Ion exchange membranes are employed in different electrochemical devices to separate the positive and negative half-cells and to prevent the permeation of the electrochemically active species (e. g. vanadium ions in all-vanadium redox flow batteries) while presenting the required ionic conductivity by non-electrochemically active ions.
In principle, anion exchange membranes (AEM) can prevent cation permeation owing to the Donnan exclusion effect, reducing significantly the cation cross-over. The Donnan exclusion is due to the presence of ionic groups grafted on the polymer backbone that repulse ions with similar charge. For example, positively charged ammonium groups in anion exchange ionomers oppose cation permeation.
Anion-conducting sulfaminated aromatic polymers were synthesized by acid functionalization. They operate in an acidic environment and may be used in vanadium flow batteries. The vanadium ion permeability of various ion-conducting polymer membranes was determined using a home-made apparatus. The study included proton-conducting sulfonated poly(ether ether ketone) (SPEEK) with various cross-linking degrees, anion-conducting membranes, such as polysulfone with quaternary ammonium groups (PSU-QA), sulfaminated PEEK (SA-PEEK), and amphoteric membranes based on PEEK containing both sulfonic and sulfonamide groups that can conduct both cations and anions. The results showed an important effect of the polymer backbone in addition to the kind of grafted ionic group (Donnan exclusion effect). The permeability measurements in acidic solution show an ultra-low vanadium permeability and excellent selectivity for highly cross-linked SPEEK membranes, anion-conducting membranes (SA-PEEK) and amphoteric membranes (SAM-PEEK).