Proton exchange membrane fuel cell (PEMFC) has high ionic conductivity, good dimensional stability, mechanical properties, and physicochemical stability. However, widespread commercial use of PEMFC is limited by the high fabrication cost, mainly due to the use of platinum-based catalysts. Moreover, PEMFCs have durability issues due to their acidic operation conditions even for noble-metal catalysts. As an alternative to PEMFC, anion exchange membrane fuel cell (AEMFC) has been developed to overcome these problems, because it allows the use of inexpensive Pt free catalysts.
Although AEMFC can be operated with a low-cost catalyst, it has several problems to be solved such as low ionic conductivity, alkaline stability issue in which chemical decomposition of polymer occurs in an alkaline environment at temperature (about 80o or more), and appropriate dimensional stability. In order to solve these problems, many researches are actively conducted such as changing the main chain structure or applying inorganic fillers.
Pyrene is a compound with four benzene rings fused, and π orbitals gather to form a huge planar π electron cloud on both sides of the molecule. Unlike a simple benzene ring, the much wider electron cloud of pyrene can exert stronger molecular interactions. Through this interaction, pyrene molecules show a special phenomenon in which molecules are stacked side by side by ''π-π stacking''. In this study, it is assumed that the microphase separation of synthesized polymers could be promoted by using this phenomenon of aggregation of pyrenes.
In this study, piperidinium having a hexagonal ring structure among heterocyclic ammonium was introduced as a functional group into the polymer main chain of ether-free and poly aromatic composition, which has been reported to have excellent alkali stability, and quaternized to prepare an anion exchange membrane. The electrochemical and the physicochemical properties were analyzed. Poly(pyrenyl-arylpiperidinium) is a resulting membrane synthesized through the superacid-catalyzed polycondensation of para-terphenyl, pyrene and 1-methyl-4-piperidone followed by a Menshutkin reaction using iodomethane. Quaternized poly(pyrenyl-aryl piperidinium) (QPPTP) was produced by quaternizing the piperidinium functional group, and those containing 10% and 20% pyrene by mole were named QPPTP-10 and QPPTP-20, respectively. In addition, in order to confirm the effect of pyrene, a quaternized poly(aryl piperidinium) (QPAP) film using only para-terphenyl and 1-methyl-4-piperidone as monomers was prepared as a comparative group.
The chemical structure of the synthesized polymer was confirmed through nuclear magnetic resonance (1H-NMR) and fourier transform infrared (FT-IR). Field emission scanning electron microscope (FE-SEM) and atomic force microscope (AFM) were performed for morphological observation, and thermal properties were analyzed through thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). As for the properties of the membrane, water uptake, swelling ratio, and ion exchange capacity (IEC) were measured, and all values decreased as pyrene was included. The ionic conductivity measurement reached a high value of 71 mS/cm for QPPTP-20 at 90 degrees. As a result, the QPPTP-20 membrane showed the highest ionic conductivity of 71.0 mS/cm at 90 °C, and QPPTP-10 and QPAP showed values of 52.0 mS/cm and 42.5 mS/cm, respectively. Single cell test results showed a high open-circuit voltage of 0.967 V and excellent durability of over 170 hours of operation. Further, the prepared anion exchange membrane was tested for chemical stability after being immersed in an alkaline solution of 2 M NaOH at 60°C for 504 hours, and showed high alkali stability.