Preparation And Properties Of High-temperature Proton Exchange Membranes Based On Polybenzimidazole-PIMs

Proton exchange membrane fuel cells(PEMFCs)have recently drawn tremendous attention as an alternative green energy source for transport,stationary and portable applications.There is currently increasing interest in developing high-temperature proton exchange membrane fuel cells(HT-PEMFCs),which can operate at high temperature(100-200 C)and low-humidity conditions.In comparison with conventional low-temperature PEMFCs(<100 C),HT-PEMFCs have many attractive features including a low probability of the anode to CO poisoning,a higher catalyst efficiency,simplified water/thermal management and better resistance to fuel impurities.As one of the core components in fuel cells,proton exchange membranes(PEMs)are responsible for proton transport from the anode to the cathode and also serve as a separator that prevents the mixing of reactants.Among the numerous membranes investigated for high operating temperature,aromatic polybenzimidazole(PBI)with exceptional thermal and chemical stability is demonstrated to be the most promising HT-PEM material.Proton conduction of the PEMs plays a crucial role in the determination of final fuel cell performance,and thus the study on the enhancements of proton conductivity is always a core subject in the field of the novel PEM preparations.During the past two decades,the investigation on the proton conduction mechanism of phosphoric acid-doped PBI(PA-PBI)membranes has been widely carried out.It has been demonstrated that the proton conduction rate of PA-PBI system under the anhydrous and high-temperature operating conditions is mainly determined by the PA doping levels(ADLs).However,most of the reported HT-PEMs can't satisfy the requirements of the practical applications because of their poor mechanical strength caused by the plasticizing effect from H3PO4 especially at high ADLs.In our previous study,a soluble arylether-type PBI bearing a benzene pendant was synthesized and it exhibited an excellent balance between proton conductivity and mechanical-dimensional stability at high ADLs.We speculated its high capacity to retain PA molecules might be associated to its large fractional free volume(FFV),which could provide more voids for PA molecules.However,it is hard to prepare PBIs having much larger FFV via synthetic means.Polymers of intrinsic microporosity(PIM-1)having the highly rigid and contorted molecular structure are known as the polymers that possess "a continuous network of interconnected intermolecular voids",and they are broadly recognized as a potential next generation membrane material for gas separations.However,only a few studies related to the PIM-1 containing alloy materials have been reported.In the work of Chapter 3,we designed and synthesized PIM-1(L-PIM and H-PIM)with two different molecular weights in order to introduce PIM-1 into PBI.These two different molecular weights of PIM-1 have a certain solubility in DMAc,which facilitates the preparation of a series of OPBI/PIM-1 alloy membranes by controlled solution casting,and then prepares PA-doped alloy membranes.Through the characterization and performance study of the new OPBI/PIM-1 alloy membranes,we noticed that PIM-1 has certain miscibility with OPBI.As the molecular weight of PIM-1 decreases,its miscibility with OPBI increases gradually,and the molecular weight of L-PIM and OPBI alloys shows better performance.By studying the relationship between the microstructure and performance of the alloys membranes,we found that the introduction of PIM-1 brought a large amount of free volume to the alloy membranes,which made the membranes exhibited a completely different effect from the original OPBI membrane.Good mechanical strength and dimensional stability at high ADLs,led to higher proton conductivity and excellent fuel cell performance.For example,after doping with phosphoric acid,the proton conductivity at 200? of L-10 membranes reached 313 mS·cm-1.A H2-O2 fuel cell based on PA doped L-10 membrane showed a peak power density of 438 mW·cm-2 at 160?,without humidification.In the work of the previous chapter,we found that the better the miscibility of PIM-1 with OPBI,the higher the performance.In the fourth chapter,we hydrolyze the cyano group to a carboxyl group by hydrolysis of PIM-1 under alkaline conditions to obtain carboxyl functionalized PIM(C-PIM)to improve its solubility in polar solvents.A series of OPBI/C-PIM alloy membranes were prepared by solution casting,we found that OPBI and C-PIM are miscibility.Due to the large free volume introduced by the PIM structure and the additional role of the carboxyl groups,we have obtained membranes with higher ADLs and better mechanical strengthes,and at same time the membranes exhibited excellent proton conductivities and fuel cell performance.After doping with phosphoric acid,the proton conductivity at 200? of C-15 membranes reached 327 mS·cm-1.A H2-O2 fuel cell based on PA doped C-15 membrane showed a peak power density of 445 mW·cm-2 at 160 ?,without humidificationIn the work of Chapter 5,we introduced sulfonic acid groups into C-PIM by sulfonation reaction of concentrated sulfuric acid to prepare sulfonated PIM(S-PIM)We prepared a series of OPBI/S-PIM alloy membranes by solution casting.We found that the acid-base interaction between S-PIM and OPBI resulted in the great improvements on the mechanical properties and dimensional stability of the PA-doped membranes.Moreover,the effect of the sulfonic acid groups on the proton conductivities gave the OPBI/S-PIM membranes higher proton conductivities at lower ADL.After doping with phosphoric acid,the proton conductivity at 200? of S-15 membranes reached 345 mS·cm-1.A H2-O2 fuel cell based on PA doped S-15 membrane showed a peak power density of 399 mW·cm-2 at 160 ?,without humidificationIn the work of Chapter 6,we try to introduce basic groups into the PIM structure The introduction of basic groups not only interacted with the OPBI basic backbone,but also enhanced the mechanical properties of the membranes.Moreover,it is possible to increase the binding site of the PA molecule,thereby permitting to dope more acid.TZ-PIM was prepared by a dipolar cycloaddition reaction that converts a cyano group into a tetrazolium structure with four nitrogen atoms.We prepared a series of OPBI/TZ-PIM alloy membranes by solution casting.We found that the molecular chains of the alloy membranes were the most compact due to the strong interaction between tetrazolium and the imidazole ring of OPBI.Compared with the primitive OPBI,the addition of tetrazolium increased the ADLs of the alloy membranes,and their mechanical properties and proton conductivity were also improved.After doping with phosphoric acid,the proton conductivity at 200? of TZ-15 membrane reached 317 mS·cm-1,without humidificationIn summary,we introduced PIM-based microporous polymers into PBI by controlling the molecular weight of PIM-1 and introducing polar functional groups,a series of PBI-based alloy membranes with large free volume were prepared.The structure-performance relationship was studied and a series of high-performance PA-PBI high-temperature proton exchange membranes were obtained.