## 1 Introduction  
Ion-exchange membranes (IEMs), which can selectively transport counter ions having the opposite charge to the fixed charge groups, and connected to the membrane matrix by means of the Donnan exclusion, have been widely utilized in many desalination processes such as electrodialysis (ED) [1; 2; 3], diffusion dialysis (DD) [4; 5], membrane capacitive deionization (MCDI) [6; 7; 8], etc. Recently, IEMs have also been successfully applied in several electrochemical energy production and storage systems, including fuel cells (FCs) [9; 10; 11; 12; 13], reverse electrodialysis (RED) [14; 15], redox flow batteries (RFBs) [16; 17; 18; 19] and so on. Particularly, proton-exchange membranes (PEMs), such as Nafion, have been widely utilized in energy conversion processes, owing to their excellent proton conductivity and chemical stability [20]. Among various energy conversion systems, the application to fuel cells has been the most actively researched. As is well known, there are several types of fuel cells, depending on the kinds of electrolytes used and operation conditions, including the proton-exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC), and direct methanol fuel cell (DMFC) [21]. Recently, alkaline direct liquid fuel cells (ADLFCs), utilizing liquid fuels and anion-exchange membranes (AEMs), have also attracted a lot of interest as one of promising energy production systems [22]. Like traditional alkaline fuel cells, ADLFCs have several attractive features, such as operation at relatively low temperature, fast electrode reaction (oxygen reduction reaction (ORR) at cathode), use of nonprecious metal catalysts, low fuel crossover, easy water management, and so on [22; 23; 24]. In addition, as liquid fuels, alcohols such as methanol, ethanol, and glycerol have been the most widely studied [24]. The use of formate alkaline solutions as liquid fuels for ADLFCs has recently attracted much attention because it can realize efficient power sources for portable electronic devices [11; 25]. They can provide several benefits over alcohols, e.g., fast oxidation kinetics, theoretically high cell potential and power densities, and low fuel crossover [25; 26]. AEMs are one of the key components determining the energy conversion performances of ADLFCs. Therefore, various studies have been conducted to develop high-performance AEMs for successful applications to alkaline membrane fuel cells. In particular, structural studies on the polymer backbone and anion-exchange groups are being carried out [27; 28]. In addition, the state-of-the-art commercial AEMs such as Tokuyama A201 and A901 exhibited excellent performance in alkaline energy conversion processes, including AFCs [28], alkaline water electrolysis [29], and alkali metal-air batteries [30]. For example, the AFCs employing Tokuyama A901 achieved high power density of around 450 mW cm\({}^{-2}\) at specific conditions [28]. Unfortunately, however, the ion conductivity of AEMs is significantly lower compared to that of PEMs (e.g., Nafion), and the chemical stability should also be further improved for practical applications. Meanwhile, RFBs are one of the prospective large-scale electricity storage technologies and require efficient membranes for separating different redox species [31]. Among many kinds of RFBs, all-vanadium redox flow batteries (VRFB), in which vanadium redox species are used as the active electrode materials, have been the most actively researched and utilized due to advantages, such as long battery cycle life, reduced cross-contamination, high electrochemical activity, and so on [32; 33]. The ideal membranes for VRFB should possess low vanadium ion permeability to reduce self-discharge and achieve high coulomb efficiency (CE), low area resistance to decrease losses in voltage efficiency (VE), and excellent chemical stability [34].