Polyelectrolytes which are composed of repeated units of electrically neutral and ionised groups attached to the polymer backbone, are used to create proton exchange membrane, also known as polymer electrolyte membrane (PEM). PEMs are semipermeable membranes that only permit protonssource https://www.openpr.com/news/2804484/perfluorosulfonic-acid-pfsa-membranes-for-fuel-cell-market