Study of oxygen evolution reaction on thermally prepared xPtOy-(100-x)IrO2 electrodes

Abstract

The mixed coupled _xPtO_y-_(100-x)IrO₂ electrodes (x = 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100) were thermally prepared at 450 °C on titanium supports. The prepared electrodes were firstly physically characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Afterwards, electrochemical characteri­zations were performed by voltammetric (cyclic and linear) methods in different electrolyte media (KOH and HClO₄). It is shown that the prepared electrodes are composed by both PtO_y (platinum and platinum oxide) and IrO₂ (iridium dioxide). For _xPtO_y-_(100-x)IrO₂ electrodes having higher content of IrO₂, more surface cracks and pores are formed, defining a higher surface area with more active sites. Higher surface area due to presence of both PtO_y and IrO₂, is for _xPtO_y-_(100-x)IrO₂ electrodes in 1 M KOH solution confirmed by cyclic voltammetry at potentials of the oxide layer region. For all prepared electrodes, voltammetric charges were found higher than for PtO_y, while the highest voltammetric charge is observed for the mixed ₄₀PtO_y-₆₀IrO₂ (x = 40) electrode. The Tafel slopes for oxygen evolution reaction (OER) in either basic (0.1 M KOH) or acid (0.1 M HClO₄) media were determined from measured linear voltammograms corrected for the ohmic drop. The values of Tafel slopes for OER at PtO_y, ₉₀PtO_y-₁₀IrO₂ and IrO₂ in basic medium are 122, 55 and 40 mV dec^-1, respectively. For other mixed electrodes, Tafel slopes of 40 mV dec^-1 were obtained. Although proceeding by different OER mechanism, similar values of Tafel slopes were obtained in acid medium, i.e., Tafel slopes of 120, 60 and 39 mV dec^-1 were obtained for PtO_y, ₉₀PtO_y-₁₀IrO₂ and IrO₂, and 40 mV dec^-1 for other mixed electrodes. The analysis of Tafel slope values showed that OER is more rapid on coupled mixed electrodes than on pure PtO_y. For mixed _xPtO_y-_(100-x)IrO₂ electrodes, OER is more rapid when the molar percent of PtO_y meets the following condition: 0 ˂ x ≤ 80. This study also showed that the mixed coupled electrodes are more electro­cata­lytically active for OER than either PtO_y or IrO₂ in these two media.