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.