Use of polyelectrolyte for incorporation of ion-exchange groups in composite membranes for vanadium redox flow battery applications

Daramic (W.R. Grace, USA), which consists of ultra high molecular polyethylene, amorphous silica and mineral oil, is cross-linked with divinylbenzene (DVB) to obtain a composite microporous separator with modified properties. In the present study, poly(sodium 4-styrenesulfonate) (PSSS) (Aldrich Chemical Company, Inc., USA, mol. wt. not, vert, similar 70 000) is used as a polyelectrolyte to impart cation-exchange capacity to the cross-linked Daramic. Incorporation of cation-exchange groups in this composite membrane results in a reduction in solvent transfer across the membrane when used in a vanadium redox cell. The ion-exchange capacity (IEC) of the new PSSS composite membrane is also evaluated. Due to the solubility of PSSS in water, the IEC of the membrane decreases initially when the membrane is soaked in water, but then stabilizes and remains unchanged after a few weeks. Water-transport measurements across the PSSS composite membrane show promising results compared with those across the untreated cross-linked Daramic separator. A detailed 13C nuclear magnetic resonance (NMR) and X-ray dispersive analysis has been carried out to confirm the incorporation of PSSS into the membrane.