Preparation And Properties Of Aromatic Polymer Electrolyte Membranes

Fuel cell is an electrochemical power generation device with high efficiency,and it has no pollution and no noise.There are many kinds of fuel cells,among which the polymer electrolyte membrane fuel cell is considered as the most potential one.Polymer electrolyte membrane is the key part of polymer electrolyte membrane fuel cell.The ion exchange membrane used in PEMFCs is PEMs that conduct protons.Under the same conditions,H~+is easier to dissociate and migrate than that of OH~-,so the ionic conductivity of PEMs is generally higher than that of AEMs.PEMs with a high degree of commercial use are generally perfluorosulfonic acid polymers,which have the disadvantages of high cost and high technical requirements for preparation.In addition,the conductivity of perfluorosulfonic acid-based membranes will drop sharply when the temperature above 80℃.Due to the high cost,complex preparation process,and low temperature resistance of commercial PEMs limit their application,a low-cost poly(dihydroxybiphenyl-biphenyl trifluoroacetophenone)based membranes was prepared by a simple preparation process.Compared with that of PEMFCs,non-noble metal catalyst can be used in AEMFCs,which greatly reduces the cost of fuel cells.In the megathermal and alkaline condition,the cationic groups in AEMs will cause Hoffmann degradation,nucleophilic degradation and other reactions,resulting in the destruction of the structure and the decline of the performance of AEMs.To solve theses problems,polyphenylene ether and poly(diphenyl-terphenyl piperidine)with long side chains were successfully,and used for the preparation of AEMs.The performance of PEMs and AEMs can be improved from the following two aspects:on the one hand,the ion transport efficiency can be improved by constructing the ion transport channel,thus improve the conductivity of membranes;On the other hand,a reliable polymer backbone is designed to keep the ionic functional groups away from the polymer backbone as far as possible to reduce their mutual influence and improve the stability of the membrane.The effects of main chain,ionic groups,the structure and lengths of side chain,and cross-linking structure on the microstructure and various properties of polymer electrolyte membrane were carefully studied.The specific research content and main conclusions of this paper are as follows:(1)Poly(dihydroxybiphenyl-biphenyl trifluoroacetophenone)(PTOH)was synthesized from biphenyl,2,2’-biphenol and trifluoroacetophenone under the catalysis of strong acid.PTOH with different hydroxyl content was prepared by controlling the ratio of 2,2’-biphenol and biphenyl.Poly(bis(1-sulfurous acid-3-propoxy)biphenyl-biphenyl trifluoroacetophenone)(PTOS)with different sulfonation degree was prepared from PTOH and 1,3-propanesultone.PTOS-50%exhibited a maximum conductivity of 82m S/cm at 80℃,while the swelling degree was only 11.5%.When PTOS-50%was soaked in Fenton’s reagent at 60℃for 48 h,the mass loss was only 7.1%.Therefore,PTOS-based membranes have excellent mechanical properties,good oxidation resistance and excellent dimensional swelling resistance.(2)Bromohexyl poly(2,6-dimethyl-1,4-phenyl ether)(PPO)was synthesized from PPO via Friedel-Crafts acylation.Cross-linked PPO(Cross-PPO)and imidazolium PPO(Im-PPO)based AEMs were prepared by using bisimidazole cross-linking agent and 1,2-dimethylimidazole.Cross-PPO-60%exhibited a maximum conductivity of 12.5 m S/cm at 60℃,which was lower than that of Im-PPO-60%.However,the tensile strength of Cross-PPO-60%is 18.4 MPa,which is much higher than that of Im-PPO-60%.At 20℃,the swelling degree of Cross-PPO-60%is only 7%.At 60℃,the conductivity of Cross-PPO-60%was only lost by 10%after being soaked in 5M KOH for 72 h.Therefore,the mechanical strength,dimensional swelling resistance and alkaline resistance stability of AEM were improved by the cross-linked structure.(3)Poly(diphenyl-terphenyl piperidine)(PDTM)was synthesized from1,2-diphenylethane,terphenyl and N-methylpiperidone under the catalysis of strong acid.Bromhexylpiperidine was synthesized and grafted onto PDTM with different diphenylethane content.The obtained PDTM-Pi-25%,PDTM-Pi-50%and PDTM-Pi-75%showed good film-forming properties.Among them,PDTM-Pi-75%exhibited a maximum conductivity of 61.7m S/cm at 80℃.The tensile strength of PDTM-Pi-75%is 28.1 MPa.The conductivity of PDTM-Pi-75%was only lost by 5.6%after being soaked in 5M KOH at 60℃for 72 h.Therefore,PDTM-Pi-75%is an AEM with high mechanical strength,high conductivity and high alkaline resistance stability.