Journal of Power Sources 195 (2010) 3950–3955
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Journal of Power Sources
j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / j p o w s o u r
Fabrication of porous nickel oxide film with open macropores by electrophoresis
and electrodeposition for electrochemical capacitors
Mao-Sung Wu
∗, Min-Jyle Wang, Jiin-Jiang Jow
Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, 415, Chien Kung Road, Kaohsiung 807, Taiwan
a r t i c l e i n f o
Article history:
Received 23 November 2009 Accepted 19 December 2009 Available online 13 January 2010 Keywords:
Electrophoretic deposition Porous nickel oxide Electrochemical capacitor Open macropores Polystyrene sphere
a b s t r a c t
The electrophoretic deposition of polystyrene sphere monolayer as a template for anodic electrodeposi-tion of interconnected nickel oxide nanoflakes is explored. Result indicates that a nickel oxide film with nanoflakes and open macropores has superior capacitive behavior. A nickel oxide film with intercon-nected nanoflakes is of great importance for electrochemical capacitors due to the high-specific surface area, fast redox reactions, and shortened diffusion path in solid phase. The open macropores may facil-itate the electrolyte penetration and ion migration, therefore increasing the utilization of nickel oxide due to the increased surface area for electrochemical reactions. The specific capacitance of a nickel oxide film with open macropores at a scan rate of 10 mV s−1reaches as high as 351 F g−1, which is 2.5 times higher than that of the bare nickel oxide film (140 F g−1).
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The nickel oxide materials have received much research atten-tion due to their potential applicaatten-tions in electrochromic films, optical materials, fuel cell electrodes, photocatalysts, electro-chemical capacitors, and batteries, etc. Most of these useful functions depend mainly on their chemical composition, config-uration, and structure. The nanostructured nickel oxides have turned out to exhibit better physical and chemical proper-ties than those of their bulk counterparts. There are numerous reports in literature on the synthesis of nanostructured nickel oxides, such as chemical precipitation[1–6], thermal decompo-sition/oxidation[7–10], hydrothermal process[11–13], surfactant template [14–17], polymer template [18–21], sol–gel method [22–24], and electrodeposition[25–31].
Most of the synthesized nickel oxide materials for electrochem-ical capacitors are nanopowders. The nickel oxide nanowhiskers or small nanowires fabricated by template are also some of the alternative structures for electrochemical capacitors[30,32]. The nickel oxide nanopowder is often difficult to well disperse in the slurry due to the strong agglomeration of nanopowder. Therefore, it becomes an important challenge for finding innovative ways to fabricate the well-dispersed nickel oxide films at low cost and in a simple manner. To reduce the above-mentioned difficulties, elec-trochemical deposition, which deposits the active material directly onto the substrate without polymer binder at room temperature,
∗ Corresponding author. Tel.: +886 7 3814526; fax: +886 9 45614423. E-mail address:ms wu@url.com.tw(M.-S. Wu).
is used for fabricating the nanostructured nickel oxide films for electrochemical capacitors.
The nanostructured materials are important for electrochemi-cal energy-storage devices because they have high-specific surface area for facilitating the electrochemical redox reactions. Generally, the ion diffusion resistance within the crystal structure of active material dominates the high-rate charging and discharging of an electrode. Diffusion resistance in the solid phase can be mitigated by shortening the diffusion path. Therefore, the nanosized mate-rials can reduce the diffusion resistance effectively. Our previous result has shown that the capacitive behavior of nickel oxide film is considerably affected by the pore size of film[33]. The larger the pore size, the higher the specific capacitance of film. The pore size of nickel oxide film can be increased with decreasing the depositing current density and potential[33]. However, the lower depositing current density (potential) may significantly increase the deposit-ing time, which is unfavorable to the fabrication process. Therefore, in this work, monolayer polystyrene (PS) sphere has been directly fabricated on the stainless steel (SS) substrate by electrophoretic deposition (EPD) as a template for anodic electrodeposition of the nickel oxide film. After removal of the PS template, a nickel oxide film with open macropores can be obtained. The deposited films with and without open macropores have been analyzed in their capacitive behavior.
2. Experimental
The monolayer PS sphere template was assembled by the EPD strategy. The monodispersed PS spheres (with negatively charged surface) of about 200 nm in diameter were suspended in water 0378-7753/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
3954 M.-S. Wu et al. / Journal of Power Sources 195 (2010) 3950–3955
Fig. 7. Galvanostatic charge/discharge curves of the nickel oxide films at a current density of 10 A g−1: (a) a film without open macropores and (b) a film with open macropores.
behavior of a nickel oxide film. The specific capacity (Q) of nickel oxide films during galvanostatic test is calculated according to the equation
Q =it
w (3)
where w is the mass of active material and i is the galvanostatic current applied for time t.
The cycle-life stability of nickel oxide films is carried out by galvanostatic charging/discharging at 10 A g−1. Fig. 8 shows the relationship between capacitance retention and cycle number of both deposited films. Two curves are almost the same, meaning that the effect of open macropores on the cycle-life stability of a nickel oxide film is very small. The capacitance retention of both films increases at the beginning of the 750 charging/discharging cycles, and then stabilizes for ongoing cycles. The capacitance retentions of both films after 3000 cycles remain almost unchanged, reflecting a high durability of nickel oxide films for electrochemical capacitor application in alkaline solution.
Fig. 8. Capacitance retention of the nickel oxide films with and without open
macro-pores during galvanostatic charge/discharge cycling.
4. Conclusion
The porous nickel oxide film with interconnected nanoflakes and open macropores has been successfully prepared by anodic electrodeposition of oxy-hydroxide onto PS template adsorbed on a SS substrate by electrophoresis. The formation of PS sphere monolayer can be achieved via transport of negatively charged PSs toward SS substrate and via deposition of PSs with charge neutralization under an applied electric field. The electrochemical performance of the nickel oxide film turns out to be significantly affected by the open macropores. The specific capacitance of nickel oxide film with open macropores is much higher than that of film without open macropores in all scan rates. Possibly, a highly porous film with bigger open pores offers large surface area for fast charge storage and redox reactions and is capable of delivering high power, so it can be discharged/recharged at high rates. A nickel oxide film with open macropores exhibits high capacity and stable capacity retention during cycling, and therefore is suitable for long-time applications in KOH solution.
Acknowledgement
The authors gratefully acknowledge the financial support from the National Science Council, Taiwan, Republic of China (Project No. NSC 98-2221-E-151-032).
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