Synthesis And Characterization Of POSS Modified Polybenzimidazole For High-temperature Proton Exchange Membrane

Polymer electrolyte fuel cell has become the most promising energy conversion device because of its green and high efficiency.Proton exchange membranes(PEMs)play an important role in blocking fuel and transferring protons in fuel cells,so it is also one of the key factors that determine the performance of fuel cells.Comparing with low and medium temperature fuel cell,high-temperature fuel cells avoid catalyst CO poisoning and simplify hydrothermal management systems and reduce costs.Therefore,the development of high-temperature fuel cell proton exchange membrane is of great significance.Polybenzimidazole has become one of the preferred materials for high temperature proton exchange membranes due to its high thermochemical stability and potential proton conductivity of imidazole rings.Pure polybenzimidazole must be doped with phosphoric acid to form a hydrogen bond network for proton hopping transmission under high temperature and anhydrous conditions.Due to the plasticization of phosphoric acid and the crystallinity of polybenzimidazole,it is difficult to maintain the dimensional stability of the films at high phosphoric acid doping level.However,the lower phosphoric acid doping does not achieve the desired level of proton conduction.Therefore,how to maintain high proton conductivity and ensure high mechanical strength is the focus and difficulty of current research in this field.Based on the above background,proton exchange membranes of polybenzimidazole with alkaline pyridine structure,non-coplanar diazofluorene structure and hydroxyl side chain were designed and prepared in this paper.The purpose is to increase the density of proton receptors and donors in the molecular chain itself,disrupt the orderly stacking of molecular chains,and thus facilitate the formation of continuous hydrogen bond networks between molecular chains and phosphoric acid.The nano-POSS was introduced into the polymer through chemical bonds,and the crystallinity of the polymer was further reduced by the plasticization of POSS,which provided a continuous hydrogen bond channel for proton hopping transport at high temperature,thus achieving a higher proton conductivity at a lower phosphoric acid doping level.The specific contents of this paper are as follows:1)The diazonium and pyridine structures were successfully introduced into the polymer,and POSS grafted polybenzimidazole was successfully prepared.The results show that the diazepine and pyridine structures and the POSS group destroy the close packing of the polymer,increase the free volume,absorb more phosphoric acid,and improve the proton conductivity.The PPBI-9PA film has a proton conductivity of 0.065 S cm-1 at 180? anhydrous.The thermal stability of the polymer was also improved and there was essentially no change in mass to 200 ?.The oxidation resistance stability was significantly improved,and the residual mass reached 60%after the 120 h Fenton reagent test.2)In order to further improve the acid retention,swelling rate and stability of the proton exchange membrane.Sulfonated cross-linked polybenzimidazole was prepared by reacting epoxy group with hydroxyl group by using epoxy POSS as cross-linking agent.The three-dimensional cross-linking structure was formed in the polymer.The dimensional stability(swelling ratio less than 45%),heat resistance(T5%>250 ?)and oxidation resistance(residual mass over 70%in Fenton test for 120 h)of the polymer were enhanced obviously,and the service life of the film was prolonged.Sulfonic groups and hydroxyl groups can form hydrogen bonds with phosphoric acid,which helps to facilitate proton transfer and achieve ideal proton conductivity at lower phosphoric acid doping.The proton conductivity of the three cross-linking membranes was above 0.01 S cm-1 under 180 ? anhydrous condition.