- Email: [email protected]
Electrodeposition of manganese oxide nanosheets on a continuous three-dimensional nickel porous scaffold for high performance electrochemical capacitors
Accepted Manuscript Electrodeposition of Manganese Oxide Nanosheets on a Continuous ThreeDimensional Nickel Porous Scaffold for High Performance Electrochemical Capacitors Junwu Xiao, Shengxiong Yang, Lian Wan, Fei Xiao, Shuai Wang PII:
S0378-7753(13)01204-4
DOI:
10.1016/j.jpowsour.2013.07.024
Reference:
POWER 17718
To appear in:
Journal of Power Sources
Received Date: 11 May 2013 Revised Date:
19 June 2013
Accepted Date: 5 July 2013
Please cite this article as: J. Xiao, S. Yang, L. Wan, F. Xiao, S. Wang, Electrodeposition of Manganese Oxide Nanosheets on a Continuous Three-Dimensional Nickel Porous Scaffold for High Performance Electrochemical Capacitors, Journal of Power Sources (2013), doi: 10.1016/j.jpowsour.2013.07.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
Three-Dimensional
of
Manganese Nickel
Oxide
Porous
Nanosheets
Scaffold
for
Electrochemical Capacitors
on
a
Continuous
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Electrodeposition
High
Performance
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Junwu Xiao,* Shengxiong Yang, Lian Wan, Fei Xiao, and Shuang Wang*
Department of Chemistry and Chemical Engineering, Hubei Key Laboratory of
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Matieral Chemistry and Service Failure, Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, Huazhong University of Science & Technology, Wuhan 430074, PR China
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Email: [email protected], [email protected]
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Highlights A facile method to synthesize a three dimensional (3D) Ni porous scaffold.
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The controllable deposition of MnO2 active materials on a 3D porous scaffold.
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The 3D porous scaffold provides a highly electrolytic accessible area of MnO2.
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The 3D porous scaffold facilitates electron and electrolyte ion transport.
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The good electrochemical capacitor performance is achieved on a 3D porous
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·
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scaffold.
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Electrodeposition of Manganese Oxide Nanosheets on a Continuous Three-Dimensional Nickel Porous Scaffold for High Performance Electrochemical Capacitors
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Junwu Xiao,* Shengxiong Yang, Lian Wan, Fei Xiao, Shuai Wang*
Department of Chemistry and Chemical Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Key Laboratory for Large-Format Battery Materials and System, Ministry of
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Email: [email protected], [email protected]
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Education, Huazhong University of Science & Technology, Wuhan 430074, PR China
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Abstract: It’s desirable to design an ideal three-dimensional (3D) interpenetrating network as the current collector for providing efficient ion and electron transport. Herein, we report a facile method
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to fabricate a novel continuous 3D Ni porous nanoarchitecture via the reduction of Ni(OH)2 nanowall precursors. The as-formed continuous 3D Ni porous network as the conductive scaffold can support a highly electrolytic accessible surface area of redox active MnO2 nanosheets, and provide reliable
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electrical connections to the MnO2 layers. In comparison with the planar conducting substrates, this 3D scaffold not only can increase the mass loading of MnO2 active materials, but also facilitate the
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facile transport of electron and electrolyte ion. Thus, the 3D (MnO2/Ni) electrode exhibited higher specific capacitance (1169.6 F g-1 at 2 A g-1, closed to the theoretical value) and better long-term cyclability (only ~ 5% loss after 1000 cycles) than that on the planar conducting substrate under the identical electrodeposition condition (611.6 F g-1 at 2 A g-1 and around 20% loss after 1000 cycles).
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These results suggest that such 3D Ni porous architecture is a promising current collector for high-performance electrochemical capacitor.
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Keywords
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Three dimensional porous scaffold; Manganese oxides; Electrodeposition; Electrochemical capacitor
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1. Introduction
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Transition metal oxides, such as RuO2,[1, 2] MnO2,[3-6] Co3O4,[7-9] NiO,[10-14] and their mixed versions,[15, 16] have been widely studied as electroactive materials for electrochemical capacitors, due to their higher theoretical specific capacitance than carbon-based materials and better
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cyclability than conducting polymers. Among them, RuO2 is notable for its specific capacitance as high as 1340 F g-1, but the sheer high cost and rareness of Ru limit its application. So, it’s essential to
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search for cheaper alternatives. MnO2 is considered as one of the promising electrode materials for next generation electrochemical capacitors, which exhibits the high theoretical specific capacitance (1370 F g-1),[17] and many intriguing characteristics, such as low cost, environment friendliness and natural abundance. However, its poor electric conductivity (10-5-10-6 S cm-1) and low utilization for
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charge storage hinders the achievement of such high theoretical capacitance. To approach the theoretical value of MnO2, in theory, the thickness of MnO2 should be infinitely small, and the scan rate should be infinitely slow, which can enhance the electron and electrolyte ion
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transport and ensure the complete utilization of MnO2 for energy storage. Actually, when MnO2 is densely packed and deposited on the current collector, the limited accessible surface area is
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participated in electrochemical charge storage process, and thus resulting in remarkably increasing the contact resistance and in turn decreasing the specific capacitance. Therefore, to maximize the utilization of MnO2 in charge storage process, it’s essential to keep its nanostructure morphology and provide reliable electrical connection to the current collector. The promising approach is to incorporate MnO2 nanostructures or nanometer-thick thin films into the highly conductive materials,[18-25] such as carbon-based materials and conducting polymers, and on a three 3
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dimensional (3D) scaffold.[26-30] A highly conductive three dimensional (3D) continuous porous scaffold not only provide high
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surface area for loading electroactive materials, but also facilitate the fast electron transport from electroactive materials to the current collector. Moreover, no binder and conductive agents are added during the preparation process of the electrodes. ZnO, SnO2 and Zn2SnO4 nanowire arrays have been
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reported as the scaffold for loading electroactive materials as electrochemical capacitor electrodes.[26-28] However, the electrical conductivity of these oxides is not as good as the stable
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metals, such as, Au and Ni. 100-nm-thick nano-porous Au films have been fabricated for enhancing the electron and electrolyte ion transport,[29] whereas this nano-porous structure severely inhibits the mass loading of active materials and the electrolyte ion transport. Additionally, the rareness and high cost of Au and Ag restrict the application. A self-assembled 3D bicontinuous Ni
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nanoarchitecture, fabricated by electrodepositing Ni on self-assembled opal templates and subsequent removing the templates, have exhibited very large battery charge and discharge rate with minimal capacity loss for lithium-ion and nickel-metal hydride batteries.[30] But the preparation
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process is so complex. Herein, we report a facile method to synthesis a continuous 3D Ni porous network as the current collector for loading electroactive materials as electrochemical capacitor
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electrode. This 3D current collector is with large surface area to increase the mass loading of active materials, with the porous structure to facilitate the fast electrolyte ion transport, and with a highly conductive network to promote the electron transport from active material layer to the current collector. 2. Experimental 2.1 Growth of MnO2 nanosheets on a continuous three dimensional (3D) Ni porous scaffold 4
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In a typical procedure, the planar conducting substrates are prepared via sputtering 10 nm Ti and followed by 100 nm Au films on one side of glass slides. Another side was glued by scotch tape.
