EQCM study of iridium anodic oxidation in H2SO4 and KOH solutions

In the present study anodic oxidation of iridium layer formed thermally on a gold-sputtered quartz crystal electrode has been investigated by electrochemical quartz crystal microgravimetry (EQCM) in the solutions of 0.5 M H2SO4 and 0.1 M KOH. The emphasis here has been put on the microgravimetric behavior of iridium as a metal, because a few previous EQCM studies reported in literature have been devoted to iridium oxide films (IROFs). The objective pursued here has been to elucidate the nature of the main voltammetric peaks, which occur at different ranges of potential in the solutions investigated. It has been found that anodic oxidation of iridium electrode in 0.5 M H,SO, and 0.1 M KOH solutions is accompanied by irregular fluctuations of the electrode mass at 0.4 V < E < 0.8 V followed by regular increase in mass at 0.8 V < E < 1.2 V. The cathodic process initially, at 1.2 V > E > 0.9 V, proceeds without any or with slight increase in electrode mass, whereas at E < 0.8 V a regular decrease in mass is observed. It has been found that mass to charge ratio characterizing the processes of interest is 2 to 3 g F-1 in acidic medium, whereas in the case of alkaline one it is 4 to 6 g F-1. The main pair of peaks seen in the voltammograms of Ir electrode in alkaline medium at E < 0.8 V is attributable to redox transition Ir(0) - Ir(III), whereas those observed in the case of acidic medium at E > 0.8 V should be related to the redox process Ir(0) Ir(IV) going via intermediate stage of Ir(III) formation. As a consequence of these redoxtransitions, the gel-like surface layer consisting of Ir(III) or Ir(IV) hydrous oxides forms on the electrode surface.