## 2 Modification of Anion-Exchange Membrane  
In the past, a conventional process \"polymerization-chloromethylation-amination\" is usually employed to prepare AEMs [16]. This process is relatively mature and provides good performance for the membrane. However, the chloromethylation agent (chloromethyl ether) being used over that time had proven to be carcinogenic and hazardous [17]. Thereof, the alternative in the co-polymerization of vinylbenzyl chloride (VBC) with divinylbenzene (DVB), the grafting of VBC or vinylpyridine onto polymer, and the copolymerization of epoxy acrylates such as glycidyl methacrylate (GMA) have been reported as the halomethyl-substituted monomers to replace the usage of chloromethyl ether [18]. Aside from that, a variety of cationic functional groups have been proposed for AEM applications such as quaternary ammonium, imidazolium, pyridinium, guanidinium, phosphonium and sulfonium [19; 20; 21; 22; 23; 24; 25; 26]. Quaternary ammonium ions have been traditionally used as the cation head group, but they are susceptible to chemical degradation via (i) Hofmann elimination, (ii) nucleophilic substitution and (iii) ylide formation [27]. One of the most important criteria in the selection of cation functional group is centered on its chemical stability under alkaline conditions. Hence, other nitrogen-containing cations (imidazolium, pyridinium and guanidinium) and nitrogen-free cations (phosphonium and sulfonium) head groups are also considered for AEM applications. Though a huge number of studies have been conducted in relation to AEMs, there are challenges in practical applications that remain to be solved, for instance, the low ionic conductivity of AEMs and low chemical stability of cationic functional groups as well as the polymer backbone in alkaline media [28; 29; 30]. Studies have shown that different fabrication methods even on the same polymer type could result in different properties. According to [PERSON] et al. [18], the final properties of the AEMs are intimately linked to the methods and materials used in fabricating an AEM. By integrating different polymer fabrication processes and altering the morphology of polymers, the performance and characteristics of AEM can be engineered. Generally, AEMs are prepared through three main steps, including the polymerization of halomethyl-substituted monomers, film casting and quaternization to create the cation head group on the polymeric film. The formation of the film is normally performed via solution casting or the sol-gel technique. Meanwhile, on the attempt to improve the thermo-mechanical, chemical stability, and ion conductivity properties of AEMs, various modification methods have been investigated by employing different polymers and reagents. Among the most commonly reported methods are direct copolymerization, plasma polymerization, the pore-filling method, and supported composite AEMs [18; 31; 32; 33]. Copolymerization, which involves the use of halomethyl-substituted monomers as copolymer, which is required to serve as the site for quaternization in the subsequent process, is one of the crucial methods to create the anions' transport sites on the membrane. For AEMs, the grafted co-polymerization technique, where the side chain grafts of the co-polymer are covalently attached to the polymer main chain, is more favorable compared to blocked copolymerization to increase the active sites for anion transport. Radiation-grafted copolymerization has emerged as the preferred technique, owing to its ease in controlling the degree of grafting, its ability to scale-up and the fact that it can minimize the residues left on the membrane during the process. This section will compare the different modification methods towards several properties of AEMs, including the ionic conductivity summarized in Table 1. Radiation-grafted AEMs will be highlighted based on their superior properties compared to other methods.