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NiCl2·6H2O (0.125 M) and hexamethylenetetramine (HMT, 0.25 M) were dissolved in 10 mL of DI water, and transferred to a 20 mL Teflon-lined stainless-steel autoclave. The planar conducting substrates were hanged in the aqueous solution. Then, Teflon-lined stainless-steel autoclave was
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sealed, maintained at 100 oC for 10 h, and allowed to cool to room temperature. The light green products on the substrates were rinsed several times with distilled water and ethanol, and dried in
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vacuum oven. Subsequently, the substrates were thermally treated under H2/Ar mixed gas (7.0 vol%) at 400 oC for 1 h with the heating rate of 5 oC min-1. Followed by electrodepositing MnO2, the substrates were as the working electrode. A platinum foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. 20 mL aqueous solution of
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Mn(CHCOO)2 (50 mM) and Na2SO4 (100 mM) was used as the electrolyte. The electrodeposition process was carried out via the linear sweep voltammetry (LSV) method ranging from 0.4 to 1.3 V at various scan rates (2, 5, 10, 30, 50 and 100 mV s-1). The as-formed samples were named as
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(MnO2/Ni)9, (MnO2/Ni)18, (MnO2/Ni)30, (MnO2/Ni)90, (MnO2/Ni)180, and (MnO2/Ni)450, according to the electrodeposition time at the scan rates of 100, 50, 30, 10, 5, and 2 mV s-1, respectively.
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According to the following equations: m=
( ∫ IdU ) M nFν
·········(I)
where I is the current, U is the potential window, ν is the scan rate, m is the electrodeposition mass of MnO2, n is electron number per mole, F is Faraday constant (96485.2 C mol-1), and M is molar mass of MnO2 (86.9 g mol-1), the MnO2 mass is directly proportional to the passed deposition charge, and thus is closely related to the scan rate and deposition current. 5
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2.2 General materials characterization. The product morphologies were directly examined by scanning electron microscopy (SEM) using
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JEOL JSM-6700F at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) observations were carried out on a JEOL 2010 microscope operating both at 200 kV. X-ray diffraction (XRD) was performed on a PANalytical X’pert Pro X-ray diffractometer with Cu kα
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irradiation (λ=1.5406 Å). The step size and scan rate are set as 0.05o and 0.025 o s-1, respectively.
2.3 Electrochemical measurements.
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The working electrode was prepared by electrodepositing MnO2 on porous Ni nanowall. The electrolyte used was 1.0 M Na2SO4 aqueous solution. The capacitive performance of the samples was evaluated on a CHI 660D electrochemical workstation using cyclic voltammetriy (CV) and chronopotentiometry test with a three-electrode cell where Pt foil serves as the counter electrode and
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Ag/AgCl (saturated KCl solution) as the reference electrode. The specific capacitance values were calculated from the galvanostatic charge and discharge curves, using the following equation: C=(I∆t)/(m∆V), where I is a charge or discharge current, ∆t is the charge or discharge time, m
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or discharge.
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indicates the mass of electroactive materials, and ∆V represents the voltage change after a full charge
3. Results and Discussion
Our strategy for electrodepositing MnO2 active materials on a continuous 3D porous Ni network is shown schematically in Figure 1. First of all, preparation of 3D continuous porous Ni network involved the precipitation of Ni(OH)2 precursors on the planar conducting substrate in aqueous solution, induced by the hydrolysis of hexamethylenetetramine (HMT) (Step I in Figure 1), and the subsequent reduction process under H2/Ar atmosphere. (Step II in Figure 1). After precipitating Ni 6
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salts on the planar conducting substrate, the light green film was found to be uniformly grown on the planar conducting substrate. As seen from SEM image (Figure 2A), the microstructure is consisted of
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the connected sheets with micro-scale length and less than 10 nm in thickness. The connected sheets grow vertically and cross-linked on the planar conducting substrate, forming a compact film with obvious holes. The corresponding XRD pattern in inset of Figure 2A exhibits a set of characteristic
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diffraction peaks, which could be indexed as a pure α-Ni(OH)2 (JCPDS 38-0715). The diffraction peaks 2θ=7.8o, 33.4o, and 60.0o should be corresponding to (002), (101), and (110) crystal planes of
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α-Ni(OH)2, respectively. After the thermal treatment under H2/Ar atmosphere, Ni(OH)2 nanowall precursors are reduced into a 3D continuous Ni porous network, in accompany with the escape of water vapor (Figure 2B), of which the XRD pattern in inset of Figure 2B is in consistent with the standard pattern of Ni (JCPDS 04-0850). The porous Ni network with around 6 µm in thickness
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uniformly grown on the substrates is composed of the connected ~100 nm Ni nanoparticles (Figure 2C and D). The component Ni nanoparticles are in single crystalline structure (Figure 2E and F). The lattice fringes distance about 0.203 nm in Figure 2F should be corresponding to the spacing of (111)
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crystal face.
Then, a continuous 3D Ni porous network as the electrodes, a thin MnO2 layer was directly and
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uniformly electrodeposited on it, of which the thicknesses could be controlled by the electrodeposition parameters (Step III in Figure 1). Figure 3 shows TEM images and EDX spectra of (MnO2/Ni)9 electrode. The as-formed MnO2 film is composed of the nanosheets, as seen from TEM image (Figure 3A). The EDX pattern in Figure 3B clearly reveals that besides Cu element from Cu grid, Mn and Ni metallic elements are also detected, which should be ascribed to MnO2 active materials and Ni scaffold, respectively. It’s found that Ni elements locate in the interior (Figure 3C), 7
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and Mn elements mainly distribute in the exterior (Figure 3D), suggesting to form MnO2-Ni core-shell nanostructure. In the SAED pattern (Figure 3E), the diffraction rings represent the
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diffraction of (301), (401) and (511) crystal planes of MnO2, whereas the diffraction dots belong to the diffraction of single crystalline Ni. The HRTEM image is in agreement with the diffraction rings in the SAED pattern, in which the lattice spacing of 0.254 and 0.208 nm match up well with the
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inter-planar spacings of (301) and (401) planes of MnO2 (Figure 3F).
The loading mass and thickness of MnO2 electrodeposited on Ni network can be adjusted via
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altering the electrodeposition charge. Figure 4 shows SEM images of a series MnO2 layers electrodeposited on 3D Ni porous network at various scan rates. At a scan rate of 30 mV s-1, MnO2 active materials just fully cover on the surface of smooth Ni nanoparticles (Figure 4A). With decreasing the scan rates, more and more MnO2 active materials are electrodeposited in the void of
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porous Ni network (Figure 4A-D). At a scanning rate of 2 mV s-1, MnO2 active materials almost fill up in the void of the 3D porous scaffold (Figure 4D). According to the equation I, the mass loading of MnO2 electrodeposited on Ni network is calculated as follows: 0.033 ((MnO2/Ni)9), 0.046
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((MnO2/Ni)18) 0.072 ((MnO2/Ni)30), 0.136 ((MnO2/Ni)90), 0.318 ((MnO2/Ni)180), and 0.714 mg cm-2 ((MnO2/Ni)450), respectively. In a control experiment, under the same electrodeposition condition, a
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uniform MnO2 layer was also deposited on the planar conducting substrate with the same morphology as that grown on 3D scaffold (Figure SI-1). The mass loading of MnO2 grown on the planar conducting substrates is 0.017, 0.028, 0.031, 0.062, 0.148, and 0.437 mg cm-2 corresponding to the electro-deposition time of 9, 18, 30, 90, 180 and 450 s, respectively. The results show that the 3D continuous porous Ni network provides much more surface area for depositing MnO2 active materials, and thus can load almost two times as much as MnO2 mass formed on the planar 8
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conducting substrate under the same electrodeposition condition. The chemical composition of the as-deposited MnO2 at various scan rates can be surveyed by
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X-ray photoelectron spectroscopy (XPS) measurement. Typical XPS spectra of Mn 2p and O 1s for MnO2 electrodeposited at the scan rates of 100 and 2 mV s-1 are shown in Figure 5. The almost identical XPS spectra indicate that the mean oxidation state of Mn is not affected by changing the
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scan rates. The broad peaks of Mn 2p3/2 and 2p1/2 are located around 642.2 eV and 653.7 eV, respectively, revealing Mn4+ ions were dominant in the products.[17, 31, 32] In addition, the
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difference of peak separation between Mn 2p3/2 and 2p1/2 is about 11.5 eV, further supporting the above statement, since the binding energy difference of Mn 2p3/2 and 2p1/2 can be used to indicate the oxidation state of Mn.[32] In the spectra of O 1s, the peaks at 529.7, 530.9, and 531.8 eV should be ascribed to the Mn-O-Mn, Mn-O-H, and H-O-H, respectively. [33, 34] The detection of Mn-O-H and
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H-O-H species reveals the hydrous nature. Above results reveal that the as-deposited active materials should be MnO2·nH2O, in accordance with the SAED and HRTEM results, and the mean oxidation state of Mn is not affected by altering the scan rates.
