Christopher M. Lew

Cross-linked polybenzimidazole with enhanced stability for high temperature proton exchange membrane fuel cells

Cross-linked polybenzimidazole membranes were obtained by heating at 160 °C, using 4,4′-diglycidyl(3,3′,5,5′-tetramethylbiphenyl) epoxy resin (TMBP) as the cross-linker. The cross-linking reaction temperature was determined by DSC and the successful completion of the cross-linking reaction was shown by FTIR and solubility tests. The cross-linked membranes showed high proton conductivity and strong mechanical properties, as well as low swelling after immersion in 85% phosphoric acid at 90 °C. For instance, the membrane with a cross-linker content weight percent of 20% (PBI-TMBP 20%) with a PA doping level of 4.1 exhibited a proton conductivity of 0.010 S cm−1 and a low swelling volume of 50%. Moreover, the cross-linked membranes showed excellent oxidative stability. The PBI-TMBP 20% cross-linked membrane tested in Fentons reagent (3% H2O2 solution, 4 ppm Fe2+, 70 °C) kept its shape for more than 480 h and did not break. In particular, the proton conductivity of the PA-PBI-TMBP 20% membrane after Fentons test (30% H2O2, 20 ppm Fe2+, 85 °C) remained at a high level of 0.009 S cm−1. This investigation proved that cross-linking is a very effective approach for improving the performance of proton exchange membranes.

Surface modification of heteropoly acid/SPEEK membranes by polypyrrole with a sandwich structure for direct methanol fuel cells

A series of SPEEK/HPW/Ppy-n composite membranes with a sandwich structure were successfully prepared by surface modification with polypyrrole (Ppy) in order to stabilize phosphotungstic acid (HPW) in poly(ether ether ketone)s (SPEEKs) and reduce the methanol crossover. Ppy coatings with a large number of secondary ammonium groups (NH2+) interact with anions of HPW to decrease HPW leaching from the membrane. In addition, the hydrophobic Ppy layers allow for little methanol transport, which leads to a significant decline in methanol crossover with reasonable levels of proton conductivity. The properties of the membranes were investigated in detail by UV, SEM, ac impedance, and TGA. As observed, Ppy-modified membranes were better at immobilizing HPW and exhibited higher selectivities than previously reported SPEEK/HPW composite membranes. All the results indicate that the SPEEK/HPW/Ppy-n composite membranes are excellent candidates for direct methanol fuel cells.

Cross-linked membranes with a macromolecular cross-linker for direct methanol fuel cells

With the goal of reducing water swelling and methanol permeability in sulfonated proton exchange membranes (PEM), bromomethylated poly(ether ether ketone) was synthesized and used as a macromolecular cross-linker. The cross-linking reaction was performed at 195 °C for 5 h and resulted in cross-linked membranes with high cross-linked density. Compared to the pristine membrane, the cross-linked membranes displayed greatly reduced water uptake and methanol permeability. Other properties of the cross-linked membranes, including proton conductivity, mechanical properties, and oxidative stability, were also investigated and compared with the pristine membrane. All the results indicated that the novel macromolecular cross-linker and the resulting cross-linked membranes are promising for fuel cell applications.