Evaluating the impact of pellet densification and graphite addition for design of on-board hydrogen storage in a fixed bed of MOF-5 pellets

On-board storage of hydrogen is a key challenge in the deployment of fuel cell technology for transportation and distributed energy generation. Hydrogen adsorption capacity of up to 6 wt% has been reported for the metal-organic framework MOF-5, at 30 bar and 77 K. However, powders of MOF-5 suffer from low volumetric storage density and poor thermal conductivity for practical use in adsorptive storage systems. Compaction of MOF-5 to form pellets and inclusion of expanded natural graphite (ENG) has been used to address these issues, but their effect on the overall refueling dynamics for a fixed-bed has not been studied. To this end, we use simulations of multiscale pellet-and-bed model (developed in a companion paper) to analyze the impact of pellet modification on the dynamics of hydrogen refueling under cryogenic conditions. We show that a fixed bed with 0.52 g/cc density pellets is recommended, compared to MOF-5 powder or lower-density pellets. In spite of some loss of gravimetric capacity, the former shows good performance in a fixed bed with improved volumetric capacity and reasonable refueling time. Although individual pellet behavior is improved by addition of ENG to the 0.52 g/cc pellets, this has only a minor effect on refueling dynamics of the fixed bed with pellet size of 6 mm or lower. Finally, the effect of pellet size, density and ENG addition is analyzed and recommendations for fixed bed adsorber design are presented.