Complications associated with kinetic studies of hydrous Ir oxide films

The kinetics of the oxidation and reduction of electrochemically formed Ir oxide films have been examined as a function of the potential limits used, oxide film thickness, solution pH and the degree of reversible aging (diminished kinetics) of the oxide. It is shown first that the oxidation kinetics of the principal Ir(III) to Ir(IV) redox reaction are diminished in acidic solutions if the lower potential limit is set so as to include the kinetically slow process which occurs in the anodic cyclic voltammetric pre-peak, Ao. Consistent with this, in alkaline solutions, little evidence for the prepeak is found, reversible aging does not occur and the kinetics do not depend significantly an the lower potential limit. Other results show that, if kinetic measurements are made employing a lower limit more positive than the Ao pre-peak and an upper potential limit below that required to generate Ir(V) and/or Ir(VI) states, the kinetics of the Ir(III)/Ir(IV) process are independent of film thickness in acidic solutions. If anodic steps are made to 1.25 V or more in acidic solutions, the Ir(V)/(VI) states generated can mediate and, hence, accelerate, the oxidation of Ir(III) to Ir(IV), as seen by the unusual shapes of the j/t transients. (C) 1999 Elsevier Science Ltd. All rights reserved.