(614f) Understanding ORR Kinetics Through Microkinetic Modeling: Revisiting the Tafel Slope

The oxygen reduction reaction (ORR), O₂ + 4H⁺ + 4e^- = 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 phenomenon is the deviation of the observed ORR kinetic behavior on Pt from the linear characteristic predicted by the electro-kinetic Tafel equation: [delta]E=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, irrespective of commonly proposed causes such as changes in rate-limiting step or interactions 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, while higher ORR turnover rates would intuitively be expected when increasing reactivity toward oxygen, a more reactive surface will also push the equilibrium-driven decomposition of H₂O to OH to lower potentials, poisoning the surface for relevant operating conditions. 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.