A model for anodic hydrous oxide growth at iridium

A detailed investigation of the electrochemistry of Ir in 0.5 M H2SO4 has been used as an experimental basis for a model for oxide growth at Ir. It appears that a compact oxide (probably IrO2) is formed initially. At potentials above + 1.2 V vs. RHE, the outer monolayer of this compact oxide is oxidised and becomes hydrated. The hydrated surface layer inhibits further oxidation of the compact oxide and therefore only one monolayer of hydrous oxide can be formed at constant potential. To obtain more hydrous oxide than this, the compact oxide must be continually reduced to Ir metal and reformed, by cycling of the potential. On each cycle, the hydrated surface layer of the compact oxide remains after reduction of the compact oxide. Thus, this material accumulates as a hydrous oxide layer.