Ultra-thin ion-selective perfluorinated sulfonic acid ionomer membrane for vanadium redox flow battery

Abstract

A perfluorinated sulfonic acid (PFSA) ionomer membrane has been used as an ion-selective membrane in various energy storage and water treatment fields such as polymer electrolyte membrane fuel cell (PEMFC), redox flow battery (RFB), water electrolysis, and capacitive deionization (CDI) due to its high ion conductivity and chemical stability. However, its randomly interconnected and relatively large ion transport channel in hydrate state induce low ion selectivity resulting in poor performance of applied systems. In this dissertation, we developed an ultra-thin membrane with highly aligned ion channels via the molecular control of PFSA ionomers on the air/water interface and apply it to vanadium redox flow battery (VRFB) as the ion-selective membrane. To begin with, using the interfacial property of PFSA ionomer, the stable and densely packed monolayer was formed on the air/water interface, and an ultra-thin PFSA ionomer membrane was prepared on the substrate by Langmuir-Blodgett method. In GISAXS and GIWAXS experiments for analysis of the nanomorphology of the ultra-thin PFSA membrane, a well-ordered structure and higher crystallinity than commercial membranes was observed. Secondly, the ion transport properties of the ultra-thin PFSA membrane were measured though the ultra-thin PFSA/PC composite membrane, and the VRFB cell test with the composite membrane was conducted to evaluate the effects of the membrane. However, unexpectedly low cell performance was observed, indicating changes in membrane properties during cell operation. Lastly, we thus identified the external force, which could affect the structure of ultra-thin PFSA membrane during the cell operation, and developed the modification methods to prevent the change of the membrane properties. The aligned structure of the ultra-thin PFSA membrane was expected to be deformed by an electric field, and to prevent this, the ultra-thin ionomer membrane was reinforced through introduction of aminosilane and heat treatment to enhance the mechanical strength and the structural stability of PFSA layers. The effects of modification were confirmed through VRFBs cell test, and the reinforced membrane showed superior cell performance compared to commercial PFSA membrane. From these results, we propose the great potential of highly ordered ultra-thin PFSA membranes with sub-50 nm as an ion-selective membrane of various energy storages.