Crosslinked And Grafted Polybenzimidazole For High-temperature Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cell(PEMFC)using hydrogen as fuel has promising applicability in stationary and mobile power sources due to their high energy conversion efficiency,net-zero emissions,and rapid response,but still faces technical barriersincluding complicated systems,high costs,and poor tolerance to CO in hydrogen fuels.Increasing the operating temperature of PEMFC can simplify the water and thermal management to reduce the system complexity,enhance the electrode reaction kinetics to reduce the application of precious metal catalysts,and improve the tolerance to CO to reduce the cost of usage.Development of high temperature proton exchange membranes is essential for high temperature PEMFCs.Phosphoric acid doped polybenzimidazole(PBI)membranes are the well-studied and commercially available high temperature proton exchange membranes.However,the degradation of mechanical properties of membranes due to phosphoric acid doping and the loss of phosphoric acid during cell operation remain problematic.Based on the above understanding,various crosslinked PBI membranes are synthesized and the effects of crosslinking modification on the physicochemical,mechanical,and electrochemical properties of phosphoric acid doped membranes are systematically investigated in this thesis.The main contents of this thesis are summarized in the following sections.(1)Covalent crosslinking of polybenzimidazole was performed by strong alkali-catalyzed activation,and the prepared samples were characterized qualitatively by infrared spectrscopy,nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy to confirm the successful crosslinking reaction.The crosslinked PBI membranes possess higher mechanical properties compared topristine PBI membrane.The acid doping level and the according proton conductivity of membranes after doping with phosphoric acid are further investigated.It has been found that the pristine PBI membrane exhibited the higher acid doping lavel but lower proton conductivity compared to the crosslinked PBI membranes due to the formed network,which faciliates the proton transfer in the membrane and in turn improves the proton conductivity.Moreover,Fenton test and thermogravimetric test demonstrate that the crosslinked PBI membranes possess higher chemical and thermal stability than pristine PBI membrane.(2)To further improve the proton conductivity of crosslinked PBI membranes,the increase in the alkaline groups inside the crosslinked membrane to further enhance the phosphoric acid doping level is propsed.To this,PBI molecules are first modified by grafting with 3-chloropropylamine hydrochloride to introduce primary amine groups on the mainchain of PB.Physicochemical characterization of the prepared samples by FTIR,~1H NMR,and XPS confirms that amine-functionalzied side chains were successfully immobilized on the mianchain of PBI.The grafting of side chians leads to an increase in the free volume inside the membrane increased and the phosphoric acid absorption accordingly increased.However,the mechanical property decreases with the increase in the degree of grafting.The proton conductivity of the thus-fromed phosphoric acid-doped PBI is much higher than that of pristine PBI membrane due to the high acid doping level.Fenton test and thermogravimetric test reveal that both chemical and thermal stability decreases with the increases in the grafting degree.The peak power density of the thus-assembled fuel cell from phosphoric acid doped modified PBI membrane is about 28%higher than that of the fuel cell using phosphoric acid-doped prisitne PBI membrane under the same operation conditions.(3)Based on the results obtained in last section,crosslinked amine-functionalized PBI is synthesized to improve the mechanical property of membranes using 1,9-dibromononane as crosslinking agent.The successful reaction is confirmed by spectroscopic investigation as some of the primary amines are transromed to secondary amines.The accordingly increased alkalinity results in the enhanced phosphoric acid doping.However,the acid doping level exhibits a non-monotonic behavior,i.e.,the acid doping level initially increases with the increase in crosslinking degree and it decreases with further increase in the crosslinking degree.The proton conductivity exhibits the same trends.Moreover,the chemical stability and thermal stability of the crosslinked membranes significantly improved compared to the un-crosslinked membrane.