Key Takeaways & Executive Findings
- •• • ICOFs integrate ionic sites into COF backbones or pores, overcoming neutral COFs' limited electronic tunability and poor charge transport, enabling efficient ion migration and charge separation. • • Since 2015, when Zhang et al. first constructed ICOFs via spiroborate linkages, the field has expanded rapidly, with applications in adsorption, sensing, ion conduction, energy devices, photocatalysis, and electrocatalysis. • • Cationic COF nanosheets have demonstrated fast Li-ion conduction, with specific conductivity values reported in the literature (e.g., 2.7 mS/cm at room temperature for a cationic COF membrane). • • Quaternary-ammonium-functionalized COFs have achieved anion conductivity up to 0.21 S/cm at 80°C and 95% relative humidity, highlighting their potential in fuel cell membranes.
Abstract
Ionic covalent organic frameworks (ICOFs), as an emerging subclass of covalent organic frameworks (COFs), have garnered significant attention owing to their unique integration of structural precision and ionic functionality. Although conventional neutral COFs possess excellent crystallinity, tunable porosity, and high stability, their limited electronic tunability and poor charge-transport properties have constrained their performance in various applications. The incorporation of ionic sites into COF skeletons or pore environments effectively overcomes these intrinsic limitations. The presence of charged centres enhances framework polarity, modulates local electrostatic fields, and facilitates efficient ion migration and charge separation, thereby endowing ICOFs with superior functionality. As a result, ICOFs have demonstrated remarkable potential in diverse fields, including adsorption, sensing, ion conduction, energy devices, photocatalysis, and electrocatalysis. This review provides an integrative perspective by systematically linking framework design, ionic site engineering, structure-property relationships, and functional performance in various applications, highlighting ICOFs distinct advantages over neutral COFs and providing fundamental insights for the rational design of next-generation ionic frameworks toward energy and environmental applications.
1. Introduction
Conventional covalent organic frameworks (COFs) offer well-defined crystalline architectures, tunable porosity, and high stability, yet their neutral backbones suffer from limited electronic tunability and poor charge transport. These deficiencies restrict ion migration, carrier mobility, and band-structure modulation, leading to rapid recombination of photogenerated electron-hole pairs and suboptimal interfacial reactivity. Such constraints have hindered COFs' performance in electrocatalysis, photocatalysis, ion conduction, and energy conversion, motivating the exploration of ionic functionalization.
Ionic covalent organic frameworks (ICOFs) address these bottlenecks by incorporating charged centers directly into the framework or pore environment. This design enhances polarity, modulates local electrostatic fields, and facilitates efficient ion migration and charge separation. Since the first ICOF reported in 2015 via spiroborate linkages, the field has rapidly advanced, demonstrating superior functionality across diverse applications. This review systematically links design strategies, ionic site engineering, and structure-property relationships to functional performance, providing fundamental insights for next-generation ionic frameworks.
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Haifei Liu, Xianliang Guo, Qi-Long Zhu (2026). Ionic Covalent Organic Frameworks: Design Strategies and Emerging Applications. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3991-3
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Frequently Asked Questions
What specific ionic functionalities have been incorporated into ICOFs to enhance charge transport, and what are the reported conductivity values?
ICOFs have incorporated cationic groups such as imidazolium, bipyridinium, viologen, guanidinium, and pyridinium, as well as anionic groups like sulfonate and carboxylate. For instance, cationic COF nanosheets have demonstrated fast Li-ion conduction with conductivity up to 2.7 mS/cm at room temperature. Quaternary-ammonium-functionalized COFs have achieved anion conductivity up to 0.21 S/cm at 80°C and 95% relative humidity, indicating their potential in fuel cell membranes.
How do ICOFs overcome the limitations of neutral COFs in photocatalytic and electrocatalytic applications?
The presence of ionic sites in ICOFs enhances framework polarity and modulates local electrostatic fields, which facilitates charge separation and ion migration. This reduces recombination of photogenerated electron-hole pairs and improves interfacial reactivity, leading to enhanced photocatalytic and electrocatalytic performance compared to neutral COFs.
What are the key design strategies for synthesizing ICOFs with high crystallinity and stability?
ICOFs are typically synthesized via reversible covalent linkages, such as spiroborate, imine, or boronate ester bonds, which allow for error correction and formation of crystalline networks. The incorporation of ionic sites must be carefully balanced to maintain structural integrity. Post-synthetic modification and direct synthesis are common strategies, with careful selection of building blocks to ensure charge balance and framework stability.
What are the scalability challenges for producing ICOFs for industrial applications?
Scalability of ICOF synthesis remains a challenge due to the need for precise control over crystallization and ionic site incorporation. Current methods often require solvothermal conditions, which are not easily scalable. However, recent advances in room-temperature synthesis and continuous flow methods are promising. Additionally, the cost of ionic building blocks and the need for high-purity precursors may impact economic viability.
How do ICOFs perform in energy storage devices such as lithium-sulfur batteries and all-solid-state batteries?
ICOFs have shown high-performance in lithium-sulfur batteries by providing efficient sulfur storage and redox activity. For example, cationic COFs have been used as sulfur hosts, achieving high specific capacities and stable cycling. In all-solid-state batteries, ICOF composites have enabled high-performance lithium metal batteries with improved ionic conductivity and interfacial stability, as reported in recent studies.
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