Recent progress in covalent organic frameworks for bifunctional oxygen electrocatalysis in rechargeable zinc-air...

Zinc-air batteries (ZABs) are regarded as one of the most promising next-generation energy storage technologies owing to their high theoretical energy density, intrinsic safety, and cost-effectiveness. However, their practical deployment is largely hindered by the sluggish kinetics of the oxygen reduction reaction (ORR) during discharge and the oxygen evolution reaction (OER) during charge. The development of robust bifunctional electrocatalysts that can efficiently and stably catalyse both reactions is therefore critical for advancing ZAB technology. Covalent organic frameworks (COFs), a class of crystalline porous polymers, have emerged as a versatile platform, and achieving intrinsic conductivity and improving electronic mobility within the framework are two benefits of customizing the electrical structure, especially by doping or adding conductive linkers. These properties are crucial for electrochemical applications. Because of their high conductivity, which promotes effective catalysis and charge transfer, as well as the arrangement and accessibility of reactive centers inside the COF, engineered COFs offer special platforms for enhanced materials design. In this review, we provide a comprehensive overview of recent progress in the rational design of bifunctional electrocatalysts, with a particular emphasis on COFs and their derived materials. We discuss design strategies, including heteroatom doping, metal coordination, pore engineering, and electronic structure modulation that enhance intrinsic catalytic activity, charge transport, and mass diffusion. Mechanistic insights from density functional theory (DFT) calculations and in-situ/operando spectroscopies are highlighted to unravel active-site structures and catalytic pathways. Furthermore, we summarize the impact of COF-based bifunctional catalysts on key ZAB performance indicators such as power density, discharge capacity, round-trip efficiency, and long-term cycling stability. Finally, we outline current challenges, including scalable synthesis, interfacial engineering, and durability and provide future perspectives on integrating machine learning and advanced characterization to accelerate the discovery of next-generation ZAB electrocatalysts. Collectively, this review underscores the pivotal role of bifunctional catalyst design in unlocking the practical potential of high-performance, sustainable, and rechargeable zinc-air batteries.