SiO2-coated polyimide nonwoven/Nafion composite membranes for proton exchange membrane fuel cells

Porous substrate-reinforced composite membranes for proton exchange membrane fuel cells (PEMFC) have drawn a great deal of attention due to their strong potential for commercialization. In this study, we develop a new reinforcing substrate based on a SiO2 nanoparticle-coated polyimide (PI) nonwoven. This substrate features low thickness, high porosity, and strong affinity for water-swollen proton conducting polymers such as Nafion®. SiO2 nanoparticles, which are interconnected by polyetherimide (PEI) binders, are introduced for the purpose of improving the mechanical strength and hydrophilicity of the electrospun PI nonwoven. The increased polarity of the nonwoven substrate is expected to allow for facile impregnation of a hydrophilic Nafion solution. In contrast, the pristine, hydrophobic PI nonwoven substrate shows very poor affinity with Nafion, and consequently fails to produce a Nafion-impregnated composite membrane. Based on this understanding of the porous substrates, the effects of the SiO2/PEI-coated PI nonwoven substrate on dimensional change and proton conductivity of a Nafion-impregnated composite membrane are investigated. A noteworthy finding is that in addition to suppressing the dimensional change, the composite membrane is effective in retarding the steep decline of proton conductivity at low humidity conditions owing to the presence of the SiO2/PEI-coated PI nonwoven substrate, which is discussed in greater detail with consideration of the state of water in the membrane.