Preparation And Properties Of High-temperature Proton Exchange Membranes Based On Polybenzimidazole/Functional Ionic Liquids

Proton exchange membrane fuel cells?PEMFCs?are convenient,stable,efficient and environment-friendly energy conversion devices.At present,the research on fuel cells has become a hot topic in the new wave of energy research.In PEMFCs,proton exchange membranes?PEMs?act as a core component blocking the fuel and transporting protons.Therefore,PEMs need to have good mechanical properties and excellent proton conductivity,and PEMs must have higher performance for high temperatures.Polybenzimidazoles?PBI?is used as HT-PEMs due to its high thermal stability and mechanical properties.However,the proton conductivity of pristine PBI membrane is low,which must be doped with inorganic acid,and phosphoric acid is widely used.Studies have shown that the proton conductivity mainly depends on the phosphoric acid doping level.The higher the phosphoric acid doping level,the higher the proton conductivity.Therefore,how to improve the phosphoric acid doping level of the membrane and to obtain higher proton conductivity has become a research problem that the relevant researchers have to solve.For this reason,various modifications on PBI membranes have been studied.In previous attempts,we discovered that ionic liquids?ILs?,as a molten salt at room temperature,can effectively increase the phosphoric acid doping ability and proton conductivity of membranes.However,some issues have also been exposed in past studies.The ILs will gradually leach out from the membranes,which is obtained by directly blending IL and polymer matrix.In this paper,we are trying to fix the IL group in the polymer matrix,resulting in increased phosphoric acid doping level and enhanced proton conductivity.First,we synthesized a siloxane-functionalized ionic liquid and prepared silica particles containing ionic liquid groups?ILS?.Then,the cross-linking between silica particles and polymer can fix the IL groups in the membranes.Following this experimental protocol,we prepared a series of silica functionalized ionic liquid/polybenzimidazole composite membranes?PBIOH-ILS?.The test results show that the performance of the composite membrane is significantly higher than that of pristine PBI membrane.In particular,PBIOH-ILS 5 exhibited the best proton conductivity(0.106 S cm^-1 at 170°C).However,the silica particles will accumulate at high amounts,which limits the further increasement of ionic liquid groups in the membranes.Thus,we have modified the polybenzimidazole membrane with another experimental protocol.First,we synthesized a double-bonded ionic liquid,and polymerizing become crosslinkable polymeric ionic liquids?cPIL?.Then,a series of polymerized ionic liquids/polybenzimidazole?6FPBI-cPIL?composite cross-linked membranes were prepared by doping the cPIL and PBI in proportion.The oxidative stability,mechanical properties,and phosphoric acid doping ability of the composite membranes were significantly enhanced with the incorporation of cPIL.In particular,the proton conductivity reached 0.116 S cm^-1 at 170°C for 6FPBI-cPIL10 membrane.And the proton conductivity of the composite membrane can still reach0.064 S cm^-1 after 96 h of long-term stability test.