Adsorption and desorption of hydrogen on metal–organic framework materials for storage applications: comparison with other nanoporous materials
Hydrogen adsorption on porous materials is one of the possible methods proposed for hydrogen storage for transport applications. High pressure experimental studies of a wide range of porous materials have obtained maximum hydrogen excess capacities of 6–8 wt% at 77 K for metal–organic frameworks (MOFs) and porous carbon materials. Grand canonical Monte Carlo (GCMC) simulation studies indicate that higher hydrogen capacities are possible for covalent organic frameworks (COFs). Currently, the maximum isosteric enthalpies of adsorption of ∼13 kJ mol−1 at 77 K have been observed experimentally for metal–organic framework materials and this is higher than for COFs, where the maximum predicted from GCMC simulations is ∼8 kJ mol−1. Metal–organic framework materials have structural diversity and scope for modification of surface chemistry to enhance hydrogen surface interactions. The synthesis of MOFs with stronger H2–surface interactions to give similar hydrogen capacities at much higher temperatures than 77 K is required and eventually, materials that have these high capacities at ambient temperatures with rapid adsorption/desorption characteristics are necessary for applications as hydrogen storage materials for transport applications. The current methods envisaged for increasing adsorption at higher temperatures involve modification of the surface chemistry, in particular, the inclusion of open metal centres to increase hydrogen surface site interactions, and utilisation of the framework flexibility are discussed.