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To test the electrochemical capacitor performance, the (MnO2/Ni) electrodes were used as the working electrodes in a three-electrode configuration. Cyclic voltammetry (CV) curves in Figure 6A
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were obtained by sweeping the voltage from 0 to 0.8 V vs Ag/AgCl (sat. KCl) in 1.0 M Na2SO4 electrolyte. The CV curves exhibit the quasi-rectangular shape, a typical capacitive behavior of MnO2, indicating the ideal electrical double-layer capacitance behavior and fast charging/discharging process characteristic. The energy storage contributions come from the surface adsorption/desorption of the electrolyte cations (H3O+ and Na+) on the Ni network and MnO2, and the reversible redox reaction by mean of the electrolyte cations expelling/intercalating from/into MnO2.[17, 35, 36] In 9
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comparison with that deposited on the planar conducting substrate, MnO2 deposited on the 3D Ni porous scaffold exhibited better quasi-linear dependence of the capacitive current density on the scan
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rate of 2 to 100 mV s-1 (Figure 6B), suggesting better rate capacitive performance.[37] It’s ascribed by that the good conductive porous Ni network structure can facilitate the transportation of electron and the electrolyte, allowing full access of the electrolyte to electrode materials and maximizing the
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utilization of MnO2.
Rate capability is one of the important factors for evaluating the power applications of
as-prepared
(MnO2/Ni)9
electrode
at
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electrochemical capacitors. Figure 6C shows the galvanostatic charge-discharge curves of the different
charge-discharge
current
densities.
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charging/discharging cycling curves have a very symmetric nature, even at a low density of 2 A g-1, indicating again that the electrode has a good electrochemical capacitive characteristic and reversible
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redox reaction. The overall specific capacitance mainly come from the contribution of MnO2 active materials, since the Ni porous network only exhibited a specific capacitance of 0.12 mF cm-2 at a current density of 1 mA cm-2 (Figure SI-2). Figure 6D shows the mass-specific capacitance of MnO2
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deposited on the planar conducting substrate and a continuous 3D Ni porous scaffold. The specific capacitance of (MnO2/Ni)9 electrode at a current density of 2 A g-1 can be up to 1169.6 F g-1 based on
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the mass of MnO2. Even at a high current density of 20 A g-1, the specific capacitance can remain at 642.9 F g-1. Prolonging the electrodeposition time increases the mass loading of MnO2 active materials, but deteriorates the specific capacitance performance, which is in the sequence of (MnO2/Ni)9 > (MnO2/Ni)18 > (MnO2/Ni)30 > (MnO2/Ni)90 > (MnO2/Ni)180 > (MnO2/Ni)450. In contrast, on the planar conducting substrates, MnO2 active materials exhibit poor specific capacitance performance under the identical electrodeposition condition (Figure 6D). The improvement of the 10
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specific capacitance can be attributed to the reduced diffusion path of electrolyte ions, highly accessible surface area and increased electrical conductivity by utilizing porous Ni scaffold for
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depositing MnO2, instead of the planar conducting substrates. In addition, it’s found that, with low mass loading of MnO2, the highly conductive porous Ni network as the current collectors can provide the good opportunity for electron transportation and ions accessibility, and maximize the
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utilization of MnO2 materials, and thus resulting in large difference in specific capacitance between that on the planar conducting substrates and the 3D Ni porous scaffold. While at high mass loading
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devices, negligible change in specific capacitance was observed at high current densities mainly due to the limited conductivity of high mass loading samples and the limited utilization of MnO2 at high current densities.
The area-specific capacitance performance of MnO2 deposited on the planar conducting
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substrate and the continuous 3D Ni porous scaffold is shown in Figure 6E. Under the identical electrodeposition conditions, the area-specific capacitances of MnO2 deposited on 3D Ni porous scaffold are much higher than that of MnO2 deposited on the planar conducting substrates, since 3D
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Ni porous scaffold with larger surface area can load more MnO2 mass and exhibit better electron and electrolyte transport. Although the area-specific capacitance of the (MnO2/Ni) electrode with the
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shortest deposition time (9 s) is as low as 11.6 mF cm-2 at a current density of 0.5 mA cm-2, it’s still over two times as that of MnO2 deposited on the planar conducting substrates (4.5 mF cm-2). With increasing the electrodeposition times, more and more MnO2 active materials are deposited on the 3D Ni porous scaffold and the planar conducting substrates, resulting in exhibiting higher and higher area-specific capacitance. The highest area-specific capacitance of 121.6 mF cm-2 is obtained from the (MnO2/Ni)450 electrode at a current density of 0.5 mA cm-2, approximately 2.5 times the value for 11
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MnO2 electrode deposited on the planar conducting substrates under the same electrodeposition condition. The area-specific capacitance performance is also higher than the reported values for
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MnO2/carbon nanoparticles,[31] MnO2/carbon nanofoam,[38] and carbon-based electrodes.[39-42] Besides the specific capacitance, another important requirement for electrochemical capacitor applications is cycling stability. The cycling stability tests over 1000 cycles at a current density of 4
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A g-1 were carried out using constant-current galvanostatic charge-discharge cycling techniques in the potential windows ranging from 0 to 0.8 V. Figure 6F shows the specific capacitance retention of
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the MnO2 electrodeposited on the 3D Ni porous network and the planar conducting substrates as a function of charge/discharge cycling numbers. MnO2 active materials grown on the 3D Ni porous network only exhibited ~ 5% loss of the specific capacitance, whereas MnO2 electrodeposited on the planar conducting substrate under the same electrodeposition condition have around 20 % loss after
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1000 repetitive charge-discharge cycles. Thus, MnO2 active materials deposited on the 3D Ni scaffold exhibited better long-term cyclability, in comparison with that grown on the planar conducting substrate.
