Electronic Structure and Growth of Electrochemically Formed Iridium Oxide Films

Abstract

The nature of the electronic structure of electrochemically formed iridium oxide films (EIROF) is investigated by in-situ conductivity measurements in an electrochemical cell and ex-situ current-sensing atomic force microscopy (CS-AFM). A direct demonstration of changes in the conductivity for electrochemically formed iridium oxide films (EIROF) with the applied potential of EIROF electrodes in an electrochemical cell is presented. The in-situ conductivity shows a single step-like change at a potential of approximately 1.2 V in 0.5 mol dm −3 H 2 SO 4 vs. a reversible hydrogen reference electrode. The change in conductivity is also reflected in results of ex-situ CS-AFM for EIROF electrodes emersed at different potentials. At an emersion potential of 0 V the CS-AFM currentvoltage characteristics are non-linear and similar to those of diodes. At an emersion potential of 1.6 V the CS-AFM current-voltage characteristics are approximately linear, consistent with metallic behavior. Mott-Schottky analysis shows that at low potentials the oxide behaves as a p-type semiconductor with a flatband potential approximately 500 mV below the transition to high conductivity from the in-situ conductivity measurements. These results allow for an interpretation of changes in the relative magnitudes of the III/IV and IV/V (or IV/VI) voltammetric peaks during film growth through a block-release behavior involving space-charge layers in the oxide. Iridium oxide 1 is relevant as a material for electrochromics, 2-4 electrocatalysis 5-19 and as a pH sensor. 20 A proposed mechanism for the electrochromic properties of the material based on the electronic structure was suggested by Granqvist. 3,4 According to this explanation the change of color with potential is due to variations in band filling associated with intercalation of protons and possibly other ions,where A − is a solution anion, and h • and electron hole. For oxygen-evolution electrocatalysts one expects the activity to depend on the binding energy of intermediates involved in the reaction according to the Sabatier principle. Since the binding energy of the various relevant intermediates appear to scale with the binding energy of oxygen, the latter quantity may serve as a descriptor of catalytic activity for the oxygen evolution reaction. Exactly how the features of the electronic structure of the catalyst are related to the binding energy appears to be less clear than in the case of metals for which the d-band theory and its refinements 22,23 appear to successfully rationalize electronic structure and (electro-) catalytic behavior. Recent work indicates that the number of electrons in the d-band of transition metal oxides is an important descriptor of catalytic activity, 24,25 although other aspects of the electronic structure may be important as well. 26Also other electrochemical properties such as the shapes of voltammograms have been attempted analyzed in terms of electronic structure. 27It is therefore of interest to characterize properties reflecting the electronic struct...

An in situ XAS study of high surface-area IrO2 produced by the polymeric precursor synthesis

“…The electronic structure of AIROF appears to be that of a semiconductor in the reduced state (reaction (2) shifted to the right) and conducting in the oxidised (reaction (2) shifted to the left) 27,58 . A direct measurement of this transition from in-situ conductivtiy and scanning-probe measurements was reported by Ilyukhina et al 26 for electrochemically formed iridium oxide (EIROF). The H-UPD region would appear at potentials at which the oxide is reduced and thus semiconducting.…”

“…In some cases the oxides are electrochromic, i.e. their color changes with the state of charge (sputtered iridium oxides (SIROF) 23 , AIROF 24,25 , and electrochemically deposited iridium oxide (EIROF) 26 ), in others not (hydrolysis) 27 .…”

The porous transport layer in proton exchange membrane water electrolysis: perspectives on a complex component

“…The electronic conductivity of IrO x , which is the most commonly used anode, electrocatalyst highly depends on anodic potential which affects its oxidation state. It has been shown that electrochemically deposited iridium oxide film behaves as a p-type semiconductor at small anodic potentials, but transitions to metallic behaviour for potentials higher than 1.2 V. 102 Although the intrinsic electronic conductivity of IrO x is high enough, the conductivity of the CL is determined by its ionomer content and the structure and percolation level of IrO x particles in the CL.…”

Operando electrochemical spectroscopic ellipsometry: Insights into electrochemical behavior of catalyst materials under realistic working conditions

“…This trend matches with the a constant to slightly increasing conductivity at low-potentials and a stronger increase at a potential of about 1.20 VRHE observed earlier by the group of Haverkamp. 25 The assumption that the changes in the ellipsometric spectra are caused by changes in resistivity has to be validated. Therefore, operando ECSE measurements were performed on a non-conductive mesoporous titanium oxide (TiOx) film in the OER regime and in the potential range between 0.40 VRHE and 1.40 VRHE.…”