1057– 1066 11
ACCEPTED MANUSCRIPT [29] Y. Zeng, M. Yu, Y. Meng, P. Fang, X. Lu, Y. Tong, Adv. Eng. Mater. 6 (2016) 1601053 [30] T. Broussea, D. Bélanger, J.W. Long, J. Electr. Soc. 162 (2015) A5185–A5189 [31] N. Wang, P. Zhao, Q. Zhang, M. Yao, W. Hu, Compos. Part B.Eng. 113 (2017) 144–151 [32] K. Xie, J. Li, Y. Lai, W. Lu, Z. Zhang, Y. Liu, L. Zhou, H. Huang, Electr. Commun 13
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(2011) 657–660 [33] E. Mitchell, F.D. Souza, R.K. Gupta, P.K. Kahol, D. Kumar, L. Dong, B.K. Gupta, Powder Technol. 272 (2015) 295-299
[34] X. Zheng, X. Yan, Y. Sun, Y. Yu, G. Zhang, Y. Shen, Q. Liang, Q. Liao, Y. Zhang, J.
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Colloid Interf. Sci. 466 (2016) 291–296
[35] X. Zhao, N. Wang, Y. Tan, Y. Liu, L. Kong, L. Kang, F. Ran, J. Alloy Compd. 710 (2017)
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72–79
[36] H. Wang, C.M.B. Holt, Z. Li, X. Tan, B.S. Amirkhiz, Z. Xu, B.C. Olsen, T. Stephenson, D. Mitlin, Nano Res. 5 (2012) 605–617
[37] T. Zhao, W. Yang, X. Ji, W. Jin, J. Hu, T. Li, Appl. Surf. Sci. (2017) https://doi.org/10.1016/j.apsusc.2017.09.046
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[38] J.M. Luo, G. Bo, X.G. Zhang, Mater Res. Bull. 43 (2008) 1119–1125
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[39] Z. Li, F. Wang, X. Wang, Small 13 (2017) 1603076
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Figure captions
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Fig.1. Modified hydrothermal system for metal electroplating: (a) schematic of the set-up, (b) inverted hydrothermal reactor, (c) Modified stainless steel supporting plate for 2 electrodes on the PTFE vessel (d) Teflon cover with 3 electrodes and (e) Teflon cover with 2 electrodes Fig.2. XRD pattern of typical porous Fe3O4 film and reference standard data for Fe3O4 of the JCPDS NO. 19-0629.
Fig.3. SEM images of (a) Fe-Zn film, (b) Fe-Zn film corroded electrochemically for 30min, (c)
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Fe-Zn film corroded electrochemically for 60min, (d) porous Fe3O4 film after oxidation of Fe.
Fig.4. HRTEM image of typical porous Fe3O4 film.
film.
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Fig.5. (a) N2 adsorption–desorption isotherms and (b) pore size distributions of porous Fe3O4
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Fig.6. CV curves of porous Fe3O4 film measured in 2M KOH electrolyte.
Fig.7. (a) GCD curves at different current densities and (b) plot of specific capacity as a function of current density of porous Fe3O4 film. Fig.8. Cycling stability of porous Fe3O4 electrode at a current density of 10A g−1.
Fig. 9. Nyquist plots obtained from the porous Fe3O4 electrode before and after the long cycle measurements, and SEM image after 10000 GCD cycles.
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ACCEPTED MANUSCRIPT Highlights • Novel hydrothermal electroplating was developed to prepare porous Fe3O4 film • The Fe3O4 film with a mesoporous structure showed a high specific surface area
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• The porous film delivered a high specific capacity of 221 C g−1 at 1 A g−1
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• The sample was very stable and only lost 4.7% capacity after 10,000 GCD cycles