## 1 Introduction
Various technologies have been employed to produce AEMs, such as solution casting and spin coating that utilize polymers dissolved in solvents, reinforcement using supports like porous PTFE or e-PTFE, and in situ cross-linked pore-filling technology utilizing porous polyolefin substrates [31; 32; 33]. Among these, the development of pore-filling type AEMs utilizing porous polyolefin substrates offers advantages in cost, high performance, chemical durability, dimensional stability, mechanical stability, and overcoming membrane production limitations with a streamlined process and low production cost [28; 34; 35; 36; 37; 38]. However, due to the limited pore volume of the porous polyolefin substrate, it is necessary to identify the optimal combination of anion-conducting monomers and cross-linking monomers that can deliver outstanding performance, while also introducing monomers with structures that enhance chemical durability [39; 40; 41]. The fabrication of pore-filling membranes involves blending an electrolyte monomer, endowed with ion-exchange capacity, with a cross-linking monomer in an optimal ratio. This homogeneously mixed monomer solution is then introduced into a porous support and subjected to ultraviolet (UV)-induced cross-linking. The resulting ion-exchange mem brane is designed to facilitate a high flux of specific ions along its thickness, a critical feature in applications like water electrolysis. To achieve this, the IEC of the ionomer must be substantial, while maintaining low water content to ensure the membrane's dimensional stability. However, IEC and water content are inversely related, necessitating careful optimization. This balance can be attained by adjusting the types and ratios of the electrolyte monomer and cross-linking agent. Recent studies indicate that even with similar IEC and water content, variations in monomer structures can significantly influence ion conductivity, as well as the performance and durability of water electrolysis systems [29; 30]. For instance, research has demonstrated that acrylamide-based pore-filling membranes outperform acrylate-based ones in current density, particularly at 1.8 V, when both lack methyl groups [29]. Additionally, hydrophilic acrylamide-based membranes have shown superior performance compared to hydrophobic benzyl-based membranes, exhibiting 25% and 41% higher ion conductivity when using cross-linkers with two and three cross-linking sites, respectively [30]. These findings underscore the importance of monomer selection and structural considerations in the design of pore-filling membranes for enhanced ion conductivity and electrolysis performance. In this study, we diverged from the previous research that primarily focused on the hydrophilicity of electrolyte monomers. Instead, we employed two distinct cross-linkers, each varying in hydrophilicity, while utilizing a standard electrolyte monomer. This approach allowed us to observe how the properties of ionomers, integrated into polypropylene (PP) substrates, were affected. Additionally, ionomer-coated electrodes are introduced to overcome interfacial resistance between the pore-filling membrane and the electrode. Our ultimate goal was to assess the resulting water electrolysis performance and durability.