## 4 Conclusions  
Double-layered membranes in the form of CEMs were successfully prepared with design strategies composed of the incorporation of crosslinked C layer containing carboxylic acid groups via the free radical polymerization under UV irradiation onto one surface of a commercially available perfluorinated NR212 membrane used for S layer containing sulfonic acid groups, and the thickness control of the C layer. After the addition of the C layer, the resulting NR212/AA-DVB membranes exhibited reduced Na\({}^{+}\) ion conductivity and improved barrier property to OH\({}^{-}\) ions, when compared with those of NR212. These trends were more significantly observed as the C layer thickness increases. The electrochemical SWE single cell performances of the resulting double-layered membranes were more affected by their Na\({}^{+}\) ion conductivity rather than OH\({}^{-}\) ion permeability. The double-layer membranes with thin C layer in the thickness range of 10 \(\upmu\)m showed single cell performances higher than the state-of-the art SWE membrane, Flemion(r). The hydrogen current efficiency was more dependent on Na\({}^{+}\) ion conduction. On the other hand, the caustic current efficiency was influenced by both OH\({}^{-}\) ion permeation and Na\({}^{+}\) ion conduction. The OH\({}^{-}\) ion diffusion through the SWE membranes lowering the NaOH production, was alleviated by increasing the C layer thickness up to 10 \(\upmu\)m. This thickness control was helpful in improving caustic current efficiency. The further increase in the C layer thickness retarded Na\({}^{+}\) conduction, and led to the highly reduced caustic current efficiency. The addition of the thin C layer contributed to extended service life of the corresponding SWE membranes. It has been proven that the proposed design strategies is very useful for preparing effective SWE membranes with high single cell performances and electrochemical efficiency values. Even the extended lifetimes of the double-layered membranes are, however, insufficient to be used as alternatives to Flemion(r) for a long-term SWE operation (e.g., at least 1-2 years). Our on-going studies are focused on the approaches to effectively incorporate a chemically robust C layer made up of PFCA ionomers on the S layer with much enhanced Na\({}^{+}\) ion conductivity (e.g., S layer made using PFSA ionomers with low equivalent weight values). We have also attempted to monitor the effect of free volume in the C layers with the identical chemical architectures on SWE membrane characteristics.  
[PERSON] synthesized and characterized the membrane materials, and wrote the manuscript in collaboration with [PERSON]; [PERSON]-[PERSON] conceived the idea and designed the experiments; [PERSON] coordinated and supervised the entire project. All authors discussed the results and commented on the manuscript. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Technology Development Program to Innovative Hydrogen Energy, and to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (2019M3E6A1064093 and 2018M1A2A2063865), respectively.  
**Acknowledgments:** We thank our colleagues for their contribution to this work.  
**Conflicts of Interest:** The authors declare no conflict of interest.