Three-dimensional nanoscale Co3O4 electrode on ordered Ni/Simicrochannel plates for electrochemical supercapacitors

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

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, 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 9 May 2014Accepted 23 June 2014Available online 30 June 2014

Keywords:Silicon microchannel plates (Si-MCPs)Cobalt oxide (Co3O4)Thin filmsEnergy storage and conversion

a b s t r a c t

Nanoscale cobalt oxide (Co3O4) particles are produced on three-dimensional nickel/silicon microchannelplates (Ni/Si-MCPs) as the active electrode materials in miniature supercapacitors. The Co3O4 nano-flakes are fabricated on the surface and interior of the Ni/Si-MCPs by annealing the nano-structured Co(OH)2/Ni/Si-MCPs, and the electrical conductivity and capacitance retention are improved. The Ni/Si-MCPs covered with nano-flaked Co3O4 supercapacitor offer smaller charging current leakage and higherdischarging voltage than Ni/Si-MCPs electrodeposited with cobalt hydroxide {Co(OH)2}. Specificcapacitance as high as 2.424 F/cm2 or 606.24 F/g is achieved from the materials and the retention ratiois 89.85% after 2000 cycles thus demonstrating excellent potential in miniature supercapacitors.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Cobalt oxide (Co3O4) has many applications in energy storage,electrochemical sensing, and catalysis due to its unique physicaland chemical properties [1]. Among the common oxides such asNiO, Ni(OH)2, Co(OH)2, MnO2, V2O5, and so on, Co3O4 is moreattractive due to its good redox activity and high theoretical speci-fic capacitance (3560 F/g) [2,3]. However, integration of Co3O4 intomicroelectromechanical systems (MEMS) for on-board powerstorage has not been explored systematically. In addition, it isimportant to produce a sufficiently large electroactive surface tofacilitate the transport of ions and electrons between the electrodeand electrolyte interface in the faradic redox reaction [1,2].

Three-dimensional silicon microchannel plates (Si-MCPs),which possess nanoscale arrays, good space management, andadequate ion transport sites, constitute the desirable substrate forthe fabrication of advanced Co3O4-based supercapacitors. Co3O4

nano-flakes serving as the active materials in the supercapacitoroffer smaller charging current leakage and higher discharging

voltage than cobalt hydroxide {Co(OH)2} electrodeposited onSi-MCPs [5]. In this work, by combining the large surface area ofthe Ni/Si-MCPs and virtues rendered by the Co3O4 nano-flakes, anelectrode with a small footprint and excellent electrochemicalproperties is demonstrated.

2. Experimental details

A nickel layer was electroless-plated on both the outer surfaceand inner side walls of the Si-MCPs and details about thefabrication of the Si-MCPs and nickel current collector can befound from Ref. [4]. The Ni/Si-MCPs were put in a buffer solutionof Triton X-100 for at least 2 min to increase the hydrophilicitybefore immersing in the electroplating solution. After electroplat-ing in the electrolyte of 0.1 M cobalt nitrate for 5 min at thecurrent density of 40 mA/cm2 [6], the Co(OH)2 was washed withde-ionized water several times. After drying at 6071 1C, theCo(OH)2/Ni/Si-MCPs were annealed for 2 h at 300 1C to producethe Co3O4/Ni/Si-MCPs. Copper wires were connected to a coppersheet using tin solder and the copper sheet was glued to the MCPsby conductive silver paste (DAD-40).

The morphology and microstructure of the cobalt hydroxide/cobalt oxide films were examined by field-emission scanningelectron microscopy (FE-SEM, Hitachi S-4800, Japan), X-raydiffraction (XRD, Rigaku, RINT2000, Japan), and X-ray photoelectronspectroscopy (XPS, Physical Electronics 5600). The electrochemical

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.06.1480167-577X/& 2014 Elsevier B.V. All rights reserved.

n 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.

nn Corresponding author.E-mail addresses: lwwang@ee.ecnu.edu.cn (L. Wang),

paul.chu@cityu.edu.hk (P.K. Chu).

Materials Letters 132 (2014) 405–408

analysis was carried out on a three-electrode electrochemicalworking station (Shanghai Chenhua CHI660D) at room temperaturein a 2 M KOH electrolyte. The as-prepared Ni/Si-MCPs, Co(OH)2/Ni/Si-MCPs and Co3O4/Ni/Si-MCPs acted as the working electrodewhile the platinum wire and saturated calomel electrode (SCE)were the counter and reference electrodes, respectively.

