Porous solids are abundant in the world. Examples include activated carbon, zeolite, and metal-organic frameworks (MOFs). Solid electrolytes in next-generation batteries have ion migration channels and hence are porous solids in a broad sense. Because the properties of porous solids are dictated by pore size, the way the pores are connected internally, and the chemical nature of the pore walls, achieving high degrees of freedom in designing these properties is crucial.
As a novel class of nanoporous solids, covalent organic frameworks (COFs) are formed by covalently and repeatedly condensing building block molecules. COFs have recently been extensively studied because they offer high design freedom in terms of function and microscopic geometries in the materials, as well as the high thermal stability that is desired for applications.
However, there has been a dilemma. COFs have more stable and harder covalent bonds than MOFs, which have weaker and softer coordination bonds. This results in an advantage and two disadvantages. An advantage is the higher stability that enhances the durability during use. The first disadvantage is the poorer topological diversity in the framework geometries so far achieved. The second disadvantage is the difficulty in obtaining COFs with high crystallinity to the extent that crystal shapes are recognizable using an optical microscope.
From:Phys.org
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