Investigation of sulfonated high-temperature polymers as proton-exchange membrane electrolytes.

摘要

The sulfonic acid derivatives of five aromatic high temperature polymers were synthesized and characterized as proton exchange membrane electrolytes. Three of the polymers, polyether(ether)ketone (PEEK), polyethersulfone (PES) and polybenzimidazole (PBI) were made by derivatization of the polymer using established methods. A fourth polymer, polyphenylquinoxaline (PPQ), was sulfonated as the already cast film using a novel method termed the soak and bake procedure. The fifth sulfonated polymer, a polyimide copolymer (PI), was prepared using a sulfonated diamine monomer, but it had very poor hydrolytic stability.; Characterization of the sulfonated films included water uptake, ionic conductivity and coulombic efficiency. The water uptake of most of the polymers ranged from roughly five to ten water molecules per sulfonic acid group, but the water uptake of the sulfonated PEEK films ranged from 22 to almost 50. The proton conductivity of these materials ranged from one to several orders of magnitude lower than Nafion®. This may be due to the lower acidity of the aromatic sulfonic acid relative to Nafion's ® perfluorosulfonic acid. Coulombic efficiency measurements were near 100% for all of the polymers tested.; The potential usefulness of these materials in direct methanol fuel cells was evaluated. Their methanol diffusion coefficients were from a factor of five to a factor of 124 lower than Nafion's®.; The thermal stability of these materials were evaluated under inert and saturated vapor conditions. Under inert conditions, sulfonation lowered the thermal stability of the aromatic polymers by as much as 150°C, but under saturated steam by only 10°C to 40°C. All of the polymers tested were not stable above 200°C in saturated vapor conditions. In one atmosphere of superheated steam, the polymers decomposed thermally. In saturated vapor, simple thermal decomposition products were not detected, but evidence of hydrolytic degradation and desulfonation was found.