Blended Nafion^®/SPEEK direct methanol fuel cell membranes for reduced methanol permeability

Sulfonated poly(ether ether ketone)s (SPEEKs) were substituted on a polymer main chain that had previously been prepared by sulfonation of poly(ether ether ketone)s in concentrated sulfuric acid for a specified time. The product was then blended with Nafion^® to create composite membranes. The blended SPEEK-containing membranes featured flaky domains dispersed in the Nafion^® matrix. These blends possessed a high thermal decomposition temperature. Additionally, owing to the more crystalline, the blended membranes had a lower water uptake compared to recast Nafion^®, the methanol permeability was reduced to 1.70 × 10^-6 to 9.09 × 10^-7 cm² s^-1 for various SPEEK concentrations, and a maximum proton conductivity of ∼0.050 S cm^-1 was observed at 30 °C. The single-cell performances of the Nafion^®/SPEEK membranes, with various SPEEK concentrations and a certain degree of sulfonation, were 15-25 mW cm^-2 for SPEEK53 and 19-27 mW cm^-2 for SPEEK63, at 80 °C. The power density and open circuit voltage were higher than those of Nafion^® 115 (power density = 22 mW cm^-2). The blended membranes satisfy the requirements of proton exchange membranes for direct methanol fuel cell (DMFC) applications.