Analysis of the anodic oxygen layer on iridium by X-ray emission, electron diffraction and electron microscopy

Abstract

Quantitative physicochemical measurements have been made of the nature and thickness of the oxygen-containing layer formed at anodic potentials on iridium electrodes in sulphuric acid solution. Continuous cycling to potential above 1.4 V results in an increase in the anodic charge in the oxygen region. This increase is associated with the growth of an oxide-hydroxide layer on the electrode surface. The topography of an oxide-covered electrode after holding at 0.06 V was studied by electron microscopic examination of replicas of the surface. Electron diffraction analysis of pieces of the surface layer removed during replication revealed that its structure was not the rutile structure of IrO 2 but was hexagonal with some ordering of the metal atoms. The average weight of the layer was determined from X-ray emission measurements and the thickness calculated. Electrodes subjected to 176 and 3,600 potential cycles to 1.5 V were found to be covered with layers of 6.5 and 22 nm thickness, respectively. From these values and the anodic charges passed on the potential sweeps it is concluded that 2 electrons are transferred per iridium atom in the process occurring during a potential cycle. This is interpreted in terms of conversion of Ir(OH) 2 to IrO 2 on the electrode surface, probably by a continuous series of reversible steps involving the removal of protons.