Novel Proton Conducting Membranes from the Combination of Sulfonated Polymers of Polyetheretherketones and Polyphosphazenes Doped with Sulfonated Single-Walled Carbon Nanotubes

Intent on developing efficient proton exchange membranes used for direct methanol fuel cells as well as hydrogen fuel cells, a series of membranes based on sulfonated polyetheretherketone and sulfonated polyphosphazene-graft copolymers is prepared by cross-linking reaction because the former material has good enough mechanical property, while the latter is excellent in the proton transfer. The cross-linked membranes combine the advantages of the two kinds of polymers. Among them, the membrane poly[(4-trifluoromethylphenoxy)(4-methylphenoxy)phosphazene]-g-poly {(styrene)₁₁-r-[4-(4-sulfobutyloxy)styrene]₃₃-sulfonated poly(ether ether ketone)75 (CF₃-PS₁₁-PSBOS₃₃-SPEEK75) shows a proton conductivity at 0.143 S cm⁻¹ under fully hydrated conditions at 80 °C and performs tensile strength about five times as much as did the sulfonated polyphosphazene membrane CF₃-PS₁₁-PSBOS₃₃. Further doping of sulfonated single-walled carbon nanotubes (S-SWCNTs) into the cross-linked membranes on the screening of additives gives composite membrane CF₃-PS₁₁-PSBOS₃₃-SPEEK75-SWCNT possessing proton conductivity of 0.196 S cm⁻¹, even higher than that of Nafion 117 and a tensile strength comparable to that of Nafion 117. However, this significance of the composite membrane in the proton conduction is not observed in the test with a H₂/air fuel cell when it shows a maximal power density of 280 mW cm⁻² at 80 °C, whereas 294 mW cm⁻² is observed for CF₃-PS₁₁-PSBOS₃₃-SPEEK75. (Figure presented.).