ConspectusCovalent organic frameworks (COFs) are a rapidly emerging class of crystalline porous polymers, characterized by their highly defined, predictable, and tunable structure, porosity, and properties. COFs can form both two-dimensional (2D) and three-dimensional (3D) architectures, each with unique characteristics and potential applications. 2D COFs have attracted particular interest due to their favorable structural and optoelectronic properties. They can be equipped with a range of different functional moieties in their backbone, ranging from acidic to basic, from hydrophilic to hydrophobic, and from metal-coordinating to redox-active functions. In addition, their crystallinity, high specific surface area, and remarkable thermal and chemical stability make them attractive for a variety of applications, including gas separation, catalysis, energy storage, and optoelectronics.This Account provides a detailed overview of our recent efforts to synthesize and apply 2D COFs. First, various synthesis routes are discussed, focusing on methods that involve reversible and irreversible linkage reactions. Reversible reactions, such as imine or boronate ester formation, are advantageous for producing highly crystalline COFs because they allow for error correction during synthesis. In contrast, irreversible reactions, such as carbon-carbon or carbon-nitrogen bond formation, yield COFs with greater chemical stability, although controlling crystallinity can be more challenging. Our group has contributed significantly to refining these methods to balance crystallinity and stability, enhancing the performance of the resulting 2D COFs.In addition to different binding patterns, we have also developed strategies to control the micro- and macromorphologies of COFs, which is crucial for optimizing their properties for specific applications. For example, we have explored the synthesis of hierarchical porous COFs by using templating techniques or by forming composites with other functional materials. These strategies enable us to fine-tune the porosity and surface properties of COFs, thereby improving their performance in applications like catalysis. Hierarchical structures in particular enhance photocatalytic efficiency by providing a larger surface area for light absorption and facilitating the transport of photogenerated charge carriers.We further examine the practical applications of 2D COFs, with a primary focus on photocatalysis. Photocatalysis uses light to enable or accelerate chemical reactions, and 2D COFs are ideal for this purpose due to their tunable band gaps and large surface areas. Our research has shown that 2D COFs are highly versatile photocatalysts that can effectively catalyze reactions such as water splitting, carbon dioxide reduction, hydrogen peroxide formation, and cross-coupling reactions. By exploiting the unique properties of 2D COFs, we have achieved significant improvement in many photocatalytic reactions.With this comprehensive overview, we aim to contribute to the further development and understanding of 2D COFs and encourage further research and innovation in this promising field.

Nitrate electroreduction to ammonia offers a dual opportunity: decarbonizing NH₃ production by replacing the energy-intensive Haber-Bosch process and remediating nitrate-contaminated wastewater. While copper-based catalysts show promise for this transformation, their practical implementation is hindered by sluggish kinetics and competing hydrogen evolution pathways. Here, we report the integration of covalent organic framework (COF) layers onto a Cu₂O surface, which enables rapid proton/electron transfer and substrate activation for efficient NH₃ electrosynthesis from NO₃^- in neutral media. By growing pyridine- or imidazole-decorated COF shells with varying thickness on Cu₂O nanocubes, we achieve precise control over the microenvironment of the electrocatalyst surface. The pyridine-COF on Cu₂O demonstrates 84% Faradaic efficiency for NH₃ production with 92.11% selectivity and a record yield of 2.3 mg h^-1 cm^-2. In situ spectroscopic investigations reveal that the COF shells have multifunctional roles: they selectively transport reactants, stabilize key intermediates through hydrogen bonding interactions, and steer the reaction along an associative pathway that bypasses common side reactions. Our findings establish COF-gated core-shell architectures as a generalizable platform for designing efficient, selective, and durable electrocatalysts for broad applications.