## 3 Results and Discussion
It infers that the crosslinked C layer acts as an  
Figure 6: Na\({}^{+}\) ion conductivity of NR212/AA-DVB membranes as a function of temperatures. effective polymeric barrier to the diffusion of OH\({}^{-}\) ions. Interestingly, the permeated OH\({}^{-}\) ions rapidly raise the pH value of the initially acidic solution to the level higher than 10 within a short period of time. After 20 min, their pH values remain in the equilibrium state. Figure 8 displays the OH\({}^{-}\) ion permeability obtained using the concentration gradient and the membrane dimension. The OH\({}^{-}\) ion permeability is linearly reduced, until the C layer thickness reaches 16 \(\upmu\)m. When a further increase in the C layer thickness is made, the reduction in the OH\({}^{-}\) ion permeability is mitigated. Because of this tendency, NR212/AA-DVB_45 and free-standing AA-DVB have almost similar concentration gradient as shown in Figure 6. The OH\({}^{-}\) permeability of NR212/AA-DVB_45 is not much different from that of free-standing AA-DVB membrane. It may mean that there are some limitations to strategies to reduce OH\({}^{-}\) ion permeation by controlling only the thickness of the C layer in double layered SWE membranes. Figure 9 exhibits the effect of the C layer thickness on the electrochemical single cell performances of NR212/AA-DVB membranes obtained via chronoamperometry at 90 \({}^{\circ}\)C. The cell voltage values  
Figure 8: OH\({}^{-}\) ion permeability of NR212/AA-DVB membranes as a function of C layer thickness. Figure 7: OH\({}^{-}\) diffusion behavior of NR212/AA-DVB membranes over time. of all the membranes increase, as the applied current density values increase. The increment in the cell voltage of Flemion(r) is much higher than that of NR212. The cell voltage of NR212 is 13.3% lower than that of Flemion(r) at 0.6 A\(\cdot\)cm\({}^{-2}\), which is often chosen in industrial SWE applications. This difference may come from the Na\({}^{+}\) conductivity of NR212 much higher than that of Flemion(r) as shown in Figure 6. After the incorporation of the C layer on NR212, the cell voltage values of NR212/AA-DVB membranes at a constant current density rise when compared with that of NR212. Particularly, the slopes at the region of current density higher than 0.1 A cm\({}^{-1}\) become faster, as the C layer thickness becomes thicker in the resulting double-layered membranes. It implies that the C layer also acts as the polymeric barrier to Na\({}^{+}\) ion transport, lowering the electrochemical SWE performances of the corresponding membranes, since their slopes in the ohmic polarization region are proportional to their ohmic overpotentials accompanied by Na\({}^{+}\) ion conduction across the membranes. The variation of electrochemical SWE performances according to the C layer thickness may also come from reduced transport rate of water molecules, maintaining ionic complexes with Na\({}^{+}\) ions, through the thickened C layer, and lowered water flux [27]. The single cell performances of NR212/AA-DVB_2 and NR212/AA-DVB_10 are still superior to that of Flemion(r). The electrochemical efficiency is a barometer to describe how the current is efficiently used for H\({}_{2}\) and NaOH generation. The comparison of the electrochemical efficiency in the terms of Faraday efficiency can be very helpful in finding out the optimum thickness of C layer which plays a key role in blocking both Na\({}^{+}\) and OH\({}^{-}\) ions as polymeric barriers. Figure 10 shows the changes of hydrogen and caustic current efficiency at the constant current density according to the C layer thickness in NR212/AA-DVB membranes. Their hydrogen and caustic efficiency trends are mostly similar, but somewhat different in NR212/AA-DVB membranes with the C layer thinner than 10 \(\upmu\)m. In the thickness range, the hydrogen current efficiency in Figure 10a is reduced as a result from lowered Na\({}^{+}\) conduction. Meanwhile, the caustic current efficiency in Figure 10b is enhanced.