## Materials and Methods
### Membrane Characterization  
The Fourier-transform infrared (FT-IR) spectroscopy was carried out using the FT-IR spectrometer (Nicolet 6700 FTIR, Bruker, Billerica, MA, USA). The water uptake and water swelling behaviors of the resulting SWE membranes were observed by measuring their weight and volumetric differences between the dried and swollen membrane coupons. For this, each membrane coupon (dimension = \(5\times 5\) cm\({}^{2}\)) was dried under vacuum at 60 \({}^{\circ}\)C for one day, and fully hydrated in deionized water at 30 \({}^{\circ}\)C for one day. The ohmic resistance of the SWE membranes (\(R\), membrane coupon size = \(1\times 4\) cm\({}^{2}\)) was measured in liquid water in the temperature range from 30 to 90 \({}^{\circ}\)C via four-point probe alternating current (a.c.) impedance spectroscopy using a VMP3 impedance spectrometer (Biologic, Seyssinet-Pariset, France) [15]. The Na\({}^{+}\) ion conductivity (\(\sigma\), S cm\({}^{-1}\)) was obtained using an equation of \(\sigma\) = \(l/(R\times S)\). Here, \(l\) and \(S\) indicate the distance of counter and working electrodes and the cross-sectional area of the membrane coupon. The concentration of OH\({}^{-}\) ions transported through the SWE membranes [sample dimension (A) = 1 cm\({}^{2}\)] was measured at 90 \({}^{\circ}\)C as a function of time (\(t\)) using the two-chamber diffusion cell [16] in **Figure** 2 where chamber A and B were filled with 31 wt % (\(\sim\)7.7 M, \(C_{A}\)) NaOH solution and 0.01 M HCl solution of the same volume (i.e., 100 mL), respectively. A SWE membrane with a constant thickness (\(L\)) was placed between the two chambers. The OH\({}^{-}\) ion concentration was obtained by monitoring the conductivity of the solution in chamber B with a potentiometric titrator (848 Titrino plus, Metrohm, Herisau, Swiss). Prior to the measurement, 9 kinds of NaOH solution with constant concentrations were made and their conductivity values were measured using the same titrator to obtain the standard curve in Figure 3. The OH\({}^{-}\) ion permeability (\(P_{OH-}\), cm\({}^{3}\) cm cm cm\({}^{-2}\) sec\({}^{-1}\)) was determined using Equation (1) with the concentration gradient (\(C_{B}(t)\)). \[P_{OH-}\Big{[}cm^{3}cm~{}cm^{-2}sec^{-1}\Big{]}=\frac{1}{A~{}[cm^{2}]}\times \frac{C_{B}(t)}{C_{A}(t-t_{0})[sec]}\times V_{B}\Big{[}cm^{3}\Big{]}\times L[cm] \tag{1}\]  
Figure 1: Chemical structure of AA-DVB polymeric network used as the C layer. The electrochemical SWE performance evaluation was conducted using a SWE cell (active area = 25 cm\({}^{2}\), CNL energy, Seoul, Republic of Korea) at 90 \({}^{\circ}\)C. The platinum-coated titanium mesh electrodes (CNL energy, Republic of Korea) were used as both anode and cathode. The 31 wt % NaOH and 25 wt % NaCl were used as catholyte and anolyte, respectively. The pH of the anolyte was controlled to 2 via HCl titration to minimize the formation of active chloride species containing chlorate [2]. The current-voltage polarization curves were obtained using EDP 3050 power supplier (PNCYS, Uiwang, Korea), which was controlled by LabVIEW (National Instruments, Austin, TX, USA). The exact amount of H\({}_{2}\) gas (molecular weight (MW\({}_{\text{H2}}\)) = 2.016 g mol\({}^{-1}\)) generated when a constant current (_I_) was applied for a constant time (_t_) was determined by measuring H\({}_{2}\) concentration using a 490 Micro GC (Agilent Technologies, Santa Clara, CA, USA). The amount of NaOH (MW\({}_{\text{NaOH}}\) = 39.997 g mol\({}^{-1}\)) was electrochemical measured via potentiometric titration using 848 Titrino plus titrator (Metrohm, Swiss), after H\({}_{2}\) removal using a gas-liquid separator. The H\({}_{2}\) and NaOH masses at time \(t\) (_m(t)_) and \(0\) (_m(0)_) were used to calculate hydrogen current efficiency (\(\eta_{H2}\)) and caustic current efficiency (\(\eta_{NaOH}\), %) with Equation (2) [5; 10; 11]. Here, the \(n\) and \(F\) mean the mole number of electron accompanied by H\({}_{2}\) and NaOH generation, and Faraday constant (96,485 C mol\({}^{-1}\)), respectively.