Iridium oxide anodes

The second form consists of Pt metal but the iridium is present as iridium dioxide. Iridium metal may or may not be present, depending on the baking temperature (14). Titanium dioxide is present in amounts of only a few weight percent. The analysis of these coatings suggests that the platinum metal acts as a binder for the iridium oxide, which in turn acts as the electrocatalyst for chlorine discharge (14). In the case of thermally deposited platinum—iridium metal coatings, these may actually form an intermetallic. Both the electrocatalytic properties and wear rates are expected to differ for these two forms of platinum—iridium-coated anodes. 

A dimensionally stable anode consisting of an electrically conducting ceramic substrate coated with a noble metal oxide has been developed (55). Iridium oxide, for example, resists anode wear experienced ia the Downs and similar electrolytic cells (see Metal anodes). 

Iridium as an electrode material has received considerable attention in the last decade not only because of its excellent catalytic properties but also in relation to the electrochromic effect observed for anodic iridium oxide films (AIROF). Electrochromism of iridium was thought to be of technical relevance for display applications and triggered several studies of the electrochemical and optical properties of AlROFs [67, 85-88],... 

Thick anodic iridium oxide films are formed by repetitive potential cycling between properly chosen anodic and cathodic limits [89]. The coloration (bleaching) transition is reflected in the cyclic voltammogram by a significant increase (decrease) of the electrode pseudo-capacity at a potential around 0.7 Vsce in acid electrolytes. At potentials above 0.7 V the thick film appears dark blue, while below 0.7 V the film is almost clear. 

Fig. 26. Cyclic voltammogram of a thick anodic iridium oxide film (AIROF) in 0.5 mol L 1 H,S04. The reaction mechanism for coloration and Oz evolution is indicated.

The number of protons extracted from the film during coloration depends on the width of the potential step under consideration. As can be seen in the formulation of Fig. 26 an additional valence state change occurs at 1.25 Vsce giving rise to another proton extraction. The second proton exchange may explain the observation by Michell et al. [91] who determined a transfer of two electrons (protons) during coloration. Equation (5) is well supported by XPS measurements of the Ir4/ and Ols levels of thick anodic iridium oxide films emersed at different electrode potentials in the bleached and coloured state. Deconyolution of the Ols level of an AIROF into the contribution of oxide (O2-, 529.6 eV) hydroxide, (OH, 531.2 eV) and probably water (533.1 eV) indicates that oxide species are formed during anodization (coloration) on the expense of hydroxide species. The bleached film appears to be pure hydroxide (Fig. 27). 

Fig. 27. Deconvoluted Ols levels of a thick anodic iridium oxide film at different potentials. After [34],...

In order to explain the changing optical properties of AIROFs several models were proposed. The UPS investigations of the valence band of the emersed film support band theory models by Gottesfeld [94] and by Mozota and Conway [79, 88]. The assumption of nonstoichiometry and electron hopping in the model proposed by Burke et al. [87] is not necessary. Recent electroreflectance measurements on anodic iridium oxide films performed by Gutierrez et al. [95] showed a shift of optical absorption bands to lower photon energies with increasing anodic electrode potentials, which is probably due to a shift of the Fermi level with respect to the t2g band [67]. 

A. Bezbaruah and T. Zhang, Fabrication of anodically electrodeposited iridium oxide film pH microelectrodes for microenvironmental studies. Anal. Chem. 74, 5726-5733 (2002). 

Ir Ir02 electrodes (commercially available from Cypress Systems, Lawrence, KS) can measure pH in harsh environments or microscopic spaces [S. A. M. Marzouk, Improved Electrodeposited Iridium Oxide pH Sensor Fabricated on Etched Titanium Substrates, Anal. Chem. 2003, 75, 1258 A. N. Bezbaruah and T. C. Zhang, Fabrication of Anodically Electrodeposited Iridium Oxide Film pH Microelectrodes for Microenvironmental Studies, Anal. Chem. 2002, 74. 5726 D. O. Wipf. F. Ge, T. W. Spaine, and J. E. Baur, Microscopic Measurement of pH with Ir02 Microelectrodes, Anal. Chem. 2000, 72, 4921]. For pH measurement in nanoscopic spaces, see X. Zhang,... 

Direct electrooxidation is theoretically possible at low potentials, before oxygen evolution, but the reaction rate usually has low kinetics that depends on the electro-catalytic activity of the anode. High electrochemical rates have been observed using noble metals such as Pt and Pd, and metal-oxide anodes such as iridium dioxide, ruthenium-titanium dioxide, and iridium-titanium dioxide (Foti et al. 1997). 

