3.4. Electrochemical responses of the membranes under direct-current stimuli
The I–V curves are important for pre-evaluating the response of membranes, such as their resistance in the ohmic region (Ror) and limiting current density (LCD) under applied DC stimuli.41 The I–V curve of the commercial CSO membrane presents three regions. Conversely, the I–V curves of the TFC, TFN-(Zr), and TFN-(Zr/Ti) membranes were approximately linear up to the current density of 100 mA cm−2 (Figure 6a). The linearity of the I–V curves indicated that the MOF-based membranes could be used at high current densities (≥100 mA cm−2) and was attributed to the presence of nanochannels in the polyamide-containing MOF layers, which minimized the concentration polarization and facilitated the unhindered and continuous flow of ions. In this study, the Ror values of the membranes were calculated by inverting the slope of the I–V curves in the current density range of 0-10 mA cm–2. The Ror of the TFN-(Zr/Ti)-2 membrane (30 Ω cm2) was lower than that of the TFN-(Zr/Ti)-1 membrane (33 Ω cm2) owing to the UiO-66(Zr/Ti)-NH2 loading of the TFN-(Zr/Ti)-2 membrane being higher than that of the TFN-(Zr/Ti)-1 membrane (Figure 6b). Conversely, the absence of Ti3+ from the TFN-(Zr)-1 and TFN-(Zr)-2 membranes caused their Ror values to be higher (34 and 37 Ω cm2, respectively) than those of the TFN-(Zr/Ti)-1 and TFN-(Zr/Ti)-2 membranes, which further confirmed the beneficial effects of the exchange of the Zr4+ions with Ti3+ ions in the MOF layers. Consequently, the low Ror of the UiO-66(Zr/Ti)-NH2layers with well-defined ion transport channels facilitated the fast ion transportation, which is a highly desirable characteristic of energy-efficient membranes.