Study On The Synthesis And Property Of Polybenzimidazole-based High-Temperature Proton Exchange Membranes

Among several energy devices,proton exchange membrane fuel cells?PEMFCs?are ideal substitute of internal combustion engine,because they have high efficiency,low emissions,compact battery design and potential contribution to environmental sustainability.As a key part of PEMFCs,the proton exchange membranes?PEMs?serve as a solid electrolyte to prevent the cross mix of the fuel and the oxidant,while still proton transport between electrodes.Nafion is a commercially available PEM at present due to its excellent mechanical properties,high conductivity and good chemical stability at moderate temperatures?<80 ^oC?.However,some fatal drawbacks such as the over-reliance on water,poor methanol crossover and high cost limit its large-scale applications.Therefore,a proton exchange membrane for high temperature application and cheap cost PEM material is imminent.Phosphoric acid doped polybenzimidazole?PA/PBI?membranes have been widely investigated,because they have outstanding thermal stability,excellent mechanical properties as well as high proton conductivity at high temperature.The PA doping level,temperature and relative humidity are important for the proton conductivity of PA-PBI membrane.However,a higher PA doping level results in decreased the mechanical strength due to the strong plasticization of PA.Therefore,to modify the membrane material and improve their overall performance will be very important.We synthesized polybenzimidazole?PBI?and ionic liquid?IL?composite membranes.The IL can be hydrolyzed to form Si-O-Si network structure under acidic conditions.PBI/ionic liquid functional Si-O-Si network?NP?composite membranes are prepared.All the PBI/NP-X composite membranes display excellent chemical stability and oxidative stability.Due to the introduction of NP,the initial decomposition temperature of the PBI/NP-X composite membrane is reduced,but the thermal stability of PBI/NP-X membranes is sufficient enough within the operation temperature of high-temperature PEMFC.Particularly,the PBI/NP-X composite membrane can absorb more phosphoric acid,which is beneficial to increase the proton conductivity,and the mechanical properties are also improved.By a comprehensive evaluation of the tradeoff between conductivity and mechanical strength,an optimum content of NP of 15%was obtained.The proton conductivity of PBI/NP-15%was 0.061 S cm^-1 at 180 ^oC and that of PBI was 0.025 S cm^-1at 180 ^oC,although the membranes were doped under the same conditions.In order to improve PA uptake and PA stability,we also synthesized benzimidazole grafted polybenzimidazole membranes?PBIm?.A series of cross-linked CPBIm-X membranes prepared by using KH560 as a cross-linker.The benzimidazole sidechains can not only increase the basic sites but also allow the membrane to achieve higher phosphoric acid uptakes.Moreover,compared with the imidazole rings in the PBI backbone,the side chains with imidazole rings are flexible,which benefit the proton transportation.The proton conductivity of CPBIm-5 is 0.092 S cm^-1 at 180 ^oC which was higher than that of the pristine PBI.Especially,the stability of phosphoric acid?PA?is enhanced because the hydrolysis of KH560 resulting silane-cross-linked network structure in the membranes.For example,CPBIm-5 still has 57%PA after standing for 312 hours in a constant temperature and humidity chamber,however,the uncross-linked PBIm has almost no residual phosphoric acid.In conclusion,the silane-cross-linked network structure not only absorbs more phosphoric acid but also prevents leaching out of the phosphoric acid.