(485b) Revisiting the Tafel Slope: Understanding ORR Kinetics Through Microkinetic Modeling

The oxygen reduction reaction (ORR), O₂ + 4H⁺ + 4e^- à 2H₂O, on Pt electrodes is known to limit performance in low-temperature hydrogen fuel cells. A mechanistic understanding of the reaction is thus critical to the rational design of improved electrode catalysts. One often-debated ORR phenomenon is the deviation of the observed kinetic behavior on Pt from the linear characteristic predicted by the electro-kinetic Tafel equation: DE=a+b log(i). An ORR Tafel plot generally displays a sharp change in slope.

    Herein, we utilize microkinetic modeling to demonstrate analytically that rapid changes in the kinetic behavior (Tafel slope and reaction orders) of electrochemical reactions are an inherent property of electrode kinetics that involve multiple elementary steps and adsorbed intermediates, even without frequently-proposed factors such as changes in rate-limiting step or repulsion between adsorbates. For the ORR on Pt, our analysis reveals the measured Tafel slope, including its characteristic shift, is consistent with a rate-limiting initial electron transfer.  We show that surface oxygen species (primarily hydroxyl) impede the rate at small overpotentials through site blocking even when their removal involves fast, quasi-equilibrated steps. Thus, higher ORR turnover rate should in principle be achieved by increasing reactivity toward oxygen, but a more active surface will drive the equilibrium-driven decomposition of H₂O to OH to lower potentials, poisoning the surface. A major breakthrough in ORR catalysis will likely require materials that can decouple the binding energies of oxygen and hydroxyl groups, which generally scale together. We additionally show that there is a family of adsorbate-substrate systems that do not follow traditional models of chemisorption and may satisfy this requirement.