Poly(ether ether ketone) (PEEK)-based graft-type polymer electrolyte membranes having high crystallinity for high conducting and mechanical properties under various humidified conditions

Poly(ether ether ketone)-based graft-type polymer electrolyte membranes (PEEK-PEMs) with wide ion exchange capacity (IEC) ranges were prepared by radiation-induced graft polymerization for investigating the relative humidity (RH) dependence of their electrochemical and mechanical properties at 80 °C for fuel cell applications. The proton conductivity ranges of 1.73-3.08 mmol g^-1 IECs at 30% and 95% RH were 0.001-0.009 and 0.108-0.431 S cm^-1, respectively. Unlike aromatic hydrocarbon-type PEMs, PEEK-PEMs' conductivity exhibited less dependence on RH. PEEK-PEMs with IECs > 3.08 mmol g^-1 exhibited a similar conductivity (0.009 S cm^-1) under 30% RH and showed 1.4 times higher tensile strength (14 MPa) under 100% RH at 80 °C in comparison to Nafion 212. The MEA fabricated by the PEEK-PEM with IEC = 2.45 mmol g^-1 showed the maximum power densities (E_max) of 860 and 826 mW cm^-2 at 2140 and 2180 mA cm^-2 under 100% and 30% RH, respectively. In particular, PEEK-PEM showed low RH dependence for E_max, which was 2.5 times higher than that of Nafion 212 at 30% RH. The X-ray scattering analyses revealed that the crystallinity of approximately 28-32% was maintained and propagated during the graft polymerization of graft-type PEEK-PEMs. The abovementioned unique structures were the origins of higher conductivity and tensile strengths compared with conventional PEMs.