3. Results and discussion

The XRD patterns acquired from the Co(OH)2/Ni/Si-MCPs andCo3O4/Ni/Si-MCPs samples are depicted in Fig. 1(A) and the XRDpattern of the Ni/Si-MCPs without deposited Co3O4 is shown forcomparison. A sharp diffraction peak with 2θ of 34.11 and thebroad peak at 11.41 are characteristic of the dominant β-Co(OH)2phase (PDF, card no. 46-0605) [6]. After annealing, the diffractionpeaks at 2θ of 31.31, 36.81, 44.81, 58.71, and 65.21 can be indexed to

the standard cubic Co3O4 phase (JCPDS card no.42-1467). The XPSdata acquired before and after annealing are shown in Fig. 1(B).The two main Co 2p peaks shift to lower binding energies, from797 eV (Co 2p1/2) and 781.1 3V (Co 2p3/2) to 795.4 eV (Co 2p1/2)and 780.4 eV (Co 2p3/2) [7,8]. The XRD and XPS patterns indicatethat Co(OH)2 is converted into Co3O4 at a high temperature.

The morphology of the Ni/Si-MCPs and Co3O4/Ni/Si-MCPssamples are displayed in Fig. 1(C). Dense and porous nickelparticles are observed to attach well onto the surface and sidewall of the channels. Hence, the nickel layer protects the Si-MCPsas shown in Fig. 1(A), (B), and (C) and the resistance of the wholestructure is around 0.5Ω. There are dense and intertwined Co3O4

nano-flakes with a semicircular shape in the Co3O4/Ni/Si-MCPs asshown in Fig. 1(D), (E), and (F). The lamellar Co3O4/Ni/Si-MCPsprovide adequate sites for charging/discharging in the solutionwhile decreasing the contact resistance. This framework facilitatesion access thus forming a favorable morphological foundation to

Fig. 1. (a) XRD patterns of Ni/Si-MCPs, Co(OH)2/Ni/Si-MCPs, and Co3O4/Ni/Si-MCPs; (b) XPS Co 2p1/2, Co 2p3/2 spectra of the as-prepared and annealed samples; (c) SEMimages of the Ni/Si-MCPs [top view (A, B) and cross-sectional view (C)], and Co3O4/Ni/Si-MCPs [top view (D, E) and cross-sectional view (F)].

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attain high specific capacitance and excellent electrochemicalperformance. The cracks on the sample surface may be causedby the long sintering process at 300 1C.

Cyclic voltammograms (CV) and chronopotentiometry mea-surements are conducted to determine the specific capacitanceand electrochemical properties of the Co3O4 electrodes. Fig. 2(a) and (b) discloses that the capacitance of Ni/Si-MCPs is negli-gible. The Co(OH)2/Ni/Si-MCPs show a considerably larger capaci-tance because of the large enclosed area in the CV curve (10 mV/S)and longer discharging time (2 mA/cm2) than the Co3O4/Ni/Si-MCPs. However, as shown in Fig. 2(b) (inset on the upper rightcorner), the Co3O4/Ni/Si-MCPs show higher discharging voltage aswell as better charging–discharging symmetry than Co(OH)2/Ni/Si-MCPs. Fig. 2(c) depicts the CV curves of the Co3O4/Ni/Si-MCPs inthe potential range between �0.6 and 0.4 V for scanning rates of10 to 20, 40, 80 and 160 mV/S. The results indicate that the Co3O4

composite has high rate capability comparable to that of thecarbon–metal oxide composite electrode reported previously [9].

According to the chronopotentiometry results in Fig. 2(d) andthe total Co3O4 mass loading of 4.1 mg, the SC values of the Co3O4/Ni/Si-MCPs are calculated to be 2.424, 2.236, 2.074, 1.773 and1.226 F/cm2 (606.24, 545.37, 432.44, 505.85 and 299.02 F/g) fordischarging current densities of 2, 4, 8, 16 and 32 mA/cm2,respectively [10]. The results obtained from the Co3O4/Ni/Si-MCPsare much larger than those of the Co3O4 hollow octahedrananostrucutures (192 F/g) [2], the relatively thick Co3O4 nanos-trucutures (354 F/g) [3] as well as nanostructured, mesoporousCo3O4 was directly deposited on a free-standing carbon substrate(1.35 F/cm2 at a scan rate of 1 mV/s) [11].

