## 2 Materials and Methods
### Characterization Methods  
The thickness of the membranes was measured by a hand-held digital micrometre (Mitutoyo Corporation (Kawasaki, Japan)) with an accuracy of 0.001 mm. All measurements were randomly performed at different sites of the membranes and the mean value was recorded. Attenuated total reflection-Fourier transform Infrared (ATR-FTIR) spectra were recorded with a Perkin-Elmer FT-IR System Spectrum BX spectrophotometer equipped with a single horizontal Golden Gate ATR cell, over the range of 600-4000 cm\({}^{-1}\) at a resolution of 4 cm\({}^{-1}\) over 32 scans. Solid-state carbon cross-polarization/magic-angle-spinning nuclear magnetic resonance (\({}^{13}\)C CP/MAS NMR) spectra were collected on a Bruker Avance III 400 spectrometer operating at a B0 field of 9.4 T using 9 kHz MAS with proton 90\({}^{\circ}\) pulse of 3 us, time between scans of 3 s, and a contact time of 2000 us. \({}^{13}\)C chemical shifts were referenced to glycine (C=O at \(\delta\) 176 ppm). The X-ray diffraction (XRD) was performed on a Phillips X'pert MPD diffractometer using Cu K\(\alpha\) radiation (\(\lambda\) = 1.541 A) with a scan rate of 0.05\({}^{\circ}\) s\({}^{-1}\). The XRD patterns were collected in reflection mode with the membranes placed on a Si wafer (negligible background signal) for mechanical support and thus avoid sample bending. Scanning electron microscopy (SEM) images of the surface and cross-section of the membranes were obtained by a HR-FESEM SU-70 Hitachi microscope operating at 4 kV. The microscope was equipped with an energy dispersive X-ray spectroscopy (EDS) Bruker QUANTAX 400 detector for elemental analysis. The membranes were previously coated with a carbon film. Thermogravimetric analysis (TGA) was carried out with a SETSYS Setaram TGA analyser equipped with a platinum cell. The samples were heated from room temperature (RT) to 800 \({}^{\circ}\)C at a constant rate of 10 \({}^{\circ}\)C min\({}^{-1}\) under a nitrogen atmosphere (200 mL min\({}^{-1}\)). Tensile tests were performed on a uniaxial Instron 5564 testing machine in the traction mode at a cross-head velocity of 10 mm min\({}^{-1}\) using a 500 N static load cell. The specimens were rectangular strips (50 \(\times\) 10 mm\({}^{2}\)) previously dried at 40 \({}^{\circ}\)C and equilibrated at RT in a 50% relative humidity (RH) atmosphere prior to testing. All measurements were performed on 5 replicates and the results were expressed as the average value. The water-uptake (_WU_) was determined via immersion of specimens with 1 \(\times\) 1 cm\({}^{2}\) in distilled water at RT for 48 h. After removing the membranes out of the water, the wet surfaces were dried in filter paper, and the wet weight (\(W_{\rm w}\)) was measured. The _WU_ is calculated by the equation: \(W\!U\left(\%\right)=\left[\left(W_{\rm w}-W_{0}\right)/W_{0}\right]\), where \(W_{0}\) is the initial weight of the dry membrane. Ion exchange capacity (IEC) measurements were carried out by soaking the samples in an aqueous solution of NaCl (0.1 M) for 24 h at RT, followed by titration with an aqueous solution of NaOH (0.005 M). The IEC was assessed by the equation: IEC (mmol g\({}^{-1}\)) = \(\left(V_{\rm NaOH}\times M_{\rm NaOH}\right)/W_{0}\), where \(V_{\rm NaOH}\) is the volume (in mL) of the NaOH aqueous solution at the equivalence point, and \(M_{\rm NaOH}\) is the molar concentration of the NaOH aqueous solution. Electrochemical impedance spectroscopy (Agilent (County of Santa Clara, CA, USA) E4980A Precision LCR meter) was used to determine the in-plane protonic conductivity (\(\sigma\)) under variable