## 4 Conclusions  
In this study, novel thin reinforced IEMs were developed by combining a porous PE substrate and an ionomer containing a fluorine moiety for VRFB application. By adjusting the ratio of VBC or Sty and OFPMA, the electrochemical and physicochemical properties of the membranes were effectively controlled. The prepared PFIEMs exhibited superior mechanical properties compared with the commercial membranes despite the thin film thickness owing to the tough physical properties of the porous substrate used. The ion conductivity of the PFIEMs, which contain a large non-ion conducting region owing to the use of the inert porous substrate, was revealed to be lower than that of the commercial membranes. However, the electrical resistance of the PFIEMs could be greatly reduced due to the thin film thickness. Meanwhile, as a result of the evaluation of oxidation stability using [PERSON]'s reagent, it was confirmed that the oxidation stability of the membranes could be greatly improved through the use of a PE support and the introduction of a fluorine moiety into the filling ionomer. It was also found that the PFIEMs having higher  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Membrane** & **Type** & **Company** & **CE (\%)** & **VE (\%)** & **EE (\%)** & **Current Density (mA/cm\({}^{2}\))** & **Ref.** \\ \hline Nafion 117 & CEM & DuPont & 85.7 & 92.5 & 79.3 & 30 & [54] \\ Nafion 212 & CEM & DuPont & 89.6 & 84.2 & 75.5 & 40 & [55] \\ NEPEM115 & CEM & Kerun & 88.6 & 85.7 & 78.5 & 60 & [11] \\ NR 212 & CEM & DuPont & 89.2 & 88.8 & 79.2 & 50 & [56] \\ N 115 & CEM & DuPont & 90.5 & 85.6 & 82.8 & 20 & [56] \\ FAP-PP-475 & AEM & Fumatech & 92.6 & 85.0 & 78.7 & 60 & [11] \\ FAP-PE-420 & AEM & Fumatech & 91.0 & 86.0 & 78.0 & 60 & [11] \\ APS & AEM & Asahi Glass & 89.3 & 87.0 & 77.7 & 60 & [11] \\ \hline \hline \end{tabular}
\end{table}
Table 7: VRFB performances of various commercial CEMs and AEMs.  
Figure 12: Spider chart for comparing the performances of the PFAEM and PFCEM in a VRFB system.  
fluorine content exhibit better chemical stability in the vanadium electrolyte, similar to the result of the Fenton oxidation. In addition, the membranes with the highest content of fluorine (i.e., PFAEM-1 and PFCEM-1) showed the lowest vanadium ion permeability, which resulted in the highest current efficiency in the VRFB tests. The PFIEMs also exhibited higher VE compared with the commercial membranes due to the relatively low mass transfer resistance. Overall, PFAEM-1 and PFCEM-1, which have the highest portion of fluorine, showed the highest energy efficiency (89.9% and 87.6%, respectively), and the VRFB performance improvement by using the thin reinforced membranes was expected to increase in a practical system with a large membrane area. Moreover, as a result of comparing the PFAEM and PFCEM, it was concluded that the PFAEM is better than the PFCEM in terms of both charging-discharging performance and durability of VRFB.  
Conceptualization, M.-S.K.; methodology, M.-S.K.; experimentation, H.-N.M. and H.-B.S.; validation, M.-S.K.; investigation, M.-S.K.; resources, M.-S.K.; writing--original draft preparation, H.-N.M., H.-B.S. and M.-S.K.; writing--review and editing, M.-S.K.; supervision, M.-S.K.; project administration, M.-S.K.; funding acquisition, M.-S.K. All authors have read and agreed to the published version of the manuscript. This work was financially supported in part by the National Research Foundation of Korea (NRF) and the Korea Environmental Industry and Technology Institute (KEITI). Not applicable. This work was supported in part by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. NRF-2019R1A2C1089286) and by the 2021 Green Convergence Professional Manpower Training Program of the Korea Environmental Industry and Technology Institute funded by the Ministry of Environment. The authors declare no conflict of interest.