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 (ΔH^#) and pre-exponential factor (A_f) as a function of overpotential and therefore the charge-transfer coefficient (α). The estimated α values suggest strong electrocatalysis on both the surfaces. While the ΔH^# 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.