Alignment of ion-channels in anion exchange membranes and their application for the energy conversion system

Abstract

In this thesis, we developed non-fluorinated anion exchange membranes (AEMs) for electrical energy systems. The role of ion channels in AEMs is transportation for specific anions with rejecting cations. In particular, ion channels for the AEM consist of the polymer backbones and negatively charged groups. These ion channels are generally entangled by following the thermodynamic theory. Consequently, transferring the anions is significantly delayed through non-orientated ion channels which lead to reducing the ion conductivity for the membrane. Meanwhile, we proved the effect of aligned ion channels by introducing the electrical treatment in the AEM which was used as the quaternary-aminated poly (2,6-dimehtyl-1,4-pheneyleneoxide) (QPPO) membranes. Specifically, the ion channels in the QPPO membrane were successfully aligned in accordance with the specific direction of the applied electric field. The aligned ion channels resulted in improving the ion conductivity for the QPPO membrane. In addition, the aligned QPPO membrane was maximally 3 times higher than the original QPPO membrane in terms of the ion conductivity. Furthermore, the deterioration of its properties (e. g., dimensional stabilities, transport number) was not observed according to the applied electric field. These phenomena revealed that the morphological structure for ion channels in QPPO membrane was regularly orientated due to the effect of the electric field. Consequently, the anion can rapidly transfer the aligned ion channels without detour in ion channels. In addition, based on the method for the electrical treatment, we optimized preparing composite QPPO (cQPPO) membranes for a non-aqueous vanadium redox flow battery. The cQPPO membranes were fabricated by combining the pore-filled method and electric field. Due to the substrate and aligned ion channels, the ion conductivity and membrane electrical resistance (MER) for the cQPPO membrane were dramatically improved. Especially, the MER of the aligned cQPPO membrane was maximally 347 times lower than that of the commercial AHA membrane. Moreover, the form of fabricated cQPPO membranes was stably maintained in non-aqueous media with improving the mechanical and dimensional stabilities. It revealed that the porous substrate having high mechanical and dimensional properties curbs the extreme swelling of the QPPO polymer in non-aqueous media. Furthermore, a single cell test was successfully operated by applying fabricated membranes. Especially, applying the aligned cQPPO membrane was operated with an average coulombic efficiency (CE) of 97.1%, energy efficiency (EE) of 69.3% and voltage efficiency (VE) of 71.3% at the operating current density of 80 mA cm-2 which was 80 times higher than the operating current density of AHA. In this study, we simply developed the QPPO membrane based on electrical treatment as a low energy consumption without additives to improve conductivities. However, we believe that membranes of different types can be easily applied by reducing development costs.