High-performance proton exchange membranes for direct methanol fuel cells based on a SPEEK/polybenzoxazine crosslinked structure

Abstract

Sulfonated poly(etheretherketone)/polybenzoxazine (SPEEK/PBa) crosslinked blends were prepared as a proton exchange membrane (PEM) for direct methanol fuel cell (DMFCs) application. SPEEK catalyzed the ring-opening polymerization of benzoxazine (Ba) monomers and then reacted with PBa to form a crosslinked membrane with greatly improved mechanical integrity. Extraction testing, Fourier-transform infrared, and X-ray photoelectron spectroscopy results indicate that the crosslinking reaction took place at 150 °C between the SPEEK and PBa. The crosslinked SPEEK/PBa structure gives a better dimensional stability, thermal stability, and mechanical integrity than the SPEEK membrane alone. The proton conductivity of the SPEEK/PBa-15% membrane is 4.25 × 10⁻³ S cm⁻¹ at 30 °C, and reaches 2.46 × 10⁻² S cm⁻¹ at 80 °C. The methanol diffusion coefficient of the SPEEK/PBa-15% is reduced by 93% when compared to a Nafion-117 membrane under the same testing conditions. The maximum selectivity at 80 °C is 4.45 × 10³ S s cm⁻³ for SPEEK/PBa-15%, which is nearly 1.3 times higher than that of a Nafion-117 membrane. The observed excellent properties of SPEEK/PBa membranes are attributed to the uniformly crosslinked structure of SPEEK/PBa and the rigid backbone of PBa.