Oxygen evolution activity and stability of iridium in acidic media. Part 1. – Metallic iridium

Abstract

A better understanding of iridium dissolution is important in the elucidation of the general mechanism of noble metals corrosion and in the design of more durable iridium based applied materials. In the current work iridium dissolution has been addressed by a complementary electrochemical and mass spectrometric technique based on a scanning flow cell (SFC) and inductively coupled plasma mass spectrometry (ICP-MS). Time- and potential-resolved iridium dissolution profiles are recorded and analyzed. It is found that during the anodic treatment dissolution is increasing constantly with potential. Anodic dissolution decreases with time, which is attributed to the buildup of a passivating oxide layer. In case a significantly low reductive potential is applied to the oxidized electrode, an additional dissolution process is observed. It is concluded that like other members of the noble metals group, e.g. gold and platinum, thoroughly studied before, dissolution of iridium is initiated by a change in the iridium oxidation state during the initial formation or reduction of a thin compact anhydrous oxide layer. Both transitions metal/oxide and oxide/metal during oxidation and reduction, respectively, result in dissolution. Thus, dissolution is a transient process. The Ir(III)/Ir(IV) transition in the thick hydrous oxide layer, responsible for iridium oxide electrochromism, do not lead, however, to any significant change in dissolution signal. At even higher anodic potentials, further destabilization is caused by a change in the oxidation number (increase and decrease) of iridium cations during oxygen evolution reaction (OER). At studied potentials OER on the metallic electrodes, covered by a thin compact anhydrous oxide layer, has Tafel slope of ca. 66mVdec−1, which is comparable to literature data on bulk IrO2 electrodes and, probably, implies on similarities in the OER mechanism on these materials.