Proton-exchange Sulfonated Poly(ether ether ketone)/Sulfonated Phenolphthalein Poly(ether sulfone) Blend Membranes in DMFCs

A sulfonated poly(ether ether ketone) (SPEEK) membrane with fairly high degree of sulfonation (DS) swells excessively and even dissolves at high temperature. To solve these problems, sulfonated phenolphthalein poly(ether sulfone) (SPES-C, DS53.7%) is blended with the SPEEK matrix (DS55.1%, 61.7%) to prepare SPEEK/SPES-C blend membrane. The decrease in swelling degree and methanol permeability of the membrane is dose-dependent. Pure SPEEK (DS61.7%) membrane dissolves completely in water at 70°C, whereas the swelling degree of the SPEEK (DS61.7%)/SPES-C (40%, by mass) membrane is 29.7% at 80°C. From room temperature to 80°C, the methanol permeability of all SPEEK (DS55.1%)/SPES-C blend membranes is about one order of magnitude lower than that of Nafion® 115. At higher temperature, the addition of SPES-C polymer increases the dimensional stability and greater proton conductivity can be achieved. The SPEEK (DS=55.1%)/SPES-C (40%, by mass) membrane can withstand temperatures up to 150°C. The proton conductivity of SPEEK (DS55.1%)/SPES-C (30%, by mass) membrane approaches 0.16 S·cm−1, matching that of Nafion® 115 at 140°C and 100% RH, while pure SPEEK (DS55.1%) membrane dissolves at 90°C. The SPEEK/SPES-C blend membranes are promising for use in direct methanol fuel cells because of their good dimensional stability, high proton conductivity, and low methanol permeability.