MIL-68(Fe) Reinforced SPEEK Membranes with Enhanced Proton Conductivity for High-Performance PEMFCs

Herein, sulfonated polyether ether ketone (SPEEK)-based composite proton exchange membranes (PEMs) containing MIL-68(Fe) were synthesized, and the effect of the additive on water management, mechanical strength, and proton transport was investigated. Structural characterizations showed that MIL-68(Fe) was successfully prepared and retained its chemical and crystalline structure after being incorporated into the SPEEK matrix. Morphological images revealed that at the optimum additive loading, MIL-68(Fe) was homogeneously distributed within the polymer, and no significant interfacial separation occurred along the cross section, supporting the idea that strong interfacial compatibility underlies the performance improvement. The water uptake capacity increased significantly with the addition of MIL-68(Fe); while the water absorption value of pristine SPEEK was 24.3% at 25 °C, it increased to approximately 53.9% with 5% MIL-68(Fe). This improvement was directly reflected in conductivity, with proton conductivity showing a ca. 3 times increase at 30 °C. A more limited but significant increase at 80 °C was observed with 5% additive compared to pristine SPEEK. Mechanical strengthening was also observed in the same membrane. Tensile strength increased by approximately 76%, and the elastic modulus also increased significantly. In contrast, at higher additive ratios (≥10%), the observed improvements decreased along with a tendency toward agglomeration. The findings demonstrate that MIL-68(Fe) additive enhances water management in SPEEK membranes, accelerates proton transport, and significantly increases mechanical strength. Thus, the resulting composite membranes are strong candidates for high-performance PEM fuel cell applications.