## 3 Results and Discussions
### Membrane Fabrication Condition Optimization  
Based on different solvent evaporation times (0.5 h, 1 h, 2 h, 3 h, 4 h, 8 h, 16 h, and 24 h), different PES membranes were fabricated. Testing results were generated to optimize the fabrication based on running conditions, like 3 g/L NaCl feed solution, 3 cm/s flow velocity, and 0.8 v/cell-pair. As shown in Figure 4a, with solvent evaporation times from 0 to 3 h, the current density increased due to decreasing electrical resistance of the membrane. When the solvent evaporation time was 3 h, the current density of PES exhibited the best performance, which was 97 A/m\({}^{2}\). Furthermore, when the solvent evaporation time increased beyond 3 h, the current density decreased, possibly due to increasing membrane density. After 8 h evaporation time, the current density decreased to 10 A/m\({}^{2}\). As shown in Figure 4b, when the solvent evaporation time was 0.5 h, the current efficiency of PES showed the best performance at 69%. However, most of the PES membranes showed a current efficiency of nearly 50%, implying that approximately 50% of the electricity was used to remove salts. As shown in Figure 4c, the best performance of salinity reduction after 60 min occurred when the PES solvent evaporation time was 3 h (24% removal). Starting from 8 h solvent evaporation time, the salinity reduction performance decreased substantially. As shown in Figure 4d, most PES membranes showed  
Figure 4: ED performance results of PES membranes in different solvent evaporation times: (**a**) current density (CD), (**b**) current efficiency (CE), (**c**) salinity reduction (SR) after 60 min running, and (**d**) normalized specific energy consumption (nSEC). a normalized specific energy consumption range of 0.040-0.050 kWh/m\({}^{3}\) per meg/L of salinity removed. Based on different solvent evaporation times (0.5 h, 1 h, 4 h, 8 h, 12 h, 16 h, 20 h, and 24 h), different sPES membranes were fabricated. Moreover, testing results were generated to optimize the fabrication based on preliminary running conditions, like 3 g/L NaCl feed solution, 3 cm/s flow velocity, and 0.8 v/cell-pair. As shown in Figure 5a, when the solvent evaporation time was within 1 h, the current density of sPES showed the best performance, which was 130 A/m\({}^{2}\). Moreover, when the solvent evaporation time increased, the current density decreased gradually. Starting from 24 h, the current density decreased obviously, and the lowest was 10 A/m\({}^{2}\) when the solvent evaporation time was 24 h. From 20 h, the current density decreased obviously, which meant the membrane resistance increased obviously. It was probably because of the membrane becoming denser, the porosity becoming small, and no porosity stopping ions from passing through [27; 28]. Compared with the current density of PES membranes, the changes of current density of sPES membranes were much smaller; this indicated that the sulfonation process significantly decreased the influence of solvent evaporation. As shown in Figure 5b, most of the sPES membranes showed a current efficiency of 48%, which meant that 48% of the electricity was used to remove salts. In addition, starting from 24 h solvation evaporation time, the current efficiency performance decreased substantially. Compared with the PES membranes, while the current density of the sPES membranes was higher, the current efficiency of sPES membranes was slightly lower than that of the PES membranes. These results indicated that the higher current density did not have effects on current efficiency. With more ions passing through sPES membranes, there may be more co-ions passing through sPES membranes too, which resulted in slightly lower current efficiency. As shown in Figure 5c, when the solvent evaporation time was  
Figure 5: ED performance results of sPES membranes in different solvent evaporation times: (**a**) current density (CD), (**b**) current efficiency (CE), (**c**) salinity reduction (SR) after 60 min running, and (**d**) normalized specific energy consumption (nSEC).