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The internal resistance of electrochemical capacitor is of great importance in energy storing devices. For low internal resistance of electrochemical capacitor, less energy will be wasted to
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produce unwanted heat and electronic transportation during charging/discharging processes. We can see a sudden IR drop at the beginning of the discharge curves, and the increase of the IR drop, in accompany with the increase of the discharge current density (Figure 6C). Figure 7A summarizes the IR drop values of the (MnO2/Ni) electrodes. With the increase of the discharge current density, the IR drop becomes larger, meaning that the energy loss during the charge/discharging process gradually increases. The smallest IR drop is observed for (MnO2/Ni)9 electrode among the (MnO2/Ni) 12
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electrodes, indicating a highly conductive characteristic. Prolonging the electrodeposition time enhances the mass loading and thickness of MnO2, and thus results in the increase of the slop of the
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plots (IR drop versus current densities). To further understand the electrochemical performance characteristics of (MnO2/Ni) electrodes, we resorted to electrochemical impedance spectroscopy (EIS) carried out at open circuit potential
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with an ac perturbation of 5 mV in the frequency range of 1000 kHz-10 mHz. Figure 7B shows the Nyquist plots thus obtained. The EIS data were fitted based on an equivalent circuit model consisting
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of bulk solution resistance Rs, charge-transfer resistance Rct, double-layer capacitance Cdl, and Warburg resistance (W), as shown in inset of Figure 7B. The bulk solution resistance Rs and charge-transfer resistance Rct can be estimated from the intercepts of the semicircle with the real axis at high and low frequencies, respectively.[43-45] The slope of the linear portion of the curve on the
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right of the semicircle is called Warburg resistance (W), which is a result of the frequency dependent ion diffusion/transport in the electrolyte toward the electrode surface.[46] A most striking difference lies in the semicircle corresponding to the charge-transfer resistance (Rct) caused by the Faradic
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reactions and the double-layer capacitance (Cdl) on the grain surface, whereas Rct increases gradually from 0.43 ((MnO2/Ni)9), 0.78 ((MnO2/Ni)18), 1.20 ((MnO2/Ni)30), 1.40 ((MnO2/Ni)90), 1.91
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((MnO2/Ni)180), to 2.82 Ω((MnO2/Ni)450) with increasing the mass loading of MnO2. The IR drop curves and EIS data clearly demonstrate that (MnO2/Ni) electrodes display and a decreasingly internal resistance and an increasingly favorable charge-transfer kinetics as the mass loading of MnO2 gradually decreases, in excellent agreement with the trend of their largely enhanced specific capacitances (Figure 6D). In comparison with that on the planar conducting substrate, the architecture structure of a thin 13
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MnO2 layer electrodeposited on a continuous 3D porous Ni network has several features for improving the electrochemical performance as follows: (I) The continuous 3D porous Ni network
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can provide much more surface area for depositing MnO2, resulting in higher mass loading under the same electrodeposition condition. (II) The porous structure of the (MnO2/Ni) electrodes is feasible for the fast transport of hydrated ion in the electrolyte. (III) The thin layer of MnO2 enables the fast
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and reversible Faradic reaction by shortening the ion diffusion path. (IV) The continuous Ni bridge may provide highly conductive channels for effective electron transport from MnO2 layer to the
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current collector.
4. Conclusions
In sum, we have successfully developed a facile method to synthesis a novel architecture by electrodepositing MnO2 nanosheets to a three-dimensional continuous porous Ni network as high
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performance electrochemical capacitor electrode. The three dimensional continuous Ni porous network are grown on the planar conducting substrate via the in-situ growth of Ni(OH)2 nanowalls and the subsequent reduction process, which as the highly conductive scaffold support a highly
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electrolytic accessible surface area of redox active MnO2 shells and provide reliable electrical connections to the MnO2 shells. The continuous three dimensional Ni porous network not only
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increases the mass loading of MnO2 around two times as much as the planar electrode, but also facilitates the facile transport of electron and electrolyte ion, and thus enhance the specific capacitance from 611.6 to 1169.6 F g-1 at a current density of 2 A g-1 and improve cycle stability from ~ 20 % to 5 % loss after the 1000 repetitive charge-discharge processes. These results suggest that such architecture is very promising for next generation high-performance electrochemical capacitor. 14
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Acknowledgments This research was supported by National Natural Science Foundation of China (Project No.
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51173055), and the Fundamental Research Funds for the Central Universities (2013QN158).
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[23] J. Liu, J. Essner, J. Li, Chemistry of Materials, 22 (2010), pp. 5022-5030. [24] J. Xiao, S. Yang, ChemPlusChem, 77 (2012), pp. 807-816. [25] J. Xiao, S. Yang, Journal of Materials Chemistry, 22 (2012), pp. 12253-12262. [26] J.A. Yan, E. Khoo, A. Sumboja, P.S. Lee, ACS Nano, 4 (2010), pp. 4247-4255. [27] L.H. Bao, J.F. Zang, X.D. Li, Nano Letter, 11 (2011), pp. 1215-1220. [28] J.P. Liu, C.W. Cheng, W.W. Zhou, H.X. Li, H.J. Fan, Chemical Communications, 47 (2011), pp. 16
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3436-3438. [29] X. Lang, A. Hirata, T. Fujita, M. Chen, Nature Nanotechnology, 6 (2011), pp. 232-236.
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Ajayan, Nature Nanotechnology, 6 (2011), pp. 496-500. [43] J. Gamby, P.L. Taberna, P. Simon, J.F. Fauvarque, M. Chesneau, Journal of Power Sources, 101
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[46] M.D. Stoller, S.J. Park, Y.W. Zhu, J.H. An, R.S. Ruoff, Nano Letters, 8 (2008), pp. 3498-3502.
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Figure captions
Figure 1. Schematic graph of the formation process of MnO2 active materials deposited on a
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continuous three dimensional Ni porous network. Step I: Ni(OH)2 nanowall network was directly grown on the planar conducting substrate. Step II: Under H2/Ar atmosphere, Ni(OH)2 nanowall precursors were reduced into a Ni porous network (black) composed of the connected Ni
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nanoparticles. Step III: Electrodepositing MnO2 thin films (blue) on the surface of the component Ni nanoparticles of the three dimensional Ni porous network. The electron could be quickly transported
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from the interface between MnO2 and electrolyte to the current collector via the continuous Ni network.
Figure 2. Characterizations of Ni(OH)2 nanowalls and Ni porous networks grown on the planar
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conducting substrates: (A) SEM image of Ni(OH)2 nanowalls (Inset: the corresponding XRD pattern); and (B-D) SEM and (E, F) TEM images of Ni porous network. (Inset of B: the
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corresponding XRD pattern)
Figure 3. (A) TEM image, (B) EDX pattern, (C, D) Ni and Mn mapping, (E) SAED pattern, and (F)
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HRTEM image of the (MnO2/Ni)9 electrode.
Figure 4. SEM images of the (MnO2/Ni) electrodes formed at various electrodeposited conditions: (A) (MnO2/Ni)30; (B) (MnO2/Ni)90; (C) (MnO2/Ni)180; and (D) (MnO2/Ni)450.
Figure 5. (A) Mn 2p and (B) O 1s XPS spectra of MnO2 deposited on a continuous three dimensional (3D) Ni porous scaffold at the scan rates of 100 and 2 mV s-1.
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Figure 6. Electrochemical characterizations of MnO2 active materials deposited on a continuous three dimensional (3D) Ni porous network in a three-electrode configuration: (A)
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CV curves of the (MnO2/Ni)9 electrode at various scan rates; (B) The quasi-linear dependence curve of the current density at 0.5 V versus the scan rate for MnO2 electrodeposited on the planar conducting substrate and 3D Ni scaffold at a scan rate of 100 mV s-1; (C) Galvanostatic
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charge-discharge curves of the (MnO2/Ni)9 electrode at various current densities; The mass (D) and area (E)-specific capacitance performance of a series of MnO2 on the planar conducting substrates
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(Solid) and the 3D Ni porous scaffold (Hollow); and (F) Normalized specific capacitance versus cycle number at a galvanostatic charge-discharge current density of 4 A g-1.
Figure 7. (A) IR drop against the discharge current densities, and (B) EIS Nyquist plots for the
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(MnO2/Ni) electrodes (Inset: Equivalent circuit diagram proposed for analysis of the EIS data).
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Supporting Information
Porous Scaffold for High Performance Electrochemical Capacitors
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Electrodeposition of Manganese Oxide Nanosheets on a Continuous Three-Dimensional Nickel
Junwu Xiao,* Shengxiong Yang, Fei Xiao, Lian Wan, Shuai Wang*
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Department of Chemistry and Chemical Engineering, Hubei Key Laboratory of Matieral Chemistry and Service Failure, Key Laboratory for Large-Format Battery Materials and System, Ministry of
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Education, Huazhong University of Science & Technology, Wuhan 430074, PR China
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Email: [email protected], [email protected]
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Figure SI-1. (A) SEM and high magnification SEM images of MnO2 film directly deposited on the planar conducting substrate.
