A facile strategy for the synthesis of guanidinium-functionalized polymer as alkaline anion exchange membrane with improved alkaline stability

A facile strategy for the synthesis of guanidinium-functionalized poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) with improved alkaline stability was developed by the reaction of bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) with 2-benzyl-1,1,3,3-tetramethylguanidine (BTMG). The chemical structure of guanidinium-functionalized polymers was confirmed by 1H NMR and FT-IR spectroscopy. A crosslinked membrane of the polymer which have an ionic exchange capacity (0.80 mmol/g) and low swelling ratio (<15%) was fabricated. The ionic conductivity of the cross-linked membrane was higher than 1.0×10−2 S cm−1 at room temperature and 5.1×10−2 S cm−1 at 70 °C. The microstructure of hydrophobic/hydrophilic phase separation of the film was observed by transmission electron microscopy (TEM). Finally, the alkaline stabilities of model guanidiniums and the cross-linked membrane were evaluated respectively. The results showed that the bulky guanidinium cation exhibited higher alkaline stability due to the steric hindrance of the substituents, and the cross-linked membrane remain the ionic conductivity of 4.2×10−2 S cm−1 at 70 °C after being immersed in 1 M NaOH/D2O solution at 60 °C for 2 days. The mechanical properties of the membranes were also measured before and after 2 days of stability testing. All results suggest that this alkaline anion exchange membrane (AAEM) functionalized by bulky guanidinium groups could be considered as a promising candidate for polymer electrolyte membrane in alkaline fuel cells.