Temperature-Dependent Oxygen Evolution Kinetics on Iridium Oxide Electrocatalysts

Abstract Iridium oxide electrocatalysts for the oxygen evolution reaction (OER) are typically evaluated at a single applied potential, with the measured current normalized by an estimate of the active site density. Reliable methods for counting active sites on these materials are not available, however, leaving performance comparisons sensitive to the choice of normalization. A further complication is that the kinetic response of a catalyst varies with applied potential; hence, the relative ordering of two catalysts at one overpotential need not hold at another. We argue that both problems can be addressed by characterizing OER kinetics through the response of current to temperature and applied potential, rather than through its absolute magnitude. We measure the OER activity of three commercial iridium oxide catalysts that differ in crystallinity, composition, and morphology, using linear sweep voltammetry in 0.1 M perchloric acid over a 35 K range. From Tafel and Eyring analyses, we extract the standard activation enthalpy and total transfer coefficient and show that the relative ordering of activity is preserved across mass-, geometric-area-, and capacitance-based normalizations. Extending the analysis across a range of overpotentials, we resolve three kinetic regimes and decompose the transfer coefficient into enthalpic and entropic contributions. Because the transfer coefficient varies with potential and differs between catalysts, which material exhibits the lowest activation barrier depends on the overpotential at which the comparison is made. Mapping the activation enthalpy and transfer coefficient across overpotential, rather than reporting either at a single condition, therefore gives a more complete and normalization-agnostic description of OER kinetics on iridium oxide.