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Figure SI-2. The galvanostatic charge-discharge curves of the three dimensional Ni porous scaffold
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at a current density of 1 mA cm-2.
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Highlights A facile method to synthesize a continuous three dimensional Ni porous scaffold.
·
The controllable deposition of MnO2 active layer on a continuous three
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·
dimensional Ni porous scaffold.
The three dimensional Ni porous scaffold electrode with high surface area
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promises high mass loading of MnO2 and with the hierarchal porous structure facilitates the fast transport of electron and electrolyte ion, thus achieving a high
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electrochemical capacitor performance.
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S0378-7753(13)01204-4
DOI:
10.1016/j.jpowsour.2013.07.024
Reference:
POWER 17718
To appear in:
Journal of Power Sources
Received Date: 11 May 2013 Revised Date:
19 June 2013
Accepted Date: 5 July 2013
Please cite this article as: J. Xiao, S. Yang, L. Wan, F. Xiao, S. Wang, Electrodeposition of Manganese Oxide Nanosheets on a Continuous Three-Dimensional Nickel Porous Scaffold for High Performance Electrochemical Capacitors, Journal of Power Sources (2013), doi: 10.1016/j.jpowsour.2013.07.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
Three-Dimensional
of
Manganese Nickel
Oxide
Porous
Nanosheets
Scaffold
for
Electrochemical Capacitors
on
a
Continuous
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Electrodeposition
High
Performance
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Junwu Xiao,* Shengxiong Yang, Lian Wan, Fei Xiao, and Shuang Wang*
Department of Chemistry and Chemical Engineering, Hubei Key Laboratory of
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Matieral Chemistry and Service Failure, Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, Huazhong University of Science & Technology, Wuhan 430074, PR China
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Email: [email protected], [email protected]
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Highlights A facile method to synthesize a three dimensional (3D) Ni porous scaffold.
·
The controllable deposition of MnO2 active materials on a 3D porous scaffold.
·
The 3D porous scaffold provides a highly electrolytic accessible area of MnO2.
·
The 3D porous scaffold facilitates electron and electrolyte ion transport.
·
The good electrochemical capacitor performance is achieved on a 3D porous
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·
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scaffold.
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Electrodeposition of Manganese Oxide Nanosheets on a Continuous Three-Dimensional Nickel Porous Scaffold for High Performance Electrochemical Capacitors
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Junwu Xiao,* Shengxiong Yang, Lian Wan, Fei Xiao, Shuai Wang*
Department of Chemistry and Chemical Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Key Laboratory for Large-Format Battery Materials and System, Ministry of
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Email: [email protected], [email protected]
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Education, Huazhong University of Science & Technology, Wuhan 430074, PR China
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Abstract: It’s desirable to design an ideal three-dimensional (3D) interpenetrating network as the current collector for providing efficient ion and electron transport. Herein, we report a facile method
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to fabricate a novel continuous 3D Ni porous nanoarchitecture via the reduction of Ni(OH)2 nanowall precursors. The as-formed continuous 3D Ni porous network as the conductive scaffold can support a highly electrolytic accessible surface area of redox active MnO2 nanosheets, and provide reliable
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electrical connections to the MnO2 layers. In comparison with the planar conducting substrates, this 3D scaffold not only can increase the mass loading of MnO2 active materials, but also facilitate the
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facile transport of electron and electrolyte ion. Thus, the 3D (MnO2/Ni) electrode exhibited higher specific capacitance (1169.6 F g-1 at 2 A g-1, closed to the theoretical value) and better long-term cyclability (only ~ 5% loss after 1000 cycles) than that on the planar conducting substrate under the identical electrodeposition condition (611.6 F g-1 at 2 A g-1 and around 20% loss after 1000 cycles).
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These results suggest that such 3D Ni porous architecture is a promising current collector for high-performance electrochemical capacitor.
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Keywords
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Three dimensional porous scaffold; Manganese oxides; Electrodeposition; Electrochemical capacitor
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1. Introduction
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Transition metal oxides, such as RuO2,[1, 2] MnO2,[3-6] Co3O4,[7-9] NiO,[10-14] and their mixed versions,[15, 16] have been widely studied as electroactive materials for electrochemical capacitors, due to their higher theoretical specific capacitance than carbon-based materials and better
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cyclability than conducting polymers. Among them, RuO2 is notable for its specific capacitance as high as 1340 F g-1, but the sheer high cost and rareness of Ru limit its application. So, it’s essential to
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search for cheaper alternatives. MnO2 is considered as one of the promising electrode materials for next generation electrochemical capacitors, which exhibits the high theoretical specific capacitance (1370 F g-1),[17] and many intriguing characteristics, such as low cost, environment friendliness and natural abundance. However, its poor electric conductivity (10-5-10-6 S cm-1) and low utilization for
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charge storage hinders the achievement of such high theoretical capacitance. To approach the theoretical value of MnO2, in theory, the thickness of MnO2 should be infinitely small, and the scan rate should be infinitely slow, which can enhance the electron and electrolyte ion
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transport and ensure the complete utilization of MnO2 for energy storage. Actually, when MnO2 is densely packed and deposited on the current collector, the limited accessible surface area is
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participated in electrochemical charge storage process, and thus resulting in remarkably increasing the contact resistance and in turn decreasing the specific capacitance. Therefore, to maximize the utilization of MnO2 in charge storage process, it’s essential to keep its nanostructure morphology and provide reliable electrical connection to the current collector. The promising approach is to incorporate MnO2 nanostructures or nanometer-thick thin films into the highly conductive materials,[18-25] such as carbon-based materials and conducting polymers, and on a three 3
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dimensional (3D) scaffold.[26-30] A highly conductive three dimensional (3D) continuous porous scaffold not only provide high
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surface area for loading electroactive materials, but also facilitate the fast electron transport from electroactive materials to the current collector. Moreover, no binder and conductive agents are added during the preparation process of the electrodes. ZnO, SnO2 and Zn2SnO4 nanowire arrays have been
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reported as the scaffold for loading electroactive materials as electrochemical capacitor electrodes.[26-28] However, the electrical conductivity of these oxides is not as good as the stable
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metals, such as, Au and Ni. 100-nm-thick nano-porous Au films have been fabricated for enhancing the electron and electrolyte ion transport,[29] whereas this nano-porous structure severely inhibits the mass loading of active materials and the electrolyte ion transport. Additionally, the rareness and high cost of Au and Ag restrict the application. A self-assembled 3D bicontinuous Ni
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nanoarchitecture, fabricated by electrodepositing Ni on self-assembled opal templates and subsequent removing the templates, have exhibited very large battery charge and discharge rate with minimal capacity loss for lithium-ion and nickel-metal hydride batteries.[30] But the preparation
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process is so complex. Herein, we report a facile method to synthesis a continuous 3D Ni porous network as the current collector for loading electroactive materials as electrochemical capacitor
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electrode. This 3D current collector is with large surface area to increase the mass loading of active materials, with the porous structure to facilitate the fast electrolyte ion transport, and with a highly conductive network to promote the electron transport from active material layer to the current collector. 2. Experimental 2.1 Growth of MnO2 nanosheets on a continuous three dimensional (3D) Ni porous scaffold 4
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In a typical procedure, the planar conducting substrates are prepared via sputtering 10 nm Ti and followed by 100 nm Au films on one side of glass slides. Another side was glued by scotch tape.
