Characterization of nanohybrid membranes for direct methanol fuel cell applications

A series of sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (S-PPO) and sulfonated poly(ether ether ketone) (S-PEEK) at various sulfonation degrees were prepared and characterized for their degree of sulfonation, water uptake, ion exchange capacity, proton conductivity and methanol permeability. Based on the obtained results, the optimum samples were determined and subsequently blended together at different compositions. A single glass transition temperature (T_g) was determined for all blend samples, which was attributed to the presence of sulfonate groups on polymer backbones resulting in the formation of electrostatic cross-linking besides phenyl–phenyl interactions. Moreover, the molecular level of mixing in blends was verified through WAXS patterns. According to the membrane selectivity and hydrolytic stability measurements, 75 wt.% of S-PPO and 25 wt.% of S-PEEK was selected as the optimum composition. Afterwards, different amounts of an organically modified montmorillonite (MMT) were incorporated into the predetermined optimum composition matrices to reduce the methanol permeability of the resulted nanocomposite proton exchange membranes. The XRD patterns of nanocomposites revealed the exfoliated microstructure of the clay nanolayers in the polymeric matrices. Transport property measurements of nanohybrid membranes showed that the maximum selectivity parameter of 75 wt.% S-PPO/25 wt.% S-PEEK composition appeared in the presence of 1.5 wt.% of MMT, which is 1.53 times higher than the corresponding value for Nafion^® 117. The DMFC single cell test of the optimum nanohybrids membrane at 5 M methanol feed showed an open circuit voltage of 0.77 V and maximum power density of 135 mW cm^? 2 in comparison with 0.67 V and 108 mW cm^? 2 for Nafion^® 117, respectively. Fabricated nanohybrid membranes, thanks to their high selectivity, desirable transport properties and tenability, could be considered as promising polyelectrolytes for direct methanol fuel cell applications.