In Situ Polymerization-Mediated Cross-Linking of the MOF Using Poly(1-vinylimidazole) in SPEEK Fuel Cells

A unique zirconium-based microporous MOF-808 constructed from Zr6 nodes (Zr6O4(OH)4) and BTC (benzene-1,3,5-tricarboxylic acid) synthesized herein has been further tailored by radical in situ polymerization of 1-vinylimidazole alongside N,N′-methylene-bis(acrylamide) as the cross-linker after their adsorption inside MOF pore channels. The cross-linked architectures were incorporated as fillers at 2 and 4 wt % in sulfonated poly (ether ether ketone) (SPEEK) solution to formulate mixed-matrix composite Proton Exchange Membranes (PEMs). The composite membranes displayed considerably higher proton conductivity, improved fuel cell performance, and thermal stability than pristine SPEEK. The highly ordered crystal-controlled 3D polymeric cross-linked networks created in the MOF channels address the undesirable leaching associated with adding low-molecular-weight azoles in membranes. Additionally, it generates highly ordered long-range proton-conducting channels through the MOF pores. The acid–base interaction between the imidazole and sulfonic functional groups causes the conductivity of protons to increase to 0.05 S/cm at 80 °C and 70% RH for the S4P3M membrane containing 4 wt % complex filler, which is 16.6-fold higher than that of pure SPEEK. The membrane retains a proton conductivity of up to 0.04 S/cm at 100 °C and 40% RH, indicating its proton-conducting abilities at low humidity conditions. Furthermore, S4P3M achieves a peak power density of 310 mW/cm2 at 60 °C and 347 mW/cm2 at 80 °C under 70% relative humidity, which are higher than those in several other reported literature studies on SPEEK-based membranes.