Selective micro-etching of duplex stainless steel for preparing manganese oxide supercapacitor electrode
Introduction
The charge storage capacity of a supercapacitor can be improved by increasing the electrode surface area. For a manganese oxide supercapacitor, the oxide film can be deposited on many substrates, which act as current collectors. Along with graphite [1], [2], [3], nickel [4], [5] and other conductive materials [6], [7], stainless steel can be used because it is inert in various supercapacitor electrolytes. Among the various grades of stainless steel, duplex stainless steels (DSSs) with a relatively high Cr content are particularly attractive because of their higher corrosion resistance in such electrolytes as KOH, Na2SO4 solutions and others.
DSS comprises austenitic (γ) and ferritic (α) phases. The former has a face-centered cubic (FCC) crystal structure while the latter is body-centered cubic (BCC). Each phase has a specific chemical composition, and therefore manifests a distinct electrochemical nature in certain electrolytes. Previous studies have demonstrated that the mixed H2SO4/HCl solutions yield two anodic peaks in the active-to-passive transition region of the potentiodynamic polarization curves of 2205 DSS. Selective (preferential) dissolution can occur at each of these characteristic potentials [8]. Accordingly, Tsai and Chen developed a novel procedure that involves selective micro-etching reaction for fabricating micro-networks or rods with an exclusively γ or α phase from dual-phase stainless steel [9]. After one of the constituent phases is selectively dissolved in a specific mixed H2SO4/HCl solution, the concave/convex microstructure of a dual-phase stainless steel can give rise to an increase in surface area. This etched stainless steel may be advantageous for use as an electrode substrate for supercapacitor applications.
Manganese oxide has been found to be a promising substitute for ruthenium oxide as the electrode material for supercapacitors [10], [11]. It can be anodically deposited under either constant potential [12] or constant current [13] conditions, to prepare a supercapacitor electrode using graphite or nickel substrate. The use of a substrate with a high specific surface area is considered to increase the specific capacitance of a manganese oxide electrode (in terms of F g−1). Coating manganese oxide onto a stainless steel substrate with a concave/convex feature, as presented in Fig. 1, may yield a high specific capacitance. Therefore, this investigation examines the feasibility of increasing the specific capacitance of a manganese oxide electrode by the selective micro-etching of a DSS substrate.
Section snippets
Specimen preparation
Table 1 presents the chemical composition of the 2205 DSS rod used in this study. After solid solution heat treatment at 1100 °C for 30 min, the α/γ volume ratio was approximately 1.13, where α was the continuous phase. Table 1 also presents the respective chemical compositions of α and γ phases, analyzed by energy dispersive spectrometry (EDS). The solution heat-treated steel rod was then cut into a 2 mm-thick disc with a cross-sectional area of 1 cm × 1 cm. This disc was connected to a copper wire,
Morphologies of etched 2205 DSS
Selective micro-etching yields a concave/convex surface on 2205 DSS. Fig. 3(a) presents some SEM micrographs of the steel after it is held at the peak potential to dissolve selectively the γ phase in the mixed 2 M H2SO4 + 0.5 M HCl solution for various periods. At the characteristic potential, the γ phase was selectively dissolved, leaving α phase extruding on the surface. Increasing the etching time increased the depth of the dissolved γ phase, as shown in Fig. 3(b). Notably, however, α phase was
Conclusions
Constituent phases selectively dissolved in 2205 DSS in 2 M H2SO4 + 0.5 M HCl solution at specific characteristic potentials. Micro-etching was thus adopted to fabricate the electrode substrate with a concave/convex surface with a high surface area. The etch depth resulting from selective dissolution increased with etching time. The etching rate of the α phase exceeded that of the γ phase at the respective characteristic potentials. Manganese oxide was successfully deposited on the micro-etched
Acknowledgement
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract no. NSC 94-2216-E-006-020.
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