Surface modification of heteropoly acid/SPEEK membranes by polypyrrole with a sandwich structure for direct methanol fuel cells

A series of SPEEK/HPW/Ppy-n composite membranes with a sandwich structure were successfully prepared by surface modification with polypyrrole (Ppy) in order to stabilize phosphotungstic acid (HPW) in poly(ether ether ketone)s (SPEEKs) and reduce the methanol crossover. Ppy coatings with a large number of secondary ammonium groups (NH₂⁺) interact with anions of HPW to decrease HPW leaching from the membrane. In addition, the hydrophobic Ppy layers allow for little methanol transport, which leads to a significant decline in methanol crossover with reasonable levels of proton conductivity. The properties of the membranes were investigated in detail by UV, SEM, ac impedance, and TGA. As observed, Ppy-modified membranes were better at immobilizing HPW and exhibited higher selectivities than previously reported SPEEK/HPW composite membranes. All the results indicate that the SPEEK/HPW/Ppy-n composite membranes are excellent candidates for direct methanol fuel cells.