Beyond the Rate-Determining Step in the Oxygen Evolution Reaction over a Single-Crystalline IrO2(110) Model Electrode: Kinetic Scaling Relations

Electrochemical water splitting is a key technology for moving toward a promising energy scenario based on renewable (regenerative) energy resources in that wind and solar energy can be stored and buffered in chemical bonds, such as in H2. The efficiency of water electrolysis is, however, limited by the sluggish oxygen evolution reaction (OER) at the anode, for which IrO2-based electrodes are considered to be the best compromise of a stable and reasonably active OER electrocatalyst in acidic medium. To improve existing OER electrocatalysts and to advance a rational search of promising alternative electrode materials, it is imperative to identify the rate-determining step (rds). We apply here the concept of the free energy diagram along the reaction coordinate to identify the rate-determining step (rds) in the oxygen evolution reaction (OER) over an IrO2(110) model anode in both acidic and basic media. The free energy diagram as a function of the applied electrode potential is constructed from experimental Tafel plots and ab initio Pourbaix diagrams. Quite in contrast to common perception, the rds for the OER over IrO2(110) at high overpotentials is identified with the decomposition of the OOH adsorbate via a decoupled electron–proton transfer to form gaseous O2. Combining linear scaling relationships with the free energy diagram approach leads to the introduction of kinetic scaling relations, which allow us to predict the rate-determining step (rds) of the OER over general transition metal oxide electrocatalysts in the high-overpotential regime by a single descriptor, namely, the free formation energy of oxygen with respect to the OH adsorbate (ΔG2) on the anode surface. On the basis of kinetic scaling relations we suggest that further improvement of the catalytic OER performance may require a decoupling of the electron–proton transfer in the rds.