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NiCl2·6H2O (0.125 M) and hexamethylenetetramine (HMT, 0.25 M) were dissolved in 10 mL of DI water, and transferred to a 20 mL Teflon-lined stainless-steel autoclave. The planar conducting substrates were hanged in the aqueous solution. Then, Teflon-lined stainless-steel autoclave was
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sealed, maintained at 100 oC for 10 h, and allowed to cool to room temperature. The light green products on the substrates were rinsed several times with distilled water and ethanol, and dried in
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vacuum oven. Subsequently, the substrates were thermally treated under H2/Ar mixed gas (7.0 vol%) at 400 oC for 1 h with the heating rate of 5 oC min-1. Followed by electrodepositing MnO2, the substrates were as the working electrode. A platinum foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. 20 mL aqueous solution of
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Mn(CHCOO)2 (50 mM) and Na2SO4 (100 mM) was used as the electrolyte. The electrodeposition process was carried out via the linear sweep voltammetry (LSV) method ranging from 0.4 to 1.3 V at various scan rates (2, 5, 10, 30, 50 and 100 mV s-1). The as-formed samples were named as
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(MnO2/Ni)9, (MnO2/Ni)18, (MnO2/Ni)30, (MnO2/Ni)90, (MnO2/Ni)180, and (MnO2/Ni)450, according to the electrodeposition time at the scan rates of 100, 50, 30, 10, 5, and 2 mV s-1, respectively.
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According to the following equations: m=
( ∫ IdU ) M nFν
·········(I)
where I is the current, U is the potential window, ν is the scan rate, m is the electrodeposition mass of MnO2, n is electron number per mole, F is Faraday constant (96485.2 C mol-1), and M is molar mass of MnO2 (86.9 g mol-1), the MnO2 mass is directly proportional to the passed deposition charge, and thus is closely related to the scan rate and deposition current. 5
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2.2 General materials characterization. The product morphologies were directly examined by scanning electron microscopy (SEM) using
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JEOL JSM-6700F at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) observations were carried out on a JEOL 2010 microscope operating both at 200 kV. X-ray diffraction (XRD) was performed on a PANalytical X’pert Pro X-ray diffractometer with Cu kα
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irradiation (λ=1.5406 Å). The step size and scan rate are set as 0.05o and 0.025 o s-1, respectively.
2.3 Electrochemical measurements.
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The working electrode was prepared by electrodepositing MnO2 on porous Ni nanowall. The electrolyte used was 1.0 M Na2SO4 aqueous solution. The capacitive performance of the samples was evaluated on a CHI 660D electrochemical workstation using cyclic voltammetriy (CV) and chronopotentiometry test with a three-electrode cell where Pt foil serves as the counter electrode and
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Ag/AgCl (saturated KCl solution) as the reference electrode. The specific capacitance values were calculated from the galvanostatic charge and discharge curves, using the following equation: C=(I∆t)/(m∆V), where I is a charge or discharge current, ∆t is the charge or discharge time, m
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or discharge.
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indicates the mass of electroactive materials, and ∆V represents the voltage change after a full charge
3. Results and Discussion
Our strategy for electrodepositing MnO2 active materials on a continuous 3D porous Ni network is shown schematically in Figure 1. First of all, preparation of 3D continuous porous Ni network involved the precipitation of Ni(OH)2 precursors on the planar conducting substrate in aqueous solution, induced by the hydrolysis of hexamethylenetetramine (HMT) (Step I in Figure 1), and the subsequent reduction process under H2/Ar atmosphere. (Step II in Figure 1). After precipitating Ni 6
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salts on the planar conducting substrate, the light green film was found to be uniformly grown on the planar conducting substrate. As seen from SEM image (Figure 2A), the microstructure is consisted of
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the connected sheets with micro-scale length and less than 10 nm in thickness. The connected sheets grow vertically and cross-linked on the planar conducting substrate, forming a compact film with obvious holes. The corresponding XRD pattern in inset of Figure 2A exhibits a set of characteristic
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diffraction peaks, which could be indexed as a pure α-Ni(OH)2 (JCPDS 38-0715). The diffraction peaks 2θ=7.8o, 33.4o, and 60.0o should be corresponding to (002), (101), and (110) crystal planes of
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α-Ni(OH)2, respectively. After the thermal treatment under H2/Ar atmosphere, Ni(OH)2 nanowall precursors are reduced into a 3D continuous Ni porous network, in accompany with the escape of water vapor (Figure 2B), of which the XRD pattern in inset of Figure 2B is in consistent with the standard pattern of Ni (JCPDS 04-0850). The porous Ni network with around 6 µm in thickness
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uniformly grown on the substrates is composed of the connected ~100 nm Ni nanoparticles (Figure 2C and D). The component Ni nanoparticles are in single crystalline structure (Figure 2E and F). The lattice fringes distance about 0.203 nm in Figure 2F should be corresponding to the spacing of (111)
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crystal face.
Then, a continuous 3D Ni porous network as the electrodes, a thin MnO2 layer was directly and
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uniformly electrodeposited on it, of which the thicknesses could be controlled by the electrodeposition parameters (Step III in Figure 1). Figure 3 shows TEM images and EDX spectra of (MnO2/Ni)9 electrode. The as-formed MnO2 film is composed of the nanosheets, as seen from TEM image (Figure 3A). The EDX pattern in Figure 3B clearly reveals that besides Cu element from Cu grid, Mn and Ni metallic elements are also detected, which should be ascribed to MnO2 active materials and Ni scaffold, respectively. It’s found that Ni elements locate in the interior (Figure 3C), 7
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and Mn elements mainly distribute in the exterior (Figure 3D), suggesting to form MnO2-Ni core-shell nanostructure. In the SAED pattern (Figure 3E), the diffraction rings represent the
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diffraction of (301), (401) and (511) crystal planes of MnO2, whereas the diffraction dots belong to the diffraction of single crystalline Ni. The HRTEM image is in agreement with the diffraction rings in the SAED pattern, in which the lattice spacing of 0.254 and 0.208 nm match up well with the
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inter-planar spacings of (301) and (401) planes of MnO2 (Figure 3F).
The loading mass and thickness of MnO2 electrodeposited on Ni network can be adjusted via
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altering the electrodeposition charge. Figure 4 shows SEM images of a series MnO2 layers electrodeposited on 3D Ni porous network at various scan rates. At a scan rate of 30 mV s-1, MnO2 active materials just fully cover on the surface of smooth Ni nanoparticles (Figure 4A). With decreasing the scan rates, more and more MnO2 active materials are electrodeposited in the void of
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porous Ni network (Figure 4A-D). At a scanning rate of 2 mV s-1, MnO2 active materials almost fill up in the void of the 3D porous scaffold (Figure 4D). According to the equation I, the mass loading of MnO2 electrodeposited on Ni network is calculated as follows: 0.033 ((MnO2/Ni)9), 0.046
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((MnO2/Ni)18) 0.072 ((MnO2/Ni)30), 0.136 ((MnO2/Ni)90), 0.318 ((MnO2/Ni)180), and 0.714 mg cm-2 ((MnO2/Ni)450), respectively. In a control experiment, under the same electrodeposition condition, a
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uniform MnO2 layer was also deposited on the planar conducting substrate with the same morphology as that grown on 3D scaffold (Figure SI-1). The mass loading of MnO2 grown on the planar conducting substrates is 0.017, 0.028, 0.031, 0.062, 0.148, and 0.437 mg cm-2 corresponding to the electro-deposition time of 9, 18, 30, 90, 180 and 450 s, respectively. The results show that the 3D continuous porous Ni network provides much more surface area for depositing MnO2 active materials, and thus can load almost two times as much as MnO2 mass formed on the planar 8
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conducting substrate under the same electrodeposition condition. The chemical composition of the as-deposited MnO2 at various scan rates can be surveyed by
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X-ray photoelectron spectroscopy (XPS) measurement. Typical XPS spectra of Mn 2p and O 1s for MnO2 electrodeposited at the scan rates of 100 and 2 mV s-1 are shown in Figure 5. The almost identical XPS spectra indicate that the mean oxidation state of Mn is not affected by changing the
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scan rates. The broad peaks of Mn 2p3/2 and 2p1/2 are located around 642.2 eV and 653.7 eV, respectively, revealing Mn4+ ions were dominant in the products.[17, 31, 32] In addition, the
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difference of peak separation between Mn 2p3/2 and 2p1/2 is about 11.5 eV, further supporting the above statement, since the binding energy difference of Mn 2p3/2 and 2p1/2 can be used to indicate the oxidation state of Mn.[32] In the spectra of O 1s, the peaks at 529.7, 530.9, and 531.8 eV should be ascribed to the Mn-O-Mn, Mn-O-H, and H-O-H, respectively. [33, 34] The detection of Mn-O-H and
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H-O-H species reveals the hydrous nature. Above results reveal that the as-deposited active materials should be MnO2·nH2O, in accordance with the SAED and HRTEM results, and the mean oxidation state of Mn is not affected by altering the scan rates.
