Covalent organic frameworks and their metal nanoparticle composites: Prospects for hydrogen storage

Covalent organic frameworks (COFs) are microporous crystalline organic frameworks with large specific surface areas. The area of COFs is rapidly developing in the direction of finding potential applications in fields like gas storage, photovoltaics, and catalysis. With respect to hydrogen storage, at first glance, COFs possess all the advantages of metal-organic frameworks (surface area, pore volume, rigidity of the structure). In addition, since the molecular frameworks of COFs are composed of light elements (C, Si, B, and O), these materials have exceptionally low densities. Due to this advantage, a lot of research activity (both theoretical and experimental) was reported in the recent literature on hydrogen-storage properties of COFs. Also, several strategies were suggested for enhancing H₂-storage capacities of COFs at cryogenic as well as room temperatures. In this feature article, we broadly discuss the scope of COFs as hydrogen storage media and also review the strategies suggested for enhanced room-temperature hydrogen-storage properties. Further, the concept of "spillover" is reviewed critically in metal@COFs and metal@MOFs systems.