Hierarchically Porous Reduced Graphene Oxide Coated with Metal-Organic Framework HKUST-1 for Enhanced Hydrogen Gas Affinity

Metal organic frameworks (MOFs) are crystalline porous materials with interconnected pores and have been actively explored for various gas storage, separation and conversion applications due to their structural tunability. While the micropores (<2 nm) in MOFs are essential for increased gas affinity, these small pores significantly decrease the mass transport kinetics. One way to address this challenge is to develop hierarchically porous MOFs with interconnected micro-, meso- and macropores. Whereas these MOFs can be formed by using soft/hard templates or by creating pores through post-modification, it can also be achieved by growing MOFs on structural templates such as porous carbons i.e., reduced graphene oxide. The latter strategy can enable the introduction of hierarchical porosity, while creating a synergistic effect to simultaneously improve both mechanical property and gas affinity by creating pores at the interface. In this direction, we demonstrated that the coating of HKUST-1 onto a porous reduced graphene oxide (HRGO) led to the formation of a hierarchically porous structure, namely, HKUST-1@HRGO with increased affinity towards H2 gas. While the isosteric heats of adsorption (Q_st) values for H2 were found to be 7.7, 6.9 and 6.7 kJ mol^-1 for HRGO, HKUST-1 and the physical mixture of HKUST-1 and HRGO, respectively, at zero coverage, that of HKUST-1@HRGO composite revealed a significant increase up to 9.26 kJ mol^-1 , thus clearly demonstrating not only the synergetic effect between HKUST-1 and the reduced graphene oxide and also the critical role interfacial pores as high affinity binding sites.