Effect of Nitrogen Doping and Oxygen Vacancy on the Oxygen Reduction Reaction on the Tetragonal Zirconia(101) Surface

Abstract

In this study, we investigated the role of defects in promoting the oxygen reduction reaction (ORR) of a zirconia-based cathode, which is motivated by the need for a stable and active electrocatalyst for proton exchange membrane fuel cells. A defective catalyst was modeled by a tetragonal ZrO2(101) slab introduced with oxygen vacancies and nitrogen dopants, and the structure of the ORR intermediates was exhaustively computed from first principles. As is typical of oxides, various adsorption structures with slightly different energies were obtained. Therefore, we obtained the free energy profile for the ORR and established that the energy barrier of the rate-determining step, or the removal of OH*, is comparable to (or only slightly lower than) that of the pristine surfaces, indicating that defects play only a minor role in enhancing the ORR activity. The measured enhancement in the ORR current could be ascribed to the enhanced carrier supply, which may be attributed to the formation of a gap state or change in the potential profile, or to other factors so far overlooked. The results of the first-principles study provide a significant insight for experimentalists to develop optimal cathode materials.