Preparation and characterization of gel polybenzimidazole proton exchange membrane with alkyl sulfonic acid side chains
Phosphoric acid(PA)doped polybenzimidazoles(PBI)has become a preferred material for high-temperature proton exchange membrane fuel cells(HT-PEMFCs)due to its excellent thermal and chemical stability,as well as its high glass transition temperature.However,the weak dissociation and diffusion rate of phosphoric acid molecules at low temperatures resulted in poor proton conductivity of the membrane and difficulties in cold start-up of the fuel cell.Therefore,developing high-temperature proton exchange membranes that can operate efficiently over a wide range of temperatures and humidities is currently a challenge.In particular,extending the low-temperature operating window and achieving cold start-up capability are of great significance for the practical applications of fuel cells,such as in vehicles.In this study,a series of PA doped gel state polybenzimidazole proton exchange membranes with flexible alkyl sulfonic acid side chains were designed and synthesized through a polyphosphoric acid sol-gel process and a lactone ring-opening reaction.The effects of introducing sulfonic acid and varying the side-chain length on PA doping level,proton conductivity,and stability under different temperatures and humidities were investigated.The results showed that the prepared gel state membranes had a porous structure with self-assembled layer stacking,which facilitated the absorption of a large amount of PA and provided fast pathways for proton transmission.Among them,PA/PS-PBI exhibited proton conductivity performance superior to other reported works over a wide temperature range.Specifically,at room temperature,its proton conductivity increased from 0.0286S/cm of the original membrane to 0.0694S/cm,which was further increased to 0.1619S/cm and 0.3578S/cm at 80℃ and 200℃,respectively.In addition,the proton conductivity values of these membranes below 80℃ under 0％relative humidity(RH)were comparable to those of Nafion membrane under 100％RH,providing a new solution for breaking the classic definition of proton exchange membranes and achieving for applications in wide temperature range(25-240℃)fuel cell operations.
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