Electrodeposition of nanostructured MnO2 electrodeon three-dimensional nickel/silicon microchannel platesfor 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, 500Dongchuan Road, Shanghai 200241, Chinab 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 2014Accepted 3 April 2014Available online 13 April 2014

Keywords:DepositionManganese dioxide (MnO2)Thin filmsSilicon 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 clearlyobserved from the channels and largely contribute to the capacitance. The compositions andmorphology 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 Na2SO4solution which preserves the structure of the substrate. The capacitance is 0.961 F/cm2 or 323.1 F/g andthe 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/conversiondevices 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 asmall 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 precautionmust be exercised to avoid materials degradation in the alkalinemedium.

MnO2 has many advantages such as low cost, high chargecapacity, and environment friendliness. Nanostructured MnO2 canbe 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 ontoSi-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 MEMSprocesses including thermal oxidation, standard photolithography,wet etching, and electrochemical etching [6]. The Si-MCPs werearranged in a square array with 200 μm deep channels, 5�5 μm2pores, and 1 μm thick wall with an intrinsic resistance on theorder of kΩ.

In order to improve the conductivity, nickel was electroplatedon the Si-MCPs to serve as the current collector and form anadhesive metal layer. The chemical reagents were of analytical(AR) grade and used without further purification. The aqueoussolutions were prepared with 18 MΩ de-ionized water and all theexperiments were carried out in a clean room at 297 K. After nickelelectroplating, the sample was dried at 90 1C in a vacuum oven for2 h for stabilization. Electrodeposition was conducted in an elec-trolyte composed of 0.1 M Mn(CH3COO)2 and 0.1 M Na2SO4. Thecurrent density was set at 0.2 A/cm2 and temperature was kept at2571 1C. After MnO2 electrodeposition for 30 min, the samplewas rinsed in deionized water for 10 min, dried at 90 1C to aconstant mass in order to determine precisely the mass of MnO2deposited on the Ni/Si-MCPs. Afterwards, the sample was fixedwith a copper sheet and glue tape, leaving approximately anexposed active area of 0.2 cm2. The experiments were repeatedon 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

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.04.0340167-577X/& 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 ChinaNormal 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

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S-4800, Japan) and the crystal structure was determined by X-raydiffraction (XRD, Rigaku, RINT2000, Japan). A three-electrode electro-chemical working station (CHI660D, Chenhua, Shanghai) was used forthe electrochemical measurements in 1 M Na2SO4 aqueous electrolyte.The as-prepared MnO2/Ni/Si-MCPs acted as the working electrodewhile 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 isdepicted in Fig. 1(a). The peaks of the Si-MCPs with high inten-sities are not displayed completely due to stray signals from othermaterials. The XRD pattern of the Ni/Si-MCPs without depositionof 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 peakat 121 is contributed by MnO2.

The morphology of MnO2/Ni/Si-MCPs and Ni/Si-MCPs samplesis shown in Fig. 1(b). Nickel covers the sidewalls of the Si-MCPssmoothly and the resistance of the whole structure is less than1Ω. There are dense and intertwined MnO2 nano-flakes with asemicircular shape in the MnO2/Ni/Si-MCPs. The lamellar MnO2/Ni/Si-MCPs provide adequate sites for charging/discharging in thesolution and decrease the contact resistance. The nanoporous thinfilm offers a short diffusion distance and large contact area. Thisframework facilitates ion access thus forming a favorable mor-phological foundation for high specific capacitance and excellentelectrochemical performance.

CV was performed at different scanning rates to gauge thecapacitive 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 ofNi/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 thepotential range between �0.6 and 0.4 V for scanning rates of 25 to50, 100, and 200 mV/s. The curves display a rectangular shape thuscorroborating the capacitive behavior and electrochemical stability.Fig. 2(b) shows that the 3D substrate is superior to planar Si inenergy storage. The MnO2/Ni/Si-MCPs provide a considerably largercapacity 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 constantcurrent rate of 2 mA. Linear and symmetrical characteristics areobserved. 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 mainlyfocused on Ni(OH)2, Co(OH)2, or other composite materials [9].The common requirement is that an alkaline electrolyte is neededto maintain the capacitive stability. In comparison, this MnO2-based electrode operates in a neutral electrolyte which does notaffect the stability of the substrate. The effective mass of MnO2deposited on the Ni/Si-MCPs is 0.6 mg and the specific capacitanceis calculated to be 0.961 F/cm2 (323.1 F/g).

CV tests are conducted at a sweeping rate of 80 mV/s on theMnO2/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 after500 cycles is only 1.4% which is much better than those observedfrom Ni(OH)2 (6.4%) and Co(OH)2 (16.1%).

EIS is conducted on the MnO2 electrode with AC perturbationof 5 mV from 0.01 Hz to 100 KHz. Fig. 3(b) shows the EIS patternobtained from the MnO2/Ni/Si-MCPs. The electrode resistance isnearly 2.4Ω indicating the highly conductive nature. The equiva-lent circuit in the inset is fitted to the impedance spectra. In thehigh-frequency region, the mechanism is controlled by diffusionrate and the penetrating distance of ions decreases making theelectrode resistive and displaying an arc shape in the Nyquist plot.As the frequency diminishes, the availability of nanoporesincreases so that the electrode is considered to be capacitive[10]. The table in Fig. 3(b) lists the elements in the equivalentcircuit, where Rs is the resistance of the solution, Cdl represents thedouble layer capacitance, Rct is the charge transfer resistance, andCPE-P reflects the diffusive resistance of ions and slope of theimpedance plot [11]. A CPE-P value of nearly 0.75 indicates fast ion

diffusion through the channels of the Si-MCPs due to nanostruc-tured MnO2 and full contact with the electrolyte, which meansthat the MnO2/Ni/Si-MCPs are a suitable electrode in miniaturesupercapacitors.

4. Conclusion

The morphology and characteristics of MnO2/Ni/Si-MCPs fabri-cated by electroplating are investigated. The nanostructured MnO2thin film retains abundant pores for ions to traverse through thechannels. The electrode has a specific capacitance of 0.961 F/cm2

(323.1 F/g) and this MnO2 electrode works in a neutral electrolytewhich protects the 3D architecture of the substrate.

Acknowledgments

The authors are grateful to financial support provided by ShanghaiNatural Sciences Foundation No. 11ZR1411000, Shanghai FundamentalKey Project under Contract number 11JC1403700, NSFC Grant number61176108, PCSIRT, Research Innovation Foundation of ECNU underContract number 78210245, Hong Kong Research Grants Council (RGC)General Research Funds (GRF) No. CityU 112510, and Guangdong—Hong Kong Technology Cooperation Funding Scheme (TCFS) GHP/015/12SZ.

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Fig. 3. (a) Long-term performance of the MnO2/Ni/Si-MCPs assessed at a scanning rate of 80 mV/s; (b) Nyquist plot of the MnO2/Ni/Si-MCPs electrode (the equivalent circuitand element values fit the impedance curve).

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Electrodeposition of nanostructured MnO2 electrode on three-dimensional nickel/silicon microchannel plates for miniature...IntroductionExperimental detailsResults and discussionConclusionAcknowledgmentsReferences