The fundamentals related to the oxygen evolution reaction and catalyst design are summarized and discussed.

Electrochemical energy storage by making H2 an energy carrier from water splitting relies on four elementary reactions, i.e., the hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein, the central objective is to recommend systematic protocols for activity measurements of these four reactions and benchmark activities for comparison, which is critical to facilitate the research and development of catalysts with high activity and stability. Details for the electrochemical cell setup, measurements, and data analysis used to quantify the kinetics of the HER, HOR, OER, and ORR in acidic and basic solutions are provided, and examples of state‐of‐the‐art specific and mass activity of catalysts to date are given. First, the experimental setup is discussed to provide common guidelines for these reactions, including the cell design, reference electrode selection, counter electrode concerns, and working electrode preparation. Second, experimental protocols, including data collection and processing such as ohmic‐ and background‐correction and catalyst surface area estimation, and practice for testing and comparing different classes of catalysts are recommended. Lastly, the specific and mass activity activities of some state‐of‐the‐art catalysts are benchmarked to facilitate the comparison of catalyst activity for these four reactions across different laboratories.

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 O , 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 Co O (x = 2, 2.5, 3), Li Mn O (x = 0.7, 1), XCo O (X = Co, Ni, Zn), and XFe O (X = Mn, Co, Ni). A general principle is concluded that the e 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.