Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the two most important reactions in rechargeable metal‐air battery, a promising technology to meet the energy requirements for various applications. The development of low‐cost, highly efficient and stable bifunctional ORR/OER catalysts is critical for a large‐scale application of this technology. In this review, the authors first introduce the fundamentals of bifunctional ORR/OER electrocatalysis in alkaline electrolyte. Various types of nanostructured materials as bifunctional ORR/OER catalysts including metal oxide, hydroxide and sulfide, functional carbon material, metal, and their composites are then reviewed. The crucial factors that can be used to tune the activity of the catalyst towards ORR/OER are summarized, including (1) phase, morphology, crystal facet, defect, mixed‐metal and strain engineering for metal oxide; (2) heteroatom doping, topological defects, and formation of metal‐N‐C structure for carbon material; (3) alloy effect for metal. These experiences lay the foundation for large scale application of metal‐air battery and can also effectively guide the rational design of catalysts for other electrocatalytic reactions.
Alongside rare‐earth metals, Ni, Fe, Co, Cu are some of the critical materials that will be in huge demand thanks to growth in clean‐energy sector. Herein scrap stainless steel wires (SSW) from worn‐out tires are employed as a support material for catalyst integration in the hydrogen evolution reaction (HER). In addition, SSW by corrosion engineering is exercised as an in situ formed freestanding robust electrode for the oxygen evolution reaction (OER). By superficial corrosion of SSW, inherent active species are unmasked in the form of Ni/FeOOH nanocrystallites displaying efficient water oxidation by reaching 500 mA cm−2 at low overpotential (η500) of 287 mV in 1 m KOH. Similarly, cathode scrap SSW with active (alloy) coatings of MoNi4 catalyzes the HER at η‐200 = 77 mV, with a low activation energy (Ea = 16.338 kJ mol−1) and high durability of 150 h. Promisingly, when used in industrial conditions, 5 m KOH, 343 K, these electrodes demonstrate abnormal activity by yielding high anodic and cathodic current density of 1000 mA cm−2 at η = 233 mV and η = 161 mV, respectively. This work may inspire researchers to explore and reutilize high‐demand metals from scrap for addressing critical material shortfalls in clean‐energy technologies.
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