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To test the electrochemical capacitor performance, the (MnO2/Ni) electrodes were used as the working electrodes in a three-electrode configuration. Cyclic voltammetry (CV) curves in Figure 6A
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were obtained by sweeping the voltage from 0 to 0.8 V vs Ag/AgCl (sat. KCl) in 1.0 M Na2SO4 electrolyte. The CV curves exhibit the quasi-rectangular shape, a typical capacitive behavior of MnO2, indicating the ideal electrical double-layer capacitance behavior and fast charging/discharging process characteristic. The energy storage contributions come from the surface adsorption/desorption of the electrolyte cations (H3O+ and Na+) on the Ni network and MnO2, and the reversible redox reaction by mean of the electrolyte cations expelling/intercalating from/into MnO2.[17, 35, 36] In 9
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comparison with that deposited on the planar conducting substrate, MnO2 deposited on the 3D Ni porous scaffold exhibited better quasi-linear dependence of the capacitive current density on the scan
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rate of 2 to 100 mV s-1 (Figure 6B), suggesting better rate capacitive performance.[37] It’s ascribed by that the good conductive porous Ni network structure can facilitate the transportation of electron and the electrolyte, allowing full access of the electrolyte to electrode materials and maximizing the
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utilization of MnO2.
Rate capability is one of the important factors for evaluating the power applications of
as-prepared
(MnO2/Ni)9
electrode
at
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electrochemical capacitors. Figure 6C shows the galvanostatic charge-discharge curves of the different
charge-discharge
current
densities.
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charging/discharging cycling curves have a very symmetric nature, even at a low density of 2 A g-1, indicating again that the electrode has a good electrochemical capacitive characteristic and reversible
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redox reaction. The overall specific capacitance mainly come from the contribution of MnO2 active materials, since the Ni porous network only exhibited a specific capacitance of 0.12 mF cm-2 at a current density of 1 mA cm-2 (Figure SI-2). Figure 6D shows the mass-specific capacitance of MnO2
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deposited on the planar conducting substrate and a continuous 3D Ni porous scaffold. The specific capacitance of (MnO2/Ni)9 electrode at a current density of 2 A g-1 can be up to 1169.6 F g-1 based on
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the mass of MnO2. Even at a high current density of 20 A g-1, the specific capacitance can remain at 642.9 F g-1. Prolonging the electrodeposition time increases the mass loading of MnO2 active materials, but deteriorates the specific capacitance performance, which is in the sequence of (MnO2/Ni)9 > (MnO2/Ni)18 > (MnO2/Ni)30 > (MnO2/Ni)90 > (MnO2/Ni)180 > (MnO2/Ni)450. In contrast, on the planar conducting substrates, MnO2 active materials exhibit poor specific capacitance performance under the identical electrodeposition condition (Figure 6D). The improvement of the 10
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specific capacitance can be attributed to the reduced diffusion path of electrolyte ions, highly accessible surface area and increased electrical conductivity by utilizing porous Ni scaffold for
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depositing MnO2, instead of the planar conducting substrates. In addition, it’s found that, with low mass loading of MnO2, the highly conductive porous Ni network as the current collectors can provide the good opportunity for electron transportation and ions accessibility, and maximize the
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utilization of MnO2 materials, and thus resulting in large difference in specific capacitance between that on the planar conducting substrates and the 3D Ni porous scaffold. While at high mass loading
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devices, negligible change in specific capacitance was observed at high current densities mainly due to the limited conductivity of high mass loading samples and the limited utilization of MnO2 at high current densities.
The area-specific capacitance performance of MnO2 deposited on the planar conducting
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substrate and the continuous 3D Ni porous scaffold is shown in Figure 6E. Under the identical electrodeposition conditions, the area-specific capacitances of MnO2 deposited on 3D Ni porous scaffold are much higher than that of MnO2 deposited on the planar conducting substrates, since 3D
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Ni porous scaffold with larger surface area can load more MnO2 mass and exhibit better electron and electrolyte transport. Although the area-specific capacitance of the (MnO2/Ni) electrode with the
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shortest deposition time (9 s) is as low as 11.6 mF cm-2 at a current density of 0.5 mA cm-2, it’s still over two times as that of MnO2 deposited on the planar conducting substrates (4.5 mF cm-2). With increasing the electrodeposition times, more and more MnO2 active materials are deposited on the 3D Ni porous scaffold and the planar conducting substrates, resulting in exhibiting higher and higher area-specific capacitance. The highest area-specific capacitance of 121.6 mF cm-2 is obtained from the (MnO2/Ni)450 electrode at a current density of 0.5 mA cm-2, approximately 2.5 times the value for 11
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MnO2 electrode deposited on the planar conducting substrates under the same electrodeposition condition. The area-specific capacitance performance is also higher than the reported values for
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MnO2/carbon nanoparticles,[31] MnO2/carbon nanofoam,[38] and carbon-based electrodes.[39-42] Besides the specific capacitance, another important requirement for electrochemical capacitor applications is cycling stability. The cycling stability tests over 1000 cycles at a current density of 4
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A g-1 were carried out using constant-current galvanostatic charge-discharge cycling techniques in the potential windows ranging from 0 to 0.8 V. Figure 6F shows the specific capacitance retention of
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the MnO2 electrodeposited on the 3D Ni porous network and the planar conducting substrates as a function of charge/discharge cycling numbers. MnO2 active materials grown on the 3D Ni porous network only exhibited ~ 5% loss of the specific capacitance, whereas MnO2 electrodeposited on the planar conducting substrate under the same electrodeposition condition have around 20 % loss after
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1000 repetitive charge-discharge cycles. Thus, MnO2 active materials deposited on the 3D Ni scaffold exhibited better long-term cyclability, in comparison with that grown on the planar conducting substrate.
