Cross-linked zwitterionic polyelectrolytes based on sulfonated poly(ether sulfone) with high proton conductivity for direct methanol fuel cells

Hybrid cross-linked membranes of SPES/TEOS/TPABS for direct methanol fuel cells (DMFCs) are synthesized by sulfonated poly(ether sulfone) (SPES), tetraethoxysilane (TEOS) and a zwitterionic silica containing sulfonic acid and ammonium groups, 3-[[3-(triethoxysilyl)-propyl]amino]butane-1-sulfonic acid (TPABS) using a sol-gel process with the goal of obtaining high proton conductivity, low methanol permeability and good stability. Increasing the amount of inorganic zwitterionic TPABS produces membranes that become denser and more uniform. The increasingly SO3-rich cross-linked networks consequently lead to higher proton conductivity and lower methanol permeability. It should be noted that the proton conductivity reaches as high as that of Nafion® 117, whereas the methanol permeability is greatly reduced by the formation of the cross-linked structures. Among these membranes, SPES/TEOS/TPABS-70 (70 wt % of TPABS to SPES in the membrane matrix), shows the best performance with a proton conductivity value of 7.24 × 10−2 S cm−1, methanol permeability value of 2.46 × 10−7 cm2 s−1, ion-exchange capacity value of 1.37 mequiv g−1 and a comparable selectivity parameter of 2.63 × 105 S cm−3 s.

Graphical abstractHybrid cross-linked membranes for direct methanol fuel cells are synthesized by the incorporation of a zwitterionic silica into sulfonated poly(ether sulfone) using a sol-gel process. The highly concentrated sulfonic acid groups and cross-linked structures create a good balance between proton conductivity and fuel permeability. These membranes possess high proton conductivity, up to 7.24 × 10−2 S cm−1, and low methanol permeability of 0.66–2.47 × 10−7 cm2 s−1, which is one order of magnitude lower in comparison with Nafion®117.Figure optionsDownload full-size imageDownload as PowerPoint slide