Kinetics of the Oxygen Evolution Reaction (OER) on Amorphous and Crystalline Iridium Oxide Surfaces in Acidic Medium

Amorphous and crystalline IrO2 catalysts are synthesized by the Adams method and characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The oxygen evolution reaction (OER) is investigated on both the catalyst surfaces in 0.5 M H2SO4 electrolyte. The Tafel slope estimated in the temperature range of 293-333 K on the two surfaces indicates a change in the rate-limiting steps. The data are also analyzed in terms of the Eyring equation to estimate the activation enthalpy (?H-#) and pre-exponential factor (A(f)) as a function of overpotential and therefore the charge-transfer coefficient (a). The estimated a values suggest strong electrocatalysis on both the surfaces. While the ?H-# plays a decisive role in the electrocatalysis on the amorphous sample, the trend of A f indicates that an increase in the entropy on the crystalline surface is pivotal in reducing the reaction barrier.