Long-time chronopotentiometry tests are conducted at the20 mA/cm2 charging–discharging current on the Co3O4/Ni/Si-MCPsto investigate the long-term capacitance stability. Fig. 2(e) indicatesthat the Co3O4/Ni/Si-MCPs perform well at a high current density

and the capacitance remains at 89.85% after 2000 cycles with noobvious loss thus confirming that their rate capability and cyclinglife time are better than those of the Co(OH)2/Ni/Si-MCPs [6]. Tofurther evaluate the commercial potential of the Co3O4/Ni/Si-MCPselectrode, an asymmetric supercapacitor device composed of theCo3O4/Ni/Si-MCPs electrode as the cathode and activated CNTs onnickel foam as the anode in quasi-solid electrolyte with one piece ofcellulose paper as the separator is prepared (Fig. 2f). They are testedby packaged in a CR2025 battery cell which indicates the excellentoverall performance from the images of the LED powered by thesupercapacitors, charge–discharge curves and Nyquist plots of thedevice.

Fig. 3 shows the EIS data obtained from the Co3O4/Ni/Si-MCPsbefore and after long-term performance test. The equivalentcircuit and results in the inset are fitted to the impedance spectra.The electrode internal resistance value of R1 is less than 2.0Ωindicating a highly conductive nature. The Faradaic charge transferresistance (R2) corresponds to the semicircle in the high frequencyrange associated with the surface properties of the electrode. Thetable in Fig. 3 shows that Co3O4/Ni/Si-MCPs has an R2 value of0.264Ω. After 2000 cycles, the value is 1.985 which is consistentwith the double-layer capacitance CPE1�n of Co3O4/Ni/Si-MCPs(0.781) and after 2000 cycles (1.017). The loose nano-flakes on theCo3O4/Ni/Si-MCPs yield a larger double layer capacitance whichmay originate from the porous structure of the sample that canfully contact the solution, but after 2000 cycles, as shown by Fig. 3(b), some flakes has been destroyed [12].

The R3 values of electrodes before and after 2000 cycles are2.835 and 3.533, respectively, suggesting that the newly madesample has more active materials and can react easily withthe electrolyte. Comparison of the index of CPE2�n shows thatthe newly made sample has a CPE2�n of 0.799 that is larger thanthat after 2000 cycles (0.599). Furthermore, the slope of the

Fig. 2. (a) CV curves of the Ni/Si-MCPs, Co(OH)2/Ni/Si-MCPs and Co3O4/Ni/Si-MCPs at a scanning rate of 10 mV/s; (b) charging and discharging curves of the Ni/Si-MCPs, Co(OH)2/Ni/Si-MCPs and Co3O4/Ni/Si-MCPs at a current density of 2 mA/cm2; (c) CV curves of Co3O4/Ni/Si-MCPs at different scanning rates; (d) charging–discharging curves ofthe Co3O4/Ni/Si-MCPs for different current densities; (e) long-term performance of the Co3O4/Ni/Si-MCPs assessed at a charging–discharging current density of 20 mA/cm2;(f) a Nyquist plot of the asymmetric supercapacitor device packaged in a CR2025 battery cell, an image of two colored LEDs powered by the 10 s large current chargedsupercapacitors, and charge–discharge curves at different current densities of the asymmetric supercapacitor.

M. Li et al. / Materials Letters 132 (2014) 405–408 407

impedance plots (CPE3�n) of the new sample is 0.837 compared to0.617 of the used sample at low frequencies. This indicates that thespecial complex microstructure with flakes and particles in newsample enables faster ion diffusion through the channel of theMCPs [13].

4. Conclusion

The effects and mechanism of cobalt oxide coated Ni/Si-MCPsfabricated by annealing are investigated. The nanostructuredCo3O4/Ni/Si-MCPs exhibit better performance than the Co(OH)2/Ni/Si-MCPs. The enhancement can be attributed to the nanostruc-ture consisting of mesopores, faster ion diffusion in the pores, aswell as Co3O4 tightly bound to the substrate. After annealing, aspecific capacitance as high as 606.24 F/g (2.424 F/cm2) is attainedat a discharging current density of 2 mA/cm2 and excellentelectrochemical stability up to 2000 cycles is demonstrated.

Acknowledgments

This work was jointly supported by the Shanghai PujiangProgram (No. 14PJ1403600), Shanghai Fundamental Key Project

(No. 11JC1403700), National Natural Science Foundation of China(No. 61176108), PCSIRT, and Research Innovation Foundation ofECNU (No. 78210245), as well as Guangdong-Hong Kong Technol-ogy Cooperation Funding Scheme (TCFS) GHP/015/12SZ.

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Fig. 3. (a) Nyquist plots of the Co3O4/Ni/Si-MCPs electrodes in a 2 M KOH solution before and after 2000 cycles in the long-term performance test with the equivalent circuitand element values fitting the impedance curve; (b) the SEM images of the Co3O4/Ni/Si-MCPs electrodes in a 2 M KOH solution after 2000 cycles.

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