Promotion of Proton Conductivity by Encapsulation of Metal-Organic Polyhedra in Metal-Organic Frameworks

A Zr-based metal-organic polyhedron (MOP) was self-assembled in a porous MOF host, DUT-68, successfully to synthesize MOP-1@DUT-68. The MOP guest (MOP-1) has a diameter of about 20 Å, larger than that of the square windows (pore sizes of ∼14 Å) of DUT-68 but smaller than that of the rhombicuboctahedral cage (27.7 Å), which means that the migration and leaching of MOP-1 could be effectively prohibited if MOP-1 is encapsulated in the MOF's cavities. The proton conductivity of MOP-1@DUT-68 is 1.14×10^-3  S cm^-1 (at 80 °C under 98 % relative humidity), which is three orders of magnitude higher than that of DUT-68. Compared with MOP-1⊂DUT-68, which was synthesized by impregnation, MOP-1@DUT-68 is more prone to form faster proton-conduction pathways and thus provides higher proton conductivity.