Covalent organic frameworks are developed by Yaghi team on the basis of metal-organic frameworks. Once reported, covalent organic frameworks have received a lot of attention in hydrogen storage materials field due to their unique mechanism and excellent properties. The skeleton of covalent organic frameworks is mostly composed of H, B, O, C, Si and other light elements, so the crystal density is low. As a new type of hydrogen storage material, covalent organic frameworks has many advantages. First, the porous structure and large surface area of covalent organic frameworks provide conditions for hydrogen storage. Second, the crystal density of covalent organic frameworks is low, which can be used to prepare light-weight hydrogen storage equipment. Thirdly, the crystal structure of covalent organic frameworks can be controlled by changing the parent. Moreover, dehydration condensation between hydroxyl groups within the molecule is the reaction used in synthesis of covalent organic frameworks. Dehydration between the same molecules will form a six-membered ring structure of B3O3, while dehydration reactions between different molecules will form a five-membered ring structure of BO2C2.
- Pure covalent organic frameworks: All the components of pure covalent organic frameworks used in hydrogen storage are light elements and do not contain metal elements. Pure covalent organic framework has a wider use as a computer model for theoretical calculations. Using density functional theory, grand canonical Monte Carlo method, Clausius-Clapperron equation and others to the theoretically calculation of the hydrogen storage properties of the materials. After a lot of research works, the understanding of pure covalent organic frameworks hydrogen storage material is more in-depth, which provides a solid foundation for future industrial application.
Figure 1. Examples of pure covalent organic frameworks.
- Doped covalent organic frameworks: Metal doping surface modification technology is a novel research method in recent studies of hydrogen storage performance. In order to improve the hydrogen storage performance of covalent organic frameworks, some research groups have studied the effect of metal doping on the hydrogen adsorption behavior of covalent organic frameworks. The simplest method is direct doping, doping elements such as Li, Ca, Mg and Ti. The results show that metal doping can significantly improve the hydrogen storage capacity of covalent organic frameworks and enhance the binding energy of covalent organic frameworks to hydrogen molecules. Another method is substitution doping, in which metal atoms are incorporated into the organic skeleton material by replacing non-metal atoms. This substitution method does not significantly increase the mass of the organic skeleton material, but the metal atoms can remain in it as part of the structure.
Figure 2. An example of doped covalent organic frameworks.
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