(470d) Ultrathin Perfluorinated Sulfonic Acid Ionomer Membranes for Vanadium Redox Flow Battery: The Effect of Ordered Nanomorphology and Annealing on Ion-Transport Properties

With the demand for large-scale energy storage systems (ESSs) for the availability of intermittent energy from renewable energy sources, a vanadium redox flow battery (VRFB) has great attention as one of the promising rechargeable batteries for ESSs because of its independent controllability of power and energy capacity, less concerns by cross-contamination between redox ion pairs (V²⁺/V³⁺ and VO²⁺/VO₂⁺ in VRFBs), and nonflammability by aqueous-based system.

In VRFBs, a membrane is one of the key components. It performs the crucial role as an ion-selective membrane, which physically separates positive and negative electrolyte for preventing cross-mixing of the redox pairs while still transfer proton to complete an electrical circuit. In general, perfluorinated sulfonic acid (PFSA) ionomer membrane, e.g., Nafion membrane, is widely used as the ion-selective membrane in a VRFB because of high proton conductivity and good chemical stability. However, a low proton/vanadium ion-selectivity caused by randomly interconnected ion channels with relatively large size in a hydrated state, and high cost impose limitations in the efficient operation of VRFB. Accordingly, a lot of previous studies have conducted focusing on improvement of the ion-selectivity, e.g., composite membrane and the morphology control of hydrophilic ion channels, but none is satisfactory yet.

In this presentation, we have utilized ultra-thin PFSA membranes as an alternative to the bulk PFSA membranes. The ultrathin PFSA membranes were fabricated at the air/water interface using only a few tens microliters of PFSA dispersion, which is 3000 times less PFSA ionomer than that of Nafion 115, a conventional membrane for VRFBs. On the air/water interface, PFSA ionomers can be absorbed and form a naturally ordered monolayer because of its amphiphilic nature. Besides, their packing structure and alignment are easily modulated by simple physical compression. With controlled microstructure, an ultra-thin PFSA membrane (~tens of nanometer) is fabricated by repeated deposition on the supporting porous polycarbonate (PC) membrane by Langmuir-Blodgett method. The stacked ultra-thin PFSA layers show a highly ordered structure to parallel direction on the support. Owing to its nanomorphology, the ion-transport properties of the membrane are dramatically changed, and the PFSA/PC composite membrane exhibits 2−3 orders of magnitude higher ion-selectivity than the commercial PFSA membrane. In addition to the nanomorpholgy, the effect of heat treatment on the ion-transport properties is also investigated according to annealing temperature. In VRFB single cell test, the composite membranes show stable cell operation over various current densities and long-term cyclic test (800 cycles), and especially, they exhibit better performance than the commercial PFSA membrane at high current density (200 mA/cm²). With dramatically improved ion-selectivity and superior cell performance, the ultra-thin PFSA membrane provides a new opportunity for an ideal ion-selective membrane for VRFBs.