Enhancing hydroxide conductivity at subzero temperature of anion exchange membranes based on imidazolium modified metal organic frameworks

Abstract

In the development of anion exchange membranes (AEMs), the most research work focused on enhancing hydroxide conductivity and improving alkaline stability. In this research, we synthesized imidazolium modified metal organic frameworks (ImMOFs) and constructed QPPO/ImMOFs series membranes based on the polymer of quaternized poly(phenylene) oxide (QPPO). In the composite membranes, ImMOFs participated in hydroxide ions conduction and resisted the hydroxyl groups continuous attacking the polymer molecular chains of QPPO. The hydroxide ions conduction behavior at subzero temperature was investigated in the consideration of long-term exposure in a colder condition causing the fracture of AEMs. Specifically, the QPPO/7%ImMOFs membrane exhibited hydroxide conductivities of 0.726 mS/cm at −25 °C and (42.1 ± 1.9) mS/cm at 80 °C. Meanwhile, the satisfactory alkaline stability was revealed from the five-cycle hydroxide conductivities of 0.87 mS/cm at −25 °C and 14.9 mS/cm at 30 °C in the range of −25 °C–30 °C. Furthermore, the stable hydroxide conductivity was obtained even if the composite membrane was immersed in 2 M KOH solution for a long-term. For QPPO/7%ImMOFs, the hydroxide conductivities retained 0.29 mS/cm and 0.26 mS/cm at −25 °C, 45.4 mS/cm and 40.5 mS/cm at 60 °C with immersing in 2 M KOH solution at RT for 768 h and at 60 °C for 433 h. The fine long-term hydroxide conductivity was derived from the component uniformity and the dimension stability. Notably, the mechanical strength of the composite membranes was enhanced with ImMOFs, such as the tensile stress of 29.1 MPa of QPPO/7%ImMOFs relative to 25.1 MPa of the pristine QPPO membrane.