Influence of the preparation method on the morphological and electrochemical properties of Ti/IrO₂-coated electrodes

Abstract

The IrO₂ layer deposited on a Ti substrate for electrocatalysis of the oxygen evolution reaction (OER) in acidic media was obtained from one of three different deposition methods consisting of painting the substrate with the corresponding solutions followed by heat treatment at a temperature range 400–600°C for 1 h. The coating solutions were prepared by dissolving IrCl₃ · 3H₂O in 15% HCl (traditional method), by dissolving iridium acetylacetonate in isopropanol and acetic acid (inorganic sol–gel method) and by dissolving IrCl₃ · 3H₂O in ethyleneglycol and citric acid (Pechini method). The redox processes occurring on these films were characterized by cyclic voltammetry at 20 mV s⁻¹ in 0.5 M aqueous H₂SO₄ at room temperature. The measured anodic voltammetric charge is related to the active area of the electrode; the electrode derived from traditional method had the highest value. The Tafel plots revealed that the electrocatalytic activities are not affected by the surface morphology, indicating that the preparation procedures produce electrocatalytic coatings with comparable activity. For an applied current density of 100 mA cm⁻² the combination of the conductivity and roughness effects caused the electrode derived from inorganic gel and heat-treated at 400°C to exhibit the lowest overpotential for the OER. On the other hand, the accelerated life tests performed at 80°C in 30% aqueous H₂SO₄ with an anodic current density of 750 mA cm⁻² showed that the stability of the coatings increases with the temperature of the heat-treatment and the Pechini-derived electrodes had a longer-life time under this condition in all the cases. The accelerated life test showed that the factor limiting the operational life of these electrodes is the dissolution of the active layer.

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.