Preparation And Characterization Of Crosslinked Anion Exchange Membranes Based On Azide Groups

As a crucial component in anion exchange membrane fuel cells?AEMFCs?,the AEMs play a dominate role in fuel cell performances.The state-of-the-art researches towards AEMs for fuel cell mainly focus on developing materials manipulating the comprehensive properties,such as high ionic conductivity,excellent chemical stability and good mechanical stability during long-term operation.Despite the increase of ionic conductivity or stability by virtue of modifications of polymer structure,including obtaining well-defined nanochannels or the steric effect,the comprehensive properties of membranes couldn't be balanced.Covalently crosslinked strategy is believed to improve the stability of membranes and also solve the problem resulted from higher ion exchange capacity?IEC?or prolonged operation under high temperature and p H conditions.The major problem is possible highly swollen of membranes or decline in mechanical property and corresponding fuel cell performance loss.Azide groups are likely to form crosslinking triggered by external conditions,or construct crosslinked architectures via azide-alkyne “click” chemistry,which hold great significance for increasing the performances of AEMs and preparing novel crosslinked membranes.On the basis of above consideration,crosslinked AEMs were prepared based on azide groups and the performances of membranes were investigated.The main contents and conclusions are summarized as follows:?1?Poly?vinylbenzyl chloride??PVBC?and azidated poly?2,6-dimethyl-phenyleneoxide?s?PPO-N₃?were chosen as the main chain of AEMs,where the PPO-N₃ was severed as the macromolecular crosslinker.The crosslinked membranes were achieved by UV-irradiation,subsequently quaternization with trimethylamine.The macromolecular crosslinker modified polystyrene AEMs exhibited increased mechanical property,swelling resistance,alkaline stability and comparable ionic conductivity.The as-obtained membranes with IEC value of 1.95 meq./g showed a lower water uptake of 22.4% in spite of in 80 oC.Moreover,excellent alkaline stability was observed for crosslinked AEMs,85% of initial hydroxide conductivity remaining after alkaline stability testing 1 M NaOH at 80 oC for 500 h.The membranes showed comparable ?-normalized hydroxide conductivity with previous reported crosslinked AEMs.?2?Following above research,novel semi-interpenetrating polymer network AEMs?sIPN AEMs?were constructed based on poly?vinylbenzyl chloride?-polystyrene?PVBC-PS?and PPO-N₃,using diyne crosslinking agent and base-catalysed azide-diyne “click” chemistry.The strategy avoided the conductivity loss caused by conventional crosslinking because of the introduction of hydrophilic triazole structures and thus an enhanced water uptake.For example,the highest hydroxide conductivity of 38 mS/cm at room temperature was achieved for the s IPN AEMs with IEC value of 1.47 meq./g in hydroxide form.Moreover,excellent alkaline stability was obtained for sIPN AEMs and membranes remained a hydroxide conductivity value of 30 mS/cm after immersing in 1 M NaOH at 80 oC for 700 h.Meanwhile,a peak power density of 110 mW/cm2 was obtained when the s IPN AEMs was used in H₂/O₂ alkaline fuel cells at 60 oC and the fuel cell durability measured at a constant current?500 mA/cm2?showed that more than 50% of initial performance remained after 24 h.?3?Following the above two sections of research,blocked polymer poly?styrene-b-?ethylene-co-butylene?-b-styrene??SEBS?and “side-chain” type structure were combined with azide-assisted crosslinking strategy,by virtue of heat treatment or addition of diyne crosslinker.The performances of conventional “side-chain” type AEMs were intent to be improved after crosslinking,and comprehensive properties of the two crosslinked membranes were compared with pristine “side-chain” type AEMs.The results indicted enhanced alkaline stability after modified by azide groups based crosslinking,the two crosslinked membranes could retain over 80% initial conductivity,after alkaline stability testing in varied NaOH concentration at 80 oC for 500 h.However,conductivity loss is inevitable for crosslinked structure formed by heat-assisted crosslinking,because of reduced water uptake;azide-diyne click chemistry based crosslinking could realize the increase of conductivity as the introduction of hydrophilic triazole structures,which hold a great prospect in fuel cell application.