![]() 4–6 Numerous sulfonated aromatic hydrocarbon-type polymers such as poly(ether ether ketone), 7,8 poly(phenylene sulfone), 9–12 poly(arylene ether ketone), 13 polyimides, 14–16 poly(phenylene), 17,18 and polybenzimidazole 19,20 have been designed and synthesized as alternatives to perfluorinated PEMs because of their rigid aromatic backbone, which gives them higher thermal stability and mechanical strength. However, these materials cause several issues such as high fuel crossover and a limited operating temperature range of up to 80 ☌ (low glass transition temperature). 2,3 Currently, sulfonated perfluoropolymers such as Nafion™ have been used mostly as PEMs. 1 The polymer electrolyte membrane (PEM) is a particularly important part of PEMFCs because it separates the two electrodes and transports protons from the anode to the cathode. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) can produce energy from hydrogen and oxygen without air pollution and have therefore attracted attention for use in next-generation energy applications such as fuel cell vehicles and stationary cogeneration systems to replace fossil fuel resources. The abovementioned unique structures were the origins of higher conductivity and tensile strengths compared with conventional PEMs. 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. 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 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 21 mA cm −2 under 100% and 30% RH, respectively. 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 ☌ in comparison to Nafion 212. Unlike aromatic hydrocarbon-type PEMs, PEEK-PEMs' conductivity exhibited less dependence on RH. 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 ☌ for fuel cell applications.
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