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In recent years, rechargeable zinc–air batteries (ZABs) have attracted much attention owing to their high theoretical specific energy density, safety, and environmental friendliness. They are also considered a viable option for powering the grid and electric vehicles in the future. The activity and the stability of the bifunctional catalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in ZABs remarkably influence the battery’s energy, power densities, and lifetime. Therefore, developing efficient and stable bifunctional catalysts is an important research direction. Previously, precious metal catalysts such as Pt/C, Ir/C, and Ru/C have been used to design efficient ORR and OER electrocatalysts. However, these electrocatalysts possess several issues, including limited natural abundance, high metal sintering and catalyst detachment rates from supports, and poor bifunctional activity, which limit their practical applications. As potential catalysts, nonprecious transition metal oxides exhibit the prominent advantages of manifold compositions, multiple valence states, environmental friendliness, high durability and abundance, and varying structures. Their disadvantages include poor electrical conductivity and a limited surface area. To address the abovementioned issues, current general research focuses on compounding transition metal oxides with carbon materials or other conductive substrates to simultaneously increase their specific surface area and electrical conductivity and control their morphology to expose more active sites. Furthermore, the intrinsic activity of the transition metal oxides must also be regulated through the most commonly used activity regulation methods (e.g., heteroatom doping and defect engineering). For example, perovskite and spinel-type transition metal oxides must meet a specific eg orbital occupancy to achieve the best bifunctional activity. Therefore, improving the catalytic performance requires the A- and B-site atom substitution with other alkaline and rare earth or transition metals or the introduction of oxygen vacancies to adjust the electronic structure. Spinel- and perovskite-type transition metals and manganese oxides are the research objects used in this work. The activity sources of different types of transition metal oxide catalysts and their performances in energy density, charge/discharge voltage, and cycle life of zinc–air batteries are introduced herein. The strategies and methods used to improve the catalytic performance of transition metal oxides in current research are summarized. Finally, the future development of transition metal bifunctional catalysts for oxygen reduction and evolution reactions is prospected.

The microstructure and properties of powder metallurgy high-speed steel prepared by hot isostatic pressing with different heat treatments have been studied. The microstructure, phase composition, effect of quenching and tempering parameters, fracture morphology, and mechanical properties of the sample are discussed in detail. The H-HSS sample presents the characteristics of the powder prior to the particle boundary and consists of carbide and ferrite, in which the carbides are fine and evenly dispersed without segregation. The bending strength and hardness of the H-HSS sample are 3112 MPa and 56.3 HRC, respectively. The Q-HSS sample is mainly composed of martensite, residual austenite, and carbides. With the increase in quenching temperature, the grain size of the matrix gradually grows, and the small carbide particles dissolve into the matrix, causing an increase in carbide size and a decrease in quantity. The bending strength and hardness of the Q-HSS sample quenched at 1210 °C achieve the maximum values of 3114 MPa and 68.8 HRC, respectively. After tempering, the martensite is transformed from a quenched lath shape to a needle shape, the residual austenite content decreases, and secondary carbides precipitate from the matrix, resulting in a secondary hardening. The T-HSS sample that is quenched at 1120 °C followed by tempering at 550 °C for 20 min has the best bending strength of 4355 MPa. However, the T-HSS sample that is quenched at 1240 °C followed by tempering at 550 °C for 120 min has a maximum hardness value of 69.5 HRC. The fracture mode of Q-HSS sample is brittle fracture, and the fracture mechanism is cleavage fracture. After tempering, the fracture mechanism of the T-HSS sample presents a transitional fracture mode between the cleavage fracture and micropore aggregation fracture.