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The internal resistance of electrochemical capacitor is of great importance in energy storing devices. For low internal resistance of electrochemical capacitor, less energy will be wasted to
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produce unwanted heat and electronic transportation during charging/discharging processes. We can see a sudden IR drop at the beginning of the discharge curves, and the increase of the IR drop, in accompany with the increase of the discharge current density (Figure 6C). Figure 7A summarizes the IR drop values of the (MnO2/Ni) electrodes. With the increase of the discharge current density, the IR drop becomes larger, meaning that the energy loss during the charge/discharging process gradually increases. The smallest IR drop is observed for (MnO2/Ni)9 electrode among the (MnO2/Ni) 12
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electrodes, indicating a highly conductive characteristic. Prolonging the electrodeposition time enhances the mass loading and thickness of MnO2, and thus results in the increase of the slop of the
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plots (IR drop versus current densities). To further understand the electrochemical performance characteristics of (MnO2/Ni) electrodes, we resorted to electrochemical impedance spectroscopy (EIS) carried out at open circuit potential
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with an ac perturbation of 5 mV in the frequency range of 1000 kHz-10 mHz. Figure 7B shows the Nyquist plots thus obtained. The EIS data were fitted based on an equivalent circuit model consisting
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of bulk solution resistance Rs, charge-transfer resistance Rct, double-layer capacitance Cdl, and Warburg resistance (W), as shown in inset of Figure 7B. The bulk solution resistance Rs and charge-transfer resistance Rct can be estimated from the intercepts of the semicircle with the real axis at high and low frequencies, respectively.[43-45] The slope of the linear portion of the curve on the
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right of the semicircle is called Warburg resistance (W), which is a result of the frequency dependent ion diffusion/transport in the electrolyte toward the electrode surface.[46] A most striking difference lies in the semicircle corresponding to the charge-transfer resistance (Rct) caused by the Faradic
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reactions and the double-layer capacitance (Cdl) on the grain surface, whereas Rct increases gradually from 0.43 ((MnO2/Ni)9), 0.78 ((MnO2/Ni)18), 1.20 ((MnO2/Ni)30), 1.40 ((MnO2/Ni)90), 1.91
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((MnO2/Ni)180), to 2.82 Ω((MnO2/Ni)450) with increasing the mass loading of MnO2. The IR drop curves and EIS data clearly demonstrate that (MnO2/Ni) electrodes display and a decreasingly internal resistance and an increasingly favorable charge-transfer kinetics as the mass loading of MnO2 gradually decreases, in excellent agreement with the trend of their largely enhanced specific capacitances (Figure 6D). In comparison with that on the planar conducting substrate, the architecture structure of a thin 13
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MnO2 layer electrodeposited on a continuous 3D porous Ni network has several features for improving the electrochemical performance as follows: (I) The continuous 3D porous Ni network
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can provide much more surface area for depositing MnO2, resulting in higher mass loading under the same electrodeposition condition. (II) The porous structure of the (MnO2/Ni) electrodes is feasible for the fast transport of hydrated ion in the electrolyte. (III) The thin layer of MnO2 enables the fast
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and reversible Faradic reaction by shortening the ion diffusion path. (IV) The continuous Ni bridge may provide highly conductive channels for effective electron transport from MnO2 layer to the
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current collector.
4. Conclusions
In sum, we have successfully developed a facile method to synthesis a novel architecture by electrodepositing MnO2 nanosheets to a three-dimensional continuous porous Ni network as high
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performance electrochemical capacitor electrode. The three dimensional continuous Ni porous network are grown on the planar conducting substrate via the in-situ growth of Ni(OH)2 nanowalls and the subsequent reduction process, which as the highly conductive scaffold support a highly
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electrolytic accessible surface area of redox active MnO2 shells and provide reliable electrical connections to the MnO2 shells. The continuous three dimensional Ni porous network not only
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increases the mass loading of MnO2 around two times as much as the planar electrode, but also facilitates the facile transport of electron and electrolyte ion, and thus enhance the specific capacitance from 611.6 to 1169.6 F g-1 at a current density of 2 A g-1 and improve cycle stability from ~ 20 % to 5 % loss after the 1000 repetitive charge-discharge processes. These results suggest that such architecture is very promising for next generation high-performance electrochemical capacitor. 14
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Acknowledgments This research was supported by National Natural Science Foundation of China (Project No.
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51173055), and the Fundamental Research Funds for the Central Universities (2013QN158).
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Figure captions
Figure 1. Schematic graph of the formation process of MnO2 active materials deposited on a
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continuous three dimensional Ni porous network. Step I: Ni(OH)2 nanowall network was directly grown on the planar conducting substrate. Step II: Under H2/Ar atmosphere, Ni(OH)2 nanowall precursors were reduced into a Ni porous network (black) composed of the connected Ni
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nanoparticles. Step III: Electrodepositing MnO2 thin films (blue) on the surface of the component Ni nanoparticles of the three dimensional Ni porous network. The electron could be quickly transported
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from the interface between MnO2 and electrolyte to the current collector via the continuous Ni network.
Figure 2. Characterizations of Ni(OH)2 nanowalls and Ni porous networks grown on the planar
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conducting substrates: (A) SEM image of Ni(OH)2 nanowalls (Inset: the corresponding XRD pattern); and (B-D) SEM and (E, F) TEM images of Ni porous network. (Inset of B: the
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corresponding XRD pattern)
Figure 3. (A) TEM image, (B) EDX pattern, (C, D) Ni and Mn mapping, (E) SAED pattern, and (F)
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HRTEM image of the (MnO2/Ni)9 electrode.
Figure 4. SEM images of the (MnO2/Ni) electrodes formed at various electrodeposited conditions: (A) (MnO2/Ni)30; (B) (MnO2/Ni)90; (C) (MnO2/Ni)180; and (D) (MnO2/Ni)450.
Figure 5. (A) Mn 2p and (B) O 1s XPS spectra of MnO2 deposited on a continuous three dimensional (3D) Ni porous scaffold at the scan rates of 100 and 2 mV s-1.
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Figure 6. Electrochemical characterizations of MnO2 active materials deposited on a continuous three dimensional (3D) Ni porous network in a three-electrode configuration: (A)
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CV curves of the (MnO2/Ni)9 electrode at various scan rates; (B) The quasi-linear dependence curve of the current density at 0.5 V versus the scan rate for MnO2 electrodeposited on the planar conducting substrate and 3D Ni scaffold at a scan rate of 100 mV s-1; (C) Galvanostatic
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charge-discharge curves of the (MnO2/Ni)9 electrode at various current densities; The mass (D) and area (E)-specific capacitance performance of a series of MnO2 on the planar conducting substrates
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(Solid) and the 3D Ni porous scaffold (Hollow); and (F) Normalized specific capacitance versus cycle number at a galvanostatic charge-discharge current density of 4 A g-1.
Figure 7. (A) IR drop against the discharge current densities, and (B) EIS Nyquist plots for the
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(MnO2/Ni) electrodes (Inset: Equivalent circuit diagram proposed for analysis of the EIS data).
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Supporting Information
Porous Scaffold for High Performance Electrochemical Capacitors
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Electrodeposition of Manganese Oxide Nanosheets on a Continuous Three-Dimensional Nickel
Junwu Xiao,* Shengxiong Yang, Fei Xiao, Lian Wan, Shuai Wang*
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Department of Chemistry and Chemical Engineering, Hubei Key Laboratory of Matieral Chemistry and Service Failure, Key Laboratory for Large-Format Battery Materials and System, Ministry of
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Education, Huazhong University of Science & Technology, Wuhan 430074, PR China
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Email: [email protected], [email protected]
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Figure SI-1. (A) SEM and high magnification SEM images of MnO2 film directly deposited on the planar conducting substrate.
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Figure SI-2. The galvanostatic charge-discharge curves of the three dimensional Ni porous scaffold
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at a current density of 1 mA cm-2.
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Highlights A facile method to synthesize a continuous three dimensional Ni porous scaffold.
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The controllable deposition of MnO2 active layer on a continuous three
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dimensional Ni porous scaffold.
The three dimensional Ni porous scaffold electrode with high surface area
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promises high mass loading of MnO2 and with the hierarchal porous structure facilitates the fast transport of electron and electrolyte ion, thus achieving a high
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electrochemical capacitor performance.
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·