Influence of Defects on Barrier Energy Formation for OOH* Intermediate in ORR on Tetragonal-ZrO₂ with Adsorbed-Hydroxyl

Accelerating the oxygen reduction reaction (ORR) is a main subject of electrocatalysis research. A key ORR step involves creating a hydroperoxyl functional group (OOH*) intermediate. This study examines the defect influence on the formation of the OOH* in a zirconia-based cathode during hydroxyl group (−OH) adsorption. Simulations on tetragonal pristine ZrO2 (111) surfaces with introduced oxygen vacancy (t-ZrO2–x) and nitrogen dopant (ZrO2–xNx) are conducted using density functional theory (DFT). Results suggest that oxynitride t-ZrO2–xNx has minimal −OH adsorption, while under-stoichiometric oxide t-ZrO2–x shows the highest affinity, potentially influencing OOH* formation. Minimum energy pathway (MEP) analysis using the nudged elastic band (NEB) approach shows the defects' significant impact on adjusting the barrier energy for OOH* formation in the presence of the −OH group. Analyzing natural bond orbital (NBO) data offers insights into electron distribution during the formation of the OOH* intermediate in reaction mechanisms. Our study demonstrates a competition between ZrO2–xNx and t-ZrO2–x with OH-adsorbed in ORR acceleration. OOH* is favored on ZrO2–xNx due to its lower barrier energy, while the stability of the OOH* is higher on the Zr site of t-ZrO2–x, potentially influencing subsequent ORR steps. These study findings provide key insights for researchers developing nonplatinum-based cathode materials.