Cations in Octahedral Sites: A Descriptor for Oxygen Electrocatalysis on Transition‐Metal Spinels

Exploring efficient and low‐cost electrocatalysts for the oxygen‐reduction reaction (ORR) and oxygen‐evolution reaction (OER) is critical for developing renewable energy technologies such as fuel cells, metal–air batteries, and water electrolyzers. A rational design of a catalyst can be guided by identifying descriptors that determine its activity. Here, a descriptor study on the ORR/OER of spinel oxides is presented. With a series of MnCo 2 O 4 , the Mn in octahedral sites is identified as an active site. This finding is then applied to successfully explain the ORR/OER activities of other transition‐metal spinels, including Mn x Co 3− x O 4 ( x = 2, 2.5, 3), Li x Mn 2 O 4 ( x = 0.7, 1), XCo 2 O 4 (X = Co, Ni, Zn), and XFe 2 O 4 (X = Mn, Co, Ni). A general principle is concluded that the e g occupancy of the active cation in the octahedral site is the activity descriptor for the ORR/OER of spinels, consolidating the role of electron orbital filling in metal oxide catalysis.