Thermally stable graft-type polymer electrolyte membranes consisting based on poly (ether ether ketone) and crosslinked graft-polymers for fuel cell applications

Abstract

The effect of crosslinking on the thermal stability of poly (styrenesulfonic acid)-grafted poly (ether ether ketone) polymer electrolyte membranes (PEEK-PEMs) was investigated in water at 120 °C. The radiation-induced graft polymerization of an ethyl 4-styrenesulfonate (ETSS) monomer onto PEEK substrates, with divinylbenzene (DVB) (0.05, 0.1, 0.2, and 0.5 wt.%) as a crosslinker, followed by hydrolysis, generated crosslinker-containing PEEK-PEMs (PEEK-×0.05, -×0.1, -×0.2, and -×0.5), because the crosslinker (up to 0.5 wt.%) did not cause any destructive effect on the on grafting and hydrolysis rates. As a result, grafting degrees of 100–114% and ion exchange capacities (IECs) of 2.2–2.7 mmol/g, respectively, were achieved. Increasing the DVB amounts (from 0 to 0.5 wt.%), the conductivity, and water uptakes decreased; the PEEK-×0.5 showed a conductivity of 0.10 S/cm and water uptake of 43%. This confirms the successful incorporation of crosslinking structures on the graft-polymers. The hydrothermal test involved the immersion of PEMs in water at 120 °C for 500 h. The no-crosslinked PEEK-PEMs with the same IEC (2.7 mmol/g) experienced a decreases in the weight and IEC to 84 and 68% levels from the initial values, respectively, while those of crosslinked PEEK-×0.05 almost maintained their initial values (100 and 96%, respectively). These results indicated that very small amounts of crosslinkers can decreases the conductivity and the water absorption tendency while increasing the hydrothermal stability at higher temperatures.