Anodic deposition of layered manganese oxide into a colloidal crystal template for electrochemical supercapacitor
Introduction
Manganese oxide stores electrochemical energy by simultaneous injection of electrons and charge-compensating cations, as with other electroactive transition metal oxides. Therefore, it is potentially useful for charge storage applications such as cathodes in secondary lithium batteries, electrochromic devices, and electrochemical supercapacitors in aqueous electrolytes. In recent years, MnO2-based thin films grown electrochemically have attracted increased interest as redox supercapacitors because of their superior electrochemical performance, environmentally friendly nature, and low cost [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. In most cases, amorphous structures have been utilized for supercapacitor applications.
Electrodeposition enables us to produce a uniform film on conductive substrates of complex shape, while one can precisely control the film thickness as desired by tuning the electrical charge delivered during electrolysis. In 2004, we demonstrated an electrochemical route to construct birnessite-type layered manganese oxides intercalated with alkaline metal and alkylammonium ions in a thin film form [11]. The process involves an anodic oxidation of aqueous Mn2+ ions in the presence of the corresponding guest cations. In general, charge storage properties of transition metal oxides are closely related to electrical conductivity in the solid phase and ionic transport within the pores [12], [13]. In this regard, a layered structure consisting of bicontinuous networks of solid and pore on the nanometer scale is an attractive candidate for application as active electrode materials. For example, Sugimoto et al. reported that layered ruthenium oxide hydrate formed by an exfoliation/reassembly process exhibits excellent pseudocapacitive behavior with large specific capacitance and good reversibility, as a result of utilization of the interlayer surface for redox capacitance [14].
A problem in our layered products grown anodically is the fact that the Mn oxide layers are lying parallel to the electrode surface, which was disclosed by cross-sectional transmission micrography and in-plane X-ray diffraction measurements [15]. This leads to a difficulty in electron conduction along a direction perpendicular to the substrate surface and does not allow easy access of charge-compensating cations in solution to the interlayer surface. To overcome such a problem, in the present study, the layered Mn oxide was deposited through a colloidal crystal template formed from polystyrene particles to fabricate ordered macropores within the Mn oxide film. Improved mass transportation resulting from the macropores will enhance the electrochemical performance. Although metals, conducting polymers, and semiconductors such as CdSe and ZnO have been electrochemically prepared through colloidal templates [16], [17], [18], [19], [20], [21], [22], to the best of our knowledge, this is the first report on the growth of Mn oxide in a colloidal crystal, irrespective of its crystalline structure. We also describe charge-storage properties of the resulting macroporous film in neutral aqueous electrolyte.
Section snippets
Experimental
All chemicals were of reagent quality and used without further purification. Monodispersed polystyrene (PS) particles with a diameter of 740 nm were obtained from Duke Scientific Corporation as a 10 vol% solution in water. To obtain a smooth surface, an indium tin oxide (ITO)-coated glass plate (R = 10 Ω cm) was used as the substrate. Prior to use, the substrate surface was hydrophilized by immersion in a mixed solution of H2O/NH3/H2O2 (17:3:1). The glass substrate was settled vertically into a PS
Results and discussion
Fig. 1 displays typical SEM images of the Mn oxide films deposited on ITO substrates without (a) and with (b) the PS template, by applying a constant anodic charge per unit geometric area (80 mC/cm2). The time necessary for this charge was different in the former (700 s) and latter (1800 s), consistent with the difference in the electroactive surface areas. A sheet-like morphology appearing for the former electrode is typical of the layered Mn oxide grown anodically [23]. On the other hand, the
Conclusions
A three-dimensionally ordered macroporous film of layered Mn oxide was prepared by electrodeposition in the interstitial spaces of a template formed from PS particles on an ITO electrode followed by removal of the PS template by dissolution in toluene. The ITO-supported macroporous film thus obtained shows a good pseudocapacitive performance in neutral electrolyte, as a result of contributions of the surface of macropores and the interlayer space of the multilayered structure.
Acknowledgements
This research was supported in part by the Japan Society of the Promotion of Science (18550179), Research Foundation of Materials Science, and Electric Technology Research Foundation of Chugoku.
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