An investigation of hydrous oxide growth on iridium in base

Abstract

The present work confirms earlier reports that hydrous oxide growth on iridium under potential cycling conditions is possible in dilute, though not strong, base (0.1 mol dm −3 and 1.0 mol dm −3 NaOH, respectively). Even in dilute base such growth is not readily observed with highly cleaned surfaces or in the presence of low levels of dissolved sulphate. The base-grown films are not particularly stable — for instance they became detached from the metal surface and dissolved readily on transfer to acid media. They also differ from acid-grown films in that under potential cycling conditions in dilute base they display signs of an acidic response during the anodic sweep, especially at fast sweep-rates. The latter effect is attributed to local pH changes within the porous layer where release of protons during the main anodic reaction creates a temporary acidic environment. The inability to produce such an environment in strong base is probably the reason why hydrous oxide grown is not readily observed with iridium in such media. Acid-grown hydrous iridium oxide films do not display local pH change effects when cycled in dilute base, possibly because of the more open texture of layers produced by cycling in solutions of low pH.