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

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 0.5wt.%) 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 0.5wt.%) 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 hours. 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

Application of graft-type poly(ether ether ketone)-based polymer electrolyte membranes to electrochemical devices – Fuel cells and electrolytic enrichment of tritium

The polymer electrolyte membrane consisting of poly(styrene sulfonic acid)-grafted poly(ether ether ketone) (PEEK-PEM) was investigated for application to two electrochemical devices; a fuel cell and electrolytic enrichment of tritium. For fuel cells, high temperature operation has been required from the viewpoints of simplification of cooling systems, heat recovery systems and so forth, and durability is one critical issue affecting practical use. We performed a long term durability test for PEEK-PEM (ion exchange capacity = 2.4 mmol/g, conductivity = 0.15 S/cm) under the condition of 110 ºC and 50% relative humidity, and achieved a lifetime of 1500 h at a constant current of 0.3 A/cm2. The cell voltage maintained 97% of initial voltage after 1300 h of operation. There have been only a few reports that PEMs exhibit longer lifetime than 1000 h at temperatures above 100 ºC. For quantitative evaluation of tritium concentration in low-level tritiated water such as environmental water, the tritium enrichment by a solid polymer electrolysis (SPE) method is required prior to the tritium concentration measurements. The SPE device composed of PEEK-PEMs with IECs of 0.9 1.2 mmol/g showed a tritium enrichment ratio of 1.35 at 30 ºC, which is 20% higher than that of Nafion. Higher tritium enrichment ratios in PEEK-PEM are explained by the smaller amount of transported water. The water transport coefficient in PEEK-PEM is ~1, which is a half value of Nafion. In addition, the water transport coefficient of PEEK-PEM shows less temperature dependence, at least, up to 60 ºC. These features have advantages in electrolytic enrichment of tritium for practical use