Ultrahigh Proton Conductivity of Encapsulating Metal–Organic Polyhedra in a Metal–Organic Framework Composite Material and Its Nanocomposite Membrane

The proton-exchange membrane fuel cell (FC) serves as an effective and environmentally sustainable energy conversion technology, which is gradually becoming an important choice for global energy transformation. In order to be used as a membrane electrode of a FC, in this work, a metal–organic polyhedra (MOP) material is encapsulated in a metal–organic framework (MOF) material, which not only improves the stability and dispersion of MOPs but also introduces a proton source and proton carrier into the MOF, thus improving the conductivity of the composite material: a copper-based MOP was successfully self-assembled within a porous MOF host (MIL-101-NH₂), resulting in the synthesis of MOP@MIL-101-NH₂. The diameter of the MOP guest is approximately 27 Å, which exceeds the dimensions of the square windows (pore sizes of approximately 12 and 16 Å) of MIL-101-NH₂, yet remains smaller than that of the rhombicuboctahedral cage (ranging from approximately 29 to 34 Å). This implies that the migration and leaching of the MOP could be effectively restricted when it is encapsulated within the cavities of the MOF. The proton conductivity of MOP@MIL-101-NH₂ is 2.55 × 10^–3 S·cm^–1 (at 95 °C under 98% relative humidity), indicating an enhancement of 6.39 times compared to that of MIL-101-NH₂. Because the proton conductivity of membrane materials used as membrane electrodes of FCs was better than that of powder materials, composite membranes were prepared by blending MOF powder with the sulfonated polyether ether ketone (SPEEK) matrix, named MOP@MIL-101-NH₂/SPEEK-X, X = 3, 5, 7, and 9, where “X” represents the mass fraction of MOP@MIL-101-NH₂ in the composite membrane. The proton conductivity of the MOP@MIL-101-NH₂/SPEEK-7 membrane reaches 3.54 × 10^–1 S·cm^–1 (at 70 °C under 98% relative humidity), which is 2 orders of magnitude higher than that of MOP@MIL-101-NH₂.