The electrochemical reduction of carbon dioxide (CO₂RR) presents a sustainable strategy for converting waste greenhouse gas into value-added fuels and chemicals. Central to the performance of CO₂RR systems is the ionomer layer, which regulates ion transport, local pH, and selectivity of different products at the catalyst–electrolyte interface. However, traditional solution-processed ionomers often suffer from non-uniform coverage, limiting their efficacy in gas diffusion electrode (GDE) configurations. This thesis introduces an all-dry, vapor-phase polymer coating strategy using initiated chemical vapor deposition (iCVD) to synthesize conformal and tunable ionomer films designed for CO₂RR. A novel ionomer, p([VIm⁺][I⁻]-co-PFOA), was synthesized by copolymerizing 1-vinylimidazole (VIm) with perfluorooctyl acrylate (PFOA), followed by post-deposition modification of the imidazole groups to introduce ionic functionalities. The iCVD platform enables precise control over film composition and thickness with customizable balances between ionic conductivity and hydrophobicity. Material characterization confirmed uniform morphology, tunable wettability, and composition-dependent ionic conductivity. When it is integrated into GDE architectures, the ionomer films exhibit stable CO₂RR activity with selectivity toward C₁ and C₂ products. However, scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) revealed that poor adhesion under operation could lead to partial delamination of the ionomer thin film. Performance testing under optimized electrolyte and current density conditions highlighted a trade-off: silane surface treatment improved mechanical stability, but excessive adhesion and film thickness limited gas access and suppressed multi-carbon product formation. These findings underscore the critical role of material interface engineering in GDE systems and establish iCVD as a powerful method for designing next-generation ionomer coatings that balance conductivity and hydrophobicity for selective and durable CO₂ electroreduction.