Combining Single Crystal Experiments and Microkinetic Modeling in Disentangling Thermodynamic, Kinetic, and Double-Layer Factors Influencing Oxygen Reduction

The volcano-shaped relation between binding energies of reaction intermediates and catalytic activities underpins the binding energy approach, a prevailing guideline, in electrocatalysis. Pt, Ir, and Rh belong to the over-adsorbing leg of the volcano plot and adsorb oxygen-containing intermediates in an intensifying order. Therefore, the binding energy approach predicts an activity order of Pt > Ir > Rh for ORR. Herein, single crystal experiments corroborate the binding strength order of Pt < Ir < Rh but give out an ORR activity order of Pt > Rh > Ir, contradicting the binding energy approach. To understand this discrepancy, we develop a microkinetic ORR model that complements the standard binding energy approach by including multistep kinetics corrected with double layer effects. Armed with this model, we extract binding energies from cyclic voltammogram, quantitatively fit the polarization curves of all three metals, and attribute the higher ORR activity of Rh than Ir to a lower activation barrier of OHad desorption because Ir possesses a higher magnitude of the surface charge, inducing a more rigid layer of adsorbed water at the metal surface. A key message delivered here is that kinetic and double layer effects are nontrivial for ORR on the single crystalline electrodes studied here.