###### Abstract  
Saling water electrolysis (SWE) is an electrochemical process to simultaneously produce hydrogen (H\({}_{2}\)), chlorine (Cl\({}_{2}\)), and sodium hydroxide (NaOH) with high purity levels (e.g., 99.999%) by applying electric power to saline water. The state-of-the art SWE membrane, Flemion(r), has excellent chemical resistance to harsh SWE conditions, but still needs to lower its energy consumption by reducing its ohmic resistance to Na\({}^{+}\) ion transport. Meanwhile, most of cation exchange membranes (CEMs) have been suffering from chemical degradation under the alkaline conditions, owing to their single layer matrices composed of sulfonic acid moieties, though they show fast Na\({}^{+}\) ion transport behavior. Here double-layered SWE membranes were prepared on the basis of design strategies composed of the incorporation of a chemically stable carboxylic acid layer (C layer) via UV irradiation onto one surface of perfluorinated Nafion(r)212 membrane chosen as one of commercially available CEMs, and the thickness control of the C layer. The resulting membranes showed excellent SWE performances and improved electrochemical service life, when compared with those of Nafion(r)212 and Flemion(r), respectively.  
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including anolyte and catholyte. Thus, the desirable membrane should be designed in the form of cation exchange membranes (CEMs) to have high Na\({}^{+}\) ion conductivity. The SWE membrane must also be able to prevent the diffusion of OH\({}^{-}\) ions as much as possible. Otherwise, the OH\({}^{-}\) ions permeated from cathode to anode through SWE membranes not only hinder NaOH production, but also reduce the corresponding energy efficiency called as caustic current efficiency. From the material point of view, permeated OH\({}^{-}\) ions induce severe chemical degradation to SWE membranes, particularly with single layer matrices made up of sulfonic acid moieties. For this, the desirable SWE membranes should have excellent resistance to alkaline attacks under strong base atmospheres. The state-of-the art SWE membrane to satisfy these tough requirements is Flemion(r) (AGC Chemicals Company, Japan). Flemion(r) is prepared in the form of double-layered CEM composed of thick perfluorinated sulfonic acid (PFSA) ionomer layer (S layer) and thin perfluorinated carboxylic acid (PFCA) ionomer layer (C-layer). Even Flemion(r) needs to be enhanced further to ensure high electrochemical SWE performances including energy efficiency.  
In this study, SWE membrane design strategies were demonstrated to develop alternative SWE membranes to Flemion(r). The strategies are based on the incorporation of a chemically robust C layer via free radical polymerization under UV irradiation on one surface of Nafion(r)212 membrane, one of commercially available CEMs, and the thickness control of the C layer, to reduce the diffusion of OH\({}^{-}\) ions, and to find out an appropriate C layer thickness considering both SWE electrochemical performances and service life, respectively. The major purpose of this study is to confirm whether the design strategies are applicable. Another objective is to disclose the effects of the C layer thickness on fundamental and electrochemical characteristics of the resulting double-layered SWE membranes. The final goal is to monitor electrochemical service life of the double-layered SWE membrane with the optimum C layer thickness.