## 2 Materials and Methods
### Characterization and Performance Assessment  
The surface morphology of the prepared membranes was observed by using a Philips scanning electron microscope XL30 FEG (SEM, Eindhoven, The Netherlands). The membrane samples were first dried in an oven and then attached on a holder before sputtering. SEM detection was working at 10 kV. Elemental analysis of the membrane surface was conducted using EDAX that was equipped with a PGT-IMIX PTS EDX system. The surface roughness information of the membranes with and without ZIF-8 were revealed by a Dimension FastScan AFM device (Bruker, Karlsruhe, Germany) with scan areas of 1 \(\mu\)m \(\times\) 1 \(\mu\)m under ambient conditions. The surface wetting nature of the membranes was characterized at room temperature via a contact angle system (OCA20, Dataphysics Instruments, Filderstadt, Germany). A circle fitting method was utilized by drop shape analysis software. The membrane resistance was determined by electrochemical impedance spectroscopy (EIS, Solartron Electrochemical System), within the frequency range from 1 kHz to 1 MHz at a 0.5 M NaCl solution. The water uptake was calculated by Equation (1):  
\[WU=\frac{W_{wet}-W_{dry}}{W_{dry}}\times 100\% \tag{1}\]  
where \(W_{wet}\) and \(W_{dry}\) are the mass of wet and dry membranes, respectively. The ion exchange capacity (IEC) of the prepared monovalent cation exchange membranes was detected by the titration method. The membrane was immersed in a 1 M HCl solution to saturate it with H\({}^{+}\), and then the membrane was transferred to a 1 M NaCl solution for 24 h to liberate the H\({}^{+}\) ions. The released H\({}^{+}\) ions concentration was quantified by a 0.01 M NaOH solution. The IEC was calculated based on the following equation:  
\[IEC=\frac{n_{H^{+}}}{W_{dry}} \tag{2}\]where \(n_{H^{+}}\) is the concentration of the released H\({}^{+}\) ions and \(W_{dry}\) is the dry membrane weight (g). Surface charge was detected by a zeta potential that was equipped with a Surpass 3 ([PERSON], Graz, Austria). The results were obtained in a 1 mM KCl electrolyte solution (pH = 6.0). Diffusion dialysis experiments were conducted using a 1 M NaCl solution and distilled water as feed side and permeate side, respectively. The effective membrane area was 13.8 cm\({}^{2}\). Both of the compartments were vigorously stirred to minimize the effect of concentration polarization. The permeate side conductivity was recorded every 10 min to determine the concentration changes. The selectivity of the monovalent cation exchange membranes was assessed by electrodialysis experiments. In order to deeply understand the transfer of cations during the electrodialysis process, single salt solution desalting experiments were conducted to evaluate the flux of single ions. Three streams were circulated in the ED stack: the concentrate, diluate and the electrode rising solution. The ED setup that was used in this experiment is the same as reported elsewhere [26]. The diluate and concentrate compartment were filled with 150 mL 2 g/L NaCl/MgCl\({}_{2}\). 1 L 20 g/L Na\({}_{2}\)SO\({}_{4}\) was used as the electrode rinsing solution with a constant flow rate of 30 L/h. The effective surface area of the membrane was 19.6 cm\({}^{2}\). The ED experiment was carried out at a current density of 15.3 mA cm\({}^{-2}\). The cation flux across the ion exchange membrane was calculated from the concentration change with time (\(dC/dt\)), based on the following equation:  
\[J_{C}(^{mol}/cm^{2,s})=\frac{V}{A}(\frac{dC}{dt}) \tag{3}\]  
where \(V\) is the volume (cm\({}^{3}\)) of the concentrate solution and \(A\) is the effective surface area of membrane (cm\({}^{2}\)). The monovalent selectivity of the membrane was measured on the same apparatus. A solution with 0.05 M MgCl\({}_{2}\) and 0.5 M NaCl was used as diluate compartment and a 0.5 M NaCl was used as concentrate compartment. The concentration of Na\({}^{+}\) and Mg\({}^{2+}\) in the diluate compartment was measured by cation chromatography after 60 min.