Iridium dioxide — Iridium oxide crystallizes in the rutile structure and is the best conductor among the transition metal oxides, exhibiting metallic conductivity at room temperature. This material has established itself as a well-known - pH sensing [i] and electrochromic [ii] material (- electrochromism) as well as a catalytic electrode in the production of chlorine and caustic [iii]. The oxide may be prepared thermally [iv] (e.g., by thermal decomposition of suitable precursors at temperatures between 300 and 500 °C to form a film on a substrate such as titanium) or by anodic electrodeposition [v]. 

Murphy et al. used a proton exchange membrane (Nafion 117) as a solid electrolyte to oxidize a mixture of 12 low-concentration (50-60 ppm TOC) organic compounds contained in deionized water [39]. Platinum 10% iridium mesh anodes and cathodes were hot-pressed onto the surface of the membrane. This membrane elec-... 

The properties of sllane-derlvatlzed Iridium and anodic Iridium oxide (AIROF) electrodes have been studied by cyclic voltammetry In tetraethylammonlum perchlorate/acetonitrile solutions. Both electrodes react with silanes such as dlchlorosilylferrocene (DCSF) to give persistently bonded silylferrocene monolayers based on geometric area. This contrasts with the behavior of anodized platinum (Pt/PtO), which gives considerable polymerization with DCSF, resulting in layers of variable and unpredictable thickness. 

In a previous communication, we gave a detailed description of the electrochemical properties of silylferrocene on iridium (6). In this paper we briefly review this work, and compare deriva-tized iridium, Pt/PtO, and anodic iridium oxide electrodes. We show that the results of derivatization can give useful and sometimes unexpected information about the nature of the oxides on these electrodes. 

Before discussing derivatlzation of anodic iridium oxide films (AIROFs), we will briefly review what is known about this material. The AIROF has been extensively studied as an electro-chromic and electrocatalyst ( l-5), however its exact composition and structure are still not known. It is amorphous, low density film which can be formed on Ir by potential cycling in aqueous acids. It exists in two oxidation states a reduced, colorless form containing Ir + and an oxidized, blue-black form containing... 

Other than the higher background current, we found the electrochemistry of silylferrocene on anodic iridium oxide to be... 

Solutions of acid copper sulfate (containing only chloride and carrier) were used as the copper electroplating bath. A piece of titanium mesh (diameter = 55 mm) coated with iridium oxide was used as an insoluble anode. The bath was pumped through the anode to the cathode under 1 l/min and controlled at 25 °C. The cathode rotating speed was maintained at 165 rpm. The copper electrodeposition tests were conducted under different electric field waveforms with an average cathodic current density of 25 to 32 ASF, which was controlled by the cell voltage. Samples were cross-sectioned with a focused ion beam scanning electron microscope (FIB-SEM) to inspect both the quality of the copper deposits in the trenches or via-holes. 

Porous anode 1, used in SPE electrolysis (fig.1) consists of mixtxjre of ruthenium dioxide (75%) and iridium oxide bound to the graphite layer. Thickness of such layer depends on the amount of aphitc, covering the anode surface unit. In particular, if this amount is 40 g/m. correspondent thickness reaches 100 0jn [12]. Current feeding to anode is held with the help of point collector 6 (the metal net can sei e as this collector). Anode space is separated from the cathode one with the help of membrane SPE (2). Platinum black, serving as an anode 1, is 100 jim v/ide. Cathode point collector 3 is connected with graphite plate 5, that maintain direct contact with cathode. 

A typical example of anodic ECMs is iridium oxide which may be rapidly and reversibly coloured in aqueous solutions containing sulphuric acid. The exact type and mechanism of the ion insertion process in this oxide are still uncertain [17]. In aqueous H2SO4, the most commonly used electrolyte, two mechanisms have been proposed, i.e. a cation mechanism involving coloration via proton extraction ... 

Electrochromic iridium oxide films can be obtained by anodic growth or by sputtering, again on an ITO-coated glass support. It has been established [18] that properly deposited iridium oxide films can provide electrochromic response times of the order of 200 ms. Such a fast response, however, is in part counterbalanced by the high cost of the material. 

Iridium oxide, Ir(OH)3, is a typical W03-complementary ECM which can be conveniently deposited on an ITO-coated glass substrate either by anodic growth or by sputtering. 

---