Mechanically robust and highly methanol-resistant sulfonated poly(ether ether ketone)/poly(vinylidene fluoride) nanofiber composite membranes for direct methanol fuel cells

As a key component of direct methanol fuel cells (DMFCs), proton exchange membranes (PEMs) are required to be mechanically robust, methanol-resistant, and conductive to protons. In this work, a novel sulfonated poly(ether ether ketone) (SPEEK) impregnated poly(vinylidene fluoride) (PVDF) electrospun nanofiber membrane was prepared. To avoid the collapse of PVDF fiber framework by the solvent of SPEEK, we designed a SPEEK/solvent/nonsolvent ternary filling system through the cloud point titration curves. Meanwhile, polydopamine (PDA), an extraordinarily adhesive and hydrophilic material, was coated on the surface of PVDF nanofibers to further improve the interface bonding between SPEEK and PVDF. The introduction of PVDF nanofibers completely changed the rigidity of SPEEK thus achieving a 5.4-fold increase of toughness when compared with that of pure SPEEK at dry state. Notably, significantly improved methanol barrier ability was also obtained due to the tortuous methanol crossover pathways caused by the embedded PVDF nanofibers. In the DMFC tests, the ultra-low methanol permeability and relatively high proton conductivity of the obtained composite membrane made it could be operated with 5 M methanol fuel and showed a peak power density of 104.0 mW cm−2, which was higher than the values of 89.3 mW cm−2 (2 M methanol) and 84.0 mW cm−2 (5 M methanol) recorded with commercial Nafion 115 membrane.