Emergence of Oxygen Reduction Activity in Zirconium Oxide-Based Compounds in Acidic Media: Creation of Active Sites for the Oxygen Reduction Reaction

To develop zirconium oxide-based electrocatalysts for the oxygen reduction reaction (ORR), we attempted to clarify the factors that affect the ORR activity of oxide-based cathodes. We prepared zirconium oxide-based electrocatalysts from oxy-zirconium phthalocyanine with multiwalled carbon nanotubes as electroconductive supports via heat treatment under a low oxygen partial pressure. We evaluated the density of the active sites, i.e., oxygen vacancies, using X-ray photoelectron spectroscopy (XPS), the average particle size of the oxide particles using a transmission electron microscopy (TEM), and the surface area utilization of the oxide particles using TEM image analysis. We successfully determined that the increase in the ORR activity was due to the increase in the density of oxygen vacancies and the effective surface area and that the decrease in the effective surface area was responsible for the decrease in the ORR activity. In addition, we performed hard X-ray photoemission spectroscopy (HAXPES) analysis to understand the process by which the active sites were created. A comparison of the XPS and HAXPES results revealed that the oxygen vacancies were concentrated near the surface during heat treatment under a low oxygen partial pressure via the oxidation of the deposited carbon. The deposited carbon plays a role in creating ORR-active oxygen vacancy sites and promoting ORR by providing local nanoscale conduction paths on the relatively insulating oxide surfaces.