*2.4. Ex Situ Accelerated Ageing*

Accelerated ex situ ageing tests were performed by treating a membrane sample (ca. 60 mg) with 20 mL of the Fenton reagent (20 ppm iron sulfate, FeSO4·7H2O, in 30 wt% hydrogen peroxide solution) for 4 h, at 75 ◦C. The membrane was then washed with deionized water and dried at room temperature. The concentration of fluoride ions in the Fenton's solution was determined using a Mettler Toledo fluoride ion-selective electrode. The pH was kept in the range of 5–7 using an electrolyte solution (TISAB) with the appropriate total ionic strength adjustment buffer [16].

## *2.5. Conductivity Measurements*

The in-plane conductivity was determined according to the four-point impedance technique on 5 cm ± 0.5 cm membrane strips at frequencies ranging from 10 Hz to 100 kHz, with 100 mV, signal amplitude using an Autolab, PGSTAT30 potentiostat/galvanostat equipped with a frequency response analyzer module, as described in ref. [45].
