Volume 14 Issue 5 - June 18, 2010 PDF
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An electrochemical preparation of a nano-structured cobalt oxide electrode with super redox activity
Ming-Jay Deng1, Fu-Lu Huang1, I-Wen Sun1*, Wen-Ta Tsai2 and Jeng-Kuei Chang2
1Department of Chemistry, National Cheng Kung University
2Department of Materials Science and Engineering, National Cheng Kung University
 
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Pseudocapacitors of which the capacitance is attributed to the continuous reversible faradaic redox reaction of electrode materials are promising energy storage devise.  Cobalt oxide is one of the attractive electrode materials due to its high redox activity and good reversibility.  Among the various approaches for preparation of pseducapacitive Co oxide, electrodeposition offers the advantages of simplicity, reliability, accuracy, versatility, and low cost.  In addition to supercapacitors, Co oxides are also useful materials  for electrochromic devices and lithium ion battery.  While bulk Co oxide exhibits a typical specific capacitance of less than 250 F g-1 [1], the capacitance can be greatly enhanced by introducing Co oxide nanostructures which provide a much higher active area allowing for higher current density.  Cao et al [2] achieved a high capacitance of 1492F g-1 by co-precipitation of a Co(OH)2/Zeolite nano-composite.  Zhou et al [3] obtained a capacitance of 1084 F g-1 by direct electrodeposition of porous Co(OH)2 onto a substrate from lyotropic liquid crystal media.  This paper explores another approach to prepare high surface Co oxide by electrodeposition of Co oxide onto a nanoporous Ni substrate which was fabricated by electrochemical etching of Cu away from an electrodeposited of NiCu alloy.  The prepared nano-structured CO oxide displays extraordinary pseudocapacitive performance.

Ni-Cu alloy film with a Ni/Cu ratio of approximately 50/50 was first electrodeposited on a Ni foil electrode (1.4 cm2) from a 1M NiSO4, 0.01 M CuSO4 and 0.5 M H3BO3 solution at 25 ˚C by passing 3 C of charge at -0.85 V versus a saturated calomel reference electrode (SCE). The scanning electron microscope (SEM) image of the NiCu film shown in Figure 1a reveals protruding structure.  A previous study [4] indicated that the NiCu films contain segregated Ni-rich and Cu-rich phases; Cu-rich phase sits in the central part of each grain surrounded by Ni-rich phase.  To prepare the nanoprous Ni film, the Cu in the NiCu film was selectively etched at 0.45 V versus SCE.  The resulted porous Ni structure after dealloying is illustrated in Figure 1b; pores with size approximately 100 nm are obvious. The nanoporous Ni was then loaded with the Co oxide in a 0.1 M Co(CH3COO)2 aqueous solution by passing 50 mC charge at 1 V versus SCE.  The morphology of the deposited Co oxide on the porous Ni is shown in Figure 1c.  As can be seen a thin oxide film was successfully deposited on the nanoporous Ni forming a nanoporous Co oxide electrode.  X-ray diffraction analysis revealed that the Co oxide film is amorphous in nature.  Deconvolution of the Co 2p3/2 XPS spectrum taken for the Co oxide film indicated the presence of Co3O4 at 780.1 eV, Co(OH)2 at 781.3 eV, and CoOOH at 782.2 eV, respectively.
Figure 1. Top-view SEM micrographs of (a) the deposited Ni-Cu film, (b) the nano-porous Ni after Cu was selectively etched away from the Ni-Cu film and (c) the nano-structure Co oxide electrode which was obtained by anodic deposition of Co oxide on the porous Ni electrode shown in (b).

The electrochemical behavior of the nano-structured Co Oxide electrode and a flat Co oxide in 1M KOH solution is illustrated by the cyclic voltammograms shown in Figure 2a.  The anodic and cathodic peaks in these voltammograms are attributed to the charging and discharging of the Co oxide electrodes as follows:

Co3O4 + OH- + H2O ←→ 3CoOOH + e-
(1)

CoOOH + OH- ←→ CoO2 + H2O + e-
(2)

As can be estimated from the area under the cyclic voltammograms in Figure 2a, the nano-structured electrode has a substantial higher charge storage capacity.  The specific pseudocapacitance of the Co oxide films was quantitatively evaluated from the cyclic voltammograms.  While the evaluated specific pseudocapacitance of the flat Co oxide is 209 F g-1, which is close to the typical values reported in the literature, the calculated capacitance of the nano-structured Co oxide is as high as 2200 F g-1. This result confirms the great improvement in the capacitance resulting from the high surface of the nano-structured Co oxide for pseducapacitive reactions.  The specific capacitance of the flat Co oxide electrode and the nano-structured Co oxide electrode was evaluated with cyclic voltammetry at various scan rates.  As shown in Figure 2b, the capacitance of the flat Co oxide electrode dropped rapidly with increasing scan rate whereas the capacitance of the nano-structured Co oxide remained relatively stable with increasing scan rate.  The high porosity nano-structure minizes bothe the ionic and electronic transport distances in the Co oxide and thus improves the electrode kinetic performance, which is crucial for high-power supercapacitor applications.
Figure 2. (a) Cyclic voltammograms of the various electrodes in 1 M KOH solution with a potential scan rate of 10 mV s-1. (b) Capacitance retained ratios of the Co oxide electrode as a function of the cyclic voltammetry scan rate.

Acknowledgment
This work was supported by the National Science Council and National Cheng Kung University.

References
  1. Srinivasan V and Weidner J W 2002 J. Power Sources 108 15.
  2. Cao L, Xu F, Liang Y Y and Li H L, 2004 Adv. Mater. 16 1853.
  3. Zhou W J, Zhang J, Xue T, Zhao D D and Li H L 2008 J. Mater. Chem. 18 905.
  4. Chang J K, Hsu S H, Sun I W and Tsai W T, 2008 J. Phys. Chem. C 112 1371.
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