Influence of the preparation method on the morphological and electrochemical properties of Ti/IrO2-coated electrodes
Introduction
Thermally prepared IrO2 coatings deposited on titanium substrates are the most widely studied anode material for industrial electrochemical applications, such as electrowinning of metals and waste-water treatment [1], [2]. Traditionally, this kind of electrode material is obtained by thermal decomposition of the corresponding metal chloride salt which promotes a surface morphology characterized by the presence of mud-flat cracking.
In recent years, the sol–gel method has appeared as an alternative to obtain pure and mixed oxide films on substrates. The chemical processing and technical applications of the sol–gel films have been reviewed by Schmidt [3] and by Sakka and Yoko [4]. The sol–gel derived oxide layer is obtained by thermal decomposition of a polymeric gel film previously deposited on the substrate surface. The gel film can be obtained using one of the following methods. In the first, the sol is prepared by dissolving the corresponding metal alkoxides or acetylacetonates in alcohol, used as the solvent, and acetic acid, as catalyst for the initial hydrolysis by residual water. After the deposition from solution on the substrate by an appropriate technique, the inorganic gel film is formed by the corresponding reactions of condensation and polymerization. In the second method (Pechini), the gel film is formed by the polymerization of organic monomers in the presence of the metal ions that are then incorporated homogeneously into the polymeric chain. In this last case, the sol is prepared by dissolving the corresponding metal chloride in citric acid and ethyleneglycol at 60°C forming the sol phase. This is followed by the deposition of the solution on the substrate and heating at 110°C to promote the polymerization and formation of the gel film [5]. One major advantage of the inorganic gel method in contrast to the Pechini and traditional methods is that it permits oxide coatings to be obtained in O2-free atmospheres. This is particularly suitable to avoid the oxidation of the substrate that can affect the substrate/active-oxide layer, as previously reported [6].
Sol–gel derived films have been widely studied as coatings for corrosion protection of stainless steels and carbon steel in acidic and neutral media [6], [7], [8], [9], [10], [11], [12], [13], [14], but little attention has been given to this method of obtaining electrocatalytic films. This method has been used to produce ultrafine RuO2–TiO2 [15], RuO2–SnO2 [16], IrO2–TaO2 [17] and IrO2–SnO2 [18] oxide particles or RuO2–TiO2 and RuO2–IrO2–TiO2 films on titanium substrates [19], [20]. Meanwhile, only recently preparation of pure and mixed sol–gel derived RuO2–IrO2 [21] and RuO2–Nb2O5 [22] deposited on Ti substrates as catalyst films for oxygen evolution reaction (OER) has been reported in the literature.
In previous studies [21], scanning electron micrographs showed a surface morphology of pure and mixed RuO2–IrO2 layers composed of well-defined grains. This is different from the morphological characteristics of surface coatings obtained by using the chloride precursor. Such observation suggests that the Ti-coated electrodes can be obtained in a different surface morphology simply by using different coating solutions. Naturally, such differences can affect the electrochemical properties of the coated electrode. Moreover, comparative electrochemical studies of the Ti-coated electrodes derived from different methods are absent from the literature, which precludes the comparison of results from several laboratories.
Thus, the aim of this work is to prepare Ti/IrO2-coated electrodes derived from the three methods above, i.e. traditional, inorganic sol–gel and Pechini methods, and to study the influence of the method on the morphological and electrochemical properties of these electrodes and to test them as anodes for the oxygen evolution reaction in acidic media.
Section snippets
Preparation of the electrodes
The Ti/IrO2 coated electrodes were obtained from three different coating solutions. Electrode A was obtained using a traditional method where the coating solution was prepared by the dissolution of IrCl3 · 3H2O (Aldrich) in 15% HCl (Merck) at room temperature. Electrode B was prepared using the inorganic sol–gel method from a coating solution prepared by the dissolution of iridium acetylacetonate (Aldrich) (10 mM total concentration) in isopropanol/acetic acid (Merck) 3/2 v/v, followed by
Results and discussion
Fig. 1 presents typical scanning electron micrographs for freshly prepared IrO2 layers. The micrographs of the layer obtained by using the traditional method (Fig. 1a) reveal a typical cracked and smooth layer of IrO2, which is frequently obtained from IrCl3 solutions. The surface morphology of the layer prepared by the inorganic sol–gel method is homogeneous, with the presence of well-defined grains (Fig. 1b) while the morphology of the layer derived from Pechini method (Fig. 1c and d) is
Conclusions
Ti/IrO2 coated electrodes with distinct morphologies can be obtained from different coating solutions. The values of the anodic voltammetric charge, which were employed here in order to evaluate the real surface areas of the electrodes, were used to normalize the current data. The ‘normalized’ Tafel plots showed that the preparation procedures produce coatings with comparable electrocatalytic activities. For the applied current density of 100 mA cm−2 the combination of the conductivity and
Acknowledgements
The authors thank the Conselho Nacional de Desenvolvimento Cientı́fico e Tecnológico (CNPq) and the Fundação Cearense de Amparo à Pesquisa (FUNCAP), Brazil, for financial assistance.
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