Composite proton exchange membrane (porous matrix + proton conducting filler) for direct methanol fuel cells (DMFCS)

Abstract

Direct Methanol Fuel Cells (DMFCs) are the promising future energy source for the portable application. However, the conventional membrane used in DMFC, Nafion® membrane, suffers from high methanol crossover and swelling effect leading to reduced performance of the fuel cell. Another type of membrane used for DMFC, sulfonated poly(styrene-ran-ethylene) (SPSE), exhibits higher proton conductivity as compared to that of conventional Nafion® membrane. However, the SPSE membrane presents the disadvantage of high methanol permeability and low mechanical strength. These undesirable properties of SPSE membrane render it an inappropriate substitution for the Nafion® membrane for DMFC application. This project investigates into a pore-filling composite membrane, composing of a porous polyimide substrate and SPSE as a filling electrolyte polymer, to resolve the current problems posed by the Nafion® membrane and SPSE membrane. Being chemically inert to methanol solution, the strong and rigid polyimide (PI) porous substrate suppresses the swelling of the membrane. The suppression of swelling enhances the durability of the fuel cell, reduces the methanol permeability and contributes to higher mechanical strength. SPSE has the advantage of high proton conductivity. By incorporating SPSE into PI, the PI/SPSE composite membrane demonstrates the advantages that are unique to both SPSE and PI. The improved performance include 30% increase in proton conductivity, 18 times decrease in methanol permeability, negligible water absorption into membrane and dimensional change, two fold increase in mechanical strength and 25% increase in maximum power density as compared to Nafion® 1135 membrane. The results demonstrate the feasibility of PI/SPSE composite to substitute the conventional Nafion® membrane for DMFC application.