## 1 Introduction
As an alternative, therefore, the use of anion-exchange membranes (AEMs) has recently attracted attention, and hydrocarbon-based IEMs are being actively developed to lower the expensive membrane cost [12; 13]. However, in the case of hydrocarbon-based IEMs, despite their excellent electrochemical characteristics, they are difficult to apply to practical systems due to their poor chemical stability, so research on this is urgently needed [2; 10; 14; 15]. In addition, a partially fluorinated IEM can be considered to improve the chemical stability and reduce the manufacturing cost of the IEMs [16; 17; 18; 19; 20]. Figure 1: Schematic diagram of a VRFB system showing the working principle. The traditional manufacturing process (i.e., \"a paste method\") of commercial IEMs is known to be complicated and expensive. In this method, typically, the IEMs are fabricated by impregnating a paste mixed with monomers and rubber into a reinforcing fabric net, radical polymerization, and then introducing ion-exchange groups through a post-treatment, such as quaternization (for AEMs) or sulfonation (for CEMs) [21]. Meanwhile, a pore-filled ion-exchange membrane (PFIEM), in which an ionomer is filled into pores of a thin porous polymer film, is shown to possess low mass transport resistance and strong mechanical strength, so it is being considered for application to various energy conversion technologies and water treatment processes [22; 23; 24]. The PFIEM, which is intermediate between a homogeneous membrane and a heterogeneous membrane, exhibits excellent electrochemical properties while lowering the manufacturing cost due to the use of inexpensive reinforcing material and a reduction in the amount of raw materials used. Figure 2 illustrates the fabrication principle of the PFIEM. In this study, novel IEMs optimized for VRFB application were developed by combining an ionomer with a porous polyethylene (PE) substrate as a reinforcing material. It was expected that the PFIEMs could possess low electrical resistance, excellent chemical and physical stabilities, and low production cost by employing a simple pore-filling method. Figure 3 shows the synthesis process of the anion- and cation-exchange polymers prepared. For the membrane fabrication, 4-vinylbenzyl chloride (VBC) or styrene (Sty), the main monomer; benzoyl peroxide (BPO), a thermal initiator; and divinylbenzene (DVB), a crosslinking agent, were filled in the pores of a porous PE substrate, and a base membrane was then prepared through in situ radical polymerization. The prepared base membrane was followed by quaternization or sulfonation post-treatment to produce a pore-filled anion-exchange membrane (PFAEM) and a pore-filled cation-exchange membrane (PFCEM), respectively. In addition, 1H,1H,5H-octafluoropentyl methacrylate (OFPMA) monomer was mixed with the monomer solution to prepare PFIEMs with a fluorine moiety. The OFPMA employed is a chemically robust fluorine monomer widely used in surface coatings to prevent oxidation, and is characterized by low \(T_{g}\) and low surface energy [25; 26; 27; 28]. The introduction of fluorine groups was expected to effectively improve the chemical stability of the PFIEMs under strongly acidic and oxidative conditions of VRFB. In this work, in particular, the effect of the content of a fluorine moiety on membrane properties and VRFB performance was systematically investigated, and the characteristics of PFAEMs and PFCEMs were also compared. Figure 2: Schematic drawing of PFIEM fabrication.