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  • Electrodeposition of nanostructured MnO2 electrode on three-dimensional nickel/silicon microchannel plates for miniature supercapacitors

    Yuwei Xu a, Shaohui Xu a,n, Mai Li a,b, Yiping Zhu a, Lianwei Wang a,b,nn, Paul K. Chu b

    a Key Laboratory of Polar Materials and Devices, Ministry of Education and Department of Electronic Engineering, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China b Department of Physics and Material Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

    a r t i c l e i n f o

    Article history: Received 8 March 2014 Accepted 3 April 2014 Available online 13 April 2014

    Keywords: Deposition Manganese dioxide (MnO2) Thin films Silicon microchannel plates (Si-MCPs) Supercapacitor

    a b s t r a c t

    Manganese-dioxide is electrodeposited on three-dimensional nickel/silicon microchannel plates (Si-MCPs) to produce miniature supercapacitors. The nanostructured MnO2 thin film can be clearly observed from the channels and largely contribute to the capacitance. The compositions and morphology are characterized by X-ray diffraction (XRD) and field emission scanning electron micro- scopy (FE-SEM). The electrochemical properties are investigated by cyclic voltammetry (CV), galvano- static charging–discharging, and electrochemical impedance spectroscopy (EIS) in a neutral 1 M Na2SO4 solution which preserves the structure of the substrate. The capacitance is 0.961 F/cm2 or 323.1 F/g and the retention ratio is 91.1% after 1000 cycles thereby demonstrating the robustness and durability.

    & 2014 Elsevier B.V. All rights reserved.

    1. Introduction

    Three-dimensional (3D) substrates with high surface-to- volume ratios are highly desirable in energy storage/conversion devices such as Li-ion batteries and supercapacitors [1–3]. Posses- sing nanoscale arrays, 3D silicon microchannel plates (Si-MCPs) offer good space management and large energy densities within a small footprint. Electrodeposition of cobalt hydroxide {Co(OH)2} and nickel hydroxide {Ni(OH)2} on Si-MCPs has been demon- strated to produce large capacitance but special precaution must be exercised to avoid materials degradation in the alkaline medium.

    MnO2 has many advantages such as low cost, high charge capacity, and environment friendliness. Nanostructured MnO2 can be deposited by many techniques and among them, electrodeposi- tion is a simple, template-free, controllable, and effective means [4,5]. In this work, manganese dioxide (MnO2) is deposited onto Si-MCPs and Na2SO4, which does no harm to the silicon skeleton, is adopted as the electrolyte to deliver stable performance.

    2. Experimental details

    Si-MCPs were fabricated on p-type silicon by a series of MEMS processes including thermal oxidation, standard photolithography, wet etching, and electrochemical etching [6]. The Si-MCPs were arranged in a square array with 200 μm deep channels, 5�5 μm2 pores, and 1 μm thick wall with an intrinsic resistance on the order of kΩ.

    In order to improve the conductivity, nickel was electroplated on the Si-MCPs to serve as the current collector and form an adhesive metal layer. The chemical reagents were of analytical (AR) grade and used without further purification. The aqueous solutions were prepared with 18 MΩ de-ionized water and all the experiments were carried out in a clean room at 297 K. After nickel electroplating, the sample was dried at 90 1C in a vacuum oven for 2 h for stabilization. Electrodeposition was conducted in an elec- trolyte composed of 0.1 M Mn(CH3COO)2 and 0.1 M Na2SO4. The current density was set at 0.2 A/cm2 and temperature was kept at 2571 1C. After MnO2 electrodeposition for 30 min, the sample was rinsed in deionized water for 10 min, dried at 90 1C to a constant mass in order to determine precisely the mass of MnO2 deposited on the Ni/Si-MCPs. Afterwards, the sample was fixed with a copper sheet and glue tape, leaving approximately an exposed active area of 0.2 cm2. The experiments were repeated on a planar silicon substrate for comparison.

    The morphology of the nanoscale manganese dioxide was exam- ined by field-emission scanning electron microscopy (FE-SEM, Hitachi

    Contents lists available at ScienceDirect

    journal homepage: www.elsevier.com/locate/matlet

    Materials Letters

    http://dx.doi.org/10.1016/j.matlet.2014.04.034 0167-577X/& 2014 Elsevier B.V. All rights reserved.

    n Corresponding author. Tel.: þ86 21 54345160; fax: þ86 21 54345119. nn Corresponding author at: Key Laboratory of Polar Materials and Devices,

    Ministry of Education and Department of Electronic Engineering, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China. Tel.: þ86 21 54345160; fax: þ86 21 54345119.

    E-mail addresses: shxu@ee.ecnu.edu.cn (S. Xu), lwwang@ee.ecnu.edu.cn (L. Wang).

