Hydrogen Storage Performance Of Metal-Organic Polyhedra/Metal-Organic Framework Composites

Hydrogen is one of the most promising and efficient energy carriers due to its environmental friendliness and high calorific value.Unfortunately,one major key factor that restricts the large-scale application of hydrogen energy is hydrogen storage.Physisorption of hydrogen molecules in porous materials has attracted increasing attention because of its fast adsorption/desorption kinetics and reversibility.Metal-organic frameworks(MOFs)with high specific surface area and porosity have demonstrated great potentials to store hydrogen.However,the hydrogen storage capacities of most MOFs cannot meet the demand for actual application.The modular nature of MOFs allows for the facile,ordered incorporation of new functionalities to enhance the hydrogen storage properties.Recently,metal-organic polyhedra(MOPs)have been widely investigated for hydrogen storage owing to their open metal sites and the discrete molecular structure.MOPs can be combined with MOFs to improve the hydrogen storage performance of composites.However,there are no reports on the hydrogen storage performance of MOPs/MOFs composites,and the influence mechanism of synthesis approach and MOPs content on the hydrogen storage performance of MOPs/MOFs composites has yet to be revealed.Therefore,MOPs/MOFs composites were prepared to explore their hydrogen storage performance in this work.In order to explore the influence of synthesis approach on hydrogen storage performance of MOPs/MOFs composites,PCN-777 with the large pore size and cage structure,and MOP-SO₃H with suitable size that is easy for preparation were chosen to synthesize MOP-SO₃H/PCN-777 through double-solvent strategy and impregnation method,respectively.The results revealed that MOP-SO₃H/PCN-777-DSS showed the higher excess hydrogen adsorption capacity(2.13wt%)at 77 K,which can be ascribed to the improved dispersion of MOP-SO₃H and the exposure of its open metal sites.Thus,the double-solvent strategy is more suitable for the preparation of MOP-SO₃H/PCN-777 composites compared with impregnation method.In order to explore the influence of MOPs contents on the hydrogen storage performance of composites,MIL-101(Cr)with the higher hydrogen storage capacity and lower cost was selected to synthesize a series of MOP-SO₃H/MIL-101(Cr)composites with different MOP-SO₃H contents via the double-solvent strategy.The results demonstrated that an appropriate addition amount of MOP-SO₃H is meaningful for increasing hydrogen uptake because of the introduction of open metal sites in MOP-SO₃H/MIL-101(Cr)composites.The excess and absolute adsorption amounts of hydrogen of MOP-SO₃H/MIL-101(Cr)-10wt%are obviously higher than MIL-101(Cr),suggesting the success of the strategy to improve the hydrogen storage performance of MOP-SO₃H/MIL-101(Cr)through integrating MOP-SO₃H with MIL-101(Cr).This work explored the impacts of synthesis approach and MOPs content on the hydrogen storage performance of MOPs/MOFs,paving the way for the exploration of the hydrogen storage performance of MOPs/MOFs composites,and could provide insight for the development of high-performance hydrogen storage materials.