Reinforced Poly(ether ether ketone)/Nafion Composite Membrane with Highly Improved Proton Conductivity for High Concentration Direct Methanol Fuel Cells

The composite membranes that combine pore-filling with a matrix having precisely assembled sulfonated groups are designed here for use in the applications of direct methanol fuel cells. A porous crystalline poly(ether ether ketone)/sulfonated poly(ether ether ketone) blended matrix with continuous pore structures is first fabricated using a phase inversion process. Solvent treatment of the porous matrix is then employed to induce the enrichment of sulfonated groups at the pore surfaces, based on the interaction between the solvent and functional groups in polymer chains. This strategy effectively shortens the proton transport path, accelerating the proton transfer rate. The pores of the matrix are then filled with Nafion ionomers, resulting in a reinforced composite membrane consisting of interconnected hydrophilic domains in a fuel-insulating rigid matrix. A single cell made using the novel composite membrane exhibits a relatively high power density of 91.7 mW/cm2 and an open circuit voltage of 0.67 V at 70 °C using 10 M methanol, significantly higher than a test device made using Nafion 117. This work supplies a simple and highly efficient strategy to address the low proton conductivity that is normally seen in composite membranes made using pore-filling technology.