Anodic iridium oxide film electrodes

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. 

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). 

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]. 

While in the previous two examples we have discussed the initial stages of oxidation, we now shall focus our attention on the optical properties of thicker oxide layers. Anodically formed iridium oxide films have attracted particular attention because of their pronounced electrochromic effect. When an Ir electrode is scanned anodically in 0.5 M H2SO4, oxidation starts at +0.6 V versus SCE. On the cathodic scan, however, the oxide layer is not reduced to the metallic state but to a low-conductivity hydroxide film facilitating further oxide formation with each anodic potential cycle. Continuous cycling of the iridium electrode between -0.25 and +1.3 V (SCE), at a frequency of 1 cps, therefore has been used as a standard treatment for the formation of thick anodic iridium oxide films. ... 

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,... 

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]. 

Considering that a DSA anode provides oxidant spiecies at higher rates, then for this hydrocarbon polluted soil electroremediation it was chosen an electrode arrangement, considering a modified DSA made of a titanium plate covered with an iridium-tantalum film (Ti I Ir02-Xa205), and two types of cathode carbon felt (CF) and a titanium plate (Ti). Also, in these set of experiments it was considered two electrode positions in the first one, a physical barrier of filter p p)er was included between soil and electrode, while in the second one the electrode was set in direct contact with soil sample. 

The electrocatalytic behavior of olefins was studied by Zanta et al. (2000) at thermally prepared ruthenium-titanium- and iridium-titanium-dioxide-coated anodes. The aliphatic olefins were shown to be inactive in the region before oxygen evolution, while aromatic ones showed one or two oxidation peaks, and the catalytic activity seemed to be the same for both substrates. However, as for platinum anodes, voltammetric studies and FTIR analyses have also shown the formation of a polymeric film that blocks the surface of the electrode and decreases its activity. 

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