    Materials Letters 126 (2014) 116–118

    www.sciencedirect.com/science/journal/0167577X www.elsevier.com/locate/matlet http://dx.doi.org/10.1016/j.matlet.2014.04.034 http://dx.doi.org/10.1016/j.matlet.2014.04.034 http://dx.doi.org/10.1016/j.matlet.2014.04.034 http://crossmark.crossref.org/dialog/?doi=10.1016/j.matlet.2014.04.034&domain=pdf http://crossmark.crossref.org/dialog/?doi=10.1016/j.matlet.2014.04.034&domain=pdf http://crossmark.crossref.org/dialog/?doi=10.1016/j.matlet.2014.04.034&domain=pdf mailto:shxu@ee.ecnu.edu.cn mailto:lwwang@ee.ecnu.edu.cn http://dx.doi.org/10.1016/j.matlet.2014.04.034

  • S-4800, Japan) and the crystal structure was determined by X-ray diffraction (XRD, Rigaku, RINT2000, Japan). A three-electrode electro- chemical working station (CHI660D, Chenhua, Shanghai) was used for the electrochemical measurements in 1 M Na2SO4 aqueous electrolyte. The as-prepared MnO2/Ni/Si-MCPs acted as the working electrode while the platinum wire electrode and saturated calomel electrode (SCE) were the counter electrode and reference electrode, respectively.

    3. Results and discussion

    The XRD pattern acquired from MnO2/Ni/Si-MCPs sample is depicted in Fig. 1(a). The peaks of the Si-MCPs with high inten- sities are not displayed completely due to stray signals from other materials. The XRD pattern of the Ni/Si-MCPs without deposition of MnO2 is shown for comparison. The two characteristic peaks of (311) and (440) at about 371 and 661 are indexed to the birnessite- type MnO2 close to γ-MnO2 (PDF# 42-1317) [7] and the weak peak at 121 is contributed by MnO2.

    The morphology of MnO2/Ni/Si-MCPs and Ni/Si-MCPs samples is shown in Fig. 1(b). Nickel covers the sidewalls of the Si-MCPs smoothly and the resistance of the whole structure is less than 1Ω. There are dense and intertwined MnO2 nano-flakes with a semicircular shape in the MnO2/Ni/Si-MCPs. The lamellar MnO2/ Ni/Si-MCPs provide adequate sites for charging/discharging in the solution and decrease the contact resistance. The nanoporous thin film offers a short diffusion distance and large contact area. This framework facilitates ion access thus forming a favorable mor- phological foundation for high specific capacitance and excellent electrochemical performance.

    CV was performed at different scanning rates to gauge the capacitive performance of the electrode and the specific capacitance

    Fig. 1. (a) XRD patterns for MnO2/Ni/Si-MCPs and Ni/Si-MCPs; (b) SEM images of Ni/Si-MCPs [top view (A), cross-sectional view (B)] and MnO2/Ni/Si-MCPs [top view (C–E) and cross-sectional view (F)].

    Fig. 2. (a) CV curves of MnO2/Ni/Si-MCPs acquired at different scanning rates; (b) CV curves of MnO2/Ni/Si-MCPs and MnO2/Ni/Si at a scanning rate of 10 mV/s; (c) charging– discharging curve of the MnO2/Ni/Si-MCPs at a current rate of 2 mA; (d) first discharge curves of MnO2/Ni/Si-MCPs at different current densities.

    Y. Xu et al. / Materials Letters 126 (2014) 116–118 117

  • of the electrode can be calculated from the charge–discharge curves [8]. Fig. 2(a) depicts the CV curves of the MnO2/Ni/Si-MCPs in the potential range between �0.6 and 0.4 V for scanning rates of 25 to 50, 100, and 200 mV/s. The curves display a rectangular shape thus corroborating the capacitive behavior and electrochemical stability. Fig. 2(b) shows that the 3D substrate is superior to planar Si in energy storage. The MnO2/Ni/Si-MCPs provide a considerably larger capacity since the enclosed area of the CV curve of the MnO2/Ni/Si- MCPs is larger than that of MnO2/Ni/Si. Fig. 2(c) shows the charging– discharging behavior of the MnO2/Ni/Si-MCPs electrode at a constant current rate of 2 mA. Linear and symmetrical characteristics are observed. Fig. 2(d) displays the first discharge curves of the MnO2/Ni/ Si-MCPs at different current densities.

    Previous studies on Si-MCPs supercapacitors have mainly focused on Ni(OH)2, Co(OH)2, or other composite materials [9]. The common requirement is that an alkaline electrolyte is needed to maintain the capacitive stability. In comparison, this MnO2- based electrode operates in a neutral electrolyte which does not affect the stability of the substrate. The effective mass of MnO2 deposited on the Ni/Si-MCPs is 0.6 mg and the specific capacitance is calculated to be 0.961 F/cm2 (323.1 F/g).

    CV tests are conducted at a sweeping rate of 80 mV/s on the MnO2/Ni/Si-MCPs to investigate the long-term capacitance stability. Our data reveal no obvious mass loss after 1000 cycles. Fig. 3 (a) indicates a 91.1% capacitance retention ratio and the loss after 500 cycles is only 1.4% which is much better than those observed from Ni(OH)2 (6.4%) and Co(OH)2 (16.1%).

    EIS is conducted on the MnO2 electrode with AC perturbation of 5 mV from 0.01 Hz to 100 KHz. Fig. 3(b) shows the EIS pattern obtained from the MnO2/Ni/Si-MCPs. The electrode

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