Surface-Densified Non-Fluorinated Proton Exchange Membrane Used for Direct Methanol Fuel Cell

Novel bifunctional polyhedral oligomeric silsesquioxane (Vi-POSS-SO3Na) and a surface densification method to fabricate the composite membrane based on sulfonated poly (fluorenyl etherketone) (SPFEK) was firstly reported for the application in direct methanol fuel cells (DMFCs). Firstly, the synthetic Vi-POSS-SO3Na implants on the SPFEK surface by swelling-filling process. Afterward, the vinyl groups on POSS are cross-linked to form a dense X-POSS layer on the membrane surface by a simply thermal treatment which is called surface densification. The crosslinked dense X-POSS with sulfonated groups on the composite membrane surface can effectively prevent the permeation of methanol and enhance the oxidative stability without the sacrificing proton conductivity. The SPFEK/POSS-0.09 membrane with an area loading of 0.09 mg cm−2 POSS exhibits enhanced oxidative stability and the lowest methanol permeability (2.12 × 10−8 cm2 s−1). Direct methanol fuel cell was assembled and its performance was evaluated. The peak power density using SPFEK/POSS-0.03 membrane reaches 65.1 mW cm−2 that is much higher than the one (24.8 mW cm−2) using pristine SPFEK membrane at 80 °C. The results demonstrate that the surface densification is an effective method to suppress methanol crossover and surface-densified SPFEK/POSS proton exchange membrane with X-POSS layer has improved the comprehensive performance of composite membrane.