J. Electroanal. Chem., 158 (1983)375-381 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands Short communication 375 EFFECT OF TEMPERATURE ON THE POTENTIODYNAMIC BEHAVIOUR OF IRIDIUM IN 0.5 M H 2SO4 M. SA, NCHEZ CRUZ, T. FERNANDEZ OTERO and S. URETA ZAlqARTU Instituto de Quimica Fisica "Rocasolano'" C.S.L C., Madrid (Spain) (Received 24th May 1982; in revised form 18th March 1983) Anodic oxides formed on Ir electrodes by continuous potential cycling show a reversible redox behaviour under moderate potential sweep speeds, as has been reported in numerous papers. However, a certain irreversibility degree of the redox process was observed, due to the incomplete reduction of the oxide surface [1-3]. The oxide layer grown on the Ir electrode under continuous potential cycling shows a "duplex" structure, after the model put forward by Burke [4] for the Rh electrode: a compact, anhydrous and extremely thin oxide film at the metal surface, over which a thick, porous and extensively hydrated oxide forms. This second layer has an open structure constituted by polynuclear species held together by hydroxy- or oxy-bridges [5,6]; its porosity and hydrated character has been confirmed by ellipsometry [7] and electron microscopy [8]. In the present work we have studied the influence of temperature on the redox behavior of the anodic oxides of iridium by means of the potentiodynamic technique already described [9]. The electrolyte was 0.5 M H2SO4, whose temperature could be fixed to within +0.5°C over the range 20-70°C. The iridium electrode was a 1.2 × 1 cm rectangle. The auxiliary electrode was a Pt mesh, and the saturated calomel electrode (SCE) was used as reference. We give in Fig. 1 the influence of temperature on the voltammogram of the Ir electrode previously activated at •20°C by continuous cycling at 2 V s -1 between -0.23 and 1.35 V. In the positive sweep, an increase in the temperature shifts the start of the anodic oxidation towards lower potentials, and increases the anodic current in the potential range extending from peak II, to the beginning of oxygen evolution. In the negative sweep peak II c decreases and • its potential, Ep(IIc), shifts towards less positive values. A new peak, I~, appears between peak II~ and the hydrogen adsorption region; it increases with increasing temperature, while its peak potential shifts to more negative values, and is unaffected by N 2 bubbling. A similar peak •was detected by Zerbino et al. [10] in 96% H2SO 4 at 90°C. It may be assumed that increasing the temperature modifies the structure and composition (degree of hydration) of the highly hydrated oxide layer formed under continuous potential cycling. The species responsible for the new reduction peak, I~, could be a partially dehydrated oxide, with a higher resistance towards reduction. 0022-0728/83/$03.00 © 1983 Elsevier Sequoia S.A.