Elsevier

Journal of Power Sources

Volume 187, Issue 1, 1 February 2009, Pages 261-267
Journal of Power Sources

Selective micro-etching of duplex stainless steel for preparing manganese oxide supercapacitor electrode

https://doi.org/10.1016/j.jpowsour.2008.10.122Get rights and content

Abstract

In this investigation, manganese oxide electrodes that incorporate selectively micro-etched duplex stainless steel (DSS) for supercapacitor applications were fabricated. The experimental results demonstrate that selective dissolution of the γ phase could result in the formation of α phase network, and vice versa. Controlling the extent of selective dissolution produces a surface with a concave–convex morphology, increasing the surface area. Manganese oxide could then be anodically deposited onto the etched dual phase steel current collector. The experimental results of cyclic voltammetry (CV) show that the specific capacitance increased with the etching depth of either the γ or the α phase. Additionally, selective dissolution occurred in α phase had a better effect on increasing the specific capacitance. The successful application of selective dissolution in fabricating a current collector was thus demonstrated.

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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