## 1 Introduction  
As energy demand is rapidly increasing around the world and environmental pollution caused by the use of fossil fuels is emerging, renewable energy is attracting attention as the energy source of the future. However, renewable energy has a disadvantage in that the output fluctuates greatly depending on the climatic environment, and to compensate for this, an energy storage system (ESS) with high capacity is required [1, 2]. ESS is a key component of a smart grid, and various types of secondary batteries that can be used for a long time and have high energy efficiency during operation are mainly used for large-capacity energy storage [1, 2, 3]. That is, lithium-ion batteries, lead-acid batteries, NaS batteries, and redox flow batteries (RFBs) are known to be efficient secondary batteries for ESS applications. Among them, RFBs possess higher availability and energy efficiency and lower capital cost requirements than other competing technologies. In addition, they are believed to have several advantages, such as low toxicity and long lifespan. In particular, the RFBs operate at room temperature, and the power and capacity of the battery can be designed independently of each other [3]. The RFB is a battery system in which an active electrode material dissolved in an electrolyte solution is oxidized and reduced to charge and discharge. In more detail, after dissolving the cathode and anode active materials in the electrolyte, they are respectively stored in an external tank and circulated through the stack using a pump when necessary, and electric energy is charged and discharged. Undesirable mixing of the electrolyte components can be prevented by independent storage of cathode and anode active materials,and a high level of stability can be obtained by using an aqueous electrolyte. As an active material for the RFB, redox couples with various potentials, such as iron/chromium, iron/titanium, all vanadium, vanadium/bromine, polysulfide bromine, zinc/bromine, and zinc/cerium, can be selected [4; 5; 6]. In the case of having different redox ion species at the cathode and the anode, such as an iron/chromium system, significant cross contamination of electrolytes can occur due to the concentration gradient of each species in the two sides of the membrane [7]. Since the crossover of these active materials causes self-discharge and limits the RFB performance, a technique using the same species as redox couples for both the anode and cathode has been proposed [7; 8]. A typical example is an all-vanadium flow battery (VRFB) using vanadium species as both the anode and the cathode redox materials. The VRFB has several advantages, such as excellent energy efficiency, long lifespan, and high cost-effectiveness. Figure 1 shows the structure and charge-discharge principle of a typical VRFB system. Meanwhile, the membrane is one of the most important components that determine the charge-discharge performance and durability of all kinds of RFB systems [9]. Although it depends on the type of redox couples, RFBs mainly employ ion-exchange membranes (IEMs) that prevent the mixing of electrolytes between the anode and cathode compartments and act as an ion conductor [2; 5]. The IEMs used in the RFB system require low electrical resistance, high selective permeability for specific ions, low diffusion coefficient for solvents, and excellent chemical and mechanical stabilities [10]. Considering the characteristics of the VRFB system operated under strongly acidic conditions, the IEMs should also have high acid and oxidation resistance and excellent selective permeability to hydrogen ions compared with vanadium cations [10; 11]. From this point of view, Nafion, a perfluorinated cation-exchange membrane (CEM), has been widely utilized as a separation membrane in VRFB systems, but it is suffering from some drawbacks, such as high membrane cost and significant vanadium crossover.